NL2018351A - Radiation System - Google Patents

Abstract

A radiation system comprising a radiation source configured to emit a main radiation beam; and a beam delivery system configured to receive the main radiation beam and direct a branch radiation beam to a beam delivery location, the branch radiation beam comprising at least a portion of the main radiation beam; wherein the beam delivery system includes a polarization adjustment apparatus arranged to receive the branch radiation beam, the polarization adjustment apparatus comprising: a plurality of reflective surfaces arranged to successively reflect the branch radiation beam, the reflective surfaces being configured to adjust the polarization state of the branch radiation beam by introducing a phase retardance between perpendicular polarization components of the branch radiation beam.

Description

Radiation System

FIELD

[0001] The present invention relates to a radiation system. In particular, but not exclusively, the radiation system may form part of a lithographic system comprising at least one lithographic apparatus.

BACKGROUND

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

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

[0004] A lithographic apparatus may be provided with EUV radiation from a radiation system. It is an object of the present invention to obviate or mitigate at least one problem of prior art techniques.

SUMMARY

[0005] According to a first aspect of the invention there is provided a radiation system comprising: a radiation source configured to emit a main radiation beam; and a beam delivery system configured to receive the main radiation beam and direct a branch radiation beam to a beam delivery location, the branch radiation beam comprising at least a portion of the main radiation beam; wherein the beam delivery system includes a polarization adjustment apparatus arranged to receive the branch radiation beam, the polarization adjustment apparatus comprising: a plurality of reflective surfaces arranged to successively reflect the branch radiation beam, the reflective surfaces being configured to adjust the polarization state of the branch radiation beam by introducing a phase retardance between perpendicular polarization components of the branch radiation beam.

[0006] The polarization adjustment apparatus allows the polarization state of a branch radiation beam to be individually adjusted and controlled such that the branch radiation beam which is provided at the beam delivery location has a desired polarization state. The branch radiation beam may, for example, be provided to a lithographic apparatus which may be situated substantially at the beam delivery location. It may be desirable to provide a lithographic apparatus with a given polarization state. The polarization adjustment apparatus allows the polarization state of the branch radiation beam to be adjusted according to the requirements of the lithographic apparatus.

[0007] A polarization adjustment apparatus comprising a plurality of reflective surfaces may advantageously be used to adjust the polarization state of a radiation beam comprising EUV radiation (which is typically absorbed by transmissive optics). The positions and/or orientations of the reflective surfaces may be arranged relative to a polarization state of the branch radiation beam so as to bring about a desired change in the polarization state of the branch radiation beam. An amount of phase retardance which is caused by the polarization adjustment apparatus may be controlled by controlling a sum of grazing incidence angles at the reflective surfaces.

[0008] The reflective surfaces may be configured to adjust the polarization state of the branch radiation beam by rotating a major axis of the polarization state of the branch radiation beam.

[0009] The major axis of a polarization state may, for example, be rotated by arranging the reflective surfaces such that p and s-polarized components at the reflective surfaces have different magnitudes. A phase retardance is introduced between the p and s-polarized components. The polarization components between which a phase retardance is introduced may be controlled by controlling the orientation of the reflective elements with respect to the polarization state of the incoming branch radiation beam. The orientation of the reflective elements relative to the incoming branch radiation beam determines a plane of incidence at the reflective elements. The reflective elements may, for example, be rotated relative to the incoming branch radiation beam so as to rotate the planes of incidence and change the polarization components between which a phase retardance is introduced.

[0010] A major axis of a polarization state is intended to refer to a major axis of a polarization ellipse, which is traced out by an electric field vector associated with the branch radiation beam.

[0011] The reflective surfaces may be configured to change an ellipse aspect ratio of the polarization state.

[0012] An ellipse aspect ratio of a polarization state is intended to refer to a parameter characterizing a polarization ellipse which is traced out by an electric field vector associated with the branch radiation beam. The ellipse aspect ratio is the length of a minor axis of the polarization ellipse divided by the length of a major axis of the polarization ellipse.

[0013] The plurality of reflective surfaces may be arranged to receive the branch radiation beam and reflect the branch radiation beam so as to define a plane of incidence at the reflective surfaces and wherein at least some of the planes of incidence are substantially parallel to each other.

[0014] At least some of the planes of incidence may lie in substantially the same plane.

[0015] If the planes of incidence at different reflective elements lie in substantially the same plane, or are at least substantially parallel to each other, then the polarization components between which a phase retardance is introduced, are substantially the same at the different reflective elements. Advantageously, this simplifies the design of the polarization adjustment apparatus. Furthermore, such an arrangement reduces the amount of radiation which is lost to absorption at the reflective surfaces, for a given phase retardance.

[0016] The reflective surfaces may be arranged to receive the branch radiation beam at grazing incidence angles of less than approximately 5°.

[0017] In some embodiments the grazing incidence angles may be less than approximately 2°. In some embodiments the grazing incidence angles may be less than approximately 1 °.

[0018] The radiation system of any preceding clause, further comprising at least one actuator operable to control the position and/or orientation of at least one of the reflective surfaces.

[0019] The at least one actuator may be operable to rotate the orientation of at least one of the reflective surfaces so as to rotate a plane of incidence at the at least one reflective surface with respect to a major axis of the polarization state of the branch radiation beam.

[0020] The at least one actuator may be operable to rotate the orientation of a plurality of the reflective surfaces by substantially the same amount.

[0021] The at least one actuator may be operable to change the orientation of at least one of the reflective surfaces so as to change a grazing incidence angle at the at least one reflective surface.

[0022] The at least one actuator may be operable to control the position and/or orientation of at least one of the reflective surfaces such that a position and pointing direction of the branch radiation beam which is output from the polarization adjustment apparatus remains substantially the same [0023] The radiation system may further comprise a controller configured to operate the at least one actuator so as to control the position and/or orientation of at least one of the reflective surfaces, thereby controlling the resulting adjustment of the polarization state.

[0024] The beam delivery system may comprise a beam splitting apparatus configured to split the main radiation beam into a plurality of branch radiation beams.

[0025] The radiation system may comprise a plurality of polarization adjustment apparatuses, each polarization adjustment apparatus being configured to receive one of the branch radiation beams.

[0026] The radiation source may be configured to emit an EUV radiation beam.

[0027] The radiation source may comprise a free electron laser.

[0028] A radiation system according to the first aspect may form part of a lithographic system comprising a radiation system and at least one lithographic apparatus arranged to receive a branch radiation beam output from the radiation system.

