Target material, high-brightness EUV source, and method for generating 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 No. 16 / 952,587, filed on November 19, 2020, which is a continuation - in - part of U.S. Patent Application No. 16 / 773,240, filed on January 27, 2020, which is a continuation - in - part of U.S. Patent Application No. 16 / 535,404, filed on August 8, 2019, which is a continuation - in - part of U.S. Patent Application No. 16 / 103,243, filed on August 14, 2018, and this application claims priority to Russian Patent Application RU2017141042, filed on November 24, 2017, and this application also claims priority to Russian Patent Application RU2022100914, filed on January 17, 2022, and the entire disclosures of all of these are hereby incorporated by reference into this specification in their entirety.

[0002] This invention relates to Li - containing target materials used in plasma sources, as well as to high - brightness extreme ultraviolet (EUV) light sources and methods for generating EUV radiation, including reducing debris particles generated by the plasma along with the radiation.

Background Art

[0003] High - brightness EUV light sources are used in many fields such as microscopes, materials science, biology and medical diagnosis, materials testing, crystal and nanostructure analysis, atomic physics, lithography, etc.

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

[0005] Instead, there is a light source that can efficiently generate a plasma in the EUV (10 - 20 nm) region by both focusing the radiation of a high-power laser onto a target and discharging. The generation of EUV light is most effectively achieved by using laser-produced plasma (LPP).

[0006] In recent years, the development of EUV light sources that emit 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 high-volume production of chips below the 7 nm node.

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

[0008] ​The production of radioactive lithium plasma uses methods including discharge in metal vapor initiated by a laser (U.S. Patent No. 7,427,766, published on September 23, 2008), generation of laser plasma using a droplet Li target (U.S. Patent No. 7,449,703, published on November 11, 2008), and generation of laser plasma using a jet Li target (U.S. Patent No. 9,476,841, published on 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 in many applications where various optical systems can be used. The optical systems are based on multilayer Mo / Si mirrors having a high (70%) reflectivity at 13.5 nm and / or Fresnel zone plates, etc. One advantage of using Li as the target material is its relatively low melting point of 160.54 °C.

[0010] However, during the generation of EUV radiation, debris particles are produced as by-products, which may deteriorate the surface of the optics integrated with the EUV light source. The debris generated as by-products of the plasma during the operation of the EUV light source takes the form of high-energy ions, neutral atoms, clusters, and microdroplets of the target material.

[0011] From the international patent application PCT / RU2012 / 000701 published under WO2013 / 122505A1 cited as a reference, a short-wavelength radiation source based on laser-induced discharge-generated plasma in a metal vapor is known. The electrodes can take the form of a liquid metal jet or a rotating disk wetted with a molten target material. By the discharge becoming curved in a banana shape, the laser beam is directed at the irradiation site on one of the electrodes. The intrinsic magnetic field of such a discharge has a gradient that determines the preferential movement of the discharge plasma flow to a region of a not-very-strong magnetic field in a direction significantly different from the emission direction of the EUV beam. This invention provides a simple and very effective suppression of charged particles in the radiation beam.

[0012] However, for the suppression of the flow of neutral particles and microdroplets, more advanced debris mitigation means are required.

[0013] In the patent application PCT / RU2018 / 000520 published on May 31, 2019 under WO2019103648, PCT / EP2020 / 061562 published on October 29, 2020 under WO2020216950, and the patent RU2743572 published on February 20, 2021, a new approach for the development of a high-brightness LPP light source based on a rapidly rotating liquid metal target with a linear velocity of about 100 m / s is proposed, and high-efficiency debris mitigation is achieved by moving the droplet part of the debris particles away from the optical collector and the incident window of the laser beam. This debris mitigation method is very effective, for example, when using a target material with a sufficiently high density ρ, such as tin with ρ = 7.31 g / cm 3 However, when using Li as the target material, the density of the target material is low (ρ = 0.534 g / cm

[0014] However, when using Li as the target material, the density of the target material is low (ρ = 0.534 g / cm 3)This is characterized by an excessively high rate of formation of the droplet portion of debris particles, and as a result of the impact of the explosive shock pulse in the interaction zone, its fragments acquire a velocity reaching 1000 m / s, which dramatically reduces the protective effect against the droplet portion of the Li target debris.

Disclosure of the Invention

[0015] Therefore, it is necessary to eliminate the above-mentioned drawbacks. In particular, an improved LPP EUV source with a high spectral brightness of 13.5 nm using lithium as the plasma-forming target material is required. It is further beneficial if the light source is compact, powerful, and can preferably provide substantially complete debris mitigation (relaxation) with high efficiency.

