In-situ Aluminum Cleaning of Enhanced Aluminum Mirrors for VUV Optical Elements Using Atomic Layer Deposition Capping Following Atomic Layer Etching

Abstract

A method for manufacturing a reinforced aluminum mirror for vacuum ultraviolet (VUV) optical elements includes a step of depositing a reflective coating made of aluminum metal on at least one surface of a substrate by physical vapor deposition (PVD) to produce a mirror having the substrate and the reflective coating. This method further includes a step of removing aluminum oxide from the outer surface of the reflective coating by performing atomic layer etching (ALE) within an atomic layer deposition (ALD) system to produce an etched surface of the reflective coating, and a step of depositing an ALD protective layer on the etched surface of the reflective coating by performing atomic layer deposition within the ALD system to produce the reinforced aluminum mirror. This reinforced aluminum mirror comprises a substrate, a reflective coating deposited on the substrate, and an ALD protective layer covering the etched surface of the reflective coating.

Description

Priority

[0001] This application claims the benefit of priority under 35 U.S.C. § 120 to U.S. Provisional Patent Application No. 63 / 354,257, filed on June 22, 2022, the content of which is relied upon and incorporated herein by reference in its entirety.

Technical Field

[0002] The present disclosure broadly relates to optical components, and more particularly to a method for manufacturing enhanced aluminium mirrors for vacuum ultraviolet (VUV).

Background Art

[0003] Optical technologies using ultraviolet light are widely used in semiconductor manufacturing. Advanced lithography techniques enable the formation of smaller feature sizes in microelectronics. With the progress of this technology, highly sensitive optical inspections that can reduce defect detection to the nanoscale range are also required. Currently, defect inspections mainly use deep ultraviolet (DUV) optical elements operating at a wavelength of about 193.4 nm. Next-generation optical elements for optical inspections are expected to mainly use vacuum ultraviolet (VUV) optical elements (e.g., wavelengths of 120 nm to 190 nm) and extreme ultraviolet (EUV) optical elements (e.g., wavelengths as small as about 13.5 nm). Although the wavelength of EUV is about 10 times shorter than that of VUV, many microelectronic defects are optically sensitive to both VUV and EUV wavelengths compared to DUV optical elements. As a result, the development of optical elements for VUV and EUV inspections has become an important focus for the semiconductor industry.

[0004] The performance of VUV optical inspection systems depends on VUV mirrors. Aluminium is recognized as a major material for manufacturing VUV reflective optical elements such as VUV mirrors. Aluminium is usually applied onto a substrate using physical vapor deposition (PVD) to deposit aluminium on the surface of the substrate to create a reflective surface. PVD-Al mirrors can provide a reflectivity exceeding 90% in the VUV wavelength range.

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, due to the deterioration over time of the PVD-Al coating by oxidation, the reflection performance of the PVD-Al coating can be significantly reduced.

Means for Solving the Problems

[0006] Therefore, there is still a need for a method for manufacturing a reinforced aluminum mirror for a VUV optical element and a method for manufacturing a reinforced aluminum mirror, in which defects are reduced and the aluminum reflective coating is protected from deterioration caused by oxidation of aluminum.

[0007] According to a first aspect of the present disclosure, a method for manufacturing a reinforced aluminum mirror for a vacuum ultraviolet (VUV) optical element can include a step of depositing a reflective coating made of aluminum metal on at least one surface of a substrate in a PVD system by physical vapor deposition (PVD) to manufacture a mirror including the substrate and the reflective coating. This method can further include a step of removing aluminum oxide from the outer surface of the reflective coating by performing atomic layer etching (ALE) in an atomic layer deposition (ALD) system to generate an etched surface of the reflective coating. This method can further include a step of depositing an ALD protective layer on the etched surface of the reflective coating by performing atomic layer deposition in an ALD system to manufacture a reinforced aluminum mirror including the substrate, the reflective coating deposited on the substrate, and the ALD protective layer covering the etched surface of the reflective coating.

[0008] A second aspect of the present disclosure can further include a step of moving a substrate including a reflective coating from a PVD system to an ALD system, and by the step of moving the substrate having the reflective coating to the ALD system, the reflective coating can be exposed to oxygen to cause oxidation of aluminum on the outer surface of the reflective coating to form aluminum oxide, and can include the first aspect.

[0009] In a third aspect of the present disclosure, the process of performing atomic layer etching in an ALD system can include the process of exposing the substrate and the reflective coating to alternating pulses of a fluorine source and a metal-organic compound, and can include either the first or the second aspect. By exposing the substrate and the reflective coating to a pulse containing a fluorine source, aluminum oxide can be converted to aluminum fluoride, and a thin layer of aluminum fluoride can be formed on the outer surface of the reflective coating. By exposing the thin layer of aluminum fluoride to a pulse containing a metal-organic compound, a volatile metal-organic compound that can be released from the outer surface of the reflective coating can be formed by reacting with the aluminum fluoride.

[0010] A fourth aspect of the present disclosure can include the third aspect, which includes the process of exposing the reflective coating to alternating pulses of a fluorine source and a metal-organic compound at a temperature of 150°C to 325°C, or 200°C to 250°C.

[0011] A fifth aspect of the present disclosure can include either the third or the fourth aspect, which includes the process of exposing the etched surface of the reflective coating to alternating pulses of a fluorine source and a metal-organic compound at 50 watts (W) to 600 W.

[0012] A sixth aspect of the present disclosure can include any one of the third to fifth aspects, which includes the process of exposing the etched surface of the reflective coating to a fluorine source for an exposure time of 1 second to 60 seconds.

[0013] A seventh aspect of the present disclosure can include the sixth aspect, and further includes the process of purging the ALD chamber of the ALD system with an inert gas for a purge time sufficient to remove at least 99% of the fluorine source and oxygen compounds remaining from the ALD chamber after the process of exposing the etched surface of the reflective coating to the fluorine source for the exposure time.

[0014] The eighth aspect of the present disclosure can include any one of the third to seventh aspects, wherein the fluorine source can include sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), trifluoroiodomethane (CF3I), hydrogen fluoride (HF), SF6 plasma, SF6·argon (Ar) plasma, NF3 plasma, NF3·Ar plasma, or a combination thereof.

[0015] The ninth aspect of the present disclosure can include any one of the third to eighth aspects, wherein the fluorine source can include SF6, SF6 plasma, or plasma containing SF6 and argon (Ar).

[0016] The tenth aspect of the present disclosure can include any one of the third to ninth aspects, wherein the organometallic compound can include trimethylaluminum (TMA), triethylaluminum (TEA), dimethylaluminum chloride (DMAC), silicon tetrachloride (SiCl4), aluminum hexafluoroacetylacetonate (Al(hfac)3), tri-i-butylaluminum (Al(iBu)3), tin(II) acetylacetonate (Sn(acac)2), tris(2,2,6,6-tetramethyl-3,5-heptanedionato)aluminum (i.e., Al(TMHD)3), or a combination thereof.

[0017] The eleventh aspect of the present disclosure can include any one of the third to tenth aspects, and includes a step of exposing a thin layer of aluminum fluoride to the organometallic compound for a total exposure time of 10 milliseconds (ms) to 60,000 ms or 10 ms to 30,000 seconds, and this total exposure time is equal to the pulse length and the closed period of the pulse of the organometallic compound.

[0018] The twelfth aspect of the present disclosure can include any one of the third to eleventh aspects, and includes a step of exposing a thin layer of aluminum fluoride to the organometallic compound at a pressure of 10 millitorr (1.33 Pa) to 100 torr (13,332 Pa).

[0019] A 13th aspect of the present disclosure can include any one of the 3rd to 12th aspects, wherein the step of exposing a thin layer of aluminum fluoride to a pulse containing an organometallic compound can include the step of injecting the organometallic compound into the ALD chamber over a pulse length and the step of closing a throttle valve of the ALD system. By the step of closing the throttle valve, the flow of material into and out of the ALD chamber can be prevented, and the thin layer of aluminum fluoride can be kept in contact with the organometallic compound for a sealing period of from 1 second to 60 seconds, or from 10 seconds to 30 seconds.

[0020] A 14th aspect of the present disclosure can include the 13th aspect, further including the step of opening the throttle valve again and the step of purging the ALD chamber with an inert gas to remove at least 99% of the remaining organometallic compound, volatile organometallic compound, or both from the ALD chamber.

[0021] A 15th aspect of the present disclosure can include any one of the 3rd to 14th aspects, wherein atomic layer etching can have an etching rate of 1.1 angstroms in thickness per cycle, and one complete cycle includes one pulse of a fluorine source and one pulse of an organometallic compound.

[0022] A 16th aspect of the present disclosure can include any one of the 1st to 15th aspects, wherein the ALD protective layer can include a metal fluoride protective coating.

[0023] A 17th aspect of the present disclosure can include the 16th aspect, wherein the metal fluoride protective coating can include one or more of aluminum trifluoride (AlF3), magnesium fluoride (MgF2), calcium fluoride (CaF2), lithium fluoride (LiF), lanthanum fluoride (LaF3), gadolinium fluoride (GdF3), or combinations thereof.

[0024] The 18th aspect of the present disclosure can include any one of the 1st to 17th aspects, where the step of applying a protective ALD coating to the outer surface of the etched aluminum layer can include the step of exposing the etched aluminum layer to alternating pulses of a metal precursor and a fluorine source.

[0025] The 19th aspect of the present disclosure can include the 18th aspect, where the fluorine source can include sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), trifluoroiodomethane (CF3I), hydrogen fluoride (HF), SF6 plasma, SF6·argon (Ar) plasma, NF3 plasma, NF3·Ar plasma, or a combination thereof.

[0026] The 20th aspect of the present disclosure can include either the 18th or 19th aspect, where the fluorine source can include SF6, SF6 plasma, or a plasma containing SF6 and argon (Ar).

[0027] The 21st aspect of the present disclosure can include any one of the 18th to 20th aspects, where the metal precursor can include an aluminum precursor selected from one or more of trimethylaluminum (TMA), triethylaluminum (TEA), dimethylaluminum isopropoxide (DMAI), [MeC(NiPr)2]AlEt2, dimethylaluminum hydride, dimethylethylamine, ethylpiperidine, dimethylaluminum hydride, or a combination thereof.

[0028] The 22nd aspect of the present disclosure can include any one of the 18th to 21st aspects, where the metal precursor can include a magnesium precursor selected from the group consisting of bis(ethylcyclopentadienyl)magnesium, bis(cyclopentadienyl)magnesium, bis(2,2,6,6-tetramethyl-3,5-heptanedionato)magnesium, bis(N,N’-di-sec-butylacetamidinate)magnesium, bis(pentamethylcyclopentadienyl)magnesium, and combinations thereof.

[0029] A 23rd aspect of the present disclosure can have an ALD growth rate for a protective ALD coating of 0.5 angstroms thickness per cycle, and one complete cycle of the ALD process can include any one of the 18th to 22nd aspects including one pulse of a fluorine source and one pulse of a metalorganic compound.

[0030] A 24th aspect of the present disclosure further includes a step of exposing the surface to a pulse containing an oxygen source after the step of exposing the surface to a pulse containing a metal precursor and before the step of exposing the surface to a pulse containing a fluorine source, and can include any one of the 18th to 23rd aspects. The metal precursor can form a ligated metal connected to the outer surface of the article. The oxygen source can cause oxidation of the ligated metal to form a metal oxide. The fluorine source can reduce the metal oxide to form a metal fluoride of the protective ALD coating.

[0031] A 25th aspect of the present disclosure can include the 24th aspect, wherein the oxygen source can include water, water plasma, oxygen, oxygen plasma, ozone, ozone plasma, hydrogen peroxide, hydrogen peroxide plasma, an oxygen-containing liquid, an oxygen-containing gas, or a combination thereof.

[0032] A 26th aspect of the present disclosure can include any one of the 1st to 25th aspects, including a step of depositing a first ALD protective layer on an etched surface of a reflective coating and a step of depositing a second ALD protective layer on an outer surface of the first ALD protective layer.

[0033] A 27th aspect of the present disclosure can include any one of the 1st to 26th aspects, wherein the ALD protective layer can be made of a high-reflectivity metal fluoride, and the high-reflectivity metal fluoride can increase the reflectivity of a reinforced aluminum mirror compared to a mirror having only a reflective coating.

[0034] The 28th aspect of the present disclosure can relate to a reinforced aluminum mirror for an ultraviolet optical system. This reinforced aluminum mirror can include a substrate having a surface and a reflective coating deposited on the surface of the substrate, and the reflective coating is made from aluminum metal deposited by physical vapor deposition. The reinforced aluminum mirror can further include an ALD protective layer deposited on the etching surface of the reflective coating. The ALD protective layer can be applied by atomic layer deposition, the reflective coating can reflect light having a wavelength in at least the vacuum ultraviolet wavelength range, and the ALD protective layer can reduce or prevent the oxidation of the aluminum of the reflective coating.

[0035] The 29th aspect of the present disclosure can include the 28th aspect, in which the reflective coating and the ALD protective layer can contain less than 5 atomic percent of oxygen atoms.

[0036] The 30th aspect of the present disclosure can include any one of the 28th or 29th aspects, in which the reinforced aluminum mirror does not have a layer of aluminum oxide disposed between the reflective coating and the ALD protective layer.

[0037] The 31st aspect of the present disclosure can include any one of the 28th to 30th aspects, in which the ALD protective layer can be made from a high-reflectivity metal fluoride, and the high-reflectivity metal fluoride can increase the reflectivity of the reinforced aluminum mirror as compared to a mirror having only the reflective coating.

[0038] The 32nd aspect of the present disclosure can include the 31st aspect, in which the high-reflectivity metal fluoride can include lanthanum fluoride, gadolinium fluoride, or both.

[0039] A 33rd aspect of the present disclosure can include any one of aspects 28 to 32, wherein the ALD protective layer can include a first ALD protective layer made from a first ALD metal fluoride and a second ALD protective layer made from a second ALD metal fluoride different from the first ALD metal fluoride.

[0040] Additional features and advantages of the enhanced aluminum mirror and the method of manufacturing an enhanced aluminum mirror described herein are set forth in the following detailed description, some of which will be readily apparent to those skilled in the art from that description, or will be recognized by practicing the embodiments described herein, which include the following detailed description, the claims, and the accompanying drawings.

[0041] It should be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and characteristics of the subject matter of the claims. The accompanying drawings are included to provide a further understanding of the various embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate the various embodiments described herein and, together with the description, serve to explain the principles and operation of the subject matter of the claims.

Brief Description of the Drawings

[0042]

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DETAILED DESCRIPTION OF THE INVENTION

[0043] Here, various embodiments of a reinforced aluminum mirror for a VUV optical element and a method of manufacturing such a reinforced aluminum mirror according to the present disclosure are described in detail. Examples of the reinforced aluminum mirror and method described herein are schematically illustrated in the accompanying drawings. Whenever possible, the same reference numbers are used throughout the drawings to refer to the same or similar parts. Referring now to FIGS. 1 and 2, a reinforced aluminum mirror 100 for a VUV optical element according to the present disclosure includes a substrate 102, a reflective coating 110 deposited on the mirror surface 104 of the substrate 102, and at least one ALD protective layer 120 deposited on the etched surface 122 of the reflective coating 110. The reflective coating 110 is aluminum metal and can be deposited on the substrate 102 by physical vapor deposition (PVD). The ALD protective layer 120 can be a metal fluoride layer applied by atomic layer deposition (ALD).

[0044] A method for manufacturing a reinforced aluminum mirror 100 can include depositing a reflective coating 110 made from aluminum metal on at least one mirror surface 104 of a substrate 102 by a PVD process within a PVD system to produce a mirror having the substrate 102 and the reflective coating 110. The method can further include removing aluminum oxide from the outer surface of the reflective coating 110 by performing atomic layer etching (ALE) within an atomic layer deposition (ALD) system to produce an etched surface 122 of the reflective coating 110, and depositing an ALD protective layer 120 on the etched surface 122 of the reflective coating 110 by performing an ALD process within the ALD system to produce a reinforced aluminum mirror 100 having the substrate 102, the reflective coating 110 deposited on the substrate 102, and the ALD protective layer 120 covering the etched surface 122 of the reflective coating 110.

