A method of forming oxynitrides is described. This method is based on a newly discovered phenomenon, namely, the incorporation of nitrogen into the silicon/SiO2 interface when the SiO2 film with silicon layer on top of ii is annealed in a nitriding ambient such as NH3 or NO. The nitrogen concentration at the silicon/SiO2 interface can be increased to 5-6 x 10(14) atoms/cm(2), which is considered to be sufficient for the suppression of the boron penetration in the pf-polysilicon gate metal oxide semiconductor (MOS) field effect transistor. The MOS structure with the oxynitride having the nitrogen distributed at the topmost surface can be fabricated by taking advantage of the phenomenon. It should be noted that the amount of nitrogen incorporated into the SiO2/substrate interface is negligible in the case of this interface nitridation through silicon (INTS) phenomenon. This term is also used to refer to the method utilizing the phenomenon. We have applied INTS to the fabrication of the MOS structure, and have investigated the interface properties. As expected from the distribution of nitrogen, the interfaces of those MOS structures have been proved to exhibit properties as good as those of interfaces of MOS structures with pure SiO2; i.e., the fixed charge density (Q(f)) of less than 1 X 10(9) cm(-2) in the number density, and lower interface trap density (D-it) than in the MOS structures with the usual NO oxynitrides.