METHOD FOR APPLYING A CARBON-BASED REFLECTIVE OVERCOATING ON A GRAZING INCIDENCE OPTICAL UNIT

Abstract

A method for applying a carbon-based reflective overcoating on a grazing incidence optical unit comprising a substrate and a coating of a high-density material chosen from the group comprising gold, platinum, iridium, palladium, rhodium, ruthenium, chrome and nickel or a low-density material such as carbon or B4C; the method comprises the step of treating the optical unit with a solution or gaseous phase containing at least one polymer precursor material to create the overcoating through absorption of the polymer material on the coating.

Claims

1. A method for applying a carbon-based reflective overcoating on a grazing incidence optical unit, the optical unit comprising a substrate and a coating of a first material, the method comprising the step of treating the optical unit with a solution or gaseous phase containing at least one organic precursor material to cause an absorption of the precursor material on said coating.

2. The method according to claim 1, wherein said at least one precursor material comprises an alkyl chain and at least one of the functional groups —CH.sub.3, —OH, —COOH, —NH.sub.2, —HC═CH.sub.2, —CH.sub.2═CHCOO.sup.−, —CH.sub.2OCH.sub.2, —SH and —CH═O.

3. The method according to claim 2, where that the precursor material is a material chosen from the group comprising alkyl mercaptans, alkyl disulphides and alkyl sulphides.

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

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

6. The method according to claim 5, wherein said at least one precursor material is an organosilane.

7. The method according to claim 4, further comprising, after the step of treatment with the solution or the gaseous phase, the step of exposing the optical unit to a source of radiation adapted to eliminate the oxygen from the precursor material.

8. The method according to claim 1, wherein said first material is a high-density material chosen from the group comprising gold, platinum, iridium, palladium, rhodium, ruthenium, chrome and nickel.

9. The method according to claim 8, wherein the optical unit comprises a monolithic shell of nickel and the first material is gold.

10. The method according to claim 9, wherein the material of the overcoating is an organic compound containing sulphur.

11. The method according to claim 8, wherein the optical unit is a module of an SPO optics constituted by at least one silicon wafer stack, and the first material is constituted by iridium.

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

13. The method according to claim 1, wherein said first material is a low-density material.

14. The method according to claim 13, wherein said first material is chosen from the group comprising carbon and B4C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] For a better understanding of the present invention, two preferred embodiments are described hereinafter, purely by way of non-limitative example and with reference to the accompanying drawings, in which:

[0034] FIG. 1 shows the reflectivity of different coating materials compared, as a function of the energy of the incident ray;

[0035] FIGS. 2 to 6 schematically show successive steps of the method of the invention for producing a monolithic shell mirror;

[0036] FIGS. 7 to 9 schematically show successive steps of the method of the invention for producing segmented optical modules (SPO).

BEST MODE FOR CARRYING OUT THE INVENTION

[0037] Referring to FIG. 2, reference numeral 1 indicates, as a whole, a monolithic shell optical unit for an astronomical X-ray mirror.

[0038] The unit 1 comprises a monolithic shell 2, for example of nickel, and an inner coating 3 of gold. The unit 1 can be made in a known manner by a replica process on a mandrel, as described in the introductory part of the description.

[0039] According to one embodiment of the present invention (FIG. 3), the unit 1 is immersed in a solution 4 or exposed to a gaseous phase containing an organic precursor material comprising one or more organic compounds containing sulphur, preferably chosen from the group comprising alkyl thiols [HS(CH2).sub.nX], alkyl disulphides [X(CH2).sub.mS—S(CH2).sub.nX] and alkyl sulphides [X(CH2).sub.mS(CH2).sub.nX], where X represents a terminal group constituted, for example, by —CH3, —OH, or —COOH.

[0040] According to another embodiment of the present invention, the precursor material comprises one or more organosilane compounds, chosen from the group comprising chlorosilanes [X(CH2).sub.nSiCl4] and alkoxysilanes [X(CH2).sub.nSi(OR′)], where X represents a terminal group constituted, for example, by —CH3, —OH, —COOH, —NH2, —HC═CH2, —CH.sub.2═CHCOO.sup.−, —CH2OCH2, —SH, —CH═O, or a combination thereof.