[0045] Various embodiments of a reinforced aluminum mirror 100 and a method for manufacturing the reinforced aluminum mirror 100 will be described herein with specific reference to the accompanying drawings.

[0046] As used herein, the term “substantially free of” a component can refer to a composition, fiber, or atmosphere that contains less than 0.01 weight percent or mole percent of that component. For example, an ALD coating substantially free of carbon can contain no more than 0.01 weight percent or mole percent of carbon.

[0047] The terms “micrometer” and “μm” are used interchangeably herein. The terms “nanometer” and “nm” are used interchangeably herein.

[0048] As used herein, the term “plasma” refers to a gas of ions that includes positive ions and electrons and is generated from a starting material by the application of heat and an electric current.

[0049] As used herein, the term "ppm" means parts per million on a molar basis and represents atomic concentration. For example, a layer of MgF2 containing 1 ppm of carbon contains 1 mole of carbon per million moles of MgF2.

[0050] As used herein, the term "conformal coating" refers to a coating that conforms to the contour of the surface of an article and has a substantially uniform thickness over all of the surface in contact with the coating.

[0051] As used herein, the term "passivation" refers to treating or coating the surface of an article to make its surface more passive, meaning making its surface non-reactive with the environment.

[0052] As used herein, the term "mirror surface" refers to the outer surface of a substrate on which a reflective coating is applied, with no intention of implying that the outer surface is mirror-finished prior to depositing the reflective coating.

[0053] As previously described, the performance of VUV and EUV optical elements for inspecting microelectronics and semiconductors can depend on the quality and reflectivity of the mirrors. These mirrors are incorporated into an optical inspection system to direct VUV and EUV wavelength light along an inspection path. Aluminum is commonly used to coat substrates for manufacturing reflective optical elements (i.e., mirrors) for VUV and EUV optics. Aluminum is typically applied to the surface of the substrate using physical vapor deposition (PVD) to deposit aluminum on the surface of the substrate to create a reflective surface. PVD - Al mirrors can provide reflectivity exceeding 90% in the VUV wavelength range.

[0054] Referring to FIG. 3, one embodiment of a typical mirror 200 for an ultraviolet optical element is schematically shown. The mirror 200 includes a substrate 102 having at least one surface, herein referred to as the mirror surface 104, and a reflective coating 110 applied to the mirror surface 104 of the substrate 102. The reflective coating 110 can be an aluminum coating consisting of, comprising, or substantially consisting of aluminum metal. The reflective coating 110 can be aluminum metal applied by physical vapor deposition (PVD) according to known methods. The reflective coating 110 can have an outer surface 112 that can reflect more than 90% of the VUV wavelength light incident on the outer surface 112 when the outer surface 112 is pure aluminum. In operation of the mirror 200, a beam including VUV wavelength light can be directed at the mirror 200. The beam of VUV wavelength light incident on the outer surface 112 of the reflective coating 110 is reflected from the outer surface 112 of the reflective coating 110 along a path different from the original beam.

[0055] The reflective coating 110 made of aluminum applied to the substrate in the PVD process can deteriorate over time due to oxidation of the aluminum. Due to the oxidation of the aluminum, the reflectivity of the aluminum reflective coating 110 can be significantly reduced. Referring to FIG. 3, a mirror 200 having only the reflective coating 110 is schematically shown. For the mirror 200 of FIG. 3, the aluminum 114 of the reflective coating 110 can oxidize to form aluminum oxide (Al2O3) on the outer surface 112 of the reflective layer 110. The aluminum oxide can form an aluminum oxide layer 116 on the aluminum 114. The aluminum oxide layer 116 can change the optical properties of the mirror 200, thereby reducing the reflectivity of the mirror 200. The reflectivity of the mirror 200 decreases as the thickness t Al2O3 increases and decreases.

[0056] Referring now to FIG. 4, the VUV reflectivity of a pure aluminum mirror as a function of wavelength is compared to that of a mirror having one or more monolayers of Al2O3 formed on an aluminum reflective layer due to surface oxidation. A monolayer of Al2O3 refers to a single layer of Al2O3 having a thickness on the order of the length or diameter of one molecule of Al2O3. Table 1 below gives the correspondence between the reference numbers of FIG. 4 and the number of Al2O3 monolayers formed on the outer surface of the aluminum reflective coating. As described above, VUV includes wavelengths from 120 nm to 190 nm. For example, at the lower end of the VUV range at a wavelength of 120 nm, the reflectivity of a pure Al mirror reaches 92%. That reflectivity decreases as oxidation of the Al surface proceeds and the thickness t Al2O3 of the Al2O3 layer increases. When the thickness t Al2O3 of the oxide layer reaches 1.2 nm, corresponding to four monolayers of Al2O3 (reference number 410 in FIG. 4), the reflectivity decreases to 50%. If the aluminum mirror is not protected or passivated, oxidation of the aluminum can result in an Al2O3 layer up to 3 nm thick. Natural oxidation corresponding to 10 Al2O3 monolayers (reference number 412) has a thickness t Al2O3 of about 3 nm, whereby, as shown in FIG. 4, the reflectivity of the aluminum mirror decreases to less than 10% at a wavelength of 120 nm.

[0057]

Table 1

[0058] To reduce or prevent oxidation of the aluminum in the reflective coating 110, the aluminum in the reflective coating 110 can be coated with a protective layer. In some cases, PVD can be used to apply a protective layer to the outer surface 112 of the reflective coating 110. The current best practice for aluminum mirrors is to apply a PVD-based metal fluoride such as, but not limited to, PVD magnesium fluoride (PVD-MgF2) or PVD aluminum fluoride (PVD-AlF3) to the outer surface 112 of the reflective coating 110 to form a protective layer. Referring now to FIG. 5, these conventional aluminum mirrors can be prepared by a method 500 that includes loading a substrate into a PVD chamber in step 502, depositing PVD aluminum (PVD-Al) to form a reflective layer in step 504, depositing an initial low-density PVD aluminum fluoride (PVD-AlF3) in step 506, and heating the mirror and depositing additional dense PVD-AlF3 over the initial low-density PVD-AlF3 in step 508. In step 504, the PVD-Al layer is deposited at a fast deposition rate under vacuum at room temperature in the PVD chamber. In step 506, the initial low-density PVD-AlF3 layer can be deposited at room temperature immediately after deposition of the PVD-Al layer. The thickness of the initial low-density PVD-AlF3 layer is typically about 15 nm. In step 508, the PVD chamber and the mirror placed therein are heated to about 200° C. for deposition of the dense PVD-AlF3 layer. The thickness of the dense PVD-AlF3 layer is typically around 10 nm. In step 510, the mirror can be removed from the PVD chamber. The initial low-density PVD-AlF3 layer protects the PVD-Al coating to some extent from oxidation, but oxidation of the aluminum still occurs at a reduced rate while the mirror is being heated to 200° C. in the PVD chamber just prior to deposition of the dense PVD-AlF3 layer in step 508 of FIG. 5. Further, the PVD-AlF3 coating can exhibit pinholes or regions of inconsistent thickness that can provide a path for air or other oxidants to oxidize the surface of the reflective layer 110.Thus, when applying a dense PVD - AlF3 coating, there is still a problem that aluminum oxide is formed on the outer surface of the aluminum reflective layer, which affects the reflectivity of the aluminum mirror.

[0059] Atomic layer deposition (ALD) can provide a metal fluoride protective layer with a lower oxygen permeation rate compared to PVD coatings, and thus can improve the passivation behavior of aluminum mirrors. Referring to FIG. 6 here, a hybrid method 600 for manufacturing an aluminum mirror can include a PVD coating of a PVD - Al reflective layer and an initial low - density PVD - AlF3 layer, followed by the deposition of an ALD protective layer. According to the hybrid method 600 of FIG. 6, first, in step 602, a substrate is loaded into the PVD chamber of the PVD process. In step 604, a PVD - Al layer is deposited in the PVD chamber at a high deposition rate under vacuum at room temperature to form a reflective layer of PVD - Al. In step 606, an initial low - density PVD - AlF3 layer can be deposited at room temperature immediately after the deposition of the PVD - Al coating to provide an initial protective coating. Following the deposition of this low - density PVD - AlF3 layer, the mirror is transferred from the PVD chamber to the ALD chamber of the ALD system in step 608. In step 610, an ALD protective layer is applied to the outer surface of the low - density PVD - AlF3 layer. Following the deposition of the ALD protective layer, in step 612, a mirror with a hybrid protective coating (e.g., low - density PVD - AlF3 layer+ALD protective layer) can be removed from the ALD chamber.

[0060] The ALD - AlF3 layer applied in step 610 of FIG. 6 has a lower oxygen permeation rate compared to the dense PVD - AlF3 layer applied in method 500 of FIG. 5. However, the accumulation of oxygen on the outer surface of the aluminum reflective layer is comparable to that of a PVD - only mirror manufactured by the method 500 of FIG. 5. Specifically, to apply the ALD coating, a mirror having a PVD - Al coating and an initial low - density PVD - AlF3 layer must be transferred from the PVD chamber of the PVD system to the ALD chamber of the ALD system. Note that the ALD coating process requires different atmospheres and operating conditions that are generally not achievable in a typical PVD system. When transferring a mirror equipped with a substrate, a PVD - Al reflective layer, and an initial low - density PVD - AlF3 layer, the partially formed mirror is exposed to air or other atmospheres containing oxygen or other oxidizing components. Oxygen can react with aluminum to form aluminum oxide on the surface of the PVD - Al reflective layer, and this can accumulate to form an aluminum oxide layer on the surface of the aluminum reflective layer. In some cases, the surface of the mirror can be cleaned with a fluorine source within the ALD chamber before applying the ALD - AlF3 coating. However, cleaning with a fluorine source within the ALD chamber is not effective in completely removing the aluminum oxide on the surface of the reflective layer made of PVD - Al.

[0061] Referring now to FIG. 7, a mirror 300 manufactured by the hybrid method 600 of FIG. 6 is schematically shown. As shown in FIG. 7, the mirror 300 includes a substrate 102 and a reflective coating 110, which may include aluminum 114 from a PVD - Al coating process. The mirror 300 further includes a low - density PVD - AlF3 layer 130 disposed on the reflective coating 110 and an ALD - AlF3 protective layer 120 deposited on the low - density PVD - AlF3 layer 130. When the mirror 300 is exposed to air when transferred from the PVD chamber to the ALD chamber, the mirror 300 may also include an aluminum oxide layer 116. The aluminum oxide layer 116 will be disposed between the aluminum 114 of the reflective coating 110 and the low - density PVD - AlF3 layer 130.

[0062] The formation of aluminum oxide in mirror 300 formed by this hybrid method can reduce the reflectivity of mirror 300 in the VUV wavelength region. Referring now to FIG. 8A, the reflectivity (Y-axis) as a function of wavelength (X-axis) is schematically shown for an aluminum mirror (reference numeral 802) manufactured by the process of FIG. 5 and an aluminum mirror (reference numeral 804 in FIG. 8A) manufactured by the process of FIG. 6. The aluminum mirror shown by reference numeral 802 in FIG. 8A included a 100 nm PVD-Al reflective coating, a 15 nm low density PVD-AlF3 layer, and a 10 nm dense PVD-AlF3 layer applied at 200 °C. The aluminum mirror shown by reference numeral 804 was prepared by the method of FIG. 6 with a fluorine cleaning treatment added prior to depositing the ALD protective layer to remove at least some of the carbon and oxygen impurities. The fluorine treatment included exposing the mirror to sulfur hexafluoride (SF6) plasma at 150 °C for a period of 30 seconds and was performed after transferring the aluminum mirror from the PVD chamber to the ALD chamber. The aluminum mirror shown by reference numeral 804 included a 100 nm PVD-Al reflective coating, a 15 nm low density PVD-AlF3 layer, and a 10 nm ALD-AlF3 protective layer. This ALD-AlF3 protective layer was applied at a temperature of 150 °C according to the method disclosed herein.

[0063] As shown in FIG. 8A, an aluminum mirror 802 with only PVD shows a significant decrease in reflectivity up to 0.70 at wavelengths between 150 nm and 180 nm, indicating the presence of aluminum oxide in various coating layers. An aluminum mirror 804 with a PVD-Al layer, a low-density PVD-AlF3 layer, and an ALD-AlF3 protective layer also showed a significant decrease in reflectivity to about 0.75 at wavelengths between 150 nm and 180 nm. The aluminum mirror 804 prepared by the hybrid method 600 of FIG. 6 provides better reflectivity performance compared to the aluminum mirror 802 with only PVD, but still shows a decrease in reflectivity performance over the VUV wavelength range due to oxidation of aluminum in the reflective coating and protective layer and accumulation of aluminum oxide. Thus, there is still a need for a method of preparing a reinforced aluminum mirror for VUV wavelength applications that reduces the presence of aluminum oxide in the reinforced aluminum mirror.

[0064] The present disclosure relates to a reinforced aluminum mirror and a process for manufacturing a reinforced aluminum mirror for VUV optical elements that further reduces the amount of aluminum oxide in the reinforced aluminum mirror and provides one or more ALD protective layers to reduce or prevent oxidation and degradation of the aluminum in the reflective layer during use of the reinforced aluminum mirror. The method of the present disclosure solves the problems in the previously described methods by including an atomic layer etching (ALE) process after transferring a mirror having a PVD-Al reflective layer from a PVD system to an ALD system. Specifically, the method of the present disclosure includes the steps of applying a PVD-Al layer to a substrate to form a protective layer, transferring the mirror having the protective layer to an ALD chamber of an ALD system, performing atomic layer etching (ALE) to remove aluminum oxide from the outer surface of the reflective layer to form an etched surface of the reflective layer, and depositing an ALD protective layer on the etched surface of the reflective layer. The ALE process removes any metal oxides that may be formed on the outer surface of the aluminum of the reflective coating before depositing the ALD protective layer.

[0065] Referring now to FIG. 9, a flow diagram of a method for manufacturing a reinforced aluminum mirror for a VUV optical element is schematically shown. First, as shown in step 902, a substrate is loaded into the PVD chamber of a PVD system. In step 904, a PVD-Al layer is subsequently deposited on the mirror surface of the substrate to form a reflective coating made of aluminum. The substrate with the PVD-Al reflective layer deposited thereon is then transferred to the ALD chamber of an ALD system as shown in step 906. The PVD-Al reflective layer can be exposed to air during the transfer to the ALD chamber, whereby some aluminum on the outer surface of the reflective layer can be oxidized. In step 908, an ALE process is performed within the ALD system to remove materials such as aluminum oxide from the surface of the reflective layer to generate an etched surface of the reflective layer. After the ALE process, the method includes a step of depositing an ALD protective layer to cover the etched surface of the reflective layer in step 910 to manufacture a reinforced aluminum mirror. After the deposition of the ALD protective layer, the reinforced aluminum mirror can be removed from the ALD chamber as shown in step 912. In some embodiments, an optional ultraviolet treatment may be performed after step 912. The ultraviolet treatment is performed at a wavelength in the optical region (e.g., between about 90 nm and about 1000 nm) and is performed, for example, to remove carbon molecules that may have accumulated on the protective layer 120. Further, in embodiments, the ultraviolet treatment may be performed for a period in the range of about 20 minutes or less, or about 15 minutes or less, or about 10 minutes or less, or about 5 minutes to about 20 minutes, or about 8 minutes to about 15 minutes. In some embodiments, the period of the ultraviolet treatment is about 10 minutes.

