Spectral reflectometry characterization of an extreme ultraviolet attenuated phase-shifting mask blank

Abstract

Background: Extreme ultraviolet (EUV) attenuated phase-shifting masks are complex structures with stringent requirements for manufacturing precision and materials properties, and they have been object of extensive research lately. Aim: We aim to characterize the optical constants (n and k) and the thickness of the layers in the mask stack with a nondestructive method. Approach: Using a spectral reflectometry approach with EUV and soft X-ray illumination at various incidence angles, different layer's properties in a photomask blank can be selectively probed. The optical constants and the thicknesses of the layers can be obtained by fitting a suitable model to the experimental reflectance. Results: The optical constants of the Pt-W alloy absorber and the thicknesses of the top three layers of the sample stack were accurately characterized. Conclusions: Stacked layer's properties can be selectively probed with the instrument (REGINE) we developed. The properties of the topmost layer can be characterized by assessing the probing depth, before investigating deeper layers with a suitable choice of illumination wavelength and angle of incidence.