This study demonstrates that the surface tension of plating solutions should be optimized to achieve a compromise between the gap-filling capability of copper electroplating and the formation of copper-void defects. The plating solution with lower surface tension has better gap filling but generates more air bubbles during copper electroplating. For a low-surface-tension electrolyte, the improvement in the gap-filling capability is caused by the enhancement in the ability of fluids to wet high-aspect-ratio features, whereas the increase in the formation of copper-void defects results from more air bubbles generated during the electroplating process. This study provides a model to describe the role of surface tension in copper electroplating. (C) 2007 American Vacuum Society.