RF MEMS Behavior, Surface Roughness and Asperity Contact
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RF MEMS Behavior, Surface Roughness and Asperity Contact O. Rezvanian1, M. A. Zikry1, C. Brown2, and J. Krim2 1 Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC, 27695 2 Department of Physics, North Carolina State University, Raleigh, NC, 27695 ABSTRACT Modeling predictions and experimental measurements were obtained to characterize the electro-mechanical response of radio frequency micro-electro mechanical system (RF-MEMS) switches due to variations in surface roughness and finite asperity deformations. A WeierstrassMandelbrot fractal representation was used to generate three-dimensional surface roughness profiles. Contact asperity deformations due to applied contact pressures, were then obtained by a creep constitutive formulation. The contact pressure is derived from the interrelated effects of roughness characteristics, material hardening and softening, temperature increases due to Joule heating, and contact forces. The numerical predictions were qualitatively consistent with the experimental measurements and observations of how contact resistance evolves as a function of deformation time history. This study provides a framework that is based on an integrated modeling and experimental measurements, which can be used in design of reliable RF MEMS devices with extended life cycles. INTRODUCTION Surface roughness and asperity behavior are critical factors that affect contact behavior at scales ranging from the nano to the micro in microelectromechanical, electronic, and photonic devices. Specifically, in MEMS devices, large surface to volume ratios underscores that it is essential to understand how asperities behave in contact devices. MEMS switches, particularly those with radio frequency (RF) applications have demonstrated significantly better performance over current electromechanical and solid-state technologies [1]. The complex physical interactions between thermo-mechanical deformation, current flow, and heating at the contact, has made it extremely difficult to obtain accurate predictions of RF MEMS behavior, such that reliable devices can be designed for significantly improved lifecycles (see, for example, [2]). Various analytical and numerical methods have been employed to study the contact mechanics of ideally smooth surfaces (see, for example, [3]). Since surface topographies are critical in MEMS devices, some probabilistic models have been proposed to account for asperity height variations (for example, see [4]). The random and multiscale nature of the surface roughness can be better described by fractal geometry [5-6]. In this study a three dimensional fractal representation of surface roughness is used with a numerical framework to obtain predictions of thermo-mechanical asperity deformations of contacting surfaces as a function of time. Contact resistance behavior is then investigated and categorized for two surface roughness models with different roughness characteristics. The resistivity of the contact material is assumed to vary by stra
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