Patent Publication Number: US-8993987-B2

Title: Target supply device and extreme ultraviolet light generation apparatus

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2012-189886 filed Aug. 30, 2012. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to target supply devices and extreme ultraviolet light generation apparatuses. 
     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 (LFP) 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 target supply device according to an aspect of the present disclosure may include a receptacle, a first electrode, a nozzle portion, a second electrode, a third electrode, a first power source, a second power source, and a third power source. The receptacle may be configured to hold a liquid target material inside the receptacle. The first electrode may be disposed within the receptacle. The nozzle portion may be provided in the receptacle. The second electrode may be provided with a first path and may be disposed facing the nozzle portion. The third electrode may be provided with a second path that, along with the first path, defines a trajectory of the liquid target material released from the nozzle portion. The first power source may be configured to take a common potential as a reference potential and apply a first potential that is higher than the common potential to the first electrode. The second power source may be configured to take the common potential as a reference potential and apply a second potential that is lower than the common potential to the third electrode. The third power source may be configured to take the common potential as a reference potential and apply a third potential that is no greater than the first potential and is no less than the second potential to the second electrode. 
     A target supply device according to another aspect of the present disclosure may include a receptacle, a first electrode, a nozzle portion, a second electrode, a third electrode, a first power source, a second power source, and a third power source. The receptacle may be configured to hold a liquid target material inside the receptacle. The first electrode may be disposed within the receptacle. The nozzle portion may be provided in the receptacle. The second electrode may be provided with a first path and disposed facing the nozzle portion. The third electrode may be provided with a second path that, along with the first path, defines a trajectory of the liquid target material released from the nozzle portion. The first power source may be configured to take the common potential as a reference potential and apply a first potential that is lower than the common potential to the first electrode. The second power source may be configured to take the common potential as a reference potential and apply a second potential that is higher than the common potential to the third electrode. The third power source may be configured to take the common potential as a reference potential and apply a third potential that is no greater than the first potential and is no less than the second potential to the second electrode. 
     An extreme ultraviolet light generation apparatus according to another aspect of the present disclosure may be configured to generate extreme ultraviolet light by irradiating a liquid target material with a pulse laser beam and turning the liquid target material into plasma, and may include a chamber, an optical system, a receptacle, a first electrode, a nozzle portion, a second electrode, a third electrode, a first power source, a second power source, and a third power source. The chamber may be provided with a through-hole. The optical system may be configured to conduct the pulse laser beam to a predetermined region in the chamber via the through-hole. The receptacle may be configured to hold the liquid target material inside the receptacle. The first electrode may be disposed within the receptacle. The nozzle portion may be provided in the receptacle. The second electrode may be provided with a first path and may be disposed facing the nozzle portion. The third electrode may be provided with a second path that, along with the first path, defines a trajectory of the liquid target material released from the nozzle portion toward the predetermined region. The first power source may be configured to take a common potential as a reference potential and apply a first potential that is higher than the common potential to the first electrode. The second power source may be configured to take the common potential as a reference potential and apply a second potential that is lower than the common potential to the third electrode. The third power source may be configured to take the common potential as a reference potential and apply a third potential that is no greater than the first potential and is no less than the second potential to the second electrode. 
     An extreme ultraviolet light generation apparatus according to another aspect of the present disclosure may be configured to generate extreme ultraviolet light by irradiating a liquid target material with a pulse laser beam and turning the liquid target material into plasma, and may include a chamber, an optical system, a receptacle, a first electrode, a nozzle portion, a second electrode, a third electrode, a first power source, a second power source, and a third power source. The chamber may be provided with a through-hole. The optical system may be configured to conduct the pulse laser beam to a predetermined region in the chamber via the through-hole. The receptacle may be configured to hold the liquid target material inside the receptacle. The first electrode may be disposed within the receptacle. The nozzle portion may be provided in the receptacle. The second electrode may be provided with a first path and may be disposed facing the nozzle portion. The third electrode may be provided with a second path that, along with the first path, defines a trajectory of the liquid target material released from the nozzle portion toward the predetermined region. The first power source may be configured to take the common potential as a reference potential and apply a first potential that is lower than the common potential to the first electrode. The second power source may be configured to take the common potential as a reference potential and apply a second potential that is higher than the common potential to the third electrode. The third power source may be configured to take the common potential as a reference potential and apply a third potential that is no less than the first potential and is no greater than the second potential to the second electrode. 
    
