CN114556167B

CN114556167B - Transmissive Diffuser

Info

Publication number: CN114556167B
Application number: CN202080071117.2A
Authority: CN
Inventors: 约斯特·安德烈·克鲁格斯特; A·尼基帕罗夫; W·J·恩格伦; 廉晋; 保罗·亚历山大·维梅伦; 哈利勒·戈凯·叶根
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2019-10-11
Filing date: 2020-09-24
Publication date: 2025-05-16
Anticipated expiration: 2040-09-24
Also published as: CN114556167A; WO2021069231A1; KR20220079853A; TW202129400A; TWI889713B

Abstract

A diffuser configured to receive and transmit radiation. The diffuser includes a scattering layer (510) configured to scatter received radiation, the scattering layer (510) including 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

Transmission type diffuser

Cross Reference to Related Applications

The present application claims priority from european application 19202644.1 filed on 10/11/2019, and the entire contents of said european application are incorporated herein by reference.

Technical Field

The present invention relates to a transmissive diffuser, i.e. a diffuser configured to receive and transmit radiation, the transmitted radiation having a varying angular distribution. The diffuser may be adapted for use with EUV radiation and may form part of a measurement system within an EUV lithographic apparatus.

Background

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

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

Known lithographic apparatus include measurement systems for determining one or more pupil function changes. The pupil function variations may include relative phase variations in the pupil plane and/or relative intensity variations in the pupil plane. Such a measurement system typically comprises an object level patterning device (e.g. a diffraction grating or a pinhole or the like), an illumination system, and an image level sensor apparatus. The illumination system is arranged to illuminate the patterning device with radiation. At least a portion of the radiation scattered by the patterning device is received by the projection system (a property of the projection system is measured), which is arranged to form an image of the patterning device on an image level sensor apparatus. It is desirable for such a measurement system that the entire entrance pupil of the projection system receives radiation from the patterning device. However, illumination systems are also typically used by lithographic apparatus for forming (diffraction limited) images of an object-level reticle or mask on an image-level substrate (e.g. a resist-coated silicon wafer), wherein it may be desirable to illuminate only one or more discrete portions of the entrance pupil of the projection system.

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

Disclosure of Invention

Described herein is a diffuser or diffuser/diffuser (diffuser) configured to receive and transmit radiation. 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, 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 would allow the entire entrance pupil of the projection system to receive radiation from the patterning device.

The void may contain a vacuum (or an environment that is substantially or functionally 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 the 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 to the received radiation (or relatively optically neutral compared to the scattering material). For example, if the received radiation is EUV radiation, the substance having the lower refractive index may have a refractive index for EUV radiation of close to 1. However, it should be appreciated that the radiation may have any wavelength (i.e. may not be EUV radiation).

In the case where the first substance is the scattering substance, the scattering substance may include 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 comprise 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 material within the void will also have a refractive index that differs significantly from the scattering material. In addition, in this way the scattering layer is particularly easy to manufacture, since no intermediate step of removing the second substance from the scattering substance has to be performed.

In the case where the voids contain the scattering material, the first material may include a porous silicon-based structure, the voids being defined by micropores of the first material.

In the case of porous masses used in the examples described herein, the micropores of the porous masses may have a range on the order of a few nanometers in at least one dimension.

The scattering material may comprise a body contacting the particles. The voids may be disposed between adjacent particles. Such diffusers can be relatively easily fabricated using a variety of deposition methods, such as liquid phase deposition methods. With respect to the term "contact particles", it is understood that each particle in the particle body is in physical contact with at least one other particle in the particle body.

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

The particles may comprise a binary mixture comprising a first material and a second material having a refractive index different from the first material. The refractive indices of the first material and the second material may be substantially different. The first material and the second material may have a low attenuation for the received radiation. The first material may comprise silicon. The second material may comprise 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.

The particles may have a range of about several nanometers in at least one dimension. The particles may vary 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 angles and/or suppression of zero order scattering.

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

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

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

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

The scattering species may include a dealloying material. The de-alloyed material will provide multiple interfaces to the voids for the scattering material.

The void may have a range on the order of 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 disposed within the first material.

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

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.

The scattering material may comprise one of molybdenum, ruthenium, niobium, rhodium, yttrium, or technetium.

The diffuser may include a plurality of scattering layers. Each of the scattering layers may be fabricated according to any of the techniques described herein or elsewhere.

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 to the received radiation.

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

The separation particles may be randomly or quasi-randomly arranged within the intermediate layer. The separation particles may comprise particles of different sizes in at least one dimension.

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 void 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 holograms may advantageously provide controlled diffusion of radiation in combination with minimal absorption of radiation. Such a diffuser may have an increased lifetime compared to known diffusers, for example due to a reduced absorption of radiation.

The hologram may have an angular intensity distribution that is at least as strong in a radially outer portion of the hologram as compared to a central region of the hologram. The angular intensity distribution may have a similar intensity in the central region compared to the 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 the central region than in the radially outer portion of the hologram.

The radially outer portion may be angularly spaced at least 9 ° from the center of the hologram. Such diffusers may have particular benefits in devices having high numerical apertures.

The first substance may include a plurality of structures having varying thicknesses. That is, the thickness distribution of the plurality of structures varies. The thickness profile may be measured in the plane of the diffuser (e.g. the plane of the surface arranged to receive the radiation). The thickness profile may vary by about several nanometers. For example, the thickness profile of the structure may vary between 0nm and 200nm in thickness.

The diffuser may be operable to form the hologram upon receiving radiation having a wavelength λ. The wavelength may be EUV wavelength λ. The holographic diffuser may have an effective refractive index n _eff. The thickness of each of the plurality of structures may beIs an integer multiple of (a). Advantageously, such a diffuser may impart a phase shift of 0, pi or 2pi to the portion of the radiation traveling therethrough.

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

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 and second substances having different real parts of their refractive indices 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 and second substances having similar imaginary parts of the refractive indices of the first and second substances may reduce attenuation through the diffuser. The combined first and second substances act to reduce the relative differences in attenuation experienced by radiation traveling through the structure and the void.

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 the nanometer scale.

The first substance may comprise one of molybdenum, ruthenium, niobium, rhodium, yttrium, or technetium. The second substance may comprise silicon.

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

Any of the diffusers described herein may further include a protective layer configured to protect the scattering layer from EUV plasma etching. The diffuser may further include a cover (cap) layer at least partially covering the scattering layer to protect the scattering layer during use.

Also described herein is a measurement system for determining an aberration map or a relative intensity map for a projection system, the measurement system including a diffuser of any of the examples described herein.

The measurement system may comprise a patterning device, an illumination system arranged to illuminate the patterning device with radiation, and a sensor apparatus. 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 apparatus is configured such that the projection system projects the received radiation onto the sensor apparatus. The diffuser may be operable to receive the radiation generated by the illumination system and alter the angular distribution of the radiation prior to the radiation illuminating the patterning device.

The diffuser may be movable between at least a first operational position, wherein the diffuser is at least partially disposed in the path of the radiation generated by the illumination system and arranged to alter the angular distribution of the radiation prior to the radiation illuminating the patterning device, and a second storage position, wherein the diffuser is disposed outside the path of the radiation generated by the illumination system.

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 the input plane of the measurement system. The input plane may comprise an input plane of a sensor device of the measurement system.

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 patterning device and to project the received radiation onto the sensor apparatus.

The diffuser may be mounted on a patterning device masking blade of the lithographic apparatus, an edge of the patterning device masking blade defining a field region of the lithographic apparatus.

A method of forming a diffuser to receive and transmit radiation is also described herein. The method includes forming an 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 species from the alloy layer and to form a scattering layer including the first species and having a plurality of voids distributed therein.

The second substance may be zinc and the dealloying may be dezincification.

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.

The porous structure may be porous silicon. The micropores may have a range on the order of a few nanometers in at least one dimension.

The scattering layer may be formed on the support layer.

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.

The second material may be a material that is relatively optically neutral to the intended radiation. For example, the second material may be relatively optically neutral to EUV radiation. For example, the second material may be silicon.

The method may further include shrinking one or more particles 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.

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 undulating 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 lying in a plane substantially perpendicular to the direction of the received radiation in use.

The method may further comprise removing the particles after depositing the scattering material.

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, 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.

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, 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 on 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 selectively etching comprises etching regions of the support layer that are in contact with the second material, or wherein the second material is a protective material and the selectively etching comprises etching regions of the support layer that are not in contact with the second material.

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 that contacts the particles. The particles may be deposited from the dispersion of the 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.

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

Depositing the plurality of particles may further comprise fusing the plurality of particles. The plurality of particles may be fused via the provision of heat and/or pressure. The plurality of particles may be fused using sintering.

The particles may comprise a binary mixture comprising a first material and a second material having a different refractive index than the first material. The first material may comprise molybdenum, ruthenium, niobium, rhodium, yttrium, or technetium. The second material may comprise silicon.

The method may further comprise forming another scattering layer on the diffuser. The further scattering layer may be formed according to the methods of any of the examples described herein.

Forming the further scattering layer may comprise depositing an intermediate layer over 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.

In any of the example methods for forming a diffuser described herein, the support layer may be formed on a carrier layer that is used to support the support layer when the diffuser is formed, and wherein the method further comprises removing the carrier layer once the first and second layers 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.

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 holograms may advantageously provide controlled diffusion of radiation in combination with minimal absorption of radiation. Such a diffuser may have an increased lifetime compared to known diffusers, for example due to a reduced absorption of radiation.

The hologram may have an angular intensity distribution that is at least as strong in a radially outer portion of the hologram as compared to a central region of the hologram. The angular intensity distribution may have a similar intensity in the central region compared to the 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 than in the radially outer portion of the hologram. The radially outer portion may be angularly spaced at least 9 ° from the center of the hologram. Such diffusers may have particular benefits in devices having high numerical apertures.

Photolithography may be used to create the plurality of structures. Each portion of the plurality of structures may haveWhere λ is the wavelength of the radiation that produced the hologram when received by the diffuser, and the holographic diffuser has an effective refractive index n _eff.

The method may further include depositing a second substance into a plurality of voids distributed within the plurality of structures. The second substance has a thickness such that a combined thickness profile 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 and second substances having different real parts of the refractive index of the first and second substances 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 and second substances having similar imaginary parts of the refractive indices of the first and second substances may reduce attenuation through the diffuser. The combined first and second substances act to reduce the relative differences in attenuation experienced by radiation traveling through the structure and the void.

The method may further include generating a thickness profile corresponding to a desired arrangement of the plurality of surface features, the desired arrangement being based on a desired angular profile of the hologram. Generating the thickness profile may include numerical methods. Generating the thickness distribution may include iteratively solving and/or performing calculations based on the optical relationship. The optical relationship may represent one or more of attenuation, refractive index, scattering angle, layer thickness, phase shift. The thickness profile generation may include a limit on the maximum and/or minimum allowable thickness. The maximum and/or minimum allowed thickness may be based on fabrication parameters. The maximum and/or minimum allowed thickness may be based on desired optical properties, such as attenuation.

