Fabrication of a Curved Row–Column Addressed Capacitive Micromachined Ultrasonic Transducer Array

Capacitive micromachined ultrasonic transducers (CMUTs) exhibit wide-bandwidth and can be made into tiny 2-D arrays that are integrable with electronics. A simple implementation of such 2-D array is the row-column addressed CMUT array. However, one critical limitation of a row-column array is its narrow field of view, which can be improved by curving the array. This paper reports a novel fabrication method that realizes a flexible 32 + 32 elements row-column addressed CMUT array using a two stage fabrication process. First, a functional row-column CMUT array is fabricated with a double silicon-on-insulator wafer bonding process. The second stage involves encapsulating the device with a polymer coating, yet leaving the bottom silicon substrate exposed for wet etching. The bottom buried oxide layer will also act as an etch stop layer. Pitch-catch experiments are presented to verify the functionality. Our final device, excluding the encapsulation layer, is less than 5 μm thick with a 12.8-mm × 12.8-mm aperture. The final flexible device exhibits a 4.5-MHz center frequency with a -3-dB fractional bandwidth of 82%.