Advances in the Characterization of Particle Size Distributions of Abrasive Particles used in CMP

ABSTRACTThe use of density gradient stabilized centrifugal sedimentation (disc centrifugation) for the characterization of abrasive particles in CMP slurries is reported. For slurries prepared with ceria and colloidal silica abrasives, the technique is demonstrated as capable of providing highly repeatable analyses of the abrasive's apparent particle size distribution (PSD). The addition of water soluble organic additives to the slurries is shown to produce large shifts in the apparent PSD relative to the PSD of the pure abrasive particles. Particle-additive interactions driving the shift in apparent PSD could not be interpreted with confidence due to the lack of accurate densities for particle-abrasive complexes formed in the slurry. To address this problem, sucrose density gradients prepared using H2O and D2O were tested as spin fluids of different densities to analyze a colloidal silica standard with a narrow PSD. Preliminary results comparing experimental disc centrifuge data with theoretical simulations of the disc sedimentation pattern suggest that this technique can potentially characterize both the particle diameter and density of abrasive particles in a CMP slurry.