FIELD OF INVENTION

[0001] The invention refers to high brightness laser-produced plasma (LPP) light sources designed to generate soft X-ray, extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) radiation at wavelengths of approximately 0.4 to 200 nm, which provide highly effective debris mitigation to ensure the long-term operation of the light source and its integrated equipment. Applications include soft X-ray and VUV metrology, microscopy, biomedical and medical diagnostics, and various types of controls, including inspection of lithographic EUV masks.

BACKGROUND OF INVENTION

[0002] High-intensity soft X-ray (0.4 - 10 nm), EUV (10 - 20 nm) and VUV (20-120 nm) light sources are used in many fields: microscopy, materials science, biomedical and medical diagnostics, materials testing, crystal and nanostructure analysis, atomic physics, and lithography. These sources are the basis of the analytical base of modern high-tech production and one of the main tools in the development of new materials and products based on them.

[0003] The light generation in in these spectral ranges is most effective with the use of laser-produced plasma. Their development in recent years has been largely stimulated by the development of projection extreme ultraviolet (EUV) lithography for high-volume manufacturing of integrated circuits (ICs) with 10-nm node and below.

[0004] EUV lithography is based on the use of radiation in the range of 13.5+/-0.135 nm, corresponding to the effective reflection of multi-layer Mo/Si mirrors. One of the most important metrological processes of modern nanolithography is the control of ICs for the absence of defects. The general trend in lithographic production is a shift from ICs inspection to the analysis of lithographic masks. The process of mask inspection is most effectively carried out with the help of its scanning by actinic radiation, i.e. radiation, the wavelength of which coincides with the working wavelength of the lithograph (the so-called Actinic Inspection). Thus, the control of lithographic mask defect-free production and operation is one of the key problems of lithography, and the creation of a device for the diagnosis of lithographic masks and its key element, the high-brightness actinic source, is one of the priorities of the development of EUV lithography.

[0005] The radiation sources for EUV lithography are using Sn - plasma generated by a powerful laser system including CO2 lasers. Such sources have the power of EUV radiation exceeding by several orders of magnitude the level of power required for the inspection of EUV masks. Therefore, their usage for mask inspection is inadequate due to the excessive complexity and cost. In this regard, there is a need for other approaches to the creation of high-brightness EUV sources for actinic inspection of EUV masks.

[0006] In accordance with one of the approaches known from the patent U.S. 8344339, issued on March 01, 2012, a known device for the generation of EUV radiation from laser produced plasma including: a vacuum chamber, which houses a rotating rod made of plasma-forming target material, an input window for the laser beam focused in the interaction zone of the laser beam and target, and an Output beam generated from the laser- produced plasma exiting an output window towards the optical collector. The device and the method of generation of EUV radiation are characterized by the fact that tin (Sn) is used as the most effective plasma-forming target material and the rod, in addition to rotation, also performs reciprocating axial movements. However, these devices and the method have a number of disadvantages, which include the non-reproducibility of the profile of the solid surface of the target from pulse to pulse during long-term continuous operation of the device, which affects the stability of the output characteristics of the LPP light source. The complexity of the design is another disadvantage, since complex movements of the target assembly and its periodic replacement are required. During production of EUV radiation, debris particles are produced as a by-product, which can degrade the optics surface. The level of debris produced in this source is too high and that severely limits the possibilities of its application.

[0007] The debris, generated as a by-product of the plasma during the radiation source operation, can be in the form of high-energy ions, neutral atoms or vapors and clusters of target material.

[0008] The magnetic mitigation technique disclosed, for example, in the U.S. patent 8519366, issued August 28, 2013, is arranged to apply a magnetic field so that charged debris particles are mitigated. In this patent the debris mitigation system for use in a short- wavelength radiation source, includes a rotatable foil trap and gas inlets for the supply of buffer gas to the foil trap so that neutral atoms and clusters of target material are effectively mitigated.

[0009] Patent application US 2013/0313423 Al, published April 3, 2013, discloses a method of mitigating debris for an LPP light source, comprising directing an ionized plasma jet across a short-wavelength beam path. The plasma charges the debris particles, after which a pulsed electric field redirects the debris particles. The method is effective for mitigating the ion/vapor fraction of debris particles, for example, in LPP light sources using xenon as a target material. However, in LPP light sources using metals as the target material, the main threat to the optical elements is the micro-droplet fraction of debris particles, against which this method is not effective.

[0010] All of the above approaches to the construction of plasma light sources, as well as to debris mitigation techniques, fail to provide for a highly efficient suppression of the micro droplet fractions of debris particles. This limits the lifetime of the equipment into which the light source is integrated due to earlier contamination of its optical elements.

[0011] One method for mitigating a micro-droplet fraction of debris particles, disclosed in US Patent Number 7,302,043, published October 27, 2007, offers one potential solution. That method is arranged to apply a rotating shutter assembly configured to permit the passage of short- wavelength radiation through at least one aperture during the first period of rotation and to thereafter rotate the shutter to obstruct passage of the debris through at least one aperture during the second period of rotation. However, the complexity of using such debris-mitigating techniques in a compact radiation source makes them technically too difficult to implement.

[0012] From the patent US9897930, issued on February 20, 2018, it is known that a membrane from carbon nano tubes (CNT) having thickness more than 20 nm and high transparency for EUV radiation is used as a mask pellicle within the lithographic apparatus. It was proposed also to use CNT-membrane as a debris trapping system for EUV lithography source.

[0013] CNT-membranes are characterized by a number of advantages, including low cost and high strength, which allows them to be produced free-standing at large (centimeter) sizes, as is known, for example, from the publication of M.Y. Timmermans, et al. "Free-standing carbon nanotube films for extreme ultraviolet pellicle application", Journal of Micro/Nanolithography, MEMS, and MOEMS 17(4), 043504 (27 November 2018).

[0014] However, the use of a CNT-membrane for trapping debris particles generated by EUV lithography source is unlikely, as the CNT-membrane is highly likely to be destroyed by such powerful radiation. For less powerful sources of radiation, there is also a limitation. As our research has shown, a small fraction of debris particles with microdroplet sizes of more than 300 nm can penetrate through the CNT-membrane, which does not ensure the purity of the LPP light source only through the use of a CNT-membrane.

[0015] Accordingly, there exists a need for improved low-debris high-brightness LPP light sources.

SUMMARY

[0016] Accordingly, there is a need to mitigate at least some of the drawbacks mentioned above. In particular, there is a need for an improved laser produced plasma light source that is compact and provides an efficient, preferably an almost complete, mitigation of the flow of debris particles in the paths of the laser beam and output beam of short- wavelength radiation.

[0017] This need is met by the features of the independent claim. The dependent claims describe embodiments of the invention.

