High-intensity laser-produced plasma sources and radiation generation and collection methods.

Abstract

The laser produced plasma light source comprises a vacuum chamber (1), a rotating target assembly (2) which delivers a target (3) to an interaction zone (4) together with a focused laser beam (5). The target is a layer of molten metal on the surface of an annular groove in the target assembly. An output beam of short wavelength radiation (7) leaves the interaction zone via debris mitigation means to a light collector (8). The linear velocity of the target is preferably greater than 100 m / s, and the vector of the linear velocity of the target in the interaction zone is directed to one side of a plane (18) passing through the interaction zone and the axis of rotation (6), and the focused laser beam and the output beam are positioned on the other side of said plane.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application is a continuation-in-part of U.S. patent application No. 16 / 952,587, filed November 19, 2020, which is a continuation-in-part of U.S. patent application No. 16 / 773,240, filed January 27, 2020, which is a continuation-in-part of U.S. patent application No. 16 / 535,404, filed August 8, 2019, which is a continuation-in-part of U.S. patent application No. 16 / 103,243, filed August 14, 2018, which claims priority to Russian patent application RU2017141042, filed November 24, 2017, which also claims priority to Russian patent application RU2021132150, filed November 4, 2021, all of which are incorporated herein by reference in their entireties.

[0002] This invention relates to high brightness laser-produced plasma (LPP) light sources designed to produce soft X-ray, extreme ultraviolet (EUV), and vacuum ultraviolet (VUV) radiation in the wavelength range of approximately 0.4 to 200 nm, and which provide highly effective debris mitigation to ensure long-term operation of the light source and its integrated equipment, as well as methods for generating and collecting radiation in a large solid angle with transmission of the image of the emitting plasma region without aberrations into the remote focus of an optical collector. [Background technology]

[0003] High-intensity light sources that generate short-wavelength radiation (short-wavelength light), including X-rays, EUV, and VUV, are used in many fields, such as microscopy, materials science, biological and medical diagnostics, materials testing, crystallography and nanostructural analysis, atomic physics, and lithography. Such light sources are the basis of the analytical infrastructure in modern high-tech production and one of the main tools in the development of new materials and products based on them.

[0004] Light generation in these spectral regions is most effectively achieved using laser-produced plasma. During light generation, debris particles are generated as a by-product and can degrade the surface of a light collector, which may consist of one or more mirrors placed near the source. This debris can take the form of high-energy ions, neutral atoms, or clusters of target material. While the deposition of droplets or particles on the collector mirror reduces its reflectivity, high-velocity particles can damage the collector mirror and possibly other parts of the optical system located downstream of the collector mirror. This underscores the need to develop high-brightness, short-wavelength light sources with enhanced debris mitigation.

[0005] In RU2670273 published on October 22, 2018, RU2709183 published on April 26, 2019, and RU2743572 published on February 20, 2021, new approaches are proposed for the development of high-brightness short-wavelength LPP light sources based on a rapidly rotating liquid metal target, where highly efficient debris mitigation is achieved by keeping the droplet fraction of debris particles away from the light collector and laser beam input window.

[0006] These LPP sources use light collectors based on multilayer normal incidence mirrors. However, such mirrors can only collect short-wavelength radiation in a relatively narrow spectral band, determined by the wavelength dependence of the mirror's reflectivity. For example, when tin or a tin-containing alloy is used as the target material, the plasma emits most efficiently in a spectral band of approximately 13.5 nm. The full spectral band of the plasma emission is approximately 6–7 nm, whereas the mirror's reflection band is only 0.54 nm. Another drawback of light collectors based on normal incidence mirrors stems from the requirement for a large collection angle and the complexity of combining highly efficient debris mitigation. A large collection angle limits the size of the area in which the debris mitigation means can be placed, adversely affecting its efficiency. Furthermore, if it is necessary to obtain an image of the source at the intermediate focus that approaches the size of the emitting plasma, aspherical mirrors, which are very complex and expensive to manufacture, must be used as normal incidence collector mirrors. The manufacturing complexity and cost increase disproportionately with the mirror size.

[0007] These drawbacks are overcome in a light collector comprising two elliptical mirror units arranged in tandem, which is used in discharge-produced plasma (DPP) EUV light sources, as known from U.S. Patent No. 6,566,668, issued May 20, 2003.

