Air-coupled aluminum nitride piezoelectric micromachined ultrasonic transducers at 0.3 MHz TO 0.9 MHz

Air-coupled piezoelectric micromachined ultrasonic transducers (PMUTs) operating at frequencies ranging from 0.3 MHz to 0.9 MHz were designed, fabricated and characterized. We increased the fractional bandwidth by 51% and improved the piezoelectric coupling over 80% by patterning the diaphragm center into a ring or structural ribs, resulting in a reduction of the PMUT's mass. Pulse-echo testing was conducted in air using PMUTs at frequencies up to 0.9 MHz and the measured acoustic loss versus pathlength was compared to theoretical models. Devices were fabricated in an industrial foundry process using wafer-level bonding of a MEMS PMUT wafer to a CMOS wafer using a conductive metal eutectic bond. This process allows for close integration of PMUT arrays and signal processing circuitry and is used here to study the effects of wafer-level packaging on acoustic performance.