Coating for optical elements

Abstract

An optical element including a substrate and a coating disposed on the substrate, the coating including a period of a first layer, a second layer, and a third layer. The first layer has a lower refractive index than the second layer, a material of the first layer is different from a material of the second layer, and the third layer has a higher density than the second layer.

Description

SP24-237COATING FOR OPTICAL ELEMENTS

[0001] This Application claims the benefit of priority to U.S. Provisional Patent Application Serial Number 63 / 691445 filed on September 6, 2024, the content of which is relied upon and incorporated herein by reference in its entirety.FIELD

[0002] The present disclosure generally relates to a coating for optical elements and, in particular, to coatings for optical elements with improved laser durability.TECHNICAL BACKGROUND

[0003] Optical systems may have various applications in research, medical procedures, and fabrication and microfabrication processes, such as photolithography, among other examples. For instance, an optical system may include one or more laser light sources, such as an excimer laser generating ultraviolet (UV) or deep ultraviolet light (DUV) light, which may be used to expose or apply laser light to a material, such as a substrate. Excimer lasers may produce light in or near the UV spectral region with relatively high peak and average powers and relatively high energies, thereby enabling, for example, photolithography procedures with improved resolution.

[0004] Such optical systems are susceptible to laser damage of the optical components and, therefore, utilize laser-durable coatings to extend the lifetimes of the optics. But as average powers and lifetime expectations increase, there is a continued need for improved laser durability of the optical coatings.SUMMARY

[0005] For microfabrication and lithography applications, stable performance of optical components such as windows, beam splitters and mirrors is critical. Therefore, it is important that such optical components maintain performance metrics such as transmission and reflection during the lifetime of the optical component.

[0006] Microfabrication and lithography systems used in the semiconductor industry, for example, utilize deep ultra-violet (DUV) laser-based light (i.e., light having a wavelength of 157 nm or in the range from 193 nm to 266 nm) or light in the broadband spectrum (i.e., lightSP24-237 having a wavelength in the range from 150 nm to 300 nm). The semiconductor industry uses deep UV (ultraviolet) light for lithography processes because of its shorter wavelength. The shorter the wavelength of light, the smaller the feature that can be created on a wafer. Deep UV light enables the creation of much smaller, more precise, and more densely packed features on a semiconductor wafer, which allows for the creation of faster and more powerful chips.

[0007] The embodiments of the present disclosure comprise a coating for DUV or broadband wavelength based optical systems. The coatings disclosed herein are disposed on an optical element to increase laser durability of the optical element.

[0008] According to a first aspect an optical element is disclosed, the optical element comprising a substrate and a coating disposed on the substrate, the coating comprising a period comprising a first layer, a second layer, and a third layer. The first layer has a lower refractive index than the second layer, a material of the first layer is different from a material of the second layer, and the third layer has a higher density than the second layer.

[0009] According to a second aspect an optical element is disclosed, the optical element comprising a substrate and a coating disposed on the substrate. The coating comprises a first layer, a second layer, and a capping layer, the first layer has a lower refractive index than the second layer, a material of the first layer is different from a material of the second layer, and the capping layer has a higher density than the second layer.

[0010] Additional features and advantages of the optical elements and methods of making same described herein 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 described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0011] It is to 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 character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments and are incorporated into 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 operations of the claimed subject matter.SP24-237BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. l is a schematic illustration of an optical element having a coating thereon, according to embodiments described herein;

[0013] FIG. 2 is a schematic illustration of another optical element having a coating thereon, according to embodiments described herein;

[0014] FIG. 3 is a schematic illustration of another optical element having a coating thereon, according to embodiments described herein;

[0015] FIG. 4 is a schematic illustration of another optical element having a coating thereon, according to embodiments described herein;

[0016] FIG. 5 is a schematic illustration of another optical element having a coating thereon, according to embodiments described herein;

[0017] FIG. 6 is a schematic illustration of another optical element having first and second coatings thereon, according to embodiments described herein;

[0018] FIG. 7 is a schematic illustration of an ultraviolet lithography system, according to embodiments described herein; and

[0019] FIG. 8 is a plot of average fluence vs. millions of pulses for an exemplary optical element and a comparative optical element.DETAILED DESCRIPTION

[0020] Reference will now be made in detail to various embodiments of coatings for optical elements having improved laser durability while maintaining environmental stability.

[0021] According to embodiments, a coating for an optical element is disclosed. The coated optical element includes, in an order moving away from a substrate, a period including a first layer and a second layer and a capping layer including SiCh, F-SiCh, or a combination thereof. In embodiments, the first layer comprises a relatively lower refractive index than the second layer. Furthermore, in some embodiments, the second layer is an undensified oxide layer. In yet some embodiments, the period further includes a third layer such that the second layer has a lower density than the third layer.SP24-237

[0022] Various embodiments of optical elements and methods of making the same will be described herein with specific reference to the appended drawings.

[0023] Ranges may 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.

[0024] Directional terms as used herein - for example up, down, right, left, front, back, top, bottom - are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0025] 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, nor that with any apparatus specific orientations be required. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or that any apparatus claim does not actually recite an order or orientation to individual components, or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, or that a specific order or orientation to components of an apparatus is not recited, it is in no way intended that an order or orientation 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, operational flow, order of components, or orientation of components; plain meaning derived from grammatical organization or punctuation, and; the number or type of embodiments described in the specification.

[0026] 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.

[0027] “Density,” as described herein, is mass density and is calculated using the following Equation (I):SP24-237(»2~1 ) _ NAa z J .(n2+2) 3M " ' ' wherein p is mass density (kg / m3) of the material, n is refractive index of the material at 193 nm, NA is the universal Avagadro’s number (6.02214076xl023particles per mole), is the molecular weight of the chemical element of the material (g / mol), and a is the mean molecular polarizability (cm3). This equation is further described in Yangang Liu, Peter H. Daum, “Relationship of refractive index to mass density and self-consistency of mixing rules for multicomponent mixtures like ambient aerosols,” Aerosol Science 39 (2008) 974-986, which is incorporated by reference herein.

[0028] The term “refractive index” refers to the refractive index at a wavelength of 193 nm.

[0029] The term “laser durability,” as used herein, refers to a material’s ability to maintain its original properties after being exposed to a laser (i.e., used), such as in an ultraviolet lithography system. “Maintaining laser durability” or “maintained laser durability,” as used herein, refer to less than 3% normalized power reduction after an accelerated lifetime.

[0030] As semiconductor processing progresses to 45 nm node processes and beyond, the application of excimer lasers (e.g., 193 nm excimer laser) with increasing power and repetition rate require laser-durable coatings for optical components. A conventional way to improve the service life of an optical coating is to reduce the inherent stresses of top layers of the coating, which may be achieved by lowering the density. However, lowering the density may allow moisture to penetrate the coating due to increased porosity, thereby causing a spectral shift (e.g., red shift) in the coating.

