Experimental and Theoretical Investigation of Contact Resistance and Reliability of Lateral Contact Type Ohmic MEMS Relays

Abstract

In this thesis, the reliability of electrostatically actuated ohmic contact type MEMS relays has been investigated. Multi-contact MEMS relays using electrostatic comb-drive actuators have been used in this study. The MEMS relays were fabricated using MetalMUMPs process, which uses 20 µm thick electroplated Nickel as the structural layer. A 3 µm thick gold layer was electroplated at the electrical contact surfaces. The overall size of the relay is approximately 3 mm x 3 mm. The relay consists of a movable main beam anchored to the substrate using two identical folded suspension springs. RF ports consist of five movable fingers connected to the movable main beam and six fixed fingers anchored to the substrate. Comb-drive actuators located at the top and bottom ends of the main beam enable bi-directional actuation of the RF contacts. An example MEMS relay with planar contacts of area 80 µm x 20 µm and a spacing of 10 µm between the movable and fixed contacting surfaces is discussed. Resistance versus applied voltage characteristics has been studied. For an applied DC bias voltage of 172 V, the movable fingers make contact with the fixed fingers. The resistance versus applied voltage characteristics have been measured for applied bias voltages in the range of 172 V to 220 V. Reliability testing has been carried out and the resistance variation of the MEMS relay over 1 x 106 actuation cycles has been measured. A statistical rough surface contact model was used to estimate the actual electrical contact resistance versus applied force curve of these devices. Two models were presented, the Greenwood and Williamson (GW) elastic model, and the Jackson and Green (JG) model. When compared to the measured results, the GW model over estimates the resistance, but the JG model is very accurate. A multi-scale contact model is also presented, and the results show good qualitative agreement with the experimental data.