Target materials, high brightness EUV sources and methods for producing EUV radiation

Abstract

The invention relates to a Li-containing target material designed to form a plasma emitting EUV radiation. The target material can be a Li-based composition with at least one additional element. By way of example, the additional element is selected from the group consisting of Au, Ag, Bi, Ba, Sr. The composition is configured to increase the density of the target material by a factor of multiple compared to the density of Li. As a result, the velocity of the droplet portion of the debris particles can be rapidly reduced compared to a Li target, and the high target velocity allows the exit direction from the plasma to be controlled. The target material is preferably used for laser-produced plasma light sources. Such light sources can have a high speed rotating target (e.g. at least 100 m / s). The target material allows the creation of compact low-debris EUV light sources with high spectral brightness designed for a wide range of applications.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This patent application is a continuation-in-part of U.S. patent application Ser. No. 16 / 952,587, filed November 19, 2020, which is a continuation-in-part of U.S. patent application Ser. No. 16 / 773,240, filed January 27, 2020, which is a continuation-in-part of U.S. patent application Ser. No. 16 / 535,404, filed August 8, 2019, which is a continuation-in-part of U.S. patent application Ser. No. 16 / 103,243, filed August 14, 2018, which claims priority to Russian patent application RU2017141042, filed November 24, 2017, and which also claims priority to Russian patent application RU2022100914, filed January 17, 2022, all of which are incorporated by reference in their entireties herein.

[0002] The present invention relates to Li-containing target materials for use in plasma sources, as well as high brightness extreme ultraviolet (EUV) light sources and methods for producing EUV radiation, including mitigation of debris particles generated by the plasma along with the radiation. [Background technology]

[0003] High-brightness EUV light sources are used in many fields, such as microscopy, materials science, biological and medical diagnostics, materials testing, crystallography and nanostructure analysis, atomic physics, and lithography.

[0004] Synchrotrons can be used as such radiation sources, but they are very expensive and not widely available.

[0005] Alternatively, there are light sources that can efficiently generate plasma in the EUV (10-20 nm) range by both focusing the radiation of a high-power laser onto a target and discharging it. EUV light is most effectively produced using laser-produced plasma (LPP).

[0006] In recent years, the development of EUV light sources emitting in the spectral region (13.5±0.135 nm) corresponding to the reflection band of multilayer Mo / Si mirrors has been greatly stimulated by the development of projection EUV lithography for mass production of chips with a 7 nm node and below.

[0007] It is known that high-temperature plasmas of many elements, especially xenon (Xe), lithium (Li), and tin (Sn), can be used to generate 13.5 nm EUV radiation with high efficiency. Unlike other materials, the use of Li as the target material produces hydrogen-like Li with a wavelength of 13.5 nm. 2+ a single resonant transition of hydrogen-like Li 2+ As is known from Schriever G, et al. Laser-produced lithium plasma as a narrow-band extended ultraviolet radiation source for photoelectron spectroscopy. Appl Opt. 1998 Mar 1; 37(7):1243-8, it is possible to obtain substantially monochromatic radiation from Li at a wavelength of 13.5 nm. 2+ The measured linewidth of the emission was limited by the spectral resolution of the spectrometer and was estimated to be λ / Δλ ≒ 1800.

[0008] Production of a radiating lithium plasma is known using methods including laser initiated discharge in a metal vapor (U.S. Patent No. 7,427,766, issued September 23, 2008), generation of laser plasma using a droplet Li target (U.S. Patent No. 7,449,703, issued November 11, 2008), and generation of laser plasma using a jet Li target (U.S. Patent No. 9,476,841, issued October 25, 2016), all of which are incorporated herein by reference.

[0009] Thus, essentially a point-like and monochromatic radiation source based on Li plasma is attractive for many applications, potentially using different optics, such as those based on multilayer Mo / Si mirrors with high (70%) reflectivity at 13.5 nm and / or those based on Fresnel zone plates. One advantage of using Li as a target material is its relatively low melting point of 160.54 °C.

[0010] However, during the production of EUV radiation, debris particles are generated as a by-product that can degrade the surfaces of optics integrated with the EUV source. Debris generated as a by-product of the plasma during the operation of an EUV source takes the form of energetic ions, neutral atoms, clusters and microdroplets of target material.

