Abstract:
In an extreme ultra violet light source apparatus having a comparatively large output power for exposing, a solid target is supplied fast and continuously while heat dissipation for irradiation of a driver laser light is performed successfully. The extreme ultra violet light source apparatus includes: a chamber in which extreme ultra violet light is generated; a target material supplying unit which coats a wire with target material, a wire supplying unit which supplies the wire coated with the target material to a predetermined position within the chamber, a driver laser which applies a laser beam onto the wire coated with the target material to generate plasma; and a collector mirror which collects the extreme ultra violet light radiated from the plasma and outputting the extreme ultra violet light.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source of exposure equipment. 
   2. Description of a Related Art 
   Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light with a wavelength of about 13 nm and reduced projection reflective optics. 
   As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography requiring power of more than several tens of watts. 
   Here, there will be explained a principle of the EUV light generation in the LPP type light source apparatus. Target material supplied into a vacuum chamber is irradiated with a laser beam, and the target material is excited into plasma state. From this plasma, light-components with various wavelengths including the EUV light are radiated. Then, a light component with a desired wavelength (e.g., component with a wavelength of 13.5 nm) is selectively reflected and collected by using an EUV collector mirror and outputted to exposure equipment. On the reflecting surface of the EUV collector mirror, for example, a multi-layered film (Mo/Si multi-layered film) is formed by alternately stacking a molybdenum (Mo) thin film and a silicon (Si) thin film. 
   In such an LPP type EUV light source apparatus, particularly in a case of using a solid target, there is a problem about influence of neutral particles or ions emitted from the plasma. Since the EUV collector mirror is disposed close to the plasma, the neutral particles emitted from the plasma attach to the reflecting surface of the EUV collector mirror to deteriorate reflectivity of the mirror. Meanwhile, the ions emitted from the plasma cut out the multi-layered film formed on the reflecting surface of the EUV collector mirror. Here, flying particles from the plasma including neutral particles and ions and remains of the target material are called as debris. 
   As a related technology, Japanese Patent Application Publication JP-P2006-244837A discloses a laser plasma radiated light generating apparatus comprising means for supplying material, which is solid at a room temperature, continuously for a long time by using a simple device operated with a simple adjustment. The laser plasma radiated light generating apparatus ejects a solution containing fine particles from a nozzle to generate a liquid jet or a liquid droplet, irradiates the liquid jet or liquid droplet with a pulse laser beam to evaporate the solvent thereof by heat, and consecutively after a delay time of 0.1 μs or more, irradiates the heated liquid jet or liquid droplet with another pulse laser beam to generate-plasma. 
   Further, Japanese Patent Application Publication JP-A-11-250842 discloses a laser plasma light source which generates little debris and has a high conversion efficiency using a solid target. The laser plasma light source uses a solid target formed with a hollow at a part thereof irradiated with a laser beam, ablates an inside wall of the hollow by using a pulse laser for ablation, irradiates the hollow with a pulse laser beam for heating, after having waited for generation of a high density portion of evaporated material in the space within the hollow, and then excites the high density portion into high temperature plasma to generate a radiation. 
   SUMMARY OF THE INVENTION 
   Generally, as the solid target material, there is used tin having high conversion efficiency from driver laser light energy to EUV light energy. However, solid tin melts and flies off at an elevated temperature by irradiation of the driver laser, and debris thereof deteriorates an efficiency to generate the EUV light. Therefore, conventionally, a target in a state of a droplet, in which fine particles of tin with diameters of about 20 μm to 200 μm are dispersed into liquid, is transferred into an irradiation space of a laser beam thereby to minimize debris generated. 
   Recently, however, it has been confirmed that combination of a CO 2  laser and tin target reduces significantly an amount of debris generated from the tin by irradiation of a laser beam. This shows a possibility of using solid tin as the target. In the past, as means for supplying a solid target continuously, it is known to reciprocate a plate-formed target, to supply and rewind a tape-formed target, or to rotate and reciprocate a rod target, and they are limitedly applied mainly to a low repetition frequency irradiation with low output power. 
