Radiation system

Abstract

A radiation system configured to generate radiation and comprising a droplet generator (3), a laser system, and a control system, the droplet generator (3) being configured to generate fuel droplets that travel towards a plasma formation region, the laser system operating to generate a pre-pulse (PP) and a main pulse (MP), wherein the pre-pulse is configured to condition the droplets to receive the main pulse, and wherein the main pulse is configured to convert the conditioned droplets into a plasma that generates radiation, the control system being configured to control a spatial offset between the pre-pulse and the droplets in a plane transverse to the propagation direction of the pre-pulse, wherein the control system is configured to adjust the spatial offset to maximize a velocity change of the conditioned droplets in a plane transverse to the propagation direction of the main pulse.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to EP application 19167341.7, filed on Apr. 4, 2019, which is incorporated herein by reference in its entirety. Field of the Invention

[0003] The present invention relates to a radiation system configured to generate radiation and related methods. Background Art

[0004] A lithographic apparatus is a machine configured 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 can project a pattern from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0005] The radiation wavelength used by a lithographic apparatus to project a pattern onto a substrate determines the minimum feature size that can be formed on the substrate. Compared to conventional lithographic apparatuses (e.g., which can use electromagnetic radiation with a wavelength of 193 nm), a lithographic apparatus using EUV radiation (i.e., electromagnetic radiation having a wavelength in the range of 4 - 20 nm) can be used to form smaller features on a substrate.

[0006] EUV radiation can be generated by converting a fuel (e.g., tin) into a plasma. One or more droplets of the fuel can be irradiated by one or more laser pulses. For example, each droplet can be irradiated with a pre-pulse for conditioning the droplet and a main pulse for converting most or all of the conditioned droplet into a plasma, thereby generating EUV radiation.

[0007] The stability and / or power of the EUV radiation can depend on the size of the conditioned droplets and / or the spatial overlap between the conditioned droplets and the main pulse. A change or variation in the spatial overlap between the conditioned droplets and the main pulse can lead to unstable generated EUV radiation, a decrease in the power of the EUV radiation, and / or a change or error in the EUV radiation dose. Summary of the Invention

[0008] According to a first aspect of the present invention, there is provided a radiation system configured to generate radiation. The radiation system may include a droplet generator. The droplet generator may be configured to generate fuel droplets that travel towards a plasma formation region. The radiation system may include a laser system. The laser system operates to generate a prepulse and a main pulse. The prepulse may be configured to condition the droplets to receive the main pulse. The main pulse may be configured to convert the conditioned droplets into a plasma that generates radiation. The radiation system may include a control system. The control system may be configured to control a spatial offset between the prepulse and the droplets in a plane transverse to the propagation direction of the prepulse. The control system may be configured to adjust the spatial offset to maximize a velocity change of the conditioned droplets in a plane transverse to the propagation direction of the main pulse.

[0009] By maximizing the velocity change, the sensitivity of the velocity change to changes in the spatial offset can be reduced or minimized. This can increase or improve the stability of a target region, such as a region in a plane transverse to the propagation direction of the main pulse, in which the conditioned droplets are converted by the main pulse into a plasma that generates radiation. This can additionally or alternatively increase or improve the spatial overlap between the conditioned droplets and the main pulse, increase the power of the generated radiation (e.g., EUV radiation), reduce radiation instabilities (e.g., EUV radiation instabilities), and / or reduce errors or fluctuations in the radiation power (e.g., EUV radiation power).

[0010] The control system may be configured to adjust, for example, an interval between the prepulse in a plane transverse to the propagation direction of the prepulse and the main pulse in a plane transverse to the propagation direction of the main pulse based on the adjusted spatial offset.

[0011] The control system may be configured to adjust, for example, a delay time between the generation of the prepulse and the generation of the main pulse based on the adjusted spatial offset.

[0012] By adjusting, for example, an interval between the prepulse in a plane transverse to the propagation direction of the prepulse and the main pulse in a plane transverse to the propagation direction of the main pulse, and / or a delay time between the generation of the prepulse and the generation of the main pulse, based on the adjusted spatial offset, the spatial overlap between the conditioned droplets and the main pulse can be increased or improved. This in turn increases the radiation power (e.g., EUV radiation power), increases the radiation stability (e.g., EUV radiation stability), and / or reduces errors or fluctuations in the radiation power (e.g., EUV radiation power).

[0013] The laser system may be configured to direct the main pulse towards a target region. The conditioned droplets may be converted into a plasma that generates radiation at the target region.

[0014] The control system can be configured to adjust a target area based on at least one of the following: an adjusted spatial offset, a speed change, an interval between a pre-pulse in a plane transverse to the propagation direction of the pre-pulse and a main pulse in a plane transverse to the propagation direction of the main pulse, and a delay time between the generation of the pre-pulse and the generation of the main pulse.

[0015] The control system can be configured to adjust the spatial offset based on one or more parameters to maximize the speed change.

[0016] The one or more parameters include the duration of the pre-pulse and / or the energy of the pre-pulse.

[0017] The one or more parameters can include at least one of the following: the position of the droplet in a plane transverse to the propagation direction of the pre-pulse, the position of the pre-pulse in a plane transverse to the propagation direction of the pre-pulse, the change in the position of the droplet in a plane transverse to the propagation direction of the pre-pulse, the change in the position of the pre-pulse in a plane transverse to the propagation direction of the pre-pulse, the position of the main pulse in a plane transverse to the propagation direction of the main pulse, and the change in the position of the main pulse in a plane transverse to the propagation direction of the main pulse.

[0018] The radiation system can include a sensor system. The sensor system can be configured to sense at least one of the one or more parameters.

[0019] The control system can be configured to control and / or adjust the spatial offset based on at least one of the sensed one or more parameters.

[0020] The adjusted spatial offset can be between approximately 20 μm and 25 μm.

[0021] The radiation system can include a sensor system. The sensor system can be configured to sense an attribute or parameter. The laser system can include a pre-pulse seed laser configured to generate a seed pre-pulse. The laser system can include a main-pulse seed laser configured to generate a seed main pulse. The laser system can include a combiner configured to place the seed pre-pulse and the seed main pulse onto a common path. The laser system can include an amplifier system. The amplifier system can be located on the common path. The amplifier system can be configured to amplify the seed pre-pulse to generate a pre-pulse and amplify the seed main pulse to generate a main pulse. The laser system can include an amplifier control system. The amplifier control system can be configured to control the amplification based on the sensed attribute or parameter. The laser system can include an additional control system. The additional control system can be configured to control the duration of the seed pre-pulse based on the sensed attribute or parameter.

[0022] By controlling the duration of the seed prepulse according to sensed properties or parameters, variations or changes in the spatial offset between the prepulse and the droplet and / or velocity variations can be reduced or compensated for. This can reduce or avoid variations or changes in the spatial overlap between the conditioned droplet and the main pulse, thereby reducing or preventing a decrease in the power of the generated radiation (e.g., EUV radiation), radiation instabilities (e.g., EUV radiation instabilities), and / or errors or fluctuations in the radiation power (e.g., EUV radiation power).

[0023] The property or parameter can include a property or parameter of the generated radiation.

[0024] The property or parameter can include at least one of the following: the power of the generated radiation, the spatial offset, and variations in the spatial offset.

[0025] An additional control system can be configured to adjust the duration of the seed prepulse laser such that, for example, when the seed prepulse and the seed main pulse are amplified according to the detected property or parameter, the velocity variation is substantially unchanged or constant (or invariant or constant).

[0026] The control system operates to maximize the velocity variation by controlling the duration of the prepulse.

[0027] By maximizing the velocity variation, the sensitivity of the velocity variation to spatial offset variations can be reduced or minimized. This can increase or improve the stability of the target region, such as a region in a plane transverse to the propagation direction of the main pulse, in which the conditioned droplet is converted by the main pulse into a plasma generating radiation. This can additionally or alternatively increase or improve the spatial overlap between the conditioned droplet and the main pulse, increase the power of the generated radiation (e.g., EUV radiation), reduce radiation instabilities (e.g., EUV radiation instabilities), and / or reduce errors or fluctuations in the radiation power (e.g., EUV radiation power).

[0028] The control system operates to adjust the duration of the prepulse in response to a change in the energy of the prepulse.

[0029] The control system can operate to adjust the duration of the prepulse based on the interval or distance between the prepulse in a plane transverse to the propagation direction of the prepulse and the main pulse in a plane transverse to the propagation direction of the main pulse.

[0030] The control system can operate to adjust the duration of the prepulse in response to a change in the interval or distance between the prepulse in a plane transverse to the propagation direction of the prepulse and the main pulse in a plane transverse to the propagation direction of the main pulse.

[0031] According to a second aspect of the present invention, a radiation system is provided. The radiation system may be configured to generate radiation. The radiation system may include a droplet generator. The droplet generator may be configured to generate fuel droplets that travel towards a plasma formation region. The radiation system may include a laser system. The laser system operates to generate a prepulse and a main pulse. The prepulse may be configured to condition the droplets to receive the main pulse. The main pulse may be configured to convert the conditioned droplets into a plasma that generates radiation. The radiation system may include a sensor system. The sensor system may be configured to sense a parameter or property. The laser system may include a prepulse seed laser configured to generate a seed prepulse. The laser system may include a main pulse seed laser configured to generate a seed main pulse. The laser system may include a combiner configured to place the seed prepulse and the seed main pulse onto a common path. The laser system may include an amplifier system. The amplifier system may be located on the common path. The amplifier system may be configured to amplify the seed prepulse to generate the prepulse and amplify the seed main pulse to generate the main pulse. The laser system may include an amplifier control system. The amplifier control system may be configured to control the amplification based on the sensed property or parameter. The laser system may include an additional control system. The additional control system may be configured to control the duration of the seed prepulse based on the sensed property or parameter. The sensed property or parameter may include at least one of the following: the power of the generated radiation, a spatial offset between the prepulse and the droplets in a plane transverse to the propagation direction of the prepulse, and a change in the spatial offset.

