Patent Publication Number: US-2022232690-A1

Title: Extreme ultraviolet light generation system and electronic device manufacturing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of Japanese Patent Application No. 2021-007564, filed on Jan. 20, 2021, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an extreme ultraviolet light generation system 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, fine processing at 70 to 45 nm and further at 32 nm or less will be required. Therefore, in order to meet the demand for fine processing of, for example, 32 nm or less, the development of an exposure apparatus that combines an extreme ultraviolet (EUV) light generation apparatus that generates EUV light having a wavelength of about 13 nm and reduced projection reflection optics is expected. 
     As an EUV light generation apparatus, three types of apparatuses have been proposed: a laser produced plasma (LPP) type apparatus using plasma generated by irradiating a target substance with pulse laser light, a discharge produced plasma (DPP) type apparatus using plasma generated by discharge, and a synchrotron radiation (SR) type apparatus using synchrotron radiation. 
     LIST OF DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: US Patent Application Publication No. 2013/099140 
         Patent Document 2: US Patent Application Publication No. 2014/353528 
       
    
     SUMMARY 
     An extreme ultraviolet light generation system according to an aspect of the present disclosure includes a target supply unit configured to supply a target substance to a first predetermined region, a laser system configured to output pulse laser light to be radiated to the target substance in the first predetermined region, a first sensor configured to detect an arrival timing at which the target substance has reached a second predetermined region between the target supply unit and the first predetermined region, an optical adjuster arranged on an optical path of the pulse laser light between the laser system and the first predetermined region, and a processor configured to control transmittance of the pulse laser light through the optical adjuster based on the arrival timing. 
     An electronic device manufacturing method according to an aspect of the present disclosure includes generating extreme ultraviolet laser light in an extreme ultraviolet light generation system, emitting the extreme ultraviolet light 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 system includes a target supply unit configured to supply a target substance to a first predetermined region, a laser system configured to output pulse laser light to be radiated to the target substance in the first predetermined region, a first sensor configured to detect an arrival timing at which the target substance has reached a second predetermined region between the target supply unit and the first predetermined region, an optical adjuster arranged on an optical path of the pulse laser light between the laser system and the first predetermined region, and a processor configured to control transmittance of the pulse laser light through the optical adjuster based on the arrival timing. 
     An electronic device manufacturing method according to an aspect of the present disclosure includes inspecting a defect of a mask by irradiating the mask with extreme ultraviolet light generated in an extreme ultraviolet light generation system, 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 system includes a target supply unit configured to supply a target substance to a first predetermined region, a laser system configured to output pulse laser light to be radiated to the target substance in the first predetermined region, a first sensor configured to detect an arrival timing at which the target substance has reached a second predetermined region between the target supply unit and the first predetermined region, an optical adjuster arranged on an optical path of the pulse laser light between the laser system and the first predetermined region, and a processor configured to control transmittance of the pulse laser light through the optical adjuster based on the arrival timing. 
    
    
     
       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  schematically shows an exemplary configuration of an LPP EUV light generation system. 
         FIG. 2  schematically shows the configuration of an EUV light generation system according to a comparative example. 
         FIG. 3  is a timing chart of laser control in the comparative example. 
         FIG. 4  schematically shows the configuration of an EUV light generation system according to a first embodiment. 
         FIG. 5  is a timing chart of laser control in the first embodiment. 
         FIG. 6  is a graph showing the relationship between a laser oscillation interval ΔTn of a laser system and pulse energy Emax(ΔTn) of pulse laser light when transmittance of pulse laser light through an optical modulator is controlled to the maximum value. 
         FIG. 7  is a graph showing the relationship between an application voltage Vn of the optical modulator and pulse energy E(Vn) of the pulse laser light. 
         FIG. 8  is a graph showing the relationship between the laser oscillation interval ΔTn and the application voltage Vn set based thereon. 
         FIG. 9  is a graph showing the relationship between the laser oscillation interval ΔTn of the laser system and the pulse energy Emax(ΔTn) of the pulse laser light when the transmittance of the pulse laser light through the optical modulator is controlled to the maximum value. 
         FIG. 10  is a graph showing the relationship between the application voltage Vn of the optical modulator and the pulse energy E(Vn) of the pulse laser light. 
         FIG. 11  schematically shows the configuration of an EUV light generation system according to a second embodiment. 
         FIG. 12  schematically shows the configuration of an exposure apparatus connected to the EUV light generation system. 
         FIG. 13  schematically shows the configuration of an inspection apparatus connected to the EUV light generation system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;Contents&gt; 
     1. Overall description of EUV light generation system  11 
         1.1 Configuration   1.2 Operation
 
2. Comparative example
       

     2.1 Configuration 
     2.2 Operation 
     2.3 Problems of comparative example 
     3. EUV light generation system  11   b  which controls transmittance of pulse laser light  31  through optical modulator OM based on target detection signal 
     3.1 Configuration and operation 
     3.2 Example of function
         3.2.1 First example   3.2.2 Second example       

     3.3 Effect 
     4. EUV light generation system  11   c  including prepulse laser device  3 P 
     4.1 Configuration 
     4.2 Operation 
     4.3 Effect 
     5. Others 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The embodiments described below show some examples of the present disclosure and do not limit the contents of the present disclosure. Also, all configurations and operation described in the embodiments are not necessarily essential as configurations and operation of the present disclosure. Here, the same components are denoted by the same reference numerals, and duplicate description thereof is omitted. 
     1. Overall Description of EUV Light Generation System  11   
     1.1 Configuration 
       FIG. 1  schematically shows an exemplary configuration of an LPP EUV light generation system  11 . An EUV light generation apparatus  1  is used together with a laser system  3 . In the present disclosure, a system including the EUV light generation apparatus  1  and the laser system  3  is referred to as the EUV light generation system  11 . The EUV light generation apparatus  1  includes a chamber  2  and a target supply unit  26 . The chamber  2  is a sealable container. The target supply unit  26  supplies a target  27  containing a target substance into the chamber  2 . The material of the target substance may include tin, terbium, gadolinium, lithium, xenon, or a combination of any two or more thereof. 
