In-Line Refractive Index In Assay Characterization Of Incoming Fresh And Effluent Spent CMP Slurry

In-line refractive index (RI) measurements are the technique of choice for qualifying the hydrogen peroxide content in CMP slurries. However, peroxide content is not the only slurry metric of interest. Typically, slurries are delivered from the manufacturer in concentrated form, then diluted with water and peroxide at the fab. Though slurry density is a critical parameter for CMP performance, incoming density can vary from batch to batch. Simply adding specified amounts of peroxide and water will give inconsistent results at the process tool. Provided the blender system can circulate incoming slurry through the refractive index unit, RI can be used to monitor the density of slurry in the blender tank, ensuring that the specified density is maintained regardless of fluctuations in the incoming material. We considered industrially-relevant slurries used for polishing copper, dielectrics, and tungsten, and found a strong correlation between density and refractive index in all three cases. Slurry density was characterized both offline and under high-volume manufacturing process conditions. The offline work showed that RI measurements reported smaller changes in water content as compared to the densitometer in all slurries tested indicating that it could provide a more precise data which would lead to a tighter control of the slurry composition. On the effluent side of the CMP tool, RI measurements were used to determine the ratio of polyurethane pad shavings to silica slurry particles after pad conditioning. Dynamic light scattering measurements were not able to report these values independently.

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