   However, in an EUV light source to be used for exposure equipment for a large scale production, a target is irradiated with driver laser light having a power of about 10 kW at a repetition frequency of about 100 kHz. Therefore, it is required to supply the target fast and continuously, and further, it becomes a problem how to dissipate heat caused by irradiation of the driver laser light having the power of about 10 kW. 
   The present invention has been achieved in view of these problems. The purpose of the present invention is to supply a solid target fast and continuously while successfully dissipating heat caused by irradiation of driver laser light in an extreme ultra violet light source apparatus having a comparatively large output power for exposing. 
   In order to accomplish the above purpose, an extreme ultra violet light source apparatus according to one aspect of the present invention is an extreme ultra violet light source apparatus for generating extreme ultra violet light by applying a laser beam onto target material, and includes: a chamber in which extreme ultra violet light is generated; a target material supplying unit which coats a wire with target material; a wire supplying unit which supplies the wire coated with the target material to a predetermined position within the chamber; a driver laser which applies a laser beam onto the wire coated with the target material to generate plasma; and a collector mirror which collects the extreme ultra violet light radiated from the plasma and outputs the extreme ultra violet light. 
   According to the present invention, a wire coated with target material is irradiated with a laser beam, and thereby, a solid target can be supplied fast and continuously while heat caused by irradiation of driver laser light is being successfully dissipated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating a configuration of an EUV light source apparatus according to a first embodiment of the present invention; 
       FIG. 2  is a diagram showing wavelengths and critical densities of a CO 2  laser and a Nd:YAG laser; 
       FIG. 3  is a diagram illustrating condition in which laser beams are reflected from the vicinity of a target; 
       FIG. 4  is a diagram illustrating a detailed configuration of a wire supplying unit and so on; 
       FIGS. 5A and 5B  are diagrams illustrating examples of a configuration for expediting heat dissipation of a wire; 
       FIGS. 6A and 6B  are diagrams illustrating examples of a configuration for preliminary cooling; 
       FIG. 7  is a diagram illustrating a first specific example of the target material supplying unit shown in  FIG. 1 ; 
       FIG. 8  is a diagram illustrating a second specific example of the target material supplying unit shown in  FIG. 1 ; 
       FIG. 9  is a diagram illustrating a third specific example of the target material supplying unit shown in  FIG. 1 ; 
       FIG. 10  is a diagram illustrating a fourth specific example of the target material supplying unit shown in  FIG. 1 ; 
       FIG. 11  is a diagram illustrating a configuration of an EUV light source apparatus according to a second embodiment of the present invention; and 
       FIG. 12  is a diagram illustrating a specific example of a pressure retaining means to be used in the second embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Here, the same constituent elements are denoted by the same reference numeral and description thereof will be omitted. 
     FIG. 1  is a schematic diagram illustrating a configuration of an EUV light source apparatus according to a first embodiment of the present invention. The EUV light source apparatus according to the present embodiment employs the laser produced plasma (LPP) type in which EUV light is generated by excitation of target material with irradiation of a laser beam. 
   As shown in  FIG. 1 , the EUV light source apparatus is provided with a vacuum chamber  10  in which the EUV light is generated, a wire supplying unit  11  for supplying a wire  1  coated with target material to a predetermined position in the vacuum chamber  10 , a surface profile forming unit  12  for forming a surface profile of the wire  1  coated with the target material, a driver laser  13  for generating an exciting laser beam  2  to be applied onto the wire  1  coated with the target material, a laser beam focusing optics  14  for focusing the exciting laser beam  2  generated by the driver laser  13 , an EUV collector mirror  15  for collecting and outputting EUV light  4  emitted from plasma  3  generated by applying the exciting laser beam  2  onto the wire  1  coated with the target material, a wire cooling unit  16  for cooling the wire  1  applied with the laser beam, and a target material supplying unit  17  for coating the wire  1  cooled by the wire cooling unit  16  with the target material. 