[0011] From the international patent application PCT / RU2012 / 000701 published under WO2013 / 122505A1, which is cited as reference, a short-wave radiation source is known based on a laser-induced discharge produced plasma in a metal vapor. The electrodes can be in the form of a liquid metal jet or in the form of a rotating disk wetted with molten target material. The laser beam is directed towards one irradiation site of the electrodes by the discharge being curved banana-shaped. The intrinsic magnetic field of such a discharge has a gradient which determines a predominant movement of the discharge plasma flow towards regions of less strong magnetic field in a direction significantly different from the exit direction of the EUV beam. The invention provides a simple and very effective suppression of charged particles in the radiation beam.

[0012] However, more advanced debris mitigation measures are required to suppress the flow of neutral particles and small droplets.

[0013] In patent applications PCT / RU2018 / 000520 published on 31 May 2019 under WO2019103648, PCT / EP2020 / 061562 published on 29 October 2020 under WO2020216950, and patent RU2743572 published on 20 February 2021, a new approach is proposed for the development of high brightness LPP light sources based on a liquid metal target with a high rotation speed, linear velocity of about 100 m / s, whereby highly efficient debris mitigation is achieved by moving the droplet part of the debris particle away from the light collector and the entrance window of the laser beam. This debris mitigation method is, for example, a method for the development of a high-brightness LPP light source based on a liquid metal target with a linear velocity of about 100 m / s, whereby highly efficient debris mitigation is achieved by moving the droplet part of the debris particle away from the light collector and the entrance window of the laser beam. 3 It is very effective when using a target material with a sufficiently high density ρ, such as tin.

[0014] However, when Li is used as the target material, the density of the target material is low (ρ = 0.534 g / cm 3) is characterized by an excessively high rate of formation of the droplet fraction of debris particles, the fragments of which can attain velocities up to 1000 m / s as a result of the impact of the explosive shock pulse in the interaction zone, which dramatically reduces the protective effect against the droplet fraction of Li target debris. DISCLOSURE OF THEINVENTION

[0015] Therefore, there is a need to overcome the above shortcomings. In particular, there is a need for an improved LPP EUV source with high spectral brightness at 13.5 nm using lithium as the plasma forming target material. It would be even more beneficial if the source were compact, powerful, and could provide highly efficient, preferably nearly complete, debris mitigation.

[0016] This need is met by the features of the independent claims. The dependent claims describe embodiments of the invention.

[0017] In one embodiment of the invention, there is provided a target material for use in a plasma source constructed and arranged to produce a radiation beam having a wavelength in the extreme ultraviolet (EUV) range, the target material comprising or consisting of a lithium (Li)-based composition having at least one further element that increases the density of the target material by several times, more than three times, compared to the density of Li. In particular, the type and amount of the at least one further element in the composition may be configured such that the density of the target material is increased by a factor of three or more compared to the density of Li.

[0018] In a preferred embodiment of the present invention, the additional element is selected from the group comprising or consisting of Ag, Au, Bi, Ba, and Sr.

[0019] In a preferred embodiment of the invention, the composition constitutes a eutectic alloy. The atomic percentage (or amount-of-substance fraction) of Li in the target material can be in the range of 60% to 90% (also written as at%).

[0020] In a preferred embodiment, the atomic fraction of said at least one additional element in the target material may be in the range of 10% to 40%, for example in the range of 15% to 30%. Preferably, the sum of the atomic fraction of Li and the atomic fraction of said at least one additional element in the target material amounts to (approximately) 100%. The term "approximately" refers to a remainder of impurities and / or contaminants that may be present depending, for example, on the purity of the raw material from which the target material is formed.

[0021] In another aspect, the invention relates to a plasma source of EUV radiation, characterized in that the plasma is generated from a target material produced according to the invention.

[0022] In a preferred embodiment of the invention, the plasma is either a laser-produced plasma (LPP) or a laser-initiated discharge produced plasma.

[0023] In a preferred embodiment of the present invention, the speed of the droplet fraction of debris particles expelled from the plasma of the target material is several times, or about an order of magnitude, smaller than the speed of the droplet fraction of debris particles expelled from the plasma of the lithium target.

