Patent Publication Number: US-2023142875-A1

Title: Euv light generation apparatus, electronic device manufacturing method, and inspection method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of Japanese Patent Application No. 2021-184363, filed on Nov. 11, 2021, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an EUV light generation apparatus, an electronic device manufacturing method, and an inspection method. 
     2. Related Art 
     Recently, miniaturization of a transfer pattern in optical lithography of a semiconductor process has been rapidly proceeding along with miniaturization of the semiconductor process. In the next generation, microfabrication at 10 nm or less will be required. Therefore, it is expected to develop a semiconductor exposure apparatus that combines an apparatus for generating extreme ultraviolet (EUV) light having a wavelength of about 13 nm with a reduced projection reflection optical system. 
     As the EUV light generation apparatus, a laser produced plasma (LPP) type apparatus using plasma generated by irradiating a target substance with laser light has been developed. 
     LIST OF DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Patent Application Publication No. 2006-128157 
         Patent Document 2: US Patent Application Publication No. 2010/0294953 
         Patent Document 3: US Patent Application Publication No. 2017/0171955 
       
    
     SUMMARY 
     An EUV light generation apparatus according to an aspect of the present disclosure generates EUV light by irradiating a target with pulse laser light to turn the target into plasma. Here, the EUV light generation apparatus includes a chamber, a target supply unit configured to supply the target to a plasma generation region in the chamber, a pulse laser device configured to generate pulse laser light to be radiated to the target, and a processor configured to change a generation frequency of the target generated by the target supply unit to a natural number multiple of an irradiation frequency of the pulse laser light based on a size of the target or related information related to the size of the target. 
     An electronic device manufacturing method according to an aspect of the present disclosure includes generating EUV light as turning a target into plasma by irradiating the target with pulse laser light using an EUV light generation apparatus, outputting the EUV light to an exposure apparatus, and exposing a photosensitive substrate to the EUV light in the exposure apparatus to manufacture an electronic device. Here, the EUV light generation apparatus includes a chamber, a target supply unit configured to supply the target to a plasma generation region in the chamber, a pulse laser device configured to generate the pulse laser light to be radiated to the target, and a processor configured to change a generation frequency of the target to a natural number multiple of an irradiation frequency of the pulse laser light based on a size of the target. 
     An inspection method according to an aspect of the present disclosure includes generating EUV light as turning a target into plasma by irradiating the target with pulse laser light using an EUV light generation apparatus, outputting the EUV light to an inspection apparatus as a light source for inspection, and exposing a mask to the EUV light to inspect the mask in the inspection apparatus. Here, the EUV light generation apparatus includes a chamber, a target supply unit configured to supply the target to a plasma generation region in the chamber, a pulse laser device configured to generate the pulse laser light to be radiated to the target, and a processor configured to change a generation frequency of the target to a natural number multiple of an irradiation frequency of the pulse laser light based on a size of the target. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure will be described below merely as examples with reference to the accompanying drawings. 
         FIG.  1    is a diagram schematically showing the configuration of an LPP EUV light generation apparatus. 
         FIG.  2    is a sectional view showing the configuration of an EUV light generation apparatus according to a comparative example. 
         FIG.  3    is a sectional view of the EUV light generation apparatus when viewed from a different point of view from  FIG.  2   . 
         FIG.  4    is a main flowchart showing the operation of the EUV light generation system. 
         FIG.  5    is a flowchart showing a target generation process shown in  FIG.  4   . 
         FIG.  6    is a diagram conceptually showing a target generation mechanism. 
         FIG.  7    is a sectional view showing the configuration of the EUV light generation apparatus according to a first embodiment. 
         FIG.  8    is an example of a flowchart showing the target generation process performed by a target generation processor based on the size of the target. 
         FIG.  9    is a schematic diagram of a case in which the generation frequency is increased when the diameter of the target is increased due to an increase in the diameter of the nozzle hole. 
         FIG.  10    is a schematic diagram of a case in which the generation frequency is decreased when the diameter of the target is decreased due to a decrease in the diameter of the nozzle hole. 
         FIG.  11    is a diagram showing an example of the configuration of the target generation processor that performs target generation process based on related information related to the size of the target. 
         FIG.  12    is an example of a reference table used in the target generation process. 
         FIG.  13    is a diagram showing the relationship between elapsed time and the diameter of the nozzle hole. 
         FIG.  14    is a flowchart showing the target generation process performed by the target generation processor based on the elapsed time. 
         FIG.  15    is a diagram showing the configuration of a target supply unit including a refill mechanism. 
         FIG.  16    is a graph showing an example of a temporal change of the total mass of solid tin replenished to a reservoir by the refill mechanism. 
         FIG.  17    is a flowchart showing the target generation process performed by the target generation processor based on the replenishment amount or the replenishment interval of target substance. 
         FIG.  18    is a diagram showing the configuration of the target supply unit including a refill mechanism different from that shown in  FIG.  15   . 
         FIG.  19    is a graph showing a supply mass and a time interval when the target substance is liquid tin. 
         FIG.  20    is a diagram schematically showing a state in which the target passes through a target detection unit. 
         FIG.  21    is a graph showing the relationship between a signal intensity and time. 
         FIG.  22    is a flowchart showing the target generation process performed by the target generation processor based on the signal intensity of a passage timing signal. 
         FIG.  23    schematically shows the configuration of the EUV light generation system according to a fifth embodiment. 
         FIG.  24    is a diagram schematically showing a state in which an image measurement unit detects the target in a mist form. 
         FIG.  25    schematically shows the configuration of an exposure apparatus connected to the EUV light generation apparatus. 
         FIG.  26    schematically shows the configuration of an inspection apparatus connected to the EUV light generation apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;Content&gt; 
     1. Overall description of EUV light generation system 
     1.1 Configuration 
     1.2 Operation 
     2. EUV light generation apparatus according to comparative example 
     2.1 Configuration 
     2.2 Operation 
     2.3 Problem 
     3. EUV light generation apparatus of first embodiment 
     3.1 Configuration 
     3.2 Operation 
     3.3 Effects 
     4. EUV light generation apparatus of second embodiment 
     4.1 Configuration 
     4.2 Operation 
     4.3 Effects 
     5. EUV light generation apparatus of third embodiment 
     5.1 Configuration 
     5.2 Operation 
     5.3 Effects 
     6. EUV light generation apparatus of fourth embodiment 
     6.1 Configuration 
     6.2 Operation 
     6.3 Effects 
     7. EUV light generation apparatus of fifth embodiment 
     7.1 Configuration 
     7.2 Operation 
     7.3 Effects 
     8. Others 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The embodiments described below show some examples of the present disclosure and do not limit the contents of the present disclosure. Also, all configurations and operation described in the embodiments are not necessarily essential as configurations and operation of the present disclosure. Here, the same components are denoted by the same reference numerals, and duplicate description thereof is omitted. 
     1. Overall Description of EUV Light Generation System 
     1.1 Configuration 
       FIG.  1    schematically shows the configuration of an LPP EUV light generation system  11 . An EUV light generation apparatus  1  is used with at least one pulse laser device (hereinafter, simply called a laser device)  3 . In the present disclosure, a system including the EUV light generation apparatus  1  and the laser device  3  is referred to as the EUV light generation system  11 . As shown in  FIG.  1    and described in detail below, the EUV light generation apparatus  1  includes a chamber  2  and a target supply unit  26 . The chamber  2  is configured sealable. The target supply unit  26  is attached, for example, to penetrate through a wall of the chamber  2 . The material of the target  27  output from the target supply unit  26  includes tin. The material of the target  27  may also include a combination of tin and terbium, gadolinium, lithium, or xenon. The target  27  has a droplet shape. 
     At least one through hole is formed in the wall of the chamber  2 . The through hole is provided with a window  21 . Pulse laser light  32  output from the laser device  3  passes through the window  21 . For example, an EUV light concentrating mirror  23  having a spheroidal reflection surface is arranged in the chamber  2 . The EUV light concentrating mirror  23  has first and second focal points. A multilayer reflection film in which, for example, molybdenum and silicon are alternately stacked is formed on a surface of the EUV light concentrating mirror  23 . The EUV light concentrating mirror  23  is arranged, for example, such that the first focal point is located in a plasma generation region  25  and the second focal point is located at an intermediate focal point (IF)  292 . A through hole  24  is formed at the center of the EUV light concentrating mirror  23 . Pulse laser light  33  passes through the through hole  24 . 
     Further, the EUV light generation apparatus  1  includes an EUV light generation processor  5 , a target sensor  4 , and the like. The target sensor  4  has an imaging function to image the target  27  and output a target image TP, and detects the presence, trajectory, position, velocity, and the like of the target  27 . 
     Further, the EUV light generation apparatus  1  includes a connection portion  29  providing communication between the inside of the chamber  2  and the inside of an external apparatus  6 . A wall  291  in which an aperture  293  is formed is arranged in the connection portion  29 . The wall  291  is arranged such that the aperture  293  is located at the second focal point of the EUV light concentrating mirror  23 . 
     Further, the EUV light generation apparatus  1  includes a laser light travel direction control unit  34 , a laser light concentrating mirror  22 , a target collection unit  28  for collecting the target  27 , and the like. The laser light travel direction control unit  34  includes an optical element for defining the travel direction of the laser light, and an actuator for adjusting the position, posture, and the like of the optical element. 
     1.2 Operation 
     As shown in  FIG.  1   , the pulse laser light  31  output from the laser device  3  passes through the window  21  through the laser light travel direction control unit  34  and enters the chamber  2  as the pulse laser light  32 . The pulse laser light  32  travels along at least one optical path in the chamber  2 , is reflected by the laser light concentrating mirror  22 , and is radiated to at least one target  27  as the pulse laser light  33 . 