[0032] By controlling the duration of the seed prepulse based on the sensed property or parameter, a change or alteration and / or a change in velocity of the spatial offset between the prepulse and the droplets can be reduced or compensated for. This can reduce or avoid a change or alteration in the spatial overlap between the conditioned droplets and the main pulse, thereby reducing or preventing a reduction in the generated radiation (e.g., EUV radiation), radiation instability (e.g., EUV radiation instability), and / or an error or fluctuation in the radiation power (e.g., EUV radiation power).

[0033] The radiation system of the second aspect may include any feature of the radiation system of the first aspect.

[0034] According to a third aspect of the present invention, a radiation system is provided. The radiation system may be configured to generate radiation. The radiation system may include a droplet generator. The droplet generator may be configured to generate fuel droplets that travel towards a plasma formation region. The radiation system may include a laser system. The laser system operates to generate a pre-pulse and a main pulse. The pre-pulse may be configured to condition the droplets to receive the main pulse. The main pulse may be configured to convert the conditioned droplets into a plasma that generates radiation. The radiation system may include a control system. The control system may be configured to or operate to adjust, control, and / or maximize the velocity change of the conditioned droplets in a plane transverse to the propagation direction of the main pulse by controlling the duration of the pre-pulse.

[0035] By adjusting, controlling, and / or maximizing the velocity change of the conditioned droplets in a plane transverse to the propagation direction of the main pulse by controlling the duration of the pre-pulse, the spatial overlap between the conditioned droplets and the main pulse in a plane transverse to the propagation direction of the main pulse can be optimized or maximized. This can increase the power of the generated radiation (e.g., the generated EUV radiation), increase the radiation stability (e.g., EUV radiation stability), and / or reduce the error or fluctuation of the radiation power (e.g., EUV radiation power).

[0036] The radiation system of the third aspect may include any features of the radiation systems of the first and / or second aspects.

[0037] According to a fourth aspect, a lithography system is provided that includes the radiation system of the first, second, and / or third aspects.

[0038] According to a fifth aspect, a method of generating radiation is provided. The method may include operating a droplet generator to generate fuel droplets that travel towards a plasma formation region. The method may include operating a laser system to generate a pre-pulse for conditioning the droplets. The method may include controlling the spatial offset between the pre-pulse and the droplets in a plane transverse to the propagation direction of the pre-pulse. The method may include adjusting the spatial offset to maximize the velocity change of the conditioned droplets in a plane transverse to the propagation direction of the main pulse. The method may include operating the laser system to generate a main pulse to convert the conditioned droplets into a plasma that generates radiation.

[0039] The method may include operating a pre-pulse seed laser to generate a seed pre-pulse. The method may include operating a main-pulse seed laser to generate a seed main-pulse. The pre-pulse seed laser and the main-pulse seed laser may be part of or included in a laser system. The seed pre-pulse and the seed main-pulse may be placed on a common path by a combiner. The method may include operating an amplifier system. The amplifier system may be located on the common path. The amplifier system may be configured to amplify the seed pre-pulse to generate a pre-pulse and amplify the seed main-pulse to generate a main-pulse. The method may include sensing a property or parameter. The method may include operating an amplifier control system. The amplifier control system may be configured to control the amplification based on the sensed property or parameter. The method may include operating an additional control system. The additional control system may be configured to control the duration of the seed pre-pulse based on the sensed property or parameter.

[0040] The property or parameter may include at least one of the following: the power of the generated radiation, a spatial offset, and a change in the spatial offset.

[0041] The method may include controlling the duration of the pre-pulse to maximize the speed change.

[0042] According to a sixth aspect of the invention, there is provided a computer program. The computer program may include computer-readable instructions configured to cause a processor to execute the method according to the fifth aspect.

[0043] According to a seventh aspect of the invention, there is provided a computer-readable medium carrying the computer program according to the sixth aspect.

[0044] According to an eighth aspect, there is provided a computer device comprising a memory storing processor-readable instructions; and a processor arranged to read and execute the instructions stored in the memory. The processor-readable instructions may include instructions arranged to control the computer to execute the method according to the fifth aspect.

[0045] It will be apparent to the person skilled in the art that the various aspects and features of the invention set out above or below may be combined with each other in various other aspects of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0047] - Figure 1 A lithography system including a lithography apparatus and a radiation system according to an embodiment of the invention is depicted;

[0048] - Figure 2 Depicts a radiation system for Figure 1 in a lithography system;

[0049] - Figure 3 Schematically depicts the interaction between fuel droplets generated by a droplet generator and pre-pulses and main pulses generated by a laser system of a radiation system; Figure 1 and 2 ;

[0050] - Figure 4A A graph depicting the velocity change in the x-direction that depends on the spatial offset between the fuel droplets and the pre-pulses in the first xy-plane;

[0051] - Figure 4B A graph depicting the measured size of the conditioned droplets that depends on the spatial offset between the fuel droplets and the pre-pulses in the xy-plane;

[0052] - Figure 5 A graph depicting the intensity of the velocity change that depends on the spatial offset; Figure 4A ;

[0053] - Figure 6A A graph depicting the intensity of the velocity change in the y-direction that depends on the x-component and y-component of the spatial offset;

[0054] - Figure 6B A graph depicting the velocity change in the y-direction that depends on the y-component of the spatial offset for the x-component of the spatial offset ranging from 0 μm to -25 μm;

[0055] - Figure 7 A graph depicting the laser system in a radiation system for; Figure 1 and / or Figure 2 ;

[0056] - Figure 8A A graph depicting the velocity change that depends on the spatial offset in the x or y direction;

[0057] - Figure 8B Another graph depicting the velocity change that depends on the spatial offset in the x or y direction;

[0058] - Figure 9 A graph depicting the laser system and control system in a radiation system for; Figure 1 and / or Figure 2 ;

[0059] - Figure 10 A graph depicting the velocity change that depends on the spatial overlap in the first xy-plane;

[0060] - Figure 11 A graph depicting the derivative of the velocity change in the x-direction with respect to the spatial offset that depends on the fluence or radiation exposure of the pre-pulses on the droplets during a pre-pulse duration that varies between 40 ns and 180 ns;

[0061] - Figure 12 A flowchart depicting a method of generating radiation;

[0062] - Figure 13 A flowchart depicting method steps that can be Figure 12 part of or included in the method depicted in

[0063] - Figure 14 A flowchart depicting method steps that can be Figure 12 and / or Figure 13 part of or included in the method depicted in Detailed Description

[0064] Figure 1 Shows a lithographic system including a radiation system RS. The lithographic system includes a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus LA includes an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident on the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include a previously formed pattern. In this case, the lithographic apparatus aligns the patterned radiation beam B with the previously formed pattern on the substrate W.

[0065] The radiation source SO, the illumination system IL, and the projection system PS can all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure lower than atmospheric pressure (e.g., hydrogen) can be provided in the radiation source SO. A vacuum can be provided in the illumination system IL and / or the projection system PS. A small amount of gas at a pressure far lower than atmospheric pressure (e.g., hydrogen) can be provided in the illumination system IL and / or the projection system PS.

[0066] Figure 1 The illustrated radiation source SO is of a type that can be referred to as a laser-produced plasma (LPP) source. A laser system 1, which can include, for example, a CO2 laser, is arranged to deposit energy into a fuel (such as tin (Sn) provided from a fuel emitter 3) via a laser beam 2. Although tin is mentioned in the following description, any suitable fuel can be used. The fuel can be, for example, in liquid form and can be, for example, a metal or an alloy. The fuel emitter 3 can be provided in the form of a droplet generator 3. The droplet generator 3 can be configured to generate fuel droplets D that travel towards a plasma formation region 4 ( Figure 1(not shown in the figure). For example, the droplet generator 3 may include a nozzle configured to guide tin, for example in the form of droplets D, along a trajectory towards the plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. Laser energy deposition into the tin generates a plasma 7 at the plasma formation region 4. Radiation (including EUV radiation) is emitted from the plasma 7 during the ion de-excitation and recombination of the plasma.

[0067] The EUV radiation is collected and focused by a near-normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multilayer structure 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 foci. As discussed below, the first focus may be at the plasma formation region 4, while the second focus may be at the intermediate focus 6.

[0068] The laser system 1 may be remote from the radiation source SO. In this case, the laser beam 2 may be transmitted from the laser system 1 to the radiation source SO with the help of a beam delivery system 1a ( Figure 2 depicted in the figure), which includes, for example, suitable steering mirrors and / or beam expanders and / or other optical devices. The laser system 1, the beam delivery system 1a, and the radiation source SO may be considered together as a radiation system RS. In other words, the laser system 1 and / or the beam delivery system 1a may be part of or included in the radiation system RS. The radiation system RS may be configured to generate radiation, such as EUV radiation, as described above. The beam delivery system 1a may be part of or included in the laser system 1.

