Origins and mitigation of spurious modes in aluminum nitride microresonators

Recently reported narrow bandwidth, <;2%, aluminum nitride microresonator filters in the 100-500 MHz range offer lower insertion loss, 100× smaller size, and elimination of large external matching networks, when compared to similar surface acoustic wave filters. While the initial results are promising, many microresonators exhibit spurious responses both close and far from the pass band which degrade the out of band rejection and prevent the synthesis of useful filters. This paper identifies the origins of several unwanted modes in overtone width extensional aluminum nitride microresonators and presents techniques for mitigating the spurious responses.