Electropolishing behaviour and microstructures of copper deposits electroplated in an acidic copper-sulphuric bath with different thiourea contents

Cu electrodeposition was performed on a rotating cylindrical Ti electrode in Cu-sulphate plating baths with different thiourea contents up to 8ppm. The hardness, microstructure and electropolishing behaviour of the Cu deposits were studied. Some sulphur-rich particles in the Cu deposits prepared from the thiourea-containing baths were identified. The sulphur-rich particles dissolved preferentially during electropolishing in a 40vol.% H₃PO₄ solution, forming a thin amorphous phase containing P in patches on the outer surface of the Cu deposit. The deposits prepared in the baths with thiourea showed higher dissolution current during polishing and formed a brightened and levelled surface with a surface roughness (R_a) lower than 30nm.