@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{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{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{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{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} }
 @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{adelegan_coutant_zhang_yamaner_oralkan_2019, title={A 2D Capacitive Micromachined Ultrasonic Transducer (CMUT) Array with Through-Glass-Via Interconnects Fabricated Using Sacrificial Etching Process}, DOI={10.1109/ultsym.2019.8925783}, abstractNote={A two-dimensional (2D) transducer array is an integral part of a three-dimensional (3D) ultrasound imaging system as well as a compact ultrasound system for neurostimulation to steer and focus the beam in a volume. In this paper, a sacrificial etching-based fabrication process for implementing a 16x16-element 2D CMUT array on a glass substrate with through-glass-interconnects is described in detail. Across the fabricated 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. The fabricated 2D CMUT array shows a 100% element yield in fabrication and excellent uniformity in device performance. The process offers the advantages of developing 2D CMUT arrays on glass substrates that do not need to be compatible with anodic bonding.}, journal={2019 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Adelegan, Oluwafemi J. and Coutant, Zachary A. and Zhang, Xiao and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2019}, month={Oct} }
 @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{seok_yamaner_sahin_oralkan_2019, title={A Sub-Millimeter Lateral Resolution Ultrasonic Beamforming System for Brain Stimulation in Behaving Animals}, DOI={10.1109/embc.2019.8857627}, abstractNote={In this paper, we present a second-generation wireless ultrasonic beamforming system, aiming for a truly wearable device for brain stimulation in small behaving animals. The fully-integrated, battery-operated system enables a self- contained untethered system. The system is partitioned into two parts for weight distribution: (1) a 1D capacitive micromachined transducer (CMUT) array on a separate head-mountable flexible printed circuit board (PCB), (2) a rigid back-mountable PCB including electronics such as a custom ASIC, a power management unit, a wireless module, and a battery. The newly developed ASIC not only enables a compact electronic system (30.5 mm x 63.5 mm) but also generates 3.4 times higher acoustic pressure (1.89 MPaPP), which corresponds to a spatial-peak pulse-average intensity (ISPPA) of 33.5 W/cm2, at a depth of 5 mm, compared to the first-generation ASIC. The full width at half maximum (FWHM) of the pressure is estimated to be 0.6 mm, achieving a sub-millimeter lateral resolution by using 5-MHz focused waves.}, journal={2019 41st Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC)}, publisher={IEEE}, author={Seok, Chunkyun and Yamaner, F. Yalcin and Sahin, Mesut and Oralkan, Omer}, year={2019}, month={Jul} }
 @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} }
 @article{coutant_adelegan_mahmud_yamaner_oralkan_2019, title={Anodically Bonded CMUT with a Two-Layer Bottom Electrode for Increased Reliability and Reduced Parasitic Capacitance}, DOI={10.1109/ultsym.2019.8926211}, abstractNote={This paper introduces a new fabrication process variation for improving the reliability and performance of capacitive micromachined ultrasonic transducers by using a two-layer bottom electrode on a glass substrate. This structure decouples the active gap from the gap at the interconnects allowing for thicker nitride in the sealing area. A larger gap at the interconnects decreases their parasitic capacitance, and thicker sealing nitride decreases the effects of charging and can prevent dielectric breakdown.}, journal={2019 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Coutant, Zachary A. and Adelegan, Oluwafemi J. and Mahmud, Marzana Mantasha and Yamaner, Feysel Yalcin and Oralkan, Omer}, year={2019}, month={Oct} }
 @article{seok_ali_yamaner_oralkan_2019, title={Ultrasound Transmission through a Flexible Printed Circuit Board Bonded to the Front Side of a Capacitive Micromachined Ultrasonic Transducer Array: Feasibility Study}, DOI={10.1109/ultsym.2019.8925606}, abstractNote={Interconnecting the inner elements of a densely populated ultrasonic transducer array with electronics poses a great challenge when pads are located in the inner area such that they are not easily accessible or wire bonding is not a viable solution. To tackle that challenge, we propose the technique of front-side flip-chip bonding capacitive micromachined ultrasonic transducer (CMUT) arrays to flexible printed circuit boards (FPCBs). As the propagation through the flex material can cause signal attenuation, we measured the pressure reduction for a reference transducer and an experimental CMUT and observed that the reference underwent a 19% pressure reduction while the experimental CMUT experienced a 33% pressure reduction after transmission through the flex. We argue that the difference can be partly attributed to inappropriate underfill in the interface between the CMUT and the FPCB. The proposed packaging approach can potentially provide a versatile interconnecting scheme for densely populated small transducers required in applications such as ultrasound neuromodulation.}, journal={2019 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Seok, Chunkyun and Ali, Ziad and Yamaner, F. Yalcin and Oralkan, Omer}, year={2019}, month={Oct} }
