NL2014430A

NL2014430A - Radiation Source.

Info

Publication number: NL2014430A
Application number: NL2014430A
Authority: NL
Inventors: Roelof Loopstra Erik; Yevgenyevich Banine Vadim; Rutgerus Bartraij Petrus; Ludovicus Donker Rilpho; Antonius Gerardus Akkermans Johannes
Original assignee: Asml Netherlands Bv
Priority date: 2014-03-13
Filing date: 2015-03-10
Publication date: 2015-11-02
Also published as: TW201539151A; WO2015135912A2; WO2015135912A3

Description

Radiation Source

FIELD

[0001] The present invention relates to a radiation source for a lithographic system. In particular it relates to an EUV radiation source that incorporates a free electron laser.

BACKGROUND

[0002] A lithographic system comprises a radiation source and at least one lithographic apparatus. 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.

[0003] 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).

[0004] A lithographic apparatus may be provided with radiation from a radiation source which forms part of a lithographic system. The radiation source may comprise at least one free electron laser which emits EUV radiation.

[0005] It is desirable to produce a radiation source suitable for providing a lithographic apparatus with radiation which obviates or mitigates one or more of the problems associated with known radiation sources.

SUMMARY

[0006] According to a first aspect there is provided a free electron laser comprising an electron source operable to emit a beam of electrons, a particle accelerator operable to accelerate the beam of electrons to relativistic energies, an undulator operable to guide the relativistic electron beam along a periodic path so as to stimulate emission of coherent radiation and a steering unit operable to alter the trajectory of the electron beam so as to direct the electron beam from a first path along which the electron beam propagates substantially in a first direction to a second path along which the electron beam propagates substantially in a second direction, wherein the first path and the second path are vertically separated from one another.

[0007] Vertically separating the first path from the second path allows the width of a region which the free electron laser occupies to be advantageously reduced. This is because the components of the free electron laser may be positioned over different vertical levels. A reduction in the width of the region which the free electron laser occupies, also allows the total area of the region to be decreased. A reduction of the width and the area of the region is advantageous in an arrangement in which the free electron laser forms part of a radiation source for a lithographic system.

[0008] The particle accelerator may be operable to accelerate the beam of electrons to energies greater than about 10 MeV.

[0009] The particle accelerator may be operable to accelerate the beam of electrons to energies greater than about 100 MeV.

[0010] The first and second directions may oppose each other.

[0011] The particle accelerator may accelerate the electron beam along the first path.

[0012] The undulator may be operable to guide the relativistic electron beam along the second path.

[0013] The linear accelerator may be positioned on a first vertical level.

[0014] The undulator may be positioned on a second vertical level.

[0015] The steering unit may be operable to alter the trajectory of the electron beams so as to direct the electron beam from the particle accelerator on the first vertical level to the undulator on the second vertical level.

[0016] The first vertical level may be situated above the second vertical level.

[0017] At least some of the components of the free electron laser may be housed within a building.

[0018] The first vertical level may correspond to a first vertical level of the building and the second vertical level may correspond to a second vertical level of the building.

[0019] The building may comprise radiation shielding walls configured to prevent harmful radiation generated by the free electron laser from propagating out of the building.

[0020] The free electron laser may further comprise a second steering unit operable to alter the trajectory of the electron beam so as to direct the electron beam from the second path to the first path.

[0021] The second steering unit may be operable to direct the electron beam which is output from the undulator to the particle accelerator.

[0022] The particle accelerator may be operable to decelerate the electron beam which is output from the undulator.

[0023] The particle accelerator may be operable to recover energy from the decelerating electron beam and use the recovered energy to accelerate the electron beam received from the electron source.

[0024] The free electron laser may further comprise a bunch compressor configured to spatially compress electrons in the electron beam.

[0025] The bunch compressor may be positioned upstream of the steering unit and downstream of the undulator.

[0026] The bunch compressor may be positioned on the same vertical level as the vertical level on which the undulator is positioned.

[0027] The components of the free electron laser which are positioned on the first vertical level and components of the free electron laser which are positioned on the second vertical level may be situated in the same room.

[0028] The particle accelerator and the undulator may be situated in the same room.

[0029] The free electron laser may further comprise a crane, wherein the crane is arranged such that it can access components of the free electron laser which are situated on the first vertical level and components of the free electron laser which are situated on the second vertical level.

[0030] The particle accelerator and the undulator may be offset from each other on a horizontal axis which is perpendicular to a longitudinal axis of the particle accelerator. For example, the majority of an extent along a longitudinal axis of the particle accelerator may be positioned at a first position on a horizontal axis, wherein the horizontal axis is perpendicular to the longitudinal axis of the particle accelerator and the majority of an extent along a longitudinal axis of the undulator may be positioned at a second position on the horizontal axis which is different to the first position.

[0031] The free electron may further comprise components configured to provide cryogenic cooling to the particle accelerator.

[0032] The cryogenic cooling components may be positioned outside of the building in which the free electron laser is housed.

[0033] The cryogenic cooling components may be housed inside the building in which the free electron laser is housed.

[0034] A portion of the building in which the cryogenic cooling components are housed may be mechanically isolated from a portion of the building in which the particle accelerator and the undulator are housed.

[0035] The free electron laser may further comprise at least one radiation shielding wall configured to shield a portion of the building in which the cryogenic cooling components are housed from a portion of the building in which the particle accelerator and the undulator are housed.

[0036] The free electron laser may further comprise electrical components configured to provide electrical power to components of the free electron laser.

[0037] The electrical components may be positioned outside of the building in which the free electron laser is housed.

[0038] The electrical cooling components may be housed inside the building in which the free electron laser is housed.

[0039] The free electron laser may further comprise at least one radiation shielding wall configured to shield a portion of the building in which the electrical cooling components are housed from a portion of the building in which the particle accelerator and the undulator are housed.

[0040] The undulator may be configured to cause the relativistic electrons to emit EUV radiation.

[0041] The particle accelerator may be a linear accelerator.

[0042] The free electron laser may comprise a plurality of particle accelerators operable to accelerate the beam of electrons.

[0043] At least one of the linear accelerators may be positioned on a different vertical level to at least one of the other linear accelerators.

[0044] The free electron laser may comprise a plurality of electron sources each operable to emit a beam of electrons.

[0045] The free electron laser may further comprise an electron beam merger configured to merge a plurality of electron beams emitted from a plurality of electron sources into a single electron beam.

[0046] The plurality of electron sources may be positioned on a different vertical level to the vertical level on which the linear accelerator is positioned.

[0047] The plurality of electron sources may be positioned on a different vertical level to the vertical level on which the undulator is positioned.

[0048] According to a second aspect there is provided a lithographic system comprising a radiation source comprising a free electron laser according to the first aspect and one or more lithographic apparatus.

[0049] The lithographic system may comprise a plurality of lithographic apparatus each arranged to receive radiation from the radiation source.

[0050] The plurality of lithographic apparatus may be located in a building and the radiation source may be located outside of the building in which the plurality of lithographic apparatus are located.

[0051] The lithographic system may further comprise a beam splitting apparatus configured to receive a radiation beam from the radiation source and split the radiation beam into branch radiation beams and which is further configured to provide a branch radiation beam to each of the plurality of lithographic apparatus.

[0052] The radiation source may comprise a plurality of free electron lasers according to the first aspect.

[0053] The radiation source may further comprise an optical system configured to receive a radiation beam from each of the plurality of free electron lasers and form a composite radiation beam from the radiation beams, and the optical system may be further configured to provide the composite radiation beam to the beam splitting apparatus.

[0054] Each of the plurality of free electron lasers may be configured to output a radiation beam on substantially the same vertical level.

[0055] The free electron lasers may be configured to output a radiation beam which propagates at an angle with respect to an optical axis of the radiation source.

[0056] The radiation source may be configured to emit a first radiation beam and a second radiation beam.

[0057] The radiation source may comprise a first free electron laser configured to emit the first radiation beam and a second free electron laser configured to emit the second radiation beam.

[0058] The radiation source may further comprise one or more optical components which are operable to split the radiation beam which is emitted from the first free electron laser so as to form the first radiation beam and the second radiation beam in the event that the second free electron laser ceases to emit radiation.

[0059] The lithographic system may comprise a plurality of rows of lithographic apparatus which each extend in a first direction.

[0060] Each row of lithographic apparatus may be provided with a beam splitting apparatus configured to receive a radiation beam, split the radiation beam into a plurality of branch radiation beams and direct each branch radiation beam to a lithographic apparatus of the row of lithographic apparatus.

[0061] The radiation source may be configured to emit a radiation beam in a second direction, wherein the second direction is not parallel with the first direction.

[0062] The second direction may be perpendicular to the first direction.

[0063] The radiation source may be configured to emit a radiation beam and the lithographic system may further comprise a beam splitting unit configured to split the radiation beam into a plurality of sub-beams wherein each sub-beam is provided to a row of lithographic apparatus.

[0064] According to a third aspect there is provided a plurality of lithographic systems, wherein each of the plurality of lithographic systems comprises a lithographic system according to the second aspect.

[0065] The plurality of lithographic systems may be arranged adjacent to one another.

[0066] According to a fourth aspect there is provided a method for producing radiation comprising emitting a beam of electrons, accelerating the beam of electrons to relativistic energies along a first path and substantially in a first direction, altering the trajectory of the relativistic electrons from the first path such that the electrons propagate along a second path and substantially in a second direction, wherein the first path and the second path are vertically separated from one another and causing the relativistic electrons to follow a periodic path thereby causing them to stimulate emission of coherent radiation.

[0067] Accelerating the beam of electrons to relativistic energies may comprise accelerating the beam of electrons to energies which are greater than about 10 MeV.

[0068] Accelerating the beam of electrons to relativistic energies may comprise accelerating the beam of electrons to energies which are greater than about 100 MeV.

[0069] The first and second directions may oppose each other.

[0070] The method may further comprise directing an electron beam from the second path to the first path.

[0071] The method may further comprise decelerating an electron beam which is directed from the second path to the first path.

[0072] The method may further comprise recovering energy from the decelerating electron beam and using the energy to accelerate an electron beam along the first path.

[0073] The method may further comprise spatially compressing electrons of the relativistic electron beam.

[0074] The relativistic electron beam may be spatially compressed after having been directed from the first path to the second path and prior to causing the relativistic electrons to follow a periodic path.

[0075] The electron beam may be accelerated along a longitudinal axis of a particle accelerator and the relativistic electrons may be caused to follow a periodic path about a longitudinal axis of an undulator.

[0076] The longitudinal axis of the particle accelerator may be offset from the longitudinal axis of the undulator on a horizontal axis which is perpendicular to the longitudinal axis of the particle accelerator.

[0077] For example, the majority of an extent of the particle accelerator may be positioned at a first position on a horizontal axis which is perpendicular to the longitudinal axis of the particle accelerator. The majority of an extent of the undulator may be positioned at a second position on the horizontal axis which is different to the first position.

[0078] The relativistic electrons may emit EUV radiation.

[0079] The first, second, third and fourth aspects of the invention may include one or more features of any of the other aspects of the invention

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 is a schematic illustration of a lithographic system comprising a radiation source and a plurality of lithographic apparatus;

Figure 2 is a schematic illustration of a lithographic apparatus that forms part of the lithographic system of Figure 1;

Figure 3 is a schematic illustration of a free electron laser;

Figure 4 is a schematic illustration of a lithographic system including a radiation source comprising two free electron lasers;

Figure 5 is a schematic illustration of an optical system;

Figure 6 is a schematic illustration of a free electron laser according to an embodiment of the invention in which the free electron laser is positioned on multiple vertical levels;

Figures 7a and 7b are schematic illustrations of front-on views of two embodiments of a free electron laser according to the invention;

Figure 8 is a schematic illustration of a free electron laser according to an alternative embodiment of the invention in which the free electron laser is positioned on multiple vertical levels;

Figure 9 is a schematic illustration of two lithographic systems according to an embodiment of the invention;

Figure 10 is a schematic illustration of two lithographic systems according to an alternative embodiment of the invention;

Figures 11A and 11B are schematic illustrations of an alternative embodiment of a free electron laser;

Figures 12A and 12B are schematic illustrations of a further alternative embodiment of a free electron laser;

Figures 13A and 13B are schematic illustrations of a still further alternative embodiment of a free electron laser;

Figures 14A and 14B are schematic illustrations of a still further alternative embodiment of a free electron laser;

Figures 15A and 15B are schematic illustrations of a still further alternative embodiment of a free electron laser;

Figures 16A and 16B are schematic illustrations of a still further alternative embodiment of a free electron laser;

Figures 17A and 17B are schematic illustrations of an embodiment of a lithographic system comprising two rows of lithographic apparatus;

Figures 18 is a schematic illustration of an embodiment of a lithographic system comprising four rows of lithographic apparatus;

Figure 19 is a schematic illustration of an alternative embodiment of a lithographic system comprising four rows of lithographic apparatus;

Figure 20 is a schematic illustration of a further alternative embodiment of a lithographic system comprising four rows of lithographic apparatus;

Figure 21 is a schematic illustration of a portion of a beam splitting apparatus; and

Figures 22A and 22B are schematic illustrations of an embodiment of an electron beam merger.

DETAILED DESCRIPTION

[0081] Figure 1 shows a lithographic system LS, comprising: a radiation source SO, a beam splitting apparatus 20 and a plurality of lithographic apparatus LA1-LA20. The radiation source SO comprises at least one free electron laser and is configured to generate an extreme ultraviolet (EUV) radiation beam B (which may be referred to as a main beam). The main radiation beam B is split into a plurality of radiation beams B^B^ (which may be referred to as branch beams), each of which is directed to a different one of the lithographic apparatus LArLA2o, by the beam splitting apparatus 20. The branch radiation beams B^B^ may be split off from the main radiation beam B in series, with each branch radiation beam being split off from the main radiation beam B downstream from the preceding branch radiation beam. The beam splitting apparatus may, for example, comprise a series of mirrors (not shown) which are each configured to split off a portion of the main radiation beam B into a branch radiation beam BrB20.

[0082] The branch radiation beams BrB20 are depicted in Figure 1 as being split off from the main radiation beam B such that the branch radiation beams BrB20 propagate in directions which are approximately perpendicular to the direction of propagation of the main radiation beam B. Flowever, in some embodiments the branch radiation beams BrB20 may instead be split off from the main radiation beam B such that an angle between the direction of propagation of each branch radiation beam B^B^ and the direction of propagation of the main radiation beam is substantially less than 90 degrees. This may allow mirrors of the beam splitting apparatus to be arranged such that the main radiation beam B is incident on the mirrors at an angle of incidence which is less than normal. This may advantageously decrease the amount of radiation which is absorbed by the mirrors and therefore increase the amount of radiation which is reflected from the mirrors and which is provided to the lithographic apparatus LA^LA^via the branch radiation beams BrB^.

[0083] The lithographic apparatus LArLA20 may all be positioned on the same vertical level. The vertical level on which the lithographic apparatus LA^LAso are positioned may be substantially the same vertical level as the vertical level on which the beam splitting apparatus 20 is positioned and on which the main beam B is received from the radiation source SO. Alternatively, the beam splitting apparatus 20 may direct at least some of the branch radiation beams to one or more different vertical levels on which at least some of the lithographic apparatus LArLA20 are positioned. For example, the main radiation beam B may be received by the beam splitting apparatus on a basement or ground floor vertical level. The beam splitting apparatus 20 may direct at least some branch radiation beams B^ B20 to a vertical level which is positioned above the beam splitting apparatus and on which at least some of the lithographic apparatus LA^U^o are positioned. The lithographic apparatus LA1-LA20 may be positioned on multiple vertical levels and as such the beam splitting apparatus 20 may direct the branch radiation beams B!-B20 to different vertical levels in order to be received by the lithographic apparatus LArLA2o.

