Aspects of the electrochemistry of additives in an acid copper plating bath

The kinetics of the Cu/Cu2+ have been defined clearly from experiments on a copper microdisc electrode. Strong inhibition of the copper deposition process occurs in the presence of polyethylene glycol and chloride ion. The synergistic action of polyethylene glycol and chloride has been confirmed and the behaviour of the system at low concentrations of chloride ion has been studied using a rotating copper disc electrode and potential step techniques. The results have been interpreted in terms of the formation of an inhibiting polyethylene glycol surface film which is stable only in the presence of chloride. Surface enhanced Raman spectroscopy was used to study, in-situ, the electrode surface during copper electroplating. The results confirm that polyethylene glycol adsorbs on a copper electrode surface. The spectroscopic experiments also indicate that the species adsorbed at open circuit is different to that present during copper electrodeposition. Experiments using a rotating copper disc electrode have shown that a surface film forms on copper in the presence of 4,5-dithiaoctane-1,8-disulphonic acid. The film forms both at open circuit and at potentials where copper is usually electroplated. The electrochemical properties of the film have been studied in detail and the data compared with the behaviour of other disulphides at a copper electrode surface. The behaviour of 4,5-dithiaoctane-1,8-disulphonic acid at carbon, mercury and platinum electrodes has also been examined. The behaviour of 4,5-dithiaoctane-1,8 disulphonic acid, at open circuit, in the plating bath has been studied by monitoring the composition of the plating solution using both voltammetry at a rotating copper disc electrode and UV/visible spectroscopy. Decomposition of the additive has been shown to occur via reaction at the copper metal to form a copper(I)-thiol complex. This complex intermediate is destroyed by reaction with oxygen. The reaction with oxygen does not, however, regenerate the 4,5-dithiaoctane-1,8-disulphonic acid. The copper(I)-thiol complex was found to have similar chemical and electrochemical and spectroscopic properties to the copper complex generated by the reaction of 3-mercaptopropylsulphonic acid with copper(II) ion in solution.