Abstract:
A laser apparatus for generating extreme ultraviolet (EUV) light at a wavelength of approximately 13 nm is provided. The laser apparatus may be combined with a reduced projection reflective optical system. Systems and methods for generating EUV light are also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from Japanese Patent Application No. 2011-073468 filed Mar. 29, 2011, and Japanese Patent Application No. 2012-007210 filed Jan. 17, 2012. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    This disclosure relates to a laser apparatus, a method for generating a laser beam, and an extreme ultraviolet light generation system. 
         [0004]    2. Related Art 
         [0005]    In recent years, semiconductor production processes have become capable of producing semiconductor devices with increasingly fine feature sizes, as photolithography has been making rapid progress toward finer fabrication. In the next generation of semiconductor production processes, microfabrication with feature sizes at 60 nm to 45 nm, and further, microfabrication with feature sizes of 32 nm or less will be required. In order to meet the demand for microfabrication at 32 nm or less, for example, an exposure apparatus is expected to be developed, in which an apparatus for generating extreme ultraviolet (EUV) light at a wavelength of approximately 13 nm is combined with a reduced projection reflective optical system. 
         [0006]    Three kinds of systems for generating EUV light are generally known, which include a Laser Produced Plasma (LPP) type system in which plasma is generated by irradiating a target material by a laser beam, a Discharge Produced Plasma (DPP) type system in which plasma is generated by an electric discharge, and a Synchrotron Radiation (SR) type system in which orbital radiation is used. 
       SUMMARY 
       [0007]    A laser apparatus according to one aspect of this disclosure may include: a plurality of master oscillators each configured to output a pulse laser beam at a different wavelength; at least one amplifier for amplifying the pulse laser beams; an optical shutter provided in a beam path of at least one of the pulse laser beams, the optical shutter being configured to adjust a transmittance of a pulse laser beam passing therethrough in accordance with a voltage applied thereto; a power source for applying the voltage to the optical shutter; a beam path adjusting unit provided in a beam path between the optical shutter and the amplifier for making beam paths of the pulse laser beams coincide with one another; and a controller configured to control the voltage to be applied to the optical shutter by the power source on a pulse-to-pulse basis for the pulse laser beam. 
         [0008]    A method according to another aspect of this disclosure for generating a laser beam in a laser apparatus that includes an amplifier containing a laser gas as a gain medium, at least two master oscillators each configured to output a pulse laser beam at a different wavelength that can be amplified in the amplifier, and at least two optical shutters provided in beam paths of the respective pulse laser beams between the master oscillators and the amplifier may include adjusting a transmittance of at least one of the two optical shutters on a pulse-to-pulse basis for the pulse laser beams from the master oscillators. 
         [0009]    An extreme ultraviolet light generation system according to yet another aspect of this disclosure may include: the aforementioned laser apparatus; a chamber; a target supply unit configured to output a target material toward a predetermined region inside the chamber; a focusing optical element for focusing a pulse laser beam from the laser apparatus in the predetermined region inside the chamber; a target detector for detecting the target material passing through a predetermined position; and a control unit configured to output a signal to cause the laser apparatus to output the pulse laser beam based on a target detection signal from the target detector. 
         [0010]    An extreme ultraviolet light generation system according to still another aspect of this disclosure may include: the aforementioned laser apparatus; a chamber; a target supply unit configured to output a target material toward a predetermined region inside the chamber; a focusing optical element for focusing a pulse laser beam from the laser apparatus in the predetermined region inside the chamber; a target detector for detecting the target material passing through a predetermined position; an extreme ultraviolet light energy detector for detecting energy of extreme ultraviolet light emitted from plasma generated when the target material is irradiated by the pulse laser beam in the predetermined region; and a control unit configured to output a signal to the controller to cause the laser apparatus to output the pulse laser beam based on a target detection signal from the target detector and to output a value of the energy required for the amplified pulse laser beam to the controller based on an extreme ultraviolet light energy detection value from the extreme ultraviolet light energy detector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Hereinafter, selected embodiments of this disclosure will be described with reference to the accompanying drawings. 
           [0012]      FIG. 1  schematically illustrates the configuration of an exemplary LPP type EUV light generation system. 
           [0013]      FIG. 2  schematically illustrates the configuration of a laser apparatus according to a first embodiment of this disclosure. 
           [0014]      FIG. 3  illustrates an example of an optical shutter that includes two polarizers and a Pockels cell according to the first embodiment. 
           [0015]      FIG. 4  shows an example of the relationship between a control voltage value of a high-voltage pulse applied to the Pockels cell shown in  FIG. 3  and transmittance of the optical shutter. 
           [0016]      FIG. 5  shows the relationship between a temporal waveform of a single pulse of a pulse laser beam and an operation timing of the optical shutter according to the first embodiment. 
           [0017]      FIG. 6  shows an example of the relationship between a gain in each amplification line and pulse energy of the pulse laser beam according to the first embodiment. 
           [0018]      FIG. 7  shows the pulse energy of an amplified pulse laser beam obtained according to the relationship shown in  FIG. 6 . 
           [0019]      FIG. 8  shows gain efficiencies in multi-line amplification and single-line amplification by an amplifier according to the first embodiment. 
           [0020]      FIG. 9  schematically illustrates the configuration of a laser apparatus according to a second embodiment of this disclosure. 
           [0021]      FIG. 10  is a timing chart showing beam intensities of pulse laser beams outputted from respective master oscillators according to the second embodiment. 
           [0022]      FIG. 11  is a timing chart showing beam intensities of the pulse laser beams transmitted through respective optical shutters for multi-line amplification according to the second embodiment. 
           [0023]      FIG. 12  is a timing chart showing beam intensities of the pulse laser beams amplified by the amplifier(s) through the multi-line amplification according to the second embodiment. 
           [0024]      FIG. 13  is a timing chart showing a beam intensity of a pulse laser beam outputted from the laser apparatus after the multi-line amplification according to the second embodiment. 
           [0025]      FIG. 14  is a timing chart showing beam intensities of pulse laser beams outputted from respective master oscillators according to the second embodiment. 
           [0026]      FIG. 15  is a timing chart showing a beam intensity of a pulse laser beam transmitted through an optical shutter for single-line amplification according to the second embodiment. 
           [0027]      FIG. 16  is a timing chart showing a beam intensity of the pulse laser beam amplified by the amplifier(s) through the single-line amplification according to the second embodiment. 
           [0028]      FIG. 17  is a timing chart showing a beam intensity of the pulse laser beam outputted from the laser apparatus after the single-line amplification according to the second embodiment. 
           [0029]      FIG. 18  is a flowchart showing an overall operation of the laser apparatus according to the second embodiment. 
           [0030]      FIG. 19  shows an example of a control voltage value calculation routine in Step S 104  of  FIG. 18 . 
           [0031]      FIG. 20  shows an example of an optical shutter switching routine in Step S 106  of  FIG. 18 . 
           [0032]      FIG. 21  schematically illustrates the configuration of an EUV light generation system according to a third embodiment of this disclosure. 
           [0033]      FIG. 22  shows a flowchart showing a portion of an overall operation of the EUV light generation system shown in  FIG. 21 . 
           [0034]      FIG. 23  shows a flowchart showing another portion of an overall operation of the EUV light generation system shown in  FIG. 21 . 
           [0035]      FIG. 24  shows a variation of the optical shutter shown in  FIG. 3 . 
           [0036]      FIG. 25  shows an example of a regenerative amplifier in the laser apparatus shown in  FIG. 9 . 
           [0037]      FIG. 26  shows a first configuration example of a beam path adjusting unit in the laser apparatus shown in  FIG. 2  and an arrangement of the master oscillators with respect to the beam path adjusting unit. 
           [0038]      FIG. 27  schematically illustrates the configuration of a seed laser device that includes a multi-longitudinal mode master oscillator. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0039]    Hereinafter, selected embodiments of this disclosure will be described in detail with reference to the accompanying drawings. The embodiments to be described below are merely illustrative in nature and do not limit the scope of this disclosure. Further, the configuration(s) and operation(s) described in each embodiment are not all essential in implementing this disclosure. Note that like elements are referenced by like reference numerals and characters, and duplicate descriptions thereof will be omitted herein. The embodiments of this disclosure will be illustrated following the table of contents below. 
       Contents 
     1. Overview 
     2. Terms 
     3. Extreme Ultraviolet Light Generation System 
     3.1 Configuration 
     3.2 Operation 
     3.3 Pulse-to-Pulse Energy Control 
     4. Laser Apparatus for Multi-line Amplification (First Embodiment) 
     4.1 Configuration 
     4.1.1 Optical Shutter (Combination of Pockels Cell and Polarizers) 
     4.2 Operation 
     4.3 Effect 
     4.4 Multi-line Amplification 
     5. Laser Apparatus Including Multiple Amplifiers (Second Embodiment 
     5.1 Configuration 
     5.2 Operation 
     5.3 Effect 
     5.4 Timing Chart 
     5.4.1 Multi-line Amplification 
     5.4.2 Single-line Amplification 
     5.5 Flowchart 
     6. Extreme Ultraviolet Light Generation System Including Laser Apparatus (Third Embodiment) 
     6.1 Configuration 
     6.2 Operation 
     6.3 Flowchart 
     7. Supplementary Descriptions 
     7.1 Variation of Optical Shutter 
     7.2 Regenerative Amplifier 
     7.3 Beam Path Adjusting Unit 
     7.4 Seed Laser Device Including Multi-Longitudinal Mode Master Oscillator and Spectroscope 
     1. Overview 
       [0040]    In one or more of the embodiments of this disclosure, the pulse energy of one or more pulse laser beams at different wavelengths entering an amplifier may be controlled for each wavelength, whereby the total energy of an amplified pulse laser beam can be controlled. 
