Method for applying a carbon-based reflective outer coating to a grazing incidence optical unit

Abstract

A method for applying a carbon-based reflective outer coating to a grazing-incident optical unit, the grazing-incident optical unit comprising a substrate and a coating of a high-density material or a low-density material, such as carbon or B4C, selected from gold, platinum, iridium, palladium, rhodium, ruthenium, chromium, and nickel; the method comprising the step of treating the optical unit with a solution or vapor phase containing at least one polymer precursor material to produce the outer coating by absorbing the polymer material onto the coating.

Description

[0001] Cross-reference to related applications

[0002] This patent application claims priority to European Patent Application No. 19191474.6, filed on August 13, 2019, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to a method for applying a carbon-based reflective outer coating to a grazing incidence optical unit. Background Technology

[0004] The present invention finds its preferred (though not exclusive) application in the manufacture of mirrors for X-ray telescopes, which will be specifically referenced below without any loss of generality.

[0005] The term "optical unit" as used here includes monolithic mirrors and segmented or perforated modules.

[0006] Due to the opacity of the Earth's atmosphere to the X-ray wavelengths under consideration, X-ray telescopes can only operate in space. However, the use of reflecting optics for X-rays can also be applied to other fields, such as medical physics, radiographic imaging, and studying materials by irradiating them with X-ray beams generated by particle accelerators (e.g., synchrotrons).

[0007] In grazing-incident X-ray mirrors used in space applications, operating in the so-called "classical" X-ray region (soft X-rays, energy levels below 10 keV), high-density reflective coatings (e.g., gold, platinum, or iridium) are typically used. This allows for the extension of the energy band in which effective reflection exists for the same reflection angle, or for the extension of the critical angle for total internal reflection for the same energy. In fact, the critical angle for total internal reflection is θ. c It is proportional to the reciprocal of the energy E of the incident X-ray, and also proportional to the square root of the density ρ of the reflecting material:

[0008]

[0009] However, due to photoelectric absorption, especially near the absorption edge (particularly in the 0.5keV to 4keV spectral region), the high density of the material (and therefore also with a higher atomic number Z) causes greater attenuation of the reflected beam.

[0010] This results in a situation where low-density films (e.g., C, B4C, and B) have higher reflectivity (close to 100%) in total reflection schemes, but are limited in band structure, while those with higher densities (Au, Ire, Pt, W, Cr, Ni) have wider band structures, but lower reflectivity.

[0011] Because dual-reflection systems are typically used, the effect of reducing the reflectivity of heavy elements is particularly unfavorable in astronomical optics due to photoelectric absorption.

[0012] To overcome this problem, it has been proposed to use a low-density material layer based on carbon or similar materials as an outer coating of the high-density metal layer. This solution is known, for example, from the following publications:

[0013] V.Cotroneo, D.Spiga, M.Barbera, R.Bruni, K.Chen et al., Carbon overcoatings for soft x-ray reflectivity enhancement, Proc.SPIE 6688, Optics for EUV, X-Ray, and Gamma-Ray Astronomy III, 66880U (20September 2007); and

[0014] V.Cotroneo, D.Spiga, R.Bruni, W.Burkert, M.Freyberg et al., New developments in light material overcoating for soft x-ray reflectivity enhancement, Proc.SPIE 7011, Space Telescopes and Instrumentation2008: Ultraviolet to GammaRay, 701119 (15July 2008).

[0015] In this way, greater reflectivity can be achieved at low energies while maintaining a wide passband. For more information, see [reference needed]. Figure 1 , Figure 1 This represents the reflectance calculated for a single-layer platinum, a single-layer carbon, and a platinum layer with a 10 nm thick outer coating.

[0016] Solutions based on carbon or B4C outer coatings have been proposed for several space missions, such as Athena (ESA), Lynx (NASA), and eXTP (CAS).

[0017] Applying an outer coating to a low-density material based on carbon (or boron, or its derivatives) is accomplished by deposition in a high vacuum via electron beam or Joule effect (physical vapor deposition) or by sputtering.

