The effect of 2,2'-dipyridyl and 3-N,N-dimethylaminodithiocarbamoyl-1-propanesulfonic acid (DPS) during Cu electroless deposition was observed in real time by measuring open-circuit potential and mass change of an electrode with a electrochemical quartz crystal microbalance. During Cu electroless deposition, the addition of 2,2'-dipyridyl reduced the deposition rate. DPS switched its role sensitively according to its concentration: deposition was accelerated at low concentrations and strongly suppressed at high concentrations. However, synergetic suppression effect was observed even at low DPS concentrations when 2,2'-dipyridyl was added together. In this system, the rate of suppression depended on DPS concentration: the higher DPS concentration decreased the deposition rate faster, while the minimum deposition rate was independent of the DPS concentration. Using the synergetic suppression effect, electroless Cu filling of 55-nm trenches was also successfully achieved. (C) 2014 The Electrochemical Society. All rights reserved.