A fundamental analysis of abrasive slurry hydrodynamics in Chemical-Mechanical Polishing (CMP)

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Chemical-mechanical polishing (CMP) is widely used in the microelectronics and semiconductor industries. Surface films (often just nanometers thick) are removed by exposing the chip, or wafer, to a flowing slurry consisting of inert abrasive particles suspended in a chemically reactive solution. The chemical components and the abrasive particles work synergistically to remove a surface layer of material. The two considerations for CMP are material removal rate (or MRR) and surface quality (i.e. roughness, scratches, gouges, pits, nonuniform material removal must be minimized). A balance between MRR and surface quality must be attained by manipulating operating conditions, e.g. abrasive particle type, slurry chemistry, polishing pressure and velocity. One of the greatest impediments to the development of a mechanistic model of the CMP process is that the flow of the slurry within the thin film separating the wafer from the pad is not well understood. Formation of abrasive particle aggregates under shear can cause non-Newtonian slurry behavior, which will substantially alter the flow pattern of the slurry, the pressure distribution between the pad and the wafer, the shear stresses experienced by the wafer, and the velocity and concentration of particles coming into contact with the surface. The proposed research program will build upon the expertise of the principal investigator in experimental multiphase flows and mechanical abrasion. The main objective of the proposed research program is to develop a new and more rigorous understanding of the slurry flow hydrodynamics in a CMP process and to show the interconnectivity among hydrodynamics, abrasive particle properties, slurry rheology, pad pressure and their combined effect on the mechanical aspects of material removal rate and surface quality. Micro-PIV (particle image velicometry) and slurry rheology measurements will be made simultaneously in a novel, custom-designed test apparatus to study directly the relationship between velocity field and particle aggregation. Additional micro-PIV experiments will be done using a bench-scale CMP machine so that the effects of pad asperities and abrasive particle aggregation on the velocity and shear fields during a polishing event can be determined. Finally, polymer composite particles designed for specific hydrodynamic conditions, and designed to minimize shear-induced aggregation, will be tested. The results of this program will be invaluable to all CMP researchers and practitioners because the slurry flow hydrodynamics really are the link between the kinetics of chemical-based polishing and mechanical abrasion. The microelectronics industry will be able to use these results to predict CMP performance and optimize their operations without using the current trial-and-error approach.