WO2013068197A1 - Particle trap for euv source - Google Patents

Abstract

Configurations for a particle trap (40), a particle trap assembly, a lithographic apparatus, a method of operating a particle trap and a device manufacturing method are disclosed. In an embodiment, the particle trap includes a rotatable hub (44), and a plurality of blades (58) extending outwards from the hub. At least one of the blades includes a slot (60) extending from an edge (62) of the blade towards an interior of the blade.

Description

PARTICLE TRAP FOR EUV SOURCE

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of US provisional application 61/558,224, 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

source, a particle trap assembly, a lithographic apparatus, a method of operating 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 e uation (1):

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) 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 of tin. [0009] The particle trap may comprise a plurality of blades which rotate around a hub (i.e. a centred part) 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.

[00010] Furthermore, due to absorption of incident radiation and ions, the blades get hot during operation. An area on the front of the blades (facing the incident radiation) tends to reach the highest temperature. This area is called a hot-spot. A temperature gradient can form between the hot-spot area and other areas of the blade. For a long life-time, the blade must be able to withstand these temperatures and gradients without risk of failure, for example due to buckling.

SUMMARY

[00011] It is desirable to increase the proportion of EUV radiation that reaches the radiation collector without allowing excessive contamination of the radiation collector by particles originating from the plasma.

[00012] 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 comprises a slot extending from an edge of the blade towards the interior of the blade.

[00013] 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 comprises an opening, the opening being surrounded by a closed path of material within the plane of the blade.

[00014] 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 rotatable hub has an extended nose portion, the extended nose portion being defined as the portion of the hub that extends towards the source from a reference longitudinal position in the hub, the reference longitudinal position being the longitudinal position at which a line that is aligned with the longest linear section on the radiation facing edge of one of the blades, or with the average longitudinal position of the radiation facing edge of one of the blades, intersects the hub; and one or more of the blades has a thermal connection element that extends from a region on the radiation facing edge of the blade to the extended nose portion and is connected thereto over a longitudinal distance that is at least 75% of the length of the extended nose portion.

[00015] According to an aspect, there is provided a particle trap for an EUV radiation source, the particle trap comprising: a rotatable hub; a plurality of blades extending outwards from the hub; and one or more gas delivery channels for providing a flow of gas onto one or more of the blades during rotation of the hub.

[00016] According to an aspect, there is provided a particle trap assembly for an EUV radiation source, the particle trap assembly comprising: a particle trap, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from the hub, the assembly further comprising: a gas source configured to supply gas onto one or more of the blades during rotation of the hub.

[00017] According to an aspect, there is provided a particle trap assembly for an EUV radiation source, the particle trap assembly comprising: a particle trap, the particle trap comprising: a rotatable hub; and a plurality of blades extending outwards from the hub, the assembly further comprising: a heater configured to heat the edges of the blades that are opposite to the radiation facing edges.

[00018] According to an aspect, there is provided a method of operating a particle trap for an 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 comprises an opening or a slot, the method comprising the following steps in order: applying molten material to the surface of the blades before using the blades to trap particles from the EUV radiation source; and rotating the blades using the hub and using the rotating blades to trap particles from the EUV radiation source.

BRIEF DESCRIPTION OF THE DRAWINGS

[00019] 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:

[0001] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention; [0002] Figure 2 is a more detailed view of the apparatus of Figure 1 ;

[0003] Figure 3 is a more detailed view of a source collector apparatus of the apparatus of

Figures 1 and 2;

[0004] Figure 4 is a schematic side sectional view of a particle trap and particles travelling from a plasma towards the particle trap;

[0005] Figure 5 is a schematic side view of a portion of a blade having slots extending from an edge towards the interior of the blade;

[0006] Figure 6 is a schematic side view of a portion of a blade that has radially extending slots;

[0007] Figure 7 is a schematic side view of a portion of a blade that has slots extending from an edge of the blade that engages with the hub;

[0008] Figure 8 is a schematic side sectional view of a hub comprising a thermal buffer member and a portion of a blade that has a front region and a rear region which are thermally isolated from each other;

[0009] Figure 9 is a schematic side view of a blade of the type depicted in Figure 8 showing an example configuration of openings within the blade;

[0010] Figure 10 is a schematic side view of a portion of a blade having openings in a sheltered portion of the blade;

[0011] Figure 11 is a schematic side view of a portion of a blade having openings forming a honeycomb structure that can be deformed radially and longitudinally;

[0012] Figure 12 is a schematic illustration of a plurality of adjacent openings defining an alternative structure that can be deformed radially and longitudinally;

[0013] Figure 13 depicts a hub having an extended nose portion and a blade having a thermal connection element connecting a front edge of the blade to the extended nose portion;

[0014] Figure 14 depicts a hub having a gas delivery channel for providing cooling gas to the blades;

[0015] Figure 15 depicts a particle trap assembly comprising a gas delivery channel that is mounted outside of the hub for providing cooling gas to the blades; and

[0016] Figure 16 depicts a particle trap assembly in which a heater is provided to heat the edges of the blades that are opposite to the radiation facing edges. DETAILED DESCRIPTION

[00020] 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.

