PATENT DOCUMENT

Abstract:
A chamber apparatus for operating with a laser apparatus includes a chamber, a target supply unit, a first optical system and a second optical system. The chamber has an inlet for introducing a laser beam thereinto. The target supply unit supplies a target material to a region inside the chamber. The first optical system focuses the laser beam in the region. The guide beam output device outputs a guide beam. The second optical system directs the guide beam such that an axis of a beam path of the guide beam substantially coincides with an axis of a beam path of the laser beam and such that the guide beam enters the focusing optical system through the region.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Japanese Patent Application No. 2011-164290 filed Jul. 27, 2011. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    This disclosure relates to a chamber apparatus, an extreme ultraviolet (EUV) light generation system, and a method for controlling the extreme ultraviolet light generation system. 
         [0004]    2. Related Art 
         [0005]    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 is needed in which a system for generating EUV light at a wavelength of approximately 13 nm is combined with a reduced projection reflective optical system. 
         [0006]    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 
       [0007]    A chamber apparatus according to one aspect of this disclosure, which may operate with a laser apparatus, may include: a chamber having an inlet for introducing a laser beam thereinto; a target supply unit configured to supply a target material to a region inside the chamber; a first optical system for focusing the laser beam in the region; a guide beam output device configured to output a guide beam; and a second optical system configured to direct the guide beam such that an axis of a beam path of the guide beam substantially coincides with an axis of a beam path of the laser beam and such that the guide beam enters the first optical system through the region. 
         [0008]    An apparatus for generating extreme ultraviolet light according to another aspect of this disclosure may include: the above-described chamber apparatus; and a laser apparatus configured to output a laser beam. 
         [0009]    A method according to yet another aspect of this disclosure for controlling an apparatus configured to generate extreme ultraviolet light, which may include a laser apparatus, a chamber, a target supply unit, a focusing optical system, a guide beam output device, an optical system, a detection unit, a first driving mechanism, and a controller, may include controlling the first driving mechanism such that an image of a guide beam detected by the detection unit lies in a desired position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Hereinafter, selected embodiments of this disclosure will be described with reference to the accompanying drawings. 
           [0011]      FIG. 1  schematically illustrates the configuration of an exemplary LPP type EUV light generation system. 
           [0012]      FIG. 2  schematically illustrates an example of the configuration of an EUV light generation system according to a first embodiment of this disclosure. 
           [0013]      FIG. 3  shows an example of an image to be detected by an image sensor in a case where the center of a guide beam and the center of a droplet do not coincide with an origin when the droplet is irradiated with a pulse laser beam in the first embodiment. 
           [0014]      FIG. 4  shows an example of an image to be detected by an image sensor after a laser beam focusing optical system is adjusted in the first embodiment. 
           [0015]      FIG. 5  shows an example of an image to be detected by an image sensor after a target supply unit is adjusted in the first embodiment. 
           [0016]      FIG. 6  shows an example of an image to be detected by an image sensor when the droplet is irradiated with the pulse laser beam in the state shown in  FIG. 5 . 
           [0017]      FIG. 7  is a flowchart showing an overall operation of an exemplary EUV light generation controller according to the first embodiment. 
           [0018]      FIG. 8  is a flowchart showing an example of a guide beam adjusting subroutine of  FIG. 7 . 
           [0019]      FIG. 9  is a flowchart showing an example of a shooting control subroutine of  FIG. 7 . 
           [0020]      FIG. 10  is a flowchart showing an example of a result determination subroutine of  FIG. 7 . 
           [0021]      FIG. 11  schematically illustrates an example of the configuration of a mirror unit of a first modification and the peripheral components thereof. 
           [0022]      FIG. 12  schematically illustrates an example of the configuration of a mirror unit of a second modification and the peripheral components thereof. 
           [0023]      FIG. 13  schematically illustrates an example of the configuration of a mirror unit of a third modification and the peripheral components thereof. 
           [0024]      FIG. 14  schematically illustrates an example of the configuration of an optical system for a guide beam in a modification of the EUV light generation system. 
           [0025]      FIG. 15  shows an image of a droplet and an image of a guide beam at a pinhole in a pinhole plate imaged on the image sensor in  FIG. 14 . 
           [0026]      FIG. 16  schematically illustrates an example of the configuration of an EUV light generation system according to a second embodiment of this disclosure. 
           [0027]      FIG. 17  shows an example of an image to be detected by an image sensor in a case where the center of a guide beam and the center of a droplet do not coincide with an origin when the droplet is irradiated with a pulse laser beam in the second embodiment. 
           [0028]      FIG. 18  shows an example of an image to be detected by an image sensor after a laser beam focusing optical system is adjusted in the second embodiment. 
           [0029]      FIG. 19  shows an example of an image to be detected by an image sensor after a target supply unit is adjusted in the second embodiment. 
           [0030]      FIG. 20  shows an example of an image to be detected by an image sensor when the droplet is irradiated with a pre-pulse laser beam in the state shown in  FIG. 19 . 
           [0031]      FIG. 21  shows an example of an image to be detected by an image sensor when a diffused target is irradiated with a main pulse laser beam after a predetermined time has elapsed since the state shown in  FIG. 20 . 
           [0032]      FIG. 22  is a flowchart showing an overall operation of an EUV light generation controller according to the second embodiment. 
           [0033]      FIG. 23  is a flowchart showing an example of a target position setting subroutine of  FIG. 22 . 
           [0034]      FIG. 24  is a flowchart showing an example of a shooting control subroutine of  FIG. 22 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Hereinafter, selected embodiments of this disclosure will be described in detail with reference to the accompanying drawings. The embodiments to be described below are merely illustrative in nature and do not limit the scope of this disclosure. Further, the configuration(s) and operation(s) described in each embodiment are not all essential in implementing this disclosure. Note that like elements are referenced by like reference numerals and characters, and duplicate descriptions thereof will be omitted herein. The embodiments of this disclosure will be illustrated following the table of contents below. 
       Contents 
     1. Overview 
     2. Terms 
     3. Overview of EUV Light Generation System 
     3.1 Configuration 
     3.2 Operation 
     4. EUV Light Generation System Including Detector for Detecting Positions of Guide Beam, Irradiation Target, and Plasma-Emitted Light: First Embodiment 
     4.1 Configuration 
     4.2 Operation 
     4.3 Effect 
     4.4 Flowchart 
     5. Specific Examples of Mirror Unit 
     5.1 First Example 
     5.1.1 Configuration 
     5.1.2 Operation 
     5.1.3 Effect 
     5.2 Second Example 
     5.2.1 Configuration 
     5.2.2 Operation 
     5.2.3 Effect 
     5.3 Third Example 
     5.3.1 Configuration 
     5.3.2 Operation 
     5.3.3 Effect 
     6. Modification of Guide Beam Optical System 
     6.1 Configuration 
     6.2 Operation 
     6.3 Effect 
       [0036]    7. EUV Light Generation System Including Pre-pulse Laser apparatus and Main Pulse Laser Apparatus: Second Embodiment 
       7.1 Configuration 
     7.2 Operation 
     7.3 Effect 
     7.4 Flowchart 
     1. Overview 
       [0037]    Embodiments to be described hereinafter relate to an LPP type EUV light generation system that includes a detector for detecting the position of a guide beam and the position of a target. 
       2. Terms 
       [0038]    Terms used in this disclosure may be interpreted as follows. The term “droplet” may refer to one or more liquid droplet(s) of a molten target material. Accordingly, the shape thereof may be substantially spherical due to its surface tension. The term “plasma generation region” may refer to a three-dimensional space in which plasma is to be generated. In a beam path of a laser beam, a direction toward the laser apparatus or a side closer to the laser apparatus is referred to as “upstream,” and a direction or a side toward which the laser beam travels from the laser apparatus is referred to as “downstream.” 
       3. Overview of EUV Light Generation System 
     3.1 Configuration 
       [0039]      FIG. 1  schematically illustrates the configuration of an exemplary LPP type EUV light generation system. An LPP type EUV light generation apparatus  1  may be used with at least one laser apparatus  3 . Hereinafter, a system that includes the EUV light generation apparatus  1  and the laser apparatus  3  may be referred to as an EUV light generation system  11 . As illustrated in  FIG. 1  and described in detail below, the EUV light generation system  11  may include a chamber  2  and a target supply unit, and so forth. The target supply unit may be a droplet generator  26 . The chamber  2  may be airtightly sealed. The target supply unit may be mounted onto the chamber  2  so as to, for example, penetrate a wall of the chamber  2 . A target material to be supplied by the target supply unit may include, but is not limited to, tin, terbium, gadolinium, lithium, xenon, or any combination thereof. 
         [0040]    The chamber  2  may have at least one through-hole or opening formed in its wall, and a pulse laser beam  32  may travel through the through-hole/opening into the chamber  2 . Alternatively, the chamber  2  may be provided with a window  21 , through which the pulse laser beam  32  may travel into the chamber  2 . An EUV collector mirror  23  having a spheroidal surface may be provided inside the chamber  2 , for example. The EUV collector mirror  23  may have a multi-layered reflective film formed on the spheroidal surface thereof. The reflective film may include a molybdenum layer and a silicon layer, which are laminated alternately. The EUV collector mirror  23  may have a first focus and a second focus, and preferably be positioned such that the first focus lies in a plasma generation region  25  and the second focus lies in an intermediate focus (IF) region  292  defined by the specification of an external apparatus, such as an exposure apparatus  6 . The EUV collector mirror  23  may have a through-hole  24  formed at the center thereof, and a pulse laser beam  33  may travel through the through-hole  24  toward the plasma generation region  25 . 
         [0041]    The EUV light generation system  11  may further include an EUV light generation controller  5  and a target sensor  4 . The target sensor  4  may have an imaging function and detect at least one of the presence, the trajectory, and the position of a droplet  27 . 
         [0042]    Further, the EUV light generation system  11  may include a connection part  29  for allowing the interior of the chamber  2  and the interior of the exposure apparatus  6  to be in communication with each other. A wall  291  having an aperture may be provided inside the connection part  29 , and the wall  291  may be positioned such that the second focus of the EUV collector mirror  23  lies in the aperture formed in the wall  291 . 
