Abstract:
A holder device for holding an optical element includes a holder having first and second members to sandwich the optical element therebetween, and a sealing member for creating a seal between the second member and the optical element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Japanese Patent Application No. 2011-259000 filed Nov. 28, 2011. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    This disclosure relates to a holder device, a chamber apparatus, and an extreme ultraviolet (EUV) 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 holder device according to one aspect of this disclosure for holding an optical element may include a holder having first and second members to sandwich the optical element therebetween, and a first sealing member for creating a seal between the second member and the optical element. 
         [0008]    A chamber apparatus according to another aspect of this disclosure may include a chamber, the aforementioned holder device, and a sealing member for sealing between the holder device and a wall of the chamber. 
         [0009]    An extreme ultraviolet light generation system according to yet another aspect of this disclosure may include the aforementioned chamber apparatus, a laser apparatus configured to output a laser beam, a target supply unit configured to supply a target material into the chamber, and an optical system configured to focus the laser beam inside the chamber through the optical element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Hereinafter, exemplary embodiments of this disclosure will be described with reference to the accompanying drawings. 
           [0011]      FIG. 1  schematically illustrates a configuration of an exemplary EUV light generation system. 
           [0012]      FIG. 2  schematically illustrates an exemplary configuration of an EUV light generation system according to a first embodiment of this disclosure. 
           [0013]      FIG. 3  is an exploded view of a holder shown in  FIG. 2 . 
           [0014]      FIG. 4  shows an exemplary configuration of a first member shown in  FIG. 3 , as viewed toward a surface thereof at which the first member comes into contact with a second member. 
           [0015]      FIG. 5  shows an exemplary configuration of a second member shown in  FIG. 3 , as viewed toward a surface thereof at which the second member comes into contact with a first member. 
           [0016]      FIG. 6  is a sectional view schematically illustrating an exemplary configuration of the holder shown in  FIG. 2 , in which a surface of the window is in direct contact with the holder. 
           [0017]      FIG. 7  a sectional view schematically illustrating an exemplary configuration of the holder shown in  FIG. 2 , in which a material having high thermal conductivity is interposed between a surface of the window and the holder. 
           [0018]      FIG. 8  is a sectional view schematically illustrating an exemplary configuration of the holder shown in  FIG. 2 , in which a surface of the window is soldered to the holder. 
           [0019]      FIG. 9  is a sectional view schematically illustrating an exemplary configuration of a holder according to a second embodiment of this disclosure. 
           [0020]      FIG. 10  shows an exemplary configuration of a first member shown in  FIG. 9 , as viewed toward a surface at which the first member comes into contact with a second member. 
           [0021]      FIG. 11  is a sectional view schematically illustrating an exemplary configuration of a holder according to a third embodiment. 
           [0022]      FIG. 12  is a perspective view of a focusing lens shown in  FIG. 11 . 
           [0023]      FIG. 13  schematically illustrates an exemplary configuration of a second member shown in  FIG. 11 . 
           [0024]      FIG. 14  shows a case where a window and a first member are in a surface contact with each other. 
           [0025]      FIG. 15  shows a case where a window and a first member are in a point contact. 
           [0026]      FIG. 16  shows a case where a window and a first member are in a line contact. 
           [0027]      FIG. 17  shows a case where a window and a first member are in a surface contact at multiple surfaces. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Hereinafter, exemplary 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, configurations and operations 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 described following the table of contents below. 
       Contents 
     1. Overview 
     2. Overview of EUV Light Generation System 
     2.1 Configuration 
     2.2 Operation 
     3. EUV Chamber Including Transmissive Optical Element 
     3.1 Configuration 
     4. Basic Structure of Optical Element Holder 
     4.1 Configuration 
     4.2 Effect 
       [0029]    5. Examples of Contact between Optical Element and Holder
 
5.1 Optical Element in Direct Contact with Holder
 
5.2 Metal Interposed between Optical Element and Holder
 
       5.3 Optical Element Soldered to Holder 
     6. Variations of Structure of Optical Element Holder 
     7. Focusing Lens as Optical Element 
     8. Types of Contact 
     8.1 Surface Contact 
     8.2 Point Contact 
     8.3 Line Contact 
     8.4 Contact at Multiple Surfaces 
     1. Overview 
       [0030]    The embodiments to be described hereinafter pertain to a holder device configured to hold a transmissive optical element. 