[0008] This source uses a debris mitigation method based solely on a counterflow of protective gas (hereafter referred to as protective gas flow), which is inefficient in terms of protecting the light collector against debris particles formed by the erosion of the source electrodes. Furthermore, electrode erosion dramatically reduces the lifetime of DPP sources. Another drawback of DPP sources is their relatively low pulse repetition frequency (a few kilohertz), which significantly limits the application field of such sources. Another significant drawback of DPP sources is the relatively large size of the radiating plasma region (more than 200 μm). First, this prevents the source from being made very bright, and second, it is unacceptable for applications, especially in X-ray and EUV microscopy. DISCLOSURE OF THE INVENTION

[0009] Therefore, there is a need to overcome the above-mentioned shortcomings, and in particular, an improved LPP source of soft x-ray, EUV, and VUV radiation using a wideband large collection angle optical collector of relatively simple design that is compact, highly efficient, and preferably provides near-complete debris mitigation.

[0010] This need is met by the features of the independent claims. The dependent claims describe embodiments of the invention.

[0011] In one embodiment of the present invention, there is provided a method for generating and collecting radiation, the method comprising the steps of forming the target as a molten metal layer on the surface of an annular groove under the action of centrifugal force in a rotating target assembly having a target surface facing an axis of rotation, irradiating the target with a focused laser beam passing through debris mitigation means at a pulse repetition rate (particularly a high pulse repetition rate, for example higher than 1 kHz or 10 kHz), generating a laser-produced plasma in the interaction region and emitting an output beam of short wavelength radiation through the debris mitigation means to an optical collector.

[0012] In one embodiment, the target is preferably rotated at a predetermined linear velocity (e.g., a high linear velocity of 100 m / s or more), whereby most of the emitted debris particles, in particular the majority of the droplet fraction of the debris particles, and the vector of the linear velocity of the target in the interaction zone, are directed to one side of a plane passing through the interaction zone and the axis of rotation, and irradiation of the target and collection of radiation from the laser-produced plasma are provided such that the focused laser beam and output beam are located on the other side of the plane. The term "most" can refer, for example, to 50% or more, 70% or more, 90% or more, or 99% or more or 99.9% or more of the droplet fraction of the debris particles.

[0013] In a preferred embodiment, the spatial distribution of a debris ejection rate from the interaction zone is estimated, e.g., calculated, and directions of a passage of both the focused laser beam and the output beam are selected in spatial regions with low debris ejection rates, which may be regions with a lower debris ejection rate than the remaining spatial regions.

[0014] In a preferred embodiment of the present invention, the spatial region through which both the focused laser beam and the output beam pass is such that the ejection rate of the droplet fraction of debris particles in said spatial region is at least 10 times greater than a maximum ejection rate of the droplet fraction of debris particles. 4 is chosen to be 2 times smaller.

[0015] In a preferred embodiment of the invention, the short wavelength radiation is collected by an optical collector comprising two elliptical mirror units arranged in tandem, with the second focal point of the first elliptical mirror unit being located at the first focal point of the second elliptical mirror unit, and the optical collector transmits an image of the emitting plasma region, preferably without distortion, to the second focal point of the second elliptical mirror unit, at a scale determined by the design of the units.

[0016] In one embodiment, the debris mitigation is provided along the entire path of the short wavelength radiation to the light collector.

[0017] In a preferred embodiment of the invention, the debris mitigation is provided by one or more debris mitigation techniques, including at least one of a protective gas flow, a magnetic mitigation, a foil trap, a debris shield, or a membrane that transmits most of the short wavelength radiation (e.g. wavelengths shorter than 200 nm or 120 nm) and has a transparency of more than 60%.

[0018] In a preferred embodiment of the invention, the target is rotated at a centrifugal acceleration of 3000 g or greater, where g is the acceleration due to gravity, and the target surface is parallel to the axis of rotation.

[0019] In another aspect, the present invention provides a laser-produced plasma light source comprising a vacuum chamber and a rotating target assembly providing a target to an interaction zone. A pulsed laser beam is focused onto a target, which is a molten metal layer on a surface of an annular groove mounted on the rotating target assembly with the target surface facing the axis of rotation of the target assembly. The laser-produced plasma light source further comprises a light collector and debris mitigation means. The debris mitigation means is configured to pass the pulsed laser beam focused on the target and to pass an output beam of short wavelength radiation exiting an interaction zone between the focused laser beam and the target to the light collector.

