Inhibition of alumina particle deposition onto SiO2 surfaces during tungsten CMP through the use of citric acid

The beneficial effects of citric acid in inhibiting alumina particle deposition onto silica areas during tungsten CMP were investigated. The electrokinetics of alumina in the presence of citric acid and the uptake of citric acid by alumina were studied experimentally. At a pH of 4 as the citric acid concentration was increased, the zeta potential of alumina became less positive and reversed sign. Slurry dip tests and small-scale polishing experiments were carried out and the surface cleanliness of contaminated oxide surfaces was characterized with a FE-SEM and image analysis. It has been demonstrated that citric acid is very effective in controlling alumina contamination on oxide surfaces when added into the slurry. Electrochemical tests showed that citric acid does not significantly attack tungsten films. A mechanism for the interaction between citric acid and alumina particles has been proposed.