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

• S-rich particles exist in Cu electroplated from thiourea-containing baths.
• An amorphous phase was developed in patches on the Cu deposit by electropolishing.
• Surface roughness of polished Cu deposits was not affected by the amorphous phase.
• The electropolished Cu deposits had a surface roughness, Ra, lower than 30 nm.

Cu electrodeposition was performed on a rotating cylindrical Ti electrode in Cu-sulphate plating baths with different thiourea contents up to 8 ppm. 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 40 vol.% 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 (Ra) lower than 30 nm.