High brightness LPP EUV light source with fast rotating target and cooling method thereof

Abstract

A laser generated plasma (LPP) light source includes a rotating target assembly (2) that supplies a target (3) together with a focused beam of high repetition frequency pulsed laser light into an interaction zone (4). The light source is efficiently cooled by thermal radiation of the peripheral portion of the rotating target assembly and by blowing gas through a serpentine gap (16) between the rotating target assembly (2) and the stationary heat exchanger (17). This achieves high brightness and high output power of the LPP light source.

Description

Technical Field

[0001] This disclosure relates to high-brightness laser-generated plasma (LPP) sources designed to generate extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) light with wavelengths from approximately 5 nm to 125 nm. These sources utilize a rapidly rotating target for efficient debris mitigation, ensuring long-term operation of the source and its integrated equipment. This disclosure also relates to a method for cooling a high-brightness LPP EUV source with a rapidly rotating target. Background Technology

[0002] High-brightness EUV light sources are used in many fields: microscopy, materials science, biomedicine and medical diagnostics, materials testing, crystal and nanostructure analysis, atomic physics, and photolithography. Furthermore, the higher the average output power of an EUV light source, the higher the performance of the equipment built upon it.

[0003] Synchrotrons can be used as such radiation sources, but they are extremely expensive and not readily available.

[0004] An alternative is provided by a source in which the radiation from a high-power laser is focused onto a target to obtain plasma that is effectively emitted in the EUV range during discharge. Using laser-generated plasma is the most efficient way to achieve EUV light generation.

[0005] During light generation, debris particles are produced as a byproduct, which can lead to surface degradation of optical collectors (including one or more mirrors located near the light source). This debris can exist in the form of high-energy ions, neutral atoms, and target material clusters. While the deposition of droplets and particles on the collector mirrors reduces their reflectivity, high-speed particles can damage the collector mirrors and potentially other components of the optical system downstream of them. This underscores the practicality of developing high-brightness, short-wavelength light sources with efficient debris mitigation capabilities.

[0006] Novel methods for developing high-brightness, short-wavelength LPP light sources based on rapidly rotating liquid metal targets have been proposed in U.S. Patent No. 10,638,588, published April 28, 2020; U.S. Patent No. 10,588,210, published October 3, 2020; and U.S. Patent No. 10,887,973, published May 1, 2021. These methods enable highly efficient debris mitigation. In fact, in various types of LPP light sources, a key factor in the contaminant collector optics is the droplet portion of debris particles ejected from the interaction zone at relatively low velocities. This phenomenon is efficiently mitigated by redirecting the vast majority of droplets laterally away from the optical collector and laser beam input window due to the rapid rotation of the target (hundreds of Hz, i.e., revolutions per second, with a linear velocity exceeding 100 m / s). Debris mitigation is further achieved through the use of various debris mitigation techniques, including: protective gas flow, magnetic mitigation, foil traps, debris barriers, and thin films that are substantially transparent to EUV radiation.

[0007] As disclosed in U.S. Patent No. 10,588,210, heat is dissipated from a rotating target assembly to a liquid-cooled heat exchanger via a liquid metal layer in a hydrodynamic bearing. The hydrodynamic bearing with liquid metal can withstand very high temperatures. The large bearing contact area and liquid metal grease enable efficient heat dissipation from the rotating target assembly. However, the viscosity of the liquid metal prevents the implementation of rapid rotation of the target assembly and high pulse repetition frequency operating modes, which limits the realization of high output power and brightness for the LPP EUV light source.

[0008] Heat can be dissipated through thermal radiation. However, radiative heat transfer also limits the ability to increase the input laser power, thereby increasing the output power and brightness of the LPP EUV light source, leading to an increase in the temperature of the rotating target assembly. The rapidly rotating disk of the target assembly is subjected to centrifugal acceleration of tens of thousands of g, and the strength properties of the disk material decrease significantly as the disk temperature rises. Furthermore, the increased temperature leads to a sharp increase in the chemical interaction between the disk material and the target material, resulting in mutual dissolution. Ultimately, the increased temperature of the rotating disk causes overheating of the bearings on the shaft of the rotation drive unit. All these arguments place particular demands on ensuring efficient cooling of the rapidly rotating target. Summary of the Invention

[0009] Therefore, it is necessary to eliminate the shortcomings mentioned above. In particular, there is a need for improved compact, commercially available, low-cost, low-debris-generating LPP EUV light sources with rapidly rotating targets, which can efficiently improve average EUV light output power and brightness.

