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    • 专利摘要:
    An electron beam chopper for forming a bunched beam of electrons, the electron beam chopper comprises a deflector and a blocking member. The deflector is operable to receive an input beam of electrons propagating along a first axis 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. The blocking member is arranged to block the output beam of electrons when it is in a first sub-range of the range of directions and to allow the output beam of electrons to pass it when it is in a second sub-range of the range of directions so as to form a bunched electron beam. 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.
    公开号:NL2017603A
    申请号:NL2017603
    申请日:2016-10-11
    公开日:2017-05-24
    发明作者:Jacobus Hendrik Brussaard Gerrit；Frederiek Dirk Stragier Xavier；Yevgenyevich Banine Vadim；Jan Luiten Otger
    申请人:Asml Netherlands Bv；
    IPC主号:
    专利说明:

    Electron Beam Chopper
    FIELD
    [0001] The present invention relates to an electron beam chopper for forming a bunched beam of electrons. In particular, the present invention may relate to a source for producing a bunched beam of electrons, and the source may form part of a free electron laser. The free electron laser may, for example, form part of a lithographic system.
    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 of the invention there is provided an electron beam chopper for forming a bunched beam of electrons, the electron beam chopper comprising: a deflector operable to receive an input beam of electrons propagating along a first axis 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; and a blocking member which is arranged to block the output beam of electrons when it is in a first sub-range of the range of directions and to allow the output beam of electrons 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.
    [0007] Such an arrangement is a convenient apparatus for forming a bunched electron beam, using the deflector to move a continuous electron beam relative to the blocking member. By arranging the deflector and the blocking member such that a rate of change of the direction of the electron beam is at a minimum when the electron beam passes the blocking member, effectively the output electron beam is being sampled at a point where its direction is slowly moving. This ensures that the emittance of the bunched electron beam formed by the first aspect of the invention is minimized. This may be advantageous for a number of reasons, depending in what the bunched electron beam is used for. In some embodiments, the electron beam chopper may form part of an electron source for a free electron laser. For such embodiments it may be desirable to minimize the emittance of the formed bunched electron beam since this may affect the gain and bandwidth of the free electron laser.
    [0008] It will be appreciated that a local minimum of the rate of change of the direction of the electron beam occurs at times for which the absolute value of the rate of change of the direction of the electron beam is greater during a period of time immediately preceding and a period of time immediately following said times. It will be appreciated that there may be more than one local minimum of the rate of change of the direction of the electron beam. Therefore the rate of change of the direction of the electron beam may also be at a local minimum at some point when the electron beam is in the first sub-range.
    [0009] The deflector may be operable to cause the direction of the output beam of electrons to oscillate between a first end direction and a second end direction.
    [0010] The oscillation of the direction of the output beam of electrons may be such that a projection of the electron beam onto a plane perpendicular to the first axis oscillates back and forth along a linear or curved path.
    [0011] The blocking member may be 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.
    [0012] Since the direction of the output beam of electrons oscillates between a first end direction and a second end direction at each of the first and second end directions, the rate of change of the direction of the electron beam may be zero.
    [0013] The oscillation between the first end direction and the second end direction may be sinusoidal.
    [0014] The range of directions may lie substantially within a plane. For example, a projection of the electron beam onto a plane perpendicular to the first axis may move linearly. Alternatively, the range of directions may not line in a plane. For example, a projection of the electron beam onto a plane perpendicular to the first axis may move alone a curved path. The curved path may, for example, be circular or elliptical.
    [0015] It will be appreciated that the deflector may be any component or combination of components that is operable to receive a beam of electrons and to alter its direction in a time dependent manner. The deflector may use electric or magnetic fields or a combination of both.
    [0016] The deflector may comprise a resonant cavity and an alternating power source.
    [0017] The alternating source may be operable to excite a transverse magnetic mode within the resonant cavity. For example, the alternating source may be operable to excite a TM110 mode within the resonant cavity.
    [0018] The blocking member may comprise a wall or a screen that is provided with one or more apertures.
    [0019] When the output beam of electrons is in the first sub-range of the range of directions it may be incident upon the wall or screen. When the output beam of electrons is in the second sub-range of the range of directions it may pass through one of the one or more apertures so as to form the bunched electron beam. The blocking member may be formed from an electrically conducting material.