    
     
       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 an exemplary configuration of an LPP type EUV light generation system. 
         FIG. 2  is a partial cross-sectional view illustrating the configuration of an EUV light generation apparatus that includes a target supply device according to a first embodiment. 
         FIG. 3A  is a partial cross-sectional view illustrating a nozzle portion and the periphery of the nozzle portion in the target supply device illustrated in  FIG. 2 . 
         FIG. 3B  is a waveform diagram illustrating potentials applied to electrodes in the target supply device illustrated in  FIG. 2 . 
         FIG. 4  is a cross-sectional view illustrating part of an EUV light generation apparatus according to a second embodiment. 
         FIG. 5  is a cross-sectional view illustrating part of an EUV light generation apparatus according to a third embodiment. 
     
    
    
     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, the configuration(s) and operation(s) 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 
     2. Terms 
     3. Overview of EUV Light Generation System 
     3.1 Configuration 
     3.2 Operation 
     4. Target Supply Device Having First to Third Electrodes 
     4.1 Configuration 
     4.2 Operation 
     4.3 Variation 
     5. EUV Light Generation Apparatus Having Target Collector as Fourth Electrode 
     6. EUV Light Generation Apparatus Having Fourth Electrode Separate from Target Collector 
     1. Overview 
     In an LPP-type EUV light generation apparatus, a target supply device may output a target so that the target reaches a plasma generation region. By irradiating the target with a pulse laser beam at the point in time when the target reaches the plasma generation region, the target can be turned into plasma and EUV light can be radiated from the plasma. 
     The target supply device may include a reservoir that holds a melted target material to serve as the material for the target, a first electrode that is electrically connected to the melted target material, and a first power source that applies a first potential to the first electrode. The target supply device may further include a second electrode disposed facing a through-hole in a nozzle portion and a third electrode disposed in the vicinity of a trajectory of the target that has passed the second electrode. The target supply device may also include a second power source that applies a second potential that is lower than the first potential to the third electrode, and a third power source that applies a third potential that is no greater than the first potential and no less than the second potential to the second electrode. 
     The target outputted from the through-hole in the nozzle portion by a potential difference between the first electrode and the second electrode may be a charged droplet. A potential slope may be formed in the trajectory of the target by the potential difference between the second electrode and the third electrode, and the target may be accelerated as a result. 
     In EUV light generation apparatuses, there can be demand for increases in the repetition rate of EUV light generation. It can be necessary to increase the repetition rate of target generation in order to increase the repetition rate of EUV light generation. In the case where the repetition rate of the target generation is increased, the interval between previous and following targets can drop if the velocity at which the targets travel from the target supply device to the plasma generation region is not to be increased. When the interval between previous and following targets is short, the travel of the following target may be affected by the plasma produced when the previous target is turned into plasma. Accordingly, it can be necessary to increase the velocity at which the targets travel. 
     In a target supply device that uses the aforementioned first to third electrodes, it can be necessary to increase the potential difference between the first electrode and the third electrode in order to increase the velocity at which the targets travel. However, in the case where the potential difference between the electrodes is increased, it can be necessary to increase the insulation breakdown voltage of the cables, feedthroughs, and so on that connect the respective power sources to the electrodes, which in turn can make it necessary to increase the size of the apparatus. 
     According to an aspect of the present disclosure, a first potential that is higher than a common potential may be applied to the first electrode, a second potential that is lower than the common potential may be applied to the third electrode, and a third potential that is no greater than the first potential and no less than the second potential may be applied to the second electrode. Through this, the potential difference between the first potential and the second potential can be greater than the potential difference between the common potential and the first potential, the potential difference between the common potential and the second potential, and so on. Accordingly, the velocity at which the targets travel can be increased by increasing the potential difference between the first potential and the second potential while suppressing insulation breakdown by suppressing the potential difference between the common potential and the first potential, the potential difference between the common potential and the second potential, and so on. Through this, the repetition rate of EUV light generation can be improved. 
     2. Terms 
     Several terms used in the present application will be described hereinafter. 
     A “trajectory” of a target may be an ideal path of a target outputted from a target supply device, or may be a path of a target according to the design of a target supply device. 
     The “trajectory” of the target may also be the actual path of the target outputted from the target supply device. 
     A “high-voltage power source  55 ” can correspond to a “first power source”. 
