CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US provisional application 61/900,848, which was filed on 06 November 2013, and EP patent application 13196302.7, which was filed 09 December 2013, which are incorporated herein in theirs entirety by reference.

FIELD

[0002] The present invention relates to a free electron laser (FEL). Particularly, but not exclusively, the present invention relates to a FEL suitable for use in a radiation source 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 5-20 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] It is desirable to produce EUV radiation sources with increased power to increase throughput of EUV lithography.

SUMMARY

[0006] According to a first aspect, there is provided a free electron laser comprising: a first electron source operable to produce a bunched electron beam, the first electron source comprising a linear accelerator arranged to impart energy to electrons in the bunched electron beam; an undulator operable to produce a periodic magnetic field and arranged so as to guide the bunched electron beam along a periodic path about a central axis of the undulator such that they interact with radiation in the undulator, stimulating emission of coherent radiation; electron beam optics arranged to direct the bunched electron beam back into the linear accelerator after it leaves the undulator so as to extract energy from electrons in the bunched electron beam. The free electron laser further comprises a deceleration mechanism arranged to further reduce the energy of the bunched electron beam after it has left the undulator.

[0007] In this way, the energy of the electron beam may be reduced beyond the level possible using only the linear accelerator.

[0008] The undulator may increase the energy of some of the electrons in the bunched electron beam and the deceleration unit may be arranged to reduce the energy of these electrons by at least the same amount. The deceleration unit may be arranged to reduce the energy of the bunched electron beam at least by an amount given by a product of a conversion efficiency of the undulator and the energy of the bunched electron beam before it enters the undulator. The deceleration unit may therefore reduce the energy of the bunched electron beam to compensate for the effect of the undulator, to return the electron beam to a condition in which it can be processed by other components of the free electron laser.

[0009] The free electron laser may introduce an energy spread in the bunched electron beam and the electron beam optics may translate the energy spread into a longitudinal spread, such that for at least some electrons in the bunched electron beam there is an energy imbalance between: the energy imparted to the bunched electron beam before it enters the undulator, and the energy extracted from the bunched electron beam after it leaves the undulator; and the deceleration unit may be arranged to reduce the energy of these electrons by an amount which is equal to or greater than the energy imbalance.

[00010] The deceleration unit may be arranged to reduce the energy of the electrons so that, once energy has been extracted by both the linear accelerator and the deceleration unit, the energy of substantially all of the electrons is equal to or less than the energy of the bunched electron beam produced by the electron source.

[00011] The energy of electrons in the bunched electron beam produced by the electron source may be around 5 to 20 MeV.

[00012] The bunched electron beam produced by the electron source may have an emittance below 1 mm mRad.

[00013] The linear accelerator may be arranged to increase the energy of electrons in the bunched electron beam produced by the electron source to around 500 to 1000 MeV. [00014] The free electron laser may further comprise: a beam dump, arranged to absorb the bunched electron beam, and the deceleration unit may be arranged to reduce the energy of at least some of the electrons in the bunched electron beam after it has left the undulator so that the energy of substantially all of the of the electrons in the bunched electron beam is below a threshold for inducing radioactivity before it enters the beam dump. The beam dump may comprise aluminium. This can aid in reducing downtime of free electron lasers, by reducing the frequency with which the beam dump requires processing to dispose of radioactive waste.

[00015] The deceleration unit may be arranged to reduce the energy of electrons in the bunched electron beam before they are decelerated by the linear accelerator.

[00016] For such embodiments, the deceleration unit and the linear accelerator may be substantially collinear. Advantageously, this reduces the sensitivity of the deceleration achieved by the linear accelerator to the deceleration unit.

[00017] The deceleration unit may be arranged to reduce the energy of electrons in the bunched electron beam after they are decelerated by the linear accelerator.

[00018] The deceleration mechanism may comprise a deceleration unit, comprising: one or more resonant cavities, wherein the bunched electron beam is directed through the one or more resonant cavities so as to excite one or more resonant standing wave modes therein; and an energy dissipation mechanism, arranged to dissipate electromagnetic energy from the one or more resonant cavities.

[00019] The resonant cavities of the deceleration unit may be provided with one or more alternating power supplies.

[00020] The energy dissipation mechanism may further comprise: a second electron source operable to produce a dummy electron beam and direct it through the one or more resonant cavities out of phase with the bunched electron beam; and a second beam dump, operable to absorb the dummy electron beam.

[00021] The current, phase and repetition rate of the dummy beam may be selected such that the rate of change of energy of the dummy beam within the one or more resonant cavities is substantially equal to the rate of change of energy of the bunched electron beam within the resonant cavities.

[00022] The current and repetition rate of the dummy electron beam may be substantially equal to that of the bunched electron beam and the dummy electron beam may be substantially 180 degrees out of phase with the bunched electron beam.

[00023] Advantageously, this makes it easier to ensure that the energy lost by the bunched electron beam matches that gained by the dummy electron beam so that the one or more resonant cavities remain in equilibrium. [00024] The energy dissipation mechanism may comprise an antenna or a waveguide coupled to the one or more resonant cavities and arranged to allow electromagnetic radiation to propagate away from the one or more resonant cavities.

[00025] The antenna or waveguide may be coupled to a radio frequency load.

[00026] According to a second aspect, there is provided a lithographic system comprising: a free electron laser according to the first aspect; and one or more lithographic tools. The lithographic tools may comprise lithographic apparatuses and/or mask inspection apparatuses, for example.

[00027] According to a third aspect, there is provided a deceleration unit for a main electron beam, comprising: one or more resonant cavities, arranged for the main electron beam to pass through so as to excite one or more resonant standing wave modes in the one or more resonant cavities; an electron source operable to produce a dummy electron beam and direct it through the one or more resonant cavities out of phase with the main electron beam; and a beam dump, operable to absorb the dummy electron beam.

[00028] According to a fourth aspect, there is provided a method of decelerating a main electron beam, comprising: producing a dummy electron beam; directing the main electron beam through one or more resonant cavities so that the main beam is decelerated and one or more standing wave modes are excited in the one or more resonant cavities; directing the dummy electron beam through the linear accelerator so it is accelerated; and directing the dummy beam towards a beam dump, to absorb it.

[00029] The third and fourth aspects allow energy to be transferred from the main electron beam (for example from the undulator of a free electron laser) to the dummy electron beam. Such arrangements provide an energy dissipation mechanism that is easy to implement because there are relatively few constraints on the dummy electron beam, i.e. it does not need to be a high quality electron beam. Since the dummy beam is dumped, it is not important to preserve its emittance.

