Long-term stability of a quasiperiodic Ta Al multilayer: Disintegration at room 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 spectroscopy (XPS) after 41 months of storage at room temperature. The specular reflectivity shows drastically suppressed diffraction peaks, whose positions and 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 that is attributed to the long spatial Fourier components of the interface profiles, 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 in 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 structural disintegration, the multilayer diffraction spectrum can still be indexed by means of the projection theory for quasiperiodic sequences, which points to a remarkable stability of quasiperiodic properties against significant disorder. We conclude that Ta/Al bilayers are apparently unsuitable for multilayer applications due to the lack of thermal stability even at room temperature, with grain boundary diffusion pointed out as a possible disintegration mechanism. The design of improved 3CF Ta/Al multilayers is discussed with regard to applications in x-ray optics. (C) 1999 American Institute of Physics. [S0021-8979(99)02212-4].