@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{zhang_ren_heitman_horton_2018, title={Summary of Advances in Heat-Pulse Methods: Measuring Near-Surface Soil Water Content}, volume={82}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2018.04.0138}, abstractNote={
Core Ideas
Describes the method for determining near‐surface water content with heat pulse sensors.
Temperature data prior to a heat‐pulse are used to reduce ambient temperature effects.
The PILS–ABC model is used to minimize errors because of the soil–air interface.
Surface layer soil water content is important for evaporation, surface energy balance, seed germination, residue decomposition, microbial activity, and many other biological, chemical, and physical processes. The standard method (i.e., the gravimetric method) for measuring soil water content requires destructive sampling and is unsuitable for continuous measurement. Techniques such as neutron thermalization and time domain reflectometry suffer relatively large errors in measuring soil water content near the surface. In a recent Methods of Soil Analysis article, the authors present the principles and procedures for using a heat‐pulse sensor to determine near‐surface soil water content.}, number={5}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Zhang, Xiao and Ren, Tusheng and Heitman, Joshua and Horton, Robert}, year={2018}, pages={1015–1015} }
@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 $16\boldsymbol {\times }16$ -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- $\boldsymbol {\Omega }$ parasitic resistance and a 20-fF shunt parasitic capacitance for 250- $\mu \text{m}$ 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- $\mu \text{m}$ diameter circular cells and a 1.5- $\mu \text{m}$ plate thickness is measured as 3.32 MHz at 15-V dc voltage (80% Vpull-in). [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{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{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{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} }
@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} }