Patent Publication Number: US-10764986-B2

Title: Extreme ultraviolet light generation apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of U.S. application Ser. No. 14/481,620 filed Sep. 9, 2014, which is a Continuation application of U.S. Ser. No. 13/474,100 filed May 17, 2012, now U.S. Pat. No. 8,872,142, which is a Continuation-in-Part application of U.S. Ser. No. 13/048,454 filed Mar. 15, 2011, now U.S. Pat. No. 8,624,208, which claims priority from Japanese Patent Application No. 2010-063358 filed Mar. 18, 2010, Japanese Patent Application No. 2011-017252 filed Jan. 28, 2011, and Japanese Patent Application No. 2011-049687 filed Mar. 7, 2011. This application further claims priority from Japanese Patent Application No. 2011-135566 filed Jun. 17, 2011. 
    
    
     BACKGROUND 
     1. Technical Field 
     This disclosure relates to an extreme ultraviolet (EUV) light generation apparatus. 
     2. Related Art 
     In recent years, semiconductor production processes have become capable of producing semiconductor devices with increasingly fine feature sizes, as photolithography has been making rapid progress toward finer fabrication. In the next generation of semiconductor production processes, microfabrication with feature sizes at 60 nm to 45 nm, and further, microfabrication with feature sizes of 32 nm or less will be required. In order to meet the demand for microfabrication with feature sizes of 32 nm or less, for example, an exposure apparatus in which a system for generating EUV light at a wavelength of approximately 13 nm combined with a reduced proj ection reflective optical system is needed. 
     Three kinds of systems for generating EUV light are known in general, which include a Laser Produced Plasma (LPP) type system in which plasma is generated by irradiating a target material with a laser beam, a Discharge Produced Plasma (DPP) type system in which plasma is generated by electric discharge, and a Synchrotron Radiation (SR) type system in which orbital radiation is used. 
     SUMMARY 
     An apparatus according to one aspect of this disclosure for generating extreme ultraviolet light, which may be used with a laser apparatus and connected to an external device so as to supply the extreme ultraviolet light thereto, may include: a chamber provided with at least one inlet through which a laser beam is introduced into the chamber; a target supply unit provided on the chamber configured to supply a target material to a predetermined region inside the chamber; a discharge pump connected to the chamber; at least one optical element provided inside the chamber; an etching gas introduction unit provided on the chamber through which an etching gas passes; and at least one temperature control mechanism for controlling a temperature of the at least one optical element. 
     An apparatus according to another aspect of this disclosure for generating extreme ultraviolet light, which may be used with a laser apparatus and connected to an external device so as to supply the extreme ultraviolet light thereto, may include: a chamber provided with at least one inlet through which a laser beam is introduced into the chamber; a target supply unit provided on the chamber configured to supply a target material to a predetermined region inside the chamber; a discharge pump connected to the chamber; a collector mirror for collecting the extreme ultraviolet light emitted from plasma of the target material; and a gas supply unit is provided with a gas outlet and is positioned so that the gas outlet is oriented toward a reflective surface of the collector mirror. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Hereinafter, selected embodiments of this disclosure will be described with reference to the accompanying drawings. 
         FIG. 1  schematically illustrates the configuration of an EUV light generation system according to a first embodiment of this disclosure. 
         FIG. 2  schematically illustrates a reaction in which hydrogen molecules are transformed into hydrogen radicals and the hydrogen radicals react with solid tin (Sn) deposited on an optical element to thereby be turned into stannane gas, and a reaction in which the stannane gas is decomposed and solid Sn is deposited. 
         FIG. 3  is a graph showing the relationship between the temperature and each of an etching reaction rate of Sn, a deposition reaction rate of Sn, and a total etching rate represented by a difference between the etching reaction rate of Sn and the deposition reaction rate of Sn. 
         FIG. 4  schematically illustrates the configuration for controlling the temperature of optical elements provided in a chamber according to the first embodiment. 
         FIG. 5  schematically illustrates the configuration for controlling the temperature of optical elements provided in a chamber of an EUV light generation system according to a second embodiment of this disclosure. 
         FIG. 6  schematically illustrates the configuration of an EUV light generation system according to a third embodiment of this disclosure. 
         FIG. 7  schematically illustrates the configuration of a trap positioned to face the reflective surface of an EUV collector mirror in the third embodiment. 
         FIG. 8  is a perspective view schematically illustrating the configuration of a trap positioned to face a gate valve in the third embodiment. 
         FIG. 9  schematically illustrates the configuration of a trap provided at a connection part between a chamber and a discharge pump in the third embodiment. 
         FIG. 10  schematically illustrates the configuration of a trap provided at a predetermined location inside the chamber in the third embodiment. 
         FIG. 11  schematically illustrates the configuration of a trap and a collection unit in an EUV light generation system according to a fourth embodiment of this disclosure. 
         FIG. 12A  schematically illustrates the configuration of a gas introduction pipe and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to a first modification of this disclosure. 
         FIG. 12B  is a sectional view, taken along XIIB-XIIB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 12A . 
         FIG. 13A  schematically illustrates the configuration of gas introduction pipes and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to a second modification of this disclosure. 
         FIG. 13B  is a sectional view, taken along XIIIB-XIIIB plane, schematically illustrating the configuration of the gas introduction pipes and the EUV collector mirror shown in  FIG. 13A . 
         FIG. 14A  schematically illustrates the configuration of gas introduction pipes and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to a third modification of this disclosure. 
         FIG. 14B  is a sectional view, taken along XIVB-XIVB plane, schematically illustrating the configuration of the gas introduction pipes and the EUV collector mirror shown in  FIG. 14A . 
         FIG. 15A  schematically illustrates the configuration of a gas introduction pipe and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to a fourth modification of this disclosure. 
         FIG. 15B  is a sectional view, taken along XVB-XVB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 15A . 
         FIG. 16A  schematically illustrates the configuration of a gas introduction pipe and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to a fifth modification of this disclosure. 
         FIG. 16B  is a sectional view, taken along XVIB-XVIB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 16A . 
         FIG. 17A  schematically illustrates the configuration of gas introduction pipes and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to a sixth modification of this disclosure. 
         FIG. 17B  is a sectional view, taken along XVIIB-XVIIB plane, schematically illustrating the configuration of the gas introduction pipes and the EUV collector mirror shown in  FIG. 17A . 
         FIG. 18A  schematically illustrates the configuration of gas introduction pipes and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to a seventh modification of this disclosure. 
         FIG. 18B  is a sectional view, taken along XVIIIB-XVIIIB plane, schematically illustrating the configuration of the gas introduction pipes and the EUV collector mirror shown in  FIG. 18A . 
         FIG. 19A  schematically illustrates the configuration of a gas introduction pipe and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to an eighth modification of this disclosure. 
         FIG. 19B  is a sectional view, taken along XIXB-XIXB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 19A . 
         FIG. 19C  is a perspective view schematically illustrating a shape of the leading end portion of the gas introduction pipe according to the eighth modification. 
         FIG. 20A  schematically illustrates the configuration of a gas introduction pipe and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to a ninth modification of this disclosure. 
         FIG. 20B  is a sectional view, taken along XXB-XXB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 20A . 
         FIG. 21A  schematically illustrates the configuration of radical generators and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to a tenth modification of this disclosure. 
         FIG. 21B  is a sectional view, taken along XXIB-XXIB plane, schematically illustrating the radical generators and the EUV collector mirror shown in  FIG. 21A . 
         FIG. 22A  schematically illustrates the configuration of filaments and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to an eleventh modification of this disclosure. 
         FIG. 22B  is a sectional view, taken along XXIIB-XXIIB plane, schematically illustrating the configuration of the filaments and the EUV collector mirror shown in  FIG. 22A . 
         FIG. 23  is a sectional view schematically illustrating the configuration of an EUV light generation system according to a fifth embodiment of this disclosure. 
         FIG. 24A  is a sectional view schematically illustrating the configuration of an EUV light generation system according to a sixth embodiment of this disclosure. 
         FIG. 24B  schematically illustrates the configuration of a filament and an EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror shown in  FIG. 24A . 
         FIG. 25A  schematically illustrates the configuration of an EUV collector mirror and gas introduction pipes, as viewed from the reflective surface side of the EUV collector mirror, according to a seventh embodiment of this disclosure. 
         FIG. 25B  is a sectional view, taken along a plane perpendicular to the reflective surface of the EUV collector mirror, schematically illustrating the configuration a laser beam focusing unit, the EUV collector mirror, and the gas introduction pipes according to the seventh embodiment. 
         FIG. 26A  is a perspective view schematically illustrating the configuration of an inner pipe and a wall unit that are integrated. 
         FIG. 26B  is a sectional view schematically illustrating the configuration of the inner pipe and the wall unit fitted into through-holes. 
         FIG. 27A  is a perspective view schematically illustrating the configuration of an outer pipe, a pipe, and a hydrogen gas supply source. 
