WO2025201796A1 - System and method for irradiating a fuel target - Google Patents

Abstract

A laser system arranged to irradiate a fuel target at a target destination with a laser pulse. The laser system is arranged to produce a plurality of sub-pulses of predetermined pulse length that are temporally combined to form the laser pulse having a specific duration. The laser system further comprises at least one optical element arranged to spatially separate each of the sub-pulses to be incident on the fuel target at corresponding target destinations.

Description

SYSTEM AND METHOD FOR IRRADIATING A FUEL TARGET

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24167180.9 which was filed on 28 March 2024 which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a laser system and a method for irradiating a fuel target at a plasma formation region. The laser system may form part of a laser produced plasma (LPP) radiation source. The LPP radiation source may produce extreme ultraviolet (EUV) radiation and may form part of a lithographic system.

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] EUV radiation for a lithographic apparatus may be produced by a laser produced plasma (LPP) radiation source. Within a LPP radiation source, a laser beam may be used to irradiate fuel droplets (a fuel target) so as to produce a plasma which will emit EUV radiation.

[0006] It is desirable that the laser beam which is used to illuminate the fuel droplets produces a high EUV source power. A seed (or drive) laser system may be used to provide a pulsed laser beam that is then amplified using optical amplifiers. The optical amplifiers increase the power of the pulsed laser beam. The amplified pulsed laser beam is incident upon fuel droplets, thereby generating EUV radiation.

[0007] CO2 drive lasers are an option, but they require one or more pre-pulse(s) to deform the fuel droplet to a more advantageous shape for optimum laser light absorption and EUV emission. Alternative drive lasers exist, such as 1 or 2pm solid-state lasers. However, they also have issues which makes them less than preferable or have challenges fortheir use. [0008] It may be desirable to provide a laser system and method of irradiating a fuel target which overcomes problems associated with the prior art, in a manner which is not disclosed or suggested by the prior art.

SUMMARY

[0009] According to a first aspect of the invention, there is provided a laser system arranged to irradiate a fuel target at a target destination with a laser pulse, wherein the laser system is arranged to produce a plurality of sub-pulses of predetermined pulse length that are temporally combined to form the laser pulse having a specific duration, and wherein the laser system further comprises at least one optical element arranged to spatially separate each of the sub-pulses to be incident on the fuel target at corresponding target destinations.

[00010] Advantageously, the sub-pulses are able to be better spatially placed in order to obtain optimum intensity distribution for EUV creation over the full propulsion trajectory of the fuel target. This may allow EUV source power to be increased, e.g. without involving improving conversion efficiencies (CE) or ramping up the pressure in the droplet emitter even more.

[00011] The fuel target may be a fuel droplet from a fuel emitter. The sub-pulses being temporally combined means that a relatively long single pulse is not required. Combining the sub-pulses means that the desired power level can be reached. The laser pulse leads to a significant propulsion of the fuel target (droplet) over the course of the laser pulse. This significant propulsion of the droplet may not be desirable with respect to EUV radiation collection for some embodiments of optics systems.

[00012] There may be a plurality of optical elements. The optical element(s) focus the sub -pulse (i.e. each sub-pulse laser beam) at a different spatial point (from other sub-pulses). This allows for movement of the fuel target over time, e.g. from one corresponding target destination (with respect to one sub-pulse) to another corresponding target destination (with respect to another sub-pulse). That is, the corresponding target destinations may be considered to be in different locations over time. The movement of the fuel target is primarily due to the effect of the sub -pulses being incident on the fuel target (i.e. z-kick). Taking the movement of the fuel target over time is necessary as the overall time (the specific duration) of the laser pulse (i.e. the combination of the sub-pulses) is longer than for previous laser systems and so the droplet moves relatively further, which needs to be taken into account accordingly.

[00013] The laser system may comprise a plurality of sub-modules that are each configured to fire respective sub-pulses at different times in sequence such that the sub-pulses are temporally combined substantially consecutively to produce the laser pulse.

[00014] The sub-pulses may be fired at different times rather than all at once towards the fuel target. The sub-modules may be not fired simultaneously. Each sub-module may be considered to be a separate laser. Each sub-module may be ON for the duration of the predetermined pulse length of the sub-pulse. Each sub-module may be turned ON at a different time and turned OFF at a different time. [00015] The laser system may comprise at least one of: a range of 2-20 sub-modules, a range of 10- 20 sub-modules, a range of 2-15 sub-modules, a range of 10-15 sub-modules, 2 sub-modules, 3 submodules and 10 sub-modules.

[00016] The sub-modules may be solid state laser modules. The sub-modules may be fiber laser systems. The sub-modules may be arranged for coherent beam combining. There may be coherent beam combining of the fiber laser systems. The laser system may be arranged for spatial beam combining or polarization beam combining between the sub-modules.

[00017] The sub-pulses may partially overlap temporally with the respective adjacent sub-pulse.

[00018] This may create a smooth overall temporal profile. The sub-pulse length and time of firing of each sub-pulse may be optimized separately to create a desired overall temporal profile.

[00019] The laser pulse may have a substantially flat top temporal profile.

[00020] The laser pulse may be considered to have a box-shape profile in the temporal domain.

[00021] The sub-pulses may have a substantially flat top spatial beam profile at the corresponding target destinations. The sub-pulses may have a Gaussian beam profile.

[00022] The substantially flat top spatial beam profile may be considered to be a constant intensity through a cross-section of the laser beam. The spatial profile and instantaneous intensity may be optimized for maximizing conversion efficiencies and output in-band EUV energy.

[00023] The at least one optical element may comprise at least one of optical elements, a telescope, a lens, lenses, a convex lens, a concave lens, a mirror, a curved mirror, and mirrors.

[00024] The optical element(s) may control beam size and divergence.

[00025] The pulse length of each sub-pulse may be at least one of: substantially the same; in a range of 10-30ns; in a range of 10-100ns; in a range of 60-100ns; in a range of 200-300ns / number of subpulses; 20 ns; and 100ns.