[0016] This need is met by the features of the independent claims. Embodiments of the invention are described in the dependent claims.

[0017] In one embodiment of the present invention, a target material for use in a plasma source configured and arranged to generate a radiation beam having a wavelength in the extreme ultraviolet (EUV) range is provided. This target material includes a lithium (Li)-based composition having at least one additional element (further element) that increases the density of the target material several times, more than three times, compared to the density of Li, or consists of a lithium (Li)-based composition. In particular, the type and amount of at least one additional element in the composition can be configured such that the density of the target material increases by more than three times 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, Sr.

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

[0020] In a preferred embodiment, the atomic fraction of the at least one additional element in the target material can 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 the at least one additional element in the target material reaches (about) 100%. The term "about" means, for example, depending on the purity of the raw materials from which the target material is formed, the remainder of the impurities and / or contaminants that may be present.

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

[0022] In a preferred embodiment of the present 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 the debris particles emitted (released) from the plasma of the target material is several times, about one order of magnitude, smaller than the speed of the droplet fraction of the debris particles emitted from the plasma of the lithium target.

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

[0025] In another aspect, the present invention relates to a method for generating EUV radiation, which includes 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 in this specification.

[0026] In one embodiment, the method is implemented in a rotating target assembly where the target surface faces the axis of rotation under the action of centrifugal force. The target is formed as a layer of molten target material on the surface of an annular groove, and preferably at a high pulse repetition rate higher than, for example, 10 kHz, the target is irradiated with a focused laser beam to generate a laser-produced plasma in the interaction zone, and the method can further include outputting an EUV radiation beam through debris mitigation means. The molten target material is composed of any of the target materials disclosed in this specification, and in particular, it is composed of a Li-based composition having 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 speed of at least 100 m / s.

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

[0028] In an embodiment of the present 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 minimized.

[0029] In a preferred embodiment of the present invention, the debris mitigation is provided by one or more debris mitigation techniques including a protective gas flow, magnetic mitigation (relaxation), foil traps, debris shields, and a film having a transparency of 60% or more, which is substantially transparent to EUV radiation.

[0030] In a preferred embodiment of the present invention, in an EUV radiation beam, narrow-band radiation spectral filters are provided in the transitions of ionized Li at a wavelength of 13.5 nm. 2+

[0031] In another aspect, the present invention relates to an EUV radiation source comprising a vacuum chamber, a laser beam focused on a target which is a layer of molten target material on the surface of an annular groove implemented in a rotating target assembly, preferably a pulsed laser beam, a rotating target assembly for supplying the target to an interaction zone with the laser beam, the rotating target assembly having a target surface facing the axis of rotation, an EUV radiation beam emerging from the interaction zone, and debris mitigation means.

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

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

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

[0035] In a preferred embodiment of the present invention, the debris reduction means is provided by one or more debris reduction techniques including a protective gas flow, magnetic reduction, foil trap, debris shield, and a film that is substantially transparent to EUV radiation.

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

[0037] According to one embodiment, even when a small amount of additional element, such as Ag (ρ = 10.5 g / cm 3 ) or Au (ρ = 19.32 g / cm 3 ), is added to a Li-based target material (Li density ρ = 0.534 g / cm 3 ) at an atomic fraction (or molar fraction) of 10% to 40%, for example, about 20%, the intensity of the 13.5 nm emission line of the lithium plasma does not substantially change.

[0038] At the same time, regarding the density of the target material, which can be called "heavy lithium", it increases by 4.7 times or 8 times respectively compared to the density of Li, and the melting point remains relatively low, about 150 °C for the alloy with Ag and about 280 °C for the alloy with Au. Furthermore, when the density of the target material increases, the velocity of the droplet part of the debris particles, which is the greatest concern, decreases by several times. Therefore, when a high target velocity (100 m / s or more) is used, it becomes possible to control the main emission (release) direction from the plasma.

[0039] It is preferable to use Ag and Au as additional elements of the target material because these materials are non-toxic, have no chemical aggressiveness, and high Li-containing ratios of eutectic fusible alloys with a target density several times higher than the density of Li (60 - 90%) can be obtained.