   The vacuum chamber  10  is provided with an input window  18  for inputting the exciting laser beam  2  and an output window  19  for outputting the EUV light radiated from the plasma  3  to exposure equipment. Here, the inside of the exposure equipment is kept in vacuum or a reduced pressure state as well as the inside of the vacuum chamber  10 . In the present embodiment, the wire supplying unit  11 , the surface profile forming unit  12 , the wire cooling unit  16 , and the target material supplying unit  17  are disposed inside the vacuum chamber  10 . 
   The wire  1  coated with the target material is transferred by the wire supplying unit  11 , formed to have a surface profile suitable for EUV light generation by the surface profile forming unit  12 , and then supplied to the predetermined position within the vacuum chamber  10 . 
   The driver laser  13  is a laser beam source capable of performing pulse-oscillation at a high repetition frequency (e.g., a pulse width of about several nanoseconds to several tens of nanoseconds, and a repetition frequency of about one kilohertz to one hundred kilohertz). Further, the laser beam focusing optics  14  is constituted from at least one lens and/or at least one mirror. The laser beam  2  focused by the laser beam focusing optics  14  irradiates the wire  1  coated with the target material at the predetermined position within the vacuum chamber  10 , and thereby, part of the target material is excited into plasma state and light components with various wavelengths are radiated from an emitting point. Here, the emitting point means a position where the plasma  3  is generated. 
   The EUV collector mirror  15  is a collecting optics for collecting a light component with a predetermined wavelength (e.g., EUV light with a wavelength near 13.5 nm) by selective reflection among the light components with various wavelengths radiated from the plasma  3 . The EUV collector mirror  15  has a concave reflecting surface, on which a multi-layered film of molybdenum (Mo) and silicon (Si) is formed to selectively reflect the EUV light with a wavelength near 13.5 nm, for example. 
   In  FIG. 1 , the EUV light is reflected in a right direction by the EUV collector mirror  15  and collected to an intermediate focusing point, and then output into the exposing device. Here, the collecting optics of the EUV light is not limited to the EUV collector mirror  15  as shown in  FIG. 1 , and may be constituted by a plurality of optical components. In this case, however, the alternative optics needs to be also a catadioptric system for suppressing EUV light absorption thereof. 
   The wire  1  irradiated with the laser beam  2  is cooled by the wire cooling unit  16 . While part of the wire  1  irradiated with the laser beam  2  lacks the target material, this is filled by the target material supplying unit  17 , and thereby, the target material can be continuously supplied. The wire  1  coated with the target material by the target material supplying unit  17  is retrieved by the wire supplying unit  11 . 
   In the present embodiment, as the driver laser  13 , a CO2 laser is used which can generate light having a comparatively long wavelength. Further, as the target  1 , tin (Sn) is used. The reason is as follows. 
   Generally, when plasma is generated by applying a laser beam onto a target, there is known a case that a melted layer of a target surface boils suddenly or part of a melted target is ejected as particles by an expanding force of plasma applied to the target (refer to Kobayashi et al. “Ablation plasma generation-control 1 (Laser)”, Journal of Plasma and Fusion Research, Vol. 76, No. 11 (November 2000), pp. 145-1150), which is incorporated herein by reference. 
   In particular, in plasma light source using a solid target, there are a high temperature low density plasma region that generates a radiation in a short wavelength band such as the EUV light and a low temperature high density plasma region that does not generate a radiation in the short wavelength band. In these regions, the low temperature high density plasma region becomes a heat source that generates a lot of debris from the target material after the laser beam irradiation. This heat source forms a melted layer on the surface of the target and a melted metal is ejected by the expanding force of the plasma to fly off, resulting in the debris generated. 
