CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of European patent application 14177904.1 , which was filed on 21 July 2014, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to radiation sources for producing a radiation generating

BACKGROUND

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

[0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 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).

[0005] EUV radiation may be produced using a radiation source arranged to generate an EUV producing plasma. An EUV producing plasma may be generated, for example, by exciting a fuel, for example liquid tin, within the radiation source. The fuel may be excited by directing a beam of initiating radiation, such as a laser beam, at a target comprising the fuel, the initiating radiation beam causing the fuel target to become an EUV generating plasma.

[0006] It is desirable to provide an EUV radiation source which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.

SUMMARY

[0007] According to a first aspect there is provided a radiation source arranged to receive an initiating radiation beam, the radiation source comprising a polarisation adjustment apparatus and a focussing unit configured to focus the initiating radiation beam to be incident on a fuel target, wherein the polarisation adjustment apparatus is operable to adjust a polarisation state of the initiating radiation beam such that the initiating radiation beam has a spatially non-uniform polarisation state in its cross-section so as to control a spatial intensity distribution of the initiating radiation beam at the fuel target.

[0008] Adjusting the polarisation state of the initiating radiation beam advantageously allows the spatial intensity profile of the initiating radiation beam which is focussed at the fuel target to be controlled such that the fuel target may be illuminated with a initiating radiation beam having a desired spatial intensity distribution. The spatial intensity distribution of the initiating radiation beam may influence one or more properties of the fuel target and/or a plasma which is formed by illuminating the fuel target. The spatial intensity distribution of the initiating radiation beam at the fuel target may be controlled in order to control the one or more properties of the fuel target and/or the plasma. For instance the spatial intensity distribution may advantageously be controlled in order to control an amount of debris which is emitted from the fuel target, a displacement and/or an elongation of the fuel target which is caused by the initiating radiation beam and/or the directions in which radiation is emitted from a plasma which is formed at the fuel target.

[0009] The polarisation adjustment apparatus may be operable to adjust the polarisation state of the initiating radiation beam such that the initiating radiation beam is linearly polarised and the orientation of the linear polarisation is different at different positions in a cross-section of the initiating radiation beam.

[00010] The polarisation adjustment apparatus may be operable to adjust the polarisation state of the initiating radiation beam such that the orientation of the linear polarisation of the initiating radiation beam has a non-zero component in an azimuthal direction.

[00011] The polarisation adjustment apparatus may be operable to adjust the polarisation state of the initiating radiation beam such that the orientation of the linear polarisation of the initiating radiation beam has a non-zero component in a radial direction.

[00012] The azimuthal direction and the radial direction may be different at different positions in the cross-section of the initiating radiation beam. The azimuthal direction may be perpendicular to a radial direction at all points of the cross-section of the initiating radiation beam. The azimuthal direction and the radial direction may be defined relative to an origin of a polar co-ordinate system. The origin of the polar co-ordinate system may coincide with a centre of the cross-section of the initiating radiation beam. The radial direction may at all points extend away from the origin of the polar co-ordinate system. The azimuthal direction may at all points be perpendicular to the radial direction.

[00013] The polarisation adjustment apparatus may comprise a plurality of polarisation adjustment elements each configured to receive a portion of the cross-section of the initiating radiation beam and adjust a polarisation state of the portion of the cross-section of the initiating radiation beam. [00014] At least one of the plurality of polarisation adjustment elements may be configured to adjust the polarisation state of a portion of the cross-section of the initiating radiation beam differently to at least one other of the polarisation adjustment elements.

[00015] Each of the polarisation adjustment elements is arranged to receive a segment of the cross-section of the initiating radiation beam.

[00016] The segments may each extend radially outward from a centre of the cross- section of the initiating radiation beam. The segments may each occupy an angular range about the centre of the cross-section of the initiating radiation beam.

[00017] Each of the polarisation adjustment elements may be configured to adjust the polarisation state of a segment of the cross-section of the initiating radiation beam such that each segment of the cross-section is linearly polarised and such that an orientation of the linear polarisation of each segment forms an angle a with a radial direction in that segment.

[00018] The angle a in a segment may be measured with respect to a radial direction which extends through the centre of an angular range which is occupied by the segment.

[00019] The angle a may be substantially the same in each segment.

[00020] Each of the polarisation adjustment elements may comprise a half-wave plate having a fast axis.

[00021] The fast axis of at least one of the half-wave plates may be orientated differently to the fast axis of at least one other of the half-wave plates.

[00022] The polarisation adjustment apparatus may comprise a first half-wave plate having a first fast axis and being configured to receive and transmit substantially the whole cross-section of the laser beam and a second half-wave plate having a second fast axis and being configured to receive substantially the whole cross-section of the laser beam from the first half-wave plate, wherein the first fast axis and the second fast axis are orientated differently to each other.

[00023] The angle between the first fast axis and the second fast axis may be controlled in order to control a rotation of the polarisation state of the initiating radiation beam which is caused by the first and second half-wave plates. The first half-wave plate and the second half-wave plate may be used in conjunction with a plurality of polarisation adjustment elements. For example, a plurality of polarisation adjustment elements may form an initiating radiation beam having a spatially non-uniform polarisation state. The first and second half-wave plates may rotate the polarisation state of each point of the cross-section of the initiating radiation beam by the same amount. The initiating radiation beam may pass through the first and second half-wave plates prior to passing through the plurality of polarisation adjustment elements or after passing through the plurality of polarisation adjustment elements. Using two half-wave plates may advantageously mean that the rotation of the polarisation state which is caused by the half-wave plates is dependent only on the relative orientation of the first and second fast axes and not on the polarisation state of the initiating radiation beam which is incident on the half-wave plates.

[00024] The initiating radiation beam may be a laser beam provided by a YAG laser.

[00025] The polarisation adjustment apparatus may be operable to adjust the polarisation state of the initiating radiation beam such that the initiating radiation beam has a substantially ring shaped spatial intensity distribution at the fuel target.

[00026] The polarisation adjustment apparatus may be operable to adjust the polarisation state of the initiating radiation beam such that the initiating radiation beam has a substantially flat-top spatial intensity distribution at the fuel target.

