WO2025201792A1 - Euv radiation generating method - Google Patents

Abstract

A method of generating extreme ultraviolet (EUV) radiation comprising directing laser pulses of a laser beam onto tin targets to generate EUV emitting plasma, wherein the laser pulses have a duration of at least 150 ns and have a wavelength between 1.6 microns and 2.5 microns.

Description

EUV RADIATION GENERATING METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63/569,362 which was filed on 25 March 2024 which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a method of generating extreme ultraviolet (EUV) radiation.

BACKGROUND

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

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses reflective mirrors instead of lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector. The multilayers may be altematingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a massproduction process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks. For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Fourthly, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle/amplitude effects.

[0006] EUV radiation may be produced by a laser produced plasma (LPP) radiation source. Within an LPP radiation source, a laser beam may be used to irradiate fuel droplets so as to produce a plasma which will emit EUV radiation.

[0007] A problem associated with LPP radiation sources is that the power of EUV radiation provided by the LPP radiation source is relatively low. This limits the throughput of the lithographic apparatus. [0008] It may be desirable to provide an LPP radiation source which is capable of providing higher power EUV radiation generation than at least some prior art LPP radiation sources.

SUMMARY

[0009] According to a first aspect of the invention there is provided A method of generating extreme ultraviolet (EUV) radiation comprising directing laser pulses of a laser beam onto tin targets to generate EUV emitting plasma, wherein the laser pulses have a duration of at least 150 ns and have a wavelength between 1.6 microns and 2.5 microns.

[00010] Advantageously, using laser pulses having a duration of at least 150 ns and a wavelength between 1.6 microns and 2.5 microns provides higher power EUV radiation than is achievable using at least some conventional LPP radiation sources. Additionally or alternatively, using laser pulses having a duration ofat least 150 ns and a wavelength between 1.6 microns and 2.5 microns may provide greater efficiency and/or less complexity than a conventional LPP radiation source.

[00011] The tin targets may be droplets having a diameter of at least 40 microns.

[00012] The EUV emitting plasma may emit at least 0.05 J of EUV radiation over a solid angle that is collected by a collector.

[00013] The tin targets may be droplets having a diameter of at least 45 microns.

[00014] The EUV emitting plasma may emit at least 0.07J of EUV radiation over a solid angle that is collected by a collector.

[00015] The tin targets may be provided with a repetition rate of 40 kHz or less.

[00016] The laser pulses may have a substantially flat temporal profile.

[00017] The laser pulses may have a top-hat spatial profile across a diameter of at least 100 microns.

[00018] In an embodiment, only one laser pulse is incident upon each tin target.

[00019] Each laser pulse may have an energy of at least 1 J.

[00020] Each laser pulse may have an energy of up to 5 J. [00021] The laser beam that is incident upon the tin targets may have an intensity of at least 0.7 x 10¹¹ W/cm².

[00022] The front of each laser pulse may have a gradually rising edge with a duration of at least 5 ns. [00023] The laser beam may have a diameter of at least 100 microns along an axial distance of at least 400 microns at a plasma formation location.

[00024] According to a second aspect of the invention there is provided a radiation source comprising a nozzle configured to direct targets of tin towards a plasma formation location, and a laser system configured to direct laser pulses having a wavelength between 1.6 microns and 2.5 microns to the plasma formation location, wherein the radiation source further comprises a controller configured to control the laser system such that the laser pulses have a duration of at least 150 ns.

[00025] Advantageously, the radiation source provides higher power EUV radiation than is achievable using at least some conventional LPP radiation sources. Additionally or alternatively, the radiation source may provide greater efficiency and/or less complexity than a conventional LPP radiation source. [00026] The nozzle may be configured to provide tin targets that are tin targets having a diameter of at least 40 microns. The nozzle may be configured to provide tin targets that are tin targets having a diameter of at least 45 microns.

[00027] The controller ma ybe configured to control the nozzle to provide the tin targets with a repetition rate of 40 kHz or less.