[0018] According to an embodiment of the invention, there is provided a laser-produced plasma light source, comprising a vacuum chamber with a rotating target assembly supplying a target into an interaction zone with a pulsed laser beam focused onto the target, which is a molten metal layer on a surface an annular groove implemented in the rotating target assembly, an output beam of short wavelength (which may also be termed useful short- wavelength radiation beam) exiting the interaction zone and means for debris mitigation.

[0019] The laser-produced plasma light source is characterized in that the means for debris mitigation include a debris shield that is rigidly mounted to surround the interaction zone, said shield comprising a first opening forming an entrance for said laser beam and a second opening forming an exit for the output beam. The debris shield is in particular stationary. In other words, the debris shield may not rotate with the target assembly, but is fixed so that the laser beams and the output beam can pass through the first and second openings.

[0020] The rotating target assembly may in particular rotate with respect to the debris shield and the laser beam. The debris shield may be stationary with respect to the path of the laser beam.

[0021] In a preferred embodiment of the invention, the molten metal layer is formed by a centrifugal force on the surface of the annular grove, the surface facing an axis of rotation of the rotating target assembly. The molten metal layer may thus in particular be an annular molten metal layer.

[0022] The output beam may be emitted by a laser-produced plasma that is produced by interaction between the pulsed laser beam and the target material.

[0023] In an embodiment, a part of the rotating target assembly is made in the form of a disk, said disk has a peripheral portion in the form of an annular barrier on an inner surface of which there is the annular groove in which the target is located. The surface of the annular groove may face the rotation axis. The disk may further have annular depressed and/or towering portions.

[0024] The annular groove may have a surface profile that is shaped so as to prevent an ejection of target material in a radial direction and in both directions along the rotation axis.

[0025] In an embodiment, the laser-produced plasma light source may be configured to operate the rotating target assembly such that a rotational speed is high enough for the surface of the target to be parallel or close to parallel to a rotation axis of the rotating target assembly.

[0026] In an embodiment, the surface of the annular groove, at least near the interaction zone, is inclined to the rotation axis of the rotating target assembly, and preferably is a conical surface. [0027] In an embodiment, the debris shield is located opposite an angular sector of the target near the interaction zone.

[0028] In an embodiment, the debris shield is circular.

[0029] In an embodiment, slit gaps separate the shield from the rotating target assembly.

[0030] In the embodiment of the invention, at least one of the first and second openings in the debris shield is conical.

[0031] In an embodiment, an axis of short wavelength radiation beam is directed at an angle of greater than 45° relative to a rotation plane of the rotating target assembly.

[0032] In an embodiment, the laser-produced plasma light source is configured to operate the rotating target assembly rotates with a linear velocity exceeding 80 m/s. The means for debris mitigation may further comprise one or a combination of the following: protective gas flows, electrostatic and magnetic mitigation, foil traps, and a membrane comprising carbon nanotubes.

[0033] In an embodiment, at least parts of the focused laser beam and the output beam are surrounded by casings in which protective gas flows are supplied. The casings may include respective gas inlets to be provided with a respective protective gas.

[0034] In an embodiment, the output beam comprises light, having wavelengths in the rage of 0.4 nm to 120 nm. The output beam may in particular comprise soft X-ray, EUV and/or VUV radiation.

[0035] In an embodiment, the molten metal comprises Sn, Li, In, Ga, Pb, Bi, Zn, and/or alloys thereof.

[0036] In an embodiment, an inductive heating system is used for starting the melting of target material.

[0037] In an embodiment, the laser-produced plasma light source comprises a nozzle, said nozzle supplying a gas flow, in particular a high-speed gas flow, to the interaction zone.

[0038] In an embodiment, the nozzle is positioned in the first opening and the laser beam is directed into the interaction zone through the nozzle.

[0039] In an embodiment, the gas comprises a noble gas.

[0040] In an embodiment, a flow velocity of the gas stream to the interaction zone is between 60 m/s and 300 m/s, and wherein a gas pressure within the interaction zone is between 5 mbar to 200 mbar.

[0041] In an embodiment, the nozzle is positioned at a distance of no greater than 2 mm from the interaction zone.

[0042] In an embodiment, the gas flow towards the interaction zone is directed to a vector of linear velocity of the target at an angle not exceeding 45 degrees. [0043] In an embodiment, the direction from interaction zone towards at least one of the first and second openings in the debris shield is significantly different from the direction of a predominant output of a droplet fraction and/or of an ion/vapor fraction of debris particles from the interaction zone.

[0044] In an embodiment, a vector of the linear velocity of the target in the interaction zone and at least one of the first and second openings in the debris shield are located on different sides of a plane passing through the interaction zone and the rotation axis. In other words, the vector of the linear velocity may point away from the side of the plane on which the first and second openings in the debris shield are located.

[0045] In an embodiment, the axis of at least one of the first and second openings in the debris shield is directed at an angle of less than 45° relative to a target surface in the interaction zone.

[0046] According to a further embodiment, a linear velocity of the target is high enough, more than 20 m/s, to influence a direction of a predominant output of microdroplet fractions of debris particles from the interaction zone.

[0047] In particular, a direction of the short- wavelength output beam that is output from the interaction zone may be different from the direction of the predominant output of the microdroplet fractions of debris particles.

[0048] In an embodiment, a replaceable membrane made of carbon nanotubes (CNT) or CNT membrane with high, more than 50% transparency in a wavelength range shorter than 20 nm, transmission is installed in a line-of-sight of the interaction zone, completely covering an aperture of the laser-produced plasma light beam.

[0049] In an embodiment, the target material is tin or its alloy. The rotating target assembly is configured to provide a linear velocity of the target that is large enough, more than 80 m/s, to suppress the output in the direction of the CNT membrane of the microdroplets with a size of more than 300 nm, which are capable of penetrating through the CNT membrane.

[0050] In an embodiment, the CNT membrane is coated on a side outside a line-of-sight of the interaction zone.

[0051] In an embodiment, the CNT membrane serves as a window between compartments of the vacuum chamber with high and medium vacuum.

[0052] In an embodiment, the pulsed laser beam consists of two parts: a pre-pulse laser beam and a main-pulse laser beam, parameters of which are chosen so as to suppress a fast ions fraction of the debris particles.

[0053] In an embodiment, a ratio of the pre-pulse laser beam energy to that of the main-pulse laser beam is less than 20%. A time delay between the pre-pulse and the main-pulse may be less than 10 ns.

[0054] In an embodiment, a laser pulse repetition rate of the pulsed laser beam is high enough to provide high-efficient evaporation of the microdroplet fractions of debris particles of a previous laser pulse by both laser-produced plasma light (i.e. by the emitted short- wavelength radiation) and fluxes of laser-produced plasma.

