Polycrystalline Silicon Germanium for Fabrication, Release, and Packaging of Microelectromechanical Systems

Polycrystalline silicon germanium has recently proven to be a compelling alternative to polysilicon for micromachining. Low temperature fabrication of micromechanical structures is possible, which enables their modular integration with conventional electronics. The deposition and crystallization temperatures are significantly lower than for polysilicon, and low-stress, low-resistivity structural films can be achieved with little or no annealing. Poly-Ge can be used as a hydrogen peroxide-soluble sacrificial layer, so a wide variety of microfabrication materials can withstand the release etch.
Several aspects of our research on poly-SiGe micromachining are presented in this dissertation. First, a “handbook” of poly-SiGe processing for MEMS is given, along with an overview of the advantages of this material system. An extensive study of the etching of poly-Ge sacrificial layers by heated hydrogen peroxide is presented. The dissolution of poly-Ge is limited by the dissolution of a GeOsurface layer, and the activation energy was determined to be 9.3 kcal/mol. The etch rate was determined to be roughly 0.5μm/minat 90C, which is 4-6 orders of magnitude faster than structural films containing 20-60% Ge. The reaction was determined to be limited partly by the reaction rate and partly by diffusion, and diffusion limits on the order of 1 mm were observed.
The fabrication of robust, high-aspect-ratio poly-SiGe structures by a thin film micromolding process (hexsil) is presented. Due to the excellent conformality of poly-Ge compared to SiO2sacrificial layers, precise replication of the mold wafer was achieved. Agimbal/microactuator fabricated in this process enabled a critical dimension to be reduced from 7 to 4.5μm when compared to a device made in a conventional process.
Poly-SiGe hexsil was also used to fabricate micromachined caps for a precision MEMS packaging technology. In this process, the hexsil caps were fabricated on a mold wafer and transferred to a target wafer by gold-to-gold thermocompression bonding. Avariation of the poly-SiGe hexsil caps in which a transparent SiO2 membrane was supportedby a poly-SiGe matrix is also presented. This technique holds promise for packaging optical MEMS devices.
Tsu-Jae King
George Johnson
Publication date: 
April 30, 2001
Publication type: 
Ph.D. Dissertation
Heck, J. M. (2001). Polycrystalline Silicon Germanium for Fabrication, Release, and Packaging of Microelectromechanical Systems. United States: University of California, Berkeley.

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