[00028] The controller may control the laser system to provide only one laser pulse per tin target.

[00029] The laser system may be configured to provide the laser pulses in a laser beam that has an intensity of at least 0.7 x 10¹¹ W/cm² at the plasma formation location.

[00030] According to a third aspect of the invention, there is provided an exposure apparatus comprising the radiation source of the second aspect of the invention and further comprising a lithographic apparatus, a metrology apparatus, or an inspection apparatus.

[00031] Features of different aspects of the invention may be combined together.

BRIEF DESCRIPTION OF THE DRAWINGS

[00032] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and further comprising a radiation source according to an embodiment of the invention;

Figure 2 schematically depicts a system for (actinic) mask inspection which comprises a radiation source according to an embodiment of the invention;

Figure 3 schematically depicts a radiation source according to an embodiment of the invention;

Figure 4 schematically depicts a temporal profile of a laser pulse used by a radiation source according to an embodiment of the invention; Figure 5 schematically depicts a spatial profile of a laser pulse used by a radiation source according to an embodiment of the invention; and

Figure 6 schematically depicts a waist of a laser beam as incident on a tin droplet at a plasma formation region of a radiation source according to an embodiment of the invention.

DETAILED DESCRIPTION

[00033] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[00034] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[00035] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[00036] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[00037] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[00038] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[00039] Figure 2 schematically depicts a system MS for (actinic) mask inspection. The mask inspection system MS can be used to identify or inspect defects in a mask to be used in a lithographic process by means of the lithographic apparatus described in figure 1. The mask inspection system MS comprises a radiation source SO, an illumination system ILM, a detection system DS and a mask stage ME.

[00040] The illumination system ILM is configured to condition an EUV radiation beam B before the EUV radiation beam B is incident upon the mask MA. The illumination system ILM may provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system ILM may comprise a plurality of mirrors 22. The illumination system ILM may comprise one or more faceted mirror devices.

[00041] The mask stage ME may be configured to move the mask MA relative to the EUV radiation beam B, so that the EUV radiation beam is incident upon different areas of the mask.

[00042] The detection system DS comprises a detector 20, and may in addition comprise a plurality of mirrors 24. The plurality of mirrors 24 may be configured to collect EUV radiation BR that has been reflected from the mask MA, and form an image of the mask MA on the detector 20 (which may be an imaging array). A processor (not depicted) may receive signals output from the detector 20 and use those signals to look for defects in the mask MA.

[00043] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1 is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator

3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region

4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[00044] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal -incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below. [00045] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser system 1, the radiation source SO and the beam delivery system (if present) may together be considered to be a radiation system.

[00046] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[00047] The radiation source SO of the mask inspection system MS may correspond with the radiation source SO depicted in Figure 1. In the same way as depicted in Figure 1, the radiation source SO of the mask inspection system MS may focus the EUV radiation beam B to form an intermediate focus 6.

[00048] Figure 3 schematically depicts a radiation source SO according to an embodiment of the invention. The radiation source SO comprises a fuel generator 3 configured to direct tin droplets to a plasma formation region 4. A tin droplet 34 is depicted at the plasma formation region 4. The fuel generator 3 includes a nozzle 36 from which tin droplets are ejected. The diameter of the tin droplets is determined by the diameter of the nozzle 36. The nozzle includes an actuator (not depicted) which modulates the nozzle 36. The modulation frequency applied to the actuator determines the repetition rate at which tin droplets are ejected from the nozzle 36. Liquid tin is held under pressure in the fuel generator 3. The pressure at which the liquid tin is held determines the velocity of tin droplets ejected from the nozzle 36.

[00049] Figure 3 also depicts a laser system 1 of the radiation source SO. The laser system 1 comprises a laser 30, a pump laser 32 and a controller CT. The laser may for example be a Tm:YLF laser. The laser 30 is configured to provide a laser beam 2. The laser beam 2 comprises laser pulses which are incident upon tin droplets 34 (as schematically depicted).

