WO2017071878A1 - Electron source, with photocathode illuminated off-axis - Google Patents

Abstract

An electron source configured to provide a beam of electrons which propagate substantially along an axis (100). The electron source comprising a photocathode (101) including a target region (105) configured to emit electrons when illuminated with radiation, wherein the target region is separated from the axis and an anode (103), wherein the photocathode and the anode are configured to generate an electric field (127) which causes electrons (106) emitted from the target region of the photocathode to be accelerated away from the photocathode and wherein the photocathode and the anode are configured such that the electric field direction is substantially the same throughout the target region, and is inclined towards the axis.

Description

ELECTRON SOURCE, WITH PHOTOCATHODE ILLUMINATED OFF-AXIS CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application no. 15192289.5, which was filed on October 30, 2015 and which is incorporated herein in its entirety be reference.

FIELD

[0002] The present invention relates to an electron source. The electron source may form part of a radiation source, such as an EUV radiation source. The radiation source may comprise a free electron laser. The radiation source may be suitable for providing radiation to a lithographic system comprising one or more lithographic apparatus.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 5-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).

[0005] EUV radiation for use in one or more lithographic apparatus may be produced by one or more free electron lasers. A free electron laser may comprise at least one electron source. It is an object of the invention to obviate or mitigate one or more of the problems associated with known electron sources.

SUMMARY

[0006] According to a first aspect of the invention there is provided an electron source configured to provide a beam of electrons which propagate substantially along an axis, the electron source comprising a photocathode including a target region configured to emit electrons when illuminated with radiation, wherein the target region is separated from the axis and an anode, wherein the photocathode and the anode are configured to generate an electric field which causes electrons emitted from the target region of the photocathode to be accelerated away from the photocathode and wherein the photocathode and the anode are configured such that the electric field direction is substantially the same throughout the target region, and is inclined towards the axis. [0007] The electric field direction being substantially the same throughout the target region ensures that electrons emitted from different parts of the target region are accelerated away from the target region in substantially the same direction. This advantageously limits the spatial extent of the electrons and produces an electron bunch having a relatively low emittance. An electron bunch having a low emittance is particularly advantageous when the electron bunch is used in a free electron laser.

[0008] The electrons which propagate along the axis may collide with gas molecules and may ionize the gas molecules leading to the formation of positively charged ions. Positively charged ions are attracted to the path of the electrons whose negative charge acts as a potential well to the positively charged ions. Ions may diffuse along the axis and may, for example, travel towards the photocathode. Positively charged ions are attracted to the photocathode and may be caused to collide with the photocathode. Ion collisions with the photocathode may damage the photocathode and may in particular reduce the quantum efficiency of the region of the photocathode with which the ions collide.

[0009] Separating the target region of the photocathode from the axis ensures that the region of the photocathode from which electrons are emitted is separated from a region of the photocathode on which ions are incident. Separating the target region of the photocathode from the axis therefore advantageously reduces any degradation of the quantum efficiency of the target region which is caused by ion collisions.

[0010] The electric field direction being inclined towards the axis causes the electrons to be accelerated towards the axis. The electrons may meet the axis, and the trajectory of the electrons may be altered such that they propagate substantially along the axis. The electron source therefore provides a beam of electrons which propagate substantially along the axis. The electrons may subsequently be provided to a linear accelerator which may further accelerate the electrons.

[0011] The term target region is intended to refer to a region of the photocathode which is illuminated with radiation and from which electrons are emitted. The axis of the electron source, is the axis along which electrons propagate as they are output from the electron source.

[0012] The target region may comprise a substantially flat surface.

[0013] A normal to the substantially flat surface of the target region may be inclined towards the axis.

[0014] The photocathode and the anode may be configured such that the electric field direction is substantially the same within a volume which lies within about 1 mm of the target region.

[0015] The photocathode and the anode may be configured such that the electric field direction is substantially the same within a volume which lies within about 1 mm of the target region.

[0016] The photocathode and the anode may be configured such that the electric field direction is substantially the same within a volume which lies within about 2 mm, about 5 mm or about 10 mm or more of the target region. [0017] The photocathode and the anode may be configured such that the electric field strength is substantially the same throughout the target region.

[0018] The photocathode may not be cylindrically symmetric about the axis.

[0019] The anode may include an opening.

[0020] The photocathode and the anode may be configured to generate an electric field which causes electrons emitted from the target region of the photocathode to pass through the opening in the anode.

[0021] The anode may have an annular shape.

[0022] The axis may extend through the opening.

[0023] The electron source may further comprise an electron steering apparatus configured to alter the trajectory of the emitted electrons so as to cause the electrons to propagate substantially along the axis.

[0024] The photocathode and the anode may be configured to generate an electric field which causes electrons emitted from the target region to be directed towards the axis and meet the axis at an axis crossing region.

[0025] The electron steering apparatus may be positioned proximate to the axis crossing region and may be configured to alter the trajectory of the electrons, near to the axis crossing region.

[0026] The beam steering apparatus may be configured to generate a magnetic field so as to alter the trajectory of the emitted electrons.

[0027] The electron source may further comprise a beam delivery system configured to direct a beam of radiation to be incident on the target region.

[0028] The electron source may further comprisie a radiation source configured to emit a radiation beam which is directed to be incident on the target region.

[0029] The radiation source may comprise a laser configured to emit a laser beam which is directed to be incident on the target region.

[0030] The radiation source may be configured to emit a pulsed radiation beam which is directed to be incident on the target region, thereby causing bunches of electrons to be emitted from the target region.

[0031] The electron source may further comprise a voltage source configured to maintain a potential difference between the photocathode and the anode.

[0032] The voltage source may be an AC voltage source.

[0033] A repetition rate of the pulsed radiation beam may be substantially matched to a frequency and phase of the AC voltage source, such that pulses of the radiation beam incident on the target region substantially coincide with peaks in the potential difference between the photocathode and the anode.

[0034] The electron source may further comprise an electron booster configured to accelerate the electrons along the axis. [0035] The electron source may further comprise a bunch compressor, configured to compress bunches of electrons towards the axis.