Generating the thickness profile may include using a Gerchberg-Saxton algorithm. Generating the thickness profile may include using a modified version of the Gerchberg-Saxton algorithm.

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

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

The methods described herein may further include etching the support layer from a surface of the support layer opposite 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.

This back-etching of the support layer allows for the use of thicker, more stable support layers during the manufacture of the diffuser. Advantageously, this may prevent damage or even breakage of the support layer. Such a final etching step may be particularly advantageous for embodiments in which a colloid is used to deposit the nanoparticles, since such a final etching step may prevent capillary forces from braking the support layer. Once the nanoparticle layer has been formed, the thickness of such layer can be ultimately determined using an etching process.

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

The term patterning device as used herein may also be referred to herein as a mask or reticle, and the term is to be interpreted as synonymous.

Drawings

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

FIG. 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;

FIG. 2 is a schematic illustration of a reflective identification;

Fig. 3A and 3B are schematic illustrations of a sensor device;

FIG. 4A shows an intensity distribution of a bipolar illumination mode of the lithographic apparatus shown in FIG. 1;

FIG. 4B depicts an intensity distribution of a four-level illumination mode of the lithographic apparatus shown in FIG. 1;

fig. 5A to 5C schematically depict intermediate stages in an example process for manufacturing a transmissive diffuser;

fig. 6A to 6C schematically depict intermediate stages in another example process for manufacturing a transmissive diffuser;

fig. 7A to 7E schematically depict intermediate stages in another example process for manufacturing a transmissive diffuser;

fig. 8A to 8D schematically depict intermediate stages in another example process for manufacturing a transmissive diffuser;

fig. 9A to 9E schematically depict intermediate stages in another example process for manufacturing a transmissive diffuser;

fig. 10A to 10E schematically depict intermediate stages in another example process for manufacturing a transmissive diffuser;

11A-11C schematically depict intermediate stages in another example process for manufacturing a transmissive diffuser;

figure 12 schematically illustrates an EUV diffuser;

figure 13 shows a plot of the extinction coefficient k for EUV radiation versus the magnitude of (1-n) for EUV radiation for some materials;

14A-14C schematically depict intermediate stages in another example process for manufacturing a transmissive diffuser;

Fig. 15A illustrates a height view of an example diffuser manufactured according to the process of fig. 14A-14C;

15B and 15C depict the scattering angle of a plane wave of radiation incident on the diffuser of FIG. 15A;

Fig. 16A and 16B schematically depict intermediate stages in another example process for manufacturing a transmissive diffuser;

FIG. 17 illustrates properties of an example diffuser manufactured according to the process of FIGS. 16A and 16B;

FIG. 18 illustrates properties of an example diffuser manufactured according to the process of FIGS. 16A and 16B;

FIG. 19 illustrates properties of an example diffuser manufactured according to the process of FIGS. 16A and 16B, and

Fig. 20 illustrates the properties of an example diffuser manufactured according to the process of fig. 16A and 16B.

Detailed Description

FIG. 1 depicts a lithographic system including a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA includes 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.

The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. In addition, illumination system IL may include a facet field mirror device 10 and a facet pupil mirror device 11. Together, facet field mirror device 10 and facet pupil mirror device 11 provide EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may also include other mirrors or devices in addition to or in place of the facet field mirror device 10 and the facet pupil mirror device 11.

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

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.

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

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.

The lithographic apparatus may be used, for example, 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 a substrate W (i.e., dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the de-magnification and image reversal characteristics of the projection system PS. The patterned beam of radiation incident on the substrate W may comprise a radiation band. The radiation band may be referred to as an exposure slit. During a scanning exposure, movement of the substrate table WT and support structure MT may cause an exposure slit to travel over an exposure field of the substrate W.

As already described above, a lithographic apparatus may be used to expose portions of the substrate W in order to form a pattern on the substrate W. To improve the accuracy of transferring the desired pattern to the substrate W, one or more properties of the lithographic apparatus LA may be measured. Such properties may be measured periodically, e.g. before and/or after exposure of each substrate W, or may be measured less frequently, e.g. as part of a calibration process. Examples of properties of the lithographic apparatus LA that may be measured include relative alignment of components of the lithographic apparatus LA and/or aberrations of components of the lithographic apparatus. For example, measurements may be made to determine the relative alignment of the support structure MT for supporting the patterning device MA and the substrate table WT for supporting the substrate W. Determining the relative alignment of the support structure MT and the substrate table WT facilitates the projection of the patterned radiation beam onto a desired portion of the substrate W. This may be particularly important when projecting the patterned radiation onto the substrate W comprising portions that have been exposed to the radiation in order to improve the alignment of the patterned radiation with previously exposed areas. Additionally or alternatively, measurements may be made to determine the deformation of the patterning device MA.

Additionally or alternatively, measurements may be made in order to determine the optical aberrations of the projection system PS. Optical aberrations are deviations of the performance of the optical system from paraxial optics and may result in blurring or distortion of the pattern being exposed at the substrate W. The aberrations of the projection system PS can be adjusted and/or taken into account in order to increase the accuracy with which the desired pattern is formed on the substrate W.

Measurements, such as alignment and aberration measurements described above, may be performed by illuminating the reflective markers 17 (as schematically shown in fig. 1) with radiation. In alternative arrangements, transmissive identification may be used. An identity is a reflective feature that appears in an image produced by an optical system when the identity 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 a measure of the nature of an image formed by an optical system. For example, radiation reflected from the reflective markers may be used to determine alignment of one or more components and/or optical aberrations of one or more components.

In the embodiment shown in fig. 1, the reflective markers 17 form part of the patterning device MA. One or more indicia 17 may be provided on the patterning device MA for performing lithographic exposure. The markers 17 may be positioned outside patterned areas of the patterning device MA, which are irradiated by radiation during lithographic exposure. In some embodiments, one or more markers 17 may additionally or alternatively be provided on the support structure MT. For example, a piece of dedicated hardware (commonly referred to as a datum) may be provided on the support structure MT. The fiducial piece may include one or more markers. For the purposes of this specification, a fiducial is considered an example of a patterning device. In some embodiments, a patterning device MA, which is specifically designed to measure one or more properties of the lithographic apparatus LA, may be placed on the support structure MT in order to perform a measurement process. The patterning device MA may comprise one or more markers 17 for performing the irradiation as part of the measurement process.

In the embodiment shown in fig. 1, the lithographic apparatus LA is an EUV lithographic apparatus and thus a reflective patterning device MA is used. The identification 17 is thus a reflective identification 17. The configuration of the identifier 17 may depend on the properties of the measurement to be made using the identifier 17. The identification may, for example, comprise one or more reflective pinhole features including a reflective region surrounded by an absorptive region, a reflective line feature, an arrangement of multiple reflective line features, and/or a reflective grating structure such as a reflective diffraction grating.

To measure one or more properties of the lithographic apparatus LA, a sensor device 19 (as schematically shown in fig. 1) is provided to measure the radiation output from the projection system PS. As shown in FIG. 1, the sensor device 19 may be provided, for example, on a substrate table WT. To perform the measurement process, the support structure MT may be positioned such that the marks 17 on the patterning device MA are illuminated by radiation. The substrate table WT may be positioned such that radiation reflected from the markers 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 from measurements made by the sensor device 19. In some embodiments, a plurality of markers 17 and/or sensor devices 19 may be provided, and properties of the lithographic apparatus LA may be measured at a plurality of different field points (i.e. fields of the projection system PS or locations in the object plane).

As described above, in some embodiments, radiation reflected from the identity may be used to determine the relative alignment of components of the lithographic apparatus LA. In such embodiments, the indicia 17 may include features that, when irradiated with radiation, impart an alignment feature to the radiation. The features may for example comprise one or more reflective patterns in the form of a grating structure.

The position of the alignment feature in the radiation beam B may be measured by a sensor device 19 located at the level of the substrate W (e.g. on the substrate table WT as shown in fig. 1). The sensor device 19 may be operable to detect the position of the alignment feature in the radiation incident on the sensor device 19. This may allow alignment of the substrate table WT relative to the marks 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 can be moved relative to each other to form a pattern at a desired location on the substrate W (using the patterned radiation beam B reflected from the patterning device MA). A separate measurement procedure may be used to determine the position of the substrate W on the substrate table.

As further described above, in some embodiments, the patterning device MA may be provided with one or more markers 17 that may be used to measure aberrations of the projection system PS. Similar to the alignment measurement described above, the aberration may be detected by measuring radiation reflected from the marker 17 with a sensor device 19 located at or near the substrate table WT. The one or more markers 17 on the patterning device MA may be illuminated with EUV radiation by an illumination system IL. Radiation reflected from one or more markers is projected by the projection system PS onto an image plane of the projection system PS. One or more sensor devices 19 are positioned at or near the image plane (e.g. on the substrate table WT as shown in fig. 1), and may measure the projected radiation in order to determine the aberrations of the projection system PS. Referring now to fig. 2 and 3A, embodiments of the identification 17 and the sensor device 19 that may be used to determine aberrations of the projection system PS will be described.

Fig. 2 is a schematic diagram of a logo 17 that may form part of a patterning device MA, according to an embodiment of the invention. The cartesian coordinate system is also shown in fig. 2. The y-direction may represent a scanning direction of the lithographic apparatus. That is, during a scanning exposure, movement of the substrate table WT and the support structure MT may be such that the patterning device MA is scanned relative to the substrate W in the y-direction. The identifier 17 generally lies in an x-y plane. That is, the indicia generally extends in a direction perpendicular to the z-direction. Although reference is made to an identifier that is generally located in a plane, it will be understood that the identifier is not entirely constrained to/limited to a plane. That is, portions of the logo may extend out of the plane in which the logo generally lies. As will be explained further below, the identification may comprise a diffraction grating. The diffraction grating may comprise a three-dimensional structure comprising portions that are not entirely in a plane but instead extend out of the plane.

The logo 17 shown in fig. 2 comprises a first portion 17a and a second portion 17b. Both the first portion and the second portion comprise a reflective diffraction grating comprising a periodic grating structure. The grating structure extends in a grating direction. The first portion 17a comprises a diffraction grating extending in a first grating direction, which is denoted u-direction in fig. 2. The second portion 17b comprises a diffraction grating extending in a second grating direction, which is denoted v-direction in fig. 2. In the embodiment of fig. 2, both the u-direction and the v-direction are aligned at approximately 45 ° relative to both the x-and y-directions and are substantially perpendicular to each other. The first portion 17a and the second portion 17b of the logo 17 may be irradiated with radiation at the same time or at different times.

Although the embodiment shown in fig. 2 includes a first portion 17a and a second portion 17b that include diffraction gratings oriented in a perpendicular grating direction, in other embodiments, the indicia 17 may be provided in other forms. For example, the indicia 17 may include reflective and absorptive regions arranged to form a checkerboard pattern. In some embodiments, the logo 17 may include an array of pinhole features. The reflective pinhole feature may include a region of reflective material surrounded by an absorptive material.

