Transmissive Diffuser

Abstract

A diffuser is configured to receive and transmit radiation. The diffuser comprises a scattering layer (510) configured to scatter the received radiation, the scattering layer (510) comprising a first substance and having a plurality of voids distributed therein. The first substance may be a scattering substance, or alternatively, at least one of the voids may contain the scattering substance, and the first substance has a lower refractive index than the scattering substance.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 19202644.1 filed on October 11, 2019, and the entire contents of said European application are incorporated herein by reference. Technical Field

[0003] The present invention relates to a transmissive diffuser, ie a diffuser configured to receive and to transmit radiation, the transmitted radiation having a changing angular distribution. The diffuser may be suitable 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 the substrate. Compared to a lithographic apparatus using radiation having a wavelength of, for example, 193 nm, 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.

[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 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. For such a measurement system, it is desirable 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 for forming a (diffraction limited) image of an object level reticle 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 discrete 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 the projection system can be altered so that the entire entrance pupil of the projection system can receive radiation from a patterning device. Summary of the invention

[0008] A diffuser or walker / diffuser configured to receive and transmit radiation is described herein. The diffuser includes a scattering layer configured to scatter the received radiation. The scattering layer includes a first substance and has a plurality of voids distributed therein. The first substance may be a scattering substance. Alternatively, at least one of the voids may contain a scattering substance, and the first substance may be a substance having a lower refractive index than the scattering substance. The scattering material is used to provide a microlens array, thereby causing scattering of the radiation received by the diffuser. Such a diffuser may be configured so as to be able to change the angular distribution of the received radiation in a manner that will cause the entire entrance pupil of the projection system to receive radiation from the pattern forming device.

[0009] The void may contain a vacuum (or an environment that is substantially or functionally a vacuum). Alternatively, the void may contain a second substance, and one of the first substance and the second substance may be a scattering substance, wherein the other of the first substance and the second substance has a lower refractive index than the scattering substance. In the case where the first substance is the scattering substance, the second substance may be an inert gas. The substance with a lower refractive index may have a refractive index close to 1 for the received radiation. Such a substance may be considered to be optically neutral (or relatively optically neutral compared to the scattering material) for the received radiation. For example, if the received radiation is EUV radiation, the substance with the lower refractive index may have a refractive index close to 1 for the EUV radiation. However, it should be understood that the radiation may have any wavelength (i.e., may not be EUV radiation).

[0010] In the case where the first substance is the scattering substance, the scattering substance may comprise a foam having micropores and the voids may be provided by the micropores. One or more of the voids may contain a vacuum or an inert gas. One or more of the voids may contain one of silicon or silicon nitride. In this way, the second substance will be optically neutral for EUV radiation. In addition, the second substance will have a low attenuation for the received radiation. The substance within the voids will also have a refractive index that is greatly different from that of the scattering substance. In addition, in this way, the scattering layer is particularly easy to manufacture because no intermediate step of removing the second substance from the scattering substance is required.

[0011] In the case where the void contains the scattering substance, the first substance may include a porous silicon-based structure, and the void is defined by micropores of the first substance.

[0012] In the case of using a porous substance in the examples described herein, the micropores of the porous substance may have a range of about several nanometers in at least one dimension.

[0013] The scattering material may include a body of contact particles. The gaps may be provided between adjacent particles. Various deposition methods, such as liquid deposition methods, may be used to relatively easily make such diffusers. With respect to the term "contact particles", it is understood that each particle in the body of particles is in physical contact with at least one other particle in the body of particles.

[0014] The particles may be fused. That is, each particle in the contact particle body may be fused with at least one other particle in the contact particle body. For example, sintering may be used to fuse the particles.

[0015] The particles may include a binary mixture including a first material and a second material having a refractive index different from the first material. The refractive indexes of the first material and the second material may be greatly different. The first material and the second material may have low attenuation for received radiation. The first material may include silicon. The second material may include molybdenum or ruthenium. It will be appreciated that one or both of the first material or the second material may be a mixture of two or more materials. For example, the first material may be molybdenum silicide.

[0016] The particles may have a range of about a few nanometers in at least one dimension. The particles may be different in size in at least one dimension. That is, the particles may be polydisperse. The particle size, particle size distribution and / or packing density of the particles may be selected based on one or more desired properties of the scattering layer, such as high scattering angle and / or suppression of zero-order scattering.

[0017] The scattering material may include a material having a ratio of a first parameter to a second parameter of 1 or less than 1, wherein the first parameter is the maximum thickness of a layer of the material that will allow 10% transmission of the received radiation and the second parameter is the minimum thickness of a layer of the material that will cause a phase shift of Pi nm.

[0018] By way of example only, the scattering material may be, for example, molybdenum, ruthenium, niobium, rhodium, yttrium, boron, molybdenum disilicide, zirconium, rhodium, or technetium.

[0019] The voids may be distributed in a plurality of layers within the first substance, each layer lying substantially in a plane which is perpendicular to the direction of propagation of the radiation during use.

[0020] The voids may be distributed in a single layer within the first substance, the layer lying substantially in a plane which, during use, is perpendicular to the direction of propagation of the radiation.

[0021] The scattering substance may include a dealloyed material, and the dealloyed material will provide the scattering material with multiple interfaces with the voids.

[0022] The voids may have an extent of about a few nanometers in at least one dimension. The voids may be polydisperse within the first material. The voids may be randomly or quasi-randomly arranged within the first material.

[0023] The scattering layer may have a thickness between 50 nm and 1000 nm.The thickness of the material is measured in the propagation direction of the received radiation during use of the diffuser.

[0024] The diffuser may be configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more. The scattering direction may preferably have a width of 9° or more.

[0025] The scattering substance may include one of: molybdenum, ruthenium, niobium, rhodium, yttrium or technetium.

[0026] The diffuser may comprise a plurality of scattering layers.Each of the scattering layers may be manufactured according to any of the techniques described herein or elsewhere.

[0027] The first scattering layer may be separated from the second scattering layer by an intermediate layer. The intermediate layer may comprise silicon, or some other material that is relatively optically neutral with respect to the received radiation.

[0028] The intermediate layer may comprise a layer of separated particles having a lower refractive index than the scattering substance.Because the particles are separated, at least a portion of the intermediate layer may be occupied by, for example, an inert gas or a vacuum, thereby reducing attenuation of the received radiation.

[0029] The separate particles may be randomly or quasi-randomly arranged within the intermediate layer. The separate particles may include particles of different sizes in at least one dimension.

[0030] The first substance and the voids therein may cooperate to produce a hologram upon receiving radiation at the surface of the scattering layer. That is, the first substance and the voids may include a holographic interference pattern. The holographic interference pattern may be selected so as to form a desired hologram given radiation of a desired wavelength. The radiation may be EUV radiation. The first substance may be a scattering substance. A diffuser operable to produce a hologram may advantageously provide controlled diffusion of radiation combined with minimal absorption of radiation. Such a diffuser may have an increased lifetime compared to known diffusers, for example due to reduced absorption of radiation.

[0031] The hologram may have an angular intensity distribution that is at least as intense in a radially outer portion of the hologram as in a central region of the hologram. The angular intensity distribution may have a similar intensity in a central region as in a radially outer portion of the hologram. The angular intensity distribution may be a top hat distribution. The angular intensity distribution has a lower intensity in a central region as in a radially outer portion of the hologram.

[0032] The radially outer portion may be angularly spaced at least 9° from the centre of the hologram.Such a diffuser may be of particular benefit in devices having a high numerical aperture.

[0033] The first material may include multiple structures with varying thicknesses. That is, the thickness distribution of the multiple structures varies. The thickness distribution may be measured in the plane of the diffuser (e.g., the plane of the surface arranged to receive radiation). The thickness distribution may vary by approximately several nanometers. For example, the thickness distribution of the structure may vary between a thickness of 0 nm and 200 nm.

[0034] The diffuser may be operable to form the hologram upon receipt of radiation having a wavelength λ. The wavelength may be an EUV wavelength λ. The holographic diffuser may have an effective refractive index n eff The thickness of each structure in the plurality of structures may be Beneficially, such a diffuser may impart a phase shift of 0, pi or 2pi to the portion of radiation travelling through it.

[0035] The void may contain a second substance. That is, the second substance may be provided so as to fill the void.

[0036] The real part of the refractive index of the second substance may be different from the real part of the refractive index of the first substance. Beneficially, the first substance and the second substance having different real parts of the refractive index of the first substance and the second substance may scatter radiation. The imaginary part of the refractive index of the second substance may be similar to the imaginary part of the refractive index of the first substance. Beneficially, the first substance and the second substance having similar imaginary parts of the refractive index of the first substance and the second substance may reduce attenuation through the diffuser. The combined first substance and the second substance serve to reduce the relative difference in attenuation experienced by radiation traveling through the structure and the void.

[0037] The combined first and second substances may have a substantially constant combined thickness profile. That is, the surface of the diffuser arranged to receive radiation is substantially smooth. The surface may be smooth on a microscale. The surface may be smooth on a nanoscale.

[0038] The first substance may include one of: molybdenum, ruthenium, niobium, rhodium, yttrium or technetium. The second substance may include silicon.

[0039] Also described herein is a holographic diffuser comprising a scattering layer comprising a plurality of structures configured to produce a hologram upon receiving extreme ultraviolet radiation at a surface of the scattering layer, wherein the hologram has an angular intensity distribution that is at least as intense in a radially outer portion of the hologram as in a central region of the hologram.

[0040] Any diffuser described herein may further include a protective layer configured to protect the scattering layer from EUV plasma etching.The diffuser may further include a cap layer at least partially covering the scattering layer to protect the scattering layer during use.

[0041] Also described herein is a measurement system for determining an aberration map or a relative intensity map for a projection system, the measurement system comprising the diffuser of any example described herein.

[0042] The measurement system may include: a patterning device; an illumination system arranged to illuminate the patterning device with radiation; and a sensor device. The illumination system and the patterning device may be configured such that the projection system receives at least a portion of the radiation scattered by the patterning device, and the sensor device is configured such that the projection system projects the received radiation onto the sensor device. The diffuser may be operable to receive the radiation generated by the illumination system and to alter the angular distribution of the radiation before the radiation illuminates the patterning device.

[0043] The diffuser can be movable between at least the following positions: a first operating position, in which 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 impinges on the pattern forming device; and a second storage position, in which the diffuser is disposed outside the path of the radiation generated by the illumination system.

[0044] When a measurement system as described herein is used with a holographic diffuser as described herein, the holographic diffuser may be designed and / or arranged such that the hologram is formed at an input plane of the measurement system. The input plane may comprise an input plane of a sensor device of the measurement system.

[0045] Also described herein is a lithographic apparatus comprising: a measurement system as described in any example herein; and a projection system configured to receive at least a portion of the radiation scattered by the pattern forming device and configured to project the received radiation onto the sensor device.

[0046] 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 area of ​​the lithographic apparatus.

[0047] Also described herein is a method of forming a diffuser for receiving and transmitting radiation. The method includes forming an alloy layer, the alloy layer including a first substance and a third substance, wherein the first substance is a scattering substance. The method also includes dealloying the alloy layer to remove the third substance from the alloy layer and to form a scattering layer including the first substance and having a plurality of voids distributed therein.

[0048] The second substance may be zinc, and the dealloying may be dezincification.

[0049] Also described herein is a method of forming a diffuser for receiving and transmitting radiation, the method comprising forming a scattering layer by infiltrating a porous structure with a scattering material.

[0050] The porous structure may be porous silicon. The micropores may have an extent of about a few nanometers in at least one dimension.

[0051] The scattering layer may be formed on a supporting layer.

[0052] A method of forming a diffuser for receiving and transmitting radiation is also described, the method comprising: depositing a plurality of particles on a surface of a support layer to form a mask; depositing a scattering material on the support layer over the mask to form a scattering layer around the plurality of particles.

[0053] The second material may be a relatively optically neutral material with respect to the intended radiation. For example, the second material may be relatively optically neutral with respect to EUV radiation. For example, the second material may be silicon.

[0054] The method may further include shrinking one or more of the plurality of particles deposited on the support layer to expose a larger area of ​​the surface of the support layer prior to depositing the scattering material.

[0055] The particles may be deposited on the support layer via vertical colloidal deposition. The particles may form a single layer deposited on the surface of the support layer, and the scattering layer forms a corrugated scattering surface on the support layer. The particles form a plurality of layers deposited on the surface of the support layer, each of the plurality of layers being in use in a plane substantially perpendicular to the direction of the received radiation.

[0056] The method may further include removing the particles after depositing the scattering material.

[0057] A method of forming a diffuser for receiving and transmitting radiation is also described, the method comprising: depositing a plurality of particles on a surface of a support layer to form a mask; depositing a second material on the surface of the support layer over the mask to form a layer of the second material around the plurality of particles; removing at least some of the plurality of particles to form pits within the layer of the second material; and depositing a scattering material into at least some of the pits within the second material to form scattering features within the layer of the second material.