         FIG. 27E  is a sectional view illustrating the configuration of the outer pipe shown in  FIG. 27A . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, selected embodiments for implementing this disclosure will be described in detail with reference to the accompanying drawings. In the subsequent description, each drawing merely illustrates shape, size, and positional relationship schematically to the extent that enables the content of this disclosure to be understood. Thus, this disclosure is not limited to the shape, the size, and the positional relationship illustrated in each drawing. In order to show the configuration clearly, part of the hatching along a section may be omitted in the drawings. Further, numerical values indicated herein are merely examples of this disclosure; thus, this disclosure is not limited to the indicated numerical values. 
     First Embodiment 
     An EUV light generation system according to a first embodiment of this disclosure will be described in detail with reference to the drawings.  FIG. 1  schematically illustrates the configuration of an EUV light generation system  1  according to the first embodiment. 
     As shown in  FIG. 1 , the EUV light generation system  1  may include an airtight chamber  11 , an exposure apparatus connection part  13 , a droplet generator  14   b  provided with a droplet controller  14   a , a pre-pulse laser PL, a main pulse laser ML, and an EUV light generation controller  10 . The chamber  11  may define a space where EUV light is generated. The exposure apparatus connection part  13  may optically connect the chamber  11  to an exposure apparatus (not shown). The droplet generator  14   b  may be configured to supply a target material, such as tin (Sn), into the chamber  11  in the form of droplets D through a nozzle (not shown). The pre-pulse laser PL may be configured to output a pre-pulse laser beam L 1   a . The main pulse laser ML may be configured to output a main pulse laser beam L 1   b . The EUV light generation controller  10  may be configured to control the pre-pulse laser PL, the main pulse laser ML, the droplet controller  14   a , and so forth. 
     The chamber  11  may be provided with windows W 1  and W 2 . The pre-pulse laser beam L 1   a  and the main pulse laser beam L 1   b  may pass through the respective windows W 1  and W 2  and enter the chamber  11 , respectively. Off-axis paraboloidal mirrors M 2  and M 3  for respectively focusing the pre-pulse laser beam L 1   a  and the main pulse laser beam L 1   b  in a plasma generation region P 1  defined inside the chamber  11  may be provided in the chamber  11 . Further, an EUV collector mirror M 1  may be provided in the chamber  11 , and the EUV collector mirror M 1  may be positioned to reflect EUV light L 2  generated in the plasma generation region P 1  such that the EUV light L 2  is focused in an intermediate focus region IF set inside the exposure apparatus connection part  13 . 
     With the above configuration, the EUV light generation controller  10  may control the droplet controller  14   a  to thereby control a timing at which the droplet D is outputted from the droplet generator  14   b . The droplet D outputted from the droplet generator  14   b  may arrive in the plasma generation region P 1 . Further, the EUV light generation controller  10  may control a timing at which the pre-pulse laser beam L 1   a  is outputted from the pre-pulse laser PL and a timing at which the main pulse laser beam L 1   b  is outputted from the main pulse laser ML. At a timing at which the droplet D arrives in the plasma generation region P 1 , the pre-pulse laser beam L 1   a  may be focused on the droplet D by the off-axis paraboloidal mirror M 2  (first-stage laser irradiation). With this, the droplet D may be diffused and turned into a state in which weak plasma, neutral particles, clusters, fragments, and the like mixedly exist. In the description to follow, a target material in this state maybe referred to as a diffused target. 
     The main pulse laser beam L 1   b  from the main pulse laser ML may be focused on the diffused target in the plasma generation region P 1  by the off-axis paraboloidal mirror M 3  (second-stage laser irradiation) through a through-hole M 1   a  formed in the EUV collector mirror M 1 . With this, the diffused target may be turned into plasma. The EUV light L 2  may be emitted when this plasma is deexcited. 
     The EUV light L 2  emitted from the plasma may be reflected by the spheroidal EUV collector mirror M 1  toward the exposure apparatus connection part  13 . The reflected EUV light L 2  may once be focused in the intermediate focus region IF, and then be outputted to the exposure apparatus through a waveguide, such as a tube (not shown). 
     In the first embodiment, the target material in the form of a droplet D, may be turned into plasma with two-stage laser beam irradiation. However, this disclosure is not limited thereto, and the target material may be turned into plasma with one-stage, or three-or-more-stage laser beam irradiation. Further, in the first embodiment, the target material may be supplied in the form of droplets. However, this disclosure is not limited thereto, and a solid target material that is rotatably set inside the chamber  11  may also be used. 
     After the EUV light L 2  is emitted, particles of the target material, Sn in this embodiment, such as ions, atoms, charged particles, and neutral particles, hereinafter, collectively referred to as Sn debris, may be emitted from the plasma generated in the plasma generation region P 1 . This Sn debris may adhere onto the optical elements, such as the EUV collector mirror M 1  and the off-axis paraboloidal mirrors M 2  and M 3 , provided inside the chamber  11 , and may be deposited thereon. 
     Therefore, the EUV light generation system  1  of the first embodiment may include radical generators  15   a  through  15   c  each configured to supply hydrogen radicals, hereinafter, referred to as H radicals or H*, into the chamber  11 , and a discharge pump  12  for discharging a gas from the chamber  11 . H radicals from the radical generators  15   a  through  15   c  may be supplied into the chamber  11  through respective gas introduction pipes  16   a  through  16   c  extending into the chamber  11 . The radical generators  15   a  through  15   c  and the gas introduction pipes  16   a  through  16   c  may be positioned to allow an etching gas, such as H radicals or H 2  gas, to flow along the surface of the optical elements provided inside the chamber  11 . With this, the etching gas may etch the Sn debris deposited on the optical elements. 
     Gas discharge ports of the respective gas introduction pipes  16   a  through  16   c  may point toward the reflective surfaces of the EUV collector mirror M 1  and the off-axis paraboloidal mirrors M 3  and M 2 , respectively. With this, the H radicals flowing along the reflective surfaces of the optical elements may react with the Sn debris deposited on the optical elements, and stannane (SnH 4 ) gas may be produced. This stannane gas is in a gaseous state approximately at or above −52° C. In this way, by allowing Sn and the H radicals to react with each other, the Sn debris deposited on the optical elements may be etched. The stannane gas produced through the etching reaction may be discharged outside the chamber  11  through the discharge pump  12 . As a result, performance degradation of the optical elements can be reduced. Note that, in the first embodiment and the other embodiments to be described later, the gas discharge port may be realized by an opening, a slit, or any other suitable form. 
     In the first embodiment, hydrogen may be supplied in the form of radicals in order to etch the Sn debris deposited on the optical elements with high efficiency. However, this disclosure is not limited thereto, and hydrogen may be supplied in the form of hydrogen molecules (H 2 ). In this case, the hydrogen molecules may be transformed into the H radicals by ultraviolet light, vacuum ultraviolet light, EUV light, and the like emitted in the plasma generation region P 1 , and the H radicals produced may react with the Sn debris. As a result, the Sn debris deposited on the optical elements may be etched, and performance degradation of the optical elements may be suppressed. In this example, the radical generators  15   a  through  15   c  may not be required, and in place of the radical generators  15   a  through  15   c , a hydrogen gas supply source may be provided. 
     Partitions  11   a  through  11   c  may be provided inside the chamber  11 . Each of the partitions  11   a  through  11   c  may, for example, be plate-shaped. The flow of a gas, such as H radicals, hydrogen gas, and stannane gas, inside the chamber  11  may be controlled with the partitions  11   a  through  11   c . As a result, the Sn debris on the optical elements may be etched efficiently, and the stannane gas may be discharged from the chamber  11  efficiently as well. An opening A 1  defined between the partitions  11   b  and  11   c  may serve as a part of a flow channel of the etching gas and as a part of a beam path of the main pulse laser beam L 1   b . An opening A 2  defined between the partitions  11   a  and  11   b  may serve as a part of a flow channel of the etching gas and as a part of a beam path of the pre-pulse laser beam L 1   a.    
     Furthermore, the flow of the gas may be controlled with the partitions  11   a  through  11   c  such that the gas flows from the optical elements toward the plasma generation region P 1 , whereby the Sn debris may be prevented from adhering onto the optical elements. With this, performance degradation of the optical elements may be suppressed more reliably. 
       FIG. 2  schematically illustrates a reaction in which hydrogen molecules are transformed into the hydrogen radicals and the hydrogen radicals react with solid Sn deposited on an optical element to thereby be turned into stannane gas, and a reaction in which the stannane gas is decomposed and solid Sn is deposited. The reaction of Sn and the H radicals may be expressed by the following chemical reaction formulae (1) and (2):
 
etching reaction:
 
Sn( s )+4H*( g )→SnH 4 ( g )  (1)
 
deposition reaction:
 
SnH 4 →Sn( s )+2H 2   (2)
 
Here, (s) and (g) indicate a solid state and a gaseous state, respectively. The reactions given by the chemical reaction formulae (1) and (2) may occur simultaneously. A total etching rate Val may be expressed as a difference between an etching reaction rate Ve and a deposition reaction rate Vd, as given by the following expression (3):
 
 Val=Ve−Vd   (3)
 
The etching reaction rate Ve, the deposition reaction rate Vd, and the total etching rate Val may vary depending on the temperature of a given optical element when the concentrations of H radicals, H 2 , and SnH 4  are constant.
