Design and fabrication of broadband EUV multilayer mirrors

Multilayer mirrors with a significantly increased bandwidth in spectral and angular reflectance have been designed and deposited with a commercial magnetron sputtering system. A non-periodic multilayer design based on the thickness optimization of each layer by a stochastic method is compared to a design which consists of 3 different stacks. The EUV reflection of the samples was investigated with synchrotron radiation at the reflectometer of the PTB (Physikalisch-Technische Bundesanstalt) at BESSY II in Berlin. A reflectivity of more than 15 percent was reached in the whole wavelength range from 13 nm to 15 nm and a reflectivity of more than 30 percent was obtained for incidence angles from 0 degrees to 20 degrees with both designs. The increase in bandwidth is unavoidably connected with a decrease of peak reflectivity. Therefore, the application of such mirrors involves areas where a maximum peak reflectivity is not required, e.g. in EUV spectroscopy and for the metrology for EUV sources. Furthermore, the use of such mirrors in combination with a broadband plasma source will result in a higher integral reflectivity.