X-ray/VUV transmission gratings for astrophysical and laboratory applications

Free-standing and membrane-supported transmission gratings are very useful for applications in VUV, X-ray, and matter-wave diffraction and spectroscopy. We will describe the techniques used to fabricate deep-submicron-period transmission gratings and review some of the applications for these diffractors. Our fabrication process begins with holographic lithography to define a master grating pattern. Spatial periods as small as 0.2 µm (5000 lines mm⁻¹) are routinely achieved, and a new technique, called achromatic holographic lithography, has achieved a period of 0.1 µm (10 000 lines mm⁻¹). Holography is followed by SiO₂ shadow-evaporation, reactive-ion etching, and gold liftoff or electroplating to transform this pattern into absorbers for C K (45 Å) or Cu L (13 Å) X-rays. This patterning is performed on the membrane of an X-ray mask, which is then printed using X-ray lithography. The X-ray lithography process transfers the master pattern into a thick-resist coated substrate, which is then gold, silver, or nickel electroplated to form the final grating pattern. These structures can be supported on 0.5-1.0 µm-thick polyimide membranes, or made free standing by the support of a metal mesh. Progress in this area has been fueled primarily by the requirements of the Advanced X-ray Astrophysics Facility (AXAF) X-ray telescope, for which thousands of square centimeters of high-quality transmission gratings are required. However, other applications, including X-ray nanolithography development, solar astronomy, X-ray and matter-wave interferometry, and laboratory VUV/X-ray spectroscopy are also driving this technology.