Optical metrology for material parameters under x-ray, EUV and VUV irradiation: methods, challenges, and future perspectives

Understanding what happens to optics when irradiated with X-ray, EUV or VUV sources is a topic that the PTB has been working on for a long time. PTB operates several beamlines at the electron storage rings BESSYII and the MLS for irradiation and characterization of optics. The determination of material parameters such as the optical constants n&k is also a growing field of research. Some materials are highly reactive and oxidize on long time scales. Even more challenging are so-called low-k materials or photoresists. The modeling of measured reflectivities to determine the optical constants of the these reactive materials becomes considerably more complex. At the same time, however, sufficient data must also be measured in order to be able to model the optical response. So many different factors need to be considered and it is obvious that the future will involve a mixture of different strategies. In addition to experimental possibilities, theoretical prediction techniques are an increasingly important option. Compressed sensing, Modern statistical evaluations, surrogates or neural networks can be useful to reduce the numerical effort or optimize the measurements.