         [0043]    The EUV light generation system  11  may also include a beam delivery unit  340 , a laser beam focusing optical system  22 , and a target collector  28  for collecting droplets  27 . The beam delivery unit  340  may include an optical element for defining the direction into which the pulse laser beam  32  travels and include an actuator for adjusting the position and the orientation (posture) of the optical element. 
       3.2 Operation 
       [0044]    With continued reference to  FIG. 1 , a pulse laser beam  31  outputted from the laser apparatus  3  may pass through the beam delivery unit  340  and be outputted therefrom as a pulse laser beam  32  after having its direction optionally adjusted. The pulse laser beam  32  may travel through the window  21  and enter the chamber  2 . The pulse laser beam  32  may travel inside the chamber  2  along at least one beam path from the laser apparatus  3 , be reflected by the laser beam focusing optical system  22 , and strike at least one droplet  27  as a pulse laser beam  33 . 
         [0045]    The target supply unit may be configured to output the droplet(s)  27  toward the plasma generation region  25  inside the chamber  2 . The droplet  27  may be irradiated with at least one pulse of the pulse laser beam  33 . Upon being irradiated with the pulse laser beam  33 , the droplet  27  may be turned into plasma, and rays of light  251  including EUV light may be emitted from the plasma. At least the EUV light included in the light  251  may be reflected selectively by the EUV collector mirror  23 . The EUV light reflected by the EUV collector mirror  23  may travel through the intermediate focus region  292  and be outputted to the exposure apparatus  6 . Here, the droplet  27  may be irradiated with multiple pulses included in the pulse laser beam  33 . 
         [0046]    The EUV light generation controller  5  may be configured to integrally control the EUV light generation system  11 . The EUV light generation controller  5  may be configured to process image data of the droplet  27  captured by the target sensor  4 . Further, the EUV light generation controller  5  may be configured to control at least one of the timing at which the droplet  27  is outputted and the direction into which the droplet  27  is outputted. Furthermore, the EUV light generation controller  5  may be configured to control at least one of the timing at which the laser apparatus  3  oscillates, the direction in which the pulse laser beam  31  travels, and the position at which the pulse laser beam  33  is focused. It will be understood that the various controls mentioned above are merely examples, and other controls may be added as necessary. 
       4. EUV Light Generation System Including Detector for Detecting Positions of Guide Beam, Irradiation Target, and Plasma-Emitted Light: First Embodiment 
     4.1 Configuration 
       [0047]      FIG. 2  schematically illustrates an example of the configuration of an EUV light generation system  11 A according to a first embodiment. As shown in  FIG. 2 , the EUV light generation system  11 A may include an EUV light generation apparatus LA and the laser apparatus  3 . 
         [0048]    The laser apparatus  3  may be configured to output the pulse laser beam  31  at a predetermined repetition rate. When the laser apparatus  3  includes CO 2  gas as a gain medium, the wavelength of the pulse laser beam  31  may be around 10.6 μm. 
         [0049]    The EUV light generation apparatus  1 A may include the beam delivery unit  340 , a beam adjusting unit  350 , a chamber  2 A, and an EUV light generation controller  5 A. 
         [0050]    The beam delivery unit  340  may include a high-reflection mirror  341  for defining a direction into which the pulse laser beam  32  travels. The high-reflection mirror  341  may be coated with a film configured to reflect the pulse laser beam  31  with high reflectance. The beam delivery unit  340  may further include an actuator (not separately shown) for adjusting the position and the orientation of the high-reflection mirror  341 . The beam delivery unit  340  may be positioned to direct the pulse laser beam  31  toward the beam adjusting unit  350  as the pulse laser beam  32 . 
         [0051]    The beam adjusting unit  350  may include a dichroic mirror  351 . The dichroic mirror  351  may be coated on a first surface thereof with a film configured to reflect the pulse laser beam  32  with high reflectance and configured to transmit a guide beam  41  with high transmittance. Further, the dichroic mirror  351  may be coated on a second surface thereof with a film configured to transmit the guide beam  41  with high transmittance. The dichroic mirror  351  may be positioned such that the pulse laser beam  32  is incident on the first surface. The dichroic mirror  351  may have a substrate which, for example, is made of diamond. 
         [0052]    The chamber  2 A may include the window  21 , the laser beam focusing optical system  22 , a target supply unit  260 , the target sensor  4 , the EUV collector mirror  23 , and the connection part  29 . The chamber  2 A may also include an etching gas supply unit  90 , manometer  93 , and an exhaust unit  94 . Further, the EUV light generation apparatus  1 A may include an optical detection unit that includes an imaging optical system  402  and an image sensor  410 . The window  21  may be coated with a film configured to reduce reflectance of the pulse laser beam  32  incident thereon. 
         [0053]    The laser beam focusing optical system  22  may include a laser beam focusing mirror  72  and a high-reflection mirror  73 . The laser beam focusing optical system  22  may further include a plate  71 , a plate moving mechanism  71   a , a mirror holder  72   a , and a holder  73   a  provided with an automatic tilt mechanism  73   b . The laser beam focusing mirror  72  may be an off-axis paraboloidal mirror, and may be mounted to the plate  71  through the mirror holder  72   a . The high-reflection mirror  73  may be mounted to the plate  71  through the holder  73   a . The plate moving mechanism  71   a  may be configured to move the laser beam focusing mirror  72  and the high-reflection mirror  73  along with the plate  71 . 
         [0054]    The plate moving mechanism  71   a  may be configured to move the plate  71  to thereby adjust the focus of the pulse laser beam  33 . The holder  73   a  may be configured to adjust the tilt angles of the high-reflection mirror  73  to thereby adjust the focus of the pulse laser beam  33 . These adjustments may be made under the control of the EUV light generation controller  5 A, which will be described in detail later. 
         [0055]    The target supply unit  260  may include the droplet generator  26  and a two-axis moving mechanism  261 . The droplet generator  26  may be positioned to output the droplets  27  toward the plasma generation region  25 . The two-axis moving mechanism  261  may be configured to move the droplet generator  26  under the control of the EUV light generation controller  5 A, to thereby adjust the position to which the droplets  27  are supplied. 
         [0056]    The chamber  2 A may further include a guide beam output device  40 , a collimator  401 , a mirror unit  101 , a beam dump  112 , a dichroic mirror  121 , and a beam dump  122 . 
         [0057]    The mirror unit  101  may include first and second reflective surfaces. The first reflective surface may be arranged upstream (i.e., toward the laser apparatus  3 ) from the second reflective surface. A through-hole may be formed in the first reflective surface. The mirror unit  101  may be supported by a mirror holder  101   a . The beam dump  112 , the guide beam output device  40 , and the collimator  401  may be housed in a sub-chamber  102 . The sub-chamber  102  is optically connected to the chamber  2 A through windows  113  and  123 . 
         [0058]    The guide beam output device  40  may be configured to output a guide beam  41 . The guide beam output device  40  may be a semiconductor laser. The guide beam output device  40  is not limited to a laser, but may be an incoherent light source, such as a light emitting diode (LED). The guide beam  41  may be a pulse beam or a continuous wave beam. The wavelength of the guide beam  41  may be shorter than the wavelength of the pulse laser beam  31 . For example, the guide beam  41  may be visible radiation, and the wavelength thereof may, for example, be around 500 nm. The guide beam  41  may preferably be at a wavelength suitable for photosensitivity of the image sensor  410 , which will be described in detail later. The collimator  401  may be provided in a beam path of the guide beam  41  outputted from the guide beam output device  40 . 
         [0059]    The imaging optical system  402  may include one or more imaging lenses. The imaging optical system  402  may be positioned to focus the guide beam  41  on the photosensitive surface of the image sensor  410 . The image sensor  410  may be a two-dimensional sensor, such as a charge-coupled device (CCD) or a position-sensitive device (PSD). 
         [0060]    In the above-described configuration, an adjustment may be made such that the axis of the beam path of the guide beam  41  reflected by the mirror unit  101  substantially coincides with the axis of the beam path of the pulse laser beam  33  traveling through the plasma generation region  25 . 
         [0061]    The EUV light generation controller  5 A may include an EUV light generation position controller  51 , a reference clock generator  52 , a target controller  53 , a target supply driver  54 , a laser beam focus position control driver  55 , and a gas controller  56 . The EUV light generation position controller  51  may be connected to the reference clock generator  52 , the laser beam focus position control driver  55 , the gas controller  56 , the target controller  53 , the laser apparatus  3 , the exposure apparatus controller  61  (of the exposure apparatus  6 ), the guide beam output device  40 , and the image sensor  410 . The target controller  53  may be connected to the target supply driver  54  and the target sensor  4 . The target supply driver  54  may be connected to the target supply unit  260 . The laser beam focus position control driver  55  may be connected to the laser beam focusing optical system  22 . The gas controller  56  may be connected to the etching gas supply unit  90 , the manometer  93 , and the exhaust unit  94 . 
         [0062]    The interior of the chamber  2 A may be divided into an upstream space  2   a  and a downstream space  2   b  by a partition  81 . The plasma generation region  25  may be set inside the downstream space  2   b . The partition  81  may serve to reduce the amount of debris of the target material, generated in the plasma generation region  25  and entering the upstream space  2   a . The partition  81  may have a communication hole  82  formed therein, through which the pulse laser beam  33  and the guide beam  41  may pass. The communication hole  82  may preferably be aligned with the through-hole  24  formed in the EUV collector mirror  23 . The EUV collector mirror  23  may be fixed to the partition  81  through a holding unit  23   a.    
       4.2 Operation 
       [0063]    The operation of the EUV light generation system  11 A shown in  FIG. 2  will now be described. The EUV light generation system  11 A may operate under the control of the EUV light generation controller  5 A. The EUV light generation controller  5 A may receive a request from the exposure apparatus controller  61  regarding a position at which the EUV light is to be generated (hereinafter, referred to as an EUV light generation request position) or the plasma generation region  25 . The EUV light generation controller  5 A may then control each of its components so that the EUV light is generated at the EUV light generation request position. Alternatively, the EUV light generation controller  5 A may control each component so that the EUV light generation request position falls within the plasma generation region  25 . 
