A module for a laser-produced plasma radiation source

Abstract

A module for a laser-produced plasma radiation source includes: first optics; second optics; and a reverse beam monitoring apparatus. The first optics are arranged to receive a first radiation beam having a first wavelength and a second radiation beam having a second wavelength and to direct the first and second radiation beams so as to form a combined radiation beam propagating along a common optical path toward the second optics. The second optics includes a polarizing beam splitter. The second optics are arranged to receive the combined radiation beam and to direct at least a first portion of the combined radiation beam towards a target region. The second optics are further arranged to receive a reflected portion of the first portion of the combined radiation beam and to direct the reflected portion towards the reverse beam monitoring apparatus. Associated methods are also disclosed.

Description

A MODULE FOR A LASER-PRODUCED PLASMA RADIATION SOURCECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 530,655 which was filed on 3- August-2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present description relates to a module for a laser-produced plasma radiation source. The present description also relates to a laser-produced plasma radiation source comprising the module. The present description also relates to a lithographic system comprising such a laser-produced plasma radiation source. The present description also relates to an associated method of directing a radiation beam to a target region within a laser-produced plasma radiation source.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 at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] One type of source of EUV radiation is a laser-produced plasma (LPP) radiation source. A laser-produced plasma radiation source comprises a fuel emitter configured to direct a stream of fuel targets (for example droplets of tin) along a trajectory towards a target region and one or more laser beams arranged to intersect the trajectory at the target region. One or more radiation beams may be arranged so as to change a topology of the fuel targets before a main radiation beam is incident on the fuel targets so as to form a plasma. The plasma may produce EUV radiation, which may be directed towards an outlet of the LPP radiation source using a mirror known as a radiation collector. One or more optical modules are provided to steer and focus the one or more laser beams at the target region.SUMMARY

[0006] It may be desirable to provide a new laser-produced plasma radiation source, or a module therefor, that at least partially addresses one or more problems associated with known arrangements,whether such problems are identified herein or otherwise.

[0007] The present description relates to a module for a laser-produced plasma radiation source. In particular, the module may be an optical module operable to direct at least two radiation beams towards a target region within the laser-produced plasma radiation source so as to intersect a stream of fuel targets. The present description also relates to an associated method of directing a first radiation beam and a second radiation beam to a target region within a laser-produced plasma radiation source.

[0008] According to an aspect of the present disclosure, there is provided a module for a laser- produced plasma radiation source, the module comprising: first optics; second optics, the second optics comprising a polarizing beam splitter; and a reverse beam monitoring apparatus, wherein the first optics are arranged to receive a first radiation beam having a first wavelength and a second radiation beam having a second wavelength and to direct the first and second radiation beams so as to form a combined radiation beam propagating along a common optical path toward the second optics, wherein the second optics are arranged to receive the combined radiation beam and to direct at least a first portion of the combined radiation beam towards a target region, and wherein the second optics are further arranged to receive a reflected portion of the first portion of the combined radiation beam and to direct the reflected portion towards the reverse beam monitoring apparatus.

[0009] The module of the present disclosure can be advantageous, as now discussed.

[0010] Within a laser-produced plasma radiation source, in use, a stream of fuel targets propagate along a trajectory through the target region. The first and second radiation beams (which may each comprise a pulsed laser beam) are incident on the fuel targets. In some embodiments, a third radiation beam (which may also comprise a pulsed laser beam) may also be directed to the target region so as to be incident on the fuel targets. There is limited space within the laser-produced plasma radiation source to accommodate separate beam steering and focusing optics for a plurality of radiation beams.

[0011] Advantageously, by combining the first and second radiation beams, they can share a common optical path while still being separable for monitoring or diagnostic purposes. This can allow, for example, for optics delivering one or more other radiation beams to the target region to have a greater numerical aperture, which may improve an efficiency of the laser-produced plasma radiation source. Furthermore, using second optics that comprise a polarizing beam splitter to direct the reflected portion of the first portion of the combined radiation beam towards the reverse beam monitoring apparatus is advantageous. In particular, this is advantageous over an arrangement that uses a fixed percentage beam splitter since the fraction of the reflected portion that is received by the reverse beam monitoring apparatus can be maximized without requiring a significant increase in input power. Advantageously, this provides the facility for reverse beam diagnostics, which can provide feedback to ensure accurate alignment of each of the first and second radiation beams (and potentially a third radiation beam) to the fuel targets.

[0012] The first and second radiation beams may be arranged so as to change a topology of the fuel targets before a third, main radiation beam is incident on the fuel targets so as to form a plasma.This may increase a conversion efficiency of input energy to EUV radiation energy. The first and second radiation beams may be referred to as a pre-pulse beam and a rarefaction beam respectively.

[0013] The module may further comprise third optics. The third optics may be arranged to receive a third radiation beam having a third wavelength and to direct the third radiation beam towards the target region.

[0014] The third radiation beam may be arranged so as to form a plasma from the fuel targets. The third radiation beam may be referred to as a main beam.

[0015] The first and second wavelengths may be different, and the first optics may comprise chromatic optics.

[0016] For example, the first optics may comprise a dichroic splitter. Advantageously, with such an arrangement, the two different wavelengths can be separated for monitoring or diagnostic purposes using suitable chromatic optics and / or filters. Advantageously, such an arrangement uses passive optics for combining the first and second radiation beams, which may be more reliable and less prone to failure than active optics.

