Characterization of Cu Reflows on Ru

As compared to conventional Cu reflows on Ta, lower temperature reflow anneals of Cu on Ru are achieved. Cu films are reflowed into 40-nm-wide features at an optimum temperature of 250°C with a measured activation energy of 0.8 eV. Although higher anneal temperatures can shorten the process time, they also tend to increase hollow metal defects (missing Cu within patterned features). Compared to the control electroplated samples, the reflow-annealed samples do not show bigger Cu grains in the final structure. The observed resistance reduction from the reflowed samples is attributed to higher purity within the Cu interconnects.