WO2016058746A1 - Source de rayonnement - Google Patents

Source de rayonnement Download PDF

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Publication number
WO2016058746A1
WO2016058746A1 PCT/EP2015/070109 EP2015070109W WO2016058746A1 WO 2016058746 A1 WO2016058746 A1 WO 2016058746A1 EP 2015070109 W EP2015070109 W EP 2015070109W WO 2016058746 A1 WO2016058746 A1 WO 2016058746A1
Authority
WO
WIPO (PCT)
Prior art keywords
radiation
radiation source
tile
aperture
shield
Prior art date
Application number
PCT/EP2015/070109
Other languages
English (en)
Inventor
Johannes Christiaan Leonardus Franken
Andrey NIKIPELOV
Pavel SEROGLAZOV
Jan Bernard Plechelmus Van Schoot
Ivo Vanderhallen
Andrei Mikhailovich Yakunin
Erik Willem BOGAART
Chuangxin ZHAO
Robertus Wilhelmus VELTMAN
Reinier Theodorus Martinus JILISEN
Original Assignee
Asml Netherlands B.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Asml Netherlands B.V. filed Critical Asml Netherlands B.V.
Publication of WO2016058746A1 publication Critical patent/WO2016058746A1/fr

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Classifications

    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/708—Construction of apparatus, e.g. environment aspects, hygiene aspects or materials
    • G03F7/70908—Hygiene, e.g. preventing apparatus pollution, mitigating effect of pollution or removing pollutants from apparatus
    • G03F7/70916—Pollution mitigation, i.e. mitigating effect of contamination or debris, e.g. foil traps
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70008—Production of exposure light, i.e. light sources
    • G03F7/70033—Production of exposure light, i.e. light sources by plasma extreme ultraviolet [EUV] sources
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70008—Production of exposure light, i.e. light sources
    • G03F7/70041—Production of exposure light, i.e. light sources by pulsed sources, e.g. multiplexing, pulse duration, interval control or intensity control
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/708—Construction of apparatus, e.g. environment aspects, hygiene aspects or materials
    • G03F7/70983—Optical system protection, e.g. pellicles or removable covers for protection of mask
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
    • G21K1/02—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diaphragms, collimators
    • G21K1/04—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diaphragms, collimators using variable diaphragms, shutters, choppers
    • G21K1/043—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diaphragms, collimators using variable diaphragms, shutters, choppers changing time structure of beams by mechanical means, e.g. choppers, spinning filter wheels
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05G—X-RAY TECHNIQUE
    • H05G2/00—Apparatus or processes specially adapted for producing X-rays, not involving X-ray tubes, e.g. involving generation of a plasma
    • H05G2/001—X-ray radiation generated from plasma
    • H05G2/003—X-ray radiation generated from plasma being produced from a liquid or gas
    • H05G2/005—X-ray radiation generated from plasma being produced from a liquid or gas containing a metal as principal radiation generating component
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05G—X-RAY TECHNIQUE
    • H05G2/00—Apparatus or processes specially adapted for producing X-rays, not involving X-ray tubes, e.g. involving generation of a plasma
    • H05G2/001—X-ray radiation generated from plasma
    • H05G2/008—X-ray radiation generated from plasma involving a beam of energy, e.g. laser or electron beam in the process of exciting the plasma

Definitions

  • the present invention relates to a radiation source that is operable to produce pulses of radiation.
  • a radiation source that is provided with a shield that is synchronized with the production of pulses of radiation so as to allow the pulses of radiation to pass through an aperture and to at least partially cover said aperture in between consecutive pulses of radiation.
  • the radiation source may form part of a lithographic system.
  • a lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate.
  • a lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs).
  • a lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation- sensitive material (resist) provided on a substrate.
  • a patterning device e.g. a mask
  • a layer of radiation- sensitive material resist
  • the wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate.
  • a lithographic apparatus which uses EUV radiation being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).
  • the radiation used by a lithographic apparatus is produced by a radiation source.
  • Known radiation sources that are operable to produce pulses of EUV radiation include laser produced plasma (LPP) and discharge produced plasma (DPP) sources.
  • LPP laser produced plasma
  • DPP discharge produced plasma
  • a plasma is formed from a fuel (for example tin) and radiation, including EUV radiation, is emitted from the plasma during de-excitation and recombination of ions of the plasma.
  • the EUV radiation is collected and focused by a radiation collector at or near to an aperture in an enclosing structure of the radiation source.
  • a radiation source operable to produce pulses of radiation
  • the radiation source comprising: a wall; an aperture in the wall; a radiation collector arranged to direct pulses of radiation along an optical axis of the radiation collector, said optical axis passing through the aperture; a shield comprising a body with one or more openings, said one or more openings forming a passageway through the body; and a drive mechanism arranged to rotate the shield about a rotation axis such that the passageway is intermittently aligned with the aperture as each pulse of radiation reaches the shield to allow the pulses of radiation to pass through the aperture in the wall, and the body at least partially covers the aperture in between consecutive pulses of radiation; wherein the rotation axis of the shield is generally perpendicular to the optical axis of the radiation collector.
  • Such an arrangement allows the drive mechanism to be disposed further from the optical axis without increasing the diameter of the body of the shield.
  • the drive mechanism (which may for example comprise a motor and bearings) to lie outside of the optical path of the radiation, which is directed generally along the optical axis of the radiation collector.
  • the diameter of the body to be decreased, relative to arrangements wherein the rotation axis is generally parallel to the optical axis. In turn, this reduces the level of internal stress that is experienced by the body at a given rotational speed.
  • this allows the shield to rotate at higher rotational speeds and allows the radiation source to operate at higher repetition rates.
  • the passageway in the shield may intersect the rotation axis of the shield.
  • Such an arrangement forms a passageway through the body that, as the body rotates will align with the optical axis of the radiation source SO twice during each full revolution of the body. Therefore, in order for the body to be synchronised with the radiation pulse generation within the radiation source, the frequency at which the body should rotate is equal to half of the repetition rate of the pulse generation.
  • such an arrangement can therefore halve the rate at which the body should rotate.
  • the shield may be provided with a plurality of openings arranged form a plurality of passageways through the body, the plurality of passageways being arranged such that as the shield rotates about the rotation axis each of the plurality of passageways is intermittently aligned with the aperture to allow the pulses of radiation to pass through the aperture in the wall.
  • the provision of multiple passageways through the body reduces the rotational speed at which it rotates, in turn reducing the internal stresses in the body.
  • the provision of a second passageway may reduce the rotational speed at which the body rotates by a factor of two.
  • An outer diameter of the body may be equal to or less than twice the diameter of the or each aperture.
  • the shield may comprise a plurality of blades, extending away from the rotation axis in a direction that is generally perpendicular to the rotation axis.
  • the body may further comprise a generally cylindrical central portion from which the plurality of blades extends.
  • the body may further comprise a generally cylindrical lower portion, a generally disc shaped upper portion, the plurality of blades extending between the lower and upper portions in a direction parallel to the rotation axis and in a direction that is generally perpendicular to the rotation axis, from a radially outer surface of the body partially towards rotation axis.
  • a smooth fillet may be provided at each intersection between the blades and the central portion or between the blades and the upper and lower portions.
  • these fillets help to distribute stresses over a larger area of the body and may aid the aerodynamics of the body.
  • the plurality of blades may be arranged symmetrically about the rotation axis.
  • the radiation source may further comprise a debris catcher arranged such that at least a portion of debris incident on the body of the shield is directed towards the debris catcher.
  • the radiation source may be a laser produced plasma source comprising a fuel emitter operable to emit droplets of fuel and a laser operable to irradiate said droplets of fuel in a plasma formation region.
  • the debris catcher may be disposed closer to the aperture than the plasma formation region.
  • the radiation source may be configured such that if rotation of the body is not synchronised with generation of the radiation pulses, generation of the radiation pulses is stopped.
  • the body of the shield may be formed from: carbon fibre reinforced polymer; graphene; a pure metal; a metal alloy; or an amorphous metal alloy.
  • the aperture may be provided with a gas flow protection system.
  • a radiation source operable to produce pulses of radiation
  • the radiation source comprising: a wall; an aperture in the wall; a radiation collector arranged to direct pulses of radiation towards the aperture in the wall; a shield comprising a body with an opening, said opening forming a passageway extending through the body; a drive mechanism arranged to rotate the shield about a rotation axis such that the passageway is intermittently aligned with the aperture as each pulse of radiation reaches the shield to allow the pulses of radiation to pass through the aperture in the wall, and the body at least partially covers the aperture in between consecutive pulses of radiation; and a debris catcher positioned so as to receive at least a portion of debris that is incident on the body of the shield and which rebounds from the body.
  • the rotation axis of the shield may be arranged at an oblique angle to an optical axis of the radiation collector.
  • the shield may be of the form of a disc shaped body and the aperture may be of the form of a circumferentially extending slit.