[0017] Alternatively, the first optics may comprise active optics arranged to combine the first and second radiation beams.

[0018] The first optics may comprise an electro-optic modulator.

[0019] The first optics may comprise an acousto-optic deflector.

[0020] In some embodiments, the first and second wavelengths may be of the order of 1 pm. In some embodiments, the first and second wavelengths may be similar such that a single plate can act as a quarter wave plate for both the first and second wavelengths. However, the first and second wavelengths may be sufficiently different that chromatic optics can be used as the first optics so as to form the combined radiation beam. For example, in some embodiments, a difference in the first and second wavelengths may be less than 100 nm. For example, in one embodiment, the first wavelength is 1064 nm and the second wavelength is 1030 nm.

[0021] The second optics may further comprise a quarter wave plate arranged to receive the first portion of the combined radiation beam from the polarizing beam splitter.

[0022] With such an arrangement, the combined radiation beam may be incident on the polarizing beam splitter, which may split the combined radiation beam into two linearly polarized components. For example, the polarized beam splitter may transmit a p-polarized component of the combined radiation beam and reflect a s-polarized component of the combined radiation beam. One of the two linearly polarized components may propagate through the quarter wave plate, which may convert the linearly polarized component into circularly polarized radiation having a first polarization state (for example right-handed polarized). Upon reflection (for example from a fuel target), the radiation will be in a second circularly polarized polarization state (for example left-handed polarized). Therefore, when the reflected radiation propagates back through the quarter wave plate it will then have an orthogonal polarization state to the forward radiation that propagated from the polarizing beam splitter to thequarter wave plate. Therefore, rather than propagating back towards the first optics, the reflected radiation beam will instead be directed to the reverse beam monitoring apparatus. In this way, the second optics may be considered to be arranged to direct a reverse beam to the reverse beam monitoring apparatus using polarization.

[0023] The reverse beam monitoring apparatus may comprise: a first detector; a second detector; and splitting optics arranged to: direct a first component of the reflected portion which originated from the first radiation beam to the first detector; and direct a second component of the reflected portion which originated from the second radiation beam to the second detector. The first and second detectors may each be operable to determine one or more parameters of the component received thereby.

[0024] The splitting optics may comprise one or more spectral filters and / or one or more chromatic optics (such as, for example, a dichroic splitter).

[0025] The module may further comprise a forward beam monitoring apparatus and the second optics may be further arranged to direct a second portion of the combined radiation beam towards the forward beam monitoring apparatus.

[0026] The forward beam monitoring apparatus may comprise: a first forward detector; a second forward detector; and splitting optics arranged to: direct a first component of the second portion which originated from the first radiation beam to the first forward detector; and direct a second component of the second portion which originated from the second radiation beam to the second forward detector. The first and second forward detectors may each be operable to determine one or more parameters of the component received thereby.

[0027] The splitting optics may comprise one or more spectral filters and / or one or more chromatic optics (such as, for example, a dichroic splitter).

[0028] The module may further comprise focusing optics arranged to focus the at least a first portion of the combined radiation beam at the target region.

[0029] Advantageously, parts of both the first radiation beam and the second radiation beam can be focused using these common focusing optics.

[0030] The module may further comprise: a first radiation source arranged to produce the first radiation beam; and a second radiation source arranged to produce the second radiation beam.

[0031] The first radiation source may comprise a laser. The second radiation source may comprise a laser.

[0032] A difference in the first and second wavelengths may be less than 100 nm.

[0033] For example, in one embodiment, the first wavelength is 1064 nm and the second wavelength is 1030 nm.

[0034] The module may further comprise a third radiation source arranged to produce the third radiation beam.

[0035] In one embodiment, the third radiation source is a carbon dioxide (CO2) laser and the third wavelength may be of the order of 10.6 pm.

[0036] According to an aspect of the present disclosure, there is provided a laser-produced plasma radiation source comprising: the module as described herein; a first radiation source arranged to produce the first radiation beam; a second radiation source arranged to produce the second radiation beam; and a fuel emitter arranged to direct a stream of fuel targets through the target region of the module such that the first portion of the combined radiation beam intersects a trajectory of the stream of fuel targets.

[0037] The fuel emitter may comprise a nozzle configured to direct a stream of fuel droplets along a trajectory towards the target region. The fuel may for example be in liquid form and may, for example, be a metal or alloy. For example, the fuel may comprise liquid tin.

[0038] The first radiation source may comprise a laser. The second radiation source may comprise a laser.

[0039] The laser-produced plasma radiation source may comprise a radiation collector arranged to receive EUV radiation from the target region and to direct it to an intermediate focus. The radiation collector may comprise a near normal incidence radiation collector (sometimes referred to more generally as a normal incidence radiation collector). The radiation collector may have a multilayer structure that is arranged to reflect EUV radiation (for example EUV radiation having a desired wavelength such as 13.5 nm). The radiation collector may have elliptical geometry having two ellipse focal points. A first focal point may be at the target region and a second focal point may be at the intermediate focus. Radiation that is reflected by the collector may form an EUV radiation beam. The EUV radiation beam may be focused at the intermediate focus so as to form an image of a plasma formed at the target region.

[0040] The laser-produced plasma radiation source may comprise an enclosing structure surrounding the target region and the radiation collector. The intermediate focus may be disposed at or near to an opening in the enclosing structure.

