Silicon Oxynitride, denoted as SiOxNy or simply as SiON, can be effective as an anti-reflective coating (ARC) in advanced integrated circuit processing typically within a 'photoresist/SiON/Si' thin film structure. However, measured values of reflectance (R) from 'SiON/Si substrate' alone cannot provide the necessary information regarding the effectiveness of SiON as an ARC layer. This is because in order to obtain an effective ARC for a given wavelength (for example at 365, 248 or 193 nm), it is necessary to reduce the standing-wave effects by minimizing the reflectance of that wavelength at the interface between the photoresist and SiON. Obviously, reflectance at this interface cannot be measured directly. R at the 'photoresist/SiON' interface must be calculated. In order to calculate R, the refractive index (n) and extinction coefficient (k) of the coating at the wavelength of interest, as well as the thickness (d) of the coating, must be known. Furthermore, n of the photoresist is also needed. However, in general, determination n and k values (especially in the DUV), as well as d, is a non-trivial problem. In this paper, we describe a new methodology, the 'n&k Method,' that simultaneously determines n and k over the 190 nm to 900 spectral range, as well as d, of all types of thin film/substrate combinations. These parameters are measured simultaneously and unambiguously, allowing rapid determination (in seconds) of the characteristics of the SiON ARC. This information can then be used to determine R at the wavelength of interest at the interface between the photoresist and SiON film.