     The target supply unit  26 A outputs the target  27  toward the plasma generation region  25  in the chamber  2 . The target  27  is irradiated with at least one pulse included in the pulse laser light  33 . The target  27  irradiated with the pulse laser light  33  is turned into plasma, and radiation light  251  is radiated from the plasma. The EUV light concentrating mirror  23  reflects EUV light  252  contained in the radiation light  251  at higher reflectance than light in other wavelength ranges. The EUV light  252  reflected by the EUV light concentrating mirror  23  is concentrated at the intermediate focal point  292  and output to an external apparatus  6 . Here, one target  27  may be irradiated with a plurality of pulses included in the pulse laser light  33 . 
     The EUV light generation processor  5  controls the entire EUV light generation system  11 . The EUV light generation processor  5  processes the image data or the like of the target  27  output by the target sensor  4 . Further, the EUV light generation processor  5  controls, for example, the timing at which the target  27  is output, the output direction of the target  27 , and the like. Furthermore, the EUV light generation processor  5  controls, for example, the oscillation timing of the laser device  3 , the travel direction of the pulse laser light  32 , the light concentration position of the pulse laser light  33 , and the like. The above-described various kinds of control are merely examples, and other control may be added as necessary. 
     2. EUV Light Generation Apparatus According to Comparative Example 
     2.1 Configuration 
       FIG.  2    is a sectional view showing the configuration of an EUV light generation apparatus  1 A according to a comparative example.  FIG.  3    is a sectional view of the EUV light generation apparatus  1 A when viewed from a different point of view from  FIG.  2   . In  FIGS.  2  and  3   , the output direction of the EUV light is represented by the Z direction, and the direction opposite to the output direction of the target  27  is represented by the Y direction. The direction perpendicular to both the Z direction and the Y direction is represented by the X direction.  FIG.  2    shows the EUV light generation apparatus  1 A viewed in the X direction.  FIG.  3    is a sectional view showing the EUV light generation apparatus  1 A viewed in the Z direction and arrangement of a target generation system  260 , a target detection unit  41 , and an image measurement unit  43 . 
     The EUV light generation apparatus  1 A includes an EUV light generation processor  5 A, a delay circuit  72 , a chamber  2 A, the target generation system  260 , a laser light travel direction control unit  34 A, the target detection unit  41 , and the image measurement unit  43 . The target detection unit  41  and the image measurement unit  43  configure the target sensor  4  shown in  FIG.  1   . 
     A light concentrating unit  22   a , the EUV light concentrating mirror  23 , the target collection unit  28 , an EUV light concentrating mirror holder  81 , plates  82 ,  83 , a stage  84 , and the connection portion  29  are provided in the chamber  2 A. 
     The plate  82  is fixed to the chamber  2 A. The plate  83  is supported by the plate  82 . The light concentrating unit  22 A includes a laser light concentrating mirror  221  and a laser light concentrating mirror  222 . 
     The stage  84  is capable of adjusting the position of the plate  83  with respect to the plate  82 . By adjusting the position of the plate  83 , the positions of the laser light concentrating mirror  221  and the laser light concentrating mirror  222  are adjusted. The positions of the laser light concentrating mirror  221  and the laser light concentrating mirror  222  are adjusted so that the pulse laser light  33  reflected by these mirrors is concentrated at the plasma generation region  25 . 
     The EUV light concentrating mirror  23  is fixed to the plate  82  via the EUV light concentrating mirror holder  81 . 
     The laser device  3 A may be a master oscillator power amplifier (MOPA) system. The laser device  3 A is configured to output pulse laser light  31 . The laser device  3 A may include a master oscillator (not shown), an optical isolator (not shown), and a plurality of CO 2  laser amplifiers (not shown). A solid-state laser device may be employed as the master oscillator. The wavelength of the pulse laser light  31  output from the master oscillator is, for example, 10.59 μm, and the repetition frequency of the pulse oscillation is, for example, 100 kHz. 
     The laser light travel direction control unit  34 A is arranged on the optical path of the pulse laser light  31  so as to reflect the pulse laser light  31  reflected by the high reflection mirrors  341 ,  342  toward the inside of the chamber  2 A. 
     As shown in  FIGS.  2  and  3   , the target generation system  260  includes the target generation processor  52 A, the target supply unit  26 A, an inert gas supply unit  290 , a heater power source  53 , and a piezoelectric power source  54 . 
     The target generation processor  52 A controls the target generation system  260 . The target generation processor  52 A is, for example, a processing device including a storage device in which a control program is stored and a central processing unit (CPU) that executes the control program. The target generation processor  52 A is specifically configured or programmed to perform various processes. 
     The target generation processor  52 A includes a memory  523  as the storage device, and initial setting information is stored in the memory  523 . The initial setting information includes various setting information such as a pressure value for controlling the inert gas supply unit  290 , temperature information for controlling the heater power source  53 , and a drive frequency for controlling the piezoelectric power source  54 . The target generation processor  52 A controls each unit of the target generation system  260  based on the initial setting information. The initial setting information may be stored in the memory  523  in advance or may be input from the EUV light generation processor  5 A. Further, the initial setting information may be stored in a storage device outside the target generation processor  52 A. Note that the EUV light generation processor  5 A is also similar to the target generation processor  52 A in being configured by a processing device including a storage device and a CPU. 
     The target supply unit  26 A includes a reservoir  267 , a heater  261 , a temperature sensor  262 , a pressure regulator  263 , a piezoelectric element  264 , a nozzle  265 , and a filter  266 . 
     The reservoir  267  stores the material of the target  27 . The heater  261  used for melting the material of the target  27  and the temperature sensor  262  for measuring the temperature of the reservoir  267  are fixed to the reservoir  267 . The output signal output from the temperature sensor  262  is input to the target generation processor  52 A. The target generation processor  52 A outputs a drive signal to the heater power source  53  based on the signal output from the temperature sensor  262 . 
     The nozzle  265  has a nozzle hole  268  through which the molten material of the target  27  in the reservoir  267  is output. The target supply unit  26 A is attached to the chamber  2 A such that the nozzle hole  268  of the nozzle  265  is arranged inside the chamber  2 A. The filter  266  is arranged on the upstream side of the nozzle  265  and removes impurities contained in the material of the target  27 . Hereinafter, the material of the target  27  may be referred to as a target substance. 
     The pressure regulator  263  is installed on a pipe between the inert gas supply unit  290  and the reservoir  267  so as to regulate the pressure of the inert gas supplied from the inert gas supply unit  290  into the reservoir  267 . The pressure regulator  263  is connected to the target generation processor  52 A. When the pressure is applied to the reservoir  267 , the target substance is output from the nozzle  265 . 
     A piezoelectric element  264  is arranged at the nozzle  265  in the vicinity of the nozzle hole  268 . The piezoelectric element  264  is connected to the piezoelectric power source  54  that supplies drive force to the piezoelectric element  264 . The target generation processor  52 A inputs an electric signal corresponding to the set drive frequency to the piezoelectric element  264  via the piezoelectric power source  54 . The piezoelectric element  264  vibrates in response to the input electric signal. The vibration of the piezoelectric element  264  is transmitted to the nozzle  265 , and the nozzle  265  vibrates. Although the vibration frequency of the nozzle  265  and the drive frequency of the piezoelectric element  264  are not accurately the same, they have a positive correlation, and the vibration frequency of the nozzle  265  increases as the drive frequency of the piezoelectric element  264  increases. 
     The vibration frequency of the nozzle  265  defines the generation frequency f of the target  27 . The plurality of targets  27  are periodically supplied to the plasma generation region  25  at intervals. The generation frequency f of the target  27  is the number of targets  27  generated by the target supply unit  26 A and supplied to the plasma generation region  25  per unit time. Although the vibration frequency of the nozzle  265  and the generation frequency f are not accurately the same as well, they have a positive correlation, and the generation frequency f increases as the vibration frequency of the nozzle  265  increases. 
     The target supply unit  26  includes an XZ stage (not shown). The EUV light generation processor  5 A adjusts a trajectory  270  of the target  27  (hereinafter referred to as the target trajectory  270 ) so that the target  27  passes through the plasma generation region  25  under the control of the XZ stage. 
     The target detection unit  41  is attached to the chamber  2 A. The target detection unit  41  is a sensor for detecting the target  27  passing through a target detection region R. The target detection region R is a predetermined region in the chamber  2 A, and is a region located at a predetermined position on the target trajectory  270  between the target supply unit  26 A and the plasma generation region  25 . 
     As shown in  FIG.  3   , the target detection unit  41  includes a light receiving unit  41 A and a light emitting unit  41 B. The light receiving unit  41 A includes a container  411 , an optical sensor  412 , and a light receiving optical system  413 . The light emitting unit  41 B includes a container  415 , a light source  416 , and an illumination optical system  417 . The light emitting unit  41 B illuminates the target  27  passing through the target detection region R. Unlike a pulse laser, the light source  416  is a continuous wave (CW) laser in which the output of the laser does not change with time and is output at a constant value. The illumination optical system  417  includes a cylindrical lens. The illumination light output from the light source  416  is concentrated by the illumination optical system  417 . The concentration position of the illumination optical system  417  is preferably on the target trajectory  270 . More specifically, as shown in  FIG.  3   , the illumination light once narrowed down is diffracted and spread by illuminating the target  27 . It is preferable that the position of the beam waist at which the light flux of the illumination light is most narrowed overlaps with the target detection region R. 
     The light receiving unit  41 A and the light emitting unit  41 B are attached to a window  21   a  and a window  21   b  which are arranged on opposite sides of the target trajectory  270 , respectively. The window  21   a  and the window  21   b  are provided at the chamber  2 A. 
     In the light receiving unit  41 A, the illumination light from the light emitting unit  41 B is concentrated by the light receiving optical system  413 , and received by the optical sensor  412 . The optical sensor  412  is configured by a photoelectric conversion element such as a photodiode, and outputs a light receiving signal having a signal intensity corresponding to the amount of received light. When the target  27  passes through the target detection region R, the output of the optical sensor  412  varies. The light receiving unit  41 A outputs a passage timing signal T 1  indicating passage of the target  27  based on the variation in the output of the optical sensor  412 . 
