Patent Publication Number: US-2013228709-A1

Title: Target supply device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2012-047983 filed Mar. 5, 2012. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to target supply devices. 
     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 which combines a system for generating EUV light at a wavelength of approximately 13 nm 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 target supply device according to one aspect of the present disclosure may include a reservoir configured to store a liquid target material, a first electrode electrically connected to the liquid target material stored in the reservoir, a nozzle having a through-hole through which the liquid target material stored in the reservoir is discharged, a first power supply configured to apply a first potential to the first electrode, a circuit electrically connected to the first electrode and configured to suppress a potential variation of the first electrode, a second electrode provided to face the through-hole in the nozzle, and a second power supply configured to apply a second potential that is different from the first 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 a configuration of an exemplary LPP-type EUV light generation system. 
         FIG. 2  is a partial sectional view illustrating an exemplary configuration of an EUV light generation apparatus including a target supply device according to a first embodiment of the present disclosure. 
         FIG. 3A  is a sectional view illustrating the target supply device shown in  FIG. 2  and peripheral components thereof. 
         FIG. 3B  is a waveform diagram showing potentials applied to electrodes in the target supply device shown in  FIG. 2 . 
         FIG. 4  is an equivalent circuit diagram showing one example of a potential variation control circuit used in the target supply device according to the first embodiment. 
         FIG. 5  is an equivalent circuit diagram showing another example of a potential variation control circuit used in the target supply device according to the first embodiment. 
         FIG. 6A  is a partial sectional view illustrating a target supply device according to a second embodiment of the present disclosure and peripheral components thereof. 
         FIG. 6B  is a waveform diagram showing potentials applied to electrodes in the target supply device shown in FIG.  6 A. 
         FIG. 7  is a partial sectional view illustrating a target supply device according to a third embodiment of the present disclosure and peripheral components thereof. 
         FIG. 8  is a diagram for describing a method for controlling the direction of a target. 
         FIG. 9  is a partial sectional view illustrating a target supply device according to a fourth embodiment of the present disclosure and peripheral components thereof. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, selected embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The embodiments described below are merely illustrative in nature and do not limit the scope of this disclosure. Further, the configuration(s) and operation(s) described in each embodiment are not all essential in implementing 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. Overview of EUV Light Generation System 
     2.1 Configuration 
     2.2 Operation 
     3. Target Supply Device Including Potential Variation Control Circuit: First Embodiment 
     3.1 Configuration 
     3.2 Operation 
     3.3 Examples of Potential Variation Control Circuit 
     4. Target Supply Device: Second Embodiment 
     5. Target Supply Device: Third Embodiment 
     5.1 Configuration 
     5.2 Operation 
     6. Target Supply Device: Fourth Embodiment 
     1. Overview 
     In an LPP type EUV light generation apparatus, a target supply device may supply a target to a plasma generation region inside a chamber, and the target may be irradiated with a pulse laser beam in the plasma generation region. Upon being irradiated with the pulse laser beam, the target may be turned into plasma, and EUV light may be emitted from the plasma. 
     The target supply device may include a reservoir for storing a target material in a molten state, a first electrode electrically connected to the molten target material, and a first power supply for applying a first potential to the first electrode. The target supply device may further include a second electrode provided to face a nozzle of the reservoir and a second power supply for applying a second potential that is different from the first potential to the second electrode. 
     A target outputted through the nozzle may be charged by a potential difference between the first and second electrodes. The speed and the trajectory of the charged target may be controlled through a potential gradient along a path from the nozzle to the plasma generation region. 
     However, plasma that emits EUV light may include charged particles such as electrons and ions of the target material. When these charged particles reach the nozzle of the target supply device, the potential of the first electrode may change unintentionally. When the potential of the first electrode changes, a charge given to the target may change as well, and the speed and the trajectory of the target may become unstable. As a result, the position at which EUV light emitting plasma is generated may change unintentionally. 
     According to one or more embodiments of the present disclosure, the target supply device may further include a circuit electrically connected to the first electrode and configured to control a variation in the potential of the first electrode. Thus, a variation in a charge given to a target may be suppressed, and the stability of the position at which EUV light is emitted may be improved. 