[00050] The pump laser 32 may for example be a laser diode, e.g. a GaAs laser diode, and is configured to emit a pump laser beam 33 which is coupled into a gain medium of the laser 30. In this way, the pump laser 32 creates a population inversion in the laser 30, which causes the laser 30 to emit the laser beam 2. In common with the laser 30, the pump laser 32 may emit a pulsed laser beam 36. A plurality of pump lasers 32 may be provided.

[00051] Optics 38 may be provided between the laser 30 and the plasma formation region 4. The optics 38 may comprise one or more focussing elements (e.g. one or more lenses). The optics 38 may form a waist of the laser beam 2 as the plasma formation region 4.

[00052] The tin droplets 34 provided by the fuel generator 3 may be spherical. The tin droplets 34 may have a diameter which is less than the diameter of the laser beam 2 at the plasma formation location 4 (i.e. the location where the laser beam is incident upon the tin droplet). For example, the tin droplet 34 may have a diameter which is less than half of the diameter of the laser beam 2 at the plasma formation region 4.

[00053] The laser beam 2 may for example have a wavelength of at least 1.6 microns. The laser beam 2 may for example have a wavelength of up to 2.5 microns. The laser beam 2 may for example have a wavelength of around 1.9 microns. The gain medium of the laser 30 may be selected to provide a desired wavelength of the laser beam 2. For example, a Tm:YLF laser 30 may provide a laser beam 2 having a wavelength of around 1.9 microns (e.g. 1.88 microns). The gain medium for Tm:YLF may for example be a slab, a disk or a fiber architecture. Other laser gain media may be used.

[00054] As noted above, the pump laser 32 may for example be a laser diode or a plurality of laser diodes. The laser diodes may for example provide the pump laser beam 33 at a wavelength of around 790 nanometres. The laser diodes may for example be GaAs laser diodes.

[00055] A combination of a pump laser diodes emitting a laser beam at a wavelength of 790nm with a Tm:YLF laser emitting a laser beam with a wavelength of 1.88 microns may be particularly advantageous. Laser diodes emitting at 790nm are commercially available with a high power and high efficiency. Tm:YLF has an absorption peak at around 790nm. Fast non-radiative cross-relaxation will populate an upper level of the Tm:YLF, thereby providing particularly effective pumping of the Tm:YLF. Furthermore, the ³F4 level of Tm:YLF which emits photons at 1.88 nm has a relatively long lifetime of around 15ms, thereby providing good energy storage and allows efficient multi -pulse extraction (i.e. multiple pulses used to fully extract a population inversion.

[00056] Each tin droplet 34 may receive a single pulse of the laser beam 2 without also receiving any laser pre-pulses. That is, unlike conventional LPP radiation sources, embodiments of the invention do not require pre-pulses. Advantageously, this simplifies the LPP radiation source SO. A pre-pulse is not used to modify the tin droplet to a disk shape before the laser beam pulse (as is done in prior art LPP radiation sources). Modification of a tin droplet to a disk shape is very sensitive to shock waves generated from vaporization of the preceding droplet. Embodiments of the invention do not use a pre- pulse and do not require the tin droplet to have a disk shape. Embodiments of the invention are more tolerant to misshapen tin droplets at the plasma formation location, and thus are more tolerant to shock waves generated by a preceding droplet. This means that a separation between successive tin droplets can be reduced (compared with prior art LPP radiation sources). For example, the separation between droplets may be as small as 1mm.

[00057] The single laser pulse converts the tin droplet 34 to EUV emitting plasma. The tin droplet transitions from liquid, to vapour with some liquid. The vapour is ionized to become a plasma. The plasma cools and self-emits light. This continues until all of the liquid tin has been used up, at which point the plasma then extinguishes itself. [00058] Figure 4 schematically depicts a pulse 40 of the laser beam. The pulse 40 is schematically depicted with intensity I as a function of time t. The duration of the pulse 40 may for example be at least 150 ns, and may for example be up to 400 ns or more.

