Effect of Thickness on the Optical Constants Determination of TiN Thin Films in the EUV/VUV Range

We measured the angle-resolved reflectance of 3 nm and 30 nm TiN films on Si using s- and p-polarized synchrotron radiation over the wavelength range of 36–120 nm. To determine the optical constants, we solved the inverse problem by employing a transfer-matrix method (TMM) forward model with Bayesian Markov Chain Monte Carlo (MCMC) inference. This enabled simultaneous extraction of n, k, and their associated uncertainties. Our results indicate that while the 30 nm film exhibits bulk-like behavior and reduced sensitivity to interfacial effects, the 3 nm film is strongly influenced by the native surface oxide and the interfacial region. In the ultrathin regime, nanometer-scale structural variations produce percent-level changes in the retrieved optical constants. These findings demonstrate that bulk optical constants are not directly applicable to ultrathin TiN films in the EUV/VUV range, where surface and interface contributions significantly modify the total optical response.