[0020] The light source, and in particular the rotating target assembly, may be configured to rotate the target at a linear velocity such that the vector of the linear velocity of the target in the interaction zone is directed to one side of a plane passing through the interaction zone and the axis of rotation, and the focused laser beam and the output beam are located on the other side of the plane. The linear velocity may be sufficiently high so that a majority of droplets of debris particles ejected from the interaction zone are directed to the same side of the plane as the linear velocity vector. The linear velocity is preferably 100 m / s or greater.

[0021] In a preferred embodiment of the present invention, the target is formed at a centrifugal acceleration of at least 3000 g, and the surface of the target is parallel to the rotation axis.

[0022] In a preferred embodiment of the invention, the light collector comprises two elliptical mirror units arranged in cascade along the path of the output beam, and the debris mitigation means may be arranged along the entire path of the short wavelength radiation to the light collector.

[0023] In a preferred embodiment of the present invention, the interaction zone is located at a first focus of a first elliptical mirror unit, and the first focus of a second elliptical mirror unit is located at a second focus of the first elliptical mirror unit.

[0024] In one embodiment of the present invention, the second elliptical mirror unit is several times, 2 to 15 times, smaller than the first elliptical mirror unit, and the light collector preferably has a magnification close to 1, for example in the range of 0.8 to 1.2.

[0025] In particular, the material of the surface of the collector mirrors is selected from the group including Mo, Ru, Rh, Pd, U, Ni, W, Fe, Nb, Al, Si, Co and BN.

[0026] In one embodiment, the debris mitigation means comprises a protective gas flow between the ellipsoidal mirror units.

[0027] In one embodiment, each of the elliptical mirror units comprises a nested set of at least two elliptical mirrors.

[0028] In one embodiment, the debris mitigation means includes a debris shield located on an axis of the optical collector outside a collection angle.

[0029] In a preferred embodiment of the invention, the debris mitigation means is provided by one or more of the following techniques: protective gas flow, magnetic mitigation, foil traps, and a membrane made of a material from the group consisting of carbon nanotubes, Ti, Al, Si, Zr, Si, BN, which may be configured to be largely transparent (e.g., greater than 70%, 80%, or 90%) to the short wavelength radiation.

[0030] In one embodiment, the membrane may be configured as a gas lock that separates volumes with different pressures to provide a flow of protective gas.

[0031] The technical results of this invention include significantly increasing the average output power, the spectral range of collected radiation, the efficiency of debris mitigation, and the lifetime of compact, high-brightness, stable sources of soft X-ray, EUV, and VUV radiation.

[0032] The following causal relationships may exist between the preferred features of this invention and the technical results achieved thereby:

[0033] In an LPP light source with a rapidly rotating liquid metal target, the use of a light collector with two elliptical mirror units (as opposed to using a normal incidence multilayer mirror) firstly allows the solid angle (collection angle) of collection of short wavelength radiation to be significantly increased, secondly the spectral range of the collected radiation to be significantly expanded, and thirdly the image of the plasma light source to be transferred to an intermediate or remote focus without aberrations.

[0034] Increasing the collection angle and spectral range of the light collector leads to a significant (several-fold) increase in the output power of the LPP source. Also, the use of grazing incidence mirrors in the light collector allows for more area in the path of the output beam for placement of debris mitigation measures, thereby extending the lifetime of the light collector.

[0035] Furthermore, the use of grazing incidence mirrors in the light collector allows for an increased area for placement of debris mitigation means along the propagation path of the output beam, thereby extending the lifetime of the light collector.

[0036] All of the above provides improved operating efficiency for high brightness LPP light sources in virtually all applications.

[0037] An embodiment of the present invention combines the advantages of laser plasma-based light sources with those of a fast-rotating target with a grazing-incidence mirror-based light collector. Indeed, the inventors found that a key role in the contamination of collector optics in various types of LPP light sources belongs to the droplet portion of debris particles that are expelled from the interaction zone at relatively slow speeds, which, according to an embodiment of the present invention, is effectively mitigated by fast target rotation (hundreds of Hz at linear velocities above 100 m / s), which redirects the overwhelming part of the droplets sideways away from the light collector and the laser beam input window. At the same time, a grazing-incidence mirror-based light collector provides highly efficient collection of short-wavelength radiation over a wide solid angle (up to a few tenths of a radian, down to a few tens of nanometers) and a wide spectral range (down to a few tens of nanometers), while providing highly effective protection of the mirror against all types of debris particles.

