Sodium Hypophosphite As The Reductant Electroless Copper Plating Study

The traditional electroless copper deposition used formaldehyde as reduction agent. This technology had many drawbacks, such as unstability of the solution and the steam of formaldehyde was harmful for human and environment, it was urgent to search for substitutes of formaldehyde. Electroless copper plating using sodium hypophosphite as reducing agent has many characteristic, such as wide operation range, long-life of the electrolyte and no deleterious steam, and it might replace the formaldehyde copper deposition. However, the quality of the copper film deposited by this method is poor. In order to resolve this problem, we study the influence of some additives on two different kinds of electroless copper plating which using sodium hypophosphite as reducing agent and sodium citrate as chelating agent, sodium citrate and ethylene diamine tetraacetic acid disodium salt (EDTA·2Na). Electrochemical methods were used to test the anodic and cathodal polarization processes which can explain the mechanism of the influence of additives.In this paper, we firstly studied the ingredients of the bath on the effect of the electroless plating in which the system of using sodium citrate as chelating agent. The results indicated that the deposition rate increase with the increase of copper sulfate, nickel sulfate and sodium hypophosphite. The deposition rate decrease with the sodium citrate used as a chelating agent. The increase of nickel sulfate can improve the deposition rate, at the same time; the content of nickel would also be higher in copper film which can increase the resistivity. The basic composition of electroless copper plating solution was follow: copper sulfate (CuSO₄·5H₂O, 10 g/L), nickel sulfate(NiSO₄·6H₂O, 0.6 g/L), sodium citrate(C₆H₅Na₃O₇·2H₂O, 15 g/L), sodium hypophosphite (NaH₂PO2·H₂O, 30 g/L), boric acid(H₃BO₃, 30 g/L), The operating temperature is 70℃and the pH of bath is 9.2. At this condition, the deposition rate was 8.4μm/h, the nickel in the film was 4.86%, and the resistivity was 15.2μΩ·cm.The additives, such as maleic acid, thiourea and allyl thiourea on the deposited copper were investigated. Maleic acid had no obvious effect on deposition rate, the content of nickel and the resistivity, but surface morphology and deposit structure was improved; Thiourea was a typical inhibitor in the bath, when its concentration was kept at 0.6 ppm, the deposition rate was reduced to 4.5μm/h, the content of nickel reach to 1.33%, and the resistivity can decrease to 5.9μΩ·cm, as well as the grains of copper was smaller, the deposition at (111) become more favorable; the effect of allyl thiourea on copper deposition was similar to thiourea, when its concentration was kept at0.8ppm, the deposition rate was reduced to 4.0μm/h, the content of nickel reach to 1.49%, and the resistivity can decrease to 6.5μΩ·cm, at the same time, the deposited copper was smaller and uniform, deposit structure was improved.Another work of this paper was to inspect the effect of sodium citrate and EDTA used as double chelating agent. The results of the basic bath were that the deposition rate was 11.26μm/h, the content of nickel was 4.59% and the resistivity was 9.56μΩ·cm. The results were better than the one in which the sodium citrate was used as sole chelating agent. The effect of maleic acid, potassium ferrocyanide and thiourea on electroless copper plating was investigated. In this bath, maleic acid was an inhibitor, when its concentration was kept at 20 mg/L, the deposition rate could reduce to 6.17μm/h, the content of nickel decreased to 1.74% and the resistivity was 7.51μΩ·cm; Potassium ferrocyanide inhibited deposition of copper, when its content was 0.5 ppm, the deposition rate decrease to 3.38μm/h, the content of nickel was 2.03%, the resistivity can reduced to 5.41μΩ·cm, the copper deposits also had higher (111) plane orientation. Thiourea was also an inhibitor in this system, when its concentration was kept at 0.8 ppm, the deposition rate was 4.72μm/h, the content of nickel in the film was 1.20%, the resistivity can reduced to 5.46μΩ·cm, at the same time, the deposition at (111) plane orientation become more favorable.Through the experiments, the behavior of the plating bath was notably changed because of the additives. The deposited rate got lower, and the grain of copper became smaller and more homogeneous. At the same time, the resistivity of the deposited copper reduced compared to the basic bath. The promotion of the copper film has very important potential application in industry.