  • the radiation source may be a laser produced plasma source comprising a fuel emitter operable to emit droplets of fuel and a laser operable to irradiate said droplets of fuel in a plasma formation region.
  • the debris catcher may be disposed closer to the aperture than the plasma formation region.
  • the radiation source may be configured such that if rotation of the body is not synchronised with generation of the radiation pulses, generation of the radiation pulses is stopped.
  • the body of the shield may be formed from: carbon fibre reinforced polymer; graphene; a pure metal; a metal alloy; or an amorphous metal alloy.
  • the aperture may be provided with a gas flow protection system.
  • a lithographic system comprising: a radiation source according to the first or second aspect of the invention; and a lithographic apparatus arranged receive radiation produced by the radiation source, impart a pattern in its cross-section to form a patterned radiation beam, and project the patterned radiation beam onto a substrate.
  • a rotatable shield for use in the radiation source of the first aspect of the invention.
  • a method for providing pulses of radiation comprising: generating a plurality of droplets of fuel; irradiating each droplet of fuel with a pulsed laser beam to produce a pulse of radiation; collecting and directing the pulses of radiation along an optical axis, said optical axis passing through an aperture in a wall; providing a shield comprising a body with one or more openings, said one or more openings forming a passageway through the body; and rotating the shield about a rotation axis such that the passageway is intermittently aligned with the aperture as each pulse of radiation reaches the shield to allow the pulses of radiation to pass through the aperture in the wall, and the body at least partially covers the aperture in between consecutive pulses of radiation, wherein the rotation axis of the shield is generally perpendicular to the optical axis of the radiation collector.
  • a radiation source comprising a fuel emitter operable to emit fuel and direct the fuel to a plasma formation region for excitation into a radiation emitting plasma at the plasma formation region and a debris collector configured to collect debris emitted from the plasma formation region
  • the debris collector comprises a first tile and a second tile, wherein the first tile is arranged between the plasma formation region and a first portion of the second tile such that there is no direct line of sight between the first portion of the second tile and the plasma formation region and wherein the second tile further includes a second portion arranged in a direct line of sight of the plasma formation region and arranged to receive debris emitted from the plasma formation region.
  • the second tile may be inclined with respect to the vertical so as to cause liquid debris which is present on the second tile to flow down the second tile.
  • the radiation source may further comprise a receptacle arranged to receive liquid debris which flows down the second tile.
  • the radiation source may further comprise a heater configured to heat the second tile.
  • the heater may be configured to heat at least a portion of the second tile to a temperature which is greater than the melting point of the fuel debris.
  • the first tile may be arranged vertically beneath at least a portion of the second tile so as to catch fuel debris which drips from the second tile.
  • the radiation source may further comprise a radiation collector configured to collect radiation which is emitted from the radiation emitting plasma, wherein the first tile is arranged between at least a portion of the radiation collector and at least a portion of the second tile such that fuel debris which drips from the second tile is caught by the first tile and prevented from being incident on the radiation collector.
  • the first tile and/or the second tile may have a cross-sectional shape which is substantially v-shaped.
  • the first tile and the second tile may be arranged to form a channel between the first tile and the second tile.
  • the radiation source may further comprise a scrubber located in the channel, wherein the scrubber is operable to clean fuel debris vapour from the environment in the radiation source.
  • the radiation source may further comprise a gas supply configured to supply a gas and cause the gas to flow along the channel.
  • the gas supply may be configured to supply a gas comprising hydrogen.
  • the first tile and the second tile may together form at least part of a column of tiles.
  • the debris collector may comprise a plurality of columns of tiles.
  • the columns of tiles may be arranged around an optical axis of the radiation source.
  • the fuel emitter may be operable to emit a fuel comprising tin.
  • a lithographic system comprising a radiation source according to the sixth aspect and a lithographic apparatus arranged receive radiation produced by the radiation source, impart a pattern in its cross-section to form a patterned radiation beam, and project the patterned radiation beam onto a substrate.
  • Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source and including a rotatable shield according to an embodiment of the invention
  • Figure 2 depicts another radiation source that may alternatively form part of the lithographic system of Figure 1 ;
  • Figure 3 is a schematic illustration of a first rotatable shield that may form the rotatable shield shown in Figures 1 and 2;
  • Figure 4 is a first embodiment of a body that may form part of the rotatable shield of Figure 3;
  • Figure 5 is a schematic illustration of a cross section of the body of Figure 4 along the line X-X;
  • Figure 6 is a second embodiment of a body that may form part of the rotatable shield of Figure 3 ;
  • Figure 7 is a third embodiment of a body that may form part of the rotatable shield of Figure 3.
  • Figure 8 is a schematic illustration of a second rotatable shield that may form the rotatable shield shown in Figures 1 and 2;
  • Figure 9 is a plan view of the body of the rotatable shield of Figure 8.
  • Figure 10 is a schematic illustration of radiation source which includes a debris collector
  • Figure 11 is a schematic illustration of a portion of the debris collector of Figure 10.
  • Figures 12A-12C are schematic illustrations of a portion of a debris collector at different stages of heating of the debris collector.
  • Figures 13A and 13B are alternative arrangements of adjacent columns of tiles which may form a portion of a debris collector.
  • FIG. 1 shows a lithographic system including a rotatable shield 100 according to one embodiment of the invention.
  • the lithographic system comprises a radiation source SO and a lithographic apparatus LA.
  • the radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B.
  • the lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS and a substrate table WT configured to support a substrate W.
  • the illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA.
  • the projection system PS is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W.
  • the substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.
  • the radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment.
  • a gas e.g. hydrogen
  • a vacuum may be provided in illumination system IL and/or the projection system PS.
  • a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.
  • the radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source.
  • a laser 1 which may for example be a C0 2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 3.
  • tin is referred to in the following description, any suitable fuel may be used.
  • the fuel may for example be in liquid form, and may for example be a metal or alloy.
  • the fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4.
  • the laser beam 2 is incident upon the tin at the plasma formation region 4.
  • the deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4.
  • Radiation including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma.
  • radiation source SO may further comprise a pre-pulse laser (not shown).
  • the pre-pulse laser may be operable to emit a pre-pulse laser beam is incident upon the fuel before the laser beam 2.
  • Such a pre-pulse laser beam may act to preheat the fuel, thereby changing a property of the fuel such as its size and/or shape before it is irradiated by laser beam 2.
  • the EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector).
  • the collector 5 may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm).
  • EUV radiation e.g. EUV radiation having a desired wavelength such as 13.5 nm.
  • the collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.
  • the laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics.
  • a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics.
  • the laser 1 and the radiation source SO may together be considered to be a radiation system.
  • Radiation that is reflected by the collector 5 forms a radiation beam B.
  • the radiation beam B propagates generally in the direction of an optical axis 12 of the radiation source SO.
  • the radiation beam B is focused at point 6 on the optical axis 12 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL.
  • the point 6 at which the radiation beam B is focused may be referred to as the intermediate focus.
  • the radiation source SO is arranged such that the intermediate focus 6 is located at or near to an aperture 8 in an enclosing structure 9 of the radiation source.
  • the radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam.
  • the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11.
  • the faceted field mirror device 10 and faceted pupil mirror device 11 together provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution.
  • the radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT.
  • the patterning device MA reflects and patterns the radiation beam B.
  • the illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 11.
  • the patterned radiation beam B enters the projection system PS.
  • the projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B onto a substrate W held by the substrate table WT.
  • the projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied.
  • the projection system PS has two mirrors in Figure 1 , the projection system may include any number of mirrors (e.g. six mirrors).
  • FIG 2 shows a laser produced plasma (LPP) radiation source SO which has an alternative configuration to the radiation source shown in Figure 1.
  • the radiation source SO includes a fuel emitter 3 which is configured to deliver fuel to a plasma formation region 4.
  • the fuel may for example be tin, although any suitable fuel may be used.
  • the pre-pulse laser beam 16 emits a pre-pulse laser beam 17 which is incident upon the fuel.
  • the pre-pulse laser beam is incident upon the fuel.
  • a main laser 18 emits a main laser beam 19 which is incident upon the fuel after the pre-pulse laser beam 17.
  • the main laser beam delivers energy to the fuel and thereby converts the fuel into an EUV radiation emitting plasma 7.
  • a radiation collector 20, which may be a so-called grazing incidence collector, is configured to collect the EUV radiation and form a radiation beam B.
  • the radiation beam B propagates generally in the direction of an optical axis 12 of the radiation source SO.
  • the radiation collector 20 focuses the EUV radiation at a point 6 along the optical axis 12, which may be referred to as the intermediate focus.
  • an image of the radiation emitting plasma 7 is formed at the intermediate focus 6.
  • the radiation source SO comprises an enclosure structure 21.
  • Enclosure structure 21 is provided with an aperture 22, which is at or near to the intermediate focus 6.
