Silicon nitride Si100Nx films were made by reactive de magnetron sputtering in the composition range between 0 less than or equal to x less than or equal to 133 (Si100N133 double left right arrow Si3N4). Electrical conduction and optical transmission of the films were measured before and after annealing. X-ray diffraction, RES, Raman and FTIR spectroscopy were used to identify the structural and compositional changes in the films. The variation of the electrical resistivity upon annealing can be explained by the number of dangling bonds, by oxidation of the film's surface, and by the formation of crystalline silicon in the films above about 800 degrees C. The change of film properties upon annealing depends on the sputtering conditions.