A 20.4-GHz Lithium Niobate A3-Mode Resonator with High Electromechanical Coupling of 6.95%

This work presents a third-order asymmetric (A3) mode Lamb wave resonator based on 128° Y-cut lithium niobate (LiNbO₃ or LN) thin film operating at a super high frequency (SHF) up to 20.4 GHz. By exploring the possible electromechanical coupling coefficient (k²) of A3-mode resonator in LiNbO₃ thin film of different orientations, we find that X-axis direction is the optimal direction to excite the A3-mode by applying horizontal electric field. Based on the analysis and simulation, spurious-free A3-mode resonator is designed and fabricated. The implemented device shows Q of 461 and k² of 6.95%, the highest reported FoM (Q·k²) for piezoelectric MEMS resonators operating in this frequency range. The SHF operation and high k² of the A3-mode resonator show great potential of acoustic resonator technology for 5G millimeter wave (mm-Wave) applications.