A first thermal treatment, which is performed at a temperature within 650
750.degree. C. for 30 240 minutes, and thereafter a second thermal
treatment, which is performed at a temperature within 900 1100.degree. C.
for 30 120 minutes, are performed as the initial thermal treatments on a
semiconductor wafer composed of silicon. Further, before forming a gate
insulating film, the temperature is increased to 1000.degree. C. at a
temperature increasing rate of 8.degree. C./min in a nitrogen ambient,
and a thermal treatment is performed at a temperature of 1000.degree. C.
for 30 minutes as a third thermal treatment.