Multi-electron transfer mechanism of dimethylamine borane in electroless gold deposition

A multi-step electron transfer mechanism is described for the oxidation of the reducing agent dimethylamine borane (DMAB) on gold in alkaline media. According to this mechanism, the active reducing species produced from DMAB in the presence of high hydroxide content is the BH₃OH^-. This species undergoes series of irreversible electrochemical oxidation producing the following intermediates BH2(OH)2-,BH(OH)3-andB(OH)4-, the latter of which is the final product. Cyclic voltammograms of DMAB in KOH at various molar ratios of DMAB:KOH indicate that the oxidation of the intermediates requires sufficient pH to proceed. The number of electrons transferred according to the proposed mechanism was studied using rotating disk electrode (RDE). The anodic polarization curves of DMAB in hydroxide electrolyte at RDE exhibited three half-wave potentials with E_1/2 of -0.778, -0.174 and 0.248 V. The proposed electrochemical mechanism is correlated with conventional redox iodimetric titration. From the standpoint of electroless gold deposition reaction, the significance of the above mechanism lies in the ability to compare the actual plating potential with the potential range in which each electroactive intermediate is oxidized. Since the gold deposition potential is in the range of the third wave, it indicates that all three intermediates are involved. This work may have far-reaching consequences in enhancing the practical applications of gold and nickel in electronics packaging applications.