Behavior of oxygen in sputtering deposited ZnO films through thermal annealing and its effect on sheet resistance of the films were investigated. The crystallinities of the ZnO film were improved by post-deposition annealing in vacuum. However, the sheet resistance of ZnO film was dramatically decreased after post-deposition annealing in vacuum at more than 300 degrees C, while O-2 desorbed from the film. The oxygen vacancies which acted as donors were formed by the thermal annealing in vacuum. The sheet resistance of the films was recovered by annealing in oxygen ambient. In this paper, O-18(2) gas as an oxygen isotope was used as the annealing ambient in order to distinguish from O-16, which was constituent atom of the ZnO films. SIMS analysis revealed that O-18 diffused into the ZnO film from the top surface by O-18(2) annealing. Therefore oxygen vacancies formed by the post-deposition annealing in vacuum could be compensated by the annealing in oxygen ambient. (C) 2010 Elsevier B.V. All rights reserved.