[0058] A method for forming a diffuser for receiving and transmitting radiation is also described, the method comprising: depositing a plurality of particles on a surface of a support layer to form a mask; depositing a second material on the surface of the support layer over the mask; selectively etching the surface of the support layer to form a plurality of structures on the surface of the support layer; depositing a scattering material onto the surface of the support layer, the scattering material being formed over the plurality of structures to form a scattering layer; wherein the second material is a catalyst and the selective etching comprises etching an area of ​​the support layer in contact with the second material, or wherein the second material is a protective material and the selective etching comprises etching an area of ​​the support layer that is not in contact with the second material.

[0059] Also described herein is a method of forming a diffuser for receiving and transmitting radiation, the method comprising: depositing a plurality of particles onto a surface of a support layer such that the particles form a body of contact particles. The particles may be deposited from a dispersion of particles in a liquid. The particles may be deposited to have a particle density such that a majority of the particles are in contact with one or more adjacent particles.

[0060] The deposition may include at least one of: vertical colloidal deposition, spin coating, and inkjet printing.Such a deposition method provides an easy diffuser fabrication method.

[0061] Depositing the plurality of particles may also include fusing the plurality of particles. The plurality of particles may be fused via providing heat and / or pressure. The plurality of particles may be fused using sintering.

[0062] The particles may include a binary mixture including a first material and a second material having a different refractive index than the first material. The first material may include molybdenum, ruthenium, niobium, rhodium, yttrium or technetium. The second material may include silicon.

[0063] The method may further include forming another scattering layer on the diffuser. The another scattering layer may be formed according to the method of any one of the examples described herein.

[0064] Forming a further scattering layer may comprise depositing an intermediate layer above the scattering layer and forming the further scattering layer on top of the intermediate layer. For example, the intermediate layer may be silicon or silicon nitride.

[0065] In any of the example methods described herein for forming a diffuser, the support layer may be formed on a carrier layer, the carrier layer being used to support the support layer while forming the diffuser, and wherein the method further comprises removing the carrier layer once the first layer and the second layer have been formed. The carrier layer may be, for example, silicon. For example, the carrier layer may be a standard silicon wafer of the type commonly used in semiconductor manufacturing.

[0066] Also described herein is a method of forming a diffuser for receiving and transmitting radiation, the method comprising producing a plurality of structures on a surface of a support layer of the diffuser, wherein the structures are arranged to produce a hologram upon receiving radiation at the surface. The radiation may be EUV radiation. A diffuser operable to produce a hologram may advantageously provide controlled diffusion of radiation combined with minimal absorption of radiation. Such a diffuser may have an increased lifetime compared to known diffusers, for example due to reduced absorption of radiation.

[0067] The hologram may have an angular intensity distribution that is at least as strong in a radially outer portion of the hologram as in a central region of the hologram. The angular intensity distribution may have a similar intensity in the central region compared to a radially outer portion of the hologram. The angular intensity distribution may be a top hat distribution. The angular intensity distribution may have a lower intensity in the central region compared to a radially outer portion of the hologram. The radially outer portion may be angularly spaced at least 9° from the center of the hologram. Such a diffuser may be of particular benefit in devices with a high numerical aperture.

[0068] The plurality of structures may be produced using photolithography. Each portion of the plurality of structures may have where λ is the wavelength of radiation that produces the hologram when the radiation is received by the diffuser, and the holographic diffuser has an effective refractive index n eff .

[0069] The method may also include depositing a second substance into a plurality of voids distributed within the plurality of structures. The second substance has a thickness such that the combined thickness distribution of the first substance and the second substance is substantially constant. That is, after providing the second substance, the surface of the diffuser operable to produce a hologram upon receiving radiation may be substantially smooth. The second substance may be a scattering substance. The real part of the refractive index of the second substance may be different from the real part of the refractive index of the first substance. Advantageously, the first substance and the second substance having different real parts of the refractive index of the first substance and the second substance may scatter radiation. The imaginary part of the refractive index of the second substance may be similar to the imaginary part of the refractive index of the first substance. Advantageously, the first substance and the second substance having similar imaginary parts of the refractive index of the first substance and the second substance may reduce attenuation through the diffuser. The combined first substance and the second substance are used to reduce the relative difference in attenuation experienced by radiation traveling through the structures and the voids.

[0070] The method may also include generating a thickness distribution corresponding to a desired arrangement of a plurality of surface features, the desired arrangement being based on a desired angular distribution of the hologram. Generating the thickness distribution may include a numerical method. Generating the thickness distribution may include iteratively solving and / or performing calculations based on an optical relationship. The optical relationship may represent one or more of the following: attenuation, refractive index, scattering angle, layer thickness, phase shift. The thickness distribution generation may include limitations on maximum and / or minimum allowed thickness. The maximum and / or minimum allowed thickness may be based on manufacturing parameters. The maximum and / or minimum allowed thickness may be based on desired optical properties, such as attenuation.

[0071] Generating the thickness distribution may comprise using a Gerchberg-Saxton algorithm. Generating the thickness distribution may comprise using a modified version of the Gerchberg-Saxton algorithm.

[0072] Any of the example methods of forming a diffuser described herein may further include etching the supporting layer from a surface of the supporting layer opposite a surface of the supporting layer supporting the scattering layer.

[0073] Any of the example methods described herein for forming a diffuser may further include providing a cover layer at least partially covering the support layer and / or the scattering layer.

[0074] The methods described herein 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.

[0075] This reverse etching of the support layer allows a thicker, more stable support layer to be used during manufacture of the diffuser. Advantageously, this can prevent damage or even breakage of the support layer. This final etching step may be particularly beneficial in embodiments where a colloid is used to deposit the nanoparticles, since this final etching step can prevent capillary forces from braking the support layer. Once the nanoparticle layer has been formed, the etching process can be used to finalize the thickness of this layer.

[0076] The methods described herein may further comprise providing a covering layer at least partially covering the support layer and / or the cover layer.

[0077] The term patterning device as used herein may also be referred to herein as a mask or a reticle, which terms are to be understood as synonymous. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0079] - Figure 1 Describing a lithographic system including a lithographic apparatus and a radiation source;

[0080] - Figure 2 is a schematic illustration of a reflective type mark;

[0081] - Figure 3A and Figure 3B is a schematic illustration of a sensor device;

[0082] - Figure 4A Show Figure 1 Intensity distribution of the bipolar illumination mode of the lithographic apparatus shown in;

[0083] - Figure 4B Show Figure 1 Intensity distribution of the four-level illumination mode of the lithography apparatus shown in;

[0084] - FIG. 5A to FIG. 5C schematically depicts intermediate stages in an example process for making a transmissive diffuser;

[0085] - FIG. 6A to FIG. 6C schematically depicts an intermediate stage in another example process for making a transmissive diffuser;

[0086] - 7A to 7E schematically depicts an intermediate stage in another example process for making a transmissive diffuser;

[0087] - FIG. 8A to FIG. 8D schematically depicts an intermediate stage in another example process for making a transmissive diffuser;

[0088] - 9A to 9E schematically depicts an intermediate stage in another example process for making a transmissive diffuser;

[0089] - FIG. 10A to FIG. 10E schematically depicts an intermediate stage in another example process for making a transmissive diffuser;

[0090] - FIG. 11A to FIG. 11C schematically depicts an intermediate stage in another example process for making a transmissive diffuser;

[0091] - Fig.12 schematically illustrates an EUV diffuser;

[0092] - Fig.13 A plot showing the extinction coefficient k for EUV radiation versus the magnitude of (1-n) for EUV radiation for some materials;

[0093] - FIG. 14A to FIG. 14C schematically depicts an intermediate stage in another example process for making a transmissive diffuser;

[0094] - Fig.15A Graphics based on FIG. 14A to FIG. 14C Height map of an example diffuser manufactured by the process of FIG.

[0095] - Fig. 15B and Fig. 15C Describe the incident Fig.15A The scattering angle of the radiated plane wave on the diffuser;

[0096] - Fig.16A and Fig. 16B schematically depicts an intermediate stage in another example process for making a transmissive diffuser;

[0097] - Fig.17 Graphics based on Fig.16A and Fig. 16B properties of the sample diffusers manufactured by the process;

[0098] - Fig.18 Graphics based on Fig.16A and Fig. 16B properties of the sample diffusers manufactured by the process;

[0099] - Fig.19 Graphics based on Fig.16A and Fig. 16B properties of an example diffuser made by the process of

[0100] - Fig. 20 Graphics based on Fig.16A and Fig. 16B Properties of the sample diffusers fabricated by the process. DETAILED DESCRIPTION

[0101] 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.

[0102] 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.

[0103] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. For the purposes described, the projection system PS may include 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 , but the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0104] The substrate W may include a previously formed pattern. In such a case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B′ with the pattern previously formed on the substrate W.

[0105] 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.

[0106] 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.

[0107] 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 while a pattern imparted 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.

[0108] As already described above, the lithographic apparatus can be used to expose parts of a substrate W in order 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 the 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 the 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 portion of the substrate W. This may be particularly important when patterned radiation is projected onto a substrate W including a portion that has been exposed to radiation in order to improve the alignment of the patterned radiation with the previously exposed area. Additionally or alternatively, measurements can be made to determine the deformation of the pattern forming device MA.

[0109] Additionally or alternatively, measurements may be taken to determine optical aberrations of the projection system PS. Optical aberrations are deviations in the performance of the optical system from paraxial optics and may result in 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.

[0110] This can be accomplished by irradiating the reflective marker 17 (e.g. Figure 1 ) to perform measurements, such as the alignment and aberration measurements described above. In an alternative arrangement, a transmissive marker may be used. A marker is a reflective feature that appears in an image produced by an optical system when the marker is placed in the field of view of the optical system. The reflective markers described herein are suitable for use as reference points and / or as measures of properties of an image formed by an optical system. For example, radiation reflected from a reflective marker can be used to determine the alignment of one or more components and / or the optical aberrations of one or more components.

[0111] exist Figure 1In 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 for measuring one or more properties of the lithographic equipment LA can be placed on the support structure MT in order to perform a measurement process. The pattern forming device MA may include one or more markers 17 for performing irradiation as part of the measurement process.

[0112] exist Figure 1 In the embodiment shown in , the lithographic apparatus LA is an EUV lithographic apparatus and thus uses a reflective pattern forming device MA. The mark 17 is thus a reflective mark 17. The configuration of the mark 17 may depend on the properties of the measurement to be performed using the mark 17. The mark may, for example, comprise one or more reflective pinhole features comprising a reflective region surrounded by an absorptive region, a reflective line feature, an arrangement of a plurality of reflective line features, and / or a reflective grating structure such as a reflective diffraction grating.

[0113] 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 1 As 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 mark 17 on the patterning device MA is illuminated by radiation. The substrate table WT may be positioned so that radiation reflected from the mark is projected by the projection system PS onto the sensor device 19. 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 marks 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).

[0114] 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 with 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.

[0115] can be provided by a device located at the level of the substrate W (e.g., Figure 1 The sensor device 19 may be operable to detect the position of the alignment feature in radiation incident on the sensor device 19. This may allow the substrate table WT to be aligned relative to a marking on the patterning device MA to be determined. Knowing the relative alignment of the patterning device MA and the substrate table WT, the patterning device MA and the substrate table WT may be moved relative to each other so as 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.

[0116] As further described above, in some embodiments the patterning device MA may be provided with one or more marks 17 that may 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 marks 17 using a sensor device 19 located at or close to the substrate table WT. The one or more marks 17 on the patterning device MA may be illuminated with EUV radiation by the illumination system IL. Radiation reflected from the one or more marks is projected by the projection system PS onto an image plane of the projection system PS. The one or more sensor devices 19 are positioned at or close to the image plane (e.g., such as a Figure 1 ) and the projected radiation can be measured in order to determine the aberrations of the projection system PS. Figure 2 and Figure 3A , embodiments of an identification 17 and a sensor device 19 that may be used to determine aberrations of the projection system PS are described.

[0117] Figure 2 is a schematic diagram of a symbol 17 that may form part of a patterning device MA according to an embodiment of the present invention. Figure 2A 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 mark 17 is typically located in the xy plane. That is, the mark typically extends in a direction perpendicular to the z-direction. Although reference is made to marks that are typically located in a plane, it will be understood that the marks are not completely constrained to / limited to a plane. That is, parts of the mark may extend outside the plane in which the mark is typically located. As will be further explained below, the mark may include a diffraction grating. The diffraction grating may include a three-dimensional structure, the three-dimensional structure including parts that are not completely in the plane but instead extend outside the plane.

[0118] Figure 2 The logo 17 shown in FIG. 1 includes a first portion 17a and a second portion 17b. Both the first portion and the second portion include a reflective diffraction grating, which includes a periodic grating structure. The grating structure extends in a grating direction. The first portion 17a includes 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.

[0119] Although Figure 2 The embodiment shown in FIG. 1 includes a first portion 17a and a second portion 17b comprising a diffraction grating oriented perpendicular to the grating direction, but in other embodiments, the mark 17 may be arranged in other forms. For example, the mark 17 may include reflective areas and absorptive areas arranged to form a checkerboard pattern. In some embodiments, the mark 17 may include an array of pinhole features. A reflective pinhole feature may include an area of ​​reflective material surrounded by an absorptive material.

