Modeling of dry stiction in micro electro-mechanical systems (MEMS)

Abstract

Stiction, a term commonly used in micro electro-mechanical systems (MEMS) to refer to adhesion, is a major failure mode in MEMS. Undesirable stiction, which results from contact between surfaces, can severely compromise the reliability of MEMS. In this paper, a model is developed to predict the dry stiction between uncharged micro parts in MEMS. In dry stiction the interacting surfaces are assumed to be either hydrophobic or placed in a dry environment. In this condition the van der Waals (vdW) and asperity deformation forces are dominant. Here a model is developed for the vdW force between rough micro surfaces, and the new model is combined with a newly developed multiple asperity point model for the elastic/plastic deformation of rough surfaces in contact to solve the equilibrium condition of the forces. This in turn will yield the equilibrium distance between micro surfaces, using which the apparent work of adhesion can be found. The theory result is compared with the available experimental data from the literature. The developed model can be easily used for design purposes. If the topographic data and material constants are known, the desirable adhesion parameters can be quickly found from the model.

A Micro–Macroapproach to Predict Stiction due to Surface Contact in Microelectromechanical Systems

“…Another function should be defined to represent the spatial horizontal correlation of ordinates. A widely used function, under the assumptions of stationary and ergodicity of the surface, is the exponential autocorrelation function (ACF), [1]. The ACF allows the determination of a characteristic length related to the spatial occurrence of given surface heights and thus, of the number of interacting asperities.…”

“…In industry [1], several applications, such as accelerometers, digital mirror devices, pressure sensors, gyroscopes or resonators have been realized using MEMS technologies and many new applications are under development. In spite of those advantages, the inherent characters of MEMS, such as, the large surface-area to volume ratio, the relatively smoothness of the surfaces, the small interfacial gaps and the small restoring forces make them particularly vulnerable to the surface forces that could lead to permanent adhesion of MEMS moving parts.…”

“…There are mainly two kinds of stiction in MEMS [1,2], the 'release stiction' and the 'in-use stiction'. Release stiction refers to the kind of stiction happening during the fabrication process at etching of the final sacrificial layer, in which the suspended structures may adhere to the substrate due to the tremendous capillary forces and to the relatively low restoring force of the structures.…”

“…In order to model the adhesive contact forces it has been revealed that in the dry environment, where the capillary forces can be neglected, the surface energies and the related surface forces (e.g. van der Waals (VDW) forces) are the key factors to be considered [1,4]. It has been shown in [4] that below 30% of relative humidity the force due to capillary condensation is reduced and that the adhesion is dominated by the molecular van der Waals forces.…”

Influence of adhesive rough surface contact on microswitches

“…In a dry and low-pressure environment, capillary forces will be relatively small, and the dominative factor of in-use stiction is the van der Waals forces between the molecules. 8,9 The use of atomic force microscopes ͑AFMs͒ and of surface force apparatuses, which allow for the accurate measurement of surface forces when the dimensions of contacting bodies are small, has shown that surface roughness plays an important role in the magnitude of the pull-out force between two bodies. The characterization of surface roughness is challenging because asperity distribution on the surface is not uniform.…”