A 6.4-GHz Spurious-Free Acoustic Filter Based on Lithium Niobate S1-Mode Resonator

This paper reports on a 6.4-GHz spurious-free filter based on first-order symmetric (S1) mode resonator fabricated in X-cut lithium niobate (LN) thin film. Specifically, we studied the electrode pitch and duty factor (DF) to suppress parasitic modes near the target resonant frequency, making it possible to better meet the requirements of spurious-free and fast roll-off in a filter. The fabricated resonator has an electromechanical coupling coefficient (k²) of 3.3% and a high quality factor (Q) of 989, while the filter exhibits low insertion loss (IL) of 2.6 dB and sharp roll-off from 2.6 dB to 40 dB within a narrow transition band of only 55 MHz (out of 6.4 GHz center frequency). The spurious-free filter design opens the possibility of S1-mode LN resonators for filter synthesis in 6G and Wi-Fi 7 applications.