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 module

Technical Field

The present invention relates to high intensity radiation sources designed to produce soft X-ray, extreme Ultraviolet (EUV) and Vacuum Ultraviolet (VUV) radiation at wavelengths of about 0.4 to 200nm, and to radiation collection methods that provide efficient debris mitigation at large collection angles to ensure long term operation of high power light sources and their integrated devices.

Background

High intensity soft X-ray, extreme Ultraviolet (EUV) and Vacuum Ultraviolet (VUV) range radiation sources are used in many fields: for microscopy, biomedical and medical diagnostics, material testing, nanostructure analysis, atomic physics and lithography.

By focusing the radiation of the high-power laser on the target and in the discharge, it is possible to obtain plasmas that are emitted efficiently in the soft X-ray range (0.4-10 nm), EUV (10-20 nm) and VUV (20-120 nm).

According to international patent application PCT/EP2013/061941, published on 22 th month 8 of 2013, a Laser Produced Plasma (LPP) EUV light source, numbered WO 2014/00071, with a collector module comprising: a collector for collecting radiation generated by the radiation-generating plasma and guiding the generated radiation; means for suppressing infrared laser radiation in the plasma beam of radiation are also included.

The LPP EUV light source has the characteristic of high brightness. However, there is a problem of protecting the optical collector from debris to ensure a long lifetime of the LPP EUV light source.

During operation of the radiation source, the debris generated as a by-product of the plasma may be in the form of energetic ions, neutral atoms or vapors, as well as clusters of plasma fuel material. The debris particles degrade the collector optics, which may consist of one or more collector mirrors located near the radiation source. In addition to 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 located behind the collector mirror. There is an urgent need to develop a debris-free high-brightness short-wavelength radiation source.

From international patent application PCT/RU2012/000701 published as WO/2013/122505 at 22, 8, 2013, a laser triggered discharge plasma EUV light source is known. The focused laser beam is directed to one of the electrodes so that the laser-triggered discharge has an asymmetric, primarily curved banana-like shape. The intrinsic magnetic field of such a 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 the direction of the optical collector. In order to obtain high radiation power, the discharge is generated at a high pulse repetition rate. The present invention provides a simple and efficient mitigation of charged particles.

However, suppression of neutral particles and clusters requires the use of more complex debris mitigation techniques.

Light generation in the soft X-ray, EUV and VUV ranges is most efficient using lasers to generate plasmas. In recent years, the development of LPP radiation sources has been greatly facilitated by the development of projection extreme ultraviolet lithography techniques for mass production of Integrated Circuits (ICs) having a node of 7nm and below.

A debris mitigation technique based on an auxiliary plasma generated along the short wavelength beam path in a special injection gas is disclosed in us patent 9268031 published in 2016, 2 and 23. Fragments that acquire charge as a result of exposure to the auxiliary plasma are then deflected by the pulsed electric field. The method is effective for protecting an optical concentrator from the ion/vapor portion of the debris, for example, in sources using xenon as a plasma fuel.

However, in sources using metal as the plasma forming material, the main threat to the optical collector element is the droplet portion of the debris particles, for which this approach is not viable.

From us patent 8519366 published at 2013, 8, 27, a method of debris mitigation in an LPP EUV radiation source using a Sn droplet target is known. The method involves magnetically mitigating charged portions of the debris particles. In addition to this, the fragmentation technique also includes foil traps and ports for providing a buffer gas shield flow, which provides a sufficiently efficient trapping of neutral atoms and clusters of liquid metal target material.

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

The known fraction mitigation method of U.S. patent 7302043 published at 11/27 2007 is partially devoid of this drawback. The method uses a fast rotating shutter (shutter) capable of transmitting short wavelength radiation through at least one opening during one rotation period and preventing debris from passing through during another rotation period of the shutter. .