[0036] According to a second aspect of the invention there is provided a free electron laser comprising an electron source according to the first aspect, a particle accelerator configured to accelerate the electron beam output from the electron source and an undulator operable to guide the accelerated electron beam along a periodic path so as to stimulate emission of radiation, thereby forming a radiation beam.

[0037] The undulator may be configured to stimulate emission of an EUV radiation beam.

[0038] According to a third aspect of the invention there is provided a lithographic system comprising a radiation source comprising a free electron laser according to the second aspect and at least one lithographic apparatus arranged to receive at least a portion of radiation provided by the radiation source.

[0039] According to a fourth aspect of the invention there is provided a method of providing a beam of electrons which propagate substantially along an axis, the method comprising illuminating a target region of a photocathode with radiation so as to cause electrons to be emitted from the target region, wherein the target region is separated from the axis and generating an electric field between the photocathode and an anode, wherein the electric field causes electrons emitted from the target region of the photocathode to be accelerated away from the photocathode and wherein the electric field direction is substantially the same throughout the target region, and is inclined towards the axis.

[0040] One or more aspects of the invention may include one or more features of any of the other aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 is a schematic illustration of a lithographic system;

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

Figure 3 is a schematic illustration of a free electron laser according to an embodiment of the invention;

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

Figure 5 is a schematic illustration of a portion of the electron source of Figure 4; and

Figure 6 is a schematic illustration of a portion of an alternative embodiment of an electron source.

DETAILED DESCRIPTION [0042] Figure 1 shows a lithographic system LS according to one embodiment of the invention. The lithographic system LS comprises a radiation source SO, a beam delivery system BDS and a plurality of lithographic apparatuses LA_a-LA_n (e.g. ten lithographic apparatuses). The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B (which may be referred to as a main beam). The radiation source SO and the beam delivery system BDS may together be considered to form a radiation system, the radiation system being configured to provide radiation to one or more lithographic apparatuses LA_a-LA_n.

[0043] The beam delivery system BDS comprises beam splitting optics and may optionally also comprise beam expanding optics and/or beam shaping optics. The main radiation beam B is split into a plurality of radiation beams B_a-B_n (which may be referred to as branch beams), each of which is directed to a different one of the lithographic apparatuses LA_a-LA_n, by the beam delivery system BDS.

[0044] In an embodiment, the branch radiation beams B_a-B_n are each directed through a respective attenuator (not shown in Figure 1). Each attenuator may be arranged to adjust the intensity of a respective branch radiation beam B_a-B_n before the branch radiation beam B_a-B_n passes into its corresponding lithographic apparatus LA_a-LA_n.

[0045] The radiation source SO, beam delivery system BDS and lithographic apparatus LA_a-LA_n may all be constructed and arranged such that they can be isolated from the external environment. A vacuum may be provided in at least part of the radiation source SO, beam delivery system BDS and lithographic apparatuses LA_a-LA_n so as to reduce 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).

[0046] Referring to Figure 2, a lithographic apparatus LA_a comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the branch radiation beam B_a that is received by that lithographic apparatus LA_a before it is incident upon the patterning device MA. The projection system PS is configured to project the radiation beam B_a' (now patterned by the patterning device MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B_a' with a pattern previously formed on the substrate W.

[0047] The branch radiation beam B_a that is received by the lithographic apparatus LA_a passes into the illumination system IL from the beam delivery system BDS though an opening 8 in an enclosing structure of the illumination system IL. Optionally, the branch radiation beam B_a may be focused to form an intermediate focus at or near to the opening 8.

[0048] The illumination system IL may include a field facet mirror 10 and a pupil facet mirror 11. The field facet mirror 10 and pupil facet mirror 11 together provide the radiation beam B_a with a desired cross-sectional shape and a desired angular distribution. The radiation beam B_a passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam to form a patterned beam B_a' . The illumination system IL may include other mirrors or devices in addition to or instead of the field facet mirror 10 and pupil facet mirror 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 1 mm across. The independently moveable mirrors may, for example, be microelectromechanical systems (MEMS) devices.

[0049] Following redirection (e.g. reflection) from the patterning device MA the patterned radiation beam B_a' enters the projection system PS. The projection system PS comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B_a' onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may, for example, be applied. Although the projection system PS has two mirrors in Figure 2, the projection system may include any number of mirrors (e.g. six mirrors).

[0050] The lithographic apparatus LA_a is operable to impart a radiation beam B_a with a pattern in its cross-section and project the patterned radiation beam onto a target portion of a substrate thereby exposing a target portion of the substrate to the patterned radiation. The lithographic apparatus LA_a may, for example, be used in a scan mode, wherein the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B_a' is projected onto a substrate W (i.e. a dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the demagnification and image reversal characteristics of the projection system PS.

[0051] Referring again to Figure 1, the radiation source SO is configured to generate an EUV radiation beam B with sufficient power to supply each of the lithographic apparatuses LA_a-LA_n. As noted above, the radiation source SO may comprise a free electron laser.

[0052] Figure 3 is a schematic depiction of a free electron laser FEL comprising an electron source 21, a linear accelerator 22, a bunch compressor 23, an undulator 24, an electron decelerator 26 and a beam dump 100. The electron source 21 may be referred to as an injector 21.

[0053] The injector 21 is arranged to produce a bunched electron beam E and comprises a photo- cathode and an accelerating electric field. Electrons in the electron beam E are further accelerated by the linear accelerator 22. In an example, the linear accelerator 22 may comprise a plurality of radio frequency cavities, which are axially spaced along a common axis, and one or more radio frequency power sources, which are operable to control 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.

[0054] Optionally, the electron beam E passes through a bunch compressor 23, disposed between the linear accelerator 22 and the undulator 24. The bunch compressor 23 is configured to spatially compress existing bunches of electrons in the electron beam E. One type of bunch compressor 23 comprises a radiation field directed transverse to the electron beam E. An electron in the electron beam E interacts with the radiation and bunches with other electrons nearby. Another type of bunch compressor 23 comprises a magnetic chicane, wherein the length of a path followed by an electron as it passes through the chicane is dependent upon its energy. This type of bunch compressor may be used to compress bunches of electrons which have been accelerated in a linear accelerator 22 by a plurality of resonant cavities.

