Application of an overlay for spurious suppression of multi-layered surface acoustic wave resonators with double busbar configuration

This paper discusses suppression of a spurious resonance called the gap mode which appears when the double busbar configuration is applied to multi-layered surface acoustic wave resonators. Two kinds of overlay structures are proposed for the gap mode mitigation. First, use of lossy ones are studied. The results indicates that the lossy overlay can effectively mitigate the gap mode response when the viscosity is adequate. The layer thickness gives negligible impact on the mitigation. Then, the discussion is extended to the application of high-acoustic-velocity materials as the overlay. The resonance frequency of gap mode can be moved to right without affecting the main resonance by simply depositing a high velocity overlay to the gap region. Finally, the choice of overlay materials is also studied and compared. The results point out that Si3N4 is the optimum choice for industrial use.