Abstract:
An apparatus for generating extreme ultraviolet light is used with a first laser device for outputting a first laser beam. The apparatus includes a second laser device for outputting a second laser beam, a beam adjusting unit for causing beam axes of the first and second laser beams to substantially coincide with each other, a chamber, a target supply unit for supplying target materials into the chamber, a laser beam focusing optical system for focusing the first laser beam on the target material for plasma generation, an optical detection system for detecting the second laser beam and light from plasma, a focus position correction mechanism for correcting a first laser beam focusing position, and a target supply position correction mechanism for correcting a target material supplying position, and a controller for the focus position correction mechanism and the target supply position correction mechanism based on the optical detection system&#39;s detection.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from Japanese Patent Application No. 2011-124531 filed Jun. 2, 2011, and Japanese Patent Application No. 2012-095735 filed Apr. 19, 2012. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    This disclosure relates to an apparatus and a method for generating extreme ultraviolet (EUV) light. 
         [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]    An apparatus according to one aspect of this disclosure for generating extreme ultraviolet light used with a first laser device configured to output a first laser beam may include: a second laser device configured to output a second laser beam; a beam adjusting unit configured to cause a beam axis of the first laser beam and a beam axis of the second laser beam to substantially coincide with each other; a chamber having a window through which the first and second laser beams are introduced into the chamber; a target supply unit configured to supply a target material to a predetermined region inside the chamber; a laser beam focusing optical system for focusing the first laser beam on the target material inside the chamber; an optical detection system for detecting the second laser beam and light emitted from plasma generated when the target material is irradiated with the first laser beam; a focus position correction mechanism configured to correct a position at which the first laser beam is focused by the laser beam focusing optical system; a target supply position correction mechanism configured to correct a position to which the target material is supplied; and a controller configured to control the focus position correction mechanism and the target supply position correction mechanism based on the detection result of the second laser beam and the light emitted from the plasma. 
         [0008]    A method according to another aspect of this disclosure for generating extreme ultraviolet light in an apparatus that is used with a first laser device configured to output a first laser beam and includes a second laser device configured to output a second laser beam, a beam adjusting unit, a chamber, a target supply unit, a laser beam focusing optical system, an optical detection system, and a controller may include: detecting the second laser beam; detecting light emitted from plasma generated when a target material is irradiated with the first laser beam; controlling a position at which the first laser beam is focused by the laser beam focusing optical system based on the detection result of the second laser beam; and controlling a position to which the target material is supplied by the target supply unit based on the detection result of the light emitted from the plasma. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Hereinafter, selected embodiments of this disclosure will be described with reference to the accompanying drawings. 
           [0010]      FIG. 1  schematically illustrates the configuration of an exemplary LPP type EUV light generation system. 
           [0011]      FIG. 2  schematically illustrates the configuration of an EUV light generation system according to an embodiment of this disclosure. 
           [0012]      FIG. 3  shows an image detected by an optical sensor when the center of a pulse laser beam coincides with the center of a target being irradiated with the pulse laser beam. 
           [0013]      FIG. 4  shows an image detected by an optical sensor when the center of a pulse laser beam does not coincide with the center of a target being irradiated with the pulse laser beam. 
           [0014]      FIG. 5  shows the relationship among the center of an image of a focused guide laser beam obtained through calculation, the center of an image of plasma-emitted light obtained through calculation, and an estimated image of the focused pulse laser beam in the state shown in  FIG. 4 . 
           [0015]      FIG. 6  schematically illustrates the configuration of a optical detection system according to a first example. 
           [0016]      FIG. 7  schematically illustrates the configuration of a optical detection system according to a second example. 
           [0017]      FIG. 8  schematically illustrates the configuration of a optical detection system according to a third example. 
           [0018]      FIG. 9  schematically illustrates the configuration of an optical system in a modification of the EUV light generation system of the embodiment of this disclosure. 
           [0019]      FIG. 10  shows the relationship among an image of a guide laser beam at a pinhole, an image of plasma-emitted light, and an image of a pulse laser beam, which are imaged on the optical sensor shown in  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    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 Detection System for Guide Laser Beam and Plasma-Emitted Light 
     4.1 Configuration 
     4.2 Operation 
     4.3 Effect 
     4.4 Examples of Optical Detection System 
     4.4.1 First Example 
     4.4.2 Second Example 
     4.4.3 Third Example 
     5. Variation 
     5.1 Configuration 
     5.2 Operation 
     5.3 Effect 
     1. OVERVIEW 
       [0021]    According to some of the embodiments of this disclosure, a guide laser beam and light emitted from plasma may be detected in an LPP type EUV light generation system, and based on the detection result, the position to which a target material is supplied and the position at which a laser beam for striking the target material is focused may be controlled. 
       2. TERMS 
       [0022]    Terms used in this application may be interpreted as follows. The term “beam path” may refer to a path along which a laser beam travels. The term “beam cross-section” may refer to a region along a plane perpendicular to the travel direction of a laser beam, in which the beam intensity is equal to or higher than a predetermined value. The term “beam axis” may refer to an axis of a laser beam which passes through substantially the center of the beam cross-section. In a beam path of a laser beam, a direction or side closer to the laser device may be referred to as “upstream,” and a direction into which the laser beam travel may be referred to as “downstream.” 
         [0023]    The term “plasma generation region” may refer to a three-dimensional space predefined as a space in which plasma is to be generated. 
         [0024]    The term “obscuration region” may refer to a three-dimensional region that is a shadow of EUV light. Typically, the EUV light that passes through the obscuration region is not used for exposure in an exposure apparatus. 
         [0025]    The term “droplet” may refer to a liquid droplet of a molten target material. Accordingly, the shape thereof may be substantially spherical due to its surface tension. 
       3. OVERVIEW OF EUV LIGHT GENERATION SYSTEM 
     3.1 Configuration 
       [0026]      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 device  3 . Hereinafter, a system that includes the EUV light generation apparatus  1  and the laser device  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 , a target supply unit  26 , and so forth. The chamber  2  may be airtightly sealed. The target supply unit  26  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  26  may include, but is not limited to, tin, terbium, gadolinium, lithium, xenon, or any combination thereof. 
         [0027]    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 being 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 . The beam cross-section of the pulse laser beam  33  may be substantially circular. 
         [0028]    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 target  27 . 
         [0029]    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  293  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  293  formed in the wall  291 . 
         [0030]    The EUV light generation system  11  may also include a laser beam direction control unit  340 , a laser beam focusing mirror  22 , and a target collector  28  for collecting targets  27 . The laser beam direction control unit  340  may include an optical element for defining the direction into which the pulse laser beam  32  travels and an actuator for adjusting the position and the orientation (posture) of the optical element. 
       3.2 Operation 
       [0031]    With continued reference to  FIG. 1 , a pulse laser beam  31  outputted from the laser device  3  may pass through the laser beam direction control 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 device  3 , be reflected by the laser beam focusing mirror  22 , and strike at least one target  27  as a pulse laser beam  33 . 