[0055] The electron beam E then passes through the undulator 24. Generally, the undulator 24 comprises a plurality of modules (not shown). 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.

[0056] 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.

[0057] 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:

K 2 \

1 +

(1) where X_em is the wavelength of the radiation, X_u is the undulator period for the undulator module that the electrons are propagating through, y 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=l, 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:

ITWXC (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.

[0058] The resonant wavelength l_em is equal to the first harmonic wavelength spontaneously radiated by electrons moving through each undulator module. The free electron laser FEL may operate in self-amplified spontaneous emission (SASE) mode. Operation in SASE mode may require a low energy spread of the electron bunches in the electron beam E before it enters each undulator module. Alternatively, the free electron laser FEL may comprise a seed radiation source, which may be amplified by stimulated emission within the undulator 24. The free electron laser FEL may operate as a recirculating amplifier free electron laser (RAFEL), wherein a portion of the radiation generated by the free electron laser FEL is used to seed further generation of radiation.

[0059] 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.

[0060] An electron which meets the resonance condition as it enters the undulator 24 will lose (or gain) energy as it emits (or absorbs) radiation, so that the resonance condition is no longer satisfied. Therefore, in some embodiments the undulator 24 may be tapered. That is, the amplitude of the periodic magnetic field and/or the undulator period l_u may vary along the length of the undulator 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 l_u within each undulator module and/or from module to module. Additionally or alternatively tapering may be achieved by varying the helicity of the undulator 24 (by varying the parameter A) within each undulator module and/or from module to module.

[0061] Radiation produced within the undulator 24 is output as a radiation beam B_FEL.

[0062] After leaving the undulator 24, the electron beam E is absorbed by a dump 100. The dump 100 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 100 with an energy below the threshold energy may produce only gamma ray showers but will not induce any significant level of radioactivity. The material may have a high threshold energy for induction of radioactivity by electron impact. For example, the beam dump may comprise aluminium (Al), which has a threshold energy of around 17 MeV. It may be desirable to reduce the energy of electrons in the electron beam E before they enter the dump 100. This removes, or at least reduces, the need to remove and dispose of radioactive waste from the dump 100. This is advantageous since the removal of radioactive waste requires the free electron laser FEL to be shut down periodically and the disposal of radioactive waste can be costly and can have serious environmental implications.

[0063] The energy of electrons in the electron beam E may be reduced before they enter the dump 100 by directing the electron beam E through a decelerator 26 disposed between the undulator 24 and the beam dump 100.

[0064] 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).

[0065] In some embodiments of a lithographic system LS the radiation source SO may comprise a single free electron laser FEL. In such embodiments the main beam B which is emitted from the radiation source SO may be a laser beam B_FEL which is emitted from the free electron laser FEL. In other embodiments, the radiation source SO may comprise a plurality of free electron lasers. A plurality of laser beams B_FEL emitted from the free electron lasers may be combined to form a single main beam B comprising radiation emitted from the plurality of free electron lasers FEL.

[0066] Figure 4 is a schematic illustration of an embodiment of the injector 21. The injector 21 is configured to provide a beam of electrons E which propagate substantially along an axis 100 of the injector 21. The axis 100, is the axis along which electrons which are provided by the injector 21, propagate along when they are output from the injector 21. As will be appreciated from the following description, electrons do not necessarily propagate along the axis 100 at all points in the injector 21. However, the electron beam which is provided by the injector 21, propagates substantially along the axis 100 as it is output from the injector 21. The injector 21 comprises a photocathode 101 and an anode 103. The photocathode 101 comprises an electron emitting surface 102 which includes a target region 105. The target region 105 is configured to emit electrons when illuminated with radiation.

[0067] The target region 105 is illuminated with a radiation beam 107, emitted from a radiation source 109. The radiation source may 109, for example, be a laser which emits a laser beam. In the embodiment which is shown in Figure 4, the radiation beam 107 is directed to be incident on the target region 105 of the photocathode 101 by a mirror 111. In some embodiments, the radiation source 109 may be separate from the injector 21 and may not form part of the injector 21. The radiation beam 107 may be delivered to be incident on the target region 105 by a beam delivery system. The beam delivery system may, for example, comprise the mirror 111 and/or other optical components not shown in Figure 4. The beam delivery system may, for example, comprise optical components configured to condition the radiation beam 107 prior to the radiation beam 107 being incident on the target region 105.

[0068] Energy from photons, which make up the radiation beam 107, which are received by the target region 105, may be absorbed by electrons in the photocathode 101. This energy may serve to excite electrons in the photocathode 101 to higher energy states. Some electrons may be excited to a high enough energy state that they are able to escape the photocathode 101 such that electrons 106 are emitted from the target region 105. Radiation stimulated emission of electrons such as this, is commonly referred to as the photoelectric effect.

[0069] The target region 105 of the photocathode may be formed from a material having a relatively high quantum efficiency. The quantum efficiency of the target region 105 is a measure of the number of electrons 106 which are emitted from the target region 105 per photon incident on the target region 105. A target region 105 which comprises a material having a high quantum efficiency advantageously allows an electron beam E having a large peak current to be provided by the injector 21.

[0070] The target region 105 may, for example, be formed from a metal. Alternatively the target region 105 may comprise a semiconductor material. For example, the target region 105 may be formed from one or more of gallium arsenide (GaAs), sodium potassium antimonide (NaKSb) and caesium potassium antimonide (CsK₂Sb). The target region 105 may, for example, have a quantum efficiency of greater than approximately 1%. In some embodiments the quantum efficiency of the target region 105 may be greater than about 5% and may, for example, be as high as about 50%. The quantum efficiency of the target region 104 may decrease with use. The photocathode 101 may be replaced at a time when the quantum efficiency of the target region 105 falls below a threshold value. For example, the photocathode 1010 may be replaced when the quantum efficiency of the photocathode falls below about 1%.

[0071] The target region 105 is used herein to refer to a region of the electron emitting surface 102, which is illuminated with a radiation beam 107 and from which electrons are emitted. In some embodiments the target region has a different composition to other regions of the electron emitting surface 102. For example, the target region 105 may comprise a material having a relatively high quantum efficiency, as described above, and other regions of the electron emitting surface 102 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 from the electron emitting surface 102 is limited to emission from the target region 105. This may prevent stray electrons emitted from other regions of the electron emitting surface 102 from interfering with the electron beam E.