[00059] Advantageously, providing the pulse 40 of the laser beam with a duration of at least 150 ns provides more energy than would be provided by a shorter pulse (for the same intensity of radiation). This allows the pulse to convert a larger droplet of tin 34 into EUV emitting plasma. The tin droplet 34 may be spherical, and may for example have a diameter of at least 40 microns, e.g. up to 50 microns or more. It may be difficult to manage tin contamination if a tin droplet with a diameter of more than 50 microns is used. In addition, the plasma emitted by a tin droplet with a diameter of more than 50 microns may be so spatially large that it is not efficiently useable by a lithographic apparatus.

[00060] Conventional LPP EUV radiation sources use a tin droplet having a diameter of around 27 microns. When a tin droplet with a larger diameter such as 40 microns is used, the volume of the tin droplet and the mass of tin that can be converted into EUV radiation increases considerably. For example, increasing the diameter from 27 microns to 40 microns increases the mass of tin by a factor of 3.4.

[00061] Using a tin droplet with a diameter of 40 microns and a laser pulse with a duration of at least 150 ns provides a very substantial increase of EUV energy from each tin droplet. This combination provides EUV energy from each tin droplet, as incident upon the collector 5, of at least 0.05J. This energy is much higher than the energy provided by a conventional EUV LPP source, and allows the throughput of the lithographic apparatus to be increased considerably.

[00062] Table 1, below, shows the EUV energy provided by a tin droplet with a diameter of 40 microns. The energy is indicated as Einband, indicating that the radiation that is referred to is within the EUV radiation wavelength range (e.g. 13.5 nm +/- 0.135nm). The energy is the amount of energy that is emitted over a solid angle that is collected by the collector 5 (see Figure 1). The intensity of the laser beam 2 is indicated as Iiaser, the energy of the pulse of the laser beam 2 that is used until vaporisation of the tin droplet is completed is indicated as Ei_aser, and the vaporisation time for the tin droplet is indicated aS Tvap.

Table 1 - EUV energy generated using a 40 micron diameter tin droplet. [00063] As may be seen from Table 1, the time T_vap taken for the tin droplet to vaporise reduces as the intensity of the laser beam increases. The duration of the laser pulse incident upon the tin droplet should be at least as long as the vaporisation time, to ensure that the tin is fully vaporised (incomplete vaporisation will cause considerable tin contamination in the radiation source). As may be seen from Table 1, an energy Einband of at least 0.05J is provided. The laser pulse duration is at least 150 ns, and may be more than 300 ns (e.g. up to 400 ns).

[00064] Table 2, below, shows the EUV energy provided by a tin droplet with a diameter of 45 microns.

Table 2 - EUV energy generated using a 45 micron diameter tin droplet.

[00065] As is the case for the 40 micron diameter tin droplet, the time T_vap taken for the 45 micron diameter tin droplet to vaporise reduces as the intensity of the laser beam increases. The duration of the laser pulse incident upon the tin droplet should be at least as long as the vaporisation time, to ensure that the tin is fully vaporised. As may be seen from Table 2, an energy E_mb_and of at least 0.07J is provided. The laser pulse duration is at least 150 ns, and may be more than 300 ns (e.g. up to 400 ns). [00066] As schematically depicted in Figure 4, in an embodiment a pulse 40 of the laser beam 2 does not have a Gaussian form or other peaked temporal form. Instead the pulse 40 has a substantially flat temporal profde. There may be a gradually rising edge at the front of the pulse 40, and may be a gradually falling edge at the back of the pulse (as schematically depicted). The gradually rising edge converts part of a tin droplet from liquid form to vapour. This is advantageous because tin vapour absorbs energy from the laser pulse more effectively than liquid tin. Converting part of the tin droplet to vapour form before the laser pulse is at its maximum intensity advantageously improves the efficiency of EUV radiation generation (compared with if the front of the laser pulse had a sharply rising edge). This is because, if the gradually rising edge was not present, then maximum intensity EUV radiation would be immediately incident on liquid tin and would be poorly absorbed by the liquid tin. The gradually rising edge may for example have a duration of at least 5 ns. The gradually rising edge may for example have a duration of up to 20 ns. A gradually rising edge of around 10 ns may provide good efficiency. [00067] The depicted gradually falling edge at the back of the pulse is not needed. A sharply falling edge may be used. Preferably, the pulse should stop as soon as possible once all of the tin droplet has been converted to EUV emitting plasma. This is because any subsequent laser radiation of the pulse does not contribute to EUV generation.

