Effects of Contact Roughness and Trapped Free Space on Characteristics of RF-MEMS Capacitive Shunt Switches

Dielectric surface roughness and top electrode metal asperities tend to affect both the lifetime reliability and the frequency response of radio frequency microelectromechanical systems (RF-MEMS) capacitive shunt switches. The downstate (OFF) capacitance of these switches is considerably affected by the interface irregularities and the free space trapped between the contacting surfaces of a MEMS switch. Attempts have been made to develop models to describe the effects of interface roughness and the trapped free space, yet no comprehensive model, including closed form analytical equations, still exists. A very large body of research has been conducted in physics to model the types of surface fluctuations and interface irregularities between two contacting media. Among these models, the self-affine fractals can be used to describe different growth and deposition techniques, such as sputtering, thermal evaporation, and molecular bean epitaxy. Based on this concept, in this paper, we developed a model that incorporates the effects of contact roughness and free space into classic equations describing the electromechanical behavior of a capacitive MEMS switch. The resulting closed form equations can properly predict the electrostatic and mechanical characteristics of a typical RF-MEMS switch. Measurement and test results from the reported works are used to confirm the validity of the model.