Fabrication, structure, and performance of a microfabricated gallium electrical switch contact
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A contact for a micromechanical switch has been fabricated using electroplated gallium (Ga) on silicon to create an electrical switch contact that can be annealed to recover its original properties after mechanical damage. The resistivity of the electroplated Ga appears to be similar to pure Ga. The resistance increased with cycling but recovered to the original value after a thermal reflow process at 120 °C for 10 min. The hardness of thermally reflowed Ga droplets was 2 MPa when the droplets were unconstrained and was up to 95 MPa for constrained droplets, suggesting that all switching in this study caused permanent deformation at room temperature and that defects formed during plastic deformation are likely candidates for the increased resistance during cycling. Up to 300 switching cycles were investigated for contacts involving up to four Ga droplets to measure contact behavior under high-current and load-switching applications. Oxidation behavior was characterized for the thermal reflow process on the Ga droplets, suggesting a passivating 30-nm oxide form at 100 °C, and electrical contact resistance nanoindentation suggests the oxide breaks during mechanical contact. I. INTRODUCTION
Microelectromechanical systems (MEMS) cover a broad base of systems that use techniques developed for microelectronics processing to fabricate devices with both electrical and mechanical functionality.1 Various types of micromechanical switches have been introduced in MEMS.2–4 These switches are useful in applications where temperature and radiation insensitivity, along with a high on–off impedance ratio, are desired. There is particular interest in having both a good electrical and thermal contact when the switch is closed. Almost all micromechanical switches have been designed with solidto-solid contacts.5 Micromechanical switches have traditionally suffered from problems such as contact bounce, high contact resistance, slow rise times, and a short operational lifetime due to adhesive failure, contact contamination, and mechanical wear.6,7 To reduce wear and decrease adhesion, one design route is to increase the hardness of the contact material, such as recent work regarding precipitation hardening in gold (Au) contacts.8 The issue for the longterm reliability of MEMS contact switches is maintaining a high current density without the increase in contact resistance during switching. In this case, the high contact resistance is one of the main issues that impact MEMS contact switches. The contact resistance depends on the surface condition, contact area due to the applied force, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.220 2428
J. Mater. Res., Vol. 26, No. 18, Sep 28, 2011
http://journals.cambridge.org
Downloaded: 14 Mar 2015
and thermal conduction.9,10 If the contact surface condition is clean, the resistance value is low. The contact resistance decreases with increases in applied force on the switch because the real contact area increases with force.9 Poor thermal conduction lea
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