Characterization of extreme ultraviolet masks by extreme ultraviolet scatterometry

The applicability of scatterometry for mask critical dimension (CD) metrology at extreme ultraviolet (EUV) wavelengths is investigated. Two different mask absorber stacks, a Cr∕SiO2 mask and a TaN∕Cr mask are patterned with periodic lines and spaces pattern and measured at EUV wavelengths in the range of λ=13.35–13.57nm. All measurements are performed by the Physikalisch-Technische-Bundesanstalt at the BESSY II storage ring. The intensity of the diffracted light is measured as a function of diffraction angle 2Θ∕2 in the range between −0.5° and 10°. The measured results are compared to simulations in the Fourier domain, using the difference method. For the Cr∕SiO2 mask, the comparison between measured and simulated results shows good agreement for nominally 800nm lines with 400nm spaces (mask level). No arbitrary scaling has to be applied for the simulated intensity of the diffraction signal. A best-fit CD value of 742nm at an absorber and buffer sidewall angle of 80° is obtained. This result is in excellent agreement with mechanical measurements using a surface nanoprofiler. Compared to a scanning electron microscope measurement, an offset of 16nm is observed. For the TaN∕Cr mask, this offset is found to be only 9nm, which can be attributed to the smaller stack thickness.