Laser beam transport system

Abstract

A new laser beam transport system comprises a structure and at least one detector. The structure comprises an inner tube, defining an input end and an output end, and an outer tube surrounding the inner tube so as to define a volume therebetween. The volume is closed such that ambient radiation cannot enter the volume. The at least one detector is configured to detect the presence of radiation in the volume and operable to generate a signal if an amount of radiation above a threshold level is detected. A new laser system comprises such a laser beam transport system and a radiation source operable to generate a radiation beam that propagates through an interior volume defined by the inner tube, from the input end to the output end. A new laser-produced plasma radiation source comprises such a laser system.

Description

LASER BEAM TRANSPORT SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 729,068 which was filed on 06 December 2024 which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a laser beam transport system. In particular, the laser beam transport system may have particular application for transporting very high power radiation beams. For example, the laser beam transport system may be suitable for radiation beams with sufficiently high power that can cut through even dense materials such as metal. The present invention also relates to a laser system comprising the laser beam transport system. The present invention also relates to a laser- produced plasma radiation (LPP) source comprising such a laser system. The LPP source may be for generating extreme ultraviolet (EUV) radiation.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] 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 (e.g., a photoresist or resist) provided on a substrate. 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 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] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses reflective mirrors instead of lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector. The multilayers may be altematingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a massproduction process. EUV mask inspection may be used for several purposes and in several differentstages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks . For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Fourthly, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0006] EUV radiation may be produced by a laser produced plasma (LPP) radiation source . Within an LPP radiation source, a laser beam may be used to irradiate fuel droplets so as to produce a plasma which will emit EUV radiation. The laser beam typically has a high power and may be delivered from a laser to the fuel droplets via a beam delivery system.

[0007] It may be desirable to provide a new beam delivery system and a laser system comprising the new beam delivery system that at least partially addresses one or more problems associated with known arrangements, whether identified herein or otherwise.SUMMARY

[0008] According to a first aspect of the present disclosure there is provided a laser beam transport system comprising: a structure, the structure comprising an inner tube defining an input end and an output end; and an outer tube surrounding the inner tube so as to define a volume therebetween, wherein the volume is closed such that ambient radiation cannot enter the volume; and at least one detector configured to detect the presence of radiation in the volume and operable to generate a signal if an amount of radiation above a threshold level is detected.

[0009] The laser beam transport system according to the first aspect of the present disclosure has particular application for transporting very high power radiation beams. In particular, radiation beams with sufficiently high power that can cut through even dense materials such as metal.

[0010] Since the volume (defined between the inner tube and the outer tube) is closed such that ambient radiation cannot enter the volume the at least one detector will not generate the signal as a result of ambient radiation.

[0011] In use, a radiation beam (for example a laser beam) can propagate through an interior volume defined by the inner tube (between the input end and the output end). In the event of a beam pointing error with such an arrangement, the laser beam may be incident on, and may cut an aperturein, a wall of the inner tube. As a result, radiation from the laser beam will enter the volume defined between the inner tube and the outer tube and can be detected by the detector. Advantageously, the outer wall can contain the laser beam and the signal can be used to trigger the laser to shut down. Therefore, advantageously, the laser beam transport system according to the first aspect of the present disclosure can be used as part of a safety system for transporting a high power laser beam.

[0012] One known safety mechanism for laser beam transport systems for high power laser beams uses one or more thermal switch-based safety apertures provided along a propagation path of the laser beam. A nominal or desired propagation path passes through the safety apertures. In the event of a pointing error, the laser beam may be incident on one of the safety apertures and may cause it to heat up. A thermal switch may activate once the temperature of the safety aperture is above a threshold level and this may be used as a trigger to shut down the laser. The laser beam transport system according to the first aspect of the present disclosure is advantageous over such known arrangements for a number of reasons. First, rather than discrete safety apertures distributed along the beam path, the structure of the laser beam transport system according to the first aspect of the present disclosure provides a continuous safety feature along the entire extent of the structure. In turn, this reduces the risk that a pointing error may occur which does not produce a signal that can be used as a trigger to shut down the laser. It also provides a simpler arrangement than such known arrangements as no complex calculations need to be performed to ensure that all possible beam pointing errors will be detected by the safety apertures. Furthermore, it avoids the use of safety apertures, which is beneficial as the construction of such safety rated apertures is difficult. Second, rather than relying upon thermal switches the laser beam transport system according to the first aspect of the present disclosure uses at least one detector configured to detect the presence of radiation (for example a photodiode or the like). Advantageously, this results in faster switching times (once radiation from the laser beam enters the volume defined between the inner tube and the outer tube) as it does not need to wait for a component to heat up before the signal can be used as a trigger.

