CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 15197311.2 which was filed on December 1, 2015 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a free electron laser (FEL). Particularly, but not exclusively, the present invention relates to a FEL suitable for use in a radiation source for a lithographic system.

BACKGROUND

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

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

[0005] It is desirable to produce EUV radiation sources with increased power to increase throughput of EUV lithography.

SUMMARY

[0006] According to a first aspect, there is provided a free electron laser comprising:

a first electron source operable to produce a bunched electron beam;

a linear accelerator arranged to impart energy to electrons in the bunched electron beam to form an accelerated electron beam;

an undulator operable to produce a periodic magnetic field and arranged so as to guide the bunched electron beam along a periodic path about a central axis of the undulator such that they interact with radiation in the undulator, stimulating emission of coherent radiation; electron beam optics arranged to direct the accelerated electron beam back into the linear accelerator after it leaves the undulator so as to extract energy from electrons in the accelerated electron beam and to form a decelerated electron beam that exits the linear accelerator in a first direction in parallel with the accelerated electron beam; and

an electron beam separator operable to separate the decelerated electron beam from the accelerated electron beam, the electron beam separator comprising a magnet system operable to generate a separator magnetic field, the separator magnetic field having a field direction that is perpendicular to the first direction and a field strength that increases in a second direction away from a point of entry of the decelerated electron beam into the separator magnetic field, the second direction being perpendicular to the field direction and at an acute angle to the first direction.

[0007] In this way, the decelerated electron beam can be decoupled from the accelerated electron beam and deflected to the beam dump without increasing the dispersion of the electrons and therefore avoiding the need to provide additional focusing elements for the deflected electron beam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0009] - Figure 1 depicts a lithographic system comprising a free electron laser according to an embodiment of the invention;

[0010] - Figure 2 depicts a lithographic apparatus that forms part of the lithographic system of Figure 1 ;

[0011] - Figure 3 is a schematic illustration of a free electron laser, which may form part of the lithographic system of Figure 1 ;

[0012] - Figure 4 is a schematic illustration of a dipole beam separator;

[0013] - Figure 5 is a schematic illustration in plan of a beam separator according to an embodiment of the invention; and

[0014] - Figure 6 is a schematic illustration of the beam separator of Figure 5 in cross-section along the line A-A.

DETAILED DESCRIPTION

[0015] Figure 1 shows a lithographic system LS, comprising: a radiation source SO, a beam splitting apparatus 20 and eight lithographic apparatuses LA1-LA8. The radiation source SO comprises a free electron laser and is configured to generate an extreme ultraviolet (EUV) radiation beam B _FEL (which may be referred to as a main beam). The main radiation beam B _FEL is split into a plurality of radiation beams B _A -B _H (which may be referred to as branch beams), each of which is directed to a different one of the lithographic apparatuses LA1-LA8, by the beam splitting apparatus 20. The branch radiation beams B _A -B _H may be split off from the main radiation beam in series, with each branch radiation beam being split off from the main radiation beam downstream from the preceding branch radiation beam. Where this is the case the branch radiation beams may for example propagate substantially parallel to each other.

[0016] The radiation source SO, beam splitting apparatus 20 and lithographic apparatuses LA1- LA8 may all be constructed and arranged such that they can be isolated from the external environment. A vacuum may be provided in at least part of the radiation source SO, beam splitting apparatus 20 and lithographic apparatuses LA1-LA8 so as to minimise the absorption of EUV radiation. Different parts of the lithographic system LS may be provided with vacuums at different pressures (i.e. held at different pressures which are below atmospheric pressure).

[0017] Referring to Figure 2, a lithographic apparatus LAI 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 LAI before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B _A ' (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LAI aligns the patterned radiation beam B _A ' with a pattern previously formed on the substrate W.

[0018] While only lithographic apparatuses are shown in Figure 2, it is to be understood that the lithographic system LS may comprise other tools, such as mask inspection apparatuses.

[0019] The branch radiation beam B _A that is received by the lithographic apparatus LAI passes into the illumination system IL from the beam splitting apparatus 20 through an opening 8 in an enclosing structure of the illumination system IL. Optionally, the branch radiation beam B _A may be focused to form an intermediate focus at or near to the opening 8.

