CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP 14199346.9 which was filed on December 19, 2014 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to an undulator for a free electron laser. In particular, but not exclusively, the free electron laser may be used in the generation of radiation for a lithographic system.

BACKGROUND

[0003] 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.

[0004] 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).

[0005] For lithography and other applications, it is desirable to be able to produce radiation beams with high and/or stable output power.

[0006] It is an object of the present invention to obviate or mitigate at least one problem of prior art techniques.

SUMMARY

[0007] According to a first aspect of the invention there is a provided a mechanical interface for an undulator module comprising: a body; a laser beam sensor attached to the body and operable to determine the position of a reference laser beam relative to part of the mechanical interface; and an alignment feature on the body arranged to allow the mechanical interface to engage with an undulator module such that the laser beam sensor is maintained in fixed relationship with a magnetic centre of the undulator module. [0008] The laser beam sensor, which is attached to the body, allows the position of the mechanical interface (and therefore the undulator module with which it is engaged) relative to a reference laser beam to be determined. Further, since the alignment feature allows the laser beam alignment sensor to be maintained in fixed relationship with a magnetic centre of an undulator module, this allows the position of the magnetic centre of the undulator module relative to the reference laser beam (at the point at which the mechanical interface is engaged) to be determined. If a plurality of undulator modules is each provided with a mechanical interface then the position of the magnetic centre of each one can be determined relative to a single reference laser beam. For example, each undulator module may be provided with two mechanical interfaces, one at each end. Such an arrangement allows a plurality of undulator modules to be aligned accurately.

[0009] Therefore a plurality of mechanical interfaces according to the first aspect of the invention may form part of an undulator alignment system that, along with a reference laser beam, allows alignment of one or more undulator modules with a desired axis or direction. The plurality of mechanical interfaces according to the first aspect may be substantially identical. For example, they may be calibrated during manufacture to eliminate manufacturing tolerances.

[0010] The laser beam sensor may comprise an extraction optic and a detector, wherein the extraction optic may be operable to receive a laser beam and to direct a first portion of the laser beam towards the detector and wherein the detector may be operable to determine a position of a beam spot formed by the first portion of the reference laser beam.

[0011] The extraction optic may, for example, comprise a beam splitter. For example, the extraction optic may comprise a partially transmissive mirror or a beam splitter cube (i.e. two triangular prisms). The detector may comprise an array of sensing elements such as, for example, a charged coupled device (CCD) array.

[0012] The mechanical interface may further comprise an electron beam sensor attached to the body, the electron beam sensor being operable to determine the position of an electron beam relative to part of the mechanical interface, wherein the alignment feature on the body is arranged to allow the mechanical interface to engage with an undulator module such that the electron beam sensor is maintained in fixed relationship with the magnetic centre of the undulator module.

[0013] The electron beam sensor, which is attached to the body, allows the position of an electron beam relative to the mechanical interface (and therefore the undulator module with which it is engaged) to be determined. Further, since the alignment feature allows the electron beam sensor to be maintained in fixed relationship with a magnetic centre of an undulator module, this allows the position of the electron beam sensor relative to the magnetic centre of the undulator module (at the point at which the mechanical interface is attached) to be determined.

[0014] The alignment feature may allow the mechanical interface to releasably engage with an undulator module such that, when engaged with an undulator module, the positions of each of the electron beam sensor and the laser beam sensor relative to the magnetic centre of the undulator module are substantially the same. The electron beam sensor and the laser beam sensor may each be positioned on the body such that when the body is engaged with an undulator module each of the electron beam sensor and the laser beam sensor is disposed at a known, specific or predetermined position relative to the magnetic centre of the undulator module.

[0015] The electron beam sensor may comprise any beam position monitor (BPM). The beam position monitor may of any type, including a button BPM, a stripline BPM or a resonant cavity BPM. The body may define an electron beam aperture for an electron beam to propagate through. The electron beam aperture may receive a beam pipe. The electron beam sensor may comprise a plurality of electrodes arranged around the electron aperture and a processor, which may be operable to determine an electron beam position from signals generated by the plurality of electrodes.

[0016] The laser beam sensor may be further operable to determine the position of a second reference laser beam relative to part of the laser beam sensor.

[0017] Advantageously, this allows for the Abbe error to be corrected for. In addition to allowing a plurality of undulator modules to be aligned (i.e. their magnetic axes may be aligned) this may allow for rotation of each undulator module about its axis to be corrected for.

[0018] The laser beam sensor may comprise a single extraction optic for both of the two reference laser beams or wherein the laser beam sensor comprises two extraction optics, one for each of the two reference laser beams.

[0019] The laser beam sensor may comprise a single detector for both of the two reference laser beams or wherein the laser beam sensor comprises two detectors, one for each of the two reference laser beams.

[0020] According to a second aspect of the present invention, there is provided an undulator module for a free electron laser comprising at least one mechanical interface according to the first aspect of the present invention.

[0021] According to a third aspect of the present invention, there is provided an undulator for a free electron laser, comprising: a plurality of undulator modules, each undulator module being operable to produce a periodic magnetic field and having a magnetic centre, the plurality of undulator modules being arranged to receive an electron beam and to guide the electron beam along a periodic path such that electrons within the electron beam interact with radiation in the undulator to stimulate emission of coherent radiation to provide a radiation beam; a plurality of actuators, each actuator operable to control the position of one of the plurality of modules; a laser operable to output a reference laser beam extending along and adjacent to the plurality of undulator modules; and a plurality of laser beam sensors, each laser beam sensor attached to one or the plurality of undulator modules, in fixed relationship with the magnetic center of that undulator module, and being operable to determine the position of the reference laser beam relative to the laser beam sensor; and a controller which is operable to control the plurality of actuators in response to the position of the magnetic centre of each of the plurality of modules relative to the reference laser beam so as to maintain alignment between the magnetic centres of each of the plurality of undulator modules.

[0022] The magnetic centre of each undulator module is a line extending between opposed ends of the undulator section. At any point along the undulator module, the magnetic centre is that point about which the magnetic field is symmetric. For an ideal undulator module, the magnetic centre may be a linear axis extending through the undulator section.