[0120] When the first part 17a and / or the second part 17b of the logo is illuminated by radiation, a plurality of diffraction orders are reflected from the logo. At least a part of the reflected diffraction orders enters the projection system PS. The projection system PS forms an image of the logo 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 3BCartesian coordinates are also shown in .

[0121] 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 markers 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.

[0122] The sensor device 19 comprises a transmissive 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 As indicated in .

[0123] Figure 3BThe diffraction grating 21 shown in 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-type 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 generally a rectangle) with rounded corners, i.e., 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.

[0124] The radiation sensor 23 is configured to detect a spatial intensity distribution 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 an image aberration, the support structure MT may be positioned such that the mark 17 is illuminated with radiation from the illumination system IL. The substrate table WT may be positioned such that radiation reflected from the mark is projected onto the sensor device 19 by the projection system PS.

[0125] 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.

[0126] 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 gradient of the wavefront along a first direction. Radiation reflected from the second portion 17b of the marker can provide information related to the gradient of the wavefront along a second direction, which is perpendicular to the first direction. In some embodiments, the first and second portions of the marker can be illuminated at different times. For example, the first portion 17a of the marker 17 can be illuminated at a first time to derive information related to the gradient of the wavefront along the first direction, and the second portion 17b of the marker 17 can be illuminated at a second time to derive information related to the gradient of the wavefront along the second direction.

[0127] In some embodiments, the patterning device MA and / or the sensor device 19 may be sequentially 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 mark 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 mark 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.

[0128] The patterning device MA and / or the sensor device 19 may be stepped by 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 in order 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 deriving information about the wavefront in two perpendicular directions, thereby allowing reconstruction of the complete wavefront.

[0129] 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 diffraction grating being illuminated (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 diffraction grating being illuminated. For example, the first part 17a of the mark 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 mark 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 diffraction grating being illuminated allows the measurements to be averaged across 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.

[0130] 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 the determination of wavefront phase changes in both the u-direction and the v-direction. The arrangement of the diffraction grating forming the identification 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 in order to determine the wavefront changes.

[0131] 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 be in 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.

[0132] Although in the embodiment 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.

[0133] 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 device PA for adjusting components of the projection system PS. For example, the adjustment device PA may adjust elements of the projection system PS so as to correct aberrations determined by the controller CN.

[0134] The projection system PS comprises a plurality of reflective lens elements 13, 14 and an adjustment device PA for adjusting the lens elements 13, 14 so as to correct aberrations. To achieve such correction, the adjustment device PA may be operable to manipulate the reflective lens elements within the projection system PS in one or more different ways. The adjustment device 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.

[0135] The projection system PS has an optical transfer function which may be inhomogeneous, which may affect the pattern imaged onto the substrate W. For unpolarized radiation, these effects may be very well described by two scalar maps or scalar diagrams which describe the transmission (apodization) and relative phase (aberrations) of the radiation exiting the projection system PS as a function of position in a pupil plane of the projection system PS. 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 appreciated 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 convenient set of basis functions for expressing these scalar maps are the Zernike polynomials, which form a set of orthogonal polynomials confined to 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 successively calculating the inner product of the measured scalar mapping with each Zernike polynomial and dividing this inner product by the square of the norm of the Zernike polynomial.

[0136] 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.

[0137] 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).

[0138] 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 referred to as a Noll index. A Zernike coefficient with a Noll index of 1 may be referred to as a first Zernike coefficient, a Zernike coefficient with a Noll index of 2 may be referred to as a second Zernike coefficient, and so on.

[0139] 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 as such, 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).

[0140] Throughout this specification, the term "aberration" is 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.

[0141] 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 separate 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 outside a patterning 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.

[0142] One or more markings 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 markings suitable for determining the alignment of the measurement patterning device relative to the substrate table WT.

[0143] In some embodiments, the same identifier can be used to determine both alignment and aberrations. For example, one or more identifiers 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, the same set of measurements can be used to simultaneously determine both alignment and aberrations.

[0144] 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 for use 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 identifiers for a measurement process. Typically, the pattern forming device MA is a removable component placed on the support structure MT in order to perform a specific process (e.g., to perform lithographic exposure and / or to 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., identifiers) for a measurement process. For example, the support structure MT may be provided with one or more fiducials including one or more identifiers. 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 identifiers herein should not be interpreted as being limited to removable pattern forming devices, but should be interpreted as including any device having a reflective identifier disposed thereon.

[0145] 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'.

[0146] 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 so as to form an image in an image plane of an object in the object plane. Within such an optical system, between each pair of field planes a pupil plane may be defined which is 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 an 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 that such a pupil plane may even be curved. This applies for 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).

[0147] 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.

[0148] 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 silicon wafer coated with resist). 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, where the illumination system PP is a local illumination pattern. 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). Ring 34 has a radius σ=1 in coordinates normalized by the numerical aperture NA of the projection system PS. Dipole distribution 30 includes two diametrically opposed polar regions 36 in which the intensity is non-zero. 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.

[0149] When the lithographic apparatus is not exposing the substrate W, one of the more reflective markers provided 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 markers to measure the alignment and / or aberrations, it can be expected that the radiation reflected from the markers 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 entrance 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 required for such an in-line or online measurement. Therefore, it is known to provide a diffuser during such a measurement, the diffuser being arranged to increase the angular spread, i.e. the angular spread, of the radiation scattered from the object horizontal pattern forming device so as to increase the proportion of the entrance pupil of the projection system PS that is filled by the radiation.

[0150] 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 in-line metrology, i.e. online 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 prior to exposing the substrate W.

[0151] At 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 membrane 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 passes after reflection through the grating membrane (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.

[0152] Another arrangement uses a reflective object, i.e. a combined diffuser and patterning device, as described in WO2017 / 207512. This arrangement is in the form of a multilayer reflective stack arranged to preferably reflect EUV radiation, to which is applied a pattern of EUV absorbing material (e.g. a diffraction grating). The layers in the multilayer reflective stack have a surface roughness such that the reflected radiation is diffused. However, while in principle such a patterning device could 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.

[0153] Embodiments of the present invention relate to novel diffusers and methods for their preparation that are particularly suitable for use with EUV measurement systems within EUV lithography apparatus of the type discussed above.

[0154] FIG. 5A to FIG. 5C (collectively, FIG. 5 ) schematically illustrates stages in a method of making a diffuser according to a first example. The diffuser may be constructed from a plurality of layers, which are referred to herein as a stack. FIG. 5A to FIG. 5C An intermediate stack of multiple layers in one example process for creating a diffuser is depicted in cross section. Figure 5A, the first intermediate stack 50 includes a support material layer 502. For example, the support material may include silicon nitride (SiN), silicon, molybdenum silicide (MoSi2). The support material layer 502 may have a thickness of about 10nm to 60nm. 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 can be considered to be relatively optically neutral for EUV radiation. The support material layer 502 is formed on a carrier layer 504, which can be used to support the support material layer 502 while forming the diffuser. For example, the carrier layer 504 can be formed of silicon, silicon nitride (SiN), porous silicon (pSi) or molybdenum silicide (MoSi). The carrier layer 504 can, for example, have any thickness suitable for providing sufficient support during manufacturing, and in some arrangements, can have a thickness of about 100μm to 500μm. For example, the carrier layer can be a standard silicon wafer. Alternatively, the carrier layer 504 and support layer 502 may be provided by a single layer of the same material.

[0155] A scattering material layer 506 is disposed on the support layer 502. The scattering material may be, for example, a substance such as molybdenum, ruthenium, or niobium, but may be other suitable scattering materials, as discussed in further detail below. Depending on the particular scattering material, the scattering material layer 506 may, for example, have a thickness (in the depicted z-direction) between approximately 50 nm and 400 nm.

[0156] Another layer 508 of another different metal is deposited on top of the intermediate stack 50 to form a second intermediate stack 52. For example, the other metal can be zinc (Zn). Layers 506 and 508 are processed to form an alloy layer (not shown) comprising an alloy of a scattering metal and another metal (e.g., a molybdenum-zinc alloy). The scattering material 506 provides a first component of the alloy, while the other metal 508 provides a second component of the alloy. For example, layers 506, 508 can be annealed. Annealing can be performed, for example, at 400 degrees. Annealing can be performed in a protective gas environment. For example, annealing can be performed in the presence of an inert gas such as argon.

[0157] The resulting alloy layer is subjected to a dealloying process to selectively corrode the second component of the alloy. For example, where the second component is zinc, the dealloying comprises a dezincification process. The dealloying may be performed by any suitable method. For example, the dealloying may include selectively dissolving zinc by immersion in an acid such as nitric acid.

[0158] After the dealloying, i.e., dealloying, process, an intermediate stack 54 is provided, which includes a porous scattering layer 510 on a support layer 502. The scattering layer 510 can be considered as a scattering substance having a plurality of voids distributed therein. The method can also include etching the support layer 504 from a surface opposite to a surface of the support layer supporting the porous scattering layer 510. In the case where the support layer 502 and the support layer 504 are separate, i.e., individual layers, the support layer 502 can provide an etch stop during this back etching process. This back etching of the support layer 504 allows the use of thicker, more stable support layers 502 and 504 during manufacturing. Advantageously, this can prevent the support layer 504 from being damaged or even broken.

[0159] Alternatively, the porous scattering layer 510 may contain another substance located within the plurality of pores (or voids). For example, the plurality of pores of the porous scattering layer 510 may be filled with an inert gas. Alternatively, the plurality of pores of the porous scattering layer 510 may be filled with a vacuum. For example, where the scattering layer 510 is subsequently capped (discussed in more detail below), the capping may be performed in an atmosphere of an inert gas, or in a vacuum. Alternatively, the porous scattering layer 510 may be infiltrated (by any suitable process, such as, for example, ALD, CVD, or sputtering) with an optically distinct material (e.g., a material that is relatively optically neutral with respect to EUV radiation, e.g., a material having a refractive index of 1 or substantially close to 1). Such infiltration or wetting of the porous scattering layer 510 may be beneficial for protection from degradation, for protecting structural integrity, and for allowing heat diffusion.

[0160] Although in practice a diffuser is likely to comprise multiple layers (such as a scattering layer and a support layer), the term diffuser herein may also refer to the scattering layer only (ie the layer configured to diffuse incident radiation).

[0161] FIG. 6A to FIG. 6C (collectively, FIG. 6 ) depict another example process for making a diffuser suitable for EUV radiation. FIG. 6A to FIG. 6C An intermediate stack of multiple layers in the example process is depicted in cross section. Fig. 6A , the first intermediate laminate 60 includes a support layer 602 and a carrier layer 604. The support layer 602 and the carrier layer 604 may be combined as described above. FIG. 5A to FIG. 5C The support layer 502 and the carrier layer 504 are described.

[0162] The intermediate stack 60 also includes a porous layer 606 formed of a material that has been processed to form a structure. For example, the porous layer 606 may include silicon or porous silicon. The process used to create the porous layer 606 may include, for example, selective etching (e.g., metal-assisted chemical etching, anodizing, selective leaching / leaching).

[0163] A scattering material is deposited onto the porous layer 602 to form the second intermediate layer 62, such that the scattering material at least partially occupies a plurality of pores (or voids) within the porous layer 602 to thereby form a scattering layer 608. Depending on the scattering material used, the scattering layer 604 may have a thickness between approximately 50 nm and 1000 nm. The scattering layer 604 may be considered to provide a first substance having voids distributed therein, at least some of the voids being filled with the scattering material.

[0164] As described with respect to the previous example process, the method may also include etching the carrier layer 604 from a surface opposite to supporting the scattering layer 604 to provide another stack 64 (which may be a final stack or may be another intermediate stack). It should be appreciated that in other examples discussed below, although a carrier layer is not depicted, a carrier layer may be provided and may be etched after the scattering layer has been provided on the support structure.

[0165] In addition, in all examples described herein, in addition to those layers shown in Figures 5 and 6, additional layers may be provided. For example, with reference to Figure 5 as an example, another layer may be provided between the support layer 502 and the scattering layer 510 or between the carrier layer 504 and the support layer 502. The additional layer may be useful in providing additional protection to the scattering layer during use, in particular protection from particles present inside the lithographic apparatus. Similarly, for the same purpose, an additional (or "cap") layer may be provided on top of the scattering layer 510. Such an additional layer may have a thickness of about 10 nm. Such an additional layer formed forms a metal oxide or a metal nitrate. For example, the additional layer may be provided from silicon nitride or molybdenum silicide.

[0166] 7A to 7E Another example process for making a diffuser suitable for EUV radiation is depicted (collectively, FIG. 7 ). In the example of FIG. 7 , an inhomogeneous layer of scattering material is deposited on a random or quasi-random arrangement of a plurality of structures (such as guide posts on the surface of a support layer or holes in the surface of a support layer). The structures may be provided according to any suitable technique. For example and as depicted in FIG. 7 , the structures may be provided by nanoparticle lithography. Alternatively, the structures may be created using normal (e.g., resist) lithography using pseudo-random masks, dealloying using selective leaching / leaching, metal-assisted chemical etching using random deposition of metal catalyst particles, and the like.

