Capacitive micromachined ultrasonic transducers for transmitting and receiving ultrasound in air

A megahertz ultrasonic transmitting and receiving system operating in air is demonstrated using capacitive micromachined ultrasonic transducers (CMUTs). These CMUTs were designed and fabricated using a low-temperature direct wafer-bonding technique with circular and rectangular shapes. The electrical characteristics of the CMUTs, including impedance, phase, resistance, and reactance, were measured using an impedance analyzer. Results showed that the CMUTs can operate as an ultrasonic device. The six-element equivalent circuits for CMUTs were presented to characterize parasitic effects based on the measured impedance characteristics. The front-end interface circuits for transmitting and receiving ultrasound waves were simulated and optimized via PSpice methods to improve response stability and to reduce noise. The CMUTs were packaged with the front-end interface circuits to achieve a dynamic range of 44.67 dB at 1.72 MHz for transmitting and receiving ultrasound in air. The signal-to-noise ratio of the received signal without averaging was 31.31 dB at the transmission distance of 23.5 mm. The maximum transmission distance was evaluated as 88.2 mm with the maximum fractional bandwidth of 15.3%. The bandwidth and gain of CMUTs could be adjusted by changing the transmitting pulse numbers and the coupled DC bias voltages pulsing AC voltages.