A Model of Chemical Mechanical Polishing
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A MODEL OF CHEMICAL MECHANICAL POLISHING Ed Paul Stockton College, Pomona NJ 08240 and NIST, Gaithersburg MD 20899 U.S.A. ABSTRACT A generic model is presented which explains the dependence of chemical mechanical polishing rates on the concentration of reacting chemicals and abrasives in the slurry. The predictions of this model are compared to data from the literature for tungsten CMP.
INTRODUCTION AND THEORY CMP has been described qualitatively as an alternation of chemical reaction and mechanical abrasion processes1. This paper provides a generic, semiquantitative description for both of these processes and links these descriptions to form a model that predicts the removal rate as a function of the concentrations of chemicals and abrasives in the slurry. Certain modifications are needed to apply the generic model to specific systems. As an example, the results will be compared to polishing data available in the literature for tungsten. In this model of CMP, the abrasive particles are small and the pad and slurry fluid support the load. Chemical reactions between the workpiece material and chemicals in the slurry form a thin film on the workpiece surface. This film, which is not tightly bonded to the bulk material, is separated from the workpiece surface by abrasive particles, which are pushed into it by the polishing pad. Fresh surface is then exposed on the workpiece, and this surface is available for reaction with the chemicals in the slurry for a subsequent CMP cycle. The chemical process of forming the reaction surface film depends on how much of the reacting chemical is in the slurry. At low chemical concentrations, the surface is only partially covered. Increases in chemical concentration will increase the surface coverage until the film is complete and additional chemical has no effect on the polishing rate. Similarly, the mechanical process of removing this film depends on the amount of abrasive in the slurry. At high abrasive loading, the pad is full and adding more abrasive will not affect the polishing rate, while at low abrasive loading there is space on the pad surface to hold more abrasive particles. In this case, additional abrasive will increase the polishing rate. A quantitative discussion of these processes is presented below. Chemical Reaction Process In the chemical process, the workpiece material reacts with chemical components of the slurry to form a thin reaction film. This surface reaction is subject to the laws of chemical equilibrium, and can be written schematically as MC*
M+C
Rxn. 1
where M and C represent the workpiece material M and the reacting chemical C while MC* represents the surface complex which forms the film. The reaction is a reversible one: the film can decompose and return chemical C to the slurry. The MC* complex is available for either dissolution into the slurry or for mechanical abrasion. When stable surface films form, their dissolution, sometimes called corrosion, is a slow process. It may be written as MC*
MC(aq) + M
E1.4.1
(Rxn. D)
where MC(aq) represents
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