Void-free damascene copper deposition process and means of monitoring thereof

A method of operating a plating bath comprising:

• providing a plating bath containing at least an accelerator;
• plating at least one metal on a substrate;
• measuring the bath concentration of at least one accelerator breakdown product (“void-formation marker, VFM”);
• measuring the bath concentration of said at least an accelerator;
• determining a VFM ratio at each of a plurality of time-points,
• wherein said VFM ratio is the concentration of said VFM divided by the concentration of said accelerator;
• counting, for each of said time-points, the number of voids in the metal plated on said substrate;
• determining a VFM threshold ratio as the highest VFM ratio at which no voids are observed;
• and
• maintaining said VFM ratio below said VFM threshold ratio by performing a bleed and feed of said plating bath to maintain said VFM ratio below said threshold VFM ratio.