Radiation Source

专利摘要:
A radiation source for an electron injector comprising a first laser configured to emit a first radiation beam with a first wavelength and a second laser configured to emit a second radiation beam with a second wavelength, an optical amplifier configured to amplify the radiation beams, one or more electrically controllable optical devices located upstream of the optical amplifier, the one or more electrically controllable optical devices being configured to switch between allowing the first radiation beam to enter the optical amplifier and allowing the second radiation beam to enter the optical amplifier, and an optical element with wavelength dependent transmission located downstream of the optical amplifier, the optical element with wavelength dependent transmission being configured to allow the amplified first radiation beam to travel towards a photocathode of the electron injector, and to prevent the amplified second radiation beam from travelling towards the photocathode of the electron injector.
公开号:NL2018274A
申请号:NL2018274
申请日:2017-02-01
公开日:2017-09-07
发明作者:Alexandrovich Nikipelov Andrey；Yevgenyevich Banine Vadim；Joep Engelen Wouter；Jacobus Hendrik Brussaard Gerrit
申请人:Asml Netherlands Bv；
IPC主号:

专利说明:

Radiation Source
FIELD
[0001] The present invention relates to a radiation source. The radiation source may provide a radiation beam for use by an electron injector. The electron injector may form part of a free electron laser. The free electron laser may form part of a lithographic system comprising one or more lithographic apparatuses.
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 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).
[0004] EUV radiation for use in one or more lithographic apparatus may be produced by one or more free electron lasers. A free electron laser may comprise at least one electron injector which is configured to receive radiation from a radiation source and emit electrons . It is an object of the invention to obviate or mitigate one or more problem associated with a radiation source for an electron injector.
SUMMARY
[0005] According to a first aspect of the invention there is provided a radiation source for an electron injector, the radiation source comprising a first laser configured to emit a first radiation beam with a first wavelength and a second laser configured to emit a second radiation beam with a second wavelength, an optical amplifier configured to amplify the first radiation beam and provide an amplified first radiation beam, and configured to amplify the second radiation beam and provide an amplified second radiation beam, one or more electrically controllable optical devices located upstream of the optical amplifier, the one or more electrically controllable optical devices being configured to switch between allowing the first radiation beam to enter the optical amplifier and allowing the second radiation beam to enter the optical amplifier, and an optical element with wavelength dependent transmission located downstream of the optical amplifier, the optical element with wavelength dependent transmission being configured to allow the amplified first radiation beam to travel towards a photocathode of the electron injector, and to prevent the amplified second radiation beam from travelling towards the photocathode of the electron injector.
[0006] Undesirable transients in the gain of the optical amplifier may be avoided by the first aspect of the invention because one of the first radiation beam or the second radiation beam is always present in the optical amplifier (thereby avoiding an increased population inversion which would occur if no radiation beam was present in the optical amplifier).
[0007] The same electrically controllable optical device may be configured to receive both the first radiation beam and the second radiation beam. This may be advantageous because it avoids any need to synchronise between different electrically controllable optical devices.
[0008] The first radiation beam may have a first polarization when it enters the electrically controllable optical device and the second radiation beam may have a second polarization when it enters the electrically controllable optical device.
[0009] The polarization of the first radiation beam may be orthogonal to the polarization of the second radiation beam when the radiation beams enter the electrically controllable optical device.
[0010] The electrically controllable optical device may be configured to change the polarizations of the first and second radiation beams, the electrically controllable optical device being switchable between a first state and a second state, with the change of polarization applied by the electrically controllable optical device to the radiation beams being determined by whether the electrically controllable optical device is in the first state or is in the second state.
[0011] The change of polarization applied by the electrically controllable optical device may be a zero change in either the first state or the second state. The change of polarization may be a polarization rotation.
[0012] The radiation source may further comprise an optical element with polarization dependent transmission provided downstream of the electrically controllable optical device and upstream of the optical amplifier.
[0013] A first electrically controllable optical device may be configured to receive the first radiation beam and a second electrically controllable optical device may be configured to receive the second radiation beam.
[0014] The radiation source may further comprise a controller configured to synchronize switching of the first and second electrically controllable optical devices.
[0015] The first electrically controllable optical device may be configured to change the polarization of the first radiation beam, and the second electrically controllable optical device may be configured to change the polarization of the second radiation beam, the electrically controllable optical devices being switchable between first states and second states, with the change of polarization applied by the electrically controllable optical devices to the radiation beams being determined by whether the electrically controllable optical devices are in the first state or are in the second state.
[0016] The change of polarization applied by the first electrically controllable optical device may be a zero change in either the first state or the second state. The change of polarization may be a polarization rotation. Similarly, the change of polarization applied by the second electrically controllable optical device may be a zero change in either the first state or the second state. The change of polarization may be a polarization rotation.
[0017] The radiation source may further comprise an optical element with polarization dependent transmission provided downstream of the first electrically controllable optical device, and an optical element with polarization dependent transmission provided downstream of the second electrically controllable optical device.
[0018] At least one of the electrically controllable optical devices may be a Pockels cell.
[0019] According to a second aspect of the invention there is provided a radiation source for an electron injector, the radiation source comprising a laser beam source configured to provide a polarized radiation beam, an electrically controllable optical device configured to change the polarization of the polarized radiation beam, the electrically controllable optical device being switchable between a first state and a second state, with the change of polarization applied by the electrically controllable optical device to the polarized radiation beam being determined by whether the electrically controllable optical device is in the first state or is in the second state, an optical amplifier configured to amplify the polarized radiation beam whilst maintaining polarization of the polarized radiation beam, the optical amplifier providing an amplified polarized radiation beam, and an optical element with polarization dependent transmission configured to allow the amplified polarized radiation beam to travel towards a photocathode of the electron injector when it has a first polarization, and to prevent the amplified polarized radiation beam from travelling towards the photocathode of the electron injector when it has a second polarization.
[0020] The change of polarization applied by the electrically controllable optical device may be a zero change in either the first state or the second state. The change of polarization may be a polarization rotation.
[0021] The second aspect of the invention is advantageous because it requires only a single laser beam source and may therefore be easier and/or cheaper to implement than the first aspect of the invention.
[0022] The laser beam source may comprise a laser and a polarizer.
[0023] The electrically controllable optical device may be a Pockels cell.
[0024] The optical amplifier may be a polarization maintaining fibre optical amplifier.
[0025] According to a third aspect of the invention there is provided a radiation source for an electron injector, the radiation source comprising a laser configured to provide a radiation beam, an optical amplifier configured to amplify the radiation beam, optics configured to expand the amplified radiation beam, an array of electrically controllable optical devices configured to receive the expanded amplified radiation beam and to output a plurality of sub-beams of radiation, the electrically controllable optical devices being configured to change the polarizations of the radiation sub-beams, the electrically controllable optical devices being switchable between first states and second states, with the change of polarization applied by the electrically controllable optical devices to the radiation sub-beams being determined by whether the electrically controllable optical devices are in the first state or in the second state, optics configured to combine the radiation sub-beams to re-form the amplified radiation beam, and an optical element with polarization dependent transmission configured to allow the amplified radiation beam to travel towards a photocathode of the electron injector when it has a first polarization, and to prevent the amplified radiation beam from travelling towards the photocathode of the electron injector when it has a second polarization.
[0026] The third aspect of the invention is advantageous because it allows the optical amplifier to continuously amplify the radiation beam and thereby avoid undesirable gain transients. The third aspect of the invention is additionally advantageous because it avoids undesirable nonlinear effects which may arise in an electrically controllable optical device if a high power radiation beam is present in that device.
[0027] The optics configured to expand the amplified radiation beam may comprise at least one grating or at least one prism.
[0028] The optics configured to expand the amplified radiation beam may comprise at least one lens.
[0029] The array of electrically controllable optical devices may be a two dimensional array.
[0030] The array of electrically controllable optical devices may be an array of Pockets cells.
[0031] According to a fourth aspect of the invention there is provided a free electron laser comprising a radiation source according to any of the first to third aspects of the invention, an electron source configured to receive a radiation beam from the radiation source and emit an electron beam, a particle accelerator configured to accelerate the electron beam output from the electron source, and an undulator operable to guide the accelerated electron beam along a periodic path so as to stimulate emission of radiation, thereby forming a radiation beam.
[0032] According to a fifth aspect of the invention there is provided a lithographic system comprising a free electron laser according to the fourth aspect of the invention and at least one lithographic apparatus arranged to receive at least a portion of radiation provided by the radiation source.
[0033] One or more aspects of the invention may include one or more features of any of the other aspects of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] 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 forms part of the lithographic system of Figure 1;
Figure 3 is a schematic illustration of a free electron laser according to an embodiment of the invention;
Figure 4 is a schematic illustration of an electron injector according to an embodiment of the invention;
Figures 5A and 5B schematically illustrate a radiation source according to an embodiment of the invention;
Figures 6A and 6B schematically illustrate a radiation source according to another embodiment of the invention;
Figures 7A and 7B schematically illustrate a radiation source according to another embodiment of the invention;
Figures 8A and 8B schematically illustrate a radiation source according to another embodiment of the invention; and
Figures 9A and 9B schematically illustrate a radiation source according to another embodiment of the invention;
