@article{riley_roeder_zinke_weisend_eidum_pinton_biliroglu_yamaner_oralkan_connolly_2024, title={Activation of primate frontal eye fields with a CMUT phased array system}, volume={402}, ISSN={["1872-678X"]}, DOI={10.1016/j.jneumeth.2023.110009}, abstractNote={There are pushes toward non-invasive stimulation of neural tissues to prevent issues that arise from invasive brain recordings and stimulation. Transcranial Focused Ultrasound (TFUS) has been examined as a way to stimulate non-invasively, but previous studies have limitations in the application of TFUS. As a result, refinement is needed to improve stimulation results. We utilized a custom-built capacitive micromachined ultrasonic transducer (CMUT) that would send ultrasonic waves through skin and skull to targets located in the Frontal Eye Fields (FEF) region triangulated from co-registered MRI and CT scans while a non-human primate subject was performing a discrimination behavioral task. We observed that the stimulation immediately caused changes in the local field potential (LFP) signal that continued until stimulation ended, at which point there was higher voltage upon the cue for the animal to saccade. This co-incided with increases in activity in the alpha band during stimulation. The activity rebounded mid-way through our electrode-shank, indicating a specific point of stimulation along the shank. We observed different LFP signals for different stimulation targets, indicating the ability to“steer” the stimulation through the transducer. We also observed a bias in first saccades towards the opposite direction. In conclusion, we provide a new approach for non-invasive stimulation during performance of a behavioral task. With the ability to steer stimulation patterns and target using a large amount of transducers, the ability to provide non-invasive stimulation will be greatly improved for future clinical and research applications.}, journal={JOURNAL OF NEUROSCIENCE METHODS}, author={Riley, Mitchell R. and Roeder, Brent M. and Zinke, Wolf and Weisend, Michael P. and Eidum, Derek M. and Pinton, Gianmarco F. and Biliroglu, Ali O. and Yamaner, Feisal Y. and Oralkan, Omer and Connolly, Patrick M.}, year={2024}, month={Feb} }
 @article{annayev_minhaj_adelegan_yamaner_dayton_oralkan_2024, title={Design and Fabrication of 1-D CMUT Arrays for Dual-Mode Dual-Frequency Acoustic Angiography Applications}, volume={71}, ISSN={["1525-8955"]}, url={https://doi.org/10.1109/TUFFC.2023.3342011}, DOI={10.1109/TUFFC.2023.3342011}, abstractNote={When microbubble contrast agents are excited at low frequencies (less than 5 MHz), they resonate and produce higher-order harmonics due to their nonlinear behavior. We propose a novel scheme with a capacitive micromachined ultrasonic transducer (CMUT) array to receive high-frequency microbubble harmonics in collapse mode and to transmit a low-frequency high-pressure pulse by releasing the CMUT plate from collapse and pull it back to collapse again in the same transmit-receive cycle. By patterning and etching the substrate to create glass spacers in the device cavity we can reliably operate the CMUT in collapse mode and receive high-frequency signals. Previously, we demonstrated a single-element CMUT with spacers operating in the described fashion. In this article, we present the design and fabrication of a dual-mode, dual-frequency 1-D CMUT array with 256 elements. We present two different insulating glass spacer designs in rectangular cells for the collapse mode. For the device with torus-shaped spacers, the 3 dB receive bandwidth is from 8 to 17 MHz, and the transmitted maximum peak-to-peak pressure from 32 elements at 4 mm focal depth was 2.12 MPa with a 1.21 MPa peak negative pressure, which corresponds to a mechanical index (MI) of 0.58 at 4.3 MHz. For the device with line-shaped spacers, the 3-dB receive bandwidth at 150 V dc bias extends from 10.9 to 19.2 MHz. By increasing the bias voltage to 180 V, the 3 dB bandwidth shifts, and extends from 11.7 to 20.4 MHz. The transmitting maximum peak-to-peak pressure with 32 elements at 4 mm was 2.06 MPa with a peak negative pressure of 1.19 MPa, which corresponds to an MI of 0.62 at 3.7 MHz.}, number={1}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, author={Annayev, Muhammetgeldi and Minhaj, Tamzid Ibn and Adelegan, Oluwafemi J. and Yamaner, Feysel Yalcin and Dayton, Paul A. and Oralkan, Omer}, year={2024}, month={Jan}, pages={191–201} }
 @article{wu_sanders_dundar_oralkan_2023, title={Deep-Learning-Based High-Intensity Focused Ultrasound Lesion Segmentation in Multi-Wavelength Photoacoustic Imaging}, volume={10}, ISSN={["2306-5354"]}, url={https://doi.org/10.3390/bioengineering10091060}, DOI={10.3390/bioengineering10091060}, abstractNote={Photoacoustic (PA) imaging can be used to monitor high-intensity focused ultrasound (HIFU) therapies because ablation changes the optical absorption spectrum of the tissue, and this change can be detected with PA imaging. Multi-wavelength photoacoustic (MWPA) imaging makes this change easier to detect by repeating PA imaging at multiple optical wavelengths and sampling the optical absorption spectrum more thoroughly. Real-time pixel-wise classification in MWPA imaging can assist clinicians in monitoring HIFU lesion formation and will be a crucial milestone towards full HIFU therapy automation based on artificial intelligence. In this paper, we present a deep-learning-based approach to segment HIFU lesions in MWPA images. Ex vivo bovine tissue is ablated with HIFU and imaged via MWPA imaging. The acquired MWPA images are then used to train and test a convolutional neural network (CNN) for lesion segmentation. Traditional machine learning algorithms are also trained and tested to compare with the CNN, and the results show that the performance of the CNN significantly exceeds traditional machine learning algorithms. Feature selection is conducted to reduce the number of wavelengths to facilitate real-time implementation while retaining good segmentation performance. This study demonstrates the feasibility and high performance of the deep-learning-based lesion segmentation method in MWPA imaging to monitor HIFU lesion formation and the potential to implement this method in real time.}, number={9}, journal={BIOENGINEERING-BASEL}, author={Wu, Xun and Sanders, Jean L. and Dundar, M. Murat and Oralkan, Omer}, year={2023}, month={Sep} }
 @article{zhou_dieffenderfer_sennik_aleem_speight_vasisht_oralkan_lee_misra_2023, title={Performance of A Monolithic E-Nose Array Integrating MEMS and ALD Processing}, ISSN={["1930-0395"]}, DOI={10.1109/SENSORS56945.2023.10325054}, abstractNote={We demonstrate a novel electronic nose (E-nose), which combines microelectromechanical systems (MEMS) and atomic layer deposition (ALD) technologies. MEMS micromachining creates a monolithic microheater array, consisting of independently controlled rows. By changing temperature profiles, a wide range of sensing surfaces are available. Sensor electrodes are arranged in crossbars with microheater rows. SnO2 thin film is deposited on this array as sensing materials by ALD. This E-nose demonstrates excellent fundamental operating characteristics such as speed and repeatability. It is ultra-sensitive against multiple volatile organic compounds (VOCs). It can also intrinsically separate VOC mixtures by tuning its operating modes.}, journal={2023 IEEE SENSORS}, author={Zhou, Yilu and Dieffenderfer, James and Sennik, Erdem and Aleem, Mahaboobbatcha and Speight, Jakob and Vasisht, Shrey and Oralkan, Omer and Lee, Bongmook and Misra, Veena}, year={2023} }
 @article{chang_belekov_wang_wong_oralkan_xu_2023, title={Photoacoustic imaging of visually evoked cortical and subcortical hemodynamic activity in mouse brain: feasibility study with piezoelectric and capacitive micromachined ultrasonic transducer (CMUT) arrays}, volume={14}, ISSN={["2156-7085"]}, DOI={10.1364/BOE.503475}, abstractNote={This study investigates the feasibility of capturing visually evoked hemodynamic responses in the mouse brain using photoacoustic tomography (PAT) and ultrasound (US) dual-modality imaging. A commercial piezoelectric transducer array and a capacitive micromachined ultrasonic transducer (CMUT) array were compared using a programmable PAT-US imaging system. The system resolution was measured by imaging phantoms. We also tested the ability of the system to capture visually evoked hemodynamic responses in the superior colliculus as well as the primary visual cortex in wild-type mice. Results show that the piezoelectric transducer array and the CMUT array exhibit comparable imaging performance, and both arrays can capture visually evoked hemodynamic responses in subcortical as well as cortical regions of the mouse brain.}, number={12}, journal={BIOMEDICAL OPTICS EXPRESS}, author={Chang, Kai-wei and Belekov, Ermek and Wang, Xueding and Wong, Kwoon y. and Oralkan, Omer and Xu, Guan}, year={2023}, month={Dec}, pages={6283–6290} }
 @article{sennik_kinoshita-millard_oh_kafer_dean_oralkan_2023, title={Plant Disease Detection Using an Electronic Nose}, ISSN={["1930-0395"]}, DOI={10.1109/SENSORS56945.2023.10325015}, abstractNote={This paper presents experimental results on differentiating between healthy wheat plants and plants infected with Fusarium Head Blight (FHB) based on sensing the ambient gases in the plant environment using a gravimetric electronic nose enabled by a functionalized capacitive micromachined ultrasonic transducer (CMUT) array and machine learning (ML) algorithms. The CMUT sensor array is functionalized with organic/inorganic materials to capture disease-related volatile signals. The sensor data is processed and analyzed using ML algorithms for accurate plant classification. Experimental results demonstrate the effectiveness of the proposed approach in achieving high accuracy for plant disease detection at the end of the 11th day after plant inoculation.}, journal={2023 IEEE SENSORS}, author={Sennik, Erdem and Kinoshita-Millard, Samuel and Oh, Yeonyee and Kafer, Christopher W. and Dean, Ralph A. and Oralkan, Omer}, year={2023} }
 @article{sanders_biliroglu_newsome_adelegan_yamaner_dayton_oralkan_2022, title={A Handheld Imaging Probe for Acoustic Angiography With an Ultrawideband Capacitive Micromachined Ultrasonic Transducer (CMUT) Array}, volume={69}, ISSN={["1525-8955"]}, url={https://doi.org/10.1109/TUFFC.2022.3172566}, DOI={10.1109/TUFFC.2022.3172566}, abstractNote={This article presents an imaging probe with a 256-element ultrawideband (UWB) 1-D capacitive micromachined ultrasonic transducer (CMUT) array designed for acoustic angiography (AA). This array was fabricated on a borosilicate glass wafer with a reduced bottom electrode and an additional central plate mass to achieve the broad bandwidth. A custom 256-channel handheld probe was designed and implemented with integrated low-noise amplifiers and supporting power circuitry. This probe was used to characterize the UWB CMUT, which has a functional 3-dB frequency band from 3.5 to 23.5 MHz. A mechanical index (MI) of 0.33 was achieved at 3.5 MHz at a depth of 11 mm. These promising measurements are then combined to demonstrate AA. The use of alternate amplitude modulation (aAM) combined with a frequency analysis of the measured transmit signal demonstrates the suitability of the UWB CMUT for AA. This is achieved by measuring only a low level of unwanted high-frequency harmonics in both the transmit signal and the reconstructed image in the areas other than the contrast bubbles.}, number={7}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Sanders, Jean L. and Biliroglu, Ali Onder and Newsome, Isabel G. and Adelegan, Oluwafemi J. and Yamaner, Feysel Yalcin and Dayton, Paul A. and Oralkan, Omer}, year={2022}, month={Jul}, pages={2318–2330} }
 @article{annayev_yamaner_oralkan_2022, title={A pre-charged CMUT structure with a built-in charge storage capacitor}, ISSN={["1948-5719"]}, DOI={10.1109/IUS54386.2022.9958201}, abstractNote={Pre-charging can eliminate the need for a DC bias in capacitive micromachined ultrasonic transducers (CMUTs) and enable energy transfer to implantables. Charging by Fowler-Nordheim (FN) tunneling through an oxide layer is challenging and requires a high electric field. We designed a pre-charged CMUT structure with a built-in charge storage capacitor. In this novel design, a floating electrode is formed between the top and bottom electrodes. Charge writing is achieved by directly contacting the floating electrode to the bottom electrode. The initial results show that the proposed structure can store electrical charges without leakage and it allows operation without a DC bias.}, journal={2022 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM (IEEE IUS)}, author={Annayev, Muhammetgeldi and Yamaner, F. Yalcin and Oralkan, Omer}, year={2022} }
 @article{zhao_annayev_oralkan_jia_2022, title={An Ultrasonic Energy Harvesting IC Providing Adjustable Bias Voltage for Pre-Charged CMUT}, volume={16}, ISSN={["1940-9990"]}, url={https://doi.org/10.1109/TBCAS.2022.3178581}, DOI={10.1109/TBCAS.2022.3178581}, abstractNote={Ultrasonic wireless power transmission (WPT) using pre-charged capacitive micromachined ultrasonic transducers (CMUT) is drawing great attention due to the easy integration of CMUT with CMOS techniques. Here, we present an integrated circuit (IC) that interfaces with a pre-charged CMUT device for ultrasonic energy harvesting. We implemented an adaptive high voltage charge pump (HVCP) in the proposed IC, which features low power, overvoltage stress (OVS) robustness, and a wide output range. The ultrasonic energy harvesting IC is fabricated in the 180 nm HV BCD process and occupies a 2 × 2.5 mm2 silicon area. The adaptive HVCP offers a 2× – 12× voltage conversion ratio (VCR), thereby providing a wide bias voltage range of 4 V–44 V for the pre-charged CMUT. Moreover, a VCR tunning finite state machine (FSM) implemented in the proposed IC can dynamically adjust the VCR to stabilize the HVCP output (i.e., the pre-charged CMUT bias voltage) to a target voltage in a closed-loop manner. Such a closed-loop control mechanism improves the tolerance of the proposed IC to the received power variation caused by misalignments, amount of transmitted power change, and/or load variation. Besides, the proposed ultrasonic energy harvesting IC has an average power consumption of 35 μW–554 μW corresponding to the HVCP output from 4 V–44 V. The CMUT device with a local surface acoustic intensity of 3.78 mW/mm2, which is well below the FDA limit for power flux (7.2 mW/mm2), can deliver sufficient power to the IC.}, number={5}, journal={IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS}, author={Zhao, Linran and Annayev, Muhammetgeldi and Oralkan, Omer and Jia, Yaoyao}, year={2022}, month={Oct}, pages={842–851} }
 @article{belekov_bautista_annayev_adelegan_biliroglu_kierski_sanders_kemal_sennik_yamaner_et al._2022, title={Performance Assessment of Ultra-Wideband and Dual-Mode 1D CMUT Arrays for Acoustic Angiography}, ISSN={["1948-5719"]}, DOI={10.1109/IUS54386.2022.9958537}, abstractNote={In this work, we have demonstrated the imaging potential of 256-element ultra-wideband (UWB) and dual-mode CMUT 1D arrays for acoustic angiography through mechanical index measurements and in-vitro imaging experiments. We have designed a custom 256-channel imaging probe with integrated low-noise amplifiers and supporting power circuitry. To improve the elevational focusing, we mounted an acoustic lens on to the array. The acoustic characterization of the CMUT array was performed by a calibrated hydrophone, with which we measured sufficiently high mechanical indices (i.e., 0.79 MI for the UWB and 0.85 MI for the dual-mode array) at the focal spot at 15-mm depth. We conducted an imaging experiment with a tissue-mimicking phantom including a 0.2-mm-diameter cellulose tube, in which microbubbles and water flowed. We demonstrated a CTR of 62.12 ± 1.06 dB for the UWB array and a CTR of 59.69 ± 0.39 dB for the dual-mode array when microbubbles were flowing through the tube. These experiments presented a strong use case for the UWB and dual-mode CMUT arrays in acoustic angiography applications.}, journal={2022 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM (IEEE IUS)}, author={Belekov, Ermek and Bautista, Kathlyne J. and Annayev, Muhammetgeldi and Adelegan, Oluwafemi J. and Biliroglu, Ali O. and Kierski, Thomas M. and Sanders, Jean L. and Kemal, Remzi E. and Sennik, Erdem and Yamaner, Feysel Y. and et al.}, year={2022} }
 @article{jones_caskey_dayton_oralkan_pinton_2022, title={Transcranial Neuromodulation Array With Imaging Aperture for Simultaneous Multifocus Stimulation in Nonhuman Primates}, volume={69}, ISSN={["1525-8955"]}, url={https://doi.org/10.1109/TUFFC.2021.3108448}, DOI={10.1109/TUFFC.2021.3108448}, abstractNote={Even simple behaviors arise from the simultaneous activation of multiple regions in the brain. Thus, the ability to simultaneously stimulate multiple regions within a brain circuit should allow for better modulation of function. However, performing simultaneous multifocus ultrasound neuromodulation introduces challenges to transducer design. Using 3-D Fullwave simulations, we have designed an ultrasound neuromodulation array for nonhuman primates that: 1) can simultaneously focus on multiple targets and 2) include an imaging aperture for additional functional imaging. This design is based on a spherical array, with 128 15-mm elements distributed in a spherical helix pattern. It is shown that clustering the elements tightly around the 65-mm imaging aperture located at the top of the array improves targeting at shallow depths, near the skull surface. Spherical arrays have good focusing capabilities through the skull at the center of the array, but focusing on off-center locations is more challenging due to the natural geometric configuration and the angle of incidence with the skull. In order to mitigate this, the 64 elements closest to the aperture were rotated toward and focusing on a shallow target, and the 64 elements farthest from the aperture were rotated toward and focusing on a deeper target. Data illustrated that this array produced focusing on the somatosensory cortex with a gain of 4.38 and to the thalamus with a gain of 3.82. To improve upon this, the array placement was optimized based on phase aberration simulations, allowing for the elements with the largest impact on the gain at each focal point to be found. This optimization resulted in an array design that can focus on the somatosensory cortex with a gain of 5.19 and the thalamus with a gain of 4.45. Simulations were also performed to evaluate the ability of the array to focus on 28 additional brain regions, showing that off-center target regions can be stimulated, but those closer to the skull will require corrective steps to deliver the same amount of energy to those locations. This simulation and design process can be adapted to an individual monkey or human skull morphologies and specific target locations within individuals by using orientable 3-D printing of the transducer case and by electronic phase aberration correction.}, number={1}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Jones, Rebecca M. and Caskey, Charles F. and Dayton, Paul A. and Oralkan, Omer and Pinton, Gianmarco F.}, year={2022}, month={Jan}, pages={261–272} }
 @article{mahmud_seok_wu_sennik_biliroglu_adelegan_kim_jur_yamaner_oralkan_2021, title={A Low-Power Wearable E-Nose System Based on a Capacitive Micromachined Ultrasonic Transducer (CMUT) Array for Indoor VOC Monitoring}, volume={21}, url={https://doi.org/10.1109/JSEN.2021.3094125}, DOI={10.1109/JSEN.2021.3094125}, abstractNote={Volatile organic compounds (VOCs) are pervasive in the environment and their real-time continuous monitoring can facilitate better understanding of their effects on human health by combining environmental factors with physiological conditions. The scope of wearable sensors for detection of VOCs is evident as the accuracy of the sensor prediction depends on its proximity to the VOC source along with the sensitivity and selectivity of the sensor itself. In this paper, we present a low-power wearable e-nose system based on a capacitive micromachined ultrasonic transducer (CMUT) array. CMUTs offer inherent benefits of excellent mass resolution, easy array fabrication, and integration with electronics, which make them an appropriate choice as a transducer element for gravimetric e-nose systems. A 5-channel CMUT sensor array was chemically functionalized and used for the detection of four volatiles, ethanol, toluene, p-xylene, and styrene. All the channels of the sensor array achieved a resolution below 10 ppm within 0.2–3% of OSHA-PEL time-weighted average (TWA) for each volatile. For each test cycle, the maximum frequency shift, the rate of adsorption, and the rate of desorption were extracted as features. Linear discriminant analysis (LDA) was applied to visualize the discrimination performance of the sensor array. The system performance was characterized using an automated testing system. The presented sensor system can be used for identification of volatiles with suitable pattern-recognition techniques.}, number={18}, journal={IEEE Sensors Journal}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Mahmud, Marzana Mantasha and Seok, Chunkyun and Wu, Xun and Sennik, Erdem and Biliroglu, Ali Onder and Adelegan, Oluwafemi Joel and Kim, Inhwan and Jur, Jesse S. and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2021}, month={Sep}, pages={19684–19696} }
 @article{sanders_biliroglu_wu_adelegan_yamaner_oralkan_2021, title={A Row-Column (RC) Addressed 2-D Capacitive Micromachined Ultrasonic Transducer (CMUT) Array on a Glass Substrate}, volume={68}, ISSN={["1525-8955"]}, url={https://doi.org/10.1109/TUFFC.2020.3014780}, DOI={10.1109/TUFFC.2020.3014780}, abstractNote={This article presents a row-column (RC) capacitive micromachined ultrasonic transducer (CMUT) array fabricated using anodic bonding on a borosilicate glass substrate. This is shown to reduce the bottom electrode-to-substrate capacitive coupling. This subsequently improves the relative response of the elements when top or bottom electrodes are used as the “signal” (active) electrode. This results in a more uniform performance for the two cases. Measured capacitance and resonant frequency, pulse-echo signal amplitude, and frequency response are presented to support this. Biasing configurations with varying ac and dc arrangements are applied and subsequently explored. Setting the net dc bias voltage across an off element to zero is found to be most effective to minimize spurious transmission. To achieve this, a custom switching circuit was designed and implemented. This circuit was also used to obtain orthogonal B-mode cross-sectional images of a rotationally asymmetric target.}, number={3}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Sanders, Jean L. and Biliroglu, Ali Onder and Wu, Xun and Adelegan, Oluwafemi J. and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2021}, month={Mar}, pages={767–776} }
 @article{seok_yamaner_sahin_oralkan_2021, title={A Wearable Ultrasonic Neurostimulator-Part I: A 1D CMUT Phased Array System for Chronic Implantation in Small Animals}, volume={15}, ISSN={["1940-9990"]}, url={https://doi.org/10.1109/TBCAS.2021.3100458}, DOI={10.1109/TBCAS.2021.3100458}, abstractNote={In this work, we present a wireless ultrasonic neurostimulator, aiming at a truly wearable device for brain stimulation in small behaving animals. A 1D 5-MHz capacitive micromachined ultrasonic transducer (CMUT) array is adopted to implement a head-mounted stimulation device. A companion ASIC with integrated 16-channel high-voltage (60-V) pulsers was designed to drive the 16-element CMUT array. The ASIC can generate excitation signals with element-wise programmable phases and amplitudes: 1) programmable sixteen phase delays enable electrical beam focusing and steering, and 2) four scalable amplitude levels, implemented with a symmetric pulse-width-modulation technique, are sufficient to suppress unwanted sidelobes (apodization). The ASIC was fabricated in the TSMC 0.18-<inline-formula><tex-math notation="LaTeX">${\mu \rm {m}}$</tex-math></inline-formula> HV BCD process within a die size of 2.5 × 2.5 <inline-formula><tex-math notation="LaTeX">${\rm {mm}^2}$</tex-math></inline-formula>. To realize a completely wearable system, the system is partitioned into two parts for weight distribution: 1) a head unit (17 mg) with the CMUT array, 2) a backpack unit (19.7 g) that includes electronics such as the ASIC, a power management unit, a wireless module, and a battery. Hydrophone-based acoustic measurements were performed to demonstrate the focusing and beam steering capability of the proposed system. Also, we achieved a peak-to-peak pressure of 2.1 MPa, which corresponds to a spatial peak pulse average intensity (<inline-formula><tex-math notation="LaTeX">$\mathrm{I_{SPPA}}$</tex-math></inline-formula>) of 33.5 <inline-formula><tex-math notation="LaTeX">${\rm {W}/\rm {cm}^2}$</tex-math></inline-formula>, with a lateral full width at half maximum (FWHM) of 0.6 mm at a depth of 3.5 mm.}, number={4}, journal={IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Seok, Chunkyun and Yamaner, Feysel Yalcin and Sahin, Mesut and Oralkan, Omer}, year={2021}, month={Aug}, pages={692–704} }
 @article{seok_adelegan_biliroglu_yamaner_oralkan_2021, title={A Wearable Ultrasonic Neurostimulator-Part II: A 2D CMUT Phased Array System With a Flip-Chip Bonded ASIC}, volume={15}, ISSN={["1940-9990"]}, url={https://doi.org/10.1109/TBCAS.2021.3105064}, DOI={10.1109/TBCAS.2021.3105064}, abstractNote={A 2D ultrasonic array is the ultimate form of a focused ultrasonic system, which enables electronically focusing beams in a 3D space. A 2D array is also a versatile tool for various applications such as 3D imaging, high-intensity focused ultrasound, particle manipulation, and pattern generation. However, building a 2D system involves complicated technologies: fabricating a 2D transducer array, developing a pitch-matched ASIC, and interconnecting the transducer and the ASIC. Previously, we successfully demonstrated 2D capacitive micromachined ultrasonic transducer (CMUT) arrays using various fabrication technologies. In this paper, we present a 2D ultrasonic transmit phased array based on a 32 × 32 CMUT array flip-chip bonded to a pitch-matched pulser ASIC for ultrasonic neuromodulation. The ASIC consists of 32 × 32 unipolar high-voltage (HV) pulsers, each of which occupies an area of 250 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m × 250 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m. The phase of each pulser output is individually programmable with a resolution of <inline-formula><tex-math notation="LaTeX">$1/f_{\mathrm{C}}/16$</tex-math></inline-formula>, where <inline-formula><tex-math notation="LaTeX">$f_{\mathrm{C}}$</tex-math></inline-formula> is less than 10 MHz. This enables the fine granular control of a focus. The ASIC was fabricated in the TSMC 0.18-<inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m HV BCD process within an area of 9.8 mm × 9.8 mm, followed by a wafer-level solder bumping process. After flip-chip bonding an ASIC and a CMUT array, we identified shorted elements in the CMUT array using the built-in test function in the ASIC, which took approximately 9 minutes to scan the entire 32 × 32 array. A compact-form-factor wireless neural stimulator system—only requiring a connected 15-V DC power supply—was also developed, integrating a power management unit, a clock generator, and a Bluetooth Low-Energy enabled microcontroller. The focusing and steering capability of the system in a 3D space is demonstrated, while achieving a spatial-peak pulse-average intensity (<inline-formula><tex-math notation="LaTeX">$\mathrm{I_{SPPA}}$</tex-math></inline-formula>) of 12.4 and 33.1 W/<inline-formula><tex-math notation="LaTeX">${\rm cm^{2}}$</tex-math></inline-formula>; and a 3-dB focal volume of 0.2 and 0.05 <inline-formula><tex-math notation="LaTeX">${\rm mm^{3}}$</tex-math></inline-formula>—at a depth of 5 mm—at 2 and 3.4 MHz, respectively. We also characterized transmission of ultrasound through a mouse skull and compensated the phase distortion due to the skull by using the programmable phase-delay function in the ASIC, achieving 10% improvement in pressure and a tighter focus. Finally, we demonstrated a ultrasonic arbitrary pattern generation on a 5 mm × 5 mm plane at a depth of 5 mm.}, number={4}, journal={IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Seok, Chunkyun and Adelegan, Oluwafemi Joel and Biliroglu, Ali Onder and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2021}, month={Aug}, pages={705–718} }
 @article{annayev_adelegan_yamaner_dayton_oralkan_2021, title={Design and Fabrication of 1D CMUT Arrays for Dual-Mode Acoustic Angiography Applications - Preliminary Results}, ISSN={["1948-5719"]}, DOI={10.1109/IUS52206.2021.9593432}, abstractNote={When microbubble contrast agents are excited at low frequencies (less than 5 MHz), they resonate and produce higher order harmonics due to their non-linear behavior. We propose a novel scheme with a capacitive micromachined ultrasonic transducer (CMUT) array to receive high-frequency microbubble harmonics in collapse mode and to transmit a low-frequency high-pressure pulse by releasing the CMUT plate from collapse and pull it back to collapse again in the same transmit-receive cycle. By patterning and etching the substrate to create glass spacers in the device cavity we can operate the CMUT in collapse mode and receive high-frequency signals. Finite element model simulation results show that the fabricated devices can transmit at low frequency (< 5 MHz) and receive echoes at high frequency (> 15 MHz), which are verified by experimental results.}, journal={INTERNATIONAL ULTRASONICS SYMPOSIUM (IEEE IUS 2021)}, author={Annayev, Muhammetgeldi and Adelegan, Oluwafemi J. and Yamaner, F. Yalcin and Dayton, Paul A. and Oralkan, Omer}, year={2021} }
 @article{ibn minhaj_adelegan_biliroglu_annayev_coutant_yamaner_oralkan_2021, title={Design and Fabrication of Single-Element CMUTs for Forming a Transcranial Array for Focused Beam Applications}, ISSN={["1948-5719"]}, DOI={10.1109/IUS52206.2021.9593499}, abstractNote={Focused ultrasound (FUS) offers numerous applications, including ablative therapies and transcranial neural stimulation. Prototypes of high-intensity FUS transducer arrays have been fabricated with the aid of rapid prototyping using piezoelectric (lead zirconate titanate, PZT) elements. However, piezoelectric transducer elements used in this process are manufactured through convoluted process steps, contain harmful element lead (Pb), and require matching layers for effective operation, which adds to the complexity and cost of the overall process. With capacitive micromachined ultrasonic transducer (CMUT) technology, such transducers can be fabricated in a substantially simplified microfabrication process. We have previously reported a three-mask process for fabricating vacuum-sealed CMUTs using anodic bonding. In this work, we designed CMUTs aiming at achieving a negative peak pressure (on the transducer surface) up to 400 kPa at 750-kHz center frequency which is required for the intended transcranial application. Later, we fabricated the designed single-element CMUT transducers and completed the initial characterization.}, journal={INTERNATIONAL ULTRASONICS SYMPOSIUM (IEEE IUS 2021)}, author={Ibn Minhaj, Tamzid and Adelegan, Oluwafemi J. and Biliroglu, Ali Onder and Annayev, Muhammetgeldi and Coutant, Zachary A. and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2021} }
 @article{adelegan_coutant_wu_yamaner_oralkan_2021, title={Design and Fabrication of Wideband Air-Coupled Capacitive Micromachined Ultrasonic Transducers With Varying Width Annular-Ring and Spiral Cell Structures}, volume={68}, ISSN={["1525-8955"]}, url={https://doi.org/10.1109/TUFFC.2021.3076143}, DOI={10.1109/TUFFC.2021.3076143}, abstractNote={Air-coupled transducers with broad bandwidth are desired for many airborne applications, such as obstacle detection, haptic feedback, and flow metering. In this article, we present a design strategy and demonstrate a fabrication process for developing improved concentric annular- and novel spiral-shaped capacitive micromachined ultrasonic transducers (CMUTs) that can generate high output pressure and provide wide bandwidth in air. We explore the ability to implement complex geometries by photolithographic definition to improve the bandwidth of air-coupled CMUTs. The ring widths in the annular design were varied so that the device can be improved in terms of bandwidth when these rings resonate in parallel. Using the same ring width parameters for the spiral-shaped design but with a smoother transition between the ring widths along the spiral, the bandwidth of the spiral-shaped device is improved. With the reduced process complexity associated with the anodic-bonding-based fabrication process, a 25- $\mu \text{m}$ vibrating silicon plate was bonded to a borosilicate glass wafer with up to 15- $\mu \text{m}$ deep cavities. The fabricated devices show an atmospheric deflection profile that is in agreement with the FEM results to verify the vacuum sealing of the devices. The devices show a 3-dB fractional bandwidth (FBW) of 12% and 15% for spiral- and annular-shaped CMUTs, respectively. We measured a 127-dB sound pressure level at the surface of the transducers. The angular response of the fabricated CMUTs was also characterized. The results demonstrated in this article show the possibility of improving the bandwidth of air-coupled devices by exploring the flexibility in the design process associated with CMUT technology.}, number={8}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Adelegan, Oluwafemi Joel and Coutant, Zachary A. and Wu, Xun and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2021}, month={Aug}, pages={2749–2759} }
 @article{annayev_adelegan_yamaner_oralkan_2021, title={Design of Pre-Charged CMUTs with a Metal Floating Gate}, ISSN={["1948-5719"]}, DOI={10.1109/IUS52206.2021.9593593}, abstractNote={Capacitive micromachined ultrasonic transducers (CMUTs) require a DC bias voltage for efficient operation. Precharged CMUTs can eliminate the requirement of DC bias voltage, thus ease the design of frontend circuitry for transmit/receive operation. We used a metal floating gate structure to trap charges instead of an oxide-nitride interface or a silicon floating island. Because the potential barrier height for metal-oxide interface is much higher than oxide-nitride and oxide-silicon barrier heights, it is possible to retain trapped charge without leakage. Also we do not expect any charge leakage on the metal-nitride side because of the vacuum gap. We used an anodic bonding based process and formed the floating metal under the silicon plate where the metal is sandwiched between silicon dioxide and silicon nitride layers. Initial results show that the proposed structure can store electrical charges to allow operation without a DC bias. The CMUTs fabricated using the described approach will be primarily used as an ultrasound-powered implantable biomedical device.}, journal={INTERNATIONAL ULTRASONICS SYMPOSIUM (IEEE IUS 2021)}, author={Annayev, Muhammetgeldi and Adelegan, Oluwafemi J. and Yamaner, F. Yalcin and Oralkan, Omer}, year={2021} }
 @article{sennik_erden_constantino_oh_dean_oralkan_2021, title={Electronic nose system based on a functionalized capacitive micromachined ultrasonic transducer (CMUT) array for selective detection of plant volatiles}, volume={341}, ISSN={["0925-4005"]}, url={https://doi.org/10.1016/j.snb.2021.130001}, DOI={10.1016/j.snb.2021.130001}, abstractNote={Here, a small, low-power, wireless gas sensor platform for selective detection of volatile organic compounds (VOCs) released from plants under different abiotic or biotic stress conditions is described. This sensor platform is implemented based on a capacitive micromachined ultrasonic transducer (CMUT) array, in which elements were functionalized with a variety of materials including polymers, phthalocyanines, and metals to improve selectivity. Input impedance measurements of the functionalized CMUT array were compared to pre-coating measurements to analyze the mechanical loading. The CMUT arrays were then exposed to VOCs known to be emitted by plants with different concentrations under dry air flow at room temperature. The results demonstrated that 1-Octanol created the strongest response across different channels and a resolution of 3-ppb was calculated for the CMUT element functionalized using silver ink when exposed to 1-Octanol. The relative responses of different channels to tested volatiles were observed to be different. The k-nearest neighbor (k-NN) algorithm was used for the gas classification by dividing the data to training and test groups. The k-NN results showed that the gases at low concentrations were successfully classified with better than 97 % accuracy. Finally, to emulate the ambient atmosphere for plants, the gas tests were repeated by adding different levels of humidity to the gas flow. With a minimum 98 % accuracy, the k-NN classifier demonstrated that the functionalized CMUT array can be used for selective detection of the group of plant VOCs used in this study, even at different relative humidity levels in the ambient atmosphere.}, journal={SENSORS AND ACTUATORS B-CHEMICAL}, publisher={Elsevier BV}, author={Sennik, Erdem and Erden, Fatih and Constantino, Nasie and Oh, YeonYee and Dean, Ralph A. and Oralkan, Omer}, year={2021}, month={Aug} }
 @article{asan_kang_oralkan_sahin_2021, title={Entrainment of cerebellar Purkinje cell spiking activity using pulsed ultrasound stimulation}, volume={14}, ISSN={["1876-4754"]}, url={https://doi.org/10.1016/j.brs.2021.03.004}, DOI={10.1016/j.brs.2021.03.004}, abstractNote={BackgroundFocused ultrasound (FUS) has excellent characteristics over other non-invasive stimulation methods in terms of spatial resolution and steering capability of the target. FUS has not been tested in the cerebellar cortex and cellular effects of FUS are not fully understood.Objective/hypothesisTo investigate how the activity of cerebellar Purkinje cells (PCs) is modulated by FUS with varying pulse durations and pulse repetition frequencies.MethodsA glass microelectrode was inserted into the cerebellar vermis lobule 6 from the dorsal side to extracellularly record single unit activity of the PCs in anesthetized rats. Ultrasonic stimulation (500 kHz) was applied through a coupling cone, filled with degassed water, from the posterior side to target the recording area with varying pulse durations and frequencies.ResultsSimple spike (SS) activity of PCs was entrained by the FUS pattern where the probability of spike occurrences peaked at around 1 ms following the onset of the stimulus regardless of its duration (0.5, 1, or 2 ms). The level of entrainment was stronger with shorter pulse durations at 50-Hz pulse repetition frequency (PRF), however, peri-event histograms spread wider and the peaks delayed slightly at 100-Hz PRF, suggesting involvement of a long-lasting inhibitory mechanism. There was no significant difference between the average firing rates in the baseline and stimulation periods.ConclusionFUS can entrain spiking activity of single cells on a spike-by-spike basis as demonstrated here in the rat cerebellar cortex. The observed modulation potentially results from the aggregate of excitatory and inhibitory effects of FUS on the entire cortical network rather than on the PCs alone.}, number={3}, journal={BRAIN STIMULATION}, publisher={Elsevier BV}, author={Asan, Ahmet S. and Kang, Qi and Oralkan, Omer and Sahin, Mesut}, year={2021}, pages={598–606} }
 @article{adelegan_coutant_minhaj_seok_biliroglu_yamaner_oralkan_2021, title={Fabrication of 32 x 32 2D Capacitive Micromachined Ultrasonic Transducer (CMUT) Arrays on a Borosilicate Glass Substrate With Silicon-Through-Wafer Interconnects Using Sacrificial Release Process}, volume={30}, ISSN={["1941-0158"]}, url={https://doi.org/10.1109/JMEMS.2021.3111304}, DOI={10.1109/JMEMS.2021.3111304}, abstractNote={Close integration of transducer arrays with supporting electronic circuits is essential in achieving efficient and compact ultrasound systems. An integral part of hybrid integration of 2D CMUT array to CMOS electronics is the introduction of through-glass-via (TGV) interconnects in glass substrates as an integral part of the 2D CMUT array fabrication. Micro-cracks around via locations, via discontinuity, and poor coplanarity between the vias and glass substrate are some of the challenges with laser-drilled, paste-filled copper-through-glass-via (Cu-TGV) interconnects. This study provides a detailed fabrication process for making $32\times 32$ -element 2D CMUT arrays on a composite glass substrate incorporating silicon-through-glass vias (Si-TGV) as interconnects using sacrificial release approach. On one column of a fabricated 2D CMUT array, we measured a mean resonant frequency of 5.6 MHz in air and an average device capacitance of 1.5 pF. With the introduction of a buried top electrode in the device structure, we achieved a collapse voltage of 93 V, which is considerably lower than the collapse voltage measured in our previously demonstrated 2D CMUT arrays with top electrode on top of the nitride plate. The fabricated array is flip-chip bonded on a custom-designed driving integrated circuit to demonstrate the complete system operation. We measured a peak-to-peak pressure of 1.82 MPa at 3.4 MHz, and 5 mm from the array surface in a 0.33 mm focal spot size. [2021-0101]}, number={6}, journal={JOURNAL OF MICROELECTROMECHANICAL SYSTEMS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Adelegan, Oluwafemi J. and Coutant, Zachary A. and Minhaj, Tamzid Ibn and Seok, Chunkyun and Biliroglu, Ali Onder and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2021}, month={Dec}, pages={968–979} }
