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 SiO_2 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, k~2, 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 LiTaO_3 (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/K) at 1 GHz, and achieve a spurious free out-of-band response as expected.
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