Nanohardness and etch rates of copper films sputter deposited onto a 30 nm tantalum coating on silicon wafers were measured after exposure to aqueous solutions containing various common chemical mechanical planarization slurry additives at different pH values. In most cases, the measured hardness values were consistent with the formation of surface films as indicated by the equilibrium potential-pH diagrams. In general, when the pH is low (< 4), hardness values are that of Cu metal or slightly higher. As the pH increases to similar to 8, the hardness decreases to less than Cu metal as hydroxides form, or to higher values than Cu metal as oxides form. Exposure to solutions with glycine or ethylenediaminetetraacetic acid caused hardness values to be less than Cu metal, as areas of the surface became porous. Exposure to H2O2 causes harder films in some areas and very porous soft films in other areas on the same surface, as passivation and dissolution of the surface occurs. (c) 2008 The Electrochemical Society.