 @article{constantino_oh_sennik_andersen_warden_oralkan_dean_2021, title={Soybean Cyst Nematodes Influence Aboveground Plant Volatile Signals Prior to Symptom Development}, volume={12}, ISSN={["1664-462X"]}, DOI={10.3389/fpls.2021.749014}, abstractNote={Soybean cyst nematode (SCN), Heterodera glycines, is one of the most destructive soybean pests worldwide. Unlike many diseases, SCN doesn't show above ground evidence of disease until several weeks after infestation. Knowledge of Volatile Organic Compounds (VOCs) related to pests and pathogens of foliar tissue is extensive, however, information related to above ground VOCs in response to root damage is lacking. In temporal studies, gas chromatography-mass spectrometry analysis of VOCs from the foliar tissues of SCN infested plants yielded 107 VOCs, referred to as Common Plant Volatiles (CPVs), 33 with confirmed identities. Plants showed no significant stunting until 10 days after infestation. Total CPVs increased over time and were significantly higher from SCN infested plants compared to mock infested plants post 7 days after infestation (DAI). Hierarchical clustering analysis of expression ratios (SCN: Mock) across all time points revealed 5 groups, with the largest group containing VOCs elevated in response to SCN infestation. Linear projection of Principal Component Analysis clearly separated SCN infested from mock infested plants at time points 5, 7, 10 and 14 DAI. Elevated Styrene (CPV11), D-Limonene (CPV32), Tetradecane (CPV65), 2,6-Di-T-butyl-4-methylene-2,5-cyclohexadiene-1-one (CPV74), Butylated Hydroxytoluene (CPV76) and suppressed Ethylhexyl benzoate (CPV87) levels, were associated with SCN infestation prior to stunting. Our findings demonstrate that SCN infestation elevates the release of certain VOCs from foliage and that some are evident prior to symptom development. VOCs associated with SCN infestations prior to symptom development may be valuable for innovative diagnostic approaches.}, journal={FRONTIERS IN PLANT SCIENCE}, author={Constantino, Nasie and Oh, Yeonyee and Sennik, Erdem and Andersen, Brian and Warden, Michael and Oralkan, Omer and Dean, Ralph A.}, year={2021}, month={Sep} }
 @article{mahmud_wu_sanders_biliroglu_adelegan_newsome_yamaner_dayton_oralkan_2020, title={An Improved CMUT Structure Enabling Release and Collapse of the Plate in the Same Tx/Rx Cycle for Dual-Frequency Acoustic Angiography}, volume={67}, url={https://doi.org/10.1109/TUFFC.2020.3001221}, DOI={10.1109/TUFFC.2020.3001221}, abstractNote={This study demonstrates, in detail, the potential of using capacitive micromachined ultrasonic transducers (CMUTs) for acoustic angiography of the microvasculature. It is known that when ultrasound contrast agents (microbubbles) are excited with moderate acoustic pressure around their resonance (2–4 MHz), they produce higher order harmonics (greater than third harmonic) due to their nonlinear behavior. To date, the fundamental challenge has been the availability of a transducer that can generate the transmit signals to excite the microbubbles at low frequencies and, in the same cycle, confocally detect harmonics in the higher frequencies. We present a novel device structure and dual-mode operation of a CMUT that operates with a center frequency of 4.3 MHz and 150% bandwidth in the conventional mode for transmitting and a center frequency of 9.8 MHz and a 125.5% bandwidth in collapse mode for receiving. Output pressure of 1.7 MPapp is achieved on the surface of a single unfocused transducer. The mechanical index at the transducer surface is 0.56. FEM simulations are performed first to show the functionality of the proposed device, and then, the device fabrication is described in detail. Finally, we experimentally demonstrate the ability to detect the microbubble signals with good contrast, and the background reflection is adequately suppressed, indicating the feasibility of the presented approach for acoustic angiography.}, number={11}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Mahmud, Marzana Mantasha and Wu, Xun and Sanders, Jean Lunsford and Biliroglu, Ali Onder and Adelegan, Oluwafemi Joel and Newsome, Isabel G. and Yamaner, Feysel Yalcin and Dayton, Paul A. and Oralkan, Omer}, year={2020}, month={Nov}, pages={2291–2302} }
 @misc{hunter_akbar_bhansali_daniele_erb_johnson_liu_miller_oralkan_hesketh_et al._2020, title={Editors' Choice-Critical Review-A Critical Review of Solid State Gas Sensors}, volume={167}, ISSN={["1945-7111"]}, DOI={10.1149/1945-7111/ab729c}, abstractNote={Solid state gas sensors are a core enabling technology to a range of measurement applications including industrial, safety, and environmental monitoring. The technology associated with solid-state gas sensors has evolved in recent years with advances in materials, and improvements in processing and miniaturization. In this review, we examine the state-of-the-art of solid state gas sensors with the goal of understanding the core technology and approaches, various sensor design methods to provide targeted functionality, and future prospects in the field. The structure, detection mechanism, and sensing properties of several types of solid state gas sensors will be discussed. In particular, electrochemical cells (solid and liquid), impedance/resistance based sensors (metal oxide, polymer, and carbon based structures), and mechanical sensing structures (resonators, cantilevers, and acoustic wave devices) as well as sensor arrays and supporting technologies, are described. Development areas for this field includes increased control of material properties for improved sensor response and durability, increased integration and miniaturization, and new material systems, including nano-materials and nano-structures, to address shortcomings of existing solid state gas sensors.}, number={3}, journal={JOURNAL OF THE ELECTROCHEMICAL SOCIETY}, author={Hunter, Gary W. and Akbar, Sheikh and Bhansali, Shekhar and Daniele, Michael and Erb, Patrick D. and Johnson, Kevin and Liu, Chung-Chiun and Miller, Derek and Oralkan, Omer and Hesketh, Peter J. and et al.}, year={2020}, month={Feb} }
 @article{adelegan_coutant_zhang_yamaner_oralkan_2020, title={Fabrication of 2D Capacitive Micromachined Ultrasonic Transducer (CMUT) Arrays on Insulating Substrates With Through-Wafer Interconnects Using Sacrificial Release Process}, volume={29}, url={https://doi.org/10.1109/JMEMS.2020.2990069}, DOI={10.1109/JMEMS.2020.2990069}, abstractNote={A critical component in a three-dimensional (3D) ultrasound imaging system is a two-dimensional (2D) transducer array. A 2D transducer array is also essential for the implementation of a compact form factor focused ultrasound system for therapeutic applications. Considering the difficulty associated with developing 2D transducer arrays using piezoelectric technology, capacitive micromachined ultrasonic transducer (CMUT) technology with the inherent advantages has emerged as a candidate to develop these devices. Previously, we demonstrated that 2D CMUT arrays can be fabricated with through-glass-via interconnects on borosilicate substrates using anodic bonding. In this paper, we present a fabrication process for implementing $16\times 16$ -element 2D CMUT arrays on an alkali-free glass substrate using the sacrificial release method. The vacuum-sealed $16\times 16$ -element 2D CMUT array is built on an SGW3 glass substrate with copper through-glass interconnects. The fabrication process developed for the 2D CMUT array is described in detail. Across the 256 elements of the 2D CMUT array, the mean resonant frequency is measured as 4.76 MHz with a standard deviation of 46.6 kHz. Also, the mean device capacitance across the array is measured as 1.17 pF with a standard deviation of 0.12 pF, and these results agree with the finite-element analysis. This study shows an alternative method to fabricate 2D CMUT arrays on glass substrates with metal interconnects, especially when the substrate is not suitable for anodic bonding. In addition to improved reliability and reduction in parasitic interconnect capacitance and resistance, this fabrication method benefits from the flexibility of developing 2D CMUT arrays on any type of insulating substrate, and still attain optimum uniformity in both yield and functionality of the fabricated devices. [2019-0246]}, number={4}, journal={Journal of Microelectromechanical Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Adelegan, Oluwafemi J. and Coutant, Zachary A. and Zhang, Xiao and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2020}, month={Aug}, pages={553–561} }
 @article{dahiya_oralkan_pratap_2020, title={Guest Editorial Special Issue on Selected Papers From the IEEE Sensors Conference 2018}, volume={20}, ISSN={["1558-1748"]}, DOI={10.1109/JSEN.2020.2984165}, abstractNote={Since its inception in 2002, the IEEE Sensors Conference has come a long way in establishing itself as a premier conference in the world on sensors and sensing technologies. With the entire world moving rapidly toward a ubiquitous intelligence society, the emphasis on deployment of trillions of sensors in the near future is driving the world of sensor technology like never before. It is reflected in the rapidly increasing research and development activities on sensors throughout the world today. The IEEE Sensors Council has done a remarkable job in promoting this area and providing a consistent platform for sharing and dissemination of knowledge related to all aspects of sensing and sensor development among researchers, manufacturers, and the users of sensors. The idea of holding the IEEE Sensors Conference in different parts of the world actively supports this promotion over different geographies. It could not have been more timely to hold SENSORS 2018, for the first time, in one of the largest and fastest developing economies of the world - India. The 2018 edition of the Sensors Conference was held on October 28–31 in New Delhi — the capital city of India that has been inhabited for six thousand years and has seen a great many upheavals in history.}, number={13}, journal={IEEE SENSORS JOURNAL}, author={Dahiya, Ravinder and Oralkan, Omer and Pratap, Rudra}, year={2020}, month={Jul}, pages={6792–6793} }
 @article{seok_mahmud_kumar_adelegan_yamaner_oralkan_2019, title={A Low-Power Wireless Multichannel Gas Sensing System Based on a Capacitive Micromachined Ultrasonic Transducer (CMUT) Array}, volume={6}, ISSN={["2327-4662"]}, url={https://doi.org/10.1109/JIOT.2018.2861330}, DOI={10.1109/JIOT.2018.2861330}, abstractNote={Detection of volatile organic compounds (VOCs), challenged by their diversity and similarity, is gaining much attention due to concerns about adverse health effects they cause, along with intensifying development efforts in wireless sensor nodes. Precise identification of volatiles may be subject to the sensitivity and selectivity of a sensor itself and the proximity of the sensor to the source, necessitating power-efficient and portable/wearable sensing systems. The metal-oxide sensors, commonly employed for detection of VOCs, are not power efficient, due to the required heating element, and lack the selectivity, thus reporting only the total VOC level. In this paper, we present a complete low-power wireless gas-sensing system based a capacitive micromachined ultrasonic transducer array, which is known to have several advantages such as high mass sensitivity, easy implementation of a multielement structure, and high selectivity upon polymer coating. We took a holistic approach to designing the sensing elements and the custom integrated circuit (IC) as well as to operating the system, resulting in a small self-contained sensor node (38-mm detect-weight diameter and 16-mm detect-weight height). The chemical-sensing capability of the system has been validated with ethanol, achieving 120-ppb limit-of-detection while the sensor array, including the IC and the power management unit, consuming 80- $\mu \text{W}$  average power with power cycling by actively taking measurements for 3 s detect-weight per minute. The presented system will eventually provide a ubiquitous tool to identify VOCs with the help of multivariate data analysis.}, number={1}, journal={IEEE INTERNET OF THINGS JOURNAL}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Seok, Chunkyun and Mahmud, Marzana Mantasha and Kumar, Mohit and Adelegan, Oluwafemi Joel and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2019}, month={Feb}, pages={831–843} }
 @article{wu_sanders_zhang_yamaner_oralkan_2019, title={An FPGA-Based Backend System for Intravascular Photoacoustic and Ultrasound Imaging}, volume={66}, ISSN={["1525-8955"]}, url={https://doi.org/10.1109/TUFFC.2018.2881409}, DOI={10.1109/TUFFC.2018.2881409}, abstractNote={The integration of intravascular ultrasound (IVUS) and intravascular photoacoustic (IVPA) imaging produces an imaging modality with high sensitivity and specificity which is particularly needed in interventional cardiology. Conventional side-looking IVUS imaging with a single-element ultrasound (US) transducer lacks forward-viewing capability, which limits the application of this imaging mode in intravascular intervention guidance, Doppler-based flow measurement, and visualization of nearly, or totally blocked arteries. For both side-looking and forward-looking imaging, the necessity to mechanically scan the US transducer limits the imaging frame rate, and therefore, array-based solutions are desired. In this paper, we present a low-cost, compact, high-speed, and programmable imaging system based on a field-programmable gate array suitable for dual-mode forward-looking IVUS/IVPA imaging. The system has 16 US transmit and receive channels and functions in multiple modes including interleaved photoacoustic (PA) and US imaging, hardware-based high-frame-rate US imaging, software-driven US imaging, and velocity measurement. The system is implemented in the register-transfer level, and the central system controller is implemented as a finite-state machine. The system was tested with a capacitive micromachined ultrasonic transducer array. A 170-frames-per-second (FPS) US imaging frame rate is achieved in the hardware-based high-frame-rate US imaging mode while the interleaved PA and US imaging mode operates at a 60-FPS US and a laser-limited 20-FPS PA imaging frame rate. The performance of the system benefits from the flexibility and efficiency provided by the low-level implementation. The resulting system provides a convenient backend platform for research and clinical IVPA and IVUS imaging.}, number={1}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Wu, Xun and Sanders, Jean L. and Zhang, Xiao and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2019}, month={Jan}, pages={45–56} }
 @inproceedings{mahmud_constantino_seok_yamaner_dean_oralkan_2018, title={A CMUT - Based Electronic Nose for Real- Time Monitoring of Volatiles Emitted by Plants: Preliminary Results}, ISBN={9781538647073}, url={http://dx.doi.org/10.1109/icsens.2018.8589740}, DOI={10.1109/icsens.2018.8589740}, abstractNote={Plants give off a unique profile of volatile organic compounds (VOCs) in response to abiotic and biotic stresses. The ability to discern specific VOC profiles provides for the early detection and subsequent treatment of insect infestation and pathogen infection. This work presents an electronic nose (e-nose) system based on a capacitive micromachined ultrasonic transducer (CMUT) with real-time monitoring capability for detecting and discriminating VOCs. Preliminary results show the almost instantaneous detection of the plant volatiles released systemically in response to physical wounding of plant tissue.}, booktitle={2018 IEEE SENSORS}, publisher={IEEE}, author={Mahmud, M. M. and Constantino, N. and Seok, C. and Yamaner, F. Y. and Dean, R. A. and Oralkan, O.}, year={2018}, month={Oct} }
 @article{zhang_adelegan_yamaner_oralkan_2018, title={A Fast-Switching (1.35-mu s) Low-Control-Voltage (2.5-V) MEMS T/R Switch Monolithically Integrated With a Capacitive Micromachined Ultrasonic Transducer}, volume={27}, ISSN={["1941-0158"]}, url={https://doi.org/10.1109/JMEMS.2017.2781255}, DOI={10.1109/jmems.2017.2781255}, abstractNote={This paper describes the design and fabrication of an electrostatic microelectromechanical systems (MEMS) switch that can be co-fabricated on the same substrate with a capacitive micromachined ultrasonic transducer (CMUT) as a transmit/receive switch. The structure of the switch is modified from a single CMUT cell. An interrupted transmission line is defined across the center of the cell with control electrodes on both sides to pull a movable plate down. The plate has an insulation layer underneath, and a metal bump is formed on the insulation layer and aligned to the transmission line gap, so that the switch could be turned ON by pulling down the plate with electrostatic force and making the metal bump close the gap in the transmission line. The switch was designed using a finite-element model and fabricated on a glass substrate using anodic bonding. A static characterization was first performed on a switch test structure, which showed that the dc switching voltage was 68 V and the ON-resistance was 50  $\Omega $ . The RFin-to-RFout isolation was measured as approximately 66 dB and insertion loss was approximately 4.85 dB for the frequency range commonly used for medical ultrasound imaging. Then, we performed the dynamic characterization in immersion. By setting the dc bias at 67 V, we found that the switch could be operated with a control-voltage as low as 2.5 V. The switching and release times are related to the rise time and fall time of the control signal, respectively. The minimum switching time was measured as 1.34  $\mu \text{s}$  with a control signal rise time of 300 ns, and the minimum release time was measured as 80 ns with a control signal fall time of 20 ns. We further demonstrated that a 1-kHz control signal with the optimized rise and fall times can be used to conduct and block a sinusoidal signal with 1-MHz frequency and 300-mVpp amplitude, as well as unipolar pulses with 5-Vpp amplitude, 500-ns pulse width, and 2-kHz repetition rate. The presented MEMS switch could potentially eliminate the high-voltage process requirement for the on-chip front-end electronics of a CMUT-based ultrasound imaging system and thus improve the overall system efficiency. [2017-0225]}, number={2}, journal={JOURNAL OF MICROELECTROMECHANICAL SYSTEMS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Zhang, Xiao and Adelegan, Oluwafemi Joel and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2018}, month={Apr}, pages={190–200} }
 @inproceedings{sanders_wu_adelegan_mahmud_yalcin yamaner_gallippi_oralkan_2018, title={A Row-Column (RC) Addressed 2D Capacitive Micromachined Ultrasonic Transducer (CMUT) Array on a Glass Substrate: Preliminary Results}, ISBN={9781538636466}, url={http://dx.doi.org/10.1109/embc.2018.8513028}, DOI={10.1109/embc.2018.8513028}, abstractNote={In this work, we present preliminary characterization results from a 32 x 32 row-column (RC) addressed 2D capacitive micromachined ultrasonic transducer (CMUT) array. The device was fabricated using anodic bonding on a borosilicate glass substrate, which eliminates the substrate - bottom electrode coupling previously observed in traditional CMUT RC arrays fabricated on silicon substrates. The characterization results were compared for the top and bottom electrodes and include impedance measurements, pulseecho impulse responses, and 2D scans of the pressure field using a calibrated hydrophone. The results showed that the array elements behave similarly when ground and hot electrodes were switched between the top and bottom electrodes for all of the measured parameters including device capacitance, center frequency, and pulse-echo response amplitude. The pressure scans verified the highly customizable nature of RC arrays by showing multiple active element configurations. A sample cross-sectional image of a metal target was also demonstrated.}, booktitle={2018 40th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC)}, publisher={IEEE}, author={Sanders, Jean L. and Wu, Xun and Adelegan, Oluwafemi J. and Mahmud, Marzana M. and Yalcin Yamaner, F. and Gallippi, Caterina M. and Oralkan, Omer}, year={2018}, month={Jul} }
 @article{zhang_wu_adelegan_yamaner_oralkan_2018, title={Backward-Mode Photoacoustic Imaging Using Illumination Through a CMUT With Improved Transparency}, volume={65}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2017.2774283}, DOI={10.1109/tuffc.2017.2774283}, abstractNote={In this paper, we describe a capacitive micromachined ultrasonic transducer (CMUT) with improved transparency for photoacoustic imaging (PAI) with backside illumination. The CMUT was fabricated on a glass substrate with indium–tin oxide bottom electrodes. The plate was a 1.5-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> silicon layer formed over the glass cavities by anodic bonding, with a 1-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> silicon nitride passivation layer on top. The fabricated device shows approximately 30%–40% transmission in the wavelength range from 700 to 800 nm and approximately 40%–60% transmission in the wavelength range from 800 to 900 nm, which correspond to the wavelength range commonly used for <italic>in vivo</italic> PAI. The center frequency of the CMUT was 3.62 MHz in air and 1.4 MHz in immersion. Two preliminary PAI experiments were performed to demonstrate the imaging capability of the fabricated device. The first imaging target was a 0.7-mm diameter pencil lead in vegetable oil as a line target with a subwavelength cross section. A 2-mm-diameter single CMUT element with an optical fiber bundle attached to its backside was linearly scanned to reconstruct a 2-D cross-sectional PA image of the pencil lead. We investigated the spurious signals caused by the light absorption in the 1.5-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> silicon plate. For pencil lead as a strong absorber and also a strong reflector, the received echo signal due to the acoustic excitation generated by the absorption in silicon is approximately 30 dB lower than the received PA signal generated by the absorption in pencil lead at the wavelength of 830 nm. The second imaging target was a “loop-shape” polyethylene tube filled with indocyanine green solution (<inline-formula> <tex-math notation="LaTeX">$50~\mu \text{M}$ </tex-math></inline-formula>) suspended using fishing lines in a tissue-mimicking material. We formed a 3-D volumetric image of the phantom by scanning the transducer in the <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula>- and <inline-formula> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula>-directions. The two experimental imaging results demonstrated that CMUTs with the proposed structure are promising for PAI with backside illumination.}, number={1}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Zhang, Xiao and Wu, Xun and Adelegan, Oluwafemi Joel and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2018}, month={Jan}, pages={85–94} }
 @inproceedings{adelegan_kemal_yamaner_dayton_oralkan_2018, title={Design and Fabrication of High-Frequency Ultra-Wideband 1D CMUT Arrays for Acoustic Angiography Applications - Preliminary Results}, ISBN={9781538634257}, url={http://dx.doi.org/10.1109/ultsym.2018.8579874}, DOI={10.1109/ultsym.2018.8579874}, abstractNote={For superharmonic imaging applications involving the use of microbubble contrast agents, transducers that can transmit energy at low frequencies (less than 5 MHz) to excite the microbubbles, and at the same time detect scattered echoes at higher harmonics (greater than 20 MHz) are essential. We explored the advantages of a thin silicon plate with an added central mass combined with a reduced bottom electrode area to further improve the bandwidth of 1D capacitive micromachined ultrasonic transducer arrays. FEM simulation results show that the fabricated devices can transmit at low frequency (<3 MHz) and receive echoes at high frequency (beyond 30 MHz). This translates into a 180% fractional bandwidth at 17 MHz for the fabricated 1D CMUT array.}, booktitle={2018 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Adelegan, Oluwafemi J. and Kemal, Remzi E. and Yamaner, Feysel Y. and Dayton, Paul A. and Oralkan, Omer}, year={2018}, month={Oct} }
 @inproceedings{adelegan_yamaner_oralkan_2018, title={Design and Implementation of Wideband CMUTs for Airborne Applications - Preliminary Results}, ISBN={9781538634257}, url={http://dx.doi.org/10.1109/ultsym.2018.8579680}, DOI={10.1109/ultsym.2018.8579680}, abstractNote={This paper describes the design and fabrication of an annular and a spiral shaped air-coupled CMUT with improved bandwidth for airborne applications. We optimized the width of each ring in the annular design and used the same parameter for the spiral design to achieve broader bandwidth air-coupled CMUTs. Using anodic bonding, a $25-\mu \text{m}$ vibrating silicon plate was bonded to a borosilicate glass wafer with up to 14.75 $\mu\text{m}$ deep cavities. The fabricated devices show an atmospheric deflection between 6 $\mu\text{m}$ to 10 $\mu\text{m}$ depending on the size of each ring which shows that the fabricated devices were vacuum sealed. The fabricated devices show a resonance frequency of 87 kHz and up to 12% 6-dB fractional bandwidth in air.}, booktitle={2018 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Adelegan, Oluwafemi J. and Yamaner, Feysel Y. and Oralkan, Omer}, year={2018}, month={Oct} }
 @inproceedings{seok_ali_yamaner_sahin_oralkan_2018, title={Towards an Untethered Ultrasound Beamforming System for Brain Stimulation in Behaving Animals}, ISBN={9781538636466}, url={http://dx.doi.org/10.1109/embc.2018.8512551}, DOI={10.1109/embc.2018.8512551}, abstractNote={In this paper, we present a wireless ultrasound transmit (TX) beamforming system, potentially enabling wearable brain stimulation for small awake/behaving animals. The system is comprised of a 16-element capacitive micromachined transducer (CMUT) array, driven by a custom phased-array integrated circuit (IC), which is capable of generating high-voltage (13.5 V) excitation signals with sixteen phase delays and four amplitude levels. In addition, a Bluetooth low-energy module and a power management unit were integrated into the system, which realizes a battery-operated self-contained unit. We validated the functionality of the system by demonstrating beamforming and steering with a hydrophone measurement setup. We achieved an acoustic pressure output of 554 kPapp at the depth of 5 mm, which corresponds to a spatial-peak pulse-average intensity (ISPPA) of 2.9 W/cm2. The measured 6-dB beamwidth (0.4 mm) is promising in that it can stimulate a specific region of the brain, especially for small animals such as mice. Further smart partitioning of the system will enable a truly wearable device for small animals.}, booktitle={2018 40th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC)}, publisher={IEEE}, author={Seok, Chunkyun and Ali, Ziad and Yamaner, F. Yalcin and Sahin, Mesut and Oralkan, Omer}, year={2018}, month={Jul} }
 @inproceedings{seok_wu_yamaner_oralkan_2017, title={A front-end integrated circuit for a 2D capacitive micromachined ultrasound transducer (CMUT) array as a noninvasive neural stimulator}, ISBN={9781538633830}, url={http://dx.doi.org/10.1109/ultsym.2017.8092055}, DOI={10.1109/ultsym.2017.8092055}, abstractNote={In this paper, we present a front-end integrated circuit (IC) for an ultrasound neurostimulation system, to be interfaced with a 16×16 2D CMUT array to realize an ultrasound field pattern (USFP) using quantized phases and amplitudes. The IC uses a pulse width modulation (PWM) technique with a three-level pulse to generate excitation signals having multi-level quantized amplitudes. For a programmable phase delay, each transmitter (TX) element has a voltage controlled delay cell (VCDL) controlled by a global delay-locked loop (DLL). A 6.75-V supply voltage is generated by an on-chip 2:1 step-down charge pump. The IC was fabricated in a 0.35-μm 13.5-V DDD process. The nonlinearity of the phase delay is characterized and shown not to significantly degrade the quality of the projected stimulation pattern. In addition, we validated the beamforming and steering capability of the IC with a 16-element 1D CMUT array operating at 6 MHz as part of our initial characterization efforts before integration with a 2D array.}, booktitle={2017 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Seok, Chunkyun and Wu, Xun and Yamaner, F. Yalcin and Oralkan, Omer}, year={2017}, month={Sep} }
 @inproceedings{sanders_zhang_wu_adelegan_yamaner_kudenov_oralkan_2017, title={A handheld 1D transparent CMUT array probe for photoacoustic imaging: Preliminary results}, ISBN={9781538633830}, url={http://dx.doi.org/10.1109/ultsym.2017.8092259}, DOI={10.1109/ultsym.2017.8092259}, abstractNote={A transparent transducer array is desired in backward-mode photoacoustic imaging (PAI). CMUT technology is especially suitable for this application because of its wide bandwidth and a wide selection of processing materials. We have previously demonstrated a single-element CMUT with an ITO bottom electrode for improved transparency. The device showed 40% to 70% optical transmission from 700 nm to 900 nm, which is the wavelength range commonly used for in-vivo PAI. In this work, we present a 1D PAI probe that integrates a 1D CMUT array, a fiber bundle, in-probe optics, and low-noise amplifiers which interface with a real-time imaging system. We also demonstrate the PAI capability of a single transparent CMUT element. In this experiment, the phantom was a polyethylene tube filled with indocyanine green (ICG) solution embedded in a tissue-mimicking material. In this setup, the light introduced from the back side of the CMUT enabled direct illumination of the imaging field. We are currently developing 1D arrays for use in PAI with high Vis-NIR transmission. As a proof of principle, we built a 128-channel handheld probe which integrates light from the back side and allows for in-probe front-end amplifiers. The center of the probe houses a compact optical design for the illuminator based on cylindrical lenses. The probe electronics consists of two printed circuit boards, each with 64 channels of low-noise amplifiers with integrated transmit/receive switching and biasing circuitry. This handheld probe has been used in initial tests with a standard nontransparent 1D CMUT array to show the basic electrical functionality.}, booktitle={2017 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Sanders, Jean L. and Zhang, Xiao and Wu, Xun and Adelegan, Oluwafemi Joel and Yamaner, F. Yalcin and Kudenov, Michael and Oralkan, Omer}, year={2017}, month={Sep} }
 @inproceedings{wu_sanders_zhang_yamaner_oralkan_2017, title={A high-frequency and high-frame-rate ultrasound imaging system design on an FPGA evaluation board for capacitive micromachined ultrasonic transducer arrays}, ISBN={9781538633830}, url={http://dx.doi.org/10.1109/ultsym.2017.8091802}, DOI={10.1109/ultsym.2017.8091802}, abstractNote={Dynamic receive beamforming (DRBF) is challenging for field-programmable gate array (FPGA)-based ultrasound (US) imaging because it is computationally intense. Work has been done to either simplify the delay calculation or precalculate the delays and store them on the FPGA. The former sacrifices image quality and the latter is challenged by limited memory resource on the FPGA. In this work, we report on the design of a compact US imaging system for capacitive micromachined ultrasonic transducer (CMUT) arrays implemented on an FPGA evaluation board. The system features high frequency and high frame rate. It transmits ultrasound pulses centered at up to 20 MHz and can receive ultrasound echo signals up to 60 MHz. When working with a 16-element 2.48-mm CMUT array centered at 4.5 MHz, the system can perform conventional phased array (CPA) imaging at 170 FPS from a depth of 2.48 mm to 17.34 mm within a 90° sector. The system features full DRBF for every pixel.}, booktitle={2017 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Wu, Xun and Sanders, Jean and Zhang, Xiao and Yamaner, F. Yalcin and Oralkan, Omer}, year={2017}, month={Sep} }
 @inproceedings{newsome_rojas_papadopoulou_lin_novell_oralkan_dayton_2017, title={Adaptation of the acoustic angiography technique for use with a capacitive micromachined ultrasound transducer (CMUT)}, ISBN={9781538633830}, url={http://dx.doi.org/10.1109/ultsym.2017.8091773}, DOI={10.1109/ultsym.2017.8091773}, abstractNote={Removed.}, booktitle={2017 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Newsome, Isabel G. and Rojas, Juan D. and Papadopoulou, Virginie and Lin, Fanglue and Novell, Anthony and Oralkan, Omer and Dayton, Paul A.}, year={2017}, month={Sep} }
 @inproceedings{zhang_adelegan_yamaner_oralkan_2017, title={An optically transparent capacitive micromachined ultrasonic transducer (CMUT) fabricated using SU-8 or BCB adhesive wafer bonding}, ISBN={9781538633830}, url={http://dx.doi.org/10.1109/ultsym.2017.8092119}, DOI={10.1109/ultsym.2017.8092119}, abstractNote={This paper describes an optically transparent capacitive micromachined ultrasonic transducer (CMUT) fabricated with two indium tin oxide (ITO) coated glass wafers bonded together using adhesive wafer bonding. Both SU-8 photoresist and photosensitive benzocyclobutene (BCB) were investigated as the adhesive layer, which also helps define the cavities and form the insulation layer in the CMUT structure. The CMUT was designed as a single transducer on a 100-mm-diameter glass wafer. The key feature of the device is a thin glass top plate with an ITO electrode, coated with an SU-8 or BCB insulation layer, adhesively bonded onto cavities that are formed by patterning SU-8 or BCB on an ITO-coated glass substrate. The fabricated CMUTs using both bonding materials demonstrated an optical transmittance of 70%–80% in the full visible wavelength range. Vacuum sealing of the device was confirmed by the atmospheric deflection of 2.8 urn in the center of a CMUT cell. The electrical input impedance was measured after establishing the electrical connections using silver epoxy. The fabricated CMUTs showed a resonance frequency of approximately 62 kHz in air and the series resistance was measured as approximately 30 Ω.}, booktitle={2017 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Zhang, Xiao and Adelegan, Oluwafemi and Yamaner, F. Yalcin and Oralkan, Omer}, year={2017}, month={Sep} }
 @article{zhang_yamaner_oralkan_2017, title={Fabrication of Vacuum-Sealed Capacitive Micromachined Ultrasonic Transducers With Through-Glass-Via Interconnects Using Anodic Bonding}, volume={26}, ISSN={1057-7157 1941-0158}, url={http://dx.doi.org/10.1109/jmems.2016.2630851}, DOI={10.1109/jmems.2016.2630851}, abstractNote={This paper presents a novel fabrication method for vacuum-sealed capacitive micromachined ultrasonic transducer (CMUT) arrays that are amenable to 3D integration. This paper demonstrates that MEMS structures can be directly built on a glass substrate with preformed through-glass-via (TGV) interconnects. The key feature of this new approach is the combination of copper through-glass interconnects with a vibrating silicon-plate structure suspended over a vacuum-sealed cavity by using anodic bonding. This method simplifies the overall fabrication process for CMUTs with through-wafer interconnects by eliminating the need for an insulating lining for vias or isolation trenches that are often employed for implementing through-wafer interconnects in silicon. Anodic bonding is a low-temperature bonding technique that tolerates high surface roughness. Fabrication of CMUTs on a glass substrate and use of copper-filled vias as interconnects reduce the parasitic interconnect capacitance and resistance, and improve device performance and reliability. A <inline-formula> <tex-math notation="LaTeX">$16\boldsymbol {\times }16$ </tex-math></inline-formula>-element 2D CMUT array has been successfully fabricated. The fabricated device performs as the finite-element and equivalent circuit models predict. A TGV interconnect shows a 2-<inline-formula> <tex-math notation="LaTeX">$\boldsymbol {\Omega }$ </tex-math></inline-formula> parasitic resistance and a 20-fF shunt parasitic capacitance for 250-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> via pitch. A critical achievement presented in this paper is the sealing of the CMUT cavities in vacuum using a PECVD silicon nitride layer. By mechanically isolating the via structure from the active cells, vacuum sealing can be ensured even when hermetic sealing of the via is compromised. Vacuum sealing is confirmed by measuring the deflection of the edge-clamped thin plate of a CMUT cell under atmospheric pressure. The resonance frequency of an 8-cell 2D array element with 78-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> diameter circular cells and a 1.5-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> plate thickness is measured as 3.32 MHz at 15-V dc voltage (80% V<sub>pull-in</sub>). [2016-0200]}, number={1}, journal={Journal of Microelectromechanical Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Zhang, Xiao and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2017}, month={Feb}, pages={226–234} }
 @inproceedings{mahmud_reese_joshipura_seok_yamaner_daniele_menegatti_oralkan_2017, title={Gravimetric biosensor based on a capacitive micromachined ultrasonic transducer functionalized with peptide ligands}, ISBN={9781509010127}, url={http://dx.doi.org/10.1109/icsens.2017.8234352}, DOI={10.1109/icsens.2017.8234352}, abstractNote={This work presents a gravimetric biosensor based on a capacitive micromachined ultrasonic transducer (CMUT). Resonant and acoustic wave devices have recently been studied extensively for chem/bio sensing applications. Small membrane mass and high resonance frequency along with high quality factor of CMUTs facilitate a highly sensitive biosensor system. The limit for the minimum detectable loaded mass per unit area achieved in this work is 0.44 ag/μm2. Here we demonstrate a selective human immune protein immunoglobulin G (IgG) sensor with hexamer peptide ligand HWRGWV.}, booktitle={2017 IEEE SENSORS}, publisher={IEEE}, author={Mahmud, M. M. and Reese, H. and Joshipura, A. and Seok, C. and Yamaner, F. Y. and Daniele, M. and Menegatti, S. and Oralkan, O.}, year={2017}, month={Oct} }
 @inproceedings{mahmud_adelegan_sanders_zhang_yamaner_dayton_oralkan_2017, title={Improved CMUT structure and method of operation for dual-frequency acoustic angiography}, ISBN={9781538633830}, url={http://dx.doi.org/10.1109/ultsym.2017.8091917}, DOI={10.1109/ultsym.2017.8091917}, abstractNote={“Acoustic angiography” is a super-harmonic contrast imaging technique, which is based on the fact that when excited with a moderate acoustic pressure near their resonance (2–4 MHz; around MI of 0.5–0.7) ultrasound contrast agents produce broadband content which extends well past 15 MHz. By detecting the higher order harmonic energy while transmitting at a low fundamental frequency, exquisite resolution and tissue-contrast (microvasculature) sensitivity can be achieved. The fundamental challenge with this technique is that it requires transducers that can transmit a low-frequency (LF) pulse and receive high-frequency (HF) harmonics. We propose a novel scheme to transmit a LF high-pressure pulse from a capacitive micromachined ultrasonic transducer (CMUT) operating in conventional mode and then switching to collapse mode to receive the HF microbubble harmonics in the same transmit-receive cycle. Previously we demonstrated vacuum-sealed CMUT arrays realized on an insulating glass substrate. Now we pattern and etch the substrate to create glass spacers inside the device cavity. In case of pull-in, the top electrode rests on these spacers that prevent electrical shorting. Dielectric charging is mitigated in these devices. The improved design allows operation in LF transmit and HF receive modes in the same pulse-echo cycle. In preliminary experiments, we demonstrated 1.75-MPapp pressure output on the transducer surface and switching the center frequency of operation from 4.8 MHz to 7.8 MHz with a higher cutoff frequency of 13 MHz.}, booktitle={2017 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Mahmud, Marzana M. and Adelegan, Oluwafemi J. and Sanders, Jean L. and Zhang, Xiao and Yamaner, Feysel Y. and Dayton, Paul A. and Oralkan, Omer}, year={2017}, month={Sep} }