[0029] According to a second aspect of the invention there is provide a polarization adjustment apparatus suitable for use in a radiation system according to the first aspect.

[0030] The polarization adjustment apparatus may be configured to adjust the polarization state of the branch radiation beam by rotating a major axis of the polarization state.

[0031] According to a third aspect of the invention there is provided a method of providing a branch radiation beam, the method comprising: emitting a main radiation beam; directing a branch radiation beam to a beam delivery location, the branch radiation beam comprising at least a portion of the main radiation beam; and adjusting the polarization state of the branch radiation beam by successive reflection from a plurality of reflective surfaces, wherein the reflective surfaces are arranged to adjust the polarization state of the branch radiation beam by introducing a phase retardance between perpendicular polarization components of the branch radiation beam.

[0032] Adjusting the polarization state of the branch radiation beam comprises rotating a major axis of the polarization state of the branch radiation beam.

[0033] Various aspects and features of the invention set out above or below may be combined with various other aspects and features of the invention as will be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

[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;

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 that forms part of the lithographic system of Figure 1;

Figure 4 is a schematic illustration of a lithographic system according to an embodiment of the invention;

Figure 5 is a schematic representation of polarization states which may be formed in the lithographic system of Figure 4;

Figure 6 is a schematic illustration of a polarization adjustment apparatus which may form part of the lithographic system of Figure 4;

Figure 7 is a schematic illustration of first and second configurations of the polarization adjustment apparatus of Figure 6;

Figure 8 is schematic representation of polarization states which may be formed by the polarization adjustment apparatus of Figures 6 and 7;

Figure 9 is a schematic illustration of an alternative embodiment of a polarization adjustment apparatus; and

Figures 10A and 10B are schematic representations of configurations of a polarization adjustment apparatus which transform a first polarization state to a second polarization state.

DETAILED DESCRIPTION

[0035] Figure 1 is a schematic illustration of a lithographic system LS. The lithographic system LS comprises a radiation source SO, a beam delivery system BDS and a plurality of lithographic apparatuses LAa-LAn (e.g. ten lithographic apparatuses). The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B (which may be referred to as a main beam). The radiation source SO and the beam delivery system BDS may together be considered to form a radiation system, the radiation system being configured to provide radiation to one or more lithographic apparatuses 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-Bn (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 radiation source SO, beam delivery system BDS and lithographic apparatus LAa-LAn may all be constructed and arranged such that they can be isolated from the external environment. A vacuum may be provided in at least part of the radiation source SO, beam delivery system BDS and lithographic apparatuses LAa-LAn so as to 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] Figure 2 is a schematic illustration of a lithographic apparatus LAa which may form part of a lithographic system LS. The lithographic apparatus LAa comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the branch radiation beam Ba that is received by that lithographic apparatus LAa before it is incident upon the patterning device MA. The projection system PS is configured to project the radiation beam Ba’ (now patterned by the patterning device MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam Ba’ with a pattern previously formed on the substrate W.

[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 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 radiation source SO may comprise a free electron laser.

[0045] Figure 3 is a schematic depiction of a free electron laser FEL comprising an injector 21, a linear accelerator 22, a bunch compressor 23, an undulator 24, an electron decelerator 26 and a beam dump 100.

[0046] The injector 21 is arranged to produce a bunched electron beam E and comprises an electron source (for example a thermionic cathode or 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 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 Aem is the wavelength of the radiation, λυ 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 /4=1, for a planar undulator that produces linearly polarized radiation /4=2, and for a helical undulator which produces elliptically polarized radiation (that is neither circularly polarized nor linearly polarized) 1</4<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 Aem 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 λυ 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 Au 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 100. The dump 100 may comprise a sufficient quantity of material to absorb the electron beam E. The material may have a threshold energy for induction of radioactivity. Electrons entering the dump 100 with an energy below the threshold energy may produce only gamma ray showers but will not induce any significant level of radioactivity. The material may have a high threshold energy for induction of radioactivity by electron impact. For example, the beam dump may comprise aluminium (Al), which has a threshold energy of around 17 MeV. It may be desirable to reduce the energy of electrons in the electron beam E before they enter the dump 100. This removes, or at least reduces, the need to remove and dispose of radioactive waste from the dump 100. This is advantageous since the removal of radioactive waste requires the free electron laser FEL to be shut down periodically and the disposal of radioactive waste can be costly and can have serious environmental implications.

[0056] The energy of electrons in the electron beam E may be reduced before they enter the dump 100 by directing the electron beam E through a decelerator 26 disposed between the undulator 24 and the beam dump 100.

[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 injector 21. The RF fields in the linear accelerator therefore serve to decelerate the electrons which are output from the undulator 24 and to accelerate electrons output from the injector 21. As the electrons decelerate in the linear accelerator 22 some of their energy is transferred to the RF fields in the linear accelerator 22. Energy from the decelerating electrons is therefore recovered by the linear accelerator 22 and may be used to accelerate the electron beam E output from the injector 21. Such an arrangement is known as an energy recovery linear accelerator (ERL).

[0058] In some embodiments of a lithographic system LS the radiation source SO may comprise a single free electron laser FEL. In such embodiments the main beam B which is emitted from the radiation source SO may be a laser beam BFEL which is emitted from the free electron laser FEL. In other embodiments, a lithographic system LS 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 schematic illustration of a lithographic system LS according to an embodiment of the invention. The lithographic system LS includes a radiation system comprising a radiation source SO and a beam delivery system BDS. The lithographic system LS further comprises a plurality of lithographic apparatuses LAa-LAj.

[0060] The radiation source SO is configured to emit a main radiation beam B. As was described above, the radiation source SO may comprise one or more free electron lasers. The radiation source SO may be configured to emit an EUV radiation beam. The beam delivery system BDS is configured to receive the main radiation beam B and direct branch radiation beams Ba-Bj to the lithographic apparatuses LAa-LAj. The locations of the lithographic apparatuses LAa-LAj may be considered to be examples of beam delivery locations. The beam delivery system BDS is configured to deliver the branch radiation beams Ba-Bj to the beam delivery locations at which the lithographic apparatuses are situated LAa-LAj.

[0061] The beam delivery system BDS comprises a beam splitting apparatus 40 configured to split the main radiation beam B into the plurality of branch radiation beams Ba-Be. The beam splitting apparatus 40 may, for example, comprise one or more diffraction gratings arranged to receive an incoming radiation beam and split the incoming radiation beam into a plurality of diffraction orders. A diffraction order may form a branch radiation beam. Alternatively a diffraction order may form an input radiation beam to a further diffraction grating, which forms further diffraction orders.