[00030] According to a fifth aspect, there is provided a method of producing a radiation beam, comprising: producing a bunched electron beam; directing the bunched electron beam through a linear accelerator so as to accelerate it; after it leaves the linear accelerator, directing the bunched electron beam through an undulator operable to produce a periodic magnetic field along a periodic path about a central axis of the undulator such that the electrons interact with radiation in the undulator, stimulating emission of coherent radiation; after it leaves the undulator, directing the bunched electron beam back through the linear accelerator so as to decelerate it; and producing a dummy electron beam; after it leaves the undulator and either before or after it is directed back through the linear accelerator: directing the bunched electron beam through one or more resonant cavities so that the bunched electron beam is decelerated and one or more standing wave modes are excited in the one or more resonant cavities; directing the dummy electron beam through the linear accelerator so it is accelerated; and directing the dummy beam towards a beam dump, to absorb it.

[00031] According to a sixth aspect, there is provided a method of producing a radiation beam, comprising: producing a bunched electron beam; directing the bunched electron beam through a linear accelerator so as to accelerate it; after it leaves the linear accelerator, directing the bunched electron beam through an undulator operable to produce a periodic magnetic field along a periodic path about a central axis of the undulator such that the electrons interact with radiation in the undulator, stimulating emission of coherent radiation; after it leaves the undulator, directing the bunched electron beam back through the linear accelerator so as to decelerate it; and producing a dummy electron beam; after it leaves the undulator and either before or after it is directed back through the linear accelerator: directing the bunched electron beam through one or more resonant cavities so that the bunched electron beam is decelerated and one or more standing wave modes are excited in the one or more resonant cavities; and dissipating energy stored in the one or more standing wave modes via an antenna or a waveguide coupled to the one or more resonant cavities.

[00032] According to a seventh aspect, there is a provided a free electron laser comprising an electron source operable to produce relativistic electrons, an undulator configured to guide the relativistic electrons and stimulate emission of coherent radiation, an electron decelerator separate from the electron source and configured to decelerate electrons output from the undulator, comprising conducting walls arranged to form a plurality of cavities, wherein each cavity is configured to support a resonant mode of a wakefield generated in the cavity by electrons passing through the cavity, and a liquid cooling system configured to cool the conducting walls of the electron decelerator.

[00033] The electron decelerator of the free electron laser of the seventh aspect allows electrons which may be travelling at relativistic speeds to be decelerated using energy from the electrons themselves. Passing the electrons through cavities comprising conducting walls leads to a transfer of energy from the electrons to electromagnetic wakefields which are generated by the electrons. This transfer of energy causes the electrons to decelerate. The cavities are configured to support a resonant mode of the wakefields so that the wakefields persist in the cavities even after the electrons have passed out of the cavities. The resonating wakefields which persist in each cavity may act to oppose the motion of further electrons which subsequently enter the cavity and therefore decelerate the further electrons. The electron decelerator therefore acts to decelerate electrons using energy which is transferred from the electrons themselves and thus the electron decelerator may act to decelerate electrons without an external power source.

[00034] Prior art free electron lasers direct electrons to be decelerated back through the electron source (e.g. through a linear accelerator of the electron source). The electrons are decelerated by externally driven electromagnetic fields in the electron source. However this prior art method may be disadvantageous because the decelerating electrons passing through the electron source produce their own electromagnetic fields which interact with the externally driven electromagnetic fields used to accelerate the electrons produced by the electron source. This destabilises the acceleration of the accelerated electrons which leads to a reduction in the efficiency with which coherent radiation is stimulated from the accelerated electrons in the undulator. By decelerating electrons with an electron decelerator which is separate from the electron source, the acceleration of electrons in the electron source remains unaffected by the electron decelerator. This leads to an increase in the efficiency of the free electron laser.

[00035] The plurality of cavities may be conducting radio frequency cavities.

[00036] The plurality of cavities may be joined by a plurality of ducts comprising conducting walls.

[00037] The liquid cooling system may comprise water cooling.

[00038] Water cooling is a simple and inexpensive way in which to cool the conducting walls of the electron decelerator which are heated by currents which flow along the conducting walls caused by the wakefields in the cavities.

[00039] The liquid cooling system may comprise cryogenic cooling. Cryogenic cooling provides cooling of conducting walls which allows the cavities to support wakefields of a high field strength. This allows the cavities to have relatively short lengths.

[00040] The plurality of cavities may be superconducting radio frequency cavities.

[00041 ] The plurality of cavities may each have a maximum diameter which is substantially equal to (N + 1/2)1 where λ is the wavelength of a wakefield generated in the cavity by electrons passing through the cavity and N is an integer including 0.

[00042] N may be greater than 0. This may cause higher resonant modes of the wakefield to be supported by the cavity. Such cavities may be referred to as overmoded cavities. In an overmoded cavity the wakefield in the cavity is spread over a larger volume which decreases the field strength of the wakefield at a given point in the cavity.

[00043] The free electron laser may further comprise an electron dump positioned to receive electrons which are output from the electron decelerator. Decelerating the electrons with the electron decelerator before they reach the electron dump reduces the levels of induced radiation and secondary particles produced in the electron dump.

[00044] The electron dump may comprise water, thereby providing an inexpensive and easily handled electron dump.

[00045] The electron decelerator may not be driven by an external power source, thereby reducing a complexity and expense of the electron decelerator.

[00046] The electron decelerator acts to decelerate electrons without being driven by an external power source. This is advantageous because external power sources may consume large amounts of energy and may therefore be expensive to operate.

[00047] However embodiments of the electron decelerator of the first aspect may also incorporate one or more external power sources. For example one or more RF power sources may be used to produce externally driven RF fields in the plurality of cavities. The externally driven RF fields may complement wakefields in the cavities which are caused by electrons passing through the cavities and may therefore act to increase the strength of the wakefields and thus increase the deceleration of electrons passing through the cavities. The power produced by a radio frequency power source may however be relatively low when compared to, for example, the power produced by a radio frequency power source used to accelerate electrons. This is because the electrons are already decelerated by the wakefields generated when they pass through the electron decelerator and thus any power from an RF power source merely serves to top-up the wakefields already produced by the electrons.

[00048] The free electron laser may further comprise a coupling device configured to couple the wakefields generated in the cavities with the electron source. This may allow energy which is transferred from the decelerating electrons to the conducting walls to be harnessed and transferred to the electron source in order to accelerate electrons in the electron source. This may improve the overall efficiency of the free electron laser.

[00049] The plurality of cavities may each have an elliptically shaped cross-section.

[00050] The undulator may be configured to guide relativistic electrons and stimulate emission of coherent EUV radiation. [00051 ] According to an eighth aspect, there is provided a lithographic system comprising a free electron laser according to the seventh aspect of the invention and one or more lithographic tool.

[00052] According to a ninth aspect, there is provided a method of producing radiation comprising producing a beam of electrons, accelerating the beam of electrons in an accelerator to relativistic speeds, guiding the beam of relativistic electrons and stimulating emission of coherent radiation, decelerating the beam of electrons in a decelerator separate from the accelerator by directing them through a plurality of cavities comprising conducting walls, thereby causing a wakefield to be generated in each of the cavities, wherein each cavity is configured to support a resonant mode of the wakefield, and cooling the conducting walls with a liquid cooling system.

