Patent Publication Number: US-11659646-B2

Title: Target supply device, extreme ultraviolet light generation apparatus, and electronic device manufacturing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of Japanese Patent Application No. 2020-191478, filed on Nov. 18, 2020, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a target supply device, an extreme ultraviolet light generation apparatus, and an electronic device manufacturing 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: U.S. Pat. No. 8,841,639 
     Patent Document 2: U.S. Pat. No. 10,225,917 
     SUMMARY 
     A target supply device according to an aspect of the present disclosure includes a tank configured to store a target substance, a pressure adjuster configured to adjust a pressure in the tank, a filter configured to filter the target substance in the tank, a nozzle configured to output a droplet of the target substance having passed through the filter, a droplet detector configured to detect outputting of the droplet from the nozzle, and a processor configured to control the pressure adjuster so that a pressure-increasing speed of the pressure in the tank is higher after detection of outputting of the droplet than before detection of outputting of the droplet, during a period in which the pressure in the tank is increased to a target pressure from a pressure at which outputting of the droplet is detected by the droplet detector for the first time after installation of the target supply device. 
     An extreme ultraviolet light generation apparatus according to an aspect of the present disclosure includes a chamber device including a plasma generation region, a target supply device configured to supply a droplet of a target substance to the plasma generation region, and a laser device configured to irradiate the droplet with laser light so that plasma is generated from the droplet in the plasma generation region. Here, the target supply device includes a tank configured to store the target substance, a pressure adjuster configured to adjust a pressure in the tank, a filter configured to filter the target substance in the tank, a nozzle configured to output the droplet of the target substance having passed through the filter, a droplet detector configured to detect outputting of the droplet from the nozzle, and a processor configured to control the pressure adjuster so that a pressure-increasing speed of the pressure in the tank is higher after detection of outputting of the droplet than before detection of outputting of the droplet, during a period in which the pressure in the tank is increased to a target pressure from a pressure at which outputting of the droplet is detected for the first time by the droplet detector after installation of the target supply device. 
     An electronic device manufacturing method according to an aspect of the present disclosure includes generating plasma by irradiating a target substance with laser light using an extreme ultraviolet light generation apparatus, emitting extreme ultraviolet light generated from the plasma to an exposure apparatus, and exposing a photosensitive substrate to the extreme ultraviolet light in the exposure apparatus to manufacture an electronic device. Here, the extreme ultraviolet light generation apparatus includes a chamber device including a plasma generation region, a target supply device configured to supply a droplet of the target substance to the plasma generation region, and a laser device configured to irradiate the droplet with the laser light so that the plasma is generated from the droplet in the plasma generation region. The target supply device includes a tank configured to store the target substance, a pressure adjuster configured to adjust a pressure in the tank, a filter configured to filter the target substance in the tank, a nozzle configured to output the droplet of the target substance having passed through the filter, a droplet detector configured to detect outputting of the droplet from the nozzle, and a processor configured to control the pressure adjuster so that a pressure-increasing speed of the pressure in the tank is higher after detection of outputting of the droplet than before detection of outputting of the droplet, during a period in which the pressure in the tank is increased to a target pressure from a pressure at which outputting of the droplet is detected for the first time by the droplet detector after installation of the target supply device. 
     An electronic device manufacturing method according to an aspect of the present disclosure includes generating plasma by irradiating a target substance with laser light using an extreme ultraviolet light generation apparatus, inspecting a defect of a mask by irradiating the mask with extreme ultraviolet light generated from the plasma, selecting a mask using a result of the inspection, and exposing and transferring a pattern formed on the selected mask onto a photosensitive substrate. Here, the extreme ultraviolet light generation apparatus includes a chamber device including a plasma generation region, a target supply device configured to supply a droplet of the target substance to the plasma generation region, and a laser device configured to irradiate the droplet with the laser light so that the plasma is generated from the droplet in the plasma generation region. The target supply device includes a tank configured to store the target substance, a pressure adjuster configured to adjust a pressure in the tank, a filter configured to filter the target substance in the tank, a nozzle configured to output the droplet of the target substance having passed through the filter, a droplet detector configured to detect outputting of the droplet from the nozzle, and a processor configured to control the pressure adjuster so that a pressure-increasing speed of the pressure in the tank is higher after detection of the outputting of the droplet than before detection of the outputting of the droplet, during a period in which the pressure in the tank is increased to a target pressure from a pressure at which the outputting of the droplet is detected for the first time by the droplet detector after installation of the target supply device. 
    
    
     
       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 schematic view showing a schematic configuration example of an entire electronic device manufacturing apparatus. 
         FIG.  2    is a schematic view showing a schematic configuration example of an entire electronic device manufacturing apparatus different from the electronic device manufacturing apparatus shown in  FIG.  1   . 
         FIG.  3    is a schematic view showing a schematic configuration example of an entire EUV light generation apparatus. 
         FIG.  4    is a schematic view showing a schematic configuration example of a pressure adjuster. 
         FIG.  5    is a graph showing the relationship between the pressure in a tank and time at which the pressure increases in a comparative example. 
         FIG.  6    is a view showing an example in which a droplet output from a nozzle is in an unstable state. 
         FIG.  7    is a view showing another example in which the droplet output from the nozzle is in an unstable state. 
         FIG.  8    is a diagram showing an example of a control flowchart of a processor according to a first embodiment. 
         FIG.  9    is a diagram showing the relationship between the pressure in the tank and time at which the pressure increases in the tank in the first embodiment. 
         FIG.  10    is a diagram showing an example of a control flowchart of the processor according to a second embodiment. 
         FIG.  11    is a diagram showing the relationship between the pressure in the tank when the droplet is re-output and time at which the pressure increases. 
         FIG.  12    is a schematic view showing a schematic configuration example of the entire EUV light generation apparatus of a third embodiment. 
         FIG.  13    is a diagram showing an example of a control flowchart of the processor according to the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     
         
         1. Overview 
         2. Description of electronic device manufacturing apparatus 
         3. Description of extreme ultraviolet light generation apparatus of comparative example 
       
    
     3.1 Configuration 
     3.2 Operation 
     3.3 Problem
     4. Description of extreme ultraviolet light generation apparatus of first embodiment   

     4.1 Configuration 
     4.2 Operation 
     4.3 Effect
     5. Description of extreme ultraviolet light generation apparatus of second embodiment   

     5.1 Configuration 
     5.2 Operation 
     5.3 Effect
     6. Description of extreme ultraviolet light generation apparatus of third embodiment   

     6.1 Configuration 
     6.2 Operation 
     6.3 Effect 
     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. Overview 
     Embodiments of the present disclosure relate to an extreme ultraviolet light generation apparatus generating light having a wavelength of extreme ultraviolet (EUV) and an electronic device manufacturing apparatus. In the following, extreme ultraviolet light is referred to as EUV light in some cases. 
     2. Description of Electronic Device Manufacturing Apparatus 
       FIG.  1    is a schematic view showing a schematic configuration example of an entire electronic device manufacturing apparatus. The electronic device manufacturing apparatus shown in  FIG.  1    includes an EUV light generation apparatus  100  and an exposure apparatus  200 . The exposure apparatus  200  includes a mask irradiation unit  210  including a plurality of mirrors  211 ,  212  that are a reflection optical system, and a workpiece irradiation unit  220  including a plurality of mirrors  221 ,  222  that are a reflection optical system different from the reflection optical system of the mask irradiation unit  210 . The mask irradiation unit  210  illuminates, via the mirrors  211 ,  212 , a mask pattern of the mask table MT with EUV light  101  incident from the EUV light generation apparatus  100 . The workpiece irradiation unit  220  images the EUV light  101  reflected by the mask table MT onto a workpiece (not shown) arranged on the workpiece table WT via the mirrors  211 ,  212 . The workpiece is a photosensitive substrate such as a semiconductor wafer on which photoresist is applied. The exposure apparatus  200  synchronously translates the mask table MT and the workpiece table WT to expose the workpiece to the EUV light  101  reflecting the mask pattern. Through the exposure process as described above, a device pattern is transferred onto the semiconductor wafer, thereby a semiconductor device can be manufactured. 
       FIG.  2    is a schematic view showing a schematic configuration example of an entire electronic device manufacturing apparatus different from the electronic device manufacturing apparatus shown in  FIG.  1   . The electronic device manufacturing apparatus shown in  FIG.  2    includes the EUV light generation apparatus  100  and an inspection apparatus  300 . The inspection apparatus  300  includes an illumination optical system  310  including a plurality of mirrors  311 ,  313 ,  315  that are a reflection optical system, and a detection optical system  320  including a plurality of mirrors  321 ,  322  that are a reflection optical system different from the reflection optical system of the illumination optical system  310  and a detector  325 . The illumination optical system  310  reflects, with the mirrors  311 ,  313 ,  315 , the EUV light  101  incident from the EUV light generation apparatus  100  to illuminate a mask  333  placed on a mask stage  331 . The mask  333  includes a mask blanks before a pattern is formed. The detection optical system  320  reflects, with the mirrors  321 ,  323 , the EUV light  101  reflecting the pattern from the mask  333  and forms an image on a light receiving surface of the detector  325 . The detector  325  having received the EUV light  101  obtains an image of the mask  333 . The detector  325  is, for example, a time delay integration (TDI) camera. Defects of the mask  333  are inspected based on the image of the mask  333  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  200 . 