[0069] The radiation reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at the point 6 to form an image of the plasma formation region 4, which serves as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused here may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near the opening 8 in the closed structure 9 of the radiation source.

[0070] The radiation beam B enters the illumination system IL from the radiation source SO, which is configured to condition the radiation beam. 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 the faceted pupil mirror device 11 together provide a radiation beam B having a desired cross-sectional shape and a desired angular intensity distribution. The radiation beam B passes through the illumination system IL and is incident on a patterning device MA held by a support structure MT. The patterning device MA reflects and patterns the radiation beam B. In addition to or instead of the faceted field mirror device 10 and the faceted pupil mirror device 11, the illumination system IL may include other mirrors or devices.

[0071] After reflection from the patterning device MA, the patterned radiation beam B enters the projection system PS. The projection system includes a plurality of mirrors 13, 14 configured to project the radiation beam B onto a substrate W held by a substrate table WT. The projection system PS may apply a reduction factor to the radiation beam to form an image having features smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 may be applied. Although in Figure 1 the projection system PS has two mirrors, the projection system may include any number of mirrors (e.g., six mirrors).

[0072] Figure 1 The radiation source SO shown may include components not shown. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive to EUV radiation but substantially blocking to radiation of other wavelengths such as infrared radiation.

[0073] Figure 2 A radiation system used in a lithography system (e.g., Figure 1 the lithography system shown) is shown. Figure 2 The radiation system RS shown may include Figure 1 any features of the radiation system RS shown. For example, Figure 2 the radiation system shown may include the droplet generator 3 described above with respect to Figure 1 .

[0074] Referring to Figures 2 to 3 , the laser system 1 operates to generate a pre-pulse PP and a main pulse MP. The pre-pulse PP is configured to condition the droplet D to receive the main pulse MP. In other words, the pre-pulse PP may be configured to heat, deform, expand, vaporize, vaporise and / or ionize the droplet D and / or generate a weak plasma. For example, as Figure 3 shown, when hit by the main pulse MP, the pre-pulse PP may be configured to deform the spherical droplet D so that its shape is closer to a disc shape (e.g., a pancake shape). It should be understood that in other embodiments, the pre-pulse may be configured to deform the droplet D into a different shape. The conditioned droplet is represented by the label "CD" in Figure 3 .

[0075] The main pulse MP is configured to convert the conditioned droplet CD into a plasma that generates EUV radiation. In other words, the main pulse MP may be configured to convert most or all of the conditioned droplet CD into a plasma, thereby generating EUV radiation.

[0076] The radiation system RS may include a control system 16 ( Figure 2as shown). The control system 16 may be configured to control a spatial offset O between the pre-pulse PP and the droplet D in a plane transverse to the propagation direction of the pre-pulse PP. The control system 16 may be configured to adjust the spatial offset O to maximize the change in the velocity of the adjusted droplet CD relative to the initial velocity of the unadjusted droplet D in a plane transverse to the propagation direction of the main pulse MP. The change in the velocity of the adjusted droplet is due to the momentum transfer of the pre-pulse PP to the unadjusted droplet D. The control system 16 may be provided in the form of a computing device. The control system 16 may be configured to communicate with other components or parts of the radiation system RS (e.g., the laser system 1, the beam delivery system 1a, and / or the droplet generator 3).

[0077] The feature "plane transverse to the propagation direction of the pre-pulse PP" may be considered to cover a plane extending in the directions indicated by the x-axis and the y-axis in Figure 3 which will be referred to as the first xy-plane in the following description.

[0078] The feature "propagation direction of the pre-pulse PP" may be considered to cover a direction parallel to (e.g., substantially parallel to) or along Figure 2 and Figure 3 the direction of the z-axis indicated in

[0079] The feature "plane transverse to the propagation direction of the main pulse MP" may be considered to cover a plane extending in the directions indicated by the x-axis and the y-axis in Figure 3 which will be referred to as the second xy-plane in the following description. It should be understood that the first xy-plane may be offset in the z-axis direction relative to the second xy-plane. This offset may be the result of the offset of the focus of the pre-pulse PP relative to the main pulse MP in the Figure 2 and Figure 3 z-axis direction shown in

[0080] The feature "propagation direction of the main pulse MP" may be considered to cover a direction parallel to (e.g., substantially parallel to) or along Figure 2 the direction of the z-axis indicated in Figure 2 as shown in

[0081] In the following description, Figure 2 and Figure 3 the direction indicated by the x-axis in Figure 3 will be referred to as the x-direction, Figure 2 and 3 the direction indicated by the y-axis in

[0082] The feature "spatial offset" can be considered as a vector located in the first xy plane and having a dimension representing the distance or interval between the droplet D (e.g., the center of the droplet D) and the pre-pulse PP (e.g., the center of the pre-pulse PP). For example, the feature "spatial offset" can be considered to cover the distance or interval between the droplet D (e.g., the center of the droplet D) and the pre-pulse PP (e.g., the center of the pre-pulse PP) in the first xy plane. The spatial offset can include an x component and / or a y component. In other words, the droplet D (e.g., the center of the droplet) can move relative to the pre-pulse PP (e.g., the center of the pre-pulse PP) or be separated from the pre-pulse PP in the x direction and / or the y direction.

[0083] It should be understood that in some embodiments, the spatial offset can include a z component. In other words, the droplet D (e.g., the center of the droplet D) can move relative to the pre-pulse (e.g., the center of the pre-pulse PP) or be separated from the pre-pulse (e.g., the center of the pre-pulse PP) in the z direction.

[0084] The spatial offset can be zero or substantially zero, for example, if the center of the droplet D overlaps (e.g., substantially overlaps) with the center of the pre-pulse PP.

[0085] It should be understood that the velocity change can occur in the x direction, y direction, or z direction or a combination thereof. Therefore, the velocity change is also a vector.

[0086] For completeness, it should be noted here that in this text, physical quantities that are vectors are represented by bold characters, and physical quantities that are scalars are represented by ordinary characters. For example, the spatial offset O is a vector, that is, a physical quantity having a direction and a magnitude, and is therefore represented in bold. As another example, the magnitude of the spatial offset O and the x component of the spatial offset are both scalars.

[0087] In Figure 3 , the spatial offset O includes a non-zero x component, while the y component is substantially zero. It should be understood that in other embodiments, the spatial offset can include an x component and a y component, as described above. Alternatively, the spatial offset can include a y component, and the x component can be substantially zero.

[0088] The velocity change can be proportional to the momentum change Δp, and the momentum change Δp is also a vector:

[0089] Δp = mv final -mv initial

[0090] where during the interaction between the droplet D and the pre-pulse PP, the mass m of the droplet can be considered constant or unchanged. The velocity change can be considered to be the velocity of the droplet D generated by the droplet generator 3 (e.g., v initial)and the velocity v of the conditioned droplet CD final The difference between them. In other words, the velocity change Δv is equal to:

[0091] Δv = v final - v initial

[0092] where the momentum change Δp and the velocity change Δv are vectors.

[0093] Figure 4A depicts a plot of the velocity change Δv in the x - direction that depends on the spatial offset between the droplet D and the prepulse PP in the first xy - plane. The velocity change Δv in the x - direction is a scalar and is denoted as Δv x . Figure 4A The light - gray dots in represent the measured values of the velocity change Δv x , the open circles represent the average value of the velocity change Δv x , and the solid line represents the curve fitting the average value of the velocity change Δv x . If the magnitude of the velocity change is maximized, for example, if the magnitude of the spatial offset is between approximately 20 μm and 25 μm, then the change δΔv x of the velocity change Δv in the x - direction associated with the change in the spatial offset x is minimized (indicated by the arrow in Figure 4A ). Note that the magnitude of the spatial offset and the magnitude of the velocity change refer to absolute values without a polarity sign. Figure 4A shows that the maximum change in the magnitude of the velocity in the x - direction occurs for spatial offsets in the range of (approx.) 20 μm to 25 μm and in the range of (approx.) - 20 μm to - 25 μm. The change (denoted as Δv x ) of the x - component of the velocity change Δv that depends on the change in the spatial offset O can be represented as a gradient G, and the gradient G is zero at the maximum of Δv x :

[0094]

[0095] where O is the spatial offset in the first xy - plane. If the magnitude of the velocity change Δv is maximized, then the sensitivity of the velocity change Δv to the change or variation of the spatial offset can be minimized. In other words, if the magnitude of the velocity change Δv is maximized, for example, at approximately 25 μm and approximately - 25 μm, the gradient G can be minimized or zero, for example, substantially zero.

[0096] As Figure 4AAs shown (and indicated by the arrow), for example, if the change in velocity Δv in the x - direction is approximately zero, a change or alteration in the spatial offset O may result in the maximum change in the velocity change Δv in the x - direction. In other words, if the magnitude of the spatial offset O is approximately zero, the gradient G can be maximum. On the other hand, for example, if the velocity change Δv is maximized, a change or alteration in the spatial offset O may result in a minimum or reduced change in the velocity change Δv in the x - direction.

[0097] By maximizing the velocity change, the sensitivity of the velocity change to changes in the spatial offset can be minimized. This can increase or improve the stability of the target region TR, such as the region in the second xy - plane where the conditioned droplet CD is converted to plasma by the main pulse MP. This can additionally or alternatively increase or improve the spatial overlap between the conditioned droplet CD and the main pulse MP, increase the power of the generated EUV radiation, reduce EUV radiation instability, and / or reduce the error or fluctuation in the EUV radiation power.