       2. Overview of EUV Light Generation System 
     2.1 Configuration 
       [0031]      FIG. 1  schematically illustrates a configuration of an exemplary Laser Produced Plasma (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  is 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. The target supply unit may be a droplet generator  26 . The chamber  2  may be airtightly sealed. The droplet generator  26  may be mounted onto the chamber  2  to, for example, penetrate a wall of the chamber  2 . A target material to be supplied by the droplet generator may include, but is not limited to, tin, terbium, gadolinium, lithium, xenon, or any combination thereof. 
         [0032]    The chamber  2  may have at least one through-hole or opening formed in its wall, and a pulse laser beam  31  may travel through the through-hole/opening into the chamber  2 . Alternatively, the chamber  2  may have a window  21 , through which the pulse laser beam  31  may travel into the chamber  2 . For example, an EUV collector mirror  23  having a spheroidal surface is provided inside the chamber  2 . 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 may 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 the pulse laser beam  31  may travel through the through-hole  24  toward the plasma generation region  25 . 
         [0033]    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 . 
         [0034]    Further, the EUV light generation system  11  may include a connection part  29  for allowing the interior of the chamber  2  to be in communication with the interior of the exposure apparatus  6 . 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 . 
         [0035]    The EUV light generation system  11  may also include a beam delivery unit  34 , a laser beam focusing mirror  22 , and a target collector  28  for collecting targets  27 . The beam delivery unit  34  may include an optical element (not separately shown) for defining the direction into which the pulse laser beam  31  travels and an actuator (not separately shown) for adjusting the position and the orientation (posture) of the optical element. 
       2.2 Operation 
       [0036]    With continued reference to  FIG. 1 , the pulse laser beam  31  outputted from the laser apparatus  3  may pass through the beam delivery unit  34  and be outputted therefrom after having its direction optionally adjusted. The pulse laser beam  31  may travel through the window  21  and enter the chamber  2 . The pulse laser beam  31  may travel inside the chamber  2  along at least one beam path from the laser apparatus  3 , be reflected by a laser beam focusing mirror  22 , and strike at least one target  27 . 
         [0037]    The target supply unit, e.g., the droplet generator  26 , may be configured to output the target(s)  27  toward the plasma generation region  25  inside the chamber  2 . The target  27  may be irradiated with at least one pulse of the pulse laser beam  31 . Upon being irradiated with the pulse laser beam  31 , 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 . EUV light  252 , which is the 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 with multiple pulses included in the pulse laser beam  31 . 
         [0038]    The EUV light generation controller  5  may be configured to 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 apparatus  3  oscillates, the direction in which the pulse laser beam  31  travels, and the position at which the pulse laser beam  31  is focused. It will be appreciated that the various controls mentioned above are merely examples, and other controls may be added as necessary. 
         [0039]    In the above-described EUV light generation system  11 , the pulse laser beam  31  to enter the chamber  2  may be a high-power laser beam of, for example, 10 kW or higher. When the pulse laser beam  31  is a high-power laser beam, optical performance of an optical element, such as the window  21 , through which the pulse laser beam  31  passes, may be reduced due to a thermal stress. Further, possible thermal expansion of the optical element may break the airtight seal between the chamber  2  and the optical element, such as window  21 . Accordingly, this disclosure proposes, but is not limited to, the following embodiments. 
       3. EUV Chamber Including Transmissive Optical Element 
     3.1 Configuration 
       [0040]      FIG. 2  schematically illustrates an exemplary configuration of an EUV light generation system according to a first embodiment of this disclosure. As shown in  FIG. 2 , an EUV light generation system  11 A may include an EUV light generation apparatus  1 A and the laser apparatus  3 . The EUV light generation apparatus  1 A may include the beam delivery unit  34 , a chamber  2 A, the connection part  29 , a cooling water circulator  130 , and an EUV light generation controller  5 A. The EUV light generation controller  5 A may include an EUV light generation control device  51  and a target controller  52 . 
