Enhanced Spatial Resolution in Solid-Coupled Ultrasonics Using Piezoelectric Micromachined Ultrasonic Transducer (pMUT)

This study examines how array size influences the resonant behavior and sensitivity of piezoelectric micromachined ultrasonic transducer (pMUT) arrays designed for ultrasonic sensing. The pMUT arrays were designed with a target resonance frequency of 1 MHz. Experimental characterization revealed a resonance near 800 kHz, consistent with post-fabrication dimensional deviations from the design geometry. The influence of array size was also evaluated, and a slight decrease in the fundamental resonant frequency was observed as the number of arrays increased. This trend is attributed to additional electrode mass associated with oxidation in larger arrays, whereas finite element analysis predicts a small upward frequency shift due to increased substrate stiffness. In both cases, the observed frequency variations are small relative to experimental uncertainty and numerical approximations. Sensitivity measurements indicate that the pMUT array response increases with the number of arrays; however, the absolute sensitivity remains below that of conventional bulk ultrasonic transducers, requiring higher acquisition gain during testing. Despite this limitation, the pMUT arrays offer a smaller effective sensor aperture, which enables improved spatial resolution and more localized defect detection.