[0023] The undulator may further comprise a sensor that is operable to determine a position of the reference laser beam and/or an intensity distribution of the reference laser beam.

[0024] Each laser beam sensor may comprise an extraction optic and a detector, wherein the extraction optic may be operable to receive a laser beam and to direct a first portion of the laser beam towards the detector and wherein the detector may be operable to determine a position of a beam spot formed by the first portion of the reference laser beam.

[0025] The detector of each laser beam sensor may comprise one or more arrays of sensing elements which are sensitive to radiation from a laser beam.

[0026] The detector may be operable to determine an edge of the laser beam, which may be used to determine a position of the reference laser beam relative to the laser beam sensor.

[0027] The edge of the laser beam may be determined by fitting a curve to data measured by the detector.

[0028] The sensor that is operable to determine a position of the reference laser beam and/or an intensity distribution of the reference laser beam may be operable to determine a shape of the reference radiation beam, which may be used in the determination of the edge of radiation beam by the detector of each laser beam sensor. [0029] The shape of the curve which is fitted to the data may be dependent on the intensity distribution of the reference laser beam.

[0030] For example, the curve that is fitted may vary with time so as to take into account the speckle of the reference laser beam (e.g. as determined by a sensor). This may increase the accu racy of the alignment system .

[0031] The sensor that is operable to determine a position of the reference laser beam and/or an intensity distribution of the reference laser beam may comprise a detector which is substantially identical to the detector of each of the laser beam sensors.

[0032] The undulator may further comprise a plurality of electron beam sensors, each of the plurality of electron beam sensors may be operable to determine the position of an electron beam relative to the electron beam sensor.

[0033] The undulator may further comprise a mechanism for refocusing an electron beam in between one or more pairs of adjacent undulator modules. For example, a quadrupole magnet may be provided between each pair of adjacent undulator modules.

[0034] The undulator may further comprise an electron beam steering unit in between each adjacent pair of undulator modules, each electron beam steering unit may be arranged to provide fine adjustment of a trajectory of an electron beam as it passes through the undulator.

[0035] Each of the plurality of electron beam sensors may be operable to output a signal to the controller which is indicative of the position of an electron beam and the controller may be operable to control the electron beam steering units in response to the signals received by from the electron beam sensors so as to maintain alignment between the electron beam and the magnetic centres of each of the plurality of undulator modules.

[0036] The undulator may be provided with one mechanical interface according to the first aspect of the present invention at each end of each undulator module.

[0037] The undulator may further comprise a mechanism for generating a second reference laser beam.

[0038] According to a fourth aspect of the present invention, there is provided a free electron laser, comprising: an electron source for producing an electron beam comprising a plurality of bunches of relativistic electrons; and an undulator according to the third aspect of the present invention arranged to receive the electron beam and guide it along a periodic path so that the electron beam interacts with radiation within the undulator, stimulating emission of radiation and providing a radiation beam.

[0039] According to a fifth aspect of the present invention, there is provided a lithographic system comprising: a free electron laser according to the fourth aspect of the present invention; and at least one lithographic apparatus, each of the at least one lithographic apparatus being arranged to receive at least a portion of at least one radiation beam produced by the free electron laser.

[0040] Various aspects and features of the invention set out above or below may be combined with various other aspects and features of the invention as will be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] 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 free electron laser according to an embodiment of the invention;

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 that forms part of the lithographic system of Figure 1 ;

Figure 4 is a schematic illustration of an undulator that forms part of the free electron laser of Figure 4;

Figure 5 is a schematic illustration of a mechanical interface for an undulator module that may form part of the undulator of Figure 4;

Figure 6 is a schematic illustration of a laser beam sensor, which forms part of the mechanical interface of Figure 5;

Figure 7 is a schematic illustration of an embodiment of a detector which may form part of the laser beam sensor of Figure 6; and

- Figure 8 illustrates, schematically, how a pointing error of reference laser beam may be corrected for.

DETAILED DESCRIPTION

[0042] Figure 1 shows a lithographic system LS according to one embodiment of the invention. The lithographic system LS comprises a radiation source SO, a beam delivery system BDS a plurality of lithographic apparatus LA _a -LA _n (e.g. eight lithographic apparatus). The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B (which may be referred to as a main beam). [0043] The beam delivery system BDS comprises beam splitting optics and may optionally also comprise beam expanding optics and/or beam shaping optics. The main radiation beam B is split into a plurality of radiation beams B _a -B _n (which may be referred to as branch beams), each of which is directed to a different one of the lithographic apparatus LA _a -LA _n , by the beam delivery system BDS.

[0044] The optional beam expanding optics (not shown) are arranged to increase the cross sectional area of the radiation beam B. Advantageously, this decreases the heat load on mirrors downstream of the beam expanding optics. This may allow the mirrors downstream of the beam expanding optics 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. For example, the beam expanding optics may be operable to expand the main beam B from approximately 100 μηι to more than 10 cm before the main beam B is split by the beam splitting optics.

[0045] In an embodiment, the branch radiation beams B _a -B _n are each directed through a respective attenuator (not shown). Each attenuator may be arranged to adjust the intensity of a respective branch radiation beam B _a -B _n before the branch radiation beam B _a -B _n passes into its corresponding lithographic apparatus LA _a -LA _n .

[0046] The radiation source SO, beam delivery system BDS and lithographic apparatus LA _a -LA _n 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 delivery system BDS and lithographic apparatuses LA _a -LA _n 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).

[0047] Referring to Figure 2, a lithographic apparatus LA _a 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 _a that is received by that lithographic apparatus LA _a before it is incident upon the patterning device MA. The projection system PS is configured to project the radiation beam B _a ' (now patterned by the patterning device MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B _a ' with a pattern previously formed on the substrate W.

[0048] The branch radiation beam B _a that is received by the lithographic apparatus LA _a passes into the illumination system IL from the beam delivery system BDS though an opening 8 in an enclosing structure of the illumination system IL. Optionally, the branch radiation beam B _a may be focused to form an intermediate focus at or near to the opening 8.

