Effects of mask roughness and condenser scattering in EUVL systems

The wavefront reflected from extreme ultraviolet lithography (EUVL) mirror and mask surfaces can contain a nonnegligible amount of phase variation due to roughness of the mirror surfaces and variations in multilayer thin-film coatings. We examine the characteristics of image and pattern formation as a function of phase variations originating at the mask surface and at condenser mirrors. A theoretical development and a Monte-Carlo simulation are used to show relationships between statistics of the phase variations and the mask pattern, coherence factor, and numerical aperture of the projection camera. Results indicate that it is possible to produce nearly 1% line-edge roughness in a photoresist pattern from moderate values of phase variations.