[0062] Additionally or alternatively the beam splitting apparatus 40 may comprise a plurality of reflective elements. The reflective elements may be arranged to receive different portions of an incoming radiation beam and reflect the different portions in different directions so as to form separate radiation beams from the incoming radiation beam. In some embodiments the reflective elements may be arranged to form facets of a facetted mirror.

[0063] The beam delivery system further comprises a plurality of polarization adjustment apparatuses 31 a-31 j. The polarization adjustment apparatuses 31 a-31 j are each arranged to receive a branch radiation beam Ba-Bj and adjust the polarization state of the branch radiation beam Ba-Bj.

[0064] The beam delivery system BDS may comprise other components not shown in Figure 4. For example, the beam delivery system BDS may comprise directing optics configured to direct one or more branch radiation beams Ba-Bj to respective beam delivery locations. Additionally or alternatively the beam delivery system BDS may comprise beam expanding optics and/or beam shaping optics.

[0065] In the embodiment which is shown in Figure 4 polarization adjustment apparatuses 31 a-31 j are positioned in the optical paths of each of the branch radiation beams Ba-Bj. The polarization adjustment apparatuses 31 a-31 j are configured to adjust the polarization states of the branch radiation beams Ba-Bj so as to provide the lithographic apparatuses LAa-LAj with branch radiation beams Ba-Bj having a desired polarization state. A lithographic apparatus LAa-LAj may have a given polarization state, which it is desirable for the lithographic apparatus LAa-LAj to receive.

[0066] A desired polarization state for a lithographic apparatus may depend on a mode of operation of the lithographic apparatus. For example, an illumination system IL which forms part of a lithographic apparatus LAa-LAj may be configured in a plurality of different illumination modes. Different illumination modes may comprise different intensity profiles in a pupil plane of the illumination system IL and may alternatively be referred to as different pupil settings. It may be desirable to provide a lithographic apparatus LAa-LAj with different polarization states when the lithographic apparatus LAa-LAj operates with different illumination modes. For example, it may be desirable to change the polarization state which is provided to a lithographic apparatus when the lithographic apparatus LAa-LAj changes the illumination mode with which it is operating. As will be described in further detail below, a configuration of a polarization adjustment apparatus 31 a-31 j may be changed so as to change the polarization state which is provided to a lithographic apparatus LAa-LAj.

[0067] The polarization states of the branch radiation beams Ba-Bj may undergo some changes other than the changes caused by the polarization adjustment apparatuses 31 a-31 j. For example, reflections at elements of the beam splitting apparatus 40, other than at a polarization adjustment apparatus 31 a-31 j, may cause some changes in the polarization states of the branch radiation beams Ba-Bj. The polarization states of the branch radiation beams Ba-Bj provided to the polarization adjustment apparatuses 31 a-31 j may therefore be different to the polarization state of the main radiation beam B which is emitted from the radiation source SO.

[0068] The polarization state of a radiation beam may be described in terms of a Jones vector J. A Jones vector J is a two component complex vector which describes the relative amplitude and relative phase of perpendicular components of the electric field vector of a radiation beam. For example, for a radiation beam propagating in a z-direction, the Jones vector J may describe the relative amplitude and relative phase of x and y-components of the electric field vector of the radiation beam, where the x, y and z-directions are perpendicular to each other.

[0069] The polarization state of the main radiation beam B, which is provided to a polarization adjustment apparatus 31 a-31 j, may be characterized with an input jones vector Jin. A change in the polarization state of a branch radiation beam Ba-Bj which is caused by the polarization adjustment apparatus may be characterized with a Jones matrix M. The optical path of each branch radiation beam Ba-Bj may be characterized by a Jones matrix M specific to the optical path of that branch radiation beam Ba-Bj. The polarization state of a branch radiation beam Ba-Bj which is output from the beam delivery system BDS and which is provided to a lithographic apparatus LAa-LAj, may be characterized with an output Jones vector Jout. Each branch radiation beam Ba-Bj may be characterized by an output Jones vector Jout specific to that branch radiation beam Ba-Bj. An output Jones vector is related to the input jones vector Jin and a Jones matrix /Wfor a given a given optical path by equation (3).

Jout=MJin (3) [0070] In order to control the polarization states of the branch radiation beams Ba-Bj provided to the lithographic apparatuses LAa-LAj, the polarization adjustment apparatuses 31a-31 j may be configured to provide a desired adjustment of the polarization states of the branch radiation beams Ba-Bj. For example, the polarization states of the branch radiation beams Ba-Bj which are input into the polarization adjustment apparatuses 31 a-31 j may be calculated or measured. An adjustment in polarization state which results in the lithographic apparatuses LAa-LAj receiving branch radiation beams Ba-Bj having a desired polarization state may then be calculated. For example, a Jones matrix M which transforms an input Jones vector Jin into a desired output Jones vector Jout may be calculated. The polarization adjustment apparatuses may be configured to provide the calculated polarization adjustment, such that the lithographic apparatuses LAa-LAj receive branch radiation beams Ba-Bj having desired polarization states.

[0071] Figure 5 is a schematic representation of two different polarization states. The representation shown in Figure 5 shows the shape in an x-y plane which is traced out over time by an electric field vector associated with a radiation beam. The x-y plane which is shown in Figure 5 is perpendicular to a z-direction along which the radiation beam propagates. A first polarization state is denoted with the reference numeral 51 in Figure 5. A second polarization state is denoted with the reference numeral 53 in Figure 5. The first polarization state 51 may, for example, represent a desired polarization state for a lithographic apparatus operating with a first illumination mode. The second polarization state 53 may represent a desired polarization state for a lithographic apparatus operating with a second illumination mode, different to the first illumination mode.

[0072] As can be seen in Figure 5, the first polarization state 51 is an elliptical polarization state characterized by an ellipse traced out by the electric field vector. The ellipse has a major axis 55 and a minor axis 57. The shape of the ellipse may be characterized by an ellipse aspect ratio a defined as the length of the minor axis 57 divided by the length of the major axis 55. In the case of circular polarization, the ellipse aspect ratio a is equal to 1. In the case of linear polarization, the ellipse aspect ratio a is equal to 0. A polarization ellipse may be further characterized by an orientation of the major axis 55. For example, the ellipse may be characterized by an angle Θ which the major axis 55 forms with a reference axis, such as the x-axis shown in Figure 5.

