In the chemical mechanical planarization (CMP) of dielectrics and metals, the presence of oversized particles in the slurry is one of the main causes of defectivity. Pumps within the slurry distribution system play a significant role in increasing both the number and distribution of oversized particles. In this study, the effect of stress induced by different types of pumps on particle agglomeration was investigated. The size of the particles in circulated silica slurries and the surface roughness of low- k samples were characterized using a single-particle optical sensor and atomic force microscope, respectively. We found the mean value of normalized oversized concentrations in the positive displacement pump system at 500 turnovers to be 6 times higher than that of a magnetically levitated centrifugal pump system. Our results indicate that the magnetically levitated centrifugal pump was a low-shear pump and did not increase the concentration of oversized particles significantly with slurry turnovers. Low- k wafers polished by circulated slurries from the magnetically levitated centrifugal pump system showed little increase in surface roughness and defectivity. Consequently, a positive correlation was established between the roughness/defect density and the degree of agglomeration.