(352j) Shear Thickening and Defect Formation of Fumed Silica CMP Slurries Using Rheo-Polishing | AIChE

(352j) Shear Thickening and Defect Formation of Fumed Silica CMP Slurries Using Rheo-Polishing

Authors 

Liberatore, M. W. - Presenter, Colorado School of Mines
Crawford, N., Colorado School of Mines
Williams, K., Colorado School of Mines



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.