[00026] The pulse length of each sub-pulse may be shorter than the duration of the laser pulse . The pulse length of the sub-pulses may be individually optimized.

[00027] The pulse length of the laser pulse may be at least one of: in a range of 100-300ns, and 200 ns.

[00028] It will be appreciated that the laser pulse referred to here is the combined laser pulse (from the combination of the plurality of sub-pulses).

[00029] The laser pulse may have a wavelength of at least one of: in a range of 1.5 - 3pm, and 2pm.

[00030] The sub-pulses may have a wavelength of at least one of: in a range of 1.5 - 3pm, and 2pm.

[00031] The laser pulse may have a power of at least one of: in a range of 60-100kW, and 50kW.

[00032] It will be appreciated that the laser pulse referred to here is the combined laser pulse (from the combination of the plurality of sub-pulses). The sub-pulse may have a power of at least one of: in a range of 6-10kW, in a range of 60-100kW / number of sub-pulses. Each sub-module may deliver a 5kW sub-pulse.

[00033] The laser system may comprise an optical arrangement configured to produce the sub- pulses from an incident pulse.

[00034] The incident pulse may be at least one of: a main pulse, a main pulse from a main -pulse module, a sub-pulse or sub-pulses, a sub-pulse or sub-pulses from at least one of the plurality of submodules.

[00035] The optical arrangement may comprise at least one electro -optical modulator configured to split the incident pulse into the sub-pulses.

[00036] The optical arrangement may comprise at least one polarization splitting optical element configured to separate the sub-pulses into different paths.

[00037] The polarization splitting optical element may comprise at least one of: a polarizing beam splitter, a thin film polarizer and a crystal polarizer.

[00038] The electro-optical modulator may be configured to switch polarization of the incident pulse, preferably substantially half-way through the incident pulse.

[00039] The electro-optical modulator may be configured to switch polarization of the incident pulse substantially instantaneously or gradually over time.

[00040] The electro-optical modulator may be configured to change divergence of the incident pulse, preferably at a predetermined time or gradually over time.

[00041] The optical arrangement may be configured to produce a respective sub-pulse from another of the sub-pulses.

[00042] The optical arrangement may comprise a plurality of electro -optical modulators, wherein at least one of the plurality of electro-optical modulators may be configured to split the other of the subpulses to produce the respective sub-pulse.

[00043] The optical arrangement may comprise a plurality of polarization splitting optical elements, wherein at least one of the plurality of polarization splitting optical elements may be configured to separate the other of the sub-pulses and the respective sub-pulse.

[00044] The optical arrangement may be configured to merge the sub-pulses onto the same path after the sub-pulses have passed through respective optical elements arranged to spatially separate the sub-pulses.

[00045] The optical arrangement may comprise at least one polarization combining optical element or at least one combining electro -optical modulator configured to merge the sub-pulses onto the same path.

[00046] The optical arrangement may comprise final optical systems located after respective optical elements arranged to spatially separate the sub-pulses, preferably wherein the final optical systems may be angled with respect to each other.

[00047] The optical arrangement may be configured to produce sub-pulses from the merged subpulses.

[00048] The optical arrangement may comprise at least two electro -optical modulators. At least one electro-optical modulator (and at least one polarization splitting optical element) may be located after (e.g. on the same optical path as) another of the at least one electro-optical modulators (and another of the at least one polarization splitting optical elements). The at least one electro -optical modulator may be configured to split the merged sub-pulses into the sub-pulses.

[00049] According to a second aspect of the invention, there is provided a laser produced plasma radiation source comprising: a fuel emitter operable to provide the fuel target at a plasma formation region; and the laser system as described above.

[00050] According to a third aspect of the invention, there is provided a lithographic system comprising: the laser produced plasma radiation source as described above; and a lithographic apparatus.

[00051] The lithographic apparatus may be an EUV lithographic apparatus.

[00052] According to a fourth aspect of the invention, there is provided a method comprising: producing a plurality of sub-pulses of predetermined pulse length; temporally combining the plurality of sub pulses to form the laser pulse having a specific duration, and spatially separating, using at least one optical element of the laser system, each of the sub-pulses to be incident on the fuel target at corresponding target destinations.

[00053] The method may further comprise firing respective sub-pulses at different times in sequence from a plurality of sub-modules such that the sub-pulses are temporally combined substantially consecutively to produce the laser pulse.

[00054] The method may further comprise producing the sub-pulses from an incident pulse using an optical arrangement.

[00055] According to a fifth aspect of the invention, there is provided a computer program comprising computer readable instructions configured to cause a processor to carry out a method as described above.

[00056] According to a sixth aspect of the invention, there is provided a computer readable medium carrying a computer program as described above.

[00057] According to an seventh aspect of the invention, there is provided a computer apparatus comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory; wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[00058] 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 comprising a lithographic apparatus and a radiation source;

Figure 2 schematically depicts a laser system according to an embodiment of the invention; Figure 3 depicts a graph of power vs. time of sub-pulses produced by the laser system according to an embodiment of the invention;

Figure 4 depicts a graph of spatial intensity of a sub-pulse produced by the laser system according to an embodiment of the invention;

Figure 5 depicts a method for using a laser system for a lithographic system according to an embodiment of the invention

Figure 6 schematically depicts a laser system according to an embodiment of the invention;

Figure 7 depicts graphs of voltage over time for an electro-optical modulator and beam intensity level over time of two sub-pulses produced by the laser system according to an embodiment of the invention;

Figure 8 depicts graphs of voltage over time for an electro-optical modulator for a different voltage switching time and beam intensity level over time of two sub-pulses produced by the laser system according to an embodiment of the invention;

Figure 9 schematically depicts a laser system according to an embodiment of the invention;

Figure 10 schematically depicts a laser system according to an embodiment of the invention;

Figure 11 schematically depicts a laser system according to an embodiment of the invention;

Figure 12 schematically depicts a laser system according to an embodiment of the invention.

