Proceedings of the 2009 4th IEEE International Conference on Nano/Micro Engineered and Molecular Systems January 5-8, 2009, Shenzhen, China A low-loss RF MEMS switch with dielectric layer on the lower surface of the bridge Kenle Chen l, Yueyang Dai 2 , Xudong Zou l, Jinwen Zhang l * 1National key laboratory ofMicro/Nano Fabrication Technology, Institute ofMicroelectronics, Peking University, Beijing 100871, P. R. China 2School ofsoftware and microelectronics, Peking University, Beijing 100871, P. R. China 3School of Electrical and Computer Engineering, Purdue University, West Lafayette 47906, IN, USA with C=C u (up-state capacitance) or Cd (down-state capacitance) depending on the position of the bridge. Figure 1. The schematic drawings of shunt capacitive RF MEMS switches: (a) plan-form, (b) up-state of conventional DOT switch, (c) up-state ofDOB switch, (d) down-state of two switches. The total capacitance of MEMS shunt capacitive switch which contains two portions: the parallel-plate capacitance II. THEORETICAL ANALYSIS A typical DOT shunt capacitive RF MEMS switch is composed by coplanar waveguide (CPW) whose characteristic impedance is Zo, a double-fixed metal bridge, and a dielectric layer which is usually fabricated on the transmission line. The t-line ofCPW is f.!m wide and the bridge is H f.!ID high and W b f.!ID wide. The dielectric layer is td f.!ID thick and Sd f.!ID2large (as shown in Fig. 1 (a) and (b)). The up-state of the new DOB switch is shown in Fig. l(c), which has the same down-state with DOT switch (as shown in Fig.l (d)). The switch capacitance primarily dominates the insertion loss and isolation, two most important microwave parameters of RF MEMS switches. They are both defined by transmission coefficient (S2f): insertion loss is I S21 Iin up-state, while isolation is I S21 in down-state. If ignoring the parasitic inductance and loss terms of t-line, S21 is given by (1) (d) 1 8 21 =------ 1+ jOJCZ o /2 (b) I. INTRODUCTION RF MEMS switches are widely used in Wireless communication and Radar systems for low insertion loss, high isolation, good linearity and very low power consumption. Although the insertion loss of a shunt capacitive MEMS switch is less than 0.2dB [1], for the whole multi-switch systems [2]-[4], it is still necessary to reduce the insertion loss of single RF MEMS switch as low as possible. Now, for achieving high-level integration of MEMS and integrated circuit (IC), the width of transmission line (t-line) is inclined to be designed as small as the IC dimensions [5]. Moreover, to make the switch impedance matches the whole t-line, the in-switch t-line is usually designed narrower than the out-switch t-line [6], [7]. However, if the t-line becomes narrow, the width of switch bridge cannot be reduced but enlarged because the contact area of bridge and t-line must be kept in order to obtain large enough down-state capacitance, which leads to sufficient isolation and low actuation voltage [8], [9]. As for these trends, the bridge would usually be much wider than t-line in the future. With this consideration, a low-loss shunt capacitive switch with the dielectric layer not conventionally on the t-line (DOT switch) but on the lower surface of the bridge (DOB switch) is proposed in this paper. By reducing the fringing capacitance of switch capacitor, the DOB structure can decrease the insertion loss of the switch without any impact on other electric per- formances, comparing with its counterparts [10]-[12] with always some effects on the isolation or actuation voltage. Keywords - RF MEMS switch, insertion loss, dielectric dayer, fringing capacitance Abstract - This paper reports a low-loss RF MEMS switch with the dielectric layer on the lower surface of bridge instead of on the transmission line as conventional switches. Analysis on the fringing capacitance of switch capacitor shows this kind of switch has much smaller fringing capacitance and lower insertion loss than conventional one when the bridge is wider than transmission line. The simulation results obtained from 3D electromagnetic simulation tools well demonstrate the theoretical analysis. As increasing compact miniaturization and highly developed integration, this kind of switch with low insertion loss is expected a broad application in the future. This project was supported by the National Natural Science Foundation ofChina (NSFC, No. 60676042). *Contacting author: Jinwen Zhang, phone: 86-10-62752536;fax: 86-10-62751789; E-mail:jwzhang@ime.pku.edu.cn. 978-1-4244-4630-8/09/$25.00 ©2009 IEEE 152