[0084] The radiation source SO, beam splitting apparatus 20 and lithographic apparatus 1_A1-LA20 may all be constructed and arranged such that they can be isolated from the external environment. A vacuum may be provided in at least part of the radiation source SO, beam splitting apparatus 20 and lithographic apparatus LArLA20 so as to minimise the absorption of EUV radiation. Different parts of the lithographic system LS may be provided with vacuums at different pressures (i.e. held at different pressures which are below atmospheric pressure).

[0085] Figure 2 is a schematic depiction of a lithographic apparatus LA! of the lithographic system LS shown in Figure 1. 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 branch radiation beam B! that is received by the lithographic apparatus LA1 before it is incident upon the patterning device MA. The projection system PS is configured to project the branch 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 B1 with a pattern previously formed on the substrate W.

[0086] The branch radiation beam B! that is received by the lithographic apparatus LA! passes into the illumination system IL from the beam splitting apparatus 20 through an opening 8 in an enclosing structure of the illumination system IL. Optionally, the branch radiation beam B! may be focused to form an intermediate focus at or near to the opening 8.

[0087] 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 to form a patterned 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 illumination system IL may for example include an array of independently moveable mirrors. The independently moveable mirrors may for example measure less than 1mm across. The independently moveable mirrors may for example be MEMS devices.

[0088] Following reflection from the patterning device MA the patterned radiation beam Bn enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam Bn 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. Although the projection system PS has two mirrors 13, 14 in Figure 2, the projection system may include any number of mirrors (e.g. six mirrors).

[0089] In some embodiments a lithographic system LS may include one or more mask inspection apparatus (not shown). A mask inspection apparatus may include optics (e.g. mirrors) configured to receive a branch radiation beam 6^20 from the beam splitting apparatus 20 and direct the branch radiation beam at a mask MA. The mask inspection apparatus may further include optics (e.g. mirrors) configured to collect radiation reflected from the mask and form an image of the mask at an imaging sensor. The image received at the imaging sensor may be used to determine one or more properties of the mask MA. The mask inspection apparatus may, for example, be similar to the lithographic apparatus LA1 shown in Figure 2, with the substrate table WT replaced with an imaging sensor.

[0090] In some embodiments a lithographic system LS may include one or more Aerial Image Measurement System (AIMS) which may be used to measure one or more properties of a mask MA. An AIMS may, for example, be configured to receive a branch radiation beam BrB2o from the beam splitting apparatus 20 and use the branch radiation beam Bt^o to determine one or more properties of a mask MA.

[0091] The radiation source SO comprises a free electron laser FEL which is operable to produce a beam of EUV radiation. Optionally, the radiation source SO may comprise more than one free electron laser FEL.

[0092] A free electron laser comprises an electron source, which is operable to produce a bunched relativistic electron beam, and a periodic magnetic field through which the bunches of relativistic electrons are directed. The periodic magnetic field is produced by an undulator and causes the electrons to follow an oscillating path about a central axis. As a result of the acceleration caused by the magnetic fields the electrons spontaneously radiate electromagnetic radiation generally in the direction of the central axis. The relativistic electrons interact with radiation within the undulator. Under certain conditions, this interaction causes the electrons to bunch together into microbunches, modulated at the wavelength of radiation within the undulator, and coherent emission of radiation along the central axis is stimulated.

[0093] Figure 3 is a schematic depiction of a free electron laser FEL comprising an electron source 21, a linear accelerator 22, a steering unit 23 and an undulator 24. The electron source 21 may alternatively be referred to as an injector.

[0094] The electron source 21 is operable to produce a beam of electrons E. The electron source 21 may, for example, comprise a photo-cathode or a thermionic cathode and an accelerating electric field. The electron beam E is a bunched electron beam E which comprises a series of bunches of electrons. The electron beam E is accelerated to relativistic energies by the linear accelerator 22. In an example, the linear accelerator 22 may comprise a plurality of radio frequency cavities, which are axially spaced along a common axis, and one or more radio frequency power sources, which are operable to control the electromagnetic fields along the common axis as bunches of electrons pass between them so as to accelerate each bunch of electrons. The cavities may be superconducting radio frequency cavities. Advantageously, this allows: relatively large electromagnetic fields to be applied at high duty cycles; larger beam apertures, resulting in fewer losses due to wakefields; and for the fraction of radio frequency energy that is transmitted to the beam (as opposed to dissipated through the cavity walls) to be increased. Alternatively, the cavities may be conventionally conducting (i.e. not superconducting), and may be formed from, for example, copper. Other types of linear accelerators may also be used. For example, the linear accelerator 22 may comprise a laser accelerator, wherein the electron beam E passes through a focused laser beam and the electric field of the laser beam causes the electrons to accelerate.

[0095] The relativistic electron beam E which exits the linear accelerator 22 enters the steering unit 23. The steering unit 23 is operable to alter the trajectory of the relativistic electron beam E so as to direct the electron beam E from the linear accelerator 22 to the undulator 24. The steering unit 23 may, for example, comprise one or more electromagnets and/or permanent magnets configured to generate a magnetic field in the steering unit 23. The magnetic field exerts a force on the electron beam E which acts to alter the trajectory of the electron beam E. The trajectory of the electron beam E upon leaving the linear accelerator 22 is altered by the steering unit 23 so as to direct the electrons to the undulator 24.

[0096] In embodiments in which the steering unit 23 comprises one or more electromagnets and/or permanent magnets, the magnets may be arranged to form one or more of a magnetic dipole, a magnetic quadrupole, a magnetic sextupole and/or any other kind of multipole magnetic field arrangement configured to apply a force to the electron beam E. The steering unit 23 may additionally or alternatively comprise one or more electrically charged plates, configured to create an electric field in the steering unit 23 such that a force is applied to the electron beam E. In general the steering unit 23 may comprise any apparatus which is operable to apply a force to the electron beam E to alter its trajectory.

[0097] The steering unit 23 directs the relativistic electron beam E to the undulator 24. The undulator 24 is operable to guide the relativistic electrons along a periodic path so that the electron beam E interacts with radiation within the undulator 24 so as to stimulate emission of coherent radiation. Generally the undulator 24 comprises a plurality of magnets, which are operable to produce a periodic magnetic field which causes the electron beam E to follow a periodic path. As a result the electrons emit electromagnetic radiation generally in the direction of a central axis of the undulator 24. The undulator 24 may comprise a plurality of sections (not shown), each section comprising a periodic magnet structure. The electromagnetic radiation may form bunches at the beginning of each undulator section. The undulator 24 may further comprise a mechanism for refocusing the electron beam E such as, for example, a quadrupole magnet in between one or more pairs of adjacent sections. The mechanism for refocusing the electron beam E may reduce the size of the electron bunches, which may improve the coupling between the electrons and the radiation within the undulator 24, increasing the stimulation of emission of radiation.

[0098] As electrons move through the undulator 24, they interact with the electric field of the electromagnetic radiation in the undulator 24, exchanging energy with the radiation. In general the amount of energy exchanged between the electrons and the radiation will oscillate rapidly unless conditions are close to a resonance condition, given by:

(1) where lem is the wavelength of the radiation, is the undulator period, γ is the Lorentz factor of the electrons and K is the undulator parameter. A is dependent upon the geometry of the undulator 24: for a helical undulator A=1, whereas for a planar undulator A=2. In practice, each bunch of electrons will have a spread of energies although this spread may be minimised as far as possible (by producing an electron beam E with low emittance). The undulator parameter K is typically approximately 1 and is given by:

(2) where q and m are, respectively, the electric charge and mass of the electrons, B0 is the amplitude of the periodic magnetic field, and c is the speed of light.

[0099] The resonant wavelength kem is equal to the first harmonic wavelength spontaneously radiated by electrons moving through the undulator 24. The free electron laser FEL may operate in self-amplified stimulated emission (SASE) mode. Operation in SASE mode may require a low energy spread of the electron bunches in the electron beam E before it enters the undulator 24. Alternatively, the free electron laser FEL may comprise a seed radiation source, which may be amplified by stimulated emission within the undulator 24.

[00100] Electrons moving through the undulator 24 may cause the amplitude of radiation to increase, i.e. the free electron laser FEL may have a non-zero gain. Maximum gain may be achieved when the resonance condition is met or when conditions are close to but slightly off resonance.

[00101] An electron which meets the resonance condition as it enters the undulator 24 will lose (or gain) energy as it emits (or absorbs) radiation, so that the resonance condition is no longer satisfied. Therefore, in some embodiments the undulator 24 may be tapered. That is, the amplitude of the periodic magnetic field and/or the undulator period ku may vary along the length of the undulator 24 in order to keep bunches of electrons at or close to resonance as they are guided though the undulator 24. Note that the interaction between the electrons and radiation within the undulator 24 produces a spread of energies within the electron bunches. The tapering of the undulator 24 may be arranged to maximise the number of electrons at or close to resonance. For example, the electron bunches may have an energy distribution which peaks at a peak energy and the tapering maybe arranged to keep electrons with this peak energy at or close to resonance as they are guided though the undulator 24. Advantageously, tapering of the undulator has the capacity to significantly increase conversion efficiency. The use of a tapered undulator may increase the conversion efficiency (i.e. the portion of the energy of the electron beam E which is converted to radiation in the radiation beam B) by more than a factor of 2. The tapering of the undulator may be achieved by reducing the undulator parameter K along its length. This may be achieved by matching the undulator period ku and/or the magnetic field strength B0 along the axis of the undulator to the electron bunch energy to ensure that they are at or close to the resonance condition. Meeting the resonance condition in this manner increases the bandwidth of the emitted radiation.

[00102] After leaving the undulator 24, the electromagnetic radiation is emitted as a radiation beam B’. The radiation beam B’ comprises EUV radiation and may form all or part of the radiation beam B which is provided to the beam splitting apparatus 20 (depicted in

Figure 1) and which forms the branch radiation beams B^o which are provided to the lithographic apparatus LA^o- [00103] In the embodiment of a free electron laser which is depicted in Figure 3, the electron beam E’ which leaves the undulator 24 enters a second steering unit 25. The second steering unit 25 alters the trajectory of the electron beam E’ which leaves the undulator 24 so as to direct the electron beam E’ back through the linear accelerator 22. The second steering unit 25 may be similar to the steering unit 23 and may, for example, comprise one or more electromagnets and/or permanent magnets. The second steering unit 25 does not affect the trajectory of the radiation beam B' which leaves the undulator 24. The steering unit 25 therefore decouples the trajectory of the electron beam E’ from the radiation beam B’. In some embodiments, the trajectory of the electron beam E’ may be decoupled from the trajectory of the radiation beam B’ (e.g. using one or more magnets) before reaching the second steering unit 25.

[00104] The second steering unit 25 directs the electron beam E’ to the linear accelerator 22 after leaving the undulator 24. Electron bunches which have passed through the undulator 24 may enter the linear accelerator 22 with a phase difference of approximately 180 degrees relative to accelerating fields in the linear accelerator 22 (e.g. radio frequency fields). The phase difference between the electron bunches and the accelerating fields in the linear accelerator 22 causes the electrons to be decelerated by the fields. The decelerating electrons E’ pass some of their energy back to the fields in the linear accelerator 22 thereby increasing the strength of the fields which accelerate the electron beam E arriving from the electron source 21. This arrangement therefore recovers some of the energy which was given to electron bunches in the linear accelerator 22 (when they were accelerated by the linear accelerator) in order to accelerate subsequent electron bunches which arrive from the electron source 21. Such an arrangement may be known as an energy recovering LINAC.

[00105] Electrons E’ which are decelerated by the linear accelerator 22 are absorbed by a beam dump 26. The steering unit 23 may be operable to decouple the trajectory of the electron beam E’ which has been decelerated by the linear accelerator 22 from the trajectory of the electron beam E which has been accelerated by the linear accelerator 22. This may allow the decelerated electron beam E’ to be absorbed by the beam dump 26 whilst the accelerated electron beam E is directed to the undulator 24. The steering unit 23 may, for example, be operable to generate a periodic magnetic field which has a substantially constant phase relationship with the electron bunches which form the accelerated electron beam E and the decelerated electron beam E’. For example at times at which electron bunches from the accelerated electron beam E enter the steering unit 23, the steering unit 23 may generate a magnetic field which acts to direct the electrons to the undulator 24. At times at which electron bunches from the decelerated electron beam E’ enter the steering unit 23, the steering unit 23 may generate a magnetic field which acts to direct the electrons to the beam dump 26. Alternatively, at times at which electron bunches from the decelerated electron beam E’ enter the steering unit 23, the steering unit 23 may generate little or no magnetic field such that the electrons pass out of the steering unit 23 and to the beam dump 26.

[00106] Alternatively the free electron laser FEL may comprise a beam splitting unit (not shown) which is separate from the steering unit 23 and which is configured to decouple the trajectory of the accelerated electron beam E from the trajectory of the decelerated electron beam E’ upstream of the steering unit 23. The beam splitting unit may, for example, be operable to generate a periodic magnetic field which has a substantially constant phase relationship with the electron bunches which form the accelerated electron beam E and the decelerated electron beam E’.

[00107] Alternatively the trajectory of the accelerated electron beam E may be decoupled from the trajectory of the decelerated electron beam E’ by generating a substantially constant magnetic field. The difference in energies between the accelerated electron beam E and the decelerated electron beam E’ causes the trajectories of the two electron beams to be altered by different amounts by the constant magnetic field. The trajectories of the two electron beams will therefore become decoupled from each other.

[00108] The beam dump 26 may, for example, include a large amount of water or a material with a high threshold for radioactive isotope generation by high energy electron impact. For example, the beam dump 26 may include aluminium with a threshold for radioactive isotope generation of approximately 15MeV. By decelerating the electron beam E’ in the linear accelerator 22 before it is incident on the beam dump 26, the amount of energy the electrons have when they are absorbed by the beam dump 26 is reduced. This reduces the levels of induced radiation and secondary particles produced in the beam dump 26. This removes, or at least reduces, the need to remove and dispose of radioactive waste from the beam dump 26. This is advantageous since the removal of radioactive waste requires the free electron laser FEL to be shut down periodically and the disposal of radioactive waste can be costly and can have serious environmental implications.

[00109] When operating as a decelerator, the linear accelerator 22 may be operable to reduce the energy of the electrons E’ to below a threshold energy. Electrons below this threshold energy may not induce any significant level of radioactivity in the beam dump 26.

[00110] In some embodiments a decelerator (not shown) which is separate to the linear accelerator 22 may be used to decelerate the electron beam E’ which has passed through the undulator 24. The electron beam E’ may be decelerated by the decelerator in addition to being decelerated by the linear accelerator 22 or instead of being decelerated by the linear accelerator 22. For example, the second steering unit 25 may direct the electron beam E’ through a decelerator prior to the electron beam E’ being decelerated by the linear accelerator 22. Additionally or alternatively the electron beam E’ may pass through a decelerator after having been decelerated by the linear accelerator 22 and before being absorbed by the beam dump 26. Alternatively the electron beam E’ may not pass through the linear accelerator 22 after leaving the undulator 24 and may be decelerated by one or more decelerators before being absorbed by the beam dump 26.

[00111] Optionally, the free electron laser FEL may comprise one or more bunch compressors (not shown). A bunch compressor may be disposed downstream or upstream of the linear accelerator 22. A bunch compressor is configured to bunch electrons in the electron beam E and spatially compress existing bunches of electrons in the electron beam E. One type of bunch compressor comprises a radiation field directed transverse to the electron beam E. An electron in the electron beam E interacts with the radiation and bunches with other electrons nearby. Another type of bunch compressor comprises a magnetic chicane, wherein the length of a path followed by an electron as it passes through the chicane is dependent upon its energy. This type of bunch compressor may be used to compress a bunch of electrons which have been accelerated in a linear accelerator 22 by a plurality of conductors whose potentials oscillate at, for example, radio frequencies.

