Optical components for deep UV sources

Abstract

An optical element for a deep ultraviolet light source includes a crystal substrate; a coating on the outer surface of the crystal substrate, the coating having a thickness along a direction extending away from the outer surface; and a structure on and / or in the coating, the structure including a plurality of features extending away from the crystal substrate along the direction. The features include amorphous dielectric materials and are arranged so that the refractive index of the structure changes along the direction.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application No. 62 / 809,983, filed on February 25, 2019, entitled “Optical Element for Deep Ultraviolet Light Source,” and U.S. Application No. 62 / 835,646, filed on April 18, 2019, entitled “Optical Element for Deep Ultraviolet Light Source,” both of which are incorporated herein by reference in their entirety. Technical Field

[0003] The present disclosure relates to an optical element for a deep ultraviolet (DUV) source. Background Art

[0004] Lithography is the process of patterning semiconductor circuit systems on substrates such as silicon wafers. The lithography light source provides deep ultraviolet (DUV) light, which is used to expose the photoresist on the wafer. A gas discharge light source used for lithography is called an excimer light source or laser. Excimer light sources generally use a gas mixture that is a combination of one or more inert gases (such as argon, krypton or xenon) and an active substance (such as fluorine or chlorine). The excimer light source is named after the fact that under appropriate conditions of electrical stimulation (supplied energy) and high pressure (of the gas mixture), a pseudo molecule called an excimer is generated, which exists only in an excited state and produces amplified light in the ultraviolet range. The excimer light source generates a light beam with a wavelength in the deep ultraviolet (DUV) range, and the light beam is used to pattern a semiconductor substrate (or wafer) in a lithography apparatus. The excimer light source can be constructed using a single gas discharge chamber or using multiple gas discharge chambers. The gas mixture in the gas discharge chamber can be discharged from one or more gas discharge chambers. Summary of the invention

[0005] In one aspect, an optical element for a deep ultraviolet light source includes a crystalline substrate; a coating on an outer surface of the crystalline substrate, the coating having a thickness along a direction extending away from the outer surface; and a structure on and / or in the coating, the structure including a plurality of features extending away from the crystalline substrate along the direction. The features include an amorphous dielectric material and are arranged such that a refractive index of the structure changes along a direction.

[0006] Implementations may include one or more of the following features.

[0007] The crystalline substrate may include calcium fluoride (CaF2). In some implementations, when in operation, deep ultraviolet (DUV) light having a wavelength of 193 nanometers (nm) is incident on the coating, the coating protects the CaF2 substrate by mitigating the removal of fluorine from the substrate, and the refractive index of the structure changes along the propagation direction of the DUV light, so that the reflection of the DUV light from the optical element is reduced.

[0008] In some implementations, the extent of the features along the direction is no greater than the thickness.All features of the structure may be within the coating.

[0009] The features and coatings may be made of the same dielectric material.

[0010] One or more features may extend outside the coating.

[0011] Amorphous dielectric materials may include fused silica or alumina.

[0012] Each of the plurality of features may be identical, and the plurality of features may be arranged relative to each other in a regular and repeating pattern.

[0013] Each feature in the plurality of features may be identical, and the plurality of features may be arranged relative to each other in a random or pseudo-random manner.

[0014] Each feature of the plurality of features may be associated with a spatial characteristic, and at least one spatial characteristic of one feature of the plurality of features is different from a spatial characteristic of at least one feature of the other features. The spatial characteristic may include any of: height, width, and shape.

[0015] The refractive index of the structure can vary linearly along a direction.

[0016] The refractive index of the structure may vary along the direction from a value substantially equal to the refractive index of the crystalline substrate to a value substantially equal to the refractive index of the fluid at the optical element. The fluid may include a nitrogen (N2) purge gas.

[0017] Features may extend on the order of 100 nanometers (nm) or less along a direction.

[0018] The direction may be substantially normal to the surface of the crystalline substrate.

[0019] The coating may completely cover the surface of the crystalline substrate.

[0020] In another aspect, a deep ultraviolet (DUV) light source includes a chamber including a housing configured to enclose a gaseous gain medium; and at least one optical element configured to transmit DUV light. The at least one optical element includes a substrate including a crystalline material configured to transmit DUV light; a coating on an outer surface of the substrate, the coating having a thickness along a direction extending away from the outer surface; and a structure on and / or in the coating, the structure including a plurality of features extending away from the outer surface along the direction. The features include amorphous material and the features are arranged such that a refractive index of the structure varies along a direction.

[0021] Implementations may include one or more of the following aspects.

[0022] One or more of the plurality of features may extend a distance along the direction, and the distance may be less than a wavelength of DUV light transmitted by the at least one optical element.

[0023] The plurality of features may be arranged relative to one another such that a spacing between any two adjacent features is within the order of a wavelength of DUV light transmitted by the at least one optical element.

[0024] The crystalline material configured to transmit DUV light may include calcium fluoride (CaF2).

[0025] The refractive index of the structure may vary along a direction from a value substantially equal to the refractive index of the crystalline substrate to a value substantially equal to the refractive index of the fluid surrounding the optical element. The fluid may include a nitrogen (N2) purge gas.

[0026] The structure may be located within the coating.

[0027] The structure may be located in and on the coating such that the structure is partially within the coating.

[0028] The chamber of the DUV light source may also include a first window on the first side of the housing; and a second window on the second side of the housing, the second side of the housing being opposite to the first side of the housing. At least one optical element includes a first window and a second window, and the coating is located on the first window and the second window. The coating on the first window and the coating on the second window may be disposed on the surface of the corresponding window outside the housing. In some implementations, the DUV light source also includes a second chamber, the second chamber including a second housing, the second housing being configured to hold a gaseous gain medium inside the second housing; a third window on the first side of the second housing; and a fourth window on the second side of the second housing, the second side of the second housing being opposite to the first side of the second housing. At least one optical element also includes a third window and a fourth window. The outer surface of the first window and the outer surface of the second window may not be perpendicular to the propagation direction of the DUV light.

[0029] The at least one optical element may include one or more of the following: a prism, a beam splitter, a lens, and an optical compensator.

[0030] The refractive index can change along the propagation direction of the DUV light.

[0031] In some implementations, in operational use, DUV light with a wavelength of 193 nanometers (nm) is incident on the coating, the coating protects the CaF2 substrate by mitigating the removal of fluorine from the substrate, and the refractive index of the structure changes along the propagation direction of the DUV light, thereby reducing the reflection of the DUV light from the optical element.

[0032] The coating may completely cover the surface of the crystalline substrate.

[0033] Implementations of any of the techniques described above and herein may include processes, devices, and / or methods. Details of one or more implementations are described in the accompanying drawings and the following description. Other features are apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1A is a perspective view of an optical element.

[0035] Figure 1B It is along Figure 1A The B-B' line is intercepted Figure 1A A side cross-sectional view of an optical element.