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

[0011] According to embodiments of the present invention, a laser-generated plasma (LPP) extreme ultraviolet (EUV) light source is provided (alternatively, an LPP source for vacuum ultraviolet (VUV) light). Hereinafter, the abbreviation EUV represents both EUV and VUV.

[0012] The source includes: a vacuum chamber having a rotating target assembly having a rotation drive unit and a component formed in the form of a disk with a barrier having an annular groove with a target material on the inner surface of the barrier, the target material being supplied to the interaction region together with a focused beam of a high repetition frequency pulsed laser, and a beam of EUV plasma light being emitted from the interaction region.

[0013] The source is characterized in that a heat exchanger with liquid cooling function is installed in a vacuum chamber and fixed relative to the vacuum chamber; a portion of the surface of the heat exchanger is separated from the surface of the rotating target assembly disk by a gap (slit gap or void).

[0014] In one embodiment, the outer surface of the peripheral portion of the rotating target assembly is formed to have a surface area exceeding 0.5 × R. 2 The surface area S is large, where R is the outer radius of the rotating target assembly (i.e., the outer radius of the assembly); and the surface has a coating with a high emissivity, exceeding 0.7 (i.e., the emissivity of the coating is higher than 0.7).

[0015] In another embodiment, the components of the heat exchanger facing each other and the disk of the rotating target assembly (i.e., the exchanger facing the component and vice versa) are equipped with concentric annular fins, and the concentric annular fins of the rotating target assembly are located between the concentric annular fins of the heat exchanger.

[0016] In yet another embodiment, the slit gap has a meandering shape in the cross-sectional plane passing through the axis of rotation of the rotating target assembly.

[0017] In another embodiment, the surfaces of the disks of the rotating target assembly located on both sides of the slit gap and the surfaces of the heat exchangers have, for example, coatings generated by micro-arc oxidation, which have a high emissivity of more than 0.7.

[0018] In another embodiment, the gas inlet is arranged to blow gas through the slit gap into the vacuum chamber at a gas pressure exceeding 20 Pa.

[0019] In another embodiment, the slit gap is less than 0.5 mm (i.e., the distance between the component and the switch).

[0020] In another embodiment, the rotary drive unit includes a shaft mounted on a bearing and a motor connected to a disk of a rotary target assembly via the shaft. The bearing and motor are located outside the vacuum chamber. A magnetic fluid seal (MFS) provides a seal between the vacuum chamber and the shaft. The bearing and MFS are equipped with an additional heat exchanger with liquid cooling functionality.

[0021] In another embodiment, the counterweight of the rotating target assembly is fixed on a shaft located outside the vacuum chamber, and the counterweight is arranged to be cooled by convective air.

[0022] In yet another embodiment, the motor is a brushless DC motor.

[0023] In the embodiments, the linear velocity of the target is at least 100 m / s, the centrifugal acceleration is at least 3000 g, where g is the standard gravitational acceleration, and the target material has fluidity under centrifugal force.

[0024] In another respect, the present invention relates to a method for cooling an LPP EUV light source (with a rapidly rotating target).

[0025] The method is characterized in that cooling is achieved by thermal radiation from the outer portion of the rotating target assembly, which is fabricated to have a thermal conductivity exceeding 0.5 × R0. 2 The surface area S, where R is the outer radius of the rotating target assembly, and cooling is further achieved by heat exchange through a slit gap located between the disk of the rotating target assembly and a liquid-cooled heat exchanger fixed relative to the vacuum chamber; and the surfaces of both the disk and the heat exchanger located on both sides of the slit gap are coated with coatings that have a high emissivity of more than 0.7.

[0026] In one embodiment, gas is blown into the vacuum chamber through a slit gap between the rotating target assembly and the heat exchanger.

