Optical element

Abstract

There is provided an optical element for an EUV utilisation apparatus, the optical element comprising alternating layers of crystalline materials having different refractive indices. Also provided is a radiation source for an EUV utilization system, said system including such an optical element. An exposure apparatus comprising such a radiation source and an EUV utilization system, said system including such an optical element, source laser system, or exposure apparatus is also provided. Also provided is a method of manufacturing an optical element, the method including depositing alternating layers of crystalline materials having different refractive indices, as well as the use of such an optical element, radiation source, exposure apparatus, EUV utilization system, and method in an EUV utilization apparatus or method.

Description

OPTICAL ELEMENTCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24183191.6 which was filed on 19 June 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to an optical element for an extreme ultraviolet (EUV) utilization apparatus. The present disclosure also relates to a radiation source for an EUV utilization system, such as a laser system, as well as an exposure apparatus. The present disclosure further relates to an EUV utilization system, a method of manufacturing an optical element, as well as the use of any of the aforesaid in an EUV utilization apparatus or method. The present disclosure has particular, but not exclusive application, to EUV exposure apparatus such as EUV utilization apparatus, for example EUV lithography apparatuses or inspection tools. The present disclosure provides for ultrahigh reflective coatings which are able to reflect light efficiently.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0005] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0006] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses reflective surfaces instead of lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector, the multilayers may be altematingly Molybdenum and Silicon, commonly referred to as a Bragg stack. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is presentis considered a requirement for a mass-production process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks. For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Forth, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0007] It is desirable for the optical elements within an EUV utilization apparatus, such as an EUV lithography apparatus, a metrology system, or an inspection apparatus to have high reflectivity, low absorption, resistance to damage within the hostile environment of an EUV utilization apparatus, and offer consistent performance. In addition, it is desirable to have optical elements which are easily handled. Existing optical elements may be susceptible to laser-induced damage or may degrade over time, resulting in lowered performance.

[0008] As requirements for next-generation EUV utilization apparatuses become ever more stringent and demanding, it is necessary to provide optical elements which address at least some of the shortcomings of existing optical elements.

[0009] The present invention has been devised in an attempt to address at least some of the problems identified above.SUMMARY

[0010] According to a first aspect of the present disclosure, there is provided an optical element for an EUV utilisation apparatus, the optical element comprising alternating layers of crystalline materials having different refractive indices.

[0011] Optical elements comprising alternating low and high refractive indices rely on constructive interference to reflect incident light. Alternately, the parameters of the layers may be tuned such that it creates a coating with a high transmissivity and good anti -reflective properties. The thickness of the alternating layers is determined by the wavelength of light which is to be reflected, with the thickness of each layer being around a quarter of the wavelength of light which is being reflected. In an EUV context, the wavelength of the light in the source laser may be around 10 microns. The materials which form such layers are based on amorphous dielectric materials, such as zinc selenide,zinc sulphide, germanium, thorium fluoride, tantalum oxide, silicon oxide, titanium oxide, hafnium oxide, and aluminium oxide. Such amorphous layers have been found to have a number of limitations. Firstly, as the power of the source laser increases, the propensity for laser induced damage is also increased. It has been found that crystalline layers are advantageous over existing amorphous layers since crystalline layers have lower absorption than corresponding amorphous layers. As such, a crystalline layer is able to handle higher laser powers than an amorphous layer. In addition, amorphous materials are hygroscopic, meaning that they absorb water even in ambient conditions, which leads to degradation in performance. The absorption coefficient of water at the wavelengths of light used in source lasers, such as around 1 to 10 microns, is high. As such, despite potentially only a small amount of water being present, it has a large effect on performance due to the propensity of water to absorb light at such wavelengths. Furthermore, crystalline materials have a higher thermal conductance than amorphous equivalents, meaning that they are more resistant to laser induced damage, which is desirable for high power applications, such as in EUV source lasers.

[0012] The optical element may be a mirror. The optical element may comprise a Bragg stack. The optical element may be a polariser. The polariser may be a reflective phase retarder. The optical element may be a transmissive window wherein the crystalline layers may be provide to enhance an Anti-reflective feature of the transmissive window.

