Recrystallization kinetics of electroplated Cu in damascene trenches at room temperature

Cu metallization for sub-0.25 μm interconnects marks not only a change in metallurgy from Al and a change in architecture from subtractive to damascene but also a major shift in deposition technology from sputtering to electroplating. A remarkable feature of electroplated Cu films is the recrystallization or grain growth process that takes place at room temperature over a period of hours to weeks after plating. While this phenomenon has been described for blanket films, the influence of substrate topography on the kinetics of recrystallization has not previously been reported. Using focused ion beam imaging we demonstrate that recrystallization of the small grained as-plated Cu is initiated at the upper corners of damascene trenches and grains continue to grow laterally, eventually transforming the entire film. Removal of overlying Cu by chemical mechanical polishing before the transformation leads to incomplete recrystallization of the Cu left in the trenches. The kinetics of the recrystallization process for trench widths of 0.3, 0.5, 0.8, 2, and 5 μm reveal a minimum time for recrystallization for the 0.8 μm trenches. An acceleration of the room temperature recrystallization rates for all trench widths is observed if the films are first cycled to −78 °C immediately after plating, and a more pronounced minimum time for recrystallization is observed for the 0.8 μm trenches. These observations lead us to propose that the initiation of this process at the upper corners of the trenches and the trench width dependence of the recrystallization rate are related to higher stress or dislocation densities.