[0027] In one embodiment, the drive unit includes a shaft mounted on a bearing and a motor connected to a disk of a rotating target assembly via the shaft. The bearing and motor are located outside the vacuum chamber and are sealed between the vacuum chamber and the shaft by a magnetic fluid seal (MFS). The bearing and MFS are equipped with an additional heat exchanger with liquid cooling capability. The counterweight of the rotating target assembly is fixed on the shaft and located outside the vacuum chamber. Heat transfer from the disk of the rotating target assembly is provided by the MFS equipped with the additional heat exchanger and by convective air cooling through the counterweight rotating on the shaft.

[0028] The following causal relationship exists between a set of basic features of the present invention and the technical results achieved.

[0029] The LPP EUV light source is characterized by achieving this goal through several methods of simultaneously using the shaft of a cooling, rapidly rotating target and a rotation drive unit.

[0030] The peripheral working components of the rotating disk are cooled by two mechanisms: (i) thermal radiation from the heated disk; and (ii) thermal conduction through the shaft of the rotation drive unit and through the high-pressure gas in contact with the rotating disk. The power of thermal radiation from the disk's surface is known to be determined by the following formula: W = σ ST 4 (1) Where σ is the emissivity coefficient of the material, and S is the surface area of ​​the disk.

[0031] As can be seen from (1), to improve the cooling efficiency of the disk, both the surface area of ​​the disk and the emissivity coefficient of the disk surface material must be increased. These two factors are used in this light source to improve the cooling efficiency.

[0032] To increase the emissivity factor of the disk surface to σ ≈ 1, the surface requires special treatment. The heat exchanger's facing surface of the rotating disk also undergoes special treatment to increase the effective surface emissivity coefficient σ. ef .in this case: σ ef = 1 / (1 / σ disk + 1 / σ heatexch - 1)(2) Where, σ disk and σ heatexch These are the emissivity coefficients of the disk and the heat exchanger, respectively.

[0033] The second mechanism, based on thermal conductivity, also achieves heat power removal from the thermal periphery of the rotating disk via two channels. The first channel is heat transfer through the gas in the narrow slit gap between the rotating disk and the stationary heat exchanger. In this case, the surfaces of both the disk and the heat exchanger, facing each other, are designed to have the maximum possible area. To increase the thermal conductivity of the gas, its pressure in the gap must be several times higher than the pressure in the vacuum chamber. The second channel is heat transfer through the disk body towards its rotational axis and subsequent cooling of the rotational axis.

[0034] In addition, the high thermal conductivity of the shaft material and the high rotational speed of the counterweight are also very effective for air convection cooling of the shaft and counterweight unit.

[0035] By simultaneously employing all the methods described above for cooling the rotating target assembly in an LPP EUV light source with a rapidly rotating target, the output power of the light source can be increased several times by enhancing the power of the laser radiation focused on the target, while ensuring that the temperatures of both the target itself and the bearings of the shaft remain within acceptable ranges. The technical result of this invention is the creation of high-power, high-brightness short-wavelength light sources that efficiently mitigate debris and effectively cool all components of the rotating target unit.

[0036] The following description, in conjunction with the accompanying drawings, will provide non-limiting examples of exemplary embodiments of the present invention, the advantages and features of which will become more apparent. Attached Figure Description

[0037] Exemplary embodiments of the present invention are illustrated in the accompanying drawings, in which: Figure 1 and Figure 2 A schematic diagram of a high-brightness LPP EUV light source according to an embodiment is shown; Figure 3 Reproduced at a higher graphics resolution Figure 1 Details in the text; and Figure 4 Reproduced at a higher graphics resolution Figure 2 Details in the text.

[0038] In the accompanying drawings, corresponding elements of the device have the same reference numerals. These drawings do not cover, and further do not limit, the entire range of options for implementing this technical solution, but are merely illustrative material representing specific scenarios of its implementation. Detailed Implementation

[0039] like Figure 1 As shown, the high-brightness LPP EUV light source contains a vacuum chamber 1 with a rotating target assembly 2, which supplies a target 3 together with a focused laser beam 5 of a high repetition rate pulsed laser to an interaction region 4.

[0040] A portion of the rotating target assembly 2 is made in the form of a disk 6 that rotates via a drive unit. The drive unit includes a rotating shaft 7 mounted on a bearing 8 and a motor 9 connected to the disk 6 of the rotating target assembly via the shaft.

[0041] The disk 6 has an outer portion in the form of an annular barrier, which has an annular groove 10 facing the axis of rotation 11.

