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

The thermal stabilization techniques for the fundamental (SH
0
) Plate acoustic wave (PAW) resonators based on LiNbO
3
are theoretically investigated in this paper. The SH
0
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 SiO
2
with positive TCF to the LiNbO
3
plate, a robust temperature compensation approach can be achieved for the SH
0
resonators on LiNbO
3
/SiO
2
bilayer and SiO
2
/LiNbO
3
/SiO
2
sandwiched structures. The propagation characteristics of the SH
0
wave propagating in the layered medium are carefully investigated. Despite the trade-off between TCF and k
²
, the SiO
2
/LiNbO
3
/SiO
2
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.