Cr/Sc multilayers have great potential as near-normal incidence reflective optics for soft X-ray applications in the energy range of 180-400 eV. The best performing mirrors are generally achieved when containing amorphous metal layers. Impurities, if present, could have dramatic effects on nano-structural evolution as well as on the optical contrast between the layers.
We have investigated the influence of residual gas elements, i.e., H, C, N, and O on the nucleation, growth, structure and performance of Cr/Sc multilayers deposited in HV conditions by a dual cathode DC magnetron sputter deposition. The multilayers were deposited with the base pressure (PB) ranging from 2.10-7- 2.10-5 Torr. Multilayer structures with the modulation periods in the range of 0.9- 4.5 nm and layer thickness ratios in the range of 0.17 – 0.83 were deposited. ERDA was used to determine the impurity content and the depth profile, while the structural investigations of the multilayers were carried out by using hard X-ray reflectivity and TEM. A series of multilayers optimized for the Sc-edge was grown and the influence on the soft X-ray reflectivity performance of these multilayers will be presented.
For the multilayers deposited with a high PB ~ 2.10-5 Torr, a N content as high as ~37 at.% was measured by ERDA. These multilayers mainly consist of nano-crystallites and showed a possibility of growing multilayered structure over extended thciknesses with very thin Sc layers ~0.3 nm, which was not observed for the multilayers grown at lower PB. This might be a consequence of ScNx formation. ERDA measurements indicate that N and O impurities are predominantly populating the interior of the Sc layers at this PB. However, a distorted layered structure and a low X-ray reflectivity was observed in the presence of high N content.
A similar content of H and O was detected as dominant impurities (~3.5 at.%) for the multilayers deposited with a PB of 2.10-7 and-2.10-6 Torr. However, different amorphous-to-crystalline transition behaviours were observed with delayed nucleation of crystalline phase in the case of 2.0-7 Torr. No preferred impurity sites in the multilayer period were observed at these PB. These multilayers exhibit smooth and abrupt layer structure with higher x-ray reflectivity
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