Multilayers with Ultra-Short Periods

Abstract

Multilayers with ultra-short periods (less than 2nm) are needed for particular applications such as mirrors for water-window X-ray microscopy and gamma-ray telescopes. Due to the extreme requirements on interface quality, manufacture is challenging even when realized with traditional material pairs. In this chapter, several types of ultra-short period multilayers, prepared by different deposition techniques from constituents with different miscibilities, are studied and compared in terms of the interface quality, structure and thermal stability. Specular/non-specular X-ray reflectometry, transmission electron microscopy and X-ray and electron diffraction were employed to obtain an insight into the interface phenomena. UHV e-beam deposition with optimized in situsubstrate heating was tested successfully as a simpler and cheaper alternative to in situion beam polishing to deposit high quality Cu/Si and Ni/C multilayers. However, true ultra-short periods could not be achieved. Replacement of elemental by compound layers in Ni/B4C multilayers using distributed electron cyclotron resonance sputtering allowed multilayer periods of less than 2 nm to be produced, with enhanced thermal stability up to 500°C compared to Ni/C multialyers. Similar excellent interface quality, ultra-short period and good thermal stability were obtained with Sc/Cr along with negligible miscibility. In both cases, the multilayer decay is controlled by the formation of a fine granular phase from amorphous layers. Vertical correlation of the interface profiles was found to be too weak to be detrimental effect to the specular imaging contrast. The results have direct implications for the targeted optimization of ultra-short period interference mirrors.