CN116195369A - Short wavelength radiation source with multi-segment collector module - Google Patents

Abstract

A radiation source comprises a collector module comprising an optical collector in a vacuum chamber with an emitting plasma, and further comprising means for debris mitigation, the means comprising at least two housings arranged to output concentric beams of short wavelength radiation without debris, the beams reaching the optical collector, preferably consisting of several identical mirrors. Each housing has a permanent magnet outside it that creates a magnetic field inside it to slow down the charged portions of the debris particles and provide a concentric beam of short wavelength radiation free of debris. Other debris mitigation techniques are also used. Preferably, the plasma is a laser generated plasma of a liquid metal target provided by a rotating target assembly to a laser beam focal region. The technical result of the present invention is a high power, high brightness, debris free short wavelength radiation source having a large collection solid angle, preferably greater than 0.25 sr.

Description

Short-wavelength radiation source with multi-segment collector modules

technical field

The present invention relates to high-brightness radiation sources designed to produce soft X-rays, extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) radiation with wavelengths of about 0.4 to 200 nm, and also to methods of radiation collection which operate at large collection angles Provide efficient debris mitigation to ensure the long-term operation of high-power light sources and their integrated equipment.

Background technique

Radiation sources in the high-intensity soft X-ray, extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) ranges are used in many fields: in microscopy, biomedicine and medical diagnostics, materials testing, nanostructural analysis, atomic physics and lithography.

By focusing the radiation of a high-power laser on the target and in the discharge, plasmas emitting efficiently in the soft X-ray range (0.4–10nm), EUV (10–20nm) and VUV (20–120nm) can be obtained.

According to the international patent application PCT/EP2013/061941, published on August 22, 2013, No. WO 2014/00071 A laser-produced plasma (LPP) EUV light source, with a collector module, comprising: a collector , for collecting radiation generated by the radiation-generating plasma, and for directing the generated radiation; and means for suppressing infrared laser radiation in the plasma radiation beam.

LPP EUV light source has the characteristics of high brightness. However, there is the issue of protecting the optical collector from debris to ensure the long lifetime of the LPP EUV light source.

During operation of the radiation source, debris generated as a by-product of the plasma can be in the form of energetic ions, neutral atoms or vapors, and clusters of plasma fuel material. Debris particles can degrade the collector optics, which may consist of one or more collector mirrors located near the radiation source. In addition to the droplets and particles deposited on the collector mirror reducing its reflectance, high velocity particles can damage the collector mirror and possibly other parts of the optical system behind the collector mirror . There is an urgent need to develop fragment-free sources of high-brightness short-wavelength radiation.

According to the international patent application PCT/RU2012/000701 published on 22 August 2013 as WO/2013/122505, a laser-triggered discharge plasma EUV light source is known. A focused laser beam is directed at one of the electrodes so that the laser-triggered discharge has an asymmetric, predominantly curved, banana-like shape. The intrinsic magnetic field of this discharge has a gradient which determines the main movement of the discharge plasma towards the region of weak magnetic field. The direction of the plasma flow is significantly different from that of the optical collector. In order to obtain high radiant power, the discharge is generated at a high pulse repetition rate. The present invention provides simple and efficient mitigation of charged particles.

However, suppressing neutrals and clusters requires the use of more sophisticated debris mitigation techniques.

Light generation in the soft X-ray, EUV and VUV ranges is most efficient using laser-generated plasmas. The development of LPP radiation sources has been largely facilitated in recent years by the development of projection EUV lithography for mass production of integrated circuits (ICs) with the 7nm node and below.

In US Patent No. 9,268,031 published on February 23, 2016, a debris mitigation technique based on an assisted plasma generated in a special injected gas along the path of a short-wavelength radiation beam is disclosed. Fragments that have acquired charge as a result of exposure to the auxiliary plasma are then deflected by the pulsed electric field. The method is effective for protecting optical concentrators from the ion/vapor fraction of debris, for example, in sources using xenon as the plasma fuel.