  • the EUV radiation passes through the aperture 22 to an illumination system of a lithographic apparatus (e.g. of the form shown schematically in Figure 1).
  • the radiation collector 20 may be a nested collector, with a plurality of grazing incidence reflectors 23, 24 and 25 (e.g. as schematically depicted).
  • the grazing incidence reflectors 23, 24 and 25 may be disposed axially symmetrically around optical axis 12.
  • the illustrated radiation collector 20 is shown merely as an example, and other radiation collectors may be used.
  • An enclosure 21 of the radiation source SO includes a window 27 through which the pre-pulse laser beam 17 can pass to the plasma formation region 4, and a window 28 through which the main laser beam 19 can pass to the plasma formation region.
  • a mirror 29 is used to direct the main laser beam 19 to the plasma formation region 4.
  • the radiation sources SO shown in Figures 1 and 2 may include components which are not illustrated.
  • a spectral filter may be provided in the radiation source.
  • the spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.
  • the tin plasma that is produced at plasma formation region 4 also generates tin debris.
  • debris comprises tin particles with a range of directions, sizes and speeds.
  • the tin debris may comprise a plurality of tin particles with a distribution of sizes, which may range in diameter from around 150 nm to around 1 ⁇ and may have an average diameter of around 350 nm.
  • the tin debris may comprise a plurality of tin particles with a distribution of speeds, which may range from around 50 m/s to around 500 m/s, or even to around 1000 m/s.
  • tin particles can hit surfaces of the radiation source SO and/or lithographic apparatus LA either directly or indirectly having rebounded from other surfaces. Any tin that is incident upon optical surfaces of the radiation source SO and/or lithographic apparatus LA will reduce the reflectivity of those optical surfaces and/or may damage the optical surfaces, reducing the performance of the lithographic apparatus LA.
  • Some tin debris may pass through the aperture 8, 22 of the radiation source and into the illumination system IL either directly or indirectly. Due to the broad distribution of tin debris momenta the rate at which tin debris reaches the aperture 8, 22 is generally constant. This is in contrast to the rate at which EUV radiation reaches the aperture 8, 22, since the EUV radiation reaches the aperture 8, 22 in temporally well resolved pulses. That is, the EUV radiation is pulsed at the aperture 8, 22 whereas the tin debris is not pulsed.
  • the aperture 8, 22 may be provided with a gas flow protection system (DGL).
  • the gas flow protection system may comprise a plurality of gas injection points disposed around the perimeter of the aperture 8, 22.
  • a gas such as, for example, hydrogen may be directed towards the optical axis 12 from each of the gas injection points.
  • the gas may be provided at low pressure.
  • the gas may flow into the enclosing structure 9, 21 of the radiation source SO, carrying some tin debris with it. Therefore the gas flow protection system can reduce the level of tin debris passing into the illumination system IL of the lithographic apparatus LA.
  • tin debris may still enter the lithographic apparatus LA.
  • Such tin debris can hit: optical elements within the illumination system IL (e.g. facetted field mirror device 10 and a facetted pupil mirror device 11); optical elements within the projection system PS (e.g. mirrors 13, 14); and/or the patterning device MA. This can significantly reduce the lifetime of these optical elements 10, 11, 13, 14, MA. Therefore, it is desirable to further reduce the level of tin debris that passes through the aperture 8, 22 and into the lithographic apparatus.
  • a rotatable shield 100 is provided at the aperture 8, 22 in the wall of the enclosure 9, 21 of each of the radiation sources SO.
  • a controller CT may be provided to control synchronisation of the rotation of rotatable shield 100 with the generation of EUV radiation pulses so as to avoid EUV losses caused by the rotatable shield 100.
  • the controller CT may be arranged to receive a signal si from the fuel emitter 3. Said signal si may be indicative of the timing of fuel droplet generation within the fuel emitter 3.
  • the controller CT may be arranged to receive a signal from a sensor (not shown) that is operable to detect the fuel droplets close to the plasma formation region 4, for example, just before they are irradiated by the laser 1, 18. Said signal may be is indicative of the timing of fuel droplets in the vicinity of the plasma formation region 4.
  • Such a signal may give a more accurate indication of the timing of EUV pulse generation (than signal si) since the average speed of fuel droplets between the fuel emitter 3 and the plasma formation region may vary.
  • the controller CT may be operable, in response to the received signal (for example signal si), to control the rotation of rotatable shield 100 so as to synchronise it with the generation of EUV radiation pulses. This control may be achieved by sending a signal s2 to the rotatable shield 100.
  • the controller CT may also, in response to the received signal (for example signal si), control the laser 1, 18 so as to synchronise the laser pulses with the fuel droplets to generate EUV radiation. This may be achieved by sending a signal s3 to the laser 1, 18.
  • a separate controller may be provided for synchronization of the laser 1, 18 with the fuel generator 3.
  • FIG 3 shows a first embodiment of a rotatable shield 200 (which may correspond with the rotatable shield 100 shown in Figures 1 and 2).
  • the rotatable shield 200 comprises a body 210, a shaft 220 and a drive mechanism 230.
  • the shaft 220 extends along a rotation axis 222 of the rotatable shield 200 between the body 210 and the drive mechanism 230.
  • the drive mechanism 230 is operable to rotate the shaft 220 and body 210 about the rotation axis 222.
  • the drive mechanism 230 may for example comprise a motor that is arranged to rotate the shaft 220.
  • the drive mechanism 230 may further comprise one or more bearings (not shown) to support the shaft 220.
  • the rotatable shield 200 is arranged such that its rotation axis is generally perpendicular to the optical axis 12 of the radiation source SO.
  • One benefit of such an arrangement is that the drive mechanism is 230 can lie outside of the optical path of the EUV radiation B produced by the radiation source SO.
  • the rotatable shield 200 is provided on the radiation source SO side of the aperture 8, 22 in the wall of the enclosure 9, 21 and may be considered to be part of the radiation source SO. It will be appreciated that the rotatable shield 200 may alternatively be provided on the lithographic apparatus LA (illuminator IL) side of the aperture 8, 22 in the wall of the enclosure 9, 21.
  • the body 210 may be similar in structure to a rotor, such as for example an impeller or a propeller, having a number of blades that extend in a direction that is generally perpendicular to the rotation axis 222. However, it will be appreciated that the body 210 may be disposed in vacuum conditions and therefore may be different in function to a rotor.
  • the body 210 may have various different shapes. Three different bodies 210a, 210b, 210c, each of which may form the body 210 of the rotatable shield 200, will now be described by way of example with reference to Figures 4 to 7.
  • FIG 4 shows a first body 210a, which may form the body 210 of the rotatable shield 200.
  • Body 210a comprises a generally cylindrical central portion 211 and two blades 214a, 214b.
  • the cylindrical central portion 211 extends along the rotation axis 222 as an extension of the shaft 220.
  • Each of the two blades 214a, 214b extends away from the central portion 211 in a direction that is generally perpendicular to the rotation axis 222.
  • a smooth fillet is provided at each intersection between the blades 214a, 214b and the central portion. These fillets help to distribute stresses over a larger area of the body 210a and may aid the aerodynamics of the body 210a.
  • An opening 212 is provided in the body 210a.
  • the opening 212 defines a passageway through the body 210a that extends in a direction that is generally perpendicular to, and passes through, the rotation axis 222. Said passageway is also generally perpendicular to each of the two blades 214a, 214b.
  • the passageway formed by the opening 212 will periodically align with the optical axis 12 of the radiation source.
  • the passageway formed by the opening 212 will align with the optical axis 12 of the radiation source SO twice during each full revolution of the body 210a.
  • the rotation of the body 210a is synchronised with the generation of EUV radiation pulses so as to avoid EUV losses caused by the rotatable shield 200. That is, the rotation is timed such that the passageway formed by the opening 212 is aligned with the optical axis 12 of the radiation source SO as each pulse of EUV radiation reaches the body 210a.
  • the rotation of body 210a is such that the time between successive alignments of the passageway formed by the opening 212 with the optical axis 12 is substantially the same as the time between consecutive pulses of EUV radiation.
  • the passageway formed by opening 212 is intermittently aligned with the aperture 8, 22 as each pulse of radiation generated by the radiation source SO reaches the shield 210. This allows the pulses of radiation to pass through the aperture 8, 22.
  • the fuel emitter 3 may comprise an oscillator such as, for example, a quartz crystal oscillator. Said oscillator may dictate the frequency of tin droplet generation and, therefore, EUV radiation production.
  • the oscillator of the fuel emitter 3 may be used to ensure that rotation of the body 210a is synchronised with the generation of EUV radiation pulses.
  • the frequency at which the body 210a should rotate is equal to half of the frequency (also known as repetition rate) of EUV pulse generation within radiation source SO. This is because the passageway formed by the opening 212 aligns with the optical axis 12 twice during each full revolution of the body 210a.
