Transmission type diffuser

Abstract

A first diffuser configured to receive and transmit radiation comprises a plurality of layers, each layer being arranged to change the angular distribution of EUV radiation passing through the layer in a different manner. A second diffuser configured to receive and transmit radiation comprises: a first layer and a second layer. The first layer is formed of a first material, the first layer comprising nanostructures on at least one surface of the first layer. The second layer is formed of a second material adjacent to the at least one surface of the first layer, such that the second layer also comprises nanostructures. The second material has a refractive index different from the refractive index of the first layer. The first diffuser and the second diffuser may be configured to receive and transmit EUV radiation.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application No. 18200620.5 filed on October 16, 2018 and European application No. 19164614.0 filed on March 22, 2019, and the entire contents of these European applications are incorporated herein by reference. Technical Field

[0003] The present invention relates to a transmissive diffuser, ie a diffuser configured to receive and transmit radiation, the transmitted radiation having a changing angular distribution.The diffuser may be adapted for use with EUV radiation and may form part of a measurement system within an EUV lithographic apparatus. Background Art

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

[0005] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of a feature that can be formed on a substrate. A lithographic apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate, compared to a lithographic apparatus using radiation having a wavelength of, for example, 193 nm.

[0006] It is known that a lithographic apparatus comprises a measurement system for determining one or more pupil function changes. The pupil function changes may include: relative phase changes in the pupil plane and / or relative intensity changes in the pupil plane. Such a measurement system typically comprises an object level pattern forming device (e.g., a diffraction grating or pinhole or the like); an illumination system; and an image level sensor device. The illumination system is arranged to illuminate the pattern forming device with radiation. At least a portion of the radiation scattered by the pattern forming device is received by a projection system (the properties of the projection system are measured), and the projection system is arranged to form an image of the pattern forming device on the image level sensor device. Such a measurement system expects that the entire entrance pupil of the projection system receives radiation from the pattern forming device. However, the illumination system is typically also used by the lithographic apparatus to form a (diffraction-limited) image of an object level mask or mask on an image level substrate (e.g., a silicon wafer coated with resist), where it may be desirable to illuminate only one or more separate portions of the entrance pupil of the projection system.

[0007] It may be desirable to provide a mechanism in which the angular distribution of an illumination beam that would otherwise illuminate one or more separate portions of an entrance pupil of a projection system can be altered so that the entire entrance pupil of the projection system can receive radiation from the patterning device. Summary of the invention

[0008] According to a first aspect of the present invention, there is provided a diffuser configured to receive and transmit radiation, wherein the diffuser comprises: a plurality of layers, each layer being arranged to alter the angular distribution of radiation passing through the layer in a different manner.

[0009] The diffuser may be configured to receive and transmit EUV radiation.

[0010] The diffuser according to the first aspect of the invention may, for example, form part of an EUV lithography apparatus. For example, the diffuser may be used in or form part of a measurement system for determining an aberration map or a relative intensity map for a projection system. Advantageously, by providing a plurality of layers, each layer being arranged to alter the angular distribution of EUV radiation passing through the layer in a different manner, the diffuser provides an arrangement whereby the EUV radiation beam may be diffused more efficiently over a desired angular range. In addition, the plurality of layers that alter the angular distribution of the EUV radiation passing through the layer in a different manner provide a greater degree of control over the angular distribution of the radiation leaving the diffuser.

[0011] At least two layers of the plurality of layers may primarily cause an angular dispersion of EUV radiation passing through the at least two layers along a single scattering direction in the plane of the diffuser, and the scattering directions of the at least two layers may be different.

[0012] The plane of the diffuser may generally be referred to as the xy plane. The propagation direction of radiation passing through the diffuser may be generally perpendicular to the plane of the diffuser and may generally be referred to as the z direction. In an exemplary embodiment, the diffuser may include two layers: a first layer, the first layer being arranged to produce an angular dispersion in the x direction; and a second layer, the second layer being arranged to produce an angular dispersion in the y direction.

[0013] At least two layers of the plurality of layers may produce different amounts of angular dispersion of EUV radiation passing through the at least two layers in corresponding scattering directions of the at least two layers.

[0014] Advantageously, this may allow the amount of angular dispersion to be adapted for specific purposes.

[0015] For example, as previously discussed, the diffuser may be used in or form part of a measurement system for determining an aberration map or relative intensity map for a projection system of a lithographic apparatus. In such a lithographic apparatus, an illumination system is typically used to illuminate an object level reticle or mask, and a projection system is used to form a (diffraction limited) image on an image level substrate (e.g., a silicon wafer coated with a resist). Typically, the projection system may apply a reduction factor in order to form an image having features that are smaller than corresponding features on the object level reticle or mask. In some systems, the projection system may have a synthetic or deformed design so that different reduction factors are applied to two orthogonal directions. With such a design, the numerical aperture may be represented by an ellipse (rather than a circle) in the entrance pupil of the projection system. With such a synthetic projection system, it may be desirable to provide, for example, a diffuser that diffuses more in a direction corresponding to the major axis of such an ellipse and less in a direction corresponding to the minor axis of such an ellipse. This may be achieved by using two layers, each of which produces a different amount of angular dispersion of the EUV radiation in the corresponding (mutually perpendicular) scattering directions of each layer.

[0016] At least one layer of the plurality of layers may include a scattering material layer having a nanostructure disposed on at least one surface of the scattering material layer.

[0017] This arrangement is particularly advantageous for use with EUV radiation (which may, for example, have a wavelength of 13.5 nm) since the nanostructures include features having dimensions comparable to or smaller than the wavelength of the radiation that is desired to be diffused. In these cases, the scattering is in the Mie-scattering regime and significant angular dispersion may be achieved.

[0018] The nanostructures may include features having dimensions in the range of 2 nm to 10 nm.

[0019] Features of this size or smaller can produce an angular divergence of about 10° (e.g., or about 7°) for EUV radiation. For example, features of this size or smaller can produce the angular scattering distribution having a width of 5° or more in at least one scattering direction. This can ensure that substantially the entire pupil plane is filled with radiation for a lithographic apparatus including a projection system having a numerical aperture of about 0.08.

[0020] The diffuser may be configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more.

[0021] It will be appreciated that, as used herein, width may mean the full width at half maximum (FWHM) of the angular scattering distribution.

[0022] The scattering material may have a refractive index n, and the magnitude of (1-n) may be greater than a threshold value of 0.06.

[0023] It will be appreciated that the diffuser according to the first aspect of the invention is a transmissive diffuser for use with EUV radiation. Due to the strong absorption of EUV radiation by most materials, EUV optical systems typically include reflective optics (rather than refractive lenses) in a vacuum. In general, to maximize the angular dispersion of the EUV radiation, it is desirable to maximize the optical contrast between the material forming the scattering nanostructure and the vacuum, i.e. it is desirable to maximize the absolute value of (1-n).

[0024] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.08. In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.1. In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.12.

[0025] The scattering material may have a relative density of less than 0.04 nm for EUV radiation. -1 The threshold extinction coefficient k.

[0026] It will be appreciated that the diffuser according to the first aspect of the invention is a transmissive diffuser for use with EUV radiation. Typically, in order to maximise the intensity of the EUV radiation output by the diffuser, it is desirable to minimise the attenuation caused by the scattering material. This can be achieved by minimising the extinction coefficient of the scattering material and / or minimising the thickness of the scattering material.

[0027] In some embodiments, the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.03 nm. -1 In some embodiments, the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.02 nm. -1 In some embodiments, the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.01 nm -1 .

[0028] It will be appreciated that for a given scattering material, in order to increase the amount of angular dispersion, it is desirable to increase the thickness of the layer; whereas in order to reduce the attenuation caused by the scattering material, it is desirable to reduce the thickness of the layer. Scattering materials with larger values ​​of (1-n) allow the thickness to be reduced (while still providing reasonable angular dispersion). Scattering materials with smaller extinction coefficients k for EUV radiation allow the thickness to be increased (while still providing reasonable transmission).

[0029] It will therefore be appreciated that suitable materials may in practice be selected to balance these two requirements.

[0030] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.06 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.01 nm -1 .

[0031] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.08 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.02 nm -1 .

[0032] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.1 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.03 nm -1 .

[0033] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.12 and the magnitude of the extinction coefficient k for EUV radiation is less than 0.04 nm. -1 The threshold value.

[0034] The scattering material may have a refractive index n and an extinction coefficient k for EUV radiation, and a ratio of a magnitude of (1-n) to (k+0.025) may be greater than a threshold value of two.

[0035] The scattering material may include one of molybdenum, ruthenium, niobium, rhodium or technetium.

[0036] The scattering material may have a thickness arranged such that EUV radiation propagating through the thickness of the scattering material produces a phase shift of (2m+1)π radians. It will be appreciated that m is an integer.

[0037] Advantageously, this suppresses zero-order (or specular) scattering. It will be appreciated that the phase shift caused by propagation through the scattering material is 2π / λ times the change in length of the optical path caused by the scattering material, where λ is the wavelength of the EUV radiation. If the scattering material is immersed in an optical medium having a refractive index of n2, the change in length of the optical path caused by the scattering material is defined as (n-n2)·t, where t is the thickness of the scattering material.

[0038] For example, for an embodiment in which the scattering material comprises molybdenum (having a refractive index of approximately 0.92) in a vacuum (i.e., n2=1), if the thickness of the material is approximately 85 nm, a phase shift of π radians can be achieved for EUV radiation having a wavelength of 13.5 nm.

[0039] The scattering material layer may be adjacent to a support material layer, and the support material may have nanostructures disposed on at least one surface of the support material.

[0040] The support material may have a refractive index close to 1 for EUV radiation and a relatively low absorption coefficient for EUV radiation. For example, the first material may include silicon or carbon. Such materials may be considered to be relatively optically neutral for EUV radiation.

[0041] The supporting material layer may include a film formed of carbon nanotubes.

[0042] The support material layer may include porous silicon.

[0043] According to a second aspect of the present invention, a diffuser is provided, which is configured to receive and transmit radiation, wherein the diffuser comprises: a first layer, the first layer is formed of a first material, the first layer comprises a nanostructure located on at least one surface of the first layer; and a second layer, the second layer is formed of a second material adjacent to the at least one surface of the first layer, so that the second layer also comprises the nanostructure, and the second material has a refractive index different from the refractive index of the first layer.

[0044] The diffuser may be configured to receive and transmit EUV radiation.

[0045] The diffuser according to the second aspect of the invention may for example form part of an EUV lithography apparatus. For example, the diffuser may be used in or form part of a measurement system for determining an aberration map or a relative intensity map for a projection system. By forming the diffuser using two layers, the first layer (which may be referred to as a support layer) may be used to apply the nanostructures to the second layer (which may be referred to as a scattering layer).

[0046] Since the second material has a refractive index different from that of the first layer, the first layer can be formed of a material that is relatively optically neutral with respect to EUV radiation (e.g., a relatively optically neutral material has a refractive index close to 1 for EUV radiation and a relatively low absorption coefficient for EUV radiation), while the second layer can be formed of a material having suitable optical properties to achieve a desired angular dispersion of EUV radiation passing through the second layer. For example, the first material can include silicon or carbon. Such a material can be considered to be relatively optically neutral with respect to EUV radiation.

[0047] Providing (via the second layer) a layer of material that causes scattering and has nanostructures formed on the second layer is particularly advantageous for use with EUV radiation (which may, for example, have a wavelength of 13.5 nm) since such nanostructures include features having a size comparable to or smaller than the wavelength of the radiation that it is desired to diffuse. In these cases, the scattering is a Mie scattering mechanism and significant angular dispersion may be achieved.

[0048] It will be understood that while the second layer may be said to be adjacent to the at least one surface of the first layer, typically the second layer will extend at least partially into the first layer. In some embodiments, the second layer may extend through the entire first layer at least at some locations in the plane of the diffuser.

[0049] The nanostructures may include features having dimensions in the range of 2 nm to 10 nm.

[0050] Features of this size or smaller can produce an angular divergence of about 10° (e.g., or about 7°) for EUV radiation. For example, features of this size or smaller can produce the angular scattering distribution having a width of 5° or more in at least one scattering direction. This can ensure that substantially the entire pupil plane is filled with radiation for a lithographic apparatus including a projection system having a numerical aperture of about 0.08.

[0051] The diffuser may be configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more.

[0052] It will be appreciated that, as used herein, width may mean the full width at half maximum (FWHM) of the angular scattering distribution.

[0053] The first layer may include a film formed of carbon nanotubes.

[0054] The first layer may include porous silicon.

[0055] The first layer may comprise a support material which has been selectively etched to form the nanostructures. The support material may, for example, comprise silicon or porous silicon.

[0056] A metal layer may be provided on the portions of the support material that have not been selectively etched.

[0057] The metal layer may include a self-assembled metal structure. The self-assembled metal structure may be formed of, for example, gold. The self-assembled metal structure may be formed by physical vapor deposition (PVD).

[0058] The second material may have a refractive index n, and the magnitude of (1-n) may be greater than a threshold value of 0.06.

[0059] It will be appreciated that the diffuser according to the second aspect of the invention is a transmissive diffuser for use with EUV radiation. Due to the strong absorption of EUV radiation by most materials, EUV optical systems typically include reflective optics (rather than refractive lenses) in a vacuum. In general, in order to maximize the angular dispersion of the EUV radiation, it is desirable to maximize the optical contrast between the material forming the scattering nanostructure and the vacuum, i.e. it is desirable to maximize the absolute value of (1-n).

[0060] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.08. In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.1. In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.12.

[0061] The second material may have a thermal conductivity less than 0.04 nm for EUV radiation. -1 The threshold extinction coefficient k.

[0062] It will be appreciated that the diffuser according to the second aspect of the invention is a transmissive diffuser for use with EUV radiation. Typically, in order to maximize the intensity of the EUV radiation output by the diffuser, it is desirable to minimize the attenuation caused by the second material (which may be referred to as the scattering material). This may be achieved by minimizing the extinction coefficient of the scattering material and / or minimizing the thickness of the scattering material.

[0063] In some embodiments, the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.03 nm. -1 In some embodiments, the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.02 nm. -1 In some embodiments, the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.01 nm -1 .

[0064] It will be appreciated that for a given scattering material, in order to increase the amount of angular dispersion, it is desirable to increase the thickness of the layer; whereas in order to reduce the attenuation caused by the scattering material, it is desirable to reduce the thickness of the layer. Scattering materials with a larger magnitude of (1-n) allow the thickness to be reduced (while still providing reasonable angular dispersion). Scattering materials with a smaller extinction coefficient k for EUV radiation allow the thickness to be increased (while still providing reasonable transmission).

[0065] It will therefore be appreciated that suitable materials may in practice be selected to balance these two requirements.

[0066] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.06 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.01 nm -1 .

[0067] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.08 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.02 nm -1 .

[0068] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.1 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.03 nm -1 .

[0069] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.12 and the magnitude of the extinction coefficient k for EUV radiation is less than 0.04 nm. -1 The threshold value.

[0070] The second material may have a refractive index n and an extinction coefficient k for EUV radiation and a ratio of a magnitude of (1-n) to (k+0.025) may be greater than a threshold value of two.

[0071] The second material may include one of molybdenum, ruthenium, niobium, rhodium or technetium.

[0072] The diffuser may have a thickness arranged such that EUV radiation propagating through the thickness of the diffuser produces a phase shift of (2m+1)π radians. It will be appreciated that m is an integer.

[0073] Advantageously, this suppresses zero-order (or specular) scattering.

[0074] The phase shift caused by propagation through the diffuser may be caused primarily by the scattering material.

[0075] It will be appreciated that the phase shift caused by propagation through the scattering material is 2π / λ times the change in length of the optical path caused by the scattering material, where λ is the wavelength of the EUV radiation. If the scattering material is immersed in an optical medium having a refractive index of n2, the change in length of the optical path caused by the scattering material is defined as (n-n2)·t, where t is the thickness of the scattering material.

[0076] For example, for an embodiment in which the scattering material comprises molybdenum (having a refractive index of approximately 0.92) in a vacuum (i.e., n2=1), if the thickness of the material is approximately 85 nm, a phase shift of π radians can be achieved for EUV radiation having a wavelength of 13.5 nm.

[0077] According to a third aspect of the present invention, a diffuser is provided, which is configured to receive and transmit EUV radiation, wherein the diffuser comprises: a support layer, which is formed of a first material; and a nanoparticle layer, which is supported by the support layer, and the nanoparticles are formed of a material that is transmissive to EUV radiation and causes refraction of EUV radiation incident on the diffuser.

[0078] The diffuser according to the third aspect of the invention may for example form part of an EUV lithography apparatus.For example, the diffuser may be used in or form part of a measurement system for determining an aberration map or a relative intensity map for a projection system.

[0079] The nanoparticle layer can act as a random array of microlenses that cause scattering of EUV radiation incident on the diffuser. The nanoparticle layer can be considered to form a nanostructure. This is particularly advantageous for use with EUV radiation (which may, for example, have a wavelength of 13.5 nm) since such nanostructures may include features having a size comparable to or smaller than the wavelength of the radiation that is desired to be diffused. In these cases, the scattering is a Mie scattering mechanism and significant angular dispersion may be achieved.

[0080] By arranging the diffuser so that the EUV radiation beam is incident on the diffuser at an oblique angle, the amount of dispersion is adapted for a particular purpose. With this arrangement, the amount of dispersion will be different in the plane of incidence and in a plane perpendicular to the plane of incidence.