However, in sources using metals as the plasma-forming material, the main threat to the optical collector elements is the droplet fraction of debris particles, which this approach does nothing about.

From US patent 8519366 published on 27 August 2013, a method of debris mitigation in LPPE UV radiation sources using Sn droplet targets is known. The method involves magnetically slowing charged portions of debris particles. In addition to this, the fragmentation technique also includes foil traps and ports for providing a shielded flow of buffer gas, which provides sufficiently efficient trapping of neutral atoms and clusters of the liquid metal target material.

However, additional, rather complex methods are required to slow down the droplet fraction of debris particles.

The debris mitigation methods known from US Patent 7,302,043, published November 27, 2007, do not partly have this drawback. The method uses a rapidly rotating shutter capable of transmitting short wavelength radiation through at least one opening during one rotation cycle and preventing the passage of debris during another rotation cycle of the shutter. .

However, using this approach to reduce debris in a compact radiation source is technically too difficult to achieve.

This deficiency largely lacks the knowledge already known from US Patent 10638588 published on April 28, 2020, US Patent 10588210 published on March 10, 2020, and US Patent Application 20200163197 published on May 5, 2020. A source of short wavelength radiation, which is incorporated by reference in its entirety into this specification. The sources disclosed in these patent documents comprise a vacuum chamber with a rotating target assembly. The complexity of debris mitigation devices includes the target rotating at high linear velocities greater than 80m/s. To suppress the ionic/vapor fraction of debris, foil traps, magnetic fields, and directed flow of protective buffer gas are provided. In an embodiment of the radiation source, a replaceable carbon nanotube membrane (CNT membrane) is installed in the path of the short wavelength radiation beam. In addition, a debris shield surrounding the emitting plasma region is fixedly mounted so that the laser beam enters the pulsed emitting plasma region and emits a short wavelength radiation beam therefrom. It is also suggested to use laser pre-pulsing to suppress the ionic portion of the fragments. Another suggested debris mitigation mechanism is the use of high repetition rate laser pulses, for example, around 1 MHz, to ensure that droplets up to 0.1 μm in size produced by previous laser pulses are evaporated by radiation and plasma of subsequent pulses.

These methods have sufficiently high debris mitigation efficiencies, however, they are aimed at collecting relatively small spatial angles of short-wavelength plasma radiation and, therefore, the average power of the short-wavelength radiation beam is insufficient for many applications.

Contents of the invention

Therefore, there is a need to eliminate at least some of the above-mentioned disadvantages. In particular, there is a need for improved light sources that are compact, high powered, have large collection angles, and provide essentially complete debris mitigation in the path of the output beam of short wavelength radiation.

The present invention aims to solve the technical problems associated with the multiplication of the average power of purely high-brightness sources of soft X-ray, EUV and VUV radiation, while ensuring their commercial availability and economical operation.

The technical result of the present invention is the creation of a high power high brightness short wavelength radiation source with efficient debris mitigation in a short wavelength radiation beam propagating at a large solid angle, preferably greater than 0.25 sr.

The object can be achieved by a plasma short-wavelength radiation source having a concentrator module comprising an optical concentrator located in a vacuum chamber, wherein the plasma emits short-wavelength radiation, further comprising means for reducing debris on the path of said short wavelength radiation to said optical collector.

The source is characterized in that the debris mitigation means comprises at least two housings arranged to output a debris-free concentric beam of short-wavelength radiation reaching the optical concentrator, each housing having a permanent magnet on the outside, generating a magnetic field inside the housing, And the magnetic field formed by the permanent magnet removes the charged part of the debris particles from the concentric beam to provide a debris-free concentric beam.

Preferably, the outer surface of each enclosure comprises two first faces extending substantially parallel to the direction of propagation of short wavelength radiation from the plasma and parallel to the vertical or another selected direction.

Preferably, each enclosure comprises two second faces extending substantially parallel to the direction of propagation of short wavelength radiation from the plasma and substantially perpendicular to the two first faces of the enclosure.

