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Novel absorber materials for EUV lithography mask

One of main issues of EUV lithography is to reduce so-called shadowing effect attributed to oblique incidence of EUV light on a mask. In order to mitigate shadowing, a thinner absorber layer thickness for EUV mask is preferred. In order to realize EUV masks with thinner absorber stack, we introduced SnO film as a high absorptive material for EUV light. Thorough actual measurement of EUV reflectance and subsequent data fitting to theoretical curve, we confirmed that SnO has large k (extinction coefficient) value. As a result, SnO absorber can do with about a half thickness compared to Ta-based absorber having the same OD value. Using SnO film, we designed a binary mask consisting of SnO/CrN-buffer pattern and an att.PSM consisting of SnO/Ru-shifter pattern. SnO has also considerable transparency in the range of DUV wavelength for use of defect inspection. We confirmed both binary mask and att.PSM have low reflectance in the range of DUV wavelength even without top AR coating. The att.PSM can realize appropriate reflectance (nearby 6%) at a phase shift of 180 deg with the total patterned (SnO/Ru) thickness of below 41 nm even with 6 nm thick SiN top coating. Furthermore, we evaluated dry etched cross sectional profile of the binary mask and the att.PSM. The initial etch profiles look encouraging.