[0072] In other embodiments, the target region 105 may not be the only region of the electron emitting surface 102 which is configured to emit electrons. For example, the entire electron emitting surface 102 may comprise a material having a relatively high quantum efficiency and may, if illuminated with a radiation beam, emit electrons. The term target region 105 is used herein to refer to a region of the electron emitting surface 102 which is illuminated by a radiation beam 107 and from which electrons 106 are emitted. In embodiments in which the entire electron emitting surface 102 comprises a material having a relatively high quantum efficiency, only the region which is illuminated by the radiation beam 107 and from which electrons 106 are emitted is therefore considered to form the target region 105.

[0073] The radiation beam 107 may be a pulsed radiation beam, thereby causing electrons 106 to be emitted from the photocathode 101 in bunches, which correspond to the pulses of the radiation beam 107. In such embodiments, the electron beam E which is provided by the electron source 21 is a bunched electron beam E. The radiation source 109 may, for example, be a picosecond laser and thus pulses in the radiation beam 107 may have a duration of approximately a few picoseconds.

[0074] A potential difference is held between the photocathode 101 and the anode 103. In particular, the photocathode 101 is held at a lower voltage than the anode 103, such that electrons 106 which are emitted from the photocathode 101 are accelerated away from the photocathode 101 by the potential difference.

[0075] The potential difference between the photocathode 101 and the anode 103 may be maintained by a voltage source (not shown) which may form part of the electron source 21 or may be separate from the electron source 21. The potential difference between the photocathode 101 and the anode 103 may, for example, be approximately several hundred kilo volts.

[0076] The voltage source may be a DC voltage source such that a substantially constant potential difference is maintained between the photocathode 101 and the anode 103. Alternatively the voltage source may be an AC voltage such that the potential difference between photocathode 101 and the anode 103 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 107, such that pulses of the radiation beam 107 which are incident on the photocathode 101 coincide with peaks in the potential difference between the photocathode 101 and the anode 103.

[0077] The potential difference between the photocathode 101 and the anode 103 establishes an electric field between the photocathode 101 and the anode 103. As will be described in more detail below, the electric field influences the trajectory of electrons 106 which are emitted from the photocathode 101. [0078] In the embodiment, which is shown in Figure 4 the anode 103 includes an opening 113. The anode 103 may, for example, have an annular shape which extends around the opening 113. The axis 100 extends through the opening 113. The photocathode 101 and the anode 103 are configured to generate an electric field which causes electrons 106 emitted from the target region 105 of the photocathode 101 to pass through the opening 113 in the anode 103. The electric field further causes electrons 106 emitted from the target region 105 to be directed towards the axis 100 and meet the axis 100 at an axis crossing region 115.

[0079] An electron steering apparatus 117 is positioned proximate to the axis crossing region 115 and is configured to alter the trajectory of the electrons 106, near to the axis crossing region 115, so as to cause the electrons to propagate substantially along the axis 110. The electron steering apparatus 117 may be configured to generate a magnetic field in and around the axis crossing region 115. For example, the electron steering apparatus 117 may comprise one or more electromagnets configured to generate a magnetic field in and around the axis crossing region 115. The magnetic field may exert a force on the electrons 106 which acts to alter the trajectory of the electrons 106. The trajectory of the electrons 106 is altered by the magnetic field until the electrons 106 are substantially coincident with the axis 100 and the electrons propagate substantially along the axis.

[0080] In embodiments in which the electron steering apparatus 117 comprises one or more electromagnets, the electromagnets 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 106. The electron steering apparatus 117 may additionally or alternatively comprise one or more electrically charged plates, configured to generate an electric field in and around the axis crossing region 115 such that a force is applied to the electrons 106. In general, the electron steering apparatus 117 may comprise any apparatus which is operable to apply a force to the electrons 106 so as to alter their trajectory such that the electrons propagate substantially along the axis 100.

[0081] Electrons 106 which propagate along the axis 100 pass through an electron booster 119. The electron booster 119 serves to accelerate the electron bunches along the axis 100. The electron booster 119 may, for example, accelerate electron bunches to relativistic energies. In some embodiments the electron booster 119 may accelerate electron bunches to energies in excess of approximately 5 MeV. In some embodiments, the electron booster 119 may accelerate electron bunches to energies which do not exceed approximately 20 MeV. However, in other embodiments the electron booster 119 may accelerate electron bunches to energies in excess of about 20 MeV.

[0082] The electron booster 119 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 121 (as depicted in Figure 4) 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 100. As bunches of electrons pass between the cavities 121, the electromagnetic fields controlled by the radio frequency power sources cause each bunch of electrons to accelerate. The cavities 121 may be superconducting radio frequency cavities. Alternatively, the cavities 121 may be conventionally conducting (i.e. not superconducting), and may be formed from, for example, copper.

[0083] In alternative embodiments other types of accelerator may be used to form the electron booster 119. 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 119 and linear accelerator 22 are provided and only one accelerator (e.g. a linear accelerator 22) may be used to accelerate the electrons.

[0084] It is desirable to accelerate the electrons 106 to relativistic energies as close as possible to the target region 105 from which they are emitted. Electrons in an electron bunch emitted from the target region 105 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.

[0085] Once electrons 106 are accelerated to relativistic energies, the relativistic effect of time dilation means that as observed in a lab frame (in which the electron source 21 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 119 and the target region 105, such that an electron bunch emitted from the target region 105 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 105 to the electron booster 119.

[0086] As was described above, in the embodiment which is shown in Figure 4, the photocathode 101 and the anode 103 are configured to generate an electric field which directs electrons emitted from the target region 105 towards the axis 100 such that the electrons meet the axis 100 at an axis crossing region 115. The trajectory of the electrons is altered by an electron steering apparatus 117 which is situated proximate to the axis crossing region 117. This allows the electrons to be guided so as to propagate substantially along the axis 100 with a single adjustment of the trajectory of the electrons by a single electron steering apparatus 117. For example, a single electromagnet positioned proximate to the axis crossing region 117 may be sufficient to direct the electrons to propagate substantially along the axis 100. This advantageously reduces the distance along the axis 100 which is required in order to steer the electrons so as to propagate substantially along the axis 100, when compared with alternative arrangements.