[0024] In a preferred embodiment of the invention, the target speed is equal to or greater than the average speed of the droplet portion of the debris particles expelled from the plasma.

[0025] In another aspect, the invention relates to a method of producing EUV radiation, comprising generating a radiation beam having a wavelength in the EUV range by a plasma source using a target material having any of the configurations described herein.

[0026] In one embodiment, the method may further comprise forming the target as a layer of molten target material on a surface of an annular groove, performed in a rotating target assembly with the target surface facing the axis of rotation under the action of centrifugal force, irradiating the target with a focused laser beam, preferably at a high pulse repetition rate, for example higher than 10 kHz, generating a laser-produced plasma in the interaction zone, and outputting a beam of EUV radiation through a debris mitigation means. The molten target material is comprised of any of the target materials disclosed herein, in particular a Li-based composition with at least one additional element configured to increase the density of the target material by a factor of three or more compared to the density of Li. The target is rotated at a high linear velocity of at least 100 m / s.

[0027] In a preferred embodiment of the invention, the centrifugal acceleration of the target is at least 10,000 g, where g is the acceleration of gravity.

[0028] In an embodiment of the invention, the spatial distribution of the debris ejection rate from the interaction zone is calculated and the passing directions of both the focused laser beam and the EUV radiation beam are selected in the spatial region where the debris ejection rate is minimal.

[0029] In a preferred embodiment of the invention, the debris mitigation is provided by one or more debris mitigation techniques including protective gas flow, magnetic mitigation, foil traps, debris shields, and membranes that are nearly transparent to EUV radiation, having a transparency of 60% or greater.

[0030] In a preferred embodiment of the present invention, the EUV radiation beam is ionized Li with a wavelength of 13.5 nm. 2+ At the transitions of the first and second electrodes, a spectral filter of narrowband emission is provided.

[0031] In another aspect, the invention relates to an EUV radiation source comprising a vacuum chamber, a laser beam focused on a target, preferably a pulsed laser beam, the target being a layer of molten target material on a surface of an annular groove mounted in the rotating target assembly, the rotating target assembly providing the target to an interaction zone with the target surface facing an axis of rotation, a beam of EUV radiation exiting the interaction zone, and debris mitigation means.

[0032] The EUV radiation source differs in that the target material is prepared according to any of the embodiments of this invention and the linear velocity of the target is preferably at least 100 m / s. The EUV radiation source can use any of the plasma sources of the configurations described in this specification.

[0033] In a preferred embodiment of the invention, a spectral purity filter is placed in the path of the EUV radiation beam.

[0034] The spectral purity filter is selected from the group consisting of a reflective filter in the form of a multi-layer Mo / Si mirror, or comprising a foil containing zirconium or beryllium.

[0035] In a preferred embodiment of the present invention, the debris mitigation means is provided by one or more debris mitigation techniques including protective gas flow, magnetic mitigation, foil traps, debris shields, membranes that are nearly transparent to EUV radiation.

[0036] The following causal relationships may exist between the preferred features of the invention and the technical results achieved:

[0037] According to one embodiment, a Li-based target material (Li density ρ=0.534 g / cm 3 ) with a small amount of additional elements, such as Ag (ρ = 10.5 g / cm 3 ) or Au (ρ = 19.32 g / cm 3 Even if 10% to 40%, say about 20%, of ZnO is added in atomic fractions (or mass fractions) of 10% to 40%, the intensity of the 13.5 nm emission line of the lithium plasma does not change substantially.

[0038] At the same time, the density of the target material, which can be called "heavy lithium", increases by a factor of 4.7 or 8 compared to that of Li, while the melting points remain relatively low, at about 150°C for alloys with Ag and 280°C for alloys with Au. Furthermore, the higher density of the target material reduces the velocity of the droplet fraction of the debris particles, which is of greatest concern, by a factor of many, making it possible to control their main ejection direction from the plasma when using high target velocities (above 100 m / s).

[0039] The use of Ag and Au as additional elements in the target material is preferred because these materials are non-toxic, non-chemically aggressive, and allow the production of eutectic fusible alloys with high Li content (60-90%) and target densities many times higher than the density of Li.

[0040] Additionally or alternatively, Bi (ρ = 9.79 g / cm) may be used as an additional element in the target material. 3 ),Ba(ρ=3.5 g / cm3 ), Sr(ρ=2.54 g / cm 3 ) can be used.

