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, comprising the steps of: (i) providing an aqueous solution comprising 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, said complexing agent having the chemical formula: COOR1-COHR2R3 wherein 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, said solution comprising no reducing agent, (ii) providing electrons from a source not being in direct contact with said solution, through transport means assuming 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 said method.