Optimum cut of LiTaO3 for high performance leaky surface acoustic wave filters

The paper describes how leaky surface acoustic wave (LSAW) propagation characteristics on LiTaO₃ depend upon its rotation angle when the Al electrode thickness is increased. It is shown that the propagation loss arising from the leaky nature changes parabolically with both the film thickness and rotation angle, and becomes zero when they are properly chosen. This means that even when relatively thick electrodes are needed, zero propagation loss is always achieved by properly determining the rotation angle for LiTO₃. When the electrode thickness is 0.07~0.1 wavelengths, for example, LSAWs on the 40~42° Y-X LiTaO₃ offer zero propagation loss without deteriorating other characteristics. Ladder filters for 800 MHz range were fabricated, which essentially need thick Al electrodes. As is predicted by theoretical calculation, experimental results showed that if the rotation angle a is increased to circa 42° from the conventional value of 36°, the insertion loss and shape factor are improved by 0.7 dB and 0.26, respectively. The result gives a valuable guideline for determining the optimal rotation angle of LiTaO₃ to be used for high performance LSAW filters, which are one of the key components in RF sections of mobile communication systems