Patent Publication Number: US-2023155343-A1

Title: Laser apparatus, wavelength control method, and electronic device manufacturing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Application No. PCT/JP2020/032816, filed on Aug. 31, 2020, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a laser apparatus, a wavelength control method, and an electronic device manufacturing method. 
     2. Related Art 
     Recently, in a semiconductor exposure apparatus, resolving power improvement has been requested along with miniaturization and high integration of a semiconductor integrated circuit. Hereinafter, the semiconductor exposure apparatus is simply referred to as an “exposure apparatus”. Thus, the wavelength of light output from an exposure light source has been shortened. Examples of a gas laser apparatus for exposure include a KrF excimer laser apparatus configured to output a laser beam having a wavelength of 248 nm approximately and an ArF excimer laser apparatus configured to output a laser beam having a wavelength of 193 nm approximately. 
     The KrF excimer laser apparatus and the ArF excimer laser apparatus have a wide spectrum line width of 350 pm to 400 pm for spontaneous oscillation light. Thus, chromatic aberration is caused by a projection lens of the exposure apparatus in some cases. As a result, resolving power potentially decreases. Thus, the spectrum line width of a laser beam output from the gas laser apparatus needs to be narrowed until chromatic aberration becomes negligible. To narrow the spectrum line width, a line narrowing module including a line narrowing element is provided in a laser resonator of the gas laser apparatus in some cases. The line narrowing element includes, for example, an etalon or a grating. Such a gas laser apparatus that achieves narrowing of the spectrum line width is referred to as a line narrowed gas laser apparatus. 
     LIST OF DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: US Patent Application Publication No. 2011/0116522 
         Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-269628 
       
    
     SUMMARY 
     A laser apparatus according to an aspect of the present disclosure includes a first optical element and a second optical element, a first actuator configured to change a first wavelength component included in a pulse laser beam by changing a posture of the first optical element, a second actuator configured to change a second wavelength component included in the pulse laser beam by changing a posture of the second optical element, a first encoder configured to measure a position of the first actuator, a second encoder configured to measure a position of the second actuator, and a processor. The processor reads a first relation between the position of the first actuator and the first wavelength component and a second relation between the position of the second actuator and the second wavelength component and performs control of the first actuator based on the first relation and the position of the first actuator measured by the first encoder and control of the second actuator based on the second relation and the position of the second actuator measured by the second encoder. 
     A wavelength control method according to an aspect of the present disclosure is a wavelength control method for a laser apparatus including a first optical element and a second optical element, a first actuator configured to change a first wavelength component included in a pulse laser beam by changing a posture of the first optical element, a second actuator configured to change a second wavelength component included in the pulse laser beam by changing a posture of the second optical element, a first encoder configured to measure a position of the first actuator, a second encoder configured to measure a position of the second actuator, and a processor. The processor reads a first relation between the position of the first actuator and the first wavelength component and a second relation between the position of the second actuator and the second wavelength component and performs control of the first actuator based on the first relation and the position of the first actuator measured by the first encoder and control of the second actuator based on the second relation and the position of the second actuator measured by the second encoder. 
     An electronic device manufacturing method according to an aspect of the present disclosure includes generating a pulse laser beam with a laser apparatus, the laser apparatus including a first optical element and a second optical element, a first actuator configured to change a first wavelength component included in the pulse laser beam by changing a posture of the first optical element, a second actuator configured to change a second wavelength component included in the pulse laser beam by changing a posture of the second optical element, a first encoder configured to measure a position of the first actuator, a second encoder configured to measure a position of the second actuator, and a processor configured to read a first relation between the position of the first actuator and the first wavelength component and a second relation between the position of the second actuator and the second wavelength component and perform control of the first actuator based on the first relation and the position of the first actuator measured by the first encoder and control of the second actuator based on the second relation and the position of the second actuator measured by the second encoder, outputting the pulse laser beam to an exposure apparatus, and exposing a photosensitive substrate to the pulse laser beam in the exposure apparatus to manufacture an electronic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure will be described below as examples with reference to the accompanying drawings. 
         FIG.  1    schematically illustrates the configuration of an exposure system in a comparative example. 
         FIG.  2    schematically illustrates the configuration of the exposure system in the comparative example. 
         FIG.  3    schematically illustrates the configuration of a line narrowing device in the comparative example. 
         FIG.  4    schematically illustrates the configuration of the line narrowing device in the comparative example. 
         FIG.  5    schematically illustrates the configuration of an optical meter in the comparative example. 
         FIG.  6    is a graph illustrating an exemplary result of interference fringe measurement by a spectroscopic sensor included in the optical meter. 
         FIG.  7    is a graph illustrating an exemplary result of interference fringe measurement when a pulse laser beam of a wavelength λ1 and a pulse laser beam of a wavelength λ2 are separately incident on the spectroscopic sensor. 
         FIG.  8    is a graph illustrating an exemplary result of interference fringe measurement when the pulse laser beam of the wavelength λ1 and the pulse laser beam of the wavelength λ2 are simultaneously incident on the spectroscopic sensor. 
         FIG.  9    schematically illustrates the configuration of a line narrowing device in a first embodiment. 
         FIG.  10    schematically illustrates the configuration of the line narrowing device in the first embodiment. 
         FIG.  11    illustrates disposition of a parallel plane substrate when the entire optical beam is incident as a first part on a grating in the first embodiment. 
         FIG.  12    illustrates disposition of the parallel plane substrate when the entire optical beam is incident as a second part on the grating in the first embodiment. 
         FIG.  13    illustrates an exemplary configuration of a first rotation stage and an encoder in the first embodiment. 
         FIG.  14    is a graph illustrating an exemplary spectrum waveform of a pulse laser beam output from a laser apparatus in the first embodiment. 
