Oxynitride films with continuous variations of the optical index with thickness were grown by electron cyclotron resonance plasma-enhanced chemical vapor deposition using silane as a silicon precursor and oxygen and nitrogen as plasma gases. Specific linear and parabolic index profiles were fabricated by computerized-control gas flow. These gradient index layers were characterized by spectroscopic ellipsometry. The data reduction was performed by polynomial analysis. The validity of the ellipsometric results was also verified by performing an in-depth analysis via the chemical etching of the gradient index layers.