Spurious-free A1-mode Lamb wave resonator with crescent-shaped busbars and high FoM of 360

In this letter, we demonstrate an effective route to suppress transverse spurious modes in A1-mode LiNbO3 Lamb wave resonators by shaping the busbars into a crescent profile. The resulting crescent-shaped excited bulk acoustic wave resonator (CS-XBAR) introduces a quadratic phase variation along the transverse direction, which establishes an energy gradient and drives destructive interference of transverse leakage waves at the busbar boundaries. A waveguide model shows that the suppression of higher-order transverse modes is maximized when a triple-busbar configuration is used and the curvature radius matches the resonator period. Devices are fabricated on a Z-cut LiNbO3-on-Si platform and exhibit a resonance frequency of 4.45 GHz with an electromechanical coupling coefficient kt2 of about 27.2%, a maximum Bode-Qmax of 1321, and a spurious-free admittance spectrum over the measured band, yielding a figure of merit kt2 × Bode-Qmax of about 360. Compared with co-fabricated straight single-busbar references, the CS-XBAR shows markedly enhanced spectral purity and is promising for low-loss N79-band 5G RF filters.