Partitioned coating method for space reflector

Abstract

The invention relates to the technical field of optical coating, in particular to a partitioned coating method for a spatial extreme ultraviolet band reflector, which aims to coat different or same films on more than two or more regions at certain intervals. The method comprises the following steps: firstly, designing more than two or more EUV wave band multilayer films, wherein the structures and the central wavelengths of the EUV multilayer films can be different; then designing an adjustable shielding coating tool which is in direct contact with the mirror surface of the reflecting mirror; the EUV wave band is plated on the surface of the cleaned reflecting mirror in a regional mode through an adjustable shielding film plating tool; and finally, testing the reflecting mirror after film coating. According to the invention, the boundary of the coating area is clear and has no shadow effect, and stray light crosstalk is reduced; and the film is good in film forming quality, small in film stress, excellent in optical performance, good in film firmness and high in space environment adaptability.

Description

Technical Field

[0001] The invention belongs to the technical field of optical coating, and in particular relates to a partition coating method for a space reflector. Background Art

[0002] At present, multi-band extreme ultraviolet (EUV) solar imaging detection has become a hot topic for research in various countries. Under strong engineering constraints such as weight and volume, the multi-band extreme ultraviolet imager realizes narrow-band monochromatic solar imaging of extreme ultraviolet multi-bands through a set of optical systems integrated inside it. The optical system usually adopts an axisymmetric structure, and the primary and secondary reflectors in the optical system are coated in different areas to achieve multi-layer thin films with different central wavelengths and bandwidths, meeting the requirements of lightweight, multi-band, and high integration.

[0003] Foreign countries started research on the scientific payload of multi-spectral EUV imagers early. Since the 1990s, they have successively developed extreme ultraviolet imaging telescopes for solar and heliospheric detectors, extreme ultraviolet cameras for the solar-terrestrial relations platform, atmospheric imaging components for the Solar Dynamics Observatory, and solar ultraviolet imagers for the U.S. Geostationary Environmental Observatory Satellite. my country has mastered the technology of single-channel EUV imagers. The extreme ultraviolet imager payload on the Fengyun-3E satellite currently in orbit is a 19.5nm channel. The extreme ultraviolet imager on the Queqiao-2 relay satellite is a 30.4nm and 83.4nm channel. However, my country currently does not have a partitioned coated solar telescope in orbit. The Fengyun-4 meteorological satellite series payload under development will soon be equipped with a multi-spectral extreme ultraviolet imager, and the solar polar orbit plan proposed by the Space Center of the Chinese Academy of Sciences will also be equipped with a multi-spectral extreme ultraviolet imager. Among them, the partitioned coating technology of the reflector is an important key technology.

[0004] Compared with visible light and infrared bands, the defects of EUV films have a more significant impact on reflectivity and stray light. EUV multilayer films are very thin in single layer, have many film layers, are difficult to prepare, and have high process requirements. Multi-spectral EUV space reflectors not only include the difficulties in preparing EUV multilayer films, but also have to overcome the partition coating technology, which makes them more difficult. Because the film bandwidth of each channel is extremely narrow, the central wavelength accuracy requirements are more stringent. High-precision EUV coatings have long been blocked by foreign technology, and my country currently does not have a high-precision partition coating method used in multi-spectral space telescopes.

[0005] Existing partition coating methods are all used for industrial production, such as lens production, and are not suitable for EUV bands. They all use simple clamping for shielding coating. There are large film stresses, and the boundary overlaps of different regions and the shadow effect of the coating will cause stray light crosstalk in different bands, resulting in inaccurate imaging accuracy. At the same time, there are many mechanisms for the generation of film stress. Excessive stress can cause film wrinkles, peeling and cracking. Among them, internal stress, thermal stress and coating tool clamping stress are the most important. Internal stress is also called intrinsic stress, which can be divided into two categories: growth stress and interface stress. The corresponding causes are the incomplete growth structure of the film, such as vacancies, impurities, dislocations, stacking faults and other defects, and lattice mismatch between the substrate and the film. Thermal stress is caused by the different thermal expansion coefficients of the substrate and the film, and the thermal effect during the preparation process causes stress. Therefore, it is necessary to design a method for partition coating of space reflectors that can eliminate coating stress, have clear boundaries of the coating area without shadow effects, have good film quality, excellent optical properties, good film firmness, and easy preparation.

[0006] In addition, the main reason for the reduction in reflectivity of multilayer films is the oxidation of the last layer of material. Generally, a cap layer is added to the outermost layer of the multilayer film (the outermost layer among the multilayer surfaces, in direct contact with the air) to reduce the oxidation effect. Commonly used cap layer materials include Si, W, Ir and Pt. Taking Si as a cap layer as an example, Si as a cap layer can not only form a smooth and stable film layer, but also effectively prevent oxidation from penetrating into the interior of the multilayer film, and has good optical properties. Si as a cap layer generally forms an oxide layer about 2nm thick in the air, and then forms a stable state. Therefore, the Si cap layer is generally thicker than 2nm to prevent oxidation from occurring inside the multilayer surface below the cap layer, destroying the multilayer film structure, and causing a significant decrease in the optical performance of the multilayer film. Summary of the invention

[0007] In view of this, the invention aims to provide a partition coating method for space reflectors. First, multiple EUV band multilayer films with different central wavelengths are designed, and then an adjustable shielding coating tool that can directly contact the surface of the reflector is designed. The device is used to perform partition coating of multiple central wavelengths on the cleaned ultra-smooth microcrystalline substrate. The invention can achieve clear boundaries of the coating area without shadow effects, reduce stray light crosstalk; and the film has good film quality, excellent optical properties, good film firmness, and strong adaptability to space environments.