     The image measurement unit  43  is arranged on the downstream side of the target detection unit  41  in the travel direction of the target  27 . The image measurement unit  43  is attached to a wall portion of the chamber  2 A in the vicinity of the plasma generation region  25 . The image measurement unit  43  images the target  27  supplied to the plasma generation region  25 , and outputs the target image TP. The image measurement unit  43  includes a light source unit  436  and an imaging unit  431 . The light source unit  436  and the imaging unit  431  are arranged to face each other across the plasma generation region  25  on the target trajectory  270 . The direction in which the light source unit  436  and the imaging unit  431  face each other is perpendicular to the target trajectory  270 . 
     The light source unit  436  outputs pulse light for imaging the target  27  that has reached the plasma generation region  25 . The light source unit  436  includes a window  437 , a light source  438 , and an illumination optical system  439 . The light source  438  may be, for example, a light source for pulse lighting such as a xenon flash lamp or a laser light source. The light source  438  is connected to the EUV light generation processor  5 A. A lighting signal LU output from the EUV light generation processor  5 A is input to the light source  438 . The light source  438  emits pulse light based on the input lighting signal LU. 
     The illumination optical system  439  is an optical system including, for example, a collimator lens. The collimator lens collimates the pulse light emitted from the light source  438 . The illumination optical system  439  guides the pulse light emitted from the light source  438  to the plasma generation region  25  on the target trajectory  270  via the window  437 . When the target  27  that has reached the plasma generation region  25  is irradiated with the pulse light, a portion of the pulse light is blocked and a projection image of the target  27  is projected onto the imaging unit  431 . 
     The imaging unit  431  images the projection image of the target  27 . The imaging unit  431  includes a window  433 , an image sensor  434 , and a transfer optical system  435 . Pulse light including the projection image of the target  27  is incident on the transfer optical system  435  in the imaging unit  431  via the window  433 . The transfer optical system  435  includes, for example, a plurality of lenses. The transfer optical system  435  forms the projection image of the target  27  on a light receiving surface of the image sensor  434 . 
     The image sensor  434  is a two dimensional image sensor such as a CCD. The image sensor  434  outputs an image signal corresponding to the projection image of the target  27  formed on the light receiving surface. The image sensor  434  includes a shutter  432 . The shutter  432  may be an electrical shutter or a mechanical shutter. An imaging timing signal TS 2  output from the EUV light generation processor  5 A is input to the imaging unit  431  via the delay circuit  72 . Opening and closing of the shutter  432  are controlled by the imaging timing signal TS 2 . The image sensor  434  images only while the shutter  432  is open. Operations of the imaging unit  431  and the light source unit  436  are synchronized by the imaging timing signal TS 2  and the lighting signal LU. 
     The EUV light generation processor  5 A controls the irradiation timing of the laser device  3  such that the target  27  that has reached the plasma generation region  25  is irradiated with the pulse laser light  33 . Upon receiving a passage timing signal TS 1  from the target detection unit  41 , the EUV light generation processor  5 A outputs, to the laser device  3 , a light emission trigger signal TR that defines the irradiation timing. The delay circuit  72  delays the light emission trigger signal TR input from the EUV light generation processor  5 A by a delay time required for the target  27  to reach the plasma generation region  25  from the target detection region R, and outputs the delayed light emission trigger signal TR. Thus, the target  27  that has reached the plasma generation region  25  is irradiated with the pulse laser light  33 . 
     Further, upon receiving the passage timing signal TS 1  from the target detection unit  41 , the EUV light generation processor  5 A outputs the imaging timing signal TS 2  to the imaging unit  431 . The delay circuit  72  delays the imaging timing signal TS 2  input from the EUV light generation processor  5 A by the above-described delay time and outputs the delayed imaging timing signal TS 2 . Thus, the imaging unit  431  can image a projection image of the target  27  at a timing when the target  27  has reached the plasma generation region  25 . 
     The EUV light generation processor  5 A receives a burst signal BT from the external apparatus  6  shown in  FIG.  2   . The burst signal BT is a signal from the external apparatus  6  to request the EUV light generation apparatus  1  to generate and stop EUV light. The EUV light generation processor  5 A generates EUV light while the burst signal BT is on and stops generating EUV light while the burst signal BT is off. 
     2.2 Operation 
       FIG.  4    is a main flowchart showing the operation of an EUV light generation system  11 A.  FIG.  5    is a flowchart showing a target generation process shown in  FIG.  4   . 
     In step S 10  of  FIG.  4   , when an activation instruction is input, the EUV light generation processor  5 A activates the EUV light generation system  11 A. After the activation, the EUV light generation processor  5 A outputs a start signal for starting target generation to the target generation processor  52 A. 
     In step S 20 , the target generation processor  52 A starts target generation when the start signal from the EUV light generation processor  5 A is input. 
     In step S 30 , the target generation processor  52 A starts EUV light generation. Specifically, the target generation processor  52 A operates the laser device  3  to start radiation of the pulse laser light  33  with respect to the target  27 . When the target  27  is irradiated with the pulse laser light  33 , the target  27  is turned into plasma, and EUV light  252  is generated. The EUV light  252  is output to the external apparatus  6 . The pulse laser light  33  has a pulse time width on the picosecond order, for example. The picosecond order means the range of 1 ps or more and less than 1 ns. The pulse laser light  33  may have a pulse time width of 1 ns or more and less than 1 μs. 
     The target generation in step S 20  is performed according to the flowchart shown in  FIG.  5   . First, in step S 210 , the target generation processor  52 A determines whether or not the initial setting has been completed. When the initial setting is not completed (NO in step S 210 ), processing proceeds to the initial setting in step S 220 . 
     In step S 220 , the target generation processor  52 A performs the initial setting based on initial setting information stored in the memory  523 . The initial setting includes temperature adjustment of the heater  261 , pressure adjustment of the reservoir  267 , adjustment of the drive frequency of the piezoelectric element  264 , and the like. 
     First, the target generation processor  52 A controls the heater  261  via the heater power source  53  based on the detection value of the temperature sensor  262  so that the target substance in the reservoir  267  becomes a predetermined temperature equal to or higher than the melting point. When tin (Sn) is used as the target substance, the predetermined temperature is between 232° C. to 300° C. When the heater  261  is driven, the target substance stored in the reservoir  267  melts into a liquid state. 
     In the initial setting, the target generation processor  52 A sets the pressure in the reservoir  267  to a target pressure via the pressure regulator  263  in order to output the target substance from the nozzle hole  268 . The pressure regulator  263  supplies and exhausts gas in the reservoir  267  based on a control signal from the target generation processor  52 A to set the pressure in the reservoir  267  to the target pressure. The pressure in the reservoir  267  defines an output pressure of the target substance output from the nozzle hole  268 , and consequently defines the velocity of the target  27  in the form of a droplet moving toward the plasma generation region  25 . The target pressure is, for example, a pressure in the range from a few MPa to 40 MPa. The target velocity of the target  27  is, for example, velocity in the range of 60 m/s to 120 m/s. 
     Further, in the initial setting, the target generation processor  52 A sets the drive frequency of the piezoelectric element  264 . As described above, the drive frequency of the piezoelectric element  264  defines the generation frequency f. The drive frequency of the piezoelectric element  264  is adjusted so that the target  27  is generated at the target generation frequency f. The initial setting of the drive frequency of the piezoelectric element  264  is performed while operating the target supply unit  26 A and supplying the target  27  to the plasma generation region  25 . When the pressure is applied to the reservoir  267  in a state in which the target substance is melted and the piezoelectric element  264  is vibrated, the nozzle  265  is vibrated and a plurality of targets  27  are periodically supplied to the plasma generation region  25 . 
     The EUV light generation processor  5 A analyzes the target image TP input from the image measurement unit  43 , and calculates the actual measurement value of the generation cycle of the targets  27  from the intervals between the plurality of targets  27  sequentially passing through the plasma generation region  25 . The EUV light generation processor  5 A calculates the difference between the calculated actual measurement value of the generation cycle and the target value of the generation cycle, and outputs the difference to the target generation processor  52 A. The target generation processor  52 A adjusts the drive frequency of the piezoelectric element  264  so that the actual measurement value of the generation cycle of the targets  27  become the target value. Thus, the generation frequency f of the target  27  is set to the target value. 
     When the initial setting is completed, the EUV light generation processor  5 A performs the basic operation of step S 230 . 
     The basic operation in step S 230  is an operation of driving the heater  261 , the pressure regulator  263 , and the piezoelectric element  264  at the temperature, the pressure, and the drive frequency adjusted in the initial setting. In the basic operation, the target  27  is supplied to the plasma generation region  25  at the target generation frequency f. 
     When the initial setting is completed in step S 210  (YES in step S 210 ), processing proceeds to step S 230  without performing the initial setting in step S 220 . 
     2.3 Problem 
     When the EUV light generation apparatus  1 A according to the comparative example is used for a long period of time, the volume of the targets  27  may change. As the reason, change in the diameter of the nozzle hole  268  of the target supply unit  26 A, clogging of the filter  266 , and the like are considered. One of the change in the diameter of the nozzle hole  268  is an increase in the diameter that occurs when the inner wall is gradually eroded by the target substance passing through the nozzle hole  268 . Another is a decrease in the diameter that occurs when a compound film of the target substance and another metal is deposited on the inner wall of the nozzle hole  268 . 
       FIG.  6    conceptually shows a target generation mechanism. The pressure in the reservoir  267  causes the molten target substance to be output from the nozzle  265 . The form of the target substance immediately after being output from the nozzle  265  is a columnar body whose axial direction extends in the output direction. When the nozzle  265  is vibrated due to the vibration applied from the piezoelectric element  264 , the target substance of the columnar body is divided into droplets  127  having a smaller volume than the target  27 . Among the plurality of droplets  127 , some adjacent droplets  127  are combined with each other in the process of traveling on the target trajectory  270 , and become the target  27  in a droplet form having a larger volume than the droplet  127 . 
     In  FIG.  6   , assuming that the diameter of the nozzle hole  268  is d, the velocity in the direction of the target trajectory  270  is V, and the generation frequency of the target  27  is f, the diameter D of the target  27  can be calculated by following Equation (1) and Equation (2). Here, d and D are both diameters. 
     
