WO2013127587A2

WO2013127587A2 - Source collector apparatus, lithographic apparatus and device manufacturing method

Info

Publication number: WO2013127587A2
Application number: PCT/EP2013/051657
Authority: WO
Inventors: Vadim Banine; Johannes Franken; Antonius Kempen; Maarten Van Kampen; Wilbert Mestrom
Original assignee: Asml Netherlands B.V.
Priority date: 2012-02-27
Filing date: 2013-01-29
Publication date: 2013-09-06
Also published as: WO2013127587A3

Abstract

A source collector apparatus includes a plasma generation apparatus arranged to excite a fuel to form a radiation emitting plasma, a collector arranged to collect the radiation, and a contamination receiving apparatus, wherein the contamination receiving apparatus is provided with a porous structure.

Description

SOURCE COLLECTOR APPARATUS, LITHOGRAPHIC APPARATUS AND DEVICE

MANUFACTURING METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US provisional application 61/603,663, which was filed on February 27 , 2012, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a radiation source collector apparatus, a lithographic apparatus and a device manufacturing method.

BACKGROUND

[0003] 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.

[0004] 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.

[0005] 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, Iq 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 Iq.

[0006] 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.

[0007] 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. Sn), 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.

[0008] An alternative source collector apparatus for producing EUV radiation may use an electrical discharge to excite fuel. For example, a very hot plasma of Sn, Li or Xe may be generated using an electrical discharge. The plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may comprise a series of grazing incidence reflective surfaces. Such a source collector apparatus is typically termed a discharge produced plasma (DPP) source. [0009] The conversion of fuel (e.g. Sn) to radiation emitting plasma is incomplete, and consequently particles of fuel material are projected from a plasma formation location when the plasma is generated. Fuel material may accumulate on surfaces of the source collector apparatus, and may reduce the efficiency of the source collector apparatus or prevent it from operating.

SUMMARY

[0010] It is desirable to provide a source collector apparatus, lithographic apparatus and device manufacturing method which overcomes or mitigates a problem associated with the prior art.

[0011] According to an aspect of the invention, there is provided a source collector apparatus comprising a plasma generation apparatus arranged to excite a fuel to form a radiation emitting plasma, a collector arranged to collect the radiation, and a contamination receiving apparatus, wherein the contamination receiving apparatus is provided with a porous structure.

[0012] The contamination receiving apparatus may be a rotating foil trap comprising a plurality of blades and a fuel catching apparatus, and wherein the porous structure is located within a fuel receiving space of the fuel catching apparatus

[0013] The porous structure may comprise fibers located within the fuel receiving space of the fuel catching apparatus.

[0014] The porous structure may comprise a layer provided on one or more surfaces of the fuel receiving space.

[0015] The layer may comprise a porous sintered metal.

[0016] The contamination receiving apparatus may be a plurality of vanes which extend from a wall of the source collector apparatus, and wherein the porous structure is located on or between the vanes.

[0017] The porous structure may comprise fibers located between the vanes.

[0018] The porous structure may comprise porous sintered metal provided on one or more of the vanes.

[0019] The fibers may be metal wool.

[0020] The metal wool may be stainless steel wool.

[0021] The contamination receiving apparatus may be a screen which extends partway from a fuel droplet generator to a plasma formation location. [0022] The porous structure may comprise a layer provided on the screen.

[0023] The plasma generation apparatus may comprise a source of fuel and electrodes arranged to apply an electrical discharge to the fuel and thereby form the plasma.

[0024] The plasma generation apparatus may comprise a source of fuel arranged to deliver fuel to a location at which a laser beam is incident upon the fuel, thereby forming the plasma.

[0025] The fuel may be tin, lithium or xenon, or may be any combination of these. The fuel may include other materials.

[0026] A covering may be provided on a surface of part of the porous structure, the covering being configured to stop fuel dripping from that part of the porous structure.

[0027] The porous structure may be wetting for the fuel.

[0028] The porous structure may be formed from metal.

[0029] According to an aspect of the invention there is provided a lithographic apparatus comprising the source collector apparatus of aspects of the invention described herein, and further comprising an illumination system configured to condition the radiation collected by the collector to form a conditioned radiation beam, a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto a target portion of the substrate.

