NL2017991A - Apparatus and Method for Producing a Beam of Electron Bunches - Google Patents

Abstract

The present application is concerned with a resonant cavity operable to receive an input beam of electrons. The resonant comprises a power source and a controller configured to cause the power source to excite an electromagnetic wave in the resonant cavity. The electromagnetic wave has a first resonant mode and a second resonant mode. The first and second resonant modes have frequencies that are integer multiples of each other.

Description

Apparatus and Method for Producing a Beam of Electron Bunches

FIELD

[0001] The present invention relates to resonant cavities and methods for their use. The present invention has particular, but not exclusive, use in resonant cavities for use in electron bunch sources which may be used with free electron lasers.

BACKGROUND

[0002] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0003] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).

[0004] A lithographic system may comprise one or more radiation sources, a beam delivery system and one or more lithographic apparatus. The one or more radiation sources may comprise a free electron laser.

[0005] It is an object of the present invention to obviate or mitigate at least one problem of prior art techniques.

SUMMARY

[0006] According to a first aspect described herein, there is provided a resonant cavity operable to receive an input beam of electrons along a longitudinal axis of the resonant cavity. The resonant cavity comprises a power source and a controller. The controller is configured to cause the power source to excite an electromagnetic wave in the resonant cavity, the electromagnetic wave having a first resonant mode and a second resonant mode, each having a magnetic field at the longitudinal axis which acts upon the input beam of electrons. The first and second resonant modes have frequencies that are integer multiples of each other.

[0007] In this way, the magnetic and/or electric fields generated within the cavity may be adjusted so as to obtain better control over the input radiation beam. It is to be understood that the first resonant mode need not be the fundamental resonant mode. The longitudinal axis may be a central axis of the resonant cavity. The magnetic field of each of the first and second modes which act upon an input electron beam may be a magnetic field having a non-zero magnitude on the longitudinal axis.

[0008] The controller may be configured to cause the power source to excite the resonant cavity to provide a longitudinal electric field along a longitudinal axis of the resonant cavity.

[0009] At a magnitude of, for example, zero, a rate of change of the magnitude of the electric field may be different compared to a rate of change of the magnitude of an electric field that would be provided by an electromagnetic wave having only the first resonant mode. In this way, a desired rate of change of the magnitude of the electric field may be provided at a desired magnitude. For example, the input electron beam may be timed such that a centre of each bunch of electrons in the input electron beam enters the cavity at a time when the magnitude of the electric field is zero.

[0010] The second resonant mode may be out of phase with the first resonant mode. The second resonant mode may have a different amplitude to the first resonant mode. By controlling the phase and/or amplitude of each of the resonant modes excited within the resonant cavity, greater control over the input beam of electrons may be provided.

[0011] The second frequency may be a third or higher harmonic of the resonant cavity.

The first and second resonant modes may be transverse magnetic resonant modes.

[0012] The controller may be configured to cause the power source to excite the resonant cavity with the electromagnetic wave such that an electric field along the longitudinal axis of the resonant cavity has a magnitude of zero.

[0013] The controller may be configured to cause the power source to excite the resonant cavity with the electromagnetic wave such that a duration for which the rate of change of the amplitude of a time dependent magnetic field within the resonant cavity may be at a local minimum may be greater than a duration for which the rate of change of the amplitude of the time dependent magnetic field within the resonant cavity would be at a local minimum if the resonant cavity were excited with an electromagnetic wave having only the first resonant mode.

[0014] The resonant cavity may be non-cylindrical. The resonant cavity may be a rectangular resonant cavity.

[0015] The resonant cavity may further comprise one or more magnets arranged to generate a constant magnetic field to cancel a time-dependent magnetic field excited within the resonant cavity at a predetermined magnitude.

[0016] The one or more magnets may be arranged such that cancelling of the time-dependent magnetic field occurs at a point of local maximum magnitude of the time-dependent magnetic field.

[0017] According to a second aspect described herein, there is provided a resonant cavity operable to receive an input beam of electrons. The resonant cavity comprises a power source and a controller. The controller is configured to cause the power source to excite an electromagnetic wave within the resonant cavity, the electromagnetic wave having at least a fundamental transverse magnetic mode. The resonant cavity further comprises one or more magnets arranged to generate a constant magnetic field to cancel a time-dependent magnetic field excited within the resonant cavity at a predetermined magnitude.

[0018] The one or more magnets are arranged such that cancelling of the time-dependent magnetic field occurs at a point of local maximum amplitude of the time-dependent magnetic field.

[0019] According to a third aspect described herein, there is provided a method for processing an electron beam. The method may comprise exciting an electromagnetic wave in the resonant cavity, the electromagnetic wave having first resonant mode and a second resonant mode. The first and second resonant modes each have a magnetic field on a longitudinal axis of the resonant cavity. The method further comprises directing the electron beam along the longitudinal axis through the excited resonant cavity such that the magnetic fields of the first and second modes act upon the electron beam.The first and second resonant modes have frequencies that are integer multiples of each other.

[0020] Exciting the resonant modes may results in a longitudinal electric field along a longitudinal axis of the resonant cavity.

[0021] At a magnitude of, for example, zero, a rate of change of the magnitude of the electric field may be different to a rate of change of the magnitude of an electric field that would be provided by an electromagnetic wave having only the first resonant mode.

[0022] The second resonant mode may be out of phase with the first resonant mode.

[0023] The second resonant mode may have a different amplitude to the first resonant mode.

[0024] The second frequency may be a third or higher harmonic of the resonant cavity.

[0025] The first and second resonant modes may be transverse magnetic resonant modes.

[0026] Exciting the resonant cavity may comprise exciting the resonant cavity such that an electric field along a longitudinal axis of the resonant cavity may have a magnitude of zero.

[0027] Exciting an electromagnetic wave within the resonant cavity may comprise exciting the resonant such that a duration for which the rate of change of the amplitude of a time dependent magnetic field within the resonant cavity is at a local minimum may be greater than a duration for which the rate of change of the amplitude of the time dependent magnetic field within the resonant cavity would be at a local minimum if the resonant cavity were excited with an electromagnetic wave having only the first resonant mode.

[0028] The electron beam may be a continuous electron beam.

[0029] The resonant cavity may be non-cylindrical.

[0030] The resonant cavity may be a rectangular resonant cavity.

[0031] The method may further comprise generating a constant magnetic field within the resonant cavity arranged to cancel a time-dependent magnetic field excited within the resonant cavity at a predetermined magnitude. In this way, the electron beam can be made to exit resonant cavity along substantially the same path as it enters the resonant cavity. As such, the need for downstream components to align and condition the electron beam may be reduced.

[0032] The cancelling of the time-dependent magnetic field occurs at a point of local maximum amplitude of the time-dependent magnetic field.

[0033] According to a fourth aspect described herein, there is provided a method of operating a resonant cavity operable to receive an input beam of electrons. The method comprises exciting an electromagnetic wave within a resonant cavity, the electromagnetic wave having at least a fundamental transverse magnetic mode, and generating a constant magnetic field within the resonant cavity configured to cancel a time-dependent magnetic field excited within the resonant cavity at a predetermined magnitude.

[0034] The cancelling of the time-dependent magnetic field occurs at a point of local maximum amplitude of the time-dependent magnetic field.

[0035] According to a fifth aspect described herein, there is provided a source for producing a beam of electron bunches, the source comprising: an electron source arranged to provide an electron beam; and a resonant cavity according to any one of the first or second aspect.

[0036] The electron source may be arranged to provide an electron beam comprising a plurality of electron bunches and the resonant cavity may be arranged to modify a phase space density between electron bunches in the electron beam. For example, the resonant cavity may be arranged to compress the electron bunches in the electron beam.