[0049] The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 1 1 . The faceted field mirror device 10 and faceted pupil mirror device 1 1 together provide the radiation beam B _a with a desired cross-sectional shape and a desired angular distribution. The radiation beam B _a 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 _a '. 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 1 1 . The illumination system IL may for example include an array of independently moveable mirrors. The independently moveable mirrors may for example measure less than 1 mm across. The independently moveable mirrors may for example be microelectromechanical systems (MEMS) devices.

[0050] Following redirection (e.g. reflection) from the patterning device MA the patterned radiation beam B _a ' enters the projection system PS. The projection system PS comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B _a ' 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 in Figure 2, the projection system may include any number of mirrors (e.g. six mirrors).

[0051] The lithographic apparatus LA _a is operable to impart a radiation beam B _a with a pattern in its cross-section and project the patterned radiation beam onto a target portion of a substrate thereby exposing a target portion of the substrate to the patterned radiation. The lithographic apparatus LA _a may, for example, be used in a scan mode, wherein the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B _a ' is projected onto a substrate W (i.e. a dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the demagnification and image reversal characteristics of the projection system PS.

[0052] Referring again to Figure 1 , the radiation source SO is configured to generate an EUV radiation beam B with sufficient power to supply each of the lithographic apparatus LA _a - LA _n . As noted above, the radiation source SO may comprise a free electron laser. [0053] Figure 3 is a schematic depiction of a free electron laser FEL comprising an injector

21 , a linear accelerator 22, a bunch compressor 23, an undulator 24, an electron decelerator 26 and a beam dump 100.

[0054] The injector 21 is arranged to produce a bunched electron beam E and comprises an electron source (for example a thermionic cathode or a photo-cathode) and an accelerating electric field. Electrons in the electron beam E are further accelerated 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 be used such as, for example, laser wake-field accelerators or inverse free electron laser accelerators.

[0055] Optionally, the electron beam E passes through a bunch compressor 23, disposed between the linear accelerator 22 and the undulator 24. The bunch compressor 23 is configured to spatially compress existing bunches of electrons in the electron beam E. One type of bunch compressor 23 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 23 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 bunches of electrons which have been accelerated in a linear accelerator 22 by a plurality of resonant cavities.

[0056] The electron beam E then passes through the undulator 24. Generally, the undulator 24 comprises a plurality of modules. Each module comprises a periodic magnet structure, which is operable to produce a periodic magnetic field and is arranged so as to guide the relativistic electron beam E produced by the injector 21 and linear accelerator 22 along a periodic path within that module. The periodic magnetic field produced by each undulator module causes the electrons to follow an oscillating path about a central axis. As a result, within each undulator module, the electrons radiate electromagnetic radiation generally in the direction of the central axis of that undulator module.

[0057] The path followed by the electrons may be sinusoidal and planar, with the electrons periodically traversing the central axis. Alternatively, the path may be helical, with the electrons rotating about the central axis. The type of oscillating path may affect the polarization of radiation emitted by the free electron laser. For example, a free electron laser which causes the electrons to propagate along a helical path may emit elliptically polarized radiation, which may be desirable for exposure of a substrate W by some lithographic apparatus.

[0058] As electrons move through each undulator module, they interact with the electric field of the radiation, 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. Under resonance conditions, the interaction between the electrons and the radiation 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 s

where X _em is the wavelength of the radiation, X _u is the undulator period for the undulator module that the electrons are propagating through, 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 that produces circularly polarized radiation A=1 , for a planar undulator A=2, and for a helical undulator which produces elliptically polarized radiation (that is neither circularly polarized nor linearly polarized) 1 <A<2. In practice, each bunch of electrons will have a spread of energies although this spread may be minimized 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:

where q and m are, respectively, the electric charge and mass of the electrons, B ₀ is the amplitude of the periodic magnetic field, and c is the speed of light.

[0059] The resonant wavelength X _em is equal to the first harmonic wavelength spontaneously radiated by electrons moving through each undulator module. The free electron laser FEL may operate in self-amplified spontaneous 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 each undulator module. Alternatively, the free electron laser FEL may comprise a seed radiation source, which may be amplified by stimulated emission within the undulator 24. The free electron laser FEL may operate as a recirculating amplifier free electron laser (RAFEL), wherein a portion of the radiation generated by the free electron laser FEL is used to seed further generation of radiation.

[0060] 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.

[0061 ] 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 X _u 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. The tapering may be achieved by varying the amplitude of the periodic magnetic field and/or the undulator period X _u within each undulator module and/or from module to module. Additionally or alternatively tapering may be achieved by varying the helicity of the undulator 24 (by varying the parameter A) within each undulator module and/or from module to module.

[0062] Radiation produced within the undulator 24 is output as a radiation beam B _FEL .

[0063] After leaving the undulator 24, the electron beam E is absorbed by a dump 1 00. The dump 1 00 may comprise a sufficient quantity of material to absorb the electron beam E. The material may have a threshold energy for induction of radioactivity. Electrons entering the dump 1 00 with an energy below the threshold energy may produce only gamma ray showers but will not induce any significant level of radioactivity. The material may have a high threshold energy for induction of radioactivity by electron impact. For example, the beam dump may comprise aluminium (Al), which has a threshold energy of around 1 7 MeV. It may be desirable to reduce the energy of electrons in the electron beam E before they enter the dump 100. This removes, or at least reduces, the need to remove and dispose of radioactive waste from the dump 100. 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. [0064] The energy of electrons in the electron beam E may be reduced before they enter the dump 1 00 by directing the electron beam E through a decelerator 26 disposed between the undulator 24 and the beam dump 1 00.

[0065] In an embodiment the electron beam E which exits the undulator 24 may be decelerated by passing the electrons back through the linear accelerator 22 with a phase difference of 1 80 degrees relative to the electron beam produced by the injector 21 . The RF fields in the linear accelerator therefore serve to decelerate the electrons which are output from the undulator 24 and to accelerate electrons output from the injector 21 . As the electrons decelerate in the linear accelerator 22 some of their energy is transferred to the RF fields in the linear accelerator 22. Energy from the decelerating electrons is therefore recovered by the linear accelerator 22 and may be used to accelerate the electron beam E output from the injector 21 . Such an arrangement is known as an energy recovering linear accelerator (ERL).