According to an embodiment of the invention, the area of the first face of each housing is larger than the rest of the surface of the housing, and the permanent magnet is substantially in contact with the first face of each housing.

According to an embodiment of the invention, the area of the first face of each housing is smaller than the area of the rest of the housing surface, and the permanent magnets are located outside the first face of the housing surface.

According to an embodiment of the invention, the angle between the two first faces of each housing is smaller than 30 degrees.

In an embodiment of the present invention, the angle between adjacent surfaces of two adjacent shells is 3 to 10 degrees.

In an embodiment of the invention, the permanent magnets located on the furthest parts of the housings furthest from each other are connected by magnetic cores.

In an embodiment of the invention, the optical collector comprises a plurality of mirrors mounted in the path of each fragment-free concentric beam of short wavelength radiation.

Preferably, the reflective surfaces of all mirrors form a sphere, one of the focal points being the plasma and the other focal point being the focal point of all mirrors of the optical concentrator.

Preferably, the debris mitigation means comprises a carbon nanotube (CNT) based membrane mounted between each housing and a light collector in the path of the short wavelength radiation beam.

In an embodiment of the invention, the debris mitigation device includes a shielding gas flow that directs the incoming plasma inside each enclosure, while each CNT film serves as an enclosure window for the exit of a fragment-free concentric beam of short-wavelength radiation, and prevents the shielding gas from The gas baffle through which it exits.

In an embodiment of the invention, permanent magnets are positioned along the entire length of the housing.

Preferably, the debris mitigation means comprises a foil plate placed in each enclosure and oriented radially with respect to the plasma, substantially perpendicular to the magnetic field lines.

In an embodiment of the invention, the plasma may be selected from the group consisting of: laser-generated plasma, z-pinch plasma, plasma focus, discharge-generated plasma, and laser-induced discharge-generated plasma.

Preferably, the plasma is laser-generated plasma provided by the rotating target assembly to the liquid metal target in the laser beam focus region.

Preferably, the target is a layer of molten metal, formed by centrifugal force on the surface of the axis of rotation of the annular groove, achieved in a rotating target assembly.

In another aspect, the invention relates to a method of collecting radiation comprising: collecting, by an optical concentrator, radiation emitted by a plasma at a location where the plasma is formed, and directing at least a portion of the radiation to a focal point, wherein the radiation emitted by the plasma The plasma radiation is directed through at least two enclosures equipped with means for debris mitigation.

Preferably, there are permanent magnets on the outside of each enclosure, a magnetic field is generated inside the enclosure, the magnetic field formed by the permanent magnets slows down the charged portion of debris particles, and other debris mitigation techniques are also used in each enclosure, including shielded airflow, foil Trap, CNT membrane to provide debris-free concentric beams.

Preferably, the optical collector comprises a plurality of mirrors mounted in each debris-free concentric beam path, all mirrors having reflective surfaces on the surface of an ellipsoid or modified ellipsoid, one of the focal points being the plasma and the other The focal point is the focal point of all the mirrors of the optical collector.

The above and other objects, advantages and features of the present invention will be more apparent from the following non-limiting description of embodiments thereof, provided as examples with reference to the accompanying drawings.

Description of drawings

The essence of the invention is illustrated by the accompanying drawings, in which:

Fig. 1 and Fig. 2 are schematic diagrams of a short-wavelength radiation source with a multi-segment optical collector module according to the present invention.

Figure 3, Figure 4 – Schematic diagram of a laser-generated plasma radiation source with a rotating target assembly.

These figures do not cover, nor limit, the entire range of options for implementing the technical solution, but merely represent illustrative material for a particular case of its implementation.

Detailed ways

According to the example of an embodiment of the invention shown in different scales in FIG. 1 , the plasma radiation source comprises a vacuum chamber 1 having a region of pulsed high-temperature plasma 2 emitting short-wavelength radiation. As a by-product, debris particles (including vapors, ions, and clusters of plasma-forming materials) are generated in the plasma region. The plasma radiation source also comprises a concentrator module comprising an optical concentrator 3 and a device for debris placed on the path of the short wavelength radiation beam 5 directed from the plasma 2 to the optical concentrator 3 Mitigation device4. Optical concentrators redirect short-wavelength radiation to an intermediate focal point and then to an optical system that operates with short-wavelength radiation.