[0073] Whilst not labelled in Figure 5, the second polarization state 53 also has a major axis and a minor axis. It will be appreciated from Figure 5, that the ellipse aspect ratio a of the second polarization state 53 is greater than the ellipse aspect ratio of the first polarization state 51. As is indicated in Figure 5, the ellipse aspect ratio a of the first polarization state 51 is approximately 0.5 and the ellipse aspect ratio a of the second polarization state 53 is approximately 0.6. It will be further appreciated from Figure 5, that the orientation of the major axis of the second polarization state 53 is different to the orientation of the major axis of the first polarization state 51. In particular, an angle Θ which the major axis of the second polarization state 53 forms with the x-axis is greater than an angle Θ which the major axis 55 of the first polarization state 51 forms with the y-axis. As is indicated in Figure 5, the angle Θ for the first polarization state 51 is approximately 30° and the angle Θ for the second polarization state 53 is approximately 40 °.

[0074] The polarization states 51,53 which are shown in Figure 5 are merely two examples of possible polarization states which may be provided to one or more lithographic apparatuses. In other embodiments, different polarization states than those shown in Figure 5 may be provided to one or more lithographic apparatuses. In general, a radiation system may be configured to provide any desired polarization state to a lithographic apparatus.

[0075] In some embodiments the polarization adjustment apparatuses 31 a-31 j may be configured to provide substantially the same adjustment to a polarization state. In other embodiments, different polarization adjustment apparatuses 31 a-31 j may be configured to provide different adjustments to a polarization state.

[0076] In general, a beam delivery system BDS may comprise one or more polarization adjustment apparatuses situation in the optical path of one or more branch radiation beams.

The one or more polarization adjustment apparatuses may be configured to adjust the polarization states of one or more branch radiation beams so as to provide lithographic apparatuses LAa-LAj with branch radiation beams Ba-Bj having desired polarization states.

[0077] A polarization adjustment apparatus 31 a-31 j may comprise a plurality of reflective surfaces arranged to successively reflect a branch radiation beam Ba-Bj. The reflective surfaces may be configured to adjust the polarization state of a branch radiation beam Ba-Bj by introducing a phase retardance between perpendicular polarization components of the branch radiation beam Ba-Bj.

[0078] Figure 6, is a schematic illustration of a polarization adjustment apparatus 31a according to an embodiment of the invention. The polarization adjustment apparatus 31a is arranged to adjust the polarization state of a branch radiation beam Ba. The direction of propagation of the branch radiation beam Ba through the polarization adjustment apparatus 31a is indicated by arrows in Figure 6. The polarization adjustment apparatus 31a comprises a first reflective element 61 having a first reflective surface 62, a second reflective element 63 having a second reflective surface 64 and a third reflective element 65 having a third reflective surface 66.

[0079] The branch radiation beam Ba is successively reflected from the first 62, second 64 and third 66 reflective surfaces in that order. During each reflection, a phase retardance is introduced between perpendicular polarization components of the branch radiation beam Ba. In particular, a phase retardance is introduced between an s-polarized component of the branch radiation beam Ba and a p-polarized component of the branch radiation beam Ba. The p-polarized component is the component of the branch radiation beam Ba having a polarization direction which is parallel to a plane of incidence. The s-polarized component is the component of the branch radiation beam Ba having a polarization direction which is perpendicular to the plane of incidence. The plane of incidence is the plane in which both the branch radiation beam Ba which is incident on a reflective surface and the branch radiation beam Ba which is reflected from the reflective surface lies. The magnitude of the p and s-polarized components at each reflective surface depends on the polarization state of the incident branch radiation beam Ba and the alignment of the polarization state relative to the plane of incidence.

[0080] In the representation which is shown in Figure 6, the polarization adjustment apparatus 31a is provided with its own co-ordinates x’ and y’. The x’ and y’ directions are perpendicular to each other and are perpendicular to a z-direction in which the branch radiation beam propagates towards the polarization adjustment apparatus. The x’ and y’ directions may be used to denote the orientation to of the polarization adjustment apparatus 31a relative to a reference co-ordinate system having x, y and z-directions. The y’ direction is substantially parallel with planes of incidence at each of the reflective surfaces 62, 64, 66. The x’ direction is substantially perpendicular to the planes of incidence at each of the reflective surfaces 62, 64, 66. As is shown in Figure 6, the polarization adjustment apparatus 31a may be rotated such that the x' and y’ directions are rotated about the z-axis with respect to the reference x and y-directions. The orientation of the x’ and y' directions relative to the x and y directions may be characterized by a polarization adjustment apparatus orientation angle φ between the x axis and the x' axis.

[0081] The polarization adjustment apparatus 31a may perform a similar function to a transmissive wave plate. The x’ axis of the polarization adjustment apparatus may be equivalent to an extraordinary axis of a transmissive wave plate. The y' axis of the polarization adjustment apparatus may be equivalent to an ordinary axis of a transmissive waveplate.

[0082] The amount of phase retardance which is introduced at a reflective surface depends, at least in part, on the composition of the reflective surface and on a grazing incidence angle at which the branch radiation beam Ba is incident on the reflective surface. In some embodiments one or more of the reflective surfaces 62, 64, 66 may be formed from a reflective coating comprising ruthenium. In other embodiments, other materials may be used. For example, a reflective coating comprising molybdenum may be used.

[0083] In general, the reflective surfaces may be arranged such that the branch radiation beam Ba is incident on the reflective surfaces at relatively small grazing incidence angles. For example, grazing incidence angles of less than about 5° may be used. In some embodiments grazing incidence angles of less than about 2° or even less than about 1 °may be used. At EUV wavelengths (e.g. a wavelength of about 13.5 nm) the amount of radiation which is lost to absorption at a reflective surface tends to increase with increasing grazing incidence angles. Using relatively small grazing incidence angles therefore advantageously reduces loss from a branch radiation beam Ba due to absorption at reflective surfaces.

[0084] At relatively small grazing incidence angles (e.g. grazing incidence angles of less than about 10°) an amount of phase retardance which is introduced between perpendicular polarization components at a reflective surface is approximately proportional to the grazing incidence angle at the reflective surface. For example, using a reflective surface comprising a ruthenium reflective coating, the phase retardance introduced at the reflective surface may be approximately equal to 0.9β at EUV wavelengths, where β is the grazing incidence angle at the reflective surface.

