Patent Publication Number: US-9894743-B2

Title: Extreme ultraviolet light generation apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/809,582, filed on Jan. 10, 2013, which claims priority from Japanese Patent Application No. 2011-076519 filed Mar. 30, 2011, Japanese Patent Application No. 2011-192971 filed Sep. 5, 2011, and Japanese Patent Application No. 2012-001052 filed Jan. 6, 2012. 
    
    
     BACKGROUND 
     1. Technical Field 
     This disclosure relates to an extreme ultraviolet (EUV) light generation apparatus. 
     2. Related Art 
     In recent years, semiconductor production processes have become capable of producing semiconductor devices with increasingly fine feature sizes, as photolithography has been making rapid progress toward finer fabrication. In the next generation of semiconductor production processes, microfabrication with feature sizes at 60 nm to 45 nm, and further, microfabrication with feature sizes of 32 nm or less will be required. In order to meet the demand for microfabrication with feature sizes of 32 nm or less, for example, an exposure apparatus 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. 
     Three kinds of systems for generating EUV light are known in general, which include a Laser Produced Plasma (LPP) type system in which plasma is generated by irradiating a target material with a laser beam, a Discharge Produced Plasma (DPP) type system in which plasma is generated by electric discharge, and a Synchrotron Radiation (SR) type system in which orbital radiation is used. 
     SUMMARY 
     An apparatus for generating extreme ultraviolet light according to one aspect of this disclosure may include: a chamber having an opening through which a laser beam is introduced into the chamber; a reference member on which the chamber is mounted; a target supply unit for supplying the target material to be irradiated by the laser beam to a predetermined region inside the chamber; a laser beam focusing optical system for focusing the laser beam in the predetermined region inside the chamber to turn the target material into plasma; and a collector mirror for collecting the extreme ultraviolet light emitted from the plasma. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Hereinafter, selected embodiments of this disclosure will be described with reference to the accompanying drawings. 
         FIG. 1  schematically illustrates the configuration of an exemplary LPP type EUV light generation system. 
         FIG. 2A  is a plan view illustrating an EUV light generation apparatus according to a first embodiment, the EUV light generation apparatus being connected to an exposure apparatus. 
         FIG. 2B  is a sectional view of the EUV light generation apparatus and the exposure apparatus shown in  FIG. 2A , taken along IIB-IIB plane. 
         FIG. 3A  is a plan view illustrating an EUV light generation apparatus according to a second embodiment. 
         FIG. 3B  is a sectional view illustrating the EUV light generation apparatus according to the second embodiment. 
         FIG. 4A  is a plan view illustrating an EUV light generation apparatus according to a third embodiment. 
         FIG. 4B  is a side view illustrating the EUV light generation apparatus according to the third embodiment. 
         FIG. 4C  is a sectional view of the EUV light generation apparatus shown in  FIG. 4A , taken along IVC-IVC plane. 
         FIG. 4D  is a plan view illustrating the EUV light generation apparatus according to the third embodiment in a state in which a reference member is moved away from an exposure apparatus. 
         FIG. 4E  is a side view illustrating the EUV light generation apparatus according to the third embodiment in a state in which the reference member is moved away from an exposure apparatus. 
         FIG. 5A  is a plan view illustrating an EUV light generation apparatus according to a fourth embodiment, which includes a laser beam path control unit for introducing a laser beam into the EUV light generation apparatus. 
         FIG. 5B  is a sectional view of the EUV light generation apparatus and the laser beam path control unit shown in  FIG. 5A . 
         FIG. 6  is a sectional view of an EUV light generation apparatus according to a fifth embodiment, in which a pre-pulse laser beam is used. 
         FIG. 7A  is a plan view illustrating an EUV light generation apparatus according to a sixth embodiment, which includes a laser beam path control unit for introducing a pre-pulse laser beam and a main pulse laser beam into the EUV light generation apparatus. 
         FIG. 7B  is a sectional view of the EUV light generation apparatus and the laser beam path control unit shown in  FIG. 7A . 
         FIG. 8A  is a plan view illustrating an EUV light generation apparatus according to a seventh embodiment, which includes a laser beam path control unit for introducing a pre-pulse laser beam and a main pulse laser beam into the EUV light generation apparatus. 
         FIG. 8B  is a sectional view of the EUV light generation apparatus and the laser beam path control unit shown in  FIG. 8A . 
         FIG. 9  is a diagram showing an configuration example of a laser beam measuring device. 
         FIG. 10  is a diagram for discussing one example of the operation of a laser beam travel direction adjusting mechanism. 