[00053] Accelerating the beam of electrons and decelerating the beam of electrons may be performed using apparatus which are distinct from one another.

[00054] The wakefields may oscillate at a radio frequency.

[00055] The liquid cooling system may cool the conducting walls by water cooling.

[00056] The liquid cooling system may cool the conducting walls by cryogenic cooling.

[00057] The wakefield in each cavity has a wavelength 1 and each cavity may have a maximum diameter which is substantially equal to (N + 1/2)1 where N is an integer including 0. N may be greater than 0, beneficially causing higher resonant modes of the wakefield to be supported by the cavity..

[00058] The method may further comprise directing the beam of electrons to an electron dump after they have been decelerated.

[00059] The beam of electrons may be decelerated without the use of an external power source.

[00060] Accelerating the beam of electrons may comprise generating radio frequency fields using a radio frequency power source.

[00061 ] The method may further comprise coupling the wakefields generated in the cavities with the radio frequency fields generated by the radio frequency power source.

[00062] Guiding the beam of relativistic electrons and stimulating emission of coherent radiation may comprise stimulating emission of EUV radiation.

[00063] Any aspect of the invention may include one or more features of any other aspect of the invention as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS [00064] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a free electron laser according to an embodiment of the invention;

- Figure 2 depicts a lithographic apparatus that forms part of the lithographic system of Figure 1 ;

Figure 3 is a schematic illustration of a free electron laser, which may form part of the lithographic system of Figure 1 ;

Figure 4 is a schematic illustration of an electron decelerator according to an embodiment of the invention;

Figures 5A and 5B are more detailed schematic illustrations of free electron lasers according to an example embodiment , which may form part of the lithographic system of Figure 1 ;

Figure 6 shows a deceleration unit according to the present invention that may form part of the free electron laser of Figures 5A and 5B; and

Figure 7 is a schematic illustration of a free electron laser according to an alternative embodiment, which may form part of the lithographic system of Figure 1 .

DETAILED DESCRIPTION

[00065] Figure 1 shows a lithographic system LS, comprising: a radiation source SO, a beam splitting apparatus 20 and eight lithographic apparatuses LA1 -LA8. The radiation source SO comprises a free electron laser and is configured to generate an extreme ultraviolet (EUV) radiation beam B _FE i_ (which may be referred to as a main beam). The main radiation beam B _F EL is split into a plurality of radiation beams B _a -B _h (which may be referred to as branch beams), each of which is directed to a different one of the lithographic apparatuses LA1 -LA8, by the beam splitting apparatus 20. The branch radiation beams B _a -B _h may be split off from the main radiation beam in series, with each branch radiation beam being split off from the main radiation beam downstream from the preceding branch radiation beam . Where this is the case the branch radiation beams may for example propagate substantially parallel to each other.

[00066] The radiation source SO, beam splitting apparatus 20 and lithographic apparatuses LA1 -LA8 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 apparatuses LA1 -LA8 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).

[00067] Referring to Figure 2, a lithographic apparatus LA1 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 LA1 before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B _a ' (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 LA1 aligns the patterned radiation beam B _a ' with a pattern previously formed on the substrate W.

[00068] While only lithographic apparatuses are shown in Figure 2, it is to be understood that the lithographic system LS may comprise other tools, such as mask inspection apparatuses.

[00069] The branch radiation beam B _a that is received by the lithographic apparatus LA1 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 _a may be focused to form an intermediate focus at or near to the opening 8.

[00070] 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 faceted field mirror device 10 and faceted pupil mirror device 1 1 may each comprise an array of independently movable mirrors. The faceted field mirror device 10 and faceted pupil mirror device 1 1 may comprise different numbers of independently movable mirrors. For example the faceted pupil mirror device 1 1 may comprise twice as many mirrors as the faceted field mirror device 10. The mirrors in the faceted field mirror device 10 and faceted pupil mirror device 1 1 may be of any suitable shape, for example, they may be generally banana shaped. 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 MEMS devices.

[00071 ] Following 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 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).

[00072] The radiation source SO comprises a free electron laser FEL according to an embodiment of the present invention, which is operable to produce a beam of EUV radiation. Optionally, the radiation source SO may comprise more than one free electron laser FEL according to an embodiment of the present invention.

[00073] The radiation source SO may further comprise optics arranged to alter the size and/or shape of the cross section of the radiation beams received from the free electron laser.

[00074] The optics may comprise beam expanding optics arranged to increase the cross sectional area of the radiation beam output by that free electron laser. 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. The beam splitting apparatus 20 may comprise a plurality of static extraction mirrors arranged in the path of the beam B _FE i_ which direct radiation from the main beam B _F EL along the plurality of branch radiation beams B _A -B _H . Increasing the size of the main beam B _F EL reduces the accuracy with which the mirrors must be located in the beam B _F EL path. Therefore, this allows for more accurate splitting of the output beam B _FEL by the splitting apparatus 20.

[00075] The radiation source SO may further comprise shape altering optics which are arranged to alter the cross sectional shape of the radiation beams received from the free electron laser. The shape altering optics may comprise one or more astigmatic or a-spherical optical elements. The shape altering optics and beam expanding optics may share common optical elements. [00076] 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 field 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.

[00077] Referring to Figure 3, a free electron laser FEL comprises an injector 21 , a linear accelerator 22, an undulator 24 and a beam dump 100.

[00078] The injector 21 is arranged to produce a bunched electron beam EB^ with a first energy The injector 21 comprises an electron source such as, for example, a thermionic cathode or photo-cathode and an accelerating electric field. Preferably the bunched electron beam EB^ has a relatively low emittance, for example below 1 mRad. The first energy E^ may be, for example, around 5-20 MeV. The first energy E^ may be around 10-15 MeV, which may be preferable since it may allow the emittance of the bunched electron beam E ^ to remain below 1 mm mRad.

[00079] The electron beam EB^ is accelerated to a second, higher energy E ₂ by the linear accelerator 22. In an example, the linear accelerator 22 may comprise a plurality of resonant 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 resonant 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 resonant cavities may be conventionally conducting (i.e. not superconducting) radio frequency cavities and may be formed from, for example, copper. Other types of linear accelerators may also be used. The injector 21 and linear accelerator 22 together form an electron source that is operable to produce relativistic electrons.

[00080] Optionally, the electron beam EE^ may pass through a bunch compressor 23. The bunch compressor 23 may be disposed downstream or upstream of the linear accelerator 22. The bunch compressor may be configured to bunch electrons in the electron beam EE^ and spatially compress existing bunches of electrons in the electron beam EE^ .

[00081] The electron beam EB^ then passes through the undulator 24. The undulator 24 comprises a plurality of magnets, which are operable to produce a periodic magnetic field and arranged so as to guide the relativistic electrons produced by the injector 21 and linear accelerator 22 along a periodic path. As a result, the electrons radiate electromagnetic radiation generally in the direction of a central axis of the undulator 24. The path followed by the electrons may be sinusoidal and planar, with the electrons periodically traversing the central axis, or 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 produce elliptically polarized radiation, which may be preferred for exposure of a substrate W by the lithographic apparatuses LA1 -LA8.

