Bandwidth and Thermal Stability Enhancement of Polymer-Integrated Piezoelectric Micromachined Ultrasonic Transducer Array for Air-Coupled Applications

Extending fractional bandwidth (FBW) and enhancing thermal stability in air-coupled Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) are critical challenges for improving spatial resolution in ultrasonic systems. This work proposes a dual approach to address these challenges in AlSc(30%)N PMUTs. First, the FBW is extended by a factor of 2 by introducing mechanical damping by adding a Parylene-C polymer layer as a mass load on the membrane. The trade-off between FBW enhancement and the inevitable loss in mechanical transmission sensitivity is also considered, to maintain an efficient transceiver functionality on the PMUT array. Second, to ensure thermal stability up to 100°C, we apply an annealing process that further increases the FBW and stabilizes the mechanical properties of the polymer coating by increasing crystallinity. Combining these techniques results in a robust PMUT design that significantly improves performance while maintaining practical operational sensitivity.