A highly reliable ultrathin tantalum oxide capacitor is fabricated by using oxygen-plasma annealing after the film deposition. A tantalum oxide film is deposited on a nitrided polycrystalline silicon surface using a mixture of penta-ethoxy-tantalum [Ta(OC2H5)5] and oxygen gas. The films are annealed in an oxygen-plasma at less than 400-degrees-C. The oxygen-plasma annealing greatly reduces the leakage current through tantalum oxide capacitors and produces better time-dependent dielectric breakdown characteristics than either dry O2 annealing or two-step annealing (oxygen-plasma + dry O2 annealings). The reason for the excellent electrical properties is examined using secondary ion mass spectrometry, extended x-ray absorption fine structure spectroscopy, and transmission electron diffraction patterns. Analytical results show that the oxygen-plasma annealed tantalum oxide films are densified by the elimination of carbon and hydrogen and by the repair by oxygen of vacant sites in the as-deposited films, as the films remain amorphous during oxygen-plasma annealing. In addition, oxygen-plasma annealing suppresses thickening of the interface layer between the tantalum oxide film and polycrystalline silicon bottom electrode. Therefore, highly reliable ultrathin tantalum oxide capacitors with an SiO2 equivalent thickness of less than 3 nm can be fabricated by using oxygen-plasma annealing.