Process for improved reflectivity uniformity in extreme ultraviolet lithography (EUVL) masks

Fabrication of EUVL masks requires formation of both a repair buffer layer and an EUV absorber layer on top of a molybdenum/silicon (Mo/Si) multilayer coated mask blank. Alteration of the Mo/Si multilayer during etch, repair or cleaning of the EUVL mask can be detrimental to the reflectivity and thus the functionality of the final mask. IBM's Next Generation Lithography (NGL) group has reported on EUVL mask fabrication based on an absorber of low stress chromium (Cr) and a buffer layer of silicon dioxide (SiO2). Due to poor etch selectivity between SiO2 and the underlying silicon capping layer, the finished masks had non-uniform and reduced EUV reflectivity after processing. This led to the development of an alternative absorber stack combination of an absorber layer of low stress TaNx on a buffer layer of low stress Cr. This paper describes the improved reflectivity uniformity of this type of mask along with several aspects of mask quality. such as CD control and image placement.