A Piezoelectric on Substrate Phononic Band Gap High-Q Micromechanical Resonator

Abstract

Micromechanically-fabricated phononic crystal (PnC) structures with phononic band gaps (PnBGs) are gaining a growing attention due to their high efficiency in controlling and confining mechanical energy in micro and nano-scale structures. Preliminary PnC devices such as waveguides and resonators based on the complete PnBG of the micro-machined PnC structures have shown a great potential to improve the characteristics of the conventional micro-mechanical (MM) devices [1–5]. Especially high-frequency, high-quality factor (Q) MM resonators are of great interest as they are main building blocks for realizing compact and complex devices such as filters, multiplexers and de-multiplexers for wireless communications and sensing applications. Therefore, development of high-Q, high-frequency PnC-based MM resonators is an important step towards realizing functional PnC-based devices with potentially better performance compared to their conventional counterparts. In this paper, we report, for the first time, a PnC slab piezoelectric-on-substrate MM resonator operating at VHF frequencies which supports high Q modes. The excitation of the resonant modes in these structures is done directly on the resonant structure (in contrast to the resonant tunneling excitation method reported earlier [5]) and therefore, no coupling from outside of the resonant structure is required. In such a structure, enough number of PnC periods can be placed around the resonant region to provide enough isolation from the surroundings; consequently the loss of the mechanical energy will be limited to material and friction losses only. We report a Fabry-Perot-type PnC slab resonator with an electrode and a piezoelectric medium directly fabricated on top of a resonant structure and show that high quality factors can be obtained in such a compact resonator. As a result, a flexural and a longitudinal mode are excited. Q’s of more than 3600 and 10,000 are obtained for the two modes with motional resistances of 1200 Ω and 5000 Ω. Such piezoelectrically excited high-Q resonators operating at such high frequencies evidence the possibility of suppressing support loss (an important source of loss) in MM resonators through the use of the especial structure of a PnC. Such PnC resonators can have a great impact on the current state-of-the-art MM devices used in wireless communication and sensing systems.