WO2013068198A2

WO2013068198A2 - Particle trap for euv source

Info

Publication number: WO2013068198A2
Application number: PCT/EP2012/070279
Authority: WO
Inventors: Ivo Vanderhallen; Johannes Franken; Robert LANSBERGEN; Lambertus VAN DEN WILDENBERG
Original assignee: Asml Netherlands B.V.
Priority date: 2011-11-10
Filing date: 2012-10-12
Publication date: 2013-05-16
Also published as: TW201319763A; WO2013068198A3

Abstract

There is disclosed a particle trap for a plasma EUV radiation source, a lithography apparatus comprising a particle trap and a device manufacturing method. In an embodiment, the particle trap includes a rotatable hub, and a plurality of blades extending outwards from the hub. Each of the blades has an end anchoring portion inserted into a complementary slot in the hub, the end anchoring portion and slot being configured to hold the blades within the hub during rotation of the hub.

Description

PARTICLE TRAP FOR EUV SOURCE

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of US provisional application 61/558,203, which was filed on November 10, 2011 and which is incorporated herein in its entirety by reference.

FIELD

[0001] The present invention relates to a particle trap for a plasma EUV radiation

lithography apparatus comprising a particle trap and a device manufacturing method.

BACKGROUND

[0002] A lithographic apparatus is a machine that applies a desired pattern onto a

substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer).

Transfer of the pattern is typically via imaging onto a layer of radiation- sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0003] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

[0004] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

CD = k *— (1)

¹ NA

where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, ki is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of ki.

[0005] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0006] EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector apparatus for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.

[0007] In other cases the source may be a discharge produced plasma EUV generator, often referred to as a DPP source.

[0008] A particle trap may be provided between the plasma (which may be discharge produced or laser produced, for example) and a radiation collector to prevent particles from the plasma reaching the radiation collector. Particles emitted from the source may comprise neutral particles and charged particles (ions). The particles may comprise particles of the fuel used to create the plasma. For example, the particles may comprise particles (or droplets) of tin.

[0009] The particle trap may comprise a plurality of blades which rotate around a hub about an axis passing through the plasma. It is challenging to design a particle trap that is effective for preventing particles from reaching the radiation collector without excessively reducing the amount of EUV radiation which passes through the radiation trap. It is also challenging to provide a particle trap that can withstand the high heat loads associated with increasing the power output from the plasma. The particle trap can limit the output power of the EUV radiation source by limiting the transmission efficiency and/or by limiting the extent to which the power output from the plasma can be increased.

[00010] The desire to withstand high heat loads may mean that special materials are desirable, such as molybdenum, tungsten and/or rhenium. Such materials tend to be heavy. Increasing the weight of the blades and/or hub may increase the mechanical stresses on the blades and/or hub (and connections between them) during rotation.

[00011] Cooling may be applied to the blades via the hub. The hub should be made as small as possible to minimize blocking of radiation by the hub. The space used for cooling may need to be relatively large for the cooling to be efficient. The space available for effecting connection between the blades and the hub may therefore be limited. A compact connection may be required. To achieve high rotation speeds and/or support heavy blades, the connection between the blades and the hub should also be strong.

[00012] The speed at which the blades can be rotated may be limited by the mechanical stress that the blade material can support. Thicker blades may be used to reduce the risks of buckling, but thicker blades will tend to be heavier and impart larger centripetal forces. Thicker blades may also reduce the transmission efficiency of the trap.

[00013] Higher plasma powers may cause buckling of the blades. Buckling is when the blade deviate from its original shape, for instance when an initial straight blade starts bending out of plane. Buckling may be caused by the development of a high thermal gradient between the front edge of the blade (facing the plasma) and the rear edge of the blade. The risk of blade buckling may be reduced by increasing the thickness of blade, but increasing the blade thickness may increase blade weight and/or reduce transmission efficiency.

[00014] More generally, it is challenging to keep critical mechanical stress levels within acceptable bounds, at the level of the hub, blade and/or the connections between them.

SUMMARY

[00015] It is desirable to increase the amount of EUV radiation that reaches the radiation collector without allowing excessive contamination of the radiation collector by particles originating from the plasma. [00016] According to an aspect, there is provided a particle trap for a plasma EUV radiation source, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from the hub, wherein each of the blades has an end anchoring portion inserted into a

complementary slot in the hub, the end anchoring portion and slot being such as to hold the blades within the hub during rotation of the hub.

[00017] According to an aspect, there is provided a particle trap for a plasma EUV radiation source, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from the hub, wherein each of the blades comprises a plurality of distinct anchoring surfaces, each anchoring surface being configured to press against a corresponding support surface within the hub in order to constrain the blade radially during rotation of the hub.

[00018] According to an aspect, there is provided a particle trap for a plasma EUV radiation source, the particle trap comprising: a rotatable hub; and a plurality of pairs of blades extending outwards from the hub, each pair of blades consisting of a single piece of material in the region where the blade is located within the hub, wherein an outer portion of the hub is segmented into a plurality of support beams, each positioned between the blades of one of the pairs of blades for holding that pair against the hub during rotation of the hub, and a plurality of intermediate beams positioned in between the support beams; and the particle trap further comprises a plurality of coupling bars, each formed separately from the hub and positioned between one of the support beams and the pair of blades that that support beam supports.

[00019] According to an aspect, there is provided a particle trap for a plasma EUV radiation source, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from the hub, wherein the hub and blades are formed by connecting together a plurality of single piece units, each single piece unit comprising one of the plurality of blades and a portion of the hub, formed together integrally, each single piece unit being connected directly to at least one other of the single piece units.

[00020] According to an aspect, there is provided a particle trap for a plasma EUV radiation source, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from openings in the hub, wherein each of the openings is defined by a continuous integral loop of material, the openings and the blades being configured so that each blade can be inserted into the hub from the side of the hub that is opposite from the opening through which that blade will protrude. [00021] According to an aspect, there is provided a particle trap for a plasma EUV radiation source, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from the hub, wherein one or more of the blades is provided with curvature to increase resistance to buckling, wherein, for a predetermined speed of rotation, the curvature is such that rays originating from a predetermined point along the axis of rotation of the hub pass the blade along lines of constant circumferential position so as not to be incident on either face of the blade.