         [0041]    The beam delivery unit  34  may include a plurality of optical elements, such as high-reflection mirrors  341  and  342 . The pulse laser beam  31  outputted from the laser apparatus  3  may be guided to the chamber  2 A by the beam delivery unit  34 . 
         [0042]    The chamber  2 A may be divided into two spaces  2   a  and  2   b  by a partition  80 . The space  2   b  may be located upstream from the space  2   a  in a beam path of the pulse laser beam  31 . A through-hole  81  may be formed in the partition  80 . 
         [0043]    As shown in  FIG. 2 , a holder  100  may be provided, and the holder  100  may include a first member  110  and a second member  120 . The holder  100  may be mounted to the outer wall of the chamber  2 A. The cooling water circulator  130  may be connected to the holder  100  through a pipe  135 . The interior of the chamber  2 A may be kept at a pressure that is lower than a pressure outside the chamber  2 A. The window  21  may be held by the holder  100  and placed between the first member  110  and the second member  120 . The holder  100  may be fixed to the outer wall of the chamber  2 A such that the window  21  covers an opening  201  (see  FIG. 3 ) formed in the chamber  2 A. The chamber  2 A and the first member  110  may be airtightly connected to each other. In addition, the first member  110  and the second member  120  may be airtightly connected to each other with the window  21  provided therebetween. Details of the holder  100  will be given later. 
         [0044]    A laser beam focusing optical system  22 A may be provided in the space  2   a . The laser beam focusing optical system  22 A may include a laser beam focusing mirror  224  and a high-reflection mirror  221 . The laser beam focusing optical system  22 A may further include a moving plate  225 , a plate moving mechanism  226 , and mirror holders  222  and  223 . The mirror holder  222  may be provided with an automatic tilt mechanism (not separately shown). The laser beam focusing mirror  224  may be an off-axis paraboloidal mirror. The laser beam focusing mirror  224  may be fixed to the moving plate  225  through the mirror holder  223 . The high-reflection mirror  221  may be fixed to the moving plate  225  through the mirror holder  222 . The plate moving mechanism  226  may be configured to move the laser beam focusing mirror  224  and the high-reflection mirror  221  along with the moving plate  225 . 
         [0045]    The EUV collector mirror  23 , the droplet generator  26 , the target collector  28 , the target sensor  4 , and a beam dump  82  may be provided in the space  2   a . The EUV collector mirror  23  may be fixed to the partition  80  through a mirror holder  23   a . The through-hole  24  in the EUV collector mirror  23  may be aligned with the through-hole  81  in the partition  80 . The beam dump  82  may be provided in a beam path of the pulse laser beam  31  downstream from the plasma generation region  25  to absorb the pulse laser beam  31 . The beam dump  82  may be fixed to the inner wall of the chamber  2 A through a support member  83 . 
       4. Basic Structure of Optical Element Holder 
     4.1 Configuration 
       [0046]      FIG. 3  is an exploded view of the holder  100  shown in  FIG. 2 .  FIG. 4  shows an exemplary configuration of the first member  110  shown in  FIG. 3 , as viewed toward a surface at which the first member comes into contact with the second member  120 .  FIG. 5  shows an exemplary configuration of the second member  120  shown in  FIG. 3 , as viewed toward a surface at which the second member comes into contact with the first member. 
         [0047]    As shown in  FIG. 3 , the holder  100  may include the first member  110 , the second member  120 , gaskets  134 ,  143 , and  144 , and bolts  101 . When the window  21  is formed of diamond, each of the first member  110  and the second member  120  may be formed of aluminum, copper, stainless steel, silicon carbide (SiC), aluminum nitride (AlN), or the like. However, this disclosure is not limited to these materials, and various other suitable materials may be used. Here, a material that has a sufficient strength, excels in thermal resistance, and has high heat conductivity may be used. 
