Characterization of Mo/Si mirror interface roughness for different Mo layer thickness using resonant diffuse EUV scattering

The throughput of extreme ultraviolet (EUV) lithography systems is presently strongly limited by the available radiant power at the wafer level. Besides increasing the power of EUV sources, also the quality of the optical elements plays a key role. With state of the art multilayer mirrors the main cause of diminished reflectance is surface and interface roughness as well as interface diffusion. Both properties lead to reduced specular reflectance while only the interface roughness causes diffuse scattering. EUV diffuse scatter thus allows to selectively assess the contribution of the interface roughness. The intensity distribution of diffusely scattered EUV radiation provides information on vertical and lateral correlations of the surface and interface roughness through the appearance of resonant diffuse scattering (RDS) sheets. The study of off-specular scattering thus serves as a natural tool for the investigation of the roughness of the interfaces. However, upon near-normal incidence impinging EUV radiation, dynamical scattering contributions from thickness oscillations (Kiessig fringes) lead to Bragg lines which intersect the RDS sheets. This causes strong resonant enhancement in the scatter cross section which we called “Kiessig-like peak" in analogy to the well known phenomenon of Bragg-like peaks appearing in hard X-ray grazing incidence measurement geometries. Thus for power spectral density studies of multilayer interface roughness, resonant dynamical scattering cannot be neglected. Theoretical simulations based on the distorted-wave Born approximation enable to separate dynamic features of the multilayer from roughness induced scattering. This allows to consistently determine an interface power spectral density (PSD). We have analyzed magnetron sputtered high-reflectance Mo/Si multilayer mirrors with different nominal molybdenum layer thicknesses from 1.7 nm to 3.05 nm crossing the Mo crystallization threshold. Our off-specular scattering measurements at multilayer samples were conducted at the PTB-EUV radiometry beamline at the Metrology Light Source (MLS) in Berlin. The samples were produced by magnetron sputtering and pre-characterized by Kα X-ray reflectivity at Fraunhofer IWS, Dresden.