When the first portion 17a and/or the second portion 17b of the marking is illuminated by radiation, a plurality of diffraction orders are reflected from the marking. At least a portion of the reflected diffraction orders enter the projection system PS. The projection system PS forms an image of the logo 17 on the sensor device 19. Fig. 3A and 3B are schematic views of the sensor device 19. Fig. 3A is a side view and fig. 3B is a top view of the sensor device. Cartesian coordinates are also shown in FIGS. 3A and 3B.

The Cartesian coordinate system used in FIGS. 2, 3A and 3B 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, in FIG. 2, the z-direction is perpendicular to the x-y plane in which patterning device MA and marking 17 typically extend. In fig. 3A and 3B, the z-direction is perpendicular to the x-y 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 a scanning exposure. The x-direction represents the non-scanning direction perpendicular to the scanning direction. It will be appreciated (e.g. from fig. 1) that in a 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 elements at each optical element in the lithographic apparatus.

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 fig. 3B and comprises a checkerboard diffraction grating. The area of the diffraction grating 21 shown as shaded black in fig. 3B represents an area of the diffraction grating 21 configured to be substantially opaque to incident radiation. The unshaded areas of the diffraction grating 21 shown in fig. 3B represent areas configured to transmit radiation. For ease of illustration, the opaque and transmissive regions of the diffraction grating 21 are not shown to scale in fig. 3B. For example, in practice, the ratio of the diffraction grating features relative to the size of the diffraction grating itself may be smaller than indicated in fig. 3B.

The diffraction grating 21 shown in fig. 3B is depicted as having a checkerboard configuration with transmissive and opaque regions that include squares. However, in practice, it may be difficult or impossible to manufacture a transmissive diffraction grating comprising perfectly square transmissive and opaque regions. The transmissive region and/or the opaque region may thus have a cross-sectional shape other than completely square. For example, the transmissive region and/or the opaque region may have a cross-sectional shape that includes a square (or more generally a rectangle) with rounded corners, i.e., rounded corners. In some embodiments, the transmissive region and/or the opaque region may have a substantially circular or elliptical cross-sectional shape. In some embodiments, the diffraction grating 21 may comprise an array of pinholes formed in an opaque material.

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 the procedure for determining aberrations, the support structure MT may be positioned such that the marking 17 is irradiated with radiation from the irradiation system IL. The substrate table WT may be positioned such that radiation reflected from the markers is projected by the projection system PS onto the sensor device 19.

As described above, a plurality of diffraction orders are formed at the mark 17. Further diffraction of the radiation occurs at the diffraction grating 21. The interaction between the diffraction orders formed at the logo 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 wave front that has propagated through the projection system. Thus, the interference pattern may be used to determine the aberrations of the projection system PS.

As described above, the first and second portions of the logo 17 comprise diffraction gratings aligned perpendicular to each other. The radiation reflected from the first portion 17a of the marker 17 may provide information about the gradient of the wavefront in the first direction. The radiation reflected from the identified second portion 17b may provide information about the gradient of the wavefront in a second direction, which is perpendicular to the first direction. In some embodiments, the first and second portions of the logo may be illuminated at different times. For example, a first portion 17a of the marker 17 may be illuminated at a first time to derive information about the gradient of the wavefront in the first direction, and a second portion 17b of the marker 17 may be illuminated at a second time to derive information about the gradient of the wavefront in the second direction.

In some embodiments, the patterning device MA and/or the sensor device 19 may be scanned sequentially 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 second portion 17b of the logo 17 is illuminated while the patterning device MA and/or the sensor device 19 can step in the u-direction and the first portion 17a of the logo 17 is illuminated while the patterning device MA and/or the sensor device 19 can step in the v-direction. 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.

The patterning device MA and/or the sensor device 19 may be stepped a distance corresponding to a fraction of the grating period of the diffraction grating. Measurements taken at different step positions may be analyzed to derive information about the wavefront in the step 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 the u-direction and the v-direction being perpendicular to each other) allows deriving information about the wavefront in both perpendicular directions, thereby allowing reconstructing the complete wavefront.

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. 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, a first portion 17a of the logo 17 is illuminated while the patterning device MA and/or the sensor device 19 may be scanned in the u-direction, and a second portion 17a of the logo 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 measurement to be averaged across the whole diffraction grating, thereby taking into account any variations in the scanning direction of the diffraction grating. 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.

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

In some embodiments, the identification 19 and/or the sensor device 19 may comprise components other than a diffraction grating. For example, in some embodiments, the logo 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 logo 17, the pinhole characteristics may include a portion of reflective material surrounded by absorptive material such that radiation is reflected from only a small portion of the logo. The single slit feature may be in the form of a single strip of reflective material surrounded by absorptive material. The pinhole feature and/or the single slit feature at the sensor device 19 may be a transmissive feature. In general, the identifier 17 may be any feature that applies to the radiation beam that may be used as a reference point or to determine a measurement of the radiation beam.

Although in the above described embodiment a single marker 17 and sensor device 19 are provided, in other embodiments a plurality of markers 17 and sensor devices 19 may be provided to measure wavefront phase variations at different field points. In general, any number and configuration of identification and sensor devices 19 may be used to provide information about the wavefront phase variation.

The controller CN (as shown in fig. 1) receives the results of the measurements made at the sensor device 19 and determines the aberrations of the projection system PS from the results of said measurements. The controller 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 one another. The controller CN may control an adjusting device PA for adjusting components of the projection system PS. For example, the adjustment device PA may adjust elements of the projection system PS to correct aberrations determined by the controller CN.

The projection system PS comprises a plurality of reflective lens elements 13, 14 and an adjustment means PA for adjusting the lens elements 13, 14 in order to correct aberrations. To achieve such correction, the adjustment device PA may be operable to manipulate the reflective lens element within the projection system PS in one or more different ways. The adjustment means PA may be operable to displace one or more lens elements, tilt one or more lens elements, and/or deform one or more lens elements in any combination.

The projection system PS has an optical transfer function that may be non-uniform, which affects the pattern imaged on the substrate W. For unpolarized radiation, these effects can be very well described by two scalar maps or scalar maps describing the transmission (apodization) and relative phase (aberration) of the radiation exiting the projection system PS as a function of position in the 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 the complete set of basis functions. It will be appreciated that the terms "transmission map" and "relative intensity map" are synonymous and that transmission map may alternatively be referred to as relative intensity map. A particularly convenient set of basis functions for expressing these scalar mappings is the Zernike (Zernike) polynomial, which forms a set of orthogonal polynomials defined on a 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 may be determined by sequentially calculating the inner product of the scalar map being measured and each zernike polynomial and dividing such inner product by the square of the norm of the zernike polynomial.

The transmission mapping and the relative phase mapping are field and system dependent. That is, typically, each projection system PS will have a different zernike expansion for each field point (i.e., for each spatial location in the image plane of the projection system PS).

Determining the aberrations of the projection system PS may comprise fitting the wavefront measurement made by the sensor device 19 to a zernike polynomial to obtain the 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).

Different zernike coefficients may provide information about different forms of aberrations caused by the projection system PS. Typically, a zernike polynomial is considered to comprise a plurality of orders, each having an associated zernike coefficient. The order and coefficients may be annotated with an index, commonly referred to as the nopal (Noll) index. A zernike coefficient having a nook index of 1 may be referred to as a first zernike coefficient, a zernike coefficient having a nook index of 2 may be referred to as a second zernike coefficient, and so on.

The first zernike coefficient is related to the average value of the measured wavefront (which 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 first zernike coefficient may not be determined using the methods described herein. The second zernike coefficient is related to the tilt of the measured wavefront in the x-direction. Tilting 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. Tilting 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 the placement in the z-direction. Higher order zernike coefficients are associated with other forms of aberrations such as astigmatism, coma, spherical aberration and other effects.

Throughout this specification, the term "aberration" is intended to include all forms of deviation of a wavefront from a perfectly spherical wavefront. That is, the term "aberration" may relate to the placement of the image (e.g., second, third, and fourth zernike coefficients) and/or to higher order aberrations, such as aberrations related to zernike coefficients having a nook index of 5 or more.

As described in detail above, one or more reflective markers 17 may be used to determine alignment and/or aberrations of a component of the lithographic apparatus LA. In some embodiments, a separate identifier 17 may be used to determine the alignment of the component with the identifier used to determine the aberrations. For example, a patterning device MA adapted for use in a lithographic exposure process may have one or more marks outside of a patterned area adapted for use in a lithographic exposure process. The one or more markers may be adapted to determine an alignment of the patterning device MA relative to the substrate table WT.

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

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

Reference herein to a patterning device MA should be construed to include any device that includes one or more features configured to modify radiation. The patterning device MA may, for example, be provided with a pattern for use during a lithographic exposure (e.g. the patterning device may be a reticle). Additionally or alternatively, the patterning device may be provided with one or more identifiers for the measurement process. Typically, the patterning device MA is a removable component that is placed on the support structure MT in order to perform a particular process (e.g. to perform a 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. logos) for a measurement process. For example, the support structure MT may be provided with one or more fiducials comprising one or more markers. In such an embodiment, the support structure MT may be regarded as an example of a patterning device, as it is provided with one or more features configured to modify radiation. Reference herein to a patterning device comprising a reflective marking should not be construed as limited to a removable patterning device, but rather should be construed to include any device having a reflective marking disposed thereon.

Referring to fig. 1, the patterning device MA may be considered to be disposed in the object plane of the projection system PS and the substrate W may be considered to be disposed 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 plane conjugated thereto, may be referred to as the field plane of the lithographic apparatus. It will be appreciated that within an optical system (e.g. a lithographic apparatus), if each point within a first plane P is imaged onto a point of a second plane P', the two planes are conjugate.

It will be appreciated that the lithographic apparatus LA comprises optics (i.e. focusing and/or diverging optics) having optical power or optical power in order to form an image in an image plane of an object in the object plane. Within such an optical system, a pupil plane may be defined between each pair of field planes as a fourier transform plane of the previous field plane and the successive field plane. The distribution of the electric field in each such pupil plane is related to the fourier transform of the object disposed in the previous field plane. It will be appreciated that the quality of such pupil planes will depend on the optical design of the system and that such pupil planes may even be curved. This applies to consider two pupil planes, the pupil plane of the illumination system and the 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 planes that are conjugate to each other. The intensity (or equivalently, the electric field intensity) distribution of the radiation in the pupil plane PP _IL of the illumination system may be referred to as the illumination mode or pupil filling and characterizes the angular distribution of the light cone at the patterning device MA (i.e. in the object plane). Similarly, the intensity (or equivalently, the electric field intensity) distribution of the radiation in the pupil plane PP _IL of the projection system characterizes the angular distribution of the light cone at the wafer level (i.e., in the image plane).