[0055] The laser pulse repetition rate of the pulsed laser beam may be of the order of 1 MHz so as to provide evaporation of the microdroplets with a size of up to 0.1 pm.

[0056] The laser-produced plasma light source may further comprise a laser system configured to generate the pulsed laser beam.

[0057] The laser-produced plasma light source may further comprise a collector mirror for collecting the output beam. For example, the CNT membrane may be arranged between the interaction zone and the collector mirror.

[0058] The technical result of the invention is the creation of X-ray, EUV and VUV radiation sources of high brightness with efficient, deep debris mitigation, characterized by increased service life, ease of operation and lower operating costs.

[0059] It should be clear that the features of the above embodiments can be combined with each other. In particular, it is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention.

[0060] 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.

BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Embodiments of the invention are explained with reference to the drawings, wherein:

[0062] Fig. 1 - Schematic diagram of high brightness LPP light source in accordance with an embodiment,

[0063] Fig. 2 - Transmission spectrum of CNT - membrane,

[0064] Fig. 3 - Simplified schematic of a high brightness LPP light source in accordance with the embodiment,

[0065] Fig. 4 - Tests results for microdroplets mitigation in the LPP light source,

[0066] Fig. 5A, Fig. 5B, Fig. 5C - SEM images, demonstrating achievement of debris mitigation effect in the high brightness LPP light source made in accordance with embodiments of the invention, [0067] Fig. 6, Fig. 7, Fig. 8 - schematic drawings of the high-brightness laser-produced plasma light source according to embodiments of this invention.

[0068] Fig. 9A and Fig. 9B - light source cross-section in the rotation plane of the rotating target assembly 2 and relative spread velocity diagrams of the droplet and ionic / vapor fractions of the debris particles for different embodiments of the invention.

[0069] Fig. 10, Fig. 11, Fig. 12 - Illustrations of debris mitigation using laser pre-pulse,

[0070] Fig. 13 - Illustration of the debris mitigation mechanism due to the high repetition rate of laser pulses.

[0071] In the drawings, the matching elements of the device have the same reference numbers.

[0072] These drawings do not cover and, moreover, do not limit the entire scope of options for implementing this technical solution, and they are only illustrative examples of particular cases of its implementation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0073] According to an example of invention embodiment illustrated in Fig. 1, the high-brightness laser-produced plasma light source comprises the vacuum chamber 1 with the rotating target assembly 2, which provides the target 3 being a layer of molten metal into the interaction zone 4. Said layer of molten metal is formed by the centrifugal force on the surface of the annular groove 6, implemented in the rotating target assembly. The laser beam 8 generated by the high pulse repetition rate laser or laser system is focused on the target in the interaction zone 4. The laser is placed outside the vacuum chamber, and the laser beam 8 is introduced through its input window 7. Interaction of the focused laser beam 8 with the target 3 in the interaction zone 4 results in producing high-temperature plasma of the target material. The high-temperature laser-produced plasma (LPP) emits light in one or more spectral ranges, which include VUV, EUV, soft X-ray, and X-ray with wavelengths of 0.4 to 120 nm. The output beam 9 of short- wavelength radiation, which leaves the interaction zone beyond the limits of the rotating target assembly, is intended for use in a device integrated with the LPP light source. The equipment using short-wavelength radiation may include a collector mirror 10 located in the clean optical compartment of vacuum chamber 1.

[0074] As a by-product, debris particles, which include micro droplets, vapors and ions of the target material, are generated in the interaction zone 4. To ensure that the high-brightness LPP light source is clean, it comprises the means for debris mitigation. Preferably, they contain casings 11, 12 which surround the laser beam 8 and the output beam 9, gas inlets 13 providing directional gas flows, sources of magnetic field, for example, in the form of permanent magnets 14 with magnetic cores, sources of electrostatic field (not shown), foil traps 15. The means for debris mitigation may in particular comprise a stationary debris shield, which is not shown in Figs. 6-9 and described further below.

[0075] The target material is kept in molten state by the power of laser radiation absorbed by the target. For initial melting of the target material the special heating system 16 may be employed, in which induction heating is used.

[0076] A part of the rotating target assembly 2 is made in the form of a disk 17 fixed to the rotation shaft 18 driven by a rotary drive 23. Said disk has a peripheral portion in the form of an annular barrier 19. On the internal surface of the annular barrier 19 facing the rotation axis 20 there is the annular groove 6, in which the target 3 is located on the surface of the annular groove facing the rotation axis 20. The annular groove configuration prevents material of the target 3 from being ejected in the radial direction and in both directions along the rotation axis 20, if the target material volume does not exceed the groove’s volume.

[0077] To ensure high stability of the target surface, a sufficiently high rotation frequency f is employed whereby the effect of the centrifugal force makes the surface of the liquid metal target 3 parallel to the rotation axis 20, i.e. it is essentially a circular cylindrical surface whose axis coincides with the rotation axis 20, Fig. 1..

[0078] A shock wave caused by the laser pulse inside the liquid metal target 3 after being reflected from the surface of the annular groove 6 can produce additional perturbations of the target surface in the interaction zone. Because of this, to ensure high stability of the target surface in the interaction zone the surface of the annular groove, at least near the interaction zone, is preferably inclined to the rotation axis.

[0079] As shown in Fig. 1, the surface of the groove 6 can be formed by a conical surface, facing the rotation axis 20, and two radial surfaces, not limited to this option.

[0080] In order to control the direction of microdroplets exiting from the interaction zone 4, the linear velocity of the target should be quite high, more than 20 m/s. Due to this the predominant direction of microdroplets exiting the interaction zone 4 becomes close to tangential. Therefore, to suppress the debris particles in the output beam 9 and in the short- wavelength beam of radiation, its direction is chosen to be significantly different from the direction of the predominant output of the microdroplets, which ensures the purity of the LPP light source.

[0081] In preferred embodiments of the invention, the means for debris mitigation include a replaceable CNT- membrane 21 with a high, more than 50%, transparency in the range of wavelengths shorter than 20 nm, installed in the line of sight of the interaction zone 4 and completely covering the aperture of the output beam 9, Fig. 1. [0082] The CNT membrane is an optical element in the form of a free-standing CNT film fixed on a frame, which has high strength, a sufficiently low absorption of radiation with a wavelength shorter than 20 nm and can be coated or filled to extend the service life or give other properties.

[0083] In order to change the CNT- membrane 21, a replacement unit 22 is introduced, for example, of the turret or cassette type, which can be driven from outside of the vacuum chamber 1 through a magnetic coupling, or through a gland, or through a miniature stepper motor installed in the vacuum chamber, not limited only to these options.

