Asymmetric surface roughness measurements and meniscus modeling of polysilicon surface micromachined flaps |
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Authors: | Xiaojie Xue Leslie M Phinney Andreas A Polycarpou |
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Affiliation: | (1) Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801, USA;(2) Present address: Sandia National Laboratories, P.O. Box 5800, MS 0834, Albuquerque, NM 87185, USA |
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Abstract: | Polycrystalline silicon (polysilicon) films are primary structural materials for microelectromechanical systems (MEMS). Due
to relatively high compliance, large surface-to-volume ratio, and small separation distances, micromachined polysilicon structures
are susceptible to surface forces which can result in adhesive failures. Since these forces depend on surface properties especially
surface roughness, three types of microhinged flaps were fabricated to characterize their roughness and adhesive meniscus
properties. The flaps enabled access to both the top and bottom surfaces of the structural polysilicon layers. Roughness measurements
using an atomic force microscope revealed that MEMS surfaces primarily exhibit non-Gaussian surface height distributions,
and for the release procedures studied, the bottom surface of the structural layers was significantly smoother and prone to
higher adhesion compared to the top surface. A non-symmetric surface roughness model using the Pearson system of frequency
curves was coupled with a capillary meniscus adhesion model to analyze the effects of surface roughness parameters (root-mean-square,
skewness, and kurtosis), relative humidity, and surface contact angle on the interfacial adhesion energy. Using the measured
roughness properties of the flaps, four different surface pairs were simulated and compared to investigate their effects on
capillary adhesion. It was found that since the base polysilicon layer (poly0) was rougher than the base silicon nitride and
the structural layer on poly0 was also rougher than that on silicon nitride, depositing MEMS devices on poly0 layer rather
than directly on silicon nitride will reduce the adhesion energy. |
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