[0036] Figure 1C is in the xy plane Figure 1A View of the optical components.

[0037] Figure 1D shows the position as a function of Figure 1A The refractive index of an optical element.

[0038] Figure 2A and Figure 2B Cross-sectional views of other optical elements are shown.

[0039] Figure 2C shows the position as a function of Figure 2B The refractive index of an optical element.

[0040] Figure 3A is a perspective view of another optical element.

[0041] Figure 3B It is along Figure 3A The line 3B-3B' is intercepted by Figure 3A Cross-sectional view of an optical element.

[0042] Figure 3C is a graph showing the refractive index of an optical element structure as a function of position.

[0043] Figure 4A is a block diagram of a lithography system.

[0044] Figure 4B It can be used for Figure 4A Block diagram of the projection optics system of a lithography system.

[0045] Figure 5 is a block diagram of another lithography system.

[0046] Fig. 6A is a block diagram of a power amplifier (PA) for a deep ultraviolet (DUV) light source.

[0047] Figure 6B It can be used for Fig. 6A Block diagram of the chamber window.

[0048] Figure 6C It is possible to Fig. 6A A block diagram of the beam steering element used in conjunction with the chamber.

[0049] Fig.6D It is possible to Fig. 6A Block diagram of the beam coupling optical system used in conjunction with the chamber. DETAILED DESCRIPTION

[0050] Reference Figures 1A to 1C , an optical element 100 is shown. Figure 1A is a perspective view of the optical element 100 . Figure 1B It is along Figure 1A A side cross-sectional view of the optical element 100 taken along line BB′. Figure 1C is a view of optical element 100 in the xy plane. Figure 1D The refractive index of the optical element 100 is shown as a function of position along the z-direction.

[0051] Optical element 100 transmits light beam 140. Light beam 140 includes a wavelength in the deep ultraviolet (DUV) range, and the optical element may be used with a DUV light source, such as Figure 4A The light source 460 or Figure 5 Light source 560 is shown. The DUV range includes wavelengths from 10 nanometers (nm) to 400 nm. In other implementations, optical element 100 can be used in various other optical systems and in combination with various other light sources.

[0052] exist Figures 1A to 1CIn the illustrated implementation, the optical element 100 includes a structure 101, which is shown as being disposed within a coating 104. However, in various implementations, the structure 101 may be partially or completely within the coating 104 or it may be on the coating 104, such as on a surface 111 of the coating 104. As discussed below, the structure 101 provides a gradient refractive index and reduces Fresnel reflection losses. Fresnel reflection losses (or reflection losses) are losses generated at input and output interfaces of a component in an optical system due to a difference between the refractive index of the component and the refractive index of one or more media at the input and output interfaces. The coating 104 is a protective coating extending outwardly from a surface 106 along a direction 109. The surface 106 is an outer surface of a crystalline substrate 108. The coating 104 is attached to the outer surface 106 of the crystalline substrate 108. The coating 104 may be attached to the outer surface 106 in a manner that allows the coating 104 to be removed from the outer surface 106. The coating 104 may cover the entire surface 106. Coating 104 has a thickness 107 along direction 109. Figures 1A to 1C In the example shown in FIG. 1 , direction 109 is parallel to the z-direction.

[0053] The crystalline substrate 108 is a crystalline material such as calcium fluoride (CaF2) or magnesium fluoride (MgF2) that transmits light in the DUV range. A crystalline material is a solid material whose components (such as atoms or ions) are arranged in a highly ordered microstructure that forms a lattice extending along the crystal axis.

[0054] The coating 104 prevents or minimizes surface degradation of the crystalline substrate 108 during operational use (e.g., when the light beam 140 interacts with the optical element 100). For example, in implementations where the light beam 140 has a wavelength of 193 nm and the crystalline substrate 108 is CaF2, the light beam 140 causes the fluorine in the crystalline substrate 108 to be depleted. In these implementations, when the light beam 140 is incident on the crystalline substrate 108, the coating 104 protects the crystalline substrate 108 by acting as a seal or barrier to prevent the fluorine from escaping. Thus, the coating 104 enables the optical element 100 to be effectively used in a DUV light source.

[0055] The coating 104 is designed in a manner to match the refractive index n3 of the crystalline substrate 108, that is, the coating 104 has a refractive index n3 that is the same as the refractive index of the crystalline material 108 or has a refractive index that is very similar to the refractive index n3. However, the refractive index n3 of the crystalline substrate 108 is different from the refractive index n1 of the medium 112 at the external surface 111 of the coating 104. Therefore, in a conventional optical element (which does not have the structure 101), Fresnel reflection losses occur at the external surface 111 (and additionally, minimal losses may occur at the surface 106). For an implementation in which n1=1 and n3=1.5018 and normal incidence angles for the s-polarization component and the p-polarization component of the light beam 140, the Fresnel reflection losses can be, for example, about 4.00% to 5% of the incident optical power at the surface 111. The Fresnel reflection losses reduce the amount of light beam 140 that can be provided to downstream optical tools (such as Figure 4A The amount of optical power of the lithography exposure device 469) can be reduced. Moreover, the reflection losses may cause various components of the DUV light source to operate beyond their efficient and / or optimal operating levels in an attempt to compensate for the reflection losses. This may result in inefficient operation, reduced performance, and / or shortened life of components in the DUV light source.

[0056] In another aspect, optical element 100 includes structure 101 that reduces or eliminates these Fresnel losses by providing a gradient or variable refractive index along direction 109. Figure 1D , the optical element 100 has a refractive index profile 110, which is the refractive index as a function of position along a direction 109. The refractive index of the structure 101 is denoted as ns. The refractive index ns is not constant along the direction 109, but varies along the direction 109 from a refractive index n3 at the surface 106 to a refractive index n1 at an outer surface 111 of the optical element 100. Figure 1B In the example of FIG. 1 , the refractive index ns varies linearly along the direction 109. In other implementations, the refractive index ns may vary along the direction 109 in other ways.

[0057] Structures such as structure 101 having features 102 generally cannot be formed directly on or in a crystalline material. Therefore, structure 101 is formed on, partially within, or completely within coating 104, which is an amorphous dielectric material. As such, in optical element 100, coating 104 performs both a protective function of protecting substrate 108 from optical degradation and a reflection reduction or elimination function due to structure 101.

[0058] Structure 101 achieves a gradient refractive index through the intentional arrangement of features 102. Features 102 are Figure 1A The feature 102 extends along the direction 109 and is shown in the yz plane of the illustration. Figure 1C The pattern arrangement shown is shown, although the features 102 may be arranged differently in other implementations. Figure 1B In FIG. 1 , structure 101 is represented as a region with dashed shading. For simplicity, Figure 1A and Figure 1C Only one of the features 102 is marked.

