Integration and optimization of the absorber stack has become a critical issue with the progress of the extreme ultraviolet lithography development because it influences many issues such as throughput, pattern fidelity, and mask yield. Simulation works to optimize an absorber stack were carried out and the results were empirically confirmed. TaN showed a great potential as an extreme ultraviolet absorber property but it did not meet the requirement for deep ultraviolet reflectivity for inspection. According to the simulation, Al2O3 was selected as an anti-reflection coating for DUV wavelength. Al2O3 ARC with optimum thickness reduces the DUV reflectivity from 42.5 to 4.4 % at 248 nm while maintaining the other properties. A novel absorber stack consisted of TaN absorber, Ru capping, and Al2O3 ARC is proposed, and the total thickness of the stack is only 47 nm and the EUV and DUV reflectivities are 0.97 % at 13.5 nm and 4.4 % at 248 nm, respectively.
The characteristics of various potential absorbers, such as Cr, Ta, and TaN materials were quantitatively investigated by calculating the optical contrast and geometrical width variation of pattern image of 25-nm-width transferred through the exposure system. The intrinsic absorber performance was evaluated by the numerical modeling of the reflectivity on the mask and the aerial image intensity on the wafer. The reflectivity on the mask was calculated for various absorber thicknesses (40-70 nm) using Fresnel equation. For the calculation of the aerial image intensity of pattern features with various absorbers, SOLID-EUV, which is capable of rigorous electromagnetic field computation, was employed. It could be reasonably concluded that the TaN absorber model showed superior optical characteristics compared to other absorber systems, whereas the best performance on the geometrical characteristics was found in the Ta absorber system.