WO2020234043A1

WO2020234043A1 - Mirror for use in a lithographic apparatus

Info

Publication number: WO2020234043A1
Application number: PCT/EP2020/063164
Authority: WO
Inventors: Johannes Hubertus Josephina Moors; Vadim Yevgenyevich Banine; Andrey Nikipelov; Marcus Adrianus Van De Kerkhof; Parham Yaghoobi
Original assignee: Asml Netherlands B.V.
Priority date: 2019-05-21
Filing date: 2020-05-12
Publication date: 2020-11-26
Also published as: KR20220008831A; CN113841071A; NL2025558A

Abstract

A mirror comprises a body and a surface defined by the body. The body comprises a plurality of layers. The plurality of layers are arranged to act as a multilayer Bragg reflector for radiation having a first wavelength when radiation having said first wavelength is incident on the surface. A local tangent plane of the surface is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers.

Description

MIRROR FOR USE IN A LITHOGRAPHIC APPARATUS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 19175714.5 which was filed on May 21, 2019 and of EP application 19184661.7 which was filed on July 5, 2019 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a mirror. In particular, the mirror may be suitable for use with extreme ultraviolet (EUV) radiation and may be used within an EUV lithographic apparatus.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] A radiation source, which generates EUV radiation to be used in a lithographic apparatus, may also generate particulate matter and radiation having a wavelength which does not correspond to EUV (“out-of-band” radiation). Such particulate matter and out-of-band radiation may propagate through a lithographic apparatus and cause a number of issues.

[0006] For example, particulate matter propagating through a lithographic apparatus may result in damage to or failure of a patterning device or other optical components (for example mirrors within the lithographic apparatus. Such particulate matter may also result in errors in a pattern transferred to a substrate. Out-of-band radiation propagating through a lithographic apparatus may result in an increased dose which is received by a substrate, which may result in errors in a pattern which is transferred to the substrate. Out-of-band radiation propagating through a lithographic apparatus may also result in an increased thermal load received by a substrate, which may result in thermal deformation of the substrate and subsequently errors in a pattern which is transferred to the substrate.

[0007] Embodiments of the present invention relate to a new design of mirror which reduce

(and may entirely prevent) the propagation of particulate matter and out-of-band radiation through the lithographic apparatus. SUMMARY

[0008] According to a first aspect of the invention there is provided a mirror. The mirror may comprise: a body comprising a plurality of layers; and a surface defined by the body. The plurality of layers may be arranged to act as a multilayer Bragg reflector for radiation having a first wavelength when radiation having said first wavelength is incident on the surface. A local tangent plane of the surface may be inclined at a non-zero angle relative to a local tangent plane of the plurality of layers.

[0009] The radiation may be electromagnetic radiation and may alternatively be referred to as light.

[00010] The mirror according to a first aspect of the invention may have a body which comprises a plurality of individual layers arranged as a multilayer Bragg reflector for radiation having a first wavelength. The plurality of layers may comprise layers formed from a first material (a“first group” of layers) interleaved with layers formed from a second material (a“second group” of layers). Extra layers may be provided between layers of the first group and layers of the second group to prevent mixing of materials during production of the body.

[00011] The body may define an inclined surface. In particular, the plurality of layers may define the inclined surface.

[00012] In general, when radiation having the first wavelength is incident on the surface, a portion of that radiation which is incident on each interface between adjacent layers of the plurality of layers may be reflected. It will be appreciated that, for the plurality of layers to act as a multilayer Bragg reflector for radiation having the first wavelength, all such reflected portions may be in phase so as to constructively interfere.

[00013] The mirror according to the first aspect of the invention is advantageous since it may act as a reflector for radiation having the first wavelength (using the plurality of layers as a multilayer Bragg reflector) and, in addition, since a local tangent plane of the surface is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers, the mirror may also act to filter out particles and radiation having different wavelengths, as now discussed.

[00014] Radiation having a larger wavelength than the first wavelength may be referred to as radiation having a second wavelength. It may be that radiation having a larger wavelength than the first wavelength is not able to resolve a structure defined by interfaces between layers of the plurality of layers of the body. Therefore, a principal interface experienced by radiation having a larger wavelength than the first wavelength, when this radiation is incident on the surface of the body, may be an interface between the surface and an environment in which the mirror is disposed (i.e., the surface). Therefore, radiation having the first wavelength will be reflected primarily from the layers of the body, whereas radiation having a larger wavelength than the first wavelength will be reflected primarily from the surface. [00015] Particulate matter may be incident on the surface of the mirror. The particulate matter, having collided with the surface of the mirror, may then be deflected from the surface in a direction determined at least in part by the local tangent plane of the surface. Therefore, radiation having the first wavelength will be reflected primarily from the layers of the body, whereas particulate matter will be reflected from the surface.

[00016] Radiation may be generated for many industrial applications (e.g., lithography). A radiation source may generate multiple wavelengths (or ranges of wavelength) of radiation where only one wavelength (or range of wavelength) of radiation is desirable. Radiation may be directed in an apparatus using one or more mirrors. Particulate debris may be present in a vicinity of said one or more mirrors. Light of an undesirable wavelength, which is directed to certain components of an apparatus downstream of a source of said light, may lower performance of and/or cause errors in the apparatus. Particulate matter, which is directed to certain components of an apparatus downstream of a source of said light, may lower performance of and or cause errors in the apparatus.

[00017] It may hence be desirable to direct one wavelength of light to a first location whilst directing other wavelengths of light to a second, different location. It may be further desirable to have a mirror which directs light to the first location without directing particulate matter to said first location. Advantageously, the mirror according to the first aspect of the invention is configured to direct light having a first wavelength in a first direction, whilst directing light having a second wavelength in another direction. Advantageously, the mirror according to the first aspect of the invention is configured to direct light having a first wavelength in a first direction, whilst directing particulate matter in another direction.

[00018] The plurality of layers of the body may be generally flat so as to form a mirror for radiation with the first wavelength having no optical power. For such embodiments, the local tangent plane of the plurality of layers is the same for all parts of the mirror. Alternatively, the plurality of layers may be curved (either concave or convex) so as to form a mirror for radiation with the first wavelength having (positive or negative) optical power. For such embodiments, the local tangent plane of the plurality of layers is dependent on position on the mirror such that the local tangent plane of the plurality of layers will, in general, be different for different parts of the mirror.

[00019] The first wavelength may correspond to extreme ultraviolet radiation.

[00020] The second wavelength may correspond to deep ultraviolet radiation.

[00021] The second wavelength may correspond to infrared radiation.

[00022] Particulate matter may refer to liquid tin droplets.

[00023] The surface may be formed from a material which transmits extreme ultraviolet radiation.

[00024] The surface may be formed from a material which transmits greater than fifty percent of incident extreme ultraviolet radiation.

[00025] The surface may be formed from one or more layers of the plurality of layers. [00026] The body may further comprise a supplementary member. The supplementary member may be disposed on the plurality of layers. The surface may be defined by the supplementary member.

[00027] As described above, the body may define the surface. When the supplementary member is provided, the surface may be defined not by the plurality of layers, but by the supplementary member. That is, the supplementary member may define a surface of which a local tangent plane is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers of the body.

[00028] The supplementary member may have a size which varies in at least one spatial dimension.

[00029] The supplementary member may be non-conformal to a surface of the plurality of layers on which the supplementary member may be disposed.

[00030] The supplementary member may be formed from a material which transmits extreme ultraviolet radiation.

[00031] The supplementary member may be formed from a material which transmits greater than fifty percent of incident extreme ultraviolet radiation.

[00032] The supplementary member may be formed, at least in part, from an element having an extinction coefficient of less than or equal to 0.1 for extreme ultraviolet radiation.

[00033] The supplementary member may be formed from at least one of carbon (C), silicon

(Si), niobium (Nb), molybdenum (Mo), ruthenium (Ru), or rhodium (Rh).

[00034] The local tangent plane of the surface may be inclined at a non-zero angle relative to the local tangent plane of the plurality of layers over a majority of the surface.

[00035] The plurality of layers and the surface may be arranged such that radiation having a second wavelength is preferentially reflected from the surface when radiation having said second wavelength is incident on the surface.

[00036] The body may be provided with a cap. The cap may be a conformal coating. The cap may be arranged to be substantially parallel to a main portion of the body. The cap may define the surface.

