Broadband laser-produced plasma light source

Abstract

A light source having a plasma emission maintained in a gas-filled chamber by a focused beam of a CW laser. The gas is an inert gas having a purity of at least 99.99%. The chamber includes a metal housing having at least one window made of MgF2 for outputting the plasma emission. Each window is placed in a hole in the housing at the end of a sleeve and soldered to the sleeve by glass cement, and each sleeve is welded to a hole in the metal housing on its outer seam. The sleeve and the housing are made of an alloy having a linear thermal expansion coefficient (CLTE) that matches the CLTE of the MgF2 crystal in the direction perpendicular to the optical axis of the MgF2 crystal. The technical result is to extend the emission spectrum of the light source into the VUV region.

Description

[Technical field]

[0001] Cross-references to related patents and applications This patent application is a continuation-in-part of U.S. patent application No. 17 / 180,063, filed on February 19, 2021, now allowed, which claims priority to Russian patent application RU2020109782, filed on March 5, 2020, and a continuation-in-part of U.S. patent application No. 16 / 986,424, now U.S. Patent No. 10,964,523, filed on August 6, 2020, which is a continuation-in-part of U.S. Patent No. 16 / 814,317, now U.S. Patent No. 10,770,282, filed on March 10, 2020, and claims priority to Russian patent application RU2021129398, filed on October 8, 2021, all of which are incorporated by reference in their entireties.

[0002] The present invention relates to a high brightness broadband light source with continuous optical discharge, a gas-filled chamber for use therein, and a method for manufacturing the same. [Background technology]

[0003] A steady gas discharge sustained by laser radiation into a previously created relatively dense plasma is known as a continuous optical discharge (COD).

[0004] COD, sustained in a gas-filled chamber by a focused beam of a continuous wave (CW) laser, has been achieved in a variety of gases, especially in Xe at high gas pressures up to 200 atm (Carlhoff et al., “Continuous Optical Discharges at Very High Pressure,” Physica 103C, 1981, pp. 439-447). COD-based light sources with plasma temperatures of about 20,000 K (Raizer, “Optical discharge,” Sov. Phys. Usp. 23(11), November 1980, pp. 789-806) are among the brightest continuous light sources in the broad spectral range from vacuum ultraviolet (VUV) to near infrared.

[0005] One of the challenges in creating a high-brightness COD-based light source is to increase the power of vacuum ultraviolet radiation, which in particular places special requirements on the short-wave boundary λb and the transparency of the optical materials used to output the COD plasma broadband radiation from the chamber.

[0006] As is known from JP 2006-010675 published on December 1, 2006, a high light output in the VUV range is achieved in the photodischarge if the purity of the inert gas in the chamber is at least 99.99%. At the same time, the shortwave boundary of the light source radiation spectrum is determined by the material of the chamber exit window, which can be lithium fluoride (LiF), magnesium fluoride (MgF2), calcium fluoride (CaF2), sapphire (Al2O3) or quartz (SiO2).

[0007] Among these materials, LiF and MgF2 have the shortest-wave boundary of transparency at about 110 nm. Moreover, among the latter two, MgF2 is the material with better mechanical and thermal properties and manufacturability, so its use is preferred to extend the radiation spectrum up to 100 nm in the VUV range.

[0008] The device described in JP 2006-010675 uses pulsed mode excitation of the photodischarge, which is a drawback of this device in that the average power and brightness of the light source is low. In the pulsed mode of photodischarge excitation, the optimal pressure in the chamber is about 1 atm, while the chamber temperature is close to room temperature, which eliminates the problem of sealing the exit window made of any of the optical materials mentioned above. However, the situation is fundamentally different for high-brightness plasma radiation sources with continuous photodischarge.

[0009] For example, as is known from US Pat. No. 1,096,523, published on March 30, 2021 and incorporated herein by reference, optimal continuous generation of COD plasma emission characterized by a spectral brightness of more than 50 mW / (mm2 nm avg) and a relative brightness instability σ of less than 0.1% is achieved by an optimal gas pressure in the chamber, preferably of 50 atm or more, a maximum possible operating temperature of the chamber inner surface of 600-900 K or more, and a chamber wall located at a distance of less than 5 mm, preferably 3 mm or less, from the plasma emission area. A sealed valve made of fused quartz used as the chamber at least partially meets these criteria.