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

This deficiency is largely devoid of short wavelength radiation sources known from U.S. patent 10638588, U.S. patent 10588210, U.S. patent application 20200163197, U.S. patent No. 2020, U.S. patent No. 28, U.S. patent No. 3, and U.S. patent application No. 5, 2020, which are disclosed in the year 2020, which are incorporated herein by reference in their entirety. The sources disclosed in these patent documents comprise a vacuum chamber having a rotating target assembly. The complexity of the debris mitigation device includes the rotation of the target at high linear speeds of greater than 80 m/s. To suppress the ion/vapor portion of the fragments, a foil trap, magnetic field and directional flow of protective buffer gas are provided. In an embodiment of the radiation source, a replaceable carbon nanotube film (CNT film) is mounted in the path of the short wavelength radiation beam. In addition, a debris shield surrounding the plasma-emitting region is fixedly mounted so that the laser beam enters the pulsed plasma-emitting region and emits a short wavelength radiation beam therefrom. Laser pre-pulsing has also been proposed to suppress the ion portion of the fragments. Another proposed debris mitigation mechanism is to use high repetition rate laser pulses, e.g. about 1MHz, to ensure that droplets up to 0.1 μm in size generated by the preceding laser pulses are vaporized by irradiation and plasma of subsequent pulses.

These methods have a sufficiently high debris mitigation efficiency, however, they aim 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.

Disclosure of Invention

Thus, there is a need to eliminate at least some of the above drawbacks. In particular, there is a need for improved light sources that are compact, high power, high collection angles, and provide substantially 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 a multiple increase in the average power of pure 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 to create a high power high intensity short wavelength radiation source with efficient debris mitigation in a short wavelength radiation beam traveling at a large solid angle, preferably greater than 0.25 sr.

The object is achieved by a plasma short wavelength radiation source having a collector module comprising an optical collector located in a vacuum chamber, wherein the plasma emits short wavelength radiation, and means for reducing debris on the path of the short wavelength radiation to the 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 collector, each housing having a permanent magnet outside, a magnetic field being generated inside the housing, and the magnetic field formed by the permanent magnets removes charged portions of debris particles from the concentric beam to provide a debris-free concentric beam.

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

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

According to an embodiment of the invention, the area of the first face of each housing is larger than the area of the rest of the housing surface, 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 less than 30 degrees.

In an embodiment of the invention, the angle between adjacent faces of two adjacent housings is 3 to 10 degrees.

In an embodiment of the invention, the permanent magnets located on the most distant portions of the housing that are most distant from each other are connected by a magnetic core.

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

Preferably, the reflective surfaces of all mirrors form spheres, wherein one focus is a plasma and the other focus is all mirror focus of the optical collector.

Preferably, the debris mitigation device comprises a Carbon Nanotube (CNT) -based film 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 comprises a shielding gas flow, a sheath window inside each sheath that directs into the plasma while each CNT film acts as an exit sheath window for the debris-free concentric beam of short wavelength radiation, and a gas barrier that prevents shielding gas from exiting therethrough.

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

Preferably, the debris mitigation means comprises a foil plate placed in each housing 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: laser-produced plasma, z-pinch plasma, plasma focus, discharge-produced plasma, and laser-induced discharge-produced plasma.

Preferably, the plasma is a laser generated plasma of a liquid metal target provided to a laser beam focusing region by a rotating target assembly.

Preferably, the object is a layer of molten metal formed by centrifugal force on the surface of the rotation shaft of the annular groove, realized in the rotation object assembly.

In another aspect, the invention relates to a method of collecting radiation, comprising: the radiation emitted by the plasma at the plasma formation location is collected by an optical collector and at least a portion of the radiation is directed to a focal point, wherein the plasma radiation emitted by the plasma radiation is directed through at least two housings, which are equipped with means for debris mitigation.

Preferably, there is a permanent magnet outside each housing, a magnetic field is generated inside the housing, the magnetic field created by the permanent magnet slows down the charged portions of the debris particles, and other debris mitigation techniques are also used in each housing, including shielding gas flow, foil traps, CNT films, to provide a debris-free concentric bundle.

Preferably, the optical collector comprises a plurality of mirrors mounted in each debris-free concentric beam path, the reflective surfaces of all mirrors being located on the surface of the ellipsoid or modified ellipsoid, one of the foci being a plasma and the other being the foci of all mirrors of the optical collector.

The above and other objects, advantages and features of the present invention will become more apparent in the following non-limiting description of its embodiments, provided as an example with reference to the accompanying drawings.

Drawings

The drawings illustrate the essence of the invention, wherein:

fig. 1 and 2 are schematic diagrams of short wavelength radiation sources with multi-segment concentrator modules according to the present invention.

Fig. 3 and 4 are schematic diagrams of laser-produced plasma radiation sources with rotating target assemblies.

These figures do not cover, and do not limit, the full scope of options for implementing the technical solution, but represent only illustrative material for the particular case in which it is implemented.