     2. Overview of EUV Light Generation System 
     2.1 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 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 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. 
     2.2 Operation 
     With continued reference to  FIG. 1 , a pulse laser beam  31  outputted from the laser apparatus  3  may pass through the laser beam direction control unit  34  and be outputted therefrom as 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  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. 
     3. Target Supply Device Including Potential Variation Control Circuit 
     3.1 Configuration 
       FIG. 2  is a partial sectional view illustrating an exemplary configuration of an EUV light generation apparatus including a target supply device according to a first embodiment of the present disclosure.  FIG. 3A  is a sectional view illustrating a target supply device shown in  FIG. 2  and peripheral components thereof.  FIG. 33  is a waveform diagram showing potentials applied to electrodes in the target supply device shown in  FIG. 2 . 
     As shown in  FIG. 2 , a laser beam focusing optical system  22   a , the EUV collector mirror  23 , an EUV collector mirror mount  41 , and plates  42  and  43  may be provided inside the chamber  2 . 
     The plate  42  may be attached to the chamber  2 , and the plate  43  may be attached to the plate  42 . The EUV collector mirror  23  may be attached to the plate  42  through the EUV collector mirror mount  41 . 
     The laser beam focusing optical system  22   a  may include an off-axis paraboloidal mirror  221 , a flat mirror  222 , and holders  223  and  224  for the respective mirrors  221  and  222 . The off-axis paraboloidal mirror  221  and the flat mirror  222  may be fixed to the plate  43  through the respective mirror holders  223  and  224  such that a pulse laser beam reflected sequentially by the mirrors  221  and  222  is focused in the plasma generation region  25 . 
     A beam steering unit  34   a  and the EUV light generation controller  5  may be provided outside the chamber  2 . The beam steering unit  34   a  may include high-reflection mirrors  341  and  342  and holders  343  and  344  for holding the respective high-reflection mirrors  341  and  342 . 
     With reference to  FIGS. 2 and 3A , the target supply device  26  may be mounted to the chamber  2 . The target supply device  26  may include a reservoir  61 , a target controller  52 , a pressure adjuster  53 , an inert gas cylinder  54 , a DC voltage power supply  55 , and a pulse voltage power supply  58 . The target supply device  26  may further include a nozzle plate  62 , a first electrode  63 , an electrically insulating member  65 , and a second electrode  66 . 
     The reservoir  61  may be configured to store a target material in a molten state. A heater (not separately shown) and a heater power supply (not separately shown) may be provided to keep the target material in a molten state. The reservoir  61  may be formed of a material that is not susceptible to reacting with a target material and that has electrically non-conductive properties. For example, when tin is used as a target material, the reservoir  61  may be formed of quartz (SiO 2 ), alumina ceramics (Al 2 O 3 ), or the like. A through-hole may be formed in the wall of the chamber  2 , and a flange  61   a  of the reservoir  61  may be fixed to surround the through-hole. 
     The nozzle plate  62  may be fixed to an output end of the reservoir  61 . The nozzle plate  62  may be formed of a material having electrically conductive properties or a material having electrically non-conductive properties. The nozzle plate  62  may have a through-hole  62   b  formed therein through which a liquid target material passes. The nozzle plate  62  may have a protrusion formed at an output side, and the aforementioned through-hole  62   b  may open at the protrusion. Thus, when a potential difference is generated between the first electrode  63  and the second electrode  66 , the electric field may be enhanced at the target material in the aforementioned through-hole  62   b.    
     The electrically insulating member  65  may be cylindrical in shape and fixed to the reservoir  61  to surround a part of the output end of the reservoir  61 . The electrically insulating member  65  may hold the nozzle plate  62  and the second electrode  66  thereinside to provide electrical insulation between the nozzle plate  62  and the second electrode  66 . The second electrode  66  may be provided to face the outer surface of the nozzle plate  62 . The second electrode  66  may have a through-hole  66   a  formed therein through which targets  27  may pass. 
     The target controller  52  may be configured to output control signals to the pressure adjuster  53 , the DC voltage power supply  55 , and the pulse voltage power supply  58 , respectively. The inert gas cylinder  54  may be connected to the pressure adjuster  53  through a pipe. The pressure adjuster  53  may be in communication with the interior of the reservoir  61  through another pipe. 
