Euv radiation generation method

Abstract

A method and system architecture for generating extreme ultraviolet (EUV) radiation. The method includes directing a laser pulse onto a tin target to generate a plasma that emits EUV radiation. In some examples, the laser pulse is controlled to have a duration of at least 80 ns and a wavelength between 1.6 microns and 2.5 microns.

Description

Cross-references to related applications

[0001] This application claims priority to US application 63 / 569,372, filed March 25, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This invention relates to a method for generating extreme ultraviolet (EUV) radiation. Background Technology

[0003] Light generated by a radiation source can be used by exposure apparatuses in semiconductor manufacturing processes. Examples of such exposure apparatuses include photolithography apparatuses, metrology or inspection apparatuses, more specifically, mask inspection apparatuses, and even more specifically, photochemical mask inspection apparatuses.

[0004] A photolithography apparatus is a machine configured to apply a desired pattern onto a substrate. Photolithography apparatuses can be used, for example, in the fabrication of integrated circuits (ICs). A photolithography apparatus can project a pattern at a patterning apparatus (e.g., a mask) onto a layer of radiation-sensitive material (e.g., photoresist or photoresist) provided on a substrate. To project the pattern onto the substrate, the photolithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Compared to a photolithography apparatus using radiation with a wavelength, for example, 193 nm, a photolithography apparatus using EUV radiation with wavelengths in the range of 4 to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0005] Photochemical mask inspection apparatus is configured to measure the dimensions of a mask or mask substrate, or to detect defects in a mask or mask substrate. EUV lithography uses mirrors instead of lenses as optical devices. The mask substrate used in EUV lithography typically has a multilayer structure that acts as a Bragg mirror. The multilayers can be alternating between molybdenum and silicon. If defects are present in this structure, the projected pattern will be distorted during the lithography process. Therefore, mask inspection to check for defects is considered a requirement for mass production processes. EUV mask inspection can be used for several purposes and at several different stages. First, it can be used to detect phase defects that may occur in the mask substrate. Such phase defects may appear during the fabrication of multilayer stacks of the mask substrate. If not detected, these phase defects will be printed onto all chips with mask portions containing the phase defects. Such phase defects can be correctly detected by using the same or similar (13.5 nm) photochemical EUV wavelength as the lithography tool. Second, mask inspection can be used for patterned mask inspection and can be used to perform quality control for EUV patterned masks. For example, mask inspection can be used to measure critical dimensions on a mask substrate. In addition to phase defects, absorption layer pattern defects on the surface can be detected. Third, mask inspection can be used to simulate exposure and determine the degradation of optical contrast of defects detected in photochemical inspection. Fourth, mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair processes to improve pattern transfer fidelity. Furthermore, it can be used to check optical contrast after defect repair. In addition to the above, mask inspection can also be used to measure microparticle / amplitude effects.

[0006] EUV radiation can be generated by a laser-generated plasma (LPP) radiation source. Within the LPP radiation source, a laser beam can be used to irradiate fuel droplets in order to generate a plasma that will emit EUV radiation.

[0007] One problem associated with conventional LPP radiation sources is that it can be difficult to increase the power of the EUV radiation provided by the LPP source. This can limit the throughput of lithography apparatuses or mask inspection apparatuses (or other metrology devices). Another problem associated with conventional LPP radiation sources is their complexity and relatively low efficiency.

[0008] It is expected that EUV radiation can be generated in a manner that is not disclosed or suggested by existing technology. Summary of the Invention

[0009] According to a first aspect of the invention, a method for generating extreme ultraviolet (EUV) radiation is provided, comprising directing a laser pulse onto a tin target to generate EUV-emitting plasma, wherein the laser pulse has a duration of at least 80 ns and a wavelength between 1.6 micrometers and 2.5 micrometers.

[0010] Advantageously, using laser pulses with a duration of at least 80 ns and a wavelength between 1.6 micrometers and 2.5 micrometers can provide higher power EUV radiation than can be achieved using conventional LPP radiation sources. Additionally or alternatively, using laser pulses with a duration of at least 80 ns and a wavelength between 1.6 micrometers and 2.5 micrometers can provide higher efficiency and / or less complexity than conventional LPP radiation sources.

[0011] The laser pulse can have a duration of at least 100 ns.

[0012] The tin target can be a droplet with a diameter of at least 40 micrometers.

[0013] Tin targets can be provided with a repetition rate of 40 kHz or less.

