Reflectivity and structural evolution of Cr/Sc and nitrogen containing Cr/Sc multilayers during thermal annealing
2008 (English)In: Journal of Applied Physics, ISSN 0021-8979, Vol. 104, no 6, 063516- p.Article in journal (Refereed) Published
It is shown that the thermal stability in vacuum of Cr/Sc multilayer thin films used as reflective optical components in soft x-ray instrumentation has substantial dependence on incorporation of N. The thermal stability is increased by incorporating 34 at.% of N in Cr/Sc multilayers. A pure Cr/Sc multilayer x-ray mirror starts a continuous degradation already at ~100 °C with a complete destruction of the multilayer at 500 °C. The resulting structure is a mixture of Cr and Sc nanocrystallites. The degradation can be described by linear diffusion theory and is suggested to be due to the formation of uniformly distributed phase-separated nanocrystallites followed by an Ostwald ripening process with an apparent activation energy of 0.5 eV. At the multilayer-substrate interface, a 7 nm thin Sc-Si layer is formed which effectively hinders indiffusion of Si and outdiffusion of Cr and Sc. A nitrided multilayer, initially consisting of crystalline fcc CrNx and fcc ScNy layers (x and y<1), is observed to improve in structural quality up to ~250 °C where it is stable for more than 12 h. At ~330 °C, the multilayer separates into regions with two multilayer periods, differing by less than 0.04 nm, which are stable at 420 °C over an extended period of time >40 h. It is proposed that the separation into the different multilayer periods is a consequence of redistribution of N within the Cr layers. Sc is observed to be stabilized in the ScN layers, which, in turn, inhibit the formation of a Sc-Si barrier layer at the substrate leading to a strong exchange of Si and Cr across the film substrate. This leads to a Cr-Si/ScN layered structure close to the substrate and chromium silicide crystallites inside the substrate. Close to the top of the multilayer, a CrN/ScN multilayer appears to be retained.
Place, publisher, year, edition, pages
2008. Vol. 104, no 6, 063516- p.
annealing, chromium, multilayers, reflectivity, scandium, thermal stability, thin films, X-ray optics, X-ray reflection
IdentifiersURN: urn:nbn:se:liu:diva-13064DOI: 10.1063/1.2980051OAI: oai:DiVA.org:liu-13064DiVA: diva2:17760