DETAILED DESCRIPTION
[0035] Figure 1 shows a lithographic system LS according to one embodiment of the invention. The lithographic system LS comprises a EUV radiation source SO, a beam delivery system BDS and a plurality of lithographic apparatuses LAa-LAn (e.g. ten lithographic apparatuses). The EUV radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B (which may be referred to as a main beam). The EUV radiation source SO and the beam delivery system BDS may together be considered to form a radiation system, the radiation system being configured to provide radiation to one or more lithographic apparatuses LAa-LAn.
[0036] The beam delivery system BDS comprises beam splitting optics and may optionally also comprise beam expanding optics and/or beam shaping optics. The main radiation beam B is split into a plurality of radiation beams Ba-B„ (which may be referred to as branch beams), each of which is directed to a different one of the lithographic apparatuses LAa-LAn, by the beam delivery system BDS.
[0037] In an embodiment, the branch radiation beams Ba-Bn are each directed through a respective attenuator (not shown in Figure 1). Each attenuator may be arranged to adjust the intensity of a respective branch radiation beam Ba-Bn before the branch radiation beam Ba-Bn passes into its corresponding lithographic apparatus LAa-LAn.
[0038] The EUV 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 EUV radiation source SO, beam delivery system BDS and lithographic apparatuses LAa-LAn so as to reduce the absorption of EUV radiation. Different parts of the lithographic system LS may be provided with vacuums at different pressures (i.e. held at different pressures which are below atmospheric pressure).
[0039] 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 Bathat 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.
[0040] 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.
[0041] The illumination system IL may include a field facet mirror 10 and a pupil facet mirror 11. The field facet mirror 10 and pupil facet mirror 11 together provide the radiation beam 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 field facet mirror 10 and pupil facet mirror 11. The illumination system IL may, for example, include an array of independently moveable mirrors. The independently moveable mirrors may, for example, measure less than 1 mm across. The independently moveable mirrors may, for example, be microelectromechanical systems (MEMS) devices.
[0042] 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).
[0043] 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.
[0044] Referring again to Figure 1, the EUV radiation source SO is configured to generate an EUV radiation beam B with sufficient power to supply each of the lithographic apparatuses LAa-LAn. As noted above, the EUV radiation source SO may comprise a free electron laser.
[0045] Figure 3 is a schematic depiction of a free electron laser FEL comprising an electron source 21, a linear accelerator 22, a bunch compressor 23, an undulator 24, an electron decelerator 26 and a beam dump 30. The electron source 21 may be referred to as an electron injector 21.
[0046] The electron injector 21 is arranged to produce a bunched electron beam E and comprises a photo-cathode and an accelerating electric field. Electrons in the electron beam E are further accelerated by the linear accelerator 22. In an example, the linear accelerator 22 may comprise a plurality of radio frequency cavities, which are axially spaced along a common axis, and one or more radio frequency power sources, which are operable to control electromagnetic fields along the common axis as bunches of electrons pass between them so as to accelerate each bunch of electrons. The cavities may be superconducting radio frequency cavities. Advantageously, this allows: relatively large electromagnetic fields to be applied at high duty cycles; larger beam apertures, resulting in fewer losses due to wakefields; and for the fraction of radio frequency energy that is transmitted to the beam (as opposed to dissipated through the cavity walls) to be increased. Alternatively, the cavities may be conventionally conducting (i.e. not superconducting), and may be formed from, for example, copper. Other types of linear accelerators may be used such as, for example, laser wake-field accelerators or inverse free electron laser accelerators.
[0047] Optionally, the electron beam E passes through a bunch compressor 23, disposed between the linear accelerator 22 and the undulator 24. The bunch compressor 23 is configured to spatially compress existing bunches of electrons in the electron beam E. One type of bunch compressor 23 comprises a radiation field directed transverse to the electron beam E. An electron in the electron beam E interacts with the radiation and bunches with other electrons nearby. Another type of bunch compressor 23 comprises a magnetic chicane, wherein the length of a path followed by an electron as it passes through the chicane is dependent upon its energy. This type of bunch compressor may be used to compress bunches of electrons which have been accelerated in a linear accelerator 22 by a plurality of resonant cavities.
[0048] The electron beam E then passes through the undulator 24. Generally, the undulator 24 comprises a plurality of modules (not shown). Each module comprises a periodic magnet structure, which is operable to produce a periodic magnetic field and is arranged so as to guide the relativistic electron beam E produced by the electron 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.
[0049] 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.
[0050] 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 λαη is the wavelength of the radiation, λ„ 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.
[0051] 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.
[0052] 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.
[0053] 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 ku 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 λ„ 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.
[0054] Radiation produced within the undulator 24 is output as a radiation beam BFEL.
[0055] After leaving the undulator 24, the electron beam E is absorbed by a dump 30. The dump 30 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 30 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 30. This removes, or at least reduces, the need to remove and dispose of radioactive waste from the dump 30. 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.
[0056] The energy of electrons in the electron beam E may be reduced before they enter the dump 30 by directing the electron beam E through a decelerator 26 disposed between the undulator 24 and the beam dump 30.
[0057] 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 electron 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 electron 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 electron injector 21. Such an arrangement is known as an energy recovery linear accelerator (ERL).
[0058] In some embodiments of a lithographic system LS the EUV radiation source SO may comprise a single free electron laser FEL. In such embodiments the main beam B which is emitted from the EUV radiation source SO may be a laser beam BFEL which is emitted from the free electron laser FEL. In other embodiments, the EUV radiation source SO may comprise a plurality of free electron lasers. A plurality of laser beams BFEL emitted from the free electron lasers may be combined to form a single main beam B comprising radiation emitted from the plurality of free electron lasers FEL.
[0059] Figure 4 is a schematic illustration of an embodiment of the electron injector 21 (which may alternatively be referred to as the electron source). The electron injector 21 is configured to provide a beam of electrons E which propagate substantially along an axis 30 of the electron injector 21. The electron injector 21 comprises a photocathode 32 and an anode 34 located in a housing 35 (which may be a vacuum housing). In an embodiment (not depicted) the housing 35 may act as the anode, in which case a separate anode is not required. The photocathode 32 comprises an electron emitting target region 36 which is configured to emit electrons when illuminated with radiation.
[0060] The target region 36 is illuminated with a radiation beam 100, emitted from a radiation source 102. Embodiments of the radiation source 102 are depicted in figures 5 to 9 and described further below. In the embodiment which is shown in Figure 4, the radiation beam 100 is directed to be incident on the target region 36 of the photocathode 32 by a mirror 38. In some embodiments, the radiation source 102 may be separate from the electron injector 21 and may not form part of the electron injector 21. The radiation beam 100 may be delivered to be incident on the target region 36 by a beam delivery system. The beam delivery system may, for example, comprise the mirror 38 and/or other optical components not shown in Figure 4. The beam delivery system may, for example, comprise optical components configured to condition the radiation beam 100 prior to the radiation beam 100 being incident on the target region 36.
[0061] Energy from photons of the radiation beam 100 is absorbed by electrons in the photocathode 32. This energy excites electrons in the photocathode 32 to higher energy states. Some electrons are excited to a high enough energy state that they are able to escape the photocathode 32 such that electrons are emitted from the target region 36. Radiation stimulated emission of electrons such as this is commonly referred to as the photoelectric effect.
[0062] The target region 36 may, for example, be formed from a metal. Alternatively the target region 36 may comprise a semiconductor material. For example, the target region 36 may be formed from one or more of gallium arsenide (GaAs), sodium potassium antimonide (NaKSb) and caesium potassium antimonide (CsK2Sb). The quantum efficiency of the target region 104 may decrease with use.
[0063] A potential difference is held between the photocathode 32 and the anode 34. In particular, the photocathode 32 is held at a lower voltage than the anode 34, such that electrons 106 which are emitted from the photocathode 32 are accelerated away from the photocathode 32 by the potential difference. The potential difference between the photocathode 32 and the anode 34 may, for example, be approximately several hundred kilovolts.
[0064] The potential difference between the photocathode 32 and the anode 34 establishes an electric field between the photocathode 32 and the anode 34. The anode 34 includes an opening 40, which may for example be annular. The axis 30 extends through the opening 40. The photocathode 32 and the anode 34 are configured to generate an electric field which causes electrons 106 emitted from the target region 36 of the photocathode 32 to pass through the opening 40 in the anode 34.
[0065] An electron booster 119 serves to accelerate the electron bunches along the axis 30, and outputs an accelerated electron beam E. The electron booster 33 may, for example, accelerate electron bunches to relativistic energies. In some embodiments the electron booster 119 may accelerate electron bunches to energies in excess of approximately 5 MeV. In some embodiments, the electron booster 119 may accelerate electron bunches to energies which do not exceed approximately 20 MeV. However, in other embodiments the electron booster 119 may accelerate electron bunches to energies in excess of about 20 MeV.
[0066] The radiation beam 100 may be a pulsed radiation beam, thereby causing electrons 106 to be emitted from the photocathode 32 in bunches, which correspond to the pulses of the radiation beam 100. In such embodiments, the electron beam E which is provided by the electron source 21 is a bunched electron beam E. The radiation source 102 may, for example, be configured to provide pulses of radiation with a duration of approximately a few picoseconds. The bunched electron beam E may for example comprise electron bunches with a charge which is between 10 and 100 pC (e.g. around 70 pC). The bunched electron beam E may for example have a bunch repetition rate which is between 100 and 1,000 MHz. For example, the repetition rate may be around 650 MHz (which corresponds with a bunch separation of around 1½ nanoseconds).
[0067] It is desirable to be able to interrupt the radiation beam 100 incident upon the photocathode 36 of the electron injector 21 without causing an undesirable transient in the radiation beam 100 when it is once again incident upon the photocathode. This in turn avoids causing undesirable transients in properties of electron bunches emitted from the photocathode 36. Interruption of the radiation beam 100 and associated interruption of the electron beam of the free electron laser may be useful for a variety of different reasons. Being able to provide this interruption without introducing undesirable transients into electron bunch properties is therefore particularly advantageous.