[0008] To achieve the above object, the technical solution created by the present invention is implemented as follows: A partition coating method for a space reflector comprises the following steps: S1: Design of multilayer films with more than 2 EUV bands; S2: Design an adjustable shielding coating tooling that is in direct contact with the mirror surface of the reflector; S3: using the adjustable shielding coating tool in step S2 to coat the EUV band multilayer film in step S1 on the surface of the cleaned reflector in different regions; S4: Test the coated reflector.

[0009] Furthermore, step S1 includes the following steps: S11: Determine the material of each EUV band multilayer film; S12: Determine the initial period thickness d0 of each EUV band multilayer film according to the following formula: ; Where m represents the Bragg diffraction order of the incident light in each EUV multilayer film, represents the central wavelength of the incident light, represents the average weighted refractive index ε of all materials in each EUV multilayer, represents the angle between the incident light and each EUV multilayer film; S13: Using a genetic algorithm to optimize the initial period thickness, obtain the period thickness range of the multilayer film in each EUV band.

[0010] Furthermore, each EUV-band multilayer film includes alternately deposited spacer layers and absorption layers; one spacer layer and one absorption layer constitute a period, the sum of the thickness of the spacer layer and the absorption layer in one period is the period thickness, and the initial period thickness is the sum of the thickness of the initial absorption layer and spacer layer in one period; the ratio of the absorption layer thickness in one period to the period thickness is the thickness ratio.

[0011] Furthermore, the adjustable shielding coating tooling in step S2 includes an embracing base, a hollow frame and a lightweight frame; wherein the reflector is placed in the embracing base; the hollow frame is located on the outside of the embracing base, and is fixed together with the embracing base on the rotating disk of the external coating machine; the lightweight frame is arranged above the hollow frame and is fixedly connected to the hollow frame; the lightweight frame adjusts the gap between the hollow frame and the reflector through no less than 4 adjustable top screws; a transparent protective cover covering the lightweight frame is also arranged above the lightweight frame, and adjustment holes corresponding to the adjustable top screws are provided on the transparent protective cover.

[0012] Furthermore, the embracing base includes a base and an embracing module; wherein the embracing module embraces the reflector and is placed inside the base together with the reflector.

[0013] Furthermore, the hollow frame includes a main frame and a hollow shielding cover; wherein the main frame is placed outside the base and is positioned and connected to the base; the lightweight frame is connected to the main frame and covers the hollow shielding cover; The hollow shielding cover is connected to the upper part of the main frame and covers the reflector; a coating area is reserved on the hollow shielding cover so that an external coating machine can coat the reflector through the coating area.

[0014] Furthermore, a cross frame is provided on the lightweight frame, and all adjustable top screws are distributed in a V shape on the cross frame. The hollow shielding cover is squeezed by rotating the adjustable top screws, thereby adjusting the distance between the hollow shielding cover and the reflector.

[0015] Further, step S3 includes the following steps: S31: cleaning the reflector; S32: Detecting the substrate surface of the reflector; S33: placing the reflector in an adjustable shielding coating tool, and using the adjustable shielding coating tool to coat the EUV band multilayer film in step S1 on the surface of the reflector in different regions by a DC power sputtering method.

[0016] Furthermore, in step S32, the surface of the reflector is checked under a strong light with a dark background to find that there are no obvious scratches and pits, and the number of surface particles of the reflector is checked line by line under a dark-field microscope, and the number of particles per square millimeter of field of view in the dark-field microscope is less than 9.

[0017] Further, in step S33, after the reflector is placed in the adjustable shielding coating tooling, a buffer layer is firstly coated on the surface of the reflector, and then an EUV band multilayer film is coated on the surface of the buffer layer.