     A “high-voltage power source  58 ” can correspond to a “second power source”. 
     A “high-voltage pulse generator  56 ” can correspond to a “third power source”. 
     A “high-voltage power source  57 ” can correspond to a “fourth power source”. 
     A “target collector  28   a ” or a “downstream electrode  69 ” can correspond to a “fourth electrode”. 
     3. Overview of EUV Light Generation System 
     3.1 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  26 . The chamber  2  may be sealed airtight. The target supply device  26  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  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 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. 
     3.2 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  26  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  33  travels, and the position at which the pulse laser beam  33  is focused. It will be appreciated that the various controls mentioned above are merely examples, and other controls may be added as necessary. 
     4. Target Supply Device Having First to Third Electrodes 
     4.1 Configuration 
       FIG. 2  is a partial cross-sectional view illustrating the configuration of an EUV light generation apparatus that includes a target supply device according to a first embodiment.  FIG. 3A  is a partial cross-sectional view illustrating a nozzle portion and the periphery of the nozzle portion in the target supply device illustrated in  FIG. 2 .  FIG. 3B  is a waveform diagram illustrating potentials applied to electrodes in the target supply device illustrated in  FIG. 2 . 
     As shown in  FIG. 2 , a laser beam focusing optical system  22   a , the EUV collector mirror  23 , the target collector  28 , an EUV collector mirror holder  41 , plates  42  and  43 , a beam dump  44 , and a beam dump support member  45  may be provided within the chamber  2 . 
     The chamber  2  may include a member (a conductive member) configured of a conductive material (metal material, for example). The chamber  2  may also include the conductive member and a member that is electrically insulative. The plate  42  may be anchored to the chamber  2 , and the plate  43  may be anchored to the plate  42 . The EUV collector mirror  23  may be anchored to the plate  42  via the EUV collector mirror holder  41 . 
     The laser beam focusing optical system  22   a  may include an off-axis paraboloid mirror  221 , a flat mirror  222 , and holders  223  and  224 . The off-axis paraboloid mirror  221  and the flat mirror  222  may be held by the holders  223  and  224 , respectively. The holders  223  and  224  may be anchored to the plate  43 . The off-axis paraboloid mirror  221  and the flat mirror  222  may be held in positions and orientations in which pulse laser beams reflected by those respective mirrors are focused at the plasma generation region  25 . 
     The beam dump  44  may be anchored to the chamber  2  via the beam dump support member  45  so as to be positioned upon a straight line extending from the optical path of the pulse laser beam reflected by the flat mirror  222 . The target collector  28  may be disposed upon a straight line extending from the trajectory of the target  27 . 
     A laser beam direction control unit  34   a  and the EUV light generation controller  5  may be provided outside the chamber  2 . The laser beam direction control unit  34   a  may include high-reflecting mirrors  341  and  342 , as well as holders  343  and  344 . The high-reflecting mirrors  341  and  342  may be held by the holders  343  and  344 , respectively. 
     The target supply device  26  may be attached to the chamber  2 . The target supply device  26  may include a reservoir  61 , a target control unit  52 , a pressure adjuster  53 , an inert gas bottle  54 , a high-voltage power source  55  (first power source), a high-voltage pulse generator  56  (third power source), and a high-voltage power source  58  (second power source). The target supply device  26  may further include a nozzle plate  62 , an electric insulation member  64 , a first electrode  65 , a second electrode (extraction electrode)  66 , an intermediate electrode  67 , and a third electrode (acceleration electrode)  68 . 
     The reservoir  61  may hold a target material in a melted state. A heater and a heater power source (not shown) may be used to melt the target material. A through-hole may be formed in a wall of the chamber  2 , and a flange portion  61   a  of the reservoir  61  may be anchored to the wall of the chamber  2  so as to cover the through-hole. The reservoir  61  may be formed of a material that does not easily react with the target material. Furthermore, the reservoir  61  may be configured of an electrically insulative material. For example, in the case where tin is used as the target material, the reservoir  61  may be configured of an electrically insulative material which does not easily react with the target material, such as silica (SiO 2 ) and alumina ceramics (Al 2 O 3 ). Alternatively, the reservoir may be configured of a conductive material. For example, in the case where tin is used as the target material, molybdenum (Mo), tungsten (W), and so on can be given as examples of materials that do not easily react with the target material and that are conductive. The reservoir that is configured of a conductive material may be attached to the conductive member of the chamber  2  via an electrically insulative member (not shown). 
     The nozzle plate  62  may be anchored to the vicinity of an output-side end of the reservoir  61 . The nozzle plate  62  may be configured of a conductive material, or may be configured of an electrically-insulative material. A through-hole may be formed in the nozzle plate  62 . In addition, the nozzle plate  62  may include a leading end portion  62   b  (see  FIG. 3A ) that protrudes in the output direction. The through-hole may be provided in the leading end portion  62   b . The target material can, as a liquid, pass through the through-hole provided in the leading end portion  62   b  and be released as the targets  27 . 