DETAILED DESCRIPTION

[00059] 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.

[00060] 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.

[00061] 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).

[00062] 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.

[00063] 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.

[00064] 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, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. 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 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 tin 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.

[00065] The EUV radiation from the plasma 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 wavelength such 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.

[00066] 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 may together be considered to be a radiation system.

[00067] 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.

[00068] The laser system 1 is depicted schematically in more detail in Figure 2 and is illustrated with a dashed line. The laser system 1 may be considered to be a seed (or drive) laser system. The laser system 1 comprises a plurality of sub-modules; a first sub-module 20A (shown uppermost in Figure 2), a second sub-module 20B (shown in the middle in Figure 2) and a third sub-module 20C (shown lowermost in Figure 2). The plurality of sub-modules 20A, 20B, 20C are each configured to fire respective sub-pulses 22 of predetermined pulse length. Each sub-module 20A, 20B, 20C may be considered to be a separate laser that fires the sub-pulses 22. The sub-pulses 22 are combined to form the laser beam 2 (which may be referred to as a laser pulse). Although three sub-modules 20A, 20B, 20C are shown in Figure 2, it will be appreciated that there may be more or less sub-modules than this. For example, there may be only two sub-modules or there may be ten sub-modules.

[00069] The laser pulse 2 is incident on a droplet 26 (more generally referred to as a fuel target) in the plasma formation region 4 (shown as dotted line in Figure 2). The laser pulse 2 may be considered to be a combined laser pulse. That is, the sub-pulses 22 are combined to form the laser pulse 2. In particular the sub-pulses 22, having a predetermined pulse length, are temporally combined to form the laser pulse 2 having a specific duration. Each of the sub-modules 20A, 20B, 20C are configured to fire respective sub-pulses 22 at different times in sequence so that sub-pulses 22 are temporally combined substantially consecutively to produce the laser pulse 2. The sub-pulses 22 from the plurality of submodules 20A, 20B, 20C being temporally combined means that there is not a relatively long single pulse from a single laser module.

[00070] The sub-pulses 22 may be fired at different times rather than all at once towards the droplet 26. In embodiments, each sub-module 20A, 20B, 20C may be ON for the duration of the predetermined pulse length of the sub-pulse 22. Each sub-module 20A, 20B, 20C is turned ON at a different time and turned OFF at a different time in order to produce the desired combined laser pulse 2.

[00071] In this embodiment, the sub-modules 20A, 20B, 20C are solid state laser modules and are fiber laser systems. The sub-modules 20A, 20B, 20C are arranged for coherent beam combining of the fiber laser systems. The laser system 1 may be arranged for spatial beam combining or polarization beam combining between the sub-modules 20A, 20B, 20C. It will be appreciated that, in other embodiments, the sub-modules may be different types of laser modules.

[00072] In embodiments, each sub-pulse 22 has a wavelength of 2 pm (substantially 2pm). Thus, the (combined) laser pulse 2 also has a wavelength of 2pm. It will be appreciated that, in other embodiments, the wavelength of the laser beams may be different. For example, the wavelength of the sub-pulse 22 and the laser pulse 2 may have a wavelength in a range of 1.5-3 pm, 1-5 pm, 1-3 pm, 1.5- 5pm, greater than 1pm, greater than 1.5pm, less than 3pm, and/or less than 5pm. A 1pm solid state drive laser may suffer from significantly lower conversion efficiencies (CE), when compared with a 10.6um CO2 drive laser (although it may work fine also albeit for lower CE). However, this is not an issue for a 2pm solid state laser drive laser. Indeed, using lasers having a wavelength of 2 pm instead of, e.g. a 10.6um CO2 drive laser, has a number of advantages. The first advantage is its higher ‘wallplug efficiency’, which is the efficiency of transforming electrical energy into laser energy. The second is the fact that a 2pm drive laser could run without any pre-pulse(s) . The third is that the 2pm drive laser is capable of significantly higher single pulse energies. This has the potential to increase EUV source power without involving improving conversion efficiencies (CE) or ramping up the pressure in the droplet emitter even more.

[00073] For a 2pm drive laser, no target preshaping is required and instead a relatively long (e.g. ~200ns) main-pulse (or laser pulse) is used when compared to the 10.6um CO2 drive laser, which may have a main pulse length of ~60ns. This relatively long pulse will lead to significant propulsion of the droplet over the course of the laser pulse leading to the plasma probing a significant path along the laser caustic. Such “z-kick” distances may go up to 800pm. However, using the plurality of sub-modules 20A, 20B, 20C and combining the sub-pulses 22 in the manner described to produce a combined laser beam 2 allows this significant propulsion to be accommodated as will be explained in more detail.

[00074] Beam steering may be used to steer the sub-pulses 22 to the plasma formation region 4. For example, in this embodiment, optical components 28 are used to align the sub-pulses 22 with each other. Since the sub-pulses 22 are separated in time, coherent combination is not possible. Therefore, either polarization combination or spatial combination may be used. For polarization combination, an electro-optic modulator (EOM) may be required if more than 2 beams (sub-pulses 22) are combined. Other components may be required for combining the sub-pulses. For simplicity of drawing, the polarization active components are omitted in Figure 2, and only generic optical components 28 are shown schematically. It will be appreciated that these optical components 28 may be any as required to combine the sub-pulses 22 with each other depending on the type of combination (e.g. polarization or spatial) used. Furthermore, in this embodiment, a mirror 30 is used to direct the (combined) laser pulse 2 to the plasma formation region 4.

[00075] The laser system 1 includes respective optical elements 32 arranged to spatially separate each of the sub-pulses 22 to be incident on the droplet 26 at corresponding target destinations. In this embodiment, the optical elements 32 form a telescope (shown schematically). The telescope comprises a convex lens and a concave lens. It will be appreciated that, in other embodiments, the optical elements 32 may be different. For example, in other embodiments, the optical elements 32 may be a lens, lenses, a mirror, a curved mirror, curved mirrors or mirrors. The optical element(s) may control beam size and divergence.