[0167] In the example depicted in Figure 7, the intermediate stack 70 includes a support layer 702. For example, the support layer 702 can take the same or similar form as the support layers 502, 602 described above with reference to Figures 5 and 6. Although not depicted in Figure 7, it should be understood that the intermediate stack 70 can include a carrier layer, which can take the same or similar form as the carrier layers 504, 604 described above.

[0168] Nanoparticle layer 704 is deposited on support layer 702 in a random or quasi-random distribution. The particles in layer 704 may be formed of polystyrene particles. Alternatively, the particles in layer 704 may be formed of another material suitable for nanosphere lithography, such as latex or silica, cellulose, etc. Fig. 7A In the example depicted in, the particles in layer 704 are polydisperse, including multiple particles with different size ranges. In particular, for example, some particles 704 may have a diameter smaller than the diameter of other particles 704. As an example, the particles may have a diameter in the range of 20nm to 300nm. The particles have a random distribution of displacements of adjacent particles. The particles in layer 704 may take the form of spheres. The particles may be applied to the support layer 702 in any suitable manner. For example, the particles may be applied using a vertical deposition process from a colloid or colloid containing the particles. For example, a Langmuir-Blodgett deposition process may be used, as generally described, for example, in Langmuir, 20042041524-1526, December 25, 2003 (https: / / doi.org / 10.1021 / la035686y). The vertical deposition process is suitable for providing a monolayer of polystyrene particles. However, it should be understood that the deposition may be based on any method that is suitable for producing a monolayer or a small number of layers. For example, deposition may be by spin coating or ink jetting the particles in a solvent. The particles may be spherical or substantially spherical (eg, the particles may be a plurality of ellipsoids). However, the particles may have other shapes.

[0169] like Figure 7B As depicted in FIG. 7 , the particles in layer 704 may be shrunk to provide a second intermediate stack 72. Shrinking of the particles in layer 704 is an optional step and may be beneficial to further expose areas of the support layer 702, thereby allowing the dimensions (size, spacing, density) of the created structures to be adjusted, as described in more detail below. For example, the particles may be processed using reactive ion etching (RIE), which causes each of the particles in layer 704 to shrink.

[0170] Whether or not the particles in layer 704 shrink, the particles in layer 704 are operable to provide a mask on the surface of support layer 702. It will be appreciated that if the arrangement of particles on the surface is random or quasi-random, the mask provided by those particles will also be random or quasi-random.

[0171] The catalyst is deposited onto the surface of the support layer 702 adjacent to the particle layer 704 so that the portion of the surface not shielded by the particles is coated with a deposit 712 of the catalyst. The catalyst can be a metal catalyst, such as gold or platinum. The particles in the layer 704 are removed, and the surface of the support layer 702 on which the catalyst is deposited is selectively etched to form a modified support material layer 714 and a third intermediate stack 74. In practice, the position of the support layer 702 in contact with the deposit 712 of the catalyst is etched to create multiple structures (or features) on the surface of the support material in contact with the catalyst. This type of metal-assisted catalytic etching is also a known and stable, i.e., robust process. The structure in this example includes multiple cavities or pits with corresponding peaks or guide pillars.

[0172] In an alternative, a process may be used whereby the mask is deposited onto the support layer 704 between the particles and the surface of the support layer that is etched in those areas not protected by the mask. Where the process depicted in FIG. 7 creates cavities in the area below the catalyst, this alternative process may be thought of as creating guide pillars in the area below the protective mask. Any suitable mask material and etching process may be used, as will be well known to those skilled in the art.

[0173] After creating the modified support structure 714, the catalyst or mask may be removed to provide the fourth intermediate layer stack 76, although it will be appreciated that this is an optional step.

[0174] A scattering material 716 may then be deposited onto the modified support structure 714, the scattering material being formed within and around the structures provided on the surface of the modified support structure 714 to provide another stack 78 (which may be the final stack or may be another intermediate stack). Due to the structures present on the modified support structure 714, the scattering material 716 acts as an array of micro-lenses, causing scattering of EUV radiation incident on the diffuser created therefrom. The lens formation is (in part) a result of shadows caused by the presence of the structures. Thus, shadows may be increased by directing the stream of particles of the scattering material 716 at an angle other than 90 degrees to the surface of the substrate.

[0175] Although not depicted, as in previous examples and where a carrier layer is provided, the carrier layer may be back etched. Additionally or alternatively, a portion of the support structure 714 (particularly from a surface opposite to the surface on which the scattering material 716 is deposited) may be etched to provide a diffuser.

[0176] FIG. 8A to FIG. 8D (collectively, FIG. 8 ) depict another example process for fabricating a diffuser suitable for use with EUV radiation. Fig. 8A A first intermediate stack 80 is depicted, comprising a support layer 802 on which is deposited a polydisperse monolayer of nanoparticles 804. The nanoparticles in layer 804 may be the same as discussed above with respect to the nanoparticles in layer 704, and may be deposited by any process. For example, nanoparticles 704 may be formed of polystyrene and may be deposited in a random or pseudo-random distribution on support layer 802 using a vertical deposition process.

[0177] The second intermediate stack 82 is created by depositing a scattering material layer 806 on the support layer 802 between the nanoparticles 804. For example, the scattering material layer 806 can be deposited by means of electrodeposition. In this way, the nanoparticles 804 form a mask on the support layer 802 so that the scattering material 806 is formed in the gaps around the particles 804 to provide an uneven scattering material layer. The nanoparticles 804 can optionally be shrunk before the scattering layer is deposited to change the spacing between the nanoparticles and expose more of the support layer 802.

[0178] The nanoparticles 804 are optionally removed to provide a third stack 84. The third stack 84 can be used to provide a diffuser (e.g., after any required backside etching of a carrier layer (not shown)). It will be appreciated that the scattering material 806 forms a rippled or wavy structure on the support layer 802. The rippled structure is defined by peaks and valleys, and the spacing between adjacent peaks is defined by the size of the nanoparticles 804 or multiple nanoparticles 804 that separate those peaks during manufacture of the diffuser. Similarly, the depth of the valley (i.e., in the direction of propagation of the radiation beam) is defined by the shape and depth of the nanoparticles 804 and the depth to which the scattering material layer 806 is deposited around the nanoparticles 804 (which can vary depending on the desired scattering / attenuation properties and the specific scattering material used).

[0179] The ripples will match the distribution of the nanoparticles 804, so that the ripples can be randomly or quasi-randomly distributed across the entire scattering layer and have multiple different ranges in each dimension. For example, some of the multiple peaks can have different ranges in any of the x, y, or z directions than other peaks in the multiple peaks, and the spacing between any pair of adjacent peaks (in the x or y direction) may be different from the spacing between any other pair of adjacent peaks. In addition, it should be understood that due to the different sizes of the nanoparticles 804, the ripples will have different curvatures (e.g., the ripples will have different gradients).

[0180] Additionally, an optional second layer of scattering material may be provided, such as Fig.8DAs depicted in FIG. 1 , in this example, an intermediate layer 808 is deposited on top of the scattering layer 806 to create another intermediate stack 86. The intermediate layer 808 can be formed of a material that is relatively optically neutral with respect to EUV radiation (e.g., has a refractive index close to 1 with respect to EUV radiation and has a relatively low absorption coefficient with respect to EUV radiation). For example, the intermediate layer 808 can be formed of silicon. The intermediate layer 808 can have a thickness in the range of 30 nm to 400 nm, and preferably in the range of 30 nm to 150 nm. The process can then be repeated. FIG. 8A to FIG. 8C 8 to form a second scattering layer 810 on the intermediate layer 808. It will be appreciated that the second scattering layer will provide additional scattering of the incident EUV radiation when used as a diffuser and will help prevent or reduce zero order scattering.

[0181] 9A to 9E (collectively, FIG. 9 ) depict another example process for making a diffuser suitable for EUV radiation. Fig.9A As shown in FIG. 1 , the first intermediate laminate 90 is formed with Fig. 8A The second intermediate stack 92 ( FIG. 8 ) is created by depositing an intermediate (or sacrificial) layer 908 between the first scattering layer 906 and another scattering layer 910. Fig. 9B ). The intermediate layer 908 may be formed of a material suitable for selective etching or any other removal process that leaves the remaining elements of the stack intact.

[0182] A third intermediate stack 94 is created by removing the nanoparticles 904, thereby leaving cavities within the layers 906, 908, 910 ( Fig. 9C ). The nanoparticles 904 may be removed by any suitable technique in the field sometimes referred to as "nanoparticle lithography", and indeed any other suitable technique as will be apparent to those skilled in the art. By way of example only, the nanoparticles 904 may be removed by heating. The fourth intermediate layer stack 96 ( Fig.9D For example, the cavities may be filled with silicon (e.g., via a silicon infiltration process using liquid silicon, via deposited silicon (some of which will fill some of the cavities), or via any other suitable method). The material within the cavities thereby forms an intermediate support structure 912 that supports the scattering layer 910 above the scattering layer 906.

[0183] The fifth stack 98 is created by removing the middle layer 908 ( Fig.9E). For example, the intermediate layer 908 can be removed by etching. The stack 98 thus includes two scattering layers 906, 910 separated and supported by a relatively sparse intermediate support structure 912 (i.e., separate particles). As in the previous example, the process of FIG. 9 can be repeated to create additional multiple layers on top of the scattering layer 910.

[0184] In an alternative arrangement, the nanoparticles 904 may be made of, for example, silicon. In such a case, the nanoparticles need not be removed prior to removing the sacrificial layer. In another alternative arrangement, both the nanoparticles and the sacrificial layer may be formed of silicon. In such a case, Fig. 9B The stack 92 depicted in can be considered the final stack and can be used to provide a diffuser (after any other required processing such as backside etching or deposition of a cap layer). That is, in some arrangements, the combination of layers 910, 908, 906 and nanoparticles 904 can together provide a scattering layer of a diffuser.

[0185] FIG. 10A to FIG. 10E Another example process for creating a diffuser suitable for EUV radiation is schematically depicted. Fig. 10A In FIG. 1 , an intermediate stack 100 is shown. The intermediate stack 100 comprises a support layer 1002 on which a nanoparticle layer 1004 is deposited. The intermediate stack 100 may be an intermediate stack 70, 80, 90, and the intermediate stack 100 may be produced according to the intermediate stacks 70, 80, 90.

[0186] The second intermediate stack 102 is created by depositing a relatively optically neutral material (e.g., silicon) onto the surfaces between and around the nanoparticles 1004. The top portions of at least some of the plurality of nanoparticles remain above the highest level of the optically neutral material. The optically neutral material thereby forms a filler layer 1006. Optionally, the nanoparticles 1004 may be treated to shrink the nanoparticles 1004 prior to the deposition of the filler layer 1006 to further expose portions of the support layer 1002.

[0187] The third intermediate layer 104 is created by removing the nanoparticles to leave a depression or cavity within the filler layer 1006. The nanoparticles 1004 may be removed according to any suitable technique as described above and will depend on their composition.

[0188] The fourth stack 106 is created by filling the cavities within the filler layer 1006 with a scattering material to form a plurality of scattering particles 1008 within the filler layer. The fourth stack 106 may be used to provide a diffuser (e.g., after any required back etching of the carrier layer and / or support layer 1002). Alternatively, the fourth stack 106 may be an intermediate stack, and another intermediate stack 108 may be created by depositing another layer 1010 of a relatively optically neutral material (which may be the same material as used for the filler layer 1006 (e.g., silicon), or may be different. The another layer 1010 provides support to create another scattering layer (e.g., using FIG. 10A to FIG. 10D or another process taught herein or elsewhere).

[0189] FIG. 11A to FIG. 11C (collectively, FIG. 11 ) depict another example process for creating a diffuser suitable for EUV radiation. Fig.11A In the embodiment of the present invention, the intermediate stack 110 includes a support layer 1102 on which a random or quasi-random multi-layer deposit of polydisperse nanoparticles 1104 (such as polystyrene particles) is provided. The multi-layer deposit of nanoparticles 1104 can be disposed on the support layer 1102 in any suitable manner as previously described herein, such as by vertical colloidal deposition. The nanoparticles 1104 can have a packing density of about 60% to 70% within the volume occupied by the nanoparticles 1104. That is, for the volume occupied by the nanoparticles, 60% to 70% of the volume can be occupied by the nanoparticles, and the remaining 30% to 40% is void space.

[0190] The second intermediate stack 112 is created by infiltrating the voids between the nanoparticles 1104 with a scattering material 1106. The scattering material 1106 can be provided according to any suitable method. Example methods of providing the scattering material 1106 include atomic laser deposition (ALD) and electrodeposition (e.g., as described in Fabrication and optical characterization of polystyrene opal templates for the synthesis of scalable, nanoporous (photo) electrocatalytic materials by electrodeposition (J. Mater. Chem. A, May 2017, 11601-11614)).