[0084] The CNT - membrane preferably has a thickness in the range of 20 to 100 nm, which ensures its high transparency in the range of wavelengths shorter than 20 nm, as illustrated in Fig. 2, which shows the transmission spectrum of the CNT - membrane with a thickness of about 100 nm measured using synchrotron radiation. It can be seen that in this range, the transparency exceeds 75%, amounting to about 90% at a wavelength of 13.5 nm. At the same time, the CNT - membrane can serve as a spectral filter that cuts off unwanted radiation, for example, as part of the laser radiation scattered in the interaction zone.

[0085] In addition, the CNT - membrane can serve as a solid base on which the coating is applied, for example, a metal foil that serves as a spectral purity filter, narrower in comparison with the CNT - membrane.

[0086] The high mechanical strength ultra-low gas permeability, high heat resistance and high durability, are the undoubted advantages of the CNT - membrane.

[0087] On one side of the CNT- membrane, a support grid with high, up to 98%, geometric transparency can be placed. In other embodiments, the CNT - membrane can be placed between two identical grids with high, up to 98%, geometric transparency, located without displacement relative to each other. It allows an increase in durability without a noticeable decrease in transparency, as well as an increase in the area of the CNT - membrane, thereby reducing the rate of its contamination and increasing its service life.

[0088] Due to its high strength and low permeability, the CNT - membrane can be used as an output window or a gas lock, for example, between the compartments of the vacuum chamber with medium and high vacuum. Thus, Fig. 1 shows a variant in which the CNT - membrane 21 serves as an output window of a LPP light source and a gas gate or lock between the casing 12 and a clean optical compartment of the vacuum chamber with a higher vacuum, in which the collector mirror 10 is placed.

[0089] At the same time, protective flows of inert buffer gas directed from both the CNT membrane 21 and input window 7 to the interaction zone 4 are supplied by means of gas inlets [0090] Fig. 3 presents a simplified diagram of a LPP light source according to the embodiment of this invention. In contrast to the design variant shown in Fig. 1, the laser beam 8 and the output beam 9 are located on both sides of the plane passing through the rotation axis 20 and the interaction zone 4. The LPP light source in accordance with the diagram Fig. 3 was used to test the debris mitigation measures. During the tests on the path of the output beam 9, a replaceable witness sample (not shown) made of mirror-polished silicon (Si) was installed.

[0091] The characteristic test parameters were as follows:

[0092] the target radius of rotation - 0.1 m.

[0093] linear target velocity - from 20 to 120 m/s

[0094] distance from the interaction zone to the Si-witness sample - 0.44 m

[0095] target material - eutectic alloy Sn/In at temperatures above 120 °C

[0096] exposure time - 5 hours or 1.08- 10 ⁹ pulses

[0097] wavelength, energy, duration and pulse repetition rate of the laser respectively - 1.06 pm, 0.44 mJ, 1.85 ns, 60 kHz.

[0098] Using a scanning electron microscope (SEM), the quantity and size of the debris particles deposited on the surface of the witness sample was calculated and determined.

[0099] In addition to the debris mitigation due to the rapid rotation of the target, it was also possible to use such debris mitigation techniques as magnetic mitigation and protective buffer gas flow.

[0100] The following tests were carried out:

[0101] 1st test: VR = 24 m/s, no other debris mitigation techniques are used,

[0102] 2nd test: VR = 24 m/s, other debris mitigation techniques are used,

[0103] 3rd test: VR = 120 m/s, other debris mitigation techniques are used except for the CNT - membrane,

[0104] 4th test: VR = 120 m/s, all debris mitigation techniques, including a CNT - membrane, are used.

[0105] In the first three tests the witness sample was installed instead of the CNT - membrane 21, in the fourth test the witness sample was installed closely behind the CNT - membrane 21.

[0106] Fig. 4 shows the results of measuring the quantity and size distribution of the microdroplets obtained in the 1st, 2nd and 3rd tests.

[0107] The results of the 1st test show that at low linear velocity without other debris mitigation techniques, microdroplets with a diameter of more than 300 nm play a major role in the deposition of Sn/In target material on the witness sample. During a week-long cycle of continuous operation, microdroplets of all sizes would cover more than 100% of the test specimen surface.

[0108] The results of the 2nd test show that the use of magnetic field and buffer gas flow were highly effective in suppressing debris such as ions and target material vapors, while the number of microdroplets with diameters greater than 300 nm is approximately 50 times less than the first test. Recalculation of the results shows that for a week-long cycle of continuous operation, microdroplets of all sizes would cover about 4% of the surface of the witness sample.

[0109] The results of the 3rd test showthat a high (V _R =120 m/s) target velocity practically fully eliminates 300+ nm microdroplets. This fact is important for highly efficient use of CNT - membranes for ultimate output beam cleaning. Recalculation of the results shows that for a week-long cycle of continuous operation, microdroplets of all sizes would cover only about 0.7% of the surface of the witness sample.

[0110] Fig. 5 A, Fig. 5B, Fig. 5C show SEM images of the witness samples obtained in the 2nd, 3rd, and 4th tests. In the 4th test the conditions were the same as in the 3rd test, but a CNT - membrane 21 was arranged in front of the witness sample. It can be seen that a low speed of rotation leads to a noticeable contamination of the sample, Fig. 5A. An increase in the linear target velocity from 24 to 120 m/s leads to a sharp increase in debris mitigation, Fig. 5B. Test results when using a CNT - membrane showed that ions and vapors of the target material do not penetrate through it. Only single microdroplets of about 400 and 500 nm in size penetrated the membrane, which indicates almost ultimate output beam cleaning, Fig. 5C.

[0111] Another result of the 4th test was the fact that the microdroplet deposition on the Si- witness sample is 45 times greater than on the CNT - membrane. This indicates that most of the microdroplets are reflected from the CNT - membrane, which is caused by non-wetting properties and high elasticity of the surface layer of the CNT - membrane. Therefore, in the case of presence of metallic or other such coatings on the CNT- membrane 21, it is preferably located on the side that is outside the line of sight of the interaction zone 4.

[0112] Based on the performed tests, it is estimated that microdroplets of more than 300 nm penetrate through the membrane with the probability of P _>3 oo, not exceeding 0.005: P _{> 300} £ 0.005. The measured S deposition rate of microdroplets of this type on the CNT - membrane corresponds to the coverage of 4- 10 ⁵ surfaces per weekly cycle of continuous operation. Accordingly, for a mirror 10 behind the CNT - membrane (Fig. 1), the rate of reflectivity loss due to the deposition of microdroplets of this size is estimated at S-P _{> 3} oo < 2· 10 ⁷ % per week of continuous operation. In other words, the degradation of 5% of the mirror surface behind the membrane is estimated to require 5- 10 ⁶ hours of continuous operation of the LPP light source.