[0066] Referring again to FIG. 2, one embodiment of a reinforced aluminum mirror 100 manufactured by the method 900 of FIG. 9 is schematically shown. As described above, the reinforced aluminum mirror 100 can include a substrate 102 having a reflective coating 110 deposited on the mirror surface 104 of the substrate 102. The reflective coating 110 can be a PVD-Al layer applied to the mirror surface 104 of the substrate 102 by a PVD process in step 904 (FIG. 9) of the method 900. Referring again to FIG. 2, the reinforced aluminum mirror 100 can further include an ALD protective layer 120 deposited on the etched surface 122 of the reflective coating 110. The reinforced aluminum mirror 100 can contain a very low concentration of oxygen in the reflective coating 110 and between the reflective coating 110 and the ALD protective layer 120.

[0067] The method of the present disclosure can produce a reinforced aluminum mirror 100 for a VUV optical element with a pinhole-free ALD protective layer 120 that reduces or prevents the oxidation and degradation of aluminum in the reflective coating 110 over time. The ALD coating process can coat all of the surface of the reinforced aluminum mirror with a single deposition performed with atomic layer precision. In other words, the ALD coating process can conformally coat the entire surface of the reinforced aluminum mirror 100 having a non-mirror surface of the substrate at the same time. The ALD coating process can produce a protective film that is atomically dense and pinhole-free even at a very small thickness, such as up to a few nanometers, such as 10 nm or less. Thus, the ALD coating process can reduce the thickness of the protective coating to less than 1 / 5 of the thickness of a PVD-based protective coating thick enough to provide the same protection. The ALD coating process can reduce the stress in the coating and extend the life of the reinforced aluminum mirror.

[0068] Furthermore, the method disclosed herein enables the use of a PVD process to apply the reflective coating 110 and also enables the use of an ALD process to produce the ALD protective layer 120. Specifically, the method disclosed herein removes the native aluminum oxide formed on the outer surface of the reflective coating 110 by an ALE process in an ALD chamber before the ALD protective layer is deposited. The strong reducing environment within the ALD chamber of the ALD system prevents further oxidation of the aluminum of the reflective coating 110 during the ALD process. Further, the ALE process and the ALD process can be performed at similar temperatures, thereby reducing the delay between the ALE step and the ALD step in this method and reducing the opportunity for further oxidation during heating or cooling steps. The ALE process and the ALD process can be performed using sulfur hexafluoride (SF6) and / or SF6 plasma, which are good alternatives to hydrogen fluoride-based fluorine sources. Further, the ALD protective layer 120 can include a plurality of different layers made of different metal fluoride materials. For example, the ALD protective layer 120 can include an aluminum fluoride ALD layer and a magnesium fluoride ALD layer, thereby reducing the roughness of the enhanced aluminum mirror 100 and improving passivation among other features. In some embodiments, the ALD protective layer 120 is made of ALD-AlF3-MgF2.

[0069] Referring again to FIG. 8A, reference numeral 806 shows the reflection as a function of wavelength for the reinforced aluminum mirror 100 fabricated by the method 900 of FIG. 9. For the reinforced aluminum mirror represented by 806, the PVD-Al layer of the reflective coating was applied up to a thickness of 100 nm. After the application of the PVD-Al layer, the mirror 100 was placed into an ALD chamber and an ALE process was performed to remove 4 nm of material from the outer surface of this reflective layer. Next, a 4 nm ALD protective layer 120 made from aluminum fluoride was applied to the etched surface according to the ALD coating method disclosed herein. As shown in FIG. 8A, the reinforced aluminum mirror represented by reference numeral 806 showed greater reflection over the wavelength range from 150 nm to 180 nm as compared to the mirrors represented by reference numerals 802 (PVD only) and 804 (PVD-Al, low density PVD-AlF3, and ALD-AlF3). Note that the method of fabricating the reinforced mirror shown by 806 did not include an optional ultraviolet treatment. The protective coating of the reinforced aluminum mirror shown by reference numeral 806 had only a 4 nm ALD protective layer, whereas the mirror corresponding to reference numeral 802 had a 25 nm PVD-AlF3 protective layer, and the mirror corresponding to reference numeral 804 had a 15 nm PVD-AlF3 protective layer and a 10 nm ALD-AlF3 protective layer. Thus, the reinforced aluminum mirror (reference numeral 806) fabricated by the method disclosed herein can provide excellent reflection performance and protection of the aluminum of the reflective coating while being able to have a thinner ALD protective layer thickness as compared to the mirrors fabricated by the conventional method 500 of FIG. 5 (reference numeral 802 in FIG. 8A) and the hybrid method 600 of FIG. 6 (reference numeral 804 in FIG. 8A).

[0070] Figure 8B also shows the reflection as a function of wavelength of the reinforced aluminum mirror manufactured by the method 900 of FIG. 9 at wavelengths shorter than those shown in FIG. 8A. Specifically, the reinforced aluminum mirror represented by 806’ was prepared by applying a 100-nm-thick layer of PVD-Al to the substrate 102. After the application of the PVD-Al layer, the mirror 100 was placed in an ALD chamber and an ALE process was performed to remove 4 nm of material from the outer surface of this reflective layer. Next, a 5-nm ALD protective layer 120 made of aluminum fluoride was applied to the etched surface according to the ALD coating method disclosed herein. Note that the method of manufacturing the reinforced mirror represented by 806’ did not include an optional ultraviolet treatment. As shown in FIG. 8B, the reinforced aluminum mirror represented by 806’ showed greater reflection over the wavelength range from 100 nm to 160 nm compared to the mirror represented by 802’ (PVD only). Specifically, the mirror represented by 802’ had a 100-nm PVD-Al reflective coating, a 10-nm low-density PVD-AlF3 layer, and a 23-nm dense PVD-AlF3 layer applied at 200 °C.

[0071] Figure 8C further shows the aluminum mirrors of reference numerals 806’ and 802’ including a 10-minute ultraviolet treatment at optical wavelengths. Specifically, the mirror of reference numeral 806” corresponds to 806’ but has been subjected to ultraviolet treatment, and the aluminum mirror of reference numeral 802” corresponds to 802’ but has been subjected to ultraviolet treatment. Comparing FIGS. 8B and 8C, including the ultraviolet treatment slightly increased the reflection of the manufactured aluminum mirrors.

[0072] The first step in the method of manufacturing the enhanced aluminum mirror 100 of the present specification for VUV optical elements includes forming a reflective coating 110 on the mirror surface 104 of the substrate 102. Referring again to FIG. 2, the mirror 100 includes a substrate 102 that provides a support structure for the reflective coating 110. The substrate 102 may include any substrate material having a hard outer surface 104 capable of supporting the reflective coating 110 and is not particularly limited. The substrate 102 can be a metal, metal fluoride (e.g., CaF2, MgF2, etc.), metal alloy, metalloid, other suitable material, or a combination of materials. In an embodiment, the substrate 102 can be aluminum metal, aluminum alloy, silicon, CaF2, MgF2, or a combination thereof.

[0073] The reflective coating 110 of the enhanced aluminum mirror 100 can be a PVD-Al layer applied to the mirror surface 104 of the substrate 102. The PVD-Al layer of the reflective coating 110 can be applied to the mirror surface 104 of the substrate 102 by a PVD process at ambient temperature according to methods known in the art. The reflective coating 110 including the PVD-Al layer can be made from aluminum metal. Referring to FIG. 2, the reflective coating 110 including the PVD-Al layer has a thickness t sufficient to ensure that the mirror surface 104 of the substrate 102 is completely covered with the PVD-Al layer and no portion of the substrate 102 on the mirror surface 104 is exposed. Al In an embodiment, the reflective coating 110 including the PVD-Al layer can have a thickness t of 50 nm or more, 80 nm or more, 90 nm or more, or even 100 nm or more. Al The reflective coating 110 including the PVD-Al layer can have a thickness t of from 50 nm to 200 nm, for example, from 70 nm to 200 nm, from 90 nm to 200 nm, from 100 nm to 200 nm, from 50 nm to 150 nm, from 70 nm to 150 nm, from 90 nm to 150 nm, from 100 nm to 150 nm, from 50 nm to 100 nm, or from 70 nm to 100 nm. Al It can have.

[0074] Referring again to FIG. 9, after the application of the PVD-Al layer, the substrate with the reflective coating is transferred from the PVD system to the ALD system, as shown in step 906. As described above, the PVD-Al layer can come into contact with air or other oxidants during the transfer to the ALD system, and due to this contact, the aluminum on the outer surface of the PVD-Al layer can be oxidized to form aluminum oxide. To remove these aluminum oxides, an ALE process is performed in the ALD system on the substrate having the reflective coating 110 to remove the layer of aluminum oxide formed by contact with an oxygen-containing atmosphere or other oxidants. Both the ALE process in step 908 of FIG. 9 and the deposition of the ALD protective layer in step 910 are performed within the ALD system.

[0075] Referring now to FIG. 10, one embodiment of an ALD system 1000 for performing the ALE process and the ALD process is schematically shown. The ALD system 1000 includes an ALD chamber 1002, which can be heated by one or more heating devices 1004 that are in thermal communication with the ALD chamber 1002. The ALD chamber 1002 has an inlet 1006 and an outlet 1008. The inlet 1006 can be operated to introduce various components used in the ALD process or the ALE process into the ALD chamber 1002. The outlet 1008 of the ALD chamber 1002 can be in fluid communication with a throttle valve 1010 disposed downstream of the ALD chamber 1002. The throttle valve 1010 can control the flow of gases and other materials exiting the ALD chamber 1002. The ALD system 1000 can further include a vacuum pump 1012 disposed downstream of the throttle valve 1010. The vacuum pump 1012 can be operated to create a vacuum within the ALD chamber 1002 to assist in drawing ALD reaction components into the ALD chamber 1002 and removing unreacted components, reaction products, inert gases, or other materials from the ALD chamber 1002.

[0076] The ALD system 1000 can further include one or more sources of reaction components in fluid communication with an inlet 1006 of the ALD chamber 1002. Specifically, the ALD system 1000 can include a fluorine source 1020, a metal precursor source 1030, an oxygen source 1040, or combinations thereof. The ALD system 1000 can include a fluorine source control valve 1022 disposed between the fluorine source 1020 and the ALD chamber 1002 and operable to control the flow rate of the fluorine source 1020 into the ALD chamber 1002. The ALD system 1000 can include a metal precursor source control valve 1032 disposed between the metal precursor source 1030 and the ALD chamber 1002 and operable to control the flow rate of the metal precursor source 1030 into the ALD chamber 1002. The ALD system 1000 can include an oxygen source control valve 1042 disposed between the oxygen source 1040 and the ALD chamber 1002 and operable to control the flow rate of the oxygen source 1040 into the ALD chamber 1002. The ALD system 1000 can also include an inert gas source 1050 and an inert gas control valve 1052 operable to control the flow of inert gas from the inert gas source 1050 into the ALD chamber 1002. A substrate 102 having a reflective coating 110 is placed within the ALD chamber 1002 of the ALD system 1000 prior to the ALE process.

[0077] Referring again to FIG. 2, as described above, the method of manufacturing the mirror 100 disclosed herein includes removing aluminum oxide from the outer surface 112 of the aluminum reflective coating 110 to generate the etched surface 122 of the reflective coating 110 by performing an ALE process within the ALD system 1000. The ALE process includes exposing the reflective coating 110 to alternating pulses of a fluorine source and a metal-organic compound. The substrate 102 and the reflective coating 110 can be contacted with or exposed to the alternating pulses of the fluorine source and the metal-organic compound under operating conditions sufficient to cause a chemical reaction on the surface of the substrate 102, the reflective coating 110, or both with the fluorine source, the metal-organic compound, or both. The method disclosed herein can include exposing the reflective coating 110 to alternating pulses of a fluorine source and a metal-organic compound at an ALE temperature of from 150°C to 325°C, from 150°C to 300°C, from 150°C to 250°C, from 200°C to 325°C, from 200°C to 300°C, or even from 200°C to 250°C. At an ALE temperature lower than about 150°C, the etching rate will be too slow. At an ALE temperature higher than about 325°C, other quality defects such as thermal changes to the aluminum of the substrate 102 or the reflective coating 110 may occur due to the high temperature. The method disclosed herein can include exposing the aluminum layer to pulses containing a fluorine source, pulses containing a metal-organic compound, or both at from 50 watts (W) to 600 W, or even from 100 W to 300 W.

[0078] When the reflective coating 110 is exposed to a pulse containing a fluorine source, aluminum oxide on the outer surface of the reflective coating 110 is converted to aluminum fluoride, and a thin layer of aluminum fluoride is formed on the outer surface of the reflective coating 110. Some aluminum metal may also react with the fluorine source to produce aluminum fluoride on the outer surface 112 of the reflective coating 110. The layer of aluminum fluoride on the outer surface 112 of the reflective coating 110 can be monolayer thick. The displaced oxygen can form one or more volatile oxygen species released into the ALD chamber 1002. The ALD chamber 1002 can be purged with an inert gas (e.g., Ar, He, Ne, etc.) after the pulse containing the fluorine source to remove volatile oxygen species and any remaining fluorine source from the ALD chamber 1002 before the next pulse of the organometallic compound is applied.

[0079] The method includes, after purging, exposing the thin layer of aluminum fluoride to a pulse containing an organometallic compound. When the aluminum fluoride layer is exposed to the organometallic compound, the aluminum fluoride can be reacted to form a volatile organometallic fluoride compound released from the outer surface 112 of the reflective coating 110 into the ALD chamber 1002. Referring again to FIG. 10, the organometallic compound can be introduced from the metal precursor source 1030 of the ALD system 1000 or by a separate organometallic compound source (not shown). The step of exposing the aluminum fluoride layer to the organometallic compound can include introducing the organometallic compound into the ALD chamber 1002 and then closing the throttle valve 1010 to seal the ALD chamber for a period sufficient for all of the AlF3 to react with the organometallic compound. After the reaction is complete, the volatile organometallic fluoride compound and any remaining unreacted organometallic compound can be removed from the ALD chamber 1002 by purging the ALD chamber 1002 with an inert gas.

[0080] Each time the ALE process is repeated, approximately one single molecular layer of aluminum oxide is removed from the surface of the reflective coating 110. The ALE process (e.g., alternating pulses of a fluorine source and an organometallic compound) can be repeated multiple times before the aluminum oxide is removed from the original outer surface 112 of the reflective layer 110 until all, or to a target depth of 1 nm to 10 nm, or 3 nm to 5 nm. In other words, the ALE process can be repeated until 1 nm to 10 nm or 3 nm to 5 nm of material is removed from the outer surface 112 of the reflective layer 110 to create the etched surface 122 of the reflective layer 110. After the ALE process, the reflective coating 110 can be substantially free of oxygen-containing compounds.

[0081] The fluorine source can be derived from a fluorine-containing precursor. The fluorine-containing precursor can be selected from the group consisting of sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), trifluoroiodomethane (CF3I), hydrogen fluoride (HF), and combinations thereof. In an embodiment, the fluorine source can be a plasma fluorine source derived from a fluorine-containing precursor or a plasma of a fluorine-containing precursor and argon (Ar). In an embodiment, the fluorine source can be a plasma containing SF6, SF6 and Ar (SF6 / Ar), or NF3 and Ar (NF3 / Ar).

[0082] HF is often used as a fluorine source in an ALD system. However, to use HF as a fluorine source, it is necessary to raise the temperature of the ALD chamber 1002 to promote the reduction of aluminum oxide to form AlF3. Subsequently, the substrate 102 and the reflective layer 110 must be cooled to a lower temperature for the ALD process. Oxidation of the etching surface 122 during this cooling period is a concern that can reduce the effectiveness of the removal of aluminum oxide during the ALE process with respect to the reflectivity of the enhanced aluminum mirror 100. Furthermore, HF is dangerous to handle and is highly corrosive, especially when in contact with water. Therefore, in an embodiment, the fluorine source can have the same process temperature for both the ALE process and the ALD process and can be a significantly safer non-HF fluorine source.