[0018] Compared with the prior art, the invention can achieve the following beneficial effects: (1) The partitioned coating method of the space reflector created by the present invention can realize partitioned coating to obtain a multi-spectral band, narrow-band space reflector, wherein the prepared thin film has good film quality, high reflectivity in multiple bands, narrow bandwidth, good adhesion, and strong adaptability to the space environment; the reflector prepared by the partitioned coating has low stress, the film uniformity is better than 2%, and the surface shape change is less than 1 / 300λ, which is conducive to high-quality imaging. At the same time, the reflector prepared by the partitioned coating has high position accuracy of the uncoated interval area in each quadrant, clear film boundaries, and no shadow effect of the film, which greatly reduces the stray light crosstalk problem between multiple bands; (2) In the partition coating method of the space reflector created by the present invention, DC power sputtering is selected, which has a stable and controllable process. By adjusting the appropriate power, the distance between the target material and the substrate and the gas pressure and other process parameters, a stable sputtering rate of about 0.5A / S is obtained, and the film accuracy can be controlled within an error of less than 0.1A, so that the thickness can be accurately controlled in the process of preparing EUV band multilayer films; (3) In the partition coating method of the space reflector created by the present invention, the internal stress can be reduced by using a genetic algorithm to reasonably optimize the thickness ratio; the DC power sputtering method used at the same time does not require heating and does not generate high temperature, and can effectively reduce thermal stress by reducing the temperature difference; at the same time, by coating a buffer layer between the reflector surface and the EUV band multilayer film, the stress caused by the difference in thermal expansion coefficient between the reflector surface and the EUV band multilayer film can be effectively reduced; and the film adhesion can also be improved; (4) The reflector prepared by the partition coating method of the space reflector created by the present invention can be used for imaging instruments for solar observation, which is of great significance to the development of multi-spectral solar observation imaging payloads in my country; the method is finally combined with effective detection means to improve the uniformity and stability of the coating, and the partition coating device effectively improves the production efficiency, increases the yield of the reflector, and reduces the cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings constituting part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation on the present invention. In the drawings: Figure 1 A schematic flow chart of a method for coating a space reflector in a partitioned manner according to an embodiment of the present invention; Figure 2 A schematic diagram of the top view of the adjustable shielding coating tooling described in an embodiment of the present invention; Figure 3 A schematic cross-sectional structure diagram of the adjustable shielding coating tooling along the AA cross section described in an embodiment of the present invention; Figure 4 A schematic cross-sectional structure diagram of the adjustable shielding coating tooling along the BB cross section described in an embodiment of the present invention; Figure 5 A schematic diagram of the structure of the adjustable shielding coating tooling as viewed from above according to an embodiment of the present invention; Figure 6 A distribution diagram of four confocal targets in a vacuum chamber of a coating machine described in an embodiment of the invention; Figure 7 The corresponding relationship between the primary and secondary reflectors of the four-band zone coating and the schematic diagram of each area; Figure 8 The reflectivity curves of four coating areas on the mirror coated with EUV multilayer film; Fig. 9 SAXRD curves of four coating areas on the mirror coated with EUV multilayer film; Fig.10 Schematic diagram of the surface changes of the secondary mirror and primary mirror before and after coating.

[0020] Description of reference numerals: 1. Surrounding base; 101. Base; 102. Surrounding module; 103. Metal top screw; 2. Hollow frame; 201. Main frame; 202. Hollow shielding cover; 203. Mounting table; 3. Lightweight frame; 4. Reflector; 5. Adjustable top screw; 6. Positioning pin; 7. Transparent protective cover; 8. Coating machine. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solution and advantages of the invention more clear, the invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described here are only used to explain the invention and do not constitute a limitation of the invention.

[0022] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0023] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0024] In the description of the invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the invention can be understood according to specific circumstances.

[0025] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments.

[0026] like Figure 1As shown, the partition coating method of the space reflector described in the embodiment of the present invention comprises the following steps: S1: Design no less than 3 EUV band multilayer films; the central wavelengths of all EUV band multilayer films may be the same or different. In the embodiment of the present invention, four EUV band multilayer films with central wavelengths of 9.4nm, 17.1nm, 21.1nm and 30.4nm are designed. Step S1 includes the following steps: S11: Determine the material of each EUV band multilayer film. Each EUV band multilayer film includes alternately deposited spacer layers and absorption layers, a spacer layer and an absorption layer are combined into a repeating unit, that is, a period, and the sum of the thickness of the spacer layer and the absorption layer of the repeating unit is the period thickness, that is: d=d1+d2; Where d represents the period thickness, d1 represents the thickness of the spacer layer, and d2 represents the thickness of the absorption layer. In a repeating unit, there is a thickness ratio τ between the absorption layer thickness d2 and the period thickness d, that is, τ=d2 / d. The layer in contact with the reflector can be a spacer layer or an absorption layer. In an EUV band multilayer film, the thickness of the spacer layer of each repeating unit can be the same or different; the thickness of the absorption layer of each repeating unit can be the same or different. The spacer layer and the absorption layer use different materials. The absorption coefficient of the spacer layer is small, and the absorption layer has a larger Fresnel coefficient. In the embodiment of the present invention, the material of each EUV band multilayer film is determined according to the paper "Sub-quarter-wave multilayer coatings with high reflectance in the extreme ultraviolet" published in the journal "Applied Optics", specifically: the material of the absorption layer in the EUV band multilayer film with a central wavelength of 9.4nm is molybdenum (Mo), and the material of the spacer layer is yttrium (Y); the material of the absorption layer in the EUV band multilayer film with central wavelengths of 17.1nm, 21.1nm and 30.4nm are all molybdenum (Mo), and the material of the spacer layer is all silicon (Si).

[0027] S12: Determine the initial period thickness d0 of each EUV band multilayer film according to the Bragg formula, that is: ; Where m represents the Bragg diffraction order of the incident light in each EUV multilayer film; represents the central wavelength of the incident light; represents the average weighted refractive index η of all materials in each EUV multilayer film, and the refractive index η is the difference between 1 and the real part of the refractive index n, that is, , ,and , represents the extinction coefficient; The above formula shows the relationship between the central wavelength and period thickness of each EUV band multilayer film.