 @article{seok_ali_yamaner_oralkan_2019, title={Ultrasound Transmission through a Flexible Printed Circuit Board Bonded to the Front Side of a Capacitive Micromachined Ultrasonic Transducer Array: Feasibility Study}, DOI={10.1109/ultsym.2019.8926143}, abstractNote={Interconnecting the inner elements of a densely populated ultrasonic transducer array with electronics poses a great challenge when pads are located in the inner area such that they are not easily accessible or wire bonding is not a viable solution. To tackle that challenge, we propose the technique of front-side flip-chip bonding capacitive micromachined ultrasonic transducer (CMUT) arrays to flexible printed circuit boards (FPCBs). As the propagation through the flex material can cause signal attenuation, we measured the pressure reduction for a reference transducer and an experimental CMUT and observed that the reference underwent a 19% pressure reduction while the experimental CMUT experienced a 33% pressure reduction after transmission through the flex. We argue that the difference can be partly attributed to inappropriate underfill in the interface between the CMUT and the FPCB. The proposed packaging approach can potentially provide a versatile interconnecting scheme for densely populated small transducers required in applications such as ultrasound neuromodulation.}, journal={2019 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Seok, Chunkyun and Ali, Ziad and Yamaner, F. Yalcin and Oralkan, Omer}, year={2019}, month={Oct} }
 @article{young_yamaner_oralkan_2019, title={Ultrasound-Based Post-Endovascular Aneurysm Repair (EVAR) Monitoring Device}, DOI={10.1109/ultsym.2019.8925770}, abstractNote={A diverse set of stimulators, transducers and data transceivers can be integrated into medically implantable devices using ultrasonic power. An application for such a device is postoperative monitoring of abdominal aortic aneurysm repair. Endoleaks are a potentially serious, but correctible, complication of aorta repair. A miniscule ultrasonic powered device incorporated into the stent-graft for endovascular delivery could provide on demand diagnostic information about the presence of endoleak by monitoring the dimensions of the aneurysm sack in real time. To power this device, a wireless ultrasonic power recovery scheme is demonstrated in oil using a single-element capacitive micromachined ultrasonic transducer, and a circuit made with commercially available discrete components. The 2-mm diameter element, a diode rectifier bridge and inductor-based boost circuit form the power conversion circuit delivering over 1 mW to the load. Other aspects of the implantable device are a pulse echo measurement of the diameter of the vessel lumen, and a data transmission method that encodes clock, data and transmission framing information in biphasic ultrasonic pulses and takes advantage of the wideband characteristics of the CMUT. This proof of concept is guiding the design of an integrated circuit implementing the key elements of the implantable device.}, journal={2019 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Young, Eric S. and Yamaner, F. Yalcin and Oralkan, Omer}, year={2019}, 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} }
 @article{seok_mahmud_kumar_adelegan_yamaner_oralkan_2018, title={A Low-Power Wireless Multichannel Gas Sensing System Based on a Capacitive Micromachined Ultrasonic Transducer (CMUT) Array}, journal={IEEE Internet of Things Journal}, author={Seok, C. and Mahmud, M.M. and Kumar, M. and Adelegan, O.J. and Yamaner, F.Y. and Oralkan, Ö.}, year={2018} }
 @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{seok_wu_yamaner_oralkan_2017, title={A front-end integrated circuit for a 2D capacitive micromachined ultrasound transducer (CMUT) array for a noninvasive neural interface to the retina}, ISBN={9781538633830}, url={http://dx.doi.org/10.1109/ultsym.2017.8092289}, DOI={10.1109/ultsym.2017.8092289}, abstractNote={A recent study showed that focused ultrasound can noninvasively create neural response in the retina in a similar way to light stimulus. To implement such functionality, we developed an algorithm for a 2D transducer array to project an image to the retina as an ultrasound field pattern (USFP) in a previous study. In this work, we report a front-end integrated circuit (IC) for a 2D CMUT array that realizes a USFP with quantized phases and amplitudes. In addition, we evaluated the projected image quality with Field II simulations considering non-linearity in phases extracted from a fabricated prototype chip.}, 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} }
 @article{sanders_zhang_wu_adelegan_yamaner_kudenov_oralkan_2017, title={A handheld 1D transparent CMUT array probe for photoacoustic imaging}, DOI={10.1109/ultsym.2017.8092108}, 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 will make a 1D PAI probe that integrates a fully packaged 1D CMUT array with improved transparency, a fiber bundle, in-probe optics, and low-noise amplifiers which will interface with a real-time imaging system.}, journal={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{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} }
