Method for applying a carbon-based reflective overcoating on a grazing incidence optical unit

Abstract

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

Description

Technical Field

[0001] Cross - reference to related applications This patent application claims the priority of European Patent Application No. 19191474.6 filed on August 13, 2019, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a method for applying a carbon - based reflective overcoating on an oblique - incidence optical unit.

Background Art

[0003] The present invention can find an exclusive but preferred application in the manufacture of mirrors for X - ray telescopes, and will be specifically referred to hereinafter without loss of generality.

[0004] In this specification, the expression "optical unit" is used to include both monolithic mirrors and modules with a segmented structure or pores.

[0005] X - ray telescopes can only operate in space considering that the Earth's atmosphere is opaque to X - ray wavelengths. However, the use of reflective optical systems for X - rays also finds applications in other fields, such as in medical physics, radiation imaging, and the study of materials by irradiation with X - ray beams generated by particle accelerators such as synchrotrons.

[0006] In the case of X - ray oblique - incidence mirrors for space applications, which operate in the so - called "classical" X - ray region (soft X - rays with an energy level of less than 10 keV), usually, high - density reflective coatings (e.g., gold, platinum, or iridium) are used. This makes it possible to widen the energy band where efficient reflection exists for the same reflection angle or to widen the critical angle of total reflection for the same energy. In fact, the critical angle θ c of total reflection is proportional to the reciprocal of the energy E of the incident X - ray and is proportional to the square root of the density ρ of the reflective material.

[0007]

Number

[0008] However, high-density materials (and thus also materials with a large atomic number Z) cause a greater attenuation of the reflected beam due to photoelectric absorption, especially in the vicinity of the absorption edge (especially in the spectral region between 0.5 and 4 keV).

[0009] Therefore, low-density films (e.g., C, B4C, and B) have a higher reflectivity (close to 100%) in the total reflection region but are limited with respect to the energy band, while high-density films (Au, Ir, Pt, W, Cr, Ni) have a wider band but a lower reflectivity.

[0010] The influence of the reduction in reflectivity of heavy elements due to photoelectric absorption is particularly disadvantageous in astronomical optical systems because a double reflection system is usually used.

[0011] To overcome this problem, it has been proposed to use a layer of a low-density material based on carbon or a similar material as an overcoating for the layer of the high-density metal. This solution is known, for example, from the following publications. V. Cotroneo, D. Spiga, M. Barbera, R. Bruni, K. Chen, etc., Carbon overcoatings for soft x-ray reflectivity enhancement, Proc. SPIE 6688, Optics for EUV, X-Ray, and Gamma-Ray Astronomy III, 66880U (September 20, 2007) and, 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 Instrumentation 2008: Ultraviolet to Gamma Ray, 701119 (July 15, 2008).

[0012] As a result, it is possible to obtain a larger reflectivity at low energy while maintaining a wide passband. Considering this, refer to Figure 1 showing the reflectivity calculated for a single layer of platinum, a single layer of carbon, and a layer of platinum with a 10-nm-thick carbon overcoating.

[0013] Solutions of overcoatings based on carbon or B4C have been proposed for several space missions such as Athena (ESA), Lynx (NASA), and eXTP (CAS).

[0014] The application of overcoatings with low-density materials based on carbon (or boron or its derivatives) is carried out by evaporation in high vacuum by electron beam or Joule effect (physical vapor deposition) or sputtering.

[0015] The aforementioned high-vacuum application methods are very expensive and cannot be applied to all types of mirrors.

