The effect of two kinds of plasma (NH3 and N2O) nitridation under plasma-enhanced chemical vapor deposition conditions on the properties of 10 nm and 20 nm thermally grown oxides was studied. It was established that both kinds of plasma induce radiation defects in the Si-SiO2 structures in the form of a fixed oxide charge and negatively charged interface states, without transformation of the oxide into an oxynitride. It has been suggested that this is due to the relatively low process temperature (643 K) as well as to the short plasma nitridation times (5-20 min). These conditions do not allow incorporation of N in the depth of the oxide. The plasma induced charges lead to degradation of the mobility and influence strongly the scattering mechanisms in the inversion channel. The interface and dielectric properties of the plasma treated Si-SiO2 structures depend on the initial oxide thickness and on the duration of the nitridation process. Degradation of the properties is stronger for thinner oxides and longer nitridation times. The N2O plasma treated oxides display a lower plasma sensitivity. The nature of plasma induced charges is connected mainly to the influence of plasma chemistry.