Laterally Excited Bulk Acoustic Wave Resonators with Rotated Electrodes Using X-Cut LiNbO₃ Thin-Film Substrates

With the development of piezoelectric-on-insulator (POI) substrates, X-cut LiNbO₃ thin-film resonators with interdigital transducers are widely investigated due to their adjustable resonant frequency (f) and effective electromechanical coupling coefficient (Keff2). This paper presents an in-depth study of simulations and measurements of laterally excited bulk acoustic wave resonators based on an X-cut LiNbO₃/SiO₂/Si substrate and a LiNbO₃ thin film to analyze the effects of electrode angle rotation (θ) on the modes, f, and Keff2. The rotated θ leads to different electric field directions, causing mode changes, where the resonators without cavities are longitudinal leaky SAWs (LLSAWs, θ = 0°) and zero-order shear horizontal SAWs (SH₀-SAWs, θ = 90°) and the resonators with cavities are zero-order-symmetry (S₀) lateral vibrating resonators (LVRs, θ = 0°) and SH₀ plate wave resonators (PAW, θ = 90°). The resonators are fabricated based on a 400 nm X-cut LiNbO₃ thin-film substrate, and the measured results are consistent with those from the simulation. The fabricated LLSAW and SH₀-SAW without cavities show a Keff2 of 1.62% and 26.6% and a Bode-Q of 1309 and 228, respectively. Meanwhile, an S₀ LVR and an SH₀-PAW with cavities present a Keff2 of 4.82% and 27.66% and a Bode-Q of 3289 and 289, respectively. In addition, the TCF with a different rotated θ is also measured and analyzed. This paper systematically analyzes resonators on X-cut LiNbO₃ thin-film substrates and provides potential strategies for multi-band and multi-bandwidth filters.