Because of their high mechanical compliance and electrical properties, the idea of using Ga and CNTs for micro electrical relay contacts has been investigated to minimize damage from switching and make good electrical contacts. Ga was electroplated into droplets on the order of 50 μm in radius on single crystal Si to create a contact for a switch that can be annealed to recover its original electrical properties after mechanical damage. CNTs were grown on Si substrates, coated with a thin Au layer, and transferred to other Si or Kapton substrates through thermocompression bonding. In the case of the Ga contact, repeated switching led to an increase in the resistance, but the resistance recovered after a thermal reflow process at 120 °C. Longer term and larger area contacts were used to measure the contact behavior under switching conditions of up to 200 A/cm2. At moderate cycling conditions (on the order of 200 cycles) the adhesion began to significantly degrade the switch. The oxidation behavior of the Ga droplets was characterized for thermal reflow, suggesting a passivating 30 nm oxide forms at 100 °C. The oxide formed by the Ga is thin and fragile as demonstrated by its use in a switch. The Ga droplets were examined with electrical contact resistance nanoindentation and the loads at fracture and the onset of electrical contact were identified. CNT turfs were also tested for making patterned electrical contacts; turfs of lateral dimensions similar to the Ga droplets were tested using electrical resistance testing during nanoindentation and as macroscopic contacts, and shown to be able to carry similar current densities. The results will be compared between the two systems, and benefits and challenges of each will be highlighted for creating compliant electrical switches and contacts.
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Kim, Y., Qiu, A., Reid, J.A. et al. Fabrication and Characterization of Two Compliant Electrical Contacts for MEMS: Gallium Microdroplets and Carbon Nanotube Turfs. MRS Online Proceedings Library 1299, 1003 (2011). https://doi.org/10.1557/opl.2011.533