Spectral shift free Anti-reflection coatings

Abstract

A method of depositing an anti-reflection (AR) coating on an optical element, such as a lens, is disclosed, the method including tailoring an AR layer thickness on the optical element by tilting / masking in physical vapor deposition and depositing a nanolaminate capping layer overtop the AR layer by atomic layer deposition. A total thickness of the nanolaminate capping layer is less than about 30 nm.

Description

SPECTRAL SHIFT FREE ANTI -REFLECTION COATINGSCROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Serial No. 63 / 617450 filed on January 4, 2024, the content of which is relied upon and incorporated herein by reference in its entirety.FIELD

[0002] The disclosure generally relates to anti-reflection coatings, and in particular to AR coatings comprising an environmentally protective cap.BACKGROUND

[0003] Anti-reflection (AR) coatings are an important aspect to precision optical elements, such as optical lenses used in laser applications. Many of these coatings, for example PVD fluoride films, can be porous and may absorb moisture when exposed to normal laboratory environments, e.g., 45% relative humidity (RH) at a room temperature. As a result, the coated lens may exhibit a spectral redshift due to changes in refractive index of the film.

[0004] What is needed is a spectral-shift-free AR coating for precision optics that maintains optical element surface reflectance to less than 0.5% for a prolonged lab exposure, i.e., a couple of days to a few weeks, instead of a few hours, and / or suitable for deep ultra-violet (DUV) precision optics objective coatings in a range from about 193 nm to about 266 nm. This extended time window can allow the completion of potting, testing, and final assembly of the DUV objectives.SUMMARY

[0005] Ina first aspect, an optical element is disclosed comprising a substrate comprising CaF2, SiO2, orF-SiO2, an AR coating deposited on the substrate, the AR coating comprising a PVD- deposited AR layer. The PVD-deposited layer comprises a plurality of discrete AR layers, the discrete AR layers comprising a first discrete AR layer comprising a high refractive index material or a low refractive index material and a second discrete AR layer deposited on the first discrete AR layer, the second discrete AR layer comprising a low refractive index material ifthe first discrete AR layer is a high refractive index if the first discrete AR layer is a low refractive index material. A nanolaminate capping layer is disposed over the PVD-deposited AR layer, the nanolaminate capping layer comprising an ALD -deposited AI2O3 layer and an ALD -deposited SiCh capping layer, the nanolaminate capping layer comprising a total thickness equal to or less than about 30 nm.

[0006] In a second aspect, the total thickness of the nanolaminate capping layer may be equal to or less than about 20 nm.

[0007] In a third aspect, the total thickness of the nanolaminate capping layer is equal to or less than about 10 nm.

[0008] In a fourth aspect, the nanolaminate capping layer comprises a plurality of capping layers and a thickness of each capping layer of the plurality of capping layers is equal to or less than about 0.5 nm.

[0009] In a fifth aspect, the AR coating may comprise an SiCh barrier layer disposed between the AR layer and the nanolaminate capping layer.

[0010] In a sixth aspect, the optical element comprises a lens comprising at least one curved surface and the AR coating is deposited on the curved surface.

[0011] In a seventh aspect, a reflectance of the optical element at an external surface of the AR coating is equal to or less than about 0.5% at an angle of incidence in a range from 0 degrees to 35 degrees at a wavelength of 193.4 nanometers.

[0012] In an eighth aspect, a reflectance of the optical element at an external surface of the AR coating is equal to or less than about 0.5% at an angle of incidence in a range from 0 degrees to 40 degrees at a wavelength of 266 nanometers.

[0013] In a ninth aspect, the SiC>2 capping layer is in contact with the AI2O3 capping layer.

[0014] In a tenth aspect, the PVD-deposited AR layer comprises a first layer of AI2F3 deposited on the substrate, a second layer of GdF3 deposited on the first layer, a third layer of AI2F3 deposited on the second layer, the optical element further comprising an SiCh barrier layer deposited on the PVD-deposited AR layer, the nanolaminate capping layer deposited on the SiO2 barrier layer.

