High Q Lateral-Field-Excited Bulk Resonator Based on Single-Crystal LiTaO₃ for 5G Wireless Communication

The paper presents the design and fabrication of lateral-field-excited (LFE) resonators based on 42° Y-cut single-crystal LiTaO3 (LT) on silicon dioxide (SiO2). A simple coplanar top electrode is defined to excite the bulk acoustic wave modes in the suspended LT/SiO2 structure, and the fabrication process that only involves two lithography steps is more simplified compared to that of commercial film bulk acoustic wave resonators. For a model structure consisting of LT(670 nm)/SiO2 (1500 nm) thin film, two types of acoustic modes are both piezoelectrically active in the LT film: the first one is the thickness-shear mode with a resonance frequency of 2.46 GHz, an electromechanical coupling ( $k_{eff}^{2}$ ) of 1.4%, a high quality factor ( ${Q}$ ) of 1690, and the second one corresponds to longitudinal mode with a resonance frequency of 4.39 GHz, ${k} _{eff}^{2}$ of 1.2%, a high ${Q}$ of 1590, which is among the highest reported for piezoelectric MEMS resonators operating at this frequency range. The excellent performance would enable application scenarios including high-resolution sensors, low-phase-noise oscillators, and low-loss, high selectivity filters in the sub-6 GHz range for the fifth-generation (5G) wireless communication.