As metal oxide semiconductor field effect transistors (MOSFETs) become highly integrated and high performance, various types of metal films are used in the fabrication of MOSFETs. From the viewpoint of low power consumption, a work function close to the energy level of the conduction band or the valence band of the material of the substrate is required for the gate electrode of the MOSFET. For example, a PMOS on a silicon (Si) substrate requires a metal film having a work function of about 5.1 eV, an NMOS on a silicon (Si) substrate requires a metal film having a work function of about 4.1 eV, and a PMOS on a germanium (Ge) substrate requires a metal film having a work function of about 4.6 eV, and an NMOS on a germanium (Ge) substrate requires a metal film having a work function of about 4.1 eV. Capacitor electrode in a DRAM requires a metal film with a higher work function and lower resistance.
Since a work function of a titanium nitride film (TiN film) on a hafnium oxide (HfO2) insulating film is about 4.9 eV, which is slightly lower than the energy in the vicinity of the valence band of silicon (Si), a metal film having a higher work function is required. A tungsten nitride film (WN film) is one of the metal films that have higher work functions. However, when tungsten in the tungsten nitride film is oxidized, oxygen is released from the oxidized tungsten in the subsequent heat treatment process, thereby adversely affecting the electrical characteristics.