[0015] In an eleventh aspect, the PVD-deposited AR layer comprises a first layer of HfCh deposited on the substrate, a second layer of SiCh deposited on the first layer, a third layer of HfO2 deposited on the second layer, and a fourth layer of SiO2 deposited on the third layer.

[0016] In a twelfth aspect, a method of making an AR -coated optical element is disclosed, comprising depositing on a substrate by PVD an AR layer comprising a high refractive indexmaterial layer and a low refractive index material or F-SiO2. The method further comprises depositing over the AR layer by atomic layer deposition a nanolaminate capping layer with a total thickness equal to or less than about 30 nm, the nanolayer capping layer comprising an AI2O3 capping layer and an SiCh capping layer.

[0017] In a thirteenth aspect, the total thickness of the nanolaminate capping layer is equal to or less than about 10 nm.

[0018] In a fourteenth aspect, the total thickness of the nanolaminate capping layer is equal to or less than about 5 nm.

[0019] In a fifteenth aspect, the method may further comprise depositing an SiCh barrier layer on the AR layer prior to the depositing the nanolaminate capping layer.

[0020] In a sixteenth aspect, a thickness of the SiO2 barrier layer is in a range from about 5 nm to about 60 nm.

[0021] In a seventeenth aspect, the SiCh capping layer is in contact with the AI2O3 capping layer.

[0022] In an eighteenth aspect, an optical axis of the optical element is tilted away from at least one of a source of the high refractive index material or a source of the low refractive index material during the depositing at least one of the AR layer or the nanolaminate capping layer.

[0023] Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0024] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments, and together with the description explain principles and operation of the various embodiments.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a plot of reflectance in percent as a function of the angle of incidence of impinging light on an optical element (e.g., lens), showing spectral performance of a 3L + QR AR-coated lens vs. a 3L AR-coated lens;

[0026] FIG. 2 is a cross-sectional view of a len^ numbered El -El 5;

[0027] FIG. 3 is a plot of reflectance in percent as a function of the angle of incidence of impinging light on an optical element (e.g., lens), showing spectral performance of a 3L + QR AR-coated lens vs. a 3L + Q enhanced AR-coated lens;

[0028] FIG. 4 is a plot of reflectance as a function of wavelength in nanometers (nm) showing a redshift due to environmental exposure;

[0029] FIG. 5 is a schematic view of a lens assembly comprising 10 lens elements numbered El to ElO;

[0030] FIG. 6 A is a schematic view of a deposition process utilizing tilted physical vapor deposition;

[0031] FIG. 6B is a schematic view of a deposition process not utilizing tilted physical vapor deposition;

[0032] FIG. 7 is a plot of reflectance as a function of wavelength in nanometers (nm) showing a redshift due to environmental exposure;

[0033] FIG. 8 is a cross-sectional side view of an exemplary calcium fluoride (CaF?) optical element comprising a fluoride-based AR coating (3L + IC + NL) comprising a 3-layer (3L) PVD-deposited fluoride AR layer and an ALD nanolaminate (NL) cap;

[0034] FIG. 9 is a plot of reflectance in percent as a function of angle of incidence in degrees for the exemplary optical element of FIG. 8, with the optical performance of a 3L+Q enhanced AR coated lens shown for comparison;

[0035] FIG. 10 is a cross-sectional side view of an exemplary optical element comprising an HPFS lens, a four layer (4L) AR layer, and an ALD deposited nanolaminate (NL) cap, with the spectral performance of a 4L AR coated optical element without capping layer shown for comparison; and

[0036] FIG. 11 is a plot of reflectance in percent as a function of angle of incidence in degrees for the exemplary optical element of FIG. 10.DETAILED DESCRIPTION

[0037] In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of various principles of the present disclosure. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that thepresent disclosure may be practiced in other emboGim mj men nvm m uvia.o disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of various principles of the present disclosure. Finally, wherever applicable, like reference numerals refer to like elements.

[0038] Ranges can be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0039] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow; plain meaning derived from grammatical organization or punctuation; the number or type of embodiments described in the specification.