[00027] The polarisation adjustment apparatus may be operable to adjust the polarisation state of the initiating radiation beam so as to control the intensity of a central minimum of a substantially ring shaped spatial intensity distribution. For example the intensity of the central minimum may be varied between zero and an intensity at which the spatial intensity distribution has a substantially flat top. The intensity of the central minimum may be controlled by controlling the angle a.

[00028] The radiation source may further comprise a fuel emitter configured to emit a fuel and direct the fuel so as to provide a fuel target on which the initiating radiation beam is incident.

[00029] The initiating radiation beam may be configured to excite the fuel target to form a plasma which emits EUV radiation.

[00030] According to a second aspect there is provided a lithographic system comprising a radiation source according the first aspect and a lithographic apparatus arranged to receive a radiation beam from the radiation source, the lithographic apparatus comprises an illumination system configured to condition the radiation beam received from the radiation source, a support structure constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate and a projection system configured to project the patterned radiation beam onto the substrate.

[00031] According to a third aspect there is provided method of providing radiation, the method comprising focussing an initiating radiation beam to be incident on a fuel target wherein the initiating radiation beam has a spatially non-uniform polarisation state in its cross-section.

[00032] The initiating radiation beam may be linearly polarised and an orientation of the linear polarisation may be different at different positions in a cross-section of the initiating radiation beam.

[00033] The orientation of the linear polarisation of the initiating radiation beam may have a non-zero component in an azimuthal direction. [00034] The orientation of the linear polarisation of the initiating radiation beam may have a non-zero component in a radial direction.

[00035] The method may further comprise adjusting the polarisation state of the initiating radiation beam such that the initiating radiation beam has a spatially non-uniform polarisation state in its cross-section so as to control a spatial intensity distribution at the fuel target.

[00036] The polarisation state of respective portions of the cross-section of the initiating radiation beam may be adjusted differently.

[00037] The polarisation state of each segment of the cross-section of the initiating radiation beam may be adjusted such that each segment of the cross-section is linearly polarised and such that the orientation of the linear polarisation forms an angle a with a radial direction in that segment.

[00038] The angle a may be substantially the same in each segment.

[00039] The initiating radiation beam at the fuel target may have a substantially ring shaped spatial intensity distribution.

[00040] The initiating radiation beam may have a substantially flat top spatial intensity distribution at the fuel target.

[00041] The initiating radiation beam may be a laser beam provided by a YAG laser.

[00042] The method may further comprise directing a fuel so as to provide the fuel target on which the initiating radiation beam is incident.

[00043] The fuel may comprise tin.

[00044] The initiating radiation beam may excite the fuel target to form a plasma which emits EUV radiation.

[00045] The method may further comprise collecting the EUV radiation and providing EUV radiation to a lithographic apparatus.

[00046] It will be appreciated that one or more aspects or features described in the preceding or following descriptions may be combined with one or more other aspects or features. BRIEF DESCRIPTION OF THE DRAWINGS

[00047] 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 comprising a lithographic apparatus and a radiation source according to an embodiment of the invention;

- Figure 2 is a schematic illustration of the radiation source of Figure 1 ;

Figures 3A-3C are representations of example intensity distributions formed at a plasma formation region of the radiation source of Figure 2; Figures 4A-4C are schematic illustrations of examples of the polarisation state of a laser beam;

Figure 5 is a schematic illustration of a polarisation adjustment apparatus of the radiation source of Figure 2;

- Figures 6A and 6B are schematic illustrations of the polarisation state of a laser beam before and after passing through the polarisation adjustment apparatus of Figure 5;

Figure 7 is a schematic illustration of a further example of a polarisation state of a laser beam;

Figure 8 is a schematic illustration of an alternative embodiment of the polarisation adjustment apparatus; and

Figures 9A-9C are representations of further example intensity distributions formed at a plasma formation region of the radiation source of Figure 2.

DETAILED DESCRIPTION

[00048] Figure 1 shows a lithographic system including a radiation source SO according to one embodiment of the invention. The lithographic system further comprises a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus LA 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 radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[00049] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[00050] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source. A laser 1 , which may for example be a YAG laser or a C0 ₂ laser, is arranged to deposit energy via a laser beam 2 into a fuel, which is provided from a fuel emitter 3. The laser beam 2 may be referred to as an initiating radiation beam. The laser beam 2 is directed to be incident on the fuel by a directing apparatus 12. The fuel may for example be in liquid form, and may for example be a metal or alloy, such as tin (Sn). Although tin is referred to in the following description, any suitable fuel may be used. The fuel emitter 3 is configured to emit a fuel and direct the fuel to a plasma formation region 4 so as to provide a fuel target at the plasma formation region 4. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards the plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin excites the tin to form a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma. The laser beam 1 may be used in a pulsed configuration, such that the laser beam 2 is a laser pulse. Where the fuel is provided as a droplet, a respective laser pulse may be directed at each fuel droplet.

[00051] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[00052] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 1 to the radiation source SO with the aid of a beam delivery system (not shown in Figure 1 ) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser 1 and the radiation source SO may together be considered to be a radiation system.

[00053] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source.

[00054] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam B. The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 1 1. The faceted field mirror device 10 and faceted pupil mirror device 1 1 together provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B 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 B. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 1 1.

[00055] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B 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 13, 14 in Figure 1 , the projection system may include any number of mirrors.

[00056] Figure 2 schematically illustrates an embodiment of the radiation source SO. Each fuel droplet provided by the fuel emitter 3 provides a fuel target at which the laser beam 2 is directed via the directing apparatus 12. The directing apparatus 12, which is shown in more detail in Figure 2, comprises a polarisation adjustment apparatus 21 and a focussing unit 23. In other embodiments the directing apparatus 12 may comprise more components than are shown in Figure 2. For example, the laser beam 2 may be reflected by one or more mirrors before being incident on the plasma formation region 4.