[0041] The laser-produced plasma radiation source may be configured such that the first radiation beam is incident on a fuel target and then subsequently the second radiation beam is incident on a fuel target.

[0042] The first and second radiation beams may be arranged so as to change a topology of the fuel targets before a third, main radiation beam is incident on the fuel targets so as to form a plasma. This may increase a conversion efficiency of input energy to EUV radiation energy. The first and second radiation beams may be referred to as a pre-pulse beam and a rarefaction beam, respectively.

[0043] The laser-produced plasma radiation source may further comprise a third radiation source arranged to produce a third radiation beam and the third radiation beam may also intersect a trajectory of the stream of fuel targets.

[0044] In one embodiment, the third radiation source is a carbon dioxide (CO2) laser and the third wavelength may be of the order of 10.6 pm.

[0045] A difference in the first and second wavelengths may be less than 100 nm.

[0046] For example, in one embodiment, the first wavelength is 1064 nm and the secondwavelength is 1030 nm.

[0047] According to an aspect of the present disclosure, there is provided a lithographic system comprising: the laser-produced plasma radiation source as described herein that is operable to produce an EUV radiation beam; and a lithographic apparatus arranged to receive the EUV radiation beam.

[0048] The lithographic apparatus may further comprise any or all of the following: an illumination system operable to illuminate an illumination region; a support structure configured to support a patterning device such that the patterning device is positionable in the illumination region; a substrate table configured to support a substrate; and a projection system operable to form an image of a patterning device supported by the support structure on a substrate supported by the substrate table. The illumination system may be configured to condition the EUV radiation beam before the EUV radiation beam is incident upon a patterning device. The illumination system may comprise any suitable optics operable to control a spatial and / or angular distribution of the EUV radiation beam in a plane of the patterning device.

[0049] According to an aspect of the present disclosure, there is provided a method of directing a first radiation beam and a second radiation beam to a target region within a laser-produced plasma radiation source, the method comprising: combining the first and second radiation beams using first optics and so as to form a combined radiation beam propagating along a common optical path; directing the combined radiation beam towards second optics comprising a polarizing beam splitter arranged to direct at least a first portion of the combined radiation beam towards a target region; and receiving a reflected portion of the first portion of the combined radiation beam with the second optics and to directing the reflected portion towards a reverse beam monitoring apparatus.

[0050] The method of the present disclosure is advantageous for similar reasons as discussed above in relation to the module according to the present disclosure. The method of the present disclosure may use the module of the present disclosure.

[0051] The first and second wavelengths may be different, and the first optics may comprise chromatic optics.

[0052] For example, the first optics may comprise a dichroic splitter. Advantageously, with such an arrangement the two different wavelengths can be separated for monitoring or diagnostic purposes using suitable chromatic optics and / or filters. Advantageously, such an arrangement uses passive optics for combining the first and second radiation beams, which may be more reliable and less prone to failure than active optics.

[0053] Alternatively, the first optics may comprise active optics arranged to combine the first and second radiation beams.

[0054] In some embodiments, the first and second wavelengths may be of the order of 1 pm. In some embodiments, the first and second wavelengths may be similar such that a single plate can act as a quarter wave plate for both the first and second wavelengths. However, the first and second wavelengths may be sufficiently different that chromatic optics can be used as the first optics so as toform the combined radiation beam. For example, in some embodiments, a difference in the first and second wavelengths may be less than 100 nm. For example, in one embodiment, the first wavelength is 1064 nm, and the second wavelength is 1030 nm.

[0055] The second optics may further comprise a quarter wave plate arranged to receive the first portion of the combined radiation beam from the polarizing beam splitter.

[0056] The method may further comprise: splitting the reflected portion into: a first component which originated from the first radiation beam, and a second component which originated from the second radiation beam; and determining one or more parameters of each of the first and second components.

[0057] The method may further comprise directing a second portion of the combined radiation beam towards a forward beam monitoring apparatus.

[0058] The method may further comprise: splitting the second portion into: a first component which originated from the first radiation beam, and a second component which originated from the second radiation beam; and determining one or more parameters of each of the first and second components.

[0059] The method may further comprise directing a third radiation beam towards the target region.

[0060] In one embodiment, the third radiation beam may have a wavelength of the order of 10.6 pm.BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0062] Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;

[0063] Figure 2 is a schematic illustration of a module for a laser-produced plasma radiation source of the type shown in Figure 1 according to an embodiment of the present invention;

[0064] Figure 3 is a schematic illustration of the module shown in Figure 2 with some additional detail and some more optional features shown; and

[0065] Figure 4 is a schematic illustration of a method of directing a first radiation beam and a second radiation beam to a target region within a laser-produced plasma radiation source according to an embodiment of the present invention.DETAILED DESCRIPTION

[0066] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LAcomprises 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.

[0067] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0068] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 and / or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0069] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0070] 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 relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0071] The radiation source SO shown in Figure 1 is of a type that may be referred to as a laser produced plasma (LPP) source. A laser 1, which may for example be a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) that is provided from a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may be, for example, in liquid form, and may be, for example, a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g., in the form of droplets, along a trajectory towards a 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 creates 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 ionsof the plasma.

[0072] 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 that 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.

[0073] In other embodiments of a laser produced plasma (LPP) source, the collector 5 may be a so-called grazing incidence collector that is configured to receive EUV radiation at grazing incidence angles and focus the EUV radiation at an intermediate focus. A grazing incidence collector may be, for example, a nested collector, comprising a plurality of grazing incidence reflectors. The grazing incidence reflectors may be disposed axially symmetrically around an optical axis.