[0038] The advantages and features of the present invention will become more apparent from the following non-limiting description of examples, illustrated by way of example with reference to the accompanying drawings.

[0039] Exemplary embodiments of the invention are illustrated by the drawings.

[0040] In the drawings, corresponding elements of the devices are numbered the same.

[0041] These drawings do not cover the entire range of options for implementing this technical solution, nor do they limit it, but merely illustrate specific cases of its implementation. [Brief explanation of the drawings]

[0042] [Figure 1] 1 is a schematic diagram of a high-brightness LPP light source having a light collector with two elliptical grazing incidence mirror units, according to one embodiment. [Figure 2] 1 is a simplified diagram of a high brightness LPP light source according to one embodiment. [Figure 3] 1 shows the calculated spatial distribution of debris ejection from the interaction region, further illustrating the selection of spatial regions for the propagation of the laser beam and the short wavelength radiation beam. DETAILED DESCRIPTION OF THE INVENTION

[0043] 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 that delivers targets 3 to an interaction zone 4 where the targets 3 interact with a focused laser beam 5. A portion of the rotating target assembly 2 is 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 the rotation axis 6. The target 3 is a layer of molten metal that is formed by centrifugal force on the surface of the annular groove of the rotating target assembly 2.

[0044] The annular groove configuration prevents the target 3 material from being ejected both radially and along the axis of rotation 6 when the volume of the target material does not exceed the volume of the groove.

[0045] To ensure high stability of both the target surface and the output parameters of the LPP light source, a high rotation speed with a centrifugal acceleration of more than 3000 g is adopted, and under the influence of centrifugal force, the surface of the liquid metal target 3 becomes parallel to the rotation axis 6, i.e., essentially a circular cylindrical surface whose axis coincides with said rotation axis.

[0046] 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 short-wavelength radiation in one or more spectral ranges including VUV, EUV, and soft X-ray. The short-wavelength radiation is directed to a light collector 8 in the form of a diverging output beam 7.

[0047] According to the invention, collection of short wavelength radiation is performed by a light collector 8 comprising two elliptical mirror units 9, 10. The elliptical mirror units 9, 10 are arranged on a common optical axis 11, preferably in tandem along the propagation path of the output beam 7. The interaction zone 4 is located at a first focal point of the first elliptical mirror unit 9, and the second focal point of the first unit 9 coincides with the first focal point of the second elliptical mirror unit 10. In this case, an image of the luminous plasma region in the interaction zone 4 is projected substantially without distortion onto the zone of the second focal point 17 of the second elliptical mirror unit 10.

[0048] On the path of the output beam 7 along the optical axis 11 of the light collector 8, debris mitigation means 12, 13, 14, 15, 16 are provided which may be provided by one or more of the following techniques:

[0049] Debris shields 14, 15 positioned outside the collection angle and cone of the focused laser beam 5

[0050] a flow of protective gas along and / or perpendicular to the optical axis 11 of the light collector 8 to suppress the vapor fraction of debris;

[0051] A foil trap is a system of plates that is highly transparent to plasma radiation and is essentially oriented radially to the plasma, effectively trapping neutral atoms and clusters of the liquid metal target material.

[0052] A magnetic field, preferably generated by a permanent magnet, that reduces the charged fraction of debris particles

[0053] A preferably replaceable membrane 16 that is essentially transparent to short wavelength radiation and impermeable to debris and gases.

[0054] Similar means for debris mitigation are placed in the propagation path of the focused laser beam 5.

[0055] An important advantage is achieved in this embodiment of the invention compared to LPP sources using a light collector based on a normal incidence mirror, where most of the area between the interaction zone and the light collector is occupied by the propagation zone of the reflected beam of short wavelength radiation. In such sources, the area for arranging debris mitigation means is very limited. According to the invention, debris mitigation means 12, 14 are arranged in essentially the entire area between the interaction zone 4 and the light collector 8. The means are arranged partly inside and partly outside a casing surrounding the laser and the beams of short wavelength radiation 5, 7. This provides very effective debris mitigation.

[0056] A debris shield 14 is rigidly mounted around the interaction zone 4 and the rotating target and is separated from the rotating target assembly 2 by a slit gap. The debris shield 14 has only two small openings (for the entrance of the focused laser beam and for the exit of the short wavelength radiation beam) through which debris particles can exit the target assembly. In a preferred embodiment of the invention, these openings are also used to direct a flow of protective gas into the interaction zone 4.