     3. Description of Extreme Ultraviolet Light Generation Apparatus of Comparative Example 
     3.1 Configuration 
     The EUV light generation apparatus  100  of a comparative example will be described. The comparative example of the present disclosure is an example recognized by the applicant as known only by the applicant, and is not a publicly known example admitted by the applicant. Further, the following description will be given with reference to the EUV light generation apparatus  100  that emits the EUV light  101  toward the exposure apparatus  200  as an external apparatus as shown in  FIG.  1   . Here, the EUV light generation apparatus  100  that emits the EUV light  101  to the inspection apparatus  300  as an external apparatus as shown in  FIG.  2    can obtain the same operation and effect as the EUV light generation apparatus  100  that emits the EUV light  101  toward the exposure apparatus  200 . 
       FIG.  3    is a schematic view showing a schematic configuration example of the entire EUV light generation apparatus  100  of the present example. As shown in  FIG.  3   , the EUV light generation apparatus  100  includes a laser device LD, a chamber device  10 , a processor  120 , and a laser light delivery optical system  30  as a main configuration. 
     The chamber device  10  is a sealable container. The chamber device  10  includes an inner wall  10   b  surrounding an internal space having a low pressure atmosphere. The chamber device  10  includes a sub-chamber  15  and a target supply device  40  is arranged in the sub-chamber  15 . The target supply device  40  is attached to penetrate through a wall of the sub-chamber  15 . The target supply device  40  includes a tank  41 , a nozzle  42 , and a pressure adjuster  43  to supply a droplet DL to the internal space of the chamber device  10 . The droplet DL is also referred to as a target. 
     The tank  41  stores therein a target substance which becomes the droplet DL. The target substance contains tin. The inside of the tank  41  is in communication with the pressure adjuster  43  which regulates the pressure in the tank  41 . A heater  44  and a temperature sensor  45  are attached to the tank  41 . The heater  44  heats the tank  41  with current applied from a heater power source  46 . Through the heating, the target substance in the tank  41  melts. The temperature sensor  45  measures the temperature of the target substance in the tank  41  through the tank  41 . The pressure adjuster  43 , the temperature sensor  45 , and the heater power source  46  are electrically connected to the processor  120 . 
     The tank  41  also includes a communication portion which communicates with the inside of the tank  41  and the nozzle  42 . The communication portion is a flow path through which the target substance flows from the inside of the tank  41  toward the nozzle  42 . The communication portion includes an enlarged diameter part having a larger diameter than another part of the communication portion, and a filter unit  51  is accommodated without a gap in the enlarged diameter part. 
     The filter unit  51  includes a filter  51   a  and a filter holder  51   b.    
     The filter  51   a  filters the target substance passing through the filter  51   a  to remove particles from the target substance. The particles are metal oxides such as tin oxide. The filter  51   a  is formed of, for example, a porous member in order to collect particles. Accordingly, numerous through holes are formed in the filter  51   a , and the diameter of the through holes is, for example, 3 μm or more and 10 μm or less. The thickness of the filter  51   a  is approximately 5 mm. The filter  51   a  may be porous glass. Alternatively, the filter  51   a  may have a structure in which a plurality of porous plate-shaped members are laminated, or may be a plurality of porous ceramics. 
     The filter  51   a  is arranged in a hollow portion of the cylindrical filter holder  51   b , and the outer circumferential surface of the filter  51   a  is in close contact with the inner circumferential surface of the filter holder  51   b  without a gap, and sealing is arranged between the outer circumferential surface and the inner circumferential surface. Further, the outer surface of the filter holder  51   b  is in close contact with the inner surface in the enlarged diameter portion without a gap, and sealing is provided between the outer surface and the inner surface. 
     The nozzle  42  is attached to the tank  41  and outputs the target substance having passed through the filter  51   a . A piezoelectric element  47  is attached to the nozzle  42 . The piezoelectric element  47  is electrically connected to a piezoelectric power source  48  and is driven by voltage applied from the piezoelectric power source  48 . The piezoelectric power source  48  is electrically connected to the processor  120 . The target substance output from the nozzle  42  is formed into the droplet DL through operation of the piezoelectric element  47 . 
     Material of the tank  41 , the nozzle  42 , and the filter holder  51   b  has low reactivity with tin as the target substance. Examples of the material include tungsten (W), molybdenum (Mo), and tantalum (Ta). 
       FIG.  4    is a schematic view showing a schematic configuration example of the pressure adjuster  43 . 
     The pressure adjuster  43  includes a pipe  43   a  communicating with the gas supply source  53  and the inside of the tank  41 , a valve  43   b  arranged in the pipe  43   a , a pipe  43   c  communicating with the pipe  43   a , a valve  43   d  arranged in the pipe  43   c , and a pressure sensor  43   e  arranged between the valve  43   b  and the tank  41  in the pipe  43   a.    
     The gas supply source  53  is a cylinder filled with inert gas such as argon (Ar) gas and helium (He) gas. The pipe  43   a  is a supply path for supplying the inert gas from the gas supply source  53  into the tank  41 . The pipe  43   a  communicates with one end of the pipe  43   c , and an exhaust port  43   f  is arranged at the other end of the pipe  43   c . The pipe  43   c  is an exhaust path for exhausting the inert gas in the tank  41  through the exhaust port  43   f.    
     The valves  43   b ,  43   d  are control valves for opening and closing the pipes  43   a ,  43   c .  FIG.  4    shows an example in which the valve  43   b  is arranged in the pipe  43   a  between the gas supply source  53  and the communication portion of the pipe  43   a  and the pipe  43   c . The valve  43   b  may be arranged upstream from the pressure sensor  43   e  in the supply passage. An actuator (not shown) is attached to each of the valves  43   b ,  43   d . Each actuator is electrically connected to the processor  120 . The respective actuators open and close the valves  43   b ,  43   d  based on signals input from the processor  120 , and the inside of the tank  41  is pressurized or depressurized by the opening and closing. In the case of pressurization, the actuator of the valve  43   d  closes the valve  43   d , and the actuator of the valve  43   b  adjusts the opening degree of the valve  43   b . In the case of depressurization, the actuator of the valve  43   b  closes the valve  43   b , and the actuator of the valve  43   d  adjusts the opening degree of the valve  43   d . The pressure-increasing speed of the pressure in the tank  41  due to the pressurization is adjusted by the opening degree of the valve  43   b , and the pressure-decreasing speed of the pressure in the tank  41  due to the depressurization is adjusted by the opening degree of the valve  43   d . Here, the inside of the tank  41  may be pressurized by opening the valve  43   b  larger than the valve  43   d , and the inside of the tank  41  may be depressurized by opening the valve  43   d  larger than the valve  43   b . The configuration of the pressure adjuster  43  is not particularly limited as long as the inside of the tank  41  is pressurized by supplying the inert gas from the gas supply source  53  and the inside of the tank  41  is depressurized by exhausting the inert gas from the inside of the tank  41 . Therefore, in the pressure adjuster  43 , instead of the valves  43   b ,  43   d , a three way valve may be arranged at the communication portion of the pipe  43   a  and the pipe  43   c.    
     The pressure sensor  43   e  measures the pressure in the tank  41  through the pipe  43   a . The pressure sensor  43   e  is electrically connected to the processor  120 . Here, the pressure sensor  43   e  may be arranged in the tank  41 . 
     Returning to  FIG.  3   , the description of the chamber device  10  will be continued. The chamber device  10  includes a target collection unit  14 . The target collection unit  14  is a box body attached to the inner wall  10   b  of the chamber device  10 . The target collection unit  14  communicates with the internal space of the chamber device  10  through an opening  10   a  continued to the inner wall  10   b  of the chamber device  10 . The target collection unit  14  and the opening  10   a  are arranged directly below the nozzle  42 . The target collection unit  14  is a drain tank to collect any unnecessary droplet DL passing through the opening  10   a  and reaching the target collection unit  14  and to accumulate the unnecessary droplet DL. 
     At least one through hole is formed in the inner wall  10   b  of the chamber device  10 . The through hole is blocked by a window  12  through which pulse laser light  90  emitted from the laser device LD passes. 
     Further, a laser light concentrating optical system  13  is located at the internal space of the chamber device  10 . The laser light concentrating optical system  13  includes a laser light concentrating mirror  13 A and a high reflection mirror  13 B. The laser light concentrating mirror  13 A reflects and concentrates the laser light  90  passing through the window  12 . The high reflection mirror  13 B reflects light concentrated by the laser light concentrating mirror  13 A. Positions of the laser light concentrating mirror  13 A and the high reflection mirror  13 B are adjusted by a laser light manipulator  13 C so that a concentrating position of the laser light  90  at the internal space of the chamber device  10  coincides with a position specified by the processor  120 . The concentrating position is adjusted to be located directly below the nozzle  42 , and when the target substance constituting the droplet DL is irradiated with the laser light at the concentrating position, plasma is generated by the irradiation, and the EUV light  101  is radiated from the plasma. In the following, the region in which plasma is generated is sometimes referred to as a plasma generation region AR. 