[0074] 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) comprising, for example, one or more 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.

[0075] 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 SO.

[0076] Although the radiation source SO shown in Figure 1 is a laser produced plasma (LPP) source, in other embodiments, the radiation source SO may comprise: a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[0077] Although shown schematically in Figure 1 as comprising one laser 1 operable to generate a single laser beam 7 that is incident on the fuel targets emitted by the fuel emitter 3, some LPP radiation sources comprise one or more additional lasers that are also incident on the fuel targets emitted by the fuel emitter 3. For example, one or more lasers may be incident on the fuel targets emitted by the fuel emitter 3 so as to shape the fuel targets before the main laser 1 is incident in the fuel targets. Some embodiments of the present disclosure relate to a module that may form part of an LPP radiation source. In particular, such a module may form part of a beam delivery system for an LPP radiation source.

[0078] Figure 2 is a schematic illustration of a module 100 for a laser-produced plasma radiation source (of the type shown in Figure 1) according to an embodiment. Figure 3 is a schematic illustration of the new module 100 shown in Figure 2 with some additional detail and some more optional featuresshown. The module 100 comprises: first optics 102; second optics 104; and reverse beam monitoring apparatus 106.

[0079] The first optics 102 are arranged to receive a first radiation beam Bi having a first wavelength and a second radiation beam B2having a second wavelength and to direct the first and second radiation beams Bi, B2so as to form a combined radiation beam Bcpropagating along a common optical path toward the second optics 104.

[0080] The first and second wavelengths (of the first and second radiation beams Bi, B2) may be different and the first optics 102 may, for example, comprise chromatic optics. For example, the first optics 104 may comprise a dichroic splitter. Advantageously, with such an arrangement the two different wavelengths can be separated for monitoring or diagnostic purposes using suitable chromatic optics and / or filters. Advantageously, such an arrangement uses passive optics for combining the first and second radiation beams Bi, B2, which may be more reliable and less prone to failure than active optics.

[0081] Alternatively, the first optics 104 may comprise active optics arranged to combine the first and second radiation beams Bi, B2. For example, the first optics 102 may comprise an electro-optic modulator and / or an acousto-optic deflector.

[0082] In some embodiments, the first and second wavelengths (of the first and second radiation beams Bi, B2) may be of the order of 1 pm. In some embodiments, the first and second wavelengths may be similar such that a single plate can act as a quarter wave plate for both the first and second wavelengths. However, the first and second wavelengths may be sufficiently different that chromatic optics can be used as the first optics 102 so as to form the combined radiation beam Bc. For example, in some embodiments, a difference in the first and second wavelengths may be less than 100 nm. For example, in one embodiment, the first wavelength is 1064 nm and the second wavelength is 1030 nm.

[0083] The second optics 104 comprises a polarizing beam splitter. The second optics 104 are arranged to receive the combined radiation beam Bcand to direct a first portion 108 of the combined radiation beam Bctowards a target region 110. The second optics 104 are further arranged to receive a reflected portion 112 (for example from the target region 110) of the first portion 108 of the combined radiation beam Bcand to direct the reflected portion 112 towards the reverse beam monitoring apparatus 106.

[0084] The second optics 104 may further comprise a quarter wave plate arranged to receive the first portion 108 of the combined radiation beam from the polarizing beam splitter.

[0085] With such an arrangement, the combined radiation beam Bcmay be incident on the polarizing beam splitter, which may split the combined radiation beam into two linearly polarized components. For example, the polarized beam splitter may transmit a p-polarized component 108 of the combined radiation beam Bcand reflect an s-polarized component 120 of the combined radiation beam Bc. One of the two linearly polarized components 108 may propagate through the quarter wave plate, which may convert the linearly polarized component into circularly polarized radiation having a firstpolarization state (for example right-handed polarization). Upon reflection (for example from a fuel target 116), the radiation will be in a second circularly polarized polarization state (for example lefthanded polarized). Therefore, when the reflected radiation 112 propagates back through the quarter wave plate it will then have an orthogonal polarization state to the forward radiation 108 that propagated from the polarizing beam splitter to the quarter wave plate. Therefore, rather than propagating back towards the first optics 102, the reflected radiation beam 112 will instead be directed to the reverse beam monitoring apparatus 106. In this way, the second optics 104 may be considered to be arranged to direct a reverse beam to the reverse beam monitoring apparatus 106 using polarization.

[0086] As discussed further below, optionally, the second portion 120 of the combined radiation beam Bcmay be directed towards a forward beam monitoring apparatus.

[0087] The target region 110 is disposed within a laser-produced plasma radiation source (and includes a region that is generally equivalent to the plasma formation region 4 shown in Figure 1 and discussed above). The target region 110 may be disposed within an enclosure 114 of the laser-produced plasma radiation source (which may be generally equivalent to the enclosure 9 shown in Figure 1 and discussed above).

[0088] As discussed above, within the laser-produced plasma radiation source SO, in use, a stream of fuel targets 116 propagate along a trajectory (indicated by arrow 118) through the target region 110. The first and second radiation beams Bi , B2(which may each comprise a pulsed laser beam) are incident on the fuel targets 116. In some embodiments, a third radiation beam (which may also comprise a pulsed laser beam) may also be directed to the target region 110 so as to be incident on the fuel targets 116. There is limited space within the laser-produced plasma radiation source SO to accommodate separate beam steering and focusing optics for a plurality of radiation beams.

