Spectral reflectance tuning of EUV mirrors for metrology applications

Abstract

Mo/Si multilayer mirrors with as well an increased as a reduced bandwidth in their spectral and angular reflectance have been designed and deposited for the wavelength about 13.5 nm. For the broadband mirrors, a non-periodic multilayer design based on the thickness optimization of each layer by a stochastic method is compared to a desing that consists of 3 different monoperiodic stacks. In addition, narrowband multilayer mirrors with a significantly reduced bandwidth based on high order reflection have been designed and fabricated. A near-normal peak reflectivity of more than 20% and 30% were achieved for the broadband mirros in the wavelength range from 13 nm to 15 nm and the incidence angles from 0° to 20°, respectively. The decrease of FWHM for Mo/Si mirrors from 0.5 nm to 0.07 nm was shown for suggested narrowband design. 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, radiation filtration and for the characterization of EUV sources. Furthermore, the use of such mirrors in combination with a broadband plasma source will result in a higher integral reflectivity.