     The output terminal of the DC voltage power supply  55  may be electrically connected to a high-voltage cable, and this high-voltage cable may be electrically connected to the first electrode  63  inside the reservoir  61  through a feedthrough  57   a  provided in the reservoir  61 . The first electrode  63  may be in contact with the target material stored in the reservoir  61 . 
     The output terminal of the pulse voltage power supply  58  may be electrically connected to the second electrode  66  through a feedthrough  58   a  provided in the chamber  2  and through a through-hole  65   a  provided in the electrically insulating member  65 . 
     3.2 Operation 
     The pressure adjuster  53  may be configured to adjust the pressure of an inert gas supplied from the inert gas cylinder  54  in accordance with a control signal from the target controller  52 . The pressure-adjusted inert gas may then be introduced into the reservoir  61  to pressurize the molten target material in the reservoir  61 . As the target material is pressurized, the target material may protrude slightly through the through-hole  62   b  at the protrusion. 
     The DC voltage power supply  55  may apply a potential P 1  to the target material through the first electrode  63  in the reservoir  61  in accordance with a control signal from the target controller  52 . The pulse voltage power supply  58  may apply a pulsed voltage P 2  to the second electrode  66  in accordance with a control signal from the target controller  52 . Accordingly, the target material may be charged, and an electric field may be generated between the target material and the second electrode  66 . As a result, the Coulomb force may be generated between the target material and the second electrode  66 . 
     In particular, as stated above, the electric field may be enhanced at the target material protruding from the through-hole  62   b  at the protrusion, and thus the Coulomb force may be enhanced between the target material protruding from the through-hole  62   b  at the protrusion and the second electrode  66 . This Coulomb force may cause a target  27  to be outputted from the through-hole  62   b  at the protrusion in the form of a charged droplet. 
     The target controller  52  may be configured to control the pressure adjuster  53  and the pulse voltage power supply  58  so that a target  27  is outputted at a timing instructed by the EUV light generation controller  5 . A target  27  outputted into the chamber  2  may be supplied to the plasma generation region  25  inside the chamber  2 . 
     A pulse laser beam outputted from the laser apparatus  3  may be reflected sequentially by the high-reflection mirrors  341  and  342 , and may enter the laser beam focusing optical system  22   a  through the window  21 . The pulse laser beam that has entered the laser beam focusing optical system  22   a  may be reflected sequentially by the off-axis paraboloidal mirror  221  and the flat mirror  222  and be focused in the plasma generation region  25 . The EUV light generation controller  5  may control the laser apparatus  3  and the target supply device  26  such that a target  27  outputted from the target supply device  26  is irradiated with the pulse laser beam at a timing at which the target  27  reaches the plasma generation region  25 . 
     With reference to  FIG. 3B , the DC voltage power supply  55  may retain a potential P 1  of the target material in the reservoir  61  at a predetermined potential Ph of, for example, 20 kV. The pulse voltage power supply  58  may first retain a potential P 2  of the second electrode  66  at a predetermined potential Ph of, for example, 20 kV, and change the potential P 2  to a potential P 0  of, for example, 0 V when a target  27  is to be outputted. Then, the pulse voltage power supply  58  may change the potential P 2  back to the potential Ph after a predetermined time ΔT elapses. The predetermined time ΔT may correspond to a pulse duration. Here, the potential Ph and the potential P 0  may satisfy a relationship of Ph&gt;P 0 . The potential P 0  may be the same as the potential of the chamber  2 , and the potential of the chamber  2  may be a ground potential of 0 V. 
     When a target  27  outputted from the target supply device  26  is irradiated with a pulse laser beam, EUV light emitting plasma may be generated from the target  27 . If charged particles contained in the plasma reach the nozzle plate  62  of the target supply device  26  or the vicinity thereof, the potential P 1  of the target material may temporarily change. In this case, even if the potential P 2  of the second electrode  66  is controlled in order to output a subsequent target  27 , a potential difference between the potential P 1  of the target material and the potential P 2  of the second electrode  66  may not be controlled to a desired potential difference. Accordingly, the speed and/or the trajectory of the subsequent target(s)  27  may vary. 