         [0032]    The target supply unit  26  may be configured to output the target(s)  27  in the form of droplets toward the plasma generation region  25  inside the chamber  2 . The target  27  may be irradiated by at least one pulse of the pulse laser beam  33 . Upon being irradiated with the pulse laser beam  33 , the target  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 target  27  may be irradiated by multiple pulses included in the pulse laser beam  33 . 
         [0033]    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 target  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 target  27  is outputted and the direction into which the target  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 device  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 appreciated that the various controls mentioned above are merely examples, and other controls may be added as necessary. 
       4. EUV LIGHT GENERATION SYSTEM INCLUDING DETECTION SYSTEM FOR GUIDE LASER BEAM AND PLASMA-EMITTED LIGHT 
     4.1 Configuration 
       [0034]    An EUV light generation system according to an embodiment will now be described in detail with reference to the drawings.  FIG. 2  schematically illustrates the configuration of an EUV light generation system  11 A. As shown in  FIG. 2 , the EUV light generation system  11 A may include an EUV light generation apparatus  1 A and the laser device  3 . 
         [0035]    The EUV light generation apparatus  1 A may include a beam delivery unit  340 , a beam adjusting unit  350 , and a chamber  2 A. Further, the EUV light generation apparatus  1 A may include a guide laser device  40  and a beam expander  401 . The EUV light generation apparatus  1 A may further include an EUV light generation controller  5 A. 
         [0036]    The laser device  3  may be configured to output the pulse laser beam  31  at a predetermined repetition rate. When the laser device  3 , for example, includes a CO 2  gas as a gain medium, the wavelength of the pulse laser beam  31  may be around 10.6 μm. The beam delivery unit  340  may include a high-reflection mirror  341  for defining the 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 shown) for adjusting the position and the orientation of the high-reflection mirror  341 . The beam delivery unit  340  may be configured to cause the pulse laser beam  32  to be introduced into a predetermined beam path. 
         [0037]    The guide laser device  40  may be configured to output a guide laser beam  41 . The guide laser device  40  may be a semiconductor laser. However, this disclosure is not limited thereto, and a light source aside from a laser, such as an incoherent light source (e.g., light emitting diode (LED)), may also be used as the guide laser device  40 . The guide laser beam  41  may be a pulsed beam or a continuous wave beam. When the guide laser beam  41  is a pulsed beam, the EUV light generation controller  5 A may synchronize the timing at which the target  27  is outputted from the target supply unit  260  and the timing of the guide laser beam  41 . In the description to follow, the guide laser beam  41  is assumed to be a continuous wave beam. The wavelength of the guide laser beam  41  may be shorter than the wavelength of the pulse laser beam  31 . The guide laser beam  41  may, for example, be visible radiation. The wavelength of the guide laser beam  41  may, for example, be around 500 nm. The guide laser beam  41  may preferably be at a wavelength suitable for photosensitivity of the optical sensor  125 , which will be described in detail later. The beam expander  401  may be provided in a beam path of the guide laser beam  41 . 
         [0038]    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 transmit a guide laser beam  42  with high transmittance. The dichroic mirror  351  may be coated on a second surface thereof with a film configured to transmit the guide laser beam  42  with high transmittance. The dichroic mirror  351  may be positioned such that the pulse laser beam  32  is incident on the first surface thereof and the guide laser beam  42  is incident on the second surface thereof. The substrate of the dichroic mirror  351  may, for example, include diamond. The beam adjusting unit  350  may be provided such that the pulse laser beam  32  reflected thereby and the guide laser beam  42  transmitted therethrough are guided toward the chamber  2 A along substantially the same beam path. This may also be applicable even when an incoherent light source is used as the guide laser device  40 . 
         [0039]    The chamber  2 A may include the window  21 , a laser beam focusing optical system  70 , a target supply unit  260 , the target sensor  4 , the EUV collector mirror  23 , and the connection part  29 . The window  21  may be coated with a film configured to reduce reflectance of the laser beams incident thereon. Further, the chamber  2 A may include an optical detection system  100 , an etching gas supply unit  90 , a manometer  93 , and a ventilation unit  94 . 
         [0040]    The laser beam focusing optical system  70  may include the laser beam focusing mirror  22  and a high-reflection mirror  72 . The laser beam focusing optical system  70  may be provided with a focus position correction mechanism. The focus position correction mechanism may include a plate  71 , a plate moving mechanism  71   a , a mirror holder  22   a , and a holder  72   a  provided with an automatic tilt mechanism. The laser beam focusing mirror  22  may be an off-axis paraboloidal mirror. The laser beam focusing mirror  22  may be mounted to the plate  71  through the mirror holder  22   a . The high-reflection mirror  72  may be mounted to the plate  71  through the holder  72   a . The plate moving mechanism  71   a  may be configured to move the laser beam focusing mirror  22  and the high-reflection mirror  72  along with the plate  71 . The laser beam focusing mirror  22  and the high-reflection mirror  72  may be positioned such that the laser beams  32  and  42  are first incident on the laser beam focusing mirror  22  and then on the high-reflection mirror  72  and such that the laser beams  33  and  43  reflected by the high-reflection mirror  72  are focused in the plasma generation region  25 . 
         [0041]    The plate moving mechanism  71   a  may be configured to move the plate  71  to thereby adjust the focus of the laser beams  33  and  43  in the Z-direction. The holder  72   a  may be configured to adjust the tilt angle of the high-reflection mirror  72  to thereby adjust the focus of the laser beams  33  and  43  along the XY-plane. The aforementioned adjustments may be controlled by the EUV light generation controller  5 A. The details of the control will be given later. 
         [0042]    The target supply unit  260  may include a target generator  26 . The target generator  26  may be provided with a two-axis moving mechanism  261 . The target generator  26  may be configured to output targets  27  in the form of droplets toward the plasma generation region  25 . The two-axis moving mechanism  261  may be configured to move the target generator  26  to thereby adjust the position to which the targets  27  are supplied from the target generator  26 . The two-axis moving mechanism  261  may be configured to move the target generator  26  in accordance with the control by the EUV light generation controller  5 A. 
         [0043]    The optical detection system  100  may include a mirror unit  101 , a beam dump  112 , a dichroic mirror  121 , a beam dump  122 , an imaging optical system  124 , and an optical sensor  125 . The mirror unit  101  may be supported by a mirror holder  101   a . The mirror unit  101  may be provided in the obscuration region. The details of the internal structure of the mirror unit  101  will be given later. The beam dump  112 , the imaging optical system  124 , and the optical sensor  125  may be housed in a sub-chamber  102  connected to the chamber  2 A. The chamber  2 A and the sub-chamber  102  may be optically connected through windows  113  and  123 . 