[0028] Since the EUV band multilayer film is an extreme ultraviolet narrowband multilayer film, the requirements for period thickness are very strict. If there is a slight deviation, the central wavelength will shift. Therefore, in the embodiment of the present invention, the initial period thickness is inferred from the central wavelength position of the EUV band multilayer film, and a method of alternately preparing two materials with different refractive indices to form a 1 / 4λ wave stack is adopted. When the period thickness is half of the wavelength, interference enhancement will be formed between different periods to increase the reflectivity. Because the average refractive index is small in the X-ray and EUV band range, is approximately 0, and the above formula can be simplified to: ; Among them, when m=1 and d0=1 / 2λ, the initial period thickness is approximately half of the central wavelength of the EUV band, so the initial period thicknesses of the four EUV band multilayer films (central wavelengths are 9.4nm, 17.1nm, 21.1nm and 30.4nm) are 4.7nm, 8.55nm, 10.55nm and 15.2nm respectively.

[0029] S13: Use a genetic algorithm to optimize the initial period thickness to obtain the period thickness range of each EUV band multilayer film. In the embodiment of the present invention, IMD software and Macleod software are used to perform spectral simulation and film layer structure optimization on EUV band multilayer films. The optical constants of the EUV band of Mo, Si and Y elements are from the handbook "Handbook of Optical Constants of Solids" written by Edward D. Palik. The optimization module in IMD software and Macleod software optimizes the initial period thickness through a genetic algorithm, and the ranges of the period thickness d and thickness ratio τ of the four EUV band multilayer films are obtained as follows: The periodic thickness d of the EUV band multilayer film with a central wavelength of 9.4 nm is 4.74-4.81 nm, and the thickness ratio τ ranges from 0.32 to 0.38; the material of the outermost thin film of the EUV band multilayer film in contact with the air is Mo, and its thickness ranges from 1.5 to 1.9 nm; when the period number N of the EUV band multilayer film is greater than 100, the theoretical reflectivity of the EUV band multilayer film is greater than 40%, and the bandwidth is 0.1 nm.

[0030] The period thickness d of the EUV band multilayer film with a central wavelength of 17.1 nm is 8.8-9.0 nm, and the thickness ratio τ ranges from 0.17 to 0.22; when the period number N of the EUV band multilayer film is greater than 35, the theoretical reflectivity of the EUV band multilayer film is greater than 53%, and the bandwidth is 0.8 nm.

[0031] The periodic thickness d of the EUV band multilayer film with a central wavelength of 21.1 nm is 10.9-11.1 nm, and the thickness ratio τ ranges from 0.14 to 0.17; when the period number N of the EUV band multilayer film is greater than 37, the theoretical reflectivity of the EUV band multilayer film is greater than 42%, and the bandwidth is 1.1 nm.

[0032] The period thickness d of the EUV band multilayer film with a central wavelength of 30.4nm is 16.1-16.5nm, and the thickness ratio τ ranges from 0.14 to 0.17; when the period number N of the EUV band multilayer film is greater than 22, the theoretical reflectivity of the EUV band multilayer film is greater than 23%, and the bandwidth is 2nm. The material of the outermost film in contact with the air of the EUV band multilayer film with central wavelengths of 17.1nm, 21.1nm and 30.4nm is Si, and its thickness ranges from 25 to 45nm.

[0033] S2: Design an adjustable shielding coating tooling that is in direct contact with the mirror surface of the reflector, and the number of coating areas of the adjustable shielding coating tooling is consistent with the number of EUV band multilayer films. For example, if 4 EUV band multilayer films need to be coated, the number of coating areas in the adjustable shielding coating tooling should be 4.

[0034] like Figures 2 to 5 As shown, the adjustable shielding coating tool in step S2 includes an embracing base 1, a hollow frame 2 and a lightweight frame 3. The reflector 4 is placed in the embracing base 1. The hollow frame 2 is placed outside the embracing base 1 and is fixed together with the embracing base 1 on the rotating disk of the external coating machine, and the lightweight frame 3 is placed above the hollow frame 2 and is fixedly connected to the hollow frame 2. The gap between the hollow frame 2 and the reflector 4 is adjusted by at least 4 adjustable top screws 5 located on the lightweight frame 3.

[0035] The embracing base 1 includes a base 101 and an embracing module 102. The embracing mold ring 102 embraces the reflector 4 and is placed inside the base 101 together with the reflector 4. On the side wall of the base 101, there are at least one top screw hole evenly arranged along the circumference of the base 101. In this embodiment, one top screw hole is preferably provided. The metal top screw 103 passes through the top screw hole and contacts the embracing module 102. By rotating the metal top screw 103 to push the embracing module 102, the base 101 and the embracing module 102 hold the back step surface of the reflector 4 tightly, thereby realizing the axial and radial positioning of the reflector 4, preventing the reflector 4 and the adjustable shielding coating tooling from relative movement during the coating process, ensuring the accuracy of the partition coating position, and the clamping process does not affect the surface shape of the reflector 4. In the embodiment of the present invention, the material of the base 101 is aluminum alloy. At the same time, in order to reduce the clamping stress of the reflector 4, the material of the surrounding module 102 is selected to be a flexible material with low outgassing and stable chemical properties. In the embodiment of the present invention, the surrounding module 102 preferably uses a polytetrafluoroethylene material.

[0036] The hollow frame 2 includes a main frame 201 and a hollow shielding cover 202. The main frame 201 is placed outside the base 101 and is coaxially positioned and connected to the base 101 through four positioning pins 6. The hollow shielding cover 202 is connected to the top of the circular main frame 201 through countersunk screws so that the hollow shielding cover 202 covers the reflector 4.

