IntroductionX-ray multilayer enables an efficient reflection of X-rays in specific wavelength, serving as an important optics for obtaining monochromatic X-ray. The period thickness of multilayers typically falls within a nano-scale, and it is generally composed of two kinds of thin films alternately due to the short wavelength of X-ray. The multilayer can realize a high reflection of specific wavelength X-ray based on the Bragg diffraction, yielding a pure monochromatic X-ray. Compared to crystal monochromators, the period of multilayer can be adjusted readily, and the multilayer can be suitable for X-rays with different energies. The energy bandwidth of reflected X-ray obtained by the multilayer is 1–2 orders of magnitude broader than that of the crystals, thus providing a higher photon flux. At present, X-ray multilayer is widely used in synchrotron radiation for applications such as X-ray imaging, and X-ray small-angle scattering. According to the working principle of X-ray mutlilayer, the period of multilayer is only a few nanometers for the efficient X-ray reflection at a larger grazing angle due to the short wavelength of X-ray. Therefore, the coating method with a high precision thickness control is essential to prepare X-ray multilayer. Magnetron sputtering is the primary preparation method for X-ray multilayers, although achieving a high precision thickness control remains a challenge. Atomic layer deposition (ALD) method can realize uniform growth of single atomic layer thickness film on substrate and the film thickness control is extremely accurate because of the special film growth process. The ALD method has unique advantages in the preparation of small-period multilayer. It is identified that HfO₂/Al₂O₃ is an effective material pair for X-ray multilayer at a wavelength of 0.154 nm and the X-ray reflectivity is acceptable.MethodsTo further investigate the properties of HfO₂/Al₂O₃ X-ray multilayer prepared by the ALD and explore its application, HfO₂/Al₂O₃ X-ray multilayer with a period of 3.8 nm and a period number of 60 was prepared by the ALD method. The surface 3D profile of the multilayer was obtained by a model ICON2-SYS atomic force microscope (AFM, Bruker Co., Germany), and the surface roughness of the multilayer was analyzed by a software named NanoScope Analysis. The microstructure of multilayer was determined by a model Talos F200s G2 transmission electron microscope (TEM, Thermo Fisher Scientific Co., USA) with a software named Digital Micrograph. The X-ray reflectivity of the multilayer was measured by a model D8 Discover X-ray diffractometer (XRD, Bruker Co., Germany) at X-ray wavelength of 0.154 nm. A software named IMD was used to fit the measured reflectance data. The fitting was based on a two-layer model, and the structure parameters of multilayer was obtained, including period and duty cycle. The multilayer was also analyzed by a modle AL-Y3500 diffractometer (Dandong Aolong Ray Instrument Group Co., Ltd., China). Before the installation of multilayer, the X-rays emitted from the source were directly illuminated on the sample by a collimator. After the installation of multilayer, the X-rays emitted from the ray source were firstly reflected by the multilayer and then irradiated on the sample. The diffraction patterns before and after installing multilayer were analyzed. This work consisted of three main parts. Firstly, a software named Fluent was used to simulate the gas flow distribution according to the structure of ALD equipment chamber. The results show that the gas flow rate is uniform, indicating that the chamber is suitable for preparing a film with uniform thickness. Secondly, HfO₂/Al₂O₃ multilayer is prepared by the ALD method, and the surface roughness, period thickness uniformity and X-ray reflectivity are characterized. Finally, the multilayer is installed on the X-ray diffractometer to test the diffraction patterns of a silicon sample. The results show that the quality of the diffraction patterns is improved after the installation of the multilayer.Results and discussionThe characterization results show that the surface root-mean-square roughness of the multilayer is 0.77 nm, the maximum deviation of the period is 0.11 nm and the maximum X-ray (i.e., 0.154 nm) reflectivity of the multilayer is approximately 45%. The multilayer is in an amorphous state, and the interface between two layers is clear and sharp. The XRD pattern of Si powder sample is obtained without and with the HfO₂/Al₂O₃ multilayer in X-ray diffractometer. The results indicate that the background intensity of the pattern with multilayer reduces, and no diffraction peaks of other X-ray appear. The reason is that the X-rays irradiated on the sample are composed of high-intensity Cu characteristic X-rays, other stray energy X-rays and low-intensity bremsstrahring radiation when the multilayer is not installed. After the multilayer is installed, the pure Cu kα characteristic X-rays are irradiated on the sample and thus there is basically no interference peak.ConclusionsThis work investigated the characteristics of HfO₂/Al₂O₃ multilayer prepared by the ALD method and explored its application in X-ray diffractometers. The results showed that HfO₂ and Al₂O₃ layers were amorphous with a maximum X-ray (0.154 nm) reflectivity of approximately 45%. The quality of the diffraction patterns after the installation of the multilayer was improved, demonstrating a potential application of the HfO₂/Al₂O₃ multilayer.