Characteristics are reported for a resist stripping process downstream of an oxygen plasma to which water vapor is added. The effects of additive water vapor are an increase in atomic oxygen concentration in the plasma, a decrease in activation energy of ashing reaction, and protection of semiconductor devices from the sodium contamination from the resist. The atomic oxygen concentration was approximately doubled by mixing 10% H2O into the oxygen plasma. The activation energy of the ashing reaction to the resist made from novolak resin decreased from about 0.5 to 0.39 eV by the addition of water vapor of more than 1%. The activation energy of hydrogen abstraction from hydrocarbon molecules by an OH radical was lower than that by a ground state oxygen atom [O(3P)], which was the dominant ashing species in the oxygen plasma downstream, and that by an atomic hydrogen was higher than that by the ground state oxygen atom. Moreover, the activation energy in the downstream ashing of the oxygen plasma added to which was 1% water vapor was lower than that of the oxygen plasma to which 3% hydrogen was added, even though the relative concentration of atomic hydrogen in each plasma was equal. Therefore the decrease in the activation energy was probably due to the OH radical generated in the plasma and the downstream. Sodium atoms in the resist were blocked from entering into the semiconductor devices in the stripping process by use of the O2+H2O plasma downstream. Thus sodium was not removed and remained on the wafer surface after resist stripping. Also, by adding N2or CF4to the O2+H2O plasma, we can increase the ashing rate without losing the above characteristics.