Temperature insensitive micromechanical resonators

This dissertation explores methods for eliminating one of the most important obstacles that hinders the use of micromechanical resonators in communication systems—the temperature-induced frequency shift due to resonator geometry and material properties. It demonstrates two effective techniques, namely geometric-stress compensation and stiffness compensation, that greatly reduce the temperature coefficient of frequency (TC_f) for micromechanical resonators to the point where the temperature stability of flexural-mode micromechanical resonators are now virtually as good as that of quartz crystals. The dissertation then introduces novel processes needed to manufacture resonators based on these design techniques.; The geometric stress compensation method utilizes strategic geometrical design of a mechanical structure to introduce temperature-dependent stresses on the resonator beam that counteract temperature-induced frequency shifts caused largely by Young's modulus temperature dependence. This technique has been demonstrated to reduce the TC_f of both surface micromachined 70kHz nickel and 13MHz polysilicon lateral μresonators by about 7 times. Meanwhile, the novel lateral sub-micron capacitive-gap process that made possible the polysilicon geometric stress compensated resonator has also benefited UHF disk resonators, multiple-port lateral free-free beam resonators, and a sensitivity variable resonant temperature sensor.; The second technique, dubbed stiffness-compensation, improves upon the above by replacing the temperature-dependent mechanical stiffness with the temperature-dependent electrical-stiffness, which has the advantage of better long-term stability and real-time adjustment of the compensation factor. This technique has achieved greatly reduced TC_f's on the order of −0.24ppm/°C, which is 67 times smaller than exhibited by uncompensated μresonators and is comparable with TC_f of quartz crystals. With this design, for the first time, TC _f's of μresonators are small enough to erase lingering concerns regarding the temperature stability of MEMS-based resonators for use in communication applications. In addition to outright better performance, this technique offers numerous other advantages over previous compensation methods, including: (1) no dc power consumption; (2) voltage-control of the temperature coefficient; (3) ease of fabrication, which makes it flexible enough to use on a variety of different resonator designs; and (4) less susceptibility to stress relaxation (which is a potential problem for the previous geometric-stress compensation method).; Both of the techniques represent significant strides towards reducing the thermal dependence of micromechanical resonators, to the point where such devices are now usable in on-chip high-Q reference oscillator applications without the need for electronic temperature compensation.