JP7798274B2 - High-intensity laser-produced plasma source and radiation generation and collection method - Google Patents

Description

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

This invention relates to high-brightness laser-produced plasma (LPP) light sources designed to generate soft X-ray, extreme ultraviolet (EUV), and vacuum ultraviolet (VUV) radiation in wavelengths between approximately 0.4 and 200 nm, and which provide highly effective debris mitigation to ensure long-term operation of the light source and its integrated equipment, as well as methods for generating and collecting radiation in a large solid angle with aberration-free transmission of the image of the emitting plasma region into the remote focus of an optical collector.

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

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

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

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

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

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

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

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

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

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

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

In a preferred embodiment of the invention, the spatial region through which both the focused laser beam and the output beam pass is selected so that the ejection rate of the droplet fraction of debris particles in said spatial region is at least ¹⁰ times smaller than a maximum ejection rate of the droplet fraction of debris particles.

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

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

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

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

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

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

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

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

In a preferred embodiment of the present invention, the interaction zone is located at the first focal point of the first elliptical mirror unit, and the first focal point of the second elliptical mirror unit is located at the second focal point of the first elliptical mirror unit.

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

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

In one embodiment, the debris mitigation means includes a protective gas flow between the ellipsoidal mirror units.

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

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

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

In one embodiment, the membrane can be configured as a gas lock that separates volumes with different pressures to provide a protective gas flow.

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

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

In an LPP light source with a rapidly rotating liquid metal target, the use of a light collector with two elliptical mirror units (as opposed to using a normal-incidence multilayer mirror) firstly allows a significant increase in the solid angle of collection of short-wavelength radiation, secondly, a significant expansion of the spectral range of the collected radiation, and thirdly, an aberration-free transfer of the image of the plasma light source to an intermediate or remote focus.

Increasing the collection angle and spectral range of the light collector allows for a significant (several-fold) increase in the output power of the LPP light source. Additionally, the use of grazing incidence mirrors in the light collector allows for a larger area in the path of the output beam for placement of debris mitigation means, thereby extending the lifetime of the light collector.

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

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

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

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

An exemplary embodiment of the present invention is shown in the drawings.

In the drawings, corresponding elements of the device are marked with the same reference numerals.

These drawings do not cover or even limit the full range of options for implementing this technical solution, but merely illustrate specific cases of its implementation.

1 is a schematic diagram of a high-brightness LPP light source having a light collector with two elliptical grazing incidence mirror units, according to one embodiment. 1 is a simplified diagram of a high brightness LPP light source, according to one embodiment. 10A-10C show the calculated spatial distribution of debris ejection from the interaction region, further illustrating the selection of spatial regions for propagation of the laser beam and the short wavelength radiation beam.

In an exemplary embodiment of the invention shown in FIG. 1, a high-brightness source of short-wavelength radiation includes a vacuum chamber 1 with a rotating target assembly 2 that delivers a target 3 to an interaction zone 4 where the target 3 interacts with a focused laser beam 5. A portion of the rotating target assembly 2 is in the form of a disk fixed to a rotating shaft. The disk has a peripheral portion in the form of an annular barrier with an annular groove facing the rotation axis 6. The target 3 is a layer of molten metal formed by centrifugal force on the surface of the annular groove of the rotating target assembly 2.

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

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

In the interaction zone 4, a pulsed, high-temperature plasma of the target material is generated under the action of the focused laser beam 5. This plasma produces short-wavelength radiation in one or more spectral ranges, including VUV, EUV, and soft x-ray. The short-wavelength radiation is directed to a light collector 8 in the form of a diverging output beam 7.

According to the invention, collection of short-wavelength radiation is performed by a light collector 8 comprising two elliptical mirror units 9, 10. The elliptical mirror units 9, 10 are arranged on a common optical axis 11, preferably in tandem along the propagation path of the output beam 7. The interaction zone 4 is located at the first focal point of the first elliptical mirror unit 9, and the second focal point of the first unit 9 coincides with the first focal point of the second elliptical mirror unit 10. In this case, an image of the luminous plasma region in the interaction zone 4 is projected substantially undistorted onto the zone of the second focal point 17 of the second elliptical mirror unit 10.

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

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

A protective gas flow along and/or perpendicular to the optical axis 11 of the light collector 8 to suppress the vapor fraction of debris.

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

A magnetic field, preferably generated by a permanent magnet, that reduces the charged portion of debris particles.

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

Similar means for debris mitigation are positioned in the propagation path of the focused laser beam 5.

Compared to LPP light sources using a normal incidence mirror-based light collector, in which the majority of the area between the interaction zone and the light collector is occupied by the propagation zone of the reflected beam of short wavelength radiation, this embodiment of the invention achieves a significant advantage. This means that the area available for arranging debris mitigation means is very limited in such light sources. According to the invention, debris mitigation means 12, 14 are arranged in essentially the entire area between the interaction zone 4 and the light collector 8. These means are arranged partially inside and partially outside the casing surrounding the laser and the beams of short wavelength radiation 5, 7. This provides very effective debris mitigation.

