We determined the optical constants of silicon oxynitride (SiOxNy) thin films using a vacuum ultraviolet spectroscopic ellipsometer. The SiOxNylayers with a nominal thickness of 25 nm ∼ 35 nm were grown on silicon substrates by using plasma enhanced chemical vapor deposition (PECVD). The precursor gases were nitrous oxide (N2O) and silane (SiH4). The ratiorof N2O flow rate to SiH4one in the deposition process was controlled from 1.16 to 3.05. The ellipsometric measurements were performed at the angle of incidence 75° for the spectral range from 0.75 eV to 8.75 eV. The complex refractive indices, optical band gaps, and thicknesses of the SiOxNylayers were determined by using Tauc‐Lorentz dispersion model. © 2003 American Institute of Physics