         [0044]    The etching gas supply unit  90  may be configured to supply an etching gas into the chamber  2 A under the control of the EUV light generation controller  5 A. When tin is used as the target material, a gas containing a hydrogen gas or hydrogen radicals may be used as the etching gas. The etching gas may be diluted with a buffer gas containing an inert gas, such as N 2 , He, Ne, and Ar. The etching gas supply unit  90  may include introduction pipes  91  and  92 . The introduction pipe  91  may be configured to introduce the etching gas toward the reflective surface of the EUV collector mirror  23 . More specifically, the gas introduction pipe  91  may be shaped such that a gas outlet of the introduction pipe  91  is orientated toward the reflective surface of the EUV collector mirror  23 , for example. The introduction pipe  92  may be configured to introduce the etching gas H* into a space  115  (see  FIG. 6 , for example) formed inside the mirror unit  101 . With this, the target material deposited on the optical elements may be etched. It should be noted that in  FIGS. 2 and 6 , the parts at which the introduction pipe  92  is connected to the mirror unit  101  differ, but the connection may be made at either part. 
         [0045]    The manometer  93  may be configured to measure the pressure inside the chamber  2 A. The manometer  93  may send the measured pressure to the EUV light generation controller  5 A. The ventilation unit  94  may discharge the gas inside the chamber  2 A under the control of the EUV light generation controller  5 A. 
         [0046]    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 target controller  53 , the laser device  3 , an exposure apparatus controller  61 , and the optical detection system  100 . The target controller  53  may be connected to the target supply driver  54 . The target supply driver  54  may be connected to the target supply unit  260  and/or the two-axis moving mechanism  261 . The laser beam focus position control driver  55  may be connected to the laser beam focusing optical system  70  and/or the focus position correction mechanism. The gas controller  56  may be connected to the etching gas supply unit  90 , the manometer  93 , and the ventilation unit  94 . 
         [0047]    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 in the downstream space  2   b . The partition  81  may serve to reduce the amount of debris of the target material generated in the space  2   b  entering the upstream space  2   a . A communication hole  82  may be formed in the partition  81 , through which the laser beams  33  and  43  from the laser beam focusing optical system  70  provided in the space  2   a  may travel into the space  2   b . The partition  81  may preferably be positioned such that the center of the communication hole  82  and the center of the through-hole  24  in the EUV collector mirror  23  are aligned in the beam path of the laser beams  33  and  43 . 
       4.2 Operation 
       [0048]    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 an instruction from the exposure apparatus controller  61  pertaining to the position at which the light  251  is to be emitted (an EUV light generation instruction position). The EUV light generation controller  5 A may control each component so that the light  251  is emitted in the EUV light generation instruction position. 
         [0049]    The EUV light generation controller  5 A may cause the guide laser device  40  to oscillate. With this, the guide laser beam  41  may be outputted from the guide laser device  40 . The guide laser beam  41  may enter the beam expander  401 , be expanded in diameter, and be outputted therefrom as a guide laser beam  42 . The guide laser beam  42  may then be transmitted through the dichroic mirror  351  of the beam adjusting unit  350 . 
         [0050]    The guide laser beam  42  may then enter the chamber  2  through the window  21  along substantially the same beam path as the pulse laser beam  32 . The guide laser beam  42  may be reflected sequentially by the laser beam focusing mirror  22  and the high-reflection mirror  72 , and as a guide laser beam  43 , may travel through the communication hole  82  and the through-hole  24 , and be focused in the plasma generation region  25 . Thereafter, the diverging guide laser beam  43  may enter the mirror unit  101  of the optical detection system  100 . 
         [0051]    Upon receiving an 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 an output signal for the target  27  to the target generator  26  through the target supply driver  54 . The target generator  26  may then output the target  27  at a timing in accordance with the inputted output signal. 
         [0052]    The target sensor  4  may be configured to detect data for calculating the position and the timing at which the target  27  may pass through the plasma generation region  25 . The detected values may be inputted to the target controller  53 . The target controller  53  may control the target supply unit  260  in accordance with the inputted detected values. Further, the target controller  53  may output the inputted detected values to the EUV light generation position controller  51 . The EUV light generation position controller  51  may send a trigger signal to the laser device  3  in accordance with the inputted detected values. The laser device  3  may output the pulse laser beam  31  at a timing delayed for a predetermined time from the trigger signal so that the target  27  is irradiated with the pulse laser beam  33  at a timing at which the target  27  reaches the EUV light generation instruction position. The laser device  3  may include a delay generator  360 . The delay generator  360  may adjustably hold a delay time of an output timing of the pulse laser beam  31  with respect to the detection timing of the target  27 . 
         [0053]    The pulse laser beam  31  outputted from the laser device  3  may be reflected by the high-reflection mirror  341  of the beam delivery unit  340  and by the dichroic mirror  351  of the beam adjusting unit  350 . Then, the pulse laser beam  32  may enter the chamber  2 A through the window  21 . The pulse laser beam  32  may then be reflected sequentially by the laser beam focusing mirror  22  and the high-reflection mirror  72 , and be focused on the target  27  in the plasma generation region  25 . 
         [0054]    Upon being irradiated with the pulse laser beam  33 , the target  27  may be turned into plasma, and the light  251  including the EUV light may be emitted from the plasma. 
         [0055]    The mirror unit  101  may include first and second reflective surfaces. The first reflective surface may be arranged upstream from the second reflective surface. A through-hole may be formed in the first reflective surface, through which the guide laser beam  43  passes. Light  34  reflected by the first reflective surface may include the pulse laser beam  33  and the light  251 . The reflected light  34  may then be transmitted through the window  113  and be absorbed by the beam dump  112 . 
         [0056]    Light  44  reflected by the second surface of the mirror unit  101  may include the guide laser beam  43 , the pulse laser beam  33 , and the light  251 . The dichroic mirror  121  provided in the path of the light  44  may transmit light  45  that includes the guide laser beam  43  and a part of the light  251  and reflect remaining light  35 . Here, in  FIG. 2 , the guide laser beam  43  and the light  44  are indicated by the same broken lines, but this does not mean that the guide laser beam  43  and the light  44  are identical. The light  35  reflected by the dichroic mirror  121  may include a part of the pulse laser beam  33  which has passed through the plasma generation region  25 . The reflected light  35  may be absorbed by the beam dump  122 . The light  45  transmitted through the dichroic mirror  121  may be transmitted through the window  123 , and be imaged on the photosensitive surface of the optical sensor  125  by the imaging optical system  124 . This image at the focus of the light  45  may include the image of the guide laser beam  43  at its focus and the image of the light  251 . The optical sensor  125  may input the detected image data to the EUV light generation position controller  51 . Here, in place of the imaging optical system  124 , a focusing mirror may be used. 