  • the repetition rate of EUV pulse generation within radiation source SO may be, for example, 50 kHz to 100 kHz. Therefore, the rotatable shield 200 will rotate at very high rotational speeds. These repetition rates set a desired refresh rate of consecutive alignments between apertures 8, 22 and opening 212 of around 10 ⁇ 8 to 20 ⁇ 8.
  • the duration of each EUV pulse may be of the order of ⁇ 0.2 ⁇ 8. With such a duty cycle, in between each pulse of EUV radiation there is a time of the order of 9.8 ⁇ 8 to 19.8 ⁇ 8 during which the rotational shield 200 can at least partially block the aperture 8, 22 at the intermediate focus 6 without attenuating the EUV radiation beam B. Instability in the fuel emitter 3 may result in a 3 sigma error in the desired refresh rate of the order of 0.5 ⁇ 8 (i.e. the refresh rate may be around 10+0.5 ⁇ 8 to 20+0.5 ⁇ ).
  • the radiation source SO may be configured such that if the rotation of the body 210a is not synchronised with the generation of EUV radiation pulses, generation of EUV radiation is stopped. This avoids the EUV radiation damaging the body 210a. EUV radiation may be stopped without stopping the fuel emitter 3 or the laser 1 , 18 by altering the timing of either or both of the fuel emitter 3 and the laser 1, 18 such that the droplets of fuel are not irradiated by the laser 1 , 18.
  • the fuel emitter 3 should be started and the rotatable shield 200 should be sped up to the same frequency as, and synchronised with, the fuel emitter 3.
  • the laser 1, 18 may be brought into synchronisation with the fuel emitter 3 to produce EUV radiation.
  • the diameter of the opening 212 through body 210a may be approximately the same as the diameter of aperture 8, 22. In some embodiments, the diameter of the opening 212 through body 210a may slightly larger than the diameter of aperture 8, 22. This can include a sufficient margin to account for short term jitter of the repetition rate of radiation source SO (for example caused by the fuel emitter 3) so as to ensure that each pulse of the EUV radiation passes through opening 212 without being attenuated. That is, the diameter of the opening 212 may be sufficiently large that if short time jitter causes any pulses of radiation to arrive at the rotatable shield 200 early (late) then the leading (trailing) edge of the opening 212 will not intersect with the radiation beam B.
  • the apertures 8, 22 may have a diameter of around 6.5 mm and the opening 212 may have a diameter of around 7-8 mm.
  • the time averaged rotational speed of the body 210a can be controlled as part of a positive feed-back loop using the frequency of droplet generation by the fuel emitter 3. This ensures synchronisation of the body 210a with the radiation source SO in spite of any slow drift of the radiation source SO pulse frequency.
  • FIG. 5 shows the body 210a of Figure 4 in situ in radiation source SO, in cross section along the line X-X.
  • each of the blades 214a, 214b sweeps out a volume 216.
  • the volume 216 is annular in shape. Therefore, in the plane of Figure 5, volume 216 appears as an annulus.
  • any tin particles that pass through the front portion 216a of the volume 216, missing the two blades 214a, 214b, may be hit by one of the blades 214a, 214b as they pass through the rear portion 216b of the volume 216.
  • this can double the stopping speed of the rotatable shield 200 (i.e. the maximum speed of particles that are stopped by the rotatable shield) or, alternatively, it allows the same stopping speed to be achieved with smaller blades 214a, 214b.
  • the rotatable shield 200 may rotate at very high rotational speeds. For example, if the repetition rate of the radiation source SO is 100 kHz then the body 210a will rotate at rate of 50,000 revolutions per second or, equivalently, 3,000,000 revolutions per minute (rpm). Such high rotational speeds will create very high internal stresses in the body 210a. In order to keep these internal stresses sufficiently low that the body 210a can survive such high rotational speeds, the body 210a should be as small as possible. Therefore, as shown in Figures 3 and 5, the body may be placed adjacent to the intermediate focus 6 (where the waist of the EUV radiation beam B reaches its minimum).
  • An alternative arrangement for a rotatable shield that has been proposed previously has a rotation axis that is parallel to the optical axis 12 of the radiation source SO but which is offset from the optical axis 12 so that the drive mechanism lies outside of the optical path of the EUV radiation.
  • Such an arrangement may be impractical since the body would have to rotate at very high speeds in order to synchronize with the radiation pulses and, at such speeds, the internal stresses in the body of the rotatable shield would be very high.
  • the rotatable shield 200 of Figure 3 having a rotation axis 222 that is perpendicular to the optical axis, the diameter of the body 210 can be decreased.
  • an outer diameter of the body 210 can be reduced to less than twice the diameter of the aperture 8, 22.
  • a rotatable shield with a rotation axis that is parallel to, but offset from, the optical axis 12 of the radiation source SO would require a body with an outer diameter that is at least twice the diameter of the aperture 8, 22.
  • the rotatable shield 200 of Figure 3 allows a design with an outer diameter of the body 210 as small as 10- 12 mm (for an aperture 8, 22 with a diameter of around 6.5 mm and an opening 212 with a diameter of around 7-8 mm). This reduction in the diameter of the body 210 significantly reduces the internal stresses that it will experience at the required rotation speeds for synchronisation with the EUV radiation generation of the radiation source SO. This makes it practical to provide a rotatable shield that will survive at such high rotational speeds.
  • the body 210a may be formed from any suitable material.
  • the material from which the body 210a is formed should be strong enough to withstand the high internal stresses that are experienced at the high rotational speeds that it experiences.
  • the internal stresses experienced by the body 210a will depend on its shape (in particular its diameter), its mass, and the rotational speed that it operates at.
  • the material from which the body 210a is formed may have a high enough ultimate tensile strength to withstand the internal stresses that it experiences.
  • material from which the body 210a is formed may have an ultimate tensile strength of greater than 1 GPa.
  • the material from which the body 210a is formed may be as light as possible, again to facilitate its rotation at high rotational speeds.
  • the material from which the body 210a is formed may have a low density or specific weight (weight per unit volume). Further, the material from which the body 210a is formed may be suitable for use at the high temperatures experienced within the radiation source SO. Further, the material from which the body 210a is formed may have a high resistance to corrosion from chemicals present within the radiation source SO such as, for example, tin, hydrogen gas and hydrogen radicals. Suitable materials may include: carbon fibre reinforced polymer such as, for example that marketed under the name TORAYCA (registered trade mark) T1100G available from Toray Industries, Inc.
  • TORAYCA registered trade mark
  • Carbon fibre reinforced polymer as marketed under the name TORAYCA (registered trade mark) T1100G has a tensile strength of around 6.6 GPa and a density of 1.79 g/cm 3 .
  • the body 210a is formed from such a material, has an outer diameter of 15 mm, and rotates at a rotational speed of the order of 3xl0 6 revolutions per minute (rpm), the body 210a would experience internal stresses of the order of 6.2 GPa, which is below its tensile strength.
  • the amorphous metal alloy as marketed under the commercial name "Liquidmetal” has a tensile strength of around 1.64 GPa and a density of 6.04 g/cm 3 .
  • FIG. 6 shows a second body 210b, which may form the body 210 of the rotatable shield 200.
  • Body 210b comprises a generally cylindrical central portion 211 and four blades 215a-215d.
  • the cylindrical central portion 211 extends along the rotation axis 222 as an extension of the shaft 220.
  • Each of the four blades 215a-215d extends away from the central portion 211 in a direction that is generally perpendicular to the rotation axis 222.
  • a smooth fillet is provided at each intersection between the blades 215a-215d and the central portion 211. These fillets help to distribute stresses over a larger area of the body 210b and may aid the aerodynamics of the body 210b.
  • Each passageway 212a, 212b extends in a direction that is generally perpendicular to, and passes through, the rotation axis 222.
  • the four blades 215a-215d are arranged symmetrically about the rotation axis 222, interspersed with the openings.
  • body 210b comprises four blades 215a-215d, as opposed to two; and body 210b is provided with two openings, which form two passageways 212a, 212b, as opposed to two. In all other ways, and in function, body 210b is similar to body 210a.
  • each of the two passageways 212a, 212b will periodically align with the optical axis 12 of the radiation source SO.
  • each passageway 212a, 212b is intermittently aligned with the aperture 8, 22.
  • a different one of the two passageways 212a, 212b is aligned with the optical axis 12 and the aperture 8, 22.
  • Each passageway 212a, 212b will align with the optical axis 12 of the radiation source SO twice during each full revolution of the body 210b.
  • body 210b can rotate at half the rotational speed of body 210a. For example, if the repetition rate of the radiation source SO is 100 kHz then the body 210b will rotate at rate of 25,000 revolutions per second or, equivalently, 1,500,000 revolutions per minute (rpm). Therefore, advantageously, the provision of multiple passageways in body 210 reduces the rotational speed at which it rotates, in turn reducing the internal stresses in the body 210.