[00057] An initial pulse of laser radiation, known as a pre-pulse, may be directed at the fuel droplet before the fuel droplet reaches the plasma formation region 4. The pre-pulse may be provided by the laser 1 , or by a separate laser (not shown). Upon the fuel target reaching the plasma formation region 4, a second pulse of laser radiation in the form of the laser beam 2, which in this case may be referred to as a main-pulse, is directed at the fuel target to generate an EUV producing plasma. The pre-pulse acts to change the shape of the fuel target so that the fuel target is in a desired shape when it reaches the plasma formation region 4. For example, the fuel target may be emitted from the fuel emitter 3 with a generally spherical distribution. A pre-pulse directed at a spherical fuel target may cause a flattening of the fuel target such that the fuel target presents a disk, or pancake-like shape at the plasma formation region 4. A pre-pulse and a main pulse may both be considered to be examples of an initiating radiation beam.

[00058] Also shown in Figure 2 is a Cartesian co-ordinate system which will be adhered to throughout the figures and the description. The laser beam 2 propagates in the z- direction when it is incident on the fuel target at the plasma formation region 4. The x- direction and the y-direction are both perpendicular to the direction of propagation of the laser beam 2 at the plasma formation region 4.

[00059] The laser beam 2 is focussed by the focussing unit 23 such that the laser beam 2 has a desired cross-section at the plasma formation region 4. For example, the laser beam 2 may be focussed by the focussing unit 23 such that the cross-section of the laser beam 2 encompasses the fuel target at the plasma formation region 4 such that the laser beam 2 is incident on substantially all of the fuel target. The focussing unit 23 which is depicted in Figure 2 comprises a transmissive lens. However, in some embodiments the focussing unit 23 may take other forms. For example, the focussing unit may comprise a plurality of lenses. In some embodiments the focussing unit 23 may comprise one or more reflective elements which are shaped to reflect the laser beam 2 so as to focus the laser beam 2. In some embodiments the focussing unit 23 may comprise a combination of transmissive and reflective elements.

[00060] Figure 3A is a representation of an example of a spatial intensity distribution of the laser beam 2 at the plasma formation region. The position which is shown on the horizontal axis of Figure 3A is the position along the x-axis of the co-ordinate system which is depicted in Figure 2. The positions shown on the horizontal axis of Figure 3A are relative to the centre of the fuel target at the plasma formation region 4. The extent of the fuel target in the x-direction is represented in Figure 2 by a double-headed arrow labelled 31 . The fuel target may, for example, have an extent in the x-direction of approximately 50 μιη. In other embodiments the fuel target may have a different extent in the x-direction.

[00061] The spatial intensity distribution of the laser beam 2 which is shown in Figure 3A is substantially Gaussian shaped and is centred on the centre of the fuel target. A spatial intensity distribution of the type which is shown in Figure 3A may be referred to as a Gaussian intensity profile. The spatial intensity distribution of the laser beam 2 in the y- direction may be similar to the spatial intensity distribution of the laser beam 2 in the x- direction. That is, a representation of the spatial intensity distribution of the laser beam 2 as a function of the position on the y-axis may be similar to the spatial intensity distribution which is shown in Figure 3A as a function of the position on the x-axis.

[00062] The Gaussian intensity profile which is shown in Figure 3A causes a greater amount of energy from the laser beam 2 to be received by a central portion of the fuel target than an outer portion of the fuel target. This may cause the fuel target to expand radially outwards from the centre of the fuel target. Additionally or alternatively, a laser beam 2 having a Gaussian intensity profile may cause the fuel target to break up into two or more separate regions. In particular a Gaussian intensity profile may cause debris to be ejected from a droplet of fuel such that the debris travels radially outwards from the centre of the fuel target.

[00063] Debris which is ejected from the fuel may be deposited on components of the radiation source SO and/or the lithographic apparatus LA thereby contaminating the components. Contamination of components of the radiation source SO and/or the lithographic apparatus LA may cause a degradation of the performance of the contaminated components. For example, debris which contaminates the radiation collector 5 may reduce the reflectivity of the radiation collector 5 thereby reducing the amount of EUV radiation which is directed to the intermediate focus 6 and into the illumination system IL. It may therefore be desirable to illuminate the fuel target 2 at the plasma formation region 4 with a laser beam 2 having a spatial intensity distribution which results in a smaller amount of debris being emitted from the fuel target. It may be further advantageous to reduce the energy of any debris which is emitted from the fuel target since a reduction in the energy of the debris may reduce a distance which the debris propagates in the radiation source SO and/or the lithographic apparatus LA.

[00064] Figure 3B is a representation of an alternative example of a spatial intensity distribution of the laser beam 2 at the plasma formation region 4. The intensity distribution which is shown in Figure 3B has a substantially flat top. The flat top of the intensity distribution may, for example, be such that the intensity of the laser beam 2 is substantially constant across substantially the whole extent of the fuel target. A spatial intensity distribution of the type which is shown in Figure 3B may be referred to as a flat-top intensity profile.

[00065] Illuminating the fuel target with a laser beam 2 having a flat-top intensity profile at the plasma formation region 4 may cause less debris to be emitted from the fuel target when compared to illuminating the fuel target with a laser beam 2 having a Gaussian intensity profile. Additionally, the energy of debris which is emitted from the fuel target may be reduced by illuminating the fuel target with a laser beam 2 having a flat-top intensity profile at the plasma formation region 4.

[00066] Figure 3C is a representation of a further alternative example of a spatial intensity distribution of the laser beam 2 at the plasma formation region. The intensity distribution which is shown in Figure 3C comprises two maxima positioned either side of the centre of the fuel target and a minimum near to the centre of the fuel target. It will be appreciated that the intensity distribution which is shown in Figure 3C forms a high intensity ring around the centre of the fuel target. A spatial intensity distribution of the type shown in Figure 3C may be referred to as a ring intensity profile. Whilst the central minimum of the intensity distribution which is shown in Figure 3C reduces to zero at the centre of the fuel target, in some embodiments the central minimum of a ring intensity profile may not reduce to zero.

[00067] Similarly to the flat-top intensity profile described above, illuminating the fuel target with a laser beam 2 having a ring intensity profile at the plasma formation region 4 may cause less debris to be emitted from the fuel target when compared to illuminating the fuel target with a laser beam 2 having a Gaussian intensity profile. Additionally, the energy of debris which is emitted from the fuel target may be reduced by illuminating the fuel target with a laser beam 2 having a ring intensity profile at the plasma formation region 4. [00068] In addition to the emission of debris from the fuel target, the intensity distribution of the laser beam 2 which illuminates the fuel target may influence other properties of the fuel and/or the plasma at the plasma formation region 4. For example, the intensity distribution of the laser beam 2 may affect the shape and/or position of the plasma which is formed at the plasma formation region 4.