[00068] Although Figure 4 schematically depicts the pulse 40 as having a constant intensity for the majority of the duration of the pulse, in practice there may be some fluctuation of the intensity of the pulse 40. The term “substantially flat temporal profile” may for example be interpreted as meaning that the intensity of the pulse does not vary more than 10% (e .g . does not vary more than 5 %) for the maj ority of the duration of the pulse. The intensity of the pulse may be within a 3 sigma standard deviation. Various methods may be used to obtain the substantially flat temporal profile, for example using a pi- shaper and/or using deformable mirrors.

[00069] Advantageously, the substantially flat temporal profile of the pulse 40, in combination with the 150 ns or more duration of the pulse 40, allows the pulse to gradually vaporize the tin droplet. This compares with for example with the vaporization caused by a pulse comprising a main peak and a tail, which does not provide gradual vaporization but instead provides an initial peak of vaporization followed by a steep decline. This initial peak is undesirable because it heats the tin droplet to a temperature which is much greater than an optimal vaporization temperature, and as a result a poor conversion efficiency to EUV is provided. Providing the pulse with a substantially flat temporal profile allows the intensity of the pulse to be selected as desired (e.g. matched to a better conversion efficiency to EUV).

[00070] In an embodiment, pulses 40 of the laser beam 2 do not have a Gaussian spatial profile or other peaked form of spatial profile. Instead, pulses 40 of the laser beam 2 may have a top-hat profile (e .g . having a substantially flat spatial profile) . An example of the top-hat spatial profile of a laser beam pulse 40 is schematically depicted in Figure 5. Figure 5 schematically depicts intensity l as a function of radial position R. The pulses 40 of the laser beam 2 may have a substantially flat spatial profile across a diameter of at least 100 microns.

[00071] Advantageously, a flat spatial profile of the laser beam pulse 40 may provide even heating fully across the tin droplet 34. This may provide better conversion efficiency than for example a laser pulse with a Gaussian spatial profile. This is because the Gaussian spatial profile will provide much more heating at the centre of the tin droplet than at edges of the tin droplet, and will thereby heat the tin droplet to a temperature which is much greater than an optimal temperature for EUV conversion efficiency.

[00072] Figure 6 schematically depicts the laser beam 2 and a tin droplet 34 at the plasma formation region 4. As schematically depicted, the laser beam 2 may be focussed to form a beam waist at the plasma formation location 4. The diameter of the laser beam 2 is greater than the diameter of the tin droplet 34. Advantageously, this means that when the tin droplet 34 expands due to heating from the laser beam 2, the tin droplet may continue to be illuminated by the laser beam. This avoids a reduction of conversion efficiency into EUV radiation that would occur if part of the expanded tin droplet fell outside of the laser beam 2. In other embodiments, the beam waist is not at the plasma formation location 4. The beam waist may be before or after the plasma formation 4, for example if a laser beam diameter which is larger than the beam waist is desired.

[00073] The tin droplet may have a trajectory which is perpendicular to the axis of the laser beam 2. The tin droplet may be pushed backwards along the axis of the laser beam 2 whilst it is being vaporized. The distance over which the tin droplet moves may for example be 400 microns or more, e.g. up to 1.2mm. Where this is the case, the plasma formation location may be said to be elongate. The laser beam may be configured to have a desired diameter over a full extent of the elongate plasma formation location (e.g. a desired diameter along a length which corresponds with the distance over which the tin droplet moves backwards). The laser beam 2 may be configured to have a diameter of at least 100 microns along an axial distance of at least 400 microns (e.g. up to 1.2 mm) at the plasma formation location. In this context the term “axial distance” may be interpreted as meaning in the direction of propagation of the laser beam.

