Decomposition of polyethylene glycol (PEG) at Cu cathode and insoluble anode during Cu electrodeposition

The mechanisms of cathodic and anodic decomposition of polyethylene glycol (PEG), a suppressor in the Cu electrodeposition process, were examined via both cyclic voltammetry stripping (CVS) and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) analysis under open-circuit, unpowered closed-circuit, and electrolysis conditions. Under the open-circuit conditions, PEG decomposition occurred only at the cathode (Cu electrode), owing to the effect of the Cu+ ions. Under the unpowered closed-circuit conditions, PEG degraded at both electrodes (Cu and Ir/IrOx on Ti/TiOx) through the galvanic proportionation reaction, owing to which Cu+ was also formed at the anode. Under the electrolysis conditions, PEG also was degraded at both electrodes, but a difference in the average PEG molecular weights (MWPEG) between the two electrodes was observed after 48 h. The PEG breakdown at the cathode appeared to be related to the Cu+ ions, while that at the anode was caused by the •OH radicals formed by water splitting. Remarkably, the PEG decomposition rate under electrolytic conditions was reduced compared with that obtained under the closed-circuit conditions, indicating a lesser extent of radical formation. Therefore, it is concluded that PEG degradation did not proceed through a direct electrochemical reaction, but rather through a radical-induced chemical reaction.