A1 Mode Lithium Niobate Solidly Mounted Filter for 5G NR and Wi-Fi 6E/7 Applications

In this work, we propose a first-order anti-symmetric (A1) mode acoustic resonator and filter based on a 128° YX-cut LiNbO3 (LN) solidly mounted resonator (SMR) structure. By adopting a non-periodic Bragg reflection layer structure, shear waves and longitudinal waves are effectively reflected at the designed frequency, reducing acoustic leakage loss and achieving a spurious-free resonator within the passband. Simulation results show that the frequency of the spurious-free A1 mode SMR can be adjusted in a wide-frequency range by optimizing the half period and metal ratio simultaneously, which is distinct from the conventional XBAR method that relies on LN film thinning. Then a 6th-order T-type A1 mode SMR filter was designed and fabricated, which exhibits a center frequency of 6.82 GHz, a minimum insertion loss of 2.2 dB, a 3dB bandwidth of 740 MHz, and out-of-band suppression exceeding 25 dB. This work demonstrates a SMR filter design method suitable for the A1 mode, and provides a feasible path and design ideas for engineering implementation of high-frequency acoustic filters.