[0037] The electron source may be arranged to provide a continuous electron beam for propagation along a first axis. The resonant cavity may form a deflector operable to receive the electron beam along the first axis and to alter the direction of the electron beam so as to form an output electron beam such that the direction of the output electron beam varies with time through a range of directions. The deflector further may comprise a blocking member which may be arranged to block the output electron beam when it may be in a first sub-range of the range of directions and to allow the output electron beam to pass it when it may be in a second sub-range of the range of directions so as to form a bunched electron beam. The deflector and the blocking member may be arranged such that a rate of change of the direction of the electron beam may be at a local minimum when the electron beam may be in the second sub-range.

[0038] The resonant cavity may be operable to cause the direction of the output electron beam to oscillate between a first end direction and a second end direction.

[0039] The blocking member may be arranged such that the second sub-range of the range of directions may comprise at least one of the first end direction or the second end direction.

[0040] The blocking member may comprise a wall or a screen that may be provided with a knife-edge or one or more apertures.

[0041] According to a sixth aspect described herein, there is provided a free electron laser comprising the source according to the fifth aspect, and an undulator arranged to receive the bunched beam of electrons and operable to cause the bunched beam of electrons to follow an oscillating path about a central axis so that a radiation beam may be emitted generally along the central axis.

[0042] According to a seventh aspect described herein, there is provided a lithographic system comprising the free electron laser of the sixth aspect, one or more lithographic apparatuses, and a beam delivery system arranged to receive the radiation beam produced by the free electron laser and to direct at least a portion of the radiation beam to at least one of the one or more lithographic apparatuses.

[0043] 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

[0044] 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 according to an embodiment of the invention; -Figure 2 is a schematic illustration of a lithographic apparatus that may form part of the lithographic system of Figure 1; -Figure 3 is a schematic illustration of a free electron laser that may form part of the lithographic system of Figure 1; -Figures 4A, 4B are schematic illustrations of example injectors which may form part of the free electron laser of Figure 3; -Figures 5A, 5B illustrate energy change of an electron caused by an electric filed within a resonant cavity of an injector; -Figures 6A, 6B, 6C are graphs illustrating changes to an electric field within a resonant cavity of an injector; -Figure 7 is a schematic illustration of an example of an electron beam chopper that may form part of the free electron laser of Figure 3; -Figure 8 is a schematic illustration of the screen of the electron beam chopper of Figure 7; -Figure 9 schematically shows an electron bunch formed by the electron beam chopper of Figure 7; -Figure 10 is a graph illustrating a magnetic field formed within a resonant cavity of the chopper of Figure 7 in accordance with an embodiment of the invention; -Figure 11 is a schematic of an electron beam chopper in accordance with an embodiment of the invention; -Figure 12 is a schematic illustration of another electron beam chopper in accordance with an embodiment of the invention; and

-Figure 13 is a schematic depiction of a magnet arranged to provide constant magnetic field. DETAILED DESCRIPTION

[0045] In the following description and figures, like integers are provided with like reference numerals.

[0046] 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 and a plurality of lithographic apparatus LAa-LAn (e.g. eight lithographic apparatus). The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam RB (which may be referred to as a main beam).

[0047] The beam delivery system BDS comprises beam splitting optics and may optionally also comprise additional beam expanding optics and/or beam shaping optics. The main radiation beam RB is split into a plurality of radiation beams Ba-Bn (which may be referred to as branch beams), each of which is directed to a different one of the lithographic apparatus LAa-LAn, by the beam delivery system BDS.

[0048] The beam delivery system BDS may comprise beam expanding optics that are arranged to increase a cross section of the main radiation beam RB and/or the branch radiation beams Ba-Bn. Advantageously, this decreases the heat load on mirrors downstream of the beam expanding optics, for example mirrors within the lithographic apparatus LAa-LAn. This may allow these mirrors 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.

[0049] In an embodiment, the branch radiation beams Ba-Bn are each directed through a respective attenuator (not shown). Each attenuator may be arranged to adjust the intensity of a respective branch radiation beam Ba-Bn before the branch radiation beam Ba-Bn passes into its corresponding lithographic apparatus LAa-LAn.

[0050] The radiation source SO, beam delivery system BDS and lithographic apparatus LAa-LAn 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 LAa-LAn 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).

[0051] Referring to Figure 2, a lithographic apparatus LAa 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 Ba that is received by that lithographic apparatus LAa before it is incident upon the patterning device MA. The projection system PS is configured to project the radiation beam Ba’ (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 Ba’ with a pattern previously formed on the substrate W.

[0052] The branch radiation beam Ba that is received by the lithographic apparatus LAa 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 Ba may be focused to form an intermediate focus at or near to the opening 8.

[0053] The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the radiation beam Ba with a desired cross-sectional shape and a desired angular distribution. The radiation beam Ba 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 Ba’. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 11. The illumination system IL may for example include an array of independently moveable mirrors. The independently moveable mirrors may for example measure less than 1mm across. The independently moveable mirrors may for example be microelectromechanical systems (MEMS) devices.

[0054] Following redirection (e.g. reflection) from the patterning device MA the patterned radiation beam Ba’ enters the projection system PS. The projection system PS comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam Ba’ 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).

[0055] The lithographic apparatus LAa is operable to impart a radiation beam Ba 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 LAa 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 Ba’ 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. The patterned radiation beam Ba’ which is incident upon the substrate W may comprise a band of radiation. The band of radiation may be referred to as an exposure slit. During a scanning exposure, the movement of the substrate table WT and the support structure MT are such that the exposure slit travels over a target portion of substrate W in a scan direction, thereby exposing the target portion of the substrate W to patterned radiation. It will be appreciated that a dose of radiation to which a given location within the target portion of the substrate W is exposed depends on the power of the radiation beam Ba’ and the amount of time for which that location is exposed to radiation as the exposure slit is scanned over the location (the effect of the pattern is neglected in this instance). The term “target location” may be used to denote a location on the substrate which is exposed to radiation (and for which the dose of received radiation may be calculated).

[0056] Referring again to Figure 1, the radiation source SO is configured to generate an EUV radiation beam RB with sufficient power to supply each of the lithographic apparatus LAa-LAn. As noted above, the radiation source SO may comprise a free electron laser.

[0057] 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 27.

[0058] The injector 21 is arranged to produce a bunched electron beam E and may comprise an electron source and an electron beam buncher. The electron source may, for example, comprise a thermionic cathode or a photo-cathode arranged to emit electrons and an accelerating electric field arranged to accelerate said electrons so as to form an electron beam. The electron beam buncher may, for example, comprise a bunch compressor and/or a chopper. Examples of bunch compressors and choppers that may form part of the injector 21 are discussed below.

[0059] 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.

[0060] 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 which may be used comprises a resonant cavity through which the electron beam E propagates. The resonant cavity is excited with a standing wave mode wherein the electric field is parallel to the propagation direction of the bunches of electrons in the electron beam E and which is arranged to spatially (longitudinally) compress the bunches of electrons in the electron beam E. 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. Other types of bunch compressor may be used.

[0061] 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.

[0062] 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.

[0063] 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 stimulated. The resonance condition may be given by:

(1) where Xem is the wavelength of the radiation, Xu 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:

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

[0064] The resonant wavelength Xem 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.

[0065] 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.

[0066] 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 Xu 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 Xu 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.

[0067] A region around the central axis of each undulator module 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, 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.

[0068] Each undulator module may have a range of acceptable initial trajectories. Electrons entering an undulator module 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 to stimulate emission of coherent radiation. In contrast, electrons entering an undulator module with other trajectories may not stimulate significant emission of coherent radiation.

[0069] For example, generally, for helical undulator modules the electron beam E should be substantially aligned with the central axis of the undulator module. A tilt or angle between the electron beam E and the central axis of the undulator module (in radians) should generally not exceed p/10, 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 should be less than 100 prad.

[0070] 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.

[0071] As electrons of the electron beam E move through a drift space between each undulator module, 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. The bunched electron beam E has a finite emittance and will therefore increase in diameter unless refocused. Therefore, the undulator 24 may further comprise a mechanism for refocusing the electron beam E in between one or more pairs of adjacent undulator modules. 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, increasing the stimulation of emission of radiation.