[00076] The optical element 32 focus the sub-pulse 22 (i.e. each sub-pulse laser beam) at a different spatial point (from the other sub-pulses) in the plasma formation region 4. This allows for movement of the droplet 26 over time, e.g. from one corresponding target destination (with respect to one subpulse) to another corresponding target destination (with respect to another sub-pulse). That is, the corresponding target destinations may be considered to be in different locations over time. [00077] This is illustrated in Figure 2 which shows a plurality of target destinations 34A, 34B, 34C, for the droplet 26 corresponding to each respective sub-pulse 22 from each respective sub-module 20A, 20B, 20C. More particularly, the first sub-module 20A has a corresponding target destination 34A (shown in dotted line) where the droplet 26 is located. The second sub-module 20B has a corresponding target destination 34B where the droplet 26 will be located at the time when the sub-pulse 22 from the second sub-module 20B arrives at this location. Finally, the third sub-module 20C has a corresponding target destination 34C where the droplet 26 will be located at the time when the sub-pulse 22 from the third sub-module 20C arrives at this location. Each of the sub-pulses 22 from the respective submodules 20A, 20B, 20C will be focused at the corresponding target destinations 34A, 34B, 34C. For example, the sub-pulse 22 from the second sub-module 20B will be focused at the target destination 34B.

[00078] In general, the first sub-module 20A will be aligned and setup to provide the desired intensity at the droplet 26 target location (i.e. at the target destination 34A) where the droplet 26 is located at the correct time. The next laser sub-modules 20B, 20C provide subsequent sub-pulses 22 at the actual target locations (i.e. at target destinations 34B, 34C) where the droplet 26 is located at the correct time. It will be appreciated that continuously increasing acceleration of the droplet 26 (i.e. the target) will need to be considered. This is because each time a sub-pulse 22 is incident on the droplet 26 it is given a further kick and thus propelled faster.

[00079] The movement of the droplet 26 over time is primarily due to the effect of the sub-pulses 22 being incident on the droplet 26 and providing a source of propulsion in the z-direction (i.e. a z- kick). Lateral movement may also be taken into consideration as the droplet 26 may be fired at significant speeds (e.g. ~100m/s) in the y-direction. This means that in the duration (e.g. 200ns) of the laser pulse 2, there is a lateral shift (-perpendicular to light propagation direction) of a certain distance (e.g. ~20um). This lateral shift is still small (though not negligible) compared to the z-shift (which may be ~800um). Thus, the movement in the z-direction, and also the y-direction, may be taken into account when focusing the sub-pulses 22 at the corresponding target destinations 34A, 34B, 34C. The effect of gravity on the droplet 26 can be ignored as the contribution of gravity to the movement of the droplet 26 is negligible. Significant propulsion of the droplet 26 may not be desirable with respect to EUV radiation collection for some embodiments of optics systems.

[00080] Taking the movement of the droplet 26 over time into account is necessary as the overall time (the specific duration) of the laser pulse 2 (i.e. the combination of the sub-pulses 22) is longer than for previous laser systems and so the droplet moves relatively further, which needs to be taken into account accordingly. For example, the specific duration of the laser pulse 2 may be ~200ns rather than e.g. ~60ns for previous laser systems. The benefit of having the plurality of sub-pulses 22, rather than a single long laser pulse (from a single laser module), means that the sub-pulses 22 are able to be better spatially placed (e.g. when compared to a single laser pulse) in order to obtain optimum intensity distribution for EUV creation over the full propulsion trajectory of the droplet 26. Using a greater number of sub-modules (e.g. ten sub-modules rather than three sub-modules as depicted in Figure 2), means that each sub-pulse may have a relatively shorter pulse length and there will be even better spatial distribution of the sub-pulses 22 in order to obtain further improvement to the optimum intensity distribution for EUV creation over the full propulsion trajectory.

[00081] In embodiments, the pulse length of each sub-pulse 22 is substantially the same length (i.e. lasts for substantially the same time). In general, the pulse length of each sub-pulse 22 is shorter than the duration of the laser pulse 2. In embodiments where there are ten sub-modules, each sub-pulse 22 lasts for e.g. ~20ns. Thus, this provides a pulse length of the (combined) laser pulse 2 of ~200ns. It will be appreciated that, in other embodiments, with a different number of sub-modules, the duration of each sub-pulse may be different and may be tailored according to the desired length of the combined laser beam. For example, the pulse length of the sub-pulse may be in a range of 10-30ns, in a range of 10-100ns, in a range of 60-100ns, or in a range of 200-300ns / number of sub-pulses. As an example, if there are only two or three 3 sub-modules, the sub-pulses from these two or three sub-modules may be in the 60ns-100ns range. More generally, the length of a sub-pulse may be approximately -200- 300ns / number of sub-pulses. The pulse length of the (combined) laser pulse may be in a range of 100- 300ns. The pulse length of the sub-pulses may be individually optimized. When different numbers of sub-modules are used (e.g. 10 or 3 sub-modules), the overall pulse length would be the same. The pulse length would be set by the initial droplet size and the laser intensity choice.

