High thermal stability of the reflectivity of Be/Al multilayer mirrors designed for extreme ultraviolet wavelength

Superior optical contrast due to the combination of beryllium and aluminum in periodic Be/Al multilayers is the reason for effective reflectivity of these mirrors at extreme ultraviolet wavelength i.e. 17 nm. Depending on the thickness of the layers and annealing temperature, microstructure of beryllium and aluminum layers in periodic multilayers was investigated by Raman scattering spectroscopy and X-ray diffraction. Thinner film of beryllium showed more ordered structure which is qualitatively determined by narrow linewidth of optical phonon. The nucleation and grain growth of beryllium and aluminum is observed at higher annealing temperature. However, the effect of annealing on the modification of microstructure of beryllium and aluminum at lower temperature 373 K is not observed. This is the reason for almost similar value of reflectivity of ∼ 55 % for as-deposited and thermally annealed mirror at 373 K. At higher annealing temperature, the complete loss of reflectivity is observed, associated with the destruction of periodic modulation of mirrors due to interdiffusion, nucleation and grain growth of beryllium and aluminum.