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

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

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

To increase the mitigation effect of the droplet portion of debris particles, the output beam 7 is directed from the linear velocity vector V _R of the target within the interaction zone 4 to the opposite side of a plane 18 passing through the interaction zone 4 and the rotation axis 6 of the target assembly, as shown in Figure 2. Both the laser beam 5 and the output beam 7 are located on one side of the plane 18. Thus, the vector V _R and the beams 7, 5 of short wavelength and laser radiation are located on different sides of the plane 18.

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

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

I Parallel to the rotation axis: θ = 0, φ is any (any)

II. Along target velocity: θ = 90°, φ = 0°

III. Normal to the target surface: θ = 90°, φ = 90°

IV. Against target velocity: θ = 90°, φ = 180°

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

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

As shown in Figure 3, the mass of debris particles is primarily concentrated within a sector along the direction of target velocity, bounded by azimuth angles φ of 0°-80° and polar angles θ of 0°-90°. The maximum debris ejection rate along the target rotation direction is 107 nm/(month-W). Figure 3 uses ellipses to indicate the spatial orientation of the laser 35 and output beam 37 cones, estimating a debris ejection rate of less than 0.2 nm/month-W in the laser cone 35, while the shorter-wavelength radiation cone 37 is less than 1 nm/month-W (the illustrated cones correspond to a solid angle of approximately 0.3 sr). The above data is provided assuming a laser radiation power of 1 W and 24/7 operation of the light source. In Figure 3, reference numeral 31 indicates the direction of the target velocity (200 m/s), with a maximum debris ejection rate of 107 nm/(month-W). Reference numeral 32 indicates the line of the level of the Sn-debris ejection rate given in nm/(month-W). Reference numeral 35 indicates debris ejection in the laser beam cone, which is less than 0.2 nm/(month-W). Reference numeral 37 indicates debris ejection in the collection angle cone, which is less than 1 nm/(month-W).

The high-brightness laser-produced plasma light source operates as described below and as shown in Figures 1, 2, and 3.

An oil-free vacuum pump system is used to evacuate the vacuum chamber 1 to a pressure of ^10-5 ... ^10-11 mbar or less, simultaneously removing gas components such as nitrogen, oxygen, and carbon that may interact with the target material and contaminate the collector mirror.

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

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

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

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

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

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

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

Debris shield placed outside the collection angle15

16 replaceable semi-permeable membranes

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

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

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

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

The method for generating and collecting laser-produced plasma radiation is carried out as described below and shown in Figures 1, 2, and 3.

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

The pulsed irradiation of the target is performed in such a way that the vector V _R of the target linear velocity in the interaction zone and the beams 5, 7 of laser and short wavelength radiation are located on different sides of a plane 18 passing through the interaction zone 4 and the axis of rotation 6, where the linear velocity of the target is above 100 m/s and is sufficiently high to prevent a large fraction of the droplets of debris particles from being directed towards the light collector 8 and the cone of the focused laser beam 5.

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

The spatial region through which both the focused laser beam 5 and the output beam 7 pass is selected so that the debris emission rate of that spatial region is at least 10 ⁴ times, preferably 10 ⁶ times, less than the maximum debris emission rate.

Optionally, additional debris mitigation measures can be used, including a protective gas flow, magnets, a foil trap, a membrane 16 that is highly transparent to short wavelength radiation, and debris shields 14, 15.

Thus, the present invention provides for the creation of an LPP source of soft X-ray, EUV, and VUV radiation characterized by high average output (power), high-intensity short-wavelength radiation, long life, and ease of use.

Industrial application fields

The proposed device is intended for many applications, including microscopy, materials science, X-ray diagnostics of materials, biomedical and medical diagnostics, inspection of nano- and microstructures, and lithography, including actinic control of lithographic EUV masks.

Claims (21)

