High-precision optical constant characterization of materials in the EUV spectral range: from large research facilities to laboratory-based instruments

Any modeling of an interaction between photons and matter is based on the optical parameters. The determination of these parameters, also called optical constants or refractive indices, is an indispensable component for the development of new optical elements such as mirrors, gratings, or lithography photomasks. Especially in the extreme ultraviolet (EUV) spectral region, existing databases for the refractive indices of many materials and compositions are inadequate or are a mixture of experimentally measured and calculated values from atomic scattering factors. Synchrotron radiation is of course ideally suited to verify such material parameters due to the tuneability of photon energy. However, due to the large number of possible compounds and alloys, the development of EUV laboratory reflectometers is essential to keep pace with the development of materials science and allow for inline or on-site quality control. Additionally, optical constants are also essential for EUV metrology techniques that aim to achieve dimensional reconstruction of nanopatterned structures with sub-nm resolution. For this purpose, we studied a TaTeN grating created on an EUV Mo/Si multilayer mirror, to mimic a novel absorber EUV photomask. We present here a first reconstruction comparison of these structures, measured by EUV scatterometry at the electron storage ring BESSYII and with a laboratory setup of a spectrally-resolved EUV reflectometer developed at RWTH Aachen University. Both approaches differ in several aspects reaching from setup size to spectral quality (brilliance, bandwidth and coherence) as well as the measured and simulated data.