[0089] Advantageously, by combining the first and second radiation beams Bi, B2, they can share a common optical path while still being separable for monitoring or diagnostic purposes. This can allow for, for example, optics delivering one or more other radiation beams to the target region to have a greater numerical aperture, which may improve an efficiency of the laser-produced plasma radiation source. Furthermore, using second optics 104 that comprise a polarizing beam splitter to direct the reflected portion 112 of the first portion 108 of the combined radiation beam Bctowards the reverse beam monitoring apparatus 106 is advantageous. In particular, this is advantageous over an arrangement that uses a fixed percentage beam splitter since the fraction of the reflected portion that is received by the reverse beam monitoring apparatus 106 can be maximized without requiring a significant increase in input power (of the first and second radiation beams Bi, B2). Advantageously, this provides the facility for reverse beam diagnostics, which can provide feedback to help ensure accurate alignment of each of the first and second radiation beams Bi, B2(and potentially a third radiation beam) to the fuel targets 116.

[0090] The first and second radiation beams Bi, B2may be arranged so as to change a topology of the fuel targets before a third, main radiation beam is incident on the fuel targets 116 so as to form aplasma. This may increase a conversion efficiency of input energy to EUV radiation energy. The first and second radiation beams Bi, B2may be referred to as a pre-pulse beam and a rarefaction beam respectively.

[0091] Optionally, the module 100 may further comprise third optics 124, which are arranged to receive a third radiation beam B3having a third wavelength and to direct the third radiation beam B3towards the target region 110. The third radiation beam B3may be arranged so as to form a plasma from the fuel targets 116. The third radiation beam may be referred to as a main beam (MP).

[0092] The reverse beam monitoring apparatus 106 comprises: a first detector 106a; a second detector 106b; and splitting optics 106c. The splitting optics 106c are arranged to: direct a first component 126 of the reflected portion 112 which originated from the first radiation beam Bi to the first detector 106a; and direct a second component 128 of the reflected portion 112 which originated from the second radiation beam B2to the second detector 106b. The first and second detectors 106a, 106b are each operable to determine one or more parameters of the component 126, 128 of radiation received thereby.

[0093] The splitting optics 106c may comprise chromatic optics such as, for example, a dichroic splitter. Alternatively, the splitting optics 106c may comprise one or more spectral filters.

[0094] As shown schematically in Figure 3, each of the first detector 106a and the second detector 106b may be provided with focusing optics (a lens or the like) and, optionally, one or more mirrors to direct the two components 126, 128 of radiation.

[0095] Optionally, the module 100 may further comprise a forward beam monitoring apparatus 122. The second optics 104 is arranged to direct a second portion 120 of the combined radiation beam Bctowards the forward beam monitoring apparatus 122.

[0096] The forward beam monitoring apparatus 122 may comprise: a first forward detector 122a; a second forward detector 122b; and splitting optics 122c. The splitting optics 122c are arranged to: direct a first component 130 of the second portion 120 which originated from the first radiation beam Bi to the first forward detector 122a; and direct a second component 132 of the second portion 120 which originated from the second radiation beam B2to the second forward detector 122b. The first and second forward detectors 122a, 122b are each operable to determine one or more parameters of the component 130, 132 received thereby.

[0097] The splitting optics 122c may comprise chromatic optics such as, for example, a dichroic splitter. Alternatively, the splitting optics 122c may comprise one or more spectral filters.

[0098] As shown schematically in Figure 3, each of the first forward detector 122a and the second forward detector 122b may be provided with focusing optics (a lens or the like) and, optionally, one or more mirrors to direct the two components 130, 132 of radiation.