[0010] However, the transparency boundary of quartz, λb≒170 nm, is inferior to other optical materials mentioned above, especially MgF2 (λb≒110 nm). At the same time, the option of replacing the valve material with MgF2 is difficult due to its mechanical properties, and the use of MgF2 windows is also problematic due to the difficulty of sealing under high temperature and pressure.

[0011] To increase the operating temperature of the chamber, U.S. Patent No. 10,109,473, published on October 23, 2018, proposes mechanically sealing the chamber window using a C-ring made of an elastic metal such as steel.

[0012] However, this solution mainly concerns the use of a sapphire window with λb ≈ 145 nm. Applying this type of sealing to a MgF2 window is problematic due to its poor mechanical strength.

[0013] In U.S. Patent No. 10,609,804 published on May 31, 2020, a laser-produced plasma light source includes a gas-filled chamber having a metal cylindrical housing consisting of two housing parts and coaxial entrance and exit windows hermetically installed at both ends of the housing. Each window is nickel-plated on its cylindrical side and placed inside a circular nickel-plated Kovar sleeve and soldered to the inner surface of the sleeve using Ag solder. Furthermore, each circular sleeve with a soldered window is soldered or welded to one of the housing parts on the outer seam. After the inner chamber parts (elliptical mirror and laser radiation blocker) are installed, the housing parts with the attached windows are welded together. After welding, the housing is evacuated and filled with gas through a welded or sealed nozzle under pressure. The coefficient of linear thermal expansion (CLTE) of the Kovar sleeve to which the windows are soldered matches that of sapphire, and therefore it is proposed that the chamber use sapphire windows.

[0014] Compared to the typically used quartz bulbs (λb ≒ 170 nm), the source is characterized by a broader emission spectrum in the VUV range when sapphire windows are used (λb ≒ 145 nm) and a more robust chamber that allows to increase the power of the laser pumping and, consequently, the power of the output radiation, including in the UV and VUV ranges.

[0015] However, further expansion of the VUV spectrum is limited due to the difficulty of applying MgF2 windows in this type of plasma source. The CLTE of the MgF2 crystal is significantly different along the optical axis and perpendicular to the optical axis, correspondingly equal to 13.7·10-6 / K and 8.48·10-6 / K. As a result, the connection seal between the isotropic metal circular sleeve and the anisotropic MgF2 crystal soldered to it is unreliable when the chamber is heated to 600-900 K, which is necessary for optimal generation of radiation from a continuous optical discharge plasma. This unreliability of the seal is due to the fact that the CLTE of the metal solder (approximately 20·10-6 / K) is also significantly different from the CLTE of MgF2. Also, the pressure of the gas on the window contributes to the shift and rupture of the sealing joint, thereby reducing its reliability. It has little effect on the broadening of the spectrum of a similar plasma source in the VUV range, also due to the fact that the plasma radiation beam is formed only by reflection of the plasma radiation by a metal mirror inside the chamber. The reflection coefficient of the metal mirror is low in the VUV range (about 20% at the wavelength of 110 nm for aluminum). The presence of the mirror inside the chamber leads to the lens focusing the CW laser beam being located outside the chamber housing, which limits the sharpness of the focusing of the CW laser beam and reduces the source brightness. The presence of the mirror also makes it impossible to minimize the dimensions of the space inside the chamber to suppress convection currents that cause instabilities in the output radiation power. A drawback of the above-mentioned design is also the propagation of the laser radiation beam towards the output window, for which special measures must be taken to block it. Summary of the Invention

[0016] Therefore, there is a need to create a brighter and highly stable light source with a broader radiation spectrum in the VUV range that does not have the above mentioned drawbacks.

[0017] The technical problem and technical result of the invention consists in expanding the radiation spectrum of laser-produced plasma light sources in the VUV range and in providing high brightness and high stability of their broadband radiation.

[0018] The invention essentially consists in using a high-tech optical material, namely MgF2, with a minimum boundary of transparency (λb≒110 nm) as the material of the window for the output of the plasma radiation beam from the chamber, which allows to extend the radiation spectrum of the laser-produced plasma source into the VUV range.