[0066] Figure 4 shows an undulator 24 according to an embodiment of the present invention. The undulator 24 comprises a plurality of undulator modules 1 01 - 1 06. In the embodiment shown in Figure 4, the undulator 24 comprises six undulator modules 1 01 - 1 06 although it will be appreciated that in alternative embodiments the undulator 24 may comprise any other number of undulator modules, for example around thirty undulator modules or more.

[0067] The undulator modules 1 01 -1 06 are arranged to receive an electron beam (e.g. electron beam E output by linear accelerator 22) and to guide the electron beam along a periodic path such that electrons within the electron beam interact with radiation in each of the undulator modules 1 01 -1 06 to stimulate emission of coherent radiation to provide a radiation beam B _F EL (see Figure 3). To achieve this, each undulator module 1 01 -1 06 is operable to produce a periodic magnetic field and has a magnetic centre.

[0068] The magnetic centre of each undulator module 1 01 - 1 06 is a line extending between opposed ends of the undulator section 1 01 - 1 06. For example, the magnetic centre may be a central axis (i.e. a straight line) extending through the undulator section 1 01 - 1 06. In the following, the term central axis may be considered synonymous with the term magnetic centre. At any point along the undulator module 1 01 -1 06, the central axis may pass through a point about which the magnetic field is symmetric.

[0069] A region around the central axis of each undulator module 1 01 - 1 06 may be considered to be a "good field region". The good field region may be a volume around the central axis wherein, for a given position along the central axis of the undulator module 1 01 -1 06, the magnitude and direction of the magnetic field within the volume are substantially constant. An electron bunch propagating within the good field region may satisfy the resonant condition of Eq. (1 ) and will therefore amplify radiation. Further, an electron beam E propagating within the good field region should not experience significant unexpected disruption due to uncompensated magnetic fields. That is, an electron propagating through the good field region should remain within the good field region.

[0070] Each undulator module 101 -106 may have a range of acceptable initial trajectories. Electrons entering an undulator module 101 -106 with an initial trajectory within this range of acceptable initial trajectories may satisfy the resonant condition of Eq. (1 ) and interact with radiation in that undulator module 101 -106 to stimulate emission of coherent radiation. In contrast, electrons entering an undulator module 101 -106 with other trajectories may not stimulate significant emission of coherent radiation.

[0071] For example, generally, for helical undulator modules the electron beam E should be substantially aligned with the central axis of the undulator module 101 -106. A tilt or angle between the electron beam E and the central axis of the undulator module 101 -106 (in micro- radians) should generally not exceed 1/10p, where p is the FEL Pierce parameter. Otherwise the conversion efficiency of the undulator module (i.e. the portion of the energy of the electron beam E which is converted to radiation in that module) may drop below a desired amount (or may drop almost to zero). In an embodiment, the FEL Pierce parameter of an EUV helical undulator module may be of the order of 0.001 , indicating that the tilt of the electron beam E with respect to the central axis of the undulator module 101 -106 should be less than 100 μ^.

[0072] For a planar undulator module, a greater range of initial trajectories may be acceptable. Provided the electron beam E remains substantially perpendicular to the magnetic field of a planar undulator module and remains within the good field region of the planar undulator module, coherent emission of radiation may be stimulated.

[0073] As electrons of the electron beam E move through a drift space between each undulator module 101 -106, the electrons do not follow a periodic path. Therefore, in this drift space, although the electrons overlap spatially with the radiation, they do not exchange any significant energy with the radiation and are therefore effectively decoupled from the radiation.

[0074] The bunched electron beam E has a finite emittance and will therefore increase in diameter unless refocused. Therefore, the undulator 24 further comprises a mechanism for refocusing the electron beam E in between one or more pairs of adjacent undulator modules 101 -106. For example, a quadrupole magnet may be provided between each pair of adjacent modules. The quadrupole magnets reduce the size of the electron bunches. This improves the coupling between the electrons and the radiation within the next undulator module 101 -106, increasing the stimulation of emission of radiation. [0075] The undulator 24 comprises an electron beam steering unit 1 1 1 -1 15 in between each adjacent pair of undulator modules 101 -106. Each electron beam steering 1 1 1 -1 15 unit is arranged to provide fine adjustment of the electron beam E as it passes through the undulator 24. For example, each beam steering unit 1 1 1 -1 15 may be arranged to ensure that the electron beam remains within the good field region and enters the next undulator module 101 -106 with a trajectory from the range of acceptable initial trajectories for that undulator module 101 -106.

[0076] To efficiently generate EUV photons within the undulator 24 and to achieve a high gain FEL, it is desirable to maximize the spatial overlap between the electron beam E and the radiation within the undulator 24 and to ensure that the electron beam is close to the central axis of each undulator module 101 -106 (i.e. within the good field region).

[0077] It is desirable for the electron beam E to propagate along substantially the same path as the radiation produced within the undulator 24. This ensures that there is good spatial overlap between the electron beam E and the radiation within the undulator 24. To achieve this, it may be desirable for the central axes of each of the undulator modules 101 -106 to be aligned. Further, it may be desirable for the electron beam E to be accurately aligned with the central axis of each of the undulator modules 101 -106. For example, it may be desirable to align the electron beam E with the central axis of each of the undulator modules 101 -106 with micron accuracy.

[0078] The undulator 24 may have a total length of the order of several meters, tens of meters or more. Alignment to micron accuracy over such lengths presents a significant challenge. The electron beam E position and the central axis of each undulator module 101 - 106 may be influenced by a range of factors, including: energy variations within the electron beam, magnetic field variation within each undulator module 101 -106 due to temperature change and magnetic damage; position stability of each of the undulator modules 101 -106.

[0079] Each undulator module 101 -106 may comprise a plurality of periodic magnetic structures. The undulator 24 is an elongate structure, extending generally along a central axis 170, which may be referred to as an axial direction. The periodic structures are arranged around the central axis, extending parallel to the central axis. Each of the periodic structures is separated from the central axis in a direction substantially perpendicular to the central axis of the undulator 24, which may be referred to as a radial direction.

