Further Investigation of Slurry Additives for Selective Polishing of SiO2 Films over Si3N4 Using Ceria Dispersions

Abstract

Following our earlier analysis of amino acids, we investigated the effectiveness of acetic acid, pyridine, and sorbitol that were chosen to represent carboxylic acid, amine and alcohol functional group families, respectively, as additives in ceria dispersions for polishing SiO2 and Si3N4 films. By comparing the speciation of the additives available and the material removal rates with respect to pH, we identified that the removal rates of nitride, but not oxide, are suppressed to <1 nm/min in the pH range where the additive species containing a protonated amino group or a neutral carboxylic group or a neutral hydroxyl group are dominant. All of these species are strong hydrogen bond donors and can form strong hydrogen bonds with Si3N4 surface while their corresponding bonding on SiO2 surface sites is weaker. The stronger hydrogen bonding hinders Si3N4 hydrolysis and suppresses Si3N4 removal. In contrast, the weakly bound additive species are easily removed from the SiO2 surface by the polishing pad and the ceria abrasives, leading to high SiO2 to Si3N4 removal rate selectivity. Similar results were also obtained with valeric acid, imidazole, glucose, sucrose, and mannitol, confirming the importance of a strong hydrogen bond formation in a broader class of additives.