     The electric insulation member  64  may have a cylindrical shape, and may be anchored to the reservoir  61  so that an end portion on the output side of the reservoir  61  is housed within the electric insulation member  64 . The electric insulation member  64  may hold the nozzle plate  62 , the second electrode  66 , the intermediate electrode  67 , and the third electrode  68  on the inside of the electric insulation member  64 . The nozzle plate  62 , the second electrode  66 , the intermediate electrode  67 , and the third electrode  68  may be electrically insulated from each other by the electric insulation member  64 . A plurality of grooves may be formed on an inner side of the electric insulation member  64 . The plurality of grooves can suppress discharges between the electrodes held on the inner side of the electric insulation member  64 . 
     The second electrode  66  may be disposed facing a surface of the nozzle plate  62  on the output side thereof. A through-hole  66   a  (first path) may be formed in the second electrode  66 . The second electrode  66  may allow the target  27  to pass therethrough via the through-hole  66   a . The through-hole  66   a  may define a trajectory of the target  27 . However, the second electrode  66  is not limited to a form in which the through-hole  66   a  is formed therein. For example, the second electrode  66  may include a plurality of members (not shown) disposed in the vicinity of the trajectory of the target  27  so as to surround the trajectory of the target  27 . The region surrounded by the plurality of members may serve as a path (the first path) for allowing the targets  27  to pass. 
     The intermediate electrode  67  may be disposed in the vicinity of the trajectory of the target  27  that has passed through the through-hole  66   a  in the second electrode  66 . A through-hole  67   a  (third path) may be formed in the intermediate electrode  67 . The intermediate electrode  67  may allow the target  27  to pass therethrough via the through-hole  67   a . The through-hole  67   a  may define the trajectory of the target  27 . However, the intermediate electrode  67  is not limited to a form in which the through-hole  67   a  is formed therein. For example, the intermediate electrode  67  may include a plurality of members (not shown) disposed in the vicinity of the trajectory of the target  27  so as to surround the trajectory of the target  27 . The region surrounded by the plurality of members may serve as a path (the third path) for allowing the targets  27  to pass. 
     The third electrode  68  may be disposed in the vicinity of the trajectory of the target  27  that has passed through the through-hole  67   a  in the intermediate electrode  67 . A through-hole  68   a  (second path) may be formed in the third electrode  68 . The third electrode  68  may allow the target  27  to pass therethrough via the through-hole  68   a . The through-hole  68   a  may define the trajectory of the target  27 . However, the third electrode  68  is not limited to a form in which the through-hole  68   a  is formed therein. For example, the third electrode  68  may include a plurality of members (not shown) disposed in the vicinity of the trajectory of the target  27  so as to surround the trajectory of the target  27 . The region surrounded by the plurality of members may serve as a path (the second path) for allowing the targets  27  to pass. 
     The target control unit  52  may be configured to output a target control signal to each of the pressure adjuster  53 , the high-voltage power source  55 , and the high-voltage power source  58 , based on an EUV control signal from the EUV light generation controller  5 . In addition, the target control unit  52  may be configured to output a trigger signal to the high-voltage pulse generator  56  based on an EUV control signal from the EUV light generation controller  5 . 
     The inert gas bottle  54  may be connected to the pressure adjuster  53  via a pipe. The pressure adjuster  53  may communicate with the interior of the reservoir  61  via another pipe. An inert gas may be supplied to the reservoir  61  from the inert gas bottle  54  via these pipes. 
     An output terminal of the high-voltage power source  55  may be electrically connected to a high-voltage cable. This high-voltage cable may be electrically connected to the first electrode  65  within the reservoir  61  via a feedthrough  55   a  (introduction terminal) provided in the reservoir  61 . The first electrode  65  may be electrically connected to the target material held within the reservoir  61  by making contact with the target material within the reservoir  61 . Alternatively, in the case where the reservoir  61  or the nozzle plate  62  is conductive, the output terminal of the high-voltage power source  55  may be electrically connected to the conductive reservoir  61  or nozzle plate  62  via the high-voltage cable. The reservoir  61  or nozzle plate  62  being conductive may function as an electrode that is electrically connected to the target material within the reservoir  61 . 
     The high-voltage power source  55  may apply a first potential that is higher than a common potential, for example, to the first electrode  65 . Here, the common potential may be a potential that is a reference potential for the high-voltage power source  55 , the high-voltage pulse generator  56 , and the high-voltage power source  58 . This potential can, for example, be a ground potential. In the case where the target supply device  26  that includes the high-voltage power source  55 , the high-voltage pulse generator  56 , and the high-voltage power source  58  is insulated from the ground, the common potential can be a different potential from the ground potential. 
