Deposition and Performance of Depth-Graded W/Si Multilayers on Silicon Pore Optics for the WXPT Mission

The development of high-performance depth-graded multilayer coatings is critical for next-generation hard X-ray astronomical telescopes, such as the proposed Wide-band X-ray Polarization Telescope (WXPT) mission. This work focuses on the fabrication and performance evaluation of W/Si depth-graded multilayers designed for WXPT. The coatings were deposited using a custom-built linear direct-current magnetron sputtering system. Grazing-incidence X-ray reflectometry (XRR) and cross-sectional transmission electron microscopy (TEM) revealed consistently smooth interfaces with widths of approximately 0.3-0.4 nm, demonstrating exceptional layer uniformity and minimal cumulative roughening even over hundreds of layers. Atomic force microscopy confirmed the ultra-smooth surface morphology (RMS roughness \((<)\) 0.3 nm) of the final coatings. The critical impact of substrate roughness on multilayer reflectivity was quantitatively demonstrated. Most importantly, hard X-ray reflectivity measurements, both from a laboratory source and synchrotron radiation, showed excellent agreement with theoretical simulations assuming an interface width of \((\sigma)\) = 0.4 nm. The coatings achieved high reflectivity across a broad energy band (e.g.,25% at 20-30 keV for a design optimized for 0.3\((^\circ)\) incidence), successfully meeting the performance targets. These results establish a robust and reliable fabrication pathway for producing high-quality W/Si multilayers, marking a significant step towards their application in SPO for the WXPT mission.