Off-plane grazing-incidence blazed grating with radial grooves as an efficient spectral purity filter for EUV lithography

It was found that the high-blaze-angle reflective diffraction grating operating in the off-plane grazing-incidence configuration allows the separation and focusing of a 2% wavelength band in the vicinity of 13.5 nm. A model of a cooled multiple grating with a fan-groove geometry was used to separate the desired spectral range and to achieve a record efficiency and stigmatic focusing of images at high dispersion of adjacent orders. The highest relative efficiency in the range of 0.96–0.99 and reflectances from 0.71 to 0.96 can be achieved for an off-plane fine-pitch grating irradiated at angles of 70°–85° with a converging unpolarized light beam from a EUV collector. Such efficiency is higher than that of the corresponding in-plane configuration. The absolute efficiency of 0.72 was achieved in the minus-first order of a Mo-coated test Si grating with 5000 grooves/mm in unpolarized light using the PCGrate^®-SX? code and the AFM-measured groove profile.