[0083] The SF6 fluorine source is significantly safer to use compared to HF and can use a similar temperature range for both the ALE process and the ALD process. In an embodiment, the fluorine source may include SF6 or a plasma derived from SF6 (i.e., an SF6-based plasma). In an embodiment, the fluorine source may comprise, consist of, or consist essentially of an SF6-based fluorine source such as SF6 or an SF6-based plasma. In an embodiment, the fluorine source may comprise, consist of, or consist essentially of SF6 and Ar (i.e., an SF6 / Ar plasma) or a plasma derived from SF6 and another inert gas. When the fluorine source includes an SF6 / Ar plasma, the flow rate ratio of Ar to SF6 can be from 0.1:1 to 10:1, from 0.1:1 to 5:1, from 0.1:1 to 2:1, from 0.5:1 to 10:1, from 0.5:1 to 5:1, from 0.5:1 to 2:1, from 1:1 to 10:1, from 1:1 to 5:1, from 1:1 to 2:1, from 2:1 to 10:1, from 2:1 to 5:1, or about 2:1, where the flow rate is a volumetric flow rate expressed in units of sccm (standard cubic centimeters per minute).

[0084] The fluorine source can be converted into plasma by heating a fluorine precursor such as SF6, although not limited to this, and exposing the heated fluorine precursor to an electric current or a strong electromagnetic field. Argon (Ar) can be added to create an SF6 / Ar plasma. The material (e.g., fluorine precursor, Ar, or a combination thereof) can be heated to the ALD process temperature and exposed to an electric current sufficient to convert the material into plasma. In an embodiment, the step of converting the material (e.g., fluorine precursor, Ar, or a combination thereof) into plasma may include the step of heating the material to a temperature of 100°C to 325°C, or 120°C to 250°C, and the step of applying an electric current having an output of 50 watts (W) to 600W, or 100 watts (W) to 300W.

[0085] The method may include the step of exposing a reflective coating 110 including a PVD-Al layer to a pulse including a fluorine source at an ALE temperature of 150°C to 325°C, such as 200°C to 250°C. This method may include the step of exposing a reflective coating 110 including a PVD-Al layer to a pulse including a fluorine source with an output of 50W to 600W, 50W to 300W, 100W to 600W, or even 100W to 300W. This method may include the step of exposing a reflective coating 110 including a PVD-Al layer to a pulse including a fluorine source for a fluorine pulse duration sufficient to react all of the aluminum oxide molecules just outside the PVD-Al layer with the fluorine source to produce a single molecular layer of aluminum fluoride. In an embodiment, the fluorine pulse duration may be from 1 second to 30 seconds, or about 7 seconds. As described above, after exposing the outer surface of the reflective layer 110 to the fluorine source for an exposure time, the ALD chamber can be purged with an inert gas for a purge time sufficient to remove at least 99% of the remaining fluorine source and oxide from the ALD chamber.

[0086] After exposure to and purging of the fluorine source, the ALE process may include exposing the aluminum fluoride layer to pulses of the organometallic compound. The organometallic compound may include one or more of trimethylaluminum (TMA), triethylaluminum (TEA), dimethylaluminum chloride (DMAC), silicon tetrachloride (SiCl4), aluminum hexafluoroacetylacetonate (Al(hfac)3), tri-i-butylaluminum (Al(iBu)3), tin(II) acetylacetonate (Sn(acac)2), tris(2,2,6,6-tetramethyl-3,5-heptanedionato)aluminum (i.e., Al(TMHD)3), or combinations thereof. In embodiments, the organometallic compound may be an organoaluminum compound selected from the group consisting of TMA, TEA, DMAC, aluminum hexafluoroacetylacetonate, tri-i-butylaluminum (Al(iBu)3), tris(2,2,6,6-tetramethyl-3,5-heptanedionato)aluminum (i.e., Al(TMHD)3), and combinations thereof. In embodiments, the organometallic compound may be TMA, and when a thin layer of aluminum fluoride on the surface of the reflective layer is exposed to TMA, a reaction occurs between TMA and aluminum fluoride to form AlF(CH3)2 gas. AlF(CH3)2 is released into the ALD chamber from the outer surface of the reflective coating 110.

[0087] Referring again to FIG. 10, in the embodiment, the step of exposing the thin layer of aluminum fluoride to the pulse containing the organometallic compound may include the step of injecting the organometallic compound into the ALD chamber 1002 over a pulse length, and the step of closing the throttle valve 1010 of the ALD system 1000 over a sealing period. When the throttle valve 1010 is closed, the flow of materials entering and leaving the ALD chamber 1002 is prevented, and the thin layer of aluminum fluoride is maintained in contact with the organometallic compound over the sealing period. The pulse length can be sufficient for the target amount of the organometallic compound to be introduced into the ALD chamber 1002. In the embodiment, the pulse length can be from about 300 ms to about 700 ms, or about 400 ms, etc., from about 10 ms to about 5000 ms. Once the throttle valve 1010 is closed, the thin layer of aluminum fluoride can be kept in contact with the organometallic compound over a sealing period, which is sufficient for the organometallic compound to react with all of the aluminum fluoride and convert the aluminum fluoride into a volatile organometallic compound. In the embodiment, the total contact time between the thin layer of aluminum fluoride and the organometallic compound can be from about 10 milliseconds (ms) to about 60,000 ms (60 seconds), or from about 10 ms to about 30 seconds, where the total contact time between the thin layer of aluminum fluoride and the organometallic compound includes the pulse length and the sealing period. In the embodiment, the ALE process may include the step of injecting the organometallic compound into the ALD chamber 1002 over a pulse length from about 10 ms to about 5,000 ms, such as from about 300 ms to about 700 ms, the step of closing the throttle valve 1010, and the step of maintaining the contact between the thin layer of aluminum fluoride and the organometallic compound over a sealing period from about 1 second to about 60 seconds or from about 10 seconds to about 30 seconds.

[0088] The method may include exposing a thin layer of aluminum fluoride to an organometallic compound at a temperature and pressure sufficient to react the organometallic compound with the aluminum fluoride to produce a volatile organometallic compound. In an embodiment, the method may include exposing the thin layer of aluminum fluoride to the organometallic compound at an ALE temperature of from 150 °C to 325 °C. The method may include exposing the thin layer of aluminum fluoride to the organometallic compound at a pressure of from 10 millitorr (1.33 Pa) to 100 torr (13,332 Pa).

[0089] Referring again to FIG. 10, after the sealing period, the ALE process may include reopening the throttle valve and evacuating the atmosphere from the ALD chamber 1002 to remove the volatile organometallic compound and any remaining various vapor species from the ALD chamber 1002. In an embodiment, the ALE process may further include purging the ALD chamber 1002 with an inert gas to remove at least 99% of the remaining organometallic compound, volatile aluminum compound, or both from the ALD chamber.

[0090] The ALE process of exposing the outer surface 112 of the reflective layer 110 to a pulse containing a fluorine source to produce a thin layer of aluminum fluoride, purging, contacting the thin layer of aluminum fluoride with an organometallic compound to convert the aluminum fluoride to a volatile organometallic compound, and purging can be performed multiple times. The etching rate of the ALE process can be about 1.1 angstroms per cycle (Å / cycle) at an ALE temperature of 225 °C. One complete cycle of the ALE process includes one pulse of the fluorine source and one pulse of the organometallic compound with a purge step therebetween. The ALE process can continue for a plurality of cycles sufficient to remove all of the aluminum oxide from the surface of the reflective coating.

[0091] Referring again to FIG. 2, as a result of the ALE process, a substrate 102 is obtained that has a reflective coating 110 with an etched surface 122 and on which no aluminum oxide layer is disposed intervening on the reflective coating 110. In an embodiment, the etched surface 122 of the reflective coating 110 can be substantially free of metal oxides. In an embodiment, the amount of oxygen at the interface between the etched surface 122 of the reflective layer 110 and the ALD protective layer 120 is less than or equal to the amount of oxygen in the aluminum of the reflective layer 110.

[0092] After the ALE process of etching and removing aluminum oxide from the surface of the reflective layer 110, an ALD protective layer 120 is then deposited on the etched surface 122 of the reflective coating 110. The ALD protective layer 120 and the ALD process for producing the ALD protective layer 120 will now be described with reference to the enhanced aluminum mirror 100 of FIG. 2 and the ALD system 1000 shown in FIG. 10. Referring now to FIG. 2, the enhanced aluminum mirror 100 includes one or more ALD protective layers 120 deposited on the etched surface 122 of the reflective coating 110 such that the ALD protective layer 120 contacts the etched surface 122 of the reflective coating 110. In an embodiment, the ALD protective layer 120 provides a barrier on the etched surface 122 of the reflective coating 110 that reduces or prevents an oxidation compound from contacting the aluminum of the reflective coating 110, thereby reducing or preventing the aluminum from oxidizing to form aluminum oxide. As described above, by reducing and / or preventing the formation of aluminum oxide on the surface of the enhanced aluminum mirror 100, the reflectivity and service life of the enhanced aluminum mirror 100 can be improved compared to an aluminum mirror by PVD alone or an aluminum mirror having a hybrid protective coating including a PVD component and an ALD component. The ALD protective layer 120 may also be deposited on other surfaces of the substrate on which the reflective coating 110 is not deposited.

[0093] The ALD protective layer 120 can be made of metal fluoride. In an embodiment, the ALD protective layer 120 can be aluminum fluoride (AlF3), magnesium fluoride (MgF2), lithium fluoride (LiF), calcium fluoride (CaF2), or a combination thereof. In addition to, or instead of, this, in an embodiment, the ALD protective layer 120 can be made of other metal fluorides, such as, but not limited to, a lanthanum fluoride (LaF3) ALD coating or a gadolinium fluoride (GdF3) ALD coating. In some embodiments, the ALD protective layer 120 is made of AlF3 and MgF2. The ALD protective layer 120 can be directly bonded to the etched surface 122 of the reflective coating 110. As used herein, the term "directly bonded to" means that the ALD protective layer 120 contacts and bonds to the etched surface of the reflective coating 110 without any intervening coating or layer disposed between the ALD protective layer 120 and the etched surface 122 of the reflective coating 110. Referring now to FIG. 11, the reinforced aluminum mirror 100 can include an ALD protective layer 120 that includes a plurality of ALD coating layers (e.g., a first ALD coating layer 140, a second ALD coating layer 150, etc.) applied to the reflective coating 110 of the reinforced aluminum mirror 100.

[0094] Referring back to FIG. 2, the ALD protective layer 120 can be applied to the etched surface 122 of the reflective coating 110 by an ALD process. The substrate 102 including the reflective coating 110 thereon is exposed to alternating pulses of one or more precursor compounds during the ALD process, whereupon exposure to the alternating pulses of precursor compounds, the ALD protective layer 120 is deposited layer by layer on the etched surface 122 of the reflective coating 110 and, optionally, on other non-reflective surfaces of the substrate 102. Each deposition layer in one cycle of the ALD process can have a thickness comparable to the size of a single molecule of the ALD coating material (e.g., monolayer coating of the surface). The ALD process enables the entire surface of the substrate 102 to be coated in a single deposition with atomic layer precision. The alternating pulses can include a metal precursor pulse and a fluorine source pulse. In an embodiment, the ALD process can include a first pulse including a metal precursor, a second pulse including an oxygen source, and a third pulse including a fluorine source.

[0095] In an embodiment, the ALD process may be a direct reduction ALD process, during which a metal precursor is deposited on the etched surface 122 of the reflective coating 110 or other surfaces of the substrate 102, and then directly reduced using a reducing agent such as a fluorine source to form the ALD protective layer 120. In an embodiment, the ALD process for applying the ALD protective layer 120 may include a step of exposing at least the etched surface 122 of the reflective coating 110 to alternating pulses of a metal precursor and a fluorine source. Referring to FIG. 10, the step of exposing the etched surface of the reflective coating to alternating pulses of a metal precursor and a fluorine source can be performed in the ALD chamber 1002 of the ALD system 1000. The pulse of the metal precursor may contain the metal precursor. In some embodiments, the pulse of the metal precursor may contain the metal precursor and one or more inert gases. The inert gas may include, but is not limited to, non-reactive gases such as noble gases (e.g., Ar, He, Ne, etc.). The inert gas may serve as a carrier for transferring the precursor to the ALD chamber. Each pulse of the metal precursor and the fluorine source may be of a sufficient duration such that the metal precursor and the fluorine source can react with at least 90%, at least 95%, at least 98%, at least 99%, or even at least 99.9% of the reaction sites on the etched surface 122 of the reflective coating 110 or the outer surface of the previously applied metal precursor or ALD protective layer during their respective pulses. Between the injection of each pulse of the metal precursor and the fluorine source, the ALD chamber 1002 may be purged with an inert gas (e.g., Ar, He, Ne, etc.) to remove any remaining metal precursor and / or fluorine source before the next pulse.

[0096] The process of depositing the ALD protective layer 120 has chemical properties similar to the ALE process, except that the order of the steps is reversed. In the ALD process, the reflective coating 110 is first exposed to a metal precursor pulse and then to a fluorine source pulse. In the ALD process, the exposure period to the metal precursor pulse is much shorter compared to the total contact time between the organometallic compound and aluminum fluoride in the ALE process. Further, in the ALD process, the throttle valve 1010 (FIG. 10) of the ALD system 1000 remains open during the pulse of the metal precursor. The ALD process can operate at the same temperature as the ALE process.

[0097] In an embodiment, the ALD protective layer 120 can be a metal fluoride coating, and the ALD process can include the steps of exposing the etched surface 122 of the reflective coating 110 to a pulse containing a metal precursor, purging the ALD chamber with an inert gas after the metal precursor pulse, and exposing the etched surface 122 of the reflective coating 110 to a subsequent pulse of a fluorine source. During the pulse of the metal precursor, the metal precursor in the form of vapor, plasma, or spray liquid may be injected into the ALD chamber containing the substrate 102 having the reflective coating 110 etched in the ALE process. The ALD process may further include the step of heating the metal precursor (e.g., aluminum precursor, magnesium precursor, lithium precursor, calcium precursor, lanthanum precursor, gadolinium precursor, etc.) to a temperature of 95 °C or higher before introducing the metal precursor into the ALD chamber. When the etched surface 122 of the reflective coating 110 is exposed to a pulse containing a metal precursor, the metal precursor may react with aluminum at the etched surface 122 of the reflective coating 110 to bond a monolayer of the coordinated metal onto the etched surface 122 of the reflective coating 110.

[0098] A monolayer of coordinated metal bonded to the surface may have a thickness approximately equal to the size of a single molecule of the coordinated metal. For a pulse of a metal precursor following the deposition of the first ALD metal fluoride layer bonded directly to the etch surface 122, the metal precursor may react with the previously deposited metal fluoride to bond a subsequent monolayer of coordinated metal to the outer surface of the previously deposited ALD metal fluoride layer. After depositing and bonding a monolayer of coordinated metal to a surface (e.g., the etch surface 122 or the surface of a previously deposited ALD metal fluoride layer), the ALD process may further include a step of stopping the exposure to the mirror's metal precursor. The step of stopping the exposure to the mirror's metal precursor may include a step of stopping the flow of the metal precursor into the ALD chamber.

[0099] The pulse of the metal precursor may have a pulse duration sufficient to react the metal precursor with at least 90%, at least 95%, at least 98%, at least 99%, or at least 99.9% of the reactive aluminum sites at the etch surface 122 of the reflective coating 110 or the reactive metal fluoride sites at the outer surface of a previously applied ALD metal fluoride layer. In an embodiment, the pulse of the metal precursor may have a pulse duration from 10 milliseconds (ms) to 10 seconds (s), or about 1 second. Factors affecting the pulse duration include the vapor pressure of the metal precursor, the flow rate of the metal precursor, the reactivity of the metal precursor with the surface, the volume of the ALD chamber, and the dimensions of the substrate 102. In an embodiment, the pulse duration of the metal precursor can be set to achieve a coverage of at least 90%, or at least 95%, or at least 98%, or at least 99%, or at least 99.9% of the total area of all surfaces of the etch surface 122 of the reflective coating 110 and, optionally, other surfaces of the substrate 102, preferably a conformal coverage. Referring again to FIG. 10, during the pulse of the metal precursor in the ALD process, the throttle valve 1010 is maintained in the open position. Next, the ALD chamber may be purged with an inert gas to remove any remaining metal precursor from the ALD chamber before continuing the ALD process.