[0191] Alternatively, the third stack 114 is created by removing the nanoparticles 1104 to leave gaps 1108 in the scattering material 1106. For example, the nanoparticles can be removed by heating the second stack 112 at a sufficiently high temperature (e.g., 500 degrees) that the nanoparticles evaporate. In the example process described in Figures 7 to 11, nanoparticles are used to create a scattering structure / layer. In the intermediate stage of the manufacturing process, the nanoparticles can be removed. For example, when the nanoparticles are polystyrene particles, the nanoparticles can be removed by heating and evaporation. As indicated above, other types of nanoparticles can be used to replace polystyrene, such as silica, cellulose, etc., which can also be removed by evaporation. As an alternative, titanium oxide (TiO2) nanoparticles can also be used, and the titanium oxide nanoparticles can be removed by, for example, selective etching.

[0192] In some example processes, the nanoparticles may not be removed.

[0193] The nanoparticles may be made of materials other than polystyrene. In another example, the nanoparticles may be made of a material that is relatively optically neutral to EUV radiation, such as silicon. In the case where the nanoparticles are made of, for example, silicon (or another optically neutral material, or a material that provides a large difference in refractive index compared to the scattering material 1106), it may be beneficial to retain the nanoparticles for the final diffuser. This provides another benefit of requiring fewer processing steps.

[0194] More generally, in the above examples, multiple stacks of materials are created to provide a diffuser suitable for EUV. As will be appreciated by those skilled in the art, the methods of creating specific material layers (e.g., scattering layers, intermediate layers, particle layers, and processes such as aerosol deposition, vertical deposition, electrodeposition, etc.) described in the context of one example can be used in any other example. In addition, the embodiments described above provide methods for creating scattering surfaces or structures in multiple stacks. It should be understood that any one or more of the processes described above can be combined to form multiple stacks with multiple scattering layers or structures. For example, as referenced Figure 8C The described scattering layers may be disposed on top of the porous scattering structure 510 described with reference to Figure 5. Any other combination of scattering layers is possible and should be understood to be within the scope of the present disclosure.

[0195] Additionally, while different layers are generally described as having different thicknesses, it will be appreciated that those thicknesses may vary depending on the materials used within the layers and the desired optical interaction of the layers with the incident EUV radiation, if any. However, in general, in each example, the diffuser layers (or scattering layers, i.e., those layers configured to scatter the incident EUV radiation) may have a total combined thickness along the direction of propagation of the received radiation of approximately between 100 nm and 1000 nm.

[0196] It will be appreciated that a diffuser manufactured according to the process described herein will be a transmissive diffuser for EUV radiation. In general, in order to maximize the intensity of EUV radiation output by the diffuser, it is desirable to minimize the attenuation caused by the scattering material layer. 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 angular dispersion, i.e. the amount of angular spread, 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. Having a scattering material with a larger value of (1-n) allows the thickness to be reduced (while still providing reasonable angular dispersion). Having a scattering material with a smaller extinction coefficient k for EUV radiation allows the thickness to be increased (while still providing reasonable transmission).

[0197] Suitable materials for the scattering material layer include molybdenum, ruthenium, yttrium, rhodium, technetium or niobium. Fig.13 Plots of the extinction coefficient k for EUV radiation versus the magnitude of (1-n) for EUV radiation are shown for some of these three materials and for carbon and silicon.

[0198] 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., Fig.13 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, Fig.13 In some embodiments, the magnitude of (1-n) of the scattering material for EUV radiation may be greater than a threshold of 0.1 (ie, Fig.13 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, Fig.13 to the right of line 66 in FIG.

[0199] 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 of Fig.13 In some embodiments, the scattering material may have an extinction coefficient k of 0.03 nm for EUV radiation. -1 The threshold of Fig.13 In some embodiments, the scattering material may have an extinction coefficient k of 0.02 nm for EUV radiation. -1 The threshold of Fig.13 In some embodiments, the scattering material may have an extinction coefficient k of 0.01 nm for EUV radiation. -1 The threshold of Fig.13 The extinction coefficient k is below line 76 in FIG.

[0200] 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. Having a scattering material with a larger magnitude of (1-n) allows the thickness to be reduced (while still providing reasonable angular dispersion). Having a scattering material with a smaller extinction coefficient k for EUV radiation allows 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.

[0201] 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 greater than 0.01 nm. -1 or the magnitude of (1-n) is greater than the threshold of 0.08 and the magnitude of the extinction coefficient k for EUV radiation is greater than 0.02 nm -1 or the magnitude of (1-n) is greater than the threshold of 0.1 and the magnitude of the extinction coefficient k for EUV radiation is greater than 0.03 nm -1 or the magnitude of (1-n) is greater than the threshold of 0.12 and the magnitude of the extinction coefficient k for EUV radiation is greater than 0.04 nm -1 The threshold of is smaller. That is, Fig.13 The material is found in the cross-hatched area.

[0202] In some embodiments, the magnitude of (1-n) and the extinction coefficient k for EUV radiation satisfy the following relationship:

[0203]

[0204] where |1-n| is the magnitude of (1-n). This is equivalent to being lower than Fig.13 Line 80 in.

[0205] Embodiments described herein provide one or more layers (referred to herein as scattering layers, scattering structures, etc.) of scattering material that causes scattering and has nanostructures formed thereon or therein. The layer (or layers) of scattering material may act as a random array of microlenses, thereby causing scattering of EUV radiation incident on a diffuser including the scattering layer. This is particularly advantageous for EUV radiation (which may, for example, have a wavelength of 13.5 nm) because such nanostructures include features having a size comparable to or less than the wavelength of the radiation desired to be diffused. Under these conditions, the scattering is in the Mie-scattering regime and significant angular dispersion may be achieved. For example, in some embodiments, the nanostructures formed in the scattering material layer include features having a size in the range of 2 nm to 220 nm.

[0206] Fig.12 The diffuser 120 is schematically shown. The diffuser 120 includes the remnants of a carrier layer 1202 that has been back-etched to allow radiation to pass through the diffuser 120. In other embodiments, the entire carrier layer may have been back-etched. The diffuser 120 also includes a support layer 1204 and a cover layer 1206. A scattering layer 1208 is between the support layer 1204 and the cover layer 1206. In the depicted example, the scattering layer 1208 takes the form of Fig. 11C 1208, but it will be appreciated that the scattering layer 1208 may take any form as described herein. In use, radiation 1210 is incident on the diffuser 120, propagating generally in the depicted z-direction. This incident radiation 1210 may correspond to the 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 incident angles, and Fig.12 The arrow 1210 shown in FIG. 12 may indicate the direction of the principal ray. The scattering layer 1208 causes this incident radiation to be spread over a larger angular range. This is schematically indicated by arrow 1212.

[0207] When in use, the diffuser 120 can be used to increase the angular range of radiation having the angular range reflected from the object horizontal mark entering the projection system PS. In particular, each portion of the diffuser 120 can be expected to cause a divergence of radiation 1210 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, which corresponds to an angular range of approximately 7°. Therefore, the microlenses provided by the scattering layer 1208 can be expected to cause a divergence of radiation 1210 of approximately 7°. This ensures that each field point on the pattern forming device MA receives radiation from approximately the entire angular range within the cone having a full angular range of approximately 7°. Equivalently, this ensures that the pattern forming device is illuminated with approximately complete pupil filling. For some applications, such as in dipole illumination (such as Figure 4A In the case of (depicted in FIG. 1 ), a diffuser may preferably be provided so that the divergence of radiation 1210 is approximately degrees to provide substantially complete pupil filling. In another embodiment, the numerical aperture of the patterning device MA (and the projection system PS) in the lithographic apparatus may be higher than 0.08, for example, a numerical aperture of 0.16 radians corresponds to an angular range of approximately 9°.

[0208] In some embodiments, the diffuser 120 may have a certain thickness (in Fig.12 The thickness is arranged such that EUV radiation 1210 propagating across the entire thickness of the diffuser 120 produces a phase shift of (2m+1)π radians. Advantageously, this suppresses zero-order (or specular) scattering.

[0209] Table 1 below lists a variety of example materials that can be used as scattering materials. In Table 1, n is the refractive index with respect to radiation having a wavelength of 13.5 nm (e.g., EUV radiation), k is the extinction coefficient of the material for radiation having a wavelength of 13.5 nm, Lt indicates the maximum thickness (in the propagation direction of the radiation) of a layer of the material that will attenuate incident radiation by at most 90%, and Lr is the minimum thickness of the layer of the material that would be required for a phase shift of pi radians. The row Lr / Lt is a ratio indicating the balance between diffusion potential and attenuation for each material.

[0210]

[0211] Table 1

[0212] As can be seen from Table 1, there are a variety of materials that will provide adequate transmission and adequate scattering for a particular material thickness. In particular, those materials having a ratio Lr / Lt of less than 1 may be considered to provide suitable candidates. Lower Lr / Lt ratios may indicate a preferred material, but it should be appreciated that other considerations may apply, such as ease of working, acquisition, longevity, etc.

[0213] As described above, some examples include a diffuser comprising a plurality of scattering layers, each layer being arranged to differently change the angular distribution of EUV radiation passing therethrough. Advantageously, by providing a plurality of layers, each layer being arranged to differently change the angular distribution of EUV radiation passing therethrough, the diffuser provides an arrangement whereby an EUV radiation beam can be more efficiently diffused across a desired angular range. Additionally, a plurality of layers that differently change the angular distribution of EUV radiation passing therethrough provides more control over the angular distribution of radiation exiting the diffuser.

[0214] FIG. 14A to FIG. 14C Another example process for making a diffuser suitable for EUV radiation is described. FIG. 14A to FIG. 14C 1 depicts an intermediate stack of multiple layers in the example process in cross-section. The first intermediate stack 140 includes a support layer 1402. For example, the support layer 1402 can take the same or similar form as the support layers 502, 602 described above with reference to FIGS. 5 and 6. FIG. 14A to FIG. 14C Not depicted, but understood, the intermediate laminate 70 may include a carrier layer, which may take the same or similar form as the carrier layers 504, 604 described above.

[0215] A random or quasi-random multilayer deposition of particles 1406 is provided to the support layer 1402 in any suitable manner as previously described herein, such as by vertical colloidal deposition. The particles are polydisperse. Each particle of the multilayer deposition of particles 1406 is in contact with one or more adjacent particles so that gaps 1408 are formed between adjacent particles. The multilayer deposition of particles 1406 can be considered to form a particle body 1406. The particles in the particle body 1406 can be referred to as contact particles, because each particle is in contact with one or more adjacent particles.

[0216] The particle body 1406 includes a first particle group 1406A of a first material and a second particle group 1406B of a second material, and can be referred to as a binary mixture of particles. The two materials are scattering materials, and an example thereof is referred to as Fig.131406A and 1406B are discussed in more detail. In particular, the first material and the second material are selected to have different refractive indices. The composition of the binary mixture (i.e., the composition of the population of the first particle population 1406A and the second particle population 1406B) is selected based on the desired properties of the diffuser. Example binary mixtures include silicon and molybdenum, ruthenium and silicon, molybdenum silicide and silicon.

[0217] In addition to the composition of the particles 1406A, 1406B, other characteristics of the particle body 1406 can also be selected based on the desired optical properties of the diffuser. For example, the angular distribution of the scattering of radiation passing through the diffuser depends on the particle size, particle size distribution, and packing density. By changing the composition, particle size, particle size distribution, and / or packing density, properties such as scattering angle, suppression of zero-order scattering, emissivity, and attenuation can be changed. Within the volume occupied by the particle body 1406, the particles 1406A, 1406B can have a packing density of approximately 60% to 70%. That is, for the volume occupied by the particles, 60% to 70% of the volume can be occupied by the particles 1406A, 1406B, with the remaining 30% to 40% being empty (i.e., including the voids 1408).

[0218] In the second intermediate laminate 142, the particle body 1406 is fixed in place, for example, by fusing the particles to form a fused particle body 1014. The process for fixing the particles 1406 can include providing heat and / or pressure. In particular, sintering can be used to fix the particles, such as laser flash sintering, spark plasma sintering, or discharge sintering. Other fixing methods are available. Fixing the particles in place can be performed as part of the deposition process or as a separate process.

[0219] In the third intermediate stack 144, a protective layer is disposed on top of the fused particle body 1410. The protective layer may provide protection for the scattering layer (i.e., the fused particle body 1410) in use (e.g., in the environment of a lithographic apparatus). Additionally or alternatively, the protective layer may provide increased emissivity for the diffuser.

[0220] FIG. 15A to FIG. 15C The diagram is based on the reference FIG. 14A to FIG. 14C Example diffusers made by the described process, and the properties of the diffusers. In particular, the particles include molybdenum silicide and silicon (MoSi and Si), and are polydisperse with a radius between 75nm and 85nm. The particles are deposited with a (quasi) random distribution. Approximately six particle layers are deposited on the support layer.

[0221] Fig.15A The resulting height map or height map of the particle body is depicted 1500. The height map 1500 omits the support layer, but in use the particle body will be supported by the support layer.

