Reflective masks for extreme ultraviolet lithography

Extreme ultraviolet lithographic masks are made by patterning multilayer reflective coatings with high normal incidence reflectivity. Masks can be patterned by depositing a patterned absorber layer above the coating or by etching the pattern directly into the coating itself. Electromagnetic simulations carried out as a part of this thesis research showed that absorber overlayer masks have superior imaging characteristics compared to etched masks. Images of absorber masks are less sensitive to variations in incident angles and pattern profiles compared to those of their etched counterparts.;In an EUVL absorber overlayer mask, defects can occur on the mask substrate, in the reflective coating, and in the absorber pattern. Electromagnetic simulations showed that substrate defects cause the most severe image degradation. To validate the simulation results, a printability study of substrate defects for absorber overlayer masks was conducted. Results showed that the printability of 25 nm high substrate defects are roughly comparable to that of defects in optical lithography.;Simulations also indicated that the manners in which the defects are covered by multilayer reflective coatings can have very significant effects on their printabilities. Coverage profiles that result in large lateral spreading of defect geometries amplify the printability of the defects by increasing their effective sizes. The coverage profiles of Mo/Si coatings deposited above programmed defects of different sizes were studied by atomic force microscopy and transmission electron microscopy. Results showed that the lateral spreading of the defect geometry is proportional to the defect's height. The presence of undercut at the edges of the defects also increases the magnitude of the lateral spreading. Reductions in defect heights were observed for 0.15 ;A long-term study of Mo/Si coating reflectivity revealed that Mo/Si coatings with Mo as the top layer suffer significant reductions in reflectivity over time. The cause of this was determined to be oxidation of the top Mo layer. Fortunately, it was also found that the high reflectivity can be maintained by simply having Si as the top layer of the multilayer coating.