[0081] As previously discussed, the diffuser may be used in or form part of a measurement system for determining an aberration map or relative intensity map for a projection system of a lithographic apparatus. In some such systems, the projection system may have a composite design in which different reduction factors are applied in two orthogonal directions. With such a composite projection system, it may be desirable to provide a diffuser that is arranged, for example, to provide a greater degree of diffusion in one direction and a lesser degree of diffusion in another perpendicular direction. This may be achieved by a suitable orientation of the diffuser.

[0082] The nanoparticles may be formed from any material having suitable optical properties in order to achieve a desired angular dispersion of EUV radiation passing through the nanoparticles.

[0083] The ratio of the average size of the nanoparticles to the average displacement of adjacent nanoparticles may be in the range of 1 to 2.

[0084] It will be understood that, as used herein, the size of a nanoparticle may mean the diameter D of the nanoparticle. It will be understood that, as used herein, the displacement of two adjacent nanoparticles may mean the distance Q between the center of one of the nanoparticles and the center of the other nanoparticle in the plane of the diffuser. By arranging the nanoparticles in the nanoparticle layer such that the average size D of the nanoparticles is av The average displacement Q of the neighboring nanoparticles av When the ratio is in the range of 1 to 2, the diffuser can be close to a diffuser based on a quasi-random diffraction structure (having a series of heights, pitches and / or widths).

[0085] The nanoparticles may include a noble metal, such as ruthenium (Ru) or rhodium (Rh).

[0086] Alternatively, the nanoparticles may be formed of yttrium (Y), zirconium (Zr), molybdenum (Mo) or diamond. Alternatively, the nanoparticles may be formed of a ceramic based on any one of ruthenium, rhodium, yttrium, zirconium, molybdenum or carbon and any one of boron (B), nitrogen (N), oxygen (O) or carbon (C).

[0087] Alternatively, for embodiments in which the nanoparticles are formed of non-noble metals, the material may be encapsulated in a shell formed of a noble metal such as, for example, ruthenium or rhodium. The thickness of such a shell may be <R / 2, where R is the radius of the nanoparticle. Such a noble metal shell improves the stability of the nanoparticle to an environment that may include EUV plasma.

[0088] The nanoparticles may include a shell of a material formed from SiO2, ZrO2, Y2O3 or other oxides, borides, nitrides, silicides or carbides. The thickness of this shell may be <R / 2, where R is the radius of the nanoparticle. This shell may improve the stability of the colloidal solution during the deposition of the nanoparticle layer.

[0089] The nanoparticle can be functionalized by polar, nonpolar or amphipathic groups to improve the stability of colloidal solution during the deposition of the nanoparticle layer.Suitable groups can include carboxyl (-COOH), polyethylene glycol (PEG) groups.Such groups can be chemically linked to the nanoparticle and can help deposit the nanoparticle layer.Such groups can form sparse sub-monolayers on the nanoparticle.

[0090] The nanoparticles may have a size in the range of 10 nm to 1 μm. In one embodiment, the nanoparticles may have a size of about 100 nm. The nanoparticles may be monodisperse (i.e., may all have a roughly uniform size) or may be polydisperse (i.e., may have a range of different sizes).

[0091] The nanoparticles may be arranged randomly or quasi-randomly.

[0092] The particle coverage of the nanoparticle layer to the support layer may range from 50% to 500% (ie, from a sub-monolayer to several monolayers stacked).

[0093] The support layer may be formed of a material having a relatively low absorption coefficient for EUV radiation. The support layer may be formed of a material suitable for an environment within an EUV lithography apparatus, which may have a relatively high temperature and may have a hydrogen plasma.

[0094] The nanoparticles may be formed from a variety of different materials. For example, a first set of nanoparticles may be formed from one material (eg, ruthenium) and a second set of nanoparticles may be formed from another material (eg, rhodium).

[0095] The support layer may be formed of a ceramic including a combination of one of metals, molybdenum, zirconium, or yttrium, and at least one of silicon, carbon, nitrogen, or boron.

[0096] For example, the support layer may be formed of a ceramic including one of MoSi, MoSiN, YSiB, SiC, ZrSi, ZrC, or MoC.

[0097] It will be understood that, as used herein, these ceramics are not precise chemical formulas, but merely indicate the elements (but not necessarily the ratios of those elements) in the ceramic.

[0098] The support layer may be formed of graphene or diamond-like carbon.

[0099] The support layer may have a thickness of about 10 nm to 1 μm. For example, in one embodiment, the support layer may have a thickness of about 100 nm.

[0100] The support layer may include a capping layer for protecting the support layer from EUV plasma etching. The capping layer may include a noble metal such as, for example, ruthenium or rhodium. The capping layer may have a thickness of less than 10 nm, for example less than 5 nm.

[0101] The support layer may comprise an intermediate layer disposed between the capping layer and the bulk of the support layer, wherein the intermediate layer is arranged to stabilize the capping layer against dewetting.

[0102] The capping layer may comprise a noble metal, such as, for example, ruthenium or rhodium. When in use, the diffuser may be at high temperatures. The intermediate layer may comprise one of the metals molybdenum, zirconium or yttrium. The intermediate layer may have a thickness of less than 5 nm, for example less than 2 nm.

[0103] The diffuser may further include a cover layer at least partially covering the support layer and / or the nanoparticle layer.

[0104] Such a cover layer can help to maintain the nanoparticles on the support layer. Additionally or alternatively, the support layer can provide a degree of protection for the support layer and / or the nanoparticle layer from the surrounding environment. When in use, the surrounding environment can be the environment in an EUV lithography apparatus, which can have a relatively high temperature and can have a hydrogen plasma.

[0105] The cover layer may be formed of a material that is transmissive to EUV radiation. The cover layer may be formed, for example, of ruthenium (Ru) or silicon dioxide (SiO2).

[0106] The diffuser may further include an inner layer disposed between the support layer and the nanoparticle layer and wherein at least some of the nanoparticles are at least partially embedded in the inner layer.

[0107] This inner layer may help adhere the nanoparticles to the support layer.This arrangement may be referred to as a micro-brazed diffuser.

[0108] The diffuser may be configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more, for example for EUV radiation.

[0109] The nanoparticles may be formed of a material having a refractive index n, and wherein the magnitude of (1-n) is greater than a threshold value of 0.06.

[0110] Typically, to maximize the angular dispersion of the EUV radiation, it is desirable to maximize the optical contrast between the material forming the nanoparticles and a vacuum, i.e., it is desirable to maximize the absolute value of (1-n). In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.08. In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.1. In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.12.

[0111] The nanoparticles may be composed of particles having a diameter less than 0.04 nm.-1 The material forms a threshold value for the extinction coefficient k for EUV radiation.

[0112] It will be appreciated that the diffuser according to the third aspect of the invention is a transmissive diffuser for use with EUV radiation. Typically, in order to maximize the intensity of the EUV radiation output by the diffuser, it is desirable to minimize the attenuation caused by the second material (the second material may be referred to as the scattering material). This may be achieved by minimizing the extinction coefficient of the scattering material (i.e., the nanoparticle layer) and / or minimizing the thickness of the scattering material (i.e., the nanoparticle layer). In some embodiments, the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.03 nm. -1 In some embodiments, the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.02 nm. -1 In some embodiments, the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.01 nm -1 .

[0113] It will be appreciated that for a given scattering material, in order to increase the amount of angular dispersion, it is desirable to increase the thickness of the nanoparticle layer, whereas in order to reduce the attenuation caused by the scattering material, it is desirable to reduce the thickness of the nanoparticle layer. Scattering materials with larger values ​​of (1-n) allow the thickness to be reduced (while still providing reasonable angular dispersion). Scattering materials with smaller extinction coefficients k for EUV radiation allow the thickness to be increased (while still providing reasonable transmission).

[0114] It will therefore be appreciated that suitable materials may in practice be selected to balance these two requirements.

[0115] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.06 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.01 nm -1 In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.08 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.02 nm -1 In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.1 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.03 nm -1 In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.12 and the magnitude of the extinction coefficient k for EUV radiation is less than 0.04 nm. -1 The threshold value.

[0116] The material forming the nanoparticles has a refractive index n and an extinction coefficient k for EUV radiation, and wherein a ratio of the magnitude of (1-n) to the extinction coefficient k for EUV radiation is greater than a threshold value of 3.5.

[0117] The transmittance of the diffuser according to the first, second or third aspect of the present invention to EUV radiation may be greater than 50%.

[0118] According to a fourth aspect of the present invention, there is provided a measurement system for determining an aberration map or a relative intensity map for a projection system comprising a diffuser according to the first, second or third aspect of the present invention.

[0119] Such a measurement system typically comprises an object level pattern forming device (e.g., a diffraction grating or pinhole or the like); an illumination system; and an image level sensor device. The illumination system is arranged to illuminate the pattern forming device with radiation. At least a portion of the radiation scattered by the pattern forming device is received by the projection system (the properties of the projection system are measured), and the projection system is arranged to form an image of the pattern forming device on the image level sensor device. Such a measurement system expects that the entire entrance pupil of the projection system receives radiation from the pattern forming device. However, the illumination system is typically also used by the lithography apparatus to form a (diffraction-limited) image of an object level mask or mask on an image level substrate (e.g., a silicon wafer coated with resist). This is the main purpose of the lithography apparatus and it is often desirable to use a local illumination mode for the exposure of the illumination mode during such image formation. For example, it may be desirable to use a multipole (e.g., dipole or quadrupole) illumination mode, in which only a limited number (e.g., two or four) of dispersed polar regions receive radiation in the pupil plane of the illumination system.

[0120] When the lithographic apparatus is not exposing and the measurement system is measuring a pupil plane function (e.g. the relative phase or intensity map of the projection system), it is possible to reconfigure the illumination system so as to fill the pupil plane of the illumination system (and hence also the entrance pupil of the projection system). However, doing so (and in order to revert to an exposure illumination mode before the next exposure) may take more time than is desirable for such in-line measurements.

[0121] Therefore, during such an inline measurement, a diffuser as described in the first, second or third aspect of the invention may be used to increase the angular spread or angular spread of the radiation scattered by the object horizontal pattern forming device so as to increase the proportion of the incident light pupil of the projection system that is filled with radiation.

[0122] The measurement system may include: a pattern forming device; an illumination system, which is arranged to illuminate the pattern forming device with radiation; and a sensor device; wherein the illumination system and the pattern forming device can be configured so that the projection system receives at least a portion of the radiation scattered by the pattern forming device, and the sensor device can be configured so that the projection system projects the received radiation onto the sensor device; and the diffuser is capable of operating to receive the radiation generated by the illumination system and change the angular distribution of the radiation before the radiation illuminates the pattern forming device.

[0123] The diffuser is capable of moving between at least a first operating position and a second storage position: in the first operating position, the diffuser is at least partially arranged in the path of the radiation generated by the irradiation system and is arranged to change the angular distribution of the radiation before the radiation irradiates the pattern forming device; and in the second storage position, the diffuser is arranged outside the path of the radiation generated by the irradiation system.

[0124] According to the fifth aspect of the present invention, a lithography device is provided, which comprises: the measurement system according to the fourth aspect of the present invention; and a projection system, which is configured to receive at least a portion of the radiation scattered by the pattern forming device and is configured to project the received radiation onto the sensor device.

[0125] The diffuser may be mounted on a patterning device shielding blade of the lithographic apparatus, the edges of the patterning device shielding blade defining a field region of the lithographic apparatus.

[0126] According to a sixth aspect of the present invention, there is provided a method for forming the diffuser according to the second aspect of the present invention, the method comprising: forming the first layer from the first material, the first layer comprising a nanostructure located on at least one surface of the first layer; and forming the second layer from the second material on the at least one surface of the first layer.

[0127] Forming the first layer may include: providing a support material layer; forming a self-assembled metal structure on a surface of the support material layer; and selectively etching a portion of the support material layer that is not in contact with the self-assembled metal structure.

[0128] The support material may, for example, comprise silicon or porous silicon (pSi).

[0129] The self-assembled metal structure can be formed of, for example, gold. The self-assembled metal structure can be formed by physical vapor deposition (PVD). The self-assembly relies on the following well-known and robust phenomenon: in the first few nanometers of the deposited film, dispersed metal islands are formed on the surface of the support material layer.

[0130] By this technique, high-aspect-ratio structures can be formed in the support material layer (i.e., in the plane of the support material layer) with a lateral resolution down to about 10 nm or less. Thus, advantageously, structures can be formed that can achieve an angular divergence of about 10° (e.g., or about 7°) for EUV radiation while still having a sufficiently thick second layer to achieve a phase shift of π radians for EUV radiation.

[0131] The layer of support material may be formed on a carrier layer, which may be used to support the layer of support material when forming the diffuser and the method may further comprise removing the carrier layer once the first and second layers have been formed.

[0132] The carrier layer may be formed of silicon nitride (SiN), porous silicon (pSi) or molybdenum silicide (MoSi). The carrier layer may be removed, for example, by etching.

[0133] According to a seventh aspect of the present invention, there is provided a method of forming a diffuser according to any one of claims 27 to 45, the method comprising: providing the support layer formed of the first material; and depositing a plurality of nanoparticles to form the nanoparticle layer supported by the support layer.

[0134] Since the process of depositing the nanoparticles to form the nanoparticle layer supported by the support layer may cause a random or quasi-random arrangement of the nanoparticles, the method according to the seventh aspect of the invention is advantageous. The method provides an alternative method of producing a diffuser that is cheap, simple and provides greater freedom in the substrate design of the support layer, so that the support layer can be better adapted to allow thermal management of the diffuser.

[0135] For example, another method for forming a quasi-random diffraction structure can be to form a plurality of (for example rectangular) features on a supporting substrate, and the supporting substrate has a quasi-random distribution of at least one in height, pitch and / or width. Such a structure requires some mechanisms to plan a quasi-random structure and then can for example use photolithography to manufacture. By contrast, the process that is usually used to deposit a plurality of nanoparticles to form a nanoparticle layer will produce a random or quasi-random structure and is usually cheaper and simpler to perform. In addition, by changing the property of the nanoparticle layer, such as the material, size, density of the nanoparticles in the layer, the property of the diffuser can be easily changed.

[0136] Depositing the plurality of nanoparticles so as to form the nanoparticle layer supported by the support layer can be achieved using aerosol deposition and may include: providing an aerosol comprising the nanoparticles suspended in a carrier gas; and depositing the plurality of nanoparticles from the aerosol so as to form the nanoparticle layer supported by the support layer.

[0137] Any suitable carrier gas may be used.

[0138] For embodiments in which spark ablation is used to form the aerosol, the carrier gas may, for example, include an inert gas. Suitable inert gases may include, for example, an inert gas (e.g., argon) or nitrogen. Such inert gases are beneficial because they can avoid unwanted reactions (e.g., oxidation) of the nanoparticles.

[0139] For embodiments in which the aerosol is formed by forming a vapor from a colloid, the carrier gas may be a gas formed from a carrier liquid of the colloid. The carrier gas or carrier liquid may be selected to ensure that the colloid and / or the aerosol is stable. The carrier fluid may be selected to ensure that the carrier fluid can evaporate completely so that no molecular residues from the carrier fluid are left after the nanoparticle layer is formed. Typically, a polar (e.g., water) carrier fluid or a non-polar (e.g., heptane or hexane) carrier fluid may be used.

[0140] This aerosol-based approach has been successfully used to form multilayer nanoparticle layers on substrates with nanoparticle sizes up to about 20 nm.

[0141] Providing an aerosol comprising the nanoparticles suspended in a carrier gas may include forming the aerosol using spark ablation.

[0142] Spark ablation is a known technique for generating aerosols. Electrodes formed of the material of the nanoparticles and a voltage source can be used to form nanoparticles of the material via spark ablation. A flow of a suitable carrier gas through (or even across) the electrodes can be used to form the aerosol.

[0143] Providing an aerosol comprising the nanoparticles suspended in a carrier gas may include forming a colloid comprising the nanoparticles suspended in a carrier liquid, and forming a vapor from the colloid.

[0144] Depositing the plurality of nanoparticles from the aerosol to form the nanoparticle layer supported by the support layer may be accomplished via impingement and / or diffusion.

[0145] The impingement may be achieved by introducing the aerosol at the support layer.

[0146] Depositing the plurality of nanoparticles so as to form the nanoparticle layer supported by the support layer can be achieved using colloidal deposition and may include: providing a colloid comprising the nanoparticles suspended in a carrier liquid; depositing the colloid on the support layer; and removing the carrier liquid via evaporation so as to deposit the plurality of nanoparticles from the colloid, thereby forming the nanoparticle layer supported by the support layer.

[0147] For example, droplets of the colloid can be deposited onto the support layer (or intermediate layer, if used) and the droplets dried. As the carrier liquid evaporates, the nanoparticles remain on the support layer. The colloid droplets can have any suitable size (e.g., the droplets can have a volume of about 1 μL to 1 mL). The colloid can have any suitable concentration of nanoparticles (e.g., the droplets can have a volume of about 1 μL to 1 mL). 10 Amount / μL to 10 12 ions / μL).

[0148] The drying pattern from such a process may produce a sparse monolayer and / or several monolayers. A random deposition pattern suitable for the diffuser may be achieved, for example, by using a polydisperse colloid and / or a colloid comprising a mixture of different shapes and / or materials. The colloid may be a polydisperse colloid. This may produce a random deposition pattern suitable for the diffuser.

[0149] Such colloid-based wet synthesis methods have been successfully used to deposit silver, gold and platinum nanoparticles (eg, having a size of about 70 nm). Similar techniques may be used by the seventh aspect of the invention to deposit nanoparticles formed of, for example, ruthenium or rhodium.

