A Model of Chemical Mechanical Polishing: The Role of Inhibitors
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A Model of Chemical Mechanical Polishing: The Role of Inhibitors Ed Paul, Stockton College, Pomona NJ 08240 and Robert Vacassy, Cabot Microelectronics, Aurora IL 60504 Abstract: A previously presented model of CMP is extended to include the role of inhibitors. In CMP, a chemical reaction forms a surface film which is removed mechanically by abrasives. When inhibitor molecules bond to the surface film, the mechanical abrasion rate is reduced. The general model will be discussed, and then applied to W-CMP explaining differences in the reduction of polishing rates for different inhibitors. Introduction: Chemical Mechanical Polishing is a complex, multi-scale process with an estimated 2 or 3 dozen input variables leading to output results. Linear dimensions can be used for an overview and can serve as a basis for modeling and understanding of separate facets of the overall process. At the 100 mm scale, fluid dynamics can describe how polishing pressure and speed, combined with the mechanical properties of the fluid, pad and wafer, result in an applied pressure that forces the pad to envelop the abrasive and push it onto the wafer surface. At the 10 µm scale, asperities on the pad surface deform under the applied pressure, increasing the contact area and leading to a constant effective pressure pushing the abrasive onto the wafer surface. Because the contact area is proportional to the applied pressure, and material removal is proportional to the contact area, it follows that the mechanical removal rate is proportional to pressure. At the 1 µm scale, abrasive particles can transfer from the slurry to the pad surface and back. Adherence of abrasive particles to the pad leads to a linear increase in the polishing rate with concentration at low concentrations, while saturation of the pad surface by abrasive particles leads to an asymptotic maximum polishing rate at high abrasive concentrations. This behavior has been observed experimentally and explained theoretically in previous work. At the 100 nm scale, the pad envelops abrasive particles and pushes them onto the wafer surface. At the 1 nm scale, chemical reactions between slurry components and the wafer form a surface film which is removed by abrasive action. The mechanism for this removal is not well characterized. It may involve plastic deformation caused by indentation of the abrasive into the surface. Or it may involve adhesion between the wafer surface film and the abrasive. The adhesion mechanism can be identified with chemical tooth1 and can be used to explain why different abrasive materials have different removal rates2. The generally accepted mechanism3 for CMP involves alternating cycles of chemical formation and mechanical removal of a surface film on the wafer. The mechanism has been successfully modeled4-6 using methods of steady state chemical kinetics, and has been used to explain how the removal rate depends on the concentrations of oxidizer and abrasive and on polishing pressure and temperature for tungsten CMP. The model is described below. Th
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