A high coupling-efficiency actuator fabricated by a wafer level piezoelectric MEMS process with optimized multilayers and improved thermal budget

Abstract Lead zirconate titanate (PZT) has been commonly used as the functional material for piezoelectric microelectromechanical system actuators, which tend to adopt multi-layer stacked structures to achieve improved actuation performance and multifunctional capabilities. However, the thermal instability of PZT has caused a great challenge in the implementation of multi-layer integrated PZT actuators using wafer-level processes. In this work, the degradation of performance of the PZT thin film was mitigated through the optimization of electrode materials and the incorporation of a protection layer. The multi-layer integration was realized through wafer-level temporary bonding and low-thermal-budget parylene bonding, both designed to preserve the functional properties of PZT. The ferroelectric and piezoelectric properties of PZT with various electrode configurations (Ti/Pt, TiW, Pt, Cr/Au), both with and without thermal sliding debonding material, were characterized after the thermal treatment to simulate the bonding process. The Cr/Au electrode was demonstrated to induce the least degradation and, therefore, was chosen as the top electrode covered by an Al 2 O 3 protection layer. More severe degradation observed at elevated temperature highlighted the importance of the parylene bonding process. The quality of the parylene bonding was subsequently evaluated. The fabricated actuators retained 85.91% of the remanent polarization and 94.30% of the saturation polarization compared to pristine PZT films. The resonant frequency deviations of the vibrators were only 0.28% and 0.576% under separate and simultaneous driving, respectively, which can be attributed to the uniform and robust anchors formed by the wafer-level parylene bonding process.