[0040] As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a “component” includes aspects having two or more such components, unless the context clearly indicates otherwise.

[0041] For brevity, ranges of values disclosed herein, including compositional ranges or attribute (performance) ranges, or series of ranges, may be appended by the phrase “including all ranges and subranges therebetween,” which is to be interpreted as including whole number or decimal subranges as though explicitly presented. Thus, by way of example, a range between 6 and 8 (units omitted) implicitly includes a subrange between 6.4 and 8, or a subrange between 6 and 7.2, or a subrange between 6 and 7, and so forth. Additionally, a series of ranges, such as “in a range from 6 to 11 or in a range from 6 to 8” implicitly includes a range from 7 to 10, or subranges therebetween, such as 7.2 to 10.4, as though explicitly presented, provided the range does not exceed the minimum or maximum endpoints of the explicitly presented range or series of ranges. Thus, for example, “in a range from 6 to 11 or in a range from 6 to 8” has as endpoints 6 and 11.

[0042] Anti-reflection coatings find use in the marlulsuch as optical lenses used in laser applications. Many of these coatings, such as physical vapor deposited (PVD) fluoride coatings, e.g., coatings applied by evaporation, can be porous and may absorb moisture when exposed to normal laboratory environments, e.g., 45% relative humidity (RH) at room temperature, e.g., 22°C. PVD is a line-of-sight deposition process and may pose a problem for steep lens surfaces wherein the coating material arrives at very large impingement angles relative to a normal to the lens surface. As a result, coating thickness and mechanical properties can be substantially different toward the edge of the lens surface compared to the center of the lens surface. For example, the coating may be thinner and have a reduced packing density toward the edge of the lens full clear aperture (CA), which is the largest aperture of a lens that meets a predetermined optical specification. As a result, a standard PVD coating on a hemispherical lens may result in a thickness uniformity as low as 20% of the coating thickness at the center of the lens. This reduced coating thickness uniformity may lead to high spectral reflectance and polarization split at the edge of the lens. Tilting of the lens during coating deposition can improve coating uniformity on steep surfaces but may reduce coating packing density toward the center of the lens, leading to increased scattering loss at the lens center. Plasma interaction with the tilted surface may be difficult when a high tilt angle is selected to achieve a desired uniformity. Moreover, the effectiveness of plasma densification is reduced at high ion impingement angles and / or high angles of deposition for precision optics and plasma ion densification is ineffective when a lens is tilted away from the plasma source. Additionally, some lens fixture design challenges may accompany tilting during coating deposition. Tilting increases the coating deposition angle (> 60deg) at the 0 CA. High deposition angle results in high porosity.

[0043] Masking is another approach that may improve PVD coating uniformity, including stationary masks and moving masks. Stationary masking may work better for large optics, whereas moving masks may improve uniformity distribution over small optics. However, both tilting and masking are based on PVD processes.

[0044] It is desirable to have an AR coating with good uniformity, particularly on curved surfaces. Atomic layer deposition (ALD) is a thin film deposition process based on sequential exposure of a substrate to self -limiting surface half -reactions. Advantages of ALD for optical coatings include the placement of conforming layers on curved surfaces enabling application of AR coatings on both sides of a lens element simultaneously, simplified lens fixture design, and reduced risk of mechanically damaging the lens element during the coating process.

[0045] Environmentally stable deep ultraviolet (D laser optics by employing a half-wave silica barrier layer overtop the AR coating. The barrier layer comprises a thin protective silica layer Q and a thicker plasma -ion densified layer R. The Q layer may be a thin F-SiCh layer deposited using PVD at about 300°C without plasma involvement. The densified R layer typically utilizes plasma-ion assisted deposition (PIAD) of an F-SiO2 layer as an outermost densified layer. By way of example, the Q and R layer thicknesses may be 10 nanometer (nm) and 52 nm, respectively, for a 3-layer (3L) fluoride AR coating at 193.4 nm and a normal (90°) angle of incidence (AOI) of incoming light rays. While the densified R layer can effectively prevent moisture penetration with full densifi cation, the Q layer protects the underlying AR coating from plasma-induced damage during deposition of the densified R layer using a PIAD process. The densified R layer may have a packing density, for example, of 100%. FIG. 1 is a plot showing a side-by-side comparison of reflectance in percent as a function of AOI for incoming light rays for 3L (without QR barrier layers) and 3L+QR AR coatings at 193.4 nm. The total optical thickness of the QR layer correspond s to a half -wave optical thickness at 193.4 nm. The environmental protection afforded by the QR barrier layer comes at the cost of reduced anti-reflection properties. The data show that for an AOI of 40 degrees the 3L anti-reflection layer provides a reflectance of about 0.5%, while the 3L+QR AR coating provided a reduced reflectance of about 3%. However, the 3L+QR AR coating produced environmentally stable optical performance: A spectral shift-free AR was confirmed even after a 6-month lab exposure.