[00022] According to an aspect, there is provided a particle trap for a plasma EUV radiation source, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from the hub, wherein at least one of the plurality of blades comprises an opening; and the blades are configured such that for a predetermined range of speeds of a particle originating from a plasma located at a predetermined point along the axis of rotation of the hub, and for a predetermined speed of rotation of the hub, when the particle avoids striking a radiation facing edge of any blade and passes through the opening the particle will strike a subsequent blade at a position where there is no opening.

[00023] According to an aspect, there is provided a particle trap for a plasma EUV radiation source, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from the hub, wherein: at least one of the blades is hollow.

[00024] According to an aspect, there is provided a particle trap for a plasma EUV radiation source, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from the hub, wherein the hub comprises channels for a coolant fluid, at least a portion of the channels comprising a linear segment angled obliquely relative to the axis of rotation of the hub.

BRIEF DESCRIPTION OF THE DRAWINGS

[00025] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[00026] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention;

[00027] Figure 2 is a more detailed view of the apparatus of Figure 1 ;

[00028] Figure 3 is a more detailed view of a source collector apparatus of the apparatus of

Figures 1 and 2; [00029] Figure 4 is a schematic side sectional view of a particle trap and particles travelling from a plasma towards the particle trap;

[00030] Figure 5 depicts a radially outer portion of a hub of the particle trap showing connection to the hub of three pairs of blades using a coupling bar between each elbow of each pair of blades and the hub;

[00031] Figure 6 depicts a radially outer portion of the hub showing connections of single blades in which end anchoring portions of the blades are folded through 180°;

[00032] Figure 7 depicts a radially outer portion of a hub and three connected blades, in which the blades each comprise a plurality of distinct anchoring surfaces;

[00033] Figure 8 is a schematic side view of a portion of material used for manufacturing a blade, in which openings have been formed for making anchoring surfaces;

[00034] Figure 9 is a schematic top view of the blade illustrated in Figure 8 in which material adjacent to each opening has been deflected to one or the other side of the blade in order to create anchoring surfaces;

[00035] Figure 10 is a schematic sectional view of a hub that comprises channels at an oblique angle to the axis of rotation of the hub;

[00036] Figure 11 is a schematic top view of a segment of a particle trap comprising a blade formed integrally with a segment of a hub;

[00037] Figure 12 is a schematic side view of the segment shown in Figure 11;

[00038] Figure 13 is a schematic side view of a portion of a hub having openings for blade insertion that are surrounded by a closed loop of integral material;

[00039] Figure 14 is a schematic illustration showing how blades can be inserted into a hub of the type illustrated in Figure 13;

[00040] Figure 15 is a schematic illustration showing the directions of propagation of radiation from a plasma past a portion of a blade of a particle trap;

[00041] Figure 16 shows schematically the front edge of a blade that is configured to have curvature perpendicular to the direction of propagation of the EUV radiation;

[00042] Figure 17 shows schematically the rear edge of the blade of Figure 16;

[00043] Figure 18 is a schematic side view of a blade having holes that are shaped and positioned to allow particles to pass through the holes but be removed by a subsequent blade; [00044] Figure 19 is a schematic side view of a blade of the type illustrated in Figure 18 except that a region containing openings is restricted to a central portion of the blade;

[00045] Figure 20 is a schematic side view of a hollow blade and of a flow of cooling gas through the interior of the hollow blade; and

[00046] Figure 21 is a schematic end view of the blade illustrated in Figure 20. DETAILED DESCRIPTION

[00047] Figure 1 schematically depicts a lithographic apparatus 100 including a

source collector apparatus SO according to one embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation); a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g. a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[00048] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[00049] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[00050] The term "patterning device" should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[00051] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[00052] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[00053] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[00054] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[00055] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source collector apparatus SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector apparatus SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector apparatus. The laser and the source collector apparatus may be separate entities, for example when a C0₂ laser is used to provide the laser beam for fuel excitation.

[00056] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source collector apparatus with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector apparatus, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[00057] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ- inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[00058] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[00059] The depicted apparatus could be used in at least one of the following modes:

1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. In scan mode, 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 target portion C (i.e. a single 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.

3. In another mode, the support structure (e.g. mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[00060] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[00061] Figure 2 shows the apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 may be formed by a discharge produced plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the very hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma 210 is created by, for example, an electrical discharge causing an at least partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[00062] The radiation emitted by the hot plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap) which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 may include a channel structure. Contamination trap 230 may also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure, as known in the art.

[00063] The collector chamber 212 may include a radiation collector CO which may be a so- called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point IF.

Alternatively or additionally, the radiation that traverses the collector CO can be focused directly in to the virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus IF is located at or near an opening 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[00064] Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the wafer stage or substrate table WT.

[00065] More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 may optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in the Figures, for example there may be 1- 6 additional reflective elements present in the projection system PS than shown in Figure 2.

[00066] Collector optic CO, as illustrated in Figure 2, is depicted as a nested collector with grazing incidence reflectors 253, 254 and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254 and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source. [00067] Alternatively, the source collector apparatus SO may be part of an LPP radiation system as shown in Figure 3. A laser LA is arranged to deposit laser energy into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), creating the highly ionized plasma 210 with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near normal incidence collector optic CO and focused onto the opening 221 in the enclosing structure 220.

[00068] Figure 4 illustrates an example of a particle trap 40 for preventing particles

originating from the plasma 210 from reaching the collector optic CO. Particles originating from the plasma 210 are illustrated schematically by arrows 42. In an embodiment, the particle trap 40 comprises a hub 44 that is rotatable about an axis 46. The axis 46 may pass through the plasma 210. The particle trap 40 may comprise a plurality of blades 58 that extend outwards (e.g. radially) from the hub 44. In the example depicted in Figure 4, the radially outermost edges of the blades 58 are not shown (see cut-off lines 50). Rotation of the hub 44 causes a corresponding rotation of the blades 58. The geometry of the blades 58 and the speed of rotation of the hub 44 are chosen so that, for all expected speeds of the particles from the source 210, when a particle reaches the region between any two blades it will be struck by a portion of one of the blades in a circumferential direction and be prevented from passing the particle trap.