 
       FIG. 2  also illustrates the configuration of a temperature control mechanism for controlling the temperature of an optical element. The temperature control mechanism may include a temperature control element  20   b , a power supply  20   a , a temperature sensor  20   c , and a temperature controller  20 . The temperature control element  20   b  may be provided on an optical element M, such as the EUV collector mirror M 1 . The power supply  20   a  may supply electric current to the temperature control element  20   b . The temperature sensor  20   c  may be provided on the optical element M. The temperature controller  20  may be configured to control the electric current to be supplied to the temperature control element  20   b  from the power supply  20   a  based on the temperature detected by the temperature sensor  20   c . An example of the temperature control element  20   b  may include a heater, and the electric current supplied to the heater may be controlled based on the detection result of the temperature sensor  20   c . By use of the temperature sensor  20   c , the temperature may be retained at a predetermined temperature. 
     The total etching rate Val with respect to a set temperature range, which in this example is from the normal temperature to the temperature at which heating control is possible, has been measured.  FIG. 3  is a graph showing the relationship between the temperature and each of the etching reaction rate of Sn, the deposition reaction rate of Sn, and the total etching rate. In  FIG. 3 , a dashed line indicates the temperature dependency of the etching reaction rate Ve, and a dot-dashed line indicates the temperature dependency of the deposition reaction rate Vd. A solid line indicates the total etching rate Val=Ve−Vd. The total etching rate Val being 0 indicates that the etching reaction rate Ve is equal to the deposition reaction rate Vd. When the etching reaction rate Ve is equal to the deposition reaction rate Vd, Sn on the optical element may not be removed. The total etching rate Val being in a positive range indicates that Sn on the optical element may be etched. Conversely, the total etching rate Val being in a negative range indicates that Sn may further be deposited on the optical element. Therefore, the electric current to be supplied to the temperature control element  20   b  from the power supply  20   a  may be controlled such that the temperature of the optical element M is retained within a target temperature range. For example, the temperature of the optical element M may be retained in a range of 40° C. to 120° C. inclusive, or in a range of 60° C. to 100° C. inclusive. When the temperature of the optical element M is retained in these ranges, the total etching rate Val may be retained above a target etching rate, and the Sn debris on the optical element may be removed. 
     As shown in  FIG. 3 , the total etching rate Val rises with the increase in the temperature while the temperature of the surface of the optical element is in a range of 0° C. to approximately 60° C. When the temperature of the surface of the optical element exceeds approximately 100° C., the total etching rate Val starts to fall. Accordingly, in the first embodiment, the temperature of the optical element provided in the chamber  11  may be controlled to fall within a temperature range of 40° C. to 120° C. inclusive, or in a temperature range of 60° C. to 100° C. inclusive. As a result, the stannane gas may be prevented from being decomposed, whereby Sn may be prevented from being deposited on the surface of the optical element. 
       FIG. 4  schematically illustrates the configuration for controlling the temperature of the optical element provided in the chamber according to the first embodiment. In  FIG. 4 , the windows W 1  and W 2 , the EUV collector mirror M 1 , and the off-axis paraboloidal mirrors M 2  and M 3 , hereinafter, simply referred to as optical elements W 1  and W 2  and M 1  through M 3 , are given as examples of the optical elements whose temperature is to be controlled. However, this disclosure is not limited to these. 
     As shown in  FIGS. 1 and 4 , the EUV light generation system  1  may include a chiller  17 . The chiller  17  may be provided outside the chamber  11 . The chiller  17  may feed, into a main supply pipe Cin, a heat carrier, for example, a liquid that is stable at the operating temperature such as temperature-controlled water or oil, to control the temperatures of the optical elements W 1  and W 2  and M 1  through M 3 . Sub-supply pipes C 1 in through C 5 in may branch off from the main supply pipe Cin to the respective optical elements W 1  and W 2  and M 1  through M 3 . With this arrangement, the temperature-controlled heat carrier fed into the main supply pipe Cin may be supplied to the optical elements W 1  and W 2  and M 1  through M 3  via the respective sub-supply pipes C 1 in through C 5 in. 
     The optical elements W 1  and W 2  and M 1  through M 3  may include respective heat carrier flow channels C 1  through C 5 . The heat carrier distributed into the sub-supply pipes C 1 in through C 5 in from the main supply pipe Cin may flow into the respective heat carrier flow channels C 1  through C 5  so as to circulate inside the respective optical elements W 1  and W 2  and M 1  through M 3 . As a result of this arrangement, the temperatures of the optical elements W 1  and W 2  and M 1  through M 3  may be controlled to fall within the target temperature range. 
     The heat carrier having circulated in the heat carrier flow channels C 1  through C 5  may then flow into sub-discharge pipes C 1 out through C 5 out. The sub-discharge pipes C 1 out through C 5 out may be connected to a main discharge pipe Cout, which is connected to the chiller  17 . With this arrangement, the heat carrier having flowed into the sub-discharge pipes C 1 out through C 5 out may return to the chiller  17  via the main discharge pipe Cout. The heat carrier having returned to the chiller  17  may have the temperature thereof readjusted and again be fed into the main supply pipe Cin. 
     The sub-supply pipes C 1 in through C 5 in and the sub-discharge pipes C 1 out through C 5 out may respectively be provided with temperature sensors T 1 in through T 5 in and T 1 out through T 5 out to detect the temperature of the heat carrier flowing through the respective pipes. The total flow rate through the temperature sensors T 1 in through T 5 in and T 1 out through T 5 out may be controlled, for example, by the EUV light generation controller  10  or by the chiller  17  equipped with a circulation pump (not shown). Using this arrangement, the heat carrier may be supplied smoothly to the optical elements W 1  and W 2  and M 1  through M 3 . 
     The sub-discharge pipes Clout through C 5 out may be provided with respective flow-rate control valves V 1  through V 5 . For example, the EUV light generation controller  10  or the chiller  17  may be configured to control the flow-rate control valves V 1  through V 5  in order to control the flow rate of the heat carrier flowing in the respective optical elements W 1  and W 2  and M 1  through M 3 . With this, the flow rate of the heat carrier flowing in the sub-discharge pipes Clout through C 5 out may be controlled, and in turn, the flow rate of the heat carrier flowing in the optical elements W 1  and W 2  and M 1  through M 3  may be controlled. With this, the temperatures of the optical elements W 1  and W 2  and M 1  through M 3  may be controlled to fall within the target temperature range. 
     As described so far, according to the first embodiment, the temperature of the optical element may be controlled so that the target material etched with the etching gas is not deposited on the optical element again. Accordingly, in the EUV light generation system according to the first embodiment, performance degradation of the optical element provided inside the chamber may be suppressed. 
     Second Embodiment 
     An EUV light generation system according to a second embodiment of this disclosure will now be described in detail with reference to the drawings. 
       FIG. 5  schematically illustrates a configuration for controlling the temperatures of the optical elements provided in the chamber of the EUV light generation system according to the second embodiment. In the second embodiment, a temperature control mechanism in which a cooling mechanism and a heating mechanism are combined may be applied to the EUV light generation system  1  of the above-described first embodiment. According to this configuration, a temperature control mechanism which includes the large-capacity chiller  17  may be used in combination with another temperature control mechanism capable of heating and cooling. In this arrangement, the temperature of the optical elements may be controlled with higher precision. Accordingly, deposition of Sn onto the optical elements may be further reduced. 
     As shown in  FIG. 5 , the EUV light generation system of the second embodiment may be similar in configuration to that shown in  FIG. 4 , but may differ in that the optical elements W 1  and W 2  and M 1  through M 3  may further be provided with respective heaters  21   b  through  25   b  and respective temperature sensors  21   c  through  25   c . The heaters  21   b  through  25   b  may be connected to respective power supplies  21   a  through  25   a . The temperature detected by the temperature sensors  21   c  through  25   c  may be inputted to respective temperature controllers  21  through  25 . The temperature controllers  21  through  25  may be configured to control electric current supplied to the heaters  21   b  through  25   b  from the power supplies  21   a  through  25   a  so that the temperatures inputted from the temperature sensors  21   c  through  25   c  fall within the target temperature range. The heaters  21   b  through  25   b  may be configured to heat the respective optical elements W 1  and W 2  and M 1  through M 3  in accordance with the electric current supplied from the power supplies  21   a  through  25   a . With this, the temperatures of the optical elements W 1  and W 2  and M 1  through M 3  may be controlled to fall within the target temperature range. In a case where the temperature of a given optical element exceeds the target temperature range, the optical element may be cooled with the heat carrier supplied from the chiller  17 . 
     As described above, according to the second embodiment, as in the first embodiment, the temperature of the optical element may be controlled so that the target material etched with the etching gas is not redeposited on the optical element. 
     Third Embodiment 
     An EUV light generation system according to a third embodiment of this disclosure will now be described in detail with reference to the drawings.  FIG. 6  schematically illustrates a configuration of the EUV light generation system according to the third embodiment. A temperature control mechanism illustrated in  FIG. 3  or  FIG. 5  may be provided in the EUV light generation system of the third embodiment. 