[0068] A first reason for interrupting the radiation beam is to provide so-called clearing gaps in the electron beam, for example when the free electron laser is operating in a high power mode (i.e. providing an electron beam which may be used to generate EUV radiation for lithographic apparatuses). These clearing gaps are interruptions in the electron beam which are sufficiently long to allow positive ions to escape from the free electron laser and thereby avoid an undesirable buildup of positive ions. In the clearing gaps the electron beam current may be reduced to zero or reduced to a very low level (e.g. less than 5% of the electron beam current during normal operation of the free electron laser). The electron beam may comprise repeating pulse trains separated by clearing gap interruptions of around one hundred nanoseconds. The pulse trains may have a duration of around 5 ps. This corresponds with a clearing gap repetition rate of around 200 kHz. It is not essential that a transition from electron bunches being present to no electron bunches being present takes place within a single pulse repetition time period. Instead, for example, the transition may take place over a period which encompasses a small number of electron bunches (e.g. five electron bunches or less). For example, switching from electron bunches being present to no electron bunches being present may take place within around ten nanoseconds or less, and may take place within less than five nanoseconds. Switching a radiation beam at this speed is achievable, for example using a Pockels cell.
[0069] In a second mode of operation, a clearing gap may be provided in the electron beam but a small number of electron bunches (e.g. between one and ten electron bunches) are provided within the clearing gap. This may be done for example to facilitate monitoring of the operation of the free electron laser. The pulses within the clearing gap, which may be referred to as witness pulses, are operated upon by components of the free electron laser without being significantly affected by other electron bunches in the free electron laser (e.g. decelerating electron bunches in the case of an energy recovering free electron laser). The witness pulses may be provided within a subset of the clearing gaps. That is, some clearing gaps may include witness pulses and other clearing gaps may include no witness pulses. The repetition rate of witness pulses may for example be between 1 kHz and 100 kHz.
[0070] In a third mode of operation, which may be referred to as a diagnostic mode, trains with small numbers of electron bunches (e.g. between one and ten electron bunches) are used. A low repetition rate between these trains of electron bunches may be used, for example up to 10 KHz, such that the power of the electron beam is relatively low. Although the power of the electron beam is relatively low the energy of the individual electron bunches is the same as during operation of the free electron laser in the high power mode. The diagnostic mode provides operation of the free electron laser with a low average current, and this allows diagnostic systems to be used to, for example, measure the position of the electron beam in the free electron laser.
[0071] Embodiments of the invention may allow these modes of operation to be achieved.
[0072] One way of interrupting the electron beam E is by interrupting the radiation beam 100 which is incident upon the photocathode 32. This may be achieved for example by controlling the radiation beam using a Pockels cell. Pockels cells are advantageous because they are capable of providing switching at a speed of ten nanoseconds or less. A Pockels cell used for switching the radiation beam 100 may have a transverse dimension of less than 5 mm in order to avoid requiring an excessively large switching voltage and/or having a large capacitance. This in turn means that the power density of the radiation beam 100 travelling through the Pockels cell is relatively high. The high power in the Pockels cell could cause non-linear effects in the Pockels cell (e.g. thermal lensing, non-linear absorption and/or thermal dependency of the birefringence index). Similar considerations apply to acousto-optical modulators. Embodiments of the invention may address this problem.
[0073] Attempting to switch a lower power radiation beam off and on using a Pockels cell (or other switching element), and then applying downstream amplification to the radiation beam may introduce undesirable transients into the electron beam E. The radiation beam may for example be amplified by an optical amplifier which is optically pumped using LEDs. These LEDs may have a switching time which is significantly slower than ten nanoseconds. Consequently, the optical amplifier will continue to be pumped when it is not amplifying the radiation beam. This will cause a population inversion of the optical amplifier to increase, and thus the gain of the optical amplifier will be increased compared with a steady state gain of the optical amplifier. As a result, when the radiation beam is unblocked and re-enters the optical amplifier it will be amplified to a higher power than during steady state operation. This transient increase of the power of the amplified radiation beam is undesirable because it will generate bigger bunches of electrons at the photocathode 32 (compared with steady state operation), and may modify other properties of the electron bunches. The transient changes of electron bunch properties will cause a fluctuation in the wavelength of radiation which is emitted from the free electron laser. This in turn may have a negative impact on the accuracy with which lithographic apparatuses LAa.n are able to expose patterns on substrates. Thus, a transient variation in the gain of the optical amplifier may have a negative impact on the accuracy with which lithography can be achieved. Embodiments of the invention may address this problem.
[0074] Figures 5A and 5B schematically depict a radiation source 102 according to an embodiment of the invention. The radiation source comprises a first laser 110 and a second laser 120. The first and second lasers 110, 120 emit radiation beams 112, 114 at different wavelengths (which may respectively be referred to as a first wavelength and a second wavelength). The lasers 110,120 may be fibre lasers, as is schematically illustrated, or may be other forms of laser. The lasers 110, 120 may for example be ytterbium doped fibre lasers. The first laser 110 may for example emit a radiation beam with a wavelength of 1020 nm and the second laser 120 may for example emit a radiation beam with a wavelength of 1030 nm. The radiation beams may for example have a bandwidth of less than 1 nm.
[0075] Polarizers 111, 121 are arranged to polarize the radiation beams 112, 114 emitted by the first and second lasers 110, 120. The polarizers 111, 121 may polarize the radiation beams 112, 114 such that they are linearly polarized and orthogonal to one another. In an alternative arrangement the first and second lasers 110, 120 may emit polarized radiation, for example with the first radiation beam 112 having a linear polarization which is orthogonal to the linear polarization of the second radiation beam 114. Where this is the case the polarizers 111, 121 may be omitted from the radiation source 102. In a further alternative arrangement the radiation beams may have polarizations other than linear polarizations, for example circular polarizations. Where this is the case one or more additional elements, such as a quarter wave plate, may be used to convert the polarizations to orthogonal linear polarizations before polarization-based separation of the radiation beams (or polarization-based merging of the radiation beams). These alternative arrangements may be used in connection with other embodiments of the invention.
[0076] A dichroic mirror 130 receives the first radiation beam 112 and the second radiation beam 114. In the depicted embodiment the second radiation beam 114 is directed to the dichroic mirror 130 by a mirror 122 whereas the first radiation beam 112 travels directly to the dichroic mirror. However, it would be appreciated that any suitable arrangement of optical elements may be used to direct the radiation beams 112, 114 to the dichroic mirror 130 (or indeed that the radiation beams may travel to the dichroic mirror without being directed by optics). The dichroic mirror 130 is an example of an optical element with wavelength dependent transmission. Other optical elements with wavelength dependent transmission may be used. For example, a grating which transmits at one wavelength and reflects at another wavelength may be used. In an alternative arrangement a polarising beam combiner may be used instead of the dichroic mirror 130. These alternative arrangements may be used in connection with other embodiments of the invention.
[0077] The dichroic mirror 130 is transmissive at the wavelength of the first radiation beam 112 (the first wavelength), and is reflective at the wavelength of the second radiation beam 114 (the second wavelength). Thus, as depicted, the first radiation beam 112 passes through the dichroic mirror 130 whereas the second radiation beam 114 is reflected by the dichroic mirror. As a result, both the first and second radiation beams 112,114 propagate in the same direction when they leave the dichroic mirror 130. The radiation beams 112, 114 may be coaxial, although they are depicted as being separated from each other in Figure 5 for ease of illustrating the invention.
[0078] A Pockels cell 131 receives the first and second radiation beams 112, 114. The Pockels cell transmits the first and second radiation beams 112, 114. The Pockels cell changes the polarization of the radiation beams when a voltage is applied across the
Pockels cell (e.g. rotates the polarization). In Figure 5A a voltage is not applied across the Pockels cell 131, whereas in Figure 5B a voltage is applied across the Pockels cell. This is indicated schematically by a 0 in the Pockels cell in Figure 5A and a 1 in the Pockels cell in Figure 5B. The Pockels cell may be referred to as being in a first state in Figure 5A and in a second state in Figure 5B. Thus, when the Pockels cell is in the first state it does not change the polarizations of the first and second radiation beams 112, 114. When the Pockels cell is in the second state it rotates the polarization of the first radiation beam 112 by 90 degrees and also rotates the polarization of the second radiation beam 114 by 90 degrees. The voltage applied across the Pockels cell is controlled by a controller CT. The Pockels cell is an example of an electrically controllable optical device, and any suitable electrically controllable optical device may be used. Another example of a suitable electrically controllable optical device is a Kerr cell. The Kerr cell may be used to control the polarizations of the first and second radiation beams 112,114 in an equivalent manner to the Pockels cell. This also applied in relation to other embodiments of the invention.
[0079] A polarizing beam splitter 132 receives the first and second radiation beams 112, 114 after they have left the Pockels cell 131. The polarizing beam splitter 132 is configured to reflect s-polarized radiation and to transmit p-polarized radiation. In this embodiment the first radiation beam 112 is p-polarized and the second radiation 114 is s-polarized. Thus, when the Pockels cell 131 is in the first state and does not change the polarization of the first and second radiation beams 112, 114, the polarizing beam splitter 132 transmits the first radiation beam 112 and reflects the second radiation beam 114. The second radiation beam 114 is directed to a beam dump 133.
[0080] An optical amplifier 140 receives the first radiation beam 112. The depicted optical amplifier 140 is a fibre optical amplifier, but any suitable form of optical amplifier may be used. The optical amplifier 140 amplifies the first radiation beam and outputs an amplified first radiation beam 113.
[0081] The amplified first radiation beam 113 travels to a second dichroic mirror 150 which is transmissive at the first wavelength but reflective at the second wavelength. Since the second dichroic mirror 150 is transmissive at the first wavelength the amplified first radiation beam 113 passes through the dichroic mirror. A frequency doubling unit 160 then receives the amplified first radiation beam 113. The frequency doubling unit 160 acts to double the frequency of the amplified first radiation beam (or equivalently to halve the wavelength of the first radiation beam). For example, the wavelength of the radiation may be halved from around 1 micron to around 500 nm. A further dichroic mirror 170 is located downstream of the frequency doubling unit 160 and is configured to transmit radiation which has been frequency doubled and to reflect radiation which has not been frequency doubled. The dichroic mirror 170 thus removes any residual radiation which was not frequency doubled by the frequency doubling unit 160. The resulting radiation beam 100 is output from the radiation source 102. The output radiation beam 100 is directed towards a photocathode 32 of an electron injector 21 in order to generate electrons, for example as depicted in Figure 4. Residual radiation which has not been frequency doubled is directed by the dichroic mirror 170 towards a beam dump 172.
