Surface modification of extreme-ultraviolet lithography mask blanks by e-beam

Abstract

A mask inspection review of pattern features and defects is normally carried out using a secondary electron microscopy technique. Ideally, such mask inspection reviews should be nondestructive; nonetheless, as reported in this paper, high-dose exposures of extreme-ultraviolet mask surfaces have resulted in significant topographical changes, which were revealed by topographical mapping of reviewed masks using atomic force microscopy. Exposures with current densities of 1 mA/cm2 and higher resulted in the formation of topographical features in and around the scanned region on mask surfaces. On the Ru-capped multilayer blanks, the topographies consisted of small or absent depressions surrounded by ridges, which were attributed to secondary-electron-emission induced hydrocarbon deposition. On the chromium-nitride backsides, the topographies were usually simple depressions, although sometimes ridges were observed. The depressions were attributed to volume compaction in the substrate, and were observed for all four mask surfaces studied, substrate compaction took place with both quartz and low thermal expansion material substrates. The height range of the topography extended up to 25 nm, whereas the lateral dimensions often exceeded the scanned area by about a micrometer. While these lateral extensions could not be explained by either beam-induced heating or stress relief, Monte Carlo simulations showed that it could be explained qualitatively by the size of the region within which the energy deposition had taken place. This interpretation suggests that the current understanding as described by Hau-Riege qualitatively describes our observations related to depression topography.