[00069] The incident radiation and particles cause heating of the hub 44 and blades 58. In order to prevent the temperature of the blades 58 or hub 44 exceeding a target temperature, a cooling system may be provided. In an embodiment, the cooling system comprises channels 52 for a coolant to flow through the hub 44. In an embodiment, the coolant is provided by a coolant outlet 54 located within the hub 44. Arrows 56 illustrate an example flow pattern for coolant within the channels 52. Alternatively, other cooling means may be provided, such as C0₂ cooling, gas flow cooling or cooling by use of a heat pipe, wherein such cooling means have the further advantage to reduce corrosion and leakage risk.

[00070] As mentioned above, it is challenging to design a particle trap that can eliminate particles effectively while at the same time allowing a large proportion of the EUV radiation to pass the particle trap.

[00071] In order to minimize the amount of radiation that is blocked by the particle trap, it is desirable to minimize the cross section of the particle trap perpendicular to the radiation.

Reducing the cross section of the trap can be achieved by reducing the cross section of the hub 44 and/or by reducing the cross section of the blades 58. Reducing the cross section of the hub 44 can be achieved by making the hub 44 smaller. Reducing the cross section of the blades 58 can be achieved by making the blades 58 thinner and/or by using fewer blades. However, reducing the number of blades may dictate that the blades 58 be rotated more quickly to provide adequate particle removal. Rotating the hub 44 more quickly may be achieved more easily if the hub 44 is made lighter. Rotation of the hub 44 may be facilitated also by reducing the weight of the blades 58. However, reducing the weight of components of the particle trap may affect the structural integrity of the particle trap. Reducing the material of the hub 44 may reduce the strength of the hub 44. Reducing the amount of material used for the blades 58 may reduce the strength of the blades 58. If the hub 44 is not sufficiently strong, failure of the hub 44 could occur. Failure of the hub 44 could involve blades 58 becoming detached from the hub 44 during use. Reducing the strength of the blades 58 could cause failure of the blades 58. For example, the blades 58 may buckle in use due to temperature gradients that develop within the blades 58 due to heating from the plasma source. Alternatively or additionally, centripetal forces applied to the blades 58 could cause failure of the blades 58.

[00072] In an embodiment, the connection between the blades 58 and the hub 44 is arranged so that the centripetal force from the blades 58 is spread evenly over the material of the hub 44. In this way, the hub 44 can be made lighter without compromising on the ability to hold the blades 58 safely during use. Figure 5 illustrates an example.

[00073] Figure 5 is a schematic sectional view of a radially outer portion of the hub 44 looking parallel to the axis of rotation of the hub 44. In this example, the hub 44 is configured to receive a plurality of pairs of blades 58A, 58B. Each pair of blades 58A, 58B consists of a single piece of material in the region of the elbow 62 connecting the blades 58 A, 58B together. In an embodiment, all of the portion of the pair of blades that is in use in the hub 44 is a single piece of material. In an embodiment, the whole of the pair of blade 58 A, 58B consists of a single piece of material. The pair of blades 58 A, 58B may be formed by bending the single piece of material at the elbow 62 in a central region of the single piece of material. The angle between the pair of blades 58 A, 58B may be 360 degrees divided by the total number of blades.

[00074] In an embodiment, the outer portion of the hub 44 is segmented into a plurality of support beams 44 A. Each of the support beams 44 A is positioned between the blades 58 A, 58B of one of the pairs of blades for holding that pair of blades against the hub 44 during rotation of the hub 44. Each of the support beams 44 A thus provides a radially inwards force against the elbow 62 of the pair of blades 58 A, 58B between which that support beam 44 A is located. In an embodiment, the hub 44 also comprises a plurality of intermediate beams 44B positioned between the support beams 44A. Such intermediate beams may be used to define the spacing between the nearest blades of directly adjacent pairs of blades 58 A, 58B. A potential problem with this arrangement is that the centripetal force applied to the blades in each pair 58 A, 58B may be transferred exclusively or predominantly to the support beams 44A. In such an arrangement, the intermediate beams 44B are not used, or are not used to a significant extent, for supporting the centripetal force from the blades 58A, 58B. The material of the hub 44 may therefore not be optimally used.

[00075] In an embodiment, the centripetal forces are spread more evenly using coupling bars 60. Each coupling bar 60 may be formed separately from the hub 44. Each coupling bar 60 may be positioned between one of the support beams 44A and the pair of blades 58 A, 58B that the support beam 44A supports. Figure 5 depicts an example arrangement.

[00076] Each coupling bar 60 may be configured to enlarge circumferentially the portion of a pair of blades 58A, 58B that is radially inside of the support beam 44A. The coupling bar 60 may be configured to cause the pair of blades 58 A, 58B to engage with one or both of the

intermediate beams 44B that surround the support beam 44A. In this way, the coupling bar 60 acts to spread the load from the pair of blades 58 A, 58B to one or both of the intermediate beams 44B. In an embodiment, the coupling bar 60 acts to enlarge the elbow 62 of the pair of blades 58 A, 58B. The elbow 62 may for example spread so that it is wider than the separation between neighbouring intermediate beams 44B. In this way, the coupling bar 60 may force the pair of blades 58 A, 58B to be brought into contact with the intermediate beams 44B. Figure 5 illustrates schematically points of engagement 64 between the blades 58 and the intermediate beams 44B. The reaction force between the blades and the intermediate beams 44B will tend to be angled inwards from either side of the intermediate beams 44B. The intermediate beams 44B will therefore tend to be compressed which may improve the stiffness of the hub 44.

[00077] In the absence of a coupling bar 60 the radially inner surfaces of the support beams 44A would have to machined more carefully, for example so as to be provided with a rounded surface. The machining could be carried out by spark erosion for example. The provision of coupling bars 60 reduces the amount of such machining that is needed and thereby facilitates manufacture.

[00078] In an embodiment, one or more of the coupling beams 60 is/are formed from a material that is softer than the material from which the support beams 44A and/or blades 58 are formed. For example, the coupling beams 60 may be formed from aluminium. Making the coupling beams 60 softer encourages deformation of the coupling beams 60. Deformation of the coupling beams 60 will tend to favor lateral expansion of the elbows 62. Deformation of the coupling beams 60 will tend to favor good thermal contact between the blades 58 and the hub 44. Improving the thermal contact between the blades 58 and the hub 44 may help ensure efficient cooling of the blades 58 by the hub 44. Deformation of the coupling beams 60 may also improve mechanical contact with the hub by spreading the contact over a larger area.