[0082] The amplified radiation beam 113 does not itself reach the electron injector 21, but instead is frequency doubled by the frequency doubling unit 160. Nevertheless, the second dichroic mirror 150 may be considered to allow the amplified first radiation beam 113 to travel towards the electron injector 21 because the amplified first radiation beam travels along a path which ultimately leads to the electron injector.
[0083] The output radiation beam 100 may be sufficiently powerful to generate a desired current of electron bunches at the photocathode 32 of the electron injector 21 (see Figure 4). The output radiation beam 100 may for example have an average power of around 10 W and may for example have a peak power of around 1 kW. These powers may also apply in relation to other embodiments of the invention.
[0084] Referring to Figure 5B, when the Pockels cell 131 is in the second state (i.e. a voltage is applied across the Pockels cell) the polarizations of the first and second radiation beams 112, 114 are rotated by 90 degrees. As a result, the first radiation beam 112 is rotated from being p-polarized to being s-polarized, and the second radiation beam 114 is rotated from being s-polarized to being p-polarized. Since the polarizing beam splitter cube 132 is reflective for s-polarized radiation, the first radiation beam 112 (which is now s-polarized) is reflected by the polarizing beam splitter 132 and is incident upon the beam dump 133. Since the second radiation beam 114 is now p-polarized, the second radiation beam 114 is transmitted by the polarizing beam splitter 132 and enters the optical amplifier 140.
[0085] The second radiation beam 114 is amplified by the optical amplifier 140. The resulting amplified second radiation beam 115 then travels to the dichroic mirror 150. The dichroic mirror 150 is reflective for the wavelength of the amplified second radiation beam 115 (the second wavelength) and thus reflects the second radiation beam towards a beam dump 151. Consequently, the amplified second radiation beam 115 does not enter the frequency doubling unit 160 and does not travel towards the photocathode of the electron injector.
[0086] The embodiment depicted in Figures 5A and 5B allows the radiation source 102 to be switched between a first state in which it provides a radiation beam that is incident upon the photocathode 32 of the electron injector (see Figure 4) and a second state in which it does not provide a radiation beam which is incident upon the photocathode. In both states a radiation beam passes through the optical amplifier 140.
[0087] An advantage of the embodiment of the invention (and other embodiments of the invention) is that the optical amplifier 140 continues to amplify a radiation beam even when no radiation beam is incident upon the photocathode 32 of the electron injector 21. Undesirable transient variation of the gain of the optical amplifier 140 is avoided in embodiments of the invention by switching between providing the first radiation beam 112 or the second radiation beam 114 to the optical amplifier 140. Since the optical amplifier 140 always receives a radiation beam, the undesirable transient variation of the gain of the optical amplifier is avoided.
[0088] Although the first and second radiation beams 112, 114 have different wavelengths, the amplification provided by the optical amplifier 140 may be substantially equal for both radiation beams. The optical amplifier 140 may for example be an ytterbium doped fibre. As mentioned above, the first and second radiation beams may have wavelengths of 1020 nm and 1030 nm respectively. The ytterbium doped fibre optical amplifier 140 will provide substantially equal amplification to the first and second radiation beams. Consequently, a build-up (or depletion) of gain of the optical amplifier and an associated subsequent undesirable transient in the output radiation beam 100 is avoided.
[0089] The dichroic mirrors 130,150 may be sufficiently wavelength specific to reflect a radiation beam with a wavelength of 1020 nm and transmit a radiation beam with a wavelength of 1030 nm (or vice versa).
[0090] The polarizing beam splitter 132 and the dichroic mirror 150 may operate imperfectly. That is, they may have an extinction ratio which is not exactly 100:0. For example, the polarizing beam splitter 132 may transmit a small proportion (e.g. 1% or less) of the radiation beam which it is intended to reflect. This may for example be due to imperfect operation of the Pockels cell 131 or imperfect operation of the polarizing beam splitter 132. Similarly, the second dichroic mirror 150 may transmit a small proportion (e.g. 1% or less) of the second radiation beam 114. As a result of these imperfections, some radiation of the second wavelength may travel to the frequency doubling unit 160. However, the frequency doubling unit 160 is a nonlinear optical element, and provides a frequency doubled output beam with a power which scales as the square of the incident radiation beam. Thus, if 1% or less of the radiation received at the frequency doubling unit 160 is the second radiation beam, then the amount of frequency doubled radiation which is generated from the second radiation beam will be «1%. This is a negligible amount which will not cause significant electron emission from the photocathode.
[0091] In an embodiment, the first dichroic mirror 130 may be replaced with a polarizing beam splitter (which may also be referred to as a polarizing beam combiner since it combines radiation beams). Since the first and second radiation beams 112, 114 have orthogonal polarizations they will be transmitted and reflected by the polarizing beam splitter in a manner which corresponds with transmission and reflection provided by the dichroic mirror 130. The polarizing beam splitter may have a lower extinction ratio than the dichroic mirror. However, the extinction ratio is not an important consideration when combining radiation beams.
[0092] The power of the first radiation beam 112 in the Pockels cell 131 may be sufficiently low to avoid undesirable nonlinear effects. For example, the power of the first radiation beam 112 may be less than 0.1 W. The peak power of the first radiation beam 112 may be less than 10 W. The same may apply for the second radiation beam 114. Similar conditions may apply in relation to other embodiments of the invention in which a Pockels cell is provided upstream of an optical amplifier).
[0093] Figures 6A and 6B schematically depict a radiation source 102 according to an alternative embodiment of the invention. The alternative embodiment is similar to the embodiment depicted in Figures 5A and 5B, and also switches between providing first and second radiation beams to an optical amplifier. However, in the embodiment depicted in Figures 6A and 6B the first and second radiation beams are switched using different Pockels cells instead of being switched using the same Pockels cell.
[0094] Referring first to Figure 6A, a first laser 210 (e.g. a fibre laser) emits a first radiation beam 212 with a first wavelength. The first radiation beam 212 passes through a polarizer 211 that transmits p-polarized radiation and then passes to a first Pockels cell 229. In Figure 6A a voltage is applied across the first Pockels cell 229 and the first Pockels cell rotates the polarization of the first radiation beam 212 by 90 degrees. The first radiation beam 212 is s-polarized when it enters the first Pockels cell 229 and is p-polarized when it leaves the first Pockels cell 229. The p-polarized first radiation beam 212 is then incident upon a first polarizing beam splitter 232. The first polarizing beam splitter 232 is transmissive for p-polarized radiation, and thus the first radiation beam 212 passes through the polarizing beam splitter. The first radiation beam 212 then enters a first dichroic mirror 230 which is transmissive at the wavelength of the first radiation beam 212. The first radiation beam 212 thus passes through the first dichroic mirror 230 and travels towards an optical amplifier 240.
[0095] The first radiation beam 212 enters the optical amplifier 240 and is amplified by the optical amplifier. An amplified first radiation beam 213 is emitted from the first optical amplifier 240 and passes to a second dichroic mirror 250. The second dichroic mirror 250 is transmissive at the wavelength of the amplified first radiation beam 213, and the amplified first radiation beam therefore passes through the second dichroic mirror. The second dichroic mirror 250 thus allows the amplified first radiation beam 213 to travel towards the electron injector.
[0096] The amplified first radiation beam 213 enters a frequency doubling unit 260 where the frequency of the amplified first radiation beam is doubled. A further dichroic mirror 270 is located downstream of the frequency doubling unit 260 and is configured to transmit radiation which has been frequency doubled and to reflect radiation which has not been frequency doubled. The further dichroic mirror 270 thus removes any residual radiation which was not frequency doubled by the frequency doubling unit 260. The resulting radiation beam 100 is output from the radiation source 102. The output radiation beam 100 is incident upon the photocathode 32 of the electron injector 21 which emits electrons (see Figure 4). Residual radiation which has not been frequency doubled is reflected by the further dichroic mirror 270 towards a beam dump 272.
[0097] A second laser 220 (e.g. a fibre laser) emits a radiation beam 214 at a second wavelength (which is different from the wavelength of the first radiation beam). The second radiation beam 214 passes through a polarizer 221 which transmits s-polarized radiation and enters a second Pockels cell 231. In Figure 6A no voltage is applied to the second Pockels cell 231 and the polarization of the second radiation beam 214 is thus not rotated by the second Pockels cell 231. The second radiation beam 214 is s-polarized when it enters the second Pockels cell 231 and is also s-polarized when it leaves the second Pockels cell. The second radiation beam 214 is incident upon a second polarizing beam splitter 236. The second polarizing beam splitter 236 is reflective for s-polarized radiation, and thus reflects the second radiation beam 214. The second radiation beam travels to a beam dump 342. Thus, the second radiation beam 214 does not enter the optical amplifier 240 when no voltage is applied to the second Pockels cell 231 (as is depicted in Figure 6A).
[0098] A controller CT controls the voltages applied to the first and second Pockels cells 229, 231. The controller CT synchronises the voltages which are applied such that when a voltage is applied to the first Pockels cell 229 no voltage is applied to the second Pockels cell 231, and when no voltage is applied to the first Pockels cell 229 a voltage is applied to the second Pockels cell 231. The synchronisation may include synchronising switching between states of the first and second Pockels cells 229, 231 such that when one Pockels cell is being switched the other Pockels cell is also being switched. The synchronisation may be sufficiently precise that the first and second radiation beams 212, 214 do not both simultaneously enter the optical amplifier 240 at their full power (this could cause an undesirable transient fluctuation of the power of the output radiation beam 100). The synchronisation may be sufficiently precise that there is no period of time during switching when the optical amplifier 240 receives no radiation beam. The synchronisation may be sufficiently precise to avoid a transient in the gain of the optical amplifier that would have a significant impact upon the power of the output radiation beam 100 and a significant impact upon the wavelength of the radiation beam emitted by the free electron laser. In this context the term ‘significant impact’ may be considered to be a wavelength change which is large enough have a measureable detrimental effect upon the accuracy with which patterns are projected onto substrates by the lithographic apparatuses LAi„n. The synchronisation may for example be to within an accuracy of around 1 ns or better.
[0099] Figure 6B depicts the embodiment after the controller CT has switched the voltages applied to the first and second Pockels cells 229, 231. Specifically, in Figure 6B no voltage is applied to the first Pockels cell 229 but a voltage is applied to the second Pockels cell 231. The first Pockels cell 229 thus does not apply a polarization rotation to the first radiation beam 212, but the second Pockels cell 231 applies a polarization rotation of 90 degrees to the second radiation beam 214. The first radiation beam 212 is s-polarized when it leaves the first Pockels cell 229 and is reflected by the polarizing beam splitter 232 to a beam dump 332. The second radiation beam 214 is p-polarized when it leaves the second Pockels cell and is transmitted by the second polarizing beam splitter 236. The second radiation beam 214 is then reflected by a mirror 322 to the first dichroic mirror 230.