       
         
           
             
               
                 
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     The velocity V of the target  27  is defined by the output pressure, and the generation frequency f of the target  27  is defined by the drive frequency of the piezoelectric element  264 . In the case in which the output pressure and the drive frequency are constant, that is, V/f is constant, when the diameter d of the nozzle hole  268  increases, the output amount of the target substance per unit time increases, and thus the diameter D of the target  27  increases. Similarly, in the case in which V/f is constant, when the diameter d of the nozzle hole  268  decreases, the output amount of the target substance per unit time d decreases, and thus the diameter D of the target  27  decreases. The larger the diameter D is, the larger the volume of the target  27  is. Further, even in the case in which the diameter d of the nozzle hole  268  does not change, when the filter  266  is clogged, the output velocity of the target substance decreases, and the volume of the target  27  decreases. 
     When the volume of the target  27  is too large, there is a case in which the amount of debris generated upon irradiation with the pulse laser light  33  is increased. Further, when the volume of the target  27  is too small, there is a case in which the output of the EUV light decreases, and a change in the volume of the target  27  may cause the output of the EUV light to be unstable. 
     3. EUV Light Generation Apparatus of First Embodiment 
     The EUV light generation apparatus  1 B of the first embodiment shown in  FIG.  7    will be described. Any component same as that described above is denoted by an identical reference sign, and duplicate description thereof is omitted unless specific description is needed. 
     3.1 Configuration 
     The difference in the configuration between the EUV light generation apparatus  1 B of the first embodiment and the EUV light generation apparatus  1 A according to the comparative example lies only on a target generation processor  52 B. Functionally, the target generation processor  52 B is different from the target generation processor  52 A of the comparative example in that the target generation processor  52 B has a function of changing the generation frequency f of the target  27  based on the size of the target  27 . In order to realize such a function, the control program and the initial setting information of the target generation processor  52 B have been improved. 
     3.2 Operation 
     The operation of the EUV light generation apparatus  1 B of the first embodiment will be described. In the EUV light generation apparatus  1 B as well, the main flowchart relating to the EUV light generation is similar to the main flowchart of the EUV light generation apparatus  1 A according to the comparative example shown in  FIG.  4   . The difference is that the content of the target generation in step S 20  is changed from the flowchart shown in  FIG.  5    to the flowchart shown in  FIG.  8    as an example. 
     The flowchart of the first embodiment shown in  FIG.  8    is different from the flowchart of the comparative example shown in  FIG.  5    in the following two points. The first difference is that the initial setting of step S 220  is changed to the initial setting of step S 220 B. The second difference is that after the basic operation of step S 230  is started, the step of the change control of the generation frequency f of the target  27  is added. 
     In step S 220 B, the initial setting for adjusting the temperature of the heater  261 , the pressure of the reservoir  267 , and the drive frequency of the piezoelectric element  264  to target values is similarly performed in the target generation processor  52 B as well. The difference is that an allowable range of the diameter D of the target  27  is set as information used for the change control of the generation frequency f. An upper limit value Dmax and a lower limit value Dmin of the allowable range are stored in the memory  523  as information of the allowable range of the diameter D. In the initial setting of step S 220 B, the target generation processor  52 B reads the allowable range of the diameter D. Further, in step S 220 B, the target generation processor  52 B calculates irradiation frequency F and frequency ratio N (=f/F) between the irradiation frequency F and the generation frequency f. When the initial setting of step S 220 B is completed, the target generation processor  52 B proceeds to step S 230 . 
     The basic operation in step S 230  is similar to the basic operation according to the comparative example shown in  FIG.  5   . The drive frequency of the piezoelectric element  264  is adjusted so that the generation frequency f of the target  27  becomes an initial target value immediately after the initial setting is completed. After starting the basic operation in step S 230 , the target generation processor  52 B starts the change control of the generation frequency f after step S 240 . 
     In step S 240 , the target generation processor  52 B first obtains the target image TP measured by the image measurement unit  43  from the EUV light generation processor  5 A. Then, a diameter D(t) of the target  27  is measured from the target image TP. The diameter D(t) is measured for each of the targets  27 . 
     In step S 250 , the target generation processor  52 B calculates an average value Dμ(t) of the diameters D(t) of the targets  27 . 
     In step S 260  and step S 270 , the target generation processor  52 B determines whether or not the diameter D of the target  27  falls within an allowable range. 
     First, in step S 260 , the target generation processor  52 B compares the average value Dμ(t) with the upper limit value Dmax of the allowable range. When the average value Dμ(t) is equal to or less than the upper limit value Dmax (YES in step S 260 ), processing proceeds to step S 270 , and when the average value Dμ(t) exceeds the upper limit value D max (NO in step S 260 ), processing proceeds to step S 261 . 
     In step S 270 , the target generation processor  52 B compares the average value Dμ(t) with the lower limit value Dmin of the allowable range. When the average value Dμ(t) is equal to or higher than the lower limit value Dmin (YES in step S 270 ), since the average value Dμ(t) is within the allowable range, the target generation processor  52 B returns to step S 30  shown in  FIG.  4    without changing the generation frequency f. On the other hand, when the average value Dμ(t) is less than the lower limit value Dmin (NO in step S 270 ), processing proceeds to step S 271 . 
     In step S 261 , since the average value Dμ(t) exceeds the upper limit value Dmax of the allowable range, the generation frequency f is increased so that the average value Dμ(t) falls within the allowable range. On the other hand, in step S 271 , since the average value Dμ(t) is less than the lower limit value Dmin of the allowable range, the target generation processor  52 B decreases the generation frequency f so that the average value Dμ(t) falls within the allowable range. 
     The target generation processor  52 B changes the generation frequency f by adjusting the drive frequency of the piezoelectric element  264  in steps S 261  and S 271 . In the initial setting of step S 220 B, the drive frequency of the piezoelectric element  264  is adjusted to the initial target value so that the generation frequency f becomes the target value. Changing the generation frequency fin step S 261  and step S 271  means to change the target value of the generation frequency f by changing the target value of the drive frequency of the piezoelectric element  264 . In the target generation, the basic operation is continued even after the target value of the generation frequency f is changed. 
     The generation frequency f is changed to a natural number multiple of the irradiation frequency F of the pulse laser light  33 . That is, in step S 261 , when increasing the generation frequency f,  1  is added to the frequency ratio N before the change, and a value obtained by multiplying the added value by the irradiation frequency F is set as the generation frequency f after the change. For example, when the target value of the generation frequency f before the change is 120 kHz and the irradiation frequency F is 20 kHz, the frequency ratio N before the change is 6. When the generation frequency f is increased, N+1=6+1=7 and f=N×F=20 kHz×7=140 kHz are obtained, and the generation frequency f is changed from 120 kHz to 140 kHz. 
     On the other hand, in step S 271 , when decreasing the generation frequency f,  1  is subtracted from the initial frequency ratio N, and a value obtained by multiplying the subtracted value by the irradiation frequency F is set as the generation frequency f after the change. For example, it is assumed that the target value of the generation frequency f before the change is 120 kHz, the irradiation frequency F is 20 kHz, and the frequency ratio N before the change is 6. When the generation frequency f is decreased, N−1=6−1=5 and f=N×F=20 kHz×5=100 kHz are obtained, and the generation frequency f is changed from 120 kHz to 100 kHz. The target generation processor  52 B repeats this process until the diameter d of the target  27  falls within the allowable range. 
     The change control of the generation frequency f shown in  FIG.  8    will be conceptually described with reference to  FIG.  9   .  FIG.  9    is a schematic diagram of a case in which the generation frequency f is increased when the diameter D of the target  27  is increased due to an increase in the diameter d of the nozzle hole  268 . 
     In  FIG.  9   , it is assumed that an initial state is at time t 1 , while the diameter of the nozzle hole  268  in the initial state is d(t 1 ) and the diameter of the target  27  is D(t 1 ). Here, f 1  is the initial target value of the generation frequency f. It is assumed that, when time elapses from time t 1  to time t 2 , the diameter d of the nozzle hole  268  becomes larger from d(t 1 ) and changes to d(t 2 ). In this case, the diameter D of the target  27  changes to D(t 2 ) larger than D(t 1 ). Also at time t 2 , since f 1  does not change, the irradiation frequency F and the generation frequency f of the pulse laser light  33  are synchronized in the same manner as in the initial state. Therefore, although the target  27  is irradiated with the pulse laser light  33 , since the volume of the target  27  is increased, the amount of generated debris is larger than that at time t 1 . 
     When the diameter D(t 2 ) of the target  27  at time t 2  exceeds the upper limit value Dmax, the target generation processor  52 B increases the generation frequency f by increasing the drive frequency of the piezoelectric element  264  as shown in step S 261  of  FIG.  8   . Here, f 2 , which is the generation frequency f after the change, is larger than f 1  before the change and is a value of a natural number multiple of the irradiation frequency F. Time t 3  is the time after changing the generation frequency f to f 2 . As the generation frequency f increases, the diameter D(t 3 ) of the target  27  decreases. Since f 2  is a natural number multiple of the irradiation frequency F, the target  27  is irradiated with the pulse laser light even after the generation frequency f is changed. Since the diameter D(t 3 ) of the target  27  at time t 3  is smaller than the diameter D(t 2 ) at time t 2 , the amount of generated debris at time t 3  is reduced compared to at time t 2 . 
     In  FIG.  9   , as the generation frequency f, both f 1  in the initial state at time t 1 , t 2  and f 2  at time t 3  after the change are natural number multiples of the irradiation frequency F. The relationship between each of f 1  and f 2  and the irradiation frequency F is expressed by Equation (3). Both n 1  and n 2  are natural numbers. With respect to the diameter D of the target  27 , the relationship between the diameter D(t 3 ) at time t 3  and the diameter D 2 (t 2 ) at time t 2  is represented by Equation (4). 
     