[0046] Lens assembly and testing is a complex and time-consuming process. Coated optical elements may sit in a testing facility (e.g., laboratory) longer than desired, wherein the optical element may be exposed to a greater humidity or temperature than desired. As a result, a well- aligned optical element, for example an objective lens in an optical assembly, may need disassembly to conduct additional ultraviolet-ozone (UVO) cleaning to bring axial transmission into specified parameters. A prolonged lab exposure may lead to a spectral redshift that results in an increase in reflectance at 193.4 nm up to about 1%. If all surfaces have 1% reflectance, an axial transmission of the system may be only 74%, which can be out of specification. This spectral redshift not only reduces the manufacturing productivity of precision optics but may impact system performance of the optical system in which the precision optical component is used.

[0047] Extending the barrier layer concept to wide angle AR coatings for high numerical aperture (NA) objectives is therefore considered. For example, a complex optical lensassembly shown in FIG. 2 comprises high purity fuo u Din ci \i n i u cinv cn z i no i ni iiio. The lens assembly comprises 15 lens elements El -El 5 and therefore 30 optical surfaces (left to right) with various surface curvatures. The angle of incidence at 100CA (or 100% clear aperture, i.e., including marginal rays) may be beyond 40 degrees for several HPFS and CaF? lens elements in the system while an axial transmission greater than 76% may be one of the system requirements. Wide-angle AR coatings have been specifically developed for precision optics, where the Q layer thickness is increased by 50% (e.g., from about 10 nm to about 15 nm). A densified R layer may or may not be used. This enhanced Q layer eliminates moisture penetration into the underlying AR (e.g., fluoride) coating layers during lens testing and assembly phases while maintaining a reflectance less than 0.5% over a wide range of angle of incidence. FIG. 3 is a plot showing a side-by-side comparison of a 3L+Q enhanced AR coating and a 3L+QR AR coating in terms of reflectance as a function of angle of incidence at 193.4 nm. The thickness of the enhanced Q layer (Q enhanced) may be about 15 nm for precision optics applications, whereas the Q layer thickness can be about 10 nm in the 3L+QR design for laser optical elements. As can be seen, the 3L+Q enhanced AR coating provides a broader angle of incidence, from about 0 degrees to about 40 degrees, when compared to an angle of incidence range of 0 degrees to about 30 degrees for a 3L+QR AR coating having a reflectance value of 0.5%. Combining masking and / or lens tilting during deposition along with ultravioletozone (UVO) cleaning, the 3L+Q enhanced AR coating can meet the system requirement. By using this 15 nm Q layer, axial transmission can be as high as 86% for the 30-surface lens assembly of FIG. 2. As an example, such an axial transmission value corresponds to an optical loss less than about 0.5% per surface for the 30-surface system.

[0048] UVO cleaning vaporizes absorbed moisture within the AR coatings. The enhanced Q layer reduces the rate of moisture re-penetration into the underlying AR coating. No noticeable spectral redshift occurs within a couple of hours after exposing a UVO -cleaned coated lens to lab conditions without N2 or dry -clean-air (DCA) purging. This operation window allows lens testing and assembly to be performed.