[0018] The above-mentioned high-vacuum application methods are very expensive and not applicable to all types of mirrors.

[0019] The first known case of application limitations for this process is that of mirrors formed from a single-piece shell, such as nickel with a gold coating produced by replication. This is one of the standard methods for producing astronomical mirrors with diameters ranging from 5 cm to 70 cm. Mirrors for the Beppo-SAX, XMM-Newton, Swift, eRosita, and Einstein Probe missions were fabricated using this method. The method provides the following steps:

[0020] • Fabricate an ultra-polished aluminum core rod with a nickel / phosphorus alloy coating to form the negative shape of the mirror;

[0021] • Deposit a gold coating approximately 100 nm thick on the mandrel;

[0022] • Electroform nickel walls onto a gold-plated core rod.

[0023] • Separate the nickel shell with a gold coating from the core rod (due to its low adhesion to the core rod, the gold layer acts not only as a reflective layer but also as a separating agent).

[0024] The use of known techniques for applying a carbon-based outer coating onto a known gold layer in conjunction with this production process is highly problematic, with both the assumption that the carbon layer is placed on the mandrel before the gold layer and the assumption that the carbon layer is placed on the gold layer after the shell is separated from the mandrel.

[0025] In fact, the carbon substrate cannot be deposited on the mandrel before the gold is deposited, because it would contaminate the evaporation chamber and, in any case, adhere to the mandrel, thus preventing replication.

[0026] Depositing a carbon base layer on the gold layer after the shell and mandrel are separated would require the use of, for example, a linear deposition source from inside the shell by sputtering; this method is only feasible for relatively large shells (diameter greater than 20 cm) and is very expensive in any case.

[0027] Monolithic mirrors can also be manufactured using other methods and materials, either through replication techniques or by adjusting the outer surface and direct ultra-cleaning. However, they also present similar challenges in applying an outer coating to a carbon-based material on a high-density reflective layer (gold, carbon, iridium, or tungsten).

[0028] Another known method for manufacturing large and small mirrors involves assemblies of mirror modules, each module consisting of stacks of silicon wafers or other materials (e.g., glass sheets) with parallel ribs to create a series of "holes". So-called silicon aperture optics (SPOs), produced by cold replication for the ATHENA (ESA) mission, are also included in this type of optics for X-rays.

[0029] If the reflector is of type Wolter-I, which consists of a parabolic portion and a hyperbolic portion connected in series, then the module is composed of two stacked wafers, namely a parabolic (SPO-P) and a hyperbolic (SPO-H), precisely bonded together.

[0030] In this context, the application of low-density layers is also problematic.

[0031] A high-density reflective coating, typically made of iridium, is applied to individual wafers by sputtering prior to assembly. The application of a carbon layer is incompatible with the assembly method because carbon contaminates the processing chamber; alternative materials to carbon, such as boron, boron carbide (B4C), or silicon carbide, have been tested, but with unsatisfactory results due to structural resistance (the tendency for outer coating delamination), application difficulties, and / or unsatisfactory optical performance. Summary of the Invention

[0032] The object of the present invention is to provide a method for applying a reflective carbon-based outer coating to a grazing incidence optical element, which solves the problems associated with the known processes described above.

[0033] The above objective is achieved by a method for applying a reflective carbon-based outer coating to a grazing incidence optical unit, the optical unit comprising a substrate and a high-density coating selected from gold, platinum and iridium, tungsten, chromium and nickel, the method comprising the step of immersing the optical unit in a solution or gas phase containing at least an organic precursor material to induce deposition of the precursor material on the high-density material coating.

[0034] In this way, even very thin layers of carbon-based materials can be deposited, typically with a thickness of about 1 nm to 2 nm. By repeating this method, layers of the desired thickness, such as 6 nm to 10 nm, can be obtained.

[0035] If desired, the method may include the step of exposing the outer coating to an ultraviolet (UV) radiation source or other radiation (laser or X-ray) or high temperature to remove oxygen and (if desired) hydrogen from the polymer chain.

