Design, fabrication and characterization of liquid-solid microelectromechanical DC-contact switches

Abstract

From critical applications such as life support systems and massive data communication centers to every day necessities like mobile phones and street lighting, people depend on the performance of electrical switches. Today, the vast majority of electrical switches are of solid-state semiconductor type. They are fast, with nanosecond response times, have the advantage of being produced by CMOS-compatible microfabrication processes, and unlike their major competitor counterparts, i.e., microelectromechanical systems (MEMS) switches, they do not suffer from mechanical contact issues like contact bounce, stiction, or contact degradation, due to the absence of solid-to-solid mechanical contacts. Therefore, electrical switches exhibit extremely long life times with superior reliability performance. However, they have a high ON-state resistance of 2-6 , low open-state impedance on the order of 105-107 , and show relatively low power-handling capabilities. In addition, their temperature and radiation-sensitive performance, limits their range of operating environment. The major counterpart switching technology, solid-to-solid DC-contact MEMS switches, on the other hand, transmit current when the two surfaces, namely dimple and contact pad, make contact. Although MEMS switches show very good RF performance, the required solid-to-solid contact is often quite non-ideal. This stems from the fact that the two contact surfaces have certain micro or nano-scale roughness including nano-scale asperities and thus cannot conform perfectly onto each other. Consequently, the actual contact area is only a small fraction of the apparent one. These devices suffer from mechanical problems such as switch bouncing, microwelding, adhesion and contact degradation, and as a result show degraded switching performance over time with long-term reliability issues. At this point, an alternative switching technology might be the proposal of liquid-to-solid MEMS (LS-MEMS) switches using movable liquid metal droplets. This promising concept enables electrical switches with higher isolation and lower insertion loss, much like conventional solid-solid MEMS switches. Moreover, since they do not have fragile moving solid parts, LS-MEMS switches potentially do not suffer from mechanical fatigue problems increased contact resistance and stiction/adhesion problems. Our aim in this study is to design and fabricate LS-MEMS switches, whereby we can characterize and examine the actuation of metallic liquid droplets, namely eutectic Ga-In (EGaIn), Gallium Indium Tin alloy (Galinstan), and mercury (Hg) using electrowetting on dielectric (EWOD) principle. We have investigated the effect of different actuation electrode geometries like rectangular, interdigitated fingers and crescent-shaped electrodes on the droplet actuation. With the application of 30-100 V voltage difference across the actuation electrode and the ground electrode, the metallic liquid droplets were observed to be actuated. With further optimization, LS-MEMS device structures demonstrated in this work might have potential applications as alternative high-performance electrical switches.