[00037] As described above, the surface of the body may be defined by the plurality of layers of the body (in which case the main portion of the body may only comprise the plurality of layers). Alternatively, the body may comprise the supplementary member and the surface may be defined by the supplementary member (in which case the main portion of the body may comprise the plurality of layers and the supplementary member). For either type of embodiment, the cap may be provided as a coating on the main portion of the body. Therefore, the cap may also be described as defining a surface of which a local tangent plane is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers of the body. That is, the cap may define the surface.

[00038] One embodiment of the mirror may comprise the plurality of layers (constituting the main portion of the body) and the cap, wherein the plurality of layers defines a surface and wherein the cap is provided as a coating on said surface which is defined by the plurality of layers. The cap may correspond to an outer surface of the body. The cap may define the surface. That is, the cap may be inclined at a non-zero angle relative to a local tangent plane of the plurality of layers.

[00039] Another embodiment may comprise the plurality of layers and the supplementary member (together constituting the main portion of the body) and the cap, wherein the supplementary member defines a surface and wherein the cap is provided as a coating on said surface which is defined by the supplementary member. The cap may correspond to an outer surface of the body. The cap may define the surface. That is, the cap may be inclined at a non-zero angle relative to a local tangent plane of the plurality of layers.

[00040] The mirror may be disposed within an environment in which the mirror is exposed to matter which corrodes or otherwise damages the mirror. For example, the mirror may be exposed to hydrogen or hydrogen plasma. Advantageously, providing a cap may protect the body from exposure to, for example, hydrogen or hydrogen plasma. This may increase a lifetime of the mirror relative to an arrangement wherein the cap is not provided.

[00041] The cap may be formed from a material which transmits extreme ultraviolet radiation.

[00042] The cap may be formed from a material which transmits greater than fifty percent of incident extreme ultraviolet radiation.

[00043] The cap may be formed, at least in part, from an element having an extinction coefficient of less than or equal to 0.1 for extreme ultraviolet radiation.

[00044] The cap may be formed from at least one of: zirconium (Zr), yttrium (Y), ruthenium

(Ru), or rhodium (Rh); any oxide, nitride, or boride ceramic formed from zirconium (Zr), yttrium (Y), ruthenium (Ru), or rhodium (Rh); or any combination thereof.

[00045] The cap may be configured such that radiation having a second wavelength is preferentially reflected from the cap when radiation having said second wavelength is incident on the cap.

[00046] The mirror may form part of an optical system within a lithographic apparatus.

[00047] The mirror may form part of a facetted mirror device.

[00048] The mirror may form part of an optical system within a radiation source.

[00049] The mirror may have optical power.

[00050] The mirror may be curved.

[00051] A composite mirror may be formed from an array of elements. Each element of the composite mirror may correspond to a mirror having any combination of features as described above.

[00052] It will be appreciated that adjacent elements within the array of elements may be formed continuously. This may make manufacture of the composite mirror simpler and cheaper. Alternatively, each individual element within the array of elements may be separate.

BRIEF DESCRIPTION OF THE DRAWINGS [00053] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;

Figure 2 depicts a mirror according to an embodiment of the present invention;

Figure 3 depicts a mirror according to an embodiment of the present invention, illustrating a modification to the design of mirror depicted in Figure 2;

Figure 4 depicts a mirror according to an embodiment of the present invention, wherein a supplementary member is provided;

Figure 5 depicts a mirror according to an embodiment of the present invention, wherein a cap is provided;

Figure 6 depicts a mirror according to an embodiment of the present invention, wherein a supplementary member and a cap are provided;

Figure 7 depicts a mirror according to an embodiment of the present invention, wherein the mirror is formed from an array of elements, each element corresponding to the mirror depicted in Figure 5;

Figure 8 depicts a cross-sectional view and a plan view of a mirror and the effect of a droplet impacting the surface of the mirror;

Figure 9 depicts a cross-sectional view and a plan view of a mirror and the effect of a droplet impacting the surface of the mirror when the mirror surface is provided with obstacles.

DETAIFED DESCRIPTION

[00054] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus FA. 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 FA. The lithographic apparatus FA comprises an illumination system IF (which may alternatively be referred to as an illuminator), 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.

[00055] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (FPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. The laser beam 2 may comprise infrared (IR) radiation. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or ahoy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7. Deep ultraviolet (DUV) radiation may also be emitted during de-excitation and recombination of electrons with ions of the plasma 7.

[00056] The EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below. With such an arrangement, an image of the plasma at the first focal point may be formed at the second focal point.

[00057] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[00058] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The

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

[00059] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source or a free electron laser (FEL) may be used to generate EUV radiation.

[00060] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident on the patterning device MA. In particular, the illumination system IL comprises optics that are arranged to provide control over the spatial and angular intensity distribution of the EUV radiation beam B in the plane of the patterning device MA.

[00061] The EUV radiation beam B enters the illumination system IL through the opening 8

(adjacent the intermediate focus 6). The illumination system IL comprises a facetted field mirror device 10 and a facetted pupil mirror device 11. The EUV radiation beam B is reflected by the facetted field mirror device 10 towards the facetted pupil mirror device 11 and then subsequently reflected by the facetted pupil mirror device 11 towards the patterning device MA. Each of the facetted field mirror device 10 and the facetted pupil mirror device 11 comprises an array of mirrors (also referred to herein as facets and facet mirrors). Each of the individual facet mirrors of the facetted field mirror device 10 and the facetted pupil mirror device 11 may be flat or may have some curvature. For example, in one embodiment, the field facets (mirrors) of the facetted field mirror device 10 may be curved and each of the pupil facets (mirrors) of the facetted pupil mirror device 11 may be flat. An orientation of each field facet may be independently controlled. This allows each field facet to direct a portion of the EUV radiation beam B which is incident thereon to any one of a plurality of pupil facets. The selected orientations of all the field facets determines the pupil shape used for illumination of the patterning device MA (i.e., an angular intensity distribution of the EUV radiation beam B in the plane of the patterning device MA).

[00062] The illumination system IL may include other mirrors or devices in addition to, or instead of, the facetted field mirror device 10 and facetted pupil mirror device 11.

[00063] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. In particular, the EUV radiation beam B may interact with only a portion of the patterning device MA. The portion of the patterning device MA with which the EUV radiation beam B interacts may be controlled by controlling a position of the patterning device relative to the EUV radiation beam B. The portion of the patterning device MA with which the EUV radiation beam B interacts may be further controlled by a shutter system 15. The shutter system 15 may comprise movable blades and the positions of these movable blades may control which portion of the patterning device MA receives energy from the EUV radiation beam B. As a result of the interaction of the EUV radiation beam B with the patterning device MA, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. In this way a typically diffraction-limited image of the patterning device MA is formed on the substrate W. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[00064] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA is operable to align the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[00065] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[00066] Figure 2 shows a cross-section through a mirror 200 according to an embodiment of the present invention. The mirror 200 comprises: a body 202; and a surface 204.

[00067] The body 202 may be referred to as a mirror body. [00068] The body 202 comprises a plurality of layers, which are arranged to act as a multilayer

Bragg reflector for radiation having a first wavelength (for example EUV radiation) when said radiation having a first wavelength is incident on the surface 204. The plurality of layers comprises layers formed from a first material (a“first group” of layers) interleaved with layers formed from a second material (a“second group” of layers). The first group and the second group are schematically depicted in Figure 2 by the light and dark horizontal lines within the body 202.

[00069] The surface 204 comprises two portions, each of which is inclined at a non-zero angle relative to a plane of the plurality of layers. It may be considered that a local tangent plane of the surface 204 is inclined at a non-zero angle relative to a plane of the plurality of layers (which in this embodiment are flat).

[00070] The mirror 200 is advantageous since it can act as a reflector for radiation having a first wavelength (using the plurality of layers as a multilayer Bragg reflector) and, in addition, since a local tangent plane of the surface 204 is inclined at a non-zero angle relative to a plane of the plurality of layers, the mirror 200 also acts to filter out particles and radiation having different wavelengths other than the first wavelength, as discussed in more detail below.

[00071] A material from which the first group is formed has a different refractive index to that of a material from which the second group is formed. The first group may be formed from molybdenum (Mo). The second group may be formed from silicon (Si). Further layers may be provided between layers of the first group and layers of the second group. These further layers may prevent mixing of a material from which the first group is formed and a material from which the second group is formed during production of the body 202.

[00072] The first group and the second group of layers within the body 202 are arranged to act as a multilayer Bragg reflector. In particular, the first group and the second group of layers within the body 202 are arranged to act as a multilayer Bragg reflector for radiation having a wavelength equal to approximately 13.5 nm (corresponding to EUV radiation). This wavelength may be described as a first wavelength.