         FIG.  15    is a flowchart illustrating operation of a laser control processor in the first embodiment. 
         FIG.  16    is a flowchart illustrating calibration processing of the wavelength λ1 in the first embodiment. 
         FIG.  17    is a flowchart illustrating calibration processing of the wavelength λ2 in the first embodiment. 
         FIG.  18    is a flowchart illustrating calibration processing of an intensity ratio R in the first embodiment. 
         FIG.  19    is a graph illustrating an exemplary result of interference fringe measurement when the pulse laser beams of the wavelengths λ1 and λ2 having a wavelength difference equal to half of a free-spectral range are separately incident on the spectroscopic sensor. 
         FIG.  20    is a graph illustrating an exemplary result of interference fringe measurement when the pulse laser beams of the wavelengths λ1 and λ2 having a wavelength difference equal to half of the free-spectral range are simultaneously incident on the spectroscopic sensor, and illustrating an exemplary result of interference fringe measurement when an intensity ratio defined as Eλ1/(Eλ1+Eλ2) is 1/2. 
         FIG.  21    is a graph illustrating an exemplary result of interference fringe measurement when pulse laser beams of the wavelengths λ1 and λ2 having a wavelength difference equal to half of the free-spectral range are simultaneously incident on the spectroscopic sensor, and illustrating an exemplary result of interference fringe measurement when the intensity ratio defined as Eλ1/(Eλ1+Eλ2) is 10/11. 
         FIG.  22    illustrates an exemplary first relation acquired through calibration of the wavelength λ1. 
         FIG.  23    illustrates an exemplary second relation acquired through calibration of the wavelength λ2. 
         FIG.  24    illustrates an exemplary third relation acquired through calibration of the intensity ratio R. 
         FIG.  25    is a flowchart illustrating outputting processing of exposure light in the first embodiment. 
         FIG.  26    is a flowchart illustrating operation of a laser control processor in a second embodiment. 
         FIG.  27    is a flowchart illustrating calibration processing of a third relation RY(f, V) in the second embodiment. 
         FIG.  28    illustrates exemplary data stored in a memory by calibration of the third relation RY(f, V). 
         FIG.  29    is a flowchart illustrating outputting processing of exposure light in the second embodiment. 
         FIG.  30    is a flowchart illustrating operation of a laser control processor in a third embodiment. 
         FIG.  31    is a flowchart illustrating setting processing of a position Y of a linear stage in the third embodiment. 
         FIG.  32    is a flowchart illustrating outputting processing of exposure light in the third embodiment. 
         FIG.  33    schematically illustrates the configuration of a line narrowing device in a fourth embodiment. 
         FIG.  34    schematically illustrates the configuration of the line narrowing device in the fourth embodiment. 
         FIG.  35    schematically illustrates the configuration of a line narrowing device in a first modification of the fourth embodiment. 
         FIG.  36    schematically illustrates the configuration of the line narrowing device in the first modification of the fourth embodiment. 
         FIG.  37    schematically illustrates the configuration of a line narrowing device in a second modification of the fourth embodiment. 
         FIG.  38    schematically illustrates the configuration of the line narrowing device in the second modification of the fourth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Contents 
     1. Comparative example
         1.1 Exposure system   1.2 Exposure apparatus  200 
           1.2.1 Configuration   1.2.2 Operation   
           1.3 Laser apparatus  100 
           1.3.1 Configuration   1.3.2 Operation   
           1.4 Line narrowing device  14 
           1.4.1 Configuration
               1.4.1.1 Prisms  41  and  42     1.4.1.2 Grating system  50     
               1.4.2 Operation   
           1.5 Optical meter  17 
           1.5.1 Configuration   1.5.2 Operation   
           1.6 Problem with comparative example
 
2. Laser apparatus configured to adjust wavelength by controlling rotation stage based on encoder output
   2.1 Configuration   2.2 Operation   2.3 Example of encoder   2.4 Example of intensity ratio R   2.5 Operation of laser control processor  130 
           2.5.1 Main flow   2.5.2 Wavelength λ1 calibration   2.5.3 Wavelength λ2 calibration   2.5.4 Intensity ratio R calibration   2.5.5 Exposure light outputting   
           2.6 Effect
 
3. Laser apparatus configured to select third relation RY in accordance with repetition frequency f and charging voltage V
   3.1 Main flow   3.2 Third relation RY(f, V) calibration   3.3 Exposure light outputting   3.4 Effect   3.5 Other exemplary configuration
 
4. Laser apparatus configured to adjust intensity ratio R without using third relation RY
   4.1 Main flow   4.2 Setting of position Y of linear stage  612     4.3 Exposure light outputting   4.4 Effect
 
5. Line narrowing device  14   d  in which a plurality of prisms  43  and  44  are disposed in XZ plane
   5.1 Configuration   5.2 Operation   5.3 First modification   5.4 Second modification       

     6. Other 
     Embodiments of the present disclosure will be described below in detail with reference to the accompanying drawings. The embodiments described below are examples of the present disclosure, and do not limit the contents of the present disclosure. Not all configurations and operations described in each embodiment are necessarily essential as configurations and operations of the present disclosure. Components identical to each other are denoted by an identical reference sign, and duplicate description thereof will be omitted. 
     1. COMPARATIVE EXAMPLE 
     1.1 Exposure System 
       FIGS.  1  and  2    schematically illustrate the configuration of an exposure system in a comparative example. The comparative example of the present disclosure is an example that the applicant recognizes as known only by the applicant, but is not a publicly known example that is recognized by the applicant. 
     The exposure system includes a laser apparatus  100  and an exposure apparatus  200 . In  FIG.  1   , the laser apparatus  100  is simplified. In  FIG.  2   , the exposure apparatus  200  is simplified. 
