The contact resistance reduction of Cu interconnects by optimizing the crystal behavior of Ta/TaN diffusion barrier

Ta/TaN bi-layer films, commonly used as diffusion barriers in Cu interconnects, have been deposited by self-ionized plasma system with various plasma conditions in this study. The variations of TaN compositions are revealed by secondary ion mass spectrometer. The structural properties of Ta/TaN films have been systematically investigated by X-ray diffraction patterns and transmission electron microscopy. On amorphous TaN film with low N concentration, Ta film consists of α-phase Ta (α-Ta) and β-phase Ta (β-Ta). The sheet resistance of Ta film with the mixture of α-Ta and β-Ta is decreased by the expansion of α-Ta content. The formation of pure α-Ta film can be achieved by increasing the N concentration of underlying amorphous TaN film. For α-Ta film, the enlargement of grain size can reduce the sheet resistance. In the practical application of Ta/TaN diffusion barrier, Kelvin contact resistance of Cu dual damascene interconnects is successfully reduced by the combination of α-Ta film with enlarged grains and amorphous TaN film with high N concentration.