[00100] The second radiation beam 214 is reflected by the first dichroic mirror 230 and enters the optical amplifier 240 where it is amplified. The resulting amplified second radiation beam 215 is then reflected by the second dichroic mirror 250 and is incident upon a beam dump 251 (it does not travel towards the electron injector). Thus, a radiation beam is not output from the radiation source 102 to the photocathode of the free electron laser (no radiation beam is output). However, although no radiation beam is output from the radiation source 102 the optical amplifier 240 continues to amplify a radiation beam (the second radiation beam 214) and thereby avoids undesirable gain fluctuations being caused by the optical amplifier 240.
[00101] Switching between states of the Pockels cells 229, 231 may take around 10 ns. During this period the polarization rotation applied by each Pockels cell may change gradually. Thus, for example, during switching the proportion of the first radiation beam 212 entering the optical amplifier 240 may reduce gradually whilst at the same time the proportion of the second radiation beam 214 entering the optical amplifier may increase gradually. Consequently, the total power of radiation entering the optical amplifier 240 may be substantially constant during switching of the Pockels cells 229,231.
[00102] In an embodiment, the first Pockels cell 229 may be in the state shown in Figure 6A for between around 1 ps and around 10 ps (e.g. around 5 ps) and may be in the state shown in Figure 6B for between 50 ns and 150 ns (e.g. around 100 ns). Similarly, the second Pockels cell 231 may be in the state shown in Figure 6A for between around 1 ps and around 10 ps (e.g. around 5 ps) and may be in the state shown in Figure 6B for between around 50 ns and 150 ns (e.g. around 100 ns).
[00103] Various modifications of the embodiment depicted in Figures 6A and 6B are possible (and are possible for other embodiments). Thus, for example, the polarizers 211, 221 are optional and may be omitted if the lasers 210, 220 emit radiation which is polarized. Although the lasers 210, 220 are depicted as fibre lasers any suitable form of laser may be used. Similarly, although the optical amplifier 240 is depicted as a fibre amplifier any suitable optical amplifier may be used. The first dichroic mirror 230 may be replaced with a polarizing beam splitter.
[00104] For brevity, the embodiment depicted in Figures 6A and 6B has been described in less detail than the embodiment depicted in Figures 5A and 5B. It will be appreciated that details of the embodiment of Figures 5A and 5B are equally applicable to the embodiment of Figures 6A and 6B (e.g. wavelengths of the first and second radiation beams). Similarly, advantages arising from the embodiment of Figures 5A and 5B may also arise from the embodiment of Figures 6A and 6B.
[00105] Figures 7A and 7B schematically depict a further alternative radiation source 102 according to an embodiment of the invention. This embodiment has similarities with previously depicted embodiments in that it uses a Pockets cell to control a radiation beam which enters an optical amplifier, but in this embodiment the Pockets cell does not switch between radiation beams of different wavelengths but instead switches the polarization of the radiation beam which enters the optical amplifier.
[00106] Referring first to Figure 7A, a laser 310 (e.g. an optical fibre laser) emits a radiation beam 312. The radiation beam 312 passes through a polarizer 311 which transmits s-polarized radiation and is then incident upon a Pockels cell 330. In Figure 7 A a voltage is applied to the Pockels cell 330 (the voltage being controlled by a controller CT) and consequently the Pockels cell 330 rotates the polarization of the radiation beam 312 by 90 degrees. The radiation beam 312 is s-polarized when it enters the Pockels cell 330 but is p-polarized when it leaves the Pockels cell. The radiation beam 312 enters an optical amplifier 340. In this embodiment the optical amplifier 340 is a polarization maintaining optical amplifier (e.g. a polarization maintaining optical fibre amplifier). This means that when a polarized radiation beam 312 enters the optical amplifier 340 the optical amplifier will emit a polarized amplified radiation beam 313. The polarization output from the optical amplifier 340 may be the same as the polarization input to the optical amplifier. Alternatively, the polarization may be rotated, for example if the optical amplifier 340 is a fibre amplifier and is twisted. For simplicity, in the illustrated embodiment the radiation beam 312 is p-polarized when it enters the optical amplifier 340 and the amplified radiation beam 313 is also p-polarized.
[00107] The amplified radiation beam 313 is incident upon a polarizing beam splitter 350. The polarizing beam splitter 350 is reflective for s-polarized radiation and transmissive for p- polarized radiation. Thus, the p-polarized radiation beam 313 passes through the polarizing beam splitter 350 (and thus travels towards the electron injector 21). The amplified radiation beam 313 is frequency doubled by a frequency doubling unit 360 and is then incident upon a dichroic mirror 370. The dichroic mirror transmits the frequency doubled radiation. This is output as radiation beam 100 from the radiation source 102 and then passes to the photocathode of the free electron laser (see Figure 4). Radiation which has not been frequency doubled is reflected by the dichroic mirror 370 to a beam dump 372.
[00108] Figure 7B depicts the radiation source in a state when a radiation beam is not required at the photocathode. In Figure 7B the laser 310 emits a radiation beam 312 which travels through the polarizer 311 and is incident as an s-polarized beam at the Pockels cell 330. However, in Figure 7B no voltage is applied to the Pockels cell 330 and thus the Pockels cell does not rotate the polarization of the radiation beam 312. The radiation beam 312 is s-polarized when it enters the Pockels cell 330 and continues to be s-polarized when it leaves the Pockels cell. The radiation beam 312 enters the optical amplifier 340. The optical amplifier 340 amplifies the radiation beam and emits an amplified radiation beam 313. The amplified radiation beam 313 is also s-polarized. The amplified radiation beam 313 is incident upon the polarizing beam splitter 350 which reflects the amplified radiation beam (dues to its s-polarization) to a beam dump 351. Thus, the amplified radiation beam 313 does not travel towards the electron injector. The amplified radiation beam 313 does not pass through the frequency doubling unit 360 and no radiation beam is output from the radiation source 102.
[00109] The embodiment depicted in Figures 7A,B is advantageous over the embodiments depicted in Figures 5A,B and 6A,B in that it uses only one laser 310 instead of using two lasers. However, the embodiment of Figure 7A,B depends upon the optical amplifier 340 maintaining the polarization of the radiation beam 312 when it amplifies that radiation beam. If the optical amplifier 340 does not maintain the polarization of the radiation beam 312 to a sufficiently accurate degree then unwanted generation of electrons at the photocathode may occur. Where there is a risk that this will happen it may be preferred to use the embodiment of Figure 5A,B or the embodiment of Figure 6A,B (or some other embodiment which uses wavelength to discriminate between two radiation beams). Although the embodiment depicted in Figure 7A,B uses a polarization maintaining optical fibre amplifier 340, other forms of optical amplifier may be used (provided that these maintain polarization of the radiation beam).
[00110] The embodiments depicted in Figures 5A,B-7A,B all have in common that a Pockels cell 130, 229, 231, 330 is used to change the polarization of a radiation beam before that radiation beam is amplified by an optical amplifier 140, 240, 340. This is desirable because once the radiation beam has been amplified by the optical amplifier its power may be so high that it causes undesirable nonlinear effects in the Pockels cell. In alternative embodiments of the invention, switching of the radiation beam may be performed after amplification by an optical amplifier, with the radiation beam being separated into a plurality of sub-beams that pass through different Pockels cells. By separating the radiation beam into a plurality of sub-beams which pass through different Pockels cells, the power of radiation passing through each Pockels cell may be reduced to a level such that it does not cause undesirable nonlinear effects in that Pockels cell.
[00111] Figures 8A and 8B schematically depict a radiation source 102 according to an embodiment in which an amplified radiation beam is expanded using a grating or prism then split into a plurality of sub-beams. Figures 9A and 9B schematically depict an embodiment in which an amplified radiation beam is expanded using lenses then separated into a plurality of sub-beams. The grating, prism and lenses are all examples of optics configured to expand the amplified radiation beam.
[00112] Referring first to Figure 8A, a laser 410 emits a radiation beam 412. The radiation beam 412 enters an optical amplifier 440 where it is amplified. The optical amplifier 440 emits an amplified radiation beam 413 which is incident upon a polarizer 411. The polarizer 411 transmits p-polarized radiation but reflects s-polarized radiation. Consequently, the amplified radiation beam 413 is p-polarized when it leaves the polarizer 411. The amplified radiation beam 413 is incident upon a semi-transparent mirror 480 (e.g. a 50% reflective mirror). The first radiation beam 413 passes through the semi-transparent mirror 480 (with reduced power) and passes to a grating 481. The grating 481 acts to expand the amplified radiation beam 413 into a fan 414.
[00113] The amplified radiation beam 413 is pulsed (for reasons explained further above). Separation of the radiation beam into the fan 414 utilises a wavelength spread which naturally arises due to the pulsed nature of the amplified radiation beam 413. The radiation beam 412, may for example have pulses with a duration of around 10ps. A radiation pulse of around 10ps will contain a non-negligible bandwidth of wavelengths. The amplified radiation beam 413 thus includes a spread of wavelengths and, as a result, when it is incident upon the grating 481 the grating expands the amplified radiation beam. The amplified radiation beam 413, which may be expanded in one direction or in two directions, is schematically depicted as the fan 414.
[00114] The expanded radiation beam 414 is incident upon a second grating 482. The second grating 482 is separated from the first grating 481 by a distance which is selected to provide a desired size of expanded radiation beam (taking into account an angle of divergence applied by the first grating). The second grating collimates the expanded radiation beam 414 and directs it towards an array of Pockels cell 430a-e.
[00115] The Pockels cells 430a-e do not provide a continuous area upon which the expanded radiation beam 414 is incident, but instead provide an array of discrete areas which are spatially separated from each other. Consequently, the Pockels cells act to separate the expanded radiation beam 414 into an array of sub-beams 414a-e when the expanded radiation beam enters the Pockels cells.
[00116] The voltages applied to the Pockels cells 430a-e are controlled by a controller CT. In Figure 8A no voltage is applied to the Pockels cells 430a-e and the Pockels cells thus do not modify the polarization of the sub-beams 414a-e. The sub-beams 414a-e are p-polarized when they enter the Pockels cells 430a-e and are p-polarized when they leave the Pockels cells. The sub-beams 414a-e are incident upon a mirror 484 and pass back through the Pockels cells 430a-e. Again, the Pockels cells 430a-e do not modify the polarization of the sub-beams 414a-e.
[00117] The sub-beams 414a-e pass back through the second grating 482 then back through the first grating 481. The first grating 481 combines the sub-beams to re-form the amplified radiation beam 413. The amplified radiation beam 413 is reflected by the semitransparent mirror 480 and passes to a polarizing beam splitter 450. The amplified radiation beam 413 is p-polarized because the polarization has not been changed by the Pockels cells 430a-e. The polarizing beam splitter 450 is transmissive for p-polarized radiation and thus transmits the amplified radiation beam 413. The amplified radiation beam thus travels towards the electron injector. A frequency doubling unit 460 doubles the frequency of the amplified radiation beam 413 and provides an output radiation beam 100. The output radiation beam 100 is directed to a photocathode of the electron injector (see Figure 4). Residual radiation which is not frequency doubled by the frequency doubling unit 460 is reflected by a dichroic mirror 470 to a beam dump 472.
