Electrochemical analysis of the effect of accelerator in methanesulfonic acid bath for copper electrodeposition

In order to realize a void-free TSV fill for Cu electrodeposition in methanesulfonic acid plating bath with high efficiency, additives are required to convert the electroplating mode to “bottom-up filling”. In this paper, UPT3360A, whose main component was Bis-(3-sodiumsulfopropyl disulfide) (SPS), was used as accelerator in MSA plating bath, and the effect of the accelerator for via filling was investigated by various electrochemical techniques including electrochemical impedance spectroscopy and chronopotentiometry. The results indicated that the reduction of Cu²⁺ to Cu⁺ was accelerated by accelerator. UPT3360A has strong acceleration effect when the concentration was around 0.5 mg/L, and the effect tended to be saturated as the concentration increased to above 1 mg/L. However, higher concentration of accelerator, up to 4 mg/L, led to an inhibiting effect rather than an accelerating one, which might probably attribute to the fixed concentration of Cl⁻ and the reduction degree of SPS. Furthermore, the coverage of accelerator on electrode was studied. According to the fitting data of Toth model, the standard Gibbs free energy was calculated to be −36.825 kJ mol⁻¹, which is lower than that of inhibitor, suggesting that accelerator absorbed on electrode surface is more likely to be replaced by inhibitor during Cu electroplating.