         FIG. 11  is a diagram for discussing another example of the operation of a laser beam travel direction adjusting mechanism. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     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. Overview of EUV Light Generation System 
     2.1 Configuration 
     2.2 Operation 
     3. EUV Light Generation Apparatus and Exposure Apparatus: First Embodiment 
     3.1 Configuration 
     3.2 Operation 
     3.3 Effect 
     4. EUV Light Generation Apparatus: Second Embodiment 
     5. EUV Light Generation Apparatus: Third Embodiment 
     6. Laser Beam Path Control Unit for Introducing Laser Beam into EUV Light Generation Apparatus: Fourth Embodiment 
     7. EUV Light Generation Apparatus in which Pre-Pulse Laser Beam is Used: Fifth Embodiment 
     8. Laser Beam Path Control Unit for Introducing Two Laser Beams into EUV Light Generation Apparatus: Sixth Embodiment 
     9. Laser Beam Path Control Unit for Introducing Two Laser Beams into EUV Light Generation Apparatus: Seventh Embodiment 
     10. Supplementary Descriptions 
     10.1 Laser Beam Measuring Device 
     10.2 Laser Beam Travel Direction Adjusting Mechanism 
     1. Overview 
     In an LPP type EUV light generation apparatus, a target material may be supplied to a plasma generation region defined inside a chamber, and the target material may be irradiated by a laser beam. Upon being irradiated by the laser beam, the target material may be turned into plasma, and EUV light may be emitted from the plasma. The emitted EUV light may be collected by an EUV collector mirror provided inside the chamber, and outputted to an external apparatus, such as an exposure apparatus. However, when the relative position of the EUV collector mirror and a laser beam focusing optical system changes, the position where the EUV light is focused before being outputted to the exposure apparatus may also change. 
     Accordingly, in one or more of the embodiments of this disclosure, at least the EUV collector mirror may be fixed to a reference member on which the chamber is mounted, in an LPP type EUV light generation apparatus. Further, in one or more of the embodiments of this disclosure, the laser beam focusing optical system for focusing a laser beam in the plasma generation region may be fixed to the reference member. Furthermore, in one or more of the embodiments of this disclosure, a laser beam introduction optical system for introducing a laser beam into the chamber may be fixed to the reference member, to which the laser beam focusing optical system is also fixed. Further, in one or more of the embodiments of this disclosure, a laser beam measuring device for measuring a laser beam guided to the laser beam introduction optical system may be fixed to the reference member. As a result of the above configuration(s), change in the relative position of the respective optical elements may be suppressed. 
     2. Overview of EUV Light Generation System 
     2.1 Configuration 
       FIG. 1  schematically illustrates the configuration of an exemplary LPP type EUV light generation system. An LPP type EUV light generation apparatus may be used with at least one laser apparatus  3 . Hereinafter, a system that includes the EUV light generation apparatus  1  and the laser apparatus  3  may be referred to as an EUV light generation system  11 . As illustrated in  FIG. 1  and described in detail below, the EUV light generation system  11  may include a chamber  2 , a target supply unit  26  and so forth. The chamber  2  may be airtightly sealed. The target supply unit  26  may be mounted to the chamber  2  so as to, for example, penetrate a wall of the chamber  2 . A target material  27  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. 
     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 disposed 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 disposed 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 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 . 
     Further, the EUV light generation system  11  may include a connection part  29  for allowing the interior of the chamber  2  and the interior of the exposure apparatus  6  to be in communication with each other. A wall  291  having an aperture may be provided inside the connection part  29 , and the wall  291  may be positioned such that the second focus of the EUV collector mirror  23  lies in the aperture formed in the wall  291 . 
     The EUV light generation system  11  may also include a laser beam direction control unit  34 , a laser beam focusing mirror  22 , and a target collector  28  for collecting targets  27 . The laser beam direction control unit  34  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. 
     2.2 Operation 
     With continued reference to  FIG. 1 , a pulse laser beam  31  outputted from the laser apparatus  3  may pass through the laser beam direction control unit  34  and be outputted therefrom as a pulse laser beam  32  after having its direction optionally adjusted. The pulse laser beam  32  may travel through the window  21  and enter the chamber  2 . The pulse laser beam  32  may travel inside the chamber  2  along at least one beam path from the laser apparatus  3 , be reflected by the laser beam focusing mirror  22 , and strike at least one target  27  as a pulse laser beam  33 . 
     The target supply unit  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 by at least one pulse of the pulse laser beam  33 . Upon being irradiated by the pulse laser beam  33 , the target  27  may be turned into plasma, and rays of light including EUV light  251  may be emitted from the plasma. The EUV light  251  may be reflected selectively by the EUV collector mirror  23 . EUV light  252  reflected by the EUV collector mirror  23  may travel through the intermediate focus region  292  and be outputted to the exposure apparatus  6 . The target  27  may be irradiated by multiple pulses included in the pulse laser beam  33 . 
     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 (e.g., the timing at which and/or direction in which the target  27  is outputted from target supply unit  26 ). 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 (e.g., by controlling the laser apparatus  3 ), the direction in which the pulse laser beam  31  travels (e.g., by controlling the laser beam direction control unit  34 ), and the position at which the pulse laser beam  33  is focused (e.g., by controlling the laser apparatus  3 , the laser beam direction control unit  34 , or the like). It will be appreciated that the various controls mentioned above are merely examples, and other controls may be added as necessary. 
     3. EUV Light Generation Apparatus and Exposure Apparatus: First Embodiment 
     3.1 Configuration 
       FIG. 2A  is a plan view illustrating an EUV light generation apparatus according to a first embodiment, the EUV light generation apparatus being connected to an exposure apparatus.  FIG. 2B  is a sectional view of the EUV light generation apparatus and the exposure apparatus shown in  FIG. 2A , taken along IIB-IIB plane. 
