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M. Schneider, M. Dorfmeister, Ph. Moll, M. Kaltenbacher, U. Schmid:
"Bi-Stable Aluminum Nitride-Based Piezoelectric Micromachined Ultrasonic Transducer (PMUT)";
Journal Of Microelectromechanical Systems, 29 (2020), 948 - 953.

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.