The microelectronics industry reduces its critical dimension by a factor of two every 4 years and is now using the KrF* excimer laser line at 248 nm as the source of photolithography steppers in production in deep UV (< 248 nm). In order to keep the critical dimension stable, the reflectivity of the resist must be minimal, or below 4%. The best way to achieve such a result is to use anti-reflective coatings (ARCs). A good choice is hydrogenated silicon oxynitride (SiON) whose refractive index can be adjusted in the deposition process. This layer is neutral and can be easily etched. It is important to characterize these materials used in the photolithographic process. The extreme deep UV capabilities of the spectroscopic ellipsometry (SE) equipment will be presented with the measurement of the optical properties (n, k) of several SiON materials. In addition, the best measurement procedure, modeling and data reduction will be discussed. By using the measured n and ii data files and our analysis software, WINELLI, it is possible to calculate the optimum properties for the AR coatings vs., energy. In addition, the thicknesses of the different layers and the absorption coefficient tolerance window can be calculated, so that the best stack can be predicted. The results of such calculations will be presented in this article.