     For example, an EUV light concentrating mirror  75  having a spheroidal reflection surface  75   a  is arranged at the internal space of the chamber device  10 . The reflection surface  75   a  reflects the EUV light  101  radiated from the plasma in the plasma generation region AR. The reflection surface  75   a  has a first focal point and a second focal point. The reflection surface  75   a  may be arranged such that, for example, the first focal point is located in the plasma generation region AR and the second focal point is located at an intermediate focal point IF. In  FIG.  3   , a straight line passing through the first focal point and the second focal point is shown as a focal line L 0 . 
     Further, the EUV light generation apparatus  100  includes a connection portion  19  providing communication between the internal space of the chamber device  10  and an internal space of the exposure apparatus  200 . A wall in which an aperture is formed is arranged inside the connection portion  19 . The wall is preferably arranged such that the aperture is located at the second focal point. The connection portion  19  is an emission port of the EUV light  101  in the EUV light generation apparatus  100 , and the EUV light  101  is emitted from the connection portion  19  and enters the exposure apparatus  200 . 
     Further, the EUV light generation apparatus  100  includes a pressure sensor  26  and a target sensor  27 . The pressure sensor  26  and the target sensor  27  are attached to the chamber device  10  and are electrically connected to the processor  120 . The pressure sensor  26  measures the pressure at the internal space of the chamber device  10 . The target sensor  27  has, for example, an imaging function, and detects the presence, trajectory, position, velocity, and the like of the droplet DL output from the nozzle hole of the nozzle  42  according to an instruction from the processor  120 . The target sensor  27  may be arranged inside the chamber device  10 , or may be arranged outside the chamber device  10  and detect the droplet DL through a window (not shown) arranged on a wall of the chamber device  10 . The target sensor  27  includes a light receiving optical system (not shown) and an imaging unit (not shown) such as a charge-coupled device (CCD) or a photodiode. In order to improve the detection accuracy of the droplet DL, the light-receiving optical system forms an image of the trajectory of the droplet DL and the periphery thereof on a light receiving surface of the imaging unit. When the droplet DL passes through a concentrating region of a light source unit (not shown) of the target sensor  27  arranged to secure the field of view of the target sensor  27 , the imaging unit detects a change of the light passing through the trajectory of the droplet DL and the periphery thereof. The imaging unit converts the detected light change into an electric signal as a signal related to the image data of the droplet DL. The imaging unit outputs the electric signal to the processor  120 . 
     The laser device LD includes a master oscillator being a light source to perform a burst operation. The master oscillator emits the pulse laser light  90  in a burst-on duration. The master oscillator is, for example, a laser device configured to emit the laser light  90  by exciting, through electric discharge, gas as mixture of carbon dioxide gas with helium, nitrogen, or the like. Alternatively, the master oscillator may be a quantum cascade laser device. The master oscillator may emit the pulse laser light  90  by a Q switch system. Further, the master oscillator may include an optical switch, a polarizer, and the like. In the burst operation, the pulse laser light  90  is continuously emitted at a predetermined repetition frequency in the burst-on duration and the emission of the laser light  90  is stopped in a burst-off duration. 
     The travel direction of the laser light  90  emitted from the laser device LD is adjusted by the laser light delivery optical system  30 . The laser light delivery optical system  30  includes a plurality of mirrors  30 A and  30 B for adjusting a travel direction of the laser light  90 . The position of at least one of the mirrors  30 A and  30 B is adjusted by an actuator (not shown). Owing to that the position of at least one of the mirrors  30 A and  30 B is adjusted, the laser light  90  can appropriately propagate to the internal space of the chamber device  10  through the window  12 . 
     The processor  120  is a processing device including a storage device  120   a  in which a control program is stored and a CPU  120   b  which executes the control program. The processor  120  is specifically configured or programmed to perform various processes included in the present disclosure. The processor  120  controls several configurations of the EUV light generation apparatus  100 . Further, the processor  120  controls the entire EUV light generation apparatus  100 . The processor  120  receives a signal related to the pressure at the internal space of the chamber device  10 , which is measured by the pressure sensor  26 , a signal related to the image data of the droplet DL captured by the target sensor  27 , a burst signal from the exposure apparatus  200 , a signal related to the pressure in the tank  41  measured by the pressure sensor  43   e , and the like. The processor  120  processes the various signals, and may control, for example, timing at which the droplet DL is output, an output direction of the droplet DL, and the like. Further, the processor  120  may control oscillation timing of the laser device LD, the travel direction of the laser light  90 , the concentrating position of the laser light  90 , and the like. Further, the processor  120  may control the opening and closing of the valves  43   b ,  43   d , the opening degree of the valves  43   b ,  43   d , and the like based on the signal from the pressure sensor  43   e . Such various kinds of control described above are merely exemplary, and other control may be added as necessary, as described later. 
     A central gas supply unit  81  for supplying an etching gas to the internal space of the chamber device  10  is arranged at the chamber device  10 . As described above, since the target substance contains tin, the etching gas is, for example, hydrogen-containing gas having a hydrogen gas concentration of 100% in effect. Alternatively, the etching gas may be, for example, a balance gas having a hydrogen gas concentration of about 3%. The balance gas contains nitrogen (N 2 ) gas and argon (Ar) gas. Tin fine particles and tin charged particles are generated when the target substance forming the droplet DL is turned into plasma in the plasma generation region AR by being irradiated with the laser light  90 . Tin constituting these fine particles and charged particles reacts with hydrogen contained in the etching gas supplied to the internal space of the chamber device  10 . Through the reaction with hydrogen, tin becomes stannane (SnH 4 ) gas at room temperature. 
     The central gas supply unit  81  has a shape of a side surface of a circular truncated cone and is called a cone in some cases. The central gas supply unit  81  is inserted through a through hole  75   c  formed in the center of the EUV light concentrating mirror  75 . 
     The central gas supply unit  81  has a central gas supply port  81   a  being a nozzle. A central gas supply port  81   a  is arranged on the focal line L 0  passing through the first focal point and the second focal point of the reflection surface  75   a . The focal line L 0  is extended along the center axis direction of the reflection surface  75   a.    
     The central gas supply port  81   a  supplies the etching gas from the center side of the reflection surface  75   a  toward the plasma generation region AR. The central gas supply port  81   a  preferably supplies the etching gas in the direction away from the reflection surface  75   a  from the center side of the reflection surface  75   a  along the focal line L 0 . The central gas supply port  81   a  is connected to a gas supply device (not shown) being a tank through a pipe (not shown) of the central gas supply unit  81  and the etching gas is supplied therefrom. The gas supply device is driven and controlled by the processor  120 . A supply gas flow rate adjusting unit being a valve (not shown) may be arranged in the pipe (not shown). 
     The central gas supply port  81   a  is a gas supply port for supplying the etching gas to the internal space of the chamber device  10  as well as an emission port through which the laser light  90  is emitted to the internal space of the chamber device  10 . The laser light  90  travels toward the internal space of the chamber device  10  through the window  12  and the central gas supply port  81   a.    
     An exhaust port  10 E is continued to the inner wall  10   b  of the chamber device  10 . Since the exposure apparatus  200  is arranged on the focal line L 0 , the exhaust port  10 E is arranged not on the focal line L 0  but on the inner wall  10   b  on the side lateral to the focal line L 0 . The direction along the center axis of the exhaust port  10 E is perpendicular to the focal line L 0 . The exhaust port  10 E is arranged on the side opposite to the reflection surface  75   a  with respect to the plasma generation region AR when viewed from the direction perpendicular to the focal line L 0 . The exhaust port  10 E exhausts residual gas to be described later at the internal space of the chamber device  10 . The exhaust port  10 E is connected to an exhaust pipe  10 P, and the exhaust pipe  10 P is connected to an exhaust pump  60 . 
     As described above, when the target substance is turned into plasma in the plasma generation region AR, the residual gas as exhaust gas is generated at the internal space of the chamber device  10 . The residual gas contains tin fine particles and tin charged particles generated through the plasma generation from the target substance, stannane generated through the reaction of the tin fine particles and tin charged particles with the etching gas, and unreacted etching gas. Some of the charged particles are neutralized at the internal space of the chamber device  10 , and the residual gas contains the neutralized charged particles as well. The residual gas is sucked to the exhaust pump  60  through the exhaust port  10 E and the exhaust pipe  10   p.    
     3.2 Operation 
     Next, operation of the EUV light generation apparatus  100  of the comparative example will be described. In the EUV light generation apparatus  100 , for example, at the time of new installation or maintenance or the like, atmospheric air at the internal space of the chamber device  10  is exhausted. At this time, purging and exhausting of the internal space of the chamber device  10  may be repeated for exhausting atmospheric components. For example, inert gas such as nitrogen or argon is preferably used for the purge gas. Thereafter, when the pressure at the internal space of the chamber device  10  becomes equal to or lower than a predetermined pressure, the processor  120  starts introduction of the etching gas from the gas supply device to the internal space of the chamber device  10  through the central gas supply unit  81 . At this time, the processor  120  may control the supply gas flow rate adjusting unit (not shown) and the exhaust pump  60  so that the pressure at the internal space of the chamber device  10  is maintained at a predetermined pressure. Thereafter, the processor  120  waits until a predetermined time elapses from the start of introduction of the etching gas. 
