Enlarging Rayleigh Elimination Window through Modulating Substrate and LiNbO3 Cut Angle for Fabricating LNOI-Based Spurious-Free Wideband SAW Filters

Lithium niobate on insulator (LNOI) platform attracts much attention in recent years for its potential to fabricate wideband surface acoustic wave (SAW) filters using the shear horizontal (SH) mode. However, the coexistence of Rayleigh mode can deteriorate the performance of SH-based filters, and thus implementing a systematical investigation of the influences of the substrates and the LN cut angles on the Rayleigh elimination window (REW) is necessary to evaluate the advantageous designing parameters for a target spurious-free filter. In this work, the REW of rotated YX-LN/SiO2/substrate (including SiC, Si, Al2O3, diamond and quartz) platform is studied through finite element method (FEM), and it is found that the LN/SiO2/SiC shows a wider REW since the Rayleigh mode can be totally eliminated in a window range (Delta h(Cu)/lambda) of Cu electrode normalized thickness of 0.08 when theta is between 18 degrees and 24 degrees. Resonators and filters with different h(Cu)/lambda were fabricated on 15 degrees, 20 degrees and 25 degrees YX-LN/SiO2/SiC and verified the simulation results, and among which the filters built on 20 degrees and 25 degrees YX-LN/SiO2/SiC are spurious-free and exhibit 3-dB fractional bandwidth (FBW) of about 20% and minimum insertion loss (ILmin) of about 1.2 dB, respectively.