       2. Terms 
       [0041]    Terms used in this application may be interpreted as follows. The term “plasma generation region” may refer to a three-dimensional space in which plasma is to be generated. The term “burst operation” may refer to an operation mode or state in which a pulse laser beam or pulse extreme ultraviolet (EUV) light is outputted at a predetermined repetition rate during a predetermined period and the pulse laser beam or the pulse EUV light is not outputted outside of the predetermined period. In a beam path of a laser beam, a direction or side closer to the laser apparatus is referred to as “upstream,” and a direction or side closer to the plasma generation region is referred to as “downstream.” The “predetermined repetition rate” does not have to be a constant repetition rate but may, in some examples, be a substantially constant repetition rate. 
         [0042]    In an optical element, the “plane of incidence” refers to a plane perpendicular to the surface on which the pulse laser beam is incident and containing the beam axis of the pulse laser beam incident thereon. A polarization component perpendicular to the plane of incidence is referred to as the “S-polarization component,” and a polarization component parallel to the plane of incidence is referred to as the “P-polarization component.” 
         [0043]    Further, in the description to follow, the term “single-line amplification” may mean that a laser beam is amplified in one amplification line (e.g., P( 20 )) of a plurality of amplification lines of a gain medium containing CO 2  gas, for example. The term “multi-line amplification” may mean that a laser beam is amplified in two or more amplification lines of the plurality of amplification lines of the gain medium. 
       3. Extreme Ultraviolet Light Generation System 
     3.1 Configuration 
       [0044]      FIG. 1  schematically illustrates the configuration of an exemplary LPP type EUV light generation system. The LPP type EUV light generation system  1  may include at least one laser apparatus  3 . As illustrated in  FIG. 1  and described in detail below, the EUV light generation system  1  may include a chamber  2 , a target supply unit  26  (a target generator, for example), and so forth. The chamber  2  may be airtightly sealed. The target supply unit  26  may be mounted to the chamber  2  so as to penetrate a wall of the chamber  2 , for example. A target material to be supplied by the target supply unit  26  may include, but is not limited to, tin, terbium, gadolinium, lithium, xenon, or any combination thereof. 
         [0045]    The chamber  2  may have at least one through-hole formed in its wall, and a pulse laser beam  32  may travel through the through-hole. Alternatively, the chamber  2  may be provided with a window  21 , through which the pulse laser beam  32  may travel into the chamber  2 . An EUV collector mirror  23  having a spheroidal surface may be disposed inside the chamber  2 , for example. The EUV collector mirror  23  may have a multi-layered reflective film formed on the spheroidal surface thereof. The reflective film may include a molybdenum layer and a silicon layer being laminated alternately, for example. The EUV collector mirror  23  may have a first focus and a second focus, and preferably be disposed such that the first focus lies in a plasma generation region  25  and the second focus lies in an intermediate focus (IF) region  292  defined by the specification of an external apparatus, such as an exposure apparatus  6 . The EUV collector mirror  23  may have a through-hole  24  formed at the center thereof, and a pulse laser beam  33  may travel through the through-hole  24  toward the plasma generation region  25 . 
         [0046]    The EUV light generation system  1  may further include an EUV light generation controller  5  and a target sensor  4 . The target sensor  4  may have an imaging function and detect at least one of the presence, the trajectory, and the position of a target. 
         [0047]    Further, the EUV light generation system  1  may include a connection part  29  for allowing the interior of the chamber  2  and the interior of the exposure apparatus  6  to be in communication with each other. A wall  291  having an aperture  293  may be provided inside the connection part  29 , and the wall  291  may be positioned such that the second focus of the EUV collector mirror  23  lies in the aperture  293  formed in the wall  291 . 
         [0048]    The EUV light generation system  1  may also include a laser beam direction control unit  34 , a laser beam focusing mirror  22 , and a target collector  28  for collecting targets  27 . The laser beam direction control unit  34  may include an optical element for defining the direction into which the laser beam travels and an actuator for adjusting the position and the orientation (posture) of the optical element. 
       3.2 Operation 
       [0049]    With continued reference to  FIG. 1 , a pulse laser beam  31  outputted from the laser apparatus  3  may pass through the laser beam direction control unit  34  and be outputted therefrom as a pulse laser beam  32  after having its direction optionally adjusted. The pulse laser beam  32  may travel through the window  21  and enter the chamber  2 . The pulse laser beam  32  may travel inside the chamber  2  along at least one beam path from the laser apparatus  3 , be reflected by the laser beam focusing mirror  22 , and strike at least one target  27  as a pulse laser beam  33 . 
         [0050]    The target generator  26  may output the targets  27  toward the plasma generation region  25  inside the chamber  2 . The target  27  may be irradiated by at least one pulse of the pulse laser beam  33 . The target  27 , which has been irradiated by the pulse laser beam  33 , may be turned into plasma, and rays of light including EUV light  251  may be emitted from the plasma. The EUV light  251  may be reflected selectively by the EUV collector mirror  23 . EUV light  252  reflected by the EUV collector mirror  23  may travel through the intermediate focus region  292  and be outputted to the exposure apparatus  6 . The target  27  may be irradiated by multiple pulses included in the pulse laser beam  33 . 
         [0051]    The EUV light generation controller  5  may be configured to integrally control the EUV light generation system  1 . The EUV light generation controller  5  may be configured to process image data of the target  27  captured by the target sensor  4 . Further, the EUV light generation controller  5  may be configured to control at least one of the timing at which the target  27  is outputted and the direction into which the target  27  is outputted (e.g., the timing at which and/or direction in which the target is outputted from target generator  26 ). Furthermore, the EUV light generation controller  5  may be configured to control at least one of the timing at which the laser apparatus  3  oscillates (e.g., by controlling laser apparatus  3 ), the direction in which the pulse laser beam  31  travels (e.g., by controlling laser beam direction control unit  34 ), and the position at which the pulse laser beam  33  is focused (e.g., by controlling laser apparatus  3 , laser beam direction control unit  34 , or the like), for example. It will be appreciated that the various controls mentioned above are merely examples, and other controls may be added as necessary. 
       3.3 Pulse-to-Pulse Energy Control 
       [0052]    An EUV light generation system for a semiconductor exposure apparatus may be required to generate EUV light in pulses at a predetermined repetition rate for exposing wafers in the exposure apparatus. In order to transfer a circuit pattern on a mask onto a resist on a wafer with high precision, an exposure amount by EUV light may preferably be controlled with high precision. 
         [0053]    For example, in an EUV light generation system including a laser apparatus, pulse energy of outputted pulsed EUV light may be controlled by controlling pulse energy of a pulse laser beam outputted from the laser apparatus. 
         [0054]    Accordingly, in one or more of the embodiments of this disclosure, a technique for controlling the energy of the pulse laser beam outputted from the laser apparatus on a pulse-to-pulse basis (hereinafter, this may be referred to as “pulse-to-pulse energy control”) will be disclosed. 
         [0055]    An EUV light generation system may include a laser apparatus that includes an amplifier containing a mixed gas including CO 2  gas as a gain medium (hereinafter, simply referred to as CO 2  gas amplifier) in order to increase output power of the pulse laser beam. However, when a master oscillator power amplifier (MOPA) method is employed in a laser apparatus that includes a CO 2  gas amplifier, the pulse-to-pulse energy control may be difficult, if not impossible, in the following respects. 
         [0056]    One of the issues is that pulse energy of a pulse laser beam amplified in a CO 2  gas amplifier may be saturated. Here, the term “saturation” may mean that the pulse energy of the pulse laser beam is in an asymptotic state at a certain value even with an increase in inputted pulse energy. In this case, even when the pulse energy of the pulse laser beam from the master oscillator is controlled on a pulse-to-pulse basis, the effect of the pulse-to-pulse energy control may hardly be reflected on the amount of change in the pulse energy of the amplified pulse laser beam. That is, the energy controllability of the amplified pulse laser beam may be low. 
         [0057]    Another issue is that even when the excitation intensity in an amplifier can be controlled on a pulse-to-pulse basis, it may be hard to control the pulse energy of the amplified pulse laser beam on a pulse-to-pulse basis with high precision. This is because the response speed of the change in a gain to the change in RF excitation energy given to the gain medium may be slow with respect to the repetition rate (e.g., 100 kHz) of the pulse laser beam. 
         [0058]    Accordingly, in this disclosure, the following embodiments will be illustrated. 
       4. Laser Apparatus for Multi-Line Amplification 
     First Embodiment 
       [0059]    A laser apparatus in which a pulse laser beam is amplified using two or more amplification lines of a CO 2  gas gain medium will be illustrated as an example. 