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

[0041] Since the plasma emission band of the additional element of the target material is separated from the 13.5 nm emission line of doubly ionized lithium, it can be easily blocked, for example, only by reflection from a Mo / Si mirror without using a complicated optical system, and the monochromaticity of the EUV light source can be substantially ensured. The monochromaticity of the EUV light source enables the use of a wide range of optical elements including multilayer Mo / Si mirrors, grazing-incidence mirrors, and Fresnel zone plates, expanding the application range of the essentially point (having a characteristic size of ~100 μm) radiation source disclosed in this invention.

[0042] Generally, this invention is applicable to many EUV light sources based on laser-produced plasma or laser-induced discharge-produced plasma.

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

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

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

[0046] The advantages and features of this invention are exemplified with reference to the accompanying drawings and will become more apparent from the following non-limiting description of the exemplary embodiments.

[0047] Exemplary embodiments of this invention are illustrated by the drawings.

[0048] In the drawings, the same elements of the device are denoted by the same reference numerals.

[0049] These drawings do not cover the entire range of options for implementing this technical solution, nor do they limit it, but rather represent only exemplary materials for specific cases of its implementation.

Brief Description of the Drawings

[0050]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Modes for Carrying Out the Invention

[0051] In an exemplary embodiment of this invention shown in FIG. 1, a high-brightness light source of short-wavelength radiation includes a vacuum chamber 1 with a rotating target assembly 2. The rotating target assembly 2 supplies a target 3 to an interaction zone 4, where the target 3 interacts with a focused laser beam 5. A part of the rotating target assembly 2 is made in the form of a disk fixed to a rotating shaft (axis). The disk has a peripheral portion in the form of an annular barrier with an annular groove facing the rotating axis 6. The target 3 is a layer of molten metal formed by centrifugal force on the surface of the annular groove of the rotating target assembly 2. For the target 3, the target material in any configuration disclosed in this specification can be used.

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

[0053] Due to the configuration of the annular groove, when the volume of the target material does not exceed the volume of the groove, the material of the target 3 is prevented from being discharged in both the radial direction and along the rotating axis 6.

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

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

[0056] On the paths of the focused laser beam 5 and the EUV radiation beam 8, debris mitigation means consisting of one or more of the following techniques are provided.

[0057] Casing 10, 11 surrounding the laser and EUV radiation beams 5, 8.

[0058] A protective gas flow introduced through the gas inlet 13 of the casings 10, 11 to reduce the vapor fraction of debris particles within the paths of the laser and EUV radiation beams.

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

[0060] Foil trap (not shown). A system of essentially plates that is very transparent to plasma radiation and radially oriented with respect to the plasma, and that effectively captures neutral atoms and clusters of the target material.

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

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

[0063] The membrane installed on 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 also arranged in the propagation path of the focused laser beam 5.

[0065] Figure 2 shows a schematic diagram of the EUV light source described above, and is a computational modeling of the spatial distribution of debris emission (release) from the interaction zone 4, executed using the RZLINE code created for applications in the field of radiative hydrodynamics of high-density high-temperature plasmas. This code uses a mathematical model based on years of experimental and theoretical research, as is 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. Kochelev, V. Ivanov, V. Medvedev et al., Journal of Micro / Nanolithography, MEMS, and MOEMS, Volume 11, Issue 2 (May 2012). This code can model the interaction of laser radiation with gases, liquids, and solid surfaces, subsequent plasma generation, and the interaction with the plasma itself.

[0066] Figure 3 shows a spatial distribution diagram of the emission (release) velocity of debris particles in experimental coordinates for Li and Li / Ag target materials (all particles in all parts of all speeds (velocities) are considered), 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 is within the interaction zone. The typical directions of the interaction zone are as follows:

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

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

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

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

[0071] The spatial distribution of the debris ejection speed shown in Fig. 3 is calculated in nm / (month·W) as the specific growth rate of the film thickness of the deposited debris particles per unit laser output on the exposure sample surface located 40 cm away from the interaction zone, with the EUV light source operating 24 hours a day, 365 days a year.

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

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

[0074] In Fig. 3, the ellipse is used to show the spatial region selected according to the embodiment where the debris ejection speed is at the minimum level and where the cones of the laser and EUV radiation beams are located.

[0075] The calculation results for the target materials of Li in Fig. 3A, 80% Li and 20% Ag (atomic fractions) in Fig. 3B show the following. That is, by increasing the density of the target material (4.7 times that of lithium in this case), it is possible to sharply (by more than one digit) reduce the ejection (emission) speed of debris particles into the spatial region of the laser and EUV beams.

[0076] Using the light source schematically shown in Fig. 1 where a test sample was placed instead of film 15, the achievement of this preferable effect of this invention was experimentally confirmed.