   This process will be described in detail. When a laser beam irradiates target material, the target material is heated and ionized by the laser beam to generate plasma. Then, the laser beam is absorbed in the plasma. A mechanism of the laser beam absorption in the plasma is an absorption mechanism that is an inverse process of bremsstrahlung in which an electromagnetic wave (laser beam) is radiated when an electron gets acceleration in an electric field of an ion, and it is called inverse bremsstrahlung. The inverse bremsstrahlung is the most basic absorption mechanism occurring in laser generation plasma, and is also called a classic absorption. Electrons vibrated by a high frequency electric field causes energy absorption while colliding with ions. 
   In plasma, an electromagnetic wave (laser beam) can be propagated only when having a higher frequency than an electron plasma frequency. That is, when an angular frequency of an laser beam is denoted by ω L  and a angular frequency of an electron plasma is denoted by ω P , the laser beam can be propagated only in a low density plasma region where ω L &gt;ω P . Here, plasma electron density N E  which provides ω L =ω P  is called as a critical density N C . 
   In cases where a solid target is irradiated with a laser beam, there exists plasma ejecting and expanding from the target surface. Therefore, the laser beam is propagated from a lower density region to a higher density region in the plasma while being absorbed, and is reflected in the critical density region. That is, the laser beam is absorbed in going paths to the critical density region and returning paths from the critical density region in the plasma. Accordingly, when the critical density is higher, higher density plasma can absorb energy, but, at the same time, there arises a greater risk of generating a low temperature high density plasma region which causes debris generation. 
   The critical density N C  is represented by the following formula.
 
 N   C (cm −3 )=1.11×10 13 /λ 2  
 
where λ represents a laser beam wavelength.
 
     FIG. 2  shows wavelengths and critical densities of a CO 2  laser and a Nd:YAG laser. The CO 2  laser has an output laser beam with a one order longer wavelength λ and thereby provides a two order lower critical density N C , compared with the Nd:YAG laser. As a result, as shown in  FIG. 3 , a laser beam output from the CO 2  laser is reflected at a high temperature low density region considerably distant from a target surface. Here, in  FIG. 3 , the horizontal axis represents plasma electron density N E  corresponding to a distance from the target surface. Further, as to the Nd:YAG laser, there is shown a case of a fundamental wave ω (wavelength of 1,064 nm) and a case of the second harmonic wave 2ω (wavelength of 532 nm). 
   Using the CO 2  laser for the driver laser suppresses generation of the low temperature high density plasma region, which becomes a heat source generating debris rather than contributing to generate the EUV light, and thereby, hinders melting of the surface of the solid target, and reduces significantly neutral particles which are emitted from the target and attach to the reflecting surface of the EUV collector mirror. On the other hand, a high-speed ion radiated also from plasma cuts off the multi-layered film formed on the reflecting surface of the EUV collector mirror. 
   In the case where tin (Sn) target is used, neutral particles generated from the target are significantly reduced, and therefore, it is verified possible to balance an amount of the neutral particles attaching to the reflecting surface of the EUV collector mirror (deposition amount) and an amount of the multi-layered film cut off from the reflecting surface of the EUV collector mirror (sputtering amount), or to make the deposition amount smaller than the sputtering amount, under a predetermined condition. This can solve a problem that debris attaches to the surface of the EUV collector mirror. 
   The condition thereof is determined mainly by the intensity and/or the pulse width of the exciting laser beam generated by the CO 2  laser. Specifically, the intensity of the exciting laser beam is determined preferably to be 3×10 9  W/cm 2  to 5×10 10  W/cm 2 , and more preferably to be 5×10 9  W/cm 2  to 3×10 10  W/cm 2 . Further, the pulse width of the exciting laser beam is preferably determined to be comparatively short as about 10 ns to 15 ns. 
   The exciting laser beam has an upper limit in the intensity thereof so as not to expand unnecessarily a melted area on a target surface by providing excessive heat to the target, and thereby, debris generation can be suppressed. On the other hand, the intensity of the exciting laser provide a great effect to an EUV conversion efficiency (CE) and thereby has a lower limit to keep the EUV conversion efficiency better than a certain level. The relationship between the exciting laser beam intensity and the EUV conversion efficiency is also disclosed in Hansson et al. “LPP EUV Source Development for HVM”, SPIE, Vol. 6151, No. 61510R (February 2006), which is incorporated herein by reference. 