     Further, the processor  120  causes the gas at the internal space of the chamber device  10  to be exhausted from the exhaust port  10 E by the exhaust pump  60 , and keeps the pressure at the internal space of the chamber device  10  substantially constant based on the signal of the pressure at the internal space of the chamber device  10  measured by the pressure sensor  26 . 
     In order to heat and maintain the target substance in the tank  41  at a predetermined temperature equal to or higher than the melting point, the processor  120  causes the heater power source  46  to apply current to the heater  44  to increase temperature of the heater  44 . In this case, the processor  120  controls the temperature of the target substance to the predetermined temperature by adjusting a value of the current applied from the heater power source  46  to the heater  44  based on an output from the temperature sensor  45 . When the target substance is tin, the predetermined temperature is equal to or higher than 231.93° C. being the melting point of tin, for example, 240° C. or higher and 290° C. or lower. 
     Further, the processor  120  causes the pressure adjuster  43  to supply the inert gas from the gas supply source  53  to the tank  41  and to adjust the pressure in the tank  41  so that the melted target substance is output through the nozzle hole of the nozzle  42  at a predetermined velocity. Under this pressure, the target substance is output through the nozzle hole of the nozzle  42  after particles are removed by the filter  51   a . The target substance output through the nozzle hole may be in the form of jet. At this time, the processor  120  causes the piezoelectric power source  48  to apply a voltage having a predetermined waveform to the piezoelectric element  47  to generate the droplet DL. The piezoelectric power source  48  applies a voltage so that the waveform of the voltage value becomes, for example, a sine wave, a rectangular wave, or a sawtooth wave. Vibration of the piezoelectric element  47  can propagate through the nozzle  42  to the target substance to be output through the nozzle hole of the nozzle  42 . The target substance is divided at a predetermined cycle by the vibration to be liquid droplets DL. 
     The target sensor  27  detects passage timing of the droplet DL passing through a predetermined position at the internal space of the chamber device  10 . The processor  120  outputs, to the laser device LD, a light emission trigger signal synchronized with the signal from the target sensor  27 . When the light emission trigger signal is input, the laser device LD emits the pulse laser light  90 . The emitted laser light  90  is incident on the laser light concentrating optical system  13  through the laser light delivery optical system  30  and the window  12 . Further, the laser light  90  travels from the laser light concentrating optical system  13  to the central gas supply unit  81  which is an emission portion. The laser light  90  is emitted along the focal line L 0  toward the plasma generation region AR from the central gas supply port  81   a , which is the emission port of the central gas supply unit  81 , and is radiated to the droplet DL in the plasma generation region AR. At this time, the processor  120  controls the laser light manipulator  13 C of the laser light concentrating optical system  13  so that the laser light  90  is concentrated in the plasma generation region AR. The processor  120  controls the timing of emitting the laser light  90  from the laser device LD based on the signal from the target sensor  27  so that the droplet DL is irradiated with the laser light  90 . Thus, the droplet DL is irradiated in the plasma generation region AR with the laser light  90  concentrated by the laser light concentrating mirror  13 A. Light including EUV light is emitted from the plasma generated through the irradiation. 
     Among the light including the EUV light generated in the plasma generation region AR, the EUV light  101  is concentrated at the intermediate focal point IF by the EUV light concentrating mirror  75 , and then is incident on the exposure apparatus  200  from the connection portion  19 . 
     When the target substance is turned into plasma, tin fine particles are generated as described above. The fine particles diffuse to the internal space of the chamber device  10 . The fine particles diffusing to the internal space of the chamber device  10  react with the hydrogen-containing etching gas supplied from the central gas supply unit  81  to become stannane. Most of the stannane obtained through the reaction with the etching gas flows into the exhaust port  10 E along with the flow of the unreacted etching gas. At least some of the unreacted charged particles, fine particles, and etching gas flow into the exhaust port  10 E. 
     The unreacted etching gas, fine particles, charged particles, stannane, and the like having flowed into the exhaust port  10 E flow as residual gas through the exhaust pipe  10 P into the exhaust pump  60  and are subjected to predetermined exhaust treatment such as detoxification. 
     In the EUV light generation apparatus  100  of the comparative example, the processor  120  pressurizes the inside of the tank  41  by the pressure adjuster  43  and outputs the droplet DL in the following procedure.  FIG.  5    is a graph showing the relationship between the pressure in the tank  41  and time at which the pressure increases. In the following, the pressure in the tank  41  may be referred to as a pressure P. A solid line L 1  shown in  FIG.  5    indicates a change of the pressure value measured by the pressure sensor  43   e , that is, a change of the pressure P with time. In the following, the time means the time from the start of pressurization. Time t 0 , t 1 , t 2 , and t 3  shown in  FIG.  5    denote 0, 10, 20, and 30 minutes. 
     At the start time of pressurization, time t 0 , the heater  44  is already heating the tank  41  by the current supplied by the heater power source  46 , and the target substance in the tank  41  is melted. Further, the valves  43   b ,  43   d  are in a closed state. 
     At time t 0 , the processor  120  outputs a signal to the actuator of the valve  43   b , and controls the opening degree of the valve  43   b  via the actuator so that the pressure P increases to a predetermined pressure P 1  at a predetermined pressure-increasing speed. The predetermined pressure-increasing speed is a speed at which the pressure P at time t 11  becomes the predetermined pressure P 1 . When the valve  43   b  is opened, the inert gas is supplied from the gas supply source  53  into the tank  41  through the pipe  43   a . Thus, the pressure adjuster  43  pressurizes the inside of the tank  41  to the predetermined pressure P 1  at the predetermined increasing speed until time t 11 . For example, time t 11  is 1 minute, and the predetermined pressure P 1  is 1 MPa. The processor  120  receives a signal related to the pressure P measured by the pressure sensor  43   e . Therefore, when the processor  120  controls the valve  43   b , the processor  120  performs feedback control based on the signal from the pressure sensor  43   e  so that the pressure P becomes the predetermined pressure P 1 . Thus, at time t 11 , the pressure P is increased to the predetermined pressure P 1 . 
     When the signal indicating that the pressure P is at the predetermined pressure P 1  is input from the pressure sensor  43   e  to the processor  120 , the processor  120  outputs a signal to the actuator of the valve  43   b  and controls the valve  43   b  to remain opened via the actuator from time t 11  to time t 12 . Thus, the pressure P remains at the predetermined pressure P 1  from time t 11  to time t 12 . For example, time t 12  is 8 minutes. From time t 0  to time t 12 , the target substance in the tank  41  permeates into the filter  51   a  in the tank  41  by the pressure increase, and is filled in the space from the filter  51   a  to the nozzle hole. Further, the processor  120  controls the opening degree of the valve  43   b  by the above-described feedback control. Accordingly, decrease in the pressure P is suppressed. 
     At time t 12 , the processor  120  outputs a signal to the actuator of the valve  43   b  and controls the opening degree of the valve  43   b  via the actuator so that the pressure P is increased at a first pressure-increasing speed from the predetermined pressure P 1  to a first target pressure P 2 . The first predetermined pressure-increasing speed is a speed at which the pressure P at time t 13  becomes the first target pressure P 2 . When the valve  43   b  is opened again, the inert gas is supplied again from the gas supply source  53  into the tank  41  through the pipe  43   a . Thus, the pressure adjuster  43  pressurizes the inside of the tank  41  from time t 12  to time t 13  to the first target pressure P 2  at the first pressure-increasing speed. For example, the first target pressure P 2  is 10 MPa, and time t 13  is 32 minutes. The first pressure-increasing speed is adjusted with the opening degree of the valve  43   b . The opening degree is controlled by the processor  120  based on the signal from the pressure sensor  43   e . Thus, at time t 13 , the pressure P is increased to the first target pressure P 2 . 
     When the signal indicating that the pressure P is the first target pressure P 2  is input from the pressure sensor  43   e  to the processor  120 , the processor  120  controls the opening degree of the valve  43   b  by the feedback control described above. As a result, decrease in the pressure P is suppressed, and the pressure P remains at the first target pressure P 2 . After time t 13  at which the pressure P becomes the first target pressure P 2 , the target supply device  40  maintains the first target pressure P 2 . 
     When the pressure P increasing at the first pressure-increasing speed after time t 12  is equal to or higher than a certain pressure, the target substance is output from the nozzle hole of the nozzle  42  due to pressurization by the pressure. The piezoelectric element  47  is driven from time t 0  and vibration of the piezoelectric element  47  can propagate through the nozzle  42  to the target substance to be output from the nozzle hole of the nozzle  42 . Accordingly, the target substance is divided at a predetermined cycle by the vibration, and output from the nozzle hole of the nozzle  42  as liquid droplets DL. In  FIG.  5   , the time at which the droplet DL is output is defined as time t 14  between time t 12  and time t 13 . Therefore, the droplet DL is not output from time t 0  to time t 14 , time t 14  is the output start time of the droplet DL, and the droplet DL continues to be output after time t 14 . For example, time t 14  is 13 minutes. The output droplet DL travels toward the plasma generation region AR. The trajectory of the droplet DL tends to be along the center axis of the circular nozzle hole. 