[0036] Brief description of the attached figures

[0037] To better understand the present invention, two preferred embodiments are described below by way of non-limiting examples and with reference to the accompanying drawings, wherein:

[0038] Figure 1 The reflectivity of different coating materials is shown as a function of incident ray energy;

[0039] Figures 2 to 6 The sequential steps of the method of the present invention for manufacturing a monolithic shell mirror are illustrated schematically.

[0040] Figures 7 to 9 The sequential steps of the method of the present invention for manufacturing a segmented optical module (SPO) are illustrated schematically.

[0041] Best mode for carrying out the present invention

[0042] Reference Figure 2 The figure shows a monolithic housing optical unit used in astronomical X-ray mirrors.

[0043] Unit 1 comprises, for example, a monolithic outer shell 2 of nickel and an inner coating 3 of gold. Unit 1 can be manufactured in a known manner by a replication process on a mandrel, as described in the introduction of the specification.

[0044] According to one embodiment of the present invention ( Figure 3 Unit 1 is immersed in solution 4 or exposed to a gas phase containing an organic precursor material, said organic precursor material comprising one or more sulfur-containing organic compounds, preferably selected from alkyl thiols [HS(CH2]]. n X], alkyl disulfides [X(CH2)] m SS(CH2) n X] and alkyl sulfides [X(CH2)] m S(CH2) n [X]. Where X represents, for example, a terminal group composed of -CH3, -OH, or -COOH.

[0045] According to another embodiment of the invention, the precursor material comprises one or more organosilane compounds selected from chlorosilanes [X(CH2)]. n SiCl4] and alkoxysilanes [X(CH2] n Si(OR')], where X represents, for example, -CH3, -OH, -COOH, -NH2, -HC=CH2, -CH=CHCOO - -CH2OCH2, -SH, -CH=O or combinations thereof.

[0046] Alternatively, different precursor materials can be used successively, such as combinations of one or more sulfur-containing organic compounds and one or more organosilane compounds, as shown above.

[0047] The precursor is dissolved in a non-aqueous solvent such as an alcohol or anhydrous saturated and unsaturated hydrocarbons, including but not limited to hexane, heptane, hexadecane, toluene, chlorobenzene, ether, carbon disulfide, and chloroform.

[0048] A thicker outer coating 5, typically 6 nm to 10 nm thick, can be produced by continuous immersion.

[0049] According to one example of the outer coating process, a wafer pre-coated with an X nm thin gold layer was immersed in 200 ml of an anhydrous ethanol solution of 1 mM undecanoic acid for 24 hours. The wafer was then removed and thoroughly rinsed with ethanol. The resulting outer coating thickness was approximately [missing value]. By repeating this process, the thickness of the outer coating can be increased.

[0050] Optionally, the wafer treated with the molecular monolayer is immersed in 200 ml of a 2% v / v solution of additional aminopropyltrimethoxysilane (APTMS) in toluene for 2 to 4 hours to form an additional monolayer of the outer coating chemically bonded to the first layer via OHN bonds. This process can be repeated to increase the thickness of the outer coating. Optionally, the wafer treated with the bilayer is immersed in 200 ml of a 1 mM solution of octadecyltrichlorosilane (OTS) in hexane for 24 hours. The wafer is then thoroughly rinsed in hexane and heated in air at 120°C for 30 minutes. The third molecular layer increases the thickness of the outer coating by approximately 2.5 nm.

[0051] The aforementioned precursor materials tend to form a nanometer-thick molecular monolayer double coating on the gold layer through absorption. Figure 4 ).

[0052] Optionally, if the precursor material molecules contain oxygen, the outer coating 5 can be exposed to UV rays (e.g., by a UV lamp 6 disposed within the housing 2, see [link]). Figure 5 ), or other suitable radiation (laser or X-ray) to obtain an oxygen-free alkyl layer 7 ( Figure 6 ).

