Investigating slurry transport beneath a wafer during chemical mechanical polishing processes

In order to better understand the role of the fluid behavior in chemical mechanical planarization, we measured slurry transport beneath a glass wafer. We quantified the slurry transport using residence time distribution techniques and two measures of slurry transport efficiency, defined as the percentage of new slurry beneath a wafer. Slurry transport efficiency depended on platen speed, flow rate, and the conditioning method. We found that the average fluid residence times under the wafer decreased roughly linearly with platen speed. A threefold increase in platen speed decreased slurry mean residence time by a factor of 3.5. Changing the flow rate from 20 to 50 mL/min decreased the slurry mean residence times by 80%. In situ conditioning generally increased the slurry mean residence times and the amount of slurry mixing. In situ conditioning also decreased gradients in the slurry composition across the wafer. Pad topography had a large effect on the slurry gradients that developed across the wafer. Finally, we found that the slurry mixing history can be accurately modeled using a simple continuous function.