[0113] The probability of P _< 3oo microdroplets with a diameter of less than 300 nm passing through the CNT membrane was estimated to be negligibly small: P _< 3oo £ 2- 10 ⁵ .

[0114] In preferred embodiments of the invention the target material is tin or its alloy and, based on the results, to ensure ultimate output beam cleaning the linear target velocity of more than 80 m/s is chosen to suppress the yield towards the CNT - membrane of microdroplets larger than 300 nm that can penetrate through it.

[0115] At a relatively small average laser power of 24 W, the EUV source brightness in the spectral band 13.5 +/-0.135 nm was B _{I3 5} = 60 W/mm ² -sr, and can be easily scaled up by increasing the laser power.

[0116] According to the invention, the debris shield 24 can be additionally used as one of the means for debris mitigation, as it is illustrated in Fig. 6. The debris shield 24 is rigidly mounted in order to surround the interaction zone 4; whereby the said shield comprises the first opening 25 forming the entrance for the laser beam 8, and the second opening 26 forming the exit for the output beam 9. In other words, the debris shield 24 is stationary and does not rotate together with the rotating target assembly. Feeding the laser beam 8 through the debris shield 24 and coupling out of the output beam 9 through the debris shield 24 is thus facilitated.

[0117] In the embodiment of invention illustrated by Fig. 6, the debris shield 24 is located opposite the angular sector of the target 3 near the interaction zone 4 and is separated from it by slit gaps 27, 28 on the ends. In other embodiments, the shield 24 may be circular.

[0118] Availability of the shield 24 results in deep mitigation of the debris particles output from the rotating target assembly and their return into the groove 6 after output from interaction zone. For deeper debris mitigation the shield 24 is separated from the rotating target assembly 2 by means of the slit gaps 27, 28, Fig. 6. In this case the target is located in the basically closed cavity 29 formed by surfaces of the groove 6 and the shield 24. Debris particles generated along with radiation in the interaction zone 4 can exit the cavity 29 only through two small openings 25, 26.

[0119] In the embodiments of invention the first and the second openings 25, 26 in the shield 24 are designed to be conical, and the apices of conical openings are located in the interaction zone 4. This allows minimizing the aperture of the openings 25, 26 in order to trap debris particles in the cavity 29 more efficiently. [0120] According to the invention, to ensure high conversion efficiency of laser energy to the output radiation at different wavelengths in the range of 0.4 to 200 nm, the target material is preferably selected from the group of nontoxic fusible metals, including Sn, Li, In, Ga, Pb, Bi, Zn and their alloys.

[0121] To obtain radiation in the wavelength range of over 20 nm, the CNT-membrane is not used, as its transparency in the said range falls significantly with increasing the radiation wavelength.

[0122] Preferably, the target linear velocity is over 80 m/s. This allows reducing ejection of the droplet fraction of debris particles out of the rotating target assembly significantly (by orders of magnitude) towards the openings 25, 26 in the shield 24 as compared to lower linear velocities.

[0123] In embodiments of the invention, one of which is schematically shown in Fig. 7, to ensure a deeper mitigation of the ionic / vapor fraction of the debris particles, the nozzle 30 is introduced and designed to form a high-speed gas flow directed into the interaction zone 4.

[0124] Part of the conical casing 11 entering the cavity 29 through the shield opening 25 can serve as the nozzle 30, as schematically shown in Fig. 7. In these embodiments of invention, the focused laser beam 8 is directed into the interaction zone 4 through the nozzle 30. As a result, deep mitigation of the ionic / vapor fraction of the debris particles is ensured in the pathway of the laser beam 8.

[0125] In other embodiments of the invention, the high-speed gas flow directed into the interaction zone 4 can be formed by the separately located nozzle 30, as illustrated by Fig. 8.

[0126] Argon or other inert gases and their mixtures are preferably used for blowing gas through the nozzle 30. Efficient mitigation of the ionic / vapor fraction of the debris particles is ensured when the speed of protective buffer gas flow directed into the interaction zone 4 is between 60 and 300 m/s and its pressure is 5 to 200 mbar. In order to ensure the above- mentioned gas flow parameters, the nozzle is preferably located at a small (not exceeding 2 mm) distance from the interaction zone 4. In the embodiment of invention illustrated by Fig. 8, the debris shield 24 is circular.

[0127] According to an embodiment, further means for debris mitigation in the pathway of the laser and output beams 8, 9 include such a configuration of the laser-produced plasma light source that the direction of the predominant output of the debris particles from the interaction zone is significantly different from the direction towards the openings 25, 26 in the debris shield 24.

[0128] In Fig. 9A and Fig. 9B the LPP light source cross-section in the rotation plane passing through the interaction zone, is schematically shown for embodiments of invention without the nozzle, Fig. 9A, and with the nozzle, Fig. 9B. Also, spread velocity diagrams of the droplet fraction 31 of the debris particles and of the ionic / vapor fraction 32 of the debris particles are schematically shown. As velocities of these two fractions can differ by as much as nearly an order of magnitude, diagrams are presented where the velocities are given relative to their maximum values.

[0129] As can be seen in Fig. 9A, the prevailing spread direction of the relatively slow droplet fraction of the debris particles 31 deviates towards the vector 33 of the target linear velocity VR in the interaction zone. Consequently, as one of the means for debris mitigation, a configuration of the light source is used, whereby the vector 33 of the target linear velocity in the interaction zone and at least one of the first and the second openings 25, 26 in the shield 24 are preferably located on different sides of the plane 34 passing through the interaction zone 4 and the rotation axis 20.

[0130] As illustrated in Fig. 9A, linear target velocity V _R has almost no effect on the spread direction of the ionic / vapor fraction 32 of debris particles. The direction of ions and vapors spread from the interaction zone 4 lies in the conically shaped area 32 whose axis is directed along the normal vector to the target surface in the interaction zone 4, and the apex angle does not exceed 90°. Consequently, in the embodiments of invention, a configuration of the light source is used as one of the means for debris mitigation, whereby at least one out of the radiation beam axes 7, 8 or of the openings 25, 26 is directed at an angle less 45 degrees to the target surface in the interaction zone. These embodiments of invention are illustrated in Fig. 6, Fig. 7, Fig. 8, where the output beam axis and, correspondingly, the axis of the second opening 26, are directed at an angle of over 45° to the rotation plane of the rotating target assembly 2 and at an angle less than 45 degrees to the target surface in the interaction zone.