 @article{zhang_yamaner_adelegan_oralkan_2017, title={An optically transparent air-coupled capacitive micromachined ultrasonic transducer (CMUT) fabricated using adhesive bonding}, DOI={10.1109/ultsym.2017.8092233}, abstractNote={Transparent transducers are desired in the applications where optics and acoustics are combined, such as integrating ultrasound sensing or parametric arrays for directional sound in display, combined optical and acoustical microparticle manipulation, and backward-mode photoacoustic imaging. Some piezoelectric materials, e.g., PVDF, LNO, and PLZT, have been investigated for such applications. Besides, a Fabry-Perot (Etalon) optical ultrasound sensor can be used as a transparent ultrasound receiver. Implementing a transparent transducer in CMUT technology is desired to take advantage of wide bandwidth, ease of fabrication, and broad selection of processing materials. We have demonstrated a CMUT with ITO bottom electrode for improved transparency fabricated using anodic bonding. The main hurdle for the transparency in the visible wavelength range was a 2-μm silicon plate. In this work, we have designed and fabricated a CMUT with a glass plate using adhesive bonding and achieved improved transparency in the full visible wavelength range.}, journal={2017 IEEE International Ultrasonics Symposium (IUS)}, publisher={IEEE}, author={Zhang, Xiao and Yamaner, Feysel Y. and Adelegan, Oluwafemi and Oralkan, Omer}, 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} }
 @article{mahmud_adelegan_sanders_zhang_yamaner_dayton_oralkan_2017, title={Improved CMUT structure and method of operation for dual-frequency acoustic angiography}, DOI={10.1109/ultsym.2017.8091920}, journal={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, F. Yalcin and Dayton, Paul A. and Oralkan, Omer}, year={2017}, month={Sep} }
 @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{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{yulug_hanoglu_yamaner_kilic_schabitz_2016, title={Focused Ultrasound and NXY-059 in Experimental Cerebral Ischemia: A New Therapeutic Opportunity?}, volume={15}, DOI={10.2174/187152731509161007122800}, number={9}, journal={CNS & Neurological Disorders - Drug Targets}, publisher={Bentham Science Publishers Ltd.}, author={Yulug, B. and Hanoglu, L. and Yamaner, F.Y. and Kilic, E. and Schabitz, W.R.}, year={2016}, pages={1010–1013} }
 @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}, ISBN={9781479982035}, url={http://dx.doi.org/10.1109/icsens.2015.7370208}, DOI={10.1109/icsens.2015.7370208}, abstractNote={In this study we extend our work on low-power CMUT chemical sensors from a single element to an array as a way to improve selectivity to volatile organic compound (VOC) analytes. A single channel of our sensor array comprises of a polymer-functionalized CMUT resonator in the feedback loop of a Colpitts oscillator, which consumes 0.76 mW, when operated continuously. Using our anodic-bonding based fabrication process, we fabricated 6-, 8-, and 15-channel prototype arrays with a standard deviation of 1 % in the parallel resonant frequency (4.5 MHz) in a 7×9-mm2 die area. We measured the response of three channels, one uncoated, one with a polyisobutylene (PIB) layer, and one with a polyvinyl alcohol (PVA) layer, to 20-ppm toluene vapor. Initial measurements show 1:13:37 ratio in the response of reference: PVA:PIB channels.}, booktitle={2015 IEEE SENSORS}, publisher={IEEE}, author={Mahmud, M. M. and Kumar, M. and Zhang, X. and Yamaner, F. Y. and Nagle, H. T. and Oralkan, O.}, year={2015}, month={Nov}, pages={181–184} }
 @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{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} }
 @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} }
 @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{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} }
 @article{yamaner_olcum_oguz_bozkurt_koymen_atalar_2012, title={High-power CMUTs: design and experimental verification}, volume={59}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2012.2318}, DOI={10.1109/tuffc.2012.2318}, abstractNote={Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with piezoelectric transducers in high-power applications. As the output pressures increase, nonlinearity of CMUT must be reconsidered and optimization is required to reduce harmonic distortions. In this paper, we describe a design approach in which uncollapsed CMUT array elements are sized so as to operate at the maximum radiation impedance and have gap heights such that the generated electrostatic force can sustain a plate displacement with full swing at the given drive amplitude. The pro- posed design enables high output pressures and low harmonic distortions at the output. An equivalent circuit model of the array is used that accurately simulates the uncollapsed mode of operation. The model facilities the design of CMUT parameters for high-pressure output, without the intensive need for computationally involved FEM tools. The optimized design requires a relatively thick plate compared with a conventional CMUT plate. Thus, we used a silicon wafer as the CMUT plate. The fabrication process involves an anodic bonding process for bonding the silicon plate with the glass substrate. To eliminate the bias voltage, which may cause charging problems, the CMUT array is driven with large continuous wave signals at half of the resonant frequency. The fabricated arrays are tested in an oil tank by applying a 125-V peak 5-cycle burst sinusoidal signal at 1.44 MHz. The applied voltage is in- creased until the plate is about to touch the bottom electrode to get the maximum peak displacement. The observed pressure is about 1.8 MPa with -28 dBc second harmonic at the surface of the array.}, number={6}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Yamaner, F. Yalcin and Olcum, Selim and Oguz, H. Kagan and Bozkurt, Ayhan and Koymen, Hayrettin and Atalar, Abdullah}, year={2012}, month={Jun}, pages={1276–1284} }
