High Frequency LLSAW Filters With Higher Order Modes Elimination Based on LiNbO ₃ /SiO ₂ /Sapphire Substrate

In this study, we focus on developing high-frequency longitudinal leaky surface acoustic wave (LLSAW) filters with efficient suppression of spurious modes. A sapphire-based platform, chosen for its appropriate acoustic velocity, was systematically optimized to enhance LLSAW excitation while minimizing spurious modes. The dispersion characteristics were evaluated by finite element analysis, and robust main mode excitation and suppression of higher order modes was achieved through electrode thickness optimization. The resulting LLSAW resonators fabricated on 32°Y-45°X LiNbO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>(LN)/SiO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>/sapphire platform demonstrates substantial suppression of higher order modes over a frequency range from 3.5 to 7 GHz and achieve a maximum quality factor (Q<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf>) of 890 for LLSAW mode. Subsequently, 3.51 GHz, 4.08 GHz and 4.82 GHz LLSAW filter with insertion loss (IL<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</inf>) of 1.59 dB, 2.43 dB and 2.13 dB were realized, exhibiting excellent out-of-band (OoB) performance up to 7 GHz.