[0020] 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 B _A with a desired cross-sectional shape and a desired angular distribution. The faceted field mirror device 10 and faceted pupil mirror device 11 may each comprise an array of independently movable mirrors. The faceted field mirror device 10 and faceted pupil mirror device 11 may comprise different numbers of independently movable mirrors. For example the faceted pupil mirror device 11 may comprise twice as many mirrors as the faceted field mirror device 10. The mirrors in the faceted field mirror device 10 and faceted pupil mirror device 11 may be of any suitable shape, for example, they may be generally banana- shaped. The radiation beam B _A passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam to form a patterned beam B _A ' . The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 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 MEMS devices.

[0021] Following reflection from the patterning device MA the patterned radiation beam B _A ' enters the projection system PS. The projection system PS comprises a plurality of mirrors 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).

[0022] The radiation source SO comprises a free electron laser FEL according to an embodiment of the present invention, which is operable to produce a beam of EUV radiation. Optionally, the radiation source SO may comprise more than one free electron laser FEL according to an embodiment of the present invention.

[0023] The radiation source SO may further comprise optics arranged to alter the size and/or shape of the cross section of the radiation beams received from the free electron laser.

[0024] The optics may comprise beam expanding optics arranged to increase the cross sectional area of the radiation beam output by that free electron laser. Advantageously, this decreases the heat load on mirrors downstream of the beam expanding optics. This may allow the mirrors downstream of the beam expanding optics to be of a lower specification, with less cooling, and therefore less expensive. Additionally or alternatively, it may allow the downstream mirrors to be nearer to normal incidence. The beam splitting apparatus 20 may comprise a plurality of static extraction mirrors arranged in the path of the beam B^ _L which direct radiation from the main beam B^ _L along the plurality of branch radiation beams B _A -B _H . Increasing the size of the main beam B _KEL reduces the accuracy with which the mirrors must be located in the beam B _FEL path. Therefore, this allows for more accurate splitting of the output beam B _FEL by the splitting apparatus 20.

[0025] The radiation source SO may further comprise shape altering optics which are arranged to alter the cross sectional shape of the radiation beams received from the free electron laser. The shape altering optics may comprise one or more astigmatic or a-spherical optical elements. The shape altering optics and beam expanding optics may share common optical elements.

[0026] A free electron laser comprises an electron source, which is operable to produce a bunched relativistic electron beam, and a periodic magnetic field through which the bunches of relativistic electrons are directed. The periodic magnetic field is produced by an undulator and causes the electrons to follow an oscillating path about a central axis. As a result of the acceleration caused by the magnetic field the electrons spontaneously radiate electromagnetic radiation generally in the direction of the central axis. The relativistic electrons interact with radiation within the undulator. Under certain conditions, this interaction causes the electrons to bunch together into microbunches, modulated at the wavelength of radiation within the undulator, and coherent emission of radiation along the central axis is stimulated.

[0027] Referring to Figure 3, a free electron laser FEL comprises an injector 21, electron beam coupler 25, a linear accelerator 22, an undulator 24, a beam separator 26 and a beam dump 100.

[0028] The injector 21 is arranged to produce a bunched electron beam EBi with a first energy Ei. The injector 21 comprises an electron source such as, for example, a thermionic cathode or photo- cathode and an accelerating electric field. Preferably the bunched electron beam EBi has a relatively low emittance, for example below 1 mm mRad. The first energy Ei may be, for example, around 5-20 MeV. The first energy Ei may be around 10-15 MeV, which may be preferable since it may allow the emittance of the bunched electron beam EBi to remain below 1 mm mRad.

[0029] The bunched electron beam EBi is coupled into a linear accelerator 22 by electron beam coupler 25. Linear accelerator 22 accelerates the electrons to a second, higher energy E ₂ to form an accelerated electron beam EB ₂ . In an example, the linear accelerator 22 may comprise a plurality of resonant cavities, which are axially spaced along a common axis, and one or more radio frequency power sources, which are operable to control the electromagnetic fields along the common axis as bunches of electrons pass between them so as to accelerate each bunch of electrons. The resonant cavities may be superconducting radio frequency cavities. Advantageously, this allows: relatively large electromagnetic fields to be applied at high duty cycles; larger beam apertures, resulting in fewer losses due to wakefields; and for the fraction of radio frequency energy that is transmitted to the beam (as opposed to dissipated through the cavity walls) to be increased. Alternatively, the resonant cavities may be conventionally conducting (i.e. not superconducting) radio frequency cavities and may be formed from, for example, copper. Other types of linear accelerators may also be used. The injector 21 and linear accelerator 22 together form an electron source that is operable to produce relativistic electrons.