     As shown in  FIGS. 2A and 2B , the EUV light generation apparatus  1  may include a moving mechanism  7 , a positioning mechanism  8 , a reference member  9 , a laser beam introduction optical system  35 , a laser beam focusing optical system  36 , a laser beam measuring device  37 , and the chamber  2 . A floor surface shown in  FIG. 2B  may serve as a reference plane, on which the EUV light generation apparatus  1  and the exposure apparatus  6  may be installed. The reference member  9  may be supported by the moving mechanism  7  installed on the floor surface serving as the reference plane. Primary components of the EUV light generation apparatus  1  may be movable along with the moving mechanism  7  with respect to the exposure apparatus  6 . The configuration and the operation of the moving mechanism  7  will be described later in detail. The reference member  9  may be positioned with respect to the exposure apparatus  6  and/or the moving mechanism  7  by the positioning mechanism  8 , and the EUV light generation apparatus  1  may be connected to the exposure apparatus  6  accordingly. 
     As shown in  FIG. 2B , the chamber  2  may have an opening  2   a  through which a laser beam is introduced into the chamber  2 . The chamber  2  may be mounted onto the reference member  9  such that the opening  2   a  is covered by the reference member  9 . For example, a slanted surface may be formed on the reference member  9 , and the chamber  2  may be mounted onto the slanted surface of the reference member  9 . The target supply unit  26  (see  FIG. 1 ) may be mounted to the chamber  2 . 
     The EUV collector mirror  23  may be provided inside the chamber  2 . The EUV collector mirror  23  may be attached to slanted surface of the reference member  9  through an EUV collector mirror mount  23   a . The laser beam introduction optical system  35 , the laser beam focusing optical system  36 , and the laser beam measuring device  37  may also be fixed to the reference member  9 . 
     The reference member  9  may include a storing chamber  9   a , which is in communication with the chamber  2  through the opening  2   a , formed in the chamber  2 , and a storing chamber  9   b  adjacent to the storing chamber  9   a . The laser beam focusing optical system  36  may be housed in the storing chamber  9   a , and the laser beam introduction optical system  35  and the laser beam measuring device  37  may be housed in the storing chamber  9   b . A window  38  may be provided between the storing chamber  9   a  and the storing chamber  9   b . With this, the chamber  2  may be airtightly sealed. 
     An optical unit  42  may be attached to the reference member  9  through a flexible pipe  44 . A beam-path tube  41  may be connected to the optical unit  42  such that a laser beam from a laser apparatus  30  (see  FIG. 5B ) may travel through the beam-path tube  41  into the optical unit  42 . At least one high-reflection mirror  43  may be provided in the optical unit  42 . The high-reflection mirror  43  may be positioned such that the laser beam guided into the optical unit  42  is reflected by the high-reflection mirror  43  so as to enter the storing chamber  9   b  through the flexible pipe  44 . 
     Inside the storing chamber  9   b , the laser beam introduction optical system  35  may be positioned to introduce the laser beam from the optical unit  42  into the storing chamber  9   a  through the window  38 . The laser beam introduction optical system  35  may, for example, include a high-reflection mirror  51 , a beam splitter  52 , a high-reflection mirror  53 , and holders for these optical elements. 
     The high-reflection mirror  51  may be positioned to reflect the laser beam from the optical unit  42  toward the beam splitter  52 . The beam splitter  52  may be positioned to transmit most of the laser beam incident thereon toward the high-reflection mirror  53  with high transmittance and to reflect a part of the laser beam toward the laser beam measuring device  37 . The high-reflection mirror  53  may be positioned to reflect the laser beam incident thereon toward the window  38  (and into the storing chamber  9   a ). 
     Inside the storing chamber  9   a , the laser beam focusing optical system  36  may be positioned to focus the laser beam introduced into the storing chamber  9   a  by the laser beam introduction optical system  35  into the plasma generation region  25 . The laser beam focusing optical system  36  may, for example, include a high-reflection mirror  61 , a laser beam focusing mirror  62 , and holders for these mirrors. 
     The high-reflection mirror  61  may be positioned to reflect the laser beam from the laser beam introduction optical system  35  toward the laser beam focusing mirror  62 . The laser beam focusing mirror  62  may, for example, be an off-axis paraboloidal mirror, and may be positioned to focus the laser beam from the high-reflection mirror  61  into the plasma generation region  25 . 
     The exposure apparatus  6  may, for example, include a mask irradiation part  6   a  and a workpiece irradiation part  6   b . The mask irradiation part  6   a  may be an optical system for irradiating a mask on a mask table MT by EUV light, and may include a plurality of high-reflection mirrors. The workpiece irradiation part  6   b  may be an optical system for projecting an image on the mask onto a workpiece (e.g., semiconductor wafer) on a workpiece table WT, and may include a plurality of high-reflection mirrors. 
     3.2 Operation 
     With the above configuration, a laser beam from a laser apparatus may be reflected by the high-reflection mirror  43  and then be incident on the high-reflection mirror  51 . The laser beam reflected by the high-reflection mirror  51  may then be incident on the beam splitter  52 . Most of the laser beam incident on the beam splitter  52  may be transmitted therethrough, and the transmitted component of the laser beam may be incident on the high-reflection mirror  53 . A part of the laser beam incident on the beam splitter  52  may be reflected thereby, and the reflected component of the laser beam may enter the laser beam measuring device  37 . The laser beam measuring device  37  may be configured to measure the cross-sectional beam intensity profile, the pointing, and the divergence of the entering laser beam. 
