Surface acoustic wave reflection characteristics of electrodes in ZnO/diamond Structures

Abstract

Zinc oxide (ZnO) films were combined with diamond to be a composite surface acoustic wave (SAW) substrate showing high velocity and high coupling coefficient. In this study, Rayleigh SAW reflection characteristics of Cu or Al electrodes in (100) or (002) ZnO films on (111) diamond structures were analyzed using the finite element method (FEM). The resonance and anti-resonance frequencies obtained using the FEM are used to compute the effective phase velocity, coupling coefficient, and reflectivity of SAWs in IDT/ZnO/diamond structures with optimized films thickness-to-wavelength ratio and different electrode thickness-to-wavelength ratios. The extracted coupling-of-mode (COM) parameters are substituted into the transmission matrix method for the design of the dual mode SAW (DMS) filter for wideband code division multiple access (WCDMA) applications. With the optimized films thickness ratio (h <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</inf> /λ = 0.42) and Cu electrode thickness ratio (h <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</inf> /λ) of 0.025, Sezawa mode in the IDT/(100) ZnO/(111) diamond structure possesses the phase velocity of 5209 m/s, coupling coefficient of 3.33%, and reflectivity of 0.15. The tailored DMS filter using these COM parameters yields a RF filter operating at 2140 MHz with 3 dB bandwidth of 60 MHz, insertion loss of 1.36 dB, shape factor of 1.27, and sideband suppression of 39.7 dB.