[0030] According to an aspect of the invention there is provided a method of generating radiation comprising exciting a fuel such that the fuel forms a radiation emitting plasma, and using a contamination receiving apparatus to collect contamination generated during formation of the plasma, wherein the contamination receiving apparatus is provided with a porous structure.

[0031] The porous structure may be formed from metal.

[0032] According to an aspect of the invention there is provided a manufacturing method comprising generating radiation using aspects of the invention described herein and using the lithographic apparatus of aspects of the invention described herein to pattern a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS [0033] 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:

[0034] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention;

[0035] Figure 2 is a more detailed view of the lithographic apparatus of Figure 1 ;

[0036] Figure 3 schematically depicts part of a source collector apparatus according to an embodiment of the invention which may form part of the lithographic apparatus of Figure 1; and

[0037] Figure 4 schematically depicts part of a source collector apparatus according to an embodiment of the invention which may form part of the lithographic apparatus of Figure 1.

DETAILED DESCRIPTION

[0038] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector apparatus SO according to an 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0045] 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.

[0046] Referring to Figure 1, the illuminator IL receives an extreme ultra violet (EUV) radiation beam from the source collector apparatus SO. Methods to produce EUV radiation 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.

[0047] In such cases, the laser is not considered to form part of the lithographic apparatus and the laser 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.

[0048] In an alternative method, often termed discharge produced plasma ("DPP"), the

EUV emitting plasma is produced by using an electrical discharge to vaporise a fuel. The fuel may be an element such as xenon, lithium or tin which has one or more emission lines in the EUV range. The electrical discharge may be generated by a power supply which may form part of the source collector apparatus or may be a separate entity that is connected via an electrical connection to the source collector apparatus.

[0049] 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.

[0050] 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 PS 1 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.

[0051] 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.

[0052] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0053] Figure 2 shows the lithographic 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 at a plasma formation location 202 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 a very hot plasma 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.

[0054] The radiation emitted by the hot plasma 210 is passed from a source chamber 211 into a collector chamber 212 via a rotating foil trap 260 and a static foil trap 261 (described further below), either or both of which may be positioned in or behind an opening in source chamber 211.

[0055] 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. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector apparatus SO 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.

[0056] 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.

[0057] 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. [0058] 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 symmetrically around an optical axis O. A collector optic CO of this type is preferably used in combination with a discharge produced plasma source (DPP source).

[0059] Part of the source collector apparatus SO is shown schematically in cross-section in more detail in Figure 3. Figure 3 shows the EUV radiation emitting plasma 210, the rotating foil trap 260 and the static foil trap 261. Unlike the first portion of the source collector apparatus SO shown in Figure 2, the optical axis O of the source collector apparatus SO is inclined relative to the horizontal. In general, the optical axis O of the source collector apparatus SO may have any suitable orientation.

[0060] The rotating foil trap 260 comprises a plurality of blades 262 (which may be foils) which extend radially from a hub 263. Although only two blades 262 are shown, many more than two blades may be provided. The hub 263 and the blades 262 are driven to rotate by a motor (not shown) and thereby intercept tin particles or other contamination. The static foil trap 261 may for example comprise a plurality of foils and a source of gas which together restrict or inhibit the passage of tin particles or other contamination. The static foil trap is represented schematically in figure 3 by dotted line 261.

[0061] A tin catching apparatus 265 extends at least partially around the circumference of the rotating foil trap 260. The tin catching apparatus 265 comprises a pair of walls 266 which extend radially inwardly from a wall of the source collector apparatus SO and define a tin receiving space 267. The walls 266 extend at least partially around the circumference of the rotating foil trap 260. Inner ends of the walls 266 may overlap in a radial direction with distal ends of the blades 262 of the rotating foil trap 260. Secondary tin droplets may be generated when tin is projected from the blades 262 of the rotating foil trap 260 onto the tin catching apparatus 265 (although the generation of secondary droplets may be suppressed by a porous structure as described below). The walls 266 act to reduce the likelihood that such secondary droplets leave the tin catching apparatus and enter other parts of the source collector apparatus SO. The tin catching apparatus 265 is provided with a heater (not shown) which maintains the tin catching apparatus at a temperature which is above the melting temperature of tin. [0062] A porous structure 268 is provided within the tin receiving space 267 of the tin catching apparatus 265. The porous structure 268 may for example be provided as a ring which extends around the tin catching apparatus 265, or may extend partially around the tin catching apparatus. The porous structure may for example be formed from metal.

