Silicon Germanium Epitaxy: A New Material for MEMS
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Silicon Germanium Epitaxy: A New Material For MEMS J.T. Borenstein*, N.D. Gerrish*†, R. White*‡, M.T. Currie** And E.A. Fitzgerald** * Charles Stark Draper Laboratory, 555 Technology Square, Cambridge, MA 02139 ** Department of Materials Science & Engineering, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139 † Present Address: Amberwave Systems Corporation, Salem, NH ‡ Present Address: Mechanical Engineering Department, University of Michigan, Ann Arbor, MI ABSTRACT A wide array of materials have been investigated as candidate fabrication templates for precision microelectromechanical structures, including boron-diffused silicon, boron-doped epitaxial silicon, polysilicon, silicon-on-insulator, and wafer-thick bulk structures. Here we present the latest fabrication results for epitaxial silicon-germanium alloys, a new class of materials which possess excellent crystalline structure, are compatible with non-toxic etchants in bulk micromachining, and are capable of on-chip integration with electronics. For MEMS applications, silicon-germanium alloy layers are grown using a graded buffer approach, resulting in very high quality micromachined structures. Very low defect densities are obtained through the use of these relaxed buffers. Original etch-stop studies determined that Ge doping provided a very weak selectivity in anisotropic etchants such as KOH and EDP. However, by extending the range of Ge concentration to over 20%, we have found extremely high etch selectivities in a variety of etchants. Unlike boron-doped layers, SiGe exhibits etch stop characteristics in the non-toxic, process compatible solution TMAH. The combination of independence from boron doping concentration and etchant compatibility make SiGe a material which is ideal for integration with on-chip electronics. In this work we present the latest fabrication data on comb-drive resonators built using SiGe epitaxial layers. Process compatibility issues related to wafer curvature, surface finish and reactive-ion-etching chemistries are addressed. An unexpected result of the fabrication process, curvature of released structures, is resolved by annealing wafers after the SiGe deposition. Changes in Young’s modulus arising from the high atomic fraction of Ge in the device can be determined by simple beam analysis based on observed resonant frequencies. Overall, build precision for these devices is excellent. We conclude by addressing the remaining challenges for wide-scale implementation of silicon-germanium epitaxial MEMS. INTRODUCTION New process technologies for MEMS, including Deep Reactive Ion Etching (DRIE), wafer bonding and Silicon-On-Insulator (SOI) materials, have greatly expanded capabilities for building large, thick and highly precise structures. Over the past few years, we have been developing silicon-germanium epitaxial materials as another technology base for high-precision MEMS structures. Early work by Draper Laboratory demonstrated bulk-micromachined inertial MEMS sensors built from highly-boron-doped crystalline silicon [1]. These device
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