[0016] The first case where there are application limitations in known methods is the case of mirrors formed by monolithic shells, for example nickel with a gold coating, and manufactured by replication, which is one of the standard methods for manufacturing astronomical mirrors with diameters of 5 - 70 cm. The mirrors of the Beppo - SAX, XMM - Newton, Swift, eRosita, Einstein Probe missions were fabricated using this method. This method is as follows · A step of fabricating a super-polished aluminum mandrel having a nickel / phosphorus alloy coating that constitutes a negative form of a mirror; · A step of depositing a gold coating about 100 nm thick on the mandrel; · A step of electroforming a nickel wall on the mandrel coated with gold; · A step of separating a nickel shell having a gold coating from the mandrel (the gold layer acts not only as a reflective layer but also as a separating agent because of its low adhesion to the mandrel); are provided.

[0017] Combined with this manufacturing method, using a known technique of applying a carbon-based overcoating to a known gold layer has very serious problems in both the premise of disposing the carbon layer on the mandrel before the gold layer and the premise of disposing the carbon layer on the gold layer after separating the shell from the mandrel.

[0018] Actually, the carbon-based layer cannot be deposited on the mandrel before gold because it will contaminate the evaporation chamber and, anyway, adhere to the mandrel and prevent replication.

[0019] After separating the shell from the mandrel, to deposit a carbon-based layer on the gold layer, for example, it is necessary to use a linear evaporation source through sputtering from inside the shell, but this method is only feasible for relatively large shells (with a diameter greater than 20 cm) and is very expensive anyway.

[0020] Monolithic mirrors can also be manufactured by other methods and materials through both replication technology and moderation and direct ultra-cleaning of the shell surface, but they have the same problems equally when applying a carbon-based overcoating to a high-density reflective layer (gold, carbon, iridium, or tungsten).

[0021] Another known method of fabricating mirrors of both large and small sizes is by assembling mirror modules, each of which consists of a stack of wafers of silicon or other materials, such as thin glass sheets, provided with parallel ribs to create a series of "pores". The so-called Silicon Pore Optics (SPO), manufactured by cold replication for the ATHENA (ESA) mission, is also included in this class of X-ray optics.

[0022] When the mirror is of the Wolter-I type, i.e., consisting of successive parabolic and hyperbolic surfaces, the modules are each composed of two wafer stacks of a parabolic surface (SPO-P) and a hyperbolic surface (SPO-H), which are precisely bonded to each other.

[0023] The application of a low-density layer is also a problem in this case.

[0024] The high-density reflective coating of the wafers is generally that of iridium and is applied to the individual wafers via sputtering before assembly. The application of a carbon layer is incompatible with the assembly method because carbon contaminates the processing chamber, and alternative materials for carbon, such as boron, boron carbide (B4C), or silicon carbide, have been tested but have not yielded satisfactory results due to reasons such as structural resistance (tendency of the overcoating to peel off), application difficulty, and / or unsatisfactory optical properties.

Prior Art Documents

Non-Patent Documents

[0025]

Non-Patent Document 1

[0026] An object of the present invention is to provide a method for applying a reflective carbon-based overcoating on an oblique incidence optical element that solves the problems associated with the above-known methods. [Means for Solving the Problems]

[0027] The above object is achieved by a method for applying a reflective carbon-based overcoating on an oblique incidence optical unit, wherein the optical unit includes a substrate and a high-density coating selected from the group consisting of gold, platinum, and iridium, tungsten, chromium, and nickel, and the method includes immersing the optical unit in a solution or gas phase containing at least an organic precursor material to cause deposition of the precursor material on the coating of the high-density material.

[0028] This makes it possible to deposit even a very thin layer of a carbon-based material, typically having a thickness of about 1 to 2 nm. By repeating this method, a layer of a desired thickness, for example 6 to 10 nm, can be obtained.

[0029] If desired, the method can include exposing the overcoating layer to ultraviolet (UV) light or other radiation (laser or X-ray) sources or high temperatures to remove oxygen and, if desired, hydrogen from the polymer chains.

[0030] For a better understanding of the present invention, two preferred embodiments will be described below by way of purely non-limiting examples with reference to the accompanying drawings.

