A 1-D Capacitive Micromachined Ultrasonic Transducer Imaging Array Fabricated With a Silicon-Nitride-Based Fusion Process

Capacitive micromachined ultrasonic transducers (CMUTs) are an alternative to the conventional method of generating ultrasound that increases bandwidths, simplifies fabrication, and facilitates the integration with necessary electronics. We report the fabrication, characterization, and initial-phased array imaging results of a 64-element array CMUT fabricated using a fusion bonding process where both the membrane and insulation layers are user deposited silicon nitride. Individual cells have a diameter of 25 μm and a membrane thickness of 500 nm. The center frequency in immersion is 6.6 MHz with a -6 dB fractional bandwidth of 123%. A 90° phased array sector scan is made of a four-wire target using a 32-element subset of the array. Pressures in excess of 2 MPa are measured. An axial resolution of 130 μm and a lateral resolution of 0.03 rad are obtained from a wire target 15 mm away from the transducer.