  • Figure 7 shows a third body 210c, which may form the body 210 of the rotatable shield 200 of Figure 3.
  • Body 210c is generally cylindrical in shape, comprising a generally cylindrical lower 213a portion, a generally disc shaped upper portion 213b and four blades 217a-217d.
  • the lower portion 213a is concentric with, and connected to, the shaft 220 and has a larger diameter than the shaft 220.
  • the upper portion 213b is concentric with, and has the same diameter as, the lower portion 213a and is spaced apart from the lower portion 213a in the direction of rotation axis 222.
  • Each of the four blades 217a-217d extends between the lower and upper portions 213a, 213b in a direction parallel to the rotation axis 222.
  • Each of the four blades 217a-217d also extends in a radial direction that is generally perpendicular to the rotation axis 222, from a radially outer surface of the body 210c partially towards rotation axis 222. That is, the blades 217a-217d do not extend all the way to rotation axis 222 which passes through the centre of the body 210c.
  • Each of the blades 217a-217d is generally cuboidal in shape.
  • a smooth fillet is provided at each intersection between the blades 217a- 217d and the upper and lower portions 213a, 213b. These fillets help to distribute stresses over a larger area of the body 210c and may aid the aerodynamics of the body 210c.
  • the four blades 217a-217d are circumferentially spaced around the body 210c and are arranged symmetrically about the rotation axis 222. [00094] Since the blades 217a-217d do not extend all the way to the centre of the body 210c, the circumferential spacing of the blades 217a-217d provides four openings, which define two mutually perpendicular passageways 212a, 212b through the body 210c. Each passageway 212a, 212b extends in a direction that is generally perpendicular to, and passes through, the rotation axis 222.
  • each passageway 212a, 212b formed by the four openings will periodically align with the optical axis 12 of the radiation source SO.
  • each passageway 212a, 212b is intermittently aligned with the aperture 8, 22.
  • a different one of the two passageways 212a, 212b is aligned with the optical axis 12 and the aperture 8, 22.
  • Each passageway 212a, 212b will align with the optical axis 12 of the radiation source SO twice during each full revolution of the body 210c.
  • body 210c can rotate at half the rotational speed of body 210a.
  • All three of the example bodies 210a, 210b, 210c described above comprises: at least two blades extending in a radial direction that is generally perpendicular to the rotation axis 222; and at least one opening through the body, which defines at least one passageway through the body that extends in a direction that is generally perpendicular to, and passes through, the rotation axis 222.
  • At least some of the tin debris which is hit by the rotating blades of the body 210 may be vaporised.
  • the resulting tin gas from such tin debris may be carried back into the enclosure 9, 21 of the radiation source SO by a gas flow protection system.
  • at least some of the tin debris may rebound from the rotating blades of the body 210. This process may be elastic or, more likely, inelastic, with the individual tin particles fragmenting into smaller tin particles.
  • a debris catcher may be provided, which may form a tin particle trap.
  • the debris catcher may be arranged such that at least a portion of the tin debris rebounding from the rotating blades of the body 210 is directed towards the debris catcher.
  • the geometry and location of the debris catcher may therefore be dependent upon the shape of body 210.
  • two debris catchers are provided. A first one of the debris catchers may be arranged to collect a portion of tin debris that rebounds from the blades of the body 210 as they rotate as they sweep through a front portion (for example 216a in Figure 5) of the volume they sweet out.
  • a second one of the debris catchers may be arranged to collect a portion of tin debris that rebounds from the blades of the body 210 as they rotate as they sweep through a rear portion (for example 216b in Figure 5) of the volume they sweet out.
  • the rotatable shield 200 which may use a range of different shaped bodies (including but not limited to bodies 210a, 210b, 210c), is arranged such that its rotation axis 222 is perpendicular to the optical axis 12 of the radiation source SO.
  • Such an arrangement has a number of benefits. For example, it allows the drive mechanism 230 to lie outside of the optical path of the EUV radiation produced by the radiation source SO. Further, it allows the diameter of the body 210 to be decreased, which in turn reduces the level of internal stress that it experiences at a given rotational speed.
  • An arrangement wherein the angle between the rotation axis 222 of the rotatable shield 200 and the optical axis 12 of the radiation source SO is close to, but not exactly, 90° may comprise larger openings through the body 210 than the above described perpendicular arrangement. This may allow such a rotatable shield to operate without attenuating the radiation beam B. Alternatively, the openings of such an arrangement may taper outwards in a radial direction extending away from rotation axis 222 to ensure that the radiation beam B is not attenuated.
  • angles are formed between any two axes. These two angles are supplementary angles (i.e. they sum to 180°). In general one of these angles will be smaller than the other angle, with first angle in the range 0° to 90° and a second angle in the range 90° to 180°. In the present application, any reference to an angle between two axes is intended to refer to the smaller of the two angles formed by those two axes unless explicitly stated otherwise.
  • the rotatable shield 200 may act as a shutter for aperture 8, 22 in the sense that it may completely block the aperture 8, 22 for a portion of each revolution of the body 210.
  • the rotatable shield 200 may only partially block the aperture 8, 22 during each revolution of the body 210.
  • tin debris that passes through the rotatable shield 200 and into the illumination system IL will tend to be inclined at a relatively large angle to the optical axis 12. Therefore, such tin debris is unlikely to hit the facetted field mirror device 10 of the illumination system IL. This is advantageous since such tin debris will not damage the facetted field mirror device 10 and may not damage any other optical elements within the lithographic apparatus LA thus extending the lifetime of the lithographic apparatus LA.
  • Body 210a comprises a single passageway (formed by opening 212) and bodies 210b, 210c each comprises two passageways 212a, 212b. It will be appreciated that in alternative embodiments, more than two passageways may be provided through the body 210. Each such passageway may extend in a direction that is generally perpendicular to, and passes through, the rotation axis 222. The more than two passageways may be arranged symmetrically about rotation axis 222.
  • Body 210a comprises two blades 214a, 214b and bodies 210b, 210c each comprises four blades 215a-215d, 217a-217d. It will be appreciated that in alternative embodiments, the body 210 may have three or more than four blades. Each of the blades may extend in a radial direction that is generally perpendicular to the rotation axis 222. The blades may be arranged symmetrically about rotation axis 222.
  • FIG 8 shows a second embodiment of a rotatable shield 300, which may form the rotatable shield 100 that is provided at the aperture 8, 22 in the wall of the enclosure 9, 21 of each of the radiation sources SO.
  • the rotatable shield 300 comprises a generally disc shaped body 310 and a drive mechanism 320.
  • the drive mechanism 320 is operable to rotate the body 310 about a rotation axis 330.
  • the drive mechanism 320 may for example comprise a motor that is arranged to rotate a shaft 322 that is connected to the body 310.
  • the drive mechanism 320 may further comprise one or more bearings (not shown) to support said shaft.
  • the rotatable shield 300 is arranged such that its rotation axis 330 is arranged at an oblique angle 332 to the optical axis 12 of the radiation source SO.
  • One benefit of such an arrangement is that the drive mechanism 320 can lie outside of the optical path of the EUV radiation produced by the radiation source SO.
  • the body 310 is provided with an opening in the form of a circumferentially extending slit 312.
  • the slit 312 forms a passageway through the body 310.
  • the body 310 is arranged such that as it rotates, the slit 312 will periodically pass through, and align with, the optical axis 12 of the radiation source SO.
  • the alignment of the slit 312 with the optical axis 12 of the radiation source SO occurs once during each full revolution of the body 310.
  • the rotation of the body 310 is synchronised with the generation of EUV radiation pulses so as to avoid EUV losses caused by the rotatable shield 300.
  • the rotation is timed such that the slit 312 is aligned with the optical axis 12 of the radiation source SO as each pulse of EUV radiation reaches the body 310.
  • the passageway formed by slit 312 is therefore intermittently aligned with the aperture 8, 22 as each pulse of radiation reaches the shield 300. This allows the pulses of radiation to pass through the aperture 8, 22.
  • the rotatable shield 300 extends across the full cross section of the radiation near the intermediate focus 6. Therefore, when no pulse is incident, the body 310 forms a mechanical shutter that blocks the aperture 8, 22 of the radiation source SO to mitigate propagation of tin debris into the illumination system IL. That is, in between consecutive pulses of radiation the body 310 covers the aperture 8, 22.
  • an oscillator of the fuel emitter 3 may be used to ensure that rotation of the body 310 is synchronised with the generation of EUV radiation pulses.
  • the frequency at which the body 310 should rotate is equal to the repetition rate of EUV pulse generation within radiation source SO. For example, if the repetition rate of the radiation source SO is 100 kHz then the body 310 will rotate at rate of 100,000 revolutions per second or, equivalently, 6,000,000 revolutions per minute (rpm).
  • the radiation source SO may be configured such that if the rotation of the body 310 is not synchronised with the generation of EUV radiation pulses, generation of EUV radiation is stopped. This avoids the EUV radiation damaging the body 310.
