Higher Order Mode Elimination for SAW Resonators Based on LiNbO₃/SiO₂/poly-Si/Si Substrate by Si Orientation Optimization

Lithium-niobate-on-insulator (LNOI) platform has emerged as a promising solution for fabricating wideband and low-loss surface acoustic wave (SAW) filters. However, it simultaneously excites higher order modes, causing out-of-band (OoB) spurious responses. In this work, the elimination condition for higher order modes in LiNbO3 (LN)/SiO2/poly-Si/Si structure was summarized from analyzing the coupling mechanism between the velocities of shear bulk acoustic waves ( V_{\mathrm {S}} ) of Si and that of higher order mode ( V_{\mathrm {p-h}} ). According to the elimination condition of V_{\mathrm {p-h}} exceeding V_{\mathrm {S}} , meticulously selecting the crystal plane and propagation angle \alpha of Si to obtain desired V_{\mathrm {S}} is necessary. First, the resonators built on 32^{\circ }Y - X LN/SiO2/poly-Si/Si platforms with typical Si (100), Si (110) and Si (111) substrates were studied by simulation, which reveals that Si (110) manifests the optimal suppression capacity with \alpha _{110} window of 18^{\circ }-60^{\circ } , followed by the Si (111) plane of \alpha _{111}= 14^{\circ }-36^{\circ } . Si (100) substrate can hardly suppress higher order modes. Furthermore, resonators were designed and prepared on the above three Si planes. In coherence with the theoretical prediction, the resonators built on Si (135°, 90°, 45°) substrate can effectively eliminate the OoB ripples, while the resonators based on Si (0°, 0°, 45°) and Si (135°, 54.74°, 60°) substrates both excite the higher order modes, whose maximum admittance ratios (AR _{\mathrm {h}} ) are 15.0 dB and 19.9 dB, respectively. This work demonstrat