 @inproceedings{jang_chang_rasmussen_moini_brenner_stephens_oralkan_khuri-yakub_2017, title={Integration of a dual-mode catheter for ultrasound image guidance and HIFU ablation using a 2-D CMUT array}, ISBN={9781538633830}, url={http://dx.doi.org/10.1109/ultsym.2017.8091627}, DOI={10.1109/ultsym.2017.8091627}, abstractNote={Image-guided high-intensity focused ultrasound (HIFU) is widely used not only for non-invasive therapy but also for a precise approach for tissue ablation. Most HIFU systems use piezoelectric transducers, which are typically bulky due to active cooling, and separate imaging and HIFU transducers, and are therefore impractical for catheter-based applications. Taking advantage of a single 2-D capacitive micromachined ultrasonic transducer (CMUT) array, we developed a dual-mode catheter that can switch between ultrasound imaging mode and HIFU ablation mode. The catheter is equipped with an application-specific integrated circuit (ASIC) and a 32 × 32-element 2-D CMUT array. Both ASIC and CMUT are flip-chip bonded to a custom-designed flexible printed circuit board (flex PCB) via 100-μm and 80-μm solder balls. Then, the flex legs are folded and terminated with pads for a micro zero insertion force (μZIF) connector, allowing easy assembly replacement without the extra cost of coaxial cable assembly. Next, the micro-coaxial cables are assembled at the end of the μZIF connectors. After integration with a 3-D printed tip and encapsulating with polydimethylsiloxane (PDMS), the catheter is finalized in a 22-mm diameter shaft. We successfully validated the functionality of both modes of the dual-mode catheter in oil. We are currently preparing the test for an animal study.}, booktitle={2017 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Jang, Ji Hoon and Chang, Chienliu and Rasmussen, Morten Fischer and Moini, Azadeh and Brenner, Kevin and Stephens, Douglas N. and Oralkan, Omer and Khuri-Yakub, Butrus}, year={2017}, month={Sep} }
 @inproceedings{wu_sanders_dundar_oralkan_2017, title={Multi-wavelength photoacoustic imaging for monitoring lesion formation during high-intensity focused ultrasound therapy}, volume={10064}, DOI={10.1117/12.2248739}, abstractNote={Photoacoustic imaging (PAI) can be used to monitor lesion formation during high-intensity focused ultrasound (HIFU) therapy because HIFU changes the optical absorption spectrum (OAS) of the tissue. However, in traditional PAI, the change could be too subtle to be observed either because the OAS does not change very significantly at the imaging wavelength or due to low signal-to-noise ratio in general. We propose a machine-learning-based method for lesion monitoring with multi-wavelength PAI (MWPAI), where PAI is repeated at a sequence of wavelengths and a stack of multi-wavelength photoacoustic (MWPA) images is acquired. Each pixel is represented by a vector and each element in the vector reflects the optical absorption at the corresponding wavelength. Based on the MWPA images, a classifier is trained to classify pixels into two categories: ablated and non-ablated. In our experiment, we create a lesion on a block of bovine tissue with a HIFU transducer, followed by MWPAI in the 690 nm to 950 nm wavelength range, with a step size of 5 nm. In the MWPA images, some of the ablated and non-ablated pixels are cropped and fed to a neural network (NN) as training examples. The NN is then applied to several groups of MWPA images and the results show that the lesions can be identified clearly. To apply MWPAI in/near real-time, sequential feature selection is performed and the number of wavelengths is decreased from 53 to 5 while retaining adequate performance. With a fast-switching tunable laser, the method can be implemented in/near real-time.}, booktitle={Photons plus ultrasound: imaging and sensing 2017}, author={Wu, Xun and Sanders, J. and Dundar, M. and Oralkan, Omer}, year={2017} }
 @inproceedings{zeshan_zhang_oralkan_yamaner_2016, title={2D CMUT array based ultrasonic micromanipulation platform}, ISBN={9781467398978}, url={http://dx.doi.org/10.1109/ultsym.2016.7728665}, DOI={10.1109/ultsym.2016.7728665}, abstractNote={In this paper, we designed and simulated a multilayer planar resonator with target frequency of 2.5 MHz which is created over a row/column-addressed 2D CMUT array. We have shown through finite element modeling and simulations that a particle can be trapped and manipulated both in lateral and axial directions inside the fluid channel by activating CMUT elements; And calculated acoustic radiation force acting on a polystyrene particle of 10-μm radius. We fabricated a 32×32-element row/column-addressed 2D CMUT array on a glass substrate using anodic bonding technology. This approach provides a cost effective and easily implementable solution to micro-particle trapping and handling.}, booktitle={2016 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Zeshan, Arooba and Zhang, Xiao and Oralkan, Omer and Yamaner, F. Yalcin}, year={2016}, month={Sep} }
 @inproceedings{zhang_zeshan_adelegan_yamaner_oralkan_2016, title={A MEMS T/R switch embedded in CMUT structure for ultrasound imaging frontends}, ISBN={9781467398978}, url={http://dx.doi.org/10.1109/ultsym.2016.7728635}, DOI={10.1109/ultsym.2016.7728635}, abstractNote={This paper describes a novel MEMS transmit/ receive (T/R) switch that could be embedded in the general structure of a capacitive micromachined ultrasonic transducer (CMUT). A MEMS switch and a CMUT element were fabricated side by side using an anodic-bonding-based fabrication process. The plates of the CMUT and the membrane-type switch were formed at the same step by anodic bonding. A single switch was tested in air for preliminary characterization. Vacuum-sealing of the switch cell was confirmed by an atmospheric deflection measurement. The switch was then biased at 59-V DC voltage and turned on and off by applying a 1-kHz, 5-Vpp square wave to the control terminal while a 1-MHz, 300-mVpp sinusoidal signal was applied at the RF input. The signal measured at the RF output demonstrates the basic switching behavior with a switch series resistance of 124 Ω. This work is important for the ultrasound imaging system efficiency and could significantly ease the high-voltage requirements of frontend circuits.}, booktitle={2016 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Zhang, Xiao and Zeshan, Arooba and Adelegan, Oluwafemi J. and Yamaner, F. Yalcin and Oralkan, Omer}, year={2016}, month={Sep} }
 @inproceedings{seok_mahmud_adelegan_zhang_oralkan_2016, title={A battery-operated wireless multichannel gas sensor system based on a capacitive micromachined ultrasonic transducer (CMUT) array}, DOI={10.1109/icsens.2016.7808803}, abstractNote={This paper reports on the design and implementation of a complete battery-operated wireless system for a mechanically resonant gas sensor based on a capacitive micromachined ultrasonic transducer (CMUT) array. A custom-designed front-end integrated circuit (IC) with eight inputs and a serial peripheral interface (SPI) was tightly integrated with a CMUT array. The power consumption of the front-end is 10 μW with a duty cycle of 1:60 corresponding to 1-s measurement time every minute. For the completeness of the system, a power management unit (PMU) was designed and interfaced with the described custom IC along with a wireless module. For multichannel operation, time-division multiplexing was adopted to minimize power consumption and prevent potential frequency locking between different channels. Multichannel wireless data acquisition with the described system was demonstrated by loading unfunctionalized sensor channels with humidity in human breath.}, booktitle={2016 ieee sensors}, author={Seok, C. and Mahmud, M. M. and Adelegan, O. and Zhang, X. and Oralkan, Omer}, year={2016} }
 @inproceedings{zhang_adelegan_yamaner_oralkan_2016, title={CMUTs on glass with ITO bottom electrodes for improved transparency}, ISBN={9781467398978}, url={http://dx.doi.org/10.1109/ultsym.2016.7728671}, DOI={10.1109/ultsym.2016.7728671}, abstractNote={In this work, we fabricated capacitive micromachined ultrasonic transducers (CMUTs) on a glass substrate with indium tin oxide (ITO) bottom electrodes for improved transparency. A 2-μm vibrating silicon plate was formed by anodic bonding. The fabrication process requires three masks. The fabricated devices show approximately 300% improvement of optical transmission in the visible to NIR wavelength range (400 nm - 1000 nm) compared to the devices with chromium/gold (Cr/Au) bottom electrodes. The measured static surface profile confirmed that the fabricated devices are vacuum-sealed. The electrical input impedance measurement shows the device has a resonant frequency of 4.75 MHz at 30-V DC voltage. The series resistance of the device is ~1 kΩ, which is mainly due to the ITO bottom electrode connections. Using a full bottom electrode or using parallel connections to the pads could reduce the resistance. The main hurdle for the transparency at shorter wavelength range is the 2-μm silicon plate. The transfer-matrix model shows the transparency could be improved to -80% across the measured spectrum, if silicon is replaced with a more transparent plate material such as ITO or silicon nitride.}, booktitle={2016 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Zhang, Xiao and Adelegan, Oluwafemi and Yamaner, F. Yalcin and Oralkan, Omer}, year={2016}, month={Sep} }
 @article{dieffenderfer_goodell_mills_mcknight_yao_lin_beppler_bent_lee_misra_et al._2016, title={Low-Power Wearable Systems for Continuous Monitoring of Environment and Health for Chronic Respiratory Disease}, volume={20}, ISSN={2168-2194 2168-2208}, url={http://dx.doi.org/10.1109/JBHI.2016.2573286}, DOI={10.1109/jbhi.2016.2573286}, abstractNote={We present our efforts toward enabling a wearable sensor system that allows for the correlation of individual environmental exposures with physiologic and subsequent adverse health responses. This system will permit a better understanding of the impact of increased ozone levels and other pollutants on chronic asthma conditions. We discuss the inefficiency of existing commercial off-the-shelf components to achieve continuous monitoring and our system-level and nano-enabled efforts toward improving the wearability and power consumption. Our system consists of a wristband, a chest patch, and a handheld spirometer. We describe our preliminary efforts to achieve a submilliwatt system ultimately powered by the energy harvested from thermal radiation and motion of the body with the primary contributions being an ultralow-power ozone sensor, an volatile organic compounds sensor, spirometer, and the integration of these and other sensors in a multimodal sensing platform. The measured environmental parameters include ambient ozone concentration, temperature, and relative humidity. Our array of sensors also assesses heart rate via photoplethysmography and electrocardiography, respiratory rate via photoplethysmography, skin impedance, three-axis acceleration, wheezing via a microphone, and expiratory airflow. The sensors on the wristband, chest patch, and spirometer consume 0.83, 0.96, and 0.01 mW, respectively. The data from each sensor are continually streamed to a peripheral data aggregation device and are subsequently transferred to a dedicated server for cloud storage. Future work includes reducing the power consumption of the system-on-chip including radio to reduce the entirety of each described system in the submilliwatt range.}, number={5}, journal={IEEE Journal of Biomedical and Health Informatics}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Dieffenderfer, James and Goodell, Henry and Mills, Steven and McKnight, Michael and Yao, Shanshan and Lin, Feiyan and Beppler, Eric and Bent, Brinnae and Lee, Bongmook and Misra, Veena and et al.}, year={2016}, month={Sep}, pages={1251–1264} }
 @inproceedings{wu_sanders_stephens_oralkan_2016, title={Photoacoustic-imaging-based temperature monitoring for high-intensity focused ultrasound therapy}, DOI={10.1109/embc.2016.7591418}, abstractNote={Temperature monitoring during high-intensity focused ultrasound (HIFU) application is necessary to ensure effective therapy while minimizing thermal damage to adjacent tissue. In this study, we demonstrate a noninvasive approach for temperature measurement during HIFU therapy based on photoacoustic imaging (PAI). Because of the dependence of photoacoustic (PA) signal amplitude on temperature of the source tissue and the linearity of the PAI system, changes in temperature will cause changes in PA image intensity. Experiments have been conducted in ex-vivo bovine tissue to characterize the linear dependence of PA image pixel values on temperature and subsequently to convert the PA image to a real-time temperature map.}, booktitle={2016 38th annual international conference of the ieee engineering in medicine and biology society (embc)}, author={Wu, Xun and Sanders, J. L. and Stephens, D. N. and Oralkan, Omer}, year={2016}, pages={3235–3238} }
 @article{novell_arena_oralkan_dayton_2016, title={Wideband acoustic activation and detection of droplet vaporization events using a capacitive micromachined ultrasonic transducer}, volume={139}, ISSN={["1520-8524"]}, DOI={10.1121/1.4953580}, abstractNote={An ongoing challenge exists in understanding and optimizing the acoustic droplet vaporization (ADV) process to enhance contrast agent effectiveness for biomedical applications. Acoustic signatures from vaporization events can be identified and differentiated from microbubble or tissue signals based on their frequency content. The present study exploited the wide bandwidth of a 128-element capacitive micromachined ultrasonic transducer (CMUT) array for activation (8 MHz) and real-time imaging (1 MHz) of ADV events from droplets circulating in a tube. Compared to a commercial piezoelectric probe, the CMUT array provides a substantial increase of the contrast-to-noise ratio.}, number={6}, journal={JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA}, author={Novell, Anthony and Arena, Christopher B. and Oralkan, Omer and Dayton, Paul A.}, year={2016}, month={Jun}, pages={3193–3198} }
 @article{yamaner_zhang_oralkan_2015, title={A Three-Mask Process for Fabricating Vacuum-Sealed Capacitive Micromachined Ultrasonic Transducers Using Anodic Bonding}, volume={62}, ISSN={["1525-8955"]}, DOI={10.1109/tuffc.2014.006794}, abstractNote={This paper introduces a simplified fabrication method for vacuum-sealed capacitive micromachined ultrasonic transducer (CMUT) arrays using anodic bonding. Anodic bonding provides the established advantages of wafer-bondingbased CMUT fabrication processes, including process simplicity, control over plate thickness and properties, high fill factor, and ability to implement large vibrating cells. In addition to these, compared with fusion bonding, anodic bonding can be performed at lower processing temperatures, i.e., 350°C as opposed to 1100°C; surface roughness requirement for anodic bonding is more than 10 times more relaxed, i.e., 5-nm rootmean- square (RMS) roughness as opposed to 0.5 nm for fusion bonding; anodic bonding can be performed on smaller contact area and hence improves the fill factor for CMUTs. Although anodic bonding has been previously used for CMUT fabrication, a CMUT with a vacuum cavity could not have been achieved, mainly because gas is trapped inside the cavities during anodic bonding. In the approach we present in this paper, the vacuum cavity is achieved by opening a channel in the plate structure to evacuate the trapped gas and subsequently sealing this channel by conformal silicon nitride deposition in the vacuum environment. The plate structure of the fabricated CMUT consists of the single-crystal silicon device layer of a silicon-on-insulator wafer and a thin silicon nitride insulation layer. The presented fabrication approach employs only three photolithographic steps and combines the advantages of anodic bonding with the advantages of a patterned metal bottom electrode on an insulating substrate, specifically low parasitic series resistance and low parasitic shunt capacitance. In this paper, the developed fabrication scheme is described in detail, including process recipes. The fabricated transducers are characterized using electrical input impedance measurements in air and hydrophone measurements in immersion. A representative design is used to demonstrate immersion operation in conventional, collapse-snapback, and collapse modes. In collapsemode operation, an output pressure of 1.67 MPa pp is shown at 7 MHz on the surface of the transducer for 60-Vpp, 3-cycle sinusoidal excitation at 30-V dc bias.}, number={5}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, author={Yamaner, F. Yalcin and Zhang, Xiao and Oralkan, Omer}, year={2015}, month={May}, pages={972–982} }
 @inproceedings{mahmud_kumar_zhang_yamaner_nagle_oralkan_2015, title={A capacitive micromachined ultrasonic transducer (CMUT) array as a low-power multi-channel volatile organic compound (VOC) sensor}, booktitle={2015 ieee sensors}, author={Mahmud, M. M. and Kumar, M. and Zhang, X. and Yamaner, F. Y. and Nagle, H. T. and Oralkan, O.}, year={2015}, pages={181–184} }
 @inproceedings{kumar_seok_mahmud_zhang_oralkan_2015, title={A low-power integrated circuit for interfacing a capacitive micromachined ultrasonic transducer (CMUT) based resonant gas sensor}, DOI={10.1109/icsens.2015.7370639}, abstractNote={In this work we present a complete end-to-end interface for a capacitive micromachined ultrasonic transducer (CMUT) intended for low-power gas sensing applications. A prototype chip was designed in a 0.18-μm BiCMOS process. Different blocks (a BJT-based Colpitts oscillator, an inverter-based oscillator, a sine-to-square wave converter, a digital frequency counter, and a parallel-to-serial converter) required for the complete system are discussed, designed, and tested for their standalone performance. Consequently, a complete system interfaced with a 3.6-MHz CMUT and providing a digital frequency output is presented. With duty cycling for one measurement per minute the system consumed 10 μW power.}, booktitle={2015 ieee sensors}, author={Kumar, M. and Seok, C. and Mahmud, M. M. and Zhang, X. and Oralkan, Omer}, year={2015}, pages={1781–1784} }
 @inproceedings{zhang_yamanery_adelegan_oralkan_2015, title={Design of high-frequency broadband CMUT arrays}, ISBN={9781479981823}, ISSN={["1948-5719"]}, url={http://dx.doi.org/10.1109/ultsym.2015.0167}, DOI={10.1109/ultsym.2015.0167}, abstractNote={In this work we demonstrate a high-frequency (29-MHz) broadband (100% FBW) CMUT 1D array. The devices are fabricated using anodic bonding with only three photolithography steps. We also discuss the design guidelines for high-frequency broadband CMUTs using the simulations. A high fill factor and a thin plate are important for the broadband design. Small cell size is required for the increased center frequency. To improve the transducer sensitivity and to keep the collapse voltage low, the gap height should be small and a high-k dielectric insulation layer should be employed. The fabrication steps we report in this paper have good potential to meet the high-frequency broadband CMUT design requirements. So far we have demonstrated that we can define a 50-nm gap, bond to a post as narrow as 2 μm, and pattern a high-k dielectric layer on the bottom electrode.}, booktitle={2015 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Zhang, Xiao and Yamanery, F. Yalcin and Adelegan, Oluwafemi and Oralkan, Omer}, year={2015}, month={Oct} }
 @article{jang_rasmussen_bhuyan_yoon_moini_chang_watkins_choe_nikoozadeh_stephens_et al._2015, title={Dual-Mode Integrated Circuit for Imaging and HIFU With 2-D CMUT Arrays}, ISSN={["1948-5719"]}, DOI={10.1109/ultsym.2015.0166}, abstractNote={Successful high intensity focused ultrasound (HIFU) operation requires a reliable guidance and monitoring method such as magnetic resonance imaging (MRI) or ultrasound imaging. However, both widely used modalities are typically separate from the HIFU system, which makes co-registration of HIFU with cross-sectional imaging difficult. In this paper, we present a dual-mode integrated circuit (IC) that can perform both ultrasound imaging and HIFU with a single 2D capacitive micromachined ultrasonic transducer (CMUT) array, combining these two systems for ease of use. The dual-mode IC consists of pulsers, transmit beamforming circuitry, and low-noise amplifiers for imaging mode and switches for HIFU mode. By turning this switching network on and off, the system can alternately operate the imaging mode and HIFU mode on demand. The dual-mode IC was designed and fabricated in the 0.18-μm HV 4LM process provided by Maxim Inc. We fabricated a 32×32-element CMUT array that has a center frequency of 5 MHz using a sacrificial release process and flip-chip bonded this CMUT array to the IC. With the back-end system, real-time volumetric imaging on the wire phantom and HIFU ablation on ex-vivo tissue were performed respectively.}, journal={2015 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM (IUS)}, author={Jang, Ji Hoon and Rasmussen, Morten Fischer and Bhuyan, Anshuman and Yoon, Hyo-Seon and Moini, Azadeh and Chang, Chienliu and Watkins, Ronald D. and Choe, Jung Woo and Nikoozadeh, Amin and Stephens, Douglas and et al.}, year={2015} }
 @inproceedings{zhang_yamanery_oralkan_2015, title={Fabrication of capacitive micromachined ultrasonic transducers with through-glass-via interconnects}, ISBN={9781479981823}, ISSN={["1948-5719"]}, url={http://dx.doi.org/10.1109/ultsym.2015.0060}, DOI={10.1109/ultsym.2015.0060}, abstractNote={This paper introduces a novel fabrication method for capacitive micromachined ultrasonic transducer (CMUT) arrays amenable to 3D integration. The work demonstrates that MEMS structures can be directly built on a through-glass-via (TGV) substrate. The key feature of this new approach is the combination of TGV interconnects with a vibrating silicon-plate structure formed by anodic bonding. This method simplifies the overall fabrication process for CMUTs with through-wafer interconnects by eliminating the need for an insulating lining for vias or isolation trenches. Fabrication of CMUTs on a glass substrate and use of copper-filled vias as interconnects can help reduce the parasitic interconnect capacitance and resistance, improving device performance and reliability. This work is especially important for fabricating 2D CMUT arrays and integrating them closely with supporting electronic circuits.}, booktitle={2015 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Zhang, Xiao and Yamanery, F. Yalcin and Oralkan, Omer}, year={2015}, month={Oct} }
 @article{misra_bozkurt_calhoun_jackson_jur_lach_lee_muth_oralkan_oeztuerk_et al._2015, title={Flexible Technologies for Self-Powered Wearable Health and Environmental Sensing}, volume={103}, ISSN={["1558-2256"]}, DOI={10.1109/jproc.2015.2412493}, abstractNote={This article provides the latest advances from the NSF Advanced Self-powered Systems of Integrated sensors and Technologies (ASSIST) center. The work in the center addresses the key challenges in wearable health and environmental systems by exploring technologies that enable ultra-long battery lifetime, user comfort and wearability, robust medically validated sensor data with value added from multimodal sensing, and access to open architecture data streams. The vison of the ASSIST center is to use nanotechnology to build miniature, self-powered, wearable, and wireless sensing devices that can enable monitoring of personal health and personal environmental exposure and enable correlation of multimodal sensors. These devices can empower patients and doctors to transition from managing illness to managing wellness and create a paradigm shift in improving healthcare outcomes. This article presents the latest advances in high-efficiency nanostructured energy harvesters and storage capacitors, new sensing modalities that consume less power, low power computation, and communication strategies, and novel flexible materials that provide form, function, and comfort. These technologies span a spatial scale ranging from underlying materials at the nanoscale to body worn structures, and the challenge is to integrate them into a unified device designed to revolutionize wearable health applications.}, number={4}, journal={PROCEEDINGS OF THE IEEE}, author={Misra, Veena and Bozkurt, Alper and Calhoun, Benton and Jackson, Thomas N. and Jur, Jesse S. and Lach, John and Lee, Bongmook and Muth, John and Oralkan, Oemer and Oeztuerk, Mehmet and et al.}, year={2015}, month={Apr}, pages={665–681} }
 @inproceedings{misra_lach_bozkurt_calhoun_datta_oralkan_2015, title={Self-powered wearable sensor platforms for wellness}, booktitle={2015 International Conference on Compilers, Architecture and Synthesis for Embedded Systems (CASES)}, author={Misra, V. and Lach, J. and Bozkurt, A. and Calhoun, B. and Datta, S. and Oralkan, O.}, year={2015}, pages={187–187} }
 @article{lee_park_oralkan_kupnik_khuri-yakub_2014, title={A Multichannel Oscillator for a Resonant Chemical Sensor System}, volume={61}, ISSN={["1557-9948"]}, DOI={10.1109/tie.2014.2300031}, abstractNote={Vapor detection using highly sensitive miniaturized resonant sensors is of great interest for many applications, including consumer, industrial, and environmental applications. An operational-amplifier-based multichannel oscillator that interfaces with a 50-MHz capacitive micromachined ultrasonic transducer array is presented for chemical sensing applications. The circuit was implemented in a 0.18-μm CMOS technology to reduce power consumption, number of wires, and active area per channel. The presented integrated circuit also addresses the potential-frequency-locking problem between channels by allowing the open-loop gain to be adjustable off-chip. The feasibility of the developed oscillator for the chemical sensing application is demonstrated. Two channels that were operated simultaneously achieved excellent volume sensitivities of 8.5 × 10-4%/Hz and 3.9 × 10-4%/Hz, respectively, to relative humidity in N2.}, number={10}, journal={IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS}, author={Lee, Hyunjoo Jenny and Park, Kwan Kyu and Oralkan, Omer and Kupnik, Mario and Khuri-Yakub, Butrus T.}, year={2014}, month={Oct}, pages={5632–5640} }
 @inproceedings{mahmud_li_lunsford_zhang_yamaner_nagle_oralkan_2014, title={A low-power gas sensor for environmental monitoring using a capacitive micromachined ultrasonic transducer}, ISBN={9781479901623}, url={http://dx.doi.org/10.1109/icsens.2014.6985089}, DOI={10.1109/icsens.2014.6985089}, abstractNote={We present a low-power gas sensor design based on a capacitive micromachined ultrasonic transducer (CMUT), for use on self-powered wearable platforms. Earlier a CMUT-based sensor, with 70-mW power consumption operating at 50 MHz, achieved ppt-level detection limit for chemical warfare agents. In this work we present a sensor operating at 4.33 MHz and consuming 0.77 mW for environmental monitoring. The sensor comprises a polymer-functionalized CMUT resonator in the feedback loop of a Colpitts oscillator. We fabricated the CMUT resonators using a novel process based on anodic bonding. The cavities and bottom electrodes are formed on a borosilicate glass wafer. The device layer of an SOI wafer bonded on glass forms the vibrating plate on top of vacuum-sealed cavities. This fabrication approach reduces process complexity and helps minimize parasitic components. CMUTs with center frequencies in the 3-50 MHz range with Q-factors as high as ~400 have successfully been fabricated. We used a 4.52-MHz device (Q=180) coated with a thin layer of polyisobutylene (PIB) for sensor demonstration.}, booktitle={IEEE SENSORS 2014 Proceedings}, publisher={IEEE}, author={Mahmud, M. M. and Li, J. and Lunsford, J. E. and Zhang, X. and Yamaner, F. Y. and Nagle, H. T. and Oralkan, O.}, year={2014}, month={Nov} }
 @article{wu_kumar_oralkan_2014, title={An Ultrasound-Based Noninvasive Neural Interface to the Retina Projection Algorithm and Frontend Integrated Circuit Architecture}, ISSN={["1948-5719"]}, DOI={10.1109/ultsym.2014.0655}, abstractNote={Focused ultrasound (FUS) is emerging as a promising technology for neural stimulation. In this study, we demonstrate the algorithm and the frontend integrated circuit (IC) architecture design for an ultrasound-based noninvasive neural interface to the retina. A digital image is provided as the input to the system, and the system calculates the excitation signal for each element in a 2D transducer array. With each element being excited accordingly, the array can “project” the image onto the retina as an ultrasound field pattern (USFP). The algorithm is based on the fast Fourier transform (FFT), which makes real-time implementation feasible.}, journal={2014 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM (IUS)}, author={Wu, Xun and Kumar, Mohit and Oralkan, Omer}, year={2014}, pages={2623–2626} }
 @article{yamaner_zhang_oralkan_2014, title={Fabrication of Anodically Bonded Capacitive Micromachined Ultrasonic Transducers with Vacuum-Sealed Cavities}, ISSN={["1948-5719"]}, DOI={10.1109/ultsym.2014.0148}, abstractNote={Capacitive micromachined ultrasonic transducers (CMUTs) have demonstrated great promise for next-generation ultrasound technology. Wafer-bonding technology particularly simplifies the fabrication of CMUTs by eliminating the requirement for a sacrificial layer and increases control over device parameters. Anodic bonding has many advantages over other bonding methods such as low temperature compatibility, high bond strength, high tolerance to particle contamination and surface roughness, and cost savings. Furthermore, the glass substrates lower the parasitic capacitance and improve reliability. The major drawback is the trapped gas inside the cavities, which occurs during bonding. Earlier CMUT fabrication efforts using anodic bonding failed to demonstrate a vacuum-sealed cavity. In this study, we developed a fabrication scheme to overcome this issue and demonstrated vacuum-backed CMUTs using anodic bonding. This new approach also simplifies the overall fabrication process for CMUTs. We demonstrated a CMUT fabrication process with three lithography steps. A vibrating plate is formed by bonding the device layer of a silicon-on-insulator (SOI) wafer on top of submicron cavities defined on a borosilicate glass wafer. The cavities and the bottom electrodes are created on the borosilicate glass wafer with a single lithography step. The recessed bottom metal layer over the glass surface allows bonding the plate directly on glass posts and therefore helps reduce the parasitic capacitance and improve the breakdown reliability. A surface roughness of 0.8 nm is achieved in the cavity using wet chemical etching. A 200-nm PECVD silicon nitride layer deposited on the 2 μm device layer of the SOI wafer prior to bonding serves as the insulation layer to prevent shorting after pull-in. The trapped gas inside the cavities is evacuated after anodic bonding by reactive ion etching. The 120-nm cavities are then sealed with PECVD silicon nitride. We measured the atmospheric deflection of the plates after fabrication, which proves the vacuum inside the cavities. Impedance and hydrophone measurements were performed both in conventional (2.8 MHz) and collapse (7.2 MHz) modes. Bonding on posts with widths as small as 2 μm was successfully demonstrated using anodic bonding which is difficult to achieve with other wafer bonding methods.}, journal={2014 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM (IUS)}, author={Yamaner, F. Yalcin and Zhang, Xiao and Oralkan, Oemer}, year={2014}, pages={604–607} }
 @inproceedings{bhuyan_choe_lee_wygant_nikoozadeh_oralkan_khuri-yakub_2013, title={3D volumetric ultrasound imaging with a 32×32 CMUT array integrated with front-end ICs using flip-chip bonding technology}, ISBN={9781467345163 9781467345156 9781467345149}, url={http://dx.doi.org/10.1109/isscc.2013.6487786}, DOI={10.1109/isscc.2013.6487786}, abstractNote={3D ultrasound imaging is becoming increasingly prevalent in the medical field. Compared to conventional 2D imaging systems, 3D imaging can provide a detailed view of tissue structures that makes diagnosis easier for the physicians. In addition, 2D image slices can be formed at various orientations to the transducer, making the examination less dependent on the skill of the sonographer. However, various challenges exist in developing a 3D imaging system, such as integration of a large number of elements, as well as post-processing of datasets received from a large number of channels. 2D transducer arrays are typically integrated with custom ICs in the probe handle to perform some intermediate beamforming and to reduce the number of cable connections to the imaging system. Capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative to piezoelectric transducers. Being a MEMS device, they greatly benefit from flexibility and ease of fabrication, and can be seamlessly integrated with electronics. Previous work demonstrates 3D stacking of CMUTs and dummy ICs with an intermediate interposer layer. However, that represents more of a mechanical demonstration of 3D integration. In this paper, we present a fully functional 3D ultrasound imaging system comprising a 32×32 2D CMUT array, 3D-stacked with front-end ICs using flip-chip bonding technology. The imaging system is capable of capturing real-time volumetric ultrasound data, and displaying 2D and 3D ultrasound images.}, booktitle={2013 IEEE International Solid-State Circuits Conference Digest of Technical Papers}, publisher={IEEE}, author={Bhuyan, A. and Choe, J. W. and Lee, Byung Chul and Wygant, I. and Nikoozadeh, A. and Oralkan, O. and Khuri-Yakub, B. T.}, year={2013}, month={Feb} }
 @inproceedings{bhuyan_chang_choe_lee_nikoozadeh_oralkan_khuri-yakub_2013, title={A 32×32 integrated CMUT array for volumetric ultrasound imaging}, ISBN={9781467356862 9781467356848}, url={http://dx.doi.org/10.1109/ultsym.2013.0141}, DOI={10.1109/ultsym.2013.0141}, abstractNote={Real-time 3D volumetric ultrasound imaging systems require transmit and receive circuitry to generate the ultrasound beam and process the received echo signals. Since a 2D array is required for 3D imaging, the complexity of building such a system is significantly higher, e.g., front-end electronics need to be interfaced to the transducer, a large number of elements need to be interfaced to the backend system and a large dataset needs to be processed. In this work, we present a 3D imaging system using capacitive micromachined ultrasonic transducer (CMUT) technology that addresses many of the challenges in building such a system. The transducer is a 5-MHz CMUT array with an 8 mm × 8 mm aperture size. The aperture consists of 1024 elements (32×32) with an element pitch of 250 μm. An integrated circuit (IC) is integrated very close to the CMUT array. It consists of a transmit beamformer and receive circuitry to improve the noise performance of the overall system. Simultaneous multi-beam transmit is also incorporated in the IC to improve the imaging frame rate. The CMUT is flip-chip bonded to the IC and the final assembly measured 9.2 mm × 9.2 mm. The assembly was then interfaced with an FPGA and a backend system (comprising of a data acquisition system and PC). The FPGA provided the digital I/O signals for the IC and the backend system was used to process the received RF echo data (from the IC) and reconstruct the volume image using a phased array imaging approach. Imaging experiments were performed using wire phantoms. Real-time volumetric images were captured at 5 volumes per second and are presented in this paper.}, booktitle={2013 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Bhuyan, Anshuman and Chang, Chienliu and Choe, Jung Woo and Lee, Byung Chul and Nikoozadeh, Amin and Oralkan, Omer and Khuri-Yakub, Butrus T.}, year={2013}, month={Jul} }
 @article{park_oralkan_khuri-yakub_2013, title={A comparison between conventional and collapse-mode capacitive micromachined ultrasonic transducers in 10-MHz 1-D arrays}, volume={60}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2013.2688}, DOI={10.1109/tuffc.2013.2688}, abstractNote={This paper presents a comprehensive comparison between a collapse-mode and a conventional-mode capacitive micromachined ultrasonic transducer (CMUT); both devices have a 1-μm-thick silicon plate and operate at 10 MHz when biased at 100 V. The radii of the circular plates and the gap heights are modified to meet the design specifications required for a fair comparison. Finite element analysis (FEA) shows that the collapse-mode CMUT has higher output pressure sensitivity (46.5 kPa/V) than the conventional CMUT (13.1 kPa/V), and achieves a 3-dB fractional bandwidth (FBW) of 124% compared with 128% for the conventional mode. These results were validated by experiments performed on devices fabricated in a 1-D phased array configuration using the local oxidation of silicon (LOCOS)/wafer-bonding process. The measured output pressure sensitivity and the FBW of the collapse-mode and the conventional CMUTs at 100 V were 26.4 kPa/V and 103% and 12.7 kPa/V and 111%, respectively. The maximum output pressure of the collapse-mode CMUT was 1.19 MPa at 10 MHz, which was much higher than the conventional CMUT (0.44 MPa). However, the second harmonic distortion (SHD) level of the collapse-mode CMUT is higher than the conventional CMUT at the same excitation condition. Even with higher electric field in the cavity, the collapse-mode CMUT was as stable as the conventional CMUT in a long-term test. A 30-h test with a total of 3.2 × 109 cycles of 30 V ac excitation resulted in no significant degradation in the performance of the collapse-mode devices.}, number={6}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Park, Kwan Kyu and Oralkan, O. and Khuri-Yakub, B. T.}, year={2013}, month={Jun}, pages={1245–1255} }
 @article{prieto_oralkan_khuri-yakub_maduke_2013, title={Dynamic Response of Model Lipid Membranes to Ultrasonic Radiation Force}, volume={8}, ISSN={1932-6203}, url={http://dx.doi.org/10.1371/journal.pone.0077115}, DOI={10.1371/journal.pone.0077115}, abstractNote={Low-intensity ultrasound can modulate action potential firing in neurons in vitro and in vivo. It has been suggested that this effect is mediated by mechanical interactions of ultrasound with neural cell membranes. We investigated whether these proposed interactions could be reproduced for further study in a synthetic lipid bilayer system. We measured the response of protein-free model membranes to low-intensity ultrasound using electrophysiology and laser Doppler vibrometry. We find that ultrasonic radiation force causes oscillation and displacement of lipid membranes, resulting in small (<1%) changes in membrane area and capacitance. Under voltage-clamp, the changes in capacitance manifest as capacitive currents with an exponentially decaying sinusoidal time course. The membrane oscillation can be modeled as a fluid dynamic response to a step change in pressure caused by ultrasonic radiation force, which disrupts the balance of forces between bilayer tension and hydrostatic pressure. We also investigated the origin of the radiation force acting on the bilayer. Part of the radiation force results from the reflection of the ultrasound from the solution/air interface above the bilayer (an effect that is specific to our experimental configuration) but part appears to reflect a direct interaction of ultrasound with the bilayer, related to either acoustic streaming or scattering of sound by the bilayer. Based on these results, we conclude that synthetic lipid bilayers can be used to study the effects of ultrasound on cell membranes and membrane proteins.}, number={10}, journal={PLoS ONE}, publisher={Public Library of Science (PLoS)}, author={Prieto, Martin Loynaz and Oralkan, Ömer and Khuri-Yakub, Butrus T. and Maduke, Merritt C.}, editor={Phillips, WilliamEditor}, year={2013}, month={Oct}, pages={e77115} }
 @article{choe_nikoozadeh_oralkan_khuri-yakub_2013, title={GPU-Based Real-Time Volumetric Ultrasound Image Reconstruction for a Ring Array}, volume={32}, ISSN={0278-0062 1558-254X}, url={http://dx.doi.org/10.1109/tmi.2013.2253117}, DOI={10.1109/tmi.2013.2253117}, abstractNote={Synthetic phased array (SPA) beamforming with Hadamard coding and aperture weighting is an optimal option for real-time volumetric imaging with a ring array, a particularly attractive geometry in intracardiac and intravascular applications. However, the imaging frame rate of this method is limited by the immense computational load required in synthetic beamforming. For fast imaging with a ring array, we developed graphics processing unit (GPU)-based, real-time image reconstruction software that exploits massive data-level parallelism in beamforming operations. The GPU-based software reconstructs and displays three cross-sectional images at 45 frames per second (fps). This frame rate is 4.5 times higher than that for our previously-developed multi-core CPU-based software. In an alternative imaging mode, it shows one B-mode image rotating about the axis and its maximum intensity projection, processed at a rate of 104 fps . This paper describes the image reconstruction procedure on the GPU platform and presents the experimental images obtained using this software.}, number={7}, journal={IEEE Transactions on Medical Imaging}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Choe, Jung Woo and Nikoozadeh, A. and Oralkan, O. and Khuri-Yakub, B. T.}, year={2013}, month={Jul}, pages={1258–1264} }
