Design of Multi-Dimensional Variable Capacitors for RF MEMS

In this work, a multi dimensional RF MEMS variable capacitor that utilizes electrostatic actuation is designed. Here we preferred Electrostatic actuation due to its low power consumption. For calculations we assumed 1 mu m thick silicon monoxide as a dielectric layer for the varactor and a movable membrane is suspended on a 2.5 mu m thick electroplated gold pedestal. The capacitance between the membrane and the bottom electrode increases as the bias voltage between the membrane and the bottom electrode is increased, eventually causing the membrane to snap down at the actuation voltage. For the varactors designed herein, the actuation voltage is approximately 30 - 90V. The RF MEMS variable capacitor is designed in a CPW topology, with multiple beams (1 to 7) on a single pedestal. For the varactors designed herein, the actuation voltage is approximately 15 - 100V. Full-wave electromagnetic simulations are performed from 1 - 25GHz to accurately predict the frequency response of the varactors. The EM simulations and the measurement results compare favorably. A series RLC equivalent circuit is used to model the varactor and used to extract the parasitic associated with the capacitor by optimizing the model with the measurement results