[0013] Bragg stacks demonstrate extremely high reflectivity for certain wavelengths of light and can be tuned by altering the thickness of the layers to reflect light having a desired wavelength, which allows reflectivity of greater than 99% of incident light.

[0014] Polarization control is important within source lasers. The basic principle to protect the seed laser from back-reflected light is based on a polarization into different components, such as a seed isolation module. Therefore, maintaining and controlling the polarization state of a laser is critical to protect the seed laser against damage and instabilities from reflected light. The component which is key to this is a phase reflective retarder, which switches light from linearly polarized light to circular polarized light, and vice versa, by introducing a 90 degree phase shift between s- and p-polarised light. Such phase reflective retarders operate with an angle of incidence (AOI) of 45 degrees. In use, an input beam is linearly polarized 45 degrees to the plane of incidence, which causes the reflected beam to be circularly polarized. Existing polarisers comprise thorium fluoride and / or zinc selenide. In next generation applications, higher powered lasers will be used and different configurations of polarizer will also be used, which may quadruple the laser load upon optical elements. As such, it is desirable to provide optical elements, such as phase reflective retarders, with low absorption and with a high damage threshold. Existing phase reflective retarders have certain limitations which are overcome by the present disclosure. In particular, existing polarisers have a phase shift between s- and p-polarised light of around ±4 degrees from the ideal 90 degree phase shift. As such, the degree of circular / 1 inear polarization is lowered, which may increase the risk of damage to sensitive components, such as the seed laser. Furthermore, existing polarisers induce approximately a 4 degree error for every degree oferror in the angle of incidence . For example, if the angle of incidence deviates from the ideal 45 degrees by 1 degree, the coating induces a 4 degree phase error. In addition, existing polarisers have a relatively high absorption of around 2%, which will become more critical as powers increase due to additional heating of the polarisers. The present disclosure provides for crystalline layers rather than the amorphous layers which are currently used. Furthermore, the use of crystalline layers in optical elements as opposed to amorphous layers provides for additional control of the thickness of the layers, for example of 0. 1% or less thickness variation for crystalline layers versus around 1% variation in the thickness of amorphous layers.

[0015] According to an aspect of the present disclosure, an Anti-Reflective, AR, coating based on coatings materials with higher refractive index than the substrate window is presented. These types of AR coatings are based on destructive interference. In particular, two different versions of AR coatings are presented. In a first embodiment of anti-reflective coating, each layer in the stack of coating comprise mono -crystalline materials, whereas in a second embodiment, each layer comprises amorphous materials. ZnSe and crystalline GaAs are used respectively.

[0016] The use of crystalline GaAs and AlGaAs for AR coatings has several advantages. These films are monocrystalline and hence intrinsically defect-free. Consequently, absorption centers, that lower the LIDT, are absent in these coatings. The two different coatings can be placed on the diamond window directly or using a thin adhesion layer, such as ZnSe. An adhesion layer might be beneficial for increased bonding between crystalline layers and diamond substrate, and extended experimental tests such as LIDT test will finally show if the adhesion layer is needed.

[0017] At least one of the crystalline materials is monocry stalline. A monocrystalline structure has the most uniform properties.

[0018] The optical element may be configured to reflect light having a wavelength of from about 1 micron to about 20 microns, from about 1 micron to about 15 microns, from about 1 micron to about 10 microns, from about 1 micron to about 5 microns, from about 1 micron to about 4 microns, or from about 1 micron to about 2 microns. In cases where the optical element is a Bragg stack, the thickness of the layers may be tuned to optimally reflect light of a given wavelength. It is known in the art how the thickness of the layers may be adjusted to optimally reflect a given wavelength.

[0019] The optical element may comprise two different crystalline materials having a lattice mismatch of 4% or less, 3% or less, 2% or less, 1% or less, or 0.5% or less. In order to minimise epitaxial strain, specifically the relaxation of strain towards higher film thicknesses, which leads to stacking faults, regions of differing stoichiometry, which can lead to disturbance of the monocrystalline structure, thereby adversely affecting performance.

