Thin tantalum oxide films (17nm) deposited by ECR PECVD were annealed in $N_2$ and $O_2$ atmospheres.
In case of annealing in $O_2$ atmosphere $TaO_2$ nonstoichiometric phase appeared above 750℃, δ-$Ta_2O_5$ appeared at 850℃ and above 900℃ only δ phase remained. Above 850℃ interfacial $SiO_2$ formed at the $TaO_x$/Si interface, which caused the increase in apparent thickness by 5.9 nm for 900℃ annealed film when measured by ellipsometer.
Electrical property was the best at 750℃ at which the nonstiochiometric $TaO_2$ started to form. Leakage current density at 1MV/cm was reduced from $5.9\times10^{-8}A/cm^2$ (as-deposited) to $2.94\times10^{-9}A/cm^2$ (750℃ annealed), and breakdown field (applied electric field at $10^{-6}A/cm^2$) was increased from 2.22 MV/cm (as-deposited) to 5.87 MV/cm (750℃ annealed). The reduction of leakage current density and the increase of breakdown field were due to $TaO_2$ phase rather than $O_2$ addition to the film, which is contrary to other reports.
In case of annealing in $N_2$ atmosphere, no nonstoichiometric phase appeared and the films crystallized at 750℃ to form δ-$Ta_2O_5$. Annealing in $N_2$ atmosphere caused severe degradation of the leakage current characteristics.
In both annealings, thickness reductions were in the range of 1.3%∼7.8%.
