Study on exposure contrast of an EUV mask

The exposure contrast (at wavelength contrast) on an extreme ultraviolet (EUV) mask has been evaluated by an experimental reflectivity measurement and an optical simulation. The experimental contrast showed good agreement with the calculated one for an EUV mask blank with TaBN absorber. The exposure contrast could be precisely estimated for an EUV mask using the simulation. Further, this simulation was used to evaluate the impact of absorber materials (TaBN, Cr and CrN) and 50-nm-thick buffer layers (CrX, SiO2, Ru and C) used to achieve thinner absorber stack. A mask composed of the TaBN absorber and the Cr-based buffer layer showed was the thinnest to achieve a contrast of 100, at 81.3-nm. The TaBN absorber and the Cr-based buffer layer were found to be more suitable materials for obtaining lower aspect ratio.