[0063] A drain 270 is connected to the tin receiving space 267 of the tin catching apparatus 265. The drain 270 is provided at a lowermost part of the tin receiving space 267 in order to facilitate effective draining of tin from the tin receiving space. The drain 270 may be connected to a receptacle (not shown) which may be periodically replaced when it is filled with tin. A pump, schematically indicated by arrow 271, may be used to draw tin into the drain 270.

[0064] In use, the rotating foil trap 260 is driven to rotate during the generation of EUV radiation emitting plasma. Contamination which is generated during formation of the plasma 210 is intercepted by blades 262 of the rotating foil trap. The contamination may be tin which is at a temperature above the melting point of tin. The temperature of the blades is also above the melting point of tin, and consequently the tin remains in liquid form when it is on the blades. The liquid tin flows towards distal ends of the blades 262 due to centrifugal force arising from rotation of the blades. The liquid tin is projected from distal ends of the blades and is received in the tin receiving space 267 of the tin catching apparatus 265. Tips of the blades may be rotating at high speed (e.g. 300 km/h) and the liquid tin may therefore have considerable speed when it is projected from the blades.

[0065] The liquid tin is received in the tin receiving space 267 of the tin catching apparatus and is absorbed by the porous structure 268. The porous structure 268 retains the liquid tin and promotes the flow of liquid tin towards the drain 270. This may be advantageous because if tin were to accumulate in the tin catching apparatus 265 then it could form a thick layer or drop, which may come into contact with the blades 262 and prevent the blades from rotating (the clearance between the blades and the tin catching apparatus may for example be a few millimetres). If this were to happen, then operation of the source collector apparatus SO may need to be interrupted in order to allow the tin catching apparatus 265 and blades 262 to be cleaned (a time consuming and hence costly process).

[0066] By preventing the formation of drops of tin in the tin catching apparatus 265, the porous structure 268 may reduce the likelihood of tin contamination passing into the static foil trap 261. This is because a tin droplet in the tin catching apparatus 265 could cause splashing of tin which may be projected onto it by the blades 262, and some of this splashed tin could pass out of the tin catching apparatus 265 into the static foil trap 261. This may gradually decrease the EUV transmission of the static foil trap 261. Furthermore, tin which enters the static foil trap 261 will solidify (the static foil trap is not heated), and could over time establish a link of solid tin between the static foil trap and the tin catching apparatus 265. If this were to happen then it may cause localized cooling of the tin catching apparatus 265, which may in turn cause a localized build up of tin that may prevent the blades 262 from turning. The porous structure 268 may reduce the likelihood of these potential disadvantageous effects from happening.

[0067] The tin flows downwards due to gravity, thus flowing towards the drain 270. The porous structure 268 promotes the flow of the liquid tin towards the drain 270 via capillary forces which are transmitted by the liquid tin through the porous structure. The pump 271 may be used to draw tin into the drain 270. Pressure applied by the pump 271 may be transmitted by the liquid tin through the porous structure. The liquid tin passes around the tin catching apparatus 265 through the porous structure 268 and enters the drain 270, from where the liquid tin passes away from the source collector apparatus SO. The liquid tin may form a continuous column which extends either side of the drain 270. Suction from the drain 270 may pull the liquid tin into the drain, the suction being transmitted by capillary forces along the column of liquid tin. This may be advantageous because it may reduce the likelihood that tin droplets are formed within the tin catching apparatus 265.

[0068] Because capillary forces are used to draw the liquid tin towards the drain 270, it may not be necessary to use gravity to cause the liquid tin to flow towards the drain. This allows the tin catching apparatus 265 (and the rotating foil trap 260) to be provided at orientations which would not be possible if only gravity was being used to cause the liquid tin to flow towards the drain 270.

[0069] Because the porous structure 268 prevents or reduces the likelihood of tin droplets forming in the tin catching apparatus 265, the clearance between the blades 262 and the walls 266 of the tin catching apparatus may be reduced. This may be advantageous because it reduces the likelihood that contamination will pass beyond the rotating foil trap 260 to the static foil trap 261. Gas may be provided in the static foil trap 261 to inhibit the passage of contamination to a collector (not shown). When embodiments of the invention are used, the pressure of this gas may be reduced because less contamination reaches the static foil trap. This may provide an increase in the efficiency of the source collector apparatus SO (the gas absorbs EUV radiation). The reduction of the passage of contamination to the collector may also increase the lifetime of the collector.