[0072] The undulator 24 may further comprise an electron beam steering unit in between each adjacent pair of undulator modules which is arranged to provide fine adjustment of the electron beam E as it passes through the undulator 24. For example, each beam steering unit may be arranged to ensure that the electron beam remains within the good field region and enters the next undulator module with a trajectory from the range of acceptable initial trajectories for that undulator module.

[0073] Radiation produced within the undulator 24 is output as a radiation beam Bfel (which may, for example, correspond to the radiation beam RB of Figure 1).

[0074] After leaving the undulator 24, the electron beam E is absorbed by a dump 27. The dump 27 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 27 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 may be desirable to reduce the energy of electrons in the electron beam E before they enter the dump 27. This removes, or at least reduces, the need to remove and dispose of radioactive waste from the dump 27. 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.

[0075] The energy of electrons in the electron beam E may be reduced before they enter the dump 27 by directing the electron beam E through a decelerator 26 disposed between the undulator 24 and the beam dump 27.

[0076] 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 180 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 recovery linear accelerator (ERL).

[0077] The radiation beam produced by a free electron laser typically has a relatively small etendue. In particular, the EUV radiation beam BFEl provided 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 BFel produced by the free electron laser FEL may have a divergence less than 500 prad, for example less than 100 prad, and may for example have a diameter of around 100 pm.

[0078] 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 one or more EUV lithographic apparatus. At these powers, since the initial diameter of the radiation beam BFel produced by the free electron laser FEL is so small the power density will be significant. Therefore the beam delivery system BDS may comprise a radiation beam expander (not shown) that is arranged to increase the cross sectional area of the radiation beam BFEl produced by the free electron laser FEL. The radiation beam expander may be located a sufficient distance from the undulator 24 to allow the beam to expand to a size with a more acceptable power density. Since the divergence of the radiation beam BFEl produced by the free electron laser FEL is so small, a distance between the undulator 24 and the radiation beam expander may be of the order of tens, or even hundreds of metres. After such a distance, the radiation beam BFEl may have a diameter of the order of 1 mm.

[0079] Figure 4A is a schematic illustration of an example injector 21 according to an embodiment. The injector 21 is configured to provide a beam of electrons E which propagate substantially along a longitudinal axis 31 of the injector 21. The injector 21 comprises an electron source 30 arranged to provide bunches of electrons to a bunch compressor in the form of a resonant cavity 35. The resonant cavity 35 is configured to introduce an energy variation to each electron bunch emitted by the electron source 30. Generally, the electrons at the front of the bunch are decelerated, while the electrons at the back of the bunch are accelerated. The variation in the energy provided to the electrons in each electron bunch as a function of the position of the electron within in the bunch depends on the time dependence of the longitudinal electric field in the resonant cavity 35. In general, an amount of bunching of an electron bunch caused by the resonant cavity 35 may be varied by varying the strength and phase of the electric field within the resonant cavity, and through selection of the cavity length.

[0080] Figure 4B is a schematic illustration of an embodiment of an injector 21 showing one example arrangement of the electron source 30 and having additional components. In Figure 4B, the electron source 30 comprises a photocathode 40 and an anode 41. The photocathode 40 comprises an electron emitting surface 42 which is configured to emit electrons when illuminated with radiation.

[0081] The photocathode 40 is illuminated with a radiation beam 43, emitted from a radiation source 44. The radiation source 44 may, for example, be a laser which emits a laser beam. In some embodiments, the radiation source 44 may be separate from the injector 21 and may not form part of the injector 21. The radiation beam 43 may be delivered to be incident on the photocathode 40 by a beam delivery system. The beam delivery system may, for example, comprise one or more mirrors and/or other optical components not shown in Figure 4B. The beam delivery system may, for example, comprise optical components configured to condition the radiation beam 43 prior to the radiation beam 43 being incident on the photocathode 40.

[0082] Energy from photons, which make up the radiation beam 43, which are received by a target region 45, may be absorbed by electrons in the photocathode 40. This energy may serve to excite electrons in the photocathode 40 to higher energy states. Some electrons may be excited to a high enough energy state that they are able to escape the photocathode 40 such that electrons are emitted. Radiation stimulated emission of electrons such as this, is commonly referred to as the photoelectric effect.

[0083] The target region 45 of the photocathode may be formed from a material having a relatively high quantum efficiency. The quantum efficiency of the target region 45 is a measure of the number of electrons which are emitted from the target region 45 per photon incident on the target region 45. A target region 45 which comprises a material having high quantum efficiency advantageously allows an electron beam E having a large peak current to be provided by the injector 21.

[0084] The target region 45 may, for example, be formed from a metal. Alternatively the target region 45 may comprise a semiconductor material. For example, the target region 45 may be formed from one or more of gallium arsenide (GaAs), sodium potassium antimonide (NaKSb) and caesium potassium antimonide (CsK2Sb). The target region 45 may, for example, have a quantum efficiency of greater than approximately 1%. In some embodiments the quantum efficiency of the target region 45 may be greater than about 5% and may, for example, be as high as about 50%. The quantum efficiency of the target region 45 may decrease with use. The photocathode 40 may be replaced at a time when the quantum efficiency of the target region 45 falls below a threshold value. For example, the photocathode 40 may be replaced when the quantum efficiency of the photocathode falls below about 1%.

[0085] The term target region is used herein to refer to a region of the photocathode 40, which is illuminated with a radiation beam 43 and from which electrons are emitted. In some embodiments the target region has a different composition to other regions of the photocathode. For example, the target region 45 may comprise a material having relatively high quantum efficiency, as described above, and other regions of the photocathode 40 may comprise other materials which are less conducive to radiation stimulated emission of electrons (and thus have a lower quantum efficiency). This may ensure that the emission of electrons is limited to emission from the target region 45. This may prevent stray electrons emitted from other regions of the photocathode from interfering with the electron beam E.

[0086] In other embodiments, the target region 45 may not be the only region of the photocathode 40 which is configured to emit electrons. For example, an entire electron emitting surface 42 may comprise a material having relatively high quantum efficiency and may, if illuminated with a radiation beam, emit electrons. In embodiments in which the entire electron emitting surface 42 comprises a material having a relatively high quantum efficiency, only the region which is illuminated by the radiation beam 43 and from which electrons are emitted is therefore considered to form the target region 45.

[0087] The radiation beam 43 may be a pulsed radiation beam, thereby causing electrons to be emitted from the photocathode 40 in bunches, which correspond to the pulses of the radiation beam 43. In such embodiments, the electron beam E which is provided by the electron source 30 is a bunched electron beam E. The radiation source 44 may, for example, be a picosecond laser and thus pulses in the radiation beam 43 may have a duration of approximately a few picoseconds.

[0088] A potential difference is held between the photocathode 40 and the anode 41. In particular, the photocathode 40 is held at a lower voltage than the anode 41, such that electrons which are emitted from the photocathode 40 are accelerated away from the photocathode 40 by the potential difference.

[0089] The potential difference between the photocathode 40 and the anode 41 may be maintained by a voltage source (not shown) which may form part of the electron source 30 or may be separate from the electron source 30. The potential difference between the photocathode 40 and the anode 41 may, for example, be approximately several hundred kilovolts.

[0090] The voltage source may be a DC voltage source such that a substantially constant potential difference is maintained between the photocathode 40 and the anode 41. Alternatively the voltage source may be an AC voltage such that the potential difference between photocathode 40 and the anode 41 alternates periodically with time. In embodiments in which the voltage source is an AC voltage source the frequency and phase of the photocathode voltage may be matched with a repetition rate of pulses of the radiation beam 43, such that pulses of the radiation beam 43 which are incident on the photocathode 40 coincide with peaks in the potential difference between the photocathode 40 and the anode 41.