   Here, the laser beam intensity is represented by the following formula.
 
Laser beam intensity (W/cm 2 ) =Laser beam energy ( J )/{Pulse width( s )·Spot area(cm 2 )}
 
In the present embodiment, since the diameter of a collected laser beam is substantially 100 μm, the spot area of the laser beam is substantially 7.85×10 −5  cm 2 , and the laser beam energy is determined to meet these conditions. For example, if the pulse width of the exciting laser beam is 12.5 ns, the laser beam energy becomes substantially 30 mJ.
 
     FIG. 4  is a diagram illustrating detailed configuration of the wire supplying unit and so on shown in  FIG. 1 . In the present embodiment, the wire supplying unit  11  ( FIG. 1 ) includes a wire drum  11   a , wire tension adjusting part  11   b , and guide pulleys  11   c  and  11   d . The wire drum  11   a , around which the loop wire  1  is wound, transfers the wire  1  and retrieves the wire  1  by rotation. The wire tension adjusting part  11   b  is constituted from, for example, a tension pulley biased with a spring and adjusts a tension of the wire  1  with a spring force. The guide pulleys  11   c  and  11   d  define trajectory of the wire  1 . 
   Rotating the wire drum  11   a  enables the wire  1  coated with the target material to be supplied continuously. Considering the wire  1  might be damaged, the wire  1  is wound more than several turns around the wire drum  11   a  for keeping a stock of the wire  1 . After being used predetermined times, the wire  1  is replaced with a new one. 
   Materials capable of being used for the wire  1  include metals having an excellent thermal conductivity such as cupper (thermal conductivity of 390 W/mK), tungsten (thermal conductivity of 130 W/mK), and molybdenum (thermal conductivity of 145 W/mK), and metals having a high melting point such as tungsten (melting point of 3,382° C.), tantalum (melting point of 2,996° C.), and molybdenum (melting point of 2,622° C.). Alternatively, a wire having a multi-layered structure may be used. For example, it is possible to use a wire made of a multi-layered coating of cupper and diamond on a stainless core wire such as used for cutting a hard material. In the present embodiment, tungsten having an excellent thermal conductivity and a high melting point is used for the material of the wire  1 . Further, the wire  1  needs to have a diameter (e.g., about several millimeters) such that the wire can be robust against deformation required for the winding around the wire drum  11   a . Also for efficient heat dissipation, it is better for the diameter of the wire  1  to be greater to some extent. 
   Further, as the surface profile forming unit  12  ( FIG. 1 ), a forming pulley  12   a  which has a plurality of protrusions meshing with a grove of the guide pulley  11   c  is provided. In the case where a tin plate is irradiated with a laser beam to generate the EUV light, it is known that the tin plate better has a grove or a hollow on the surface thereof. Therefore, the forming pulley  12   a  rotates together with the guide pulley  11   c  when the wire  1  is transferred, and pushes the plurality of protrusions to the wire  1 , and thereby, forms V-shape grooves or hollows having a predetermined profile on the surface of the tin coated on the wire  1 . Thereby, the generation efficiency of EUV light is improved and a highly efficient EUV light source can be realized. 
   In that case, a transfer speed of the wire  1  and a pitch of the protrusions of the forming pulley  12   a  need to be arranged such that a repetition period of the driver laser  13  ( FIG. 1 ) and a pitch of the grooves or hollows formed on the wire  1  correspond to each other. Further, a profile of the protrusions of the forming pulley  12   a  is arranged so as to increase the generation efficiency of the EUV light. For example, the protrusion of the forming pulley  12   a  has a cylindrical shape and the diameter or the height thereof is optimized. 