 @inproceedings{choe_nikoozadeh_oralkan_khuri-yakub_2013, title={GPU-based real-time imaging software suite for medical ultrasound}, ISBN={9781467356862 9781467356848}, url={http://dx.doi.org/10.1109/ultsym.2013.0525}, DOI={10.1109/ultsym.2013.0525}, abstractNote={We developed a GPU-based real-time imaging software suite for medical ultrasound imaging to provide a fast real-time imaging platform for various probe geometries and imaging schemes. The imaging software receives raw RF data from a data acquisition system, and processes them on GPU to reconstruct real-time images. The most general-purpose imaging program in the suite displays three cross-sectional images for arbitrary probe geometry and various imaging schemes including conventional beamforming, synthetic beamforming, and plane-wave compounding. The other imaging programs in the software suite, derived from the general-purpose imaging program, are optimized for their own purposes, such as displaying a rotating B-mode plane and its maximum intensity projection (MIP), photoacoustic imaging, and real-time volume-rendering. Real-time imaging was successfully demonstrated using each of the imaging programs in the software suite.}, booktitle={2013 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Choe, Jung Woo and Nikoozadeh, Amin and Oralkan, Omer and Khuri-Yakub, Butrus T.}, year={2013}, month={Jul} }
 @inbook{khuri-yakub_oralkan_nikoozadeh_2013, title={Innovations in Ultrasound Instrumentation for Image Guidance}, ISBN={9781461476566 9781461476573}, url={http://dx.doi.org/10.1007/978-1-4614-7657-3_11}, DOI={10.1007/978-1-4614-7657-3_11}, booktitle={Intraoperative Imaging and Image-Guided Therapy}, publisher={Springer New York}, author={Khuri-Yakub, Pierre and Oralkan, Ömer and Nikoozadeh, Amin}, year={2013}, month={Nov}, pages={163–171} }
 @article{bhuyan_choe_lee_wygant_nikoozadeh_oralkan_khuri-yakub_2013, title={Integrated Circuits for Volumetric Ultrasound Imaging With 2-D CMUT Arrays}, volume={7}, ISSN={["1940-9990"]}, DOI={10.1109/tbcas.2014.2298197}, abstractNote={Real-time volumetric ultrasound imaging systems require transmit and receive circuitry to generate ultrasound beams and process received echo signals. The complexity of building such a system is high due to requirement of the front-end electronics needing to be very close to the transducer. A large number of elements also need to be interfaced to the back-end system and image processing of a large dataset could affect the imaging volume rate. In this work, we present a 3-D imaging system using capacitive micromachined ultrasonic transducer (CMUT) technology that addresses many of the challenges in building such a system. We demonstrate two approaches in integrating the transducer and the front-end electronics. The transducer is a 5-MHz CMUT array with an 8 mm × 8 mm aperture size. The aperture consists of 1024 elements (32 × 32) with an element pitch of 250 μm. An integrated circuit (IC) consists of a transmit beamformer and receive circuitry to improve the noise performance of the overall system. The assembly was interfaced with an FPGA and a back-end system (comprising of a data acquisition system and PC). The FPGA provided the digital I/O signals for the IC and the back-end system was used to process the received RF echo data (from the IC) and reconstruct the volume image using a phased array imaging approach. Imaging experiments were performed using wire and spring targets, a ventricle model and a human prostrate. Real-time volumetric images were captured at 5 volumes per second and are presented in this paper.}, number={6}, journal={IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS}, author={Bhuyan, Anshuman and Choe, Jung Woo and Lee, Byung Chul and Wygant, Ira O. and Nikoozadeh, Amin and Oralkan, Omer and Khuri-Yakub, Butrus T.}, year={2013}, month={Dec}, pages={796–804} }
 @article{kamaya_vaithilingam_chung_oralkan_khuri-yakub_2013, title={Photoacoustic Imaging of the Bladder A Pilot Study}, volume={32}, ISSN={["1550-9613"]}, DOI={10.7863/ultra.32.7.1245}, abstractNote={Photoacoustic imaging is a promising new technology that combines tissue optical characteristics with ultrasound transmission and can potentially visualize tumor depth in bladder cancer. We imaged simulated tumors in 5 fresh porcine bladders with conventional pulse‐echo sonography and photoacoustic imaging. Isoechoic biomaterials of different optical qualities were used. In all 5 of the bladder specimens, photoacoustic imaging showed injected biomaterials, containing varying degrees of pigment, better than control pulse‐echo sonography. Photoacoustic imaging may be complementary to diagnostic information obtained by cystoscopy and urine cytologic analysis and could potentially obviate the need for biopsy in some tumors before definitive treatment.}, number={7}, journal={JOURNAL OF ULTRASOUND IN MEDICINE}, author={Kamaya, Aya and Vaithilingam, Srikant and Chung, Benjamin I. and Oralkan, Omer and Khuri-Yakub, Butrus T.}, year={2013}, month={Jul}, pages={1245–1250} }
 @article{menz_oralkan_khuri-yakub_baccus_2013, title={Precise Neural Stimulation in the Retina Using Focused Ultrasound}, volume={33}, ISSN={0270-6474 1529-2401}, url={http://dx.doi.org/10.1523/jneurosci.3521-12.2013}, DOI={10.1523/jneurosci.3521-12.2013}, abstractNote={Focused ultrasound is a promising noninvasive technology for neural stimulation. Here we use the isolated salamander retina to characterize the effect of ultrasound on an intact neural circuit and compared these effects with those of visual stimulation of the same retinal ganglion cells. Ultrasound stimuli at an acoustic frequency of 43 MHz and a focal spot diameter of 90 μm delivered from a piezoelectric transducer evoked stable responses with a temporal precision equal to strong visual responses but with shorter latency. By presenting ultrasound and visual stimulation together, we found that ultrasonic stimulation rapidly modulated visual sensitivity but did not change visual temporal filtering. By combining pharmacology with ultrasound stimulation, we found that ultrasound did not directly activate retinal ganglion cells but did in part activate interneurons beyond photoreceptors. These results suggest that, under conditions of strong localized stimulation, timing variability is largely influenced by cells beyond photoreceptors. We conclude that ultrasonic stimulation is an effective and spatiotemporally precise method to activate the retina. Because the retina is the most accessible part of the CNSin vivo, ultrasonic stimulation may have diagnostic potential to probe remaining retinal function in cases of photoreceptor degeneration, and therapeutic potential for use in a retinal prosthesis. In addition, because of its noninvasive properties and spatiotemporal resolution, ultrasound neurostimulation promises to be a useful tool to understand dynamic activity in pharmacologically defined neural pathways in the retina.}, number={10}, journal={Journal of Neuroscience}, publisher={Society for Neuroscience}, author={Menz, M. D. and Oralkan, O. and Khuri-Yakub, P. T. and Baccus, S. A.}, year={2013}, month={Mar}, pages={4550–4560} }
 @article{kothapalli_ma_vaithilingam_oralkan_khuri-yakub_gambhir_2012, title={Deep Tissue Photoacoustic Imaging Using a Miniaturized 2-D Capacitive Micromachined Ultrasonic Transducer Array}, volume={59}, ISSN={0018-9294 1558-2531}, url={http://dx.doi.org/10.1109/tbme.2012.2183593}, DOI={10.1109/tbme.2012.2183593}, abstractNote={In this paper, we demonstrate 3-D photoacoustic imaging (PAI) of light absorbing objects embedded as deep as 5 cm inside strong optically scattering phantoms using a miniaturized (4 mm × 4 mm × 500 μm), 2-D capacitive micromachined ultrasonic transducer (CMUT) array of 16 × 16 elements with a center frequency of 5.5 MHz. Two-dimensional tomographic images and 3-D volumetric images of the objects placed at different depths are presented. In addition, we studied the sensitivity of CMUT-based PAI to the concentration of indocyanine green dye at 5 cm depth inside the phantom. Under optimized experimental conditions, the objects at 5 cm depth can be imaged with SNR of about 35 dB and a spatial resolution of approximately 500 μm. Results demonstrate that CMUTs with integrated front-end amplifier circuits are an attractive choice for achieving relatively high depth sensitivity for PAI.}, number={5}, journal={IEEE Transactions on Biomedical Engineering}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Kothapalli, S. and Ma, Te-Jen and Vaithilingam, S. and Oralkan, O. and Khuri-Yakub, B. T. and Gambhir, S. S.}, year={2012}, month={May}, pages={1199–1204} }
 @article{stephens_truong_nikoozadeh_oralkan_seo_cannata_dentinger_thomenius_de la rama_nguyen_et al._2012, title={First In Vivo Use of a Capacitive Micromachined Ultrasound Transducer Array-Based Imaging and Ablation Catheter}, volume={31}, ISSN={0278-4297}, url={http://dx.doi.org/10.7863/jum.2012.31.2.247}, DOI={10.7863/jum.2012.31.2.247}, abstractNote={ObjectivesThe primary objective was to test in vivo for the first time the general operation of a new multifunctional intracardiac echocardiography (ICE) catheter constructed with a microlinear capacitive micromachined ultrasound transducer (ML‐CMUT) imaging array. Secondarily, we examined the compatibility of this catheter with electroanatomic mapping (EAM) guidance and also as a radiofrequency ablation (RFA) catheter. Preliminary thermal strain imaging (TSI)‐derived temperature data were obtained from within the endocardium simultaneously during RFA to show the feasibility of direct ablation guidance procedures.}, number={2}, journal={Journal of Ultrasound in Medicine}, publisher={Wiley}, author={Stephens, Douglas N. and Truong, Uyen T. and Nikoozadeh, Amin and Oralkan, Ömer and Seo, Chi Hyung and Cannata, Jonathan and Dentinger, Aaron and Thomenius, Kai and de la Rama, Alan and Nguyen, Tho and et al.}, year={2012}, month={Feb}, pages={247–256} }
 @inproceedings{nikoozadeh_choe_kothapalli_moini_sanjani_kamaya_oralkan_gambhir_khuri-yakub_2012, title={Photoacoustic imaging using a 9F microLinear CMUT ICE catheter}, ISBN={9781467345620 9781467345613 9781467345606}, url={http://dx.doi.org/10.1109/ultsym.2012.0007}, DOI={10.1109/ultsym.2012.0007}, abstractNote={This work presents our preliminary results on developing a multi-modality imaging catheter enabling combined ultrasound and photoacoustic imaging. We have developed an optical fiber ring catheter for use with our previously demonstrated 9F, real-time, forward-looking intracardiac ultrasound imaging catheter. Our custom software provides realtime ultrasound and photoacoustic imaging on a PC-based imaging platform. The promising phantom and in vivo imaging results presented here demonstrate the utility of a fully integrated catheter that provides both anatomical and functional information through co-registered ultrasound and photoacoustic imaging capabilities.}, booktitle={2012 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Nikoozadeh, Amin and Choe, Jung Woo and Kothapalli, Sri-Rajasekhar and Moini, Azadeh and Sanjani, Sahinaz S. and Kamaya, Aya and Oralkan, Omer and Gambhir, Sanjiv S. and Khuri-Yakub, Pierre T.}, year={2012}, month={Oct} }
 @article{prieto_oralkan_khuri-yakub_maduke_2012, title={Ultrasound-Induced Currents in Planar Lipid Blayers: Origins and Potential Physiological Significance}, volume={102}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2011.11.212}, DOI={10.1016/j.bpj.2011.11.212}, abstractNote={Low-intensity focused ultrasound shows great promise for non-invasive, spatially resolved modulation of neural activity in vivo. To determine the mechanisms involved in ultrasonic modulation of neural activity and guide the development of this technology, we have been investigating the effects of ultrasound on protein-free planar lipid bilayers. Previously, we reported that ultrasound causes decaying current oscillations in planar bilayers at the onset and offset of the stimulus. These on and off responses are of opposite polarity but otherwise identical. Here, we report that if the rise time of the ultrasound pulse is prolonged, the on response is resolved into two distinct components: a sigmoidal component during the rise time and a damped oscillating component once the pulse reaches its final value. This result suggests that changes in ultrasound intensity during the rise time of the pulse may be important in determining the response to ultrasound in vivo, and is consistent with the observation that pulsed ultrasound is more effective than continuous ultrasound in modulating neural activity. To investigate further the origins of the on/off behavior, we used an optical interferometer to measure the velocity of the ultrasound-induced movement (acoustic streaming) in the solution surrounding the bilayer. We find that the time course of the ultrasound-induced current matches the time course of the streaming velocity, with a ratio of 162 pA/(mm/s). This acoustic streaming is probably due to the action of ultrasonic radiation force. To explore the potential physiological relevance of these effects, and to obtain further mechanistic insight, we are investigating the response of planar bilayers to ultrasound under current-clamp. In preliminary experiments, we find that ultrasound pulses with intensity comparable to those used in vivo produce voltage changes that would be sufficient to initiate an action potential.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Prieto, Martin L. and Oralkan, Omer and Khuri-Yakub, Butrus T. and Maduke, Merritt C.}, year={2012}, month={Jan}, pages={34a} }
 @article{choe_oralkan_nikoozadeh_gencel_stephens_o'donnell_sahn_khuri-yakub_2012, title={Volumetric real-time imaging using a CMUT ring array}, volume={59}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2012.2310}, DOI={10.1109/tuffc.2012.2310}, abstractNote={A ring array provides a very suitable geometry for forward-looking volumetric intracardiac and intravascular ultrasound imaging. We fabricated an annular 64-element capacitive micromachined ultrasonic transducer (CMUT) array featuring a 10-MHz operating frequency and a 1.27-mm outer radius. A custom software suite was developed to run on a PCbased imaging system for real-time imaging using this device. This paper presents simulated and experimental imaging results for the described CMUT ring array. Three different imaging methods-flash, classic phased array (CPA), and synthetic phased array (SPA)-were used in the study. For SPA imaging, two techniques to improve the image quality-Hadamard coding and aperture weighting-were also applied. The results show that SPA with Hadamard coding and aperture weighting is a good option for ring-array imaging. Compared with CPA, it achieves better image resolution and comparable signal-tonoise ratio at a much faster image acquisition rate. Using this method, a fast frame rate of up to 463 volumes per second is achievable if limited only by the ultrasound time of flight; with the described system we reconstructed three cross-sectional images in real-time at 10 frames per second, which was limited by the computation time in synthetic beamforming.}, number={6}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Choe, Jung Woo and Oralkan, Omer and Nikoozadeh, Amin and Gencel, Mustafa and Stephens, Douglas N. and O'Donnell, Matthew and Sahn, David J. and Khuri-Yakub, Butrus T.}, year={2012}, month={Jun}, pages={1201–1211} }
 @article{park_lee_kupnik_oralkan_ramseyer_lang_hegner_gerber_khuri-yakub_2011, title={Capacitive micromachined ultrasonic transducer (CMUT) as a chemical sensor for DMMP detection}, volume={160}, ISSN={0925-4005}, url={http://dx.doi.org/10.1016/j.snb.2011.09.036}, DOI={10.1016/j.snb.2011.09.036}, abstractNote={We present a chemical sensor based on a capacitive micromachined ultrasonic transducer (CMUT) configured as a resonant mass sensor with a chemically selective polymer coating. The sensing unit of the CMUT consists of 100s to 1000s of resonators connected in parallel and acts as a single resonator. The high resonant frequency (18.2 MHz) and the small mass (296 pg) enable the CMUT to have a good mass sensitivity of 130 zg/Hz/μm2. We functionalized the CMUT with polyisobutylene (PIB) as the sorbent film, which targets dimethyl methylphosphonate (DMMP), a simulant for nerve agent, sarin. An oscillator circuit was populated to trace the resonant frequency of the CMUT with a fast response time and low noise. We characterized the noise performance of the sensor system and identified the optimal gate time of the read-out frequency counter. Based on the noise measurement, the calculated limit of detection (LOD) of mass loading is 0.192 ag/μm2 (3-σ confidence level). Chemical experiments were performed on the CMUT sensor with several analytes including DMMP, water, and ethanol. The limit of detection of the CMUT chemical sensor to DMMP vapor was measured as 56 ppb (3-σ confidence level).}, number={1}, journal={Sensors and Actuators B: Chemical}, publisher={Elsevier BV}, author={Park, Kwan Kyu and Lee, Hyunjoo and Kupnik, Mario and Oralkan, Ömer and Ramseyer, Jean-Pierre and Lang, Hans Peter and Hegner, Martin and Gerber, Christoph and Khuri-Yakub, Butrus T.}, year={2011}, month={Dec}, pages={1120–1127} }
 @article{khuri-yakub_oralkan_2011, title={Capacitive micromachined ultrasonic transducers for medical imaging and therapy}, volume={21}, ISSN={0960-1317 1361-6439}, url={http://dx.doi.org/10.1088/0960-1317/21/5/054004}, DOI={10.1088/0960-1317/21/5/054004}, abstractNote={Capacitive micromachined ultrasonic transducers (CMUTs) have been subject to extensive research for the last two decades. Although they were initially developed for air-coupled applications, today their main application space is medical imaging and therapy. This paper first presents a brief description of CMUTs, their basic structure and operating principles. Our progression of developing several generations of fabrication processes is discussed with an emphasis on the advantages and disadvantages of each process. Monolithic and hybrid approaches for integrating CMUTs with supporting integrated circuits are surveyed. Several prototype transducer arrays with integrated front-end electronic circuits we developed and their use for 2D and 3D, anatomical and functional imaging, and ablative therapies are described. The presented results prove the CMUT as a micro-electro-mechanical systems technology for many medical diagnostic and therapeutic applications.}, number={5}, journal={Journal of Micromechanics and Microengineering}, publisher={IOP Publishing}, author={Khuri-Yakub, Butrus T and Oralkan, Ömer}, year={2011}, month={Apr}, pages={054004} }
 @article{lee_park_kupnik_oralkan_khuri-yakub_2011, title={Chemical Vapor Detection Using a Capacitive Micromachined Ultrasonic Transducer}, volume={83}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac201626b}, DOI={10.1021/ac201626b}, abstractNote={Distributed sensing of gas-phase chemicals using highly sensitive and inexpensive sensors is of great interest for many defense and consumer applications. In this paper we present ppb-level detection of dimethyl methylphosphonate (DMMP), a common simulant for sarin gas, with a ppt-level resolution using an improved capacitive micromachined ultrasonic transducer (CMUT) as a resonant chemical sensor. The improved CMUT operates at a higher resonant frequency of 47.7 MHz and offers an improved mass sensitivity of 48.8 zg/Hz/μm(2) by a factor of 2.7 compared to the previous CMUT sensors developed. A low-noise oscillator using the CMUT resonant sensor as the frequency-selective device was developed for real-time sensing, which exhibits an Allan deviation of 1.65 Hz (3σ) in the presence of a gas flow; this translates into a mass resolution of 80.5 zg/μm(2). The CMUT resonant sensor is functionalized with a 50-nm thick DKAP polymer developed at Sandia National Laboratory for dimethyl methylphosphonate (DMMP) detection. To demonstrate ppb-level detection of the improved chemical sensor system, the sensor performance was tested at a certified lab (MIT Lincoln Laboratory), which is equipped with an experimental chemical setup that reliably and accurately delivers a wide range of low concentrations down to 10 ppb. We report a high volume sensitivity of 34.5 ± 0.79 pptv/Hz to DMMP and a good selectivity of the polymer to DMMP with respect to dodecane and 1-octanol.}, number={24}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Lee, Hyunjoo J. and Park, Kwan Kyu and Kupnik, Mario and Oralkan, Ö. and Khuri-Yakub, Butrus T.}, year={2011}, month={Dec}, pages={9314–9320} }
 @inproceedings{park_oralkan_khuri-yakub_2011, title={Comparison of conventional and collapse-mode CMUT in 1-D array configuration}, ISBN={9781457712524 9781457712531 9781457712517}, url={http://dx.doi.org/10.1109/ultsym.2011.0245}, DOI={10.1109/ultsym.2011.0245}, abstractNote={This paper presents a comparison between two types of capacitive micromachined transducers (CMUTs), one operating in the conventional mode and the other one in the collapse-mode. Both types of devices are designed to have the same membrane properties (silicon, 1-μm thick), the same peak frequency (10 MHz) and similar operating bias voltages (100 V - 130 V). In the finite element analysis (FEA) of transmit pressure, both types of devices have similar fractional bandwidth (FBW) more than 120%, whereas, the collapse-mode CMUT has higher output pressure (46.5 kPa/V) than that of the conventional CMUT (13.1 kPa/V). To validate the FEA results, these two devices are fabricated using LOCOS/wafer-bonding process. The fabricated devices have a phased array configuration with an element pitch of 75 μm. The output pressure and the FBW of the conventional CMUT is 110% and 12.7 kPa/V, respectively, which match well with FEA results. The collapse-mode CMUT has a FBW of 102.5% and output pressure of 26.4 kPa/V. With a large AC excitation and high DC bias, the conventional CMUT has an output pressure of 1.25 MPap-p with 56.4-Vp-p AC and 180-V DC. The collapse-mode CMUT has an output pressure of 2.37 MPap-p with 56.2-Vp-p AC, and 150-V DC.}, booktitle={2011 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Park, K. K. and Oralkan, O. and Khuri-Yakub, B. T.}, year={2011}, month={Oct} }
 @inproceedings{cristman_oralkan_mandella_solgaard_contag_khuri-yakub_2011, title={Interdigitated annular CMUT arrays for ultrasound assisted delivery of fluorescent contrast agents}, ISBN={9781457712524 9781457712531 9781457712517}, url={http://dx.doi.org/10.1109/ultsym.2011.6293695}, DOI={10.1109/ultsym.2011.6293695}, abstractNote={Detection of early stage cancer is critical to successful treatment. Molecular imaging contrast agents offer great promise for early stage cancer detection. For improved image quality and diagnostics there is a need to improve the delivery, activation, and uptake of molecular imaging contrast agents. Low intensity focused ultrasound is one option to improve the delivery of molecular imaging agents. Here we investigate a circularly symmetric interdigitated capacitive micromachined ultrasonic transducer for improved delivery of fluorescent contrast agents to be imaged by a miniature dual axis confocal microscope. In order to apply ultrasound energy in the field of view of the optical microscope interface waves must be exploited. The device clearly focuses the ultrasound energy on the surface and at the center of the device. Overlapping the field of view of the optical microscope is required for improved in vivo imaging. Reasonable intensities of about 145 mW/cm2 have been demonstrated for enhanced delivery of the contrast agents.}, booktitle={2011 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Cristman, P. and Oralkan, O. and Mandella, M. and Solgaard, O. and Contag, C. and Khuri-Yakub, B. T.}, year={2011}, month={Oct} }
 @inbook{nikoozadeh_wygant_lin_oralkan_thomenius_dentinger_wildes_akopyan_shivkumar_mahajan_et al._2011, title={Intracardiac Forward-Looking Ultrasound Imaging Catheters Using Capacitive Micromachined Ultrasonic Transducers}, ISBN={9789048132546 9789048132553}, ISSN={0270-5117}, url={http://dx.doi.org/10.1007/978-90-481-3255-3_24}, DOI={10.1007/978-90-481-3255-3_24}, abstractNote={Atrial fibrillation is the most common sustained arrhythmia that now affects approximately 2.2 million adults in the United States alone. Minimally invasive catheter-based electrophysiological interventions have revolutionized the management of cardiac arrhythmias. We are developing forward-viewing ultrasound imaging catheters based on two types of transducer arrays using the capacitive micromachined ultrasonic transducer technology: A 10-MHz, 24-element MicroLinear (ML) array with a footprint of 1.7 mm × 1.3 mm, and a 10-MHz, 64-element annular ring array with an outside diameter of 2.6 mm and inner diameter of 1.6 mm. Both arrays are integrated with custom-designed front-end electronic circuitry to overcome the performance degradation associated with long cables in the catheter. The ML and ring arrays provide real-time 2-D and 3-D images, respectively, in front of the catheter tip. Using the ML array, we demonstrated ex-vivo images of the left atrial appendage in an isolated Langendorff-perfused rabbit heart model and in-vivo images of heart through the open chest in a porcine animal model. We used the ring array to demonstrate 3-D images of coronary stents and an anatomic cast of a left atrial model.}, booktitle={Acoustical Imaging}, publisher={Springer Netherlands}, author={Nikoozadeh, A. and Wygant, I.O. and Lin, D.-S. and Oralkan, Ö. and Thomenius, K. and Dentinger, A. and Wildes, D. and Akopyan, G. and Shivkumar, K. and Mahajan, A. and et al.}, year={2011}, pages={203–210} }
 @inproceedings{bhuyan_choe_lee_cristman_oralkan_khuri-yakub_2011, title={Miniaturized, wearable, ultrasound probe for on-demand ultrasound screening}, ISBN={9781457712524 9781457712531 9781457712517}, url={http://dx.doi.org/10.1109/ultsym.2011.0260}, DOI={10.1109/ultsym.2011.0260}, abstractNote={There are several applications in the medical field that require periodic monitoring of blood vessels or organ functions. Ultrasound, being one of the preferred imaging modalities, is often used for such applications. However, present day ultrasound probes are bulky and inconvenient. We present a low profile, wearable ultrasound probe that can be taped onto the patient's body for periodic or constant monitoring of organ functions. The small form factor is key for such applications. Therefore, CMUT technology is ideal for development of such probes. The probe consists of a 64-element 1D linear array CMUT operating at 5 MHz. Front-end electronics were integrated with the CMUT to improve the SNR of the acquired image. The final assembly measures 6 cm × 3.5 cm × 0.35 cm. A PDMS lens lay on top of the assembly to allow focusing in the elevation plane. The probe is interfaced to a backend system (Verasonics data acquisition system, Verasonics, Inc., Redmond, WA). Verasonics provides the high voltage pulses and also digitizes the received RF echo data for image reconstruction. Field characterization of the probe was performed using a calibrated hydrophone and was compared to Field II simulations. Finally, imaging experiments were performed on a commercially available phantom as well as the human neck. Images were also acquired with a commercially available probe for the sake of comparison. The CMUT probe provided with comparable image quality to that of the commercial probe. Real time images were also acquired by the CMUT probe at 30 frames per second.}, booktitle={2011 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Bhuyan, Anshuman and Choe, Jung Woo and Lee, Byung Chul and Cristman, Paul and Oralkan, Omer and Khuri-Yakub, Butrus T.}, year={2011}, month={Oct} }
 @inproceedings{choe_oralkan_nikoozadeh_bhuyan_lee_gence_khuri-yakub_2011, title={Real-time volumetric imaging system for CMUT arrays}, ISBN={9781457712524 9781457712531 9781457712517}, url={http://dx.doi.org/10.1109/ultsym.2011.0261}, DOI={10.1109/ultsym.2011.0261}, abstractNote={We designed and implemented a flexible real-time volumetric ultrasound imaging system for capacitive micromachined ultrasonic transducer (CMUT) arrays, consisting of an ultrasound data acquisition system, an FPGA board, and a host PC. The system works with arbitrary-shaped CMUT arrays and non-standard beamforming methods, as well as with regular-shaped CMUT arrays and conventional beamforming methods. In this paper, we present the system design and real-time imaging results obtained using this system with a ring array, a rectangular array, and a linear array. In synthetic phased array (SPA) imaging with a 64-element ring array, we could display 3 image planes with a total of about 70,000 pixels in real time, at a frame rate of 9 frames per second (fps) which was limited by the computational load on the CPU required for synthetic beamforming. On the other hand, the frame rate in classic phased array (CPA) imaging is limited by the data transfer time. In CPA imaging with a 16×16-element rectangular array, a frame rate of 5.4 fps was achieved for 1,250 acquisitions per frame and a 2.5-cm imaging depth. The frame rate can be increased by reducing the number of pixels processed in SPA, or by reducing the number of beams received in CPA, at the expense of degraded image quality or reduced field of view.}, booktitle={2011 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Choe, Jung Woo and Oralkan, Omer and Nikoozadeh, Amin and Bhuyan, Anshuman and Lee, Byung Chul and Gence, Mustafa and Khuri-Yakub, Butrus T.}, year={2011}, month={Oct} }
 @inproceedings{hyung seo_stephens_cannata_dentinger_lin_park_wildes_thomenius_chen_nguyen_et al._2011, title={Regulating energy delivery during intracardiac radiofrequency ablation using thermal strain imaging}, ISBN={9781457712531 9781457712524}, url={http://dx.doi.org/10.1109/ultsym.2011.0470}, DOI={10.1109/ultsym.2011.0470}, abstractNote={Tissue temperature is critically related to the success or failure of catheter ablation procedures. Temperature imaging using ultrasound techniques is attractive because of the potential to provide real-time information at low cost. The signal-processing methods used here were developed to investigate the feasibility of monitoring ablative therapy by identifying the point at which the slope of the thermal strain curve changes sign caused primarily by speed of sound variations with temperature. Previously, we have demonstrated the feasibility of this method in-vivo using porcine models. In this paper, we present recent results with temperature validation for this method in-vivo using an integrated intracardiac echocardiography (ICE) probe. Also preliminary results on thermal strain imaging using a cMUT array integrated into the ICE probe are presented.}, booktitle={2011 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Hyung Seo, Chi and Stephens, Douglas and Cannata, Jonathan and Dentinger, Aaron and Lin, Feng and Park, Suhyun and Wildes, Douglas and Thomenius, Kai and Chen, Peter and Nguyen, Tho and et al.}, year={2011}, month={Oct} }
 @article{seo_stephens_cannata_dentinger_lin_park_wildes_thomenius_chen_nguyen_et al._2011, title={The feasibility of using thermal strain imaging to regulate energy delivery during intracardiac radio-frequency ablation}, volume={58}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2011.1960}, DOI={10.1109/tuffc.2011.1960}, abstractNote={A method is introduced to monitor cardiac ablative therapy by examining slope changes in the thermal strain curve caused by speed of sound variations with temperature. The sound speed of water-bearing tissue such as cardiac muscle increases with temperature. However, at temperatures above about 50°C, there is no further increase in the sound speed and the temperature coefficient may become slightly negative. For ablation therapy, an irreversible injury to tissue and a complete heart block occurs in the range of 48 to 50°C for a short period in accordance with the well-known Arrhenius equation. Using these two properties, we propose a potential tool to detect the moment when tissue damage occurs by using the reduced slope in the thermal strain curve as a function of heating time. We have illustrated the feasibility of this method initially using porcine myocardium in vitro. The method was further demonstrated in vivo, using a specially equipped ablation tip and an 11-MHz microlinear intracardiac echocardiography (ICE) array mounted on the tip of a catheter. The thermal strain curves showed a plateau, strongly suggesting that the temperature reached at least 50°C.}, number={7}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Seo, Chi Hyung and Stephens, D. N. and Cannata, J. and Dentinger, A. and Lin, Feng and Park, Suhyun and Wildes, D. and Thomenius, K. E. and Chen, P. and Nguyen, Tho and et al.}, year={2011}, month={Jul}, pages={1406–1417} }
 @inproceedings{moini_nikoozadeh_oralkan_choe_sarioglu_stephens_de la rama_chen_chalek_dentinger_et al._2011, title={Volumetric intracardiac imaging using a fully integrated CMUT ring array: Recent developments}, ISBN={9781457712524 9781457712531 9781457712517}, url={http://dx.doi.org/10.1109/ultsym.2011.0168}, DOI={10.1109/ultsym.2011.0168}, abstractNote={Atrial fibrillation, the most common type of cardiac arrhythmia, now affects more than 2.2 million adults in the US alone. Currently, electrophysiological interventions are performed under fluoroscopy guidance, a procedure that introduces harmful ionizing radiation without providing adequate soft-tissue resolution. Intracardiac echocardiography (ICE) provides real-time, high-resolution anatomical information, reduces fluoroscopy time, and enhances procedural success. We have previously developed a forward-looking, volumetric ICE catheter using a ring-shaped, 64-element capacitive micromachined ultrasonic transducer (CMUT) array with a 10MHz center frequency. The Ring array was flip-chip bonded to a flexible PCB along with 8 identical custom ASICs providing a total of 64 dedicated preamplifiers. The flex was then reshaped for integration with the catheter shaft. In the second-generation catheter, 72 micro-coaxial cables (reduced from 100) are terminated on a newly designed flex to provide the connection between the array electronics and the imaging system. The reduced number of cables enhances the catheter's steerability. Furthermore, the new flex allows grounding of the top CMUT electrode through proper level-shifting of the ASIC supplies without additional circuitry. This feature enables complete ground shielding of the catheter, which improves its noise susceptibility and is an important safety measure for its clinical use. Beyond real-time, forward-looking imaging capability, the Ring catheter provides a continuous central lumen, enabling convenient delivery of other devices such as HIFU transducers, RF ablation catheters, etc. Using a PC-based imaging platform from Verasonics and a commercial Vivid7 imaging system from GE, we have demonstrated the in vivo, volumetric, real-time imaging capability of the finalized Ring catheter in a pig heart.}, booktitle={2011 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Moini, Azadeh and Nikoozadeh, Amin and Oralkan, Omer and Choe, Jung Woo and Sarioglu, A. Fatih and Stephens, Douglas N. and de la Rama, Alan and Chen, Peter and Chalek, Carl and Dentinger, Aaron and et al.}, year={2011}, month={Oct} }
 @inproceedings{ma_kothapalli_vaithilingam_oralkan_kamaya_wygant_zhuang_gambhir_jeffrey_khuri-yakub_et al._2010, title={3-D Deep penetration photoacoustic imaging with a 2-D CMUT array}, ISBN={9781457703829}, url={http://dx.doi.org/10.1109/ultsym.2010.5935647}, DOI={10.1109/ultsym.2010.5935647}, abstractNote={In this work, we demonstrate 3-D photoacoustic imaging of optically absorbing targets embedded as deep as 5 cm inside a highly scattering background medium using a 2-D capacitive micromachined ultrasonic transducer (CMUT) array with a center frequency of 5.5 MHz. 3-D volumetric images and 2-D maximum intensity projection images are presented to show the objects imaged at different depths. Due to the close proximity of the CMUT to the integrated frontend circuits, the CMUT array imaging system has a low noise floor. This makes the CMUT a promising technology for deep tissue photoacoustic imaging.}, booktitle={2010 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Ma, Te-Jen and Kothapalli, Sri Rajasekhar and Vaithilingam, Srikant and Oralkan, Omer and Kamaya, Aya and Wygant, Ira O. and Zhuang, Xuefeng and Gambhir, Sanjiv S. and Jeffrey, R. Brooke and Khuri-Yakub, Butrus T. and et al.}, year={2010}, month={Oct} }
 @inproceedings{choe_oralkan_khuri-yakub_2010, title={Design optimization for a 2-D sparse transducer array for 3-D ultrasound imaging}, ISBN={9781457703829}, url={http://dx.doi.org/10.1109/ultsym.2010.5935854}, DOI={10.1109/ultsym.2010.5935854}, abstractNote={In 3-D ultrasound imaging where 2-D transducer arrays with more than hundreds of elements are used, sparse arrays can be used to reduce the number of active ultrasound channels. Under a restriction of desired number of active channels, we can maximize the image quality by optimally choosing the positions of active elements. Here we use the method of simulated annealing to find the optimal configuration of a 2-D sparse array. This algorithm tries to minimize the value of an objective function defined as the energy ratio between the non-focal and focal regions in the point spread function (PSF). Optimal configurations were found for the cases of choosing 16, 20, 24, 28, and 32 transmit and receive elements from a 16×16-element rectangular transducer array. With only 32 transmit and 32 receive elements, we could achieve an energy ratio of 16%, compared to 6% of the full array, which is the gold standard utilizing all the 256 elements for both transmit and receive. Using Field II, we simulated imaging with the optimal sparse arrays, for off-axis targets as well as on-axis targets, and the resulting images were compared with those from some other configurations, such as full-transmit full-receive, full-transmit x-receive, x-transmit boundary-receive, and so on.}, booktitle={2010 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Choe, Jung Woo and Oralkan, Omer and Khuri-Yakub, Pierre T.}, year={2010}, month={Oct} }
 @inproceedings{nikoozadeh_oralkan_gencel_choe_stephens_de la rama_chen_lin_dentinger_wildes_et al._2010, title={Forward-looking intracardiac imaging catheters using fully integrated CMUT arrays}, ISBN={9781457703829}, url={http://dx.doi.org/10.1109/ultsym.2010.5935817}, DOI={10.1109/ultsym.2010.5935817}, abstractNote={Atrial fibrillation, the most common type of cardiac arrhythmia, now affects more than 2.2 million adults in the US alone. Currently, electrophysiological interventions are performed under fluoroscopy guidance, which besides its harmful ionizing radiation does not provide adequate soft-tissue resolution. Intracardiac echocardiography (ICE) provides realtime anatomical information that has proven valuable in reducing the fluoroscopy time and enhancing procedural success. We developed two types of forward-looking ICE catheters using capacitive micromachined ultrasonic transducer (CMUT) technology: MicroLinear (ML) and ring catheters. The ML catheter enables real-time forward-looking 2-D imaging using a 24-element 1-D CMUT phased-array that is designed for a center frequency of 10 MHz. The ring catheter uses a 64-element ring CMUT array that is also designed for a center frequency of 10 MHz. However, this ring-shaped 2-D array enables real-time forward-looking volumetric imaging. In addition, this catheter provides a continuous central lumen that enables convenient delivery of other devices such as RF ablation catheter, EP diagnostic catheter, biopsy devices, etc. Both catheters are equipped with custom front-end IC's that are integrated with the CMUT arrays at the tip of the catheters. The integration of the IC's with the CMUT arrays was accomplished using custom flexible PCB's. We also developed several image reconstruction schemes for the ring catheter on a PC-based imaging platform from VeraSonics. We performed a variety of bench-top characterizations to validate the functionality and performance of our fully integrated CMUT arrays. Using both catheters, we demonstrated in vivo images of the heart in a porcine animal model. We have successfully prototyped the first CMUT-based ICE catheters and proven the capabilities of the CMUT technology for implementing high-frequency miniature transducer arrays with integrated electronics.}, booktitle={2010 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Nikoozadeh, Amin and Oralkan, Omer and Gencel, Mustafa and Choe, Jung Woo and Stephens, Douglas N. and de la Rama, Alan and Chen, Peter and Lin, Feng and Dentinger, Aaron and Wildes, Douglas and et al.}, year={2010}, month={Oct} }