[0019] The gas in the chamber belongs to inert gas with a purity of at least 99.99%, which eliminates the self-absorption of VUV radiation caused by impurities.

[0020] Crystalline magnesium fluoride is anisotropic and characterized by weak double refraction. According to the present invention, in order to eliminate the double refraction of the plasma radiation beam, the surface of the end of the axially symmetric sleeve and the surface of the adjacent MgF2 exit window are essentially perpendicular to the optical axis of the MgF2 crystal.

[0021] In order to provide high brightness and high stability of the light source, the possibility of operating at high temperatures of at least 600 K and at pressures of about 50 atm and above is achieved by sealing the chamber window by soldering with glass cement. According to the invention, the glass cement soldering process involves the application of a single-stage annealing of the joint at a temperature of at least 400° C., which results in the possibility of operating the joint at temperatures of up to 900 K. The window is soldered to a separate metal part of the housing designed as a sleeve. After annealing, the metal parts of the chamber housing are joined by welding, in such a way that the seal joint is not exposed to another annealing that may reduce the reliability of the seal joint.

[0022] To provide a reliable seal for the MgF2 exit window, the sleeve and housing are fabricated from an iron-nickel alloy, e.g., 47ND alloy, having a predetermined CLTE that matches the CLTE of the crystalline magnesium fluoride in a direction perpendicular to the optical axis of the crystal.

[0023] To prevent window cracking caused by irregular cooling, instead of soldering to the complex shaped housing parts of the chamber, the windows are soldered to the ends of axisymmetric metal sleeves with a length of about 1 cm or more. The soldering is performed using the sealed joint components with matching linear thermal expansion coefficients (CLTE) and optimally positioned in terms of gravity. The sleeves with the soldered windows are then welded to the housing on the outside seam. In another embodiment, the sleeves with the soldered windows are welded to the housing parts and the housing is permanently welded together after the internal chamber elements are installed. At the same time, the axisymmetric sleeves compensate for the irregularities in heating and cooling of the assembled chamber structure.

[0024] According to the invention, a window is installed inside the gas-filled chamber. On the one hand, the reliability of the seal is improved due to the high pressure of the gas in the chamber, which compresses the sealing elements. On the other hand, the possibility of producing a chamber with optimally minimized dimensions is realized, if the chamber wall, including its optical elements, is located at a distance of less than 5 mm from the plasma emission area. This provides high stability of the radiation source by suppressing convection turbulence in the chamber.

[0025] The internal chamber element includes a lens for focusing the CW laser beam. Said focusing lens, preferably of aspherical design, is arranged between the entrance window and the plasma emission region, improving the brightness of the light source due to the sharpest possible focusing of the CW laser beam. For the same purpose, at least one retroreflector, for example in the form of a spherical mirror centered in the plasma emission region, can be mounted in the chamber, located opposite the exit window and / or on the axis of the focused laser beam. The exit window can also be a lens designed to reduce aberrations that distort the path of the plasma emission beam passing through the exit window and / or to reduce the angular aperture of the exit plasma emission beam.

[0026] To prevent the generation of ozone and the absorption of the plasma radiation beam, a vacuum or gas environment that does not absorb VUV radiation having a wavelength of 110 nm or more can be placed outside the MgF2 exit window. For this purpose, in one embodiment of the invention, the chamber can be hermetically connected to an outer chamber having an object, through which the plasma radiation beam is transported, filled with a vacuum or gas environment that does not absorb the plasma radiation exiting the chamber through the MgF2 window. Since the optimum temperature can be as high as 600 K or more, the chamber can be hermetically connected to the outer chamber by a branch pipe made to have the function of a thermal bridge between the chamber and the outer chamber. Furthermore, said branch pipe can be equipped with a cooling radiator to prevent heating of said outer chamber.

[0027] Another aspect of the present invention aims to further increase the brightness and stability of laser-produced plasma radiation sources and improve their performance.

[0028] The above and other objects, advantages and features of the present invention will become more apparent in the following non-limiting description of embodiments thereof, provided by way of example with reference to the accompanying drawings.

[0029] The essence of the invention is illustrated by the drawings.

[0030] In the drawings, identical device elements are given the same reference numerals.