     The laser beam reflected by the high-reflection mirror  53  may then be reflected sequentially by the high-reflection mirror  61  and the laser beam focusing mirror  62 , and may enter the chamber  2  through the through-hole formed in the reference member  9  and the opening  2   a . Further, the laser beam that has entered the chamber  2  may pass through a through-hole  24  as shown in  FIG. 1 , formed in the EUV collector mirror  23  and be focused in the plasma generation region  25 . 
     The laser beam may be focused on a target  27  formed of target material supplied by the target supply unit  26  in the plasma generation region  25 . With this, the target material may be turned into plasma, and EUV light may be emitted from the plasma. The emitted EUV light may be reflected by the EUV collector mirror  23  so as to be focused in the intermediate focus region  292  and then be outputted to the exposure apparatus  6 . 
     Inside the exposure apparatus  6 , a mask on the mask table MT may be irradiated by the EUV light through the mask irradiation part  6   a . Further, the EUV light reflected by the mask may be imaged on a workpiece (e.g., semiconductor wafer) on the workpiece table WT through the workpiece irradiation part  6   b . Here, by transitionally moving the mask table MT and the workpiece table WT simultaneously, the pattern on the mask may be transferred onto the workpiece. 
     3.3 Effect 
     With the configuration according to the first embodiment, change in the relative position and orientation of the respective optical elements may be suppressed. Fixing the optical unit  42 , the laser beam introduction optical system  35 , and the laser beam focusing optical system  36  to the reference member  9  may allow a beam path of a laser beam from a laser apparatus to be stabilized, and a position at which the laser beam is focused inside the chamber  2  may be stabilized accordingly. Further, fixing the EUV collector mirror  23  to the reference member  9 , to which the laser beam introduction optical system  36  and the laser beam focusing optical system  36  are fixed, may allow the focus of the laser beam to overlap the first focus of the EUV collector mirror  23  with high precision. With this, the precision with which the EUV light is focused in the intermediate focus region  292  may be improved. Consequently, by positioning the reference member  9  with respect to the exposure apparatus  6 , a stable EUV light may be supplied to the exposure apparatus  6 . 
     4. EUV Light Generation Apparatus: Second Embodiment 
       FIG. 3A  is a plan view illustrating an EUV light generation apparatus according to a second embodiment.  FIG. 3B  is a sectional view illustrating the EUV light generation apparatus according to the second embodiment. In the second embodiment, guide rails  71  and wheels  72  may be used as the moving mechanism  7 , and a positioning block  81 , a fixing plate  82 , and bolts (or pins)  83  may be used as the positioning mechanism  8  of the first embodiment shown in  FIGS. 2A and 2B . Here, the wheels  72  may be replaced by casters provided with wheels. Other configurations and operations may be similar to those of the first embodiment. 
     As shown in  FIGS. 3A and 3B , two guide rails  71  may be installed on a floor which serves as the reference plane. The wheels  72  may be rotatably attached to the reference member  9 . The wheels  72  may rotate on the guide rails  71 , whereby the primary components of the EUV light generation apparatus  1  may be movable with respect to the exposure apparatus  6 . In this way, using the guide rails  71  and the wheels  72  may allow the reference member  9  to be moved with ease. 
     Here, the reference member  9  may be positioned by making a first surface (right-side surface in  FIG. 3A ) of the reference member  9  facing the exposure apparatus  6  abut against the positioning block  81 . In addition, the reference member  9 , after being positioned, may be prevented from moving by making a second surface of the reference member  9  opposite to the first surface abut against the fixing plate  82  and fixing the fixing plate  82  to the guide rails  71  by the bolts (or the pins)  83 . 
     5. EUV Light Generation Apparatus: Third Embodiment 
       FIG. 4A  is a plan view illustrating an EUV light generation apparatus according to a third embodiment.  FIG. 4B  is a side view illustrating the EUV light generation apparatus according to the third embodiment.  FIG. 4C  is a sectional view of the EUV light generation apparatus shown in  FIG. 4A , taken along IVC-IVC plane. 
     In the third embodiment, the guide rails  71  and a carriage  74  that includes the wheels  72  and air cylinders  73  may be used as the moving mechanism  7  of the first embodiment shown in  FIGS. 2A and 2B . Further, a positioning stage  84  may be used as the positioning mechanism  8  of the first embodiment shown in  FIGS. 2A and 2B . The reference member  9  may be disposed on the positioning stage  84  via at least three kinematic mounts  100 . Further, as shown in  FIG. 4C , an 0-ring seal  93  may be used at the connection between the chamber  2  and the reference member  9 . 
     As shown in  FIGS. 4A and 4B , two guide rails  71  may be installed on a floor which serves as the reference plane. The reference member  9  may be disposed on the air cylinders  73  of the carriage  74 . The wheels  72  may be rotatably attached to the carriage  74 . The wheels  72  may rotate on the guide rails  71 , whereby the primary components of the EUV light generation apparatus  1  may be movable with respect to the exposure apparatus  6 . 
     In the third embodiment, when the reference member  9  is to be disposed on the positioning stage  84 , the reference member  9  may be lowered using the air cylinders  73  of the carriage  74 . With this, the reference member  9  may be supported by the positioning stage  84  via the kinematic mounts  100 . 