       
         
           
             
               
                 
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     Since  FIG.  9    shows an example in which the generation frequency f is to be increased, n 2  is larger than n 1 . Further, at time t 3 , the number of the targets  27  not to be irradiated with the pulse laser light  33  increases owing to that the generation frequency f is higher than that at time t 1 , t 2 . In  FIG.  9   , the targets  27  to be irradiated with the pulse laser light  33  are each denoted by a reference sign  27 A, and the targets  27  not to be irradiated with the pulse laser light  33  are each denoted by a reference sign  27 B. In  FIG.  9   , as an example, when the generation frequency f is f 1 , the number of the targets  27 B existing between two targets  27 A is one, but after the change to f 2 , the number of the targets  27 B increases to two. Thus, when the generation frequency f increases, the number of the targets  27 B not to be irradiated with the pulse laser light  33  increases. 
     The example of  FIG.  9    corresponds to step S 261  of  FIG.  8   , and is an example of increasing the generation frequency f. In contrast to step S 261 , as shown in step S 271 , when the size of the target  27  decreases due to a decrease in the diameter d of the nozzle hole  268 , the generation frequency f is decreased in order to increase the size of the target  27 . In this case, in Equation (3) and Equation (4) described above, n 2  is smaller than n 1 , and f 2  is smaller than f 1 . 
     In contrast to the example of  FIG.  9   ,  FIG.  10    is a schematic diagram of a case in which the generation frequency f is decreased when the diameter D of the target  27  is decreased. Further, unlike the example of  FIG.  9   , the example of  FIG.  10    is an example of a case in which the cause of the diameter D of the target  27  becoming decreased is not an increase in the diameter d of the nozzle hole  268  but clogging of the filter  266 . 
     In  FIG.  10   , at time t 2  when time elapses from time t 1  in the initial state, deposition of impurities progresses, and a part of the filter  266  is blocked. The diameter d of the nozzle hole  268  remains d(t 1 ) without being changed between time t 1  and time t 2 . When the filter  266  is partially blocked, the output velocity of the target substance output from the nozzle hole  268  decreases. Accordingly, since the output amount of the target substance per unit time decreases, the diameter D of the target  27  decreases, and V/f, which is the distance that the target  27  travels per unit time, also decreases. In the example of  FIG.  10   , the velocity V of the target  27  decreases from V(t) at time t 1  to velocity V(t 2 ) at time t 2 . However, since the generation frequency f 1  does not change from f 1  at time t 1  and time t 2 , the irradiation frequency F of the pulse laser light and the generation frequency f are synchronized. However, when the volume decreases by the decrease of the diameter D of the target  27 , the output of the EUV light is reduced. 
     When the diameter D(t 2 ) of the target  27  at time t 2  becomes less than the lower limit value Dmin, the target generation processor  52 B decreases the generation frequency f by decreasing the drive frequency of the piezoelectric element  264  as shown in step S 271  of  FIG.  8   . Here, f 2 , which is the generation frequency f after the change, is smaller than f 1  before the change and is a value of a natural number multiple of the irradiation frequency F. Time t 3  is the time after changing the generation frequency f to f 2 . In this case, as the generation frequency f decreases, the diameter D(t 3 ) of the target  27  increases. Since f 2  is a natural number multiple of the irradiation frequency F, even after the generation frequency f is changed, the target  27  is irradiated with the pulse laser light  33 . In the example of  FIG.  10   , since the diameter D(t 3 ) of the target  27  at time t 3  is larger than the diameter D(t 2 ) at time t 2 , the output of EUV light at time t 3  is increased compared to at time t 2 . In contrast to the example of  FIG.  9   , in the example of  FIG.  10   , n 2  is smaller than n 1 , and f 2  is smaller than f 1  in Equation (3) and Equation (4) described above. 
     3.3 Effects 
     As described above, the EUV light generation apparatus  1 B of the present embodiment is the EUV light generation apparatus to generate EUV light by irradiating the target  27  with the pulse laser light  33  to turn the target  27  into plasma including the chamber  2 A, the target supply unit  26 A configured to supply the target  27  to the plasma generation region  25  in the chamber  2 A, the laser device  3 A configured to generate the pulse laser light  33  to be radiated to the target  27 , and the target generation processor  52 B configured to change the generation frequency f of the target  27  generated by the target supply unit  26 A to a natural number multiple of the irradiation frequency F of the pulse laser light  33  based on the size of the target  27 . Here, the diameter D of the target  27  is an example of the size of the target of the present disclosure, and the target generation processor  52 B is an example of the processor of the present disclosure. 
     According to the EUV light generation apparatus  1 B of the present embodiment as described above, even when the volume of the target  27  varies due to the change of the diameter d of the nozzle hole  268 , clogging of the filter  266 , or the like, the following effects can be obtained by changing the generation frequency f. That is, there are an effect of suppressing an increase in the amount of generated debris and an effect of suppressing decrease in the output of EUV light. 
     The first embodiment further includes the image measurement unit  43  for imaging the image of the target  27  and outputting the target image TP, and the target generation processor  52 B measures the size of the target  27  from the target image TP. Since the size of the target  27  is directly measured from the target image TP, the accuracy of determining whether or not the size of the target  27  is within the allowable range may be improved as compared with a case in which the target image TP is not used. 
     In the first embodiment, an allowable range of the size of the target  27  is set in advance, and the target generation processor  52 B increases the generation frequency f when the size of the target  27  exceeds the upper limit value Dmax of the allowable range, and decreases the generation frequency f when the size of the target  27  falls below the lower limit value Dmin of the allowable range. By setting the allowable range, since the generation frequency f is not changed within the allowable range, it is possible to suppress the load of the control compared to, for example, control in which the generation frequency f is adjusted to one threshold value instead of the allowable range. 
     In the first embodiment, the target generation processor  52 B compares the average value Dμ of the diameters D, which is an example of the measured size of the plurality of targets  27 , with the upper limit value Dmax and the lower limit value Dmin. Thus, even when there is a variation in the measurement value, relatively stable control can be performed. Here, the average value Dμ is an example of the representative value of the present disclosure. As the representative value, a median value of a plurality of measurement values may be used other than the average value Dμ. 
     Further, although the diameter D of the target  27  is used as the size of the target  27 , the volume of the target  27  may be used. In this case, for example, a plurality of image measurement units  43  having different imaging directions are provided. Then, the diameter D is measured from each of the target images TP imaged by the image measurement units  43 , and the volume of the target  27  is obtained by calculation from the plurality of measured diameters D. Further, a pressure sensor that measures the pressure at the time of collision of the target  27  may be provided in the target collection unit  28 , and the volume of the target  27  may be obtained by calculation from the pressure value measured by the pressure sensor. As a method of calculating the volume from the pressure value, for example, the mass of the target  27  is calculated from the pressure value, and the volume is obtained from the mass and the density of the target substance. 
     4. EUV Light Generation Apparatus of Second Embodiment 
     Next, the EUV light generation apparatus of a second embodiment will be described. Any component same as that described above is denoted by an identical reference sign, and duplicate description thereof is omitted unless specific description is needed. 
     4.1 Configuration 
     Since the basic configuration of the EUV light generation apparatus according to the second embodiment is substantially the same as that of the EUV light generation apparatus  1 B according to the first embodiment, description of the entire configuration is omitted. The difference in the configuration between the EUV light generation apparatus according to the second embodiment and the EUV light generation apparatus according to the first embodiment lies only on a target generation processor  52 C shown in  FIG.  11   . The target generation processor  52 C is the same as that of the EUV light generation apparatus  1 B according to the first embodiment in that the generation frequency f of the target  27  is changed based on the size of the target  27 , but the actualization method is different. In response to the modification of the actualization method, the target generation processor  52 C has been improved in the control program and in the initial setting information. 
     That is, the target generation processor  52 C of the second embodiment changes the generation frequency f of the target  27  based not on the size of the target  27  but on the related information related to the size of the target  27 . More specifically, in the second embodiment, the related information is an elapsed time t from a preset time point, and the target generation processor  52 C changes the generation frequency f based on the elapsed time t. As shown in the first embodiment, the diameter d of the nozzle hole  268  varies with the elapse of time, and clogging of the filter  266  proceeds with the passage of time as well. Therefore, the elapsed time t can be understood to be related information related to the size of the target  27 . 
     As shown in  FIG.  11   , the target generation processor  52 C includes a timer  522 . The timer  522  is an example of an elapsed time measurement unit for measuring the elapsed time t from the preset time point. In the present example, the cumulative value of the operation time of the target supply unit  26 A is used as the elapsed time t. The target generation processor  52 C causes the timer  522  to measure the operation time of the target supply unit  26 A, and updates the elapsed time t in the memory  523  based on the measurement value. 
     In addition to the initial setting information and the elapsed time t, a reference table  524  is stored in the memory  523 . As shown in  FIG.  12   , the reference table  524  records a correspondence relationship between a plurality of reference times RX compared with the elapsed time t and a plurality of generation frequencies f set corresponding to the plurality of reference times RX. In the present example, the memory  523  which is a storage device in the target generation processor  52 C stores the correspondence relationship, but a storage device outside the target generation processor  52 C may store the correspondence relationship. 
     For example, as shown in  FIG.  13   , the reference table  524  is created on the assumption that the diameter d of the nozzle hole  268  increases as the elapsed time t increases. On the assumption of the relationship shown in  FIG.  13   , since the diameter D of the target  27  increases as the elapsed time t increases, it is necessary to increase the generation frequency fin order to decrease the diameter D. Therefore, in the reference table  524 , the generation frequency f increases as the reference time RX increases. The reference time RX and the generation frequency f have one-to-one correspondence. The reference number RN is a number assigned to each combination of the reference time RX and the generation frequency f. 
     The reference table  524  is created by, for example, actually measuring a change in the size of the target  27  according to the elapsed time t and assigning an appropriate generation frequency f according to the elapsed time t based on the actual measurement value. 
     4.2 Operation 
     Next, with reference to the flowchart shown in  FIG.  14   , the operation of the EUV light generation apparatus of the second embodiment will be described. 