[0031] Disclosed herein are coatings and methods of making an optical element having a coating thereon that mitigate the aforementioned problems. Specifically, the coatings disclosed herein comprise at least first and second layers that provide improved laser durability.

[0032] With reference to FIG. 1, a coated optical element 100 is disclosed, which comprises a substrate 110 and a coating 120 disposed thereon. As discussed further below, coating 120 comprises a period 130 of a first layer 122 and a second layer 124. In embodiments, coating 120 comprises repeating units of period 130 so that coating comprisesSP24-237 alternating layers of 122 and 124. Furthermore, coating 120 comprises a capping layer 126 to provide increased laser exposure durability.

[0033] Substrate 110 may be comprised of, for example, glass, glass-ceramic, or ceramic such as, for example, silicate glass or fused quartz. Exemplary glass substrates include, but are not limited to, HPFS® fused silica sold by Corning Incorporated of Coming, New York under glass codes 7980, 7979, and 8655. In some embodiments, the glass or glass ceramic has 50 wt.% or more, 60 wt.% or more, 70 wt.% or more, 80 wt.% or more, 90 wt.% or more, or 95 wt.% or more silica content by weight on an oxide basis. In embodiments, substrate 110 comprises a metal or metal fluoride. For example, substrate 110 may comprise calcium fluoride (CaF2), magnesium fluoride (MgF2), aluminum fluoride (AIF3), strontium fluoride (SrF2), barium fluoride (BaF2), and / or lithium fluoride (LiF). In some exemplary embodiments, substrate 110 is comprised of an aluminum metal, an aluminum alloy, silicon, or combinations of these materials. In some exemplary embodiments, substrate 110 is comprised of silica (SiCh) or a fluorine doped silica (F-SiCh) or a combination thereof.

[0034] Substrate 110 may comprise, for example, a lens, window, objective, prism, beam splitter, filter, and / or mirror. Coating 120 may be a protective coating that provides a barrier on surface(s) of substrate 110. In embodiments, coating 120 may prevent or reduce deterioration and / or erosion of the material of substrate 110. As discussed further below, in embodiments, coating 120 is an anti-reflective (AR) coating that provides anti -reflective properties to substrate 110.

[0035] In embodiments, substrate 110 has a thickness of about 1 mm or greater, or about 2 mm or greater, or about 4 mm or greater, or about 6 mm or greater, or about 8 mm or greater, or about 10 mm or greater. Additionally or alternatively, substrate 110 has a thickness of about 10 mm or less, or about 8 mm or less, or about 6 mm or less, or about 4 mm or less, or about 2 mm or less, or about 1 mm or less. In embodiments, the thickness is from about 1 mm to about 10 mm, or about 2 mm to about 8 mm, or about 4 mm or about 6 mm, or any range encompassing these endpoints.

[0036] Coating 120 has a total thickness of about 50 nm or greater, or about 75 nm or greater, or about 100 nm or greater, or about 150 nm or greater or about 200 nm or greater, or about 250 nm or greater, or about 300 nm or greater, or about 350 nm or greater, or about 400 nm or greater, or about 450 nm or greater, or about 500 nm or greater. Additionally or alternatively, coating 120 has a total thickness of about 500 nm or less, or about 450 nm orSP24-237 less, or about 400 nm or less, or about 350 nm or less, or about 300 nm or less, or about 250 nm or less or about 200 nm or less, or about 150 nm or less, or about 100 nm or less, or about 75 nm or less, or about 50 nm or less. In embodiments, the total thickness is from about 50 nm to about 500 nm, or about 75 nm to about 450 nm, or about 100 nm to about 400 nm, or about 150 nm to about 350 nm, or about 200 nm to about 300 nm, or about 200 nm to about 300 nm, or about 250 nm to about 300, or any range encompassing these endpoints.

[0037] As discussed above, coating 120 may comprise repeating units of period 130, such that each period 130 comprises first layer 122 and second layer 124. In embodiments, coating 120 may comprise 1 or more periods, or 2 or more periods, or 3 or more periods, or 4 or more periods, or 5 or more periods, or 6 or more periods, or 7 or more periods, or 8 or periods, or 9 or more periods, or 10 or more periods. In embodiments, the periods are arranged such that first layer 122 is directly adjacent to and in direct contact with substrate 110.

[0038] First layer 122 may comprise a metal fluoride such as, for example, magnesium fluoride (MgF2), aluminum fluoride (A1F3), lithium fluoride (LiF), sodium fluoride (NaF), yttrium fluoride (YF3), strontium fluoride (SrF2), barium fluoride (BaF2), or a combination thereof. The inclusion of MgF2 in coating 120 is advantageous, in embodiments, due to its low refractive index, high transmittance, and chemical and mechanical stability at, for example, wavelengths within the DUV range. Additionally or alternatively, first layer 122 may comprise an oxide such as, for example, silica (SiCh), beryllium oxide (BeO), fluorine doped silica (F-SiCh), aluminum oxide (AI2O3), nitrogen doped silica (N-SiCh), or a combination thereof. In embodiments, first layer 122 has a refractive index at a wavelength of 193 nm of about 1.90 or less, or about 1.85 or less, or about 1.80 or less, or about 1.75 or less, or about 1.70 or less, or about 1.65 or less, or about 1.60 or less, or about 1.58 or less, or about 1.56 or less, or about 1.54 or less, or about 1.52 or less, or about 1.50 or less, or about1.48 or less, or about 1.46 or less, or about 1.44 or less, or about 1.42 or less, or about 1.40 or less, or about 1.38 or less, or about 1.35 or less, or any range encompassing these endpoints.In some embodiments, first layer 122 is a low refractive index layer with a refractive index of about 1.60 or less at a wavelength of 193 nm. In embodiments, the refractive index of layer 122 is in a range from about 1.35 to about 1.80, or about 1.35 to about 1.58, or about 1.38 to about 1.60, or about 1.40 to about 1.58, or about 1.42 to about 1.56, or about 1.44 to about 1.54, or about 1.46 to about 1.52, or about 1.48 to about 1.50, or about 1.44 to about 1.50, or any range encompassing these endpoints, at a wavelength of 193 nm. In yet someSP24-237 embodiments, first layer 122 comprises MgF2 with a refractive index of 1.45 at a wavelength of 193 nm.

