Magnesium-silicide-based multilayers for soft x-ray optics

Using a diode rf-sputtering technique, different magnesium silicide based multilayer systems have been deposited in very thin films for optical applications in the soft x-ray range. A detailed structural analysis of the different multilayers has been made using in-situ kinetic ellipsometry, ex-situ grazing x-ray reflection at the copper K-(alpha) line and transmission electron microscopy. The multilayer performances have been measured by synchrotron radiation at the magnesium K-(alpha) and L-(alpha) lines and related to the structural characteristics. For short wavelength, the W/Mg2Si system shows characteristics very similar to those of the more common W/Si system. Non-negligible interdiffusion and limited interface roughness allow the layer thicknesses to be reduced to very low values. Well-defined Bragg peaks are observed even when the double period is as low as 44 angstrom. First Bragg peak reflectivity as high as 31 has been measured at 9.89 angstrom for a multilayer with a double period equal to 84 angstrom and a limited number of periods. This preliminary result is very promising for future applications in the field of x-ray fluorescence analysis. W/Mg2Si and Si3N4/Mg2Si multilayers have also been fabricated for use at higher wavelengths around the Mg L-(alpha) line (286 angstrom). In the case of the W/Mg2Si multilayers have also been fabricated for use at higher wavelengths around the Mg L-(alpha) line (286 angstrom). In the case of the W/Mg2Si system, the tungsten layers are crystallized due to their higher thickness and consequently the interface roughness is slightly higher. In spite of this, more than 20 reflectivity at the first Bragg peak has been measured at normal incidence on different W/Mg2Si samples, with a selectivity two times better than conventional Mo/Si mirrors ((lambda) /(delta) (lambda) approximately equals 20). When we replace tungsten by a thin silicon nitride layer deposited by reactive sputtering, we increase the selectivity up to (lambda) /(delta) (lambda) approximately equals 30, and the thermal stability is drastically improved ( 800 degree(s)C).