     A through hole is formed in a wall of the chamber  2 . The through hole is blocked by a window  21  through which pulse laser light  32  emitted from the laser system  3  passes. An EUV light concentrating mirror  23  having a spheroidal reflection surface is arranged in the chamber  2 . The EUV light concentrating mirror  23  has first and second focal points. A multilayer reflection film in which molybdenum and silicon are alternately stacked is formed on a surface of the EUV light concentrating mirror  23 . The EUV light concentrating mirror  23  is arranged such that the first focal point is located in a plasma generation region  25  and the second focal point is located at an intermediate focal point  292 . The plasma generation region  25  corresponds to the first predetermined region in the present disclosure. A through hole  24  is formed at the center of the EUV light concentrating mirror  23 , and pulse laser light  33  passes through the through hole  24 . 
     The EUV light generation apparatus  1  includes a processor  5 , a target sensor  4 , and the like. The processor  5  is a processing device including a memory  501  in which a control program is stored, and a central processing unit (CPU)  502  which executes the control program. The processor  5  is specifically configured or programmed to perform various processes included in the present disclosure. The target sensor  4  detects at least one of the presence, trajectory, position, and velocity of the target  27 . The target sensor  4  may have an imaging function. 
     Further, the EUV light generation apparatus  1  includes a connection portion  29  providing communication between the internal space of the chamber  2  and the internal space of an EUV light utilization apparatus  6 . An example of the EUV light utilization apparatus  6  will be described later with reference to  FIGS. 12 and 13 . A wall  291  in which an aperture is formed is arranged in the connection portion  29 . The wall  291  is arranged such that the aperture is located at the second focal point of the EUV light concentrating mirror  23 . 
     Furthermore, the EUV light generation apparatus  1  includes a laser light transmission device  34 , a laser light concentrating mirror  22 , a target collection unit  28  for collecting the target  27 , and the like. The laser light transmission device  34  includes an optical element for defining a transmission state of laser light, and an actuator for adjusting the position, posture, and the like of the optical element. 
     1.2 Operation 
     Operation of the EUV light generation system  11  will be described with reference to  FIG. 1 . Pulse laser light  31  emitted from the laser system  3  enters, via the laser light transmission device  34 , the chamber  2  through the window  21  as the pulse laser light  32 . The pulse laser light  32  travels along a laser light path in the chamber  2 , is reflected by the laser light concentrating mirror  22 , and is radiated to the target  27  as the pulse laser light  33 . 
     The target supply unit  26  outputs the target  27  toward the plasma generation region  25  in the chamber  2 . The target  27  is irradiated with the pulse laser light  33 . The target  27  irradiated with the pulse laser light  33  is turned into plasma, and radiation light  251  is radiated from the plasma. EUV light contained in the radiation light  251  is reflected by the EUV light concentrating mirror  23  with higher reflectance than light in other wavelength ranges. Reflection light  252  including the EUV light reflected by the EUV light concentrating mirror  23  is concentrated at the intermediate focal point  292  and output to the EUV light utilization apparatus  6 . Here, one target  27  may be irradiated with a plurality of pulses included in the pulse laser light  33 . 
     The processor  5  controls the entire EUV light generation system  11 . The processor  5  processes a detection result of the target sensor  4 . Based on the detection result of the target sensor  4 , the processor  5  controls the timing at which the target  27  is output, the output direction of the target  27 , and the like. Furthermore, the processor  5  controls the oscillation timing of the laser system  3 , the travel direction of the pulse laser light  32 , the concentrating position of the pulse laser light  33 , and the like. The above-described various kinds of control are merely examples, and other control may be added as necessary. 
     2. Comparative Example 
     2.1 Configuration 
       FIG. 2  schematically shows the configuration of an EUV light generation system  11   a  according to a comparative example. 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. 
     In the EUV light generation system  11   a  according to the comparative example, the laser system  3  includes a master oscillator MO and an amplifier PA. At least one of the master oscillator MO and the amplifier PA is an yttrium aluminum garnet (YAG) laser device including, as a laser medium, a YAG crystal or a YAG crystal doped with impurities such as neodymium. Alternatively, the laser system  3  may be a laser device including, as the laser medium, an Nd:YVO 4  crystal or an optical fiber doped with a rare earth such as Yb. The YAG laser device, the Nd:YVO 4  laser device, or the fiber laser device further includes an excitation light source such as a laser diode (not shown). The excitation light source excites the laser medium by outputting continuous oscillation laser light. 
     An optical modulator OM is arranged on an optical path of the pulse laser light  31  between the laser system  3  and the plasma generation region  25 . The optical modulator OM is an example of the optical adjuster in the present disclosure. The optical modulator OM includes an acoustic optical element (not shown) and transmittance of the pulse laser light  31  is controlled by an application voltage applied to the acoustic optical element. The optical modulator OM may include an electric optical element or an attenuator instead of the acoustic optical element, and the transmittance of the pulse laser light  31  may be controlled by the application voltage to the electric optical element or the attenuator. In the present disclosure, the application voltage applied to the acoustic optical element, the electric optical element, or the attenuator is referred to as the application voltage of the optical modulator OM. 