[0098] Figure 4B A graph showing the measured size of the conditioned droplet CD as a function of the magnitude of the spatial offset O between the droplet D and the prepulse PP in the first xy - plane is shown. As described above, the prepulse PP can be configured to, for example, expand the droplet D. In Figure 4B , the size of the conditioned droplet CD is measured at about 3 μs after the prepulse PP starts to impinge on the droplet D, for example. Figure 4B The light - gray dots in Figure 4B represent the measured values of the size of the conditioned droplet CD, the open circles represent the average value of the measured size of the conditioned droplet CD, and the solid line represents the curve fitting the average value of the measured size of the conditioned droplet CD. As can be seen from Figure 4B , the size of the conditioned droplet CD depends on the spatial offset O between the droplet D and the prepulse PP. For a spatial offset O with a size greater than about 25 μm (or less than about - 25 μm), the size of the conditioned droplet CD can decrease. This may be due to the decreasing spatial overlap between the droplet D and the prepulse PP as the spatial offset O increases. The size of the conditioned droplet CD can be considered to be maximized for a spatial offset O in the range of approximately - 25 μm to 25 μm. Although

[0099] shows the distribution of the measured size and the average size of the conditioned droplet within this range, the size of the conditioned droplet CD can be considered to be largely independent of or remain unchanged with respect to the magnitude of the spatial offset O in the range of approximately - 25 μm to 25 μm. In other words, for a spatial offset with a size in the range of about - 25 μm to 25 μm, the change in the size of the conditioned droplet CD can be considered to be reduced. This can improve the spatial overlap between the conditioned droplet CD and the main pulse MP.

[0099] Referring again to Figure 2, the control system 16 can be configured to adjust the spatial offset O, for example, by adjusting the relative position between the droplet D and the pre-pulse PP. For example, the control system 16 can be configured to communicate with the droplet generator 3. The droplet generator 3 can be configured to release the droplet D in response to a signal from the control system 16. Alternatively, or in combination with the previous adjustment options, the droplet generator 3 can be configured to slightly change the direction of the trajectory of the droplet in response to a signal from the control system 16. The control system 16 can be configured to communicate with the laser system 1 and / or the beam delivery system 1a. For example, the control system 16 can be configured to transmit a signal to the laser system 1 and / or the beam delivery system 1a. In response to the signal transmitted by the control system 16, the laser system 1 and / or the beam delivery system 1a can be configured to set or adjust the position, timing, shape, and / or power of the pre-pulse PP in the first xy plane and / or the main pulse MP in the second xy plane. This can allow the control system 16 to adjust the spatial offset O between the droplet D and the pre-pulse PP.

[0100] The control system 16 can be configured to set the spatial offset O, for example, such that the change in the velocity of the adjusted droplet CD is maximized in the second xy plane relative to the velocity of the unadjusted droplet D. The control system 16 can be configured to maintain the spatial offset O, for example, by setting the spatial offset, for example, by adjusting the relative position between the pre-pulse PP and the droplet D, as described above.

[0101] The control system 16 can be configured to adjust the interval or distance S (which is indicated in Figure 3 between the pre-pulse PP in the first xy plane and the main pulse MP in the second xy plane based on the adjusted spatial offset. For example, during maintenance and / or calibration operations of the lithography system or a part thereof, the position of the droplet D relative to the pre-pulse PP can be adjusted such that the magnitude of the spatial offset O is zero, for example, the center of the droplet overlaps (e.g., substantially overlaps) with the center of the pre-pulse PP. Subsequently, as described above, the control system 16 can be configured to adjust the spatial offset to maximize the magnitude of the change in velocity Δv of the adjusted droplet CD in the second xy plane. Based on the adjusted spatial offset, the control system 16 can be configured to adjust the interval or distance S between the pre-pulse PP in the first xy plane and the main pulse MP in the second xy plane, for example, to optimize or maximize the spatial overlap between the adjusted droplet CD and the main pulse MP.

[0102] The control system 16 can operate to transmit a signal to the laser system 1 and / or the beam delivery system 1a. In response to the signal transmitted by the control system 16, the laser system 1 and / or the beam delivery system 1a can operate to adjust the relative position between the pre-pulse PP in the first xy plane and the main pulse MP in the second xy plane to adjust the interval or distance S between the pre-pulse PP and the main pulse MP.

[0103] The control system 16 can be configured to adjust the delay time based on the adjusted spatial offset O, e.g., the time between the generation of the pre-pulse PP and the main pulse MP, the time between the generation of the pre-pulse and the generation of the main pulse MP. For example, the control system 16 can operate to transmit a signal to the laser system 1 and / or the beam delivery system 1a. In response to the signal transmitted by the control system 16, the laser system 1 and / or the beam delivery system 1a can adjust the time between the generation of the pre-pulse PP and the generation of the main pulse MP.

[0104] By adjusting the interval or distance S and / or the delay time between the pre-pulse PP in the first xy-plane and the main pulse MP in the second xy-plane based on the adjusted spatial offset, the spatial overlap between the adjusted droplet CD and the main pulse MP can be optimized or maximized. This can in turn increase the EUV radiation power, increase the EUV radiation stability, and / or reduce the error or fluctuation of the EUV radiation power.

[0105] The laser system 1 and / or the beam delivery system 1a can be configured to direct the main pulse MP towards the target region TR. The control system 16 can be configured to adjust the target region TR. For example, the control system 16 can be configured to adjust the target region TR depending on at least one of the following: the adjusted spatial offset O, the velocity change Δv, the interval S between the pre-pulse PP in the first xy-plane and the main pulse MP in the second xy-plane, and the delay time between the generation of the pre-pulse and the generation of the main pulse MP. In other words, the position of the adjusted droplet CD relative to the main pulse MP can be adjusted or tuned (or adjustable or tunable) by adjusting at least one of the following: the spatial offset O, the velocity change Δv, the interval S between the pre-pulse PP in the first xy-plane and the main pulse MP in the second xy-plane, and the delay time between the generation of the pre-pulse PP and the generation of the main pulse MP. The control system 16 can operate to transmit a signal to the laser system 1 and / or the beam delivery system 1a. In response to the signal, the laser system 1 and / or the beam delivery system 1a can direct the main pulse MP towards the adjusted target region TR.

[0106] The spatial overlap between the conditioned droplet CD and the main pulse MP can be determined or dependent on the interval S between the prepulse PP in the first xy plane and the main pulse MP in the second xy plane, the delay time between the generation of the prepulse PP and the generation of the main pulse MP, and / or the trajectory of the conditioned droplet CD. The trajectory of the conditioned droplet CD can be determined by the spatial offset O and / or the velocity change Δv. Thus, by adjusting one or all of the interval S between the prepulse PP in the first xy plane and the main pulse MP in the second xy plane, the delay time between the generation of the prepulse and the generation of the main pulse MP, and the trajectory of the conditioned droplet CD, the spatial overlap between the conditioned droplet CD and the main pulse MP can be optimized or maximized. This can increase the generated EUV radiation power, increase the EUV radiation stability, and / or reduce the error or fluctuation of the EUV radiation power.

[0107] The control system 16 can be configured to adjust the spatial offset O under the control of one or more parameters to maximize the magnitude of the velocity change Δv. For example, the one or more parameters can include the duration and / or the energy of the prepulse PP. As will be described in more detail below, the velocity change Δv can be varied by changing or adjusting the duration and / or the energy of the prepulse PP.

[0108] The one or more parameters can include at least one of the following: the position of the droplet in the first xy plane, the position of the prepulse PP in the first xy plane, the position change of the droplet D in the first xy plane, the position change of the prepulse in the first xy plane, the position of the main pulse in the second xy plane, and the position change of the main pulse in the second xy plane.

[0109] The radiation system RS can include a sensor system 18. The sensor system 18 can be configured to sense at least one of the one or more parameters. The sensor system 18 can include a first sensor 18a. The first sensor 18a can be provided, for example, in the form of a four-cell, which includes an arrangement of four photodiodes behind a pinhole or the like. The first sensor 18a can be configured to sense the position of the prepulse PP in the xy plane. As previously described, the prepulse PP can include wavelengths of 10.6 μm, 10.26 μm, 10.207 μm, and / or 1 μm. A part of the sensor system 18 (e.g., the first sensor 18a) can be provided or included in a focal volume measurement unit (not shown) of the radiation system RS.

[0110] The sensor system 18 can be operative to sense the position of the droplet D in a first xy-plane and / or the position of the conditioned droplet CD in a second xy-plane. For example, the first sensor 18a can be operative and / or arranged to sense a pre-pulse PP (or a portion thereof) and / or at least a portion of the pre-pulse PP reflected from the droplet D, e.g., after conditioning the droplet by the pre-pulse PP. The sensed pre-pulse PP (or a portion thereof) and / or the sensed reflected portion of the pre-pulse PP can be used to determine the position of the droplet D in the first xy-plane, e.g., the position of the droplet D in the first xy-plane relative to the pre-pulse PP, and / or the position of the conditioned droplet CD in the second xy-plane. Additionally or alternatively, the sensed pre-pulse PP (or a portion thereof) and / or the sensed reflected portion of the pre-pulse PP can be used to determine a change in position of the pre-pulse PP and / or the droplet in the first xy-plane. For example, a change in intensity of the sensed pre-pulse PP (or a portion thereof) and / or the sensed reflected portion of the pre-pulse PP passing through the first sensor 18a can be used to determine a change in position of the pre-pulse PP and / or the droplet D in the first xy-plane. For more background information regarding alignment and other metrology aspects of laser pulses (pre-pulse PP and main pulse MP), see, e.g., U.S. Patent 9,000,405 to Fleurov et al. (assigned to ASML and incorporated herein by reference); U.S. Patent 8,872,144 to Fomenkov (assigned to ASML and incorporated herein by reference); U.S. Patent 8,648,999 to Graham et al. (assigned to Cymer, a subsidiary of ASML and incorporated herein by reference).