     The laser apparatus  100  includes a laser control processor  130 . The laser apparatus  100  outputs a pulse laser beam toward the exposure apparatus  200 . 
     1.2 Exposure Apparatus  200   
     1.2.1 Configuration 
     As illustrated in  FIG.  1   , the exposure apparatus  200  includes an illumination optical system  201 , a projection optical system  202 , and an exposure control processor  210 . 
     The illumination optical system  201  illuminates a reticle pattern of a non-illustrated reticle disposed on a reticle stage RT with the pulse laser beam incident from the laser apparatus  100 . 
     The pulse laser beam having transmitted through the reticle is imaged on a non-illustrated workpiece disposed on a workpiece table WT by reduced projection through the projection optical system  202 . The workpiece is a photosensitive substrate such as a semiconductor wafer to which a resist film is applied. 
     The exposure control processor  210  is a processing device including a memory  212  in which a control program is stored and a central processing unit (CPU)  211  configured to execute the control program. The exposure control processor  210  is specially configured or programmed to execute various kinds of processing included in the present disclosure. The exposure control processor  210  collectively controls the exposure apparatus  200  and transmits and receives various kinds of data and various signals to and from the laser control processor  130 . 
     1.2.2 Operation 
     The exposure control processor  210  sets various parameters related to exposure conditions and controls the illumination optical system  201  and the projection optical system  202 . 
     The exposure control processor  210  transmits data of a target wavelength, data of a pulse energy target value, and a trigger signal to the laser control processor  130 . The laser control processor  130  controls the laser apparatus  100  in accordance with those data and signals. 
     The exposure control processor  210  translates the reticle stage RT and the workpiece table WT in directions opposite to each other in synchronization. Accordingly, the workpiece is exposed to the pulse laser beam reflected by the reticle pattern. 
     Through such an exposure process, the reticle pattern is transferred to the semiconductor wafer. Thereafter, an electronic device can be manufactured through a plurality of processes. 
     1.3 Laser Apparatus  100   
     1.3.1 Configuration 
     As illustrated in  FIG.  2   , the laser apparatus  100  includes a laser chamber  10 , a charger  12 , a pulse power module (PPM)  13 , a line narrowing device  14 , an output coupling mirror  15 , an optical meter  17 , and a shutter  18  in addition to the laser control processor  130 . The line narrowing device  14  and the output coupling mirror  15  constitute an optical resonator. 
     The laser chamber  10  is disposed on the optical path of the optical resonator. The laser chamber  10  is provided with windows  10   a  and  10   b.    
     The laser chamber  10  includes a pair of electrodes  11   a  and  11   b  inside and houses laser gas as a laser medium. The laser medium is, for example, F 2 , ArF, KrF, XeCl, or XeF. 
     The charger  12  stores electric energy to be supplied to the pulse power module  13 . The pulse power module  13  includes a switch  13   a.    
     The line narrowing device  14  includes wavelength selection elements such as prisms  41  and  42  and gratings  51  and  52  to be described later. The line narrowing device  14  is called a line narrowing module in some cases. 
     The output coupling mirror  15  is a partially reflective mirror. 
     The optical meter  17  includes a beam splitter  17   a  and a sensor unit  17   b . The beam splitter  17   a  is disposed on the optical path of a pulse laser beam output from the output coupling mirror  15 . The beam splitter  17   a  transmits part of the pulse laser beam at high transmittance and reflects another part of the pulse laser beam into the sensor unit  17   b . The sensor unit  17   b  includes an energy sensor  17   e  and spectroscopic sensors  17   f  and  17   g  to be described later with reference to  FIG.  5   . 
     The shutter  18  is disposed on the optical path of the pulse laser beam having transmitted through the beam splitter  17   a . When the shutter  18  is closed, the pulse laser beam having transmitted through the beam splitter  17   a  is cut off not to enter the exposure apparatus  200 . When the shutter  18  is opened, the pulse laser beam having transmitted through the beam splitter  17   a  is not cut off but enters the exposure apparatus  200 . 
     The laser control processor  130  is a processing device including a memory  132  in which a control program is stored and a CPU  131  configured to execute the control program. The laser control processor  130  is specially configured or programmed to execute various kinds of processing included in the present disclosure. The memory  132  corresponds to a storage device in the present disclosure. 
     1.3.2 Operation 
     The laser control processor  130  acquires data of the target wavelength from the exposure control processor  210 . The laser control processor  130  transmits an initial setting signal to the line narrowing device  14  based on the target wavelength. After the output of the pulse laser beam is started, the laser control processor  130  receives wavelength measured data from the optical meter  17  and transmits a feedback control signal to the line narrowing device  14  based on the target wavelength and the wavelength measured data. 
     The laser control processor  130  acquires data of the pulse energy target value from the exposure control processor  210 . The laser control processor  130  transmits an initial setting signal of a charging voltage V to the charger  12  based on the pulse energy target value. After the output of the pulse laser beam is started, the laser control processor  130  receives pulse energy measured data from the optical meter  17  and transmits a feedback control signal of the charging voltage V to the charger  12  based on the pulse energy target value and the pulse energy measured data. 
     The laser control processor  130  receives the trigger signal from the exposure control processor  210 . The laser control processor  130  transmits an oscillation trigger signal based on the trigger signal to the switch  13   a  of the pulse power module  13 . 
     When having received the oscillation trigger signal from the laser control processor  130 , the switch  13   a  is turned on. When the switch  13   a  is turned on, the pulse power module  13  generates high voltage in pulses from the electric energy stored in the charger  12 . The pulse power module  13  applies the high voltage to the electrodes  11   a  and  11   b.    