[0100] After purging the chamber, the mirror may be exposed to a pulse containing a fluorine source. During the pulse of the fluorine source, the fluorine source may be injected into the ALD chamber containing the mirror. The pulse of the fluorine source may contain a fluorine source, or a fluorine source combined with an inert gas such as any of the inert gases described herein. By exposing the layer of coordinated metal bonded to the surface of the mirror to a subsequent pulse containing the fluorine source, the fluorine source may react with the coordinated metal and reduce the coordinated metal to form a metal fluoride (for example, through a chemical reduction reaction between the fluorine source and the coordinated metal, substituting the ligand with fluorine to generate the metal fluoride of the ALD protective layer 120). The injection of the fluorine source may be stopped at the end of the pulse when at least 90%, at least 95%, at least 98%, at least 99%, or even at least 99.9% of the metal coordinated on the surface of the mirror has reacted with the fluorine source to form a metal fluoride. In an embodiment, the fluorine source pulse may have a pulse duration of 10 ms to 30 s, for example, 10 ms to 20 s, 10 ms to 10 s, 1 s to 30 s, 1 s to 20 s, 1 s to 10 s, 3 s to 30 s, 3 s to 20 s, or 3 s to 10 s. The fluorine source pulse may be stopped by stopping the flow of the fluorine source into the ALD chamber. The ALD process may be repeated multiple times with a series of alternating pulses of a metal precursor and a fluorine source to further add an ALD metal fluoride layer and increase the thickness of the ALD protective layer 120.

[0101] In an embodiment, the ALD protective layer 120 may include an ALD aluminum fluoride (ALD-AlF3) layer. When the ALD protective layer 120 includes an ALD-AlF3 layer, the metal precursor may be a metal ligand complex containing aluminum. In an embodiment, the metal precursor may include one or more of trimethylaluminum (TMA), triethylaluminum (TEA), dimethylaluminum isopropoxide (DMAI), dimethylaluminum hydride:dimethyl ethylamine, ethyl piperidine:dimethylaluminum hydride, dimethylaluminum chloride (DMAC), aluminum hexafluoroacetylacetonate (Al(hfac)3), tri-i-butylaluminum (Al(iBu)3), tris(2,2,6,6-tetramethyl-3,5-heptanedionato)aluminum (Al(TMHD)3), or a combination thereof. Other aluminum compounds may also be suitable for use as the metal precursor. In an embodiment, the ALD protective layer 120 may include an ALD magnesium fluoride (ALD-MgF2) layer. In the case of the ALD-MgF2 layer, the metal precursor may be a metal ligand complex containing magnesium as the metal. In an embodiment, the metal precursor may be selected from the group consisting of bis(ethylcyclopentadienyl)magnesium, bis(cyclopentadienyl)magnesium(II), bis(2,2,6,6-tetramethyl-3,5-heptanedionato)magnesium, bis(N,N'-di-sec-butylacetamidinato)magnesium, bis(pentamethylcyclopentadienyl)magnesium, and combinations thereof. Other magnesium-containing compounds may also be suitable for use as the metal precursor for forming the MgF2 ALD layer.

[0102] In an embodiment, the ALD protective layer 120 can be a metal fluoride having a metal other than aluminum or magnesium. In these cases, similar metal ligand complexes different from aluminum or magnesium may be used. For example, in an embodiment, the metal of the metal precursor may be calcium (Ca), lithium (Li), or a combination thereof. In an embodiment, the ALD protective layer 120 may include an ALD calcium fluoride (ALD-CaF2) layer. When the ALD protective layer 120 is an ALD-CaF2 layer, the metal precursor may be selected from the group consisting of Ca(2,2,6,6-tetramethyl-3,5-heptanedionato)2, bis(N,N'-diisopropylformamidinato)calcium(II), bis(N,N'-diisopropylacetamidinato)calcium(II), [Ca3(2,2,6,6-tetramethyl-3,5-heptanedionate)6], Ca(1,2,4-triisopropylcyclopentadienyl)2, and combinations thereof. In an embodiment, the ALD protective layer 120 may include an ALD lithium fluoride (ALD-LiF) coating. When the ALD protective layer 120 includes an ALD-LiF layer, the metal precursor may be selected from the group consisting of lithium tert-butoxide, lithium 2,2,6,6-tetramethyl-3,5-heptanedionate, and combinations thereof.

[0103] In an embodiment, the ALD protective layer 120 may include a high reflectivity fluoride ALD layer such as, but not limited to, lanthanum fluoride (LaF3), gadolinium fluoride (GdF3), or a combination thereof. When the ALD protective layer 120 includes an ALD-LaF3 layer, the metal precursor may be one or more lanthanum precursors selected from the group consisting of tris(N,N'-diisopropylformamidinato)lanthanum, tris[N,N-bis(trimethylsilyl)amide]lanthanum(III), (2,2,6,6-tetramethyl-3,5-heptanedionato)lanthanum, tris(tetramethylcyclopentadienyl)lanthanum(III), lanthanum precursors of the LANA (trademark) brand from Air Liquide, and combinations thereof. When the ALD protective layer 120 includes an ALD-GaF3 layer, the metal precursor may be one or more gadolinium precursors selected from the group consisting of gadolinium tris(N,N'-isopropylacetamidinato), tris(isopropyl-cyclopentadienyl)gadolinium(III) (Gd(iPrCp)3), tris(OCMe2CH2OMe)gadolinium(III) (Gd(mmp)3), tris(2,3-dimethyl-2-butoxy)gadolinium(III) (Gd(DMB)3), tris(2,2,6,6-tetramethyl-3,5-heptanedionato)gadolinium(III) (Gd(thd)3), gadolinium precursors of the GANBETTA (trademark) brand available from Air Liquide, gadolinium precursors of the GAUDI (trademark) brand available from Air Liquide, and combinations thereof.

[0104] The metal precursor may be in the form of a vapor, plasma, liquid, or spray liquid. The metal precursor pulse may include a metal precursor, or a mixture of a metal precursor and an inert gas, and the inert gas may be any of the inert gases described earlier herein. The inert gas may be used as a carrier gas for transporting the metal precursor to the ALD chamber.

[0105] In an embodiment, the ALD protective layer 120 may be a metal fluoride compound containing a plurality of different metals. In an embodiment, the ALD protective layer 120 has the general formula A X M Y F Z wherein A is a first metal selected from the group consisting of Mg, Ca, Li, and Al, M is a second metal different from the first metal A, selected from the group consisting of Mg, Ca, Li, and Al, X is the number of moles of the first metal A, Y is the number of moles of the second metal, and Z is the number of moles of fluorine (F). In an embodiment, the ALD protective layer 120 is Li X Al Y F Z or Ca X Al Y F Z wherein X is the number of moles of Li or Ca respectively, Y is the number of moles of Al, and Z is the number of moles of F. Other metal fluorides containing a mixture of different metals are conceivable. A metal fluoride ALD coating containing a plurality of different metals may be made by exposing an optical component to metal precursor pulses having a plurality of different metal precursors, each having a different metal.

[0106] The fluorine source can be derived from a fluorine-containing precursor selected from the group consisting of sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), ammonium fluoride (NH4F), trifluoroiodomethane (CF3I), hydrogen fluoride (HF), and combinations thereof. In an embodiment, the ALD process can be a plasma-assisted ALD process in which the fluorine source is a fluorine-containing precursor or a plasma fluorine source derived from a fluorine-containing precursor and argon (Ar) plasma. In an embodiment, the fluorine source can be a plasma containing SF6, SF6 and Ar (SF6 / Ar), or NF3 and Ar (NF3 / Ar). In an embodiment, the fluorine source can be derived from one or more organic fluorine sources such as, but not limited to, hexafluoroacetylacetone or other fluorine-containing organic compounds. However, for organic fluorine sources, additional pulse steps such as long ozone pulses will be required in the ALD process to remove carbon compounds attributable to the organic fluorine source from the ALD coating.

[0107] HF is commonly used as a fluorine source in ALD coating operations. However, HF is dangerous to handle and highly corrosive, especially when in contact with water. Therefore, a safer alternative to HF is desirable. The SF6 fluorine precursor is significantly safer to use compared to HF and is more productive than organic fluorine sources, which require a four-step process and long ozone pulses to form a metal fluoride ALD protective layer. In embodiments, the fluorine source may include SF6 or a plasma derived from SF6 (i.e., an SF6-based plasma). In embodiments, the fluorine source may comprise, consist of, or consist essentially of an SF6-based fluorine source such as SF6 or an SF6-based plasma. In embodiments, the fluorine source may comprise, consist of, or consist essentially of a plasma derived from SF6 and Ar (i.e., an SF6 / Ar plasma) or SF6 and another inert gas. When the fluorine source includes an SF6 / Ar plasma, the flow rate ratio of Ar to SF6 may be from 0.1:1 to 10:1, from 0.1:1 to 5:1, from 0.1:1 to 2:1, from 0.5:1 to 10:1, from 0.5:1 to 5:1, from 0.5:1 to 2:1, from 1:1 to 10:1, from 1:1 to 5:1, from 1:1 to 2:1, from 2:1 to 10:1, from 2:1 to 5:1, or about 2:1, where the flow rate is a volumetric flow rate expressed in units of sccm (standard cubic centimeters per minute).

[0108] As described above, the ALD process may be a direct reduction process in which the ALD protective layer 120 is deposited by binding a coordinated metal to the surface of the optical component and then directly reducing the coordinated metal with a fluorine source to produce an ALD metal fluoride layer. However, when SF6, an SF6 plasma, or an SF6 / Ar plasma is used as the fluorine source, the resulting ALD coating may have a high concentration of carbon impurities originating from the ligands of the coordinated metal. Without intending to be bound by any particular theory, it is believed that sulfur from SF6 reacts with the ligand and damages or cleaves the ligand during the reaction of the coordinated metal with the fluorine source to form the metal fluoride, thus leaving the carbon or carbon-containing fragments of the ligand within the ALD protective layer.

[0109] When SF6 is used to provide a fluorine source, the concentration of carbon deposits in the ALD protective layer 120 can be reduced or eliminated by performing an oxide formation step between the metal precursor pulse and the fluorine source pulse. The oxide formation step may include exposing a mirror having a layer of coordinated metal deposited on its surface to an oxygen source for a pulse duration sufficient to oxidize or convert the coordinated metal to a metal oxide. By exposing the coordinated metal to a pulse containing an oxygen source, the ligands of the coordinated metal react with the oxygen of the oxygen source to replace the ligands with oxygen, which will bond to the metal (e.g., the coordinated metal undergoes an oxidation reaction to convert the layer of coordinated metal into a metal oxide layer). After the metal oxide is formed, the remaining oxygen source in the ALD chamber may be purged. Then, the mirror having a layer of metal oxide on its outermost surface can be exposed to a pulse containing a fluorine source. When the metal oxide is exposed to the fluorine source, the metal oxide is converted to a metal fluoride of the ALD protective layer 120. The oxygen source may include water (H2O), H2O plasma, ozone (O3), O3 plasma, oxygen (O2), O2 plasma, hydrogen peroxide, other oxygen-containing gases, other oxygen-containing liquids, or combinations thereof. The oxygen source may be in a liquid state, a gaseous state, or a plasma state. In embodiments, the oxygen source pulse may include an oxygen source, or an oxygen source combined with one or more inert gases, where the inert gas may be any of those previously described herein.

[0110] When an SF6-based fluorine source is used, an ALD process that includes first converting metal ligands to a metal oxide with an oxygen source pulse and then converting the metal oxide to a metal fluoride with a fluorine source pulse may result in an ALD protective layer having a lower concentration of carbon compared to the direct reduction of metal ligands by the fluorine source. Without intending to be bound by any particular theory, it is believed that the oxidation of the ligands of the coordinated metal may completely remove the ligands from the metal without decomposing the ligands, thereby eliminating or significantly reducing fragments of organic (carbon-containing) components that would otherwise remain bonded to the metal or present in the ALD protective layer 120.

[0111] In an embodiment, the method disclosed herein includes exposing an etched surface 122 of a mirror's reflective layer 110 to a pulse containing a metal precursor to form a metal ligand layer, exposing the metal ligand layer to a pulse containing an oxygen source to form a metal oxide layer, and then exposing the metal oxide layer to a pulse containing a fluorine source to convert the metal oxide to a metal fluoride of an ALD protective layer 120. The ALD process of the method disclosed herein may first include exposing a mirror to a metal precursor. During a pulse of the metal precursor, the metal precursor, in the form of vapor, plasma, or a spray liquid, may be introduced into an ALD chamber containing a mirror (i.e., a substrate 102 including the reflective layer 110 after atomic layer etching) by injection or the like. When the surface of the mirror is exposed to a pulse containing a metal precursor, the metal precursor may react with aluminum at the etched surface 122 of the mirror's reflective coating 110 to bond a monolayer (monomolecular layer) of coordinated metal to the etched surface 122 of the reflective coating 110. The metal precursor may be any of the metal precursors described previously herein. The metal precursor pulse may have a duration sufficient for the metal precursor to react with at least 90%, at least 95%, at least 98%, at least 99%, or even at least 99.9% of the reactive aluminum sites on the etched surface 122 of the reflective layer 110. The metal precursor pulse may have a pulse duration from 10 ms to 10 seconds, or about 1 second. The monolayer of coordinated metal bonded to the surface may have a thickness equal to the size of a single molecule of the metal ligand. For a pulse of the metal precursor following the first ALD metal fluoride layer, the metal precursor may react with the previously deposited ALD metal fluoride layer to bond a subsequent monolayer of coordinated metal to the outer surface of the ALD metal fluoride layer. After depositing and bonding a monolayer of coordinated metal to the outer surface of the mirror (e.g., the etched surface 122 or the outer surface of the previously deposited ALD metal fluoride layer), the ALD coating process may further include stopping the exposure of the mirror to the metal precursor. Stopping the exposure of the mirror to the metal precursor may include stopping the flow of the metal precursor into the ALD chamber.Next, prior to continuing the ALD process, the ALD chamber may be purged with an inert gas to remove any residual metal precursor from the ALD chamber.

[0112] After exposing the optical component to the metal precursor and purging the ALD chamber, the ALD process portion of the methods disclosed herein can include exposing the mirror having the coordinated metal layer bonded thereto to an oxygen source. The oxygen source can include water, water plasma, oxygen, oxygen plasma, ozone, ozone plasma, hydrogen peroxide, hydrogen peroxide plasma, an oxygen-containing liquid, an oxygen-containing gas, or combinations thereof. The oxygen source can be in a liquid state, a gaseous state, or a plasma state. The step of exposing the metal layer coordinated on the surface of the mirror to the oxygen source can include introducing a pulse containing the oxygen source into the ALD chamber containing the mirror. In an embodiment, the oxygen source pulse can include an oxygen source or an oxygen source combined with one or more inert gases, where the inert gas can be any of the inert gases previously described herein. By exposing the mirror to the oxygen-containing pulse, the coordinated metal may be oxidized, and a metal oxide may be formed on the surface of the mirror. The oxygen source pulse can have a pulse duration sufficient for the oxygen source to react with at least 90%, at least 95%, at least 98%, at least 99%, or even at least 99.9% of the coordinated metal bonded to the surface of the mirror. The oxygen source pulse can have a pulse duration of from 0.1 second to 1 second, or about 0.3 second. The ALD process can further include stopping the exposure of the mirror to the oxygen source pulse, such as by stopping the flow of the oxygen source into the ALD chamber, at the end of the oxygen source pulse. In an embodiment, the ALD chamber may then be purged with an inert gas after the oxygen source pulse, thereby removing any residual oxygen and organic compounds from the ALD chamber.