[00079] Figure 6 depicts an alternative arrangement for connecting blades 58 to the hub 44. Figure 6 is a schematic sectional view of a radially outer portion of the hub 44 viewed parallel to the axis of rotation of the hub 44. In this embodiment, the blades 58 are each provided as separate components rather than in pairs connected together by and/or formed from a single piece of material (as in embodiments of the type shown in Figure 5). In order to provide an efficient anchoring of the blades 58 within the hub 44, end portions of the blades 58 are folded for example through 180° in order to form end anchor portions 66. The end anchor portions 66 thereby have a thickness that is twice the thickness of the portion of the blade outside of the hub. In the folding through 180°, a first portion 66A of the end anchor portion 66 may define a reference direction. A subsequent portion 66B may be folded through 90° with respect to the reference direction. A portion 66C that is subsequent to the portion 66B may be folded through a further 90° to achieve the total folding angle of 180°. The blades 58 with folded end anchor portions 66 may be inserted into the hub 44 in a direction parallel to the axis of rotation of the hub 44 or to the outer surface of the hub 44. Where the hub 44 is cone shaped, the blades 58 may be inserted in a direction parallel to the outer surface of the cone. A plurality of slots 68 may be formed in the hub 44. The shapes of the slots 68 may be complementary to the shapes of the blades 58. The slots 68 may be such as to prevent the blades 58 from being pulled radially out of the hub 44 by centripetal forces during rotation of the hub. For example, the slots 68 may present anchoring surfaces 72 against which the end anchor portions 66 of the blades 58 engage. [00080] In an embodiment, a first set 74 of the plurality of blades 58 have end anchor portions 66 located at a first radius relative to the axis of rotation of the hub 44. A second set of the plurality of blades 58 may be configured to have end anchor portions 66 that are located at a second radius relative to the axis of rotation of the hub 44. The first radius may be larger than the second radius. Arranging the blades 58 to have end anchor portions 66 at different radii helps to ensure that, for a given blade spacing, there is sufficient hub material surrounding the end anchor portions 66 to ensure that the end anchor portions 66 can be held reliably. If the end anchor portions 66 were all located at the same radius the separation between adjacent end anchor portions 66 would be smaller. The amount of hub material surrounding the end anchor portions 66 would therefore be less. The strength of the materials surrounding the end anchor portions 66 would therefore tend to be lower. This approach makes it possible to have more closely spaced blades without compromising on the quality of connection between the blades 58 and the hub 44.

[00081] The anchoring strength of the end anchor portions 66 within the hub 44 may be improved further by tilting the end anchor portions 66 relative to the radial direction. In the arrangement of Figure 6, the end anchor portions 66 are tilted relative to a radial direction 78 through an angle 80. Tilting the end anchor portions 66 is optionally performed at an angle that distributes the end anchor portions 66 evenly within the hub material. Distributing the end anchor portions 66 evenly within the hub helps to distribute the centripetal forces applied by the end anchor portions 66 evenly over the hub material 44. Tilting the end anchor portions 66 also helps to improve the anchoring function of the hub material by ensuring that the material of the end anchor portions 66 is more evenly distributed to both circumferential sides of the axis of the blades 58. The tilting increases the surface area of the end anchor portions 66 that is pressed against hub material by centripetal forces, which may improve anchoring and/or the quality of the thermal connection between the end anchor portions 66 and the hub.

[00082] In the example shown in Figure 6 the end anchor portions 66 are arranged along two different radii. However, in alternative embodiments, the end anchor portions 66 may be distributed over more than two radii. In the arrangement of Figure 6, the end anchor portions 66 are tilted and arranged along different radii. In alternative embodiments, the end anchor portions 66 are tilted but arranged along the same radii. In alternative embodiments, the end anchor portion 66 are arranged along different radii but are not tilted. [00083] Figure 7 illustrates an alternative mechanism for anchoring blades 58 within the hub 44. Figure 7 is a schematic sectional view in a direction parallel to the axis of rotation of the hub 44. In this example, each of the blades 58 comprises a plurality of distinct anchoring surfaces 82-87. Each of the anchoring surfaces 82-87 is configured to press against a corresponding support surface within the hub 44 to constrain the blade 58 radially during rotation of the hub 44. In an embodiment, each of the blades 58 has anchoring surfaces 82-87 that are positioned at different radial separations from the axis of rotation of the hub 44. In the example shown, anchoring surfaces 82 and 83 are at a larger radial separation than anchoring surfaces 84 and 85. Anchoring surfaces 84 and 85 are at a larger radial separation than anchoring surfaces 86 and 87. The provision of multiple anchoring surfaces at different radii helps to improve the distribution of anchoring forces within the hub 44. In the embodiments shown, the anchoring surfaces 82-87 are distributed over three different radii. However, in alternative embodiments, the anchoring surfaces may be distributed over two radii or over more than three radii. In the example shown, the anchoring surfaces of adjacent blades 58 are positioned at the same radii. However, in alternative embodiments, the anchoring surfaces of adjacent blades 58 may be positioned at different radii.

[00084] In an embodiment, the anchoring surfaces 82-87 have surface areas that vary according to their radial separation from the axis of rotation of the hub 44. In the example shown, the surface areas of the anchoring surfaces at larger radii are larger than the surface areas of anchoring surfaces at smaller radii. Anchoring surfaces 82 and 83 are larger than anchoring surfaces 84 and 85. Anchoring surfaces 84 and 85 are larger than anchoring surfaces 86 and 87. This arrangement helps to maintain a uniform separation between the blades 58 and their anchoring portions 82-87 within the hub 44.

[00085] In the example shown, the anchoring surfaces 82-87 are configured to provide an anchoring force on one circumferential side of the blade 58 that is substantially equal to the anchoring force provided on the other circumferential side of the blade 58. In the example shown, this is achieved by arranging for anchoring surfaces 82, 84 and 86 to be substantially the same, respectively, to anchoring surfaces 83, 85 and 87 on the opposite circumferential side of the blade 58.

[00086] In an embodiment, the cross sectional shape of the anchoring portions of the blades 58 are substantially constant over the length of the hub 44 in which the blade 58 is positioned. In this way, the blades 58 can be inserted into the hub longitudinally into slots that are substantially complementary in shape to the blades 58.

