Suppression of spurious modes in mirror-type thin film BAW resonators using an appropriate shape of the active area

Abstract

F ilm bulk acoustic resonators (FBARs) have emerged as an important new technology for realization of GHz filter components. Two different technologies are commonly used: bridge-type and mirror-type resonator layer stacks. Both resonator configurations are susceptible to unwanted lateral modes beside the main resonance. Filters composed of FBARs, e.g. bandpass ladder-type filters, suffer from those spurious modes: they show a higher insertion loss, a lower bandwidth and several ripples in their passbands. Therefore, a practical method is needed to suppress spurious modes in FBARs. In the past, using membrane-type resonators with non-parallel edges of the active area (apodization) has been successfully applied to suppress lateral spurious modes. In contrast, for mirror-type FBARs the focus has been on mode suppression by means of a lateral edge design (frame-like resonators). In this paper we investigate in detail, if apodization works for acoustically more complicated mirror-type FBARs. We have processed a lot of eight 200-mm-wafers using a special test reticle set comprising resonators and filters with both distinct shapes of the active area, lateral edge design as well as combinations thereof. Our measurement results clearly single out optimum electrode shapes for apodization, but also manifest the superior performance of the lateral edge design. I. INTRODUCTION