Copper bottom-up deposition in 200 nm trenches by an acid-copper sulfate with only two additives [poly(ethylene glycol) (PEG) and Cl-] is achieved. The inhibiting effect of electrodeposition by PEG is strongly related to Cl- concentration. Secondary-ion mass spectroscopy measurements show that Cl- is consumed in the electroplating process. The explanation of bottom-up deposition realized in copper superfilling, in which the decrease of Cl- concentration causes rapid electrodeposition on trench bottoms, is verified experimentally. (C) 2002 The Electrochemical Society.