[00021] 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.

[00022] 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.

[00023] 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.

[00024] 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.

[00025] 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.

[00026] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[00027] 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.

[00028] 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.

[00029] 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.

[00030] 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.

[00031] 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.

[00032] 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.

[00033] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[00034] 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.

[00035] 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.

[00036] 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.

[00037] 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.

[00038] 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.

[00039] 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.

[00040] 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.

[00041] 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 outer most 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 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 prevented from passing the particle trap.

[00042] 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.

[00043] As mentioned above, it is challenging to design a particle trap 40 that can eliminate particles effectively while at the same time allowing a large proportion of the EUV radiation to pass the particle trap.

[00044] Figure 5 is a schematic side view of a portion of a blade 58 adjacent to the hub 44 in which the blade 58 is provided with slots 60 (slot being a narrow opening; a groove or slit) extending from an edge of the blade 58 towards an interior of the blade 58. Extended from an edge means opening out at the edge. In this embodiment the slots 60 extend from the front edge 62 of the blade 58. The front edge 62 of the blade may also be referred to as the "radiation facing" edge. In use, the front edge 62 will face the radiation that is emitted from the plasma 210. In other embodiments, slots may be provided that extend from the rear edge 64 of the blade instead of or in addition to slots extending from the front edge 62 of the blade 58. In the example shown, each of the slots 60 has the same rectangular shape. However, this is not essential. The slots may have different shapes to each other. The slots may have shapes other than rectangular shapes. The slots may have curved walls, for example. In the example shown, the slots 60 extend in a direction parallel to the longitudinal axis of rotation of the hub 44.

However, this is not essential. The slots 60 could extend in other directions. [00045] The slots 60 help the blade 58 accommodate thermal expansions and/or contractions within the plane of the blade 58. For example, the absence of blade material within the slots 60 provides space for the blade material to expand. Thermal expansions and/or contractions may occur in use due to heating from the particles, ions and/or radiation incident on the particle trap from the plasma 210 and variations in the rate of such heating. Where the thermal expansions and/or contractions are non-uniform due to temperature gradients, buckling or deformation perpendicular to the plane of the blade 58 can occur. The slots 60 can reduce the risk and/or extent of such buckling or deformations. Buckling or deformations perpendicular to the plane of the blade 58 may be particularly damaging because they increase the extent to which the blade 58 blocks radiation from the plasma 210. Increasing the amount of radiation blocked by the blade 58 undesirably reduces the output of the plasma radiation source. Increasing the amount of radiation blocked may also increase the rate of heating of the blade 58. Increasing the rate of heating of the blade 58 may lead to further mechanical failure of the blade 58 and/or particle trap 40. For example, where the blade is made of Molybdenum, the temperature of the blade should preferably be kept below 800 degrees C at all points on the blade 58 because Molybdenum starts to recrystallize above this temperature. Recrystallization causes the material to become brittle and can lower the maximum allowed stress. If the maximum allowed stress is exceeded by stresses caused by centripetal forces, the blade might rupture.

[00046] The slots 60 facilitate reduction of the thickness of the blades 58 while keeping the risk and/or extent of buckling within acceptable bounds. Reducing the thickness of the blades 58 may reduce the amount of radiation that is blocked and increase output power.

[00047] It is possible to predict how the temperature of the blade 58 will vary as a function of position during use of the particle trap 40. It is therefore possible to predict which portions of the blade 58 will be hottest in use. It is possible to verify such predictions by measuring the temperature of the blades during use. It is expected that the hottest point on the blade 58 will be located at or near the front edge 62 of the blade 58. In an embodiment, the slots 60 are located at or near the predicted hotspot on the blade 58. In this way, the slots 60 are provided in the region where the blade 58 will need to expand most. The region surrounding the hotspot may also be subject to particularly large temperature gradients. Arranging for slots 60 to be present where temperature gradients are largest helps to avoid buckling or deformation perpendicular to the plane of the blade 58. In the example shown in Figure 5, the hotspot is expected in the region of the front edge 62 where the slots 60 have been arranged to be longest. At positions on the front edge 62 that are separated from the position of the expected hotspot the length of the slots 60 is gradually reduced.

[00048] The slots 60 intersect the hotspot region and therefore prevent centripetal forces from being supported directly by material in the hotspot region. The centripetal forces tend to be supported to a greater extent by material outside of the hotspot region. This effect may further help to avoid buckling or deformation in the hotspot region. Additionally or alternatively, by lowering the stresses in the hotspot region, failure due to weakening of the material in the hotspot region (caused for example by recrystallization effects) may be reduced.