     When the high voltage is applied to the electrodes  11   a  and  11   b , discharging occurs between the electrodes  11   a  and  11   b . The laser gas in the laser chamber  10  is excited by energy of the discharging and transitions to a higher energy level. Thereafter, when transitioning to a lower energy level, the excited laser gas discharges light of a wavelength in accordance with the difference between the energy levels. 
     The light generated in the laser chamber  10  is output from the laser chamber  10  through the windows  10   a  and  10   b . The light output through the window  10   a  is incident as an optical beam on the line narrowing device  14 . Light having a wavelength near a desired wavelength in the light incident on the line narrowing device  14  is reflected by the line narrowing device  14  and returned to the laser chamber  10 . 
     The output coupling mirror  15  transmits and outputs part of the light output out through the window  10   b  and reflects and returns another part thereof to the laser chamber  10 . 
     In this manner, light output from the laser chamber  10  reciprocates between the line narrowing device  14  and the output coupling mirror  15 . The light is amplified each time the light passes through a discharge space between the pair of electrodes  11   a  and  11   b . In this manner, light provided with laser oscillation and line narrowing is output as a pulse laser beam from the output coupling mirror  15 . 
     The pulse laser beam output from the laser apparatus  100  enters the exposure apparatus  200 . 
     1.4 Line Narrowing Device  14   
     1.4.1 Configuration 
       FIGS.  3  and  4    schematically illustrate the configuration of the line narrowing device  14  in the comparative example. An X axis, a Y axis, and a Z axis that are orthogonal to one another are illustrated in each of  FIGS.  3  and  4   .  FIG.  3    illustrates the line narrowing device  14  viewed in the positive Y direction, and  FIG.  4    illustrates the line narrowing device  14  viewed in the negative X direction. The positive Y direction and the negative Y direction match directions in which the electrodes  11   a  and  11   b  (refer to  FIG.  2   ) face each other. The negative Z direction matches the traveling direction of an optical beam output through the window  10   a . The positive Z direction matches the traveling direction of a pulse laser beam output from the window  10   b  and then output through the output coupling mirror  15 . 
     The line narrowing device  14  includes the prisms  41  and  42  and a grating system  50 . 
     1.4.1.1 Prisms  41  and  42   
     The prism  41  is disposed on the optical path of the optical beam output through the window  10   a . The prism  41  is supported by a holder  411 . 
     The prism  42  is disposed on the optical path of the optical beam having passed through the prism  41 . The prism  42  is supported by a holder  421 . 
     The prisms  41  and  42  are disposed such that surfaces of the prisms  41  and  42  that the optical beam is incident on and output from are all parallel to the Y axis. 
     1.4.1.2 Grating System  50   
     The grating system  50  includes the gratings  51  and  52 . The gratings  51  and  52  are disposed at positions different from each other in the direction of the Y axis on the optical path of the optical beam having passed through the prism  42 . The direction of grooves of the gratings  51  and  52  matches the direction of the Y axis. The positions of the gratings  51  and  52  are set such that the optical beam having passed through the prism  42  is incident across the gratings  51  and  52 . 
     The gratings  51  and  52  are supported by a holder  511 . The grating  51  is rotatable about an axis parallel to the Y axis by a first rotation stage  512 . The grating  52  is rotatable about an axis parallel to the Y axis by a second rotation stage  522 . 
     1.4.2 Operation 
     The traveling direction of the optical beam output through the window  10   a  is changed in a plane parallel to an XZ plane that is orthogonal to the Y axis by each of the prisms  41  and  42 , and the beam width of the optical beam is expanded in the plane parallel to the XZ plane. The traveling direction of the optical beam having passed through both the prisms  41  and  42  and traveling toward the gratings  51  and  52  substantially matches the negative Z direction, for example. 
     The light incident on the gratings  51  and  52  from the prism  42  is reflected by the grooves of each of the gratings  51  and  52  and is diffracted in a direction in accordance with the wavelength of the light. Accordingly, the light reflected by the grooves of each of the gratings  51  and  52  is dispersed in a plane parallel to the XZ plane. The grating  51  is disposed in Littrow arrangement such that the incident angle of the optical beam incident on the grating  51  from the prism  42  matches the diffracting angle of diffracted light of a desired wavelength λ1. The grating  52  is disposed in Littrow arrangement such that the incident angle of the optical beam incident on the grating  52  from the prism  42  matches the diffracting angle of diffracted light of a desired wavelength λ2. When the incident angles of the optical beams incident on the gratings  51  and  52  from the prism  42  are different from each other, a wavelength difference occurs between the wavelength λ1 of the diffracted light returned from the grating  51  to the prism  42  and the wavelength λ2 of the diffracted light returned from the grating  52  to the prism  42 . 
     Although only optical beams in a direction from the prism  41  to the gratings  51  and  52  are illustrated with dashed line arrows in  FIGS.  3  and  4   , an optical beam of a wavelength selected by the line narrowing device  14  travels from the gratings  51  and  52  toward the prism  41  through paths opposite to the dashed line arrows. 
     The prisms  42  and  41  reduce the beam width of light returned from the gratings  51  and  52  in a plane parallel to the XZ plane and return the light into the laser chamber  10  through the window  10   a.    
     The first rotation stage  512  and the second rotation stage  522  are controlled by the laser control processor  130 . 
     When the first rotation stage  512  slightly rotates the grating  51 , the incident angle of the optical beam incident on the grating  51  from the prism  42  slightly changes. As a result, the wavelength λ1 changes. 
     When the second rotation stage  522  slightly rotates the grating  52 , the incident angle of the optical beam incident on the grating  52  from the prism  42  slightly changes. As a result, the wavelength λ2 changes. 