         [0057]    The EUV light generation position controller  51  may calculate the size (e.g., the width and/or the area) and the center of the image of the guide laser beam  43  at its focus from the inputted data. The EUV light generation position controller  51  may control the focus position correction mechanism such that the center of the image of the guide laser beam  43  at its focus coincides with the EUV light generation instruction position received from the exposure apparatus controller  61 . Here, the coordinate system of the image inputted from the optical sensor  125  may be converted as necessary so that the EUV light generation instruction position can be specified. The EUV light generation position controller  51  may also be configured to control the laser beam focusing optical system  70  so that the size of the image of the guide laser beam  43  at its focus becomes a predetermined size. The predetermined size may be held in the EUV light generation position controller  51  or may be given from the exposure apparatus controller  61 . The EUV light generation position controller  51  may control the focus position correction mechanism through the laser beam focus position control driver  55 . The laser beam focus position control driver  55  may send driving signals to the holder  72   a  and the plate moving mechanism  71   a  under the control of the EUV light generation position controller  51 . For example, the EUV light generation position controller  51  may modify the tilt angles of the high-reflection mirror  72  in two directions through the holder  72   a  based on the information on the center of the image of the guide laser beam  43  at its focus. One of the two directions may be a rotational direction about the Y-axis, and the other direction may be a rotational direction about an axis that is perpendicular to the Y-axis and that lies on a plane parallel to the reflection surface of the high-reflection mirror  72 . Further, the EUV light generation position controller  51  may move the plate  71  in the Z-direction through the plate moving mechanism  71   a  based on the information on the size of the image of the guide laser beam  43  at its focus. The movement of the plate  71  may, for example, be controlled as follows. First, a difference between the size of the image of the guide laser beam  43  at its focus and the predetermined size may be calculated. Then, the plate  71  may be moved in one direction along the Z-direction for a predetermined amount, and the difference may be calculated again. At this time, if the difference is larger than the difference calculated first, the plate  71  may be moved in the other direction along the Z-direction for an amount that is slightly larger than the aforementioned predetermined amount. If the difference becomes smaller, the plate  71  may further be moved in the same direction for a smaller amount. Such an operation may be repeated until the difference becomes equal to or smaller than a predetermined amount. In this way, the focus of the guide laser beam  43  may be adjusted, and in turn the focus of the pulse laser beam  33  may be adjusted. 
         [0058]    Further, the EUV light generation position controller  51  may calculate the size (e.g., the width and/or the area) and the center of the image from the image data of the light  251 . The EUV light generation position controller  51  may control the target supply unit  260  and the laser device  3  such that the center of the image of the light  251  coincides with the EUV light generation instruction position received from the exposure apparatus controller  61 . Further, the EUV light generation position controller  51  may be configured to control the two-axis moving mechanism  261  such that the size of the image of the light  251  becomes a predetermined size. The predetermined size may be held in the EUV light generation position controller  51  or may be given from the exposure apparatus controller  61 . The EUV light generation position controller  51  may control the target supply unit  260  through the target supply driver  54 . 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 . For example, the EUV light generation position controller  51  may move the target generator  26  in the Y-direction through the two-axis moving mechanism  261  based on the information on the center of the light  251 . Further, the EUV light generation position controller  51  may output a signal to the laser device  3  to correct the delay time for the output timing of the pulse laser beam  31  with respect to the output timing of the target  27  based on the information on the center of the light  251 . Based on this signal, the laser device  3  may correct the delay time held in the delay generator  360 . Further, the EUV light generation position controller  51  may move the target generator  26  in the Z-direction through the two-axis moving mechanism  261  based on the information on the size of the light  251 . The control of the movement of the target generator  26  in the Z-direction may, for example, be similar to the above-described control of the plate  71 . In this way, the position to which the target  27  is supplied may be corrected. 
         [0059]    The gas controller  56  may control the etching gas supply unit  90  and the ventilation unit  94  based on the value inputted from the manometer  93 . With this, the gas pressure inside the chamber  2 A may be retained at a predetermined low pressure, and at the same time a sufficient amount of the etching gas may be introduced into the chamber  2 A. 
         [0060]    Here, the image of the guide laser beam  43  at its focus imaged on the optical sensor  125  and the image of the light  251  will be discussed. 
         [0061]      FIG. 3  shows an example of an image  1001  to be detected by an optical sensor when the center of the pulse laser beam coincides with the center of the target being irradiated with the pulse laser beam. In  FIG. 3 , the center of an image  1011  of the guide laser beam  43  at its focus may substantially coincide with the center of an image  1012  of the light  251 , and the respective centers may be detected around an ideal position. As stated above, information on the position at which the light  251  is to be generated may be given from the exposure apparatus controller  61 . The EUV light generation position controller  51  may determine through calculation which position the specified generation position corresponds to in the coordinate system of the image obtained by the optical sensor  125 , and store the determined position as the ideal position in a memory (not shown) or the like. In  FIGS. 3 through 5 , the intersection of the dotted lines may be set as the ideal positions, respectively. As shown in  FIG. 3 , it may be preferable that the image  1011  and the image  1012  are captured in the same image. For example, when the optical sensor  125  is configured of a CCD, the image  1011  and the image  1012  may be captured in the same image by appropriately selecting the capture timing and the exposure time. Alternatively, the image  1011  and the image  1012  may be captured at different timings, and the two images may be made into a composite image. Further alternatively, the image  1011  and the image  1012  may be captured at different timings, and the respective centers of the images  1011  and  1012  may be calculated separately. When the image  1011  and the image  1012  are captured at different timings, the respective capture timings may preferably be close to each other. 
         [0062]      FIG. 4  shows an example of an image  1002  to be detected by an optical sensor when the center of the pulse laser beam does not coincide with the center of the target being irradiated with the pulse laser beam. As shown in  FIG. 4 , in this case, the center of an image  1021  of the guide laser beam  43  at its focus may not coincide with the center of an image  1022  of the light  251 , and the center of the image  1022  may be offset from the ideal position. 
         [0063]      FIG. 5  shows an example of an image  1002   a  of the relationship among the center of the image of the guide laser beam at its focus obtained through calculation, the center of the image of the plasma-emitted light obtained through calculation, and the estimated image  1023  of the pulse laser beam in the state shown in  FIG. 4 . The EUV light generation position controller  51  may control the focus of the pulse laser beam  33  and the position to which the target  27  is supplied based on such data as shown in  FIG. 5 . With this, the center  1021   a  of the image  1021  may coincide with the center  1022   a  of the image  1022 . More specifically, in  FIG. 5 , for example, the center  1021   a  of the image  1021  is detected around the ideal position; thus, it is speculated that the pulse laser beam  33  is appropriately focused at the ideal position. On the other hand, the center  1022   a  of the image  1022  is not detected around the ideal position; thus, it is speculated that the target  27  is not supplied to the ideal position. Thus, the direction and the degree to which the center  1022   a  is offset from the ideal position may, for example, be calculated, and based on the calculation result, the position to which the target  27  has been supplied at the time of irradiation may be estimated. In the case shown in  FIG. 5 , since the center  1022   a  is offset to the upper left from the ideal position, it is speculated that the target  27  has been irradiated with the pulse laser beam  33  at a position offset to the lower right from the ideal position. Accordingly, the position to which a subsequent target  27  is to be supplied may be adjusted toward the upper left. The amount of adjustment to be made may be determined based on the relative position of and the relative distance among at least the target supply unit  260 , the mirror unit  101 , and the optical sensor  125 . Without being limited to the above example, the determination may be made in accordance with the system to be implemented. Here, as in the control of the supply position of the target  27 , the focus of the pulse laser beam  33  may be controlled similarly. Further, in place of the centers of the respective images, the centroids of the respective images may be obtained. 
