The plasma-enhanced chemical vapor deposition (PECVD) technique is well suited for fabricating optical filters with continuously variable refractive index profiles; however, it is not clear how the optical and structural properties of thin films differ when deposited on different substrates. Herein, silicon nitride films were deposited on silicon, fused silica, and glass substrates by PECVD, using silane and ammonia, to investigate the effects of the substrate used on the optical properties and structures of the films. All of the deposited films were amorphous. Further, the types and amounts of Si-centered tetrahedral Si–Si_vN_4-v bonds formed were based upon the substrates used; Si–N₄ bonds with higher elemental nitrogen content were formed on Si substrates, which lead to obtaining higher refractive indices, and the Si–SiN₃ bonds were mainly formed on glass and fused silica substrates. The refractive indices of the films formed on the different substrates had a maximum difference of 0.05 (at 550 nm), the refractive index of SiN_x films formed on silicon substrates was 1.83, and the refractive indices of films formed on glass were very close to those formed on fused silica. The deposition rates of these SiN_x films are similar, and the extinction coefficients of all the films were lower than 10^?4.