  • the fuel emitter 3 should be started and the rotatable shield 300 should be speed up to the same frequency as, and synchronised with, the fuel emitter 3.
  • the laser 1, 18 may be brought into synchronisation with the fuel emitter 3 to produce EUV radiation.
  • the diameter of the body 310 may, for example, be in the range 50-200 mm.
  • the size and/or shape of the slit 312 may be dependent upon the stability of the repetition rate of radiation source SO (which, in turn, may be dependent upon the stability of droplet generation within fuel emitter 3).
  • the extent of the slit 312 in a direction perpendicular to the optical axis 12 of the radiation source SO may be approximately equal to, or slightly larger than, the diameter of aperture 8, 22. It will be appreciated that the extent of the slit 312 in a direction perpendicular to the optical axis 12 of the radiation source SO is given by the radial extent of the slit 312 (i.e.
  • the apertures 8, 22 may for example have a diameter of around 6 mm and the extent of the slit 312 in a direction perpendicular to the optical axis 12 of the radiation source SO may be around 6-10 mm.
  • the circumferential extent of the slit 312 may be slightly larger than the diameter of aperture 8, 22 and may include a sufficient margin to account for short term jitter of the repetition rate of radiation source SO so as to ensure that each pulse of the EUV radiation passes through the slit 312 without being attenuated.
  • the apertures 8, 22 may have a diameter of around 6 mm and the circumferential extent of the slit 312 may be around 6-20 mm.
  • a mitigation factor of the rotatable shield 300 may be defined as the fraction of tin that is prevented from passing through the aperture 8, 22. Assuming a generally constant rate of tin incident on the aperture 8, 22, the mitigation factor is the fraction of time that the aperture 8, 22 is blocked. In general, this depends on the diameter of the body 310 (or at least the radial position of the slit 312) and the circumferential extent of the slit.
  • the body 310 has a diameter of 100 mm, the extent of the slit 312 in the direction perpendicular to the optical axis 12 of the radiation source SO is 6 mm, the circumferential extent of the slit 312 is 10 mm and the slit is disposed at a radius of 40 mm.
  • the slit will be aligned with the aperture 8, 22 for a fraction of time given by the ratio of the circumferential extent of the slit 312 to the circumference of a circle 314 on which the slit 312 lies (given by 2 ⁇ , where r is the radius of the circle 314).
  • the slit will be aligned with the aperture 8, 22 for a fraction of time given by 10/(2 ⁇ 40) ⁇ 4%. Therefore, assuming a generally constant rate of tin incident on the aperture 8, 22, the mitigation factor of this exemplary embodiment is approximately 96%.
  • the body 310 may be formed from any suitable material.
  • the material from which the body 310 is formed should be strong enough to withstand the high internal stresses that are experienced at the high rotational speeds that it experiences.
  • the internal stresses experienced by the body 310 will depend on its shape (in particular its diameter), its mass, and the rotational speed that it operates at.
  • the material from which the body 310 is formed may have a high enough ultimate tensile strength to withstand the internal stresses that it experiences.
  • material from which the body 310 is formed may have an ultimate tensile strength of greater than 1 GPa.
  • the material from which the body 310 is formed may be as light as possible, again to facilitate its rotation at high rotational speeds.
  • the material from which the body 310 is formed may have a low density or specific weight (weight per unit volume). Further, the material from which the body 310 is formed may be suitable for use at the high temperatures experienced within the radiation source SO. Further, the material from which the body 310 is formed may have a high resistance to corrosion from chemicals present within the radiation source SO such as, for example, tin, hydrogen gas and hydrogen radicals. Suitable materials may include: carbon fibre reinforced polymer such as, for example that marketed under the name TORAYCA (registered trade mark) T1100G available from Toray Industries, Inc.
  • TORAYCA registered trade mark
  • Liquidmetal a metal alloy such as, for example, steel, which may be tempered to increase its strength
  • Liquidmetal amorphous metal alloy
  • the rotatable shield 300 is provided with a debris catcher 340, which forms a tin particle trap.
  • the debris catcher is positioned such that at least a portion of the tin debris rebounding from the rotating blades of the body 310 is received by the debris catcher.
  • the placement of the debris catcher 340 depends on the angle 332 between rotation axis 330 and the optical axis 12 of the radiation source SO.
  • the debris catcher 340 may be disposed such that a line joining the debris catcher 340 to the surface of the body 310 forms approximately the same angle with the normal to the surface of the body 340 as does the optical axis 12 of the radiation source SO.
  • Debris catcher may comprise, for example, a molybdenum (Mo) plate.
  • the molybdenum plate may be shielded by tin.
  • the molybdenum plate may be heated above the melting point of tin, for example, to a temperature of around 300°C.
  • the molybdenum plate may be arranged such that tin rebounding from body 310 is incident on the molybdenum plate at a grazing incidence angle.
  • a debris catcher with any or all of the above mentioned features may be provided in combination with the rotatable shield 200 shown in Figure 3.
  • rotatable shield 300 may be used in combination with a gas flow protection system 400.
  • the gas flow protection system may comprise a plurality of gas injection points 410 disposed around the perimeter of the aperture 8, 22.
  • a gas 420 such as, for example, hydrogen is directed towards the optical axis 12 from each of the gas injection points 410.
  • the gas 420 may be provided at low pressure.
  • the gas 420 flows into the enclosing structure 9, 21 of the radiation source SO, carrying some tin debris with it. Therefore the gas flow protection system can help to reduce the level of tin debris passing into the illumination system IL of the lithographic apparatus LA.
  • the body 310 may be provided with a plurality of openings in the form of a plurality of circumferentially extending slits.
  • the plurality of circumferentially extending slits may be distributed evenly about rotation axis 330.
  • Each slit may form a passageway through the body 310.
  • the body 310 may be arranged such that as it rotates, each of the plurality of slits will periodically pass through, and align with, the optical axis 12 of the radiation source SO. The alignment of each slit with the optical axis 12 of the radiation source SO occurs once during each full revolution of the body 310.
  • the rotation is timed such that one of the plurality of slits is aligned with the optical axis 12 of the radiation source SO as each pulse of EUV radiation reaches the body 310.
  • the passageway formed by each slit is therefore intermittently aligned with the aperture 8, 22 as a pulse of radiation reaches the shield 300. This allows the pulses of radiation to pass through the aperture 8, 22.
  • the body 310 covers the aperture 8, 22.
  • the frequency at which the body 310 should rotate is equal to the repetition rate of EUV pulse generation within radiation source SO divided by the number of slits provided in body 310. Therefore the provision of a plurality of slits will decrease the rotational speed that the body 310 should rotate at to ensure synchronization with the radiation source. For example, if the repetition rate of the radiation source SO is 100 kHz and the body 310 comprises two slits then the body 310 will rotate at rate of 50,000 revolutions per second or, equivalently, 3,000,000 revolutions per minute (rpm).
  • the optical axis of a radiation source SO may be the optical axis of its radiation collector 5, 20.
  • the optical axis of a radiation collector may be a line along which the radiation collector has some degree of rotational symmetry. Additionally or alternatively, the optical axis of a radiation collector may be a line which passes through the centre of the radiation collector, along which radiation is directed by the radiation collector.
  • the term “intermittently” may be considered to mean alternatively starting and stopping. Therefore the phrase “the passageway is intermittently aligned with the aperture” may be considered to mean that the passageway alternates between being aligned with the aperture and not being aligned with the aperture. Similarly, the term “periodically” may be considered to mean recurring at regular intervals of time. Therefore the phrase “the passageway periodically aligns with the optical axis of the radiation source” may be considered to mean that the passageway aligns with the optical axis of the radiation source at regular intervals of time.
  • the term "the body at least partially covers the aperture” may be considered to mean that the body extends over a portion of the aperture such that particles and/or light cannot pass through said portion of the aperture.
  • a radiation source SO are operable to produce a series of pulses of radiation.
  • the term in between "consecutive pulses of radiation” may be considered to mean two adjacent pulses in said series, i.e. one pulse of radiation and the next pulse of radiation.
  • Embodiments of a radiation source SO have been described above which include a shield which is arranged to reduce an amount of tin debris which passes through an aperture 8, 22 of the radiation source SO.
  • Tin debris is generally emitted from the plasma formation region 4 at which a tin plasma is produced.
  • tin debris may be emitted in other directions.
  • tin debris may be emitted in any direction from the plasma formation region 4 and may be incident on any surface of the radiation source SO.
  • Tin debris which is incident on a surface of the radiation source SO may rebound from the surface and may be directed towards another component of the radiation source SO.
  • tin debris which has rebounded from a surface of the radiation SO may be directed towards an aperture 8, 22 in the radiation source SO or a radiation collector 5, 20, either directly or indirectly having rebounded further from one or more other surfaces.
  • FIG. 10 is a schematic illustration of a radiation source SO which includes a debris collector 401.
