Colloidchemical Interactions of Silica Particles in the Cu-CMP-Process

摘要

Polishing slurries for the copper chemical mechanical planarization (CMP) process consist of a complex composition of highly stable nanoparticle suspensions in the presence of chemical additives for etching and protecting the surface. The target of such dispersions is to achieve perfectly smooth surfaces of low topography. Key factors are the surface roughness, local geometry and total flatness of the whole wafer. Surface defects like scratches, dishing, etching and erosion e.g. due to strong particle adsorption, aggregates or chemical impact have to be avoided. Particle wafer interactions between silica particles of a silica CMP-dispersion and a copper wafer surface were analyzed by electron cpectroscopy for chemical analysis (ESCA), scanning electon microscopy (SEM) and zetapotential-measurements. The colloidal stability of the silica dispersion over a broad pH-range of 2.3 to 9.8 was analyzed in the presence of Na~+-, Mg~(2+)-, Cu~(2+)-, and Al~(3+)-ions in terms of the critical coagulation concentration (CCC), change in particle size and pH. Silica particles strongly and irreversibly adsorbed to the predominantly oxidic copper wafer surface at pH 2.3 but not at pH > 4. Over the whole pH- range a high colloidal stability was observed with a maximum at pH 2.3. CCC-values of 100-300 mmol/L versus Cu~(2+) were obtained. Even at high pH of 9.8, the behaviour could not be explained by the Derjaguin-Landauer-Verley-Overbeek (DLVO) theory. In the presence of Cu~(2+)-ions a higher colloidal stability compared to divalent Mg~(2+)-ions was observed at high initial slurry-pH of 9.8. Due to the acidic reaction of Cu~(2+) in the aqueous environment, the pH was reduced to 2-3, where colloidal silica showed the highest stability. Even at high removal rates in the polishing process of 1000 nm/min, the released Cu~(2+)-concentration (40 mmol/L) was lower than the critical coagulation concentration (100 mmol/L).