[0020] The optical element may have a refractive index of 1 or more, 1.5 or more, 2 or more, 2.5 or more, 3 or more, 3.5 or more, or 4 or more. By having a large refractive index, the coating can provide high performance as regards reflectivity. The individual crystalline materials of the alternating may have a high contrast in refractive index in order to create a structure which is highly reflective.

[0021] The optical element may include one or more of GaAs, AlGaAs, InGaAsP, InP, InGaAs, AlGaP, GaP, CdTe, HgCdTe, MgCdTe, MgZnTe, AlSb, GaSb, and CdZnTe. These materials may be combined in layers such that the layers of different materials have different refractive indices to thereby form a reflective material. As specific examples of layered crystalline materials, the optical element may comprise: i) InGaAsP and InP; ii) InGaAs and InP; iii) AlGaP and GaP; iv) CdTe and HgCdTe; v) HgCdTe and CdZnTe; or vi) GaAs and AlGaAs.

[0022] These materials are eminently suitable for use in optical elements as described herein. For example, cadmium telluride has low absorption. Mercury cadmium telluride can be grown lattice matched with cadmium zinc telluride. Cadmium zinc telluride can be lattice matched to mercury cadmium telluride. Indium gallium arsenide, indium gallium arsenide phosphide can be stoichiometrically tuned to lattice match indium phosphide. Gallium phosphide can be lattice matched with silicon, and has very low absorption up to 1 micron. Aluminium antimonide and gallium antimonide in layers has a large mismatch in refractive index. In addition, these materials are oxide free, which means that the issue of absorption of water is mitigated. Similarly, these materials are thorium free, meaning that they are not radioactive and do not require special handling for radioactive materials.

[0023] The optical element may include a base layer. The base layer may be an amorphous layer. As such, there may be a stack of alternating crystalline materials on top of an amorphous layer. As such, the entirety of the optical element does not have to be crystalline. The optical element may therefore be a hybrid of crystalline layers and one or more amorphous layers. In such cases, the layers are arranged such that, in use, incident light is incident on the crystalline layers first.

[0024] The crystalline material of the optical element may be thorium-free, barium-free, and / or oxide-free. By not including these materials, issues such as absorption of water and radioactivity are avoided.

[0025] The optical element may have a reflectivity of 99% or greater, 99.1% or greater, 99.2% or greater, 99.3% or greater, 99.4% or greater, 99.5% or greater, 99.6% or greater, 99.7% or greater, 99.8% or greater, 99.9% or greater, or 99.94% or greater. Where the optical element is a mirror, the reflectivity is advantageously 99.6% or greater, 99.7% or greater, 99.8% or greater, 99.9% or greater, or 99.94% or greater. Where the optical element is a polarisers, the reflectivity is advantageously 99% or greater, 99.1% or greater, 99.2% or greater, 99.3% or greater, 99.4% or greater, 99.5% or greater, 99.6% or greater, 99.7% or greater, 99.8% or greater, 99.9% or greater, or 99.94% or greater. The reflectivity may be measured at predetermined wavelengths. The optical element may be a polariser having 1%or less absorption. For example, the reflectivity or absorption is such values with regards to light having a wavelength of from about 1 micron to about 10 microns.

[0026] The crystalline materials may be supported by a support substrate. The support substrate may comprise one or more of Copper, silicon and silicon oxide. The support substrate may be the base layer or may be a layer additional to the base layer.

[0027] The optical element maybe a polarizer having a phase error of 1 degree of deviation in angle of incidence of 1 degree, or a phase error of 5 degrees or less of deviation in angle of incidence of 2 degrees. Existing polarisers have phase errors much greater than this and the crystalline structure of the presently described polariser improves performance of the polariser.

[0028] It will be appreciated that the layers of the optical element each have a thickness. The variation in thickness of the crystalline layers may be less than 1%, 0.9% or less, 0.8% or less, 0.7% or less, 0.6% or less, 0.5% or less, 0.4% or less, 0.3% or less, 0.2% or less, or 0.1% or less. It has been found that a further advantage of crystalline layers over amorphous layers is the improved uniformity in the thickness of the layers, with amorphous layers differing in thickness by more than 1%. Better control of layer thickness allows for improves efficiency since the thickness of the layers can be more accurately tuned to provide constructive interference between reflected light waves.

