Systematic Investigation of Wafer-Level Performance in AlScN-Based PMUTs

The systematic investigation of wafer-level performance in scandium-doped aluminum nitride (AlScN)-based piezoelectric micromachined ultrasonic transducers (PMUTs) is crucial for high-yield mass production and application development. In this work, pinned structures with 600- and 1000- $\mu $ m diaphragm diameters (Pinned 600U and Pinned 1000U) were prepared based on an 8-in SOI wafer. The optimization of micro-electromechanical system (MEMS) process parameters and the analysis of resonant behavior across multiple wafer-level devices based on the neutral axis model have been carried out systematically. The MEMS process parameters included the distribution of piezoelectric film thickness, piezoelectric film stress, diaphragm diameter, and residual silicon oxide after diaphragm release across the wafer. Pinned 600U exhibited greater wafer-level performance uniformity by optimizing key parameters that were mentioned. The uniformity of within-wafer and wafer-to-wafer was achieved with less than 1.5% and 4.5%, respectively, in resonant frequency and electromechanical coupling coefficient. The laser Doppler vibrometer (LDV) tests showed a highly consistent (1.56%) amplitude in resonant frequency across devices. High wafer-level performance uniformity enables high-yield mass production and thereby reduces costs. This study offers valuable wafer-level guidance, facilitating both high-yield mass production and the performance enhancement of PMUTs and other MEMS devices based on AlScN, particularly for array applications.