     A target timing sensor  4   a , an EUV energy sensor  7   a , and a laser light concentrating optical system  22   a  are arranged in the chamber  2 . The target timing sensor  4   a  corresponds to the first sensor in the present disclosure, and the EUV energy sensor  7   a  corresponds to the second sensor in the present disclosure. The target timing sensor  4   a  includes a light source, a transfer optical system, and an optical sensor which are not illustrated. The light source illuminates the target  27  having reached a detection region  35  between the target supply unit  26  and the plasma generation region  25 . The detection region  35  corresponds to the second predetermined region in the present disclosure. The transfer optical system images a part of an image of the target  27  illuminated by the light source on the optical sensor. The optical sensor detects a change in light intensity when the target  27  passes through the detection region  35 . The optical sensor may be a line sensor or an image sensor. The EUV energy sensor  7   a  is arranged at a position where a part of the EUV light generated in the plasma generation region  25  is incident. The processor  5  includes a modulation signal generating unit  51  and a timing signal generating unit  52 . 
     2.2 Operation 
     The master oscillator MO performs laser oscillation and outputs pulse laser light. The output timing of the pulse laser light from the master oscillator MO is defined by a trigger timing signal output from the timing signal generating unit  52  to the master oscillator MO. The amplifier PA amplifies the pulse laser light incident from the master oscillator MO. Thus, the laser system  3  outputs the pulse laser light  31 . 
     The optical modulator OM transmits the pulse laser light  31  at the transmittance corresponding to the application voltage. The application voltage of the optical modulator OM is defined by a modulation signal output from the modulation signal generating unit  51  to the optical modulator OM. The timing for changing the application voltage of the optical modulator OM is defined by a modulation timing signal output from the timing signal generating unit  52  to the optical modulator OM. 
     The laser light transmission device  34  guides the pulse laser light  31  incident from the optical modulator OM to the laser light concentrating optical system  22   a  as the pulse laser light  32 . The laser light concentrating optical system  22   a  concentrates the pulse laser light  32  incident from the laser light transmission device  34  to the plasma generation region  25  as the pulse laser light  33 . 
     The target supply unit  26  supplies the target  27  in a droplet form to the plasma generation region  25  by outputting the target  27  toward the plasma generation region  25 . The target timing sensor  4   a  detects the arrival timing at which the target  27  has reached the detection region  35 , and outputs a target detection signal indicating the arrival timing to the timing signal generating unit  52 . The pulse laser light  33  is radiated to the target  27  in the plasma generation region  25 . The EUV energy sensor  7   a  detects pulse energy of the EUV light generated by irradiating the target  27  with the pulse laser light  33 , and outputs the detection result to the modulation signal generating unit  51 . The pulse energy of the EUV light corresponds to the second pulse energy in the present disclosure. 
     The modulation signal generating unit  51  outputs a modulation signal for controlling the application voltage of the optical modulator OM based on the pulse energy of the EUV light received from the EUV energy sensor  7   a . The modulation signal includes a feedback control signal FB EUV  based on the pulse energy of the EUV light. For example, when the pulse energy of the EUV light is lower than a target value, the transmittance of the pulse laser light  31  through the optical modulator OM may be increased by increasing the application voltage of the optical modulator OM. Since the pulse energy of the target  27  radiated with the pulse laser light  33  is increased by increasing the transmittance of the pulse laser light  31 , higher energy is applied to the target  27 . Accordingly, the pulse energy of the EUV light is increased and is allowed to approach the target value. 
     Based on the target detection signal received from the target timing sensor  4   a , the timing signal generating unit  52  outputs the trigger timing signal to the master oscillator MO and outputs the modulation timing signal to the optical modulator OM. In the present disclosure, the trigger timing signal and the modulation timing signal may be collectively referred to as a timing signal. 
       FIG. 3  is a timing chart of laser control in a comparative example. The target detection signal output from the target timing sensor  4   a  to the timing signal generating unit  52  includes a plurality of pulses. For example, the rising timing of each pulse indicates the arrival timing at which the target  27  has reached the detection region  35 . Time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . from the rising of one pulse to the rising of the next pulse correspond to the time intervals of the targets  27 . 
     The trigger timing signal output from the timing signal generating unit  52  to the master oscillator MO includes a first trigger having a delay time t A  with respect to the rising of a pulse in the target detection signal. Since the delay time t A  is constant, the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the first trigger are equal to the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27 , respectively. Since the master oscillator MO starts laser oscillation at the timing of receiving the first trigger, a laser oscillation interval ΔTn is equal to each of the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the first trigger. The delay time t A  corresponds to the difference between the time required for the target  27  to reach the plasma generation region  25  after reaching the detection region  35  and the time required for the pulse laser light  33  to reach the plasma generation region  25  after the master oscillator MO starts laser oscillation. 
     The trigger timing signal may further include a second trigger having a delay time t A +t B  with respect to the rising of a pulse in the target detection signal. The laser system  3  starts excitation of the laser medium for the next laser oscillation at the timing of receiving the second trigger. The excitation energy of the laser medium is accumulated until the master oscillator MO receives the first trigger for the next laser oscillation. 
     The delay time t A  may be defined by the timing of the falling of the trigger timing signal and the delay time t A +t B  may be defined by the timing of the rising of the trigger timing signal. 
     The modulation timing signal output from the timing signal generating unit  52  to the optical modulator OM includes a third trigger having a delay time t D  with respect to the rising of a pulse in the target detection signal. The delay time t D  is longer than the delay time t A . The difference between the delay time t D  and the delay time t A  is shorter than the time required for the pulse laser light  31  to reach the optical modulator OM after the master oscillator MO starts laser oscillation. 
     When receiving the modulation timing signal including the third trigger from the timing signal generating unit  52 , the optical modulator OM changes the transmittance of the pulse laser light  31  through the optical modulator OM by changing the application voltage in accordance with the modulation signal received from the modulation signal generating unit  51 . 
     2.3 Problems of Comparative Example 
     As shown in  FIG. 3 , the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  appearing in the target detection signal may vary. Variation in the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  occurs due to changes in the mechanical conditions of the target supply unit  26  and the like. In  FIG. 3 , variation in the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  is depicted in an exaggerated manner. 