     As shown in  FIG. 6 , an EUV light generation system  3  according to the third embodiment may be similar in configuration to the EUV light generation system  1  shown in  FIG. 1 , but may differ in that traps  31  through  36  may further be provided in the chamber  11  to trap Sn deposited when the stannane gas is decomposed. The trap  31  may, for example, be positioned to face the reflective surface of the EUV collector mirror M 1 , and may trap Sn deposited from the stannane gas produced from Sn on the surface of the EUV collector mirror M 1 . The trap  32  may, for example, be provided in the chamber  11  so as to face a gate valve W 3 , and may trap Sn deposited from the stannane gas flowing toward the exposure apparatus connection part  13  via the gate valve W 3 . The trap  33  may, for example, be provided at the connection between the chamber  11  and the discharge pump  12 , and may trap Sn deposited from the stannane gas flowing into the discharge pump  12  from the interior of the chamber  11 . The traps  34  through  36  may each be provided at a predetermined position inside the chamber  11 , such as a position where Sn debris is likely to reach, and may trap Sn deposited from the stannane gas present in the chamber  11 . 
     The temperatures of the traps  31  through  36  may be controlled to be equal to or higher than a temperature at which the stannane gas is decomposed and Sn is deposited, for example, 120° C. As a result, the concentration of the stannane gas inside the chamber  11  may be reduced, and the concentration of the stannane gas near the surface of a Sn layer deposited on the optical element may be reduced as well. Therefore, the total etching rate may be increased. Further, retaining the temperatures of the traps  31  through  36  below the melting point of Sn, 232° C., may allow Sn deposited from the stannane gas to be fixed on the surfaces of the traps  31  through  36  in a solid state. The traps  31  through  36  may be formed of a material having low reactivity with Sn, such as molybdenum (Mo), titanium (Ti) alumina (Al 2 O 3 ), or the like. 
     Hereinafter, examples of the traps according to the third embodiment will be described in detail with reference to the drawings. 
     Trap  31   
       FIG. 7  is a perspective view schematically illustrating the configuration of a trap positioned to face the reflective surface of the EUV collector mirror in the third embodiment. As shown in  FIG. 7 , the trap  31  may generally be column-shaped with both ends thereof being open. Specifically, the trap  31  may have a double-ring structure including an outer ring  31 A and a laser-beam passing ring  31 E. The laser-beam passing ring  31 B may be provided at the center of the outer ring  31 A. The outer ring  31 A and the laser-beam passing ring  31 B may be arranged substantially coaxially. The inner space of the laser-beam passing ring  31 B may serve as a laser-beam passing hole  31   a   1 , through which a laser beam (e.g., the main pulse laser beam L 1   b ) may pass. A space defined between the outer ring  31 A and the laser-beam passing ring  31 B may serve as EUV-light passing holes  31   b   1 , through which the EUV light L 2  may pass. Here, the EUV light L 2  reflected by the EUV collector mirror M 1  may travel through a space outside the outer ring  31 A or inside the laser-beam passing ring  31 B aside from the EUV-light passing holes  31   b   1 . 
     Trapping blades  31 C may radially extend between the outer surface of the laser-beam passing ring  31 B and the inner surface of the outer ring  31 A. Each of the trapping blades  31 C may be plate-shaped. The trapping blades  31 C may be configured to fix the laser-beam passing ring  31 B to the outer ring  31 A and define the EUV-light passing holes  31   b   1 . Sn deposited from the stannane gas passing through the EUV-light passing holes  31   b  may be trapped by the trapping blades  31 C. 
     The trap  31  configured as such may be positioned such that the axis of the laser-beam passing ring  31 B substantially coincides with the beam axis of a laser beam (e.g., the main pulse laser beam L 1   b ) focused in the plasma generation region P 1  via the through-hole M 1   a  in the EUV collector mirror M 1 . Further, the axis of the outer ring  31 A may substantially coincide with the axis of the EUV light L 2  reflected by the EUV collector mirror M 1 . With this, the trap  31  may allow the laser beam to pass through the laser-beam passing hole  31   a   1  without blocking the laser beam, and may allow the EUV light L 2  reflected by the EUV collector mirror M 1  to pass through the EUV-light passing holes  31   b   1  without reducing the energy of the EUV light L 2 . 
     The trap  31  may be heated, for example, to a temperature equal to or higher than the temperature at which the stannane gas is decomposed and Sn is deposited, for example, 120° C., as described above. The trap  31  may be provided with a heater  40   b , and a power supply  40   a  may be connected to the heater  40   b  to supply electric current thereto. The trap  31  may further be provided with a temperature sensor  40   c , and the temperature detected by the temperature sensor  40   c  may be inputted to a temperature controller  40 . The temperature controller  40  may control the electric current to be supplied to the heater  40   b  from the power supply  40   a  based on the temperature detected by the temperature sensor  40   c . With this, the trap  31  may be heated to a temperature equal to or higher than the aforementioned temperature, and the stannane gas passing through the trap  31  may be heated, whereby Sn may be deposited on the trap  31 . That is, apart of Sn contained in the stannane gas produced from Sn on the surface of the EUV collector mirror M 1  may be collected by the trap  31 . As a result, the concentration of the stannane gas inside the chamber  11  may be reduced, and the concentration of the stannane gas near the surface of the Sn layer on the optical element may be reduced. Therefore, the total etching rate may be increased. Further, retaining the temperature of the trap  31  below the melting point of Sn may allow Sn to be fixed on the trap  31  in a solid state. 
     Trap  32   
       FIG. 8  is a perspective view schematically illustrating the configuration of a trap positioned to face the gate valve in the third embodiment. As shown in  FIG. 8 , a trap  32  may be generally frustoconical in shape with both ends being open. Specifically, the trap  32  may include an outer ring  32 A, a core  32 B, and trapping blades  32 C. The outer ring  32 A may be frustoconical in shape. The core  32 B may be rod-shaped and positioned at the center of the outer ring  32 A. The trapping blades  32 C may radially extend between the core  32 B and an inner surface of the outer ring  32 A. The core  32 B may serve to tie the trapping blades  32 C together. The core  32 B may be omitted. The inner space of the outer ring  32 A may serve as EUV-light passing holes  32   b   1 , through which the EUV light L 2  may pass. Here, the EUV light L 2  reflected by the EUV collector mirror M 1  may travel through a space outside the outer ring  32 A aside from the EUV-light passing holes  32   b   1 . 
     The trapping blades  32 C radially extending between the core  32 B and the inner surface of the outer ring  32 A may define the EUV-light passing holes  32   b   1 . The trapping blades  32 C may serve to trap Sn deposited from the stannane gas passing through the EUV-light passing holes  32   b   1 . 
     The trap  32  configured as such may be positioned such that the axis of the outer ring  32 A substantially coincides with the axis of the EUV light L 2  reflected by the EUV collector mirror M 1 . With this, the trap  32  may allow the EUV light L 2  reflected by the EUV collector mirror M 1  to pass therethrough without reducing the energy of the EUV light L 2 . 
     The trap  32  may be heated, for example, to a temperature equal to or higher than the temperature at which the stannane gas is decomposed and Sn is deposited, as described above. The configuration and the operation for heating the trap  32  may be similar to those of the trap  31  described above. Thus, detailed description thereof is omitted here. The trap  32  may be heated to a temperature equal to or higher than the aforementioned temperature, and the stannane gas passing through the trap  32  may be heated and decomposed, whereby Sn may be deposited on the trap  32 . That is, Sn contained in the stannane gas flowing into the exposure apparatus connection part  13  via the gate valve W 3  may be collected by the trap  32 . As a result, the stannane gas may be prevented from flowing into the exposure apparatus. Further, retaining the temperature of the trap  32  below the melting point of Sn may allow Sn to be fixed on the trap  32  in a solid state. 
     Trap  33   
       FIG. 9  schematically illustrates the configuration of a trap provided at a connection between the chamber and the discharge pump of the third embodiment. As shown in  FIG. 9 , a trap  33  may include a plurality of plate-shaped blades  33 B arranged substantially parallel to the direction in which the gas flows from the chamber  11  toward the discharge pump  12 . The blades  33 B may be arranged such that a cross-section thereof has a lattice pattern. 
     The trap  33  may be heated, for example, to a temperature equal to or higher than the temperature at which the stannane gas is decomposed and Sn is deposited, as described above. The configuration and the operation for heating the trap  33  may be similar to those of the trap  31  described above. Thus, detailed description thereof is omitted here. The trap  33  may be heated to a temperature equal to or higher than the aforementioned temperature, and the stannane gas flowing into the discharge pump  12  may be heated and decomposed when passing through the trap  33 , whereby Sn may be deposited on the trap  33 . That is, Sn contained in the stannane gas flowing into the discharge pump  12  may be collected by the trap  33 . As a result, the stannane gas may be prevented from flowing into the discharge pump  12 . Further, retaining the temperature of the trap  33  below the melting point of Sn may allow Sn to be fixed on the trap  33  in a solid state. 
     Traps  34  through  36   
       FIG. 10  schematically illustrates the configuration of a trap provided at a predetermined location inside the chamber of the third embodiment. As shown in  FIG. 10 , each of traps  34  through  36  may include wires or ribbons woven in a three-dimensional net or lattice form. 