       4.1 Configuration 
       [0060]      FIG. 2  schematically illustrates the configuration of a laser apparatus  3 A according to a first embodiment. As shown in  FIG. 2 , the laser apparatus  3 A may include a seed laser device  100 , a laser controller  110 , and an amplifier  120 . The amplifier  120  may be a CO 2  gas amplifier, but this disclosure is not limited thereto. Further the amplifier  120  may be provided in plurality. When a plurality of amplifiers  120  is used, these amplifiers may be connected serially. 
         [0061]    The seed laser device  100  may include master oscillators  101   1  through  101   n , optical shutters  102   1  through  102   n , and a beam path adjusting unit  103 . Each of the master oscillators  101   1  through  101   n  may, for example, be a semiconductor laser (e.g., quantum cascade laser), a solid-state laser, or the like. Each of the master oscillators  101   1  through  101   n  may be configured to oscillate in a single-longitudinal mode and at a different wavelength from one another. In that case, the master oscillators  101   1  through  101   n  may output respective pulse laser beams L 1   1  through L 1   n , each having an extremely narrow wavelength spectrum. However, this disclosure is not limited thereto. Each of the master oscillators  101   1  through  101   n  may, for example, be configured to oscillate in a multi-longitudinal mode. Alternatively, a pulse laser beam outputted from a single master oscillator configured to oscillate in multi-longitudinal mode may be split into a plurality of such single-longitudinal mode pulse laser beams L 1   1  through L 1   n  as shown in  FIG. 2 , using a prism, a grating, or the like. This split of a multi-longitudinal mode pulse laser beam will be described in detail later with an example. 
         [0062]    The master oscillators  101   1  through  101   n  may preferably be configured to output the respective pulse laser beams L 1   1  through L 1   n  at respective wavelengths that are contained in any one of the amplification lines in the amplifier  120 . 
         [0063]    The optical shutters  102   1  through  102   n  may be provided downstream from the respective master oscillators  101   1  through  101   n . The optical shutters  102   1  through  102   n  may be provided between the respective master oscillators  101   1  through  101   n  and the beam path adjusting unit  103 . Switching of the optical shutters  102   1  through  102   n  may be controlled by the laser controller  110 . The laser controller  110  may preferably be configured to be capable of controlling the opening (transmittance) of each of the optical shutters  102   1  through  102   n  independently from one another. The opening may be a ratio of the pulse energy of the outputted laser beam with respect to the inputted laser beam. The opening being large may mean that the transmittance of the pulse laser beams L 1   1  through L 1   n  entering the respective optical shutters  102   1  through  102   n  is high. Accordingly, the pulse energy (e.g., beam intensity) of pulse laser beams L 2   1  through L 2   n  transmitted through the respective optical shutters  102   1  through  102   n  may depend on the transmittance (opening) of the respective optical shutters  102   1  through  102   n . 
         [0064]    The pulse laser beams L 2   1  through L 2   n  transmitted through the respective optical shutters  102   1  through  102   n  may then enter the beam path adjusting unit  103 , have their respective beam paths adjusted thereby so as to substantially coincide with one another (i.e., into a single predetermined beam path), and be outputted as a pulse laser beam L 2  from the seed laser device  100 . The pulse laser beam L 2  may then enter the amplifier  120  and be amplified in the amplifier  120 . An excitation control signal S 5  may be sent from the laser controller  110  to an RF power source (not shown) of the amplifier  120  in synchronization with a timing at which an amplification region in the amplifier  120  is filled with the pulse laser beam L 2 , for example. Upon receiving the excitation control signal S 5 , the RF power source may supply excitation power to the amplifier  120 . With this, the pulse laser beam L 2  passing through the amplification region inside the amplifier  120  may be amplified. 
       4.1.1 Optical Shutter (Combination of Pockels Cell and Polarizers) 
       [0065]    An example of the optical shutter according to the first embodiment will now be described in detail with reference to the drawings.  FIG. 3  illustrates an example of an optical shutter  102  that includes two polarizers  102   a  and  102   b  and a Pockels cell  102   c . Here, each of the polarizers  102   a  and  102   b  is of a transmissive type. 
         [0066]    In the configuration shown in  FIG. 3 , the polarizer  102   a  may be positioned so as to transmit a polarization component in the Y-direction of a laser beam incident thereon and block a polarization component in the X-direction thereof. Meanwhile, the polarizer  102   b  may be positioned so as to transmit, for example, the polarization component in the X-direction of a laser beam incident thereon and block the polarization component in the Y-direction thereof. In this way, the polarizers  102   a  and  102   b  may be positioned so as to transmit polarization components in different directions. In this example, the polarizers  102   a  and  102   b  may be positioned such that the polarization directions of the transmitted laser beam may differ by 90 degrees. 
         [0067]    A high-voltage pulse may be applied to the Pockels cell  102   c  by a high-voltage power source  102   d  under the control of the laser controller  110 . The Pockels cell  102   c  may modulate the phase of an entering laser beam in accordance with a voltage (control voltage value) of the high-voltage pulse applied thereto. Accordingly, the pulse energy of a pulse laser beam L 2   0  outputted from the optical shutter  102  may be controlled on a pulse-to-pulse basis by controlling the control voltage value applied to the Pockels cell  102   c  as appropriate. In other words, by controlling the control voltage value of the high-voltage pulse applied to the Pockels cell  102   c , the transmittance (opening) of the optical shutter  102  may be controlled. 
         [0068]      FIG. 4  shows an example of the relationship between the control voltage value (V) applied to the Pockels cell  102   c  and the transmittance (T) of the optical shutter  102 . As shown in  FIG. 4 , the optical shutter  102  may be configured such that the control voltage value (V) and the transmittance (T) may be in the relationship of one-to-one correspondence. Thus, the control voltage value (V) may be calculated from the transmittance (T) required of the optical shutter  102 , and a high-voltage pulse of this control voltage value (V) may be applied to the Pockels cell  102   c . With this, the pulse energy of the pulse laser beam L 2   0  outputted from the optical shutter  102  may be controlled by controlling the control voltage value (V). This may also be applicable in a case where each of the polarizers  102   a  and  102   b  is of a reflective type. 
         [0069]    A pulse laser beam L 1   0  entering the optical shutter  102  may first be incident on the polarizer  102   a . The polarizer  102   a  may transmit a polarization component in the Y-direction of the pulse laser beam L 1   0  incident thereon. The component of the pulse laser beam L 1   0  transmitted through the polarizer  102   a  may then enter the Pockels cell  102   c.    
         [0070]    When a high-voltage pulse is not applied to the Pockels cell  102   c , the component of the pulse laser beam L 1   0  having entered the Pockels cell  102   c  may be outputted from the Pockels cell  102   c  without being subjected to phase modulation, and then be incident on the polarizer  102   b . The component of the pulse laser beam L 1   0 , which is polarized in the Y-direction, may be absorbed by the polarizer  102   b . As a result, the pulse laser beam L 1   0  may be blocked by the optical shutter  102 . 
         [0071]    On the other hand, when the high-voltage pulse is applied to the Pockels cell  102   c , the phase of the pulse laser beam L 1   0  entering the Pockels cell  102   c  may be modulated in accordance with the control voltage value. As a result, an elliptically-polarized pulse laser beam L 1   0  having a phase that has been modulated in accordance with the control voltage value may be outputted from the Pockels cell  102   c , and then be incident on the polarizer  102   b . A polarization component in the X-direction of the elliptically-polarized pulse laser beam L 1   0  may be transmitted through the polarizer  102   b  and outputted as a pulse laser beam L 2   0 . In this way, the pulse laser beam L 2   0  whose pulse energy has been adjusted in accordance with the control voltage value of the high-voltage pulse applied to the Pockels cell  102   c  may be outputted from the optical shutter  102 . In other words, the pulse laser beam L 2   0  having a pulse energy that has been adjusted in accordance with the transmittance corresponding to the control voltage value may be outputted from the optical shutter  102 . After the pulse laser beam L 2   0  is outputted from the optical shutter  102 , the application of the high-voltage pulse may be stopped. For example, the control voltage value may be set to 0 V, to thereby close the optical shutter  102 . 
         [0072]    When the high-voltage pulse is applied to the Pockels cell  102   c  in accordance with a passing timing of a single pulse in the pulse laser beam L 1   0 , a self-oscillation beam or a returning beam from an amplifier disposed downstream therefrom may be suppressed. Further, switching the optical shutter  102  while allowing the master oscillators  101   1  through  101   n  to oscillate continually at a predetermined repetition rate may allow the pulse laser beam L 2   0  to be outputted in burst. That is, the optical shutter  102  may fulfill the functions of both suppressing the self-oscillation beam or the returning beam and generating a burst output. 
         [0073]      FIG. 5  shows an operation of the optical shutter on a single pulse in the pulse laser beam according to the first embodiment. As shown in  FIG. 5 , when, for example, a duration (pulse width) of the pulse laser beam L 1   0  is 20 ns, preferably a high-voltage pulse with such a duration that can absorb some timing jitter of the pulse laser beam L 1   0  (for example, 40 ns) may be applied to the Pockels cell  102   c  of the optical shutter  102 . Here, when the duration of the high-voltage pulse is too long, the returning beam may not be blocked by the optical shutter  102  in some cases. Accordingly, the duration of the high-voltage pulse may preferably be set appropriately. Further, a Pockels cell typically has a few-nanosecond-responsiveness. Thus, it may be suitably used for an optical shutter in a laser apparatus where high-speed switching is required. 
