Technical And Mechanistic Investigation For Electroless Copper Plating Using Glyoxylic Acid As Reducing Agent

Electroless copper plating have been extensively applied in various fields. Until now, formaldehyde has almost always been the reducing agent in electroless copper plating, which is harm to environment and human body so that it is essential to find a substitute for formaldehyde. Moreover, the development of electroless copper deposition remains to face many challenges such as lower deposition rate, less stable solution, higher resistance of the copper deposits due to mixing the impurities of Cu₂O and others. In this work the technical and mechanistic investigation was carried out for electroless copper plating using glyoxic acid as reducing agent, the focus was put on the following:(1) The effects of technical parameters on the deposition rate and the structure of the deposites, especially the influences of pH value and temperature of bath on the deposition rate and solution stability, morphology and structure of the deposites, as well as the influnces of using 2,2ËŠ-dipyridine and K₄Fe(CN)₆ as additives. The electrical resistivity of the deposites was measured so as to study its affecting factors .(2) Electrochemical studies of electroless copper plating. The electrochemical processes of CHOCOOH oxidation and Cu²⁺ reduction, as well as the functions of additives on the electrochemical oxidation and reduction were studied. The results will be helpful to explore the mechanism of electroless copper plating.The composition of the bath used is as follows: CuSO₄·5H₂O 28g/L (0.112mol/L),complexing agent Na₂EDTA 44g/L (0.118 mol/L),glyoxic acid 9.2g/L (0.1mol/L),2,2ËŠ-dipyridine 10mg/L and K₄Fe(CN)₆ 10mg/L。The kinetics of the deposition processes was determined by traditional technical methods and electrochemical methods, the structures , components and morphologies of the deposits obtained in different conditions were examined by X-ray diffraction, XPS and SEM,respectively. The main results are outlined as follows:(1) In electroless copper plating using glyoxic acid as reducing agent, the components and pH value of the bath have a remarkable effect on the deposition rate. The better copper deposits can be got. only when the pH value is controlled at 11～13 , increasing pH value will make the solution unstable. The reaction orders of [Cu²⁺] was estimated about 0.63 when the [Cu²⁺] was varied from 0.03～0.2 mol/L. Excess of glyoxic acid leads to decreasing deposition rate, its reaction orders was estimated about–0.07 when its concentration was higher than 0.1 mol/L. In the solution containing excessive EDTA, the deposition rates decrease with rising EDTA concentration, and .its reaction orders was estimated about–0.1 if EDTA concentration was higher than 0.118 mol/L.Increasing the temperature of the bath will enhance the deposition rate, the apparent heat of activation of deposition reaction was estimated as 37kJ/mol.Although a little amount of 2,2ËŠ-dipyridine 10mg/L and K4Fe(CN)6 were used as additives, both their kinetic and morphological effects are evident. Appropriate amount of 2,2ËŠ-dipyridine 10mg/L and K4Fe(CN)6 will not only increase the deposition rate, but also enhance the stability of the bath.(2) the deposits obtained in this work was found to be polycrystalline f.c.c copper , in which no Cu2O or CuO was detected. Its electrical resistivity is about 2.4μΩ·cm^-1, depending on the deposition conditions used.(3) from the electrochemical experiments it was found that the oxidation of CHOCOOH on copper electrode occurred in the potential range of -0.8～-0.4V, the voltammetric curves of the oxidation were affected with the changes in pH values of the solution. The oxidation of CHOCOOH was controlled by diffusion. when E > -0.55V, and other reaction appeared simultanously when the potentials shifted toward more positive. The reduction of CHOCOOH on the copper electrode takes place if E < -0.8V. The presence of EDTA2- in the solution inhabits the oxidation of CHOCOOH on copper electrode, attributed to the co-adsorption of the complexing agent with CHOCOOH .Increasing the bath tempreture promotes the oxidation of CHOCOOH, and from the relationship of the oxidation current and temperature, the apparent heat of activation for the oxidation of CHOCOOH was estimated as 25kJ/mol(4) In a system containing Cu2+ and EDTA2-, the reduction current is caused by free Cu²⁺ ions when E >-1.0V, while the reduction current is caused by complex ions of Cu²⁺ when E <-1.0V. The presence of complexing agent in the bath prevents Cu2+ from precipitation in alkaline solution, however, excess of the ligand will decrease the reduction rate released of cupric complex. Increasing the bath temperature will enhance the reduction and from the relationship of the reduction current and temperature, the apparent heat of activation of the reduction was estimated as 17 kJ/mol.(5) When 2,2ËŠ-dipyridine or K₄Fe(CN)₆ was added in the solution alone, they would inhabit the oxidation of CHOCOOH, but affect less on the reduction of cupric ions. When these two additives co-exist in the solution, the oxidation of CHOCOOH are more strongly inhabited, thereby the stability of the bath further raised, on the other hand, the reduction of cupric ions was accelerated. The mechanism how the two additives exert the effect in cooperation with each other needs to be further studied.