Capacitive micromachined ultrasonic transducers for generation of highly directional sound with a parametric array

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) with vacuum‐sealed cavities have been fabricated and used to project directional sound using parametric arrays. The wafer bonding approach used to fabricate the CMUTs provides good control over device dimensions, a single‐crystal silicon membrane that has predictable mechanical properties, and the capability to fabricate CMUTs with large‐diameter membranes and deep cavities. The fabricated CMUTs are about 8 cm in diameter and comprise 284 circular membranes, each 4 mm in diameter. Testing of CMUTs with 40‐μm and 60‐μm‐thick membranes shows they have center frequencies of 46 kHz and 55 kHz and 3‐dB bandwidths of 1.9 kHz and 5.3 kHz, respectively. With application of DC bias voltages of 380 V and 350 V and an AC excitation of 200 V peak‐to‐peak, the devices generate effective source levels of 139 dB and 131 dB (re 20 μPa), respectively. We used the CMUT design with a 60‐μm‐thick membrane to produce 5 kHz sound at 3 m with a 6‐dB beamwidth of 8.7 deg and a sound pressure level of 58 dB. The results demonstrate that large‐area CMUTs, which produce high pressure ultrasound, can be fabricated for transmitting directional sound with parametric arrays. [Work supported by DARPA.]