On the Stiction of MEMS Materials

Stiction is a serious problem in microelectromechanical systems (MEMS) due to their large surface area-to-volume ratio. Stiction is closely related to surface forces, which greatly depend on the materials used, surface topography and surface treatment process. In this paper, we investigate surface energies and stiction of commonly used MEMS materials by contact angle measurements and atomic force microscopy (AFM). Dispersive and polar components of surface energies are calculated by Owens-Wendt-Rabel-Kaelble method. Silicon and silicon-related materials have higher polar surface energies than SU-8 and poly-methylmethacrylate (PMMA), thereby have larger surface energies and enhanced tendency for stiction. The nano-scale adhesion forces between Si3N4 tip and surfaces obtained by AFM further verified that silicon wafer with native oxide has 3-4 times higher adhesion force than SU-8 and PMMA. It has been shown that the materials with higher surface energy have higher sticton/adhesion forces. The topography of surface influences the contact angle and stiction, and is also discussed in the paper.