[00087] Figure 8 is a schematic side view of a sheet of material to be formed into a blade 58 illustrating how anchoring surfaces 90, 91 may be formed according to an embodiment. In this arrangement, a plurality of holes 88 are cut into a sheet of material. A laser may be used to cut the material for example, using techniques that are well known in the art. The holes 88 are shaped so that material adjacent to the holes 88 can be deformed to create the anchoring surfaces 90, 91. The holes 88 may be shaped so as to avoid sharply curved geometries which might weaken the blades by concentrating stresses. In the example shown, the material adjacent to each of the holes is deflected into or out of page to create an anchoring surface 90, 91. Such an anchoring surface 90, 91 is an example of an anchoring surface that extends to a first

circumferential side (e.g. into the page) to a greater extent than to the circumferential side opposite to the first circumferential side (e.g. out of the page) over a first portion of the length of the hub 44. In the example of Figure 8 the first portion of the length of the hub 44 may correspond to the uppermost opening 88. In an embodiment, a further anchoring surface 91 is provided that extends to the circumferential side opposite to the first circumferential side (e.g. out of the page) to a greater extent than to the first circumferential side (e.g. into the page) over a second portion of the length of the hub 44 in which the blade 58 is positioned. The second portion of the length of the hub 44 may correspond to the portion of the blade 58 containing the second highest opening 88 in Figure 8, for example. In an embodiment, the anchoring surfaces 90 and 91 alternate in this manner from the top of the blade 58 to the bottom of the blade 58. For example, this may result in half of the openings 88 having an anchoring surface 90 formed by deflection in the first circumferential direction and half of the openings 88 having an anchoring surface 91 formed by deflection in the other circumferential direction. This approach creates a set of anchoring surfaces 90, 91 that provide a balanced anchoring force.

[00088] Figure 9 is a schematic top view of a blade 58 formed according to the structure of Figure 8 with the anchoring portions 90 and 91 deflected to both circumferential sides.

Arrangements of this type may be configured such that the positions of the first and second portions of the length of the hub 44 are the same for at least two neighbouring blades 58. This approach helps to distribute the forces within the hub 44 effectively. This approach helps to ensure that the anchoring portions 90, 91 for adjacent blades 58 are evenly spaced relative to each other.

[00089] Improving the efficiency of the anchoring of the blades 58 within the hub 44 makes it possible to reduce the amount of hub material that is used for the anchoring. Improving the efficiency of anchoring may make more space available within the hub 44. The extra space could be exploited for improving the cooling of the hub 44, for example.

[00090] Figure 10 illustrates an example arrangement for a hub 44 in which the cooling of the hub 44 has been improved by providing channels for coolant 92 that are angled relative to the axis of rotation of the hub 44. In the example shown, the channels 92 comprise a linear segment that is angled obliquely relative to the axis of rotation of the hub 44. Providing segments of channel 92 that are angled obliquely makes it possible to position the channels closer to the surface of the hub 44 and thereby additionally improve the heat exchange between the coolant and the blades 58. For example, the channels 92 may be arranged to be at a more uniform depth from the radially outer surface of the hub 44 relative to the case where the channels for coolant are parallel or perpendicular to the axis of rotation of the hub 44 (as in the embodiment depicted in Figure 4, for example). Optionally, segments of channel 92 are arranged to be approximately parallel to at least a portion of the radially outer surface of the hub 44.

[00091] Figures 11 and 12 illustrate an alternative approach to implementing the connection between the blades 58 and the hub. In an embodiment the hub and blades 58 are formed by connecting together a plurality of single piece units 94. Each single piece unit 94 comprises one of the plurality of blades 58 making up the particle trap and a portion 96 of the hub. The single piece units 94 are configured so as to be connectable together to form the particle trap. Each single piece unit 94 may be configured to be connected directly to at least one other of the single piece units 94. In an embodiment, each of the single piece units 94 is configured to be directly connectable to two other identical single piece units 94 arranged on either side in the

circumferential direction. The single piece units 94 may be configured so that, when arranged next to each other in the circumferential direction, a complete, circumferentially continuous hub is formed. In an embodiment, one or more constraining members are provided to hold the plurality of single piece units 94 together. For example, a constraining member forming a closed loop may be provided. The closed loop may encircle the plurality of single piece units 94 in order to hold the single piece units 94 together during rotation of the hub. [00092] In an embodiment the transition between the blade and the hub segment in the single piece units 94 is providing smoothly or gradually so as to reduce stress concentrations and distribute the load evenly.

[00093] Forming the blades integrally with the hub, so that the transition between hub and blade is provided without any material interfaces, increases the strength of the trap. The blades can therefore be rotated more quickly. Rotating the blades more quickly means that fewer blades may be used to block particles to the desired extent. Fewer blades may reduce the extent to which radiation is blocked by the blades and therefore increase output power.

[00094] In an embodiment, the single piece units 94 are formed by sintering.

[00095] Figure 13 is a schematic radially inward view of a portion of a hub 44 according to an embodiment. In the arrangement shown, openings 104 are provided in the outer surface of the hub 44. The openings 104 are each surrounded by a continuous integral loop of material. The openings 104 and the blades 58 are configured so that each blade 58 can be inserted into the hub 44 from the side of the hub 44 that is opposite to the opening 104 through which that blade 58 will protrude. The method of blade insertion for this type of embodiment is therefore different from embodiments in which the blades 58 are inserted into slots in a direction parallel, or approximately parallel, to the axis of rotation of the hub 44. The use of openings having a continuous integral loop of material surrounding the openings provides greater structural strength. In an embodiment, the hub 44 is a single, integral piece of material.

[00096] In the portion of the hub 44 illustrated in Figure 13, there are five pairs of blades 58 protruding out of the page. Each of the pairs of blades 58 in this embodiment are formed from a single piece of material that is bent at an elbow. The elbows are not visible in the diagram because the elbows are behind the supporting columns 98 of the hub 44. During use, when the hub 44 is rotating, the elbows of the pairs of blades 58 will press in a radially outward direction against the support columns 98.

