Long-term stability of a quasiperiodic Ta Al multilayer: Disintegration atroom temperature analyzed by grazing angle x-ray scattering and photoelectron spectroscopy

A three-component Fibonacci (3CF) Ta/Al multilayer has been reinvestigated by specular and diffuse x-ray reflectivity and x-ray photoelectron spectros copy (XPS) after 41 months of storage at room temperature. The specular ref lectivity shows drastically suppressed diffraction peaks, whose positions a nd intensities are explained by severe interdiffusion of the Ta/Al bilayers building the 3CF sequence. Nonspecular reflectivity scans still indicate a high degree of interfacial roughness correlation in the growth direction t hat is attributed to the long spatial Fourier components of the interface p rofiles, which are substantially less affected by interdiffusion. The angle -resolved XPS spectra show that the Ta capping layer is completely oxidized and interdiffused by Al, whereas below the oxide layer Ta and Al coexist i n metallic form in the same film. Both x-ray reflectivity and XPS yield an oxide layer of similar to 30 Angstrom thickness. Despite the severe structu ral disintegration, the multilayer diffraction spectrum can still be indexe d by means of the projection theory for quasiperiodic sequences, which poin ts to a remarkable stability of quasiperiodic properties against significan t disorder. We conclude that Ta/Al bilayers are apparently unsuitable for m ultilayer applications due to the lack of thermal stability even at room te mperature, with grain boundary diffusion pointed out as a possible disinteg ration mechanism. The design of improved 3CF Ta/Al multilayers is discussed with regard to applications in x-ray optics. (C) 1999 American Institute o f Physics. [S0021-8979(99)02212-4].