     Thus, a potential variation control circuit  59  (see  FIG. 3A ) may be connected to the first electrode  63 . The potential variation control circuit  59  may be connected to the first electrode  63  through a wire connecting the first electrode  63  to the DC voltage power supply  55 . In one embodiment, the potential variation control circuit  59  may be connected to a wire between the first electrode  63  and the DC voltage power supply  55  at a position closer to the first electrode  63 . 
     3.3 Examples of Potential Variation Control Circuit 
       FIG. 4  is an equivalent circuit diagram showing one example of a potential variation control circuit used in the target supply device according to the first embodiment. In  FIG. 4 , the liquid target material in contact with the first electrode  63  and the wall of the chamber  2  connected to the potential P 0  are electrically insulated from each other, and thus the target supply device  26  is expressed as a single capacitor. 
     A potential variation control circuit  59   a  may include a capacitor  59   c  connected at one terminal to the first electrode  63  and at the other terminal to the ground potential. The capacitor  59   c  may have capacitance in a range from 5 nF to 10 nF inclusive. When the potential of the first electrode  63  is to change, the capacitor  59   c  of the potential variation control circuit  59   a  may supply a charge to the first electrode  63  or accept a charge from the first electrode  63 . Accordingly, a variation in the potential of the first electrode  63  may be controlled. That is, the potential variation control circuit  59   a  that includes the capacitor  59   c  may function as a low pass filter for suppressing a high-frequency component of a potential variation at the first electrode  63 . 
     The above-described configuration may allow a potential difference between the potential P 1  of the target material and the potential P 2  of the second electrode  66  to be controlled to a desired potential difference. Accordingly, a variation in a charge given to a target  27  may be suppressed. As a result, a variation in the speed and/or the trajectory of targets  27  may be reduced. 
       FIG. 5  is an equivalent circuit diagram showing another example of a potential variation control circuit used in the target supply device according to the first embodiment. In  FIG. 5  as well, the target supply device  26  is expressed as a single capacitor. 
     A potential variation control circuit  59   b  may include the capacitor  59   c  and a resistor  59   r  that is connected at one terminal to the first electrode  63  and at the other terminal to the ground potential. The resistor  59   r  may have a resistance in a range from 50 kΩ to 200 kΩ inclusive. With the resistor  59   r  being further included, a time constant of the potential variation control circuit  59   b  may be adjusted, and the potential variation of the first electrode  63  may further be suppressed. 
     4. Target Supply Device 
     Second Embodiment 
       FIG. 6A  is a partial sectional view illustrating a target supply device according to a second embodiment of the present disclosure and peripheral components thereof.  FIG. 6B  is a waveform diagram showing potentials applied to electrodes in the target supply device shown in  FIG. 6A . In the second embodiment, a reservoir  61  may be formed of a material that is not susceptible to reacting with the target material and that has electrically conductive properties such as molybdenum (Mo) and tungsten (W). A third electrode  67  may further be provided downstream from the second electrode  66  in the direction in which the target  27  travels. The third electrode  67  may be held inside the electrically insulating member  65 . 
     A through-hole may be formed in the wall of the chamber  2 , and a flange  84  may be fixed to cover the through-hole in the chamber  2 . A through-hole may be formed in the flange  84 , and the reservoir  61  of the target supply device  26  may be fixed to the flange  84  to pass through the through-hole. The flange  84  may have electrically non-conductive properties. This configuration allows the reservoir  61  having electrically conductive properties to be electrically insulated from the chamber  2  having electrically conductive properties. 
     The feedthrough  57   a  (see  FIG. 3A ) may not be provided in the reservoir  61 . In the second embodiment, the reservoir  61  may serve as the first electrode  63  and may be electrically in contact with the liquid target material. 