     The laser oscillation interval ΔTn also varies in accordance with variation in the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27 . 
     The excitation intensity I 1 , I 2 , I 3 , . . . of the laser system  3  may vary in accordance with the laser oscillation interval ΔTn. For example, in the laser system  3  that excites the laser medium with continuous oscillation laser light, the excitation intensity I 1 , I 2 , I 3 , . . . can be higher when the laser oscillation interval ΔTn is long than that when the laser oscillation interval ΔTn is short. 
     The laser light intensity of the pulse laser light  33  varies in accordance with the excitation intensity I 1 , I 2 , I 3 , . . . of the laser system  3 , and the pulse energy E 10 , E 20 , E 30 , . . . varies in accordance with the laser light intensity. For example, when the transmittance of the pulse laser light  31  is set the same for a plurality of pulses by setting the application voltage of the optical modulator OM, the pulse energy E 10 , E 20 , E 30 , . . . of the pulse laser light  33  increases as the excitation intensity of the laser system  3  I 1 , I 2 , I 3 , . . . increases. 
     If the pulse energy E 10 , E 20 , E 30 , . . . of the pulse laser light  33  varies, the pulse energy of the EUV light may become unstable. 
     3. EUV Light Generation System  11   b  which Controls Transmittance of Pulse Laser Light  31  Through Optical Modulator OM Based on Target Detection Signal 
     3.1 Configuration and Operation 
       FIG. 4  schematically shows the configuration of an EUV light generation system  11   b  according to a first embodiment.  FIG. 5  is a timing chart of laser control in the first embodiment. 
     In the first embodiment, the timing signal generating unit  52  calculates the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  based on the detection time difference of the target detection signal sequentially received from the target timing sensor  4   a . The timing signal generating unit  52  may include a timer that measures the detection time difference. The timing signal generating unit  52  outputs the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  as the laser oscillation interval ΔTn to the modulation signal generating unit  51 . 
     The target detection signal, the trigger timing signal, and the excitation intensity I 1 , I 2 , I 3 , . . . of the laser system  3  shown in  FIG. 5  may be similar to those described in the comparative example. 
     The modulation signal generating unit  51  calculates the application voltages V 1 , V 2 , V 3 , . . . of the optical modulator OM based on the laser oscillation interval ΔTn. In the present disclosure, the application voltages V 1 , V 2 , V 3 , . . . of the optical modulator OM calculated based on the laser oscillation interval ΔTn may be collectively referred to as an application voltage Vn. 
     The modulation signal generating unit  51  controls the transmittance R 1 , R 2 , R 3 , . . . of the pulse laser light  31  through the optical modulator OM by controlling the application voltages V 1 , V 2 , V 3 , . . . of the optical modulator OM. 
     Thus, the processor  5  controls the transmittance R 1 , R 2 , R 3 , . . . of the pulse laser light  31  through the optical modulator OM based on the arrival timing detected by the target timing sensor  4   a.    
     The application voltage Vn is given by a function of the laser oscillation interval ΔTn. 
         Vn=f (Δ Tn )
 
     Specific examples of the function will be described later with reference to  FIGS. 6 to 10 . 
     For example, when the time interval of the targets  27  is the first time interval ΔT 1 , the application voltage V 1  is set so that the transmittance of the pulse laser light  31  through the optical modulator OM is the first transmittance R 1 . When the time interval of the targets  27  is the second time interval ΔT 2  shorter than the first time interval ΔT 1 , the application voltage V 2  is set so that the transmittance is the second transmittance R 2  higher than the first transmittance R 1 . 
     Thus, a second variation in the pulse energy of the pulse laser light  33  transmitted through the optical modulator OM and radiated to the target  27  is smaller than a first variation in the pulse energy of the pulse laser light  31  incident on the optical modulator OM. The first variation corresponds to variation in the excitation intensity I 1 , I 2 , I 3 , . . . . The second variation corresponds to variation in the pulse energy E 1 , E 2 , E 3 , . . . shown in  FIG. 5 . 
     The processor  5  performs the following operation within a period after the target  27  reaches the detection region  35  until the target  27  reaches the plasma generation region. 
     1. The timing signal generating unit  52  receives the target detection signal indicating the time interval ΔT 1  of the targets  27  from the target timing sensor  4   a , and transmits the time interval ΔT 1  of the targets  27  as the laser oscillation interval ΔTn to the modulation signal generating unit  51 . 
     2. The timing signal generating unit  52  outputs the trigger timing signal based on the time interval ΔT 1  of the targets  27  to the master oscillator MO. 
     3. Before the pulse laser light output from the master oscillator MO reaches the optical modulator OM in accordance with the trigger timing signal based on the time interval ΔT 1  of the targets  27 , the modulation signal generating unit  51  controls the transmittance R 1  of the pulse laser light  31  through the optical modulator OM based on the time interval ΔT 1  of the targets  27 . 
     Thus, for each pulse, the transmittance R 1  is controlled based on the time interval ΔT 1  of the targets  27  within the period after the target  27  reaches the detection region  35  and until the target  27  reaches the plasma generation region  25 . 
     The same applies to another target  27  reaching the detection region  35  thereafter. 
     The modulation signal generating unit  51  may control the transmittance R 1 , R 2 , R 3 , . . . using both the application voltage Vn calculated based on the laser oscillation interval ΔTn and the feedback control signal FB EUV  based on the pulse energy of the EUV light. In this case, the application voltage Vn calculated based on the laser oscillation interval ΔTn may be corrected based on the pulse energy of the EUV light, or the feedback control signal FB EUV  based on the pulse energy of the EUV light may be corrected based on the laser oscillation interval ΔTn. 