[0014] Laser pulses can have a basically flat time distribution.

[0015] Laser pulses can have a flat-top spatial distribution spanning a diameter of at least 80 micrometers.

[0016] In this embodiment, only one laser pulse is incident on each tin target.

[0017] Each laser pulse can have 1J or more of energy.

[0018] Each laser pulse may have a gradually rising edge with a duration of at least 5 ns at the beginning.

[0019] The laser pulse can be provided by a Tm:YLF laser.

[0020] According to a second aspect of the invention, a radiation source is provided, comprising a nozzle configured to guide molten tin droplets to a plasma formation location, and a laser system configured to guide laser pulses having wavelengths between 1.6 micrometers and 2.5 micrometers 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 80 ns.

[0021] Advantageously, using laser pulses with a duration of at least 80 ns and a wavelength between 1.6 micrometers and 2.5 micrometers can provide higher power EUV radiation than can be achieved using conventional LPP radiation sources. Additionally or alternatively, using laser pulses with a duration of at least 80 ns and a wavelength between 1.6 micrometers and 2.5 micrometers can provide higher efficiency and / or less complexity than conventional LPP radiation sources.

[0022] The laser pulse can have a duration of at least 100 ns.

[0023] The nozzle can be configured to provide a tin target, which is a tin target with tin droplets having a diameter of at least 40 micrometers.

[0024] The controller can be configured to control the nozzle to deliver a repetition rate of 40 kHz or less to the tin target.

[0025] The controller can control the laser system to provide only one laser pulse per tin target.

[0026] The laser system can be configured to provide laser pulses with a gradually rising edge and a duration of at least 5 ns.

[0027] Laser systems may include Tm:YLF lasers.

[0028] According to a third aspect of the invention, a photolithography system is provided, including a radiation source according to the second aspect, and further including a photolithography apparatus.

[0029] According to a fourth aspect of the invention, a photolithography tool is provided, comprising a radiation source according to the second aspect.

[0030] Different aspects of the present invention can be combined together. Attached Figure Description

[0031] Embodiments of the invention will now be described by way of example only with reference to the accompanying schematic diagrams, wherein: - Figure 1 A lithography system including a lithography apparatus and a radiation source according to an embodiment of the present invention is schematically depicted; - Figure 2 A system for (photochemical) mask inspection, including a radiation source, is schematically depicted according to an embodiment of the present invention; - Figure 3 A radiation source according to an embodiment of the present invention is schematically depicted; - Figure 4 The temporal distribution of laser pulses used by a radiation source according to an embodiment of the present invention is schematically depicted; - Figure 5The spatial distribution of laser pulses used by a radiation source according to an embodiment of the present invention is schematically depicted; and - Figure 6 The diagram schematically depicts the waist of a laser beam incident on a tin droplet in the plasma formation region of a radiation source, according to an embodiment of the invention. Detailed Implementation

[0032] Figure 1 A lithography system including a radiation source SO and a lithography apparatus LA is shown. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an irradiation system IL, a support structure MT configured to support a pattern forming apparatus MA (e.g., a mask), a projection system PS, and a substrate stage WT configured to support a substrate W.

[0033] The irradiation system IL is configured to adjust the EUV radiation beam B before it is incident on the pattern forming apparatus MA. Therefore, the irradiation system IL may include a faceted field mirror assembly 10 and a faceted pupil mirror assembly 11. Together, the faceted field mirror assembly 10 and the faceted pupil mirror assembly 11 provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. In addition to, or in place of, the faceted field mirror assembly 10 and the faceted pupil mirror assembly 11, the irradiation system IL may include other mirrors or devices.

[0034] After such adjustments, the EUV radiation beam B interacts with the patterning apparatus MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. A projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. For this purpose, the projection system PS may include a plurality of mirrors 13, 14, configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate stage WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thereby forming an image with features smaller than the corresponding features on the patterning apparatus MA. For example, a reduction factor of 4 or 8 may be applied. Although in Figure 1 The projection system PS is illustrated as having only two mirrors 13 and 14, but the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0035] The substrate W may include a previously formed pattern. In this case, the photolithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0036] A relative vacuum (i.e., a small amount of gas (e.g., hydrogen) at a pressure much less than atmospheric pressure) can be provided in the radiation source SO, the irradiation system IL, and / or the projection system PS.