 @inproceedings{lee_park_kupnik_oralkan_khuri-yakub_2010, title={Highly sensitive detection of DMMP using a CMUT-based chemical sensor}, ISBN={9781424481705}, url={http://dx.doi.org/10.1109/icsens.2010.5690493}, DOI={10.1109/icsens.2010.5690493}, abstractNote={We present ppt-level detection of dimethyl methylphosphonate (DMMP), a common simulant used in detector calibrations for sarin gas, using a new capacitive micromachined ultrasonic transducer (CMUT) design with a mass sensitivity improved from 130 zg/Hz/µm2 to 48.8 zg/Hz/µm2. A low-noise oscillator using the CMUT as the frequency selective component exhibits an Allan deviation of 0.55 Hz at the presence of air flow. The CMUT resonant sensor was functionalized with a 50-nm thick proprietary polymer layer (Sandia National Laboratory, Albuquerque, NM). Our sensor performance was reliably measured at the MIT Lincoln Laboratory where the accurate delivery of low concentrations of gases was ensured through stringent calibrations. Our sensor response to various concentrations of DMMP in air (10 ppb-1 ppm) showed an excellent volume sensitivity of 30.6 pptv/Hz. Based on the system noise floor, we achieved a mass resolution of 26.9 zg/µm2 and a limit of detection of 16.8 pptv. In addition, the sensor showed good selectivity to DMMP.}, booktitle={2010 IEEE Sensors}, publisher={IEEE}, author={Lee, H J and Park, K K and Kupnik, M and Oralkan, O and Khuri-Yakub, B T}, year={2010}, month={Nov} }
 @inproceedings{khuri-yakub_oralkan ömer_nikoozadeh_wygant_zhuang_gencel_choe_stephens_de la rama_chen_et al._2010, title={Miniaturized ultrasound imaging probes enabled by CMUT arrays with integrated frontend electronic circuits}, ISBN={9781424441235 9781424441242}, url={http://dx.doi.org/10.1109/iembs.2010.5627580}, DOI={10.1109/iembs.2010.5627580}, abstractNote={Capacitive micromachined ultrasonic transducer (CMUT) arrays are conveniently integrated with frontend integrated circuits either monolithically or in a hybrid multichip form. This integration helps with reducing the number of active data processing channels for 2D arrays. This approach also preserves the signal integrity for arrays with small elements. Therefore CMUT arrays integrated with electronic circuits are most suitable to implement miniaturized probes required for many intravascular, intracardiac, and endoscopic applications. This paper presents examples of miniaturized CMUT probes utilizing 1D, 2D, and ring arrays with integrated electronics.}, booktitle={2010 Annual International Conference of the IEEE Engineering in Medicine and Biology}, publisher={IEEE}, author={Khuri-Yakub, B T and Oralkan Ömer and Nikoozadeh, A and Wygant, I O and Zhuang, S and Gencel, M and Choe, Jung Woo and Stephens, D N and de la Rama, A and Chen, P and et al.}, year={2010}, month={Aug} }
 @inproceedings{seo_stephens_cannata_dentinger_lin_park_wildes_thomenius_chen_nguyen_et al._2010, title={Monitoring radiofrequency catheter ablation using thermal strain imaging}, ISBN={9781457703829}, url={http://dx.doi.org/10.1109/ultsym.2010.5935567}, DOI={10.1109/ultsym.2010.5935567}, abstractNote={A method to monitor ablative therapy by examining slope changes in the thermal strain curve caused by speed of sound with temperature is introduced. The variation of sound speed with temperature rise for most soft tissue follows a similar pattern to that of water. Unlike most liquids, the sound speed of tissue increases with temperature. However, at temperatures above about 50 °C, there is no further increase in the sound speed and the temperature coefficient may become slightly negative. For ablation therapy, an irreversible injury to tissue and a complete heart block occurs in the range of 48–50 °C for a short period in accordance with the well known Arrhenius equation. Using these two properties, we propose a potential tool to detect the moment when tissue damage occurs using the reduced slope in the thermal strain curve as a function of heating time. Using a prototype intracardiac echocardiography (ICE) array for imaging and a catheter for RF ablation, we were able to observe an obvious slope change in the thermal strain curve in an excised tissue sample. The method was further tested in-vivo, using a specially equipped ablation tip and an 11 MHz microlinear (ML) ICE array mounted on the tip of a catheter. As with in-vitro experiments, the thermal strain curve showed a plateau and a change in the sign of the slope.}, booktitle={2010 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Seo, Chi Hyung and Stephens, Douglas and Cannata, Jonathan and Dentinger, Aaron and Lin, Feng and Park, Suhyun and Wildes, Douglas and Thomenius, Kai and Chen, Peter and Nguyen, Tho and et al.}, year={2010}, month={Oct} }
 @inproceedings{lin_zhuang_wodnicki_woychik_kupnik_oralkan_khuri-yakub_2010, title={Packaging of large and low-pitch size 2D ultrasonic transducer arrays}, ISBN={9781424457618 9781424457649}, url={http://dx.doi.org/10.1109/memsys.2010.5442455}, DOI={10.1109/memsys.2010.5442455}, abstractNote={The successful packaging and electronics integration of large 2D array devices with small pitch-sizes, such as fully populated 2D ultrasonic transducer arrays, require a flexible, simple, and reliable integration approach. One example for such electronics integration is based on through silicon vias (TSVs) with under-bump metallization (UBM) stack for solder bumping. In this paper, we demonstrate such an approach by successfully integrating a fully populated 2D ultrasonic transducer array. Our integration is based on a previously reported TSV technology (trench-frame technology), based on trench-isolated interconnects with supporting frame. We successfully combined the trench-frame technology with a simple UBM preparation technique - electro plating or chemical plating techniques with passivation layers for UBM pad definition are not required. Our results show high shear strength (26.5 g) of the UBM, which is essential for successful flip-chip bonding. The yield of the interconnections is 100% with excellent solder-ball-height uniformity (¿ = 0.9 ¿m). As demonstrated in this paper, this allows for a large-scale assembly of a tiled array by using an interposer. A design guideline for finer element-pitch design was developed suggesting that fusion bonding strength and the length of pillars are the main design parameters.}, booktitle={Proceedings of the 2010 IEEE 23rd International Conference on Micro Electro Mechanical Systems (MEMS)}, publisher={IEEE}, author={Lin, Der-Song and Zhuang, Xuefeng and Wodnicki, Robert and Woychik, Charles. G. and Kupnik, Mario and Oralkan, Ö and Khuri-Yakub, Butrus T.}, year={2010}, month={Jan}, pages={508–511} }
 @inproceedings{baek_oralkan_kupnik_willatzen_khuri-yakub_jensen_2010, title={Simulating capacitive micromachined ultrasonic transducers (CMUTs) using field II}, ISBN={9781457703829 9781457703812}, url={http://dx.doi.org/10.1109/ultsym.2010.5935580}, DOI={10.1109/ultsym.2010.5935580}, abstractNote={Field II has been a recognized simulation tool for piezoceramic medical transducer arrays for more than a decade. The program has its strength in doing fast computations of the spatial impulse response (SIR) from array elements by dividing the elements into smaller mathematical elements (ME)s from which it calculates the SIR responses. The program features predefined models for classical transducer geometries, but currently none for the fast advancing CMUTs. This work addresses the assumptions required for modeling CMUTs with Field II. It is shown that rectangular array elements, populated with cells, can be well approximated by neglecting the cells. Further, it is demonstrated that scaling of the SIR translates into better computational efficiency.}, booktitle={2010 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Baek, David and Oralkan, Omer and Kupnik, Mario and Willatzen, Morten and Khuri-Yakub, Butrus T. and Jensen, Jorgen Arendt}, year={2010}, month={Oct} }
 @inproceedings{stephens_cannata_seo_jeong_sun_cao_nikoozadeh_oralkan_de la rama_nguyen_et al._2010, title={Ultrasound compatible RF ablation electrode design for catheter based guidance of RF ablation — In vivo results with thermal strain imaging}, ISBN={9781457703812 9781457703829 9781457703805}, url={http://dx.doi.org/10.1109/ultsym.2010.5935664}, DOI={10.1109/ultsym.2010.5935664}, abstractNote={Currently the feedback guidance of intracardiac radiofrequency ablation (RFA) is very limited, offering only a catheter electrode (not tissue) temperature estimation and a means to titrate radiofrequency (RF) power delivery to the tissue. Our "MicroLinear" (ML) forward imaging ultrasound catheter design, now at a true 9F (3mm) in size, has been optimized with several features to simultaneously permit, a) high quality intracardiac steering and imaging, b) tracking of 3D position with electroanatomical mapping, c) RF ablation, and d) tissue thermal strain (TS) estimation for direct tissue temperature feedback. Two types of ML catheters have been built and tested in 3 porcine animal models. The first type, in its third generation, is based on a PZT transducer array; the second type, in its second generation, is based on a CMUT array with custom integrated interface circuitry. Both types of devices are true 9F in size and performed well in imaging tests in recent in vivo studies. Both the ML-PZT and ML-CMUT arrays, as described previously, have a fine pitch (65 and 63 micron respectively) 24 element phased arrays operating at 14 MHz which project a B-mode plane directly out from the tip of the catheter. Intracardiac imaging performance was documented to show that the very small array apertures of the ML design (1.2mm × 1.58mm, and 1.1mm × 1.4mm) permit good, high resolution imaging to depths as great as 4 cm. The ML-PZT catheter was equipped with a special low profile ablation tip which allowed simultaneous imaging and ablation at the distal end of the catheter. TS data were acquired during tissue ablations in right atrium (RA) and right ventricle (RV). The TS data of the RF ablations were processed off line. In vivo use of this new technology has shown for the first time the very substantial potential for a single, low profile catheter to simultaneously image within the heart and perform intracardiac ablation therapy with tissue temperature guidance produced from the incorporation of TS imaging. Work is underway to further assess the temperature estimation accuracy and to integrate the TS processing for real time displays.}, booktitle={2010 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Stephens, Douglas N. and Cannata, Jonathan and Seo, Chi Hyung and Jeong, Jong-Seob and Sun, Enwei and Cao, Wenwu and Nikoozadeh, Amin and Oralkan, Omer and de la Rama, Alan and Nguyen, Tho and et al.}, year={2010}, month={Oct} }
 @inproceedings{park_kupnik_lee_oralkan_khuri-yakub_2010, title={Zero-bias resonant sensor with an oxide-nitride layer as charge trap}, ISBN={9781424481705}, url={http://dx.doi.org/10.1109/icsens.2010.5690742}, DOI={10.1109/icsens.2010.5690742}, abstractNote={We report on a capacitive resonant sensor with an oxide-nitride (ON) layer used as charge trap. The main idea is that we intentionally inject charges into the ON layer by biasing the device for 30 s with 160% of the pull-in voltage. We use a capacitive micromachined ultrasonic transducer (CMUT) to demonstrate this idea. The CMUT is fabricated via high temperature assisted direct wafer bonding after a local oxidation of silicon (LOCOS) to form evacuated cavities (vacuum gaps), and, thus, the device inherently has the ON layer beneath single-crystal silicon plates and vacuum gaps. Therefore, this device is ideal for this work. It allows us to test an elegant charge injection mechanism. By simply pulling in the plate a high electric field strength (∼ 8.9 MV/cm) is created in the ON layer for a designated time, which results in charge injection. These charges stay trapped in the ON layer and create an intrinsic electric field in the vacuum gaps, which would otherwise require an external dc bias voltage of 44% of the pull-in voltage. We successfully implemented a 5.3-MHz oscillator with this zero-bias resonator and achieved excellent noise performance of 0.06 Hz of Allan deviation.}, booktitle={2010 IEEE Sensors}, publisher={IEEE}, author={Park, Kwan Kyu and Kupnik, Mario and Lee, Hyunjoo J and Oralkan, O and Khuri-Yakub, Butrus T}, year={2010}, month={Nov} }
 @inproceedings{cristman_oralkan_zhuang_ma_vaithilingam_carver_wygant_khuri-yakub_2009, title={A 2D CMUT hydrophone array: Characterization results}, ISBN={9781424443895}, url={http://dx.doi.org/10.1109/ultsym.2009.5441543}, DOI={10.1109/ultsym.2009.5441543}, abstractNote={As the use of ultrasonic transducers has increased so has the need to measure and characterize their pressure fields. Currently available hydrophones are limited by long scan times, and multiple source pulses to measure large area fields. Using a 2D capacitive micro-machined ultrasonic transducer (CMUT) array and associated electronics we are able to improve on these limitations. The CMUT inherently allows for a tunable sensitivity by changing DC bias. Motivated by the need to improve the characterization of megasonic cleaners used in semiconductor processing we were able to implement such an area hydrophone.}, booktitle={2009 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Cristman, P. and Oralkan, O. and Zhuang, X. and Ma, T.-J. and Vaithilingam, S. and Carver, T. and Wygant, I. and Khuri-Yakub, B. T.}, year={2009}, month={Sep} }
 @inproceedings{lee_park_cristman_oralkan_kupnik_khuri-yakub_2009, title={A Low-Noise Oscillator based on a Multi-Membrane CMUT for High Sensitivity Resonant Chemical Sensors}, ISBN={9781424429776}, url={http://dx.doi.org/10.1109/memsys.2009.4805494}, DOI={10.1109/memsys.2009.4805494}, abstractNote={We present 17.5-MHz and 42.7-MHz low-noise Colpitts oscillators employing capacitive micromachined ultrasonic transducers (CMUTs), each composed of a thousand resonator cells electrically connected in parallel. The massive parallelism lowers the motional impedance, and thus, reduces frequency noise and provides better matching to low-noise oscillator topologies. The 42.7-MHz oscillator achieved a phase noise of -105 dBc/Hz and -148 dBc/Hz at offset frequencies of 1 kHz and 1 MHz, respectively in air. The performance is comparable to MEMS oscillators based on resonators with high Q in vacuum. The lowest Allan deviation of the oscillator was measured to be 4.7 × 10-9 implying a mass resolution of 0.96 attogram per membrane. In addition, using the 17.5-MHz CMUT resonator, the performance of the Colpitts topology is compared to that of the amplifier based oscillator topology.}, booktitle={2009 IEEE 22nd International Conference on Micro Electro Mechanical Systems}, publisher={IEEE}, author={Lee, H. J. and Park, K. K. and Cristman, P. and Oralkan, O. and Kupnik, M. and Khuri-Yakub, B. T.}, year={2009}, month={Jan} }
 @inproceedings{sahn_stephens_cannata_kirk shung_oralkan_nikoozadeh_khuri-yakub_nguyen_chen_dentinger_et al._2009, title={A family of intracardiac ultrasound imaging devices designed for guidance of electrophysiology ablation procedures}, ISBN={9781424432967}, url={http://dx.doi.org/10.1109/iembs.2009.5332380}, DOI={10.1109/iembs.2009.5332380}, abstractNote={Our Bioengineering Research Partnership grant, ldquoHigh Frequency Ultrasound Arrays for Cardiac Imagingrdquo, including the individuals cited at the end of this paper - Douglas N. Stephens (UC Davis), Matthew O'Donnell (UW Seattle), Kai Thomenius (GE Global Research), Aaron M. Dentinger (GE Global Research), Douglas Wildes (GE Global Research), Peter Chen (St. Jude Medical), K. Kirk Shung (University of Southern California), Jonathan M. Cannata (University of Southern California), Butrus (Pierre) T. Khuri-Yakub (Stanford University), Omer Oralkan (Stanford University), Aman Mahajan (UCLA School of Medicine), Kalyanam Shivkumar (UCLA School of Medicine) and David J. Sahn (Oregon Health & Science University) - is in its sixth year of NIH funding, having proposed to develop a family of high frequency miniaturized forward and side-looking ultrasound imaging devices equipped with electrophysiology mapping and localization sensors and eventually to include a family of capacitive micromachined ultrasonic transducer (cMUT) devices - a forward-looking cMUT MicroLinear array and a ring array capable of 3-dimensional imaging and a 5Fr lumen large enough to admit an electrode and ablation devices.}, booktitle={2009 Annual International Conference of the IEEE Engineering in Medicine and Biology Society}, publisher={IEEE}, author={Sahn, D.J. and Stephens, D.N. and Cannata, J.M. and Kirk Shung, K. and Oralkan, O. and Nikoozadeh, A. and Khuri-Yakub, B.T. and Nguyen, H. and Chen, P. and Dentinger, A.M. and et al.}, year={2009}, month={Sep} }
 @article{wygant_jamal_lee_nikoozadeh_oralkan_karaman_khuri-yakub_2009, title={An integrated circuit with transmit beamforming flip-chip bonded to a 2-D CMUT array for 3-D ultrasound imaging}, volume={56}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2009.1297}, DOI={10.1109/tuffc.2009.1297}, abstractNote={State-of-the-art 3-D medical ultrasound imaging requires transmitting and receiving ultrasound using a 2-D array of ultrasound transducers with hundreds or thousands of elements. A tight combination of the transducer array with integrated circuitry eliminates bulky cables connecting the elements of the transducer array to a separate system of electronics. Furthermore, preamplifiers located close to the array can lead to improved receive sensitivity. A combined IC and transducer array can lead to a portable, high-performance, and inexpensive 3-D ultrasound imaging system. This paper presents an IC flip-chip bonded to a 16times16-element capacitive micromachined ultrasonic transducer (CMUT) array for 3-D ultrasound imaging. The IC includes a transmit beamformer that generates 25-V unipolar pulses with programmable focusing delays to 224 of the 256 transducer elements. One-shot circuits allow adjustment of the pulse widths for different ultrasound transducer center frequencies. For receiving reflected ultrasound signals, the IC uses the 32-elements along the array diagonals. The IC provides each receiving element with a low-noise 25-MHz-bandwidth transimpedance amplifier. Using a field-programmable gate array (FPGA) clocked at 100 MHz to operate the IC, the IC generated properly timed transmit pulses with 5-ns accuracy. With the IC flip-chip bonded to a CMUT array, we show that the IC can produce steered and focused ultrasound beams. We present 2-D and 3-D images of a wire phantom and 2-D orthogonal cross-sectional images (Bscans) of a latex heart phantom.}, number={10}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Wygant, I. and Jamal, N. and Lee, H. and Nikoozadeh, A. and Oralkan, O. and Karaman, M. and Khuri-yakub, B.}, year={2009}, month={Oct}, pages={2145–2156} }
 @inproceedings{woychik_wodnicki_fritzsch_ehrmann_jordan_glaw_fisher_thomenius_zhuang_oralkan_et al._2009, title={Challenges of solder bumping capacitive micro-machined ultrasonic transducer (cMUT) trenched devices}, booktitle={Proceedings of SMTA International Conference & Exhibition}, author={Woychik, CG and Wodnicki, R and Fritzsch, T and Ehrmann, O and Jordan, R and Glaw, V and Fisher, R and Thomenius, K and Zhuang, X and Oralkan, Ö and et al.}, year={2009}, pages={1–11} }
 @inproceedings{de la zerda_liu_zavaleta_bodapati_teed_vaithilingam_ma_oralkan_chen_khuri-yakub_et al._2009, title={Enhanced sensitivity carbon nanotubes as targeted photoacoustic molecular imaging agents}, url={http://dx.doi.org/10.1117/12.809601}, DOI={10.1117/12.809601}, abstractNote={Photoacoustic imaging of living subjects offers high spatial resolution at increased tissue depths compared to purely optical imaging techniques. We have recently shown that intravenously injected single walled carbon nanotubes (SWNTs) can be used as targeted photoacoustic imaging agents in living mice using RGD peptides to target αvβ3 integrins. We have now developed a new targeted photoacoustic imaging agent based on SWNTs and Indocyanine Green (SWNT-ICG) with absorption peak at 780nm. The photoacoustic signal of the new imaging agent is enhanced by ~20 times as compared to plain SWNTs. The particles are synthesized from SWNT-RGD that noncovalently attach to multiple ICG molecules through pi-pi stacking interactions. Negative control particles had RAD peptide instead of RGD. We measured the serum stability of the particles and verified that the RGD/RAD conjugation did not alter the particle's absorbance spectrum. Finally, through cell uptake studies with U87MG cells we verified that the particles bind selectively to αvβ3 integrin. In conclusion, the extremely high absorption of the SWNT-ICG particles shows great promise for high sensitivity photoacoustic imaging of molecular targets in-vivo. This work lays the foundations for future in-vivo studies that will use the SWNT-ICG particles as imaging agents administered systemically.}, booktitle={Photons Plus Ultrasound: Imaging and Sensing 2009}, publisher={SPIE}, author={de la Zerda, Adam and Liu, Zhuang and Zavaleta, Cristina and Bodapati, Sunil and Teed, Robert and Vaithilingam, Srikant and Ma, Te-Jen and Oralkan, Omer and Chen, Xiaoyuan and Khuri-Yakub, Butrus T. and et al.}, editor={Oraevsky, Alexander A. and Wang, Lihong V.Editors}, year={2009}, month={Feb} }
 @article{stephens_o'donnell_thomenius_dentinger_wildes_chen_shung_cannata_khuri-yakub_oralkan_et al._2009, title={Experimental Studies With a 9F Forward-Looking Intracardiac Imaging and Ablation Catheter}, volume={28}, ISSN={0278-4297}, url={http://dx.doi.org/10.7863/jum.2009.28.2.207}, DOI={10.7863/jum.2009.28.2.207}, abstractNote={Objective. The purpose of this study was to develop a high‐resolution, near‐field‐optimized 14‐MHz, 24‐element broad‐bandwidth forward‐looking array for integration on a steerable 9F electrophysiology (EP) catheter. Methods. Several generations of prototype imaging catheters with bidirectional steering, termed microlinear (ML), were built and tested as integrated catheter designs with EP sensing electrodes near the tip. The wide‐bandwidth ultrasound array was mounted on the very tip, equipped with an aperture of only 1.2 by 1.58 mm. The array pulse echo performance was fully simulated, and its construction offered shielding from ablation noise. Both ex vivo and in vivo imaging with a porcine animal model were performed. Results. The array pulse echo performance was concordant with Krimholtz‐Leedom‐Matthaei model simulation. Three generations of prototype devices were tested in the right atrium and ventricle in 4 acute pig studies for the following characteristics: (1) image quality, (2) anatomic identification, (3) visualization of other catheter devices, and (4) for a mechanism for stabilization when imaging ablation. The ML catheter is capable of both low‐artifact ablation imaging on a standard clinical imaging system and high–frame rate myocardial wall strain rate imaging for detecting changes in cardiac mechanics associated with ablation. Conclusions. The imaging resolution performance of this very small array device, together with its penetration beyond 2 cm, is excellent considering its very small array aperture. The forward‐looking intracardiac catheter has been adapted to work easily on an existing commercial imaging platform with very minor software modifications.}, number={2}, journal={Journal of Ultrasound in Medicine}, publisher={Wiley}, author={Stephens, Douglas N. and O'Donnell, Matthew and Thomenius, Kai and Dentinger, Aaron and Wildes, Douglas and Chen, Peter and Shung, K. Kirk and Cannata, Jonathan and Khuri-Yakub, Pierre and Oralkan, Omer and et al.}, year={2009}, month={Feb}, pages={207–215} }
 @inproceedings{nikoozadeh_oralkan_gencel_choe_stephens_de la rama_chen_thomenius_dentinger_wildes_et al._2009, title={Forward-looking volumetric intracardiac imaging using a fully integrated CMUT ring array}, ISBN={9781424443895}, url={http://dx.doi.org/10.1109/ultsym.2009.5441600}, DOI={10.1109/ultsym.2009.5441600}, abstractNote={Atrial fibrillation is the most common type of cardiac arrhythmia that now affects over 2.2 million adults in the United States alone. Currently fluoroscopy is the most common method for guiding interventional electrophysiological procedures. We are developing a 9-F forward-looking intracardiac ultrasound catheter for real-time volumetric imaging. We designed and fabricated a 64-element 10-MHz CMUT ring array with through-wafer via interconnects. We also designed custom front-end electronics to be closely integrated with the CMUT array at the tip of the catheter for improved SNR. This integrated circuit (IC) is composed of preamplifiers and protection circuitry, and can directly interface a standard imaging system. This multi-channel IC is capable of passing up to ±50-V bipolar pulses. An 8-channel front-end IC was fabricated based on this circuit topology. Additionally, a flexible PCB was designed for the integration of ring array with front-end electronics. We have acquired a PC-based real-time imaging platform and demonstrated real-time imaging with the ring array. We have also shown volume images using off-line full synthetic aperture image reconstruction method. The presented experimental results demonstrate the performance of our forward-looking volumetric intracardiac imaging approach. We are currently working on the final catheter integration and further development of our real-time imaging methods.}, booktitle={2009 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Nikoozadeh, Amin and Oralkan, Omer and Gencel, Mustafa and Choe, Jung Woo and Stephens, Douglas N. and de la Rama, Alan and Chen, Peter and Thomenius, Kai and Dentinger, Aaron and Wildes, Douglas and et al.}, year={2009}, month={Sep} }
 @article{karaman_wygant_oralkan_khuri-yakub_2009, title={Minimally Redundant 2-D Array Designs for 3-D Medical Ultrasound Imaging}, volume={28}, ISSN={0278-0062 1558-254X}, url={http://dx.doi.org/10.1109/tmi.2008.2010936}, DOI={10.1109/tmi.2008.2010936}, abstractNote={In real-time ultrasonic 3-D imaging, in addition to difficulties in fabricating and interconnecting 2-D transducer arrays with hundreds of elements, there are also challenges in acquiring and processing data from a large number of ultrasound channels. The coarray (spatial convolution of the transmit and receive arrays) can be used to find efficient array designs that capture all of the spatial frequency content (a transmit-receive element combination corresponds to a spatial frequency) with a reduced number of active channels and firing events. Eliminating the redundancies in the transmit-receive element combinations and firing events reduces the overall system complexity and improves the frame rate. Here we explore four reduced redundancy 2-D array configurations for miniature 3-D ultrasonic imaging systems. Our approach is based on 1) coarray design with reduced redundancy using different subsets of linear arrays constituting the 2-D transducer array, and 2) 3-D scanning using fan-beams (narrow in one dimension and broad in the other dimension) generated by the transmit linear arrays. We form the overall array response through coherent summation of the individual responses of each transmit-receive array pairs. We present theoretical and simulated point spread functions of the array configurations along with quantitative comparison in terms of the front-end complexity and image quality.}, number={7}, journal={IEEE Transactions on Medical Imaging}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Karaman, M. and Wygant, I.O. and Oralkan, O. and Khuri-Yakub, B.T.}, year={2009}, month={Jul}, pages={1051–1061} }
 @inproceedings{wodnicki_woychik_byun_fisher_thomenius_lin_zhuang_oralkan_vaithilingam_khuri-yakub_et al._2009, title={Multi-row linear cMUT array using cMUTs and multiplexing electronics}, ISBN={9781424443895}, url={http://dx.doi.org/10.1109/ultsym.2009.5442074}, DOI={10.1109/ultsym.2009.5442074}, abstractNote={A large area reconfigurable imaging array for research purposes is being developed with co-integrated cMUTs and control electronics. The goal is a 2.5cm 2D tileable module with ≫16,000 transducer sub-elements spaced at a pitch of 185um in X and Y dimensions. As a prototype demonstration of some of the goals of this effort, a multi-row linear array using cMUTs and external multiplexing electronics was designed and fabricated. In this paper the challenges of trenched cMUT attach to a laminate interposer as part of a tileable module will be discussed. The architecture of the tileable module build-up for manufacturability, reliability, acoustic planarity, and reduced spacing between tiles and cMUT chips will also be addressed. Finally, a first prototype will be shown and experimental acoustic results with the new cMUT-based probe will be presented.}, booktitle={2009 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Wodnicki, Robert and Woychik, Charles G. and Byun, Albert T. and Fisher, Rayette and Thomenius, Kai and Lin, Der-Song and Zhuang, Xuefeng and Oralkan, Omer and Vaithilingam, Srikant and Khuri-Yakub, Butrus T. and et al.}, year={2009}, month={Sep} }
 @article{khuri-yakub_oralkan_kupnik_2009, title={Next-gen ultrasound}, volume={46}, ISSN={0018-9235}, url={http://dx.doi.org/10.1109/mspec.2009.4907385}, DOI={10.1109/mspec.2009.4907385}, abstractNote={Almost invariably, a new baby's photo album begins with a grainy black-and-white picture taken months before birth - a prenatal ultrasound image, which is often detailed enough to inspire comments about the child's resemblance to various members of the family. But jokes about balding uncles notwithstanding, such scans serve a serious purpose and can prove immensely important, as when they allow doctors to diagnose and sometimes even repair a congenital malformation while the baby is still in the womb.}, number={5}, journal={IEEE Spectrum}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Khuri-yakub, Butrus and Oralkan, Omer and Kupnik, Mario}, year={2009}, month={May}, pages={44–54} }
 @inproceedings{de la zerda_zavaleta_keren_vaithilingam_bodapati_teed_liu_levi_smith_ma_et al._2009, title={Photoacoustic molecular imaging using single walled carbon nanotubes in living mice}, url={http://dx.doi.org/10.1117/12.806497}, DOI={10.1117/12.806497}, abstractNote={Photoacoustic molecular imaging is an emerging technology offering non-invasive high resolution imaging of the molecular expressions of a disease using a photoacoustic imaging agent. Here we demonstrate for the first time the utility of single walled carbon nanotubes (SWNTs) as targeted imaging agents in living mice bearing tumor xenografts. SWNTs were conjugated with polyethylene-glycol-5000 connected to Arg-Gly-Asp (RGD) peptide to target the αvβ3 integrin that is associated with tumor angiogenesis. In-vitro, we characterized the photoacoustic spectra of the particles, their signal linearity and tested their uptake by αvβ3-expressing cells (U87MG). The photoacoustic signal of SWNTs was found not to be affected by the RGD conjugation to the SWNTs and was also found to be highly linear with concentration (R2 = 0.9997 for 25-400nM). The cell uptake studies showed that RGD-targeted SWNTs gave 75% higher photoacoustic signal than non-targeted SWNTs when incubated with U87MG cells. In-vivo, we measured the minimal detectable concentration of SWNTs in living mice by subcutaneously injecting SWNTs at increasing concentrations. The lowest detectable concentration of SWNTs in living mice was found to be 50nM. Finally, we administered RGDtargeted and non-targeted SWNTs via the tail-vein to U87MG tumor-bearing mice (n=4 for each group) and measured the signal from the tumor before and up to 4 hours post-injection. At 4 hours post-injection, tumors of mice injected with RGD-targeted SWNTs showed 8 times higher photoacoustic signal compared with mice injected with non-targeted SWNTs. These results were verified ex-vivo using a Raman microscope that is sensitive to the SWNTs Raman signal.}, booktitle={Photons Plus Ultrasound: Imaging and Sensing 2009}, publisher={SPIE}, author={de la Zerda, Adam and Zavaleta, Cristina and Keren, Shay and Vaithilingam, Srikant and Bodapati, Sunil and Teed, Robert and Liu, Zhuang and Levi, Jelena and Smith, Bryan R. and Ma, Te-Jen and et al.}, editor={Oraevsky, Alexander A. and Wang, Lihong V.Editors}, year={2009}, month={Feb} }
 @article{vaithilingam_ma_furukawa_wygant_zhuang_de la zerda_oralkan_kamaya_gambhir_jeffrey_et al._2009, title={Three-dimensional photoacoustic imaging using a two-dimensional CMUT array}, volume={56}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2009.1329}, DOI={10.1109/tuffc.2009.1329}, abstractNote={In this paper, we describe using a 2-D array of capacitive micromachined ultrasonic transducers (CMUTs) to perform 3-D photoacoustic and acoustic imaging. A tunable optical parametric oscillator laser system that generates nanosecond laser pulses was used to induce the photoacoustic signals. To demonstrate the feasibility of the system, 2 different phantoms were imaged. The first phantom consisted of alternating black and transparent fishing lines of 180 μm and 150 μm diameter, respectively. The second phantom comprised polyethylene tubes, embedded in chicken breast tissue, filled with liquids such as the dye indocyanine green, pig blood, and a mixture of the 2. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.8 cm from the CMUT array. Two-dimensional cross-sectional slices and 3-D volume rendered images of pulse-echo data as well as photoacoustic data are presented. The profile and beamwidths of the fishing line are analyzed and compared with a numerical simulation carried out using the Field II ultrasound simulation software. We investigated using a large aperture (64 x 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller CMUT array (16 x 16 elements). Two-dimensional transducer arrays overcome many of the limitations of a mechanically scanned system and enable volumetric imaging. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large, fully populated 2-D arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. A CMUT based photoacoustic system is proposed as a viable alternative to a piezoelectric transducer based photoacoustic systems.}, number={11}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Vaithilingam, S. and Ma, T-J. and Furukawa, Y. and Wygant, I.O. and Zhuang, X. and De La Zerda, A. and Oralkan, Omer and Kamaya, Aya and Gambhir, Sanjiv and Jeffrey, R. and et al.}, year={2009}, month={Nov}, pages={2411–2419} }
 @article{zhuang_wygant_lin_kupnik_oralkan_khuri-yakub_2009, title={Wafer-bonded 2-D CMUT arrays incorporating through-wafer trench-isolated interconnects with a supporting frame}, volume={56}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2009.1018}, DOI={10.1109/tuffc.2009.1018}, abstractNote={This paper reports on wafer-bonded, fully populated 2-D capacitive micromachined ultrasonic transducer (CMUT) arrays. To date, no successful through-wafer via fabrication technique has been demonstrated that is compatible with the wafer-bonding method of making CMUT arrays. As an alternative to through-wafer vias, trench isolation with a supporting frame is incorporated into the 2-D arrays to provide through-wafer electrical connections. The CMUT arrays are built on a silicon-on-insulator (SOI) wafer, and all electrical connections to the array elements are brought to the back side of the wafer through the highly conductive silicon substrate. Neighboring array elements are separated by trenches on both the device layer and the bulk silicon. A mesh frame structure, providing mechanical support, is embedded between silicon pillars, which electrically connect to individual elements. We successfully fabricated a 16 times 16-element 2-D CMUT array using wafer bonding with a yield of 100%. Across the array, the pulse-echo amplitude distribution is uniform (sigma = 6.6% of the mean amplitude). In one design, we measured a center frequency of 7.6 MHz, a peak-to-peak output pressure of 2.9 MPa at the transducer surface, and a 3-dB fractional bandwidth of 95%. Volumetric ultrasound imaging was demonstrated by chip-to-chip bonding one of the fabricated 2-D arrays to a custom-designed integrated circuit (IC). This study shows that through-wafer trench-isolation with a supporting frame is a viable solution for providing electrical interconnects to CMUT elements and that 2-D arrays fabricated using wafer-bonding deliver good performance.}, number={1}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Zhuang, Xuefeng and Wygant, I. and Lin, Der-Song and Kupnik, M. and Oralkan, O. and Khuri-Yakub, B.}, year={2009}, month={Jan}, pages={182–192} }
 @inproceedings{lee_park_oralkan_kupnik_khuri-yakub_2008, title={CMUT as a chemical sensor for DMMP detection}, ISBN={9781424417940}, url={http://dx.doi.org/10.1109/freq.2008.4623034}, DOI={10.1109/freq.2008.4623034}, abstractNote={We present an 18-MHz capacitive micromachined ultrasonic transducer (CMUT), used as a chemical sensor for detection of a common simulant for chemical weapons, dimethyl methylphosphonate (DMMP), in air. CMUTs are attractive for chemical sensor applications because of their unprecedented mass sensitivity per membrane area, low motional impedance, and high quality factor compared to other flexural-mode resonators. The device is composed of 1000 individual cells operating in parallel, which provides a robust operation with low false alarm rate and low motional impedance comparable to that of Quartz crystal resonators. We designed a CMUT-based oscillator with a phase noise of −84.8 dBc/Hz and −136.6 dBc/Hz at offset frequencies of 1 kHz and 1 MHz, respectively. The oscillator exhibits an Allan deviation of 0.6 Hz with a one-sigma error of 0.037 Hz, which translates to a theoretical limit of mass detection of 16.2 ag. The 18-MHz CMUT is functionalized with a 50-nm thick polymer layer, polyisobutylene (PIB). The described CMUT sensor demonstrates a volume sensitivity of 37.38 ppb/Hz to DMMP.}, booktitle={2008 IEEE International Frequency Control Symposium}, publisher={IEEE}, author={Lee, Hyunjoo J. and Park, Kwan Kyu and Oralkan, Omer and Kupnik, Mario and Khuri-Yakub, Butrus T.}, year={2008}, month={May} }
 @inproceedings{park_lee_kupnik_oralkan_khuri-yakub_2008, title={Capacitive micromachined ultrasonic transducer as a chemical sensor}, ISBN={9781424425808}, url={http://dx.doi.org/10.1109/icsens.2008.4716369}, DOI={10.1109/icsens.2008.4716369}, abstractNote={We present a resonant chemical sensor based on a capacitive micromachined ultrasonic transducer (CMUT) technology. Depending on the frequency of the devices (18 to 32 MHz), the mass sensitivity per unit area ranges from 73 to 130 zg/Hz/mum2. We functionalized the 18-MHz device with polyisobutylene (PIB) to detect dimethyl methylphosphonate (DMMP), a common simulant for the sarin nerve agent. Even with only a 50-nm thick coating layer, our sensor has a high volume sensitivity of 37 ppbv/Hz to DMMP in air. Taking advantage of multiple CMUT cells (100 to 2240), all resonating in parallel, the sensor achieves an equivalent volume resolution of 21 ppbv (parts per 109 by volume) to DMMP. In addition, 200 test cycles with DMMP applied over 26 hours revealed a zero false alarm rate and a 4.7% (3-sigma) variation of volume sensitivity to DMMP. By using principal component analysis (PCA), we successfully classified all analytes in 21 experiments, and we present the results of pattern recognition. This work demonstrates that CMUT has a great potential for the sensitive, reliable, and yet portable chemical sensing systems.}, booktitle={2008 IEEE Sensors}, publisher={IEEE}, author={Park, Kwan Kyu and Lee, Hyunjoo J. and Kupnik, Mario and Oralkan, Omer and Khuri-Yakub, Butrus T.}, year={2008}, month={Oct} }