[00074] As noted above, the laser beam 2 (and thus pulses 40 of the laser beam) may have a top-hat spatial profile. The diameter of the laser beam 2 may be considered to be the diameter of the substantially flat top portion of the laser beam 2.

[00075] The pulses 40 of the laser beam 2 may have a substantially flat spatial profile across a diameter of at least 100 microns (e.g. up to 150 microns).

[00076] The laser beam 2 may have a diameter which is at least 2 times the diameter of the tin droplet 34. During buming/vaporization of the tin droplet 34, the tin droplet or some of the tin may expand. Providing the laser beam 2 with a diameter at least 2 times the diameter of the tin droplet 34 may ensure that the laser beam 2 is incident upon tin which has expanded, thereby converting that expanded tin into EUV radiation. The laser beam 2 may have a diameter of up to 3 times the diameter of the tin droplet 34. The laser beam 2 may have a diameter of more than 3 times the diameter of the tin droplet 34 (e.g. up to 4 times the diameter of the tin droplet). However, such a large diameter may mean that an outer portion of the diameter does not interact with tin and thus does not contribute to EUV radiation generation (thereby reducing the conversion efficiency to EUV radiation).

[00077] The pulses 40 of the laser beam 2 may for example have an energy of at least 1 J or more. The pulses 40 of the laser beam may for example have an energy of 2 J or more, e.g. up to 4 J. Pulses with an energy of more than 4 J, e.g. up to 5 J, may provide only a small increase of the amount of EUV radiation that is generated. This may apply for example for atin droplet having a diameter of 40 microns or more, e.g. up to 50 microns. This is because the additional energy is not needed to vaporize the tin, and in addition may cause undesirable heating of the tin above a desirable temperature for EUV radiation generation.

[00078] The tin droplets 34 may be spherical with a diameter of at least around 40 microns, e.g. 45 microns or more. A conventional LPP radiation source may use spherical tin droplets with a diameter of around 30 microns. A spherical tin droplet with a diameter of 40 microns has a mass (and volume) which is 2.4 times greater than the mass of a 30 micron tin droplet. A spherical tin droplet with a diameter of 45 microns has a mass (and volume) which is 3.4 times greater than the mass of a 30 micron tin droplet. These larger than conventional tin droplets are able to generate significantly more EUV radiation than 30 micron tin droplets. Advantageously, this may allow the LPP radiation source SO to provide a higher power of EUV radiation than conventional LPP radiation sources.

[00079] When a laser pulse 40 having a duration of at least 150 ns and a wavelength between 1.6 microns and 2.5 microns is directed onto each tin droplet, efficient EUV radiation generation can be achieved using only one laser pulse per droplet. This is different to a conventional LPP arrangement, in which one or two initial laser pulses are incident upon a tin droplet before a laser pulse having a wavelength of around 10 microns. In a conventional LPP arrangement the initial laser pulses are used to change the shape of the tin droplet and reduce the density of the tin droplet, in order to improve efficiency of EUV generation. When a laser pulse having a duration of at least 150 ns and a wavelength between 1.6 microns and 2.5 microns is used, there is no need to change the shape or reduce the density of the tin droplet using initial laser pulses

[00080] The laser beam 2 may have an intensity of at least 0.7xl0ⁿ W/cm², e.g. 1.4xlOⁿ W/cm² or more (1.4xlOⁿ W/cm² or more provides a considerably higher EUV power than 0.7 xlO¹¹ W/cm²). The intensity of the laser beam, which is higher than the intensity of a laser beam in a conventional LPP radiation source allows more EUV radiation to be generated from a tin droplet than in a conventional LPP radiation source.