         [0048]    As shown in  FIGS. 3 and 4 , the first member  110  may be disc-shaped and have first and second flat surfaces. The first and second surfaces may be parallel to each other or may be inclined with respect to each other. A circular through-hole  111  may be formed at substantially the center of the first member  110 , and thus, the first and second surfaces of the first member  110  may be substantially annular in shape. 
         [0049]    A flow channel  112  may be formed inside the first member  110 . The flow channel  112  may be substantially annular in shape to follow along the inner circumferential surface of the first member  110 . The cross-sectional shape of the flow channel  112  may be circular, elliptical, or polygonal. The flow channel  112  may be discontinuous, and an end at the discontinuous portion may be connected to a flow channel  113 . The flow channel  113  may open into the outer circumferential surface of the first member  110 . The flow channel  113  may then be connected to the pipe  135  through a joint  131  (see  FIG. 2 ). When multiple flow channels  113  are provided as shown in  FIGS. 3 and 4 , one of the flow channels  113  may serve as a flow-in channel through which cooling water from the cooling water circulator  130  flows into the flow channel  112 , and another flow channel  113  may serve as a flow-out channel through which the cooling water that has circulated in the follow channel  112  flows into the pipe  135 . 
         [0050]    The joint  131  may include a through-hole  132  and a gasket  133 . The joint  131  may be attached to the first member  110  using bolts (not separately shown). Alternatively, the joint  131  may include threads (not separately shown), and the threads may be inserted into respective threaded holes (not separately shown) formed in the first member  110 , to thereby be fixed to the first member  110 . The gasket  133  may airtightly seal between the joint  131  and the first member  110  when the joint  131  is attached to the first member  110 . The through-hole  132  may be in communication with the flow channel  113  when the joint  131  is attached to the first member  110 . 
         [0051]    Further, threaded holes  115  may be formed in the first surface of the first member  110 , and bolts  101  may be inserted into the respective threaded holes  115  to fix the second member  120  to the first member  110 . The threaded holes  115  may not penetrate the first member  110 . Further, the first member  110  may have through-holes  116  formed therein, into which bolts  101  are inserted to fix the first member  100  to the chamber  2 A. The bolts  101  may be inserted into the respective through-holes  116  with respective washers (not separately shown) provided therebetween. The bolts  101  inserted into the respective through-holes  116  may then be inserted into respective threaded holes  206  formed in the outer wall of the chamber  2 A. The through-hole  111  in the first member  110  may be aligned with the opening  201  in the chamber  2 A. 
         [0052]    A groove  114  may be formed in the second surface of the first member  110 , and the gasket  134  may be fitted into the groove  114 . The groove  114  may be deep enough for at least part of the gasket  134  to protrude from the groove  114 . The gasket  134  may be elastically deformable to come into close contact with the outer surface of the chamber  2 A and with the bottom of the groove  114  when the first member  110  is fixed to the outer wall of the chamber  2 A with the bolts  101 . Hence, the first member  110  and the chamber  2 A may be airtightly connected to each other. For example, gasket  134  is an O-ring. The gasket  134  may be formed of fluorine rubber or metal, such as copper. However, this disclosure is not limited to these materials, and various other suitable materials may be used to form the gasket  134 . 
         [0053]    As shown in  FIGS. 3 and 5 , the second member  120  may be disc-shaped and have first and second flat surfaces. The first and second flat surfaces may be parallel to each other. A circular through-hole  121  may be provided at substantially the center of the second member  120 , and each of the first and second surfaces of the second member  120  may be substantially annular in shape. 
         [0054]    A groove  122  may be formed in the second surface of the second member  120 , and the window  21  may be fitted into the groove  122 . The groove  122  may be formed to be concentric with the through-hole  121 . The depth of the groove  122  may be substantially the same as the thickness of the window  21 . A groove  123  may be formed in the bottom surface of the groove  122 . The gasket  143  may be fitted into the groove  123 . The groove  123  may be deep enough for at least part of the gasket  143  to protrude from the groove  123 . 