[0059] Features 102 are arranged in a pattern such that the refractive index of structure 101 varies or is not constant. For example, when the dimensions of features 102 along the x, y, and / or z directions are small compared to the wavelength of light beam 140, features 102 exhibit a variable and gradually changing refractive index ns along the propagation direction of light beam 140. By making the refractive index vary, Fresnel reflection losses caused when light beam 140 interacts with optical element 100 are reduced.

[0060] Further, structure 101 is suitable for use with light in the DUV light range. A typical approach to reducing Fresnel reflection losses at the interface of an optical element is to place an anti-reflection (AR) coating on the interface. AR coatings are typically formed by stacking dielectric materials with different refractive indices, and may also be referred to as multilayer dielectric stacks or Bragg reflectors. However, these types of AR coatings are generally not suitable for use in the DUV light range, and / or the AR coating material degrades due to the relatively high photon energy of the DUV light, and generally have a shorter life than coating 104. For example, AR coatings for use at 193nm for AR stacks may include fluorides such as lanthanum trifluoride (LaF3), gadolinium fluoride (GdF3), aluminum fluoride (AlF3), cerium (III) fluoride (CeF3), lithium fluoride (LiF), and magnesium fluoride (MgF2). However, these materials tend to form porous structures when deposited, and attempts to densify the coatings for robustness at 193nm increase their absorption at 193nm. Uncorrectable porosity results in a shorter service life. Moreover, densified forms of these materials often have unacceptable heat absorption and coating stress issues, which can lead to degradation (e.g., blistering and delamination).Further, dielectric coating stacks are often subject to manufacturing variability, and this can lead to inconsistent lifetime performance.

[0061] On the other hand, the structure 101 is formed of the same material or the same type of material as the coating 104 or of a similar amorphous dielectric, and thus its lifetime is generally similar to that of the coating 104 .

[0062] The coating 104 and the feature 102 are made of an amorphous dielectric solid material that transmits light in the DUV range. An amorphous solid material is a solid material that lacks the long-range order that is characteristic of crystalline materials. The coating 104 and the feature 102 can be made of, for example, glass, fused quartz, borosilicate glass, plastic, silicon, or aluminum oxide. The coating 104 and the feature 102 can be made of the same amorphous dielectric material or different dielectric materials.

[0063] Each of features 102 has a shape that does not change after optical element 100 is formed. The shape of a feature defines the volume of space occupied by the feature. In some implementations, all features have the same shape. In other implementations, the shapes of features 102 vary (e.g., semi-randomly) and are not identical. Figure 1A and Figure 1C However, feature 102 may have other shapes. For example, feature 102 may be conical, with the widest portion of the cone closest to surface 106.

[0064] exist Figures 1A to 1C In the example of FIG. 1 , structure 101 is shown as being completely within coating 104. In other words, none of features 102 extend beyond outer surface 111 in the z-direction. Features 102 may be formed into coating 104 using, for example, a dry or wet etching process or a chemical etching process.

[0065] The structure can be coextensive with the coating 104. For example, the extent 113 of the feature 102 along the z-direction can be the same as the thickness 107 of the coating 104. The extent 113 is shown at Figure 1A In some implementations, the extent 113 is less than the thickness 107 of the coating 104. Also, in some implementations, the extent 113 is greater than the thickness 107, such that the structure 101 is partially within or on and in the coating 104.

[0066] refer to Figure 1C , the features have a center-to-center spacing 115 along the x-direction and a center-to-center spacing 116 along the y-direction. The spacings 115, 116 represent the distances between two neighbor features 102. The features 102 can be arranged in a rectilinear grid. In these implementations, the spacings 115 and 116 are uniform throughout the structure 101, so that all features are separated from the nearest features by the same spacing 116 along the y-direction and the same spacing 115 along the x-direction. Moreover, the spacings 116 and 115 can be the same. In other implementations, the features 102 are arranged in a random or pseudo-random manner. In these implementations, the spacings 115 and 116 are non-uniform, and the distance between any two neighbor features 102 varies throughout the structure 101.

[0067] The range 113 may be smaller than the wavelength of the light beam 140, and the spacings 115, 116 may be on the order of the wavelength of the light beam 140. For example, in an implementation where the wavelength of the light beam 140 is 193 nm, the range 113 may be between 30 nm and 50 nm, and the spacings 115, 116 may be between 100 nm and 200 nm. The structure 101 may be a nanostructure, which is a structure having components (such as features 102) having dimensions no greater than a few hundred nanometers (nm) in spatial extent.

[0068] exist Figures 1A to 1C In the example of FIG. 1 , the light beam 140 propagates in the medium 112 along the -z direction (entering Figure 1C ). Light beam 140 impinges on outer surface 111 of optical element 100. Due to structure 101, the difference between the refractive index n1 of medium 112 and the refractive index ns of coating 104 at outer surface 111 is reduced or eliminated, and the difference between the refractive index n2 at surface 106 and the refractive index n1, if any, is reduced or eliminated.

[0069] exist Figures 1A to 1C In the optical element 100 shown, the surfaces 106, 111 are substantially flat in the xy plane, and the crystalline substrate 108 and the coating 104 are plate-like structures. For example, the optical element 100 can be used as a window. The substrate 108 and the coating 104 can have different shapes, such as Figure 2A Furthermore, the optical element 100 may be oriented at an angle relative to the propagation direction of the light beam 140, such as Figure 2B shown.

[0070] Figure 2A and Figure 2B Cross-sectional views of optical elements 200A and 200B are shown, respectively. Optical elements 200A and 200B are examples of other implementations of optical element 100. The cross-sectional views of optical elements 200A and 200B are in the yz plane.

[0071] Optical element 200A includes coating 204A that extends radially outward from surface 206A of crystalline substrate 208A along direction 209A. Structure 201A (indicated by dotted shading) is within coating 204A. Although structure 201A is within coating 204A, in other implementations, structure 201A may be on or partially within coating 204A. Figures 1A to 1D The structure 101 discussed above is similar. Structure 201A is within coating 204A. Structure 201A has a refractive index that varies along direction 209A, thereby reducing Fresnel reflection losses of light beam 140. Figure 2AIn an implementation of , structure 201A includes features extending perpendicular to surface 206A. Surface 206A is curved such that not all features extend in the same direction.

[0072] refer to Figure 2B , optical element 200B includes a crystalline substrate 208B and a coating 204B extending outwardly from a surface 206B of the crystalline substrate 208B. Coating 204B has a thickness 207B along a direction perpendicular to surface 206B. Optical element 200B is similar in shape to optical element 100. However, optical element 200B is intended to be positioned at an angle θ relative to the propagation direction (z) of incident light beam 140. Structure 201B (shown as dotted shading) is within coating 204B. Structure 202B includes features 201B that are similar to those described above with respect to Figures 1A to 1D The features in structure 201B are arranged such that the refractive index ns of structure 201B varies along direction 209B. For example, feature 201B may extend perpendicular to surface 206B and the refractive index ns of structure 201B may have a profile 210 that varies along direction 209B, such as Figure 2C Direction 209B is parallel to but opposite to the propagation direction of light beam 140. Thus, the refractive index of structure 201B changes as light beam 140 propagates, even though optical element 200B is positioned at an angle θ relative to the propagation direction of light beam 140.