[0041] Alternatively, different precursor materials can be used in succession, such as one or more organic compounds containing sulphur in combination with one or more organosilane compounds, as shown above.

[0042] The precursor is dissolved in non-aqueous solvents such as alcohol or anhydrous saturated and unsaturated hydrocarbons comprising, but not limited to, hexane, heptane, hexadecane, toluene, chlorobenzene, ethers, carbon disulphide and chloroform.

[0043] It is possible to make thicker layers of overcoating 5, conveniently in the order of 6-10 nm thick, by means of successive immersions.

[0044] According to one example of overcoating treatment, 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. Afterwards, the wafer is removed and abundantly rinsed with ethanol. The overcoating thickness obtained is approximately 10 Å. By repeating the process, it is possible to increase the thickness of the overcoating.

[0045] Optionally, the wafer treated with a molecular monolayer is immersed in 200 ml of another 2% v/v solution of aminopropyltrimethoxysilane (APTMS) in toluene for 2-4 hours, to form a further monolayer of overcoating chemically bonded to the first by an O—H—N bond. The process can be repeated to increase the thickness of the overcoating. Optionally, the wafer treated with a double molecular layer is immersed in 200 ml of a 1 mM solution of octadecyltrichlorosilane (OTS) in hexane for 24 hours. The wafer is abundantly rinsed in hexane and heated in air at 120° C. for 30 minutes. The third molecular layer adds approximately 2.5 nm to the thickness of the overcoating.

[0046] The aforementioned precursor materials tend to form, through absorption, a molecular monolayer di overcoating of nanometre thickness on the gold layer (FIG. 4).

[0047] Optionally, if the molecule of the precursor material contains oxygen, the layer of overcoating 5 can be exposed to UV rays (for example, by a UV lamp 6 arranged inside the shell 2, see FIG. 5), or other suitable radiation (laser or X-rays) so as to obtain an oxygen-free alkyl layer 7 (FIG. 6).

[0048] A similar method, mutatis mutandis, can be used to produce segmented optics, if necessary, assembled in stacks so as to form so-called “pore” optics (Silicon Pore Optics—SPO, if based on silicon substrates).

[0049] In this case, instead if immerging the monolithic shell in the solution, the individual segments or modules formed by segment stacks 10 (FIG. 8) constituting the optics are immerged.

[0050] According to one example of treatment, a wafer pre-coated with a thin layer of iridium of X nm is activated by exposure to ozone plasma for 30 seconds. Then, the wafer is immersed in 200 ml of a 1 mM solution of octadecyltrichlorosilane (OTS) in hexane for 24 hours. The wafer is abundantly rinsed with hexane and heated in air at 120° C. for 30 minutes. The monolayer thus obtained has a thickness of approximately 2.5 nm.

[0051] According to a further example of treatment, a wafer pre-coated with a thin layer of iridium of X nm is activated by exposure to ozone plasma for 30 seconds. Then, the wafer is immersed in 200 ml of a 1 mM solution of octadecyltrichlorosilane (OTS) in hexane for 7 days. The wafer is abundantly rinsed with hexane and heated in air at 120° C. for 30 minutes. The monolayer thus obtained has a thickness of approximately 9 nm.

[0052] If the material contains oxygen, the module can be exposed to a source of UV rays, laser or X-rays for elimination of the oxygen (FIG. 9).

[0053] At the end of the process (FIG. 11), an oxygen-free alkyl layer of overcoating on the iridium is thus obtained.

[0054] From an examination of the characteristics of the described process, the advantages that can be achieved therewith are evident.

[0055] Due to the technique of coating by immersion, all of the problems related to known vacuum deposition processes (e-beam, physical vapour deposition and sputtering) are eliminated.

[0056] Low-density overcoatings with optimal optical and physical characteristics are thus obtained at substantially reduced cost with respect to the known techniques.

[0057] Moreover, the process of the invention is extremely simple and does not suffer from the described application limits, being utilizable for any type of monolithic or segmented mirror. If necessary, the method could be employed to even cover a carbon or B4C film already deposited with a high-vacuum process (for example, sputtering or evaporation by Joule effect) with a carbon-based film, to protect the underlying films and make them more stable.