     The stannane gas flowing inside the chamber  11  may pass through any of the traps  34  through  36  or the vicinities thereof. Thus, the traps  34  through  36  may be heated, for example, to a temperature equal to or higher than the temperature at which the stannane gas is decomposed and Sn is deposited, as described above. The configuration and the operation for heating each of the traps  34  through  36  may be similar to those of the trap  31  described above. Thus, detailed description thereof is omitted here. Each of the traps  34  through  36  may be heated to a temperature equal to or higher than the aforementioned temperature, and the stannane gas flowing inside the chamber  11  may be heated and decomposed when passing through any of the traps  34  through  36 , whereby Sn may be deposited on any of the traps  34  through  36 . With this, Sn contained in the stannane gas inside the chamber  11  may be collected by the traps  34  through  36 . Further, retaining the temperature of the traps  34  through  36  below the melting point of Sn may allow Sn to be fixed on the traps  34  through  36  in a solid state. Here, each of the traps  34  through  36  may be formed of a porous material, such as sponge. 
     Fourth Embodiment 
     An EUV light generation system according to a fourth embodiment of this disclosure will now be described in detail with reference to the drawings. In the above-described third embodiment, the temperatures of the traps  31  through  36  are controlled to be equal to or higher than the temperature at which the stannane gas is decomposed and Sn is deposited, for example, 120° C., and lower than the melting point of Sn, 232° C., whereby Sn trapped in the traps  31  through  36  is fixed thereon in a solid state. On the other hand, in the fourth embodiment, Sn trapped in the traps  31  through  36  may be liquefied so as to flow out of the traps  31  through  36  by heating the traps  31  through  36  to a temperature equal to or higher than the melting point of Sn. Molten Sn that has flowed out of each of the traps  31  through  36  may be collected in a collection unit, such as a bucket. 
       FIG. 11  schematically illustrates the configuration of a trap and a collection unit in the EUV light generation system of the fourth embodiment. Note that, in  FIG. 11 , the trap provided at a connection between the chamber  11  and the discharge pump  12  is shown as an example. 
     As shown in  FIG. 11 , in the EUV light generation system of the fourth embodiment, a connection part between the chamber  11  and the discharge pump  12  may be bent in an L-shape. A part of the connection part on the side of the chamber  11  may extend vertically. A part of the connection part on the side of the discharge pump  12  may branch off from the connection part and extend horizontally. The trap  33  may be provided in the part of the connection part extending vertically from the chamber  11 . The trap  33  may be provided with the temperature controller  40 , the power supply  40   a , the heater  40   b , and the temperature sensor  40   c , and the operation of these components may be similar to that of the third embodiment. With this configuration, the trap  33  may be heated to a temperature equal to or higher than the melting point of Sn, and molten Sn DD may flow out of the trap  33 . 
     A collection unit  40 A may be provided at the bottom of the connection part extending vertically from the chamber  11  to collect molten Sn DD flowing out of the trap  33 . With this, molten Sn DD flowing out of the trap  33  may be prevented from contaminating the optical elements inside the chamber  11 . By forming a connecting portion between the connection part extending from the chamber  11  and the collection unit  40 A in a frustoconical shape, molten Sn DD flowing out of the trap  33  may be collected reliably into the collection unit  40 A. 
     In the fourth embodiment, the trap  33  provided at the connection part between the chamber  11  and the discharge pump  12  is shown as an example. However, this disclosure is not limited thereto, and a similar configuration may be applied to any of the traps  31  through  36  of the third embodiment as well. That is, a heater, a power supply, a temperature sensor, a temperature controller, and a collection unit may be provided to each of the traps  31  through  36 , whereby Sn trapped in each trap may be collected in a liquid state. 
     First Modification 
     Modifications of the gas introduction pipe in the above-described embodiments will now be described in detail with reference to the drawings. In the description to follow, a gas introduction pipe for making H radicals or H 2  gas flow along the reflective surface of the EUV collector mirror M 1  will be shown as an example. 
     A gas introduction pipe according to a first modification will be described in detail with reference to the drawings.  FIG. 12A  schematically illustrates the configuration of the gas introduction pipe and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the first modification.  FIG. 12B  is a sectional view, taken along XIIB-XIIB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 12A . 
     As shown in  FIGS. 12A and 12B , in the first modification, a gas introduction pipe  16 - 1  from a radical generator  15  may extend so as to surround the reflective surface of the EUV collector mirror M 1 . The gas introduction pipe  16 - 1  may have multiple holes formed therein at substantially equal intervals, as shown by the arrows. The gas introduction pipe  16 - 1  may be positioned such that the holes are opened toward the reflective surface of the EUV collector mirror M 1 . The H radicals fed into the gas introduction pipe  16 - 1  from the radical generator  15  may be blown out through the holes provided in the gas introduction pipe  16 - 1 . With this, the H radicals may flow substantially uniformly along the reflective surface of the EUV collector mirror M 1  from the periphery of the reflective surface of the EUV collector mirror M 1 . As a result, Sn deposited particularly on the reflective surface of the EUV collector mirror M 1  may be etched. Here, the radical generator  15  may be replaced by a hydrogen gas supply source. In that case, the hydrogen gas may be made to flow along the reflective surface of the EUV collector mirror M 1 . 
     The gas introduction pipe  16 - 1  may, for example, have a gap in a part thereof, so that the droplet D outputted from the droplet generator  14   b  is not prevented from traveling toward the plasma generation region P 1 . 
     Second Modification 
     A gas introduction pipe according to a second modification will be described in detail with reference to the drawings.  FIG. 13A  schematically illustrates the configuration of the gas introduction pipes and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the second modification.  FIG. 13B  is a sectional view, taken along XIIIB-XIIIB plane, schematically illustrating the configuration of the gas introduction pipes and the EUV collector mirror shown in  FIG. 13A . 
     As shown in  FIGS. 13A and 13B , a plurality of radical generators  15 A and  15 B may be provided for the EUV collector mirror M 1  in the second modification. In the present example, two radical generators  15 A and  15 B are shown, although other embodiments may use more than two. Alternatively, in place of the radical generators  15 A and  15 B, hydrogen gas supply sources may be used. The radical generators  15 A and  15 B may be positioned symmetrically about the center of the reflective surface of the EUV collector mirror M 1 . A semiarc-shaped gas introduction pipe  16 - 2   a  may extend from the radical generator  15 A so as to surround a half of the reflective surface of the EUV collector mirror M 1 . Similarly, a semiarc-shaped gas introduction pipe  16 - 2   b  may extend from the radical generator  15 B so as to surround the other half of the reflective surface of the EUV collector mirror M 1 . In this way, the reflective surface of the EUV collector mirror M 1  may be surrounded by the two semiarc-shaped gas introduction pipes  16 - 2   a  and  16 - 2   b . Each of the gas introduction pipes  16 - 2   a  and  16 - 2   b  may have multiple holes formed therein at substantially equal intervals, as is shown by the arrows. Each of the gas introduction pipes  16 - 2   a  and  16 - 2   b  may be positioned such that the holes are opened toward the reflective surface of the EUV collector mirror M 1 . The H radicals fed into the gas introduction pipes  16 - 2   a  and  16 - 2   b  from the respective radical generators  15 A and  15 B may be blown out through the holes provided in the gas introduction pipes  16 - 2   a  and  16 - 2   b . With this, the H radicals may flow substantially uniformly along the reflective surface of the EUV collector mirror M 1  from the periphery of the reflective surface of the EUV collector mirror M 1 . As a result, Sn deposited on the reflective surface of the EUV collector mirror M 1  may be etched. 
     The gas introduction pipes  16 - 2   a  and  16 - 2   b  may, for example, be positioned to form gaps therebetween, so that a droplet D outputted from the droplet generator  14   b  is not prevented from traveling toward the plasma generation region P 1 , and so that the droplet D having passed through the plasma generation region P 1  is not prevented from traveling toward the droplet collection unit  14   d.    
     Third Modification 
     A gas introduction pipe according to a third modification will be described in detail with reference to the drawings.  FIG. 14A  schematically illustrates the configuration of the gas introduction pipes and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the third modification.  FIG. 14B  is a sectional view, taken along XIVB-XIVB plane, schematically illustrating the configuration of the gas introduction pipes and the EUV collector mirror shown in  FIG. 14A . 
