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A model of copper chemical mechanical polishing ͑CMP͒ based on methods of chemical kinetics is presented which includes both chemical and mechanical processes. This CMP model explains observed patterns in removal rates for peroxide and nonperoxidebased slurries as a function of oxidizer concentration, polishing pressure and speed, etchant concentration, and pH.

A general model of chemical mechanical polishing ͑CMP͒ has been derived which shows the dependence of the polishing rate on the concentration of chemicals and abrasives in the slurry. This paper applies the model to tungsten CMP and describes the chemistry of tungsten CMP in some detail.In the preceding paper, 1 a model of chemical mechanical polishing ͑CMP͒ was presented describing it as an alternation of chemical and mechanical processes. The chemical process starts with a reversible reaction leading to the formation of a thin surface film, which can decompose, dissolve into the slurry solution, or be mechanically abraded. The mechanical process links material removal to abrasive particles in the slurry. This paper applies the model to the specific case of tungsten CMP. The chemical process for tungsten polishing is discussed in detail below, and then combined with the mechanical process to predict the removal rate as a function of chemical and abrasive concentrations in the slurry.Chemical process.-In the chemical portion of CMP, tungsten atoms on the workpiece surface react with oxidizers in the slurry to form a complex surface film. Different oxidizers follow different chemical mechanisms to form either different films or to form the same film at different rates. Detailed studies 2-3 show that in many cases oxidation occurs in two steps. The slower first step forms a monolayer oxide, and the faster second step thickens this surface layer by an oxygen anion vacancy diffusion process 4 to form a mature film that is about 10 layers thick. 3 At that thickness, the diffusion rate becomes too slow to thicken the film further.Tafel curves from electrochemical measurements have been used to correlate limiting currents and potentials, for different slurry chemicals, to CMP polishing rates. 5 While these curves are qualitatively similar for different oxidizers, they show clear differences that are related to the nature of the surface layer and its rates of formation and dissolution, all of which are determined by the detailed chemistry of the system.In particular, studies have shown that oxidation with Fe ϩ3 ions in ferric nitrate leads to a FeWO 4 surface film 6 while KIO 3 and H 2 O 2 lead to a complicated nonstoichiometric WO 2 /WO 3 duplex oxide. 3 Further, this oxide layer is significantly more soluble in the presence of H 2 O 2 than in its absence. 5 Some electrochemical studies 2 recorded the electron transfer current as a function of time, giving insight into the kinetics of surface film formation. Although rate constants for these reactions have not been determined, the curves qualitatively show a slow initial drop in current with time, followed by a fast decay. The observed patterns reinforce the model of an oxide layer formed in a two-step mechanism.Chemical differences lead to different possible chemical processes, and knowledge of these differences can lead to understanding variations in CMP behavior. A simple kinetic mechanism for the two step process between tungsten surface atoms W and slurry oxidizing...

A model of chemical mechanical polishing is presented which quantitatively correlates the polishing rate with the slurry concentrations of both chemicals and abrasives. The model predicts that as the concentration of either chemicals or abrasives is increased, an initial steep rise in the polishing rate is followed by an asymptotic approach to a maximum rate. The causes and implications of this behavior are discussed.Chemical mechanical polishing ͑CMP͒ has been described qualitatively as an alternation of chemical reaction and mechanical abrasion processes. 1 Although the removal rate for different systems has been characterized 2 experimentally in terms of the applied pad pressure and velocity, changes in concentrations of slurry chemicals and abrasive, and the composition of the pads and abrasives, 3-6 fundamental modeling of the system has been lacking. This paper provides a generic, semiquantitative description for both chemical and mechanical processes, and links these descriptions to form an overall model which predicts the removal rate as a function of the concentrations of chemicals and abrasives in the slurry.In conventional polishing, abrasive particles span a gap between the workpiece and the polishing pad. These particles support the externally applied load, which pushes the abrasive into the moving workpiece forming furrows and removing material. In conventional polishing, the removal rate is independent of both abrasive size and concentration, while the root mean square ͑rms͒ surface roughness increases with increasing abrasive size. 7 By contrast, in CMP, the polishing rate depends on abrasive concentration while the rms surface roughness is independent of abrasive size. 7 Understanding these differences involves a focus on the surface material that is removed.In conventional polishing, the removed material is identical to the bulk workpiece. In CMP, reactions between the workpiece and slurry chemicals produce a surface film. Material is removed by the abrasive separation of this film from the workpiece surface. Modeling this process involves describing the nature of the surface material formed, the chemical/workpiece reactions, and the abrasive/film interactions. Chemical Reaction ProcessIn the chemical process, the workpiece material reacts with chemical components of the slurry to form a thin reaction film. For a given workpiece surface, reactions with different chemicals can form different films, or can form similar films at different rates. Once formed, the surface films may decompose, or, they may dissolve or dissociate into the slurry at different rates. These chemical processes are independent of polishing and can be investigated under lab conditions without polishing abrasive present. Their reaction rates should provide insight into choosing practical CMP slurry components.The surface film formed in CMP is the result of a reaction between the workpiece material M and some chemical component C of the solution. This reversible surface reaction is subject to the laws of chemical equilibrium, ...