A Parametric Model of Mems Capacitive Switch Operating at Microwave Frequencies

This paper is focused on the creation of an efficient electromagnetic model of MEMS switches which operates at microwave frequencies. The switches are first characterized using a full wave analysis based on finite element method aiming to extract the S-parameters of the switches for different geometrical dimensions. From the S-parameter data base, a scalable lumped circuit model is extracted to allow easy implementation of the switch model into available microwave CAD software. The simulated results are compared with published measured data as validation of our model. INTRODUCTION T h e r e c e n t d e v e l o p m e n t s o f microelectromechanical systems (MEMS) switch and their use at microwave frequencies have promoted exciting advancements in the field of microwave switching. In comparison with other switches, realized by FET's or p-i-n diodes, MEMS switches exhibit low-loss performance, zero power consumption and very low intermodulation distortion [1]. To our knowledge, there is very little work which has been done to describe accurately their behaviors at microwave frequencies [2-4]. Due to the articulate geometry of such switches, a full wave analysis is needed to characterize those switches. The result of the full wave analysis will provide scattering parameters for different switch geometries, which will allow to construct an equivalent lumped circuit model for different geometries. Based on this data base, the equivalent lumped circuit model of the switch is determined. The values of the circuit elements are related to the physical dimensions of the switch as final result. THE ELECTROMAGNETIC MODEL In this paper, we will focus on the shunt capacitive MEMS switch, which consists of a thin metal membrane bridge suspended over the center conductor of a coplanar waveguide (CPW) and fixed on the ground conductor of the CPW, as schematically shown in Fig. 1(a). The parameters L and H indicate the length and the height of the membrane bridge in this figure. The full wave electromagnetic simulation of the switch is done using Ansoft High Frequency Structure Simulator (HFSS). In the simulation a box size 1200 x 600 x 600 μm is used and boundary radiation conditions are imposed on the six sides of the box. After the full wave analysis is performed, S-parameters are extracted in the frequency range going from 1 GHz to 60 GHz for different heights of the switch. The substrate is assumed to be lossless with relative dielectric constant of 9.8 (correspondent to Alumina). The thickness of the substrate is 600μm and the CPW conductors and MEMS switch are treated as perfect conductors. The central conductor of the CPW is assumed to be coated with silicon nitrate (Si3N4) having relative dielectric constant of 7 and thickness of 0.1 μm. Figure 1(b) presents the first order equivalent circuit model obtained for the capacitive MEMS switch.