[0042] In this embodiment, the target 3 is a layer of molten metal formed by centrifugal force on the surface of the annular groove 10 of the rotating target assembly 2. The annular groove configuration prevents the target 3 from being ejected in both the radial direction and along the rotation axis 11. The rotating target assembly 2 is provided with a fixed heating system 12 for the target material. To keep the target material in a molten state inside the rotating target assembly, the heating system 12 needs to provide non-contact induction heating. The fixed heating system 12 may have the option to maintain the temperature of the molten metal within an optimal temperature range. The heating system is turned off in the maximum output power mode of the LPP EUV source.

[0043] Within the interaction region 4, pulsed high-temperature plasma of the target is generated at a high repetition rate in the range of hundreds to thousands of kHz under the action of a focused laser beam 5. The plasma generates short-wavelength light in one or more spectral ranges, including EUV and VUV. The short-wavelength light to be used exits the interaction region 4 as a divergent output beam 13, which is directed towards the optical collector 14.

[0044] To ensure high stability of both the target surface and output parameters of the LPP EUV light source, the rapid rotation speed must exceed 100 m / s and the centrifugal acceleration must be no less than 3000 g. The centrifugal force thus makes the surface of the liquid metal target 3 parallel to the rotation axis 11. That is, the surface is basically a cylindrical surface, and the axis of the cylindrical surface coincides with the rotation axis.

[0045] Due to the rapid rotation of the target, the droplet portion of the debris particles ejected from the interaction zone acquires a significant tangential velocity component comparable to the linear target velocity. Consequently, the acquired droplet velocity vector will be significantly redirected away from the cone of the focused laser beam 5 and the output beam 13.

[0046] Devices (not shown) for debris mitigation, provided by one or more technologies, are present in the path from the output beam 13 to the optical collector 14. These technologies include: a debris barrier mounted outside the focusing angle and cone of the focused laser beam 5; a protective gas flow arranged to suppress the vapor portion of debris; a foil trap, highly transparent to plasma light, which is essentially a plate system oriented radially relative to the plasma, thereby enabling sufficiently efficient trapping of neutral atoms and clusters of the liquid metal target; a magnetic field, preferably generated by a permanent magnet, for mitigating the charged portion of debris particles; and a replaceable thin film, substantially transparent to EUV light and impermeable to debris and gas. Similar devices for debris mitigation are placed in the propagation path of the focused laser beam 5.

[0047] The outer surface 15 of the peripheral portion of the rotating target assembly 2 is formed to have a diameter exceeding 0.5 × R. 2Heat transfer in a rapidly rotating target is achieved through thermal radiation over a large surface area (S), where R is the outer radius of the rotating target assembly. To enhance heat transfer, the outer surface of the disk portion of the rotating target assembly is coated with a high emissivity exceeding 0.7. This coating can be generated by micro-arc oxidation or plasma electrolytic oxidation.

[0048] Heat transfer to the target is also achieved via a narrow submillimeter slit gap 16 between the disk 6 of the rotating target assembly and the stationary heat exchanger 17 in which the cooling liquid 18 flows. The size of the slit gap is less than 0.5 mm, preferably about 0.3 mm.

[0049] To improve heat exchange, the surfaces of the disk 6 of the rotating target assembly located on both sides of the slit gap 16 and the surface of the heat exchanger 17 are coated with a high emissivity exceeding 0.7. For the same purpose, the components of the heat exchanger facing each other and the disk of the rotating target assembly are equipped with concentric annular fins 19 and 20, such that the concentric annular fins 19 of the rotating target assembly are located between the concentric annular fins 20 of the heat exchanger. In this case, the slit gap 16 has a meandering shape in the cross-sectional plane passing through the rotation axis 11.

[0050] To further improve the cooling efficiency of the rotating target assembly, the heat exchanger is equipped with a gas inlet 21, which is arranged to blow gas through a slit gap into the vacuum pump 22 at a gas pressure exceeding 20 Pa. This gas can be the same as the protective gas, such as argon, but is not limited to this option.

[0051] A vacuum pump is installed to allow a flow of blowing gas to pass through the bearings 8, thereby cooling the bearings. In addition, the bearings 8 are equipped with an additional heat exchanger 23 in which a cooling liquid 24 flows.

