Silicon oxynitride (SiOxNy) films have been grown by a low‐pressure chemical vapor deposition (LPCVD) process from mixtures of SiH2Cl2, N2O, and NH3at 820 °C. The overall layer composition can be varied by adjusting the N2O/NH3gas flow ratio. Rutherford backscattering and Auger analysis of the films indicated a uniform composition throughout the layer, irrespective of the nature of the substrate. Both the thickness and the composition of these oxynitride films can conveniently be measured with ellipsometry; the oxygen to nitrogen ratio can be derived reliably from the value of the refractive index. It is inferred that LPCVD oxynitrides are homogeneous on an atomic scale, i.e., the silicon atoms are randomly surrounded by oxygen and nitrogen atoms, and are therefore not to be conceived of as a physical two phase mixture of silicon oxide and silicon nitride. Their stability in metal–oxynitride–oxide–silicon structures is found to improve with increasing oxygen content as regards flatband voltage shift upon temperature‐bias stress.