Validation of optical constants in the EUV spectral range

After the introduction to high volume manufacturing, continuous development of EUVL systems and components will require the use of novel engineering materials, for example as absorber layer on next generation photomasks. In the EUV or soft X-ray spectral range the optical parameters of many materials are often not well known or based on theoretical calculations and interpolations. Thus, highly accurate measurements of the optical constants of thin layers obtained from different material compositions are necessary for the realistic modelling of new photomasks designs. Reflectometry is a widely used method for the determination of film thickness, especially in the X-ray spectral range. The same approach can also be used in EUV with a different focus. The aim is then to reconstruct the optical material parameters, the real and imaginary part of the refractive index, from the reflectivity measurements with a well-defined layer thickness. In this study, we will present the feasibility of determining the optical constants for candidate materials for EUV photomask absorbers using EUV reflectometry.