[0031] These drawings do not cover the entire scope of embodiments of this technical solution, and furthermore, do not limit the entire scope of embodiments of this technical solution, but are merely illustrative examples of specific cases in which this technical solution is implemented. [Brief description of the drawings]

[0032] [Figure 1] FIG. 1 is a cross-sectional view of a broadband laser pumping light source according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a cross-sectional view of a broadband laser pumping light source according to an embodiment of the present invention. [Diagram 3] FIG. 1 is an external view of a broadband laser excitation light source. [Figure 4] FIG. 1 is a diagram of a broadband laser pump source according to an embodiment of the present invention. [Diagram 5] FIG. 1 is a diagram of a broadband laser pump source according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] In an example embodiment of the invention shown in Figure 1, a broadband laser pumped source comprises a chamber 1 filled with high pressure gas, within which a plasma emission region 2 is maintained by a focused beam 3 of a continuous wave (CW) laser 4. The chamber 1 comprises a metal housing 5 with a window 6a for introducing the CW laser beam into the chamber, and at least one window 6b for outputting a plasma emission beam 8 from the chamber for subsequent use.

[0034] The light source also includes means for initiating plasma ignition. As means for plasma ignition a pulsed laser system 9 can be used which generates at least one focused pulsed laser beam 10 in the chamber area designed to sustain the plasma emission 2. In another embodiment of the invention igniting electrodes can be used as plasma ignition means.

[0035] According to the invention, the CW laser beam is directed into the chamber by a dichroic mirror 11 and focused using a lens 12 located in the chamber between the window 6a and the plasma emission region 2, providing sharper focusing of the CW laser beam and thereby increasing the source brightness. The lens 12 can be used simultaneously to focus the pulsed laser beam 10 at the onset of plasma ignition.

[0036] The source brightness is increased by ensuring the sharpest possible focusing of the CW laser beam using an optical system with a window 6a and a focusing lens 12, preferably of aspherical design, which minimizes the total aberrations of said optical system. The focusing lens 12 is preferably positioned at the smallest possible distance from the plasma emission area 2, said distance not exceeding 5 mm. To facilitate the chamber design, the window 6a can be produced using simple manufacturing techniques, for example in the form of a plate or a lens with a spherical surface. The aspherical lens 12 can be made from glass or quartz to facilitate its manufacture.

[0037] At least one window 6b, through which the plasma radiation beam 8 leaves the chamber, is made of crystalline magnesium fluoride (MgF2). MgF2 is characterized by high productivity and at the same time has the shortest-wave boundary of transparency among optical materials. The short-wave boundary of the spectrum of the plasma radiation beam leaving the chamber is therefore determined by the MgF2 transmission limit in the vacuum ultraviolet (VUV) region, which is about 110 nm. Furthermore, said gas belongs to inert gases or is a mixture thereof with a purity of at least 99.99%, eliminating the self-absorption of VUV radiation by gas impurities. This allows the radiation spectrum of the light source to be extended into the vacuum ultraviolet region.

[0038] In Fig. 1, the plasma radiation beam 8 is directed straight and without reflection from the plasma emission region 2 into the MgF2 window 6b. In contrast to sources in which the plasma radiation beam is formed by an in-chamber metal mirror, which has a low reflection coefficient in the VUV range (<20% at λ = 110 nm), this ensures that there is no cut-off or suppression of the VUV component in the spectrum of the plasma radiation beam.

[0039] Each of the windows 6a, 6b is located inside the chamber at one end of the sleeve 7a, 7b closest to the plasma emission region 2. Each of the windows 6a, 6b is soldered to one of the sleeves 7a, 7b using glass cement 13. The window soldering, which is performed in a process of annealing, ensures the sealed joint and the handleability of the chamber assembly at temperatures up to 900K, which is optimal for achieving high brightness and high stability of the light source.

[0040] Each of the sleeves 7a, 7b with the soldered window 6a, 6b is placed in one of the holes of the housing 5 and welded to said hole of the housing 5 on the outer weld seam 14. Furthermore, the inner part of the axisymmetric sleeve 6a, 6b is the outer part of the chamber and is not in contact with the filling gas, which improves the reliability of the sealing joint and facilitates the sealing of the optical element due to the high pressure of the gas in the chamber which compresses the sealing material (glass cement 13) along with the placement of the window in the chamber.

