Fabrication process for 200 nm-pitch polished freestanding ultrahigh aspect ratio gratings

A fully integrated fabrication process has been developed to fabricate freestanding, ultrahigh aspect ratio silicon gratings with potassium hydroxide (KOH)-polished sidewalls. The gratings are being developed for wavelength-dispersive, soft x-ray spectroscopy on future space telescopes. For this application, the grating needs to have a large open-area fraction and smooth sidewalls (roughness < 1 nm) to maximize efficiency. The prototype gratings fabricated with the process presented here have been tested on a synchrotron beamline and have demonstrated an absolute diffraction efficiency greater than 30% for 2 nm-wavelength x-rays in blazed orders. This efficiency is greater than twice the efficiency of previously fabricated gratings. The fabrication process utilizes silicon-on-insulator wafers where the grating and a cross support are etched in the device layer, and an additional structural support is etched in the handle layer. The device layer and handle layer are both etched via deep reactive-ion etching using a Bosch process. The buried SiO₂ layer stops both etches and is removed at the end of the process to create a freestanding structure. The gratings have a pitch of 200 nm, a depth of 4 μm, and the bars are polished via KOH. The polishing process reduces both the roughness and the grating-bar thickness. The finished gratings span an area of approximately 10 by 30 mm, supported by 1 mm-wide hexagons in the handle layer.