[0039] Similarly, second layer 124 may comprise an oxide such as, for example, silica (SiCh), beryllium oxide (BeO), fluorine doped silica (F-SiCh), aluminum oxide (AI2O3), nitrogen doped silica (N-SiCh), or a combination thereof. Additionally or alternatively, second layer 124 may comprise a metal fluoride such as, for example, magnesium fluoride (MgF2), aluminum fluoride (A1F3), lithium fluoride (LiF), sodium fluoride (NaF), yttrium fluoride (YF3), strontium fluoride (SrF2), barium fluoride (BaF2), or a combination thereof. In embodiments, second layer 124 has a refractive index at a wavelength of 193 nm of about 1.90 or less, or about 1.85 or less, or about 1.80 or less, or about 1.75 or less, or about 1.70 or less, or about 1.65 or less, or about 1.60 or less, or about 1.58 or less, or about 1.56 or less, or about 1.54 or less, or about 1.52 or less, or about 1.50 or less, or about 1.48 or less, or about 1.46 or less, or about 1.44 or less, or about 1.42 or less, or about 1.40 or less, or about 1.38 or less, or any range encompassing these endpoints. In some embodiments, second layer 124 is a low refractive index layer with a refractive index of about 1.60 or less at a wavelength of 193 nm. In embodiments, the refractive index of layer 124 is in a range from about 1.38 to about 1.90, or about 1.40 to about 1.80, or about 1.40 to about 1.58, or about 1.42 to about 1.56, or about 1.44 to about 1.54, or about 1.42 to about 1.60, or about 1.44 to about 1.60, or any range encompassing these endpoints, at a wavelength of 193 nm. In yet some embodiments, second layer 124 comprises F-SiCh with a refractive index of 1.55 at a wavelength of 193 nm.

[0040] In some embodiments, both first layer 122 and second layer 124 are low refractive index layers with a refractive index of 1.60 or less at a wavelength of 193 nm. Furthermore, in embodiments, second layer 124 may have a higher refractive index than first layer 122. The difference between the refractive index of second layer 124 and first layer 122 may be about 0.50 or less, or about 0.40 or less, or about 0.30 or less, or about 0.20 or less, or about 0.10 or less, or about 0.09 or less, or about 0.08 or less, or about 0.07 or less, or about 0.06 or less, or about 0.05 or less, or about 0.04 or less, or about 0.03 or less, or about 0.02 or less, or about 0.01 or less, or any range encompassing these endpoints. In some embodiments, the difference between the refractive index of second layer 124 and first layer 122 is about 0.01.

[0041] In embodiments, the material of first layer 122 is different from the material of second layer 124. Therefore, first layer 122 does not comprise the same material as second layer 124. In some embodiments, first layer 122 comprises MgF2 and second layer 124SP24-237 comprises F-SiCh. In embodiments, MgF2 is advantageous as the material of first layer 122 when second layer 124 comprises a metal oxide (such as, for example F-SiCh) because the MgF2 of first layer 122 prevents or reduces oxygen diffusion from the metal oxide of second layer 124 into substrate 110. This is advantageous especially when substrate 110 is comprised of CaF2, as CaF2 may be converted to CaO due to the diffusion of oxygen and CaO is a strongly absorbing compound at the DUV wavelength of 193 nm.

[0042] In some embodiments, first layer comprises MgF2 and second layer 124 comprises BeO. The inclusion of beryllium oxide (BeO), in embodiments, has several advantages over other metal oxides. BeO is one of the hardest materials with a hardness of 9 on the Mohs hardness scale so that inclusion of BeO provides structure and stability to coating 120. BeO is also very dense so that it functions as a superior blocking layer to reduce ingress of oxygen and water as well as egress of fluorine from surrounding layers and materials. Furthermore, any diffusion of fluorine from a surrounding layer into the BeO layer produces BeF2, which is a chemically stable solid and transparent at a wavelength of 193 nm. Therefore, any diffusion of fluorine into the BeO layer will not create excess durability loss in the BeO layer. In some embodiments, as disclosed above, second layer 124 comprises BeO and first layer 122 comprises a different material.

[0043] A thickness of each of first layer 122 and second layer 124 is about 2 nm or greater, or about 5 nm or greater, or about 10 nm or greater, or about 15 nm or greater, or about 20 nm or greater, or about 25 nm or greater, or about 30 nm or greater, or about 35 nm or greater, or about 40 nm or greater, or about 45 nm or greater, or about 50 nm or greater. Additionally or alternatively, the thickness of each of first layer 122 and second layer 124 is about 50 nm or less, or about 45 nm or less, or about 40 nm or less, or about 35 nm or less, or about 30 nm or less, or about 25 nm or less or about 20 nm or less, or about 15 nm or less, or about 10 nm or less, or about 5 nm or less, or about 2 nm or less. In embodiments, the thickness of each layer 122 and 124 is in a range from about 2 nm to about 50 nm, or about 5 nm to about 45 nm, or about 10 nm to about 40 nm, or about 10 nm to about 35nm, or about 15 nm to about 40 nm, or about 20 nm to about 35 nm, or about 25 nm to about 30 nm, or about 8 nm to about 15 nm, or about 9 nm to about 12 nm, or any range encompassing these endpoints.

[0044] First layer 122 may have the same or different thickness from second layer 124. In embodiments, first layer 122 has a smaller thickness than that of second layer 124. In other embodiments, first layer 122 has a larger thickness than that of second layer 124. In theSP24-237 embodiments in which coating 120 comprises multiple periods 130, the multiple first layers 122 in the different periods 130 may all have the same or different thickness values. For example, in an embodiment in which coating 120 comprises 3 periods 130 (such that coating 120 comprises 3 first low layers 122 between the different periods), one or more of those 3 first layers 122 may have a different thickness from one or more other first layers 122. Similarly, in an embodiment in which coating 120 comprises 3 periods 130 (such that coating 120 comprises 3 second layers 124 between the different periods), one or more of those 3 second layers 124 may have a different thickness from one or more other second layers 122.

[0045] Furthermore, in embodiments in which coating 120 comprises multiple periods 130, one or more first layers 122 may have the same or different thickness values from one or more second low layers 124 in the different periods. In some embodiments, the difference in thickness values between first layer 122 and second layer 124 is about 30 nm or less, or about 25 nm or less, or about 20 nm or less, or about 15 nm or less, or about 10 nm or less, or about 5 nm or less, or about 2 nm or less, or about 1 nm or less, or any range encompassing these endpoints.

[0046] In some embodiments, each of the first layers 122 and each of the second layers 124, between the different periods, comprises a different thickness so that each individual layer has a unique thickness in coating 120.

[0047] Capping layer 126 may be disposed on and outward of periods 130 of coating 120. In embodiments, capping layer 126 comprises, for example, an oxide such as, for example, silica (SiCh), beryllium oxide (BeO), fluorine doped silica (F-SiCh), aluminum oxide (AI2O3), nitrogen doped silica (N-SiCh), or a combination thereof. Capping layer 126 may provide many benefits to coating 120 including improved laser exposure durability and environmental stability.