[00069] During a pulse of the laser beam 2, a fuel target which is illuminated by the pulse of the laser beam 2 may be elongated along the z-axis by the laser beam 2. An elongation of the fuel target along the z-axis during a pulse of the laser beam 2 may expand the region from which EUV radiation is emitted.

[00070] Additionally or alternatively a pulse of the laser beam 2 may cause a displacement of the fuel target. For example, the fuel target may be displaced in the direction of propagation of the laser beam 2 along the z-axis. Additionally or alternatively the fuel target may be displaced in the x and/or the y-directions such that the fuel target no longer lies on the z-axis. A displacement of the fuel target during a pulse of the laser beam 2 causes a displacement of the region from which EUV radiation is emitted.

[00071] It may be desirable for the position and the extent of a region from which EUV radiation is emitted to remain relatively stable throughout a pulse of the laser beam 2 and throughout the resulting pulse of EUV radiation which is emitted from the plasma. A stable fuel target may result in the emission of a pulse of EUV radiation having a relatively low etendue. A pulse of EUV radiation having a relatively low etendue may in particular be desirable when the pulse of EUV radiation is provided to a lithographic apparatus LA.

[00072] The extent to which fuel at the plasma formation region 4 elongates and/or is displaced due to being illuminated by a pulse of the laser beam 2 may depend on the intensity distribution of the laser beam 2 which is incident on the fuel target at the plasma formation region 4. For example, illuminating a fuel target at the plasma formation region 4 with a laser beam 2 which has a flat-top intensity profile or a ring intensity profile may result in less elongation and/or displacement of the fuel at the plasma formation region 4 than illuminating a fuel target at the plasma formation region 4 with a laser beam 2 which has a Gaussian intensity profile.

[00073] The intensity distribution of the laser beam 2 at the plasma formation region 4 may be additionally or alternatively influence the direction in which EUV radiation is emitted from the plasma formation region 4. In general EUV radiation is emitted from the plasma formation region 4 anisotropically. That is, more EUV radiation is emitted in some directions than in other directions. It may be desirable for EUV radiation to be preferentially emitted towards the radiation collector 5. EUV radiation which is incident on the radiation collector 5 is directed to the intermediate focus 6 and forms the radiation beam B which is provided to the lithographic apparatus LA. Increasing the portion of EUV radiation which is emitted towards the radiation collector 5 therefore increases the amount of EUV radiation which is directed to the intermediate focus 6 and which is provided to the lithographic apparatus LA.

[00074] Illuminating a fuel target at the plasma formation region 4 with a laser beam 2 which has a flat-top intensity profile or a ring intensity profile may increase the amount of EUV radiation which is emitted towards the radiation collector 5 when compared to illuminating a fuel target with a laser beam 2 having a Gaussian intensity profile.

[00075] The intensity profile of the laser beam 2 at the plasma formation region 4 may additionally influence other factors. For example, the conversion efficiency (i.e. the efficiency with which energy from the laser beam 2 is converted to EUV radiation at the plasma formation region 4) may be affected by the intensity distribution of the laser beam 2.

[00076] In order to bring about desired properties of the fuel target, the plasma and/or the emission of EUV radiation from the plasma, the intensity distribution of the laser beam 2 at the plasma formation region 4 may be controlled. For example, the intensity distribution of the laser beam 2 at the plasma formation region 4 may be controlled so as to illuminate a fuel target with a flat-top intensity profile. Alternatively the intensity distribution of the laser beam 2 at the plasma formation region 4 may be controlled so as to illuminate a fuel target with a ring intensity profile. As was described above, illumination of a fuel target with a flattop intensity profile or a ring intensity profile may bring about one or more desired effects. In alternative embodiments a fuel target may be illuminated with a laser beam 2 having another intensity profile which brings about one or more desired effects.

[00077] The intensity distribution of the laser beam 2 at the plasma formation region 4 is controlled by controlling the polarisation state of the laser beam which is focussed by the focussing unit 23. The polarisation state of the laser beam may be controlled by the polarisation adjustment apparatus 21 .

[00078] Figures 4A-C are schematic depictions of three different embodiments of the laser beam 2 before the laser beam 2 is focussed by the focussing unit 23. The laser beams 2 are shown in Figures 4A-C as viewed along the direction of propagation of the laser beam 2 (i.e. along the z-axis). Also shown in Figures 4A-C are arrows 32 which indicate the polarisation state of the laser beam 2 at different regions of the cross-section of the laser beam 2. The arrows 32 represent electric field vectors at different regions of the cross- section of the laser beam 2.

[00079] The laser beam 2 which is shown in Figure 4A has a spatially uniform linear polarisation state. That is, the polarisation state of the laser beam 2 is substantially the same over the entire cross-section of the laser beam 2. In some embodiments the laser 1 may emit a laser beam 2 having an approximately spatially uniform linear polarisation state such as the one shown in Figure 4A. [00080] The laser beam 2 which is shown in Figure 4B has a spatially non-uniform polarisation state. That is, the polarisation state of the laser beam 2 is different at different positions over the cross-section of the laser beam 2. In particular, the electric field vector at each position over the cross-section of the laser beam 2 extends in a radial direction relative to the centre of the cross-section of the laser beam 2. The polarisation state which is shown in Figure 4B may be referred to as a radial polarisation state.

[00081] The laser beam 2 which is shown in Figure 4C also has a spatially non-uniform polarisation state. The electric field vector at each position over the cross-section of the laser beam 2 extends in an azimuthal direction. That is, each electric field vector 32 extends in a direction which is perpendicular to the radial direction (i.e. perpendicular to the direction in which the electric field vector extends at the corresponding position of the laser beam 2 of Figure 4B). The polarisation state which is shown in Figure 4C may be referred to as an azimuthal polarisation state.

