An RF MEMS variable capacitor, consisting of two MEMS bridges, was simulated with CST MICROWAVE STUDIO® (CST MWS) and the results then compared with actual measurements. Figure 1 shows the structure as constructed in CST MWS. The width of the central conductor of the coplanar waveguide was 100 um, which is also equal to the distance between it and the ground planes. The bridges connecting the ground planes functioned as varactors between the RF signal and the ground. The 500 um thick silicon was used as a substrate, 1000nm thick Molybdenium as a metal of the transmission line, and MEMS bridges were constructed of 1000 nm Aluminium.
The capacitance of the varactor can be changed between 120 fF (up-state) and 360 fF (down-state). As the MEMS bridge represents a parallel plate capacitor, the height of the bridge could be easily calculated. The height of the bridges of the up-state was 1.1 um, and of the down-state 0.2 um. The simulation results of the S11- and S21-parameters of the varactor in the up-state are plotted in Figure 2 and Figure 3 respectively....
From figures 2 and 3 it can be seen that the curves are very close to the measured ones . The measured results have slightly higher losses but the measured loss also contains loss arising from the contact resistance between the probe tip and the aluminium contact pads . In the down-state the reflection becomes higher and the resonance frequency shifts from 44GHz to 34GHz as can be seen from the figure 4.
Figure 4 shows the simulated S11-parameter of the varactor in the down-state and also shows good correlation with measurement results .
 Ylönen M., Vähä-Heikkilä T., Kattelus H., "Amorphous Metal Alloy Based MEMS for RF Applications", VTT Information Technology, Finland, September 2005.