Shear Thickening and Defect Formation of Fumed Silica CMP Slurries Using Rheo-Polishing

During the chemical mechanical polishing (CMP) process, it is believed that shear thickening of the slurry, caused by particle agglomeration, results in a significant increase in surface defects (i.e, scratches, gouges, pits, etc.). These CMP-induced defects then cost the semiconductor industry billions of dollars annually in lost production. We have developed a methodology for the synchronized measurement of rheological behavior while polishing a semiconductor wafer, the first of its kind (a technique termed rheo-polishing). We investigated the shear thickening of a 25 wt% fumed silica slurry with 0.15 M added KCl (100-200 nm particle diameter). The thickened slurry displayed a ~5-fold increase in viscosity with increasing shear rate. As the shear rate was reduced to zero, the slurry continued to thicken showing a final viscosity that was ~100x greater than the initial viscosity. Optical microscopy and non-contact profilometry were then utilized to directly link slurry thickening behavior to more severe surface scratching. The thickened slurry generated up to 7x more surface scratches than a non-thickened slurry. Both slurry thickening and surface scratching were associated with a dramatic increase in the population of “large” particles (>300 nm) which were undetectable in the non-thickened slurry. These “large” and potentially scratch-generating particles are believed to instigate measurable surface damage.