Brief Description of the Drawings

[0031]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

DETAILED DESCRIPTION OF THE INVENTION

[0032] Referring to FIG. 2, reference numeral 1 indicates a monolithic shell optical unit for an astronomical X-ray mirror as a whole.

[0033] The unit 1 includes a monolithic shell 2 made of, for example, nickel and an inner coating 3 made of gold. The unit 1 can be manufactured by a method known by the replica method on a mandrel as described at the beginning of this specification.

[0034] According to an embodiment of the present invention (FIG. 3), the unit 1 contains sulfur and preferably represents a terminal group in which X is composed of, for example, -CH3, -OH or -COOH, and is immersed in a solution 4 containing an organic precursor material containing one or more organic compounds selected from the group consisting of alkyl thiol [HS(CH2) n X], alkyl disulfide [X(CH2) m S-S(CH2) n X] and alkyl sulfide [X(CH2) m S(CH2) n X], or is exposed to the gas phase.

[0035] According to another embodiment of the present invention, the precursor material has a terminal group in which X is composed of, for example, -CH3, -OH, -COOH, -NH2, -HC=CH2, -CH 2 =CHCOO - , -CH2OCH2, -SH, -CH=O or a combination thereof, and contains one or more organosilane compounds selected from the group consisting of chlorosilane [X(CH2) n SiCl4] and alkoxysilane [X(CH2) n Si(OR')].[[]END]]

[0036] Alternatively, different precursor materials can be used continuously, for example, one or more sulfur-containing organic compounds as shown above can be used in combination with one or more organosilane compounds.

[0037] The precursor is dissolved in a non-aqueous solvent, such as alcohol or anhydrous saturated and unsaturated hydrocarbons including, but not limited to, hexane, heptane, hexadecane, toluene, chlorobenzene, ether, carbon disulfide, and chloroform.

[0038] By continuous dipping, it is possible to create a thicker layer of the overcoating 5, advantageously about 6 - 10 nm thick.

[0039] According to an example of the overcoating process, a wafer pre-coated with a thin gold layer of X nm is immersed in 200 ml of a 1 mM solution of mercaptoundecanoic acid in absolute ethanol for 24 hours. Then, the wafer is taken out and thoroughly washed with ethanol. The thickness of the resulting overcoating is about 10 Å. By repeating this method, it is possible to increase the thickness of the overcoating.

[0040] Optionally, the wafer treated with a monolayer is immersed in another 200 ml of a 2% v / v solution of aminopropyltrimethoxysilane (APTMS) in toluene for 2 - 4 hours to form a further monolayer of the overcoating chemically bonded to the first one by O - H - N bonds. By repeating this method, the thickness of the overcoating can be increased. Optionally, the wafer treated with a bilayer is immersed in 200 ml of a 1 mM solution of octadecyltrichlorosilane (OTS) in hexane for 24 hours. This wafer is thoroughly washed in hexane and heated in air at 120 °C for 30 minutes. The third molecular layer adds about 2.5 nm to the thickness of the overcoating.

[0041] The aforementioned precursor material has a tendency to form a molecular monolayer double overcoating of nanometer thickness on the gold layer by absorption (Figure 4).

[0042] Optionally, if the molecules of the precursor material contain oxygen, the layer of overcoating 5 can be exposed to ultraviolet light (e.g., by an ultraviolet lamp 6 disposed within the shell 2, see Figure 5) or other suitable radiation (laser or X-ray) so as to obtain an oxygen-free alkyl layer 7 (Figure 6).

[0043] Using a similar method, with necessary modifications, a segmented optical system assembled in a stack as needed can be manufactured to form a so-called "pore" optical system (when based on a silicon substrate, silicon pore optical system - SPO).

[0044] In this case, instead of immersing the monolithic shell in a solution, the individual segments or modules 10 (Figure 8) formed by the segment stack constituting the optical system are immersed.

