Method of manufacturing a semiconductor device, in which a silicon wafer is provided at its surface with field oxide regions

A method of manufacturing semiconductor devices having defect free areas below field oxide layers comprising the steps of

• forming an oxidation preventing mask locally on a surface of a silicon wafer, and
• oxidizing unmasked portions of said silicon wafer by carrying out in sequence the steps of
• heating said silicon wafer to a temperature range of 950.degree. C. to 1050.degree. C.,
• initially passing a wet oxidizing gas mixture over said silicon wafer in said temperature range to initially oxidize said unmasked portions of said silicon wafer to a thickness of up to at least 50 nm, said wet oxidizing gas mixture having a water content of at most 30% by volume to prevent defects from forming in the underlying silicon wafer,
• then adding additional water to said oxidizing gas mixture in said temperature range to bring said water content to an amount of more than 30% by volume, and
• continuing passing said oxidizing gas mixture with the added water over said silicon wafer until a desired thickness of oxide is formed on said silicon wafer.