         [0064]    The EUV light generation controller  5 A may cause the guide beam output device  40  to oscillate, so that the guide beam output device  40  may output the guide beam  41 . The guide beam  41  may then be incident on the collimator  401  and be collimated by the collimator  401 . The collimated guide beam  41  may enter the chamber  2 A through the window  123 . The guide beam  41  having entered the chamber  2 A may be reflected toward the plasma generation region  25  by the second surface of the mirror unit  101 . The second surface of the mirror unit  101  may be configured such that the guide beam  41  reflected thereby is focused in the plasma generation region  25 . Here, the axis of the beam path of the guide beam  41  reflected by the second surface may substantially coincide with the axis of the beam path of the pulse laser beam  33  to be focused in the plasma generation region  25 . 
         [0065]    Thereafter, the guide beam  41  may enter the laser beam focusing optical system  22  and be collimated thereby. Then, the guide beam  41  may enter the beam adjusting unit  350 . The guide beam  41  that has entered the beam adjusting unit  350  may be transmitted through the dichroic mirror  351 , and be focused on the photosensitive surface of the image sensor  410  by the imaging optical system  402 . Thus, the guide beam  41  may be imaged on the photosensitive surface of the image sensor  410 . This image detected by the image sensor  410  may include an image of the droplet  27 . The image sensor  410  may send this image data to the EUV light generation position controller  51 . Here, the spot size of the guide beam  41  may preferably be adjusted such that the image of the droplet  27  overlaps the image of the guide beam  41  so as to be detected inside an image of the guide beam  41 . The spot size of the guide beam  41  may be equal to or larger than the spot size of the pulse laser beam  33 . 
         [0066]    The image detected by the image sensor  410  may also include an image of the light  251 . That is, a part of the light  251  emitted in the plasma generation region  25  may enter the image sensor  410 , through the through-hole  24  in the EUV collector mirror  23 , the laser beam focusing optical system  22 , the window  21 , the beam adjusting unit  350 , and the imaging optical system  402 . The image sensor  410  may send the image data of the detected light  251  to the EUV light generation position controller  51 . 
         [0067]    Upon receiving the image data of the guide beam  41  from the image sensor  410 , the EUV light generation position controller  51  may calculate the center of the image of the guide beam  41 . The EUV light generation position controller  51  may then control the laser beam focusing optical system  22  through the laser beam focus position control driver  55  such that the center of the image of the guide beam  41  coincides with a predetermined target position (the EUV light generation request position). 
         [0068]    The laser beam focus position control driver  55  may send driving signals to the automatic tilt mechanism  73   b  and the plate moving mechanism  71   a , respectively, under the control of the EUV light generation position controller  51 . The automatic tilt mechanism  73   b  may control the tilt angles of the high-reflection mirror  73  in θx and θy directions based on the driving signal received from the laser beam focus position control driver  55 . The plate moving mechanism  71   a  may move the plate  71  in the Z-direction based on the driving signal from the laser beam focus position control driver  55 . 
         [0069]    When the image of the droplet  27  is contained in the image of the guide beam  41 , the EUV light generation position controller  51  may calculate the center of the image of the droplet  27 . The EUV light generation position controller  51  may then control the target supply unit  260  through the target controller  53  and the target supply driver  54  such that the center of the droplet  27  coincides with the EUV light generation request position. The target supply driver  54  may send a driving signal to the two-axis moving mechanism  261  under the control of the target controller  53 . The two-axis moving mechanism  261  may move the droplet generator  26  in the X- and Y-directions based on the driving signal from the target supply driver  54 . Further, the target supply driver  54  may adjust the timing at which an output signal of the droplet  27  is sent to the droplet generator  26  based on a timing control from the target controller  53 . 
         [0070]    The EUV light generation position controller  51  may also receive the image of the light  251  from the image sensor  410 . Upon receiving the image of the light  251 , the EUV light generation position controller  51  may calculate the center of the image of the light  251 . Then, the EUV light generation position controller  51  may compare the calculated center of the image of the light  251  with the EUV light generation request position. Based on a result of the comparison, the EUV light generation position controller  51  may control the EUV light generation system  11 A. 
         [0071]    The EUV light generation controller  5 A may receive an EUV light generation request signal from the exposure apparatus controller  61 . Upon receiving the EUV light generation request signal, the EUV light generation controller  5 A may input the EUV light generation request signal to the target controller  53 . Upon receiving the EUV light generation request signal, the target controller  53  may send the output signal of the droplet  27  to the droplet generator  26  of the target supply unit  260  through the target supply driver  54 . 
         [0072]    The target sensor  4  may detect the position and the timing at which the droplet  27  passes through a predetermined region. The detected position and timing values may be inputted to the target controller  53 . The target controller  53  may then control the target supply unit  260  through the target supply driver  54  in accordance with the inputted position and timing values. Further, the target controller  53  may send the inputted values to the EUV light generation position controller  51 . The EUV light generation position controller  51  may send a trigger signal to the laser apparatus  3  in accordance with the inputted values so that the droplet  27  is irradiated with the pulse laser beam  33  at a timing at which the droplet  27  reaches EUV light generation request position. The laser apparatus  3  may be configured to output the pulse laser beam  31  at a timing delayed for a predetermined time from the trigger signal. 
         [0073]    The pulse laser beam  31  may travel through the beam delivery unit  340  and the beam adjusting unit  350 , and then enter the chamber  2 A through the window  21 . The pulse laser beam  32  may then be focused on the droplet  27  in the plasma generation region  25  by the laser beam focusing optical system  22 . 
         [0074]    Upon being irradiated with the pulse laser beam  33 , the droplet  27  may be turned into plasma, and the light  251  that includes the EUV light may be emitted from the plasma. The EUV collector mirror  23  may selectively reflect the EUV light of the light  251 . The reflected EUV light may be focused in the intermediate focus region  292  and be outputted to the exposure apparatus  6 . 
         [0075]    A part of the pulse laser beam  33 , the light  251 , and the EUV light may be reflected by the first reflective surface of the mirror unit  101  as light  34 . The light  34  may then be transmitted through the window  113  and be absorbed by the beam dump  112 . 
         [0076]    Another part of the pulse laser beam  33 , the light  251 , and the EUV light may be reflected by the second reflective surface of the mirror unit  101  as light  35 . The dichroic mirror  121  provided in the path of the light  35  may reflect a part of the light  35 . The reflected part of the light  35  may be absorbed by the beam dump  122 . 
         [0077]    When the target material includes metal, debris of the target material may be generated when the target material is turned into plasma. The debris may be deposited on the EUV collector mirror  23 , the mirror unit  101 , and so forth. The etching gas supply unit  90  may be configured to supply an etching gas for etching the deposited debris through introduction pipes  91  and  92  toward the reflective surface of the EUV collector mirror  23  or into the mirror unit  101 . When tin (Sn) is used as the target material, a gas containing a hydrogen gas or hydrogen radicals may be used as the etching gas. 
         [0078]    The etching gas supply unit  90  may be configured to supply the etching gas into the chamber  2 A under the control of the EUV light generation controller  5 A. The etching gas may be introduced toward the reflective surface of the EUV collector mirror  23  through the introduction pipe  91 . Similarly, the etching gas may be introduced into the mirror unit  101  through the introduction pipe  92 . 
         [0079]    The manometer  93  may be configured to measure the pressure inside the chamber  2 A. The manometer  93  may send the measured pressure value to the EUV light generation controller  5 A. The exhaust unit  94  may discharge the gas inside the chamber  2 A under the control of the EUV light generation controller  5 A. 
         [0080]    The gas controller  56  may control the etching gas supply unit  90  and the exhaust unit  94  based on the pressure value inputted from the manometer  93  so that the gas pressure inside the chamber  2 A is retained at a predetermined pressure while ensuring that a sufficient amount of gas is introduced into the chamber  2 A. 
         [0081]    Hereinafter, the image of the guide beam  41  and the image of the light  251 , which are imaged on the image sensor  410  will be discussed. In the description to follow, it is assumed that the axis of the beam path of the guide beam  41  is adjusted to coincide with the axis of the beam path of the pulse laser beam  33 . Further, a target position at which the droplet  27  is to be irradiated with the pulse laser beam  33  is the intersection (i.e., origin o) of the X-axis and the Y-axis in each of  FIGS. 3 through 6 . 
         [0082]      FIG. 3  shows an example of an image to be detected by the image sensor  410  in a case where the center of the guide beam  41  and the center of the droplet  27  do not coincide with the origin o when the droplet  27  is irradiated with the pulse laser beam  33 . In the example shown in  FIG. 3 , the center G of an image G 41  of the guide beam  41  does not coincide with the origin o. Similarly, the center D of an image D 27  of the droplet  27  does not coincide with the origin o. The position of the center G and the position of the center D may, for example, be obtained through various methods (e.g., by calculating the centers from the beam intensity distribution in the images acquired by the image sensor  410 ). Alternatively, the centroids may be used in place of the centers. 
         [0083]      FIG. 4  shows an example of an image to be detected by the image sensor  410  after the laser beam focusing optical system  22  is adjusted. As shown in  FIG. 4 , after the laser beam focusing optical system  22  is adjusted, the center G of the image G 41  may substantially coincide with the origin o. 
         [0084]      FIG. 5  shows an example of an image to be detected by the image sensor  410  after the target supply unit  260  is adjusted. As shown in  FIG. 5 , after the target supply unit  260  is adjusted, the center D of the image D 27  may substantially coincide with the origin o. 
         [0085]      FIG. 6  shows an example of an image to be detected by the image sensor  410  when the droplet  27  is irradiated with the pulse laser beam  33  in the state shown in  FIG. 5 . As shown in  FIG. 6 , after the laser beam focusing optical system  22  and the target supply unit  260  are adjusted, the center G of the image G 41  and the center D of the image D 27  may substantially coincide with the origin o. Accordingly, the center E of an image E 251  of the light  251  obtained when the droplet  27  is irradiated with the pulse laser beam  33  under the aforementioned state may be at or around the origin o. Since the image E 251  of the light  251  can be obtained with the above-described configuration, the EUV light may be generated at or around the origin by repeatedly adjusting the laser beam focusing optical system  22  and the target supply unit  260 . 