[0070] In an embodiment, porous sintered metal (or other porous material) may be provided as a layer on one or more walls 266 of the tin catching apparatus 265 (or on any surface of the tin receiving space 267). This may be in addition to or instead of a porous structure which is provided as a separate entity (e.g. metal wool).

[0071] The porous structure may be formed from metal or any other suitable material.

The porous structure may, for example, be a ceramic which has an open cell structure.

[0072] In an embodiment, a surface of the porous structure 268 may be provided with a covering such as metal foil. The covering may be arranged to prevent tin from dripping from the porous structure 268, which in some instances might be possibility. The covering may be provided at locations where it is expected that tin will not be incident upon the porous structure 268 (otherwise the covering may prevent the tin being received and retained by the porous structure).

[0073] In an embodiment, the source collector apparatus SO may be part of an LPP radiation system, as shown in Figure 4. A fuel droplet generator 201 is configured to generate droplets of fuel and direct them towards a plasma formation location 202. The fuel may for example be xenon (Xe), tin (Sn) or lithium (Li), or may be some other suitable material. A laser LA is arranged to deposit laser energy into the fuel droplets, creating a highly ionized plasma 210 with electron temperatures of several 10's of eV. EUV radiation generated during de- excitation and recombination of these ions is emitted from the plasma 210, collected by a near normal incidence collector optic CO and focused onto the opening 221 in the enclosing structure 220. Conversion of a fuel droplet into plasma 210 may be incomplete, and particles of tin may be projected from the plasma formation location 202.

[0074] A screen 203 partially encloses a trajectory along which fuel droplets travel from the fuel droplet generator 201 to the plasma formation location 202. The screen 203 extends partway from the fuel droplet generator 201 to the plasma formation location 202. The screen 203 is seen viewed from one side in the main part of Figure 4. A cross-sectional view of the screen 203 along line AA is shown towards the bottom of Figure 4. In this embodiment the screen 203 is U-shaped in cross-section. However, the screen may have any suitable shape. [0075] Some of the particles of tin (or other fuel) that are projected from the plasma formation location 202 during plasma formation may initially travel in the general direction of the collector CO, but may then be pushed away from the collector by gas flow (the gas is used to protect the collector from contamination). Dashed arrow 204 schematically shows an example of a tin particle travelling along such a trajectory. The screen 203 prevents the tin particle from colliding with a fuel droplet which is travelling from the fuel droplet generator 201 to the plasma formation location 202. If such a collision were to occur then it could change the trajectory of the fuel droplet, and as a result the fuel droplet might not be accurately delivered to the plasma formation location 202.

[0076] The screen 203 also protects fuel droplets from gas flow which could otherwise change the trajectory of the fuel droplets and which could interfere with coalescence of smaller fuel droplets into larger fuel droplets (such coalescence may be used to generate fuel droplets having desired diameters).

[0077] The enclosing structure 220 includes tapering portions 222 which taper towards the opening 221. The tapering portions are provided with vanes 223 which extend from the tapering portions. The vanes 223 may extend in the general direction of the plasma formation location 202, or may be substantially perpendicular to the optical axis O (or may have any other suitable configuration). Particles of tin which are projected from the plasma formation location 202 are received between the vanes 223 and are retained there. This prevents those particles from adhering to optical surfaces of the lithographic apparatus and reducing their efficiency. It also prevents tin from accumulating on non-optical surfaces to such an extent that part of a radiation path between the plasma 210 and the intermediate focus IF is blocked (or reduces the rate at which this may happen).

[0078] The temperature of the vanes 223 and the tapering portion 222 of the enclosing structure may be higher than the melting point of tin. Consequently, tin which is present on the vanes (or between the vanes) will be in liquid form. In order to prevent tin from flowing down the vanes 223 and dripping from the vanes, a porous structure 225 is located between the vanes. The liquid tin is drawn into the porous structure 225 and is thus retained by the porous structure. Retaining the liquid tin and thereby preventing the liquid tin from dripping from the vanes 223 is advantageous because drips could obscure EUV radiation and thus reduce the efficiency of the source collector apparatus SO. Dripping tin may, for example, cool as it moves away from a vane, causing it to solidify as it leaves the vane. Over time this may lead to tin formations which resemble stalactites and which intersect with EUV radiation propagating towards the intermediate focus IF.