       4.3 Effect 
       [0064]    With the above configuration and operation, the guide laser beam  43  and the light  251  may be detected by the optical sensor  125 . Through this, the focus of the pulse laser beam  33  and the position of the target  27  when irradiated with the pulse laser beam  33  may be detected. 
         [0065]    Based on this detection result, the position at which the pulsed laser beam  33  is focused and the position to which the target  27  is supplied may be controlled. Accordingly, generation of the light  251  may be controlled with high precision. 
         [0066]    Further, when a continuous wave laser beam is used as the guide laser beam  43 , the focus of the pulse laser beam  33  may be controlled without outputting the pulse laser beam  31 . 
       4.4 Examples of Optical Detection System 
     4.4.1 First Example 
     4.4.1.1 Configuration 
       [0067]      FIG. 6  schematically illustrates the configuration of an optical detection system  100 A of a first example. As shown in  FIG. 6 , the optical detection system  100 A may include a mirror unit  101 A, the window  113 , the beam dump  112 , the dichroic mirror  121 , the beam dump  122 , the window  123 , the imaging optical system  124 , and the optical sensor  125 . The optical detection system  100 A may further include baffles  114  and  127 . 
         [0068]    The mirror unit  101 A may include mirror blocks  110  and  120 , a lens block  118 , a 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 . 
         [0069]    The lens block  118  may be provided between the mirror block  110  and the mirror block  120 . The lens  128  and the baffle  129  may be fixed to the lens block  118 . The lens block  118  may be hollow so as not to block the guide laser beam  43 . The lens block  118  may be provided with a heat carrier pipe (not shown), through which a heat carrier may circulate. With this, a rise in temperature of the lens block  118  caused by the irradiation with the laser beam or the scattered rays of the laser beam may be suppressed. 
         [0070]    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 shown), through which a heat carrier may circulate. With this, a rise in temperature of the respective mirror blocks  110  and  120  caused by the irradiation with the laser beam or the scattered rays of the laser beam may be suppressed. 
         [0071]    The mirror block  110  may include an off-axis paraboloidal mirror  110   a . A space  115  may be formed in the mirror block  110  along the direction in which the guide laser beam  43  may travel. The mirror block  110  may be positioned such that the focus of the off-axis paraboloidal mirror  110   a  substantially coincides with the plasma generation region  25 . 
         [0072]    The light  34  reflected by the mirror block  110  may enter the sub-chamber  102  through a 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 may be coated with anti-reflective films for the wavelength corresponding to the wavelength of the laser beams 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. The cylindrical baffle  114  may be provided on the inner wall of the chamber  2 A so as to surround the window  113 . With this, deposition of debris onto the window  113  may be reduced. The baffle  114  may be provided with an introduction pipe (not shown), through which the etching gas may be supplied from the etching gas supply unit  90 . The inner diameter of the baffle  114  may preferably be larger than the beam diameter of the light  34  reflected by the off-axis paraboloidal mirror  110   a  of the mirror block  110 . 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 heat carrier (not shown) may circulate in the beam dump  112 . A commercially available laser power meter head may be used as the beam dump  112 . 
         [0073]    The mirror block  120  may be positioned such that the guide laser beam  43  is reflected at an angle of approximately 45 degrees by a reflective surface  120   a . The lens  128 , the dichroic mirror  121 , the window  123 , the filter  126 , the imaging optical system  124 , and the optical sensor  125  may be arranged in this order along the path of the light  44  reflected by the mirror block  120 . 
         [0074]    The lens  128  may be positioned such that the focus thereof along the beam path of the guide laser beam  43  substantially coincides with the plasma generation region  25 . The lens  128  may collimate the light  44 . The lens  128  may be made of diamond. The cylindrical baffle  129  may be provided on the outer wall of the lens block  118  so as to surround the lens  128 . With this, deposition of debris onto the lens  128  may be reduced. The baffle  129  may be provided with an introduction pipe (not shown), through which the etching gas may be supplied from the etching gas supply unit  90 . 
         [0075]    The light  44  transmitted through the lens  128  may be incident on the dichroic mirror  121 . The dichroic mirror  121  may be configured to transmit the guide laser beam  43  and a part of the light  251  and reflect the remaining light  35 . The wavelength of the part of the light  251  which is transmitted through the dichroic mirror  121  may be in the range of visible radiation. The dichroic mirror  121  may be made of diamond. The light  35  reflected by the dichroic mirror  121  may be absorbed by the beam dump  122 . A heat carrier (not shown) may circulate in the beam dump  122 . 
         [0076]    The light  45  transmitted through by the dichroic mirror  121  may enter the sub-chamber  102  through the 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 may be coated on both sides thereof with anti-reflective films for the wavelength sensitive to the optical sensor  125 . The window  123  may be held by the 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 . With this, deposition of debris onto the window  123  may be reduced. The baffle  127  may be provided with an introduction pipe (not shown), through which the etching gas may be supplied from the etching gas supply unit  90 . Further, a through-hole  122   a  may be formed in the baffle  127 , through which the light  35  reflected by the dichroic mirror  121  may travel toward the beam dump  122 . 
         [0077]    The filter  126 , the imaging optical system  124 , and the optical sensor  125 , collectively serving as an optical detection unit, may be provided inside the sub-chamber  102 . The filter  126  may be an optical bandpass filter which allows a part of the guide laser beam  43  and a part of the light  251  (see  FIG. 2 ) to be transmitted therethrough. For example, the filter  126  may be configured to transmit visible radiation. The imaging optical system  124  may include a convex lens  124   a  and a concave lens  124   b . The optical sensor  125  may be positioned such that the imaging plane of the imaging optical system  124  lies on the photosensitive surface of the optical sensor  125 . The optical sensor  125  may be a two-dimensional sensor, such as a CCD or a PSD. 
