<title>Flatness correction of NZTE mask blank substrates</title>

Abstract

For EUV lithography mask blank substrates of the 6025 type will be made from Near Zero Thermal Expansion (NZTE) materials. The low thermal expansion of EUV substrates shall provide for the required better thermal stability during mask-writing and lithography exposure. The manufacturing of such NZTE mask blank substrates requires modified finishing processes in comparison to standard mask blanks. Super polishing takes place to provide high spatial frequency roughness of less than 2 Angstroms RMS. To improve the flatness and the roughness at the mid spatial frequencies another correction step is required. Such step can be Ion Beam Figuring for example. We evaluate the results for the different spatial frequencies by interferometry and atomic force microscopy. We have a closer look how the additional finishing steps improve the flatness and mid spatial frequency roughness. The impact of these newly introduced finishing steps for the high spatial frequency roughness is studied. We examine the processes for two different substrate materials with near zero thermal expansion (ZERODUR and ULE).