[0222] Fig. 15B and Fig. 15C The scattering angles for a plane wave of EUV radiation incident on an example diffuser are plotted. In particular, Fig. 15B and Fig. 15C collaborate to illustrate the beam profile of scattered EUV radiation, wherein Fig. 15B plotting the intensity of the scattered EUV radiation across a range of angles in a plane orthogonal to the direction of travel of the scattered EUV radiation, and Fig. 15C Cross-sectional representation depicting the beam profile. EUV radiation experiences scattering over a wide angular range of up to 40°. EUV radiation experiences scattering with relatively constant intensity over an angular distribution of approximately 10°. As such, such an example diffuser can provide an effective diffuser for a high numerical aperture patterning device.

[0223] Fig.16A and Fig. 16B Another example process for making a diffuser suitable for EUV radiation is described. In particular, Fig.16A and Fig. 16B The diffuser in is a holographic diffuser suitable for EUV radiation. The intermediate stack 160, 162 in the example process is depicted in cross section.

[0224] The first intermediate layer stack 160 includes a support layer 1602. For example, the support layer 1602 may take the same or similar form as the support layers 502, 602 described above with reference to FIGS. 5 and 6. Fig.16A and Fig. 16B Not depicted, but understood, the intermediate laminate 70 may include a carrier layer, which may take the same or similar form as the carrier layers 504, 604 as described above.

[0225] The structure 1604 is disposed on top of the support layer 1602. The structure 1604 may be disposed according to any suitable technique. Fig.16A and Fig. 16B As depicted in FIG. 16, the structure 1604 may be provided by photolithography using an electron beam mask. Alternatively, the structure may be created using electron beam lithography or nanoimprint lithography or the like.

[0226] The voids 1605 are formed between the structures 1604. That is, within the volume of the space occupied by the structures 1604, there is a volume that does not contain the structures and therefore includes the voids 1605.

[0227] The structures 1604 (and therefore the voids 1605) are arranged in a holographic interference pattern so that when illuminated by radiation, the radiation is diffracted to form a hologram. The structural arrangement is selected so that a desired hologram is produced. The hologram can be produced at an input plane of a measurement system (e.g., a measurement system as described above).

[0228] In an example arrangement, a holographic interference pattern is selected to form a hologram having an angular distribution (i.e., angular intensity distribution) that is substantially constant across a selected angular distribution (e.g., 10°). A substantially constant angular distribution may be referred to as a top-hat distribution. In another example arrangement, the holographic interference pattern is selected so as to form a hologram with a stronger angular distribution in a radially outer portion of the hologram than in a radially inner portion of the hologram. That is, the holographic diffuser diffuses the EUV radiation such that the light is scattered with higher intensity for larger scattering angles.

[0229] The structures 1604 are arranged in a particular arrangement on the plane of the support layer 1602 (e.g., in the xy plane). Each portion of each structure 1604 has a certain height, and each portion extends from the support layer 1602 to the height. The height can be referred to as the thickness of the portion of the structure 1604. The arrangement of the structures 1604 on the support layer 1602 includes a combination of the position and thickness of each structure 1604 on the support layer 1602, and can be referred to as the thickness distribution L(x, y) of the holographic interference pattern. The method of determining the thickness distribution L(x, y) is described in more detail further below.

[0230] The second intermediate stack 162 depicts the step of disposing a filler or infill layer 1606 atop the support layer 1602 and / or the structure 1604. The filler layer 1606 may be disposed according to any suitable technique, such as atomic laser deposition (ALD) or electrodeposition.

[0231] A filler layer 1606 is provided to fill the previously included gap 1605 (e.g., Fig.16A 1604). The filler layer 1606 has a certain thickness so that the combined thickness of the structure 1604 and the filler layer 1606 in the z direction (i.e., extending from the support structure 1604) is approximately constant. The filler layer 1606 can be considered to level the structure, thereby providing a smooth surface (e.g., substantially smooth on a microscale or nanoscale).

[0232] The filler layer 1606 includes a material having a different refractive index than the material of the structure 1604. In particular, the filler layer 1606 includes a material having a real part (n structure ) compared to the different real parts of the refractive index (n padding ) materials.

[0233] The differential refractive index δn can be used padding ,δn structure To quantify the real part of the refractive index n through layer 1606 padding , and the real part of the refractive index n of structure 1604 structure The amount of deviation from 1. When the filler layer 1606 and the structure 1604 are combined into a thin layer (such as Fig. 16B ), the thin layer has an effective real part of the refractive index n approximated using equation (2) eff For simplicity, the effective real part of the refractive index may be simply referred to as the effective refractive index.

[0234] n eff =δn padding -δn structure (2)

[0235] The real part of the refractive index n padding 、n structure The filler layer 1606 is selected so that it has a refractive index n which is equal to the real part of the refractive index of the structure 1604. structure Compared to the real part of the higher refractive index n padding .

[0236] Filler layer 1606 also includes a similar imaginary part (k structure ) compared to the similar imaginary part of the refractive index (k padding ) material. When the filler layer 1606 and the structure 1604 are combined into a thin layer (such as Fig. 16B ), the thin layer has an effective imaginary part of the refractive index n approximated using equation (3) eff .

[0237] k eff =δk structure -δk padding (3)

[0238] The effective imaginary part of the refractive index k eff Thus, by providing the filler layer 1606 and the structure 1604 with similar imaginary parts of the refractive index, they each have comparable, ie, equivalent, attenuations.

[0239] The effective layer thickness L of the thin layer can be approximated using equation (4).

[0240]

[0241] Radiation traveling through the diffuser experiences a phase shift and attenuation based on the thickness of each of the filler layer 1606 and the structure 1604 at the location of the diffuser through which the radiation travels. When radiation passes through a region of the diffuser where the filler layer 1606 has a thickness of zero (0) and the structure 1604 has a thickness of L, a phase shift of zero (0) is experienced. When radiation passes through a region of the diffuser where the filler layer 1606 and the structure 1604 each have a thickness of L, a phase shift of zero (0) is experienced. When the radiation passes through the region of the diffuser where the thickness of the packing layer 1606 is L and the thickness of the structure 1604 is zero (0), it experiences a phase shift of two pi (2π). By limiting the thickness of the packing layer 1606 and the structure 1604 to If the thickness is a multiple of π, the phase shift can be controlled to be a multiple of the pi (π) phase shift, thereby providing controlled phase modulation. Using such a controlled thickness in a holographic diffuser can be referred to as binary phase modulation or ternary phase modulation.

[0242] It should be noted that the holographic diffuser as described herein employs controlled phase modulation due to the controlled selection of thickness and arrangement within the scattering layer. This is different from other diffusers described herein (e.g., reference FIG. 14A to FIG. 14C This is in contrast to the diffuser described herein which uses random phase modulation due to the (quasi) random arrangement of the nanoparticles in the scattering layer.

[0243] The thickness of the filler layer 1606 and parts of the structure 1604 may be further limited to a minimum thickness, such as 50 nm or 0 nm, for example based on manufacturing limitations. The thickness of the filler layer 1606 and parts of the structure 1604 may be further limited to a maximum thickness, such as 200 nm, for example to limit attenuation.

[0244] An example method for determining the arrangement and thickness of the structures 1604 is as follows. Considering a thin diffuser layer extending in a plane denoted as plane (x, y), the scattering of light through the diffuser layer can be approximated according to equation (5).

[0245]

[0246] M(x,y) quantifies the scattering angle experienced by a radiation ray having a wavelength λ traveling through a particular position (in x and y) of the diffuser layer. The calculated scattering of light M(x,y) may be referred to as an angular distribution M(x,y). The diffuser layer has a thickness distribution L(x,y) representing the effective thickness of the diffuser layer at each position in x and y. Δn represents the deviation of the real part of the refractive index of the diffuser layer from 1, and k is the imaginary part of the refractive index.

[0247] Given the approximation in equation (4), it can be obtained by performing a Fourier transform according to equation (5) to approximate the spatial distribution S(fx, fy) of the diffused light associated with the diffuser layer (ie, the spatial intensity distribution).

[0248]

[0249] The desired angular distribution M can be selected D (x, y). Expected angular distribution M D (x, y) may include, for example and as described above, a top hat distribution. D (x, y), approximations such as those in equations (4) and (5) can be used to determine the thickness distribution L(x, y) that will produce a desired angular distribution M given radiation of a particular wavelength λ. D It will be appreciated that, in addition or alternatively, similar methods may be used to determine a refractive index profile (e.g., the deviation Δn of the real part of the refractive index from 1 and / or the imaginary part of the refractive index) that will produce a hologram having a desired angular distribution M given radiation of a particular wavelength. D Furthermore, to determine the corresponding thickness distribution, the desired spatial intensity distribution at a certain distance from the holographic diffuser can be selected instead of the desired angular distribution M. D (x, y). However, in the example described herein, the determination of the thickness distribution L(x, y) is described for simplicity.

[0250] In a specific example, the determination of the thickness distribution L(x, y) is performed numerically using the Gerchberg-Saxton algorithm. The algorithm receives a desired angular distribution M of a selected material (e.g., one of the scattering substances described above with respect to Table 1). D (x, y), wavelength λ, refractive index and / or deviation Δn. The algorithm then iteratively performs calculations, such as modified versions of equations (2) and (3), to determine the thickness distribution L(x, y). The determination may be an estimate. The number of iterations may be predetermined. Alternatively, the number of iterations may be selected based on a quality metric associated with the estimated thickness distribution L(x, y). The algorithm may use height limits, for example to limit the thickness distribution so that an area of ​​the diffusion layer does not exceed a maximum thickness and / or is not thinner than a minimum thickness. Such maximum and minimum thicknesses may be selected based on a manufacturing method, such as a resolution limit of the manufacturing method. It should be understood that other methods may be used to determine the thickness distribution L(x, y) and / or the refractive index distribution, for example analytical methods may be used or different numerical methods may be used.

[0251] Back to Fig.16A and Fig. 16B By arranging the structure 1604 corresponding to the determined thickness distribution L(x, y), it is possible to produce a structure with a desired angle distribution M D (x, y) Holographic diffuser for scattered radiation.

[0252] Fig.17 , Fig.18 and Fig.19 Example holographic diffusers each comprising molybdenum, ruthenium, and molybdenum silicide and their performance are illustrated. Fig.17 , Fig.18 and Fig.19 An example holographic diffuser of does not include a filler layer, but instead includes voids between structures thereon.

[0253] Each holographic diffuser comprises a structure 1604 arranged according to the determined thickness distribution L(x, y). For a desired angular distribution M comprising a top-hat distribution with an angular distribution of 9° D (x, y), the thickness distribution L(x, y) of each holographic diffuser is determined using the method described above. The thickness distribution L(x, y) for each holographic diffuser is determined using the refractive index data of the material (i.e., molybdenum, ruthenium, and molybdenum silicide) included for each corresponding holographic diffuser.

[0254] Fig.17 The thickness distribution L(x, y) 172 determined for a holographic diffuser including a molybdenum structure is shown. The thickness distribution L(x, y) includes a height measure 0, Or a quasi-random arrangement of the structures of L, where L is calculated for molybdenum.

[0255] Fig.17 Also shown are a phase shift distribution 170 and a transmission distribution 174 for a holographic diffuser comprising molybdenum structures. The phase shift distribution 170 illustrates how the phase shift experienced by EUV radiation transmitted through the holographic diffuser at different locations across the diffuser is -π, 0, or π (i.e., equivalent to 0, π, and 2π) in a quasi-random pattern that corresponds roughly to the quasi-random arrangement of structures. The transmission distribution 174 illustrates how the transmittance of EUV radiation through the holographic diffuser ranges from 0.6 to 1 in a quasi-random pattern that corresponds roughly to the quasi-random arrangement of structures. The average transmittance of the holographic diffuser is roughly 78%.

[0256] Fig.17 Also shown is the angular distribution 176 of EUV radiation diffused by the holographic diffuser comprising a molybdenum structure. The angular distribution 176 is substantially constant within an angular distribution of 9°. That is, the angular distribution 176 substantially corresponds to the desired angular distribution M D(x, y). The angular distribution 176 does not exactly correspond to the desired angular distribution M D (x, y) because there is some inhomogeneity in the angular distribution at 9°. In particular, there is a bright spot 177 at 0°, indicating some zeroth order scattering. In addition, some EUV radiation is scattered above 9°, which is evident by the appearance of a "halo" 178 of scattered light at angles above 9°.

[0257] Fig.18 The thickness distribution L(x, y) 182 determined for a holographic diffuser including a ruthenium structure is shown. The thickness distribution L(x, y) includes a thickness distribution L(x, y) having a height measure of 0, Or a quasi-random arrangement of the structures of L, where L is calculated for ruthenium.

[0258] Fig.18 Also shown are a phase shift distribution 180 and a transmission distribution 184 for a holographic diffuser comprising ruthenium structures. The phase shift distribution 180 illustrates how the phase shift experienced by EUV radiation transmitted through the holographic diffuser at different locations across the diffuser is -π, 0, or π (i.e., equivalent to 0, π, and 2π) in a quasi-random pattern that corresponds roughly to the quasi-random arrangement of structures. The transmission distribution 184 illustrates how the transmittance of EUV radiation through the holographic diffuser ranges from 0.4 to 1 in a quasi-random pattern that corresponds roughly to the quasi-random arrangement of structures. The average transmittance of the holographic diffuser is roughly 66%.

