The proceeding scaling inmicroelectronic devices requires smaller and smaller copper wires for energy transfer in integrated devices. Voids in the copper wires lead to a resistance increase and damage of the wiring. Copper wires are fabricated by electrochemical deposition as it enables a bottom-up, void free copper growth, so-called superfilling. The present work focuses on the mechanism of the electrochemical deposition with the goal of understanding and describing the superfilling. Electrochemical measurements and partial fill experiments under production-like conditions are carried out to study the effects of bath additives. The co-adsorption theory is adopted to explain additive interaction which is presumed to be the key for superfilling. It is shown that superfilling is a result of the synergetic adsorption behavior of at least two organic additives and a kinetic balancing between additive accumulation and copper deposition rate. (C) The Author(s) 2016. Published by ECS. All rights reserved.