Objective Multilayer mirrors are widely used as X-ray monochromators in synchrotron radiation facilities. Compared with crystal monochromators, multilayers have variable period thicknesses and can be applied at different energies. At the same time, the energy bandwidth of the multilayers is 1-2 orders of magnitude larger than that of the crystals, which can provide higher photon flux. As mirrors in synchrotron radiation beamlines operate under grazing incidence conditions, larger mirrors are usually required to fully receive the beam. In addition, a double-channel multilayer composed of two different structural material pairs is usually deposited on the surface of the mirror to make the beamline cover a wider energy range. In recent years, China's synchrotron radiation facilities have been continuously upgraded and built, including the Shanghai Synchrotron Radiation Facility (SSRF) and Beijing High Energy Photon Source ( HEPS). In some beamlines, single-channel multilayer mirrors are no longer sufficient, and double-channel multilayer mirrors are required. Driven by these applications, a large-size double- channel multilayer mirror is developed in this paper. Methods The double- channel multilayers used a combination of W/ Si and Ru/ C multilayers, and Ru/C multilayers and W/ Si multilayers work in the energy range of 10-18 keV and 18-25 keV, respectively. The W/Si and Ru/ C multilayer samples are fabricated in a linear magnetron sputtering system. The base pressure before the deposition is 9. 5x10- 5 Pa and the working gas uses high-purity argon ( volume fraction of 99. 999%). A series of experiments are first carried out on Si wafers mainly to optimize the quality and thickness uniformity of the multilayers. The uniformity in the length direction can be ensured as long as the stability of the motion rate is guaranteed, and the uniformity in the width direction can be controlled by installing a crescent-shaped mask in front of the target. Then, W/ Si and Ru/C double-channel multilayers are deposited on the surface of a high-precision Si plane mirror based on the optimized results. The areas of the two multilayer stripes are both 320 mmx20 mm, and the interval is less than 3 nm. After deposition, the multilayer samples are characterized by grazing incidence X-ray reflectometry (GIXR) at 8. 04 keV using an X-ray diffractometer. The GIXR curve is fitted by IMD software to obtain thickness, density, and interface width. The non- specular scattering tests of the multilayers are also conducted on an X- ray diffractometer. The surface morphologies of the multilayers are measured by atomic force microscopy (AFM) and then one-dimensional power spectrum density (PSD) functions are calculated. Results and Discussions The average interface width of the deposited W/ Si multilayers is 0. 30 nm, and the thickness varies from 2. 894 nm to 2. 918 nm over the 320 mm length (Fig. 4). In the range of 320 mm length and 20 mm width, the root mean square (RMS) error of the thickness is 0. 30% and 0. 19%, respectively. The average interface width of the Ru/ C multilayers is 0. 32 nm, and the thickness varies from 3. 060 nm to 3. 095 nm over the 320 mm length (Fig. 7). In the range of 320 mm length and 20 mm width, the RMS error of the thickness is 0. 39% and 0. 20%, respectively. The surface roughness of the W/Si multilayers and the Ru/ C multilayers is 0. 098 nm and 0. 139 nm, respectively ( Fig. 8). In the spatial frequency range of 1-7 mu m- 1, the PSD of the Ru/C multilayers is less different from that of the W/Si multilayers, while in the spatial frequency range of 7- 50 mu m-1, the PSD of the Ru/C multilayers is significantly higher than that of the W/Si multilayers. The surface roughness of the Ru/ C multilayers is larger than that of the W/Si multilayers, which is consistent with the GIXR test results. The non-specular scattering results indicate that the interface roughness of the W/Si multilayers is smaller than that of the Ru/C multilayers (Fig. 9). Based on the above researches, W/ Si and Ru/ C double- channel multilayers are deposited on the surface of a Si plane mirror with a size of 350 mmx 60 mmx50 mm. The measured thicknesses of both multilayer stripes are around 3. 06 nm. The estimated reflectivity of the W/Si multilayers at 8. 04 keV is 68%, and the reflectivity at 18. 00 keV, 21. 50 keV, and 25. 00 keV is 70%, 76%, and 81%, respectively. The reflectivity of the Ru/ C multilayers at 8. 04 keV is 65%, and the reflectivity at 10. 00 keV, 14. 00 keV, and 18. 00 keV is 72%, 79%, and 82%, respectively. Conclusions A W/Si and Ru/C double-channel multilayer mirror is fabricated in this paper. After process optimization, within the range of 320 mm length and 20 mm width, the RMS error of the thickness of the W/Si multilayer is 0. 30% and 0. 19%, and that of the Ru/C multilayers is 0. 39% and 0. 20%, which has almost reached the world- class level. Finally, on the basis of the optimized experimental results, W/Si and Ru/C multilayers are deposited on a high-precision Si plane mirror with a size of 350 mmx60 mm in two stripes, and the estimated reflectivity ( 8. 04 keV) is 68% and 65%, respectively. The multilayer mirrors can meet the requirements of the beamline and are successfully applied in the membrane protein beamline of SSRF. In future research, uniformity can be improved by increasing mask fabrication, mounting accuracy, and substrate movement rate stability.