[00049] The slots 60 may allow the temperature in the hotspot to be higher without risking failure of the blade. The output power of the plasma may therefore be increased without risk of failure of the particle trap due to the increased heating of the blades. For Molybdenum blades, it is thought that the slots would enable the blades to remain functional even up to about 1200 degrees C.

[00050] Figure 6 depicts an alternative arrangement in which openings 68 are provided within the blade 58. In contrast to the slots 60 of embodiments of the type shown in Figure 5, the openings 68 are surrounded by a closed path of material within the plane of the blade 58. The openings 68 do not intersect with any of the outer peripheral edges of the blade 58. The openings 68 may operate in a similar manner to the slots 60 of embodiments of the type shown in Figure 5. The absence of material within the openings 68 makes it easier for the blade 58 to expand or contract without buckling in the direction perpendicular to the plane of the blade 58. In the example arrangement shown in Figure 6, a plurality of the openings 68 that are aligned in a direction that is roughly parallel to the direction of intersection between the blade 58 and the hub 44 are provided. This sequence of openings 68 enables the blade 58 to expand or contract particularly effectively along this direction. However, other arrangements are possible. For example, the openings 68 could be distributed non-uniformly across the surface of the blade 58. For example, the openings 68 may be arranged so that they are not aligned with each other in any direction. In the embodiment shown in Figure 6, the openings 68 are elongated parallel to the direction that the blade 58 extends away from the hub 44 (i.e. parallel to the radial direction). However, other arrangements are possible. For example, the openings 68 may not be elongated. Alternatively or additionally, the openings 68 may be elongated but with their long axes aligned in different directions. Alternatively, the openings 68 may be elongated and have long axes that are all aligned in a given direction that is not parallel to the radial direction.

[00051] Figure 7 illustrates an embodiment in which a plurality of slots 70 that extend from the edge 72 of the blade 58 that engages with the hub 44. The absence of material within the slots 70 provides space for expansion of the blade material near the edge 72 of the blade 58 in a direction parallel to the edge 72. Furthermore, the material between the slots may be bent at the end provided in the hub 44 (bending provided up to 180 degrees) as to provide more contact area and implicitly increase in thermal conductance. This arrangement may therefore reduce the extent to which the geometry of the edge of the blade 72 will vary due to different amounts of thermal expansion near the edge 72 (which may occur in use due to temperature gradients within the blade). It could be said that in such configuration the contact stress will be spread more evenly due to several "springs" in parallel that can deform independently. Arrangements of this type may therefore help to reduce the possibility of buckling or deformation of the blade 58 perpendicular to the plane of the blade 58. In addition, the reduction in the variation of the geometry of the edge 72 may help to ensure that the thermal connection between the blade 58 and the hub 44 remains uniform along the edge 72. Alternatively or additionally, the slots 70 may allow the "legs" between the slots to flex individually so as to adapt more easily to any variation in geometry of the edge 72 when the edge 72 is pressed into the hub 44. The individual leg flexing will help to increase the contact area between the material of the blade and the material of the hub. The uniformity of the thermal connection may therefore be increased.

Maintaining a uniform thermal contact between the edge 72 and the hub 44 may help to reduce the extent of temperature gradients within the region of the blade 58 near to the edge 72.

Reducing temperature gradients further reduces the risk of buckling or deformation

perpendicular to the plane of the blade 58. Furthermore, when the slots are made narrow enough they may act as capillaries holding liquefied tin, which will help conducting heat without negative mechanical influence.

[00052] In the example shown in Figure 7 the slots are straight and parallel. "Legs" between the slots are rectangular. However, other configurations are possible (in any pattern). The slots may have other forms and/or be non-parallel with other slots. The "legs" may be other than rectangular. [00053] Figure 8 depicts an embodiment in which the blade 58 is divided into a front region 74 and a rear region 76. The front region 74 is adjacent to the front edge 62. The rear region 76 is adjacent to the rear edge 64. The front and rear regions are separated from each other by a region or line 78 of reduced thermal conductivity. The front and rear regions are therefore at least partially thermally isolated from each other. The region or line 78 of low thermal conductivity may be formed from a material having lower thermal conductivity than the material of the rest of the blade 58. Alternatively or additionally, the region or line 78 may be formed from a material that is thinner than material elsewhere in the blade 58. Alternatively or additionally, the region or line 78 may comprise one or more thermally isolating openings. The use of thinner material or openings reduces the amount of material available to conduct heat between the front region 74 and the rear region 76. The shapes of the front and rear regions 74 and 76 may take any form. In the example shown, the front and rear regions 74 and 76 are formed by a straight line 78 of low thermal conductivity that is parallel to the front and rear edges 62 and 64. However, this is not essential. The region or line 78 may be aligned in different directions and/or be curved.