[0013] As used here, a tube is intended to mean an elongate hollow structure. Such a tube can have any cross sectional shape such as, for example, circular or rectangular. In some embodiments, cross sectional shape of the inner tube may be similar to that of the outer tube. It will be appreciated that the inner tube may be supported by the outer tube via one or more support members extending between the inner tube and the outer tube. The inner tube and the outer tube may be generally concentric.

[0014] In some embodiments, at least part of an outer surface of the inner tube and / or the inner surface of the outer tube may be reflective for radiation having wavelength between 1.5 pm and 2.5 pm.

[0015] Additionally or alternatively, at least part of an outer surface of the inner tube and / or the inner surface of the outer tube may be reflective for radiation having wavelength of the order of 1 pm or 10 pm (for example a wavelength of 10.59 pm).

[0016] For example, the outer surface of the inner tube and / or the inner surface of the outer tube may be coated with a suitable metal coating. With such an arrangement in the event of failure of the inner tube at least a portion of a radiation beam propagating through an interior of the inner tube will be guided through the volume defined between the inner tube and the outer tube such that it is received at a plurality of different locations within the volume defined between the inner tube and the outer tube. Advantageously, this may allow for such a failure to be detected efficiently with a reduced number of detectors.

[0017] In some embodiments, at least part of an outer surface of the inner tube and / or the inner surface of the outer tube may comprise a texture.

[0018] With such an arrangement, in the event of failure of the inner tube at least a portion of a radiation beam propagating through an interior of the inner tube may be scattered in such a way that the power of a reflected portion the radiation beam may be spread out over a larger volume. That is, the texture may promote a diffuse reflection rather than specular reflection. Advantageously, this may (a) increase an amount of time before the outer tube will fail (increasing the time in which the laser can be shut down before failure of the outer tube); and / or (b) reduce the risk that the at least one detector will be damaged before it can be used to trigger a shut down of the laser.

[0019] Any suitable texture may be used. For example, the texture may comprise ridges, ribs or a stipple pattern defined on at least part of an outer surface of the inner tube and / or the inner surface of the outer tube.

[0020] In some embodiments, the at least one detector may comprise a safety-rated photo diode system.

[0021] In some embodiments, the laser beam transport system may comprise a plurality of detectors.

[0022] Advantageously, this provides some redundancy.

[0023] In some embodiments, the laser beam transport system may further comprise a pressure control mechanism that is operable to reduce a pressure of the volume defined between the inner tube and the outer tube relative to an interior volume defined by the inner tube.

[0024] The pressure control mechanism may comprise a pump that is operable to reduce a pressure of the volume defined between the inner tube and the outer tube. Additionally or alternatively, the pressure control mechanism may comprise a pump that is operable to increase a pressure in an interior volume defined by the inner tube.

[0025] Advantageously, with such an arrangement, in the event of failure of the inner tube particles, fumes or other debris will tend to be directed away from the interior volume defined by the inner tube and so away from the beam path.

[0026] In some embodiments, the laser beam transport system may further comprise a purge gas system operable to provide a purge gas to an interior volume defined by the inner tube.

[0027] The purge gas may comprise nitrogen. This purge gas system may be considered to form part of a pressure control mechanism.

[0028] In some embodiments, the structure may comprise a plurality of modules, each module defining a portion of the inner tube and a portion of the outer tube.

[0029] That is, the structure may be segmented. Advantageously, with such an arrangement in the event of failure of the inner tube only a subset of the plurality of modules need to be replaced.

[0030] In some embodiments, at least one detector of the at least one detector may be disposed in the volume defined between the portions of the inner tube and the outer tube defined by each of the plurality of modules.

[0031] In some embodiments, the plurality of modules may be arranged such that radiation can propagate between the volumes defined between the portions of the inner tube and the outer tube defined by each pair of adjacent modules.

[0032] With such an arrangement there may be fewer detectors than modules. Advantageously, this can reduce the complexity and cost of the laser beam transport system.

[0033] In some embodiments, a wall thickness of the inner tube may be smaller than a wall thickness of the outer tube.

[0034] In some embodiments, the inner tube and / or the outer tube may comprise aluminium.

[0035] In some embodiments, the outer tube may comprise copper.

[0036] In some embodiments, the inner tube and / or the outer tube may comprises stainless steel.

[0037] In some embodiments, the outer tube may have a thickness of the order of 40 mm or larger.

[0038] According to a second aspect of the present disclosure there is provided a laser system comprising: a radiation source operable to generate a radiation beam; and a laser beam transport system according to the first aspect of the present disclosure arranged such that the radiation beam propagates through an interior volume defined by the inner tube, from the input end to the output end.

[0039] In some embodiments, the radiation beam may have a power of the order of 50 kW or more.