[0080] All of the periodic magnetic structures of a given undulator module 101 -106 may be substantially similar in structure. In particular, each of the periodic magnetic structures may have substantially the same undulator period, A _u . [0081] Each periodic structure may be generally of the form of a Halbach array. That is, each periodic structure may comprise a linear array of permanent dipole magnets, arranged such that the magnetic fields of the permanent dipole magnets interfere constructively on one side of the periodic array and destructively on an opposite side of the array.

[0082] Alternatively, each of the periodic structures may comprise a plurality of (active) dipole magnets and a plurality of (passive) ferromagnetic (e.g. iron) poles. The plurality of magnets of a given periodic magnetic structure may be arranged alternately with the iron poles of that periodic magnetic structure in a line extending in an axial direction. Each of the plurality of magnets may have a substantially constant polarization direction, which may be generally in either the positive or negative axial direction. The plurality of magnets of a given periodic structure may be arranged such that along a length of the periodic magnetic structure the polarizations of the magnets alternate between the positive and negative axial directions. Each such periodic magnetic structure produces a periodic magnetic field, with the period A _u being the length of two magnets and two iron poles. Each of the iron poles may act as a passive ferromagnetic element. Each iron pole may separate an adjacent pair of magnets and may extend farther towards the central axis than each of the magnets.

[0083] In some embodiments, each undulator module 101 -106 may comprise a pair of periodic structures arranged on opposite sides of the central axis. The periodic structures may be out of phase by 180°. Such an arrangement may result in a magnetic field along the central axis which generally points in a single direction and which has a magnitude which varies generally sinusoidally along the central axis. Such an arrangement may be referred to as a planar undulator module and may result in radiation which is linearly polarized.

[0084] In some embodiments, each undulator module may comprise four periodic structures arranged around the central axis. The four periodic structures may be formed from two pairs of periodic structures arranged on opposite sides of the central axis, which are out of phase by 180° (i.e. two planar undulator modules). One pair of periodic structures may be rotated relative to the other pair of periodic structures about the central axis of the undulator 24 by 90 °. One pair of periodic structures may be shifted axially relative to the other pair of periodic structures such that the two pairs of periodic structures are out of phase. The amount of the shift may determine the polarization of radiation produced by the undulator module. For example, one pair of periodic structures may be shifted axially relative to the other pair of periodic structures by a quarter of the undulator period A _u . Such an arrangement may be referred to as a helical undulator and may produce circularly polarized radiation as the electron beam E propagates through it. [0085] The undulator 24 comprises a beam pipe (not shown) for the electron beam E, which extends through all of the undulator modules 101 -106. The beam pipe is arranged such that, in use, the electron beam E enters one end of the beam pipe, passes through it, substantially along the central axis 170 of the undulator 24, and exits an opposite end of the beam pipe. In use, the beam pipe may be held under vacuum conditions. As such, the beam pipe may be formed from a material such as stainless steel, which does not suffer from outgassing. Alternatively, the beam pipe may be formed from aluminium. For embodiments wherein the beam pipe is formed from aluminium, the beam pipe may be provided with a coating of non- evaporable getters (NEG) material. The coating of NEG material may prevent outgassing and may improve the vacuum quality within the beam pipe. The beam pipe may for example be generally circular in cross section, in a plane perpendicular to the axis of the undulator 24.

[0086] An interior of the beam pipe provides a suitable environment for the electron beam E to propagate through. In particular, it is held under vacuum conditions. In an alternative embodiment, the undulator 24 does not comprise a beam pipe but rather the electron beam propagates through a channel defined by the plurality of magnetic structures. In such an embodiment, the entire undulator 24 may maintain a suitable environment for the electron beam E to propagate through.

[0087] In order to maintain alignment of the central axis of each of the undulator modules 101 -106 and the electron beam E, the undulator 24 is provided with an alignment system, as now described.

[0088] Figure 5 shows a mechanical interface 200 for an undulator module 101 -106, which forms part of an alignment system for undulator 24. The mechanical interface 200 comprises: a body 210 in the form of a plate, a laser beam sensor 230 and an electron beam sensor 240.

[0089] The mechanical interface 200 further comprises an alignment feature on the body 210 in the form of three fixing apertures 220-222. The fixing apertures 220-222 are arranged to allow the mechanical interface 200 to engage with an undulator module such that the laser beam sensor 230 and the electron beam sensor 240 are both maintained in fixed relationship with the central axis of that undulator module.

[0090] The laser beam sensor 230 is rigidly attached to the body 210 and is operable to determine the position of a reference laser beam relative to part of the body 210. The electron beam sensor 240 is rigidly attached to the body 210 and is operable to determine the position of an electron beam relative to part of the body 210.

[0091] The undulator 24 comprises a plurality of mechanical interfaces 200 substantially as described with reference to Figure 4. In particular, one mechanical interface 200 is provided at each end of each undulator module 101 -106. Each mechanical interface 200 is attached to its undulator module 101 -106 via the fixing holes 220-222. For example, screws or bolts or the like may pass through the fixing holes 220-222 and engage with corresponding features on the undulator module 101 -106.

[0092] The plurality of mechanical interfaces 200 are substantially identical. For example, they may be calibrated during manufacture to eliminate manufacturing tolerances, or to reduce manufacturing tolerances such that they are below a specified level. All of the undulator modules 101 -106 are generally aligned such that all of the laser beam sensors 230 are generally aligned and all of the electron beam sensors 240 are generally aligned.

[0093] The alignment features 220 allow the mechanical interfaces 200 to releasably engage with an undulator module 101 -106 such that, when engaged with the undulator module 101 -106, the positions of each of the electron beam sensors 240 and the laser beam sensors 230 relative to the central axis of the undulator module 101 -106 are substantially the same. The electron beam sensor 240 and the laser beam sensor 230 may each be positioned on the body 210 such that when the body 210 is engaged with an undulator module 101 -106 the electron beam sensor 240 and the laser beam sensor 230 is disposed at a known, specific or predetermined position relative to the central axis of the undulator module 101 -106.