The illumination system IL may change the intensity distribution of the beam in a pupil plane of the illumination system. This can be achieved by appropriately configuring the facet field mirror device 10 and the facet pupil mirror device 11.

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

When the lithographic apparatus is not exposing the substrate W, one of the more reflective markers provided on the patterning device MA may be used in a measurement process, for example to determine alignment and/or aberrations associated with the lithographic apparatus LA. When using reflections from the markers to measure alignment and/or aberrations, it may be desirable for the radiation reflected from the markers to fill a larger portion of the pupil of the projection system PS. To achieve such filling, in principle, the illumination system IL may be reconfigured to fill the pupil plane of the illumination system (and thus also the entrance pupil of the projection system). However, to do so (and to revert to the exposure illumination mode prior to the next exposure) may take more time than is required for such online or on-line measurements. It is therefore known to provide a diffuser during such measurements, which is arranged to increase the angular spread, i.e. angular spread, of radiation scattered from the object horizontal patterning device in order to increase the proportion of the entrance pupil of the projection system PS filled with radiation.

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 lithographic apparatus to be operable to perform semi-continuous in-line metrology, i.e. inline metrology, which in turn may be used to maintain optimal dynamic settings of the projection system PS, the support structure MT and the substrate table WT. In addition, such a measurement system may be used to align the patterning device MA with the substrate W prior to exposing the substrate W.

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

Another arrangement uses a reflective object, i.e. a combined diffuser and patterning device, as described in WO 2017/207512. Such an arrangement is in the form of a multilayer reflective stack arranged to preferably reflect EUV radiation, the arrangement being applied with a pattern of EUV absorbing material (e.g. a diffraction grating). Each layer in the multilayer reflective stack is provided with a surface roughness such that the reflected radiation is diffused. However, although in principle such patterning means may be provided on the reticle, it is significantly more complicated to manufacture patterning means having such built-in surface roughness. Thus, in practice, such patterning devices are more likely to be formed on the reference.

Embodiments of the invention relate to a novel diffuser and a method for its preparation, which is particularly suitable for use with an EUV measurement system within an EUV lithographic apparatus of the type discussed above.

Fig. 5A to 5C (collectively, i.e. fig. 5) schematically show stages in a method of making a diffuser according to a first example. The diffuser may be constructed from multiple layers, referred to herein as a stack. An intermediate stack of layers in one example process for creating a diffuser is depicted in cross-section in fig. 5A-5C. Referring to fig. 5A, the first intermediate laminate 50 includes a layer 502 of support material. For example, the support material may include silicon nitride (SiN), silicon, molybdenum silicide (MoSi ₂). The support material layer 502 may have a thickness of about 10nm to 60 nm. In some embodiments, the support material is a material having a refractive index close to 1 for EUV radiation and a relatively low absorption coefficient for EUV radiation. For such embodiments, the support material may be considered to be relatively optically neutral to EUV radiation. A layer of support material 502 is formed on a carrier layer 504 that may be used to support the layer of support material 502 while the diffuser is being formed. For example, the carrier layer 504 may be formed of silicon, silicon nitride (SiN), porous silicon (pSi), or molybdenum silicide (MoSi). The carrier layer 504 may, for example, have any thickness suitable to provide adequate support during manufacture, and in some arrangements may have a thickness of about 100 μm to 500 μm. For example, the carrier layer may be a standard silicon wafer. Alternatively, the carrier layer 504 and the support layer 502 may be provided by a single layer of the same material.

A layer 506 of scattering material 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 material, 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) of between about 50nm and 400 nm.

Another layer 508 of a different metal is deposited on top of the intermediate stack 50 to form a second intermediate stack 52. For example, the other metal may be zinc (Zn). Layers 506 and 508 are treated to form an alloy layer (not shown) comprising an alloy of a scattering metal and another metal, such as 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, the layers 506, 508 may be annealed. The annealing may be performed, for example, at 400 degrees. Annealing may be performed in a protective gas atmosphere. For example, annealing may be performed in the presence of an inert gas such as argon.

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

After the dealloying, i.e., dealloying, process, an intermediate stack 54 is provided that includes a porous scattering layer 510 on a support layer 502. The scattering layer 510 may be considered as a scattering substance having a plurality of voids distributed therein. The method may further include etching the carrier layer 504 from a surface opposite the surface of the support layer supporting the porous scattering layer 510. In the case where the carrier layer 502 and the support layer 504 are separate, i.e., separate layers, the carrier layer 502 may provide an etch stop during this back side etching process. This back-side etching of the carrier layer 504 allows for the use of thicker, more stable carriers 502 and support layers 504 during fabrication. Advantageously, this may prevent the support layer 504 from being damaged or even broken.

Alternatively, the porous scattering layer 510 may contain another substance located within a plurality of pores (or voids). For example, the plurality of holes of the porous scattering layer 510 may be filled with an inert gas. Alternatively, the plurality of holes of the porous scattering layer 510 may be filled with 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 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 a material that is optically significantly different (e.g., a material that is relatively optically neutral to EUV radiation, e.g., a material having a refractive index of 1 or approximately close to 1). Such infiltration or wetting of the porous scattering layer 510 may be beneficial to protect against degradation, to protect structural integrity, to allow for thermal diffusion.

While in practice, the diffuser is likely to comprise multiple layers (such as a scattering layer and a support layer), the term diffuser herein may also refer to only a scattering layer (i.e., a layer configured to diffuse incident radiation).

Fig. 6A to 6C (collectively, i.e. fig. 6) depict another example process for manufacturing a diffuser suitable for EUV radiation. The intermediate stack of layers in the example process is depicted in cross-section in fig. 6A-6C. Referring to fig. 6A, a 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 as described above in connection with the support layer 502 and the carrier layer 504 of fig. 5A-5C.

The intermediate laminate 60 also includes a porous layer 606, the porous layer 606 being 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 treatment to create the porous layer 606 may include, for example, selective etching (e.g., metal-assisted chemical etching, anodization, selective leaching/leaching).

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

As described with respect to the previous example process, the method may further include etching the carrier layer 604 from a surface opposite the support for 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 etched after the scattering layer has been provided on the support structure.

In addition, in all examples described herein, additional layers may be provided in addition to those shown in fig. 5 and 6. For example, referring to fig. 5 as an example, another layer may be disposed 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 beneficial to provide additional protection for the scattering layer during use, in particular protection from particles present inside the lithographic apparatus. Similarly, an additional (or "cap") layer may be provided atop the scattering layer 510 for the same purpose. Such additional layers may have a thickness of about 10 nm. This additional layer formed forms a metal oxide or metal nitrate. For example, an additional layer may be provided from silicon nitride or molybdenum silicide.

Fig. 7A to 7E (collectively, i.e. fig. 7) depict another example process for manufacturing a diffuser suitable for EUV radiation. In the example of fig. 7, a non-uniform layer of scattering material is deposited on a random or quasi-random arrangement of structures (such as pillars on the surface of the support layer or holes in the surface of the support layer). The structure may be provided according to any suitable technique. For example, and as depicted in fig. 7, structures may be provided by nanoparticle lithography. Alternatively, the structure may be created using normal (e.g., resist) lithography with a pseudo-random mask, dealloying with selective leaching/leaching, metal-assisted chemical etching using random deposition of metal catalyst particles, and the like.

In the example depicted in fig. 7, the intermediate stack 70 includes a support layer 702. For example, the support layer 702 may take the same or similar form as the support layers 502, 602 described above with reference to fig. 5 and 6. Although not depicted in fig. 7, it should be appreciated that 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.

The nanoparticle layer 704 is deposited on the support layer 702 in a random or quasi-random distribution. The particles in layer 704 may be formed from polystyrene particles. Alternatively, the particles in layer 704 may be formed of another material suitable for nanosphere lithography, such as latex or silica, cellulose, and the like. In the example depicted in fig. 7A, the particles in layer 704 are polydisperse, including multiple particles having different size ranges. In particular, for example, it may be that some particles 704 have a smaller diameter than the diameter of other particles 704. As an example, the particles may have a diameter in the range of 20nm to 300 nm. The particles have a random distribution of the displacement 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 colloid or a vertical deposition process of colloid from the containing particles. For example, a Langmuir-Blodgett deposition process may be used, as generally described, for example, in 20042041524-1526 (https:// doi. Org/10.1021/la035686 y) on month 12 2003 of Langmuir. The vertical deposition process is suitable for providing a monolayer of polystyrene particles. It should be appreciated, however, that the deposition may be according to any method that can be adapted to produce a single layer or a small number of layers. For example, deposition may be by spin coating or inkjet particles in a solvent. The particles may be spherical or substantially spherical (e.g., the particles may be a plurality of ellipsoids). However, the particles may have other shapes.

As depicted in fig. 7B, the particles in layer 704 may be shrunk to provide a second intermediate stack 72. Shrinkage of the particles in layer 704 is an optional step and may be beneficial to further expose areas of support layer 702, allowing for adjustment of the dimensions (size, spacing, density) of the created structure, as described in more detail below. For example, the particles may be treated using Reactive Ion Etching (RIE), which causes each of the particles in layer 704 to shrink.

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 the arrangement of particles on the surface will be random or quasi-random, as will the mask provided by those particles.

The catalyst is deposited onto the surface of the support layer 702 adjacent to the particle layer 704 such that the portions of the surface not obscured by the particles are coated with a deposit 712 of catalyst. The catalyst may be a metal catalyst such as gold or platinum. Particles in layer 704 are removed and the surface of the support layer 702 with the catalyst deposited thereon is selectively etched to form a modified support material layer 714 and a third intermediate stack 74. In effect, the locations of the support layer 702 in contact with the catalyst deposit 712 are etched to create a plurality of 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 robust process. The structure in this example includes a plurality of cavities or pockets with corresponding peaks or pillars.

In the alternative, a process may be used whereby the mask is deposited onto the support layer 704 between the particles and the etched surfaces of the support layer in those areas not protected by the mask. In the case where the process depicted in fig. 7 creates a cavity in the area under the catalyst, such an alternative process may be considered to create a guide post in the area under the protective mask. Any suitable masking material and etching process may be used, as will be well known to those skilled in the art.

After creating the modified support structure 714, the catalyst or mask may be removed to provide the fourth intermediate stack 76, but it should be understood that this is an optional step.

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

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

Fig. 8A to 8D (collectively, i.e. fig. 8) depict another example process for manufacturing a diffuser suitable for EUV radiation. Fig. 8A depicts a first intermediate stack 80 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 on support layer 802 in a random or pseudo-random distribution using a vertical deposition process.

The second intermediate stack 82 is created by depositing a layer 806 of scattering material on the support layer 802 between the nanoparticles 804. For example, the scattering material layer 806 may be deposited by means of electrodeposition. In this manner, the nanoparticles 804 form a mask on the support layer 802 such that the scattering material 806 forms in the gaps around the particles 804 to provide a non-uniform layer of scattering material. The nanoparticles 804 may optionally be shrunk prior to the deposition of the scattering layer to alter the spacing between the nanoparticles and expose more of the support layer 802.