[0040] Safety measures, such as provided by the at least one detector, may be especially important for such high power laser systems, which pose a significant risk of damage if the laser radiation were able to escape the housing. In some embodiments, the radiation beam may have a power of the order of 100 kW or more.

[0041] In use, the output radiation beam of such embodiments may form part of a laser-produced plasma radiation source. In particular, the radiation beam of such embodiments may be directed so as to be incident on a target (for example a tin droplet) so as to generate a tin plasma.

[0042] In some embodiments, the radiation source may be operable to generate radiation with a wavelength between 1.5 pm and 2.5 pm.

[0043] Such radiation may be generally referred to as 2 pm radiation. In some embodiments, the radiation source (for example a laser module) may be operable to generate radiation with a wavelength of approximately 2 pm.

[0044] Additionally or alternatively, the radiation source (for example a laser module) may be operable to generate radiation with a wavelength of the order of 10 pm (for example 10.59 pm).

[0045] In some embodiments, the laser system may further comprise a controller that is configured to receive the signal from the or each at least one detector and is operable to stop operation of the radiation source upon receipt of a signal indicative that an amount of radiation above a threshold level is detected by at least one detector.

[0046] For example, the controller may be operable to send one or more control signals so as to close a safety shutter of the radiation source upon detection of light between the inner and outer tube.

[0047] The controller may be operable to send one or more control signals to the radiation source so as to control operation thereof. For example, the controller may be operable to control: at least one power supply so as to power a pump generator of a radiation source. Additionally or alternatively, the controller may be operable to control: a coolant system of the radiation source. Additionally or alternatively, the controller may be operable to control: a purge gas system of the radiation source.

[0048] In some embodiments, the outer tube may be such that it would take 0.5 seconds or more for a wall of the outer tube to be penetrated by the radiation beam.

[0049] According to a third aspect of the present disclosure there is provided a laser-produced plasma radiation source comprising: a fuel generator operable to generate a stream of fuel targets along a fuel target trajectory; the laser system according to the second aspect of the present disclosure, wherein the output radiation beam is directed to intersect the fuel target trajectory at a plasma formation region; and collector optics arranged to direct radiation originating from the plasma formation region to an output so as to form a second output radiation beam.

[0050] Features of different aspects of the present disclosure may be combined together.BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 schematically depicts a lithographic system according to an embodiment of the present disclosure, which comprises a lithographic apparatus and a radiation source;Figure 2 schematically depicts a system for (actinic) mask inspection according to an embodiment of the present disclosure, which comprises a radiation source;Figure 3A schematically shows a first cross section of a new laser beam transport system according to an embodiment of the present disclosure;Figure 3B schematically shows a second cross section of the new laser beam transport system shown in Figure 3 A (in an orthogonal plane);Figure 4 schematically shows the new laser beam transport system shown in Figures 3A and 3B in the event of a beam pointing error and an associated failure of the inner tube of the laser beam transport system;Figure 5 schematically shows one known safety mechanism for laser beam transport systems for high power laser beams that uses one or more thermal switch -based safety apertures provided along a propagation path of a laser beam;Figure 6 shows an enlarged portion of the inner tube and the outer tube of the new laser beam transport system shown in Figures 3A and 3B according to one type of embodiment wherein an outer surface of the inner tube and the inner surface of the outer tube comprise a texture;Figure 7 shows a first variant of the laser beam transport system shown in Figures 3A and 3B wherein the structure comprises a plurality of modules, each module defining a portion of an inner tube and a portion of an outer tube and wherein each of the modules is generally of the form of the structure shown in Figure 3A;Figure 8 shows a second variant of the laser beam transport system shown in Figures 3A and 3B wherein the structure comprises a plurality of modules, each module defining a portion of an inner tube and a portion of an outer tube and wherein each of the modules is arranged such that radiation can propagate between the volumes defined between the portions of the inner tube and the outer tube defined by each pair of adjacent modules;Figure 9 schematically shows a new laser system according to an embodiment of the present disclosure, the new laser system comprising: a radiation source and the new laser beam transport system shown in any one of Figures 3A to 8; the new laser system may, for example, form part of the lithographic system shown in Figure 1 ; andFigure 10 schematically shows a new laser-produced plasma radiation source according to an embodiment of the present disclosure which comprises a new laser system of the type shown in Figure 9; the new laser-produced plasma radiation source may, for example, form part of the lithographic system shown in Figure 1 .DETAILED DESCRIPTION

[0052] 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 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.

[0053] 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 faceted field mirror device 10 and a faceted 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 mayinclude other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0054] 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 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).

[0055] 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.

[0056] 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.

[0057] The lithographic apparatus LA and radiation source SO described herein can be used for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from the radiation source SO to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0058] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1 is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region4. 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 electrons with ions of the plasma 7.