[0111] Additionally or alternatively, the sensor system 18 can include a second sensor 18b. The second sensor 18b can be operative to sense the duration and / or energy of the pre-pulse PP. The second sensor 18b can be provided in the form of a photo-electromagnetic sensor.

[0112] The control system 16 can be configured to determine a spatial offset O between the droplet D and the pre-pulse PP based on at least one sensed parameter among one or more parameters. The control system 16 can be configured to control and / or adjust the spatial offset O according to at least one sensed parameter among one or more parameters. For example, the sensor system 18 can be configured to communicate with the control system 16. The sensor system 18 can be configured to transmit a signal to the control system 16. The signal can indicate at least one or all of the one or more parameters.

[0113] Figure 5 An intensity map showing the magnitude of the velocity change Δv that depends on the magnitudes of the x and y components of the spatial offset O is shown. Figure 5The circles indicated by solid lines in the figure represent the values of the x-component and / or y-component of the spatial offset O for which the magnitude of the velocity change Δv can be considered to be maximized and / or the gradient G can be considered to be minimized. In this embodiment, the magnitude of each of the x-component and y-component of the adjusted spatial offset can be between approximately 20 μm and 25 μm:

[0114] |O| = √(x 2 + y 2 ) ≈ 25 μm.

[0115] The amount of the velocity change Δv is represented in gray scale in arbitrary units. For example, the darker the region in the intensity map, the smaller the velocity change, and the brighter the region, the larger the velocity change.

[0116] In an example where the x-component and y-component of the spatial offset O are adjusted to increase beyond 25 μm, the magnitude of the velocity change Δv can decrease towards zero. This may be due to the pre-pulse PP missing the droplet D. Additionally or alternatively, for example, if the spatial offset O (e.g., its x-component and / or y-component) is adjusted to increase beyond 25 μm, the expansion of the droplet D (e.g., caused by the pre-pulse PP) may be reduced. This, in turn, can reduce the size of the adjusted droplet CD, which may affect the spatial overlap between the adjusted droplet CD and the main pulse MP. In other words, the spatial offset O between the droplet D and the pre-pulse PP can be considered to affect the size of the adjusted droplet, as described above. In an example where the x-component and y-component of the spatial offset O are adjusted to decrease towards zero, the amount of the velocity change Δv can decrease towards zero. This may be due to the reduction of the spatial offset O between the droplet D and the pre-pulse PP.

[0117] Figure 6A An intensity map depicting the velocity change Δv in the y-direction that depends on the x-component and y-component of the spatial offset O is shown. In Figure 6A the example depicted, the spatial offset O in the y-direction is assumed to be zero. For a fixed value of the spatial offset O in the x-direction, Figure 6A each of the dotted lines in represents a series of different values of the spatial offset O in the y-direction. As can be seen from Figure 6A the change in the spatial offset O in the y-direction can cause or result in a change in the velocity change Δv in the y-direction.

[0118] Figure 6B A graph depicting the velocity change Δv in the y-direction that depends on the y-component of the spatial offset O for the x-component of the spatial offset ranging from 0 μm to -25 μm (where the dotted line represents the x-component of the spatial offset of -25 μm, the dashed line represents the x-component of the spatial offset of -15 μm, and the solid line represents the x-component of the spatial offset of zero) is shown. As can be seen from Figure 6B the gradient G changes as the x-component of the spatial offset O changes. For example, in Figure 6BIn it, the magnitude of the gradient G decreases as the x-component of the spatial offset O increases. In other words, as the x-component of the spatial offset O changes (e.g., increases), the derivative of the velocity change in the y-direction with respect to the y-component of the spatial offset may change (e.g., decrease), for example independently of the y-component of the spatial offset O.

[0119] Figure 7 A laser system 1 for a radiation system RS is shown. It can be used Figure 7 The shown laser system 1 can be used as part of or included in the radiation system RS as described above with respect to Figure 1 and / or Figure 2 described radiation system RS. Figure 7 The shown laser system 1 can include any features of the laser system 1 as described above.

[0120] The laser system 1 can include a prepulse seed laser 20. The prepulse seed laser 20 can be configured to generate a seed prepulse SPP. The laser system 1 can include a main pulse seed laser 22. The main pulse seed laser 22 can be configured to generate a seed main pulse SMP. Each of the prepulse seed laser 20 and the main pulse seed laser 22 can be provided in the form of a wavelength-tunable seed laser, such as a CO2 laser. It should be understood that the prepulse seed laser and / or the main pulse seed laser disclosed herein is not limited to being provided in the form of a wavelength-tunable seed laser, such as a CO2 laser, and in other embodiments, other suitable seed lasers can be used. For example, at least one of the prepulse seed laser and the main pulse seed laser can be provided in the form of a YAG (yttrium-aluminum-garnet) laser, which can provide radiation at a wavelength of about 1 μm. The seed prepulse SPP and the seed main pulse SMP can include different wavelengths. For example, one of the seed prepulse SPP and the seed main pulse SMP can include a wavelength of about 10.26 μm or 10.207 μm, while the other of the seed prepulse SPP and the seed main pulse SMP can include a wavelength of about 10.6 μm. This can allow different paths to be provided to the prepulse PP and the main pulse MP, for example using one or more optical elements, such as one or more beam splitters, one or more dispersive optical elements, dichroic mirrors, or combinations thereof, for example, as described above, before interacting with the droplet D and the conditioned droplet CD. Exemplary seed lasers are disclosed in US 2013 / 0321926 A1 and US 2014 / 0233055 A1 (both in the name of Cymer, Inc., a subsidiary of ASML), the contents of which are incorporated herein by reference.

[0121] The laser system 1 may include a combiner 24, such as a beam path combiner. The combiner 24 may be provided in the form of a dichroic mirror. The combiner 24 may be configured to place the seed pre-pulse SPP and the seed main pulse SMP onto a common path 26. Exemplary combiners are disclosed in US 2013 / 0321926 A1 (in the name of Cymer, Inc., a subsidiary of ASML), the content of which is incorporated herein by reference.

[0122] The laser system 1 may include an amplifier system 28. The amplifier system 28 may be located on the common path 26. The amplifier system 28 may be configured to amplify the seed pre-pulse SPP to generate a pre-pulse PP, and amplify the seed main pulse SMP to generate a main pulse MP. The amplifier system 28 may include one or more optical or laser amplifiers. For example, the amplifier system 28 may include a pre-amplifier and four power amplifiers. It should be understood that the amplifier system disclosed herein is not limited to including a pre-amplifier and four power amplifiers, and another suitable amplifier arrangement may be used.

[0123] As described above, the radiation system RS may include a sensor system 18. The sensor system 18 may be configured to sense an attribute or parameter. The attribute or parameter may be part of or included in one or more parameters. The attribute or parameter may indicate the generated EUV radiation. For example, the attribute or parameter may indicate the power of the generated EUV radiation. The sensor system 18 may include a third sensor 18c. The third sensor 18c may be configured to sense the power of the generated EUV radiation. The third sensor 18c may be provided in the form of an EUV sensor. The third sensor 18c may be arranged in the radiation source SO or a part thereof. The radiation system RS may include a plurality of third sensors 18c ( Figure 7 only one third sensor is shown in the figure), and the plurality of third sensors 18c may be arranged at different angles in the radiation source, for example, to sense the attributes or parameters of the generated EUV radiation. It should be understood that Figure 7 the exemplary sensor system 18 shown may also include a second sensor 18b (for example, Figure 2 shown in the figure), and the second sensor 18b is not shown in Figure 7 for clarity. It should be understood that in some embodiments, the attributes or parameters of the generated radiation (for example, the power of the EUV radiation) may be sensed by a sensor arranged near the patterning device MA. In other words, in some embodiments, the third sensor may be arranged at or be part of the support structure MT that supports the patterning device MA.

[0124] The laser system 1 may include an amplifier control system 32. The amplifier control system 32 may be configured to control amplification, for example, depending on a sensed property or parameter. The amplifier control system 32 may be provided in the form of, or include, a radio frequency (RF) controller. The RF controller 32 may be configured to apply RF power to the amplifier system 28 to cause amplification of the seed pre-pulse SPP and the seed main pulse SMP. The RF controller 32 may be configured to control and / or regulate the duty cycle of the amplifier system 28. In other words, the RF controller may be configured to control and / or regulate a portion of the period during which RF power is applied to the amplifier system 28 for amplifying the seed pre-pulse SPP and the seed main pulse SMP. For example, the RF controller 32 may use a drive laser gain command (DLGC), which may be a modulation of the RF power of the amplifier system 28. An exemplary RF controller is disclosed in US 2014 / 0233005 A1 (in the name of Cymer, Inc., a subsidiary of ASML), the content of which is incorporated herein by reference.

