Structure and thermal stability of graded Ta-TaN diffusion barriers between Cu and SiO2

Sputter deposited Ta and TaN single layers of 10 nm thickness as well as graded TaN/Ta and Ta/TaN/Ta layer stacks that act as diffusion barriers for Cu metallization were investigated after annealing at temperatures between Tan=300 and 700 deg. C. By means of glancing angle X-ray diffraction, glow discharge optical emission spectroscopy and transmission electron microscopy, results of microstructure and phase characterization were correlated with diffusion phenomena. For the pure Ta barrier, Ta diffusion through the Cu cap layer to the sample surface is observed at Tan=500 deg. C, and the transformation of initially grown metastable β-Ta into the equilibrium α-Ta phase occurs at Tan=600 deg. C. In contrast, a fcc TaN layer remains stable at least up to Tan=700 deg. C. In the case of the graded layer stacks, first signs of N diffusion out of the TaN film into the adjacent Ta layers are observed after annealing at Tan=300 deg. C, and formation of hexagonal Ta2N starts at Tan=500 deg. C. Whereas in the course of thermal treatments for the threefold graded Ta/TaN/Ta barrier all TaN reacts with Ta to form Ta2N, some fcc TaN remains in the twofold graded TaN/Ta barrier