       4.3 Effect 
       [0086]    With the above-described configuration and operation, the axis of the beam path of the pulse laser beam  33  may be made to substantially coincide with the axis of the beam path of the guide beam  41  at a predetermined position. Further, the image of the guide beam  41  may be detected by the image sensor  410 , and this image may include the image of the droplet  27 . Accordingly, the focus of the pulse laser beam  33  and the position of the droplet  27  at the time of being irradiated with the pulse laser beam  33  may be identified from the image of the guide beam  41 . Thus, the focus of the pulse laser beam  33  and the position to which and the timing at which the droplet  27  is supplied may be controlled based on a result of the detection. As a result, generation of the EUV light may be controlled with high precision. 
         [0087]    Further, the guide beam output device  40  may output the guide beam  41  even while the laser apparatus  3  is not in operation. Thus, the focus of the pulse laser beam  33  may be controlled without putting the laser apparatus  3  into operation. 
         [0088]    According to the first embodiment, the guide beam  41  may be focused in the target position at which the droplet  27  is to be irradiated with the pulse laser beam  33 . Further, the image of the guide beam  41  at the target position may be detected through the laser beam focusing optical system  22  and the imaging optical system  402 . As a result, the position at which the pulse laser beam  33  is focused, the position of the droplet  27  being irradiated with the pulse laser beam  33 , and the target position may be detected simultaneously. Then, the focus of the laser beam focusing optical system  22  and the position of the droplet  27  may be controlled based on the detection result. Accordingly, the focus of the laser beam focusing optical system  22  and the position of the droplet  27  may be controlled with high precision to the desired target position. As a result, the droplet  27  may be irradiated with the pulse laser beam  33  stably, and the EUV light may be generated at the desired target position with high precision. 
       4.4 Flowchart 
       [0089]    The operation of the EUV light generation controller  5 A of the first embodiment will now be described in detail with reference to the drawings.  FIG. 7  is a flowchart showing the overall operation of the EUV light generation controller  5 A of the first embodiment.  FIG. 8  is a flowchart showing an example of a guide beam adjusting subroutine of  FIG. 7 .  FIG. 9  is a flowchart showing an example of a shooting control subroutine of  FIG. 7 .  FIG. 10  is a flowchart showing an example of a result determination subroutine of  FIG. 7 . 
         [0090]    The operation shown in  FIG. 7  may be carried out when the EUV light generation controller  5 A receives an instruction for a burst operation from an external apparatus, such as the exposure apparatus controller  61 , or when the EUV light generation controller  5 A is started. 
         [0091]    As shown in  FIG. 7 , the EUV light generation controller  5 A may first carry out the guide beam adjusting subroutine (Step S 101 ). Then, the EUV light generation controller  5 A may carryout the shooting control subroutine to generate the light  251  (Step S 102 ). Subsequently, the EUV light generation controller  5 A may carry out the result determination subroutine to determine whether or not the generation result of the light  251  through the shooting control subroutine in Step S 102  falls within a permissible range (Step S 103 ). When the generation result of the light  251  is determined not to fall within the permissible range based on the result from Step S 103  (Step S 104 ; NO), the EUV light generation controller  5 A may return to Step S 101  and repeat the subsequent steps. When the generation result of the light  251  is determined to fall within the permissible range based on the result from Step S 103  (Step S 104 ; YES), the EUV light generation controller  5 A may determine whether or not to stop the shooting control resulting in the generation of the light  251  (Step S 105 ). When the shooting control is to be stopped (Step S 105 ; YES), the EUV light generation controller  5 A may terminate the operation shown in  FIG. 7 . On the other hand, when the shooting control is not to be stopped (Step S 105 ; NO), the EUV light generation controller  5 A may return to Step S 102  and repeat the subsequent steps. 
         [0092]    With reference to  FIG. 8 , in the guide beam adjusting subroutine in Step S 101 , the EUV light generation controller  5 A may first turn on the guide beam output device  40  (Step S 111 ). Then, the EUV light generation controller  5 A may operate the image sensor  410  to detect the image G 41  of the guide beam  41  (Step S 112 ). Subsequently, the EUV light generation controller  5 A may analyze the image inputted from the image sensor  410  to calculate a distance L 1  between the center G of the image G 41  and the origin o (Step S 113 ). 
         [0093]    Then, the EUV light generation controller  5 A may determine whether or not the distance L 1  falls within a permissible range ΔL 1  (Step S 114 ). The permissible range ΔL 1  may be set in advance or may be inputted from an external apparatus, such as the exposure apparatus controller  61 . When the distance L 1  does not fall within the permissible range ΔL 1  (Step S 114 ; NO), the EUV light generation controller  5 A may actuate the laser beam focusing optical system  22  so that the center G of the image G 41  coincides with the origin o (Step S 115 ). Thereafter, the EUV light generation controller  5 A may return to Step S 111 . 
         [0094]    On the other hand, when the distance L 1  falls within the permissible range ΔL 1  (Step S 114 ; YES), the EUV light generation controller  5 A may actuate the target supply unit  260  to output the droplet  27  (Step S 116 ). Subsequently, the EUV light generation controller  5 A may turn on and off the guide beam output device  40  in synchronization with the planned irradiation timings with the pulse laser beam  33  (Step S 117 ). Then, the EUV light generation controller  5 A may detect the image D 27  of the droplet  27  from the image G 41  of the guide beam  41  inputted by operating the image sensor  410  (Step S 118 ). Subsequently, the EUV light generation controller  5 A may analyze the image D 27  of the droplet  27  to calculate a distance L 2  between the center D of the image D 27  and the origin o (Step S 119 ). 
         [0095]    Then, the EUV light generation controller  5 A may determine whether or not the distance L 2  falls within a permissible range ΔAL 2  (Step S 120 ). The permissible range ΔAL 2  may be set in advance or may be inputted from an external apparatus, such as the exposure apparatus controller  61 . When the distance L 2  does not fall within the permissible range ΔAL 2  (Step S 120 ; NO), the EUV light generation controller  5 A may actuate the two-axis moving mechanism  261  of the target supply unit  260  so that the center D of the image D 27  coincides with the origin o (Step S 121 ). At this point, the EUV light generation controller  5 A may also correct the timing at which the droplet  27  is outputted from the droplet generator  26 . Thereafter, the EUV light generation controller  5 A may return to Step S 116 . On the other hand, when the distance L 2  falls within the permissible range ΔAL 2  (Step S 120 ; YES), the EUV light generation controller  5 A may return to the operation shown in  FIG. 7 . 
         [0096]    Through the guide beam adjusting subroutine shown in  FIG. 8 , the focus of the laser beam focusing optical system  22  and the position of the droplet  27  may be adjusted to the origin o. 
         [0097]    With reference to  FIG. 9 , in the shooting control subroutine in Step S 102  of  FIG. 7 , the EUV light generation controller  5 A may first cause the droplet  27  to be outputted (Step S 131 ). Subsequently, the EUV light generation controller  5 A may turn on and off the guide beam output device  40  in synchronization with the planned irradiation timing with the pulse laser beam  33  (Step S 132 ). Then, the EUV light generation controller  5 A may detect the image G 41  of the guide beam  41  and the image D 27  of the droplet  27  from the image inputted by operating the image sensor  410  (Step S 133 ). Then, the EUV light generation controller  5 A may analyze the image G 41  and the image D 27 . Thus, the EUV light generation controller  5 A may calculate the distance L 1  between the center G of the image G 41  and the origin o and the distance L 2  between the center D of the image D 27  and the origin o (Step S 134 ). 
         [0098]    Subsequently, the EUV light generation controller  5  may determine whether or not the calculated distances L 1  and L 2  fall within the permissible ranges ΔL 1  and ΔAL 2 , respectively (Step S 135 ). When the distances L 1  and L 2  do not fall within the respective permissible ranges ΔL 1  and ΔAL 2  (Step S 135 ; NO), the EUV light generation controller  5 A may actuate the laser beam focusing optical system  22  so that the center G of the image G 41  coincides with the origin o (Step S 136 ). Further, the EUV light generation controller  5 A may actuate the two-axis moving mechanism  261  of the target supply unit  260  so that the center D of the image D 27  coincides with the origin o (Step S 115 ). At this point, the EUV light generation controller  5 A may also correct the timing at which the droplet  27  is outputted from the droplet generator  26  (Step S 137 ). Thereafter, the EUV light generation controller  5 A may return to Step S 131 . Note that only one of the Steps S 136  and S 137  may be carried out as necessary. 
         [0099]    On the other hand, when the distances L 1  and L 2  fall within the respective permissible ranges ΔL 1  and ΔAL 2  (Step S 135 ; YES), the EUV light generation controller  5 A may actuate the laser apparatus  3  so that the droplet  27  is irradiated with the pulse laser beam  33  (Step S 138 ). Thus, the light  251  may be generated at the desired target position. Thereafter, the EUV light generation controller  5 A may return to the operation shown in  FIG. 7 . 
         [0100]    With reference to  FIG. 10 , in the result determination subroutine in Step S 103  of  FIG. 7 , the EUV light generation controller  5 A may first operate the image sensor  410 , so that the image E 251  of the light  251  generated through the shooting control subroutine may be detected (Step S 141 ). Subsequently, the EUV light generation controller  5 A may analyze the image E 251  of the light  251  to calculate a distance L 3  (e.g., L 3 =√{square root over (x 2 +y 2 )}) between the center E of the image E 251  and the origin o (Step S 142 ). 
         [0101]    Then, the EUV light generation controller  5 A may determine whether or not the distance L 3  falls within a permissible range ΔL 3  (Step S 143 ). The permissible range ΔL 3  may be set in advance or may be inputted from an external apparatus, such as the exposure apparatus controller  61 . When the distance L 3  falls within the permissible range ΔL 3  (Step S 143 ; YES), the EUV light generation controller  5 A may make a determination that the distance L 3  is within the permissible range (Step S 144 ), and return to the operation shown in  FIG. 7 . On the other hand, when the distance L 3  does not fall within the permissible range ΔL 3  (Step S 143 ; NO), the EUV light generation controller  5 A may make a determination that the distance L 3  does not fall within the permissible range (Step S 145 ), and return to the operation shown in  FIG. 7 . 