[0259] Fig.18 Also shown is the angular distribution 186 of EUV radiation diffused by the holographic diffuser including the ruthenium structure. The angular distribution 186 is substantially constant within an angular distribution of 9°. That is, the angular distribution 186 substantially corresponds to the desired angular distribution M D (x, y). The angular distribution 186 does not exactly correspond to the desired angular distribution M D (x, y) because there is some inhomogeneity in the angular distribution at 9°. In particular, there is a bright spot 187 at 0°, indicating some zeroth order scattering. In addition, some EUV radiation is scattered above 9°, which is evident by the appearance of a "halo" 188 of scattered light at angles above 9°.

[0260] Fig.19 The thickness distribution L(x, y) 192 determined for a holographic diffuser including a molybdenum silicide structure is shown. The thickness distribution L(x, y) includes a height measurement of 0, Or a quasi-random arrangement of the structure of L, where L is calculated for molybdenum silicide.

[0261] Fig.19Also shown are a phase shift distribution 190 and a transmission distribution 194 for a holographic diffuser comprising molybdenum silicide structures. The phase shift distribution 190 illustrates how the phase shift experienced by EUV radiation transmitted through the holographic diffuser at different locations across the diffuser is -π, 0, or π (i.e., equivalent to 0, π, and 2π) in a quasi-random pattern that corresponds roughly to the quasi-random arrangement of structures. The transmission distribution 194 illustrates how the transmittance of EUV radiation through the holographic diffuser ranges from 0.45 to 1 in a quasi-random pattern that corresponds roughly to the quasi-random arrangement of structures. The average transmittance of the holographic diffuser is roughly 68%.

[0262] Fig.19 Also shown is the angular distribution 196 of EUV radiation diffused by the holographic diffuser including the molybdenum silicide structure. The angular distribution 196 is substantially constant within an angular distribution of 9°. That is, the angular distribution 196 substantially corresponds to the desired angular distribution M D (x, y). The angular distribution 196 does not exactly correspond to the desired angular distribution M D (x, y) because there is some inhomogeneity in the angular distribution at 9°. In particular, there is a bright spot 197 at 0°, indicating some zeroth order scattering. In addition, some EUV radiation is scattered above 9°, which is evident by the appearance of a "halo" 198 of scattered light at angles above 9°.

[0263] Fig. 20 The thickness distribution (x, y) 2002 of the ruthenium structure as determined for a holographic diffuser comprising a ruthenium structure and a silicon oxide filler layer is shown. The thickness distribution (x, y) 2002 is shown before the deposition of the filler layer. The thickness distribution (x, y) 2002 includes a height measurement of 0, or a quasi-random arrangement of structures of L, wherein L is calculated for ruthenium. After providing the filler layer, the resulting thickness distribution is approximately equal to L without any substantial thickness variation.

[0264] Fig. 20 Also shown are a phase shift distribution 2000 and a transmission distribution 2004 for a holographic diffuser comprising ruthenium structures and a silicon oxide filler layer. The phase shift distribution 2000 illustrates how the phase shift experienced by EUV radiation transmitted through the holographic diffuser at different locations across the diffuser is -π, 0, or π (i.e., equivalent to 0, π, and 2π) in a quasi-random pattern that corresponds roughly to the quasi-random arrangement of structures. The transmission distribution 2004 illustrates how the transmittance of EUV radiation through the holographic diffuser ranges from 0.3 to 0.5 in a quasi-random pattern that corresponds roughly to the quasi-random arrangement of structures. The average transmittance of the holographic diffuser is approximately 39%.

[0265] Fig. 20Also shown is the angular distribution 2006 of EUV radiation diffused by the holographic diffuser including the molybdenum silicide structure. The angular distribution 2006 is substantially constant within an angular distribution of 9°. That is, the angular distribution 2006 substantially corresponds to the desired angular distribution M D (x, y). The angular distribution 2006 does not exactly correspond to the expected angular distribution M D (x, y) because there is some inhomogeneity within the angular distribution at 9°. However, the inhomogeneity is lower than that of the previous example holographic diffuser without the filler layer. In particular, there is no bright spot at 0°, indicating reduced zero-order scattering.

[0266] A holographic diffuser comprising a filler layer may be beneficially used in applications where a highly uniform scattering profile is desired. A holographic diffuser without a filler layer may be beneficially used in applications where a high EUV transmission is desired.

[0267] 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.

[0268] In use, the diffuser is arranged such that it 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 of the angular distribution of radiation in a field plane of a lithographic apparatus LA, downstream of which the optical device is. 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). 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.

[0269] The lithographic apparatus LA has four reticle shielding blades, which may also be referred to as patterning device shielding blades, defining the extent of an illuminated field on the patterning device MA. The illumination system IL is operable to illuminate a substantially rectangular area of ​​an object (e.g., patterning 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.

[0270] Each of the shielding blades is arranged close to the plane of the support structure MT, but slightly outside said plane. The x shielding blades are arranged in a first plane and the y shielding blades are arranged in a second plane.

[0271] Each of the shielding blades defines an edge of a rectangular field region in the plane of the object where radiation is received. Each blade may be 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.

[0272] The x-direction may correspond to a non-scanning direction of the lithography apparatus LA, and the y-direction may correspond to a scanning direction of the lithography apparatus LA. That is, the object (and the substrate W in the image plane) may be capable of moving through the field area in the y-direction 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.

[0273] The diffuser may be mounted on a patterning device shield blade of the lithographic apparatus LA. In particular, during scanning exposure, the diffuser may be positioned such that when the shield blades are arranged at positions within their nominal range of motion, the diffuser is not arranged substantially in the path of the radiation beam.

[0274] The diffuser 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 or Gaussian angular power distribution (as a function of the scattering angle). The diffuser may have an absorptivity of less than 90%, for example less than 50%, for EUV radiation (for a single pass, i.e., a single stroke). The diffuser may have a lifetime, i.e., a service life, of more than 7 years in a lithographic apparatus (e.g., with an illumination duty cycle of about ~0.1% to 1%). The diffuser may be capable of operating to withstand about 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.

[0275] Where reference is made to vertical colloidal deposition as deposition method, the following procedures may additionally or alternatively be used: inkjet printing and spin coating.

[0276] According to another embodiment, the transmissive diffuser comprises a support structure comprising a porous structure having pores. The support structure may be a network of nanotubes, such as bundles of carbon nanotubes, multi-walled carbon nanotubes, single-walled carbon nanotubes, boron nitride or MoS2 nanotubes as core fibers. The nanotubes may be randomly aligned, thereby providing structural support for the optically active diffuser material deposited onto the tubes.

[0277] A scattering layer at least partially covers the support structure, the scattering layer being configured to scatter the received radiation. The scattering layer comprises at least one of Mo, Y, Zr, Nb, Ru. The scattering layer provides an optically active material to diffuse the light into a desired light profile. Ideally, the scattering layer has a relatively low absorption of EUV light and a high refractive index contraction compared to a vacuum. The scattering layer has a thickness of at least 10 nm, optionally at least 20 nm, optionally at least 40 nm, optionally at least 100 nm. The thickness determines the absorbance.

[0278] Optionally, the scattering layer supports a top layer, the top layer comprising at least one MoO3, Y2O3, ZrO2, Al2O3, HfO2, ZrO2, Ru, W, metal, the thickness of the top layer being at least 0.3nm, optionally at least 1nm. Such a top layer may provide plasma and high temperature tolerance and mitigation. The diffuser may be a holographic diffuser.

[0279] The support structure described in this embodiment can also be used in other embodiments.

[0280] Although the use of lithographic equipment in IC manufacturing may be specifically mentioned herein, it will be understood 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, etc.

[0281] Although embodiments of the invention may be specifically mentioned herein in the context of lithographic equipment, embodiments of the invention may be used in other equipment. Embodiments of the invention may form part of a mask inspection device, a metrology device, or any device that measures or processes a target such as a wafer (or other substrate) or a mask (or other pattern forming device). These devices may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0282] 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. Thus, 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 aspects set forth below.

[0283] Aspect 1. A diffuser configured to receive and transmit radiation, wherein the diffuser comprises: a scattering layer, the scattering layer being configured to scatter the received radiation, the scattering layer comprising a first substance and having a plurality of voids distributed therein, wherein: the first substance is a scattering substance, or at least one of the voids contains a scattering substance and the first substance has a lower refractive index than that of the scattering substance.

[0284] Aspect 2. The diffuser according to aspect 1, wherein the first substance is the scattering substance.

[0285] Aspect 3. The diffuser according to aspect 2, wherein the scattering substance comprises a foam having micropores and the voids are provided by the micropores and the voids contain a vacuum or an inert gas.

[0286] Aspect 4. The diffuser according to aspect 2, wherein the void comprises one of silicon or silicon nitride.

[0287] Aspect 5. The diffuser according to aspect 1, wherein the void contains the scattering substance.

[0288] Aspect 6. The diffuser according to Aspect 5, wherein the first substance comprises a porous silicon-based structure, and the voids are defined by micropores of the first substance.

[0289] Aspect 7. The diffuser according to any one of aspects 1 to 4, wherein the scattering substance comprises a body of contact particles, and the gaps are provided between adjacent particles.

[0290] Aspect 8. The diffuser of aspect 7, wherein each particle within the body of contact particles is fused with at least one other particle in the body of contact particles.

[0291] Aspect 9. The diffuser of aspect 7 or 8, wherein the particles comprise a binary mixture comprising a first material and a second material having a different refractive index than the first material.

[0292] Aspect 10. The diffuser of aspect 9, wherein the first material comprises silicon.

[0293] Aspect 11. The diffuser according to aspect 9 or 10, wherein the second material comprises molybdenum or ruthenium.

[0294] Aspect 12. The diffuser according to any one of aspects 7 to 11, wherein the particles have a range of approximately a few nanometers in at least one dimension.

[0295] Aspect 13. The diffuser according to any one of aspects 7 to 12, wherein the particles differ in size in at least one dimension.

[0296] Aspect 14. A diffuser according to any of the preceding aspects, wherein the scattering material comprises a material having a ratio of a first parameter to a second parameter of 1 or less than 1, wherein the first parameter is the maximum thickness of a layer of the material that will allow 10% transmission of the received radiation, and the second parameter is the minimum thickness of a layer of the material that will cause a phase shift of Pi.

[0297] Aspect 15. A diffuser according to any preceding aspect, wherein the voids are distributed in a plurality of layers within the first substance, each layer lying substantially in a plane perpendicular to a direction of propagation of the radiation during use.

[0298] Aspect 16. A diffuser according to any preceding aspect, wherein the voids are distributed in a single layer within the first substance, the layer lying substantially in a plane that is perpendicular to the direction of propagation of the radiation during use.

[0299] Aspect 17. The diffuser according to any preceding aspect, wherein the scattering substance comprises a dealloyed material.

[0300] Aspect 18. The diffuser according to any preceding aspect, wherein the voids have an extent of approximately a few nanometers in at least one dimension.

[0301] Aspect 19. The diffuser according to any preceding aspect, wherein the voids are polydisperse within the first material.

[0302] Aspect 20. The diffuser according to any preceding aspect, wherein the voids are randomly or quasi-randomly arranged within the first material.

[0303] Aspect 21. The diffuser according to any preceding aspect, wherein the scattering layer has a thickness between 50 nm and 1000 nm.

[0304] Aspect 22. The diffuser according to any preceding aspect, the diffuser being configured such that the angular scattering distribution in at least one scattering direction has a width of 5° or more.

[0305] Aspect 23. The diffuser according to any preceding aspect, wherein the scattering substance comprises one of the following: molybdenum, ruthenium, niobium, rhodium, yttrium or technetium.

[0306] Aspect 24. The diffuser according to any preceding aspect, comprising a plurality of scattering layers.

[0307] Aspect 25. The diffuser according to aspect 24, wherein the first scattering layer is separated from the second scattering layer by an intermediate layer.

[0308] Aspect 26. The diffuser of aspect 25, wherein the intermediate layer comprises silicon.

[0309] Aspect 27. The diffuser according to aspect 25 or 26, wherein the intermediate layer comprises a layer of separate particles having a lower refractive index than the scattering substance.

[0310] Aspect 28. The diffuser according to aspect 27, wherein the separate particles are randomly or quasi-randomly arranged within the intermediate layer.

[0311] Aspect 29. The diffuser of aspect 27 or 28, wherein the separated particles comprise particles of different sizes in at least one dimension.

[0312] Aspect 30. The diffuser of aspect 1, wherein the first substance and the voids cooperate to produce a hologram upon receiving radiation at a surface of the scattering layer.

[0313] Aspect 31. The diffuser of aspect 30, wherein the hologram has an angular intensity distribution that is at least as intense in a radially outer portion of the hologram as it is in a central region of the hologram.

[0314] Aspect 32. The diffuser of aspect 31, wherein the radially outer portion is angularly spaced at least 9° from the center of the hologram.