[0030] Optionally, the bunched electron beam EBi and/or the accelerated electron beam EB ₂ may pass through a bunch compressor (not shown). The bunch compressor may be disposed downstream or upstream of the linear accelerator 22. The bunch compressor may be configured to bunch electrons and spatially compress existing bunches of electrons.

[0031] The accelerated electron beam EB ₂ then passes through the undulator 24. The undulator 24 comprises a plurality of magnets, which are operable to produce a periodic magnetic field and arranged so as to guide the relativistic electrons produced by the injector 21 and linear accelerator 22 along a periodic path. As a result, the electrons radiate electromagnetic radiation generally in the direction of a central axis of the undulator 24. The path followed by the electrons may be sinusoidal and planar, with the electrons periodically traversing the central axis, or may be helical, with the electrons rotating about the central axis. The type of oscillating path may affect the polarization of radiation emitted by the free electron laser. For example, a free electron laser which causes the electrons to propagate along a helical path may produce elliptically polarized radiation, which may be preferred for exposure of a substrate W by the lithographic apparatuses LA1-LA8.

[0032] The undulator 24 comprises a plurality of sections, each section comprising a periodic magnet structure. The undulator 24 may further comprise a mechanism for refocusing the accelerated electron beam EB ₂ such as, for example, a quadrupole magnet in between one or more pairs of adjacent sections. The mechanism for refocusing the accelerated electron beam EB ₂ may reduce the size of the electron bunches, which may improve the coupling between the electrons and the radiation within the undulator 24, increasing the stimulation of emission of radiation.

[0033] As electrons move through the undulator 24, they interact with the electric field of the radiation, exchanging energy with the radiation. In general the amount of energy exchanged between the electrons and the radiation will oscillate rapidly unless conditions are close to a resonance condition, given by:

(1)

where X _em is the wavelength of the radiation, X _u is the undulator period, γ is the Lorentz factor of the electrons and K is the undulator parameter. A is dependent upon the geometry of the undulator 24: for a helical undulator A=l, whereas for a planar undulator A=2. In practice, each bunch of electrons will have a spread of energies although this spread may be minimised as far as possible (by producing an electron beam EBi with low emittance). The undulator parameter K is typically approximately 1 and is given b

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

[0034] The resonant wavelength X _em is equal to the first harmonic wavelength spontaneously radiated by electrons moving through the undulator 24. The free electron laser FEL may operate in self-amplified stimulated emission (SASE) mode. Operation in SASE mode may require a low energy spread of the electron bunches in the accelerated electron beam EB ₂ before it enters the undulator 24. Alternatively, the free electron laser FEL may comprise a seed radiation source, which may be amplified by stimulated emission within the undulator 24.

[0035] Electrons moving through the undulator 24 may cause the amplitude of radiation to increase, i.e. the free electron laser FEL may have a non-zero gain. When the resonance condition of Eq. (1) is satisfied, the gain of the free electron laser FEL may be zero. Maximum gain may be achieved when conditions are close to but slightly off resonance.

[0036] The interaction between the electrons and radiation within the undulator 24 produces a spread of energies within the electron bunches in the accelerated electron beam EB ₃ . The electron beam exiting the undulator 24 may be considered to be a different electron beam with a spread of energies, and is referred to herein as the downstream electron beam EB ₃ . The energy spread in the downstream electron beam EB ₃ is dependent upon the conversion efficiency of the undulator 24. Quantitatively, the width of the energy spread in the downstream electron beam EB ₃ may be given by a product of a conversion efficiency of the undulator 24 and the second energy E ₂ .

[0037] An electron which meets the resonance condition as it enters the undulator 24 will lose (or gain) energy as it emits (or absorbs) radiation, so that the resonance condition is no longer satisfied. Therefore, in some embodiments the undulator 24 may be tapered. That is, the amplitude of the periodic magnetic field and/or the undulator period X _u may vary along the length of the undulator in order to keep bunches of electrons at or close to resonance as they are guided though the undulator 24. Note that the interaction between the electrons and radiation within the undulator 24 produces a spread of energies within the electron bunches. The tapering of the undulator 24 may be arranged to maximise the number of electrons at or close to resonance. For example, the electron bunches may have an energy distribution which peaks at a peak energy and the tapering maybe arranged to keep electrons with this peak energy at or close to resonance as they are guided though the undulator 24. Advantageously, tapering of the undulator has the capacity to significantly increase conversion efficiency. The use of a tapered undulator may increase the conversion efficiency (i.e. the portion of the energy of the electron beam EBi which is converted to radiation in the radiation beam BKE _L ) by more than a factor of 2. The tapering of the undulator may be achieved by reducing the undulator parameter K along its length. This may be achieved by matching the undulator period X _u and/or the magnetic field strength B ₀ along the axis of the undulator to the electron bunch energy to ensure that they are at or close to the resonance condition. Meeting the resonance condition in this manner increases the bandwidth of the emitted radiation.