     The DC voltage power supply  55  may retain a potential P 1  of the target material in the reservoir  61  at a predetermined potential Ph of, for example, 20 kV. The pulse voltage power supply  58  may first retain a potential P 2  of the second electrode  66  at a potential Pm of, for example, 10 kV, and change the potential P 2  to a potential P 0  of, for example, 0 V when a target  27  is to be outputted. Then, the pulse voltage power supply  58  may change the potential P 2  back to the potential Pm after a predetermined time ΔT elapses. The predetermined time ΔT may correspond to a pulse duration. Here, the potential Ph, the potential Pm, and the potential P 0  may satisfy a relationship of Ph≧Pm&gt;P 0 . The potential P 0  may be the same of the potential of the chamber  2 , and the potential of the chamber  2  may be a ground potential of 0 V. 
     Thus, a positively charged target  27  may be pulled out through the through-hole  62   b  in the nozzle plate  62 . The positively charged target  27  may be pulled out toward the second electrode  66  to which a potential P 2  that is lower than the potential P 1  is applied, and may pass through the through-hole  66   a  in the second electrode  66 . 
     A potential P 3  of the third electrode  67  may be retained at the potential P 0 . Accordingly, the target  27  that has passed through the through-hole  66   a  may be accelerated toward the third electrode  67  to which the potential P 0  that is lower than the potential of the second electrode  66  is applied. 
     In this way, the target  27  may be accelerated through a potential gradient formed along a path from the nozzle plate  62  to the third electrode  67 , and may pass through a through-hole  67   a  formed in the third electrode  67 . Along the path of the target  27  that has passed through the through-hole  67   a , the potential gradient may be gradual since the potential of the chamber  2  is the ground potential. Accordingly, after passing through the through-hole  67   a , the target  27  may travel inside the chamber  2  with momentum at the time of passing through the through-hole  67   a.    
     In the second embodiment as well, since the potential variation control circuit  59  is connected to the reservoir  61 , the potential variation of the reservoir  61  may be suppressed. According to this configuration, since a potential difference between the potential P 1  of the target material and the potential P 2  of the second electrode  66  may be controlled to a desired potential difference, a variation in a charge given to a target  27  may be suppressed. Accordingly, a variation in the speed of the target  27  accelerated through the third electrode  67  may be suppressed. The third electrode  67  may also be included in the target supply device according to the first embodiment. 
     5. Target Supply Device 
     Third Embodiment 
     5.1 Configuration 
       FIG. 7  is a partial sectional view illustrating a target supply device according to a third embodiment of the present disclosure and peripheral components thereof. In the third embodiment, the target supply device  26  may further include a cover  85  and fourth electrodes  70 . 
     As shown in  FIG. 7 , the main constituent elements of the target supply device  26  such as the reservoir  61  may be housed in a shielding container formed by the cover  85  and a lid  86  attached to seal the cover  85 . The cover  85  may be mounted to the wall of the chamber  2 . The cover  85  may have a through-hole  85   a  formed therein through which targets  27  pass. The reservoir  61  may be mounted to the lid  86 . 
     The cover  85  may be formed of an electrically conductive material such as metal and may have electrically conductive properties. The cover  85  may be electrically connected to the chamber  2 . Alternatively, the cover  85  may be electrically connected to the chamber  2  through an electrically conductive connection member such as a wire. The chamber  2  may be electrically connected to the ground potential. An electrically non-conductive material such as mullite may be used as a material for the lid  86 . Accordingly, the cover  85  and the reservoir  61  may be electrically insulated from each other. The cover  85  may serve to shield electrically non-conductive members such as the electrically insulating member  65  from charged particles emitted from plasma generated in the plasma generation region  25 . 
     A plurality of fourth electrodes  70  may be provided downstream from the third electrode  67  in the direction in which the target  27  travels. In the third embodiment, two pairs of fourth electrodes  70  may be provided. The fourth electrodes  70  may be held by the electrically insulating member  65  to be electrically insulated from one another. 
     Wires for the fourth electrodes  70  may be electrically connected to a power supply  57  through a through-hole in the electrically insulating member  65  and through a feedthrough  90   c  provided in the lid  86 . A wire for the third electrode  67  may be electrically connected to the cover  85  through another through-hole in the electrically insulating member  65 . A wire for the second electrode  66  may be electrically connected to the pulse voltage power supply  58  through yet another through-hole in the electrically insulating member  65  and through a feedthrough  90   a.    