[00112] A bunch compressor may in particular be disposed between the steering unit 23 and the undulator 24. An electron bunch which is accelerated by the linear accelerator 22 may have a spread of energies. For example, some electrons in an electron bunch may have energies which are higher than an average energy of the electron bunch and some electrons in the bunch may have energies which are lower than the average energy. An alteration to the trajectory of an electron which is caused by the steering unit 23 may be dependent on the energy of the electrons (e.g. when the trajectory is altered by a magnetic field). Electrons of different energies therefore have their trajectories altered by different amounts by the steering unit 23. A spread of energies of an electron bunch leaving the linear accelerator 22 may therefore lead to a spread in position of the electron bunch in the steering unit 23. It may be desirable for electron bunches entering the undulator 24 to be tightly bunched and therefore have a very small spread in position. It may therefore be desirable to compress the electron bunches before they pass into the undulator 24 using one or more bunch compressors in order to reduce the spread in position of electron bunches in the undulator 24.

[00113] The free electron laser FEL shown in Figure 3 is housed within a building 31. The building 31 may comprise walls which do not substantially transmit radiation which is generated in the free electron laser FEL whilst the free electron laser FEL is in operation. For example, the building 31 may comprise thick concrete walls (e.g. walls which are approximately 4 metres thick). The walls of the building 31 may be further provided with radiation shielding materials such as, for example, lead and/or other materials which are configured to absorb neutrons and/or other radiation types. Providing walls of a building 31 with radiation absorbing materials may advantageously allow the thickness of the walls of the building 31 to be reduced. However adding radiation absorbing materials to a wall may increase the cost of constructing the building 31. A relatively cheap material which may be added to a wall of the building 31 in order to absorb radiation may, for example, be a layer of earth.

[00114] In addition to providing walls of the building 31 which have radiation shielding properties. The building 31 may also be configured to prevent radiation generated by the free electron laser FEL from contaminating ground water below the building 31. For example, the base and/or foundations of the building 31 may be provided with radiation shielding materials or may be sufficiently thick to prevent radiation from contaminating ground water below the building 31. In an embodiment the building 31 may be positioned at least partly underground. In such an embodiment ground water may surround portions of the exterior of the building 31 as well as being below the building 31. Radiation shielding may therefore be provided around the exterior of the building 31 in order to prevent radiation from contaminating ground water which surrounds the building 31.

[00115] In addition to or as an alternative to shielding radiation at the exterior of the building 31, radiation shielding may also be provided inside of the building 31. For example, radiation shielding may be provided inside the building 31 at locations proximate to portions of the free electron laser FEL which emit large amounts of radiation.

[00116] The building 31 has a width W and a length L. The width W and the length L of the building 31 is partly determined by the size of a loop 32 which the electron beam E follows through the free electron laser FEL. The loop 32 has a length 33 and a width 35.

[00117] The length 33 of the loop 32 is determined by the length of the linear accelerator 22 and the length of the undulator 24. A given length of linear accelerator 22 may, for example, be required in order to accelerate the electron beam E to high enough energies such that the electrons emit EUV radiation in the undulator 24. For example, a linear accelerator 22 may have a length of greater than about 40 metres. In some embodiments a linear accelerator 22 may have a length of up to about 80 metres. Additionally a given length of undulator 24 may be required in order to stimulate emission of coherent radiation in the undulator 24. For example, an undulator 24 may have a length of greater than about 40m. In some embodiments an undulator 24 may have a length of up to about 60 metres.

[00118] The width of the loop is determined by the radius of curvature with which the steering unit 23 adjusts the trajectory of the electron beam E. The radius of curvature of the electron beam E in the steering unit 23 may depend, for example, on the velocity of the electrons in the electron beam E and on the strength of a magnetic field which is generated in the steering unit 23. An increase in the strength of a magnetic field which is generated in the steering unit 23 will decrease the radius of curvature of the electron beam E whereas an increase in the velocity of the electrons will increase the radius of curvature of the electron beam E. The radius of curvature of the electron beam E through the steering unit 23 may, for example, be approximately 12m. In some embodiments the radius of curvature of the electron beam E through the steering unit 23 may be less than 12m. For example, the radius of curvature of an electron beam E through the steering unit 23 may be approximately 7m.

[00119] The loop 32 which the electron beam E follows through the free electron laser FEL may have a length 33 which is greater than about 60 metres. In some embodiments the loop 32 may have a length 33 which is up to about 120 metres. The loop 32 may have a width 35 which is greater than about 12 metres. In some embodiments the loop 32 may have a width 35 which is up to about 25 metres.

[00120] The building 31 may also house other components. For example, electrical cabinets 37 which contain electrical components which supply electrical power to, for example, the undulator 24, the steering units 23, 25 and/or other components of the free electron laser FEL may be housed within the building 31. It may be advantageous to provide the electrical cabinets 37 in close proximity to the undulator 24 as is shown in Figure 3. However electrical cabinets 37 may be positioned in other positions relative to the components of the free electron laser FEL.

[00121] Additionally cryogenic cooling cabinets 39 which contain apparatus which is configured to provide cryogenic cooling to components of the free electron laser FEL may be housed within the building 31. Cryogenic cooling may, for example, be provided to the linear accelerator 22 and may cool superconducting cavities of the linear accelerator 22. It may be advantageous to provide the cryogenic cooling cabinets 39 in close proximity to the linear accelerator 22. This may reduce any energy loss between the cryogenic cooling cabinets 39 and the linear accelerator 22.

[00122] It may be desirable to provide electrical cabinets 37 and cryogenic cooling cabinets 39 on the outside of the loop 32 which the electron beam E follows through the free electron laser FEL (as is shown in Figure 3). Providing the cabinets 37, 39 on the outside of the loop 32 may allow easy access to the cabinets, for example, to monitor, control, maintain and/or repair components which are housed within the cabinets 37, 39. As will be appreciated from Figure 3, positioning the cabinets 37, 39 on the outside of the loop 32 may increase the minimum width W of the building 31 which is required to house the components of the free electron laser FEL within the building 31. The building 31 may also house other components which are not shown in Figure 3 which may also determine the dimensions of the building 31.

[00123] As is shown in Figure 3, a wall 47 is positioned between the loop 32 which the electron beam follows through the free electron laser FEL and the electric cabinets 37. A wall 47 is also positioned between the loop 32 and the cryogenic cooling cabinets 39. The walls 47 may shield the electric cabinets 37 and the cryogenic cabinets 39 from radiation which is generated by the electron beam E in the free electron laser FEL. This protects the components in the cabinets 37, 39 from being damaged by radiation and may allow maintenance workers to access the cabinets 37, 39 whilst the free electron laser FEL is in operation without being exposed to dangerous levels of radiation.

[00124] In the embodiment depicted in Figure 3 the cabinets 37, 39 are shown as being housed in the same building 31 as the loop 32 which the electron beam follows through the free electron laser FEL whilst being shielded from the loop 32 by the walls 47. The cryogenic cooling components which are housed within the cabinets 39 may generate vibrations which may be transferred to components of the free electron laser FEL and may adversely affect components of a free electron laser FEL which are sensitive to vibrations. In order to prevent vibrations which are generated by cryogenic cooling components from transferring to sensitive parts of the free electron laser, a portion of the building 31 in which the cryogenic cooling cabinets 39 are housed may be mechanically isolated from the portion of the building in which sensitive components are housed. For example, the cryogenic cooling cabinets 39 may be mechanically isolated from the linear accelerator 22, the steering unit 23 and the undulator 24. In order to provide mechanical isolation the portion of the building 31 in which the cryogenic cooling cabinets 39 are housed may, for example, have separate foundations to a portion of the building in which the linear accelerator 22, the steering unit 23 and the undulator 24 are housed.

[00125] Alternatively the cryogenic cooling cabinets 39 and/or the electrical cabinets 37 may be housed in one or more buildings which are separate from the building 31. This may ensure that the cabinets 37, 39 are shielded from radiation which is produced by the electron beam E and that sensitive components of the free electron laser FEL are mechanically isolated from the cryogenic cooling cabinets 39.

[00126] A lithographic system LS may comprise a single free electron laser FEL. The free electron laser FEL may supply an EUV radiation beam to a beam splitting apparatus 20 which provides branch radiation beams to a plurality of lithographic apparatus. The radiation source SO may comprise an optical system which includes dedicated optical components configured to direct a radiation beam B’ output from a free electron laser FEL to a beam splitter 20 of a lithographic system LS. Since EUV radiation is generally well absorbed by all matter, reflective optical components are generally used (rather than transmissive components) so as to minimise losses. The dedicated optical components of the optical system may adapt the properties of the radiation beam produced by the free electron laser FEL so that it is suitable for acceptance by the illumination systems IL of the lithographic apparatus LA!-LA20 and/or a mask inspection apparatus.

[00127] Alternatively a radiation source SO may comprise a plurality of free electron lasers (e.g. two free electron lasers) which may each provide an EUV radiation beam to an optical system which also forms part of the radiation source SO. The optical system may receive a radiation beam from each of a plurality of free electron lasers and may combine the radiation beams into a composite radiation beam which is provided to a beam splitting apparatus 20 in order to provide branch radiation beams BrE^o to lithographic apparatus LA1-LA20· [00128] Figure 4 is a schematic depiction of a lithographic system LS which includes a radiation source SO comprising a first free electron laser FEL’ and a second free electron laser FEL”. The first free electron laser FEL’ outputs a first EUV radiation beam B’ and the second free electron laser FEL” outputs a second EUV radiation beam B”. The first free electron laser FEL’ is housed within a first building 31’. The second free electron laser FEL” is housed within a second building 31”.

[00129] The first and second radiation beams B’, B” are received by an optical system 40. The optical system 40 comprises a plurality of optical elements (e.g. mirrors) which are arranged to receive the first radiation beam B’ and the second radiation beam B” and output a main radiation beam B. At times at which both the first and second free electron lasers are operating, the main radiation beam B is a composite radiation beam which comprises radiation from both the first and second radiation beams B’, B”. The composite radiation beam B is provided to the beam splitting apparatus 20 which provides branch radiation beams Βί^ο to lithographic apparatus LArLA20.

[00130] The arrangement which is depicted in Figure 4 in which two free electron lasers are arranged to provide radiation beams B’, B” to form a main radiation beam B, may allow one of the free electron lasers to be turned off whilst radiation is continuously provided to the lithographic apparatus LAT-LA20· For example, one of the free electron lasers may be taken out of operation in order to, for example, allow the free electron laser to be repaired or to undergo maintenance. In this event the other free electron laser may continue to provide a radiation beam which is received by the optical system 40. In the event that only one of the free electron lasers provides radiation to the optical system 40, the optical system 40 is operable to form a main radiation beam B which comprises radiation from the free electron laser which is providing radiation to the optical system 40. This allows for continuous operation of the lithographic apparatus LArLA20 even when one of the free electron lasers is taken out of operation.

[00131] Figure 5 is a schematic depiction of an embodiment of an optical system 40 according to an embodiment of the invention which is arranged to receive a beam of radiation B’, B” from each of the free electron lasers FEL’, FEL” and to output an output radiation beam B. The radiation beam B that is output by the optical system 40 is received by the beam splitting apparatus 20 (see Figure 1).

[00132] The optical system 40 comprises four optical elements: first and second optical elements 132, 134 associated with a first one of the free electron lasers FEL’; and first and second optical elements 136, 138 associated with a second one of the free electron lasers FEL”. The optical elements 132, 134, 136, 138 are arranged to alter the size and shape of the cross section of the radiation beams B’, B” from the free electron lasers FEL’, FEL”.

[00133] In particular, the first optical elements 132, 136 are convex mirrors, which act to increase the cross sectional area of the radiation beams B’, B” from the free electron lasers FEL’, FEL”. Although in Figure 5 the first optical elements 132, 136 appear to be substantially flat in the x-y plane they may be convex both in this plane and in the z direction. Since the first optical elements 132, 136 are convex, they will increase the divergence of the EUV radiation beams B’, B”, thereby decreasing the heat load on mirrors downstream of them. The first optical element 132 is therefore a diverging optical element arranged to increase the cross sectional area of the radiation beam B’ received from the first free electron laser FEL’. The first optical element 136 is a diverging optical element arranged to increase the cross sectional area of the radiation beam B” received from the second free electron laser FEL. This may allow mirrors downstream to be of a lower specification, with less cooling, and therefore less expensive. Additionally or alternatively, it may allow the downstream mirrors to be nearer to normal incidence. In practice, the radiation beam B output by the radiation source SO may be split by a plurality of consecutive, static, knife edge mirrors arranged in series in the path of the beam B. Increasing the size of the beam B (by, for example, using convex mirrors as the first optical elements 132, 136) reduces the accuracy with which the mirrors must be located in the beam B path. Therefore, this allows for more accurate splitting of the output beam B by the splitting apparatus 20.

[00134] The second optical elements 134, 138 are concave and are complementary in shape to the first optical elements such that the beams leaving the second optical elements 134, 138 have substantially zero divergence. Therefore, downstream of the second optical elements 134, 138 the beams are substantially collimated. Again, although in Figures 5 the second optical elements 134, 138 appear to be substantially flat in the x-y plane they are in fact concave both in this plane and in the z direction.

[00135] It may be preferable for the output beam B, which is received by the beam splitting apparatus 20, to have a different shape and/or intensity distribution to that output by the free electron lasers FEL’, FEL”. For example, a rectangular shape may be preferable to a circular beam for consecutive knife edge extraction mirrors within the beam splitting apparatus 20. Therefore, in addition to increasing the cross sectional area of the radiation beams B’, B”, the optical elements 132, 134, 136, 138 may act to alter the cross sectional shape of the radiation beams B’, B”. In particular, the optical elements 132, 134, 136, 138 may be astigmatic or aspherical and may be shaped so as to ensure that the radiation beams B’, B” leaving the second optical elements 134, 138 are more rectangular in shape than the radiation beams B’, B” produced by the free electron lasers FEL’, FEL”. For example, the optical elements may be shaped so that the beams B’, B” leaving the second optical elements 134, 138 are generally rectangular but with rounded corners, although other shapes are also possible. The two dimensions of such a rectangular shape may be related to radii of curvature of the optical elements in two perpendicular directions such as, for example, in the x-y plane and in the z direction. Advantageously, this allows the mirrors that are used to split the output radiation beam B into branch radiation beams BrB2o (see Figure 1) before they enter the lithographic apparatuses LA1-LA20, to be identical or at least very similar. This is especially beneficial from a manufacturing point of view.

[00136] When both of the free electron lasers FEL’, FEL” are on, the optical system 40 is operable to combine their radiation beams B’, B” to form a composite radiation beam B. In this embodiment, this is achieved by offsetting the first and second optical elements 132, 134 of the first free electron laser FEL’ from those 136, 138 of the second free electron laser FEL” in the x-direction so that the beams B’, B” leaving the second optical elements 134, 138 are both adjacent to each other and mutually parallel. In particular, the first and second optical elements 132, 134 of the first free electron laser FEL’ are disposed “downstream” (with respect to the direction of propagation of the laser beams B’, B”) of those 136, 138 of the second free electron laser FEL”.

[00137] In such an arrangement, the optical system 40 is operable to combine the two radiation beams B’, B” to form a composite radiation beam. The composite beam is the output radiation beam B output by the optical system 40. It will be appreciated that Figure 5 is merely exemplary and that the optical system 40 may be implemented other than as shown in Figure 5.

