Update on optical material properties for alternative EUV mask absorber materials

The application of EUV lithography at the 7 nm node and below requires, among others, to reduce 3D mask effects1 like shadowing e.g. by introducing a thinner absorber structure. A search for new potential absorber materials with improved optical properties is done within the ECSEL JU project SeNaTe2,3,4,5. The standard Ta-based absorber system of the actual photomasks has been optically characterized in detail as the benchmark. The results are in agreement with the optical data presently used in the optical modeling of EUV photomasks. As candidates for an alternative absorber material, Ni, Co, NiAl are investigated. For the investigation of the alternative EUV mask absorber materials, metal layers of several 10 nm were deposited on silicon wafers. At PTB the spectral reflectance was measured in the angular range from normal incidence to grazing incidence in a wavelength band from 10 nm to 16 nm using PTB’s lubricationfree Ellipso-Scatterometer at the soft X-ray radiometry beamline. The measured reflectance is then fitted using Fresnel’s equations to a layer model accounting for thickness and roughness of the metal layer and additional top-oxide and a SiO2 layer on the Si-substrate surface. We present here an update on the optical constants of Ni, Co and NiAl layers.