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

摘要

Abstract: Using a diode rf-sputtering technique, differentmagnesium silicide based multilayer systems have beendeposited in very thin films for optical applicationsin the soft x-ray range. A detailed structural analysisof the different multilayers has been made usingin-situ kinetic ellipsometry, ex-situ grazing x-rayreflection at the copper K-$alpha line and transmissionelectron microscopy. The multilayer performances havebeen measured by synchrotron radiation at the magnesiumK-$alpha and L-$alpha lines and related to thestructural characteristics. For short wavelength, theW/Mg$-2$/Si system shows characteristics very similarto those of the more common W/Si system. Non-negligibleinterdiffusion and limited interface roughness allowthe layer thicknesses to be reduced to very low values.Well-defined Bragg peaks are observed even when thedouble period is as low as 44 angstrom. First Braggpeak reflectivity as high as 31% has been measured at9.89 angstrom for a multilayer with a double periodequal to 84 angstrom and a limited number of periods.This preliminary result is very promising for futureapplications in the field of x-ray fluorescenceanalysis. W/Mg$- 2$/Si and Si$-3$/N$-4$//Mg$-2$/Simultilayers have also been fabricated for use at higherwavelengths around the Mg L-$alpha line (286 angstrom).In the case of the W/Mg$-2$/Si multilayers have alsobeen fabricated for use at higher wavelengths aroundthe Mg L-$alpha line (286 angstrom). In the case of theW/Mg$-2$/Si system, the tungsten layers arecrystallized due to their higher thickness andconsequently the interface roughness is slightlyhigher. In spite of this, more than 20% reflectivity atthe first Bragg peak has been measured at normalincidence on different W/Mg$-2$/Si samples, with aselectivity two times better than conventional Mo/Simirrors ($lambda@/$delta $lambda $APEQ 20). When wereplace tungsten by a thin silicon nitride layerdeposited by reactive sputtering, we increase theselectivity up to $lambda@/$delta $lambda $APEQ 30, andthe thermal stability is drastically improved ($GRT800$DGR@C). !23