[00138] Referring again to Figure 4, the buildings 31’, 31” are configured to substantially prevent radiation (other than the radiation beams B’, B”) which is generated by an operating free electron laser from propagating out of the buildings 31’, 31”. Housing the first and second free electron lasers inside separate buildings therefore allows maintenance and/or repair to be safely carried out on one of the free electron lasers whilst the other free electron laser continues to operate. For example, the first electron laser FEL’ may be taken out of operation in order to allow the first free electron laser FEL’ to be repaired or to undergo maintenance. During this time the second free electron laser FEL” may continue to operate in order to provide radiation to the optical system 40 and to the lithographic apparatus LAr LA20. Radiation will therefore be generated in the second building 31” due to the operation of the second free electron laser FEL”. Dangerous levels of radiation do not however leave the second building 31” and do not enter the first building 31’ due to the radiation shielding which is provided by the walls of the second building 31”. The first building may therefore be safely entered by maintenance workers in order to repair or carry out maintenance to the first free electron laser FEL’ [00139] It will be appreciated from Figure 4, that the size of the first and second buildings means that the first and second radiation beams B’, B” which are output from the first and second free electron lasers FEL’, FEL” are separated from each other. It is however advantageous for the first and second radiation beams B’, B” to have a relatively small separation (or no separation) between them at the point at which they are received by the optical system 40. This may be advantageous for the stability of the optical system 40 and may reduce the amount of radiation which may be lost by the optical system 40 in forming the composite radiation beam B. In order for the seperation between the first and second radiation beams B’, B” to be relatively small at the optical system 40 and to be smaller than the separation of the radiation beams B’, B” at the point at which they are output from the free electron lasers FEL’, FEL” the first and second buildings 31’, 31” are disposed at angles β’ and β” relative to an optical axis O of the radiation source SO. The optical axis O of the radiation source SO corresponds with the axis along which the main radiation beam B propagates from the optical system 40 to the beam splitting apparatus 20. The angles β’ and β” may for example be approximately the same as each other.

[00140] Alternatively the buildings 31', 31” may be aligned approximately parallel to the optical axis O and the radiation beams B’, B” may be output from the buildings 31’, 31” at angles β’ and β” relative to the optical axis O. For example, at least a section of the undulators 24 in the free electron lasers FEL’, FEL” may be disposed at angles β’, β” relative to the optical axis O.

[00141] It will be appreciated from Figure 4 that the width W, and the length L of the first and second buildings 31’, 31” and the angles β’, β” at which the buildings 31’, 31” are disposed relative to the optical axis O cause the radiation source SO to have a considerable extent in the y-direction which is indicated in Figure 4. The extent in the y-direction of the radiation source SO is considerably greater than the extent in the y-direction of the beam splitting apparatus 20 and the lithographic apparatus LA1-LA20. The difference in extent in the y-direction between the radiation source SO and the beam splitting apparatus 20 and the lithographic apparatus LArLA20 may be increased when, for example, the lithographic apparatus LArLA20 are positioned on different vertical levels since this may decrease the extent in the y-direction of the beam splitting apparatus 20 and the lithographic apparatus LA1-LA20· [00142] In some embodiments it may be disadvantageous for there to be a large difference between the extent in the y-direction of the radiation source SO and the beam splitting apparatus 20 and the lithographic apparatus LA1-LA20. For example, in some applications it may be desirable to position a plurality of lithographic systems LS adjacent to one another. In particular, it may be desirable to position a plurality of lithographic systems LS at different positions on the y-axis and at approximately the same positions on the x-axis. In such an arrangement it will be appreciated that for the lithographic system LS of Figure 4 (where the extent in the y-direction of the radiation source SO is significantly larger than the extent in the y-direction of the beam splitting apparatus 20 and the lithographic apparatus LArLA20), the separation between adjacent lithographic systems LS and the total extent in the y-direction of a plurality of lithographic systems is determined by the extent in the y-direction of the radiation source SO of each lithographic system LS.

[00143] It is desirable to reduce the extent in the y-direction of a radiation source SO of a lithographic system LS. This advantageously allows the separation between adjacent lithographic systems LS to be reduced and allows the total extent in the y-direction of a plurality of adjacent lithographic systems LS to be reduced. Reducing the extent in the y-direction of a radiation source SO also provides greater flexibility to a designer of lithographic systems LS in the relative arrangement of the lithographic systems LS.

[00144] Figure 6 is a schematic side view of a free electron laser FEL which is arranged such that the extent of the free electron laser FEL in the y-direction may be reduced (compared to, for example, the extent in the y-direction of the free electron lasers depicted in Figures 3 and 4). The free electron laser FEL which is depicted in Figure 6 is arranged on a plurality of vertical levels. The linear accelerator 22 is positioned on a first vertical level 41 and is supported by a floor 42 of the building 31 within which the free electron laser FEL is housed. The electron beam propagates along a first path 60 and in a first direction 61 through the linear accelerator 22. The undulator 24 is positioned on a second vertical level 43. The electron beam E propagates along a second path 62 and substantially in a second direction 63 through the undulator 24. The first path 60 and the second path 62 are vertically separated from each other. As explained above the undulator 24 is operable to guide the electron beam E along a periodic path. However, the electron beam E may still be considered to be propagating in substantially the second direction 63 through the undulator 24. In the embodiment depicted in Figure 6, the first direction 61 and the second direction 63 oppose one another.

[00145] The second vertical level 43 on which the undulator 24 is positioned may, for example, be at a ground floor level of the building 31 in which the free electron laser FEL is housed. Alternatively, the second vertical level 43 may be at a basement level of the building 31. The first vertical level 41 on which the linear accelerator 22 is positioned may, for example, be at a first floor level of the building 31. Alternatively, the first vertical level 41 may be at a ground floor level of the building 31 (e.g. when the second vertical level 43 is at a basement level of the building 31).

[00146] In the embodiment of the free electron laser FEL which is depicted in Figure 6, the first vertical level 41 on which the linear accelerator 22 and the first path 60 are positioned above the second vertical level 43 on which the undulator 24 is positioned and the second path 62. However in some embodiments the first vertical level 41 and the first path 60 may be below the second vertical level 43 and the second path 62.

[00147] The steering unit 23 includes a portion which is positioned on the first vertical level 41 and a portion which is positioned on the second vertical level 43. The steering unit 23 therefore extends between the first and second vertical levels. The steering unit 23 is operable to alter the trajectory of the electron beam E so as to direct the electron beam E from the first path 60 and the second path 62. In the embodiment depicted in Figure 6, the steering unit 23 directs the electron beam E from the first vertical level 41 to the second vertical level 43 such that the electron beam E is directed from the linear accelerator 22 to the undulator 24.

[00148] The second steering unit 25 also extends between the first and second vertical levels 41, 43. The second steering unit 25 is operable to alter the trajectory of the electron beam E’ which leaves the undulator 24 so as to direct the electron beam E’ from the second path 62 to the first path 64. In the embodiment of Figure 6, the second steering unit 25 directs the second vertical level 43 to the first vertical level 41 such that the electron beam E’ is directed from the undulator 24 to the linear accelerator 22. As was described above with reference to the free electron laser FEL depicted in Figure 3, the second steering unit 25 is an optional feature of a free electron laser FEL. In some embodiments the electron beam E’ which is output from the undulator 24 is not directed back into the linear accelerator 22. The electron beam E’ may instead be directed to a separate decelerator which may be housed within the building 31 or may be separate from the building 31.

[00149] In the embodiment shown in Figure 6, the electron source 21 and the beam dump 26 are housed within the building 31 and are positioned on the first vertical level 41. However, in some embodiments the electron source 21 and the beam dump 26 may be positioned at other locations. For example, the electron source 21 may be positioned on a third vertical level. The third vertical level may, for example, be positioned above the first and second vertical levels, between the first and second vertical levels or below the first and second vertical levels. Alternatively the electron source 21 may be positioned on the second vertical level 43. Alternatively the electron source 21 may be positioned outside of the building 31 and may direct an electron beam E into the building 31.

[00150] An electron source 21 which is positioned on a vertical level other than the first vertical level 41 (on which the linear accelerator 22 is positioned) may be used in conjunction with one or more steering units which direct an electron beam E which is output from the electron source 21 to the first vertical level 41 such that the electron beam E enters the linear accelerator 22.

[00151] The beam dump 26 may be positioned on the first vertical level 41 as shown in Figure 6 or on a different vertical level. For example, the beam dump 26 may be positioned on the second vertical level 43 or on a third vertical level (which may be different to or the same as a third vertical level on which the electron source 21 may be positioned). Alternatively the beam dump 26 may be positioned outside of the building 31. For example, a lithographic system LS may include a beam dump 26 which absorbs multiple decelerated electron beams E' which are output from more than one free electron laser. Such a beam dump 26 may be positioned outside of the building 31.

[00152] A free electron laser which is arranged on multiple vertical levels (such as the free electron laser FEL depicted in Figure 6) may include additional or alterative components to those depicted in Figure 6 and described above. For example, a free electron laser may include one or more decelerators which are separate from the linear accelerator 22. A decelerator may for example decelerate the electron beam E’ which is output from the undulator 24 prior to being absorbed by the beam dump 26. A decelerator may decelerate the electron beam E’ in addition to or instead of the linear accelerator 22. In an embodiment in which a decelerator decelerates the electron beam E’ in addition to a deceleration in the linear accelerator 22, a decelerator may be positioned upstream, downstream or both upstream and downstream of the linear accelerator 22. A decelerator may be positioned on the first vertical level 41, on the second vertical level 43 or on a different vertical level.

[00153] A free electron laser FEL which is arranged on multiple vertical levels may include one or more bunch compressors which are configured to bunch electrons in an electron beam E and spatially compress existing bunches of electrons in the electron beam E. A bunch compressor may be positioned on the first vertical level 41, on the second vertical level 43 or on a different vertical level.

[00154] It may be desirable to provide one or more overhead cranes in a building 31 which houses a free electron laser FEL. An overhead crane may be used, for example, to move and/or replace components of the free electron laser FEL or in the maintenance and repair of a free electron laser FEL. In the embodiment depicted in Figure 6, an overhead crane 44 is positioned in each of the first and second vertical levels 41,43. The overhead cranes 44 are attached to beams 45 which are suspended from a ceiling of the vertical levels 41, 43. The overhead cranes may be movable along the beams 45 in order to access different positions along the length of the vertical levels 41,43 of the free electron laser FEL. The overhead cranes 44 may additionally or alternatively be moveable along the width of the vertical levels 41, 43 (i.e. into and out of the page of Figure 6).

[00155] It may be desirable to provide additional apparatus which provides access to the different vertical levels in the building 31. For example one or more staircases may be provided in the building 31 which provide access to different components of the free electron laser FEL. A building 31 may be provided with, for example, height adjustable work platforms which may be used to access components of the free electron laser FEL which are positioned on different vertical levels.

[00156] Figures 7a and 7b are schematic front-on views of two different embodiments of a free electron laser FEL which are arranged on multiple vertical levels. Figure 7a depicts an embodiment in which the free electron laser FEL is housed within a building 31 and comprises a first vertical level 41 and a second vertical level 43. The linear accelerator 22 is positioned on the first vertical level 41 and the undulator 24 is positioned on the second vertical level 43. The undulator 24 outputs a radiation beam B’ on the second vertical level 43. In the arrangement shown in Figure 7a the radiation beam B’ propagates out of the page. The free electron laser FEL further comprises a steering unit (not shown) which is operable to alter the trajectory of an electron beam E output from the linear accelerator 22 so as to direct the electron beam E from the first vertical level 41 to the second vertical level 43 (and thus from the linear accelerator 22 to the undulator 24).

[00157] The building 31 comprises a floor 42 which separates the first and second vertical levels 41, 43. The building 31 further comprises a wall 47 which extends vertically throughout both the first and second vertical levels 41, 43. On the first vertical level the wall 47 separates the linear accelerator 22 from cryogenic cooling cabinets 39 which contain apparatus which is configured to provide cryogenic cooling to components of the free electron laser FEL (e.g. the linear accelerator 22). On the second vertical level 43, the wall 47 separates the undulator 24 from electrical cabinets 37 which contain electrical components which supply electrical power to components of the free electron laser FEL (e.g. the undulator 24).

[00158] The wall 47 may have radiation shielding properties and may therefore prevent dangerous radiation levels which may be generated by an electron beam E in the free electron laser FEL from reaching portions of the building 31 in which the cabinets 37, 39 are housed. This may allow the cabinets 37, 39 to be accessed by maintenance workers whilst the free electron laser FEL is in operation without the maintenance workers being exposed to dangerous levels of radiation.

[00159] In the embodiment depicted in Figure 7a an overhead crane 44 is provided on each of the vertical levels 41, 43. The overhead cranes 44 provide access to the linear accelerator 22, the undulator 24 and other components of the free electron laser FEL which are positioned on the first and second vertical levels 41,43.

[00160] Figure 7b depicts an embodiment in which the free electron laser FEL is housed within a building 31 and comprises a first vertical level 41 and a second vertical level 43. The linear accelerator 22 is positioned on the first vertical level 41 and the undulator 24 is positioned on the second vertical level 43. The linear accelerator 22 and the undulator 24 are offset from each other on a horizontal x-axis which is depicted in Figure 7b. The linear accelerator 22 has a longitudinal axis which extends along the length of the linear accelerator 22. The horizontal x-axis extends perpendicular to the longitudinal axis of the linear accelerator 22.

[00161] In some embodiments the longitudinal axis of the linear accelerator 22 may extend in a direction which is not parallel with a longitudinal axis of the undulator 24 and therefore the longitudinal axis of the undulator 24 may not be perpendicular to the x-axis. In such an embodiment the majority of the extent of the linear accelerator 22 along its longitudinal axis may be positioned at a first position on the x-axis and the majority of the extent of the undulator 24 may be positioned at a second position on the x-axis which is different to the first position on the x-axis at which the linear accelerator 22 is positioned.

[00162] The electron beam E which is directed from the first vertical level 41 to the second vertical level 43 by a steering unit (not shown) therefore propagates at an angle a with respect to the vertical. A floor 42 is provided across a horizontal portion of the building 31 and supports the linear accelerator 22. A wall 47 separates the portion of the building 31 in which the floor 42 is provided from the portion of the building in which the floor 42 is not provided. On the side of the wall 42 on which the floor 42 is provided a second floor 48 is provided on the second vertical level 43. The second floor 48 splits a portion of the second vertical level 43 into vertical sub-levels 43a and 43b. Electrical cabinets 37 are provided on a first vertical sub-level 43a and cryogenic cooling cabinets 39 are provided on a second vertical sub-level 43b.

[00163] The wall 47 and the floors 42, 48 may have radiation shielding properties and may therefore prevent dangerous radiation levels which may be generated by an electron beam E in the free electron laser FEL from reaching portions of the building 31 in which the cabinets 37, 39 are housed. This may allow the cabinets 37, 39 to be accessed by maintenance workers whilst the free electron laser FEL is in operation without the maintenance workers being exposed to dangerous levels of radiation.

[00164] In the embodiment in Figure 7b, the components of the free electron laser FEL which are positioned on the first vertical level 41 and components of the free electron laser FEL which are positioned on the second vertical level 43 are situated in the same room of the building 31. That is, the linear accelerator 24 which is positioned on the first vertical level 41 and the undulator 24 which is positioned on the second vertical level are situated in the same room of the building 31. This means that components which are positioned on the first vertical level and components which are positioned on the second vertical level can be accessed by a single piece of equipment. For example, the arrangement which is shown in Figure 7b allows components which are positioned on the first vertical level 41 and components which are positioned on the second vertical level 43 to be accessed from above. The reduced extent of the floor 42 across a horizontal portion of the building 31 advantageously allows a single crane 44 to access both components of the free electron laser FEL which are positioned on the first vertical level 41 and components which are positioned on the second vertical level 43. This advantageously allows the crane 44 to move components of the free electron laser FEL between the first and second vertical levels 41, 43.

