Thin Film Resonator Technology

The thin film resonator technology has been under development for over forty years in one form or another. Although the basic approach is derived from the desire to reach higher frequencies than those readily achieved by thinning bulk crystals, there have always been competing technologies or fundamental material or processing problems that have impeded the development. Finally, a point was reached in the wireless market wherein competing technologies appeared unable to meet the demands of modern wireless applications and thin film approaches began to receive some emphasis. This paper will survey the thin film resonator technology. Every effort will be made to provide an objective analysis of the technology in relation to applications and competing technologies, and point out obstacles and promises, as known, for further technology advancement to high frequencies. Thin film piezoelectric transducers using CdS or ZnO, were first used in microwave delay lines as a means of generating the high frequency wide bandwidth time delays required by radar signal processing applications. These delay lines required piezoelectric plates bonded to the delay medium (which was often sapphire) for transduction. For UHF and microwave frequencies, piezoelectric thin films were a viable approach to obtaining high frequency microwave acoustic transduction signals and held that niche application for a considerable time.