Resistivity reduction of Cu interconnects

Although Cu was found to be attractive as interconnect materials for ultra-large scale intergrated (ULSI) Si devices, the grain boundary scattering primarily increases the resistivity of the Cu interconnects and resistivity increase due to the barrier layers becomes significantly large upon reducing the line width of the sub-100 nm Cu interconnects. Thus, large grained Cu interconnects and ultra thin barrier layers are essential to realize low resistance nano-scale Cu interconnects in the future ULSI devices. For development of a fabrication technique of large-grained Cu interconnects, grain growth mechanism of Cu thin films was understood. A new grain growth model for Cu thin films was proposed based on the strain energy criterion model at temperatures where dislocation glide was the dominant strain relaxation mechanism. Based on this model, the grains of the Cu films under tensile or compressive strain were explained to grow primarily to reduce the elastic strain energy by dislocation glide, because the dislocations are easily introduced into large grains upon introduction of strain into the films. The fabrication technique to self-form the thin barrier layers in Cu(Ti) alloy films by annealing at relatively low temperatures was demonstrated. Oxygen contained in Ar atmosphere facilitated Ti segregation at the surface, reduced the Ti concentration in the alloy films, and formed relatively large grains in the alloy films, which are essential for low-resistivity alloy films. For the self-formation of the Ti-rich barrier layer, it was found that the selection of a substrate that is reactive to Ti atoms (forming Ti compounds) was essential. A strong reaction of the Ti atoms in the alloy films with SiN compared with that with SiO₂ was observed. The excess Si atoms yielded by the strong reaction diffused into the alloy films causing the resistivity increase. Thus, low Ti concentration in the alloy films is essential for low-resistivity alloy films.