     In the EUV light generation apparatus according to the second embodiment as well, the main flowchart relating to the EUV light generation is similar to the main flowchart of the EUV light generation apparatus  1 A according to the comparative example shown in  FIG.  4   . The difference is that the content of the target generation in step S 20  is changed from the flowchart shown in  FIG.  5    to the flowchart shown in  FIG.  14    as an example. 
     The flowchart of the second embodiment shown in  FIG.  14    is different from the flowchart of the first embodiment shown in  FIG.  8    in the following two points. The first difference is that the initial setting of step S 220 B is changed to the initial setting of step S 220 C. The second difference is that the change control of the generation frequency f of the target  27  in step S 330  and thereafter is different. 
     In the initial setting in step S 220 C, the initial setting for adjusting the temperature of the heater  261 , the pressure of the reservoir  267 , and the drive frequency of the piezoelectric element  264  to target values is performed in the target generation processor  52 C as well similarly to the first embodiment. The difference is that the target generation processor  52 C reads the reference table  524  instead of the allowable range. When the initial setting of step S 220 C is completed, the target generation processor  52 C proceeds to step S 230 . 
     The basic operation in step S 230  is similar to that in the first embodiment. After starting the basic operation in step S 230 , the target generation processor  52 C proceeds to step S 310 . 
     In step S 310 , the target generation processor  52 C selects the reference number RNk of the reference table  524 . The elapsed time t is the cumulative value of the operation time. Therefore, when the EUV light generation apparatus is stopped, the target generation processor  52 C stores the value k of the reference number RNk at the time of stopping in the memory  523 . When the EUV light generation apparatus is restarted, the target generation processor  52 C proceeds to NO in step S 210 , and proceeds to step S 310  via steps S 220 C and S 230 . In step S 310 , the target generation processor  52 C reads the value of k stored in the memory  523  at the time of the previous stop, and selects the reference number RNk corresponding to the value of k. Then, the combination of the reference time RTk and the generation frequency fk corresponding to the reference number RNk is read. 
     In step S 320 , the target generation processor  52 C activates the timer  522  and starts measuring the elapsed time t. After step S 320 , the target generation processor  52 C starts the change control of the generation frequency fin step S 330  and thereafter. 
     In step S 330 , the target generation processor  52 C compares the elapsed time t with the reference time RXk corresponding to the selected reference number RNk. When the elapsed time t does not reach the reference time RXk, processing returns to step S 30  shown in  FIG.  4    without changing the generation frequency f. When the elapsed time t reaches the reference time RXk (YES in S 330 ), the target generation processor  52 C proceeds to step S 340 . 
     In step S 340 , the target generation processor  52 C changes the generation frequency f to the generation frequency fk corresponding to the reference time RXk. When the generation frequency f is changed, processing proceeds to step S 350 . 
     In step S 350 , the target generation processor  52 C updates the reference number RNk by adding 1 to the value of k. After step S 350 , step S 30  of  FIG.  4   . The target generation processor  52 C repeats the above process until the EUV light generation apparatus is stopped. 
     4.3 Effects 
     As described above, in the first embodiment, the generation frequency f is changed based on the size of the target  27  based on the image data. However, in the second embodiment, the generation frequency f is changed based on the elapsed time t from the preset time point without measuring the size of the target  27 . Also in the second embodiment, the effect of suppressing an increase in the amount of generated debris and the effect of stabilizing the output of EUV light are the same as those in the first embodiment. 
     Further, in the second embodiment, the cumulative value of the operation time of the target supply unit  26 A is used as the elapsed time t. As described above, the change in the diameter D of the target  27  is considered to be caused due to the change in the diameter d of the nozzle hole  268  and clogging of the filter  266 . These are both considered to be highly correlated with the operation time of the target supply unit  26 A. Therefore, it is considered that by using the cumulative value of the operation time of the target supply unit  26 A as the elapsed time t, it is possible to appropriately perform change control of the generation frequency f as compared with a case in which the cumulative value of the operation time is not used. 
     The elapsed time t may include a stop period during which the target supply unit  26 A is stopped. Even when the target supply unit  26 A is stopped, there is a case in which variation of the diameter d of the nozzle hole  268 , clogging of the filter, or the like progresses. Including the stop period in the elapsed time t is effective in such a case. 
     In the second embodiment, the target generation processor  52 C increases the generation frequency f as the elapsed time t increases. Therefore, as compared with the case of increasing and decreasing the generation frequency f, the change control of the generation frequency f may be simplified. Naturally, in a case in which clogging of the filter  266  is dominant as the cause of the change in the size of the target  27 , the generation frequency f may be decreased as the elapsed time t increases. 
     Further, the second embodiment adopts the reference table  524  in which a correspondence relationship between a plurality of reference times RX compared with the elapsed time t and a plurality of generation frequencies f set corresponding to the plurality of reference times RX is recorded. In the second embodiment, the target generation processor  52 C includes a memory  523  in which the reference table  524  is stored in advance, and changes the generation frequency f according to the elapsed time t with reference to the reference table  524 . Thus, by using the correspondence relationship such as the reference table  524 , more flexible correspondence may be easily performed compared to a case in which the relationship between the elapsed time t and the generation frequency f is defined by a mathematical expression. For example, when the diameter D of the target  27  varies due to complex factors such as a temporal change in the diameter d of the nozzle hole  268  and a temporal change in clogging of the filter  266 , the relationship between the elapsed time t and the generation frequency f may become complicated. In such a case, a method of using the reference table  524  or the like in which the correspondence relationship is recorded is effective. In addition, the reference table  524  may be easier to perform maintenance such as correction and update than the mathematical expression. 
     5. EUV Light Generation Apparatus of Third Embodiment 
     Next, the EUV light generation apparatus of a third embodiment will be described. Any component same as that described above is denoted by an identical reference sign, and duplicate description thereof is omitted unless specific description is needed. 
     5.1 Configuration 
     The configuration of the EUV light generation apparatus according to the third embodiment is different from the EUV light generation apparatus  1 B according to the first embodiment in that a refill mechanism  91 A is provided as shown in  FIG.  15   . In addition, the third embodiment is similar to the first embodiment in that the generation frequency f is changed based on the diameter D of the target  27 , but is different in a method of measuring the diameter D of the target  27 . In the first embodiment, the diameter D of the target  27  is measured from the target image TP. On the other hand, in the third embodiment, the diameter D of the target  27  is calculated from the replenishment amount of the refill mechanism  91 A. In other words, in the first embodiment, the target  27  is directly measured, whereas in the third embodiment, the diameter D of the target  27  is indirectly measured based on the replenishment amount. Hereinafter, the difference will be mainly described. 
     As shown in  FIG.  15   , the refill mechanism  91 A is a mechanism for replenishing the target substance to the reservoir  267  of a target supply unit  26 D. The refill mechanism  91 A includes a tank  97 A, a measuring unit  96 A, a pipe  98 A, a load lock chamber  92 A, and a liquid level sensor  73 A. The tank  97 A contains solid tin, which is a solid target substance, as a material of the target  27 . The solid tin is, for example, spherical. The measuring unit  96 A measures the mass of the solid tin supplied from the tank  97 A to the pipe  98 A. The measuring unit  96 A counts, for example, the number of pieces of the solid tin to be replenished. Since the mass of solid tin per piece is known, the mass of the solid tin to be supplied can be calculated based on the mass and the number of pieces thereof to be supplied. 
     The load lock chamber  92 A is arranged on the downstream side of the measuring unit  96 A in the supply direction of the solid tin. The load lock chamber  92 A is connected to the reservoir  267  through an openable/closable supply port, and temporarily holds the solid tin transferred from the measuring unit  96 A. The load lock chamber  92 A opens the supply port in a state in which the inside of the chamber and the tank  97 A have the same pressure and replenishes the solid tin into the reservoir  267 . 
     The liquid level sensor  73 A detects the liquid level of the liquid target substance in the reservoir  267 . The liquid level sensor  73 A of the present example has a rod shape, and has a detection region for detecting the liquid level at one location in the longitudinal direction. The liquid level sensor  73 A detects whether or not the position of the liquid level of the target substance reaches the detection region. For example, the liquid level sensor  73 A outputs a detection signal to a target generation processor  52 D when the position of the liquid level reaches the detection region, and does not output the detection signal when the position of the liquid level does not reach the detection region. The liquid level sensor  73 A is arranged such that the detection region is located at the height of the liquid level where the target substance is filled to the target level in the reservoir  267 . Therefore, although the detection signal is output to the target generation processor  52 D from the liquid level sensor  73 A in a state in which the liquid level position of the target substance in the reservoir  267  exceeds the height of the target liquid level, the detection signal is not output to the target generation processor  52 D from the liquid level sensor  73 A when the liquid level position of the target substance becomes lower than the height of the target liquid level. The target generation processor  52 D determines that replenishment of the target substance is unnecessary in the state in which the detection signal is received, and determines that replenishment of the target substance is necessary in the state in which the detection signal is not received. 
     When the target substance is output from the nozzle  265 , the liquid level in the reservoir  267  is lowered. Then, when the target generation processor  52 D turns into the state in which the detection signal from the liquid level sensor  73 A is not received, the target generation processor  52 D operates the refill mechanism  91 A to replenish the solid tin into the reservoir  267  until the detection signal is received again. That is, the target generation processor  52 D causes the refill mechanism  91 A to replenish the solid tin so that the liquid level of the target substance in the reservoir  267  is maintained at the target liquid level position or higher. 
     Here, the output amount of the target substance from the reservoir  267  is defined as Qout, and the supply amount from the refill mechanism  91 A to the reservoir  267  is defined as Qin. The replenishment amount Qin is the mass of the solid tin supplied to the reservoir  267  during replenishment. Assuming that Qin and Qout are substantially equal to each other, the relationship represented by following Equation (5) and Equation (6) is satisfied between the replenishment amount Qin of the refill mechanism  91 A and the diameter D of the target  27 . Therefore, if Qin is known, the target generation processor  52 D can calculate the diameter D of the target  27  based on Equation (5) and Equation (6). 
     