[0125] The sensor system 18 (e.g., sensor 18c) may be configured and / or arranged to communicate with the RF controller 32. The sensor system 18 (e.g., sensor 18c) may be configured to transmit a signal to the RF controller 32. The signal may indicate a sensed property or parameter, e.g., the power of the generated radiation. For example, when the radiation power is sensed or measured to be reduced, the RF controller 32 may increase the duty cycle of the amplifier system 28. This may cause amplification of the seed pre-pulse SPP and the seed main pulse SMP. This, in turn, may result in an amplified pre-pulse PP and main pulse MP.

[0126] The laser system 1 may include an additional control system 34. The additional control system 34 may be configured to control the duration of the seed pre-pulse SPP based on a sensed property or parameter. The property or parameter may alternatively or additionally indicate a spatial offset and / or a change in the spatial offset. It should be understood that the additional control system 34 may alternatively or additionally be configured to control the duration of the seed main pulse SMP, for example, based on a sensed property or parameter. The additional control system 34 may include a switching device 34a. The switching device 34a may be provided in the form of an electro-optic modulator (EOM). Exemplary EOMs are disclosed in US2013 / 0321926 A1 and US 2014 / 0233055 A1 (both in the name of Cymer, Inc., a subsidiary of ASML), the contents of which are incorporated herein by reference. The EOM may be regarded as acting as a shutter. The EOM may be configured to allow the leading edge of the seed pre-pulse SPP to pass through and then may be configured to close in order to cut off the trailing edge of the seed pre-pulse SPP at a desired point. In other words, the switching device 34a may be arranged and / or configured to adjust the duration of the seed pre-pulse SPP, for example, in response to a signal from the additional control system 34. The additional control system 34 may include an additional switching device 34b. The additional switching device may be the same as the switching device 34a. The additional switching device 34b may be arranged and / or configured to adjust the duration of the seed main pulse SMP.

[0127] For example, when the sensor system 18 (e.g., sensor 18c) senses a reduction in the generated radiation, the RF controller 32 may operate to increase the duty cycle of the amplifier system 28. This may increase the amplification of both the seed pre-pulse SPP and the seed main pulse SMP, since the amplifier system 28 is arranged on the common path 26. The amplification of the seed pre-pulse SPP may cause a change or alteration in the spatial offset O between the pre-pulse PP and the droplet D. It should be understood that other factors may alternatively or additionally cause a change or alteration in the spatial offset. For example, fluctuations, variations, or changes in the position of the droplet D and / or the pre-pulse PP in the first xy plane (such as random fluctuations, variations, or changes) may cause a change or alteration in the offset O. Alternatively or additionally, a change or alteration in the energy of the pre-pulse PP may cause a change or alteration in the spatial offset, as will be described below. The change or alteration in the spatial offset may cause a change or alteration in the velocity change Δv. The change or alteration in the velocity change Δv may cause a change or alteration in the spatial overlap between the conditioned droplet CD and the main pulse MP, which in turn may cause a change or alteration in the generated EUV radiation, for example, a reduction.

[0128] By controlling the duration of the seed pre-pulse SPP according to the sensed property or parameter, variations or changes in the spatial offset and / or the velocity change Δv can be reduced or compensated for. This can reduce or avoid variations or changes in the spatial overlap between the conditioned droplet CD and the main pulse MP, thereby reducing or preventing a decrease in the power of the generated EUV radiation, EUV radiation instability, and / or errors or fluctuations in the EUV radiation power.

[0129] The sensor system 18 can be configured to sense the spatial offset O or its change between the droplet and the pre-pulse PP. The change in the spatial offset may be due to the amplification of the seed pre-pulse SPP and the seed main pulse SMP by the amplifier system, and / or due to fluctuations, variations, or changes (such as random fluctuations, variations, or changes) in the position of the droplet D and / or the pre-pulse PP in the first xy plane, as described above.

[0130] The sensor system 18 (e.g., the first sensor 18a) can be configured and / or arranged to communicate with the RF controller 32. The sensor system 18 (e.g., the first sensor 18a) can send a signal to the RF controller 32. The signal can indicate the sensed property or parameter, e.g., the spatial offset O or its change. The additional control system 34 can be configured to adjust the duration of the seed pre-pulse according to the sensed property or parameter (e.g., the spatial offset or its change). For example, the additional control system 34 can be configured and / or arranged to communicate with the RF controller 32. The additional control system 34 can be configured to adjust the duration of the seed pre-pulse SPP in response to a signal from the RF controller 32. As described above, the laser system 1 can include a beam delivery system 1a. The beam delivery system 1a can be configured to separate the pre-pulse PP and the main pulse MP, e.g., using one or more optical elements, such as one or more beam splitters, one or more dispersive optical elements, dichroic mirrors, or a combination thereof, as described above. The beam delivery system 1a can be configured to direct the pre-pulse PP and the main pulse MP towards the plasma formation region 4. The beam delivery system 1a can determine the position of the pre-pulse PP in the first xy plane, the position of the main pulse in the second xy plane, and / or the interval S between the pre-pulse PP in the first xy plane and the main pulse MP in the second xy plane. For example, in response to a signal from a control system (e.g., the control system 16), the beam delivery system 1a can adjust the position of the pre-pulse PP in the first xy plane, the position of the main pulse in the second xy plane, and / or the interval S between the pre-pulse PP in the first xy plane and the main pulse MP in the second xy plane. Alternatively or additionally, the RF controller 32 and / or the additional control system 34 can be arranged to communicate with the beam delivery system 1a.

[0131] Figure 8A and 8BEach figure shows the magnitude of the velocity change Δv that depends on the spatial offset O in the first xy plane. For convenience in these two figures, the y - component of the spatial offset is considered to be zero. It should be understood that in other embodiments, the y - component of the spatial offset can be greater than or less than zero.

[0132] Figure 8A A figure depicts the magnitude of the velocity change Δv that depends on the spatial offset O in the x or y direction. Figure 8A It shows the change or alteration in the magnitude of the velocity change Δv, which may be caused by the amplification of the above - mentioned seed pre - pulse SPP and seed main - pulse SMP. Figure 8A The solid line in [[]] shows the magnitude of the velocity change Δv that depends on the spatial offset for the nominal amplification of the seed pre - pulse SPP (e.g., the amplification of the seed pre - pulse SPP that can be applied during the normal operation of the laser system 1 (e.g., the radiation system RS)). Figure 8A The dashed line in [[]] shows the magnitude of the velocity change Δv according to the spatial offset for an increased (e.g., increased relative to the nominal amplification) amplification of the seed pre - pulse SPP. It can be seen that for the increased amplification of the seed pre - pulse SPP, a change or alteration in the spatial offset O may lead to an increase in the velocity change Δv. This increase in the size of the velocity change Δv may cause a change or alteration in the spatial overlap between the conditioned droplet CD and the main pulse MP. This, in turn, may lead to a decrease in the power of the generated EUV radiation, EUV radiation instability, and / or an error or fluctuation in the EUV radiation power. It should be understood that the increased velocity change ( Figure 8A the dashed line in [[]]) can alternatively or additionally be due to fluctuations, changes, or alterations (e.g., random fluctuations, changes, or alterations) in the position of the droplet D and / or the pre - pulse PP in the first xy plane.

[0133] Figure 8B Another figure depicts the magnitude of the velocity change Δv that depends on the spatial offset in the x or y direction. Figure 8B Similar to Figure 8A . Figure 8B It also shows the magnitude of the velocity change Δv that depends on the spatial offset O after adjusting the duration of the seed pre - pulse SPP, which is represented by the light - gray dashed line. In Figure 8BIn the example shown, for instance, the duration of the seed pre-pulse SPP is adjusted (reduced) by an additional control system 34. It should be understood that the disclosed additional control system is not limited to reducing the duration of the seed pre-pulse, and for example, in other embodiments, the duration of the seed pre-pulse can be increased. By reducing the duration of the seed pre-pulse SPP, the magnitude of the velocity change Δv can be kept substantially constant or similar to the magnitude of the velocity change Δv before the nominal amplification of the seed pre-pulse SPP and / or fluctuations, variations, or changes (e.g., random fluctuations, variations, or changes) in the position of the droplet D and / or the pre-pulse PP in the first xy plane. This can prevent or reduce changes or alterations in the spatial overlap between the conditioned droplet CD and the main pulse MP. In other words, the additional control system 34 can be configured to adjust the duration of the seed pre-pulse SPP such that the magnitude of the velocity change Δv remains unchanged, e.g., substantially constant or constant, for example when the seed pre-pulse SPP is amplified according to a sensed property or parameter and / or fluctuations, variations, or changes (e.g., random fluctuations, variations, or changes) in the position of the droplet D and / or the pre-pulse PP in the first xy plane exist or occur. This can allow compensation for the reduction in radiation in response to the amplification of the seed pre-pulse and / or due to fluctuations, variations, or alterations (e.g., random fluctuations, variations, or changes) in the position of the droplet D and / or the pre-pulse PP in the first xy plane.

[0134] It should be understood that Figure 7 the laser system 1 shown can be used in a radiation system that does not include the above-described control system 16. In other words, the duration of the seed pre-pulse SPP can be controlled independently of the spatial offset O or without adjusting the spatial offset O to maximize the velocity change Δv of the conditioned droplet CD in the second xy plane. Alternatively, Figure 7 the laser system 1 shown can be used in combination with a radiation system RS (e.g., the control system 16), as Figure 2 shown and described above.

