Fabrication and Hot Switching Behavior of Electroplated Gallium Spheres for MEMS
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Fabrication and Hot Switching Behavior of Electroplated Gallium Spheres for MEMS Yoonkap Kim and David F. Bahr Mechanical and Materials Engineering, Washington State University, Pullman WA USA ABSTRACT Liquid metal microscale switches, often using mercury, are sometimes used in place of solid-solid contact switches because of the ability to minimize damage from switching and the ability to make good contacts for electrical and thermal conductivity. However, mercury has potential health and safety problems, and is difficult to use at high frequency (kHz) operation due to poor adhesion between the liquid-solid contacts. One alternative to the mechanical and chemical problems of a liquid mercury switch is using soft metals, such as gallium or tin, as a solid metal sphere for switches that can melt at moderate temperatures. Ga micro-spheres for switching operations were deposited on a substrate consisting of photolithographically patterned W films on SiO2 and Si substrates by electroplating, and the applicability for use in a microscale switch was investigated by characterizing the macro structure, hardness, and electrical performance during switching. The resistivity of the electroplated Ga droplets was similar to the theoretical value for pure Ga, and suggests that the electrodeposited Ga will be suitable for a solid MEMS switch. The hardness of the Ga sphere was 5.7 MPa. This suggests a maximum of ~40 µN can be applied to each 50µm radius Ga contact in the current configuration for switching applications. When the Ga spheres were investigated for electrical performance during hot switching, the resistance increased over six switching cycles, but the original lower resistivity was recovered after a 393 K thermal reflow process. INTRODUCTION Micro electro mechanical systems (MEMS) basically consist of using processing developed for microelectronics processing to fabricate devices with both electrical and mechanical functionality [1,2]. Various types of micromechanical switches have been introduced since MEMS were developed [3-7]. These micromechanical switches are useful in applications for wide operating temperature ranges, radiation insensitivity, and usually have a high on-off impedance ratio. There is particular interest in having both a good thermal and electrical contact when the switch is closed. Almost all micromechanical switches have been designed with solidto-solid contacts [8]. Micromechanical switches have various problems such as contact bounce, high contact resistance, noise, slow rise times, and a short operational lifetime due to mechanical wear and tear. These problems can be solved by using liquid metals because they show a fast signal rise time, high contact area, and low contact resistance, and can reduce wear and tear since it is a liquid phase [9]. There are several pure low melting point liquid metals that could be used for MEMS switches such as gallium, mercury, sodium, potassium, phosphorous, and tin. The representative liquid metal is mercury for micro electrical switches becaus
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