Patent Publication Number: US-2013228695-A1

Title: Device for collecting extreme ultraviolet light

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2012-045455 filed Mar. 1, 2012, and Japanese Patent Application No. 2012-261425 filed Nov. 29, 2012. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a device for collecting extreme ultraviolet (EUV) light. 
     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 to generate plasma. 
     SUMMARY 
     A device for collecting EUV light emitted at a plasma generation region according to one aspect of the present disclosure may include a first EUV collector mirror having a first spheroidal reflective surface and arranged such that a first focus of the first spheroidal reflective surface lies in the plasma generation region and a second focus of the first spheroidal reflective surface lies in a predetermined intermediate focus region, and a second EUV collector mirror having a second spheroidal reflective surface and arranged a third focus of the second spheroidal reflective surface lies in the plasma generation region and a fourth focus of the second spheroidal reflective surface lies in the predetermined intermediate focus region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Hereinafter, selected embodiments of the present disclosure will be described with reference to the accompanying drawings. 
         FIG. 1  schematically illustrates a configuration of an exemplary LPP type EUV light generation apparatus. 
         FIG. 2  is a sectional view, taken along a YZ plane, schematically illustrating an exemplary configuration of an EUV light generation apparatus to which a device for collecting EUV light is applied according to a first embodiment of the present disclosure. 
         FIG. 3  schematically illustrates a state where radiation is reflected by first and second EUV collector mirrors. 
         FIG. 4  schematically illustrates first and second far field patterns to be formed in an exposure apparatus. 
         FIG. 5  is a sectional view, taken along a YZ plane, schematically illustrating an exemplary configuration of an EUV light generation apparatus to which a device for collecting EUV light is applied according to a second embodiment of the present disclosure. 
         FIG. 6  schematically illustrates exemplary configurations of first and second adjustment stages. 
         FIG. 7A  shows radiation reflected by a first EUV collector mirror entering a focus detection unit. 
         FIG. 7B  shows an example of a result to be obtained by the focus detection unit shown in  FIG. 7A . 
         FIG. 8A  shows radiation reflected by a second EUV collector mirror entering a focus detection unit. 
         FIG. 8B  shows an example of a result to be obtained by the focus detection unit shown in  FIG. 8A . 
         FIG. 9  is a flowchart showing a main flow of an operation in which an EUV light generation controller controls a focus state at the intermediate focus. 
         FIG. 10  is a flowchart showing a subroutine of an operation in which an adjustment controller controls the posture of the first EUV collector mirror. 
         FIG. 11  is a flowchart showing a subroutine of an operation in which an adjustment controller controls the posture of the first EUV collector mirror. 
         FIG. 12  is a flowchart showing a subroutine of an operation in which an adjustment controller controls the posture of the second EUV collector mirror. 
         FIG. 13  is a flowchart showing a subroutine of an operation in which an adjustment controller controls the posture of the second EUV collector mirror. 
         FIG. 14  is a sectional view, taken along a YZ plane, schematically illustrating an exemplary configuration of an EUV light generation apparatus to which a device for collecting EUV light is applied according to a third embodiment of the present disclosure. 
         FIG. 15  is a sectional view schematically illustrating an exemplary configuration of the EUV light generation apparatus, taken along an XZ plane. 
         FIG. 16A  shows an example of radiation reflected by the first EUV collector mirror entering a focus detection unit. 
         FIG. 16B  shows an example of a result to be obtained by the focus detection unit shown in  FIG. 16A . 
         FIG. 17  is a sectional view, taken along a YZ plane, schematically illustrating an exemplary configuration of an EUV light generation apparatus to which a device for collecting EUV light is applied according to a fourth embodiment of the present disclosure. 
         FIG. 18  schematically illustrates an exemplary configuration of a controller. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, selected embodiments of the present 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 the present disclosure. Further, configurations and operations described in each embodiment are not all essential in implementing the present disclosure. Note that like elements are referenced by like reference numerals and characters, and duplicate descriptions thereof will be omitted herein. 
     Contents 
     1. Overview of EUV Light Generation System 
     1.1 Configuration 
     1.2 Operation 
     2. EUV Light Generation Apparatus Including Device for Collecting EUV Light 
     2.1 Terms 
     2.2 Overview 
     2.3 First Embodiment 
     2.3.1 Configuration 
     2.3.2 Operation 
     2.4 Second Embodiment 
     2.4.1 Configuration 
     2.4.1 Operation 
     2.5 Third Embodiment 
     2.5.1 Configuration 
     2.5.2 Operation 
     2.6 Fourth Embodiment 
     2.6.1 Configuration 
     2.6.2 Operation 
     3. Configuration of Controller 
     1 Overview of EUV Light Generation System 
     1.1 Configuration 
       FIG. 1  schematically illustrates an exemplary configuration of an LPP type EUV light generation system. An EUV light generation apparatus  1  may be used with at least one laser apparatus  3 . Hereinafter, a system that includes the EUV light generation apparatus  1  and the laser apparatus  3  may be referred to as an EUV light generation system  11 . As shown in  FIG. 1  and described in detail below, the EUV light generation system  11  may include a chamber  2  and a target supply device  7 . The chamber  2  may be sealed airtight. The target supply device  7  may be mounted onto the chamber  2 , for example, to penetrate a wall of the chamber  2 . A target material to be supplied by the target supply device  7  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 have 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, for example, be provided in 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 alternately laminated. 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 specifications 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 so that 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, trajectory, position, and speed 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  to be in communication with the interior of the exposure apparatus  6 . A wall  291  having an aperture  293  may be provided in the connection part  29 . The wall  291  may be positioned such that the second focus of the EUV collector mirror  23  lies in the aperture  293  formed in the wall  291 . 
     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 (not separately shown) for defining the direction into which the pulse laser beam  32  travels and an actuator (not separately shown) for adjusting the position and the orientation or posture of the optical element. 
     1.2 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 the 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 device  7  may be configured to output the target(s)  27  toward the plasma generation region  25  in the chamber  2 . The target  27  may be irradiated with at least one pulse of the pulse laser beam  33 . Upon being irradiated with the pulse laser beam  33 , the 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  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 when 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 when the laser apparatus  3  oscillates, the direction in which the pulse laser beam  31  travels, and the position at which the pulse laser beam  33  is focused. It will be appreciated that the various controls mentioned above are merely examples, and other controls may be added as necessary. 
