Electrode Design Based on Strain Mode Shapes for Configurable PMUTs

Piezoelectric micromachined ultrasonic transducers (PMUTs) with multiple electrodes can be utilized as multi-frequency transducers by exciting selected vibration modes of the diaphragm providing configurability that may be beneficial in modern ultrasound imaging and therapeutic techniques. This requires judicious arrangement of the electrode configuration to excite the desired modes or combination of modes. Optimization of the electrode pattern can be done using full electroacoustic Finite Element Method (FEM) simulations, but this is computationally intensive and gives limited insight into the underlying physics. This paper presents a simple and efficient approach based on the in-vacuum strain mode shapes of the PMUT diaphragm to optimize the electrode configurations for an arbitrary PMUT cell resonating at any flexural mode. Strain mode shapes are obtained both from an analytical model as well as FEM. The proposed method is compared to and verified by full electroacoustic FEM simulations of PMUTs radiating into water. The optimal electrode patterns for the first few flexural modes are found for rectangular PMUTs with three different length-to-width aspect ratios, and examples of configurable PMUTs are given by combining the optimal electrodes at two different modes.