[00073] The surface 204 is defined by the body 202. The surface 204 may be described as an inclined surface. In particular, a local tangent plane of the surface 204 is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers within the body 202.

[00074] The first group and the second group of layers within the body 202 are arranged to act as a multilayer Bragg reflector for radiation having a first wavelength (such as EUV radiation) when radiation having said first wavelength is incident on the surface 204.

[00075] In use, radiation having the first wavelength may be incident on the surface 204.

Radiation having the first wavelength will at least partially propagate into layers within the body 202. Propagation of incident radiation having the first wavelength is schematically depicted in Figure 2 by the arrow 18a. A portion of the radiation 18a which is incident on each interface between adjacent layers of the plurality of layers will be reflected. It will be appreciated that for the plurality of layers to act as a multilayer Bragg reflector for radiation having the first wavelength, all such reflected portions may be in phase so as to constructively interfere. The first group of layers and the second group of layers are hence dimensioned such that portions of the radiation 18a which are reflected from interfaces between adjacent layers are in phase so as to constructively interfere. Reflected radiation having the first wavelength is schematically depicted in Figure 2 by the arrow 18b.

[00076] Radiation having a larger wavelength than the first wavelength (such as IR radiation or

DUV radiation) may be referred to as radiation having a second wavelength. In use, radiation having a second wavelength may be incident on the surface 204. Propagation of incident radiation having a second wavelength is schematically depicted in Figure 2 by the arrow 19a. The radiation 19a is generally unable to resolve a structure defined by interfaces between layers within the body 202. Therefore, a principal interface experienced by the radiation 19a, when this radiation 19a is incident on the surface 204, is an interface between the surface 204 and an environment in which the mirror 200 is disposed. The radiation 19a is therefore reflected from the surface 204 in a direction determined at least in part by a local tangent plane of the surface 204. Reflected radiation having the second wavelength is schematically depicted in Figure 2 by the arrow 19b.

[00077] Therefore, radiation having the first wavelength will be reflected primarily from the layers of the body 202 (see arrows 18a, 18b), whereas radiation having a larger wavelength than the first wavelength (such as radiation having a second wavelength) will be reflected primarily from the surface 204 (see arrows 19a, 19b).

[00078] In use, particulate matter may be incident on the surface 204. A trajectory of incident particulate matter is schematically depicted in Figure 2 by the arrow 19a. The particulate matter 19a, having collided with the surface 204, may then be deflected from the surface 204 in a direction determined at least in part by the local tangent plane of the surface 204. Deflected particulate matter is schematically depicted in Figure 2 by the arrow 19b.

[00079] Therefore, radiation having the first wavelength will be reflected primarily from the layers of the body 202 (see arrows 18a, 18b), whereas particulate matter will be reflected from the surface 204 (see arrows 19a, 19b).

[00080] It will be appreciated that the optical properties of the mirror 200 (for the radiation having the first wavelength) are defined by the geometry of the plurality of layers in the body 202. The plurality of layers within the body 202 may be generally flat (as shown in Figure 2) so as to form a mirror (for radiation having the first wavelength) having no optical power. For such embodiments, a local tangent plane of the plurality of layers is the same for all parts of the mirror. Alternatively, the plurality of layers may be curved (either concave or convex) so as to form a mirror (for radiation having the first wavelength) having positive or negative optical power. For example the mirror 200 may be one of the field facet mirrors of the facetted field mirror device 10. For such embodiments, the local tangent plane of the plurality of layers is dependent on position on the mirror such that the local tangent plane of the plurality of layers will, in general, be different for different parts of the mirror 200. [00081] In contrast, the optical properties of the mirror 200 for radiation not having the first wavelength are defined by the geometry of the surface 204.

[00082] Radiation may be generated for many industrial applications (e.g., lithography). A radiation source may generate multiple wavelengths (or ranges of wavelength) of radiation where it may be that only one wavelength (or range of wavelength) of radiation is desirable. Radiation may be directed in an apparatus using one or more mirrors. Mirrors may be particularly beneficial if the radiation is well absorbed by material such as, for example, EUV radiation (for which the use of refractive lenses may be impractical. Particulate matter may be present in a vicinity of said one or more mirrors. Radiation of an undesirable wavelength, which is directed to certain components of an apparatus downstream of a source of said radiation, may lower performance of and/or cause errors in the apparatus. Particulate matter, which is directed to certain components of an apparatus downstream of a source of said radiation, may lower performance of and or cause errors in the apparatus.

[00083] It may be that tin droplets on which the laser 2 is incident (which may be referred to as

“LPP tin targets”) in the source SO (see Figure 1) are not completely converted to the plasma 7. Tin droplets which are not completely converted to the plasma 7 may scatter from the plasma formation region 4 after receiving energy from the laser beam 2. Upon interacting with a surface within the source SO, scattering of particulate matter (particularly scattered liquid tin spheres) may be approximated as ray reflection. This approximation may be valid due to surface tension of the particulate matter on impact and recoil of the particulate matter after impact, which may demonstrate some degree of energy conservation and momentum conservation upon colliding with a surface. Hence, tin droplets which are not completely converted to the plasma 7 may bounce off and/or propagate along inner walls of the enclosing structure 9 of the radiation source SO. Further, tin droplets which are not completely converted to the plasma 7 may be reflected by the collector 5.

[00084] The radiation which constitutes the laser beam 2 may be IR radiation. IR radiation from the laser beam 2 may be reflected by tin droplets which are not completely converted to the plasma 7. IR radiation may be reflected by the collector 5. DUV radiation may be emitted during de-excitation and recombination of electrons with ions of the plasma 7. This DUV radiation may be reflected by the collector 5.

[00085] It may therefore be possible for particulate matter (particularly tin droplets) to propagate from the source SO, through the opening 8 in the enclosing structure 9, into the illumination system IF. Particulate matter which propagates from the source SO into the illumination system IF may consist primarily of material which forms FPP tin targets (i.e., molten tin). Particles constituting such particulate matter may have a diameter of the order of 1 um and may travel at a speed of the order of 10 m s ¹. It may also be possible for IR radiation to propagate from the source SO, through the opening 8 in the enclosing structure 9, into the illumination system IF. It may also be possible for DUV radiation to propagate from the source SO, through the opening 8 in the enclosing structure 9, into the illumination system IF. [00086] Tin which enters the illumination system IL may propagate through the lithographic apparatus LA and subsequently lower the performance of and/or cause errors in the lithographic apparatus LA. IR and or DUV radiation which enters the illumination system IL may propagate through the lithographic apparatus LA and subsequently lower the performance of and/or cause errors in the lithographic apparatus LA.

[00087] It may hence be desirable to direct one wavelength of radiation (such as EUV radiation) to a first location whilst directing other wavelengths of radiation (such as IR and DUV radiation) to a second, different location. It may be further desirable to direct one wavelength of radiation (such as EUV radiation) to the first location without directing particulate matter (such as tin droplets) to said first location.

[00088] Referring again to Figure 2, advantageously, the mirror 200 is configured to direct radiation having a first wavelength 18a (such as EUV radiation) in a first direction 18b, whilst directing radiation having a second wavelength 19a (such as IR and DUV radiation) in another direction 19b. Advantageously, the mirror 200 is configured to direct radiation having a first wavelength 18a (such as EUV radiation) in a first direction 18b, whilst directing particulate matter 19a (such as tin droplets) in another direction 19b. To alleviate the problems of tin and radiation having a second wavelength propagating through the lithographic apparatus LA, one or more mirrors within the lithographic apparatus LA may be of the type of the mirror 200.

[00089] Referring to Figure 1, there may be different optical requirements and/or tolerances for propagation of a radiation wave front for a radiation beam propagating through the illumination system IL (such as the EUV radiation beam B) and a radiation beam propagating through the projection system PS (such as the patterned EUV radiation beam B’). In particular, there may be more stringent optical requirements (such as lower tolerances) in the projection system PS compared with the illumination system IL. It may therefore be desirable to modify optical elements within the illumination system IL (for example so as to incorporate the mirror 200) rather than optical elements within the projection system PS when incorporating the mirror 200 into the lithographic apparatus.

[00090] Each of the facetted field mirror device 10 and the facetted pupil mirror device 11 (both of which are disposed within the illumination system IL) comprises individual facets (individual mirrors). To alleviate the problems of tin and radiation having a second wavelength propagating through the lithographic apparatus LA, one or more of the facets of the facetted field mirror device 10 and/or the facetted pupil mirror device 11 may be of the type of the mirror 200. Such a usage of the mirror 200 is now described.