[00165] In each of the embodiments of a free electron laser FEL depicted in Figures 7a and 7b, a portion of the building 31 in which the cabinets 37, 39 are housed may be mechanically isolated from a portion of the building 31 in which the free electron laser FEL is housed. For example the portion of the building 31 in which the cabinets 37, 39 are housed may have separate foundations to the portion of the building in which the free electron laser FEL is housed. This may prevent vibrations which may be generated in the cabinets 37, 39 from being transferred to components of the free electron laser FEL which may be sensitive to vibrations. In particular, components which are housed in the cryogenic cooling cabinets 39 may generate vibrations which may adversely affect sensitive components of a free electron laser FEL. It is therefore particularly advantageous for the cryogenic cooling cabinets 39 to be mechanically isolated from the free electron laser FEL.

[00166] Although Figures 7a and 7b depict embodiments in which the electrical cabinets 37 and the cryogenic cooling cabinets 39 are housed within the same building 31 as the electron beam E propagates, in alternative embodiments one or more of the cabinets 37, 39 may be housed outside of the building 31. For example, one or more of the electrical cabinets 37 and/or the cryogenic cooling cabinets 39 may be housed in one or more separate buildings. The one or more buildings in which cabinets 37, 39 are housed may, for example, be positioned in close proximity to the building 31. Flousing one or more cabinets 37, 39 in a separate building from the building 31 in which the electron beam E propagates may advantageously shield the cabinets 37, 39 from radiation generated by the electron beam E. Additionally vibrations generated by components which are housed in the cabinets 37, 39 may be prevented from being transferred to sensitive components of the free electron laser FEL.

[00167] Although specific embodiments of a free electron laser FEL which is positioned on multiple vertical levels are depicted in Figures 6 and 7 and described above, it will be appreciated that other arrangements may instead be used without departing from the scope of the invention. For example, in the embodiments described above a free electron laser FEL comprises a single linear accelerator 22. Flowever in some embodiments a free electron may include a plurality of linear accelerators 22. A plurality of linear accelerators 22 may be used to accelerate an electron beam E prior to the electron beam E passing through the undulator 24. For example, an electron beam E may be accelerated by a first linear accelerator and accelerated by a second linear accelerator before being directed to an undulator 24. One or more steering units may be configured to direct the electron beam E from the first linear accelerator to the second linear accelerator. The trajectory of the electron beam E may, for example, be altered so as to guide the electron beam E around a loop (e.g. a loop which is analogous to the loop 32 depicted in Figure 6) between the first linear accelerator and the second linear accelerator.

[00168] A first linear accelerator and a second linear accelerator may be positioned on substantially the same vertical level. For example, in the embodiments depicted in Figures 7a and 7b a second linear accelerator may be positioned on the first vertical level 41. The first and second linear accelerators may be disposed at different horizontal positions on the x-axis shown in Figure 7. For example, a first and second linear accelerator may be positioned side-by-side to each other. Such an arrangement may result in an increase in the width W of a building 31 in which a free electron laser FEL is housed.

[00169] Alternatively a first and second linear accelerator may be positioned on different vertical levels, for example, a second linear accelerator may be positioned on a third vertical level (not shown). The third vertical level may, for example, be disposed above the first and second vertical levels, in between the first and second vertical levels or below both the first and second vertical levels. Positioning a second linear accelerator on a third vertical level which is different to the first vertical level 41 on which a first linear accelerator is positioned, may allow the first and second linear accelerators to be disposed at approximately the same position on the horizontal x-axis. This may allow the width W of the building 31 to remain substantially the same. Flowever such an arrangement may result in an increase in the height of a building 31. In some embodiments a free electron laser FEL may comprise more than two linear accelerators.

[00170] Figure 8 schematically depicts a side on view of an arrangement of a free electron laser FEL in which the linear accelerator 22 and the undulator 24 are positioned on substantially the same vertical level 43 and in a linear arrangement. In the embodiment of Figure 8 an electron beam E is emitted from an electron source 21. The electron beam E propagates along a first path 60 and substantially in a first direction 61 through a linear accelerator 22 and an undulator 24. The first path is positioned on a second vertical level 43 of a building 31 in which the free electron laser FEL is housed. Although the undulator 24 guides the electron beam E along a periodic path the electron beam E may still be considered to propagate in substantially the first direction through the undulator 24. A first steering unit 23 is operable to alter the trajectory of the electron beam E’ which is output from the undulator 24 so as to direct the electron beam E’ from the first path 60 to a second path 62 along which the electron beam E’ propagates substantially in a second direction 62. The second path 62 is positioned on a first vertical level 41 of the building 31. The first and second paths 60, 62 are therefore vertically separated from one another. In the embodiment of Figure 8 the first and second directions 61, 63 oppose each other.

[00171] In the embodiment of Figure 8 the electron beam E’ propagates along the second path 62 to a second steering unit 25 which is operable to direct the electron beam E’ from the second path 62 back to the first path 60. The electron beam E’ propagates along the first path 60 through the linear accelerator 22 which may act to decelerate the electron beam E’. The linear accelerator 22 may recover energy from the decelerating electron beam E’. After being decelerated by the linear accelerator 22 the electron beam E' may be directed to a beam dump 26 (not shown in Figure 8) before reaching the undulator 24.

[00172] It will be appreciated that in the embodiment of Figure 8 the vertical separation between the first path 60 and the second path 62 along which the electron beam E propagates in the free electron laser FEL, advantageously allows the width W of the building 31 to be reduced when compared to, for example, the free electron lasers shown in Figures 3 and 4. Flowever, in the embodiment of Figure 8, the electron source 21, the linear accelerator 22 and the undulator 24 are all positioned on the same vertical level 43 of a building 31. It will be appreciated that this arrangement increases the length L of the building 31 and therefore increases the surface area which the building 31 takes up. In other embodiments the electron source 21 may be positioned on a different vertical level to the linear accelerator 22 and the undulator 24. For example, the electron source 21 may be positioned on the first vertical level 41 of the building 31 shown in Figure 8 which may allow the length L of the building 31 to be reduced. Flowever, in general positioning the linear accelerator 22 and the undulator 24 on the same vertical level increases the length L of the building when compared to, for example, the embodiment shown in Figure 6.

[00173] It will be appreciated that other arrangements of components of a free electron laser FEL over vertical levels are possible. In general it is advantageous to provide a steering unit 23, 25 which is operable to alter the trajectory of an electron beam E so as to direct the electron beam E from a first path 60 along which the electron beam E propagates substantially in a first direction 61 to a second path 62 along which the electron beam E propagates substantially in a second direction 63, where the first path 60 and the second path 62 are vertically separated from one another.

[00174] It will be appreciated from, for example, Figure 7 that such an arrangement allows the width W of a building 31 in which the free electron laser FEL is housed to be advantageously reduced, since the width W of the building 31 no longer depends on the size of the loop 32 which an electron beam E follows through the free electron laser. A reduction in the width W of the building 31 also allows the area which the building 31 occupies to be decreased. A reduction of the width W and the area of a building 31 in which a free electron laser is housed is advantageous in an arrangement in which the free electron laser FEL forms part of a radiation source SO for a lithographic system LS.

[00175] Figure 9 is a schematic depiction of a first lithographic system LSi and a second lithographic system LS2 which are disposed adjacent to one another in the y-direction. Each of the first and second lithographic systems LSi, LS2 include a radiation source SO which comprise two free electron lasers FEL which are positioned on multiple vertical levels (e.g. the free electron lasers depicted in Figures 6 and 7). Positioning the free electron lasers FEL on multiple vertical levels allows the width W of buildings 31 in which the free electron lasers are housed to be reduced. This advantageously allows the extent in the y-direction of the radiation sources SO to be reduced. For example, the extent in the y-direction of a radiation source SO may be reduced such that it is substantially equivalent to the extent in the y-direction of the beam splitting apparatus 20 and the lithographic apparatus LAi-LA20 to which the radiation source SO provides radiation (as is depicted in Figure 9). In this case the extent in the y-direction of a lithographic system LS may no longer be determined by the extent in the y-direction of the radiation source SO and may instead be determined by the extent in the y-direction of the beam splitting apparatus 20 and the lithographic apparatus LAi-LA20of the lithographic system LS.

[00176] Reducing the extent in the y-direction of the radiation sources SO allows the separation (in the y-direction) between the first and second lithographic systems LSi, LS2 to be reduced and allows a reduction in the area which is occupied by the lithographic systems LSi, LS2. This is advantageous because it allows more lithographic systems LS to be positioned in a given area. This reduces the area of land which is required to position a given number of lithographic systems which may decrease a cost associated with procuring and maintaining the land. Reducing the extent in the y-direction of a lithographic system LS and the total area which is occupied the lithographic system LS additionally provides greater flexibility in the arrangement of one or more lithographic systems LS.

[00177] In each of the lithographic systems LS!, LS2 depicted in Figure 9, the buildings 31 which house the free electron lasers FEL are disposed at angles β’, β” with respect to an optical axis O of the radiation source SO of that lithographic system LS!, LS2. This allows the separation between the radiation beams B’, B” which are output from the free electron lasers FEL to be greater at a position on the x-axis at which they are output from the free electron lasers than at a position on the x-axis at which they are received by the optical system 40. This is advantageous for the stability of the optical system 40 and may reduce the amount of radiation which may be lost by the optical system 40 in forming the composite radiation beam B. However, it will be appreciated that by disposing the buildings 31 at angles β’, β” with respect to the optical axis O of a radiation source, the extent in the y-direction of a building 31 (having a given width W) is increased.

[00178] Figure 10 is a schematic depiction of an alternative embodiment of first and second lithographic systems LS^ LS2. The lithographic systems LSi, LS2 each comprise free electron lasers FEL which are housed in buildings 31 that are disposed parallel to the optical axes O of the radiations sources SO. In each free electron laser FEL the undulator 24 is disposed at an angle with respect to the optical axis O such that radiation beams B’, B” which are output from the free electron lasers FEL propagate at an angle β’, β” with respect to the optical axis O. This allows the separation between the radiation beams B’, B” which are output from the free electron lasers FEL to be greater at a position on the x-axis at which they are output from the free electron lasers than a position on the x-axis at which they are received by the optical system 40, without disposing the buildings 31 at an angle with respect to the optical axis O. Disposing the buildings 31 parallel to the optical axis O may reduce the extent of the buildings 31 in the y-direction and thus may allow the separation between adjacent buildings to be reduced. This may allow the extent of a radiation source SO to be further reduced in the y-direction.

[00179] However it will be appreciated that disposing an undulator 24 within a building 31 at an angle β’, β” with respect to the optical axis O may require the width W of the building 31 to be increased. The degree to which the width W of a building 31 may need to be increased may depend on the dimensions of the undulator 24 and on the angle at which the undulator 24 is disposed.

[00180] In the embodiments of lithographic systems LS^ LS2 which are depicted in Figures 9 and 10 each of the free electron lasers comprise a linear accelerator 22 which is positioned on a first vertical level 41 and an undulator 24 which is positioned on a second vertical level 43. Each free electron laser FEL emits a radiation beam from the second vertical level 43 on which the undulator 24 is positioned. As was described above with reference to Figures 6 and 7 the second vertical level 43 may be either above or below the first vertical level 41. However it is advantageous for all of the free electron lasers FEL which make up a given radiation source SO to include undulators 24 which are positioned on the substantially the same vertical level. This ensures that all of the radiation beams B’, B” which are directed towards an optical system 40 are output on the same vertical level and thus an optical system 40 only receives radiation beams B', B” on a single vertical level. This is advantageous for the stability of the optical system 40 and may simplify the design of the optical system 40.

[00181] It should be appreciated that the dimensions and arrangements of components of lithographic systems which are depicted in the Figures are not to scale. In particular in the lithographic systems depicted in Figures 4, 9 and 10, the separation between the free electron lasers FEL and the optical systems 40 in the x-direction may in practice be considerably greater than is suggested by the Figures. For example a free electron laser FEL may be separated from an optical system 40 by a distance of approximately 50 metres. In some embodiments the separation between a free electron laser FEL and an optical system may be greater than 50 metres (e.g. 100 metres). This may allow the diameter of a radiation beam B’ which is output from a free electron laser to expand before reaching an optical system 40 in order to reduce the energy density of the radiation beam B’ on the optical system 40. A radiation beam B’ may, for example, have a beam diameter which expands to approximately 5 mm when incident on an optical system 40.

[00182] It will further be appreciated that the angles β’, β” with which features are disposed relative to the optical axes O may be smaller than is suggested by the Figures. For example the angles β’, β” may be between 0 and 15°. However in some embodiments the angles β', β” may be greater than 15° and may, for example, be as large as 90°.

[00183] Although embodiments of a free electron laser have been described as including electrical cabinets 37 and cryogenic cooling cabinets 39 housed inside buildings 31, it should be appreciated that in some embodiments one or more the electrical cabinets 37 and/or the cryogenic cooling cabinets 39 may be housed outside of a building 31 in which other components of a free electron laser FEL are housed. In some embodiments a free electron laser may include further cabinets which house other components of a free electro laser FEL. For example, a free electron laser FEL may comprise auxiliary cabinets containing electrical and/or thermal components of a free electron laser FEL. One or more auxiliary cabinets may be housed inside a building 31 or may be housed outside of a building 31.

[00184] Various embodiments of a free electron laser FEL have been described above which each comprise a single electron source 21 and a single linear accelerator 22. However in other embodiments a free electron laser FEL may comprise a plurality of electron sources 21 and/or a plurality of linear accelerators 22. In such embodiments various different arrangements of the components of the free electron laser FEL across one or more vertical levels are possible. Figures 11-16 are schematic illustrations of various alternative embodiments of a free electron laser FEL which comprises a plurality of electron sources 21 and a plurality of linear accelerators 22.

[00185] Figure 11A is a schematic illustration of a top view of an embodiment of a free electron laser FEL comprising two electron sources 21a, 21b and six linear accelerators 22a-22f. Figure 11B is a schematic illustration of a side view of the embodiment of a free electron laser FEL which is shown in Figure 11 A. The two electron sources 21a, 21b are positioned on a first vertical level 41 which is located above a second vertical level 43. The two electron sources 21a, 21b each emit an electron beam E. The electron beams which are emitted by the two electron sources 21a, 21b are emitted in opposing directions. The electron beams E which are emitted from the electron sources 21a, 21b are merged by an electron beam merger 301 so as to form a single electron beam E. The electron beam merger 301 is located on the first vertical level 41.

[00186] The merged electron beam E is directed from the first vertical level 41 to the second vertical level 43 on which the linear accelerators 22a-22f and an undulator 24 are situated. The merged electron beam E is accelerated by the linear accelerators 21 a-21 f before being provided to the undulator 24. The electron beam E is caused to follow a periodic path in the undulator 24 which causes the emission of a radiation beam B from the undulator 24. As is shown in Figure 11A the electron beam E which leaves the undulator 24 passes back through the linear accelerators 22a-22f which may act to decelerate the electron beam E. The decelerated electron beam E may be directed to an electron dump (not shown in Figures 11A and 11B). Whilst not shown in Figures 11A and 11B, the free electron laser may comprise one or more beam steering units which are configured to direct the electron beam E between the components of the free electron laser FEL.

[00187] Providing a free electron laser FEL with two electron sources 21a, 21b as is shown in Figures 11A and 11B advantageously provides redundancy in case one of the electron sources 21a, 21b stops working or is taken offline (e.g. for maintenance). In the event that one of the electron sources 21a, 21b stops working or is taken offline (e.g. for maintenance) the other electron source may continue to emit an electron beam and the free electron laser FEL can continue operating.

