Innovations in relevant micro-contact areas are highlighted, these include, design, contact resistance modeling, contact materials, performance and reliability. For each area the basic theory and relevant innovations are explored. A brief comparison of actuation methods is provided to show why electrostatic actuation is most commonly used by radio frequency microelectromechanical systems designers. An examination of the important characteristics of the contact interface such as modeling and material choice is discussed. Micro-contact resistance models based on plastic, elastic-plastic and elastic deformations are reviewed. Much of the modeling for metal contact micro-switches centers around contact area and surface roughness. Surface roughness and its effect on contact area is stressed when considering micro-contact resistance modeling. Finite element models and various approaches for describing surface roughness are compared. Different contact materials to include gold, gold alloys, carbon nanotubes, composite gold-carbon nanotubes, ruthenium, ruthenium oxide, as well as tungsten have been shown to enhance contact performance and reliability with distinct trade offs for each. Finally, a review of physical and electrical failure modes witnessed by researchers are detailed and examined.

This paper presents the micro-contact performance comparison between contact pairs of Au/Au and composite contact pairs Au/Au-CNT. The Au/Au-CNT micro-contact's planarlower contact interface is an Au-CNT composite film with encapsulated CNTs. Micro-contact performance is affected by factors such as applied micro-contact force, temperature, current density, etc. At the micro-contact interface, asperities provide localized points for current flow. Increased temperature at these localized points may soften the contact metal and lead to bridge transfer. Prior work revealed that an Au/CNT contact pair performed poorly compared to an Au/Au contact pair, with two orders of magnitude difference in contact resistance. To maintain microcontact performance and to reduce thermal effects, a Au/Au-CNT micro-contact was designed and fabricated. This design allows the micro-contact interface to remain Au/Au with the embedded CNTs acting as a thermal conduction conduit below the lower Au contact interface. The upper micro-contact support structure is an Au micromachined fixed-fixed beam with hemisphere-shaped upper contact geometry. The micro-contacts were studied under repeated cycles using an external, calibrated load, resulted in repeatable resistance of approximately 1Ω for nearly 40 million cycles. This research revealed that including a CNT composite film in the lower contact extend the operating life and lower contact resistance as compared to similarly constructed microcontacts.

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This paper reports the contact resistance evolution results of thin film, sputtered gold with encapsulated Ag colloids, micro-contacts dynamically tested up to 3kHz. The upper contact support structure consists of a sputtered gold surface micromachined, fixed-fixed beam designed with sufficient restoring force to overcome adhesion. The hemisphere-upper and planar-lower contacts are mated with a calibrated, external load resulting in approximately 100µN's of contact force. Contact resistance is measured, in-situ, using a crossbar configuration and the entire apparatus is isolated from external vibration and housed in an enclosure to minimize contamination due to ambient environment. Additionally, contact cycling and data collection are automated using a computer and LabVIEW. The results showed that Au-Au micro-contact had a final resistance of 14Ωs after 10 million cycles and 81Ωs after 8 million cycles with Au-Ag micro-contacts.