An integrated optical detection method for capacitive micromachined ultrasonic transducers

Capacitive micromachined ultrasonic transducers (cMUTs) have so far relied on traditional detection methods where the capacitance change due to membrane deflection is measured under constant voltage. Sensitivity of this method is proportional to the frequency of the acoustic signal and the capacitance of the device. This limits the low-frequency operation and the maximum power output of the cMUTs. This paper describes an integrated optical detection method, which offers electrical isolation between transmit-receive electronics and displacement sensitivity independent of the gap thickness and acoustic frequency. These features alleviate the limitations on the transmit signal and DC bias levels. The method is demonstrated on a cMUT made of 1.3 mum thick, 100 mum diameter aluminum membranes fabricated on a fused quartz substrate. The aluminum back electrode of each membrane is shaped in the form of an optical diffraction grating. Thus the cMUT membrane and the diffraction fingers on the substrate form a phase sensitive diffraction grating. For displacement detection, the grating is illuminated through the quartz substrate by a laser and the intensity of reflected diffraction orders are monitored to provide the sensitivity of an optical interferometer. Displacement measurements on the cMUT array elements and ultrasonic pulse-echo experiments in air at 1.87MHz are presented.