Experimental investigation of thin film spreading resistance in micro-contacts

This paper experimentally explores thin film micro-contact pairs of Au-Au, Au-Ru and Au-RuO₂ through the utilization of thin-film contact resistance and spreading resistance theory. Contacts of various combinations of material pairs, including evaporated Au lower planar contact and sputtered Au, Ru or Au-RuO₂ upper hemispherical contact were fabricated and their performance monitored through many cycles of actuation. The micro-contacts were actuated using an external, calibrated point load and cycled between 1 and 10⁷ cycles in a controlled atmosphere. To examine the micro-contact performance, the contact resistance and force required to close the contacts were monitored simultaneously throughout testing. Overall performance of these devices followed current models reasonably well, but did show some degree of variation from predicted behavior using standard elastic and plastic material deformation model based predictions of contact resistance. Thin film spreading resistance theories provide a possible explanation for these observed variations and using this theory may also allow for explanations of other observations such as material transfer and micro-contact failure. Better understanding of the physics driving this and the manner in which these devices behave is a necessary step for optimizing micro-switch designs to survive greater cycling and provide more predictable and reliable operations. When compared to data that is free from error induced by other factors (such as contamination film) addition of spreading resistance theory improves the ability to predict contact resistance. In the case of Au-Ru contact pairs values have been measured as low as ~ 0.2Ω. Without spreading resistance, current theory predicts values for this experiment to be around 1.5-3.5Ωs. Addition of spreading resistance reduces the expected range to 0.8-1.5Ω. Similar results were shown with Au-RuO₂ pairs while Au-Au were inconclusive.