[0053] If necessary, a similar method (with necessary modifications) can be used to produce stacked, segmented optics, thereby forming so-called "hole" optics (silicon hole optics—SPO, if based on a silicon substrate).

[0054] In this case, conversely, if the monolithic shell is immersed in the solution, the individual segments or modules formed by the stack of segments 10 constituting the optical device ( Figure 8 It was submerged.

[0055] In one example of the process, a wafer pre-coated with an X nm iridium layer was activated by exposure to ozone plasma for 30 seconds. The wafer was then immersed in 200 ml of a 1 mM solution of octadecyltrichlorosilane (OTS) in hexane for 24 hours. The wafer was thoroughly rinsed with hexane and heated in air at 120°C for 30 minutes. The resulting monolayer had a thickness of approximately 2.5 nm.

[0056] According to another example of the process, a wafer pre-coated with an iridium layer of X nm was activated by exposure to ozone plasma for 30 seconds. The wafer was then immersed in 200 ml of a 1 mM solution of octadecyltrichlorosilane (OTS) in hexane for 7 days. The wafer was thoroughly rinsed with hexane and heated in air at 120°C for 30 minutes. The resulting monolayer had a thickness of approximately 9 nm.

[0057] If the material contains oxygen, the module can be exposed to UV rays, lasers, or X-ray sources to eliminate the oxygen. Figure 9 ).

[0058] At the end of the method (Figure 11), an oxygen-free alkyl outer coating is thus obtained on iridium.

[0059] The advantages that can be achieved by examining the characteristics of the method are obvious.

[0060] By employing immersion coating technology, all problems associated with known vacuum deposition processes (electron beam, physical vapor deposition, and sputtering) are eliminated.

[0061] Therefore, a low-density outer coating with optimal optical and physical properties is obtained at a significantly reduced cost compared to known technologies.

[0062] Furthermore, the method of the present invention is very simple and is not limited by the aforementioned applications; it can be used for any type of monolithic or segmented mirror. If desired, the method can be used to cover carbon or B4C films already deposited using high-vacuum processes (e.g., by Joule sputtering or evaporation) with a carbon-based film to protect the underlying films and make them more stable.

Claims

1. A method for applying a carbon-based reflective overcoat on a grazing incidence optical unit (1; 10) for X-rays, said optical unit (1; 10) comprising a substrate (2) and a coating (3) of a first high-density material selected from the group consisting of gold, platinum, iridium, palladium, rhodium, ruthenium, chromium and nickel, said method comprising the step of immersing said optical unit (1; 10) in a liquid solution (4) containing at least one organic precursor material, so as to cause absorption of said precursor material on said coating (3).

2. The method of claim 1, wherein, The at least one precursor material comprises an alkyl chain and at least one of the functional groups -CH3, -OH, -COOH, -NH2, -HC=CH2, -CH=CHCOO - , -CH2OCH2, -SH, and -CH=O.

3. The method of claim 2, wherein, Said precursor material is a material selected from the group consisting of alkyl mercaptans, alkyl disulfides and alkyl sulfides.

4. The method of claim 1, wherein, Said precursor material comprises oxygen.

5. The method of claim 1, wherein, Said precursor material comprises silicon.

6. The method of claim 5, wherein, Said at least one precursor material is an organosilane.

7. The method of claim 4, wherein, Said method comprises, after the step of immersing in said liquid solution, the step of exposing said optical unit (1; 10) to a source of radiation (6) suitable for removing oxygen from said precursor material.

8. The method of claim 1, wherein, Said optical unit (1) comprises a monolithic housing (2) of nickel and said first material is gold.

9. The method of claim 8, wherein, The material of said overcoat (5, 7) is a sulfur-containing organic compound.

10. The method of claim 1, wherein, Said optical unit is a module (10) of an SPO optical device consisting of a stack of at least one silicon wafer and said first material consists of iridium.

11. The method of claim 10, wherein, The material of said overcoat is a silicon-containing organic compound.

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