Spurious Free SAW Resonators on Layered Substrate with Ultra-High Q, High Coupling and Small TCF

Abstract

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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> over coat, on traditional piezoelectric substrates through a higher quality factor (Q), higher electromechanical coupling factor (k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , 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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (LT)piezoelectric thin film. Fabricated SAW resonators on LT/sapphire confirm ultra-high Q (>7,000), high k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (9.9%)and small TCF (-23 ppm/K)at 1 GHz, and achieve a spurious free out-of-band response as expected.