[0037] The embodiment of the present invention takes the four-quadrant zone coating of the reflector 4 as an example, and a 1 / 4 quadrant coating area is reserved on the hollow shielding cover 202, so that the external coating machine performs four-quadrant zone coating on the reflector 4 through the coating area. It should be noted that the adjustable shielding coating tooling provided in the embodiment of the present invention only takes the four-quadrant zone coating as a specific embodiment, and corresponds to a 1 / 4 quadrant coating area reserved on the hollow shielding cover 202. The division and setting of the coating area can be adaptively adjusted according to actual conditions, including but not limited to three-area, five-area, six-area, N-area coating, etc. The coating area reserved on the hollow shielding cover 202 can be correspondingly adjusted to 3 quadrants, 5 quadrants, 6 quadrants, N quadrants, etc. (N=3,4,5,...).

[0038] The bottom of the main frame 201 is provided with no less than three mounting platforms 203 along the circumference of the main frame 201. All the mounting platforms 203 are connected to the rotating disk of the external coating machine by screws, so that the hollow frame 2 as a whole together with the surrounding base 1 are installed on the rotating disk of the external coating machine to ensure that the coating tooling and the reflector 4 do not produce relative displacement during the coating rotation process to cause position deviation.

[0039] In the embodiment of the present invention, the main frame 201 and the base 101 are made of the same material, which is aluminum alloy. At the same time, the embodiment of the present invention preferably uses polyimide to make the hollow shielding cover 202, and the hollow part of the hollow shielding cover 202 has a cutting edge design, and its curved surface is consistent with the surface curvature of the reflector 4. Polyimide has good mechanical processing performance, moderate hardness, and elasticity. It can deform slightly under external pressure; the amount of degassing under vacuum is very small, which does not affect the coating quality. Even if the reflector 4 is accidentally touched, scratches, pitting and other defects will not be generated on the mirror surface of the reflector 4, which is friendly to the mirror surface of the reflector 4.

[0040] The lightweight frame 3 is connected to the main frame 201 through the countersunk screws on the side wall of the lightweight frame 3 and covers the hollow shielding cover 202. A cross frame is provided on the lightweight frame 3, and all the adjustable top screws 5 are evenly distributed on the cross frame in a V shape. By rotating the adjustable top screws 5 to squeeze the hollow shielding cover 202, the hollow shielding cover 202 undergoes uniform elastic deformation, and the micrometer-level gap between the hollow shielding cover 202 and the mirror surface of the reflector 4 is controlled, or the mirror surface of the reflector 4 is infinitely close to eliminate the coating shadow effect. The angle of the cross frame is adjusted according to the coating area on the hollow shielding cover 202. For example, when four areas are coated, the angle of the cross frame is 90°; when five areas are coated, the angle of the cross frame is 72°; when six areas are coated, the angle of the cross frame is 60°. The adjustable shielding coating tooling described in the embodiment of the invention also includes a dustproof transparent protective cover 7, which covers the lightweight frame 3 and is provided with an adjustment hole corresponding to the adjustable top screw 5. When the gap between the hollow shielding cover 202 and the mirror surface of the reflector 4 needs to be adjusted, a screwdriver is inserted into the adjustment hole to screw the corresponding adjustable top screw 5. The transparent protective cover 7 has the advantage of visual adjustment, which keeps the mirror surface of the reflector 4 clean and protects the safety of the mirror surface, preventing the screwdriver from accidentally scratching the mirror surface. If coating is required, the transparent protective cover 7 needs to be removed before coating.

[0041] S3: Use the adjustable shielding coating tool in step S2 to coat the EUV band multilayer film in step S1 on the surface of the cleaned reflector in different regions. The material of the reflector can be fused quartz, microcrystals or silicon, etc. In order to improve the reflectivity at the center wavelength of the EUV multilayer film and suppress stray light, the reflector needs to be ultra-smoothly polished, and the roughness (RMS) of the mirror surface is better than 0.3nm. The partition coating process includes three parts: mirror cleaning, mirror inspection and mirror partition coating. These three steps are completed in a Class 1000 clean room.

[0042] Specifically, step S3 specifically includes the following steps: S31: Cleaning the reflector. In the embodiment of the present invention, a dust-free cloth is used with AR-grade anhydrous ethanol and acetone to wipe off stubborn stains on the reflector, and then ultrapure water with a resistivity of >18MΩ·cm mixed with a neutral detergent is used to clean the reflector in an ultrasonic wave for 5-8 minutes, and finally a centrifuge is used to spin dry the reflector at 300-1500r / min, and no water stains are left.

[0043] S32: Detect the surface of the reflector. In the embodiment of the present invention, the reflector is detected under a dark background and strong light in a Class 1000 clean room to ensure that there are no obvious scratches, pits or other defects on the surface. Then, a dark field microscope is used to detect the number of particles in the field of view of the reflector. The reflector can be used only when the number of particles per square millimeter in the field of view of the reflector is less than 9.