[0091] In the embodiment, which is shown in Figure 4B the anode 41 includes an opening 46. The anode 41 may, for example, have an annular shape which extends around the opening 46. The axis 31 of the injector 21 extends through the opening 46. The photocathode 40 and the anode 41 are configured to generate an electric field which causes electrons emitted from the target region 45 of the photocathode 40 to pass through the opening 46 in the anode 41.

[0092] An electron steering apparatus 48 may be provided to steer the trajectory of the electrons so as to cause the electrons to propagate substantially along the axis 31. The electron steering apparatus 48 may be configured to generate a magnetic field. For example, the electron steering apparatus 48 may comprise one or more electromagnets configured to generate a magnetic field. The magnetic field may exert a force on the electrons which acts to alter the trajectory of the electrons. The trajectory of the electrons is altered by the magnetic field until the electrons are substantially coincident with the desired axis and the electrons propagate substantially along the axis 31.

[0093] In embodiments in which the electron steering apparatus 48 comprises one or more electromagnets, the electromagnets (such as a solenoid) may be arranged to form one or more of a magnetic dipole, a magnetic quadrupole, a magnetic sextupole and/or any other kind of multipole magnetic field arrangement configured to apply a force to the electrons. The electron steering apparatus 48 may additionally or alternatively comprise one or more electrically charged plates, configured to generate an electric field such that a force is applied to the electrons. In general, the electron steering apparatus 48 may comprise any apparatus which is operable to apply a force to the electrons so as to alter their trajectory such that the electrons propagate substantially along the axis 31, and/or to focus/defocus the electron beam.

[0094] The electrons pass through the resonant cavity 35 as described above with reference to Figure 4A. The electrons may additionally pass through an electron booster 49. The electron booster 49 serves to accelerate the electron bunches along the axis 31. The electron booster 49 may, for example, accelerate electron bunches to relativistic energies. In some embodiments the electron booster 49 may accelerate electron bunches to energies in excess of approximately 5 MeV. In some embodiments, the electron booster 49 may accelerate electron bunches to energies which do not exceed approximately 20 MeV. Flowever, in other embodiments the electron booster 49 may accelerate electron bunches to energies in excess of about 20 MeV.

[0095] The electron booster 49 may be similar to the linear accelerator 22 which was described above with reference to Figure 3 and may, for example, comprise a plurality of radio frequency cavities 50 (as depicted in Figure 4b) and one or more radio frequency power sources (not shown). The radio frequency power sources may be operable to control electromagnetic fields along the axis 31. As bunches of electrons pass between the cavities 50, the electromagnetic fields controlled by the radio frequency power sources cause each bunch of electrons to accelerate. The cavities 50 may be superconducting radio frequency cavities. Alternatively, the cavities 50 may be conventionally conducting (i.e. not superconducting), and may be formed from, for example, copper.

[0096] In alternative embodiments other types of accelerator may be used to form the electron booster 49. Other suitable types of accelerators may, for example, include laser wake-field accelerators and/or inverse free electron lasers. In some embodiments, no separate electron booster 49 and linear accelerator 22 are provided and only one accelerator (e.g. a linear accelerator 22) may be used to accelerate the electrons.

[0097] Electrons in an electron bunch emitted from the target region 45 are each repelled away from each other by repulsive electrostatic forces which act between the electrons. This may cause an electron bunch to spread and is sometimes referred to as the space charge effect. The spread of an electron bunch in position and momentum phase space may be characterised by the emittance of the electron beam E. Spreading out of electron bunches due to the space charge effect increases the emittance of the electron beam E. It may be desirable for the electron beam E to have a low emittance in the linear accelerator 22 and the undulator 24 since this may increase the efficiency with which energy from the electrons is converted to radiation in the undulator 24.

[0098] Once the electrons within the electron bunches are accelerated to relativistic energies, the relativistic effect of time dilation means that as observed in a lab frame (in which the electron source 30 and the rest of the free electron laser is situated) the time which is experienced by the electrons is slowed down relative to the lab frame. The space charge effect which forces the electrons apart from each other is therefore slowed down, as observed in the lab frame, when the electrons are accelerated to relativistic energies. Accelerating an electron bunch to relativistic energies therefore has the effect of substantially freezing the emittance of the electron bunch, as observed in the lab frame. It is therefore advantageous to reduce the distance between the electron booster 49 and the target region 45, such that an electron bunch emitted from the target region 45 is accelerated to relativistic energies (and the emittance substantially frozen) before the emittance of the electron bunch can be significantly increased as the electrons propagate from the target region 45 to the electron booster 49.

[0099] The photocathode 40 and the anode 41 are positioned inside an enclosing structure 51 in which vacuum pressure conditions may be maintained. For example, one or more vacuum pumps (not shown) may be used to pump down the environment inside of the enclosing structure 51. The enclosing structure 51 extends into a beam pipe 52 along which the electrons propagate. The beam pipe 52 may extend throughout a free electron laser FEL. For example, the beam pipe 52 may extend through the linear accelerator 22, the bunch compressor 23 and the undulator 24 which are shown in Figure 3. The environment inside the beam pipe 52 may also be maintained at vacuum pressure conditions.

[00100] The injector 21 may include other components which are not shown in Figures 4a, 4b. More generally, it will be understood that the examples of injectors 21 shown in Figures 4a, 4b are merely exemplary and that other configurations of injector 21 may be used. For example, the electron source may be a resonant frequency (RF) cavity, or any other suitable source of electrons.

[00101] The resonant cavity 35 may comprise a hollow body extending along the axis 31 of the resonant cavity 35 (along the z- direction in Figures 4A, 4B). The resonant cavity 35 is provided with a power source (not shown) that is arranged to excite a longitudinal mode within the resonant cavity 35. The power source is a radio frequency (RF) source and may comprise an antenna that is arranged to emit electromagnetic radiation. The antenna may be disposed within the resonant cavity 35 or, alternatively, the antenna may be located outside of the resonant cavity 35 and may be coupled to resonant cavity 35, for example, by a waveguide.

[00102] The body of the resonant cavity 35 is formed from a material that is an electrical conductor. The material from which the body is formed may be superconducting. 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 dissipated through the walls of the body to be decreased. Alternatively, the material from which the body is formed may be conventionally conducting (i.e. not superconducting), and may comprise, for example, copper.

[00103] The resonant modes of resonant cavity 35 are dependent on the geometry (i.e. shape) of the resonant cavity 35 and the material inside the resonant cavity 35. The resonant modes of resonant cavity 35 are also dependent on the size and position of the antenna of the alternating power source although this is often treated as a small disturbance or perturbation. The resonant modes of resonant cavity 35 are also dependent on the conductivity of the material from which the body is formed although this is also usually a small correction.

[00104] As described above, the resonant cavity 35 acts to provides different amounts of energy to the electrons in each electron bunch as a function of the time a particular electron enters and exits the cavity (which is itself dependent on the position of the particular electron within the electron bunch).

[00105] Figure 5A illustrates how the resonant cavity 35 changes the energy of the electrons within an electron bunch when only the fundamental longitudinal mode is excited within resonant cavity 35 to provide an electric field that varies sinusoidally in time with a constant electric field in the direction of propagation of the electrons. The resonant cavity 35 is taken to have a length such that an electron traveling at a given velocity traverses the resonant cavity 35 in half of an oscillation period. The energy gained by an electron within an electron bunch as it passes through the resonant cavity 35 is given by the integral of the electric field experienced by the electron over the time it takes the electron to traverse the resonant cavity 35. A plot 55 shows how the magnitude of the electric field within the resonant cavity 35 changes with time. In the depicted example, an electron that enters resonant cavity 35 at the minimum of the electric field will experience a net energy gain of zero.

