Electrodeposition of Alloys from Deep Eutectic Solvents

Thin films of copper alloys are of interest for electronic applications. These are routinely produced by electrodeposition using aqueous solutions. The solvent strongly affects composition and morphology of the deposit which consequently affects the electrical and mechanical properties. The novelty of this thesis is that copper alloys are deposited from Deep Eutectic Solvents, DESs, which are forms of ionic liquids. The work shows that novel morphologies and compositions can be obtained using this approach. The electrodeposition of copper alloys is described from a solution of the metal salts in choline chloride based ionic liquids using urea and ethylene glycol as hydrogen bond donors. The thesis is split into three sections focussing on Cu alloys with Ag, Sn and P. These were chosen for their different phase behaviour. It is shown that the composition and morphology can be varied by altering the deposition potential and copper ion concentration of the plating bath. A variety of analytical techniques have been used to probe the deposition of these alloys. Although many of these are relatively standard for such studies this project uses an electrochemical quartz crystal microbalance (EQCM) for the first time to determine alloy composition in real time. From this data, the two systems Cu-Ag and Cu-Sn are shown to be close to 100% current efficiency, but the Cu-P does not fellow this trend. Analysis of the chronoamperometric transient behaviour during electrodeposition suggests that pure copper electrodeposition proceeds via three-dimensional instantaneous nucleation with diffusion-controlled growth. However, the deposition of most alloys does not fit either progressive or instantaneous nucleation models well. The alloy phases formed during deposition were analysed using X-ray diffraction and the surface morphologies and the compositions of the electrodeposited Cu alloys were analysed using SEM/EDAX. Thin films of Cu and Cu-P were electroformed onto Ti substrates and then peeled off. The mechanical properties of these films were tested and it was found that the incorporation of phosphorous was found to have a significant effect on the stress-strain curves. The final part of this study involved the dissolution of copper using cyclic voltammetry and EQCM and it was shown that diffusion of the chloride ligand to the electrode limits the rate of the anodic process.