[0041] The plasma emission bands of the additional elements of the target material are separated from the 13.5 nm emission line of doubly ionized lithium and can therefore be easily blocked without the use of complex optics, e.g., by reflection from a Mo / Si mirror, essentially ensuring the monochromaticity of the EUV source. The monochromaticity of the EUV source allows the use of a wide range of optical elements, including multilayer Mo / Si mirrors, grazing-incidence mirrors, and Fresnel zone plates, expanding the range of applications of the essentially point (having a characteristic size of ∼100 μm) radiation source disclosed in this invention.

[0042] In general, the invention is applicable to many EUV light sources based on laser-produced or laser-induced discharge produced plasmas.

[0043] In one aspect, the invention relates to a low debris LPP EUV light source having a rapidly rotating liquid metal target made of "heavy lithium", i.e., any of the target materials of the disclosed configurations.

[0044] Therefore, this invention can provide the possibility to create a 13.5 nm low-debris high-brightness monochromatic radiation source with wide range of applications.

[0045] The features of the aspects and embodiments disclosed in this specification may be combined with each other unless stated to the contrary.

[0046] The advantages and features of the present invention will become more apparent from the following non-limiting description of exemplary embodiments thereof, given by way of example with reference to the accompanying drawings, in which:

[0047] Exemplary embodiments of the present invention are illustrated in the drawings.

[0048] In the drawings, identical elements of a device are given the same reference numerals.

[0049] These drawings do not cover or limit the entire range of options for implementing this technical solution, but only represent exemplary materials of specific cases of its implementation. [Brief description of the drawings]

[0050] [Figure 1] 1 is a schematic diagram of a high brightness LPP EUV light source with a fast rotating target according to one embodiment. [Diagram 2] 1 is a schematic diagram of a high brightness LPP EUV light source according to the above embodiment. [Diagram 3] 3A and 3B show the calculated spatial distribution of droplet ejection from the interaction zone for target materials with compositions having atomic fractions of Li (80%) and Ag (20%). [Figure 4] 4A and 4B show SEM images showing the debris mitigation effect achieved by replacing the Li target material with a composition of 80% Li + 20% Ag in a high brightness LPP EUV source. [Diagram 5] 5A and 5B show the spectra of an EUV radiation source for target materials with compositions of Li and 80% Li+20% Ag. [Figure 6] 4 shows the spectral reflectance curve of a Mo / Si mirror used as the reflecting mirror. [Figure 7] 7A and 7B show the emission spectra after reflection from a Mo / Si mirror for target materials with compositions Li and 80% Li+20% Ag. [Figure 8] 8A, 8B, 8C and 8D show schematic diagrams of an EUV light source according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] In an exemplary embodiment of the invention shown in Figure 1, a high brightness source of short wavelength radiation includes a vacuum chamber 1 with a rotating target assembly 2 which delivers targets 3 to an interaction zone 4 where said targets 3 interact with a focused laser beam 5. A portion of said rotating target assembly 2 is made in the form of a disk fixed to a rotating shaft. The disk has a peripheral portion in the form of an annular barrier with an annular groove facing said rotation shaft 6. The target 3 is a layer of molten metal formed by centrifugal force on the surface of the annular groove of said rotating target assembly 2. The target 3 can be a target material in any configuration disclosed herein.

[0052] The target material is melted and maintained at a predetermined optimum temperature range using a heating system 7 .

[0053] The annular groove configuration prevents the material of the target 3 from being expelled both radially and along the axis of rotation 6 if the volume of the target material does not exceed the volume of the groove.

[0054] In the interaction zone 4, a pulsed high-temperature plasma of the target material is generated under the action of the focused laser beam 5. This plasma produces radiation in the EUV spectral range. The produced EUV radiation leaves the interaction zone 4 in the form of a diverging beam 8 of EUV radiation.

[0055] In one embodiment of the invention the beam of EUV radiation is directed to a light collector 9 in the form of a Mo / Si mirror with a maximum reflection at a wavelength of 13.5 nm. In other cases collectors based on optical elements in the form of glazing incidence mirrors or Fresnel zone plates can be used.