     3.3 Problem 
     In the target supply device  40  of the comparative example, the pressure-increasing speed remains the same as the first pressure-increasing speed before and after the outputting of the droplet DL. When the pressure P in the tank  41  is increased at the first pressure-increasing speed from time t 14  to time t 13 , the droplet DL output from the nozzle hole may be in an unstable state. As an unstable state, as shown in  FIG.  6   , the trajectory of the droplet DL deviates from the stable state along the center axis of the nozzle hole as shown by a broken line, and deviates from the stable state as shown by a dotted chain line as the droplet DL travels. Alternatively, as shown in  FIG.  7   , the droplet DL may be sprayed from the nozzle hole and scattered as fine splashes  400 . Although the shape of the splashes  400  is shown as a circle, the shape is not particularly limited. In  FIG.  7   , the trajectory of the droplet DL in the stable state is also shown by a broken line. 
     The unstable state does not always continue to occur after time t 14 , but tends to be gradually resolved over time. Therefore, in the process after the droplet DL is output at time t 14 , the deviation of the trajectory of the droplet DL shown in  FIG.  6    and the scattering of the droplet DL shown in  FIG.  7    are gradually eliminated, and the droplet DL tends to shift from the unstable state to the stable state in which the trajectory of the droplet DL is along the center axis of the nozzle hole. In view of the above, it is presumed that the unstable state occurs during a predetermined time from time t 14 . 
     In the unstable state, as described above, the trajectory of the droplet DL deviates from the center axis of the nozzle hole, or the droplet DL scatters. As a result, the droplet DL may adhere to and contaminate the reflection surface  75   a  of the EUV light concentrating mirror  75  and the window (not shown) of the target sensor  27 . Accordingly, the reflectivity of the reflection surface  75   a  may decrease, or the detection sensitivity of the target sensor  27  may decrease. Such contamination of the structural components at the internal space of the chamber device  10  may cause failure of the chamber device  10 . Here, the longer the time during which the droplet DL is in the unstable state, the more the contamination may spread. 
     Therefore, in the following embodiments, the target supply device  40  capable of suppressing failure of the EUV light generation apparatus  100  due to the unstable state of the droplet DL by shortening the time during which the droplet DL is in the unstable state is exemplified. 
     4. Description of Extreme Ultraviolet Light Generation Apparatus of First Embodiment 
     Next, the configuration of the EUV light generation apparatus  100  of a first 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 
     The configuration of the EUV light generation apparatus  100  of the present embodiment is the same as the configuration of the EUV light generation apparatus  100  of the comparative embodiment, and therefore description thereof is omitted. 
     4.2 Operation 
     Next, operation of the processor  120  for controlling the pressure P in the tank  41  in the present embodiment will be described. 
       FIG.  8    is a diagram showing an example of a control flowchart of the processor  120  according to the present embodiment. As shown in  FIG.  8   , the control flow of the present embodiment includes steps SP 11  to SP 17 . The control flow of the present embodiment is used when the target supply device  40  is installed in the EUV light generation apparatus  100  for the first time, the target substance is filled in the tank  41  for the first time in the EUV light generation apparatus  100  and permeates the filter  51   a , and the nozzle  42  outputs the droplet DL for the first time after the installation of the target supply device  40 . 
       FIG.  9    is a diagram showing the relationship between the pressure P in steps SP 11  to SP 15  and the time at which the pressure increases. A solid line L 2  shown in  FIG.  9    indicates a change of the pressure P in steps SP 11  to SP 15 . In  FIG.  9   , in order to compare the present embodiment with the comparative example, the change of the pressure P indicated by the solid line L 1  in  FIG.  5    is shown by a broken line L 1 . 
     The state at start shown in  FIG.  8    is the same as that at time t 0  in the comparative example. Further, in the state at start of the present embodiment, a signal is input from the target sensor  27  to the processor  120 . 
     (Step SP 11 ) 
     In this step, the processor  120  controls the pressure adjuster  43  so that the change of the pressure P from time t 0  to time t 12  is the same as that in the comparative example. Therefore, the pressure P becomes the predetermined pressure P 1  by being increased from time t 0  to time t 11 , and remains at the predetermined pressure P 1  from time t 11  to time t 12 . After the processor  120  controls the pressure adjuster  43  as described above, the control flow proceeds to step SP 12 . 
     (Step SP 12 ) 
     In this step, at time t 12 , similarly to the comparative example, the processor  120  outputs a signal to the actuator of the valve  43   b  and controls the opening degree of the valve  43   b  via the actuator so that the pressure P is increased at a first pressure-increasing speed from the predetermined pressure P 1  to the first target pressure P 2 . Therefore, the pressure adjuster  43  pressurizes the inside of the tank  41  from time t 12  at the first pressure-increasing speed. In the target supply device  40  of the present embodiment, it is preferable that the first pressure-increasing speed is approximately 0.002 MPa/s or higher and 0.0067 MPa/s or lower, but may be lower than 0.002 MPa/s. The first pressure-increasing speed is stored in the storage device  120   a  in advance, and the processor  120  may read out the first pressure-increasing speed from the storage device  120   a . When the first pressure-increasing speed is 0.002 Mpa/s or higher and 0.0067 MPa/s or lower, generation of bubbles in the target substance in the tank  41  is suppressed, and due to the suppression, generation of particles in the target substance in the tank  41  is suppressed. Further, when the generation of particles is suppressed, clogging of the nozzle hole due to the particles that have passed through the filter  51   a  is suppressed. After the processor  120  controls the pressure adjuster  43  as described above, the control flow proceeds to step SP 13 . 
     (Step SP 13 ) 
     In this step, when a signal indicating detection of outputting of the droplet DL is input from the target sensor  27  to the processor  120 , the processor  120  advances the control flow to step SP 14 . At time t 14 , when the droplet DL is output for the first time after the installation of the target supply device  40 , in the target supply device  40  of the present embodiment, the droplet DL is detected for the first time by the target sensor  27 . The detection region of the target sensor  27  is located directly below the nozzle hole, and the target sensor  27  is a droplet detector that detects the droplet DL by imaging the droplet DL immediately after output according to an instruction from the processor  120 . Therefore, time t 14  can be regarded as the time when the target sensor  27  has detected the droplet DL for the first time after the installation of the target supply device  40 . In  FIG.  5   , the pressure in the tank  41  at time t 14  is set as a pressure P 3 , so that the droplet DL is output for the first time at the pressure P 3  and the droplet DL continues to be output at the pressure P 3  or higher. For example, the pressure P 3  is 3 MPa. 
     Further, in this step, when a signal not indicating the detection of outputting of the droplet DL is input to the processor  120  from the target sensor  27 , the processor  120  advances the control flow to step SP 16 . Since the droplet DL is not output from time t 12  to time t 14 , the processor  120  advances the control flow to step SP 16  in a period from time t 12  to time t 14 . Here, the droplet DL is a liquid droplet, and the liquid droplets are output at intervals. The interval is approximately 0.5 mm or more and 1 mm or less. When the droplet DL enters the detection region of the target sensor  27  which is sufficiently larger than the interval, the target sensor  27  detects at least one droplet DL. Therefore, the state in which the target sensor  27  does not detect the droplet DL indicates the state before the droplet DL is output from the nozzle hole or before the output droplet DL enters the detection region of the target sensor  27 . 
     (Step SP 14 ) 
     In this step, the processor  120  outputs a signal to the actuator of the valve  43   b  and controls the opening degree of the valve  43   b  via the actuator so that the pressure P is increased at a second pressure-increasing speed after time t 14  to the first target pressure P 2  from the pressure P 3  at time t 14 . Therefore, the pressure adjuster  43  pressurizes the inside of the tank  41  from time t 14  at the second pressure-increasing speed. The second pressure-increasing speed being higher than the first pressure-increasing speed is a speed at which the pressure P becomes the first target pressure P 2  at time t 15  earlier than time t 13 . It is preferable that the second pressure-increasing speed is approximately 0.2 MPa/s or higher and 1 Mpa/s or lower, but may exceed 1 MPa/s. The second pressure-increasing speed is stored in the storage device  120   a  in advance, and the processor  120  may read out the second pressure-increasing speed from the storage device  120   a . For example, time t 15  is 16 minutes. The processor  120  of the present embodiment increases the pressure-increasing speed of the pressure P to be higher than that before the detection of the outputting of the droplet DL after a predetermined time elapses since the signal indicating the detection of the outputting of the droplet DL is input from the target sensor  27  to the processor  120   g . The predetermined time is approximately 1 ms or more and 1 s or less. Since the predetermined time is much shorter than the time during which the pressure P is increased at the first pressure-increasing speed and the second pressure-increasing speed, the timing of switching the pressure-increasing speed overlaps with time t 14  in  FIG.  9   . As described above, since the predetermined time is very short and the pressure increase in the predetermined time is small, the pressure at which the pressure-increasing speed is switched in the present embodiment can be generally regarded as the pressure P 3 . As described above, in the target supply device  40  of the present embodiment, during the period in which the pressure P is increased to the first target pressure P 2  from the pressure P 3  at which the outputting of the droplet DL is detected by the target sensor  27  for the first time after the installation of the target supply device  40 , the pressure-increasing speed of the pressure P becomes higher after the detection of the outputting of the droplet DL than before the detection of the outputting of the droplet DL, and the pressure P increases faster after the detection than before the detection. 
