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.