[00082] In embodiments, each sub-module 20A, 20B, 20C delivers a power of 5kW. In embodiments where there are e.g. ten sub-modules, and each sub-module delivers a power of 5kW, then the (combined) laser pulse 2 has a power of 50kW. It will be appreciated that, in other embodiments, e.g. with a different number of sub-modules, the power provided by each sub-module may be different and may be tailored according to the desired power of the combined laser beam. For example, the subpulses 22 may have a power in a range of 6-10kW. Thus, the laser pulse 2 may have a power in a range of 60-100kW. More generally, the power of a sub-pulse may be approximately ~60-100kW / number of sub-pulses. It will be appreciated that the laser pulse 2 referred to here is the combined laser pulse 2 (from the combination of the plurality of sub-pulses 22) - this may be referred to as the overall continuous wave (CW) power. This is distinct from the power of the sub -pulses 22 - which may be referred to as pulse intensity. In an embodiment, there may be (only) two sub-pulses, e.g. from two 40kW sub-modules (lasers), which are combined into one 80kW laser pulse. Each of the two 40kW sub-modules may produce a 100ns sub-pulse which is combined into one 200ns laser pulse with two focal points. It will be appreciated that, in some embodiments, the sub-pulses may be formed from a plurality of incident sub-pulses. These incident sub-pulses may be produced by incident sub-modules. For example, five incident sub-pulses from five 5kW incident sub-modules may be combined to form a 25kW sub-pulse of 100ns (effectively forming a 25kW sub-module) which is then combined with another 25kW sub-pulse of 100ns (effectively forming a second 25kW sub-module) to form a 50kW laser pulse of 200ns. [00083] Figure 3 shows a graph of power of sub-pulses 22 over time. There are four rather than three sub-pulses 22 in this embodiment (i.e. there are four sub-modules). The graph illustrates how the separate sub-pulses 22 can be temporally overlapped to create a smooth overall temporal intensity profde. In this embodiment, the shape formed by each sub-pulse 22 is an isosceles trapezoid and it can be seen that the first sub-pulse begins before the second sub-pulse has finished and there is an overlapping section O (shown by dotted line). Thus, for example, where the power of the first subpulse has decreased to an approximate midpoint, the power of the second sub-pulse has increased to an approximate midpoint. Thus, the combination of both the first sub-pulse and the second sub-pulse at this point, and across the overlapping section O, add up to approximately the same power level as each sub-pulse 22 when at its maximum power level. Therefore, the (combined) laser pulse 2 will have a substantially flat top temporal profile. The (combined) laser pulse 2 may be considered to have an overall box-shape profile in the temporal domain, more particularly in this example, forming an overall isosceles trapezoid shape (shown by dashed line). It will be appreciated that, in other embodiments, the shape of the sub-pulses in the temporal domain may be different and may be optimised for the particular situation.

[00084] Figure 4 shows a graph of spatial intensity of the sub-pulse 22. Only one sub-pulse is shown in Figure 4 but it will be appreciated that this is also applicable to the other sub-pulses. It can be seen that the sub-pulse 22 has a substantially flat top spatial beam profile at the droplet 26 located at e.g. the corresponding target destination 34A. That is, the intensity of the sub-pulse is substantially level over the diameter of the droplet 26. Although the substantially flat top spatial beam profile of the sub-pulse 22 is shown in Figure 2 as being substantially the same diameter as the droplet, it will be appreciated that this is just an example, and it may have a diameter that is substantially larger than the droplet diameter, e.g. up to 2 times or even 3 times larger. The substantially flattop spatial beam profile may be considered to be a constant intensity through a cross-section of the laser beam. The spatial profile and instantaneous intensity may be optimized for maximizing conversion efficiencies and output in-band EUV energy. It will be appreciated that, the sub-pulses 22 do not need to have a substantially flat top spatial beam profile. In other embodiments, the sub-pulses may have a different profile, e.g. a Gaussian profile (i.e. be a Gaussian beam). The laser pulse 2 may be of a flat top-type with relatively good beam quality and resolution. As an example, given a 100pm flattop with, e.g. 20pm resolution on its edges at 1.88pm then an NA (numerical aperture) of order 0.1 (resolution ~ lambda/NA) may be required. This would translate to a depth of field DoF (DoF ~ lambda/2NA2=100um) after which most of the advantageous properties of the flattop beam delivery would disappear.

[00085] Figure 5 illustrates a method 100 for irradiating the fuel target (e.g. the droplet 26) at the target destination 34A, 34B, 34C with the laser pulse 2.

[00086] In a first step 102, a plurality (three) of sub-pulses 22 of predetermined pulse length are produced by the three sub-modules 20A, 20B, 20C. [00087] In a second step 104, the plurality of sub-pulses 22 are temporally combined to form the (combined) laser pulse 2 having a specific duration.

[00088] In a third step 106, each of the three sub-pulses 22 are spatially separated to be incident on the fuel target (droplet 26) at corresponding target destinations 30A, 30B, 30C from the respective submodules 20A, 20B, 20C using the respective optical elements 32 of the laser system 1.

[00089] Figure 6 illustrates another embodiment of the invention. Specifically, Figure 6 shows a laser system 40 comprising a main pulse system 42 (which may be considered to be a main module or main pulse module). The main pulse module 42 is configured to fire a main pulse 44 (which may be referred to as an incident pulse). The main pulse module 42 may be considered to be a laser that fires the incident pulse 44.

[00090] It will be understood that aspects of this embodiment may be combined with other embodiments as previously described. For example, incident pulse 44 may be a result of previously described sub-modules. The laser system 40 includes an optical arrangement that is configured to produce sub-pulses from the incident pulse 44. In particular, in this embodiment, there is provided an electro-optical modulator (EOM) 46 to split the incident pulse 44 into sub-pulses, a first sub-pulse 48A and a second sub-pulse 48B, and a polarizing beam splitter (PBS) 50 configured to separate the subpulses 48A, 48B into different paths. The first sub-pulse 48A is shown as a solid line and the second sub-pulse 48B is shown as a dashed line. More generally, the polarizing beam splitter (PBS) may be considered to be a polarization splitting optical element. It will be appreciated that the polarizing beam splitter (PBS) is just an example and other polarization splitting optical elements may be used, such as thin-film polarizers and crystal polarizers etc.