     2. EUV Light Generation Apparatus Including Device for Collecting EUV Light 
     2.1 Terms 
     When a wall of an EUV generation chamber shown in  FIGS. 2 ,  5 ,  14 ,  15 , and  17  is identified, a wall extending in a direction perpendicular to the +Y direction may be referred to as an “upper wall,” a wall extending in a direction perpendicular to the −Y direction may be referred to as a “lower wall,” a wall extending in a direction perpendicular to the +Z direction may be referred to as a “left wall,” a wall extending in a direction perpendicular to the −Z direction may be referred to as a “right wall,” a wall extending in a direction perpendicular to the +X direction may be referred to as a “front wall,” and a wall extending in a direction perpendicular to the −X direction may be referred to as a “rear wall.” 
     2.2 Overview 
     In an LPP-type EUV light generation apparatus, a collector mirror having a large solid angle may be used in order to improve efficiency of collecting EUV light. In order to increase a solid angle of a collector mirror, a reflective surface thereof may, for example, be extended in a direction along the rotation axis of a spheroid. However, if the reflective surface is to be extended in the direction of the rotation axis, a distance in which tools for processing the reflective surface are moved in the rotation axis direction may be increased, and an existing member for holding the tools may not withstand such load. Thus, it may be difficult to process the entire reflective surface of such a collector mirror having an extended reflective surface. 
     In one or more embodiments of the present disclosure, a device for collecting EUV light may include first and second EUV collector mirrors arranged confocally with each other. This configuration may make it possible to secure a greater reflective region that, in total, has a large solid angle. 
     2.3 First Embodiment 
     2.3.1 Configuration 
       FIG. 2  is a sectional view, taken along a YZ plane, schematically illustrating an exemplary configuration of an EUV light generation apparatus to which a device for collecting EUV light is applied according to a first embodiment of the present disclosure.  FIG. 3  schematically illustrates a state where radiation is reflected by first and second EUV collector mirrors.  FIG. 4  schematically illustrates first and second far field patterns to be formed in an exposure apparatus. 
     As shown in  FIG. 2 , an EUV light generation apparatus  1 A may include an chamber  2 A and a target supply device  7 . The target supply device  7  may include a target generation unit  70  and a target controller  80 . 
     The target generation unit  70  may include a target generator  71  and a pressure adjuster (not separately shown). The target generator  71  may include a tank  711  for storing a target material  270  thereinside. The tank  711  may be cylindrical in shape. The tank  711  may include a nozzle  712 , and the target material  270  stored inside the tank  711  may be outputted through the nozzle  712  into the chamber  2 A as targets  27 . A nozzle opening may be formed at a tip of the nozzle  712 . The target generator  71  may be mounted to the chamber  2 A such that the tank  711  is located outside the chamber  2 A and the nozzle  712  is located inside the chamber  2 A. The aforementioned pressure adjuster may be connected to the tank  711 . 
     A first through-hole  200 A serving as a laser beam inlet may be formed in the right wall of the chamber  2 A, and the pulse laser beam  33  may enter the chamber  2 A through the first through-hole  200 A. The first through-hole  200 A may be covered by the window  21 . Further, a second through-hole  201 A may be formed in the upper wall of the chamber  2 A. The nozzle  712  may be fitted in the second through-hole  201 A such that targets  27  are introduced into a space formed between a first EUV collector mirror  90 A and a second EUV collector mirror  91 A. 
     As shown in  FIGS. 2 and 3 , an EUV light collection device  9 A may be provided inside the chamber  2 A. The EUV light collection device  9 A may include the first EUV light collector mirror  90 A and the second EUV collector mirror  91 A. The first EUV collector mirror  90 A may include a first reflective surface  901 A. The first reflective surface  901 A may be spheroidal in shape and positioned such that a first focus lies in the plasma generation region  25  and a second surface lies in the intermediate focus region  292 . To be more specific, with reference to  FIG. 3 , the first reflective surface  901 A may have a shape corresponding to a part of a spheroid  900 A that has a first focus  908 A, which may coincide with the plasma generation  25  in the description to follow, and a second focus  909 A, which may coincide with the intermediate focus region  292  in the description to follow. 
     Referring back to  FIG. 2 , the first EUV collector mirror  90 A may be arranged toward the right wall of the chamber  2 A and attached to a first holder  92 A. A through-hole  902 A may be formed in the first EUV collector mirror  90 A to penetrate the first EUV collector mirror  90 A in the major axis direction, and the pulse laser beam  33  may travel through the through-hole  902 A toward the plasma generation region  25 . A through-hole  921 A may be formed in the first holder  92 A and aligned with the through-hole  902 A coaxially, so that the pulse laser beam  33  may travel through the through-hole  921 A toward the plasma generation region  25 . 
     The second EUV collector mirror  91 A may include a second reflective surface  911 A. The second reflective surface  911 A may be spheroidal in shape and positioned confocally with the first EUV collector mirror  90 A. To be more specific, with reference to  FIG. 3 , the second reflective surface  911 A may have a shape corresponding to another part of the spheroid  900 A, the part being different from that of the first reflective surface  901 A. 
     Referring back to  FIG. 2 , the second EUV collector mirror  91 A may be fixed to the chamber  2 A through a second holder  93 A. The second EUV collector mirror  91 A may be provided on the side of the left wall relative to the position of the first EUV collector mirror  90 A such that a space that contains the plasma generation region  25  is secured between the first EUV collector mirror  90 A and the second EUV collector mirror  91 A. 
     With the above-described arrangement, radiation  250 A may be incident on the first reflective surface  901 A at an angle smaller than an angle at which radiation  260 A is incident on the second reflective surface  911 A. Here, the radiation  250 A and the radiation  260 A may include EUV light emitted from plasma generated in the plasma generation region  25 . The first reflective surface  901 A may be formed of a multi-layered reflective film that includes a molybdenum layer and a silicon layer which are alternately laminated. The multi-layered reflective film configured as such may selectively reflect EUV light included in the radiation  250 A incident thereon at a small angle. Meanwhile, the second reflective surface  911 A may be formed of a single layer reflective film that includes a ruthenium layer. The second reflective surface  911 A configured as such may selectively reflect EUV light included in the radiation  260 A incident thereon at a large angle. 
     Further, as shown in  FIG. 2 , an opening  293 A may be defined in the connection part  29  and the connection part  29  may be connected to the exposure apparatus  6  through the opening  293 A. Radiation  251 A reflected by the first EUV collector mirror  90 A and radiation  261 A reflected by the second EUV collector mirror  91 A may be outputted to the exposure apparatus  6  from the chamber  2 A through the opening  293 A. 
     Further, the EUV light generation apparatus  1 A may include the laser beam direction control unit  34  and a laser beam focusing optical system  22 A. The laser beam direction control unit  34  may include a first optical element  341  and a second optical element  342  for defining a direction in which the pulse laser beam  32  travels. The laser beam focusing optical system  22 A may comprise a single mirror instead of a lens as shown in  FIG. 2 . 