[0041] According to the invention, the surface of the end of the sleeve 7b and the surface of the MgF2 exit window 6b adjacent thereto are essentially perpendicular to the optical axis of the MgF2 crystal. The coefficients of linear thermal expansion (CLTE) of the glass cement 13, the material of the sleeves 7a, 7b and the housing 5 match the CLTE of the crystalline magnesium fluoride in the direction perpendicular to the optical axis of the MgF2 crystal. All the above provides high reliability and longer life of the window and chamber assembly. Preferably, the sleeve and chamber housing are made from 47ND iron-nickel alloy which meets these requirements.

[0042] The chamber 1 is filled with high pressure gas through soldered welded piping or through gas ports 15 designed to control the pressure of the gas in the chamber and / or the composition of the gas in the chamber.

[0043] Therefore, the present invention provides for the fabrication of reliable chambers with MgF2 windows operating under high pressure (about 50 atm) and high temperature (about 900°K) and the creation of brighter and more stable COD-based light sources with the broadest radiation spectrum in the VUV range.

[0044] According to an embodiment of the invention shown in Fig. 1, a vacuum environment or a gas environment such as helium, argon, which does not absorb VUV radiation having a wavelength of 110 nm or more, is arranged outside the MgF2 exit window 6b, which outputs the plasma radiation beam 8 from the chamber. For this purpose, the chamber 1 may be hermetically connected by means of a branch pipe 16 to an outer chamber 17, which comprises an object through which the plasma radiation beam 8 is transported.

[0045] In this case, the beam is delivered without generating ozone and without losing the VUV component of the plasma radiation.

[0046] High stability and high brightness of plasma emission in continuous operation mode is achieved when the gas pressure in the chamber is about 50 atm or more and the chamber temperature is about 600 K or more. Due to the high temperature of the chamber 1, the branch pipe 16 is designed with the function of a thermal bridge between the chamber 1 and the outer chamber 17. For this purpose, at least a part of the branch pipe 16 is made, for example from thin stainless steel, so as to have a low thermal conductivity. To cool the part of the branch 16 spaced apart from the window 6b, it is designed as a cooling radiator 18 to prevent heating of the outer chamber 17. The sealed joint of the branch pipe 16 to the chamber 1 and the outer chamber 17 can be provided by means of a sealing gasket 19, which can be made of copper, at least on the side of the heated chamber 1.

[0047] In the embodiment of the invention shown in Fig. 1, all of the axially symmetric sleeves 7a, 7b with the windows 6a, 6b soldered thereto are welded to a single common housing part 5. Furthermore, the plasma emission area 2 is positioned in a cavity of the housing 5 formed by the intersection of at least two holes, in each of which one of the sleeves 7a, 7b with one of the windows 6a, 6b is located. The sleeves 7a, 7b have a variable outside diameter, and the windows 6a, 6b are located at the ends of the sleeves with the smaller outside diameter.

[0048] The broadband laser pump source works as follows. First, a light source chamber 1 is manufactured with a metal housing 5 having at least two windows 6, 6b, as shown in FIG. 1. At least one window 6b is made of MgF2. The material of at least one window 6a can be glass with a CLTE matching that of MgF2. The chamber housing is manufactured from 47ND precision alloy, also with a CLTE matching that of MgF2. Each of the windows 6a, 6b is soldered to one of the sleeves 7a, 7b using glass cement 13, applying annealing at a temperature of at least 400°C. Each of the sleeves with the soldered windows is welded to a hole in the metal housing 5. The chamber is filled with high pressure gas either through sealed piping or gas port 15.