[0087] For example, in an alternative arrangement the electric field between the photocathode and the anode may not serve to direct electrons emitted from the target region towards the axis 100. In such an arrangement, the electrons may be directed such that they propagate substantially along the axis 100 by two or more steering apparatuses. For example, a first steering apparatus may initially direct the electrons towards the axis 100 and a second steering apparatus may adjust the trajectory of the electrons at an axis crossing region such that the electrons propagate substantially along the axis 100. In a further alternative arrangement an electron steering apparatus may not be positioned proximate an axis crossing region. In such an arrangement the electrons may be directed such that they propagate substantially along the axis 100 by two or more steering apparatuses.

[0088] In arrangements in which two or more electron steering apparatuses are used, in order to direct electrons to propagate substantially along the axis 100, the distance along the axis 100 which is required in order to position the electron steering apparatuses is greater than embodiments in which a single electron steering apparatus is used (e.g. as is shown in the embodiment of Figure 4). An electron source in which electrons emitted from a target region 105 are directed towards the axis 100 such that they meet the axis 100 at an axis crossing region 115, and in which an electron steering apparatus 117 is located proximate to the axis crossing region 115 and is configured to alter the trajectory of the electrons near to the axis crossing region 115 (such as the electron source 21 show in Figure 4), therefore advantageously reduces a distance along the axis 100 which is required in order to position an electron steering apparatus. This advantageously allows a distance between the target region 105 of the photocathode 101 and the electron booster 119 to be reduced. As was explained above, a reduction in this distance limits an increase in emittance of an electron bunch which occurs prior to the electron bunch being accelerated to relativistic energies by the electron booster 119.

[0089] The injector 21 may include other components which are not shown in Figure 4. For example, the injector may include a bunch compressor (not shown in the figures) which is configured to compress bunches of electrons which pass through it. A bunch compressor which forms part of the injector 21 may, for example, be similar to the bunch compressor 23, which was described above with reference to Figure 3.

[0090] The photocathode 105 and the anode 103 are positioned inside an enclosing structure 123 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 123. The enclosing structure 123, extends into a beam pipe 125 along which the electrons propagate. The beam pipe 125 may extend throughout a free electron laser FEL. For example, the beam pipe 125 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 125 may also be maintained at vacuum pressure conditions.

[0091] Despite the vacuum pressure conditions inside the enclosing structure 123 and the beam pipe 125, residual gas molecules may remain in the enclosing structure 123 and/or the beam pipe 125. Electrons in the electron beam E may collide with residual gas molecules and may ionize the gas molecules, thereby creating positively charged ions. Positively charged ions may be formed by electron collisions at any position in the free electron laser FEL. For example, positively charged ions may be formed within the electron source 21, the linear accelerator 23, the bunch compressor 23, the undulator 24 and/or the beam pipe 125 which connects these components.

[0092] Positively charged ions throughout the free electron laser FEL are attracted to the path of the electron beam E whose negative charge acts as a potential well to the positively charged ions. For much of the path of the electron beam E, the electron beam E is substantially coincident with the axis 100. Positively charged ions may therefore accumulate along the axis 100. Ions have a substantially higher mass than electrons and consequently in contrast to the electrons, the ions are not substantially accelerated by the linear accelerator 22 or by the electron booster 119. Instead the ions may diffuse along the axis 100 and may, for example, travel towards the photocathode 101. The positive charge of the ions causes them to be attracted to the photocathode 101 and may be caused to collide with the photocathode 101.

[0093] Ion collisions with the photocathode 101 may damage the photocathode 101. In particular, the collision of ions with the photocathode 101 may cause sputtering of material from the photocathode 101. Damage to the photocathode 101 may cause a change in composition of the photocathode 101. Additionally or alternatively damage of the photocathode 101 by ion collisions may cause an increase in the surface roughness of the photocathode 101. Damage to the photocathode, which is caused by ion collisions may, for example, serve to increase the emittance of electron bunches which are emitted from the photocathode 101 and/or may lead to a reduction in the quantum efficiency of the photocathode 101.

[0094] As was described above, the negative charge of the electron beam E which propagates along the axis 100, acts as a potential well to the positively charged ions. The majority of positively charged ions which collide with the photocathode 101, therefore do so within a collision region 128 which surrounds an intersection of the axis 100 with the photocathode 101. The majority of damage which is caused to the photocathode 101 due to ion collisions thus occurs within the collision region 128.

[0095] As was explained above, with reference to Figure 4, the target region 105 from which electrons 106 are emitted is separated from the axis 100 and therefore from the collision region 128. Separation of the target region 105 from the axis 100 limits any ion collisions which may occur at the target region 105. Any damage to the target region 105 which results from ion collisions with the photocathode 101 is therefore reduced by separating the target region 105 from the axis 100. This advantageously increases the lifetime over which electron bunches having desired properties can be emitted from the target region 105. For example, the lifetime over which electron bunches having a desired emittance and/or peak current, can be emitted from the target region 105 is increased. This increases the overall lifetime of the photocathode 101 and reduces the frequency with which a photocathode 101 requires replacement.

[0096] Replacing a photocathode 101 in an electron source 21 may be a time consuming process during which the electron source 21 and the free electron laser FEL are not operational. Increasing the lifetime of the photocathode 101 therefore advantageously reduces any downtime of the electron source 21 and the free electron laser FEL which results from replacement of a photocathode 101.

[0097] Figure 5 is a schematic illustration of a portion of an electron source 21 which includes the photocathode 101 and the anode 103. Also shown in Figure 5 is a representation of electric field lines 127 which extend between the photocathode 101 and the anode 103.