[0150] Any suitable carrier fluid can be used. The carrier fluid can be selected to ensure that the colloid remains stable (i.e., so that the nanoparticles remain dispersed and will not dissolve and / or coalesce). The carrier fluid can be selected to ensure that the carrier fluid can evaporate completely so that no molecular residues from the carrier fluid are left after forming the nanoparticle layer. Typically, polar (e.g., water) carrier fluids or non-polar (e.g., heptane or hexane) carrier fluids can be used. Those skilled in the art will understand which carrier fluids can be suitable for a given type of nanoparticle.

[0151] The method may further comprise heating the support layer and / or the nanoparticle layer so as to weld or sinter the nanoparticle layer to the support layer.

[0152] This may also enhance the retention of the nanoparticle layer on the support layer.Heating the support layer and / or the nanoparticle layer may be achieved, for example, via electron beam exposure or thermal annealing.

[0153] The diffuser may further include an inner layer disposed between the support layer and the nanoparticle layer, the inner layer having a lower melting point than the support layer and the nanoparticle layer, and the method may further include heating the inner layer above the melting point of the inner layer, and cooling the inner layer so as to braze the support layer to the nanoparticle layer.

[0154] This may also enhance retention of the nanoparticle layer on the support layer.Heating the inner layer may be achieved, for example, via electron beam exposure or thermal annealing.

[0155] The method may further include etching the support layer from a surface of the support layer opposite to a surface of the support layer supporting the nanoparticle layer once the plurality of nanoparticles have been deposited to form the nanoparticle layer supported by the support layer.

[0156] This reverse etching of the support layer allows the use of a thicker, more stable support layer during the deposition of the nanoparticles. Advantageously, this can prevent damage or even fracture of the support layer. This final etching step may be particularly useful in embodiments where a colloid is used to deposit the nanoparticles, because this final etching step can prevent capillary forces from braking the support layer. Once the nanoparticle layer has been formed, the thickness of this layer can be finally determined using an etching process.

[0157] The method may further comprise providing a cover layer at least partially covering the support layer and / or the nanoparticle layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0158] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0159] - Figure 1 Describing a lithographic system including a lithographic apparatus and a radiation source;

[0160] - Figure 2 is a schematic diagram of a reflective marker;

[0161] - Figure 3A and Figure 3B is a schematic diagram of the sensor device;

[0162] - Figure 4A Show Figure 1 Intensity distribution of a bipolar illumination mode of a lithographic apparatus shown in;

[0163] - Figure 4B Show Figure 1 Intensity distribution of a four-level illumination mode of a lithographic apparatus shown in;

[0164] - Figure 5A According to the first embodiment of the present invention, it can be formed Figure 1 A schematic cross-sectional view of a diffuser of a portion of a lithography system is shown in;

[0165] - Figure 5B yes Figure 5A A schematic plan view of a diffuser shown in FIG.

[0166] - Figure 6 A graph showing the extinction coefficient k for EUV radiation of some materials versus the magnitude of (1-n) for EUV radiation;

[0167] - Figure 7 Shows the scattered radiation as used for Figure 5A and Figure 5B The intensity distribution as a function of the scattering angle in the scattering direction of a diffuser of the type described is shown in;

[0168] - Fig. 8A is a schematic cross-sectional view of a portion of an intermediate stack of layers from which a diffuser according to a second embodiment of the invention is formed;

[0169] - Figure 8B yes Fig. 8A A schematic plan view of a portion of the intermediate stacking layer shown in FIG.

[0170] - Fig. 9 The support material layer in the middle stacking layer is selectively etched by Fig. 8A and Figure 8B Schematic cross-sectional view of a portion of a second intermediate stacking layer formed by the intermediate stacking layer shown in ;

[0171] - Fig.10 The scattering material layer is formed on the modified support material layer in the second intermediate stack layer. Fig. 9 A schematic cross-sectional view of a portion of a third intermediate stacking layer formed by the second intermediate stacking layer shown in FIG.

[0172] - Fig.11 According to the second embodiment of the present invention, Fig.10 A schematic cross-sectional view of a portion of a diffuser formed by a third intermediate stacked layer shown in FIG.

[0173] - Appendix 12A is a schematic diagram of a diffuser based on a quasi-random refractive structure disposed on a support layer, the refractive structure comprising a plurality of (e.g. rectangular) features having a uniform pitch L and a quasi-randomly distributed height H;

[0174] - Fig. 12B is a schematic diagram of a diffuser based on a quasi-random refractive structure disposed on a support layer, the refractive structure comprising a plurality of (e.g. rectangular) features having a uniform height H and a quasi-randomly distributed pitch P or width N, M;

[0175] - Fig.13A is a schematic diagram of a diffuser according to an embodiment of the present invention, the diffuser comprising a monodisperse layer of nanoparticles supported by a supporting layer;

[0176] - Fig. 13B is a schematic diagram of a diffuser according to an embodiment of the present invention, the diffuser comprising a nanoparticle polydisperse layer supported by a support layer;

[0177] - Fig. 13C is a schematic diagram of a diffuser according to an embodiment of the present invention, the diffuser comprising a nanoparticle polydisperse layer comprising two different types of materials;

[0178] - Fig.14A Show Fig.13A A variation of the diffuser shown in , wherein the diffuser comprises a covering layer at least partially covering the support layer and / or the nanoparticle layer;

[0179] - Fig. 14B Show Fig.13A A variation of the diffuser shown in , in which the nanoparticle layer has been welded or sintered to the support layer; and

[0180] - Fig. 14C Show Fig.13A A variant of the diffuser is shown in , in which the nanoparticle layer has been micro-brazed to the support layer by an internal layer located between the support layer and the nanoparticle layer, the internal layer having a lower melting point than the support layer and the nanoparticle layer. DETAILED DESCRIPTION

[0181] Figure 1 A lithographic system is shown comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate and supply an 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.

[0182] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on said patterning device MA. In addition, the illumination system IL may comprise a faceted field mirror arrangement 10 and a faceted pupil mirror arrangement 11. The faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 together provide a desired cross-sectional shape and a desired intensity distribution to the EUV radiation beam B. The illumination system IL may also comprise other mirrors or arrangements in addition to or instead of the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11.

[0183] After being adjusted thus, 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 the purposes described, the projection system PS may comprise a plurality of mirrors 13, 14 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′, thereby forming an image having 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 Figure 1 1 , but the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0184] The substrate W may include a previously formed pattern. In such a case, the lithographic apparatus LA aligns an image formed by the patterned EUV radiation beam B′ with a pattern previously formed on the substrate W.

[0185] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure substantially below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL and / or in the projection system PS.

[0186] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source capable of producing EUV radiation.

[0187] The lithographic apparatus may, for example, be used in a scanning mode in which the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously as a pattern applied to the radiation beam is projected onto the substrate W (i.e. dynamic exposure). The speed and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the reduction ratio and image reversal characteristics of the projection system PS. The patterned radiation beam incident on the substrate W may include a band of radiation. The band of radiation may be referred to as an exposure slit. During scanning exposure, movement of the substrate table WT and the support structure MT may cause the exposure slit to travel over an exposure field of the substrate W.

[0188] As already described above, the lithographic apparatus can be used to expose parts of a substrate W to form a pattern on the substrate W. In order to improve the accuracy of transferring the desired pattern to the substrate W, one or more properties of the lithographic apparatus LA can be measured. Such properties can be measured regularly, for example before and / or after exposure of each substrate W, or can be measured less frequently, for example as part of a calibration process. Examples of properties of the lithographic apparatus LA that can be measured include the relative alignment of components of the lithographic apparatus LA and / or aberrations of components of the lithographic apparatus. For example, measurements can be made to determine the relative alignment of a support structure MT for supporting the pattern forming device MA and a substrate table WT for supporting the substrate W. Determining the relative alignment of the support structure MT and the substrate table WT helps to project a patterned radiation beam onto a desired part of the substrate W. This may be particularly important when projecting patterned radiation onto a substrate W including a portion that has already been exposed to radiation in order to improve the alignment of the patterned radiation with the previously exposed area.

[0189] Additionally or alternatively, measurements may be taken to determine optical aberrations of the projection system PS. Optical aberrations are deviations from the performance of the optical system of the paraxial optics and may cause blurring or distortion of the pattern exposed at the substrate W. Aberrations of the projection system PS may be adjusted and / or accounted for in order to increase the accuracy with which a desired pattern is formed on the substrate W.

[0190] This can be accomplished by irradiating the reflective marker 17 (e.g. Figure 1The reflective markers described herein are suitable for use as reference points and / or for use as measurements of properties of an image formed by an optical system. For example, radiation reflected from the reflective markers can be used to determine the alignment of one or more components and / or the optical aberrations of one or more components.

[0191] exist Figure 1 In the embodiment shown in , the reflective marker 17 forms part of the pattern forming device MA. One or more markers 17 can be arranged on the pattern forming device MA for performing lithographic exposure. The marker 17 can be positioned outside the patterned area of ​​the pattern forming device MA, and during the lithographic exposure, the patterned area is irradiated by radiation. In some embodiments, one or more markers 17 can be arranged on the support structure MT in addition or alternatively. For example, a special hardware (commonly referred to as a reference) can be arranged on the support structure MT. The reference can include one or more markers. For the purpose of this specification, the reference is regarded as an example of a pattern forming device. In some embodiments, a pattern forming device MA specially designed to measure one or more properties of the lithographic equipment LA can be placed on the support structure MT to perform a measurement process. The pattern forming device MA can include one or more markers 17 for irradiation as part of the measurement process.

[0192] exist Figure 1 In the embodiment shown in , the lithographic apparatus LA is an EUV lithographic apparatus and therefore uses a reflective pattern forming device MA. The marker 17 is therefore a reflective marker 17. The configuration of the marker 17 may depend on the properties of the measurement to be performed using the marker 17. The marker may, for example, comprise one or more reflective pinhole features comprising a reflective region surrounded by an absorptive region, a reflective line feature, a plurality of reflective line features and / or an arrangement of a reflective grating structure such as a reflective diffraction grating.

[0193] In order to measure one or more properties of the lithographic apparatus LA, a sensor device 19 (e.g. Figure 1 ) to measure the radiation output from the projection system PS. Figure 1As shown, the sensor device 19 may, for example, be arranged on the substrate table WT. In order to perform a measurement process, the support structure MT may be positioned so that the marker 17 on the patterning device MA is illuminated by radiation. The substrate table WT may be positioned so that radiation reflected from the marker is projected onto the sensor device 19 by the projection system PS. The sensor device 19 is in communication with a controller CN, which may determine one or more properties of the lithographic apparatus LA based on measurements performed by the sensor device 19. In some embodiments, a plurality of markers 17 and / or sensor devices 19 may be provided and the properties of the lithographic apparatus LA may be measured at a plurality of different field points (i.e. locations in the field or object plane of the projection system PS).

[0194] As described above, in some embodiments, radiation reflected from a marker may be used to determine the relative alignment of components of the lithographic apparatus LA. In such embodiments, the marker 17 may include features that, when illuminated by radiation, impart alignment features to the radiation. The features may, for example, include one or more reflective patterns in the form of a grating structure.

[0195] This can be accomplished by being located at the level of the substrate W (e.g., at Figure 1 The sensor device 19 (shown in Figure 1 on the substrate table WT) measures the position of the alignment features in the radiation beam B. The sensor device 19 is operable to detect the position of the alignment features in radiation incident on the sensor device 19. This may allow the substrate table WT to be aligned relative to a marker on the patterning device MA to be determined. With an understanding of the relative alignment of the patterning device MA to the substrate table WT, the patterning device MA and substrate table WT may be moved relative to each other to form a pattern at a desired location on the substrate W (using a patterned radiation beam B reflected from the patterning device MA). The position of the substrate W on the substrate table may be determined using a separate measurement process.

[0196] As further described above, in some embodiments the patterning device MA may be provided with one or more markers 17 that can be used to measure aberrations of the projection system PS. Similar to the alignment measurements described above, aberrations may be detected by measuring radiation reflected from the markers 17 with a sensor device 19 located at or near the substrate table WT. The one or more markers 17 on the patterning device MA may be irradiated with EUV radiation by the illumination system IL. Radiation reflected from the one or more markers is projected by the projection system PS onto an image plane of the projection system PS. The one or more sensor devices 19 are arranged at or near the image plane (e.g. as Figure 1 The projected radiation can be measured to determine the aberrations of the projection system PS. Figure 2and 3 describe embodiments of markers 17 and sensor devices 19 that may be used to determine aberrations of the projection system PS.

[0197] Figure 2 is a schematic diagram of a marker 17 that may form part of a patterning device MA according to an embodiment of the present invention. Figure 2 A Cartesian coordinate system is also shown in . The y-direction may represent the scanning direction of the lithographic apparatus. That is, during scanning exposure, the movement of the substrate table WT and the support structure MT may cause the pattern forming device MA to be scanned relative to the substrate W in the y-direction. The marker 17 is typically located in the xy plane. That is, the marker typically extends in a direction perpendicular to the z-direction. Although reference is made to a marker typically located in a plane, it will be understood that the marker is not completely confined to a plane. That is, portions of the marker may extend outside the plane in which the marker is typically located. As will be further explained below, the marker may include a diffraction grating. The diffraction grating may include a three-dimensional structure including portions that are not completely in the plane but instead extend outside the plane.

[0198] Figure 2 The marker 17 shown in FIG. 1 comprises a first portion 17a and a second portion 17b. Both the first portion and the second portion comprise a reflective diffraction grating comprising a periodic grating structure. The grating structure extends in a grating direction. The first portion 17a comprises a diffraction grating extending in a first grating direction. The first grating direction is Figure 2 The second portion 17b includes a diffraction grating extending in a second grating direction. Figure 2 In the v direction. Figure 2 In an embodiment, both the u-direction and the v-direction are aligned at approximately 45° relative to both the x-direction and the y-direction and are substantially perpendicular to each other. The first portion 17a and the second portion 17b of the marker 17 may be irradiated at the same time or at different times.

[0199] Although Figure 2 The embodiment shown in FIG. 1 includes a first portion 17a and a second portion 17b comprising a diffraction grating oriented in a perpendicular grating direction, but in other embodiments, the marker 17 may be provided in other forms. For example, the marker 17 may include reflective areas and absorptive areas arranged to form a checkerboard pattern. In some embodiments, the marker 17 may include an array of pinhole features. A reflective pinhole feature may include an area of ​​reflective material surrounded by an absorptive material.

[0200] When the first part 17a and / or the second part 17b of the marker is illuminated by radiation, a plurality of diffraction orders are reflected from the marker. At least a part of the reflected diffraction orders enters the projection system PS. The projection system PS forms an image of the marker 17 on the sensor device 19. Figure 3A and Figure 3B is a schematic diagram of the sensor device 19. Figure 3A is a side view of the sensor device and Figure 3B is a top view of the sensor device. Figure 3A and Figure 3B Cartesian coordinates are also shown in .

[0201] Used for Figure 2 , Figure 3A and Figure 3B The Cartesian coordinate system in is intended as a coordinate system for radiation propagating through the lithographic apparatus. At each reflective optical element, the z direction is defined as the direction perpendicular to the optical element. That is, at Figure 2 In , the z direction is perpendicular to the xy plane in which the patterning device MA and the marker 17 generally extend. Figure 3A and Figure 3B In , the z-direction is perpendicular to the xy plane in which the diffraction grating 19 and the radiation sensor 23 generally extend. The y-direction represents the scanning direction in which the support structure MT and / or the substrate table WT are scanned relative to each other during scanning exposure. The x-direction represents the non-scanning direction perpendicular to the scanning direction. It will be appreciated that (for example, according to Figure 1 ), in the lithographic apparatus, the z-direction at the patterning device MA is not aligned with the z-direction at the substrate W. As explained above, the z-direction is defined as being perpendicular to the optical element at each optical element in the lithographic apparatus.

[0202] The sensor device 19 comprises a transmission diffraction grating 21 and a radiation sensor 23. At least some of the radiation 25 output from the projection system PS passes through the diffraction grating 21 and is incident on the radiation sensor 23. The diffraction grating 21 is shown in more detail in Figure 3B and includes a checkerboard diffraction grating. The diffraction grating 21 is Figure 3B The areas shown shaded black in FIG. 2 represent areas of the diffraction grating 21 that are configured to be substantially opaque to incident radiation. Figure 3B The non-shaded areas of the diffraction grating 21 shown in FIG. 2 represent areas that are configured to transmit radiation. For ease of illustration, Figure 3B The opaque and transmissive regions of the diffraction grating 21 are not shown to scale. For example, in practice, the ratio of the diffraction grating features to the size of the diffraction grating itself may be smaller than Figure 3B The proportions indicated in .

[0203] Figure 3BThe diffraction grating 21 shown in FIG. 2 is depicted as having a checkerboard configuration including square transmission areas and opaque areas. However, in practice, it may be difficult or impossible to manufacture a transmission diffraction grating including completely square transmission areas and opaque areas. The transmission areas and / or opaque areas may therefore have a cross-sectional shape other than a completely square. For example, the transmission areas and / or opaque areas may have a cross-sectional shape including a square (or more typically a rectangle) with rounded corners. In some embodiments, the transmission areas and / or opaque areas may have a substantially circular or elliptical cross-sectional shape. In some embodiments, the diffraction grating 21 may include an array of pinholes formed in an opaque material.

[0204] The radiation sensor 23 is configured to detect a spatial intensity profile of radiation incident on the radiation detector 23. The radiation detector 23 may, for example, comprise an array of individual detector elements. For example, the radiation detector 23 may comprise a CCD or CMOS array. During a process for determining image aberrations, the support structure MT may be positioned such that the marker 17 is illuminated with radiation from the illumination system IL. The substrate table WT may be positioned such that radiation reflected from the marker is projected onto the sensor device 19 by the projection system PS.