     3.2 Example of Function 
     3.2.1 First Example 
       FIG. 6  is a graph showing the relationship between the laser oscillation interval ΔTn of the laser system  3  and pulse energy Emax(ΔTn) of the pulse laser light  33  when the transmittance of the pulse laser light  31  through the optical modulator OM is controlled to the maximum value. The pulse energy Emax(ΔTn) corresponds to the first pulse energy in the present disclosure. In  FIG. 6 , the longer the laser oscillation interval ΔTn is, the higher the pulse energy Emax(ΔTn) is. The modulation signal generating unit  51  controls the application voltage Vn of the optical modulator OM based on the relationship between the laser oscillation interval ΔTn and the pulse energy Emax(ΔTn) shown in  FIG. 6 . 
       FIG. 7  is a graph showing the relationship between the application voltage Vn of the optical modulator OM and the pulse energy E(Vn) of the pulse laser light  33 .  FIG. 7  is obtained from the relationship between the application voltage Vn of the optical modulator OM and the transmittance of the pulse laser light  31  through the optical modulator OM. 
     The pulse energy Emax(ΔTn) of the pulse laser light  33  when the transmittance of the pulse laser light  31  through the optical modulator OM is controlled to the maximum value differs in accordance with the laser oscillation interval ΔTn. The value of the pulse energy Emax(ΔTn) is determined using the relationship shown in  FIG. 6 . 
     As shown in  FIG. 7 , the pulse energy E(Vn) varies by changing the transmittance according to the application voltage Vn of the optical modulator OM. When the application voltage Vn is equal to or less than the threshold voltage Vth, the pulse energy E(Vn) becomes 0, and when the application voltage Vn is equal to or more than the maximum voltage Vmax, the pulse energy E(Vn) becomes Emax(ΔTn). As the application voltage Vn increases from the threshold voltage Vth to the maximum voltage Vmax, the pulse energy E(Vn) gradually increases from 0 to Emax(ΔTn). Between the threshold voltage Vth and the maximum voltage Vmax, the pulse energy E(Vn) varies in proportion to a square value of the sine of the application voltage Vn. By determining the application voltage Vn based on the properties shown in  FIG. 7 , it is possible to control the transmittance of the pulse laser light  31  through the optical modulator OM and to control the pulse energy E(Vn) of the pulse laser light  33 . 
     A first example of the function for calculating the application voltage Vn based on the laser oscillation interval ΔTn is given by the following expression. 
         Vn =(2/ n )( V max− Vth )sin −1 [√{ E target/ E max(Δ Tn )}]+ Vth    (Expression 1)
 
     Here, X=sin −1 [Y] represents an inverse function of a sine function, and √{Z} represents a positive square root of Z. n is the circular constant. Etarget is the target value of the pulse energy E(Vn). 
     For example, it is assumed that the threshold voltage Vth is 1 V, the maximum voltage Vmax is 5 V, and the target value Etarget of the pulse energy E(Vn) is 11.5 mJ. Further, it is assumed that the pulse energy Emax(ΔTn) of the pulse laser light  33  when the transmittance of the pulse laser light  31  through the optical modulator OM is controlled to the maximum value is 16.7 mV. Then, the application voltage Vn is calculated to be about 3.49 V from Expression 1. 
       FIG. 8  is a graph showing the relationship between the laser oscillation interval ΔTn and the application voltage Vn set based thereon.  FIG. 8  exemplarily shows the case where the target value Etarget of the pulse energy E(Vn) is 11.5 mJ. Even when the laser oscillation interval ΔTn varies due to variation in the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the target  27 , the pulse energy E(Vn) can be set close to the target value Etarget by controlling the application voltage Vn of the optical modulator OM. 
     3.2.2 Second Example 
       FIG. 9  is a graph showing the relationship between the laser oscillation interval ΔTn of the laser system  3  and the pulse energy Emax(ΔTn) of the pulse laser light  33  when the transmittance of the pulse laser light  31  through the optical modulator OM is controlled to the maximum value.  FIG. 9  shows the differential coefficient dEmax(ΔTc)/dΔTn (i.e., the gradient) of the pulse energy Emax(ΔTn) at a reference interval ΔTc in the same graph as  FIG. 6 . 
     Here, it is assumed that the laser oscillation interval ΔTn of the laser system  3  varies in the vicinity of the reference interval ΔTc, and that the change rate of the pulse energy Emax(ΔTn) at that time is approximated by the differential coefficient dEmax(ΔTc)/dΔTn. This is the first assumption. 
       FIG. 10  is a graph showing the relationship between the application voltage Vn of the optical modulator OM and the pulse energy E(Vn) of the pulse laser light  33 . In  FIG. 10 , the laser oscillation interval ΔTn is the reference interval ΔTc shown in  FIG. 9 . That is, the pulse energy of the pulse laser light  33  when the transmittance of the pulse laser light  31  through the optical modulator OM is controlled to the maximum value is Emax(ΔTc).  FIG. 10  further shows the differential coefficient dE(Vc)/dVn (i.e., the gradient) of the pulse energy E(Vn) at a reference voltage Vc. 
     Here, it is assumed that the application voltage Vn of the optical modulator OM is controlled to vary in the vicinity of the reference voltage Vc, and that the change rate of the pulse energy E(Vn) at that time is approximated by the differential coefficient dE(Vc)/dVn. This is the second assumption. 
     To the extent that both the first and second assumptions hold, a second example of the function for calculating the application voltage Vn based on the laser oscillation interval ΔTn is given by the following expression. 
         Vn =( dE max(Δ Tc )/ dΔTn )( dVn/dE ( Vc ))(Δ Tc−ΔTn )+ Vc  
 
     Here, dVn/dE(Vc) is the inverse of the differential coefficient dE(Vc)/dVn shown in  FIG. 10 . 
     Here, the reference voltage Vc is given by the following expression using the reference interval ΔTc and the target value Etarget of the pulse energy E(Vn). 
