A FRACTAL ANALYSIS OF STICTION IN MICROELECTROMECHANICAL SYSTEMS

The strong adherence (stiction) of adjacent surfaces is a major design concern in microelectromechanical systems (MEMS). Advances in microma chine technology greatly depend on basic understanding of microscale s tiction phenomena. An analysis of the different stiction micromechanis ms and the elastic deformation of asperities at MEMS interfaces is dev eloped using a two-dimensional fractal description of the surface topo graphy. The fractal contact model is scale independent since it is bas ed on parameters invariant of the sample area size and resolution of m easuring instrument. The influence of surface roughness, relative humi dity, applied voltage, and material properties on the contributions of the van der Waals, electrostatic, and capillary forces to the total s tiction force is analyzed in eight of simulation results. It is shown that the effects of surface roughness and applied voltage on the maxim um stiction force are significantly more pronounced than that of mater ial properties. Results for the critical pull-off stiffness versus sur face roughness are presented for different material properties and mic rostructure stand-free surface spacings. The present analysis can be u sed to determine the minimum stiffness of microdevices required to pre vent stiction in terms of surface roughness, apparent contact area, re lative humidity, applied voltage, and material properties.