[00097] To allow for insertion of all of the pairs of blades 58 into position within the hub 44, the openings 104 should be made relatively large. In particular, it should be possible to insert a pair of blades 58 through an opening 104 when the support columns 98 defining that opening 104 both have pairs of blades 58 mounted on them. This is illustrated by the single pair of blades that is depicted in hatched form in Figure 13. This pair of blades 58 is being inserted in a direction into the page and will be mounted on a support column 98 on the side of the hub 44 that is opposite to the side of the hub 44 shown in Figure 13. In order for the hatched pair of blades 58 to pass through the opening 104 the opening should have a width that is at least four times the circumferential thickness of an individual blade 58. It may also be desirable to have large openings 104 to help avoid sharply curved geometries which may weaken the hub 44 by concentrating stresses.

[00098] Figure 14 is a schematic illustration showing the insertion process. Arrow 102 shows a direction of insertion of the hatched pair of blades 58. Arrows 100 show the direction of opening of the hatched pair of blades 58 that will be required for mounting of the pair of blades 58 onto the support column 98 that will receive the hatched pair of blades 58. In the diagram of Figure 14, the upper row of support columns 98 represents a portion of the hub 44 on a first side of the hub 44. The lower row of support columns 98 represents the support columns on a side of the hub 44 that is opposite. Broken line 105 represents a break in the figure to show that the interior of the hub is not represented.

[00099] Out of plane buckling or deformation of a blade 58 is particularly damaging because it increases the extent to which the blade 58 blocks EUV radiation. Blocking more radiation undesirably reduces the power output of the radiation source. The rate at which heat is transferred to the buckled blade may also increase. The increase in heating may cause further buckling or other failure modes of the blade. One approach for avoiding buckling is to increase the thickness of the blades to increase their strength. However, increasing the thickness of the blades makes the blades heavier, which may make it more difficult to rotate the hub at high speed. Thicker blades may increase blocking of radiation by the front edges of the blades, thus reducing output power.

[000100] Figures 15 to 17 illustrate an example embodiment in which the blade is provided with curvature to increase the resistance to buckling of the blade 58 without increasing the thickness of the material forming the blade. In an embodiment, the curvature of the blade 58 is arranged so that, for a predetermined speed of rotation, rays originating from a predetermined point along the axis of rotation of the hub pass the blade along lines of zero curvature or lines of constant circumferential position. The circumferential position of the blade surface along a line of closest approach between the path of a given ray and the blade, at a given instance, is constant.

[000101] The predetermined point may correspond to an expected position of the plasma 210 for example. When such a blade 58 is used in a particle trap positioned such that the plasma is at the predetermined point and the hub is rotated at the predetermined speed of rotation, the curvature of the blade 58 will not cause any additional blocking of radiation, except insofar as the curvature may increase the length of the front edge of the blade for a given particle trap diameter.

[000102] The arrangement is illustrated schematically in Figures 15 to 17. In Figure 15, a plasma 210 is provided at the predetermined point along the axis of rotation of the hub 44. Rays are shown by dotted lines 106. The solid lines 108 represent a given feature of the particular curvature that is applied. The given feature may be a peak 108, as in the example shown, a trough or any other characteristic feature of the curvature. As can be seen, the characteristic feature of the curvature is aligned with the direction of rays 106 from the plasma 210.

[000103] Figure 16 illustrates schematically how a portion of the plasma facing (front) edge of the blade 58 would appear when viewed parallel to the axis of rotation. Figure 17 depicts schematically how a corresponding portion of the edge opposite to the plasma facing edge (the rear edge) would appear when viewed along the axis of rotation. Arrows 109 and 111 indicate the correspondence between peaks on the front and rear edges. Each pair of arrows 109,111 point from the vertical broken line passing through a peak on the front edge to the vertical broken line passing through the corresponding peak on the rear edge. The portion of the rear edge corresponding to the larger curvature periodicity shown towards to the right hand side of the portion of the front edge illustrated in Figure 16 is not visible in Figure 17 because it is "off the page" to the right.

[000104] The curvature is shown in a highly exaggerated form for illustrative purposes. In practice, the curvature would be much smaller in amplitude. When viewed along the axis of rotation, the curvature would cause the faces of the blade 58 to be visible. However, when the blade 58 is viewed from the position of the plasma 210, only the front edge of the blade 58 would be visible due to the alignment of the curvature in the manner depicted in Figure 15.

[000105] When manufacturing the blades 58 it is desirable to consider the speed of rotation at which the blades will be used in order to predict the degree of elongation of the blades that will occur during rotation. The elongation will determine the form of curvature that is required to ensure that the rays pass the blade along lines of constant circumferential position (i.e. so that rays from the plasma 210 are not incident on a face of any blade due to the curvature). [000106] In an embodiment, the blades having curvature are manufactured using a die-mold. The curvature may optionally be adapted to allow efficient migration of liquid Sn over the surface of the blades 58 due to centripetal forces. Sharp rectangular corners or other structures which might tend to trap Sn particles may be avoided.

[000107] Figures 18 and 19 are schematic side views of embodiments in which the blade 58 is provided with one or more openings 110. The blades 58 may be configured such that for a predetermined range of speeds of a particle originating from a plasma located at a predetermined point along the axis of rotation of the hub 44, and for a predetermined speed of rotation of the hub 44, when the particle avoids striking a front edge of any blade and passes through one of the openings 110 the particle will strike a subsequent blade at a position where there is no opening. Embodiments of this type make use of the fact that when a particle passes through one of the openings 110, given the speed of the particle it is possible to calculate where the particle will strike a subsequent blade. By calculation it is possible to ensure that for all possible particle trajectories and speeds (or for selected ranges of particle trajectories and/or speeds), the particles will be stopped by the particle trap, even if the particles pass through one or more openings in the blades 58 of the trap. The calculation may take into account the restriction in the number of possible particle trajectories onto the faces of the blades caused by the finite thickness of the blades 58. Many particle trajectories will cause the particle to strike the front edge of the blade 58 and be captured by the blade 58 without having to impact against a portion of a blade face that does not have an opening. Taking the blade thickness into account may facilitate the provision of more and/or larger openings without reducing the extent to which particles are stopped by the trap to below an acceptable threshold.

[000108] The provision of openings 110 makes it possible for the blades 58 to be lighter. In order to compensate for the reduction in efficiency of the blades 58 due the presence of the openings 110, it may be necessary to extend the overall length of the blade slightly but it will generally still be possible to reduce the overall amount of material that is required for the blades 58.