[0056] Provided on the path of the focused laser beam 5 and the EUV radiation beam 8 are means for debris mitigation which consist of one or more of the following techniques:

[0057] A casing 10,11 enclosing the laser and the beam of EUV radiation 5,8.

[0058] A protective gas flow is directed through gas inlets 13 in the casings 10, 11 to mitigate the vapor fraction of debris particles in the path of the laser and EUV radiation beams.

[0059] A debris shield 12 is separated from the rotating target assembly 2 by a slit gap and has only two small holes (for the input of the focused laser beam and for the output of the EUV radiation beam), through which debris particles can exit the target assembly.

[0060] Foil traps (not shown): Essentially a system of plates oriented radially to the plasma that are highly transparent to the plasma radiation and are quite effective at trapping neutral atoms and clusters of the target material.

[0061] A magnetic field, preferably generated by a permanent magnet, to reduce the charged portion of the debris particles.

[0062] A preferably replaceable membrane 15 that is essentially transparent to short wavelength radiation and impermeable to debris and gases.

[0063] The membrane, which is placed in the path of the EUV radiation beam, is preferably made of a material belonging to the group including carbon nanotubes (CNT), Ti, Al, Si, ZrSi, BN.

[0064] Similar debris mitigation means are placed in the propagation path of the focused laser beam 5 .

[0065] Figure 2 shows a simplified diagram of the above-mentioned EUV source and a computational modeling of the spatial distribution of debris ejection (emission) from the interaction zone 4, carried out with the RZLINE code, created for applications in the field of radiation hydrodynamics of dense and hot plasmas. This code uses mathematical models based on many years of experimental and theoretical research, as known, for example, from the paper "Return-to-zero line code modeling of distributed tin targets for laser-produced plasma sources of extreme ultraviolet radiation" by K. Koshelev, V. Ivanov, V. Medvedev et al., Journal of Micro / Nanolithography, MEMS, and MOEMS, Vol. 11, No. 2 (May 2012). The code allows modeling the interaction of laser radiation with gas, liquid and solid surfaces, the subsequent plasma generation, as well as the interaction with the plasma itself.

[0066] Figure 3 shows the spatial distribution of the ejection velocity of debris particles in the experimental coordinates (particles at all speeds are considered) for Li and Li / Ag target materials, where θ is the angle with respect to the axis of rotation and φ is the azimuthal angle lying in the plane of the figure. The origin of the coordinates lies within the interaction zone. Typical directions of the interaction zone are:

[0067] I Parallel to the rotation axis: θ=0, φ is any

[0068] II Along target speed: θ=90°, φ=0°

[0069] III Normal to the target surface: θ=90°, φ=90°

[0070] IV Against target speed: θ=90°, φ=180°

[0071] The spatial distribution of the debris emission rate shown in Figure 3 was calculated as a specific growth rate per unit laser power of the thickness of the deposited debris particles on the surface of the exposed sample, located 40 cm away from the interaction zone, in nm / (month·W) with the EUV light source operating 24 / 7.

[0072] This distribution was obtained for typical values ​​of the source parameters: laser emission wavelength of 1-2 μm, laser pulse energy of a few mJ with pulse duration of a few ns, focal spot diameter of a few tens of μm, and target linear velocity of 200 m / s. No debris mitigation techniques were used apart from the high speed rotation of the target.

[0073] As shown in Figure 3, the mass of debris particles is concentrated primarily within a sector along the direction of the target velocity bounded by azimuth angles φ of 0°-80° and polar angles θ of 0°-90°. The maximum debris ejection rate along the target rotation direction is 10 7 nm / (month W).

[0074] In FIG. 3, an ellipse is used to indicate a spatial region selected according to an embodiment where the debris emission velocity is at a minimum level, in which the cone of the laser and EUV radiation beam is located.

[0075] Calculation results for target materials with compositions Li in Figure 3A and 80% Li and 20% Ag (atomic fractions) in Figure 3B show that an increase in the density of the target material (4.7 times that of lithium in this case) can drastically reduce (by more than one order of magnitude) the ejection (emission) rate of debris particles into the spatial region of the laser and EUV beam.

[0076] The achievement of this favorable effect of the present invention was experimentally confirmed using a light source as shown diagrammatically in FIG. 1 in which a test sample was placed in place of the film 15 .