[00106] A time at which a first electron of an electron bunch enters the resonant cavity 35 is shown with the label Fenter, the time at which the first electron exits the resonant cavity 35 is depicted with the label FeXit. It can be seen that in the example of Figure 5, the first electron enters the resonant cavity 35 just ahead of the minimum magnitude and exits the resonant cavity a little ahead of the maximum amplitude. The time at which the last electron within an electron bunch enters the resonant cavity 35 is depicted with the label Benter and the time at which the last electron exits the resonant cavity 35 is depicted with the label BeXit. It can be seen that the last electron enters the resonant cavity 35 just after the minimum magnitude and exits the resonant cavity 35 just after the maximum magnitude.

[00107] The amount of energy gained by an electron changes with the time the electron enters the resonant cavity 35 and follows a simple sine. A line 56, illustrated below the plot 55 between Fenter and Fexit, schematically illustrates energy imparted to electrons at the front of a bunch, while a line 57, illustrated above the plot 55 between Benter and Bexit, schematically illustrates energy imparted to electrons at the back of a bunch. It can be seen that electrons at the front of the bunch lose energy and are therefore decelerated (i.e. the integral of the plot 55 is negative) while electrons at the back of the bunch gain energy and are accelerated (i.e., the integral of the plot 55 is positive).

[00108] Generally, where only the fundamental mode is excited in the resonant cavity 35, the resonant cavity 35 provides only a generally linear time-energy gained relationship between the first electron of a bunch to enter the resonant cavity 35 and the last electron of the bunch to enter the resonant cavity 35 where the time between the first electron entering the resonant cavity 35 and the last electron entering the resonant cavity 35 is short with respect to the period of oscillation of the electric field and the overall energy gained by the bunch as a whole is close to zero. This is depicted in Figure 5B, which depicts the energy gained by electrons entering the resonant cavity 35 between Fenter and Benter· From Figure 5B, it can be seen that for electrons entering the resonant cavity 25 between Fenter and Benter, the time-energy gain distribution is general linear.

[00109] There is now described a resonant cavity that can provide an arbitrary time-energy gain relationship, thereby providing additional control over the compression of the electron bunches within the resonant cavity of the injector.

[00110] In an embodiment, the resonant cavity 35 is excited to resonant mode that includes higher harmonics. It is to be understood by that by the term harmonic, it is meant that the frequency is an integer multiple of the fundamental frequency. By adding one or more higher harmonics to the fundamental frequency of the electric field of the resonant cavity 35, an arbitrary time-energy gain relationship may be provided.

[00111] Figure 6 illustrates an example in which the longitudinal electric field 62 within the resonant cavity 35 is made up of a third harmonic 60 with a phase delay of π to, and relative amplitude of 1/3 of, the fundamental frequency 61. The resultant electric field remains periodic at the fundamental frequency but has a time-energy gain relationship that results in a rate of change of the amplitude of the electric field of substantially zero at a time when the amplitude is zero (which, in the depicted example substantially coincides with the centre of the bunch), as depicted at 63.

[00112] By adding one or more additional harmonics to the fundamental frequency within the same cavity 35, a non-linear, arbitrary, time-energy gain relationship may be obtained within the cavity 35 to obtain more effective bunching of the electron bunches. For example, with reference to Figure 4B, where the injector 21 is used with an FEL, influencing the longitudinal phase space between the electron source 30 and the booster 49 will result in a larger fraction of the original electron bunch participating in the FEL (lasing) process. This will in turn provide a higher power or equal power with a reduced electron bunch charge. Where a FEL is used with lithographic apparatuses, with a higher power FEL, more lithography apparatuses may be coupled to the FEL (as shown in Figure 1). Reduced bunch charges, on the other hand, provides better reliability with lower radiation, reduced laser demand and reduced demands on a high-voltage power supply. While Figure 6 depicts an example in which the rate of change of the amplitude of the electric field is made to be substantially zero when the amplitude of the electric field is zero, it will be appreciated that other changes to the electric field may be used.

For example, it may be desirable in some embodiments to provide an electric field that has a constantly changing rate of amplitude change. Figures 6B and 6C depict other examples of electric fields which may be excited within the resonant cavity.

[00113] Generally, prior art resonant cavities for electron sources are cylindrical. Flowever, where a resonant cavity is excited with one or more harmonics in additional to the fundamental, cylindrical cavities may not be preferred as the higher order modes are not integer multiples of the fundamental frequency. As such, rectangular or square cavities may be preferred. Rectangular and square cavities both allow for the generation of electric fields on the axis of propagation that are periodic at the fundamental frequency but also contain higher harmonics. It will be appreciated, however, that other resonant cavity geometries may be used.

[00114] Where a harmonic mode is excited in a perfect rectangular resonant cavity, a second (and higher) harmonic mode at exactly half the wavelength may be automatically excited. For example, where a TMxyz = TM120 mode may have a resonant mode at 2x its frequency: that is a TM2x2yz = TM240. Flowever, the automatically excited mode does not have an on-axis magnetic field and therefore does not influence the electrons passing through the resonant cavity. Additionally, one can avoid automatically exciting the additional resonant mode by appropriately positioning the antenna or waveguide that couples the power into the resonant cavity.

[00115] In another embodiment, electron bunches may be provided through the use of an electron beam chopper. Figure 7 is a schematic illustration of an electron beam chopper 70, which may form part of the injector 21 of the free electron laser of Figure 3. The electron beam chopper 70 is arranged to form a bunched beam of electrons. The electron beam chopper 70 comprises a resonant cavity 65 and a screen 69.

[00116] The resonant cavity 65 is provided with an alternating power source (not shown) that is arranged to excite a transverse magnetic mode within the resonant cavity 65. The resonant mode(s) excited within the resonant cavity 65 may be dependent upon the frequency content (spectrum) of the electromagnetic radiation emitted by the antenna.

[00117] The alternating power source is a radio frequency (RF) source and may comprise an antenna that is arranged to emit electromagnetic radiation. The antenna may be disposed within the resonant cavity 65 or, alternatively, the antenna may be located outside of the resonant cavity 65 and may be coupled to resonant cavity 65, for example, by a waveguide.

[00118] A body 71 of the resonant cavity 65 is formed from a material that is an electrical conductor. The material from which the body 71 is formed may be superconducting. Advantageously, this allows: relatively large electromagnetic fields to be applied at high duty cycles; larger apertures 72, 73, resulting in fewer losses due to wakefields; and for the fraction of radio frequency energy that is dissipated through the walls of the body to be decreased. Alternatively, the material from which the body 71 is formed may be conventionally conducting (i.e. not superconducting), and may comprise, for example, copper.

[00119] The resonant modes of resonant cavity 65 are dependent on the geometry (i.e. shape) of the resonant cavity 65 and the material inside the resonant cavity 65. The resonant modes of resonant cavity 65 are also dependent on the size and position of the antenna of the alternating power source although this is often treated as a small disturbance or perturbation. The resonant modes of resonant cavity 65 are also dependent on the conductivity of the material from which the body 71 is formed although this is also usually a small correction.

[00120] With such an arrangement, an oscillating magnetic field is generated within the resonant cavity. The magnetic field is perpendicular to the plane of Figure 7, i.e. in the x-direction and oscillates with time. As an input beam of electrons propagates into the resonant cavity 65 through aperture 72, it experiences a Lorentz force that is perpendicular to its trajectory and the magnetic field. The direction of the electron beam as it exits the resonant cavity 65 through aperture 73 is dependent on the magnetic field during the time that it was in the resonant cavity 65. The trajectory of the electron beam as it exits through aperture 73 remains in the z-y plane in Figure 7 and oscillates with time through a range of directions between a first end direction 74 and a second end direction 75.

[00121] The resonant cavity 35 may therefore be considered to be a deflector that is operable to receive an input beam of electrons propagating along an axis (the z-direction in Figure 4A) and to alter the direction of the beam of electrons so as to form an output beam of electrons such that the direction of the output beam of electrons varies with time through a range of directions.