[00082] In order to bring about a desired spatial intensity profile of the laser beam 2 at the plasma formation region 4, it may be desirable for the laser beam 2 to have a spatially nonuniform polarisation state prior to being focussed by the focussing unit 23. For example, the laser beam 2 may have a radial polarisation state or an azimuthal polarisation state before it is focussed by the focussing unit 23. As will be described further below, in some embodiments it may be desirable for the laser beam 2 to have a polarisation state which is a combination of a radial polarisation state and an azimuthal polarisation state prior to being focussed by the focussing unit 23. Laser beams which have a radial polarisation state, an azimuthal polarisation state or a polarisation state which is a combination of a radial and an azimuthal polarisation state, all have a polarisation state which is spatially non-uniform in its cross-section. These polarisation states are all examples of linear polarisation states, where the orientation of the linear polarisation is different at different positions in the cross-section of the laser beam.

[00083] As was mentioned above, the laser beam 2 which is emitted from the laser 1 may have a spatially uniform polarisation state such as the spatially uniform linear polarisation state which is shown in Figure 4A. It may therefore be desirable to adjust the polarisation state of the laser beam 2 before the laser beam is incident on the focussing unit 23. In particular the polarisation state of the laser beam 2 may be adjusted differently at different regions of the cross-section of the laser beam 2 so as to form a laser beam 2 which has a spatially non-uniform polarisation state.

[00084] In the embodiment of Figure 2 the polarisation state of the laser beam 2 is adjusted with the polarisation adjustment apparatus 21 . Figure 5 is a schematic depiction of an embodiment of the polarisation adjustment apparatus 21 as viewed along the z-axis. The polarisation adjustment apparatus 21 comprises a segmented polarisation adjuster 22. The segmented polarisation adjuster 22 comprises eight polarisation adjusting elements 25a- 25h. Each polarisation adjustment element 25a-25h forms a segment of the segmented polarisation adjuster 22. Each polarisation adjustment element 25a-25h is configured to independently adjust the polarisation state of a portion of the laser beam 2 which is incident on the polarisation adjustment element 25a-25h. At least some of the polarisation adjustment elements 25a-25h are configured to adjust the polarisation state differently such that different portions of the laser beam 2 which are incident on different polarisation adjustment elements 25a-25h have their polarisation state adjusted differently. The segmented polarisation adjuster 22 therefore forms a laser beam 2 which has a spatially non-uniform polarisation state.

[00085] Whilst the segmented polarisation adjuster 22 is shown in Figure 5 as comprising eight polarisation adjustment elements 25a-25h, it will be appreciated that a segmented polarisation adjuster 22 may comprise more or fewer than eight polarisation adjustment elements 25a-25h.

[00086] The polarisation adjustment elements 25a-25h may, for example, comprise wave plates. The wave plates which form the polarisation adjustment elements 25a-25h may be configured differently to each other such that each wave plate adjusts the polarisation state of a portion of the laser beam 2 which is incident upon it differently.

[00087] In the embodiment which is depicted in Figure 5 each polarisation adjustment element 25a-25h comprises a half-wave plate. Each half-wave plate has a fast axis which is depicted by a double-headed arrow 27. A portion of the laser beam 2 which is incident on a half-wave plate has its polarisation state rotated about the fast axis of the half-wave plate. For example, a portion of a laser beam 2 which is incident on a half-wave plate and has an electric field vector which forms an angle β with the fast axis of the half-wave plate will be transmitted by the half -wave plate such that the transmitted portion of the laser beam 2 has an electric field vector which forms an angle of -β with the fast axis of the half-wave plate.

[00088] In the embodiment which is depicted in Figure 5 each half-wave plate which forms the polarisation adjustment elements 25a-25h has a fast axis 27 which is orientated differently to the fast axes of each of the other half-wave plates. The orientation of the fast axes are represented in Figure 5 by angles 0 _a -0 _h which the fast axes 27 of each respective polarisation adjustment elemens 25a-25h forms with a radial direction. The values of the angles 0 _a -0 _h in the embodiment of Figure 5 are 0 _a =0°, 0 _b =22.5 °, 0 _C =45 °, 0 _d =67.5 °, 0 _e =90 °, 12.5 °, 0g=135 ° and 0 _h =157.5 °. Each half-wave plate is orientated such that the value of Θ of each adjacent half-wave plate is separated by 22.5 °.

[00089] Figure 6A is a schematic depiction of the polarisation state of a laser beam 2 prior to being incident on the polarisation adjustment apparatus 21 . The laser beam 2 is shown as being separated into a plurality of segments 28a-28h. Each segment 28a-28h of the laser beam 2 is incident on a respective polarisation adjustment element 25a-25h of the polarisation adjustment apparatus 21 which is shown in Figure 5. The polarisation state of each segment 28a-28h of the laser beam 2 is represented by an arrow 32 in each segment 28a-28h. The polarisation state of each of the segments 28a-28h of the laser beam 2 is the same. In particular, each of the arrows 32 point in the negative x-direction. The laser beam 2 therefore has a spatially uniform polarisation state.

[00090] Figure 6B is a schematic depiction of the polarisation state of the laser beam 2 of Figure 6A after the laser beam 2 has passed through the polarisation adjustment apparatus which is shown in Figure 5. The polarisation state of each segment 28a-28h of the laser beam 2 is represented by an arrow 32' in each segment 28a-28h. The polarisation state of each segment 28a-28h of the laser beam 2 is rotated about the fast axis 27 of each of the half-wave plates which form the polarisation adjustment elements 25a-25h. It can be seen from Figure 6B that the effect of the segmented polarisation adjuster of Figure 5 is to form a laser beam 2 whose polarisation state is approximately an azimuthal polarisation state. It will be appreciated that the polarisation state of the laser beam 2 of Figure 6B is not truly azimuthal since the polarisation state of the laser beam 2 is uniform within each respective segment 28a-28h. Each segment 28a-28h of the laser beam 2 therefore includes regions which are proximate adjacent segments in which the polarisation direction of the laser beam 2 differs slightly from a truly azimuthal direction. The laser beam 2 which is shown in Figure 6B may however be considered to have a substantially azimuthal polarisation state.

