The use of low pure ammonia pressures (≤ 10^-1 mbar) for silicon and silicon dioxide nitridation has the unique advantage to result in a better control of both the kinetics of the nitridation first stages of Si(100) and the concentration of nitrogen at the Si/SiO₂ interface. The kinetics of thermal nitridation first stages of Si(100) have been investigated using low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS). In the high temperature regime (> 800 °C), the kinetics obey a layer-by-layer mode of growth. Moreover the formation of the first reacted layer is proportional to the ammonia pressure and limited by the molecular dissociation rate on the surface. Further growth upon nitridation is limited by diffusion of the reacting species through the reacted layer. The nitride thin film / silicon interface is quite abrupt. Surface nitridation of SiO₂ thin films (~13 nm) has been achieved by thermal activation (900 °C-1050 °C) and studied by means of AES, secondary ion mass spectroscopy (SIMS), X-ray photoemission (XPS), Raman (RS) and infrared absorption (IRS) spectroscopies. The extent of nitridation increases with pressure, temperature and time. The nitriding species appear to be NH_x (0<x<3) amine groups and their apparent diffusion coefficient estimated using rough assumptions decreases when the nitrogen content in the surface region of SiO₂ increases. Electrical characterization of metal-insulator-silicon structures shows that the SiO₂/Si interfacial quality is not damaged as long as the interfacial nitrogen concentration remains negligible. A new process of Si or SiO₂ nitridation at ambient temperature has been worked out on the basis of electron-beam-enhanced reaction.