POLYIMIDE PLANARIZATION FOR INCREASED TRANSMISSION OF LARGE-AREA X-RAY WINDOWS

The substrate roughness has previously set the limit for the minimum t hickness of X-ray windows based on technology where planar tungsten gr ids 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 etchst op removal, which allows further reduction in the membrane thickness. A polyimide resin has been selected for the planarizing film, which ca n be removed either by solvents or by plasma etching. The fabrication process presented is capable of producing pressure-enduring X-ray wind ow 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. T he test windows prepared are analyzed by pressure endurance and leak m easurements and their X-ray transmission properties are presented.