    [0020] The electron beam chopper may further comprise electron optics arranged to alter the size and/or shape of the bunched electron beam.
    [0021] It will be appreciated that the term electron optics is intended to include any system that produces electromagnetic fields that may be arranged to influence the bunched electron beam. The electron optics may be arranged to alter a shape of the bunched electron beam.
    [0022] The electron optics may comprise focusing optics arranged to reduce a divergence of the bunched electron beam.
    [0023] The focusing optics may be arranged to substantially collimate the bunched electron beam. That is, the focusing optics may be arranged to reduce the divergence of the bunched electron beam to substantially zero. The focusing optics may comprise a dipole magnetic field. Additionally or alternatively, the focusing optics may comprise a resonant cavity.
    [0024] The electron optics may be arranged such that the size and/or shape of the bunched electron beam is substantially the same as that of the input electron beam.
    [0025] The electron optics may be arranged to at least partially correct for a curvature of individual electron bunches of the bunched electron beam caused by the deflector and blocking member.
    [0026] According to a second aspect of the invention there is provided a source for producing a bunched beam of electrons, the source comprising: an electron source operable to produce a beam of electrons; and an electron beam chopper according to the first aspect of the invention, wherein the deflector of the electron beam chopper is arranged to receive the beam of electrons produced by the electron source and the electron beam chopper is arranged to output a bunched beam of electrons.
    [0027] According to a third aspect of the invention there is provided a free electron laser comprising a source according to the second aspect of the invention; 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.
    [0028] According to a fourth aspect of the invention there is provided a lithographic system comprising: the free electron laser of the third aspect of the invention; 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.
    [0029] 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
    [0030] 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;
    Figure 4A is a schematic illustration of an embodiment of an electron beam chopper that may form part of the free electron laser of Figure 3;
    Figure 4B is a schematic illustration of another embodiment of an electron beam chopper that may form part of the free electron laser of Figure 3;
    Figure 5A shows the screen of the electron beam chopper of Figure 4A;
    Figure 5B shows the screen of the electron beam chopper of Figure 4B;
    Figure 6 shows an electron bunch formed by the electron beam chopper of Figure 4B; Figure 7 is a schematic illustration of a variant of the electron beam chopper of Figures 4A or 4B, which comprises an additional dipole magnet;
    Figure 8 is a schematic illustration of a variant of the electron beam chopper of Figures 4A or 4B, which comprises an additional resonant cavity; and
    Figure 9 is a schematic illustration of a variant of the electron beam chopper of Figures 4A or 4B, which comprises an additional dipole magnet and an additional resonant cavity.
    DETAILED DESCRIPTION
    [0031] 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).
    [0032] 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.
    [0033] 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.
    [0034] 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.
    [0035] 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).
    [0036] 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.
    [0037] 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.
    [0038] 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.
    [0039] 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).
    [0040] 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).
    [0041] 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.
    [0042] 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.
    [0043] The injector 21 is arranged to produce a bunched electron beam E and may comprise an electron source and an electron beam chopper. 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. An electron chopper that may form part of the injector 21 is discussed below with reference to Figure 4.
    [0044] 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.
    [0045] 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 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.
    [0046] 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.
    [0047] 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.
    [0048] 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, ku 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=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.
    [0049] The resonant wavelength kem 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.
    [0050] 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.
    [0051] 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 ku 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.
    [0052] 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.
    [0053] 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.
    [0054] 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.
    [0055] 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.
    [0056] 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.
    [0057] 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.
    [0058] 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).
    [0059] 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.
    [0060] 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.
    [0061] 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).
    [0062] 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 grad, and may for example have a diameter of around 100 pm.
    [0063] 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.
    [0064] Figure 4A is a schematic illustration of an electron beam chopper 100 according to an embodiment of the invention, which may form part of the injector 21 of the free electron laser of Figure 3. The electron beam chopper 100 is arranged to form a bunched beam of electrons. The electron beam chopper 100 comprises a resonant cavity 110 and a screen 120.