     An output terminal of the high-voltage power source  58  may be electrically connected to a high-voltage cable. This high-voltage cable may be electrically connected to the third electrode  68  via a feedthrough  58   a  (introduction terminal) provided in a wall of the chamber  2  and a through-hole  68   b  provided in a side surface of the electric insulation member  64 . The high-voltage power source  58  may apply a second potential to the third electrode  68 . Here, in the case where the first potential is a higher potential than the common potential, the second potential may be a lower potential than the common potential. Conversely, in the case where the first potential is a lower potential than the common potential, the second potential may be a higher potential than the common potential. 
     An output terminal of the high-voltage pulse generator  56  may be electrically connected to a high-voltage cable. This high-voltage cable may be electrically connected to the second electrode  66  via a feedthrough  56   a  (introduction terminal) provided in a wall of the chamber  2  and a through-hole  66   b  provided in a side surface of the electric insulation member  64 . The high-voltage pulse generator  56  may apply a third potential that is between the first potential and the common potential to the second electrode  66 . For example, in the case where the first potential is a higher potential than the common potential, the third potential may be a potential that is no higher than the first potential and no lower than the common potential. Conversely, in the case where the first potential is a lower potential than the common potential, the third potential may be a potential that is no lower than the first potential and no higher than the common potential. The third potential may be a potential that changes in pulses between a potential V 1  and a potential V 2  (mentioned later), which are potentials between the first potential and the common potential. 
     The intermediate electrode  67  may be electrically connected to the common potential (ground potential, for example) via a through-hole  67   b  provided in a side surface of the electric insulation member  64  and a feedthrough  57   a  (introduction terminal) provided in a wall of the chamber  2 . 
     4.2 Operation 
     The pressure adjuster  53  may adjust the pressure of the inert gas supplied to the reservoir  61  from the inert gas bottle  54  based on the target control signal outputted from the target control unit  52 . The inert gas introduced into the reservoir  61  may pressurize the melted target material within the reservoir  61 . As a result of the inert gas pressurizing the target material, the target material may protrude slightly from the leading end portion  62   b  of the nozzle plate  62 , in which the through-hole is provided. 
     As illustrated in  FIG. 3B , the high-voltage power source  55  may apply a first potential V H  to the target material via the first electrode  65  in the reservoir  61 , and hold that potential, based on the target control signal outputted from the target control unit  52 . 
     The potential of the intermediate electrode  67  may be held at a common potential Vc (ground potential, for example). 
     The high-voltage power source  58  may apply a second potential V L  to the third electrode  68  and hold that potential, based on the target control signal outputted from the target control unit  52 . In the case where the common potential Vc is the ground potential (0 V), the second potential V L  may have the opposite polarity to the first potential V H . The absolute value of the second potential V L  may be substantially the same potential as the first potential V H  (V L =−V H ). 
     The high-voltage pulse generator  56  may apply the third potential that changes in pulses to the second electrode  66  based on the trigger signal outputted from the target control unit  52 . The third potential may be a potential that changes between the potential V 1  obtained when the high-voltage pulse generator  56  is not receiving the trigger signal and the potential V 2  that is held for a predetermined amount of time in the case where the high-voltage pulse generator  56  has received the trigger signal. The potentials V H , V 1 , V 2 , Vc, and V L  may be in a relationship where V H ≧V 1 &gt;V 2 ≧Vc&gt;V L . Conversely, in the case where the first potential is a lower potential than the common potential, the potentials may be in a relationship where V H ≦V 1 &lt;V 2 ≦Vc&lt;V L . 
     Depending on a potential difference between the first electrode  65  and the second electrode  66 , an electrical field can be generated between the target material in the reservoir  61  and the second electrode  66 , and a Coulomb force can be produced between the target material and the second electrode  66  as a result. 
     The electrical field concentrates particularly in the vicinity of the target material that protrudes from the leading end portion  62   b  under the pressure of the inert gas as described above, and thus a stronger Coulomb force can be produced between the target material protruding from the leading end portion  62   b  and the second electrode  66 . Under this Coulomb force, the targets  27  can be released from the leading end portion  62   b  as charged droplets. In the case where the first potential V H  is higher than the common potential Vc, that is, in the case where V H ≧V 1 &gt;V 2 ≧Vc, the targets  27  can take on a positive charge. Conversely, in the case where the first potential V H  is lower than the common potential Vc, the targets  27  can take on a negative charge. 
     The targets  27  that have been charged and released from the leading end portion  62   b  can pass through the through-hole  66   a  of the second electrode  66 , and can pass through the through-hole  67   a  in the intermediate electrode  67  having been further accelerated by the Coulomb force produced by the potential difference between the second electrode  66  and the intermediate electrode  67 . When the electrical charge of the target  27  is taken as “e” and the common potential Vc is 0 V, a potential energy E 1  of the target  27  released from the leading end portion  62   b , as viewed from the through-hole  67   a , can be expressed through the following formula.
 