     2.3.2 Operation 
     With reference to  FIG. 2 , the pulse laser beam  31  outputted from the laser apparatus  3  may reach the plasma generation region  25  as the pulse laser beam  33  through the laser beam direction control unit  34 , the laser beam focusing optical system  22 A, and the window  21 . Further, a target  27  may be outputted from the target generator  70  toward the plasma generation region  25  and irradiated with the pulse laser beam  33 . Upon being irradiated with the pulse laser beam  33 , the target  27  may be turned into plasma, and the radiation  250 A and the radiation  260 A may be emitted therefrom. Here, for the sake of convenience, the radiation  250 A may refer to a part of isotropic radiation from the plasma emitted toward the first EUV collector mirror  90 A, and the radiation  260 A may refer to another part of the isotropic radiation from the plasma emitted toward the second EUV collector mirror  260 A. 
     The radiation  250 A may be reflected by the first reflective surface  901 A of the first EUV collector mirror  90 A and outputted as the radiation  251 A to the exposure apparatus  6  through the intermediate focus region  292 . Similarly, the radiation  260 A may be reflected by the second reflective surface  911 A of the second EUV collector mirror  91 A and outputted as the radiation  261 A to the exposure apparatus  6  through the intermediate focus region  292 . 
     To be more specific, with reference to  FIG. 3 , a part of the radiation  251 A which is reflected by an outer peripheral portion of the first reflective surface  901 A may be focused in the intermediate focus region  292  as radiation  252 A. A part of the radiation  251 A which is reflected by an edge of the first reflective surface  901 A around the through-hole  902 A may be focused in the intermediate focus region  292  as radiation  253 A. In this way, the first EUV collector mirror  90 A may focus the radiation  250 A incident on the first reflective surface  901 A in the intermediate focus region  292 . 
     Further, a part of the radiation  261 A which is reflected by an edge of the second reflective surface  911 A on the side of the intermediate focus region  292  may be focused in the intermediate focus region  292  as radiation  262 A. Another part of the radiation  261 A which is reflected by an edge of the second reflective surface  911 A on the side of the first EUV collector mirror  90 A may also be focused in the intermediate focus region  292  as radiation  263 A. In this way, the second EUV collector mirror  91 A may focus the radiation  260 A incident on the second reflective surface  911 A in the intermediate focus region  292 . 
     Then, as shown in  FIG. 4 , an annular first far field pattern  101 A of the radiation  251 A from the first EUV collector mirror  90 A may be seen inside the exposure apparatus  6 . The inner circumference of the first far field pattern  101 A may be defined by the radiation  253 A, and the outer circumference thereof may be defined by the radiation  252 A. Further, an annular second far field pattern  102 A of the radiation  261 A from the second EUV collector mirror  91 A may be formed to surround the first far field pattern  101 A. The inner circumference of the second far field pattern  102 A may be defined by the radiation  263 A, and the outer circumference thereof may be defined by the radiation  262 A. An annular dark section  103 A may be formed between the first far field pattern  101 A and the second far field pattern  102 A. 
     The dark section  103 A may be a region that is not irradiated with the radiation  251 A and the radiation  261 A. A dimension Pa 1  of the annular dark section  103 A will be described. With respect to a straight line that connects the first focus  908 A and the second focus  909 A, an angle formed with a path of the radiation  252 A is designated as  81   a , and an angle formed with a path of the radiation  263 A is designated as θ 2   a . The dimension Pa 1  may correspond to a difference θda between the angles θ 1   a  and θ 2   a  as expressed through θ 2   a −θ 1   a =θda. This difference θda may correspond to a dimension Pa 2  of spacing between the first EUV collector mirror  90 A and the second EUV collector mirror  91 A. 
     As described above, the EUV light collection device  9 A may includes the first EUV collector mirror  90 A and the second EUV collector mirror  91 A for focusing the radiation  251 A and the radiation  261 A, respectively, in the intermediate focus region  292  and guiding into the exposure apparatus  6 . The first and second EUV collector mirrors  90 A and  91 A may be arranged confocally. With the above-described configuration, even if a solid angle of each of the first reflective surface  901 A and the second reflective surface  911 A is small, a reflective region having, overall, a large solid angle may be formed with the first reflective surface  901 A and the second reflective surface  911 A combined together. 
     Further, the EUV light collection device  9 A may reflect the radiation  250 A and the radiation  260 A only once by the first and second reflective surfaces  901 A and  911 A, respectively, toward in the intermediate focus region  292 . This may allow the number of times the radiation  250 A and the radiation  260 A are reflected to be kept to be the minimum, and the absorption by the first and second reflective surfaces  901 A and  911 A may be kept to be the minimum. 
     2.4 Second Embodiment 
     2.4.1 Configuration 
       FIG. 5  is a sectional view, taken along a YZ plane, schematically illustrating an exemplary configuration of an EUV light generation apparatus to which a device for collecting EUV light is applied according to a second embodiment of the present disclosure.  FIG. 6  schematically illustrates exemplary configurations of first and second adjustment stages.  FIG. 7A  shows radiation reflected by a first EUV collector mirror entering a focus detection unit.  FIG. 7B  shows an example of a result to be obtained by the focus detection unit shown in  FIG. 7A .  FIG. 8A  shows radiation reflected by a second EUV collector mirror entering a focus detection unit.  FIG. 8B  shows an example of a result to be obtained by the focus detection unit shown in  FIG. 8A . 
     As shown in  FIG. 5 , an EUV light generation apparatus  1 C of the second embodiment may differ from the EUV light generation apparatus  1 A of the first embodiment in that an EUV light generation controller  5 C is provided in place of the EUV light generation controller  5  and an EUV light collection device  9 C is provided in place of the EUV light collection device  9 A. 
     The EUV light collection device  9 C may further include a first mirror adjuster  94 C, a second mirror adjuster  95 C, a focus detection unit  96 C, and an adjustment controller  97 C in addition to those of the EUV light collection device  9 A of the first embodiment. 
     The first mirror adjuster  94 C may be configured to adjust the posture of the first EUV collector mirror  90 A. The first mirror adjuster  94 C may include a first adjustment stage  940 C for holding the first EUV collector mirror  90 A and a first stage controller  945 C for controlling an operation of the first adjustment stage  940 C. The first adjustment stage  940 C may be a so-called five-axis stage. As shown in  FIGS. 5 and 6 , the first adjustment stage  940 C may include a fixed plate  941 C, a movable plate  942 C, and six actuators  943 C. The fixed plate  941 C may have an annular shape and may be fixed to the right wall of the chamber  2 C. The movable plate  942 C may also have an annular shape and may hold the first EUV collector mirror  90 A through a first holder  92 C. The six actuators  943 C may connect the fixed plate  941 C with the movable plate  942 C at six points. Each of the actuators  943 C may be configured to be deformable. Each of the actuators  943 C may be electrically connected to the first stage controller  945 C. The first stage controller  945 C may be electrically connected to the adjustment controller  97 C and may cause each of the actuators  943 C to deform under the control of the adjustment controller  97 C. 
