Characterization and fabrication of high-performance B₄C-coated mirrors for X-ray free-electron lasers

The internal layer structure and optical performance of B₄C coating for X-ray free-electron laser (XFEL) applications were studied. Surface morphology analysis shows that the B₄C layer growth at 5 mTorr replicates the substrate morphology at low and mid frequencies but adds slight high-frequency roughness, with the RMS value increasing from 0.13 to 0.33 nm. To investigate the internal physical and chemical structure of B₄C, angle-resolved X-ray photoelectron spectroscopy (ARXPS) and depth profiling were performed. Together with soft X-ray (SXR) and hard X-ray (HXR) reflectivity measurements and the fitted results, a three-sublayer model with different composition and density was built for the 50 nm B₄C coating. The developed B₄C coating was further deposited on a 600 mm length mirror with a thickness variation of 0.7 nm (peak-to-valley). The radius of curvature changed slightly from 181 km before coating to 128 km after coating, and the slope errors were maintained at around 0.07 µrad, indicating a high-performance and large-size B₄C coating for the XFEL.