[00091] As discussed above, the first group and the second group of layers within the body 202 are dimensioned such that the mirror 200 acts as a multilayer Bragg reflector for EUV radiation. Further, each mirror 200 included within the lithographic apparatus LA may be arranged such that the“first direction” referred to above (for each mirror 200) corresponds to EUV radiation propagating through the illumination system IL and projection system PS of the lithographic apparatus LA as described above with reference to Figure 1 (i.e., in an intended manner). That is, each mirror 200 included within the lithographic apparatus LA may be arranged such that the“first direction” corresponds to EUV radiation propagating through the lithographic apparatus LA as schematically depicted by the beams B, B’. It will be appreciated that, as used here, for a curved mirror of the type shown in Figure 2 (i.e., having a plurality of layers that are curved), the term first direction may be intended to refer to a direction of a chief ray of a portion of the radiation beam B that is scattered from said curved mirror.

[00092] The patterning device MA may form part of a patterning device assembly, which may comprise: the support structure MT; the patterning device MA; and a protective membrane, which may be known as a pellicle 17. Tin droplets which enter the illumination system IL may propagate through the illumination system IL so as to be incident on the patterning device assembly MT, MA, 17. This may lead to errors in a pattern transferred to the patterned EUV radiation beam B’ and subsequently to the substrate W. Further, if tin reaches the patterning device assembly MT, MA, 17, at least a portion of the patterning device assembly MT, MA, 17 may be damaged. This may ultimately lead to failure of at least a portion of the patterning device assembly MT, MA, 17.

[00093] The substrate W may be provided with a coating of photoresist (a substance which undergoes a chemical reaction upon receipt of a nominal dose of radiation). The photoresist may be configured so as to undergo said chemical reaction upon receipt of EUV radiation (via the patterned EUV radiation beam B’) for a predefined length of time. DUV radiation which propagates though the lithographic apparatus LA may propagate so as to be incident on the substrate W. This may increase a dose which is received by the substrate W. This may result in errors in a pattern which is transferred to the substrate W (for example, this may affect critical dimension uniformity).

[00094] Integrated circuitry may be formed from the substrate W. In order to form integrated circuitry form the substrate W, the substrate W may be provided with patterned radiation (via the patterned EUV radiation beam B’) multiple times. Thus, the substrate W may be described as comprising layers. It is desirable to ensure that all of these layers are well aligned. Misalignment of a pattern that is projected on to the substrate W relative to a pattern exposed to layers of the substrate W that have already been formed (or are yet to be formed) may be referred to as“overlay”. Overlay generally has a detrimental effect on lithographic performance. An increase in overlay may correspond to a decrease in quality, or even in failure, of integrated circuitry produced using the lithographic apparatus LA. IR radiation which propagates though the lithographic apparatus LA may propagate so as to be incident on the substrate W. This may result in a thermal load on the substrate W. This may result in thermal deformation of the substrate W. This may increase overlay.

[00095] IR and/or DUV radiation which propagates though the lithographic apparatus LA may propagate so as to be incident on the patterning device assembly MT, MA, 17. This may result in a thermal load on the pellicle 17. This may reduce the lifetime of or result in failure of the pellicle 17.

[00096] Each mirror 200 included within the lithographic apparatus LA may be arranged such that tin droplets which enter the illumination system IL and which are incident on the surfaces 204 of the mirrors 200 (which may constitute facets of the facetted field mirror device 10 and/or of the facetted pupil mirror device 11) are deflected from the mirrors 200 in a direction which does not correspond to the direction of the EUV radiation beam B within the illumination system IL. That is, the“second direction” (the direction in which incident tin droplets may be deflected) does not correspond to the direction of the EUV radiation beam B within the illumination system IL. Therefore, advantageously, tin droplets are not directed towards the patterning device MA.

[00097] Each mirror 200 included within the lithographic apparatus LA may be arranged such that IR and/or DUV radiation which enters the illumination system IL and which is incident on the surfaces 204 of the mirrors 200 (which may constitute facets of the facetted field mirror device 10 and/or of the facetted pupil mirror device 11) are deflected from the mirrors 200 in a direction which does not correspond to the direction of the EUV radiation beam B within the illumination system IL. That is, the“second direction” (the direction in which incident IR and/or DUV radiation may be deflected) does not correspond to the direction of the EUV radiation beam B within the illumination system IL. Therefore, advantageously, IR and or DUV radiation is not incident on the patterning device MA (or any other components of the lithographic apparatus LA downstream of the illumination system IL, including the substrate W).

[00098] Propagation of IR and/or DUV radiation within the illumination system IL may be shifted from the direction of propagation of the EUV radiation beam B shown in Ligure 1 such that IR and/or DUV radiation is incident on blades of the shutter system 15 whilst the EUV radiation beam B propagates through an aperture of the shutter system 15 (defined by said blades). Propagation of IR and/or DUV radiation within the illumination system IL may be shifted further such that IR and/or DUV radiation is incident on a portion 16 of a wall of the illumination system IL. The portion 16 of the wall of the illumination system IL may be shaped so as to have a relatively large surface area. Lor example, the portion 16 of the wall of the illumination system IL may comprise grooves and/or fins. The portion 16 of the wall of the illumination system IL may comprise a heatsink. This may be advantageous for dissipating a thermal load received by the portion 16 of the wall of the illumination system IL from the IR and/or DUV radiation.

[00099] By incorporating mirrors of the form of the mirror 200 shown in Ligure 2 into the lithographic apparatus LA (for example, as described above), propagation of tin droplets through the lithographic apparatus LA is reduced or prevented. Advantageously, issues associated with tin droplets propagating towards the patterning device assembly MT, MA, 17 (such as: errors in a pattern transferred to the patterned EUV radiation beam B’ and subsequently to the substrate W; at least a portion of the patterning device assembly MT, MA, 17 becoming damaged; failure of at least a portion of the patterning device assembly MT, MA, 17) are alleviated.

[000100] By incorporating mirrors of the form of the mirror 200 shown in Ligure 2 into the lithographic apparatus LA (for example, as described above), propagation of IR and DUV radiation through the lithographic apparatus LA is reduced or prevented. Advantageously, issues associated with DUV radiation being incident on the substrate W (such as increasing a dose which is received by the substrate W, which may result in errors in a pattern which is transferred to the substrate W) are alleviated. Advantageously, issues associated with IR radiation being incident on the substrate W (such as increasing a thermal load received by the substrate W, which may result in thermal deformation and subsequently an increase in overlay of the substrate W) are alleviated. Advantageously, issues associated with IR and DUV radiation being incident on the pellicle 17 (such as increasing a thermal load on the pellicle 17, which may reduce the lifetime of or result in failure of the pellicle 17) are alleviated.

[000101] Mirrors of the form of the mirror 200 shown in Figure 2 may be incorporated into the lithographic apparatus LA as described in detail above (by forming part or ah of one or more facets of the facetted field mirror device 10 or the facetted pupil mirror device 11). However, it will be appreciated that the mirror 200 may be incorporated into the lithographic system in another arrangement. For example, the mirror 200 may form part of the collector 5. Alternatively, the mirror 200 may be incorporated into the lithographic system in another arrangement. It will be further appreciated that the mirror 200 may be incorporated into an entirely different apparatus.

[000102] The mirror 200 may offer advantages in any optical system in which particulate matter is present. The mirror 200 may offer advantages in any optical system in which multiple wavelengths of radiation are present and in which only a subset of said multiple wavelengths of radiation are desired.

[000103] Figure 3 shows a cross-section through a mirror 300 according to an embodiment of the present invention. The mirror 300 comprises: a body 302; and a surface 304.

[000104] The body 302 may be referred to as a mirror body.

[000105] The body 302 (Figure 3) is formed from generally the same materials as the body 202

(Figure 2). That is, the body 302 comprises a first group and second group of layers which are arranged to act as a multilayer Bragg reflector.

[000106] The surface 304 is defined by the body 302 (Figure 3), similarly to how the surface 204 is defined by the body 202 (Figure 2). The surface 304 may be described as an inclined surface, similarly to the surface 204, but the shape of the surfaces 204, 304 are different. Nonetheless, a local tangent plane of each of the surfaces 204, 304 is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers within each of the bodies 202, 302, respectively.

