Fresnel zone plate (FZP) is one of the most important components in X-ray microscopic imaging systems, which can realize high-efficiency and high-resolution three-dimensional (3D) nondestructive imaging by diffraction principle. It is considered as one of the most potential devices to improve the quality of X-ray microscopic imaging and has been widely used in metal flaw detection, cell imaging, material testing, and other fields. High X-ray microscopic imaging quality means the high imaging resolution and diffraction efficiency wihch expects X-ray FZP with a large aspect ratio, that is, small outermost zone width and large thickness. The conventional fabrication technique of X-ray FZP is based on the most accurate lithography technique, namely e-beam lithography (EBL). However, the combination of large FZP thickness and small outermost zone width is usually out of the limits of EBL. Multilayer FZP (ML-FZP) is a promising solution to fabricate high-aspect-ratio FZP due to the unique sputter-sliced technique. Especially, the technique of atomic layer deposition (ALD) and focused ion beam (FIB) has emerged as an attractive combination to fabricate ML-FZP. Due to self-limiting surface reaction, ALD can realize atomic-scale precision in layer thickness and excellent conformality on the cylindrical substrate and is capable of coating many substrates simultaneously. The slicing and polishing processes both can be completed with FIB equipment. In order to meet the demand of a large aspect ratio of the zone plate for X-ray application, in this paper, multilayer films with high precision control of thickness were grown on a smooth surface of metal wire by ALD, and then ML-FZP with a large aspect ratio was obtained by FIB slicing.

First, The X-ray ML-FZP with Al₂O₃/HfO₂ as bright and dark ring materials was designed by the complex amplitude superposition method. Then, Al₂O₃/HfO₂ multilayer films were alternately deposited on the surface of gold wire with a diameter of 72 μm by ALD. The layers were grown in a double-cavity rapid ALD equipment at the substrate temperature of 150 ℃ with a chamber pressure of 0.2 Torr. Al₂O₃ and HfO₂ depositions were carried out by taking trimethyl aluminum (TMA) and tetrakis (dimethylamino) hafnium (TDMAH) as metal sources and H₂O as the oxygen source. TDMAH was heated at 75 ℃ during the deposition process. The deposition processes for Al₂O₃(HfO₂) were performed with TMA and TDMAH pulses of 0.02 and 0.08 s, N₂ purge of 10 and 15 s, H₂O pulse of 0.015 s, and another N₂ purge of 10 s, respectively. During the whole process, the deposition was stopped many times to clean the chamber or check the film growth rate due to the extremely long growth time. Some silicon slices were put in the chamber around the gold wires during the deposition. By monitoring the growth of multilayer films on silicon slices, the film growth rate was corrected in time. After the deposition of all the Al₂O₃/HfO₂ layers, the surface of the multilayer fiber was coated by a layer of Cr which was used to protect the multilayer structure from damage in subsequent processes. The large focused Ga⁺ beam was used for slicing, and the beam current was 65 nA. Then the small Ga⁺ beam of 5 nA was used for polishing the two surfaces of the zone plate. Finally, the X-ray FZP with a specific thickness was obtained. The prepared ML-FZP was applied to the soft X-ray imaging line station of Shanghai Synchrotron Radiation Facility (BL08U1A), and the focusing imaging function was realized under the X-ray of 1.2 keV. The resolving capability of the ML-FZP was analyzed by a gold-plated star test sample.

The uniformity of the Al₂O₃ and HfO₂ films deposited by ALD was below ±1% (Fig. 3). The multilayer film consisted of 356 layers with an extremely large zone thickness of 10.11 μm and outermost zone width of 25 nm, and for each layer, the film thickness control precision was below 2 nm (Fig. 4). Through slicing and polishing of FIB, the ML-FZP with a height 1.08 μm and an aspect ratio of 43∶1 was obtained. As shown in Fig. 7(f), the imaging resolution was about 800 nm. Although the ML-FZP showed imaging capacity, the properties could be further improved by optimizing the structure of ML-FZP. As the total zone thickness increased, special attention was required to minimize the imperfection of ML-FZP. In the next step, we will optimize the combination of multilayer films and use films with positive and negative stresses alternately to reduce or eliminate the adverse effect caused by stress, so as to improve the imaging resolution.

In this paper, the design and fabrication method of large-size X-ray ML-FZP and the imaging test were completed. First, the characteristics of Al₂O₃ and HfO₂ films grown by ALD were studied. The results showed that the uniformity of the films was below ±1%. Al₂O₃/HfO₂ multilayers with a thickness of 10.11 μm were alternately deposited on the surface of gold wire with a diameter of 72 μm, and a foil monitoring mechanism was proposed to achieve accurate control of each band width so that the error of the band width will not exceed 2 nm, and the outermost ring width will reach 25 nm. Then FIB technology was used for slicing and polishing, and an X-ray ML-FZP with an aspect ratio of 43∶1 was finally obtained. The imaging test of the zone plate was carried out at the BL08U1A line station, and the focusing imaging resolution of about 800 nm was achieved. The feasibility and great potential of preparing large-size X-ray zone plates by ALD and FIB slicing were successfully verified. Future priorities include searching for the appropriate way of film stress release to avoid cracks and the precise thickness control of all multilayer films for realizing imaging performance.