According to the invention, the device 4 for debris mitigation comprises at least two casings 6 arranged to output a debris-free concentric beam 7 of short-wavelength radiation reaching an optical collector 3, preferably consisting of a plurality of mirrors 8 . A typical plasma size is about 0.1mm (measured as the FWHM of the free electron density or the FWHM of the brightness distribution of the luminous plasma region), therefore, the plasma radiation source can be considered as a punctual point, and the radiation beam emanating from it is concentric of.

On the outside of each housing 6 there is a permanent magnet 9 which generates a magnetic field inside the housing 6, the magnetic field formed by the permanent magnet 9 removes charged parts of debris particles from the concentric beam 7 to provide a debris free concentric beam.

The outer surface of each enclosure 6 comprises two first faces 10 substantially parallel to the direction of propagation of the short wavelength radiation from the plasma 2 and extending parallel to the vertical or another selected direction.

On the outside of each housing 6 there is a permanent magnet 9 which generates a magnetic field inside the housing 6 whose magnetic induction vector is substantially perpendicular to the optical axis of the housing.

Preferably, the permanent magnets 9 are positioned along the entire length of the housing 6 .

Compared to known solutions, the debris mitigation device 4 according to the invention is a multi-segment system that allows a significant increase in the solid angle of collection of short-wavelength plasma radiation while maintaining efficient debris mitigation. The increase in the collection solid angle makes it possible to significantly (several times) increase the collection power of short-wavelength radiation, thereby increasing the efficiency of using such radiation sources in almost all fields of application.

In a single-section system, a simple increase in the lateral dimension of the enclosure can lead to a drastic decrease in the effectiveness of the magnetic protection against charged particles. This is due to the fact that the larger the size of the enclosure along the field lines of the magnetic field, the lower the magnetic induction value in the enclosure volume, which causes charged particles to propagate through the enclosure from the plasma region emitting short wavelengths to the collector mirror The lateral speed of 8 is reduced. Therefore, during said segment of motion, the particles cannot be deflected far enough to avoid hitting the mirror. Experiments have shown that in order to effectively operate the magnetic protection, the magnetic induction value at the center of the enclosure about 40mm from the plasma region emitting short wavelength must not be less than 0.5T, and the angle at which the magnet is located should not exceed 30 degrees.

Therefore, using a multi-stage debris mitigation system (where the plane angle between the enclosure surfaces does not exceed 30 degrees), a constant magnetic field of sufficient magnitude can be generated in each enclosure for efficient magnetic mitigation of charged particles.

According to a preferred embodiment of the invention, the permanent magnets 9 located on the first face 10 of the housing 6 furthest from each other are connected by a magnetic core 11 . The magnetic core 11 is preferably made of soft magnetic steel. By concentrating the magnetic field in the magnetic core, the loss of the magnetic field due to scattering can be reduced, thereby increasing the volume of each shell, thereby improving the efficiency of magnetic debris mitigation.

In one embodiment of the invention, each enclosure 6 comprises two second faces 12 extending substantially parallel to the direction of propagation of the short-wavelength radiation from the plasma 2 and substantially perpendicular to the two first faces of the enclosure 10.

The radial orientation of the first and second faces 10, 12 relative to the plasma 2 provides high geometric transparency for the multi-stage debris mitigation system. In an embodiment of the invention, the angle between adjacent faces of two adjacent housings 6 is in the range of 3 to 10 degrees.

In a preferred embodiment of the invention, the area of the first face 10 of each housing 6 is larger than the rest of the surface of the housing 6 , and the permanent magnet 9 is substantially in contact with the first face in each housing 6 .