[0077] Figure 4 shows scanning electron microscope (SEM) images of witness samples obtained for the target materials with Li (Figure 4A) and with 80Li and 20%Ag compositions (Figure 4B). The frame size of the SEM images is 125 μm. The tests were carried out under the following conditions:

[0078] The linear velocity of the target is 150m / s, the centrifugal acceleration is 23000g,

[0079] The laser beam energy on the target is 3.3mJ / pulse, and the focal spot size is 1 / e 2 Intensity level (1 / e 2 Intensity level) is 120μm,

[0080] Pulse repetition rate (frequency) is 25kHz

[0081] The exposure times for the witnessed samples were 2.5 hours (Figure 4A) and 10 hours (Figure 4B).

[0082] It can be seen that the increased density of the target material according to the present invention provides a dramatic (sharp) debris mitigation effect.

[0083] Based on the tests carried out, it has been shown that the relatively large droplets of 300 nm or more that penetrate the CNT membrane are completely suppressed. Thus, by replacing the test sample with an exchangeable membrane, especially one made of carbon nanotubes, it is possible to ultrapurify the 13.5 nm high-brightness monochromatic radiation source produced according to this invention.

[0084] Figures 5A and 5B show the spectra of an EUV light source when the target material is Li and when the composition is 80%Li+20%Ag, respectively. The spectrum shown in Figure 5 was measured for radiation incident directly from the plasma (without reflection from mirror 9 and without using film 15) using the EUV light source shown diagrammatically in Figure 1. Even when the target material is replaced from Li to a composition of 80%Li+20%Ag, the 13.5 nm Li 2+ It can be seen that the emission intensity of the 13.5 nm Li line does not change substantially. At the same time, the presence of Ag ions in the plasma 2+ An emission band appears with a peak at a wavelength of 17 nm that does not overlap with the emission lines of the , but this can be easily filtered out using, for example, a Mo / Si mirror 9 (Figure 1).

[0085] Figure 6 shows the spectral reflectivity curve of a Mo / Si mirror with a reflection band in which the radiation from the Ag plasma does not fall. This determines the possibility of using a multi-layer Mo / Si mirror as a reflection filter according to the invention, but is not limited to this option only. Similarly, other types of filters can be used, for example made from foils containing zirconium or beryllium.

[0086] As can be seen from Figures 7A and 7B, the spectrum of the EUV light source after reflection from the Mo / Si mirror is substantially consistent when the target material is Li and when the target material is 80% Li + 20% Ag.

[0087] The radiation behind the multilayer mirror is monochromatic and has a bandwidth of λ / Δλ≧1300. This value corresponds to the spectral line Li 2+ (1s-4p), 11.39 nm Li +2 (1s-3p), and 13.5 nm Li 2+ (1s-2p) as estimated from the measured bandwidth (using a spectral resolution λ / Δλ of up to 500) (Figure 5A).

[0088] Maximum conversion efficiency CE of laser radiation energy into in-band EUV radiation (13.5±0.135 nm) at a spatial angle of 2π sr for an EUV source 13.5 was 2% for all Li-based target materials.

[0089] In general, there is a series of parameters, including the size of the laser spot, the energy and duration of the laser pulse, which can minimize the amount of debris along with a high CE.

[0090] The method of generating radiation using an EUV light source, as shown diagrammatically in Figures 1, 2 and 3, is carried out as described below.

[0091] Under the action of centrifugal force a target is formed in the form of a layer of molten metal on the surface of the annular groove of the rotating target assembly 2, which surface faces the rotation axis 6. The target 3 is irradiated by a pulsed focused laser beam 5 and a plasma is formed in the interaction zone 4. An output beam 8 of EUV radiation is generated which passes through debris mitigation means 11, 12, 13, 14, 15 into a light collector 9.

[0092] The rotation of the target is performed at a high linear velocity of more than 100 m / s. According to the embodiments disclosed herein, a target material is used that comprises a lithium composition with at least one additional element, which allows the density of the material of the target to be increased by a factor of several, for example by a factor of three or more, compared to the density of Li, thereby making it possible to dramatically reduce the ejection speed of the droplet part of the debris particles. Due to the high speed rotation of the target, the droplet part of the debris particles acquires a significant tangential component of velocity comparable to the ejection speed of the droplets, which according to the invention is drastically reduced by increasing the density of the Li-containing target material. As a result, the velocity vector of the droplet part of the debris particles is directed away from the beams 6, 8 of laser and EUV radiation, and its propagation path is selected in a spatial region where the debris ejection speed is at a minimum level.