     As shown in  FIGS. 14A and 14B , in the third modification, a plurality of radical generators  15 A through  15 D may be provided for the EUV collector mirror M 1 . In the present example, four radical generators  15 A through  15 D are shown, although other embodiments may use more than four. Alternatively, in place of the radical generators  15 A through  15 D, hydrogen gas supply sources may be used. The radical generators  15 A through  15 D may be positioned symmetrically at equal intervals about the center of the reflective surface of the EUV collector mirror M 1 . A quarter-arc-shaped gas introduction pipe  16 - 3   a  may extend from the radical generator  15 A so as to surround a quarter of the reflective surface of the EUV collector mirror M 1 . Similarly, each of quarter-arc-shaped gas introduction pipes  16 - 3   b  through  16 - 3   d  may extend from the respective radical generators  15 B through  15 D so as to surround a quarter of the reflective surface of the EUV collector mirror M 1 , each. In this way, the reflective surface of the EUV collector mirror M 1  may be surrounded by the four quarter-arc-shaped gas introduction pipes  16 - 3   a  through  16 - 3   d . Each of the gas introduction pipes  16 - 3   a  through  16 - 3   d  may have multiple holes formed therein at substantially equal intervals, as is shown by the arrows. Each of the gas introduction pipes  16 - 3   a  through  16 - 3   d  may be positioned such that the holes are opened toward the reflective surface of the EUV collector mirror M 1 . The H radicals fed into the gas introduction pipes  16 - 3   a  through  16 - 3   d  from the respective radical generators  15 A through  15 D may be blown out through the holes formed in the gas introduction pipes  16 - 3   a  through  16 - 3   d . With this, the H radicals may flow substantially uniformly along the reflective surface of the EUV collector mirror M 1  from the periphery of the reflective surface of the EUV collector mirror M 1 . As a result, Sn deposited particularly on the reflective surface of the EUV collector mirror M 1  may be etched. 
     The gas introduction pipes  16 - 3   a  and  16 - 3   d  may, for example, be positioned to form gaps therebetween so that the droplet D outputted from the droplet generator  14   b  is not prevented from traveling toward the plasma generation region P 1 , and so that the droplet D having passed through the plasma generation region P 1  is not prevented from traveling toward the droplet collection unit  14   d.    
     A magnetic field B may be generated so as to pass through the plasma generation region P 1 . Then, Sn debris, such as ions, generated in the plasma generation region P 1  may be collected by the magnetic field and into ion collection units  18   a  and  18   b . In this case, the gas introduction pipes  16 - 3   a  through  16 - 3   d  may be positioned to form gaps therebetween in order to allow the debris traveling in the magnetic field to pass through the gaps into the ion collection unit  18   a  and  18   b.    
     Fourth Modification 
     A gas introduction pipe according to a fourth modification will be described in detail with reference to the drawings.  FIG. 15A  schematically illustrates the configuration of the gas introduction pipe and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the fourth modification.  FIG. 15B  is a sectional view, taken along XVB-XVB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 15A . 
     As shown in  FIGS. 15A and 15B , a gas introduction pipe  16 - 4  of the fourth modification may be curved in an arch and be positioned such that the arch follows the reflective surface of the EUV collector mirror M 1  and such that the gas introduction pipe  16 - 4  extends linearly across the reflective surface of the EUV collector mirror M 1  as viewed from the reflective surface side of the EUV collector mirror M 1 . A curved portion  16 - 4   a  in a semiarc shape may be formed at substantially the middle of the gas introduction pipe  16 - 4 . The gas introduction pipe  16 - 4  may be positioned so that the curved portion  16 - 4   a  does not overlap the through-hole M 1   a  in the EUV collector mirror M 1 . 
     The gas introduction pipe  16 - 4  having such a shape may be provided within an obscuration region E of EUV light L 2 . The obscuration region may refer to a region corresponding to a predetermined angular range, and a part of EUV light collected by the EUV collector mirror corresponding to the obscuration region may not be used for exposure. That is, the obscuration region is a three-dimensional region included in the angular range of the EUV light which is not used for exposure. 
     The gas introduction pipe  16 - 4  may have multiple holes formed therein on two opposite sides at substantially equal intervals, as is shown by the arrows. The gas introduction pipe  16 - 4  may be positioned so that the holes are opened to allow the H radicals to flow along the reflective surface of the EUV collector mirror M 1 . 
     The H radicals fed into the gas introduction pipe  16 - 4  from the radical generator  15  may be blown out through the holes formed in the gas introduction pipe  16 - 4 . With this, the H radicals may flow substantially uniformly along the surface of the reflective surface of the EUV collector mirror M 1  from the center line passing through the through-hole M 1   a  across the reflective surface of the EUV collector mirror M 1 . As a result, Sn deposited particularly on the reflective surface of the EUV collector mirror M 1  may be etched. Here, the radical generator  15  may be replaced by a hydrogen gas supply source. In that case, the hydrogen gas may be made to flow along the reflective surface of the EUV collector mirror M 1 . 
     Since the gas introduction pipe  16 - 4  is curved along the reflective surface of the EUV collector mirror M 1 , the droplet D outputted from the droplet generator  14   b  may not be prevented from traveling toward the plasma generation region P 1 . 
     Fifth Modification 
     A gas introduction pipe according to a fifth modification will be described in detail with reference to the drawings.  FIG. 16A  schematically illustrates a configuration of the gas introduction pipe and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the fifth modification.  FIG. 16B  is a sectional view, taken along XVIB-XVIB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 16A . 
     As shown in  FIGS. 16A and 16B , a gas introduction pipe  16 - 5  of the fifth modification may be curved in an arch and be positioned such that the arch follows the reflective surface of the EUV collector mirror M 1  from one edge toward the through-hole M 1   a  and such that the gas introduction pipe  16 - 4  extends linearly across the reflective surface of the EUV collector mirror M 1  as viewed from the reflective surface side of the EUV collector mirror M 1 . A circular portion  16 - 5   a  may be formed at a leading end of the gas introduction pipe  16 - 5 , and the gas introduction pipe  16 - 5  may be positioned such that the circular portion  16 - 5  surround the through-hole M 1   a  so as not to overlap the through-hole M 1   a . The gas introduction pipe  16 - 5  of the fifth modification may be provided in the obscuration region E of the EUV light L 2 . 
     The circular portion  16 - 5   a  may have multiple holes formed therein at substantially equal intervals, as shown by the arrows. The gas introduction pipe  16 - 5  may be positioned such that the holes are opened toward the periphery so as to make the H radicals flow along the reflective surface of the EUV collector mirror M 1 . The H radicals fed into the gas introduction pipe  16 - 5  from the radical generator  15  may be blown out through the holes formed in the circular portion  16 - 5   a . As a result, the H radicals may flow substantially uniformly along the reflective surface of the EUV collector mirror M 1  from the vicinity of the through-hole M 1   a , and Sn deposited particularly on the reflective surface of the EUV collector mirror M 1  may be etched. Here, the radical generator  15  may be replaced by a hydrogen gas supply source, in which case the hydrogen gas may flow along the reflective surface of the EUV collector mirror M 1 . 
     Since the gas introduction pipe  16 - 5  may be curved along the reflective surface of the EUV collector mirror M 1 , the droplet D outputted from the droplet generator  14   b  may not be prevented from traveling toward the plasma generation region P 1 . 
     Sixth Modification 
     A gas introduction pipe according to a sixth modification will be described in detail with reference to the drawings.  FIG. 17A  schematically illustrates the configuration of the gas introduction pipes and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the sixth modification.  FIG. 17B  is a sectional view, taken along XVIIB-XVIIB plane, schematically illustrating the configuration of the gas introduction pipes and the EUV collector mirror shown in  FIG. 17A . 
     As shown in  FIGS. 17A and 17B , gas introduction pipes  16 - 6   a  through  16 - 6   d  of the sixth modification may each be curved at a portion thereof and be positioned so as to project from a rear surface side of an EUV collector mirror M 1  toward the reflective surface side through the through-hole M 1   a . The gas introduction pipes  16 - 6   a  through  16 - 6   d  may be arranged symmetrically about the center of the reflective surface of the EUV collector mirror M 1 . 
     Each of leading end portions of the respective gas introduction pipes  16 - 6   a  through  16 - 6   d  projecting through the through-hole M 1   a  may have multiple holes formed therein, as shown by the arrows. The gas introduction pipes  16 - 6   a  through  16 - 6   d  may be positioned so that the holes are opened toward the reflective surface of the EUV collector mirror M 1 . The H radicals fed into the gas introduction pipes  16 - 6   a  through  16 - 6   d  from the radical generator  15 , such as shown in  FIG. 16A , may be blown out through the holes in the respective leading end portions. With this, the H radicals may flow substantially uniformly along the reflective surface of the EUV collector mirror M 1  from the vicinity of the through-hole M 1   a . As a result, Sn deposited particularly on the reflective surface of the EUV collector mirror M 1  may be etched. Here, the radical generator  15  may be replaced by a hydrogen gas supply source, in which case the hydrogen gas may flow along the reflective surface of the EUV collector mirror M 1 . 
     Seventh Modification 
     A gas introduction pipe according to a seventh modification will be described in detail with reference to the drawings.  FIG. 18A  schematically illustrates the configuration of the gas introduction pipes and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the seventh modification.  FIG. 18E  is a sectional view, taken along XVIIIB-XVIIIB plane, schematically illustrating the configuration of the gas introduction pipes and the EUV collector mirror shown in  FIG. 18A . 
