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This paper presents a new method to improve the performance of SAW devices based on transferred lithium niobate thin films. Amorphous silicon replaces the single-crystal silicon to eliminate a conductive layer generated in the thin-film transfer processes. As a result, the electromechanical coupling of the surface-guided acoustic wave is maximized, and the substrate loss is reduced in the amorphous-silicon-based hybrid wafer. The fabricated resonator shows an extracted electromechanical coupling coefficient of 22% with over 60 dB impedance radio, which is 30 dB higher than the resonator with parasitic surface conductive effect. The fabricated delay lines show excellent propagation properties of the excited surface-guided acoustic wave with a minimum propagation loss of 18.1 dB/mm at 2.35 GHz, which further demonstrate the elimination of the conductive layer and outstanding acoustic energy confinement between lithium niobate and amorphous silicon. This work has shown the strong potential of the silicon-SAW platform for front-end signal processing. © 2022 IEEE.

This work presents an acoustic platform using solidly mounted thin-oriented lithium niobate (LiNbO3) film on silicon (Si). A thin layer of amorphous Si eliminates a conductive layer generated in the thin-film bonding processes and contributes to acoustic energy confinement and thermal frequency stability. The gigahertz operating frequency is achieved by resorting to the guided acoustic wave shear-horizontal wave (SH0) in a 400-nm-thick X-cut LiNbO3 thin film. Due to the elimination of the parasitic surface conduction (PSC) effect, the electromechanical coupling of the guide acoustic wave is maximized in the Si-based heterogeneous wafer. Due to the minimized acoustic impedance mismatch between surface material and substrate, longitudinal and higher order spurious modes are suppressed. Due to the stiff substrate with a small temperature expansion coefficient (TEC), the solidly mounted structure features a reduced temperature coefficient of frequency (TCF) and improved power handling. The fabricated resonator shows an extracted electromechanical coupling coefficient of 22.8%, a high loaded Q of 1208 at 1.6 GHz, and a TCF of -36 p/min/K. The compressed filter is demonstrated with a minimum insertion loss (IL) of 0.6 dB, a fractional bandwidth (FBW) of 8%, an out-of-hand rejection of 30 dB, a TCF of -38.5 p/min/K at roll-off, and a miniaturized footprint of 0.4 mm(2). The performance has shown the strong potential of the LiNbO3-Si platform for front-end applications in 5G.