[00081] Conversion efficiency to EUV radiation is good (e.g. >3%) without initial laser pulses being needed for a tin droplet before the laser pulse which generates EUV emitting plasma. The conversion efficiency is good because the wavelength between 1.6 microns and 2.5 microns is able to more effectively vaporize a liquid tin droplet (compared with a conventional 10 micron laser pulse). Thus, in embodiments of the invention no initial pulses are used. The tin receives radiation from only one laser pulse. Thus, the next laser pulse is incident upon the next fuel droplet.

[00082] The duration of the pulses 40 of the laser beam 2 may be selected based upon the diameter of the tin droplet (and thus also based upon the mass of the tin droplet). The diameter of the tin droplet is a known parameter which is determined by the diameter of the nozzle 36 of the fuel generator 3 as explained further above. For a given diameter of tin droplet, the pulse duration may be selected to provide conversion of the entire tin droplet to EUV emitting plasma (and thereby maximize EUV generation). The selection of the pulse duration takes into account the intensity of the laser beam. This is reflected in Table 1 and Table 2 further above.

[00083] In order to increase an amount of EUV radiation from an LPP radiation source, the number of tin droplets converted into EUV radiation per second may be increased. This may be referred to as the repetition rate of the tin droplets. Increasing the repetition rate of the tin droplets however, means that (for a given droplet speed) the droplets are closer to each other as they travel to the plasma formation region. When a tin droplet is vaporized at the plasma formation region, it generates a shock wave (which may comprise debris from the plasma, e.g. ions). This shock wave can change the path of the next tin droplet, such that the next tin droplet does not arrive accurately at the plasma formation region. [00084] Embodiments of the invention advantageously avoid this problem by using larger tin droplets (compared with conventional LPP radiation sources) instead of increasing the repetition rate of the tin droplets. The larger tin droplets generate more EUV radiation (compared with smaller tin droplets), without requiring an increase of the repetition rate. Thus, embodiments of the invention may provide tin droplets with a repetition rate of 40 kHz or less (e.g. 30 kHz or less).

[00085] The controller CT of the laser system 1 may control the laser system to operate with any of the parameters mentioned above. The controller CT may also control modulation of the nozzle 36 of the fuel generator 3, and thus may control the repetition rate of the tin droplets. The controller CT may control other parts of the radiation source, such as the optics 38. The controller CT may include a processor.

[00086] Although embodiments of the invention have been described in connection with tin droplets which are spherical, the tin droplets may have some other shape. For example, a tin droplet may be non-spherical or misshapen as a result of forces exerted by vaporization of a preceding tin droplet. For example, the tin droplet may be ellipsoid. In a case where the tin droplet is non-spherical, a diameter mentioned herein in connection with a tin droplet may be interpreted as meaning the largest dimension of the non-spherical droplet. In general, the tin droplets are examples of tin targets.

[00087] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[00088] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical, and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world. The following clauses are also part of the present disclosure.

1. A method of generating extreme ultraviolet (EUV) radiation comprising directing laser pulses of a laser beam onto tin targets to generate EUV emitting plasma, wherein the laser pulses have a duration of at least 150 ns and have a wavelength between 1.6 microns and 2.5 microns.

2. The method of clause 1, wherein the tin targets are droplets having a diameter of at least 40 microns.

3. The method of clause 2, wherein the EUV emitting plasma emits at least 0.05 J of EUV radiation over a solid angle that is collected by a collector.

4. The method of clause 1, wherein the tin targets are droplets having a diameter of at least 45 microns.

5. The method of clause 4, wherein the EUV emitting plasma emits at least 0.07J of EUV radiation over a solid angle that is collected by a collector.

6. The method of any preceding clause, wherein the tin targets are provided with a repetition rate of 40 kHz or less.

7. The method of any preceding clause, wherein the laser pulses have a substantially flat temporal profde.

8. The method of any preceding clause, wherein the laser pulses have a top-hat spatial profde across a diameter of at least 100 microns.