In another embodiment (not shown), the area of the first face 10 of each shell 6 can be smaller than the area of the remaining surface of the shell, and the permanent magnet 9 can be located on the surface of the shell 6, on the first face 10 thereof. Also, for example, on the second large face 12 of each housing 6 .

The debris mitigation device 4 preferably comprises a carbon nanotube film 13 mounted between each housing 6 in the path of the concentric beam 7 and the mirror 8 of the optical collector 3 . The thickness of the CNT film is preferably in the range of 20 to 100 nm, which ensures its high strength and high transparency in the wavelength range of less than 20 nm. Thus, the CNT film 13 provides for the exit of the concentric beam 7 due to its high transparency in the wavelength range shorter than 20 nm. At the same time, the CNT membrane 13 prevents the passage of debris particles, thereby providing a debris-free concentric beam 7 of short-wavelength radiation.

In addition to this, the debris mitigation device includes protective gas flows that are guided into the plasma within each enclosure 6, while each CNT film 13 simultaneously acts as an enclosure window for a fragment-free concentric beam of short-wavelength radiation 7 outlet, and a gas baffle that prevents the shielding gas from passing through its outlet.

With a shielding gas pressure of about 20 Pa, providing an average vacuum in the enclosure increases the number of collisions between gas molecules and debris particles scattered from the plasma region, causing them to deviate from linear motion. At the same time, the use of the CNT membrane as a gas seal allows the use of increased pressure only within the enclosure, rather than along the entire propagation path of the concentric beam 7 to the consumer optics. This reduces the loss of short wavelength radiation due to absorption in the gas.

In order to obtain radiation in the wavelength range greater than 20 nm, the CNT film 13 is not used, since its transparency in the indicated range decreases sharply with increasing radiation wavelength.

In the preferred embodiment shown in FIG. 2 , the optical collector 3 comprises a plurality of mirrors 8 , while the reflective surfaces of all mirrors belong to spheroids or in other words, spheres 15 , one of the focal points of which is the pulsed emission plasma 2 The other focal point is the focal point 16 of the mirror 8 of the optical concentrator 3 . Such mirrors are very expensive to produce because the collector mirror substrate has a roughness of only 0.2–0.3 nm, and the cost of such mirrors, especially with aspheric profiles, increases by 2- 3-fold regularity, increasing as its size increases. Thus, the use of several identical mirrors 8 significantly reduces the cost of the optical collector.

The pulsed emission plasma may be selected from the group consisting of: laser generated plasma, z-pinch plasma, plasma focus, discharge generated plasma, laser triggered discharge plasma.

In a preferred embodiment, the pulsed high-temperature plasma is a laser plasma of a liquid metal target material delivered by a rotating target assembly to the focal region of the laser beam, as described in patent application 20200163197 published on May 21, 2020, which The application is hereby incorporated by reference in its entirety.

According to a preferred embodiment of the invention shown schematically in FIG. 3 , the target 17 is a layer of molten metal formed by centrifugal force on the surface of the annular groove 19 of the rotating target assembly 20 facing the axis of rotation 18 . Figure 4 schematically shows an isometric view of a preferred embodiment of the invention.

Operation of the high-brightness short-wavelength radiation source in the preferred embodiment of the laser-generated plasma using the liquid metal target shown in Figures 3 and 4 proceeds as follows. The vacuum chamber 1 is pumped by an oil-free pumping system to a pressure below 10 ^-5 ... 10 ^-8 mbar to remove gaseous components that can interact with the liquid metal target material, such as nitrogen, oxygen, carbon, etc.

The target 17, whose material belongs to the group of non-toxic low-melting point metals, including Sn, Li, In, Ga, Pb, Bi, Zn and their alloys, is delivered to the interaction zone together with the focused laser beam 21 by rotating the target assembly. The target is exposed to a focused pulsed laser beam 21 with a high pulse repetition rate in the range of 1 kHz to 1 MHz. Depending on the target material and the laser power density on the target, short-wavelength radiation of the laser plasma is generated in the soft X-ray and/or EUV and/or VUV spectral range.