       
         
           
             
               
                 
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       FIG.  16    is a graph showing an example of a temporal change of the total mass M of the solid tin supplied to the reservoir  267  by the refill mechanism  91 A. Assuming that the mass of the replenishment amount Qin per unit time is ΔM, ΔM 2  becomes larger than ΔM 1  with the elapse of time. In  FIG.  16   , time durations Δt 1 , Δt 2  are the same.  FIG.  16    shows an example in which the output amount Qout changes when the diameter d of the nozzle hole  268  or the output velocity of the target substance changes, and the replenishment amount Qin changes accordingly. In addition, assuming that the mass of the replenishment amount Qin per unit time Δt is ΔM and the density of liquid tin which is a material of the target substance in the reservoir  267  is ρ, the replenishment amount Qin can be calculated by following Equation (7). 
     
       
         
           
             
               
                 
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     5.2 Operation 
     Next, with reference to the flowchart shown in  FIG.  17   , the operation of the EUV light generation apparatus of the third embodiment will be described. The flowchart shown in  FIG.  17    is different from the flowchart of the first embodiment shown in  FIG.  8    only in step S 540  and step S 550 . Since the others are the same, the difference will be described. 
     In step S 540 , the target generation processor  52 D measures the mass ΔM per unit time Δt by the measuring unit  96 A, and calculates the replenishment amount Qin from ΔM based on Equation (7). After step S 540 , processing proceeds to step S 550 . 
     In step S 550 , the target generation processor  52 D calculates the diameter D(t) of the target  27  at time t from the replenishment amount Qin based on Equation (6). Steps S 250  and thereafter are the same as in the first embodiment, and the target generation processor  52 D changes the generation frequency f based on the diameter D(t) of the target  27 . 
     (Modification of Third Embodiment) 
       FIGS.  18  and  19    show a modification of the third embodiment. In the example shown in  FIGS.  15  to  17   , the diameter D of the target  27  is calculated from the replenishment amount Qin, but in the modification shown in  FIGS.  18  and  19   , the diameter D of the target  27  is calculated from the replenishment interval. 
     As shown in  FIG.  18   , a refill mechanism  91 B of the modification includes a tank  97 B, a heater  281 , a pipe  98 B, a valve  92 B, and a liquid level sensor  73 B. The tank  97 B can be heated by the heater  281 , and the target substance is contained in the tank  97 B. The target substance is, for example, liquid tin. The tank  97 B is connected to the reservoir  267  via the pipe  98 B. The valve  92 B for starting and stopping replenishment is arranged in the middle of the pipe  98 B. 
     The target substance in the reservoir  267  is also liquid tin, and the liquid level sensor  73 B detects the liquid level of the target substance in the reservoir  267 . The liquid level sensor  73 B is similar to the liquid level sensor  73 A shown in  FIG.  15    in having only one detection region for detecting the liquid level. However, unlike the liquid level sensor  73 A shown in  FIG.  15   , the liquid level sensor  73 B is arranged such that the detection region is located at a height of the lower limit of the liquid level of the liquid tin set in advance. The liquid level sensor  73 B outputs a detection signal when the liquid level of the liquid tin is at the lower limit or higher, and does not output the detection signal when the liquid level of the liquid tin is lower than the lower limit. The refill mechanism  91 B determines that the liquid level of the liquid tin in the reservoir  267  is lower than the lower limit when the detection signal from the liquid level sensor  73 B is not received. When the liquid level becomes lower than the lower limit value, the refill mechanism  91 B replenishes a preset amount of the target substance. The replenishment amount Qin of the refill mechanism  91 B of one time is constant. 
     As shown in  FIG.  19   , even when the diameter d of the nozzle hole  268  or the like is changed in the modification, the mass m of the liquid tin corresponding to the replenishment amount Qin of one time is constant, but the replenishment interval td changes.  FIG.  19    shows an example in which the replenishment interval td is shortened from td 1  to td 2  due to the increase of the diameter d of the nozzle hole  268 . 
     In the case of the modification, the volume of the liquid tin corresponding to the replenishment amount Qin of one time can be calculated by dividing the mass m by the density ρ. The replenishment amount Qin of the modification is calculated by further dividing the calculated value of m/p by the replenishment interval td. The diameter D of the target  27  can be calculated by applying the replenishment amount Qin to Qin of Equation (6) described above. In other respects, the modification is similar to the flowchart of  FIG.  17   . 
     5.3 Effects 
     As described above, the EUV light generation apparatus of the third embodiment further includes the refill mechanism  91 A,  91 B that replenishes the target supply unit exemplified by the target supply unit  26 D,  26 E with the materials of the target  27 , and the target generation processor  52 D changes the generation frequency f based on the replenishment amount Qin of the refill mechanism  91 A or the replenishment interval td of the refill mechanism  91 B. The third embodiment is effective, for example, when the target image TP cannot be obtained as in the first embodiment. 
     The replenishment amount Qin is the replenishment amount Qin per unit time of the refill mechanism  91 A, and the replenishment interval td is the replenishment interval td when the replenishment amount Qin of one time of the refill mechanism  91 B is fixed. The target generation processor  52 D calculates the size of the target  27  based on the replenishment amount Qin or the replenishment interval td. In the third embodiment, by converting the replenishment amount Qin or the replenishment interval td into the size of the target  27 , the common portions of the first embodiment can be used. 
     Further, since the third embodiment uses the allowable range of the size of the target  27 , the same effect as the example using the allowable range in the first embodiment is obtained. 
     Further, in the third embodiment, the size of the target  27  is measured from the replenishment amount Qin or the replenishment interval td, but the size of the target  27  may not be measured. For example, as in the second embodiment, the reference table recording the correspondence relationship between the replenishment amount Qin and the generation frequency f may be used, and the change control of the generation frequency f may be performed using the reference table. The replenishment amount Qin or the replenishment interval td correlates with the size of the target  27 . Therefore, the replenishment amount Qin or the replenishment interval td is also an example of the “related information related to the size of the target” of the present disclosure. 
     6. EUV Light Generation Apparatus of Fourth Embodiment 
     Next, the EUV light generation apparatus of a fourth embodiment will be described. Any component same as that described above is denoted by an identical reference sign, and duplicate description thereof is omitted unless specific description is needed. 
     6.1 Configuration 
     Since the configuration of the EUV light generation apparatus according to the fourth embodiment is similar to that of the EUV light generation apparatus  1 B according to the first embodiment, description thereof is omitted. The EUV light generation apparatus according to the fourth embodiment includes a target detection unit  41  that optically detects the target  27  moving from the target supply unit  26 A toward the plasma generation region  25  and outputs a passage timing signal having a signal intensity corresponding to the size of the passing target  27 . Since the entire configuration diagram of the EUV light generation apparatus according to the fourth embodiment is omitted, the target generation processor of the fourth embodiment is denoted by the same reference numeral as that of the first embodiment and will be described as the target generation processor  52 B. As described below, the target generation processor  52 B according to the fourth embodiment is different from the target generation processor  52 B according to the first embodiment in the content of the change control of the generation frequency f. The target generation processor  52 B of the fourth embodiment changes the generation frequency f based on the signal intensity. The target generation processor  52 B is different from the EUV light generation apparatus  1 B according to the first embodiment in that the generation frequency f of the target  27  is changed based on the signal intensity of the passage timing signal output by the target detection unit  41 . 
     The fourth embodiment will be described with reference to  FIGS.  20  to  22   .  FIG.  20    is a diagram schematically showing a state in which the target  27  passes through the target detection unit  41 . As described in the first embodiment, the optical sensor  412  of the target detection unit  41  outputs a light receiving signal having a signal intensity corresponding to the amount of received light. When the target  27  passes through the target detection region R, the target  27  blocks the illumination light traveling from the light emitting unit  41 B to the light receiving unit  41 A, and thus the signal intensity of the light receiving signal that is the output of the optical sensor  412  changes. The target detection unit  41  detects the target  27  based on the change in the signal intensity when the target  27  passes therethrough. 
       FIG.  21    is a graph showing a temporal change in the signal intensity I of the light receiving signal output from the optical sensor  412  when the target  27  passes through the target detection region R. In  FIG.  21   , the signal intensity I 0  is the signal intensity output by the optical sensor  412  when the target  27  is not passing, and the signal intensity I 0  is the baseline. Since the illumination light is not blocked by the target  27  when the target  27  is not passing, the signal intensity I 0  of the baseline indicates the maximum value of the signal intensity I. When the target  27  passes through the target detection region R, since a portion of the illumination light is blocked by the target  27 , the signal intensity I decreases. As the size of the target  27  is larger, the signal intensity I decreases, and the amount of change ΔI from the signal intensity I 0  which is the baseline increases. On the other hand, as the size of the target  27  is smaller, the decrease in the signal intensity I is smaller, and the change amount ΔI from the signal intensity I 0  which is the base line decreases. 
     Since there is a correlation between the size of the target  27  and the magnitude of the change amount ΔI, the change amount ΔI is an example of the “related information related to the size of the target” in the present disclosure. In the fourth embodiment, the target generation processor  52 B changes the generation frequency f based on the signal intensity I. Specifically, in this example, the change amount ΔI of the signal intensity I from the baseline is calculated, and the generation frequency f is changed based on the change amount ΔI. 