[0098] Electrons 106 which are emitted from the target region 105 of the photocathode 101 may be emitted in any direction. The directions in which electrons are emitted from the target region 105 may follow a probability distribution, such as a cosine distribution whose peak coincides with a normal to the emitting surface of the target region 105. However, the initial kinetic energy of an electron as it is emitted from the target region 105 may be relatively small when compared to the kinetic energy which is gained by acceleration by the electric field between the photocathode 101 and the anode 103. For example, the initial kinetic energy of an electron which is emitted from the target region 105 may be of the order of about 1 eV or less. After the electron has been accelerated by the electric field in between the photocathode 101 and the anode 103, the electron may have a kinetic energy which is greater than about 30 keV (e.g. 100-500 keV). The direction with which emitted electrons propagate may therefore be almost entirely dependent on the electric field between the photocathode 101 and the anode 103 and may have little or no dependence on the initial kinetic energy of the electrons.

[0099] At the surface of the photocathode 101, the electric field extends perpendicular to the surface of the photocathode 101. In the embodiment which is shown in Figure 5, the surface 102 of the photocathode 101 is tilted with respect to the axis 100. That is, the surface 102 is neither parallel with or perpendicular to the axis 100. In particular, the surface of the photocathode 101 is arranged such that a normal 129 to the surface of the target region 105 is inclined towards the axis 100. Consequently the electric field lines 127 which extend out of the target region 105 are also inclined towards the axis 100.

[00100] The acceleration of electrons 106 emitted from the target region 105 follows the electric field lines 127 and thus the electrons are initially accelerated in a direction which is perpendicular to the surface of the target region 105, from which the electrons are emitted (or equivalently parallel to the normal 129 to the surface). As was explained above, the initial kinetic energy of the electrons is relatively small when compared to the energy gained by acceleration by the electric field. The direction in which the electrons propagate away from the target region 105 will therefore initially closely follow the electric field lines which extend out of the target region 105. As was explained above the normal 129 to the emitting surface of the target region 105 is inclined towards the axis 100 and thus the electrons will be directed towards the axis 100.

[00101] The opening 113 in the anode 103 causes the electric field lines 127 to bend away from the axis 100 in the vicinity of the anode 103. As the electrons travel towards the anode 103 and the electric field lines 127 bend away from the axis 100, the electrons will experience an electromagnetic force whose direction follows the electric field lines (and thus bends away from the axis 100). However, by the time the electrons reach the vicinity of the anode 103, where the electric field lines 127 bend away from the axis 100, the electrons have gained a large amount of kinetic energy due to the acceleration of the electrons away from the photocathode 101 and towards the axis 100. The electromagnetic force which pulls the electrons towards the anode 103, in the vicinity of the anode 103, therefore has a smaller impact on the trajectory of the electrons than the initial direction of acceleration of the electrons away from the photocathode 101. Consequently the electrons continue substantially along the trajectory along which they were initially accelerated away from the photocathode 101.

[00102] For example, electrons may pass the anode 103 at a distance of approximately 1-2 mm from the axis 100. The electric field in the vicinity of the anode 103 may cause an angular deflection, in the direction of propagation of the electrons, of approximately 0.3-0.6°. The electrons may be emitted from the photocathode 101 such that their direction of propagation forms an angle of approximately 5° with the axis 100. The angular deflection which is caused by the electric field in the vicinity of the anode 103 may therefore be of the order of approximately 10% of the initial angle which the electrons trajectory forms with the axis 100.

[00103] As was described above, the direction in which the electrons propagate is predominantly determined by the electric field direction in the target region 105 and in the volume immediately adjacent to the target region 105. The photocathode 101 and the anode 103 are configured such that the electric field direction throughout the target region 105 is inclined towards the axis 100. Electrons emitted from the target region 105 are therefore accelerated towards the axis 100 and follow a trajectory towards the axis 100 such that they meet the axis 100 at an axis crossing region 115.

[00104] In an embodiment, a separation x between the target region 105 and the anode 103 may be greater than about 3 cm. For example, the separation x may be about 5 cm. A diameter d of the opening 113 in the anode 103 may be approximately 1 cm. The target region 105 which is illuminated by a radiation beam 107 (not shown in Figure 5) may be approximately circular and may have a diameter of approximately 2 mm. In general, the diameter d of the opening 113 is greater than the diameter of the target region 105. A separation y between the axis 100 and the target region 105 may be approximately 5 mm. In such an embodiment the direction of propagation of the electrons towards the axis 100 may be largely determined by the electric field direction in a volume which lies within about 1 mm of the target region 105. The photocathode 101 and the anode 103 may therefore be configured such that the electric field direction is inclined towards the axis 100 at least in the volume which lies within about 1 mm of the target region.

[00105] In some embodiments, the photocathode 101 and the anode 103 may be configured such that the electric field direction is inclined towards the axis 100 in a volume which lies within about 2mm, about 5 mm, about 10 mm, or about 50 mm or more of the target region 105.

[00106] The target region may be tilted with respect to the axis 100, such that a normal 129 to the surface of the target region 105 forms an angle of approximately 5° with the axis 100. The initial trajectory of the electrons away from the photocathode may therefore form an angle of approximately 5°. In some embodiments, the target region may be tilted with respect to the axis 100, such that the initial trajectory of the electrons would cause the electrons to cross the axis 100 at approximately the same position along the axis 100 at which the anode 103 is positioned. However, as was mentioned above, the electric field in the vicinity of the anode 103 may cause the electrons to be deflected slightly off their initial trajectory. For example, the electrons may undergo an angular deflection of approximately 5°. This may cause the electrons to pass the anode 103 at a position which is approximately 1 mm from the axis 100. The electrons may meet the axis 100 at a location which is approximately 1.5 cm along the axis 100, from the anode 103.

[00107] It will be appreciated from the above example dimensions that the representations which are displayed in Figures 4 and 5 may not be to scale. For example, for ease of illustration, the separation y between the target region 105 and the axis 100 has been exaggerated in the Figures.

[00108] As has been described above with reference to Figure 5, the electric field direction at the target region 105 is important for determining the direction in which electrons propagate away from the photocathode 101. As was also described above, it may be desirable for an electron bunch having a relatively low emittance to be emitted from the target region 105. The emittance of an electron bunch depends on a spread of position of electrons which form a bunch and on a spread of momentum which the electrons in a bunch have.