[0099] Optionally, the module 100 may further comprise focusing optics 134 arranged to focus the first portion 108 of the combined radiation beam Bcat the target region 110. Advantageously, parts of both the first radiation beam Bi and the second radiation beam B2can be focused using these commonfocusing optics 134.[000100] The first radiation beam Bi may be produced by a first radiation source 136. The first radiation 136 source may comprise a laser. The second radiation beam B2may be produced by a second radiation source 138. The second radiation source 138 may comprise a laser. For embodiments comprising third optics 124 arranged to direct a third radiation beam B3towards the target region 110, the third radiation beam B3may be produced by a third radiation source 140. The third radiation source 140 may comprise a laser. In one embodiment, the third radiation source 140 is a carbon dioxide (CO2) laser and the third wavelength may be of the order of 10.6 pm.[000101] The first, second and / or third radiation source 136, 138, 140 may form part of the module 100. Alternatively, the first, second and / or third radiation source 136, 138, 140 may be separate from the module 100.[000102] Beam transport optics 142 may be disposed between the first optics 102 and the first, second and third radiation source 136, 138, 140. Beam transport optics 142 may comprise any desirable optics such as, for example, steering optics, expansion optics or the like.[000103] As shown schematically in Figure 3, an optical path of each of the first and second radiation beams Bi, B2may be provided with adjustable focusing optics 144, 150 (for example a lens or the like). Each of the adjustable focusing optics 144, 150 may be movable so as to adjust a position of the focus of the pre-pulse (PP) beam and the rarefaction pulse (RP) beam, as indicated schematically by arrows 146, 152. Optionally, each of the first and second radiation beams Bi, B2may be provided with one or more mirrors 148 to direct the first and second radiation beams Bi, B2to the first optics 102.[000104] Optionally, the module 100 may further comprise beam steering optics disposed between the first optics 102 and the second optics 104. As shown schematically the beam steering optics may, for example, comprise two rotatable mirrors 154, 156 disposed in the optical path between the first optics 102 and the second optics 104.[000105] The third optics 124 may comprise: one or more mirrors 156, 158, 160 arranged to direct the third radiation beam B3towards the target region 110. One or more of these mirrors 160 may be adjustable or rotatable to provide some beam steering functionality.[000106] The third optics 124 may further comprise: a beam splitter 162; a forward diagnostics detector 164; and a reverse diagnostics detector 166. The beam splitter 162 may be a fixed ratio beam splitter (for example a plate beam splitter). The beam splitter 162 is arranged to direct a portion 168 of the third radiation beam B3to the forward diagnostics detector 164. The beam splitter 162 is arranged to direct a portion 170 of the third radiation beam B3that is reflected (for example from the target region 110) to the reverse diagnostics detector 166.[000107] As shown schematically in Figure 3, each of the forward diagnostics detector 164 and the reverse diagnostics detector 166 may be provided with focusing optics (a lens or the like) and, optionally, one or more mirrors to direct the two portions 168, 170 of the third radiation beam B3towards the forward diagnostics detector 164 and the reverse diagnostics detector 166 respectively.[000108] Some embodiments of the present disclosure relate to a laser-produced plasma radiation source (of the type shown in Figure 1) comprising the module 100 described above with reference to Figures 2 and 3. In general, the laser-produced plasma radiation source may comprise the first, second and third radiation sources 136, 138, 140.[000109] The laser-produced plasma radiation source further comprises a fuel emitter (for example similar to the fuel emitter 3 shown in Figure 1) that is arranged to direct a stream of fuel targets through the target region 110 of the module 100 such that the first portion 108 of the combined radiation beam Bcintersects a trajectory of the stream of fuel targets 116.[000110] The laser-produced plasma radiation source may be configured such that the first radiation beam Bi is incident on a fuel target 116 and then, subsequently, the second radiation beam B2is incident on a fuel target 116.[000111] The first and second radiation beams Bi, B2may be arranged so as to change a topology of the fuel targets 116 before a third, main radiation beam B3is incident on the fuel targets 116 so as to form a plasma. This may increase a conversion efficiency of input energy to EUV radiation energy. The first and second radiation beams Bi, B2may be referred to as a pre-pulse beam and a rarefaction beam respectively.[000112] Some embodiments of the present disclosure relate to a lithographic system (of the type shown in Figure 1) comprising the module 100 described above with reference to Figures 2 and 3 and a lithographic apparatus LA.[000113] Some embodiments of the present disclosure relate to a method of directing a first radiation beam Bi and a second radiation beam B2to a target region 110 within a laser-produced plasma radiation source SO. Such a method 200 is shown schematically in Figure 4.[000114] The method 200 comprises a step 202 of combining the first and second radiation beams Bi, B2using first optics 102 and so as to form a combined radiation beam Bcpropagating along a common optical path.[000115] The method 200 further comprises a step 204 of directing the combined radiation beam Bctowards second optics 104 comprising a polarizing beam splitter arranged to direct at least a first portion 108 of the combined radiation beam Bctowards a target region 110.[000116] The method 200 further comprises a step 206 of receiving a reflected portion 112 of the first portion 108 of the combined radiation beam Bcwith the second optics 104 and to directing the reflected portion 112 towards a reverse beam monitoring apparatus 106.[000117] The method 200 shown in Figure 4 is advantageous for similar reasons as discussed above in relation to the module 100 shown in Figures 2 and 3. The method 200 shown in Figure 4 may use the module 100 shown in Figures 2 and 3.[000118] As discussed above, the first and second wavelengths may be different and the first optics 102 may comprise chromatic optics. For example, the first optics 102 may comprise a dichroic splitter. Advantageously, with such an arrangement, the two different wavelengths can be separated formonitoring or diagnostic purposes using suitable chromatic optics and / or filters. Advantageously, such an arrangement uses passive optics for combining the first and second radiation beams Bi, B2, which may be more reliable and less prone to failure than active optics. Alternatively, the first optics 102 may comprise active optics arranged to combine the first and second radiation beams Bi, B2.[000119] In some embodiments, the first and second wavelengths may be of the order of 1 pm. In some embodiments, the first and second wavelengths may be similar such that a single plate can act as a quarter wave plate for both the first and second wavelengths. However, the first and second wavelengths may be sufficiently different that chromatic optics can be used as the first optics so as to form the combined radiation beam Bc. For example, in some embodiments, a difference in the first and second wavelengths may be less than 100 nm. For example, in one embodiment, the first wavelength is 1064 nm and the second wavelength is 1030 nm.[000120] The second optics 104 may further comprise a quarter wave plate arranged to receive the first portion of the combined radiation beam Bcfrom the polarizing beam splitter.[000121] Optionally, the method 200 may further comprise a step 208 of splitting the reflected portion 112 into: a first component 126 which originated from the first radiation beam Bi; and a second component 128 which originated from the second radiation beam Bi; and determining one or more parameters of each of the first and second components 126, 128.[000122] Optionally, the method 200 may further comprise a step 210 of directing a second portion 120 of the combined radiation beam Bctowards a forward beam monitoring apparatus.[000123] Optionally, the method 200 may further comprise a step 212 of splitting the second portion 120 into: a first component 130 which originated from the first radiation beam Bi; and a second component 132 which originated from the second radiation beam B2; and determining one or more parameters of each of the first and second components 130, 132.