4E-3 Spurious Mode Suppression in BAW Resonators

Designing bulk acoustic wave resonators for RF filter applications is governed by the need for a high quality factor (Q-value), a large piezoelectric coupling and the purity of the main resonance. The most powerful method to suppress spurious modes is terminating the edges of the resonators by a specific border region with a different eigenresonance. In this paper, we demonstrate the design constraints according to the dispersion diagrams and discuss specific realizations of this border regions, particularly for resonators of dispersion type II. Contrary to mirror type resonators with monotonic dispersion branches, membrane resonators comprising AlN as the piezoelectric layer show a negative slope in the main dispersion branch. Consequently, a border region of higher eigenresonance frequency is required for spurious mode suppression. Based on dispersion calculations and FEM simulations, we demonstrate how it has to be designed and discuss the sensitivity to processing induced layer thickness variations. The additional modes emerging between series and parallel resonance are analyzed and related to the respective branches of the dispersion diagram