EUV/soft x-ray multilayer optics

Abstract

Most applications of Mo/Si multilayer optics in Extreme ultraviolet lithography (EUVL) require a high normal incidence reflectivity. Using dc magnetron sputtering we achieved R = 68.8 % @ λ = 13.45 nm. High-reflective Mo/Si/C and high-temperature stable Mo/C/Si/C multilayer mirrors with reflectivity of 69.6 % and 61.0 % at 13.5 nm were developed. Microstructure and optical properties of the multilayers have been investigated by small and large angle Cu-Kα scattering and characterized by EUV reflectivity. Beside the periodic multilayer design, Mo/Si multilayer mirrors with increased as well as reduced bandwidth in their spectral and angular reflectance have been designed and deposited. A reflectivity of more than 20 % was achieved in the wavelength range from 13 nm to 15 nm. In addition, narrowband multilayer mirrors with a significantly reduced band-width (FWHM = 0.077 nm) basing on high order reflection have been designed and fabricated. Both the increase and the reduction of the reflection bandwidth are unavoidably connected with a decrease of peak reflectivity. Therefore, the application of such specially designed mirrors involves areas where a maximum peak reflectivity is not required, e.g. in EUV spectroscopy and for the metrology of EUV sources. According to the optics requirements of an EUVL tool, the accurate deposition of high reflective and laterally graded multilayers on ultraprecise polished substrates can be regarded as one of the major challenges of EUVL development today. To meet these requirements, a new dc magnetron sputtering system has been developed.