Optimum leaky-SAW cut of LiTaO3 for minimised insertion loss devices

The paper describes how the propagation loss α of leaky surface acoustic wave (LSAW) is dependent upon the Al thickness h of grating electrodes and rotation angle θ for rotated Y-X LiTaO₃. Since the energy leak of LSAW originates from the coupling of the slow-shear field component with other components through the surface boundary conditions, zero α can be realised when the slow-shear component is decoupled. This means that the optimal θ giving zero α should change with h as well as the surface electrical boundary conditions. Since relatively large h of 0.07-0.1 wavelengths is needed for present LSAW devices, the optimal θ shifts to 40-42° from the conventional value of 36°. Thanks to the minimised propagation loss, experimental results showed that device performances are markedly improved only by increasing θ to circa 42° from 36°. We also describe the state-of-the-art of the performances obtainable in commercially available LSAW devices based on 42°YX-LiTaO₃