Method for fabricating semiconductor integrated circuit device

A method for fabricating a semiconductor integrated circuit device, comprising the steps of:

• (a) forming element isolation grooves in a silicon surface on a first main surface of a wafer;
• (b) forming an insulating film from outside on the element isolation grooves;
• (c) flattening said silicon surface to expose a portion on which a thermal oxidation film is to be formed on said silicon surface;
• (d) after the (c) step, preparing moisture from oxygen and hydrogen by use of a catalyst;
• (e) keeping the thus prepared moisture in a gaseous state and feeding it to an oxidation unit;
• and
• (f) forming a silicon oxide film, which arrives at said element isolation grooves in the silicon surface on the first main surface of said wafer and serves as a gate insulating film of a field effect transistor, in an oxidative atmosphere, which contains oxygen gas as a main component and in which a ratio of a partial pressure of the fed moisture to the total atmospheric pressure is in the range of 0.1% to 40%, in the moisture-fed oxidation unit by thermal oxidation treatment wherein the first main surface of the wafer is heated to 700° C. or over.