Soft x-ray performance and fabrication of flight-like blazed transmission gratings for the x-ray spectrometer on arcus probe

Abstract

In response to Astro2020 recommendations, NASA has issued an announcement of opportunity for an astrophysics probe explorer. Arcus-Probe is a mission concept featuring two co-aligned high-resolution spectrometers, one for the soft x-ray band, and one for the far ultraviolet. This combination of instruments will provide unprecedented performance for the exploration of feedback that shapes the structure and influences the dynamics of our universe across many scales, ranging from galaxy clusters down to individual stars. In the soft x-ray band the x-ray spectrometer features four parallel channels, comprised of arrays of Silicon Pore Optics (SPO) and Critical-Angle Transmission (CAT) gratings. The channels will provide a combined effective area ⪆350 cm2 near O VII (approximately 470 cm2 expected), and resolving power R = λ/Δλ ⪆2500 (approximately 3500 expected) across the approximately 12 − 50 Å wavelength range. Light-weight, high-efficiency, blazed and alignment-insensitive CAT gratings have been shown to meet or exceed Arcus requirements for diffraction efficiency, open area, and effective resolving power. A total of 864 to approximately 32 × 32.5 mm2 CAT grating facets must be fabricated for Arcus, implying efficient volume production. We describe the x-ray testing and performance of the most recent flight-like grating facets, made from 200 mm Silicon-on-Insulator (SOI) wafers using tools compatible with volume production. Newly acquired SOI wafers with 100 nm device layer thickness control are expected to increase fabrication yields, and ongoing post-fabrication thinning of freestanding CAT grating bars promises increases in diffraction efficiency.