Large-Range Spurious Mode Elimination for Wideband SAW Filters on LiNbO₃/SiO₂/Si Platform by LiNbO₃ Cut Angle Modulation
Huiping Xu, Sulei Fu, Junyao Shen, Zengtian Lu, Rongxuan Su, Ruikang K. Wang, Cheng Song, Fei Zeng, Weibiao Wang, Feng Pan
Abstract
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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta $ </tex-math></inline-formula> ) of LN. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}^{{2}}$ </tex-math></inline-formula> 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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${h}_{\text {Cu}}/\lambda $ </tex-math></inline-formula> ) ranging from 0.1 to 0.19 when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta $ </tex-math></inline-formula> is between 19° and 22° on the LN/SiO2/Si platform, in contrast to the quite narrow REW on bulk LN restricted to some specific <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${h}_{\text {Cu}}/\lambda$ </tex-math></inline-formula> . To verify the simulation results, resonators were prepared on 15°YX-LN/SiO2/Si, 20°YX-LN/SiO2/Si, bulk 15°YX-LN, and bulk 20°YX-LN. In addition, the typical spurious-free wideband SAW filter with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${h}_{\text {Cu}} = {200}$ </tex-math></inline-formula> nm based on the 20°YX-LN/SiO2/Si platform demonstrates high performance with a center frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{\text {c}}$ </tex-math></inline-formula> ) of 1.27 GHz, a minimum insertion loss (ILmin) of 0.7 dB, and a 3-dB fractional bandwidth (FBW) of ~20.1%. This work provides a workable solution in fabricating spurious-free wideband and low-loss SAW filters for fifth-generation (5G) applications.