[00082] The undulator 24 comprises a plurality of sections, each section comprising a periodic magnet structure. The undulator 24 may further comprise a mechanism for refocusing the electron beam EE^ such as, for example, a quadrupole magnet in between one or more pairs of adjacent sections. The mechanism for refocusing the electron beam EB _! 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.

[00083] As electrons move through the undulator 24, 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, given by:

where l _em is the wavelength of the radiation, l _u is the undulator period, γ is the Lorentz factor of the electrons and κ 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 EE^ with low emittance). The undulator parameter K is typically approximately 1 and is given by: _{K =} <?4A

2mnc ' (2) 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.

[00084] The resonant wavelength l _em 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 EB^ 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.

[00085] 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. When the resonance condition of Eq. (1 ) is satisfied, the gain of the free electron laser FEL may be zero. Maximum gain may be achieved when conditions are close to but slightly off resonance.

[00086] The interaction between the electrons and radiation within the undulator 24 produces a spread of energies within the electron bunches in the electron beam EE^ . The electron beam EB ₂ exiting the undulator 24 may be considered to be a different electron beam with a spread of energies. The energy spread in the electron beam EB ₂ exiting the undulator 24 is dependent upon the conversion efficiency of the undulator 24. Quantitatively, the width of the energy spread in the electron beam EB ₂ exiting the undulator 24 may be given by a product of a conversion efficiency of the undulator 24 and the second energy E ₂ .

[00087] 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 l _u may vary along the length of the undulator 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 EB^ which is converted to radiation in the radiation beam B _F EL) 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 l _u and/or the magnetic field strength B ₀ 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.

[00088] A beam of radiation B _FEL propagates from the undulator 24. The radiation beam B _F EL comprises EUV radiation. The beam of EUV radiation B _F EL output by the free electron laser FEL may have a substantially circular cross section and a Gaussian intensity profile. The radiation beam produced by an EUV free electron laser typically has a relatively small etendue. In particular, the EUV radiation beam B _FEL produced by the free electron laser FEL has a significantly smaller etendue than an EUV radiation beam that would be generated by a laser produced plasma (LPP) source or a discharge produced plasma (DPP) source (both of which are known in the prior art). For example, the radiation beam B _FE i_ produced by the free electron laser FEL may have a divergence less than 50(^rad, for example less than 10(^rad, and may for example have a diameter of around 50μιη as it leaves the undulator 24.

[00089] The output power of the free electron laser FEL may be of the order of tens of kilowatts, in order to support high throughput for the eight EUV lithographic apparatus LA1 -LA8. At these powers, since the initial diameter of the radiation beam B _FEL produced by the free electron laser FEL is so small, the power density of the radiation beams B _A . _H will be significant.

[00090] In order for the output power of the free electron laser FEL to be sufficient to support high throughput for a plurality of EUV lithographic apparatus LA1 -LA8, the free electron laser FEL may have certain properties. For example, the second energy E ₂ that the linear accelerator 22 accelerates the electron beam EB^ to may be around 500 to 1000 MeV. The power of the electron beam EB^ may be of the order of 1 to 100 MW. The power of the electron beam EB^ may be dictated by a desired power of the output beam of EUV radiation B _FEL and the conversion efficiency of the undulator 24. For a given output power of the free electron laser FEL, the higher the conversion efficiency of the undulator 24 is, the lower the current of the injector 21 will be. Higher undulator 24 conversion efficiencies and lower injector 21 currents may be highly desirable. [00091] After leaving the undulator 24, the electron beam EB ₂ is absorbed by a dump 100. The dump 100 may comprise a sufficient quantity of material to absorb the electron beam EB ₂ . The material may have a threshold energy for induction of radioactivity. Electrons entering the dump 100 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 17 MeV. It is desirable to reduce the energy of electrons in the electron beam E ₂ 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.

[00092] Before entering the dump 100, energy is extracted from the electron beam EB ₂ . In order to reduce the energy of the electrons before they are absorbed by the dump 100, an electron decelerator 28 is disposed between the undulator 24 and the beam dump 100. The electron decelerator 28 reduces the amount of energy the electrons have when they are absorbed by the beam dump 100 and will therefore reduce the levels of induced radiation and secondary particles produced in the beam dump 100. This removes, or at least reduces, the need to remove and dispose of radioactive waste from the beam 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.

[00093] The electron decelerator 28 may be operable to reduce the energy of the electrons to below 7MeV and, preferably, below 5MeV. Advantageously, electrons below this energy do not induce any significant level of radioactivity in the beam dump 100. During operation of the free electron laser FEL, gamma radiation will be present but when the electron beam E is switched off, the beam dump 100 will be safe to handle.

[00094] One known method to decelerate electrons leaving the undulator 24 is to use the linear accelerator 22. That is, the linear accelerator 22 that is used to accelerate electrons output from the injector 21 may also be used for deceleration. The electron bunches that leave the undulator 24 may be injected into the linear accelerator 22 with a phase difference of around 180 degrees relative to the radio frequency (RF) field in the linear accelerator 22. Such an arrangement is known as an energy recovering LINAC (ERL). However, there is a limit to the spread of electron energies within an electron bunch that such an arrangement can accept. The undulator 24 will introduce a spread in the energy of the electron beam EB^ as it passes through the undulator 24. This will result in imperfect deceleration of electron bunches that are injected into the linear accelerator 22 with a phase difference of 180 degrees relative to radio frequency (RF) field. As a result, some of the electrons in the electron beam EB ₂ may have an energy as they leave the linear accelerator 22 in excess of, for example, the desired threshold of 7 or 5 MeV. As a result, a mechanism to further reduce the energy of these electrons is beneficial.

[00095] Decelerating the electrons by injecting the electron bunches into the linear accelerator 22 may be additionally disadvantageous since the decelerating electrons passing through the linear accelerator 22 generate their own RF field as they pass through the linear accelerator 22. This RF field may disturb an externally driven RF field in the linear accelerator 22 which is designed to accelerate the bunched electron beam EB^ provided by the injector 21 . The acceleration of the bunched electron beam EBT output from the injector 21 may be destabilized by an RF field generated by the decelerating electrons. This may spread the energies and/or positions of an accelerated electron bunch which may reduce the efficiency with which energy from the electron bunch is converted to EUV radiation in the undulator 24.

[00096] In order to overcome the above mentioned disadvantages of using the linear accelerator 22 to decelerate the electron beam EB ₂ output from the undulator 24, a separate electron decelerator 28 is placed between the undulator 24 and the electron dump 100, as is depicted in Figure 3.

[00097] The electron decelerator 28 may for example comprise superconducting RF cavities and one or more RF power sources. The RF power sources may be operable to control electromagnetic fields in the RF cavities so as to decelerate bunches of electrons passing through the electron decelerator 28. Such a decelerator may therefore be similar to the linear accelerator 22 but may be operated such that the relative phase between the electromagnetic fields in the RF cavities and the electron bunches is such that the electromagnetic fields oppose the motion of the electron bunches and hence cause the electron bunches to decelerate.

