Investigation of transverse optical phonon of thin Si films embedded in periodic Mo/Si and W/Si multilayer mirrors

Abstract

Physical properties and stress analysis of Si layers embedded in bi-layer periodic Mo/Si and W/Si multilayer mirrors operating in extreme ultraviolet and X-ray region of wavelengths were investigated by Raman scattering. The study showed that alternate Si layers in these multilayers were exist in the amorphous phase. This was determined from the peak shift and linewidth of the transverse optical (TO) phonon mode. This mode was blueshifted with an increase in the thickness of amorphous Si (a-Si) layers embedded in the Mo/Si multilayers and redshifted upon thermal annealing. In contrast, the redshift of this mode in a-Si with an increase of thickness was observed for W/Si multilayers, and blueshifted upon thermal annealing. The behavior of the TO phonon mode of a-Si was also influenced by the microstructure of metallic layers and the composite stress of multilayers. Thermally annealed multilayers showed a reduction of the period thickness due to defect annihilation of a-Si layers which lead to the change of reflection properties of mirrors.