[00091] The EOM 46 is configured to switch polarization of the incident pulse 44 and this allows the PBS 50 to split the sub-pulses 48A, 48B into different paths as the PBS 50 transmits P-polarized light, and reflects S-polarized light. More particularly, the EOM 46 switches the polarization substantially (approximately) half-way through the incident pulse 44 which means the first half of the incident pulse 44 (which becomes the first sub-pulse 48A) is P or S polarized light and the second half of the incident pulse 44 (which becomes the second sub-pulse 48B) is the other of P or S polarized light. It will be appreciated that, in other embodiments, the polarization may be switched at a different time during the incident pulse 44, e.g. part way through the first half or partway through the second half of the incident pulse.

[00092] The first sub-pulse 48A is transmitted through the PBS 50 on a first path towards an optical system 52A (which may more generally be referred to as an optical element or elements). The second sub-pulse 52B is reflected by the PBS 50 on a second path (different from the first path) towards an optical system 52B (which may more generally be referred to as an optical element or elements). In other words, the second sub-pulse 48B is diverted into a different (which is shown as a lower) path than the first sub-pulse 48A. The optical systems 52A, 52B are arranged to spatially separate each of the sub-pulses 48A, 48B to be incident on a droplet 26 (more generally referred to as a fuel target) at corresponding respective target destinations, first image il and second image i2 (i.e. different image positions). The optical systems 52A, 52B provide similar functionality to the optical elements 32 of the laser system 1 of the embodiment of Figure 2.

[00093] A polarizing beam combiner (PBC) 54 is provided to merge the second sub-pulse 48B back onto the same path as the first sub-pulse 48A after the sub-pulses 48A, 48B have passed through the optical systems 52A, 52B. More generally, the polarizing beam combiner (PBC) may be considered to be a polarization combining optical element. It may be considered that, at the PBC 54, the sub-pulses 48A, 48B (of predetermined pulse length) are temporally combined to form a laser pulse 58 having a specific duration. The sub-pulses 48A, 48B together may be considered to be the (main) laser pulse 58. [00094] Mirrors 56 (more generally optical components) are provided to steer the second sub-pulse 48B to the optical system 52B and to the PBC 54.

[00095] A final optical system 60 is provided to focus both the sub-pulses 48A, 48B (now on the same path and forming the laser pulse 58) to their respective target destinations i 1 , i2. It will be apparent that the second sub-pulse 48B (which has followed the lower path) has a different position along the z- axis. Since this will be in effect for a later time during the laser pulse 58, this will align better with the droplet 26, which will have moved toward this position during the z-kick during the laser pulse 58. Thus, switching the polarization using the EOM 46 allows for a time resolved position, following the droplet 26 as it is kicked by the (main) laser pulse 58.

[00096] Figure 7 (upper graph) shows voltage over time for the EOM 46 and (lower graph) shows the beam intensity level over time for the first sub-pulse 48A and the second sub-pulse 48B.

[00097] As shown in the upper graph of Figure 7, the polarization of the incident beam 44 is achieved by switching the voltage of the EOM 46 from a low value (e.g. 0) to a high value (e.g. 7k V). It will be appreciated that 7kV is just an example and the high voltage will depend on the EOM used. In some embodiments, the polarization may be achieved by switching the voltage from a high value to a low value. In this embodiment, the switching of the voltage from the low value to the high value is substantially instantaneous (i.e. the switch from one polarization to the other is discrete) or at least as quickly as can be achieved. However, as can be seen in the lower graph in Figure 7, there is some overlap of the first sub-pulse 48A (solid line) with the second sub-pulse 48B (dashed line) in this switching time.

[00098] Figure 8 (upper graph) shows voltage over time for the EOM 46 and (lower graph) shows the beam intensity level over time for the first sub-pulse 48A and the second sub-pulse 48B for a different voltage switching timing. That is, as shown in the upper graph of Figure 8, the polarization of the incident beam 44 is achieved by switching the voltage of the EOM 46 from a low value (e.g. 0) to a high value gradually (i.e. the voltage is slowly ramped up on the EOM 46 over time). This means that the ratio between the intensity at the two different image positions il, i2 can be slowly changed as well. Thus, the switch from one polarization to the other does not need to be discrete. This aspect may provide more control over the main pulse properties over the z-kick of the droplet 26 (interference is avoided as the two images have opposite polarization).

[00099] Figure 9 shows an embodiment where four image positions i 1 -i4 are provided instead of the two image positions il, i2 shown in Figure 6. Here, the laser system 40’ includes an optical arrangement with three EOMs 46 (also referred to as Pl, P2, P3) and three corresponding PBSs 50. Similarly, there are three corresponding PBCs 54.

[000100] The first EOM 46 (i.e. Pl) splits the incident pulse 44 into the first sub-pulse 48A) and the second sub-pulse 48B whilst the corresponding PBS 50 separates the first sub-pulse 48A and the second sub-pulse 48B into different paths. Similarly, the second EOM 46 (i.e. P2) splits the first sub-pulse 48A to produce a third sub-pulse 48C whilst the corresponding PBS 50 separates the first sub-pulse 48A and the third sub-pulse 48C into different paths. Likewise, the third EOM 46 (i.e. P3) splits the second sub-pulse 48B to produce a fourth sub-pulse 48D whilst the corresponding PBS 50 separates the second sub-pulse 48B and the fourth sub-pulse 48D into different paths.

[000101] There are corresponding optical systems 52A, 52B, 52C, 52D for each of the sub-pulses 48A, 48B, 48C, 48D.

[000102] In this embodiment, there are a further two EOMs (which will be referred to as combining electro-optical modulators CEOMs 46’, also referred to as P2 (mirrored) and P3 (mirrored)) after the optical systems 52A-52D that have opposite effects on the polarization, which, once again, allows the sub-pulses 48A, 48B, 48C, 48D to be merged onto a single path. The laser system 40’ can thus give four different focus (image) positions i 1 -i4 for the (main) laser pulse 58, giving even more control of the beam properties over z-position.