[0037] The photolithography apparatus LA and radiation source SO described herein can be used in a method for performing a circuit layout patterning process. The circuit layout patterning method includes receiving a substrate having a photoresist layer. The method further includes directing EUV radiation from the radiation source to the photoresist layer to form a patterned photoresist layer. The method also includes developing and etching the patterned photoresist layer to form a circuit layout.

[0038] Figure 2 A system MS for (photochemical) mask inspection is schematically depicted. The mask inspection system MS can be used by means of... Figure 1 The described lithography apparatus is used to identify or inspect defects in masks to be used in a lithography process. The mask inspection system MS includes a radiation source SO, an illumination system ILM, an inspection system DS, and a mask stage ME.

[0039] The illumination system ILM is configured to adjust the EUV radiation beam B before it is incident on the mask MA. The illumination system ILM can provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system ILM may include multiple mirrors 22. The illumination system ILM may include one or more multifaceted mirror assemblies.

[0040] The mask stage ME can be configured to move the mask MA relative to the EUV radiation beam B so that the EUV radiation beam is incident on different regions of the mask.

[0041] The detection system DS includes a detector 20 and may also include multiple mirrors 24. The multiple mirrors 24 can be configured to collect EUV radiation BR 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) can receive signals output from the detector 20 and use those signals to locate defects in the mask MA.

[0042] Figure 1The radiation source SO shown is of the type that can be referred to as a laser-generated plasma (LPP) source, for example. The laser system 1 is arranged to deposit energy into a fuel (i.e., a target material) via a laser beam 2, such as tin (Sn) supplied, for example, by a fuel generator 3. 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 generator 3 may include a nozzle configured to guide the fuel, for example, in droplet form, along a trajectory to the plasma-forming region 4. The laser beam 2 is incident on the fuel at the plasma-forming region 4. The laser energy is deposited into the tin to generate plasma 7 at the plasma-forming region 4. During the de-excitation and recombination of electrons with ions in the plasma 7, radiation, including EUV radiation, is emitted from the plasma 7.

[0043] EUV radiation from plasma 7 is collected and focused by collector 5. Collector 5 includes, for example, a near-normal incident radiation collector 5 (sometimes more generally referred to as a near-normal radiation collector). Collector 5 may have a multi-layer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation with a desired wavelength, such as 13.5 nm). Collector 5 may have an ellipsoidal configuration with two foci. The first foci may be located at plasma formation region 4, and the second foci may be located at intermediate foci 6, as described below.

[0044] Laser system 1 can be spatially separated from radiation source SO. In this case, laser beam 2 can be transmitted from laser system 1 to radiation source SO by means of a beam transmission system (not shown), which includes, for example, suitable directional mirrors and / or beam expanders, and / or other optical devices. Laser system 1, radiation source SO, and beam transmission system (if present) can be considered together as a radiation system.

[0045] The radiation reflected by collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image of the plasma present in plasma formation region 4. The image at intermediate focus 6 serves as a virtual radiation source for irradiating system IL. Radiation source SO is arranged such that intermediate focus 6 is located at or near opening 8 in the surrounding structure 9 of radiation source SO.

[0046] The radiation source SO of the mask inspection system MS can be related to... Figure 1 The depicted radiation source SO corresponds to... Figure 1 In the same manner described, the radiation source SO of the mask inspection system MS can focus the EUV radiation beam B to form an intermediate focal point 6.

[0047] Figure 3A radiation source SO according to an embodiment of the present invention is schematically depicted. The radiation source SO includes a fuel generator 3 configured to guide molten tin droplets to a plasma formation region 4. Molten tin droplets 34 are depicted at the plasma formation region 4. The fuel generator 3 includes a nozzle 36 from which molten tin droplets are ejected. The diameter of the molten tin droplets is determined by the diameter of the nozzle 36. The nozzle includes an actuator (not depicted) for modulating the nozzle 36. The modulation frequency applied to the actuator determines the repetition rate of the molten tin droplets ejected from the nozzle 36. The liquid tin is held under pressure in the fuel generator 3. The pressure at which the liquid tin is held determines the velocity of the molten tin droplets ejected from the nozzle 36.

[0048] Figure 3 A laser system 1 for a radiation source SO is also depicted. Laser system 1 includes a laser 30, a pump laser 32, and a controller CT. The laser may be, for example, a Tm:YLF laser. Laser 30 is configured to provide a laser beam 2. Laser beam 2 comprises laser pulses (as schematically depicted) incident on a tin droplet 34. Laser beam 2 may be referred to as a laser beam because it is emitted from laser 30.