[00118] In Figure 8B the controller CT has applied voltages to the Pockels cells 430a-e. Thus, the Pockels cells 430a-e change the polarization of the radiation sub-beams 414a-e when the sub-beams pass through the Pockels cells. In this embodiment the Pockels cells 430a-e apply a quarter wave polarization change to the sub-beams (although other changes of polarization may be applied). The quarter wave polarization change modifies the p-polarization of the sub-beams 414a-e to circular polarization.
[00119] The sub-beams 414a-e are incident upon the mirror 484 and are reflected back through the Pockels cells 430a-e. The Pockels cells 430a-e apply a further quarter wave change to the sub-beams 414a-e. This changes the polarization of the sub-beams from circular polarization to s-polarization. Thus, a cumulative polarization rotation of around 90 degrees is applied to the sub-beams 414a-e by the Pockels cells 430a-e.
[00120] The sub-beams 414a-e pass back through the gratings 482, 481 and are reflected by the semi-transparent mirror 480 in the manner described above. The resulting re-formed radiation beam 413 is incident upon the polarizing beam splitter 450. However, the radiation beam 413 is now s-polarized and so is reflected by the polarizing beam splitter 450 and does not travel to the electron injector. The radiation beam 413 is instead incident upon a beam dump 451. Thus, when the radiation beam is s-polarized no output radiation beam is provided from the radiation source 102.
[00121] Therefore, in common with other embodiments, the controller CT, by controlling the voltage applied to the Pockets cells 430a-e, switches between a radiation beam 100 being output from the radiation source 102 and no radiation beam being output from the radiation source. Because the optical amplifier 440 continuously amplifies the radiation beam 412 without interruption, undesirable transients which would be caused by such interruptions are avoided in the radiation beam 100 output from the radiation source 102.
[00122] The gratings 481, 482 may for example correspond with gratings configured for use in a chirped pulse compressor. The gratings 481,482 may be reflective gratings or may be transmissive gratings. In an embodiment, prisms may be used instead of either or both of the gratings 481,482.
[00123] The polarization change which is provided by the Pockets cells 430a-e may include some wavelength dependency. Where this is the case, the controller CT may apply different voltages to different Pockels cells to compensate for this wavelength dependency.
[00124] Although five Pockels cells 430a-e are depicted in Figure 8 other numbers of Pockels cells may be used. The number of Pockels cells which is used may be selected such that the power in each sub-beam is low enough to avoid causing undesirable nonlinear effects in the Pockels cells. For example, an array of up to 100 Pockels cells may be used. An array having more than 100 Pockels cells may be used.
[00125] Figures 9A and 9B schematically depict a radiation source 102 according to a further alternative embodiment of the invention. In this embodiment lenses are used to expand an amplified radiation beam which is then passed through an array of Pockels cells. The resulting sub-beams are used to re-form the amplified radiation beam which in turn is used to provide an output radiation beam 100.
[00126] Referring first to Figure 9A, a laser 510 emits a radiation beam 512. The radiation beam 512 is amplified by an optical amplifier 540 which outputs an amplified radiation beam 513. The amplified radiation beam 513 passes through a polarizer 511. In this embodiment the polarizer 511 only transmits p-polarized radiation. Consequently, the amplified radiation beam 513 is p-polarized after it passes through the polarizer 511.
[00127] A pair of lenses 580, 581 expand the amplified radiation beam 513 to form an expanded radiation beam 514. The expanded radiation beam 514 may be collimated by the lenses 580, 581. The expanded radiation beam 514 is incident upon an array of Pockels cells 530a-e. The Pockels cells 530a-e provide an array of discrete areas which are spatially separated from each other. Consequently, the Pockels cells 530a-e act to separate the expanded radiation beam 514 into an array of sub-beams 514a-e when the expanded radiation beam enters the Pockels cells.
[00128] In Figure 9A a controller CT does not apply a voltage to the Pockels cells 530a-e and thus the polarization of the sub-beams 514a-e is not changed by the Pockels cells. Each Pockels cell 530a-e thus transmits a sub-beam 514a-e which is p-polarized.
[00129] A pair of lenses 582, 583 receives the sub-beams 514a-e and combines them to re-form a radiation beam 513. The radiation beam 513 is reflected by a mirror 590 to a polarizing beam splitter 550. The polarizing beam splitter 550 is transmissive for p-polarized radiation and thus transmits the radiation beam 513. The radiation beam 513 thus travels towards the electron injector. The radiation beam 513 enters a frequency doubling unit 560 which doubles the frequency of the radiation beam to provide an output radiation beam 100. The output radiation beam 100 passes through a dichroic mirror 570 and is output from the radiation source 102. The output radiation beam 100 is directed to a photocathode of the free electron laser (see Figure 4). Residual radiation which is not frequency doubled by the frequency doubling unit 560 is reflected by a dichroic mirror 570 to a beam dump 572.
[00130] In Figure 9B the controller CT applies voltages to the Pockels cells 530a-e such that they change the polarizations of the sub-beams 514a-e. In this embodiment the change of polarization is a rotation of polarization by 90 degrees, although other changes of polarization may be used. The re-formed radiation beam 513 is thus s-polarized. When the s-polarized radiation beam 513 is incident upon the polarizing beam splitter 550 it is reflected by the polarizing beam splitter 550 and is incident upon a beam dump 551 (it does not travel towards the electron injector). Thus, when the controller CT applies voltages to the Pockels cells 530a-e the radiation source 102 does not provide an output radiation beam.
[00131] Irrespective of whether or not the radiation source 102 outputs a radiation beam 100, the optical amplifier 540 continues to amplify a radiation beam 512. Because the optical amplifier 540 continuously amplifies the radiation beam 512 without interruption, undesirable transients which would be caused by such interruptions are avoided in the radiation beam 100 output from the radiation source 102.
[00132] The lenses 580, 581 which are used to expand the radiation beam 513 are schematically illustrated as a pair of lenses. However, any configuration of lenses may be used to expand the radiation beam. The lenses may for example be provided in a telescope configuration in a known manner. The same applies to the lenses 582, 583 which are used to recombine the sub-beams 514a-e. Although five Pockels cells are depicted in Figure 9, any number of Pockels cells may be used. The number of Pockels cells may be selected depending upon the extent to which the power of the amplified radiation beam 513 needs to be reduced in order to avoid nonlinear effects occurring in the Pockels cells (as discussed above). For example, an array of up to 100 Pockels cells may be used. An array having more than 100 Pockels cells may be used.
[00133] In the embodiments depicted in Figures 8A,B and 9A,B the Pockels cells 430a-e, 530a-e are arranged such that the radiation source 102 provides an output beam 100 when no voltage is applied to the Pockels cells by the controller CT and does not provide an output radiation beam is output when voltages are applied to the Pockels cells. However, it will be appreciated that in an embodiment the Pockels cells may have an opposite configuration such that a radiation beam 100 is output from the radiation source 102 when voltages are applied to the Pockels cells and no radiation beam is output when no voltages are applied to the Pockels cells. This may for example be achieved by replacing the polarizers 411, 511 with polarizers which transmit s-polarized radiation.
[00134] Similarly, other embodiments of the invention may be configured such that they operate in the opposite manner to that depicted in the figures.
[00135] In some of the above described embodiments of the invention an unpolarized radiation beam is emitted by a laser and is passed through a polarizer which polarizes the radiation beam. In an alternative approach a laser which emits a polarized radiation beam may be used. The term “laser beam source” as used in this document encompasses a laser and polarizer in combination and also encompasses a laser without a polarizer. In general, where a polarizer is depicted or described for an embodiment of the invention, that polarizer may be omitted if a polarized radiation beam is already present.
[00136] In illustrated embodiments of the invention a Pockels cell switches from a state in which no polarization rotation is applied to a state in which polarization rotation is applied. However, the Pockels cells may switch from a state in which a first polarization rotation is applied (e.g. 45° rotation) to a state in which a second polarization rotation is applied (e.g. 135° rotation). In general, a Pockels cell of an embodiment is configured to change the polarization of a radiation beam and is switchable between a first state and a second state. The change of polarization applied by the Pockels cell to the radiation beam is determined by whether the Pockels cell is in the first state or is in the second state. The change of polarization applied by the Pockels cell to the radiation beam may be zero in one of the first or second states.
[00137] Embodiments of the invention are described with reference to a first state in which no voltage is applied to a Pockels cell and a second state in which a voltage is applied to a Pockels cell (or vice versa). However, it will be appreciated that these are merely examples. In general, in a first state a first voltage may be applied to a Pockels cell and in a second state a second (different) voltage may be applied to the Pockels cell.
[00138] The Pockels cell is an example of an electrically controllable optical device. Any suitable electrically controllable optical device may be used. Another example of a suitable electrically controllable optical device is a Kerr cell. The Kerr cell may be used to control the polarizations of the first and second radiation beams 112,114 in an equivalent manner to the Pockels cell. Another example of a suitable electrically controllable optical device is an acousto-optic modulator.
[00139] Embodiments of the invention include one or more dichroic mirrors. It will be appreciated that the dichroic mirrors are examples optical elements with wavelength dependent transmission. Other optical elements with wavelength dependent transmission may be used. For example, a grating which transmits at one wavelength and reflects at another wavelength may be used.
[00140] The term “downstream” in this document is intended to refer to a location which receives a radiation beam after an “upstream” location (and vice versa). In other words, a radiation beam will pass through an upstream optical element before passing through a downstream optical element.
[00141] downstream of another optical element is intended to mean that [00142] Although embodiments of a free electron laser have been described as comprising a linear accelerator 22, it should be appreciated that a linear accelerator 22 is merely an example of a type of particle accelerator which may be used to accelerate electrons in a free electron laser. A linear accelerator 22 may be particularly advantageous since it allows electrons having different energies to be accelerated along the same trajectory. However in alternative embodiments of a free electron laser other types of particle accelerators may be used.
[00143] It should be appreciated that a radiation source which comprises a free electron laser FEL may comprise any number of free electron lasers FEL. For example, a radiation source may comprise more than one free electron laser FEL. For example, two free electron lasers may be arranged to provide EUV radiation to a plurality of lithographic apparatus. This is to allow for some redundancy. This may allow one free electron laser to be used when the other free electron laser is being repaired or undergoing maintenance.
[00144] A lithographic system LS may comprise any number of lithographic apparatuses. The number of lithographic apparatuses which form a lithographic system LS may, for example, depend on the amount of radiation which is output from a EUV radiation source SO and on the amount of radiation which is lost in a beam delivery system BDS. The number of lithographic apparatuses which form a lithographic system LS may additionally or alternatively depend on the layout of a lithographic system LS and/or the layout of a plurality of lithographic systems LS.
