Investigation of Mems Resonator Characteristics during Long-term and Wide Temperature Variation Operation

Two types of single-crystal silicon micromechanical resonators having resonant frequencies at 150 kHz and 130 kHz were tested under harsh environment to investigate stability. To observe long-term stability, the main characteristics, such as resonant frequency and quality factor were measured over 2,500 hrs continuously while maintaining constant environmental temperature at 25°C±0.1°C. A separate experiment was also initiated to show stability during temperature cycling from -50°C to 80°C. In both experiments, the total change in resonant frequency were less than 10 ppm and quality factor less than 10%, which demonstrates the stability of encapsulated micromechanical resonators upon exposure to harsh environments. INTRODUCTION Micromechanical silicon resonators are increasingly important devices due to potential applications as frequency references and filters[1-3]. However, commercialization of these silicon resonators is only beginning. The main focus of our current MEMS resonator research efforts is the stability, reliability, packaging, and tunability of these devices. In order to be used as a frequency reference, MEMS resonators should be able to maintain stability of the resonant frequency under various environment effects such as wide temperature range or long-term operation. While some preliminary studies have been reported[4, 5], there is a lack of good data regarding long-term performance of MEMS resonators. The purpose of this research is to enable our own studies of the characteristics of MEMS resonators upon exposure to various environment conditions. FABRICATION The resonators are fabricated by using a wafer-scale encapsulation process[6, 7], which was developed from our previous work in collaboration with researchers from Robert Bosch Palo Alto Research and Technology Center. The resonators are fabricated in silicon-based and IC-compatible MEMS process. Starting with SOI wafer, the device structure is etched with a plasma etch. Then a layer of epitaxial silicon is deposited to encapsulate and the resonator is released with vapor phase HF etch. At the end, a second layer of epitaxial silicon is deposited to complete the final seal. Figure 1 A cross-section of an encapsu lated resonator. A resonator made with an SOI wafer is encapsulated by our epipolysilicon process to maintain atmosphere inside the cavity. Si (Substrate) Single Crystalline Si Polysilicon Seal