         Vc =(2/π)( V max− Vth )sin −1 [√{Etarget/ E max(Δ Tc )}]+ Vth  
 
     3.3 Effect 
     (1) According to the first embodiment, the EUV light generation system  11   b  includes the target supply unit  26  which supplies the target  27  to the plasma generation region  25 , and the laser system  3  which outputs the pulse laser light  31  to  33  to be radiated to the target  27  in the plasma generation region  25 . The EUV light generation system  11   b  further includes the target timing sensor  4   a  which detects the arrival timing at which the target  27  has reached the detection region  35  between the target supply unit  26  and the plasma generation region  25 , and the optical modulator OM arranged on the optical path of the pulse laser light  31  between the laser system  3  and the plasma generation region  25 . The EUV light generation system  11   b  further includes the processor  5  which controls the transmittance of the pulse laser light  31  through the optical modulator OM based on the arrival timing. 
     Accordingly, even when the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  vary, it is possible to suppress variation in the pulse energy of the pulse laser light  33  radiated to the target  27 . Therefore, it is possible to stabilize the pulse energy of the EUV light. 
     (2) According to the first embodiment, when the arrival timing detected by the target timing sensor  4   a  is the first time interval ΔT 1 , the optical modulator OM transmits the pulse laser light  31  at the first transmittance R 1 . Further, when the arrival timing detected by the target timing sensor  4   a  is the second time interval ΔT 2  shorter than the first time interval ΔT 1 , the optical modulator OM transmits the pulse laser light  31  at the second transmittance R 2  higher than the first transmittance R 1 . 
     Accordingly, even when the time intervals ΔT 1 , ΔT 2  of the targets  27  are short, it is possible to suppress decrease of the pulse energy of the pulse laser light  33  radiated to the target  27 . 
     (3) According to the first embodiment, the transmittance of the pulse laser light  31  through the optical modulator OM is controlled so that the second variation in the pulse energy of the pulse laser light  33  transmitted through the optical modulator OM and radiated to the target  27  is smaller than the first variation in the pulse energy of the pulse laser light  31  incident on the optical modulator OM. 
     Accordingly, even when the pulse energy of the pulse laser light  31  output from the laser system  3  varies, it is possible to suppress variation in the pulse energy of the pulse laser light  33  radiated to the target  27 . 
     (4) According to the first embodiment, the processor  5  controls the transmittance of the pulse laser light  31  through the optical modulator OM by controlling the application voltage Vn of the optical modulator OM. 
     Accordingly, the transmittance of the pulse laser light  31  can be controlled with high response performance. 
     (5) According to the first embodiment, the processor  5  controls the transmittance of the pulse laser light  31  through the optical modulator OM based on the arrival timing of the target  27  to the detection region  35  within the period after the target  27  reaches the detection region  35  until the target  27  reaches the plasma generation region. 
     At the timing when the target  27  reaches the plasma generation region  25 , the pulse laser light  33  reaches the plasma generation region  25 . Therefore, by controlling the transmittance before the target  27  reaches the plasma generation region  25 , the target  27  can be irradiated with the pulse laser light  33  having the pulse energy adjusted. 
     (6) According to the first embodiment, the processor  5  outputs the trigger timing signal to the laser system  3  based on the arrival timing at which the target  27  has reached the detection region  35 . 
     Accordingly, the laser system  3  can perform laser oscillation at an appropriate timing for irradiating the target  27  with the pulse laser light  33 . 
     (7) According to the first embodiment, the processor  5  outputs different timing signals respectively to the laser system  3  and the optical modulator OM based on the arrival timing at which the target  27  has reached the detection region  35 . 
     Accordingly, the timing of the laser oscillation of the laser system  3  and the timing for changing the transmittance through the optical modulator OM can be controlled separately. 
     (8) According to the first embodiment, the transmittance is controlled based on the relationship between the laser oscillation interval ΔTn of the laser system  3  and the pulse energy Emax(ΔTn) of the pulse laser light  33  when the transmittance of the pulse laser light  31  through the optical modulator OM is controlled to the maximum value. 
     Accordingly, even when the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  vary, it is possible to suppress variation in the pulse energy of the pulse laser light  33  due to variation in the laser oscillation interval ΔTn. 
     (9) According to the first embodiment, the transmittance is controlled based on both of the relationship between the laser oscillation interval ΔTn and the pulse energy Emax(ΔTn) and the relationship between the application voltage Vn of and the transmittance through the optical modulator OM. 
     Accordingly, the transmittance of the pulse laser light  31  through the optical modulator OM can be controlled in accordance with variation in the pulse energy of the pulse laser light  31  due to variation in the laser oscillation interval ΔTn, so that the pulse energy of the pulse laser light  33  is stabilized. 
     (10) According to the first embodiment, the transmittance is controlled based on both of the change rate of the pulse energy Emax(ΔTn) when the laser oscillation interval ΔTn of the laser system  3  varies and the change rate of the transmittance when the application voltage Vn of the optical modulator OM varies. 
     Accordingly, even without performing complex calculations such as an inverse function of a sine function for each pulse, it is possible to perform control using the change rate calculated in advance, and thus the load of the processor  5  for each pulse can be reduced. 
     (11) According to the first embodiment, the target supply unit  26  supplies the target  27  in a droplet form to the plasma generation region  25 . 
     Accordingly, it is possible to reduce variation in the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27 . Even when the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  vary slightly, the pulse energy of the pulse laser light  33  can be stabilized by controlling the optical modulator OM. 
     (12) According to the first embodiment, the laser system  3  includes the YAG laser device. 
     By irradiating the target  27  with the pulse laser light  33  generated using the YAG laser device, EUV light can be efficiently generated. Even when the pulse energy of the pulse laser light  31  varies, the pulse energy of the pulse laser light  33  can be stabilized by controlling the optical modulator OM. 