[0057] The output beam 7, having passed through the section of the debris mitigation means 12, strikes the collector mirror of the first unit 9. This collector mirror may comprise several embedded coaxial ellipsoid mirrors so that their focuses coincide. In the zone between the first and second ellipsoidal mirror units, there is a second section of the debris mitigation means 13, which is used to supply protective gas, preferably argon, by means of a nozzle array arranged around the optical axis. This makes this zone a pressurized zone, which serves as an additional gas shield in the path of debris propagation to the second ellipsoidal mirror unit 10 and the equipment integrated with the LPP light source. This gas is discharged through an annular branch pipe (not shown), which is also arranged in this zone. Furthermore, a membrane 16 may be arranged here, which, on the one hand, additionally restricts the flow of contaminant particles and, on the other hand, separates zones of protective gas at different pressures. The membrane is preferably made from a material belonging to the group including carbon nanotubes (CNT), Ti, Al, Si, ZrSi, BN.

[0058] After being reflected by the mirrors of the first unit 9, the short wavelength radiation is focused to a second focal spot of the first unit 9 and strikes the mirror of the second unit 10, where it is reflected towards the second focal spot 17 of the second elliptical mirror unit 10.

[0059] In order to increase the mitigation effect of the droplet portion of the debris particles, the output beam 7 is aligned with the linear velocity vector of the target within the interaction zone 4, as shown in FIG. V R (In the original text, the superscripted right-pointing arrow is underlined; the same applies below) to the opposite side of a plane 18 passing through the interaction zone 4 and the rotation axis 6 of the target assembly. Both the laser beam 5 and the output beam 7 are located on one side of the plane 18. Therefore, the vector V R The beams 7, 5 of short wavelength and laser radiation are located on different sides of the plane 18.

[0060] This positive effect is confirmed by the results of computational modeling of the spatial distribution of debris emissions from the interaction zone 4, performed using the RZLINE code, developed for applications in the field of radiation hydrodynamics of dense, high-temperature plasmas. The code uses mathematical models based on many years of experimental and theoretical research, as known, for example, from K. Koshelev, V. Ivanov, V. Medvedev et al., "Return-to-zero line code modeling of distributed tin targets for laser-produced plasma sources of extreme ultraviolet radiation," Journal of Micro / Nanolithography, MEMS, and MOEMS, Vol. 11, Issue 2 (May 2012). The code allows modeling the interaction of laser radiation with gases, the interaction of liquid and solid surfaces with subsequent plasma generation, and the interaction of the plasma itself.

[0061] Figure 3 shows the spatial distribution of ejection rates of debris particles (particles at all speeds and all fractions are considered) in experimental coordinates, where θ is the angle relative to the rotation axis and φ is the azimuthal angle lying in the plane of the figure. The origin of the coordinates is within the interaction zone. Typical directions in the interaction zone are:

[0062] I Parallel to the rotation axis: θ=0, φ is any

[0063] II Along the target velocity: θ=90°, φ=0°

[0064] III Normal to the target surface: θ=90°, φ=90°

[0065] IV Against target velocity: θ=90°, φ=180°

[0066] The spatial distribution of the debris emission rate is calculated as a specific growth rate of the film thickness of deposited debris particles on a surface of the exposed sample located at a distance of 40 cm from the interaction zone per unit of laser power, in nm / (month·W), as shown in Figure 3. No debris mitigation techniques were used other than high-speed target rotation.

[0067] This distribution was obtained for typical values ​​of the source parameters: target material is tin, laser emission wavelength is ~1-2 μm, laser pulse energy is several mJ with pulse duration of several ns, focal spot diameter is several tens of μm, and target linear velocity is 200 m / s.

[0068] As shown in Figure 3, the debris particle mass is primarily concentrated 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 107 nm / (month·W). Figure 3 uses ellipses to indicate the spatial orientation of the laser 35 and output beam 37 cones, estimating a debris ejection rate of less than 0.2 nm / month·W in the laser cone 35, while it is less than 1 nm / month·W in the short-wavelength radiation cone 37 (the illustrated cones correspond to a solid angle of approximately 0.3 sr). The data presented assumes a laser radiation power of 1 W and 24 / 7 operation of the light source. In Figure 3, reference numeral 31 indicates the direction of the target velocity (200 m / s velocity), with a maximum debris ejection rate of 107 nm / (month·W). Reference numeral 32 indicates the line of the level of the Sn-debris ejection rate given in nm / (month·W). Reference numeral 35 indicates the debris ejection in the cone of the laser beam, which is less than 0.2 nm / (month·W). Reference numeral 37 indicates the debris ejection in the cone of the collection angle, which is less than 1 nm / (month·W).