[00091] It will be appreciated that the polarisation state of each of the segments 27a-27e of the laser beam 2 may be adjusted by adjusting the orientation of the fast axes 27 of each of the half-wave plates which form the polarisation adjustment elements 25a-25h. For example, the fast axes 27 may be orientated such that a laser beam 2 which has an approximately radial polarisation state is formed by the polarisation adjustment apparatus. In order to form a laser beam 2 having an approximately radial polarisation state (from the laser beam 2 which is shown in Figure 6A) the orientation of the fast axes 27 of the polarisation adjustment elements 25a-25h of Figure 5 may be orientated such that the angles 0 _a -0 _h are approximately 0 _a =45°, 0 _b =67.5°, 0 _C =90°, 0 _d =112.5°, 0 _e =135°, 0 _f =157.5°, 0 _g =180° and 0 _h =202.5°.

[00092] Alternatively the fast axes 27 may be orientated such that a laser beam 2 which has a polarisation state which is a combination of an azimuthal polarisation state and a radial polarisation state is formed by the segmented polarisation adjuster 22.

[00093] Figure 7 is a schematic depiction of the polarisation state of a laser beam 2 which has a polarisation state which may be considered to be a combination of an azimuthal polarisation state and a radial polarisation state. The electric field vector (denoted by arrows 32') in each segment 28a-28h has a component which extends in the azimuthal direction and a component which extends in the radial direction. It will be appreciated that the laser beam 2 which is shown in Figure 7 may be formed by rotating the fast axes 27 of each half- wave plate of the segmented polarisation adjuster of Figure 5 and passing the laser beam 2 of Figure 6A through the segmented polarisation adjuster.

[00094] In the example depicted in Figure 7 the electric field vector 32' forms an angle a with the radial direction in each of the segments 28a-28h of the laser beam 2. The angle a which is shown in Figure 7 is approximately 45°. Such a laser beam 2 may, for example, be formed with a segmented polarisation adjuster 22 whose polarisation adjustment elements 25a-25h have fast axes 27 which are orientated such that the angles 0 _a -0 _h are approximately 0 _a =22.5°, e _b =45°, 0 _C =67.5°, 0 _d =90°, 0 _e =1 12.5°, θ,=135°, 0 _g =157.5° and 0 _h =180°.

[00095] The laser beam 2 which is shown in Figure 7 is an example of a laser beam 2 which has a spatially non-uniform polarisation state. Other examples of laser beams 2 which have spatially non-uniform polarisation states include the azimuthal polarisation state shown in Figure 6B and the radial polarisation state shown in Figure 4B. Each of these polarisation states are linear polarisation states in which the orientation of the linear polarisation is different at different positions in the cross-section of the laser beam 2. A polarisation state of a laser beam 2 having a spatially non-uniform polarisation state may be quantified by an angle a which the electric field vector forms with the radial direction at each point in the laser beam 2. The angle a which is formed between the electric field vector and the radial direction is shown in each segment 28a-28h of the laser beam 2 in Figure 7. In the embodiment which is shown in Figure 7, whilst the polarisation state is different in each segment 28a-28h the angle a is the same in each segment. In particular, in the embodiment of Figure 7 the angle a is approximately 45° in each segment 28a-28h.

[00096] It will be appreciated that for a laser beam 2 which has a radial polarisation state, the angle a is equal to zero in each segment 28a-28h of the laser beam 2. It will be further appreciated that for a laser beam 2 which has an azimuthal polarisation state the angle a is equal to 90° in each segment 28a-28h of the laser beam 2. The angle a may therefore be used to quantify the component of the polarisation state which is radially polarised and the component of the polarisation state which is azimuthally polarised. The component of azimuthal polarisation increases with increasing values of a and the component of radial polarisation decreases with increasing values of a.

[00097] The value of a may be adjusted by adjusting the orientation of the fast axes 27 of each of the half-wave plates which form the polarisation adjustment elements 25a-25h. Additionally or alternatively the laser beam 2 may be passed through one or more optical elements which are configured to adjust the polarisation state of the laser beam 2. For example, the laser beam 2 may be passed through one or more wave plates (e.g. one or more half-wave plates) before or after having been transmitted by the polarisation adjustment apparatus 21 of Figure 5. The one or more wave plates may rotate the polarisation states of the laser beam segments 28a-28h by the same amount. For example, the one or more wave plates may serve to rotate the angle a of each of the segments 28a- 28h by the same amount. In this way a radially polarised radiation beam may be adjusted to form an azimuthally polarised laser beam (or vice versa) by rotating the angle a through 90°. Alternatively the angle a of each of the segments 28a-28h may be rotated to form a laser beam having a polarisation state which is a combination of a radial polarisation state and an azimuthal polarisation state.

[00098] Figure 8 is a schematic illustration of an embodiment of the polarisation adjustment apparatus 21 comprising a segmented polarisation adjuster 22, a first wave plate 24a and a second wave plate 24b. The segmented polarisation adjuster 22 may, for example, be similar to the segmented polarisation adjuster 22 which is shown in Figure 5. The segmented polarisation adjuster 22 may form a laser beam 2 having a spatially nonuniform polarisation state which may be quantified with an angle a which is formed between the electric field vector and a radial direction in each segment of the laser beam 2. The first wave plate 24a and the second wave plate 24b may serve to alter the polarisation state of the laser beam 2 by rotating the angle a. The first and second wave plates 24a, 24b may, for example, be half-wave plates having fast axes 241 and 242 respectively. The fast axis 241 of the first half-wave plate 24a is orientated differently to the fast axis of the second half- wave plate 24b such that an angle Φ is formed between the two fast axes. The first and second half-wave plates 24a, 24b together serve to rotate the polarisation direction of the laser beam (and thus the angle a) through an angle which is equal to 2Φ. For example, if the angle Φ between the fast axes is approximately 45 ° then the angle a will be rotated by approximately 90°. Such a configuration may be used, for example, to form a radially polarised laser beam from an azimuthally polarised laser beam (or vice versa). The angle Φ between the fast axes 241 , 242 of first and second wave plates may be adjusted in order to adjust the polarisation state of the laser beam 2 which is output from the polarisation adjustment apparatus 21 .