     As shown in  FIGS. 18A and 18B , gas introduction pipes  16 - 7   a  through  16 - 7   d  of the seventh modification may each be curved at a portion thereof and be positioned so as to project from the rear surface side of an EUV collector mirror M 1  toward the reflective surface side through the through-hole M 1   a . Portions of the respective gas introduction pipes  16 - 7   a  through  16 - 7   d  projecting through the through-hole M 1   a  may each extend toward the edge of the reflective surface of the EUV collector mirror M 1  and be curved to follow along the reflective surface of the EUV collector mirror M 1 . The gas introduction pipes  16 - 7   a  through  16 - 7   d  may be arranged symmetrically about the center of the reflective surface of the EUV collector mirror M 1 . 
     The portions of the gas introduction pipes  16 - 7   a  through  16 - 7   d  projecting through the through-hole M 1   a  may have multiple holes formed therein, as shown by the arrows. The gas introduction pipes  16 - 7   a  through  16 - 7   d  may be positioned so that the holes are opened toward the reflective surface of the EUV collector mirror M 1 . The H radicals fed into the gas introduction pipes  16 - 7   a  through  16 - 7   d  from the radical generator  15 , such as shown in  FIG. 16A , may be blown out through the holes formed therein. With this, the H radicals may flow substantially uniformly along the reflective surface of the EUV collector mirror M 1 . As a result, Sn deposited particularly on the reflective surface of the EUV collector mirror M 1  may be etched. Here, the radical generator  15  may be replaced by a hydrogen gas supply source, in which case the hydrogen gas may flow along the reflective surface of the EUV collector mirror. 
     Eighth Modification 
     A gas introduction pipe according to an eighth modification will be described in detail with reference to the drawings.  FIG. 19A  schematically illustrates the configuration of the gas introduction pipe and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the eighth modification.  FIG. 19B  is a sectional view, taken along XIXB-XIXB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 19A .  FIG. 19C  is a perspective view schematically illustrating the shape of the leading end portion of the gas introduction pipe according to the eighth modification. 
     As shown in  FIGS. 19A through 19C , a gas introduction pipe  16 - 8  of the eighth modification may include a generally conical outer plate  16 - 8   a  and a generally conical inner plate  16 - 8   b . The outer plate  16 - 8   a  may be shaped such that the leading end thereof is folded back in a dome-shape. Similarly, the inner plate  16 - 8   b  may be shaped such that the leading end thereof is folded back in a dome-shape. The dome-shaped end of the inner plate  16 - 8   b  may project from the opening in the outer plate  16 - 8   a  so as to cover the dome-shaped end of the outer plate  16 - 8   a  with a gap therebetween. The gas introduction pipe  16 - 8  may be positioned such that the leading end of the gas introduction pipe  16 - 8  projects from the rear surface side of the EUV collector mirror M 1  toward the reflective surface side through the through-hole M 1   a . A gap  16 - 81  may be defined between the outer plate  16 - 8   a  and the inner plate  16 - 8   b , and may open up at one end toward the reflective surface of the EUV collector mirror M 1 . The H radicals may flow into the gap  16 - 81  from the rear surface side of the EUV collector mirror M 1 , and then be blown out through the opening at the end facing toward the reflective surface of the EUV collector mirror M 1 . With this, the H radicals may flow substantially uniformly along the reflective surface of the EUV collector mirror M 1  from the vicinity of the through-hole M 1   a . As a result, Sn deposited particularly on the reflective surface of the EUV collector mirror M 1  may be etched. 
     Ninth Modification 
     A gas introduction pipe according to a ninth modification will be described in detail with reference to the drawings. In the above-described embodiments and modifications, H 2  is turned into H radicals, and the H radicals are made to flow along the surfaces of the optical elements. In contrast, in the ninth modification, instead of the H radicals, H 2  gas may be blown against the optical elements in the chamber  11  (see  FIG. 1 or 6 ), in particular against the EUV collector mirror M 1  that is irradiated with EUV light L 2  more intensely.  FIG. 20A  schematically illustrates the configuration of the gas introduction pipe and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the ninth modification.  FIG. 20B  is a sectional view, taken along XXB-XXB plane, schematically illustrating the configuration of the gas introduction pipe and the EUV collector mirror shown in  FIG. 20A . 
     As shown in  FIGS. 20A and 20B , a gas introduction pipe  16 - 9  of the ninth modification may be in any shape that does not substantially block the EUV light L 2  reflected by the EUV collector mirror M 1 . The gas introduction pipe  16 - 9  may be positioned such that the H 2  gas flows along the reflective surface of the EUV collector mirror M 1 . 
     The H 2  gas flowing along the reflective surface of the EUV collector mirror M 1  may be irradiated with short-wavelength light, such as ultraviolet light, vacuum ultraviolet light, and EUV light L 2  generated in the plasma generation region P 1 , whereby the H 2  gas may be turned into H radicals. As a result, hydrogen may flow in the form of H radicals along the reflective surface of the EUV collector mirror M 1 , and Sn deposited particularly on the reflective surface of the EUV collector mirror M 1  may be etched. 
     Tenth Modification 
     A radical generator according to a tenth modification will be described in detail with reference to the drawings. In the above-described embodiments and modifications, the H radicals or the H 2  gas are/is made to flow along the surfaces of the optical elements via a gas introduction pipe. However, this disclosure is not limited thereto. A radical generator may be provided to directly apply the H radicals or the H 2  gas on the surfaces of the optical elements.  FIG. 21A  schematically illustrates the configuration of the radical generator and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the tenth modification.  FIG. 21B  is a sectional view, taken along XXIB-XXIB plane, schematically illustrating the radical generator and the EUV collector mirror shown in  FIG. 21A . 
     As shown in  FIGS. 21A and 21B , in the tenth modification, radical generators  15 - 10   a  and  15 - 10   b  may be provided for the EUV collector mirror M 1 . Alternatively, in place of the radical generators  15 - 10   a  and  15 - 10   b , hydrogen gas supply sources may be used. The radical generators  15 - 10   a  and  15 - 10   b  may be arranged symmetrically about the center of the EUV collector mirror M 1  above the reflective surface of the EUV collector mirror M 1 . Radical output ports of the respective radical generators  15 - 10   a  and  15 - 10   b  may be oriented such that the H radicals flow along the reflective surface of the EUV collector mirror M 1 . With this, the H radicals from the radical generators  15 - 10   a  and  15 - 10   b  may flow substantially uniformly along the reflective surface of the EUV collector mirror M 1  from the periphery of the reflective surface of the EUV collector mirror M 1 . As a result, Sn deposited on the reflective surface of the EUV collector mirror M 1  may be etched. 
     Eleventh Modification 
     An eleventh modification will be described in detail with reference to the drawings. In the above-described embodiments and modifications, H radicals or H 2  gas are/is made to flow along the surfaces of the optical elements via a gas introduction pipe. However, this disclosure is not limited thereto. Filaments may be provided in the vicinity of the optical element to turn the H 2  gas into the H radicals. In this configuration, the chamber  11  may be filled with the H 2  gas.  FIG. 22A  schematically illustrates the configuration of the filaments and the EUV collector mirror, as viewed from the reflective surface side of the EUV collector mirror, according to the eleventh modification.  FIG. 22B  is a sectional view, taken along XXIIB-XXIIB plane, schematically illustrating the configuration of the filaments and the EUV collector mirror shown in  FIG. 22A . 
     As shown in  FIGS. 22A and 22B , in the eleventh modification, a plurality of filaments  16 - 11  may be provided for the EUV collector mirror M 1 . The filaments  16 - 11  may be arranged symmetrically about the center of the EUV collector mirror M 1  outside the reflective surface of the EUV collector mirror M 1 . The H 2  gas may be turned into the H radicals as the H 2  gas receives energy when passing near the filaments  16 - 11 . Then, the H radicals may be incident on the reflective surface of the EUV collector mirror M 1 . As a result, Sn deposited particularly on the reflective surface of the EUV collector mirror M 1  may be etched. 
     Fifth Embodiment 
     An EUV light generation system according to a fifth embodiment of this disclosure will now be described in detail with reference to the drawing. In the above-described embodiments and modifications, Sn deposited on the optical element may be etched by making the H radicals or the H 2  gas flow along the reflective surface while the EUV light generation system is in operation. In contrast, in the fifth embodiment, Sn deposited on the optical element may be etched by making the H radicals or the H 2  gas flow over the entire reflective surface of the optical element while the EUV light generation system is not in operation.  FIG. 23  is a sectional view schematically illustrating the configuration of the EUV light generation system according to the fifth embodiment. 
     As shown in  FIG. 23 , a chamber  11 A of the fifth embodiment may have a gate W 4   a  formed in the wall, and a radical generator  16 - 12  may be introduced into or retracted from a predetermined position facing the reflective surface of the EUV collector mirror M 1  through the gate W 4   a . The gate W 4   a  may be sealed airtightly with a shutter W 4 . With this configuration, while the EUV light generation system is not in operation, the shutter W 4  may be moved to open the gate W 4   a , and the radical generator  16 - 12  may be introduced into the chamber  11 A. Then, the H radicals or the H 2  gas may be blown against the entire reflective surface of the EUV collector mirror M 1  from the radical generator  16 - 12 . As a result, Sn deposited on the EUV collector mirror M 1  may be etched. 