[0048] In embodiments, capping layer 126 has a refractive index at a wavelength of 193 nm of about 1.90 or less, or about 1.85 or less, or about 1.80 or less, or about 1.75 or less, or about 1.70 or less, or about 1.68 or less, or about 1.66 or less, or about 1.64 or less, or about 1.62 or less, or about 1.60 or less, or about 1.58 or less, or about 1.56 or less, or about 1.54 or less, or about 1.52 or less, or about 1.50 or less, or about 1.48 or less, or any range encompassing these endpoints. In some embodiments, the refractive index is in a range from about 1.48 to about 1.70, or about 1.50 to about 1.68, or about 1.52 to about 1.66, or about 1.54 to about 1.64, or about 1.56 to about 1.62, or about 1.58 to about 1.60, or about 1.52 toSP24-237 about 1.58, or any range encompassing these endpoints, at a wavelength of 193 nm. In embodiments, the refractive index of capping layer 126 is greater than the refractive index of each of first layer 122 and second layer 124.

[0049] Capping layer 126 may have a thickness of about 10 nm or greater, or about 15 nm or greater, or about 20 nm or greater, or about 25 nm or greater, or about 30 nm or greater, or about 35 nm or greater, or about 40 nm or greater, or about 45 nm or greater, or about 50 nm or greater, or about 55 nm or greater, or about 60 nm or greater, or about 65 nm or about 70 nm or greater, or about 75 nm or greater, or about 80 nm or greater, or about 85 nm or greater, or about 90 nm or greater, or about 95 nm or greater, or about 100 nm or greater. Additionally or alternatively, the thickness is about 100 nm or less, or about 95 nm or less, or about 90 nm or less, or about 85 nm or less, or about 80 nm or less, or about 75 nm or less, or about 70 nm or less, or about 65 nm or less, or about 60 nm or less, or about 55 nm or less, or about 50 nm or less, or about 45 nm or less, or about 40 nm or less, or about 35 nm or less, or about 30 nm or less, or about 25 nm or less, or about 20 nm or less, or about 15 nm or less, or about 10 nm or less. In embodiments, the thickness is in a range from about 10 nm to about 100 nm, or about 15 nm to about 95, or about 20 nm to about 90, or about 25 nm to about 85 nm, or about 30 nm to about 80 nm, or about 35 nm to about 75 nm, or about 40 nm to about 70 nm, or about 45 nm to about 65 nm, or about 50 nm to about 60 nm, or about 55 nm to about 60 nm, or any range encompassing these endpoints. In embodiments, the thickness of capping layer 126 is greater than the thickness of first layer 122 and greater than the thickness of second layer 124. In embodiments, the thickness of capping layer 126 is greater than the thickness of period 130 (i.e., greater than the thickness of both first layer 122 and second layer 124). In other embodiments, the thickness of capping layer 126 is less than the thickness of period 130.

[0050] FIG. 2 shows an embodiment in which coating 120 comprises 3 periods 130, such that each period is comprised of a first layer 122 and a second layer 124, as discussed above. Thus, optical element 100 comprises in an order moving away from substrate 110, a first layer 122’, a second layer 124’, another first layer 122”, another second layer 124”, another first layer 122’”, another second layer 124’”, and capping layer 126. As shown in FIG. 2, first layer 122’ and second layer 124’ form a first period 130’, first layer 122” and second layer 124” form a second period 130”, and first layer 122’” and second layer 124’” form a third period 130’”. As discussed above, each of the first layers 122’, 122”, 122’” may haveSP24-237 the same or different thickness values from each other, and each of the second layers 124’, 124”, 124’” may have the same or different thickness values from each other.

[0051] In some embodiments in which coating 120 comprises multiple periods 130, the second layer 124 immediately adjacent and directly contacting capping layer 126 comprises a smaller thickness than the other second layers 124 in coating 120. Therefore, with reference to FIG. 2, in this embodiment, second layer 124’” comprises a smaller thickness than either second layer 124’ and second layer 124”. Furthermore, in embodiments, the second layer124 immediately adjacent and directly contacting capping layer 126 may comprise the smallest thickness of all the layers in coating 120. Therefore, in these embodiments, this second layer 124 is smaller than the remaining second layers 124 and smaller than each of the first layers 122 and smaller than capping layer 126. With reference to FIG. 2, in these embodiments, second layer 124’” comprises a smaller thickness than each of the remaining layers 122’, 122’”, 122’”, 124’, 124”, and 126 in coating 120.

[0052] Each period 130 may comprise one or more additional layers in addition to first layer 122 and second layer 124. FIG. 3 shows another embodiment in which each period 130 comprises, in an order moving away from substrate 110, first layer 122, second layer 124, and a third layer 125. However, it is also contemplated in embodiments that layers 122, 124,125 may be positioned in different arrangements within a period 130. For example, a single period 130 may comprise the following different arrangements of layers such that the following arrangements are provided in an order moving away from substrate 110: (i) 122,124, 125; (ii) 122, 125, 124; (iii) 124, 122, 125; (iv) 124, 125, 122, (iv) 125, 122, 124; (v)125, 124, 122.

[0053] Third layer 125 may comprise, for example, an oxide such as, for example, silica (SiCh), beryllium oxide (BeO), fluorine doped silica (F-SiCh), aluminum oxide (AI2O3), nitrogen doped silica (N-SiCh), or a combination thereof. In some embodiments, third layer 125 comprises the same material as capping layer 126. In yet some embodiments, both third layer 125 and capping layer 126 comprise F-SiCh. Third 125 provides a barrier layer to the underlying first and second layers 122, 124.

[0054] In embodiments, third layer 125 has a refractive index at a wavelength of 193 nm of about 1.90 or less, or about 1.85 or less, or about 1.80 or less, or about 1.75 or less, or about 1.70 or less, or about 1.68 or less, or about 1.66 or less, or about 1.64 or less, or about 1.62 or less, or about 1.60 or less, or about 1.58 or less, or about 1.56 or less, or about 1.54 orSP24-237 less, or about 1.52 or less, or about 1.50 or less, or about 1.48 or less, or any range encompassing these endpoints. In some embodiments, the refractive index is in a range from about 1.48 to about 1.70, or about 1.50 to about 1.68, or about 1.52 to about 1.66, or about 1.54 to about 1.64, or about 1.56 to about 1.62, or about 1.58 to about 1.60, or about 1.52 to about 1.58, or any range encompassing these endpoints, at a wavelength of 193 nm. In embodiments, the refractive index of third layer 125 is greater than the refractive index of each of first layer 122 and second layer 124. In embodiments, both third layer 125 and capping layer 126 have the same refractive index.