[000109] Alternatively or additionally, the provision of openings 110 may facilitate deformation of the blades 58 in a way which does not involve buckling due to temperature gradients developing within the blades 58 and/or centripetal forces. [000110] In the arrangement shown in Figure 18, substantially all of the surface area of the blade 58 is available for openings 110. In the arrangement of Figure 19, the provision of openings 110 is restricted to a central region 112. This approach makes it possible to provide an ion absorption section 114 along a radiation facing edge of the blade 58, for example. In an embodiment, the ion absorption section 114 does not comprise any openings. Additionally or alternatively, the blade 58 may be provided with a fast particle compensation band 116 along a rear edge of the blade 58. In an embodiment, the fast particle compensation band 116 does not comprise any openings. In the arrangements shown in Figure 19, both an ion absorption section 114 and a fast particle compensation band 116 are provided. However, in alternative embodiments the ion absorption section 114 may be provided without a fast particle

compensation band 116. In an alternative embodiment, the fast particle compensation band 116 may be provided without an ion absorption section 114.

[000111] In an embodiment, as shown in the example of Figure 19, one or more of the blades 58 may comprise one or more support beams 118. The one or more support beams 118 may run continuously from the front edge of the blade to the rear edge. In an embodiment, the one or more support beams 118 do not comprise any openings. In the example shown in Figure 19, two support beams 118 are provided. However, in other embodiments, fewer than two support beams, for example one support beam, may be provided. In alternative embodiments, more than two support beams 118 may be provided.

[000112] Figures 20 and 21 illustrate an embodiment in which the particle trap comprises at least one blade 58 that is hollow. Figure 20 is a schematic side view of such a blade 58. Figure 21 is a schematic radially inward view of such a blade 58. The provision of a hollow blade 58 makes it possible to supply a cooling gas or liquid to the inside of the blade 58 for example during use. The provision of such a coolant flow helps to improve the efficiency with which the temperature of the blade 58 is regulated. The development of temperature gradients within the blade 58, which can cause buckling of the blade, can be reduced, for example. Arrows 120 illustrate an example flow pattern for coolant within the hollow blade 58. However, other flow patterns could be used. The hollow blade structure may provide greater resistance to buckling due to the improved mechanical resistance to buckling provided by the hollow structure 58 relative to a completely flat structure using a similar amount of material in the cross section of the blade. [000113] The hollow blade may be formed by sintering for example. For example, the sintering powder may comprise particles that will evaporate during heating. Cavities can be created by locating such particles where the cavities are required and applying heat to remove the particles.

[000114] The hub 44 may be machined using a variety of techniques. For example, Electrical Discharge Machining (EDM) may be used to cut openings, slots etc. into the hub 44. Where slots are formed in the hub 44 for receiving blades 58, the slots may be formed so as to be substantially parallel to the outer contour of the hub 44. In an embodiment, the hub may have a cone shape. The slots may therefore be angled so as to be parallel to the angle of the outer surface of the cone. The slots may be open at one longitudinal end or at both longitudinal ends to allow insertion of blades 58. Apparatus may be provided to block one or both of the longitudinal ends of the slots after the blades 58 have been inserted, to secure the blades longitudinally.

[000115] The insertion of blades into slots or openings allows replacement of damaged blades without having to replace all of the blades or the whole trap.

[000116] In an embodiment, the slots in the hub are longer in the longitudinal direction than the blades to allow for expansion of the blades on heating. The risk of buckling of the blades is thereby reduced.

[000117] Sintering may be used to form either or both of the hub and blades.

[000118] The hub and/or blades may be formed from Molybdenum, Tungsten and/or Rhenium for example. Alternatively or additionally, the hub and/or blades may be formed from carbon Silicon Carbide. Carbon Silicon Carbide is substantially less dense than Molybdenum, Tungsten and Rhenium. The use of carbon Silicon Carbide may therefore allow high rotational speeds to be obtained by lowering centripetal forces.

[000119] Any of the embodiments or concepts described can be combined in any compatible combination. For example, any of the described configurations for the hub can be combined with any of the described configurations for the blades. Any of the described configurations for connecting the blades to the hub can be combined with any of the described configurations for the hub and/or any of the described configurations for the blades.

[000120] 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, such as 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. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[000121] Although specific reference may have been made above to the use of

embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[000122] The term "lens", where the context allows, may refer to any one or

combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[000123] While specific embodiments of the invention have been described above,

it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. 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

CLAIMS:

1. A particle trap for a plasma EUV radiation source, the particle trap comprising:

a rotatable hub; and

a plurality of blades extending outwards from the hub,

wherein each of the blades has an end anchoring portion inserted into a complementary slot in the hub, the end anchoring portion and slot being configured to hold the blades within the hub during rotation of the hub.

2. A particle trap according to claim 1, wherein:

a first set of the plurality of blades have end anchor portions located at a first radius relative to the axis of rotation of the hub;

a second set of the plurality of blades have end anchor portions located at a second radius relative to the axis of rotation of the hub; and

the first radius is larger than the second radius.

3. A particle trap according to claim 1 or 2, wherein:

the end anchor portions of either or both of the first and second sets of blades are tilted away from the radial direction to make the separation between end anchor portions of neighboring end anchor portions more uniform.

4. A particle trap according to any one of claims 1 to 3, wherein:

the end anchor portion is formed by folding an end portion of the blade through 180 degrees.

5. A particle trap for a plasma EUV radiation source, the particle trap comprising:

a rotatable hub; and

a plurality of blades extending outwards from the hub,

wherein each of the blades comprises a plurality of distinct anchoring surfaces, each anchoring surface being configured to press against a corresponding support surface within the hub in order to constrain the blade radially during rotation of the hub.

6. A particle trap according to claim 5, wherein:

a first one of the anchoring surfaces of each of the blades is separated from the axis of rotation of the hub by a smaller distance than a second one of the anchoring surfaces of the same blade.

7. A particle trap according to claim 6, wherein:

the first anchoring surface has a smaller surface area than the second anchoring surface.

8. A particle trap according to claim 6 or 7, wherein:

either or both of the first and second anchoring surfaces are configured to provide an anchoring force on one circumferential side of the blade that is substantially equal to the anchoring force provided on the other circumferential side of the blade.

