The effect of 3–mercapto–1–propanesulfonate sodium salt (MPS), polyethylene glycol (PEG), thiourea(TU) and ethylenethiourea (ETU) on copper electrodeposition from acidic sulfate electrolyte with lowCu2+ concentration was investigated by linear sweep voltammetry (LSV), and chronoamperometric andelectrochemical impedance spectroscopy (EIS) measurements. The LSV experimental results showedthat all additives have inhibitory effects on copper electrodeposition. The results of thechronoamperometric experiments suggested that copper electrodeposition was related to threedimensional instantaneous nucleation in the diffusion-controlled regime regardless of the presence ofthe additives. In agreement with the LSV results, EIS experiments revealed the influence of theadditives on the electric double layer during the electrodeposition process. MPS and PEG were foundto inhibit the electrodeposition of copper through direct adsorption on the cathode surface, while TUand ETU produce stronger inhibition through complex intermediate reactions. Scanning electronmicroscopy images showed that the addition of TU and ETU resulted in flatter and finer-grainedcopper electrodeposits. TU and its derivatives are considered to be suitable additives for direct copperelectrodeposition at low copper ion concentration..
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