The nanoparticles 804 are optionally removed to provide a third stack 84. The third stack 84 may be used to provide a diffuser (e.g., after any desired back side etching of a carrier layer (not shown). It should be appreciated that the scattering material 806 forms a corrugated or undulating structure on the support layer 802. The undulating structure is defined by peaks and valleys, and the spacing between adjacent peaks is defined by the size of the nanoparticle 804 or nanoparticles 804 separating those peaks during manufacture of the diffuser. Similarly, the depth of the valleys (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 layer of scattering material 806 is deposited around the nanoparticles 804 (which may vary depending on the desired scattering/attenuation properties and the particular scattering material used).

The corrugations will match the distribution of nanoparticles 804 such that the corrugations may be randomly or quasi-randomly distributed across the scattering layer and have a plurality of different ranges in each dimension. For example, some of the plurality of peaks may have a different range in any of the x, y, or z directions than others of the plurality of 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. Additionally, it should be appreciated that the corrugations will have different curvatures (e.g., the corrugations will have different gradients) due to the different sizes of the nanoparticles 804.

Additionally, an optional second layer of scattering material may be provided, as depicted in fig. 8D, in this example, an intermediate layer 808 is deposited atop the scattering layer 806 to create another intermediate stack 86. The intermediate layer 808 may be formed of a material that is relatively optically neutral to EUV radiation (e.g., has a refractive index close to 1 for EUV radiation and a relatively low absorption coefficient for EUV radiation). For example, the intermediate layer 808 may be formed of silicon. The intermediate layer 808 may have a thickness in the range of 30nm to 400nm, and preferably in the range of 30nm to 150 nm. The process depicted in fig. 8A-8C may then be repeated 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 incident EUV radiation when used as a diffuser, and will help prevent or reduce zero order scattering.

Fig. 9A to 9E (collectively, i.e. fig. 9) depict another example process for manufacturing a diffuser suitable for EUV radiation. As shown in fig. 9A, the first intermediate stack 90 takes the same form as the intermediate stack 80 of fig. 8A, with a nanoparticle layer 904 deposited on top of the support layer 902. Similar to the process depicted in fig. 8, a first scattering layer 906 is deposited on the support layer 902 between the nanoparticles 904. In contrast to the method depicted in fig. 8, a second intermediate stack 92 (fig. 9B) is created by depositing an intermediate (or sacrificial) layer 908 between the first scattering layer 906 and another scattering layer 910. 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.

The third intermediate stack 94 is created by removing the nanoparticles 904, leaving cavities within the layers 906, 908, 910 (fig. 9C). The nanoparticles 904 may be removed by any suitable technique within the field sometimes referred to as "nanoparticle lithography," and indeed any other suitable technique as will be apparent to those skilled in the art. For example only, the nanoparticles 904 may be removed by heating. The fourth intermediate stack 96 is created by filling the cavity with a material that is relatively optically neutral to EUV radiation (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 cavities), or via any other suitable method). The material within the cavity thus forms an intermediate support structure 912 supporting the scattering layer 910 above the scattering layer 906.

A fifth stack 98 is created by removing the intermediate layer 908 (fig. 9E). For example, the intermediate layer 908 may be removed by etching. The stack 98 thus comprises two scattering layers 906, 910 separated and supported by a relatively sparse intermediate support structure 912 (i.e., separated particles). As in the previous example, the process of fig. 9 may be repeated to create additional layers on top of the scattering layer 910.

In alternative arrangements, 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, the stack 92 depicted in fig. 9B may be considered the final stack and may be used to provide a diffuser (after any other desired processing such as back etching or deposition of a cap layer). That is, in some arrangements, the combination of layers 910, 908, 906 and nanoparticles 904 may together provide a scattering layer of the diffuser.

Fig. 10A to 10E schematically depict another example process for creating a diffuser suitable for EUV radiation. In fig. 10A, an intermediate stack 100 is shown. The intermediate stack 100 includes a support layer 1002 having a nanoparticle layer 1004 deposited thereon. The intermediate stack 100 may be an intermediate stack 70, 80, 90, and the intermediate stack 100 may be generated from the intermediate stack 70, 80, 90.

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

The third intermediate stack 104 is created by removing the nanoparticles to leave pockets or cavities within the filler layer 1006. Nanoparticles 1004 may be removed according to any suitable technique as described above and will depend on their composition.

The fourth stack 106 is created by filling 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 desired back-side 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 relatively optically neutral material, which may be the same as the material (e.g., silicon) used for the filler layer 1006, or may be different. The further layer 1010 provides support to create a further scattering layer (e.g., using the process set forth in fig. 10A-10D, or another process taught herein or elsewhere).

Fig. 11A to 11C (collectively, i.e. fig. 11) depict another example process for creating a diffuser suitable for EUV radiation. In fig. 11A, the intermediate stack 110 includes a support layer 1102 provided with a random or quasi-random multi-layer deposit of polydisperse nanoparticles 1104, such as polystyrene particles. The multilayer deposit of nanoparticles 1104 may be disposed on the support layer 1102 in any suitable manner as previously described herein, such as by vertical colloidal deposition. The nanoparticles may 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 may be occupied by the nanoparticles, and the remaining 30% to 40% are voids.

The second intermediate stack 112 is created by penetrating the interstices between the nanoparticles 1104 with scattering material 1106. The scattering material 1106 may 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,2017, month 5, 11601-11614)).

Optionally, the third stack 114 is created by removing the nanoparticles 1104 to leave voids 1108 within the scattering material 1106. For example, nanoparticles may 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 set forth in fig. 7-11, nanoparticles are used to create scattering structures/layers. The nanoparticles may be removed at an intermediate stage in the manufacturing process. For example, in the case where the nanoparticles are polystyrene particles, the nanoparticles may be removed by heating and evaporation. As indicated above, other types of nanoparticles may be used instead of polystyrene, such as silica, cellulose, etc., which may also be removed by evaporation. Alternatively, titanium oxide (TiO ₂) nanoparticles may also be used, and the titanium oxide nanoparticles may be removed by, for example, selective etching.

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

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 greatly different refractive index than the scattering material 1106), it may be beneficial to retain the nanoparticles for the final diffuser. This provides another benefit in that fewer processing steps are required.

More generally, in the above example, 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 a particular material layer (e.g., scattering layer, intermediate layer, particle layer, and other materials such as aerosol deposition, vertical deposition, electrodeposition, etc.) described in the context of one example may 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 appreciated that any one or more of the processes set forth above may be combined to form a multiple stack having multiple scattering layers or structures. For example, a scattering layer as described with reference to fig. 8C may be disposed atop the porous scattering structure 510 described with reference to fig. 5. Any other combination of scattering layers is possible and should be understood to be within the scope of the present disclosure.

In addition, although the different layers are generally described as having different thicknesses, it should be appreciated that those thicknesses may vary depending on the materials used within the layers and the desired optical interaction of the layers with incident EUV radiation, if present. 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 of approximately between 100nm and 1000nm along the propagation direction of the received radiation.

It will be appreciated that a diffuser manufactured according to the process described herein will be a transmissive diffuser for EUV radiation. In general, to maximize the intensity of EUV radiation output by the diffuser, it is desirable to minimize the attenuation caused by the layer of scattering material. This may be achieved by minimizing the extinction coefficient of the scattering material and/or minimizing the thickness of the scattering material. Furthermore, it will be appreciated that for a given scattering material, it is desirable to increase the thickness of the layer in order to increase the angular dispersion, i.e. the amount of angular spread, whereas it is desirable to decrease the thickness of the layer in order to reduce the attenuation caused by the scattering material. Having a scattering material with a larger magnitude with (1-n) allows for a reduced thickness (while still providing a reasonable angular dispersion). A scattering material with a smaller extinction coefficient k for EUV radiation allows for an increased thickness (while still providing reasonable transmission).

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

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 of 0.06 (i.e., to the right of line 60 in fig. 13). In some embodiments, the magnitude of (1-n) of the scattering material for EUV radiation may be greater than a threshold of 0.08 (i.e., to the right of line 62 in 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 (i.e., to the right of line 64 in fig. 13). In some embodiments, the magnitude of (1-n) of the scattering material for EUV radiation may be greater than a threshold of 0.12 (i.e., to the right of line 66 in fig. 13).

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 for EUV radiation that is smaller than a threshold of 0.04nm ^-1 (i.e., below line 70 in fig. 13). In some embodiments, the scattering material may have an extinction coefficient k for EUV radiation that is smaller than a threshold of 0.03nm ^-1 (i.e., below line 72 in fig. 13). In some embodiments, the scattering material may have an extinction coefficient k for EUV radiation that is smaller than a threshold of 0.02nm ^-1 (i.e., below line 74 in fig. 13). In some embodiments, the scattering material may have an extinction coefficient k for EUV radiation that is smaller than a threshold of 0.01nm ^-1 (i.e., below line 76 in fig. 13).

It will be appreciated that for a given scattering material, it is desirable to increase the thickness of the layer in order to increase the amount of angular dispersion, whereas it is desirable to decrease the thickness of the layer in order to reduce the attenuation caused by the scattering material. A scattering material having a magnitude with a larger (1-n) allows for a reduced thickness (while still providing a reasonable angular dispersion). A scattering material with a smaller extinction coefficient k for EUV radiation allows for an increased thickness (while still providing reasonable transmission). It will therefore be appreciated that in practice suitable materials may be selected to balance these two requirements.

In some embodiments, (1-n) is greater in magnitude than a threshold of 0.06 and the extinction coefficient k for EUV radiation is less in magnitude than a threshold of 0.01nm ^-1, or (1-n) is greater in magnitude than a threshold of 0.08 and the extinction coefficient k for EUV radiation is less in magnitude than a threshold of 0.02nm ^-1, or (1-n) is greater in magnitude than a threshold of 0.1 and the extinction coefficient k for EUV radiation is less in magnitude than a threshold of 0.03nm ^-1, or (1-n) is greater in magnitude than a threshold of 0.12 and the extinction coefficient k for EUV radiation is less in magnitude than a threshold of 0.04nm ^-1. That is, the material can be found in the cross-hatched area of fig. 13.

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

Where |1-n| is the magnitude of (1-n). This is equivalent to line 80 below in fig. 13.

Embodiments described herein provide one or more layers of scattering material (referred to herein as scattering layers, scattering structures, etc.) that cause scattering and have nanostructures formed thereon or therein. The layer (or layers) of scattering material may act as a random array of microlenses, causing scattering of EUV radiation incident on a diffuser comprising the scattering layer. This is particularly advantageous for EUV radiation (which may for example have a wavelength of 13.5 nm) since such nanostructures comprise features having dimensions comparable to or smaller than the wavelength of the radiation desired to be diffused. Under these conditions, scattering is in the Mie-scattering (Mie-scattering) system, and significant angular scattering can be achieved. For example, in some embodiments, the nanostructures formed in the scattering material layer include features having dimensions in the range of 2nm to 220 nm.