[0093] Ag or Au is preferably selected as the additional element of the target material, which, even with a small atomic fraction, e.g., 20%, can increase the density of the target material by many times (approximately 5 and 8 times, respectively) compared to the density of Li. In a preferred embodiment of the invention, the target material is a eutectic alloy with an atomic fraction of Li in the range of 60% to 90%, which ensures the homogeneity and relatively low melting point of the target material.

[0094] Bi, Ba, and Sr can also be used as additional elements in the target material.

[0095] Li at 13.5 nm in EUV radiation beam 8 2+ Spectral filtering of the narrowband line emission is performed, for example, by means of a Mo / Si mirror 9 acting as a reflective filter. In particular, foils containing zirconium or beryllium can also be used as filters.

[0096] When EUV radiation is generated debris mitigation means 11, 12, 13, 14, 15 are used which consist of protective gas flow, magnets, foil traps, a membrane 15 which is highly transparent at 13.5 nm, a debris shield 12 placed outside the propagation area of ​​the laser and EUV radiation beams 5, 8.

[0097] With a high target speed of more than 100 m / s and a high-speed target rotation with a centrifugal acceleration of at least 10000 g, the surface of the target 4 becomes cylindrical, which results in a stable shape of the target surface 3 in the interaction zone 4, and the long-term stability of the EUV radiation source is achieved by the continuous circulation and renewal of the target material in the interaction zone, as well as the restoration of the shape of the target surface after the next laser pulse.

[0098] In general, the target material produced according to the invention is widely applicable to various types of EUV light sources known from the prior art, both based on laser-produced and discharge-produced plasmas.

[0099] For example, FIG. 8 illustrates an embodiment of an EUV light source using target material fabricated in accordance with an embodiment of the present invention.

[0100] In Fig. 8A, the LPP EUV source has a target 3 in the form of a liquid metal jet circulating at high speed in an interaction zone 4 along a closed loop 16 with a nozzle 17 and a high pressure pump 18 for transporting the molten target material. A debris shield 12, at a temperature above the melting point of the target material, is placed around the jet. The jet may be continuous or may consist of individual target droplets following each other at high speed. The other parts of the device in this embodiment are similar to those in the previous embodiment (Figs. 1, 2), are given the same reference numerals in Fig. 8 and their detailed description will be omitted here and below.

[0101] In another embodiment shown in FIG. 8B, two high-velocity liquid metal jets are used as electrodes 19 connected to a pulsed power source 20 to generate a laser-induced discharge plasma 21 during evaporation of the target material of the target 3.

[0102] In another embodiment shown in Figure 8C, an LPP EUV source is shown in which the target material of the target 3 is delivered to the interaction zone 4 by a rapidly rotating disk 22 with a casing 23 having holes for the entrance of the laser beam 5 and the exit of the EUV radiation beam 8. Outside the interaction zone 4, the disk 22 is wetted by molten target material 24.

[0103] Similarly, in the embodiment shown in FIG. 8D, a rapidly rotating disk 22 wetted by the molten target material is used as an electrode 19 for generating a laser-induced discharge produced plasma 21 during evaporation of the target material of the target 3 .

[0104] Therefore, this invention makes it possible to create a monochromatic EUV light source with a wavelength of 13.5 nm, characterized by high spectral brightness and average output, long life, and ease of use.

[0105] The proposed device is envisioned for many applications, including microscopy, materials science, X-ray diagnostics of materials, biological and medical diagnostics, inspection of nano- and micro-structures, and lithography, including actinic control of lithographic EUV masks.

Claims

1. 1. A target material for use in a plasma source constructed and arranged to produce a radiation beam (8) having a wavelength in the extreme ultraviolet (EUV) region, comprising: a lithium (Li)-based composition having at least one additional element, the composition configured to increase the density of the target material by at least a factor of three compared to the density of Li; Target material.

