Smoothing of interfacial micron-scale roughness in a Ni/C X-ray multilayer mirror

Correlation between the roughness of neighboring interfaces (roughness cross correlation) in a Ni/C X-ray multilayer mirror (XMM) prepared by laser ablation was studied by measuring X-ray diffuse scattering (XDS). The XDS intensities in the vicinity of the first Bragg reflection were measured at different photon energies: slightly below (8.325 keV) and slightly above (8.350 keV) the nickel photoabsorption $K$ edge. The effective screening of the contribution from the deep layers to the XDS cross section due to the strong damping of the wave field at a photon energy higher than the photoabsorption edge allowed information on the character of the in-depth roughness cross correlation in the sample to be obtained. In particular, the characteristic lateral correlation length of the roughness was $0.35\mathrm{\mu m}$ at a photon energy of 8.325 keV (the contribution to the XDS cross section of the entire XMM volume), and it increased to $0.4 \mathrm{\mu m}$ at a photon energy of 8.350 keV (predominantly the contribution from the upper layers). These data give direct evidence for the mechanism of smoothing of the interfacial roughness in the process of Ni/C XMM growth on anomalously large (up to micron) spatial scales. It was found that only rough large-scale defects with sizes of $\geq 10\mathrm{\mu m}$ are reproduced reasonably well from layer to layer. The processes of viscous flow and (or) reevaporation of high-energy target ions during deposition, which is characteristic of the laser method of XMM preparation, may serve as a possible explanation of the observed phenomenon.