[0045] According to an example of the process, a wafer pre-coated with a thin layer of iridium of X nm is activated by being exposed to ozone plasma for 30 seconds. Next, the wafer is immersed in 200 ml of a 1 mM solution of octadecyltrichlorosilane (OTS) in hexane for 24 hours. The wafer is thoroughly washed with hexane and heated in air at 120 °C for 30 minutes. The monolayer thus obtained has a thickness of about 2.5 nm.

[0046] According to a further example of the process, a wafer pre-coated with a thin layer of iridium of X nm is activated by being exposed to ozone plasma for 30 seconds. Next, the wafer is immersed in 200 ml of a 1 mM solution of octadecyltrichlorosilane (OTS) in hexane for 7 days. The wafer is thoroughly washed with hexane and heated in air at 120 °C for 30 minutes. The monolayer thus obtained has a thickness of about 9 nm.

[0047] If the material contains oxygen, the module can be exposed to a source of ultraviolet light, laser or X-rays to remove the oxygen (Figure 9).

[0048] Thus, at the end of the process (Figure 11), an overcoating of an oxygen-free alkyl layer on iridium is obtained.

[0049] From the examination of the features of the above method, the advantages that can be achieved thereby are obvious.

[0050] By the technique of coating by dipping, all the problems associated with known vacuum deposition methods (electron beam, physical vapor deposition and sputtering) are eliminated.

[0051] In this way, a low-density overcoating having optimal optical and physical properties is obtained at a significantly lower cost compared to known techniques.

[0052] Furthermore, the method of the present invention is extremely simple, not subject to the above application limitations, and can be used for any type of monolithic mirror or segmented mirror. If necessary, the method can be used to cover carbon films or B4C films already deposited by high-vacuum methods (e.g., sputtering or evaporation by Joule effect) with carbon-based films in order to protect the underlying films and make them more stable.

Explanation of symbols

[0053] 1 Monolithic shell optical unit for astronomical X-ray mirrors 2 Monolithic shell made of nickel 3 Inner coating made of gold 4 Solution 5 Overcoating 6 Ultraviolet lamp 7 Oxygen-free alkyl layer 10 Individual segments or modules formed by a segment stack constituting an optical system

Claims

1. 1. A method for applying a carbon-based reflective overcoating (5, 7) onto a grazing incidence optical unit (1; 10), said optical unit (1; 10) comprising a substrate (2) and a coating (3) of a first material, said method comprising the step of immersing said optical unit (1; 10) in a solution (4) comprising at least one organic precursor material to cause absorption of said precursor material onto said coating (3).

2. The at least one precursor material may comprise an alkyl chain and one or more of the following: -CH3, -OH, -COOH, -NH2, -HC=CH2, -CH 2 =CHCOO - 2. The method of claim 1, characterized in that the compound contains at least one of the following functional groups: -CH2OCH2, -SH and -CH=O.

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

4. 4. The method according to claim 1, wherein the precursor material comprises oxygen.

5. 5. The method according to claim 1, wherein the precursor material comprises silicon.

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

7. 5. The method according to claim 4, characterized in that it comprises, after the step of treatment with a solution, a step of exposing the optical unit (1; 10) to a radiation source (6) adapted to remove oxygen from the precursor material.

8. 8. The method according to claim 1, wherein the first material is a high density material selected from the group consisting of gold, platinum, iridium, palladium, rhodium, ruthenium, chromium and nickel.

9. 9. The method according to claim 8, characterized in that the optical unit (1) comprises a monolithic shell (2) of nickel and the first material is gold.

10. 10. The method according to claim 9, characterized in that the material of the overcoating (5, 7) is an organic compound containing sulfur.

11. 9. The method according to claim 8, characterized in that the optical unit is a module (10) of SPO optics made of at least one silicon wafer stack and the first material is made of iridium.

12. 12. The method of claim 11, wherein the material of the overcoating is an organic compound containing silicon.

13. 8. The method according to claim 1, wherein the first material is selected from the group consisting of carbon and B4C.

Images