In-band spurious suppression in first-order antisymmetric-mode resonators using air holes and half electrode

Laterally excited bulk acoustic resonator (XBAR) exhibits a high electromechanical coupling coefficient ( k t 2 ) due to their intrinsically superior piezoelectric properties, which is highly advantageous for the subsequent design and implementation of acoustic filters. However,it is not yet suitable for commercial application due to its numerous spurious modes and low quality factor ( Q value). This paper presents an exhaustive analysis of the origins of in-band spurious modes and the method to suppress based on the first-order antisymmetric (A1) mode XBAR. Simulation results indicate that these spurious modes primarily originate from acoustic wave scattering along unintended propagation paths and standing waves caused by crosstalk between electrodes. These spurious modes lead to energy dissipation, thereby reducing the resonator’s Q value. To effectively suppress them without increasing fabrication complexity while significantly enhancing the Q value, an etching-hole strategy is proposed, introducing the transverse holes that create acoustic impedance mismatch based on velocity differences. Additionally, the adoption of a half electrode configuration optimizes the electric field and stress redistribution within the resonator, thereby effectively reducing interfacial losses at the electrode-piezoelectric layer interface and enhancing the Q value by approximately 3.5 times. The fabricated XBAR, integrating holes and half electrode configuration, exhibits an in-band spurious-free response, a Bode- Q value of 1954, a k t 2 of 27.5 % and an exceptionally high figure of merit ( FOM = k t 2 × Bode- Q ) of 537.