[0113] The ALD process may further include a step of exposing a mirror having a layer of metal oxide deposited on its surface to a fluorine source after an oxygen source pulse. The step of exposing the mirror to the fluorine source may include introducing a pulse containing the fluorine source into the ALD chamber containing the mirror. The fluorine source may be any of the compositions described hereinabove for the fluorine source. In embodiments, the fluorine source is an SF6-based fluorine source such as, but not limited to, SF6, SF6 plasma, SF6 / Ar plasma, or combinations thereof. Fluorine from the fluorine source may reduce the metal oxide to form a metal fluoride of the ALD protective layer 120 on the surface of the mirror to produce a reinforced aluminum mirror 100. The fluorine source pulse may have a pulse duration sufficient for fluorine to react with at least 90%, at least 95%, at least 98%, at least 99%, or even at least 99.9% of the metal oxide on the surface of the mirror to produce the reinforced aluminum mirror 100. The fluorine source pulse may have a pulse duration of from 10 ms to 30 s, for example, from 10 ms to 20 s, from 10 ms to 10 s, from 1 s to 30 s, from 1 s to 20 s, from 1 s to 10 s, from 3 s to 30 s, from 3 s to 20 s, or from 3 s to 10 s. This process may further include a step of stopping the exposure of the reinforced aluminum mirror 100 to the fluorine source, such as by stopping the flow of the fluorine source into the ALD chamber at the end of the fluorine source pulse. As described above, exposing the reinforced aluminum mirror 100 to an oxygen source after exposure to the metal precursor and before exposure to the fluorine source will result in a lower concentration of carbon in the ALD protective layer 120 applied to the reflective coating 110 of the reinforced aluminum mirror 100 compared to the case of alternating pulses of the metal precursor and the fluorine source without a pulse containing the oxygen source.

[0114] A mirror with a substrate 102 and a reflective coating 110 may be contacted with or exposed to a metal precursor, a fluorine source, an oxygen source, or a combination thereof under operating conditions sufficient for a chemical reaction to occur on the surface of the mirror and deposit an ALD protective layer 120 on the reflective coating 110. The ALD process may be performed at a process temperature sufficient for a metal precursor, an oxygen source, a fluorine source, or a combination thereof to react on the surface of the optical component. In embodiments, the ALD process may include depositing an ALD coating on the surface of the optical component at a process temperature of from 120 °C to 250 °C.

[0115] In embodiments, the ALD process may be a plasma-assisted ALD process in which a plasma material is utilized in one or more of a metal precursor pulse, an oxygen source pulse, a fluorine source pulse, or a combination thereof. A metal precursor, an oxygen source, a fluorine source, or a combination thereof may be converted to a plasma by heating the material and exposing the material to an electric current or a strong electromagnetic field. The material (e.g., a metal precursor, an oxygen source, a fluorine source, or a combination thereof) may be heated to the ALD process temperature and exposed to an electric current sufficient to convert the material to a plasma. In embodiments, the step of converting the material (e.g., a metal precursor, an oxygen source, a fluorine source, or a combination thereof) to a plasma may include heating the material to a temperature of from 100 °C to 325 °C, or from 120 °C to 250 °C, and applying an electric current having an output of from 100 watts (W) to 300 W, or about 200 W.

[0116] The ALD process may be repeated multiple times to increase the thickness of the ALD protective layer 120. With each iteration of the ALD process, another molecular layer of ALD metal fluoride may be added to the ALD protective layer 120. The thickness of the ALD protective layer 120 can be controlled by controlling the number of iterations of the ALD process, i.e., by controlling the number of molecular layers of ALD metal fluoride present in the ALD protective layer 120. The growth rate of the ALD process for manufacturing the ALD protective layer 120 can be about 0.5 angstroms per cycle, where a cycle includes a series of metal precursors followed by a fluorine source, or a complete series of metal precursor - oxygen source - fluorine source.

[0117] In an embodiment, the ALD protective layer 120 may comprise a laminate of different ALD metal fluoride layers, where each of the different ALD metal fluoride layers is made from a metal fluoride having a different metal from the metal fluoride in an adjacent layer of the laminate. In an embodiment, the ALD protective layer 120 may comprise a laminate of metal fluorides including, but not limited to, an amorphous ALD - AlF3 layer disposed between layers of polycrystalline ALD metal fluoride layers such as an ALD - CaF2 layer, an ALD - MgF2 layer, or an ALD - LiF2 layer. Other combinations of different metal fluoride layers formed in the laminate are also contemplated.

[0118] Referring back to FIG. 2, the enhanced aluminum mirror 100 of the present disclosure includes a substrate 102, a reflective coating 110 made of PVD - Al deposited on the mirror surface of the substrate 102, and one or more ALD protective layers 120 deposited on the reflective coating 110. The ALD protective layer 120 can bond to the etching surface 122 of the reflective coating 110. In an embodiment, the ALD protective layer 120 can also be bonded to other surfaces of the substrate 102 that do not have the reflective coating 110, in which case the ALD protective layer 120 can be directly bonded to the surface of the substrate 102. The ALD protective layer 120 can include, but is not limited to, metal fluorides such as AlF3, MgF2, CaF2, LiF, or combinations thereof. In an embodiment, the ALD protective layer 120 can include an ALD metal fluoride layer that contains 90% or more, 95% or more, 98% or more, 99% or more, or 99.9% or more metal fluoride based on the total mass of the ALD protective layer 120.

[0119] In an embodiment, the ALD protective layer 120 can be an ALD metal fluoride and may contain sulfur in addition to the metal fluoride. In an embodiment, the ALD protective layer may include an ALD metal fluoride layer and may have a sulfur content in the ALD metal fluoride layer that is greater than zero parts per million (ppm), for example, greater than zero ppm to 300 ppm, or greater than 1 ppm to 250 ppm, or greater than 5 ppm to 200 ppm, or greater than 10 ppm to 150 ppm, or greater than 25 ppm to 125 ppm.

[0120] In an embodiment, the ALD protective layer 120 may be made of an ALD metal fluoride and may be substantially carbon-free. In an embodiment, the ALD protective layer 120 may have a carbon concentration of 10,000 ppm or less, or 5,000 ppm or less, or 1,000 ppm or less, or less than 500 ppm in the ALD protective layer 120. In an embodiment, the reflective coating 110, the ALD protective layer 120, or both may have less than 5 atomic percent (at%) of oxygen, for example, 4 at% or less, 3 at% or less, 2 at% or less, or even 1 at% or less of oxygen, where the atomic percent of oxygen is the number of oxygen atoms in the structure divided by the total number of atoms in the structure. In an embodiment, the reflective coating 110, the ALD protective layer 120, or both may have from 0.1 at% to about 5 at% of oxygen atoms.

[0121] Referring to FIG. 2, the ALD protective layer 120 has a total thickness t sufficient to cover the etch surface 122 of the reflective coating 110 without substantially exposing the atmosphere within the aluminum-coated area of the reflective coating 110. ALD In an embodiment, the ALD protective layer has a total thickness t of 40 nm or less, 30 nm or less, 25 nm or less, 20 nm or less, or even 10 nm or less. ALD In an embodiment, the ALD protective layer 120 may have a total thickness t of 1 nm or more, 3 nm or more, 5 nm or more, or even 10 nm or more. ALD In an embodiment, the ALD protective layer 120 has a total thickness t of from 1 nm to 40 nm, from 1 nm to 30 nm, from 1 nm to 25 nm, from 1 nm to 20 nm, from 1 nm to 10 nm, from 3 nm to 40 nm, from 3 nm to 30 nm, from 3 nm to 25 nm, from 3 nm to 20 nm, from 3 nm to 10 nm, from 5 nm to 40 nm, from 5 nm to 30 nm, from 5 nm to 25 nm, from 5 nm to 20 nm, from 5 nm to 10 nm, from 10 nm to 40 nm, from 10 nm to 30 nm, from 10 nm to 25 nm, from 10 nm to 20 nm, from 20 nm to 40 nm, or from 20 nm to 30 nm. ALDmay have. In embodiments, the ALD protective layer may include a number of ALD metal fluoride layers, where each of the number of ALD metal fluoride layers is made from a different coating material. In these embodiments, each of the number of ALD metal fluoride layers may have a thickness of from 1 nm to 30 nm, from 1 nm to 25 nm, from 1 nm to 20 nm, from 1 nm to 10 nm, from 3 nm to 30 nm, from 3 nm to 25 nm, from 3 nm to 20 nm, from 3 nm to 10 nm, from 5 nm to 30 nm, from 5 nm to 25 nm, from 5 nm to 20 nm, from 5 nm to 10 nm, from 10 nm to 30 nm, from 10 nm to 25 nm, from 10 nm to 20 nm, or from 10 nm to 30 nm. The total thickness t of the ALD protective layer 120 ALD may be the sum of the thicknesses of each of the individual ALD metal fluoride layers for the ALD protective layer 120 made from a plurality of different ALD metal fluorides.

[0122] The ALD protective layer 120 may be a conformal coating having a uniform thickness across the entire coated surface. In embodiments, the ALD protective layer 120 may have a thickness that varies by no more than 5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or even 0.5% or less from the average thickness of the ALD protective layer 120. The average thickness of the ALD protective layer 120 is the total thickness t of the ALD protective layer 120 averaged over the entire surface area of the surface in contact with the ALD protective layer 120 ALD is.

[0123] The enhanced aluminum mirror 100 can have a light reflectivity of 77% or more, 80% or more, 81% or more, or even 82% or more in the wavelength range of 110 nm to 180 nm (or 150 nm to 180 nm). The reflectivity was measured by using a commercially available VUV spectrophotometer. In embodiments, the enhanced aluminum mirror 100 can include a high-reflectivity ALD layer, such as, but not limited to, a lanthanum fluoride ALD layer, a gadolinium fluoride ALD layer, or other high-reflectivity layers. When the ALD protective layer 120 includes a high-reflectivity ALD layer, the enhanced aluminum mirror 100 can have a reflectivity of VUV wavelength light greater than that of a substrate having a pure aluminum reflective coating, such as greater than 90% or even greater than 92%.

[0124] Referring to FIG. 2, the ALD coating process can deposit an ALD protective layer uniformly on not only the mirror surface 104 having the reflective coating 110, but also one, a plurality, or all of the surfaces of the substrate. In embodiments, the enhanced aluminum mirror 100 can include an ALD protective layer 120 deposited on and / or in contact with at least 95%, at least 98%, at least 99%, or even at least 99.5% of the surface of the enhanced aluminum mirror intended to be coated (e.g., the etched surface 122 of the reflective coating 110 and other surfaces of the substrate 102). The surface of the enhanced aluminum mirror 100 intended to be coated is a surface that is not intentionally masked to prevent ALD coating. The ALD protective layer 120 may also be deposited on and / or in contact with other reflective surfaces of the optical component, such as surfaces where laser beams or light rays are not expected to be incident.

[0125] Referring now to FIG. 11, as described above, the reinforced aluminum mirror 100 may have an ALD protective layer 120 including a plurality of ALD protective layers, such as, but not limited to, a first ALD protective layer 140, a second ALD protective layer 150, etc., applied to the surface of the reinforced aluminum mirror 100. In an embodiment, the reinforced aluminum mirror 100 may have a first ALD protective layer 140 that directly contacts and is directly bonded to the etched surface 122 of the reflective layer 110, and at least one second ALD protective layer 150 applied over the first ALD protective layer 140. The second ALD protective layer 150 may be of a material different from that of the first ALD protective layer 140. The second ALD protective layer 150 may be directly bonded to the first ALD protective layer 140 such that the inner surface of the second ALD protective layer 150 contacts and bonds to the outer surface of the first ALD protective layer 140 without any intervening layer or coating disposed between the first ALD protective layer 140 and the second ALD protective layer 150. One or more additional ALD protective layers may be deposited over the second ALD protective layer 150 to provide a stack of ALD protective layers. Each of the first ALD protective layer 140, the second ALD protective layer 150, subsequent ALD coating layers, or combinations thereof may have a thickness of up to 40 nm, for example, from 1 nm to 40 nm, or from 5 nm to 30 nm. In an embodiment, one or more of the ALD protective layers may have a thickness greater than 40 nm. In an embodiment, the thickness t ALD1 of the first ALD protective layer 140, the thickness t ALD2 of the second ALD protective layer 150, or both may be from 1 nm to 40 nm, from 3 nm to 30 nm, or from 5 nm to 20 nm.

[0126] The reinforced aluminum mirror 100 with a plurality of ALD protective layers 120 can be prepared by depositing an aluminum reflective layer 110 on the mirror surface 104 of a substrate using a PVD process, etching the outer surface 112 of the reflective layer 110 by an ALE process to generate an etched surface 122 of the reflective layer 110, depositing a first ALD protective layer 140 on the etched surface 122 of the reflective layer 110, and then depositing a second ALD protective layer 150 on the outer surface of the first ALD protective layer 140. As a result, the first ALD protective layer 140 is disposed between the reflective layer 110 and the second ALD protective layer 150. After depositing the second ALD protective layer 150, subsequent ALD protective layers can be applied until the desired ALD coating structure is obtained. The first ALD protective layer 140, the second ALD protective layer 150, and any other subsequent ALD protective layers can be deposited on one or more surfaces of the reinforced aluminum mirror 100 by any of the ALD processes described earlier in this specification.

[0127] The reinforced aluminum mirror 100 can include a substrate 102, a reflective layer 110 made of PVD - Al, a first ALD protective layer 140 made of ALD - AlF3 deposited on the etched surface 122 of the reflective layer 110, and a second ALD protective layer 150 made of ALD - MgF2 deposited on the outer surface of the first ALD protective layer. In an embodiment, the first ALD protective layer 140 made of ALD - AlF3 can have a thickness from 5 nm to 25 nm, and the second ALD protective layer 150 made of ALD - MgF2 can have a thickness from 5 nm to 25 nm. In an embodiment, the reinforced aluminum mirror 100 can include a first ALD protective layer 140 made of ALD - AlF3 having a thickness of 5 nm and a second ALD protective layer 150 made of ALD - MgF2 having a thickness of 23 nm.

[0128] In an embodiment, the reinforced aluminum mirror 100 can include an ALD protective layer 120 including a plurality of alternating ALD-AlF3 layers and ALD-MgF2 layers. In an embodiment, the reinforced aluminum mirror can include a first ALD protective layer made of 5 nm of ALD-AlF3, a second ALD protective layer made of 9 nm of ALD-MgF2, a third ALD protective layer made of 5 nm of ALD-AlF3, and a fourth ALD protective layer made of 9 nm of ALD-MgF2. In an embodiment, the reinforced aluminum mirror 100 can include a first ALD protective layer made of 5 nm of ALD-AlF3, a second ALD protective layer made of 2 nm of ALD-MgF2, a third ALD protective layer made of 5 nm of ALD-AlF3, a fourth ALD protective layer made of 2 nm of ALD-MgF2, a fifth ALD protective layer made of 5 nm of ALD-AlF3, and a sixth ALD protective layer made of 2 nm of ALD-MgF2. Other combinations of ALD materials, the number of ALD protective layers, and the total thickness are conceivable.

[0129] In an embodiment, the reinforced aluminum mirror 100 can include a substrate 102, a reflective coating 110, a first ALD protective layer 140, and a second ALD protective layer 150, where the second ALD protective layer 150 is made of a high-reflectivity metal fluoride that can improve the reflectivity of the reinforced aluminum mirror 100 compared to an aluminum mirror that does not include a high-reflectivity metal fluoride. In an embodiment, the ALD protective layer 120 of the reinforced aluminum mirror 100 can include a first ALD protective layer made of ALD-AlF3 and a second ALD protective layer 150 made of lanthanum fluoride (LaF3), gadolinium fluoride (GdF3), or other high-reflectivity metal fluorides. In an embodiment, the reinforced aluminum mirror 100 can include an aluminum reflective layer 110 having a thickness of 70 nm to 100 nm, a first ALD protective layer 140 made of 10 nm to 30 nm of ALD-AlF3, and a second ALD protective layer 150 made of 5 nm to 20 nm of ALD-LaF3.