Fig. 12 schematically illustrates a diffuser 120. The diffuser 120 includes residues of the carrier layer 1202 that have been back etched to allow radiation to pass through the diffuser 120. In other embodiments, the carrier layer as a whole may have been back etched. The diffuser 120 further includes a support layer 1204 and a cover layer 1206. A scattering layer 1208 is between the support layer 1204 and the cap layer 1206. In the depicted example, the scattering layer 1208 takes the form of the scattering layer 1106 of fig. 11C, but it should 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. Such incident radiation 1210 may correspond to the radiation beam B output by the illumination system IL. It should be appreciated that the incident radiation may include radiation having a range of different angles of incidence, and that arrow 1210 shown in fig. 12 may represent the direction of the primary ray. The scattering layer 1208 spreads this incident radiation over a wide range of angles. This is schematically indicated by arrow 1212.

In use, the diffuser 120 may be used to increase the angular range with which reflected radiation from the object level indication enters the projection system PS. In particular, it may be desirable for each portion of the diffuser 120 to result in a divergence of the radiation 1210, the divergence of the radiation 1210 being approximately the angular range of radiation received by the patterning device MA in the lithographic apparatus LA. For example, in one embodiment, the numerical aperture of the patterning device MA (and projection system PS) in the lithographic apparatus may be about 0.08, with a numerical aperture of about 0.08 corresponding to an angular range of approximately 7. Accordingly, the microlenses provided by the scattering layer 1208 may be expected to cause a divergence of about 7 ° of the radiation 1210. This ensures that each field point on the patterning device MA receives radiation from substantially the entire angular range within the cone having a full angular range of about 7 °. Equivalently, this may ensure that the patterning device is illuminated with a substantially complete pupil filling. For some applications, such as in the case of dipole illumination (as depicted in fig. 4A), a diffuser may preferably be provided such that the divergence of the radiation 1210 is aboutTo provide a 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, e.g. a numerical aperture of 0.16 radians corresponds to an angular range of approximately 9.

In some embodiments, the diffuser 120 may have a thickness (in the z-direction in fig. 12) arranged such that EUV radiation 1210 propagating across the entire thickness of the diffuser 120 produces a phase shift of (2m+1) pi radians. Advantageously, this suppresses zero-order (or specular) scattering.

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

TABLE 1

As can be seen from table 1, there are a number 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. A lower Lr/Lt ratio may indicate a preferred material, but it should be appreciated that other considerations may apply, such as ease of operation, acquisition, longevity, etc.

As described above, some examples include a diffuser comprising a plurality of scattering layers, each layer being arranged to differently alter the angular distribution of EUV radiation passing therethrough. Advantageously, by providing a plurality of layers, each arranged to differently alter the angular distribution of EUV radiation passing therethrough, the diffuser provides an arrangement whereby the EUV radiation beam may be diffused more effectively throughout a desired angular range. In addition, the multiple layers that differently alter the angular distribution of EUV radiation passing therethrough provide more control over the angular distribution of radiation exiting the diffuser.

Fig. 14A-14C depict another example process for manufacturing a diffuser suitable for EUV radiation. The intermediate stack of layers in the example process is depicted in cross-section in fig. 14A-14C. The first intermediate stack 140 includes a support layer 1402. For example, the support layer 1402 may take the same or similar form as the support layers 502, 602 described above with reference to fig. 5 and 6. Although not depicted in fig. 14A-14C, it should be appreciated that the intermediate laminate 70 may include a carrier layer that may take the same or similar form as the carrier layers 504, 604 described above.

Random or quasi-random multilayer deposits of particles 1406 are 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 multi-layer deposit of particles 1406 is in contact with one or more adjacent particles such that a void 1408 is formed between adjacent particles. The multi-layer deposit of particles 1406 may be considered to form a particle body 1406. The particles in the particle body 1406 may be referred to as contact particles because each particle is in contact with one or more adjacent particles.

The particle body 1406 includes a first particle population 1406A of a first material and a second particle population 1406B of a second material, and may be referred to as a binary mixture of particles. The two materials are scattering materials, examples of which are discussed in more detail with reference to fig. 13 and table 1. 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 populations of the first population of particles 1406A and the second population of particles 1406B) is selected based on the desired properties of the diffuser. Exemplary binary mixtures include silicon and molybdenum, ruthenium and silicon, molybdenum silicide and silicon.

In addition to the composition of the particles 1406A, 1406B, other characteristics of the particle body 1406 may 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. Properties such as scatter angle, suppression of zero order scattering, emissivity, and attenuation are achieved by altering composition, particle size distribution, and/or packing density. The particles 1406A, 1406B may have a packing density of about 60% to 70% within the volume occupied by the particle body 1406. That is, for the volume occupied by the particles, 60% to 70% of the volume may be occupied by the particles 1406A, 1406B, with the remaining 30% to 40% being empty (i.e., including the void 1408).

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

In the third intermediate laminate 144, a protective layer is disposed atop the fused particle body 1410. The protective layer may provide protection for the scattering layer (i.e., 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.

Fig. 15A-15C illustrate example diffusers, and the performance of the diffusers, manufactured according to the processes described with reference to fig. 14A-14C. In particular, the particles comprise molybdenum silicide and silicon (MoSi and Si) and are polydisperse with a radius between 75nm and 85 nm. The particles are deposited in a (quasi-) random distribution. Approximately six particle layers were deposited on the support layer.

Fig. 15A depicts a resulting height map or height map 1500 of a particle body. The height map 1500 omits the support layer but in use the particle body will be supported by the support layer.

Fig. 15B and 15C depict scattering angles of plane waves for EUV radiation incident on an example diffuser. In particular, fig. 15B and 15C cooperate to illustrate a beam profile of scattered EUV radiation, wherein fig. 15B depicts 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 depicts a cross-sectional representation of the beam profile. EUV radiation undergoes scattering over a wide angle range of up to 40 °. EUV radiation undergoes scattering with a relatively constant intensity within an angular distribution of approximately 10 °. As such, such an example diffuser may provide an effective diffuser for high numerical aperture patterning devices.

Fig. 16A and 16B depict another example process for manufacturing a diffuser suitable for EUV radiation. In particular, the diffuser in fig. 16A and 16B is a holographic diffuser suitable for EUV radiation. The intermediate stacks 160, 162 in the example process are depicted in cross-section.

The first intermediate 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 fig. 5 and 6. Although not depicted in fig. 16A and 16B, it should be appreciated that the intermediate laminate 70 may include a carrier layer that may take the same or similar form as the carrier layers 504, 604 as described above.

Structure 1604 is disposed atop support layer 1602. The structure 1604 may be provided according to any suitable technique. For example, and as depicted in fig. 16A and 16B, the structure 1604 may be provided lithographically using an e-beam mask. Alternatively, electron beam lithography or nanoimprint lithography or the like may be used to create the structure.

Voids 1605 are formed between the structures 1604. That is, within the volume of space occupied by the structure 1604, there is a volume that does not contain structure, and thus includes voids 1605.

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

In an example arrangement, a holographic interference pattern is selected that forms a hologram having an angular distribution (i.e., angular intensity distribution) that is substantially constant across a selected angular distribution (e.g., 10 °). The 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 having 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 EUV radiation such that light is scattered at a higher intensity for a larger scattering angle.

The structures 1604 are arranged in a particular arrangement on the plane of the support layer 1602 (e.g., in the x-y plane). Each portion of each structure 1604 has a height up to which it extends from the support layer 1602. The height may be referred to as the thickness of the portion of structure 1604. The arrangement of structures 1604 on the support layer 1602 includes a combination of the location and thickness of each structure 1604 on the support layer 1602 and may be referred to as a thickness profile 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.

Second intermediate stack 162 depicts the step of disposing filler or shim layer 1606 atop support layer 1602 and/or structure 1604. The filler layer 1606 may be provided according to any suitable technique, such as Atomic Laser Deposition (ALD) or electrodeposition.

The filler layer 1606 is provided so as to fill the volume previously including the void 1605 (e.g., as shown in fig. 16A). The filler layer 1606 is provided with a thickness such that the combined thickness of the structure 1604 and filler layer 1606 in the z-direction (i.e., extending from the support structure 1604) is substantially constant. Filler layer 1606 may be considered to level, i.e., level, the structure, providing a smooth surface (e.g., substantially smooth on a microscale or nanoscale).

The filler layer 1606 includes a material having a different refractive index than the material of the structure 1604. In particular, filler layer 1606 includes a material having a different real part of the refractive index (n _padding) than the real part of the refractive index (n _structure) of structure 1604.

The differential refractive index δn _padding、δn_structure may be used to quantify the amount by which the real part of the refractive index n _padding of the pass layer 1606 and the real part of the refractive index n _structure of the structure 1604 deviate from 1. When filler layer 1606 and structure 1604 are combined into a thin layer (as depicted in fig. 16B), the thin layer has an effective real part n _eff of the refractive index approximated using equation (2). For simplicity, the effective real part of the refractive index may be referred to simply as the effective refractive index.

n_eff＝δn_padding-δn_structure (2)

The real part of the refractive index n _padding、n_structure affects the refraction of the radiation and thus controls the scattering properties of the diffuser. The filler layer 1606 is selected such that it has a higher real part of the refractive index n _padding than the real part of the refractive index n _structure of the structure 1604.

Filler layer 1606 also includes a material having a similar imaginary part (k _padding) of the refractive index as compared to the similar imaginary part (k _structure) of the refractive index of structure 1604. When the filler layer 1606 and structure 1604 are combined into a thin layer (as depicted in fig. 16B), the thin layer has an effective imaginary part n _eff of refractive index approximated using equation (3).

k_eff＝δk_structure-δk_padding (3)

The effective imaginary part k _eff of the refractive index affects the attenuation of the diffuser. Thus, by providing filler layer 1606 and structure 1604 with similar imaginary parts of refractive index, they each have comparable, i.e., comparable, attenuation.

Equation (4) can be used to approximate the effective layer thickness L of the thin layer.

Radiation traveling through the diffuser undergoes a phase shift and attenuation based on the thickness of each of the filler layer 1606 and structure 1604 at the location of the diffuser through which the radiation travels. As radiation passes through the region of the diffuser where the thickness of the filler layer 1606 is zero (0) and the thickness of the structure 1604 is L, a phase shift of zero (0) is experienced. Wherein the filler layer 1606 and the structure 1604 each have a thickness when radiation is transmitted through the diffuserIs subjected to pi (pi) phase shift. As radiation passes through the region of the diffuser where the thickness of the filler layer 1606 is L and the thickness of the structure 1604 is zero (0), a phase shift of two pi (2pi) is experienced. By limiting the thickness of the filler layer 1606 and structure 1604 toThe phase shift can be controlled to be a multiple of pi (pi) phase shift, thereby providing controlled phase modulation. The use of such a controlled thickness in a holographic diffuser may be referred to as binary phase modulation or ternary phase modulation.