         [0078]    A gas outlet of the introduction pipe  92  connected to the etching gas supply unit  90  (see  FIG. 2 ) may be positioned in the space  115  inside the mirror unit  101 A. The etching gas H* may be introduced into the space  115 , whereby debris deposited on the reflective surface  120   a  of the mirror block  120  and the surface of the lens  128  may be removed. Alternatively, an inert gas may be introduced into the space  115  from an inert gas supply unit (not shown) in order to prevent dust or the like from adhering onto the optical elements. The inert gas may be a noble gas, such as N 2 , He, Ne, or Ar. In either case, a discharge port (not shown) may preferably be provided in the sub-chamber  102  so as to discharge the introduced gas. When the etching gas H* is introduced into the space  115 , an appropriate scrubber may preferably be connected to the discharge port. When the substance to be etched is Sn and the etching gas H* is hydrogen, stannane (SnH 4 ) may be produced through the etching reaction. 
       4.4.1.2 Operation 
       [0079]    The general operation of the optical detection system  100 A shown in  FIG. 6  will now be described. The beam axis of the guide laser beam  43  may substantially coincide with the beam axis of the pulse laser beam  33 . The guide laser beam  43  may once be focused in the plasma generation region  25 , and then the diverging guide laser beam  43  may travel through the space  115  in the mirror block  110 . The guide laser beam  43  may then be incident on the reflective surface  120   a  of the mirror block  120  at substantially 45 degrees. The guide laser beam  43  reflected by the mirror block  120  may then be collimated through the lens  128 . The collimated guide laser beam  43  may be transmitted through the dichroic mirror  121  and the window  123 , and enter the optical detection unit inside the sub-chamber  102 . 
         [0080]    The center portion of the pulse laser beam  33  that has passed through the plasma generation region  25  may also travel through the space  115  and be reflected by the reflective surface  120   a , as in the guide laser beam  43 . The reflected pulse laser beam  33  may be transmitted through the lens  128 , be reflected by the dichroic mirror  121  with high reflectance, and enter the beam dump  122 . 
         [0081]    The peripheral portion (aside from the aforementioned center 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 . 
         [0082]    The guide laser beam  43  that has entered the optical detection unit may be transmitted through the filter  126  and the imaging optical system  124 . With this, the guide laser beam  43  may be imaged onto the optical sensor  125  by the imaging optical system  124 . 
         [0083]    The light  251  emitted from the plasma generated in the plasma generation region  25  may also travel through the space  115 , as in the guide laser beam  43 . The light  251  may then be incident on the reflective surface  120   a  at substantially 45 degrees. The light  251  reflected by the reflective surface  120   a  may be transmitted through the lens  128 . The lens  128  may collimate the light  251 . The collimated light  251  may be transmitted through the dichroic mirror  121  and the window  123 , and enter the optical detection unit. 
         [0084]    The light  251  that has entered the optical detection unit may be incident on the filter  126 . The filter  126  may transmit, of the light  251 , at least light at a predetermined wavelength. The light  251  transmitted through the filter  126  may then enter the imaging optical system  124 . The imaging optical system  124  may image the entering light  251  onto the photosensitive surface of the optical sensor  125 . With this, the image of the light  251  at the plasma generation region  25  may be transferred onto the optical sensor  125 . 
         [0085]    The etching gas H* supplied into the space  115  through the introduction pipe  92  from the etching gas supply unit  90  may flow into the chamber  2 A along the surfaces of the optical elements provided in the beam path in the mirror unit  101 A. The optical elements provided in the beam path in the mirror unit  101 A may, for example, include the reflective surface  120   a  of the mirror block  120 , the lens  128 , and so forth. With this, debris deposited on the surfaces of the optical elements may be etched by the etching gas H*. 
       4.4.1.3 Effect 
       [0086]    According to the first example, the guide laser beam  43  and the light  251  emitted from the plasma may be detected by the single optical sensor  125 . With this, the focus of the pulse laser beam  33  and the position to which the target  27  is supplied may be detected with high precision. 
         [0087]    Further, debris deposited on the surfaces of the optical elements may be etched. With this, the guide laser beam  43  and the light  251  may be detected stably for a relatively long time. 
         [0088]    Here, when tin (Sn) is used as the target material, a hydrogen gas or hydrogen radicals may be used as the etching gas H*. The hydrogen gas or the hydrogen radicals may etch deposited Sn through the following chemical reaction: 
         [0000]      Sn (solid)+2H 2  (gas)-&gt;SnH 4  (gas) 
         [0089]    However, when the temperature reaches or exceeds 100° C., the reverse reaction may occur, and Sn may be deposited. Accordingly, the temperature of each optical element (e.g., the 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 faster 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 a heat carrier circulating in the mirror unit  101 A based on the detection result of a temperature sensor (not shown) attached to the mirror unit  101 A. The flow rate and/or the temperature of the heat carrier may be regulated by controlling a flow controller (not shown) or a chiller (not shown) connected to a flow channel (not shown) of the heat carrier. 
       4.4.2 Second Example 
     4.4.2.1 Configuration 
       [0090]      FIG. 7  schematically illustrates the configuration of an optical detection system  100 B of a second example. As shown in  FIG. 7 , the optical detection system  100 B may differ from the optical detection system  100 A in that the mirror unit  101 A is replaced by a mirror unit  101 B. Further, in the optical detection system  100 B, the dichroic mirror  121  and the beam dump  122  may be omitted. 
         [0091]    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 . 
         [0092]    The mirror block  110  and the lens block  118  may be configured similarly to those shown in  FIG. 6 . A dichroic mirror  132  may be fixed to the dichroic mirror block  138 . The dichroic mirror  132  may be coated with a film configured to transmit the pulse laser beam  33  with high transmittance and reflect the guide laser beam  43  and a part of the light  251  (see  FIG. 2 ) with high reflectance. The substrate of the dichroic mirror  132  may, for example, be made of diamond. 
         [0093]    Here, the lens  128  fixed to the lens block  118  may be made of a material that transmits the guide laser beam  43  and the light  251 . The beam dump block  133  may include a conical surface  133   a  so that the pulse laser beam  33  is absorbed efficiently. The beam dump block  133  may be provided with a flow channel (not shown), through which a heat carrier may circulate to suppress a rise in temperature due to the energy of the laser beam. The introduction pipe  92  from the etching gas supply unit  90  (see  FIG. 2 ) may be connected to the mirror unit  101 B such that the etching gas H* flows along the respective surfaces of the dichroic mirror  132  and the lens  128 . 
       4.4.2.2 Operation 
       [0094]    The general operation of the optical detection system  100 B shown in  FIG. 7  will now be described. The beam axis of the guide laser beam  43  may substantially coincide with the beam axis of the pulse laser beam  33 . The guide laser beam  43  may once be focused in the plasma generation region  25 , and then the diverging guide laser beam  43  may travel through the space  115  in the mirror block  110 . The guide laser beam  43  that has traveled through the space  115  may be incident on the dichroic mirror  132  at substantially 45 degrees. The guide laser beam  43  reflected by the dichroic mirror  132  may be collimated through the lens  128 . The collimated guide laser beam  43  may pass through the window  123 , and enter the optical detection unit inside the sub-chamber  102 . 