     Each of the kinematic mounts  100  may include a housing member  85 , a housing member  91 , and internal spherical body  92 . The housing member  85  may be integrated into or attached to the positioning stage  84 , and the housing member  91  may be integrated into or attached to the reference member  9 . The internal spherical body  92  may be provided between the housing member  85  and the housing member  91 . Here, a v-shaped groove may be formed in the housing member  85  (or  91 ). With this, the kinematic mounts  100  may absorb displacement caused by structural deformation, thermal deformation, or the like generated between the positioning stage  84  and the reference member  9 . Accordingly, the reference member  9  may remain positioned precisely with respect to the positioning stage  84 . 
       FIG. 4D  is a plan view illustrating the EUV light generation apparatus according to the third embodiment in a state in which the reference member is moved away from the exposure apparatus.  FIG. 4E  is a side view illustrating the EUV light generation apparatus according to the third embodiment in a state in which the reference member is moved away from the exposure apparatus. When the chamber  2  is to be detached from the exposure apparatus  6  for maintenance work, the reference member  9  may be raised using the air cylinders  73  of the carriage  74 , and the housing member  91  along with the internal spherical body  92  may be detached from the housing member  85 . Thereafter, the carriage  74  on which the reference member  9  is placed may be moved horizontally. In this example, the internal spherical body  92  may move along with the housing member  91 , but the configuration may be such that the internal spherical body  92  remains on the housing member  85 . 
     6. Laser Beam Path Control Unit for Introducing Laser Beam into EUV Light Generation Apparatus: Fourth Embodiment 
     A laser beam path control unit may be configured to control a beam path of a laser beam introduced into the EUV light generation apparatus.  FIG. 5A  is a plan view illustrating an EUV light generation apparatus according to a fourth embodiment which includes a laser beam path control unit.  FIG. 5B  is a sectional view of the EUV light generation apparatus and the laser beam path control unit shown in  FIG. 5A . In the fourth embodiment, the laser beam path control unit may be used to control the beam path of the laser beam from the laser apparatus  30  to the EUV light generation apparatus  1 . The EUV light generation apparatus  1  may be similar in configuration to any of the first through third embodiments. As shown in  FIG. 5B , the EUV light generation apparatus  1  may be installed on a clean room floor, and the laser apparatus  30  may be installed on a sub-fab floor. 
     The laser beam from the laser apparatus  30  may be reflected sequentially by the high reflection mirrors,  45 ,  46 ,  47 , and  43 , and enter the reference member  9 . The high-reflection mirrors  45  and  46  may respectively be provided with mirror holders  45   a  and  46   a  which respectively include actuators  45   b  and  46   b . A mechanism including the high-reflection mirrors  45  and  46 , the mirror holders  45   a  and  46   a , and the actuators  45   b  and  46   b  may be referred to herein as a laser beam travel direction adjusting mechanism. 
     Drivers  48  and  49  may be configured to drive the respective actuators  45   b  and  46   b , whereby the orientation (posture) of the high-reflection mirrors  45  and  46  may be adjusted by the respective actuators  45   b  and  46   b . A laser beam axis controller  39  may calculate the center in the cross-section of the laser beam from the measurement result of the laser beam measuring device  37 . The laser beam axis controller  39  may control the drivers  48  and  49  based on the calculation result, whereby the orientation (posture) of the high-reflection mirrors  45  and  46  may be controlled, and the laser beam may be guided to a predetermined position inside the reference member  9  at a predetermined angle. Here, the laser beam measuring device  37 , the laser beam axis controller  39 , the drivers  48  and  49 , and the above-described laser beam travel direction adjusting mechanism may constitute the laser beam path control unit. 
     7. EUV Light Generation Apparatus in which Pre-Pulse Laser Beam is Used: Fifth Embodiment 
       FIG. 6  is a sectional view of an EUV light generation apparatus according to a fifth embodiment, in which a pre-pulse laser beam is used. The fifth embodiment may be a modification of any of the first through third embodiments. The fifth embodiment to be described below is a modification of the EUV light generation apparatus of the third embodiment which includes at least three kinematic mounts. 
     In the fifth embodiment, a primary target may be irradiated by a pre-pulse laser beam to be turned into a secondary target, and the secondary target may be irradiated by a main pulse laser beam to be turned into plasma. For example, a yttrium aluminum garnet (YAG) laser apparatus may be used to generate the pre-pulse laser beam, and a carbon dioxide (C02) gas laser apparatus may be used the generate the main pulse laser beam. 
     As shown in  FIG. 6 , a laser beam introduction optical system  35   a , the laser beam focusing optical system  36 , and a laser beam measuring device  37   a  may be provided inside the reference member  9 . The laser beam introduction optical system  35   a  shown in  FIG. 6  may differ from the laser beam introduction optical system  35  shown in  FIG. 4C  in that a high-reflection mirror  54  for introducing the pre-pulse laser beam (broken line) is additionally provided, and the high-reflection mirror  51  is provided to introduce the main pulse laser beam (solid line) 
     The high-reflection mirror  54  may be configured to reflect the pre-pulse laser beam with high reflectance. The high-reflection mirror  51  may be configured to reflect the main pulse laser beam with high reflectance. The pre-pulse laser beam reflected by the high-reflection mirror  54  may be incident on a first surface (the right side surface in  FIG. 6 ) of a beam splitter  52   a , and the main pulse laser beam reflected by the high-reflection mirrors  51  may be incident on a second surface (the left side surface in  FIG. 6 ) of the beam splitter  52   a.    