 @article{de la zerda_zavaleta_keren_vaithilingam_bodapati_liu_levi_smith_ma_oralkan_et al._2008, title={Carbon nanotubes as photoacoustic molecular imaging agents in living mice}, volume={3}, ISSN={1748-3387 1748-3395}, url={http://dx.doi.org/10.1038/nnano.2008.231}, DOI={10.1038/nnano.2008.231}, abstractNote={Photoacoustic imaging of living subjects offers higher spatial resolution and allows deeper tissues to be imaged compared with most optical imaging techniques1,2,3,4,5,6,7. As many diseases do not exhibit a natural photoacoustic contrast, especially in their early stages, it is necessary to administer a photoacoustic contrast agent. A number of contrast agents for photoacoustic imaging have been suggested previously8,9,10,11,12,13,14,15, but most were not shown to target a diseased site in living subjects. Here we show that single-walled carbon nanotubes conjugated with cyclic Arg-Gly-Asp (RGD) peptides can be used as a contrast agent for photoacoustic imaging of tumours. Intravenous administration of these targeted nanotubes to mice bearing tumours showed eight times greater photoacoustic signal in the tumour than mice injected with non-targeted nanotubes. These results were verified ex vivo using Raman microscopy. Photoacoustic imaging of targeted single-walled carbon nanotubes may contribute to non-invasive cancer imaging and monitoring of nanotherapeutics in living subjects16. Photoacoustic imaging offers higher spatial resolution than most optical imaging techniques, but contrast agents are needed because many diseases in their early stages do not display a natural photoacoustic contrast. Using single-walled carbon nanotubes conjugated with a peptide as a contrast agent allows the non-invasive photoacoustic imaging of tumours in animals.}, number={9}, journal={Nature Nanotechnology}, publisher={Springer Nature}, author={De La Zerda, Adam and Zavaleta, Cristina and Keren, Shay and Vaithilingam, Srikant and Bodapati, Sunil and Liu, Zhuang and Levi, Jelena and Smith, Bryan R. and Ma, Te-Jen and Oralkan, Omer and et al.}, year={2008}, month={Aug}, pages={557–562} }
 @inproceedings{yang_vaithilingam_ma_salehi-had_oralkan_khuri-yakub_guccione_2008, title={Development of nanoparticle-based gold contrast agent for photoacoustic tomography}, volume={1}, booktitle={Technical Proceedings of NSTI Nanotech}, author={Yang, Y.S. and Vaithilingam, S. and Ma, T.-J. and Salehi-Had, S. and Oralkan, Ö. and Khuri-Yakub, B.T. and Guccione, S.}, year={2008}, pages={708–711} }
 @inproceedings{park_lee_kupnik_oralkan_khuri-yakub_2008, title={Fabricating capacitive micromachined ultrasonic transducers with direct wafer-bonding and LOCOS technology}, ISBN={9781424417926 9781424417933}, ISSN={1084-6999}, url={http://dx.doi.org/10.1109/memsys.2008.4443662}, DOI={10.1109/memsys.2008.4443662}, abstractNote={We present an improved fabrication method for capacitive micromachined ultrasonic transducers (CMUTs). Recently, a process was developed to fabricate CMUTs using direct wafer-bonding instead of the traditional sacrificial release method. This paper presents a method based on local oxidation of silicon (LOCOS) and direct wafer-bonding to improve the controllability of gap heights and the parasitic capacitance. Critical vertical dimensions are determined by a thermal oxidation process, which allows tight vertical tolerances (< 10 nm) with unmatched uniformity over the entire wafer. Using this process we successfully fabricated CMUTs with gap heights as small as 40 nm with a uniformity of plusmn 2 nm over the entire wafer.}, booktitle={2008 IEEE 21st International Conference on Micro Electro Mechanical Systems}, publisher={IEEE}, author={Park, K. K. and Lee, H. J. and Kupnik, M. and Oralkan, O. and Khuri-Yakub, B. T.}, year={2008}, month={Jan} }
 @article{zhuang_lin_oralkan_khuri-yakub_2008, title={Fabrication of Flexible Transducer Arrays With Through-Wafer Electrical Interconnects Based on Trench Refilling With PDMS}, volume={17}, ISSN={1057-7157 1941-0158}, url={http://dx.doi.org/10.1109/jmems.2008.918381}, DOI={10.1109/jmems.2008.918381}, abstractNote={Flexible transducer arrays are desired to wrap around catheter tips for side-looking intravascular ultrasound imaging. We present a technique for constructing flexible capacitive micromachined ultrasonic transducer (CMUT) arrays by forming polymer-filled deep trenches in a silicon substrate. First, we etch deep trenches between the bottom electrodes of CMUT elements on a prime silicon wafer using deep reactive ion etching. Second, we fusion-bond a silicon-on-insulator (SOI) wafer to the prime silicon wafer. Once the silicon handle and buried oxide layers are removed from the back side of the SOI wafer, the remaining thin silicon device layer acts as a movable membrane and top electrode. Third, we fill the deep trenches with polydimethylsiloxane, and thin the wafer down from the back side. The 16 by 16 flexible 2-D arrays presented in this paper have a trench width that varies between 6 and 20 ; the trench depth is 150 ; the membrane thickness is 1.83 ; and the final substrate thickness is 150 . We demonstrate the flexibility of the substrate by wrapping it around a needle tip with a radius of 450 (less than catheter size of 3 French). Measurements in air validate the functionality of the arrays. The 250- by 250- transducer elements have a capacitance of 2.29 to 2.67 pF, and a resonant frequency of 5.0 to 4.3 MHz, for dc bias voltages ranging from 70 to 100 V.}, number={2}, journal={Journal of Microelectromechanical Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Zhuang, Xuefeng and Lin, Der-Song and Oralkan, O. and Khuri-Yakub, B.T.}, year={2008}, month={Apr}, pages={446–452} }
 @article{nikoozadeh_wygant_lin_oralkan_ergun_stephens_thomenius_dentinger_wildes_akopyan_et al._2008, title={Forward-looking intracardiac ultrasound imaging using a 1-D CMUT array integrated with custom front-end electronics}, volume={55}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2008.980}, DOI={10.1109/tuffc.2008.980}, abstractNote={Minimally invasive catheter-based electrophysiological (EP) interventions are becoming a standard procedure in diagnosis and treatment of cardiac arrhythmias. As a result of technological advances that enable small feature sizes and a high level of integration, nonfluoroscopic intracardiac echocardiography (ICE) imaging catheters are attracting increasing attention. ICE catheters improve EP procedural guidance while reducing the undesirable use of fluoroscopy, which is currently the common catheter guidance method. Phased-array ICE catheters have been in use for several years now, although only for side-looking imaging. We are developing a forwardlooking ICE catheter for improved visualization. In this effort, we fabricate a 24-element, fine-pitch 1-D array of capacitive micromachined ultrasonic transducers (CMUT), with a total footprint of 1.73 mm x 1.27 mm. We also design a custom integrated circuit (IC) composed of 24 identical blocks of transmit/ receive circuitry, measuring 2.1 mm x 2.1 mm. The transmit circuitry is capable of delivering 25-V unipolar pulses, and the receive circuitry includes a transimpedance preamplifier followed by an output buffer. The CMUT array and the custom IC are designed to be mounted at the tip of a 10-Fr catheter for high-frame-rate forward-looking intracardiac imaging. Through-wafer vias incorporated in the CMUT array provide access to individual array elements from the back side of the array. We successfully flip-chip bond a CMUT array to the custom IC with 100% yield. We coat the device with a layer of polydimethylsiloxane (PDMS) to electrically isolate the device for imaging in water and tissue. The pulse-echo in water from a total plane reflector has a center frequency of 9.2 MHz with a 96% fractional bandwidth. Finally, we demonstrate the imaging capability of the integrated device on commercial phantoms and on a beating ex vivo rabbit heart (Langendorff model) using a commercial ultrasound imaging system.}, number={12}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Nikoozadeh, A. and Wygant, I.O. and Lin, Der-Song and Oralkan, O. and Ergun, A.S. and Stephens, D.N. and Thomenius, K.E. and Dentinger, A.M. and Wildes, D. and Akopyan, G. and et al.}, year={2008}, month={Dec}, pages={2651–2660} }
 @article{wygant_zhuang_yeh_oralkan_ergun_karaman_khuri-yakub_2008, title={Integration of 2D CMUT arrays with front-end electronics for volumetric ultrasound imaging}, volume={55}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2008.652}, DOI={10.1109/tuffc.2008.652}, abstractNote={For three-dimensional (3D) ultrasound imaging, connecting elements of a two-dimensional (2D) transducer array to the imaging system's front-end electronics is a challenge because of the large number of array elements and the small element size. To compactly connect the transducer array with electronics, we flip-chip bond a 2D 16 times 16-element capacitive micromachined ultrasonic transducer (CMUT) array to a custom-designed integrated circuit (IC). Through-wafer interconnects are used to connect the CMUT elements on the top side of the array with flip-chip bond pads on the back side. The IC provides a 25-V pulser and a transimpedance preamplifier to each element of the array. For each of three characterized devices, the element yield is excellent (99 to 100% of the elements are functional). Center frequencies range from 2.6 MHz to 5.1 MHz. For pulse-echo operation, the average -6-dB fractional bandwidth is as high as 125%. Transmit pressures normalized to the face of the transducer are as high as 339 kPa and input-referred receiver noise is typically 1.2 to 2.1 rnPa/ radicHz. The flip-chip bonded devices were used to acquire 3D synthetic aperture images of a wire-target phantom. Combining the transducer array and IC, as shown in this paper, allows for better utilization of large arrays, improves receive sensitivity, and may lead to new imaging techniques that depend on transducer arrays that are closely coupled to IC electronics.}, number={2}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Wygant, I.O. and Zhuang, Xuefeng and Yeh, D.T. and Oralkan, O. and Ergun, A.S. and Karaman, M. and Khuri-Yakub, B.T.}, year={2008}, month={Feb}, pages={327–342} }
 @inproceedings{vaithilingam_ma_furukawa_oralkan_kamaya_torashima_kupnik_wygant_zhuang_brooke jeffrey jr_et al._2008, title={Investigating large 2D arrays for photoacoustic and acoustic imaging using CMUT technology}, ISBN={9781424424283}, url={http://dx.doi.org/10.1109/ultsym.2008.0299}, DOI={10.1109/ultsym.2008.0299}, abstractNote={In this paper, we investigate using a large aperture (64 times 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller array (16 times 16 elements) of capacitive micromachined ultrasonic transducers (CMUTs). We show results from the imaging of: 1) A fishing-line phantom. 2) Tubes embedded in chicken breast tissue containing the contrast agent indocyanine green (ICG), pig blood and a mixture of the two. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.9 cm from the CMUT array. Three-dimensional volume rendered images of traditional pulse-echo data as well as photoacoustic data are shown.}, booktitle={2008 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Vaithilingam, Srikant and Ma, Te-Jen and Furukawa, Yukio and Oralkan, Omer and Kamaya, Aya and Torashima, Kazutoshi and Kupnik, Mario and Wygant, Ira O. and Zhuang, Xuefeng and Brooke Jeffrey Jr, R. and et al.}, year={2008}, month={Nov} }
 @misc{park_lee_kupnik_oralkan_khuri-yakub_2008, title={Miniaturized chemical sensor array for detection of warfare agents}, author={Park, K.K. and Lee, H.J. and Kupnik, M. and Oralkan, Ö. and Khuri-Yakub, B. T.}, year={2008}, month={May} }
 @article{stephens_cannata_liu_zhao_shung_nguyen_chia_dentinger_wildes_thomenius_et al._2008, title={Multifunctional catheters combining intracardiac ultrasound imaging and electrophysiology sensing}, volume={55}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2008.834}, DOI={10.1109/tuffc.2008.834}, abstractNote={A family of 3 multifunctional intracardiac imaging and electrophysiology (EP) mapping catheters has been in development to help guide diagnostic and therapeutic intracardiac EP procedures. The catheter tip on the first device includes a 7.5 MHz, 64-element, side-looking phased array for high resolution sector scanning. The second device is a forward-looking catheter with a 24-element 14 MHz phased array. Both of these catheters operate on a commercial imaging system with standard software. Multiple EP mapping sensors were mounted as ring electrodes near the arrays for electrocardiographic synchronization of ultrasound images and used for unique integration with EP mapping technologies. To help establish the catheters' ability for integration with EP interventional procedures, tests were performed in vivo in a porcine animal model to demonstrate both useful intracardiac echocardiographic (ICE) visualization and simultaneous 3-D positional information using integrated electroanatomical mapping techniques. The catheters also performed well in high frame rate imaging, color flow imaging, and strain rate imaging of atrial and ventricular structures. The companion paper of this work discusses the catheter design of the side-looking catheter with special attention to acoustic lens design. The third device in development is a 10 MHz forward-looking ring array that is to be mounted at the distal tip of a 9F catheter to permit use of the available catheter lumen for adjunctive therapy tools.}, number={7}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Stephens, D.N. and Cannata, J. and Liu, Ruibin and Zhao, Jian Zhong and Shung, K.K. and Nguyen, Hien and Chia, R. and Dentinger, A. and Wildes, D. and Thomenius, K.E. and et al.}, year={2008}, month={Jul}, pages={1570–1581} }
 @inproceedings{park_lee_crisman_kupnik_oralkan_khuri-yakub_2008, title={Optimum design of circular CMUT membranes for high quality factor in air}, ISBN={9781424424283}, url={http://dx.doi.org/10.1109/ultsym.2008.0123}, DOI={10.1109/ultsym.2008.0123}, abstractNote={This paper presents optimum design of circular membrane for high quality factor. A quality factor of CMUT, we designed for resonant chemical sensor, is dominated by Q<sub>air</sub> and Q<sub>support</sub>. We investigated these two factors independently. We calculate Q<sub>air</sub> of circular silicon membrane analytically, and numerically. Calculated Q<sub>air</sub> is compared to measured value. In order to find Q<sub>support</sub> empirically, we measured quality factor of CMUT in vacuum chamber. Q<sub>air</sub> is proportional to (radius/thickness)<sup>-2</sup> and Q<sub>support</sub> is proportional to (radius/thickness)<sup>3</sup>. Thus the optimum aspect ratio of membrane exists for maximum quality factor.}, booktitle={2008 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Park, K. K. and Lee, H. J. and Crisman, P. and Kupnik, M. and Oralkan, O. and Khuri-Yakub, B. T.}, year={2008}, month={Nov} }
 @inproceedings{lee_park_cristman_oralkan_kupnik_khuri-yakub_2008, title={The effect of parallelism of CMUT cells on phase noise for chem/bio sensor applications}, ISBN={9781424424283}, url={http://dx.doi.org/10.1109/ultsym.2008.0481}, DOI={10.1109/ultsym.2008.0481}, abstractNote={We investigated the effects of electrically connecting multiple microresonators on the frequency noise with a goal to improve the resolution of a chemical sensor based on the capacitive micromachined ultrasonic transducer (CMUT) technology. We fabricated twenty-two 50-MHz CMUTs with varying number of cells and measured the input impedance characteristics. The impedance measurement results show a linear increase in quality factor as the number of cells increases. Further, a phase noise simulation of Colpitts oscillators employing these CMUTs verifies that the phase noise of the oscillator in the 1/f2 regime are influenced by the quality factor while the phase noise in the white noise regime are primarily affected by the motional impedance. The oscillator based on the CMUT with 1027 cells has 8.8 dB and 11.4 dB lower phase noise than that based on the CMUT with 397 cells at offset frequencies of 1 kHz and 5 MHz, respectively. Therefore, we demonstrated that electrically connecting multiple microresonators is an effective technique to improve the sensor resolution.}, booktitle={2008 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Lee, Hyunjoo J. and Park, Kwan Kyu and Cristman, Paul and Oralkan, Omer and Kupnik, Mario and Khuri-Yakub, Butrus T.}, year={2008}, month={Nov} }
 @inbook{khuri-yakub_ergun_oralkan_yaralioglu_2008, title={Ultrasonic Transduction}, ISBN={9780444521903}, url={http://dx.doi.org/10.1016/b978-044452190-3.00046-x}, DOI={10.1016/b978-044452190-3.00046-x}, abstractNote={Piezoelectricity has long been the dominant transduction mechanism to detect and generate ultrasound. Recently, capacitive micromachined ultrasonic transducers (cMUTs) based on electrostatic transduction have emerged as an alternative to piezoelectric transducers. Wide bandwidth in immersion operation, ease of fabricating large arrays, and potential for integration with electronics are some of the advantages offered by cMUT technology. This chapter explains the theory and modeling of this modern type of electrostatic transducers. Both analytical and numerical models are presented. Fabrication processes based on surface and bulk micromachining techniques are explained. Examples of imaging systems based on this new technology are also described.}, booktitle={Comprehensive Microsystems}, publisher={Elsevier}, author={Khuri-Yakub, B.T. and Ergun, A.S. and Oralkan, O. and Yaralioglu, G.G.}, year={2008}, pages={517–539} }
 @inproceedings{nikoozadeh_wygant_lin_oralkan_ergun_thomenius_dentinger_wildes_akopyan_shivkumar_et al._2007, title={10C-6 Fully Integrated CMUT-Based Forward-Looking Intracardiac Imaging for Electrophysiology}, ISBN={9781424413836 9781424413843}, ISSN={1051-0117}, url={http://dx.doi.org/10.1109/ultsym.2007.230}, DOI={10.1109/ultsym.2007.230}, abstractNote={Minimally invasive percutaneous electrophysiological mapping of the heart chambers is becoming a standard procedure to diagnose and treat cardiac arrhythmias. Due to advances in technology that enable small feature sizes and a high level of integration, non-fluoroscopic intracardiac imaging is attracting more attention to better guide electrophysiologal (EP) interventions. In this effort, we are developing a forward-looking intracardiac ultrasound imaging catheter, which is also equipped with several EP electrode sensor bands and a metal RF ablation tip enclosure. A 24-element fine-pitch (63 mum) 1-D array, based on capacitive micromachined ultrasonic transducer (CMUT) technology, has been fabricated for high-frame-rate imaging. Through-wafer vias are incorporated in the device to connect the signal and ground electrodes to the flip-chip bond pads on the backside of the array. The total footprint of the array measures 1.73 mm x 1.27 mm. Also a custom-designed integrated circuit (IC) has been fabricated to be closely integrated with the CMUT array for improved SNR. This IC comprises some of the important front- end electronics of an ultrasound imaging system. It measures 2 mm x 2 mm and is composed of 24 individual transmit/receive blocks. The transmit circuitry is capable of delivering 25 -V unipolar pulses. The receive circuitry includes a transimpedance preamplifier followed by a line driver buffer. A CMUT array was flip-chip bonded directly on to the IC for initial testing. All of the 24 elements of the array and the IC are functional. Array uniformity was tested by measuring the resonant frequency in air. A standard deviation of 0.37 percent was measured around the mean value of 17.9 MHz. The same array operates at 9.2 MHz in immersion with a 104 percent fractional bandwidth. Imaging performance of the described front-end was tested on a commercial phantom and also in ex- vivo environment on an isolated perfused rabbit heart (Langendorfl). The final goal is to integrate the CMUT array and the front-end electronics at the tip of a 10 -F catheter. A flexible printed circuit board (PCB) has been designed and the first sub-assembly is ready for cable attachment and final catheter integration.}, booktitle={2007 IEEE Ultrasonics Symposium Proceedings}, publisher={IEEE}, author={Nikoozadeh, A. and Wygant, I.O. and Lin, Der-Song and Oralkan, O. and Ergun, A.S. and Thomenius, K. and Dentinger, A. and Wildes, D. and Akopyan, G. and Shivkumar, K. and et al.}, year={2007}, month={Oct} }
 @inproceedings{wygant_jamal_lee_nikoozadeh_zhuang_oralkan_ergun_karaman_khuri-yakub_2007, title={2C-3 An Integrated Circuit with Transmit Beamforming and Parallel Receive Channels for 3D Ultrasound Imaging: Testing and Characterization}, ISBN={9781424413836 9781424413843}, ISSN={1051-0117}, url={http://dx.doi.org/10.1109/ultsym.2007.20}, DOI={10.1109/ultsym.2007.20}, abstractNote={The cost and complexity of medical ultrasound imaging systems can be reduced by integrating the transducer array with an integrated circuit (IC). By incorporating some of the system's front-end electronics into an IC, bulky cables and costly system electronics can be eliminated. Here we present an IC for 3D intracavital imaging that requires few electrical connections but uses a large fraction of a 16times16-element 2D transducer array to transmit focused ultrasound. To simplify the receive and data acquisition electronics, only the 32 elements along the array diagonals are used as receivers. The IC provides a preamplifier for each receiving element. Each of the 224 transmitting elements is provided an 8-bit shift register, a comparator, and a 25-V pulser. To transmit, a global counter is incremented from 1 to 224; each pulser fires when its stored register value is equal to the global count value. Electrical testing of the fabricated IC shows that it works as designed. The IC was flip-chip bonded to a two-dimensional capacitive micromachined ultrasonic transducer (CMUT) array. A two-dimensional image of a wire target phantom was acquired.}, booktitle={2007 IEEE Ultrasonics Symposium Proceedings}, publisher={IEEE}, author={Wygant, I. O. and Jamal, N. and Lee, H. J. and Nikoozadeh, A. and Zhuang, X. and Oralkan, O. and Ergun, A. S. and Karaman, M. and Khuri-Yakub, B. T.}, year={2007}, month={Oct} }
 @inproceedings{vaithilingam_ma_furukawa_zerda_oralkan_kamaya_keren_gambhir_jeffrey_khuri-yakub_2007, title={A co-axial scanning acoustic and photoacoustic microscope}, booktitle={Proceedings of the IEEE Ultrasonics Symposium}, author={Vaithilingam, S. and Ma, T.J. and Furukawa, Y. and Zerda, A. and Oralkan, Ö. and Kamaya, A. and Keren, S. and Gambhir, S.S. and Jeffrey, R.B., Jr. and Khuri-Yakub, B.T.}, year={2007}, pages={2413–2416} }
 @article{park_lee_yaralioglu_ergun_oralkan_kupnik_quate_khuri-yakub_braun_ramseyer_et al._2007, title={Capacitive micromachined ultrasonic transducers for chemical detection in nitrogen}, volume={91}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2776348}, DOI={10.1063/1.2776348}, abstractNote={The authors present the prototype of a chemical sensor using a capacitive micromachined ultrasonic transducer array. Each element in the array consists of a large number of resonating membranes connected in parallel. A five-channel oscillator circuit operates at the resonant frequency around 6MHz in this prototype. The surface of the elements in the array is coated by polymers such as polyallylamine hydrochloride, polyethylene glycol, and polyvinyl alcohol to detect different chemicals. By measuring shift in oscillation frequencies due to the mass-loading effect, analytes, e.g., water and isopropanol, with concentrations around 20ppbv (parts per 109 by volume) range can be detected.}, number={9}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Park, K. K. and Lee, H. J. and Yaralioglu, G. G. and Ergun, A. S. and Oralkan, Ö. and Kupnik, M. and Quate, C. F. and Khuri-Yakub, B. T. and Braun, T. and Ramseyer, J.-P. and et al.}, year={2007}, month={Aug}, pages={094102} }
 @inproceedings{wong_ergun_yaralioglu_oralkan_kupnik_pauly_khuri-yakub_2007, title={Design of HIFU CMUT Arrays for Treatment of Liver and Renal Cancer}, ISSN={0094-243X}, url={http://dx.doi.org/10.1063/1.2744251}, DOI={10.1063/1.2744251}, abstractNote={We present the development of a capacitive micromachined ultrasonic transducer (CMUT) array for noninvasive focused ultrasound ablation of lower abdominal cancers under MR‐guidance. While piezoelectric transducers have been traditionally used for HIFU, recent advances in CMUT design have made them highly competitive. Not only are CMUTs cost effective, they allow fabrication flexibility and advantages in efficiency and bandwidth. Current imaging CMUTs have shown capability of HIFU operation through high power and continuous wave operation. In this paper, we will present the development of CMUT membranes designed specifically for HIFU. These membranes are piston‐like membranes fabricated by placing a thick layer of silicon or gold at the center of the membrane. The width of the piston layer is usually 60–85% of the membrane width and allows the membrane mass and elasticity to be controlled independently. It also increases the average displacement and average output pressure of the membrane. We patterned the...}, booktitle={AIP Conference Proceedings}, publisher={AIP}, author={Wong, Serena H. and Ergun, Arif Sanli and Yaralioglu, Goksen G. and Oralkan, Omer and Kupnik, Mario and Pauly, Kim Butts and Khuri-Yakub, B. T.}, year={2007} }
 @article{bayram_kupnik_yaralioglu_oralkan_ergun_lin_wong_khuri-yakub_2007, title={Finite element modeling and experimental characterization of crosstalk in 1-D CMUT arrays}, volume={54}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2007.256}, DOI={10.1109/tuffc.2007.256}, abstractNote={Crosstalk is the coupling of energy between the elements of an ultrasonic transducer array. This coupling degrades the performance of transducers in applications such as medical imaging and therapeutics. In this paper, we present an experimental demonstration of guided interface waves in capacitive micromachined ultrasonic transducers (CMUTs). We compare the experimental results to finite element calculations using a commercial package (LS-DYNA) for a 1-D CMUT array operating in the conventional and collapsed modes. An element in the middle of the array was excited with a unipolar voltage pulse, and the displacements were measured using a laser interferometer along the center line of the array elements immersed in soybean oil. We repeated the measurements for an identical CMUT array covered with a 4.5-mum polydimethyl-siloxane (PDMS) layer. The main crosstalk mechanism is the dispersive guided modes propagating in the fluid-solid interface. Although the transmitter element had a center frequency of 5.8 MHz with a 130% fractional bandwidth in the conventional operation, the dispersive guided mode was observed with the maximum amplitude at a frequency of 2.1 MHz, and had a cut-off frequency of 4 MHz. In the collapsed operation, the dispersive guided mode was observed with the maximum amplitude at a frequency of 4.0 MHz, and had a cut-off frequency of 10 MHz. Crosstalk level was lower in the collapsed operation (-39 dB) than in the conventional operation (-24.4 dB). The coverage of the PDMS did not significantly affect the crosstalk level, but reduced the phase velocity for both operation modes. Lamb wave modes, A0 and S0, were also observed with crosstalk levels of -40 dB and -65 dB, respectively. We observed excellent agreement between the finite element and the experimental results}, number={2}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Bayram, Baris and Kupnik, Mario and Yaralioglu, Goksen and Oralkan, Omer and Ergun, Arif and Lin, Der-song and Wong, Serena and Khuri-yakub, Butrus}, year={2007}, month={Feb}, pages={418–430} }
 @inproceedings{zhuang_lin_oralkan_khuri-yakub_2007, title={Flexible transducer arrays with through-wafer electrical interconnects based on trench refilling with PDMS}, url={http://dx.doi.org/10.1109/memsys.2007.4433016}, DOI={10.1109/memsys.2007.4433016}, abstractNote={This paper reports on a method to fabricate flexible one-dimensional (1D) and two-dimensional (2D) micromachined transducer arrays that are electrically connected to flip-chip bond pads on the back side of the array. In our case, the transducers are capacitive micromachined ultrasonic transducers (CMUT) intended for medical ultrasound imaging. For ultrasound imaging, flexible arrays conform to the body part being imaged. Flexible arrays are also desired for certain catheter and fixed-focus array geometries. Electrical connection to bond pads on the back side of the array is provided for flip-chip bonding to an integrated circuit or flexible PCB. The arrays are made flexible by etching through-wafer trenches and filling the trenches with polydimethylsiloxane (PDMS). The flexibility of the substrate is demonstrated by wrapping it around a needle tip with a radius of 650 mum (French catheter size of 4).}, booktitle={2007 IEEE 20th International Conference on Micro Electro Mechanical Systems (MEMS)}, publisher={IEEE}, author={Zhuang, Xuefeng and Lin, Der-Song and Oralkan, Omer and Khuri-Yakub, Butrus T.}, year={2007}, month={Jan} }
 @article{zhuang_ergun_huang_wygant_oralkan_khuri-yakub_2007, title={Integration of trench-isolated through-wafer interconnects with 2d capacitive micromachined ultrasonic transducer arrays}, volume={138}, ISSN={0924-4247}, url={http://dx.doi.org/10.1016/j.sna.2007.04.008}, DOI={10.1016/j.sna.2007.04.008}, abstractNote={This paper presents a method to provide electrical connection to a 2D capacitive micromachined ultrasonic transducer (CMUT) array. The interconnects are processed after the CMUTs are fabricated on the front side of a silicon wafer. Connections to array elements are made from the back side of the substrate via highly conductive silicon pillars that result from a deep reactive ion etching (DRIE) process. Flip-chip bonding is used to integrate the CMUT array with an integrated circuit (IC) that comprises the front-end circuits for the transducer and provides mechanical support for the trench-isolated array elements. Design, fabrication process and characterization results are presented. The advantages when compared to other through-wafer interconnect techniques are discussed.}, number={1}, journal={Sensors and Actuators A: Physical}, publisher={Elsevier BV}, author={Zhuang, Xuefeng and Ergun, Arif S. and Huang, Yongli and Wygant, Ira O. and Oralkan, Omer and Khuri-Yakub, Butrus T.}, year={2007}, month={Jul}, pages={221–229} }
 @inbook{khuri-yakub_oralkan_kupnik_2007, place={New York}, title={Micromachined ultrasonic transducers}, booktitle={McGraw-Hill Yearbook of Science and Technology 2008}, publisher={McGraw-Hill Professional}, author={Khuri-Yakub, B.T. and Oralkan, Ö. and Kupnik, M.}, year={2007}, pages={207–210} }
 @inproceedings{khuri-yakub_park_lee_yaralioglu_ergun_oralkan_kupnik_quate_braun_lang_et al._2007, title={The capacitive micromachined ultrasonic transducer (CMUT) as a chem/bio sensor}, booktitle={Proceedings of the IEEE Ultrasonics Symposium}, author={Khuri-Yakub, B.T. and Park, K.K. and Lee, H.J. and Yaralioglu, G. and Ergun, A.S. and Oralkan, Ö. and Kupnik, M. and Quate, C.F. and Braun, T. and Lang, H. and et al.}, year={2007}, pages={472–475} }
 @inproceedings{zhuang_wygant_lin_kupnik_oralkan_khuri-yakub_2007, title={Trench-isolated CMUT arrays with a supporting frame: characterization and imaging results}, booktitle={Proceedings of the IEEE Ultrasonics Symposium}, author={Zhuang, X. and Wygant, I. O. and Lin, D. S. and Kupnik, M. and Oralkan, Ö. and Khuri-Yakub, B. T.}, year={2007}, pages={507–510} }
 @inproceedings{vaithilingam_wygant_sifferman_zhuang_furukawa_oralkan_keren_gambhir_khuri-yakub_2006, title={1E-3 Tomographic Photoacoustic Imaging Using Capacitive Micromachined Ultrasonic Transducer (CMUT) Technology}, ISBN={1424402018}, url={http://dx.doi.org/10.1109/ultsym.2006.112}, DOI={10.1109/ultsym.2006.112}, abstractNote={Photoacoustic imaging is performed using a two-dimensional array of capacitive micromachined ultrasonic transducers (CMUTs) with integrated front-end electronics. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. One of the goals of this research is to build a complete photoacoustic imaging system with an inward-looking cylindrical array that can enclose the target. As steps toward this goal two experiments are presented in this paper - photoacoustic imaging with planar scanning of the target and photoacoustic imaging with circular scanning of the target. Pulse-echo and photoacoustic images are presented. Image resolution and sensitivity of the setup are discussed and photoacoustic images of chicken tissue ablation are shown}, booktitle={2006 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Vaithilingam, S. and Wygant, I. O. and Sifferman, S. and Zhuang, X. and Furukawa, Y. and Oralkan, O. and Keren, S. and Gambhir, S. S. and Khuri-Yakub, B. T.}, year={2006} }
 @inproceedings{zhuang_khuri-yakub_yaralioglu_kupnik_oralkan_wong_ergun_butts-pauly_2006, title={1I-2 Capacitive Micromachined Ultrasonic Transducers for High Intensity Focused Ablation of Upper Abdominal Tumors}, ISBN={1424402018}, url={http://dx.doi.org/10.1109/ultsym.2006.219}, DOI={10.1109/ultsym.2006.219}, abstractNote={We present the development of a capacitive micromachined ultrasonic transducer (CMUT) array for noninvasive focused ultrasound ablation of lower abdominal cancers under MR-guidance. While piezoelectric transducers have been traditionally used for high intensity focused ultrasound (HIFU), recent advances in capacitive micromachined ultrasonic transducers (CMUTs) have made them highly competitive with regard to costs, fabrication flexibility, and performance. Even current imaging CMUTs have shown capability of HIFU operation through high power and continuous wave operation. In this paper, we will show our experiments with current imaging CMUTs operated in HIFU mode. In addition, we will show the design and development of CMUT membranes and a transducer array specifically for HIFU ablation lower abdominal cancers}, booktitle={2006 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Zhuang, X. and Khuri-Yakub, B. T. and Yaralioglu, G. G. and Kupnik, M. and Oralkan, O. and Wong, S. H. and Ergun, A. S. and Butts-Pauly, K.}, year={2006} }
 @article{yeh_oralkan_wygant_o'donnell_khuri-yakub_2006, title={3-D ultrasound imaging using a forward-looking CMUT ring array for intravascular/intracardiac applications}, volume={53}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2006.1642519}, DOI={10.1109/tuffc.2006.1642519}, abstractNote={Forward-viewing ring arrays can enable new applications in intravascular and intracardiac ultrasound. This work presents compelling, full-synthetic, phased-array volumetric images from a forward-viewing capacitive micromachined ultrasonic transducer (CMUT) ring array wire bonded to a custom integrated circuit front end. The CMUT ring array has a diameter of 2 mm and 64 elements each 100 /spl mu/m /spl times/ 100 /spl mu/m in size. In conventional mode, echo signals received from a plane reflector at 5 mm had 70% fractional bandwidth around a center frequency of 8.3 MHz. In collapse mode, 69% fractional bandwidth is measured around 19 MHz. Measured signal-to-noise ratio (SNR) of the echo averaged 16 times was 29 dB for conventional operation and 35 dB for collapse mode. B-scans were generated of a target consisting of steel wires 0.3 mm in diameter to determine resolution performance. The 6 dB axial and lateral resolutions for the B-scan of the wire target are 189 /spl mu/m and 0.112 radians for 8 MHz, and 78 /spl mu/m and 0.051 radians for 19 MHz. A reduced firing set suitable for real-time, intravascular applications was generated and shown to produce acceptable images. Rendered three-dimensional (3-D) images of a Palmaz-Schatz stent also are shown, demonstrating that the imaging quality is sufficient for practical applications.}, number={6}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Yeh, D.T. and Oralkan, O. and Wygant, I.O. and O'Donnell, M. and Khuri-Yakub, B.T.}, year={2006}, month={Jun}, pages={1202–1211} }
 @inproceedings{yeh_oralkan_wygant_o'donnell_khuri-yakub_2006, title={3-D ultrasound imaging using forward viewing CMUT ring arrays for intravascular and intracardiac applications}, ISBN={0780393821}, url={http://dx.doi.org/10.1109/ultsym.2005.1602968}, DOI={10.1109/ultsym.2005.1602968}, abstractNote={Forward-viewing ring arrays can enable new appli- cations in intravascular and intracardiac ultrasound. We have demonstrated full synthetic phased array volumetric ultrasound imaging using a forward-viewing CMUT ring array with 64 elements, in both the conventional (8 MHz) and collapse (19 MHz) regimes of operation. Measured SNR of an echo from a plane reflector at 5 mm is 29 dB for 8 MHz and 35 dB for 19 MHz. The 6-dB axial and lateral resolutions for the B-scan of the wire target is 189µm and 0.112 radians for 8 MHz, and 78µm and 0.051 radians for 19 MHz. Rendered 3-D images of a Palmaz-Schatz stent are also shown, demonstrating that the imaging quality is sufficient for clinical applications.}, booktitle={IEEE Ultrasonics Symposium, 2005.}, publisher={IEEE}, author={Yeh, D.T. and Oralkan, O. and Wygant, I.O. and O'Donnell, M. and Khuri-Yakub, B.T.}, year={2006}, month={Mar} }
 @inproceedings{stephens_cannata_liu_shung_oralkan_nikoozadeh_khuri-yakub_nguyen_chia_dentinger_et al._2006, title={5G-6 Forward Looking Intracardiac Imaging Catheters for Electrophysiology}, ISBN={1424402018}, url={http://dx.doi.org/10.1109/ultsym.2006.191}, DOI={10.1109/ultsym.2006.191}, abstractNote={Minimally invasive electrophysiology interventions to treat cardiac arrhythmias are increasing worldwide due to advances in technologies that enable more effective clinical procedures. A forward imaging ultrasound catheter design has been developed and tested to advance the methods of integration of intracardiac imaging and electrophysiology sensing. The first catheters built have been constructed with a 9F (3 mm) shaft and a large (15F) tip to support experimental wire ports adjacent to a 24 element phased array operating at 14 MHz. The final tip design construction size will be 9F and possess an integrated metal electrode at the catheter distal end. Two forward looking array designs have been developed in parallel to produce two contrasting fine pitch (65 microns) 24 element phased array construction approaches. The first array has been assembled with a standard 2-2 composite PZT technology with the flex circuit mounted on the front facing side, and the second design is a cMUT version with a flip-chip bonded silicon die bonded to a backside flex circuit. The cMUT design requires a special interface to assure a safe element biasing scheme while enhancing the array's linearity and sensitivity. The first PZT array prototypes, built without explicit matching layers, have been characterized and agree with FEA and KLM analyses in operation at 14 MHz. Matching layer variations have been used on the front layer flex circuit to optimize the sensitivity and bandwidth of the PZT arrays while minimizing the thermal boundary layer. Specially designed assembly approaches addressed the challenging forward looking array configurations that utilize interconnection flex circuits with bend radii at 250 microns. Animal studies have been performed utilizing beam forming adaptations for the forward looking imaging catheter operation on a Vingmed Vivid-7 system. The first piezoceramic array devices were used successfully to image the myocardium of the right atrium of a pig while simultaneous tissue ablation was performed}, booktitle={Proceedings of the 2006 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Stephens, D. N. and Cannata, J. and Liu, R. and Shung, K. K. and Oralkan, O. and Nikoozadeh, A. and Khuri-Yakub, P. and Nguyen, H. and Chia, R. and Dentinger, A. and et al.}, year={2006}, pages={702–705} }
 @inproceedings{khuri-yakub_yaralioglu_bayram_kupnik_wong_ergun_oralkan_lin_2006, title={5I-6 Finite Element Analysis of Fabrication Related Thermal Effects in Capacitive Micromachined Ultrasonic Transducers}, ISBN={1424402018}, url={http://dx.doi.org/10.1109/ultsym.2006.254}, DOI={10.1109/ultsym.2006.254}, abstractNote={The accurate prediction of the static operation point of capacitive micromachined ultrasonic transducer (CMUT) membranes is essential for dynamic performance modeling, device design, and device fabrication. We evaluate whether fabrication-related stress effects in CMUT cells cause significant deviations observed between membrane deflection measurements and calculations. Our finite element analysis (FEA) considers the thermal structural interaction associated with the temperature stress history during device fabrication. Specific boundary conditions enable thermal expansion in all directions. Compared to conventional FEAs with one static solution step, we use seven static solution steps using the element birth and death feature of ANSYS to assemble the CMUT cell layer by layer at the appropriate deposition temperatures. We use measured static deflection profiles of metallized and non-metallized membranes with different diameters and thicknesses to extract all unknown parameters for the model. This approach improves the prediction of membrane deflections significantly, and it explains why present models often overestimate the collapse voltage}, booktitle={2006 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Khuri-Yakub, B. T. and Yaralioglu, G. G. and Bayram, B. and Kupnik, M. and Wong, S. H. and Ergun, A. S. and Oralkan, O. and Lin, D.}, year={2006} }