[00122] The screen 69 comprises a knife edge 76. As the trajectory of the electron beam exiting through aperture 73 oscillates with time through the range of directions, at times the electron beam is blocked (absorbed) by the screen and at times the electron beam passes over the knife edge 76. Since the oscillation of the trajectory of the electron beam exiting the resonant cavity 65 is periodic, the portions of the electron beam that pass over the knife edge 76 form temporally discrete bunches 77 of electrons. The screen 69 may therefore be considered to be a blocking member which is arranged to block the beam of electrons exiting the resonant cavity 65 when it is in a first sub-range of the range of directions (i.e. when the beam of electrons is incident on the screen 69) and to allow the beam of electrons to pass it when it is in a second sub-range of the range of directions (i.e. when the beam of electrons passes over the knife edge 76) so as to form a bunched electron beam.

[00123] In an alternative arrangement, the screen 69 that forms a blocking member may comprise one or more apertures instead of a knife edge, with the position of the aperture defining the first and second sub-ranges of the range of directions.

[00124] For embodiments wherein the alternating source excites a TM110 mode within a cylindrical resonant cavity, the resonant cavity 65, the magnetic field in the resonant cavity 65 oscillates sinusoidally with time. Therefore, as shown in Figure 8, in the plane of the screen 69, the beam spot 78 of the electron beam exiting aperture 73 oscillates along a linear path 79. This oscillation is sinusoidal. As shown in Figure 8, in the plane of the screen 69, the beam spot 78 of the electron beam exiting aperture 73 undergoes simple harmonic motion.

[00125] The knife edge 76 of the screen 69 is disposed proximate to one end of the linear path 79, which corresponds to the electron beam propagating along the first end direction 74, and where the rate of change of the direction of the electron beam that exits the resonant cavity 65 is at a local minimum. Therefore, the resonant cavity 65 and the screen 69 are arranged such that a rate of change of the direction of the electron beam that exits the resonant cavity 65 is at a local minimum when the electron beam is in the second sub-range and passes above the knife edge 76.

[00126] Note that the rate of change of the direction of the electron beam that exits the resonant cavity 65 is also at a local minimum when the electron beam is propagating along the second end direction 75, which corresponds to the other end of the linear path 79 the electron beam. Therefore, the knife edge 76 (or aperture) may be proximate to either end of the linear path 79 traced by the electron beam spot. Alternatively, a knife edge (or aperture) may be provided at each end of the linear path 79 traced by the electron beam spot. Such an arrangement produces two (out of phase) bunched electron beams, which may, for example, each form part of an injector 21 of a different free electron laser.

[00127] The electron beam chopper 70 is a convenient apparatus for forming a bunched electron beam, using the resonant cavity 65 to move a continuous electron beam relative to the screen 69. Since the magnetic field in the resonant cavity 65 oscillates with time, so too does the force exerted on electrons within the resonant cavity 65. Furthermore, since the bunches 77 of electrons that pass past knife edge 76 have a non-zero temporal length, this means that, in general, different electrons within each bunch 77 experience different forces as they pass through the resonant cavity 65. As a result, the bunched electron beam which passes past knife edge 76 is divergent (i.e. has a greater spread of directions). Furthermore, the normalized emittance of the bunched electron beam that passes past knife edge 76 is greater than the normalized emittance of the electron beam that enters the electron beam chopper 70. That is, the normalized emittance of the bunched electron beam is increased by the electron beam chopper 70.

[00128] The normalized emittance growth ε induced by the electron beam chopper 70 is given by:

(3) where R is the radius of the input electron beam, vz is the initial velocity of the electron beam (in the z direction), c is the speed of light, 5 is the width of the slit (in the y direction), L is the distance between the resonant cavity 65 and the screen 69, and φ is a phase of the electromagnetic wave within resonant cavity 65 when as the centre of the electron bunch 77 passes through the centre of the resonant cavity 65. The phase φ is defined such that: φ=0,π when the centre of the electron bunch 77 passes through the centre of the resonant cavity 65 as the magnetic field is zero; and φ=π/2,3π/2 when the centre of the electron bunch 77 passes through the centre of the resonant cavity 65 as the strength of the magnetic field is maximum. Eq. (3) is correct assuming no space charge forces and starting from zero emittance (i.e. a perfectly collimated input electron beam).

[00129] In electron beam chopper 70 the resonant cavity 65 and the screen 69 are arranged such that a rate of change of the direction of the electron beam is at a minimum when the electron beam passes past knife edge 76 of the screen 69. As a result, effectively the electron beam is being sampled at a point where its direction is slowly moving. With this arrangement, wherein the rate of change of the direction of the electron beam that exits the resonant cavity 65 is at a local minimum when the electron beam is in the second sub-range and passes past knife edge 76, the phase φ-π/2,3π/2. As can be seen from Eq. (3), this ensures that the emittance of the bunched electron beam formed by the beam chopper 70 is minimized. In fact, Eq. (3) is a simplification that assumes that the electron beam which enters the resonant cavity 65 is collimated and that the temporal length of the electron bunches 77 is significantly smaller than the time period of the oscillation of the electromagnetic radiation within the resonant cavity 65. With these assumptions, as can be seen from Eq. (3), no increase in emittance can be expected. In practice, some small increase in emittance can be expected but, by effectively sampling the electron beam at a point where its direction is slowly moving, the emittance of the bunched electron beam formed by the beam chopper 70 is minimized.

[00130] This minimization of the emittance of the bunched electron beam formed by the beam chopper 70 is advantageous for a number of reasons, as now discussed. The electron beam chopper 70 may form part of an injector 21 for a free electron laser FEL. For such embodiments it is desirable to minimize the emittance of the formed bunched electron beam since this may affect the gain and bandwidth of the free electron laser.

[00131] The electron beam chopper 70 may form part of the injector 21 of the free electron laser of Figure 3, along with an electron source arranged to produce a beam of electrons that is directed into the resonant cavity 65. The electron source may, for example, comprise a thermionic cathode or a photo-cathode arranged to emit electrons and an accelerating electric field arranged to accelerate said electrons so as to form an electron beam.

[00132] As now described with reference to Figure 9, due to the non-zero extent of the electron bunches 77 formed, the electron beam chopper 70 described above results in curved electron bunches. The extent of this distortion is dependent on the duty cycle of the electron beam chopper 70, i.e. the ratio of the temporal length of the electron bunches 77 to the time period of the oscillation of the electromagnetic radiation within the resonant cavity 65. The higher the duty cycle of the electron beam chopper 70, the greater the distortion will be.

[00133] As explained above, the trajectory of the electron beam as it exits through aperture 73 remains in the z-y plane in Figure 7 and oscillates with time through a range of directions between a first end direction 74 and a second end direction 75. The screen 69 is arranged to block the beam of electrons exiting the resonant cavity 65 when it is in a first sub-range of the range of directions (i.e. when the beam of electrons is incident on the screen 69) and to allow the beam of electrons to pass it when it is in a second sub-range of the range of directions (e.g. when the beam of electrons passes past the knife edge 76) so as to form a bunched electron beam. The second sub-range of the range of directions is defined between the first end direction 74 (which is dependent on the resonant cavity 65 and the input electron beam) and an intermediate direction 74’ (which is dependent on the screen 69, for example, the size and the position of the knife edge 76).

[00134] A single bunch 77 of electrons formed by the electron beam chopper 70 is shown in Figure 9. Also indicated in Figure 9 are the front 77a, the centre 77b and the rear 77c of the electron bunch 77. Electrons at the front 77a and the rear 77c of the bunch 77 propagate along the intermediate direction 74’ whereas the electrons at the centre of the bunch propagate along the first end direction 74. As a result, the electron bunch 77 is curved or banana-shaped. Furthermore, the bunch 77 is divergent and continues to increase in size as it propagates away from the screen 69.

