Effects of K4Fe(CN)6 on electroless copper plating using hypophosphite as reducing agent

Abstract

K4Fe(CN)6 was used to improve the microstructure and properties of copper deposits obtained from hypophosphite baths. In electroless copper plating solutions using hypophosphite as the reducing agent, nickel ions (0.0038 M with Ni2+/Cu2+ mole ratio 0.12) was used to catalyze hypophosphite oxidation. However, the color of the copper deposits was dark or brown and its resistivity was much higher than that obtained in formaldehyde baths. The effects of K4Fe(CN)6 on the deposit composition, resistivity, structure, morphology and the electrochemical reactions of hypophosphite (oxidation) and cupric ion (reduction) have been investigated. The deposition rate and the resistivity of the copper deposits decreased significantly with the addition of K4Fe(CN)6 to the plating solution and the color of the deposits changed from dark-brown to copper-bright with improved uniformity. The nickel and phosphorus content in the deposits also decreased slightly with the use of K4Fe(CN)6. Smaller crystallite size and higher (111) plane orientation were obtained by addition of K4Fe(CN)6. The electrochemical current–voltage results show that K4Fe(CN)6 inhibited the catalytic oxidation of hypophosphite at active nickel sites and reduced the reduction reaction of cupric ions on the deposit surface by adsorption on the electrode. This results in lower deposition rate and a decrease in the mole ratio of NaH2PO2/CuSO4 consumed during plating.