Unveiling electron scattering mechanism at Al-doped grain boundaries in Cu interconnects

• Abnormal resistivity decrease was found in fine-grained Cu after Al-doping. • Al dopants were observed to segregate at Cu grain boundaries. • Atomic density of Cu at grain boundaries is increased by Al-doping. • Extra Cu atoms fill voids at low-symmetry grain boundaries and reduce scattering. Doping by capping layers followed by thermal diffusion is an effective way to improve electromigration reliability in Cu interconnects and extend the lifetime of electronic devices. However, the dopant atoms will cause extra electron scattering, exacerbating the interconnect resistivity issues. Herein, we focused on clarifying the influence of Al dopants on the resistivity of Cu interconnects. We prepared Al-doped fine-grained Cu and nanotwinned Cu with a high and low proportion of low-symmetry grain boundaries, respectively. Interestingly, we observed an abnormal resistivity decrease after Al-doping in fine-grained Cu, where the dopants segregate at Cu grain boundaries. Furthermore, we revealed that doping at grain boundaries will lead to opposite effects on resistivity, depending on the grain-boundary structures. After doping, the Cu atomic density is increased at grain-boundary planes by the lattice distortion from dopants. For low-symmetry void-containing grain boundaries, the extra Cu atoms fill the voids, reducing grain-boundary scattering and the overall resistivity; while for void-free grain boundaries, the extra atoms perturb the regular grain-boundary structure and intensify scattering. The findings not only provide a possible approach to gain both reliability and resistivity benefits in Cu interconnects, but also give a new insight of the electron scattering mechanism at doped grain boundaries.