     The exposure control processor  210  transmits the values of a target wavelength λ1t of the wavelength λ1 and a target wavelength λ2t of the wavelength λ2 to the laser control processor  130 . The wavelengths λ1 and λ2 are wavelengths with which, for example, images are formed at two positions on the upper and bottom surfaces, respectively, of the resist film applied to the semiconductor wafer. 
     The laser control processor  130  controls the first rotation stage  512  based on the target wavelength λ1t. Accordingly, the first rotation stage  512  changes the posture of the grating  51 , thereby adjusting the incident angle of the optical beam on the grating  51 . 
     The laser control processor  130  controls the second rotation stage  522  based on the target wavelength λ2t. Accordingly, the second rotation stage  522  changes the posture of the grating  52 , thereby adjusting the incident angle of the optical beam on the grating  52 . 
     With the above-described configuration and operation, light of the wavelength λ1 and light of the wavelength λ2 in an optical beam output through the window  10   a  of the laser chamber  10  are selected and returned into the laser chamber  10 . Accordingly, the laser apparatus  100  can perform two-wavelength oscillation. The wavelengths λ1 and λ2 may be separately set by controlling the first rotation stage  512  and the second rotation stage  522 . 
     A pulse laser beam subjected to the two-wavelength oscillation and output from the laser apparatus  100  includes two wavelength components of the wavelengths λ1 and λ2. The pulse laser beam includes pulses in which a first pulse laser beam having the wavelength λ1 and a second pulse laser beam having the wavelength λ2 temporally and spatially overlap. Alternatively, the first pulse laser beam having the wavelength λ1 and the second pulse laser beam having the wavelength λ2 may temporally overlap but not spatially overlap. 
     A focal length in the exposure apparatus  200  (refer to  FIG.  1   ) depends on the wavelength of a pulse laser beam. A pulse laser beam subjected to the two-wavelength oscillation and output from the laser apparatus  100  can be imaged at two different positions in the direction of the optical path axis of the pulse laser beam on the workpiece table WT of the exposure apparatus  200 , and a focal point depth can be increased in effect. For example, even when a resist film having a large film thickness is exposed, it is possible to maintain imaging performance in the thickness direction of the resist film. 
     1.5 Optical Meter  17   
     1.5.1 Configuration 
       FIG.  5    schematically illustrates the configuration of the optical meter  17  in the comparative example. The sensor unit  17   b  included in the optical meter  17  includes beam splitters  17   c  and  17   d , the energy sensor  17   e , and the spectroscopic sensors  17   f  and  17   g . The beam splitter  17   c  is disposed on the optical path of a pulse laser beam reflected by the beam splitter  17   a , reflects part of the pulse laser beam, and transmits another part thereof. 
     The energy sensor  17   e  is disposed on the optical path of the pulse laser beam reflected by the beam splitter  17   c . The energy sensor  17   e  includes a diffusion plate  17   h  and a photodiode  17   i.    
     The beam splitter  17   d  is disposed on the optical path of the pulse laser beam having transmitted through the beam splitter  17   c , reflects part of the pulse laser beam, and transmits another part thereof. 
     The spectroscopic sensor  17   f  is disposed on the optical path of the pulse laser beam reflected by the beam splitter  17   d . The spectroscopic sensor  17   f  includes a diffusion plate  17   j , an etalon  17   k , a light condensing lens  17   m , and an image sensor  17   n.    
     The spectroscopic sensor  17   g  is disposed on the optical path of the pulse laser beam having transmitted through the beam splitter  17   d . The spectroscopic sensor  17   g  includes a diffusion plate  17   o , an etalon  17   p , a light condensing lens  17   q , and an image sensor  17   r.    
     1.5.2 Operation 
     In the energy sensor  17   e , the pulse laser beam having transmitted through the diffusion plate  17   h  is incident on the photodiode  17   i . The photodiode  17   i  generates current in accordance with the light intensity of the pulse laser beam. The pulse energy of the pulse laser beam is obtained by integrating the current for a time corresponding to one pulse of the pulse laser beam. 
     In the spectroscopic sensor  17   f , the pulse laser beam having transmitted through the diffusion plate  17   j , the etalon  17   k , and the light condensing lens  17   m  in the stated order forms an interference fringe on the image sensor  17   n.    
       FIG.  6    is a graph illustrating an exemplary result of interference fringe measurement by the spectroscopic sensor  17   f  included in the optical meter  17 . In  FIG.  6    and  FIGS.  7 ,  8 , and  19  to  21    to be described later, the horizontal axis represents a radius L of the interference fringe, and the vertical axis represents light intensity I. Units on the axes are optional. The central wavelength of the pulse laser beam can be calculated from a value on the L axis at which the light intensity I has a peak. 
     Operation of the spectroscopic sensor  17   g  is the same as operation of the spectroscopic sensor  17   f . In each of the spectroscopic sensors  17   g  and  17   f , the radius L of the interference fringe periodically changes in accordance with wavelength change of the pulse laser beam. The range of wavelength change corresponding to one period of the periodic change is referred to as a free-spectral range (FSR). When the free-spectral range of the spectroscopic sensor  17   g  is larger than the free-spectral range of the spectroscopic sensor  17   f , wavelength measurement in a wide range can be performed by the spectroscopic sensor  17   g  and wavelength measurement at high accuracy can be performed by the spectroscopic sensor  17   f . These wavelength measurements can be combined to perform wavelength measurement in a wide range at high accuracy. 
     1.6 Problem with Comparative Example 
       FIG.  7    is a graph illustrating an exemplary result of interference fringe measurement when a pulse laser beam of the wavelength λ1 and a pulse laser beam of the wavelength λ2 are separately incident on the spectroscopic sensor  17   f . When the wavelengths λ1 and λ2 are close to each other but separately measured, the wavelengths can be accurately measured by specifying peaks of the light intensity I. 
