Suppression of copper electrodeposition by PEG in methanesulfonic acid electrolytes

We investigate the suppression behavior of poly(ethylene glycol) (PEG) in methanesulfonic acid (MSA) Cu plating baths using electrochemical methods, normal Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and electrochemical quartz crystal microbalance (QCM) measurements. Suppression of Cu electrodeposition by PEG in H₂SO₄ electrolytes only occurs in the presence of Cl⁻, whereas Cl− is not required in MSA electrolytes. SERS measurements of MSA electrolytes without PEG at a Cu surface show MSA molecules undergo a re-orientation at ca. −0.15 V vs. Ag/AgCl, as evidenced by potential-dependent symmetry changes. The re-orientation of MSA in MSA + PEG electrolyte does not occur until −0.3 V vs. Ag/AgCl. At potentials negative of −0.3 V, MSA re-orients and PEG leaves the surface, in coordination with onset of Cu reduction current, suggesting the suppression interaction of PEG at a Cu surface is facilitated by MSA. QCM measurements demonstrate a similar departure of PEG mass at potentials negative of the MSA re-orientation.