[0038] A beam of radiation BKE _L propagates from the undulator 24. The radiation beam BKE _L comprises EUV radiation. The beam of EUV radiation BKE _L output by the free electron laser FEL may have a substantially circular cross section and a Gaussian intensity profile. The radiation beam produced by an EUV free electron laser typically has a relatively small etendue. In particular, the EUV radiation beam BKE _L produced by the free electron laser FEL has a significantly smaller etendue than an EUV radiation beam that would be generated by a laser produced plasma (LPP) source or a discharge produced plasma (DPP) source (both of which are known in the prior art). For example, the radiation beam BFE _L produced by the free electron laser FEL may have a divergence less than

500|irad, for example less than 100|irad, and may for example have a diameter of around 50μηι as it leaves the undulator 24. [0039] The output power of the free electron laser FEL may be of the order of tens of kilowatts, in order to support high throughput for the eight EUV lithographic apparatus LA1-LA8. At these powers, since the initial diameter of the radiation beam B _FEL produced by the free electron laser FEL is so small, the power density of the radiation beams B _a _ _h will be significant.

[0040] In order for the output power of the free electron laser FEL to be sufficient to support high throughput for a plurality of EUV lithographic apparatus LA1-LA8, the free electron laser FEL may have certain properties. For example, the second energy E ₂ that the linear accelerator 22 accelerates the electrons of the bunched electron beam EBi to may be around 500 to 1000 MeV. The power of the accelerated electron beam EB ₂ may be of the order of 1 to 100 MW. The power of the accelerated electron beam EB ₂ may be dictated by a desired power of the output beam of EUV radiation B _KEL and the conversion efficiency of the undulator 24. For a given output power of the free electron laser FEL, the higher the conversion efficiency of the undulator 24 is, the lower the current of the injector 21 will be. Higher undulator 24 conversion efficiencies and lower injector 21 currents may be highly desirable.

[0041] After leaving the undulator 24, energy is extracted from the downstream electron beam EB ₃ , i.e. the electrons are decelerated, and then the downstream electron beam EB ₃ is absorbed by a dump 100. The dump 100 may comprise a sufficient quantity of material to absorb the electron beam EB ₃ . 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. Thus it is desirable to reduce the energy of electrons in the electron beam E ₂ before they enter the dump 100. This removes, or at least reduces, the need to remove and dispose of radioactive waste from the dump 100. This is advantageous since the removal of radioactive waste requires the free electron laser FEL to be shut down periodically and the disposal of radioactive waste can be costly and can have serious environmental implications.

[0042] The energy of the electrons may be reduced to below 10 MeV and, preferably, below 5MeV. Advantageously, electrons below this energy do not induce any significant level of radioactivity in the beam dump 100. During operation of the free electron laser FEL, gamma radiation will be present but when the electron beam E is switched off, the beam dump 100 will be safe to handle.

[0043] The linear accelerator 22 that is used to accelerate electrons output from the injector 21 is also used for deceleration. The electron bunches of downstream electron beam EB ₃ are injected by electron beam coupler 25 into the linear accelerator 22 with a phase difference of around 180 degrees relative to the radio frequency (RF) field in the linear accelerator 22. Such an arrangement is known as an energy recovering LIN AC (ERL). However, there is a limit to the spread of electron energies within an electron bunch that such an arrangement can accept. The undulator 24 will introduce a spread in the energy of the electrons as they pass through the undulator 24. Electron bunches are injected into the linear accelerator 22 with a phase difference of 180 degrees relative to radio frequency (RF) field and so electrons with slightly higher energy than average are decelerated more than electrons with slightly lower than average energy. As a result, the absolute distribution of energies of electrons in the downstream electron beam EB ₃ after deceleration is reduced but the relative distribution of energies is increased because of the large decrease in average energy. The downstream electron beam after deceleration may also be referred to as a decelerated electron beam.

