Wavelength selection for multilayer coatings for the lithography generation beyond EUVL

Key in designing the next generation of EUVL optics, i.e. at the wavelength of 6.x nm, will be matching its optimum operational wavelength to that of the candidate light sources. Light sources might be based on Tb or Gd, and the published spectra of plasmas created from these materials show highest intensities at 6.5 and 6.8 nm respectively. The reflective properties of the La/B4C coating, one of the more promising multilayer compositions for 6.x nm, are mainly determined by the B optical constants, near its K absorption edge (6.6 nm). We have experimentally assessed the critical material properties required for making the choice on the next stage lithography wavelength. Calculations of the maximum reflectance for B-based multilayers show significant gain near the B absorption edge when fine tuned with experimentally obtained optical data from appropriate multilayer structures. Results notably differ from tabulated optical data. The wavelength dependency of B-based multilayer reflectivity has been experimentally studied for various La-, B-based multilayer structures, including La/B4C, LaN/B4C, La/B and LaN/B, together with predictions of their peak reflectance. These results are of direct relevance for the final wavelength choice and the evaluation of the beyond EUV perspective.