Patent ID: 7635653

Claim:
A method of fabricating a semiconductor integrated circuit device, comprising the steps of: forming a device isolation structure on a surface of a semiconductor substrate so as to define at least a first active region, a second active region and a third active region; introducing, after said step of forming said device isolation structure, an impurity element that suppresses oxidation of said semiconductor substrate into said first active region; applying, after said step of introducing said impurity element, a thermal oxidation process to said surface of said semiconductor substrate to form a first insulation film so as to cover said semiconductor substrate surface in said first active region with a first thickness and a second insulation film so as to cover said semiconductor substrate surface in said second and third active regions with a second thickness; forming, after said step of applying said thermal oxidation process, an oxidation-resistant film on said surface of said semiconductor substrate so as to cover at least said first active region and said second active region and said third active region; exposing, after step of forming said oxidation-resistant film, said surface of said semiconductor substrate by removing said oxidation-resistant film and said second insulation film from said third active region while leaving said oxidation-resistant film on said first and second active regions; and applying, after said step of exposing said surface of said semiconductor substrate, a thermal oxidation process to said semiconductor substrate to form a third thermal oxide film on said third active region with a third thickness and to increase a thickness of said second thermal oxide film simultaneously, after said step of forming said device isolation region, said third active region experience an HF etching process only once, while said first and second active regions does not experience an HF-etching process after said step of forming said device isolation region.