Fabrication, thermal stability, and reflectivity measurements of Mo/Si multilayers as x-ray mirrors and other optical components

Abstract

For the wavelength region above the Si-L edge normal incidence soft X-ray mirrors are produced with peak reflectivities around 55 percent. The Mo/Si multilayer systems are fabricated by electron beam evaporation in ultrahigh vacuum. Analysis of the quality of the stack is made by using an in situ monitoring system measuring the reflection of the C-K line and ex situ grazing X-ray reflection of the Cu-K-alpha line. A smoothing of the boundaries and thereby a drastic enhancement of the reflectivity can be obtained by thermal treatment of the multilayer system during growth. The microstructure of the multilayer systems is investigated by means of Rutherford Backscattering spectroscopy and Sputter/AES technique. Baking the final stack after deposition up to 900 C is applied to study the thermal stability of the soft X-ray mirror. Near normal incidence mirrors even for short wavelengths, e.g., the water window (2.4 - 4.4 nm), are produced with a Mo/Si bilayer thickness of 2.6 nm. An improvement in the quality of the interfaces for such ultrathin multilayer systems can be obtained by bombardment of the deposited layers with Ar(+) ions as well as by thermal treatment of the multilayer system and mixing of Mo and Si in the absorber layer during the deposition run. We report on reflectivity measurements of the mirrors and their behavior as polarizers and analyzers and on the diffraction efficiencies of laterally structured multilayer systems as gratings.