[000103] Figure 10 shows an embodiment of a laser system 40” where at least some of the subpulses 48A-48D may not be merged (i.e. not spatially overlapped but separated with a small angular difference). More particular, first and third sub-pulses 48A, 48C are not merged with second and fourth sub-pulses 48B, 48D. In this embodiment, there are two different final optical systems 60A, 60B which are angled with respect to each other. This means that CEOMs, and at least one of the PBCs, may not be required. This would be electronically simpler, but more complex optically/mechanically than the embodiment of Figure 9. The sub-pulses 48A-48D of Figure 10 may not be merged in the same sense as in Figure 9 but they are overlapped in space by virtue of the small angular alignment. So, it may be considered that, the final optical systems 60A, 60B carry out this functionality to this extent.

[000104] Figure 11 shows an embodiment of a laser system 40’” where optical systems are in series. That is, the optical arrangement allows four sub-pulses 48A-48D to be made using only two EOMs 46. This would be electronically simpler, but would make the different foci depend on all optical systems, and thus more complex optically than other embodiments.

[000105] The optical arrangement is arranged such that first and second sub-pulses 48A, 48B that have been formed using a first EOM 46 (i.e. Pl) are each passed through optical systems 52A, 52B. The sub-pulses 48A, 48B from the first EOM 46 are then merged before being passed through a second EOM 46 (i.e. P2). This, then forms third and fourth sub-pulses 48C, 48D, which are in turn respectively passed through optical systems 52C, 52D and then merged before the final optical system 60 focusses all four sub-pulses 48A-D (now on the same path and forming the laser pulse 58) to their respective target destinations il, i2, i3, i4. It will be understood that, effectively, first and second sub-pulses 48A, 48B are also passed through optical systems 52C, 52D respectively.

[000106] Figure 12 shows an embodiment of a laser system 40”” where the properties of an EOM is used in a different way. Since a voltage applied to the EOM changes its refractive index, it can also be used as a type of lens when a divergent/convergent beam passes through it. Thus, in this embodiment, the EOM 46 changes the divergence of the beam at a certain time (or gradually), which changes the beam properties at the position of the droplet 26. In this embodiment, two sub-pulses 48A, 48B are formed with two corresponding image positions il, i2.

[000107] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[000108] 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.

[000109] The following clauses are part of the present disclosure.

1. A laser system arranged to irradiate a moving fuel target with a laser pulse, wherein the laser system is arranged to produce a plurality of sub-pulses of predetermined pulse length that are temporally combined to form the laser pulse, and wherein the laser system further comprises at least one optical element arranged to spatially separate the sub-pulses to be incident on the moving fuel target at different points along a path of the moving fuel target. 2. The laser system of clause 1, wherein the laser system comprises a plurality of sub-modules that are each configured to fire respective sub-pulses at different times in sequence such that the subpulses are temporally combined substantially consecutively to produce the laser pulse.

3. The laser system of clause 2, comprising at least one of: a range of 2-20 sub-modules, a range of 10-20 sub-modules, a range of 2-15 sub-modules, a range of 10-15 sub-modules, 2 sub-modules, 3 sub-modules and 10 sub-modules.

4. The laser system of any preceding clause, wherein the sub-pulses partially overlap temporally with the respective adjacent sub-pulse.

5. The laser system of any preceding clause, wherein the laser pulse has a substantially flat top temporal profile.

6. The laser system of any preceding clause, wherein the sub-pulses have a substantially flat top spatial beam profile at the corresponding target destinations.

7. The laser system of any preceding clause, wherein the at least one optical element comprises at least one of optical elements, a telescope, a lens, lenses, a convex lens, a concave lens, a mirror, a curved mirror, and mirrors.

8. The laser system of any preceding clause, wherein the pulse length of each sub-pulse is at least one of: substantially the same; in a range of 10-30ns; in a range of 10-100ns; in a range of 60-100ns; in a range of 200-300ns / number of sub-pulses; 20 ns; and 100ns.

9. The laser system of any preceding clause, wherein the pulse length of the laser pulse is at least one of: in a range of 100-300ns, and 200 ns.

10. The laser system of any preceding clause, wherein the laser pulse has a wavelength of at least one of: in a range of 1 - 5pm, 1 - 3pm, 1.5 - 5pm, 1.5 - 3pm, greater than 1pm, greater than 1.5pm, less than 3pm, less than 5pm, and substantially 2pm.

11. The laser system of any preceding clause, wherein the laser pulse has a power of at least one of: in a range of 60-100kW, and 50kW.

12. The laser system of any preceding clause, wherein the laser system comprises an optical arrangement configured to produce the sub-pulses from an incident pulse.

13. The laser system of clause 12, wherein the optical arrangement comprises at least one electro- optical modulator configured to split the incident pulse into the sub-pulses.

14. The laser system of clause 13, wherein the optical arrangement comprises at least one polarization splitting optical element configured to separate the sub-pulses into different paths.

15. The laser system of either of clauses 13 or 14, wherein the electro-optical modulator is configured to switch polarization of the incident pulse, preferably substantially half-way through the incident pulse.

16. The laser system of clause 15, wherein the electro-optical modulator is configured to switch polarization of the incident pulse substantially instantaneously or gradually over time.

17. The laser system of either of clauses 13 or 14, wherein the electro-optical modulator is configured to change divergence of the incident pulse, preferably at a predetermined time or gradually overtime.

18. The laser system of any of clauses 12-17, wherein the optical arrangement is configured to produce a respective sub-pulse from another of the sub-pulses.

19. The laser system of clause 18, wherein the optical arrangement comprises a plurality of electro- optical modulators, wherein at least one of the plurality of electro -optical modulators is configured to split the other of the sub-pulses to produce the respective sub-pulse

20. The laser system of clause 19, wherein the optical arrangement comprises a plurality of polarization splitting optical elements, wherein at least one of the plurality of polarization splitting optical elements is configured to separate the other of the sub-pulses and the respective sub-pulse.

21. The laser system of any of clauses 12-20, wherein the optical arrangement is configured to merge the sub-pulses onto the same path after the sub-pulses have passed through respective optical elements arranged to spatially separate the sub-pulses.

