Tribology of MEMS
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Tribology of MEMS M.P. de Boer and T.M. Mayer Because of large surface-to-volume ratios and low restoring forces, unwanted adhesion and friction can dominate the performance of microelectromechanical systems (MEMS) devices. To guarantee the function and reliability of MEMS devices, tribologists must understand the origins of adhesion, friction, and wear over a broad range of length scales from the macroscopic to the molecular. In this article, we present an overview of challenges, successes, and initial steps toward a fundamental understanding. Polycrystalline silicon (polysilicon) is the material of choice in surface-micromachined MEMS because it is compatible with integrated-circuit technology, can be stressrelieved to less than 10 MPa, and deposits conformally by low-pressure chemical vapor deposition (CVD), allowing the fabrication of sophisticated hub geometries.1 (For more on the technique of surface micromachining, see the article by Mehregany and Zorman in this issue.) However, silicon oxidizes readily to form a hydrophilic surface, making it susceptible to adhesion. Most metals also oxidize, so the problem is relevant to other micromachining technologies. Oxide surfaces are also prone to accumulating static charge, which can interfere with capacitive-sensing circuitry. Given these surface properties, we must consider aspects of device design, materials selection, and processing, and specific surface modifications to minimize the effects of adhesion, friction, and wear. Even in applications where contact is never intended, adhesion arises as a significant problem. For example, in accelerometers and gyroscopes, compliant mechanisms are freely suspended above the substrate. During fabrication, a sacrificial material surrounds the structural films and must be removed to render the films freestanding. Sacrificial films are etched or dissolved in liquid in a so-called release step. To avoid capillary collapse,2 a supercritical carbon dioxide drying process has been developed to circumvent the associated surface tension.3 However, strain gradients through the thickness of the films can cause structural members to curl and contact the substrate, resulting in adhesion. Proper deposition and annealing sequences4 must
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be implemented to guarantee that freestanding films exhibit low curvature, and sufficient design tolerance must be built into the device to avoid contact and adhesion due to handling after release of the freestanding structures. Contacts are often inherent to the function of a device. Optical switching devices require rapid contact and disengagement, creating a dynamic adhesive interface. Microrelays have conflicting demands of low contact resistance and low adhesion. In some applications, switches may remain in contact for extended periods. In all of these, long-term changes in adhesive energy due to the stability of the interface and environmental factors are of great concern. Low-surface-energy, hydrophobic coatings applied to oxide surfaces are promising for minimizing adhesion and static-char
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