9. A particle trap according to any one of claims 6 to 8, wherein:

the cross-sectional shape of either or both of the first and second anchoring surfaces is/are substantially constant over the length of the hub in which the blade is positioned.

10. A particle trap according to any one of claims 5 to 9, wherein one or more of the blades has an anchoring surface that extends to a first circumferential side to a greater extent than to the circumferential side opposite to the first circumferential side over a first portion of the length of the hub in which the blade is positioned and a further anchoring surface that extends to the circumferential side opposite to the first circumferential side to a greater extent than to the first circumferential side over a second portion of the length of the hub in which the blade is positioned.

11. A particle trap according to claim 10, wherein the positions of the first and second portions of the length of the hub are the same for at least two neighboring blades.

12. A particle trap according to any one of claims 5 to 11, wherein each blade is formed by cutting a plurality of holes into a sheet of material and deforming material adjacent to the holes in order to form the anchoring surfaces.

13. A particle trap according to claim 12, wherein the material adjacent to the holes is deformed in one circumferential direction for a first set of the holes and in the other

circumferential direction for a second set of the holes.

14. A particle trap for a plasma EUV radiation source, the particle trap comprising:

a rotatable hub;

a plurality of pairs of blades extending outwards from the hub, each pair of blades consisting of a single piece of material in the region where the blade is located within the hub, wherein an outer portion of the hub is segmented into a plurality of support beams, each positioned between the blades of one of the pairs of blades for holding that pair against the hub during rotation of the hub, and a plurality of intermediate beams positioned in between the support beams; and

a plurality of coupling bars, each formed separately from the hub and positioned between one of the support beams and the pair of blades that that support beam supports.

15. A particle trap according to claim 14, wherein each coupling bar is configured to enlarge the radially innermost portion of the pair of blades between which the coupling bar is positioned so that the pair of blades engages with one or both of the intermediate beams surrounding the support beam.

16. A particle trap according to claim 14 or 15, wherein:

one or more of the coupling beams is/are formed from a material that is softer than the material from which the support beams and/or blades are formed.

17. A particle trap for a plasma EUV radiation source, the particle trap comprising:

a rotatable hub; and

a plurality of blades extending outwards from the hub, wherein the hub and blades are formed by connecting together a plurality of single piece units, each single piece unit comprising one of the plurality of blades and a portion of the hub, formed together integrally, each single piece unit being connected directly to at least one other of the single piece units.

18. A particle trap according to claim 17, further comprising:

a constraining member forming a closed loop around the plurality of single piece units to hold the single piece units together during rotation of the hub.

19. A particle trap for a plasma EUV radiation source, the particle trap comprising:

a rotatable hub; and

a plurality of blades extending outwards from openings in the hub,

wherein each of the openings is defined by a continuous integral loop of material, the openings and the blades being configured so that each blade can be inserted into the hub from the side of the hub that is opposite from the opening through which that blade will protrude.

20. A particle trap according to claim 19, wherein the hub consists of a single, integral piece of material.

21. A particle trap according to claim 19 or 20, wherein:

the blades are provided in pairs, each pair of blades consisting of a single piece of material in the region located within the hub, the single piece of material being hinged,

the pairs of blades can be inserted into the hub from one side by pressing the blades together until the blades are substantially parallel with each other, and

the pair of blades can be located in position at the other side of the hub by opening them out and inserting them through their respective openings, the hinge being located in between their respective openings to act as an anchor to hold the pair of blades within the hub on rotation of the hub.

22. A particle trap according to claim 21, wherein:

the width of each opening is equal to or greater than four times the thickness of one of the blades.

23. A particle trap for a plasma EUV radiation source, the particle trap comprising:

a rotatable hub; and

a plurality of blades extending outwards from the hub,

wherein one or more of the blades is provided with curvature to increase resistance to buckling, wherein, for a predetermined speed of rotation, the curvature is such that rays originating from a predetermined point along the axis of rotation of the hub pass the blade along lines of constant circumferential position so as not to be incident on either face of the blade.

24. A particle trap for a plasma EUV radiation source, the particle trap comprising:

a rotatable hub; and

a plurality of blades extending outwards from the hub,

wherein at least one of the plurality of blades comprises an opening, and wherein the blades are configured such that for a predetermined range of speeds of a particle originating from a plasma located at a predetermined point along the axis of rotation of the hub, and for a predetermined speed of rotation of the hub, when the particle avoids striking a radiation facing edge of any blade and passes through the opening the particle will strike a subsequent blade at a position where there is no opening.

25. A particle trap according to claim 24, wherein at least one of the blades comprises an ion absorption section along a radiation facing edge of the blade, the ion absorption section not comprising any openings.

26. A particle trap according to claim 24 or 25, wherein at least one of the blades comprises a fast particle compensation band along an edge of the blade that is opposite to the plasma, the fast particle compensation band not comprising any openings.

27. A particle trap according to any one of claims 24 to 26, wherein at least one of the blades comprises one or more support beams running continuously from the edge of the blade facing the plasma source to the opposite edge, the one or more support beams not comprising any openings.

28. A particle trap for a plasma EUV radiation source, the particle trap comprising:

a rotatable hub; and

a plurality of blades extending outwards from the hub,

wherein at least one of the blades is hollow.

29. A particle trap according to claim 28, wherein:

the hub comprises channels for coolant fluid, the channels being configured to connect with the cavity or cavities of the one or more hollow blades in order to allow the coolant fluid to flow within the one or more hollow blades.

30. A particle trap according to any one of the preceding claims, wherein one or more of the blades and/or hub are formed from carbon silicon carbide.

31. A particle trap for a plasma EUV radiation source, the particle trap comprising:

a rotatable hub; and

a plurality of blades extending outwards from the hub,

wherein the hub comprises channels for a coolant fluid, at least a portion of the channels comprising a linear segment angled obliquely relative to the axis of rotation of the hub.

32. A lithography apparatus comprising an EUV radiation source and a particle trap according to any one of claims 1 to 31.

33. A device manufacturing method, comprising:

supplying a beam of radiation using an EUV radiation source;

using a particle trap according to any one of claims 1 to 31 to prevent particles from the EUV radiation source from contaminating optics downstream from the EUV source; and

using the supplied beam to project a patterned beam of radiation onto a substrate.