Effects of alloying and local order in AuNi contacts for Ohmic radio frequency micro electro mechanical systems switches via multi-scale simulation
BE Gaddy and AI Kingon and DL Irving, JOURNAL OF APPLIED PHYSICS, 113, 203510 (2013).
Ohmic RF-MEMS switches hold much promise for low power wireless communication, but long-term degradation currently plagues their reliable use. Failure in these devices occurs at the contact and is complicated by the fact that the same asperities that bear the mechanical load are also important to the flow of electrical current needed for signal processing. Materials selection holds the key to overcoming the barriers that prevent widespread use. Current efforts in materials selection have been based on the material's (or alloy's) ability to resist oxidation as well as its room-temperature properties, such as hardness and electrical conductivity. No ideal solution has yet been found via this route. This may be due, in part, to the fact that the in-use changes to the local environment of the asperity are not included in the selection criteria. For example, Joule heating would be expected to raise the local temperature of the asperity and impose a non-equilibrium thermal gradient in the same region expected to respond to mechanical actuation. We propose that these conditions should be considered in the selection process, as they would be expected to alter mechanical, electrical, and chemical mechanisms in the vicinity of the surface. To this end, we simulate the actuation of an Ohmic radio frequency micro electro mechanical systems switch by using a multi-scale method to model a current-carrying asperity in contact with a polycrystalline substrate. Our method couples continuum solutions of electrical and thermal transport equations to an underlying molecular dynamics simulation. We present simulations of gold-nickel asperities and substrates in order to evaluate the influence of alloying and local order on the early stages of contact actuation. The room temperature response of these materials is compared to the response of the material when a voltage is applied. Au-Ni interactions are accounted for through modification of the existing Zhou embedded atom method potential. The modified potential more accurately captures trends in high-temperature properties, including the enthalpy of mixing and melting temperatures. We simulate the loading of a contacting asperity to several substrates with varying Ni alloying concentrations and compare solid solution strengthening to a phase-separated system. Our simulations show that Ni concentration and configuration have an important effect on contact area, constriction resistance, thermal profiles, and material transfer. These differences suggest that a substrate with 15 at. % Ni featuring phase segregation has fewer early markers that experimentally have indicated long-term failure. (C) 2013 AIP Publishing LLC.
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