[0073] refer to Figure 3A and Figure 3B , another implementation of the optical element 100 is shown. Figure 3A is a perspective view of the optical element 300 . Figure 3B yes Figure 3A Along Figure 3B-3B ' is a cross-sectional view of the optical element 300 taken along line '.

[0074] Optical element 300 includes a crystalline substrate 308 and a protective coating 304 attached to (e.g., coated on) a surface 306 of the crystalline substrate 308. The crystalline substrate 308 is a crystalline material that transmits DUV light. The protective coating 304 is an amorphous dielectric material that extends from the surface 306 along a direction 309. The protective coating 304 has a thickness 307 along the direction 309. The thickness 307 is the distance from the surface 306 to an end 319 of the coating. The end 319 is at Figure 3B Use dashed line style to depict.

[0075] Optical element 300 also includes structure 301 having features 302. Features 302 are made of an amorphous dielectric material, which may be the same or different than the amorphous dielectric material used for protective coating 304. Similar to features 201 on structure 101, features 302 are arranged such that the refractive index ns of structure 301 varies along direction 309. Direction 309 is parallel to and opposite to the propagation direction of light beam 140. Therefore, the refractive index ns also varies along the propagation direction of light beam 140. Figure 3C A profile 310 representing the refractive index of structure 301 as a function of position along direction 309 is shown.

[0076] Feature 302 extends outside of coating 304 along direction 309. Figure 3A and 3B In the example shown, feature 302 extends beyond end 319 of protective coating 304 along direction 309, and extent 313 of feature 302 along direction 309 is greater than thickness 307 of coating 304. Feature 302 may be formed by photolithographic etching or by building feature 302 on coating 304. For example, feature 302 may be built by a chemical etching process (e.g., dry gas etching) and then protective coating 304 may be deposited on top of feature 302.

[0077] In other implementations, features 302 and coating 304 may be used in conjunction with various other optical elements.

[0078] Figure 4A and Figure 5 Examples of DUV light sources 460 and 560 that may use optical elements 100, 200A, 200B, and / or 300, respectively, are provided.

[0079] refer to Figure 4A and Figure 4B , the lithography system 450 includes a DUV light source 460 that provides a light beam 441 to a lithography exposure device 469 that processes a wafer 470 received by a wafer holder or stage 471. The DUV light source 460 includes a discharge chamber 465 that encloses a gain medium 461, a cathode 462a, and an anode 462b. The gain medium 461 is a gaseous gain medium. The discharge chamber 465 is sealed so that the gain medium 461 remains in and is contained by the discharge chamber 465. Figure 4A Only one gas discharge chamber 465 is shown in FIG. However, the light source 460 may include more than one discharge chamber, such as Figure 5 shown.

[0080] The DUV light source 460 also includes a gas management system 479. The gas management system 479 is in fluid communication with the interior 478 of the DUV light source 460. The gas management system 479 may include equipment for managing pressure and / or fluid substances in the interior 478. For example, the gas management system 479 may include pumps, fans, filters, and / or other equipment capable of managing gas and debris. The gas management system 479 may remove unwanted chemicals, elements, or mixtures from the interior 478. For example, the gas management system 479 may use another chemical (in the form of a gas) (such as, for example, nitrogen (N2) or helium (He)) to remove oxygen from the interior 478. The gas used by the gas management system 479 to remove unwanted substances is called a purge gas 412. Although the purge gas 412 is located in the interior 478 and may surround the discharge chamber 465, the purge gas 412 does not penetrate the discharge chamber 465 and does not interfere with or change the chemical composition of the gain medium 461. The light beam 441 propagates in the interior 478 and therefore propagates in the purge gas 412.

[0081] The light beam 441 may be a pulsed light beam including light pulses separated from each other in time. The photolithography exposure device 469 includes a projection optical system 475 through which the light beam 441 passes before reaching the wafer 470, and a measurement system 472. The measurement system 472 may include, for example, a camera or other device capable of capturing an image of the wafer 470 and / or the light beam 441 at the wafer 470, or an optical detector capable of capturing data describing the characteristics of the light beam 441 (such as the intensity of the light beam 441 at the wafer 470 in the xy plane). The photolithography exposure device 469 may be an immersion system or a dry system. The photolithography system 450 also includes a control system 480 to control the light source 460 and / or the photolithography exposure device 469.

[0082] Microelectronic features are formed on wafer 470 by exposing a layer of radiation-sensitive photoresist material on wafer 470, for example, using beam 441. Figure 4B , projection optical system 475 includes slit 476, mask 474, and a projection objective including lens system 477. Lens system 477 includes one or more reflective or refractive optical elements capable of interacting with light in the DUV range. Light beam 441 enters optical system 475 and impinges on slit 476, and at least a portion of light beam 441 passes through slit 476. Figure 4A and Figure 4B In the example of , slit 476 is rectangular and shapes light beam 441 into an elongated rectangular beam. Mask 474 includes a pattern, and the pattern determines which portions of the shaped light beam are transmitted by mask 474 and which portions are blocked by mask 474. The design of the pattern is determined by the specific microelectronic circuit design to be formed on wafer 470.

[0083] refer to Figure 5 , a block diagram of a lithography system 550 is shown. System 550 is a block diagram of system 450 ( Figure 4A ). For example, in the lithography system 550, the light source 560 is used as the light source 460 ( Figure 4A ). The light source 560 generates a pulsed light beam 541, which is provided to the photolithography exposure device 469. The photolithography system 550 also includes a control system 580. Figure 5 In the example of FIG. 5 , the control system 580 is connected to components of the light source 560 and the photolithography exposure apparatus 469 to control various operations of the system 550. In other implementations, the control system 580 may be implemented as two independent control systems, one control system controlling various aspects of the light source 560 and the other control system controlling the photolithography exposure apparatus 469. In still other implementations, various other control systems 580 may be implemented.

[0084] exist Figure 5 In the example shown, the light source 560 is a two-stage laser system that includes a master oscillator (MO) 567 that provides a seed beam 542 to a power amplifier (PA) 568. The MO 567 and the PA 568 can be considered a subsystem of the light source 560 or a system that is part of the light source 560. The PA 568 receives the seed beam 542 from the MO 567 and amplifies the seed beam 542 to generate a beam 541 for use in the lithography exposure device 469. For example, in some implementations, the MO 567 can emit a pulsed seed beam, where the seed pulse energy is about 1 millijoule (mJ) per pulse, and these seed pulses can be amplified to about 10mJ to 15mJ by the PA 568.