[0044] S33: placing the reflector in an adjustable shielding coating tool, first coating the surface of the reflector with a buffer layer, and then using the adjustable shielding coating tool on the surface of the buffer layer, using a DC power sputtering method to coat the EUV band multilayer film in step S1 on the surface of the reflector in different regions. Among them, adding a buffer layer to the multilayer film structure is an effective method for controlling stress. The buffer layer is generally located between the substrate (i.e., the surface of the reflector) and the deposited EUV band multilayer film, and its function is to effectively reduce the stress caused by the difference in thermal expansion coefficient between the substrate and the multilayer film; it can also improve the adhesion of the film and enhance the firmness of the substrate and the EUV band multilayer film directly; it can also adjust the growth conditions of the multilayer film, adjust the lattice structure, and help the film form a more uniform microstructure. There are many materials that can be used as a buffer layer. In the embodiment of the present invention, the material of the buffer layer is commonly used Cr material, which has the characteristics of lattice structure matching, high thermal stability and enhanced adhesion. In some other specific embodiments, Ti can also be used as a buffer layer. Ti buffer layer is usually used between metal film and non-metal substrate to increase the adhesion between the film and substrate, and can be used as a seed layer to prepare ultra-smooth multilayer films.

[0045] Combination Figures 2 to 6 In the embodiment of the present invention, the regional coating process is specifically as follows: First, install the reflector 4 in the adjustable shielding coating tooling. Specifically: the reflector 4 is placed in the base 101, and the metal top screw 103 is rotated to squeeze the embracing module 102, so that the base 101, the embracing module 102 and the reflector 4 are locked. Then the hollow shielding cover 202 is connected to the main frame 201, and the base 101 is connected to the main frame 201 through the positioning pin 6. The positioning pin 6 can be used for precise positioning of the installation and adjustment, and the coating area is strictly divided evenly. Then the lightweight frame 3 is connected to the main frame 201 and covers the hollow shielding cover 202, and the adjustable top screws 5 are distributed and installed on the cross frame. Finally, the transparent protective cover 7 is installed on the lightweight frame 3 according to the positional relationship between the adjustable top screws 5 and the adjustment holes. After the reflector 4 is installed in the adjustable shielding coating tooling, the interior of the adjustable shielding coating tooling is evacuated. In the embodiment of the present invention, the vacuum degree is set to be higher than 5×10 -4 Pa.

[0046] When starting to coat the reflector 4 in different regions, remove the transparent protective cover 7, and install the adjustable shielding coating tooling provided in the embodiment of the present invention on the rotating disk of the external coating machine through screws, and introduce argon gas Ar with a purity of 99.999% into the adjustable shielding coating tooling. The working pressure during coating is less than 0.1 Pa. First, coat the EUV band multilayer film with central wavelengths of 17.1nm, 21.1nm and 30.4nm, and then coat the EUV band multilayer film with a central wavelength of 9.4nm. In the embodiment of the present invention, the coating is carried out by using a vacuum chamber of a coating machine combined with a four-target confocal DC power sputtering method, such as Figure 6 As shown, there are 4 coating targets in the vacuum chamber of the coating machine, marked as target #1, target #2, target #3 and target #4, corresponding to the four coating areas of the adjustable shielding coating tooling. Since the two materials in the four coating areas are symmetrically spaced, target #1 and target #3 are plated with a spacing layer of Si material or Y material, and target #2 and target #4 are plated with an absorption layer of Mo material. During coating, the rotating disk drives the adjustable shielding coating tooling and the reflector 4 to rotate at a speed of 30-60rad / min, so that the uniformity of the period thickness of the reflector is better than 2%.

[0047] When a multilayer film of EUV band with a central wavelength of 17.1nm, 21.1nm and 30.4nm is plated on the surface of the reflector 4 by a DC power sputtering method, targets #1 and #3 are plated with a spacer layer of Si material, targets #2 and #4 are plated with an absorption layer of Mo material, the purities of Si material and Mo material are 99.999% and 99.99% respectively, and the powers are 50W and 75W respectively; when a multilayer film of EUV band with a central wavelength of 9.4nm is plated on the surface of the reflector 4 by a DC power sputtering method, targets #1 and #3 are plated with a spacer layer of Y material, targets #2 and #4 are plated with an absorption layer of Mo material, the purities of Y material and Mo material are 99.95% and 99.99% respectively, and the powers are 50W and 25W respectively.

[0048] The hollow shielding cover 202 in the adjustable shielding coating tool has 1 / 4 of the hollow left, and the coating area on the hollow shielding cover 202 is rotated to achieve regional coating of EUV band multilayer films with specific central wavelengths. After the EUV band multilayer film of the current area is plated, the hollow shielding cover 202 is adjusted to change the shielding area, and the films of the four bands are plated in four times. The effect after coating is as follows Figure 7 As shown, Figure 7 (a) shows the effect after coating the main mirror in the optical system, the surface of the first coating area I of the main mirror is coated with an EUV band multilayer film with a central wavelength of 9.4nm, the surface of the second coating area II of the main mirror is coated with an EUV band multilayer film with a central wavelength of 17.1nm, the surface of the third coating area II of the main mirror is coated with an EUV band multilayer film with a central wavelength of 30.4nm, and the surface of the fourth coating area IV of the main mirror is coated with an EUV band multilayer film with a central wavelength of 21.1nm. Figure 7 (b) shows the effect after coating the secondary mirror in the optical system, the surface of the first coating area I of the secondary mirror is coated with an EUV band multilayer film with a central wavelength of 17.1nm, the surface of the second coating area II of the secondary mirror is coated with an EUV band multilayer film with a central wavelength of 9.4nm, the surface of the third coating area III of the secondary mirror is coated with an EUV band multilayer film with a central wavelength of 21.1nm, and the surface of the fourth coating area IV of the secondary mirror is coated with an EUV band multilayer film with a central wavelength of 30.4nm.

