Method of fabricating semiconductor device with MIS structure

A fabrication method of a semiconductor device with the MIS structure is provided, which prevents the boron penetration phenomenon from occurring even if a gate insulator film is as thin as approximately 3 nm or less. After a silicon nitride film is formed on a semiconductor substrate, oxygen is doped into the silicon nitride film by a suitable process such as a thermal oxidation, ion implantation or plasma doping process, thereby forming an oxygen-doped silicon nitride film having an oxygen-rich region that extends along an interface between the oxygen-doped silicon nitride film and the substrate. The oxygen-rich region is higher in oxygen concentration than the remainder of the oxygen-doped silicon nitride film. At least part of the oxygen-doped silicon nitride film serves as a gate insulator film of a MISFET. Next, a gate electrode of the MISFET is formed on the oxygen-doped silicon nitride film. A dopant is selectively introduced into the substrate to form a pair of source/drain regions of the MISFET in the substrate at each side of the boron-doped gate electrode. Finally, the substrate is heat-treated to activate or anneal the dopant introduced into the substrate.