Abstract
In chemical-mechanical polishing (CMP), even the soft pad asperities may, under certain conditions, generate scratches on the relatively hard surfaces being polished. In the present study, contact mechanics models of pad-induced scratching are formulated, and the effects of the hardness of the surface layers and of pad asperities as well as the interfacial friction are elucidated. Additionally, scratch-regime maps are proposed to provide criteria for scratching hard surface layers by the softer pad asperities. Furthermore, scratching indexes are introduced to predict the proportion of asperities in contact that are likely to scratch. The contact mechanics models of scratching have been validated by sliding experiments with two commercial CMP pads (Pad A and IC1000) and various thin-films (Al, Cu, SiO2, Si3N4, TiN and three low-k dielectrics) using deionized water as a “lubricant.” Both the theoretical models and the experimental results show that the number of scratches increases as the scratching index exceeds 0.33. Al and Cu layers are found to be more susceptible to pad scratching due to their low hardness and high interfacial friction. The scratch-regime maps provide practical guidelines for mitigating pad scratching in CMP.
Highlights
The MIT Faculty has made this article openly available
The Chemical-mechanical polishing (CMP) process is widely used in the manufacture of integrated circuits (IC), computer hard disks, optical glass, and micro-electromechanical systems (MEMS).[1,2,3,4,5]
The maximum hardness values of pad asperities may be estimated as log(Ha,max*) = log(Ha,avg* + 3σ*), the estimated value of the IC1000 pad was 19 GPa, which is unrealistic for a polyurethane polymer
Summary
Creative Commons Attribution 4.0 International License http://creativecommons.org/licenses/by/4.0/. Assuming that the magnitudes of maximum von Mises stress and maximum shear stress lie between the values of the two extremes cases, the normalized average contact pressure at the onset of yielding in the surface layer under elastic-plastically deformed asperity, can be approximated to be the pressure distribution of Hertzian or uniform. In the extreme case of fully plastic asperity deformation (δf–p ≤ δ), from Eqs. 4, 9 and 12, the scratch criteria can be given as σy,a σy,l In this extreme case, too, the condition whether an asperity can scratch the surface depends only on the yield strengths of the asperity and of the surface layer, and the friction coefficient. If the pad-to-layer hardness and the friction coefficient fall in the ‘scratch regime,’ an elastically deformed asperity, at the onset of yielding, will scratch the surface layer. To simplify Eqs. 31 through 33, scratching indexes for elastically and plastically deformed asperities, αe and αp respectively, are introduced as: αe
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