Design and Modeling of Liquid Gallium Contact RF MEMS Switch

Due to the high power density and local temperature increase on nanoscopic asperities of solid metal contacts, traditional MEMS contact switches suffer from contact welding, pitting, electromigration and oxidation. Particularly, when MEMS switches are used to handle high power, solid metal contacts pose serious limitation on the contact reliability. A self-healing RF MEMS switch, which utilizes liquid gallium contacts to take the place of the traditional solid metal-to-metal contacts, is proposed in this paper. Electrostatic actuation is used to drive a silicon nitride bridge with upper electrodes. When the bridge is pulled down, liquid gallium droplets work as an interface between the upper and lower contact electrodes. The loss of the gallium droplets can be avoided due to the unwettability of the material surrounding the contact electrodes. The switch is fabricated using a surface micromachining process. A coupled-field finite element analysis (FEA) is used to model the electric current, heating and thermal conduction of the contacts. The model includes deformable gallium droplets with 4 μm base diameter. The two sides of the droplets are connected to the upper and lower solid metal contact electrodes, respectively. By using the FEA models, the electric and thermal characteristics of the gallium droplets featuring a variety of geometric parameters have been studied. 1 A current handling capability of the liquid gallium contact is verified by the FEA models.