Ultrahigh-Sensitivity Coupled Cantilever and Dual-Cavity Piezoelectric Micromachined Ultrasonic Transducers

Abstract

The round-trip sensitivity is a key metric for ultrasonic transducers. It is well known that the bimorph transducer design can effectively improve the sensitivity of a piezoelectric micromachined ultrasonic transducer (PMUT). However, inevitable process variations will lead to degraded sensitivity and frequency control issues in fabricated bimorph PMUTs. This letter presents a coupled cantilever and dual-cavity PMUT (CCDP) based on a bimorph structure. The design is insensitive to fabrication process variations, because the coupled cantilevers can deform to release residual stress while suppressing the sound pressure cancellation from misalignment. Moreover, dual cavities are adopted to precisely define the cantilever boundary and reduce the dimensional inconsistency at anchor among cantilevers. The typical measured transmitting and receiving sensitivities are 1.38 Pa/V/mm2 at a distance of 10 cm and 1.5 mV/Pa, respectively. The fabricated CCDP demonstrated the highest round-trip sensitivity per unit area among previously reported airborne PMUTs, and the sensitivity was more than 10 times greater than that of conventional ceramic ultrasonic transducers. Furthermore, the CCDP shows a resonant frequency uniformity of 4.9% across an 8-inch wafer. It is believed that even greater round-trip sensitivity could be achieved by combining the CCDP design with higher piezoelectric coefficient materials, such as PZT and AlScN.