   Alternatively, as the surface profile forming unit  12  ( FIG. 1 ), a hollow may be formed on the surface of the target material coated on the wire  1  by using a laser instead of the forming pulley  12   a . For example, it is possible to irradiate repeatedly the wire  1  with a laser beam from the driver laser  13  shown in  FIG. 1  by controlling the rotation direction and the rotation speed of the wire drum  11   a , and thereby, the hollow may be formed by the first laser beam irradiation and the plasma  3  may be generated by the second laser beam irradiation. 
   In order to cool the wire  1 , the temperature of which has been increased by the laser beam irradiation, a cooling pulley  16   a  cooled with cooling water is provided as the wire cooling unit  16  ( FIG. 1 ). Here, the wire  1  and the cooling pulley  16   a  are disposed in a vacuum, and therefore, there is a possibility that insufficient contact between the wire  1  and the cooling pulley  16   a  causes a kind of thermal insulation to prevent the heat of the wire  1  from being dissipated. 
   Accordingly, as shown in  FIG. 5A , a nozzle  16   b  may be provided near the cooling pulley  16   a , and a low temperature cooling gas such as argon (Ar) or helium (He) may be made to flow from the nozzle  16   b  toward the wire  1  and the cooling pulley  16   a , and thereby, the heat dissipation of the wire  1  will be expedited. Alternatively, as shown in  FIG. 5B , gas ejection holes  16   c  may be formed in the cooling pulley  16   a , and the low temperature cooling gas such as argon or helium may be made to flow from the gas ejection holes  16   c  toward the wire  1 , and thereby, the heat dissipation of the wire  1  will be expedited. 
   The cooling pulley  16   a  may be disposed far from the plasma generation point or may be disposed close to the plasma generation point. In an extreme case, the cooling pulley  16   a may be disposed on the rear side of the part of the wire  1  irradiated with the laser beam. In that case, since the cooling pulley  16   a  is disposed in an EUV light path, the cooling pulley  16   a  is desired to be made thinner so as not to interrupt the EUV light. Further, a plurality of cooling pulleys may be disposed. 
   Even in the case where the wire  1  has a high heat resistance, when the temperature of the wire  1  exceeds 232° C., there is a case that coated tin melts and thereby reduces the EUV conversion efficiency (CE) or a case that the melted tin flies off to attach to the other members. To solve these problems, it is desirable to control an ultimate temperature in the temperature rise of the wire  1  to be equal to or less than substantially 230° C. lower than the melting point of tin (232° C.) For this purpose, it may be effective to increase the transfer speed of the wire  1 , but a more effective way is a preliminary cooling method in which the wire  1  is preliminarily cooled and the cooled wire  1  is supplied into the plasma generation space. 
     FIGS. 6A and 6B  are diagrams illustrating configuration examples for performing the preliminary cooling. As shown in  FIG. 6A , the wire  1  is cooled to −150° C. by the cooling pulley  16   a , and then, the wire  1  is transferred into the plasma generation space, and thereby, a temperature rise margin up to the melting point of tin becomes substantially 380° C. By this method, even in the case where the wire  1  is irradiated with a laser beam from a high power laser, tin does not melt as far as the temperature rise of the wire  1  is 380° C. or less and tin in the original solid state can be supplied into the plasma generation space. Thereby, the EUV light can be generated stably. 
   Alternately, as shown in  FIG. 6B , an upstream side nozzle  16   d  and a downstream side nozzle  16   e  may be provided to feed the wire  1  through the insides thereof. A low temperature cooling gas such as argon or helium may be supplied from a gas inlet  16   f  provided at a predetermined position of the upstream side nozzle  16   d  into the inside of the upstream side nozzle  16   d , and the low temperature cooling gas maybe sprayed to the periphery of the wire  1 . Since the evaporation temperature of the argon gas is substantially −180° C. and the evaporation temperature of the helium is substantially −268° C., the use of the helium gas for the cooling gas can make a cooling effect greater. 
