Cu is promising material for the future interconnection in Si-based integrated circuits due to its lower resistivity and better electromigration resistance as compared to aluminium. However Cu is very mobile in Si at elevated temperature and creates the deep trap level that degrades device reliability. To solve this problem many materials such as refractory metals, intermetallic alloys, or compounds, nitrides, and ternary metal-Si-N amorphous alloys. Among these, tantalum nitride is one of the important candidates because of its chemical inertness with Cu.
In this study reactively sputtered Ta-N thin films have been investigated as a diffusion barrier between Cu and Si. By changing the ratio of $N_2$ to Ar +$N_2$ gas flow rate at a total pressure of 5 mtorr with RF power 300W, 300Å Ta-N films with different composition were deposited. Cu was deposited successively without breaking vacuum after deposition of Ta-N. The thickness of the Cu film was 1000Å. There was no additional substrate heating during deposition. Prepared films were annealed at temperatures ranging from 500℃ to 850℃ for 1h in 10% $H_2$/Ar forming gas ambient. Barrier properties, before and after annealing were examined by RBS, sheet resistance measurement, XRD, SEM, C-V measurement.
All of Ta-N films were deposited as amorphous phase. $Ta_{0.73}N_{0.27}, Ta_{0.68}N_{0.32}$ were crystallized as $Ta_2N$ after 550℃ annealing. In the Si/ $Ta_{0.73}N_{0.27}$/Cu structure, sheet resistance measurement, XRD, SEM results revealed copper silicide formation after 600℃ annealing. There was no new phase formation after 700℃ annealing in the Si/$Ta_{0.68}N_{0.32}$/Cu structure. $Ta_{0.57}N_{0.43}, Ta_{0.48}N_{0.52}$ amorphous phases were stable at elevated temperature. $Ta_{0.57}N_{0.43}$ thin film was crystallized as TaN after 700℃ annealing forming copper silicide. $Ta_{0.48}N_{0.52}$ was the most stable barrier. There was no copper silicide formation even after 800℃ annealing. Electrical properties of Ta-N barrier were investigated by C-V measurement. As a results, barrier failure was detected at temperature 50~100℃ lower than at failure temperature detected by other measurement methods.