    [0065] The resonant cavity 110 comprises a generally cylindrical hollow body 112 extending along an axis of the resonant cavity 110 (along the x direction in Figure 4A). Although in this example embodiment, the hollow body 112 of the resonant cavity 110 is generally cylindrical, it will be appreciated that body 112 may alternatively have another shape. For example, in an alternative embodiment the body 112 of the resonant cavity 110 may be rectangular or square in cross section. Two apertures 114, 116 are defined by the body 112, one on each of two opposite sides of the body 112. The resonant cavity 110 is therefore operable to receive an input beam of electrons propagating along the axis of the resonant cavity 110 (i.e. along the x direction) through one aperture 114. The beam of electrons may exit the resonant cavity 110 through the other aperture 116.The resonant cavity 110 is provided with an alternating power source (not shown) that is arranged to excite a transverse magnetic mode within the resonant cavity 110. 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 110 or, alternatively, the antenna may be located outside of the resonant cavity 110 and may be coupled to resonant cavity 110, for example, by a waveguide.
    [0066] The body 112 is formed from a material that is an electrical conductor. The material from which the body 112 is formed may be superconducting. Advantageously, this allows: relatively large electromagnetic fields to be applied at high duty cycles; larger beam apertures 114, 116, 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 112 is formed may be conventionally conducting (i.e. not superconducting), and may comprise, for example, copper.
    [0067] The resonant modes of resonant cavity 110 are dependent on the geometry (i.e. shape) of the resonant cavity 110 and the material inside the resonant cavity 110. The resonant modes of resonant cavity 110 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 110 are also dependent on the conductivity of the material from which the body 112 is formed although this is also usually a small correction.
    [0068] The alternating source is operable to excite a transverse magnetic mode within the resonant cavity 110. For example, the alternating source may be operable to excite a TM110 mode within the resonant cavity 110. The resonant mode(s) excited within the resonant cavity 110 may be dependent upon the frequency content (spectrum) of the electromagnetic radiation emitted by the antenna.
    [0069] With such an arrangement, an oscillating magnetic field is generated within the resonant cavity. The magnetic field is perpendicular to the plane of Figure 4A, i.e. in the z-direction and oscillates with time. As an input beam of electrons propagates into the resonant cavity 110 through aperture 114, 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 110 through aperture 116 is dependent on the magnetic field during the time that it was in the resonant cavity 110. The trajectory of the electron beam as it exits through aperture 116 remains in the x-y plane in Figure 4A and oscillates with time through a range of directions between a first end direction 130 and a second end direction 132.
    [0070] The resonant cavity 110 may therefore be considered to be a deflector that is operable to receive an input beam of electrons propagating along an axis (the x-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.
    [0071] The screen 120 defines an aperture 122. As the trajectory of the electron beam exiting through aperture 116 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 through the aperture 122. Since the oscillation of the trajectory of the electron beam exiting the resonant cavity 110 is periodic, the portions of the electron beam that pass through the aperture 122 for temporally discrete bunches 134 of electrons. The screen 120 may therefore be considered to be a blocking member which is arranged to block the beam of electrons exiting the resonant cavity 110 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 120) 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 through the aperture 122) so as to form a bunched electron beam.
    [0072] The arrangement of Figure 4A comprises a screen 120 which extends in the y-direction on both sides of the aperture 122 that defines the second sub-range of the range of directions. An alternative embodiment of screen 120 that forms a blocking member is shown in Figure 4B. In the arrangement of Figure 4B, the screen 120 comprises a knife edge 123. It is the position of the knife edge 123 which defined the first and second sub-ranges of the range of directions.
    [0073] For embodiments wherein the alternating source excites a TM110 mode within the resonant cavity 110, the magnetic field in the resonant cavity 110 oscillates sinusoidally with time. Therefore, as shown in Figures 5A and 5B, in the plane of the screen 120, the beam spot 136 of the electron beam exiting aperture 116 oscillates along a linear path 138. This oscillation is sinusoidal. As shown in Figures 5A and 5B, in the plane of the screen 120, the beam spot 136 of the electron beam exiting aperture 116 undergoes simple harmonic motion.
    [0074] As shown in Figure 5A (which corresponds to the screen shown in Figure 4A), the aperture 122 in the screen 120 is disposed at one end of the linear path 138, which corresponds to the electron beam propagating along the first end direction 130, and where the rate of change of the direction of the electron beam that exits the resonant cavity 110 is at a local minimum. Therefore, the resonant cavity 110 and the screen 120 are arranged such that a rate of change of the direction of the electron beam that exits the resonant cavity 110 is at a local minimum when the electron beam is in the second sub-range and passes through the aperture 122.