     The beam splitter  52   a  be positioned to reflect the pre-pulse laser beam incident on the first surface thereof toward the high-reflection mirror  53  with high reflectance and to transmit a part of the pre-pulse laser beam incident thereon toward the laser beam measuring device  37   a . Further, the beam splitter  52  may be positioned to transmit the main pulse laser beam incident on the second surface thereof toward the high-reflection mirror  53  with high transmittance and to reflect a part of the main pulse laser beam incident thereon toward the laser beam measuring device  37   a.    
     The pre-pulse laser beam reflected by the beam splitter  52   a  and the main pulse laser beam transmitted through the beam splitter  52   a  may both be incident on the high-reflection mirror  53 . The high-reflection mirror  53  may be positioned to reflect the pre-pulse laser beam and the main pulse laser beam incident thereon toward the laser beam focusing optical system  36 . 
     The pre-pulse laser beam transmitted through the beam splitter  52   a  and the main pulse laser beam reflected by the beam splitter  52   a  may enter the laser beam measuring device  37   a . The laser beam measuring device  37   a  may be configured to measure the cross-sectional beam intensity profile, the pointing, and the divergence of each of the pre-pulse laser beam and the main pulse laser beam entering the laser beam measuring device  37   a . Here, the laser beam measuring device  37   a  may include a pre-pulse laser beam measuring device for measuring the pre-pulse laser beam and a main pulse laser beam measuring device for measuring the main pulse laser beam. 
     In this way, the pre-pulse laser beam and the main pulse laser beam may be adjusted to travel into substantially the same direction by the beam splitter  52   a . In this regard, the beam splitter  52   a  may serve as a beam combiner to make the travel direction of the pre-pulse laser beam and the travel direction of the main pulse laser beam coincide with each other. The beam splitter  52   a  may, for example, be formed of diamond. The first surface of the beam splitter  52   a  may be coated with an optical thin-film that reflects the pre-pulse laser beam with high reflectance and transmit the main pulse laser beam with high transmittance. 
     Each of the pre-pulse laser beam and the main pulse laser beam that have entered the laser beam focusing optical system  36  may be reflected sequentially by the high-reflection mirror  61  and the laser beam focusing mirror  62  with high reflectance. With this, the pre-pulse laser beam may be focused on a primary target in the plasma generation region  25 , and the main pulse laser beam may be focused on a secondary target in the plasma generation region  25 . 
     Alternatively, the pre-pulse laser beam may be introduced into the reference member  9  via the high-reflection mirror  51 , and the main pulse laser beam may be introduced into the reference member  9  via the high-reflection mirror  54 . In that case, the beam splitter  52   a  may be positioned to transmit the pre-pulse laser beam incident on the second surface thereof toward the high-reflection mirror  53  with high transmittance and to reflect a part of the pre-pulse laser beam incident thereon toward the laser beam measuring device  37   a . Further, the beam splitter  52   a  may be positioned to reflect the main pulse laser beam incident on the first surface thereof toward the high-reflection mirror  53  with high reflectance and to transmit a part of the main pulse laser beam incident thereon toward the laser beam measuring device  37   a.    
     According to the fifth embodiment, even in the case where the pre-pulse laser beam and the main pulse laser beam are introduced into the reference member  9  and into the chamber  2 , fixing the respective optical elements to the reference member  9  may yield similar effects as the first embodiment. 
     8. Laser Beam Path Control Unit for Introducing Two Laser Beams into EUV Light Generation Apparatus: Sixth Embodiment 
       FIG. 7A  is a plan view illustrating an EUV light generation apparatus according to a sixth embodiment, which includes a laser beam path control unit for introducing a pre-pulse laser beam and a main pulse laser beam into the EUV light generation apparatus.  FIG. 7B  is a sectional view of the EUV light generation apparatus and the laser beam path control unit shown in  FIG. 7A . In the sixth embodiment, the laser beam path control unit may be used to control the beam path of the pre-pulse laser beam (broken line) from a pre-pulse laser apparatus  131  and the beam path of the main pulse laser beam (solid line) from a main pulse laser apparatus  132  to the EUV light generation apparatus  1 . As shown in  FIG. 7B , the EUV light generation apparatus  1  may be installed on a clean room floor, and the pre-pulse laser apparatus  131  and the main pulse laser apparatus  132  may be installed on a sub-fab floor. 
     With reference to  FIGS. 7A and 7B , the main pulse laser beam from the main pulse laser apparatus  132  may be reflected sequentially by the high-reflection mirrors,  45 ,  46 ,  47 , and  43 , and enter the reference member  9 . The high-reflection mirrors  45  and  46  may be provided with the respective mirror holders  45   a  and  46   a  which include the respective actuators  45   b  and  46   b . The high-reflection mirrors  45  and  46 , the mirror holders  45   a  and  46   a , and the actuators  45   b  and  46   b  may constitute a main pulse laser beam travel direction adjusting mechanism. 
     A driver (for the main pulse laser beam)  148  may be configured to drive the actuators  45   b  and  46   b , whereby the actuators  45   b  and  46   b  may adjust the orientation (posture) of the respective high-reflection mirrors  45  and  46 . The laser beam axis controller  39  may be configured to control the driver  148  based on the measurement result of the laser beam measuring device  37   a , and the orientation (posture) of the high-reflection mirrors  45  and  46  may be controlled accordingly. With this, the main pulse laser beam may be supplied to a predetermined position inside the reference member  9  at a predetermined angle. 