[0049] The pump laser 32 may be, for example, a laser diode, such as a GaAs laser diode, and is configured to emit a pump laser beam 33 coupled to the gain medium of the laser 30. In this manner, the pump laser 32 produces population inversion in the laser 30, causing the laser 30 to emit a laser beam 2. Similar to the laser 30, the pump laser 32 may emit a pulsed laser beam 36. Multiple pump lasers 32 may be provided.

[0050] Optical element 38 may be provided between laser 30 and plasma forming region 4. Optical element 38 may include one or more focusing elements (e.g., one or more lenses). Optical element 38 may form the beam waist of laser beam 2 as plasma forming region 4.

[0051] The controller CT is configured to control the operation of pump laser 32 and / or laser 30. The controller CT can control other parts of the radiation source, such as optics 38. The controller CT may include a processor.

[0052] The tin droplet 34 provided by the fuel generator 3 can be spherical. The tin droplet 34 can have a smaller diameter than the diameter of the laser beam 2 at the plasma formation position 4 (i.e., the position where the laser beam is incident on the tin droplet). For example, the tin droplet 34 can have a smaller diameter than half the diameter of the laser beam 2 at the plasma formation region 4.

[0053] Laser beam 2 may, for example, have a wavelength of at least 1.6 micrometers. Laser beam 2 may, for example, have a wavelength of up to 2.5 micrometers. Laser beam 2 may, for example, have a wavelength of approximately 1.9 micrometers. The gain medium of laser 30 can be selected to provide the desired wavelength of laser beam 2. For example, Tm:YLF laser 30 can provide laser beam 2 with a wavelength of approximately 1.9 micrometers (e.g., 1.88 micrometers). The gain medium for Tm:YLF may, for example, be a flat panel, a disk, or an optical fiber structure. Other laser gain media may be used.

[0054] As described above, the pump laser 32 can be, for example, a laser diode or multiple laser diodes. The laser diode can, for example, provide a pump laser beam 33 with a wavelength of approximately 790 nanometers. The laser diode can, for example, be a GaAs laser diode.

[0055] A combination of a pump laser diode emitting a 790 nm laser beam and a Tm:YLF laser emitting a 1.88 μm laser beam may be particularly advantageous. High-power, high-efficiency 790 nm emitting laser diodes are commercially available. Tm:YLF has an absorption peak at approximately 790 nm. Fast nonradiative cross-relaxation will fill the upper energy level of the Tm:YLF, thus providing particularly efficient pumping for the Tm:YLF. Furthermore, the Tm:YLF emitting photons at 1.88 nm... 3 The F4 level has a relatively long lifetime of about 15 ms, which provides good energy storage and allows for efficient multi-pulse pumping (i.e., using multiple pulses to completely pump population inversion).

[0056] Each tin droplet 34 can receive a single pulse of the laser beam 2 without simultaneously receiving any laser pre-pulse. That is, unlike conventional LPP radiation sources, embodiments of the present invention do not require a pre-pulse. Advantageously, this simplifies the LPP radiation source SO. Before the laser beam pulse, the tin droplets are not modified into a disk shape using a pre-pulse (as done in prior art LPP radiation sources). Modifying the tin droplets into a disk shape is highly sensitive to the shock wave generated by the vaporization of the previous droplet. Embodiments of the present invention do not use a pre-pulse and do not require the tin droplets to have a disk shape. Embodiments of the present invention are more tolerant of malformed tin droplets at the plasma formation location, and therefore more tolerant of the shock wave generated by the previous droplet. This means that the spacing between consecutive tin droplets can be reduced (compared to prior art LPP radiation sources). For example, the spacing between droplets can be as low as 1 mm.

[0057] A single laser pulse transforms a droplet 34 of molten tin into EUV-emitting plasma. The tin droplet changes from a liquid to a vapor containing some liquid. The vapor is ionized into plasma. The plasma cools and becomes self-illuminating. This process continues until all the liquid tin has been used up, at which point the plasma then self-extinguishes.

[0058] Figure 4 A laser beam pulse 40 is schematically depicted. Pulse 40 is schematically depicted as having intensity I As time t The duration of pulse 40 can be at least 80 ns. The duration of pulse 40 can be, for example, at least 100 ns, at least 150 ns, and, for example, up to 200 ns or more.

