Silicon nitride films were prepared on polished silicon wafers by radio-frequency reactive sputtering using a silicon target in nitrogen-argon plasma. The deposition rate was controlled from 2 to 7 nm/minute. The refractive index of the film decreased continuously and was varied from 2.8 to 1.96. The process described here is simple and does not require knowledge of the absolute magnitudes of partial pressures of the gases used. This method is well suited for fabricating multilayer structures of silicon nitride films with different refractive indices.