[0055] In some embodiments, first layer 122, second layer 124, and third layer 125 are all low refractive index layers with a refractive index of 1.60 or less. Furthermore, in embodiments, third layer 125 has a higher refractive index than the refractive index of first layer 122 and higher refractive index than the refractive index of second layer 124. The difference between the refractive index of third layer 125 and either first layer 122 or second layer 124 may be about 1.50 or less, or about 1.40 or less, or about 1.30 or less, or about 1.20 or less, or about 1.10 or less, or about 1.00 or less, or about 0.90 or less, or about 0.80 or less, or about 0.70 or less, or about 0.60 or less, or about 0.50 or less, or about 0.40 or less, or about 0.30 or less, or about 0.20 or less, or about 0.10 or less, or about 0.09 or less, or about 0.08 or less, or about 0.07 or less, or about 0.06 or less, or about 0.05 or less, or about 0.04 or less, or about 0.03 or less, or about 0.02 or less, or about 0.01 or less, or any range encompassing these endpoints. In some embodiments, the difference between the refractive index of third layer 125 and first layer 122 is about 0.13, and the difference between the refractive index of third layer 125 and second layer 124 is about 0.03.

[0056] Third layer 125 may have a thickness of about 2 nm or greater, or about 5 nm or greater, or about 10 nm or greater, or about 15 nm or greater, or about 20 nm or greater, or about 25 nm or greater, or about 30 nm or greater, or about 35 nm or greater, or about 40 nm or greater, or about 45 nm or greater, or about 50 nm or greater. Additionally or alternatively, the thickness of third layer 125 is about 50 nm or less, or about 45 nm or less, or about 40 nm or less, or about 35 nm or less, or about 30 nm or less, or about 25 nm or less or about 20 nm or less, or about 15 nm or less, or about 10 nm or less, or about 5 nm or less, or about 2 nm or less. In embodiments, the thickness of third layer 125 is in a range from about 2 nm to about 50 nm, or about 5 nm to about 45 nm, or about 10 nm to about 40 nm, or about 15 nm to about 40 nm, or about 20 nm to about 35 nm, or about 25 nm to about 30 nm,SP24-237 or any range encompassing these endpoints. In some embodiments, third layer 125 comprises the same or different thickness as each of first layer 122 and second layer 124.

[0057] First 122, second layer 124, third layer 125, and / or capping layer 126 may each be deposited on substrate 110 using any well-known deposition process including, for example, physical vapor deposition, atomic layer deposition, thermal evaporation in vacuum, either through resistively or electron beam heating, reactive sputter process, and / or ion-assisted plasma. In embodiments, first layer 122, second layer 124, third layer 125, and / or capping layer 126 are each deposited on substrate 110 at a temperature from about 200°C to about 300°C, or about 225°C to about 275°C, or about 250°C to achieve a desired thermal densification while controlling the overall coating stress. In some embodiments, first layer 122, second layer 124, third layer 125, and capping layer 126 are all deposited on substrate 110 at a temperature of about 250°C.

[0058] In embodiments, during the deposition of third layer 125 using any of the abovedisclosed processes, third layer 125 is also deposited with an ion assisted plasma to further compact and densify this layer. In embodiments, during the deposition of capping layer 126 using any of the above-disclosed processes, capping layer 126 is also deposited with an ion assisted plasma to further compact and densify this layer. However, in embodiments, neither first layer 122 nor second layer 124 are deposited with such ion assisted plasma. Therefore, in these embodiments, third layer 125 and capping layer 126 may be referred to herein as “densified” layers, while first layer 122 and second layer 124 may be referred to herein as “undensified” layers. Accordingly, layers 125 and 126 are each more compact and have a relatively higher density than each of layers 122 and 124.

[0059] As discussed above, in embodiments second layer 124 may comprise the same material as third layer 125. For example, in embodiments, both second layer 124 and third layer 125 comprise F-SiCh. However, in embodiments, third layer 125 may comprise a densified layer, such as a densified F-SiCh layer, while second layer 124 may comprise an undensified layers, such as an undensified F-SiCh layer. In some embodiments, third layer 125 comprises a densified layer with a density from about 2.10 g / cm3to about 2.39 g / cm3, or about 2.15 g / cm3to about 2.35 g / cm3, or about 2.20 g / cm3to about 2.30 g / cm3, or about 2.24 g / cm3, to about 2.30 g / cm3, or about 2.26 g / cm3to about 2.30 g / cm3, or about 2.26 g / cm3to about 2.28 g / cm3, or any range encompassing these endpoints. In some embodiments, second layer 124 comprises an undensified layer with a density from about 1.80 g / cm3to about 2.20 g / cm3, or about 1.85 g / cm3to about 2.20 g / cm3, or about 1.90 g / cm3to about 2.20 g / cm3, orSP24-237 about 1.95 g / cm3to about 2.20 g / cm3, or about 2.00 g / cm3to about 2.20 g / cm3, or about 2.05 g / cm3to about 2.20 g / cm3, or about 2.10 g / cm3to about 2.18 g / cm3, or about 2.12 g / cm3to about 2.16 g / cm3, or about 2.12 g / cm3to about 2.14 g / cm3, or any range encompassing these endpoints. In these embodiments, it is noted that the densified layer of third layer 125 has a higher density than the undensified layer of second layer 124. In embodiments, the density of third layer 125 is greater than the density of second layer 124 by about 0.50 g / cm3or less, or about 0.45 g / cm3or less, or about 0.40 g / cm3or less, or about 0.35 g / cm3or less, or about 0.30 g / cm3or less, or about 0.25 g / cm3or less, or about 0.20 g / cm3or less, or about 0.15 g / cm3or less, or about 0.10 g / cm3or less, or about 0.05 g / cm3or less, or about 0.02 g / cm3or less, or about 0.01 g / cm3or less, or any range encompassing these endpoints.

[0060] In some particular embodiments, third layer 125 comprises a densified layer of F- SiCh with a density of about 2.28 g / cm3 and second layer 124 comprises an undensified layer of F-SiCh with a density of about 2.14 g / cm3.

[0061] FIG. 4 shows yet another embodiment in which coating 120 comprises three periods, 130’, 130”, 130’”, such that the first period 130’ comprises a first layer 122’, a second layer 124’, and a third layer 125’ and the second period 130” also comprises a first layer 122”, a second layer 124”, and a third layer 125”. However, in this embodiment the third period 130’” comprises a first layer 122’” and a second layer 124’” but does not comprise a third layer 125.

[0062] In the embodiment of FIG. 5, coating 120 is covered with an encapsulating layer 140. As shown in FIG. 5, encapsulating layer 140 may be disposed outward of coating 120 such that encapsulating layer 140 coats and covers top and side surfaces of coating 120.Thus, coating 120 is completely enclosed and encapsulated and sealed between encapsulating layer 140 and substrate 110. Encapsulating layer 140 comprises, for example, an oxide such as, for example, silica (SiCh), beryllium oxide (BeO), fluorine doped silica (F-SiCh), aluminum oxide (AI2O3), nitrogen doped silica (N-SiCh), or a combination thereof. Encapsulating layer 140 provides additional durability to coating 120. Although encapsulating layer 140 is shown with reference to FIG. 5, it is noted that any of the embodiments disclosed herein may comprise such an encapsulating layer 140.

