Based on the theory of colloid, a new KOH-free silica slurry is developed for chemical mechanical polishing (CMP) to planarize the interlayer dielectric films in ULSI's. The ammonium salt addition to the slurries of pH=6-7 promotes the silica particle agglomeration in the slurry, and enlarges the effective abrasive particle size. The particle agglomeration yields a high polishing rate due to increasing of the mechanical effect, and good particle removability with scrub cleaning is achieved. The ammonium salt-added slurry provides the high-speed CMP without alkaline-metal contamination to the LSI fabrication line.