[0079] Although the source collector apparatus SO shown in Figure 4 is oriented such that the optical axis O is horizontal, the source collector apparatus may have any suitable orientation. For example, the source collector apparatus may be oriented such that the optical axis subtends an angle to the horizontal. Where this is the case, one or more of the vanes 223 may be located over the collector CO, and tin which dripped from that vane could therefore fall onto the collector CO. This is undesirable because it may cause localized reflection loss on the collector, which may in turn case black spots to be seen in the EUV radiation in the far field. By retaining tin, the porous structure 225 prevents tin from dripping onto the collector CO and therefore prevents localized reflection losses.

[0080] The porous structure 225 may be removed and replaced periodically in order to prevent the porous structure from becoming so full of tin that it is no longer able to absorb tin effectively. Removal and replacement of the porous structure 225 may for example be performed once per year (or at any other suitable interval). A sufficient quantity of the porous structure 225 may be provided to allow up to 10 liters, or even up to 20 liters or more of tin to be absorbed before the porous structure needs to be removed and replaced.

[0081] In an embodiment, the porous structure 225 may be connected to a drain (not shown) which is configured to receive tin that has been collected by the porous structure. The drain may be provided with a pump which is configured to draw tin into the drain. When a drain is used in this manner it may not be necessary to periodically remove and replace the porous structure 225. Alternatively, the drain may extend the period of time before the porous metal structure 225 is removed and replaced.

[0082] In an embodiment, the porous structure may be provided as a layer on the vanes

223. This may be done in addition to or instead of providing the porous structure between the vanes 223.

[0083] The temperature of the screen 203 may be higher than the melting point of tin.

Thus, it may be possible for tin which is incident upon the screen 203 to drip from the screen onto other parts of the source collector apparatus SO. In order to prevent tin from dripping from the screen 203 (or reduce the extent to which tin drips from the screen), the screen 203 may include a porous structure. Liquid tin on the screen is drawn into the porous structure and is thus retained by the porous structure. The porous structure may for example be provided as a layer on the screen 203.

[0084] The porous structure 225, 268 used by the illustrated embodiments (or any other embodiments) may for example comprise fibers (e.g. metal fibers). Spaces between adjacent fibers may provide porosity to the porous structure 225, 268. The metal fibers may be metal wool. The porous structure 225, 268 may for example be stainless steel wool.

[0085] The porous structure 225, 268 used by the illustrated embodiments (or any other embodiments) may for example be a porous sintered metal (e.g. Molybdenum). The porosity may be provided by pores in the porous sintered metal (the pores may be spaces between adjacent grains of the sintered metal). A porous sintered metal may be made for example by pressing and/or melting metal powder together. The porous sintered metal may for example be stainless steel or bronze. The porous sintered metal may for example be a high porosity metal which is available from GKN Sinter Metal Filters GmbH of Radevormwald, Germany. The porous sintered metal may for example be a high porosity metal which is available from Purolator Facet, Inc. of North Carolina, USA.

[0086] The porous structure may, for example, be metal or may be a non-metal material.

The porous structure may for example be a ceramic with an open cell structure, or may be any other suitable material.

[0087] In an embodiment, instead of providing a component such as a vane 223 or screen

203 (or other component) with a layer of porous material, the component may be formed from the porous material (i.e. the material used to form the component may be the porous structure). The component may for example be made from porous metal.

[0088] The porous structure may be wetting for tin. That is, the porous structure may be such that tin wets the porous structure. The porous structure may be such that a droplet of tin forms a contact angle of less than 90° on the surface of the material.

[0089] The porous structure 225, 268 may be formed from two or more layers of porous material. Each porous material layer may have pores of a different size. For example, an outer porous material layer may have pores which are larger than pores of an inner porous material layer. This arrangement may cause tin to be drawn from the outer porous material layer to the inner porous material layer. Effects arising from providing a two-layer porous structure are discussed in "Microdroplet Absorption By Two-Layer Porous Media"; Yu. D. Varlamov, Yu. P. Meshcheryakov, M. R. Predtechensky, and S. N. Ul'yankin; Journal of Applied Mechanics and Technical Physics, Vol. 48, No. 1, pp. 101-108, 2007, which is incorporated herein by reference.