[0029] According to a second aspect of the present disclosure, there is provided a radiation source for an EUV utilization system, said system including an optical element according to the first aspect of the present disclosure. The radiation source may include a laser system. A lithographic system may comprise a radiation source and a lithographic apparatus. The source laser system may be arranged to irradiate fuel droplets with an amplified laser beam, also referred to as a drive laser, so as to create EUV radiation, the source laser system comprises a plurality of reflective surfaces arranged to transport the laser beam wherein the reflective surfaces are formed as a stack of layers of a crystalline material.

[0030] According to a third aspect of the present disclosure, there is provided an exposure apparatus comprising the radiation source according to the second aspect of the present disclosure or optical element according to the first aspect of the present disclosure.

[0031] According to a fourth aspect of the present disclosure, there is provided an EUV utilization system, said system including an optical element, source laser system, or exposure apparatus according to any of the first to third aspects of the present disclosure.

[0032] The EUV utilization system may comprise an EUV lithography apparatus, a metrology apparatus, or an inspection apparatus, such as a mask inspection apparatus.

[0033] According to a fifth aspect of the present disclosure, there is provided a method of manufacturing an optical element, the method including depositing alternating layers of crystalline materials having different refractive indices.

[0034] The method may include depositing such alternating layers on an amorphous substrate.

[0035] The method may include depositing the alternating layers via molecular beam epitaxy. Whilst the process of manufacturing amorphous layers is relatively straightforward, the difficulty inproviding crystalline layers and the lack of realisation of the advantages of crystalline layers in an optical element has previously prevented the use of crystalline layers in optical elements.

[0036] According to a sixth aspect of the present disclosure, there is provided use of an optical element according to the first aspect, a radiation source according to the second aspect, an exposure apparatus according to the third aspect, an EUV utilization system according to the fourth aspect, or a method according to the fifth aspect in an EUV utilization apparatus or method.

[0037] It will be appreciated that features described in respect of one embodiment may be combined with any features described in respect of another embodiment and all such combinations are expressly considered and disclosed herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 schematically depicts a system for (actinic) mask inspection;Figure 3 depicts an alternative radiation source; andFigures 4a and 4b depict exemplary structures of a hybrid optical element (Fig. 4a) and a crystalline optical element (Fig. 4b).

[0039] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elementsDETAILED DESCRIPTION

[0040] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0041] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 andfaceted pupil mirror device 11. Any one of the optical elements within any of the figures may be the optical element according to the present disclosure.

[0042] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0043] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0044] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0045] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process . A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0046] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0047] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal -incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelengthsuch as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0048] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0049] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0050] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a free electron laser (FEL) or a discharge produced plasma (DPP) source may be used to generate EUV radiation.

[0051] Figure 2 depicts an alternative radiation source. For generating plasma target material, like tin or xenon may be provided to a rotating element (19), like for example rotating wheels, cylinder, or a drum or variations thereof. Target material may be provided in liquified form to the one or more rotating elements, e.g. by means of a target material bath. Alternatively, target material may also be provided in solid or frozen form (e.g. Xenon). Gaseous target material may be sprayed onto the rotating element to replace target material transformed to plasma. According to embodiments the rotating element would be cold enough to solidify the target material. An excitation device (20) may be used to assist in plasma formation. According to embodiments this excitation device is a laser like a solid-state or gas laser. Reflective optics, like one or more mirrors can be used to reflect the EUV light generated to the intermediate focus. According to embodiments, the reflective optics are a collector mirror. Buffer gas flow may be provided to mitigate debris present in the radiation source. A vacuum pump 21 may be provided.

[0052] Figure 3 depicts a system for (actinic) mask inspection. A mask inspection system can be used to identify or inspect defects in a mask to be used in a lithographic process by means of an apparatus described in figure 1. The mask inspection system comprises a radiation source (SO) and an illumination system (IL) and a detection system (DS). A mask (16) is placed on a mask stage (17) and illuminated by the illumination system (IL) reflecting radiation incident from the radiation source. The radiation coming from the illuminated mask is reflected by the detection system. In this way an image is formed on a detector 15. Again the optical elements, such as the mirrors 18, may be the optical element as described herein.