[0059] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, 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 mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelengthsuch as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0060] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system (if present) may together be considered to be a radiation system.

[0061] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. 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.

[0062] Figure 2 schematically depicts a mask inspection system MS for (actinic) mask inspection. The mask inspection system MS can be used to identify or inspect defects in a mask to be used in a lithographic process by means of the lithographic apparatus described in figure 1. The mask inspection system MS comprises a radiation source SO, an illumination system ILM, a detection system DS and a mask stage ME.

[0063] The illumination system ILM is configured to condition an EUV radiation beam B before the EUV radiation beam B is incident upon a mask MA supported by the mask stage ME. The illumination system ILM may provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system ILM may comprise a plurality of mirrors 22. The illumination system ILM may comprise one or more faceted mirror devices.

[0064] The mask stage ME may be configured to support and move a mask MA relative to the EUV radiation beam B, so that the EUV radiation beam is incident upon different areas of the mask.

[0065] The detection system DS comprises a detector 20, and may in addition comprise a plurality of mirrors 24. The plurality of mirrors 24 may be configured to collect EUV radiation BRthat has been reflected from the mask MA, and form an image of the mask MA on the detector 20 (which may be an imaging array). A processor (not depicted) may receive signals output from the detector 20 and use those signals to look for defects in the mask MA.

[0066] The radiation source SO of the mask inspection system MS may correspond with the radiation source SO depicted in Figure 1. In the same way as depicted in Figure 1, the radiation source SO of the mask inspection system MS may focus the EUV radiation beam B to form an intermediate focus 6.

[0067] Some embodiments of the present disclosure relate to new laser beam transport systems. Examples of such new laser beam transport systems are now discussed with reference to Figures 3A to8. These new laser beam transport systems may be used, for example, as part of a beam delivery system for delivering the laser beam 2 from the laser system 1 to the radiation source SO shown in Figure 1.

[0068] Figures 3A and 3B schematically show two cross sections of an embodiment of a new laser beam transport system 100 in two orthogonal planes. The laser beam transport system 100 comprises a structure 102 and at least one detector 104. The structure 102 comprises an inner tube 106 defining an input end 108 and an output end 110. The structure 102 further comprises an outer tube 112 surrounding the inner tube 106 so as to define a volume 114 therebetween.

[0069] The volume 114 is closed such that ambient radiation cannot enter the volume 114. In Figure 3 A, this is indicated schematically by two generally annular walls 116, 118 extending between the inner tube 106 and the outer tube 112, proximate to the input end 108 and the output end 110 respectively.

[0070] The at least one detector 104 is configured to detect the presence of radiation in the volume 114 and is operable to generate a signal if an amount of radiation above a threshold level is detected.

[0071] The laser beam transport system 100 shown in Figures 3A and 3B has particular application for transporting very high power radiation beams. In particular, radiation beams with sufficiently high power that can cut through even dense materials such as metal.

[0072] Since the volume 114 (defined between the innertube 106 and the outertube 112) is closed such that ambient radiation cannot enter the volume 114 the at least one detector 102 will not generate the signal as a result of ambient radiation.

[0073] In use, a radiation beam 120 (for example a laser beam) can propagate through an interior volume 122 defined by the inner tube 106 (between the input end 108 and the output end 110). In the event of a beam pointing error with such an arrangement, the laser beam 120 may be incident on, and may cut an aperture in, a wall of the inner tube 106. Such an arrangement is shown schematically in Figure 4, which shows the same cross section to that shown in Figure 3A in the event of such a beam pointing error and associated failure of the inner tube 106.

[0074] As a result, radiation from the laser beam 120 will enter the volume 114 defined between the inner tube 106 and the outer tube 112 and can be detected by the at least one detector 104. Advantageously, the outer tube 112 can contain the laser beam 120 and the signal generated by the at least one detector 104 can be used to trigger the laser to shut down. Therefore, advantageously, the laser beam transport system 100 can be used as part of a safety system for transporting a high power laser beam.