[00099] Whilst a polarisation adjustment apparatus 21 is shown in Figure 8 as comprising two half-wave plates 24a, 24b it will be appreciated that other numbers of half-wave plates may instead be used. For example the polarisation state of the laser beam 2 may be rotated using a single half-wave plate. However, rotating the polarisation state of the laser beam 2 with a pair of half-wave plates 24a, 24b as is shown in Figure 8 advantageously means that the rotation of the polarisation state of the laser beam 2 is independent of the initial polarisation state of the laser beam 2 and depends only on the angle Φ between the fast axes 241 , 242 of the first and second wave plates 24a, 24b. [000100] One or more wave plates through which the laser beam 2 passes (e.g. the first and second half-wave plates 24a, 24b) may be considered to form part of the polarisation adjustment apparatus 21 . In general the polarisation adjustment apparatus 21 may comprise any apparatus which is configured to adjust the polarisation state of the laser beam 2. The polarisation adjustment apparatus 21 may include apparatus which is configured to adjust the polarisation of different portions of the laser beam differently (e.g. the polarisation adjustment apparatus shown in Figure 5). This may allow a laser beam 2 having a spatially non-uniform polarisation state to be formed. The polarisation adjustment apparatus 21 may additionally or alternatively comprise apparatus which is configured to adjust the polarisation of all regions of the laser beam 2 in the same way.

[000101] As was described above, the polarisation state of the laser beam 2 before it is focussed by the focussing element 23 may influence the spatial intensity distribution of the laser beam 2 at the plasma formation region 4. In particular, if the laser beam 2 which is incident on the focussing element 23 has a spatially non-uniform polarisation state then the angle a which the electric field vector forms with the radial direction in each segment 28a- 28h of the laser beam 2 may affect the spatial intensity distribution of the laser beam 2 at the plasma formation region 4. In an embodiment, the angle a may be controlled in order to control the spatial intensity distribution of the laser beam 2 at the plasma formation region 4.

[000102] Figure 9A is a representation of an embodiment of the spatial intensity distribution of a laser beam 2 at the plasma formation region 4. The spatial intensity distribution which is shown in Figure 9A is formed at the plasma formation region 4 when a laser beam 2 having a radial polarisation state is focussed to the plasma formation region 4 by the focussing unit 23. In an embodiment in which a laser beam 2 having a radial polarisation state is focussed to the plasma formation region 4, the net electric field which is formed at the plasma formation region has a component which is parallel with the direction of propagation of the laser beam 2 (and thus parallel with the z-axis) and a component which is perpendicular to the direction of propagation of the laser beam 2. A component of the electric field which is parallel with the direction of propagation of the laser beam 2 may be referred to as a longitudinal component of the electric field. A component of the electric field which is perpendicular to the direction of propagation of the laser beam 2 may be referred to as a transverse component of the electric field. The longitudinal component and the transverse component combine to give a total electric field at the plasma formation region 4.

[000103] The transverse component of the laser beam 2 which is formed at the plasma formation region is shown in Figure 9A with a dotted line labelled 41 . The longitudinal component of the laser beam 2 which is formed at the plasma formation region 4 is shown in Figure 9A with a dashed line labelled 43. The longitudinal component and the transverse component combine to give a total spatial intensity distribution which is shown in Figure 9A with a solid line labelled 45. It can be seen from Figure 9A that focussing a laser beam 2 having a radial polarisation state forms a Gaussian intensity profile at the plasma formation region 4.

[000104] Figure 9B is a representation of the spatial intensity distribution at the plasma formation region 4 when a laser beam 2 having an azimuthal polarisation state is focussed to the plasma formation region 4 by the focussing unit 23. When a laser beam having an azimuthal polarisation state is focussed at the plasma formation region 4, the resulting electric field at the plasma formation region consists only of a transverse component and no longitudinal component is formed. For this reason the transverse and longitudinal components are not shown separately in Figure 9B and only the total spatial intensity distribution is shown with a solid line labelled 46. It can be seen from Figure 9B that focussing a laser beam 2 having an azimuthal polarisation state forms a ring intensity profile at the plasma formation region 4.

[000105] Figure 9C is a representation of the spatial intensity distribution at the plasma formation region 4 when a laser beam 2 having both an azimuthal polarisation component and a radial polarisation component is focussed to the plasma formation region 4 by the focussing unit 23. In the embodiment which is depicted in Figure 9C the laser beam 2 which is focussed by the focussing unit has an angle a between the electric field vectors and the radial direction in each segment 28a-28h of the laser beam 2 of approximately 24 °. The laser beam 2 therefore contains a component which is azimuthally polarised and a component which is radially polarised. The intensity of the azimuthally polarised component at the plasma formation region 4 is shown in Figure 9C with a dash-dot line labelled 47. The intensity of the radially polarised component at the plasma formation region 4 is shown in Figure 9C with a dash-dot-dot line labelled 49. The combination of radially and azimuthally polarised components forms a net electric field at the plasma formation region 4 which has both a transverse component and a longitudinal component. The transverse component is shown in Figure 9C with a dotted line 50. The longitudinal component is shown in Figure 9C with a dashed line labelled 51 . The total spatial intensity distribution which is formed at the plasma formation region 4 is shown in Figure 9C with a solid line labelled 52. It can be seen from Figure 9C that focussing a laser beam 2 having an azimuthally polarised component and a radially polarised component and an angle a of approximately 24° forms a flat-top intensity profile at the plasma formation region 4.

[000106] As was described above it is advantageous to control the spatial intensity distribution of the laser beam 2 at the fuel target so as to obtain a desired intensity distribution. As was further described above, in some situations, it may be desired to form a ring intensity profile and/or a flat-top intensity profile, rather than forming a Gaussian intensity profile at the plasma formation region 4. With reference to Figures 9B and 9C a ring intensity profile may be formed by focussing a laser beam 2 having an azimuthal polarisation state and a flat-top intensity profile may be formed by focussing a laser beam 2 having both an azimuthal polarisation component and a radial polarisation component.

[000107] It will be appreciated from Figures 9A-9C that the intensity distribution which is formed at the plasma formation region 4 may be controlled by controlling the angle a between the electric field vector and the radial direction in each segment 28a-28h of the laser beam 2. In the embodiment depicted in Figure 9A, the angle a is equal to zero, which results in a Gaussian intensity profile being formed at the plasma formation region. In the embodiment which is depicted in Figure 9B the angle a is equal to 90° which results in a ring intensity profile being formed at the plasma formation region 4. In the embodiment which is depicted in Figure 9C the angle a is equal to 24 ° which results in a flat-top intensity profile being formed at the plasma formation region 4.

