Analytical model for EUV mask diffraction field calculation

This paper proposes an analytical model to describe the mask diffraction in EUV lithography. The model is used to improve the understanding of the EUV mask performance and to analyze relevant mask topography effects. The multilayer and absorber constituting the EUV mask are simulated separately in this model. The light incident on the mask is first diffracted by the absorber, and then reflected by the multilayer and propagated upwards through the absorber again. The multilayer reflection is calculated by a mirror approximation, and the absorber transmission is calculated by a modified Kirchhoff model, where the absorber is considered to be thin and located in a certain plane. Moreover, an analytical expression of the diffraction spectrum of masks with arbitrary pattern orientation is der ived. Comparisons with rigorous simulation are used to validate the accuracy of the developed model. It predicts mask diffraction of 16nm wide line and space features. For 0.35 NA EUV systems with an incidence angle of 6° the simulated CD errors are below 0.5 nm, with a pattern pitch ranging from 32nm to 250nm.