It should be noted that holographic diffusers as described herein employ controlled phase modulation due to controlled selection of thicknesses and arrangements within the scattering layer. This is in contrast to other diffusers described herein (e.g., the diffusers described with reference to fig. 14A-14C) that use random phase modulation due to the (quasi) random arrangement of nanoparticles in the scattering layer.

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

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

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

Given the approximation in equation (4), the Fourier transform can be performed by following equation (5)To approximate the spatial distribution S (fx, fy) (i.e., the spatial intensity distribution) of the diffused light associated with the diffusion layer.

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

In a particular 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 _D (x, y), wavelength λ, refractive index, and/or deviation Δn of a selected material (e.g., one of the scattering species described above with respect to table 1). The algorithm then iteratively performs calculations, such as modified versions of equations (2) and (3), to determine the thickness profile 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 a height limit, for example to limit the thickness profile so that the region 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 the manufacturing method, e.g., the resolution limit of the manufacturing method. It should be appreciated that other methods may be used to determine the thickness profile L (x, y) and/or the refractive index profile, for example, analytical methods may be used or different numerical methods may be used.

Returning to fig. 16A and 16B, by arranging the structures 1604 corresponding to the determined thickness distribution L (x, y), a holographic diffuser may be fabricated that scatters radiation at a desired angular distribution M _D (x, y).

Fig. 17, 18 and 19 illustrate example holographic diffusers and their performance, respectively, each comprising molybdenum, ruthenium and molybdenum silicide. The example holographic diffusers of fig. 17, 18, and 19 do not include a filler layer, but instead include voids between structures thereon.

Each holographic diffuser comprises a structure 1604 arranged according to the determined thickness distribution L (x, y). The thickness profile L (x, y) of each holographic diffuser is determined using the method described above for the desired angular profile M _D (x, y) comprising a top hat profile with an angular profile of 9 °. The refractive index data for the materials (i.e., molybdenum, ruthenium, and molybdenum silicide) included for each respective holographic diffuser is used to determine the thickness profile L (x, y) for each holographic diffuser.

Fig. 17 illustrates a thickness distribution L (x, y) 172 determined for a holographic diffuser comprising a molybdenum structure. The thickness profile L (x, y) comprises a profile having a height measure of 0,Or a quasi-random arrangement of structures of L, where L is calculated for molybdenum.

Fig. 17 also shows 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-pi, 0, or pi (i.e., equivalent to 0, pi, and 2 pi) in a quasi-random pattern that corresponds generally to a quasi-random arrangement of structures. The transmission distribution 174 illustrates how the transmittance of EUV radiation through the holographic diffuser is in the range from 0.6 to 1 in a quasi-random pattern that corresponds generally to a quasi-random arrangement of structures. The average transmission of the holographic diffuser is approximately 78%.

Fig. 17 also shows an angular distribution 176 of EUV radiation diffused by a holographic diffuser comprising a molybdenum structure. The angular distribution 176 is substantially constant over an angular distribution of 9 °. That is, the angular distribution 176 generally 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 of some non-uniformity within the angular distribution of 9 °. In particular, there is a bright spot 177 at 0, indicating some zero order scattering. Furthermore, some EUV radiation is scattered more than 9 °, as is evident by the occurrence of a "halo" 178 of scattered light at angles greater than 9 °.

Fig. 18 illustrates a thickness profile L (x, y) 182 determined for a holographic diffuser comprising a ruthenium structure. The thickness profile L (x, y) comprises a profile having a height measure of 0,Or a quasi-random arrangement of structures of L, where L is calculated for ruthenium.

Fig. 18 also shows 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 positions across the diffuser is-pi, 0, or pi (i.e., equivalent to 0, pi, and 2 pi) in a quasi-random pattern that corresponds generally to a quasi-random arrangement of structures. The transmission distribution 184 illustrates how the transmittance of EUV radiation through the holographic diffuser is in the range from 0.4 to 1 in a quasi-random pattern that corresponds generally to a quasi-random arrangement of structures. The average transmission of the holographic diffuser is approximately 66%.

Fig. 18 also shows an angular distribution 186 of EUV radiation diffused by a holographic diffuser comprising a ruthenium structure. The angular distribution 186 is substantially constant over an angular distribution of 9 °. That is, the angular distribution 186 generally 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 of some non-uniformity within the angular distribution of 9 °. In particular, there is a bright spot 187 at 0 °, indicating some zero order scattering. Furthermore, some EUV radiation is scattered more than 9 °, as is evident by the occurrence of a "halo" 188 of scattered light at angles greater than 9 °.

Fig. 19 illustrates a thickness distribution L (x, y) 192 determined for a holographic diffuser comprising a molybdenum silicide structure. The thickness profile L (x, y) comprises a profile having a height measure of 0,Or a quasi-random arrangement of structures of L, where L is calculated for molybdenum silicide.

Fig. 19 also shows a phase shift profile 190 and a transmission profile 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 entire diffuser is-pi, 0, or pi (i.e., equivalent to 0, pi, and 2 pi) in a quasi-random pattern that corresponds generally to a quasi-random arrangement of structures. The transmission profile 194 illustrates how the transmittance of EUV radiation through the holographic diffuser is in the range from 0.45 to 1 in a quasi-random pattern that corresponds generally to a quasi-random arrangement of structures. The average transmission of the holographic diffuser is approximately 68%.

Fig. 19 also shows an angular distribution 196 of EUV radiation diffused by a holographic diffuser comprising molybdenum silicide structures. The angular distribution 196 is substantially constant over an angular distribution of 9 °. That is, the angular distribution 196 generally 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 of some non-uniformity within the 9 ° angular distribution. In particular, there is a bright spot 197 at 0 °, indicating some zero order scattering. Furthermore, some EUV radiation is scattered more than 9 °, as is evident by the occurrence of a "halo" 198 of scattered light at angles greater than 9 °.

Fig. 20 illustrates a thickness distribution (x, y) 2002 of a ruthenium structure as determined for a holographic diffuser comprising the ruthenium structure and a silica filler layer. The thickness profile (x, y) 2002 is shown prior to deposition of the filler layer. The thickness profile (x, y) 2002 includes a profile having a height measure of 0,Or a quasi-random arrangement of structures of L, where L is calculated for ruthenium. After the filler layer is provided, the resulting thickness profile is approximately equal to L without any substantial thickness variation.

Fig. 20 also shows a phase shift profile 2000 and a transmission profile 2004 for a holographic diffuser comprising a ruthenium structure and a silica filler layer. The phase shift distribution 2000 illustrates how the phase shift experienced by EUV radiation transmitted through the holographic diffuser at different positions across the diffuser is-pi, 0, or pi (i.e., equivalent to 0, pi, and 2 pi) in a quasi-random pattern that corresponds generally to a quasi-random arrangement of structures. The transmission profile 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 generally to a quasi-random arrangement of structures. The average transmission of the holographic diffuser is approximately 39%.

Fig. 20 also shows an angular distribution 2006 of EUV radiation diffused by a holographic diffuser comprising molybdenum silicide structures. The angular distribution 2006 is substantially constant over an angular distribution of 9 °. That is, the angular distribution 2006 generally corresponds to the desired angular distribution M _D (x, y). The angular distribution 2006 does not exactly correspond to the desired angular distribution M _D (x, y) because of some non-uniformity within the 9 ° angular distribution. However, the non-uniformity is low compared to the non-uniformity of the previous example holographic diffuser without the filler layer. In particular, the absence of bright spots at 0 ° indicates reduced zero order scattering.

Holographic diffusers including filler layers can be advantageously used in applications where highly uniform scattering distribution is desired. Holographic diffusers without a filler layer are advantageously used in applications where high EUV transmission is desired.

According to some embodiments of the invention, a measurement system for determining an aberration map or a relative intensity map for a projection system PS is provided, 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.

In use, the diffuser is arranged such that the diffuser can be moved into and out of the optical path of radiation between the illumination system IL and the projection system PS. Such an optical device provides control of the angular distribution of radiation in the field plane of the lithographic apparatus LA downstream of said lithographic apparatus LA. Such field planes include 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 the radiation between the illumination system IL and the projection system PS, the diffuser can be mounted on a patterning device masking blade of the lithographic apparatus LA, as will now be discussed.

The lithographic apparatus LA has four reticle-masking blades (which may also be referred to as patterning device-masking blades) that define a range of the illuminated field on the patterning device MA. The illumination system IL is operable to illuminate a generally rectangular region of an object (e.g., patterning device MA) disposed on the support structure MT. Such a generally rectangular region may be referred to as a slit of the illumination system IL and is defined by four reticle masking blades. The extent of the generally rectangular region in the first direction (the first direction may be referred to as the x-direction) is defined by a pair of x-masking blades. The extent of the generally rectangular region in the second direction (the second direction may be referred to as the y-direction) is defined by a pair of y-masking blades.

Each of the masking blades is arranged close to the plane of the support structure MT, but slightly out of said plane. The x masking blades are disposed in a first plane and the y masking blades are disposed in a second plane.

Each of the masking blades defines one edge of a rectangular field area in the plane of the object that receives radiation. Each blade may be independently movable between a retracted position (in which the blade is not disposed 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 blades into the path of the radiation beam, the radiation beam B may be truncated (in x and/or y directions), thus limiting the extent of the field area receiving the radiation beam B.

The x-direction may correspond to a non-scanning direction of the lithographic apparatus LA and the y-direction may correspond to a scanning direction of the lithographic apparatus LA. That is, the object (and the substrate W in the image plane) may be a larger target area that can be moved in the y-direction through the field area in order to expose the object (and the substrate W) in a single dynamic scanning exposure. During such dynamic scanning exposure, the y-masking blade is moved to control the field region so as to ensure that portions of the substrate W outside the target region are not exposed. At the beginning of a scanning exposure, one of the y-masking blades is arranged in the path of the radiation beam B to act as a mask so that part of the substrate W does not receive radiation. At the end of the scanning exposure, another y-masking blade is disposed in the path of the radiation beam B to act as a mask so that portions of the substrate W do not receive radiation.

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

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 for EUV radiation of less than 90%, for example less than 50% (for a single pass, i.e. a single pass). The diffuser may have a lifetime, i.e., a lifetime of more than 7 years (e.g., have an illumination duty cycle of about 0.1% to 1%) in a lithographic apparatus. The diffuser may be operable to withstand unattenuated EUV power densities of about 1W/cm ² to 10W/cm ². The diffuser may have a size of about 1 to 3mm ² x1 to 3mm ².

In the case of referring to vertical colloidal deposition as a deposition method, additionally or alternatively, a process of inkjet printing and spin coating may be used.

According to another embodiment, a transmissive diffuser includes a support structure including a porous structure having pores. The support structure may be a network of nanotubes, such as carbon nanotubes, multiwall carbon nanotubes, bundles of single-walled carbon nanotubes, boron nitride or MoS2 nanotubes as core fibers. The nanotubes may be randomly aligned to provide structural support to the optically active diffuser material deposited onto the tubes.