[00054] Figure 9 illustrates an example configuration for an embodiment of the type shown in Figure 8 in which the region or line 78 of low thermal conductivity is formed from a plurality of thermally isolating openings 82. The thermally isolating openings 82 reduce the average amount of blade material per unit area in the plane of the blade 58 in a continuous band extending front the hub 44 to the radially outermost edge of the blade 58 to less than 20% of the average amount of blade material per unit area in the rear region 76 of the blade 58, optionally less than 10%, optionally less than 5%. In the example shown, the thermally isolating openings 82 are elongate and aligned with the front and rear edges 62 and 64. However, other shapes and orientations of openings could be used.

[00055] Either or both of the front and rear regions 74 and 76 may be provided with openings and/or slots, for example of the type illustrated in Figures 5 and 6. These openings or slots will help to accommodate thermal expansions and contractions and avoid buckling or deformation perpendicular to the plane of the blade 58. The thermal isolation between the front and rear regions 74 and 76 may be configured such that heat flow is concentrated in certain regions. In such arrangements, more openings may be provided in the vicinity of these regions of concentrated heat flow than in other regions. [00056] Figure 9 illustrates an example embodiment in which a plurality of expansion openings 86 are provided in the front region 74. The plurality of expansion openings 86 may comprise a plurality of groups 81, 83, 85, 87. Each group 81, 83, 85, 87 may comprise expansion openings 86 that are closer to each other than to expansion openings 86 in any other group 81, 83, 85, 87. In an embodiment, each of the groups 81, 83, 85, 87 is centred on a gap 84 between two radially adjacent ones of the thermally isolating openings 82. The gap 84 represents a point of concentrated heat flow because heat can travel more easily through the gap 84 than through the empty space of the thermally isolating openings 82. The openings 86 are thus provided preferentially in those regions where thermal expansion and contraction will be maximal.

[00057] In the example shown, the openings 86 adjacent to the gaps 84 are only provided in the front region 74. However, this is not essential. In alternative embodiments, openings may be provided also in the rear region 76 instead of or in addition to the openings 86 in the front region 74.

[00058] It is additionally advantageous if cooling is provided to the hub 44 by any cooling means, such as C0₂ cooling, cooling using a heat pipe (two phase cooling), etc. In an

embodiment, the hub 44 may comprise channels 52 for coolant fluid, as shown in Figure 4. A thermal buffer member 80 may be provided to reduce the rate of transfer of heat to the coolant from the rear region 76 of the blade 58 relative to the rate of transfer of heat to the coolant from the front region 74 of the blade 58. An example configuration for such a thermal buffer member 80 is illustrated in Figures 8 and 9. In an embodiment, the thermal buffer member 80 may be positioned adjacent to a radially outermost wall of one of the channels 52. For example, the thermal buffer member 80 may be provided within the channel 52 and in contact with the outermost wall of the channel 52. Alternatively or additionally, the thermal buffer member 80 may be provided within the material of the hub 44 outside of the channel 52. The thermal buffer member 80 may form part of the external wall of the channel 52. For example, an inner surface of the thermal buffer member 80 may be flush with an outer surface of the channel 52.

Alternatively or additionally, the thermal buffer member 80 may be encapsulated by the material of the hub 44 so that none of the coolant fluid comes into direct contact with the thermal buffer member 80. [00059] In an embodiment, the thermal buffer member 80 is positioned radially adjacent to the rear region 76 of the blade and not radially adjacent to the front region 74 of the blade 58. In an embodiment, the thermal buffer member 80 may extend over a range of longitudinal positions that overlaps with the range of longitudinal positions of the rear region 76 to a greater extent than the range of longitudinal positions of the front region 74. The examples of Figures 8 and 9 illustrate such configurations.

[00060] In an embodiment, the material of the thermal buffer member 80 has a thermal conductivity that is lower than the thermal conductivity of the material of the rest of the hub 44. The thermal buffer member 80 may be formed by means of a cavity within the hub 44, for example. The gas or vacuum within the cavity may have a thermal conductivity that is lower than the thermal conductivity of the material of the hub 44.

[00061] The thermal buffer member 80 reduces the efficiency with which the rear region 76 is cooled relative to the front region 74. This will tend to increase the average temperature of the rear region 76 and/or decrease the average temperature of the front region 74. The temperature gradients that exist between the front region 74 and the rear region 76 will tend therefore to be reduced. The risk and/or extent of buckling or deformation perpendicular to the plane of the blade 58 will therefore tend to be reduced. In the example embodiments of Figures 8 and 9, the thermal buffer member 80 is used in combination with a blade 58 that has been divided into front and rear regions 74 and 76 by a region or line 78 of reduced thermal conductivity. However, the thermal buffer member 80 may also be provided in configurations which do not have a blade 58 that is divided in this manner.

