Investigation of PEG adsorption on copper in Cu ²⁺-free solution by SERS and AFM

The surface adsorption behavior of polyethyleneglycols (PEG), a wide-used additive in damascene copper plating, was investigated in acidic Cu ²⁺-free electrolyte in the presence of PEG and Cl ^- by in situ surface enhanced Raman spectroscopy (SERS) and atomic force microscopy (AFM) under potentiostatic conditions. Both SERS and AFM studies indicate that the adsorption amount of PEG gradually increases within ∼30 min immersion time at all the examined potentials prior to reaching steady status. It is demonstrated that, at relatively negative potentials in the examined range (-0.15 V to -0.35 V(SCE)), more PEG are adsorbed on the surface and form less-compact clusters in piles. In situ AFM images show that, in the damascene plating electrolyte containing PEG, Cl ^- and large amount of H ₂SO ₄, the adsorbed PEG appear to be flat-cone shaped clusters with a bottom diameter of several tens of nanometers, and a height of several nanometers. The diameters of the adsorbed clusters increase with time and the cathodic polarization, while the heights of the adsorbed clusters do not increase obviously. The present results suggest that PEG mainly contribute to the slow deposition rate at the external surface of nanometer-sized vias and trenches, for the dimensions of adsorbed PEG clusters are too big to move into the tiny structures.