[000107] In use, the mirror 300 (Figure 3) provides similar functional behaviour to that provided by the mirror 200 (Figure 2).

[000108] Radiation having the first wavelength may be incident on the surface 304. Propagation of incident radiation having the first wavelength is schematically depicted in Figure 3 by the arrow 18 a. Propagation of reflected radiation having the first wavelength is schematically depicted in Figure 3 by the arrow 18b.

[000109] Radiation having the second wavelength may be incident on the surface 304.

Propagation of incident radiation having a second wavelength is schematically depicted in Figure 3 by the arrow 19a. Propagation of reflected radiation having the second wavelength is schematically depicted in Figure 3 by the arrow 19b.

[000110] Particulate matter may be incident on the surface 304. A trajectory of incident particulate matter is schematically depicted in Figure 3 by the arrow 19a. Propagation of deflected particulate matter is schematically depicted in Figure 3 by the arrow 19b.

[000111] The mirror 300 is thus configured to direct radiation having a first wavelength 18a

(such as EUV radiation) in a first direction 18b, whilst directing radiation having a second wavelength 19a (such as IR and DUV radiation) in another direction 19b. The mirror 300 is further configured to direct radiation having a first wavelength 18a (such as EUV radiation) in a first direction 18b, whilst directing particulate matter 19a (such as tin droplets) in another direction 19b. Therefore, the mirror 300 (Figure 3) provides the same advantageous features as those provided by the mirror 200 (Figure 2).

[000112] The mirror 300 is included as an exemplary embodiment of the present invention to demonstrate an alternative design of mirror which provides the same advantageous features as those described in detail above with reference to the mirror 200 of Figure 2.

[000113] Figure 4 shows a cross-section through a mirror 400 according to an embodiment of the present invention. The mirror 400 comprises: a body 402; a surface 404; and a supplementary member 406.

[000114] The body 402 may be referred to as a mirror body. The supplementary member 406 may be referred to as an auxiliary layer.

[000115] The body 402 (Figure 4) is formed from generally the same materials as the body 202

(Figure 2). That is, the body 402 comprises a first group and second group of layers which are arranged to act as a multilayer Bragg reflector.

[000116] The supplementary member 406 is disposed on the body 402. The supplementary member 406 is generally triangular in cross-section. Differently shaped supplementary members may be used in alternative embodiments, but a size of a supplementary member will generally vary in at least one spatial dimension. The supplementary member 406 may be described as being non-conformal to a surface of the plurality of layers on which the supplementary member is disposed.

[000117] The supplementary member 406 may be formed from a material having a relatively low extinction coefficient (e.g., less than or equal to 0.1) and low refractive index for the first wavelength of radiation. The supplementary member 406 may be formed from a material having a relatively high transmittance (e.g., 50% or higher) for EUV radiation. The supplementary member 406 is formed from a material having a relatively high reflectance to the second wavelength of radiation. The supplementary member 406 may be formed from a material having a relatively high reflectance to IR and/or DUV radiation. The supplementary member 406 may be formed from at least one of carbon (C), silicon (Si), niobium (Nb), molybdenum (Mo), ruthenium (Ru), or rhodium (Rh).

[000118] The surface 404 of the body 402 is defined by a surface of the supplementary member

406. The surface 404 may be described as an inclined surface. In particular, a local tangent plane of the surface 404 is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers within the body 402.

[000119] In use, the mirror 400 (Figure 4) provides similar functional behaviour to that provided by the mirror 200 (Figure 2).

[000120] A surface which is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers within the body 402 is, in the mirror 400, defined by the surface 404 of the supplementary member 406. The body 402 may, in some instances, be described as comprising the plurality of layers of the body 402 and the supplementary member 406.

[000121] Radiation having the first wavelength may be incident on the surface 404. Propagation of incident radiation having the first wavelength is schematically depicted in Figure 4 by the arrow 18 a. As the supplementary member 406 has relatively high transmittance to the first wavelength of radiation, incident radiation having the first wavelength 18a generally propagates through the supplementary member 406 towards the plurality of layers of the body 402. Incident radiation having the first wavelength 18a is then reflected by the plurality of layers of the body 402 which acts as a multilayer Bragg reflector. Propagation of reflected radiation having the first wavelength is schematically depicted in Figure 4 by the arrow 18b.

[000122] Radiation having a second wavelength may be incident on the surface 404. Propagation of incident radiation having a second wavelength is schematically depicted in Figure 4 by the arrow 19a. As the supplementary member 406 has relatively high reflectance to the second wavelength of radiation, incident radiation having the second wavelength 19a is generally reflected from the supplementary member 406. Propagation of reflected radiation having the second wavelength is schematically depicted in Figure 4 by the arrow 19b. Note that any (small) fraction of incident radiation having the second wavelength of radiation which propagates into the supplementary member 406 will, in general, not be well reflected by the plurality of layers of the body 402 since this is specifically tuned to act as a multilayer Bragg reflector for the first wavelength (not the second wavelength).

[000123] Particulate matter may be incident on the surface 404. A trajectory of incident particulate matter is schematically depicted in Figure 4 by the arrow 19a. As the supplementary member 406 is solid, incident particulate matter will generally be deflected from the supplementary member 406. Propagation of deflected particulate matter is schematically depicted in Figure 4 by the arrow 19b.

[000124] The mirror 400 is thus configured to direct radiation having a first wavelength 18a

(such as EUV radiation) in a first direction 18b, whilst directing radiation having a second wavelength 19a (such as IR and DUV radiation) in another direction 19b. The mirror 400 is further configured to direct radiation having a first wavelength 18a (such as EUV radiation) in a first direction 18b, whilst directing particulate matter 19a (such as tin droplets) in another direction 19b. Therefore, the mirror 400 (Figure 4) provides the same advantageous features as those provided by the mirror 200 (Figure 2). [000125] The mirror 400 is included as an example embodiment of the present invention to demonstrate an alternative design of mirror which provides the same advantageous features as those described in detail above with reference to the mirror 200 of Figure 2.

[000126] Figure 5 shows a cross-section through a mirror 500 according to an embodiment of the present invention. The mirror 500 comprises: a body 502; a surface 504; and a cap 508.

[000127] The body 502 may be referred to as a mirror body.

[000128] The body 502 and the surface 504 of Figure 5 are identical to the body 202 and the surface 204 of Figure 2. The mirror 500 of Figure 5 may therefore be described as comprising: the mirror 200 of Figure 2; and the cap 508.

[000129] The cap 508 is disposed on the body 502. The cap 508 generally follows the shape of the surface 504. The cap 508 may be described as a coating which coats the surface 504. The cap 508 may be described as being conformal to the surface 504. The cap 508 may be described as being substantially parallel to the surface 504. A thickness of the cap 508 may be less than 100 nm. A thickness of the cap may be less than 10 nm. It will be appreciated that, for illustration purposes, a thickness of the cap 508 shown in Figure 5 may be exaggerated relative to other components of the mirror 500.

[000130] The cap 508 is formed from a material having a relatively low extinction coefficient

(e.g., less than or equal to 0.1) and low refractive index for the first wavelength of radiation. The cap 508 is formed from a material having a relatively high transmittance to the first wavelength of radiation. The cap 508 may be formed from a material having a relatively high transmittance (e.g., 50% or higher) to EUV radiation. The cap 508 is formed from a material having a relatively high reflectance to the second wavelength of radiation. The cap 508 may be formed from a material having a relatively high reflectance to IR and/or DUV radiation. The cap 508 may be formed from a metallic material, such as ruthenium (Ru) or rhodium (Rh). The cap 508 may be formed from a stable oxide of an EUV-transparent metal, such as zirconium oxide (ZrCT) or yttrium oxide (Y2O3. The cap 508 may be formed from a ceramic material, such as a metal nitride, boride, or oxide. The cap 508 may be formed from a combination of any of the example materials given.

[000131] The surface 504 may be described as an inclined surface. In particular, a local tangent plane of the surface 504 is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers within the body 502. As the cap 508 generally follows the shape of the surface 504, an outer surface 510 of the cap 508 may also be described as an inclined surface. The outer surface 510 of the cap 508 may be generally parallel to the surface 504. In particular, a local tangent plane of the surface 510 is inclined at a non- zero angle relative to a local tangent plane of the plurality of layers within the body 502. Hence, both the surface 504 and the outer surface 510 are both inclined at a non-zero angle relative to a local tangent plane of the plurality of layers.