[00098] An embodiment of the electron decelerator 28 comprising superconducting RF cavities and one or more RF power sources may however be expensive to manufacture and operate. For example RF power sources may consume large amounts of energy in order to drive the deceleration of the electrons. It may therefore be advantageous to provide an electron decelerator 28 which does not require an externally driven power source such as an RF power source.

[00099] Figure 4 is a schematic depiction of an embodiment of an electron decelerator 28 which is not driven by an external power source. The electron decelerator 28 comprises conducting walls 31 which are shaped to form a plurality of cavities 33 which are elliptically shaped in their cross-section. The conducting walls 31 may, for example, comprise aluminum, copper or another conducting material. The cavities 33 are joined by sections of the conducting walls 31 which form a plurality of ducts 35 along the length of the electron decelerator 28.

[000100] The ducts 35 and the cavities 33 together define a passage 39 which is bound by the conducting walls 31 . The passage 39 has a diameter d which varies along its length. For example the passage 39 has a minimum diameter d' in a duct 35 and a maximum diameter d" at a position along the passage 39 coinciding with the widest point of a cavity 33.

[000101] The electron beam EB ₂ which is output from the undulator 24 is directed through the passage 39. Each electron which passes through the passage 39 has an associated electromagnetic field. As an electron passes from a duct 35 into a cavity 33, for example at point 41 in Figure 4, its associated electromagnetic field is perturbed by the increase in diameter of the passage 39 at this point. The electromagnetic field associated with the electron is perturbed again at the point 43 in Figure 4, where the electron passes from the cavity 33 into the duct 35 and the diameter of the passage 39 decreases. The passage of electrons through the electron decelerator 28 therefore leads to changing electromagnetic fields within the electron decelerator 28.

[000102] In general the electromagnetic field of an electron passing through the passage 39 is perturbed at points at which the diameter d of the passage 39 changes. As bunches of electrons pass through the passage 39 this causes an oscillating electromagnetic field to exist within the cavities. This oscillating electromagnetic field caused by the passage of electrons through the passage 39 may be referred to as a wakefield.

[000103] Certain shapes and dimensions of cavities 33 may support resonant modes of a wakefield. For example a cavity 33 may have a maximum diameter d" which is approximately equal to half of the wavelength of a wakefield caused by a passing bunch of electrons. Such a cavity supports one or more resonant modes of the wakefield causing the wakefield to persist even after the electron bunch, which caused the wakefield, has passed out of the cavity 33. [000104] As an electron bunch passes through a cavity 33 and causes a wakefield, energy is transferred from the electrons to the wakefield. This transfer of energy causes the electrons in the bunch to decelerate and thus the electrons are decelerated as they pass through the passage 39. In addition to this effect, the dimensions of the passage 39 may be set according to the timing of the electron bunches entering the electron decelerator 28 such that subsequent bunches of electrons entering a cavity 33 do so at a time at which a wakefield in the cavity (formed by a previous electron bunch) acts to oppose the motion of the electron bunch entering the cavity 33. Electron bunches may therefore be further decelerated by a wakefield in a cavity 33 which was caused by previous electron bunches which passed through the cavity 33. The passing of such an electron bunch through the cavity 33 also results in a further transfer of energy to the wakefield in the cavity 33, thereby causing the wakefield to persist such that it may act to decelerate further electron bunches which subsequently enter the cavity 33.

[000105] Through the mechanisms described above the electron decelerator 28 acts to decelerate electrons which pass through it without being driven by an external power source. Energy from decelerating electron bunches is instead transferred to resonating wakefields in the cavities 33 which act to decelerate subsequent electrons entering the cavities 33. The electrons passing out of the electron decelerator 28 therefore have much smaller velocities than electrons entering the electron decelerator 28 and thus levels of induced radiation and secondary particles produced in the beam dump 100 by the electrons passing out of the electron decelerator 28 is reduced.

[000106] The wakefields caused by the electrons passing through the cavities 33 cause electrical currents to flow in the conducting walls 31. These electrical currents cause the conducting walls 31 to be heated. Excessive heating of the conducting walls 31 may cause undesirable damage to the conducting walls 31 . In order to reduce heating, the conducting walls 31 are therefore cooled using a liquid cooling system (not shown). The liquid cooling system may comprise cryogenic cooling. For example the conducting walls 31 may be immersed in one or more baths of a cryogenic fluid such as, for example, liquid helium or liquid nitrogen. Alternatively a cryogenic fluid such as liquid helium or liquid nitrogen may be passed through coolant channels which are positioned in proximity to the conducting walls 31 . Use of cryogenic cooling may cause the cavities 33 to be superconducting RF cavities.

[000107] The inventors have realised that as an alternative to cryogenic cooling, water cooling may be used to provide sufficient cooling to the conducting walls 31 . For example, the liquid cooling system may comprise coolant channels which are positioned in proximity to the conducting walls 31 . Water may be passed through the coolant channels such that heat is transferred from the conducting walls 31 to the water. The water may then carry the heat through the coolant channels and away from the conducting walls 31 , thereby cooling the conductive walls 31 . Other coolant fluids may be used instead of water.

[000108] Water cooling (or cooling using other coolant fluids) and cryogenic cooling may be considered as two alternative embodiments of a liquid cooling system. Cryogenic cooling is capable of cooling the electron decelerator to much lower temperatures than for example water cooling. However cryogenic cooling is inherently more complex and expensive to provide than water cooling and thus providing a decelerator 28 with water cooling may be advantageous for cost and practicality reasons.

[000109] The amount of cooling which is applied to the electron decelerator 28 may determine the maximum field strength of wakefields which may be supported by a cavity 33 of the electron decelerator 28 without the heat generated by the wakefields causing damage to conducting walls 31 . For example a water cooled cavity 33 may only be able to support wakefields up to a strength of approximately 5 MV m ^"1 before the heating caused by the wakefield causes damage to the conducting material 31 . A cryogenically cooled cavity 33 may however be able to support wakefields up to a strength of approximately 50 MV m ^"1 before the heating caused by the wakefield causes damage to the conducting walls 31 .

[000110] The amount and type of cooling which is provided to the electron decelerator 28 therefore imposes a limit on the field strength which a cavity 33 may support without causing damage to the conducting walls 31 . However by increasing the length L of a cavity 33 a given wakefield in the cavity 33 is spread over a greater volume and thus the field strength at a given point in the cavity 33 is reduced. Water cooled cavities may, for example, therefore have a longer length L than cryogenically cooled cavities since water cooled cavities can only support a lower field strength without causing damage to the conducting walls 31 . Since each cavity of a water cooled electron decelerator may have a longer length L than cavities of a cryogenically cooled electron decelerator then the total length of a water cooled electron decelerator which decelerates electrons by a given amount may be longer than that of a cryogenically cooled electron decelerator which decelerates electrons by an equivalent amount.