       4.2 Operation 
       [0074]    The overall operation of the laser apparatus  3 A shown in  FIG. 2  will now be described. The laser controller  110  may be configured to send an oscillation trigger S 3  to each of the master oscillators  101   1  through  101   n  in accordance with an oscillation trigger S 1  from an external device  5 A. The external device  5 A may, for example, be the EUV light generation controller  5  shown in  FIG. 1 . Upon receiving the oscillation trigger S 3 , each of the master oscillators  101   1  through  101   n  may oscillate continually at a predetermined repetition rate. As mentioned earlier, the master oscillators  101   1  through  101   n  may be configured to output the respective pulse laser beams L 1   1  through L 1   n  having central wavelengths that are contained in the amplification lines in the amplifier  120 . Timings at which the master oscillators  101   1  through  101   n  output the respective pulse laser beam L 1   1  through L 1   n  may be synchronized with one another. 
         [0075]    Further, the laser controller  110  may be configured to control the transmittance (opening) of the optical shutters  102   1  through  102   n  based on a laser beam energy instruction value Ptm (see  FIG. 18 ) from the external device  5 A. Here, the relationship between the laser beam energy instruction value Ptm and the transmittance of the optical shutters  102   1  through  102   n  may be held in a table prepared in advance. Alternatively, a formula for calculating the transmittance of the optical shutters  102   1  through  102   n  from the laser beam energy instruction value Ptm may be prepared in advance. The table or the formula may be obtained through experiments, simulations, or the like. Further, the relationship between the transmittance required of the optical shutters  102   1  through  102   n  and the control voltage values of high-voltage pulses S 4   1  through S 4   n  to be applied to the respective optical shutters  102   1  through  102   n  may be stored in a table prepared in advance, as in the aforementioned relationship. Alternatively, a formula for calculating the control voltage value from the required transmittance may be prepared in advance. The table or the formula may be held in a memory (not shown) or the like, and the laser controller  110  may load the table or the formula from the memory as necessary. 
         [0076]    Each of the master oscillators  101   1  through  101   n  may be a so-called continuous wave (CW) laser. In this case, the laser controller  110  may cause the master oscillators  101   1  through  101   n  to oscillate continuously with constant output power. Then, the laser controller  110  may control the transmittance (opening) and the opening duration of the respective optical shutters  102   1  through  102   n  based on the laser beam energy instruction value Ptm from the external device  5 A, whereby the pulse laser beams L 2   1  through L 2   n  may be generated. With such control, the CW laser beams outputted from the respective master oscillators  101   1  through  101   n  at respectively differing wavelengths may be transmitted through the optical shutter  102   1  through  102   n , respectively, whereby the pulse laser beams L 2   1  through L 2   n  at respectively different wavelengths and with predetermined pulse energy may be generated. 
       4.3 Effect 
       [0077]    With the above configuration and operation, the pulse energy of the pulse laser beams L 2   1  through L 2   n  entering the amplifier  120  may be controlled on a pulse-to-pulse basis by the optical shutters  102   1  through  102   n . Here, the pulse energy of the pulse laser beams L 2   1  through L 2   n  entering the amplifier  120  may preferably be controlled within a range where the pulse energy of each of the pulse laser beams L 2   1  through L 2   n  amplified in a given amplification line does not saturate. With this, the pulse-to-pulse energy control of the pulse laser beams L 2   1  through L 2   n  may be reflected on the pulse energy of the pulse laser beam  31  amplified in the amplifier  120 . This may make it possible to control the pulse energy of the amplified pulse laser beam  31  to be outputted from the laser apparatus  3 A to be controlled with high precision. Further, an energy controllable range (dynamic range) of the pulse laser beam  31  from the laser apparatus  3 A may be broadened, as compared to the case of single-line amplification using a single amplification line P( 20 ) (see  FIG. 8 ), for example, of the amplifier  120 . 
       4.4 Multi-line Amplification 
       [0078]    The multi-line amplification by the amplifier  120  will now be discussed.  FIG. 6  shows an example of the relationship between gains S 18  through S 30  of the respective amplification lines P( 18 ) through P( 30 ) in the amplifier  120  and the pulse energy of the pulse laser beams L 2   1  through L 2   5  transmitted through the respective optical shutters  102   1  through  102   5 . Here, the gains S 18  through S 30  are shown to indicate gain properties in the respective amplification lines.  FIG. 7  shows the pulse energy of components L 3   1  through L 3   5  at respectively different wavelengths contained in the amplified pulse laser beam  31 . 
         [0079]    As shown in  FIG. 6 , the transmittance of the optical shutters  102   1  through  102   5  may, for example, be controlled in accordance with the gains S 18  through S 30  of the respective amplification lines P( 18 ) through P( 30 ). With this, as shown in  FIG. 7 , the pulse energy of the components L 3   1  through L 3   5  amplified in the respective amplification lines P( 18 ) through P( 30 ) can become substantially equal. 
         [0080]    Adjusting the pulse energy of the pulse laser beams L 2   1  through L 2   5  by controlling the transmittance of the respective optical shutters  102   1  through  102   5  may make it possible to control the pulse energy of the components L 3   1  through L 3   5 . As a result, the pulse energy of the pulse laser beam  31  outputted from the laser apparatus  3 A may be controlled as desired (e.g., to a value requested in the laser beam energy instruction value Ptm) with high precision. 
         [0081]    Here, carrying out the pulse-to-pulse energy control using primarily the amplification line P( 20 ), which has a relatively high power conversion efficiency, may lead to energy savings. 
         [0082]      FIG. 8  shows the gain efficiencies in the multi-line amplification and the single-line amplification using the amplifier  120 . In  FIG. 8 , a line C 1  shows the gain efficiency in the single-line amplification using the amplification line P( 20 ), and a line C 2  shows the gain efficiency in the multi-line amplification using the amplification lines P( 20 ) through P( 28 ). 
         [0083]    As may be apparent from the comparison between the lines C 1  and C 2  shown in  FIG. 8 , the multi-line amplification where there is substantially no saturation in the amplification lines may yield 1.5 times higher output pulse energy than the single-line amplification where there is substantially no saturation in the amplification line. This suggests that the multi-line amplification can yield a 1.5 times broader dynamic range than that of the single-line amplification. Here, the output pulse energy shown in  FIG. 8  may be the pulse energy of the pulse laser beam  31  outputted from the laser apparatus  3 A. 
       5. Laser Apparatus Including Multiple Amplifiers 
     Second Embodiment 
       [0084]    A laser apparatus including a plurality of amplifiers will now be described in detail as a second embodiment with reference to the drawings. 
       5.1 Configuration 
       [0085]      FIG. 9  schematically illustrates the configuration of a laser apparatus  3 B according to the second embodiment. The laser apparatus  3 B shown in  FIG. 9  may be similar in configuration to the laser apparatus  3 A shown in  FIG. 2 . However, the laser apparatus  3 B may include a regenerative amplifier  120   R  and a plurality of amplifiers  120   1  through  120   n . As in the first embodiment, single-longitudinal mode semiconductor lasers may be used as the master oscillators  101   1  through  101   n , and each of the semiconductor lasers may be a quantum cascade laser (QCL). The regenerative amplifier  120   R  may be provided between the seed laser device  100  and the first-stage amplifier  120   1 . Each of the regenerative amplifier  120   R  and the amplifiers  120   1  through  120   n  may be a CO 2  gas amplifier. 
         [0086]    At least one of the master oscillators  101   1  through  101   n  may be configured to output a pulse laser beam at a different wavelength from the rest of the master oscillators. The master oscillators  101   1  through  101   n  may preferably be configured to output the pulse laser beam L 1   1  through L 1   n  at respective wavelengths contained in any of the amplification lines of the gain bandwidth of the regenerative amplifier  120   R  and the amplifiers  120   1  through  120   n . 
       5.2 Operation 
       [0087]    The overall operation of the laser apparatus  3 B shown in  FIG. 9  will now be described. In the second embodiment, the operation of the seed laser device  100  and the operation of the laser controller  110  on the seed laser device  100  may be similar to those in the first embodiment described above with reference to  FIG. 2 . 
         [0088]    The pulse laser beam L 2  outputted from the seed laser device  100  may first be amplified in the regenerative amplifier  120   R . The amplification in the regenerative amplifier  120 R may be the multi-line amplification. At this point, the pulse width may be adjusted. Thereafter, an amplified pulse laser beam L 2   a  may be sequentially amplified in the amplifiers  120   1  through  120   n . The amplification in each of the amplifiers  120   1  through  120   n  may also be the multi-line amplification. Here, the laser controller  110  may send excitation control signals S 5   R  and S 5   1  through S 5   n  to the RF power sources of the regenerative amplifier  120   R  and the amplifiers  120   1  through  120   n , preferably in synchronization with timings at which amplification regions in the regenerative amplifier  120   R  and the amplifiers  120   1  through  120   n  are respectively filled with the pulse laser beam L 2  or L 2   a.    
       5.3 Effect 
       [0089]    With the above configuration and operation, effects similar to those of the first embodiment may be obtained. As in the first embodiment, when the semiconductor lasers, such as QCLs, are used for the master oscillators  101   1  through  101   n  and these master oscillators  101   1  through  101   n  are controlled to oscillate continually at a predetermined repetition rate, heat loads on the master oscillators  101   1  through  101   n  may not fluctuate, which in turn may stabilize the pulse energy of the pulse laser beam L 1   1  through L 1   n . As a result, the pulse energy of the pulse laser beams L 2  and L 2   a  to be amplified may be stabilized as well, and in turn the pulse energy of the pulse laser beam  31  outputted from the laser apparatus  3 B may be stabilized. 