[0094] Referring again to Figure 4, a laser 150 is provided which is operable to output a reference laser beam 155 extending along and adjacent to the plurality of undulator modules 101 -106 of undulator 24. As shown in Figure 4, the laser 150 may be provided separate from other parts of the undulator 24, for example in a separate room. The laser may be coupled, for example via an optical fibre 152, to optics 154 which are arranged to direct the reference laser beam 155 along and adjacent to the plurality of undulator modules 101 -106 of undulator 24. Advantageously, this allows the laser 150 to be located in an environment that is less exposed to radiation from the undulator 24. This can improve the lifetime of the laser 150 and/or its accessibility for maintenance. In alternative embodiments, the laser may be disposed where the optics 154 are shown in Figure 4. The reference laser beam 155 passes through the laser beam sensor 230 of each of the mechanical interfaces 200. The reference laser beam 155 may propagate through a guiding tube, which may provide a low pressure or vacuum environment. This may increase the accuracy of the alignment system.

[0095] A sensor 160 is provided at an opposite axial end of the undulator 24. The sensor 160 is operable to determine the position of the reference laser beam 155. The sensor 160 may be further operable to determine an intensity distribution of the reference laser beam 155. In particular, the sensor 160 may be operable to monitor speckle of the reference laser beam 155, i.e. any time dependent variations in the intensity distribution. Sensor 160 may be disposed to receive the reference laser 155 directly, as shown in Figure 4. Alternatively, the sensor 160 may be located separate from other parts of the undulator 24, for example in a separate room. For such embodiments, the sensor 160 may be coupled to optics which are arranged to receive the reference laser beam 155, for example via an optical fibre. Advantageously, this allows the sensor 160 to be provided in an environment that is less exposed to radiation from the undulator 24. This can improve the lifetime of the sensor 160 and/or its accessibility for maintenance.

[0096] Referring to Figure 6, the laser beam sensor 230 provided on each mechanical interface 200 comprises an extraction optic 232 and a detector 234. The extraction optic 232 is disposed in the path of the reference laser beam 155. The extraction optic 232 is arranged to receive the reference laser beam 155 and to direct a first portion 155a of the reference laser beam 155 towards the detector 234. A second portion 155b of the reference laser beam 155 is transmitted by the extraction optic 232. The extraction optic 232 may, for example, comprise any suitable type of beam splitter. For example, the extraction optic 232 may comprise a partially transmissive mirror, a beam splitter cube (i.e. two triangular prisms) or a Brewster plate.

[0097] As shown in Figure 6, the detector 234 may be located separate from other parts of the undulator 24, for example in a separate room. For example, the detector 234 may be coupled via an optical fibre 234b, to optics 234a which are arranged to receive the first portion 155a of the reference laser beam 155. Advantageously, this allows the detector 234 to be provided in an environment that is less exposed to radiation from the undulator 24. This can improve the lifetime of the laser 234 and/or its accessibility for maintenance. In alternative embodiments, the detector 234 may be disposed where the optics 234a are shown in Figure 6.

[0098] The detector 234 is operable to determine the position of the mechanical interface 200 with respect to the reference laser beam 155. To achieve this, the detector may be operable to determine a position of a beam spot formed by the first portion 155a of the reference laser beam 155. The extraction optic 232 may comprise one or more reference markers and the determined position may be relative to said reference markers. The position of a beam spot formed by the first portion 155a may be determined in one of a plurality of different ways.

[0099] The detector 234 may comprise one or more arrays of sensing elements which are sensitive to radiation from laser beam 155a. The detector 234 may be operable to determine an edge of the laser beam 155a, which may be used to determine its position. It will be appreciated that laser beams typically do not have sharp edges, for example the intensity profile of a laser beam may be Gaussian-like, and therefore the edge of the laser beam 155a may be defined in a number of different ways. For example, the edge of the laser beam 155a may be defined as the point at which the intensity of the laser beam 155a falls below a threshold value. If the intensity distribution of the reference laser beam 155 is rotationally symmetric then the edge of the radiation beam 155 (and the edge of the first portion 155a of the radiation beam 155 which is directed towards the detector 234) will be a circle. Intensity variations in reference radiation beam 155 may result in the edge being distorted in shape such that it is no longer circular. If the position of the radiation beam 155a is determined assuming that the reference laser beam 155 is rotationally symmetric (and the edge of radiation beam 155a is a circle) then such intensity variations will result in an error in the determination of the position of the radiation beam 155a.

[00100] The sensor 160 may comprise a detector, which may be substantially identical to the detector 234 of each of the laser beam sensors 230. In particular, the detector of sensor 160 may comprise one or more arrays of sensing elements which are sensitive to radiation from laser beam 155. The detector of sensor 160 can be used to determine the shape of the radiation beam 155, which can be used in the determination of the position of radiation beam 155a by each detector 234. By using a substantially identical detector in sensor 160 and in each of the laser beam sensors 230, time dependent variations in the intensity distribution (i.e. speckle) may be automatically taken into account at each mechanical interface 200.

[00101 ] Two arrangements for determining a position of a beam spot formed by the first portion 155a of the reference laser beam 155 will now be described.

[00102] In a first arrangement, the detector 234 comprises a two-dimensional array of sensing elements which are sensitive to radiation from laser beam 155a. For example, the two- dimensional array of sensing elements may comprise a charged coupled device (CCD) array. The array of sensing elements will produce a signal which is related to the intensity distribution of the laser beam 155a. The edge of the laser beam 155a may be determined by fitting a curve to the data measured by the sensing array (e.g. using a least squares fit or other fitting procedure). The position of the laser beam 155a may be given by the centre of the laser beam 155a and may be determined from the fitted shape. For example, in one embodiment, a circle may be fitted to the data and the position of the laser beam may be determined to be the centre of the circle. Such an embodiment may be used if the intensity distribution of the reference laser beam 155 is rotationally symmetric. In an alternative embodiment, the shape of the curve which is fitted is dependent upon the intensity distribution of the reference laser beam 155 as determined by the sensor 160. For example, the curve that is fitted may vary with time so as to take into account any time dependent intensity variations of the reference laser beam 155 as determined by the sensor 160. This may increase the accuracy of the alignment system.

