Magnetron sputtered EUV mirrors with high thermal stability

Many applications of multilayers in the EUV spectral region require not only high normal incidence reflectivity but also high thermal stability. We investigated the thermal stability of Mo/Si muitilayers in comparison with the new material combination Mo2C/Si in the temperature range from 200 degrees C to 700 degrees C. Additionally, we deposited and studied Mo/Si multilayers having Mo2C diffusion barriers with 0.6 nm single layer thickness. The multilayer mirrors were designed for normal incidence reflectivity at about 13 nm wavelength and were deposited by de magnetron sputtering. X-ray scattering, transmission electron microscopy and atomic force microscopy were used for characterization of the multilayer structures. The results are correlated to the measured normal incidence reflectivity using synchrotron radiation. We achieved maximal normal incidence reflectivities of 61.8 % @ 13.0 nm wavelength for Mo2C/Si and 59.9 % @ 133 nm for Mo/Si multilayers having Mo2C diffusion barriers. While the reflectivity of Mo/Si multilayers decreased considerably after annealing above 300 degrees C the Mo2C/Si multilayers showed a superior thermal stability up to 600 degrees C.