         [0102]    With the above-described operation, the position at which the light  251  is generated may be controlled to fall within the permissible range. 
       5. Specific Examples of Mirror Unit 
     5.1 First Example 
     5.1.1 Configuration 
       [0103]      FIG. 11  schematically illustrates an example of the configuration of a mirror unit  101 A as a first example and the peripheral components thereof. As shown in  FIG. 11 , the mirror unit  101 A may include mirror blocks  110  and  120 , a lens block  118 , a focusing lens  128 , and a baffle  129 . The mirror block  110  may be provided upstream from the mirror block  120 , that is, toward the plasma generation region  25 . 
         [0104]    The lens block  118  may be provided between the mirror block  110  and the mirror block  120 . The focusing lens  128  and the baffle  129  may be fixed to the lens block  118 . The lens block  118  may be hollow in shape so that the lens block  118  does not block the guide beam  41 . The lens block  118  may be provided with a heat carrier pipe (not separately shown). A heat carrier may circulate inside the heat carrier pipe via a cooling device (not separately shown) and a pump (not separately shown) to suppress a rise in the temperature of the lens block  118  caused by the pulse laser beam and/or scattered rays of the pulse laser beam. 
         [0105]    The base material of the mirror blocks  110  and  120  may be a material with high heat-conductivity, such as copper (Cu). Further, each of the mirror blocks  110  and  120  may be coated with a material, such as molybdenum (Mo), having low reactivity with the target material. Each of the mirror blocks  110  and  120  may be provided with a heat carrier pipe (not separately shown). A heat carrier may circulate inside the heat carrier pipe via a cooling device (not separately shown) and a pump (not separately shown) to suppress a rise in temperature of the mirror blocks  110  and  120 . 
         [0106]    One of the surfaces of the mirror block  110  may be processed into a reflective surface (first reflective surface), and may serve as an off-axis paraboloidal mirror  110   a . A through-hole  110   b  may be formed in the center of the off-axis paraboloidal mirror  110   a  in a direction in which the guide beam  41  travels. A space  115 , which is in communication with the through-hole  110   b , may be defined by the lens block  118  and the mirror block  120 . The mirror unit  101 A may be positioned such that the focus of the off-axis paraboloidal mirror  110   a  substantially coincides with the plasma generation region  25 . 
         [0107]    Referring to  FIG. 11 , the light  34  reflected by the mirror block  110  may enter the sub-chamber  102  through the communication hole  116  formed in the chamber  2 A. The communication hole  116  may be covered by the window  113 . The window  113  may be formed of diamond, and coated with anti-reflective films for the wavelength of the pulse laser beam  33  on both sides thereof. The window  113  may be held by the window holder  113   a  attached to the outer wall of the chamber  2 A. Further, the window  113  may preferably be arranged so that the light  34  is not incident normally thereon. A cylindrical baffle  114  may be provided on the inner wall of the chamber  2 A so as to surround the window  113 . Thus, deposition of debris onto the window  113  may be reduced. The baffle  114  may be provided with an introduction pipe (not separately shown) connected to the etching gas supply unit  90  (see  FIG. 2 ), through which the etching gas flows. The inner diameter of the baffle  114  may preferably be larger than the diameter of the light  34  reflected by the off-axis paraboloidal mirror  110   a . The light  34  that has entered the sub-chamber  102  through the window  113  may be absorbed by the beam dump  112 . The beam dump  112  may be provided with an energy sensor for detecting the energy of the entering laser beam. A commercially available laser power meter head may be used as the beam dump  112 . Cooling water (not separately shown) may circulate in the beam dump  112 . 
         [0108]    One of the surfaces of the mirror block  120  may be processed into a reflective surface (second reflective surface)  120   a , and may be positioned to reflect the guide beam  41  at 45 degrees. The collimator  401 , the window  123 , the dichroic mirror  121 , and the focusing lens  128  may be arranged in this order in a path of the guide beam  41  from the guide beam output device  40 . A baffle  127  may preferably be provided so as to surround the window  123  and the dichroic mirror  121 . 
         [0109]    Referring to  FIG. 11 , the mirror block  120  may be positioned such that the guide beam  41  transmitted through the focusing lens  128  and reflected by the reflective surface  120   a  is focused in the plasma generation region  25 . A part of the pulse laser beam  33 , the light  251  (see  FIG. 2 ), and the EUV light that have passed through the plasma generation region  25  may be reflected by the second reflective surface  120   a  as the light  35 . The focusing lens  128  may serve to collimate the light  35 . The focusing lens  128  may be made of diamond. A cylindrical baffle  129  may be provided on the outer wall of the lens block  118  so as to surround the focusing lens  128 , so that deposition of debris onto the focusing lens  128  may be reduced. The baffle  129  may be provided with an introduction pipe (not separately shown) connected to the etching gas supply unit  90  (see  FIG. 2 ), through which the etching gas flows. 
         [0110]    The light  35  transmitted through the focusing lens  128  may be incident on the dichroic mirror  121 . The dichroic mirror  121  may be coated with a film configured to transmit the guide beam  41  with high transmittance and reflect the light  35  with high reflectance. The dichroic mirror  121  may be made of diamond. The light  35  reflected by the dichroic mirror  121  may enter the beam dump  122  through a through-hole  122   a  formed in the baffle  127 , and be absorbed by the beam dump  122 . Cooling water (not separately shown) may circulate in the beam dump  122 . 
         [0111]    Referring to  FIG. 11 , the guide beam output device  40  and the collimator  401  may be provided inside the sub-chamber  102 . The collimator  401  may collimate the guide beam  41  outputted from the guide beam output device  40 . The guide beam  41  transmitted through the collimator  401  may enter the chamber  2 A through a communication hole  117  formed in the chamber  2 A. The communication hole  117  may be covered by the window  123 . The window  123  may be formed of diamond, and coated with anti-reflective films for the wavelength sensitive to the image sensor  410  on both sides thereof. The window  123  may be held by a window holder  123   a  attached to the outer wall of the chamber  2 A. The cylindrical baffle  127  may be provided on the inner wall of the chamber  2 A so as to surround the window  123 , so that deposition of debris onto the window  123  may be reduced. The baffle  127  may be provided with an introduction pipe (now shown) connected to the etching gas supply unit  90  (see  FIG. 2 ), through which the etching gas flows. 
         [0112]    The guide beam  41  transmitted through the dichroic mirror  121  may be incident on the reflective surface  120   a  of the mirror block  120  through the focusing lens  128 . The guide beam  41  may be reflected by the reflective surface  120   a  and focused in the plasma generation region  25 . 
         [0113]    Referring to  FIG. 11 , a gas outlet of the introduction pipe  92  connected to the etching gas supply unit  90  (see  FIG. 2 ) may be arranged in the space  115  inside the mirror unit  101 A. The etching gas may be introduced into the space  115 , whereby debris deposited on the reflective surface  120   a  and on a surface of the focusing lens  128  may be removed. Alternatively, an inert gas may be introduced into the space  115  from an inert gas supply unit (not separately shown) in order to prevent dust or the like from adhering onto the optical elements. In either case, a discharge port (not separately shown) may be provided in the sub-chamber  102  to discharge the introduced gas(es). When the etching gas is introduced into the space  115 , an appropriate scrubber may be connected to the discharge port. 
       5.1.2 Operation 
       [0114]    The operation of the configuration shown in  FIG. 11  will now be described. The axis of the beam path of the guide beam  41  may coincide with the axis of the beam path of the pulse laser beam  33 . The guide beam  41  outputted from the guide beam output device  40  may be collimated through the collimator  401 . Thereafter, the guide beam  41  may be transmitted through the dichroic mirror  121  and the focusing lens  128 , and be incident on the reflective surface  120   a  of the mirror block  120 . The guide beam  41  reflected by the reflective surface  120   a  may pass through the space  115 , be focused in the plasma generation region  25 , and then enter the laser beam focusing optical system  22  (see  FIG. 2 ). 
         [0115]    Referring to  FIG. 11 , the guide beam  41  may be collimated through the laser beam focusing optical system  22 . Thereafter, the guide beam  41  may be transmitted through the imaging optical system  402  (see  FIG. 2 ). The cross-sectional image of the guide beam  41  at its focus may be transferred onto the photosensitive surface of the image sensor  410  (see  FIG. 2 ) by the imaging optical system  402 . 
         [0116]    The center portion of the pulse laser beam  33  that has passed through the plasma generation region  25  may pass through the space  115 , and be reflected by the reflective surface  120   a . The reflected pulse laser beam  33  may be incident on the dichroic mirror  121  through the focusing lens  128 , be reflected by the dichroic mirror  121  with high reflectance, and enter the beam dump  122 . 
         [0117]    The peripheral portion of the pulse laser beam  33  that has passed through the plasma generation region  25  may be reflected by the off-axis paraboloidal mirror  110   a , and enter the beam dump  112  inside the sub-chamber  102  through the window  113 . 
         [0118]    Referring to  FIG. 11 , a part of the light  251  (see  FIG. 2 ) emitted from plasma generated in the plasma generation region  25  may enter the laser beam focusing optical system  22  (see  FIG. 2 ). The light  251  may be collimated through the laser beam focusing optical system  22 . Thereafter, the light  251  may be transmitted through the imaging optical system  402  (see  FIG. 2 ). The cross-sectional image of the light  251  at its focus may be transferred onto the photosensitive surface of the image sensor  410  through the imaging optical system  402 . 
         [0119]    Referring to  FIG. 11 , the etching gas supplied into the space  115  through the introduction pipe  92  from the etching gas supply unit  90  (see  FIG. 2 ) may flow along the surfaces of the optical elements of the mirror unit  101 A to the outside of the space  115 . The surfaces of the optical elements may include the reflective surface  120   a  of the mirror block  120 , the surface of the focusing lens  128 , and so forth. Debris deposited on the surfaces of the optical elements may be etched by the etching gas. 
