Quasi-closed diaphragm based piezoelectric micromachined ultrasonic transducer with reduced Q and stress sensitivity for in-air rangefinding

To improve the performance of the Piezoelectric Micromachined Ultrasonic Transducer (PMUT) based rangefinder and decrease its stress sensitivity, a novel design with quasi-closed structure is proposed. It adopts a circular piezoelectric composite diaphragm structure with clamped boundary, in which all the deposited stack layers in its central region are intentionally removed and additional cross-slits are created into the remaining silicon device layer. Due to the reduced mass and the enhanced thermal-viscous damping at slits, a 35.2 % decrease in quality factor Q has been achieved in the proposed PMUT when compared with the conventional design, resulting in a distinctly reduced blind area from 231.3 mm to 170.7 mm. At the same time, the proposed quasi-closed PMUT facilitates the release of accumulated stress in the device structure during fabrication and operation. As a result, an approximate 50 % reduction in frequency deviation between different as-fabricated PMUTs across the same wafer has been successfully obtained. Moreover, due to the increased linear operation range, the developed bare PMUT chip demonstrates a maximum detection distance of 3 m at the operation frequency of 71.5 kHz under 40 V_pp driving voltage. Given the advantages of lower Q, insensitivity to stress, good fabrication consistency and large linear operation range, the proposed quasi-closed PMUT design can well address the requirements on small blind area and large detection range for distance sensing applications.