Solution chemical constraints in the chemical-mechanical polishing of copper: Aqueous stability diagrams for the Cu-H_?O and Cu-NH₃-H₂O systems

A general graphical approach to the solution chemistry of chemical mechanical polishing is presented with the aid of a variety of aqueous stability diagrams, such as Eh-pH (Pourbaix), log{Metal}-pH, and log{Ligand}-pH diagrams. The common thermodynamic origin of these diagrams is highlighted. The important role played by concentration gradients in effecting chemical mechanical polishing is stressed and illustrated with the aid of stability diagrams generated for the Cu-H₂O and Cu-NH₃-H₂O model systems. It is demonstrated that chemical mechanical polishing is feasible when the following two conditions are satisfied simultaneously: (a) at the metal surface dissolved metal concentration is high and/or ligand concentration is low (this favors oxide film formation), and (b) in the bulk aqueous phase the metal concentration is low and/or the ligand concentration is high (this favors the dissolution of film fragments).