Patent ID: 7432216

Claim:
A manufacturing method of a semiconductor device, in which a field effect transistor, including a gate insulator formed on a substrate, the gate insulator being formed of a silicon oxynitride film with an equivalent SiO 2 thickness of 2.5 nm or less and a gate electrode formed on said gate insulator is formed, the method comprising the steps of: forming, on said substrate, a silicon nitride film with a physical thickness of 0.9 nm; and forming said silicon oxynitride film by performing a thermal treatment of said silicon nitride film in a NO or N 2 O atmosphere, wherein the physical thickness of said silicon oxynitride film is increased 1.9 times or more of said silicon nitride film, wherein the thermal treatment in said NO or N 2 O atmosphere comprises the steps of: forming a first silicon oxynitride film by replacing nitrogen in said silicon nitride film by oxygen; and forming a second silicon oxynitride film by oxynitriding said substrate, wherein said silicon oxynitride film composed of said first and second silicon oxynitride film is formed.