The short-wavelength radiation beam 5 emitted by the plasma 2 passes through the housing 6 , preferably through the CNT film 13 , is converted into a fragment-free concentric beam, and is directed towards the mirror 8 of the optical collector 3 . Thus, the permanent magnet 9 (Fig. 4) generates a constant magnetic field, preferably perpendicular to the axis of the concentric beam. Under the action of the Lorentz force, the charged parts of the debris particles (mainly ions) deviate from the linear motion along the axis of the concentric beam 7 and collide with the inner wall of the housing 6 or a plate 22 specially placed in the housing, as shown in Fig. 3, debris particles are captured by it. A plate 22 mounted in the housing 6 is directed radially towards the plasma 2 and preferably perpendicular to the magnetic field lines generated by the magnet 9 . The plate 22 allows a more efficient trapping of high-speed charged particles, since the higher the speed of the particles, the smaller is the lateral distance that they are deflected under the influence of the magnetic field. Together with this, the protective gas flow prevents the movement of the ionic/vapour fraction of the debris particles, depositing it on the walls of the housing 6 and plate 22, protecting the CNT film 13 from debris. Due to the high transparency of the CNT film in the wavelength range shorter than 20 nm, the CNT film provides the exit of the short wavelength light beam to the mirror 8 of the collector 3 . At the same time, the CNT film 13 prevents debris from passing through, thereby providing reliable protection for each mirror 8 . Furthermore, effective debris mitigation in the housing 6 is ensured by organizing the directional flow of the shielding gas supplied through the gas inlet 14 . The shielding gas flow protects the CNT membrane 13 from the ionic/vapor portion of the debris, thereby prolonging its lifetime.

Along the path of the laser beam 21, similar debris mitigation methods are used.

The apparatus described above enables a particular embodiment of the invention, one aspect of which relates to a method of collecting radiation. The method comprises collecting short wavelength radiation emitted by the plasma 2 at the plasma formation location by an optical collector 3 and directing at least a portion of the radiation to the focal point 16 of FIG. 2 . The radiation beam 5 emitted by the plasma 2 is directed through at least two enclosures 6 integrated with the means for debris mitigation 4 and arranged to form a debris-free concentric beam 7 of short-wavelength radiation exiting the enclosures 6, reaching The optical collector 3.

Outside each enclosure, permanent magnets 9 that generate a magnetic field inside the enclosure 6 are used to slow down the charged portion of the debris particles, other debris mitigation technologies including shielded airflow, foil traps, CNT membranes are also used in each enclosure to provide Concentric bundles of fragments7.

The optical collector 3 preferably comprises a plurality of mirrors 8 mounted in the path of each debris-free concentric beam 7, and the reflective surfaces of all mirrors lie on the surface of an ellipsoid 15 or a modified ellipsoid, one of the focal points is the plasma 2 and the other focal point 16 is the focal point of all mirrors 8 of the optical concentrator 3 . An improved ellipsoidal shape may be used to provide improved intensity uniformity of radiation collected in the far field compared to a perfect ellipsoidal shape.

Thus, the present invention enables the creation of fragment-free, powerful, high-brightness sources of soft X-ray, EUV and VUV radiation that are long-lasting and easy to use.

Industrial applicability

The proposed device is suitable for a variety of applications, including microscopy, materials science, materials X-ray diagnostics, biomedical and medical diagnostics, nano- and microstructural inspection, including photomask defect inspection for EUV lithography.

Claims (20)

1. A plasma short wavelength radiation source having a collector module, comprising: an optical collector (3) located in a vacuum chamber (1) having a plasma (2) emitting short wavelength radiation, and further comprising means (4) for reducing debris on the short wavelength radiation path to the optical collector (3), wherein

The device (4) for debris mitigation comprises at least two housings (6) arranged to output a debris-free concentric beam (7) of the short wavelength radiation reaching the optical collector (3),

and outside each housing (6) there is a permanent magnet (9) which generates a magnetic field inside the housing (6) and the magnetic field formed by the permanent magnets (9) removes the charged portions of the debris particles from the concentric bundle (7) to provide a debris-free concentric bundle.