         [0120]    The baffles  114 ,  129 , and  127  may respectively serve to reduce the debris to be deposited on the surfaces of the window  113 , the focusing lens  128 , the dichroic mirror  121 , and the window  123 . The etching gas supply unit  90  (see  FIG. 2 ) may cause the etching gas to flow along the surfaces of the optical elements through a pipe (not separately shown). Thus, debris deposited on the surfaces of the optical elements may be etched. 
       5.1.3 Effect 
       [0121]    According to the first example, the beam path of the guide beam  41  and the beam path of the pulse laser beam  33  may be made to substantially coincide with each other. Further, the guide beam  41  and the light  251  may be detected by a single image sensor  410 . 
         [0122]    In addition, debris deposited on the surfaces of the optical elements in the mirror unit  101 A may be etched. Thus, the guide beam  41  and the light  251  may be detected stably for a relatively long time. 
         [0123]    When tin (Sn) is used as the target material, a hydrogen gas or hydrogen radicals may be used as the etching gas. The hydrogen gas or the hydrogen radicals may etch deposited Sn through the following chemical reaction: 
         [0000]      Sn(solid)+2H 2 (gas)→SnH 4 (gas)
 
         [0124]    However, when the temperature reaches or exceeds 100° C., a reverse reaction may occur, and Sn may be deposited. Thus, the temperature of each optical element (e.g., mirror unit  101 A) may preferably be controlled to fall within a range of 30° C. to 80° C., where the etching reaction rate is greater than the deposition reaction rate. The temperature of the mirror unit  101 A may, for example, be controlled by controlling at least one of the temperature and the flow rate of the heat carrier circulating in the mirror unit  101 A based on the detection value in a temperature sensor (not separately shown) attached to the mirror unit  101 A. 
       5.2 Second Example 
     5.2.1 Configuration 
       [0125]      FIG. 12  schematically illustrates an example of the configuration of a mirror unit  101 B as a second example and the peripheral components thereof. As shown in  FIG. 12 , the mirror unit  101 B may include the mirror block  110 , the lens block  118 , a dichroic mirror block  138 , and a beam dump block  133 . 
         [0126]    Referring to  FIG. 12 , the mirror block  110  and the lens block  118  may be configured similarly to those shown in  FIG. 11 . A dichroic mirror  132  may be fixed to the dichroic mirror block  138 . The space  115  may be formed inside the mirror unit  101 B. The dichroic mirror  132  may be coated with a film configured to transmit the pulse laser beam  33  and a part of the light  251  (see  FIG. 2 ) with high transmittance and reflect the guide beam  41  with high reflectance. The dichroic mirror  132  may preferably be made of diamond. 
         [0127]    Here, referring to  FIG. 12 , the focusing lens  128  fixed to the lens block  118  may be made of a material that transmits the guide beam  41 . A conical protrusion may be formed on an inner surface of the beam dump block  133  in order to absorb the pulse laser beam  33  and a part of the light  251  efficiently. The beam dump block  133  may be provided with a pipe (not separately shown), through which a heat carrier may circulate to suppress a rise in temperature of the beam dump block  133 . The introduction pipe  92  from the etching gas supply unit  90  (see  FIG. 2 ) may be connected to the mirror unit  101 B so that the etching gas flows along the surfaces of the dichroic mirror  132  and the focusing lens  128 . 
       5.2.2 Operation 
       [0128]    The operation of the configuration shown in  FIG. 12  will now be described. The axis of the beam path of the guide beam  41  may coincide with the axis of the beam path of the pulse laser beam  33 . The guide beam  41  outputted from the guide beam output device  40  may be collimated through the collimator  401 . Then, the guide beam  41  may be transmitted through the focusing lens  128 , and be incident on the dichroic mirror  132 . The guide beam  41  reflected by the dichroic mirror  132  may pass through the space  115 , be focused in the plasma generation region  25 , and then enter the laser beam focusing optical system  22  (see  FIG. 2 ). 
         [0129]    The guide beam  41  may be collimated through the laser beam focusing optical system  22 . Thereafter, the guide beam  41  may be transmitted through the imaging optical system  402 . The cross-sectional image of the guide beam  41  at its focus may be transferred onto the photosensitive surface of the image sensor  410  by the imaging optical system  402  (see  FIG. 2 ). 
         [0130]    The center portion of the pulse laser beam  33  that has passed through the plasma generation region  25  may pass through the space  115 , be transmitted through the dichroic mirror  132 , and be incident on the conical surface  133   a  of the beam dump block  133 . 
         [0131]    The peripheral portion of the pulse laser beam  33  that has passed through the plasma generation region  25  may be reflected by the off-axis paraboloidal mirror  110   a , and enter the beam dump  112  inside the sub-chamber  102  through the window  113 . 
         [0132]    Referring to  FIG. 12 , a part of the light  251  (see  FIG. 2 ) emitted from plasma generated in the plasma generation region  25  may enter the laser beam focusing optical system  22 . The light  251  may be collimated through the laser beam focusing optical system  22  (see  FIG. 2 ). Thereafter, the light  251  may be transmitted through the imaging optical system  402  (see  FIG. 2 ). The cross-sectional image of the light  251  at its focus may be transferred onto the photosensitive surface of the image sensor  410  (see  FIG. 2 ) by the imaging optical system  402 . 
         [0133]    The etching gas supplied into the space  115  through the introduction pipe  92  from the etching gas supply unit  90  may flow along the surfaces of the optical elements in the mirror unit  101 B to the outside of the space  115 . The surfaces of the optical elements may include the surface of the dichroic mirror  132 , the surface of the focusing lens  128 , and so forth. Debris deposited on the surfaces of the optical elements may be etched by the etching gas. 
         [0134]    The baffles  114 ,  129 , and  127  may respectively reduce the debris to be deposited on the surfaces of the window  113 , the focusing lens  128 , and the window  123 . The etching gas supply unit  90  may cause the etching gas to flow along the surfaces of the optical elements through a pipe (not separately shown). Thus, debris deposited on the surfaces of the optical elements may be etched. 
       5.2.3 Effect 
       [0135]    According to the second example, referring to  FIG. 12 , the dichroic mirror  132  and the beam dump block  133  may be provided in the mirror unit  101 B. Accordingly, the amount of the pulse laser beam  33  and the light  251  (see  FIG. 2 ) incident on the focusing lens  128  may be reduced. As a result, the focusing lens  128  need not have durability against the high power pulse laser beam  33 , and need not be formed of diamond, which is relatively expensive. 
       5.3 Third Example 
     5.3.1 Configuration 
       [0136]      FIG. 13  schematically illustrates an example of the configuration of a mirror unit  101 C as a third example and the peripheral components thereof. As shown in  FIG. 13 , the mirror unit  101 C may include the mirror block  110 , the lens block  118 , the dichroic mirror block  138 , and a guide beam output device housing  143 . 
         [0137]    The mirror block  110  and the lens block  118  may be configured similarly to those shown in  FIG. 11 . The dichroic mirror  132  may be fixed to the dichroic mirror block  138 . The space  115  may be formed inside the mirror unit  101 C. The dichroic mirror  132  may be coated with a film configured to reflect the pulse laser beam  33  and a part of the light  251  (see  FIG. 2 ) with high reflectance and transmit the guide beam  41  with high transmittance. The dichroic mirror  132  may preferably be made of diamond. 
         [0138]    Referring to  FIG. 13 , the guide beam output device  40 , a collimator  401   a , and a focusing lens  401   b  may be housed in the guide beam output device housing  143 . The guide beam  41  outputted from the guide beam output device  40  may be incident on the collimator  401   a , and be collimated through the collimator  401   a . Thereafter, the guide beam  41  may be incident on the focusing lens  401   b , and be focused in the plasma generation region  25  by the focusing lens  401   b  through the dichroic mirror  132 . Then, the guide beam  41  may enter the laser beam focusing optical system  22  (see  FIG. 2 ). 
         [0139]    Here, the lens block  118  to which the focusing lens  128  is fixed may be provided with a pipe (now shown), through which a heat carrier circulates to suppress a rise in temperature of the lens block  118  caused by the energy of the laser beam passing through the focusing lens  128 . The introduction pipe  92  from the etching gas supply unit  90  (see  FIG. 2 ) may be connected to the mirror unit  101 C so that the etching gas flows along the surfaces of the dichroic mirror  132  and the focusing lens  128 . 
         [0140]    The beam dump  142  may be provided inside the sub-chamber  102 . The pulse laser beam  33  and the light  251  (see  FIG. 2 ) reflected by the dichroic mirror  132  (collectively, the light  35 ) may enter the beam dump  142  through the window  123 . 
       5.3.2 Operation 
       [0141]    The operation of the configuration shown in  FIG. 13  will now be described. The axis of the beam path of the guide beam  41  may coincide with the axis of the beam path of the pulse laser beam  33 . The guide beam  41  outputted from the guide beam output device  40  may be transmitted through the collimator  401   a  and the focusing lens  401   b . Then, the guide beam  41  may be transmitted through the dichroic mirror  132 , pass through the space  115 , and be focused in the plasma generation region  25 . Thereafter, the guide beam  41  may enter the laser beam focusing optical system  22  (see  FIG. 2 ). 
         [0142]    Referring to  FIG. 13 , the guide beam  41  may be collimated through the laser beam focusing optical system  22  (see  FIG. 2 ). Thereafter, the guide beam  41  may be transmitted through the imaging optical system  402  (see  FIG. 2 ). The cross-sectional image of the guide beam  41  at its focus may be transferred onto the photosensitive surface of the image sensor  410  (see  FIG. 2 ) by the imaging optical system  402 . 
         [0143]    The center portion of the pulse laser beam  33  that has passed through the plasma generation region  25  may pass through the space  115 , and be reflected by the dichroic mirror  132 . The reflected pulse laser beam  33  may be transmitted through the window  123 , and enter the beam dump  142  inside the sub-chamber  102 . 
         [0144]    Referring to  FIG. 13 , the peripheral portion of the pulse laser beam  33  that has passed through the plasma generation region  25  may be reflected by the off-axis paraboloidal mirror  110   a , and enter the beam dump  112  inside the sub-chamber  102  through the window  113 . 
