Recrystallization kinetics of electroplated Cu in damascene trenches at room temperature

Abstract

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.

Increasing the mean grain size in copper films and features

“…Several mechanisms that could drive such a transformation have been proposed, including defect, grain-boundary, and surface energies, but also chemical gradients in the material that can cause diffusion-induced grain-boundary migration [84]. During the last decade, microstructural evolution or self-annealing has been observed in electroplated copper films that are stored at room temperature [44,[85][86][87][88][89]. Although this behaviour of electroplated copper is intriguing, the kinetics are hard to study, since the as-deposited grain structure and impurity content of the film strongly depend on the electroplating bath chemistry.…”

In situ investigation of the microstructure evolution in nanocrystalline copper electrodeposits at room temperature

“…1,2 Since the era of copper interconnects this socalled self-annealing effect came into focus. [3][4][5] Selfannealing has been recognized as an unwanted side effect in the successful manufacturing of copper thin films for applications in microelectronics and postdeposition annealing at elevated temperatures for stabilizing the microstructure has been introduced as an integrated part of the manufacturing process. Even so self-annealing can be prevented by intentional annealing at elevated temperatures, the phenomenon remains of scientific and technological interest, since understanding the kinetics and mechanisms of self-annealing is the essence of tailoring the microstructure.…”

Rapid trench initiated recrystallization and stagnation in narrow Cu interconnect lines

“…10 Several publications, however, have suggested that in narrow lines, high dislocation density, seed layer texture, and minimization of the Cu-Ta interfacial energy cause coarsening of grains in the trenches prior to the overburden. 8,11,12 In this work, TEM analysis of samples during and after the transformation is presented supporting the trench-initiated recrystallization hypothesis, and goes further to show that the microstructure inside the trenches is stable well before the transformation in the overburden is complete.…”