[0052] like Figure 1 As shown, for this type of cooling, the gas conductivity and contact area are sufficient to remove up to 1.5 kW of heat power.

[0053] Meanwhile, other cooling methods can also be used for the rotating target assembly. The following embodiments of the invention focus on further improving the apparatus complex for cooling the rotating target assembly.

[0054] Figure 2 An embodiment is shown in which the bearing 8 of the shaft 7 and the motor 9 can be located outside the vacuum chamber 1, exposed to the surrounding atmosphere. In this embodiment, a magnetic fluid seal (MFS) 25 provides a seal between the vacuum chamber and the rotating shaft 7, and the bearing 8 and the MFS 25 are equipped with an additional heat exchanger 26 in which a cooling liquid 24 flows.

[0055] The submillimeter layer of MFS liquid achieves high thermal conductivity, thereby enabling efficient cooling of the rotating target assembly 2 through the nominal temperature range of shaft 7 and bearing 8.

[0056] Meanwhile, the counterweight 27 of the rotating target assembly is fixed to the shaft and located outside the vacuum chamber. The counterweight is designed to simultaneously suppress vibrations of the rotating target assembly and enable convective air cooling, ensuring further heat dissipation from the rotating target assembly via shaft 7. This vibration suppression of the rotating target assembly enhances spatial stability and ensures high brightness for the LPP EUV light source.

[0057] Preferably, motor 9 is a brushless DC motor. Compared with brushed motors, brushless DC motors have the following advantages: higher efficiency, longer lifespan, lower maintenance costs, higher torque-to-weight ratio, more precise speed control, quieter operation, reduced electromagnetic interference, higher power density, improved heat dissipation, smoother operation, higher speed, digital control, higher starting torque, improved energy efficiency, better thermal management, compact design, faster response, no brush dust, higher overload capacity, higher durability, lower electromagnetic noise, wider temperature range, lower inertia, digital feedback, remote operation capability, improved reliability, reduced vibration, higher stall torque, reduced eddy current losses, soft start, lower total cost of ownership, enhanced speed stability, integrated electronics, and reduced emissions.

[0058] The operation method of a high-brightness laser-generated plasma source is described and shown below. Figure 1 and Figure 2 middle.

[0059] The vacuum chamber 1 is evacuated to a pressure below 10 using an oil-free vacuum pump system 22. -5 ...10 -11 Millibars. Simultaneously, it removes gaseous components that can interact with the target material and contaminate the collector mirror, such as nitrogen, oxygen, and carbon.

[0060] Using a fixed heating system 12 that can employ induction heating, a target material from a group of non-toxic fusible metals (including Sn, Li, In, Ga, Pb, Bi, Zn, Ag, Au and their alloys) is converted into a molten state within a predefined optimal temperature range.

[0061] In embodiments of the present invention, the spectral brightness of the EUV light source is maximum at a wavelength of 13.5 nm, but it is not limited thereto.

[0062] In another embodiment of the invention, the spectral brightness of the EUV radiation source can be maximized in the wavelength range of 6.6 nm to 6.8 nm, and the target material contains gadolinium (Gd) or terbium (Tb) or their compounds, and is present in the form of refractory powder.

[0063] A rotary drive unit is used to actuate the rotating target assembly 2. Under centrifugal force, the target 3 forms a layer of molten metal on the surface of the annular groove facing the axis of rotation 6. Under a centrifugal acceleration of at least 3000g, the target surface is substantially parallel to the axis of rotation.

[0064] The target 3 is exposed to a focused laser beam 5 at a high pulse repetition frequency, which can be in the range of 10 kHz to 100 kHz or higher. The focused laser beam 5 heats the target material to the plasma formation temperature, thereby generating short-wavelength light. Depending on the laser radiation power density at the focal point and the type of target material, the plasma generated by the laser emits light in a short wavelength range, from 5 nm to 120 nm.

[0065] An output beam 7 is emitted from the high-temperature plasma via a device for debris mitigation, and directed into an optical collector 14. Due to the rapid rotation of the target, the droplet portion of the debris particles ejected from the interaction zone acquires a significant tangential velocity component comparable to the linear target velocity. Consequently, the acquired droplet velocity is significantly redirected away from the cone of the laser beam 5 and the output beam 7.

