Wavelength selection for multilayer coatings for the lithography generation beyond EUVL

The spectral properties of LaN/B and LaN/B4C multilayer mirrors have been investigated in the 6.5-6.9 nm wavelength range, based on measured B and B4C optical constants. We show that the wavelength of optimal single mirror reflectance for boron based optics is between 6.63 and 6.65 nm, depending on the boron chemical state. The wavelength of the maximum reflectance of the LaN/B4C multilayer system is confirmed experimentally. Calculations of the wavelengthintegrated reflectance for ideal 10-multilayer-mirror stacks show that a B-based optical column can be optimized for a wavelength larger than 6.65 nm.