Friction and Wear Properties of ALD Coated MEMS
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Friction and Wear Properties of ALD Coated MEMS Corina Nistorica, Igor Gory, George D. Skidmore Zyvex Corporation, 1321 N. Plano Road, Richardson, Texas 75081 Fadziso M. Mantiziba, Bruce E. Gnade Department of Electrical Engineering, University of Texas at Dallas, Richardson, Texas 75083 ABSTRACT A comparative study of the microtribological properties of native oxide covered single crystal silicon and silicon coated with atomic layer deposited (ALD) alumina films is presented. The dry friction and wear behavior were investigated using a novel microelectromechanical system (MEMS) tribotester. The coefficient of friction for alumina coated surfaces and for silicon uncoated surfaces was monitored before and after wear. The friction versus normal load curves of uncoated silicon can be described by a Johnson-Kendall-Roberts model with pressure dependent shear strength while for the alumina coated surfaces, a linear dependence between the friction force and the normal load was found. Both uncoated silicon surfaces and alumina coated surfaces showed a decrease of the friction force with the number of sliding cycles. INTRODUCTION For commercial viability and industrial growth, MEMS have to be built with high yields and must exhibit reliable and reproducible device properties over the device life. While for most MEMS sensors, friction and wear are minimal, in the case of actuators, wear and friction are a major design concern and a key to the success of MEMS. Measurements of dynamic friction and static friction at the nano and micro scale have been reported recently, primarily using scanning probe microscope (SPM) techniques [1] and measurements using MEMS structures [2, 3]. The frictional forces and the loads in the later experiments were obtained using electrostatic actuation. One disadvantage in using electrostatically actuated devices is that the actuators need to be characterized before starting the friction experiments. The displacement of the actuators have a quadratic dependence on the voltage applied across them, and the proportionality factor has to be determined experimentally since there are discrepancies between the theoretical and experimental values due to process variation. As a general trend, the coefficient of friction (COF) measured using MEMS structures is larger than that obtained in SPM experiments [4]. This is most probably due to the larger interacting areas of the microstructures compared to the SPM tips, hence stronger surface forces contributing to the load forces. In order to mitigate the effects of friction and wear, organic and inorganic coating films have been investigated [5, 6]. Self assembled monolayers (SAM) such as octadecyltrichlorosilane (ODTS), perfluorodecyltrichlorosilane (FDTS), perfluoropolyether (PFPE) work well for contact friction, but not for sliding friction where coated surfaces exhibit high wear after moderate periods of applied shear force. Many SAMS will thermally degrade in air at temperatures above 200 ºC and at relatively
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higher humidity [6, 7]. Line
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