Stress engineering design for improved frequency stability and amplitude in air-coupled PMUTs

Piezoelectric Micromachined Ultrasonic Transducers (PMUT) are widely used in distance sensing, medical imaging, non-destructive testing, and underwater communication due to their small size, low power consumption, and ease of arraying. However, PMUT design still faces challenges such as low amplitude and limited sound pressure. Moreover, during actual fabrication and operation, residual and accumulated stresses often cause uneven membrane deformation, frequency drift, and amplitude attenuation, which significantly impact the consistency of devices in the array and the overall acoustic performance of the system. To address these issues, we propose a staged stress optimization structural design method. This approach can significantly reduce the frequency deviation between devices from the same batch without adding process complexity. Experimental statistical analysis confirms that the optimized structure cuts the frequency standard deviation to approximately one-fourth of the original (from 1.39 kHz to 0.35 kHz) and nearly doubles the center displacement at the resonance frequency. Experimental validation with Doppler laser vibrometer (LDV) and optical microphone measurements confirmed excellent linearity up to 13V. Using CMOS-compatible piezoelectric aluminum nitride (AlN) thin films, this design enables seamlessly transfers to the fabrication of arrayed PMUT and offers a scalable and process-friendly stress engineering path for achieving high frequency consistency and high sound pressure output in ultrasonic arrays, providing valuable guidance for high-performance ultrasonic systems design and manufacturing.