[0131] In embodiments of the invention illustrated in Fig. 9B, the gas flow out of the nozzle 30 is directed into the interaction zone 4 at an angle to the vector 33 of the target linear velocity VR (i.e. to the tangential vector) that does not exceed 45 degrees. Due to this, the direction of the predominant output of ionic / vapor fraction 32 from the interaction zone also deviates towards the vector 33 of the target linear velocity VR and, as well as the droplet fraction 31, is not directed towards the first and the second openings 25, 26 in the debris shield 24.

[0132] The following embodiments of the invention are focused at further improving the complex of means for debris mitigation.

[0133] In an embodiment of the invention, illustrated by Fig. 10, the laser beam 8 consists of two parts: a pre-pulse laser beam with relatively low energy and the main pulsed laser beam, which is delayed for some time relative to the pre- pulse. In accordance with the invention, the laser pulse parameters are selected in such a way as to mitigate the fractions of fast ions of debris particles.

[0134] The RZLINE code was used for computational modeling. In one particular case, the laser energy in the pre-pulse is 0.4 mJ, the energy in the main pulse is 4 mJ, the delay between them is 5 ns, the size of the laser spot is 70 pm, the wavelength of the laser radiation is 1 pm, and the target material is tin. For this case, Fig. 11 shows the distribution of vapor density of the target material along the optical axis of the laser beam, created by the laser pre-pulse at a time of 6 ns. A laser beam with a wavelength of about 1 pm is absorbed at an atomic density of -3- 10 ¹⁹ cm ³ , i.e. inside an atomic cloud created at the target surface by a pre-pulse. This means that the plasma expanding from this point encounters more distant atoms from the target, thus reducing its velocity and losing kinetic energy.

[0135] The resulting ion energy distribution in the direction of normal to the target surface 6 ns after the start of the laser pulse is shown in Fig. 12. The simulation results are presented for various cases: without a pre-pulse and with a pre-pulse of different energy at a delay of 5 ns. Fig. 12 shows that the presence of the pre-pulse leads to a decrease in the maximum energy of ions by several times. A pre-pulse energy of 0.2 mJ at a delay of 5 ns is optimal in the case under consideration. In general, it is preferred to have a ratio of the pre-pulse laser beam energy to that of main-pulse less than 20% and a time delay between the pre-pulse and the main-pulse less than 10 ns.

[0136] In accordance with another embodiment of the invention, laser pulse repetition rate is chosen sufficiently high enough to provide highly efficient evaporation of the microdroplet fraction of the debris particles of the previous pulse by both short-wavelength radiation and fluxes of laser-produced plasma, Fig. 13. In accordance with this embodiment of the invention, at a sufficiently high pulse repetition rate, the microdroplet fraction of debris particles from the previous impulse does not have time to fly away from the interaction point a sufficient distance, so that short- wavelength radiation and plasma streams from the next impulse will effectively evaporate it.

[0137] Denoting laser pulse repetition rate as /, average laser power as P, and part of the laser energy used to generate short- wavelength radiation and plasma fluxes as c, drop velocity as V _d , target atom sublimation energy as f¾, and target atomic density as N _t , the evaporation condition of a microdroplet with diameter d can be written as follows:

[0138] where Q=P/fx - energy emitted as a result of one laser pulse in the form of short-wavelength radiation and plasma fluxes; L=V _d /f - distance travelled by a drop between two pulses. From (1) follows the limitation on the laser frequency:

f > (2/1.5)·p·ά·N,·E'·nϊc/(R·c) .

[0139] Taking reasonable estimates of parameters for a liquid Sn- target: N _t = 3.5- 10 ²² cm ³ , -£! _s -=3- 1.6- 10 ^-19 J/atom, P = 10 ³ W, c ~ 0.5, V _d = 3- 10 ⁴ cm/s will have:

/>10 ^u d [cm] sec ¹ .

[0140] This means, in particular, that at a laser pulse repetition rate of /> 10 ⁶ sec ¹ = 1 MHz it is possible to evaporate microdroplets with diameter d up to 0.1 p = 10 ⁵ cm. As seen from Fig. 4, the bulk of the droplets are characterized by a size of less than 0.1 microns. Thus, a high repetition rate of laser pulses in excess of 1 MHz provides a deep suppression of droplet fractions of debris particles.

[0141] The high-brightness laser-produced plasma light source is operated as described below and illustrated by Fig. 1, Fig. 6, Fig. 7, and Fig. 8.

[0142] The vacuum chamber 1 is evacuated using the oil-free vacuum pump system (not shown) to the pressure below 10 ⁵ -10 ⁸ mbar. At the same time, gas components, such as nitrogen, oxygen, carbon, etc., capable of interacting with the target material, are removed. After light source power-on the target material is transferred into molten state using the fixed heating system 16 which may employ induction heating.

[0143] The rotating target assembly 2 is actuated using the rotating drive unit 23, for example, an electric motor with a magnetic coupling, which ensures cleanliness of the vacuum chamber 1. Under the action of the centrifugal force, the target 3 is formed as the molten metal layer on the surface of the annular groove 6 facing the rotation axis 20.

[0144] The target 3 is exposed to the focused laser beam 8 with a high pulse repetition rate that can be in the range of 1 kHz to more than 1 MHz. Short- wavelength radiation is generated by the laser beam 8 heating the target material to a plasma-forming temperature. The laser-produced plasma emits light in the short- wavelength range including wavelengths of 0.4 to 120 nm. Depending on the laser radiation power density in the focal spot and the target material, short-wavelength radiation is generated mainly in the soft X-ray (0.4-10 nm) and / or EUV (10-20 nm) and / or VUV (20-120 nm) range.

[0145] Heat transfer from the target is ensured via the narrow (-0.2-0.4 mm) gap between the rotating target assembly 2 and the fixed water-cooled heat exchanger (not shown) through which gas is blown at the pressure of -1 mbar. Gas conductivity and area of contact are sufficient to remove up to 1.5 kW of thermal power for this type of cooling. At the same time, other cooling methods may be used for the rotating target assembly 2.

[0146] The output beam 9 is coming out of the high-temperature laser- produced plasma generated in the interaction zone 4, preferably, via the CNT-membrane 21 installed in the line-of-sight area of the interaction zone 4 and completely overlapping the aperture of the output beam 9. The CNT-membrane 21 ensures the output of short- wavelength radiation due to its high (>50%, preferably 80-90 %) transparency in the wavelength range below 20 nm. Simultaneously, the CNT-membrane 21 prevents debris particles from passing through, which ensures deep debris mitigation in the pathways to the collector mirror 10.