[0077] Fig. 4 shows scanning electron microscope (SEM) images of witness samples obtained for the target materials in the case of Li (Fig. 4A) and in the case of the composition of 80Li and 20% Ag (Fig. 4B). The frame size of the SEM image is 125 μm. The tests were conducted under the following conditions.

[0078] The linear speed of the target was 150 m / s and the centrifugal acceleration was 23000 g.

[0079] The energy of the laser beam was 3.3 mJ / pulse on the target, and the focal size was 120 μm at the 1 / e 2 intensity level (1 / e 2 intensity level).

[0080] The pulse repetition rate (frequency) was 25 kHz.

[0081] The exposure time of the witness samples was 2.5 hours (Fig. 4A) and 10 hours (Fig. 4B).

[0082] It can be seen that increasing the density of the target material according to this invention brings about a dramatic (sharp) debris reduction effect.

[0083] Based on the conducted tests, it has been shown that relatively large droplets with a size of 300 nm or more passing through the CNT film are completely suppressed. Thus, by replacing the test sample with an exchangeable film, particularly a film made of carbon nanotubes, it becomes possible to ultra-purify the 13.5 nm high-brightness monochromatic radiation source manufactured according to this invention.

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

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

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

[0087] The radiation behind the multilayer mirror is monochromatic, with a bandwidth of λ / Δλ ≧ 1300. This value is estimated from the measured bandwidths (using a spectral resolution of λ / Δλ up to 500) of the spectral lines of 10.8 nm Li 2+ (1s-4p), 11.39 nm Li +2 (1s-3p), and 13.5 nm Li 2+ (1s-2p) (Figure 5A).

[0088] In the EUV light source, the maximum conversion efficiency CE of the laser radiation energy in the spatial angle of 2π sr to the EUV radiation within the band (13.5 ± 0.135 nm) 13.5 was 2% for any Li-based target material.

[0089] Generally, there is a series of parameters including the size of the laser spot, the energy and duration of the laser pulse, by which the amount of debris can be minimized while achieving a high CE.

[0090] The method for generating radiation using the EUV light source schematically shown in FIGS. 1, 2, and 3 is implemented as described below.

[0091] By 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, the surface of which 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 and enters the optical collector 9 through the debris reduction means 11, 12, 13, 14, 15.

[0092] The rotation of the target is performed at a high linear velocity of 100 m / s or more. A target material including a lithium composition having at least one additional element is used according to the embodiments disclosed in this specification, whereby the density of the target material can be increased several times, for example, 3 times or more compared to the density of Li, thereby enabling a dramatic reduction in the ejection speed of the droplet portion of debris particles. The droplet portion of the debris particles acquires a significant tangential component of a speed comparable to the ejection speed of the droplets due to the high-speed rotation of the target. According to this invention, this is sharply reduced by increasing the density of the Li-containing target material. As a result, the velocity vector of the droplet portion of the debris particles moves away from the laser and EUV radiation beams 6, 8, and its propagation path is selected in the spatial region where the debris ejection speed is at the minimum level.

[0093] Ag or Au is preferably selected as an additional element of the target material, whereby even if it is, for example, 20% atomic fraction, the density of the target material can be increased several times (about 5 times and about 8 times, respectively) compared to the density of Li. In a preferred embodiment of this invention, the target material is a eutectic alloy with a Li atomic fraction in the range of 60% to 90%, thereby ensuring the uniformity and relatively low melting point of the target material.

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

[0095] In the EUV radiation beam 8, spectroscopic filtering of the narrow-band line emission of Li at 13.5 nm is performed using, for example, a Mo / Si mirror 9 that serves as a reflection filter. In particular, a foil containing zirconium or beryllium can also be used as a filter. 2+

[0096] ​Debris reduction means 11, 12, 13, 14, 15 are used when EUV radiation is generated, which consists of a protective gas flow, a magnet, a foil trap, a film 15 that is highly transparent at 13.5 nm, a laser, and a debris shield 12 installed outside the propagation regions of the EUV radiation beams 5, 8.

[0097] When using a high-speed target rotation with a target speed of 100 m / s or more and a centripetal acceleration of at least 10,000 g, the surface of the target 4 becomes cylindrical. As a result, a stable shape of the target surface 3 in the interaction zone 4 is ensured, 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] Generally, the target materials manufactured in accordance with this invention are widely applicable to various types of EUV light sources known from the prior art, based on both laser-generated plasmas and discharge-generated plasmas.