[0049] The broadband emission of the COD plasma is generated as follows: A focused beam 3 of a CW laser 4 is directed into the region 2 of the chamber with the intention of sustaining the plasma emission. Preferably, high purity inert gases and mixtures thereof are used as said gas. At least one pulsed laser beam 10 is generated by a pulsed laser system 9. A beam of the CW laser and a pulsed laser beam are introduced into the chamber 1 through a window 6a. At the same time, an optical system comprising the window 6a and a focusing lens 12 provides a sharp focusing of the laser beam. The pulsed laser system 9 is used to provide optical breakdown and generate a starting plasma having a density above the threshold density of the COD plasma having a value of about 1018 electrons / cm3. The concentration and volume of said starting plasma are sufficient for reliable sustainment of a continuous optical discharge by the focused beam of the CW laser 3 at a relatively low power not exceeding 300 W. In stationary mode, a broadband high-intensity emission is output from the plasma emission region 2 of the continuous optical discharge using at least one plasma emission beam 8. The shortwave boundary of the spectrum of the plasma radiation leaving the chamber is determined by the MgF2 transmission limit, which is about 110 nm. The beam 8 leaving the chamber through the MgF2 exit window 7b is intended for further use, for example in an outer chamber 17. The chamber 1 can be hermetically connected to the outer chamber 17, which is filled with a vacuum or gas environment that does not absorb the VUV radiation leaving the chamber 1. In the operating mode, the temperature of the chamber 1 is preferably about 600 K or higher. Furthermore, thermal insulation between the chamber 1 and the outer chamber 17 is provided by a branch pipe 17 designed to have a thermal bridge function and equipped with a cooling radiator 19.

[0050] In the embodiment of the invention shown in FIG. 2, the chamber 1 comprises a welded metal housing 5 with at least two housing parts 5a, 5b to which are welded respective sleeves 7a, 7b to which windows 6a, 6b are soldered.

[0051] After the internal chamber elements including the focusing lens 12 with the mount or casing 20 and the insert 21 are installed, the housing parts 5a, 5b having the windows 6a, 6b are welded at the weld seam 22. During welding of the housing parts 5a, 5b, the axisymmetric sleeves 7a, 7b welded together with the windows 6a, 6b compensate for irregular heating and cooling of the assembled chamber 1.

[0052] An external view of the welded housing of the light source is shown diagrammatically in FIG.

[0053] To simplify the chamber design, the welds 14 , 22 are located on the exterior surface of the housing 5 .

[0054] 4 shows a schematic representation of another embodiment, in which the MgF2 window 6b for outputting the plasma radiation beam 8 from the chamber is a lens designed to reduce the angular aperture of the plasma radiation beam or to reduce aberrations that distort the path of the plasma radiation rays as they pass through the window 6b. Typically, the window 6b is designed as a meniscus or another type of matching lens. This increases the brightness of the radiation source, minimizes the dimensions of the source and improves its ease of operation.

[0055] For the same purpose of increasing the source brightness, retroreflectors 23, 24 designed as spherical mirrors centered on the plasma emission region 2 are mounted in the source chamber of Fig. 4. The retroreflectors 23 and 24 are arranged opposite the MgF2 window 6b and are positioned on the axis of the focused laser beam 3.

[0056] In order to exclude the undesired presence of CW laser radiation in the plasma radiation beam, the direction of the plasma radiation beam 8 differs from the direction of the beam of the CW laser 3 passed through the plasma radiation area 2. This prerequisite is easily implemented in the design of the chamber 1, whose housing is designed as a cube or rectangular prism as shown in figures 1, 2, 3 and 4, in which case the focused beam of the CW laser 3 and the respective plasma radiation beam 8 are arranged on mutually orthogonal axes that intersect in the plasma radiation area 2.

[0057] In a preferred embodiment of the invention, in Figs. 1, 2 and 4, the axis of the focused beam of the CW laser 3 is directed vertically upwards, i.e. against gravity or close to vertical. With this proposed design the highest stability of the output of the laser pumped source radiation is achieved. This is due to the fact that typically the plasma emission region 2 is slightly shifted from the focus towards the focused beam 3 of the CW laser to a cross section of the focused laser beam where the intensity of the focused beam 3 of the CW laser is still sufficient to maintain the plasma emission region 2. With the focused beam 3 of the CW laser directed from below to above, the plasma emission region 2, which contains the hottest plasma with the lowest mass density, tends to float under the influence of buoyancy forces. The rise of the plasma emission region 2 ends up at the position closest to the focus, where the cross section of the focused beam 3 of the CW laser becomes smaller and the laser radiation intensity is higher. This on the one hand increases the brightness of the plasma emission and on the other hand equalizes the forces acting on the plasma emission region, ensuring high stability of the radiation output of the high brightness laser pumped source.

