Manufacturing method of semiconductor integrated circuit device

摘要

The invention is directed to a semiconductor integrated circuit device having a plurality of gate insulation films of different thicknesses where reliability of the gate insulation films and characteristics of MOS transistors are improved. A photoresist layer is selectively formed on a SiO₂ film in first and third regions, and a SiO₂ film in a second region is removed by etching. After the photoresist layer is removed, a silicon substrate is thermally oxidized to form a SiO₂ film having a smaller thickness than a first gate insulation film in the second region. Then, the SiO₂ film in the third region is removed by etching. After a photoresist layer is removed, the silicon substrate is thermally oxidized to form a SiO₂ film having a smaller thickness than a second gate insulation film in the third region.