[00062] Figure 10 illustrates an embodiment in which openings 90 are provided in a sheltered region 88 of the blade 58. The sheltered region 88 is a region where substantially no particles hit the blade 58 during use. The sheltered region 88 may be in close proximity to the point of intersection between the rear edge 64 of the blade 58 and the hub 44 for example. In regions near the axis of the hub 44, it is difficult for particles to penetrate as far as in regions further away from the hub 44. This is because the finite thickness of the front edge 62 of the blade 58 makes up a larger proportion of the area through which the particle has to travel than in regions further away from the hub. In regions near to the hub 44 many more particles will therefore strike the front edge 62 for a given incident flux of particles than at regions further away from the hub 44. This effect combines with the fact that a particle is more likely to be hit by a blade the further the particle has to travel between the blades to define a sheltered region 88 having a form of the type illustrated schematically in Figure 10. The form of the sheltered region can be predicted by computer simulation. Alternatively or additionally the form of the sheltered region can be determined by inspecting blades during or after use.

[00063] The sheltered region 88 does not participate in stopping particles from getting through the trap. Therefore, it is possible to form openings 90 in the sheltered region 88 without risk of these openings 90 reducing the particle stopping efficiency of the trap 40.

[00064] The openings 90 in the sheltered region 88 may be larger than openings provided elsewhere on the blade 58 because they do not need to contribute to trapping of particles. The openings 90 may therefore be optimized to perform other functions. The openings 90 may be configured, for example, to reduce the thermal conductance between the blade 58 and the hub 44 in regions of the blade 58 adjacent to the rear edge 64 of the blade. As discussed above, reducing the efficiency of heat exchange between regions adjacent to the rear edge 64 and the hub 44 can help to reduce temperature gradients in the blade 58. This is because regions adjacent to the front edge 62 of the blade 58 will tend to be heated more than the regions adjacent to the rear edge 64. Alternatively or additionally, the openings 90 may be configured to accommodate expansions and/or contractions and thereby avoid buckling or deformation perpendicular to the plane of the blade 58.

[00065] In the example shown in Figure 10, the material between the openings 90 comprises linear elements that are connected to each other at elbows. The deformation of the shape of the openings 90 causes changes in the angles between neighbouring linear elements. In the example shown in Figure 10, the openings 90 have a hexagonal form, such that the structure resembles a honeycomb. However, this is not essential. Other structures are possible. Figure 12 illustrates a alternative structure in which linear elements 92 are connected together at elbows 94. The linear elements 92 and elbows 94 define a structure comprising openings 96 that are similar in shape to each other but alternate in orientation.

[00066] In the arrangement of Figure 10, the openings 90 are restricted to the sheltered region 88. However, this is not essential. Figure 11 illustrates an example arrangement in which the openings 90 are distributed over a larger region of the blade 58. In embodiments of this type, the size and/or distribution of the openings 90 may be configured such that particles will not penetrate through the openings 90 during use. In an embodiment, the openings 90 are prevented from allowing particle penetration by filling the openings 90 with liquid prior to use. For example, the openings 90 can be filled with a molten form of the expected composition of the material ejected from the plasma 210. For example, in the case where the plasma is based on Sn, molten Sn can be applied to the blades 58 prior to use. The molten material may fill the openings 90 by capillary action, for example. This approach makes it possible to provide a large number of openings. For example, the number and sizes of the openings may be such that the total surface area of the openings is greater than 20% of the total surface area of the blade, optionally greater than 40%, optionally greater than 75%. The weight of the liquid material filling the openings 90 may be substantially less than the weight of the blade material that would have filled the openings 90 if the openings were not present. This approach therefore

additionally enables the blades 58 to be considerably lighter in use. Reducing the weight of the blades 58 enables the particle trap to be rotated more quickly and/or reduces the centripetal forces applied by blades 58 to the hub 44.

[00067] Where the openings 90 are filled with liquid (e.g. Sn) prior to use the thermal conductivity across the openings 90 may be larger than when the openings 90 are empty.

However, the liquid may still have a thermal conductivity that is substantially lower than the thermal conductivity of the material (e.g. Mo) forming the rest of the blade 58. The thermal conductivity in the region of liquid- filled openings 90 may therefore still be substantially lower than would be the case if the openings 90 were not present at all. The liquid- filled openings 90 can therefore still be used to reduce temperature gradients within the blade 58 (for example by being configured to favor heat exchange between the hub and regions adjacent to the front edge of the blade relative to heat exchange between the hub and regions adjacent to the rear edge of the blade).

[00068] Figure 13 illustrates an embodiment in which the hub 44 has an extended nose portion 98. The extended nose portion 98 may be defined as the portion of the hub 44 that extends towards the plasma 210 from a reference longitudinal position 100 in the hub 44. The reference longitudinal position 100 may be defined as the position at which a line 102 that is aligned with the longest linear section 104 on one of the blades 58 or with the average longitudinal position of the front edge 62 of one of the blades 58 intersects the hub 44. In the example of Figure 13, these two criteria yield the same position 100. In other embodiments, the two criteria may yield slightly different positions. [00069] In an embodiment, one or more of the blades 58 has a thermal connection element 106 that extends from a region on the front edge 62 of the blade 58 to the extended nose portion 98. The thermal connection element 106 may be connected to the extended nose portion 98 over a longitudinal distance 1 10 that is at least 75% of the length 108 of the extended nose portion 98, optionally at least 85%, optionally at least 95%.