[00145] Embodiments of a lithographic system LS may also include one or more mask inspection apparatuses MIA and/or one or more Aerial Inspection Measurement Systems (AIMS). In some embodiments, the lithographic system LS may comprise a plurality of mask inspection apparatuses to allow for some redundancy. This may allow one mask inspection apparatus to be used when another mask inspection apparatus is being repaired or undergoing maintenance. Thus, one mask inspection apparatus is always available for use. A mask inspection apparatus may use a lower power radiation beam than a lithographic apparatus. Further, it will be appreciated that radiation generated using a free electron laser FEL of the type described herein may be used for applications other than lithography or lithography related applications.
[00146] It will be further appreciated that a free electron laser comprising an undulator as described above may be used as a radiation source for a number of uses, including, but not limited to, lithography.
[00147] 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.
[00148] The lithographic apparatus which have been described herein may be used in the manufacture of ICs. Alternatively, the lithographic apparatuses described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.
[00149] Different embodiments may be combined with each other. Features of embodiments may be combined with features of other embodiments.
[00150] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below. Other aspects of the invention are set-out as in the following numbered clauses. 1. A radiation source for an electron injector, the radiation source comprising: a first laser configured to emit a first radiation beam with a first wavelength and a second laser configured to emit a second radiation beam with a second wavelength; an optical amplifier configured to amplify the first radiation beam and provide an amplified first radiation beam, and configured to amplify the second radiation beam and provide an amplified second radiation beam; one or more electrically controllable optical devices located upstream of the optical amplifier, the one or more electrically controllable optical devices being configured to switch between allowing the first radiation beam to enter the optical amplifier and allowing the second radiation beam to enter the optical amplifier; and an optical element with wavelength dependent transmission located downstream of the optical amplifier, the optical element with wavelength dependent transmission being configured to allow the amplified first radiation beam to travel towards a photocathode of the electron injector, and to prevent the amplified second radiation beam from travelling towards the photocathode of the electron injector. 2 The radiation source of clause 1, wherein the same electrically controllable optical device is configured to receive both the first radiation beam and the second radiation beam. 3. The radiation source of clause 2, wherein the first radiation beam has a first polarization when it enters the electrically controllable optical device and the second radiation beam has a second polarization when it enters the electrically controllable optical device. 4. The radiation source of clause 3, wherein the polarization of the first radiation beam is orthogonal to the polarization of the second radiation beam when the radiation beams enter the electrically controllable optical device. 5. The radiation source of clause 3 or clause 4, wherein the electrically controllable optical device is configured to change the polarizations of the first and second radiation beams, the electrically controllable optical device being switchable between a first state and a second state, with the change of polarization applied by the electrically controllable optical device to the radiation beams being determined by whether the electrically controllable optical device is in the first state or is in the second state. 6. The radiation source of clause 5, wherein the radiation source further comprises an optical element with polarization dependent transmission provided downstream of the electrically controllable optical device and upstream of the optical amplifier. 7. The radiation source of clause 1, wherein a first electrically controllable optical device is configured to receive the first radiation beam and a second electrically controllable optical device is configured to receive the second radiation beam. 8. The radiation source of clause 7, wherein the radiation source further comprises a controller configured to synchronize switching of the first and second electrically controllable optical devices. 9. The radiation source of clause 7 or clause 8, wherein the first electrically controllable optical device is configured to change the polarization of the first radiation beam, and the second electrically controllable optical device is configured to change the polarization of the second radiation beam, the electrically controllable optical devices being switchable between first states and second states, with the change of polarization applied by the electrically controllable optical devices to the radiation beams being determined by whether the electrically controllable optical devices are in the first state or are in the second state. 10. The radiation source of clause 9, wherein the radiation source further comprises an optical element with polarization dependent transmission provided downstream of the first electrically controllable optical device, and an optical element with polarization dependent transmission provided downstream of the second electrically controllable optical device. 11. The radiation source of any preceding clause, wherein at least one of the electrically controllable optical devices is a Pockels cell. 12. A radiation source for an electron injector, the radiation source comprising: a laser beam source configured to provide a polarized radiation beam; an electrically controllable optical device configured to change the polarization of the polarized radiation beam, the electrically controllable optical device being switchable between a first state and a second state, with the change of polarization applied by the electrically controllable optical device to the polarized radiation beam being determined by whether the electrically controllable optical device is in the first state or is in the second state; an optical amplifier configured to amplify the polarized radiation beam whilst maintaining polarization of the polarized radiation beam, the optical amplifier providing an amplified polarized radiation beam; and an optical element with polarization dependent transmission configured to allow the amplified polarized radiation beam to travel towards a photocathode of the electron injector when it has a first polarization, and to prevent the amplified polarized radiation beam from travelling towards the photocathode of the electron injector when it has a second polarization. 13. The radiation source of clause 12, wherein the laser beam source comprises a laser and a polarizer. 14. The radiation source of clause 11 or clause 12, wherein the electrically controllable optical device is a Pockels cell. 15. A radiation source for an electron injector, the radiation source comprising: a laser configured to provide a radiation beam; an optical amplifier configured to amplify the radiation beam; optics configured to expand the amplified radiation beam; an array of electrically controllable optical devices configured to receive the expanded amplified radiation beam and to output a plurality of sub-beams of radiation, the electrically controllable optical devices being configured to change the polarizations of the radiation sub-beams, the electrically controllable optical devices being switchable between first states and second states, with the change of polarization applied by the electrically controllable optical devices to the radiation sub-beams being determined by whether the electrically controllable optical devices are in the first state or in the second state; optics configured to combine the radiation sub-beams to re-form the amplified radiation beam; and an optical element with polarization dependent transmission configured to allow the amplified radiation beam to travel towards a photocathode of the electron injector when it has a first polarization, and to prevent the amplified radiation beam from travelling towards the photocathode of the electron injector when it has a second polarization. 16. The radiation source of clause 15, wherein the optics configured to expand the amplified radiation beam comprise at least one grating or at least one prism. 17. The radiation source of clause 15, wherein the optics configured to expand the amplified radiation beam comprise at least one lens 18. The radiation source of any of clauses 15 to 17, wherein the array of electrically controllable optical devices is a two dimensional array. 19. The radiation source of any of clauses 15 to 18, wherein the array of electrically controllable optical devices is an array of Pockels cells. 20. A free electron laser comprising a radiation source according to any preceding clause, an electron source configured to receive a radiation beam from the radiation source and emit an electron beam, a particle accelerator configured to accelerate the electron beam output from the electron source, and an undulator operable to guide the accelerated electron beam along a periodic path so as to stimulate emission of radiation, thereby forming a radiation beam. 21. A lithographic system comprising a free electron laser according to clause 20 and at least one lithographic apparatus arranged to receive at least a portion of radiation provided by the radiation source. 22. A method of operating a radiation source for an electron injector, the method comprising: using a first laser to emit a first radiation beam with a first wavelength and using a second laser to emit a second radiation beam with a second wavelength; using an optical amplifier to amplify the first radiation beam and provide an amplified first radiation beam, or to amplify the second radiation beam and provide an amplified second radiation beam; using one or more electrically controllable optical devices located upstream of the optical amplifier to switch between allowing the first radiation beam to enter the optical amplifier and allowing the second radiation beam to enter the optical amplifier; and using an optical element with wavelength dependent transmission located downstream of the optical amplifier to allow the amplified first radiation beam to travel towards a photocathode of the electron injector, and to prevent the amplified second radiation beam from travelling towards the photocathode of the electron injector. 23. A method of operating a radiation source for an electron injector, the method comprising: using a laser beam source to provide a polarized radiation beam; using an electrically controllable optical device to change the polarization of the polarized radiation beam, the electrically controllable optical device switching between a first state and a second state, with the change of polarization applied by the electrically controllable optical device to the polarized radiation beam being determined by whether the electrically controllable optical device is in the first state or is in the second state; using an optical amplifier to amplify the polarized radiation beam whilst maintaining polarization of the polarized radiation beam, the optical amplifier providing an amplified polarized radiation beam; and using an optical element with polarization dependent transmission to allow the amplified polarized radiation beam to travel towards a photocathode of the electron injector when it has a first polarization, and to prevent the amplified polarized radiation beam from travelling towards the photocathode of the electron injector when it has a second polarization. 24. A method of operating a radiation source for an electron injector, the method comprising: using a laser to provide a radiation beam; using an optical amplifier to amplify the radiation beam; using optics to expand the amplified radiation beam; using an array of electrically controllable optical devices to receive the expanded amplified radiation beam and to output a plurality of sub-beams of radiation, the electrically controllable optical devices changing the polarizations of the radiation sub-beams, the electrically controllable optical devices being switchable between first states and second states, with the change of polarization applied by the electrically controllable optical devices to the radiation sub-beams being determined by whether the electrically controllable optical devices are in the first state or in the second state; using optics to combine the radiation sub-beams to re-form the amplified radiation beam; and using an optical element with polarization dependent transmission to allow the amplified radiation beam to travel towards a photocathode of the electron injector when it has a first polarization, and to prevent the amplified radiation beam from travelling towards the photocathode of the electron injector when it has a second polarization.

