Cryogenic Characterization of Low-Loss Thin-Film Lithium Niobate on Sapphire Shear Horizontal Surface Acoustic Wave Devices

This paper reports a cryogenic study on wideband shear horizontal surface acoustic wave (SH-SAW) devices based on an emerging Y-cut LiNbO₃/SiO₂/Sapphire (LNOS) platform. To perform a comprehensive study, low-loss acoustic delay lines (ADLs) equipped with unidirectional transducers were designed with a wavelength (λ) of 4 μm (900 MHz) and a wide fractional bandwidth (FBW) of around 7%, featuring various physical delays ranging from 5λ to 200λ as testing structures. By cooling the temperature down to 5 K, the insertion loss (IL) of the longest ADL and the extracted propagation loss (PL) were characterized as 4.1 dB and 3.5 dB/mm, respectively. Compared with an IL of 5.78 dB and a PL of 4.37 dB/mm at 275 K, the temperature-dependent acoustic losses diminish at low temperatures, with the overall PL dominated by the acoustic waveguide formed by the acoustic velocity mismatch between layers. Furthermore, a one-port resonator (λ = 2.8 μm) with a large perceived effective electromechanical coupling greater than 40% was also characterized using the same technique, showing a 2X boost in the maximum Bode-Q at cryogenic temperatures. This study not only characterized the acoustic properties of wideband LNOS SH-SAW devices but also validated their excellent performance across a wide temperature range, suggesting their potential applications in cryogenic phononic circuits.