[00188] Providing a free electron laser FEL with a plurality of linear accelerators 22a-22f may allow each linear accelerator 22a-22f to be configured to perform a specific role. For example, a first linear accelerator 22a may be configured to provide an initial acceleration to the relatively low energy electrons which are emitted from the electron sources 21a, 21b. Subsequent linear accelerators 22b-22f may be configured to accelerate electrons of increasing energy so as to further accelerate the electron beam E.

[00189] Figure 12A is a schematic illustration of a top view of an alternative embodiment of a free electron laser FEL comprising two electron sources 21a, 21b and a plurality of linear accelerators 22a-22f. Figure 12B is a schematic illustration of a side view of the embodiment of a free electron laser FEL which is shown in Figure 12A. The embodiment which is shown in Figures 12A and 12B is the same as the embodiment which is shown in Figures 11A and 11B except that the electron sources 21a, 21b are arranged such that the electron beams E which are emitted by the electron sources 21a, 21b are not emitted in opposing directions. The electron beams E are merged by an electron beam merger 301 which directs the merged electron beam to the second vertical level 43. The electron beam merger 301 is configured to direct the merged electron beam to the second vertical level 43 such that the merged electron beam forms a first angle 303 with the electron beam which propagates through the linear accelerators 22a-22f. The first angle 303 may, for example, be approximately 30°.

[00190] Figure 13A is a schematic illustration of a top view of an alternative embodiment of a free electron laser FEL comprising two electron sources 21a, 21b and a plurality of linear accelerators 22a-22f. Figure 13B is a schematic illustration of a side view of the embodiment of a free electron laser FEL which is shown in Figure 13A. The embodiment which is shown in Figures 13A and 13B is the same as the embodiment which is shown in Figures 12A and 12B except that the electron beams which are emitted from the electron sources 21a, 21b are merged by an electron beam merger 301 which is located on the second vertical level 43. The electron beams E which are emitted by the electron sources 21a, 21b are directed to the second vertical level 43 where they are merged by the electron beam merger 301.

[00191] Figure 14A is a schematic illustration of a top view of an alternative embodiment of a free electron laser FEL comprising two electron sources 21a, 21b and a plurality of linear accelerators 22a-22f. Figure 14B is a schematic illustration of a side view of the embodiment of a free electron laser FEL which is shown in Figure 14A. In the embodiment which is shown in Figures 14A and 14B a first linear accelerator 22a and a second linear accelerator 22b are positioned on the first vertical level. The electron beam which is output from the second linear accelerator 22b is directed (e.g. with one or more beam steering units) to the second vertical level 43 on which the remaining linear accelerators 22c-22f are positioned.

[00192] Figure 15A is a schematic illustration of a top view of an alternative embodiment of a free electron laser FEL comprising two electron sources 21a, 21b and a plurality of linear accelerators 22a-22f. Figure 15B is a schematic illustration of a side view of the embodiment of a free electron laser FEL which is shown in Figure 15A. In the embodiment which is shown in Figures 15A and 15B all of the linear accelerators 22a-22f are positioned on the first vertical level 41. The electron beam which is output from a sixth linear accelerator 22f is directed (e.g. with one or more beam steering units) to the second vertical level 43 on which the undulator 24 is positioned.

[00193] Figure 16A is a schematic illustration of a top view of an alternative embodiment of a free electron laser FEL comprising two electron sources 21a, 21b and a plurality of linear accelerators 22a-22f. Figure 16B is a schematic illustration of a side view of the embodiment of a free electron laser FEL which is shown in Figure 16A. In the embodiment which is shown in Figures 16A and 16B the first linear accelerator 22a is located on the first vertical level 41 and the remaining linear accelerators 22b-22f are located on the second vertical level 43.

[00194] Although embodiments of a free electron laser have been described as comprising a linear accelerator 22, it should be appreciated that a linear accelerator 22 is merely an example of a type of particle accelerator which may be used to accelerate electrons in a free electron laser. A linear accelerator 22 may be particularly advantageous since it allows electrons having different energies to be accelerated along the same trajectory. Flowever in alternative embodiments of a free electron laser other types of particle accelerators may be used to accelerate electrons to relativistic energies.

[00195] Embodiments of a free electron laser have been described in which an electron beam propagates along a first path and substantially in a first direction and along a second path and substantially in a second direction, wherein the first path and the second path are vertically separated from one another. Whilst embodiments have been described and depicted in which the first and second paths are substantially parallel with each other and are substantially parallel with a horizontal direction, other arrangements may instead be used. For example, in some embodiments the first path and/or the second path may be disposed at a non-zero angle with respect to the horizontal whilst remaining vertically separated from each other. In some embodiments the first and second paths may form different angles with respect to the horizontal and may therefore be disposed at a non-zero angle with respect to each other.

[00196] Whilst embodiments of a radiation source SO have been described and depicted as comprising two free electron lasers FEL, it should be appreciated that a radiation source may comprise any number of free electron lasers FEL. For example, a radiation source may comprise a single free electron laser FEL or may comprise a number of free electron lasers which is greater than two.

[00197] Whilst embodiments of a radiation source SO have been described and depicted as comprising an optical system 40, it should be appreciated that some embodiments of a radiation source SO may not include an optical system 40. For example, a free electron laser may provide a radiation beam B’ directly to a beam splitting apparatus 20 of a lithographic system LS without first being directed to an optical system 40. Alternatively a free electron laser may provide a radiation B' directly to a lithographic apparatus LA.

[00198] Various embodiments of lithographic systems have been described above in which the radiation which is emitted from one or more free electron lasers FEL is provided to a plurality of lithographic apparatus. It will be appreciated, from the above described embodiments, that radiation which is emitted from a free electron laser FEL may be directed to a lithographic apparatus by one or more reflective elements at which the radiation undergoes a reflection so as to rotate the direction of propagation of the radiation.

[00199] During each reflection which a radiation beam undergoes at a reflective element, a portion of the radiation beam may be absorbed at the reflective element and will therefore be lost from the radiation beam. Absorption of radiation at one or more reflective elements at which radiation is reflected during its optical path from a free electron laser to a lithographic apparatus may therefore reduce the amount of radiation which is provided to a lithographic apparatus. The amount of radiation which is lost due to absorption along an optical path from a free electron laser FEL to a lithographic apparatus depends on the number of reflections which the radiation undergoes during the optical path and the amount of radiation which is lost to absorption during the reflections.

[00200] The amount of radiation which is absorbed during reflection at a reflective element may depend on the angle of incidence at which the radiation is incident on the reflective element. For example, the amount of radiation which is absorbed during a reflection may increase with increases in the angle of incidence with which radiation is incident on a reflective element. The angle of incidence with which radiation is incident on a reflective element depends on the rotation of the direction of propagation which the reflective element is arranged to cause. For example, a reflective element may be arranged to rotate the direction of propagation of a radiation beam through approximately 90°. In order to rotate the direction of propagation of a radiation beam through approximately 90° the angle of incidence of the radiation beam on a reflective element may be approximately 45°, which may result in a relatively large amount of radiation being lost due to absorption at the reflective element.

[00201] Alternatively, the direction of propagation of a radiation beam may be rotated by approximately 90° by reflection at a plurality of reflective elements which are each arranged to rotate the direction of propagation of a radiation beam by an angle which is less than 90°. In an embodiment in which a radiation beam is incident on a plurality of reflective elements which each rotate the direction of propagation of a radiation beam by an angle which is less than 90°, the amount of absorption which occurs during each reflection may be less than the amount of absorption which occurs during a single reflection which rotates the direction of propagation of a radiation beam by 90°. Flowever, in order to rotate the direction of propagation of a radiation beam by 90“the number of reflections which the radiation beam undergoes (during which radiation is absorbed) is increased.

[00202] In general the amount of radiation which is lost from a radiation beam by absorption at reflective elements therefore depends on the amount by which the direction of propagation of the radiation beam is rotated during reflections. It may therefore be desirable to arrange the components of a lithographic system so as to reduce the amount by which the direction of propagation of a radiation beam is rotated during reflections, so as to reduce the amount of radiation which is lost from a radiation beam by absorption at reflective elements.

[00203] Figure 17A is a schematic illustration of a lithographic system LS which includes a first row of lithographic apparatus 203 and a second row of lithographic apparatus 205, as viewed from above. In the embodiment which is shown in Figure 11, the first row of lithographic apparatus 203 comprises ten lithographic apparatus LArLA10 and the second row of lithographic apparatus 205 comprises ten lithographic apparatus LAn-UW However, in other embodiments the first 203 and/or the second row 205 of lithographic apparatus may comprise more or less than ten lithographic apparatus.

[00204] The first and second rows of lithographic apparatus are provided with radiation from a radiation source SO. In the embodiment which is shown in Figure 17A, the radiation source SO emits a first radiation beam 201 and a second radiation beam 202. The radiation source may comprise one or more free electron lasers. For example, the radiation source may comprise a single free electron laser which emits a radiation beam which is split into the first and second radiation beams 201, 202. Alternatively, the radiation source SO may comprise a plurality of free electron lasers. For example, the radiation source may comprise a first free electron laser which emits the first radiation beam 201 and a second free electron laser which emits the second radiation beam 202. Alternatively, the first radiation beam 201 and the second radiation beam 202 may each comprise a combination of radiation which is emitted from a plurality of free electron lasers.

[00205] In some embodiments the radiation source SO may comprise a first free electron laser which in normal operation emits the first radiation beam 201 and a second free electron laser which in normal operation emits the second radiation beam 202. The radiation source SO may further comprise optical components which are configured to provide the first and second radiation beams in the event that one of the first and second free electron lasers stops working or is taken offline (e.g. for maintenance). For example, in the event that one of the free electron lasers stops working or is taken offline, one or more optical components may be moved into the optical path of the radiation beam which continues to be emitted. The one or more optical components may be configured to split the radiation which continues to be emitted into the first and second radiation beams 201, 202. Such an arrangement may ensure that both the first and second radiation beams 201, 202 continue to be emitted even when one of the free electron lasers stops working or is taken offline.

[00206] The first radiation beam 201 is provided to the first row of lithographic apparatus 203 and the second radiation beam 202 is provided to the second row of lithographic apparatus 205. In the embodiment which is shown in Figure 17A, the lithographic apparatus LA1-LA20 are arranged in two straight lines along which the first and second radiation beams 201,202 propagate.

[00207] Figure 17B is a schematic illustration of a portion of the lithographic system LS of Figure 17A as viewed from the side. For ease of illustration only the first row of lithographic apparatus 203 is shown in Figure 17A. The second row of lithographic apparatus 205 may be arranged similarly to the arrangement of the first row of lithographic apparatus which is shown in Figure 17B. As can be seen in Figure 17B, the lithographic system LS includes a beam splitting apparatus 220 which receives the first radiation beam 201 and splits the first radiation beam 201 into a plurality of branch radiation beam BrB10. The beam splitting apparatus 220 directs each branch radiation beam to a lithographic apparatus in the first row of lithographic apparatus 203. The radiation source SO and the beam splitting apparatus 220 are positioned on a first vertical level 211 and the first row of lithographic apparatus 203 are positioned on a second vertical level 212. The beam splitting apparatus 220 directs the branch radiation beams Βτ-Βκ, from the first vertical level 211 to the second vertical level 212. The beam splitting apparatus 220 may, for example, comprise a series of mirrors (not shown) which are each configured to split off a portion of the first radiation beam 201 into a branch radiation beam BrB10.

[00208] In the embodiment of a lithographic system LS which is shown in Figure 17A and 17B, each branch radiation beam which is provided to a lithographic apparatus undergoes a single reflection in the beam splitting apparatus 220 before being provided to the lithographic apparatus. The single reflection may serve to change the direction of propagation of the branch radiation beam by approximately 90° as is shown in Figure 17B. Each branch radiation beam Bt^o therefore experiences a single rotation in its direction of propagation of approximately 90° on its optical path from the radiation source SO to a lithographic apparatus. The amount of radiation which is lost to absorption during the rotation of the direction of propagation of each branch radiation beam may be relatively small.

[00209] In the embodiment of a lithographic system LS which is shown in Figures 17A and 17B a radiation source SO emits two radiation beams which each provide radiation to a single row of lithographic apparatus. The rows of lithographic apparatus extend parallel to the direction of propagation of the radiation beams which are emitted from the radiation source SO. Advantageously this allows each lithographic apparatus to be provided with a branch radiation beam whose direction of propagation has undergone only a single rotation of approximately 90° before being provided to the lithographic apparatus. Consequently the amount of radiation which is lost from each branch radiation beam due to absorption is relatively small.

[00210] In other embodiments, the number of rows of lithographic apparatus which are to be provided with radiation is greater than the number of radiation beams which are emitted from the radiation source. For example, in some embodiments it may be desirable to provide radiation to more than two rows of lithographic apparatus.

[00211] Figure 18 is a schematic illustration of an alternative embodiment of a lithographic system LS in which radiation is provided to four rows of lithographic apparatus. Similarly to the embodiment of a lithographic system which is shown in Figures 17A and 17B, the lithographic system LS of Figure 18 includes a radiation source SO which emits a first radiation beam 201 and a second radiation beam 202. As was described above with reference to the embodiment shown in Figure 17A and 17B, the radiation source SO may comprise one or more free electron lasers. The first and second radiation beams 201, 202 may each comprise radiation which is emitted from a single free electron laser or may comprise a combination of radiation which is emitted from a plurality of free electron lasers.

[00212] The lithographic system LS which is shown in Figure 18 further comprises a first row of lithographic apparatus 203, a second row of lithographic apparatus 205, a third row of lithographic apparatus 207 and a fourth row of lithographic apparatus 209. The first, second, third and fourth rows of lithographic apparatus 203, 205, 207 and 209 each extend parallel to each other. In order to provide a radiation beam to each of the rows of lithographic apparatus 203, 205, 207, 209, the first and second radiation beams which are emitted by the radiation source SO are split into sub-beams by a first beam splitting unit 231 and a second beam splitting unit 232. The first beam splitting unit 231 splits the first radiation beam 201 into a first sub-beam 2011 which is provided to the first row of lithographic apparatus 203 and a third sub-beam 2012 which is provided to the third row of lithographic apparatus 207. The second beam splitting unit 232 splits the second radiation beam 202 into a second subbeam 2021 which is provided to the second row of lithographic apparatus 207 and a fourth sub-beam 2022 which is provided to the fourth row of lithographic apparatus 209.

[00213] The first beam splitting unit 231 and the second beam splitting unit 232 may, for example, comprise a mirror or a reflective grating which is configured to split a radiation beam into respective sub-beams. It can be seen from Figure 18 that the directions of propagation of the third sub-beam 2012 and the fourth sub-beam 2022 are rotated by 90° in the first and second beam splitting units 231, 232 respectively. The rotation of the direction of propagation of the third and fourth sub-beams in the beam splitting units means that the third and fourth sub-beams propagate away from the beam splitting units in a direction which is perpendicular to the direction in which the third and fourth rows of lithographic apparatus extend.

[00214] In order to provide the third sub-beam 2012 to the third row of lithographic apparatus 207 a first reflector 233 is arranged to reflect the third sub-beam 2012 so as to rotate the direction of propagation of the third sub-beam 2012 by approximately 90°. The third sub-beam 2012 which is reflected from the first reflector propagates along the third row of lithographic apparatus 207 such that the third sub-beam 2012 may be split into branch radiation beams, each branch radiation beam being provided to a lithographic apparatus LA2i-LA30 of the third row of lithographic apparatus 207. The third sub-beam may be split into branch radiation beams by a beam splitting apparatus (not shown in Figure 18) which may be similar to the beam splitting apparatus 220 which is shown in Figure 17B.

[00215] As is the case in the beam splitting apparatus 220 which is shown in Figure 17B, the direction of propagation of each branch radiation beam which is provided to a lithographic apparatus LA2i-LA30 of the third row of lithographic apparatus 207 may undergo a rotation of approximately 90° in a beam splitting apparatus before being provided to a lithographic apparatus LA2rLA30. The directions of propagation of each branch radiation beam which is provided to a lithographic apparatus LA2i-LA30 of the third row of lithographic apparatus 207 therefore undergoes three rotations of approximately 90° between the radiation source SO and a lithographic apparatus LA21-LA30.