    [0075] As shown in Figure 5B (which corresponds to the screen shown in Figure 4B), the knife edge 123 of the screen 120 is disposed proximate to one end of the linear path 138, which corresponds to the electron beam propagating along the first end direction 130, and where the rate of change of the direction of the electron beam that exits the resonant cavity 110 is at a local minimum. Therefore, the resonant cavity 110 and the screen 120 are arranged such that a rate of change of the direction of the electron beam that exits the resonant cavity 110 is at a local minimum when the electron beam is in the second sub-range and passes above the knife edge 123.
    [0076] Note that the rate of change of the direction of the electron beam that exits the resonant cavity 110 is also at a local minimum when the electron beam is propagating along the second end direction 132, which corresponds to the other end of the linear path 138 the electron beam. Therefore, the aperture 122 or knife edge 123 may be proximate to either end of the linear path 138 traced by the electron beam spot. Alternatively, an aperture (or knife edge) may be provided at each end of the linear path 138 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.
    [0077] The electron beam chopper 100 is a convenient apparatus for forming a bunched electron beam, using the resonant cavity 110 to move a continuous electron beam relative to the screen 120. Since the magnetic field in the resonant cavity 110 oscillates with time, so too does the force exerted by on electrons within the resonant cavity 110. Furthermore, since the bunches 134 of electrons that pass through the aperture 122 (or past knife edge 123) have a non-zero temporal length, this means that, in general, different electrons within each bunch 134 experience different forces as they pass through the resonant cavity 110. As a result, the bunched electron beam which passes through the aperture 122 (or past knife edge 123) is divergent (i.e. has a greater spread of directions). Furthermore, the normalized emittance of the bunched electron beam that passes through the aperture 122 (or past knife edge 123) is greater than the normalized emittance of the electron beam that enters the electron beam chopper 100. That is, the normalized emittance of the bunched electron beam is increased by the electron beam chopper 100.
    [0078] The normalized emittance growth ε induced by the electron beam chopper 100 is given by:
    (3) where R is the radius of the input electron beam, vx is the initial velocity of the electron beam (in the x direction), c is the speed of light, s is the width of the slit (in the y direction), L is the distance between the resonant cavity 110 and the screen 120, and <p is a phase of the electromagnetic wave within resonant cavity 110 when as the centre of the electron bunch 134 passes through the centre of the resonant cavity 110. The phase φ is defined such that: φ=0,π when the centre of the electron bunch 134 passes through the centre of the resonant cavity 110 as the magnetic field is zero; and φ=π/2,3π/2 when the centre of the electron bunch 134 passes through the centre of the resonant cavity 110 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).
    [0079] In electron beam chopper 100 the resonant cavity 110 and the screen 120 are arranged such that a rate of change of the direction of the electron beam is at a minimum when the electron beam passes through the aperture 122 (or past knife edge 123) of the screen 120. 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 110 is at a local minimum when the electron beam is in the second sub-range and passes through the aperture 122 (or past knife edge 123), 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 100 is minimized. In fact, Eq. (3) is a simplification that assumes that the electron beam which enters the resonant cavity 110 is collimated and that the temporal length of the electron bunches 134 is significantly smaller than the time period of the oscillation of the electromagnetic radiation within the resonant cavity 110. 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 100 is minimized.
    [0080] This minimization of the emittance of the bunched electron beam formed by the beam chopper 100 is advantageous for a number of reasons, as now discussed. The electron beam chopper 100 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.
    [0081] The electron beam chopper 100 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 110. 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.
    [0082] As now described with reference to Figure 6, due to the non-zero extent of the electron bunches 134 formed, the electron beam chopper 100 described above results in curved electron bunches. The extent of this distortion is dependent on the duty cycle of the electron beam chopper 100, i.e. the ratio of the temporal length of the electron bunches 134 to the time period of the oscillation of the electromagnetic radiation within the resonant cavity 110. The higher the duty cycle of the electron beam chopper 100, the greater the distortion will be.