[0063] Encapsulating layer 140 may be deposited on coating 120 using any well-known deposition process including, for example, physical vapor deposition, atomic layer deposition, thermal evaporation in vacuum, either through resistively or electron beamSP24-237 heating, reactive sputter process, and / or ion assisted plasma. In embodiments, encapsulation layer 140 is deposited on coating 120 using atomic layer deposition.

[0064] FIG. 6 shows an optical element 100, according to some embodiments, in which both a first surface 112 and an opposing second surface 114 of substrate 110 are coated with coating 120. In particular, a first coating 120’ coats first surface 112 of substrate 110 and a second coating 120” coats second surface 114 of substrate 110. It is also contemplated that only one surface (either first surface 112 or second surface 114) of substrate 110 is coated with coating 120.

[0065] Coating 120 may comprise certain properties (e.g., partial reflective properties, anti- reflective properties) based upon the components and materials of the coating. For example, the number of layers 122, 124, and / or 125 and / or the material of each layer may be modified in coating 120 to achieve a desired reflectivity. In embodiments, a partial-reflective coating and an anti-reflective coating may both comprise the same materials in each of their layers, however, the partial-reflective coating may include more layers 122, 124, and / or 125 overall than the anti -reflective coating, thereby resulting in partial-reflectivity instead of the antireflectivity.

[0066] In embodiments, coating 120 may comprise an anti -reflective coating comprising a reflectance of less than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of the endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of the endpoints. In embodiments, the anti -reflective coatings disclosed herein comprise a reflectance of less than or equal to 0.5%, as measured at every wavelength within the range from 150 nm to 300 nm, inclusive of the endpoints, and at an angle of incidence within the range from 0 degrees to 75 degrees, inclusive of the endpoints. In embodiments, the anti -reflective coating may comprise a reflectance less than or equal to 0.5%, less than or equal to 0.4%, less than or equal to 0.3%, less than or equal to 0.2%, or even less than or equal to 0.1%, as measured at a wavelength within the range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within the range from 0 degrees to 75 degrees, inclusive of endpoints. One skilled in the art would appreciate that the reflectance achieved at a given angle is dependent on the design of the coating.

[0067] In embodiments, coating 120 may comprise a reflective coating such as a partial- reflective coating. In embodiments, coating 120 may comprise a partial-reflective coating comprising a reflectance of greater than or equal to 0.5%, as measured at a wavelength withinSP24-237 a range from 150 nm to 300 nm, inclusive of the endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of the endpoints. In embodiments, the partial-reflective coatings disclosed herein may comprise a reflectance of greater than or equal to 0.5%, as measured at every wavelength within the range from 150 nm to 300 nm, inclusive of the endpoints, and at an angle of incidence within the range from 0 degrees to 75 degrees, inclusive of the endpoints. In embodiments, the partial-reflective coating may comprise a reflectance greater than or equal to 0.5%, greater than or equal to 1%, greater than or equal to 5%, greater than or equal to 10%, greater than or equal to 25%, greater than or equal to 50%, greater than or equal to 75%, or even greater than or equal to 95%, as measured at a wavelength within the range from 150 nm to 300 nm, inclusive of the endpoints, and at an angle of incidence within the range from 0 degrees to 75 degrees, inclusive of the endpoints. In embodiments, the partial-reflective coating may comprise a reflectance greater than or equal to 95%, as measured at a wavelength within the range from 150 nm to 300 nm, inclusive of the endpoints, and at an angle of incidence within the range from 0 degrees to 75 degrees, inclusive of the endpoints, thereby forming a beam splitter.

[0068] In embodiments, coating 120 may be highly reflective (e.g., reflectance of about 100%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of the endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of the endpoints), thereby forming a mirror. In embodiments, highly reflective coatings disclosed herein may comprise a reflectance of about 100%, as measured at every wavelength within the range from 150 nm to 300 nm, inclusive of the endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of the endpoints, thereby forming the mirror.

[0069] In embodiments, coating 120 may comprise an anti -reflective coating comprising a reflectance less than or equal to 0.5%, as measured at a wavelength of 193 nm and at an angle of incidence of 45 degrees. In embodiments, coating 120 may comprise a partial-reflective coating comprising a reflectance greater than or equal to 0.5%, as measured at a wavelength of 193 nm and at an angle of incidence of 45 degrees. In embodiments, coating 120 may be highly reflective (e.g., reflectance of about 100%, as measured at a wavelength of 193 nm and at an angle of incidence of 45 degrees).

[0070] With reference again to FIG. 6, optical element 100 may comprise first coating 120’ on first surface 112 of substrate 110 and second coating 120” on second surface 114 ofSP24-237 substrate 110. In embodiments, first coating 120’ may comprise a partial-reflective coating and second coating 120” may comprise an anti-reflective coating. In other embodiments, both first coating 120’ and second coating 120” comprise anti-reflective coatings, thereby forming a laser window such that optical element 100 reflects radiation. It is noted that the first anti -reflective coating 120’ may be the same or different from the second anti -reflective coating 120”.

[0071] According to the embodiments disclosed herein, an ultraviolet lithography system may comprise optical element 100. For example, referring now to FIG. 7, an ultraviolet lithography system is shown at 200, which comprises two optical systems 202 and 204 such that system 202 is an illumination system and system 204 is a projection system. A radiation source 206 (e.g., an excimer laser), emits radiation 208 at a specific wavelength, for example, at 248 nm, 193 nm, or 157 nm. The radiation 208 emitted by the radiation source 206 may be conditioned with the aid of the illumination system 202 such that a mask 210 (e.g., reticle) may thereby be illuminated. For this purpose, illumination system 202 may include at least one transmissive optical element. The optical element 220, for example, concentrates the radiation 208.

[0072] Mask 210 has on its surface a structure that is transferred to an element 222 to be exposed, for example, a wafer in the context of production of semiconductor components with the aid of projection system 204. As shown in FIG. 7, projection system 204 also comprises at least one transmissive optical element. In the example illustrated here, two transmissive optical elements 230, 240, for example, reduce the structures on mask 210 to the size desired for the exposure of element 222. In projection system 204, a wide variety of optical elements may be combined with one another in a known manner.