       FIG.  8    is a graph illustrating an exemplary result of interference fringe measurement when the pulse laser beam of the wavelength λ1 and the pulse laser beam of the wavelength λ2 are simultaneously incident on the spectroscopic sensor  17   f . The waveform illustrated in  FIG.  8    is substantially equivalent to a synthesis waveform of the waveform of the interference fringe of the wavelength λ1 and the waveform of the interference fringe of the wavelength λ2 in  FIG.  7   . When the wavelengths λ1 and λ2 are close to each other, it is difficult to specify peaks of the light intensity I based on the measurement result illustrated in  FIG.  8    in some cases. In such a case, it is difficult to measure the wavelengths λ1 and λ2. 
     In embodiments described below, encoders  513  and  523  are attached to the first rotation stage  512  that changes the wavelength λ1 and the second rotation stage  522  that changes the wavelength λ2, respectively. A first relation λ1θ1 between a position θ1 of the first rotation stage  512  and the wavelength λ1 and a second relation λ2θ2 between a position θ 2  of the second rotation stage  522  and the wavelength λ2 are stored in the memory  132 . The first rotation stage  512  is controlled based on the first relation λ1θ1 and the position θ 1  of the first rotation stage  512  measured by the encoder  513 , and the second rotation stage  522  is controlled based on the second relation λ2θ2 and the position θ 2  of the second rotation stage  522  measured by the encoder  523 . 
     2. LASER APPARATUS CONFIGURED TO ADJUST WAVELENGTH BY CONTROLLING ROTATION STAGE BASED ON ENCODER OUTPUT 
     2.1 Configuration 
       FIGS.  9  and  10    schematically illustrate the configuration of a line narrowing device  14   a  in a first embodiment.  FIG.  9    illustrates the line narrowing device  14   a  viewed in the positive Y direction, and  FIG.  10    illustrates the line narrowing device  14   a  viewed in the negative X direction. 
     The line narrowing device  14   a  includes a grating system  50   a  in place of the grating system  50 . The line narrowing device  14   a  includes a beam separation optical system  60   a  between the prism  42  and the grating system  50   a.    
     In the grating system  50   a , the encoder  513  is attached to the first rotation stage  512  of the grating  51 . The encoder  523  is attached to the second rotation stage  522  of the grating  52 . The first rotation stage  512  and the encoder  513  are separated from each other in  FIG.  10    but may be integrated as described later with reference to  FIG.  13   . This is the same for the second rotation stage  522  and the encoder  523 . 
     In the first embodiment, the grating  51  corresponds to a first optical element in the present disclosure, and the grating  52  corresponds to a second optical element in the present disclosure. The first rotation stage  512  corresponds to a first actuator in the present disclosure, and the second rotation stage  522  corresponds to a second actuator in the present disclosure. The encoder  513  corresponds to a first encoder in the present disclosure, and the encoder  523  corresponds to a second encoder in the present disclosure. 
     The beam separation optical system  60   a  includes a parallel plane substrate  61 . 
     The parallel plane substrate  61  can be disposed at a position partially overlapping a section of the optical path of the optical beam having passed through the prism  42 . The parallel plane substrate  61  is supported by a holder  611 . The parallel plane substrate  61  can be moved in the negative Y direction and the positive Y direction by a linear stage  612 . An encoder  613  is attached to the linear stage  612 . 
     In the first embodiment, the linear stage  612  corresponds to a third actuator in the present disclosure. The encoder  613  corresponds to a third encoder in the present disclosure. 
     The parallel plane substrate  61  has an incident surface  615  on which part of the optical beam having passed through the prism  42  is incident, and an output surface  614  through which the light incident on the parallel plane substrate  61  through the incident surface  615  is output from inside the parallel plane substrate  61  toward the grating  52  (refer to  FIG.  10   ). The incident surface  615  and the output surface  614  are both parallel to the X axis and tilted relative to the Y axis. The incident surface  615  and the output surface  614  are parallel to each other. 
     The parallel plane substrate  61  also has an end face  616  on the positive Y side. The end face  616  has an acute angle relative to the output surface  614 . The end face  616  may be parallel to the XZ plane. 
     2.2 Operation 
     As illustrated in  FIG.  10   , a first part B1 of the optical beam having passed through the prism  42  passes outside the parallel plane substrate  61  and is incident on the grating  51 , and a second part B2 of the optical beam transmits inside the parallel plane substrate  61  and is incident on the grating  52 . In this case, the parallel plane substrate  61  shifts the optical path axis of the second part B2 of the optical beam in the negative Y direction relative to the optical path axis of the first part B1. 
     The amount of light incident on the grating  52  increases as the linear stage  612  moves the parallel plane substrate  61  in the positive Y direction to increase the second part B2 of the optical beam, which is incident on the parallel plane substrate  61 . 
     The amount of light incident on the grating  52  decreases as the linear stage  612  moves the parallel plane substrate  61  in the negative Y direction to decrease the second part B2 of the optical beam, which is incident on the parallel plane substrate  61 . 
     In this manner, the proportion between the first part B1 and the second part B2 changes and an intensity ratio R between the wavelengths λ1 and λ2 changes as the linear stage  612  changes the position of the parallel plane substrate  61  in the direction of the Y axis. 
     The direction in which the parallel plane substrate  61  is moved by the linear stage  612  may be different from the direction of the Y axis. The linear stage  612  may move the parallel plane substrate  61  in a direction intersecting the XZ plane orthogonal to the Y axis. 
     The exposure control processor  210  transmits the target wavelength λ1t, the target wavelength λ2t, and a target intensity ratio Rt to the laser control processor  130 . 
     The laser control processor  130  controls the first rotation stage  512  based on the target wavelength λ1t. Accordingly, the first rotation stage  512  changes the posture of the grating  51 , thereby adjusting the wavelength λ1 selected by the grating  51 . 