[0058] The stability of the output characteristics of the laser pumped light source is also influenced by the size of the pulse obtained under the action of buoyancy due to the heated gas in the plasma emission region 2. The pulse and convection turbulence obtained by the gas are less the closer the plasma emission region 2 is to the upper wall of the chamber. Therefore, to ensure more stable output characteristics of the light source, the upper wall of the chamber housing is located (positioned) at a distance of 5 mm or less from the plasma emission region 2.

[0059] Suppression of convection turbulence in the chamber and improvement of the stability of the light source output characteristics are achieved by reducing the internal volume of the chamber. For this purpose, in a preferred embodiment of the present invention, the chamber wall, as well as the focusing lens 13 and each window 6b for outputting the plasma emission beam are located (positioned) at a distance of 5 mm or less from the plasma emission region.

[0060] 5 shows diagrammatically a further embodiment of the light source according to the invention, in which the chamber housing comprises several windows 6b, 6c for outputting from the chamber 1 several plasma radiation beams 8 as required for a particular application of the light source.

[0061] Preferably, a highly efficient diode near infrared laser is used as CW laser 4, which outputs its radiation into an optical fiber 25. At the exit of the optical fiber 25, the divergent laser beam is directed towards a collimator 26, for example in the form of a collecting lens. After the collimator 26 and a dichroic deflection mirror 11, the divergent beam of the CW laser is directed into the chamber 1. The optics window 6a and the focusing lens 12 ensure a sharp focusing of the beam 3 of the CW laser, which is necessary to achieve a high brightness of the light source.

[0062] In the embodiment of the invention of Fig. 5, the ignition initiation of the plasma is provided by a solid-state laser system comprising a first laser 27 generating a first laser beam 28 in Q-switched mode and a second laser 29 generating a second laser beam 30 in free-running mode. The pulsed laser with active element 31 comprises an optical pumping source, for example in the form of a flash lamp 32, and preferably comprises a common cavity mirror 33, 34. The first laser 27 is equipped with a Q-switch 35.

[0063] In Fig. 5, two pulsed laser beams 28, 30 are directed into the chamber and focused with the intention of maintaining a plasma emission region 2. The first laser beam 28 is intended to initiate plasma ignition or optical breakdown. The second laser beam 30 is intended to create a plasma, the volume and density of which is high enough for the constant maintenance of the plasma emission region 2 by the focused beam 3 of a CW laser.

[0064] Preferably, the wavelength λCW of the CW laser is different from the wavelengths λ1, λ2 of the first and second pulsed laser beams 28, 30. For example, the wavelength of the CW laser can be λCW=0.808 μm or 0.976 μm, and the pulsed laser can have an emission wavelength λ1=λ2=1.064 μm. This allows the use of a dichroic mirror 11 for introducing the laser beam 36 of the CW laser 4 and the pulsed laser beams 28, 30. Furthermore, a tilt mirror 37 can be used to transmit the pulsed laser beams 28, 30 of FIG. 5.

[0065] This embodiment of the invention offers the reliability of laser ignition and the ease of use of the light source. In contrast to sources that use electrodes to initiate plasma ignition, the possibility to optimize the chamber geometry, reduce convection turbulence in the chamber, and minimize optical aberrations is achieved.

[0066] Otherwise, the equipment parts of this embodiment are the same as those of the previously described embodiment, so they are given the same reference numerals in FIG. 5 and detailed descriptions of them are omitted.

[0067] In summary, the proposed invention makes it possible to extend the radiation spectrum in the VUV spectral range and ensure high brightness and stability of the laser-produced plasma radiation source. [Industrial Applicability]

[0068] The high-brightness, high-stability laser-excited light source designed according to this invention can be used in various projection systems for spectrochemical analysis, spectral microanalysis of biological objects in biology and medicine, microcapillary liquid chromatography, inspection of photolithography processes, spectrophotometry and other purposes.