         [0145]    Referring to  FIG. 13 , a part of the light  251  (see  FIG. 2 ) emitted from plasma generated in the plasma generation region  25  may enter the laser beam focusing optical system  22  (see  FIG. 2 ). The light  251  may be collimated through the laser beam focusing optical system  22 . Thereafter, the light  251  may be transmitted through the imaging optical system  402  (see  FIG. 2 ). The cross-sectional image of the light  251  at its focus may be transferred onto the photosensitive surface of the image sensor  410  (see  FIG. 2 ) by the imaging optical system  402 . 
         [0146]    The etching gas supplied into the space  115  through the introduction pipe  92  from the etching gas supply unit  90  (see  FIG. 2 ) may flow along the surfaces of the optical elements in the mirror unit  101 C to the outside of the space  115 . The surfaces of the optical elements may include the surface of the dichroic mirror  132 , the surface of the focusing lens  128 , and so forth. Debris deposited on the surfaces of the optical elements may be etched by the etching gas. 
         [0147]    Referring to  FIG. 13 , the baffles  114 ,  129 , and  127  may respectively reduce the debris to be deposited on the surfaces of the window  113 , the focusing lens  128 , and the window  123 . The etching gas supply unit  90  (see  FIG. 2 ) may cause the etching gas to flow along the surfaces of the optical elements through a pipe (not separately shown). Thus, debris deposited on the surfaces of the optical elements may be etched. 
       5.3.3 Effect 
       [0148]    According to the third example, the beam dumps  112  and  142 , which are subjected to a large heat load as they absorb the pulse laser beam  33 , may be provided inside the sub-chamber  102 . Thus, the beam dumps  112  and  142 , which may emit radiation heat, may be kept away from the mirror unit  101 C. As a result, thermal deformation in the mirror unit  101 C may be suppressed, and the beam path of the guide beam  41  may be stabilized. 
       6. Modification of Guide Beam Optical System 
     6.1 Configuration 
       [0149]      FIG. 14  schematically illustrates an example of the configuration of an optical system relating to the guide beam  41  in a modification of the EUV light generation system  11 A (see  FIG. 2 ). In  FIG. 14 , only an example of the primary optical systems is illustrated, and omitted elements may be similar to those of the above-described configuration. 
         [0150]    As shown in  FIG. 14 , in the modification, a pinhole plate  411  and a collimator  412  in a mirror unit  100  may be provided in place of the collimator  401  in  FIG. 2 . The pinhole plate  411  may be provided at the focus of the collimator  412 . The pinhole plate  411  may be configured such that the pinhole formed therein is smaller in diameter than the guide beam  41  from the guide beam output device  40 . Alternatively, the diameter of the pinhole may be set approximately to the spot size of the pulse laser beam  33  in the plasma generation region  25 . 
       6.2 Operation 
       [0151]    With reference to  FIG. 14 , the guide beam  41  outputted from the guide beam output device  40  may be incident on the pinhole plate  411 . The guide beam  41  that has passed through the pinhole in the pinhole plate  411  may be diverged and be incident on the collimator  412 . Thus, the guide beam  41  may be collimated by the collimator  412 . 
         [0152]    The collimated guide beam  41  may be transmitted through window  123  and the dichroic mirror  121 , and enter the mirror unit  101 . The mirror unit  101  may reflect the guide beam  41  toward the plasma generation region  25 . Thus, the axis of the beam path of the guide beam  41  may substantially coincide with the axis of the beam path of the pulse laser beam  33 . 
         [0153]    The guide beam  41  reflected by the mirror unit  101  may be focused in the plasma generation region  25 . At this time, the image of the guide beam  41  at the pinhole in the pinhole plate  411  may be imaged as a shadow of the guide beam  41  at the focus of the laser beam focusing optical system  22  in the plasma generation region  25 . For example, the focal distance of the collimator  412  for the wavelength of the guide beam  41  may be adjusted to match the focal distance of the focusing lens  128 . Thus, the image of the guide beam  41  at the pinhole may be transferred with the same magnification in the plasma generation region  25 . 
         [0154]    Referring to  FIG. 14 , the guide beam  41  that has passed through the plasma generation region  25  may enter the image sensor  410  provided at the focus of the imaging optical system  402  through the laser beam focusing optical system  22 , the beam adjusting unit  350  (see  FIG. 2 ), and the imaging optical system  402 . The image of the guide beam  41  at the pinhole may be imaged as the shadow of the guide beam  41  on the photosensitive surface of the image sensor  410 . The image data of the guide beam  41  at the pinhole may be sent to the EUV light generation position controller  51  (see  FIG. 2 ). 
         [0155]      FIG. 15  shows an image P 411  of the guide beam  41  at the pinhole in the pinhole plate  411  imaged on the image sensor  410  in  FIG. 14  and the image D 27  of the droplet  27 . In  FIG. 15 , a cross-sectional image B 33  of the pulse laser beam  33  at its focus in the plasma generation region  25  is shown as well. 
         [0156]    As shown in  FIG. 15 , in the modification, the image P 411  of the guide beam  41  may be substantially the same in size as the cross-sectional image B 33  of the pulse laser beam  33 . Referring to  FIG. 14 , the guide beam  41  may travel along substantially the same path as the pulse laser beam  33  from the mirror unit  101  to the dichroic mirror  351 . Accordingly, the image P 411  of the guide beam  41  may reflect the focus position and the beam diameter of the pulse laser beam  33 . Further, as in the first embodiment, the image sensor  410  may detect the image E 251  of the light  251 , and calculate the center E of the image E 251 . Accordingly, the EUV light generation position controller  51  may control the focus of the pulse laser beam  33  and the position to which the droplet  27  is supplied such that the center P of the image P 411  coincides with the center E of the image E 251 . At this time, the focus of the pulse laser beam  33  and the position to which the droplet  27  is supplied may be controlled so that the centers of the respective images coincide with a predetermined target position (e.g., the origin o). Here, in place of the centers of the respective images, the centroids of the respective images may be obtained. 
       6.3 Effect 
       [0157]    According to the modification of  FIGS. 14 and 15 , the beam path of the guide beam  41  and the beam path of the pulse laser beam  33  may be made to substantially coincide with each other. Further, the image of the guide beam  41  at the pinhole may be transferred in the plasma generation region  25 . Accordingly, without actually outputting the pulse laser beam  33 , the center and the beam diameter of the pulse laser beam  33  may be detected based on the detection result of the image P 411  of the guide beam  41 . 
       7. EUV Light Generation System Including Pre-Pulse Laser Apparatus and Main Pulse Laser Apparatus: Second Embodiment 
     7.1 Configuration 
       [0158]      FIG. 16  schematically illustrates an example of the configuration of an EUV light generation system  11 B according to a second embodiment. As shown in  FIG. 16 , the EUV light generation system  11 B may be similar in configuration to the EUV light generation system  11 A shown in  FIGS. 2 and 14 , but may further include a pre-pulse laser apparatus  150  and a high-reflection mirror  360 . The high-reflection mirror  341  in the beam delivery unit  340  may be replaced by a dichroic mirror  342 . Hereinafter, the laser apparatus  3  may be referred to as a main pulse laser apparatus  3 , and the pulse laser beams  31  through  33  may be referred to as main pulse laser beam  31  through  33 . 
         [0159]    The pre-pulse laser apparatus  150  may, for example, be a YAG laser. A pre-pulse laser beam  151  outputted from the pre-pulse laser apparatus  150  may be reflected by the high-reflection mirror  360 . The reflected pre-pulse laser beam  151  may be incident on the dichroic mirror  342  of the beam delivery unit  340 . The dichroic mirror  342  may be coated with a film configured to transmit the pre-pulse laser beam  151  with high transmittance and reflect the main pulse laser beam  31  with high reflectance. The pre-pulse laser beam  151  incident on the dichroic mirror  342  may be transmitted therethrough. Thus, the axis of the beam path of the pre-pulse laser beam  151  and the axis of the beam path of the main pulse laser beam  32  may be made to substantially coincide with each other. Alternatively, the configuration may be such that the axis of the beam path of the pre-pulse laser beam  152  and the axis of the beam path of the main pulse laser beam  32  are set to be in a predetermined positional relationship. 
         [0160]    In the EUV light generation system  11 B, the guide beam output device  40  inside the sub-chamber  102  may be provided with a three-axis moving mechanism  420 . The three-axis moving mechanism  420  may be configured to move the guide beam output device  40  under the control of the EUV light generation position controller  51 . At this time, the three-axis moving mechanism  420  may move the pinhole plate  411  along with the guide beam output device  40 . 
       7.2 Operation 
       [0161]    The operation of the EUV light generation system  11 B shown in  FIG. 16  will now be described. Hereinafter, the operation of the EUV light generation system  11 B will be described using the image P 411  (see  FIG. 15 ) of the guide beam  41  at the pinhole in the pinhole plate  411  imaged on the photosensitive surface of the image sensor  410 , the image D 27  (see  FIG. 15 ) of the droplet  27  or an image F 27  of the diffused target, the estimated cross-sectional image B 33  (see  FIG. 15 ) of the main pulse laser beam  33  at its focus, and the image E 251  (see  FIG. 15 ) of the light  251  containing the EUV light. 
         [0162]    In the description to follow, the focus of a pre-pulse laser beam  153  and the focus of the main pulse laser beam  33  may substantially coincide with each other, or may be set to be in a predetermined positional relationship. Further, the pinhole in the pinhole plate  411  may be configured so that the diameter of the image P 411  (see  FIG. 15 ) of the guide beam  41  is substantially the same in size as the spot size of the main pulse laser beam  33 . 
         [0163]      FIG. 17  shows an example of an image to be detected by the image sensor  410  in a state where the pre-pulse laser beam  151  and the main pulse laser beam  31  are not outputted.  FIG. 17  shows an example where the position of the center P of the image P 411  and the position of the center D of the image D 27  do not coincide with the origin o. In the example shown in  FIG. 17 , the center P of the image P 411  doe not coincide with the origin o. Similarly, the center D of the image D 27  does not coincide with the origin o. The position of the center P and the position of the center D may, for example, be obtained through various methods, such as by calculating the centers from the beam intensity distribution in the images acquired by the image sensor  410 . Alternatively, the centroids may be used in place of the centers. 