 E   1   =eV   H  
 
     Note that V H  represents a potential difference between the first electrode  65  and the intermediate electrode  67 . 
     Meanwhile, when the mass of the target  27  is taken as m and the velocity of the target  27  when passing through the through-hole  67   a  in the intermediate electrode  67  is taken as v 1 , a kinetic energy E 2  of the target  27  when passing through the through-hole  67   a  can be expressed through the following formula.
 
 E   2   =mv   1   2 /2
 
     Based on the law of energy conservation, E 1  can be equal to E 2 . Accordingly, the velocity v 1  of the target  27  when passing through the through-hole  67   a  in the intermediate electrode  67  can be expressed through the following formula.
 
 v   1 =(2 eV   H   /m ) 1/2  
 
     The target  27  that has passed through the through-hole  67   a  in the intermediate electrode  67  can pass through the through-hole  68   a  in the third electrode  68  having been further accelerated by the Coulomb force produced by the potential difference between the intermediate electrode  67  and the third electrode  68 . Assuming that the potential V L  of the third electrode  68  is approximately equal to −V H , a potential energy E3 of the target  27  that has passed through the through-hole  67   a , as viewed from the through-hole  68   a , can be expressed through the following formula.
 
 E   3   =eV   H  
 
     Note that V H  represents the potential difference between the intermediate electrode  67  and the third electrode  68 . 
     Meanwhile, when the velocity of the target  27  when passing through the through-hole  68   a  in the third electrode  68  is taken as v 2 , a kinetic energy E 4  of the target  27  when passing through the through-hole  68   a  can be expressed through the following formula.
 
 E   4   =mv   2   2 /2
 
     Based on the law of energy conservation, E 1 +E 3  can be equal to E 4 . Accordingly, the velocity v 2  of the target  27  when passing through the through-hole  68   a  in the third electrode  68  can be expressed through the following formula. 
     
       
         
           
             
               v 
               2 
             
             = 
             
               
                 
                   ( 
                   
                     
                       2 
                       · 
                       2 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     e 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         V 
                         H 
                       
                       / 
                       m 
                     
                   
                   ) 
                 
                 
                   1 
                   ⁢ 
                   
                     / 
                   
                   ⁢ 
                   2 
                 
               
               = 
               
                 
                   2 
                   
                     1 
                     ⁢ 
                     
                       / 
                     
                     ⁢ 
                     2 
                   
                 
                 · 
                 
                   v 
                   1 
                 
               
             
           
         
       
     
