A novel flexible capacitive micromachined ultrasonic transducer (CMUT) array with isolated metallic islands riveted to a polymer film

This paper reports a method for fabricating CMUT arrays using a novel rivet structure to fasten isolated metal islands to the flexible polymer film. Previously, a stretchable CMUT array was reported from our group, in which both the membrane and the cavity were fabricated using polydimethylsiloxane (PDMS) then bonded together by O2 plasma when electrodes were made by screen-printing of liquid metal alloy. Comparing to the PDMS membrane, the nickel membrane used here has higher resonant frequency for ultrasonic imaging. In this CMUT array, nickel over-plating is employed to form the rivet structure for securing metal islands to the polymer film, which can solve the adhesion problem between metal and polymer. EPON 1002F photoresist can be patterned to form via holes and also serves as the polymer film. The calculation result shows that the concave bottom electrode can increase the device capacitance compared to the rectangular bottom. The preliminary experimental result shows a resonant frequency at around 6.25MHz by using an impedance analyzer.