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Research Papers

Inelastic Contact Behavior of Crystalline Asperities in rf MEMS Devices

[+] Author and Article Information
O. Rezvanian, M. A. Zikry

Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695-7910

J. Eng. Mater. Technol 131(1), 011002 (Dec 15, 2008) (10 pages) doi:10.1115/1.3026545 History: Received July 31, 2008; Revised September 17, 2008; Published December 15, 2008

Microelectromechanical systems (MEMS), particularly those with radio frequency (rf) applications, have demonstrated significantly better performance over current electromechanical and solid-state technologies. Surface roughness and asperity microcontacts are critical factors that can affect contact behavior at scales ranging from the nano to the micro in MEMS devices. Recent investigations at the continuum level have underscored the importance of microstructural effects on the inelastic behavior of asperity microcontacts. Hence, a microstructurally based approach that accounts for the inhomogeneous deformation of the asperity microcontacts under cyclic loading and that is directly related to asperity physical scales and anisotropies can provide a detailed understanding of the deformation mechanisms associated with asperity microcontacts so that guidelines can be incorporated in the design and fabrication process to effectively size critical components and forces for significantly improved device durability and performance. A physically based microstructural representation of fcc crystalline materials that couples a multiple-slip crystal plasticity formulation to dislocation densities is used in a specialized finite-element modeling framework. The asperity model and the loading conditions are based on realistic service conditions consistent with rf MEMS with metallic normal contacts. The evolving microstructure, stress fields, contact width, hardness, residual effects, and the localized phenomena that can contribute to failure initiation and evolution in the flattening of single crystal gold asperity microcontacts are characterized for a loading-unloading cycle. It is shown that the nonuniform loading conditions due to asperity geometry and contact loading and the size effects due to asperity dimensions result in significant contribution of the geometrically necessary dislocation densities to stress, deformation, and microstructural evolution of crystalline asperities. This is not captured in modeling efforts based on von Mises continuum plasticity formulations. Residual strains and stresses are shown to develop during the cyclic loading. Localized tensile stress regions are shown to develop due to stress reversal and strain hardening during both loading and unloading regimes. Hardness predictions also indicate that nano-indentation hardness values of the contact material can overestimate the contact force in cases, where a rigid flat surface is pressed on a surface roughness asperity.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) A close-up view of a gold contact surface in a rf MEMS switch with normal contacts. (b) Magnified view of the contact surface topography showing the surface roughness and asperities.

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Figure 2

Single asperity and thin film geometry, boundary conditions, and global coordinate axes

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Figure 3

Contact force per micrometer into the plane thickness versus nominal compressive strain during the first loading-unloading cycle

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Figure 4

Contact width due to asperity flattening versus nominal compressive strain during loading

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Figure 5

Contact force versus contact area during loading

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Figure 6

Contours of the normalized stresses in the z direction (a) at nominal compressive strain of 10% and (b) at the end of the first loading-unloading cycle

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Figure 7

Contours of the normalized lateral stresses (y direction) at nominal compressive strains of (a) 2%, (b) 10%, and (c) at the end of the first loading-unloading cycle. (a) and (b) share the same legend.

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Figure 8

Contours of the shear slips on slip system 3 at nominal compressive strains of (a) 6%, (b) 10%, and (c) at the end of the first loading-unloading cycle. (a)–(c) share the same legend.

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Figure 9

Contours of the lattice rotations at nominal compressive strains of (a) 6%, (b) 10%, and (c) at the end of the first loading-unloading cycle. (a)–(c) share the same legend.

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Figure 10

Contour of the statistical dislocation densities on slip system 3 at 10% nominal strain. Initial SSD density on all slip systems is 1.0E+10m−2.

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Figure 11

Contours of the GND densities (m−2) of edge type on slip system 3 at nominal compressive strains of (a) 2%, (b) 6%, and (c) 10%, and the schematic configurations of the positive and negative GNDs of edge type resulting in (d) downward slip and (e) upward slip. (a)–(c) share the same legend.

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