[0049] Still, there may be a problem in practice. Sometimes, the coated lens elements may sit in a lab longer than expected, during which time a coated lens may be exposed to a humidity or a temperature higher than desired. FIG. 4 plots an initial reflectance (dashed curve) as a function of wavelength at 0 CA of E15R1 (where R1 denotes the first surface of the 15thlens element, left side in the figure) with the 3L+Q enhanced AR coating. Initial reflectance is determined after UVO cleaning to remove absorbed moisture. The solid curve showsreflectance as a function of wavelength after several num o cn iwni iviii vi ci Luu ., emu 45% RH. The data show, for example, that the brief environmental exposure resulted in an approximately 12 nm shift in minimum reflectance, illustrating the wavelength dependence of the AR coating. A prolonged lab exposure may lead to spectral redshift that results in a reflectance increase to about 1% at 193.4 nm. If all surfaces have 1% reflectance, an axial transmission of the system is only 74%, which may be out of specification.

[0050] There is a need, therefore, for a spectral-shift-free AR coating for precision optics that maintains reflectance to less than about 0.5% for a prolonged lab exposure, e.g., a couple of days to a few weeks, instead of a few hours. This extended time window allows completion of potting, testing, and final assembly of DUV optical assemblies.

[0051] Also needed is a spectral-shift-free PIAD-oxide AR coating for tilt-applied coatings for precision optical assemblies at 266 nm. PIAD-SiCL and PI AD-HfChd-layer AR coatings may be used to address this challenge and to explain why there is a need even for PIAD-oxide AR coated lens. FIG. 5 is a cross-sectional optical diagram of a 266 nm optical assembly comprising 10 HPFS lenses. The first surface of lens element 9 (E9R1) has a convex shape with steep surface slopes. This surface is tilted from the optical axis at an angle of 47 degrees for the AR coating deposition. Also included is E1R1, a relatively flat surface, for comparison.

[0052] The advantages of PVD and atomic level deposition (ALD) are combined to provide high performance AR coatings for demanding precision optical systems at wavelengths in a range from about 190 nm to about 270 nm, e.g., 193 nm, 213 nm, and / or 266 nm. PVD is used to tailor a desired AR coating layer thickness distribution and ALD is used to deposit an AI2O3- SiO2 nanolamination barrier layer to seal the PVD -deposited AR coating, thereby providing a spectral-shift-free AR coating.

[0053] FIGS. 6A and 6B are schematic diagrams showing a tilted AR coating application for E9R1 (FIG. 6A) and a non-tilted AR coating forElRl (FIG. 6B). BothE9Rl and E1R1 AR coatings use a 4-layer alternating PIAD-HfCh and PIAD-SiCh coating. The AR coating on the E1R1 lens surface is spectral-shift-free, whereas the AR coating on the E9R1 lens surface is sensitive to lab exposure. Accordingly, exposure of E9R1 to a lab environment may lead to a spectral redshift in optical performance.

[0054] FIG. 7 plots initial (dashed curved) and redshifted (solid curve) spectral reflectance of E9R1 at 0 CA and 8 degrees AOI. As shown, the redshift moves the minimum spectral reflectance from 290 nm to 345 nm. The 55 nm red-shift leads to an increase of reflectance from 0.3% to 2.5% at 266 nm.

[0055] The present disclosure combines the adviim^^ emu uvuwi mgu performance AR coatings for demanding precision optical systems at 193 nm, 213 nm, and 266 nm. PVD is used to tailor a desired coating layer thickness distribution and ALD derived AhOs-SiO? nanolamination is used to seal the PVD AR coating, leading to spectral-shift-free AR coatings for precision optics.