         [0055]    Further, through-holes  125  may be formed in the second member  120 , and the bolts  101  may be inserted into the respective through-holes  125  to fix the second member  120  to the first member  110 . The bolts  101  may be inserted into the respective through-holes  125  with respective washers (not separately shown) provided therebetween. The bolts  101  inserted into the respective through-holes  125  may then be inserted into the respective threaded holes  115  formed in the first surface of the first member  110 . Here, the through-hole  121  in the second member  120  may be aligned with the through-hole  111  in the first member  110 . 
         [0056]    A groove  124  may be formed in the second surface of the second member  120 , and the gasket  144  may be fitted into the groove  124 . The groove  124  may be deep enough for at least part of the gasket  144  to protrude from the groove  124 . The gasket  144  may be elastically deformable to come into close contact with the first surface of the first member  110  and with the bottom of the groove  124  when the second member  120  is fixed to the first surface of the first member  110  with the bolts  101 . Hence, the first member  110  and the second member  120  may be airtightly connected to each other. Here, the gasket  143  fitted in the groove  123  may also be elastically deformable to come into close contact with a surface of the window  21  and with the bottom of the groove  123 . Hence, the window  21  and the second member  120  may be airtightly connected to each other. Each of the gaskets  143  and  144  may be an O-ring. Each of the gaskets  143  and  144  may be formed of fluorine rubber or metal, such as copper. However, this disclosure is not limited to these materials, and various other suitable materials may be used to form the gaskets  143  and  144 . 
         [0057]    The window  21  may be provided to be in contact with the first surface of the first member  110  when the second member  120  is fixed to the first surface of the first member  110  with the bolts  101 . A shape of mating surfaces between the window  21  and the first surface of the first member  110  may be symmetrical about the axis of the through-hole  111  and/or the window  21 . Here, the window  21  and the first member  110  may or may not be connected airtightly to each other. 
         [0058]    When the EUV light generation system  11 A is in operation, the interior of the chamber  2 A may be kept at a pressure that is lower than that of the exterior of the chamber  2 A. Under the aforementioned state, the holder  100  may be attached to the chamber  2 A such that a first surface of the window  21  faces the exterior of the chamber  2 A and a second surface of the window  21  faces the interior of the chamber  2 A. The gasket  143  to airtightly connect the window  21  to the second member  120  may be provided toward the first surface of the window  21 . The second surface of the window  21  may be in contact with a member, such as the first member  110  in this example, having relatively high thermal conductivity, which will be later described with specific examples. The cooling water may cool the first member  110  which is in contact with the second surface of the window  21 . Here, an area between at which the first member  110  and the second surface of the window  21  are in contact with each other may preferably be large. 
       4.2 Effect 
       [0059]    As described above, since the first member  110  and the window  21  are in contact through a structure leading to relatively high thermal conductivity, the window  21  heated by absorbing apart of the pulse laser beam  31  passing therethrough may be cooled effectively. Accordingly, reduction in optical performance of the window  21  may be suppressed. 
         [0060]    Further, while the EUV light generation system  11 A is in operation, a pressure difference between the interior and the exterior of the chamber  2 A may help the window  21  stay in close contact with the first member  110 . Accordingly, the window  21  may be cooled efficiently. At that point, since the elastic gasket  143  may create an airtight seal between the first surface of the window  21  and the second member  120 , the chamber  2 A may be kept airtight. 
         [0000]    5. Examples of Contact between Optical Element and Holder
 
5.1 Optical Element in Direct Contact with Holder
 
         [0061]    A case where the second surface of the window  21  is in direct contact with the holder  100  will be described with reference to  FIG. 6 . 
         [0062]    As shown in  FIG. 6 , there is a direct contact portion  211  where the second surface of the window  21  may be in direct contact with the first surface of the first member  110 . The direct contact portion  211  may or may not airtightly seal between the first member  110  and the window  21 . When the first member  110  and the window  21  are in direct contact, heat in the window  21  may efficiently be conducted to the first member  110 . Accordingly, the window  21  may be cooled efficiently. 
