  A refined model of an extreme ultraviolet (EUV) mask stack consisting of the
Mo/Si multilayer coated by a Ru protective layer and a TaBN/TaBO absorber layer
was developed to facilitate accurate simulations of EUV mask performance for
high-NA EUV photo-lithography (EUVL) imaging. The model is derived by combined
analysis of the measured EUV and X-ray reflectivity of a state-of-the-art mask
blank. These two sets of measurements were analyzed using a combined free-form
analysis procedure that delivers high-resolution X-ray and EUV optical constant
depth profiles based on self-adapted sets of sublayers as thin as 0.25nm
providing a more accurate description of the reflectivity than obtained from
only EUV reflectivity. 'Free-form analysis' means that the shape of the
layer-interfaces in the model is determined experimentally and is not given a
priori by the structure model. To reduce the numerical effort for EUV imaging
simulations a low-resolution model of the multilayer and absorber stack with
sublayer thicknesses larger than 2nm, that fits to only the EUV reflectance,
was derived from the high-resolution model. Rigorous high-NA EUVL simulations
were done to compare the performance of the new model to our previous work.