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.

类似技术:

---

公开号 | 公开日 | 专利标题

---

JP6417418B2|2018-11-07|Electron injector, free electron laser, lithography system, electron beam generation method, and radiation generation method

---

JP2011508965A|2011-03-17|Driving laser for EUV light source

---

NL2013014A|2014-12-22|Lithographic method.

---

US10222702B2|2019-03-05|Radiation source

---

WO2014119199A1|2014-08-07|Laser device and extreme ultraviolet generating device

---

JPWO2014119198A1|2017-01-26|Laser apparatus and extreme ultraviolet light generator

---

TW201524053A|2015-06-16|Free electron laser

---

NL2018274A|2017-09-07|Radiation Source

---

US10468225B2|2019-11-05|Electron source for a free electron laser

---

US10736205B2|2020-08-04|Electron beam transport system

---

TWI704736B|2020-09-11|Free electron laser

---

JP2020519934A|2020-07-02|Laser generated plasma source

---

NL2017695A|2017-06-07|Free electron laser

---

Alejo et al.2022|Demonstration of kilohertz operation of hydrodynamic optical-field-ionized plasma channels

---

NL2025612B1|2021-02-16|Extreme ultraviolet light generation system and electronic device manufacturing method

---

JP2022503714A|2022-01-12|Optical modulator control

---

KR20210148361A|2021-12-07|Apparatus and method for generating multiple laser beams

---

NL2018351A|2017-09-19|Radiation System

---

Martens et al.2015|Developments of optical resonators and optical recirculators for Compton X/γ ray machines

---

NL2017475A|2017-05-19|Electron Source

同族专利:

---

公开号 | 公开日

---

引用文献:

---

公开号 | 申请日 | 公开日 | 申请人 | 专利标题

---

---

法律状态:

优先权:

---

申请号 | 申请日 | 专利标题

---

EP16158184|2016-03-02|

---