 @inproceedings{meynier_yanamer_canney_nguyen-dinh_carpentier_chapelon_2012, title={Performance assessment Of CMUTs in dual modality imaging/HIFU applications}, ISBN={9781467345620 9781467345613 9781467345606}, url={http://dx.doi.org/10.1109/ultsym.2012.0020}, DOI={10.1109/ultsym.2012.0020}, abstractNote={In this paper, multi-element arrays based on cMUT and piezoelectric technologies, using the same geometry, have been realized. The first part of the paper is focused on comparing both in terms of imaging performances. The CMUT is shown to be lower in sensivity but better in terms of bandwidth and resolution. The second part of the paper investigates the ability of the CMUT array for HIFU applications. The dual imaging-HIFU capability of the cMUT array is demonstrated. This is a new feature of the CMUT technology, as piezoelectric transducers are designed with a trade-off between bandwidth and transduction efficiency.}, booktitle={2012 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Meynier, Cyril and Yanamer, Yalcin and Canney, Michael and Nguyen-Dinh, An and Carpentier, Alexandre and Chapelon, Jean-Yves}, year={2012}, month={Oct} }
 @article{olcum_yamaner_bozkurt_koymen_atalar_2011, title={An equivalent circuit model for transmitting capacitive micromachined ultrasonic transducers in collapse mode}, volume={58}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2011.1966}, DOI={10.1109/tuffc.2011.1966}, abstractNote={The collapse mode of operation of capacitive micromachined ultrasonic transducers (CMUTs) was shown to be a very effective way to achieve high output pressures. However, no accurate analytical or equivalent circuit model exists for understanding the mechanics and limits of the collapse mode. In this work, we develop an equivalent nonlinear electrical circuit that can accurately simulate the mechanical behavior of a CMUT with given dimensions and mechanical parameters under any large or small signal electrical excitation, including the collapse mode. The static and dynamic deflections of a plate predicted from the model are compared with finite element simulations. The equivalent circuit model can estimate the static deflection and transient behavior of a CMUT plate to within 5% accuracy. The circuit model is in good agreement with experimental results of pulse excitation applied to fabricated CMUTs. The model is suitable as a powerful design and optimization tool for collapsed and uncollapsed CMUTs.}, number={7}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Olcum, S. and Yamaner, F. Y. and Bozkurt, A. and Koymen, H. and Atalar, A.}, year={2011}, month={Jul}, pages={1468–1477} }
 @article{olcum_yamaner_bozkurt_koymen_atalar_2011, title={CMUT array element in deep-collapse mode}, DOI={10.1109/ultsym.2011.0027}, abstractNote={Collapse and deep-collapse mode of operations have boosted the pressure outputs of capacitive micromachined ultrasonic transducers (CMUTs) considerably. In this work, we demonstrate a CMUT element operating in the deep-collapse mode with 25 V pulse excitation and without the effects of charge trapping. The fabricated CMUT element consists of 4 by 4 circular cells with 20 μm radius and 1 μm thick plates suspended over a 50 nm cavity. The overall size of the element is 0.190 mm by 0.19 mm. The collapse voltage of the plates is measured to be approximately 3V. By driving the CMUTs with 25V pulses in the deep-collapse mode without any bias, we achieved 1.2 MPa peak-to-peak pressure output on the surface of the CMUT element with a center frequency of 9 MHz and 100% fractional bandwidth. We applied 1000 consecutive electrical pulses with alternating polarity to the element and witnessed no change in the transmitted acoustic pulse.}, journal={2011 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Olcum, Selim and Yamaner, F. Yalcin and Bozkurt, Ayhan and Koymen, Hayrettin and Atalar, Abdullah}, year={2011}, month={Oct} }
 @article{olcum_yamaner_bozkurt_atalar_2011, title={Deep-collapse operation of capacitive micromachined ultrasonic transducers}, volume={58}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2011.2104}, DOI={10.1109/tuffc.2011.2104}, abstractNote={Capacitive micromachined ultrasonic transducers (CMUTs) have been introduced as a promising technology for ultrasound imaging and therapeutic ultrasound applications which require high transmitted pressures for increased penetration, high signal-to-noise ratio, and fast heating. However, output power limitation of CMUTs compared with piezoelectrics has been a major drawback. In this work, we show that the output pressure of CMUTs can be significantly increased by deep-collapse operation, which utilizes an electrical pulse excitation much higher than the collapse voltage. We extend the analyses made for CMUTs working in the conventional (uncollapsed) region to the collapsed region and experimentally verify the findings. The static deflection profile of a collapsed membrane is calculated by an analytical approach within 0.6% error when compared with static, electromechanical finite element method (FEM) simulations. The electrical and mechanical restoring forces acting on a collapsed membrane are calculated. It is demonstrated that the stored mechanical energy and the electrical energy increase nonlinearly with increasing pulse amplitude if the membrane has a full-coverage top electrode. Utilizing higher restoring and electrical forces in the deep-collapsed region, we measure 3.5 MPa peak-to-peak pressure centered at 6.8 MHz with a 106% fractional bandwidth at the surface of the transducer with a collapse voltage of 35 V, when the pulse amplitude is 160 V. The experimental results are verified using transient FEM simulations.}, number={11}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Olcum, S. and Yamaner, F. Y. and Bozkurt, A. and Atalar, A.}, year={2011}, month={Nov}, pages={2475–2483} }
