Recrystallization effects in Cu electrodeposits used in fine line damascene structures

Resistance transformations in blanket films and damascene lines of electroplated Cu were investigated to gain insight regarding the recrystallization kinetics of electrochemically deposited Cu in constrained deep submicron features. The study examined grain transformations in blanket Cu films with thicknesses ranging from 0.8 to 3.3 μm and Cu damascene lines with widths from 0.25 μm to approximately 1.0 μm. Bulk film resistivity changes were found to be directly related to new grain formation and growth (recrystallization) observed in the films. By monitoring the electrical properties of the continuous and patterned films, it was observed that damascene lines of Cu required longer times to achieve the same degree of bulk resistivity change when compared with blanket films. For example, at room temperature, a 0.35 μm wide×0.8 μm high Cu line underwent only half the resistive transformation over an 865 h period (36 days) that was completed in 120 h (5 days) in a 0.8 μm blanket Cu film. Microstructural images of damascene structures, using a focused ion-beam microscope, clearly indicated that wider Cu lines transform much faster than narrower ones. To investigate the recrystallization kinetics, samples, prepared under identical conditions, were annealed at different temperatures and times. The recrystallization activation energies derived using an Arrhenius model (under isoresistivity conditions) were found to be approximately 0.8 eV for all Cu films and lines regardless of geometry. The study indicates that although the overall rate of recrystallization of Cu was very different and dependant on physical geometrical constraints, the grain growth mechanism was the same for all Cu films and lines.