     The reservoir  61  having electrically conductive properties may serve as the first electrode  63  to apply a potential to the target material. Alternatively, when the nozzle plate  62  has electrically conductive properties, the nozzle plate  62  may serve as the first electrode  63 . A wire for the reservoir  61  may be electrically connected to the DC voltage power supply  55  through the feedthrough  90   a.    
     The target supply device  26  may further include a heater  64 , a heater power supply  51 , a temperature sensor  73 , and a temperature controller  56 . The heater power supply  51  may be connected to the heater  64  with two wires through the feedthrough  90   c . The temperature sensor  73  may be connected to the temperature controller  56  with two wires through the feedthrough  90   c.    
     The heater  64  may be mounted to the outer surface of the reservoir  61  to heat the reservoir  61 . The temperature sensor  73  may measure the temperature of the reservoir  61  and output a signal indicative of a measurement result. A signal from the temperature sensor  73  may be inputted to the temperature controller  56 . 
     A control signal from the target controller  52  may be inputted to the temperature controller  56 . The temperature controller  56  may output a drive signal to the heater power supply  51  in accordance with a signal from the temperature sensor  73  and a control signal from the target controller  52 . The heater power supply  51  may supply power to the heater  64  in accordance with a drive signal from the temperature controller  56 . Thus, the reservoir  61  may be heated by the heater  64  to a temperature equal to or higher than the melting point of the target material. As a result, the target material may be stored in the reservoir  61  in a molten state. 
     Each of the target controller  52 , the pressure adjuster  53 , the power supply  57 , the temperature controller  56 , and the heater power supply  51  may be connected to the secondary of an isolation transformer  100  and supplied with power from the isolation transformer  100 . The primary of the isolation transformer  100  may be connected to an AC power supply  101 . Each of the target controller  52 , the pressure adjuster  53 , the power supply  57 , the temperature controller  56 , and the heater power supply  51  may be retained at a potential equivalent to that of the target material. That is, each of the target controller  52 , the pressure adjuster  53 , the power supply  57 , the temperature controller  56 , and the heater power supply  51  may be electrically insulated from the chamber  2  and/or the EUV light generation controller  5 . The target controller  52  and the EUV light generation controller  5  may be connected through an optical fiber for transmitting signals therebetween. 
     A wire connecting the reservoir  61  to the DC voltage power supply  55  may be connected to one of the two wires connecting the temperature controller  56  to the temperature sensor  73 . The wire connecting the first electrode  63  to the DC voltage power supply  55  may further be connected to one of the two wires connecting the heater power supply  51  to the heater  64 . Accordingly, an electric discharge between wires may be suppressed. 
     5.2 Operation 
     The target controller  52  may be configured to output control signals to the pressure adjuster  53 , the DC voltage power supply  55 , and the pulse voltage power supply  58 , respectively. Thus, a charged target  27  may be pulled out through the through-hole  62   b  formed in the nozzle plate  62 , and may pass through the through-hole  66   a  in the second electrode  66 . The target  27  that has passed through the through-hole  66   a  may be accelerated through an electric field between the second electrode  66  and the third electrode  67  that is connected to the ground potential, and pass through the through-hole  67   a  in the third electrode  67 . 
     The two pairs of fourth electrodes  70  may cause an electric field to act on the target  27  that has passed through the through-hole  67   a  to deflect the travel direction of the target  27 . When a target  27  needs to be deflected, the target controller  52  may output a control signal to the power supply  57  to control a potential difference between each pair of the fourth electrodes  70 . The power supply  57  may be configured to apply a potential difference between each pair of the fourth electrodes  70 . 
     A target  27  may be deflected based on a control signal from the EUV light generation controller  5 . Various signals may be transmitted between the EUV light generation controller  5  and the target controller  52 . For example, the EUV light generation controller  5  may obtain information on the trajectory of a target  27  from a target sensor (not separately shown), and calculate a difference between the obtained trajectory and an ideal trajectory of a target  27 . Then, the EUV light generation controller  5  may send a signal to the target controller  52  to control a voltage applied between each pair of the fourth electrodes  70  to bring the aforementioned difference closer to zero. A target  27  that has passed through the two pairs of the fourth electrodes  70  may then pass through the through-hole  85   a  in the cover  85 . 