     Sixth Embodiment 
     An EUV light generation system according to a sixth embodiment of this disclosure will now be described in detail with reference to the drawings. In the sixth embodiment, the radical generator  16 - 12  of the above-described fifth embodiment may be replaced by a gas introduction pipe and a filament. The H 2  gas may be blown out through the gas introduction pipe, and then the H 2  gas may be turned into the H radicals through the filament.  FIG. 24A  is a sectional view schematically illustrating the configuration of the EUV light generation system according to the sixth embodiment.  FIG. 24B  schematically illustrates the configuration of the filament and the EUV collector mirror shown in  FIG. 24A , as viewed from the reflective surface side of the EUV collector mirror. 
     As shown in  FIGS. 24A and 24E , in the sixth embodiment, a gas introduction pipe  16 - 13   a  may be introduced into a chamber  11 A. The gas introduction pipe  16 - 13   a  may be connected to an H 2  gas cylinder (not shown). A filament  16 - 13  may be provided at a gas output port of the gas introduction pipe  16 - 13   a  to turn H 2  gas into H radicals. While the EUV light generation system is not in operation, the shutter W 4  may be moved to open a gate W 5 , and the gas introduction pipe  16 - 13   a  may be introduced into the chamber  11 A. Then, electric current may be supplied to the filament  16 - 13  and the H 2  gas may be made to flow into the gas introduction pipe  16 - 13   a . With this, the H 2  gas from the gas introduction pipe  16 - 13   a  may be turned into the H radicals, which then may be blown against the entire reflective surface of the EUV collector mirror M 1 . As a result, Sn deposited on the EUV collector mirror M 1  may be etched. 
     Seventh Embodiment 
     An EUV light generation system according to a seventh embodiment of this disclosure will now be described in detail with reference to the drawings.  FIG. 25A  schematically illustrates the configuration of an EUV collector mirror and gas introduction pipes, as viewed from the reflective surface side of the EUV collector mirror, according to the seventh embodiment.  FIG. 25B  is a sectional view schematically illustrating the configuration of a laser beam focusing unit, the EUV collector mirror, and the gas introduction pipes according to the seventh embodiment, taken along a plane perpendicular to the reflective surface of the EUV collector mirror. 
     As shown in  FIGS. 25A and 258 , the gas introduction pipes of the seventh embodiment may include an outer pipe  263 , an inner pipe  264 , and a wall unit  265 . The EUV collector mirror M 1  may be fixed onto the plate H 1 . The plate H 1  may be fixed inside the chamber  11 . 
     The laser beam focusing optical unit  210  may include a window W 2 , an off-axis paraboloidal convex mirror  212 , and a spheroidal concave mirror  213  arranged in this order in the direction in which the pre-pulse laser beam L 1   a  and/or the main pulse laser beam L 1   b , here shown generically as L 1 , travel (s). 
     The off-axis paraboloidal convex mirror  212  and the spheroidal concave mirror  213  may be provided inside a sub-chamber  211 . The sub-chamber  211  may be in communication with a hydrogen gas supply source  251  through a pipe  261 . The hydrogen gas supply source  251  may be replaced by a radical generator. The pipe  261  may be positioned so that the H 2  gas from the pipe  261  flows along the surface of the window W 2 . The sub-chamber  211  may be fixed onto the plate H 1 . Alternatively, the sub-chamber  211  may be provided inside the chamber  11 , or may be fixed to the outer wall of the chamber  11 . 
     The sub-chamber  211 , the plate H 1 , and the EUV collector mirror M 1  may, respectively, have through-holes  211   a , H 1   a , and M 1   a  formed therein, through which the laser beam L 1  travels toward the plasma generation region P 1 . The sub-chamber  211  may generally be sealed airtightly except at the connection part between the sub-chamber  211  and the pipe  261  and at the through-hole  211   a . The inner pipe  264  and the wall unit  265  may be fitted into the through-hole  211   a.    
       FIG. 26A  is a perspective view schematically illustrating the configuration of the inner pipe  264  and the wall unit  265  that are integrated.  FIG. 26B  is a sectional view schematically illustrating the configuration of the inner pipe  264  and the wall unit  265  fitted into the through-holes  211   a , H 1   a , and M 1   a.    
     As shown in  FIGS. 26A and 26B , the wall unit  265  may be a frustconical hollow member having openings  265   a  and  265   b  formed at the ends. The hollow part of the wall unit  265  may serve as a path through which the pre-pulse laser beam L 1   a  and/or the main pulse laser beam L 1   b  travel(s). The wall unit  265  may be positioned such that the axis passing through the centers of the respective openings  265   a  and  265   b  coincide with the beam axis of the pre-pulse laser beam L 1   a  and/or the main pulse laser beam L 1   b  reflected by the spheroidal concave mirror  213 . The solid angle formed by the inner surface of the wall unit  265  may be substantially equal to the focusing solid angle of the pre-pulse laser beam L 1   a  and/or the main pulse laser beam L 1   b  reflected by the spheroidal concave mirror  213 . With this configuration, even when a part of the pre-pulse laser beam L 1   a  and/or the main pulse laser beam L 1   b  strikes the inner surface of the wall unit  265 , the angle of incidence of the laser beam may be relatively large; thus, the damage to the wall unit  265  may be kept small. 
     The inner pipe  264  may include first and second members. Each of the first and second members may include a frustconical hollow body part and a trumpet-shaped folded part. The inner diameter of the body part of the first member may be larger than the outer diameter of the body part of the second member. The first and second members may be fixed to each other with a spacer or the like (not shown) provided therebetween so as to forma substantially uniform gap. The assembled first and second members may form an opening  264   a , through which the H 2  gas flows into the inner pipe  264 , and an opening  264   b , through which the H 2  gas flows out of the inner pipe  264 . 
     The body part of the second member may be fixed onto the wall unit  265  at a bottom portion of the wall unit  265 . The opening  264   a  in the inner pipe  264  may be positioned on the same plane as the opening  265   a  in the wall unit  265 . The body part of the first member may be fixed to the periphery of the through-hole  211   a  in the sub-chamber  211 . The inner pipe  264  may be positioned so that the H 2  gas that flows out through the opening  264   b  flows radially along the reflective surface of the EUV collector mirror M 1  from the center to the periphery thereof. 
       FIG. 27A  is a perspective view schematically illustrating the configuration of the outer pipe  263 , a pipe  262 , and a hydrogen gas supply source  252 .  FIG. 27B  is a sectional view illustrating the configuration of the outer pipe  263  shown in  FIG. 27A . 
     As shown in  FIGS. 27A and 27B , the outer pipe  263  may be annular in shape. The outer pipe  263  may be connected to the hydrogen gas supply source  252  through the pipe  262 . The hydrogen gas supply source  252  may be replaced by a radical generator. A slit  263   a  may be formed in the inner side of the outer pipe  263  to serve as an outlet for the H 2  gas. The slit  263   a  may be formed so as to surround the inner side of the outer pipe  263 . The slit  263   a  may, for example, be formed to face the reflective surface of the EUV collector mirror M 1 , as shown in  FIG. 27B . The H 2  gas from the hydrogen gas supply source  252  may be blown out through the slit  263   a  in the outer pipe  263  via the pipe  262 . The outer pipe  263  may be positioned such that the center of the annular outer pipe  263  substantially coincides with the center of the EUV collector mirror M 1 . With this, the H 2  gas may flow along the reflective surface of the EUV collector mirror M 1  from the periphery toward the center thereof. Here, the center of the EUV collector mirror M 1  may be the rotational axis of the spheroidal surface. 
     According to the seventh embodiment, the H 2  gas may flow along the reflective surface of the EUV collector mirror radially from the center toward the periphery and also from the periphery toward the center. With this, debris generated in the plasma generation region P 1  may be prevented from being deposited on the reflective surface of the EUV collector mirror M 1 . Further, even when the debris is deposited on the reflective surface, the deposited debris may be etched. 
     The H 2  gas supplied into the sub-chamber  211  may flow along the surface of each optical element. With this, the debris may be prevented from being deposited on the optical elements. Further, even when the debris is deposited on the optical elements, the deposited debris may be etched. 
     The above-described embodiments and the modifications thereof are merely examples for implementing this disclosure, and this disclosure is not limited thereto. Making various modifications according to the specification or the like is within the scope of this disclosure, and other various embodiments are possible within the scope of this disclosure. For example, the modifications illustrated for particular embodiments can be applied to other embodiments as well including the other embodiments described herein. 
     As a device for controlling the temperature of an optical element, an example in which a temperature-controlled heat carrier is made to flow in the substrate of the optical element or an example in which the heater and the chiller are used in combination has been shown. However, this disclosure is not limited thereto, and any system capable of heating and cooling may be applied. For example, a Peltier element may be used, and the temperature of the optical element may be controlled with high precision by controlling electric current supplied to the Peltier element. 
     The terms used in this specification and the appended claims should be interpreted as “non-limiting.” For example, the terms “include” and “be included” should be interpreted as “including the stated elements but not limited to the stated elements.” The term “have” should be interpreted as “having the stated elements but not limited to the stated elements.” Further, the modifier “one (a/an)” should be interpreted as “at least one” or “one or more.”