 @article{yamaner_olcum_bozkurt_koymen_atalar_2011, title={Design and implementation of capacitive micromachined ultrasonic transducers for high power}, DOI={10.1109/ultsym.2011.6293596}, abstractNote={Capacitive micromachined ultrasonic transducers (CMUTs) have a strong potential to compete piezoelectric transducers in high power applications. In a CMUT, obtaining high port pressure competes with high particle velocity: a small gap is required for high electrostatic force while particle displacement is limited by the gap height. On the other hand, it is shown in [1] that CMUT array exhibits radiation impedance maxima over a relatively narrow frequency band. In this paper, we describe a design approach in which CMUT array elements resonate at the frequency of maximum impedance and have gap heights such that the generated electrostatic force in uncollapsed mode, can sustain particle displacement peak amplitude up to the gap height. The CMUT parameters are optimized for around 3 MHz of operation, using both a SPICE model and FEM. The optimized parameters require a thick membrane and low gap heights to get maximum displacement without collapsing membrane during the operation. We used anodic bonding process to fabricate CMUT arrays. A conductive 100 μm silicon wafer is bonded to a glass wafer. Before the bonding process, the silicon wafer is thermally oxidized to create an insulating layer which prevents break down in the operation. Then, the cavities are formed on the insulating layer by a wet etch. The gap height is set to 100 nm. Meanwhile, the glass wafer is dry etched by 120 nm and the etched area is filled by gold evaporation to create the bottom electrodes. The wafers are dipped into piranha solution and bonding process is done afterwards. The fabricated CMUTs are tested in an oil tank. To eliminate the DC voltage which may cause charging problem in the operation, we tried to drive the CMUT array with large continuous wave signals at half of the operating frequency. We observed 1MPa peak to peak pressure with -23 dB second harmonic at the surface of the array (Fig. 1). The proposed design further extends the operation of CMUTs. Observing low harmonic distortions at high output pressure levels, without any charging problem, make CMUT a big candidate for high power applications.}, journal={2011 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Yamaner, F. Yalcin and Olcum, Selim and Bozkurt, Ayhan and Koymen, Hayrettin and Atalar, Abdullah}, year={2011}, month={Oct} }
 @inproceedings{olcum_yamaner_bozkurt_koymen_atalar_2010, title={An equivalent circuit for collapse operation mode of CMUTs}, ISBN={9781457703829}, url={http://dx.doi.org/10.1109/ultsym.2010.5935697}, DOI={10.1109/ultsym.2010.5935697}, abstractNote={Collapse mode of operation of the capacitive mi-cromachined ultrasonic transducers (CMUTs) was shown to be a very effective way for achieving high output pressures. However, no accurate model exists for understanding the mechanics and limits of the collapse mode. In this work, we extend the analyses made for CMUTs working in uncollapsed mode to collapsed mode. We have developed an equivalent nonlinear electrical circuit that can accurately simulate the mechanical behavior of a CMUT under any large signal electrical excitation. The static and dynamic deflections of a membrane predicted by the model are compared with the finite element simulations. The equivalent circuit model can estimate the static deflection within 1% and the transient behavior of a CMUT membrane within 3% accuracy. The circuit model is also compared to experimental results of pulse excitation applied to fabricated collapse mode CMUTs. The model is suitable as a powerful design and optimization tool for the collapsed as well as the uncollapsed case of CMUTs.}, booktitle={2010 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Olcum, Selim and Yamaner, F. Yalcin and Bozkurt, Ayhan and Koymen, Hayrettin and Atalar, Abdullah}, year={2010}, month={Oct} }
