X-ray optical components are ones applied to the X-ray range and are widely employed in synchrotron radiation, free-electron lasers, high-energy astronomical observation, laboratory X-ray detection, and other scientific instruments. Among them, X-ray multilayers are important reflective optical components. Due to the short wavelength of X-rays, the multilayer period is usually in the order of a few to several tens of nanometres. In the case that the incident angle remains unchanged, the multilayer period decreases with the wavelength of X-rays. When the multilayer period is reduced to a few nanometres, the defects such as interface width and roughness will significantly reduce the X-ray reflectivity. Therefore, high-precision film preparation techniques are essential for fabricating X-ray multilayers with small periods. Several methods including ion beam sputtering, magnetron sputtering, and atomic layer deposition (ALD) have been adopted to prepare X-ray multilayers. Compared with other techniques, ALD shows advantages in achieving highly conformal films with precise control of film thicknesses on the order of angstroms. Thus, it has great potential for preparing multilayers with small periods. We study the preparation of an X-ray multilayer with small periods by the ALD method. Based on the film types that can be prepared by ALD, we calculate the X-ray (0.154 nm) reflectivity of four multilayers which consist of HfO₂/Al₂O₃, Ir/Al₂O₃, Ru/Al₂O₃, and W/Al₂O₃ respectively. We also further analyze the effects of the structural parameters of multilayers on the reflectivity including periodic thickness, duty ratios, and number of periods. Based on these results, the HfO₂/Al₂O₃ multilayer with period of 4 nm, number of periods of 60, and duty ratio of 0.5 is designed and prepared by ALD.

In the theoretical part, we adopt the Fresnel coefficient recursion method to calculate the X-ray reflectivity of multilayers with different layer materials, periodic thickness, duty ratios, and number of periods. The influence of these parameters on the X-ray reflectivity is investigated. Based on the calculated results, the HfO₂/Al₂O₃ X-ray multilayer with periodic thickness of 4 nm, number of periods of 60, and duty ratio of 0.5 is designed. In the experimental part, ALD is applied to achieve HfO₂ and Al₂O₃ films. For each film, in a growth cycle, two reactants are employed as precursors and they react to form films on the substrate surface in a surface self-limiting growth mode. The film thickness is controlled by the cycle numbers. As for testing methods, ThermoFisher's Scios 2 dual-beam system is adopted to obtain a cross-section sample of the multilayer that is suitable for transmission electron microscope (TEM) observation. Meanwhile, the structure of the multilayer film is observed by JEOL JEM-2100F TEM. The X-ray reflectivity of the multilayer is tested on the Beijing synchrotron radiation 1W1A line station with an X-ray wavelength of 0.154 nm. Before the test, the multilayer is placed on a horizontal stage, and the positions of the sample stage and detector are adjusted. The data of X-ray intensity at different grazing angles are acquired and fitted by IMD software. The parameters and X-ray reflectivity of the multilayer are obtained from the fitted results accordingly.

Figure 7 shows the TEM images of the cross-section of the HfO₂/Al₂O₃ multilayer at different magnifications. The interface between HfO₂ and Al₂O₃ is relatively sharp. However, the thickness of HfO₂ is slightly larger than that of Al₂O₃ in one period, which indicates that a small interdiffusion exists between the layers. The results of measured and fitted X-ray reflectivity of the HfO₂/Al₂O₃ multilayer are shown in Fig. 8. We find that two Bragg diffraction peaks appear at 1.15° and 2.23° respectively and the widths of the diffraction peaks are small, which reveals that the film deposition rate is stable and the thicknesses of each layer in the multilayer keep almost the same. By analyzing the fitted data, the X-ray reflectivity of the multilayer film is about 43%, which is a little lower than the theoretical value. The main reasons probably are the relatively large roughness of the Si substrate and the interdiffusion between the layers. For example, the roughness of the Si substrate can be transferred to the layers accumulatively, which leads to an increase in the scattering of X-rays and a decrease in the reflectivity.

We study the preparation of X-ray multilayers by ALD technique. X-ray (0.154 nm) reflectivity of the multilayer in ideal conditions with different layer materials and structural parameters is calculated. Additionally, we also discuss the effects of layer materials, periodic thickness, duty ratios, and number of periods on the X-ray reflectivity in detail. The calculated results show that the X-ray reflectivity of HfO₂/Al₂O₃ multilayer with periodic thickness of 4 nm, duty ratio of 0.5, and number of periods of 60 is 53%. On this basis, the HfO₂/Al₂O₃ multilayer film is prepared by ALD. TEM results of the multilayer show a relatively sharp interface between the layers. X-ray reflectivity results indicate that the X-ray reflectivity of the multilayer is about 43%, which shows the great potential of the ALD method for preparing X-ray multilayers with small periods.