[0315] Aspect 33. The diffuser according to any one of aspects 30 to 32, wherein the first substance comprises a plurality of structures having varying thicknesses perpendicular to the surface of the scattering layer.

[0316] Aspect 34. The diffuser according to aspect 33, wherein: the diffuser is operable to form the hologram upon receiving radiation having a wavelength λ; the holographic diffuser has an effective refractive index n eff ; and the thickness of each structure in the plurality of structures is An integer multiple of .

[0317] Aspect 35. The diffuser of any one of Aspects 30 to 34, wherein the voids contain a second substance.

[0318] Aspect 36. The diffuser according to aspect 35, wherein a real part of the refractive index of the second substance is different from the real part of the refractive index of the first substance, and an imaginary part of the refractive index of the second substance is similar to the imaginary part of the refractive index of the first substance.

[0319] Aspect 37. The diffuser of aspect 35 or 36, wherein the combined first substance and second substance have a substantially constant combined thickness profile.

[0320] Aspect 38. The diffuser according to any one of aspects 30 to 37, wherein the first substance comprises one of the following: molybdenum, ruthenium, niobium, rhodium, yttrium, or technetium.

[0321] Aspect 39. The diffuser of any one of aspects 35 to 38, wherein the second substance comprises silicon.

[0322] Aspect 40. A holographic diffuser, comprising a scattering layer, the scattering layer comprising a plurality of structures configured to produce a hologram upon receiving extreme ultraviolet radiation at a surface of the scattering layer, wherein the hologram has an angular intensity distribution that is at least as intense in a radially outer portion of the hologram as in a central region of the hologram.

[0323] Aspect 41. The diffuser according to any preceding aspect, further comprising a protective layer configured to protect the scattering layer from EUV plasma etching.

[0324] Aspect 42. The diffuser according to any preceding aspect, wherein the diffuser further comprises a cover layer at least partially covering the scattering layer to protect the scattering layer during use.

[0325] Aspect 43. A measurement system for determining an aberration map or a relative intensity map for a projection system, the measurement system comprising a diffuser according to any preceding aspect.

[0326] Aspect 44. A measurement system according to Aspect 43, 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 capable of operating 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.

[0327] Aspect 45. A measurement system according to Aspect 44, wherein the diffuser is capable of moving between at least the following positions: a first operating position, wherein 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 impinges on the pattern forming device; and a second storage position, wherein the diffuser is disposed outside the path of the radiation generated by the illumination system.

[0328] Aspect 46. The measurement system according to any one of aspects 43 to 45, when comprising a diffuser according to any one of aspects 30 to 40, wherein the hologram is formed at an input plane of the measurement system.

[0329] Aspect 47. A lithographic device, comprising: a measurement system according to any one of Aspects 43 to 46; 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.

[0330] Clause 48. The lithographic apparatus according to clause 47, 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 area of ​​the lithographic apparatus.

[0331] Aspect 49. A method for forming a diffuser according to Aspects 1 to 3 or 14 to 20 for receiving and transmitting radiation, the method comprising: forming an alloy layer, the alloy layer comprising a first substance and a third substance, wherein the first substance is a scattering substance; dealloying the alloy layer so as to remove the third substance from the alloy layer and so as to form a scattering layer comprising the first substance and having a plurality of voids distributed therein.

[0332] Aspect 50. A method of forming a diffuser for receiving and transmitting radiation, the method comprising: forming a scattering layer by infiltrating a porous structure with a scattering material.

[0333] Aspect 51. The method according to aspect 50, wherein the scattering layer is formed on a support layer.

[0334] Aspect 52. A method of forming a diffuser for receiving and transmitting radiation, the method comprising: depositing a plurality of particles on a surface of a support layer to form a mask; depositing a scattering material on the support layer over the mask to form a scattering layer around the plurality of particles.

[0335] Aspect 53. The method according to aspect 52, further comprising shrinking one or more of the plurality of particles deposited on the support layer to expose a larger area of ​​the surface of the support layer before depositing the scattering material.

[0336] Aspect 54. The method according to aspect 52 or 53, wherein the particles are deposited on the support layer via vertical colloidal deposition.

[0337] Aspect 55. The method according to aspect 52, 53 or 54, wherein the particles form a single layer deposited on the surface of the support layer, and the scattering layer forms a corrugated scattering surface on the support layer.

[0338] Aspect 56. A method according to aspect 52, 53 or 54, wherein the particles form a plurality of layers deposited on the surface of the support layer, each of the plurality of layers being in a plane substantially perpendicular to the direction of the received radiation in use.

[0339] Aspect 57. The method according to any one of aspects 52 to 56, further comprising removing the particles after depositing the scattering material.

[0340] Aspect 58. A method for forming a diffuser for receiving and transmitting radiation, the method comprising: depositing a plurality of particles on a surface of a support layer to form a mask; depositing a second material on the surface of the support layer over the mask to form a layer of the second material around the plurality of particles; removing at least some of the plurality of particles to form pits within the layer of the second material; and depositing a scattering material into at least some of the pits within the second material to form scattering features within the layer of the second material.

[0341] Aspect 59. A method for forming a diffuser for receiving and transmitting radiation, the method comprising: depositing a plurality of particles on a surface of a support layer to form a mask; depositing a second material on the surface of the support layer above the mask; selectively etching the surface of the support layer to form a plurality of structures on the surface of the support layer; depositing a scattering material onto the surface of the support layer, the scattering material being formed above the plurality of structures to form a scattering layer; wherein the second material is a catalyst and the selective etching comprises etching an area of ​​the support layer in contact with the second material, or wherein the second material is a protective material and the selective etching comprises etching an area of ​​the support layer that is not in contact with the second material.

[0342] Aspect 60. A method of forming a diffuser for receiving and transmitting radiation, the method comprising: depositing a plurality of particles onto a surface of a support layer such that the particles form a body of contact particles.

[0343] Aspect 61. The method according to aspect 50, wherein depositing comprises at least one of: vertical colloidal deposition, spin coating, and inkjet printing.

[0344] Aspect 62. The method according to aspect 60 or 61, wherein depositing comprises fusing the plurality of particles.

[0345] Aspect 63. The method according to aspect 62, wherein the plurality of particles are fused via providing heat and / or pressure.

[0346] Aspect 64. The method according to aspect 62 or 63, wherein the plurality of particles are fused using sintering.

[0347] Aspect 65. The method according to any one of Aspects 60 to 64, wherein the particles comprise a binary mixture comprising a first material and a second material having a different refractive index than the first material.

[0348] Aspect 66. The method according to any one of aspects 52 to 65, further comprising forming another scattering layer.

[0349] Aspect 67. The method according to aspect 66, wherein the another scattering layer is formed according to the method according to any one of aspects 33 to 40.

[0350] Aspect 68. The method according to aspect 66 or 67, wherein forming another scattering layer comprises depositing an intermediate layer over the scattering layer and forming the another scattering layer on top of the intermediate layer.

[0351] Aspect 69. A method according to any one of Aspects 52 to 68, wherein the support layer is formed on a carrier layer, the carrier layer being used to support the support layer when forming the diffuser, and wherein the method further comprises removing the carrier layer once the first layer and the second layer have been formed.

[0352] Aspect 70. A method of forming a diffuser for receiving and transmitting radiation, the method comprising creating a plurality of structures on a surface of a support layer of the diffuser, wherein the structures are arranged to create a hologram upon receiving radiation at the surface.

[0353] Aspect 71. The method according to aspect 70, wherein the hologram has an angular intensity distribution that is at least as intense in a radially outer portion of the hologram as in a central region of the hologram.

[0354] Aspect 72. A method according to any one of aspects 69 to 71, wherein the plurality of structures are produced using photolithography.

[0355] Aspect 73. The method according to any one of Aspects 69 to 72, further comprising depositing a second substance into a plurality of voids distributed within the plurality of structures.

[0356] Aspect 74. The method according to any one of Aspects 70 to 73, further comprising generating a thickness distribution corresponding to a desired arrangement of a plurality of surface features, the desired arrangement being based on a desired angular distribution of the hologram.

[0357] Aspect 75. The method according to Aspect 74, wherein generating the surface distribution includes using a Gerchberg-Saxton algorithm.

[0358] Aspect 76. The method according to any one of aspects 52 to 75, further comprising etching the support layer from a surface of the support layer opposite to a surface of the support layer supporting the scattering layer.

[0359] Aspect 77. The method according to any one of aspects 52 to 76, further comprising providing a cover layer at least partially covering the support layer and / or the scattering layer.

[0360] Aspect 78. A diffuser configured to receive and transmit radiation, wherein the diffuser comprises: a support structure comprising a porous structure having pores; and a scattering layer at least partially covering the support structure and configured to scatter the received radiation.

[0361] Aspect 79. The diffuser of Aspect 78, wherein the support structure comprises nanotubes.

[0362] Aspect 80. The diffuser according to any one of aspects 78 to 79, wherein the scattering layer comprises at least one of the following: molybdenum, ruthenium, niobium, rhodium, yttrium, zirconium, or technetium.

[0363] Aspect 81. The diffuser according to any one of aspects 78 to 80, wherein the scattering layer has a thickness of at least 10 nm, optionally at least 20 nm, optionally at least 40 nm, optionally at least 100 nm.

[0364] Aspect 82. A diffuser according to any one of Aspects 78 to 81, wherein the scattering layer supports a top layer comprising at least one of the following: MoO3, Y2O3, ZrO2, Al2O3, HfO2, ZrO2, Ru, W, metal, the top layer having a thickness of at least 0.3 nm, optionally at least 1 nm.

[0365] Aspect 83. A diffuser according to any one of Aspects 78 to 82, wherein the diffuser has a porosity fraction of at least 10%, optionally at least 20%, optionally at least 30%, optionally at least 40%, optionally at least 50%.

[0366] Aspect 84. The diffuser according to any preceding aspect, wherein the diffuser is a transmissive diffuser.

Claims

1. A diffuser configured to receive and transmit radiation, wherein the diffuser comprises: a scattering layer configured to scatter the received radiation, The scattering layer includes a first substance and has a plurality of voids distributed therein, Wherein: the first substance is a scattering substance, or At least one of the voids contains a scattering substance and the first substance has a lower refractive index than the scattering substance; and wherein the first substance and the void comprise a holographic interference pattern, the holographic interference pattern being selected to form a hologram given radiation of a desired wavelength, and the hologram having an angular intensity distribution which is a top-hat shaped distribution having a lower intensity in a central region than in a radially outer portion of the hologram, the radially outer portion of the hologram being angularly spaced at least 9° from the center of the hologram. 2 . The diffuser according to claim 1 , wherein the first substance is the scattering substance, wherein the scattering substance comprises a foam having micropores and the voids are provided by the micropores and the voids contain a vacuum or an inert gas.

3. The diffuser of claim 2, wherein the voids comprise one of silicon or silicon nitride.

4. The diffuser of claim 1, wherein a void contains the scattering substance, wherein the first substance comprises a porous silicon-based structure, and the void is defined by micropores of the first substance. 5 . The diffuser of claim 1 , wherein the scattering material comprises a body of contact particles, and the voids are disposed between adjacent particles.

6. The diffuser of claim 5, wherein the particles comprise a binary mixture comprising a first material and a second material having a different refractive index than the first material.

7. The diffuser of claim 6, wherein the first material comprises silicon and the second material comprises molybdenum or ruthenium.

8. The diffuser according to any one of claims 1 to 7, further comprising a support structure, wherein the scattering layer at least partially covers the support structure, wherein the support structure comprises nanotubes.

9. The diffuser of claim 1, wherein the first substance and the voids cooperate to produce a hologram upon receiving radiation at a surface of the scattering layer.

10. The diffuser of claim 9, wherein the void contains a second substance, wherein a real part of the refractive index of the second substance is different from the real part of the refractive index of the first substance, and an imaginary part of the refractive index of the second substance is similar to the imaginary part of the refractive index of the first substance.

11. The diffuser of claim 1, wherein the first substance comprises at least one of: molybdenum, ruthenium, niobium, rhodium, yttrium, zirconium, or technetium.

12. The diffuser of claim 10, wherein the second substance comprises silicon.

13. A holographic diffuser, comprising a scattering layer, the scattering layer comprising a plurality of structures configured to produce a hologram upon receiving extreme ultraviolet radiation at a surface of the scattering layer, wherein the hologram has an angular intensity distribution, the angular intensity distribution being a top-hat distribution having a lower intensity in a central region than a radially outer portion of the hologram, the radially outer portion of the hologram being angularly spaced at least 9° from the center of the hologram.

14. A lithographic apparatus comprising: a measurement system for determining an aberration map or a relative intensity map for a projection system, the measurement system comprising a diffuser according to any preceding claim; and A 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.

15. A method of forming a diffuser for receiving and transmitting radiation, the method comprising producing a plurality of structures on a surface of a support layer of the diffuser, wherein the structures are arranged to produce a hologram upon receiving radiation at the surface, wherein the hologram has an angular intensity distribution which is a top-hat distribution having a lower intensity in a central region compared to a radially outer portion of the hologram, the radially outer portion of the hologram being angularly spaced at least 9° from the center of the hologram.

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