[0085] Using this principle, the amount of phase retardance which is introduced by a polarization adjustment apparatus may be controlled by controlling the sum of grazing incidence angles at the reflective surfaces 62, 64, 66. In the embodiment which is shown in Figure 6, the reflective surfaces 63, 64, 66 are orientated with respect to the branch radiation beam Ba such that the planes of incidence at the reflective surfaces 62, 64, 66 each lie substantially in the same plane. Consequently the phase retardance which is introduced at each of the reflective surfaces 62, 64, 66, is introduced between the same polarization components at each reflective surface 62, 64, 66. That is, the p-polarized component is substantially the same at each reflective surface 62, 64, 66 and the s-polarized component is substantially the same at each reflective surface 62, 64, 66. The total phase retardance introduced between s and p-polarized components by the polarization adjustment apparatus 31a as a whole may therefore be approximated as a sum of the phase retardances introduced at the reflective surfaces 62, 64, 66.

[0086] For example, in the embodiment which is shown in Figure 6 the grazing incidence angle β may be approximately equal to a value ε at both the first 62 and third 66 reflective surfaces. The grazing incidence angle β at the second reflective surface 64 may be approximately equal to 2ε. As explained above for reflective surfaces comprising a ruthenium reflective coating, and at EUV wavelengths, the retardance at each reflective surface may be approximately 0.9β. In the presently described example, the total phase retardance R introduced by the polarization adjustment apparatus is given approximately by Β=0.9ε+(0.9χ2ε)+0.9ε=3.6ε. The total phase retardance which is introduced by the polarization adjustment apparatus 31 a may be controlled by controlling the grazing incidence angles β at the reflective surfaces. For example, the orientation and/or position of the reflective surfaces may be changed in order to change the value of ε, thereby changing the grazing incidence angles β at the reflective surfaces.

[0087] Figure 7 is a schematic illustration of alternative configurations of the polarization adjustment apparatus 31a as viewed in the x-direction (Cartesian co-ordinates indicated in Figure 7 are consistent with those indicated in Figure 6). A first configuration of the polarization adjustment apparatus 31a is illustrated with solid lines. The first, second and third reflective elements are denoted with the reference numerals 61, 63 and 65 in the first configuration. A second configuration of the polarization adjustment apparatus 31a is illustrated with dotted lines. The first, second and third reflective elements are denoted with reference numerals 61 63’ and 65’.

[0088] As can be seen from Figure 7, the first and third reflective elements are rotated in the second configuration relative to the first configuration. The rotation of the reflective elements is indicated with double-headed arrows in Figure 7. The rotation of the reflective elements between the first and second configurations serves to increase the grazing incidence angles β at the reflective surfaces. Increasing the grazing incidence angle at the first reflective element 61 changes the direction in which the branch radiation beam Ba is reflected. To account for this change in direction the second reflective element 63 is translated in the y-direction between the first and second configurations. Translation of the second reflective element 63 ensures that the second reflective element 63 continues to be situated in the optical path of the branch radiation beam Ba and continues to direct the branch radiation beam Ba to be incident on the third reflective element 65.

[0089] Moving the reflective elements 61,63, 65 between their positions and orientations in the first configuration and their positions and orientations in the second configuration, serves to increase the grazing incidence angle at each of the reflective elements. Consequently the sum of the grazing incidence angles is increased and the phase retardation which is introduced by the polarization adjustment apparatus 31a is increased. Controlling the position and/or the orientation of the reflective elements and consequently the reflective surfaces allows the phase retardation which is caused by the polarization adjustment apparatus 31a to be controlled.

[0090] In the embodiment which is shown in Figure 7, the polarization adjustment apparatus comprises actuators 71, 72, 73. A first actuator 71 is operable to change the orientation and/or the position of the first reflective element 61. The second actuator 72 is operable to change the orientation and/or the position of the second reflective element 63. The third actuator is operable to change the orientation and/or the position of the third reflective element 65. The actuators 71,72, 73 are controlled by a controller 74. The controller is configured to operate the actuators 71,72, 73 so as to control the position and/or the orientation of the reflective elements 71,72, 73 so as to control the phase retardation which is caused by the polarization adjustment apparatus 31a. For example, the controller 74 may operate the actuators 71, 72, 73 so as to move the reflective elements between the first and second configurations shown in Figure 7. In general, the controller 74 may operate the actuators 71, 72, 73 to move the reflective elements so as to bring about any desired change in the polarization state of the branch radiation beam Ba- [0091] In the embodiment which is shown in Figure 7, movement between the first and second configurations of the polarization adjustment apparatus does not cause any change in the position or pointing direction of the branch radiation beam Ba which is reflected from the third reflective element 65. Advantageously, this means that the branch radiation beam Ba may be delivered to the same beam delivery location without requiring a change in configuration of other components of the beam delivery system BDS. In the embodiment which is shown in Figure 7, the branch radiation beam Ba which is incident on the first reflective element 61 is substantially coincident with the branch radiation beam Ba which is reflected from the third reflective element 65. However, in other embodiments this need not necessarily be the case.

[0092] In the embodiment which is shown in Figure 7, the arrangement of the reflective elements 61, 63, 65 is substantially symmetric. For example, the grazing incidence angles ε at the first and third reflective elements 61,65 are substantially the same. The grazing incidence angle 2ε at the second reflective element 63 is substantially twice the grazing incidence angles at the first and third reflective elements 61, 65. Such an arrangement allows the second reflective element 63 to translate between different configurations (e.g. the first and second configurations shown in Figure 7). The second actuator 72 may therefore be configured to translate the second reflective element 63 but not rotate the second reflective element 63. In other embodiments, a different arrangement of reflective elements may be used. For example, the arrangement of reflective elements may be asymmetric. For example, the grazing incidence angles at first and third reflective elements 61,65 may be different.

[0093] The controller 74 may, for example, cause the actuators 71, 72, 73 to change the position and/or orientation of the reflective elements 61, 63, 65 when an illumination mode used by a lithographic apparatus is changed. Changing the position and/or orientation of the reflective elements 61, 63, 65 serves to change the polarization state of the branch radiation beam Ba which is output from the polarization adjustment apparatus 31a. For example, the position and/or orientation of the reflective elements 61, 63, 65 may be changed such that the polarization state of the branch radiation beam Ba is substantially a desired polarization state for a new illumination mode, with which a lithographic apparatus (which receives the branch radiation beam Ba) operates after a change in illumination mode.