22. The laser system of clause 21, wherein the optical arrangement comprises at least one polarization combining optical element or at least one combining electro -optical modulator configured to merge the sub-pulses onto the same path.

23. The laser system of any of clauses 12-22, wherein the optical arrangement comprise final optical systems located after respective optical elements arranged to spatially separate the sub-pulses, preferably wherein the final optical systems are angled with respect to each other.

24. The laser system of any of clauses 21-23, wherein the optical arrangement is configured to produce sub-pulses from the merged sub-pulses.

25. A laser produced plasma radiation source comprising: a fuel emitter operable to provide the fuel target at a plasma formation region; and the laser system of any of clauses 1 to 24.

26. A lithographic system comprising: the laser produced plasma radiation source of clause 25; and a lithographic apparatus.

27. The lithographic system of clause 26, wherein the lithographic apparatus is an EUV lithographic apparatus.

28. A method of irradiating a fuel target at a target destination with a laser pulse, the method comprising: producing a plurality of sub-pulses of predetermined pulse length; temporally combining the plurality of sub pulses to form the laser pulse having a specific duration, and spatially separating, using at least one optical element of the laser system, each of the sub- pulses to be incident on the fuel target at corresponding target destinations.

29. The method of clause 28, further comprising firing respective sub-pulses at different times in sequence from a plurality of sub-modules such that the sub-pulses are temporally combined substantially consecutively to produce the laser pulse.

30. The method of either of clauses 28 or 29, further comprising producing the sub-pulses from an incident pulse using an optical arrangement. 31. A computer program comprising computer readable instructions configured to cause a processor to carry out a method according to any of clauses 28-30.

32. A computer readable medium carrying a computer program according to clause 31.

33. A computer apparatus comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory;

[000110] wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any of clauses 28-30.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

1. A laser system arranged to irradiate a moving fuel target with a laser pulse, wherein the laser system is arranged to produce a plurality of sub-pulses of predetermined pulse length that are temporally combined to form the laser pulse, and wherein the laser system further comprises at least one optical element arranged to spatially separate the sub-pulses to be incident on the moving fuel target at different points along a path of the moving fuel target.

2. The laser system of claim 1, wherein the laser system comprises a plurality of sub-modules that are each configured to fire respective sub-pulses at different times in sequence such that the sub-pulses are temporally combined substantially consecutively to produce the laser pulse.

3. The laser system of any preceding claim, wherein the sub-pulses partially overlap temporally with the respective adjacent sub-pulse.

4. The laser system of any preceding claim, wherein the sub-pulses have a substantially flat top spatial beam profile at the corresponding target destinations.

5. The laser system of any preceding claim, wherein the pulse length of each sub-pulse is at least one of: substantially the same; in a range of 10-30ns; in a range of 10-100ns; in a range of 60-100ns; in a range of 200-300ns / number of sub-pulses; 20 ns; and 100ns.

6. The laser system of any preceding claim, wherein the pulse length of the laser pulse is at least one of: in a range of 100-300ns, and 200 ns.

7. The laser system of any preceding claim, wherein the laser pulse has a wavelength of at least one of: in a range of 1 - 5pm, 1 - 3pm, 1.5 - 5pm, 1.5 - 3pm, greater than 1pm, greater than 1.5pm, less than 3pm, less than 5pm, and substantially 2pm.

8. The laser system of any preceding claim, wherein the laser system comprises an optical arrangement configured to produce the sub-pulses from an incident pulse.

9. The laser system of claim 8, wherein the optical arrangement comprises at least one of:

- an electro-optical modulator configured to split the incident pulse into the sub-pulses; and

- a polarization splitting optical element configured to separate the sub-pulses into different paths.

10. The laser system of claim 9 , wherein the electro -optical modulator is configured to:

- switch polarization of the incident pulse, preferably substantially half-way through the incident pulse;

- switch polarization of the incident pulse substantially instantaneously or gradually over time.

11. The laser system of claim 9, wherein the electro-optical modulator is configured to change divergence of the incident pulse, preferably at a predetermined time or gradually overtime.

12. The laser system of any of claims 8-11, wherein the optical arrangement is configured to produce a respective sub-pulse from another of the sub-pulses.

13. The laser system of claim 12, wherein the optical arrangement comprises a plurality of electro- optical modulators, wherein at least one of the plurality of electro -optical modulators is configured to split the other of the sub-pulses to produce the respective sub-pulse, and/or wherein the optical arrangement comprises a plurality of polarization splitting optical elements, wherein at least one of the plurality of polarization splitting optical elements is configured to separate the other of the sub -pulses and the respective sub-pulse..

14. The laser system of any of claims 8-13, wherein the optical arrangement is configured to merge the sub-pulses onto the same path after the sub-pulses have passed through respective optical elements arranged to spatially separate the sub-pulses, wherein the optical arrangement comprises at least one polarization combining optical element or at least one combining electro -optical modulator configured to merge the sub-pulses onto the same path.

15. The laser system of any of claims 8-14, wherein the optical arrangement comprise final optical systems located after respective optical elements arranged to spatially separate the sub -pulses, preferably wherein the final optical systems are angled with respect to each other.

16. The laser system of any of claims 14-15, wherein the optical arrangement is configured to produce sub-pulses from the merged sub-pulses.

17. A laser produced plasma radiation source comprising: a fuel emitter operable to provide the fuel target at a plasma formation region; and the laser system of any of claims 1 to 16.

18. A lithographic system comprising: the laser produced plasma radiation source of claim 17; and a lithographic apparatus, wherein the lithographic apparatus is an EUV lithographic apparatus.

19. A method of irradiating a fuel target at a target destination with a laser pulse, the method comprising: producing a plurality of sub-pulses of predetermined pulse length; temporally combining the plurality of sub pulses to form the laser pulse having a specific duration, and spatially separating, using at least one optical element of the laser system, each of the sub- pulses to be incident on the fuel target at corresponding target destinations.