     (13) According to the first embodiment, the laser system  3  includes the excitation light source which outputs continuous oscillation laser light to excite the laser medium of the laser system  3 . 
     Accordingly, the pulse energy of the pulse laser light  33  can be stabilized by controlling the optical modulator OM using the relationship between the laser oscillation interval ΔTn and the pulse energy of the pulse laser light  31 . 
     (14) According to the first embodiment, the optical modulator OM includes any one of an acoustic optical element, an electric optical element, and an attenuator. 
     Accordingly, the transmittance of the pulse laser light  31  through the optical modulator OM can be controlled with high response. 
     (15) According to the first embodiment, the EUV light generation system  11   b  further includes the EUV energy sensor  7   a  which detects the pulse energy of the EUV light generated by irradiating the target  27  with the pulse laser light  33 . The processor  5  controls the transmittance of the pulse laser light  31  through the optical modulator OM based on both of the arrival timing at which the target  27  has reached the detection region  35  and the pulse energy of the EUV light. 
     Accordingly, it is possible to stabilize the pulse energy of the EUV light by performing control using the measurement value of the pulse energy of the EUV light in addition to suppressing variation in the pulse energy of the pulse laser light  33  according to variation in the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27 . 
     (16) According to the first embodiment, the processor  5  performs feedback control on the transmittance of the pulse laser light  31  through the optical modulator OM based on the pulse energy of the EUV light. 
     Accordingly, it is possible to stabilize the pulse energy of the EUV light with the feedback control using the measurement value of the pulse energy of the EUV light. In other respects, the first embodiment is similar to the comparative example. 
     4. EUV Light Generation System  11   c  Including Prepulse Laser Device  3 P 
     4.1 Configuration 
       FIG. 11  schematically shows the configuration of an EUV light generation system  11   c  according to a second embodiment. In the second embodiment, the laser system  3  includes a prepulse laser device  3 P and a main pulse laser device  3 M. The prepulse laser device  3 P includes a master oscillator MOP and an amplifier PAP, and the main pulse laser device  3 M includes a master oscillator MOM and an amplifier PAM. 
     A first optical modulator OMP is arranged on an optical path of prepulse laser light  31 P between the prepulse laser device  3 P and the laser light transmission device  34 . The first optical modulator OMP corresponds to the first optical adjuster in the present disclosure. 
     A second optical modulator OMM is arranged on an optical path of main pulse laser light  31 M between the main pulse laser device  3 M and the laser light transmission device  34 . The second optical modulator OMM corresponds to the second optical adjuster in the present disclosure. 
     4.2 Operation 
     The master oscillators MOP, MOM perform laser oscillation and output pulse laser light, respectively. The output timing of the pulse laser light by the master oscillator MOP is controlled by a first trigger timing signal. The output timing of the pulse laser light by the master oscillator MOM is controlled by a second trigger timing signal. 
     Each of the first trigger timing signal and the second trigger timing signal is similar to the trigger timing signal in the comparative example. However, the delay time t AP  of the first trigger included in the first trigger timing signal is shorter than the delay time t AM  of the first trigger included in the second trigger timing signal. As a result, the prepulse laser light  31 P is generated earlier than the main pulse laser light  31 M. 
     The first trigger timing signal may further include the second trigger having a delay time t AP +t BP  with respect to the rising of a pulse in the target detection signal. The second trigger timing signal may further include the second trigger having a delay time t AM +t BM  with respect to the rising of a pulse in the target detection signal. 
     The first optical modulator OMP transmits the prepulse laser light  31 P at transmittance corresponding to an application voltage Vn P . The second optical modulator OMM transmits the main pulse laser light  31 M at transmittance corresponding to an application voltage Vn M . 
     A delay time t DP  given to the first optical modulator OMP is longer than the delay time t AP . The difference between the delay time t CP  and the delay time t AP  is shorter than the time required for the prepulse laser light  31 P to reach the first optical modulator OMP after the master oscillator MOP starts laser oscillation. 
     A delay time t DM  given to the second optical modulator OMM is longer than the delay time t AM . The difference between the delay time t CM  and the delay time t AM  is shorter than the time required for the main pulse laser light  31 M to reach the second optical modulator OMM after the master oscillator MOM starts laser oscillation. 
     The laser light transmission device  34  guides the prepulse laser light  31 P and the main pulse laser light  31 M incident from the first and second optical modulators OMP, OMM to the laser light concentrating optical system  22   a . The prepulse laser light  31 P and the main pulse laser light  31 M are incident on the laser light concentrating optical system  22   a  as the pulse laser light  32 . 
     The prepulse laser light  31 P is radiated to the target  27  in a droplet form as the pulse laser light  33 . The target  27  irradiated with the prepulse laser light  31 P is broken and diffused by the energy of the prepulse laser light  31 P. Variation in the pulse energy of the prepulse laser light  31 P may affect the diffused state of the target  27 . When the diffused state of the target  27  is an undesirable state, the pulse energy of the EUV light may not be a desirable value even when the target  27  is irradiated with the main pulse laser light  31 M. 
     Therefore, it is desirable that the pulse energy of the prepulse laser light  31 P is stable without depending on variation in the laser oscillation interval ΔTn. Further, in order to suppress variation in the diffused state of the target  27  due to variation in the pulse energy of the EUV light, the pulse energy of the prepulse laser light  31 P may not be feedback-controlled based on the pulse energy of the EUV light. 
     Then, the modulation signal generating unit  51  controls the transmittance of the prepulse laser light  31 P through the first optical modulator OMP using the application voltage Vn P  calculated based on the laser oscillation interval ΔTn. 