[0135] Figure 9 A laser system 1 and a control system 16 for use in a radiation system are depicted. The laser system 1 and the control system 16 shown can be used Figure 9 as part of or included in the radiation system RS described above with respect to Figure 1 and / or Figure 2 described. Figure 9 The laser system 1 shown is the same as Figure 7 the laser 1 shown. It should be understood that in other embodiments, different laser systems or their arrangements can be used. The laser system 1 can be configured and / or arranged to communicate with the above-described control system 16. Figure 9The control system 16 shown may include any features of the control system 16 described above. The control system 16 may be arranged to communicate with at least a portion of the sensor system 18 (e.g., the first sensor 18a), as described above. It should be understood that in other embodiments, the control system may be arranged to communicate with the second sensor and / or the third sensor additionally or alternatively.

[0136] Reference Figure 9 , the control system 16 may operate to maximize the magnitude of the velocity change Δv by controlling the duration of the prepulse PP. As described above, the duration of the prepulse PP may be adjusted by adjusting the duration of the seed prepulse SPP. For example, the control system 16 may communicate with an additional control system 34. As described above, the additional control system 34 may include a switching device 34a and / or an additional switching device 34b. The control system 16 may transmit a signal to the additional control system 34. The signal may indicate the duration of the seed prepulse SPP. The switching device 34a may adjust the duration of the seed prepulse SPP, e.g., in response to a signal from the additional control system 34. The duration of the prepulse PP (e.g., the seed prepulse SPP) may vary between, for example, 40 ns and 180 ns. It should be understood that in some embodiments, the additional control system 34 and / or the switching devices 34a, 34b may be a part of or included in the control system 16.

[0137] In addition to or instead of being configured to adjust the spatial offset O to maximize the magnitude of the velocity change Δv of the conditioned droplet CD in the second xy plane as described above, the control system 16 may operate to maximize the magnitude of the velocity change Δv by controlling the duration of the prepulse PP.

[0138] Additionally or alternatively, the control system 16 may operate to control and / or adjust the velocity change by controlling the duration of the prepulse PP. For example, the control system 16 may operate to control and / or adjust the velocity change by controlling the duration of the prepulse PP such that the spatial overlap between the conditioned droplet CD and the main pulse MP in the second xy plane is optimized or maximized. The control system 16 may be configured to maintain the velocity change by controlling the duration of the prepulse PP, e.g., such that the spatial overlap between the conditioned droplet CD and the main pulse MP in the second xy plane is optimized or maximized. This may increase the generated EUV radiation power, increase the EUV radiation stability, and / or reduce the error or fluctuation of the EUV radiation power.

[0139] The control system 16 may operate to adjust the duration of the prepulse PP based on the interval S between the prepulse PP in the first xy plane and the main pulse MP in the second xy plane (see Figure 3)。The interval S between the pre-pulse PP in the first xy-plane and the main pulse MP in the second xy-plane can be changed or varied in the x-direction and / or the y-direction, for example due to thermal induction effects in the radiation system RS (e.g., laser system 1). The control system 16 can be operated to adjust the duration of the pre-pulse PP based on the interval S between the pre-pulse PP in the first xy-plane and the main pulse MP in the second xy-plane, for example if the spatial offset O between the droplet D and the pre-pulse PP is fixed or set. By adjusting the duration of the pre-pulse PP based on the interval S between the pre-pulse PP in the first xy-plane and the main pulse MP in the second xy-plane, the spatial overlap between the conditioned droplet CD and the main pulse MP can be adjusted, for example to optimize or maximize the spatial overlap between the conditioned droplet CD and the main pulse MP.

[0140] Figure 10 A graph depicting the magnitude of the velocity change Δv that depends on the spatial overlap in the first xy-plane is shown. Figure 10 The light gray dots in [graph] represent the measured values of the velocity change, the hollow circles represent the average values of the measured velocity changes, and the solid line represents the curve that fits the average values of the velocity changes. In this example, the spatial offset O has been chosen such that the value of the y-component of the spatial offset O is fixed at, for example, zero and the value of the x-component can vary. In other words, in this example, there is a spatial offset O in the x-direction and no spatial offset in the y-direction. However, it should be understood that in other embodiments, the y-component can be chosen to vary, for example to be greater than or less than zero, the x-component can be chosen to be fixed, for example zero, or both the x-component and the y-component can be chosen to be variable.

[0141] In Figure 10 [graph], the gray solid line shows the velocity change that depends on the spatial offset in the first xy-plane for an increased duration of the pre-pulse PP, relative to the velocity change indicated by the circles and the black solid line. As can be seen from Figure 10 [graph], by controlling the duration of the pre-pulse PP, for a given spatial offset O, the magnitude of the velocity change Δv can be varied, for example increased or decreased. In other words, the velocity change Δv can be changed by controlling the duration of the pre-pulse PP, for example independently of the spatial offset O or without changing the spatial offset O. It can also be seen that, for example, by adjusting (e.g., increasing) the duration of the pre-pulse PP and setting the y-component of the spatial offset O to, for example, zero, the velocity change Δv in the x-direction can be changed or adjusted (e.g., increased) independently of the velocity change Δv in the y-direction. It should be understood that in other embodiments, by adjusting (e.g., increasing) the duration of the pre-pulse PP and setting the x-component of the spatial offset O to, for example, zero, the velocity change in the y-direction can be adjusted (e.g., increased) independently of the velocity change in the x-direction.

[0142] The control system 16 can be operated to adjust the duration of the pre-pulse PP in response to a change in the energy of the pre-pulse PP. For example, the change in the energy of the pre-pulse PP may be due to the degradation of the amplification medium, which can be part of or included in the amplifier system 28. Alternatively or additionally, the change in the energy of the pre-pulse can be due to a change in the beam path of the seed pre-pulse SPP, the seed main pulse SMP, the pre-pulse PP, and / or the main pulse MP, e.g., absorption of these pulses or a change thereof. The change in the energy of the pre-pulse PP may cause a change or alteration in the velocity change Δv, which in turn may cause a change or alteration in the spatial overlap between the conditioned droplet CD and the main pulse MP.

[0143] Figure 11 A graph depicting the derivative of the velocity change in the x direction with respect to the magnitude of the spatial offset O (or the magnitude of the gradient G), which derivative is a function of the fluence or radiation exposure of the pre-pulse PP on the droplet D. Figure 11 A graph including pre-pulse PP durations having values of 40 ns, 70 ns, 130 ns, and 180 ns. In Figure 11 it, the fluence or radiation exposure of the pre-pulse PP is represented as the ratio of the energy of the pre-pulse to the square of the beam size of the pre-pulse. It can be seen from Figure 11 that the magnitude of this derivative increases with an increase in the duration and an increase in the energy of the pre-pulse PP. The velocity change Δv in the x direction can be considered to be based on the duration of the pre-pulse PP and / or the energy of the pre-pulse PP. It should be understood that in other embodiments, the derivative of the velocity change in the y direction and / or the z direction with respect to the magnitude of the spatial offset may alternatively or additionally vary (e.g., increase) with a change (e.g., increase) in the duration and / or energy of the pre-pulse PP. Alternatively or additionally, the velocity change itself can be considered to be based on the duration of the pre-pulse and / or the energy of the pre-pulse, as well as the spatial overlap.

[0144] The control system 16 can be operated to adjust the duration of the pre-pulse in response to a change in the interval S between the pre-pulse PP in the first xy plane and the main pulse MP in the second xy plane. This can allow the spatial overlap between the conditioned droplet CD and the main pulse MP to remain unchanged and / or compensate for the drift in the position of the pre-pulse PP in the first xy plane and / or the main pulse MP in the second xy plane.

[0145] Figure 12A flowchart depicting a method of generating EUV radiation is shown. The method may include operating a droplet generator 3 to generate fuel droplets that travel towards a plasma formation region 4 (step 1200). The method may include operating a laser system 1 to generate a prepulse PP for conditioning the droplets D (step 1205). In step 1210, the method may include controlling a spatial offset O between the prepulse PP and the droplets in a plane transverse to the propagation direction of the prepulse PP (e.g., a first xy plane). The method may include adjusting the spatial offset O to maximize a velocity change Δv of the conditioned droplets in a plane transverse to the propagation direction of the main pulse MP (step 1215). In step 1220, the method may include operating the laser system 1 to generate a main pulse MP to convert the conditioned droplets CD into a plasma that generates EUV radiation.

[0146] Figure 13 A flowchart depicting method steps that may be Figure 12 part of or included in the method depicted in Figure 13 Only some of the method steps depicted in Figure 12 may be part of or included in the method depicted in Figure 13 Alternatively, all or none of the method steps depicted in Figure 12 may be part of or included in the method depicted in Figure 13 In other words, the method steps depicted in Figure 12 may be used separately from some or all of the method steps depicted in Figure 12 or in combination with at least one or all of the method steps depicted in

[0147] In step 1300, the method may include operating a prepulse seed laser 20 to generate a seed prepulse SPP. The method may include operating a main pulse seed laser 22 to generate a seed main pulse SMP (step 1305). The prepulse seed laser 20 and the main pulse seed laser 22 may be part of or included in the laser system 1. The seed prepulse SPP and the seed main pulse SMP may be placed on a common path 26 by a combiner 24. The method may include operating an amplifier system 18 located on the common path 26 (step 1310). The amplifier system 28 may be configured to amplify the seed prepulse SPP to generate a prepulse PP and amplify the seed main pulse SMP to generate a main pulse MP. In step 1315, the method may include as described above with reference to Figure 7 and 9The sensed property or parameter being discussed. The method may include operating an amplifier control system 32 (step 1320). The amplifier control system 32 may be configured to control amplification based on the sensed property or parameter. The method may include operating an additional control system 34, for example, the additional control system 34 includes a switching device 34a and is configured to control the duration of a seed pre-pulse SPP based on the sensed property or parameter (1325).