         [0063]    Here, airtightness between the window  21  and the holder  100  may be retained by the gaskets  134 ,  143 , and  144 . That is, the gasket  143  may create an airtight seal between the second member  120  and the window  21 , the gasket  144  may create an airtight seal between the second member  120  and the first member  110 , and the gasket  134  may create an airtight seal between the first member  110  and the chamber  2 A. As a result, even when the window  21  is biased against the first member  110  due to, for example, a pressure difference, the deformable gasket  143  may retain the airtight seal between the window  21  and the second member  120 . As a result, the chamber  2 A may be kept airtight. 
         [0000]    5.2 Metal Interposed between Optical Element and Holder 
         [0064]    A case where a material having high thermal conductivity is provided between the second surface of the window  21  and the holder  100  will now be described with reference to  FIG. 7 . 
         [0065]    As shown in  FIG. 7 , a contact member  212  may be provided between the second surface of the window  21  and the first surface of the first member  110 . The contact member  212  may be in direct contact with the second surface of the window  21  and with the first surface of the first member  110 . The contact member  212  and the window  21 , and the contact member  212  and the first member  110 , respectively, may or may not be connected airtightly to each other. 
         [0066]    The contact member  212  may be formed of a material having relatively high thermal conductivity, such as metal. Accordingly, heat in the window  21  may be efficiently conducted to the first member  110 . As a result, the window  21  may be cooled efficiently. 
         [0067]    Further, the contact member  212  may be formed of a relatively soft material, such as gold (Au), indium (In), and tin (Sn). Then, an area at which the contact member  212  and the window  21  are in contact with each other and an area at which the contact member  212  and the first member  110  are in contact with each other may be increased. As a result, thermal conduction efficiency from the window  21  to the first member  110  may be increased, and the window  21  may be cooled efficiently. Further, airtightness between the contact member  212  and the window  21  and airtightness between the contact member  212  and the first member  110  may be enhanced. 
       5.3 Optical Element Soldered to Holder 
       [0068]    A case where the second surface of the window  21  is soldered to the holder  100  will now be described with reference to  FIG. 8 . 
         [0069]    As shown in  FIG. 8 , a soldered portion  213  may be formed between the second surface of the window  21  and the first surface of the first member  110 . A material used for soldering may, for example, include solder or silver solder. Then, an area at which the window  21  is in contact with the holder  100  may be increased compared to a case where the second surface of the window  21  and the holder  100  are in direct contact. As a result, thermal conduction efficiency from the window  21  to the first member  110  may be increased, and the window  21  may be cooled efficiently. Further, airtightness between the window  21  and the first member  110  may be enhanced. 
       6. Variations of Structure of Optical Element Holder 
       [0070]    In the above-described embodiment, the holder  100  may be attached on the outer wall of the chamber  2 A. However, this disclosure is not limited to this configuration, and the holder  100  may be attached on the inner surface of the chamber  2 A.  FIG. 9  is a sectional view schematically illustrating a configuration of a holder attached on an inner wall of a chamber according to a second embodiment of this disclosure.  FIG. 10  shows an exemplary configuration of a first member shown in  FIG. 9 , as viewed toward a surface at which the first member comes into contact with a second member. In the second embodiment, the second member  120  may, for example, be configured similarly to the second member  120  shown in  FIG. 5 . 
         [0071]    As shown in  FIGS. 9 and 10 , a first member  110 A may be fixed at a first surface thereof to the inner wall of the chamber  2 A using the bolts  101 . A space between the inner wall of the chamber  2 A and the first member  110 A may be airtightly sealed by the gasket  134  fitted into a groove  114   a.    
         [0072]    The first member  110 A may have a groove  117  formed in the first surface thereof to accommodate the second member  120 . The diameter of the groove  117  may be either smaller or larger than the diameter of the opening  201  in the chamber  2 A. When the diameter of the groove  117  is larger than the diameter of the opening  201 , the depth of the groove  117  may be greater than the thickness of the second member  120 . The second member  120  may be fixed on the bottom of the groove  117  using the bolts  101 , as in the holder  100  described above. 