[0073] The optical elements 220, 230, and 240 illustrated in FIG. 7 may each comprise an optical element 100 according to embodiments described herein. Moreover, in embodiments, radiation source 206 may include an optical element, such as a beam splitter, therein.Examples

[0074] In order that various embodiments be more readily understood, reference is made to the following examples, which are intended to illustrate various embodiments of the optical elements according to embodiments described herein.SP24-237

[0075] Table 1 provides an example of a first exemplary example of an anti -reflective coating according to the embodiments disclosed herein. As shown in Table 1, the first exemplary example comprises three periods, each with a first layer and a second layer.Table 1 : First Exemplary Example

[0076] Table 2 provides an example of a second exemplary example of a parti al -reflective coating according to the embodiments disclosed herein. As shown in Table 2, the second exemplary example comprises four periods, each with a first layer and a second layer.Table 2: Second Exemplary Example

[0077] Table 3 below provides an example of a third exemplary example of an anti-reflective coating according to the embodiments disclosed herein. As shown in Table 3, the third exemplary example comprises two periods, each with a first layer, a second layer, and a third layer, and a third period with a first layer and a second layer.SP24-237Table 3: Third Exemplary Example

[0078] Table 4 below provides an example of a fourth exemplary example of an anti- reflective coating according to the embodiments disclosed herein. As shown in Table 4, the fourth exemplary example comprises three periods, each with a first layer and a second layer.Table 4: Fourth Exemplary Example

[0079] Table 5 below provides an example of a fifth exemplary example of a partial reflective coating according to the embodiments disclosed herein. As shown in Table 5, the fifth exemplary example comprises four periods, each with a first layer and a second layer.Table 5: Fifth Exemplary ExampleSP24-237

[0080] Table 6 below provides a comparative example of a coating that does not comprise the disclosed layers. Instead, the comparative example includes alternating layers of magnesium fluoride (MgF2) and gadolinium fluoride (GdF3), wherein GdF3is a high refractive index material with a refractive index of 1.67 at 193 nm.Table 6: Comparative Example

[0081] With reference to FIG. 8, the optical elements of the third exemplary example (Table 3) and the comparative example (Table 6) were subjected to an accelerated laser damage test. In particular, the third exemplary example and the comparative example were each exposed to an elevated high laser fluence (i.e., to accelerate test time) until a 3% loss of transmitted power was recorded in the optical element. In particular, the coated optical element of each example was exposed to millions of pulses by the high laser fluence until the coated optical element experienced a 3% loss of transmission. The pulse of the high laser fluence had a wavelength of 193 nm, a run rate of 3 kilohertz, and a pulse width of 40-45 nanoseconds. As shown in FIG. 8, the average fluence (mJ / cm2) is plotted vs. millions of pulses for each example. Due to the exemplary optical coating provided on the third exemplary example, this example experienced less loss of transmission when exposed to the millions of high laser fluence pulses as compared to the comparative example. Thus, the third exemplary example had a longer accelerated lifetime as compared to the comparative example.

[0082] According to a first aspect, an optical element is disclosed that comprises a substrate and a coating disposed on the substrate, the coating comprising a period comprising a first layer, a second layer, and a third layer. The first layer has a lower refractive index than the second layer, a material of the first layer is different from a material of the second layer, and the third layer has a higher density than the second layer.SP24-237

[0083] According to a second aspect, the optical element of the first aspect, wherein a difference between the refractive index of the first layer and the second layer is about 0.50 or less.

[0084] According to a third aspect, the optical element of the second aspect, wherein the difference is about 0.10 or less.

[0085] According to a fourth aspect, the optical element of any one of the first through third aspects, wherein the refractive index of the first layer is lower than the refractive index of the third layer.

[0086] According to a fifth aspect, the optical element of any one of the first through fourth aspects, wherein the first layer comprises a metal fluoride.

[0087] According to a sixth aspect, the optical element of the fifth aspect, wherein the metal fluoride is magnesium fluoride (MgF2).

[0088] According to a seventh aspect, the optical element of any one of the first through sixth aspects, wherein the second layer comprises a metal oxide.

[0089] According to an eighth aspect, the optical element of the seventh aspect, wherein the metal oxide is fluorine doped silica (F-SiCh).

[0090] According to a ninth aspect, the optical element of the seventh aspect, wherein the metal oxide is beryllium oxide (BeO).

[0091] According to a tenth aspect, the optical element of any one of the first through ninth aspects, wherein the third layer comprises a metal oxide.

[0092] According to an eleventh aspect, the optical element of the tenth aspect, wherein the metal oxide is fluorine doped silica (F-SiCh).

[0093] According to a twelfth aspect, the optical element of any one of the first through eleventh aspects, wherein the second layer and the third layer comprise the same material.

[0094] According to a thirteenth aspect, the optical element of any one of the first through twelfth aspects, wherein the refractive index of the first layer is in a range from about 1.35 to about 1.58.

[0095] According to a fourteenth aspect, the optical element of any one of the first through thirteenth aspects, wherein the refractive index of the second layer is in a range from about 1.40 to about 1.80.SP24-237

[0096] According to a fifteenth aspect, the refractive index of any one of the first through fourteenth aspects, wherein the density of the second layer is from about 2.10 g / cm3to about 2.39 g / cm3.

[0097] According to a sixteenth aspect, the optical element of any one of the first through fifteenth aspects, wherein the density of the third layer is greater than the density of the second layer by about 0.50 g / cm3or less.

[0098] According to a seventeenth aspect, the optical element of the sixteenth aspect, wherein the density of the third layer is greater than the density of the second layer by about 0.30 g / cm3or less.

[0099] According to an eighteenth aspect, the optical element of any one of the first through seventeenth aspects, wherein the optical element comprises two or more periods.

[0100] According to a nineteenth aspect, the optical element of any one of the first through eighteenth aspects, wherein the coating further comprises a capping layer.

[0101] According to a twentieth aspect, the optical element of the nineteenth aspect, wherein a density of the capping layer is greater than the density of the second layer.

[0102] According to a twenty-first aspect, the optical element of the nineteenth or twentieth aspects, wherein the third layer and the capping layer comprise the same material.

[0103] According to a twenty-second aspect, the optical element of any one of the first through twenty-first aspects, wherein the first layer has a different thickness than the second layer.

[0104] According to a twenty-third aspect, the optical element of the twenty-second aspect, wherein the thickness of the first layer and the thickness of the second layer are both within a range from about 10 nm to about 35 nm.

[0105] According to a twenty-fourth aspect, the optical element of any one of the first through twenty -third aspects, wherein each of the first layer, the second layer, and the third layer have a different thickness.

[0106] According to a twenty-fifth aspect, the optical element of any one of the first through twenty-fourth aspects, wherein the coating comprises an anti -reflective coating, the anti -reflective coating comprising a reflectance less than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints.

[0107] According to a twenty-sixth aspect, the optical element of any one of the first through twenty-fourth aspects, wherein the coating comprises a partial-reflective coating, the partial-reflective coating comprising a reflectance greater than or equal to 0.5%, as measuredSP24-237 at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints.

[0108] According to a twenty-seventh aspect, the optical element of any one of the first through twenty-fourth aspects, wherein the coating comprises a first anti -reflective coating disposed on a first surface of the substrate and a second anti -reflective coating disposed on a second surface of the substrate opposite the first major surface, each of the first anti -reflective coating and the second anti -reflective coating comprising a reflectance less than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints.

[0109] According to a twenty-eighth aspect, the optical element of the twenty-seventh aspect, wherein the optical element comprises a laser window.

