Impact of surface periodic grating on FBAR structures to spurious transverse resonances

This paper discusses applicability of the Bragg reflection in a periodic grating placed on the top electrode of the film bulk acoustic resonator (FBAR) structure in order to forbid lateral propagation of the spurious Lamb mode. The finite element analysis is performed for two representative structures, namely, Mo/ZnO/Mo and Ru/AlN/Ru, on which the first-order symmetric Lamb mode exhibits the “Type-I” and “Type-II” dispersion, respectively. It is demonstrated theoretically that for both cases, the stop band is generated by the Bragg reflection, and a spurious free resonance is obtainable provided that the grating period and height are set so that the stop band covers the frequency range where the lateral mode resonances occur.