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INTRODUCTION
Compound solid state switches such as GaAs MESFETs and PIN diodes are widely used in microwave and millimeter wave integrated circuits (MMICs) for telecommunications applications including signal routing, impedance matching networks, and adjustable gain amplifiers. However, these solid-state switches have a large insertion loss (typically 1 dB) in the on state and poor electrical isolation in the off state. The recent developments of micro-electro-mechanical systems (MEMS) have been continuously providing new and improved paradigms in the field of microwave applications. Different configured micromachined miniature switches have been reported. Among these switches, capacitive membrane microwave switching devices present lower insertion loss, higher isolation, better nonlinearity and zero static power consumption. In this presentation, we describe the design, fabrication and performance of a surface micromachined capacitive microwave switch on glass substrate using electroplating techniques.
RF MEMS TECHNOLOGY
Basically RF MEMS switches are of two configurations-:
 RF series contact switch
 RF shunt capacitive switch
Currently, both series and shunt RF MEMS switch configurations are under development, the most common being series contact switches and capacitive shunt switches.
RF Series Contact Switch
An RF series switch operates by creating an open or short in the transmission line, as shown in Figure 1. The basic structure of a MEMS contact series switch consists of a conductive beam suspended over a break in the transmission line. Application of dc bias induces an electrostatic force on the beam, which lowers the beam across the gap, shorting together the open ends of the transmission line . Upon removal of the dc bias, the mechanical spring restoring force in the beam returns it to its suspended (up) position. Closed-circuit losses are low (dielectric and I2R losses in the transmission line and dc contacts) and the open-circuit isolation from the ~100 μm gap is very high through 40 GHz. Because it is a direct contact switch, it can be used in low-frequency applications without compromising performance. An example of a series MEMS contact switch, the Rockwell Science Center MEMS relay, is shown in Figure 2.
Figure 1. Circuit equivalent of RF MEMS series contact switch.
Figure 2. Structure and operation of MEMS dc series switch .
RF Shunt Capacitive Switch
A circuit representation of a capacitive shunt switch is shown in Figure 3. In this case, the RF signal is shorted to ground by a variable capacitor. Specifically, for RF MEMS capacitive shunt switches, a grounded beam is suspended over a dielectric pad on the transmission line (see Figure 4). When the beam is in the up position, the capacitance of the line-dielectric-air-beam configuration is on the order of ~50 fF, which translates to a high impedance path to ground through the beam [IC=1/(C)]. However, when a dc voltage is applied between the transmission line and the electrode, the induced electrostatic force pulls the beam down to be coplanar with the dielectric pad, lowering the capacitance to pF levels, reducing the impedance of the path through the beam for high frequency (RF) signal and shorting the RF to ground. Therefore, opposite to the operation of the series contact switch, the beam in the up position corresponds to a low-loss RF path to the output load, while the beam in the down position results in RF shunted to ground and no RF signal at the output load . While the shunt configuration allows hot-switching and gives better linearity, lower insertion loss than the MEMS series contact switch, the frequency dependence of the capacitive reactance restricts high quality performance to high RF signal frequencies (5-100 GHz), whereas the contact switch can be used from dc levels.