    [0083] As explained above, the trajectory of the electron beam as it exits through aperture 116 remains in the x-y plane in Figure 4A and oscillates with time through a range of directions between a first end direction 130 and a second end direction 132. The screen 120 is arranged to block the beam of electrons exiting the resonant cavity 110 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 120) 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 through the aperture 122 or past the knife edge 123) so as to form a bunched electron beam. The second sub-range of the range of directions is defined between the first end direction 130 (which is dependent on the resonant cavity 110 and the input electron beam) and an intermediate direction 133 (which is dependent on the screen 120, for example, the size and position of the aperture 122 or the position of the knife edge 123).
    [0084] A single bunch 134 of electrons formed by the electron beam chopper 100 is shown in Figure 6. Also indicated in Figure 6 are the front 134a, the centre 134b and the rear 134c of the electron bunch 134. Electrons at the front 134a and the rear 134c of the bunch 134 propagate along the intermediate direction 133 whereas the electrons at the centre of the bunch propagate along the first end direction 130. As a result, the electron bunch 134 is curved or banana-shaped. Furthermore, the bunch 134 is divergent and continues to increase in size as it propagates away from the screen 120.
    [0085] In some embodiments, the electron beam chopper 100 may further comprise electron optics arranged to alter the size and/or shape of the bunched electron beam, as now discussed. The electron optics may be arranged to at least partially correct for the curved shape of the electron beam and/or the increase in the divergence of the beam. It will be appreciated that the term electron optics is intended to include any system that produces electromagnetic fields that may be arranged to influence the bunched electron beam.
    [0086] The electron optics may comprise focusing optics arranged to reduce a divergence of the bunched electron beam. For example, the focusing optics may be arranged to substantially collimate the bunched electron beam. That is, the focusing optics may be arranged to reduce the divergence of the bunched electron beam to substantially zero.
    [0087] The focusing optics may comprise a dipole magnetic field, as now described with reference to Figure 7. As shown in Figure 7, in one embodiment the electron beam chopper 100 further comprises a dipole magnet 140. The dipole magnet 140 is disposed downstream of the screen 120 such that the electrons in the electron bunches 134 that pass the screen 120 travel through the magnetic field of the dipole magnet 140. The dipole magnet 140 produces a substantially uniform magnetic field in the same direction as the magnetic field of the resonant cavity 110, in this case in the z-direction (perpendicular to the plane of Figure 7). The substantially uniform magnetic field generated by the dipole magnet 140 is generated in a generally trapezium shaped region in the x-y plane. Although trapezium shaped in this embodiment, it will be appreciated that in alternative embodiments the region in the x-y plane in which the substantially uniform magnetic field is generated may have a different shape.
    [0088] The magnetic field of the dipole magnet 140 is perpendicular to the trajectories of each of the electrons in the electron bunch 134. Therefore, each of the electrons in the electron bunch 134 follows a curved trajectory through the dipole magnet 140, the curved trajectory being a circular arc. Therefore, the trajectory of each of the electrons in the electron bunches 134 is altered. Electrons that were propagating along the first end direction 130 as they enter the dipole magnet 140 propagate along a different trajectory 142 as they leave the dipole magnet 140. Similarly, electrons that were propagating along the intermediate direction 133 as they enter the dipole magnet 140 propagate along a different trajectory 144 as they leave the dipole magnet 140.
    [0089] The dipole magnet 140 is arranged such that the electrons in the centre of the bunch 134b pass through a greater length of the generally trapezium shaped region within which the substantially constant magnetic field is produced than the electrons at the front and the rear of the bunch 134a, 134c. As a result, the trajectory of the electrons in the centre of the bunch 134b is curved more than that of the electrons at the front and the rear of the bunch 134a, 134c. The difference in curvature is such that the trajectories of all of the electrons in the electron bunch 134 are substantially mutually parallel.
    [0090] Additionally or alternatively, the focusing optics may comprise a resonant cavity, as now described with reference to Figures 8 and 9. As shown in Figure 8, in one embodiment the electron beam chopper 100 further comprises a second resonant cavity 150. The second resonant cavity 150 is disposed downstream of the screen 120 such that the electrons in the electron bunches 134 that pass the screen 120 travel through the second resonant cavity 150.
    [0091] The second resonant cavity 150 may be substantially identical to the resonant cavity 110. It particular, it may comprise a generally cylindrical hollow body 152 and two apertures 154, 156 may be defined by the body 152, one on each of two opposite sides of the body 152. The second resonant cavity 150 is arranged to receive the bunched electron beam that passes the screen 120 through one aperture 154. The bunched electron beam exits the second resonant cavity 150 through the other aperture 156.