[000132] A surface which is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers within the body 502 is, in the mirror 500, defined by the outer surface 510 of the cap 508. The body 502 may, in some instances, be described as comprising the plurality of layers of the body 502 and the cap 508.

[000133] In use, the mirror 500 (Figure 5) provides similar functional behaviour to that provided by the mirror 200 (Figure 2).

[000134] Radiation having the first wavelength may be incident on the outer surface 510 of the cap 508. Propagation of incident radiation having the first wavelength is schematically depicted in Figure 5 by the arrow 18a. As the cap 508 has relatively high transmittance to the first wavelength of radiation, incident radiation having the first wavelength 18a generally propagates through the cap 508 towards the body 502. Incident radiation having the first wavelength 18a is then reflected by the body 502 which acts as a multilayer Bragg reflector. Propagation of reflected radiation having the first wavelength is schematically depicted in Figure 5 by the arrow 18b.

[000135] Radiation having the second wavelength may be incident on the outer surface 510 of the cap 508. Propagation of incident radiation having the second wavelength is schematically depicted in Figure 5 by the arrow 20a. A portion of incident radiation having the second wavelength 20a is reflected from the outer surface 510 of the cap 508. A portion of incident radiation having the second wavelength 20a is transmitted by the cap 508 and is subsequently incident on the surface 504. Propagation of the portion of radiation having the second wavelength reflected from the outer surface 510 of the cap 508 is schematically depicted in Figure 5 by the arrow 20b. Propagation of the portion of radiation having the second wavelength transmitted by the cap 508 is schematically depicted in Figure 5 by the arrow 20c. The transmitted portion of radiation having the second wavelength 20c may be generally unable to resolve a structure defined by interfaces between layers within the body 502 (due to the second wavelength being larger than a spacing between layers within the body 502). Therefore, a principal interface experienced by the transmitted portion of radiation having the second wavelength 20c is the surface 504. The transmitted portion of radiation having the second wavelength 20c is therefore generally reflected from the surface 504. Propagation of the portion of radiation having the second wavelength reflected from the surface 504 is schematically depicted in Figure 5 by the arrow 20d.

[000136] Particulate matter may be incident on the outer surface 510 of the cap 508. A trajectory of incident particulate matter is schematically depicted in Figure 5 by the arrow 20a. As the cap 508 is solid, incident particulate matter will generally be deflected from the cap 508. Propagation of deflected particulate matter is schematically depicted in Figure 5 by the arrow 20b.

[000137] The mirror 500 is thus configured to direct radiation having a first wavelength 18a

(such as EUV radiation) in a first direction 18b, whilst directing radiation having a second wavelength 20a (such as IR and DUV radiation) in another direction 20b, 20d. The mirror 500 is further configured to direct radiation having a first wavelength 18a (such as EUV radiation) in a first direction 18b, whilst directing particulate matter 20a (such as tin droplets) in another direction 20b. Therefore, the mirror 500 (Figure 5) provides the same advantageous features as those provided by the mirror 200 (Figure 2). [000138] Both the surface 504 and the outer surface 510 are configured to provide at least the same advantages as the surface 204 of Figure 2.

[000139] A mirror may be exposed to matter which corrodes or otherwise damages said mirror.

For example, a mirror may be exposed to hydrogen or hydrogen plasma. Advantageously, the mirror 500 comprises a cap 508, which may protect the body 502 of the mirror 500 from exposure to, for example, hydrogen or hydrogen plasma. This may increase a lifetime of the mirror 500 (relative to an arrangement wherein the cap 508 is not provided).

[000140] As discussed above, incident radiation having the second wavelength 20a is reflected partly by the outer surface 510 of the cap 508, and radiation having the second wavelength 20c which is transmitted by the cap 508 is generally reflected by the surface 504 of the body 202. There are therefore two surfaces 504, 510 from which radiation having the second wavelength may be reflected. This may result in a relatively high proportion of radiation having the second wavelength being reflected in the second direction 20b, 20d. In particular, use of the mirror 500 may result in a higher proportion of radiation having the second wavelength being reflected in the second direction, compared with a mirror (such as the mirror 200 of Figure 2) which does not comprise a cap (such as the cap 508 of Figure 5). Therefore, advantageously, the mirror 500 may provide better effective filtering of a radiation beam comprising radiation having the first and second wavelengths, compared with a mirror (such as the mirror 200 of Figure 2) which does not comprise a cap (such as the cap 508 of Figure 5). That is, the mirror 500 may better reflect radiation having the first wavelength and radiation having the second wavelength in different directions.

[000141] The mirror 400 (Figure 4) and the mirror 500 (Figure 5) may be described as providing an object between incident radiation and the plurality of layers. In particular, the mirror 400 (Figure 4) provides the supplementary member 406 between incident radiation 18a, 19a and the plurality of layers of the body 402, and the mirror 500 (Figure 5) provides the cap 508 between incident radiation 18a, 20a and the plurality of layers of the body 502. As described above, each of the supplementary member 406 and the cap 508 is formed from a material which exhibits high transmittance to EUV radiation. The supplementary member 406 is generally thicker than the cap 508. Hence, it may be particularly important to select a material from which to form the supplementary member 406 which is highly transmissive to EUV radiation. In other words, to achieve a same nominal level of transmittance to EUV radiation for both the supplementary member 406 and the cap 508, the cap 508 may be formed from a material which is less transmissive to EUV radiation than a material chosen to form the supplementary member 406.

[000142] It will be appreciated that a cap (such as the cap 508 of Figure 5) may be provided on any mirror (in particular, the mirror 500 of Figure 5 corresponds to the mirror 200 of Figure 2 provided with a cap). It is also possible to provide other mirrors (such as the mirrors 300, 400 of Figures 3, 4, respectively) with a cap. [000143] Any mirror which is provided with a cap, but particularly a mirror comprising an inclined surface, may offer some or all of the advantages of using a cap as described above with reference to Figure 5. By way of example, Figure 6 shows a cross-section through a mirror 600 according to an embodiment of the present invention. The mirror 600 comprises: a body 602; a surface 604; a supplementary member 606; and a cap 608. The surface 604 may be described as an inclined surface. The body 602 may be referred to as a mirror body. The body 602, the surface 604, and the supplementary member 606 of Figure 6 are identical to the body 402, the surface 404, and the supplementary member 406 of Figure 4. The mirror 600 of Figure 6 may therefore be described as comprising: the mirror 400 of Figure 4; and the cap 608. The cap 608 (Figure 6) is identical to the cap 508 (Figure 5), and therefore provides at least the same advantages. In use, the mirror 600 (Figure 6) provides similar functional behaviour to that provided by the mirror 400 (Figure 4), but with the further advantages associated with using the cap 608 as described above with reference to the cap 508 (Figure

5).

[000144] A surface which is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers within the body 602 is, in the mirror 600, defined by an outer surface of the cap 608. The body 602 may, in some instances, be described as comprising the plurality of layers of the body 602, the supplementary member 606, and the cap 608.

[000145] Figure 7 shows a mirror 700 comprising an array of elements. The mirror 700 may be referred to as a composite mirror. The mirror 700 may be considered to comprise a plurality of elements (although the plurality of elements may be mutually continuously formed together, as shown in Figure 7). Each element of the mirror 700 generally corresponds to the mirror 500 (Figure 5). In use, radiation 702 is generally incident on outer surfaces of the cap of each element of the mirror. The mirror 700 may provide all the same advantages as those of the mirror 500 described above with reference to Figure 5. However, the mirror 700 (Figure 7) also provides further advantages, which are now described.

[000146] A spacing 704 between repeated peaks or troughs on a radiation-facing surface of the mirror 700 defines a reflective diffraction grating. This spacing 704 may be referred to as a“pitch” of the reflective diffraction grating. An incident radiation beam 702 may thus interact with the mirror 700 such that there is a zeroth order diffracted beam 706, first order diffracted beams 708a, 708b, and potentially more diffracted beams (not shown).

[000147] After interacting with the radiation-facing surface of the mirror 700, different wavelengths of radiation are diffracted in different directions by the reflective diffraction grating. The spacing 704 between elements of the mirror 700 may be configured such that, after interacting with the radiation-facing surface of the mirror 700, EUV radiation is diffracted in a different direction to the direction in which DUV and/or IR radiation is diffracted. In other words, the spacing 704 of elements of the mirror 700 may be configured to further filter out radiation having a second wavelength (such as DUV and or IR radiation) from radiation having a first wavelength (such as EUV radiation) relative to using a single mirror corresponding to the mirror 500 (Figure 5). [000148] It is known to include a reflective diffraction grating (e.g., in the form of grooves) on a mirror. The mirror 700 schematically depicts how such a known arrangement may be achieved whilst also adopting the principles of the invention disclosed herein (i.e., by using any of the mirrors 200, 300, 400, 500, 600).