[00216] In order to provide the fourth sub-beam 2022 to the fourth row of lithographic apparatus 209 a second reflector 234 is arranged to reflect the fourth sub-beam 2022 so as to rotate the direction of propagation of the fourth sub-beam 2022 by 90°. The fourth subbeam 2022 which is reflected from the second reflector 234 propagates along the fourth row of lithographic apparatus 209 such that the fourth sub-beam 2022 may be split into branch radiation beams, each branch radiation beam being provided to a lithographic apparatus of the fourth row of lithographic apparatus 209. The fourth sub-beam 2022 may be split into branch radiation beams by a beam splitting apparatus (not shown in Figure 18) which may be similar to the beam splitting apparatus 220 which is shown in Figure 17B. As is the case in the beam splitting apparatus 220 which is shown in Figure 17B, the direction of propagation of each branch radiation beam which is provided to a lithographic apparatus LA31-LA40 of the fourth row of lithographic apparatus 209 may undergo a rotation of approximately 90° in a beam splitting apparatus before being provided to a lithographic apparatus LA31-LA40. The directions of propagation of each branch radiation beam which is provided to a lithographic apparatus LA31-LA40 of the fourth row of lithographic apparatus 209 therefore undergoes three rotations of approximately 90° between the radiation source SO and a lithographic apparatus LA31-LA40.

[00217] As has been described above with reference to Figure 18, the direction of propagation of each branch radiation beam which is provided to a lithographic apparatus of the third row of lithographic apparatus 207 and the fourth row of lithographic apparatus 209, undergoes three rotations of approximately 90° before the branch radiation beam is provided to a lithographic apparatus. In contrast to the branch radiation beams which are provided to the third and fourth rows of lithographic apparatus 207, 209, the directions of propagation of each branch radiation beam which is provided to a lithographic apparatus of the first row of lithographic apparatus 203 and the second row of lithographic apparatus 205, undergoes a single rotation of approximately 90° before the branch radiation beam is provided to a lithographic apparatus. Since radiation is lost from a radiation beam by absorption each time it undergoes a rotation of its direction of propagation, more radiation is lost from the branch radiation beams which are provided to the third and fourth rows of lithographic apparatus than is lost from the branch radiation beams which are provided to the first and second rows of lithographic apparatus. It is therefore desirable to arrange the components of a lithographic system so as to reduce the amount of radiation which is lost from branch radiation beams which are provided to the third and fourth rows of lithographic apparatus.

[00218] In the embodiments of a lithographic system which are shown in Figures 17A, 17B and 18, the direction of propagation of the branch radiation beams which are provided to lithographic apparatus of the first and second rows of lithographic apparatus undergo only a single rotation of approximately 90° between the radiation source SO and a lithographic apparatus. However, in other embodiments the direction of propagation of the branch radiation beams which are provided to lithographic apparatus of the first and second rows of lithographic apparatus may undergo more than one rotation. For example, if the first and second radiation beams which are emitted from the radiation source SO are not aligned with the first and second rows of lithographic apparatus (as they are in Figure 17A and 18) then the direction of propagation of the radiation beams which are provided to the first and second radiation beams may need to be rotated at least twice (e.g. two rotations of approximately 90°) in order to align their direction of propagation with the first and second rows of lithographic apparatus. Consequently the amount of radiation which is lost from the radiation beams which are provided to the first and second rows of lithographic apparatus may be increased. It may therefore be further desirable to arrange the components of a lithographic system so as to reduce the amount of radiation which is lost due to absorption.

[00219] Figure 19 is a schematic illustration of an embodiment of a lithographic system LS in which the components of the lithographic system LS are arranged so as to reduce the amount of radiation which is lost from radiation beams due to absorption. The lithographic system LS which is shown in Figure 19 comprises a radiation source SO and first, second, third and fourth rows of lithographic apparatus 203, 205, 207, 209. The first, second, third and fourth rows of lithographic apparatus 203, 205, 207, 209 each extend parallel to each other. The radiation source SO emits a first radiation beam 201 and a second radiation beam 202. In the embodiment which is shown in Figure 19, the radiation source SO is arranged such that the first and second radiation beams are emitted such that they propagate in a direction which is perpendicular to the direction in which the first, second, third and fourth rows of lithographic apparatus extend.

[00220] The lithographic system LS comprises a first beam splitting unit 231 and a second beam splitting unit 232. The first beam splitting unit 231 is arranged to split the first radiation beam into a first sub-beam 2011 which is provided to the first row of lithographic apparatus 203 and a third sub-beam 2012 which is provided to the third row of lithographic apparatus 207. The second beam splitting unit 232 is arranged to split the second radiation beam into a second sub-beam 2021 which is provided to the second row of lithographic apparatus 205 and a fourth sub-beam 2022 which is provided to the fourth row of lithographic apparatus 209. A first reflector 233 is arranged to direct the third sub-beam to the third row of lithographic apparatus 207. A second reflector 234 is arranged to direct the fourth sub-beam to the fourth row of lithographic apparatus 209.

[00221] The directions of propagation of the first and second sub-beams 2011, 2021 are rotated by approximately 90° at the first and second beam splitting units 231, 232 respectively. The directions of propagation of the third and fourth sub-beams 2012, 2022 are rotated by approximately 90° at the first and second reflectors 233, 234 respectively. The first, second, third and fourth sub-beams are therefore provided to the first, second, third and fourth rows of lithographic apparatus having undergone a single rotation in their direction of propagation.

[00222] The first, second, third and fourth sub-beams are each split into a plurality of branch radiation beams by a beam splitting apparatus (not shown in Figure 19). The branch radiation beams are provided to a lithographic apparatus in the rows of lithographic apparatus. As has been described above splitting a sub-beam into a branch radiation beam may be performed by rotating the direction of propagation of a branch radiation beam by approximately 90°. In the embodiment which is shown in Figure 19, each branch radiation beam which is provided to a lithographic apparatus may therefore have undergone two rotations of its direction of propagation (each of approximately 90°) before being provided to the lithographic apparatus. The first rotation of the direction of propagation of a branch radiation beam occurs at a beam splitting unit 231, 232 or a reflector 233, 234 and the second rotation of the direction of propagation of a branch radiation beam occurs at a beam splitting apparatus.

[00223] An embodiment of a lithographic system LS in which the radiation source SO emits radiation beams which propagate perpendicular to the direction in which the rows of lithographic apparatus extend (e.g. as shown in Figure 19), may therefore advantageously reduce the number of rotations in the direction of propagation of a branch radiation beam which occurs before the branch radiation beam is provided to a lithographic apparatus. For example, in the embodiment of Figure 19 the branch radiation beams which are provided to lithographic apparatus in the third and fourth rows of lithographic apparatus have undergone two rotations in their direction of propagation before being provided to a lithographic apparatus. This is fewer rotations than occurs, for example, in the embodiment which is shown in Figure 18, in which each branch radiation beam undergoes three rotations in its direction of propagation before being provided to a lithographic apparatus in the third or fourth row of lithographic apparatus. As was described above, reducing the number of rotations of the direction of propagation of a branch radiation beam before it is provided to a lithographic apparatus advantageously reduces the amount of radiation which is lost from the branch radiation beam due to absorption.

[00224] Figure 20 is a further alternative embodiment of a lithographic system LS comprising a radiation source SO and four rows of lithographic apparatus. The components which form the lithographic system LS of Figure 20 are the same as the components which form the optical system of Figure 19 and will not be described again in detail with reference to Figure 20. In the embodiment of Figure 20 the radiation source SO is arranged so as to emit the first and second radiation beam 201, 202 such that they propagate in a direction which is neither perpendicular to or parallel with the direction in which the rows of lithographic apparatus extend. As is shown in Figure 20 the direction in which the first and second radiation beams which are emitted from the radiation source SO propagate forms an angle Θ with the direction in which the rows of lithographic apparatus extend. The angle Θ is greater than 0° and less than 90°. For example, the angle Θ may be between about 10° and about 60°. In an embodiment the angle Θ may be approximately 45°. In some embodiments the angle Θ may be less than about 45°.

[00225] As is shown in Figure 20, the first and second radiation beams are split into subbeams at a first beam splitting unit 231 and a second beam splitting unit 232. A first subbeam 2011 is provided to the first row of lithographic apparatus 203 by the first beam splitting unit 231. A second sub-beam 2021 is provided to the second row of lithographic apparatus 205 by the second beam splitting unit 231. As can be seen in Figure 20, the rotation of the direction of propagation of the first sub-beam 2011 at the first beam splitting unit 231 and the rotation of the direction of propagation of the second sub-beam 2021 at the second beam splitting unit 232 is less than 90°.

[00226] A third sub-beam 2012 is directed to the third row of lithographic apparatus 207 by reflection at a first reflector 233. A fourth sub-beam 2022 is directed to the fourth row of lithographic apparatus 209 by reflection at a second reflector 234. As can be seen in Figure 20, the rotation of the direction of propagation of the third sub-beam 2012 at the first reflector 233 and the rotation of the direction of propagation of the fourth sub-beam 2022 at the second reflector 234 is less than 90°.

[00227] As was described above with reference to Figure 19 each of the sub-beams are split into branch radiation beams by rotating the direction of propagation of branch radiation beams by approximately 90° in a beam splitting apparatus (not shown in Figure 20). As was the case in the embodiment which is shown in Figure 19, each branch radiation beam in the embodiment of Figure 20 therefore undergoes two rotations in its direction of propagation before being provided to a lithographic apparatus. Flowever in contrast to the embodiment of Figure 19, in the embodiment of Figure 20 one of the rotations in the direction of propagation of the branch radiation beams is less than 90°. For example, the rotation of the direction of propagation which occurs at the first or second beam splitting unit 231, 232 or the rotation of the direction of propagation which occurs at the first or second reflector 233, 234 is less than 90°.

[00228] The amount of radiation which is lost due to absorption during a rotation of the direction of propagation of a radiation beam may decrease with the decreases in the amount by which the direction of propagation of the radiation beam is rotated. The arrangement which is shown in Figure 20 therefore advantageously reduces the amount of radiation which is lost due to absorption by reducing the amount by which the direction of propagation of sub-beams is rotated at the beam splitting units 231, 232 and the reflectors 233, 234. The amount by which the direction of propagation of the radiation beam is rotated decreases with decreases in the angle Θ.

[00229] The amount of radiation which is lost due to absorption may additionally or alternatively be reduced by splitting the sub-beams into branch radiation beams such that the rotation of the direction of propagation of the branch radiation beams is less than 90°. Figure 21 is a schematic illustration of a portion of a beam splitting apparatus configured to split a sub-beam 2011 into a branch radiation beam E^. The branch radiation beam is provided to a lithographic apparatus LA^ The branch radiation beam Bt may, for example, be formed by a reflective element (not shown in Figure 21) which is arranged in a portion of the cross-section of the sub-beam 2011. The reflective element reflects the portion of the sub-beam 2011 which is incident on the reflective element so as to form and direct a branch radiation beam into the lithographic apparatus LAt through an opening 251 in an enclosing structure of the lithographic apparatus U^. In the example, which is shown in Figure 21 the rotation of the direction of propagation of the branch radiation beam B, which causes the branch radiation beam Bi to be directed to the lithographic apparatus LAi is less than 90°. Advantageously this may reduce the amount of radiation which is lost from the branch radiation beam.

[00230] Figure 22A is a schematic illustration of a top view of an embodiment of an electron beam merger 301 for merging two input electron beams 2201 and 2202, e.g. from two electron sources 21a, 21b (not shown) at a first vertical level, into a single output electron beam 2212 at a second vertical level. Figure 22B is a schematic illustration of a side view of the embodiment of an electron beam merger 301 which is shown in Figure 22A. The embodiment of the electron beam merger 301 as shown in Figures 22A and 22B is advantageous in that it minimizes space-charge induced emittance growth of the merged electron beam by minimizing length and bending angle of the electron beamline. The electron beam merger 301 receives two incoming electron beams 2201 and 2202, e.g. from electron sources 21a and 21b. Electron beam 2201 is first bend slightly in two directions by a dipole magnet 2203. Next, the electron beam is focused and defocused by three quadrupole magnets 2205, again bend slightly in two directions by a dipole magnet 2207, again focused or defocused by a quadrupole magnet 2208, bend slightly in one direction by a dipole magnet 2209, again focused or defocused by three quadrupole magnets 2210 and again bend slightly in one direction by a dipole magnet 2211 to finally arrive at output 2212. Electron beam 2202 is first bend slightly in two directions by a dipole magnet 2204. Next, the electron beam is focused and defocused by three quadrupole magnets 2206, again bend slightly in two directions by the dipole magnet 2207, again focused or defocused by the quadrupole magnet 2208, bend slightly in one direction by the dipole magnet 2209, again focused or defocused by the three quadrupole magnets 2210 and again bend slightly in one direction by the dipole magnet 2211 to finally arrive at output 2212.

[00231] The dipole magnet 2203 and quadrupole magnets 2205 on the one hand side and the dipole magnet 2204 and quadrupole magnets 2206 on the other hand side rotate the input electron beams 2201 and 2202, respectively, by the same angle but in opposite directions. The dipole magnet 2207 can select one or the other electron beam by applying a correspondingly rotated dipole magnetic field. Dipole magnet 2207 can select either one of the electron beams 2201 and 2202 as bend by dipole magnets 2203 and 2204, respectively, and (de)focused by quadrupole magnets 2205 and 2206, respectively, for processing by magnets 2208 to 2211. To this end, dipole magnet 2207 may be a dipole magnet with a rotatable dipole field. To rotate the dipole field, the dipole magnet 2207 itself may be rotated. Alternatively, the dipole magnet 2207 can be equipped with two sets of coils to selectively generate one out of two mutually rotated magnetic fields. Also, the dipole magnet 2207 may have two separate and mutually rotated magnets that are placed behind each other and that can be switched on or off in order to select either one of the electron beams 2201 and 2202 as bend by dipole magnets 2203 and 2204, respectively, and (de)focused by quadrupole magnets 2205 and 2206, respectively, for processing by magnets 2208 to 2211.

[00232] The electron beams E which are emitted by the electron sources 21a, 21b are directed to the second vertical level 43 where they are merged by the electron beam merger 301.

[00233] Embodiments of a lithographic system LS have been described above with reference to Figures 17-20, in which a radiation source SO is configured to emit two radiation beams. In other embodiments the radiation source may be configured to emit a different number of radiation beams. For example, the radiation source SO may be configured to emit a single radiation beam or may be configured to emit more than two radiation beams.

[00234] Typically, lithographic apparatus, such as the lithographic apparatus which form the rows of lithographic apparatus, which are shown in Figures 11-20 are housed in a semiconductor fabrication plant, which may be referred to as a fab. In some embodiments a radiation source which provides radiation to lithographic apparatus may be positioned outside of a fab. For example, a radiation source SO may be positioned in a separate building to a fab. In some embodiments a radiation source which is positioned outside of a fab may emit one or more radiation beams on a different vertical level to the vertical level of the fab. For example, the row of lithographic apparatus which is shown in Figure 17B may be located inside a fab which is positioned on the second vertical level 212. The branch radiation beams may enter the fab through a floor of the fab. Alternatively a radiation beam which is emitted from a radiation source SO may be directed through the floor of a fab and the radiation may be split into branch radiation beams within the fab.

