Study of friction and wear with AFM in CMP process of selective layer

Abstract

Atomic force microscopy (AFM) allows one to examine the effects of applying highly localised stress to a surface. In the presence of solutions, tribochemical friction and wear can be investigated. We present the results of fundamental studies of the simultaneous application of chemical agents and mechanical stress using a single asperity model and a solid surface. At the same time, we show the consequences of combining highly localised mechanical stress (due to contact with AFM tip) and exposure to solutions of known pH. The experiment simulates several features of a single particle–substrate–slurry interaction in chemical mechanical polishing (CMP). To optimise CMP process, one needs to get information on the interaction between the abrasive slurry particles and the surface being polished. To study such interaction, we used AFM. Surface analysis of selective layer using the AFM revealed detailed surface characteristics obtained by CMP. In studying the selective layer CMP, which is predominated by copper (in proportion of over 85%), we found that the AFM scanning removes the surface oxide layer in different rates depending on the depth of removal and the pH of the solution. It was found that removal mechanisms depend also on the slurry chemistry, potential, percentage of oxidiser and the applied load. We show that linear and raster scans display significantly different material removal rates. Oxide removal happens considerably faster than the copper CMP removal from the selective layer. This is in agreement with generally accepted models of copper CMP. Quantitative models are presented to explain the observed nanometre scale surface modifications. Both load force and the friction forces acting between the AFM tip and surface during the polishing process were measured. One big advantage of using the AFM tip (of radius of ∼50 nm) as abrasive silica particle is that we can measure forces acting between the particle tip and the surface being polished. Here, we report measurement of the friction force while scratching and polishing. The correlation between those forces and the removal rate is discussed. At the same time, this paper complements recent observations of tip induced wear and growth in a number of inorganic surfaces in solution.