[00135] In an embodiment, one or more harmonics are added to the fundamental frequency of the magnetic field of the resonant cavity 65. In this way, the duration for which the change in the magnitude of the magnetic field (and therefore the rate of change of the direction of the electron beam) is at a local minimum, or is at or close to zero may be extended. Figure 10 shows an example of how adding a second harmonic component, with an amplitude of 1/4 of the fundamental component and with an appropriate phase difference can extend the duration that the change in the magnitude of the magnetic field is near zero. In Figure 10, a line 100 depicts how the component of the magnetic field which oscillates at the fundamental frequency varies with time, a line 101 depicts how the component of the magnetic field which oscillates at the second harmonic varies over time, and a line 102 depicts how the overall magnetic field within the resonant cavity 65 varies with time. From Figure 10, it can be seen that a time for which the rate of change of the magnitude of magnetic field (and therefore the electron beam) is at a local minimum (marked at A-A) is longer in duration than duration (B-B) when only the fundamental mode is excited within the resonant cavity 65.

[00136] Because the rate of change of the magnitude of the magnetic field is lower for a “useful portion”, the curvature of the electron bunches is reduced. While Figure 10 illustrates the addition of the second harmonic to the magnetic field, additional higher harmonics (e.g., a third harmonic) may be added to the magnetic field within the resonant cavity 65 to increase the useful portion of the magnetic field even further. It will also be appreciated that through the addition of one or more additional harmonics, with appropriate phases and amplitudes with respect to the fundamental, a period of nearly constant deflection can be generated at any desired moment of time. For example, a period of constant deflection can be provided near to the zero-crossing of the magnitude of the magnetic field. For example, a period of constant deflection near to the zero-crossing may be preferred so as to provide no net deflection of the electron beam, thereby allowing the electron beam to continue to propagate in a straight line. Additionally, by providing a period of nearly constant deflection near to the zero-crossing, a bunch repetition rate of double the frequency (there are 2 zero-crossings per period) may be provided.

[00137] For resonant cavities which are to be used with electron beam choppers (such as the resonant cavity 65), it is desired for the longitudinal electric field to have a magnitude of zero on the axis 31 along which the electron beam propagates. As described above with reference to Figure 4, where a resonant cavity is excited with one or more harmonics in additional to the fundamental frequency of the magnetic field, cylindrical cavities may not be preferred. Assuming that the cavity 65 is a rectangular cavity and that the electron beam propagates along the z-axis, examples of the lowest modes that could be used within the cavity 65 to provide a zero magnitude electric field on the central axis are the TM210 mode (using the notation TMxyz) and the TM120 modes.

[00138] Where excited to the second harmonic transverse magnetic mode, it may also be desirable for the second harmonic to have an electric field on the axis 31 of zero magnitude. An example of a mode satisfying these requirements is the TM320 mode, which, when the ratio of the dimensions of the cavity satisfy

where a is the width of the cavity in the x- direction, bthe width of the cavity in the y-direction, is the second harmonic of the TM120 mode.

[00139] For a fundamental frequency of, for example, 650 MHz (and a second harmonic of 1.3 GHz), with vacuum inside the resonant cavity, the resonant cavity may have dimensions of: a = 377 mm, b = 584 mm. The length of the resonant cavity (z-direction) may be chosen freely, avoiding lengths which would result in other modes being excited at or close to the same frequencies (650 MHz or 1.3 GHz in this example). It will be appreciated from the above that the dimensions of the resonant cavity 65 may be adjusted to account for the effect of the physical presence of an antenna. The resonant cavity may also be filed, to a large extent with a dielectric material in order to reduce the size of the cavity and the power consumption.

[00140] Figure 11 is a schematic illustration of an electron beam chopper 110 which may form part of the injector 21 of the free electron laser of Figure 3 in an embodiment of the invention. The electron beam chopper 110 takes the general form of the electron beam chopper 70, but comprises a resonant cavity 111. The resonant cavity 111 is provided with an alternating power source (not shown) that is arranged to excite both the TM120 and TM320 magnetic modes simultaneously within the resonant cavity 111, with respective amplitudes and at a desired phase difference, to provide a magnetic field within the resonant cavity 111 as depicted in Figure 10. The power source may be, for example, a single power source providing both frequencies to the resonant cavity, or a plurality of power sources, to provide respective frequencies to the resonant cavity. The one or more power sources may feed power into the resonant cavity 35 through either a single or a plurality of antennae.

[00141] As an input beam of electrons 112 propagates into the resonant cavity 111 through an aperture 113, it experiences a Lorentz force that is perpendicular to its trajectory and the magnetic field. The direction of the electron beam as it exits the resonant cavity 65 through aperture 114 is dependent on the magnetic field during the time that it was in the resonant cavity 111. The trajectory of the electron beam as it exits through aperture 73 is depicted in Figure 11. As in Figure 7, the electron beam oscillates with time through a range of directions between a first end position 115 and a second end position 116. In comparison to Figure 7, however, the addition of the second harmonic to the magnetic field within the resonant cavity 111 is such that the electron beam remains at or close to the first end position for a longer duration.

[00142] As indicated above, where multiple magnetic modes are stimulated simultaneously within a single resonant cavity, non-cylindrical cavities may be preferred, such as, for example, rectangular or square cavities.

[00143] In an embodiment, a constant magnetic field is added to the resonant cavity of an electron beam chopper (e.g., the resonant cavities 65, 111) so as to cancel the time-dependent magnetic field of the resonant cavity 65, 111 at a desired point. For example, as shown in Figure 12, the time-dependent magnetic field of a resonant cavity 120 may be cancelled at a point at which the rate of change of the magnitude of the magnetic field is at a local minimum. Figure 12 schematically illustrates the effect of providing such a constant magnetic field to the arrangement of Figure 11 (although it is to be understood that a constant magnetic field may equally be applied in the arrangement of Figure 7). In the arrangement of Figure 12, the resonant cavity 111 is replaced with a resonant cavity 120. The resonant cavity 120 is provided with a power source (not shown) arranged to excite a plurality of time-dependent magnetic modes (e.g. the modes TM120, TM320) simultaneously within the resonant cavity 120 (as described above with reference to Figure 11) and to provide a constant magnetic field which counteracts (cancels) the resulting magnetic field within the resonant cavity 120 at the time of maximum amplitude (at which point the rate of change of the magnitude of the magnetic field is at a local minimum).

[00144] To provide a constant magnetic field, the resonant cavity 120 may be placed inside a dipole magnetic field. For example, referring to Figure 13, a dipole magnet 130 arranged to generate a dipole field 131, inside which the resonant cavity 120 may be placed.

[00145] From Figure 12 it can be seen that by providing a constant magnetic field selected so as to cancel the time-dependent magnetic field at the maximum amplitude, the electron beam can be made to exit resonant cavity 120 along substantially the same path 122 as the incoming electron beam 112. In particular, the electron beam exiting the resonant cavity 120 oscillates between the direction 122 and a direction 121. Causing the electron beam to exit the resonant cavity 120 in a straight line reduces the need for downstream components to align and condition the electron beam. Furthermore, the core of each electron bunch exiting electron beam chopper 110 is centred around the axis of propagation both before and exiting the resonant cavity 120.

[00146] Generally, the control over the electron bunch properties, as provided by the embodiments described herein allows for a higher fraction of the charge of each electron bunch to contribute to the lasing process in the FEL.

[00147] While square and rectangular resonant cavities have generally been described above, it will be appreciated that other resonant cavity geometries may be used.

[00148] 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.

[00149] 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.

[00150] 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.

[00151] 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 (511 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.

[00152] 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. Furthermore, while injectors and resonant cavities of the type described herein may be used with FELs, it will be appreciated that this is simply one example usage and that the concepts described herein are more generally applicable.

[00153] The term “EUV radiation” may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.

[00154] The lithographic apparatuses LAa to LAn may be used in the manufacture of ICs. Alternatively, the lithographic apparatuses LAa to LAn 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.

[00155] Different embodiments may be combined with each other. Features of embodiments may be combined with features of other embodiments.

