invited paper for International Conference on Fracture, to be held Dec. 3-7, 2001, Honolulu, Hawaii, Hilton Hotel EFFECT OF NANOTEXTURING ON INTERFACIAL ADHESION IN MEMS M. P. de Boer1, J. A. Knapp2 and P. J. Clews3 Sandia National Laboratories 1Intelligent Micromachining, 2Radiation Solid-Interactions, 3Silicon Processing Dept. www.mdl.sandia.gov/Micromachine ABSTRACT We demonstrate that the interfacial adhesion between microelectromechanical systems (MEMS) surfaces is in a regime not previously considered by standard models of rough surfaces such as the Fuller-Tabor [1] or Maugis [2] extensions of the theory of elastic contact of rough surfaces [3]. Our experiments and models show that at small roughness values, adhesion is mainly due to van der Waals forces across extensive non-contacting areas and is proportional to 1/(average surface separation)2. At large roughness values, asperities that nearly bridge the gap become the dominating contributor to the adhesion. These van der Waals contributions to adhesion have been ignored in the above models. They cannot be ignored in MEMS because the surfaces are in close proximity over a long range as a result of the planar deposition technology. KEYWORDS microcantilevers, interfacial adhesion, surface roughness, van der Waals forces, self-assembled monolayers INTRODUCTION MEMS is a recently developed technology in which free standing polycrystalline silicon (polysilicon) thin film structures are actuated electrostatically to form micron-scale complex mechanisms such as resonating sensors, gears, linear racks, pop up mirrors, and mechanical logic [4]. Because of the large surface-to-volume ratio in this regime, surface forces can dominate over inertial forces, causing mechanisms to adhere rather than perform their intended function. It is well known that surface roughness reduces adhesion of two contacting bodies. Models describing the effect of roughness on adhesion consider only the adhesion at [1] or near [2] areas of real contact. A reduction in adhesion due to enhanced roughness has also been observed in MEMS. For example, polysilicon roughening techniques have been used to reduce the tendency towards adhesion under wet conditions [5,6]. Free standing cantilevers were actuated under dry conditions and the transition from adhered to free cantilevers was detected to estimate adhesion values [7,8]. It was observed that adhesion decreases with increasing surface roughness, and the authors suggested that 2 ~ /12 o A dp G , where Gis the adhesion (J/m2), A is the Hamaker constant representing van der Waals forces, and d o is the sum of the root mean square (rms) roughness of the two surfaces as measured by atomic force microscopy (AFM). Theoretical analysis considering the fractal nature of surfaces has also shown that adhesive forces decrease with increasing roughness [9,10]. In this work, we employ a joint experimental and modeling approach to address the effect of roughness on adhesion under dry conditions, allowing us to quantitatively address the following outstanding
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