Probable role of abrasion in chemo-mechanical polishing of tungsten

The chemo-mechanical polishing process (CMP) has received much attention recently because of its importance as a nano-scale finishing process for high value-added large silicon wafers, which are used in the production of integrated circuits. The process is not well understood at present. It is clearly a synergistic interaction of the components of the slurry and its chemistry with the surface of the material being polished. It is not immediately obvious which of the processes of abrasion or erosion, electrochemistry, tribochemistry, or fluid dynamics is the rate controlling process and the prime determinant in achieving the required removal rates and nano-scale planarity. In this study, we estimate the probable contributions of abrasion in CMP of tungsten metal, which is one of the metals used in integrated circuits. The low-pH slurry contains nano-scale alumina particles, which may or may not be in contact with the polymeric pad and the metal surface at the same time. Thus, they may interact with the surface under either erosive or two-body or three-body abrasive conditions. We have applied extensions of commonly known abrasion and erosion models, together with electrochemical considerations, to actual polishing data from laboratory and industrial scale CMP processes. The conclusions are that while the actual removal rates are within the range which is expected to be typical of abrasion of tungsten metal by very fine abrasives, energy considerations suggest that material is removed in the form of a weak surface film. Formation of this film is consistent with the electrochemical conditions of the tungsten-slurry system. The results provide tentative explanations for the planarization effect and possibly also for the occasional growth of surface defects during polishing. (C) 1999 Elsevier Science S.A. All rights reserved.