         [0095]    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 . 
         [0096]    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  of the mirror block  110 , and enter the beam dump  112  inside the sub-chamber  102  through the window  113 . 
         [0097]    The guide laser beam  43  that has entered the optical detection unit may be transmitted through the filter  126  and the imaging optical system  124 . With this, the guide laser beam  43  may be imaged on the photosensitive surface of the optical sensor  125  by the imaging optical system  124 . 
         [0098]    A part of the light  251  emitted from the plasma generated in the plasma generation region  25  may travel through the space  115 , as in the guide laser beam  43 . The light  251  may then be incident on the dichroic mirror  132  at substantially 45 degrees. The light  251  reflected by the dichroic mirror  132  may be transmitted through the lens  128 . The lens  128  may collimate the light  251 . The collimated light  251  may be transmitted through the window  123  and enter the optical detection unit. 
         [0099]    The light  251  that has entered the optical detection unit may be incident on the filter  126 . The filter  126  may transmit, of the light  251 , at least light at a predetermined wavelength. The light  251  transmitted through the filter  126  may then enter the imaging optical system  124 . With this, the image of the light  251  at the plasma generation region  25  may be transferred onto the optical sensor  125 . 
         [0100]    The operation of etching the debris deposited on the optical elements provided in the beam path in the mirror unit  101 B may be similar to that of the first example. Thus, detailed description thereof will be omitted. 
       4.4.2.3 Effect 
       [0101]    According to the second example, the dichroic mirror  132  and the beam dump block  133  may be provided in the mirror unit  101 B. Thus, the pulse laser beam  33 , the guide laser beam  43 , and the light  251  may be separated prior to passing through the lens  128 . As a result, the lens  128  need not have durability against the high power pulse laser beam  33 , and thus need not be formed of diamond, which is relatively expensive. 
       4.4.3 Third Example 
     4.4.3.1 Configuration 
       [0102]      FIG. 8  schematically illustrates the configuration of an optical detection system  100 C of a third example. As shown in  FIG. 8 , the optical detection system  100 C may differ from the optical detection system  100 A in that the mirror unit  101 A is replaced by a mirror unit  101 C. In the mirror unit  101 C, the lens  128  and the lens block  118  for holding the lens  128  may be omitted. Further, in the optical detection system  100 C, the beam dump  122  for the light  35  and the beam dump  112  for the light  34  may be replaced by a common beam dump unit  212 . 
         [0103]    More specifically, the mirror unit  101 C may include the mirror block  110  and a mirror block  220 . The mirror block  110  may be configured similarly to the mirror block  110  shown in  FIG. 6 . However, a flow channel  281  may preferably be provided inside the mirror block  110 , through which a heat carrier supplied from a chiller (not shown) may flow. 
         [0104]    The mirror block  220  may be configured similarly to the mirror block  120  shown in  FIG. 6 . However, in place of the introduction pipe  92  shown in  FIG. 6 , a channel  272  through which the etching gas H* supplied from the etching gas supply unit  90  flows via a pipe (not shown) may be formed inside the mirror block  220 . Further, a flow channel  282  may preferably be provided inside the mirror block  220 , as in the mirror block  110 , through which a heat carrier supplied from a chiller (not shown) may flow. 
         [0105]    The optical detection system  100 C may include the dichroic mirror  121 , the window  123 , the imaging optical system  124 , and the optical sensor  125 . The window  123  may be held by a window holder  223   a . The window holder  223   a  may be provided such that the window  123  covers a communication hole  217  formed in the chamber  2 A. A flow channel  284  may preferably be provided in the window holder  223   a , through which a heat carrier supplied from a chiller (not shown) may flow. Here, the window holder  223   a  and the mirror holder  221  may be formed integrally. 
         [0106]    The dichroic mirror  121  may be held by the mirror holder  221 . The mirror holder  221  may be provided so as project into the chamber  2 A. The mirror holder  221  may hold the dichroic mirror  121  such that the dichroic mirror  121  is inclined with respect to the travel direction of the light  44  reflected by the reflective surface  120   a  of the mirror block  220 . A flow channel  283  may preferably be provided in the mirror holder  221 , through which a heat carrier supplied from a chiller (not shown) may flow. A baffle  227  may be provided on the dichroic mirror  121  to reduce the debris being deposited on the surface thereof on which the light  44  is incident. A through-hole  227   a  may be formed in the baffle  227 , through which the light  35  reflected by the dichroic mirror  121  may travel toward the beam dump  212 . Further, the interior space of the baffle  227  may be in communication with the etching gas supply unit  90  through a pipe  273 . With this, the etching gas H* may be supplied from the etching gas supply unit  90  through the pipe  273  toward a surface of the dichroic mirror  121  which is exposed to a space in the chamber  2 A. 
         [0107]    The filter  126 , the imaging optical system  124 , and the optical sensor  125  may be provided inside a sub-chamber  202 . The sub-chamber  202  may project to the outside of the chamber  2 A. The positional relationship among the window  123 , the filter  126 , the imaging optical system  124 , and the optical sensor  125  may be similar to that in the optical detection system  100 A shown in  FIG. 6 . A flow channel  285  may preferably be provided in the sub-chamber  202 , through which a heat carrier supplied from a chiller (not shown) may flow. 
         [0108]    The beam dump unit  212  may be provided so as to cover a communication hole  216  formed in the chamber  2 A. A V-shaped recess  212   a  may be formed in the beam dump unit  212  at a portion on which the light  34  and the light  35  may be incident. A flow channel  286  may preferably be provided near the recess  212   a  in the beam dump unit  212 , through which a heat carrier supplied from a chiller (not shown) may flow. 
       4.4.3.2 Operation 
       [0109]    The general operation of the optical detection system  100 C shown in  FIG. 8  will now be described. The beam axis of the guide laser beam  43  may substantially coincide with the beam axis of the pulse laser beam  33 . The guide laser beam  43  may once be focused in the plasma generation region  25 , and then the diverging guide laser beam  43  may travel through the space  115  in the mirror block  110 . The guide laser beam  43  that has traveled through the space  115  may be incident on the reflective surface  120   a  of the mirror block  220  at substantially 45 degrees. The guide laser beam  43  reflected by the reflective surface  120   a  may pass through an opening  220   a  in the mirror block  220 , be transmitted through the dichroic mirror  121  and the window  123 , and enter the optical detection unit inside the sub-chamber  202 . 
         [0110]    The center portion of the pulse laser beam  33  that has passed through the plasma generation region  25  may also travel through the space  115 , be reflected by the reflective surface  120   a , and pass through the opening  220   a , as in the guide laser beam  43 . The pulse laser beam  33  that has passed through the opening  220   a  may be incident on the dichroic mirror  121  and be reflected thereby. 
         [0111]    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  of the mirror block  110 , and enter the beam dump unit  212 . 
