Method of manufacturing semiconductor device

A method of manufacturing a semiconductor device, comprising:

• forming a gate insulating film and a gate electrode film composed of polysilicon on a silicon substrate in this sequence, and forming a hard mask composed of a first film on said gate electrode film;
• selectively etching said gate insulating film and said gate electrode film by employing said hard mask to form a gate electrode;
• and
• removing said hard mask,
• wherein said removing the hard mask includes:
• forming a first silicon oxide film by oxidizing upper surfaces of regions and side surfaces of said gate electrode, except for an upper surface of said formed gate electrode having said first film;
• removing at least portions of said first film and said first silicon oxide film with a first chemical solution;
• forming a second silicon oxide film by oxidizing upper surfaces of regions and the side surfaces of said gate electrode, except for the upper surface of said formed gate electrode having said first film;
• and
• removing said first film and said second silicon oxide film with a second chemical solution.