[000108] Other values of the angle a may be used to form other intensity distributions at the plasma formation region 4. In particular the angle a may be varied so as to form ring intensity profiles whose central minimums reduce to different intensities. The ring intensity profile which is shown in Figure 9B has a central minimum in which the intensity substantially reduces to zero. However decreasing the value of the angle a below 90° may lead to the formation of a ring intensity profile whose central minimum does not reduce to zero. Decreasing the value of the angle a below 90° may cause the minimum intensity of the central minimum to increase with decreasing values of a until the flat-top intensity profile shown in Figure 9B is formed.

[000109] It will be appreciated that the intensity distribution which is formed at the plasma formation region 4 may additionally depend on factors other than the polarisation state of the laser beam 2 which is focussed by the focussing unit 23. For example, the intensity distribution which is formed at the plasma formation region 4 also depends on the intensity profile of the laser beam 2 which is received by the polarisation adjustment apparatus 21 . The intensity profiles at the plasma formation region 4 which are depicted in Figures 9A-9C assume that the intensity profile of the laser beam 2 which is incident on the polarisation adjustment apparatus 21 has a substantially Gaussian intensity distribution. It will be appreciated that if the laser beam 2 which is incident on the polarisation adjustment apparatus 21 has a different intensity distribution then this will affect the intensity distribution which is formed at the plasma formation region 4. However similar principles as were described above may still be used to control the intensity distribution which is formed at the plasma formation region 4. That is the polarisation state of a laser beam 2 having a non- Gaussian intensity distribution may still be adjusted in order to bring about a desired intensity distribution at the plasma formation region 4 in the manner that was described above. However the adjustment of the polarisation state of the laser beam 2 which is performed by the polarisation adjustment apparatus 21 may be different for laser beams 2 having different intensity distributions.

[000110] The intensity distribution of the laser beam 2 at the plasma formation region 4 may additionally depend on the focussing of the laser beam 2 by the focussing unit 21 . For example, a focal length of the focussing unit 21 , the diameter of the laser beam 2 which is incident on the focussing unit 21 and/or the diameter of a lens which may form part of the focussing unit 21 may all influence the intensity distribution of the laser beam 2 which is focussed at the plasma formation region 4. One or more of these parameters and/or other parameters may be controlled and/or adjusted in order to achieve a desired intensity distribution at the plasma formation region 4.

[000111] Whilst embodiments have been described above in the context of the laser beam 2 which is incident on the polarisation adjustment unit 21 having a spatially uniform linear polarisation state it will be appreciated that the laser beam 2 which is incident on the polarisation adjustment apparatus 21 may have another polarisation state. The polarisation adjustment apparatus 21 adjusts the polarisation state of the laser beam 2 such that after having passed through the polarisation adjustment apparatus 21 the laser beam 2 has a spatially non-uniform polarisation state in its cross-section so that the laser beam 2 has a desired spatial intensity distribution at the plasma formation region 4. The laser beam 2 which is incident on the polarisation adjustment apparatus may have any polarisation state and the adjustment to the polarisation state which is brought about by the polarisation adjustment apparatus 21 may be dependent on the initial polarisation state of the laser beam 2. For example, the laser beam 2 which is incident on the polarisation adjustment apparatus 21 may already have a spatially non-uniform polarisation state in its cross-section and the polarisation adjustment apparatus 21 may adjust this polarisation state so that a desired spatial intensity distribution is formed at the plasma formation region 4.

[000112] Whilst embodiments have been described above and depicted in the figures in which the polarisation state of the laser beam 2 is adjusted prior to the laser beam 2 being focussed by the focussing unit 23, in some embodiments the polarisation state may be adjusted after the laser beam 2 is focussed by the focussing unit 23. For example, the polarisation adjustment apparatus 21 may be positioned downstream of the focussing unit 23 such that the laser beam 2 which is incident on the polarisation adjustment apparatus 21 has already been focussed by the focussing unit 23.

[000113] Whilst the polarisation state and the spatial intensity distribution of a laser beam 2 which excites a fuel to form an EUV radiation emitting plasma and which may be referred to as a main-pulse laser beam have been described above, the invention may also be used to control the polarisation state and the spatial intensity distribution of a pre-pulse laser beam which acts to change the shape of a fuel target. In general the polarisation adjustment apparatus 21 and the focussing unit 23 may be operable to adjust the polarisation state of an initiating radiation beam and focus the initiating radiation beam to be incident on the fuel target. The initiating radiation beam may be a main pulse laser beam. Additionally or alternatively the initiating radiation beam may be a pre-pulse laser beam.

[000114] In an embodiment, the invention may form part of a mask inspection apparatus. The mask inspection apparatus may use EUV radiation to illuminate a mask and use an imaging sensor to monitor radiation reflected from the mask. Images received by the imaging sensor are used to determine whether or not defects are present in the mask. The mask inspection apparatus may include optics (e.g. mirrors) configured to receive EUV radiation from an EUV radiation source and form it into a radiation beam to be directed at a mask. The mask inspection apparatus may further include optics (e.g. mirrors) configured to collect EUV radiation reflected from the mask and form an image of the mask at the imaging sensor. The mask inspection apparatus may include a processor configured to analyse the image of the mask at the imaging sensor, and to determine from that analysis whether any defects are present on the mask. The processor may further be configured to determine whether a detected mask defect will cause an unacceptable defect in images projected onto a substrate when the mask is used by a lithographic apparatus.

[000115] In an embodiment, the invention may form part of a metrology apparatus. The metrology apparatus may be used to measure alignment of a projected pattern formed in resist on a substrate relative to a pattern already present on the substrate. This measurement of relative alignment may be referred to as overlay. The metrology apparatus may for example be located immediately adjacent to a lithographic apparatus and may be used to measure the overlay before the substrate (and the resist) has been processed.

[000116] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[000117] 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.

[000118] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus 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.

[000119] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.