Metrology for quality control and alignment of CAT grating spectrometers

Arcus, a mission proposed as a Medium Size Explorer for high-resolution x-ray spectroscopy, requires unprecedented sensitivities: high resolving power (λ/Δλ >; 2500) and large collecting area (~ 350 cm2). The core instruments on Arcus are Critical-Angle Transmission (CAT) grating spectrometers consisting of hundreds of co-aligned diffraction gratings. The gratings require thorough quality control along the entire manufacturing process: from bare silicon wafers to CAT grating petal assembly. Period variation, grating bar tilt angles, misalignment, and grating film buckling are potential errors of interest which could degrade the performance of the x-ray grating spectrometer. We present progress towards development of metrology techniques to measure and manage aforementioned errors during the entire alignment and integration processes: starting right after fabrication of CAT grating membranes to their assembly into large arrays. A scanning laser reflection tool (SLRT) was developed to measure period variations, alignment, and area percentage of pinched grating bars. An array of four CAT gratings was successfully aligned to satisfy Arcus alignment allocations for a grating window alignment test (GWAT). No discernible signal was found from an effort to measure a ‘half’ diffraction order to characterize stiction between grating bars. A metrology protocol was developed to measure grating bar tilt angle variations and average bar tilt angles relative to the grating surface normal, based on small-angle x-ray scattering (SAXS, Cu-Kα) and an optical surface normal measurement (OSNM) setup. A grating holder was designed with integrated slits to relate independent measurements from two different setups using visible and x-ray beams. Bar tilt variations of 1 degree and average bar tilt angles of ~0.3 degree were observed for seven different CAT grating samples. Bar tilt angle variations induced from buckled grating films were also measured. We discuss implications for a more demanding CAT grating spectrometer for the proposed Lynx X-ray Surveyor mission to be presented to the next Astrophysics Decadal Survey.