[0099] For example, FIG. 8 shows an embodiment of an EUV light source using a target material fabricated in accordance with an embodiment of this invention.

[0100] In FIG. 8A, the LPP EUV light source has a target 3 in the form of a liquid metal jet that circulates at high speed within the interaction zone 4 along a closed loop 16 equipped with a nozzle 17 and a high-pressure pump 18 for transporting the molten target material. A debris shield 12 at a temperature exceeding the melting point of the target material is installed around the jet. The jet may be continuous or may consist of individual target droplets that follow each other at high speed. The other components of the apparatus in this embodiment are the same as those in the above embodiments (FIGS. 1, 2), are labeled with the same reference numerals in FIG. 8, and their detailed descriptions are omitted here and below.

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

[0102] In another embodiment shown in FIG. 8C, an LPP EUV light source is shown, and the target material of the target 3 is sent to the interaction zone 4 by a high-speed rotating disk 22 provided with a casing 23 having holes for the incidence of the laser beam 5 and the emission of the EUV radiation beam 8. Outside the interaction zone 4, the disk 22 is wetted by the molten target material 24.

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

[0104] Therefore, the present invention enables the creation of a monochromatic EUV light source with a wavelength of 13.5 nm, characterized by high spectral brightness, average output, long life, and ease of use. Industrial Application Fields

[0105] The proposed device assumes many applications, such as microscopes, materials science, X-ray diagnosis of materials, biological and medical diagnosis, inspection of nano-microstructures, lithography including actinic control of EUV masks for lithography.

Claims

1. An EUV radiation plasma source configured to generate plasma as either laser - produced plasma or laser - induced discharge - produced plasma, and comprising a target material for generating the plasma, wherein the target material comprises a lithium (Li) - based composition having at least one additional element, the composition is an alloy, the at least one additional element is selected from the group consisting of Ag, Au, Bi, Ba, Sr, and the composition is configured to increase the density of the target material by three times or more compared to the density of Li. Plasma source.

2. The plasma source according to claim 1, wherein the composition forms a eutectic alloy.

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

4. The plasma source 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%, and the sum of the atomic fraction of Li and the atomic fraction of the at least one additional element in the target material is approximately 100%.

5. Configured such that the velocity of the droplet portion of the debris particles discharged from the plasma of the target material is smaller than the velocity of the droplet portion of the debris particles discharged from the plasma of a lithium target. The plasma source according to claim 1.

6. The plasma source according to claim 1, wherein the velocity of the target (3) including the target material is equal to or greater than the average speed of the droplet portion of the debris particles discharged from the plasma.

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

8. The laser-produced plasma EUV source according to claim 7, wherein a spectral purity filter is installed in the path of the EUV radiation beam.

9. The laser-produced plasma EUV source according to claim 8, wherein the spectral purity filter is selected from the group consisting of a multilayer Mo / Si mirror (9) and a reflection filter in the form of a foil containing zirconium or beryllium.

10. The laser-produced plasma EUV source according to claim 7, wherein the debris mitigation means is provided by one or more debris mitigation techniques consisting of a protective gas flow, magnetic mitigation, foil trap, debris shield, and a film that transmits most of the EUV radiation.

11. Generating a radiation beam having a wavelength in the EUV range with the plasma source according to any one of claims 1 to 6, A method for generating extreme ultraviolet (EUV) radiation.

12. Under the action of centrifugal force, the target (3) is formed as a layer of molten target material on the surface of an annular groove mounted on a rotating target assembly (2) having a target surface facing the axis of rotation (6) of the rotating target assembly (2), irradiating the target (3) with a focused laser beam (5), generating a laser-produced plasma in the interaction zone (4), outputting an EUV radiation beam (8) through debris mitigation means (10, 11, 12, 13, 14, 15), where the target (3) is rotating at a linear velocity of 100 m / s or more. The method according to claim 11.

13. wherein the centripetal acceleration of the target (3) is at least 10,000 g, where g is the acceleration due to gravity, The method according to claim 12.

14. 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 minimized. The method according to claim 12.

15. The debris mitigation is provided by one or more debris mitigation techniques consisting of a protective gas flow (13), magnetic mitigation (14), foil trap, debris shield (12), and a film that transmits most of the EUV radiation (15) with a transmissivity exceeding 60%. The method according to claim 12.

16. In the EUV radiation beam (8), a spectral filter for narrow-band radiation is provided in the ionization Li transition at a wavelength of 13.5 nm ２＋ thereof The method according to claim 12.

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