Bi-Stable Aluminum Nitride-Based Piezoelectric Micromachined Ultrasonic Transducer (PMUT)

Abstract

This work presents a new type of piezoelectric micromachined ultrasonic transducer (PMUT) based on a bi-stable MEMS membrane structure with integrated aluminum nitride (AlN) actuators. By switching the membrane between both stable ground states, large, design-dependent membrane center displacements in the range of 10 µm can be achieved, which result in sound pressure levels of up to 96.8 dB in the ultrasonic range at frequencies between 30 and 60 kHz at a distance of 10.5 mm. This bi-stable operation mode can be triggered by moderate voltage levels of up to 40 V, which is not uncommon for PMUT devices. The usage of AlN compared to the more commonly utilized lead zirconate titanates (PZT) significantly simplifies the device integration process and ensures CMOS compatibility. A comprehensive comparison with similar devices in literature shows that despite the lower piezoelectric coefficients of AlN compared to PZT, this bi-stable actuation approach features exceptionally high stroke levels with a correspondingly high overall sound pressure level.