Acoustics and Underwater Technologies Research Center (BASTA)

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    Beam steering in a half-frequency driven airborne CMUT transmitter array
    (IEEE Computer Society, 2019) Khan, Talha Masood; Taşdelen, Akif Sinan; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
    An airborne Capacitive Micromachined Ultrasonic Transducer (CMUT) transmit array was designed using electromechanical modelling for unbiased airborne operation. The array elements are designed for maximum swing at 10V p-p unbiased drive, whereas conventional practice is to bias CMUT close to the collapsed voltage to achieve higher swing. The devices were fabricated using a customized single photolithographic process with a combination of wet and dry etching. The wafer level fabrication enabled the usage of 2x2 and 3x3 arrays. Driving CMUTs in an unbiased mode at half frequency drives the ‘static pressure’ depressed silicon membrane at a larger swing without letting it collapse. The 2x2 array displays 3.375 kHz bandwidth when characterized in air. The phase and amplitude differences due to the dispersion of resonance frequencies of the elements are compensated for beamformed and beamsteered airborne operation.
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    Designing transmitting CMUT cells for airborne applications
    (Institute of Electrical and Electronics Engineers Inc., 2014) Ünlügedik, A.; Taşdelen, A.; Atalar, Abdullah; Köymen, Hayrettin
    We report a new mode of airborne operation for capacitive micromachined ultrasonic transducers (CMUT), in which the plate motion spans the entire gap without collapsing and the transducer is driven by a sinusoidal voltage without a dc bias. We present equivalent-circuit-based design fundamentals for an airborne CMUT cell and verify the design targets using fabricated CMUTs. The performance limits for silicon plates are derived. We experimentally obtain 78.9 dB//20 μPa@1 m source level at 73.7 kHz, with a CMUT cell of radius 2.05 mm driven by 71 V sinusoidal drive voltage at half the frequency. The measured quality factor is 120. We also study and discuss the interaction of the nonlinear transduction force and the nonlinearity of the plate compliance. © 1986-2012 IEEE.
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    High-Intensity airborne CMUT transmitter array with beam steering
    (IEEE, 2020) Khan, Talha Masood; Taşdelen, Akif Sinan; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
    A 2×2 high-intensity CMUT transmit array that is capable of two-dimensional beam steering is presented. The device uses an ac drive voltage at half the ultrasound frequency without any dc bias, enabling the usage of the entire gap height. The device is designed using a large signal equivalent model approach. A fabrication method that requires a single lithographic mask has been used. The fabricated devices are operated at 76 kHz to beam steer at various angles. An equivalent element pressure of 144 dB// 20 μ Pa at the transducer surface was measured. The entire half-space can be steered without any sidelobes and the beam obtained from the array is in excellent agreement with the theoretical predictions. [2020-0253]
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    Mic-in-CMOS: CMUT as a sealed-gap capacitive microphone
    (IEEE, 2020) Köymen, Hayrettin; Ahiska, Y.; Atalar, Abdullah; Köymen, I.; Taşdelen, A .Sinan; Yılmaz, Mehmet
    The design and production of a CMOS compatible, watertight and ingress-proof CMUT (capacitive micromachined ultrasonic transducer) microphone, mic-in-CMOS, with vacuum-gap is described. We present an analytical model-based approach for the design of mic-in-CMOS, where a basis for quantitative comparison of performance trade-offs is provided. The sealed vacuum gap of the mic-in-CMOS is basically a lossless sensor, free of mechanical noise. Its SNR is determined by the noise of the pre-amplification electronics (the noise contributor in a CMUT with vacuum gap is essentially the radiation resistance, which is less than 0 dBA for audio band for a 1 mm2 device). The design of mic-in-CMOS involves many multilateral trade-offs such as gap height vs membrane thickness vs sensitivity vs need for linear operation vs bias voltage and atmospheric depression, to name few. The mic-inCMOS design can be mass produced using CMOS film stacks only, as such the fabrication process can be carried out entirely in a CMOS processes production line complemented with CMOS compatible post-processing approaches. Mic-inCMOS has the advantage of low production cost with minimal packaging requirement and on-die EMI / EMC.
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    Optimization of a collapsed mode CMUT receiver for maximum off-resonance sensitivity
    (Institute of Electrical and Electronics Engineers, 2018-07-28) Khan, M.; Khan, T. M.; Taşdelen, A. S.; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
    We propose an airborne collapse capacitive micromachined ultrasonic transducer (CMUT) as a practical viable ultrasound transducer capable of providing a stable performance at the off-resonance frequencies. Traditional practice is to bias the CMUT plate close to collapse voltage to achieve high coupling coefficient and sense the incoming ultrasound as an open-circuit receive voltage signal of the transducer or short-circuit receive current (SCRC). Maintaining CMUT plate in the vicinity of collapse threshold is rather difficult. In this paper, an analytic approach to design an airborne collapsed-mode CMUT for maximum off-resonance sensitivity is presented. We use small-signal circuit model to evaluate the performance of a collapsed CMUT for varying operating conditions. CMUT operational parameters that yield the highest off-resonance SCRC are directly obtained from performance design curves. Collapsed CMUT plate is then biased in a critical biasing region that produces a stable and maximum off-resonance sensitivity. We experimentally verify and measure a stable sensitivity of a fabricated collapsed CMUT cell of -60 dB V/Pa at 100 kHz when biased between 50 to 65 V. We characterize our linear circuit model performance against the measured performance of collapsed CMUT in air within 4-dB tolerance. [2018-0058]
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    Transmitting CMUT arrays without a DC bias
    (IEEE Computer Society, 2019) Enhoş, Kerem; Taşdelen, Akif Sinan; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
    This study focuses on design, simulation, fabrication and measurement of transmitting CMUT arrays with unbiased mode of operation. We presented a design procedure considering the radiation pattern, Rayleigh distance and the parameters of the large signal equivalent circuit, which can be applied for different operation frequencies and applications. Large signal equivalent circuit model is used for lumped-element simulations. Harmonic balance and transient analyses are carried out with Gaussian enveloped tone burst, sinusoidal and pulse width modulation (PWM) signals with different duty cycles. Outputs of these simulations are fed in beamforming toolboxes for further verification. According to the design specifications transmitting CMUT arrays are fabricated. Corresponding experimental impedance measurements are conducted.
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    Unbiased charged circular CMUT microphone: lumped-element modeling and performance
    (Institute of Electrical and Electronics Engineers, 2018) Köymen, Hayrettin; Atalar, Abdullah; Güler, S.; Köymen, I.; Taşdelen, A. S.; Ünlügedik, A.
    An energy-consistent lumped-element equivalent circuit model for charged circular capacitive micromachined ultrasonic transducer (CMUT) cell is derived and presented. It is analytically shown and experimentally verified that a series dc voltage source at the electrical terminals is sufficient to model the charging in CMUT. A model-based method for determining this potential from impedance measurements at low bias voltages is presented. The model is validated experimentally using an airborne CMUT, which resonates at 103 kHz. Impedance measurements, reception measurements at resonance and off-resonance, and the transient response of the CMUT are compared with the model predictions.