Silicon oxynitride thin films, deposited by r.f. magnetron sputtering, art : used to synthesize multilayer stacks such as high reflectivity mirrors. The SiOxNy refractive index and the optical performances of these layers are measured at 1064 nm and the two compositions SiO2 and Si3N4 were chosen to realize the multilayers. The losses (absorption, scattering) are measured and compared to the performances of the Ta2O5/SiO2 mirrors deposited by dual ion beam sputtering. The absorption level (40 ppm) is eight times higher than that of the Ta2O5/SiO2 mirrors. This is due to the high absorption coefficient of Si3N4. In contrast, the scattering is not governed by the material properties but by the deposition process. The r.f. sputtering increases the sample RMS roughness. Solutions are proposed to optimize optical losses of Si3N4/SiO2 mirrors.