CMP characteristics of quartz glass substrate by aggregated colloidal ceria slurry

Colloidal ceria is expected to be a candidate of alternative slurry for glass substrate CMP because the slurry particles are small and have a regular shape comparing to conventional calcined ceria particles. Thanks to the characteristics, the colloidal ceria can generate a high quality surface in glass substrate CMP, but removal rate (RR) is smaller than that of calcined ceria. In order to improve the RR, KOH was added to the colloidal ceria. KOH additives make colloidal ceria particles aggregate and improve RR close to that of calcined ceria. By evaluating the extent of aggregation with and without ultrasonic dispersion, it was found there was a strong correlation with KOH concentration to aggregation strength and RR. To obtain further RR and to reduce the consumption of slurry, influence of polishing pressure and slurry concentration on polishing characteristics was investigated. As the polishing pressure increased, the RR improved, while slurry residue occurred partly on the edge of the substrate surface. The slurry residue was restrained by decreasing slurry concentration. Furthermore, although scratches in the nanometer order increase slightly under high polishing pressure, they are smaller than those by calcined ceria slurry. By controlling the degree of aggregation of colloidal ceria, we have achieved both high RR and high quality surface.