[0085] MO 567 includes a discharge chamber 565_1 having two elongated electrodes 562a_1 and 562b_1, a gain medium 561_1 as a gas mixture, and a fan (not shown) for circulating the gas mixture between the electrodes 562a_1, 562b_1. A resonator is formed between a line narrowing module 586 on one side of the discharge chamber 565_1 and an output coupler 581 on a second side of the discharge chamber 565_1.

[0086] The discharge chamber 565_1 includes a first chamber window 563_1 and a second chamber window 564_1. The first chamber window 563_1 and the second chamber window 564_1 are located on opposite sides of the discharge chamber 565_1. The first chamber window 563_1 and the second chamber window 564_1 transmit light in the DUV range and allow the DUV light to enter and leave the discharge chamber 565_1.

[0087] The line narrowing module 586 may include a diffractive optical device, such as a grating to fine tune the spectral output of the discharge chamber 565_1. The light source 560 also includes a line center analysis module 584 and a beam coupling optical system 583 that receive the output beam from the output coupler 581. The line center analysis module 584 is a measurement system that can be used to measure or monitor the wavelength of the seed beam 542. The line center analysis module 584 can be placed at other locations in the light source 560, or it can be placed at the output of the light source 560.

[0088] The gas mixture as gain medium 561_1 can be any gas suitable for producing a beam of wavelength and bandwidth required for the application. For an excimer source, the gas mixture 561_1 can contain an inert gas (rare gas) such as, for example, argon or krypton, a halogen (such as, for example, fluorine or chlorine), and a trace amount of xenon in addition to a buffer gas (such as helium). Specific examples of gas mixtures include argon fluoride (ArF) emitting light at a wavelength of about 193nm, krypton fluoride (KrF) emitting light at a wavelength of about 248nm, or xenon chloride (XeCl) emitting light at a wavelength of about 351nm. Therefore, in this implementation, the beams 541 and 542 include wavelengths in the DUV range. The excimer gain medium (gas mixture) is pumped using short (e.g., nanosecond) current pulses in a high voltage discharge by applying a voltage to the elongated electrodes 562a_1, 562b_1.

[0089] PA 568 includes a beam coupling optical system 583 that receives the seed beam 542 from MO 567 and directs the seed beam 542 through the discharge chamber 565_2 and reaches a beam steering optical element 582 that modifies or changes the direction of the seed beam 542 so that it is sent back into the discharge chamber 565_2. The beam steering optical element 582 and the beam coupling optical system 583 form a circular closed loop optical path, in which the input of the ring amplifier intersects the output of the ring amplifier at the beam coupling optical system 583.

[0090] The discharge chamber 565_2 includes a pair of elongated electrodes 562a_2, 562b_2, a gain medium 561_2, and a fan (not shown) for circulating the gain medium 561_2 between the electrodes 562a_2, 562b_2. The gas mixture forming the gain medium 561_2 may be the same as the gas mixture forming the gain medium 561_1.

[0091] The discharge chamber 565_2 includes a first chamber window 563_2 and a second chamber window 564_2. The first chamber window 563_2 and the second chamber window 564_2 are located on opposite sides of the discharge chamber 565_2. The first chamber window 563_2 and the second chamber window 564_2 transmit light in the DUV range and allow the DUV light to enter and leave the discharge chamber 565_2.

[0092] The output beam 541 can be directed through a beam preparation system 585 before reaching the lithographic exposure device 469. The beam preparation system 585 can include a bandwidth analysis module that measures various parameters (such as bandwidth or wavelength) of the beam 541. The beam preparation system 585 can also include a pulse stretcher (not shown) that stretches each pulse of the output beam 541 in time. The beam preparation system 585 can also include other components that can act on the beam 541, such as, for example, reflective and / or refractive optical elements (such as, for example, lenses and mirrors), filters, and optical apertures (including automated shutters).

[0093] The DUV light source 560 also includes a gas management system 479 that is in fluid communication with the interior 578 of the DUV light source 560. As discussed above, the gas management system 479 provides the purge gas 412 to the interior 578. Figure 5 In the example of FIG. 5 , the purge gas 412 surrounds the chambers 565_1 and 565_2 and also surrounds optical components of some of the subsystems of the DUV light source 560. For example, the purge gas 412 surrounds optical components in the line narrowing module 586, the output coupler 581, the line center analysis module 584, the beam coupling optical system 583, and the beam steering optical element 582. Although the purge gas 412 is in the interior 578 and surrounds the discharge chambers 565_1 and 565_2 and various other optical components, the purge gas 412 does not penetrate the discharge chambers 565_1 and 565_2 and does not interfere with or change the chemical composition of the gain media 561_1 and 561_2.

[0094] The photolithography system 550 also includes a control system 580. The control system 580 can control when the light source 560 emits a light pulse or a burst of light pulses including one or more light pulses by sending one or more signals to the light source 560. The control system 580 is also connected to the photolithography exposure device 469. Therefore, the control system 580 can also control various aspects of the photolithography exposure device 469. For example, the control system 580 can control the exposure ( Figure 4A ), and thus can be used to control how electronic features are printed on wafer 470. In some implementations, control system 580 can control the scanning of wafer 470 by controlling the movement of slit 476 in the xy plane ( Figure 4B). In addition, the control system 580 can be connected to the measurement system 472 and / or the optical system 475 ( Figure 4B ) to exchange data.

[0095] The photolithography exposure apparatus 469 may also include, for example, temperature control equipment (such as air conditioning equipment and / or heating equipment) and / or power supplies for various electronic components. The control system 580 may also control these components. In some implementations, the control system 580 is implemented to include more than one sub-control system, wherein at least one sub-control system (photolithography controller) is dedicated to controlling aspects of the photolithography exposure apparatus 469. In these implementations, the control system 580 may be used to control aspects of the photolithography exposure apparatus 469 as an alternative to the photolithography controller, or in addition to using the photolithography controller.

[0096] When the gain medium 561_1 or 561_2 is pumped by applying a voltage to the electrodes 562a_1, 562b_1 or 562a_2, 562b_2, respectively, the gain medium 561_1 and / or 561_2 emits light. When the voltage is applied to the electrodes at regular time intervals, the light beam 541 is pulsed. Therefore, the repetition rate of the pulsed light beam 541 is determined by the rate at which the voltage is applied to the electrodes. For various applications, the repetition rate of the pulses can range between approximately 500 Hz and 6,000 Hz. In some implementations, the repetition rate can be greater than 6,000 Hz, and can be, for example, 12,000 Hz or higher, but other repetition rates can be used in other implementations.