[0205] As described above, a plurality of diffraction orders are formed at the marker 17. Further diffraction of the radiation occurs at the diffraction grating 21. The interaction between the diffraction orders formed at the marker 17 and the diffraction pattern formed at the diffraction grating 21 produces an interference pattern formed on the radiation detector 23. The interference pattern is related to the derivative of the phase of the wavefront that has propagated through the projection system. Therefore, the interference pattern can be used to determine the aberrations of the projection system PS.

[0206] As described above, the first and second portions of the marker 17 include diffraction gratings aligned perpendicularly to each other. Radiation reflected from the first portion 17a of the marker 17 can provide information related to the wavefront in a first direction. Radiation reflected from the second portion 17b of the marker can provide information related to the wavefront in a second direction, the second direction being perpendicular to the first direction. In some embodiments, the first and second portions of the marker can be irradiated at different times. For example, the first portion 17a of the marker 17 can be irradiated at a first time to derive information related to the wavefront in the first direction, and the second portion 17b of the marker 17 can be irradiated at a second time to derive information related to the wavefront in the second direction.

[0207] In some embodiments, the patterning device MA and / or the sensor device 19 may be continuously scanned and / or stepped in two perpendicular directions. For example, the patterning device MA and / or the sensor device 19 may be stepped relative to each other in the u-direction and the v-direction. The patterning device MA and / or the sensor device 19 may be stepped in the u-direction while the second portion 17b of the marker 17 is illuminated, and the patterning device MA and / or the sensor device 19 may be stepped in the v-direction while the first portion 17a of the marker 17 is illuminated. That is, the patterning device MA and / or the sensor device 19 may be stepped in a direction perpendicular to the grating direction of the illuminated diffraction grating.

[0208] The patterning device MA and / or the sensor device 19 may be stepped with a distance corresponding to a fraction of the grating period of the diffraction grating. The measurements made at the different stepping positions may be analyzed to derive information about the wavefront in the stepping direction. For example, the phase of the first harmonic of the measured signal may contain information about the derivative of the wavefront in the stepping direction. Thus, stepping the patterning device MA and / or the sensor device 19 in both the u-direction and the v-direction (both being perpendicular to each other) allows to derive information about the wavefront in two perpendicular directions, thereby allowing to reconstruct the full wavefront.

[0209] In addition to the stepping of the patterning device MA and / or the sensor device 19 in a direction perpendicular to the grating direction of the illuminated diffraction grating (as described above), the patterning device MA and / or the sensor device 19 may also be scanned relative to each other. The scanning of the patterning device MA and / or the sensor device 19 may be performed in a direction parallel to the grating direction of the illuminated diffraction grating. For example, the first part 17a of the marker 17 is illuminated while the patterning device MA and / or the sensor device 19 may be scanned in the u direction, and the second part 17a of the marker 17 is illuminated while the patterning device MA and / or the sensor device 19 may be scanned in the v direction. Scanning the patterning device MA and / or the sensor device 19 in a direction parallel to the grating direction of the illuminated diffraction grating allows averaging the measurements over the entire diffraction grating, thereby taking into account any variations of the diffraction grating in the scanning direction. The scanning of the patterning device MA and / or the sensor device 19 may be performed at a different time than the stepping of the patterning device MA and / or the sensor device 19 described above.

[0210] As described above, the diffraction grating 21 forming part of the sensor device 19 is configured in the form of a chessboard. This may allow the sensor device 19 to be used during determination of wavefront phase changes in both the u-direction and the v-direction. The arrangement of the diffraction grating forming the marker 17 and the sensor device 19 is presented only as an exemplary embodiment. It will be appreciated that a variety of different arrangements may be used to determine wavefront changes.

[0211] In some embodiments, the marker 19 and / or the sensor device 19 may include components other than a diffraction grating. For example, in some embodiments, the marker 17 and / or the sensor device 19 may include a single slit or one or more pinhole features through which at least a portion of the radiation beam may propagate. In the case of the marker 17, the pinhole feature may include a portion of a reflective material surrounded by an absorptive material so that radiation is reflected only from a smaller portion of the marker. The single slit feature may have the form of a single strip of reflective material surrounded by an absorptive material. The pinhole feature and / or the single slit feature at the sensor device 19 may be a transmissive feature. In general, the marker 17 may be any feature that applies a feature to the radiation beam that can be used as a reference point or to determine a measurement of the radiation beam.

[0212] Although in the embodiments described above, a single marker 17 and sensor device 19 are provided, in other embodiments, multiple markers 17 and sensor devices 19 may be provided to measure wavefront phase changes at different field points. In general, any number and configuration of markers and sensor devices 19 may be used to provide information related to wavefront phase changes.

[0213] Controller CN (such as Figure 1 The controller CN may also be configured to control one or more components of the lithographic apparatus LA. For example, the controller CN may control a positioning device operable to move the substrate table WT and / or the support structure MT relative to each other. The controller CN may control an adjustment member PA for adjusting components of the projection system PS. For example, the adjustment member PA may adjust an element of the projection system PS so as to correct an aberration determined by the controller CN.

[0214] The projection system PS includes a plurality of reflective lens elements 13, 14 and an adjustment member PA for adjusting the lens elements 13, 14 so as to correct aberrations. To achieve such correction, the adjustment member PA may be operable to manipulate the reflective lens elements within the projection system PS in one way or in a plurality of different ways. The adjustment member PA may be operable to perform any combination of: shifting one or more lens elements; tilting one or more lens elements; and / or deforming one or more lens elements.

[0215] The projection system PS has an optical transfer function that may be non-uniform, which may affect the pattern imaged onto the substrate W. For unpolarized radiation, these effects can be very well described by two scalar maps or scalar diagrams that describe the transmission (apodization) and relative phase (aberrations) of the radiation exiting the projection system PS as a function of the position in the pupil plane of the radiation. These scalar maps, which may be referred to as transmission maps and relative phase maps, may be expressed as linear combinations of a complete set of basis functions. It will be understood that the terms "transmission map" and "relative intensity map" are synonymous and that the transmission map may alternatively be referred to as a relative intensity map. A particularly suitable set of basis functions for expressing these scalar maps are the Zernike polynomials, which form a set of orthogonal polynomials confined on the unit circle. The determination of each scalar map may involve determining coefficients in such an expansion. Since the Zernike polynomials are orthogonal on the unit circle, the Zernike coefficients can be determined by computing the inner product of the measured scalar mapping with each Zernike polynomial in turn and dividing this inner product by the square of the norm of the Zernike polynomial.

[0216] The transmission map and the relative phase map are field and system dependent. That is, in general, each projection system PS will have a different Zernike expansion for each field point (ie for each spatial location in the image plane of the projection system PS).

[0217] Determining aberrations of the projection system PS may include fitting wavefront measurements made by the sensor device 19 to Zernike polynomials to obtain Zernike coefficients. Different Zernike coefficients may provide information about different forms of aberrations caused by the projection system PS. The Zernike coefficients may be determined independently at different locations in the x and / or y direction (i.e., at different field points).

[0218] Different Zernike coefficients can provide information about different forms of aberrations caused by the projection system PS. Typically, Zernike polynomials are considered to include multiple orders, each order having an associated Zernike coefficient. The orders and coefficients can be labeled with an index, which is usually called a Noll index. A Zernike coefficient with a Noll index of 1 may be called a first Zernike coefficient, a Zernike coefficient with a Noll index of 2 may be called a second Zernike coefficient, and so on.

[0219] The first Zernike coefficient is related to the average value of the measured wavefront (which average value may be referred to as the "piston"). The first Zernike coefficient may not be related to the performance of the projection system PS, and thus, the method described herein may not be used to determine the first Zernike coefficient. The second Zernike coefficient is related to the tilt of the measured wavefront in the x-direction. The tilt of the wavefront in the x-direction is equivalent to placement in the x-direction. The third Zernike coefficient is related to the tilt of the measured wavefront in the y-direction. The tilt of the wavefront in the y-direction is equivalent to placement in the y-direction. The fourth Zernike coefficient is related to the defocus of the measured wavefront. The fourth Zernike coefficient is equivalent to placement in the z-direction. Higher-order Zernike coefficients are related to other forms of aberrations (e.g., astigmatism, coma, spherical aberration, and other effects).

[0220] Throughout this specification, the term "aberration" shall be intended to include all forms of deviation of the wavefront from a perfectly spherical wavefront. That is, the term "aberration" may be associated with the placement of an image (e.g., second, third, and fourth Zernike coefficients) and / or with higher order aberrations, such as aberrations associated with Zernike coefficients having a Noelle index of 5 or greater.

[0221] As described in detail above, one or more reflective markers 17 may be used to determine the alignment and / or aberrations of components of the lithographic apparatus LA. In some embodiments, a discrete marker 17 may be used to determine the alignment of a component with a marker for determining aberrations. For example, a patterning device MA suitable for a lithographic exposure process may have one or more markers disposed outside a patterned area suitable for a lithographic exposure process. The one or more markers may be suitable for determining the alignment of the patterning device MA relative to the substrate table WT.

[0222] One or more markers 17 suitable for determining aberrations may be provided on a measurement patterning device which is separate from a patterning device MA (e.g. a reticle) for performing the lithographic exposure. The measurement patterning device MA may, for example, be provided on the support structure MT for the purpose of performing aberration measurements. The measurement patterning device MA may comprise other features suitable for determining other properties of the projection system PS. For example, the measurement patterning device may additionally comprise markers suitable for determining the alignment of the measurement patterning device relative to the substrate table WT.

[0223] In some embodiments, the same marker can be used to determine both alignment and aberrations. For example, one or more markers in the form of a reflective grating structure (e.g., a diffraction grating) can be used to determine both alignment and aberrations. In some embodiments, both alignment and aberrations can be determined simultaneously using the same set of measurements.

[0224] References to pattern forming devices MA herein should be interpreted as including any device including one or more features configured to modify radiation. The pattern forming device MA may, for example, be provided with a pattern used during lithographic exposure (e.g., the pattern forming device may be a mask). Additionally or alternatively, the pattern forming device may be provided with one or more markers for a measurement process. Typically, the pattern forming device MA is a removable component placed on the support structure MT to perform a specific process (e.g., perform lithographic exposure and / or perform one or more measurement processes). However, in some embodiments, the lithographic apparatus LA itself may be provided with one or more patterned features. For example, the support structure MT may be provided with one or more patterned features (e.g., markers) for a measurement process. For example, the support structure MT may be provided with one or more reference objects including one or more markers. In such an embodiment, since the support structure MT is provided with one or more features configured to modify radiation, the support structure MT itself may be considered as an example of a pattern forming device. References to pattern forming devices including reflective markers herein should not be interpreted as being limited to removable pattern forming devices, but should be interpreted as including any device having reflective markers disposed thereon.

[0225] refer to Figure 1, the patterning device MA may be considered to be arranged in the object plane of the projection system PS and the substrate W may be considered to be arranged in the image plane of the projection system PS. In the context of such a lithographic apparatus, the object plane of the projection system PL (in which the patterning device MA is arranged), the image plane of the projection system PL (in which the substrate W is arranged) and any planes conjugate thereto may be referred to as field planes of the lithographic apparatus. It will be understood that in an optical system (e.g. a lithographic apparatus), two planes are conjugate if every point in a first plane P is imaged onto a point in a second plane P′.

[0226] It will be understood that the lithographic apparatus LA comprises optics (i.e. focusing and / or diverging optics) having optical power or optical focal length to form an image in an image plane of an object in an object plane. Within such an optical system, between each pair of field planes a pupil plane may be defined as a Fourier transform plane of a previous field plane and a successive field plane. The distribution of the electric field within each such pupil plane is related to the Fourier transform of the object arranged in the previous field plane. It will be understood that the quality of such a pupil plane will depend on the optical design of the system and such a pupil plane may even be curved. This applies to considering two such pupil planes: a pupil plane of the illumination system and a pupil plane of the projection system. The pupil plane of the illumination system and the pupil plane of the projection system (and any other pupil planes) are mutually conjugate planes. Radiation in pupil plane PP of the illumination system IL The intensity (or equivalently, electric field strength) distribution in can be referred to as the illumination pattern or pupil filling and characterizes the angular distribution of the light cone at the pattern forming device MA (i.e., in the object plane). Similarly, the radiation is radiated in the pupil plane PP of the projection system IL The intensity (or equivalently, the electric field strength) distribution in φ characterizes the angular distribution of the light cone at the wafer level (ie, in the image plane).

[0227] The illumination system IL may change the intensity distribution of the beam in a pupil plane of the illumination system. This may be achieved by configuring the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 appropriately.

[0228] During exposure of substrate W, illumination system IL and projection system PS are used to form a (diffraction-limited) image of object-level patterning device MA on image-level substrate W (e.g., a resist-coated silicon wafer). During such exposure, for the illumination pattern, it may be desirable to use a local illumination pattern. For example, it may be desirable to use a multipole (e.g., dipole or quadrupole) illumination pattern, in which illumination system PP is used to form a (diffraction-limited) image of object-level patterning device MA on image-level substrate W (e.g., a resist-coated silicon wafer). IL In the pupil plane of the image, only a limited number (eg, two or four) of discrete polar regions receive radiation. Figure 4A and Figure 4BTwo examples of such illumination modes are shown in FIG. For example, the illumination mode may be as follows: Figure 4A The dipole distribution 30 shown in Figure 4B The quadrupole distribution 32 is shown in FIG. Figure 4A and Figure 4B Also shown is a ring 34 which represents the limit of what can be physically captured by the projection system PS and imaged onto the image plane (this represents the numerical aperture NA or the sine of the maximum angle that can be captured by the projection system PS). In coordinates normalized by the numerical aperture NA of the projection system PS, the ring 34 has a radius σ=1. The dipole distribution 30 includes two diametrically opposed polar regions 36 in which the intensity is non-zero. The quadrupole distribution 32 includes something like Figure 4A and a second dipole distribution rotated by π / 2 radians relative to the first dipole distribution but otherwise identical to the first dipole distribution. Thus, quadrupole distribution 32 includes four polar regions 34 in which the intensity is non-zero.

[0229] When the lithographic apparatus is not exposing the substrate W, one of more reflective markers arranged on the pattern forming device MA can be used in the measurement process, for example to determine the alignment and / or aberrations associated with the lithographic apparatus LA. When using reflections from the marker to measure the alignment and / or aberrations, it can be expected that the radiation reflected from the marker fills a larger part of the pupil of the projection system PS. In order to achieve such filling, in principle, the illumination system IL can be reconfigured to fill the pupil plane of the illumination system (and therefore also the incident pupil of the projection system). However, to do this (and to return to the exposure illumination mode before the next exposure) may take more time than is desired for such an inline measurement. Therefore, it is known to provide a diffuser during such a measurement, which is arranged to increase the angular spread of radiation scattered from the object horizontal pattern forming device so as to increase the proportion of the incident pupil of the projection system PS that is filled with radiation.

[0230] Such a diffuser may be placed in the path of the radiation beam during these metrology measurements but not during exposure of the substrate W. This allows the EUV lithography apparatus to be operated to perform semi-continuous inline metrology, which in turn may be used to maintain an optimal dynamic setup of the projection system PS, support structure MT and substrate table WT. In addition, such a metrology system may be used to align the patterning device MA to the substrate W before exposing the substrate W.

[0231] At the object level, some existing measurement systems use a combined diffuser and patterning device (e.g., a one-dimensional diffraction grating). One arrangement uses a three-dimensional structure mounted on a support structure MT, the three-dimensional structure comprising a recessed diffuser and a grating film arranged in two different planes. The EUV radiation beam B leaves the illumination system IL, reflects from the recessed diffuser (which increases the angular spread of the radiation), and then after reflection passes through the grating film (which scatters the radiation, some of which is captured by the projection system). Such a three-dimensional arrangement cannot be formed on a reticle and therefore on a reference.

[0232] As described in WO2017 / 207512, another arrangement uses a reflective object, i.e. a combined diffuser and patterning device. This arrangement has the form of a multi-layer reflective stack arranged to preferably reflect EUV radiation, to which a pattern of EUV absorbing material (e.g. a diffraction grating) is applied. The multi-layer reflective stack has a surface roughness such that the reflected radiation is diffused. However, although in principle such a patterning device can be provided on a reticle, it is significantly more complicated to manufacture a patterning device with such a built-in surface roughness. Therefore, in practice, such a patterning device is more likely to be formed on a reference piece.

[0233] Embodiments of the present invention relate to novel diffusers that are particularly suitable for use with EUV measurement systems within EUV lithography apparatus of the type described above.

[0234] Figure 5A and Figure 5B Schematically shown in FIG. 1 is a diffuser 50 according to a first embodiment of the present invention. The diffuser 50 comprises a film 52 formed of carbon nanotubes and two scattering material layers 54 applied to two opposite surfaces of the carbon nanotube film 52. The carbon nanotube film 52 can be regarded as forming a support material layer having nanostructures disposed on two opposite surfaces of the support material layer.

[0235] The carbon nanotube film 52 has a refractive index close to 1 for EUV radiation and a relatively low absorption coefficient for EUV radiation. Therefore, the carbon nanotube film 52 can be considered to be relatively optically neutral with respect to EUV radiation.

[0236] In contrast, the scattering material layer 54 is formed of a material having suitable optical properties so as to produce a large angular dispersion for EUV radiation (while also having a substantial transmittance for EUV radiation).

[0237] Therefore, the scattering material layer 54 is formed of a refractive index different from that of the carbon nanotube film 52. In particular, the scattering material can be formed of a material having a refractive index n such that the magnitude of (1-n) is as large as possible. In addition, the scattering material can be formed of a material having an extinction coefficient k as small as possible for EUV radiation.

