Improved efficiency critical-angle transmission gratings for high-resolution soft x-ray spectroscopy

High resolving power soft x-ray spectroscopy has been confirmed by the Astro2020 Decadal Survey as a highpriority strategic measurement technique with resolving power R = λ/Δλ up to 7500 for some science cases. Examples are the characterization of highly ionized gases in galaxy halos and within and around galaxy clusters, accretion onto supermassive black holes, stellar coronal mass ejections and coronal heating. Arcus, a recently proposed high-resolution x-ray and FUV grating spectrometer Probe class mission, exceeds current capabilities by far, with a minimum R of 2500 (∼ 3500 expected) and effective area up to 500 cm² in the 10-50 Å band, covered by the X-ray Spectrometer (XRS) instrument. The XRS relies on light-weight, high-efficiency, blazed and alignment-insensitive critical-angle transmission (CAT) gratings for dispersion and calls for hundreds of ∼ 30 × 30 mm² gratings. Recent gratings have been fabricated from 200-mm silicon-on-insulator (SOI) wafers using commercial tools from the semiconductor and MEMS industries compatible with volume production. Previously we reported x-ray results from quasi-fully illuminated co-aligned CAT gratings showing record-high R up to 1.3×10⁴ in 18th and 21st diffraction orders at Al-K (∼ 1.5 keV), and diffraction efficiency in agreement with synchrotron measurements and model predictions at O-K. We were recently able to chemically reduce the width of the freestanding, 200 nm-period, ultra-high aspect ratio CAT grating bars post-fabrication, and we report on the resulting increase in diffraction efficiency.