[00070] The thermal connection element 106 may be integrally formed with the blade 58. There may be no interface between the thermal connection element 106 and the blade 58.

Alternatively, the thermal connection element 106 may be formed from a separate piece of material that is not integral with the blade 58. An interface may be present between the thermal connection element 106 and the blade 58. In an embodiment, the thermal connection element 106 forms an oblique angle to the longest linear section 104. The oblique angle may be formed in the region of a hotspot 112. The hotspot 112 may be defined as the region on the blade 58 that will become hottest during use of the particle trap 40. The thermal connection element 106 may be configured to provide an improved thermal connection between the hotspot 112 and the hub 44 relative to an otherwise equivalent arrangement in which the thermal connection element 106 is absent (for example where the longest linear section 104 of the blade 58 extends all the way to the hub 44).

[00071] The thermal connection element 106 may improve the thermal connection between the front edge 62 of the blade 58 and the hub 44. The front edge 62 of the blade 58 can therefore be cooled efficiently. Cooling the front edge 62 of the blade 58 more efficiently reduces the extent of temperature gradients between regions adjacent to the front edge 62 and regions further away from the front edge 62. Reducing such temperature gradients reduces the risk of buckling or deformation perpendicular to the plane of the blade 58.

[00072] In an embodiment, an apparatus is provided for providing a flow of gas onto one or more of the blades 58 during rotation of the hub 44.

[00073] Figure 14 illustrates an example configuration of a gas delivery channel 114 that is formed within the hub 44 itself. The channel 114 may be connected to a gas source for providing the gas to be blown onto the blades 58. The channel 114 may comprise one or more outlets 116. The outlets 116 may be located adjacent to regions of the blade 58 to which a higher degree of cooling is desirable. For example, the outlets 116 may be provided adjacent to a hotspot 112. Alternatively or additionally, the outlets 116 may be configured to direct gas preferentially towards those regions where enhanced cooling is desirable. For example, the outlets 116 may be configured to direct gas towards a hotspot 112. In the example shown in Figure 14, three outlets 116 are provided for a single channel 114. However, other

configurations are possible. For example, fewer than three outlets 116 may be provided for a given channel 114 or more than three outlets 116 may be provided for a given channel 114. In an embodiment a single outlet per channel is provided. In an embodiment, one or more of the outlets may be flared.

[00074] Figure 15 illustrates an alternative configuration in which a gas delivery channel 114 for supplying gas is provided outside of the hub 44. In embodiments of this type, the channel 114 and any outlets 116 of the channel 114 are stationary in use. The channel 114 and outlets 116 do not rotate with the hub 44. As in the arrangement of Figure 14, the outlets 116 may be provided so as to be in close proximity to regions of the blades 58 where enhanced cooling is desirable. For example, the outlets 116 may be provided adjacent to a hotspot 112.

Alternatively or additionally, the outlets 116 may be configured to direct the flow of gas preferentially onto the regions of the blades 58 where enhanced cooling is desirable, for example onto a hotspot 112. In the arrangement shown in Figure 15, three outlets 116 are provided, but fewer than three or more than three could be provided. The portion of the gas delivery channel 114 that intersects radiation from the plasma 210 may be aligned so that only radiation that would have been incident on a supporting structure of the collector, rather than on the collector itself, is blocked by the delivery channel 114. In this way, it is possible to provide the gas delivery channels 114 without reducing the output power of the EUV radiation source.

[00075] The gas that is applied via the gas delivery channels of any of the embodiments described above may be argon for example.

[00076] Figure 16 illustrates an embodiment in which temperature gradients within the blades 58 are reduced by applying heat from the rear edge 64 side of the blades 58. In the example arrangement shown, a heater 118 directs heating onto the blades 58 from the rear edge 64 side of the blades 58. The heater 118 may comprise a laser, for example. The laser may be an infrared laser, for example. A heater control system 120 may be provided from controlling the position of the heater 118, the direction of heat output from the heater 118 and/or the power output of the heater 118. A temperature measurement system 122 may be provided for measuring the temperature of one or more of the blades 58 and providing the measured temperature as input to the heater control system 120. The heater control system 120 may be configured to respond to the measured temperature or input power to source or pinch. For example the heater control system 120 may respond by varying one or more of the position of the heater 118, the direction of heat output from the heater 118 and/or the power output of the heater 118. When heating is done using a laser, varying beam diameter focusing may also be done. The use of a heater according to this embodiment can be combined with any of the various configurations for the hub and blades described above.

