The fabrication of optically transparent silicon oxynitride x-ray masks using CVD process on silicon is reported in this paper. The etch rate in the ethylenediamine, pyrocatechol, and water mixture used for substrate removal has shown a strong dependence on the layer composition. Because of the notable etch rate of SixOyNz films, it is of advantage to cover them with a thin (?500 Å) Si3N4 layer. Thin Si3N4/SixOyNz/Si3N4 composite membrane foils have been fabricated with a total thickness of 6000 Å and a window size of 2.5×2.5 cm2. Fizeau interference measurements showed that the window geometry has a significant influence on the total geometrical distortion of the thin mask membrane.