Polyimide planarization for increased transmission of large-area X-ray windows

Abstract

The substrate roughness has previously set the limit for the minimum thickness of X-ray windows based on technology where planar tungsten grids are used for the membrane support, because the window membrane is built directly onto the substrate. Here this restriction is precluded by using a planarizing layer on the foil combined with retarded etchstop removal, which allows further reduction in the membrane thickness. A polyimide resin has been selected for the planarizing film, which can be removed either by solvents or by plasma etching. The fabrication process presented is capable of producing pressure-enduring X-ray window membranes in large formats down to the membrane polyimide thickness of 250 nm. The fabrication procedures of the windows are presented in detail to correlate the observed behaviour to the window structure. The test windows prepared are analyzed by pressure endurance and leak measurements and their X-ray transmission properties are presented.