     The pre-pulse laser beam from the pre-pulse laser apparatus  131  may be reflected sequentially by high-reflection mirrors,  145 ,  146 ,  147 , and  143 , and enter the reference member  9 . The high-reflection mirrors  145  and  146  may be provided with respective mirror holders which include respective actuators, as in the high-reflection mirrors  45  and  46 . The high-reflection mirrors  145  and  146 , the mirror holders, and the actuators may constitute a pre-pulse laser beam travel direction adjusting mechanism. 
     A driver (for the pre-pulse laser beam)  149  may be configured to drive the actuators for the high-reflection mirrors  145  and  146 , whereby the actuators may adjust the orientation (posture) of the respective high-reflection mirrors  145  and  146 . The laser beam axis controller  39  may be configured to control the driver  149  based on the measurement result of the laser beam measuring device  37   a , and the orientation (posture) of the high-reflection mirrors  145  and  146  may be controlled accordingly. With this, the pre-pulse laser beam may be supplied to a predetermined position inside the reference member  9  at a predetermined angle. Here, the laser beam measuring device  37   a , the laser beam axis controller  39 , the drivers  148  and  149 , the main pulse laser beam travel direction adjusting mechanism, and the pre-pulse laser beam travel direction adjusting mechanism may constitute the laser beam path control unit. 
     The optical systems in the EUV light generation apparatus  1  for guiding the pre-pulse laser beam and the main pulse laser beam to the plasma generation region  25  may be similar to those described in the fifth embodiment. 
     As shown in  FIG. 7A , an optical unit  142  may be connected to the reference member  9  through a flexible pipe  144  to introduce the pre-pulse laser beam into the reference member  9 . A beam-path tube  141  may be connected to the optical unit  142 , and at least one high-reflection mirror  143  may be provided in the optical unit  142 . With this configuration, the pre-pulse laser beam from the pre-pulse laser apparatus  131  (see  FIG. 7B ) may travel through the beam-path tube  141  toward the optical unit  142 . In the optical unit  142 , the pre-pulse laser beam may be reflected by the high-reflection mirror  143  toward the high-reflection mirror  54  provided inside the reference member  9 . With this, the pre-pulse laser beam may enter the laser beam introduction optical system. 
     The optical unit  42  may be connected to the reference member  9  through the flexible pipe  44  to introduce the main pulse laser beam into the reference member  9 . The beam-path tube  41  may be connected to the optical unit  42 , and at least one high-reflection mirror  43  may be provided in the optical unit  42 . The main pulse laser beam from the main pulse laser apparatus  132  may travel through the beam-path tube  41  toward the optical unit  42 . In the optical unit  42 , the main pulse laser beam may be reflected by the high-reflection mirror  43  toward the high-reflection mirror  51  provided inside the reference member  9 . With this, the main pulse laser beam may enter the laser beam introduction optical system. 
     9. Laser Beam Path Control Unit for Introducing Two Laser Beams into EUV Light Generation Apparatus: Seventh Embodiment 
       FIG. 8A  is a plan view illustrating an EUV light generation apparatus according to a seventh embodiment, which includes a laser beam path control unit for introducing a pre-pulse laser beam and a main pulse laser beam into the EUV light generation apparatus.  FIG. 8B  is a sectional view of the EUV light generation apparatus and the laser beam path control unit shown in  FIG. 8A . 
     In the seventh embodiment, a beam path adjusting mirror  133  may be positioned to make the beam path of the pre-pulse laser beam from the pre-pulse laser apparatus  131  and the beam path of the main pulse laser beam from the main pulse laser apparatus  132  coincide with each other inside a laser unit  130  that includes the pre-pulse laser apparatus  131  and the main pulse laser apparatus  132 . Further, the laser beam path control unit may be used to control the beam path of the pre-pulse laser beam from the pre-pulse laser apparatus  131  and the beam path of the main pulse laser beam from the main pulse laser apparatus to the EUV light generation apparatus  1 . As shown in  FIG. 8B , the EUV light generation apparatus  1  may be installed on a clean room floor, and the laser unit  130  may be installed on a sub-fab floor. 
     The beam path adjusting mirror  133  may be positioned to reflect the pre-pulse laser beam from the pre-pulse laser apparatus  131  toward the high-reflection mirror  45  with high reflectance and to transmit the main pulse laser beam from the main pulse laser apparatus  132  toward the high-reflection mirror  45  with high transmittance. With this, the beam path of the pre-pulse laser beam and the beam path of the main pulse laser beam may be adjusted to coincide with each other. Here, the beam path adjusting mirror  133  may be similar in configuration to the beam splitter  52   a  shown in  FIG. 6 . 
     The pre-pulse laser beam and the main pulse laser beam may then be reflected sequentially by the high-reflection mirrors,  45 ,  46 ,  47 , and  43 , and enter the reference member  9 . The high-reflection mirrors  45  and  46  may be provided with the respective mirror holders  45   a  and  46   a  which include the respective actuators  45   b  and  46   b . The high-reflection mirrors  45  and  46 , the mirror holders  45   a  and  46   a , and the actuators  45   b  and  46   b  may constitute a laser beam travel direction adjusting mechanism. 
