Stabilization of alumina slurries in presence of oxidizers for tungsten chemical mechanical polishing

Planarization of tungsten surfaces is achieved through a chemical mechanical polishing (CMP) process which uses slurries containing both a particulate abrasive and an oxidizing agent. In some systems, the combination of these components results in an unstable slurry. Formation of large particle agglomerates was found to be primarily responsible for destabilization of slurries. In this paper, it is reported that slurries containing alumina (Al₂O₃) as the abrasive particle and potassium ferricyanide (K₃Fe(CN)₆) as the oxidizer have been stabilized using a combination of surfactants. The surfactant system which was found to stabilize the slurry contains both an anionic surfactant and a nonionic surfactant. Slurry stability was determined through visual observation of settling as well as particle size and zeta potential measurements. The use of surfactant-stabilized alumina slurries was found to decrease the surface roughness of a polished tungsten wafer when compared to a slurry without surfactant. Surface roughness was characterized with atomic force microscopy (AFM). The polishing rate of tungsten was found to decrease slightly when using a surfactant-stabilized slurry. Also, scanning electron microscopy (SEM) results showed a decrease in adsorption of alumina particles to both a tungsten and silica surface when a surfactant-stabilized slurry is compared to a slurry without surfactant. These results can be explained by assuming that, in a stable system, there is a smaller mean particle size and that a lubricating organic layer of surfactant has adsorbed on the alumina particle surface.