EUV multilayer mirrors with tailored spectral reflectivity

Abstract

EUV multilayer mirrors with as well an increased as a reduced bandwidth in their spectral and angular reflectance have been designed and deposited with a commercial magnetron sputtering system. Concerning the broadband mirrors, a non-periodic multilayer design based on the thickness optimization of each layer by a stochastic method is compared to a design that consists of 3 different stacks. In addition, narrowband multilayer mirrors with a significantly reduced bandwidth based on high order reflection have been designed and fabricated. 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% was reached in the whole wavelength range from 13 nm to 15 nm and a reflectivity of more than 30% was obtained for incidence angles from 0° to 20° with both designs. Both the increase and the reduction of the reflection bandwidth are 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.