Novel Chemistry for Surface Engineering in Mems
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NOVEL CHEMISTRY FOR SURFACE ENGINEERING IN MEMS X.-Y. Zhu*,a Y. Jun, a V. Boiadjiev, a R. Major, a H. I. Kim,b J. E. Houstonb a Department of Chemistry, University of Minnesota, Minneapolis, MN 55455 b Sandia National Laboratories, Albuquerque, NM 87185 * [email protected] ABSTRACT
It is well recognized that controlling surface forces is one of the key issues in the design, fabrication, and operation of microelectromechanical systems (MEMS). In this report we present a novel strategy for the efficient assembly of organic monolayers onto silicon surfaces to control surface energy. This is achieved by the reaction between an alcohol functional group and a chlorinated Si surface. The resulting molecular monolayers are thermally and chemically stable. Surface adhesion energy on silicon is reduced by a factor of 40 by the monolayer coating and friction coefficient of the coated surface is only 0.013. The coatings are successfully demonstrated in adhesion reduction in a model MEMS structure: cantilever beam array (CBA). Polycrystalline beams with length up to 1.5 mm can be released. INTRODUCTION
Controlling surface forces is one of the key issues in the design, fabrication, and operation of micro-machined devices [1]. This is a consequence of the scaling law: the surface-to-volume ratio scales with 1/length; surface forces dominate at length scales < 1mm [2]. A well-known problem in the fabrication of MEMS devices from surface micromachining is stiction, which occurs when surface adhesion forces overcome the mechanical restoring force of the micro-structure. Microstructures such as beams are usually formed from wet etching of an underlying sacrificial layer. When a device is removed from the aqueous solution, the liquid meniscus formed on hydrophilic surfaces pulls the microstructure towards the substrate and stiction occurs. While this problem may be alleviated by drying techniques such as supercritical CO2 drying or freezesublimation, a more difficult problem is stiction during operation when microstructures come into contact (intentionally or accidentally). A number of approaches have been explored for the engineering of MEMS surfaces to combat the stiction problem [1]. These reported processes include a) plasma deposition of fluorocarbon films; b) texturing surfaces of contact; c) hydrogen termination on silicon surfaces; and d) formation of alkyltrichlorosilane self-assembled monolayers (SAMs). Other processes developed at industrial laboratories include the formation of fluorinated fatty acid SAMs [3] and the thermal evaporation of silicone polymer materials at the packaging stage [4]. Due to the limited diffusion length of reactive species in a plasma, approach a) is limited to simple structures where conformal coating is possible. Texuring may reduce the actual surface area of contact and, thus, reduce stiction and friction; however, this approach can only reduce stiction to a limited extent and reduce friction for relatively light loads. Hydrogen-terminated silicon surfaces have low surface energies, but these s
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