     Note that X 1/2  represents the positive square root of X. 
     As described thus far, in the case where the first potential V H  has been applied to the first electrode  65  and the 2nd potential V L  (where V L =−V H ) has been applied to the third electrode  68 , a potential difference of 2V H  can be produced between the first electrode  65  and the third electrode  68 . In this case, the insulation breakdown voltage of the cables, terminals, and so on that connect the respective power sources and electrodes may be V H , which is the potential difference between the common potential Vc (where Vc=0) and the first potential V H  or the second potential V L . Through this, a velocity of approximately 2 1/2  times (approximately 1.4 times) the velocity achieved when only the V H  potential difference is applied between the first electrode  65  and the third electrode  68  can be imparted on the target  27 . 
     The target control unit  52  may control the pressure adjuster  53  and the high-voltage pulse generator  56  so that the target  27  is outputted at a timing provided by the EUV light generation controller  5 . The target  27  outputted into the chamber  2  may be supplied to the plasma generation region  25  within the chamber  2 . 
     The pulse laser beam outputted from the laser apparatus  3  may be reflected by the high-reflecting mirrors  341  and  342  and may enter the laser beam focusing optical system  22   a  via the window  21 . The pulse laser beam that has entered into the laser beam focusing optical system  22   a  may be reflected by the off-axis paraboloid mirror  221  and the flat mirror  222 . The EUV light generation controller  5  may carry out control so that the target  27  outputted from the target supply device  26  is irradiated with the pulse laser beam at the timing when the target  27  reaches the plasma generation region  25 . 
     The potential of the second electrode  66  may be held at V 1  in the case where the output of the targets is to be temporarily stopped. Furthermore, the potentials of the first electrode  65 , the second electrode  66 , and the third electrode  68  may be controlled to take on the common potential Vc (ground potential, for example) in the case where the output of the targets is to be stopped for a longer period of time. 
     4.3 Variation 
     The intermediate electrode  67  is provided in the first embodiment that has been described with reference to  FIGS. 2 to 3B . Even if the potential of the second electrode  66  varies in pulses, variations in the slope of the potential between the intermediate electrode  67  and the third electrode  68  can be suppressed by providing the intermediate electrode  67 . For example, when a target is outputted and passes through the through-hole  67   a  in the intermediate electrode  67  and a pulse for outputting the next target is then applied to the second electrode  66 , the influence of that pulse on the target that has passed through the through-hole  67   a  in the intermediate electrode  67  can be suppressed. 
     However, the present disclosure is not limited to a case where the intermediate electrode  67  is provided. Even in the case where the intermediate electrode  67  is not provided, the travel velocity of the target can be increased while suppressing insulation breakdown in the case where a higher potential than the common potential Vc is applied to the first electrode  65  and a lower potential than the common potential Vc is applied to the third electrode  68 . Likewise, even in the case where the intermediate electrode  67  is not provided, the travel velocity of the target can be increased while suppressing insulation breakdown in the case where a lower potential than the common potential Vc is applied to the first electrode  65  and a higher potential than the common potential Vc is applied to the third electrode  68 . 
     In the case where the intermediate electrode  67  is not provided, the third potential applied to the second electrode  66  may be a potential that varies between the potentials V 1  and V 2 . Here, the potentials V H , V 1 , V 2 , Vc, and V L  may be in a relationship where V H ≧V 1 &gt;V 2 ≧V L  and V H &gt;Vc&gt;V L . Conversely, in the case where the first potential is a lower potential than the common potential, the relationship may be V H ≦V 1 &lt;V 2 ≦V L  and V H &lt;Vc&lt;V L . 
     In addition, although the first embodiment describes a case where the intermediate electrode  67  is electrically connected to the common potential Vc, the present disclosure is not limited thereto. A fourth potential V 4  that is between the first potential V H  and the second potential V L  may be applied to the intermediate electrode  67 . In this case, the potentials V H , V 1 , V 2 , V 4 , Vc, and V L  may be in a relationship where V H ≧V 1 &gt;V 2 ≧V 4 &gt;V L  and V H &gt;Vc&gt;V L . Conversely, in the case where the first potential is a lower potential than the common potential, the relationship may be V H ≦V 1 &lt;V 2 ≦V 4 &lt;V L  and V H &lt;Vc&lt;V L . A fourth power source (described later) configured to apply the fourth potential V 4  to the intermediate electrode  67  may be further provided. 
     5. EUV Light Generation Apparatus Having Target Collector as Fourth Electrode 
       FIG. 4  is a cross-sectional view illustrating part of an EUV light generation apparatus according to a second embodiment. In the second embodiment, the target collector  28   a  may be configured of a conductive material, and the output terminal of the high-voltage power source  58  may be electrically connected to the target collector  28   a  (fourth electrode). 
     In the aforementioned first embodiment, a slope of the potential from the leading end portion  62   b  of the nozzle plate  62  to the third electrode  68  and a slope of the potential from the third electrode  68  to the plasma generation region  25  can take on opposite potential slopes in the case where the chamber  2  is connected to the common potential Vc. Accordingly, even if the target  27  is accelerated in the area from the first electrode  65  to the third electrode  68 , the target  27  may decelerate to a certain extent in the area from the third electrode  68  to the plasma generation region  25 . 