     In the fourth embodiment, the allowable range of the change amount ΔI is set in the memory  523  of the target generation processor  52 B. As shown in  FIG.  21   , the lower limit value of the allowable range of the change amount ΔI is ΔImin, and the upper limit value is ΔImax. 
     6.2 Operation 
     Next, the operation of the EUV light generation apparatus according to the fourth embodiment will be described. Specifically, with reference to  FIG.  22   , the operation of the target generation processor  52 B according to the fourth embodiment will be described. 
       FIG.  22    is a flowchart of the fourth embodiment. Step S 210  and step S 230  are similar to those in the first embodiment. In the fourth embodiment, in the initial setting of step S 220 E, the upper limit value ΔImax and the lower limit value ΔImin are read as the allowable range of the change amount ΔI. 
     When step S 230  is completed, the target generation processor  52 B performs the change control of the generation frequency fin step S 650  and thereafter. In step S 650 , the target generation processor  52 B obtains the passage timing signal for each target  27 , and calculates the change amount ΔI of the signal intensity I from the baseline. The calculated change amount ΔI is stored in the memory  523 . 
     In step S 660 , the target generation processor  52 B calculates the average value ΔIμ of the plurality of change amounts ΔI for the plurality of targets  27 . 
     In step S 670 , the target generation processor  52 B compares the average value ΔIμ with the upper limit value ΔImax. When the average value ΔIμ exceeds the upper limit value (NO in step S 670 ), it is considered that the target  27  is too large. In this case, the target generation processor  52 B increases the generation frequency f. As a result, the size of the target  27  can be decreased. In step S 670 , when the average value ΔIμ is equal to or less than the upper limit value ΔImax (YES in step S 670 ), processing proceeds to step S 680 . 
     In step S 680 , the target generation processor  52 B compares the average value ΔIμ with the lower limit value ΔImin. When the average value ΔIμ is less than the lower limit value ΔImin (NO in step S 680 ), it is considered that the target  27  is too small. In this case, the target generation processor  52 B decreases the generation frequency f. As a result, the size of the target  27  can be increased. In step S 680 , when the average value ΔIμ is equal to or larger than the lower limit value (YES in step S 680 ), processing returns to step S 30  of  FIG.  4    without changing the generation frequency f. 
     6.3 Effects 
     As described above, the EUV light generation apparatus of the fourth embodiment includes the target detection unit  41  for detecting the passage timing signal, and includes the target generation processor  52 B that changes the generation frequency f based on the signal intensity I of the passage timing signal of the target  27 . Therefore, the fourth embodiment is effective when the size of the target  27  cannot be measured from the target image TP. 
     In the fourth embodiment, the allowable range is used as shown in  FIG.  22   . Therefore, the effect of using the allowable range is obtained similarly as in the first embodiment. In the fourth embodiment, the representative value of the change amount ΔI of the plurality of signal intensities I is compared with the upper limit value Imax or the lower limit value Imin of the allowable range. Accordingly, the same effect therewith as in the first embodiment is also obtained. 
     In the fourth embodiment, an example has been described in which the change amount ΔI of the signal intensity I is used as the related information, and the generation frequency f is changed based on the change amount ΔI. However, the generation frequency f may be changed using a reference table in which the correspondence relationship between the signal intensity I or the change amount ΔI and the generation frequency f is recorded. 
     7. EUV Light Generation Apparatus of Fifth Embodiment 
     Next, an EUV light generation apparatus  1 F of a fifth embodiment will be described. Any component same as that described above is denoted by an identical reference sign, and duplicate description thereof is omitted unless specific description is needed. 
     7.1 Configuration 
       FIG.  23    is a diagram schematically showing the configuration of the EUV light generation apparatus  1 F according to the fifth embodiment. The EUV light generation apparatus  1 F according to the fifth embodiment is different from the EUV light generation apparatus  1 B according to the first embodiment in that a laser device  3 F and a laser light travel direction control unit  34 F are arranged instead of the laser device  3 A and the laser light travel direction control unit  34 A. 
     The laser device  3 F includes a prepulse laser  3 P and a main pulse laser  3 M. The prepulse laser  3 P is configured to output prepulse laser light  31 P. The main pulse laser  3 M is configured to output main pulse laser light  31 M. The prepulse laser  3 P is, for example, a YAG laser device or a laser device using Nd:YVO4. The main pulse laser  3 M is, for example, a CO 2  laser device. The main pulse laser  3 M may be a YAG laser device or a laser device using Nd:YVO4. 
     The laser light travel direction control unit  34 F includes high reflection mirrors  343 ,  344 ,  345  and a combiner  346 . The high reflection mirrors  343 ,  344  are arranged on an optical path of the main pulse laser light  31 M. The high reflection mirror  345  is arranged on an optical path of the prepulse laser light  31 P. 
     The combiner  346  is located on an optical path of the prepulse laser light  31 P reflected by the high reflection mirror  345  and an optical path of the main pulse laser light  31 M reflected by the high reflection mirror  344 . The combiner  346  is configured to reflect the prepulse laser light  31 P at high reflectance and transmit the main pulse laser light  31 M at high transmittance. The high reflection mirror  344  and the combiner  346  are configured to reflect the main pulse laser light  31 M and the prepulse laser light  31 P toward the inside of the chamber  2 A. The combiner  346  is configured to substantially cause the optical path axes of the prepulse laser light  31 P and the main pulse laser light  31 M to be matched to each other. 
     The fifth embodiment includes the image measurement unit  43  similar to that of the first embodiment.  FIG.  24    is a diagram schematically showing a state in which the image measurement unit  43  detects the target in a mist form. The image measurement unit  43  images, instead of the target  27  in the droplet form as in the first embodiment, a target  27 A in the mist form that has been irradiated with the prepulse laser light  31 P and has become in the diffused state. The target generation processor  52 B measures the size of the target  27 A in the mist form from the target image TP using the target image TP output from the image measurement unit  43 . The other points are the same as those of the first embodiment. 
     7.2 Operation 
     Next, the operation of the EUV light generation apparatus IF according to the fifth embodiment is substantially the same as that of the first embodiment. The difference is that the generation frequency f is changed based on the diameter D of the target  27  in the droplet form in the first embodiment, whereas the generation frequency f is changed based on the diameter D of the target  27 A in the mist form in the fifth embodiment. 
     7.3 Effects 
     The EUV light generation apparatus IF according to the fifth embodiment can also obtain the same effects as those of the first embodiment. The fifth embodiment may be combined with any of the second to fourth embodiments. 
     8. Others 
       FIG.  25    schematically shows the configuration of an exposure apparatus  6 A connected to the EUV light generation apparatus  1 B. In  FIG.  24   , the exposure apparatus  6 A as the external apparatus  6  includes a mask irradiation unit  68  and a workpiece irradiation unit  69 . The mask irradiation unit  68  illuminates, via a reflection optical system, a mask pattern of the mask table MT with the EUV light incident from the EUV light generation apparatus  1 B. The workpiece irradiation unit  69  images the EUV light reflected by the mask table MT onto a workpiece (not shown) arranged on a workpiece table WT via the reflection optical system. The workpiece is a photosensitive substrate such as a semiconductor wafer on which photoresist is applied. The exposure apparatus  6 A synchronously translates the mask table MT and the workpiece table WT to expose the workpiece to the EUV light reflecting the mask pattern. Through the exposure process as described above, a device pattern is transferred onto the semiconductor wafer, thereby an electronic device can be manufactured. 
       FIG.  26    schematically shows the configuration of an inspection apparatus  6 B connected to the EUV light generation apparatus  1 B. In  FIG.  26   , the inspection apparatus  6 B as the external apparatus  6  includes an illumination optical system  63  and a detection optical system  66 . The EUV light generation apparatus  1 B outputs, as a light source for inspection, EUV light to the inspection apparatus  6 B. The illumination optical system  63  reflects the EUV light incident from the EUV light generation apparatus  1 B to illuminate a mask  65  placed on a mask stage  64 . Here, the mask  65  conceptually includes a mask blanks before a pattern is formed. The detection optical system  66  reflects the EUV light from the illuminated mask  65  and forms an image on a light receiving surface of a detector  67 . The detector  67  having received the EUV light obtains the image of the mask  65 . The detector  67  is, for example, a time delay integration (TDI) camera. Defects of the mask  65  are inspected based on the image of the mask  65  obtained by the above-described process, and a mask suitable for manufacturing an electronic device is selected using the inspection result. Then, the electronic device can be manufactured by exposing and transferring the pattern formed on the selected mask onto the photosensitive substrate using the exposure apparatus  6 A. 
     In the inspection apparatus  6 B, the above-described EUV light concentrating mirror  23  may be a grazing incidence type. Further, in  FIGS.  25  and  26   , instead of the EUV light generation apparatus  1 B, any one of the EUV light generation apparatus of the second to fifth embodiments may be used. 
     The description above is intended to be illustrative and the present disclosure is not limited thereto. Therefore, it would be obvious to those skilled in the art that various modifications to the embodiments of the present disclosure would be possible without departing from the spirit and the scope of the appended claims. 
     The terms used throughout the present specification and the appended claims should be interpreted as non-limiting terms. For example, terms such as “comprise”, “include”, “have”, and “contain” should not be interpreted to be exclusive of other structural elements. Further, indefinite articles “a/an” described in the present specification and the appended claims should be interpreted to mean “at least one” or “one or more.” Further, “at least one of A, B, and C” should be interpreted to mean any of A, B, C, A+B, A+C, B+C, and A+B+C as well as to include combinations of the any thereof and any other than A, B, and C.