[00156] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below. Other aspects of the invention are set-out as in the following numbered clauses. 1. A resonant cavity operable to receive an input beam of electrons along a longitudinal axis of the resonant cavity, comprising: a power source; a controller configured to cause the power source to excite an electromagnetic wave in the resonant cavity, the electromagnetic wave having a first resonant mode and a second resonant mode each having a magnetic field at the longitudinal axis of the resonant cavity which acts upon the input beam of electrons, wherein the first and second resonant modes have frequencies that are integer multiples of each other. 2. The resonant cavity of clause 1, wherein the controller is configured to cause the power source to excite the resonant cavity to provide a longitudinal electric field along the longitudinal axis of the resonant cavity. 3. The resonant cavity of clause 2, wherein at a magnitude of zero, a rate of change of the magnitude of the electric field is different to a rate of change of the magnitude of an electric field that would be provided by an electromagnetic wave having only the first resonant mode. 4. The resonant cavity of any one of clauses 2 or 3, wherein the second resonant mode is out of phase with the first resonant mode. 5. The resonant cavity of any one of clauses 2 to 4, wherein the second resonant mode has a different amplitude to the first resonant mode. 6. The resonant cavity of any one of clauses 2 to 5, wherein the second frequency is a third or higher harmonic of the resonant cavity. 7. The resonant cavity of any preceding clause, wherein the first and second resonant modes are transverse magnetic resonant modes. 8. The resonant cavity of clause 7, wherein the controller is configured to cause the power source to excite the resonant cavity with the electromagnetic wave such that an electric field along the longitudinal axis of the resonant cavity has a magnitude of zero. 9. The resonant cavity of clause 7 or 8, wherein the controller is configured to cause the power source to excite the resonant cavity with the electromagnetic wave such that a duration for which the rate of change of the amplitude of a time dependent magnetic field within the resonant cavity is at a local minimum is greater than a duration for which the rate of change of the amplitude of the time dependent magnetic field within the resonant cavity would be at a local minimum if the resonant cavity were excited with an electromagnetic wave having only the first resonant mode. 10. The resonant cavity of any preceding clause, wherein the resonant cavity is non-cylindrical. 11. The resonant cavity of clause 10, wherein the resonant cavity is a rectangular resonant cavity. 12. The resonant cavity of any one of clauses 7 to 11, further comprising one or more magnets arranged to generate a constant magnetic field to cancel a time-dependent magnetic field excited within the resonant cavity at a predetermined magnitude. 13. The resonant cavity of clause 12, wherein the one or more magnets are arranged such that cancelling of the time-dependent magnetic field occurs at a point of local maximum magnitude of the time-dependent magnetic field. 14. A resonant cavity operable to receive an input beam of electrons along a longitudinal axis of the resonant cavity, comprising: a power source; and a controller configured to cause the power source to excite an electromagnetic wave within the resonant cavity, the electromagnetic wave having at least a fundamental transverse magnetic mode; and one or more magnets arranged to generate a constant magnetic field to cancel a time-dependent magnetic field excited within the resonant cavity at a predetermined magnitude. 15. The resonant cavity of clause 14, wherein the one or more magnets are arranged such that cancelling of the time-dependent magnetic field occurs at a point of local maximum amplitude of the time-dependent magnetic field. 16. A method for processing an electron beam, comprising: exciting an electromagnetic wave in the resonant cavity, the electromagnetic wave having first resonant mode and a second resonant mode, each of the first and second resonant modes having a magnetic field at a longitudinal axis of the resonant cavity; and directing the electron beam through the excited resonant cavity along the longitudinal axis; wherein the first and second resonant modes have frequencies that are integer multiples of each other. 17. The method of clause 16, wherein exciting the resonant modes results in a longitudinal electric field along the longitudinal axis of the resonant cavity. 18. The method of clause 17, wherein at a magnitude of zero, a rate of change of the magnitude of the electric field is different to a rate of change of the magnitude of an electric field that would be provided by an electromagnetic wave having only the first resonant mode. 19. The method of any one of clauses 17 or 18, wherein the second resonant mode is out of phase with the first resonant mode. 20. The method of any of clauses 17 to 19, wherein the second resonant mode has a different amplitude to the first resonant mode. 21. The method of any one of clauses 17 to 20, wherein the second frequency is a third or higher harmonic of the resonant cavity. 22. The method of any one of clauses 16 to 21, wherein the first and second resonant modes are transverse magnetic resonant modes. 23. The method of clause 22, wherein exciting the resonant cavity comprises exciting the resonant cavity such that an electric field along the longitudinal axis of the resonant cavity has a magnitude of zero. 24. The method of clause 22 or 23, wherein exciting an electromagnetic wave within the resonant cavity comprises exciting the resonant such that a duration for which the rate of change of the amplitude of a time dependent magnetic field within the resonant cavity is at a local minimum is greater than a duration for which the rate of change of the amplitude of the time dependent magnetic field within the resonant cavity would be at a local minimum if the resonant cavity were excited with an electromagnetic wave having only the first resonant mode. 25. The method of any of clauses 22 to 24, wherein the electron beam is a continuous electron beam. 26. The method of any of clauses 16 to 25, wherein the resonant cavity is non-cylindrical. 27. The method of clause 26, wherein the resonant cavity is a rectangular resonant cavity. 28. The method of any of clauses 22 to 27, further comprising generating a constant magnetic field within the resonant cavity arranged to cancel a time-dependent magnetic field excited within the resonant cavity at a predetermined magnitude. 29. The method of clause 28, wherein the cancelling of the time-dependent magnetic field occurs at a point of local maximum amplitude of the time-dependent magnetic field. 30. A method of operating a resonant cavity operable to receive an input beam of electrons, comprising: exciting an electromagnetic wave within a resonant cavity, the electromagnetic wave having at least a fundamental transverse magnetic mode; and generating a constant magnetic field within the resonant cavity configured to cancel a time-dependent magnetic field excited within the resonant cavity at a predetermined magnitude. 31. The method of clause 30, wherein the cancelling of the time-dependent magnetic field occurs at a point of local maximum amplitude of the time-dependent magnetic field. 32. A source for producing a beam of electron bunches, the source comprising: an electron source arranged to provide an electron beam; and a resonant cavity according to any one of clauses 1 to 15. 33. The source of clause 32, wherein the electron source is arranged to provide an electron beam comprising a plurality of electron bunches; and wherein the resonant cavity is arranged to modify a phase space density between electron bunches in the electron beam. 34. The source of clause 33, wherein the resonant cavity is arranged to compress the electron bunches in the electron beam. 35. The source of clause 32, wherein the electron source is arranged to provide a continuous electron beam for propagation along a first axis; wherein the resonant cavity forms a deflector operable to receive the electron beam along the first axis and to alter the direction of the electron beam so as to form an output electron beam such that the direction of the output electron beam varies with time through a range of directions; and wherein the deflector further comprises a blocking member which is arranged to block the output electron beam when it is in a first sub-range of the range of directions and to allow the output electron beam to pass it when it is in a second sub-range of the range of directions so as to form a bunched electron beam, wherein the deflector and the blocking member are arranged such that a rate of change of the direction of the electron beam is at a local minimum when the electron beam is in the second sub-range. 36. The source of clause 35, wherein the resonant cavity is operable to cause the direction of the output electron beam to oscillate between a first end direction and a second end direction. 37. The source of clause 36, wherein the blocking member is arranged such that the second sub-range of the range of directions comprises at least one of the first end direction or the second end direction. 38. The source of any of clauses 35 to 37, wherein the blocking member comprises a wall or a screen that is provided with a knife-edge or one or more apertures. 39. A free electron laser comprising: the source of any one of clauses 32 to 38; and an undulator arranged to receive the bunched beam of electrons and operable to cause the bunched beam of electrons to follow an oscillating path about a central axis so that a radiation beam is emitted generally along the central axis. 40. A lithographic system comprising: the free electron laser of clause 39; one or more lithographic apparatuses; and a beam delivery system arranged to receive the radiation beam produced by the free electron laser and to direct at least a portion of the radiation beam to at least one of the one or more lithographic apparatuses.

Claims (1)

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