Tungsten passivation layer (WO3) formation mechanisms during chemical mechanical planarization in the presence of oxidizers

Abstract

Effects of single and mixed oxidants of Fe(NO<inf>3</inf>)<inf>3</inf> and H<inf>2</inf>O<inf>2</inf> containing acidic silica slurries were studied to investigate the mechanism of tungsten (W) chemical mechanical planarization (CMP). The W polishing rate obtained from the CMP test depicted high W polishing rate in the presence of mixed oxidants of Fe(NO<inf>3</inf>)<inf>3</inf> and H<inf>2</inf>O<inf>2</inf> as compared to a single oxidant of either H<inf>2</inf>O<inf>2</inf> or Fe(NO<inf>3</inf>)<inf>3</inf>. The formation of a passive layer of tungsten oxide (WO<inf>3</inf>) and W dissolution could be the reason for these results as confirmed by XPS. Further investigation revealed that the generation of much stronger oxidants of hydroxyl radicals ([rad]OH) was solely responsible for WO<inf>3</inf> layer formation. Quantitative evaluation of [rad]OH generation was estimated using a UV–visible spectrophotometer and confirmed that in-situ generation of hydroxyl radicals ([rad]OH) could be a main driving force for the high W polishing rate by converting a hard W film into a soft passive film of WO<inf>3</inf>. WO<inf>3</inf> film formation was further confirmed using potentiodynamic polarization studies, which showed a smaller value of corrosion current density (I<inf>corr</inf>) in mixed oxidants of Fe(NO<inf>3</inf>)<inf>3</inf> and H<inf>2</inf>O<inf>2</inf> as compared to the large values of I<inf>corr</inf> observed for H<inf>2</inf>O<inf>2</inf> alone. This study revealed that a single oxidizer of either Fe(NO<inf>3</inf>)<inf>3</inf> or H<inf>2</inf>O<inf>2</inf> was not capable of achieving a high W removal rate. Rather, only mixed oxidants of Fe(NO<inf>3</inf>)<inf>3</inf> and H<inf>2</inf>O<inf>2</inf> could cause a high W polishing rate due to excessive in-situ generation of [rad]OH radicals during the W CMP process. © 2020