The effect of O2+H2O plasma downstream ashing on sodium contamination from resists has been studied through measurement of flatband voltage shifts of metal–oxide–semiconductor (MOS) diodes after bias‐stress treatment and measurement by atomic absorption spectroscopy of the amount of sodium in the SiO2layer after resist stripping. The flatband voltage shift of the MOS diodes on which a resist layer was stripped by the O2+H2O plasma downstream was almost the same as that treated by the O2+H2O plasma downstream without resist and smaller than that where the resist was removed by other dry ashing methods such as O2plasma and O2plasma downstream. In addition, the amount of sodium that existed in the SiO2layer after resist ashing by the O2+H2O plasma downstream was also nearly the same as that in the SiO2layer as grown. This sodium passivation by the O2+H2O plasma downstream was most effective at the H2O percentage of 40%–60%, and did not depend on wafer temperature below 200 °C or over‐ashing time. These results support our original idea of sodium passivation, i.e., OH radicals generated in the O2+H2O plasma and its downstream react with sodium to prevent sodium invasion into the SiO2layer.