     As each of the actuators  943 C deforms in accordance with the control of the first stage controller  945 C, the posture of the movable plate  942 C relative to the fixed plate  941 C may be adjusted. In more detail, provided that a face of the fixed plate  941 C lies along the XY plane and a line normal thereto coincides with the Z-axis, the movable plate  942 C has the posture thereof adjusted along the total of five axes, which includes translation in the X-axis, in the Y-axis, and in the Z-axis, and rotation about the X-axis (θx) and the Y-axis (θy). That is, in relation to the fixed plate  941 C, the movable plate  942 C translates in the vertical, lateral and longitudinal directions, and tilts along the lateral direction and along the longitudinal direction. 
     The second mirror adjuster  95 C may be provided to adjust the posture of the second EUV collector mirror  91 A and may include a second adjustment stage  950 C for holding the second EUV collector mirror  91 A and a second stage controller  955 C for controlling an operation of the second adjustment stage  950 C. The second adjustment stage  950 C may include a fixed plate  951 C, a movable plate  952 C, and actuators  953 C. The fixed plate  951 C may be fixed to an inner wall of the chamber  2 C. The movable plate  952 C may hold the second EUV collector mirror  91 A through a second holder  93 C. Each of the actuators  953 C may be electrically connected to the second stage controller  955 C. The second stage controller  955 C may be electrically connected to the adjustment controller  97 C and may cause each of the actuators  953 C to deform under the control of the adjustment controller  97 C. Through the control of the second stage controller  955 C, the posture of the second adjustment stage  950 C may be adjusted in five axes, as in the first adjustment stage  940 C. 
     As shown in  FIG. 5 , the focus detection unit  96 C may include a splitting optical element  960 C and an IF detector  961 C. The splitting optical element  960 C may be provided between the plasma generation region  25  and the intermediate focus region  292 . The splitting optical element  960 C may be positioned and configured to reflect a part of the radiation  251 A and a part of the radiation  261 A toward the IF detector  961 C as radiation  254 C and radiation  264 C, respectively. The splitting optical element  960 C may be a plate in which a plurality of openings is formed and may serve as a spectral purity filter. The IF detector  961 C may be provided such that the radiation  254 C and the radiation  264 C from the splitting optical element  960 C enter the IF detector  961 C. As shown in  FIG. 7A , the IF detector  961 C may include a shield switching unit  962 C, a fluorescent screen  963 C, a transfer optical system  964 C, and an image sensor  965 C. 
     The shield switching unit  962 C may selectively shield either of the radiation  254 C and the radiation  264 C. As shown in  FIG. 7A , the shield switching unit  962 C may be electrically connected to the adjustment controller  97 C. The shield switching unit  962 C may set a first light shielding plate  966 C in a path of the radiation  264 C to shield the radiation  264 C and allow the radiation  254 C to pass through under the control of the adjustment controller  97 C. Similarly, as shown in  FIG. 8A , the shield switching unit  962 C may set a second light shielding plate  967 C in a path of the radiation  254 C to shield the radiation  254 C and allow the radiation  264 C to pass through. As shown in  FIGS. 7A and 8A , the fluorescent screen  963 C may be provided along a predetermined focal plane of the radiation  254 C and the radiation  264 C that have passed through the shield switching unit  962 C. The fluorescent screen  963 C may be positioned such that a distance between the splitting optical element  960 C and the intermediate focus region  292  is substantially the same as a distance between the splitting optical element  960 C and the fluorescent screen  963 C. As the radiation  254 C and the radiation  264 C are incident on the fluorescent screen  963 C, the fluorescent screen  963 C may emit visible light  255 C and visible light  265 C, respectively. The transfer optical system  964 C may be provided in paths of the visible light  255 C and the visible light  265 C. The transfer optical system  964 C may be positioned and configured to focus the visible light  255 C and the visible light  265 C on the photosensitive surface of the image sensor  965 C. That is, the transfer optical system  964 C may be positioned to transfer an image of each of the visible light  255 C and the visible light  265 C along the plane where the fluorescent screen  963 C is provided onto the photosensitive surface of the image sensor  965 C. 
     When the visible light  255 C is incident on the photosensitive surface of the image sensor  965 C, a first image P IF1  as shown in  FIG. 7B  may be formed on the photosensitive surface of the image sensor  965 C. Data on the first image P IF1  may be sent to the adjustment controller  97 C. The image sensor  965 C may be electrically connected to the adjustment controller  97 C. Upon receiving the data from the image sensor  965 C, the adjustment controller  97 C may calculate an intensity distribution of the visible light  255 C. Further, the adjustment controller  97 C may calculate a center C IF1  and a diameter D IF1  of the first image P IF1  from the calculated intensity distribution. As described later, P IFt  shown in  FIG. 7B  indicates a target position of the center C IF1 . 
     Further, when the visible light  265 C is incident on the photosensitive surface of the image sensor  965 C, a second image P IF2  as shown in  FIG. 8B  may be formed on the photosensitive surface of the image sensor  965 C. Data on the second image PIF 2  may be sent to the adjustment controller  97 C. Upon receiving the data from the image sensor  965 C, the adjustment controller  97 C may calculate an intensity distribution of the visible light  265 C. Further, the adjustment controller  97 C may calculate a center C IF2  and a diameter D IFS  of the second image P IF2  from the calculated intensity distribution. As described later, P IFt  shown in  FIG. 8B  indicates a target position of the center C IF2 . An operation for bringing the center C IF2  to approach P IFt  will be described later. 
     As shown in  FIG. 5 , the adjustment controller  97 C may be housed in a case  20 C of the chamber  2 C together with the first stage controller  945 C and the second stage controller  955 C. The adjustment controller  97 C may be electrically connected to the EUV light generation controller  5 C. The adjustment controller  97 C may be configured to control the first stage controller  945 C and the second stage controller  955 C based on a result of the aforementioned calculation. 
     2.4.2 Operation 
       FIG. 9  is a flowchart showing a main flow of an operation in which an EUV light generation controller controls a focus state at the intermediate focus.  FIGS. 10 and 11  are flowcharts showing a subroutine of an operation in which an adjustment controller controls the posture of the first EUV collector mirror.  FIGS. 12 and 13  are flowcharts showing a subroutine of an operation in which an adjustment controller controls the posture of the second EUV collector mirror. The operation shown in these flowcharts can be performed when the EUV light generation apparatus is in operation to maintain the posture of the first EUV collector mirror to be optimum or when the apparatus is under maintenance. 
