Friction Measurement in MEMS Using a New Test Structure
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ab B.T. Croziera, M.P. de Boer , J.M. Redmond , D.F. Bahra, and T.A. Michalskeb a Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164 b Sandia National Laboratories, Albuquerque, NM 87111
ABSTRACT A MEMS test structure capable of measuring friction between polysilicon surfaces under a variety of test conditions has been refined from previous designs. The device is applied here to measuring friction coefficients of polysilicon surfaces under different environmental, loading, and surface conditions. Two methods for qualitatively comparing friction coefficients (gi)using the device are presented. Samples that have been coated with a self-assembled monolayer of the lubricating film perfluorinated-decyltrichlorosilane (PFTS) have a coefficient of friction that is approximately one-half that of samples dried using super-critical CO 2 (SCCO 2) drying. Qualitative results indicate that gxis independent of normal pressure. Wear is shown to increase ýt for both supercritically dried samples and PFL'S coated samples, though the mechanisms appear to be different. Super critically dried surfaces appear to degrade continuously with increased wear cycles, while PFTS coated samples reach a steady state friction value after about 105 cycles. INTRODUCTION Adhesion [1-3], friction [4-6], and frictional wear [7-9] represent important failure mechanisms in MEMS but have not been extensively characterized. An improved knowledge of surface interactions, frictional forces, and lubricating coatings will help to improve the lifetime and reliability of MEMS devices that have rubbing surfaces. A variety of methods for evaluating interfacial friction and coating performance exist, with each method having distinct advantages and disadvantages. MEMS reciprocating comb driven friction devices [5-7] have proven useful. The surface roughness, coatings, and environmental history of the device are representative of adjacent, functional micro systems on the same chip. Also, the mechanics analysis to measure friction is straightforward. However, their relatively large area consumption on a wafer limits their potential use as on-chip surface diagnostic devices. Furthermore, the small force provided by comb drives limits the range over which pressure can be measured to about one order of magnitude. With such devices, several methods for reducing friction and wear have been explored, with a large effort directed at the application of low surface energy coatings, in particular silane monolayers [5,8]. Investigators employing electrostatic comb-driven friction devices have reported sliding friction coefficients around 0.4-0.5 [6] for polysilicon surfaces with a hydrophilic oxide. Static friction coefficients have been reported between 2 [5] and 5 [10] for supercritically dried, oxide coated surfaces, and as low as 0.1 for surfaces coated with hydrophobic films [5]. Frictional wear has been observed in operating devices under a variety of conditions [7-9]. Other researchers have successfully employed atomic force microscopy
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