Developing reflective multilayer coatings, an enabling component of Extreme Ultraviolet Lithography and beyond

EUVL at 6.x nm is one of the candidates for the next generation photolithography. Its design requires matching the optimal wavelength of the optics to that of the light sources. Light sources might be based on Tb or Gd, 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 most 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 from the multilayer optics point of view. Calculations of the maximum reflectance for B-based multilayers show gain near the B absorption edge. Critical wavelength was calculated using experimentally obtained optical data for B and B4C. Results notably differ from obtained using tabulated CXRO data. The wavelength dependency of 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 optimal wavelength choice and the evaluation of the beyond EUV perspective.