[0066] The method for cooling the LPP EUV light source is described and illustrated below. Figure 1 and Figure 2 middle.

[0067] Several methods are employed to ensure heat transfer in a rapidly rotating target that absorbs high-average-power laser radiation. These methods include thermal radiation from the outer surface 15 of the peripheral portion of the rotating target assembly 2 (which is configured to have the largest possible surface area). To enhance heat transfer, the surface of the peripheral portion of the disk of the rotating target assembly is coated with a coating having a high emissivity exceeding 0.7.

[0068] Heat transfer to the target is also achieved via a narrow slit gap 16 between the disk 6 of the rotating target assembly and the liquid-cooled heat exchanger 17, which is rigidly mounted in the vacuum chamber and fixed relative to the vacuum chamber.

[0069] To enhance cooling by increasing the surface area involved in heat transfer, the slit gap 16 needs to be meandering in the cross-sectional plane passing through the rotation axis 11, and the slit gap size needs to be less than 0.5 mm. To improve heat exchange, the surfaces of the disk 6 of the rotating target assembly located on both sides of the slit gap 16 and the surface of the heat exchanger 17 are coated with coatings that have a high emissivity, exceeding 0.7.

[0070] To further improve the cooling efficiency of the rotating target assembly, the heat exchanger is equipped with a gas inlet 21, which is arranged to blow gas through the slit gap at a gas pressure exceeding 20 Pa. The gas can be argon, but is not limited to this option.

[0071] For this type of cooling, the gas conductivity and contact area are sufficient to remove up to 1.5 kW of heat power.

[0072] exist Figure 2 In the illustrated embodiment, the cooling efficiency of the rapidly rotating target is further improved because the magnetic fluid seal (MFS) 25 seals the rotating shaft 7. Meanwhile, the bearing 8, motor 9, and counterweight 27 of the shaft 7 are located outside the vacuum chamber 1 and in the surrounding atmosphere. The bearing 8 and MFS 25 are equipped with an additional liquid-cooled heat exchanger 26.

[0073] The submillimeter layer of MFS liquid achieves high thermal conductivity, thereby enabling efficient cooling of the rotating target assembly 2 through the nominal temperature range of shaft 7 and bearing 8.

[0074] Another method for cooling the rotating target assembly via a shaft is through convective heat exchange between the disc counterweight 26 mounted on the shaft and the atmospheric air. The high thermal conductivity of the shaft material and the high linear velocity of the counterweight are also very effective for this cooling method.

[0075] By using all the methods described above for cooling the rotating target assembly in the LPP EUV source, it is possible to increase the output power of the light source several times by increasing the laser input power, while ensuring that the temperature of both the target itself and the bearings of the shaft are within acceptable ranges.

[0076] Overall, cooling the rotating target assembly according to the invention enables the LPP EUV light source to operate at an average laser power of several kilowatts at the target.

[0077] Therefore, the present invention can form LPP EUV light sources, which are characterized by high average power, high brightness, long life and ease of use.

[0078] Figure 3 Reproduce at higher graphics resolution Figure 1 The details are shown again on the right side of the figure. Figure 1 The device is shown in a magnified circular frame, but with the addition of a partial enlargement; and the disk 6 (rotating) and heat exchanger 17 (fixed, non-rotating) as well as the slit gap 16 (i.e., the gap between component 6 and component 17) are shown in magnified form on the right side of the figure.

[0079] Figure 4 Reproduce at higher graphics resolution Figure 2 Details.

[0080] Regarding industrial applications, the disclosed equipment is intended for use in a variety of applications, including microscopy, materials science, X-ray diagnostics of materials, biomedical and medical diagnostics, inspection of nanostructures and microstructures, and photolithography (including illumination control of photolithography EUV masks).

Claims

1. A laser produced plasma (LPP) source for extreme ultraviolet (EUV) or vacuum ultraviolet (VUV) light, comprising: Vacuum chamber (1) with a rotating target assembly (2) having a rotating drive unit and a component made in the form of a disc (6) with a barrier on the inner surface of which there is an annular groove (10) with target material supplied to an interaction zone (4) together with a focused beam (5) of a high-repetition-rate pulsed laser and a beam (13) of EUV plasma light is emitted from the interaction zone (4), wherein a heat exchanger (17) with a liquid cooling function is mounted in the vacuum chamber (1) fixed relative to the vacuum chamber (1); and wherein a part of the heat exchanger surface is separated from the surface of the rotating target assembly disc (6) by a slit gap (16) or an interspace.

