A Model of Cu-CMP
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A Model of Cu-CMP Ed Paul1, Vlasta Brusic2, Fred Sun2, Jian Zhang2, Robert Vacassy2 and Frank Kaufman2 1 Stockton College, Pomona NJ 08240 USA 2 Cabot Microelectronics, Aurora IL 60504 USA CMP has been described qualitatively1 in terms of alternating cycles of chemical formation and mechanical removal of a thin layer on the wafer surface. A quantitative model of CMP has been developed2-7 which is based on mechanisms for surface kinetics, treating mechanical removal as one step in the mechanism. This model has been used successfully to explain removal rates for tungsten and thermal oxide CMP. In particular, for tungsten CMP the removal rate increases steeply with increasing oxidizer concentration at low concentrations, and then approaches an asymptotic maximum removal rate at high concentrations. The model explains this by starting with the assumption that mechanical abrasion removes only tungsten oxide but not tungsten metal. It then focuses on the fraction of wafer surface covered by a tungsten oxide layer. At low oxidizer concentrations, the oxide formation rate is small compared the removal rate, so only a small fraction of the surface is oxidized and the removal rate is small. At high oxidizer concentrations, the oxide formation rate is large compared to the removal rate, so most of the surface is oxidized and the removal rate is large. Increasing the oxidizer concentration in the high oxidizer concentration region does not significantly increase the surface fraction of tungsten oxide, and the removal rate approaches a constant value. The same dependence of removal rate on oxidizer concentration is observed for copper CMP with many different oxidizers, including iodate8, dichromate9, ferricyanide10 and nitric acid10. Thus the same qualitative and quantitative model can be used to understand Cu-CMP. The equation2 for the removal rate R as a function of oxidizer concentration [Ox], for fitting constants a and b which depend on the mechanical conditions, is R = a [Ox] / (b + [Ox]). This equation has been used to fit the removal rate data8 in Fig. 1. However, when hydrogen peroxide is used as an oxidizer, the dependence of the removal rate on the oxidizer concentration is qualitatively different. As the concentration of peroxide increases at very low peroxide concentrations, there is a very rapid rise in the removal rate. The removal rate reaches a maximum in slurries below about 0.5 % H2O2. As the peroxide concentration increases beyond this point, the removal rate drops and then levels off at some asymptotic value well below the maximum rate. Typical cases11,12 are illustrated as part of the discussion below, with similar examples available in the literature13,14,15 under different conditions. Clearly, copper CMP using peroxide based slurries is different in some important ways from other metal CMP processes. Peroxide is a preferred oxidizer because its reduction product is water, leaving no residues or environmental contaminants. Etchants are often used for copper CMP with peroxide slurries to increase th
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