[0053] Figure 4a is a schematic depiction of an optical element according to the present disclosure . The optical element comprising a stack 22 of alternating crystalline materials having different refractive indices. The stack 22 of crystalline materials is depicted as having four layers, but the invention is not particularly limited to this number of layers and it will be appreciated that the skilled person may select the number of layers required as appropriate. The stack 22 of crystalline layers is provided on top of a stack of amorphous materials 23. The stack of amorphous materials 23 may be a base layer. Again, the number of layers is exemplary and the invention is not particularly limited to the specific number of layers depicted. Similarly, the relative thicknesses of the layers is exemplary. The optical element may include a support layer 24. The support layer 24 may comprise one or both of silicon and silicon oxide.

[0054] Figure 4b is similar to Figure 4a, except that the stack comprises crystalline layers rather than a combination of crystalline and amorphous layers. It will be appreciated that there may be a capping layer (not depicted) on top of the uppermost layer of the stack 22 of crystalline layers (in either Figure 4a or Figure 4b) which provides protection from hydrogen plasma.

[0055] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0056] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools or exposure apparatus. Such a lithographic tool may use vacuum conditions or ambient (non -vacuum) conditions.

[0057] The following clauses are also part of the present disclosure:1. An optical element for an EUV utilisation apparatus, the optical element comprising alternating layers of crystalline materials having different refractive indices.2. The optical element according to clause 1, wherein the optical element is a mirror or a polariser, optionally wherein the optical element comprises a Bragg stack.3. The optical element according to clause 2, wherein the polariser is a reflective phase retarder.4. The optical element according to clause 1 or clause 2, wherein at least one of the crystalline materials is monocrystalline.5. The optical element according to any preceding clause, wherein the optical element is configured to reflect light having a wavelength of from about 1 micron to about 20 microns, from about 1 micron to about 15 microns, from about 1 micron to about 10 microns, from about 1 micron to about 5 microns, from about 1 micron to about 4 microns, or from about 1 micron to about 2 microns.6. The optical element according to any preceding clause, wherein the optical element comprises two different crystalline materials having a lattice mismatch of 4% or less, 3% or less, 2% or less, 1% or less, or 0.5% or less.7. The optical element according to any preceding clause, wherein the optical element has a refractive index of 1 or more, 1.5 or more, 2 or more, 2.5 or more, 3 or more, 3.5 or more, or 4 or more.8. The optical element according to any preceding clause, wherein the optical element includes one or more of GaAs, AlGaAs, InGaAsP, InP, InGaAs, AlGaP, GaP, CdTe, HgCdTe, MgCdTe, MgZnTe, AlSb, GaSb, and CdZnTe.9. The optical element according to clause 8, wherein the optical element comprises: i) InGaAsP and InP; ii) InGaAs and InP; iii) AlGaP and GaP; iv) CdTe and HgCdTe; v) HgCdTe and CdZnTe; or vi) GaAs and AlGaAs.10. The optical element according to any preceding clause, wherein the optical element includes a base layer, optionally wherein the base layer is an amorphous layer.11. The optical element according to any preceding clause, wherein the crystalline material of the optical element is thorium -free, barium -free, and / or oxide-free.12. The optical element according to any preceding clause, wherein the optical element has a reflectivity of 99.1% or greater, 99.2% or greater, 99.3% or greater, 99.4% or greater, 99.5% or greater, 99.6% or greater, 99.7% or greater, 99.8% or greater, 99.9% or greater, or 99.94% or greater.13. The optical element according to any preceding clause, wherein the crystalline materials are supported by a support substrate, optionally wherein the support substrate comprises one or more of Copper, silicon and silicon oxide.14. The optical element according to any preceding clause, wherein the optical element is a polariser having 1% or less absorption.15. The optical element according to any preceding clause, wherein the optical element is a polariser having a phase error of 1 degree of deviation in angle of incidence of 1 degree, or a phase error of 5 degrees or less of deviation in angle of incidence of 2 degrees.16. The optical element according to any preceding clause, wherein a variation in thickness of crystalline layers is less than 1%, 0.9% or less, 0.8% or less, 0.7% or less, 0.6% or less, 0.5% or less, 0.4% or less, 0.3% or less, 0.2% or less, or 0.1% or less.17. A radiation source for an EUV utilization system, said system including an optical element according to any preceding clause, optionally wherein the radiation source is a laser system.18. An exposure apparatus comprising the radiation source according to clause 17.19. An EUV utilization system, said system including an optical element, source laser system, or exposure apparatus according to any preceding clause.20. The EUV utilization system according to clause 18, wherein the EUV utilization system comprises an EUV lithography apparatus, a metrology apparatus, or an inspection apparatus, such as a mask inspection apparatus.21. A method of manufacturing an optical element, the method including depositing alternating layers of crystalline materials having different refractive indices.22. The method according to clause 21, wherein the method includes depositing such alternating layers on an amorphous substrate. 23. The method according to clause 21 or 22, wherein the method includes depositing the alternating layers via molecular beam epitaxy.24. The use of an optical element according to any of clauses 1 to 16, a radiation source according to clause 17, an exposure apparatus according to clause 18, an EUV utilization system according to clause 19 or clause 20, or a method according to clause 21 to 23 in an EUV utilization apparatus or method.