[0059] A longer laser pulse provides more energy (for the same radiation intensity) compared to a shorter pulse. This allows the pulse to convert a larger tin droplet 34 into EUV-emitting plasma. The tin droplet 34 can be spherical and, for example, can have a diameter of at least 40 micrometers, such as up to 50 micrometers or more. If tin droplets with a diameter greater than 50 micrometers are used, tin contamination may be difficult to manage. Furthermore, the plasma emitted by tin droplets with a diameter greater than 50 micrometers may be so spatially large that it cannot be used efficiently by a photolithography apparatus.

[0060] Traditional LPP EUV radiation sources use molten tin droplets with a diameter of approximately 27 micrometers. When molten tin droplets with larger diameters (such as 40 micrometers) are used, the volume of the molten tin droplets and the mass of tin that can be converted into EUV radiation increase significantly. For example, increasing the diameter from 27 micrometers to 40 micrometers increases the mass of tin by 3.4 times. Example molten tin droplet diameters, along with their associated mass and volume, are listed in Table 1 below: Table 1 – Mass and volume of tin droplets for various droplet diameters

[0061] Preferably, tin droplets with a diameter larger than the conventional 27 micrometer diameter are provided. Preferably, a laser pulse 40 is provided to this tin droplet, the laser pulse 40 being long enough to completely vaporize the tin droplet larger than the conventional tin droplet. For example, a laser pulse 40 with a duration of at least 100 ns is preferred. Using tin droplets larger than conventional tin droplets and laser pulses with a duration of at least 100 ns provides a very significant increase in EUV energy from each tin droplet. This allows the EUV LPP source to deliver significantly higher EUV power to the substrate exposed by the lithography apparatus, which allows for an increase in the throughput of the lithography apparatus. For a given desired EUV power, increasing the size of the tin droplet will allow for a reduction in the repetition rate of the tin droplets.

[0062] Tin droplets with a conventional diameter of 27 micrometers (or similar) can be used. In this case, shorter laser pulses 40 can be used, for example, with a duration of 80 ns or more. In such cases, the tin droplets can be completely vaporized and will emit EUV radiation. However, the amount of EUV radiation emitted is significantly less than that emitted for tin droplets with a larger diameter (e.g., 40 micrometers or more). This means that the EUV energy from each tin droplet is lower than the EUV energy for larger tin droplets, and it means that the EUV power is correspondingly lower (for a given droplet repetition rate).

[0063] like Figure 4 In a schematic depiction, in this embodiment, the pulse 40 of the laser beam 2 does not have a Gaussian or other peak-time pattern. Instead, pulse 40 has a substantially flat time distribution. A gradually rising edge may be present at the beginning of pulse 40, and a gradually falling edge may be present at the end of the pulse (as schematically depicted). The gradually rising edge converts a portion of the tin droplet from liquid form into vapor. This is advantageous because tin vapor absorbs energy from the laser pulse more efficiently than liquid tin. Converting a portion of the tin droplet into vapor form before the laser pulse reaches its maximum intensity advantageously improves the efficiency of EUV radiation generation (compared to if the laser pulse had a steep rising edge at the beginning). This is because, without the gradually rising edge, the EUV radiation of maximum intensity would immediately strike the liquid tin and be poorly absorbed by it. 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 approximately 10 ns can provide good efficiency.

[0064] The gradually falling edge depicted at the end of the pulse is not necessary. A steep falling edge can be used. Preferably, the pulse should be stopped as quickly as possible once all the tin droplets have been converted into EUV-emitting plasma. This is because any subsequent laser radiation from the pulse does not contribute to EUV generation.

[0065] although Figure 4 The illustration schematically depicts pulse 40 with a constant intensity for most of its duration; however, in practice, the intensity of pulse 40 may fluctuate. The term "substantially flat time distribution" can be interpreted, for example, as meaning that the pulse intensity varies by no more than 10% (e.g., no more than 5%) for most of its duration. The pulse intensity can be within a 3σ standard deviation. Various methods can be used to obtain a substantially flat time distribution, such as using a π shaper and / or a deformable mirror.

[0066] Advantageously, the substantially flat time distribution of pulse 40, combined with a pulse duration of 80 ns or more, allows the pulse to gradually vaporize the tin droplet. This contrasts with vaporization, for example, caused by pulses including a main peak and a tail, which instead provides an initial vaporization peak followed by a sharp decline. This initial peak is undesirable because it heats the tin droplet to temperatures far above the optimal vaporization temperature, and as a result, provides poorer EUV conversion efficiency. Providing a substantially flat time distribution to the pulse allows the pulse intensity to be matched to a desired level of EUV conversion efficiency.