[0148] Figure 14 A flowchart depicting other method steps, which method steps may be Figure 12 and / or Figure 13 part of or included in the method depicted in Figure 14 The method steps depicted in may be used separately from each other, in combination with Figure 12 and / or Figure 13 the method (or method steps) depicted in or separately from or in combination with at least one or all of the method steps depicted in Figure 12 and / or Figure 13

[0149] In step 1400, the property or parameter includes at least one of the following: the power of the generated EUV radiation, a spatial offset, a change in the spatial offset.

[0150] In step 1405, the method may include controlling the duration of a pre-pulse PP to maximize a velocity change Δv.

[0151] It should be understood that the term "duration" of a pre-pulse may be considered to cover the length of the pre-pulse PP.

[0152] It should be understood that the term "radiation" may be considered to cover EUV radiation and these terms may be used interchangeably.

[0153] Although embodiments of the present invention may be specifically referred to herein in the context of a lithographic apparatus, embodiments of the present invention may be used in other devices. Embodiments of the present invention may form part of a mask inspection device, a metrology device, or any device for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). Such devices may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0154] The term "EUV radiation" may be considered to cover electromagnetic radiation having a wavelength in the range of 4 - 20 nm (e.g., in the range of 13 - 14 nm). The wavelength of EUV radiation may be less than 10 nm, for example, in the range of 4 - 10 nm, such as 6.7 nm or 6.8 nm.

[0155] ​Although specific reference may be made herein 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, the guidance and detection of patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0156] 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 disk storage media; optical storage media; flash memory devices; electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.). Additionally, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only and that such actions are in fact caused by computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

[0157] Although specific embodiments of the invention have been described above, it should be understood that the invention may be practiced in a manner different from that described. The foregoing description is intended to be illustrative, not limiting. Thus, it will be apparent to those skilled in the art that various modifications may be made to the invention without departing from the scope of the claims set forth below.

Claims

1. A radiation system configured to generate radiation and comprising: A droplet generator configured to generate fuel droplets traveling towards a plasma formation region at an initial velocity; A laser system operative to generate a prepulse and a main pulse, wherein the prepulse is configured to condition the droplets to receive the main pulse, and wherein the main pulse is configured to convert the conditioned droplets into a plasma that generates the radiation; And A control system configured to control a spatial offset between the prepulse and the droplets in a plane transverse to the propagation direction of the prepulse; Wherein the control system is configured to adjust the spatial offset to maximize a velocity change of the conditioned droplets relative to the initial velocity in a plane transverse to the propagation direction of the main pulse, wherein the velocity change of the conditioned droplets is produced by momentum transfer of the prepulse to the unconditioned droplets.

2. The radiation system according to claim 1, wherein the control system is configured to adjust an interval between the prepulse in a plane transverse to the propagation direction of the prepulse and the main pulse in a plane transverse to the propagation direction of the main pulse based on the spatial offset.

3. The radiation system according to claim 1 or 2, wherein the control system is configured to adjust a delay time between generation of the prepulse and generation of the main pulse based on the spatial offset.

4. The radiation system according to claim 1 or 2, wherein the laser system is configured to direct the main pulse towards a target region where the conditioned droplets are converted into a plasma that generates radiation.

5. The radiation system according to claim 4, wherein the control system is configured to adjust the target region depending on at least one of: The spatial offset, The velocity change, An interval between the prepulse in a plane transverse to the propagation direction of the prepulse and the main pulse in a plane transverse to the propagation direction of the main pulse, and A delay time between generation of the prepulse and generation of the main pulse.

6. The radiation system according to claim 1 or 2, wherein the control system is configured to adjust the spatial offset depending on one or more parameters to maximize the velocity change.

7. The radiation system according to claim 6, wherein the one or more parameters include a duration of the prepulse and / or an energy of the prepulse.

8. The radiation system according to claim 6, wherein the one or more parameters include at least one of: A position of the droplets in a plane transverse to the propagation direction of the prepulse, A position of the prepulse in a plane transverse to the propagation direction of the prepulse, A change in a position of the droplets in a plane transverse to the propagation direction of the prepulse, A change in a position of the prepulse in a plane transverse to the propagation direction of the prepulse, A position of the main pulse in a plane transverse to the propagation direction of the main pulse, and The variation of the position of the main pulse in a plane transverse to the propagation direction of the main pulse.

9. The radiation system according to claim 6, wherein the radiation system comprises a sensor system configured to sense at least one of the one or more parameters.

10. The radiation system according to claim 9, wherein the control system is configured to control and / or adjust the spatial offset depending on the sensed at least one of the one or more parameters.

11. The radiation system according to claim 1 or 2, wherein the spatial offset at which the velocity variation is maximized is between 20 μm and 25 μm.

12. The radiation system according to claim 1 or 2, wherein the radiation system comprises a sensor system configured to sense an attribute or parameter; And wherein the laser system comprises: A pre-pulse seed laser configured to generate a seed pre-pulse; A main-pulse seed laser configured to generate a seed main-pulse; A combiner configured to place the seed pre-pulse and the seed main-pulse on a common path; An amplifier system located on the common path and configured to amplify the seed pre-pulse to generate the pre-pulse and amplify the seed main-pulse to generate the main-pulse; An amplifier control system configured to control the amplification of the seed pre-pulse and the amplification of the seed main-pulse depending on sensed properties or parameters; An additional control system configured to control the duration of the seed pre-pulse depending on sensed properties or parameters.

13. The radiation system according to claim 12, wherein the property or parameter comprises a property or parameter of the generated radiation.

14. The radiation system according to claim 12, wherein the property or parameter comprises at least one of the following: The power of the generated radiation, The spatial offset, and The variation of the spatial offset.

15. The radiation system according to claim 12, wherein the additional control system is configured to adjust the duration of the seed pre-pulse laser such that when the seed pre-pulse and the seed main-pulse are amplified depending on detected properties or parameters, the velocity variation is substantially unchanged or constant.

16. The radiation system according to claim 1 or 2, wherein the control system operates to maximize the velocity variation by controlling the duration of the pre-pulse.

17. The radiation system according to claim 1 or 2, wherein the control system operates to adjust the duration of the pre-pulse in response to a change in the energy of the pre-pulse.

18. The radiation system according to claim 16, wherein the control system operates to adjust the duration of the pre-pulse based on the spacing or distance between the pre-pulse in a plane transverse to the propagation direction of the pre-pulse and the main-pulse in a plane transverse to the propagation direction of the main-pulse.

19. The radiation system according to claim 16, wherein the control system operates to adjust the duration of the pre-pulse in response to a change in the spacing or distance between the pre-pulse in a plane transverse to the propagation direction of the pre-pulse and the main-pulse in a plane transverse to the propagation direction of the main-pulse.

20. A lithography system comprising a radiation system according to any one of claims 1 to 19.

21. A method of generating radiation, the method comprising: operating a droplet generator to generate fuel droplets travelling towards a plasma formation region; operating a laser system to generate a pre-pulse for conditioning the droplets; controlling a spatial offset between the pre-pulse and the droplets in a plane transverse to the propagation direction of the pre-pulse; adjusting the spatial offset to maximise a velocity change of the conditioned droplets in a plane transverse to the propagation direction of a main pulse; and operating the laser system to generate the main pulse to convert the conditioned droplets into a plasma generating radiation.

22. The method according to claim 21, wherein the method comprises: operating a pre-pulse seed laser to generate a seed pre-pulse; operating a main-pulse seed laser to generate a seed main-pulse, the pre-pulse seed laser and the main-pulse seed laser being part of or included in the laser system, the seed pre-pulse and the seed main-pulse being placed on a common path by a combiner; operating an amplifier system located on the common path and configured to amplify the seed pre-pulse to generate the pre-pulse and amplify the seed main-pulse to generate the main-pulse; sensing a property or parameter; operating an amplifier control system configured to control the amplification of the seed pre-pulse and the amplification of the seed main-pulse depending on the sensed property or parameter; operating an additional control system configured to control the duration of the seed pre-pulse depending on the sensed property or parameter.

23. The method according to claim 22, wherein the property or parameter comprises at least one of the following: the power of the generated radiation, the spatial offset, and a change in the spatial offset.

24. The method according to any one of claims 21 to 23, wherein the method comprises controlling the duration of the pre-pulse to maximise the velocity change.

25. A computer program comprising computer-readable instructions configured to cause a processor to execute the method according to any one of claims 21 to 24.

26. A computer-readable medium carrying the computer program according to claim 25.

27. A computer device comprising: a memory storing processor-readable instructions; and a processor arranged to read and execute the instructions stored in the memory; wherein the processor-readable instructions comprise instructions arranged to control the computer to execute the method according to any one of claims 21 to 24.

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