 @inproceedings{wygant_zhuang_yeh_vaithilingam_nikoozadeh_oralkan_ergun_karaman_khuri-yakub_2006, title={An endoscopic imaging system based on a two-dimensional CMUT array: real-time imaging results}, ISBN={0780393821}, url={http://dx.doi.org/10.1109/ultsym.2005.1602970}, DOI={10.1109/ultsym.2005.1602970}, abstractNote={Real-time catheter-based ultrasound imaging tools are needed for diagnosis and image-guided procedures. The continued development of these tools is partially limited by the difficulty of fabricating two-dimensional array geometries of piezoelectric transducers. Using capacitive micromachined ultrasonic transducer (CMUT) technology, transducer arrays with widely varying geometries, high frequencies, and wide bandwidths can be fabricated. A volumetric ultrasound imaging system based on a two-dimensional, 16×16-element, CMUT array is presented. Transducer arrays with operating frequencies ranging from 3 MHz to 7.5 MHz were fabricated for this system. The transducer array including DC bias pads measures 4 mm by 4.7 mm. The transducer elements are connected to flip-chip bond pads on the array back side with 400-µm long through-wafer interconnects. The array is flip-chip bonded to a custom- designed integrated circuit (IC) that comprises the front-end electronics. Integrating the front-end electronics with the transducer array reduces the effects of cable capacitance on the transducer's performance and provides a compact means of connecting to the transducer elements. The front-end IC provides a 27-V pulser and 10-MHz bandwidth amplifier for each element of the array. An FPGA-based data acquisition system is used for control and data acquisition. Output pressure of 230 kPa was measured for the integrated device. A receive sensitivity of 125 mV/kPa was measured at the output of the amplifier. Amplifier output noise at 5 Mhz is 112 nV/√Hz. Volumetric images of a wire phantom and vessel phantom are presented. Volumetric data for a wire phantom was acquired in real-time at 30 frames per second. Keywords-ultrasound imaging, catheter, capacitive micromachined ultrasonic transducer, CMUT, integrated electronics, volumetric, real-time}, booktitle={IEEE Ultrasonics Symposium, 2005.}, publisher={IEEE}, author={Wygant, I.O. and Zhuang, X. and Yeh, D.T. and Vaithilingam, S. and Nikoozadeh, A. and Oralkan, O. and Ergun, A.S. and Karaman, M. and Khuri-Yakub, B.T.}, year={2006}, month={Mar} }
 @inproceedings{wygant_karaman_oralkan_khuri-yakub_2006, title={Beamforming and hardware design for a multichannel front-end integrated circuit for real-time 3D catheter-based ultrasonic imaging}, url={http://dx.doi.org/10.1117/12.673786}, DOI={10.1117/12.673786}, abstractNote={We are working on integrating front-end electronics with the ultrasound transducer array for real-time 3D ultrasound imaging systems. We achieve this integration by flip-chip bonding a two-dimensional transducer array to an integrated circuit (IC) that comprises the front-end electronics. The front-end IC includes preamplifiers, multiplexers, and pulsers. We recently demonstrated a catheter-based real-time ultrasound imaging system based on a 16x16-element capacitive micromachined ultrasonic transducer (CMUT) array. The CMUT array is flip-chip bonded to a front-end IC that includes a pulser and preamplifier for each element of the array. To simplify the back-end processing and signal routing on the IC for this initial implementation, only a single array element is active at a time (classic synthetic aperture (CSA) imaging). Compared with classic phased array imaging (CPA), where multiple elements are used on transmit and receive, CSA imaging has reduced signal-to-noise ratio and prominent grating lobes. In this work, we evaluate three array designs for the next generation front-end IC. The designs assume there are 16 receive channels and that numerous transmit pulsers are provided by the IC. The designs presented are: plus-transmit x-receive, boundary-transmit x-receive with no common elements, and full-transmit x-receive with no common elements. Each design is compared with CSA and CPA imaging. We choose to implement an IC for the full-transmit x-receive with no common elements (FT-XR-NC) design for our next-generation catheter-based imaging system.}, booktitle={Medical Imaging 2006: Ultrasonic Imaging and Signal Processing}, publisher={SPIE}, author={Wygant, Ira O. and Karaman, Mustafa and Oralkan, Ömer and Khuri-Yakub, Butrus T.}, editor={Emelianov, Stanislav and Walker, William F.Editors}, year={2006}, month={Mar} }
 @inproceedings{wong_wygant_yeh_zhuang_bayram_kupnik_oralkan_ergun_yaralioglu_khuri-yakub_2006, title={Capacitive micromachined ultrasonic transducer arrays for integrated diagnostic/therapeutic catheters}, volume={829}, number={1}, booktitle={AIP Conference Proceedings}, author={Wong, S.H. and Wygant, I.O. and Yeh, D.T. and Zhuang, X. and Bayram, B. and Kupnik, M. and Oralkan, Ö. and Ergun, A. S. and Yaralioglu, G. G. and Khuri-Yakub, B. T.}, year={2006}, pages={395–399} }
 @inproceedings{vaithilingam_wygant_kuo_zhuang_oralkan_olcott_khuri-yakub_2006, title={Capacitive micromachined ultrasonic transducers (CMUTs) for photoacoustic imaging}, url={http://dx.doi.org/10.1117/12.659459}, DOI={10.1117/12.659459}, abstractNote={In photoacoustic (optoacoustic) medical imaging, short laser pulses irradiate absorbing structures found in tissue, such as blood vessels, causing brief thermal expansions that in turn generate ultrasound waves. These ultrasound waves which correspond to the optical absorption distribution were imaged using a two dimensional array of capacitive micromachined ultrasonic transducers (CMUTs). Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large two dimensional arrays, arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. In this study, a phantom consisting of three 0.86-mm inner diameter polyethylene tubes inside a tissue mimicking material was imaged using a 16 x 16 element CMUT array. The center tube was filled with India-ink to provide optical contrast. Traditional pulse-echo data as well as photoacoustic image data were taken. 2D cross-sectional slices and 3D volume rendered images are shown. Simple array tiling was attempted, whereby a 48 x 48 element array was simulated, to illustrate the advantages of larger arrays. Finally, the sensitivity of the photoacoustics setup to the concentration of ink in the tube was also explored. For the sensitivity experiment a different phantom consisting of only one 1.14-mm inner diameter polyethylene tube inside a tissue mimicking material was used. The concentration of the ink inside the tube was varied and images were taken.}, booktitle={Photons Plus Ultrasound: Imaging and Sensing 2006: The Seventh Conference on Biomedical Thermoacoustics, Optoacoustics, and Acousto-optics}, publisher={SPIE}, author={Vaithilingam, Srikant and Wygant, Ira O. and Kuo, Paulina S. and Zhuang, Xuefeng and Oralkan, Ömer and Olcott, Peter D. and Khuri-Yakub, Butrus T.}, editor={Oraevsky, Alexander A. and Wang, Lihong V.Editors}, year={2006}, month={Feb} }
 @inproceedings{bayram_kupnik_yaralioglu_oralkan_lin_zhuang_ergun_sarioglu_wong_khuri-yakub_et al._2006, title={Characterization of cross-coupling in capacitive micromachined ultrasonic transducers}, ISBN={0780393821}, url={http://dx.doi.org/10.1109/ultsym.2005.1602924}, DOI={10.1109/ultsym.2005.1602924}, abstractNote={This paper analyzes element-to-element and cell- to-cell cross-coupling in capacitive micromachined ultrasonic transducers (cMUTs) using an interferometer. In a 1-D linear cMUT array immersed in oil, a single element was excited, and membrane displacements were measured at different positions along the array with an interferometer. Electrical measurements of the received voltage on each array element were also performed simultaneously to verify the optical measurements. The array was then covered with a polydimethylsiloxane (PDMS) layer, and the cross-coupling measurements were repeated. The cross-coupling levels for conventional and collapsed operation of the cMUT were compared. Since the cMUTs were immersed in oil, the optical measurements were corrected for acousto-optic interaction, and the results were reviewed in time-spatial and frequency- spatial domains. The main cross-coupling mechanism was due to the dispersive guided modes supported by the membrane periodicity. In both modes of operation, cross-coupling dispersion curves predicted a gradual reduction in phase velocity at higher frequencies. At lower frequencies, this phase velocity tended to approach 1480 m/s asymptotically. Better cross-coupling suppression was observed in the collapsed (-34 dB) than the conventional operation (-23 dB). The element-to-element cross-coupling experiments showed that a 5-µm PDMS layer reduced the measured cross- coupling levels down to -39 dB in the collapsed operation. were corrected to eliminate the acousto-optic interaction due to the refractive index of the oil and the pressure created in the oil (4). The optical time domain measurements were analyzed in the wave number-frequency (k-w) domain for the multi-mode wave propagation (5). Conventional and collapsed operations of the cMUT were compared, and the influence of a 5-µm polydimethylsiloxane (PDMS) layer covering the cMUT was investigated. The main cross-coupling mechanism was due to the dispersive guided modes. Interface waves (Stoneley-Scholte) and surface waves (Rayleigh) were relatively weak in cross-coupling (3). The dispersive guided modes were determined for conventional and collapsed operations and corresponding k-w diagrams were analyzed.}, booktitle={IEEE Ultrasonics Symposium, 2005.}, publisher={IEEE}, author={Bayram, B. and Kupnik, M. and Yaralioglu, G.G. and Oralkan, Omer and Lin, Dersong and Zhuang, Xuefeng and Ergun, A.S. and Sarioglu, A.F. and Wong, S.H. and Khuri-Yakub, B.T. and et al.}, year={2006}, month={Mar} }
 @article{oralkan_bayram_yaralioglu_ergun_kupnik_yeh_wygant_khuri-yakub_2006, title={Experimental characterization of collapse-mode CMUT operation}, volume={53}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2006.1665109}, DOI={10.1109/tuffc.2006.1665109}, abstractNote={This paper reports on the experimental characterization of collapse-mode operation of capacitive micromachined ultrasonic transducers (CMUTs). CMUTs are conventionally operated by applying a direct current (DC) bias voltage less than the collapse voltage of the membrane, so that the membrane is deflected toward the bottom electrode. In the conventional regime, there is no contact between the membrane and the substrate; the maximum alternating current (AC) displacement occurs at the center of the membrane. In collapse-mode operation, the DC bias voltage is first increased beyond the collapse voltage, then reduced without releasing the collapsed membrane. In collapse-mode operation, the center of the membrane is always in contact with the substrate. In the case of a circular membrane, the maximum AC displacement occurs along the ring formed between the center and the edge of the membrane. The experimental characterization presented in this paper includes impedance measurements in air, pulse-echo experiments in immersion, and one-way optical displacement measurements in immersion for both conventional and collapse-mode operations. A 205-mum times 205-mum 2-D CMUT array element composed of circular silicon nitride membranes is used in the experiments. In pulse-echo experiments, a custom integrated circuit (IC) comprising a pulse driver, a transmit/receive switch, a wideband low-noise preamplifier, and a line driver is used. By reducing the parasitic capacitance, the use of a custom IC enables pulse-echo measurements at high frequencies with a very small transducer. By comparing frequency response and efficiency of the transducer in conventional and collapse regimes, experimental results show that a collapsed membrane can be used to generate and detect ultrasound more efficiently than a membrane operated in the conventional mode. Furthermore, the center frequency of the collapsed membrane can be changed by varying the applied DC voltage. In this study, the center frequency of a collapsed transducer in immersion is shown to vary from 20 MHz to 28 MHz with applied DC bias; the same transducer operates at 10 MHz in the conventional mode. In conventional mode, the maximum peak-to-peak pressure is 370 kPa on the transducer surface for a 40-ns, 25-V unipolar pulse excitation. In collapse mode, a 25-ns, 25-V unipolar pulse generates 590 kPa pressure at the surface of the transducer}, number={8}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Oralkan, O. and Bayram, B. and Yaralioglu, G.G. and Ergun, A.S. and Kupnik, M. and Yeh, D.T. and Wygant, I.O. and Khuri-Yakub, B.T.}, year={2006}, month={Aug}, pages={1513–1523} }
 @inproceedings{yeh_oralkan_wygant_ergun_wong_khuri-yakub_2006, title={High-resolution imaging with high-frequency 1-D linear CMUT arrays}, ISBN={0780393821}, url={http://dx.doi.org/10.1109/ultsym.2005.1602940}, DOI={10.1109/ultsym.2005.1602940}, abstractNote={High frequency ultrasound arrays allow dynamic focusing and shorter scan times, but the dimensions required at high frequencies present stringent challenges to current ul- trasound array fabrication technology. Using CMUTs facilitates building high frequency linear arrays, as demonstrated by a collapse-mode array (20 MHz) with 50-µm element pitch and two conventional-mode arrays (26 MHz and 41 MHz) with 36- µm element pitch. The fractional bandwidths are 85%, 52%, and 32% respectively; the narrow bandwidths are due to low fill factor. Thirty-two elements of the arrays were used for imaging line targets. In addition, B-scans of a rabbit eye are shown. I. INTRODUCTION}, booktitle={IEEE Ultrasonics Symposium, 2005.}, publisher={IEEE}, author={Yeh, D.T. and Oralkan, O. and Wygant, I.O. and Ergun, A.S. and Wong, J.H. and Khuri-Yakub, B.T.}, year={2006}, month={Mar} }
 @article{steen_baldewsing_degertekin_emelianov_frijlink_furukawa_goertz_karaman_khuri-yakub_kim_et al._2006, title={IVUS beyond the horizon}, volume={2}, number={1}, journal={EuroIntervention}, author={Steen, A. F. W. van der and Baldewsing, R. A. and Degertekin, F. L. and Emelianov, S. and Frijlink, M. E. and Furukawa, Y. and Goertz, D. and Karaman, M. and Khuri-Yakub, P. T. and Kim, K. and et al.}, year={2006}, month={May}, pages={132–142} }
 @inproceedings{zhuang_ergun_oralkan_wygant_khuri-yakub_2006, title={Interconnection and Packaging for 2D Capacitive Micromachined Ultrasonic Transducer Arrays Based on Through-Wafer Trench Isolation}, ISBN={0780394755}, url={http://dx.doi.org/10.1109/memsys.2006.1627788}, DOI={10.1109/memsys.2006.1627788}, abstractNote={A new process for connecting elements of a 2D capacitive micromachined ultrasonic transducer (CMUT) array to flip-chip bond pads on the back side of the transducer array is presented. Array elements are isolated from one another by trenches etched into the highly conductive silicon substrate using deep reactive ion etching (DRIE). Gold is deposited on the back side of the array to create the flip-chip bond pads. The array is then flip-chip bonded to an integrated circuit that comprises the front-end circuitry for the transducer and provides mechanical support for the isolated array elements. Because the interconnects are fabricated after the CMUT membranes, this technique is suitable for CMUT arrays regardless of the fabrication techniques used to form the membranes.}, booktitle={19th IEEE International Conference on Micro Electro Mechanical Systems}, publisher={IEEE}, author={Zhuang, Xuefeng and Ergun, A.S. and Oralkan, O. and Wygant, I.O. and Khuri-Yakub, B.T.}, year={2006}, month={May} }
 @inproceedings{zhuang_oralkan_khuri-yakub_ergun_lin_2006, title={P2P-6 Trench-Isolated CMUT Arrays with a Supporting Frame}, ISBN={1424402018}, url={http://dx.doi.org/10.1109/ultsym.2006.494}, DOI={10.1109/ultsym.2006.494}, abstractNote={We present a trench-isolated CMUT process with a supporting mesh frame for a fully populated 2D array. In this process, the CMUT array is built on a silicon-on-insulator (SOI) wafer. Electrical interconnections to array elements are provided through the highly conductive silicon substrate. Neighboring array elements are separated from one another by trenches on both the device layer and the bulk silicon. A mechanically supporting frame is designed as a mesh structure between the silicon pillars providing electrical connections to the individual elements. Like the frameless trench isolation process, the framed trench isolation is compatible with both wafer-bonded and surface-micromachined CMUTs. In addition, this process eliminates the need for attaching the device wafer to a carrier wafer for the required mechanical support during the deep trench etching and flip-chip bonding steps, which presents difficulties during the release of the carrier wafer}, booktitle={2006 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Zhuang, X. and Oralkan, O. and Khuri-Yakub, B. T. and Ergun, A. S. and Lin, D.-S.}, year={2006} }
 @inproceedings{khuri-yakub_karaman_oralkan_nikoozadeh_wygant_lee_yeh_2006, title={P3H-6 An Integrated Circuit with Transmit Beamforming and Parallel Receive Channels for Real-Time Three-Dimensional Ultrasound Imaging}, ISBN={1424402018}, url={http://dx.doi.org/10.1109/ultsym.2006.551}, DOI={10.1109/ultsym.2006.551}, abstractNote={We present the design of an integrated circuit (IC) that will be flip-chip bonded to a 16times16-element CMUT array. The IC provides 16 receive channels which can be configured to receive along either of the array diagonals or on any single row of the array. On transmit, all 256 elements can be used to transmit arbitrarily focused beams. Focused transmission with the full array is made possible by on-chip pulsers and memory. A 25-V pulser and 8-bit shift register is provided for each element of the array. Prior to each transmit, new values are loaded into the shift registers. Current-controlled one-shots control the transmit pulse widths. Circuit simulations and the IC layout are presented. Simulations predict that delay values can be loaded in less than 1.3 mus and show the generation of precisely timed pulses. The IC is being prepared for submission to National Semiconductor for fabrication in a high-voltage BiCMOS process}, booktitle={2006 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Khuri-Yakub, B. T. and Karaman, M. and Oralkan, O. and Nikoozadeh, A. and Wygant, I. and Lee, H. and Yeh, D. T.}, year={2006} }
 @inproceedings{khuri-yakub_karaman_oralkan_wygant_2006, title={P3H-7 Volumetric Imaging Using Fan-Beam Scanning with Reduced Redundancy 2D Arrays}, ISBN={1424402018}, url={http://dx.doi.org/10.1109/ultsym.2006.552}, DOI={10.1109/ultsym.2006.552}, abstractNote={Phased array processing with a fully populated 2D array produces the best image quality but requires excessive number of active parallel front-end channels. Here we explore four array designs with reduced redundancy in spatial frequency contents. To minimize the number of firings we employ fan-beam processing, where 1D arrays are used to insonify 2D planar slices of the volume at successive firing events; echo signals are collected by the receive array elements. The array designs are compared based on simulated point spread functions, frame rate, motion susceptibility, and signal-to-noise ratio}, booktitle={2006 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Khuri-Yakub, B. T. and Karaman, M. and Oralkan, O. and Wygant, I.}, year={2006} }
 @inproceedings{wygant_zhuang_kuo_yeh_oralkan_khuri-yakub_2006, title={Photoacoustic imaging using a two-dimensional CMUT array}, ISBN={0780393821}, url={http://dx.doi.org/10.1109/ultsym.2005.1603249}, DOI={10.1109/ultsym.2005.1603249}, abstractNote={Photoacoustic imaging is a promising complement to pulse-echo ultrasound imaging because it provides contrast between areas with different light absorption characteristics. Specifically, regions with higher blood concentration can be identified, which is useful for imaging vascularization and the early detection of cancer. Here we present volumetric photoacoustic images of a vessel-like phantom. The phantom consists of three 1.3-mm diameter tubes inside a tissue mimicking material. The center tube is filled with ink to provide optical contrast. A two-dimensional capacitive micromachined ultrasonic transducer (CMUT) array is used for acoustic detection. The use of a two-dimensional transducer array eliminates the drawbacks of a mechanically scanned system and enables volumetric imaging. CMUT technology enables new types of transducer arrays that would benefit photoacoustic imaging. Fully populated two-dimensional arrays, annular ring- arrays, and high-frequency arrays have all been demonstrated using CMUT technology and have advantages for photoacoustic imaging systems. Other advantages of CMUT technology for photoacoustic imaging include a wider bandwidth than comparable piezoelectric devices and ease of integration with electronics. Keywords-photoacoustics, capacitive micromachined ultrasonic transducer, CMUT, three-dimensional, integrated electronics}, booktitle={IEEE Ultrasonics Symposium, 2005.}, publisher={IEEE}, author={Wygant, I.O. and Zhuang, X. and Kuo, P.S. and Yeh, D.T. and Oralkan, O. and Khuri-Yakub, B.T.}, year={2006}, month={Mar} }
 @inproceedings{zhuang_ergun_oralkan_huang_wygant_yaralioglu_yeh_khuri-yakub_2006, title={Through-wafer trench-isolated electrical interconnects for CMUT arrays}, ISBN={0780393821}, url={http://dx.doi.org/10.1109/ultsym.2005.1602894}, DOI={10.1109/ultsym.2005.1602894}, abstractNote={Through-wafer trench-isolated interconnects provide electrical access to elements in a capacitive micromachined ultrasonic transducer (CMUT) array. The series resistance of these interconnects is low (4.5 Ω), and the element-to-element electrical crosstalk is insignificant (less than -53 dB at 5 MHz). This interconnect technique is compatible with both surface- micromachined and wafer-bonded CMUTs. Compared with the existing through-wafer via interconnect process, through- wafer trench isolation can be achieved by a simpler fabrication process and enables the implementation of curved CMUT arrays.}, booktitle={IEEE Ultrasonics Symposium, 2005.}, publisher={IEEE}, author={Zhuang, Xuefeng and Ergun, A.S. and Oralkan, O. and Huang, Yongli and Wygant, I.O. and Yaralioglu, G.G. and Yeh, D.T. and Khuri-Yakub, B.T.}, year={2006}, month={Mar} }
 @inproceedings{wygant_yeh_zhuang_nikoozadeh_oralkan_ergun_karaman_khuri-yakub_2005, title={A miniature real-time volumetric ultrasound imaging system}, url={http://dx.doi.org/10.1117/12.596019}, DOI={10.1117/12.596019}, abstractNote={Progress made in the development of a miniature real-time volumetric ultrasound imaging system is presented. This system is targeted for use in a 5-mm endoscopic channel and will provide real-time, 30-mm deep, volumetric images. It is being developed as a clinically useful device, to demonstrate a means of integrating the front-end electronics with the transducer array, and to demonstrate the advantages of the capacitive micromachined ultrasonic transducer (CMUT) technology for medical imaging. Presented here is the progress made towards the initial implementation of this system, which is based on a two-dimensional, 16x16 CMUT array. Each CMUT element is 250 um by 250 um and has a 5 MHz center frequency. The elements are connected to bond pads on the back side of the array with 400-um long through-wafer interconnects. The transducer array is flip-chip bonded to a custom-designed integrated circuit that comprises the front-end electronics. The result is that each transducer element is connected to a dedicated pulser and low-noise preamplifier. The pulser generates 25-V, 100-ns wide, unipolar pulses. The preamplifier has an approximate transimpedance gain of 500 kOhm and 3-dB bandwidth of 10 MHz. In the first implementation of the system, one element at a time can be selected for transmit and receive and thus synthetic aperture images can be generated. In future implementations, 16 channels will be active at a given time. These channels will connect to an FPGA-based data acquisition system for real-time image reconstruction.}, booktitle={Medical Imaging 2005: Ultrasonic Imaging and Signal Processing}, publisher={SPIE}, author={Wygant, Ira O. and Yeh, David T. and Zhuang, Xuefeng and Nikoozadeh, Amin and Oralkan, Omer and Ergun, Arif S. and Karaman, Mustafa and Khuri-Yakub, Butrus T.}, editor={Walker, William F. and Emelianov, Stanislav Y.Editors}, year={2005}, month={Apr} }
 @inproceedings{oralkan_hansen_bayram_yaralioglu_ergun_khuri-yakub_2005, title={CMUT ring arrays for forward-looking intravascular imaging}, ISBN={0780384121}, url={http://dx.doi.org/10.1109/ultsym.2004.1417748}, DOI={10.1109/ultsym.2004.1417748}, abstractNote={The paper describes an annular CMUT ring array designed and fabricated for the tip of a catheter used for forward-looking intravascular imaging. A 64-element, 2-mm average diameter array was fabricated as an experimental prototype. A single element in the array is connected to a single-channel custom front-end integrated circuit for pulse-echo operation. In conventional operation, the transducer operates at around 10 MHz. In the collapsed regime, the operating frequency shifts to 25 MHz and the received echo amplitude is tripled. The SNR is measured as 23 dB in a 50-MHz measurement bandwidth for an echo signal from a plane reflector at 1.5 mm. We also performed a nonlinear dynamic transient finite element analysis for the transducer, and found these results to be in good agreement with experimental measurements, both for conventional and collapsed operation.}, booktitle={IEEE Ultrasonics Symposium, 2004}, publisher={IEEE}, author={Oralkan, O. and Hansen, S.T. and Bayram, B. and Yaralioglu, G.G. and Ergun, A.S. and Khuri-Yakub, B.T.}, year={2005}, month={Apr} }
 @article{bayram_oralkan_ergun_haeggstrom_yaralioglu_khuri-yakub_2005, title={Capacitive micromachined ultrasonic transducer design for high power transmission}, volume={52}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2005.1406558}, DOI={10.1109/tuffc.2005.1406558}, abstractNote={Capacitive micromachined ultrasonic transducers (cMUTs) were developed to meet the demands of the ultrasonic industry. To achieve maximum efficiency, the conventional operation of the cMUT requires a bias voltage close to the collapse voltage. Total acoustic output pressure is limited by the efficiency of the cMUT and the maximum-allowed pulse voltage on the membrane. In this paper, we propose the collapse-snapback operation of the cMUT: the membrane is collapsed onto the substrate in the collapsing cycle, and released in the snapback cycle. The collapse-snapback operation overcomes the above-mentioned limitations of the conventional operation. The collapse-snapback operation utilizes a larger range of membrane deflection profiles (both collapsed and released profiles) and generates higher acoustic output pressures. The static finite element calculations were performed to design cMUTs with specific collapse and snapback voltages by changing the electrode parameters (radius (r/sub e/), position (d/sub e/), and thickness (t/sub e/)). These designs were refined for optimum average displacement per cycle. An electrode radius greater than 60% of the membrane radius significantly improved the displacement per volt. Moderately thick membranes (t/sub e//spl sim/0.2 /spl mu/m) were preferred, as thicker membranes reduced the displacement per volt. Under proper bias conditions, the collapse-snapback operation, designed for high-power transmission, allowed the application of pulse voltages larger than the difference of collapse and snapback voltages. Dynamic finite element calculations of an infinite cMUT array on the substrate loaded with acoustic fluid medium were performed to determine the dynamic response of the cMUT. Commercially available FEM packages ANSYS and LSDYNA were used for static and dynamic calculations, respectively. The cMUTs were fabricated for optimal performance in the collapse-snapback operation. The transmit experiments were performed on a 2-D cMUT array using a calibrated hydrophone. Taking into account the attenuation and diffraction losses, the pressure on the cMUT surface was extracted. The cMUT generated 0.47 MPa (6 kPa/V) and 1.04 MPa (11 kPa/V) in the conventional and collapse-snapback operations, respectively. Therefore, collapse-snapback operation of the cMUTs was superior for high-power transmission.}, number={2}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Bayram, B. and Oralkan, O. and Ergun, A.S. and Haeggstrom, E. and Yaralioglu, G.G. and Khuri-Yakub, B.T.}, year={2005}, month={Feb}, pages={326–339} }
 @inproceedings{ergun_zhuang_huang_oralkan_yaralioglu_khuri-yakub_2005, title={Capacitive micromachined ultrasonic transducer technology for medical ultrasound imaging}, url={http://dx.doi.org/10.1117/12.595692}, DOI={10.1117/12.595692}, abstractNote={Capacitive micromachined ultrasonic transducer (cMUT) technology has been recognized as an attractive alternative to the more traditional piezoelectric transducer technology in medical ultrasound imaging for several years now. There are mainly two reasons for the interest in this technology: Micromachining is derived from the integrated circuit technology and therefore shares the well-known advantages and experience of it. Also, capacitive transduction using thin membranes has fundamental superiorities over the piezoelectric transduction mechanism such as wide frequency bandwidth. Capacitive micromachined ultrasonic transducers are essentially capacitor cells where the two plates of the capacitor, the membrane and the substrate, are separated with a vacuum sealed cavity. Typically, a cMUT is made of many micro-scale capacitor cells operating in parallel. This paper describes a new fabrication technique for building cMUTs which is called the wafer-bonding method. In this method, the cavity and the membrane are defined on separate wafers and brought together by wafer-bonding in vacuum. The wafer-bonding method has several advantages over the traditional sacrificial release method of cMUT fabrication. It allows greater flexibility in the cMUT design which means better device performance. It reduces the number of process steps, device turn-around time, and increases the overall uniformity, reliability. and repeatability. Device examples of one-dimensional and two-dimensional arrays designed to work in the 1 to 50 MHz range with 100% fractional bandwidth highlight the advantages of this method, and show that cMUT technology is indeed the better candidate for next generation ultrasonic imaging arrays.}, booktitle={Medical Imaging 2005: Ultrasonic Imaging and Signal Processing}, publisher={SPIE}, author={Ergun, Arif S. and Zhuang, Xuefeng and Huang, Yongli and Oralkan, Omer and Yaralioglu, Goksen G. and Khuri-Yakub, Butrus T.}, editor={Walker, William F. and Emelianov, Stanislav Y.Editors}, year={2005}, month={Apr} }
 @article{erguri_huang_zhuang_oralkan_yarahoglu_khuri-yakub_2005, title={Capacitive micromachined ultrasonic transducers: fabrication technology}, volume={52}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2005.1563267}, DOI={10.1109/tuffc.2005.1563267}, abstractNote={Capacitive micromachined ultrasonic transducer (MUT) technology is a prime candidate for next generation imaging systems. Medical and underwater imaging and the nondestructive evaluation (NDE) societies have expressed growing interest in cMUTs over the years. Capacitive micromachined ultrasonic transducer technology is expected to make a strong impact on imaging technologies, especially volumetric imaging, and to appear in commercial products in the near future. This paper focuses on fabrication technologies for cMUTs and reviews and compares variations in the production processes. We have developed two main approaches to the fabrication of cMUTs: the sacrificial release process and the recently introduced wafer-bonding method. This paper gives a thorough review of the sacrificial release processes, and it describes the new wafer-bonding method in detail. Process variations are compared qualitatively and quantitatively whenever possible. Through these comparisons, it was concluded that wafer-bonded cMUT technology was superior in terms of process control, yield, and uniformity. Because the number of steps and consequent process time were reduced (from six-mask process to four-mask process), turn-around time was improved significantly.}, number={12}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Erguri, A.S. and Huang, Yongli and Zhuang, Xuefeng and Oralkan, O. and Yarahoglu, G.G. and Khuri-Yakub, B.T.}, year={2005}, month={Dec}, pages={2242–2258} }
 @article{johnson_oralkan_ergun_demirci_karaman_khuri-yakub_2005, title={Coherent array imaging using phased subarrays. Part II: simulations and experimental results}, volume={52}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2005.1397350}, DOI={10.1109/tuffc.2005.1397350}, abstractNote={For pt.I, see ibid., vol.52, no.1, p.37-50 (2005). The basic principles and theory of phased subarray (PSA) imaging provides the flexibility of reducing the number of front-end hardware channels between that of classical synthetic aperture (CSA) imaging - which uses only one element per firing event - and full-phased array (FPA) imaging - which uses all elements for each firing. The performance of PSA generally ranges between that obtained by CSA and FPA using the same array, and depends on the amount of hardware complexity reduction. For the work described in this paper, we performed FPA, CSA, and PSA imaging of a resolution phantom using both simulated and experimental data from a 3-MHz, 3.2-cm, 128-element capacitive micromachined ultrasound transducer (CMUT) array. The simulated system point responses in the spatial and frequency domains are presented as a means of studying the effects of signal bandwidth, reconstruction filter size, and subsampling rate on the PSA system performance. The PSA and FPA sector-scanned images were reconstructed using the wideband experimental data with 80% fractional bandwidth, with seven 32-element subarrays used for PSA imaging. The measurements on the experimental sector images indicate that, at the transmit focal zone, the PSA method provides a 10% improvement in the 6-dB lateral resolution, and the axial point resolution of PSA imaging is identical to that of FPA imaging. The signal-to-noise ratio (SNR) of PSA image was 58.3 dB, 4.9 dB below that of the FPA image, and the contrast-to-noise ratio (CNR) is reduced by 10%. The simulated and experimental test results presented in this paper validate theoretical expectations and illustrate the flexibility of PSA imaging as a way to exchange SNR and frame rate for simplified front-end hardware}, number={1}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Johnson, J.A. and Oralkan, O. and Ergun, S. and Demirci, U. and Karaman, M. and Khuri-Yakub, B.T.}, year={2005}, month={Jan}, pages={51–64} }
 @inproceedings{bayram_yaralioglu_ergun_oralkan_khuri-yakub_2005, title={Dynamic FEM analysis of multiple cmut cells in immersion}, ISBN={0780384121}, url={http://dx.doi.org/10.1109/ultsym.2004.1417714}, DOI={10.1109/ultsym.2004.1417714}, abstractNote={This paper reports on the accurate modeling of immersion capacitive micromachined ultrasonic transducers (cMUTs) using a time-domain, nonlinear FEM package. A cMUT device consists of many cells. In this paper, a square membrane was used as the unit cell to cover the transducer area by periodic replication on the surface. The FEM calculations were used to analyze the nonlinear operation regimes of the cMUT. Nonlinear operation regimes (collapsed and collapse-snapback) provided higher acoustic output pressures than the conventional operation. The FEM calculations were compared to transmit experiment results performed with a hydrophone, and good agreement was observed.}, booktitle={IEEE Ultrasonics Symposium, 2004}, publisher={IEEE}, author={Bayram, B. and Yaralioglu, G.G. and Ergun, A.S. and Oralkan, O. and Khuri-Yakub, B.T.}, year={2005}, month={Apr} }
 @article{bayram_yaralioglu_kupnik_ergun_oralkan_nikoozadeh_khuri-yakub_2005, title={Dynamic analysis of capacitive micromachined ultrasonic transducers}, volume={52}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2005.1563269}, DOI={10.1109/tuffc.2005.1563269}, abstractNote={Electrostatic transducers are usually operated under a DC bias below their collapse voltage. The same scheme has been adopted for capacitive micromachined ultrasonic transducers (cMUTs). DC bias deflects the cMUT membranes toward the substrate, so that their centers are free to move during both receive and transmit operations. In this paper, we present time-domain, finite element calculations for cMUTs using LS-DYNA, a commercially available finite element package. In addition to this DC bias mode, other new cMUT operations (collapse and collapse-snapback) have recently been demonstrated. Because cMUT membranes make contact with the substrate in these new operations, modeling of these cMUTs should include contact analysis. Our model was a cMUT transducer consisting of many hexagonal membranes; because it was symmetrical, we modeled only one-sixth of a hexagonal cell loaded with a fluid medium. The finite element results for both conventional and collapse modes were compared to measurements made by an optical interferometer; a good match was observed. Thus, the model is useful for designing cMUTs that operate in regimes where membranes make contact with the substrate.}, number={12}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Bayram, B. and Yaralioglu, G.G. and Kupnik, M. and Ergun, A.S. and Oralkan, O. and Nikoozadeh, A. and Khuri-Yakub, B.T.}, year={2005}, month={Dec}, pages={2270–2275} }
 @inproceedings{yeh_oralkan_ergun_zhuang_wygant_khuri-yakub_2005, title={High-frequency CMUT arrays for high-resolution medical imaging}, url={http://dx.doi.org/10.1117/12.595918}, DOI={10.1117/12.595918}, abstractNote={The paper describes high-frequency 1D CMUT arrays designed and fabricated for use in electronically scanned high-resolution ultrasonic imaging systems. Two different designs of 64-element linear CMUT arrays are presented. A single element in each array is connected to a single-channel custom front-end integrated circuit for pulse-echo operation. The first design has a resonant frequency of 43 MHz in air, and operates at 30 MHz in immersion. The second design exhibits a resonant frequency of 60 MHz in air, and operates at 45 MHz in immersion. Experimental results are compared to simulation results obtained from the equivalent circuit model and nonlinear dynamic finite element analysis; a good agreement is observed between these results. The paper also briefly discusses the effects of the area fill factor on the frequency characteristics of CMUTs, which reveals that the transducer active area should be maximized to obtain a wideband response at high frequencies.}, booktitle={Medical Imaging 2005: Ultrasonic Imaging and Signal Processing}, publisher={SPIE}, author={Yeh, David T. and Oralkan, Omer and Ergun, Arif S. and Zhuang, Xuefeng and Wygant, Ira O. and Khuri-Yakub, Butrus T.}, editor={Walker, William F. and Emelianov, Stanislav Y.Editors}, year={2005}, month={Apr} }
 @inproceedings{oralkan_hansen_bayram_yaralglu_ergun_khuri-yakub_2005, title={High-frequency CMUT arrays for high-resolution medical imaging}, ISBN={0780384121}, url={http://dx.doi.org/10.1109/ultsym.2004.1417747}, DOI={10.1109/ultsym.2004.1417747}, abstractNote={The paper describes high-frequency 1D CMUT arrays designed and fabricated for use in electronically scanned high-resolution ultrasonic imaging systems. Two different designs of 64-element linear CMUT arrays are presented. A single element in each array is connected to a single-channel custom front-end integrated circuit for pulse-echo operation. The first design has a resonant frequency of 43 MHz in air, and operates at 30 MHz in immersion. The second design exhibits a resonant frequency of 60 MHz in air, and operates at 45 MHz in immersion. Experimental results are compared to simulation results obtained from the equivalent circuit model and nonlinear dynamic finite element analysis; a good agreement is observed between these results. The paper also briefly discusses the effects of the area fill factor on the frequency characteristics of CMUTs, which reveals that the transducer active area should be maximized to obtain a wideband response at high frequencies.}, booktitle={IEEE Ultrasonics Symposium, 2004}, publisher={IEEE}, author={Oralkan, O. and Hansen, S.T. and Bayram, B. and Yaralglu, G.G. and Ergun, A.S. and Khuri-Yakub, B.T.}, year={2005}, month={Apr} }