Detailed Description

According to an example of embodiment of the invention, which is shown in fig. 1 to a different scale, the plasma radiation source comprises a vacuum chamber 1, said vacuum chamber 1 having a region of pulsed high temperature plasma 2 emitting short wavelength radiation. As a by-product, debris particles (including clusters of vapor, ions, and plasma-forming materials) are generated in the plasma region. The plasma radiation source further comprises a collector module comprising an optical collector 3 and means 4 for debris mitigation placed in the path of the short wavelength radiation beam 5 directed from the plasma 2 to the optical collector 3. The optical concentrator redirects short wavelength radiation to an intermediate focus and then to an optical system operating with the short wavelength radiation.

According to the invention, the means 4 for debris mitigation comprise at least two shells (casing) 6 arranged to output a debris-free concentric beam 7 of short wavelength radiation reaching the 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 light emitting plasma region), and therefore the plasma radiation source can be regarded as a quasi-point and the radiation beam emitted therefrom is concentric.

The exterior of each housing 6 has a permanent magnet 9, the permanent magnet 9 generating a magnetic field inside the housing 6, the magnetic field formed by the permanent magnet 9 removing charged portions of the debris particles from the concentric bundle 7 to provide a debris-free concentric bundle.

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

The exterior of each housing 6 has a permanent magnet 9 which generates a magnetic field inside the housing 6 with a magnetic induction vector which 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.

In contrast to known solutions, the debris mitigation device 4 according to the invention is a multi-stage system allowing to significantly increase the solid angle of collection of short wavelength plasma radiation while maintaining efficient debris mitigation. The increase in collection solid angle makes it possible to significantly (several times) increase the collection power of short wavelength radiation, thereby improving the efficiency of using such radiation sources in almost all application areas.

In a single-segment system, a simple increase in the lateral dimensions of the housing can lead to a dramatic decrease in the effectiveness of the magnetic protection of the charged particles. This is due to the fact that the larger the size of the sheath along the magnetic field lines, the lower the magnetic induction value in the sheath volume, which results in a reduced lateral velocity of the charged particles propagating from the plasma region emitting the short wavelength 3 through the sheath to the collector mirror 8. Thus, during the segment movement, the particles cannot deflect a sufficient distance to avoid striking the mirror. Experiments have shown that in order to operate magnetic protection effectively, the magnetic induction value at the center of the housing, which is about 40mm from the region of the plasma where the short wavelength is emitted, must not be less than 0.5T, and the angle over which the magnet is located should not exceed 30 degrees.

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

According to a preferred embodiment of the invention, the permanent magnets 9 located on the first sides 10 of the housings 6 furthest from each other are connected by means of magnetic cores 11. The magnetic core 11 is preferably made of soft magnetic steel, and 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 housing and thus improving the efficiency of the magnetic fragment mitigation.

In one embodiment of the invention, each housing 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 10 of the housing.

The radial orientation of the first and second faces 10, 12 relative to the plasma 2 provides a 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 area of the rest of the housing 6 surface, and the permanent magnets 9 are 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 housing 6 may be smaller than the area of the remaining surfaces of the housing, and the permanent magnets 9 may be located on the surface of the housing 6 outside its first face 10, e.g. on the large second 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 100nm, which ensures high strength and high transparency in the wavelength range of less than 20 nm. Thus, the CNT film 13 provides 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 film 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 comprises a shielding gas flow, which is guided into the plasma inside each enclosure 6, while each CNT film 13 simultaneously acts as an enclosure window, an outlet for the debris-free concentric beam 7 of short wavelength radiation, and a gas barrier preventing shielding gas from passing through its outlet.

At a shielding gas pressure of about 20Pa, providing an average vacuum in the enclosure increases the number of collisions between gas molecules and debris particles scattered from the plasma region, thereby causing them to deviate from rectilinear motion. At the same time, using CNT films as a gas seal allows the use of increased pressure only within the enclosure, rather than along the entire propagation path of concentric beams 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 20nm, the CNT film 13 is not used, since its transparency in the indicated range decreases drastically with increasing radiation wavelength.