    [0092] The second resonant cavity 150 is provided with an alternating power source (not shown) that is arranged to excite a transverse magnetic mode within the second resonant cavity 150, in a manner similar to the resonant cavity 110. For example, a TM110 mode may be excited in the second resonant cavity 150. The resonant cavity 110 and the second resonant cavity 150 are excited with transverse magnetic modes of the same frequency and are phase locked to each other, in a known manner. To achieve this, the resonant cavity 110 and the second resonant cavity 150 may share a common alternating power source. In particular, the phase of the second resonant cavity 150 is set relative to that of the resonant cavity 110 so that the middle 134b of each electron bunch 134 passes through the middle of the second resonant cavity 150 when the magnetic field is at its maximum.
    [0093] Each of the electrons in the electron bunch 134 follows a curved trajectory through the second resonant cavity 150. Therefore, as with the dipole magnet 140 of Figure 7, the trajectory of each of the electrons in the electron bunches 134 is altered. Electrons that were propagating along the first end direction 130 as they enter the second resonant cavity 150 propagate along a different trajectory 142 as they leave the second resonant cavity 150. Similarly, electrons that were propagating along the intermediate direction 133 as they enter the second resonant cavity 150 propagate along a different trajectory 144 as they leave the second resonant cavity 150.
    [0094] The phase of the second resonant cavity 150 is such that the electrons in the centre of the bunch 134b experience a greater magnetic field than the electrons at the front and the rear of the bunch 134a, 134c. As a result, the trajectory of the electrons in the centre of the bunch 134b is curved more than that of the electrons at the front and the rear of the bunch 134a, 134c. As with the dipole magnet 140 of Figure 7, the difference in curvature is such that the trajectories of all of the electrons in the electron bunch 134 are substantially mutually parallel.
    [0095] Using a second cavity 150 that is identical to the resonant cavity 110, set at the appropriate relative phases, ensures (to first order and using a small angle approximation) that the trajectories of the electrons at the front 134a, centre 134b and rear 134c of the bunches 134 are all parallel and that the electrons at the front 134a and the rear 134c of the bunch 134 follow the same path.
    [0096] The arrangement of Figure 7 (using a dipole magnet 140) and the arrangement of Figure 8 (using a second resonant cavity 150) both correct for the increase in the divergence of the bunched electron beam, such that the bunched electron beam is substantially collimated.
    [0097] The electron optics may be arranged to at least partially correct for a curvature of individual electron bunches 134 of the bunched electron beam caused by the resonant cavity 110 and the screen 120. The electron optics may be arranged such that the size and/or shape of the bunched electron beam is substantially the same as that of the input electron beam.
    [0098] The focusing optics may comprise a dipole magnetic field and a second resonant cavity, as now described with reference to Figure 9. As shown in Figure 9, in one embodiment the electron beam chopper 100 further comprises a dipole magnet 160 and a second resonant cavity 170.
    [0099] The dipole magnet 160 is disposed downstream of the screen 120 such that the electrons in the electron bunches 134 that pass the screen 120 travel through the magnetic field of the dipole magnet 160. The dipole magnet 160 is similar to the dipole magnet 140 shown in
    Figure 7 and described above. In particular, the dipole magnet 160 produces a substantially uniform magnetic field in the same direction as the magnetic field of the resonant cavity 110, in this case in the z-direction (perpendicular to the plane of Figure 9). The substantially uniform magnetic field generated by the dipole magnet 160 is generated in a generally trapezium shaped region in the x-y plane.
    [00100] As with the dipole magnet 140 of Figure 7, the dipole magnet 160 of Figure 9 causes a focusing of the electron beam as a result of different electrons having different path lengths through the dipole magnet 160. However, the dipole magnet 160 has greater focusing power such that rather than collimating the bunched electron beam, the dipole magnet 160 is arranged to image each electron bunch 134 onto the second cavity 170.
    [00101] The second resonant cavity 170 is disposed downstream of the dipole magnet 160 such that the electrons in the electron bunches 134 exit the dipole magnet 160 travel through the second resonant cavity 170.
