Low energy (3-100 eV) electrons are shown to be efficient promoters of the superficial nitridation of thin (15 nm) SiO2 films using low pure NH3 pressures (less than or equal to 2 x 10(-4) mbar) at ambient temperature. The ultrahigh vacuum experimental set-up permits us to uncouple two important effects both present in a thermally-assisted (700-950 degrees C) r.f. NH3 plasma nitridation process : the role played by low energy (3-5 eV) electrons and the temperature. It is found that the existence of charged particles near the sample surface must be taken into account in the plasma reaction process and that the main effect of thermal activation is to enhance the reaction and the diffusion of nitrogen species through the SiO, layer down to the SiO3/Si interface. Auger sputter depth profiles of the plasma nitrided SiO, films have been measured before and after bombardment with energetic electrons (1, 3, or 5 keV). Electron irradiation of the film induced a nitrogen depletion at the vacuum/dielectric interface and a nitrogen pile-up at the dielectric/silicon interface. Our results indicate that this phenomenon is a maximum for the lowest primary electron energy. This behavior can be understood on the basis of an electron-stimulated Si-N bond decomposition and the creation of positive nitrogen ions. The released nitrogen ions were found to either escape in vacuum or migrate through the dielectric layer and pile-up at the dielectric/silicon interface because of the existence inside the layer of a non-uniform electric field, in agreement with recent theoretical predictions.