Air-coupled CMUTs operating at ambient pressures ranging from 1 to 20 atm

We present impedance and pitch-catch measurements of capacitive micromachined ultrasonic transducers (CMUTs) in permanent contact mode with improved mechanical strength that demonstrate functionality up to 20 atm ambient pressure. Changes in device design and fabrication are made to improve the mechanical strength of the CMUT plates, including using smaller deflection to thickness ratio (9 - 33%), choosing better quality SOI wafers (bowing <; 20 μm), and designing a much larger bonding area (300 - 700 μm overlap in radial direction) for each cell. As a result, all designs with 2000 μm radius, 65-μm-thick plates, 7.74 μm gap heights and with 300, 500, and 700 μm wide bonding area overlap for the plate, performed from 1 - 20 atm without a single failure. Despite larger bonding area, pitch-catch measurements with these CMUTs (700 μm bonding width biased at 250 V dc) still give received signal with good SNR even at 20 atm. Our results support that such CMUTs are reliable and efficient over a wide pressure range.