[0130] The enhanced aluminum mirror 100 of the present disclosure having a reflective coating 110 made of PVD-Al and an ALD protective layer 120 including an ALD metal fluoride coating can be used as a mirror in various VUV or EUV applications, such as a VUV or EUV lithography or inspection system for manufacturing and / or inspecting microelectronics. The enhanced aluminum mirror 100 can be used for lasers having wavelengths within the VUV range, such as a beam having a wavelength from 110 nm to 180 nm, although not limited thereto. In embodiments, the enhanced aluminum mirror 100 can be used for beams having wavelengths longer than 180 nm.

Example

[0131] Embodiments of the coated optical component described herein and the ALD process for manufacturing the coated optical component will become further apparent by the following examples.

[0132] Example 1 - AlF 3 ALD protective layer made therefrom - 4nm In Example 1, a reinforced aluminum mirror was prepared according to the method disclosed herein and shown in the flowchart of FIG. 9. For the reinforced aluminum mirror of Example 1, first, the substrate was coated with an aluminum reflective layer by a PVD process. The initial thickness of this reflective layer was 100 nm. Next, the substrate having the reflective layer was transferred to the ALD chamber of the ALD system. The substrate and the reflective layer were subjected to ALE in the ALD chamber to remove a 4-nm reflective layer. The ALE process included the step of applying alternating pulses of a fluorine source and an organometallic compound to the substrate and the reflective layer. For Example 1, the fluorine source was SF6 / Ar plasma, and the organometallic compound was trimethylaluminum (TMA). The ALE was carried out at 225°C. For each fluorine source pulse, the substrate and the reflective coating were exposed to SF6 / Ar plasma for a 7-second exposure period with an ICP output of 300 W. After the fluorine pulse, the ALD chamber was purged with Ar. For each organometallic pulse, the exposure period to TMA was 400 ms, and then, for a 10-second sealing period, the throttle valve of the ALD chamber was closed. After sealing, the throttle valve was opened, and the ALD chamber was purged with Ar again. The ALE process was repeated until a 4-nm reflective coating was removed. The etching rate was 1.1 angstroms per cycle for the ALE process.

[0133] After ALE, the etched surface of the reflective layer was coated with an ALD protective layer made of 4-nm AlF3. The etched surface of the protective layer was exposed to alternating pulses of a metal precursor (TMA) and a fluorine source, and an ALD process was carried out by starting the pulse of TMA. This ALD process was carried out at a temperature of 225°C. For the TMA pulse, the exposure time was 40 ms, and the throttle valve was maintained in the open position during the TMA pulse. The fluorine pulse was carried out with the same exposure time and output as the ALE process. The growth rate of the ALD protective layer was 0.5 angstroms per cycle. The resulting reinforced aluminum mirror comprised a substrate, an aluminum metal reflective layer having a thickness of 96 nm, and an ALD protective layer made of AlF3 having a thickness of 4 nm.

[0134] Comparative Example 2 - PVD only For Comparative Example 2, an aluminum reflective coating was deposited by a PVD process, and then the reflective coating was passivated by depositing a low-density PVD-AlF3 layer and a dense PVD-AlF3 layer according to the method shown in the flowchart of FIG. 5, thereby preparing an aluminum mirror. The aluminum mirror of Comparative Example 2 included a 100-nm PVD-Al reflective coating, a 15-nm low-density PVD-AlF3 layer, and a 10-nm dense PVD-AlF3 layer. The reflective coating made of PVD-Al was applied at room temperature by the PVD process. After deposition of the aluminum reflective coating, a low-density PVD-AlF3 layer was formed at room temperature using the PVD process. After deposition of the low-density PVD-AlF3 layer, the aluminum mirror was heated to a temperature of 200° C. and a dense PVD-AlF3 layer was applied. The total thickness of the PVD passivation layer was 25 nm.

[0135] Comparative Example 3 - Hybrid For Comparative Example 3, an aluminum reflective coating was deposited by a PVD process, followed by deposition of a low-density PVD-AlF3 layer, and passivation of the reflective coating by depositing a dense ALD-AlF3 protective layer by an ALD process to prepare an aluminum mirror according to the method shown in the flowchart of FIG. 6. The aluminum mirror of Comparative Example 3 included a 100-nm PVD-Al reflective coating, a 15-nm low-density PVD-AlF3 layer, and a 10-nm ALD-AlF3 layer. The reflective coating made from PVD-Al was applied at room temperature by a PVD process. After deposition of the aluminum reflective coating, a low-density PVD-AlF3 layer was formed at room temperature using a PVD process. After deposition of the PVD-AlF3 layer, the mirror was transferred from the PVD system to the ALD chamber of the ALD system. In the ALD chamber, the aluminum mirror with the reflective coating and the low-density PVD-AlF3 layer was subjected to a fluorine wash to remove carbon and oxygen impurities as much as possible. This fluorine wash treatment included exposing the reflective coating and the low-density PVD-AlF3 layer in the ALD chamber to sulfur hexafluoride (SF6) plasma at 150° C. for a period of 30 seconds. After the fluorine wash treatment, an ALD protective layer made from ALD-AlF3 was deposited on the outer surface of the low-density PVD-AlF3 layer. For the ALD process, the metal precursor of the metal precursor pulse was TMA and the fluorine source was SF6 / Ar plasma. The ALD process of Comparative Example 3 was carried out at a temperature of 150° C. The sequence, pulse duration, and output were the same as those described above for the ALD process of Example 1.

[0136] Example 4 - Reflectivity In Example 4, the reflectivities of the reinforced aluminum mirror of Example 1 and the aluminum mirrors of Comparative Examples 2 and 3 were evaluated according to the method disclosed herein. In Example 4, the reflectivity was measured by using a commercially available VUV spectrophotometer. Referring again to FIG. 8A, for the reinforced aluminum mirror (806) of Example 1, the aluminum mirror (802) of Comparative Example 2, and the aluminum mirror (804) of Comparative Example 3, the reflectivity (Y-axis) as a function of wavelength (X-axis) is shown in a graph. As shown in FIG. 8A, the PVD-only aluminum mirror (082) of Comparative Example 2 was shown to have a significantly reduced reflectivity of up to 0.70 at wavelengths between 150 nm and 180 nm. The aluminum mirror (804) of Comparative Example 3, which includes a PVD-Al layer, a low-density PVD-AlF3 layer, and an ALD-AlF3 protective layer, also showed a significant decrease in reflectivity to about 0.75 at wavelengths between 150 nm and 180 nm. The aluminum mirror (804) of Comparative Example 3 provides better reflectivity performance compared to the PVD-only aluminum mirror (802) of Comparative Example 2, but still shows a decrease in reflectivity performance over the VUV wavelength range due to aluminum oxidation and the accumulation of aluminum oxide in the reflective coating and the protective layer.

[0137] As shown in FIG. 8A, the reinforced aluminum mirror (806) of Example 1 showed higher reflectivity over the wavelength range from 150 nm to 180 nm compared to the mirrors of Comparative Example 2 (802) and Comparative Example 3 (804). Further, note that the protective coating of the reinforced aluminum mirror of Example 1 has only a 4-nm ALD protective layer, while the mirrors of Comparative Examples 2 and 3 had a passivation layer (PVD-AlF3, ALD-AlF3, or both) with a total thickness of 25 nm. Thus, the reinforced aluminum mirror (806) of Example 1 provides excellent reflection performance and protection of the aluminum in the reflective coating, while being able to make the thickness of the ALD protective layer much thinner compared to the PVD-only mirror of Comparative Example 2 and the mirror manufactured by the hybrid PVD-ALD process in Comparative Example 3.

[0138] Example 5 - Reinforced aluminum mirror with two ALD protective layers For Example 5, a reinforced aluminum mirror having a first ALD protective layer made of ALD - AlF3 and a second ALD protective layer made of ALD - MgF2 was prepared. Using a PVD process, an aluminum reflective coating having a thickness of 100 nm was deposited. After PVD, the mirror was transferred to the ALD chamber of the ALD process, and an ALE process was performed to remove a 4 - nm reflective coating. The steps, materials, and conditions of the ALE process were the same as those previously described in Example 1. The final thickness of the reflective coating after etching was 96 nm. According to the process described in Example 1, a first ALD protective layer made of ALD - AlF3 was deposited. The thickness of the first ALD protective layer was 12 nm.

[0139] Next, a second ALD protective layer was deposited on the first ALD protective layer. For the second ALD protective layer, the ALD chamber was cooled to a temperature of 150 °C. The magnesium precursor was (EtCp)2Mg (i.e., bis(ethylcyclopentadienyl)magnesium). The temperature of the bubbler was set at 92 °C, and the ICP plasma output was 200 W. The pulse duration of the metal precursor pulse containing (EtCp)2Mg was 1 second, and then a 9 - second purge with an inert gas was performed. After the metal precursor pulse and the purge, a water pulse having a duration of 40 milliseconds was performed, and a purge with an inert gas was performed for 8 seconds. After the water pulse and the purge, the optical component was subjected to a fluorine source pulse containing a mixture of SF6 and argon. The SF6 / Ar flow ratio was 30 / 15, and the duration of the fluorine source pulse was 7 seconds. After the fluorine source pulse, a purge pulse was performed. A series of metal precursor pulses / water pulses / fluorine source pulses were repeated until the thickness of the MgF2 ALD coating reached 9 nm.

[0140] Example 6 - SIMS Regarding Example 6, the reinforced aluminum mirrors of Examples 1 and 5 and the aluminum mirrors of Comparative Examples 2 and 3 were analyzed using secondary ion mass spectrometry (SIMS) to evaluate the relative amounts of aluminum, aluminum fluoride, oxygen, carbon, and hydrogen in various coating layers. The secondary ion mass spectrometry was performed using a secondary ion mass spectrometer. The SIMS analysis was carried out by sputtering the surface of the mirror with cesium ions (Cs + ) having a kinetic energy of 2 kV and analyzing in the positive mode of Bi 3+ ions at 30 kV. In FIGS. 12 to 15, the vertical dashed lines represent the interfaces between various coatings and layers, and each layer is shown in the space above the graph.

[0141] Referring to FIG. 12 here, the SIMS analysis of the reinforced aluminum mirror of Example 1 is shown in the graph. Reference numeral 1200 indicates the transition point between PVD-Al of the reflective coating and ALD-AlF3 of the ALD protective layer. In FIG. 12, the various normalized amounts (Y-axis) in the coating are plotted as a function of the depth (X-axis) from the outer surface of the ALD protective layer. Table 2 below gives the cross-reference between the reference numerals in FIG. 12 and the species corresponding thereto.

[0142]

Table 2

[0143] As shown in FIG. 12, the normalized amount of oxygen in the coating does not increase at the interface 1200 between the aluminum reflective layer and the ALD protective layer made of ALD-AlF3. This indicates that all of the natural oxide formed on the surface of the aluminum reflective coating was effectively removed by the ALE process. Furthermore, it was not observed that interfacial oxide continued to accumulate between the reflective layer 110 and the ALD protective layer 120 in the reinforced aluminum mirror of Example 1.

[0144] Referring now to Figure 13, the SIMS analysis of the enhanced aluminum mirror of Example 5 is shown in a graph. Reference numeral 1300 indicates the transition point between PVD-Al of the reflective coating and ALD-AlF3 of the first ALD protective layer, and reference numeral 1302 indicates the transition point between ALD-AlF3 of the first protective layer and ALD-MgF2 of the second ALD protective layer. In Figure 13, various normalized amounts (Y-axis) during the coating are plotted as a function of the depth (X-axis) from the outer surface of the second ALD protective layer. Table 2 gives a cross-reference between the reference numerals in Figure 13 and the species corresponding thereto. As shown in Figure 13, the normalized amount of oxygen during the coating does not increase at the interface 1300 between the aluminum reflective layer and the ALD protective layer made of ALD-AlF3, and in fact, it decreases in the ALD-AlF3 layer. This indicates that all of the native oxide formed on the surface of the aluminum reflective coating has been effectively removed by the ALE process. Furthermore, it was not observed that interfacial oxides subsequently accumulated between the reflective layer and the first ALD protective layer in the enhanced aluminum mirror of Example 5. Further, no accumulation of carbon or oxygen was observed near the interface 1302 between the first ALD coating and the second ALD coating. This indicates that no oxidation occurred during the cooling of the ALD chamber during the transition from ALD-AlF3 to ALD-MgF2.

[0145] Referring now to Figure 14, the SIMS analysis regarding the PVD-only aluminum mirror of Comparative Example 2 is shown in a graph. Reference numeral 1400 is aluminum oxide (AlO formed on the outer surface of the PVD-Al of the reflective coating and the reflective layer x) shows the transition points between the layers, and reference numeral 1402 indicates the transition point between the aluminum oxide layer and the passivation layer made of PVD - AlF₃. In FIG. 14, various normalized amounts (Y - axis) during coating are plotted as a function of the depth (X - axis) from the outer surface of the PVD - AlF₃ layer. Table 2 gives a cross - reference between the reference numerals in FIG. 14 and the species corresponding thereto. As shown in FIG. 14, the normalized oxygen content increases in the region of interface 1400 and then decreases on the other hand at interface 1402. This local increase in aluminum oxide bonding indicates the accumulation of oxygen species on the surface of the PVD - Al reflective layer, and thus shows the reaction of residual oxygen in the PVD chamber with aluminum during the deposition of the PVD - AlF₃ passivation layer.

[0146] Referring now to FIG. 15, the SIMS analysis for the PVD - only aluminum mirror of Comparative Example 2 is added to the SIMS analysis for the aluminum mirror of Comparative Example 3. Reference numeral 1500 indicates the transition point between the PVD - Al of the reflective coating and the aluminum oxide (AlO x ) layer formed on the outer surface of the reflective layer, and reference numeral 1502 indicates the transition point between the aluminum oxide layer and the hybrid passivation layer made of PVD - AlF₃ and ALD - AlF₃. In FIG. 15, the normalized amounts (Y - axis) of oxygen, aluminum, and AlF₃ during coating are plotted as a function of the depth (X - axis) from the outer surface of the ALD - AlF₃ layer. Table 2 gives a cross - reference between the reference numerals in FIG. 15 and the species corresponding thereto. As shown in FIG. 15, the aluminum mirror of Comparative Example 3 shows an increase in oxide at the transition point between the PVD - Al of the reflective layer and the AlF₃ of the protective layer. This is similar to the increase in oxide observed in the PVD - only aluminum mirror of Comparative Example 2. Without intending to be bound by any particular theory, the oxidation of the aluminum surface of the reflective layer in Comparative Example 3 is thought to occur mainly during the deposition of the low - density PVD - AlF₃ layer at room temperature. Since the ALD conditions are more reducing than the PVD conditions compared to PVD - AlF₃, the oxygen content of ALD - AlF₃ is lower.

[0147] Example 7 - UV exposure In Example 7, the reinforced aluminum mirror of Example 1 and the PVD-only aluminum mirror with a PVD-AlF3 passivation layer were exposed to UV ozone to evaluate the effectiveness of the protective layer in reducing or preventing oxidation of the reflective layer. As described above, the reinforced aluminum mirror of Example 1 was made from ALD-AlF3 and had an ALD protective layer with a thickness of 4 nm. The PVD-only aluminum mirror had a protective layer with a thickness of 18 nm made from PVD-AlF3. The exposure time was increased from 0 to 99 minutes, 2×99 minutes, and 3×99 minutes.