     Accordingly, in the second embodiment, the high-voltage power source  58  may apply the second potential V L , which is the same potential as that applied to the third electrode  68 , to the target collector  28   a . Through this, the slope of the potential from the third electrode  68  to the plasma generation region  25  can be softened and the target  27  can be suppressed from decelerating. 
     The target collector  28   a  may be a cylindrical receptacle having a closed base. An electric insulation member  70  may be disposed between the target collector  28   a  and the conductive member of the chamber  2 . The conductive member of the chamber  2  may be connected to the common potential Vc (ground potential, for example). A plurality of grooves may be formed on an outer surface of the electric insulation member  70 . This plurality of grooves can suppress insulation breakdown between the target collector  28   a  and the conductive member of the chamber  2 . 
     The configuration may be the same as that described in the first embodiment in other respects. 
     According to the second embodiment, the target  27  that has passed through the through-hole  68   a  in the third electrode  68  can reach the plasma generation region  25  while being suppressed from decelerating due to the potential difference between the third electrode  68  and the conductive member of the chamber  2 . EUV light can be generated when the target  27  that has reached the plasma generation region  25  is irradiated with the pulse laser beam. Targets  27  that are not irradiated with the pulse laser beam upon reaching the plasma generation region  25  can pass through the plasma generation region  25  and be collected by the target collector  28   a.    
     Note that a fifth potential that is different from the second potential V L  may be applied to the target collector  28   a . Here, in the case where the first potential V H  is a higher potential than the potential applied to the conductive member of the chamber  2  (common potential Vc, for example), the fifth potential may be a lower potential than the potential applied to the conductive member of the chamber  2 . Alternatively, in the case where the first potential V H  is a lower potential than the potential applied to the conductive member of the chamber  2  (common potential Vc, for example), the fifth potential may be a higher potential than the potential applied to the conductive member of the chamber  2 . A fifth power source (not shown) configured to apply the fifth potential to the target collector  28   a  may be further provided. 
     6. EUV Light Generation Apparatus Having Fourth Electrode Separate from Target Collector 
       FIG. 5  is a cross-sectional view illustrating part of an EUV light generation apparatus according to a third embodiment. In the third embodiment, a downstream electrode  69  (fourth electrode) may be disposed in the vicinity of an area downstream from the plasma generation region  25  in the trajectory of the target  27 , separate from the target collector  28   a . The output terminal of the high-voltage power source  58  may be electrically connected to the downstream electrode  69 . Through this, the second potential V L , which is the same as the potential applied to the third electrode  68 , can be applied to the downstream electrode  69  as well. Accordingly, the slope of the potential from the third electrode  68  to the plasma generation region  25  can be softened and the target  27  can be suppressed from decelerating. The downstream electrode  69  may be anchored to the conductive member of the chamber  2  through an electric insulation member (not shown). 
     Note that the fifth potential that is different from the second potential V L  may be applied to the downstream electrode  69 . The magnitude of the fifth potential may be as described in the aforementioned second embodiment. The fifth power source (not shown) configured to apply the fifth potential to the downstream electrode  69  may be further provided. 
     In the third embodiment, the second potential V L  or the fifth potential may be applied to the target collector  28   a . Alternatively, the target collector  28   a  may be connected to the common potential Vc. 
     In addition, in the third embodiment, the fourth potential V 4  that is between the first potential V H  and the second potential V L  may be applied to the intermediate electrode  67 . The magnitude of the fourth potential V 4  may be as described in the aforementioned first embodiment. The fourth power source (the high-voltage power source  57 ) configured to apply the fourth potential V 4  to the intermediate electrode  67  may be further provided. 
     The present disclosure is not limited to the case where the fourth potential V 4  is applied to the intermediate electrode  67 . As described in the first embodiment, the intermediate electrode  67  may be connected to the common potential Vc. 
     The configuration may be the same as that described in the second embodiment in other respects. 
     In addition, the present disclosure is not limited to the case where the target collector  28   a  or the downstream electrode  69  (which both correspond to the fourth electrode) is provided. The travel velocity of the target at the plasma generation region  25  can be increased while suppressing insulation breakdown even in the case where the fourth electrode is not provided and the conductive member of the chamber  2  is connected to the common potential. This is because the velocity of the target  27  at the point in time when the target  27  passes through the plasma generation region  25  can be greater than the velocity of the target  27  at the point in time when the target  27  passes near the intermediate electrode  67  connected to the common potential. 
     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.”