[000111] In some applications of an electron decelerator 28 it may be desirable to limit the total length of the decelerator 28, for example for space conservation reasons. In such applications a cryogenically cooled electron decelerator 28 may therefore be advantageous.

[000112] A water cooled electron decelerator 28 of shorter length may however be produced by increasing the maximum diameter d" of the cavities 33 of the electron decelerator 28. Increasing the maximum diameter d" of a cavity 33 causes a given wakefield in the cavity to be spread over a larger volume and thus the field strength at a given point in the cavity 33 is reduced. Increasing the maximum diameter d" of a cavity 33 may therefore allow the length L of the cavity to be decreased without increasing the field strength at a given point in the cavity.

[000113] As was described above, it is desirable to provide a cavity 33 with a maximum diameter d" which supports resonant modes of a wakefield which is set up in the cavity due to electron bunches passing through the cavity. The maximum diameter d" of a cavity 33 may therefore be limited to values which support resonant modes of a given wakefield. For a given wakefield of wavelength λ the diameter d" may for example be approximately equal to ^λ / ₂ , ^3λ /2> ^5/ί /2 ^{or 7λ} /2- ^{l n} 9 ^{eneral tne} diameter d" of a cavity 33 may be set according to equation 3. Where N is an integer including 0, i.e. N = 0, 1, 2, 3, 4, 5, 6 etc. A cavity 33 having a diameter d" according to equation 1 in cases where N > 0 may support higher resonant modes of the wakefield and may be referred to as an overmoded cavity.

[000114] An electron decelerator 28 comprising overmoded cavities may allow the length L of the cavities to be reduced without significantly reducing the deceleration caused by the cavities. The total length of a decelerator 28 which decelerates electrons by a given amount and comprises overmoded cavities may therefore be shorter than the total length of a decelerator 28 which decelerates electrons by an equivalent amount but that does not comprise overmoded cavities. This may be particularly advantageous in an embodiment of a decelerator 28 comprising water cooled overmoded cavities which may allow the total length of the electron decelerator 28 to be reduced to an equivalent length of a cryogenically cooled decelerator. Overmoded cavities may however also be advantageously employed in a cryogenically cooled decelerator.

[000115] Embodiments of an electron decelerator 28 have been described above which are not driven by an external power source. However embodiments of an electron decelerator 28 according to the invention may also incorporate one or more external power sources. For example one or more RF power sources may be used to produce externally driven RF fields in the cavities 33. The externally driven RF fields may complement wakefields in the cavities which are caused by electrons passing through the cavities and may therefore act to increase the strength of the wakefields and thus increase the deceleration of electrons passing through the cavities.

[000116] One or more RF power sources arranged to produce externally driven RF fields may be used in conjunction with a water cooled electron decelerator 28 and/or in conjunction with a cryogenically cooled electron decelerator 28. The power produced by an RF power source may however be relatively low when compared to, for example, the power produced by an RF power source used to accelerate electrons in the linear accelerator 22. This is because the electrons are already decelerated by the wakefields generated when they pass through the electron decelerator 28 and thus any power from an RF power source merely serves to top-up the wakefields already produced by the electrons.

[000117] Wakefields produced by the electrons are themselves a source of RF power. According to some embodiments of the invention the RF power produced by the wakefields may be harnessed and used as an RF power source for other applications. For example the RF power produced by a wakefield may be coupled to the linear accelerator 22 and used in conjunction with RF fields generated by one or more RF power sources to accelerate the electron beam E passing through the linear accelerator 22. Such an embodiment may reduce the amount of power required from the one or more RF power sources in order to produce a given acceleration in the linear accelerator 22. This may be advantageous since it may increase the overall efficiency of a free electron laser FEL.

[000118] The RF power generated by a wakefield in an electron decelerator 28 may be coupled to the linear accelerator 22, for example, via a coupling device (not shown) such as a waveguide or a coaxial cable in order to transport the RF field from the electron decelerator 28 to the linear accelerator 22. In alternative embodiments RF power generated by a wakefield in an electron decelerator may be coupled, via a coupling device, to RF loads other than the linear accelerator 22.

[000119] Although the embodiment of an electron decelerator 28 in Figure 4 comprise cavities 33 which have elliptically shaped cross-sections, an electron decelerator 28 according to an embodiment of the invention may comprise cavities 33 having other shapes which support resonant modes of wakefields generated in the cavities. [000120] Referring to Figure 5A, an alternative arrangement of a free electron laser FEL1 is schematically illustrated. In Figure 5A, the linear accelerator 22 operates as an energy recovery LINAC (ERL). Electron beam optics (not shown) are used to direct the electron beam EB ₂ leaving the undulator 24 to a beam merging unit 25. The beam merging unit 25 combines the electron beam EB ₂ from the undulator 24 with the electron beam EE^ from the injector 21 such that the two electron beams EB^ EB ₂ are out of phase by approximately 180 degrees. Both electron beams EB^ EB ₂ are directed through the linear accelerator 22 in the same direction. The timing is such that the electrons from the injector 21 are accelerated and the electrons from the undulator 24 are decelerated. Advantageously, this allows energy to be extracted from the electron beam EB ₂ leaving the undulator 24, and for that energy to be at least partially recycled to accelerate electron beam EB^ from the injector 21 . After exiting the linear accelerator 22, both electron beams EB^ EB ₂ are directed through a beam splitting unit 26 that directs the electron beam EB ₂ from the undulator 24 towards the dump 100 and directs the electron beam EB^ from the injector 21 towards the undulator 24. Such an arrangement reduces the amount of energy the electrons have when absorbed by the dump 100 and, therefore, will reduce the levels of induced radiation and secondary particles produced in the dump 100.

[000121 ] Electrons in the electron beam EB^ from the injector 21 are accelerated by the linear accelerator 22 from the first energy E^ to the second energy E ₂ . If the electrons in the electron beam EB ₂ from the undulator 24 had the second energy E ₂ and it was possible to ensure that they pass through the linear accelerator 22 exactly 180 degrees out of phase with the electrons from the injector 21 , they would be decelerated back down to the first energy E^ However, this is not the case for at least two reasons.

[000122] First, the spread in the energy of the electron beam EB ₂ induced by the undulator 24 will mean that the electrons in electron beam EB ₂ will not all have the second energy E ₂ . In particular, there will be some electrons in the electron beam EB ₂ that exceed the second energy E ₂ by an amount given by a product of a conversion efficiency of the undulator 24 (typically of the order of 1 %) and the second energy E ₂ . After deceleration by the linear accelerator such electrons will exceed the first energy by this amount.