[0069] The high brightness laser-produced plasma light source operates as described below and as shown in FIGS.

[0070] Vacuum chamber 1 is 100mW using an oil-free vacuum pump system. -5 ...10 -11The atmosphere is evacuated to a pressure of less than 100 mbar, simultaneously removing gas components such as nitrogen, oxygen, and carbon that may interact with the target material and contaminate the collector mirror.

[0071] A target material belonging to the group of non-toxic soluble metals, including Sn, Li, In, Ga, Pb, Bi, Zn and their alloys, is brought to a molten state and maintained at a predetermined optimum temperature range using a stationary heating system, which may employ induction heating.

[0072] The rotating target assembly 2 is operated by a rotary drive unit, e.g., an electric motor with a magnetic coupling, which keeps the vacuum chamber 1 clean. Under the action of centrifugal force, the target 3 forms a molten metal layer on the surface of the annular groove facing the rotation axis 6. At centrifugal accelerations of at least 3000 g, the target surface is substantially parallel to the rotation axis.

[0073] The target 3 is exposed to a focused laser beam 5 with a high pulse repetition rate (speed) that can range from 1 kHz to 5 MHz. Short wavelength radiation is generated by the focused laser beam 5, which heats the target material to plasma formation temperature. The laser-produced plasma emits light in the short wavelength range, including wavelengths from 0.4 to 120 nm. Depending on the laser emission power density at the focused spot and the target material, short wavelength radiation is generated, primarily in the soft x-ray (0.4-10 nm), and / or EUV (10-20 nm), and / or VUV (20-120 nm) ranges.

[0074] Heat transfer from the target is ensured through a narrow gap between the rotating target assembly 2 and the stationary water-cooled heat exchanger (not shown), through which gas is blown at a pressure of ~1 mbar. The gas conductivity and contact area are sufficient for this type of cooling to remove a thermal power of up to 1.5 kW. At the same time, other cooling methods may be used for the rotating target assembly 2.

[0075] A high density, high temperature, laser-produced plasma formed in the interaction zone 4 emits short wavelength radiation. The output beam 7 from the high temperature plasma is emitted through debris mitigation means 12, 13, 14, 15, 16 onto a light collector based on two coaxial elliptical mirror units 9, 10. Due to the high speed target rotation, the droplet fraction of debris particles emitted from the interaction zone acquires a significant tangential velocity component comparable to the linear target velocity (Fig. 2). Consequently, the linear velocity of the target in the interaction zone V R Since the beams 5, 7 are located on different sides of the plane 18 passing through the interaction zone 4 and the rotation axis 6, the droplet velocity vector is significantly redirected from the cone of the laser beam 5 and the output beam 7.

[0076] The first elliptical mirror unit 9 transmits an image of the luminous plasma region from the interaction zone located at the first focus of the first elliptical mirror to the second focus. Due to different gain coefficients of different parts of the elliptical mirror, the resulting image of the luminous plasma is strongly distorted at the intermediate focus between the two mirror units 9, 10. In this regard, the second elliptical mirror unit 10, whose first focus coincides with the second focus of the first elliptical mirror unit 9, can eliminate the image distortion. That is, at the second focus of the second elliptical mirror unit, the plasma image is projected without distortion and with a scale factor determined by the design of the first and second elliptical mirror units 9, 10.

[0077] In an embodiment of the present invention, the light collector 8 has a magnification close to or equal to 1, in the range of 0.8 to 1.2. In a preferred embodiment, the second elliptical mirror unit 10 is several times smaller than the first elliptical mirror unit 9, between 2 and 15 times smaller. This is because, despite the same collection angle, there is no need to reserve much space in front of the second mirror for the debris mitigation means. This configuration allows the light collector assembly to be more compact and less expensive. The debris mitigation means that can be used to protect the second mirror unit 10 and downstream optics can be configured as follows:

[0078] Debris shield 15 located outside the collection angle

[0079] Replaceable semi-permeable membrane 16

[0080] A protective gas flow is supplied before the second focal point of the first mirror unit 9 through a nozzle and an annular branch pipe for gas discharge, which are preferably arranged circumferentially with respect to the optical axis 11. A gas curtain is formed in the zone before the second focal point of the first mirror unit.