[0056] FIG. 8 is a schematic diagram of an ALD nanolaminate (NL) capping layer over a 3L fluoride-based AR coating (3L + IC + NL) 100 on a calcium fluoride (CaF?) lens 102, the AR coating comprising a 3-layer PVD-fluoride AR layer (3L) 104 including a first AR layer 106 of aluminum fluoride (AIF3), a second AR layer 108 of gadolinium fluoride (GdFs), and athiid AR layer 110 of AIF3. A 5 nm PVD-Q (SiCh) barrier layer 112 (IC) is deposited overtop AR layer 104, and a nanolaminate (NL) capping layer 114 is deposited overtop the barrier layer, such as in contact with the barrier layer, NL capping layer 114 comprising at least a first capping layer 116 comprising AI2O3 and optionally a second capping layer 118 comprising SiO2 overtop the AI2O3 capping layer. NL capping layer 114 is deposited by ALD. A total thickness of NL capping layer 114 may be equal to or less than about 30 nm, for example equal to or less than about 20 nm, such as equal to or less than about 10 nm, equal to or less than about 5 nm, or even equal to or less than about 1 nm, including all ranges and subranges therebetween. The NL capping layer thickness may be measured, for example, by spectroscopic ellipsometry. As described, NL capping layer 114 may comprise a plurality of individual discrete capping layers. Individual capping layers may each be equal to or less than about 0.5 nm, for example equal to or less than about 0.3 nm. While two capping layers are shown, NL capping layer 114 may have more than two capping layers. By way of example, lens surface 118 is the E14R1 surface from the 30-surface lens system of FIG. 2 and the reflectance curve in percent reflectance as a function of AOI at 193.4 nm is plotted in FIG. 9. A curve of a 3L + Q enhanced coated lens is shown for comparison. FIG. 9 shows a reflectance of the optical element at an external surface of the AR coating equal to or less than about 0.5% at an angle of incidence in a range from 0 degrees to equal to or greater than 35 degrees at a wavelength of 193.4 nanometers after exposure of the optical element to a temperature of 22 °C and 45% RH for 48 hours. The ALD nanolaminate-capped AR coating 100 achieves similar spectral reflectance as that of the initial PVD 3L+Q enhanced AR coating at 193.4 nm but enables a substantially spectral-shift-free performance.

[0057] Fig. 10 is a cross-sectional schematic diagram of a 4L ALD deposited nanolaminate (NL) capped AR coating 200 on an HPFS lens 202 similar to that shown in FIG. 8. AR coating200 comprises a 4-layer PIAD-oxide AR layer 202v-r^ . m iuumg u / w ,aVv, of hafnium oxide (HfCL), a second AR layer 208 of silica (SiCL), a third AR layer 210 of HfCL, and a fourth AR layer 212 of silica. A nanolaminate (NL) capping layer 214 is deposited overtop AR layer 202, for example in contact with AR layer 202, NL capping layer 214 comprising a first capping layer 216 of AI2O3 and a second capping layer 218 of SiCh overtop the AI2O3 such that the silica capping layer is exposed to the external environment. NL capping layer 214 is deposited by ALD. A total thickness of NL capping layer 214 is equal to or less than about 30 nanometers, for example equal to or less than about 20 nm, such as equal to or less than about 10 nm, equal to or less than about 5 nm, or equal to or less than about 1 nm. NL capping layer 214 may comprise a single capping layer or a plurality of individual capping layers. Individual capping layers may be equal to or less than about 0.5 nm, for example equal to or less than about 0.3 nm. While two capping layers are shown, NL capping layer 214 may have fewer than two layers or more than two capping layers. The lens surface 220 is the E9R1 surface from the 10-lens system previously described and the reflectance curve at 193.4 nm is plotted in FIG. 11. FIG. 11 shows a reflectance of the optical element at an external surface of the AR coating is equal to or less than about 0.5% at an angle of incidence in a range from 0 degrees to equal to or greater than 40 degrees at a wavelength of 266 nanometers after exposure of the optical element to a temperature of 22°C and 45% RH for 48 hours. A 47-degree tilt was employed in both cases for the PVD process. A curve of a 4L AR coated lens is shown for comparison. Again, the ALD nanolaminate capped AR coating achieves similar spectral reflectance as that of the initial PVD 4L AR at 266 nm but enables spectral -shift-free performance.

