Interfacial properties for real rough MEMS/NEMS surfaces by incorporating the electrostatic and Casimir effects–a theoretical study

Abstract

This paper studied the adhesive properties of real rough micro/nano-electromechanical systems (MEMS/NEMS) surfaces by considering the electrostatic force and the Casimir force theoretically, and an improved model has been proposed. A statistical approach for characterizing surface topography was used by taking the surface standard deviation, the asperity density and the radius of curvature into account. The effects of surface roughness on the electrostatic force and the Casimir force were analysed individually, and a comparison between the proposed model and existing models has been conducted. The whole adhesive force increases with the surface standard deviation, and the prediction by the proposed model becomes more in agreement with the one by existing models when the surface standard deviation is increased. The contribution of the Casimir force to the total adhesive force tends to vanish when the surface standard deviation is relatively large. The electrostatic force and the Casimir force contribute more to the total adhesive forces calculated based on the proposed model with the increase of the asperity density and the radius of curvature. Copyright c 2009 John Wiley & Sons, Ltd. Keywords: MEMS/NEMS; interfacial properties; electrostatic force; Casimir force; theoretical analysisAdhesion is a serious problem specific to micro/nanoelectromechanical systems (MEMS/NEMS) due to their large surface area-to-volume ratio. The study on adhesion problems is of great importance to the normal operation and resolving the tribological problems in static micro-components (e.g. microbeams) and dynamic devices (e.g. rotors in micro-motors). [1 -4] A lot of attention has been given to the adhesion behaviour of smooth surfaces in earlier years, and increasingly more studies have been conducted for MEMS surfaces with rough features since real micro-components possess some roughness, and configurations of multi-asperities exist universally in MEMS systems. [5,6] Although the roughness of some devices is on the nanoscale, its effect on adhesion is still significant due to the influence of surface and size effect. [7] Hence, to investigate the adhesion problems based on real rough surfaces is more meaningful from practical perspectives. [8,9] It has been revealed that surface energies and related surface forces (e.g. van der Waals (VDW) force and electrostatic force, etc.) are the key factors in determining the adhesive and contact deformations. Real contact area tends to increase and the contact deformation could set in due to the presence of surface forces. [3,10,11] Two earlier theories of adhesion models for the contact between an elastic sphere and a flat surface have been proposed: one is the Johnson-Kendall-Roberts (JKR) model proposed by Johnson et al. [12] and the model can be applied to soft materials with a larger contact curvature and a higher adhesion energy, e.g. rubber; the other is the Derjaguin-MullerToporov (DMT) model which can be applied to hard materials with a smaller contact curvature and a lower adhesion ...

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