[0081] If the integral amplification of the light collector is close to 1, an undistorted image of the plasma source with the same size is obtained at the second focal point 17 of the second unit of the light collector.

[0082] Depending on the wavelength operating range, one of Mo, Ru, Rh, Pd, U, Ni, W, Fe, Nb, Al, Si, Co and BN, which have relatively high grazing incidence reflectivity, can be selected as the reflective material for the surface of the elliptical mirror.

[0083] Depending on the working range of wavelength, one of Sn, Li, In, Ga, Pb, Bi, Zn and their alloys can be selected as the working medium of the source.

[0084] The method for generating and collecting laser-produced plasma radiation is carried out as described below and as shown in FIGS.

[0085] Under the action of centrifugal force, a target is formed in the form of a molten metal layer on the surface of the annular groove of the rotating target assembly 2, with its surface facing the rotation axis. The target is irradiated with a pulsed focused laser beam 5, resulting in the formation of plasma in the interaction zone 4. An output beam is generated and passes through debris mitigation means 12, 13, 14, 15, 16 onto a light collector 8. The short-wavelength radiation is collected using the light collector 8, which comprises two essentially distortion-free coaxial elliptical mirror units 9 and 10, and an image of the radiating plasma region is transmitted (passed) to a second focal point of a second elliptical mirror unit 17. The second elliptical mirror unit 17 is arranged such that the second focal point of the first elliptical mirror unit coincides with the first focal point of the second elliptical mirror unit.

[0086] The pulsed irradiation of the target is caused by the vector of the target linear velocity in the interaction zone. V R The laser and short-wavelength radiation beams 5, 7 are positioned on different sides of a plane 18 passing through the interaction zone 4 and the rotation axis 6, where the linear velocity of the target is sufficiently high, above 100 m / s, to prevent a large portion of the droplet fraction of debris particles from being directed towards the light collector 8 and the cone of the focused laser beam 5.

[0087] The spatial distribution of the debris emission rate from the interaction zone 4 is calculated and the passage directions of both the focused laser beam 5 and the output beam 7 of short wavelength radiation are selected within a spatial region with a low debris emission rate.

[0088] The spatial region through which both the focused laser beam 5 and the output beam 7 pass is such that the debris emission rate of the spatial region is at least 10 times greater than the maximum debris emission rate. 4 times, preferably 10 6 is chosen to be 2 times smaller.

[0089] Optionally, additional debris mitigation measures may be used, including protective gas flow, magnets, foil traps, membranes 16 that are highly transparent to short wavelength radiation, and debris shields 14, 15.

[0090] Thus, the present invention provides for the formation of an LPP source of soft X-ray, EUV and VUV radiation, characterized by high average power, high brightness short wavelength radiation, long life and ease of use.

[0091] The proposed device is intended for many applications, such as microscopy, materials science, X-ray diagnostics of materials, biomedical and medical diagnostics, inspection of nano- and micro-structures, and lithography, including actinic control of lithographic EUV masks.

Claims

1. 1. A method of generating and collecting radiation, comprising: forming a target as a molten metal layer on a surface of an annular groove in a rotating target assembly under the action of centrifugal force, the target having a target surface facing an axis of rotation of the rotating target assembly; irradiating the target at a predetermined pulse repetition rate with a focused laser beam passing through debris mitigation means; generating a laser-produced plasma in an interaction zone between the focused laser beam and the target; and emitting an output beam of short wavelength radiation that is directed through the debris mitigation means to a light collector, The target is rotated at a sufficiently high linear velocity, so that the majority of droplets of debris particles ejected from the interaction zone are aligned with the linear velocity vector (V Ｒ ) is directed to one side of a plane passing through the interaction zone and the axis of rotation, and illumination of the target and collection of radiation from the laser-produced plasma is provided such that the focused laser beam and output beam are located to the other side of the plane; a spatial distribution of the debris emission rate from the interaction zone is estimated by calculating a growth rate of the film thickness of deposited debris particles at a predetermined distance from the interaction zone, and based on the estimated spatial distribution of the debris emission rate, directions of both the focused laser beam and the output beam of short wavelength radiation are selected to pass through a spatial region that results in a debris emission rate that is at least 10 times less than the maximum debris emission rate; method.

