Design and fabrication of 128°Y-cut LiNbO₃ XBARs with acoustic scattering hole arrays achieving FOM of 220

This paper proposes a laterally-excited bulk acoustic resonator (XBAR) based on a suspended 128ºY-cut lithium niobate thin film, which has a high quality factor (Q value) achieved by acoustic scattering hole arrays. To improve the Q value of XBAR, the length of the gap region was optimized in transverse direction so that the acoustic wave can be reflected back to the active region of the resonator. Also, in lateral direction, a structure employing acoustic scattering hole arrays with varying pitches, named SA-XBAR, was introduced. It leverages non-parallel boundary formed by acoustic impedance mismatch to further suppress the leakage of acoustic wave energy. The measured electromechanical coupling coefficient (kt2) values of the resonators were around 35%. The SA-XBAR demonstrated a Bode-Q value reaching 627 and the Figure of Merit (FOM=kt2•Q) value of 220, which achieved 1.8× improvement, showing a great potential for application in 5G/6G communications.