Theoretical Study of Thermally Stable Large-Coupling Lithium Niobate SH₀ Plate Wave Resonator

The thermal stabilization techniques for the fundamental (SH0) Plate acoustic wave (PAW) resonators based on LiNbO3 are theoretically investigated in this paper. The SH0 mode offers an ultra-large coupling coefficient (k(2)) at a certain crystal cut angle and normalized plate thickness, but its poor temperature coefficient of frequency (TCF) hinders its application in RF systems and needs further improvement urgently. By adding SiO2 with positive TCF to the LiNbO3 plate, a robust temperature compensation approach can be achieved for the SH0 resonators on LiNbO3/SiO2 bilayer and SiO2/LiNbO3/SiO2 sandwiched structures. The propagation characteristics of the SH0 wave propagating in the layered medium are carefully investigated. Despite the trade-off between TCF and k(2), the SiO2/LiNbO3/SiO2 structure provides large k(2) and near-zero TCF for wider thickness combinations. Furthermore, the periodic structure dispersion of the simplest zero-TCF stack is provided and the critical threshold IDT thickness is given.