In the preferred embodiment shown in fig. 2, the optical collector 3 comprises a plurality of mirrors 8, and the reflecting surfaces of all mirrors belong to a sphere 15 of revolution or in other words, one of the foci is the area of the pulsed emitting plasma 2 and the other is the focal point 16 of the mirror 8 of the optical collector 3. Such mirrors are very expensive to produce, since the roughness of the collector mirror substrate is only 0.2-0.3nm, and the cost of such mirrors, in particular mirrors with aspherical profiles, increases with increasing size according to the law of increasing area by a factor of 2-3. Thus, the use of several identical mirrors 8 significantly reduces the cost of the optical collector.

The pulsed emission plasma may be selected from: 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 metallic target material delivered by a rotating target assembly to a laser beam focal region, as described in published patent application 20200163197, month 5 and 21 of 2020, which is incorporated herein by reference in its entirety.

According to a preferred embodiment of the invention, schematically illustrated 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 rotating shaft 18. Figure 4 schematically shows an isometric view of a preferred embodiment of the invention.

The operation of the high brightness short wavelength radiation source in the preferred embodiment of laser generated plasma using the liquid metal target shown in fig. 3 and 4 proceeds as follows. The vacuum chamber 1 is pumped to 10 by an oil-free pumping system ^-5 …10 ^-8 Pressures below millibars remove gaseous components such as nitrogen, oxygen, carbon, etc. that are capable of interacting with the liquid metal target material.

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 its alloys, is transported to the interaction zone by rotating the target assembly together with the focused laser beam 21. The target is exposed to a focused pulsed laser beam 21 having a high pulse repetition rate in the range of 1kHz to 1 MHz. Short wavelength radiation of the laser plasma is generated in the soft X-ray and/or EUV and/or VUV spectral range depending on the target material and the laser power density on the target.

The short wavelength radiation beam 5 emitted by the plasma 2 is converted into a debris-free concentric beam by passing through the housing 6, preferably through the CNT film 13, towards the mirror 8 of the optical collector 3. The permanent magnet 9 (fig. 4) thus generates a constant magnetic field, preferably perpendicular to the axis of the concentric beam. The charged portions of the debris particles (mainly ions) deviate from a linear motion along the axis of the concentric beam 7 under the action of the lorentz force, collide with the inner wall of the housing 6 or the plate 22 placed in particular in the housing, as shown in fig. 3, by which the debris particles are captured. The 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 magnets 9. The plate 22 allows more efficient capturing of high-speed charged particles because the higher the speed of the particles, the smaller the lateral distance they deflect under the influence of the magnetic field. Together with this, the protective gas flow prevents the movement of the ionic/vapor portion of the debris particles, depositing them on the walls of the housing 6 and the plate 22, protecting the CNT film 13 from the debris. Due to the high transparency of the CNT film in the wavelength range shorter than 20nm, the CNT film provides an exit of the short wavelength light beam to the mirror 8 of the light collector 3. At the same time, the CNT film 13 prevents the passage of debris, thereby providing reliable protection for each mirror 8. Furthermore, the directional flow of shielding gas supplied through the gas inlet 14 by the tissue ensures efficient debris mitigation in the housing 6. The protective gas flow protects the CNT film 13 from the ionic/vapor portions of the fragments, thereby extending its useful life.

A similar debris mitigation method is also used along the path of the laser beam 21.

The above-described apparatus implements 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 a plasma formation location by an optical collector 3 and directing at least a portion of the radiation to a focal point 16 of fig. 2. The radiation beam 5 emitted by the plasma 2 is guided through at least two housings 6, the housings 6 being integrated with the means 4 for debris mitigation and being arranged to form a debris-free concentric beam 7 of short wavelength radiation exiting from the housings 6 to the optical collector 3.

Outside each housing, a permanent magnet 9 generating a magnetic field inside the housing 6 is used to slow down the charged part of the debris particles, other debris mitigation techniques including a protective gas flow, foil traps, CNT films are also used in each housing to provide a debris-free concentric bundle 7.

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 are located on the surface of an ellipsoid 15 or modified ellipsoid, with one focus being the plasma 2 and the other focus 16 being the focus of all mirrors 8 of the optical collector 3. The improved ellipsoidal shape can be used to provide improved intensity uniformity of the radiation collected in the far field as compared to a perfect ellipsoidal shape.

Therefore, the invention can create a soft X-ray, EUV and VUV radiation source with no fragments, strong and high brightness, and has long service life and easy use.

INDUSTRIAL APPLICABILITY

The proposed device is suitable for a variety of applications including microscopy, material science, material X-ray diagnostics, biomedical and medical diagnostics, nano-and microstructure 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.

Images