The tantalum oxide thin films with a thickness of 14 nm were deposited at 95 degrees C by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD), and annealed at various temperatures (700 similar to 850 degrees C) in O-2 and N-2 ambients. The microstructure and composition of the tantalum oxide thin films and the growth of interfacial silicon oxide layer were investigated and were related to the electrical characteristics of the film. Annealing in an O-2 ambient led to a high dielectric constant (epsilon(r)(Ta2O5) = 24) as well as a small leakage current (E(bd) = 2.3 MV/cm), which were due to the improved stoichiometry and the decreased impurity carbon content. Annealing in an N-2 ambient resulted in poor and nonuniform leakage current characteristics. The as-deposited tantalum oxide films were crystallized into delta-Ta2O5 after annealing at above 750 degrees C regardless of the ambient. The leakage current of the film abruptly increased after annealing at 850 degrees C probably because of the stress caused by thermal expansion or contraction.