       5.4 Timing Chart 
       [0090]    The overall operation of the laser apparatus  3 B shown in  FIG. 9  will now be described with reference to the timing charts. 
       5.4.1 Multi-line Amplification 
       [0091]    Hereinafter, the overall operation of the laser apparatus  3 B including five master oscillators and configured for the multi-line amplification will be described.  FIGS. 10  through  13  are timing charts showing the overall operation of the laser apparatus  3 B for the multi-line amplification. In the description to follow, a case where the pulse energy of the components L 3   1  through L 3   5  contained in the pulse laser beam  31  is made substantially equal to one another will be discussed, as described with reference to  FIGS. 5 and 6 .  FIG. 10  is a timing chart showing the beam intensity of the pulse laser beams L 1   1  through L 1   5  outputted from the respective master oscillators  101   1  through  101   5 .  FIG. 11  is a timing chart showing the beam intensity of the pulse laser beams L 2   1  through L 2   5  transmitted through the respective optical shutters  102   1  through  102   5 .  FIG. 12  is a timing chart showing the beam intensity of the components L 3   1  through L 3   5  contained in the pulse laser beam  31  amplified in the amplifier  120   n .  FIG. 13  is a timing chart showing the beam intensity of the pulse laser beam  31  outputted from the laser apparatus  3 B. 
         [0092]    As shown in  FIG. 10 , the master oscillators  101   1  through  101   5  may be configured to output the respective pulse laser beams L 1   1  through L 1   5  with the same beam intensity and at the same timing T 1 . Here, the pulse laser beams L 1   1  through L 1   5  shown in  FIG. 10  may be outputted from the master oscillators  101   1  through  101   5  continually at a predetermined repetition rate. This may make it possible to thermally stabilize the master oscillators  101   1  through  101   5 . 
         [0093]    Meanwhile, high-voltage pulses S 4   1  through S 4   5  of the respective control voltage values may be applied to the respective optical shutters  102   1  through  102   5  at timing T 2  (see  FIG. 11 ). Here, the control voltage values may be determined in accordance with the gains of the amplifications lines P( 20 ) through P( 28 ) corresponding to the wavelengths of the respective pulse laser beams L 1   1  through L 1   5  entering the respective optical shutters  102   1  through  102   5 . With this, the transmittance (opening) of the optical shutters  102   1  through  102   5  may preferably be controlled to the transmittance in accordance with the gains of the corresponding amplification lines P( 20 ) through P( 28 ). The timing T 2  at which the high-voltage pulses S 4   1  through S 4   5  are applied to the respective optical shutters  102   1  through  102   5  may be adjusted to the timing at which the pulse laser beams L 1   1  through L 1   5  enter the respective optical shutters  102   1  through  102   5 . As a result, as shown in  FIG. 11 , the pulse laser beams L 2   1  through L 2   5  whose beam intensity has been adjusted may be outputted from the respective optical shutters  102   1  through  102   5  substantially simultaneously at the timing T 2 . 
         [0094]    The pulse laser beams L 2   1  through L 2   5  transmitted through the optical shutters  102   1  through  102   5  may then enter the beam path adjusting unit  103  to have their beam paths made to coincide with one another and be outputted as the pulse laser beam L 2 . Thereafter, the pulse laser beam L 2  may undergo the multi-line amplification in the regenerative amplifier  120   R  and the amplifiers  120   1  through  120   n . Here, the pulse width of the pulse laser beam  31  to be outputted from the laser apparatus  3 B may be adjusted by adjusting the operation timing of the regenerative amplifier  120   R . As shown in  FIG. 12 , the components L 3   1  through L 3   5  with substantially the same beam intensity contained in the pulse laser beam  31  may be outputted from the amplifier  120   n  at substantially the same timing T 3 . As a result, as shown in  FIG. 13 , the pulse laser beam  31  with the beam intensity Em may be outputted from the laser apparatus  3 B at a timing T 4 . 
         [0095]    In this example, the pulse laser beams L 1   1  through L 1   5  are outputted at the same timing T 1 , whereby the peak of the pulse energy of the pulse laser beam  31  is made higher. However, this disclosure is not limited thereto. For example, by offsetting the timings at which the pulse laser beams L 1   1  through L 1   5  are outputted, respectively, by a predetermined duration, a pulse laser beam having a larger pulse width may be outputted from the laser apparatus  3 B. Even if that is the case, the pulse energy of the pulse laser beam  31  outputted from the laser apparatus  3 B can satisfy the laser beam energy instruction value Ptm from the external device  5 A. 
       5.4.2 Single-line Amplification 
       [0096]    The overall operation of the laser apparatus  3 B configured for the single-line amplification will now be described.  FIGS. 14 through 17  show the overall operation of the laser apparatus  3 B configured for the single-line amplification. Here, a case where only the pulse laser beam L 1   1  outputted from the master oscillator  101   1  is amplified will be shown as an example.  FIG. 14  is a timing chart showing the beam intensity of the pulse laser beams L 1   1  through L 1   5  outputted from the respective master oscillators  101   1  through  101   5 .  FIG. 15  is a timing chart showing the beam intensity of the pulse laser beam L 2   1  transmitted through the optical shutter  102   1 .  FIG. 16  is a timing chart showing the beam intensity of the component L 3   1  contained in the pulse laser beam  31  amplified in the amplifier  120 .  FIG. 17  is a timing chart showing the beam intensity of the pulse laser beam  31  outputted from the laser apparatus  3 B. 
         [0097]    As shown in  FIG. 14 , the master oscillators  101   1  through  101   5  may be configured to output the pulse laser beams L 1   1  through L 1   5  with the same beam intensity and at the same timing T 1 , as in the case shown in  FIG. 10 . Here, the pulse laser beams L 1   1  through L 1   5  may be outputted from the master oscillators  101   1  through  101   5  continually at a predetermined repetition rate. This may make it possible to thermally stabilize the master oscillators  101   1  through  101   5 . 
         [0098]    Meanwhile, as for the optical shutters  102   1  through  102   5 , only the high-voltage pulse S 4   1  may be applied to the optical shutter  102   1  for opening the optical shutter  102   1 . At this point, the transmittance of the optical shutters  102   2  through  102   5  may preferably be set to 0. As a result, as shown in  FIG. 15 , the pulse laser beam L 2   1  whose beam intensity has been adjusted may be outputted from the optical shutter  102   1  at the timing T 2 . Here, in section (a) of  FIG. 15 , the transmittance of the optical shutter  102   1  is set higher, compared to section (a) of  FIG. 11 . 
         [0099]    The pulse laser beam L 2   1  transmitted through the optical shutter  102   1  may then enter the beam path adjusting unit  103  to have its beam path adjusted to a predetermined beam path and be outputted as the pulse laser beam L 2 . The pulse laser beam L 2  may then undergo the single-line amplification in the regenerative amplifier  120   R  and the amplifiers  120   1  through  120   n . At this point, the pulse width of the pulse laser beam  31  to be outputted from the laser apparatus  3 B may be adjusted by adjusting the operation timing of the regenerative amplifier  120   R . As shown in  FIG. 16 , the component L 3   1  amplified in the amplification line P( 20 ) may be outputted from the final-stage amplifier  120  at the timing T 3 . As a result, as shown in  FIG. 17 , the pulse laser beam  31  with the beam intensity Es may be outputted from the laser apparatus  3 B at a timing T 4 . 
         [0100]    Here, as can be seen from the comparison between  FIG. 13  and  FIG. 17 , the beam intensity Em of the pulse laser beam  31  obtained through the multi-line amplification may be 1.5 times higher than the beam intensity Es of the pulse laser beam  31  obtained through the single-line amplification using the amplification line P( 20 ) which has the highest power conversion efficiency. This suggests that the multi-line amplification may yield a 1.5 times wider dynamic range of the pulse energy control than the single-line amplification. In this way, with the multi-line amplification, the controllability on the pulse energy of the amplified pulse laser beam  31  outputted from the laser apparatus  3 B may be improved. 
       5.5 Flowchart 
       [0101]    The operation of the laser apparatus  3 B shown in  FIG. 9  will now be described with reference to the flowcharts.  FIG. 18  is a flowchart showing the overall operation of the laser apparatus  3 B. The flowchart in  FIG. 18  shows the operation of the laser control  110 . 
         [0102]    As shown in  FIG. 18 , the laser controller  110  may first start sending oscillation triggers S 3  to each of the master oscillators  101   1  through  101   n  at a predetermined repetition rate for controlling the master oscillators  101   1  through  101   n  to oscillate with predetermined pulse energy (Step S 101 ). With this, the master oscillators  101   1  through  101   n  may start outputting the respective pulse laser beams L 1   1  through L 1   n  continually at a predetermined repetition rate. Here, the laser controller  110  may be configured to control the optical shutters  102   1  through  102   n  to be closed (Step S 102 ). This may be achieved by, for example, keeping the control voltage values for the respective optical shutters  102   1  through  102   n  to 0 V. With this, the pulse laser beams L 1   1  through L 1   n  may be blocked by the respective optical shutters  102   1  through  102   n . At this point, each of the amplifiers  120   1  through  120   n  may be brought into an operable state. Here, Step S 102  may be carried out prior to Step S 101  or simultaneously with Step S 101 . 