[0044] At the exit of the linear accelerator 22 it is necessary to separate the accelerated electron beam EB ₂ from the downstream electron beam EB ₃ . The accelerated electron beam EB ₂ and the downstream electron beam EB ₃ propagate through the linear accelerator 22 along the same path. Therefore, an electron beam separator 26 is provided so that the downstream electron beam EB ₃ can be sent to the dump 100 and the accelerated beam EB ₂ passes to the undulator 24.

[0045] Conventionally, it has been proposed to separate the downstream electron beam EB ₃ from the accelerated electron beam EB ₂ using a dipole separator. A dipole separator simply applies a uniform magnetic field across the beam path causing the electron beams EB ₂ , EB ₃ to bend. The degree of deflection to the electron beams caused by a dipole separator is dependent upon the energies of the electrons in the respective beams. Thus, as shown in Figure 4, the accelerated electron beam EB ₂ is deflected by dipole separator DS by only a small amount, whilst the downstream electron beam EB ₃ is deflected by a greater angle and so is soon physically separate from the accelerated electron beam EB ₂ and can then be further steered into the dump 100 without affecting the accelerated electron beam EB ₂ . However, due to the distribution of energies of electrons in the downstream electron beam EB ₃ , deflected electron beam EB ₄ is divergent rather than collimated. Therefore, additional mechanisms must be provided to collect, refocus and collimate the deflected electron beam EB ₄ so as to be able to transport it to the dump.

[0046] According to an embodiment of the present invention, an achromatic electron beam separator 200 as depicted in Figures 5 and 6, forms the electron beam separator 26 shown in Figure 3. The electron beam separator 200 comprises a magnet system comprising two pole pieces 201, 202 configured to generate a non-uniform magnetic field. The non-uniform magnetic field, referred to herein as a separator magnetic field, is arranged such that all electrons of the downstream electron beam EB ₃ are deflected by substantially the same angle and therefore exit the electron beam separator 200 along substantially the same path. Therefore the electron beam separator can be described as achromatic. As depicted in Figure 5 however, not all electrons follow the same path through the achromatic beam separator 200, higher energy electrons will be deflected with a higher radius of curvature.

[0047] Of course accelerated electron beam EB ₂ is also deflected by electron beam separator 200, as also depicted in Figure 5. However, since the electrons of accelerated electron beam EB ₂ are much faster than the electrons of downstream electron EB ₃ , the amount of deflection of accelerated electron beam EB2 is much less than that of downstream electron beam EB ₃ . In an embodiment of the present invention, the extent of the separator magnetic field in a plane parallel to the direction of propagation of downstream electron beam EB ₃ and the direction of propagation of deflected electron beam EB ₄ is greater than twice the radius of curvature of the fastest electrons in downstream electron beam EB ₃ but smaller than the radius of curvature of the slowest electrons in accelerated electron beam EB ₂ . In an embodiment the width and/or length of the separator magnetic field in that plane is in the range of 200 mm to 1 m.

[0048] The deflection of accelerated electron beam EB ₂ by electron beam separator 200 can be taken account of in the physical position of undulator 24 relative to electron beam separator 200 or an additional steering magnet system can be provided to reverse the deflection imparted by electron beam separator 200. If necessary, an additional focusing magnet system can be provided to counteract any undesired divergence (dispersion) that may have been introduced to the accelerated electron beam EB ₂ by electron beam separator 200. In an embodiment, two or more dipoles are used advantageously to form a chicane for the accelerated electron beam EB ₂ which corrects both the path and dispersion of the accelerated electron beam EB ₃ .

[0049] The exact form and strength of the separator magnetic field depends on the desired angle of deflection (i.e. the angle between the directions of propagation of downstream electron beam EB ₃ and deflected electron beam EB ₄ ) and the speed of the electrons. In an embodiment of the invention, the separator magnetic field can be defined in a Cartesian co-ordinate system (x, y, z) with the directions of propagation of downstream electron beam EB ₃ and deflected electron beam EB ₄ being substantially parallel to the xy plane. In this co-ordinate system, the separator magnetic field is substantially uniform in y and z but varies in x. The field strength of the separator magnetic field is substantially proportional to x ⁿ , where n is the field index. In an embodiment, the field index is in the range of from 0.6 to 1, desirably 0.7 to 0.9, desirably about 0.8. In an embodiment of the present invention, the projection of the downstream electron EB ₃ onto the xy plane enters the separated field at the origin O and at an angle of incidence to the x axis in the range of from 30 degrees to 60 degrees, desirably 40 degrees to 50 degrees, desirably about 45 degrees. In an embodiment of the present invention, the field index n is about 0.8 and the angle of incidence of the downstream electron beam EB ₃ is about 45 degrees. This results in a deflection (angle between downstream electron beam EB ₃ and deflected electron beam EB ₄ ) of about 90 degrees.