 @inproceedings{yamaner_olcum_bozkurt_koymen_atalar_2010, title={Optimizing CMUT geometry for high power}, ISBN={9781457703829}, url={http://dx.doi.org/10.1109/ultsym.2010.5935942}, DOI={10.1109/ultsym.2010.5935942}, abstractNote={Capacitive micromachined ultrasonic transducers (CMUTs) have demonstrated various advantages over piezoelectric transducers. However, current CMUT designs produce low output pressures with high harmonic distortions. Optimizing the transducer parameters requires an iterative solution and is too time consuming using finite element (FEM) modelling tools. In this work, we present a method of designing high output pressure CMUTs with relatively low distortion. We analyze the behavior of a membrane under high voltage continuous wave operation using a nonlinear electrical circuit model. The radiation impedance of an array of CMUTs is accurately represented using an RLC circuit in the model. The maximum membrane swing without collapse is targeted in the transmit mode. Using SPICE simulation of the parametric circuit model, we design the CMUT cell with optimized parameters such as the membrane radius (a), thickness (tm), insulator thickness (ti) and gap height (tg). The model also predicts the amount of second harmonic at the output. To verify the accuracy of the results, we built a FEM model with the same CMUT parameters. The design starts by choosing ti for the given input voltage level. First, a is selected for the maximum radiation resistance of the array at the operating frequency. Second, tm is found for the resonance at the input frequency. Third, tg is chosen for the maximum membrane swing. Under this condition, a frequency shift in the resonant frequency occurs. Second and third steps are repeated until convergence. This method results in a CMUT array with a high output power and with low distortion.}, booktitle={2010 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Yamaner, F. Yalcin and Olcum, Selim and Bozkurt, Ayhan and Koymen, Hayrettin and Atalar, Abdullah}, year={2010}, month={Oct} }
 @article{guldiken_zahorian_yamaner_degertekin_2009, title={Dual-electrode CMUT with non-uniform membranes for high electromechanical coupling coefficient and high bandwidth operation}, volume={56}, ISSN={0885-3010}, url={http://dx.doi.org/10.1109/tuffc.2009.1169}, DOI={10.1109/tuffc.2009.1169}, abstractNote={In this paper, we report measurement results on dual-electrode CMUT demonstrating electromechanical coupling coefficient (k2) of 0.82 at 90% of collapse voltage as well as 136% 3 dB one-way fractional bandwidth at the transducer surface around the design frequency of 8 MHz. These results are within 5% of the predictions of the finite element simulations. The large bandwidth is achieved mainly by utilizing a non-uniform membrane, introducing center mass to the design, whereas the dual-electrode structure provides high coupling coefficient in a large dc bias range without collapsing the membrane. In addition, the non-uniform membrane structure improves the transmit sensitivity of the dual-electrode CMUT by about 2dB as compared with a dual electrode CMUT with uniform membrane.}, number={6}, journal={IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Guldiken, R.O. and Zahorian, J. and Yamaner, F. and Degertekin, F.}, year={2009}, month={Jun}, pages={1270–1276} }
 @inproceedings{yamaner_bozkurt_2009, title={Modeling the pulse-echo response of a 2D CMUT array element}, ISBN={9781424443895}, url={http://dx.doi.org/10.1109/ultsym.2009.5441944}, DOI={10.1109/ultsym.2009.5441944}, abstractNote={In this paper, we present an equivalent circuit model for a 2D CMUT array element to asses the overall pulse-echo response. The proposed circuit models the propagation medium, and incorporates reactive effects due to transducer dimensions which are comparable to the acoustic wavelength, and the effects of diffraction and loss. The model has been verified experimentally using hydrophone and pulse-echo measurements on a set-up that incorporates a high-voltage pulse generator and receive amplifier IC. The presented methodology can be used to evaluate the performance of a front-end IC within the EDA environment in which the circuit is designed.}, booktitle={2009 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Yamaner, Feysel Yalcin and Bozkurt, Ayhan}, year={2009}, month={Sep} }
 @inproceedings{olcum_oguz_senlik_yamaner_bozkurt_atalar_koymen_2009, title={Wafer bonded capacitive micromachined underwater transducers}, ISBN={9781424443895}, url={http://dx.doi.org/10.1109/ultsym.2009.5441699}, DOI={10.1109/ultsym.2009.5441699}, abstractNote={In this work we have designed, fabricated and tested CMUTs as underwater transducers. Single CMUT membranes with three different radii and 380 microns of thickness are fabricated for the demonstration of an underwater CMUT element. The active area of the transducer is fabricated on top of a 3" silicon wafer. The silicon wafer is bonded to a gold electrode coated glass substrate wafer 10 cm in diameter. Thermally grown silicon oxide layer is used as the insulation layer between membrane and substrate electrodes. Electrical contacts and insulation are made by epoxy layers. Single CMUT elements are tested in air and in water. Approximately 40% bandwidth is achieved around 25 KHz with a single underwater CMUT cell. Radiated pressure field due to second harmonic generation when the CMUTs are driven with high sinusoidal voltages is measured.}, booktitle={2009 IEEE International Ultrasonics Symposium}, publisher={IEEE}, author={Olcum, Selim and Oguz, Kagan and Senlik, Muhammed N. and Yamaner, F. Yalcin and Bozkurt, Ayhan and Atalar, Abdullah and Koymen, Hayrettin}, year={2009}, month={Sep} }