         [0103]    Then, the laser controller  110  may stand by until it receives the laser beam energy detection value Ptm required for the pulse laser beam  31  from the external device  5 A (Step S 103 ; NO). Upon receiving the laser beam energy instruction value Ptm (Step S 103 ; YES), the laser controller  110  may execute a control voltage value calculation routine (Step S 104 ). In the control voltage value calculation routine, the control voltage values of the high-voltage pulses S 4   1  through S 4   n  to be applied to the respective optical shutters  102   1  through  102   n  may be calculated from the laser beam energy instruction value Ptm. 
         [0104]    Then, the laser controller  110  may stand by until it receives a burst output signal S 2  requesting a burst output of the pulse laser beam  31  from the external device  5 A (Step S 105 ; NO). Upon receiving the burst output signal S 2  (Step S 105 ; YES), the laser controller  110  may execute an optical shutter switching routing for switching the optical shutters  102   1  through  102   n  based on the control voltage values calculated in Step S 104  (Step S 106 ). In Step S 106 , the optical shutters  102   1  through  102   n  may be switched on a pulse-to-pulse basis for the respective pulse laser beams L 1   1  through L 1   n  (pulse-to-pulse energy control). 
         [0105]    Thereafter, the laser controller  110  may determine whether or not it has received a burst pause signal requesting the burst output of the pulse laser beam  31  to be paused from the external device  5 A (Step S 107 ). When the burst pause signal has been received (Step S 107 ; YES), the laser controller  110  may terminate this operation. On the other hand, when the burst pause signal has not been received (Step S 107 ; NO), the laser controller  110  may return to Step S 106  and repeat the subsequent steps. 
         [0106]    With the above operation, the pulse energy of the pulse laser beam L 2  entering the amplifiers  120   1  through  120   n  may be controlled on a pulse-to-pulse basis. This in turn may make it possible to control the pulse energy of the amplified pulse laser beam  31  outputted from the laser apparatus  3 B to be controlled with high precision. Further, an energy controllable range (dynamic range) of the pulse laser beam  31  outputted from the laser apparatus  3 B may be broadened compared to the case of the single-line amplification using a single amplification line (e.g., P( 20 )) in each of the amplifiers  120   1  through  120   n . 
         [0107]    The control voltage value calculation routine in Step S 104  of  FIG. 18  will now be described in detail with reference to  FIG. 19 . As shown in  FIG. 19 , in the control voltage value calculation routine, the laser controller  110  may obtain the transmittances T 1  through T n  of the respective optical shutters  102   1  through  102   n  such that the pulse energy of the amplified pulse laser beam  31  satisfies the laser beam energy instruction value Ptm (Step S 141 ). The relationship between the laser beam energy instruction value Ptm and the transmittances T 1  through T n  may be held in a table prepared in advance as stated above. Alternatively, a formula for calculating the transmittances T 1  through T n  of the respective optical shutters  102   1  through  102   n  from the laser beam energy instruction value Ptm may be prepared in advance. The table or the formula may be obtained through experiments, simulations, or the like. 
         [0108]    Then, the laser controller  110  may calculate control voltage values V 1  through V n  of the high-voltage pulses S 4   1  through S 4   n  to be applied to the respective optical shutters  102   1  through  102   n  from the obtained transmittances T 1  through T n  of the optical shutters  102   1  through  102   n  (Step S 142 ). Thereafter, the laser controller  110  may return to the operation shown in  FIG. 18 . Here, the formula used in Step S 142  may be prepared in advance based on experiments, simulations, or the like. Alternatively, the relationship between the transmittances and the control voltage values may be stored in a table prepared in advance. 
         [0109]    The optical shutter switching routine in Step S 106  of  FIG. 18  will now be described in detail with reference to  FIG. 20 . As shown in  FIG. 20 , in the optical shutter switching routine, the laser controller  110  may stand by until a predetermined delay time from an output of the oscillation trigger S 3  to each of the master oscillators  101   1  through  101   n  elapses (Step S 161 ; NO). The predetermined delay time may be a period from an input of the oscillation trigger S 3  into each of the master oscillators  101   1  through  101   n  until the pulse laser beams L 1   1  through L 1   n  enter the respective optical shutters  102   1  through  102   n . 
         [0110]    The determination of whether or not the predetermined delay time has elapsed from the output of the oscillation trigger S 3  may be made by, for example, measuring an elapsed time by a timer (not shown). Alternatively, in place of measuring an elapsed time using the timer, a delay circuit may be provided for achieving a predetermined stand-by time from the output of the oscillation trigger S 3 . In that case, the processing in Step S 161  may be realized using hardware. Therefore, the operation of the laser controller  110  may be simplified. 
         [0111]    When the predetermined delay time elapses (Step S 161 ; YES), the laser controller  110  may apply the high-voltage pulses S 4   1  through S 4   n  of the control voltage values V 1  through V n  to the respective optical shutters  102   1  through  102   n  (Step S 162 ). With this, the optical shutters  102   1  through  102   n  may be opened in synchronization with the timing at which the pulse laser beams L 1   1  through L 1   n  reach the respective optical shutters  102   1  through  102   n . 
         [0112]    Then, the laser controller  110  may stand by until a predetermined time elapses from the application of the high-voltage pulses S 4   1  through S 4   n  (Step S 163 ; NO). This predetermined time may be a period required for the pulse laser beams L 1   1  through L 1   n  to pass through the respective optical shutters  102   1  through  102   n . 
         [0113]    The determination of whether or not the predetermined time has elapsed from the application of the high-voltage pulses S 4   1  through S 4   n  may be made by, for example, measuring an elapsed time by a timer (not shown). Alternatively, in place of measuring the elapsed time using the timer, a delay circuit may be provided for achieving a predetermined stand-by time from the application of the high-voltage pulses S 4   1  through S 4   n . In this case, the processing in Step S 163  may be realized using hardware. Therefore, the operation of the laser controller  110  may be simplified. 
         [0114]    When the predetermined time elapses (Step S 163 ; YES), the laser controller  110  may stop the application of the high-voltage pulses S 4   1  through S 4   n  to the respective optical shutters  102   1  through  102   n , to thereby close the optical shutters  102   1  through  102   n  (Step S 164 ). Thereafter, the laser controller  110  may return to the operation shown in  FIG. 18 . 
       6. Extreme Ultraviolet Light Generation System Including Laser Apparatus 
     Third Embodiment 
       [0115]    An EUV light generation system  1 C that includes the laser apparatus  3 B will be described in detail as a third embodiment with reference to the drawings. 
       6.1 Configuration 
       [0116]      FIG. 21  schematically illustrates the configuration of the EUV light generation system  1 C according to the third embodiment. The EUV light generation system  1 C shown in  FIG. 21  may be similar in configuration to the EUV light generation system  1  shown in  FIG. 1 , but may differ in that a target controller  260  and an EUV light energy detector  262  are added and that the target sensor  4  and the laser apparatus  3  may respectively be replaced by a target detector  261  and the laser apparatus  3 B. 
       6.2 Operation 
       [0117]    The overall operation of the EUV light generation system  1 C shown in  FIG. 21  will now be described. The EUV light generation controller  5  may first receive an EUV light energy instruction value Pte (see  FIG. 22 ) required for EUV light  252  and a burst output signal from an exposure apparatus controller  61 . The EUV light generation controller  5  may send a target output signal to the target generator  26  via the target controller  260 . With this, a target  27  may be outputted from the target generator  26 . 
         [0118]    The target detector  261  may detect the target  27  outputted from the target generator  26  passing a predetermined position inside the chamber  2 . Here, the predetermined position may be set to any position in a trajectory of the target  27  between the target generator  26  and the plasma generation region  25 . Upon detecting the target  27 , the target detector  261  may output a target detection signal. This target detection signal may be sent to the EUV light generation controller  5  via the target controller  260 . 
         [0119]    The EUV light generation controller  5  may send the laser beam energy instruction value Ptm to the laser controller  110  based on the EUV light energy instruction value Pte received from the exposure apparatus controller  61  or on a detected value reflecting the energy of the EUV light  252  received from the EUV light energy detector  262 , which will be described later. 
         [0120]    Then, the EUV light generation controller  5  may send an oscillation trigger S 1  to the laser controller  110  so that the target  27  is irradiated by the pulse laser beam  33  when the target  27  arrives in the plasma generation region  25 . The timing here may be adjusted based on the burst output signal of the EUV light  252  received from the exposure apparatus controller  61  or on the target detection signal received from the target controller  260 . 
         [0121]    The laser controller  110  may send the oscillation triggers S 3  to the master oscillators  101   1  through  101   n  and apply the high-voltage pulses S 4   1  through S 4   n  to the respective optical shutters  102   1  through  102   n . With this, the pulse laser beam  31  may be outputted from the laser apparatus  3 B. 
         [0122]    The pulse laser beam  31  outputted from the laser apparatus  3 B may travel through the laser beam direction control unit  34 , and enter the chamber  2  through the window  21 . Then, the pulse laser beam  31  may be reflected by the laser beam focusing mirror  22 , and be focused as the pulse laser beam  33  on the target  27  passing through the plasma generation region  25  inside the chamber  2 . With this, the target  27  may be turned into plasma, and the light  251  including the EUV light  252  may be emitted from the plasma. 