9. The method of any preceding clause, wherein only one laser pulse is incident upon each tin target.

10. The method of any preceding clause, wherein each laser pulse has an energy of at least 1 J.

11. The method of any preceding clause, wherein each laser pulse has an energy of up to 5 J.

12. The method of any preceding clause, wherein the laser beam that is incident upon the tin targets has an intensity of at least 0.7 x 10¹¹ W/cm².

13. The method of any preceding clause, wherein the front of each laser pulse has a gradually rising edge with a duration of at least 5 ns.

14. The method of any preceding clause, wherein the laser beam has a diameter of at least 100 microns along an axial distance of at least 400 microns at a plasma formation location.

15. A radiation source comprising a nozzle configured to direct targets of tin towards a plasma formation location, and a laser system configured to direct laser pulses having a wavelength between 1.6 microns and 2.5 microns to the plasma formation location, wherein the radiation source further comprises a controller configured to control the laser system such that the laser pulses have a duration of at least 150 ns.

16. The radiation source of clause 15, wherein the nozzle is configured to provide tin targets that are tin targets having a diameter of at least 40 microns or at least 45 microns.

17. The radiation source of clause 15 or clause 16, wherein the controller is configured to control the nozzle to provide the tin targets with a repetition rate of 40 kHz or less. 18. The radiation source of any of clauses 15 to 17, wherein the controller controls the laser system to provide only one laser pulse per tin target.

19. The radiation source of any of clauses 15 to 18, wherein the laser system is configured to provide the laser pulses in a laser beam that has an intensity of at least 0.7 x 10¹¹ W/cm² at the plasma formation location.

20. An exposure apparatus comprising the radiation source of any of clauses 15 to 19 and further comprising a lithographic apparatus, a metrology apparatus, or an inspection apparatus.

[00089] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A method of generating extreme ultraviolet (EUV) radiation comprising directing laser pulses of a laser beam onto tin targets to generate EUV emitting plasma, wherein the laser pulses have a duration of at least 150 ns and have a wavelength between 1.6 microns and 2.5 microns.

2. The method of claim 1, wherein the tin targets are droplets having a diameter of at least 40 microns.

3. The method of claim 2, wherein the EUV emitting plasma emits at least 0.05 J of EUV radiation over a solid angle that is collected by a collector.

4. The method of claim 1, wherein the tin targets are droplets having a diameter of at least 45 microns.

5. The method of claim 4, wherein the EUV emitting plasma emits at least 0.07J of EUV radiation over a solid angle that is collected by a collector.

6. The method of any preceding claim, wherein the tin targets are provided with a repetition rate of 40 kHz or less.

7. The method of any preceding claim, wherein the laser pulses have a substantially flat temporal profde.

8. The method of any preceding claim, wherein the laser pulses have a top-hat spatial profde across a diameter of at least 100 microns.

9. The method of any preceding claim, wherein only one laser pulse is incident upon each tin target.

10. The method of any preceding claim, wherein each laser pulse has an energy of at least 1 J, and/or wherein each laser pulse has an energy of up to 5J.

11. The method of any preceding claim, wherein the laser beam that is incident upon the tin targets has an intensity of at least 0.7 x 10¹¹ W/cm².

12. The method of any preceding claim, wherein the front of each laser pulse has a gradually rising edge with a duration of at least 5 ns.

13. The method of any preceding claim, wherein the laser beam has a diameter of at least 100 microns along an axial distance of at least 400 microns at a plasma formation location.

14. A radiation source comprising a nozzle configured to direct targets of tin towards a plasma formation location, and a laser system configured to direct laser pulses having a wavelength between 1.6 microns and 2.5 microns to the plasma formation location, wherein the radiation source further comprises a controller configured to control the laser system such that the laser pulses have a duration of at least 150 ns.

15. The radiation source of claim 14, wherein the nozzle is configured to provide tin targets that are tin targets having a diameter of at least 40 microns or at least 45 microns.

16. An exposure apparatus comprising the radiation source of any of claims 14 to 15 and further comprising a lithographic apparatus, a metrology apparatus, or an inspection apparatus.