3^rd type of FBARs?

Abstract

Different «layered structures» were proposed in the past combining the advantageous features of substrates (e.g. low SAW attenuation in sapphire) and high quality of piezoelectric (e.g. AlN) films for the development of acoustic wave guides. Unfortunately, the achievable electromechanical coupling constant K ² is in most cases very small. Meanwhile in FBARs AlN films excited in thickness mode demonstrated K ² > 6%. We propose here a transducer structure comprising: 1) a substrate suitable for propagation of surface acoustic waves in the X-direction with low loss, 2) a metal layer deposited on said substrate and 3) micro-acoustic transducers (resonators) created in the form of piezoelectric (AlN, ZnO, etc.) bars or ridges with cross-section dimension in X-direction smaller than a half wavelength of any acoustic wave in the substrate material at operation frequencies and having top conducting electrodes. The thickness of the piezoelectric layer is close to λ p /4 or λ p /2 of the bulk wave excited in said transducers/resonators. Said transducers being placed periodically and separated (centre-to-centre) by a distance in the X-direction smaller than 1/2 wavelength of any acoustic wave in the substrate material are connected to the balanced source of voltage +/− V/2. We have simulated both cases. The principal results are that the resonator grating always provides larger coupling factors than the solid layer one, and that a mode can occur exhibiting a coupling of at least 4.2% depending on the layer thickness. In this mode the resonances in narrow FBARs are coupled through the substrate. Actually a new type of FBAR is described, wherein the isolation from the substrate is provided by destructive interference of the radiated bulk waves because of the resonators periodicity < λ Bulk /2. The coupling can be easily adjusted. The mode we see may be treated as not a SAW, but as a vibration in a system of coupled FBARs