1. A method of generating and collecting radiation, comprising: forming a target as a molten metal layer on a surface of an annular groove in a rotating target assembly under the action of centrifugal force, the target having a target surface facing an axis of rotation of the rotating target assembly; irradiating the target at a predetermined pulse repetition rate with a focused laser beam passing through debris mitigation means; generating a laser-produced plasma in an interaction zone between the focused laser beam and the target; and emitting an output beam of short wavelength radiation that is directed through the debris mitigation means to a light collector,
the target is rotated at a sufficiently high linear velocity such that a majority of droplets of debris particles ejected from the interaction zone and a linear velocity vector (V _R ) of the target in the interaction zone are directed to one side of a plane passing through the interaction zone and the rotation axis, and irradiation of the target and collection of radiation from the laser-produced plasma are provided such that the focused laser beam and output beam are located on the other side of the plane;
a spatial distribution of debris emission rate from the interaction zone is estimated by calculating a growth rate of a film thickness of deposited debris particles at a predetermined distance from the interaction zone, and based on the estimated spatial distribution of debris emission rate, directions of both the focused laser beam and the output beam of short wavelength radiation are selected to pass through a spatial region that results in a debris emission rate that is at least 10 times less than a maximum debris emission rate ^;
method. the directions of both the focused laser beam and the output beam of short wavelength radiation are selected to pass through a region of space where the debris emission rate is lower than the remaining region of space;
The method of claim 1. The method of claim 1, wherein the linear velocity is a high linear velocity of 100 m/s or more. the short wavelength radiation is collected by a light collector consisting of two elliptical mirror units arranged in cascade, the light collector transmitting an image of the luminous plasma region to a second focal point of a second elliptical mirror unit, the second focal point of the first elliptical mirror unit being located at the first focal point of the second elliptical mirror unit;
The method of claim 1. the light collector transmits an image of the luminous plasma region without distortion, and/or the scale factor of the image is determined by the design of the two elliptical mirror units;
The method of claim 4. the debris mitigation is provided along the entire path of the short wavelength radiation to the light collector;
The method of claim 1. the debris mitigation is provided by one or more debris mitigation techniques selected from the group consisting of protective gas flow, magnetic mitigation, foil traps, debris shields, and membranes that are largely transparent to short wavelength radiation with a transmission of 60% or more;
The method of claim 1. The target is rotated at a centrifugal acceleration of 3000 g or more, where g is the acceleration of gravity, and the target surface is parallel to the rotation axis.
The method of claim 1. 1. A laser-produced plasma light source comprising: a vacuum chamber; a rotating target assembly configured to deliver a target to an interaction zone, the target being a layer of molten metal on a surface of an annular groove provided in the rotating target assembly with a target surface facing a rotation axis of the rotating target assembly; a light collector; and debris mitigation means configured to pass a pulsed laser beam focused on the target and to pass an output beam of short wavelength radiation exiting an interaction zone between the focused laser beam and the target to the light collector;
the rotating target assembly is configured to rotate the target at a predetermined linear velocity, the linear velocity vector (V _R ) of the target in the interaction zone is oriented on one side of a plane passing through the interaction zone and the axis of rotation, and the focused laser beam and the output beam are positioned on the other side of the plane;
the debris mitigation means estimates a spatial distribution of debris emission rate from the interaction zone by calculating a growth rate of a film thickness of deposited debris particles at a predetermined distance from the interaction zone, and based on the estimated spatial distribution of debris emission rate, the directions of both the focused laser beam and the output beam of short wavelength radiation are selected to pass through a spatial region that results in a debris emission rate that is at least ¹⁰⁴ times less than a maximum debris emission rate;
light source. The light source described in claim 9, wherein the target speed is 100 m/s or more. the rotating target assembly is configured to form the target such that a surface of the target is parallel to the axis of rotation at a centrifugal acceleration of at least 3000 g;
11. A light source according to claim 9 or 10. the light source further comprises a light collector comprising two elliptical mirror units arranged in cascade along the path of the output beam, and/or the debris mitigation means is arranged along the entire path of short wavelength radiation to the light collector.
10. The light source of claim 9. the interaction zone is located at a first focal point of the first elliptical mirror unit, and the first focal point of the second elliptical mirror unit is located at a second focal point of the first elliptical mirror unit;
13. The light source of claim 12. the second elliptical mirror unit being several times smaller than the first elliptical mirror unit, for example, 2 to 15 times smaller, and the light collector having a magnification between 0.8 and 1.2;
14. A light source according to claim 12 or 13. the material of the surface of the mirror of the light collector is selected from the group consisting of Mo, Ru, Rh, Pd, U, Ni, W, Fe, Nb, Al, Si, Co, and BN;
13. The light source of claim 12. the debris mitigation means including a protective gas flow between the elliptical mirror units;
13. The light source of claim 12. Each of the elliptical mirror units comprises a nested set of at least two elliptical mirrors;
13. The light source of claim 12. the debris mitigation means includes a debris shield positioned on the axis of the light collector outside the collection angle;
10. The light source of claim 9. the debris mitigation means is provided by one or more of the following techniques: protective gas flow, magnetic mitigation, foil traps, and membranes made of materials from the group consisting of carbon nanotubes, Ti, Si, ZrSi, and BN;
10. The light source of claim 9. the membrane is configured to provide a gas lock separating volumes having different pressures providing a protective gas flow;
20. The light source of claim 19. the light collector includes a grazing incidence mirror;
10. The light source of claim 9.