2. The source of claim 1, wherein the outer surface of each housing comprises two first faces (10) extending substantially parallel to the direction of propagation of short wavelength radiation from the plasma (2), and optionally the two first faces are parallel to the vertical direction.

3. The source of claim 2, wherein each housing (6) comprises two second faces (12) extending substantially parallel to a direction of propagation of short wavelength radiation from the plasma (2) and substantially perpendicular to the two first faces (10) of the housing.

4. A source according to claim 2, wherein the area of the first face (10) of each housing (6) is larger than the area of the remaining surface of the housing (6), and the permanent magnet (9) is substantially in contact with the first face (10) of each housing (6).

5. A source according to claim 2, wherein the first face (10) of each housing (6) has an area smaller than the area of the remaining surface of the housing (6), and the permanent magnet (9) is located on the surface of the housing (6) outside its first face (10).

6. The source of claim 2, wherein the angle between the two first faces (10) of each housing (6) is less than 30 degrees.

7. A source according to claim 2, wherein the angle between adjacent faces of the two adjacent housings (6) is 3 to 10 degrees.

8. A source according to claim 1, wherein the permanent magnets (9) located on the mutually most distant parts of the mutually most distant housing (6) are connected by means of a magnetic core (11).

9. The source of claim 1, wherein the optical collector (3) comprises a plurality of mirrors (8) mounted in the path of each of the debris-free concentric beams (7).

10. A source according to claim 9, wherein the reflecting surfaces of all mirrors (8) form spheres (15), one of the foci being the plasma (2) and the other focus (16) being the focus of all mirrors of the optical collector.

11. The source according to claim 1, wherein the means (4) for debris mitigation comprise a membrane (13) based on Carbon Nanotubes (CNTs) mounted between each housing (6) and the optical collector (3).

12. The source according to claim 11, wherein the means (4) for debris mitigation comprises a flow of shielding gas which is guided into the plasma inside each enclosure (6), while each CNT film (13) simultaneously acts as an enclosure window for exit of the debris-free concentric beam (7) of short wavelength radiation and as a gas barrier for preventing exit of the shielding gas therethrough.

13. A source according to claim 1, wherein the permanent magnet (9) is positioned along the entire length of the housing.

14. A source according to claim 1, wherein the means (4) for debris mitigation comprise a foil plate (22) placed in each of the housings (6) and oriented in a radial direction with respect to the plasma (2), substantially perpendicular to the magnetic field lines.

15. The source of claim 1, wherein the plasma may be selected from: laser-produced plasma, z-pinch plasma, plasma focus, discharge-produced plasma, and laser-triggered discharge plasma.

16. The source of claim 1, wherein the plasma is a laser generated plasma of a liquid metal target (17) provided by a rotating target assembly (20) to a focal region of a laser beam (21).

17. The source according to claim 16, wherein the target (17) is a layer of molten metal formed by centrifugal force on a surface of an annular groove (19) facing the rotation axis (18), which is embodied in the rotating target assembly (20).

18. A method of collecting radiation, comprising: collecting radiation emitted by the plasma by an optical collector at a plasma formation location and directing at least a portion of the radiation emitted by the plasma radiation to a focal point, wherein

Radiation emitted by the plasma radiation is directed through at least two housings provided with means for debris mitigation and arranged to form a debris-free concentric beam of short wavelength radiation exiting the housings to the optical collector.

19. The method of claim 18, wherein a permanent magnet generating a magnetic field inside the enclosure is used to slow down the charged portion of the debris particles outside each enclosure, and optionally includes a shielding gas flow, foil trap, CNT film debris mitigation element.

20. The method of claim 18, wherein the optical collector comprises a plurality of mirrors mounted in the path of each of the debris-free concentric beams, and the reflective surfaces of all mirrors are located on the surface of an ellipsoid or modified ellipsoid, with one focus being the plasma.

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