     FIG. 7  is a diagram illustrating a specific example of the target material supplying unit shown in  FIG. 1 . The target material supplying unit  17  has a container  17   a  for storing melted tin and a pulley  17   b  rotatably held inside the container  17   a . Solid tin is put into the container  17   a  kept at not less than substantially 235° C. higher than the melting point of tin (232° C.), and melted to form a tin bath. Since tin has a low vapor pressure in a vacuum, little tin vapor is generated by melting tin. Accordingly, the container  17   a  is not required to be hermetic and can be disposed in a vacuum in an open state. Further, the container  17   a  is easily replenished with tin. For expediting re-melting of tin, it is desirable to control the temperature of melted tin to, for example, 500° C. higher than the melting point of tin (232° C.) and lower than the vaporization temperature of tin (2,602° C.). 
   As the material of the pulley  17   b , stainless steel (SUS) can be used, for example. In order to repair the wire  1  which has a damaged surface state because of tin lacking caused by the laser beam irradiation, the wire  1  is fed through the melted tin in the container  17   a  guided by the pulley  17   b , and thereby, tin on the surface of the wire  1  is melted and reattachment of tin is carried out. In this manner, the wire  1  is put into the melted tin, and tin on the surface thereof is once melted and tin reattaches from the melted tin to the surface of the wire  1 . Then the wire  1  to which tin has attached is cooled, and thereby, the tin target, which always has a new surface state, can be supplied. 
   As to problems, impurities such as tin oxide floating on the surface layer of the melted tin attaches to the wire  1  resulting in an adverse effect to the EUV generation from the plasma, and tin having attached to the wire  1  is not melted and the diameter of the wire  1  after the reattachment becomes non-uniform. For the former problem, it is effective to provide means for preventing the tin oxide from generating by replacing the inside of the container  17   a  with a gas such as hydrogen. Alternatively, as shown in  FIG. 8 , as means for removing the impurities floating on a surface layer (liquid level) of the melted tin, a pipe  17   c  having a hole diameter slightly larger than the diameter of the wire  1  is provided on an output side of the wire  1  in the container  17   a , and thereby, an amount of the impurities attaching to the wire can be reduced. Here, the lower end of the pipe  17   c  is positioned on the lower side of the liquid level of the melted tin, and the upper end of the pipe  17   c  is positioned on the upper side of the liquid level of the melted tin. 
   For the latter problem, it is effective to control the temperature of the melted tin to be increased up to degree of 1,000° C. Alternatively, as shown in  FIG. 9 , as means for removing mechanically the tin coated on the wire  1  in the melted tin, a scraper  17   a  may be provided in the container  17   d . Here, the scraper  17   d  is positioned on the lower side of the liquid level of the melted tin. 
   Further, in order to expedite and stabilize attachment of tin onto the wire  1  in the melted tin, the surface roughness of the wire  1  may be intentionally increased, or the surface of the wire  1  may be applied with a finishing like knurling. Furthermore, as the material of the wire  1 , a material having a good attachment property for tin such as cupper may be used. Moreover, in order to stabilize the diameter of the wire  1  including reattached tin layer, a forming pulley may be provided. Thereby, it is possible to keep uniform the diameter of the wire including the attached tin layer. 
   Since an amount of the melted tin in the container  17   a  decreases gradually, it is necessary to replenish tin appropriately. As shown in  FIG. 10 , a liquid level detector  17   e  monitoring the liquid level of the melted tin is provided in the container  17   a , and, when the liquid level becomes lower than a predetermined level, solid tin is put into the melted tin and the tin replenishing is carried out. As the liquid level detector  17   e , for example, a thermo-couple for detecting the liquid level by temperature or a laser displacement meter for detecting the liquid level by laser light reflection can be used. According to the present embodiment, only a surface part irradiated with the laser beam flies off from the tin coated on the wire  1 , and tin consumption can be smaller compared with a case using a tin droplet as the target. 
   Next, a second embodiment of the present invention will be described. 
     FIG. 11  is a diagram illustrating a configuration of an EUV light source apparatus according to the second embodiment of the present invention. In the EUV light source apparatus according to the present embodiment, a wire cooling unit  16  is disposed inside a vacuum chamber  10 , while a wire supplying unit (wire drum  11   a , wire tension adjusting part  11   b , and guide pulleys  11   c  and  11   d ), a surface profile forming unit  12 , and a target material supplying unit  17  are disposed outside the vacuum chamber  10  (in the air atmosphere). 