[000149] Adjacent elements within the array of elements may be formed continuously. This may make manufacture of the composite mirror simpler and cheaper. Alternatively, each individual element within the array of elements may be separate.

[000150] Figure 8 schematically depicts a cross-section through a mirror 800 and illustrates the effect of a droplet 901 when it impacts a substantially flat surface of the mirror. The mirror 800 comprises a body 801 and a supplementary member 802 arranged on the body 801. In the embodiment as shown, the body 801 is arranged on a substrate 803. The body 801 may be referred to as a mirror body. The supplementary member 802 may be referred to as an auxiliary layer, it may e.g. be a cap layer or capping layer.

[000151] The body 801 can e.g. be formed from generally the same materials as the body 202

(Figure 2) or the body 402 (Figure 4). That is, the body 801 can comprise a first group and second group of layers which are arranged to act as a multilayer Bragg reflector.

[000152] When such a mirror is applied in an EUV lithographical apparatus, whereby the EUV radiation is generated by impacting tin droplets by a laser beam, the apparatus, in particular the mirrors as applied in the apparatus, may become contaminated by tin droplets, propagating through the apparatus as already described above. When considering a typical outer surface of a mirror as applied in an EUV lithographical apparatus, it can be pointed out that a curvature of such a mirror, e.g. a mirror as applied in an illumination system (or illuminator) of the apparatus, will be substantially larger than a diameter than an incident droplet. In such case, the reflection of a droplet will follow ray optics. On the left side of Figure 8 this is illustrated by a droplet 901 having a diameter D which, upon impact with the surface of the supplementary member 802 of the mirror 800, will flatten to a droplet 902, and subsequently bounce off, away from the surface, whereby an incident angle of the droplet, denoted as A, will substantially correspond to the reflection angle A of the reflected droplet 903, having a diameter D, that moves away from the surface of the supplementary member 802.

[000153] With respect to the characteristics of the tin droplets that may appear at or near the optics as applied in the EUV lithographical apparatus, the following can be said: it is expected that tin droplets propagating through the apparatus, e.g. through the illuminator of the apparatus, have a diameter in the range of 3 - 30 pm. Such particles may travel with a speed in a range of 10 - 50 m/s. When a droplet impacts a surface, a substantially flat surface, it will flatten, before it retracts again and bounces off the surface. When balancing surface energy and kinetic energy of such a droplet, it can be shown that a flattened droplet on the surface of the mirror may have a diameter, indicated as D_p in Figure 8, in a range of 10 -100 pm. The thickness, indicated as H_p in figure 8 of such a flattened droplet may e.g. be in a range of 0.2 to 2 pm. The right side of Figure 8 illustrates the position 810 of an incident droplet on the surface of member 802, and the maximum spread 912 of the flattened droplet.

[000154] It is an objective of the present invention to reduce or mitigate the propagation of particles such as tin droplets through the lithographic apparatus. As illustrated in Figures 2 to 7, this can be obtained by taking the measure of ensuring that a local tangent plane of the surface of the mirror is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers of the body of the mirror.

[000155] An alternative measure, or a measure that can be applied in addition, is schematically illustrated in Figure 9. Figure 9 schematically shows a cross-sectional view of a mirror 900 according to the present invention, the mirror 900 comprising a body 801, which can be similar or the same as the body 801 of mirror 800, and a supplementary member 802, which can be similar or the same as the supplementary member 802 of mirror 800. In addition, mirror 900 comprises a plurality of obstacles 1000 that are arranged on the surface of the supplementary member 802 that can be affected by particles such as tin droplets. In accordance with an embodiment of the present invention, the obstacles can be small structures that are distributed across the surface, i.e. the outer surface of the mirror 900. In accordance with the invention, these obstacles are configured to interrupt the spreading and retracting of the flow of a tin droplet impacting the surface. With other words, the obstacles or structures 1000 as provided on the surface are provided to disrupt the process described with reference to figure 8, whereby a droplet 901 impacts the surface, flattens to a flat droplet 902, retracts again and is reflected as a droplet 903 having substantially the same size as droplet 901 and having a reflection angle substantially equal to the angle of incidence.

[000156] In order to interrupt this process, obstacles or structures 1000 are distributed and sized in such manner that when a droplet impacts the surface and flattens, it will at least, during the flattening process, encounter one obstacle or structure. On the right side of Figure 9, a distribution of the obstacles 1000 is shown, together with an expected flow or expansion 1011 of a droplet 1010 impacting the surface and expanding to a flattened droplet having a diameter D_p. In Figure 9, the height of the structures or obstacles is indicated as H_o, whereas D_o refers to a width or diameter of the structures or obstacles. Distance S is used to denote the distance between adjacent obstacles in the cross-sectional view on the left side of Figure 9. Due to the encounter of the expanding droplet with one or more of the obstacles 1000, a scattering or breaking of the droplet is stimulated. As a result, the bouncing back of the droplet will no longer be symmetrical, i.e. the scattered or broken down droplet will be reflected by the surface at a reflection angle that is different from the angle of incidence. In particular, as illustrated on the left side of Figure 9, the initial droplet 1001 may, due to the interaction of the expanding droplet with the obstacles 1000, break down in various small or smaller droplets such as droplets 1003 and 1004, which bounce off the surface at a different reflecting angle, e.g. angle B, compared to the incidence angle A of the droplet 1001. In Figure 9, D denotes the diameter of the droplet 1001, i.e. droplet impacting the surface, whereas d denotes the smaller diameter of a droplet 1003 reflected by the surface. In order for the obstacles to effectively scatter or break down the incident droplets, it may be advantageous to select a height H_o of the obstacles to be in the same range as the expected height of the thinned or flattened droplet, i.e. H_o being in a range of 0.1 to 1 pm. Preferably, the obstacles may be distributed across the surface in such manner that an average distance between the obstacles is smaller than the diameter D_p of a droplet when said droplet would be allowed to fully flatten or expand. Referring to the right side of Figure 9, distances S or W between adjacent obstacles should thus be smaller than 100 pm, preferably smaller than 30 pm, e.g. in an range of 3 to 30 pm. In a preferred embodiment, the obstacles may have a size D_o in a range of 0.1 to 1 pm. By keeping the size of the obstacles small, a scattering or transmission loss of the mirror can be kept low. In this respect, it can be pointed out that the scattering or transmission loss scales with the ratio of the integrated obstacle area over the mirror area, so e.g. proportional to (D_o/S)².

[000157] In an embodiment, the obstacles or structures 1000 may be arranged in a regular grid or array, a quasi-regular grid or array or an irregular array. It can also be pointed out that the height of the obstacles need not be the same for all obstacles. A variation of the height of +/- 50% of the average height may be acceptable. Phrased differently, there are no strict tolerances w.r.t. height or relative positions of the obstacles.

[000158] In an embodiment, the obstacles 1000 can be made from EUV or hydrogen plasma tolerant materials. Suitable materials would therefore be Ru, Zr, ZrO(x), Mo, MoO(x), Fe, Ni, other metals, metal oxides, metal nitrides or metal borides, or a mix of such materials. In case a supplementary member is used, such as supplementary member 802, it may be preferred to select a material that is also compatible with the material of said member (e.g. Ru, Zr, ZrO(x), Mo, MoO(x)), in manufacturing or in use.

[000159] As illustrated, the use of obstacles or structures 1000 on a surface of a mirror used in an EUV lithographic apparatus, enables to change the trajectory of a particle, e.g. a tin droplet, imparting on the mirror. By doing so, a propagation of such a particle into or in the apparatus can be limited or hindered. In particular, the application of the obstacles as described is configured to at least partly scatter or break down a particle, e.g. a tin droplet, that impacts an outer surface of the mirror.

[000160] In the embodiment as shown in Figure 9, the plurality of obstacles is applied onto a supplementary member 802 of the mirror. Such a supplementary member 802 can e.g. be a capping layer of the mirror. It can be pointed out however that, in the absence of such a supplementary member 802, the plurality of obstacles may also be arranged directly on the outer surface of the body 801 of the mirror.