[000124] Optionally, the method 200 may further comprise a step 214 of directing a third radiation beam B3towards the target region 110. In one embodiment, the third radiation beam has wavelength of the order of 10.6 pm.[000125] In the embodiments discussed above the second optics 104 comprises a polarizing beam splitter and splits the combined radiation beam and its reflection using polarization. However, in some alternative arrangements, the second optics 104 may comprise a fixed percentage beam splitter.[000126] In the embodiments discussed above, the first and second radiation beams Bi, B2are combined to form a combined radiation beam Bcsuch that radiation from the first and second radiation beams Bi, B2share common optics. However, in some alternative arrangements, the first and second radiation beams Bi, B2may be directed to the target region 110 using different optics by having an angular separation between the first and second radiation beams Bi, B2.[000127] 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 opticalsystems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-fdm magnetic heads, etc.[000128] 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.[000129] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.[000130] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.[000131] 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.[000132] Clauses1. A module for a laser-produced plasma radiation source, the module comprising: first optics; second optics, the second optics comprising a polarizing beam splitter; and a reverse beam monitoring apparatus, wherein the first optics are arranged to receive a first radiation beam having a first wavelength and a second radiation beam having a second wavelength and to direct the first and second radiationbeams so as to form a combined radiation beam propagating along a common optical path toward the second optics, wherein the second optics are arranged to receive the combined radiation beam and to direct at least a first portion of the combined radiation beam towards a target region, and wherein the second optics are further arranged to receive a reflected portion of the first portion of the combined radiation beam and to direct the reflected portion towards the reverse beam monitoring apparatus.2. The module of clause 1, further comprising third optics arranged to receive a third radiation beam having a third wavelength and to direct the third radiation beam towards the target region.3. The module of clause 1 or clause 2, wherein the first and second wavelengths are different and wherein the first optics comprises chromatic optics.4. The module of any preceding clause, wherein the first optics comprises an electro-optic modulator.5. The module of any preceding clause, wherein the first optics comprises an acousto-optic deflector.6. The module of any preceding clause, wherein the second optics further comprise a quarter wave plate arranged to receive the first portion of the combined radiation beam from the polarizing beam splitter.7. The module of any preceding clause, wherein the reverse beam monitoring apparatus comprises: a first detector; a second detector; and splitting optics arranged to direct a first component of the reflected portion which originated from the first radiation beam to the first detector, and direct a second component of the reflected portion which originated from the second radiation beam to the second detector, wherein the first and second detectors are each operable to determine one or more parameters of the component received thereby.8. The module of any preceding clause, further comprising forward beam monitoring apparatus and wherein the second optics is further arranged to direct a second portion of the combined radiation beam towards the forward beam monitoring apparatus.9. The module of clause 8, wherein the forward beam monitoring apparatus comprises: a first forward detector; a second forward detector; and splitting optics arranged to direct a first forward beam monitoring apparatus component of the second portion which originated from the first radiation beam to the first forward detector, and direct a second forward beam monitoring apparatus component of the second portion which originated from the second radiation beam to the second forward detector,wherein the first and second forward detectors are each operable to determine one or more parameters of the forward beam monitoring apparatus component received thereby.10. The module of any preceding clause, further comprising focusing optics arranged to focus the at least a first portion of the combined radiation beam at the target region.11. The module of any preceding clause, further comprising: a first radiation source arranged to produce the first radiation beam; and a second radiation source arranged to produce the second radiation beam.12. The module of any preceding clause, wherein a difference in the first and second wavelengths is less than 100 nm.13. The module of any preceding clause when dependent either directly or indirectly on clause 2, further comprising a third radiation source arranged to produce the third radiation beam.14. A laser-produced plasma radiation source comprising: the module of any of clauses 1 to 12; a first radiation source arranged to produce the first radiation beam; a second radiation source arranged to produce the second radiation beam; and a fuel emitter arranged to direct a stream of fuel targets through the target region of the module such that the first portion of the combined radiation beam intersects a trajectory of the stream of fuel targets.15. The laser-produced plasma radiation source of clause 14, configured such that the first radiation beam is incident on a fuel target and then subsequently the second radiation beam is incident on a fuel target.16. The laser-produced plasma radiation source of clause 14 or clause 15 , further comprising a third radiation source arranged to produce a third radiation beam and wherein the third radiation beam also intersects a trajectory of the stream of fuel targets.17. The laser-produced plasma radiation source of any of clauses 14 to 16, wherein a difference in the first and second wavelengths is less than 100 nm.18. A lithographic system comprising: the laser-produced plasma radiation source of any of clauses 14 to 17 operable to produce an EUV radiation beam; and a lithographic apparatus arranged to receive the EUV radiation beam.19. A method of directing a first radiation beam and a second radiation beam to a target region within a laser-produced plasma radiation source, the method comprising: combining the first and second radiation beams using first optics and so as to form a combined radiation beam propagating along a common optical path; directing the combined radiation beam towards second optics comprising a polarizing beam splitter arranged to direct at least a first portion of the combined radiation beam towards the target region; andreceiving a reflected portion of the first portion of the combined radiation beam with the second optics and directing the reflected portion towards a reverse beam monitoring apparatus.20. The method of clause 19, wherein the first and second wavelengths are different and wherein the first optics comprises chromatic optics.21. The method of clause 19 or clause 20, wherein the second optics further comprises a quarter wave plate arranged to receive the first portion of the combined radiation beam from the polarizing beam splitter.22. The method of any of clauses 19 to 21, further comprising: splitting the reflected portion into a first component which originated from the first radiation beam and into a second component which originated from the second radiation beam; and determining one or more parameters of each of the first and second components.23. The method of any of clauses 19 to 22, further comprising directing a second portion of the combined radiation beam towards a forward beam monitoring apparatus.24. The method of clause 23, further comprising: splitting the second portion into a first forward beam monitoring apparatus component which originated from the first radiation beam and into a second forward beam monitoring apparatus component which originated from the second radiation beam; and determining one or more parameters of each of the first and second forward beam monitoring apparatus components.25. The method of any of clauses 19 to 24, further comprising directing a third radiation beam towards the target region.