     The lower limit of the pressure P 3  at time t 14  at which the droplet DL is output for the first time depends on the conductance from the inlet of the filter  51   a  to the nozzle hole. The lower limit is generally 2.83 MPa, but varies depending on the target supply device  40 . Therefore, it is preferable to adopt, as an index of the timing of switching the pressure-increasing speed, the detection of the outputting of the droplet DL as described above rather than the use of the lower limit value. 
     Next, the reason why the first pressure-increasing speed is adopted before the second pressure-increasing speed will be described. 
     A space is arranged between the filter  51   a  and the nozzle hole. If the second pressure-increasing speed is adopted without adopting the first pressure-increasing speed before the detection of the outputting of the droplet DL, the pressure P is increased fast compared to the case where the first pressure-increasing speed is adopted, and the target substance may vigorously enter the space. Due to this entering, part of the atmospheric air in the space is discharged from the nozzle hole, but the remaining part of the atmospheric air may enter the target substance as bubbles. As a result, there is a concern that the droplet DL being output may become unstable. However, when the first pressure-increasing speed is adopted, the target substance enters the space more slowly than when the second pressure-increasing speed is adopted, permeates into the filter  51   a  in the tank  41 , and is filled in the space from the filter  51   a  to the nozzle hole. As a result, the atmospheric air in the space is discharged from the nozzle hole, and the entering of the bubbles into the target substance is suppressed. Therefore, occurrence of the unstable state of the droplet DL is suppressed. 
     Further, if the second pressure-increasing speed is adopted without adopting the first pressure-increasing speed before the detection of the outputting of the droplet DL, the pressure P is increased fast compared to the case where the first pressure-increasing speed is adopted. The pressure increase causes a pressure difference between the upstream side and the downstream side of the filter  51   a , and an impact may be suddenly applied to the filter  51   a  due to the pressure difference. However, when the first pressure-increasing speed is adopted, the occurrence of the pressure difference is suppressed, and the sudden impact to the filter  51   a  is suppressed. 
     After the processor  120  controls the pressure adjuster  43  as described above, the control flow proceeds to step SP 15 . 
     (Step SP 15 ) 
     In this step, the processor  120  returns the control flow to step SP 15  when the pressure P indicated by the signal input from the pressure sensor  43   e  is lower than the first target pressure P 2 . Thus, the pressure adjuster  43  pressurizes the inside of the tank  41  to the first target pressure P 2  continuously at the second pressure-increasing speed. On the other hand, when the pressure P becomes the first target pressure P 2 , the processor  120  controls the opening degree of the valve  43   b  by the above-described feedback control. As a result, decrease in the pressure P is suppressed, and the pressure P remains at the first target pressure P 2 . After time t 15  at which the pressure P becomes the first target pressure P 2 , the target supply device  40  maintains the first target pressure P 2 . After controlling the valve  43   b  as described above, the processor  120  ends the control flow. 
     (Step SP 16 ) 
     In this step, the processor  120  returns the control flow to step SP 13  when the pressure P indicated by the signal input from the pressure sensor  43   e  is lower than the second target pressure P 4 . The second target pressure P 4  is higher than the pressure P 3  that is assumed when the outputting of the droplet DL is detected by the target sensor  27  for the first time after the installation of the target supply device  40 , and is equal to or lower than the first target pressure P 2 . The second target pressure P 4  is, for example, 5 MPa. The second target pressure P 4  is stored in the storage device  120   a , and the processor  120  reads out the second target pressure P 4 . When the droplet DL is not output even when the pressure P becomes equal to or higher than the second target pressure P 4 , it is assumed that, for example, the nozzle hole is clogged with particles. 
     In this step, the processor  120  advances the control flow to step SP 17  when the pressure P indicated by the signal input from the pressure sensor  43   e  is the second target pressure P 4  or higher. 
     (Step SP 17 ) 
     In this step, the processor  120  outputs a signal to the actuator of the valve  43   b  and closes the valve  43   b  via the actuator. The processor  120  also outputs a signal to the actuator of the valve  43   d  and controls the opening degree of the valve  43   d  via the actuator so that the pressure P is decreased. When the valve  43   b  is closed, supply of the inert gas from the gas supply source  53  through the pipe  43   a  into the tank  41  is stopped. Further, when the valve  43   d  is opened, the inert gas in the tank  41  is exhausted through the pipes  43   a ,  43   c . Thus, the pressure adjuster  43  depressurizes the inside of the tank  41 . For example, if the pressure P becomes equal to or higher than the second target pressure P 4  in a state in which the droplet DL is not output due to clogging of the nozzle hole with particles or the like, there is a concern that the target supply device  40  has operation failure. In this case, there is a possibility that the clogging of the nozzle hole is solved only by an overhaul. As another possibility, the droplet DL is pushed out from the nozzle hole together with particles by the pressure P equal to or higher than the second target pressure P 4  and is output together with the particles, but there is a possibility that the droplet DL is scattered. When the droplet DL is scattered, there is a concern that structural components at the internal space of the chamber device  10  are contaminated. In either case, it is not preferable to set the pressure P to be equal to or higher than the second target pressure P 4  in the state in which the droplet DL is not output. Therefore, in this step, the processor  120  controls the opening degree of the valve  43   d  via the actuator of the valve  43   d  so that the pressure P is decreased to be lower than the pressure P 3  that is assumed when the outputting of the droplet DL is detected by the target sensor  27  for the first time after the installation of the target supply device  40 . The pressure P 3  is stored in the storage device  120   a , and the processor  120  reads out the pressure P 3 . Thus, the pressure-increasing operation is stopped. After the processor  120  controls the pressure adjuster  43  as described above, the control flow ends. 
     4.3 Effect 
     The target supply device  40  of the present embodiment includes the tank  41  for storing the target substance, the pressure adjuster  43  for adjusting the pressure P inside the tank  41 , the filter  51   a  for filtering the target substance in the tank  41 , and the nozzle  42  for outputting the droplet DL of the target substance having passed through the filter  51   a . The target supply device  40  includes the target sensor  27  that detects the outputting of the droplet DL from the nozzle  42 , and the processor  120  that controls the pressure adjuster  43  so that the pressure-increasing speed of the pressure P is higher after the detection of the outputting of the droplet DL than before the detection of the outputting of the droplet DL during the period in which the pressure P is increased to the first target pressure P 2  from the pressure P 3  at which the outputting of the droplet DL is detected by the target sensor  27  for the first time after the installation of the target supply device  40 . 
     With the above-described configuration, the pressure P is increased faster after the detection of the outputting of the droplet DL than before the detection of the outputting of the droplet DL, as compared with the case where the pressure-increasing speed is the same before and after the detection of the outputting of the droplet DL or the case where the pressure-increasing speed is lower after the detection of the outputting of the droplet DL than before the detection of the outputting of the droplet DL. When the pressure P is increased fast, the droplet DL is vigorously output from the nozzle hole, the trajectory of the droplet DL is along the center axis of the nozzle hole, and the time during which the droplet DL is in the unstable state may be shortened. When the unstable time is short, contamination of the structural components at the internal space of the chamber device  10  can be suppressed, and occurrence of the failure of the chamber device  10  can be suppressed. 
     It is considered that the unstable state is caused by the wet state or the like of the target substance at the edge of the nozzle hole, but it is difficult to predict the occurrence of the unstable state. Therefore, as described above, the target sensor  27  detects the outputting of the droplet DL from the nozzle  42 , and the pressure-increasing speed is switched after the detection, so that the prediction may be unnecessary. 
     Even if the first pressure-increasing speed is adopted by the time immediately before the detection of the outputting of the droplet DL, the atmospheric air in the space from the filter  51   a  to the nozzle hole may enter the target substance as bubbles, and the trajectory of the droplet DL to be output may be disturbed by the bubbles. In particular, when the droplet DL is output by the pressure P which is increased at the second pressure-increasing speed in a state where the bubbles enter the target substance, there is a concern that the trajectory of the droplet DL to be output is greatly disturbed and the contamination of the structural components at the internal space of the chamber device  10  spreads as compared with a case where the bubbles do not enter the target substance. However, in the target supply device  40  of the present embodiment, the processor  120  sets the pressure-increasing speed of the pressure P to be higher than that before the detection of the outputting of the droplet DL after the elapse of the predetermined time from the detection of the outputting of the droplet DL by the target sensor  27  for the first time after the installation of the target supply device  40 . Owing to that the predetermined period is secured, the atmospheric air in the space is discharged from the nozzle hole, and the entering of the bubbles into the target substance is suppressed. When the entering is suppressed, even in a case where the droplet DL is output by the pressure P which is increased at the second pressure-increasing speed, the disturbance of the trajectory can be suppressed, and the spread of the contamination of the structural components at the internal space of the chamber device  10  can be suppressed. 