[00077] Any of the slots or openings described above may have a shape and size which is such that particles from the plasma will not pass through the slots or openings in use. For example, in the case of elongated openings or slots, the width of the openings or slots may be too thin to allow particles from the plasma to pass through the slots in use. The prevention of particle penetration may be achieved because the particles are always larger than the slots or openings or because the material of the particles is such as to clog up the slots or openings in use and prevent penetration of subsequent particles. Slots or openings descried herein also include pockets (openings which partially extend through the width of the blade) which may be provided by partial perforation of the blade 58 on one or both blade surfaces (such that the blade is not completely perforated but some blade material is still left to delimit the pocket).

[00078] In any of the embodiments described above the blades may be configured to be, individually or in groups, detachable from the hub. Making the blades detachable makes it possible to replace individual blades (e.g. due to damage of the blade) without replacing all of the blades. Making the blades detachable avoids potentially costly manufacturing steps such as brazing of the blades to the hub.

[00079] Any of the embodiments or concepts described can be combined in any compatible combination. For example, any of the described configurations for the hub and/or cooling mechanisms for the hub can be combined with any of the described configurations for the blades.

[00080] 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.

[00081] 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.

[00082] 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.

[00083] 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

WHAT IS CLAIMED IS:

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 at least one of the blades comprises a slot extending from an edge of the blade towards an interior of the blade.

2. A particle trap according to claim 1, wherein the slot is shaped so that particles from the plasma EUV source that are incident on the slot in use will not pass through the slot.

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

the blade comprises a plurality of the slots.

4. A particle trap according to any one of claims 1 to 3, wherein the slot extends from a radiation facing edge of the blade.

5. A particle trap according to claim 4, wherein:

the length of each slot varies as a function of the distance between the slot and a point on the blade that is expected to become hottest during use.

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

the slot length decreases as a function of increasing distance between the slot and the point of the blade that is expected to become hottest during use.

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

the slot extends from an edge of the blade that is inserted into the hub.

8. 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 comprises an opening, the opening being surrounded by a closed path of material within the plane of the blade.

9. A particle trap according to claim 8, wherein:

the opening is shaped so that particles from the plasma EUV source incident on the opening in use will not pass through the opening.

10. A particle trap according to claim 8 or 9, wherein:

the blade comprises a plurality of the openings.

11. A particle trap according to any one of claims 8 to 10, wherein:

the openings are located in a sheltered region of the blade, the sheltered region being defined as the region where substantially no particles hit the blade during use.

12. A particle trap according to claim 11, wherein:

the sheltered region of the blade extends outwards from a point where the edge of the blade that is opposite to the plasma meets the hub over a distance parallel to the axis of rotation of the hub that is less than the length of the edge that is inserted into the hub, and over a distance perpendicular to the axis of rotation of the hub that is less than the length of the edge that is opposite to the plasma.

13. A particle trap according to any one of claims 8 to 12, wherein:

the material between the openings form a region having a structure that can expand or contract, radially or axially, by deformation of the shape of the openings.

14. A particle trap according to claim 13, wherein:

the structure comprises a plurality of linear elements that are connected to each other, the deformation of the shape of the openings causing changes to the angles between neighboring linear elements.

15. A particle trap according to any one of claims 10 to 14, wherein the openings are shaped such that if particles from the plasma were incident on the openings in use, the particles would pass through the openings.

16. A particle trap according to any one of claims 10 to 15, wherein:

the total surface area of the openings in a blade is greater than 20% of the total surface of the blade.

17. A particle trap according to any one of claims 8 to 16, wherein:

the opening or openings is/are configured to be filled with a liquid metal in use that spans the opening and prevents penetration of the blade through the opening by a particle from the plasma.

18. A particle trap according to any one of claims 8 to 17, wherein:

the opening or openings is/are elongated.

19. A particle trap according to claim 18, wherein:

the opening or openings is/are aligned substantially perpendicularly to the axis of rotation of the hub.

20. A particle trap according to any one of claims 8 to 19, wherein:

at least one of the blades comprises one or more thermally isolating openings that are configured to thermally isolate a front region of the blade that is adjacent to the radiation facing edge of the blade from a rear region of the blade that is adjacent to the edge opposite to the radiation facing edge.

21. A particle trap according to claim 20, wherein the thermally isolating openings reduce the average amount of blade material per unit area in the plane of the blade in a continuous band extending from the hub to the radially outermost edge of the blade to less than 20% of the average amount of blade material per unit area in the rear region of the blade.

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

the front region comprises a plurality of expansion openings to allow expansion of the material of the front region on heating.

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

the plurality of expansion openings comprises a plurality of groups, each group comprising expansion openings that are closer to each other than to expansion openings in any other group.

24. A particle trap according to claim 23, wherein:

each of the groups of expansion openings is located adjacent to or centered on a gap between two radially adjacent ones of the thermally isolating openings.