     A driver  134  may be configured to drive the actuators  45   b  and  46   b , whereby the actuators  45   b  and  46   b  may adjust the orientation (posture) of the respective high-reflection mirrors  45  and  46 . The laser beam axis controller  39  may be configured to control the driver  134  based on the measurement result of the laser beam measuring device  37   a , and the orientation (posture) of the high-reflection mirrors  45  and  46  may be controlled accordingly. With this, the pre-pulse laser beam and the main pulse laser beam may be supplied to a predetermined position inside the reference member  9  at a predetermined angle. Here, the laser beam measuring device  37   a , the laser beam axis controller  39 , the driver  134 , and the laser beam travel direction adjusting mechanism may constitute the laser beam path control unit. 
     The laser beam introduction optical system may include the high-reflection mirror  54 , a beam path adjusting mirror (beam splitter)  52   b , the high-reflection mirror  53 , and the laser beam measuring device  37   a.    
     The high-reflection mirror  54  may be configured to reflect the pre-pulse laser beam and the main pulse laser beam with high reflectance. The pre-pulse laser beam and the main pulse laser beam reflected by the high-reflection mirror  54  may be incident on the beam path adjusting mirror  52   b . The beam path adjusting mirror  52   b  may be configured to reflect the pre-pulse laser beam and the main pulse laser beam incident thereon toward the high-reflection mirror  53  with high reflectance and to transmit a part of the pre-pulse laser beam and a part of the main pulse laser beam toward the laser beam measuring device  37   a.    
     The pre-pulse laser beam and the main pulse laser beam reflected by the beam path adjusting mirror  52   b  may be reflected by the high-reflection mirror  53  toward the laser beam focusing optical system with high reflectance. The configuration and the operation past the laser beam focusing optical system may be similar to the configuration and the operation in the fifth embodiment. 
     The pre-pulse laser beam and the main pulse laser beam transmitted through the beam path adjusting mirror  52   b  may enter the laser beam measuring device  37   a . The laser beam measuring device  37   a  may be configured to measure the cross-sectional beam intensity profile, the pointing, and the divergence of the pre-pulse laser beam and the main pulse laser beam that have entered the laser beam measuring device  37   a.    
     According to the seventh embodiment, the beam path of the pre-pulse laser beam and the beam path of the main pulse laser beam may be adjusted to coincide with each other inside the laser unit  130 . Accordingly, compared to the sixth embodiment, the configuration of the beam delivery path between the laser unit  130  and the reference member  9  and the configuration of the laser beam introduction optical system may be simplified. 
     10. Supplementary Descriptions 
     10.1 Laser Beam Measuring Device 
       FIG. 9  is a diagram showing an example of a configuration of a laser beam measuring device. As shown in  FIG. 9 , the laser beam measuring device  37  may include a beam splitter  101 , lenses  102  and  103 , and beam profilers  104  and  105 . The beam splitter  101  may be wedge-shaped. 
     A part of a laser beam transmitted through the beam splitter  101  may be incident on the beam profiler  104  via the lens  102 . Another part of the laser beam reflected by the beam splitter  101  may be incident on the beam profiler  105  via the lens  103 . As shown in  FIG. 9 , the beam profiler  104  may be provided at a position displaced from the focus of the lens  102  and configured to measure the cross-sectional beam intensity profile of the laser beam incident thereon. The beam profiler  105  may provided at the focus of the lens  103  having a focal distance F and configured to measure the beam profile (pointing and divergence) of the laser beam at its focus. 
     The center in the cross-section of the laser beam and the size of the cross-section of the laser beam may be calculated from the measurement result of the cross-sectional beam intensity profile of the laser beam. Further, the divergence (wavefront curvature) and the travel direction of the laser beam may be obtained from the measurement result of the beam profile of the laser beam at its focus. 
     10.2 Laser Beam Travel Direction Adjusting Mechanism 
       FIG. 10  is a diagram for discussing one example of the operation of a laser beam travel direction adjusting mechanism. The beam path of a laser beam may be adjusted to a desired beam path by controlling tilt angles ( 8   x ,  8   y ), such as shown in  FIG. 11 , of each of the high-reflection mirrors  45  and  46 . Here, the direction of the angle  8   x  may be perpendicular to the direction of the angle  8   y . For example, each of the mirror holders  45   a  and  46   a  (as shown, for example, in  FIG. 7B ), to which the respective mirrors  45  and  46  are mounted, may have a gimbal mechanism, which is a type of swivel with which an object can be rotated about two axes that are orthogonal to each other. 
       FIG. 11  is a diagram for discussing another example of the operation of a laser beam travel direction adjusting mechanism. In place of the high-reflection mirrors  45  and  46  shown in  FIG. 10 , two wedge-shaped substrates  111  and  112 , through which a laser beam may be transmitted, may be used. The substrates  111  and  112  may be provided in a beam path of a laser beam, and the substrates  111  and  112  may be rotated about the center of the beam path, whereby the beam path may be adjusted to a desired beam path. Even in that case, actuators  113  and  114 , which may be driven by a driver, may be provided to control the tilt angles ( 8   x ,  8   y ) of the substrates  111  and  112 . Alternatively, the high-reflection mirrors  45  and  46  shown in  FIG. 10  may be provided with the actuators  113  and  114 , and the tilt angles of the high-reflection mirrors  45  and  46  may be controlled by driving the actuators  113  and  114 . 
     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). 
     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.”