Characterization of soft x-ray critical-angle transmission gratings for the REDSoX polarimetry sounding rocket

The Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX) is a sounding rocket instrument that was selected by NASA to make the first measurement of the linear x-ray polarization of an astrophysical source in the 0.2-0.4 keV band. The optical design employs Wolter-I focusing mirrors, followed by critical-angle transmission (CAT) gratings to disperse x-rays onto dispersion-matched laterally-graded mulitlayers, which in turn reflect x-rays near the Brewster angle for a broadband, polarization-sensitive response. The instrument contains three parallel channels with 120-degree azimuthal rotation between them to achieve polarization modulation factors over 90%. About 50 CAT gratings (∼ 38×11mm2 each) are being fabricated to cover the required aperture. Over 40 gratings have been produced to date and are at various stages of metrology and characterization. Here we present our methods and the current status. CAT gratings feature 200 nm-period, freestanding, ultra-high aspect ratio grating bars. They are fabricated from 200mm silicon-on-insulator wafers using optical projection lithography, deep reactive-ion etching, KOH polishing, critical-point drying, and oxide removal via HF vapor. Grating bar tilt is measured using small-angle x-ray scattering and used for grating alignment. Diffraction efficiency performance in the soft x-ray band is characterized on our in-house beamline. We also present recent diffraction efficiency improvements for deeper CAT gratings with reduced support structure blockage.