         [0164]      FIG. 18  shows an example of an image to be detected by the image sensor  410  after the laser beam focusing optical system  22  is adjusted from the state shown in  FIG. 17 . As shown in  FIG. 18 , after the laser beam focusing optical system  22  is adjusted, the center P of the image P 411  may substantially coincide with the origin o. 
         [0165]      FIG. 19  shows an example of an image to be detected by the image sensor  410  after the target supply unit  260  is adjusted from the state shown in  FIG. 18 . As shown in  FIG. 19 , after the target supply unit  260  is adjusted, the center D of the image D 27  may substantially coincide with the origin o. 
         [0166]      FIG. 20  shows an example of an image to be detected by the image sensor  410  when the droplet  27  is irradiated with the pre-pulse laser beam  153  in the state shown in  FIG. 19 . As shown in  FIG. 19 , after the laser beam focusing optical system  22  and the target supply unit  260  are adjusted, both the center P of the image P 411  and the center D of the image D 27  may substantially coincide with the origin o. Since the focus of the pre-pulse laser beam  153  substantially coincides with the focus of the main pulse laser beam  33 , the image F 27  of the diffused target to be generated when the droplet  27  is irradiated with the pre-pulse laser beam  153  under the state shown in  FIG. 19  may be in the visual field of the image sensor  410 . Further, the center F of the image F 27  may be at or around the origin o. 
         [0167]      FIG. 21  shows an example of an image to be detected by the image sensor  410  when the diffused target is irradiated with the main pulse laser beam  33  after a predetermined time has elapsed since the state shown in  FIG. 20 . As shown in  FIG. 21 , the center E of the image E 251  may be at or around the origin o. That is, although the diffused target generated around the origin o may move slightly while the predetermined time elapses, the diffused target may still be irradiated with the main pulse laser beam  33  within the visual field of the image sensor  410 . As a result, the light  251  may be generated around the origin o. 
       7.3 Effect 
       [0168]    According to the second embodiment, the guide beam  41  may be focused in the position at which the droplet  27  is to be irradiated with the pre-pulse laser beam  153 . Further, the image of the guide beam  41  at this position may be detected through the laser beam focusing optical system  22  and the imaging optical system  402 . As a result, the position at which the pre-pulse laser beam  153  is focused, the position of the droplet  27 , and the position at which the light  251  is generated may be detected simultaneously. Then, the focus of the laser beam focusing optical system  22  and the position of the droplet  27  may be controlled based on the detection result, whereby the focus of the laser beam focusing optical system  22  and the position of the droplet  27  may be controlled with high precision to the desired target position. Thus, the droplet  27  may be irradiated with the pre-pulse laser beam  153  stably. As a result, the diffused target may be generated at or around the desired target position with high precision. 
         [0169]    Further, the position at which the diffused target is generated may be detected using the guide beam  41 . Accordingly, the positional relationship between the diffused target and the main pulse laser beam  33  may be estimated before the diffused target is irradiated with the main pulse laser beam  33 . Then, the diffused target may be irradiated with the main pulse laser beam  33  within the visual field of the image sensor  410 . 
       7.4 Flowchart 
       [0170]    The operation of an EUV light generation controller  5 B of the second embodiment will now be described in detail with reference to the drawings.  FIG. 22  is a flowchart showing an overall operation of the EUV light generation controller  5 B of the second embodiment.  FIG. 23  is a flowchart showing an example of a target position setting subroutine of  FIG. 22 .  FIG. 24  is a flowchart showing an example of a shooting control subroutine of  FIG. 22 . 
         [0171]    The operation shown in  FIG. 22  may be carried out when the EUV light generation controller  5 B receives an instruction for a burst operation from an external apparatus, such as the exposure apparatus controller  61 , or when the EUV light generation controller  5 B is started. 
         [0172]    As shown in  FIG. 22 , the EUV light generation controller  5 B may receive an instruction regarding the generation position of the light  251  from the exposure apparatus controller  61 , and carry out the target position setting subroutine for setting the received generation position to the target plasma generation position (Step S 201 ). Then, the EUV light generation controller  5 B may carry out the guide beam adjusting subroutine (Step S 101 ). The guide beam adjusting subroutine may be similar to that shown in  FIG. 8 . 
         [0173]    Then, the EUV light generation controller  5 B may carry out the shooting control subroutine to generate the light  251  (Step S 202 ). Subsequently, the EUV light generation controller  5 B may carry out the result determination subroutine to determine whether or not the generation result of the light  251  through the shooting control subroutine in Step S 202  falls within a permissible range (Step S 103 ). The result determination subroutine may be similar to that shown in  FIG. 10 . 
         [0174]    When the generation result of the light  251  is determined not to fall within the permissible range based on the result of Step S 103  (Step S 104 ; NO), the EUV light generation controller  5 B may return to Step S 101  and repeat the subsequent steps. When the generation result of the light  251  is determined to fall within the permissible range based on the result of Step S 103  (Step S 104 ; YES), the EUV light generation controller  5 B may determine whether or not to stop the shooting control (Step S 105 ). When shooting control is to be stopped (Step S 105 ; YES), the EUV light generation controller  5 B may terminate the operation shown in  FIG. 22 . On the other hand, when the shooting control is not to be stopped (Step S 105 ; NO), the EUV light generation controller  5 B may return to Step S 202  and repeat the subsequent steps. 
         [0175]    With reference to  FIG. 23 , in the target position setting subroutine shown in Step S 201  of  FIG. 22 , the EUV light generation controller  5 B may first receive a relative amount of change in the target position at which the light  251  is to be generated from the current position (or the initial position) from the exposure apparatus controller  61  (Step S 211 ). 
         [0176]    Then, the EUV light generation controller  5 B may actuate the three-axis moving mechanism  420  to control the position of the guide beam output device  40  so that the center P of the image P 411  of the guide beam  41  changes by the amount corresponding to the aforementioned relative amount of change. At this time, the position of the pinhole plate  411  may be controlled along with the position of the guide beam output device  40  (Step S 212 ). Thereafter, the EUV light generation controller  5 B may return to the operation shown in  FIG. 22 . 
         [0177]    With reference to  FIG. 24 , in the shooting control subroutine in Step S 202  of  FIG. 22 , the EUV light generation controller  5 B may first cause the droplets  27  to be outputted (Step S 221 ). Subsequently, the EUV light generation controller  5 B may turn on and off the guide beam output device  40  in synchronization with the planned irradiation timing with the pre-pulse laser beam  153  (Step S 222 ). Then, the EUV light generation controller  5 B may operate the image sensor  410  to detect the image P 411  of the guide beam  41  and the image D 27  of the droplet  27  (Step S 223 ). Then, the EUV light generation controller  5 B may analyze the image P 411  and the image D 27 . Thus, the EUV light generation controller  5 B may calculate a distance L 4  between the center P of the image P 411  and the origin o and the distance L 2  between the center D of the image D 27  and the origin o (Step S 224 ). 
         [0178]    Subsequently, the EUV light generation controller  5 B may determine whether or not the calculated distances L 4  and L 2  fall within the permissible ranges ΔL 4  and ΔAL 2 , respectively (Step S 225 ). The permissible ranges ΔL 4  and ΔAL 2  may be set in advance or inputted from an external apparatus, such as the exposure apparatus controller  61 . When the distances L 4  and L 2  do not fall within the respective permissible ranges ΔL 4  and ΔAL 2  (Step S 225 ; NO), the EUV light generation controller  5 B may actuate the laser beam focusing optical system  22  so that the center P of the image P 411  coincides with the origin o (Step S 226 ). Further, the EUV light generation controller  5 B may actuate the two-axis moving mechanism  261  of the target supply unit  260  so that the center D of the image D 27  coincides with the origin o. Here, the EUV light generation controller  5 B may also correct the timing at which the droplet  27  is outputted from the droplet generator  26  (Step S 227 ). Thereafter, the EUV light generation controller  5 B may return to Step S 221 . Note that only one of the Steps S 226  and S 227  may be carried out as necessary. 
         [0179]    On the other hand, when the distances L 4  and L 2  fall within the respective permissible ranges ΔL 4  and ΔAL 2  (Step S 225 ; YES), the EUV light generation controller  5 B may actuate the pre-pulse laser apparatus  150  so that the droplet  27  is irradiated with the pre-pulse laser beam  153  (Step S 228 ). Thus, the droplet  27  may be turned into the diffused target at the desired target position. 
         [0180]    Subsequently, the EUV light generation controller  5 B may turn on and off the guide beam output device  40  in synchronization with the planned irradiation timing with the main pulse laser beam  33  (Step S 229 ). Then, the EUV light generation controller  5 B may operate the image sensor  410  to detect the image P 411  of the guide beam  41  and the image F 27  of the diffused target (Step S 230 ). Thereafter, the EUV light generation controller  5 B may analyze the image P 411  and the image F 27 . Thus, the EUV light generation controller  5 B may determine whether or not the image F 27  is contained in the image P 411  (Step S 231 ). The state where the image F 27  is contained in the image P 411  means that the image F 27  is detected within the image P 411 , as shown in  FIG. 20 . 
         [0181]    Based on the determination result in Step S 231 , when the image F 27  is not contained in the image P 411  (Step S 231 ; NO), the EUV light generation controller  5 B may return to Step S 221  and repeat the subsequent steps. On the other hand, when the image F 27  is contained in the image P 411  (Step S 231 ; YES), the EUV light generation controller  5 B may actuate the main pulse laser apparatus  3  and cause the diffused target to be irradiated with the main pulse laser beam  33  (Step S 232 ). Thus, the light  251  may be generated. Thereafter, the EUV light generation controller  5 B may return to the operation shown in  FIG. 22 . 
         [0182]    With the above-described operation, the position at which the light  251  is generated may be kept within the permissible range. 
         [0183]    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 specifications 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 ones of the embodiments can be applied to other embodiments as well (including the other embodiments described herein). 
         [0184]    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.”