Effects of mask absorber structures on the extreme ultraviolet lithography

In this paper, the authors present the results of an investigation of the dependence of mask absorber thickness on the extreme ultraviolet lithography (EUVL) and suggest a new mask structure to minimize shadowing effects. For this purpose, several patterned masks with various TaN absorber thicknesses are fabricated using in-house Ru-capped EUVL mask blanks. According to the simulation using practical refractive indices, which are obtained at EUV wavelengths, the absorber thickness can be reduced to that of out-of-phase $(ΔΦ=180°)$ ranges without loss of image contrast and normalized image log slope. Thickness to meet out-of-phase in real mask can be obtained by comparing field spectrum intensity ratio using the EUV coherent scattering microscopy (CSM). $52.4nm$ in thickness is close to $ΔΦ=180°$ for TaN absorber since it shows the highest 1st/0th order intensity ratio as well as the best resolution in the microfield exposure tool (MET) test. When we apply $40-nm$-thick TaN instead of $80-nm$-thick TaN, the amounts of $H-V$ bias reduction in wafer scale correspond to 80% $(2.46–0.48nm)$ by CSM and 70% $(2.23–0.65nm)$ by MET test results. Considering the fact that $H-V$ bias in the MET is similar with that of simulation using the resist model, the degree of $H-V$ bias in the alpha demo tool (ADT) is supposed to be much higher than that of MET due to its higher incident angle $(θ=6°)$⁠. Our final goal is to develop a thin absorber EUVL mask which has a low $H-V$ bias, high EUV printability and DUV contrast, and sufficient optical density at the border. To achieve this, blind layer treatment and integration with anti-reflective coating layer are in progress.