Surface-Micromachined CMUT Using Low-Temperature Deposited Silicon Carbide Membranes for Above-IC Integration

This paper presents a surface-micromachining technology to fabricate silicon carbide (SiC)-based capacitive micromachined ultrasonic transducers (CMUTs). The use of dc-sputtered amorphous SiC as a structural layer allows the fabrication process to limit the temperature to a thermal budget of 200 °C, which is the lowest reported to date, making this technology ideally suited for above-IC integration. The high Young´s modulus of the deposited SiC film, along with its very low residual stress, results in high strength and resilient CMUT membranes. The placement of the suspended aluminum electrode directly at the bottom side of the membrane reduces the effective size of the electrostatic transduction gap, resulting in superior electro-mechanical coupling. Fabricated transducers are tested in air with both continuous-wave and pulsed signals, using a pitch-and-catch configuration. The transducer pair, composed of 110-μm-diameter membrane arrays, exhibits a resonant frequency of 1.75 MHz, a 3 dB-bandwidth of 0.15 MHz, and a transmission gain of -38 dB. The CMUT prototypes showcase the versatility of low-temperature dc-sputtered SiC films applied in the field of MEMS.