[0097] Optical elements 100, 200A, 200B and / or 300 may be used with light source 460 or light source 560 to reduce Fresnel reflection losses. For example, any or all of chamber windows 563_1, 564_1, 563_2, 564_2 may be formed by optical element 200B ( Figure 2B ) are formed by similar optical elements. Moreover, any or all of the optical components used as beam steering optical element 582, beam coupling optical system 583, line center analysis module 584, output coupler 581, and line narrowing module 586 may include a structure (such as structure 101 including feature 102) that produces a gradient refractive index to reduce Fresnel reflection features.

[0098] In various other implementations, optical elements 100, 200A, 200B, and / or 300 may be used with various other suitable light sources and in conjunction with various other photolithography systems.

[0099] Fig. 6A This is the block diagram of PA 668. PA 668 is a PA 568 ( Figure 5). PA 668 can replace PA 568 for DUV light source 560 ( Figure 5 ). PA 668 includes a discharge chamber 662_2, a beam steering optical element 682 and a beam coupling optical system 683. Fig.6D The beam coupling optical system 683 is shown in more detail in FIG.

[0100] The discharge chamber 662_2 encloses the gain medium 561_2 ( Figure 5 ). The discharge chamber 662_2 includes a first chamber window 663_2 and a second chamber window 664_2. The first chamber window 663_2 and the second chamber window 664_2 allow DUV light to enter and leave the discharge chamber 662_2. The DUV light enters the second chamber window 664_2 and propagates on a path 666 through the gain medium 561_2 and exits through the first chamber window 663_2.

[0101] In the example shown, the first chamber window 663_2 is on the left wall 687 of the discharge chamber 662_2, and the second chamber window 664_2 is on the right wall 688 of the discharge chamber 662_2. Walls 687 and 688 extend parallel to each other and are on opposite sides of the discharge chamber 662_2. Other implementations and other relative orientations of the windows 663_2, 664_2 are possible. The windows 663_2, 664_2 are at an angle θ relative to the propagation direction of the intermediate beam i. Figure 6B The angle θ is shown in FIG.

[0102] In addition, reference Fig.6D , the beam coupling optical system 683 receives the seed beam 542 at the folding mirror 690, which reflects the seed beam 542 to the partial reflector 691 (also called input / output coupler 691). The input / output coupler 691 is the entrance and exit of the ring resonator formed by the beam coupling optical system 683 and the beam steering optical element 682.

[0103] The input / output coupler 691 has a surface 698a that transmits DUV light and a partially reflective surface 698b (e.g., 20% to 40% reflective of DUV light), so that at least a portion of the seed beam 542 passes through the input / output coupler 691, and at least some of the light incident on the surface 698b of the input / output coupler 691 is reflected back to the discharge chamber 662_2, thereby providing regenerative feedback. Other degrees of reflectivity can be used in other implementations. The light reflected back to the discharge chamber 662_2 and the portion of the seed beam 542 that passes through the input / output coupler 691 are referred to as intermediate beam i in the discussion below.

[0104] The intermediate beam i passes through prisms 693 and 694. Prisms 693 and 694 together form a beam compression system that compresses the intermediate beam i horizontally to substantially match the lateral dimensions of the gain medium 561_2, which may be, for example, less than a few millimeters (mm). Prism 694 aligns the intermediate beam i with the second chamber window 664_2, the first chamber window 663_2, and the beam steering optical element 682. The intermediate beam i passes through prism 694 and the second chamber window 664_2, propagates in path 666 through the gain medium 561_2, and is reflected by the beam steering optical element 682. The intermediate beam i reenters the chamber 662_2 through the first chamber window 663_2, propagates through the gain medium 561_2, and passes through the second chamber window 664_2, and then passes through prism 694, which transfers the beam to prism 695, which transfers the intermediate beam i to the input / output coupler 691.

[0105] The first chamber window 663_2 and / or the second chamber window 664_2 may be implemented as an optical element 200B ( Figure 2B ). Figure 6B A second chamber window 664_2 implemented using features of optical element 200B is shown. In this implementation, second chamber window 664_2 includes a protective coating 204B extending outward from surface 206B of a crystalline substrate 208B. Structures 201B (shown using dotted shading) are in or on protective coating 204B to reduce Fresnel reflections. Structures 201B have a refractive index ns that varies along direction 209B, which is parallel to and opposite to direction p. Thus, the refractive index ns varies along the propagation direction of the intermediate beam i. Figure 2C The refractive index ns is shown as a function of position along the direction 209B. In the example where the optical element 200B is the second chamber window 664_2, n1 is the refractive index of the purge gas 412, and n3 is the refractive index of the crystalline substrate 208B.

[0106] Reference again Figure 6B , the intermediate beam i propagates through the purge gas 412 in the p direction. The intermediate beam i is incident on the surface 211B and passes through the structure 201B and through the crystal substrate 208B. Although the refractive index of the protective coating 204B is different from the refractive index of the purge gas 412, the gradient refractive index in the structure 201B reduces or eliminates Fresnel reflection losses that would otherwise occur at the surface 211B. By reducing the Fresnel reflection losses, more of the intermediate beam i can be used for amplification without increasing the amount of energy supplied to the electrodes 562a_2 and 562b_2. Therefore, using the optical element 200B as the chamber window 663_2 and / or 664_2 increases the lifetime of the chamber 665_2.

[0107] Moreover, the use of optical elements including structures such as structures 101, 201A, 201B, or 301 is beneficial to the entire DUV light source 550 by reducing the total energy passing through and / or applied to the DUV light source 550 to generate an output (such as output beam 541) with specifications required by the end user. In the absence of optical elements including structures such as structures 101, 201A, 201B, or 301, the DUV light source 550 generates more light to compensate for the Fresnel reflection losses generated by interacting with the optical element. This additional light is called compensation light. The use of optical elements including structures such as structures 101, 201A, 201B, or 301 reduces the Fresnel reflection losses, so that the compensation light can be minimized or eliminated. Because less total light is generated to provide the same amount of light in the output beam 541, a lower operating voltage can be applied to the electrodes 562a_1, 562b_1, 562a_2, and / or 562b_2. Moreover, by reducing the amount of compensation light, there is a lower thermal load on the optical elements that interact with the intermediate beam i and / or the seed beam 542. Therefore, when the power load of the DUV light source 550 changes rapidly (for example, when the operating state of the DUV light source changes), the thermal transients of these optical elements are lower and less severe. Further, reducing the amount of compensation light results in a reduced impact on bulk substrates and coatings, thereby extending the service life of these components. Additionally, because features in structures such as structures 101, 201A, 201B, or 301 can be arranged to reduce Fresnel reflections at expected incident angles, using optical elements having structures such as structures 101, 201A, 201B, or 301 can have greater flexibility in adjusting optical incident angles to meet design goals without having to worry too much about Fresnel reflection losses, and can also produce greater design flexibility. In addition, by reducing Fresnel reflection losses, scattered stray light is reduced. Thus, the overall performance of DUV light source 550 is improved by using one or more optical elements including structures such as structures 101 , 201A, 201B, or 301 .