[0238] It will be appreciated that diffuser 50 is a transmissive diffuser for use with EUV radiation. In general, in order to maximize the intensity of EUV radiation output by diffuser 50, it is desirable to minimize the attenuation caused by scattering material layer 54. This can be achieved by minimizing the extinction coefficient of the scattering material and / or minimizing the thickness of the scattering material. In addition, it will be appreciated that for a given scattering material, in order to increase the amount of angular dispersion, it is desirable to increase the thickness of the layer; whereas in order to reduce the attenuation caused by the scattering material, it is desirable to reduce the thickness of the layer. Scattering materials having a larger value of (1-n) allow the thickness to be reduced (while still providing reasonable angular dispersion). Scattering materials having a smaller extinction coefficient k for EUV radiation allow the thickness to be increased (while still providing reasonable transmission).

[0239] Suitable materials for the scattering material layer 54 include molybdenum, ruthenium, or niobium. Figure 6 Plots of the extinction coefficient k for EUV radiation versus the magnitude of (1-n) for EUV radiation are shown for these three materials and for carbon and silicon.

[0240] As stated, it is desirable to maximize the magnitude of (1-n) of the scattering material for EUV radiation. In some embodiments, the magnitude of (1-n) of the scattering material for EUV radiation may be greater than a threshold value of 0.06 (i.e., Figure 6 In some embodiments, the magnitude of (1-n) of the scattering material for EUV radiation may be greater than a threshold value of 0.08 (ie, Figure 6 In some embodiments, the magnitude of (1-n) of the scattering material for EUV radiation may be greater than a threshold value of 0.1 (ie, Figure 6 In some embodiments, the magnitude of (1-n) of the scattering material for EUV radiation may be greater than a threshold value of 0.12 (ie, Figure 6 to the right of line 66 in FIG.

[0241] As stated, it is desirable to minimize the extinction coefficient k of the scattering material for EUV radiation. In some embodiments, the scattering material may have an extinction coefficient k of less than 0.04 nm for EUV radiation. -1 The threshold value (i.e., Figure 6 In some embodiments, the scattering material may have an extinction coefficient k of less than a threshold value of 0.03 nm for EUV radiation. -1(Right now, Figure 6 In some embodiments, the scattering material may have an extinction coefficient k of less than a threshold value of 0.02 nm for EUV radiation. -1 (Right now, Figure 6 In some embodiments, the scattering material may have an extinction coefficient k of less than a threshold value of 0.01 nm for EUV radiation. -1 (Right now, Figure 6 The extinction coefficient k is below line 76 in FIG.

[0242] It will be appreciated that for a given scattering material, in order to increase the amount of angular dispersion, it is desirable to increase the thickness of the layer; however, in order to reduce the attenuation caused by the scattering material, it is desirable to reduce the thickness of the layer. Scattering materials with a larger value of (1-n) allow the thickness to be reduced (while still providing reasonable angular dispersion). Scattering materials with a smaller extinction coefficient k for EUV radiation allow the thickness to be increased (while still providing reasonable transmission). It will therefore be appreciated that in practice a suitable material can be selected to balance these two needs.

[0243] In some embodiments, the magnitude of (1-n) is greater than a threshold value of 0.06 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.01 nm -1 ; or the magnitude of (1-n) is greater than a threshold value of 0.08 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.02 nm -1 ; or the magnitude of (1-n) is greater than a threshold value of 0.1 and the magnitude of the extinction coefficient k for EUV radiation is less than a threshold value of 0.03 nm -1 ; or the value of (1-n) is greater than the threshold value 0.12 and the value of the extinction coefficient k for EUV radiation is less than 0.04nm -1 That is, in Figure 6 The material is found in the cross-hatched area.

[0244] In some embodiments, the magnitude of (1-n) and the extinction coefficient k for EUV radiation satisfy the following relationship:

[0245]

[0246] where |1-n| is the magnitude of (1-n). This is equivalent to being lower than Figure 6 Line 80 in.

[0247] By forming the diffuser 50 using multiple layers, a carbon nanotube film 52 (the carbon nanotube film 52 may be referred to as a support layer) may be used to apply the nanostructure to a scattering material layer 54 (the scattering material layer 54 may be referred to as a scattering layer).

[0248] Providing a material layer 54 that causes scattering and has nanostructures formed thereon is particularly advantageous for use with EUV radiation (which may, for example, have a wavelength of 13.5 nm) because such nanostructures include features having sizes comparable to or smaller than the wavelength of the radiation for which diffusion is desired. In these cases, scattering is a Mie scattering mechanism and significant angular dispersion may be achieved. For example, in some embodiments, the nanostructures formed in both the carbon nanotube film 52 and the scattering material layer 54 include features having sizes in the range of 2 nm to 10 nm.

[0249] The carbon nanotube film 52 can be formed in a variety of ways, either as a randomly oriented nanotube collection or as a structured nanotube array. In particular, the structured array type works well as a diffraction / scattering device: the nanotubes have a typical diameter of 2nm to 10nm, forming a grating structure with a size well below the EUV wavelength of 13.5nm. Figure 5A and Figure 5B In the diffuser 50 shown in FIG. 5 , the diffuser 50 is substantially planar, lying substantially in the xy plane and the membrane 52 comprises a structured array of nanotubes extending substantially in the x direction. It will be understood that as used herein, an object is described as being substantially planar meaning that the size of the object is in one dimension ( Figure 5A and Figure 5B The z-direction in the image is significantly smaller than in the other two dimensions.

[0250] like Figure 5A As more schematically shown in FIG. 1 , in use, radiation 56 is incident on the diffuser 50, the radiation propagating generally in the z-direction. This incident radiation 56 may correspond to a radiation beam B output by the illumination system IL. It will be appreciated that the incident radiation may include radiation having a range of different angles of incidence and Figure 5A The arrow 56 shown in FIG. 5 may indicate the direction of the principal ray. The scattering material layer 54 causes this incident radiation to be spread over a larger angular range. This is schematically indicated by arrow 58.

[0251] When in use, the diffuser 50 can be used to increase the angular range of radiation having the angular range reflected from the object horizontal marker entering the projection system PS. In particular, it can be expected that each portion of the diffuser 50 causes a divergence of radiation 58 that is approximately the angular range of radiation received by the pattern forming device MA in the lithographic apparatus LA. For example, in one embodiment, the numerical aperture of the pattern forming device MA (and the projection system PS) in the lithographic apparatus can be approximately 0.08, and a numerical aperture of approximately 0.08 corresponds to an angular range of approximately 7°. Therefore, it can be expected that each lens formed by the second portion 105 causes a divergence of radiation 56 of approximately 7°. This can ensure that each field point on the pattern forming device MA receives radiation from approximately the entire angular range within the cone having a complete angular range of approximately 7°. Equivalently, this can ensure that the pattern forming device is illuminated by a substantially complete pupil fill.

[0252] Figure 7 The intensity distribution 90 of scattered radiation 58 is shown as a function of the scattering angle in the scattering direction of diffuser 50 for an embodiment in which the carbon nanotubes in carbon nanotube film 52 have a diameter of about 10 nm and scattering material layer 54 includes molybdenum with a thickness of 4 nm. The carbon nanotube density of carbon nanotube film 52 for this embodiment is about 50%. Figure 7 As can be seen, the width 92 (eg, full width at half maximum) of such an angular distribution 90 may be approximately 7°.

[0253] In some embodiments, diffuser 50 may have a thickness (in Figure 5A and Figure 5B The thickness is arranged such that EUV radiation 56 propagating through the thickness of the diffuser 50 produces a phase shift of (2m+1)π radians. Advantageously, this suppresses zero-order (or specular) scattering.

[0254] Since the carbon nanotube film 52 is relatively optically neutral with respect to EUV radiation, the phase shift caused by propagation through the diffuser 50 can be primarily caused by the scattering material layer 54. It will be appreciated that the phase shift caused by propagation through the scattering material is 2π / λ times the change in length of the optical path caused by the scattering material, where λ is the wavelength of the EUV radiation. If the scattering material is immersed in an optical medium having a refractive index of n2, the change in length of the optical path caused by the scattering material is defined as (n-n2)·t, where t is the thickness of the scattering material.

[0255] For example, for an embodiment where the scattering material comprises molybdenum (having a refractive index of approximately 0.92) in a vacuum (i.e., n,=1), if the thickness of the material is approximately 85 nm, a phase shift of π radians can be achieved for EUV radiation having a wavelength of 13.5 nm. If the molybdenum is formed on the carbon nanotube film 52 in multiple layers each having a thickness of approximately 8 nm, a phase shift of approximately π radians can be achieved with 10 layers. The transmittance of such a structure for EUV radiation is >50%.

[0256] will understand, Figure 5A and Figure 5B The diffuser 50 shown in FIG. 5 forms a diffraction grating in which the grating lines extend substantially in the y-direction. Thus, Figure 5A and Figure 5B The diffuser 50 shown in 3 causes an angular dispersion of EUV radiation 56 passing through the diffuser 50 primarily in the x-direction (the x-direction may be referred to as a scattering direction of the diffuser 50 in the plane of the diffuser 50 ).

[0257] According to some embodiments of the present invention, there is provided a method comprising: Figure 5A and Figure 5B A diffuser of the type shown in FIG. 5 includes at least two diffusers 50 , the scattering directions of which are different.

[0258] For example, in one embodiment, the diffuser may include Figure 5A and Figure 5B Two diffusers 50 of the type shown in FIG. 5 are: a first diffuser 50 arranged to produce an angular dispersion in the x-direction, and a second diffuser 50 arranged to produce an angular dispersion in the y-direction.

[0259] By adding more Figure 5A and Figure 5B Diffusers 50 of the type shown in FIG. 1 , each having a different scattering direction, can achieve scattering in all directions. For example, additional diffusers 50 can be added, and the scattering directions of the additional diffusers are arranged at angles of +45° and -45° relative to the x-direction. In addition, additional diffusers 50 can be added, and the scattering directions of the additional diffusers 50 are arranged at angles of +30°, +60°, -30°, -60°, etc. relative to the x-direction.

[0260] In this manner, some embodiments of the present invention involve a plurality of layers (each layer comprising, for example, Figure 5A and Figure 5B50) of the type shown in , each layer is arranged to change the angular distribution of EUV radiation passing through the layer in a different way. Advantageously, by providing a plurality of layers, each layer being arranged to change the angular distribution of EUV radiation passing through the layer in a different way, the diffuser provides an arrangement in which the EUV radiation beam can be more effectively diffused throughout a desired angular range. In addition, the plurality of layers that change the angular distribution of EUV radiation passing through the plurality of layers in different ways provide a greater degree of control over the angular distribution of radiation leaving the diffuser.

[0261] In some embodiments, the individual layers (each layer is Figure 5A and Figure 5B The scattering properties of a diffuser 50 of the type shown in Figure 5 can be individually tuned. This can be achieved, for example, by changing parameters of the carbon nanotube film 52, such as (for example) the carbon nanotube diameter and / or the carbon nanotube packing density. In this way, such a multilayer diffuser can be adapted to the entrance pupil of the projection system PS within the lithographic apparatus. For example, the scattering properties can be tuned so as to fill the entrance pupil of the projection system PS; thereby minimizing gradients in the entire entrance pupil and / or minimizing the amount of radiation scattered outside the numerical aperture of the projection system PS.

[0262] For example, at least two of the multiple layers of such a multilayer diffuser may be arranged to produce different amounts of angular dispersion of EUV radiation passing through the at least two layers in corresponding scattering directions of the at least two layers. Advantageously, this may allow the amount of dispersion to be adapted for a particular purpose. For example, the projection system PS of the lithography apparatus LA may apply a reduction factor in order to form an image having features that are smaller than corresponding features on an object level reticle or mask. In some systems, the projection system PS may have a synthetic design in which different reduction factors are applied in two orthogonal directions. With such a design, the numerical aperture may be represented by an ellipse (rather than a circle) in the entrance pupil of the projection system PS. With such a synthetic projection system PS, it may be desirable to provide a diffuser that has, for example, a greater degree of dispersion in a direction corresponding to the major axis of such an ellipse and a lesser degree of dispersion in a direction corresponding to the minor axis of such an ellipse. This may be accomplished by using two layers, each of which is Figure 5A and Figure 5B ), each of the two layers produces a different amount of angular dispersion of the EUV radiation in its corresponding (mutually perpendicular) scattering direction.

[0263] Although in the above described embodiments, the support layer for the scattering material layer 54 is provided by a carbon nanotube film, in alternative embodiments, the support layer may include any material provided with nanostructures. In some embodiments, the support layer may be formed of a material that is relatively optically neutral for EUV radiation (i.e., having a refractive index close to 1 for EUV radiation and a relatively low absorption coefficient for EUV radiation). In one embodiment, the support layer is alternatively provided by porous silicon.

[0264] Reference FIG. 8A to FIG. 11 A diffuser and a process for forming the diffuser according to a second embodiment of the present invention are discussed.

[0265] A diffuser according to a second embodiment of the invention comprises a layer of support material comprising nanostructures on at least one surface, the nanostructures having been formed by self-assembled metal-assisted chemical etching of the support material. Such a layer of support material is coated with a scattering material. FIG. 8A to FIG. 9 The formation of nanostructures in a support material by self-assembling metal-assisted chemical etching of the support material is discussed.

[0266] First, the intermediate stacking layer 98 is formed. Fig. 8A is a schematic cross-sectional view of a portion of the intermediate stacking layer 98, and Figure 8B is a schematic plan view of a portion of the intermediate stacking layer 98.

[0267] The intermediate stacking layer 98 includes a support material layer 100. The support material may, for example, include silicon or porous silicon (pSi). The support material layer 100 may have a thickness of about 60 nm. In some embodiments, the support material is a material having a refractive index close to 1 for EUV radiation and a relatively low absorption coefficient for EUV radiation. For such embodiments, the support material may be considered to be relatively optically neutral for EUV radiation.

[0268] The support material layer 100 is formed on a carrier layer 102, which can be used to support the support material layer 100 when forming a diffuser. The carrier layer 102 can be formed of silicon nitride (SiN), porous silicon (pSi) or molybdenum silicide (MoSi). For example, the carrier layer 102 can have a thickness of about 100 nm.

[0269] The intermediate stacking layer 98 also includes a self-assembled metal structure formed on the surface of the support material layer 100. The self-assembled metal structure can be formed, for example, from gold. The self-assembled metal structure can be formed by physical vapor deposition (PVD). The self-assembly relies on the following well-known and well-established phenomenon: in the first few nanometers of the deposited film, dispersed metal islands 104 are formed on the surface of the support material layer 100 (see in particular Figure 8B ). This is explained, for example, in X. Yu et al., "Coalescence and percolation in thin metal-films", Phys. Rev. B 44 (23), 13163-13166 (1991), which is incorporated herein by reference in its entirety.

[0270] If the reference Fig. 9 As described, a second intermediate stacking layer 108 is subsequently formed from the intermediate stacking layer 98. Then, the support material layer 100 in the intermediate stacking layer 98 is selectively etched to form a modified support material layer 106, such as Fig. 9 In cross-section. In fact, the dispersed metal islands 104 shield those parts of the support material layer 100 that are in contact with the etchant. This type of anisotropic catalytic etching of crystalline silicon in direct contact with such metal islands is a known and stable process that has been very well studied for several metals and etchants. This is described in, for example, J. Duran et al., "Fabrication of ultrahigh aspect ratio silicon nanostructures using self-assembled gold metal-assisted chemical etching", J. Micro / Nanolith. MEMS MOEMS 16 (1), 014502 (2017), which is incorporated herein by reference in its entirety.

[0271] By this technique, higher aspect ratio structures can be formed in the modified support material layer 106 with a lateral resolution down to about 10 nm or less (i.e., in the plane of the support material layer 106, i.e., in the xy plane). Thus, the modified support material layer 106 can be considered to be a layer including nanostructures on at least one surface thereof.

[0272] It will be appreciated that the support material layer 100 in the intermediate stack 98 is selectively etched down to the interface between the support material layer 100 and the carrier layer 102. In an alternative embodiment, an etch stop layer may be provided between the support material layer 100 and the carrier layer 102. For example, such an etch stop layer may include a 2 nm layer of ruthenium. This may facilitate cooling of the luminescent light of the diffuser. Furthermore, by using an etch stop layer, a wider range of etchants may be allowed to be used and better control may be provided over the etching process.

[0273] If the reference Fig.10As described, the third intermediate stacking layer 110 is then formed from the second intermediate stacking layer 108. Fig.10 , a scattering material layer 112 is formed on the modified support material layer 106. The scattering material layer 112 can be considered to be adjacent to the surface of the modified support material layer 106. Thus, the scattering material layer 112 also includes nanostructures. The scattering material layer 112 can include a 5 nm ruthenium coating.

[0274] In such an embodiment, the dispersed metal islands 104 are sealed between the scattering material layer 112 and the modified support material layer 106. This can assist in the radiative cooling of the diffuser. However, in alternative embodiments, the dispersed metal islands 104 can be removed after etching and before the scattering material layer 112 is formed on the modified support material layer 106.

[0275] Finally, the carrier layer 102 is etched back to form the diffuser 114 (see Fig.11 ).