     Further, in the target supply device  40  of the present embodiment, the processor  120  controls the pressure adjuster  43  so that the pressure P is decreased in a case where the outputting of the droplet DL is not detected by the target sensor  27  and the pressure P is equal to or higher than the second target pressure P 4 . When the pressure P becomes equal to or higher than the second target pressure P 4  while the droplet DL is not output, there is a concern that the target supply device  40  has operation failure. However, in the target supply device  40  of the present embodiment, with the above-described configuration, the pressure-increasing operation is stopped, and the occurrence of the operation failure of the chamber device  10  can be suppressed. 
     Further, in the target supply device  40  of the present embodiment, the processor  120  controls the pressure adjuster  43  so that the pressure P is decreased to be lower than the pressure P 3  that is assumed when the outputting of the droplet DL is detected by the target sensor  27  for the first time after the installation of the target supply device  40 . With this configuration, it is possible to further suppress the occurrence of the operation failure of the chamber device  10 . 
     In the target supply device  40  of the present embodiment, as described above, the processor  120  switches the pressure-increase speed after a predetermined time elapses since the signal indicating the detection of the outputting of the droplet DL by the target sensor  27  for the first time after the installation of the target supply device  40  is input from the target sensor  27  to the processor  120 , but it is not limited thereto. 
     The switching timing will be described below. 
     In the target supply device  40  of the present embodiment, the processor  120  may increase the pressure-increasing speed of the pressure P to be higher than that before the detection of the outputting of the droplet DL until the pressure P is increased from the pressure P 3  to approximately 90% of the first target pressure P 2  at the latest. With this configuration as well, the time during which the droplet DL is in the unstable state can be shorter than in the case where the pressure P is increased to the first target pressure P 2  at the first pressure-increasing speed. Here, the pressure at which the pressure-increasing speed is switched is set to 90% of the first target pressure P 2 , but the numerical value is not particularly limited. 
     Alternatively, in the target supply device  40  of the present embodiment, the processor  120  may increase the pressure-increasing speed of the pressure P to be higher than that before the detection of the outputting of the droplet DL until the pressure P is increased from the pressure P 3  to approximately 130% of the pressure P 3  at the latest. With this configuration as well, the time during which the droplet DL is in the unstable state can be shorter than in the case where the pressure P is increased to the first target pressure P 2  at the first pressure-increasing speed. The processor  120  may increase the pressure-increasing speed of the pressure P higher than that before the detection of the outputting of the droplet DL when the pressure P is increased to a pressure equal to or higher than approximately 130% of the pressure P 3 . In the above, the pressure at which the pressure-increasing speed is switched is set to 130% of the pressure P 3 , but the numerical value is not particularly limited. 
     The pressure at which the pressure increasing speed is switched may be 100% or higher of the pressure P 3  and lower than 100% of the first target pressure P 2 . Therefore, the processor  120  may increase the pressure-increasing speed of the pressure P after detecting the outputting of the droplet DL than before detecting the outputting of the droplet DL, before the pressure P is increased to the first target pressure P 2  from the pressure P 3  at which the outputting of the droplet DL is detected by the target sensor  27  for the first time after the installation of the target supply device  40 . 
     Further, in the target supply device  40  of the present embodiment, the speed at which the pressure in the tank  41  is increased from time t 12  to time t 14  is described as the first pressure-increasing speed, but it is not limited thereto. The first pressure-increasing speed may be the speed immediately before the outputting of the droplet DL is detected by the target sensor  27  for the first time after the installation of target supply device  40 . Therefore, if the pressure P is gradually increased from time t 0  to time t 14 , the pressure-increasing speed of the pressure P from time t 0  to time t 14  is the first pressure-increasing speed. Further, after time t 15  at which the pressure P becomes the first target pressure P 2 , the target supply device  40  does not necessarily need to maintain the first target pressure P 2 . 
     Further, in the target supply device  40  of the present embodiment, the above-described control flow may be used when the nozzle  42  outputs the droplet DL for the first time after the installation of the target supply device  40  in order to check the output state of the target supply device  40 , or may be used when the nozzle  42  outputs the droplet DL in order to generate the EUV light  101 . 
     Further, in the target supply device  40  of the present embodiment, it is preferable that the imaging unit of the target sensor  27  images the droplet DL toward the travel direction of the droplet DL output from the nozzle hole or images the droplet DL toward the direction substantially perpendicular to the trajectory of the droplet DL rather than imaging the droplet DL toward the nozzle hole. The target sensor  27  including the imaging unit may further include a magnifying lens system, a laser curtain, and the like. The imaging unit may be configured to include an image sensor such as a CCD or a (CMOS), but may be configured to include a light receiving element such as a line sensor. The target sensor  27  as a droplet detector may include a non-contact proximity switch instead of the light receiving optical system and the imaging unit. Here, although the target sensor  27  is used as the droplet detector in the above description, a droplet detector may be arranged separately from the target sensor  27 . 
     5. Description of Extreme Ultraviolet Light Generation Apparatus of Second Embodiment 
     Next, the configuration of the EUV light generation apparatus  100  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. 
     5.1 Configuration 
     In the EUV light generation apparatus  100  of the present embodiment, the configuration of the storage device  120   a  differs from the configuration of the storage device  120   a  of the first embodiment. 
     The storage device  120   a  stores output information of the target supply device  40 . The output information includes an output count after the installation of the target supply device  40 . One output indicates that the pressure P in the tank  41  has reached the first target pressure P 2  as described in step SP 15  after the pressure increase at time t 0 . When the outputting of the droplet DL is performed once, the storage device  120   a  increments the current output count by one. If the output count is 0, the droplet DL is to be output for the first time after the installation of the target supply device  40 , and if the output count is 1 or more, the droplet DL is to be re-output. The re-outputting of the droplet DL does not indicate a condition in which the target substance stored after the tank  41  is emptied is output. In the re-outputting of the droplet DL, before the tank  41  becomes empty, the pressure P is decreased to be lower than the pressure P 3  after reaching the first target pressure P 2  as described above, and is increased from the pressure lower than the pressure P 3  to the pressure P 3  or higher after the pressure decrease. Here, in the re-outputting of the droplet DL, before the tank  41  becomes empty, the pressure P may be increased to the pressure P 3  or higher, then decreased to lower than the pressure P 3 , and then increased from the pressure lower than the pressure P 3  to the pressure P 3  or higher. The output information may include information indicating whether or not the droplet DL has been output in the past, instead of the output count. Further, in the output information, the installation date and time of the target supply device  40  to the chamber device  10  and the output start date and time by the pressure adjuster  43  into the tank  41  may be further stored. 
     5.2 Operation 
     Next, operation of the processor  120  for controlling the pressure P in the tank  41  in the present embodiment will be described. 
       FIG.  10    is a diagram showing an example of a control flowchart of the processor  120  according to the present embodiment. As shown in  FIG.  10   , the control flow of the present embodiment includes steps SP 31  to SP 34 . As will be described later, the control flow of the present embodiment further includes steps SP 11  to SP 17  described in the first embodiment.  FIG.  11    is a diagram showing the relationship between the pressure P in steps SP 31  to SP 34  and the time at which the pressure increases. A solid line L 3  shown in  FIG.  11    indicates a change of the pressure P in steps SP 31  to SP 34 . In  FIG.  11   , in order to compare the present embodiment with the first embodiment, the change of the pressure P indicated by the solid line L 2  in  FIG.  9    is shown by a broken line L 2 . 
     The start state shown in  FIG.  10    is the start state in the first embodiment and corresponds to time t 0  immediately after the start of pressurization. 
     (Step SP 31 ) 
     In this step, the processor  120  reads out the output information from the storage device  120   a . When the output count is 0 in the output information, the target supply device  40  is to output the droplet DL for the first time after the installation of the target supply device  40 , and the processor  120  advances the control flow to step SP 11  described in the first embodiment. The control flow after step SP 11  includes steps SP 12  to SP 17  described in the first embodiment and shown in  FIG.  8   , so that the illustration is omitted in  FIG.  10    and description thereof is also omitted in the following. When the output count is 1 or more, the target supply device  40  is to re-output the droplet DL, and the processor  120  advances the control flow to step SP 32 . 
     (Step SP 32 ) 
     In this step, the processor  120  outputs a signal to the actuator of the valve  43   b  and controls the opening degree of the valve  43   b  via the actuator so that the pressure P is increased at the second pressure-increasing speed to the first target pressure P 2  after time t 0 . Thus, when the target supply device  40  re-outputs the droplet DL, unlike when the target supply device  40  outputs the droplet DL for the first time after the installation of the target supply device  40 , the pressure adjuster  43  pressurizes the inside of the tank  41  at the second pressure-increasing speed from time t 0 , regardless of whether or not the outputting of the droplet DL is detected by the target sensor  27 . Thus, the processor  120  sets the pressure-increasing speed at the second pressure-increasing speed when the pressure P is increased from the pressure lower than the pressure P 3 . 
