Numerical analysis of DUV scatterometry on EUV masks

We investigated the potential, applicability, and advantages of DUV scatterometry including DUV ellipsometry for the dimensional characterisation of the absorber structures on EUV photomasks. By means of numerical investigations on the basis of rigorous diffraction calculations we demonstrate the sensitivity of DUV scatterometry with regard to dimensions and geometry of the absorber structures. Further we show, that in contrast to at-wavelength scatterometry in the EUV scatterometry in the DUV spectral range is nearly insensitive to perturbations of the reflecting MoSi multilayer. As for EUV scatterometry the separation of absorber structure and multilayer parameters is sometimes difficult the application of DUV scatterometry allows for an easy separation. It is further on verified, that for this reason a quite good reconstruction of geometry and size of the absorber structures from measured data is possible. Finally we prove the sensitivity of DUV ellipsometric measurements, a feature, which is not (yet) available in the EUV spectral range. Because of these results we estimate DUV scatterometry to be an excellent metrology method for in-line monitoring and process optimization in the fabrication process of EUV masks.