[0050] As illustrated in Figure 6, another desirable property of the separator magnetic field is that the deflected electron beam EB ₄ is displaced in the z direction relative to downstream electron beam EB ₃ . In an embodiment, if the downstream electron beam EB ₃ enters the separator field a distance dl away from the mid-plane 203, then the deflected electron beam EB ₄ will exit the separator field a distance d2 on the opposite side of the mid-plane 203 where dl is substantially equal to d2. This vertical deflection is desirable because it minimises or avoids interaction between the incoming downstream electron beam EB ₃ and the outgoing deflected electron beam EB ₄ , which cross in the xy plane. Because the beam diameters of accelerated electron beam EB ₂ and downstream electron beam EB ₃ are small, only a few mm or a few 10s of mm of vertical displacement within electron beam separator 200 is sufficient to avoid interaction between deflected beam EB ₄ and the input beams: accelerated electron beam EB ₂ and downstream electron beam EB ₃ .

[0051] As shown in Figure 5, a shielding plate 204 is provided to limit the extent of separator magnetic field and is provided with an aperture to allow entry and exit of the various electron beams.

[0052] In an embodiment of the present invention, the average field strength of the separator magnetic field may be in the range of from about 10 mT to 1 T, e.g. from 100 mT to 500 mT. Such a field strength is not difficult to achieve through permanent magnets, electromagnets or a combination of permanent magnets and electromagnets. In an embodiment of the present invention, the separator magnetic field is generated using permanent magnets to create most of the field strength and electromagnets to provide fine-tuning of the magnetic field and achieve the exact desired field profile. Such an arrangement can provide a desired magnetic field with high efficiency. The pole pieces 201, 202 may be joined by a yoke to close the field lines and confine the field to the desired region. The yoke can be positioned to avoid interfering with the paths of the various electron beams.

[0053] The theory behind the achromatic beam separator is described in H. Enge, Rev. Sci. Instrum. 34 (1963) 385-389. The beam separator is sometimes referred to as an Enge magnet, alpha magnet or pretzel magnet.

[0054] Electron beam coupler 25, which couples the electron beam that has left undulator 24 and the bunched electron beam EBi into the linear accelerator 24 with the appropriate phase relationship, can be a dipole magnet system. In an embodiment, the electron beam coupler 25 is an achromatic beam separator as described above operating in reverse. In other words, the incoming beams enter the separator magnetic field (which in this usage could be called a coupler magnetic field) at different positions and from different directions and leave the coupler magnetic field at or near the origin and travelling in the same direction.

[0055] Although the described embodiment of lithographic system LS comprises eight lithographic apparatuses LA1-LA8, a lithographic system according to an embodiment of the invention may comprise any number of lithographic apparatuses.

[0056] A lithographic system according to an embodiment of the invention may further comprise one or more mask inspection apparatuses. The beam splitting apparatus 20 may direct a portion of the main radiation beam B _FEL to the mask inspection apparatus. The mask inspection apparatus may use this radiation to illuminate a mask and uses an imaging sensor to monitor radiation reflected from the mask MA. The mask inspection apparatus may include optics (e.g. mirrors) configured to receive a branch radiation beam from beam splitting apparatus 20 and direct the radiation beam at a mask. The mask inspection apparatus may further include optics (e.g. mirrors) configured to collect radiation reflected from the mask and form an image of the mask at the imaging sensor. The mask inspection apparatus may be similar to the lithographic apparatus LAI shown in Figure 2, with the substrate table WT replaced with an imaging sensor. In some embodiments, the lithographic system may comprise two mask inspection apparatuses to allow for some redundancy. This may allow one mask inspection apparatus to be used when the other mask inspection apparatus is being repaired or undergoing maintenance. Thus, one mask inspection apparatus is always available for use. A mask inspection apparatus may use a lower power radiation beam than a lithographic apparatus.

[0057] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.

[0058] The lithographic apparatuses LA1-LA8 may be used in the manufacture of ICs.

Alternatively, the lithographic apparatuses LA1-LA8 described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin- film magnetic heads, etc..

[0059] 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.