[0090] An potential advantage which may be provided by embodiments of the invention is that even when the porous structure 225, 268 is filled with tin, it may be less reflective for contamination particles (e.g. tin droplets) than a flat surface which has been coated with tin. Thus, contamination particles are less likely to be reflected to parts of the source collector apparatus where tin is not desirable (e.g. an optical surface).

[0091] Although some embodiments of the invention have been described with tin being used as a fuel material, embodiments of the invention may be used for any suitable fuel material (e.g. Li or Xe).

[0092] Although the rotating foil trap 260 has been described in the context of a DPP source, it may be used in connection with an LPP source. Although the vanes 223 have been described in the context of an LPP source, they may be used in connection with a DPP source.

[0093] The rotating foil trap 260, vanes 223 and screen 203 may all be considered to be examples of contamination receiving apparatus. In general, a contamination receiving apparatus may be any component or surface of the source collector apparatus which receives contamination during operation of the source collector apparatus.

[0094] 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.

[0095] 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.

[0096] 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 bove, 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 source collector apparatus comprising:

a plasma generation apparatus arranged to excite fuel to form a radiation emitting plasma; a collector arranged to collect the radiation; and

a contamination receiving apparatus provided with a porous structure.

2. The source collector apparatus of claim 1, wherein the contamination receiving apparatus is a rotating foil trap comprising a plurality of blades and a fuel catching apparatus, and wherein the porous structure is located within a fuel receiving space of the fuel catching apparatus.

3. The source collector apparatus of claim 2, wherein the porous structure comprises fibers located within the fuel receiving space of the fuel catching apparatus.

4. The source collector apparatus of claim 2 or claim 3, wherein the porous structure comprises a layer provided on one or more surfaces of the fuel receiving space.

5. The source collector apparatus of claim 4, wherein the layer comprises a porous sintered metal.

6. The source collector apparatus of claim 1, wherein the contamination receiving apparatus is a plurality of vanes which extend from a wall of the source collector apparatus, and wherein the porous structure is located on or between the vanes.

7. The source collector apparatus of claim 6, wherein the porous structure comprises fibers located between the vanes.

8. The source collector apparatus of claim 6 or claim 7, wherein the porous structure comprises porous sintered metal provided on one or more of the vanes.

9. The source collector apparatus of claim 3 or claim 7, wherein the fibers are metal wool.

10. The source collector apparatus of claim 9, wherein the metal wool is stainless steel wool.

11. The source collector apparatus of claim 1, wherein the contamination receiving apparatus is a screen which extends partway from a fuel droplet generator to a plasma formation location.

12. The source collector apparatus of claim 11, wherein the porous structure comprises a layer provided on the screen.

13. The source collector apparatus of any preceding claim, wherein the plasma generation apparatus comprises a source of fuel and electrodes arranged to apply an electrical discharge to the fuel and thereby form the plasma.

14. The source collector apparatus of any of claims 1 to 12, wherein the plasma generation apparatus comprises a source of fuel arranged to deliver fuel to a location at which a laser beam is incident upon the fuel, thereby forming the plasma.

15. The source collector apparatus of claim 2 or claim 3, wherein a covering is provided on a surface of part of the porous structure, the covering being configured to stop fuel dripping from that part of the porous structure.

16. The source collector apparatus of any preceding claim, wherein the porous structure is wetting for the fuel.

17. The source collector apparatus of any preceding claim, wherein the porous structure is formed from metal.

18. A lithographic apparatus comprising:

the source collector apparatus of any preceding claim;

an illumination system configured to condition the radiation collected by the collector to form a conditioned radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate; and

a projection system configured to project the patterned radiation beam onto a target portion of the substrate.

19. A method of generating radiation comprising:

exciting a fuel such that the fuel forms a radiation emitting plasma; and

collecting contamination generated during formation of the plasma using a contamination receiving apparatus, wherein the contamination receiving apparatus is provided with a porous structure.

20. A device manufacturing method comprising generating radiation using the method of claim 19 and using the lithographic apparatus of claim 18 to pattern a substrate.