     With reference to  FIG. 5 , the EUV light generation controller  5 C may control the laser apparatus  3  and the target controller  80  to generate the radiation  250 A and the radiation  260 A. The radiation  250 A may be reflected by the first reflective surface  901 A and outputted as the radiation  251 A to the exposure apparatus  6 . The radiation  260 A may be reflected by the second reflective surface  911 A and outputted as the radiation  261 A to the exposure apparatus  6 . The splitting optical element  960 C may be provided in a path of the radiation  251 A, and thus a part of the radiation  251 A may be split by the splitting optical element  960 C and may enter the IF detector  961 C as the radiation  254 C. The remaining part of the radiation  251 A may be transmitted through the splitting optical element  960 C and outputted to the exposure apparatus  6 . Similarly, a part of the radiation  261 A may be reflected by the splitting optical element  960 C and may enter the IF detector  961 C as the radiation  264 C. The remaining part of the radiation  261 A may be transmitted through the splitting optical element  960 C and outputted to the exposure apparatus  6 . 
     With reference to  FIG. 9 , the EUV light generation controller  5 C may output an adjustment start signal to the adjustment controller  97 C to carry out a control to adjust the focus state of the radiation. This control may be started after the radiation  250 A and the radiation  260 A are generated. Upon receiving an adjustment start signal, the adjustment controller  97 C may carry out a subroutine to control the posture of the first EUV collector mirror  90 A (Step S 1 ). Through this control, the posture of the first EUV collector mirror  90 A may be adjusted, and thus the radiation  251 A from the first EUV collector mirror  90 A may be focused in the intermediate focus region  292  in a predetermined state. 
     With reference to  FIG. 10 , the adjustment controller  97 C may set the first light shielding plate  966 C in the shield switching unit  962 C (Step S 11 ). Here, the adjustment controller  97 C may output a first light shielding plate set signal to the shield switching unit  962 C. Upon receiving the first light shielding plate set signal, the shield switching unit  962 C may either keep the first light shielding plate  966 C if the first light shielding plate  966 C is already set or may switch from the second light shielding plate  967 C to the first light shielding plate  966 C if the second light shielding plate  967 C is already set. 
     When the first light shielding plate  966 C is set in the shield switching unit  962 C, the radiation  254 C may pass through the shield switching unit  962 C, as shown in  FIG. 7A , and the radiation  254 C may be incident on the fluorescent screen  963 C. The fluorescent screen  963 C on which the radiation  254 C is incident may emit the visible light  255 C, and the emitted visible light  255 C may be transferred onto the photosensitive surface of the image sensor  965 C by the transfer optical system  964 C. Referring back to  FIG. 10 , the image sensor  965 C may obtain data, or a first image P IF1 , indicative of an intensity distribution of the visible light  255 C incident on the photosensitive surface thereof (Step S 12 ), and may send the obtained data to the adjustment controller  97 C. Upon receiving the data from the image sensor  965 C, the adjustment controller  97 C may calculate the center C IF1  and the diameter D IF1  of the first image P IF1  (Step S 13 ). At this point, the adjustment controller  97 C may also load a target position P IFt  from a memory. 
     The adjustment controller  97 C may then control the posture of the first EUV collector mirror  90 A so that the center C IF1  approaches the target position P IFt  (Step S 14 ) through the first mirror adjuster  94 C. When the center C IF1  is located at the position shown in  FIG. 7B , the adjustment controller  97 C determines that the center C IF1  should be moved toward the lower left in the drawing. Then, the adjustment controller  97 C may output a first XY adjustment signal to the first stage controller  945 C to adjust the rotation angles θx and θy of the first EUV collector mirror  90 A so that the center C IF1  moves toward the lower left in the drawing. Upon receiving the first XY adjustment signal, the first stage controller  945 C may drive each of the actuators  943 C in accordance with the first XY adjustment signal. When each of the actuators  943 C is driven, the posture of the first EUV collector mirror  90 A may change, and in turn the position of the center C IF1  to be detected by the image sensor  965 C may change accordingly. 
     Thereafter, the image sensor  965 C may again obtain data on the visible light  255 C after the above-described adjustment, and the adjustment controller  97 C may calculate the intensity distribution of the visible light  255 C (Step S 15 ). Then, based on this calculation result, the adjustment controller  97 C may again calculate the center C IF1  and the diameter D IF1  of the first image P IF1  (Step S 16 ). The adjustment controller  97 C may then determine whether or not a distance between the center C IF1  and the target position P IFt  falls within a predetermined permissible range (Step S 17 ). In Step S 17 , when the adjustment controller  97 C determines that the aforementioned difference does not fall within the predetermined permissible range (Step S 17 ; NO), the adjustment controller  97 C may return to Step S 14  to repeat the subsequent steps. When the adjustment controller  97 C determines that the aforementioned difference falls within the predetermined permissible range (Step S 17 ; YES), the adjustment controller  97 C may then control the position of the first EUV collector mirror  90 A in the Z-axis direction so that the diameter D IF1  of the first image P IF1  is reduced, as shown in  FIG. 11  (Step S 18 ). The adjustment controller  97 C may output a first Z adjustment signal to the first stage controller  945 C to move the first EUV collector mirror  90 A in the Z-axis direction so that the diameter D IF1  is reduced. Upon receiving a first Z adjustment signal, the first stage controller  945 C may drive each of the actuators  943 C in accordance with the received first Z adjustment signal. As each of the actuators  943 C is driven, the position of the first EUV collector mirror  90 A in the Z-axis direction may change, and in turn the diameter D IF1  to be obtained by the image sensor  965 C may change accordingly. 
     Thereafter, the image sensor  965 C may again obtain data on the visible light  255 C and send the data to the adjustment controller  97 C. Upon receiving the data from the image sensor  965 C, the adjustment controller  97 C may again calculate the intensity distribution of the visible light  255 C (Step S 19 ), and may also calculate the center C IF1  and the diameter D IF1  of the first image P IF1  (Step S 20 ). Then, the adjustment controller  97 C may determine whether or not a difference between the calculated diameter D IF1  and a target diameter falls within a predetermined permissible range and a distance between the center C IF1  and the target position P IFt  falls within a predetermined permissible range (Step S 21 ). Here, the adjustment controller  97 C may load the aforementioned target diameter from a memory. In Step S 21 , when the adjustment controller  97 C determines that at least one of the center C IF1  and the diameter D IF1  does not meet to the aforementioned conditions (Step S 21 ; NO), the adjustment controller  97 C may return to Step S 14 . At this time, in a case where the diameter D IF1  calculated by the adjustment controller  97 C is greater than a previous instance of the diameter D IF1  as a result of changing the position of the first EUV collector mirror  90 A in the Z-axis direction, the direction in which the first EUV collector mirror  90 A is to be moved in the Z-axis direction for the next instance may be reversed. In Step S 21 , when the adjustment controller  97 C determines that both the center C IF1  and the diameter D IF1  meet the aforementioned conditions, the adjustment controller  97 C may terminate the control to adjust the posture of the first EUV collector mirror  90 A. 
