Most modern electronic devices use frequency references based on quartz crystal resonators. The difficulty of miniaturizing the current quartz crystal resonator packaging technology provides an opportunity for wafer-scale packaged micromachined silicon resonators to displace quartz resonators in the frequency reference market. However, the higher temperature sensitivity of silicon's material properties results in higher frequency variation and limits the ability of silicon resonators to compete with quartz crystal resonators in ultra-high precision applications.

This work describes the design, fabrication, and testing of composite resonators made of single-crystal silicon with a silicon dioxide coating. These composite resonators have a temperature sensitivity of less than 4% that of silicon resonators, which makes them comparable to quartz crystal resonators. Single-anchor, flexural-mode, double-ended tuning fork and ring resonators were designed in the frequency range of 700 kHz to 1.3 MHz. The minimum achieved frequency variation is 200 parts per million over a -55 C to 125 C temperature range. The frequency behavior of the resonator exhibits quadratic temperature dependence with a quadratic temperature coefficient of frequency of approximately -20 parts per billion per C² and a turnover temperature at which the linear temperature coefficient of frequency is zero. This turnover temperature can be designed to be at the desired operating temperature, which is attractive for integration with active temperature compensation methods.

The resonators were packaged using a modified wafer-scale encapsulation technology described by Candler et al. in Journal of Microelectromechanical Systems in 2006. The MEMS-first, CMOS-compatible, packaging process yields composite resonators which are hermetically sealed in a low pressure environment, exhibit quality factors of 10,000 to 30,000, and have sub-mm³ volume. In conjunction with active temperature compensation schemes, this technology enables silicon-based frequency references to meet the needs of high-precision applications at a reduced size.