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-S) of Si and that of higher order mode (Vp-h). According to the elimination condition of Vp-h exceeding V-S, meticulously selecting the crystal plane and propagation angle alpha of Si to obtain desired V-S is necessary. First, the resonators built on 32 degrees Y-XLN/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 degrees-60 degrees, followed by the Si (111) plane of alpha(111)=14 degrees-36 degrees. 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 degrees, 90 degrees, 45 degrees) substrate can effectively eliminate the OoB ripples, while the resonators based on Si (0 degrees, 0 degrees, 45 degrees)and Si (135 degrees, 54.74 degrees, 60 degrees) substrates both excite the higher order modes, whose maximum admittance ratios (AR(h))are 15.0 dB and 19.9 dB, respectively. This work demonstrates a valid methodology for constructing spurious-free filters meeting 5G requirements.