[0058] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. An optical element for an EUV utilisation apparatus, the optical element comprising alternating layers of crystalline materials having different refractive indices.

2. The optical element according to claim 1, wherein the optical element is a mirror or a polariser, optionally wherein the optical element comprises a Bragg stack.

3. The optical element according to claim 1 or claim 2, wherein at least one of the crystalline materials is monocrystalline.

4. The optical element according to any preceding claim, wherein the optical element is configured to reflect light having a wavelength of from about 1 micron to about 20 microns, from about 1 micron to about 15 microns, from about 1 micron to about 10 microns, from about 1 micron to about 5 microns, from about 1 micron to about 4 microns, or from about 1 micron to about 2 microns.

5. The optical element according to any preceding claim, wherein the optical element comprises two different crystalline materials having a lattice mismatch of 4% or less, 3% or less, 2% or less, 1% or less, or 0.5% or less.

6. The optical element according to any preceding claim, wherein the optical element has a refractive index of 1 or more, 1.5 or more, 2 or more, 2.5 or more, 3 or more, 3.5 or more, or 4 or more.

7. The optical element according to any preceding claim, wherein the optical element includes one or more of GaAs, AlGaAs, InGaAsP, InP, InGaAs, AlGaP, GaP, CdTe, HgCdTe, MgCdTe, MgZnTe, AlSb, GaSb, and CdZnTe.

8. The optical element according to claim 7, wherein the optical element comprises: i) InGaAsP and InP; ii) InGaAs and InP; iii) AlGaP and GaP; iv) CdTe and HgCdTe; v) HgCdTe and CdZnTe; or vi) GaAs and AlGaAs.

9. The optical element according to any preceding claim, wherein the optical element includes a base layer, optionally wherein the base layer is an amorphous layer.

10. The optical element according to any preceding claim, wherein the optical element has a reflectivity of 99.1% or greater, 99.2% or greater, 99.3% or greater, 99.4% or greater, 99.5% or greater, 99.6% or greater, 99.7% or greater, 99.8% or greater, 99.9% or greater, or 99.94% or greater.

11. The optical element according to any preceding claim, wherein the crystalline materials are supported by a support substrate, optionally wherein the support substrate comprises one or more of Copper, silicon and silicon oxide.

12. The optical element according to any preceding claim, wherein the optical element is a polariser having 1% or less absorption.

13. The optical element according to any preceding claim, wherein the optical element is a polariser having a phase error of 1 degree of deviation in angle of incidence of 1 degree, or a phase error of 5 degrees or less of deviation in angle of incidence of 2 degrees.

14. The optical element according to any preceding claim, wherein a variation in thickness of crystalline layers is less than 1%, 0.9% or less, 0.8% or less, 0.7% or less, 0.6% or less, 0.5% or less, 0.4% or less, 0.3% or less, 0.2% or less, or 0. 1% or less.

15. A method of manufacturing an optical element, the method including depositing alternating layers of crystalline materials having different refractive indices.

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