     As described thus far, by adjusting the posture of the first EUV collector mirror  90 A such that the difference between the diameter D in  and the target diameter of the first image P IFt  falls within the predetermined permissible range and the distance between the center C IF1  and the target position P IFt  falls within the predetermined permissible range, the radiation  251 A from the first EUV collector mirror  90 A may be focused appropriately at the intermediate focus region  292 . 
     Referring back to  FIG. 9 , the adjustment controller  97 C may then control the posture of the second EUV collector mirror  91 A (Step S 2 ). Through this control, the posture of the second EUV collector mirror  91 A may be adjusted, and thus the radiation  261 A reflected by the second EUV collector mirror  91 A may be focused in the intermediate focus region  292  in a preset state. 
     With reference to  FIG. 12 , the adjustment controller  97 C may set the second light shielding plate  967 C in the shield switching unit  962 C (Step S 31 ). Here, the adjustment controller  97 C may output a second light shielding plate set signal to the shield switching unit  962 C. Upon receiving the second light shielding plate set signal, the shield switching unit  962 C may either keep the second light shielding plate  967 C if the second light shielding plate  967 C is already set or may switch from the first light shielding plate  966 C to the second light shielding plate  967 C if the first light shielding plate  966 C is already set. As shown in  FIG. 8A , when the second light shielding plate  967 C is set in the shield switching unit  962 C, the radiation  264 C may pass through the shield switching unit  962 C, and may be incident on the fluorescent screen  963 C. The fluorescent screen  963 C on which the radiation  264 C is incident may emit the visible light  265 C, and the emitted visible light  265 C may be transferred onto the photosensitive surface of the image sensor  965 C by the transfer optical system  964 C. Referring back to  FIG. 12 , the image sensor  965 C may obtain data, or a second image P IF2  indicative of the intensity distribution of the visible light  265 C (Step S 32 ), and send the obtained data to the adjustment controller  97 C. Upon receiving the data from the image sensor  965 C, the adjustment controller  97 C may calculate a center C IF2  and a diameter D IF2  of the second image P IF2  (Step S 33 ). 
     The adjustment controller  97 C may control the posture of the second EUV collector mirror  91 A through the second mirror adjuster  95 C so that the center C IF2  approaches the target position P IFt  (Step S 34 ). When the center C IF2  is located at a position shown in  FIG. 8B , the adjustment controller  97 C determines that the center C IF2  should be moved toward the lower right in the drawing. Then, the adjustment controller  97 C may output a second XY adjustment signal to the second stage controller  9550  to adjust the rotation angles θx and θy of the second EUV collector mirror  91 A so that the center C IF2  moves toward the lower right in the drawing. Upon receiving a second XY adjustment signal, the second stage controller  955 C may drive each of the actuators  953 C in accordance with the received second XY adjustment signal. As each of the actuators  953 C is driven, the posture of the second EUV collector mirror  91 A may change, and in turn the position of the center C IF2  to be detected by the image sensor  965 C may change accordingly. 
     Thereafter, the image sensor  965 C may again obtain data indicative of the intensity distribution of the visible light  265 C and sent the data to the adjustment controller  97 C. Upon receiving the data, the adjustment controller  97 C may again calculate the intensity distribution of the visible light  265 C (Step S 35 ). Further, the adjustment controller  97 C may again calculate the center C IF2  and the diameter D IF2  from the calculated intensity distribution (Step S 36 ). Then, the adjustment controller  97 C may determine whether or not a distance between the center C IF2  and the target position P IFt  falls within a predetermined permissible range based on a calculation result (Step S 37 ). In Step S 37 , when the adjustment controller  97 C determines that the aforementioned difference does not fall within the predetermined permissible range (Step S 37 ; NO), the adjustment controller  97 C may return to Step S 34  to repeat the subsequent steps. When the adjustment controller  97 C determines that the aforementioned difference falls within the predetermined permissible range (Step S 37 ; YES), the adjustment controller  97 C may then control the position of the second EUV collector mirror  91 A in the Z-axis direction so that the diameter D IF2  of the second image P IF2  is reduced, as shown in  FIG. 13  (Step S 38 ). To be more specific, the adjustment controller  97 C may output a second Z adjustment signal to the second stage controller  955 C to move the second EUV collector mirror  91 A in the Z-axis direction so that the diameter D IF2  is reduced. The second stage controller  955 C may drive each of the actuators  953 C in accordance with the received second Z adjustment signal. As each of the actuators  953 C is driven, the position of the second EUV collector mirror  91 A in the Z-axis direction may change, and in turn the diameter D IF2  to be detected by the image sensor  965 C may change accordingly. 
     Thereafter, the image sensor  965 C may again obtain data on the visible light  265 C, and send the data to the adjustment controller  97 C. Upon receiving the data, the adjustment controller  97 C may calculate the intensity distribution of the visible light  265 C (Step S 39 ), and may again calculate the center C IF2  and the diameter D IF2  from the calculated intensity distribution (Step S 40 ). Then, the adjustment controller  97 C may determine whether or not a difference between the diameter D IF2  and a target diameter and a distance between the center C IF2  and the target position P IFt  fall within predetermined permissible ranges, respectively (Step S 41 ). In Step S 41 , when the adjustment controller  97 C determines that at least one of the aforementioned conditions is not met, the adjustment controller  97 C may return to Step S 34 . At this time, in a case where the diameter D IF2  detected by the image sensor  965 C is greater than a previous instance of the diameter D IF2  as a result of changing the position of the second EUV collector mirror  91 A in the Z-axis direction, the direction in which the second EUV collector mirror  91 A is to be moved in the Z-axis direction for the next instance may be reversed. In Step S 41 , when the adjustment controller  97 C determines that both the center C IF2  and the diameter D IF2  meet the aforementioned conditions, the adjustment controller  97 C may terminate the control to adjust the posture of the second EUV collector mirror  91 A. 
     As described above, by adjusting the posture of the second EUV collector mirror  91 A such that the difference between the diameter D IF2  and the target diameter of the second image P IF2  falls within the predetermined permissible range and the distance between the center C IF2  and the target position P IFt  falls within the predetermined permissible range, the radiation  261 A reflected by the second EUV collector mirror  91 A may be focused appropriately at the intermediate focus region  292 . 
