Method for preparing an electroplating bath and related copper plating process

The present invention is related to a method for the preparation of a composition for electroplating a copper-containing layer on a substrate. This method makes use of an aqueous solution that has at least: a source of copper Cu (II) ions, an additive to adjust the pH to a predetermined value, and a complexing agent for complexing Cu (II) ions. The complexing agent has the chemical formula: COOR1-COHR2R3in which R1 is an organic group covalently bound to the carboxylate group (COO), R2 is either hydrogen or an organic group, and R3 is either hydrogen or an organic group. The solution has no reducing agent. The method involves providing electrons from a source not in direct contact with the solution, through transport means that provides the contact between said source and said solution. The present invention is also related to a process for forming a copper-containing layer on a substrate in an electroplating bath prepared according to the foregoing method.