[0075] One known safety mechanism for laser beam transport systems for high power laser beams is shown schematically in Figure 5 and uses one or more thermal switch-based safety apertures 30 provided along a propagation path of a laser beam 32. A nominal or desired propagation path passes 34 through the safety apertures 30. In the event of a pointing error, the laser beam may be incident on one of the safety apertures 30 and may cause it to heat up. A thermal switch may activate once the temperature of the safety aperture 30 is above a threshold level and this may be used as a trigger to shutdown the laser. The new laser beam transport system shown in Figures 3A to 4 is advantageous over such known arrangements for a number of reasons. First, rather than discrete safety apertures 30 distributed along the beam path 34, the structure 102 of the new laser beam transport system 100 provides a continuous safety feature along the entire extent of the structure 102. In turn, this reduces the risk that a pointing error may occur which does not produce a signal that can be used as a trigger to shut down the laser. It also provides a simpler arrangement than such known arrangements as no complex calculations need to be performed to ensure that all possible beam pointing errors will be detected by the safety apertures 30. Furthermore, it avoids the use of safety apertures 30, which is beneficial as the construction of such safety rated apertures is difficult. Second, rather than relying upon thermal switches 30 the new laser beam transport system 100 uses at least one detector 104 configured to detect the presence of radiation (for example a photodiode or the like). Advantageously, this results in faster switching times (once radiation from the laser beam 120 enters the volume 114 defined between the inner tube 106 and the outer tube 112) as it does not need to wait for a component to heat up before the signal can be used as a trigger.

[0076] As used here, a tube (for example the inner tube 106 and the outer tube 112) is intended to mean an elongate hollow structure. Such a tube 106, 112 can have any cross sectional shape such as, for example, circular (as shown in Figure 3B) or rectangular. In some embodiments, cross sectional shape of the inner tube 106 may be similar to that of the outer tube 112. It will be appreciated that the inner tube 106 may be supported by the outer tube 112 via one or more support members extending between the inner tube 106 and the outer tube 112. For example, the two generally annular walls 116, 118 may be considered to be such support members . The inner tube 106 and the outer tube 112 may be generally concentric.

[0077] In some embodiments, at least part of an outer surface of the inner tube 106 and / or the inner surface of the outer tube 112 may be reflective for radiation 120 that the laser beam transport system 100 will, in use, be used to transport. It will be appreciated that the laser beam transport system 100 may be used for any wavelength of radiation that it is desired to transport. In some embodiments, at least part of an outer surface of the inner tube 106 and / or the inner surface of the outer tube 112 may be reflective for radiation having wavelength between 1.5 pm and 2.5 pm. Additionally or alternatively, at least part of an outer surface of the inner tube 106 and / or the inner surface of the outer tube 112 may be reflective for radiation having wavelength of the order of 1 pm or 10 pm (for example a wavelength of 10.59 pm).

[0078] For example, the outer surface of the inner tube 106 and / or the inner surface of the outer tube 112 may be coated with a suitable metal coating. With such an arrangement in the event of failure of the inner tube 106 at least a portion of a radiation beam 120 propagating through an interior 122 of the inner tube 106 will be guided through the volume 114 defined between the inner tube 106 and the outer tube 108 such that it is received at a plurality of different locations within said volume 114.Advantageously, this may allow for such a failure to be detected efficiently with a reduced number of detectors 104.

[0079] Figure 6 which shows an enlarged portion of the inner tube 106 and the outer tube 112 according to one type of embodiment. As shown schematically in Figure 6, in some embodiments, at least part of an outer surface 124 of the inner tube 106 and / or the inner surface 126 of the outer tube 112 may comprise a texture.

[0080] With such an arrangement, in the event of failure of the inner tube 106 at least a portion of a radiation beam 120 propagating through an interior 122 of the inner tube 106 may be scattered in such a way that the power of a reflected portion the radiation beam 120 may be spread out over a larger volume. That is, the texture may promote a diffuse reflection rather than specular reflection. Advantageously, this may (a) increase an amount of time before the outer tube 112 will fail (increasing the time in which the laser can be shut down before failure of the outer tube 112); and / or (b) reduce the risk that the at least one detector 104 will be damaged before it can be used to trigger a shut down of the laser.

[0081] Any suitable texture may be used. For example, the texture may comprise ridges, ribs or a stipple pattern defined on at least part of an outer surface 124 of the inner tube 106 and / or the inner surface 126 of the outer tube 112.

[0082] In some embodiments, the at least one detector 104 comprises a safety-rated photo diode system.

[0083] In some embodiments, the laser beam transport system 100 may comprise a plurality of detectors 104. Advantageously, this provides some redundancy.

[0084] In some embodiments, the laser beam transport system 100 may further comprise a pressure control mechanism 128 that is operable to reduce a pressure of the volume 114 defined between the inner tube 106 and the outer tube 112 relative to an interior volume 122 defined by the inner tube 106.

[0085] The pressure control mechanism 128 may comprise a pump 130 that is operable to reduce a pressure of the volume 114 defined between the inner tube 106 and the outer tube 112. Additionally or alternatively, the pressure control mechanism 128 may comprise a pump that is operable to increase a pressure in an interior volume 122 defined by the inner tube 106.

[0086] Advantageously, with such an arrangement, in the event of failure of the inner tube particles, fumes or other debris will tend to be directed away from the interior volume 122 defined by the inner tube 106 and so away from the beam 120 path.

