Revisiting the Polyethylene Glycol-Chloride Adsorption Structure on Cu Electrode in Sulfuric Acid Solution by Wide-Frequency ATR-SEIRAS

Polyethylene glycol (PEG) is a typical suppressor in the presence of chloride anions for Cu interconnect electroplating in the field of microelectronics manufacture, yet its adsorption structure and its correlation with the inhibition effect remain controversial and unclear. In this work, wide-frequency attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is utilized to clarify the interfacial adsorption structure of PEG with Cl– on a Cu electrode, providing direct molecular-level evidence for the inhibition effect. Time-resolved spectroscopic results show that the OC–CO subunit of PEG transforms from trans to gauche conformation with the addition of Cl– in a way favoring the hydrocarbon sections of PEG to lean toward the Cl–-covered Cu surface, as demonstrated by the evolution of characteristic peaks at 1134, 1090, and 848 cm–1 as well as that of interfacial water stretching vibration bands (ca. 3400–3650 cm–1). Moreover, a comparison of spectra for adsorbed PEG and dissolved PEG shows that more OC–CO and CC-OC gauche conformations are present in the former, in favor of PEG chain coiling to form mounds on the Cl–-covered Cu electrode. The steric hindrance of as-formed PEG mounds may account for the strong interfacial inhibition effect of the PEG-Cl– adstructure, as supported by the electrochemical quartz crystal microbalance (EQCM) measurement. A series of potential-dependent ATR-SEIRAS spectra reveal that with decreasing potential, the coiled PEG chains become loosened due to gradual desorption of the underlying Cl– adlayer.