High-Performance SAW Resonator With Simplified LiTaO₃/SiO₂ Double Layer Structure on Si Substrate

In response to the increased mobile data traffic, there is a growing need for more low-loss RF band filters with steep frequency characteristics, and high-quality (Q)-factor and low-temperature coefficient of frequency (TCF) resonators are required to achieve this. We previously reported that for a surface acoustic wave (SAW) resonator on a three-layer structure, which is composed of a thin LiTaO 3 (LT) plate whose orientation is 50° rotated YX propagation, SiO 2 layer, and AlN layer on a Si substrate, a Q-factor several times higher than that of an SAW resonator on a standard 42° rotated YX propagation LiTaO 3 (42YX-LT) substrate could be obtained. In this study, we investigated this layer structure and found that a two-layer structure, in which the AlN layer is removed, achieves a high Q-factor. Numerical analyses using a finite element method showed that the acoustic wave energy can be confined to the surface of the two-layer substrate, and the TCF and electromechanical coupling coefficient (k 2 ) were improved by optimizing the thickness of each layer. We fabricated and evaluated prototype one-port resonators with the two-layer structure and the standard 42YX-LT SAW substrate with resonant frequencies from 0.95 to 3.6 GHz. An improvement of the Q-factor of 3 to 4 times compared with that of the resonator with standard 42YX-LT substrate was observed for the two-layer structure, which means that a reduction of complexity of the layer structure could be obtained without performance loss. The two-layer structure was applied to a 2.4-GHz band Wi-Fi filter to achieve high performances such as low-loss, better steepness, and high attenuation.