Leaky Surface Acoustic Wave with Velocity 10 km/s and Suppressed Leakage in LiNbO3 Plate Bonded to Sapphire

The high acoustic wave velocities required for high-frequency SAW devices can be combined with strong piezoelectric coupling and low propagation losses if a thin plate of LiNbO₃ (LN) is bonded to a high-velocity sapphire substrate. This paper presents a numerical investigation of acoustic modes propagating in an LN/sapphire substrate under a periodic Al grating, demonstrating the occurrence of longitudinal leaky SAWs (LLSAWs) with a unique combination of high velocities exceeding 10 000 m/s, an electromechanical coupling coefficient of 4.5%, and negligible leakage into the substrate. The leakage was suppressed by optimizing the LN plate and Al electrode thicknesses. Compared with conventional SAW substrates, the LN/sapphire substrate offers 2- to 2.5-fold higher frequencies for periodic electrode structures and enables the production of 6-GHz devices by standard photolithographic processes while the required LN plate and Al electrode thicknesses remain compatible with existing wafer bonding technologies. In the optimal LN/sapphire structure, LLSAW behaves as a perfect Rayleigh SAW unperturbed by interactions with spurious modes in a wide range of frequencies.