Structures of ultra-thin atomic-layer-deposited TaNx films

Atomic layer deposition (ALD) is an attractive technique in fabrication of microelectronics presently and in the future, for its accurate thickness control in atomic scale, excellent conformality, and uniformity over large areas at low temperature. It has been adapted and used in deposition of ultrathin TaNx films as diffusion barriers for Cu metallization. In this study, composition, structure, and stability of ultra-thin (1.5–10 nm) atomic layer deposited films are characterized by a set of complementary analytical techniques. The results indicate that the N to Ta atomic concentration ratio in the ALD TaNx films is approximately 2, independent of the film thickness and annealing up to 750 °C. Hydrogen, oxygen, and carbon are detected as impurities within the as-deposited films. The as-deposited ALD TaNx films have an fcc NaCl-type nanocrystalline structure even when the film thickness is 1.5 nm. Following thermal anneal at 600 °C and higher, the films do not undergo a structural change except for an increase in grain size and a decrease in the lattice constant. X-ray photoelectron spectra results indicate that all the Ta atoms in the films are bonded ionically with the surrounding N atoms. An ex situ thermal treatment at 600 °C for 1 h removes the O, which penetrated the layers, by a reduction reaction with the residual H and results in densification of the ALD films. Our analysis of the experimental results indicates that the excess of N atoms of the ALD TaNx films is mainly due to Ta vacancies in the fcc NaCl-type structure. The structural and compositional characteristics of the films explain why the films serve as good diffusion barriers to Cu metallization.