     The laser control processor  130  controls the second rotation stage  522  based on the target wavelength λ2t. Accordingly, the second rotation stage  522  changes the posture of the grating  52 , thereby adjusting the wavelength λ2 selected by the grating  52 . 
     The laser control processor  130  controls the linear stage  612  based on the target intensity ratio Rt. Accordingly, the linear stage  612  adjusts the position of the parallel plane substrate  61 , thereby adjusting the intensity ratio R between the wavelengths λ1 and λ2. 
       FIG.  11    illustrates disposition of the parallel plane substrate  61  when the entire optical beam is incident as the first part B1 on the grating  51  in the first embodiment. The linear stage  612  may set the proportion of the second part B2 to the first part B1 to zero by retracting the parallel plane substrate  61  from the optical path of the optical beam. 
       FIG.  12    illustrates disposition of the parallel plane substrate  61  when the entire optical beam is incident as the second part B2 on the grating  52  in the first embodiment. The linear stage  612  may set the proportion of the first part B1 to the second part B2 to zero by disposing the parallel plane substrate  61  at a position that the entire optical beam passes. 
     2.3 Example of encoder  FIG.  13    illustrates an exemplary configuration of the first rotation stage  512  and the encoder  513  in the first embodiment. The first rotation stage  512  and the encoder  513  may be housed in a common housing  519 . 
     The first rotation stage  512  includes a rotor  514  and a stator  515 . The rotor  514  is fixed to a rotation shaft A supported to a bearing  516  and rotates with the rotation shaft A. Although details of the rotor  514  and the stator  515  are not illustrated, for example, the rotor  514  includes a permanent magnet and the stator  515  includes an electromagnet. The rotor  514  rotates as a magnetic field generated by the electromagnet of the stator  515  is switched. 
     The encoder  513  includes a circular disk  517  and a measurement head  518 . The circular disk  517  is fixed to the rotation shaft A and rotates with the rotation shaft A. Although details of the circular disk  517  and the measurement head  518  are not illustrated, the circular disk  517  includes a hologram diffracting lattice, for example. The measurement head  518  includes, for example, a laser diode, an optical system in which positive first-order diffracted light and negative first-order diffracted light of light output from the laser diode, which are generated through the hologram diffracting lattice, interfere with each other, and a photo detector configured to measure the interference light. 
     As the rotation shaft A is rotated by the first rotation stage  512 , the circular disk  517  rotates and the hologram diffracting lattice moves relative to the measurement head  518  in the circumferential direction of the circular disk  517 . The intensity of the interference light relative to a distance by which the hologram diffracting lattice moves has a change pattern that is substantially the same as a sine curve. Thus, a movement distance shorter than the length of one pitch of the hologram diffracting lattice can be measured at high accuracy by measuring the intensity of the interference light. 
     A disclosed optical system is devised such that brightness and darkness of the interference light are repeated a plurality of times for each movement by one pitch of the hologram diffracting lattice. Further resolution improvement can be achieved by using such an optical system. 
     Although the example of the first rotation stage  512  and the encoder  513  is illustrated in  FIG.  13   , the same description applies to the second rotation stage  522  and the encoder  523 . 
     The linear stage  612  is different from the first rotation stage  512  in that the linear stage  612  linearly moves the parallel plane substrate  61  instead of rotating the grating  51 , and the encoder  613  is different from the encoder  513  in that the hologram diffracting lattice moves straight instead of moving in the circumferential direction. However, the same principle of measurement of the movement distance of the hologram diffracting lattice by the encoder  513  applies to the encoder  613 , and thus detailed description thereof is omitted. 
     2.4 EXAMPLE OF INTENSITY RATIO R 
       FIG.  14    is a graph illustrating an exemplary spectrum waveform of a pulse laser beam output from the laser apparatus  100  in the first embodiment. In  FIG.  14   , the horizontal axis represents a wavelength λ, and the vertical axis represents the light intensity I. The pulse laser beam includes a wavelength component of the wavelength λ1 and a wavelength component of the wavelength λ2. The wavelength λ1 corresponds to a first wavelength component in the present disclosure, and the wavelength λ2 corresponds to a second wavelength component in the present disclosure. 
     When Eλ1 represents pulse energy E of the wavelength component of the wavelength λ1 and Eλ2 represents pulse energy E of the wavelength component of the wavelength λ2, the intensity ratio R between the wavelength component of the wavelength λ1 and the wavelength component of the wavelength λ2 is defined by an expression below. 
         R=Eλ 1/( Eλ 1+ Eλ 2) 
     Alternatively, the intensity ratio R may be defined by an expression below. 
         R=Eλ 1/ Eλ 2 
     Alternatively, the intensity ratio R may be expressed as the ratio Eλ1:Eλ2. 
     In the spectrum waveform illustrated in  FIG.  14   , Sλ1 represents an integration value S of the light intensity I of a wavelength component having a central wavelength at the wavelength λ1, and Sλ2 represents an integration value S of the light intensity I of a wavelength component having a central wavelength at the wavelength λ2. The intensity ratio R may be defined by an expression below. 
         R=Sλ 1/( Sλ 1+ Sλ 2) 
     Alternatively, the intensity ratio R may be defined by an expression below. 
         R=Sλ 1/ Sλ 2 
     Alternatively, the intensity ratio R may be expressed as the ratio Sλ1:Sλ2. 
     When the wavelength component having a central wavelength at the wavelength λ1 and the wavelength component having a central wavelength at the wavelength λ2 have the same spectrum line width, the peak value of the light intensity I and the integration value S are proportional to each other for the wavelength components. Thus, when Iλ1 represents the peak value of the light intensity I of the wavelength component of the wavelength λ1 and Iλ2 represents the peak value of the light intensity I of the wavelength component of the wavelength λ2, the intensity ratio R may be defined by an expression below. 
