@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{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{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} }
 @inproceedings{seok_mahmud_adelegan_zhang_oralkan_2016, title={A battery-operated wireless multichannel gas sensor system based on a capacitive micromachined ultrasonic transducer (CMUT) array}, DOI={10.1109/icsens.2016.7808803}, abstractNote={This paper reports on the design and implementation of a complete battery-operated wireless system for a mechanically resonant gas sensor based on a capacitive micromachined ultrasonic transducer (CMUT) array. A custom-designed front-end integrated circuit (IC) with eight inputs and a serial peripheral interface (SPI) was tightly integrated with a CMUT array. The power consumption of the front-end is 10 μW with a duty cycle of 1:60 corresponding to 1-s measurement time every minute. For the completeness of the system, a power management unit (PMU) was designed and interfaced with the described custom IC along with a wireless module. For multichannel operation, time-division multiplexing was adopted to minimize power consumption and prevent potential frequency locking between different channels. Multichannel wireless data acquisition with the described system was demonstrated by loading unfunctionalized sensor channels with humidity in human breath.}, booktitle={2016 ieee sensors}, author={Seok, C. and Mahmud, M. M. and Adelegan, O. and Zhang, X. and Oralkan, Omer}, year={2016} }
 @inproceedings{kumar_seok_mahmud_zhang_oralkan_2015, title={A low-power integrated circuit for interfacing a capacitive micromachined ultrasonic transducer (CMUT) based resonant gas sensor}, DOI={10.1109/icsens.2015.7370639}, abstractNote={In this work we present a complete end-to-end interface for a capacitive micromachined ultrasonic transducer (CMUT) intended for low-power gas sensing applications. A prototype chip was designed in a 0.18-μm BiCMOS process. Different blocks (a BJT-based Colpitts oscillator, an inverter-based oscillator, a sine-to-square wave converter, a digital frequency counter, and a parallel-to-serial converter) required for the complete system are discussed, designed, and tested for their standalone performance. Consequently, a complete system interfaced with a 3.6-MHz CMUT and providing a digital frequency output is presented. With duty cycling for one measurement per minute the system consumed 10 μW power.}, booktitle={2015 ieee sensors}, author={Kumar, M. and Seok, C. and Mahmud, M. M. and Zhang, X. and Oralkan, Omer}, year={2015}, pages={1781–1784} }