[0067] In this embodiment, the pulses 40 of the laser beam 2 do not have a Gaussian spatial distribution or other peak-form spatial distribution. Instead, the pulses 40 of the laser beam 2 may have a flat-top distribution (e.g., a substantially flat spatial distribution). An example of a flat-top spatial distribution of the laser beam pulses 40 is... Figure 5 It is depicted schematically in the middle. Figure 5 The intensity is depicted schematically. I As radial position R The function of the laser beam 2. The pulse 40 of the laser beam 2 can have a substantially flat spatial distribution across a diameter of at least 80 micrometers. The pulse 40 of the laser beam 2 can have a substantially flat spatial distribution across a diameter of at least 100 micrometers.

[0068] Advantageously, the flat spatial distribution of the laser beam pulses 40 can provide uniform heating across the entire tin droplet 34. This can provide better conversion efficiency than, for example, laser pulses with a Gaussian spatial distribution. This is because a Gaussian spatial distribution will provide much more heating at the center of the tin droplet than at its edges, thus heating the tin droplet to temperatures far exceeding the optimal temperature for EUV conversion efficiency.

[0069] Figure 6 A laser beam 2 and a molten tin droplet 34 are schematically depicted at a plasma formation region 4. As schematically depicted, the laser beam 2 can be focused to form a waist at the plasma formation location 4. The diameter of the laser beam 2 is larger than the diameter of the molten tin droplet 34. Advantageously, this means that the molten tin droplet 34 can continue to be irradiated by the laser beam as it expands due to heating from the laser beam 2. This avoids the reduction in conversion efficiency to EUV radiation that would occur if a portion of the expanding molten tin droplet falls outside the laser beam 2. In other embodiments, the waist is not at the plasma formation location 4. The waist can be before or after the plasma formation location 4, for example, if a laser beam diameter larger than the waist is desired.

[0070] As described above, laser beam 2 (and therefore laser beam pulse 40) can have a flat-top spatial distribution. The diameter of laser beam 2 can be considered as the diameter of the basically flat top portion of laser beam 2.

[0071] The pulses 40 of the laser beam 2 can have a substantially flat spatial distribution spanning a diameter of at least 80 micrometers. The pulses 40 of the laser beam 2 can have a substantially flat spatial distribution spanning a diameter of at least 100 micrometers (e.g., up to 150 micrometers).

[0072] The laser beam 2 can have a diameter at least twice the diameter of the molten tin droplet 34. During the combustion / vaporization of the molten tin droplet 34, the molten tin droplet, or some of the tin within it, may expand. Providing the laser beam 2 with a diameter at least twice the diameter of the molten tin droplet 34 ensures that the laser beam 2 is incident on the expanded tin, thereby converting the expanded tin into EUV radiation. The laser beam 2 can have a diameter up to three times the diameter of the molten tin droplet 34. The laser beam 2 can have a diameter more than three times the diameter of the molten tin droplet 34 (e.g., up to four times the diameter of the molten tin droplet). However, such a large diameter may mean that the outer portion of the diameter does not interact with the tin, and therefore cannot contribute to EUV radiation generation (thus reducing the conversion efficiency to EUV radiation).

[0073] The pulse 40 of laser beam 2 can, for example, have an energy of 1 J or more.

[0074] The tin droplet 34 can be spherical, having a diameter of at least about 40 micrometers, such as 45 micrometers or more. Conventional LPP radiation sources can use spherical tin droplets with a diameter of about 30 micrometers. A spherical tin droplet with a diameter of 40 micrometers has a mass (and volume) 2.4 times greater than that of a 30-micrometer tin droplet. A spherical tin droplet with a diameter of 45 micrometers has a mass (and volume) 3.4 times greater than that of a 30-micrometer tin droplet. These larger tin droplets can generate significantly more EUV radiation compared to 30-micrometer tin droplets. Advantageously, this allows the LPP radiation source SO to operate at a lower repetition rate than conventional LPP radiation sources and / or provide higher EUV radiation power than conventional LPP radiation sources.

[0075] Embodiments of the present invention can provide more EUV radiation than conventional LPP radiation sources.

