Manufacture and performance of blazed soft x-ray transmission gratings for Arcus and Lynx

The soft x-ray band covers the characteristic lines of the highly ionized low-atomic-number elements, providing diagnostics of the warm and hot plasmas in star atmospheres, interstellar dust, galaxy halos and clusters, and the cosmic web. High-resolution spectroscopy in this band is best performed with grating spectrometers. Soft x-ray grating spectroscopy with R = λ / Δ λ = > 104 has been demonstrated with critical-angle transmission (CAT) gratings. CAT gratings combine the relaxed alignment and temperature tolerances and the low mass of transmission gratings with high diffraction efficiency blazed in high orders. They are an enabling technology for the proposed Arcus grating explorer and were selected for the Lynx Design Reference Mission grating spectrometer instrument. Both Arcus and Lynx require the manufacture of hundreds to perhaps ~2000 large-area CAT gratings. We are moving toward CAT grating volume manufacturing using 200 mm silicon-on-insulator wafers, 4X optical projection lithography tools, deep reactive-ion etching, and KOH polishing. We have, for the first time, produced high-throughput 200 nm-period CAT gratings ~50% deeper than previously fabricated. X-ray diffraction efficiency is significantly improved in the ~1:25 - 1.75 nm wavelength range, peaking above 40% (sum of blazed orders). A new grating-to-grating alignment technique utilizing cross-support diffraction of visible light is presented, as well as the results of CAT grating emissivity measurements.λ