2. The directions of both the focused laser beam and the output beam of short wavelength radiation are selected to pass through a region of space where the debris emission rate is lower than the remaining region of space, The method of claim 1.

3. 2. The method according to claim 1, wherein the linear velocity is a high linear velocity of 100 m / s or more.

4. the short wavelength radiation is collected by a light collector consisting of two elliptical mirror units arranged in cascade, the light collector transmitting an image of the luminous plasma region to a second focal point of a second elliptical mirror unit, the second focal point of the first elliptical mirror unit being located at the first focal point of the second elliptical mirror unit; The method of claim 1.

5. the light collector transmits an image of the luminous plasma region without distortion, and / or the scale factor of the image is determined by the design of the two elliptical mirror units; The method of claim 4.

6. the debris mitigation is provided along the entire path of the short wavelength radiation to the light collector; The method of claim 1.

7. the debris mitigation is provided by one or more debris mitigation techniques selected from the group consisting of protective gas flow, magnetic mitigation, foil traps, debris shields, and membranes that are largely transparent to short wavelength radiation with a transmission of 60% or more; The method of claim 1.

8. The target is rotated at a centrifugal acceleration of 3000 g or more, where g is the acceleration of gravity, and the target surface is parallel to the rotation axis. The method of claim 1.

9. 1. A laser-produced plasma light source comprising: a vacuum chamber; a rotating target assembly configured to deliver a target to an interaction zone, the target being a layer of molten metal on a surface of an annular groove provided in the rotating target assembly with a target surface facing a rotation axis of the rotating target assembly; a light collector; and debris mitigation means configured to pass a pulsed laser beam focused on the target and to pass an output beam of short wavelength radiation exiting an interaction zone between the focused laser beam and the target to the light collector; The rotating target assembly is configured to rotate the target at a predetermined linear velocity, and the linear velocity vector (V Ｒ ) is directed to one side of a plane passing through the interaction zone and the axis of rotation, and the focused laser beam and the output beam are positioned on the other side of the plane; the debris mitigation means estimates a spatial distribution of debris emission rate from the interaction zone by calculating a growth rate of a film thickness of deposited debris particles at a predetermined distance from the interaction zone, and based on the estimated spatial distribution of debris emission rate, the directions of both the focused laser beam and the output beam of short wavelength radiation are selected to pass through a spatial region that results in a debris emission rate that is at least 10 times less than the maximum debris emission rate; light source.

10. 10. The light source of claim 9, wherein the target has a velocity of 100 m / s or greater.

11. the rotating target assembly is configured to form the target such that a surface of the target is parallel to the axis of rotation at a centrifugal acceleration of at least 3000 g; 11. A light source according to claim 9 or 10.

12. the light source further comprises a light collector comprising two elliptical mirror units arranged in cascade along the path of the output beam, and / or the debris mitigation means is arranged along the entire path of short wavelength radiation to the light collector.

10. The light source of claim 9.

13. the interaction zone is located at a first focal point of the first elliptical mirror unit, and the first focal point of the second elliptical mirror unit is located at a second focal point of the first elliptical mirror unit; 13. The light source of claim 12.

14. the second elliptical mirror unit being several times smaller than the first elliptical mirror unit, for example, 2 to 15 times smaller, and the light collector having a magnification between 0.8 and 1.2; 14. A light source according to claim 12 or 13.

15. the material of the surface of the mirror of the light collector is selected from the group consisting of Mo, Ru, Rh, Pd, U, Ni, W, Fe, Nb, Al, Si, Co and BN; 13. The light source of claim 12.

16. the debris mitigation means including a protective gas flow between the elliptical mirror units; 13. The light source of claim 12.

17. Each of the elliptical mirror units comprises a nested set of at least two elliptical mirrors; 13. The light source of claim 12.

18. the debris mitigation means includes a debris shield positioned on the axis of the light collector outside the collection angle; 10. The light source of claim 9.

19. the debris mitigation means is provided by one or more of the following techniques: protective gas flow, magnetic mitigation, foil traps, and membranes made of materials from the group consisting of carbon nanotubes, Ti, Si, ZrSi, and BN; 10. The light source of claim 9.

20. the membrane is configured to provide a gas lock separating volumes having different pressures providing a protective gas flow; 20. The light source of claim 19.

21. the light collector includes a grazing incidence mirror; 10. The light source of claim 9.

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