A special method to create gratings of variable line density by low voltage electron beam lithography

Abstract

XUV radiation and soft X-rays can be focussed using optical elements that represent extremely smooth and planar substrates with diffracting structures on them. Optical elements of this kind can generally be described as reflection gratings with straight or curved lines of variable line density. Such arbitrary line shapes are easily generated with the help of electron beam lithography. Continuous path control applied in conjunction with data fracturing into BÉZIER curves appears to be particularly suited for this purpose. The method avoids breaking the individual lines down into polygons, which implies an inevitably greater edge roughness. The use of low electron energies allows the individual grid lines to be generated by superpositioning of two path curves without any need to consider the proximity effect. The exposures were carried out on an electron beam lithography system LION-LV11. Measurements performed on BESSY for the first diffraction order showed the resulting optical element to have an efficiency of up to four percent depending on the particular wavelength.