    [00102] The second resonant cavity 170 may be substantially identical to the resonant cavity 110. It particular, it may comprise a generally cylindrical hollow body 172 and two apertures 174, 176 may be defined by the body 172, one on each of two opposite sides of the body 172. The second resonant cavity 170 is arranged to receive the bunched electron beam that exits the dipole magnet 160 through one aperture 154. The bunched electron beam exits the second resonant cavity 170 through the other aperture 156.
    [00103] The second resonant cavity 170 is provided with an alternating power source (not shown) that is arranged to excite a transverse magnetic mode within the second resonant cavity 170, in a manner similar to the resonant cavity 110. For example, a TM110 mode may be excited in the second resonant cavity 170. The resonant cavity 110 and the second resonant cavity 170 are excited with transverse magnetic modes of the same frequency and are phase locked to each other, in a known manner. To achieve this, the resonant cavity 110 and the second resonant cavity 170 may share a common alternating power source. In particular, the phase of the second resonant cavity 170 is set relative to that of the resonant cavity 110 so that the middle 134b of each electron bunch 134 passes through the middle of the second resonant cavity 170 when the magnetic field is at its maximum.
    [00104] Each of the electrons in the electron bunch 134 follows a curved trajectory through the second resonant cavity 170. The phase of the second resonant cavity 170 is such that the electrons in the centre of the bunch 134b experience a greater magnetic field than the electrons at the front and the rear of the bunch 134a, 134c. As a result, the trajectory of the electrons in the centre of the bunch 134b is curved more than that of the electrons at the front and the rear of the bunch 134a, 134c.
    [00105] With the above arrangement, the trajectories of all of the electrons in the electron bunch 134 as it exits the electron beam chopper 100 (i.e. downstream of the second resonant cavity 170) are substantially mutually parallel and the curvature of the electron bunch 134 is corrected.
    [00106] Furthermore, the above arrangement (as shown in Figure 9) corrects for another form of distortion that is caused by the combination of the resonant cavity 110 and the screen 120, as now described. Although, as described above with reference to Figure 6, the electrons at the front 134a and the rear 134c of the bunch 134 propagate along substantially to same direction (the intermediate direction 133 shown in Figure 6), electrons at the front 134a and the rear 134c of an electron bunch 134 following slightly different directions since the electromagnetic field that they experience within the resonant cavity 110 is slightly different. In particular, electrons towards the front 134a of each electron bunch 134 are subject to a smaller deflection near the entrance aperture 114 of the resonant cavity 110 and a larger deflection near the exit aperture 116 of the cavity 110. In contrast, electrons towards the rear 134c of each electron bunch 134 are subject to a larger deflection near the entrance aperture 114 and smaller deflection near the exit aperture 116 of the cavity 110. As a result, the electrons towards the front 134a of each electron bunch 134 exit the resonant cavity 110 at a slightly different angle to the electrons towards the rear 134c of each electron bunch 134. In Figure 9, the trajectory followed by the electrons at the front 134a of each electron bunch 134 is shown as a dotted line; the trajectory followed by the electrons in the centre 134b of each electron bunch 134 is shown as a dashed line; and the trajectory followed by the electrons at the rear 134c of each electron bunch 134 is shown as a dot-dashed line. As each electron bunch 134 exits the resonant cavity 110, electrons at the front 134a of the bunch 134 propagate along a first intermediate direction 133a, electrons at the centre of the bunch propagate along the first end direction 130, and electrons at the rear 134c of the bunch 134 propagate along a second intermediate direction 133c. The electrons towards the front 134a of each electron bunch 134 appear to originate from a slightly different point to the electrons towards the rear 134c of each electron bunch 134. Advantageously, the use of the arrangement shown in Figure 9, wherein a dipole magnet 160 is arranged to image the electron bunches onto a second resonant cavity 170 also corrects for this effect.
    [00107] The electron beam chopper 100 described above is operable to produce a bunched electron beam wherein a single electron bunch 134 is formed during each cycle of the RF mode excited within the resonant cavity 110. With such an arrangement, the chopping frequency (and therefore the frequency of the bunches 134) is equal to the frequency of the RF mode excited within the resonant cavity 110. Such an arrangement may be advantageous over an arrangement wherein the electron beam is sampled more than once during each cycle of the RF mode excited within the resonant cavity 110. This is because bunches formed while the trajectory of the electron beam is moving in one direction may differ from those formed while the trajectory of the electron beam is moving in the opposite direction.