[0276] By this technique, higher aspect ratio structures can be formed in the modified support material layer 106, and in turn in the scattering material layer 112 with a lateral resolution down to about 10 nm or less (i.e., in the plane of the diffuser 114, i.e., in the xy plane). This lateral resolution is far beyond that which can be achieved by other means, such as, for example, photolithography or electron beam patterning. As discussed above, providing such a scattering material layer 112 that produces scattering and has nanostructures formed thereon is particularly advantageous for use with EUV radiation (which may, for example, have a wavelength of 13.5 nm) because such nanostructures include features having a size that is smaller than or comparable to the wavelength of the radiation that is desired to be diffused. In these cases, the scattering is a Mie scattering mechanism, and significant angular dispersion can be achieved.

[0277] This level of lateral resolution of the structures formed in the scattering material layer 112 allows the diffuser 114 to achieve scattering angles > 10°. In addition, the higher aspect ratio allows for a sufficient thickness of the scattering material layer 112 to ensure sufficient phase effects for the diffuser 114. For example, as explained above, the diffuser 114 may be desired (in Fig.11 The diffuser 114 may have a thickness (in the z-direction in FIG. 1 ) arranged to produce a phase shift of (2m+1)π radians to EUV radiation propagating through the thickness of the diffuser 114. Advantageously, this suppresses zero-order (or specular) scattering.

[0278] If the scattering material is immersed in an optical medium having a refractive index of n2, the change in length of the optical path caused by the scattering material is defined as (n-n2) t, where t is the thickness of the scattering material. For example, for an embodiment in which the scattering material comprises molybdenum (having a refractive index of approximately 0.92) in a vacuum (i.e., n2=1), if the thickness of the material is approximately 85 nm, a phase shift of π radians can be achieved for EUV radiation having a wavelength of 13.5 nm.

[0279] It will be appreciated that the support material layer 100 may be formed from any suitable material that is relatively optically neutral with respect to EUV radiation and that may be etched to form the modified support material layer 106. Suitable materials include silicon and porous silicon (pSi).

[0280] It will be appreciated that the scattering material layer 112 is formed of a material having suitable optical properties so as to produce a large angular dispersion for EUV radiation (while also having a substantial transmittance for EUV radiation). It will be appreciated that the scattering material layer 112 may have Figure 5A and Figure 5B In addition, when selecting a suitable material for the scattering material layer 112, the properties of the scattering material layer 54 in the diffuser 50 may also be considered. Figure 5A and Figure 5B Suitable materials for the scattering material layer 54 in the diffuser 50 shown in FIG. 5 may take into account the same considerations as discussed above.

[0281] Therefore, the scattering material layer 112 may be formed of a material having a refractive index different from that of the support material layer 100 (the support material layer may, for example, include silicon). In particular, the scattering material may be formed of a material having a refractive index n such that the magnitude of (1-n) is as large as possible. In addition, the scattering material may be formed of a material having an extinction coefficient k for EUV radiation that is as small as possible.

[0282] Suitable materials for the scattering material layer 112 include molybdenum, ruthenium or niobium. Figure 6 Plots of the extinction coefficient k for EUV radiation versus the magnitude of (1-n) for EUV radiation are shown for these three materials and for carbon and silicon.

[0283] will understand, Fig.11 The diffuser 114 shown in FIG. 1 forms a two-dimensional nanostructure. Therefore, Fig.11 The diffuser 14 shown in gives rise to an angular dispersion of EUV radiation passing through the diffuser 14 in two directions in the plane of the diffuser, ie in the x-direction and the y-direction.

[0284] A diffuser and a process for forming the diffuser according to a third embodiment of the present invention will now be discussed with reference to FIGS. 12 to 14 .

[0285] Fig. 12A and Fig. 12B is a schematic illustration of two diffusers 120, 122 based on quasi-random refractive structures 124, 126 arranged on a support layer 128. Fig. 12A and Fig. 12B In each of the figures, a flat wavefront W, X is shown incident on the diffuser 120, 122. In addition, a deformed wavefront Wref, Xref is also shown, which is generated by the incident flat wavefront W, X after being refracted by the refractive structures 124, 126. Fig. 12A and Fig. 12B As schematically shown in , each of the diffusers 120, 122 generates some scattering a, b. The scattering a, b can be estimated based on the phase delay distribution over a typical pitch of the structure.

[0286] Fig. 12A A refractive structure 124 is shown comprising a plurality of (eg rectangular) features having a uniform pitch L and a quasi-randomly distributed height H. The scatter α (in radians) can be estimated as:

[0287]

[0288] where ΔH is a measure of the diffusion of the height and n is the refractive index of the medium from which the refractive structure 124 is formed.

[0289] Fig. 12B A refractive structure 126 is shown, which comprises a plurality of (e.g., rectangular) features having uniform height H, pitch P, and widths N, M. By means of this general structure (e.g., Fig. 12B ), in the case where the pitch P is variable and the widths M and N are uniform, the scatter b (in radians) can be estimated as:

[0290]

[0291] where ΔP is a measure of the spread of the pitch, P av is the average pitch, n is the refractive index of the medium from which the refractive structure 126 is formed, and H is the uniform height. Fig. 12B ), where the pitch P is uniform and the refractor width M is variable, the scatter b (in radians) can be estimated as:

[0292]

[0293] Where ΔM is a measure of the spread of the width, P is the uniform pitch, n is the refractive index of the medium from which the refractive structures 126 are formed, and H is the uniform height. It is desirable to keep the total thickness of the refractive structures 124, 126 relatively small in order to avoid excessive absorption of EUV radiation. For example, the thickness of the structure is kept moderate so that:

[0294]

[0295] where k is the extinction coefficient of the material, H is a typical height scale, and λ is the wavelength of the EUV radiation (e.g., 13.5 nm). In addition, to achieve good mixing, for a fixed scattering c (in radians), it is desirable to choose a typical length scale such that:

[0296]

[0297] where L is the length scale, λ is the wavelength of the EUV radiation (e.g. 13.5 nm) and c is the amount of scattering in radians (the amount of scattering may be given, for example, by α as given by equation (2) or by b as given by equation (3) or (4)).

[0298] It may be desirable to achieve a scattering angle of approximately 5° (or 0.1 radians).The greatest amount of scattering will be achieved using materials with a maximum value of (n-1), such as, for example, ruthenium and rhodium (n~0.9 for ruthenium and rhodium).

[0299] With such materials (ruthenium or rhodium), it is possible to use Fig. 12A For example, L~50nm and The refractive structure 124 (which is consistent with inequalities (5) and (6)) achieves a scattering angle of approximately 5°.

[0300] With such a material (ruthenium or rhodium), if the height H is equal to the average pitch P av The ratio is about 2 (i.e., ), then a reasonable fill factor (e.g., with approximately The average width M av With the average pitch P av The ratio of randomly placed Fig. 12B A refractive structure 126 of the type shown in 1 achieves a scattering angle of about 5°. This can be achieved, for example, by a pitch of about P-50 nm and a height of about H-100 nm (the height being consistent with inequalities (5) and (6)).

[0301] The inventors have realized Fig. 12A and Fig. 12BThe random refractive structures 124, 126 shown in FIG. 1 and can be realized by a randomly placed layer of nanoparticles (e.g., made of ruthenium or rhodium), provided that the typical size of the nanoparticles corresponds to the typical displacement of adjacent nanoparticles in the layer. For example, assuming that the average size D of the nanoparticles is similar to Q av The ratio of the average displacements of neighboring nanoparticles is in the range of 1 to 2.

[0302] FIG. 13A to FIG. 13C Schematically showing diffusers 130 , 132 , 134 according to a third embodiment of the present invention.

[0303] The diffusers 130, 132, 134 are configured to receive and transmit EUV radiation. Each diffuser 130, 132, 134 includes a support layer 136 formed of a first material and a nanoparticle layer supported by the support layer 136. The nanoparticles are formed of a material that is transmissive to EUV radiation and refracts EUV radiation incident on the diffusers 130, 132, 134.

[0304] exist Fig.13A In the embodiment shown in FIG. 1 , the nanoparticle layer is monodisperse, comprising a plurality of nanoparticles 138 all having substantially uniform size. In particular, the nanoparticles 138 have a diameter D and have a random distribution of displacements of adjacent nanoparticles, with the displacements of adjacent nanoparticles being located from a minimum displacement Q min To maximum displacement Q max within the range.

[0305] exist Fig. 13B In the embodiment shown in FIG. 1 , the nanoparticle layer is polydisperse, comprising a plurality of nanoparticles 138 all having a range of different sizes. In particular, a first set of nanoparticles 138 have a diameter D, and a second set of nanoparticles 140 have a diameter d (d being less than D). The nanoparticles 138, 140 have a random distribution of displacements of adjacent nanoparticles, with the displacements of adjacent nanoparticles being located from a minimum displacement Q min To the maximum displacement Q max within the range.

[0306] exist Fig. 13CIn the embodiment shown in , the nanoparticle layer. The nanoparticles are formed by a variety of different materials. In particular, the first set of nanoparticles 138, 140 (represented by black circles) are formed by a material (e.g., ruthenium), and the second set of nanoparticles 142, 144 (represented by gray circles) are formed by another material (e.g., rhodium). The first set of nanoparticles 138, 140 and the second set of nanoparticles 142, 144 are both polydisperse, including multiple nanoparticles 138 that all have a series of different sizes. In particular, some of the nanoparticles 138 of the first set have a diameter D, and some of the nanoparticles 140 of the first set have a diameter d (d is less than D). Similarly, some of the nanoparticles 142 of the second set have a diameter D, and some of the nanoparticles 144 of the second set have a diameter d (d is less than D). Nanoparticles 138, 140, 142, 144 have a random distribution of displacements of neighboring nanoparticles, with the displacements of neighboring nanoparticles being located from a minimum displacement Q min To the maximum displacement Q max within the range.

[0307] In each of the diffusers 130, 132, 134, the nanoparticle layer may act as a random array of microlenses that cause scattering of EUV radiation incident on the diffuser. The nanoparticle layer may be viewed as forming a nanostructure. This is particularly advantageous for use with EUV radiation (which may, for example, have a wavelength of 13.5 nm) since such nanostructures may include features having a size smaller than or comparable to the wavelength of the radiation that is desired to be diffused. In these cases, the scattering is a Mie scattering mechanism and significant angular dispersion may be achieved.

[0308] By arranging the diffusers 130, 132, 134 so that the EUV radiation beam is incident on the diffuser at an oblique angle, the amount of dispersion is adapted for a particular purpose. With this arrangement, the amount of dispersion will be different in the plane of incidence and in a plane perpendicular to the plane of incidence.

[0309] As previously discussed, the diffusers 130, 132, 134 may be used in or form part of a measurement system for determining an aberration map or a relative intensity map for a projection system of a lithographic apparatus. In some such systems, the projection system may have a composite design in which different reduction factors are applied in two orthogonal directions. With such a composite projection system, it may be desirable to provide diffusers 130, 132, 134 that are arranged, for example, to provide a greater degree of diffusion in one direction and a lesser degree of diffusion in another perpendicular direction. This may be achieved by suitable orientation of the diffusers 130, 132, 134.

[0310] In each of the diffusers 130, 132, 134, the ratio of the average size of the nanoparticles to the average displacement of adjacent nanoparticles is in the range of 1 to 2. It will be understood that, as used herein, the size of a nanoparticle may mean the diameter D of the nanoparticle. It will be understood that, as used herein, the displacement of two adjacent nanoparticles may mean the distance Q between the center of one of the nanoparticles and the center of the other nanoparticle in the plane of the diffuser. By arranging the nanoparticles in the nanoparticle layer such that the average size D of the nanoparticles is av The average displacement Q of the neighboring nanoparticles av With a ratio in the range of 1 to 2, the diffuser can be close to a diffuser based on a quasi-random diffractive structure (having a range of heights, pitches and / or widths).

[0311] The nanoparticles 38 , 40 , 42 , 44 may be formed of any material having suitable optical properties in order to achieve a desired angular dispersion of EUV radiation passing through the nanoparticles 38 , 40 , 42 , 44 .

[0312] In some embodiments, the nanoparticles 38 , 40 , 42 , 44 are formed of a noble metal, such as ruthenium (Ru) or rhodium (Rh).

[0313] Alternatively, the nanoparticles 38, 40, 42, 44 may be formed of yttrium (Y), zirconium (Zr), molybdenum (Mo) or diamond. Alternatively, the nanoparticles 38, 40, 42, 44 may be formed of a ceramic based on any one of ruthenium, rhodium, yttrium, zirconium, molybdenum or carbon and any one of boron (B), nitrogen (N), oxygen (O) or carbon (C).

[0314] Alternatively, for embodiments in which the nanoparticles 38, 40, 42, 44 are formed of non-noble metals, the material may be encapsulated in a shell formed of a noble metal such as, for example, ruthenium or rhodium. The thickness of such a shell may be <R / 2, where R is the radius of the nanoparticle 38, 40, 42, 44. Such a noble metal shell improves the stability of the nanoparticles 38, 40, 42, 44 to an environment that may include EUV plasma.

[0315] Alternatively, the nanoparticles 38, 40, 42, 44 may include a shell of a material formed of SiO2, ZrO2, Y2O3 or other oxides, borides, nitrides, silicides or carbides. The thickness of such a shell may be <R / 2, where R is the radius of the nanoparticle 38, 40, 42, 44. Such a shell may improve the stability of the colloidal solution during deposition of the nanoparticle 38, 40, 42, 44 layer.

[0316] Alternatively, nanoparticles 38,40,42,44 can be functionalized by one or more polar, nonpolar or amphipathic groups to improve the stability of colloidal solutions during the deposition of nanoparticles 38,40,42,44 layers.Suitable groups can include carboxyl (-COOH), polyethylene glycol (PEG) groups.Such groups can be chemically connected to nanoparticles 38,40,42,44 and can help deposit nanoparticles 38,40,42,44 layers.Such groups can form sparse sub-monolayers on nanoparticles 38,40,42,44.

[0317] The nanoparticles 38, 40, 42, 44 may have any suitable size to achieve a desired level of scattering for a desired wavelength. For example, the nanoparticles 38, 40, 42, 44 may have a size in the range of 10 nm to 1 μm.

[0318] The nanoparticles 38, 40, 42, 44 may be arranged randomly or quasi-randomly.The particle coverage of the layer of nanoparticles 38, 40, 42, 44 to the support layer 136 may range from 50% to 500% (ie, sub-monolayer to several monolayers of the stack).

[0319] The support layer 136 may be formed of a material having a relatively low absorption coefficient for EUV radiation. The support layer 136 may be formed of a material suitable for the environment within an EUV lithography apparatus, which may have a relatively high temperature and may have a hydrogen plasma (also referred to as EUV induced plasma or just EUV plasma).

[0320] The support layer 136 may be formed of a ceramic including a combination of one of the metals molybdenum, zirconium, or yttrium and at least one of silicon, carbon, nitrogen, or boron. For example, the support layer may be formed of a ceramic including one of the following: MoSi, MoSiN, YSiB, SiC, ZrSi, ZrC, or MoC. It will be understood that, as used herein, these ceramics are not precise chemical formulas, but merely indicate the elements in the ceramic (but not necessarily the ratios of those elements).

[0321] Alternatively, the support layer 136 may be formed of graphene or diamond-like carbon.

[0322] The support layer 136 may have any suitable thickness to provide sufficient self-support and to provide support for the layers of nanoparticles 138, 140, 142, 144. For example, the support layer 136 may have a thickness of about 10 nm to 1 μm. For example, in one embodiment, the thickness of the support layer 136 may be ˜100 nm.

[0323] Optionally, the support layer 136 may include a capping layer to protect the support layer from EUV plasma etching. The capping layer may include a noble metal, such as (for example) ruthenium or rhodium. The capping layer may completely seal the bulk of the support layer 136. The capping layer may have a thickness of less than 10nm (for example, less than 5nm). Optionally, the support layer 136 may also include an intermediate layer disposed between the capping layer and the bulk of the support layer 136. The intermediate layer may be arranged to stabilize the capping layer from dewetting (for example, the intermediate layer may be formed of a suitable material to reduce the dewetting of the capping layer). When in use, the diffusers 130, 132, 134 may be at high temperatures, in which case the probability of such dewetting may be increased. The intermediate layer may include one of the metals molybdenum, zirconium or yttrium. The intermediate layer may have a thickness of less than 5nm, for example, less than 2nm.

[0324] In some embodiments, one or more measures may be taken to better adhere the nanoparticle layers 138, 140, 142, 144 to the support layer 136, as shown in FIG. FIG. 14A to FIG. 14C Discussed. FIG. 14A to FIG. 14C Show Fig.13A However, it will be understood that it is also possible to Fig. 13B and Fig. 13C Diffusers 132 , 134 of the type shown in FIG. 1 provide these features.

[0325] like Fig.14A As schematically shown in FIG. 1 , the diffuser 130 may further include a cover layer 146 that at least partially covers the support layer 136 and / or the layer of nanoparticles 138 .

[0326] Such a capping layer 146 may help retain the nanoparticles 138 on the support layer 136. Additionally or alternatively, the support layer 146 may provide a degree of protection from the surrounding environment for the support layer 136 and / or the layer of nanoparticles 138. When in use, the surrounding environment may be the environment within an EUV lithography apparatus, which may have a relatively high temperature and may have a hydrogen plasma.

[0327] The capping layer 146 may be formed of a material that is transmissive to EUV radiation. The capping layer 146 may be formed of, for example, ruthenium (Ru) or silicon dioxide (SiO2).

[0328] like Fig. 14B136. It will be appreciated that this can be achieved by heating the support layer 136 and / or the layer of nanoparticles 138 so as to weld or sinter the layer of nanoparticles 138 to the support layer 136. This is schematically illustrated by the deformed portions 148 of the nanoparticles 138, which have been formed by such heating and are used to bond the nanoparticles to the support layer 136 (and to each other). This can also enhance the retention of the layer of nanoparticles 138 on the support layer 136. Heating the support layer 136 and / or the layer of nanoparticles 138 can be achieved, for example, via electron beam exposure or heat annealing.

