Design and Fabrication of A1 Mode Lamb Wave Solidly Mounted Resonator Operating Above 6 GHz Based on Single-Crystalline LiNbO₃ Thin Film

Laterally excited bulk acoustic wave resonators (XBAR) using the first-order anti-symmetric (A1) mode Lamb wave based on lithium niobate (LiNbO3) have shown great potential in 5G and future 6G communication band for its large electromechanical coupling coefficient. Solidly mounted resonator (SMR) structure using Bragg reflector can both improve the structural stability and suppress spurious wave modes effectively. In this work, A1 mode Lamb wave SMR is designed. The effects of LiNbO3 cut angle, LiNbO3 film thickness, and number of the Bragg reflection layer have been studied in simulations. In experiments, a 4-in Y128-cut single-crystal LiNbO3 film was fabricated with the designed SiO2/Ta2O5 Bragg reflector. Then, the designed A1 mode Lamb wave resonator is prepared with the LiNbO3 single crystal film. The measured resonant frequency of the prepared SMR is 6.231 GHz, while the anti-resonant frequency is 6.610 GHz. The calculated electromechanical coupling coefficient is 15.46% and the maximum quality factor is 491.34. This work demonstrates that A1 mode Lamb wave resonators with Bragg reflector can achieve high performance to meet the requirements of 5G and 6G communications for filters.