 @inproceedings{yamaner_cenkeramaddi_bozkurt_2008, title={Front-end IC design for intravascular ultrasound imaging}, ISBN={9781424419838}, url={http://dx.doi.org/10.1109/rme.2008.4595774}, DOI={10.1109/rme.2008.4595774}, abstractNote={Capacitive micromachined ultrasonic transducers (cMUT) technology is a new trend for intravascular ultrasound (IVUS) imaging. Large bandwidth, high sensitivity and compatibility to CMOS processes makes the cMUT a better choice compared to the conventional piezoelectric transducer. To exploit the merits of cMUT technology, an accurately designed front end circuit is required. The circuit functions as an output pulse driver for the generation of the acoustic signal and buffers the return echo. For an accurate evaluation before tape-out, the circuit has to be simulated using the post-layout extracted netlist of the IC with the electrical equivalent circuit that models the transducer pulse-echo behavior. In this paper, we present two different designs of front-end IC for 2D cMUT arrays that can be used for intravascular ultrasound imaging system. To simulate the response of the front-end circuit, we first developed a pulse-echo model for an array element using Mason Equivalent Circuit. The model is then combined with the front-end circuit using Cadence Spectre. The simulation results are verified by comparing them to experimental data obtained from the manufactured front-end IC. The results show that the front-end circuit tested with the equivalent circuit model of the cMUT elements is promising for the optimization of the overall system performance before manufacturing.}, booktitle={2008 Ph.D. Research in Microelectronics and Electronics}, publisher={IEEE}, author={Yamaner, F. Yalcin and Cenkeramaddi, Linga Reddy and Bozkurt, Ayhan}, year={2008}, month={Jun} }
 @inproceedings{harput_bozkurt_yamaner_2008, title={Ultrasonic phased array device for real-time acoustic imaging in air}, ISBN={9781424424283}, url={http://dx.doi.org/10.1109/ultsym.2008.0148}, DOI={10.1109/ultsym.2008.0148}, abstractNote={A real-time acoustic imaging system is developed as a prototype electronic travel aid (ETA) device. The design is implemented on a field programmable gate array (FPGA). A 6 channel transmit and 4 channel receive digital beamforming algorithm with dynamic focusing is accommodated in a FPGA. The developed system consists of a FPGA, pulser and receiver circuitry and separate transmitter and receiver arrays, which are constructed by using commercially available transducers. The transducer elements have a physical dimension of 1.9 wavelengths and a half-power beamwidth of 43deg at 40.8 kHz center frequency. The transmitter array is formed by aligning the transducers with minimum spacing between the elements, which is 2 wavelengths. Obviously, more than one wavelength inter-element spacing leads to the occurrence of grating lobes in the array response and decreases the Field of View (FOV) below the half-power beamwidth of transducers. To extend the FOV and eliminate the grating lobe, the receiver array is formed with 3 wavelength inter-element spacing. The non-identical element spacing makes the grating lobes of transmitter and receiver array to appear at different places. The described placement strategy and the functionality of the system is tested with several experiments. The results of these experiments prove the grating lobe suppression capability of the applied placement strategy.}, booktitle={2008 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Harput, Sevan and Bozkurt, Ayhan and Yamaner, Feysel Yalcin}, year={2008}, month={Nov} }
 @inproceedings{reddy_bozkurt_yamaner_ytterdal_2007, title={A Low Noise Capacitive Feedback Analog Front-end for CMUTs in IVUS Imaging}, booktitle={Proceedings of the 2007 IEEE Ultrasonics Symposium}, author={Reddy, C. Linga and Bozkurt, Ayhan and Yamaner, F.Y. and Ytterdal, Trond}, year={2007} }
 @inproceedings{yamaner_bozkurt_2007, title={Front-End IC Design for 2D cMUT Arrays: Modeling and Experimental Verification}, booktitle={Proceedings of the 2007 IEEE Ultrasonics Symposium}, author={Yamaner, F.Y. and Bozkurt, A.}, year={2007} }
 @article{cenkeramaddi_bozkurt_yamaner_ytterdal_2007, title={P4M-6 A Low Noise Capacitive Feedback Analog Front-End for CMUTs in Intra Vascular Ultrasound Imaging}, DOI={10.1109/ultsym.2007.539}, abstractNote={In this paper, we present the capacitive feedback analog front-end for intra vascular ultrasound (IVUS) imaging as opposed to the conventional resistive feedback analog front-ends. In our proposed capacitive feedback architecture, floating input node of the amplifier is dynamically biased during the transmit mode of the CMUTs (capacitive micromachined ultrasound transducers). During the reception mode, the biased voltage at the floating input node of the amplifier is stored on the gate of input transistor of the amplifier. A high voltage pulser circuit with small output capacitance is integrated on-chip with the proposed low-noise capacitive feedback receiver. The proposed capacitive feedback analog front-end circuit is designed using the 0.35 mum high-voltage CMOS technology library of Austria Microsystems Corporation. Based on post- layout simulation results, we were able to achieve an overall noise figure of less than 2 dB with the proposed capacitive feedback analog front end for the amplification of signals generated by a 100times200 mum2 CMUT array element.}, journal={2007 IEEE Ultrasonics Symposium Proceedings}, publisher={IEEE}, author={Cenkeramaddi, L. R. and Bozkurt, A. and Yamaner, F. Y. and Ytterdal, T.}, year={2007}, month={Oct} }
 @article{yamaner_bozkurt_2007, title={P4M-7 Front-End IC Design for 2D cMUT Arrays: Modeling and Experimental Verification}, DOI={10.1109/ultsym.2007.540}, abstractNote={In this paper, we present the modeling, design and test of a front-end IC for 2D cMUT arrays. In the modeling phase, 

present simulation results for a front-end circuit integrated with an array transducer element, and compare these with the experimental result for a front-end IC for 2D cMUT array.