[0087] In some embodiments, the laser beam transport system 100 may further comprise a purge gas system 132 operable to provide a purge gas to an interior volume 122 defined by the inner tube 106. The purge gas may comprise nitrogen. This purge gas system 132 may be considered to form part of a pressure control mechanism 128, as indicated schematically in Figure 3A.

[0088] In some embodiments of the laser beam transport system 100, the structure 102 may comprise a plurality of modules, each module defining a portion of the inner tube and a portion of theouter tube. That is, the structure 102 may be segmented. Advantageously, with such an arrangement in the event of failure of the inner tube 106 only a subset of the plurality of modules need to be replaced. Such embodiments are now discussed with reference to Figures 7 and 8.

[0089] In one example, as shown schematically in Figure 7, the structure 102 may comprise a plurality of modules 102a, 102b, 102c, each of which is generally of the form of the structure 102 shown in Figure 3A and described above. For such embodiments, at least one detector 104 is disposed in the volume 114 defined between the portions of the inner tube 106 and the outer tube 112 defined by each of the plurality of modules 102a, 102b, 102c.

[0090] In another example, as shown schematically in Figure 8, the structure 102 may comprise a plurality of modules 102a, 102b, 102c that are arranged such that radiation can propagate between the volumes 114 defined between the portions of the inner tube 106 and the outer tube 112 defined by each pair of adjacent modules 102a, 102b, 102c. In order to achieve this, some of the generally annular walls 116, 118 may be at removed (or may be partially removed to allow for radiation to propagate between the volumes 114 defined between the portions of the inner tube 106 and the outer tube 112 defined by each pair of adjacent modules 102a, 102b, 102c). Note that the modules 102a, 102c at each end of the structure 102 may comprise a wall 116, 118 at an end of that module 102a, 102c that is distal to its adjacent module 102b. This may ensure that the volume 114 defined between the inner tube 106 and the outer tube 112 is closed such that ambient radiation cannot enter the volume 114.

[0091] With such an arrangement, as shown in Figure 8, there may be fewer detectors 104 than modules 102a, 102b, 102c. For example, in this simple example, the laser beam transport system 100 is shown with three modules 102a, 102b, 102c and only a single radiation detector 104. Advantageously, this can reduce the complexity and cost of the laser beam transport system 100.

[0092] In some embodiments of the laser beam transport system 100, a wall thickness of the inner tube 106 may be smaller than a wall thickness of the outer tube 112.

[0093] In some embodiments of the laser beam transport system 100, the inner tube 106 and / or the outer tube 112 may comprise aluminium. In some embodiments of the laser beam transport system 100, the outer tube 112 may comprise copper. In one embodiment, the outer tube 112 may comprise a layer of aluminium and a layer of copper.

[0094] In some embodiments of the laser beam transport system 100, the outer tube 112 may have a thickness of the order of 40 mm or larger.

[0095] Some embodiments of the present disclosure relate to a new laser system comprising a new laser beam transport system 100 of the type described above with reference to Figures 3 A to 8. Examples of such new laser systems are now discussed with reference to Figure 9. Such a new laser systems may be used, for example, as the laser system 1 and at least a part of a beam delivery system for delivering the laser beam 2 from the laser system 1 to the radiation source SO, as shown in Figure 1 and described above.

[0096] Figure 9 schematically shows an embodiment of a new laser system 200. The laser system 200 comprises: a radiation source 202 and the new laser beam transport system 100 described above with reference to Figures 3 A to 8.

[0097] The radiation source 202 is operable to generate a radiation beam 204. The laser beam transport system 100 is arranged such that the radiation beam 204 propagates through an interior volume 122 defined by the inner tube 106, from the input end 108 to the output end 110.

[0098] In some embodiments of the laser system 200, the radiation beam 204 may have a power of the order of 50 kW or more. Safety measures, such as provided by the at least one detector 104, may be especially important for such high power laser systems, which pose a significant risk of damage if the laser radiation 204 were able to escape the interior volume 122 of the inner tube 106. In some embodiments, the radiation beam 204 may have a power of the order of 100 kW or more.

