Model for nitridation of nanoscale SiO ₂ thin films in pulsed inductively coupled N ₂ plasma

As nitration of SiO2 gate dielectric can increase the film's dielectric constant and reduce boron penetration into the Si channel during ion implantation, plasma nitridation is of considerable interest for the fabrication of semiconductor devices. A coupled plasma equipment-surface physics model is used in conjunction with an experimental analysis of nitrided SiO2 thin films to understand the mechanism of SiO2 plasma nitridation. This investigation is conducted in a pulsed inductively coupled N2 plasma. Computational results show that N atoms and N2+ ions are the primary species in the N2 plasma that contribute to the nitridation of SiO2 thin film. N atoms adsorb at the SiO2 surface and diffuse into the bulk film, and most nitrogen near the surface is due to these adsorbed N atoms. N2+ ions, on the other hand, penetrate deeper into the SiO2 film in an ion-implantation-like manner, and these ions are responsible for the observed tail in the nitrogen concentration profile. Nitrogen concentration in the film can be increased by enhancing the plasma source power or the nitridation time. However, once the dielectric surface starts saturating with nitrogen, further nitrogen adsorption is inhibited and nitridation rate tapers off. As the fluxes of atomic N and N2+ ions to the wafer decrease with increasing gas pressure, the nitridation rate decreases with gas pressure. For the range of SiO2 film thickness examined (13-15 Å), the nitrogen transport and reaction properties in the film are film thickness dependent, probably due to the nonuniform density of the initial SiO2 thin film or to interfacial stresses.