Large Scale High Voltage 192+192 Row-Column Addressed CMUTs Made with Anodic Bonding

This work presents the fabrication process of anodically bonded 192+192 2D row-column addressed (RCA) capacitive micromachined ultrasonic transducer (CMUT) arrays. Four large 2.1 by 2.1 cm
²
arrays with a resonance frequency of 7.5 MHz in water and λ/2 element pitch at 95 μm are shown successfully fabricated. The arrays exhibit a 100% bond yield by anodically bonding an SOI wafer to a borosilicate glass wafer structured with CMUT cavities. A silicon nitride layer has been used at the wafer bonding interface to insulate the top and bottom electrodes for high voltage operation to at least ±200V DC bias. Electrical measurements have been performed on 192+192 RCA arrays and smaller 16+16 RCA arrays. These showed a resonance frequency of approximately 18 MHz in air at a bias of -200 V, corresponding to 80% of the pull-in voltage and 50 mV AC, with a coupling coefficient of 26.8 %.