This paper describes electroless Cu filling of trenches with different widths ranging from 130 to 300 nm, using a concentration-dependent effect of 3-N,N-dimethylaminodithiocarbamoyl-1-propanesulfonic acid (DPS). With a fixed DPS concentration, it is shown that these trenches with different dimensions cannot be superfilled simultaneously. This is presumably caused by different surface concentrations of the adsorbed additive, which depends on the feature size and surface area. A two-step filling method is employed to superfill those trenches, which is also effective in control of the deposited Cu amounts to obtain uniform growth front regardless of the trench dimensions. (C) 2007 The Electrochemical Society.