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

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Abstract
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, leads 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 (θ) of LN. The K2 dispersion characteristics of the main mode (shear horizontal wave) and spurious mode (Rayleigh wave) on LN/SiO2/Si substrates were systematically analyzed by 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 (hCu/λ) ranging from 0.1 to 0.19 when θ is between 19° and 22° on LN/SiO2/Si platform, in contrast to the quite narrow REW on bulk LN restricted to some specific hCu/λ. 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. Additionally, the typical spurious-free wideband SAW filter with hCu = 200 nm based on the 20°YX-LN/SiO2/Si platform demonstrates high performance with a center frequency (fc) 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 5G applications.
Funding Information
  • National Key Research and Development Program of China (2020YFB0408900)
  • National Natural Science Foundation of China (52002205)
  • China Postdoctoral Science Foundation (2020M690577)
  • Special Project for Research and Development in Key areas of Guangdong Province (2020B0101040002)

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