    [00108] Furthermore, because the output electron beam is sampled at a point where its direction is slowly moving (i.e. the rate of change of the direction of the electron beam is at a minimum when the electron beam passes the screen 120) the duty cycle of the electron beam can be increased whilst the emittance of the bunched electron beam remains low.
    [00109] The electron beam chopper 100 according to embodiments of the present invention allow the use of a continuous electron emitter as a source for a free electron laser. This has advantages over using a bunched electron beam source of the type using a photocathode that is irradiated by a pulsed laser beam to produce a bunched electron beam. For example a bunched electron source using the electron beam chopper 100 does not require either: (a) a pulsed laser; or (b) an ultra-high vacuum environment, which is required in photoemission guns for semiconductor cathodes. Furthermore, the electron beam chopper 100 according to embodiments of the present invention has an improved lifetime relative to bunched electron beam sources of the type using a photocathode that is irradiated by a pulsed laser beam to produce a bunched electron beam.
    [00110] Whilst embodiments of electron beam chopper 100 according to embodiments of the present invention have been described which use a resonant cavity 110 (e.g. a TM110 cavity) as a deflector, it will be appreciated that other deflectors may alternatively be used. The deflector may comprise any component or combination of components that is operable to receive a beam of electrons and to alter its direction in a time dependent manner. The deflector may use electric or magnetic fields or a combination of both.
    [00111] 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.
    [00112] 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.
    [00113] 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.
    [00114] 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.
    [00115] 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.
    [00116] 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.
    [00117] 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.
    [00118] Different embodiments may be combined with each other. Features of embodiments may be combined with features of other embodiments.
    [00119] 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. An electron beam chopper for forming a bunched beam of electrons, the electron beam chopper comprising: a deflector operable to receive an input beam of electrons propagating along a first axis 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; and a blocking member which is arranged to block the output beam of electrons when it is in a first sub-range of the range of directions and to allow the output beam of electrons 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. 2. The electron beam chopper of clause 1 wherein the deflector is operable to cause the direction of the output beam of electrons to oscillate between a first end direction and a second end direction. 3. The electron beam chopper of clause 2 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. 4. The electron beam chopper of clause 2 or clause 3 wherein the oscillation between the first end direction and the second end direction is sinusoidal. 5. The electron beam chopper of any preceding clause wherein the range of directions lies substantially within a plane. 6. The electron beam chopper of any preceding clause wherein the deflector comprises a resonant cavity and an alternating power source. 7. The electron beam chopper of any preceding clause wherein the blocking member comprises a wall or a screen that is provided with one or more apertures. 8. The electron beam chopper of any preceding clause further comprises electron optics arranged to alter the size and/or shape of the bunched electron beam. 9. The electron beam chopper of clause 8 wherein the electron optics comprises focusing optics arranged to reduce a divergence of the bunched electron beam. 10. The electron beam chopper of clause 9 wherein the focusing optics is arranged to substantially collimate the bunched electron beam. 11. The electron beam chopper of any one of clauses 8 to 10 wherein the electron optics is arranged such that the size and/or shape of the bunched electron beam is substantially the same as that of the input electron beam. 12. The electron beam chopper of any one of clauses 8 to 11 wherein the electron optics is arranged to at least partially correct for a curvature of individual electron bunches of the bunched electron beam caused by the deflector and blocking member. 13. A source for producing a bunched beam of electrons, the source comprising: an electron source operable to produce a beam of electrons; and the electron beam chopper of any preceding clause, wherein the deflector of the electron beam chopper is arranged to receive the beam of electrons produced by the electron source and the electron beam chopper is arranged to output a bunched beam of electrons. 14. A free electron laser comprising: the source of clause 13; 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. 15. A lithographic system comprising: the free electron laser of clause 14; 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.
    权利要求:
    Claims (1)
    [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.
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    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    FR2248586B1|1973-10-19|1976-10-01|Cig R Mev|
    WO2015067467A1|2013-11-06|2015-05-14|Asml Netherlands B.V.|Free electron laser|
    法律状态:
    优先权:
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    EP15193219|2015-11-05|
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