The pulse-echo model for the transducer is a modified version of the Mason Equivalent Circuit where the radiation impedance term has been replaced by an RLC network to include the effects of finite transducer size and diffraction loss. The model has been verified by running transient simulations using ANSYS. 


The circuit was composed of a high voltage (50 Volt) pulse driver, a NMOS protection switch and a trans impedance amplifier. The IC was manufactured by AustriaMicroSystems AG, Graz, Austria, in 0.35 μm high-voltage CMOS technology. We wire bonded the IC to a cMUT element to test the overall circuit performance. The cMUT elements that we used in the experiments had an operating frequency of 10 MHz and consisted of 49 CMUT cells with an overall transducer area of 200×200 µm2. The applied DC bias was 70 Volts. The cMUTs were driven by a 40 Volts unipolar pulse. We first performed hydrophone measurements to verify the functionality of the driver circuit. A droplet of vegetable oil was used as the propagation medium for pulse-echo measurements. The echo was observed from the air-oil interface. The results show that the performance of the circuit was consistent with the simulations. We were able to receive an echo from the surface of an oil layer of thickness less than 0.5 mm. (approximately 1 μs round-trip flight time.) The overall layout size of the manufactured circuit is 170×170 µm2 and it is suitable for integration to 3-5 MHz cMUT elements.}, journal={2007 IEEE Ultrasonics Symposium Proceedings}, publisher={IEEE}, author={Yamaner, F. Y. and Bozkurt, A.}, year={2007}, month={Oct} }
 @inproceedings{yamaner_bozkurt_2006, title={A Lumped Circuit Model for the Mutual Radiation Impedance of Acoustic Array Elements}, booktitle={Proceedings of the 2006 IEEE Ultrasonics Symposium}, author={Yamaner, F.Y. and Bozkurt, A.}, year={2006} }
 @article{bozkurt_yamaner_2006, title={P3R-6 A Lumped Circuit Model for the Mutual Radiation Impedance of Acoustic Array Elements}, DOI={10.1109/ultsym.2006.295}, abstractNote={Closely spaced transducer elements used in ultrasonic imaging suffer from cross-coupling via the fluid medium. Circuit models for transducer elements in ultrasonic arrays have to account for the coupling effect for an accurate representation of their radiation impedance. In this paper, we present a circuit model for the cross-coupling effects among capacitive micromachined ultrasonic transducer (cMUT) elements. Using the finite element method (FEM) we first show that the mutual radiation impedance of cMUT cells with a physical distance much smaller that the acoustic wavelength obey the analytic results derived for plane piston transducers. Then, we show that this mutual impedance can be modeled by an electrical RLC tank circuit suitable to be used in general circuit simulators. The circuit shows 80% percent accuracy in between 5 to 15 MHz for a cMUT resonant frequency of 11.8 MHz. Found element values for R, L and, C components are 0.268 mOmega, 4.156 pH and, 49.946 muF respectively. We then combine this circuit with a previously proposed model for the self radiation impedance of a cMUT cell to yield an equivalent circuit representation that accounts for the cross-coupling effect. The proposed equivalent circuit proves itself to be useful in the analysis of transceiver front-end integrated circuits where an accurate transducer model is compulsory for optimizing circuit performance}, journal={2006 IEEE Ultrasonics Symposium}, publisher={IEEE}, author={Bozkurt, A. and Yamaner, F. Y.}, year={2006} }
 @article{engin_yamaner_engin_2005, title={A biotelemetric system for human ECG measurements}, volume={38}, ISSN={0263-2241}, url={http://dx.doi.org/10.1016/j.measurement.2005.04.001}, DOI={10.1016/j.measurement.2005.04.001}, abstractNote={The use of telemedicine capabilities to therapy chronically ill patients is becoming more and more clinically relevant and economically cost effective. This paper presents own designed a prototype telemedicine system which provides human electrocardiogram (ECG) signals transferring via a mobile phone. System also covers the management of electronic records of patient and access to databases on the hospital side. The parts of system include an ECG amplifier, a communicator for data transferring and automated software which can receive and monitor ECG data.}, number={2}, journal={Measurement}, publisher={Elsevier BV}, author={Engin, Mehmet and Yamaner, Yalçın and Engin, Erkan Zeki}, year={2005}, month={Sep}, pages={148–153} }