[00103] In a second arrangement, as shown in Figure 7, the detector 234 comprises two one- dimensional arrays 234a, 234b of sensing elements which are sensitive to radiation from laser beam 155a. For example, the two one-dimensional arrays of sensing elements 234a, 234b may each comprise a charged coupled device (CCD) array. The two one-dimensional arrays 234a, 234b are generally mutually perpendicular (although it will be appreciated that they may be arranged at any other angle to each other) and may cross each other. Again, the sensing elements will produce a signal which is related to the intensity distribution of the laser beam 155a. Each of the one-dimensional arrays 234a, 234b of sensing elements can be used to determine the edges (one on each side of the radiation beam 155a) of the radiation beam 155a. For example, a first one-dimensional array 234a may be aligned with a z-axis and it may determine two points z _{1 ;} z ₂ along its length which pass through the edge of the radiation beam 155a. Similarly, a second one-dimensional array 234b may be aligned with an y-axis and it may determine two points yi , y ₂ along its length which pass through the edge of the radiation beam 155a. In one embodiment, the radiation beam 155a may be assumed to be rotationally symmetric. For such embodiments, the position of the beam spot of radiation beam 155a in a direction along each of the one-dimensional arrays 234a, 234b may be determined by the midpoint between each of the two edges determined by each of the one-dimensional arrays 234a, 234b. For example, the centre of the laser beam 155a in the z and y directions may be determined as (z ₂ +Zi)/2 and (y ₂ +yi)/2 respectively. The position of the laser beam 155a may be given by the centre of the laser beam 155a and may be determined as the intersection of the two midpoints. In an alternative embodiment, the position of the beam spot of radiation beam 155a in a direction along each of the one-dimensional arrays 234a, 234b may be determined in dependence on the measurements made by sensor 160 and may therefore take into account time dependent intensity variations of reference beam 155.

[00104] The laser beam sensor 230, which is attached to the body 210, allows the position of each mechanical interface 200 (and therefore the undulator module 101 -106 with which it is engaged) relative to the reference laser beam 155 to be determined. The alignment feature allows the laser beam alignment sensor 230 to be maintained in fixed relationship with a central axis of the undulator module. Therefore, the laser beam sensor 230 allows the position of the central axis of the undulator module 101 -106 to be determined relative to the reference laser beam 155 at the point at which the mechanical interface is engaged (e.g. one end of the undulator module 101 -106). The position of the central axis of each of the plurality of undulator modules 101 -106 at each end of the undulator module 101 -106 can therefore be determined relative to a single reference laser beam.

[00105] The alignment feature on the mechanical interface 200 may for example comprise holes 220-222 which are positioned to engage with protrusions that extend from an undulator module 101 -106. Alternatively, the alignment feature may for example comprise protrusions which are configured to be received in holes or recesses provided in the undulator module. The alignment feature provided on the mechanical interface 200 may for example comprise a combination of holes and protrusions.

[00106] The plurality of mechanical interfaces 200 and the reference laser beam 155 therefore form part of an undulator alignment system that allows alignment of one or more undulator modules with a desired axis or direction.

[00107] It may be desirable to align the axis of each of the plurality of undulator modules 101 - 106 with a reference axis, for example central axis 170. The reference axis may be defined by two points in space. For example, the central axis 170 may be defined by two points 172, 174 at either end of the undulator 24. Additionally or alternatively, a first optical element which receives the radiation beam B of EUV radiation which is output by the undulator 24 may define the reference axis.

[00108] As explained above, the plurality of mechanical interfaces 200 and the reference laser beam 155 allow the position of each end of the central axis of each undulator module 101 - 106 to be determined relative to the reference laser beam 155. The laser 150 is arranged such that the reference laser beam 155 is generally parallel to the central axis 170. In order to allow the position of each end of the central axis of each undulator module 101 -106 to be determined relative to the central axis 170, the position of the reference laser beam 155 relative to the central axis 170 may be determined. Any variation in the pointing direction of the reference laser beam 155 (e.g. relative to the central axis 170) can then be corrected by calculation, as will be described with reference to Figure 8. In order to determine the position of the reference laser beam 155 relative to the central axis 170, the position of the laser beam relative to each of the two reference points 172, 174 may be determined. This may be achieved using a sensor which is fixed relative point 172 and a sensor which is fixed relative to point 174. Alternatively, the laser 150 may be mechanically fixed relative to one reference point 172 and the sensor 160 may be fixed relative to the other reference point 174.

[00109] A nominal position 156 of the reference laser beam 155 is shown in Figure 8. This nominal position 156 may, for example, be perfectly parallel to the central axis 170 of the undulator 24. A deviation dx1 from this nominal position 156 at an end of the undulator 24 which is opposite to the laser 150 may be determined, for example by sensor 160 (which is mechanically fixed relative to reference point 174). If the distance between the laser 150 and the sensor 160 is L1 then the deviation dx2 of the reference laser beam 155 from the nominal position 156 at a point which is a distance L2 from the laser 150 will be given by (L1/L2)dx1 .

[00110] Each undulator module 101 -106 is provided with an actuator 181 -186, which is operable to control its position. Each actuator 181 -186 is operable to control the position of each end of its undulator module 101 -106 independently. In some embodiments, each actuator 181 -186 may comprise a plurality of actuators at each end of the undulator module 101 -106, which each allow the position of that end of the undulator module 101 -106 to be controlled in a different direction. The undulator alignment system comprises a controller. Each of the laser beam sensors 230 is operable to output a signal to the controller which is indicative of the position of the position of central axis of the undulator module 101 -106 with which it is engaged relative to reference laser beam 155. The controller is operable to control the actuators 181 -186 in response to the signals received from the laser beam sensors 230 so as to maintain alignment between the central axes of each of the plurality of undulator modules 101 -106.

[00111 ] Therefore, the laser beam sensors 230, controller and the actuators 181 -186 form a feed-back loop which allows the central axes of all of the undulator modules 101 -106 to be aligned accurately.

[00112] Each electron beam sensor 240, which is attached to the body 210, allows the position of an electron beam E relative to the mechanical interface 200 (and therefore the undulator module 101 -106 with which it is engaged) to be determined. The alignment feature (e.g. fixing holes 220-222) allows the electron beam sensor 240 to be maintained in fixed relationship with a central axis of an undulator module 101 -106. This allows the position of the electron beam sensor 240 relative to the central axis of the undulator module 101 -106 at the point at which the mechanical interface 200 (i.e. one of the ends of the undulator module 101 - 106) to be determined.

