Analysis of longitudinal surface acoustic waves on LiNbO3 piezoelectric film bonded to diamond substrate

With the rapid popularization of the fifth-generation (5G) mobile communication system, surface acoustic wave (SAW) filters with high frequency, wide bandwidth, and low insertion loss are increasingly in demand. In this paper, a LiNbO3/diamond layered structure is designed to convert leaky longitudinal SAWs to low-loss guided waves, achieving high phase velocity, high electromechanical coupling coefficient, and a high quality factor. A 2.5-dimensional model is constructed using the finite element method (FEM). The crystal orientation is described by Euler angles. The influence of the crystal orientation and thickness of piezoelectric films on the propagation characteristics of SAWs is systematically analyzed through parametric sweep simulation. Subsequently, the electrode thickness and metallization ratio are further optimized to achieve a high performance longitudinal SAW (LSAW). The optimized parameters for the LiNbO3/diamond structure are ultimately determined: X-cut LiNbO3 with Euler angles (39°, 90°, 30°), a piezoelectric layer thickness of 0.52λ, an electrode thickness of 0.06λ, and a metallization ratio of 0.5. The device achieves an electromechanical coupling coefficient as high as 24.06% and a phase velocity of 6301 m/s. In addition, a relatively large quality factor exceeding 1100 is achieved under idealized simulation conditions that considered only limited loss mechanisms.