[0094] As was described above with reference to Figures 6 and 7, an amount of phase retardance which is introduced between perpendicular polarization components of a branch radiation beam Ba may be altered by altering the grazing incidence angles at which the branch radiation beam Ba is incident on reflective surfaces of a polarization adjustment apparatus. Figure 8 is a schematic representation of different polarization states which result from different amounts of phase retardance being introduced to a radiation beam.

[0095] In the example shown in Figure 8, the polarization states are shown relative to x' and y’ directions which are co-ordinates of a polarization adjustment apparatus (e.g. the polarization adjustment apparatus 31a which is shown in Figure 6). The y’-direction is parallel with planes of incidence at reflective surfaces of the polarization adjustment apparatus. The x’-direction is perpendicular to planes of incidence at reflective surfaces of the polarization adjustment apparatus. Components of the polarization states which extend in the y’-direction are therefore the p-polarized components. Components of the polarization states which extend in the x’-direction are the s-polarized components. As was explained above a polarization adjustment apparatus introduces a phase retardance between p and s-polarized components.

[0096] In the example, shown in Figure 8 the radiation beam which is input into a polarization adjustment apparatus has a linear polarization state denoted by a solid line 81 in Figure 8. The linear polarization state 81 is orientated at approximately 45° with respect to both the x' and the y’ directions. The linear polarization state 81 therefore has s and p-components of substantially equal magnitude. The linear polarization state 81 may be considered to have a major axis corresponding to the solid line 81 and a polarization ellipse aspect ratio a of 0.

[0097] Also shown in Figure 8 are several elliptical polarization states which result from introducing phase retardances of 0.2 π, 0.4 π, 0.6 π and 0.8 π between the s and p-polarized components of the radiation beam. As can be seen in Figure 8 increasing the amount of phase retardance to 0.2 π and then 0.4 π causes an increase in the ellipse aspect ratio of the polarization state without changing the orientation of the major axis of the polarization state. Whilst not shown in Figure 8, a phase retardance of 0.5 π would result in a substantially circular polarization state, having a polarization ellipse aspect ratio of 1.

[0098] For increasing phase retardations of greater than 0.5 tt, the polarization ellipse aspect ratio a decreases with increasing phase retardation (up to a phase retardation of tt). For phase retardations of between 0.5 π and π, the major axis of the polarization ellipse is rotated by 90° with respect to the major axis of polarization states which result from a phase retardance of between 0 and 0.5 π.

[0099] Figure 8 is presented merely as an example, of different changes in polarization state which may be caused by a polarization adjustment apparatus. The different phase retardances shown in Figure 8 may be achieved, for example, by arranging reflective surfaces such that a branch radiation beam undergoes reflections at grazing incidence angles which result in the different phase retardances. For example, the positions and/or orientations of reflective surfaces of a polarization adjustment apparatus may be varied in order to vary the sum of grazing incidence angles at the reflective surfaces. The sum of grazing incidence angles may be controlled in order to control the total phase retardance caused by a polarization adjustment apparatus.

[00100] The amount of radiation which is lost to absorption at a reflective surface typically increases with increasing grazing incidence angle. Increasing the grazing incidence angle at a reflective surface in order to increase a phase retardance introduced at the reflective surface, may therefore result in an increase in an amount of radiation lost due to absorption at the reflective surface. In embodiments in which the reflective surfaces comprise ruthenium coatings, the transmittance T of a polarization adjustment apparatus may be approximated by: T = e (—0.75 £ β) = e(—0.83R) (4) where Σβ is the sum of grazing incidence angles at the reflective surfaces and R is the total phase retardance introduced by a polarization adjustment apparatus (approximated by R = 0.9Σβ tor ruthernium coated reflective surfaces). Both the phase retardance Rand the grazing incidence angles β are in radians. Arranging a polarization adjustment apparatus to cause a phase retardance of, for example, 0.26 radians may therefore result in a transmittance T of approximately 0.80.

[00101] In the embodiments described with reference to Figure 8, a polarization adjustment apparatus is orientated with respect to the polarization state of a branch radiation beam, such that p and s-polarized components have substantially the same magnitude. In particular, the polarization adjustment apparatus may be orientated such that planes of incidence at reflective elements are orientated at approximately 45° to the orientation of the major axis of the incoming polarization state 81. Consequently phase retardation introduced by the polarization adjustment apparatus serves to change the polarization ellipse aspect ratio but does not change the orientation of the major axis of the polarization state (except to cause the major axis to rotate by 90“for phase retardances of greater than 0.5 π).

[00102] In other embodiments, reflective surfaces of a polarization adjustment apparatus may be orientated such that p and s-polarized components at the reflective surfaces have different magnitudes. Consequently a phase retardance will be introduced between polarization components having different magnitudes. Introducing a phase retardance between polarization components having different magnitudes will result in a change in the orientation Θ of the major axis of the polarization state and may additionally change the ellipse aspect ratio a of the polarization state. For example, the major axis of a polarization state may be rotated by differing amounts by rotating the orientation of a polarization adjustment apparatus with respect to an incoming polarization state.

[00103] In the embodiment which is shown in Figure 6, the y’ and x’ axes of the polarization adjustment apparatus may be rotated about the z-axis. The y’-direction corresponds to a direction which is parallel to planes of incidence at the reflective surfaces 62, 64, 66. The x’- direction corresponds to a direction which is perpendicular to the planes of incidence at the reflective surfaces 62, 64, 66. Rotating the y’ and x’ axes amounts to a rotation of the reflective surfaces 62, 63, 64 so as to rotate the planes of incidence at the reflective surfaces with respect to the major axis of an incoming polarization state. Rotation of the planes of incidence may result in a rotation of the major axis of the polarization state which is output from the polarization adjustment apparatus (for a given incoming polarization state). The reflective surfaces 62, 64, 66 may be rotated by one or more actuators (e.g. the actuators 71, 72, 73 shown in Figure 7).

[00104] In general, the reflective surfaces 62, 63, 66 may be configured to bring about any desired change in polarization state. The polarization adjustment which is caused by a polarization adjustment apparatus may be controlled by controlling the orientation of planes of incidence of the reflective elements with respect to a major axis of an incoming polarization state. This controls the polarization components between which a phase retardance is introduced. The polarization adjustment may additionally be controlled by controlling the grazing incidence angles at the reflective elements. This controls the amount of phase retardance which is introduced between the polarization components.

