Wide pressure range operation of air-coupled CMUTs

We present measurement results of capacitive micromachined ultrasonic transducers (CMUTs) in permanent contact mode over a wide pressure range (1 - 8 atm). The CMUT plates are in contact with the bottom of the cavities due to atmospheric pressure, even without any dc bias voltage. The electrical input impedance at various dc bias voltages are measured at elevated pressure to characterize individual devices. The open and short circuit resonant frequencies are extracted from the impedance data, and the acoustic performance of pairs of devices is evaluated by performing pitch-catch measurements. A frequency matching method is proposed and used to determine the optimal dc bias voltages for the transmitting and receiving CMUTs individually. Our electrical impedance results show good agreement with the finite element model results (modal and harmonic analysis performed with ANSYS) over the entire pressure range. Moreover, the pitch-catch measurement results validate the proposed frequency matching method for an optimal biasing scheme, and a received signal with good signal-to-noise ratio of 45 dB was observed at a pressure of 7 atm. In conclusion, the behavior of CMUTs in permanent contact mode can be predicted well with our FEA, and they are indeed a promising solution in providing ultrasonic transducers that can operate over a wide pressure range.