[00109] The spread of positions and momentums in an electron bunch will, at least in part, depend on the electric field which different electrons in a bunch experience. In order to limit the spread in position and momentum of electrons in a bunch (and therefore the emittance of the bunch) it may be desirable for all electrons which form a bunch to experience substantially the same electric field as they are emitted from the target region 105. In the embodiment which is shown in Figures 4 and 5, the photocathode 101 and the anode 103 are configured such that the electric field direction is substantially the same throughout the target region 105. Electrons which are emitted from different parts of the target region 103 are therefore accelerated away from the photocathode in substantially the same direction. This advantageously ensures that electrons emitted from different parts of the target region 103 remain relatively tightly bunched and thus a spread in position of electrons in the bunch is relatively small. [00110] In addition to the direction of the electric field being substantially the same throughout the target region 105, the photocathode 101 and the anode 103 may also be configured such that the magnitude of the electric field is substantially the same throughout the target region. Electrons which are emitted from different parts of the target region 103 are therefore accelerated away from the photocathode by substantially the same amount. This advantageously ensures that electrons emitted from different parts of the target region 103 remain relatively tightly bunched and may ensure that the electrons which form a bunch have similar momentums.

[00111] As was described above, the arrangement of the photocathode 101 and the anode 103 may cause electrons emitted from the target region 103 to experience an electric field having substantially the same direction and/or magnitude, so as to limit a spread of position and momentum in an electron bunch. However, in practice the space charge effect may cause a significant spreading out of an electron bunch after it has been emitted from the target region 103. The emittance of an electron bunch accounts both for the spread in position and momentum and for the correlation between the position and (transverse) momentum. The shape of the target region 105, which is illuminated with the radiation beam 107, may be such that the transverse momentum of electrons in a bunch is approximately linear with transverse position in the bunch. This may limit any increase in the emittance of the bunch despite there being a spread in position and momentum within the bunch.

[00112] In the embodiment which is shown in Figures 4 and 5 the target region 105 comprises a substantially flat surface. The target region is tilted with respect to the axis 100, such that a normal 129 to the substantially flat surface of the target region 105 is inclined towards the axis 100. Since the electric field direction is perpendicular to the surface of the target region 105 (and parallel with the normal 129) at the surface 105, the electric field direction is substantially the same throughout the target region 105. Since the normal 129 to the substantially flat surface of the target region is inclined towards the axis 100, the electric field direction throughout the target region 105 is inclined towards the axis 100 and the electrons are accelerated towards the axis 100.

[00113] As can be seen in Figures 4 and 5, the electron emitting surface 102 of the photocathode 101 and the target region 105 are tilted with respect to the axis 100 (that is, they are neither parallel with or perpendicular to the axis 100). The photocathode 101 which is shown in Figures 4 and 5 is not therefore cylindrically symmetrical about the axis 100.

[00114] In some embodiments the photocathode 101 may have a different form to the photocathode 101 which is shown in Figures 4 and 5. Figure 6 is a schematic illustration of an alternative embodiment of a photocathode 1101. The photocathode 1101 which is shown in Figure 6 comprises a first section 1101a and a second section 1101b. The first section 1101a meets the second section 1101b at the axis 100. The axis 100 which is shown in Figure 6 is the same as the axis 100 which is shown in Figures 4 and 5. The axis 100 is an axis along which electrons, which are provided by an injector 21 comprising the photocathode 1101, propagate when leaving the injector 21. [00115] The first section 1101a of the photocathode 1101 includes a first surface 1102a which is tilted towards the axis 100. The second section 11002b includes a second surface 1102b which is also tilted towards the axis 100 but which is not parallel with the first surface 1102a. The first region 1101a includes a target region 1105 which is illuminated with a radiation beam (not shown in Figure 6) and which emits electrons 106 when illuminated by the radiation beam. The target region 1105 comprises a substantially flat surface, wherein a normal 129 to the surface of the target region 1105 is inclined towards the axis 100.

[00116] Also shown in Figure 6 is an anode 1103. The anode 1103 which is shown in Figure 6 is similar to the anode 103 which is shown in Figures 4 and 5 and comprises an opening 1113. Electric field lines 127 extend between the photocathode 1101 and the anode 1103. The electric field lines 127 in the vicinity of the target region 1105, and which are shown in Figure 6, are similar to the electric field lines in the vicinity of the target region 105 of the embodiment which is shown in Figures 4 and 5. The photocathode 1101 of Figure 6 therefore provides all of the advantageous effects which were described above with reference to Figures 4 and 5. In particular, the photocathode 1101 and the anode 1103, which are shown in Figure 6, are configured such that the electric field direction is substantially the same throughout the target region 1105 and is inclined towards the axis 100. Electrons 106 which are emitted from the target region are directed towards the axis 100 and meet the axis 100 at an axis crossing region 115.

[00117] In the embodiment which is shown in Figure 6, the target region 1105 is located in the first section 1101a of the photocathode. It will be appreciated, that the electric field lines 127 in the vicinity of the second section 1101b are substantially a mirror image of the electric filed lines 127 in the vicinity of the first section 1101a. In an alternative embodiment, the target region 1105 may be located in the second section 1101b.

[00118] In some embodiments, the photocathode 1101 may include at least one region in the first section 1101a which is suitable for emitting electrons when illuminated with radiation and at least one region in the second section which is suitable for emitting electrons. At some times a target region 1105 in the first section 1101a may be illuminated with radiation so as to cause emission of electrons from the first section 1101a and at other times a target region 1105 in the second section 1101b may be illuminated with radiation so as to cause emission of electrons from the second section 1101b. A change in location of the target region 1105 which is illuminated with radiation, from the first section 1101a to the second section 110b (or vice versa) may require a change in the force which is applied by an electron steering apparatus (not shown in Figure 6) in order to alter the trajectory of the electrons such that they propagate substantially along the axis 100.

[00119] The embodiments of a photocathode which are shown in Figures 4, 5 and 6 have a number of features in common. For example, both embodiments include a target region having a substantially flat surface, where a normal to the substantially flat surface is inclined towards the axis 100. Both embodiments of the photocathode are also not cylindrically symmetric about the axis 100. [00120] In an alternative embodiment a photocathode may be substantially cylindrically symmetric about the axis 100. For example, in an embodiment a photocathode may include a substantially conical surface which includes a target region. The conical surface may be substantially cylindrically symmetric about the axis 100. All regions of the conical surface may therefore be tilted towards the axis.