[00105] In general a polarization adjustment apparatus according to embodiments of the invention may be considered to have two degrees of freedom. The first degree of freedom is the sum of grazing incidence angles at reflective surfaces of the polarization adjustment apparatus. Controlling the first degree of freedom controls the amount of phase retardation which is introduced between perpendicular polarization components. The second degree of freedom is the orientation of the reflective surfaces with respect to an incoming polarization state. Controlling the second degree of freedom controls the perpendicular polarization components between which a phase retardance is introduced. Any given incoming polarization state may be transformed to a desired output polarization state with a suitable selection of the first and second degrees of freedom.

[00106] Embodiments have been described above in which a polarization adjustment apparatuses comprises three reflective surfaces. In other embodiments, a polarization adjustment apparatus may comprise more than three reflective surfaces. Figure 9 is a schematic illustration of an alternative embodiment of a polarization adjustment apparatus 131a. The polarization adjustment apparatus 131a shown in Figure 9 is similar to the polarization adjustment apparatus 31a depicted in Figures 6 and 7 except that it comprises four reflective elements 161, 163, 165, 167. A first reflective element 161 includes a first reflective surface 162, a second reflective element 163 includes a second reflective surface 164, a third reflective element 165 includes a third reflective surface and a fourth reflective element 167 includes a fourth reflective surface 168. A branch radiation beam Ba is successively reflected at the reflective surfaces 162, 164, 166, 168. At each reflective surface 162, 164, 166, 168 a phase retardance is introduced between perpendicular polarization components of the branch radiation beam Ba, thereby adjusting the polarization state of the branch radiation beam Ba.

[00107] The polarization adjustment apparatus 131a shown in Figure 9 may include any of the features described above with reference to the polarization adjustment apparatus 31a shown in Figures 6 and 7. For example, the polarization adjustment apparatus may include at least one actuator operable to control the position and/or orientation of one or more of the reflective surfaces 162,164,166,168. The position and/or orientation of the reflective surfaces may be varied in order to vary the change in polarization state which is caused by the polarization adjustment apparatus 131a.

[00108] The position and/or orientation of the reflective surfaces may be changed in such a way that the position and/or the pointing direction of the branch radiation beam Ba which is output from the polarization adjustment apparatus 131a remains substantially the same. For example, the first 161 and the fourth 167 reflective elements may remain in substantially the same position and may be rotated about the x-direction. The second 163 and third 165 reflective elements may be translated in the y-direction in order to accommodate the change in direction in which the branch radiation beam Bais reflected from the first reflective surface 162. Such a change in position and/or orientation of the reflective surfaces may change the grazing incidence angles at which the branch radiation beam Ba is incident on the reflective surfaces. Consequently the amount of phase retardance which is caused by the polarization adjustment apparatus 131a is changed.

[00109] Additionally or alternatively, the reflective elements may be rotated about a z-direction. For example, the reflective elements may be rotated about an axis along which the branch radiation beam Ba which is input to the polarization adjustment apparatus 131a propagates. Such a rotation may rotate planes of incidence at the reflective surfaces and therefore change the polarization components between which a phase retardance is introduced.

[00110] In the embodiment of Figure 9 (as was the case in the embodiment of Figures 6 and 7) the reflective surfaces may be arranged such that planes of incidence at each of the reflective surfaces lie in substantially the same plane or are at least parallel with each other. Advantageously such an arrangement reduces the amount of loss due to absorption which occurs for a given amount of phase retardance which is caused by a polarization adjustment apparatus. Furthermore, such an arrangement simplifies the design of a polarization adjustment apparatus. For example, such an arrangement allows the amount of phase retardance which is introduced to be adjusted simply by adjusting a sum of grazing incidence angles at the reflective elements. Independently of this, the polarization components between which a phase retardance is introduced may be adjusted by adjusting the orientation of the planes of incidence. In an arrangement in which planes of incidence at different reflective surfaces do not lie substantially in the same plane, there may be an interaction between the amount of phase retardance which is introduced and the polarization components between which a retardance is introduced.

[00111] In other embodiments, a polarization adjustment apparatus may comprise fewer than three reflective surfaces. However, in embodiments comprising fewer than three reflective surfaces it may not be possible to arrange the reflective elements so as to allow a change in the position and/or orientation of the reflective elements in such a way that the position and/or the pointing direction of the branch radiation beam Ba which is output from the polarization adjustment apparatus remains substantially the same.

[00112] Figures 10A and 10B are schematic representations of configurations of a polarization adjustment apparatus which transforms a first polarization state into a second polarization state. Figures 10A and 10B are both contour plots with the angle Θ shown on the horizontal-axis and the polarization ellipse aspect ratio a shown on the vertical axis. As was described above, the angle Θ denotes the orientation of the major axis of the polarization ellipse with respect to a reference axis (such as the x-axis shown in Figures 5-9). Different positions on the plots of Figures 10A and 10B therefore relate to different polarization states characterized by different values of the parameters a and Θ.

[00113] In the representations shown in Figures 10A and 10B, the center of the contour plots represent a first polarization state 51 which is input to a polarization adjustment apparatus. The first polarization state 51 is characterized by a major axis orientation angle Θ of 30° and a polarization aspect ratio a of 0.5. The first polarization state 51 used for the purposes of Figures 10A and 10B therefore corresponds to the first polarization state 51 shown in Figure 5. The second polarization state 53 of Figure 5 (θ=40°, a=0.6) is also labelled in Figures 10A and 10B.

[00114] Figure 10A shows contours which represent the polarization adjustment apparatus orientation angle φ which transforms the first polarization state 51 into other polarization states. The contours shown in Figure 10A join polarization states which can be output from a polarization adjustment apparatus using a given value of the orientation angle φ (labelled for each contour). Different positions on a contour may be reached by causing different amounts of phase retardance (whilst keeping the orientation angle φ constant).

[00115] Figure 10B shows contours which represent the amount of phase retardance (labelled in degrees) needed to transform the first polarization state 51 into other polarization states. The contours shown in Figure 10B join polarization states which can be output from a polarization adjustment apparatus by introducing a given amount of phase retardance (labelled for each contour). Different positions on a contour may be reached by using different polarization adjustment apparatus orientation angles φ (whilst keeping the amount of phase retardance constant).

[00116] It will be appreciated that from a given input first polarization state 51, an output polarization state may be realized by choosing an appropriate value of the polarization adjustment apparatus angle φ using the contour plot of Figure 10A and by choosing an appropriate phase retardance value using the contour plot of Figure 10B. For example, in order to transform the first polarization state 51 (θ=30°, a=0.5) into the second polarization state 53 (0=40°, a=0.6) a polarization adjustment apparatus may be configured with an orientation angle φ of approximately -38° and may be configured to introduce a phase retardance of approximately 15°. As was described above, the amount of phase retardance which is introduced by a polarization adjustment apparatus may be controlled by controlling the sum of grazing incidence angles at reflective elements of the polarization adjustment apparatus.