[0147] An important component of the technology to suppress the droplet fraction of debris particles is using a high linear velocity of the target which gives significant tangential velocity components to the droplets. This allows redirecting the bulk of the droplets sideways of the optical axes of the laser and output beams 7, 8. In an embodiment, the target material is tin (Sn) or its alloy. Using Sn or Sn alloy is preferable for achieving high brightness at 13.5 nm while having high conversion efficiency (CE _{I3 5} ) of laser energy into in-band EUV energy within 13.5 nm+/- 0.135nm spectral band used in manufacturing and metrology processes of EUV lithography. For a Sn- target the maximum spreading speed of the droplets is less than 100 m/s. Consequently, the target linear velocity over 80 m/s is used in embodiments of the invention, which, as demonstrated by calculations and measurements, achieves highly efficient mitigation of the droplet fraction of debris particles.

[0148] According to the invention, the interaction zone 4 is covered by the fixed (not rotating) debris shield 24 separated by the minimum slit gaps 27, 28 from the rotating target assembly and forming the basically closed cavity 29 around the target 3, the cavity having only two openings 25, 26 for the laser and output beams 7, 8. Due to the fact that the droplets velocity vector is mainly directed along the vector of the target linear velocity away from the openings 25, 26, the largest part of the droplet fraction of the debris particles after bouncing off the walls of the cavity 29 multiple times, remains inside it. At the same time, it is ensured that the largest part of the debris particles is returned into the annular groove 6, as the temperature of the debris shield 24 near the interaction zone is maintained above the target material melting temperature by the plasma and radiation generated in the interaction zone 4.

[0149] According to an embodiment, output of short- wavelength radiation from the interaction zone at an angle over 45° to the rotation plane helps reduce the flow of the droplet fraction of the debris particles by several times, and the flow of ionic / vapor fraction - by an order of magnitude. This happens because the indicatrix of debris particles spread from the interaction zone is a heterogeneous one. At the same time, intensity of the short-wavelength radiation under these angles varies insignificantly in relation to the angles of emission in the range of 0° to 45°.

[0150] In embodiments of the invention, the high-speed gas flow out of the nozzle 30 is used. It can be essentially a part of the casing 18 surrounding the laser beam, Fig. 7, or be a separate device as shown in Fig. 8, Fig. 9B.

[0151] In the embodiments of the invention, the gas flow is directed into the interaction zone 4 through the nozzle 30 at a small angle to the linear velocity vector 33 of the target 3, not exceeding 45 degrees, Fig. 8, Fig. 9B.

[0152] The nozzle 30 is located near the interaction zone at a small distance of 1-2 mm, so that the gas jet has a comparable momentum value relative to the momentum values of plasma and vapors spreading from the interaction zone. As a result of the interaction with the gas jet, the prevailing direction of ions and vapors spreading is deflected from the original direction, and the contaminant flow is not directed towards the pathways of the laser and output beams 7, 8. On the one hand, pressure in the gas flow must be high enough to efficiently deflect the spreading plasma and vapors, and on the other hand, it must not exceed the pressure under which an excessively high (over 20 %) short- wavelength radiation absorption is observed near the interaction zone 4. As demonstrated by evaluations, such a compromise is achieved if the appropriate speed of the vacuum chamber 1 evacuation is ensured.

[0153] In the preferable embodiments of the invention, in the fixed casings 11, 12 that surround a part of the laser beam 8 and a part of the output beam 9, using the gas inlets 13, protective buffer gas is continuously blown between the CNT- membrane 21 and the input window 7 towards the interaction zone 4, Fig. 1, Fig. 6. The gas flows protect the input window 7 and the CNT-membrane 21 from the ionic / vapor fraction of the debris particles, depositing debris particles on the walls of casings 11, 12 or of the foil traps 15, shown in, Fig. 1, Fig. 6.

[0154] Charged particles are also deposited on the surfaces of casings 11, 12 and of foil traps 15 using the magnetic field generated by the permanent magnets 14 located on the external surface of the casings 11, 12. The magnetic fields are preferably oriented cross-wise to the axis of the output beam 9 and the laser beam 8 which allows deflecting the charged particles from rectilinear motion to the CNT-membrane 21 and to the input window 7. This helps to increase the lifetime of the CNT-membrane 21 and of the input window 7 before replacement.

[0155] Overall, according to embodiments of the invention, one or more debris mitigation techniques such as magnetic mitigation, protective gas flows, foil traps 15, a replaceable membrane 21 comprising carbon nanotubes, the debris shield 24, a nozzle 30 supplying a high-speed gas flow to the interaction zone 4, a laser prepulse and a high laser pulse repetition rate, of order of 100 kHz or 1 MHz, are used to ensure a high cleanliness of the LPP light source. It should be clear that each of these mitigation techniques may be used in isolation, or two or more of the these mitigation techniques may be combined.

[0156] Thus, embodiments of the invention allow the creation of low-debris soft X-ray, EUV and VUV radiation sources with the highest brightness, long lifetime, and high ease of use.

[0157] Particular aspects of the subject-matter disclosed herein are set out in the following numbered clauses. The claims of the present disclosure or of any divisional application might be directed to one or more of these aspects.

[0158] 1. A high-brightness short- wavelength radiation source, containing a vacuum chamber with a rotating target assembly supplying a target into an interaction zone; an energy beam focused on the target in the interaction zone; and a useful short-wavelength radiation beam coming out of the interaction zone, wherein the rotating target assembly is made with an annular groove, the target is a layer of a target material being molten metal formed by a centrifugal force on a surface of the annular groove facing a rotation axis, and the energy beam is either a pulsed laser beam or an electron beam.

[0159] 2. The source according to clause 1, wherein the rotating target assembly is a disk with a peripheral part in a form of a ring barrier, on an inner surface of which, facing the axis of rotation, there is the annular groove with a surface profile preventing a release of the target material in a radial direction and in both directions along the axis of rotation.

[0160] 3. The source according to clause 1, wherein the short- wavelength radiation is generated by the energy beam heating the target material to a plasma-forming temperature.

[0161] 4. The source according to clause 1, wherein the energy beam is the electron beam, the rotating target assembly is a rotating anode of an electron gun, and the short- wavelength radiation is an X-ray radiation generated by an electron bombardment of the target.

[0162] 5. The source according to clause 1, wherein the target material is selected from fusible metals, including Sn, Li, In, Ga, Pb, Bi, Zn and their alloys.

[0163] 6. The source according to clause 1, additionally containing a replaceable membrane made of carbon nanotubes or CNT- membrane, which is installed in a line-of-sight of the interaction zone, completely covering an aperture of the short- wavelength radiation beam.