  • the radiation source SO comprises a fuel emitter 3, a radiation collector 5, and the debris collector 401.
  • the radiation source SO is housed within an enclosing structure 9.
  • the radiation collector 5 which is shown in Figure 10 is a normal incidence radiation collector 5 of the type which is shown in the embodiment of Figure 1.
  • the fuel emitter 3 emits droplets of fuel 31 and directs the fuel 31 towards a plasma formation region 4.
  • the fuel 31 may, for example comprise tin (Sn). Although tin is referred to in the following description, any suitable fuel 31 may be used.
  • a laser 1 is arranged to deposit energy via a laser beam 2 into the tin 31 at the plasma formation region 4.
  • the deposition of laser energy into the tin 31 creates a plasma 7 at the plasma formation region 4.
  • Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma.
  • Radiation which is emitted from the plasma 7 is reflected at the radiation collector 5 to form a radiation beam B.
  • the radiation beam B is focused at an intermediate focus 6.
  • the radiation source SO is arranged such that the intermediate focus 6 is located at or near to an aperture 8 in the enclosing structure 9 of the radiation source SO.
  • the intermediate focus 6 and the plasma formation region both lie on an optical axis 12 of the radiation source SO.
  • the optical axis 12 is arranged at an angle a with respect to the vertical V. In other embodiments the angle a between the vertical V and the optical axis 12 may be different to the angle a which is shown in Figure 10.
  • the radiation source SO which is shown in Figure 10 may include one or more features which were described above in connection with other embodiments of a radiation source.
  • the radiation source SO may further include a shield which is configured to reduce the amount of tin debris which passes through the aperture 8 in the enclosing structure 9.
  • the debris collector 401 comprises a plurality of columns of tiles 403.
  • the columns of tiles 403 are arranged around the optical axis 12 and around the radiation beam B so as to allow the radiation beam B to pass from the radiation collector 5 to the aperture 8.
  • the columns of tiles 403 are configured to collect debris which is emitted from the plasma formation region 4.
  • the debris may be directly incident on the debris collector 401 or may travel to the debris collector 401 indirectly having rebounded further from one or more other surfaces.
  • FIG 11 is a schematic illustration of a portion of a column of tiles 403 which may form part of the debris collector 401.
  • the portion of a column of tiles 403 which is shown in Figure 10 comprises a first tile 405a, a second tile 405b and a third tile 405c.
  • each of the tiles 405a-405c have a generally v-shaped cross-section such that they include a ridge 409.
  • one or more of the tiles 405a-405c may have a cross- sectional shape other than a v-shape.
  • tiles 405a-405c may form an intertwined array (a column) of L-shaped tiles which are an optically closed, yet physically open. Tin particles expelled from the plasma location will collide and be redirected or will attach to the surface of the tiles where multiple particles may coalescence or merge (aggregate) and drip-off downwards onto a tin debris catcher positioned so as to receive at least a portion of debris.
  • the tiles 405a-405c may have other shapes such as S-, C-, I-, J- shaped, which may be staggered together to form a labyrinth-like passage.
  • the tiles are staggered predominantly in a radial direction.
  • several tiles can be fitted with holes, ribs, extensions or other features of any shape (on edges on either side), to allow control of a gas flow (speed and/or direction) through the physically open tiles.
  • An advantage of such an optically closed and physically open array of tiles is to allow blowing gas (for example via a center-cone flow and/or a perimeter flow) to easily be removed from the source vessel (e.g. sideways), therefore not allowing tin-contaminated gas to (re)circulate in the vessel, especially above the collector. Also, the use of an open array of tiles will results in a lighter structure.
  • the configuration of the array of tiles is rotationally symmetric, in which case in any direction seen from the plasma location there is no direct (linear) path to the outside (behind) of the array.
  • the configuration of the array of tiles can have a non-rotational symmetry of any shape, such as oval or square shaped wherein the array of tiles has an "optically closed, physically open" layout.
  • the plasma formation region 4 is not shown in Figure 11, however its approximate position is indicated by arrows which depict tin debris 407 travelling directly from the plasma formation region 4 and towards the column of tiles 403.
  • the tiles 405a-405c are arranged relative to each other and the plasma formation region 4 such that at least some of the tiles 405a-405c include a portion which is shielded from the plasma formation region 4 by another tile.
  • the first tile 405a is arranged between the plasma formation region 4 and a first portion 405b' of the second tile 405b.
  • the arrangement of the first tile 405a therefore results in there being no direct line of sight between the first portion 405b' of the second tile 405b and the plasma formation region 4.
  • the first portion 405b' of the second tile 405b therefore receives little or no tin debris directly from the plasma formation region since the first portion 405b' is shielded from the plasma formation region 4 by the first tile 405a.
  • the second tile 405b also includes a second portion 405b" which is arranged in a direct line of sight of the plasma formation region 4.
  • the second portion 405b" of the second tile 405b therefore receives tin debris directly from the plasma formation region 4.
  • the third tile 405c includes a first portion 405c' and a second portion 405c".
  • the first portion 405c' of the third tile 405c is shielded from the plasma formation region 4 by the second tile 405b such that there is no direct line of sight between the plasma formation region 4 and the first portion 405c' of the third tile 405c'.
  • the second portion 405c" of the third tile is arranged in a direct line of sight of the plasma formation region 4.
  • the second portion 405b" of the second tile 405b therefore receives tin debris directly from the plasma formation region 4.
  • the overlapping arrangement of the tiles 405a-405c forms channels 411a, 411b between the tiles 405a-405c.
  • a first channel 411a is formed between the first tile 405a and the second tile 405b and a second channel 411b is formed between the second tile 405b and the third tile 405c.
  • Tin debris which is incident on the tiles 405a-405c may be in a solid form, a liquid form and/or a gaseous form (e.g. tin vapour).
  • Liquid and/or solid tin which is incident on the tiles 405a-405c may be deposited on the tiles 405a-405c. Additionally and/or alternatively tin vapour may condense on the tiles 405a-405c to form tin droplets on the tiles.
  • Liquid tin which is present on the tiles 405a-405c may flow under the force of gravity along the tiles 405a-405c so as to run down the channels 41 la, 41 lb between the tiles 405a-405c.
  • the tiles 405a-405c are tilted with respect to the vertical V in order to facilitate the flow of liquid tin along the tiles 405a-405c under the force of gravity. Tin which flows down the channels 41 la, 41 lb may advantageously be collected and removed from the radiation source SO.
  • FIGS 12A-12C are schematic illustrations of the first 405a and second 405b tiles at different stages of heating of the second tile 405b.
  • Figure 12A depicts the tiles 405a, 405b when the second tile 405b is not being heated.
  • Tin 413 in a solid and/or a liquid form is deposited on the second tile 405b.
  • Tin 413 may also be deposited on the first tile 405a, however for ease of illustration tin 413 is only depicted as being present on the second tile 405b.
  • Tin 413 may, for example, reach the first portion 405b' of the second tile 405b having rebounded off another surface. Additionally or alternatively tin vapour may reach the first portion 405b' and may condense onto the first portion 405b' of the second tile 405b.
  • the temperature of the second tile 405b may be less than the melting point temperature of the tine 413.
  • the tin 413 which is present on the second tile 405b may therefore be in a solid state.
  • FIG 12B is a schematic illustration of the first and second tiles 405a, 405b after heating of the second tile 405b has commenced.
  • the second tile 405b is heated by a heater which is not shown in the Figures.
  • the heater may take any suitable form.
  • the second tile 405b is heated to a temperature which is greater than the melting point of tin 413 thereby causing tin 413 which is present on the second tile 405b to be melted.
  • the tin 413 melts it flows down the surface of the second tile 405b under the force of gravity.
  • the tin 413 may flow down the second tile 405b towards the ridge 409. Droplets of tin 413 may meet at the ridge 409 and coalesce into larger droplets.
  • two larger droplets 415 of tin which are formed by coalescence of smaller droplets are shown on the ridge 409 in Figure 12B.
  • FIG. 12C is a schematic illustration of the first and second tiles 405a, 405b after further heating of the second tile 405b.
  • droplets of tin continue to flow down the tile and coalesce into larger droplets.
  • the tin has coalesced into a single large droplet 415 which flows down the ridge 409 under the force of gravity.
  • the large droplet 415 may continue to flow down the ridge 409 and may be collected in a receptacle (not shown) which is arranged to receive tin which flows down the first channel 411a between the first and second tiles 405a, 405b. Collection of the tin may allow the tin to be removed from the radiation source SO.
  • tin droplets which drip from the second tile 405b may be caught by the first tile 405a.
  • tin droplets which drip from the second tile 405b may be caught by a backside 417a (as labelled in Figure 12C) of the first tile 405 a. This advantageously prevents the tin droplet from leaving the debris collector and contaminating other components of the radiation source SO.