         [0073]    A flow channel  113 A to connect the flow channel  112  in the first member  110 A to the joint  131  may include a first channel  113   a  extending from an end of the flow channel  112  and a second channel  113   b  extending from the first channel  113   a  and having an opening in the surface of the first member  110 A. The joint  131  may be attached to the first member  110 A at the opening of the second channel  113   b  so that the second channel  113   b  is in communication with the through-hole  132 . A through-hole may be formed in the chamber  2 A, and the joint  131  may be fitted into that through-hole. 
         [0074]    The groove  114   a , into which the gasket  134  is to be fitted, may be provided to surround the groove  117  and the connection part between the joint  131  and the second channel  113   b . This configuration may create an airtight seal between the first member  110 A and the chamber  2 A. Further, even if cooling water leaks from the connection part between the second channel  113   b  and the joint  131 , the cooling water may be prevented from flowing into the chamber  2 A by the gasket  134 . Here, the groove  117  may be provided in the first surface of the first member  110 A or in the inner wall of the chamber  2 A. 
       7. Focusing Lens as Optical Element 
       [0075]    The above-described embodiments may be applied when the window  21  is formed of a parallel plate or a wedge substrate. However, this disclosure is not limited thereto. For example, a focusing lens having at least one convex surface may be used in place of the window  21 . 
         [0076]      FIG. 11  is a sectional view schematically illustrating an exemplary configuration of a holder where a focusing lens is used as an optical element according to a third embodiment. As shown in  FIG. 11 , a holder  100 A may include a second member  120 A in place of the second member  120 . The holder  100 A may be configured to hold a focusing lens  21 B. The first member  110  may be similar to the first member  110  of the holder  100 . The holder  100 A may be fixed to a deformable flange  241  provided on the chamber  2 A. 
         [0077]      FIG. 12  is a perspective view schematically illustrating an exemplary configuration of the focusing lens shown in  FIG. 11 . As shown in  FIG. 12 , the focusing lens  21 B may have at least one convex surface  21   a . The focusing lens  21 B may, for example, be formed of diamond. 
         [0078]      FIG. 13  is a perspective view schematically illustrating an exemplary configuration of the second member shown in  FIG. 11 . As shown in  FIGS. 11 and 13 , the bottom of the groove  122  formed in the second member  120 A may be curved to follow the curved surface of the convex surface  21   a  of the focusing lens  21 B. The gasket  134  may be fitted into the groove  124  formed in the bottom of the groove  122   a  to create an airtight seal between the second member  120 A and the focusing lens  21 B. 
         [0079]    Further, as shown in  FIG. 11 , the focusing lens  21 B and the first member  110  may be in contact with each other in any of the contact modes described with reference to  FIGS. 6 through 8 . The first member  110  may be fixed on a ring-shaped substrate  242  provided at a first end of the deformable flange  24  through a method similar to that between the first member  110  and the chamber  2 A. 
         [0080]    The deformable flange  241  may be a bellows. A second of the deformable flange  241  may be fixed to the chamber  2 A to surround the opening  201  formed in the chamber  2 A. The deformable flange  241  may be deformed by a driving mechanism (not separately shown) to displace the ring-shaped substrate  242  three-dimensionally. 
         [0081]    With the above-described configuration, even when the focusing lens  21 B is used in place of the window  21 , a similar effect to that of the above-described embodiments may be obtained. 
       8. Types of Contact 
       [0082]    Types of contact between the window  21  or the focusing lens  21 B and the first member  110  or  110 A or the contact member  212  will now be described with examples. In the description to follow, a case where the window  21  and the first member  110  are in direct contact with each other will be illustrated, but this disclosure is not limited thereto, and other combinations may be configured similarly. 
       8.1 Surface Contact 
       [0083]    A case where the window  21  and the first member  110  are in surface contact will be described first with reference to FIG.  14 . As shown in  FIG. 14 , the window  21  and the first member  110  may form a surface contact  231 . In that case, at least a portion of the second surface of the window  21  which comes into contact with the first member  110  may have relatively high flatness. The flatness of this portion may, for example, be enhanced by grinding. Similarly, at least a portion of the first surface of the first member  110  which comes into contact with the window  21  may have relatively high flatness, and the flatness of this portion may, for example, be enhanced by grinding. 
