Analytical and Numerical Analysis of Pull in Characteristics of an RF MEMS Switch in the Presence of Surface Roughness

Abstract

RF MEMS (Radio Frequency micro electromechanical systems) switches are revolutionizing the commercial and military wireless communication and satellite systems. However, the reliability of switches still remains a matter of concern. The electromechanical characteristics of an RF MEMS switch are studied with the assumptions of ideally smooth contacting surfaces. In spite of that, recent researches have exhibited that the effect of surface properties like roughness cannot be ignored on performance of the MEMS devices. So, the main aim of the thesis is to carry out a detailed analytical study and numerical analysis of effects of surface roughness on pull in characteristics of an RF MEMS switches. A closed form analytical model constituting spheres of constant radius is adopted for the presentation of surface roughness. The model considers the effect of sphere radius on the electromechanical characteristics like capacitance, pull in voltage, switching time and pull in gap of an RF MEMS switches. The analytical model is validated through coupled field electrostatic-structural FEM simulations. The results clearly demonstrate that value of pull in gap and capacitance increases whereas the switching time and pull in voltage decreases for an increase in radius of the spheres. The situation becomes worse when air gap present between the suspended plate and bottom dielectric is very small. The results in the thesis shows that value of pull in voltage degrades by 27 V, 15 V and 5 V for an air gap of 3 µm, 2 µm, and 1 µm with roughness scale of 10 nm from its smooth surfaced values whereas switching time value decrements by 24% and 42% with an air gap of 3 µm and 0.4 µm for roughness scale of 100 nm respectively. Similarly, the capacitance shows a normalized value of 1.82 and 1.59 for an air gap of 3 µm and 0.4 µm with roughness scale of 100 nm. All the presented results create a clear understanding of the fact that the roughness effect on the electromechanical properties of an RF MEMS switch cannot be neglected and proper consideration must be given to study of surface properties in the design and development phases of the switches.