[000123] Second, since the bending angle for an electron in a given magnetic dipole field is inversely proportional to energy of that electron, the spread in the energy of the electron beam EB ₂ induced by the undulator 24 will result in chromatic aberrations. These can be corrected for by higher order multipole fields such as, for example, a sextupole magnetic field. However, the combination of a dipole field and a sextupole field will produce a longitudinal spread in the bunches of the electron beam EB ₂ . If the length of bunches within the electron beam EB ₂ becomes significant on the scale of the wavelength of the radio frequency field in the linear accelerator 22 then at least some of the electrons will not be exactly 180 degrees out of phase with the electrons from the injector 21 . Therefore, these electrons will not lose the energy corresponding to the full accelerating potential (E^) of the linear accelerator 22.

[000124] As a result of these two effects, following deceleration by the linear accelerator 22, the electron beam EB ₂ from the undulator 24 will have an energy distribution wherein the vast majority of the electrons will have an energy close to the first energy E^ but there will be a high energy tail, which may extend beyond the threshold of production of radioactive isotopes in the dump 100. The tail of the distribution of electron energies extend beyond the first energy E^ by an amount that is dependent upon the conversion efficiency of the undulator 24 and the second energy E ₂ . Therefore, for high conversion efficiencies the tail of the distribution of electron energies is more likely to extend beyond the threshold of production of radioactive isotopes in the dump 100.

[000125] As a result, in the ERL arrangement of Figure 5A, a deceleration unit 28' is arranged to reduce the energy of electrons in the electron beam EB ₂ from the undulator 24 before they enter the beam merging unit 25. As described above with reference to the deceleration unit 28, the deceleration unit 28' comprises one or more resonant cavities 28a. The deceleration unit 28' may also comprises an energy dissipation mechanism 28b.

[000126] The one or more resonant cavities 28a may be as described above with reference to Figures 3 and 4. That is, the resonant cavities are arranged so that the electron beam EB ₂ from the undulator 24 passes through them, exciting one or more resonant standing wave modes in the resonant cavities 28a. The resonant cavities 28a may be superconducting radio frequency cavities. Alternatively, the resonant cavities 28a may be conventionally conducting (i.e. not superconducting), and may be formed from, for example, copper. The resonant cavities 28a may be provided with one or more alternating power supplies (not shown), which may be radio frequency (RF) supplies. The supplies may help to ensure that the electron beam EB ₂ from the undulator 24 excites one or more resonant standing wave modes in the resonant cavities 28a. This may be especially useful at the start and end of operation of the free electron laser FEL1 . During steady state operation of the deceleration unit 28, the power supplied to the one or more resonant cavities 28a by the alternating power supplies may be reduced or completely eliminated.

[000127] The energy dissipation mechanism 28b is arranged to dissipate the electromagnetic energy within the one or more resonant cavities 28a. The energy from the decelerating beam can be dissipated in various ways.

[000128] Referring to Figure 6, in a first embodiment, the energy dissipation mechanism 28b comprises an electron source 42, a merging unit 44, a demerging unit 46 and a beam dump 48. The electron source 42 may comprise a thermionic or photo- injection electron gun which is operable to produce a dummy bunched electron beam EB ₃ . The dummy beam EB ₃ may be generated to have the same repetition rate as the electron beam EB ₂ from the undulator 24. The merging unit 44 combines the dummy electron beam EB ₃ with the electron beam EB ₂ from the undulator 24 such that the two electron beams EB ₂ , EB ₃ are out of phase by 180 degrees. Therefore, both electron beams EB ₂ , EB ₃ are directed through the one or more resonant cavities 28a in the same direction. The timing is such that the dummy electron beam EB ₃ is accelerated by one or more resonant standing wave modes in the resonant cavities 28a excited by the electrons from the undulator 24, thus absorbing the energy that was removed from electron beam EB ₂ . The demerging unit 46 is arranged to split the two electron beams EB ₂ , EB ₃ after they exit the one or more resonant cavities 28a. The demerging unit 46 directs the electron beam EB ₂ from the undulator 24 towards beam merging unit 25 and directs the dummy electron beam EB ₃ towards the beam dump 48.

[000129] The beam dump 48 may comprise a sufficient quantity of material to absorb the dummy electron beam EB ₃ and may be similar to dump 100 described above. The deceleration unit 28' is arranged so that the energy of the dummy beam EB ₃ is sufficiently low that it is below a threshold for producing radioactive isotopes in beam dump 48. In one embodiment, the beam dump comprises aluminium and the energy of the dummy beam EB ₃ after it leaves the one or more resonant cavities 28a is around 1 - 10 MeV.

[000130] In an embodiment, the energy dissipation mechanism 28b allows energy to be transferred from the electron beam EB ₂ to the dummy electron beam EB ₃ . The current of the dummy beam EB ₃ may be substantially equal to that of the electron beam EB ₂ from the undulator 24 and the beams EB ₂ , EB ₃ may have a phase difference of substantially 180 degrees. Advantageously, this can help ensure that the energy lost by the electron beam EB ₂ from the undulator 24 matches that gained by the dummy electron beam EB ₃ and the one or more resonant cavities 28a remain in equilibrium. [000131] However, the current of the dummy electron beam EB ₃ need not be equal to that of the electron beam EB ₂ in order to retain energy gain/loss equilibrium within the cavities 28a. An amplitude of the electric field component along the axis of an RF cavity changes sinusoidally with time. The energy that an electron bunch gains or loses within an RF cavity therefore depends on the phase of electric field when that electron bunch enters the RF cavity. The energy that an electron bunch loses or receives from an RF cavity is given by

^Ebunch NEg _a i _nea - (4)

[000132] where N is the number of electrons in the electron bunch and E _gained is the energy gained by each electron as it passes through the cavity. By way of illustration only, an electron beam EB ₂ comprising electron bunches each having a charge of 1 nC may be considered. In this illustration the electron beam EB ₂ has a repetition rate and phase such that each electron bunch enters an RF cavity such that each electron in the electron bunch loses 1 MeV as it passes through the cavity. Considering a dummy electron beam EB ₃ having electron bunches each with a charge 100 pC and with the same repetition rate as the electron beam EB ₂ , energy equilibrium may be maintained by setting the phase of the dummy beam EB ₃ such that each electron bunch enters the RF cavity such that each electron gains 10 MeV as it passes through the cavity. In this case, the energy lost by the each bunch in the electron beam EB ₂ is equal to 1 mJ, while the energy gained by each bunch in the dummy beam EB ₃ is also equal to 1 mJ. As such, a steady state is provided within the cavity.

[000133] It can be seen, therefore, that a dummy beam EB ₃ having a different current to the electron beam EB ₂ can provide a current dilution deceleration of the electron beam EB ₂ by adjusting the phase of the dummy electron beam EB ₂ . Generally, it is desirable that the rate of change of energy of the dummy beam EB ₃ is equal to the rate of change of energy of the electron beam EB ₂ .

[000134] It will be appreciated from the above that while in some embodiments, the repetition rate may be the same both the dummy beam EB ₃ and the electron beam EB _2i this is not essential. The repetition rate of one of the electron beams may be an integer multiple of the repetition rate of the other one of the electron beams, with the higher repetition rate being determined by the frequency of the electric filed in the RF cavity.

