Large-Range Spurious Mode Elimination for Wideband SAW Filters on LiNbO3/SiO2/Si. Platform by LiNbO3 Cut Angle Modulation

A LiNbO3 (LN)/SiO2/Si multilayered structure was recently reported as a new platform for achieving wideband radio frequency (RF) filters. However, the in-band ripples in filters resulting from the spurious Rayleigh mode lead to deteriorated performance, and thus, a wide Rayleigh elimination window (REW) is highly desired for realizing spurious-free wideband surface acoustic wave (SAW) filters with a wide design space and good process tolerance. Here, we investigated the spurious mode suppression on the LN/SiO2/Si platform theoretically and experimentally through modulating the cut angle (theta) of LN. The K(2 )dispersion characteristics of the main mode (shear-horizontal wave) and spurious mode (Rayleigh wave) on LN/SiO2/Si substrates were systematically analyzed by the finite-element method (FEM), along with bulk LN for comparison. It is found that the REW is wider on LN/SiO2/Si than bulk LN, as Rayleigh wave can be totally eliminated with Cu electrode normalized thickness (h(Cu)/lambda) ranging from 0.1 to 0.19 when theta is between 19 degrees and 22 degrees on the LN/SiO2/Si platform, in contrast to the quite narrow REW on bulk LN restricted to some specific h(Cu)/lambda. To verify the simulation results, resonators were prepared on 15 degrees YX-LN/SiO2/Si, 20 degrees YXLN/SiO2/Si, bulk 15 degrees YX-LN, and bulk 20 degrees YX-LN. In addition, the typical spurious-free wideband SAW filter with h(Cu) 200 nm based on the 20 degrees YX-LN/SiO2/Si platform demonstrates high performance with a center frequency (f(c)) of 1.27 GHz, a minimum insertion loss (ILmin) of 0.7 dB, and a 3-dB fractional bandwidth (FBW) of similar to 20.1%. This work provides a workable solution in fabricating spurious-free wideband and low-loss SAW filters for fifth-generation (5G) applications.