Spurious Free Saw Resonators On Layered Substrate With Ultra-High Q, High Coupling And Small Tcf

A new type of surface acoustic wave (SAW) configuration consisting of a very thin single crystal piezoelectric film bonded onto a support substrate (layered substrate) has been actively developed in recent years. This layered SAW structure exhibits superior performance compared to conventional SAW or temperature compensated (TC) SAW, with SiO2 over coat, on traditional piezoelectric substrates through a higher quality factor (Q), higher electromechanical coupling factor (k(2)) and smaller temperature coefficient of frequency (TCF). However, without a careful design of the substrate, the layered SAW potentially has spurious responses in the out-of band frequencies due to higher order modes guided in the layer. This paper focuses on the design of layered substrate not only to optimize its narrow band characteristics (Q, k2, TCF) but also for eliminating the out-of-band spurious responses. By using a finite element method / boundary element method (FEM/BEM) approach, requirements for the substrate velocity and piezoelectric layer thickness are derived to avoid the presence of spurious modes. Based on the analyses, a new orientation of quartz is proposed as a spurious free support substrate. Sapphire as well as new quartz is selected as a demonstration support substrate and bonded wafers are fabricated using LiTaO3 (LT) piezoelectric thin film. Fabricated SAW resonators on LT/sapphire confirm ultra-high Q (>7,000), high k(2) (9.0%) and small TCF (-2 ppm/K) at 1 GHz, but have several out-of-band spurious responses. LT/new quartz SAW resonators also show ultra-high Q (>6,000), high k(2) (9.9%) and small TCF (-23 ppm/10 at 1 GHz, and achieve a spurious free out-of-band response as expected.