         [0123]    The EUV light energy detector  262  may detect a value reflecting the energy of at least the EUV light  252  included in the light  251 . For example, the EUV light energy detector  262  may detect an energy value of the EUV light component contained in the light  251  emitted from the plasma. The detected energy value may be sent to the EUV light generation controller  5 . 
       6.3 Flowchart 
       [0124]    The overall operation of the EUV light generation system  1 C shown in  FIG. 21  will now be described with reference to the drawings.  FIGS. 22 and 23  show a flowchart of the overall operation of the EUV light generation system  1 C. Here, the flowchart in  FIGS. 22 and 23  shows the operation of the EUV light generation controller  5 . 
         [0125]    As shown in  FIG. 22 , the EUV light generation controller  5  may first stand by until it receives an exposure preparation signal from the exposure apparatus controller  61  instructing the preparation for exposure (Step S 201 ; NO). The exposure preparation signal may be inputted to the EUV light generation controller  5  in order for the EUV light generation system  1 C to be brought into a state where the exposure operation can be started immediately after receiving the burst output signal. Upon receiving the exposure preparation signal (Step S 201 ; YES), the EUV light generation controller  5  may start outputting the oscillation trigger S 1  to the laser controller  110  at a predetermined repetition rate for controlling the master oscillators  101   1  through  101   n  to oscillate with predetermined pulse energy (Step S 202 ). The laser controller  110  may then output the oscillation triggers S 3  to the master oscillators  101   1  through  101   n  at a predetermined repetition rate in accordance with the oscillation trigger S 1 . At this point, the master oscillators  101   1  through  101   n  may start oscillating at a predetermined repetition rate so as to facilitate thermal stability. The master oscillators  101   1  through  101   n  may preferably be controlled to operate under a constant operation condition. 
         [0126]    Further, the EUV light generation controller  5  may control the laser controller  110  to close the optical shutters  102   1  through  102   n  (Step S 203 ). With this, the pulse laser beams L 1   1  through L 1   n  may be blocked by the respective optical shutters  102   1  through  102   n . At this point, each of the amplifiers  120   1  through  120   n  may be brought into an operable state. Here, Step S 203  may be carried out prior to Step S 202  or simultaneously with Step S 202 . Further, the EUV light generation controller  5  may control the target controller  260  to send the target output signal to the target generator  26  for causing the target generator  26  to output a target  27  (Step S 204 ). With this, targets  27  may be outputted from the target generator  26  at a predetermined repetition rate toward the plasma generation region  25 . Here, the target generator  26  may be of a continuous-jet type configured to output targets  27  continuously at a predetermined repetition rate. Alternatively, the target generator  26  may be of an on-demand type configured to output a target  27  in accordance with an instruction from the target controller  260 . 
         [0127]    Then, the EUV light generation controller  5  may stand by until it receives a burst output signal from the exposure apparatus controller  61  for requesting a burst output of the EUV light  252  (Step S 205 ; NO). Upon receiving the burst output signal (Step S 205 ; YES), the EUV light generation controller  5  may determine whether or not it has received the EUV light energy instruction value Pte from the exposure apparatus controller  61  specifying the energy required for the EUV light  252  (Step S 206 ). When the EUV light energy instruction value Pte has been received (Step S 206 ; YES), the EUV light generation controller  5  may send a control voltage value calculation command to the laser controller  110  for causing the laser controller  110  to execute the control voltage value calculation routine (Step S 207 ). Thereafter, the EUV light generation controller  5  may proceed to Step S 208 . The laser controller  110  may execute the control voltage value calculation routine in response to the control voltage value calculation command. Here, the control voltage value calculation routine may be similar to the operation shown in  FIG. 19 . Thus, a detailed description thereof will be omitted here. 
         [0128]    On the other hand, when the EUV light energy instruction value Pte has not been received (Step S 206 ; NO), the EUV light generation controller  5  may proceed to Step S 208 . However, when the EUV light energy instruction value Pte has never been received since the EUV light generation system  1 C is started, the EUV light generation controller  5  may load the EUV light energy instruction value Pte stored in a memory (not shown) or the like, and send the control voltage value calculation command to the laser controller  110  based on the loaded EUV light energy instruction value Pte. 
         [0129]    In Step S 208 , the EUV light generation controller  5  may stand by until it receives a target detection signal from the target detector  261  (Step S 208 ; NO). Upon receiving the target detection signal (Step S 208 ; YES), the EUV light generation controller  5  may stand by until a predetermined time elapses from the reception of the target detection signal (Step S 209 ; NO). Here, the predetermined time may be a delay time for adjusting the timing at which the pulse laser beam  31  is outputted so that the detected target  27  can be irradiated by the pulse laser beam  33  in the plasma generation region  25 . The determination of whether or not the predetermined time has elapsed from the reception of the target detection signal may be made by, for example, measuring an elapsed time by a timer (not shown). Alternatively, in place of measuring the elapsed time using the timer, a delay circuit may be provided for delaying the oscillation triggers S 3  to be outputted to the master oscillators  101   1  through  101   n  in Step S 210  to follow (see  FIG. 23 ) for a predetermined time. In this case, the processing in Step S 209  may be realized using hardware. Therefore, the operation of the laser controller  110  may be simplified. 
         [0130]    When the predetermined time has elapsed after the target detection signal is received (Step S 209 ; YES), the EUV light generation controller  5  may cause the laser controller  110  to output new oscillation triggers S 3 , which are different from the oscillation triggers S 3  at the predetermined repetition rate, to the master oscillators  101   1  through  101   n  to control the master oscillators  101   1  through  101   n  to oscillate in synchronization with the target detection signals. Here, the new oscillation triggers S 3  may include information, such as amplitude and pulse width, for adjusting the output energy of the master oscillators  101   1  through  101   n  in accordance with the EUV light energy instruction value Pte. Through the operation in Step S 210 , an output of the targets  27  and an output of the pulse laser beams L 1   1  through L 1   n  from the respective master oscillators  101   1  through  101   n  may be synchronized. Then, the EUV light generation controller  5  may send an optical shutter switching command to the laser controller  110  for causing the laser controller  110  to execute the optical shutter switching routine for switching the optical shutters  102   1  through  102   n  in accordance with the control voltage values calculated in response to the control voltage value calculation command in Step S 207  (Step S 211 ). The laser controller  110  may execute the optical shutter switching routine in response to the optical shutter switching command. Here, the optical shutter switching routine may be similar to the operation shown in  FIG. 20 . Thus, a detailed description thereof will be omitted here. 
         [0131]    Subsequently, the EUV light generation controller  5  may stand by until it receives an energy detection value from the EUV light energy detector  262  (Step S 212 ; NO). Upon receiving the energy detection value (Step S 212 ; YES), the EUV light generation controller  5  may determine whether or not the energy of the detected EUV light  252  satisfies the EUV light energy instruction value Pte (Step S 213 ). When the energy of the detected EUV light  252  satisfies the EUV light energy instruction value Pte (Step S 213 ; YES), the EUV light generation controller  5  may proceed to Step S 215 . Here, the case where the energy of the detected EUV light  252  satisfies the EUV light energy instruction value Pte may mean that the energy of the detected EUV light  252  falls between predetermined upper and lower limits of the EUV light energy instruction value Pte. On the other hand, when the energy of the detected EUV light  252  does not satisfy the EUV light energy instruction value Pte (Step S 213 ; NO), the EUV light generation controller  5  may again send the control voltage value calculation command to the laser controller  110  (Step S 214 ). Thereafter, the EUV light generation controller  5  may proceed to Step S 215 . The laser controller  110  may again execute the control voltage value calculation routine in response to the control voltage value calculation command, to thereby recalculate the control voltage values of the high-voltage pulses S 4   1  through S 4   n  to be applied to the respective optical shutters  102   1  through  102   n . The recalculated control voltage values of the high-voltage pulses S 4   1  through S 4   n  may be reflected on the currently executed optical shutter switching routine. 
         [0132]    In Step S 215 , the EUV light generation controller  5  may determine whether or not it has received a burst pause signal from the exposure apparatus controller  61  for requesting the burst output of the EUV light  252  to be paused (Step S 215 ). When the burst pause request has not been received (Step S 215 ; NO), the EUV light generation controller  5  may return to Step S 206  of  FIG. 22  and repeat the subsequent steps. 
         [0133]    On the other hand, when the burst pause signal has been received (Step S 215 ; YES), the EUV light generation controller  5  may, as in Step S 202 , output the oscillation triggers S 1  to the laser controller  110  at a predetermined repetition rate to cause the master oscillators  101   1  through  101   n  to oscillate with predetermined pulse energy (Step S 216 ). The laser controller  110  may output the oscillation triggers S 3  to the master oscillators  101   1  through  101   n  at a predetermined repetition rate in accordance with the oscillation triggers S 1 . Further, the EUV light generation controller  5  may, as in Step S 203 , control the laser controller  110  to close the optical shutters  102   1  through  102   n  (Step S 217 ). With this, the pulse laser beams L 1   1  through L 1   n , may be blocked by respective the optical shutters  102   1  through  102   n . At this point, each of the amplifiers  120   1  through  120   n  may be brought into an unoperated state. Here, Step S 217  may be carried out prior to Step S 216  or simultaneously with Step S 216 . 