[0058] It will be apparent to those skilled in the art that various modifications and variations can be made to embodiments of the present disclosure without departing from the spirit and scope of the disclosure. Thus, it is intended that the present disclosure cover such modifications and variations provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. An optical element, comprising: a substrate comprising CaF2, SiCh, or F-SiCh; an AR coating deposited on the substrate, the AR coating comprising: a PVD-deposited AR layer comprising a plurality of discrete AR layers, the discrete AR layers comprising: a first discrete AR layer comprising a high refractive index material or a low refractive index material; a second discrete AR layer deposited on the first discrete AR layer, the second discrete AR layer comprising a low refractive index material if the first discrete AR layer is a high refractive index material or a high refractive index material if the first discrete AR layer is a low refractive index material; and a nanolaminate capping layer disposed over the PVD-deposited AR layer, the nanolaminate capping layer comprising an ALD -deposited of AI2O3 capping layer and an ALD-deposited SiChcapping layer, the nanolaminate capping layer comprising a total thickness equal to or less than about 30 nm.

2. The optical element of claim 1, wherein the total thickness of the nanolaminate capping layer is equal to or less than about 20 nm.

3. The optical element of claim 1, wherein the total thickness of the nanolaminate capping layer is equal to or less than about 10 nm.

4. The optical element of any one of claims 1 to 3, wherein the nanolaminate capping layer comprises a plurality of capping layers and a thickness of each capping layer of the plurality of capping layers is equal to or less than about 0.5 nm.

5. The optical element of any one of claims 1 to 4, wherein the AR coating comprises an SiCh barrier layer disposed between the AR layer and the nanolaminate capping layer.

6. The optical element of any one of claims 1 to lens comprising at least one curved surface and the AR coating is deposited on the curved surface.

7. The optical element of any one of claims 1 to 6, wherein a reflectance of the optical element at an external surface of the AR coating is equal to or less than about 0.5% at an angle of incidence in a range from 0 degrees to 35 degrees at a wavelength of 193.4 nanometers.

8. The optical element of any one of claims 1 to 6, wherein a reflectance of the optical element at an external surface of the AR coating is equal to or less than about 0.5% at an angle of incidence in a range from 0 degrees to 40 degrees at a wavelength of 266 nanometers.

9. The optical element of any one of claims 1 to 7, wherein the SiCh capping layer is in contact with the AI2O3 capping layer.

10. The optical element of any one of claims 1 to 9, wherein the PVD -deposited AR layer comprises a first layer of AI2F3 deposited on the substrate, a second layer of GdF3 deposited on the first layer, a third layer of AI2F3 deposited on the second layer, the optical element further comprising an SiO2 barrier layer deposited on the PVD-deposited AR layer, the nanolaminate capping layer deposited on the SiCh barrier layer.

11. The optical element of any one of claims 1 to 9, wherein the PVD-deposited AR layer comprises a first layer of HfO2 deposited on the substrate, a second layer of SiO2 deposited on the first layer, a third layer of HfCh deposited on the second layer, and a fourth layer of SiCh deposited on the third layer.

12. A method of making an AR -coated optical element, comprising: depositing on a substrate by PVD an AR layer comprising a high refractive index material layer and a low refractive index material layer, the substrate comprising CaF2, SiCh, or F-SiCh; depositing over the AR layer by atomic layer deposition a nanolaminate capping layer with a total thickness equal to or less than about 30 nm, the nanolayer capping layer comprising an AI2O3 capping layer and an SiO2 capping layer.

13. The method of claim 12, wherein the total thickness of the nanolaminate capping layer is equal to or less than about 10 nm.

14. The method of claim 12, wherein the total thickness of the nanolaminate capping layer is equal to or less than about 5 nm.

15. The method of any one of claims 12 to 14, further comprising depositing an SiCh barrier layer on the AR layer prior to the depositing the nanolaminate capping layer.

16. The method of claim 15, wherein a thickness of the SiCh barrier layer is in a range from about 5 nm to about 60 nm.

17. The method of any one of claims 12 to 16, wherein the SiCh capping layer is in contact with the AI2O3 capping layer.

18. The method of any one of claims 12 to 17, wherein an optical axis of the optical element is tilted away from at least one of a source of the high refractive index material or a source of the low refractive index material during the depositing at least one of the AR layer or the nanolaminate capping layer.

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