     In this step, since the target supply device  40  is in the state of re-outputting the droplet DL, the target substance permeates the filter  51   a  and the space between the filter  51   a  and the nozzle hole is already filled with the target substance. Therefore, even when the pressure P is increased at the second pressure-increasing speed after time t 0 , generation of the bubbles in the target substance in the tank  41  is suppressed, and due to the suppression, generation of particles in the target substance in the tank  41  is suppressed. In addition, when the generation of particles is suppressed, clogging of the nozzle hole due to the particles is suppressed. Further, since the space from the filter  51   a  to the nozzle hole is filled with the target substance as described above, it is possible to prevent the bubbles, which are part of the atmospheric air in the space, from entering the target substance and to prevent the trajectory of the droplet DL to be output from being disturbed. Further, since the space is filled with the target substance as described above, even when the second pressure-increasing speed is adopted, the occurrence of the pressure difference between the upstream side and the downstream side of the filter  51   a  is suppressed and a sudden impact on the filter  51   a  is suppressed. 
     After the processor  120  controls the pressure adjuster  43  as described above, the control flow proceeds to step SP 33 . 
     (Step SP 33 ) 
     In this step, the droplet DL is output by the pressure increase of the pressure P, and when a signal indicating the detection of outputting of the droplet DL is input from the target sensor  27  to the processor  120 , the processor  120  advances the control flow to step SP 34 . In  FIG.  11   , the time when the droplet DL is detected is defined as time t 21 . Since the second pressure-increasing speed is adopted at time t 21 , time t 21  is earlier than time t 14  and is, for example, 0.4 minutes. 
     (Step SP 34 ) 
     In this step, the processor  120  returns the control flow to step SP 34  when the pressure P indicated by the signal input from the pressure sensor  43   e  is lower than the first target pressure P 2 . Thus, the pressure adjuster  43  pressurizes the inside of the tank  41  to the first target pressure P 2  continuously at the second pressure-increasing speed. On the other hand, when the pressure P becomes the first target pressure P 2 , the processor  120  controls the opening degree of the valve  43   b  by the above-described feedback control. As a result, decrease in the pressure P is suppressed, and the pressure P remains at the first target pressure P 2 . In  FIG.  11   , the time when the pressure P reaches the first target pressure P 2  at the second pressure-increasing speed is defined as time t 22 . Time t 22  is earlier than time t 15  and is, for example, 0.8 minutes. Therefore, the pressure P remains at the first target pressure P 2  after time t 22 . The processor  120  maintains the first target pressure P 2  by controlling the valve  43   b  as described above. After the processor  120  controls the pressure adjuster  43  as described above, the control flow ends. 
     Further, in step SP 33 , when a signal not indicating the detection of the outputting of the droplet DL is input to the processor  120  from the target sensor  27 , the processor  120  advances the control flow to step SP 16 . The control flow after step SP 16  includes step SP 17  described in the first embodiment and described above, so that description thereof is omitted. 
     5.3 Effect 
     In the target supply device  40  of the present embodiment, in a state where the detection of the outputting of the droplet DL by the target sensor  27  is not the detection for the first time after the installation of the target supply device  40 , the processor  120  sets the pressure-increasing speed to the second pressure-increasing speed when the pressure P in the tank  41  is to be increased from the pressure lower than the pressure at which the outputting of the droplet DL is detected by the target sensor  27  for the first time after the installation of the target supply device  40 . 
     As described above, it is considered that the unstable state of the droplet DL is caused by a wet state or the like at the edge of the nozzle hole. When the droplet DL is to be re-output, the edge of the nozzle hole is formed in a more wet state than that at the previous output. Therefore, in the case where the droplet DL is re-output, the unstable state of the droplet DL can be suppressed as compared with the case where the droplet DL is output for the first time after the installation of the target supply device  40 . Further, if the droplet DL is output once, the bubbles in the space from the filter  51   a  to the nozzle hole are discharged from the nozzle hole, and the entering of the bubbles into the target substance during the re-outputting of the droplet DL is suppressed. In this state, the pressure-increasing speed of the pressure P becomes the second pressure-increasing speed immediately after the start of pressurization, and thus the pressure P can be increased to the first target pressure P 2  regardless of the detection of the outputting of the droplet DL by the target sensor  27 . Further, in the target supply device  40  according to the present embodiment, the pressure P can be increased to the first target pressure P 2  in a shorter time than in the case where the pressure P is increased at the first pressure-increasing speed and second pressure-increasing speed. Therefore, in the target supply device  40  of the present embodiment, the droplet DL can be immediately supplied to the plasma generation region AR when the droplet DL is re-output. 
     When the output count is one or more, the processor  120  sets the pressure-increasing speed of the pressure P to the second pressure-increasing speed immediately after the start of pressurization by the pressure adjuster  43 , but it is not limited thereto. The processor  120  may set the pressure-increasing speed of the pressure P to the second pressure-increasing speed before the signal indicating the detection of the outputting of the droplet DL is input from the target sensor  27  to the processor  120 . 
     Here, although the storage device  120   a  of the processor  120  is used as the storage device for storing the output information, the storage device may be arranged outside the processor  120  as a device different from the storage device  120   a . In this case, the storage device is electrically connected to the processor  120 . The storage device is, for example, a non-transitory recording medium, and is preferably a semiconductor recording medium such as a random access memory (RAM) or a read only memory (ROM). However, the storage device may include a recording medium of an arbitrary format such as an optical recording medium or a magnetic recording medium. The non-transitory recording medium includes all computer-readable recording media except for transitory propagation signals, and does not exclude volatile recording media. 
     6. Description of Extreme Ultraviolet Light Generation Apparatus of Third Embodiment 
     Next, the configuration of the EUV light generation apparatus  100  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. 
     6.1 Configuration 
       FIG.  12    is a schematic view showing a schematic configuration example of the entire EUV light generation apparatus  100  of the third embodiment. In the EUV light generation apparatus  100  of the present embodiment, the configuration of the droplet detector differs from that of the droplet detector of the first embodiment. 
     The droplet detector of the present embodiment is not the target sensor  27  but the gas detector  55  that detects the inert gas discharged from the nozzle hole to the internal space of the chamber device  10 . The inert gas stays in the space between the filter  51   a  and the nozzle hole before the target substance is filled in the space, is pushed out from the nozzle hole to the internal space of the chamber device  10  by the outputting of the droplet DL, and is discharged together with the droplet DL. The gas detector  55  is arranged at the internal space of the chamber device  10 , detects the inert gas at the internal space of the chamber device  10 , and detects the outputting of the droplet DL by detecting the inert gas. The gas detector  55  is electrically connected to the processor  120 . The processor  120  detects increase of the inert gas at the internal space of the chamber device  10  based on a signal from the gas detector  55 . 
     The gas detector  55  is, for example, a gas analyzer or a vacuum gauge. The vacuum gauge is, for example, a Pirani vacuum gauge or an ion gauge. 
     6.2 Operation 
     Next, operation of the processor  120  for controlling the pressure P in the tank  41  in the present embodiment will be described. 
       FIG.  13    is a diagram showing an example of a control flowchart of the processor  120  according to the present embodiment. The control flow of the present embodiment differs from the control flow of the first embodiment in that step SP 41  is included instead of step SP 13  in the control flow of the first embodiment. Further, in the start state of the present embodiment, unlike the first embodiment, the signal is input from the gas detector  55  to the processor  120 . 
     (Step SP 41 ) 
     In this step, when the signal is input from the gas detector  55  to the processor  120  and the amount of the inert gas at the internal space of the chamber device  10  is increased, the processor  120  advances the control flow to step SP 14 . 
     Further, in this step, when the inert gas at the internal space of the chamber device  10  is not increased, the processor  120  advances the control flow to step SP 16 . 
     6.3 Effect 
     In the target supply device  40  of the present embodiment, the gas detector  55  being the droplet detector detects the inert gas at the internal space of the chamber device  10 . When the inert gas is discharged from the tank  41  into the internal space of the chamber device  10  by the outputting of the droplet DL, the amount of the inert gas at the internal space of the chamber device  10  increases. Therefore, even when the gas detector  55  is used, the outputting of the droplet DL can be detected. When the internal space of the chamber device  10  is maintained at a low pressure, even if a small amount of the inert gas is discharged from the nozzle  42 , the inert gas tends to instantaneously diffuse to the internal space of the chamber device  10 . In this case, the gas detector  55  may be arranged anywhere at the internal space of the chamber device  10  as long as it can detect the inert gas. Therefore, the degree of freedom of arrangement can be increased as compared with the case where the arrangement position of the droplet detector is determined to one position. 
     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. Further, it would be also obvious to those skilled in the art that embodiments of the present disclosure would be appropriately combined. 
     The terms used throughout the present specification and the appended claims should be interpreted as non-limiting terms unless clearly described. 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 any thereof and any other than A, B, and C.