The leakage current mechanism of the chemical vapor deposition Ta2O5 film has been investigated. In the case of an as-deposited amorphous film, the presence of impurities such as carbon and hydrogen remaining in the film leads to the Poole-Frenkel type leakage current. The oxidation of these impurities results in a reduction in leakage current. O-2 plasma is especially effective for oxidizing impurities, leading to a drastic reduction of the leakage current. However O-2 plasma cannot reduce the leakage current of the Ta2O5 film crystallized at 700 degrees C. This leakage current is not due to C and H, but rather to Si penetraed into the Ta2O5 film from the underlying poly-Si electrode. Therefore, the amorphous Ta2O5 film treated by O-2 plasma is most suitable for DRAM use.