  • catching tin droplets which drip from the second tile 405b advantageously prevents the tin droplets from dripping onto the radiation collector 5 and contaminating the radiation collector 5.
  • Tin droplets which are caught by the backside 417a of the first tile 405a may flow down the backside 417a of the first tile 405a and may be collected by a receptacle (not shown).
  • the first 405 a and/or the second 405b tiles may also be heated.
  • the location at which one or more tiles are heated and/or the rate at which the tiles are heated may be configured to bring about a desired flow of tin down the tiles. For example, more heat energy may be delivered to regions of a tile which receive the largest amount of tin debris and less heat energy may be delivered to regions of a tile which receive a smaller amount of tin debris.
  • One or more tiles may be heated continuously or may be heated intermittently.
  • a tile may be periodically heated so as to remove tin from the tile. In periods in between heating of a tile, tin may be allowed to accumulate on the tile before being removed during a subsequent heating period.
  • a debris collector 401 which comprises a plurality of tiles as described above may include a thermal mass, which is heated to a temperature which is greater than the melting point of tin, which is smaller than an equivalent thermal mass in a prior art debris collector.
  • a reduced thermal mass advantageously reduces the heating power and/or time which is needed to heat the mass to a temperature which is greater than the melting point of tin.
  • the radiation source SO may not be operational during cleaning of debris from the debris collector 401 by heating the debris collector 401.
  • the radiation source SO may be placed in an offline state in which no radiation beam B is provided in order to heat the debris collector 401 and clean debris from the debris collector. Reducing the amount of time which it takes to heat portions of the debris collector 401 to a temperature which is greater than the melting point of tin advantageously reduces the amount of time during which the radiation source SO is in an offline state.
  • Channels which are formed between tiles provide a space which may be used for other purposes.
  • one or more tin scrubbers may be located between adjacent tiles and in a channel between the tiles.
  • a tin scrubber is operable to clean tin vapour from the environment in the radiation source SO and may be used to remove tin vapour from the radiation source SO.
  • Positioning a tin scrubber in a channel between adjacent tiles advantageously shields the scrubber from solid and/or liquid tin debris which travels from the plasma formation location 4.
  • Shielding the scrubber from solid and/or liquid tin debris advantageously reduces any clogging of the scrubber with solid and/or liquid tin.
  • the channels between tiles may provide an increased amount of space in which to position scrubbers (when compared to prior art radiation sources SO).
  • the total surface area of the scrubbers may therefore be increased which advantageously allows the rate at which tin vapour is cleaned from the radiation source SO to be increased.
  • Channels between tiles may additionally or alternatively be used to introduce a gas flow in to the radiation source SO.
  • gas supplies may be configured to introduce a gas flow along one or more channels between tiles.
  • the gas may be suitable for cleaning components of the radiation source.
  • a flow of hydrogen gas and/or hydrogen radicals may be introduced into the radiation source SO.
  • Hydrogen may act to clean contamination from optical components of the radiation source SO.
  • hydrogen may clean tin from the radiation collector 5.
  • a debris collector may comprise a plurality of columns of tiles 403.
  • Figures 13A and 13B are schematic illustrations of two example arrangements of three adjacent columns of tiles 403a, 403b, 403c.
  • a second row of tiles 403b is located between a first row of tiles 403a and a third row of tiles 403c.
  • the position of the tiles which form the second row of tiles 403b is vertically offset relative to the tiles which form the first 403a and third 403c rows of tiles.
  • adjacent tiles in the first 403a, second 403b and third 403c rows of are located at the same vertical level.
  • the tiles may be arranged differently than has been described above and is depicted in the Figures.
  • the size, shape, position and/or orientation of the tiles may be configured to optimise the amount of debris which is collected by the debris collector 401.
  • the tiles may be configured to allow tin to flow along the tiles for removal from the radiation source SO.
  • the size, shape and/or orientation of the tiles may be different at different positions on the debris collector 401.
  • tiles which are located further away from the plasma formation region 4 may have a different size, shape and/or orientation to tiles which are located closer to the plasma formation region 4.
  • tiles which are located further away from the plasma formation region 4 may be smaller than tiles which are located closer to the plasma formation region 4.
  • the tilt of the tiles with respect to the vertical V may be different at different locations in the debris collector 401. For example, tiles which are located further away from the plasma formation region 4 may be tilted at a different angle with respect to the vertical V to tiles which are located closer to the plasma formation region 4. In some embodiments the tilt of the tiles may vary at different angular positions around the debris collector 401 with respect to the optical axis 12. As was described above with reference to Figure 10, the optical axis may be tilted at an angle a with respect to the vertical V. In some embodiments the tilt of the tiles with respect to the vertical V may be approximately the same at different angular positions around the optical axis 12 such that liquid tin may flow along each of the tiles under the force of gravity. In embodiments in which the optical axis 12 is tilted with respect to the vertical V, the tilt of the tiles with respect to the optical axis 12 may therefore be different at different angular positions around the optical axis 12.
  • Embodiments of a debris collector 403 have been described above with reference to its use in a radiation source SO comprising a normal incidence radiation collector 5 (as shown in Figure 10).
  • a debris collector 403 of the type described herein may additionally or alternatively be used in a radiation source SO which comprises a grazing incidence radiation collector (e.g. of the type shown in Figure 2).
  • a debris collector 403 may be suitable for use in any form of radiation source.
  • a debris collector Whilst embodiments of a debris collector have been described above in the context of collecting tin debris, some embodiments may be suitable for collecting debris other than tin debris. For example, in a radiation source SO in which a fuel other than tin is used the debris collector may be suitable for collecting fuel debris which is emitted from the fuel.
  • the invention may form part of a mask inspection apparatus.
  • the mask inspection apparatus may use EUV radiation to illuminate a mask and use an imaging sensor to monitor radiation reflected from the mask. Images received by the imaging sensor are used to determine whether or not defects are present in the mask.
  • the mask inspection apparatus may include optics (e.g. mirrors) configured to receive EUV radiation from an EUV radiation source and form it into a radiation beam to be directed at a mask.
  • the mask inspection apparatus may further include optics (e.g. mirrors) configured to collect EUV radiation reflected from the mask and form an image of the mask at the imaging sensor.
  • the mask inspection apparatus may include a processor configured to analyse the image of the mask at the imaging sensor, and to determine from that analysis whether any defects are present on the mask.
  • the processor may further be configured to determine whether a detected mask defect will cause an unacceptable defect in images projected onto a substrate when the mask is used by a lithographic apparatus.
  • the invention may form part of a metrology apparatus.
  • the metrology apparatus may be used to measure alignment of a projected pattern formed in resist on a substrate relative to a pattern already present on the substrate. This measurement of relative alignment may be referred to as overlay.
  • the metrology apparatus may for example be located immediately adjacent to a lithographic apparatus and may be used to measure the overlay before the substrate (and the resist) has been processed.
  • Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.
  • EUV radiation may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.
  • Figures 1 and 2 depict the radiation source SO as a laser produced plasma LPP source
  • any suitable source may be used to generate temporally well resolved pulses of EUV radiation.
  • EUV emitting plasma may be produced by using an electrical discharge to convert fuel (e.g. tin) to a plasma state.
  • a radiation source of this type may be referred to as a discharge produced plasma (DPP) source.
  • the electrical discharge may be generated by a power supply which may form part of the radiation source or may be a separate entity that is connected via an electrical connection to the radiation source SO.

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Abstract

Une source de rayonnement (SO), qui est utilisable pour produire des impulsions de rayonnement, comprend : une paroi (9,21) avec une ouverture (8,22) ; un collecteur de rayonnement; un écran rotatif (200); et un mécanisme d'entraînement (230) agencé pour faire tourner l'écran autour d'un axe de rotation (222). La source de rayonnement peut être une source de plasma produit par laser. Le collecteur de rayonnement est agencé de manière à diriger des impulsions de rayonnement le long d'un axe optique du collecteur de rayonnement et les impulsions de mise au point de rayonnement au niveau d'un foyer intermédiaire, ledit axe optique passant à travers l'ouverture et ledit foyer intermédiaire étant dans le voisinage de l'ouverture. L'écran comprend un corps (210) avec une ou plusieurs ouvertures, lesdites une ou plusieurs ouvertures formant une voie de passage s'étendant à travers le corps. Le mécanisme d'entraînement est agencé de façon à tourner l'écran de telle sorte que la voie de passage est alignée par intermittence avec l'ouverture au fur et à mesure que chaque impulsion de rayonnement atteint l'écran pour permettre aux impulsions de rayonnement de passer à travers l'ouverture dans la paroi, et le corps recouvre au moins partiellement l'ouverture dans entre des impulsions consécutives de rayonnement. L'axe de rotation de l'écran est généralement perpendiculaire à l'axe optique du collecteur de rayonnement.
PCT/EP2015/070109 2014-10-13 2015-09-03 Source de rayonnement WO2016058746A1 (fr)

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