Discrete Analysis of Gold Surface Asperities Deformation under Spherical Nano-Indentation Towards Electrical Contact Resistance Calculation

MEMS ohmic switches have demonstrated interesting performances due in part to their low contact resistance which depends on multiple contributions, one amongst them is the roughness of the contact area. In fact, the contact area is composed by clusters asperities that have different sizes, heights and curvature radii. In this work, we investigate the discrete mechanical deformation of asperities at the nano-scale, in the micro-switch pressure range. Loads from 250 µN up to 2 mN are applied by a nano-indenter with a spherical diamond tip (48.5 µm curvature radius). The resulting contact areas are investigated by AFM topography measurements and treated by digital image processing. As a result for each applied loads, the asperities in plastic deformation mode are sorted and used to determine a value of "surface hardness", coherent with the hardness measured by Berkovich nano-indentation. Finally, the asperities identified in plastic deformation mode are used as inputs for the calculation of the electrical contact resistances of equivalent micro-switches.