Claims

CLAIMS1. A module for a laser-produced plasma radiation source, the module comprising: first optics; second optics, the second optics comprising a polarizing beam splitter; and a reverse beam monitoring apparatus, wherein the first optics are arranged to receive a first radiation beam having a first wavelength and a second radiation beam having a second wavelength and to direct the first and second radiation beams so as to form a combined radiation beam propagating along a common optical path toward the second optics, wherein the second optics are arranged to receive the combined radiation beam and to direct at least a first portion of the combined radiation beam towards a target region, and wherein the second optics are further arranged to receive a reflected portion of the first portion of the combined radiation beam and to direct the reflected portion towards the reverse beam monitoring apparatus.

2. The module of claim 1, further comprising third optics arranged to receive a third radiation beam having a third wavelength and to direct the third radiation beam towards the target region.

3. The module of claim 1 or claim 2, wherein the first and second wavelengths are different and wherein the first optics comprises chromatic optics.

4. The module of any preceding claim, wherein the first optics comprises an electro-optic modulator and / or an acousto-optic deflector.

5. The module of any preceding claim, wherein the second optics further comprise a quarter wave plate arranged to receive the first portion of the combined radiation beam from the polarizing beam splitter.

6. The module of any preceding claim, wherein the reverse beam monitoring apparatus comprises: a first detector; a second detector; and splitting optics arranged to direct a first component of the reflected portion which originated from the first radiation beam to the first detector, and direct a second component of the reflected portion which originated from the second radiation beam to the second detector, wherein the first and second detectors are each operable to determine one or more parameters of the component received thereby.

7. The module of any preceding claim, further comprising forward beam monitoring apparatus and wherein the second optics is further arranged to direct a second portion of the combined radiation beam towards the forward beam monitoring apparatus.

8. The module of claim 7, wherein the forward beam monitoring apparatus comprises: a first forward detector; a second forward detector; and splitting optics arranged to direct a first forward beam monitoring apparatus component of the second portion which originated from the first radiation beam to the first forward detector, and direct a second forward beam monitoring apparatus component of the second portion which originated from the second radiation beam to the second forward detector, wherein the first and second forward detectors are each operable to determine one or more parameters of the forward beam monitoring apparatus component received thereby.

9. The module of any preceding claim, further comprising focusing optics arranged to focus the at least a first portion of the combined radiation beam at the target region.

10. The module of any preceding claim, further comprising: a first radiation source arranged to produce the first radiation beam; and a second radiation source arranged to produce the second radiation beam.

11. The module of any preceding claim, wherein a difference in the first and second wavelengths is less than 100 nm.

12. The module of any preceding claim when dependent either directly or indirectly on claim 2, further comprising a third radiation source arranged to produce the third radiation beam.

13. A laser-produced plasma radiation source comprising: the module of any of claims 1 to 12; a first radiation source arranged to produce the first radiation beam; a second radiation source arranged to produce the second radiation beam; and a fuel emitter arranged to direct a stream of fuel targets through the target region of the module such that the first portion of the combined radiation beam intersects a trajectory of the stream of fuel targets.

14. A lithographic system comprising:the laser-produced plasma radiation source of claim 13 operable to produce an EUV radiation beam; and a lithographic apparatus arranged to receive the EUV radiation beam.

15. A method of directing a first radiation beam and a second radiation beam to a target region within a laser-produced plasma radiation source, the method comprising: combining the first and second radiation beams using first optics and so as to form a combined radiation beam propagating along a common optical path; directing the combined radiation beam towards second optics comprising a polarizing beam splitter arranged to direct at least a first portion of the combined radiation beam towards the target region; and receiving a reflected portion of the first portion of the combined radiation beam with the second optics and directing the reflected portion towards a reverse beam monitoring apparatus.

16. The method of claim 15, further comprising: splitting the reflected portion into a first component which originated from the first radiation beam and into a second component which originated from the second radiation beam; and determining one or more parameters of each of the first and second components.

17. The method of any of claims 15 to 16, further comprising directing a second portion of the combined radiation beam towards a forward beam monitoring apparatus.

18. The method of claim 17, further comprising: splitting the second portion into a first forward beam monitoring apparatus component which originated from the first radiation beam and into a second forward beam monitoring apparatus component which originated from the second radiation beam; and determining one or more parameters of each of the first and second forward beam monitoring apparatus components.

19. The method of any of claims 15 to 18, further comprising directing a third radiation beam towards the target region.

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