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Planar optical waveguides on thermally oxidized Si(111) substrates have been made with rf magnetron sputtering deposition from a SiO_2 target in a N_2 and Ar reactive atmosphere. Reproducible guiding layers of silicon oxynitride with refractive index in the 1.6–1.9 range have been obtained changing deposition parameters. A detailed study of the film characteristics in terms of optical and chemical properties is reported. The films are a mixture of SiO_2 and silicon oxynitride, with an extended intermixed region at the thermal SiO_2 buffer layer interface. Further annealing in N_2 atmosphere (600°C < T < 1000°C) resulted in waveguide attenuation values lower than 1 dB/cm for λ = 0.633 μm.