A scattering layer at least partially covers the support structure, the scattering layer configured to scatter the received radiation. The scattering layer includes at least one of Mo, Y, zr, nb, ru. The scattering layer provides an optically active material to diffuse light into a desired light profile. Ideally, the scattering layer has a relatively low EUV light absorption and a high refractive index shrinkage compared to the case of vacuum. The scattering layer has a thickness of at least 10nm, alternatively at least 20nm, alternatively at least 40nm, alternatively at least 100 nm. The thickness determines the absorbency.

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

The support structures described in such embodiments may also be used in other embodiments.

Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it will be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the fabrication of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid Crystal Displays (LCDs), thin film magnetic heads, and the like.

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

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative, and not restrictive. Thus, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the aspects set out below.

Aspect 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 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 comprises a scattering substance and the first substance has a lower refractive index than the scattering substance.

Aspect 2. The diffuser of aspect 1, wherein the first substance is the scattering substance.

Aspect 3. The diffuser of aspect 2, wherein the scattering material comprises a foam having micro-pores and the voids are provided by the micro-pores and the voids comprise a vacuum or an inert gas.

Aspect 4. The diffuser of aspect 2, wherein the voids comprise one of silicon or silicon nitride.

Aspect 5. The diffuser of aspect 1, wherein the voids contain the scattering species.

Aspect 6. The diffuser of aspect 5, wherein the first substance comprises a porous silicon-based structure, the voids being defined by micropores of the first substance.

Aspect 7. The diffuser of any one of aspects 1 to 4, wherein the scattering material comprises a body that contacts particles, and the voids are disposed between adjacent particles.

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.

Aspect 9. The diffuser of aspects 7 or 8, wherein the particles comprise a binary mixture comprising a first material and a second material having a refractive index different from the first material.

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

Aspect 11. The diffuser of aspects 9 or 10, wherein the second material comprises molybdenum or ruthenium.

Aspect 12. The diffuser of any one of aspects 7 to 11, wherein the particles have a range of about several nanometers in at least one dimension.

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

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

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

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

Aspect 17. The diffuser of any preceding aspect, wherein the scattering species comprises a dealloying material.

Aspect 18. The diffuser of any preceding aspect, wherein the void has a range of about several nanometers in at least one dimension.

Aspect 19. The diffuser of any preceding aspect, wherein the voids are polydisperse within the first material.

Aspect 20. The diffuser of any preceding aspect, wherein the voids are randomly or quasi-randomly disposed within the first material.

Aspect 21. The diffuser of any preceding aspect, wherein the scattering layer has a thickness of between 50nm and 1000 nm.

Aspect 22. The diffuser of 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.

Aspect 23. The diffuser of any preceding aspect, wherein the scattering material comprises one of molybdenum, ruthenium, niobium, rhodium, yttrium, or technetium.

Aspect 24. The diffuser of any preceding aspect, comprising a plurality of scattering layers.

Aspect 25. The diffuser of aspect 24, wherein the first scattering layer is separated from the second scattering layer by an intermediate layer.

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

Aspect 27. The diffuser of aspects 25 or 26, wherein the intermediate layer comprises a layer of discrete particles having a lower refractive index than the scattering material.

Aspect 28. The diffuser of aspect 27, wherein the separation particles are randomly or quasi-randomly disposed within the intermediate layer.

Aspect 29. The diffuser of aspects 27 or 28, wherein the separated particles comprise particles that differ in size in at least one dimension.

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

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

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

Aspect 33 the diffuser of any one of aspects 30-32, wherein the first substance comprises a plurality of structures of varying thickness perpendicular to the surface of the scattering layer.

Aspect 34 the diffuser of 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 of the plurality of structures isIs an integer multiple of (a).

Aspect 35 the diffuser of any one of aspects 30-34, wherein the void comprises a second substance.

Aspect 36. The diffuser of aspect 35, wherein the 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 the imaginary part of the refractive index of the second substance is similar to the imaginary part of the refractive index of the first substance.

Aspect 37. The diffuser of aspects 35 or 36, wherein the combined first and second substances have a substantially constant combined thickness profile.

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

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

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

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

Aspect 42. The diffuser of 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.

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.

Aspect 44 the measurement system of aspect 43, comprising a patterning device, an illumination system arranged to illuminate the patterning device with radiation, and a sensor arrangement, wherein the illumination system and patterning device are configured such that the projection system receives at least a portion of the radiation scattered by the patterning device and the sensor arrangement is configured such that the projection system projects the received radiation onto the sensor arrangement, and wherein the diffuser is operable to receive the radiation generated by the illumination system and to alter an angular distribution of the radiation prior to the radiation illuminating the patterning device.

Aspect 45 the measurement system of aspect 44, wherein the diffuser is movable between at least a first operational position, wherein the diffuser is at least partially disposed in a path of the radiation generated by the illumination system and arranged to alter an angular distribution of the radiation prior to the radiation illuminating the patterning device, and a second storage position, wherein the diffuser is disposed outside the path of the radiation generated by the illumination system.

Aspect 46 the measurement system of 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.

Aspect 47. A lithographic apparatus comprising a measurement system according to any one of aspects 43 to 46, and a projection system configured to receive at least a portion of the radiation scattered by the patterning device and to project the received radiation onto the sensor apparatus.

Aspect 48 the lithographic apparatus according to aspect 47, wherein the diffuser is mounted on a patterning device masking blade of the lithographic apparatus, an edge of the patterning device masking blade defining a field region of the lithographic apparatus.

Aspect 49. A method of forming a diffuser according to aspects 1 to 3 or 14 to 20 to receive and transmit radiation, the method comprising forming an alloy layer comprising a first substance and a third substance, wherein the first substance is a scattering substance, dealloying the alloy layer to remove the third substance from the alloy layer and to form a scattering layer comprising the first substance and having a plurality of voids distributed therein.

Aspect 50. A method of forming a diffuser for receiving and transmitting radiation includes forming a scattering layer by infiltrating a porous structure with a scattering material.

Aspect 51. The method of aspect 50, wherein the scattering layer is formed on a support layer.

Aspect 52. A method of forming a diffuser for receiving and transmitting radiation includes 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.

Aspect 53 the method of aspect 52, further comprising shrinking one or more particles 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.

Aspect 54. The method of aspects 52 or 53, wherein the particles are deposited on the support layer via vertical colloidal deposition.

Aspect 55 the method of aspects 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 undulating scattering surface on the support layer.

Aspect 56 the method of aspects 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 in use lying in a plane substantially perpendicular to the direction of the received radiation.

Aspect 57 the method of any one of aspects 52-56, further comprising removing the particles after depositing the scattering material.

Aspect 58. A method of forming a diffuser for receiving and transmitting radiation includes 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.

Aspect 59. A method of forming a diffuser for receiving and transmitting radiation includes 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 on 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 selectively etching includes etching regions of the support layer that are in contact with the second material, or wherein the second material is a protective material and the selectively etching includes etching regions of the support layer that are not in contact with the second material.

Aspect 60. A method of forming a diffuser for receiving and transmitting radiation includes depositing a plurality of particles onto a surface of a support layer such that the particles form a body that contacts the particles.

Aspect 61. The method of aspect 50, wherein depositing comprises at least one of vertical colloidal deposition, spin coating, and inkjet printing.

Aspect 62. The method of aspects 60 or 61, wherein depositing comprises fusing the plurality of particles.

Aspect 63. The method of aspect 62, wherein the plurality of particles are fused via the provision of heat and/or pressure.

Aspect 64 the method of aspects 62 or 63, wherein the plurality of particles are fused using sintering.

Aspect 65 the method of any 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.

Aspect 66 the method of any one of aspects 52 to 65, further comprising forming another scattering layer.

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

Aspect 68 the method of aspects 66 or 67, wherein forming another scattering layer comprises depositing an intermediate layer over the scattering layer and forming the other scattering layer atop the intermediate layer.

Aspect 69 the method of any one of aspects 52 to 68, wherein the support layer is formed on a carrier layer that is used to support the support layer when the diffuser is formed, and wherein the method further comprises removing the carrier layer once the first and second layers have been formed.

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.

Aspect 71 the method of aspect 70, wherein the hologram has an angular intensity distribution that is at least as strong in a radially outer portion of the hologram as compared to a central region of the hologram.

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

Aspect 73 the method of any one of aspects 69 to 72, further comprising depositing a second substance into a plurality of voids distributed within the plurality of structures.

Aspect 74 the method of any one of aspects 70-73, further comprising generating a thickness profile corresponding to a desired arrangement of a plurality of surface features, the desired arrangement being based on a desired angular profile of the hologram.

Aspect 75. The method of aspect 74, wherein generating the surface distribution includes using a Gerchberg-Saxton algorithm.

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

Aspect 77 the method of 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.

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

Aspect 79. The diffuser of aspect 78, wherein the support structure comprises nanotubes.

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

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

Aspect 82. The diffuser of any of aspects 78-81, wherein the scattering layer supports a top layer comprising at least one of MoO3, Y2O3, zrO2, al2O3, hfO2, zrO2, ru, W, metal, the top layer having a thickness of at least 0.3nm, optionally at least 1 nm.

Aspect 83 the diffuser of any one of aspects 78 to 82, wherein the diffuser has a porosity fraction of at least 10%, alternatively at least 20%, alternatively at least 30%, alternatively at least 40%, alternatively at least 50%.

Aspect 84. The diffuser of 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 received radiation,

The scattering layer comprises 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 comprises a scattering material and the first material has a lower refractive index than the scattering material, and

Wherein the first substance and the void comprise holographic interference patterns selected to form a hologram given a desired wavelength of radiation, and the hologram has an angular intensity distribution that is a top hat distribution having a lower intensity in a central region than a radially outer portion of the hologram that is angularly spaced from a center of the hologram by at least 9 °.

2. The diffuser of claim 1, wherein the first substance is the scattering substance, wherein scattering substance comprises a foam having micro-pores and the voids are provided by the micro-pores and the voids comprise a vacuum or an inert gas.

3. The diffuser of claim 2, wherein the void comprises one of silicon or silicon nitride.

4. The diffuser of claim 1, wherein voids contain the scattering material, wherein the first material comprises a porous silicon-based structure, the voids being defined by micropores of the first material.

5. A diffuser according to claim 1, wherein the scattering material comprises a body contacting 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 of any of claims 1-7, 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 void cooperate to create a hologram upon receiving radiation at a surface of the scattering layer.

10. The diffuser of claim 9, wherein the void comprises a second substance, wherein the 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 the 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 comprising a plurality of structures configured to produce a hologram upon receipt of extreme ultraviolet radiation at a surface of the scattering layer, wherein the hologram has an angular intensity distribution that is 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 from a center of the hologram by at least 9 °.

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 configured to receive at least a portion of the radiation scattered by the patterning device and configured 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 that is a top hat distribution having a lower intensity in a central region than a radially outer portion of the hologram that is angularly spaced from a center of the hologram by at least 9 °.