         [0084]    By forming the surface contact  231  at mating surfaces of the window  21  and the first member  110 , an area at which the window  21  and the first surface  110  are in contact with each other may be increased. Accordingly, the window  21  may be cooled efficiently. 
         [0085]    A shape of the surface contact  231  between the window  21  and the first member  110  may be symmetric about the axis of the window  21  and/or the through-hole  111 . Since the contact surface  231  is symmetric, the window  21  may be cooled uniformly from the periphery thereof. As a result, optical performance of the window  21  may be stabilized. Here, the window  21  may be positioned such that the beam axis of the pulse laser beam  31  passing through the window  21  substantially coincides with the axis of the window  21 . Then, the window  21  may be cooled more uniformly, and optical performance thereof may further be stabilized. 
       8.2 Point Contact 
       [0086]    A case where the window  21  and the first member  110  are in point contact at multiple points will now be described with reference to  FIG. 15 . As shown in  FIG. 15 , the window  21  and the first member  110  may form a point contact  232  at multiple points. In that case, multiple protrusions may be formed on a portion of a surface of at least one of the window  21  and the first member  110 . The protrusions may, for example, be formed through sandblast. 
         [0087]    The protrusions may be distributed symmetrically about the axis of the window  21  and/or the through-hole  111 . Since the protrusions are distributed symmetrically, the window  21  may be cooled uniformly from the periphery thereof. As a result, optical performance of the window  21  may be stabilized. Here, the window  21  may be positioned such that the beam axis of the pulse laser beam  31  passing through the window  21  substantially coincides with the axis of the window  21 . Then, the window  21  may be cooled more uniformly, and optical performance thereof may further be stabilized. 
       8.3 Line Contact 
       [0088]    A case where the window  21  and the first member  110  are in line contact at multiple lines will now be described with reference to  FIG. 16 . As shown in  FIG. 16 , the window  21  and the first member  110  may form a line contact  233  at multiple lines that are concentric. In that case, multiple circular protrusions may be formed concentrically on a portion of a surface of at least one of the window  21  and the first member  110 . 
         [0089]    The protrusions may be formed symmetrically about the axis of the window  21  and/or the through-hole  111 . Since the protrusions are formed symmetrically, the window  21  may be cooled uniformly from the periphery thereof. As a result, optical performance of the window  21  may be stabilized. Here, the window  21  may be positioned such that the beam axis of the pulse laser beam  31  passing through the window  21  substantially coincides with the axis of the window  21 . Then, the window  21  may be cooled more uniformly, and optical performance thereof may further be stabilized. 
       8.4 Contact at Multiple Surfaces 
       [0090]    A case where the window  21  and the first member  110  are in surface contact at multiple discontinuous surfaces will now be described with reference to  FIG. 17 . As shown in  FIG. 17 , the window  21  and the first member  110  may form a surface contact  234  at multiple discontinuous mating surfaces. In that case, island-like protrusions (surface contacts  234 ) may be formed in a portion of a surface of at least one of the window  21  and the first member  110 . 
         [0091]    The upper surface of each of the protrusions may have relatively high flatness. Further, the upper surfaces of the multiple protrusions may be flush with each other. The flatness of the upper surfaces may, for example, be enhanced by grinding. Further, a portion of the surface of one of the window  21  and the first member  100  where the protrusions are not formed may have relatively high flatness, and the flatness of this portion may, for example, be enhanced by grinding. 
         [0092]    The island-like protrusions may be distributed symmetrically about the axis of the window  21  and/or the through-hole  111 . Since the protrusions are distributed symmetrically, the window  21  may be cooled uniformly from the periphery thereof. As a result, optical performance of the window  21  may be stabilized. Here, the window  21  may be positioned such that the beam axis of the pulse laser beam  31  passing through the window  21  substantially coincides with the axis of the window  21 . Then, the window  21  may be cooled more uniformly, and optical performance thereof may further be stabilized. 
         [0093]    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). 
         [0094]    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.”