     Referring back to  FIG. 9 , the EUV light generation controller  5 C may determine whether or not the control of the focus state of the radiation is to be terminated (Step S 3 ). For example, the EUV light generation controller  5 C may determine whether or not the EUV light generation controller  5 C has been notified of a termination of the control by an operator, through a signal by the exposure apparatus  6 , or through a signal from a detector or a controller in the EUV light generation system. When the EUV light generation controller  5 C does not receive a termination signal (Step S 3 ; NO), the EUV light generation controller  5 C may return to Step S 1 . When the EUV light generation controller  5 C receives the termination signal (Step S 3 ; YES), the EUV light generation controller  5 C terminates the control. 
     As described above, under the control of the EUV light generation controller  5 C, the adjustment controller  97 C may adjust the postures of the first EUV collector mirror  90 A and the second EUV collector mirror  91 A, respectively, based on detection results of the visible light  255 C and the visible light  265 C by the image sensor  965 C. 
     Here, adjusting the posture of one of the first EUV collector mirror  90 A and the second EUV collector mirror  91 A may be omitted (see, e.g., the third embodiment discussed below). Further, although the configuration for adjusting the rotation angles θx and θy and the position in the Z-axis direction of the first or second EUV collector mirror  90 A or  91 A is shown above, at least one of the above may be adjusted. 
     2.5 Third Embodiment 
     2.5.1 Configuration 
       FIG. 14  is a sectional view, taken along a YZ plane, schematically illustrating an exemplary configuration of an EUV light generation apparatus to which a device for collecting EUV light is applied according to a third embodiment of the present disclosure.  FIG. 15  is a sectional view schematically illustrating an exemplary configuration of the EUV light generation apparatus, taken along an XZ plane.  FIG. 16A  shows an example of radiation reflected by the first EUV collector mirror entering a focus detection unit.  FIG. 16B  shows an example of a result to be obtained by the focus detection unit shown in  FIG. 16A . 
     As shown in  FIGS. 14 through 16A , an EUV light generation apparatus  1 D of the third embodiment may differ from the EUV light generation apparatus  1 C of the second embodiment in that an EUV light generation controller  5 D is provided in place of the EUV light generation controller  5 C and an EUV light collection device  9 D is provided in place of the EUV light collection device  9 C. The EUV light collection device  9 D may differ from the EUV light collection device  9 C in that a focus detection unit  96 D and an adjustment controller  97 D are provided in place of the focus detection unit  96 C and the adjustment controller  97 C and in that the second mirror adjuster  95 C is not provided. Here, in  FIGS. 14 and 15 , the pulse laser beam  31  and the laser beam direction control unit  34  are not depicted, but these components may also be provided as in the configuration shown in  FIG. 5 . 
     With reference to  FIGS. 14 and 15 , the focus detection unit  96 C may include a splitting optical element  960 D and an IF detector  961 D. The splitting optical element  960 D may be held by a holder  969 D such that the splitting optical element  960 D is arranged between the plasma generation region  25  and the intermediate focus region  292  in an obscuration region  202 D. The obscuration region  202 D may be such a solid angle region that radiation traveling therethrough into the exposure apparatus  6  is not used for exposure in exposure apparatus  6 . Although a region corresponding to the obscuration region  202 D is indicated as a belt-shaped region in the far field pattern in  FIGS. 14 and 15 , the shape of the obscuration region  202 D and the corresponding region in the far field pattern are not limited thereto. The splitting optical element  960 D may be arranged in accordance with the shape of the obscuration region  202 D. The splitting optical element  960 D may be positioned and configured to reflect the radiation  251 A with high reflectance toward the IF detector  961 D as radiation  254 D. 
     As shown in  FIG. 16A , the IF detector  961 D may include the fluorescent screen  963 C, the transfer optical system  964 C, and the image sensor  965 C. The fluorescent screen  963 C may be positioned such that a distance between the splitting optical element  960 D and the intermediate focus region  292  is substantially the same as a distance between the splitting optical element  960 D and the fluorescent screen  963 C. The transfer optical system  964 C may be positioned such that an image of visible light  255 D along a plane where the fluorescent screen  963 C is arranged is transferred onto the photosensitive surface of the image sensor  965 C. 
     Referring back to  FIG. 15 , the adjustment controller  97 D may be housed in a case  20 C of a chamber  2 D together with the first stage controller  945 C. The adjustment controller  97 D may be electrically connected to the EUV light generation controller  5 D, the first stage controller  945 C, and the image sensor  965 C. The adjustment controller  97 D may be configured to control the first stage controller  945 C in accordance with a calculation result of data obtained from the image sensor  965 C. 
     2.5.2 Operation 
     With reference to  FIGS. 14 and 15 , the radiation  250 A generated in accordance with the control of the EUV light generation controller  5 D may be reflected by the first reflective surface  901 A and outputted to the exposure apparatus  6  (see  FIG. 5 ) as the radiation  251 A. The radiation  260 A may be reflected by the second reflective surface  911 A and outputted as the radiation  261 A to the exposure apparatus  6 . 
     The splitting optical element  960 D may be provided in a path of the radiation  251 A, as shown in  FIG. 15 , and thus a part of the radiation  251 A traveling through the obscuration region  202 D may be reflected by the splitting optical element  960 D and directed toward the IF detector  961 D as the radiation  254 D. Another part of the radiation  251 A traveling through a region aside from the obscuration region  202 D may be outputted to the exposure apparatus  6 . 
     Accordingly, the first far field pattern  101 A, the second far field pattern  102 A, and the dark section  103 A may be formed inside the exposure apparatus  6 . Further, an obscuration region  104 D extending in the Y-axis direction may be formed to pass through the centers of the first far field pattern  101 A and the second far field pattern  102 A. As stated above, radiation traveling in the obscuration region  202 D may not be used for exposure in the exposure apparatus  6 , and thus even if the radiation in the obscuration region  202 D is sampled by the splitting optical element  960 D, the exposure performance or throughput of the exposure apparatus  6  is rarely affected. 
     The EUV light generation controller  5 D may output an adjustment start signal to the adjustment controller  97 D to carry out the operation shown in  FIGS. 9 through 11 . Here, Step S 2  in  FIG. 9  may be omitted from the operation in the third embodiment. As the aforementioned operation is carried out, the difference between the diameter D IF1  and the target diameter of the first image P IF1  may fall within the predetermined permissible range and the distance between the center C IF1  and the target position P IFt  may fall within the predetermined permissible range. Thus, the radiation  251 A reflected by the first EUV collector mirror  90 A may be focused appropriately in the intermediate focus region  292 . 
