Study of the Effect of Surface Roughness on the Performance of RF MEMS Capacitive Switches Through 3-D Geometric Modeling

Surface roughness is an important factor that influences the reliability and performance of radio frequency microelectromechanical systems switches. The presence of surface roughness impedes a perfect contact in the down-state condition of the switch which in turn affects the following parameters-resonant frequency, down-state capacitance, and isolation at resonance. The result is a significant deviation from the electromagnetic characteristics simulated by assuming flat surfaces making a perfect contact. However, creating a rough topography manually is cumbersome. A Visual Basic Script code for generating rough surfaces of any average value and standard deviation has been developed. The code runs on High Frequency Structure Simulator platform. This paper focuses on the simulations that include the effect of surface roughness on both the beam and the dielectric layer using a 3-D geometric model by representing surface asperities as an array of pyramids, the heights of which follow a Gaussian distribution. This assumption takes into account of the statistical nature of surface roughness. The results of simulation indicate that the presence of surface roughness reduces down-state capacitance and isolation and shifts the resonant frequency from the X-band (8-12 GHz) for which it was originally designed.