25. A particle trap according to any one of claims 1 to 24, wherein the hub further comprises cooling means.

26. A particle trap according to any one of claims 1 to 25, wherein the hub further comprises: channels for coolant fluid; and

a thermal buffer member positioned to reduce the rate of transfer of heat to the coolant fluid from the rear region of the blade relative to rate of transfer of heat to the coolant fluid from the front region of the blade.

27. A particle trap according to claim 26, wherein:

the thermal buffer member is adjacent to a radially outermost wall of one of the channels.

28. A particle trap according to claim 26 or 27, wherein:

the thermal buffer member extends over a range of longitudinal positions that overlaps with the range of longitudinal positions of the rear region to a greater extent than with the range of longitudinal positions of the front region.

29. A particle trap according to any one of claims 26 to 28, wherein:

the material of the thermal buffer member has a thermal conductivity that is lower than the thermal conductivity of the material of the hub.

30. 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 rotatable hub has an extended nose portion, the extended nose portion being defined as the portion of the hub that extends towards the source from a reference longitudinal position in the hub, the reference longitudinal position being the longitudinal position at which a line that is aligned with the longest linear section on the radiation facing edge of one of the blades, or with the average longitudinal position of the radiation facing edge of one of the blades, intersects the hub; and wherein one or more of the blades has a thermal connection element that extends from a region on the radiation facing edge of the blade to the extended nose portion and is connected thereto over a longitudinal distance that is at least 75% of the length of the extended nose portion.

31. A particle trap according to claim 30, wherein:

the thermal connection element is integrally formed with the blade.

32. A particle trap according to claim 30 or 31 , wherein:

the thermal connection element forms an oblique angle to the longest linear section.

33. A particle trap according to claim 32, wherein:

the oblique angle is formed in the region of a hotspot, the hotspot being defined as the region on the blade that will become hottest during use of the particle trap.

34. A particle trap according to any one of claims 30 to 33, wherein:

the thermal connection element provides an improved thermal connection between a hotspot, the hotspot being defined as the region on the blade that will become hottest during use of the particle trap, and the hub relative to an arrangement in which the longest linear section of the blade extends all the way to the hub.

35. A particle trap for an EUV radiation source, the particle trap comprising:

a rotatable hub;

a plurality of blades extending outwards from the hub; and

one or more gas delivery channels configured to provide a flow of gas onto one or more of the blades during rotation of the hub.

36. A particle trap assembly for an EUV radiation source, the particle trap assembly comprising:

a particle trap, the particle trap comprising

a rotatable hub, and

a plurality of blades extending outwards from the hub; and

a gas source configured to supply gas onto one or more of the blades during rotation of the hub.

37. An assembly according to claim 36, wherein:

the hub comprises one or more gas delivery channels configured to provide a flow of gas onto one or more of the blades during rotation of the hub; and

the gas source is configured to supply gas to the one or more gas delivery channels.

38. An assembly according to claim 36 or 37, wherein:

the gas source comprises a gas delivery channel mounted outside of the hub, the gas delivery channel having an outlet configured to direct gas towards the radiation facing edge of the blades.

39. An assembly according to claim 38, wherein the portion of the gas delivery channel that intersects radiation is aligned so that only radiation that would have been incident on a supporting structure of the collector, rather than the collector itself, is blocked.

40. A particle trap assembly for an EUV radiation source, the particle trap assembly comprising:

a particle trap, the particle trap comprising

a rotatable hub, and

a plurality of blades extending outwards from the hub; and

a heater configured to heat the edges of the blades that are opposite to the radiation facing edges.

41. An assembly according to claim 40, wherein:

the heater comprises a laser.

42. An assembly according to claim 40 or 41, further comprising:

a heater control system configured to control the position of the heater, the direction of heat output of the heater, and the power output of the heater.

43. An assembly according to claim 42, wherein the assembly further comprises a temperature measurement system configured to measure the temperature of one or more of the blades and to provide the measured temperature as input to the heater control system.

44. A lithography apparatus comprising an EUV radiation source and a particle trap according to any one of claims 1 to 35.

45. A lithography apparatus comprising an EUV radiation source and a particle trap assembly according to any one of claims 36 to 43.

46. A method of operating a particle trap for an 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 comprises an opening and/or a slot, the method comprising:

applying molten material to a surface of the blades before using the blades to trap particles from the EUV radiation source; and

rotating the blades using the hub to trap particles from the EUV radiation source.

47. A method according to claim 46, wherein the molten material has the same composition as the particles from the plasma.

48. An EUV radiation source comprising a particle trap according to any one of claims 1 to 35 or a particle trap assembly according to any one of claims 36 to 43.

49. A device manufacturing method comprising:

supplying a beam of radiation using an EUV radiation source;

using a particle trap or a particle trap assembly according to any one of claims 1 to 43 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.