2. 2. The target material of claim 1, wherein the at least one additional element is selected from the group consisting of Ag, Au, Bi, Ba, and Sr.

3. 3. The target material of claim 1, wherein the composition constitutes a eutectic alloy.

4. 4. The target material according to claim 1, wherein the atomic fraction of Li in the target material is in the range of 60% to 90%.

5. 5. The target material according to claim 1, wherein the atomic fraction of the at least one additional element in the target material is in the range of 10% to 40%, preferably 15% to 30%, and preferably the sum of the atomic fraction of Li and the atomic fraction of the at least one additional element in the target material is about 100%.

6. 1. A plasma source of EUV radiation, comprising: A target material according to any one of claims 1 to 5 is provided for generating plasma. Plasma source.

7. The plasma source of claim 6 , wherein the plasma source is configured to generate the plasma as either a laser produced plasma or a laser induced discharge produced plasma.

8. the velocity of the droplet portion of the debris particles ejected from the plasma of the target material is configured to be smaller, preferably several times smaller, more preferably about an order of magnitude smaller, than the velocity of the droplet portion of the debris particles ejected from the plasma of the lithium target; A plasma source according to claim 6 or 7.

9. 9. A plasma source according to any one of claims 6 to 8, wherein the velocity of the target (3) containing the target material is equal to or greater than the average speed of droplets of debris particles expelled from the plasma.

10. 1. A laser produced plasma EUV source comprising: a rotating target assembly (2) configured to provide a target (3) within an interaction zone (4) with a laser beam (5) focused on said target (3), said target (3) being a layer of molten target material on a surface of an annular groove mounted in the rotating target assembly (2) having a target surface facing a rotation axis (6) of said rotating target assembly (2), said laser produced plasma EUV source configured to pass an EUV radiation beam (8) exiting said interaction zone (4), and comprising debris mitigation means (10, 11, 12, 13, 14, 15); The target material is manufactured according to any one of claims 1 to 5, The linear velocity of the target is preferably 100 m / s or more. Laser-produced plasma EUV source.

11. 11. The laser produced plasma EUV source according to claim 10, wherein a spectral purity filter is positioned in the path of the EUV radiation beam.

12. Laser produced plasma EUV source according to claim 11, wherein the spectral purity filter is selected from the group consisting of a multi-layer Mo / Si mirror (9), a reflective filter in the form of a foil containing zirconium or beryllium.

13. 13. The laser produced plasma EUV source according to any of claims 10 to 12, wherein the debris mitigation means is provided by one or more of the debris mitigation techniques consisting of protective gas flow, magnetic mitigation, foil traps, debris shields, membranes that are mostly transparent to EUV radiation.

14. Producing a radiation beam having a wavelength in the EUV range by a plasma source using a target material according to any one of claims 1 to 5. A method for producing extreme ultraviolet (EUV) radiation.

15. forming a target (3) as a layer of molten target material on a surface of an annular groove mounted in a rotating target assembly (2) with a target surface facing a rotation axis (6) of the rotating target assembly (2) under the action of centrifugal force; Irradiating the target (3) with a focused laser beam (5); generating a laser-produced plasma in an interaction zone (4); outputting a beam of EUV radiation (8) through debris mitigation means (10, 11, 12, 13, 14, 15), wherein said target (3) is rotating at a high linear velocity, preferably greater than 100 m / s; The method of claim 14.

16. The centrifugal acceleration of the target (3) is at least 10,000 g, where g is the acceleration of gravity. The method of claim 15.

17. The spatial distribution of the debris ejection velocity from the interaction zone (4) is calculated and the passing directions of both the focused laser beam (5) and the EUV radiation beam (8) are selected in the spatial region where the debris ejection velocity is at a minimum.

17. The method according to claim 15 or 16.

18. 18. The method according to any of claims 15 to 17, wherein the debris mitigation is provided by one or more debris mitigation techniques consisting of a protective gas flow (13), a magnetic mitigation (14), a foil trap, a debris shield (12), a membrane that is mostly transparent to EUV radiation (15) with a transmission of more than 60%.

19. In the EUV radiation beam (8), ionized Li ２＋ A spectral filter is provided with narrow-band radiation at the transition of 19. The method according to any one of claims 15 to 18.

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