Investigating Nonlinear Dynamic Response of Zinc Oxide Piezoelectric Micromachined Ultrasonic Transducers

Ultrasonic transducers have shown tremendous potential with wide-ranging applications in medical diagnostics, sensing and energy harvesting. Piezoelectric micromachined ultrasonic transducers (PMUTs) are ideal due to their miniature size, low power consumption, ease of fabricating transducer arrays, and integration with CMOS circuitry. We utilize highly oriented ZnO thin films grown by the microwave-assisted hydrothermal method as the piezo material to fabricate ZnO-based PMUTs. The PMUTs achieve high vibrational amplitude and depict nonlinearity.

In this work, we investigate the effect of fabrication-induced curvature in the circular PMUTs on the dynamics and nonlinearity of the diaphragm. We observe a clear transition from hardening to softening nonlinearity for devices of identical radial diameter (500μm) and increasing pre-curvature. Increasing the driving strength leads to a mixed nonlinear response depicting softening followed by hardening nonlinearity. The ability to change the curvature of a device through annealing and its associated transition in nonlinear response further confirms our observation of tuning of nonlinearity through curvature. We verify our observed experimental results through numerical simulations of the device response using a reduced-order model.