2. The LPP EUV light source according to claim 1, wherein, The outer surface of the peripheral portion of the rotating target assembly (2) is made to have a relatively large surface area S exceeding 0.5 x R 2 , where R is the outer radius of the rotating target assembly; and where the outer surface has a coating with a relatively high emissivity, exceeding 0.

7.

3. The LPP EUV light source according to any of the preceding claims, wherein, The components of the heat exchanger facing each other and the disc (6) of the rotating target assembly are each equipped with concentric annular fins (19, 20), and the concentric annular fins (19) of the rotating target assembly (2) are located between the concentric annular fins (20) of the heat exchanger (17).

4. The LPP EUV light source according to any of the preceding claims, wherein, The slit gap (16) has a meandering shape in a cross-sectional plane passing through the rotation axis (11) of the rotating target assembly (2).

5. The LPP EUV light source according to any of the preceding claims, wherein, The surfaces of the disc (6) of the rotating target assembly (2) and the heat exchanger (17) located on both sides of the slit gap (16) have a coating generated by micro-arc oxidation, the coating having a high emissivity, exceeding 0.

7.

6. The LPP EUV light source according to any of the preceding claims, wherein, A gas input is arranged to blow gas through the slit gap (16) at a gas pressure exceeding 20 Pa.

7. The LPP EUV light source according to any of the preceding claims, wherein, The slit gap (16) has a size of less than 0.5 mm.

8. The LPP EUV light source according to any of the preceding claims, wherein, The rotating drive unit comprises a shaft (7) mounted on bearings (8) and a motor (9) connected to the disc (6) of the rotating target assembly (2) through the shaft, the bearings and the motor being located outside the vacuum chamber (1), wherein a seal is provided between the vacuum chamber (1) and the shaft (7) by a magnetic fluid seal (25), and the bearings (8) and the magnetic fluid seal (25) are equipped with an additional heat exchanger (26) with a liquid cooling function.

9. The LPP EUV light source of claim 8, wherein, A counterweight (27) of the rotating target assembly (2) is fixed on the shaft (7) outside the vacuum chamber (1), and the counterweight (27) is arranged to be cooled by convection air.

10. The LPP EUV light source according to any one of claims 8 or 9, wherein, The motor (9) is a brushless DC motor.

11. The LPP EUV light source according to any of the preceding claims, wherein, The linear speed of the rotating target assembly is at least 100 m / s, the centrifugal acceleration is at least 3000 g, where g is the standard gravitational acceleration, and the target material has fluidity under centrifugal force.

12. A method for cooling an LPP EUV light source according to any of claims 1 to 11, wherein, The cooling is achieved by thermal radiation of the peripheral part of the rotating target assembly, which is made with a surface area S exceeding 0.5 x R 2 , where R is the outer radius of the rotating target assembly (2); and further by heat exchange of the slit gap (16) between the disc (6) of the rotating target assembly (2) and the heat exchanger (17) with the liquid cooling function fixed relative to the vacuum chamber (1), while the surfaces of both the disc and the heat exchanger on both sides of the slit gap have a coating with a high emissivity exceeding 0.

7.

13. The method of cooling an LPP EUV light source according to claim 12, wherein, Gas is blown into the vacuum chamber through the slit gap between the rotating target assembly and the heat exchanger.

14. The method of cooling an LPP EUV light source of claim 12, wherein, The drive unit comprises a shaft (7) mounted on bearings (8) and a motor (9) connected to the disc (6) of the rotating target assembly through said shaft, said bearings and said motor being located outside the vacuum chamber, a magnetic fluid seal (MFS) providing a seal between said vacuum chamber and said shaft, said bearings and MFS (25) being equipped with an additional heat exchanger (26) with said liquid cooling function, and a counterweight (27) of the rotating target assembly being fixed on said shaft, outside the vacuum chamber; and heat transfer from the disc of the rotating target assembly is provided by convection air cooling through said MFS equipped with said additional heat exchanger and through the rotating counterweight on said shaft.

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