[0099] In use, the output radiation beam 204 of such embodiments may form part of a laser- produced plasma radiation source. In particular, the radiation beam 204 of such embodiments may be directed so as to be incident on a target (for example a tin droplet) so as to generate a tin plasma.[000100] In some embodiments of the laser system 200, the radiation source 202 may be operable to generate radiation 204 with a wavelength between 1.5 pm and 2.5 pm.[000101] Such radiation 204 may be generally referred to as 2 pm radiation. In some embodiments, the radiation source 202 (for example a laser module) may be operable to generate radiation 204 with a wavelength of approximately 2 pm.[000102] Alternatively, in some embodiments of the laser system 200, the radiation source 202 may be operable to generate radiation 204 with a wavelength of the order of 10 pm (for example 10.59 pm). [000103] In some embodiments, the laser system 200 may further comprise a controller 206. The controller 206 may be configured to receive the signal 134 from the or each at least one detector 104. The controller 206 may be operable to stop operation of the radiation source 202 upon receipt of a signal 134 indicative that an amount of radiation above a threshold level is detected by at least one detector 104.[000104] For example, the controller 206 may be operable to send one or more control signals 208 so as to close a safety shutter of the radiation source 202 upon detection of light between the inner tube 106 and the outer tube 112.[000105] The controller 206 may be operable to send one or more control signals 208 to the radiation source 202 so as to control operation thereof. For example, the controller 206 may be operable to control: at least one power supply so as to power a pump generator of a radiation source 202. Additionally or alternatively, the controller 206 may be operable to control: a coolant system of the radiation source 202. Additionally or alternatively, the controller 206 may be operable to control: a purge gas system of the radiation source 202.[000106] Additionally or alternatively, the controller 206 may be operable to control a shutter that, when closed, is configured to block radiation from entering the interior 122 of the inner tube 106. Inparticular, the shutter may be configured such that, when closed, it blocks radiation 204 from entering the interior 122 of the inner tube 106 form the radiation source 202. When the shutter is open it may allow the radiation beam 204 output by the radiation source 202 to propagate through an interior volume 122 defined by the inner tube 106, from the input end 108 to the output end 110, as described above. Such a shutter may form part of the radiation source 202 and / or the laser beam transport system 100. Alternatively, such a shutter may be separate form the radiation source 202 and the laser beam transport system 100.[000107] In some embodiments of the laser system 200, the outer tube 112 of the structure 102 may be such that it would take 0.5 seconds or more for the wall of the outer tube 112 to be penetrated by the radiation beam 204. That is, a thickness and material of the outer tube 112 may be selected with reference to the power of the radiation beam 204 to ensure that it would take 0.5 seconds or more for the wall of the outer tube 112 to be penetrated by the radiation beam 204.[000108] Some embodiments of the present disclosure relate to a new laser-produced plasma radiation source comprising a new laser system 200 of the type described above with reference to Figure 9. Such a new laser-produced plasma radiation source may be used, for example, as the radiation source SO shown in Figure 1 and described above.[000109] Figure 10 schematically shows such a new laser-produced plasma radiation source 300.[000110] The new laser-produced plasma radiation source 300 comprises: a fuel generator 302; the laser system 200 described above with reference to Figure 9 and collector optics 304.[000111] The a fuel generator 302 is operable to generate a stream of fuel targets along a fuel target trajectory 306. The laser-produced plasma radiation source 300 is arranged such that the output radiation beam 204 of the laser system 200 is directed to intersect the fuel target trajectory 306 at a plasma formation region 308. The collector optics 304 is arranged to direct radiation 310 originating from the plasma formation region 308 to an output 312 so as to form a second output radiation beam 314.[000112] 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 substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.[000113] 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, 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.[000114] The following clauses also form part of the disclosure:I . A laser beam transport system comprising: a structure, the structure comprising an inner tube defining an input end and an output end; and an outer tube surrounding the inner tube so as to define a volume therebetween, wherein the volume is closed such that ambient radiation cannot enter the volume; and at least one detector configured to detect the presence of radiation in the volume and operable to generate a signal if an amount of radiation above a threshold level is detected.2. The laser beam transport system of clause 1 wherein at least part of an outer surface of the inner tube and / or the inner surface of the outer tube is reflective for radiation having wavelength between 1.5 pm and 2.5 pm.3. The laser beam transport system of any preceding clause wherein at least part of an outer surface of the inner tube and / or the inner surface of the outer tube comprises a texture.4. The laser beam transport system of any preceding clause wherein the at least one detector comprises a safety-rated photo diode system.5. The laser beam transport system of any preceding clause comprising a plurality of detectors.6. The laser beam transport system of any preceding clause further comprising a pressure control mechanism that is operable to reduce a pressure of the volume defined between the inner tube and the outer tube relative to an interior volume defined by the inner tube.7. The laser beam transport system of any preceding clause further comprising a purge gas system operable to provide a purge gas to an interior volume defined by the inner tube.8. The laser beam transport system of any preceding clause wherein the structure comprises a plurality of modules, each module defining a portion of the inner tube and a portion of the outer tube.9. The laser beam transport system of clause 8 wherein at least one detector of the at least one detector is disposed in the volume defined between the portions of the inner tube and the outer tube defined by each of the plurality of modules.10. The laser beam transport system of clause 8 or clause 9 wherein the plurality of modules are arranged such that radiation can propagate between the volumes defined between the portions of the inner tube and the outer tube defined by each pair of adjacent modules.I I . The laser beam transport system of any preceding clause wherein a wall thickness of the inner tube is smaller than a wall thickness of the outer tube.12. The laser beam transport system of any preceding clause wherein the inner tube and / or the outer tube comprises aluminium.13. The laser beam transport system of any preceding clause wherein the outer tube comprises copper.14. The laser beam transport system of any preceding clause wherein the outer tube has a thickness of the order of 40 mm or larger.15. A laser system comprising : a radiation source operable to generate a radiation beam; and a laser beam transport system according to any preceding clause arranged such that the radiation beam propagates through an interior volume defined by the inner tube, from the input end to the output end.16. The laser system of clause 15 wherein the radiation beam has a power of the order of 50 kW or more.17. The laser system of clause 15 or clause 16 wherein the radiation source is operable to generate radiation with a wavelength between 1.5 pm and 2.5 pm.18. The laser system of any one of clauses 15 to 17 further comprising a controller that is configured to receive the signal from the or each at least one detector and is operable to stop operation of the radiation source upon receipt of a signal indicative that an amount of radiation above a threshold level is detected by at least one detector.19. The laser system of any one of clauses 15 to 18 wherein the outer tube is such that it would take 0.5 seconds or more for a wall of the outer tube to be penetrated by the radiation beam.20. A laser-produced plasma radiation source comprising: a fuel generator operable to generate a stream of fuel targets along a fuel target trajectory; the laser system of any one of clauses 15 to 19, wherein the output radiation beam is directed to intersect the fuel target trajectory at a plasma formation region; and collector optics arranged to direct radiation originating from the plasma formation region to an output so as to form a second output radiation beam.[000115] 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.