[0164] 7. A high-brightness short- wavelength radiation source, comprising a vacuum chamber with a rotating target assembly supplying a target into an interaction zone with a pulsed laser beam focused onto the target, which is a molten metal layer as a target material, the layer being formed by a centrifugal force on a surface of an annular groove, implemented in the rotating target assembly, and means for debris mitigation on the path of the short- wavelength radiation beam output wherein a linear velocity of the target is high enough, more than 20 m/s, to influence a direction of a predominant output of microdroplet fractions of debris particles from the interaction zone, a direction of a short- wavelength beam output from the interaction zone is different from the direction of the predominant output of the microdroplet fractions of debris particles, a replaceable membrane made of carbon nanotubes or CNT membrane with high, more than 50% transparency in a wavelength range shorter than 20 nm, transmission is installed in a line-of-sight of the interaction zone, completely covering an aperture of the short- wavelength radiation beam.

[0165] 8. The source according to clause 7, wherein the target material is tin or its alloy, the linear velocity of the target is large enough, more than 80 m/s, to suppress the output in the direction of the CNT membrane of the microdroplets with a size of more than 300 nm, which are capable of penetrating through the CNT membrane.

[0166] 9. The source according to clause 6, wherein one or more debris mitigation techniques such as electrostatic and magnetic mitigation, protective gas flows and foil traps are additionally used.

[0167] 10. The source according to clause 7, wherein the CNT membrane is coated on a side outside a line-of- sight of the interaction zone.

[0168] 11. The source according to clause 7, wherein the CNT membrane serves as a window between compartments of the vacuum chamber with high and medium vacuum.

[0169] 12. The source according to clause 7, wherein the pulsed laser beam consists of two parts: a pre-pulse laser beam and a main-pulse laser beam, parameters of which are chosen so as to suppress a fast ions fraction of the debris particles.

[0170] 13. The source according to clause 12, wherein a ratio of the pre-pulse laser beam energy to that of the main-pulse laser beam is less than 20% and a time delay between the pre-pulse and the main-pulse is less than 10 ns.

[0171] 14. The source according to clause 7, wherein a laser pulse repetition rate is high enough to provide high-efficient evaporation of the microdroplet fractions of debris particles of a previous pulse by both short- wavelength radiation and fluxes of laser-produced plasma.

[0172] 15. A high brightness X-ray source with a rotating anode, containing a vacuum chamber in which an electron beam produced by an electron gun, is directed to an interaction zone with a target, which is a layer of molten metal formed by a centrifugal force on a surface of an annular groove of a rotating anode.

[0173] 16. The source according to clause 15, containing a means for debris mitigation.

[0174] 17. The source according to clause 16, wherein a CNT membrane is installed on a path of an X-ray beam output. [0175] 18. The source according to clause 15, wherein the rotating anode is equipped with a liquid cooling system.

[0176] 19. The source according to clause 15, wherein a size of a focal spot of the electron beam on the target is less than 50 microns.

[0177] 20. The source according to clause 15, wherein a linear velocity of the target is more than 80 m/s.

[0178] 21. A laser-produced plasma light source, comprising a vacuum chamber with a rotating target assembly providing a target in an interaction zone with a laser beam focused on the target, which is a molten metal, a useful short-wavelength radiation beam exiting the interaction zone and means for debris mitigation, characterized in that a debris shield is rigidly mounted to surround the interaction zone, said shield comprising a first opening forming an entrance for said laser beam and a second opening forming an exit for a short- wavelength radiation beam.

[0179] 22. The laser-produced plasma light source according to clause 21, wherein the target is the molten metal layer, formed by a centrifugal force on a facing to an axis of rotation surface of an annular groove, implemented in the rotating target assembly.

[0180] 23. The laser-produced plasma light source according to clause 21, wherein the debris shield is circular.

[0181] 24. The laser-produced plasma light source according to clause 21, wherein slit gaps, separate the shield from the rotating target assembly.

[0182] 25. The laser-produced plasma light source according to clause 21, wherein at least one of the first and second openings in the debris shield is conical.

[0183] 26. The laser-produced plasma light source according to clause 21, wherein an axis of short wavelength radiation beam is directed at an angle of greater than 45° relative to a rotation plane of the rotating target assembly.

[0184] 27. The laser-produced plasma light source according to clause 21, wherein the rotating target assembly rotates the target with a linear velocity exceeding 80 m/s and one or more debris mitigation techniques such as protective gas flows, electrostatic and magnetic mitigation, foil traps and a membrane comprising carbon nanotubes are additionally used.

[0185] 28. The laser-produced plasma light source according to clause 21, wherein at least parts of the focused laser beam and the short- wavelength radiation beam are surrounded by casings, in which a protective gas flows are supplied.

[0186] 29. The laser-produced plasma light source according to clause 21, wherein the short- wavelength radiation beam comprises light, having wavelengths in the rage of 0.4 nm to 120 nm. [0187] 30. The laser-produced plasma light source according to clause 21, wherein the molten metal comprises Sn, Li, In, Ga, Pb, Bi, Zn, and/or alloys thereof.

[0188] 31. The laser-produced plasma light source according to clause 21, further comprising a nozzle, said nozzle supplying a high-speed gas flow to the interaction zone.

[0189] 32. The laser-produced plasma light source according to clause 31, wherein the nozzle is positioned in the first opening and the laser beam is directed into the interaction zone through the nozzle.

[0190] 33. The laser-produced plasma light source according to clause 31, wherein the gas comprises a noble gas.

[0191] 34. The laser-produced plasma light source according to clause 31, wherein a gas flow velocity to the interaction zone is between 60 m/s and 300 m/s, and wherein a gas pressure within the interaction zone is between 5 mbar to 200 mbar.

[0192] 35. The laser-produced plasma light source according to clause 31, wherein the nozzle is positioned at a distance of no greater than 2 mm from the interaction zone.

[0193] 36. The laser-produced plasma light source according to clause 31, wherein the gas flow towards the interaction zone is directed to a vector of linear velocity of the target at an angle not exceeding 45 degrees.

[0194] 37. The light source according to clause 21, wherein the direction from interaction zone towards at least one of the first and second openings, in the debris shield is significantly different from the direction of a predominant output of a droplet fraction and/or of ion/vapor fraction of debris particles from the interaction zone.

[0195] 38. The laser-produced plasma light source according to clause 36, wherein a vector of the linear velocity of the target in the interaction zone and at least one of the first and second openings, in the debris shield are located on different sides of a plane passing through the interaction zone and the axis of rotation.

[0196] 39. The laser-produced plasma light source according to clause 36, wherein the axis of at least one of the first and second openings in the debris shield is directed at an angle of less than 45° relative to a target surface in the interaction zone.

[0197] While specific embodiments are disclosed herein, various changes and modifications can be made without departing from the scope of the invention. The present embodiments are to be considered in all respects as illustrative and non-restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

INDUSTRIAL APPLICABILITY [0198] The proposed devices are designed for a number of applications, including microscopy, materials science, diagnostics of materials, biomedical and medical diagnostics, inspection of nano- and microstructures, including actinic inspection of lithographic EUV masks.