[00235] In other embodiments a radiation source SO comprising one or more free electron lasers may be positioned within a fab. The radiation source SO may therefore be positioned inside the same building as a plurality of lithographic apparatus which are provided with radiation from the radiation source SO. Typically the temperature inside a fab in which lithographic apparatus are located is accurately controlled such that the lithographic apparatus are held at a stable temperature. Positioning a radiation source SO comprising a free electron laser inside a fab may therefore advantageously lead to the components of the radiation source also being held at a stable temperature.

[00236] Holding the components of a radiation source at a stable temperature may assist in controlling the relative alignment of the components of the radiation source. For example, if the temperature of components of a radiation source were to vary over time then some components may expand and/or contract with changes in their temperature. Expansion and/or contraction of components of a radiation source may disadvantageously alter the relative alignment of components. Holding the components of a radiation source SO at a stable temperature may advantageously assist in reducing any undesirable variation in the relative alignment of components of a radiation source SO.

[00237] Positioning a radiation source SO inside a fab may circumvent a need to construct one or more extra buildings in which a radiation source SO is positioned. For example, an existing fab building which contains a plurality of lithographic apparatus may be updated by positioning a radiation source SO comprising a free electron laser inside the fab building. Updating an existing fab by positioning a radiation source SO inside the fab building may circumvent a need to construct one or more extra buildings in which a radiation source SO is positioned. This may be a cheaper alternative to constructing a new building and may be particularly advantageous in situations in which there is no space available near to a fab in which to position a radiation source SO. Circumventing a need to construct one or more extra buildings may additionally make it easier to gain regulatory approval for updating a fab. For example, the construction of one or more extra buildings in which a radiation source SO is positioned may require regulatory approval which may be difficult to obtain.

[00238] In embodiments in which a radiation source SO comprising a free electron laser is positioned inside a fab, one or more components of a free electron laser may be surrounded by walls which do not substantially transmit radiation which is generated in the free electron laser whilst the free electron laser is in operation. For example, components of a free electron laser may be surrounded by thick concrete walls (e.g. walls which are approximately 4 metres thick). The walls may be further provided with radiation shielding materials such as, for example, lead, steel, boron and/or other materials which are configured to absorb neutrons and/or other radiation types.

[00239] In embodiments in which a radiation source SO comprising a free electron laser is positioned inside a fab some components may be positioned outside of the fab. For example, cryogenic cooling cabinets which contain apparatus which is configured to provide cryogenic cooling to components of a free electron laser may be housed outside of the fab. Cryogenic cooling components which are housed within cryogenic cooling cabinets may generate vibrations. If the cryogenic cooling cabinets were to be positioned inside a fab then the vibrations generated by the cryogenic cooling components may disadvantageously be transferred to one or more lithographic apparatus housed within the fab which may adversely affect the operation of the lithographic apparatus. Positioning cryogenic cooling components outside of the fab may advantageously reduce the impact of vibrations generated by the cryogenic cooling components on the lithographic apparatus housed within the fab. In some embodiments one or more other components may be located outside of a fab.

[00240] Although the described embodiments of a lithographic system LS comprise twenty lithographic apparatuses LA1-LA20, a lithographic system LS may comprise any number of lithographic apparatus. The number of lithographic apparatus which form a lithographic system LS may, for example, depend on the amount of radiation which is output from a radiation source SO and on the amount of radiation which is lost in a beam splitting apparatus 20. The number of lithographic apparatus which form a lithographic system LS may additionally or alternatively depend on the layout of a lithographic system LS and/or the layout of a plurality of lithographic systems LS.

[00241] Embodiments of a lithographic system LS may also include one or more mask inspection apparatus MIA and/or one or more Aerial Inspection Measurement Systems (AIMS). In some embodiments, the lithographic system LS may comprise two mask inspection apparatuses to allow for some redundancy. This may allow one mask inspection apparatus to be used when the other mask inspection apparatus is being repaired or undergoing maintenance. Thus, one mask inspection apparatus is always available for use. A mask inspection apparatus may use a lower power radiation beam than a lithographic apparatus. Further, it will be appreciated that radiation generated using a free electron laser FEL of the type described herein may be used for applications other than lithography or lithography related applications.

[00242] The term “relativistic electrons” should be interpreted to mean electrons which have been accelerated to relativistic energies by a particle accelerator. An electron may be considered to have a relativistic energy when its kinetic energy is comparable to or greater than its rest mass energy (5.11 keV). In practice a particle accelerator which forms part of a free electron laser may accelerate electrons to energies which are much greater than its rest mass energy. For example a particle accelerator may accelerate electrons to energies of >10 MeV, >100 MeV, >1GeV or more.

[00243] Embodiments of the invention have been described in the context of a free electron laser FEL which outputs an EUV radiation beam. However a free electron laser FEL may be configured to output radiation having any wavelength. Some embodiments of the invention may therefore comprise a free electron which outputs a radiation beam which is not an EUV radiation beam.

[00244] The term “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.

[00245] The lithographic apparatus LAT-LA20 may be used in the manufacture of ICs. Alternatively, the lithographic apparatuses LA1-LA20 described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[00246] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below. Other aspects of the invention are set-out as in the following numbered clauses. 1. A free electron laser comprising: an electron source operable to emit a beam of electrons; a particle accelerator operable to accelerate the beam of electrons to relativistic energies; an undulator operable to guide the relativistic electron beam along a periodic path so as to stimulate emission of coherent radiation; and a steering unit operable to alter the trajectory of the electron beam so as to direct the electron beam from a first path along which the electron beam propagates substantially in a first direction to a second path along which the electron beam propagates substantially in a second direction, wherein the first path and the second path are vertically separated from one another. 2. The free electron laser of clause 1, wherein the particle accelerator is operable to accelerate the beam of electrons to energies greater than about 10 MeV. 3. The free electron laser of clause 2, wherein the particle accelerator is operable to accelerate the beam of electrons to energies greater than about 100 MeV. 4. The free electron of any preceding clause, wherein the first and second directions oppose each other. 5. The free electron laser of any preceding clause, wherein the particle accelerator accelerates the electron beam along the first path. 6. The free electron laser of any preceding clause, wherein the undulator is operable to guide the relativistic electron beam along the second path. 7. The free electron laser of any preceding clause, wherein the linear accelerator is positioned on a first vertical level. 8. The free electron laser of any preceding clause, wherein the undulator is positioned on a second vertical level. 9. The free electron laser of clause 8 as dependent on clause 7, wherein the steering unit is operable to alter the trajectory of the electron beams so as to direct the electron beam from the particle accelerator on the first vertical level to the undulator on the second vertical level. 10. The free electron laser of clause 9, wherein the first vertical level is situated above the second vertical level. 11. The free electron laser of any preceding clause, wherein at least some of the components of the free electron laser are housed within a building. 12. The free electron laser of clause 11 as dependent from clause 9 or 10, wherein the first vertical level corresponds to a first vertical level of the building and the second vertical level corresponds to a second vertical level of the building. 13. The free electron laser of clause 11 or 12, wherein the building comprises radiation shielding walls configured to prevent harmful radiation generated by the free electron laser from propagating out of the building. 14. The free electron laser of any preceding clause, further comprising a second steering unit operable to alter the trajectory of the electron beam so as to direct the electron beam from the second path to the first path. 15. The free electron laser of clause 14, wherein the second steering unit is operable to direct the electron beam which is output from the undulator to the particle accelerator. 16. The free electron laser of clause 15, wherein the particle accelerator is operable to decelerate the electron beam which is output from the undulator. 17. The free electron laser of clause 16, wherein the particle accelerator is operable to recover energy from the decelerating electron beam and use the recovered energy to accelerate the electron beam received from the electron source. 18. The free electron laser of any preceding clause, further comprising a bunch compressor configured to spatially compress electrons in the electron beam. 19. The free electron laser of clause 18, wherein the bunch compressor is positioned upstream of the steering unit and downstream of the undulator. 20. The free electron laser of clause 18 or 19, wherein the bunch compressor is positioned on the same vertical level as the vertical level on which the undulator is positioned. 21. The free electron laser of any preceding clause, wherein components of the free electron laser which are positioned on the first vertical level and components of the free electron laser which are positioned on the second vertical level are situated in the same room. 22. The free electron laser of clause 21, wherein the particle accelerator and the undulator are situated in the same room. 23. The free electron laser of clause 21 or clause 22, further comprising a crane, wherein the crane is arranged such that it can access components of the free electron laser which are situated on the first vertical level and components of the free electron laser which are situated on the second vertical level. 24. The free electron laser of any preceding clause, wherein the particle accelerator and the undulator are offset from each other on a horizontal axis which is perpendicular to a longitudinal axis of the particle accelerator. 25. The free electron laser of any preceding clause, further comprising components configured to provide cryogenic cooling to the particle accelerator. 26. The free electron laser of clause 25 as dependent on any of clauses 11-13, wherein the cryogenic cooling components are positioned outside of the building in which the free electron laser is housed. 27. The free electron laser of clause 25 as dependent on any of clauses 11-13, wherein the cryogenic cooling components are housed inside the building in which the free electron laser is housed. 28. The free electron laser of clause 27, wherein a portion of the building in which the cryogenic cooling components are housed is mechanically isolated from a portion of the building in which the particle accelerator and the undulator are housed. 29. The free electron laser of clause 27 or 28, further comprising at least one radiation shielding wall configured to shield a portion of the building in which the cryogenic cooling components are housed from a portion of the building in which the particle accelerator and the undulator are housed. 30. The free electron laser of any preceding clause, further comprising electrical components configured to provide electrical power to components of the free electron laser. 31. The free electron laser of clause 30 as dependent on any of clauses 11-13, wherein the electrical components are positioned outside of the building in which the free electron laser is housed. 32. The free electron laser of clause 30, and any of clauses 11-13, wherein the electrical cooling components are housed inside the building in which the free electron laser is housed. 33. The free electron laser of clause 32, further comprising at least one radiation shielding wall configured to shield a portion of the building in which the electrical cooling components are housed from a portion of the building in which the particle accelerator and the undulator are housed. 34. The free electron laser of any preceding clause, wherein the undulator is configured to cause the relativistic electrons to emit EUV radiation. 35. The free electron laser of any preceding clause, wherein the particle accelerator is a linear accelerator. 36. The free electron laser of any preceding clause, wherein the free electron laser comprises a plurality of particle accelerators operable to accelerate the beam of electrons. 37. The free electron laser of clause 36, wherein at least one of the linear accelerators is positioned on a different vertical level to at least one of the other linear accelerators. 38. The free electron laser of any preceding clause, wherein the free electron laser comprises a plurality of electron sources each operable to emit a beam of electrons. 39. The free electron laser of clause 38, further comprising an electron beam merger configured to merge a plurality of electron beams emitted from a plurality of electron sources into a single electron beam. 40. The free electron laser of clause 38 or 39, wherein the plurality of electron sources are positioned on a different vertical level to the vertical level on which the linear accelerator is positioned. 41. The free electron laser of any of clauses 38-40, wherein the plurality of electron sources are positioned on a different vertical level to the vertical level on which the undulator is positioned. 42. A lithographic system comprising: a radiation source comprising a free electron laser as claimed in any of clauses 1-36; and one or more lithographic apparatus. 43. The lithographic system of clause 42, wherein the lithographic system comprises a plurality of lithographic apparatus each arranged to receive radiation from the radiation source. 44. The lithographic system of clause 43, wherein the plurality of lithographic apparatus are located in a building and the radiation source is located outside of the building in which the plurality of lithographic apparatus are located. 45. The lithographic system of clause 43 or 44, further comprising a beam splitting apparatus configured to receive a radiation beam from the radiation source and split the radiation beam into branch radiation beams and which is further configured to provide a branch radiation beam to each of the plurality of lithographic apparatus. 46. The lithographic system of any of clauses 42-45, wherein the radiation source comprises a plurality of free electron lasers as claimed in any of clauses 1 -41. 47. The lithographic system of clause 46, wherein the radiation source further comprises an optical system configured to receive a radiation beam from each of the plurality of free electron lasers and form a composite radiation beam from the radiation beams. 48. The lithographic system of clause 47 and 43, wherein the optical system is further configured to provide the composite radiation beam to the beam splitting apparatus. 49. The lithographic system of clause and of clauses 46-48, wherein each of the plurality of free electron lasers is configured to output a radiation beam on substantially the same vertical level. 50. The lithographic system of any of clauses 46-49, wherein the free electron lasers are configured to output a radiation beam which propagates at an angle with respect to an optical axis of the radiation source. 51. The lithographic system of any of clauses 43-50, wherein the radiation source is configured to emit a first radiation beam and a second radiation beam. 52. The lithographic system of clause 51, wherein the radiation source comprises a first free electron laser configured to emit the first radiation beam and a second free electron laser configured to emit the second radiation beam. 53. The lithographic system of clause 52, wherein the radiation source further comprises one or more optical components which are operable to split the radiation beam which is emitted from the first free electron laser so as to form the first radiation beam and the second radiation beam in the event that the second free electron laser ceases to emit radiation. 54. The lithographic system of any of clauses 43-53, wherein the lithographic system comprises a plurality of rows of lithographic apparatus which each extend in a first direction. 55. The lithographic system each row of lithographic apparatus is provided with a beam splitting apparatus configured to receive a radiation beam, split the radiation beam into a plurality of branch radiation beams and direct each branch radiation beam to a lithographic apparatus of the row of lithographic apparatus. 56. The lithographic system of clause 55, wherein the radiation source is configured to emit a radiation beam in a second direction, wherein the second direction is not parallel with the first direction. 57. The lithographic system of clause 56, wherein the second direction is perpendicular to the first direction. 58. The lithographic system of any of clauses 54-57, wherein the radiation source is configured to emit a radiation beam and the lithographic system further comprises a beam splitting unit configured to split the radiation beam into a plurality of sub-beams wherein each sub-beam is provided to a row of lithographic apparatus. 59. A plurality of lithographic systems, wherein each of the plurality of lithographic systems comprises a lithographic system as claimed in any of clauses 42-58. 60. The plurality of lithographic systems of clause 59, wherein the plurality of lithographic systems are arranged adjacent to one another. 61. A method for producing radiation comprising: emitting a beam of electrons; accelerating the beam of electrons to relativistic energies along a first path and substantially in a first direction; altering the trajectory of the relativistic electrons from the first path such that the electrons propagate along a second path and substantially in a second direction, wherein the first path and the second path are vertically separated from one another; and causing the relativistic electrons to follow a periodic path thereby causing them to stimulate emission of coherent radiation. 62. The method of clause 61, wherein accelerating the beam of electrons to relativistic energies comprises accelerating the beam of electrons to energies which are greater than about 10 MeV. 63. The method of clause 62, wherein accelerating the beam of electrons to relativistic energies comprises accelerating the beam of electrons to energies which are greater than about 100 MeV. 64. The method of any of clauses 61-63, wherein the first and second directions oppose each other. 65. The method of any of clauses 61-64, further comprising directing an electron beam from the second path to the first path. 66. The method of clause 65, further comprising decelerating an electron beam which is directed from the second path to the first path. 67. The method of clause 66, further comprising recovering energy from the decelerating electron beam and using the energy to accelerate an electron beam along the first path. 68. The method of any of clauses 61-67, further comprising spatially compressing electrons of the relativistic electron beam. 69. The method of clause 68, wherein the relativistic electron beam is spatially compressed after having been directed from the first path to the second path and prior to causing the relativistic electrons to follow a periodic path. 70. The method of any of clauses 61-69, wherein the electron beam is accelerated along a longitudinal axis of a particle accelerator and the relativistic electrons are caused to follow a periodic path about a longitudinal axis of an undulator; and wherein the longitudinal axis of the particle accelerator is offset from the longitudinal axis of the undulator on a horizontal axis which is perpendicular to the longitudinal axis of the particle accelerator. 71. The method of any of clauses 61-70, wherein the relativistic electrons emit EUV radiation.

Claims

A lithography device comprising: an exposure device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.