Selective layer chemical mechanical planarization process mimicked with atomic force microscope

Abstract

To understand the mechanisms of chemical mechanical planarization (CMP), an atomic force microscope (AFM) was used to characterize polished layer surfaces formed by selective transfer after a series of polishing experiments. The AFM allows one to examine the effects of applying highly localized 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 localized mechanical stress (due to contact with AFM tip) and exposure to aqueous solutions of known pH. The experiment simulates several features of a single particle–substrate–slurry interaction in CMP. To optimize the CMP process, one needs to obtain information on the interaction between the slurry abrasive particles and the polished surface. To study such interactions, we used AFM. An AFM tip of radius of about 50 nm was used to mimic a single abrasive particle, typical of those found in CMP slurry. Surface analysis of selective layer using the AFM revealed detailed surface characteristics obtained by CMP. Studying the selective layer CMP, of which the predominant one is copper (in proportion of over 85 per cent), 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 oxidizer, and the applied load. We show that linear scans 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. Both long-range and the friction forces acting between the AFM tip and surface during the polishing process were measured. The correlation between those forces and removal rate is discussed. At the same time, this article complements recent observations of tip-induced friction and wear and growth in a number of inorganic surfaces in aqueous solutions.