[0049] After completing the above EUV band multilayer film operation, the EUV band multilayer film is annealed. Annealing the EUV band multilayer film is an effective post-processing method, which is mainly affected by process conditions such as annealing temperature, annealing time, atmosphere control and cooling method. A reasonable annealing process can repair and reduce defects in the deposition process of the multilayer film. The grains in the multilayer film have the opportunity to recrystallize and rearrange, which helps to release the stress of the multilayer film, improve the crystal structure of the multilayer film, improve the uniformity of the multilayer film and reduce the roughness.

[0050] S4: Testing the coated reflector. In the embodiment of the present invention, the coated reflector is subjected to spectral testing, film quality testing, stress testing, adhesion testing, and space environment adaptability testing. The adjustable shielding coating tooling provided in the embodiment of the present invention is combined with a variety of effective detection methods to improve the uniformity and stability of the coating. The partitioned coating device effectively improves production efficiency, improves the yield of the reflector, and reduces costs.

[0051] When the reflector after coating is subjected to spectrum test, the embodiment of the present invention adopts the U27 spectrum radiation standard of Hefei Synchrotron Radiation Laboratory and the measurement terminal station to perform spectrum test. The test results are as follows: Figure 8 As shown in the figure, the reflectivity of the coating area corresponding to the central wavelength of 21.1nm on the reflector is 30.3%, and the bandwidth is 1.2nm; the reflectivity of the coating area corresponding to the central wavelength of 17.1nm on the reflector is 42.3%, and the bandwidth is 0.8nm; the reflectivity of the coating area corresponding to the central wavelength of 9.4nm is 26.6%, and the bandwidth is 0.12nm; the reflectivity of the coating area corresponding to the central wavelength of 30.4nm is 23.1%, and the bandwidth is 2.7nm. The reflectivity and bandwidth of the four coating areas on the reflector have reached international standards.

[0052] When the film quality of the coated reflector is tested, the embodiment of the present invention uses an X-ray diffractometer (XRD, the test conditions are tube voltage of 40KV, tube current of 200mA), with a copper target of wavelength λ=0.154nm (Cu-Kα), to perform a small angle scan (SAXRD) from 0.5° to 0.01° in a grazing incidence manner on the coated reflector. The test results are as follows: Fig. 9 As shown, Fig. 9 (a) shows the XRD small-angle diffraction curve of the EUV multilayer film with a central wavelength of 21.1 nm. Fig. 9 (b) shows the XRD small-angle diffraction curve of the EUV multilayer film with a central wavelength of 30.4 nm. Fig. 9 (c) shows the XRD small-angle diffraction curve of the EUV multilayer film with a central wavelength of 9.4 nm. Fig. 9 (d) shows the XRD small-angle diffraction curve of the EUV multilayer film with a central wavelength of 17.1 nm. Fig. 9 It can be seen that the SAXRD curves of the four coating areas measured have high intensity, clear peaks, narrow half-height widths, and many diffraction orders, indicating that the film quality of each coating area is good, the film structure is clear, and the diffusion between film layers is small, so the reflectivity is high, which is consistent with Figure 8 The reflectivity curve in is consistent.

[0053] When stress testing is performed on the coated reflector, the ZYGO interferometer is used to measure the surface data of the reflector before and after coating. The test results are as follows: Fig.10 shown. Fig.10 (a) shows the bare mirror surface of the secondary mirror in the optical system. Fig.10 (b) shows the surface shape of the secondary mirror after the secondary mirror is coated in different areas. Fig.10 (c) shows the bare mirror surface of the primary mirror in the optical system. Fig.10 (d) shows the surface shape of the primary mirror after the area coating is performed. Fig.10From (a) and (c) in Figure 1, it can be seen that the RMS of the secondary mirror and the primary mirror are 0.016λ (where λ represents the wavelength of the laser emitted by the ZYGO interferometer) and 0.013λ respectively when they are not coated. Fig.10 (b) shows that the RSM of the four coating areas on the secondary mirror are 0.018λ, 0.018λ, 0.017λ and 0.013λ, and the RMS changes are 0.002λ, 0.002λ, 0.001λ and -0.003λ respectively; Fig.10 (d) shows that the RSM of the four coating areas on the primary mirror are 0.014λ, 0.017λ, 0.014λ and 0.015λ, respectively, and the RMS changes are 0.01λ, 0.03λ, 0.01λ and 0.02λ, respectively. It can be seen that the surface shape changes of the primary mirror and the secondary mirror before and after coating are less than 1 / 300λ, and the coating stress is extremely small. It can be seen that the coating effect of the partition coating method provided in the embodiment of the present invention is good.

[0054] Since the load has a large acceleration and strong vibration when the rocket takes off, the film is required to have good mechanical properties and strong adhesion, so it is necessary to conduct vibration, impact and adhesion tests on the coated reflector. The specific process of the adhesion test is to use a tape with a width of 12~13mm and a peeling strength of not less than 6.86N according to GB / T 26331-2010 Test Method for Environmental Adaptability of Optical Thin Film Components. First, slowly pull up the coated reflector three times, and then quickly pull up the coated reflector three times. Use a strong light to detect the surface of the reflector, and it is found that the film on the coated reflector does not fall off or peel off, and the surface morphology is good, indicating that the film adhesion is qualified.