[00117] The contour plots of Figures 10A and 10B represent adjustments to a specific input polarization state 51. It will be appreciated that, corresponding plots may be formed for other input polarization states and the configuration of a polarization adjustment apparatus which results in a desired output polarization state may be determined.

[00118] Throughout this description, the polarization state of a radiation beam has been described in the context of a radiation beam being completely polarized. Such a polarization state may be characterized by two parameters. For example, a completely polarized radiation beam may be characterized by the two components of a Jones vector. Alternatively, a completely polarized radiation beam may be characterized by the polarization ellipse aspect ratio a and major axis orientation 0 described above. In some embodiments a radiation beam may not be completely polarized but may instead be partially polarized. A partially polarized radiation beam may be characterized by the two parameters described above and a parameter describing the degree of polarization. For example, the degree of polarization may be represented with a value between 0 and 1. A degree of polarization value of 1 represents completely polarized radiation. A degree of polarization value of less than 1 and greater than 0 represents partially polarized radiation. A degree of polarization value of 0 represents unpolarized radiation.

[00119] In general, a polarization adjustment apparatus as described herein will only serve to alter a polarized component of a radiation beam and will not alter an unpolarized component of a radiation beam. References made herein to adjusting the polarization state of a radiation beam should be interpreted to mean adjusting the polarized component of a radiation beam. Such references should be interpreted to encompass adjusting the polarized component of a partially polarized radiation beam, without adjusting the unpolarized component of a partially polarized radiation beam.

[00120] Whilst embodiments of a radiation source SO have been described and depicted as comprising a free electron laser FEL, a radiation source SO may include a source of radiation other than a free electron laser FEL

[00121] 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.

[00122] A lithographic system LS may comprise any number of lithographic apparatuses. The number of lithographic apparatuses which form a lithographic system LS may, for example, depend on the amount of radiation which is output from a radiation source SO and on the amount of radiation which is lost in a beam delivery system BDS. The number of lithographic apparatuses which form a lithographic system LS may additionally or alternatively depend on the layout of a lithographic system LS and/or the layout of a plurality of lithographic systems LS.

[00123] 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.

[00124] 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.

[00125] 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.

[00126] 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.

[00127] Different embodiments may be combined with each other. Features of embodiments may be combined with features of other embodiments.

[00128] 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 system comprising: a radiation source configured to emit a main radiation beam; and a beam delivery system configured to receive the main radiation beam and direct a branch radiation beam to a beam delivery location, the branch radiation beam comprising at least a portion of the main radiation beam; wherein the beam delivery system includes a polarization adjustment apparatus arranged to receive the branch radiation beam, the polarization adjustment apparatus comprising: a plurality of reflective surfaces arranged to successively reflect the branch radiation beam, the reflective surfaces being configured to adjust the polarization state of the branch radiation beam by introducing a phase retardance between perpendicular polarization components of the branch radiation beam. 2. The radiation system of clause 1, wherein the reflective surfaces are configured to adjust the polarization state of the branch radiation beam by rotating a major axis of the polarization state of the branch radiation beam. 3. The radiation system of clause 1 or 2, wherein the reflective surfaces are configured to change an ellipse aspect ratio of the polarization state. 4. The radiation system of any preceding clause, wherein the plurality of reflective surfaces are arranged to receive the branch radiation beam and reflect the branch radiation beam so as to define a plane of incidence at the reflective surfaces and wherein at least some of the planes of incidence are substantially parallel to each other. 5. The radiation system of clause 4, wherein at least some of the planes of incidence lie in substantially the same plane. 6. The radiation system of any preceding clause, wherein the reflective surfaces are arranged to receive the branch radiation beam at grazing incidence angles of less than approximately 5°. 7. The radiation system of any preceding clause, further comprising at least one actuator operable to control the position and/or orientation of at least one of the reflective surfaces. 8. The radiation system of clause 7, wherein the at least one actuator is operable to rotate the orientation of at least one of the reflective surfaces so as to rotate a plane of incidence at the at least one reflective surface with respect to a major axis of the polarization state of the branch radiation beam. 9. The radiation system of clause 8, wherein the at least one actuator is operable to rotate the orientation of a plurality of the reflective surfaces by substantially the same amount. 10. The radiation system of any of clauses 7-9, wherein the at least one actuator is operable to change the orientation of at least one of the reflective surfaces so as to change a grazing incidence angle at the at least one reflective surface. 11. The radiation system of any of clauses 7-10, wherein the at least one actuator is operable to control the position and/or orientation of at least one of the reflective surfaces such that a position and pointing direction of the branch radiation beam which is output from the polarization adjustment apparatus remains substantially the same 12. The radiation system of any of clauses 7-11, further comprising a controller configured to operate the at least one actuator so as to control the position and/or orientation of at least one of the reflective surfaces, thereby controlling the resulting adjustment of the polarization state. 13. The radiation system of any preceding clause, wherein the beam delivery system comprises a beam splitting apparatus configured to split the main radiation beam into a plurality of branch radiation beams. 14. The radiation system of clause 13, wherein the radiation system comprises a plurality of polarization adjustment apparatuses, each polarization adjustment apparatus being configured to receive one of the branch radiation beams. 15. The radiation system of any preceding clause, wherein the radiation source is configured to emit an EUV radiation beam. 16. The radiation system of any preceding clause, wherein the radiation source comprises a free electron laser. 17. A polarization adjustment apparatus suitable for use in a radiation system according to any of clauses 1-16. 18. The polarization adjustment apparatus of clause 17, wherein the polarization adjustment apparatus is configured to adjust the polarization state of the branch radiation beam by rotating a major axis of the polarization state. 19. A method of providing a branch radiation beam, the method comprising: emitting a main radiation beam; directing a branch radiation beam to a beam delivery location, the branch radiation beam comprising at least a portion of the main radiation beam; and adjusting the polarization state of the branch radiation beam by successive reflection from a plurality of reflective surfaces, wherein the reflective surfaces are arranged to adjust the polarization state of the branch radiation beam by introducing a phase retardance between perpendicular polarization components of the branch radiation beam. 20. The method of clause 21, wherein adjusting the polarization state of the branch radiation beam comprises rotating a major axis of the polarization state of the branch radiation beam.

Claims (1)

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