[00121] A target region which is situated on the conical surface will not be exactly flat and will include some curvature. The size of a target region may, however be sufficiently small when compared to the local curvature of the conical surface that the electric field direction may be substantially the same throughout the target region. The curvature of the target region will mean that there are small differences in the direction of the electric field across the target region. However, the curvature may be sufficiently small across the size of the target region that the direction of the electric field may be considered to be substantially the same throughout the target region.

[00122] Since the conical surface is, at all points, tilted towards the axis, the direction of the electric field in the target region is inclined towards the axis. Electrons emitted from the target region are therefore directed towards the axis.

[00123] Whilst specific embodiments of the photocathode and anode have been described herein, in general the photocathode and the anode may take any form which causes an electric field to be established where the direction of the electric field is substantially the same throughout a target region, and is inclined towards the axis.

[00124] Although embodiments of a free electron laser have been described as comprising a linear accelerator 22, it should be appreciated that a linear accelerator 22 is merely an example of a type of particle accelerator which may be used to accelerate electrons in a free electron laser. A linear accelerator 22 may be particularly advantageous since it allows electrons having different energies to be accelerated along the same trajectory. However in alternative embodiments of a free electron laser other types of particle accelerators may be used.

[00125] It should be appreciated that a radiation source which comprises a free electron laser FEL 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.

[00126] A lithographic system LS may comprise any number of lithographic apparatuses. The number of lithographic apparatuses 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 apparatuses 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. [00127] Embodiments of a lithographic system LS may also include one or more mask inspection apparatuses 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.

[00128] It will be further appreciated that a free electron laser comprising an undulator as described above may be used as a radiation source for a number of uses, including, but not limited to, lithography.

[00129] 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.

[00130] The lithographic apparatus which have been described herein may be used in the manufacture of ICs. Alternatively, the lithographic apparatuses described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

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

[00132] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. An electron source configured to provide a beam of electrons which propagate substantially along an axis, the electron source comprising:

a photocathode including a target region configured to emit electrons when illuminated with radiation, wherein the target region is separated from the axis; and

an anode;

wherein the photocathode and the anode are configured to generate an electric field which causes electrons emitted from the target region of the photocathode to be accelerated away from the photocathode; and

wherein the photocathode and the anode are configured such that the electric field direction is substantially the same throughout the target region, and is inclined towards the axis.

2. The electron source of claim 1, wherein the target region comprises a substantially flat surface.

3. The electron source of claim 2, wherein a normal to the substantially flat surface of the target region is inclined towards the axis.

4. The electron source of any preceding claim, wherein the photocathode and the anode are configured such that the electric field direction is substantially the same within a volume which lies within about 1 mm of the target region.

5. The electron source of any preceding claim, wherein the photocathode and the anode are configured such that the electric field strength is substantially the same throughout the target region.

6. The electron source of any preceding claim, wherein the photocathode is not cylindrically symmetric about the axis.

7. The electron source of any preceding claim, wherein the anode includes an opening.

8. The electron source of claim 7, wherein the photocathode and the anode are configured to generate an electric field which causes electrons emitted from the target region of the photocathode to pass through the opening in the anode.

9. The electron source of claim 7 or 8, wherein the anode has an annular shape.

10. The electron source of any of claims 7-9, wherein the axis extends through the opening.

11. The electron source of any preceding claim, further comprising an electron steering apparatus configured to alter the trajectory of the emitted electrons so as to cause the electrons to propagate substantially along the axis.

12. The electron source of any preceding claim, wherein the photocathode and the anode are configured to generate an electric field which causes electrons emitted from the target region to be directed towards the axis and meet the axis at an axis crossing region.

13. The electron source of claims 11 and 12, wherein the electron steering apparatus is positioned proximate to the axis crossing region and is configured to alter the trajectory of the electrons, near to the axis crossing region.

14. The electron source of claim 11 or any claim dependent thereon, wherein the beam steering apparatus is configured to generate a magnetic field so as to alter the trajectory of the emitted electrons.

15. The electron source of any preceding claim, further comprising a beam delivery system configured to direct a beam of radiation to be incident on the target region.

16. The electron source of any preceding claims, further comprising a radiation source configured to emit a radiation beam which is directed to be incident on the target region.

17. The electron source of claim 16, wherein the radiation source comprises a laser configured to emit a laser beam which is directed to be incident on the target region.

18. The electron source of claim 16 or 17, wherein the radiation source is configured to emit a pulsed radiation beam which is directed to be incident on the target region, thereby causing bunches of electrons to be emitted from the target region.

19. The electron source of any preceding claim, further comprising a voltage source configured to maintain a potential difference between the photocathode and the anode.

20. The electron source of claim 19, wherein the voltage source is an AC voltage source.

21. The electron source of claim 18 and 20, wherein a repetition rate of the pulsed radiation beam is substantially matched to a frequency and phase of the AC voltage source, such that pulses of the radiation beam incident on the target region substantially coincide with peaks in the potential difference between the photocathode and the anode.

22. The electron source of any preceding claim, further comprising an electron booster configured to accelerate the electrons along the axis.

23. The electron source of any preceding claim, further comprising a bunch compressor, configured to compress bunches of electrons towards the axis.

24. A free electron laser comprising:

an electron source according to any preceding claim;

a particle accelerator configured to accelerate the electron beam output from the electron source; and

an undulator operable to guide the accelerated electron beam along a periodic path so as to stimulate emission of radiation, thereby forming a radiation beam.

25. The free electron laser of claim 24, wherein the undulator is configured to stimulate emission of an EUV radiation beam.

26. A lithographic system comprising:

a radiation source comprising a free electron laser of claim 24 or 25; and

at least one lithographic apparatus arranged to receive at least a portion of radiation provided by the radiation source.

27. A method of providing a beam of electrons which propagate substantially along an axis, the method comprising:

illuminating a target region of a photocathode with radiation so as to cause electrons to be emitted from the target region, wherein the target region is separated from the axis; and

generating an electric field between the photocathode and an anode, wherein the electric field causes electrons emitted from the target region of the photocathode to be accelerated away from the photocathode and wherein the electric field direction is substantially the same throughout the target region, and is inclined towards the axis.