[0329] like Fig. 14C As schematically shown in FIG. 1 , the diffuser 130 may further include an inner layer 150 disposed between the support layer 136 and the layer of nanoparticles 138 . At least some of the nanoparticles 138 are at least partially embedded in the inner layer 150 .

[0330] The interior layer 150 may have a lower melting point than the support layer 136 and the layer of nanoparticles 138 and such an arrangement, wherein at least some of the nanoparticles 138 are at least partially embedded in the interior layer 150, may be achieved by heating the interior layer 150 above the melting point of the interior layer 150 and cooling the interior layer 150 so as to braze the support layer 136 to the layer of nanoparticles 138. This may also enhance the retention of the layer of nanoparticles 138 on the support layer 136. Heating the interior layer 150 may be achieved, for example, via electron beam exposure or heat annealing.

[0331] This inner layer 150 can help adhere the nanoparticles 138 to the support layer 136. This arrangement can be referred to as a micro-brazed diffuser 130.

[0332] References FIG. 13A to FIG. 14C The described diffusers 130 , 132 , 134 may be configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more.

[0333] Typically, the nanoparticles 138, 140, 142, 144 may be formed of a material having a refractive index n such that the magnitude of (1-n) is as large as possible (e.g., greater than a threshold value of 0.06). Typically, the nanoparticles 138, 140, 142, 144 may be formed of a material having an extinction coefficient k for EUV radiation that is as small as possible (e.g., less than 0.04 nm). -1 threshold value).

[0334] Now, the discussion of the FIG. 13A to FIG. 14C A method of diffusers 130 , 132 , 134 of the type described.

[0335] Usually, used to form the above reference FIG. 13A to FIG. 14C A method of making a diffuser 130 , 132 , 134 of the type described includes providing a support layer 136 formed of a first material; and depositing a plurality of nanoparticles 138 , 140 , 142 , 144 to form a layer of nanoparticles 138 , 140 , 142 , 144 supported by the support layer 136 .

[0336] In general, the process of depositing nanoparticles 138, 140, 142, 144 to form a layer of nanoparticles 138, 140, 142, 144 supported by a support layer 136 can produce a random or quasi-random arrangement of nanoparticles 138, 140, 142, 144 (which is suitable for use as a diffractor). The method provides an alternative method of producing diffusers 130, 132, 134 that is inexpensive, simple and provides greater freedom in the substrate design of the support layer, so that the support layer 136 can be better adapted to allow thermal management of the diffusers 130, 132, 134.

[0337] For example, another method for forming a quasi-random diffraction structure may be to form a plurality of (e.g., rectangular) features on a support substrate having ( Fig. 12A and Fig. 12B 140,142,144 layers. In some embodiments, the quasi-random distribution of at least one of the height, pitch and / or width of the type shown in the figure. Such a structure requires some mechanisms to plan the quasi-random structure and can then be manufactured, for example, using photolithography. In contrast, the process usually used to deposit a plurality of nanoparticles 138,140,142,144 to form nanoparticles 138,140,142,144 layers will produce random or quasi-random structures and is usually cheaper and simpler to perform. In addition, by changing the properties of nanoparticles 138,140,142,144 layers, such as (for example) the material, size, density of the nanoparticles 138,140,142,144 in the layer, the properties of the diffusers 130,132,143 can be easily changed.

[0338] As presently discussed, the plurality of nanoparticles 138 , 140 , 142 , 144 may be deposited onto the support layer 136 using an aerosol or colloid that confines the nanoparticles 138 , 140 , 142 , 144 .

[0339] In some embodiments, aerosol deposition can be used to achieve deposition of the plurality of nanoparticles 138, 140, 142, 144 so as to form a layer of nanoparticles 138, 140, 142, 144 supported by the support layer 136. For example, deposition can include: providing an aerosol including the nanoparticles 138, 140, 142, 144 suspended in a carrier gas; and depositing the plurality of nanoparticles 138, 140, 142, 144 using the aerosol so as to form a layer of nanoparticles 138, 140, 142, 144 supported by the support layer 136.

[0340] Any suitable carrier gas may be used.

[0341] In some embodiments, an aerosol comprising nanoparticles 138, 140, 142, 144 suspended in a carrier gas may include forming the aerosol using spark ablation. Spark ablation is a known technique for generating aerosols. Electrodes formed of a conductive material and a voltage source may be used to form nanoparticles 138, 140, 142, 144 of the material via spark ablation. A suitable carrier gas flow through (or even across) the electrodes may be used to form the aerosol.

[0342] For embodiments in which spark ablation is used to form the aerosol, the carrier gas may, for example, include an inert gas. Suitable inert gases may include, for example, an inert gas (e.g., argon) or nitrogen. Such inert gases are beneficial because they can avoid unwanted reactions (e.g., oxidation) of the nanoparticles.

[0343] This aerosol-based approach has been successfully used to form multilayer nanoparticle layers on substrates with nanoparticles up to about 20 nm in size.

[0344] Alternatively, an aerosol comprising nanoparticles 138, 140, 142, 144 suspended in a carrier gas can be formed by forming a colloid comprising nanoparticles suspended in a carrier liquid and forming a vapor from the colloid. For such embodiments (wherein the aerosol is formed by forming a vapor using a colloid), the carrier gas can be a gas formed by a carrier liquid (the carrier liquid can be referred to as a carrier fluid) of the colloid. The carrier gas or carrier liquid can be selected to ensure that the colloid and / or aerosol is stable. The carrier fluid can be selected to ensure that the carrier fluid can be completely evaporated so that after forming the nanoparticle 138, 140, 142, 144 layer, no molecular residues from the carrier fluid are left. Typically, a polar (e.g., water) carrier fluid or a non-polar (e.g., heptane or hexane) carrier fluid can be used.

[0345] Deposition of the plurality of nanoparticles 138, 140, 142, 144 from the aerosol onto the support layer 136 may be achieved by impingement and / or diffusion. Impingement may be achieved by introducing the aerosol at the support layer 136.

[0346] Alternatively, direct ("wet") colloidal deposition can be used to achieve the deposition of the plurality of nanoparticles 138, 140, 142, 144 onto the support layer 136. For example, a colloid including the nanoparticles 138, 140, 142, 144 suspended in a carrier liquid can be formed and then deposited directly onto the support layer 136. The carrier liquid can then be removed by evaporation to deposit the plurality of nanoparticles 138, 140, 142, 144 using the colloid, thereby forming a layer of nanoparticles 138, 140, 142, 144 supported by the support layer 136.

[0347] For example, droplets of the colloid can be deposited onto the support layer 136 (or the intermediate layer if an intermediate layer is used) and the droplets are dried. As the carrier liquid evaporates, the nanoparticles 138, 140, 142, 144 remain on the support layer 136. The colloid droplets can have any suitable size (e.g., the droplets can have a volume of about 1 μL). The colloid can have any suitable concentration of the nanoparticles 138, 140, 142, 144 (e.g., the droplets can have a volume of about 1 μL). 10 Up to 10 12 ppm / μl concentration).

[0348] The drying pattern from such a process can produce a sparse monolayer and / or several monolayers. A random deposition pattern suitable for diffusers 130, 132, 134 can be achieved, for example, by using a poly-dispersed colloid and / or a colloid comprising a mixture of different shapes and / or materials. The colloid can be a poly-dispersed colloid. This can produce a random deposition pattern suitable for diffusers 130, 132, 134.

[0349] Such colloid-based wet synthesis methods have been successfully used to deposit silver, gold and platinum nanoparticles (eg, having a size of about 70 nm). Similar techniques can be used to deposit nanoparticles formed of, for example, ruthenium or rhodium.

[0350] Any suitable carrier fluid can be used. The carrier fluid can be selected to ensure that the colloid remains stable (i.e., so that the nanoparticles 138, 140, 142, 144 remain dispersed and do not dissolve and / or agglomerate). The carrier fluid can be selected to ensure that the carrier fluid can be completely evaporated so that after the nanoparticle 138, 140, 142, 144 layer is formed, no molecular residues from the carrier fluid are left. Typically, a polar (e.g., water) carrier fluid or a non-polar (e.g., heptane or hexane) carrier fluid can be used. Those skilled in the art will understand which carrier fluids may be suitable for a given type of nanoparticle 138, 140, 142, 144.

[0351] The method of forming the diffuser 130, 132, 134 may also include, once the layer of nanoparticles 138, 140, 142, 144 has been formed on the support layer 136, back etching the support layer 136 (i.e., etching the surface of the support layer 136 opposite the surface of the support layer 136 supporting the layer of nanoparticles 138, 140, 142, 144). This back etching of the support layer 136 allows for the use of a thicker, more stable support layer during the deposition of the nanoparticles 138, 140, 142, 144. Advantageously, this may prevent damage or even breakage of the support layer 136. This final etching step may be particularly beneficial in embodiments where a colloid is used to deposit the nanoparticles 138, 140, 142, 144, as this final etching step may prevent capillary forces from braking the support layer 136. Once the nanoparticle 138, 140, 142, 144 layer has been formed, the thickness of this support layer 136 may be finalized using an etching process.

[0352] The method of forming the diffusers 130, 132, 134 may also include implementing one of the above measures to better adhere the nanoparticle layers 138, 140, 142, 144 to the support layer 136, as described above with reference to FIG. 14A to FIG. 14C For example, the method may further include providing a covering layer 146 that at least partially covers the support layer 136 and / or the nanoparticle layer 138 (see Fig.14A Additionally or alternatively, the method may further include welding or sintering the nanoparticles 138, 140, 142, 144 to the support layer 136 (see Fig. 14B This can be accomplished by heating the support layer 136 and / or the layer of nanoparticles 138 (e.g., using electron beam exposure or heat annealing) to weld or sinter the layer of nanoparticles 138 to the support layer 136. Additionally or alternatively, at least some of the nanoparticles 138 can be at least partially embedded in the interior layer 150 (see Fig. 14C). This can be accomplished by heating the inner layer 150 above the melting point of the inner layer 150 and allowing the inner layer 150 to cool so as to braze the support layer 136 to the layer of nanoparticles 138. Heating the inner layer 150 can be accomplished, for example, via electron beam exposure or heat annealing.

[0353] According to some embodiments of the invention, there is provided a measurement system for determining an aberration map or a relative intensity map for a projection system PS, the measurement system comprising one of the diffusers described above. According to some embodiments of the invention, there is provided a lithographic apparatus comprising such a measurement system.

[0354] In use, a diffuser (e.g., comprising Figure 5A and Figure 5B At least one diffuser 50 of the type shown or Fig.11 A diffuser 114 of the type shown or FIG. 13A to FIG. 14C A diffuser 130, 132, 134 of the type shown in Figure 1 is arranged so that the diffuser can be moved into and out of the optical path of radiation between the illumination system IL and the projection system PS. Such an optical device provides control over the angular distribution of radiation in a field plane of the lithographic apparatus LA, which is downstream of the lithographic apparatus LA. Such a field plane comprises the plane of the support structure MT (i.e. the plane of the patterning device MA) and the plane of the substrate table WT (i.e. the plane of the substrate W). In order to ensure that the diffuser can be moved into and out of the optical path of radiation between the illumination system IL and the projection system PS, the diffuser may be mounted on a patterning device shielding blade of the lithographic apparatus LA, as now discussed.

[0355] The lithographic apparatus LA is provided with four reticle shielding blades which define the extent of the illuminated field on the pattern forming device MA. The illumination system IL is operable to illuminate a substantially rectangular area of ​​an object (e.g., the pattern forming device MA) disposed on the support structure MT. Such a substantially rectangular area may be referred to as a slit of the illumination system IL and is defined by the four reticle shielding blades. The extent of the substantially rectangular area in a first direction (the first direction may be referred to as the x-direction) is defined by a pair of x shielding blades. The extent of the substantially rectangular area in a second direction (the second direction may be referred to as the y-direction) is defined by a pair of y shielding blades.

[0356] Each of the shielding blades is arranged close to the plane of the support structure MT, but slightly outside of said plane. The x shielding blades are arranged in a first plane and the y shielding blades are arranged in a second plane.

[0357] Each of the shielding blades defines one edge of a rectangular field region in the plane of the object where radiation is received. Each blade is independently movable between a retracted position in which the blade is not arranged in the path of the radiation beam and an inserted position in which the blade at least partially blocks the radiation beam projected onto the object. By moving the shielding blade into the path of the radiation beam, the radiation beam B may be intercepted (in the x and / or y direction), thereby limiting the extent of the field region where the radiation beam B is received.

[0358] The x-direction may correspond to a non-scanning direction of the lithographic apparatus LA, and the y-direction may correspond to a scanning direction of the lithographic apparatus LA. That is, the object (and the substrate W in the image plane) can be moved in the y-direction through the field area so as to expose a larger target area of ​​the object (and the substrate W) in a single dynamic scanning exposure. During such a dynamic scanning exposure, the y shielding blade is moved to control the field area so as to ensure that portions of the substrate W outside the target area are not exposed. At the start of the scanning exposure, one of the y shielding blades is disposed in the path of the radiation beam B to act as a shield so that portions of the substrate W do not receive radiation. At the end of the scanning exposure, the other y shielding blade is disposed in the path of the radiation beam B to act as a shield so that portions of the substrate W do not receive radiation.

[0359] A diffuser (e.g., comprising Figure 5A and Figure 5B At least one diffuser 50 of the type shown or Fig.11 A diffuser 114 of the type shown or FIG. 13A to FIG. 14C A diffuser 130, 132, 134 of the type shown in Figure 1 is mounted on a patterning device shield blade of a lithographic apparatus LA. In particular, during scanning exposure, the diffuser may be positioned such that when the shield blade is disposed at a position within its nominal range of motion, the diffuser is not disposed substantially in the path of the radiation beam.

[0360] Diffuser (e.g., including Figure 5A and Figure 5B At least one diffuser 50 of the type shown or Fig.11 A diffuser 114 of the type shown or FIG. 13A to FIG. 14CDiffusers 130, 132, 134 of the type shown in FIG. 1 may have any of the following properties. The diffuser may produce an angular scattering distribution having a width of 5° to 10° or more in at least one scattering direction. The diffuser may produce a uniform power distribution or a Gaussian angular power distribution (as a function of the scattering angle). The diffuser may have an absorptivity (for a single stroke) of less than 90%, for example less than 50%, for EUV radiation. The diffuser may have a useful life of greater than 7 years in a lithographic apparatus (e.g., and an illumination duty cycle of about ~0.1 to 1%). The diffuser may be capable of operating to withstand approximately 1 W / cm 2 Up to 10W / cm 2 The diffuser may have a diameter of about 1 to 3 mm. 2 ×1 to 3 mm 2 size.

[0361] Although specific reference may be made herein to the use of lithographic equipment in IC manufacturing, it will be appreciated that the lithographic equipment described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0362] Although reference may be made herein specifically to embodiments of the invention in the context of 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 for measuring or processing an object such as a wafer (or other substrate) or a mask (or other pattern forming device). These apparatus may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0363] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be appreciated by those skilled in the art that modifications may be made to the present invention as described without departing from the scope of the claims set forth below.

[0364] aspect

[0365] 1. A diffuser configured to receive and transmit radiation, wherein the diffuser comprises: a plurality of layers, each layer being arranged to alter the angular distribution of radiation passing through the layer in a different manner.

[0366] 2. The diffuser according to clause 1, wherein at least two layers of the plurality of layers primarily cause angular dispersion of EUV radiation passing through the at least two layers along a single scattering direction in the plane of the diffuser, the scattering directions of the at least two layers being different.

[0367] 3. The diffuser of clause 2, wherein at least two layers of the plurality of layers cause EUV radiation passing through the at least two layers to have different amounts of angular dispersion in respective scattering directions of the at least two layers.

[0368] 4. The diffuser of any preceding aspect, wherein at least one layer of the plurality of layers comprises a scattering material layer having nanostructures disposed on at least one surface of the scattering material layer.

[0369] 5. The diffuser of aspect 4, wherein the nanostructures comprise features having dimensions in the range of 2 nm to 10 nm.

[0370] 6. The diffuser according to any one of the preceding aspects, the diffuser being configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more.

[0371] 7. A diffuser according to any one of aspects 4 to 6, wherein the scattering material has a refractive index n, and wherein the magnitude of (1-n) is greater than a threshold value of 0.06.

[0372] 8. The diffuser according to any one of clauses 4 to 7, wherein the scattering material has an extinction coefficient k for EUV radiation, the extinction coefficient k being less than 0.04 nm. -1 The threshold value.

[0373] 9. A diffuser according to any one of clauses 4 to 8, wherein the scattering material has a refractive index n and an extinction coefficient k for EUV radiation, and wherein the ratio of the magnitude of (1-n) to (k+0.025) is greater than a threshold of 2.

[0374] 10. The diffuser of any one of aspects 4 to 9, wherein the scattering material comprises one of molybdenum, ruthenium, niobium, rhodium or technetium.

[0375] 11. A diffuser according to any of clauses 4 to 10, wherein the scattering material has a thickness arranged such that EUV radiation propagating through the thickness of the scattering material produces a phase shift of (2m+1)π radians.

[0376] 12. The diffuser of any one of aspects 4 to 11, wherein the layer of scattering material is adjacent to a layer of support material, the support material having nanostructures disposed on at least one surface of the support material.

[0377] 13. The diffuser of aspect 12, wherein the layer of support material comprises a film formed of carbon nanotubes.

[0378] 14. The diffuser of clause 12, wherein the layer of support material comprises porous silicon.