[0108] Additionally, using optical elements having structures 101, 201A, 201B, or 301 in PA 568 improves the efficiency of PA 568. The improved efficiency enables the PA 568 cavity to produce the same amount of power with a lower power within the cavity and inter-cavity beam width (the beam width of the light beam propagating in PA 568). The smaller inter-cavity beam width allows the use of smaller cavity optics, which can reduce the overall size and / or cost of PA 568. Finally, the improved efficiency allows the use of more reflective optics in coupler 691. The use of more reflective optics allows PA 568 to produce the same output power, but with longer pulse lengths.

[0109] Other optical elements may be implemented to include structures that produce a gradient refractive index to reduce or eliminate Fresnel reflections, such as structures 101, 201A, 201B, and 301. For example, beam steering optical element 582 may be implemented as a crystal structure (such as CaF2) processed into a prism having two reflective surfaces 596a, 596b, such as Figure 6C In these implementations, the intermediate light beam i is transmitted through the transmission surface 596c into the beam steering optical element 582. The transmission surface includes a protective coating 604C similar to the protective coatings 104, 204A, 204C, and 304 and a structure 601C similar to the structures 101, 201B, 201C, and 301.

[0110] According to other implementations, various other arrangements of optical elements may be used (such as FIG. 6A to FIG. 6D arrangement shown).

[0111] Moreover, other transmissive optical components in light source 560 may include structures similar to structures 101, 201B, 201C, 301. For example, one or more of prisms 693, 694, and 695 may include structures similar to structures 101, 201B, 201C, 301.

[0112] Still other implementations are within the scope of the following claims.

[0113] Further aspects of the invention are set out in the following numbered clauses.

[0114] 1. An optical element for a deep ultraviolet light source, the optical element comprising:

[0115] Crystal substrate;

[0116] a coating on the outer surface of the crystalline substrate, the coating having a thickness along a direction extending away from the outer surface; and

[0117] A structure on and / or in the coating, the structure comprising a plurality of features extending away from the crystalline substrate along the direction, wherein the features comprise an amorphous dielectric material and are arranged such that a refractive index of the structure varies along the direction.

[0118] 2. An optical element according to clause 1, wherein the crystalline substrate comprises calcium fluoride (CaF2).

[0119] 3. An optical element according to clause 1, wherein the extent of the features along the direction is no greater than the thickness.

[0120] 4. An optical element according to clause 3, wherein all features of the structure are within the coating.

[0121] 5. An optical element according to clause 1, wherein the features and the coating are made of the same dielectric material.

[0122] 6. An optical element according to clause 1, wherein one or more of the features extend outside the coating.

[0123] 7. An optical element according to clause 1, wherein the amorphous dielectric material comprises fused silica or alumina.

[0124] 8. The optical element of clause 1, wherein each feature of the plurality of features is identical, and the plurality of features are arranged relative to each other in a regular and repeating pattern.

[0125] 9. The optical element of clause 1, wherein each feature of the plurality of features is identical, and the plurality of features are arranged relative to each other in a random or pseudo-random manner.

[0126] 10. An optical element according to claim 1, wherein each feature of the plurality of features is associated with a spatial characteristic, and at least one spatial characteristic of one feature of the plurality of features is different from the spatial characteristic of at least one feature of the other features, and wherein the spatial characteristic includes any of the following: height, width, and shape.

[0127] 11. An optical element according to clause 1, wherein the refractive index of the structure varies linearly along the direction.

[0128] 12. An optical element according to clause 1, wherein the refractive index of the structure varies along a direction from a value substantially equal to the refractive index of the crystalline substrate to a value substantially equal to the refractive index of the fluid at the optical element.

[0129] 13. An optical element according to clause 12, wherein the fluid comprises a nitrogen (N2) purge gas.

[0130] 14. An optical element according to clause 1, wherein the extent of the features along the direction is 100 nanometers (nm) or less.

[0131] 15. An optical element according to clause 1, wherein the direction is substantially normal to a surface of the crystalline substrate.

[0132] 16. An optical element according to claim 2, wherein in operational use, deep ultraviolet (DUV) light having a wavelength of 193 nanometers (nm) is incident on the coating, the coating protects the CaF2 substrate by reducing the removal of fluorine from the substrate, and the refractive index of the structure changes along the propagation direction of the DUV light, thereby reducing the reflection of the DUV light from the optical element.

[0133] 17. An optical element according to clause 1, wherein the coating completely covers the surface of the crystalline substrate.

[0134] 18. A deep ultraviolet (DUV) light source, comprising:

[0135] a chamber comprising a housing configured to enclose a gaseous gain medium; and

[0136] At least one optical element configured to transmit DUV light, wherein the at least one optical element comprises:

[0137] a substrate comprising a crystalline material configured to transmit DUV light;

[0138] a coating on the outer surface of the substrate, the coating having a thickness along a direction extending away from the outer surface; and

[0139] A structure on and / or in the coating, the structure comprising a plurality of features extending along a direction away from the external surface, wherein the features comprise an amorphous material and the features are arranged such that a refractive index of the structure varies along the direction.

[0140] 19. The DUV light source of clause 18, wherein one or more of the plurality of features extend a distance along the direction, and the distance is less than a wavelength of the DUV light transmitted by the at least one optical element.

[0141] 20. The DUV light source of clause 18, wherein the plurality of features are arranged relative to one another such that a spacing between any two adjacent features is within the order of a wavelength of DUV light transmitted by the at least one optical element.

[0142] 21. The DUV light source of clause 18, wherein the crystalline material configured to transmit DUV light comprises calcium fluoride (CaF2).

[0143] 22. A DUV light source according to clause 18, wherein the refractive index of the structure varies along a direction from a value substantially equal to the refractive index of the crystalline substrate to a value substantially equal to the refractive index of the fluid surrounding the optical element.

[0144] 23. A DUV light source according to clause 22, wherein the fluid comprises a nitrogen (N2) purge gas.

[0145] 24. A DUV light source according to clause 18, wherein the structure is located within the coating.

[0146] 25. A DUV light source according to clause 18, wherein the structures are located in and on the coating such that the structures are partially within the coating.

[0147] 26. The DUV light source of clause 18, wherein the chamber further comprises:

[0148] a first window on a first side of the housing; and

[0149] A second window is located on a second side of the housing, the second side of the housing being opposite to the first side of the housing, and wherein the at least one optical element includes the first window and the second window, and the coating is on the first window and the second window.

[0150] 27. A DUV light source according to clause 26, wherein the coating on the first window and the coating on the second window are provided on surfaces of the respective windows external to the housing.