[00113] The electron beam sensor 240 may comprise any beam position monitor (BPM). The beam position monitor may of any type, including a button BPM, a stripline BPM or a resonant cavity BPM. The body 210 may define an electron beam aperture 242 for an electron beam E to propagate through. The electron beam aperture 242 may form part of a beam pipe for the undulator 24. The electron beam sensor 240 may comprise a plurality of electrodes 244a-244d arranged around the electron aperture 242 and a processor (not shown). The processor may be operable to determine an electron beam position from signals generated by the plurality of electrodes 244a-244d as an electron beam E passes through the electron beam aperture 242.

[00114] Each of the electron beam sensors 240 is operable to output a signal to the controller which is indicative of the position of an electron beam E passing through its electron beam aperture 242. The controller is operable to control the electron beam steering units 1 1 1 -1 15 in response to the signals received by from the electron beam sensors 240 so as to maintain alignment between electron beam E and the central axes of each of the plurality of undulator modules 101 -106. The controller may for example control each beam steering unit 1 1 1 -1 15 so as to ensure that the electron beam E remains within the good field region and enters the next undulator module 101 -106 and with a trajectory falling within a range of acceptable initial trajectories for that undulator module 101 -106.

[00115] Therefore, the electron beam sensors 240, controller and the electron beam steering units 1 1 1 -1 15 form a feed-back loop which allows the electron beam E to be accurately aligned with the central axes of each of the plurality of undulator modules 101 -106.

[00116] When the magnetic center (central axis) of each undulator module 101 -106 and the electron beam is aligned with central axis 170, the EUV radiation beam B will also be aligned with the central axis 170. Therefore, for embodiments wherein a first optical element which receives the radiation beam B of EUV radiation which is output by the undulator 24 defines the reference axis, the alignment of the radiation beam B with the first optical element may be improved.

[00117] Each laser beam sensor 230 is operable to measure a displacement between the reference laser beam 155 and the magnetic centre of the undulator module 101 -106 in two orthogonal directions (e.g. the x and y directions in Figure 4). If an undulator module 101 -106 is rotated about its axis (z direction in Figure 4) by an angle ΔΘ then this will result in a change in the determined displacements between the reference laser beam 155 and the magnetic centre of the undulator module 101 -106 in the two orthogonal directions. This is known as an Abbe error, or cosine error.

[00118] In some embodiments, the undulator alignment system may comprise two or more reference laser beams. The two or more reference laser beams can each be generated by a different laser. Alternatively, the two or more reference laser beams may be generated by the single laser 150 in combination with optics arranged to split the laser beam into two or more reference laser beams. For example, a beam splitter may be positioned adjacent to the laser 150 and may split an output of the laser 150 to produce two or more parallel reference beams. Alternatively, a corner reflector may be positioned at an opposite end of the undulator 24 to the laser 150 and may reflect the reference laser beam 155 back along a parallel path to generate a further reference laser beam.

[00119] The use of two or more reference laser beams allows the Abbe error to be eliminated. Each laser beam sensor 230 may comprise a single extraction optic for each of the reference laser beams. Alternatively, each laser beam sensor 230 may comprise two or more extraction optics; one for each of the reference laser beams. Each laser beam sensor 230 may comprise a single detector for each of the reference laser beams. Alternatively, each laser beam sensor 230 may comprise two or more detectors, one for each of the reference laser beams.

[00120] In some embodiments, all reference beams may pass through the same guiding tube and the mechanical interfaces 200 may be substantially as shown in Figure 5. Alternatively, in other embodiments the reference beams may propagate along opposite sides of the central axis 170. For such embodiments, two guiding tubes may be provided. The reference beams may be equidistant from the central axis 170. The mechanical interfaces for such an embodiment may therefore be different to the arrangement shown in Figure 5. In particular, the electron beam sensor 240 may be provided towards the centre of the body 210 of mechanical interface 200 and the laser beam sensor 230 may be provided in two parts at opposite ends of the body 210, each part being equidistant from the electron beam sensor 240.

[00121 ] Instead of or in addition to laser beam sensors 230 the mechanical interfaces 200 can be equipped with reflective gratings, the position of which can be accurately measured with a position-measuring device as disclosed in US 2007/0058173 A1 .

[00122] The alignment system according to the present invention has a number of advantages. It allows accurate alignment of the undulator modules 101 -106 and the electron beam E. In turn, this ensures that the output power of the free electron laser FEL is stable and constant. It is simple and relatively fast to install and maintain the system. Further, it is a cheaper system than, for example, a stretched wire based alignment system due to lower stringent mechanical tolerances.

[00123] Whilst embodiments of a radiation source SO have been described and depicted as comprising a free electron laser 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 more than one free electron laser FEL. For example, two free electron lasers may be arranged to provide EUV radiation to a plurality of lithographic apparatus. This is to allow for some redundancy. This may allow one free electron laser to be used when the other free electron laser is being repaired or undergoing maintenance. [00124] Although the described embodiment of a lithographic system LS comprises eight lithographic apparatuses LA _a -LA _n , 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 delivery system BDS. 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.

[00125] 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 a plurality of mask inspection apparatuses to allow for some redundancy. This may allow one mask inspection apparatus to be used when another 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.

[00126] It will be further appreciated that a free electron laser comprising an undulator as described above may be used as a radiation source for a number of uses, including, but not limited to, lithography.

[00127] The term "relativistic electrons" should be interpreted to mean electrons which have relativistic energies. An electron may be considered to have a relativistic energy when its kinetic energy is comparable to or greater than its rest mass energy (51 1 keV in natural units). 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, >1 GeV or more.

[00128] 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.

[00129] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 5-20 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 5-10 nm such as 6.7 nm or 6.8 nm.

[00130] The lithographic apparatuses LA _a to LA _n may be used in the manufacture of ICs. Alternatively, the lithographic apparatuses LA _a to LA _n 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.

[00131 ] Different embodiments may be combined with each other. Features of embodiments may be combined with features of other embodiments.

[00132] 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 claims set out below.