An electrochemical investigation of electroless deposition : the copper-DMAB system

Abstract

An electrochemical study of the copper electroless deposition process, using dimethylamine borane as a reducing agent, has been performed, in order to gain further understanding of the mechanism and kinetics of electroless deposition. An in-depth study of the electro-oxidation of dimethylamine borane (DMAB) was additionally carried out, due to its increasing relevance, not only in electroless deposition, but also in fuel cell technology. DMAB oxidation was studied using different experimental techniques such as voltammetry, chronoamperometry, single-crystal electrochemistry and in-situ Fourier Transform infrared spectroscopy, which demonstrated that adsorption of the DMAB molecule, and its intermediates, plays an important role in the oxidation mechanism and kinetics. The initial dissociation process is catalysed by the presence of metallic surfaces and the applied potential. On gold surfaces, DMAB undergoes a three-electron transfer at low overpotentials, with a further oxidation process of up to six electrons occurring at high overpotentials. Chemical interactions with gold oxide produce further oxidation of the DMAB molecule. In the potential region of gold oxide formation, in highly alkaline media, the dimethylamine is also oxidised. The voltammetric behaviour of bipolar cells was studied using model reversible and quasi-reversible redox couples, in conjunction with numerical simulations of the system. DMAB oxidation and copper electrodeposition were studied separately and together using the bipolar cell, providing useful information of the 'coupling' effects between the cathodic and anodic processes of electroless deposition. The ability to quantify side reactions associated with electroless plating, namely hydrogen evolution in the copper-DMAB system, was also demonstrated. The kinetics of the copper-DMAB electroless system was studied in detail, using the electroless bath and a galvanic cell configuration. The fact that the rate of deposition decreased upon the physical separation of the two half-reactions, as well as the observed catalysis of the oxidation of DMAB by copper surfaces, lead to the conclusion that the mixed potential theory (MPT) does not apply to this system. Faradaic efficiencies never reached 100% due to the parasitic side reactions mentioned above; the latter were especially prominent in the early stages of deposition. Crystalline copper films were obtained, with a higher fraction of Cu (111) than expected for polycrystalline copper, while the roughness of the deposits was found to increase with deposition time.EThOS - Electronic Theses Online ServiceGBUnited Kingdo