[0151] 28. The DUV light source of clause 26, further comprising a second chamber comprising:

[0152] a second housing configured to retain a gaseous gain medium within an interior of the second housing;

[0153] a third window on the first side of the second housing; and

[0154] A fourth window is located on a second side of the second housing, the second side of the second housing being opposite to the first side of the second housing, wherein the at least one optical element further comprises a third window and a fourth window.

[0155] 29. The DUV light source of clause 26, wherein an outer surface of the first window and an outer surface of the second window are not perpendicular to a propagation direction of the DUV light.

[0156] 30. The DUV light source of clause 18, wherein the at least one optical element comprises one or more of: a prism, a beam splitter, a lens, and an optical compensator.

[0157] 31. A DUV light source according to clause 18, wherein the refractive index varies along the propagation direction of the DUV light.

[0158] 32. A DUV light source according to claim 18, wherein in operational use, DUV light having a wavelength of 193 nanometers (nm) is incident on the coating, the coating protects the CaF2 substrate by mitigating the removal of fluorine from the substrate, and the refractive index of the structure changes along the propagation direction of the DUV light, thereby reducing the reflection of the DUV light from the optical element.

[0159] 33. A DUV light source according to clause 18, wherein the coating completely covers the surface of the crystalline substrate.

Claims

1. An optical element for a deep ultraviolet light source, the optical element comprising: a crystalline substrate configured to transmit deep ultraviolet light; a coating on an outer surface of the crystalline substrate, the coating having a thickness along a direction extending away from the outer surface; as well as A structure located on and / or in the coating, the structure comprising a plurality of features extending away from the crystalline substrate along the direction, wherein the features comprise an amorphous dielectric material and are arranged such that a refractive index of the structure varies along the direction.

2. The optical element of claim 1, wherein the crystalline substrate comprises calcium fluoride (CaF2). The optical element of claim 1 , wherein the extent of the feature along the direction is no greater than the thickness.

4. An optical element according to claim 3, wherein all features of the structure are within the coating.

5. The optical element of claim 1, wherein the features and the coating are made of the same dielectric material.

6. The optical element of claim 1, wherein one or more of the features extend outside of the coating.

7. The optical element of claim 1, wherein the amorphous dielectric material comprises fused silica or alumina.

8. The optical element of claim 1, wherein each feature of the plurality of features is identical, and the plurality of features are arranged relative to each other in a regular and repeating pattern.

9. The optical element of claim 1, wherein each feature of the plurality of features is identical and the plurality of features are arranged relative to each other in a random or pseudo-random manner.

10. An optical element according to claim 1, wherein each feature of the plurality of features is associated with a spatial characteristic, and at least one spatial characteristic of one feature of the plurality of features is different from the spatial characteristic of at least one feature of the other features, and wherein the spatial characteristic comprises any one of the following: height, width, and shape. The optical element according to claim 1 , wherein the refractive index of the structure varies linearly along the direction.

12. The optical element of claim 1, wherein the refractive index of the structure varies along the direction from a value substantially equal to the refractive index of the crystalline substrate to a value substantially equal to the refractive index of the fluid at the optical element.

13. The optical element of claim 12, wherein the fluid comprises a nitrogen (N2) purge gas.

14. The optical element of claim 1, wherein the features have an extent of 100 nanometers (nm) or less along a direction.

15. The optical element of claim 1, wherein the direction is substantially normal to a surface of the crystalline substrate.

16. The optical element of claim 2, wherein in operational use, deep ultraviolet (DUV) light having a wavelength of 193 nanometers (nm) is incident on the coating, the coating protects the substrate comprising CaF2 by mitigating the removal of fluorine from the substrate, and the refractive index of the structure varies along a propagation direction of the DUV light such that reflection of the DUV light from the optical element is reduced.

17. The optical element according to claim 1, wherein the coating completely covers the surface of the crystal substrate.

18. A deep ultraviolet (DUV) light source, comprising: a chamber including a housing configured to enclose a gaseous gain medium; as well as at least one optical element configured to transmit DUV light, wherein the at least one optical element comprises: a crystalline substrate configured to transmit DUV light; a coating on an outer surface of the substrate, the coating having a thickness along a direction extending away from the outer surface; and A structure located on and / or in the coating, the structure comprising a plurality of features away from the external surface along the direction, wherein the features comprise amorphous material and are arranged such that a refractive index of the structure varies along the direction.

19. The DUV light source of claim 18, wherein one or more of the plurality of features extend a distance along the direction, and the distance is less than a wavelength of DUV light transmitted by the at least one optical element.

20. The DUV light source of claim 18, wherein the plurality of features are arranged relative to each other such that a spacing between any two adjacent features is within the order of a wavelength of DUV light transmitted by the at least one optical element.

21. The DUV light source of claim 18, wherein the crystalline substrate configured to transmit DUV light comprises calcium fluoride (CaF2).

22. The DUV light source of claim 18, wherein the refractive index of the structure varies along the direction from a value substantially equal to the refractive index of the crystalline substrate to a value substantially equal to the refractive index of a fluid surrounding the optical element.

23. The DUV light source of claim 22, wherein the fluid comprises a nitrogen (N2) purge gas.

24. The DUV light source of claim 18, wherein the structure is located within the coating.

25. The DUV light source of claim 18, wherein the structures are located in and on the coating such that the structures are partially within the coating.

26. The DUV light source of claim 18, wherein the chamber further comprises: a first window on a first side of the housing; as well as A second window is located on a second side of the housing, the second side of the housing being opposite to the first side of the housing, and wherein the at least one optical element includes the first window and the second window, and the coating is on the first window and the second window.

27. The DUV light source of claim 26, wherein the coating on the first window and the coating on the second window are disposed on surfaces of the respective windows outside the housing.

28. The DUV light source of claim 26, further comprising a second chamber, the second chamber comprising: a second housing configured to retain a gaseous gain medium within an interior of the second housing; a third window located on the first side of the second housing; as well as A fourth window is located on a second side of the second housing, the second side of the second housing being opposite to the first side of the second housing, wherein the at least one optical element further comprises the third window and the fourth window.

29. The DUV light source of claim 26, wherein an outer surface of the first window and an outer surface of the second window are not perpendicular to a propagation direction of the DUV light.

30. The DUV light source of claim 18, wherein the at least one optical element comprises one or more of: a prism, a beam splitter, a lens, and an optical compensator.

31. The DUV light source of claim 18, wherein the refractive index varies along a propagation direction of the DUV light.

32. The DUV light source of claim 18, wherein in operational use, DUV light having a wavelength of 193 nanometers (nm) is incident on the coating, the coating protects the substrate comprising CaF2 by mitigating removal of fluorine from the substrate, and the refractive index of the structure varies along a propagation direction of the DUV light such that reflection of the DUV light from the optical element is reduced.

33. The DUV light source of claim 18, wherein the coating completely covers the surface of the crystalline substrate.

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