New electrostatically-excited single crystal silicon resonator vibrating in a Thickness-Extensional mode

This paper presents a new patent-pending structure of BAW resonator built on standard silicon wafer and driven by electrostatic force. Thickness-Extensional (TE) modes are exploited, yielding a fundamental frequency near 10 MHz with a 400μm-thick Si wafer. The device is based on a one-port design featuring a 1μm gap submitted to a superimposition of a DC voltage bias and a lesser-amplitude AC excitation. In contrast with conventional bar and plate MEMS resonators relying on structural resonances, our resonators are designed to use an energy-trapping of the TE waves to optimize the Q factor. The structure essentially consists of a single (100) Silicon plate, p-doped and bonded onto a Corning glass substrate by standard anodic bonding. The thin gap required for the electrostatic excitation is machined in the glass layer by Reactive Ion Etching. Extensive electrical and mechanical characterisations were performed. They match rather well the behavior predicted by the theoretical analysis. A compensation of the large static capacitance is needed to give a better access to the motional parameters. Q factors near 9000 have been observed on the fundamental TE-mode, with a sufficient coupling for frequency source applications. The stabilization of a RF oscillator using the resonators is still under development.