Effect of sodium 3,3'-dithiodipropane sulfonate in a dual-ligand electroless copper system

The process of electroless copper plating in an EDTA/THPED dual-ligand system using sodium 3,3'-dithiodipropane sulfonate (SPS) as an additive was studied by the electrochemical method. The mixed potential of the system was measured as a function of time, and results indicate that addition of SPS gradually shifts the mixed potential toward the negative direction without saltation. The dual-ligand electroless copper system was then tested by linear sweep voltammetry, and SPS is found to accelerate both cathodic and anodic polarization. The additive mainly influences the anodic polarization of the formaldehyde oxidation process and the rate of copper deposition is improved to a certain extent. The surface morphology and texture of the resulting plated copper were also analyzed by scanning electron microscope, energy dispersive spectrometer and X-ray diffraction, and a high-purity product without Cu₂O inclusions is confirmed. Moreover, the X-ray diffraction results of the electroless copper layers show that addition of SPS favors the formation of the preferred orientation on the (200) lattice plane.