[000161] It can further be pointed out that the measure to apply a plurality of obstacles as described onto an outer surface of a mirror can be applied as a stand-alone measure to limit or hinder the propagation of particles inside an EUV lithographic apparatus. In a further embodiment, the application of the plurality of obstacles can also be combined with the measure of ensuring that a local tangent plane of the surface of the mirror is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers of the body of the mirror.

[000162] It has been described above, in detail, how the mirror 200 (Figure 2) may be incorporated into the lithographic system and the advantages that such an arrangement would provide. The same holds for the mirror 900 of Figure 9. In particular, the mirror 200 or the mirror 900 may form one or more facets of the facetted field mirror device 10 or the facetted pupil mirror device 11. Mirrors 300, 400, 500, 600, 700 (of Figures 3, 4, 5, 6, and 7) illustrate modifications to the mirror 200 (Figure 2) which provide at least the same general advantages as those provided by the mirror 200. It will therefore be appreciated that any of the mirrors 300, 400, 500, 600, 700 (of Figures 3, 4, 5, 6, and 7) may be incorporated into a lithographic system in a similar manner to that described above with reference to the mirror 200 (Figure 2), and at least the same general advantages as those provided by the mirror 200 (discussed within the context of a lithographic system) will be provided.

[000163] The term“radiation having a/the first wavelength” (or variants thereof) has generally been used herein to refer to EUV radiation. It will be appreciated that, in other embodiments,“radiation having a/the first wavelength” may refer to other wavelengths of radiation.

[000164] The term “radiation having a/the second wavelength” (or variants thereof) has generally been used herein to refer to IR radiation or to DUV radiation or to both. It will be appreciated that, in other embodiments,“radiation having a/the second wavelength” may refer to other wavelengths of radiation.

[000165] The term“particulate matter” (or variants thereof) has generally been used herein to refer to tin droplets or molten tin. It will be appreciated that, in other embodiments,“particulate matter” may refer to other matter.

[000166] Features described in the context of embodiments disclosed herein may be combined whilst remaining within the scope of the invention as set out in the Claims. Features described in the context of embodiments disclosed herein may be used in different configurations to those presented herein whilst remaining within the scope of the invention as set out in the Claims.

[000167] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[000168] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions. [000169] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A mirror comprising:

a body comprising a plurality of layers; and

a surface defined by the body;

wherein the plurality of layers are arranged to act as a multilayer Bragg reflector for radiation having a first wavelength when radiation having said first wavelength is incident on the surface; and wherein a local tangent plane of the surface is inclined at a non-zero angle relative to a local tangent plane of the plurality of layers.

2. A mirror according to claim 1, wherein the surface is formed from a material which transmits extreme ultraviolet radiation.

3. A mirror according to claim 2, wherein the surface is formed from a material which transmits greater than fifty percent of incident extreme ultraviolet radiation.

4. A mirror according to any of claim 1 to claim 3, wherein the surface is formed from one or more layers of the plurality of layers.

5. A mirror according to any of claim 1 to claim 3, wherein the body further comprises a supplementary member which is disposed on the plurality of layers and wherein the surface is defined by the supplementary member.

6. A mirror according to claim 5, wherein the supplementary member has a size which varies in at least one spatial dimension.

7. A mirror according to claim 5 or claim 6, wherein the supplementary member is non-conformal to a surface of the plurality of layers on which the supplementary member is disposed.

8. A mirror according to any of claim 5 to claim 7, wherein the supplementary member is formed from a material which transmits extreme ultraviolet radiation.

9. A mirror according to any of claim 5 to claim 8, wherein the supplementary member is formed from a material which transmits greater than fifty percent of incident extreme ultraviolet radiation.

10. A mirror according to any of claim 5 to claim 9, wherein the supplementary member is formed, at least in part, from an element having an extinction coefficient of less than or equal to 0.1 for extreme ultraviolet radiation.

11. A mirror according to any of claim 5 to claim 8, wherein the supplementary member is formed from at least one of carbon (C), silicon (Si), niobium (Nb), molybdenum (Mo), ruthenium (Ru), or rhodium (Rh).

12. A mirror according to any preceding claim, wherein the local tangent plane of the surface is inclined at a non-zero angle relative to the local tangent plane of the plurality of layers over a majority of the surface.

13. A mirror according to any preceding claim, wherein the plurality of layers and the surface are arranged such that radiation having a second wavelength is preferentially reflected from the surface when radiation having said second wavelength is incident on the surface.

14. A mirror according to any preceding claim, wherein the body is provided with a cap, the cap being a conformal coating which is arranged to be substantially parallel to a main portion of the body, and wherein the cap defines the surface.

15. A mirror according to claim 14, wherein the cap is formed from a material which transmits extreme ultraviolet radiation.

16. A mirror according to claim 15, wherein the cap is formed from a material which transmits greater than fifty percent of incident extreme ultraviolet radiation.

17. A mirror according to any of claim 14 to claim 16, wherein the cap is formed, at least in part, from an element having an extinction coefficient of less than or equal to 0.1 for extreme ultraviolet radiation.

18. A mirror according to any of claim 14 to claim 17, wherein the cap is formed from at least one of: zirconium (Zr), yttrium (Y), ruthenium (Ru), or rhodium (Rh); any oxide, nitride, or boride ceramic formed from zirconium (Zr), yttrium (Y), ruthenium (Ru), or rhodium (Rh); or any combination thereof.

19. A mirror according to any of claim 14 to claim 18, wherein the cap is configured such that radiation having a second wavelength is preferentially reflected from the cap when radiation having said second wavelength is incident on the cap.

20. A mirror according to any preceding claim, wherein the mirror forms part of an optical system within a lithographic apparatus.

21. A mirror according to claim 20, wherein the mirror forms part of a facetted mirror device.

22. A mirror according to any preceding claim, wherein the mirror forms part of an optical system within a radiation source.

23. A mirror according to any preceding claim, wherein the mirror has optical power.

24. A mirror according to claim 23, wherein the mirror is curved.

25. A mirror according to any of the preceding claims, whereby the surface is provided with a plurality of obstacles configured to at least partly scatter or break down a particle that impacts the surface of the mirror.

26. The mirror according to claim 25, whereby the obstacles are configured to at least partly scatter or break down the particle into multiple particles, a reflecting angle of the multiple particles off the surface being different from an incident angle of the particle.

27. The mirror according to claim 26, whereby a height of an obstacle of the plurality of obstacles is in a range from 0.1 to 1 pm.

28. The mirror according to claim 26 or 27, whereby an average distance between adjacent obstacles of the plurality of obstacles is smaller than 100 pm, preferably smaller than 30 pm, preferably in an range of 3 to 30 pm.

29. The mirror according to claim 26, 27 or 28, whereby an obstacle of the plurality of obstacles has a size in a range of 0.1 to 1 pm.

30. The mirror according to any of the claims 26 to 29, whereby the plurality of obstacles is formed from Ru, Zr, ZrO(x), Mo, MoO(x), Fe, Ni, a metal oxide, a metal nitride or a metal borides, or a combination thereof.

31. A mirror compri sing :

a body comprising a plurality of layers; and

a surface defined by the body;

wherein the surface is provided with a plurality of obstacles configured to at least partly scatter or break down a particle that impacts the surface of the mirror.

32. The mirror according to claim 31, whereby the obstacles are configured to at least partly scatter or break down the particle into multiple particles, a reflecting angle of the multiple particles off the surface being different from an incident angle of the particle.

33. The mirror according to any of claim 31 to claim 32, wherein the surface is formed from one or more layers of the plurality of layers.

34. A mirror according to any of claim 31 to claim 33, wherein the body further comprises a supplementary member which is disposed on the plurality of layers and wherein the surface is defined by the supplementary member.

35. The mirror according to any of the claims 31 to 34, whereby a height of an obstacle of the plurality of obstacles is in a range from 0.1 to 1 pm.

36. The mirror according to any of the claims 31 to 35, whereby an average distance between adjacent obstacles of the plurality of obstacles is smaller than 100 pm, preferably smaller than 30 pm, preferably in an range of 3 to 30 pm.

37. The mirror according to any of the claims 31 to 36, whereby an obstacle of the plurality of obstacles has a size in a range of 0.1 to 1 pm.

38. The mirror according to any of the claims 31 to 37, whereby the plurality of obstacles is formed from Ru, Zr, ZrO(x), Mo, MoO(x), Fe, Ni, a metal oxide, a metal nitride or a metal boride, or a combination thereof.

39. A composite mirror formed from an array of elements, wherein each element of the composite mirror corresponds to a mirror according to any preceding claim.