Claims

1. 1. A laser-produced plasma light source comprising a chamber filled with high pressure gas, plasma ignition means, and a plasma emission region maintained within said chamber by a focused beam of a continuous wave (CW) laser, said chamber including a metal housing having a window for introducing said beam of the CW laser into said chamber and at least one window for outputting said plasma emission beam from said chamber, said laser-produced plasma light source comprising: the beam of the CW laser is focused by a lens located within the chamber between the window and the plasma emission region; The gas belongs to the inert gas family or is a mixture of inert gases having a purity of at least 99.99%, At least one window for outputting said plasma radiation beam is made of crystalline magnesium fluoride (MgF ２ ) Each window is disposed inside the chamber at an end of a sleeve closest to the plasma emission region, the sleeve being disposed in a bore in the housing; each window being soldered to the sleeve by glass cement, and the sleeve with the window soldered thereto being welded to the hole in the metal housing; the chamber is hermetically connected by a branch pipe to an outer chamber comprising an object irradiated with plasma radiation through the MgF 2 window, the branch pipe being made as a thermal bridge; Laser-produced plasma light source.

2. The surface of the end face of the sleeve and the MgF ２ The adjacent surface of the window is the above MgF ２ The light source of claim 1 , wherein the light source is substantially perpendicular to the optical axis of the crystal.

3. Each sleeve and the housing are made of the MgF ２ 10. The light source of claim 1, fabricated from a nickel-iron alloy having a coefficient of linear thermal expansion (CLTE) in a direction perpendicular to the optical axis of the crystal that matches the CLTE of said crystalline magnesium fluoride.

4. The short-wave boundary of the spectrum in the plasma radiation beam is MgF in the vacuum ultraviolet (VUV) region. ２ 2. The light source of claim 1, wherein the transmission boundary is determined by a transmission boundary equal to 110 nm.

5. The above MgF is placed in a vacuum or gas environment that does not absorb VUV light with a wavelength of 110 nm or more. ２ 10. The light source of claim 1, disposed outside the window.

6. A light source as described in claim 5, wherein the branch pipe is equipped with a cooling radiator.

7. The plasma emission beam is guided from the plasma emission region to the MgF ２ 10. The light source of claim 1, which is aimed directly at the window without reflection.

8. 2. The light source of claim 1, wherein all sleeves are axisymmetric sleeves to which said window is soldered, said axisymmetric sleeves being welded to said housing made in a single piece.

9. 2. The light source of claim 1, wherein the plasma emission region is disposed within a housing cavity formed by the intersection of at least two holes, and a sleeve having a window is present at each of the at least two holes.

10. 2. The light source of claim 1, wherein at least one of said sleeves is disposed within said bore of said housing, said sleeve having a variable outer diameter, and said window is disposed at an end of said sleeve having a smaller outer diameter.

11. 10. The light source of claim 1, wherein the housing includes at least two housing parts having the window, the housing parts being welded together after an internal chamber part is installed.

12. 12. The light source according to claim 11, wherein at least one retroreflector designed as a spherical mirror is disposed in the chamber.

13. 10. The light source of claim 1, wherein a weld is on the exterior of the housing.

14. 2. The light source of claim 1, wherein the plasma ignition means is a solid-state laser system generating two pulsed laser beams in a Q-switching mode and a free-running mode, and wherein in a continuous operation mode the gas pressure in the chamber is greater than about 50 atm and the temperature of the inner surface of the chamber is at least 600 K.

15. 2. The light source of claim 1, wherein the focused beam of the CW laser is directed vertically upward into the chamber and the top wall of the housing is positioned 5 mm or less from the plasma emission region.

16. 2. The light source of claim 1, wherein the lens for focusing the beam of the CW laser and the window for outputting the plasma emission beam are located at a distance of 5 mm or less from the plasma emission region.

17. 2. The light source of claim 1, wherein the window is a lens positioned to reduce aberrations that distort the path of rays of the plasma radiation beam passing through the window and to reduce the angular aperture of the plasma radiation beam exiting the chamber.

18. 2. The light source of claim 1, wherein a direction of said plasma emission beam is different from a direction of said CW laser beam that passed through said plasma emission region.

19. 2. The light source according to claim 1, wherein the chamber housing is designed as a rectangular parallelepiped, and the focused beam of the CW laser and the plasma emission beam have mutually orthogonal axes that intersect within the plasma emission region.

20. 10. The light source of claim 1, wherein the housing includes either a sealed gas inlet or a gas port designed to fill the chamber with gas and to control the pressure and composition of the gas within the chamber.

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