     The target  27  diffused by being irradiated with the prepulse laser light  31 P is irradiated with the main pulse laser light  31 M as the pulse laser light  33 . The target  27  irradiated with the prepulse laser light  31 P and the main pulse laser light  31 M is turned into plasma, and EUV light is radiated from the plasma. The pulse energy of the main pulse laser light  31 M may affect the pulse energy of the EUV light. For example, when the pulse energy of the main pulse laser light  31 M is low, the pulse energy of the EUV light may be low. 
     Therefore, it is preferable that the pulse energy of the main pulse laser light  31 M is stable without depending on variation in the laser oscillation interval ΔTn. Further, in order to control the pulse energy of the EUV light into the vicinity of the target value, the pulse energy of the main pulse laser light  31 M may be feedback-controlled based on the pulse energy of the EUV light. 
     Then, the modulation signal generating unit  51  controls the transmittance of the main pulse laser light  31 M through the second optical modulator OMM using both of the application voltage Vn M  calculated based on the laser oscillation interval ΔTn and the feedback control signal FB EUV  based on the pulse energy of the EUV light received from the EUV energy sensor  7   a.    
     That is, the modulation signal generating unit  51  controls the transmittance so that the pulse energy of the EUV light has a greater influence on the transmittance of the main pulse laser light  31 M through the second optical modulator OMM than the transmittance of the prepulse laser light  31 P through the first optical modulator OMP. 
     4.3 Effect 
     (17) According to the second embodiment, the EUV light generation system  11   c  includes the EUV energy sensor  7   a  which detects the pulse energy of the EUV light generated by irradiating the target  27  with the pulse laser light  33 . The laser system  3  includes the prepulse laser device  3 P which outputs the prepulse laser light  31 P, and the main pulse laser device  3 M which outputs the main pulse laser light  31 M. The prepulse laser light  31 P is radiated to the target  27 , and the main pulse laser light  31 M is radiated to the target  27  to which the prepulse laser light  31 P has been radiated. The EUV light generation system  11   c  includes, as the optical adjuster, the first optical modulator OMP arranged on the optical path between the prepulse laser device  3 P and the plasma generation region  25  and the second optical modulator OMM arranged on the optical path between the main pulse laser device  3 M and the plasma generation region  25 . The processor  5  controls the transmittance of the prepulse laser light  31 P through the first optical modulator OMP based on the arrival timing at which the target  27  has reached the detection region  35 . In addition, the processor  5  controls the transmittance of the main pulse laser light  31 M through the second optical modulator OMM based on both of the arrival timing and the pulse energy of the EUV light. 
     Accordingly, since the main pulse laser light  31 M is radiated to the target  27  having been irradiated with the prepulse laser light  31 P and diffused, the target  27  can be efficiently turned into plasma. Further, even when the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  vary, variation in the pulse energy of the prepulse laser light  31 P radiated to the target  27  can be suppressed, so that the diffused state of the target  27  can be stabilized. Further, since the pulse energy of the main pulse laser light  31 M radiated to the target  27  is controlled based on both of the time intervals ΔT 1 , ΔT 2 , ΔT 3 , . . . of the targets  27  and the pulse energy of the EUV light, the pulse energy of the EUV light can be stabilized. 
     (18) According to the second embodiment, the processor  5  controls the transmittance so that the pulse energy of the EUV light has a greater influence on the transmittance of the main pulse laser light  31 M through the second optical modulator OMM than the transmittance of the prepulse laser light  31 P through the first optical modulator OMP. 
     Accordingly, since influence of the pulse energy of the EUV light on the transmittance of prepulse laser light  31 P through the first optical modulator OMP can be reduced, it is possible to suppress variation in the diffused state of the target  27  due to variation in the pulse energy of the EUV light. 
     In other respects, the second embodiment is similar to the first embodiment. 
     5. Others 
       FIG. 12  schematically shows the configuration of an exposure apparatus  6   a  connected to the EUV light generation system  11   b.    
     In  FIG. 12 , the exposure apparatus  6   a  as the EUV light utilization apparatus  6  (see  FIG. 1 ) includes a mask irradiation unit  68  and a workpiece irradiation unit  69 . The mask irradiation unit  68  illuminates, via a reflection optical system, a mask pattern of a mask table MT with the EUV light incident from the EUV light generation system  11   b . The workpiece irradiation unit  69  images the EUV light reflected by the mask table MT onto a workpiece (not shown) arranged on a workpiece table WT via the reflection optical system. The workpiece is a photosensitive substrate such as a semiconductor wafer on which photoresist is applied. The exposure apparatus  6   a  synchronously translates the mask table MT and the workpiece table WT to expose the workpiece to the EUV light reflecting the mask pattern. Through the exposure process as described above, a device pattern is transferred onto the semiconductor wafer, thereby an electronic device can be manufactured. 
       FIG. 13  schematically shows the configuration of an inspection apparatus  6   b  connected to the EUV light generation system  11   b.    
     In  FIG. 13 , the inspection apparatus  6   b  as the EUV light utilization apparatus  6  (see  FIG. 1 ) includes an illumination optical system  63  and a detection optical system  66 . The Illumination optical system  63  reflects the EUV light incident from the EUV light generation system  11   b  to illuminate a mask  65  placed on a mask stage  64 . Here, the mask  65  conceptually includes a mask blank before a pattern is formed. The detection optical system  66  reflects the EUV light from the illuminated mask  65  and forms an image on a light receiving surface of a detector  67 . The detector  67  having received the EUV light obtains the image of the mask  65 . The detector  67  is, for example, a time delay integration (TDI) camera. Defects of the mask  65  are inspected based on the image of the mask  65  obtained by the above-described process, and a mask suitable for manufacturing an electronic device is selected using the inspection result. Then, the electronic device can be manufactured by exposing and transferring the pattern formed on the selected mask onto the photosensitive substrate using the exposure apparatus  6   a.    
     In  FIG. 12  or  FIG. 13 , the EUV light generation system  11   c  may be used instead of the EUV light generation system  11   b.    
     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.