Stability and performance of wafer-scale thin-film encapsulated MEMS resonators

Silicon resonator is one of the most promising devices due to their potential application as frequency references in electronic circuits. Reduced size and batch fabrication will make silicon resonators cost effective compared to quartz oscillators which are most widely used as circuit frequency references. While there have been many breakthroughs in the field of MEMS resonators, the problem of packaging is yet to be solved. The stability of the resonant frequency over time is absolutely essential for use as a frequency reference, and the frequency stability depends on the quality of the package environment.; This work presents the stability and performance of MEMS resonators packaged in a wafer-scale thin-film encapsulation process, called 'epi-seal'. This encapsulation is formed by depositing polycrystalline silicon in a CMOS-clean and extremely high temperature (∼980°C) environment. The mechanical robustness of the encapsulation provides MEMS resonators with extremely high yield even after harsh post processing, such as wafer sawing and wire bonding.; During more than one year of operation, resonant frequencies of these encapsulated resonators were stable to ppm levels of drift. This high level of stability was achieved by the cleanliness and hermeticity of the 'epi-seal' encapsulation. For further optimization of encapsulation design, diffusive gas species and diffusion paths were investigated by a 400°C accelerated diffusion experiment.; In addition, other efforts to develop commercial level high performance MEMS resonators are presented. Quality Factor, Q, is a description of the energy loss of resonators, which is very important for designing oscillator circuits with the resonators. The temperature dependence of various energy losses is investigated. The quality factors of MEMS resonators can be engineered to be either strong or weak functions of temperature. Especially for 'oven-based' active temperature compensation, strongly temperature dependent Q can be used as an effective, direct, and delayless measure of the temperature of the resonators.; To achieve temperature stability, silicon dioxide, which becomes stiffer as temperature increases while silicon becomes softer, can be used as a compensating material. Si-SiO2 composite resonators were successfully fabricated inside a modified 'epi-seal' encapsulation. These encapsulated Si-SiO 2 composite resonators showed more than 20x improvement in temperature stability.