[0076] When a laser pulse 40 with a duration of at least 80 ns and a wavelength between 1.6 μm and 2.5 μm is directed onto each molten tin droplet, efficient EUV radiation generation can be achieved using only one laser pulse per droplet. This differs from conventional LPP arrangements, where one or two initial laser pulses are incident on the molten tin droplets before a laser pulse with a wavelength of approximately 10 μm. In conventional LPP arrangements, the initial laser pulse is used to alter the shape and reduce the density of the molten tin droplets to improve EUV generation efficiency. When a laser pulse with a duration of at least 80 ns and a wavelength between 1.6 μm and 2.5 μm is used, it is not necessary to use an initial laser pulse to alter the shape or reduce the density of the molten tin droplets.

[0077] Laser beam 2 can have a size of at least 0.6 x 10⁻⁶. 11 W / cm 2 The intensity is higher than that of the laser beam in a conventional LPP radiation source, allowing more EUV radiation to be generated from the molten tin droplet than in a conventional LPP radiation source.

[0078] Without the need for an initial pulse, the conversion efficiency to EUV radiation is good (e.g., >3%) because the laser pulse is able to vaporize denser tin droplets more efficiently (compared to a 10-micron laser pulse). Therefore, an initial pulse is not used in embodiments of the invention. The tin receives radiation from only one laser pulse. Thus, the next laser pulse is incident on the next fuel droplet.

[0079] The duration of pulse 40 of laser beam 2 can be selected based on the diameter of the tin droplet (and therefore also based on the mass of the tin droplet). The diameter of the tin droplet is a known parameter, determined by the diameter of nozzle 36 of fuel generator 3, as further described above. For a given tin droplet diameter, the pulse duration can be selected to provide the entire tin droplet for conversion to the EUV-emitting plasma (and thus maximize EUV generation). The selection of the pulse duration can take into account the intensity of the laser beam.

[0080] To increase the amount of EUV radiation from the LPP radiation source, the number of molten tin droplets converted into EUV radiation per second can be increased. This can be referred to as the molten tin droplet repetition rate. However, increasing the molten tin droplet repetition rate means (for a given droplet velocity) that the droplets travel closer to each other as they enter the plasma-forming region. When the molten tin droplet is vaporized in the plasma-forming region, it generates a shock wave (which can include debris from the plasma, such as ions). This shock wave can alter the path of the next molten tin droplet, preventing it from reaching the plasma-forming region precisely.

[0081] Embodiments of the present invention advantageously avoid this problem by using larger tin droplets (compared to conventional LPP radiation sources) instead of increasing the repetition rate of the tin droplets. Larger tin droplets (compared to smaller tin droplets) generate more EUV radiation without requiring an increased repetition rate. Therefore, embodiments of the present invention can provide tin droplets with a repetition rate of 40 kHz or less (e.g., 30 kHz or less).

[0082] The controller CT of laser system 1 can control the operation of the laser system using any of the parameters mentioned above. The controller CT can also control the modulation of the nozzle 36 of fuel generator 3, and thus control the repetition rate of molten tin droplets.

[0083] Although embodiments of the invention have been described in conjunction with spherical molten tin droplets, molten tin droplets can have other shapes. For example, as a result of the force applied during the vaporization of a previous molten tin droplet, the molten tin droplet may be non-spherical or deformed. For example, the molten tin droplet may be an ellipsoid. In the case of a non-spherical molten tin droplet, the diameter mentioned herein in conjunction with molten tin droplets can be interpreted as referring to the maximum size of the non-spherical droplet. Typically, molten tin droplets are examples of tin targets.

[0084] Although embodiments of the invention are specifically referred to herein in the context of a lithography apparatus, these embodiments can be used in other apparatuses. Embodiments of the invention can form part of a mask inspection apparatus, a measurement apparatus, or any apparatus for measuring or processing an object (such as a wafer (or other substrate) or a mask (or other patterning apparatus)). These apparatuses are generally referred to as lithography tools. Such lithography tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0085] Where the context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present 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, acoustic, and other media. Further, firmware, software, routines, and instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only, and such actions are actually caused by a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., and doing so enables the actuator or other device to interact with the physical world.

[0086] Although specific embodiments of the invention have been described above, it should be understood that the invention can be practiced in ways other than those described. The description above is intended to be illustrative and not restrictive. Therefore, it will be apparent to those skilled in the art that modifications can be made to the described invention without departing from the scope of the claims set forth below.