[0379] 15. A diffuser configured to receive and transmit radiation, wherein the diffuser comprises: a first layer, the first layer is formed of a first material, the first layer includes nanostructures located on at least one surface of the first layer; and a second layer, the second layer is formed of a second material adjacent to the at least one surface of the first layer, so that the second layer also includes nanostructures, and the second material has a refractive index different from the refractive index of the first layer.

[0380] 16. The diffuser of aspect 15, wherein the nanostructures comprise features having dimensions in the range of 2 nm to 10 nm.

[0381] 17. The diffuser according to aspect 15 or aspect 16, the diffuser being configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more.

[0382] 18. The diffuser of any one of aspects 15 to 17, wherein the first layer comprises a film formed of carbon nanotubes.

[0383] 19. The diffuser of any one of aspects 15 to 17, wherein the first layer comprises porous silicon.

[0384] 20. The diffuser of any of aspects 15 to 17, wherein the first layer comprises a support material that has been selectively etched to form the nanostructures.

[0385] 21. The diffuser of aspect 20, wherein a metal layer is provided on portions of the support material that have not been selectively etched.

[0386] 22. The diffuser of any one of aspects 15 to 21, wherein the second material has a refractive index n, and wherein the magnitude of (1-n) is greater than a threshold value of 0.06.

[0387] 23. The diffuser according to any one of clauses 15 to 22, wherein the second material has an extinction coefficient k for EUV radiation, the extinction coefficient k being less than 0.04 nm. -1 The threshold value.

[0388] 24. A diffuser according to any one of clauses 15 to 23, wherein the second material has a refractive index n and an extinction coefficient k for EUV radiation, and wherein the ratio of the magnitude of (1-n) to (k+0.025) is greater than a threshold of 2.

[0389] 25. The diffuser of any one of aspects 15 to 24, wherein the second material comprises one of molybdenum, ruthenium, niobium, rhodium or technetium.

[0390] 26. A diffuser according to any of clauses 15 to 25, wherein the diffuser has a thickness arranged such that EUV radiation propagating through the thickness of the diffuser produces a phase shift of (2m+1)π radians.

[0391] 27. A diffuser configured to receive and transmit EUV radiation, wherein the diffuser comprises: a support layer formed of a first material; and a nanoparticle layer supported by the support layer, the nanoparticles being formed of a material that is transmissive to EUV radiation and causes refraction of EUV radiation incident on the diffuser.

[0392] 28. The diffuser of aspect 27, wherein a ratio of an average size of the nanoparticles to an average displacement of adjacent nanoparticles is in the range of 1 to 2.

[0393] 29. The diffuser according to aspect 27 or aspect 28, wherein the nanoparticles can include a noble metal, such as ruthenium (Ru) or rhodium (Rh).

[0394] 30. A diffuser according to any one of aspects 27 to 29, wherein the nanoparticles include a shell of a material formed of SiO2, ZrO2, Y2O3 or other oxides, borides, nitrides, silicides or carbides.

[0395] 31. The diffuser of any one of aspects 27 to 29, wherein the nanoparticles are functionalized with polar, non-polar or amphiphilic groups to increase the stability of the colloidal solution during deposition of the nanoparticle layer.

[0396] 32. The diffuser of any one of aspects 27 to 31, wherein the nanoparticles have a size in the range of 10 nm to 1 μm.

[0397] 33. A diffuser according to any one of aspects 27 to 32, wherein the nanoparticles are arranged randomly or quasi-randomly.

[0398] 34. The diffuser of any one of aspects 27 to 33, wherein the nanoparticles are formed from a plurality of different materials.

[0399] 35. The diffuser of any one of aspects 27 to 34, wherein the support layer is formed of a ceramic comprising a combination of one of the metals molybdenum, zirconium or yttrium and at least one of silicon, carbon, nitrogen or boron.

[0400] 36. The diffuser of any one of aspects 27 to 35, wherein the support layer is formed of graphene or diamond-like carbon.

[0401] 37. The diffuser of any one of aspects 27 to 36, wherein the support layer has a thickness of about 10 nm to 1 μm.

[0402] 38. The diffuser of any one of clauses 27 to 37, wherein the support layer comprises a cap layer for protecting the support layer from EUV plasma etching.

[0403] 39. The diffuser of aspect 38, wherein the support layer comprises an intermediate layer disposed between the capping layer and a mass of the support layer, wherein the intermediate layer is arranged to stabilize the capping layer against dewetting.

[0404] 40. The diffuser of any one of aspects 27 to 39, wherein the diffuser further comprises a cover layer at least partially covering the support layer and / or the nanoparticle layer.

[0405] 41. A diffuser according to any one of aspects 27 to 40, wherein the diffuser further comprises an inner layer disposed between the support layer and the nanoparticle layer, and wherein at least some of the nanoparticles are at least partially embedded in the inner layer.

[0406] 42. The diffuser of any one of aspects 27 to 41, the diffuser being configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more.

[0407] 43. The diffuser of any one of aspects 27 to 42, wherein the nanoparticles are formed of a material having a refractive index n, and wherein the magnitude of (1-n) is greater than a threshold value of 0.06.

[0408] 44. The diffuser of any one of clauses 27 to 43, wherein the nanoparticles are formed of a material having an extinction coefficient k for EUV radiation that is less than 0.04 nm. -1 The threshold value.

[0409] 45. A diffuser according to any one of aspects 27 to 44, wherein the material forming the nanoparticles has a refractive index n and an extinction coefficient k for EUV radiation, and wherein the ratio of the magnitude of (1-n) to the extinction coefficient k for EUV radiation is greater than a threshold value of 3.5.

[0410] 46. ​​The diffuser of any preceding clause, wherein the transmittance of the diffuser to EUV radiation is greater than 50%.

[0411] 47. A measurement system for determining an aberration map or a relative intensity map of a projection system, the measurement system comprising a diffuser according to any of the preceding aspects.

[0412] 48. A measurement system according to aspect 47, the measurement system comprising: a pattern forming device; an illumination system, the illumination system being arranged to illuminate the pattern forming device with radiation; and a sensor device; wherein the illumination system and the pattern forming device are configured so that the projection system receives at least a portion of the radiation scattered by the pattern forming device and the sensor device is configured so that the projection system projects the received radiation onto the sensor device; and wherein the diffuser is operable to receive the radiation generated by the illumination system and to change the angular distribution of the radiation before the radiation illuminates the pattern forming device.

[0413] 49. A measurement system according to aspect 48, wherein the diffuser is capable of moving between at least a first operating position and a second storage position: in the first operating position, the diffuser is at least partially disposed in the path of the radiation generated by the illumination system and is arranged to change the angular distribution of the radiation before the radiation irradiates the pattern forming device; and in the second storage position, the diffuser is disposed outside the path of the radiation generated by the illumination system.

[0414] 50. A lithography device, comprising: a measurement system according to any one of Aspects 47 to 49; and a projection system, which is configured to receive at least a portion of the radiation scattered by the pattern forming device and is configured to project the received radiation onto the sensor device.

[0415] 51. The lithographic apparatus of clause 50, wherein the diffuser is mounted on a patterning device shielding blade of the lithographic apparatus, an edge of the patterning device shielding blade defining a field region of the lithographic apparatus.

[0416] 52. A method of forming a diffuser according to any one of aspects 15 to 26, the method comprising: forming the first layer from the first material, the first layer comprising nanostructures located on at least one surface of the first layer; and forming the second layer from the second material on the at least one surface of the first layer.

[0417] 53. A method according to aspect 52, wherein forming the first layer comprises: providing a support material layer; forming a self-assembled metal structure on a surface of the support material layer; and selectively etching a portion of the support material layer that does not contact the self-assembled metal structure.

[0418] 54. A method according to aspect 53, wherein the layer of support material is formed on a carrier layer, the carrier layer being used to support the layer of support material when forming the diffuser, and wherein the method further comprises: once the first layer and the second layer have been formed, removing the carrier layer.

[0419] 55. A method of forming a diffuser according to any one of aspects 27 to 45, the method comprising: providing the support layer formed of the first material; and depositing a plurality of nanoparticles to form the nanoparticle layer supported by the support layer.

[0420] 56. A method according to aspect 55, wherein depositing the plurality of nanoparticles so as to form the nanoparticle layer supported by the support layer is achieved using aerosol deposition and comprises: providing an aerosol comprising the nanoparticles suspended in a carrier gas; and depositing the plurality of nanoparticles from the aerosol so as to form the nanoparticle layer supported by the support layer.

[0421] 57. The method of aspect 56, wherein providing an aerosol comprising the nanoparticles suspended in a carrier gas comprises forming the aerosol using spark ablation.

[0422] 58. The method of aspect 56, wherein providing an aerosol comprising the nanoparticles suspended in a carrier gas comprises forming a colloid comprising the nanoparticles suspended in a carrier liquid and forming a vapor from the colloid.

[0423] 59. The method of any one of aspects 56 to 58, wherein depositing the plurality of nanoparticles from the aerosol to form the nanoparticle layer supported by the support layer is achieved via impaction and / or diffusion.

[0424] 60. A method according to aspect 55, wherein depositing the plurality of nanoparticles so as to form the nanoparticle layer supported by the support layer is achieved using colloidal deposition and comprises: providing a colloid comprising the nanoparticles suspended in a carrier liquid; depositing the colloid on the support layer; and removing the carrier liquid via evaporation so as to deposit the plurality of nanoparticles from the colloid to form the nanoparticle layer supported by the support layer.

[0425] 61. The method according to any one of aspects 55 to 60, further comprising: heating the support layer and / or the nanoparticle layer to weld or sinter the nanoparticle layer to the support layer.

[0426] 62. A method according to any one of aspects 55 to 60, wherein the diffuser also includes an internal layer arranged between the support layer and the nanoparticle layer, the internal layer having a lower melting point than the support layer and the nanoparticle layer, and wherein the method also includes: heating the internal layer above the melting point of the internal layer, and cooling the internal layer so as to braze the support layer to the nanoparticle layer.

[0427] 63. A method according to any one of Aspects 55 to 62, further comprising: once the plurality of nanoparticles have been deposited to form the nanoparticle layer supported by the support layer, etching the support layer from a surface of the support layer opposite to a surface of the support layer supporting the nanoparticle layer.

[0428] 64. The method according to any one of aspects 55 to 63, further comprising: providing a covering layer, the covering layer at least partially covering the support layer and / or the nanoparticle layer.

Claims

1. A diffuser configured to receive and transmit radiation, wherein: The diffuser comprises: A plurality of layers, each layer being arranged to modify the angular distribution of radiation passing through the layer in a different manner, wherein at least two layers of the plurality of layers primarily cause an angular dispersion of EUV radiation passing through the at least two layers in a plane of the diffuser along a single scattering direction, the scattering directions of the at least two layers being different.

2. The diffuser according to claim 1, wherein: At least two layers of the plurality of layers cause EUV radiation passing through the at least two layers to have different amounts of angular dispersion in respective scattering directions of the at least two layers.

3. The diffuser according to claim 1 or 2, wherein: At least one layer of the plurality of layers includes a scattering material layer having nanostructures disposed on at least one surface of the scattering material layer. 4 . The diffuser according to claim 1 , wherein the diffuser is configured such that an angular scattering distribution in at least one scattering direction has a width of 5° or more.

5. The diffuser according to claim 3, wherein: The scattering material has a refractive index n, and wherein the magnitude of (1-n) is greater than a threshold value of 0.06, or / and wherein the scattering material has an extinction coefficient k for EUV radiation, the extinction coefficient k being less than 0.04 nm -1 The threshold value.

6. The diffuser according to claim 3, wherein: The scattering material has a refractive index n and an extinction coefficient k for EUV radiation, and wherein the ratio of the magnitude of (1-n) to (k+0.025) is greater than a threshold of 2, or / and wherein the scattering material comprises one of molybdenum, ruthenium, niobium, rhodium or technetium.

7. The diffuser according to claim 3, wherein: The scattering material has a thickness arranged such that EUV radiation propagating through the thickness of the scattering material produces a phase shift of (2m+1)π radians.

8. The diffuser according to claim 3, wherein: The layer of scattering material is adjacent to a layer of support material, the support material having nanostructures disposed on at least one surface of the support material.

9. The diffuser according to claim 8, wherein: The support material layer comprises a film formed of carbon nanotubes, or / and wherein the support material layer comprises porous silicon.

10. A diffuser configured to receive and transmit radiation, wherein: The diffuser comprises: a first layer, the first layer being formed of a first material, the first layer comprising a nanostructure located on at least one surface of the first layer; and a second layer formed of a second material adjacent to the at least one surface of the first layer, such that the second layer also includes nanostructures, the second material having a refractive index different from that of the first layer, Therein, the first layer and the second layer primarily cause an angular dispersion of EUV radiation passing through the first layer and the second layer in a plane of the diffuser along a single scattering direction, the scattering directions of the first layer and the second layer being different.

11. The diffuser according to claim 10, wherein: The nanostructures comprise features having dimensions in the range of 2 nm to 10 nm, or / and wherein the angular scattering distribution in at least one scattering direction has a width of 5° or more.

12. The diffuser according to any one of claims 10 to 11, wherein The first layer comprises a film formed of carbon nanotubes, or / and wherein the first layer comprises porous silicon.

13. The diffuser according to any one of claims 10 to 11, wherein The first layer includes a support material that has been selectively etched to form the nanostructures, wherein the metal layer is disposed on portions of the support material that have not been selectively etched.

14. The diffuser according to any one of claims 10 to 11, wherein The second material has a refractive index n, and wherein the magnitude of (1-n) is greater than a threshold value of 0.06, or / and wherein the second material has an extinction coefficient k for EUV radiation, the extinction coefficient k being less than 0.04 nm -1 The threshold value.

15. The diffuser according to any one of claims 10 to 11, wherein The second material has a refractive index n and an extinction coefficient k for EUV radiation, and wherein a ratio of a magnitude of (1-n) to (k+0.025) is greater than a threshold value of two.

16. The diffuser according to any one of claims 10 to 11, wherein The second material comprises one of molybdenum, ruthenium, niobium, rhodium or technetium, or / and wherein the diffuser has a thickness arranged such that EUV radiation propagating through the thickness of the diffuser produces a phase shift of (2m+1)π radians.

17. A diffuser configured to receive and transmit EUV radiation, wherein: The diffuser comprises: a support layer formed of a first material; and a layer of nanoparticles, the layer of nanoparticles being supported by the support layer, the nanoparticles being formed of a material that is transmissive for EUV radiation and causes refraction of EUV radiation incident on the diffuser, Therein, the support layer and the nanoparticle layer primarily cause an angular dispersion of EUV radiation passing through the support layer and the nanoparticle layer in a plane of the diffuser along a single scattering direction, the scattering directions of the support layer and the nanoparticle layer being different.

18. The diffuser according to claim 17, wherein: The ratio of the average size of the nanoparticles to the average displacement of adjacent nanoparticles is in the range of 1 to 2.

19. A diffuser according to claim 17 or claim 18, wherein The nanoparticles include precious metals, or / and wherein the nanoparticles include a shell of a material formed of SiO2, ZrO2, or Y2O3.

20. A diffuser according to claim 17 or claim 18, wherein The nanoparticles are functionalized with polar, nonpolar or amphiphilic groups to improve the stability of the colloidal solution during deposition of the nanoparticle layer.

21. A diffuser according to claim 17 or claim 18, wherein The nanoparticles have a size in the range of 10 nm to 1 μm, or / and wherein the nanoparticles are arranged randomly or quasi-randomly.

22. A diffuser according to claim 17 or claim 18, wherein The nanoparticles are formed of a plurality of different materials, or / and wherein the support layer is formed of a ceramic comprising a combination of one of the metals molybdenum, zirconium or yttrium and at least one of silicon, carbon, nitrogen or boron.

23. A diffuser according to claim 17 or claim 18, wherein The support layer is formed of graphene or diamond-like carbon.

24. A diffuser according to claim 17 or claim 18, wherein The support layer has a thickness of 10 nm to 1 μm, or / and wherein the support layer comprises a cap layer for protecting the support layer from EUV plasma etching.

25. A diffuser according to claim 17 or claim 18, wherein The diffuser further comprises a covering layer at least partially covering the support layer and / or the nanoparticle layer, or / and wherein the diffuser comprises an inner layer arranged between the support layer and the nanoparticle layer, and wherein at least some of the nanoparticles are at least partially embedded in the inner layer.

26. A diffuser according to claim 17 or claim 18, configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more.

27. A diffuser according to claim 17 or claim 18, wherein The nanoparticles are formed of a material having a refractive index n, and wherein the magnitude of (1-n) is greater than a threshold value of 0.06, or / and wherein the nanoparticles are formed of a material having an extinction coefficient k for EUV radiation, the extinction coefficient k being less than 0.04 nm -1 The threshold value.

28. A diffuser according to claim 17 or claim 18, wherein the material forming the nanoparticles has a refractive index n and an extinction coefficient k for EUV radiation, and wherein the ratio of the magnitude of (1-n) to the extinction coefficient k for EUV radiation is greater than a threshold value of 3.5, or / and wherein, The transmittance of the diffuser to EUV radiation is greater than 50%.

29. A lithographic apparatus, comprising: a measurement system for determining an aberration map or a relative intensity map for a projection system comprising a diffuser and a patterning device according to any one of the preceding claims; and The projection system is configured to receive at least a portion of the radiation scattered by the patterning device and to project the received radiation onto a sensor device.

30. A method of forming a diffuser according to any one of claims 17 to 28, the method comprising: providing the support layer formed of the first material; as well as A plurality of nanoparticles are deposited to form the nanoparticle layer supported by the support layer.

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