[000135] In another embodiment, the energy dissipation mechanism 28b comprises a coupling out antenna or a waveguide coupled to the one or more resonant cavities. The electromagnetic radiation may propagate along the antenna or waveguide, carrying RF energy away from the one or more cavities 28a. The electromagnetic radiation may be coupled to an RF power load which dissipates the energy.

[000136] In a third embodiment, the energy dissipation mechanism 28b comprises both a dummy beam generator and a coupling out antenna or waveguide.

[000137] The amount of energy that is extracted from the electron beam EB ₂ from the undulator 24 may be of the order of the product of the accelerating potential of the linear accelerator 22 and the conversion efficiency of the undulator 24. This may be around 1 -10 MeV. Such a change in energy may be achievable using one or more resonant cavities 28a that are significantly shorter in length than those of the linear accelerator 22. For example, they may be an order of magnitude shorter. The implementation of this embodiment of the energy dissipation mechanism 28b is significantly easier than the deceleration provided by the linear accelerator 22 since it is not important to preserve the emittance of the dummy electron beam EB ₃ . In contrast, within the linear accelerator 22 it is important to preserve the emittance of the electron beam E ^ from the injector 21 .

[000138] The deceleration unit 28' benefits from low relative energy spread and low divergence in the electron beam EB ₂ from the undulator 24. The deceleration unit 28' may affect the deceleration achieved in the linear accelerator 22. For example, this is the case if the electron beam EB ₂ changes direction between the deceleration unit 28' and linear accelerator 22. This is because any variation in the amount of deceleration achieved in the deceleration unit 28' will be translated into different path lengths for the bunches of the electron beam EB ₂ sent to the linear accelerator 22. Potentially, this may reduce the efficiency of deceleration in the linear accelerator since some electrons may be out of a desired phase range. Therefore, it is advantageous to arrange the deceleration unit 28' and the linear accelerator 22 such that they are substantially collinear, as shown in Figure 5B. Such an arrangement reduces the sensitivity of the deceleration achieved by the linear accelerator to the deceleration unit 28'.

[000139] Referring to Figure 7, an alternative embodiment of a free electron laser FEL2 according to the invention is substantially similar to the free electron laser FEL1 shown in Figures 5A and 5B expect that it comprises a different deceleration unit. In particular, the deceleration unit 28' of the FEL1 has been replaced by a deceleration unit 29 arranged to reduce the energy of electrons in the electron beam EB ₂ from the undulator 24 after they exit the beam splitting unit 26. The deceleration unit 29 comprises one or more resonant cavities 29a and an energy dissipation mechanism 29b. The deceleration unit 29 may be substantially similar to, and may share common features with, the deceleration unit 28' described above in relation to the FEL1 . [000140] The deceleration unit 29 further comprises a chromatic aberration correction unit (not shown). This is arranged to correct for a relatively large angular spread in electrons in the electron beam EB ₂ from the undulator 24 that results from a bending of electron beam EB ₂ by the splitting unit 26 and the energy spread introduced by the undulator 24.

[000141 ] The one or more resonant cavities 29a may be substantially similar to the one or more resonant cavities 28a described above. The energy dissipation mechanism 29b may comprise any of the embodiments of the dissipation mechanism 28b described above. For embodiments wherein the energy dissipation mechanism 29b comprises an electron source operable to produce a dummy bunched electron beam with the same repetition rate as the electron beam EB ₂ from the undulator 24, the energy dissipation mechanism 29b may not require a demerging unit. Rather, both the dummy electron beam and the electron beam EB ₂ from the undulator 24 may be directed towards dump 100.

[000142] Advantageously, the deceleration unit 29 does not affect the deceleration achieved in the linear accelerator 22 since the electron beam EB ₂ from the undulator 24 passes through the latter first.

[000143] The above described embodiments of the invention provide a free electron laser FEL1 , FEL2 design which can operate in energy recovery (ERL) mode with an undulator with high conversion efficiency and an injection with a high first energy E^ whilst ensuring that there is no significant build-up of radioactivity in the dump 100.

[000144] More generally, the above described embodiments of the invention provide a free electron laser FEL, FEL1 , FEL2 design which eliminates, or significantly reduces, build-up of radioactivity in the dump 100 due to the electron energy spread introduced by the undulator 24. In addition, the above described embodiments of the invention provide a free electron laser FEL, FEL1 , FEL2 design which allows the injector 21 to produce a bunched electron beam EB^ with a first energy E^ which exceeds a threshold energy for induction of radioactivity of the dump 100.

[000145] For embodiments wherein the deceleration units 28', 29 utilise a dummy beam the elements are simple and can be easily implemented since it is not necessary to preserve the emittance of the dummy electron beam. The dummy electron source may comprise a thermionic gun and a buncher based injector. Advantageously, these components are relatively cheap and reliable. Embodiments wherein the deceleration units 28', 29 utilise an antenna or waveguide coupled to the one or more resonant cavities, the deceleration units 28', 29 also provide simple and cheap designs. Advantageously, for such embodiments, the energy extracted from the electron beam EB ₂ may be utilised by connecting the antenna or waveguide to an RF load.

[000146] Embodiments of the invention may comprise a combination of the three arrangements FEL, FEL1 , FEL2 described above. For example, an embodiment of the invention may comprise: (a) a first deceleration unit arranged to reduce the energy of electrons in the electron beam EB ₂ from the undulator 24 before they enter the beam merging unit 25; and (b) a second deceleration unit arranged to reduce the energy of electrons in the electron beam EB ₂ from the undulator 24 after they exit the beam splitting unit 26.

[000147] As used in this document, the term "dummy" in the expressions "dummy bunched electron beam", "dummy electron beam" and "dummy beam" may be considered to imply that the beam is produced to absorb a quantity of energy before being dumped. The dummy electron beam is not used to generate laser radiation.

[000148] Although the described embodiment of lithographic system LS comprises eight lithographic apparatuses LA1 -LA8, a lithographic system according to an embodiment of the invention may comprise any number of lithographic apparatuses.

[000149] A lithographic system according to an embodiment of the invention may further comprise one or more mask inspection apparatuses. The beam splitting apparatus 20 may direct a portion of the main radiation beam B _F EL to the mask inspection apparatus. The mask inspection apparatus may use this radiation to illuminate a mask and uses an imaging sensor to monitor radiation reflected from the mask MA. The mask inspection apparatus may include optics (e.g. mirrors) configured to receive a branch radiation beam from beam splitting apparatus 20 and direct the radiation beam at a mask. 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 the imaging sensor. The mask inspection apparatus may be similar to the lithographic apparatus LA1 shown in Figure 2, with the substrate table WT replaced with an imaging sensor. In some embodiments, the lithographic system 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.

[000150] 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.

[000151] The lithographic apparatuses LA1 -LA8 may be used in the manufacture of ICs. Alternatively, the lithographic apparatuses LA1 -LA8 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.

[000152] 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.