Optimal Orientations of Lithium Niobate for Lateral-and Thickness-Field-Excitation PMUTs

This work identifies the optimal orientation of lithium niobate (LiNbO3, LN) for piezoelectric micromachined ultrasonic transducers (PMUTs) operating in lateral-field excitation (LFE) and thickness-field excitation (TFE) modes. Geometry-independent material figures of merit (FoMs), representing the round-trip signal-to-noise ratio (SNR), are evaluated by sweeping rotated material tensors across the full orientation space. Finite element method (FEM) simulation is then used to quantify the electromechanical coupling $k_t^2$ under consistent device stacks and electrode layouts. The FoMs peak at 140°Y-cut LiNbO3 (≈120% of PZT-5H); the best commercial TFE option, 128°Y-cut, attains ~65% of that maximum. Under the shared baseline design, the highest $k_t^2$ is achieved with X-cut LiNbO3 (≈7.2%) using elongated rectangular membranes, about 70% of the PZT-5H reference. Our results provide clear design guidance for LiNbO3 PMUTs to maximize performance: optimal cut, in-plane rotation, and membrane geometry.