Chemical-Mechanical Polishing (CMP) is a process used in the semiconductor industry to planarize layers of material that are deposited on wafers during the fabrication of integrated circuits. In CMP, the wafer is pressed against a rotating polishing pad whose roughened surface is saturated with a chemically- reactive and abrasive slurry. The chemical activity attacks and softens a thin layer on the wafer surface, which is then removed by the abrasive action of slurry particles that are trapped between the wafer and pad asperities. High regions are removed faster due to larger mechanical pressures, thus causing the surface to planarize.
New measurements of the fluid pressures at the pad/wafer interface taken at Georgia Tech show suction pressures under most of the wafer, with a small positive pressure region at the trailing edge. While the suction pressures can be understood using standard lubrication theory based on the Reynolds Equation, the positive pressures are difficult to explain given the known geometry of the system. Furthermore, experiments involving changes in the pad rotation rate show dramatic history effects that indicate that viscoelastic properties of the polishing pad play a major role in the process.
This seminar will summarize the basic experimental results that require explanation. Several simple, related 1D models based on load and moment balance and the Reynolds Equation with a source term will be described that have been used to characterize the Georgia Tech data. Results suggest that fluid is injected from the lubrication layer into pores in the top few microns of the pad surface under the leading ~2/3 of the wafer, then is returned to the lubrication layer under the trailing third. The calculated wafer/pad separation distance decreases and the wafer tilt angle increases with time, consistent with a viscoelastic model of the pad.