[0087] The implementation method can be further described using the following terms. 1. A method for generating extreme ultraviolet (EUV) radiation, comprising directing a laser pulse onto a tin target to generate EUV-emitting plasma, wherein the laser pulse has a duration of at least 80 ns and a wavelength between 1.6 micrometers and 2.5 micrometers. 2. The method according to Clause 1, wherein the tin target is a droplet with a diameter of at least 40 micrometers. 3. The method according to Clause 1 or Clause 2, wherein the tin target is provided with a repetition rate of 40 kHz or less. 4. The method according to any one of the preceding clauses, wherein the laser pulse has a substantially flat time distribution. 5. The method according to any one of the preceding clauses, wherein the laser pulses have a flat-top spatial distribution spanning a diameter of at least 80 micrometers. 6. The method according to any one of the preceding clauses, wherein only one laser pulse is incident on each tin target. 7. The method according to any one of the preceding clauses, wherein each laser pulse has an energy of 1 J or more. 8. The method according to any one of the preceding clauses, wherein the beginning of each laser pulse has a gradually rising edge with a duration of at least 5 ns. 9. The method according to any one of the preceding clauses, wherein the laser pulse is provided by a Tm:YLF laser. 10. A radiation source comprising a nozzle configured to guide molten tin droplets to a plasma formation location, and a laser system configured to guide laser pulses having wavelengths between 1.6 micrometers and 2.5 micrometers 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 80 ns. 11. The radiation source according to Clause 10, wherein the nozzle is configured to provide a tin target, the tin target being a tin droplet having a diameter of at least 40 micrometers. 12. The radiation source according to Clause 10 or Clause 11, wherein the controller is configured to control the nozzle to deliver a repetition rate of 40 kHz or less to the tin target. 13. The radiation source according to any one of Clauses 10 to 12, wherein the controller controls the laser system to provide only one laser pulse per tin target. 14. The radiation source according to any one of Clauses 10 to 13, wherein the laser system is configured to provide a laser pulse with a gradually rising edge having a duration of at least 5 ns. 15. The radiation source according to any one of Clauses 10 to 14, wherein the laser system comprises a Tm:YLF laser. 16. A lithography system comprising a radiation source according to any one of Clauses 10 to 15, and further comprising a lithography apparatus. 17. A photolithography tool comprising a radiation source according to any one of Clauses 10 to 15.

Claims

1. A method for generating extreme ultraviolet (EUV) radiation, comprising directing a laser pulse onto a tin target to generate EUV-emitting plasma, wherein the laser pulse has a duration of at least 80 ns and a wavelength between 1.6 micrometers and 2.5 micrometers.

2. The method of claim 1, wherein the tin target is a droplet with a diameter of at least 40 micrometers.

3. The method of claim 1, wherein the tin target is provided with a repetition rate of 40 kHz or less.

4. The method according to claim 1, wherein the laser pulse has a substantially flat time distribution.

5. The method of claim 1, wherein the laser pulses have a flat-top spatial distribution spanning a diameter of at least 80 micrometers.

6. The method of claim 1, wherein only one laser pulse is incident on each tin target.

7. The method of claim 1, wherein each laser pulse has an energy of 1 J or more.

8. The method of claim 1, wherein the beginning of each laser pulse has a gradually rising edge with a duration of at least 5 ns.

9. The method of claim 1, wherein the laser pulse is provided by a Tm:YLF laser.

10. A radiation source comprising a nozzle configured to guide molten tin droplets to a plasma formation location, and a laser system configured to guide laser pulses having wavelengths between 1.6 micrometers and 2.5 micrometers 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 80 ns.

11. The radiation source of claim 10, wherein the nozzle is configured to provide a tin target, the tin target being a tin droplet having a diameter of at least 40 micrometers.

12. The radiation source of claim 10, wherein the controller is configured to control the nozzle to provide a repetition rate of 40 kHz or less to the tin target.

13. The radiation source of claim 10, wherein the controller controls the laser system to provide only one laser pulse per tin target.

14. The radiation source of claim 10, wherein the laser system is configured to provide laser pulses with a gradually rising edge having a duration of at least 5 ns.

15. The radiation source of claim 10, wherein the laser system comprises a Tm:YLF laser.

16. A lithography system comprising a radiation source according to claim 10, and further comprising a lithography apparatus.

17. A photolithography tool comprising the radiation source according to claim 10.

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