[0055] Since the payload is surrounded by a cold black vacuum space when it is on orbit, and the temperature changes greatly, the reflector after coating is tested for adaptability to the space environment, specifically including vacuum high and low temperature thermal cycle tests, space particle irradiation tests (proton, electron, total dose irradiation), and ultraviolet irradiation. After the test, it was found that the EUV band multilayer film prepared by the embodiment of the present invention has basically unchanged bandwidth, reflectivity attenuation amplitude less than 20%, central wavelength position basically unchanged, and XRD small angle diffraction peak position after ultraviolet irradiation, electron irradiation, proton irradiation, and vacuum thermal cycle space environment, indicating that the film structure is complete, the multilayer film structure is not damaged after irradiation, and the optical performance is good; at the same time, it was found that the reflectivity, bandwidth and surface shape of the reflector after coating with EUV band multilayer film did not change significantly, meeting the requirements.

[0056] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the disclosure of the present invention can be performed in parallel, sequentially or in different orders, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and this document does not limit this.

[0057] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A partition coating method for a space reflector, characterized in that: The following steps are involved: S1: Design of multilayer films with more than 2 EUV bands; S2: Design an adjustable shielding coating tooling that is in direct contact with the mirror surface of the reflector; S3: using the adjustable shielding coating tool in step S2 to coat the EUV band multilayer film in step S1 on the surface of the cleaned reflector in different regions; S4: Test the coated reflector.

2. The partition coating method of a space reflector according to claim 1, characterized in that: Step S1 includes the following steps: S11: Determine the material of each EUV band multilayer film; S12: Determine the initial period thickness d0 of each EUV band multilayer film according to the following formula: ； Where m represents the Bragg diffraction order of the incident light in each EUV multilayer film, represents the central wavelength of the incident light, represents the average weighted refractive index ε of all materials in each EUV multilayer, represents the angle between the incident light and each EUV multilayer film; S13: Optimizing the initial period thickness using a genetic algorithm to obtain a range of period thickness and thickness ratio of the multilayer film in each EUV band.

3. The partition coating method of a space reflector according to claim 2, characterized in that: Each EUV band multilayer film includes alternately deposited spacer layers and absorption layers, one spacer layer and one absorption layer constitute a period, the sum of the thickness of the spacer layer and the absorption layer in one period is the period thickness, the initial period thickness is the sum of the thickness of the initial absorption layer and spacer layer in one period; the ratio of the absorption layer thickness in one period to the period thickness is the thickness ratio.

4. The method for coating a space mirror by partitioning according to claim 1, characterized in that: The adjustable shielding coating tooling in step S2 includes an embracing base, a hollow frame and a lightweight frame; wherein the reflector is placed in the embracing base; the hollow frame is located on the outside of the embracing base and is fixed together with the embracing base on the rotating disk of the external coating machine; the lightweight frame is arranged above the hollow frame and is fixedly connected to the hollow frame; the lightweight frame adjusts the gap between the hollow frame and the reflector through no less than 4 adjustable top screws; a transparent protective cover covering the lightweight frame is also arranged above the lightweight frame, and adjustment holes corresponding to the adjustable top screws are provided on the transparent protective cover.

5. The method for coating a space mirror according to claim 4, characterized in that: The embracing base includes a base and an embracing module; wherein the embracing module embraces the reflector and is placed inside the base together with the reflector.

6. The method for partition coating of a space reflector according to claim 4, characterized in that: The hollow frame includes a main frame and a hollow shielding cover; wherein the main frame is placed outside the base and is positioned and connected to the base; the lightweight frame is connected to the main frame and covers the hollow shielding cover; The hollow shielding cover is connected above the main frame and covers the reflector; the coating area is reserved on the hollow shielding cover, so that the external coating machine coats the reflector through the coating area.

7. The method for coating a space mirror by partitioning according to claim 6, characterized in that: A cross frame is provided on the lightweight frame, and all adjustable top screws are distributed in a V shape on the cross frame. The hollow shielding cover is squeezed by rotating the adjustable top screws, thereby adjusting the distance between the hollow shielding cover and the reflector.

8. The method for partition coating of a space reflector according to claim 1, characterized in that: Step S3 includes the following steps: S31: cleaning the reflecting mirror; S32: Detecting the surface of the reflector; S33: placing the reflector in the adjustable shielding coating tooling, and using the adjustable shielding coating tooling to coat the EUV band multilayer film in step S1 on the surface of the reflector in different regions by a DC power sputtering method.

9. The method for partition coating of a space reflector according to claim 8, characterized in that: In step S32, the surface of the reflector is checked under a strong light with a dark background to ensure that there are no obvious scratches or pits, and the number of surface particles of the reflector is checked line by line under a dark field microscope, where the number of particles per square millimeter of field of view in the dark field microscope is less than 9.

10. The method for partition coating of a space reflector according to claim 8, characterized in that: In step S33, after the reflector is placed in the adjustable shielding coating tooling, a buffer layer is firstly coated on the surface of the reflector, and then the EUV band multilayer film is coated on the surface of the buffer layer.

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