       FIG. 8  is a diagram for discussing a method for deflecting a target. In an example below, the direction of a charged target  27  moving in the Z-direction may be deflected through an electric field in the X-direction using a pair of flat electrodes  70   a  and  70   b.    
     A charged target  27  having a charge Q may be subjected to the Coulomb force F expressed in the following expression through an electric field E between the flat electrodes  70   a  and  70   b:    
     
       
      
       F=QE  
      
     
     The following description assumes that the electric lines of force between the flat electrodes  70   a  and  70   b  are substantially parallel to one another at any given locations between the electrodes. The electric field E may be expressed in the following expression through a potential difference (Pa−Pb) between a potential Pa applied to the flat electrode  70   a  and a potential Pb applied to the flat electrode  70   b , and a gap length G between the flat electrodes  70   a  and  70   b:    
         E =( Pa−Pb )/ G    
     When a target  27  enters the electric field with an initial speed V 0 , the target  27  may be subjected to the Coulomb force in the X-direction, and thus the direction of the target  27  may be deflected. The target  27  may be accelerated in the X-direction by the Coulomb force F while moving in the Z-direction with a Z-direction velocity component V z  (V z =V 0 ). The target  27  is subjected to the Coulomb force F while moving in the electric field. An acceleration a in the Z-direction at this time may be obtained from the expression below when a mass m of the target  27  is known: 
     
       
      
       F=ma  
      
     
     Further, an X-direction velocity component V x  when the target  27  exits the electric field may be obtained through the following expression: 
     
       
      
       V 
       x 
       =aL/V 
       z  
      
     
     Here, L is the length of the electrodes  70  in the Z-direction. 
     A speed V of the target  27  when the target  27  exits the electric field is expressed in the following expression by the Z-direction velocity component V z  and the X-direction velocity component V x : 
         V =( V   z   2   +V   x   2 ) 1/2    
     In this way, providing a potential difference (Pa−Pb) to cause an electric field to act on a part of the trajectory of the target  27  may make it possible to deflect the direction of the target  27 . Further, adjusting the potential difference (Pa−Pb) may make it possible to control the deflection amount. Through this control, the target  27  that exits the electric field may move at a speed V and arrives at a position at which the target  27  is to be irradiated with a pulse laser beam. Similarly, with respect to the Y-direction, the direction of the target  27  may be controlled by disposing a pair of flat electrodes in the Y-direction. 
     In the third embodiment as well, since the potential variation control circuit  59  is connected to the reservoir  61 , the potential variation of the reservoir  61  may be suppressed. Accordingly, a potential difference between the potential P 1  of the target material and the potential P 2  of the second electrode  66  may be controlled to a desired potential difference, and a variation in a charge given to the target  27  may be suppressed. Thus, a variation in the speed of the target  27  accelerated through the third electrode  67  may be suppressed. Further, the trajectory of the target  27  may be controlled to a desired trajectory through the fourth electrodes  70 . Here, the fourth electrodes  70  may be included in the target supply device according to the first or second embodiment. 
     6. Target Supply Device 
     Fourth Embodiment 
       FIG. 9  is a partial sectional view illustrating a target supply device according to a fourth embodiment of the present disclosure and peripheral component thereof. 
     In the fourth embodiment, one of the terminals of a potential variation control circuit  59   d  may be in contact with the reservoir  61  having electrically conductive properties and may be electrically connected to a liquid target material through the reservoir  61 . The reservoir  61  may serve as the first electrode  63 . Accordingly, the one of the terminals of the potential variation control circuit  59   d  may be directly connected to the reservoir  61 , instead of being connected to a wire connecting the DC voltage power supply  55  to the reservoir  61 . The other terminal of the potential variation control circuit  59   d  may be connected to the cover  85 . 
     In the fourth embodiment as well, since the potential variation control circuit  59   d  is connected to the reservoir  61 , the potential variation of the reservoir  61  may be suppressed. Accordingly, a variation in the speed of the target  27  accelerated through the third electrode  67  may be suppressed. Further, the trajectory of the target  27  may be controlled to a desired trajectory through the fourth electrodes  70 . 
     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.”