Piezoelectric micromachined ultrasonic transducer with increased output pressure via concentric venting rings

In a conventional piezoelectric micromachined ultrasonic transducer (PMUT), only half the acoustic output is used, because pressure emerging from the back of the PMUT is wasted. Here, we demonstrate a novel method to recycle the back-side acoustic pressure by redirecting it to the front-side through concentric venting rings. The ring diameter determines the phase-shift between the sound emerging from the front-side port and the ring, and can be adjusted to either amplify the far-field sound pressure level (SPL) or change the directivity of the output beam. 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. We have designed, fabricated, and characterized nine new venting designs, and we achieved a 4.5 dB increase in SPL for a design with a 400 μm radius venting ring.