PURPOSE: To repair pinholes which are induced in a gate insulating film at the cleaning of its surface so as to obtain an LSI of high reliability where a gate insulating film is prevented from deteriorating by a method wherein a gate insulating film is annealed in an atmosphere of O2, O3, or Ar after the cleaning of its surface before a gate electrode film is formed.

CONSTITUTION: An element isolation insulating film 2 is formed on a silicon substrate 1, then a gate oxide film 3 is formed, and foreign objects or contaminants present on the surface of the gate oxide film 3 are removed with HF, when pinholes 4 are induced in a weak part of the gate oxide film. Thereafter, the substrate 1 is subjected to an oxidation treatment at a temperature of 950°C in an atmosphere of O2 for 40 minutes to mend the pinholes 4. A polycrystalline silicon 5 is formed through a CVD method, and a gate wiring 5, a source 6, and a drain 6 are provided, whereby a MOSFET can be obtained. At this point, when the above oxidation treatment is carried out in an atmosphere of mixed gas composed of O2 and N2 or Ar or O3 at a temperature of 600-1000°C, a MOSFET can be improved in gate breakdown strength, and if the treatment is executed in an atmosphere of Ar or N2, a gate can be also improved in breakdown strength by increasing a treatment temperature to 800-1100°C.