 @inproceedings{wygant_zhuang_yeh_nikoozadeh_oralkan_ergun_karaman_khuri-yakub_2005, title={Integrated ultrasonic imaging systems based on CMUT arrays: recent progress}, ISBN={0780384121}, url={http://dx.doi.org/10.1109/ultsym.2004.1417745}, DOI={10.1109/ultsym.2004.1417745}, abstractNote={The paper describes the development of an ultrasonic imaging system based on a two-dimensional capacitive micromachined ultrasonic transducer (CMUT) array. The transducer array and front-end electronics are designed to fit in a 5-mm endoscopic channel. A custom-designed integrated circuit, which comprises the front-end electronics, is connected with the transducer elements via through-wafer interconnects and flip-chip bonding. FPGA-based signal-processing hardware provides real-time three-dimensional imaging. The imaging system is being developed to demonstrate a means of integrating the front-end electronics with the transducer array and to provide a clinically useful technology. Integration of the electronics can improve signal-to-noise ratio, reduce the number of cables connecting the imaging probe to a separate processing unit, and provide a means of connecting electronics to large two-dimensional transducer arrays. The paper describes the imaging system architecture and the progress we have made on implementing each of its components: a 16/spl times/16 CMUT array; custom-designed integrated circuits; a flip-chip bonding technique; signal-processing hardware.}, booktitle={IEEE Ultrasonics Symposium, 2004}, publisher={IEEE}, author={Wygant, I.O. and Zhuang, X. and Yeh, D.T. and Nikoozadeh, A. and Oralkan, O. and Ergun, A.S. and Karaman, M. and Khuri-Yakub, B.T.}, year={2005}, month={Apr} }
 @inproceedings{wygant_yeh_zhuang_vaithilingam_nikoozadeh_oralkan_ergun_yaralioglu_khuri-yakub_2005, title={Integrated ultrasound imaging systems based on capacitive micromachined ultrasonic transducer arrays}, booktitle={Proceedings of the IEEE Conference on Sensors}, author={Wygant, I.O. and Yeh, D.T. and Zhuang, X. and Vaithilingam, S. and Nikoozadeh, A. and Oralkan, Ö. and Ergun, A.S. and Yaralioglu, G.G. and Khuri-Yakub, B.T.}, year={2005}, pages={704–707} }
 @inbook{ergun_yaralioglu_oralkan_khuri-yakub_2005, place={New York}, series={MEMS/NEMS Handbook}, title={Techniques and Applications of Capacitive Micromachined Ultrasonic Transducers}, volume={2}, booktitle={Fabrication Techniques for MEMS/NEMS}, publisher={Springer}, author={Ergun, A.S. and Yaralioglu, G.G. and Oralkan, Ö. and Khuri-Yakub, B.T.}, editor={Leondes, C.T.Editor}, year={2005}, pages={223–280}, collection={MEMS/NEMS Handbook} }
 @inproceedings{zhuang_wygant_yeh_nikoozadeh_oralkan_ergun_cheng_huang_yaralioglu_khuri-yakub_et al._2005, title={Two-dimensional capacitive micromachined ultrasonic transducer (CMUT) arrays for a miniature integrated volumetric ultrasonic imaging system}, url={http://dx.doi.org/10.1117/12.594702}, DOI={10.1117/12.594702}, abstractNote={We have designed, fabricated, and characterized two-dimensional 16x16-element capacitive micromachined ultrasonic transducer (CMUT) arrays. The CMUT array elements have a 250-μm pitch, and when tested in immersion, have a 5 MHz center frequency and 99% fractional bandwidth. The fabrication process is based on standard silicon micromachining techniques and therefore has the advantages of high yield, low cost, and ease of integration. The transducers have a Si3N4 membrane and are fabricated on a 400-μm thick silicon substrate. A low parasitic capacitance through-wafer via connects each CMUT element to a flip-chip bond pad on the back side of the wafer. Each through wafer via is 20 μm in diameter and 400 μm deep. The interconnects form metal-insulator-semiconductor (MIS) junctions with the surrounding high-resistivity silicon substrate to establish isolation and to reduce parasitic capacitance. Each through-wafer via has less than 0.06 pF of parasitic capacitance. We have investigated a Au-In flip-chip bonding process to connect the 2D CMUT array to a custom integrated circuit (IC) with transmit and receive electronics. To develop this process, we fabricated fanout structures on silicon, and flip-chip bonded these test dies to a flat surface coated with gold. The average series resistance per bump is about 3 Ohms, and 100% yield is obtained for a total of 30 bumps.}, booktitle={Medical Imaging 2005: Ultrasonic Imaging and Signal Processing}, publisher={SPIE}, author={Zhuang, Xuefeng and Wygant, Ira O. and Yeh, David T. and Nikoozadeh, Amin and Oralkan, Omer and Ergun, Arif S. and Cheng, Ching-Hsiang and Huang, Yongli and Yaralioglu, Goksen G. and Khuri-Yakub, Butrus T. and et al.}, editor={Walker, William F. and Emelianov, Stanislav Y.Editors}, year={2005}, month={Apr} }
 @inproceedings{oralkan_ergun_cheng_johnson_karaman_khuri-yakub_2005, title={Underwater acoustic imaging using capacitive micromachined ultrasonic transducer arrays}, ISBN={0780375343}, url={http://dx.doi.org/10.1109/oceans.2002.1191996}, DOI={10.1109/oceans.2002.1191996}, abstractNote={Capacitive micromachined ultrasonic transducers (CMUTs) have recently emerged as an alternative technology to piezoelectric transducers, offering advantages such as wide bandwidth, ease of fabricating large arrays and potential for integration with electronic circuits. In this paper, we present 2D and 3D pulse-echo imaging results using ID linear and 2D rectangular CMUT arrays, respectively. The aim of this paper is to demonstrate the viability of CMUTs for underwater acoustic imaging. For imaging experiments, we have fabricated ID and 2D CMUT arrays, and built an experimental setup allowing us to transmit and receive ultrasound signals from individual transducer elements. The image quality obtained shows that CMUTs are a strong alternative to conventional piezoelectric transducer arrays for the design of future generations of underwater acoustic imaging systems.}, booktitle={Oceans '02 MTS/IEEE}, publisher={IEEE}, author={Oralkan, O. and Ergun, A.S. and Cheng, C.-H. and Johnson, J.A. and Karaman, M. and Khuri-Yakub, B.T.}, year={2005}, month={Aug} }
 @article{demirci_ergun_oralkan_karaman_khuri-yakub_2004, title={Forward-viewing CMUT arrays for medical imaging}, volume={51}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2004.1320749}, DOI={10.1109/tuffc.2004.1320749}, abstractNote={This paper reports the design and testing of forward-viewing annular arrays fabricated using capacitive micromachined ultrasonic transducer (CMUT) technology. Recent research studies have shown that CMUTs have broad frequency bandwidth and high-transduction efficiency. One- and two-dimensional CMUT arrays of various sizes already have been fabricated, and their viability for medical imaging applications has been demonstrated. We fabricated 64-element, forward-viewing annular arrays using the standard CMUT fabrication process and carried out experiments to measure the operating frequency, bandwidth, and transmit/receive efficiency of the array elements. The annular array elements, designed for imaging applications in the 20 MHz range, had a resonance frequency of 13.5 MHz in air. The immersion pulse-echo data collected from a plane reflector showed that the devices operate in the 5-26 MHz range with a fractional bandwidth of 135%. The output pressure at the surface of the transducer was measured to be 24 kPa/V. These values translate into a dynamic range of 131.5 dB for 1-V excitation in 1-Hz bandwidth with a commercial low noise receiving circuitry. The designed, forward-viewing annular CMUT array is suitable for mounting on the front surface of a cylindrical catheter probe and can provide Doppler information for measurement of blood flow and guiding information for navigation through blood vessels in intravascular ultrasound imaging.}, number={7}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Demirci, U. and Ergun, A.S. and Oralkan, O. and Karaman, M. and Khuri-Yakub, B.T.}, year={2004}, month={Jul}, pages={887–895} }
 @inbook{khuri-yakub_ergun_huang_cheng_oralkan_johnson_yaralioglu_karaman_2004, title={Micromachined Ultrasonic Transducers and Their use for 2D and 3D Imaging}, ISBN={9789048166527 9781402024023}, ISSN={0270-5117}, url={http://dx.doi.org/10.1007/978-1-4020-2402-3_1}, DOI={10.1007/978-1-4020-2402-3_1}, abstractNote={Capacitive micromachined ultrasonic transducers (cMUTs) have proven to have remarkable features such as wide bandwidth and high sensitivity allowing the implementation of systems with wide dynamic range. This paper will review the theory and implementation of CMUTs that enable performance that is superior to piezoelectric transducers. In particular, the paper will discuss one-dimensional and two-dimensional arrays, broad frequency of operation (10 kHz to 50 MHz), and high coupling coefficient (k T 2 value as high as 0.85), and the use of these arrays in imaging applications.Traditionally, the number of transmit and receive processing channels in a full phased array imaging system is equal to the number of transducers in an ultrasound imaging system. Certain applications limit the number of processing channels such that there are fewer channels than transducer elements. For these cases, phased subarray imaging can be used to reduce the number of transducer elements required for each firing event. Experimental images using the full phased array and phased subarray imaging techniques are compared. Initial results indicate that imaging using subarrays leads to a major reduction in hardware with a small reduction in frame rate and signal to noise ratio.}, booktitle={Acoustical Imaging}, publisher={Springer Netherlands}, author={Khuri-Yakub, B. T. and Ergun, A. S. and Huang, Y. and Cheng, C. H. and Oralkan, O. and Johnson, J. and Yaralioglu, G. G. and Karaman, M.}, year={2004}, pages={1–9} }
 @inproceedings{johnson_oralkan_ergun_demirci_karaman_khuri-yakub_2004, title={Phased subarray imaging for low-cost, wideband coherent array imaging}, ISBN={0780379225}, url={http://dx.doi.org/10.1109/ultsym.2003.1293280}, DOI={10.1109/ultsym.2003.1293280}, abstractNote={The front-end hardware complexity of conventional full phased array (FPA) imaging is proportional to the number of array elements. Phased subarray (PSA) imaging has been proposed as a method of reducing the hardware complexity $and therefore system cost and size - while achieving near-FPA image quality. A new method is presented for designing the subarray-dependent interpolation filters suitable for wideband PSA imaging. The method was tested experimentally using pulse-echo data of a wire target phantom acquired using a 3.2-cm, 128-element capacitive micromachines ultrasonic transducer (CMUT) array with 85% fractional bandwidth at 3 MHz. A specific PSA configuration using seven 32-element subarrays was compared to FPA imaging, representing a 4-fold reduction in front-end hardware complexity and a 43% decrease in frame rate. For targets near the fixed transmit focal distance, the mean 6-dB lateral resolution was identical to that of FPA, the axial resolution improved by 4%, and the SNR decreased by 5 dB. Measurements were repeated for 10 different PSA configurations with subarray sizes ranging from 4 to 60. The lateral and axial resolutions did not vary significantly with subarray size; both the SNR and contrast-to-noise ration (CNR) improved with increased subarray size.}, booktitle={IEEE Symposium on Ultrasonics, 2003}, publisher={IEEE}, author={Johnson, J.A. and Oralkan, O. and Ergun, A.S. and Demirci, U. and Karaman, M. and Khuri-Yakub, B.T.}, year={2004}, month={Jul} }
 @article{kaviani_oralkan_khuri-yakub_wooley_2003, title={A multichannel pipeline analog-to-digital converter for an integrated 3-d ultrasound imaging system}, volume={38}, ISSN={0018-9200}, url={http://dx.doi.org/10.1109/jssc.2003.813294}, DOI={10.1109/jssc.2003.813294}, abstractNote={An 8-channel 10-bit pipeline analog-to-digital converter, designed for use in an integrated three-dimensional ultrasound imaging system, has been implemented in a 0.25-/spl mu/m CMOS technology. Two parallel multiplexing sample-and-hold stages are employed to multiplex a total of eight adjacent ultrasound channels, each sampled at 20 MHz. The sampled and multiplexed signals are fed into two parallel time-interleaved pipeline paths, each operating at 80 MHz. The two parallel pipelines are subsequently multiplexed into a single pipeline operating at 160 MHz to conserve area and reduce complexity. An experimental prototype of the proposed architecture occupies less than 4 mm/sup 2/ of active silicon area and shows a peak signal-to-noise-plus-distortion ratio more than 54 dB for a 2.1-MHz input signal, while dissipating only 20 mW of analog power per input channel from a 2.5-V supply.}, number={7}, journal={IEEE Journal of Solid-State Circuits}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Kaviani, K. and Oralkan, O. and Khuri-Yakub, P. and Wooley, B.A.}, year={2003}, month={Jul}, pages={1266–1270} }
 @inproceedings{ergun_huang_cheng_oralkan_johnson_jagannathan_demirci_yaralioglu_karaman_khuri-yakub_et al._2003, title={Broadband capacitive micromachined ultrasonic transducers ranging from 10 kHz to 60 MHz for imaging arrays and more}, ISBN={0780375823}, url={http://dx.doi.org/10.1109/ultsym.2002.1192473}, DOI={10.1109/ultsym.2002.1192473}, abstractNote={Capacitive micromachined ultrasonic transducers (CMUTs) have long been studied. Past research has shown that CMUTs indeed have remarkable features such as wide bandwidth and high efficiency. This paper introduces an inclusion to the CMUT technology that uses the wafer-bonding technique to fabricate membranes on silicon. This new technology enables the fabrication of large membranes with large gaps, and expands the frequency span of CMUTs to 10 kHz in the low end. CMUT devices with different frequency spans are fabricated using both technologies, and tested. Electromechanical coupling efficiency, k/sub T//sup 2/, value as high as 0.85 and fractional immersion bandwidth as wide as 175 % are measured.}, booktitle={2002 IEEE Ultrasonics Symposium, 2002. Proceedings.}, publisher={IEEE}, author={Ergun, A.S. and Huang, Yongli and Cheng, C.-H. and Oralkan, Omer and Johnson, J. and Jagannathan, H. and Demirci, U. and Yaralioglu, G.G. and Karaman, M. and Khuri-Yakub, B.T. and et al.}, year={2003}, month={Dec} }
 @inproceedings{oralkn_jin_degertekin_khuri-yakub_2003, title={Simulation and experimental characterization of a 2-D, 3-MHz capacitive micromachined ultrasonic transducer (CMUT) array element}, ISBN={0780357221}, url={http://dx.doi.org/10.1109/ultsym.1999.849200}, DOI={10.1109/ultsym.1999.849200}, abstractNote={In this paper, a 400-/spl mu/m/spl times/400-/spl mu/m, 2-D capacitive micromachined ultrasonic transducer array element is experimentally characterized, and the results are found to be in agreement with theoretical predictions. As a receiver the transducer has a 1.8/spl times/10/sup -7/ nm//spl radic/Hz displacement sensitivity, and, as a transmitter, it produces 16.4 kPa/V of output pressure at the transducer surface at 3 MHz. The transducer also has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging. The radiation pattern measurements indicate a 3-dB acceptance angle of /spl plusmn/35 degrees in agreement with the theoretical predictions.}, booktitle={1999 IEEE Ultrasonics Symposium. Proceedings. International Symposium (Cat. No.99CH37027)}, publisher={IEEE}, author={Oralkn, O. and Jin, X.C. and Degertekin, F.L. and Khuri-Yakub, B.T.}, year={2003}, month={Jan} }
 @inproceedings{oralkan_ergun_cheng_johnson_karaman_lee_khuri-yakub_2003, title={Volumetric imaging using 2D capacitive micromachined ultrasonic transducer arrays (CMUTs): initial results}, ISBN={0780375823}, url={http://dx.doi.org/10.1109/ultsym.2002.1192483}, DOI={10.1109/ultsym.2002.1192483}, abstractNote={This paper presents the first volumetric images obtained using a 2D CMUT array with through-wafer via interconnects. An 8/spl times/6-element portion of a 32/spl times/64-element array flip-chip bonded onto a glass fanout chip was used in the experiments. This study experimentally demonstrates that 2D CMUT arrays can be fabricated with high yield using silicon micromachining processes, individual electrical connections can be provided using through-wafer interconnects, and the flip-chip bonding technique can be used to integrate the dense 2D arrays with electronic circuits for practical imaging applications.}, booktitle={2002 IEEE Ultrasonics Symposium, 2002. Proceedings.}, publisher={IEEE}, author={Oralkan, O. and Ergun, A.S. and Cheng, Ching-hsiang and Johnson, J.A. and Karaman, M. and Lee, T.H. and Khuri-Yakub, B.T.}, year={2003}, month={Dec} }
 @article{oralkan_ergun_cheng_johnson_karaman_lee_khuri-yakub_2003, title={Volumetric ultrasound imaging using 2-D CMUT arrays}, volume={50}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2003.1251142}, DOI={10.1109/tuffc.2003.1251142}, abstractNote={Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a candidate to overcome the difficulties in the realization of 2-D arrays for real-time 3-D imaging. In this paper, we present the first volumetric images obtained using a 2-D CMUT array. We have fabricated a 128/spl times/128-element 2-D CMUT array with through-wafer via interconnects and a 420-/spl mu/m element pitch. As an experimental prototype, a 32/spl times/64-element portion of the 128/spl times/128-element array was diced and flip-chip bonded onto a glass fanout chip. This chip provides individual leads from a central 16/spl times/16-element portion of the array to surrounding bondpads. An 8/spl times/16-element portion of the array was used in the experiments along with a 128-channel data acquisition system. For imaging phantoms, we used a 2.37-mm diameter steel sphere located 10 mm from the array center and two 12-mm-thick Plexiglas plates located 20 mm and 60 mm from the array. A 4/spl times/4 group of elements in the middle of the 8/spl times/16-element array was used in transmit, and the remaining elements were used to receive the echo signals. The echo signal obtained from the spherical target presented a frequency spectrum centered at 4.37 MHz with a 100% fractional bandwidth, whereas the frequency spectrum for the echo signal from the parallel plate phantom was centered at 3.44 MHz with a 91% fractional bandwidth. The images were reconstructed by using RF beamforming and synthetic phased array approaches and visualized by surface rendering and multiplanar slicing techniques. The image of the spherical target has been used to approximate the point spread function of the system and is compared with theoretical expectations. This study experimentally demonstrates that 2-D CMUT arrays can be fabricated with high yield using silicon IC-fabrication processes, individual electrical connections can be provided using through-wafer vias, and flip-chip bonding can be used to integrate these dense 2-D arrays with electronic circuits for practical 3-D imaging applications.}, number={11}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Oralkan, O. and Ergun, A.S. and Cheng, Ching-Hsiang and Johnson, J.A. and Karaman, M. and Lee, T.H. and Khuri-Yakub, B.T.}, year={2003}, month={Nov}, pages={1581–1594} }
 @inproceedings{kaviani_oralkan_khuri-yakub_wooley_2002, title={A multichannel, pipeline analog-to- digital converter for an integrated 3-D ultrasound imaging system}, booktitle={Proceedings of the European SolidState Circuits Conference (ESSCIRC)}, author={Kaviani, K. and Oralkan, Ö. and Khuri-Yakub, B.T. and Wooley, B.A.}, year={2002}, pages={263–266} }
 @inproceedings{denmirci_oralkan_johnson_ergun_karaman_khuri-yakub_2002, title={Capacitive micromachined ultrasonic transducer arrays for medical imaging: experimental results}, ISBN={0780371771}, url={http://dx.doi.org/10.1109/ultsym.2001.991878}, DOI={10.1109/ultsym.2001.991878}, abstractNote={Capacitive micromachined ultrasonic transducer (cMUT) arrays provide broad bandwidth, high sensitivity, low mechanical impedance, and potential for electronic integration, and thus are promising for medical imaging applications. We have designed and fabricated 1D and 2D MUT arrays of various sizes using standard integrated circuit fabrication processes. We improved the device parameters for medical imaging applications to achieve fully functional 64- and 128-element linear 1D cMUT arrays. We have also built a computer controlled experimental setup for collecting pulse-echo data from the test phantoms using MUT arrays. In this paper the design and optimization of the immersion cMUTs for medical imaging system are discussed, and the phased array B-scan sector images taken by 1D MUT arrays are presented.}, booktitle={2001 IEEE Ultrasonics Symposium. Proceedings. An International Symposium (Cat. No.01CH37263)}, publisher={IEEE}, author={Denmirci, U. and Oralkan, O. and Johnson, J.A. and Ergun, A.S. and Karaman, M. and Khuri-Yakub, B.T.}, year={2002}, month={Nov} }
 @article{oralkan_ergun_johnson_karaman_demirci_kaviani_lee_khuri-yakub_2002, title={Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?}, volume={49}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2002.1049742}, DOI={10.1109/tuffc.2002.1049742}, abstractNote={Piezoelectric materials have dominated the ultrasonic transducer technology. Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative technology offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with electronics. The aim of this paper is to demonstrate the viability of CMUTs for ultrasound imaging. We present the first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array. We fabricated 64- and 128-element 1-D CMUT arrays with 100% yield and uniform element response across the arrays. These arrays have been operated in immersion with no failure or degradation in performance over the time. For imaging experiments, we built a resolution test phantom roughly mimicking the attenuation properties of soft tissue. We used a PC-based experimental system, including custom-designed electronic circuits to acquire the complete set of 128/spl times/128 RF A-scans from all transmit-receive element combinations. We obtained the pulse-echo frequency response by analyzing the echo signals from wire targets. These echo signals presented an 80% fractional bandwidth around 3 MHz, including the effect of attenuation in the propagating medium. We reconstructed the B-scan images with a sector angle of 90 degrees and an image depth of 210 mm through offline processing by using RF beamforming and synthetic phased array approaches. The measured 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively. The electronic noise floor of the image was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk among array elements on the image quality. In the near field, some artifacts were observable extending out from the array to a depth of 2 cm. A tail also was observed in the point spread function (PSF) in the axial direction, indicating the existence of crosstalk. The relative amplitude of this tail with respect to the mainlobe was less than -20 dB.}, number={11}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Oralkan, O. and Ergun, A.S. and Johnson, J.A. and Karaman, M. and Demirci, U. and Kaviani, K. and Lee, T.H. and Khuri-Yakub, B.T.}, year={2002}, month={Nov}, pages={1596–1610} }
 @inproceedings{oralkan_ilker karsilayan_ali tan_2002, title={Design of all-pole low-pass ladder filters using current-mode damped integrators}, ISBN={0780330730}, url={http://dx.doi.org/10.1109/iscas.1996.539880}, DOI={10.1109/iscas.1996.539880}, abstractNote={A method for operational simulation of all-pole low-pass LC ladders filters by using current-mode damped integrators is introduced. The circuits obtained by this method need only current mirrors and capacitors and are convenient for realization in CMOS technology as well as can be used in other technologies.}, booktitle={1996 IEEE International Symposium on Circuits and Systems. Circuits and Systems Connecting the World. ISCAS 96}, publisher={IEEE}, author={Oralkan, O. and Ilker Karsilayan, A. and Ali Tan, M.}, year={2002}, month={Dec} }
 @inproceedings{jackson_robinson_oralkan_dumin_brown_2002, title={Differentiation between electric breakdowns and dielectric breakdown in thin silicon oxides}, ISBN={0780339851}, url={http://dx.doi.org/10.1109/ipfa.1997.638180}, DOI={10.1109/ipfa.1997.638180}, abstractNote={It has been known for some time that non-destructive electric breakdowns precede destructive thermal dielectric breakdown. We have been studying both processes in oxides between 5 nm and 80 nm in thickness. We have shown that the electric breakdowns can trigger dielectric breakdown under certain conditions. This triggering of dielectric breakdown causes TDDB distributions to be non-unique. The TDDB distributions could be shifted to shorter times if (a) the impedance of the test equipment was lowered and/or (b) the capacitance of the test equipment was raised. The implications of this work are discussed in terms of electric/dielectric breakdown models and practical circuit and device operation.}, booktitle={Proceedings of the 1997 6th International Symposium on the Physical and Failure Analysis of Integrated Circuits}, publisher={IEEE}, author={Jackson, J.C. and Robinson, T. and Oralkan, O. and Dumin, D.J. and Brown, G.A.}, year={2002}, month={Nov} }
 @article{johnson_oralkan_demirci_ergun_karaman_khuri-yakub_2002, title={Medical imaging using capacitive micromachined ultrasonic transducer arrays}, volume={40}, ISSN={0041-624X}, url={http://dx.doi.org/10.1016/s0041-624x(02)00161-0}, DOI={10.1016/s0041-624x(02)00161-0}, abstractNote={We are investigating the use of capacitive micromachined ultrasonic transducers (cMUT's) for use in medical imaging. We propose an ultrasound probe architecture designed to provide volumetric ultrasound imaging from within an endoscope channel. A complete automated experimental system has been implemented for testing the imaging performance of cMUT arrays. This PC-based system includes custom-designed circuit boards, a software interface, and resolution test phantoms. We have already fabricated 1D and 2D cMUT arrays, and tested the pulse-echo imaging characteristics of 1D arrays. Beamforming and image formation algorithms that aim to reduce the complexity of data acquisition hardware are tested via numerical simulations and using real data acquired from our system.}, number={1-8}, journal={Ultrasonics}, publisher={Elsevier BV}, author={Johnson, Jeremy and Oralkan, Ömer and Demirci, Utkan and Ergun, Sanlı and Karaman, Mustafa and Khuri-Yakub, Pierre}, year={2002}, month={May}, pages={471–476} }
 @inproceedings{jackson_oralkan_robinson_dumin_brown_2002, title={The non-uniqueness of breakdown distributions in silicon oxides}, ISBN={0780342054}, url={http://dx.doi.org/10.1109/irws.1997.661871}, DOI={10.1109/irws.1997.661871}, abstractNote={Time-dependent-dielectric-breakdown (TDDB) distributions obtained from oxides of the same physical geometry and stressed at the same electric field were found to shift to shorter times when the amount of energy available to flow through electric breakdowns was increased. This paper shows that TDDB distributions are nonunique and that for a breakdown model to accurately describe the reliability of an oxide during actual use conditions, the oxide thermal geometry must be taken into account. An accurate method of obtaining electric breakdown distributions is also presented which allows the use of smaller sample sizes to obtain time-dependent-electric-breakdown (TDEB) distributions which are similar to TDDB distributions.}, booktitle={1997 IEEE International Integrated Reliability Workshop Final Report (Cat. No.97TH8319)}, publisher={IEEE}, author={Jackson, J.C. and Oralkan, O. and Robinson, T. and Dumin, D.J. and Brown, G.A.}, year={2002}, month={Nov} }
 @article{johnson_oralkan_kaviani_demirci_karaman_khuri-yakub_2001, title={An ultrasonic volumetric scanner for image-guided surgery}, volume={1230}, ISSN={0531-5131}, url={http://dx.doi.org/10.1016/s0531-5131(01)00035-8}, DOI={10.1016/s0531-5131(01)00035-8}, abstractNote={The design of a two-dimensional ultrasonic array for providing real-time volumetric imaging during minimally invasive surgery is presented. The array is fully populated with capacitive micromachined ultrasonic transducers. A custom-designed experimental system is used to capture A-scans from the transducers. Pulse-echo images from a 16-element 1D array have been constructed. Point spread functions have been simulated for both 1D and 2D arrays.}, journal={International Congress Series}, publisher={Elsevier BV}, author={Johnson, Jeremy and Oralkan, Ömer and Kaviani, Kambiz and Demirci, Utkan and Karaman, Mustafa and Khuri-Yakub, Pierre}, year={2001}, month={Jun}, pages={190–196} }
 @article{jin_oralkan_degertekin_khuri-yakub_2001, title={Characterization of one-dimensional capacitive micromachined ultrasonic immersion transducer arrays}, volume={48}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/58.920706}, DOI={10.1109/58.920706}, abstractNote={We report on the characterization of 1D arrays of capacitive micromachined ultrasonic transducers (cMUT). A 275/spl times/5600 /spl mu/m 1D CMUT array element is experimentally characterized, and the results are found to be in agreement with theoretical predictions. As a receiver, the transducer has a 0.28-fm//spl radic/Hz displacement sensitivity, and, as a transmitter, it produces 5 kPa/V of output pressure at the transducer surface at 3 MHz with a DC bias of 35 V. The transducer has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging. The radiation pattern of isolated single elements, as well as those of array elements are measured, and two major sources of acoustical crosstalk are identified. A weakly dispersive non-leaky interface wave (Stoneley wave) is observed to be propagating at the silicon substrate-fluid interface at a speed close to the speed of sound in the fluid. This wave causes internal reflections, spurious resonance, and radiation from the edges of the silicon substrate. The large lateral component of the particle velocity generated by the membranes at the edge of the cMUT array elements is found to be the source of this interface wave. Lowest order Lamb waves in the silicon substrate are also found to contribute to the crosstalk between elements. These waves are excited at the edges of individual vibrating membranes, where they are anchored to the substrate, and result in a narrowing of the beam profile of the array elements. Several methods, such as trench isolation and wafer thinning, are proposed and implemented to modify the acoustical cross coupling between array elements.}, number={3}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Jin, Xuecheng and Oralkan, O. and Degertekin, F.L. and Khuri-Yakub, B.T.}, year={2001}, month={May}, pages={750–760} }
 @inproceedings{khuri-yakub_karaman_cheng_ergun_yaralioglu_bayram_demirci_hansen_badi_oralkan_2001, title={Micromachined capacitor ultrasonic transducers}, booktitle={Program and Extended Abstracts of The 10th US-Japan Seminar on Dielectric and Piezoelectric Ceramics}, author={Khuri-Yakub, B.T. and Karaman, M. and Cheng, C.-H. and Ergun, S. and Yaralioglu, G. and Bayram, B. and Demirci, U. and Hansen, S. and Badi, M. and Oralkan, Ö.}, year={2001}, pages={391–399} }
 @inproceedings{oralkan_jin_kaviani_ergun_degertekin_karaman_khuri-yakub_2000, title={Initial pulse-echo imaging results with onedimensional capacitive micromachined ultrasonic transducer arrays}, volume={1}, DOI={10.1109/ULTSYM.2000.922700}, abstractNote={In this paper, the first experimental results of pulse-echo imaging employing a 1-D capacitive micromachined ultrasonic transducer array are presented. A 1-D array consisting of 16 elements is used in the experiment. A wire phantom consisting of seven steel wires immersed in vegetable oil is used as the imaging target. A B-scan image with a sector angle of 90 degrees and an image depth of 150 mm is reconstructed by employing RF beamforming and synthetic phase array approaches. The reconstructed image is displayed at different display dynamic ranges. The measured near and far side lobe levels are around /spl sim/25 dB and /spl sim/35 dB, respectively. The noise floor of the image is below /spl sim/50 dB.}, booktitle={Proceedings of the IEEE Ultrasonics Symposium}, author={Oralkan, Ö. and Jin, X.C. and Kaviani, K. and Ergun, A.S. and Degertekin, F.L. and Karaman, M. and Khuri-Yakub, B.T.}, year={2000}, pages={959–962} }
 @article{khuri-yakub_cheng_degertekin_ergun_hansen_jin_oralkan_2000, title={Silicon Micromachined Ultrasonic Transducers}, volume={39}, ISSN={0021-4922 1347-4065}, url={http://dx.doi.org/10.1143/jjap.39.2883}, DOI={10.1143/jjap.39.2883}, abstractNote={ 
  This paper reviews capacitor micromachined ultrasonic 
  transducers (cMUTs). Transducers for air-borne and immersion 
  applications are made from parallel-plate capacitors whose 
  dimensions are controlled through traditional integrated circuit 
  manufacturing methods. Transducers for airborne ultrasound 
  applications have been operated in the frequency range of 
  0.1–11 MHz, while immersion transducers have been operated in the 
  frequency range of 1–20 MHz. The Mason model is used to represent 
  the cMUT and highlight the important parameters in the design of 
  both airborne and immersion transducers. Theory is used to compare 
  the dynamic range and the bandwidth of the cMUTs to piezoelectric 
  transducers. It is seen that cMUTs perform at least as well if not 
  better than piezoelectric transducers. Examples of single-element 
  transducers, linear-array transducers, and two-dimensional arrays of 
  transducers will be presented. 
  }, number={Part 1, No. 5B}, journal={Japanese Journal of Applied Physics}, publisher={Japan Society of Applied Physics}, author={Khuri-Yakub, Butrus T. and Cheng, Ching-Hsiang and Degertekin, Fahrettin-Levent and Ergun, Sanli and Hansen, Sean and Jin, Xue-Cheng and Oralkan, Omer}, editor={Zakharia, M.E. and Chevret, P. and Dubail, P.Editors}, year={2000}, month={May}, pages={2883–2887} }
 @inproceedings{khuri-yakub_cheng_degertekin_ergun_hansen_jin_oralkan_2000, place={New York}, title={Silicon micromachined ultrasonic transducers}, booktitle={Proceedings of the ASME Noise Control and Acoustic Division}, publisher={The American Society of Mechanical Engineers}, author={Khuri-Yakub, B.T. and Cheng, C.-H. and Degertekin, F.L. and Ergun, S. and Hansen, S. and Jin, X.C. and Oralkan, Ö.}, editor={Zakharia, M.E. and Chevret, P. and Dubail, P.Editors}, year={2000}, pages={153–171} }
 @article{jackson_oralkan_dumin_brown_1999, title={Electric breakdowns and breakdown mechanisms in ultra-thin silicon oxides}, volume={39}, ISSN={0026-2714}, url={http://dx.doi.org/10.1016/s0026-2714(98)00236-4}, DOI={10.1016/s0026-2714(98)00236-4}, abstractNote={It was found that the breakdown times measured using time-dependent-dielectric-breakdown (TDDB) distributions could be shifted to shorter times when the amount of energy available during the breakdown event was increased. The TDDB distributions were non-unique and breakdown models must account for both electrical breakdowns and dielectric breakdown. A novel approach for obtaining breakdown distributions will be presented. This approach uses a small number of oxides to obtain a time-dependent-electric-breakdown (TDEB) distribution, which will be shown to provide complementary information to that obtained from (TDDB) distributions. While the observation of dielectric breakdown in ultra-thin dielectrics may be difficult using standard test conditions, it will be shown that electric breakdowns are relatively easy to observe.}, number={2}, journal={Microelectronics Reliability}, publisher={Elsevier BV}, author={Jackson, J.C and Oralkan, Ö and Dumin, D.J and Brown, G.A}, year={1999}, month={Feb}, pages={171–179} }
 @inproceedings{jin_cheng._oralkan_calmes_degertekin_khuri-yakub_1999, title={Recent progress in capacitive micromachined ultrasonic immersion transducer array}, booktitle={Proceedings of the 8th International Symposium on Integrated Circuits, Devices and Systems}, author={Jin, X.C. and Cheng., C.H. and Oralkan, Ö. and Calmes, S. and Degertekin, F.L. and Khuri-Yakub, B.T.}, year={1999}, pages={159–162} }
 @article{oralkan_jin_degertekin_khuri-yakub_1999, title={Simulation and experimental characterization of a 2-D capacitive micromachined ultrasonic transducer array element}, volume={46}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/58.808855}, DOI={10.1109/58.808855}, abstractNote={In this letter, a 400-/spl mu/m/spl times/400-/spl mu/m 2-D capacitive micromachined ultrasonic transducer (cMUT) array element is experimentally characterised, and the results are found to be in good agreement with theoretical predictions. As a receiver, the transducer has a 1.8/spl times/10/sup -7/ nm//spl radic/(Hz) displacement sensitivity, and, as a transmitter, it produces 16.4 kPa/V of output pressure at the transducer surface at 3 MHz. The transducer also has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging.}, number={6}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Oralkan, O. and Jin, Xuecheng and Degertekin, F.L. and Khuri-Yakub, B.T.}, year={1999}, month={Nov}, pages={1337–1340} }
 @article{jackson_robinson_oralkan_dumin_brown_1998, title={Differentiation Between Electric Breakdowns and Dielectric Breakdown in Thin Silicon Oxides}, volume={145}, ISSN={0013-4651}, url={http://dx.doi.org/10.1149/1.1838384}, DOI={10.1149/1.1838384}, abstractNote={In several models of dielectric breakdown, nondestructive electric breakdowns precede destructive thermal dielectric breakdown. Both processes in oxides between 5 nm and 80 nm thick have been studied. The two breakdown phenomena have been differentiated, and the electric breakdowns have been separated from the dielectric breakdown. During constant voltage stressings, prior to dielectric breakdown, transient voltage spikes were measured and spots formed on the surface of the wafers due to electric breakdowns. Similar transient spikes occurred when measuring ramped breakdown voltages. It was found that the time dependent dielectric breakdown (TDDB) distributions measured on a series of identical oxides at the same voltages depended on the resistance and capacitance of the measurement test equipment due to the thermal nature of the dielectric breakdown. The TDDB distributions were shifted to shorter times if (i) the impedance of the test equipment was lowered and/or (ii) the capacitance of the test equipment was raised. The implications of this work are discussed in terms of electric and dielectric breakdown models and practical circuit and device operation.}, number={3}, journal={Journal of The Electrochemical Society}, publisher={The Electrochemical Society}, author={Jackson, J.C. and Robinson, T. and Oralkan, Ö. and Dumin, D.J. and Brown, G.A.}, year={1998}, pages={1033} }
 @article{chen_1998, title={The Search for Cathode and Anode Traps in High-Voltage Stressed Silicon Oxides}, volume={145}, ISSN={0013-4651}, url={http://dx.doi.org/10.1149/1.1838453}, DOI={10.1149/1.1838453}, abstractNote={When silicon oxide is stressed at high voltages, traps are generated inside the oxide and at the oxide's interfaces. The traps are negatively charged near the cathode and positively charged near the anode. The charge state of the traps can be easily changed by application of low voltages. Several models of trap generation have been proposed. These models involve either electron impact ionization processes or high field generation processes. We have attempted to determine the relative trap locations inside the oxides for oxides between 5 and 80 nm thick, in order to determine which processes are most likely. No evidence for a higher density of traps near the anode in any of these oxides was found, casting doubt on the efficiency of the impact ionization process in trap generation, even in thicker oxides. These data would support a trap generation model controlled by the high fields inside the oxides.}, number={4}, journal={Journal of The Electrochemical Society}, publisher={The Electrochemical Society}, author={Chen, L}, year={1998}, pages={1292} }
 @inproceedings{jackson_robinson_oralkan_dumin_brown_1997, title={Differentiation between electric breakdowns and dielectric breakdown in thin silicon oxides}, booktitle={Proceeding of the Symposium on Silicon Nitride and Silicon Dioxide Thin Insulating Films}, author={Jackson, J.C. and Robinson, T. and Oralkan, Ö. and Dumin, D.J. and Brown, G.A.}, year={1997}, pages={20–33} }
 @article{jackson_robinson_oralkan_dumin_brown_1997, title={Nonuniqueness of time-dependent-dielectric-breakdown distributions}, volume={71}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.120480}, DOI={10.1063/1.120480}, abstractNote={The time-dependent-dielectric-breakdown (TDDB) distributions measured on a series of identical oxides at the same voltages have been shown to depend on the resistance and capacitance of the measurement test equipment. The TDDB distributions were shifted to shorter times if the impedance of the test equipment was lowered and/or the capacitance of the test equipment was raised. The lower resistances and higher capacitances allowed the nonshorting early electric breakdowns to develop into shorting, thermal, dielectric breakdowns.}, number={25}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Jackson, J. C. and Robinson, T. and Oralkan, O. and Dumin, D. J. and Brown, G. A.}, year={1997}, month={Dec}, pages={3682–3684} }
 @inproceedings{chen_kang_oralkan_dumin_brown_bellutti_1997, title={The search for cathode and anode traps in high-voltage stressed silicon oxides}, booktitle={Proceedings of the Symposium on Silicon Nitride and Silicon Dioxide Thin Insulating Films}, author={Chen, L. and Kang, C.-S. and Oralkan, Ö. and Dumin, D.J. and Brown, G.A. and Bellutti, P.}, year={1997}, pages={88–98} }