[0148] Referring now to FIG. 16, the reflectance (Y-axis) of the PVD-only aluminum mirror as a function of wavelength (X-axis) shows a decrease in reflectance for wavelengths in the range of 150 to 180 nm, indicating that oxidation occurs even without exposure to UV ozone. Referring now to FIG. 17, a graph shows the reflectance (Y-axis) as a function of wavelength (X-axis) for the reinforced aluminum mirror of Example 1. Table 3 below provides a cross-reference between the reference numbers in FIG. 17 and the exposure time of the mirror to UV ozone.

[0149]

Table 3

[0150] As shown in FIG. 17, for each of reference numbers 1702, 1704, and 1706, the reflectance exceeded 0.8 over the wavelength range of 150 to 180 nm. When the exposure to UV ozone was 3×99 minutes, the reinforced aluminum mirror showed a decrease in reflectance indicating oxidation. However, FIG. 17 demonstrates that even an ALD-AlF3 protective layer with a small thickness of about 4 nm provides good protection to the PVD-Al mirror.

[0151] Example 8 - Reinforced aluminum mirror with a high reflectivity layer In Example 8, a predictive example of a reinforced aluminum mirror can comprise a first ALD protective layer and a second ALD protective layer made of a high-reflectivity fluoride. For Example 8, the high-reflectivity fluoride is lanthanum fluoride (LaF3). The aluminum reflective coating is a PVD aluminum coating and has a thickness of 100 nm. The first ALD protective layer is made of ALD-AlF3 and has a thickness of 28 nm. The second ALD protective layer is made of ALD-LaF3. The ALD-LaF3 of the second ALD protective layer can be prepared by subjecting the mirror to an ALD process involving alternating pulses of a lanthanum precursor and a fluorine source. The ALD process for depositing ALD-LaF3 can be carried out at a temperature in the range of 250 °C to 350 °C. The lanthanum precursor can be any of the lanthanum precursors disclosed herein, such as, but not limited to, tris(N,N'-diisopropylformamidinato)lanthanum, and each of the lanthanum precursor pulses can have a pulse duration of about 2 seconds. The fluorine pulse can be carried out as described previously herein. The thickness of the second ALD protective layer made of ALD-LaF3 is 10 nm.

[0152] The theoretical reflectivity of the reinforced aluminum mirror of Example 8 is calculated based on the properties of the materials in each of the reflective coating of the reinforced aluminum mirror of Example 8 and the ALD protective layers. For comparison, the theoretical reflectivity of an aluminum mirror having only a reflective coating made of PVD-Al as a function of wavelength is also calculated. Referring now to FIG. 18, the reflectivity (Y-axis) as a function of wavelength (X-axis) for the reinforced aluminum mirror of Example 8 and the mirror having only the aluminum reflective layer is shown in a graph. In FIG. 18, the solid line (reference numeral 1802) is the reflectivity of the mirror of aluminum only, and the dashed line (reference numeral 1804) is the reflectivity of the reinforced aluminum mirror of Example 8. As shown in FIG. 18, the presence of the high-reflectivity ALD-LaF3 layer can improve the theoretical reflectivity performance of the reinforced aluminum mirror compared to a mirror having only a PVD-Al reflective layer.

[0153] Humidity test Referring again to FIG. 8A, a humidity test was performed on each of the aluminum mirrors of reference numerals 806 and 802 to determine the stability of the reflective coating on each mirror. FIG. 19A shows the results of the aluminum mirror of reference numeral 806, and FIG. 19B shows the results of the aluminum mirror of reference numeral 802. The humidity test included exposing the aluminum mirror to an 80% humid environment at a temperature of 80° C. for a period of 1 hour. The reflectivity of each mirror was measured before and after the humidity test. As shown in FIG. 19A, the enhanced aluminum mirror of 806 had substantially the same reflectivity after the humidity test as before, and thus demonstrated the durability and stability of the enhanced aluminum mirror 806. In contrast, as shown in FIG. 19B, the aluminum mirror 802 had a much lower reflectivity after the humidity test than before.

[0154] It will be apparent to those skilled in the art that various modifications and changes can be made to the embodiments described herein without departing from the spirit and scope of the subject matter of the claims. That is, this specification is intended to cover various modifications and changes to the various embodiments described herein, provided that such modifications and changes fall within the scope of the appended claims and their equivalents.

[0155] Hereinafter, preferred embodiments of the present invention will be described item by item.

[0156] Embodiment 1 In a method for manufacturing an enhanced aluminum mirror for a vacuum ultraviolet (VUV) optical element, depositing a reflective coating made of aluminum metal on at least one surface of a substrate in a PVD system by physical vapor deposition (PVD) to produce a mirror comprising the substrate and the reflective coating, removing aluminum oxide from the outer surface of the reflective coating by performing atomic layer etching (ALE) in an atomic layer deposition (ALD) system to produce an etched surface of the reflective coating, and A step of manufacturing a reinforced aluminum mirror including the substrate, the reflective coating deposited on the substrate, and the ALD protective layer covering the etched surface of the reflective coating by depositing an ALD protective layer on the etched surface of the reflective coating by performing atomic layer deposition within the ALD system. A method including the above.

[0157] Embodiment 2 The method according to Embodiment 1, further including a step of moving the substrate including the reflective coating from the PVD system to the ALD system, and exposing the reflective coating to oxygen by moving the substrate including the reflective coating to the ALD system, causing oxidation of aluminum on the outer surface of the reflective coating to form aluminum oxide.

[0158] Embodiment 3 The step of performing atomic layer etching within the ALD system includes a step of exposing the substrate and the reflective coating to alternating pulses of a fluorine source and an organometallic compound. By exposing the substrate and the reflective coating to a pulse containing the fluorine source, aluminum oxide is converted to aluminum fluoride, forming a thin layer of aluminum fluoride on the outer surface of the reflective coating. The method according to Embodiment 1, wherein by exposing the thin layer of aluminum fluoride to a pulse containing the organometallic compound, it reacts with the aluminum fluoride to form a volatile organometallic compound released from the outer surface of the reflective coating.

[0159] Embodiment 4 The method according to Embodiment 3, further including a step of exposing the reflective coating to alternating pulses of the fluorine source and the organometallic compound at a temperature of 150°C to 325°C and an ICP output of 50 watts (W) to 600 W.

[0160] Embodiment 5 The method according to Embodiment 3, further comprising the step of exposing the etched surface of the reflective coating to the fluorine source for an exposure time of from 1 second to 60 seconds.

[0161] Embodiment 6 The method according to Embodiment 3, wherein the fluorine source comprises SF6, SF6 plasma, or a plasma containing SF6 and argon (Ar), and the organometallic compound comprises trimethylaluminum (TMA), triethylaluminum (TEA), dimethylaluminum chloride (DMAC), silicon tetrachloride (SiCl4), aluminum hexafluoroacetylacetonate (Al(hfac)3), tri-i-butylaluminum (Al(iBu)3), tin(II) acetylacetonate (Sn(acac)2), tris(2,2,6,6-tetramethyl-3,5-heptanedionato)aluminum (i.e., Al(TMHD)3), or a combination thereof.

[0162] Embodiment 7 The method according to Embodiment 3, further comprising the step of exposing the thin layer of aluminum fluoride to the organometallic compound for a total exposure time of from 10 milliseconds (8 ms) to 60,000 ms, the total exposure time being equal to the pulse length and the closed period of the pulse of the organometallic compound.

[0163] Embodiment 8 The method according to Embodiment 3, further comprising the step of exposing the thin layer of aluminum fluoride to the organometallic compound at a pressure of from 10 millitorr (1.33 Pa) to 100 torr (13,332 Pa).

[0164] Embodiment 9 The step of exposing the thin layer of aluminum fluoride to the pulse containing the organometallic compound comprises injecting the organometallic compound into the ALD chamber for the pulse length, and A step of closing the throttle valve of the ALD system, wherein by the step of closing the throttle valve, the flow of materials into and out of the ALD chamber is blocked, and the thin layer of aluminum fluoride remains in contact with the organometallic compound for a sealing period of 1 second to 60 seconds, the step of closing the throttle valve. The method according to Embodiment 3, comprising.

[0165] Embodiment 10 A step of opening the throttle valve again, and A step of purging the ALD chamber with an inert gas to remove at least 99% of the remaining organometallic compound, the volatile organometallic compound, or both from the ALD chamber. The method according to Embodiment 9, further comprising.

[0166] Embodiment 11 The ALD protective layer includes a metal fluoride protective coating containing one or more of aluminum trifluoride (AlF3), magnesium fluoride (MgF2), calcium fluoride (CaF2), lithium fluoride (LiF), lanthanum fluoride (LaF3), gadolinium fluoride (GdF3), or combinations thereof. The method according to Embodiment 1, wherein the step of applying a protective ALD coating to the outer surface of the etched aluminum layer includes exposing the etched aluminum layer to alternating pulses of a metal precursor and a fluorine source.

[0167] Embodiment 12 The method according to Embodiment 11, wherein the fluorine source includes SF6, SF6 plasma, or a plasma containing SF6 and argon (Ar).

[0168] Embodiment 13 The method according to embodiment 11, wherein the metal precursor comprises an aluminum precursor selected from one or more of trimethylaluminum (TMA), triethylaluminum (TEA), dimethylaluminum isopropoxide (DMAI), [MeC(NiPr)2]AlEt2, dimethylaluminum hydride, dimethylethylamine, ethylpiperidine, dimethylaluminum hydride, or combinations thereof.

[0169] Embodiment 14 The method according to embodiment 11, wherein the ALD protective layer is made of magnesium fluoride (MgF2).

[0170] Embodiment 15 A step of depositing a first ALD protective layer on the etched surface of the reflective coating, and A step of depositing a second ALD protective layer on the outer surface of the first ALD protective layer, The method according to embodiment 1, comprising:

[0171] Embodiment 16 The method according to embodiment 1, wherein the ALD protective layer is made of a high-reflectivity metal fluoride, and the high-reflectivity metal fluoride increases the reflectivity of the reinforced aluminum mirror as compared to a mirror having only the reflective coating.

[0172] Embodiment 17 In a reinforced aluminum mirror for an ultraviolet optical system, A substrate having a surface, A reflective coating deposited on the surface of the substrate, the reflective coating being made of aluminum metal deposited by a physical vapor deposition method, and An ALD protective layer deposited on the etched surface of the reflective coating, Comprising: The ALD protective layer is applied by atomic layer deposition, The reflective coating reflects light having a wavelength in at least the vacuum ultraviolet wavelength range, The ALD protective layer is a reinforced aluminum mirror that reduces or prevents oxidation of the aluminum in the reflective coating.

[0173] Embodiment 18 The reinforced aluminum mirror according to Embodiment 17, wherein the reflective coating and the ALD protective layer contain less than 5 atomic percent of oxygen atoms.

[0174] Embodiment 19 The reinforced aluminum mirror according to Embodiment 17, wherein the ALD protective layer is made of a high-reflectivity metal fluoride, and the high-reflectivity metal fluoride increases the reflectivity of the reinforced aluminum mirror compared to a mirror having only the reflective coating.

[0175] Embodiment 20 The reinforced aluminum mirror according to Embodiment 17, wherein the ALD protective layer includes a first ALD protective layer made of a first ALD metal fluoride and a second ALD protective layer made of a second ALD metal fluoride different from the first ALD metal fluoride.

Description of Reference Numerals

[0176] 100 Reinforced aluminum mirror 102 Substrate 104 Mirror surface of the substrate 110 Reflective coating 112 Outer surface of the reflective coating 114 Aluminum 116 Aluminum oxide layer 120 ALD protective layer 122 Etching surface of the reflective coating 130 Low-density PVD - AlF3 layer 140 First ALD coating layer 150 Second ALD coating layer 200 Mirror having only the reflective coating 1000 ALD system 1002 ALD chamber 1004 Heating device Inlet of the ALD chamber Outlet of the ALD chamber Throttle valve Vacuum pump Fluorine source Fluorine source control valve Metal precursor source Metal precursor source control valve Oxygen source Oxygen source control valve Inert gas source Inert gas control valve

Claims

1. A method for manufacturing a reinforced aluminum mirror for a vacuum ultraviolet (VUV) optical element, a step of depositing a reflective coating made of aluminum metal on at least one surface of a substrate in a PVD system by physical vapor deposition (PVD) to manufacture a mirror including the substrate and the reflective coating, a step of removing aluminum oxide from the outer surface of the reflective coating by performing atomic layer etching (ALE) in an atomic layer deposition (ALD) system to generate an etched surface of the reflective coating, and a step of depositing an ALD protective layer on the etched surface of the reflective coating by performing atomic layer deposition in the ALD system to manufacture a reinforced aluminum mirror including the substrate, the reflective coating deposited on the substrate, and the ALD protective layer covering the etched surface of the reflective coating, a method including the above steps.

2. The method according to claim 1, further including a step of moving the substrate including the reflective coating from the PVD system to the ALD system, wherein by moving the substrate including the reflective coating to the ALD system, the reflective coating is exposed to oxygen, causing oxidation of aluminum on the outer surface of the reflective coating to form aluminum oxide.

3. The step of performing atomic layer etching in the ALD system includes a step of exposing the substrate and the reflective coating to alternating pulses of a fluorine source and an organometallic compound, by exposing the substrate and the reflective coating to a pulse including the fluorine source, the aluminum oxide is converted to aluminum fluoride, forming a thin layer of aluminum fluoride on the outer surface of the reflective coating, and by exposing the thin layer of aluminum fluoride to a pulse including the organometallic compound, reacting with the aluminum fluoride to form a volatile organometallic compound released from the outer surface of the reflective coating.

4. The method according to claim 3, further including a step of exposing the reflective coating to alternating pulses of the fluorine source and the organometallic compound at a temperature of 150 °C to 325 °C and an ICP output of 50 watts (W) to 600 W.

5. The method according to claim 3, further including a step of exposing the etched surface of the reflective coating to the fluorine source for an exposure time of 1 second to 60 seconds.

6. The fluorine source is SF ６ , SF ６ plasma, or SF ６ and a plasma containing argon (Ar), and the organometallic compound is trimethylaluminum (TMA), triethylaluminum (TEA), dimethylaluminum chloride (DMAC), silicon tetrachloride (SiCl ４ ), aluminum hexafluoroacetylacetonate (Al(hfac) ３ ), tri-i-butylaluminum (Al(iBu) ３ ), tin(II) acetylacetonate (Sn(acac) ２ ), tris(2,2,6,6-tetramethyl-3,5-heptanedionato)aluminum (i.e., Al(TMHD) ３ ), or a combination thereof, the method according to claim 3.

7. The method according to any one of claims 3 to 6, further comprising a step of exposing the thin layer of aluminum fluoride to the organometallic compound for a total exposure time of 10 milliseconds (8 ms) to 60,000 ms, the total exposure time being equal to the pulse length and the sealing period of the pulse of the organometallic compound.

8. The method according to any one of claims 3 to 6, further comprising a step of exposing the thin layer of aluminum fluoride to the organometallic compound at a pressure of 10 millitorr (1.33 Pa) to 100 torr (13,332 Pa).

9. The step of exposing the thin layer of aluminum fluoride to the pulse containing the organometallic compound is a step of injecting the organometallic compound into the ALD chamber over a pulse length, and a step of closing the throttle valve of the ALD system, the step of closing the throttle valve preventing the flow of materials into and out of the ALD chamber and leaving the thin layer of aluminum fluoride in contact with the organometallic compound for a sealing period of 1 second to 60 seconds. The method according to any one of claims 3 to 6, comprising

10. In a reinforced aluminum mirror for an ultraviolet optical system, a substrate having a surface, a reflective coating deposited on the surface of the substrate, the reflective coating being made of aluminum metal deposited by physical vapor deposition, and an ALD protective layer deposited on the etched surface of the reflective coating, comprising the ALD protective layer being applied by atomic layer deposition, the reflective coating reflecting light having a wavelength in at least the vacuum ultraviolet wavelength range, and the ALD protective layer reducing or preventing oxidation of the aluminum of the reflective coating, a reinforced aluminum mirror.

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