Claims

CLAIMS1. A laser beam transport system comprising: a structure, the structure comprising an inner tube defining an input end and an output end; and an outer tube surrounding the inner tube so as to define a volume therebetween, wherein the volume is closed such that ambient radiation cannot enter the volume; and at least one detector configured to detect the presence of radiation in the volume and operable to generate a signal if an amount of radiation above a threshold level is detected.

2. The laser beam transport system of claim 1 wherein at least part of an outer surface of the inner tube and / or the inner surface of the outer tube is reflective for radiation having wavelength between 1.5 pm and 2.5 pm.

3. The laser beam transport system of any preceding claim wherein the at least one detector comprises a safety-rated photo diode system.

4. The laser beam transport system of any preceding claim further comprising a pressure control mechanism that is operable to reduce a pressure of the volume defined between the inner tube and the outer tube relative to an interior volume defined by the inner tube.

5. The laser beam transport system of any preceding claim further comprising a purge gas system operable to provide a purge gas to an interior volume defined by the inner tube.

6. The laser beam transport system of any preceding claim wherein the structure comprises a plurality of modules, each module defining a portion of the inner tube and a portion of the outer tube.

7. The laser beam transport system of claim 6 wherein at least one detector is disposed in the volume defined between the portions of the inner tube and the outer tube defined by each of the plurality of modules.

8. The laser beam transport system of claim 6 or claim 7 wherein the plurality of modules are arranged such that radiation can propagate between the volumes defined between the portions of the inner tube and the outer tube defined by each pair of adjacent modules.

9. The laser beam transport system of any preceding claim wherein the inner tube and / or the outer tube comprises aluminium.

10. The laser beam transport system of any preceding claim wherein the outer tube comprises copper.

11. The laser beam transport system of any preceding claim wherein the outer tube has a thickness of the order of 40 mm or larger.

12. A laser system comprising: a radiation source operable to generate a radiation beam; and a laser beam transport system according to any preceding claim arranged such that the radiation beam propagates through an interior volume defined by the inner tube, from the input end to the output end.

13. The laser system of claim 12 wherein the radiation source is operable to generate radiation with a wavelength between 1.5 pm and 2.5 pm.

14. The laser system of any one of claims 12 to 13 further comprising a controller that is configured to receive the signal from the, or each of, at least one detector and is operable to stop operation of the radiation source upon receipt of a signal indicative that an amount of radiation above a threshold level is detected by at least one detector.

15. The laser system of any one of claims 12 to 14 wherein the outer tube is such that it would take 0.5 seconds or more for a wall of the outer tube to be penetrated by the radiation beam.

16. A laser-produced plasma radiation source comprising: a fuel generator operable to generate a stream of fuel targets along a fuel target trajectory; the laser system of any one of claims 12 to 15, wherein the output radiation beam is directed to intersect the fuel target trajectory at a plasma formation region; and collector optics arranged to direct radiation originating from the plasma formation region to an output so as to form a second output radiation beam.

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