Stabilization of gamma alumina slurry for chemical–mechanical polishing of copper

Stabilization of γ  -alumina suspension for chemical–mechanical polishing (CMP) of copper was investigated. Citric acid and poly(acrylic acid) (PAA) (Mw=5000Mw=5000) were used as dispersant. The stability of suspension was evaluated from the changes in viscosity, particle size and zeta potential. It appears that metastable γ-alumina mainly due to its high specific surface area and to the presence of aluminol groups on its surface is progressively transformed to bayerite (β-Al(OH)3) by hydration procedure. Citric acid molecules were adsorbed onto γ-alumina surface effectively and exhibited the excellent hydration inhibition effect. Although citrate–alumina surface complexes give barrier to the flocculation, the repulsion potential is based mainly on the electrostatic repulsion, thereby steric hindrance caused by the adsorption of these small molecules is very weak. The electrosteric repulsion, which provides more effective dispersion stability than electrostatic repulsion force, can be expected by using polyelectrolyte such as PAA; however, adsorbed layers of PAA onto solid/liquid interface are loosely formed. Therefore, a large amount of PAA was required to inhibit the surface hydration of γ-alumina suspension, thereby the excess addition of PAA decreased the electrosteric repulsion and re-bridging of the dispersant between particles caused an increase in suspension viscosity. Therefore, synergistic effect can be expected in mixed dispersant system of citric acid and PAA, since small citric acid molecules are adsorbed faster than PAA, inhibiting the progress of surface hydration, and then adsorbed PAA layers exhibit the effective electrosteric repulsion interaction between particles with a small amount compared with PAA alone. It was revealed that the γ-alumina slurry dispersed by mixed dispersant exhibited the improved removal rate of Cu layer by CMP polishing test.

Proposed adsorption model of mixed monolayer coverage with poly(acrylic acid) and citric acid molecules onto the γ-alumina surface in aqueous slurry for Cu CMP.Figure optionsDownload as PowerPoint slide