         [0134]    Subsequently, the EUV light generation controller  5  may determine whether or not it has been notified of the end of the exposure from the exposure apparatus controller  61  (Step S 218 ). When the end of the exposure has not been notified (Step S 218 ; NO), the EUV light generation controller  5  may return to Step S 205  of  FIG. 22  and repeat the subsequent steps. On the other hand, when the end of the exposure has been notified (Step S 218 ; YES), the EUV light generation controller  5  may stop outputting the oscillation triggers S 1  to the laser controller  110  (Step S 219 ). Further, the EUV light generation controller  5  may stop sending the target output signal to the target controller  260  (Step S 220 ). With this, the output of the pulse laser beams L 1   1  through L 1   n  from the respective master oscillators  101   1  through  101   n  and the output of the target  27  from the target generator  26  may be stopped. Thereafter, the EUV light generation controller  5  may terminate this operation. 
       7. Supplementary Descriptions 
     7.1 Variation of Optical Shutter 
       [0135]      FIG. 24  shows a variation of the above-described optical shutter  102 . As illustrated in  FIG. 24 , an optical shutter  102 A may include, for example, two reflective polarizers  102   e  and  102   f  and the Pockels cell  102   c . Even with such reflective polarizers  102   e  and  102   f , functionality similar to that of the optical shutter  102  may be achieved by operating the optical shutter  102 A similarly to the optical shutter  102  shown in  FIG. 3 . Further, when the reflective polarizers  102   e  and  102   f  are used, the optical shutter  102 A which is more resistive to a heat load may be obtained, compared to the case where the transmissive polarizers  102   a  and  102   b  are used. The reflective polarizers  102   e  and  102   f  may each be an Absorbing Thin-Film Reflector (ATFR), for example. Here, being resistive to a head load may mean that the optical shutter is less likely to be heated, or can operate more stably against a rise in temperature. 
       7.2 Regenerative Amplifier 
       [0136]    The regenerative amplifier  120   R  will now be described in detail.  FIG. 25  schematically illustrates the configuration of the regenerative amplifier  120   R . The regenerative amplifier  120   R  may include a polarization beam splitter  121 , a CO 2  gas amplification part  122 , Pockels cells  123  and  126 , a quarter-wave plate  124 , and resonator mirrors  125  and  127 . 
         [0137]    The polarization beam splitter  121  may be a thin-film polarizer, for example. The polarization beam splitter  121  may reflect the S-polarization component of a laser beam incident thereon and transmit the P-polarization component thereof. The pulse laser beam L 2  which has entered the regenerative amplifier  120   R  may first be incident on the polarization beam splitter  121  mostly as the S-polarization component and be reflected thereby. With this, the pulse laser beam L 2  may be introduced into a resonator formed by the resonator mirrors  125  and  127 . The pulse laser beam L 2  taken into the resonator may be amplified as it passes through the CO 2  gas amplification part  122 . Then, the pulse laser beam L 2  may pass through the Pockels cell  123 , to which a voltage is not applied. Further, the pulse laser beam L 2  may be transmitted through the quarter-wave plate  124 , reflected by the resonator mirror  125 , and again transmitted through the quarter-wave plate  124 , whereby the polarization direction of the pulse laser beam L 2  may be rotated by 90 degrees. 
         [0138]    The pulse laser beam L 2  may then pass through the Pockels cell  123  again, to which a voltage is not applied. At this point, a predetermined voltage may be applied to the Pockels cell  123  by a power source (not shown) after the pulse laser beam L 2  passes therethrough. The Pockels cell  123 , to which the predetermined voltage is applied, may give a quarter-wave phase shift to a laser beam passing therethrough. Thus, while the predetermined voltage is applied to the Pockels cell  123 , the polarization direction of the pulse laser beam L 2  incident on the polarization beam splitter  121  may be retained in a direction parallel to the plane of incidence, and therefore the pulse laser beam L 2  may be trapped in the resonator. 
         [0139]    Thereafter, at a timing at which the pulse laser beam L 2   a  is to be outputted, a predetermined voltage may be applied to the Pockels cell  126  by a power source (not shown). The pulse laser beam L 2  traveling back and forth in the resonator may be transmitted through the polarization beam splitter  121  and then be subjected to a quarter-wave phase shift when passing through the Pockels cell  126 . Then, the pulse laser beam L 2  may be reflected by the resonator mirror  127  and pass through the Pockets cell  126  again, to thereby be converted into a linearly-polarized laser beam that may be incident on the polarization beam splitter  121  mostly as the S-polarization component. The pulse laser beam L 2  incident on the polarization beam splitter  121  mostly as S-polarization component may be reflected by the polarization beam splitter  121 , and be outputted from the regenerative amplifier  120   R  as the pulse laser beam L 2   a . Here, controlling the duration for which the voltage is applied to the Pockels cell  126  may allow the pulse width of the pulse laser beam L 2  (or L 2   a ) to be controlled. 
       7.3 Beam Path Adjusting Unit 
       [0140]      FIG. 26  shows an example of the beam path adjusting unit  103  and an arrangement of the master oscillators  101   1  through  101   n  with respect to the beam path adjusting unit  103 . In  FIG. 26 , the optical shutters  102   1  through  102   n  are not depicted. 
         [0141]    As illustrated in  FIG. 26 , the beam path adjusting unit  103  may include a reflective grating  103   a . The master oscillators  101   1  through  101   n  may, for example, be positioned with respect to the grating  103   a  such that rays diffracted at the same order (e.g., −1st order) of the respective laser beams L 1   1  through L 1   n  from the respective master oscillators  101   1  through  101   n  are outputted from the grating  103   a  at the same angle in the same direction. The master oscillators  101   1  through  101   n  may preferably be positioned with respect to the grating  103   a  so as to satisfy Expression (1) below. In Expression (1), N is the number of grooves per unit length, λ 1  through λ n  are central wavelengths of the respective pulse laser beams L 1   1  through L 1   n , β is a diffraction angle, and α 1  through α n  are incident angles of the respective pulse laser beams L 1   1  through L 1   n . 
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         [0142]    By positioning the master oscillators  101   1  through  101   n  with respect to the reflective grating  103   a  in the above-described manner, the beam paths of the pulse laser beams L 1   1  through L 1   n  may be made to coincide with one another with ease using a compact optical element (i.e., grating  103   a ). Here, the reflective grating  103   a  has been used in this example, but a transmissive grating may be used instead. 
       7.4 Seed Laser Device Including Multi-Longitudinal Mode Master Oscillator and Spectroscope 
       [0143]    In one or more of the embodiments, when a pulse laser beam is to be subjected to the multi-line amplification, a seed laser device  100 A that includes a multi-longitudinal mode master oscillator may be used in place of the seed laser device  100 .  FIG. 27  schematically illustrates the configuration of the seed laser device  100 A. 
         [0144]    As shown in  FIG. 27 , the seed laser device  100 A may include a master oscillator  101   m , a spectroscope  103 A, the optical shutters  102   1  through  102   n , and the beam path adjusting unit  103 . The optical shutters  102   1  through  102   n  and the beam path adjusting unit  103  may be similar to the optical shutters  102   1  through  102   n  and the beam path adjusting unit  103  shown in  FIG. 2 . 
         [0145]    The reflective grating  103   a  shown in  FIG. 26  may be used as the spectroscope  103 A. However, this disclosure is not limited thereto, and a transmissive grating or the like may be used instead. Further, when the grating  103   a  is used as the spectroscope  103 A, the spectroscope  103 A may further include an optical system, such as a mirror, for adjusting the beam paths (output directions) of the diffracted rays. 
         [0146]    The master oscillator  101   m  may, for example, output a multi-longitudinal mode laser beam L 1   m  at wavelengths contained in at least two of the amplification lines of the amplifier  120 . The spectroscope  103 A may split the pulse laser beam L 1   m  into the pulse laser beams L 1  through L 1   n  for respective longitudinal modes (wavelengths). The optical shutters  102   1  through  102   n  may be provided in beam paths of the respective pulse laser beams L 1   1  through L 1   n  which have been split by and outputted from the spectroscope  103 A. The pulse laser beams L 2   1  through L 2   n  transmitted through the respective optical shutters  102   1  through  102   n  may then enter the beam path adjusting unit  103 . The beam path adjusting unit  103  may make the beam paths of the pulse laser beams L 2   1  through L 2   n  substantially coincide with one another and be outputted as the pulse laser beam L 2 . 
         [0147]    The above-described embodiments and the modifications thereof are merely examples for implementing this disclosure, and this disclosure is not limited thereto. Making various modifications according to the specifications or the like is within the scope of this disclosure, and other various embodiments are possible within the scope of this disclosure. For example, the modifications illustrated for particular embodiments may be applied to other embodiments as well (including the other embodiments described herein). 
         [0148]    The terms used in this specification and the appended claims should be interpreted as “non-limiting.” For example, the terms “include” and “be included” should be interpreted as “including the stated elements but not limited to the stated elements.” The term “have” should be interpreted as “having the stated elements but not limited to the stated elements.” Further, the modifier “one (a/an)” should be interpreted as “at least one” or “one or more.”