The effect of bath composition on the initial stages of copper electrodeposition onto Ru substrates is reported. The impact of additives and bulk cupric-ion concentration is reported for both potentiostatic and galvanostatic operation. The presence of chloride ions and polyethylene glycol leads to an increase in the density of Cu particles, while bis (3-sulfopropyl) disulfide leads to a decrease in particle density for potentiostatic comparisons and a slight increase or perhaps remains constant when results at the same current density are compared. (c) 2005 The Electrochemical Society.