Microelectromechanical systems for applications in extreme temperature environments

The development of solid state transducers for application in extreme temperature environments has been limited largely due to the unacceptable degradation of material properties of typical transducer materials at the extreme temperatures. The miniature size, low cost, and sophisticated functionality benefits of MEMS technology may be extended towards the development of solid state transducers for extreme temperature environments using two general strategies. The first strategy requires the development of packaging schemes that allow the use of existing transducer materials at extreme temperatures, while the second strategy requires the replacement of existing transducer materials with alternate materials that exhibit enhanced material characteristics at the extreme temperatures. This thesis exploits the packaging strategy for the development of ice detection sensors for application in icing environments down to −20°C, as well as, the alternate material strategy for the development of polycrystalline silicon carbide polySiC) as a transducer material for high temperature operation up to 950°C.; Ice detection sensors based on silicon and PZT transducers are developed using suitable packaging and electronic interface circuits to ensure that the icing environment does not interfere with sensor operation. The silicon transducer utilizes a microfabricated diaphragm as the sensing element and capacitance detection to discriminate between ice and water films ∼0.5–1.5 mm thick. The PZT ice detection sensor monitors changes in resonant frequency of a miniature disc element to discriminate between ice and water films 0.06–0.45 mm thick.; PolySiC grown on polysilicon is investigated as an alternate transducer material for high temperature transducers. A microstructural analysis of polySiC grown on polysilicon by atmospheric pressure chemical vapor deposition (APCVD) at 1280°C indicates grain-to-grain epitaxy between the polySiC film and underlying polysilicon layer. The Young's modulus, residual stress, and burst strength of polySiC films are determined using bulk micromachined diaphragms and the influence of diaphragm geometry and film microstructure on mechanical properties is examined. PolySiC resonators are fabricated by a surface micromachining and packaged using ceramic-based materials nickel wirebonding procedures. The devices are successfully tested over a wide range of pressures (<10 −5–760 Torr) and temperatures (22–95°C).