         [0112]    The guide laser beam  43  that has entered the optical detection unit may be transmitted through the filter  126  and the imaging optical system  124 , as in the case shown in  FIG. 6 . With this, the image of the guide laser beam  43  at its focus may be imaged onto the optical sensor  125 . 
         [0113]    The light  251  (see  FIG. 2 ) emitted from the plasma generated in the plasma generation region  25  may be reflected by the mirror block  220 , be transmitted through the dichroic mirror  121  and the window  123 , and enter the optical detection unit, as in the guide laser beam  43 . 
         [0114]    The etching gas H* supplied into the space  115  through the pipe  272  from the etching gas supply unit  90  may flow into the chamber  2 A along the surfaces of the optical elements provided in the beam path in the mirror unit  101 C. The optical elements provided in the mirror unit  101 C may, for example, include the reflective surface  120   a  of the mirror block  220 . With this, debris deposited on the surfaces of the optical elements may be etched by the etching gas H*. 
       4.4.3.3 Effect 
       [0115]    According to the third example, the single beam dump unit  212  may be provided to absorb both the light  35  and the light  34 . Further, since the light  35  and the light  34  may enter the beam dump unit  212  without being transmitted through the windows, heat generated from unnecessary light may be processed with a simple configuration. 
         [0116]    Further, according to the third example, heat carriers may be made to flow in locations where the temperature may rise, such as the mirror unit  101 A, the window holder  223   a , the sub-chamber  202 , and the beam dump unit  212 . Accordingly, the deterioration in performance of the optical detection system  100 C caused by the heat may be suppressed. 
       5. VARIATION 
     5.1 Configuration 
       [0117]      FIG. 9  schematically illustrates the configuration of an optical system in a modification of the EUV light generation system  11 A. In  FIG. 9 , only the primary optical elements are illustrated. The omitted elements may be similar to those shown in  FIG. 2 ,  6 ,  7 , or  8 . 
         [0118]    As shown in  FIG. 9 , in the modification, a pinhole plate  411  and a lens  412  may be provided in place of the beam expander  401  in  FIG. 2 . Other configurations may be similar to those shown in  FIG. 2 . The pinhole plate  411  may be provided at the focus of the lens  412 . The pinhole in the pinhole plate  411  may be smaller than the beam diameter of the guide laser beam  41  outputted from the guide laser device  40 . Alternatively, the diameter of the pinhole may be set to the spot size of the pulse laser beam  33  in the plasma generation region  25 . 
       5.2 Operation 
       [0119]    With reference to  FIG. 9 , the guide laser beam  41  outputted from the guide laser device  40  may first be incident on the pinhole plate  411 . Apart of the guide laser beam  41  which has passed through the pinhole in the pinhole plate  411  may be diverged and be incident on the lens  412 . The lens  412  may collimate the guide laser beam  41 . The beam diameter of a collimated guide laser beam  42 A may substantially coincide with the beam diameter of the pulse laser beam  32 . 
         [0120]    The guide laser beam  42 A may be transmitted through the dichroic mirror  351  of the beam adjusting unit  350  (see  FIG. 2 ). The pulse laser beam  32  may be reflected by the dichroic mirror  351 . With this, the beam axis of the pulse laser beam  32  may substantially coincide with the beam axis of the guide laser beam  42 A. The guide laser beam  42 A may travel through substantially the same beam path as the pulse laser beam  32 , and be focused by the laser beam focusing optical system  70  in the plasma generation region  25  as a guide laser beam  43 A. At this time, the image of the guide laser beam  41  at the pinhole in the pinhole plate  411  may be imaged at the focus of the laser beam focusing optical system  70  in the plasma generation region  25 . For example, the image of the guide laser beam  41  at the pinhole in the pinhole plate  411  may be transferred with the same magnification in the plasma generation region  25  by adjusting the focal distance of the lens  412  for the wavelength of the guide laser beam  41  to the focal distance of the laser beam focusing optical system  70 . 
         [0121]    The guide laser beam  43 A that has once been focused in the plasma generation region  25  may then enter the mirror unit  101  of the optical detection system  100 . The diverging guide laser beam  43 A may be reflected by one of the reflective surfaces of the mirror unit  101  as a guide laser beam  44 A. The reflected guide laser beam  44 A may be collimated through the lens  128 , be transmitted through the dichroic mirror  121  and the window  123 , and enter the imaging optical system  124 . Thereafter, the guide laser beam  44 A may be incident on the optical sensor  125  provided such that the photosensitive surface thereof lies at the focus of the imaging optical system  124 . With this, the image of the guide laser beam  41  at the pinhole in the pinhole plate  411  may be imaged on the photosensitive surface of the optical sensor  125 . The data on this image may be sent to the EUV light generation position controller  51 . 
         [0122]      FIG. 10  shows an image  2002   a  as an example of the relationship among the image of the guide laser beam  44 A, the image of the light  251 , and the image of the pulse laser beam  33 . Here,  FIG. 10  shows the image  1022  of the light  251  to be detected by the optical sensor  125  when the center of the target  27  coincides with the center of the pulse laser beam  33  at the time of being irradiated with the pulse laser beam  33  and an image  2021  of the guide laser beam  41  at the pinhole in the pinhole plate  411 . 
         [0123]    As shown in  FIG. 10 , in the modification, the image  2021  may substantially coincide with an image  1023  of the pulse laser beam  33 . Since the guide laser beam  42 A/ 43 A may travel through substantially the same beam path as the pulse laser beam  32 / 33  and have substantially the same beam diameter as the pulse laser beam  32 / 33 , the image  2021  may reflect the spot size of the pulse laser beam  33 . Further, in the image  2002   a , the center  2021   a  of the image  2021  and the center  1022   a  of an image  1022  of the light  251  may be calculated. The EUV light generation position controller  51  may control the focus of the pulse laser beam  33  and the position to which the target  27  is supplied based on the calculated data. With this, the center  2021   a  of the image  2021  may coincide with the center  1022   a  of the image  1022 . Here, the focus of the pulse laser beam  33  and the position to which the target  27  is supplied may be controlled so that the centers of the respective images are at the ideal position (e.g., the intersection of the broken lines in the image  2002   a ). Here, in place of the centers of the respective images, the centroids of the respective images may be obtained. 
       5.3 Effect 
       [0124]    According to the modification, the beam diameter of the guide laser beam  42 A and the beam diameter of the pulse laser beam  32  may be made to substantially coincide with each other. Further, the image of the guide laser beam  41  at the pinhole in the pinhole plate  411  may be imaged in the plasma generation region  25 . Accordingly, the center and the beam diameter of the pulse laser beam  33  may be detected based on the detection result of the image  2021  of the guide laser beam  41 . As a result, the positional relationship between the pulse laser beam  33  and the light  251  may be detected with high precision. 
         [0125]    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). 
         [0126]    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.”