   Accordingly, in transferring a wire  1  from air to a vacuum and back to the air, it is necessary to provide pressure retaining means for retaining a vacuum of the vacuum chamber  10 . In  FIG. 11 , the pressure retaining means is provided at a wire inputting part  21  and a wire outputting part  22  of the vacuum chamber  10 . The other points are the same as the first embodiment. 
     FIG. 12  is a diagram illustrating a specific example of the pressure retaining means to be used in the second embodiment of the present invention. As shown in  FIG. 12 , a member (multistage labyrinth)  23 , which is constituted by arranging a plurality of plates in parallel, each of the plates having an opening with a diameter slightly larger than that of the wire  1 , is used for separating an air atmosphere transferred through and a vacuum atmosphere. When the wire  1  is let into the opening of the multistage labyrinth  23 , a micro-gap is generated between each of the plates and the wire  1 . Accordingly, by vacuum pumping of spaces among the plates by using exhausting pumps  24  and  25 , it is possible to keep a pressure difference between the air atmosphere and the vacuum atmosphere. 
   Although the multi-stage labyrinth  23  is preferably not contact with the wire  1 , the multi-stage labyrinth  23  may come into contact with the wire  1  when the multi-stage labyrinth  23  is made from a flexible material such as rubber, for example. Further, the opening of the plate may be formed by piercing a pipe shaped member through the plate, instead of forming the hole in the plate. According to the present embodiment, the wire supplying unit and so on are disposed outside the vacuum chamber  10 . Thereby, the wire  1  is easily exchanged, and the mechanisms such as the wire drum  11   a  need not be accommodated within a vacuum, resulting in a low cost production of an EUV light source apparatus. 
   Next, as a target of the EUV light source apparatus, there will be compared three cases of: a case of using a wire coated with tin as described in the first and second embodiments of the present invention, a case of using a tin plate, and a case of using a tin droplet. 
   Regarding continuity in the transfer direction of the target, the use of the tin-coated wire and the use of the tin plate have advantages. In these cases, it is possible to select arbitrarily a repetition frequency of the driver laser light. On the other hand, in the case of using the tin droplet, the repetition frequency of the driver laser light is limited depending on a droplet generation frequency, and a control for synchronization thereof is required to make the apparatus complicated. 
   Regarding the EUV conversion efficiency (CE), the use of the tin coated wire and the use of the tin plate have advantages. In the case of using the tin droplet, a pre-pulse laser is necessary for increasing the CE and the cost becomes higher. 
   Regarding an EUV light capturing efficiency, the use of the tin coated wire and the use of the tin droplet have advantages. In the case of using the tin plate, an area where the EUV light is interrupted by the target becomes larger and the EUV light capturing efficiency becomes reduced. 
   Regarding repetition easiness of the target supplying, the use of the tin-coated wire has an advantage. In that case, the wire can be irradiated repeatedly with the driver laser light and a wire supply speed of degree of 10 m/s is sufficiently high. On the other hand, supplying the tin plate in a speed of 10 m/s makes handling of the tin plate difficult and requires a great amount of tin material. Further, in the case of using the tin droplet, a supply speed of the droplet is required to be degree of 100 m/s for making the repetition frequency of the driver laser to be 100 kHz. 
   Regarding heat dissipation easiness of the target, the use of the tin coated wire and the use of the tin droplet have advantages. In the case of using the tin coated wire, the heat dissipation is easy just like a rotating electrode and, when the core material thereof is tungsten or the like, the wire is not cut even at a temperature where tin melts. On the other hand, in the case of using the tin plate, a cooling plate is necessary to be provided in the back of the tin plate. 
   Regarding debris generation, although the use of the tin droplet has an advantage, the debris generation can be suppressed by selection of the conditions as described above also in the case of using the solid tin.