[0110] According to a twenty -ninth aspect, the optical element of any one of the first through twenty-fourth aspects, wherein the coating comprises a partial-reflective coating disposed on a first surface of the substrate and an anti -reflective coating disposed on a second surface of the substrate, the partial-reflective coating comprising a reflectance greater than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints, the anti -reflective coating comprising a reflectance less than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints.

[0111] According to a thirtieth aspect, the optical element of the twenty-ninth aspect, wherein the optical element comprises a beam splitter.

[0112] According to a thirty-first aspect, an ultraviolet lithography system comprising the optical element of any one of the first through thirtieth aspects.

[0113] According to a thirty-second aspect, an optical element is disclosed that comprises a substrate and a coating disposed on the substrate. The coating comprises a first layer, a second layer, and a capping layer, the first layer has a lower refractive index than the second layer, a material of the first layer is different from a material of the second layer, and the capping layer has a higher density than the second layer.

[0114] According to a thirty -third aspect, the optical element of the thirty-second aspect, wherein the first layer comprises a metal fluoride.SP24-237

[0115] According to a thirty-fourth aspect, the optical element of the thirty -third aspect, wherein the metal fluoride is magnesium fluoride (MgF2).

[0116] According to a thirty-fifth aspect, the optical element of any one of the thirty- second through thirty-fourth aspects, wherein the second layer comprises a metal oxide.

[0117] According to a thirty-sixth aspect, the optical element of the thirty-sixth aspect, wherein the metal oxide is fluorine doped silica (F-SiCh).

[0118] According to a thirty-seventh aspect, the optical element of any one of the thirty- second through thirty-sixth aspects, wherein the second layer and the capping layer both comprise fluorine doped silica (F-SiCh).

[0119] According to a thirty-eighth aspect, an ultraviolet lithography system comprising the optical element of any one of the thirty-second through thirty-seventh aspects.

[0120] It will be apparent to those skilled in the art that various modifications and variations may be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus, it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.

Claims

SP24-237CLAIMSWhat is claimed is:

1. An optical element comprising: a substrate; and a coating disposed on the substrate, the coating comprising a period comprising a first layer, a second layer, and a third layer, wherein: the first layer has a lower refractive index than the second layer, a material of the first layer is different from a material of the second layer, and the third layer has a higher density than the second layer.

2. The optical element of claim 1, wherein a difference between the refractive index of the first layer and the second layer is about 0.50 or less.

3. The optical element of claim 2, wherein the difference is about 0.10 or less.

4. The optical element of any one of claims 1-3, wherein the refractive index of the first layer is lower than the refractive index of the third layer.

5. The optical element of any one of claims 1-4, wherein the first layer comprises a metal fluoride.

6. The optical element of claim 5, wherein the metal fluoride is magnesium fluoride (MgF2).

7. The optical element of any one of claims 1-6, wherein the second layer comprises a metal oxide.

8. The optical element of claim 7, wherein the metal oxide is fluorine doped silica (F- SiO2).

9. The optical element of claim 7, wherein the metal oxide is beryllium oxide (BeO).SP24-23710. The optical element of any one of claims 1-9, wherein the third layer comprises a metal oxide.

11. The optical element of claim 10, wherein the metal oxide is fluorine doped silica (F- SiO2).

12. The optical element of any one of claims 1-11, wherein the second layer and the third layer comprise the same material.

13. The optical element of any one of claims 1-12, wherein the refractive index of the first layer is in a range from about 1.35 to about 1.58.

14. The optical element of any one of claims 1-13, wherein the refractive index of the second layer is in a range from about 1.40 to 1.80.

15. The optical element of any one of claims 1-14, wherein the density of the second layer is from about 2.10 g / cm3to about 2.39 g / cm3.

16. The optical element of any one of claims 1-15, wherein the density of the third layer is greater than the density of the second layer by about 0.50 g / cm3or less.

17. The optical element of claim 16, wherein the density of the third layer is greater than the density of the second layer by about 0.30 g / cm3or less.

18. The optical element of any one of claims 1-17, wherein the optical element comprises two or more periods.

19. The optical element of any one of claims 1-18, wherein the coating further comprises a capping layer.

20. The optical element of claim 19, wherein a density of the capping layer is greater than the density of the second layer.SP24-23721 The optical element of claim 19 or claim 20, wherein the third layer and the capping layer comprise the same material.

22. The optical element of any one of claims 1-21, wherein the first layer has a different thickness than the second layer.

23. The optical element of claim 22, wherein the thickness of the first layer and the thickness of the second layer are both within a range from about 10 nm to about 35 nm.

24. The optical element of any one of claims 1-23, wherein each of the first layer, the second layer, and the third layer have a different thickness.

25. The optical element of any one of claims 1-24, wherein the coating comprises an anti- reflective coating, the anti -reflective coating comprising a reflectance less than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints.

26. The optical element of any one of claims 1-24, wherein the coating comprises a partial-reflective coating, the partial-reflective coating comprising a reflectance greater than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints.

27. The optical element of any one of claims 1-24, wherein the coating comprises a first anti -reflective coating disposed on a first surface of the substrate and a second anti -reflective coating disposed on a second surface of the substrate opposite the first major surface, each of the first anti -reflective coating and the second anti -reflective coating comprising a reflectance less than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints.

28. The optical element of claim 27, wherein the optical element comprises a laser window.SP24-23729. The optical element of any one of claims 1-24, wherein the coating comprises a partial-reflective coating disposed on a first surface of the substrate and an anti -reflective coating disposed on a second surface of the substrate, the partial-reflective coating comprising a reflectance greater than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints, the anti -reflective coating comprising a reflectance less than or equal to 0.5%, as measured at a wavelength within a range from 150 nm to 300 nm, inclusive of endpoints, and at an angle of incidence within a range from 0 degrees to 75 degrees, inclusive of endpoints.

30. The optical element of claim 29, wherein the optical element comprises a beam splitter.

31. An ultraviolet lithography system comprising the optical element of any one of claims 1-30.

32. An optical element comprising: a substrate; and a coating disposed on the substrate, wherein: the coating comprises a first layer, a second layer, and a capping layer, the first layer has a lower refractive index than the second layer, a material of the first layer is different from a material of the second layer, and the capping layer has a higher density than the second layer.

33. The optical element of claim 32, wherein the first layer comprises a metal fluoride.

34. The optical element of claim 33, wherein the metal fluoride is magnesium fluoride (MgF2).

35. The optical element of any one of claims 32-34, wherein the second layer comprises a metal oxide.SP24-23736. The optical element of claim 35, wherein the metal oxide is fluorine doped silica (F- SiO2).

37. The optical element of any one of claims 32-36, wherein the second layer and the capping layer both comprise fluorine doped silica (F-SiO2).

38. An ultraviolet lithography system comprising the optical element of any one of claims 32-37.

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