Mechanics and Tribology of MEMS Materials - prod.sandia.gov ...

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6.9 Song JF, Vorburger TV and Rubert, P “Comparison Between Precision Roughness Master Specimens and Their Electroformed Replicas”, Precision Engineering-Journal of thee American Society for Precision Engineering, (v14, 1992), pp84-90. 58
7 Impact of Silane Degradation Due to Water Vapor and Radiation Exposure on Tribological Behavior 7.1 ABSTRACT Microelectromechanical systems (MEMS) with high out-of-plane stiffness are less susceptible to adhesion than more compliant structures, but reliable operation of sliding contacts still requires surfaces that exhibit adequate friction and wear performance after long periods of storage. Alkylsilane monolayers are popular surface treatments for silicon devices, and there has been some research to understand the performance of monolayers as a function of environment. However, there have been limited investigations of the tribological behavior of these surface treatments after exposure to harsh environments. There is a need to quantitatively determine the effects of storage environments on the performance of MEMS interfaces, rather than verifying device functionality alone. To this end, surface micromachined (SMM) structures that contain isolated tribological contacts have been used to investigate interface performance of alkylsilane monolayers after storage in inert environments, and after exposure to a variety of thermal and radiation environments. Results show that both octadecyltrichlorosilane (ODTS) and perfluorodecyltrichlorosilane (PFTS) exhibit little change in hydrophobicity or friction after Co- 60 radiation exposures at a total dose of up to 500 krad. However, exposure to temperature cycles consistent with packaging technologies, in the presence of low levels of water vapor, produces degradation of hydrophobicity and increase in static friction for ODTS films while producing no significant degradation in PFTS films. 7.2 INTRODUCTION Alkylsilane coupling agents have been used to create hydrophobic surfaces on silicon MEMS and thus prevent adhesion of structures due to adsorption of water and the creation of capillary forces between surfaces in contact [7.1-7.2]. Hydrophobic films are typically terminated by a methyl group, which creates a low energy surface. These films were originally applied to silicon surface micromachined MEMS in order to alleviate adhesion after the aqueous release etch used to remove sacrificial oxide layers and free the movable structures (release adhesion) [7.2]. They also reduce the tendency of surfaces to adhere during operation (in-use adhesion). More recently, in devices that rely on regular contact and shear between contacting surfaces, alkylsilane films have been called upon to reduce static and dynamic friction between silicon surfaces. Static and dynamic friction coefficients for the methyl-terminated films on MEMS devices are typically below 0.1 in the as-deposited condition [7.3]. Srinivasan et al. [7.4] demonstrated that exposure of alkylsilane films to water vapor at elevated temperatures, similar to those that may be present during back-end-of-line processes such as packaging, can cause treated silicon surfaces to become less hydrophobic. These experiments were performed on blanket films of polycrystalline silicon, treated with octadecyltrichlorosilane (chemical formula CH3(CH2)17SiCl3, abbreviated ODTS) or perfluorodecyltrichlorosilane (chemical formula CF3(CF2)7(CH2)2SiCl3, abbreviated PFTS), and then exposed to nitrogen or ambient air. The water vapor content of the air was not specified, but was probably near 50% relative humidity. Samples were exposed to these environments for 59
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SANDIA REPORT SAND2004-1319 Unlimit
Page 3 and 4:
SAND2004-1319 Unlimited Release Pri
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Contents ACKNOWLEDGEMENTS..........
Page 7 and 8: 6.5 Summary........................
Page 9 and 10: List of Figures Figure 1.1 SEM micr
Page 11 and 12: Figure 6.2 Contact between an elast
Page 13 and 14: Preface Our team was involved in a
Page 15 and 16: strength. The subsequent chapter di
Page 17 and 18: Fig. 1.3. SEM image of cylindrical
Page 19 and 20: Fig. 1.5. Schematic of primary feat
Page 21 and 22: Fig 1.7 Double-hinged pull-tab tens
Page 23 and 24: Fig 1.10. SEM image of optical forc
Page 25 and 26: 2 Strength Distributions in SUMMiT
Page 27 and 28: 2.2 Critical Flaw Size Evaluation T
Page 29 and 30: Fig. 2.5. SEM micrograph of pull-ta
Page 31 and 32: (1) Differences in surface roughnes
Page 33 and 34: 3 The Role Of Microstructure In SUM
Page 35 and 36: crystallographic orientation of a g
Page 37 and 38: direction. Also, maximum stress loc
Page 39 and 40: 3.4 References 3.1 Randle, V., Micr
Page 41 and 42: has the advantage of distributing t
Page 43 and 44: and servovalve at a strain rate of
Page 45 and 46: 4.6 References 4.1. TJ Garino, AM M
Page 47 and 48: microscopy studies. As a first step
Page 49 and 50: Fig. 5.2. Orientation imaging of a
Page 51 and 52: Fig. 5.5. Orientation imaging of a
Page 53 and 54: 6 Novel Techniques for Measurement
Page 55 and 56: of adhesion, and, (iv) a smaller co
Page 57: Force (mN) Fig. 6.5. Typical load-d
Page 61 and 62: All samples were coated with alkyls
Page 63 and 64: the friction structure and the cali
Page 65 and 66: 7.4 RESULTS 7.4.1 Simulation of dos
Page 67 and 68: Dose (MeV/g) Fig. 7.4. Results of X
Page 69 and 70: heating on contact angle for PFTS.
Page 71 and 72: the C-C backbone of the PFTS molecu
Page 73 and 74: The increase in static friction for
Page 75 and 76: 8 Friction and Wear of Selective Tu
Page 77 and 78: During processing of microelectrome
Page 79 and 80: actuator. After unloading, the devi
Page 81 and 82: 9 Conclusions and Recommendations T
Page 83: 10 Distribution 3 MS 0889 M.T. Dugg
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