     As described above, the splitting optical element  960 D may be provided in the obscuration region  202 D. The IF detector  961 D may detect whether or not the radiation  251 A is focused in the intermediate focus region  292  based on a result of detecting the radiation  254 D reflected by the splitting optical element  960 D. The adjustment controller  97 D may control the first mirror adjuster  94 C based on a result detected by the IF detector  961 D so that the radiation  251 A is focused in the intermediate focus region  292 . In this way, by arranging the splitting optical element  960 D in the obscuration region  202 D, a loss in the radiation  251 A to be used for exposure, which is caused by reflecting a part of the radiation  251 A, may be reduced. As a result, without leading to a drop in the efficiency of collecting the radiation  251 A used for exposure, the posture of the first EUV collector mirror  90 A may be adjusted to focus the radiation  251 A appropriately in the intermediate focus region  292 . 
     2.6 Fourth Embodiment 
     2.6.1 Configuration 
       FIG. 17  is a sectional view, taken along a YZ plane, schematically illustrating an exemplary configuration of an EUV light generation apparatus to which a device for collecting EUV light is applied according to a fourth embodiment of the present disclosure. 
     A second through-hole  201 E may be formed in a corner of a chamber  2 E of an EUV light generation apparatus  1 E, and the target generator  71  may be mounted onto the chamber  2 E such that the nozzle  712  is located inside the chamber  2 E passing through the second through-hole  201 E. 
     An EUV light collection device  9 E may be provided inside the chamber  2 E. The EUV light collection device  9 E may include a first EUV collector mirror  90 E having a first reflective surface  901 E and a second EUV collector mirror  91 E having a second reflective surface  911 E. Each of the first reflective surface  901 E and the second reflective surface  911 E may be off-axis spheroidal in shape, and may be arranged such that the first reflective surface  901 E and the second reflective surface  911 E follows along distinct parts of the spheroid  900 A. The first EUV collector mirror  90 E may be attached to the chamber  2 E through a first holder  92 E. The second EUV collector mirror  91 E may be attached to the chamber  2 E through a second holder  93 E. 
     2.6.2 Operation 
     As the target  27  is irradiated with the pulse laser beam  33 , radiation including components in EUV range may be emitted isotropically from the plasma generation region  25 . Of such radiation, radiation  250 E may be reflected by the first reflective surface  901 E and focused in the intermediate focus region  292  as radiation  251 E. Further, radiation  260 E may be reflected by the second reflective surface  911 E and focused in the intermediate focus region  292  as radiation  261 E. The radiation  251 E and the radiation  261 E focused in the intermediate focus region  292  may then be outputted to the exposure apparatus  6 . 
     As shown in  FIG. 17 , the second EUV collector mirror  91 E may be arranged closer to the opening  293 A than the first EUV collector mirror  90 E. This configuration may make it possible to secure a reflective region that overall has a large solid angle without increasing a dimension of the first EUV collector mirror  90 E and the second EUV collector mirror  91 E in the major axis direction. Accordingly, with the first reflective surface  901 E and the second reflective surface  911 E each being relatively easy to process with high precision, the radiation  251 E and the radiation  261 E may be focused in the intermediate focus region  292 . 
     3. Configuration of Controller 
     Those skilled in the art will recognize that the subject matter described herein may be implemented by a general purpose computer or a programmable controller in combination with program modules or software applications. Generally, program modules include routines, programs, components, data structures, and so forth that can perform process as discussed in the present disclosure. 
       FIG. 18  is a block diagram showing an exemplary hardware environment in which various aspects of the disclosed subject matter may be implemented. An exemplary environment  100  in  FIG. 18  may include, but not limited to, a processing unit  1000 , a storage unit  1005 , a user interface  1010 , a parallel input/output (I/O) controller  1020 , a serial I/O controller  1030 , and an analog-to-digital (A/D) and digital-to-analog (D/A) converter  1040 . 
     The processing unit  1000  may include a central processing unit (CPU)  1001 , a memory  1002 , a timer  1003 , and a graphics processing unit (GPU)  1004 . The memory  1002  may include a random access memory (RAM) and a read only memory (ROM). The CPU  1001  may be any of various commercially available processors. Dual microprocessors and other multi-processor architectures may also be employed as the CPU  1001 . 
     These components in  FIG. 18  may be interconnected to one another to perform the processes discussed in the present disclosure. 
     In operation, the processing unit  1000  may load programs stored in the storage unit  1005  to execute them, read data from the storage unit  1005  in accordance with the programs, and write data in the storage unit  1005 . The CPU  1001  may execute the programs loaded from the storage unit  1005 . The memory  1002  may be a work area to temporally store programs to be executed by the CPU  1001  and data to be used for the operations of the CPU  1001 . The timer  116  may measure time intervals to provide the CPU  1001  with a measured result in accordance with the execution of the program. The GPU  1004  may process image data and provide the CPU  1001  with a processing result, in accordance with a program to be loaded from the storage unit  1005 . 
     The parallel I/O controller  1020  may be coupled to parallel I/O devices such as the image sensor  965 C, the EUV light generation controllers  5 ,  5 C, and  5 D, the adjustment controllers  97 C and  97 D, the first stage controller  945 C, the second stage controller  955 C, and the target controller  80 , which can communicate with the processing unit  1000 , and control communication between the processing unit  1000  and those parallel I/O devices. The serial I/O controller  1030  may be coupled to serial I/O devices such as the image sensor  965 C, the shield switching unit  962 C, the first adjustment stage  940 C, and the second adjustment stage  950 C, which can communicate with the processing unit  1000 , and control communication between the processing unit  1000  and those serial I/O devices. The A/D and D/A converter  1040  may be coupled to analog devices such as a temperature sensor, a pressure sensor, and a vacuum gauge, through analog ports. 
     The user interface  1010  may display progress of executing programs by the processing unit  1000  for an operator so that the operator can instruct the processing unit  1000  to stop execution of the programs or to execute an interruption routine. 
     The exemplary environment  100  can be applicable to implement each of the EUV light generation controllers  5 ,  5 C, and  5 D, the adjustment controllers  97 C and  97 D, the first stage controller  945 C, the second stage controller  955 C, and the target controller  80  in the present disclosure. Persons skilled in the art will appreciate that those controllers can be implemented in distributed computing environments where tasks are performed by processing units that are linked through any type of a communications network. As discussed in the present disclosure, the EUV light generation controllers  5 ,  5 C, and  5 D, the adjustment controllers  97 C and  97 D, the first stage controller  945 C, the second stage controller  955 C, and the target controller  80  can be connected to each other through a communication network such as the Ethernet (these controller can be parallel I/O devices as discussed above, when they are connected to each other). In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
     The above-described embodiments and the modifications thereof are merely examples for implementing the present disclosure, and the present disclosure is not limited thereto. Making various modifications according to the specifications or the like is within the scope of the present disclosure, and other various embodiments are possible within the scope of the present 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.”