         R=Iλ 1/( Iλ 1+ Iλ 2) 
     Alternatively, the intensity ratio R may be defined by an expression below. 
         R=I λ1/ Iλ 2
 
     Alternatively, the intensity ratio R may be expressed as the ratio Iλ1:Iλ2. 
     2.5 Operation of Laser Control Processor  130   
     2.5.1 Main Flow 
       FIG.  15    is a flowchart illustrating operation of the laser control processor  130  in the first embodiment. As described below, after having acquired data for control of the wavelengths λ1 and λ2 and control of the intensity ratio R, the laser control processor  130  controls the laser apparatus  100  so that exposure light is output. 
     At S 11 , the laser control processor  130  closes the shutter  18 . Accordingly, no pulse laser beam is incident on the exposure apparatus  200 . 
     At S 12 , the laser control processor  130  stores the positions of various actuators measured by various encoders. The various actuators include the first rotation stage  512 , the second rotation stage  522 , and the linear stage  612 . The various encoders include the encoders  513 ,  523 , and  623 . The position θ1 of the first rotation stage  512  is defined as the rotation angle of the first rotation stage  512  with reference to a predetermined position. The position θ1 is measured by the encoder  513 . The rotation angle of the second rotation stage  522  with reference to a predetermined position is defined as the position θ2 of the second rotation stage  522 . The position θ2 is measured by the encoder  523 . The movement distance of the linear stage  612  with reference to a predetermined position is defined as a position Y of the linear stage  612 . The position Y is measured by the encoder  613 . 
     At S 13 , the laser control processor  130  performs calibration of the wavelength λ1. The laser control processor  130  acquires, through the calibration of the wavelength λ1, data for controlling the position θ1 to adjust the wavelength λ1. Details of the calibration of the wavelength λ1 will be described later with reference to  FIG.  16   . 
     At S 14 , the laser control processor  130  performs calibration of the wavelength λ2. The laser control processor  130  acquires, through the calibration of the wavelength λ2, data for controlling the position θ2 to adjust the wavelength λ2. Details of the calibration of the wavelength λ2 will be described later with reference to  FIG.  17   . 
     At S 16 , the laser control processor  130  performs calibration of the intensity ratio R. The laser control processor  130  acquires, through the calibration of the intensity ratio R, data for controlling the position Y to adjust the intensity ratio R. Details of the calibration of the intensity ratio R will be described later with reference to  FIG.  18   . 
     At S 18 , the laser control processor  130  returns the various actuators to the positions stored at S 12 . 
     At S 19 , the laser control processor  130  opens the shutter  18  and transmits a preparation OK signal to the exposure control processor  210  of the exposure apparatus  200 . 
     At S 20 , the laser control processor  130  reads parameters stored through various calibrations. The various calibrations include the calibration of the wavelength λ1, the calibration of the wavelength λ2, and the calibration of the intensity ratio R. The parameters stored through the various calibrations include parameters of the first relation λ1θ1, parameters of the second relation λ2θ2, and parameters of a third relation RY. The various calibrations and parameters will be described later with reference to  FIGS.  16  to  18   . 
     At S 21 , the laser control processor  130  performs exposure light outputting in accordance with various signals received from the exposure control processor  210 . Details of the exposure light outputting will be described later with reference to  FIG.  25   . 
     At S 22 , the laser control processor  130  determines whether a stop signal has been received from the exposure control processor  210 . The stop signal notifies that exposure is not to be performed, for example, when reticles or semiconductor wafers are to be exchanged in the exposure apparatus  200 . 
     When the stop signal has not been received (NO at S 22 ), the laser control processor  130  returns the processing to S 21 . 
     When the stop signal has been received (YES at S 22 ), the laser control processor  130  advances the processing to S 23 . 
     At S 23 , the laser control processor  130  transmits a preparation NOK signal to the exposure control processor  210 . Thereafter, the laser control processor  130  returns the processing to S 11 . 
     2.5.2 Wavelength λ1 Calibration 
       FIG.  16    is a flowchart illustrating calibration processing of the wavelength λ1 in the first embodiment. The processing illustrated in  FIG.  16    corresponds to a subroutine of S 13  in  FIG.  15   . 
     At S 131 , the laser control processor  130  sets the position Y of the linear stage  612  to a first position Ymin. Accordingly, the parallel plane substrate  61  moves to a position retracted from the optical path of an optical beam as illustrated in  FIG.  11   . Thus, the proportion of the wavelength component of the wavelength λ1 in a pulse laser beam becomes larger than the proportion of the wavelength component of the wavelength λ2. 
     At S 132 , the laser control processor  130  sets the position θ1 of the first rotation stage  512  to θ1 min. The value θ1 min is, for example, the minimum value of θ1 in the movable range of the first rotation stage  512 . 
     At S 133 , the laser control processor  130  transmits the oscillation trigger signal to the switch  13   a  of the pulse power module  13  (refer to  FIG.  2   ) so that the laser apparatus  100  performs laser oscillation. The laser control processor  130  outputs the oscillation trigger signal without receiving the trigger signal from the exposure control processor  210 . 
     At S 134 , the laser control processor  130  measures the wavelength λ1 by using the optical meter  17 . 
     At S 135 , the laser control processor  130  stores the position θ1 and the wavelength λ1 in association with each other in the memory  132 . 
     At S 136 , the laser control processor  130  sets the position θ1 to the next position θ1+Δθ1. The value Δθ1 is a movement amount of the position θ1 for the calibration of the wavelength λ1. 
     At S 137 , the laser control processor  130  determines whether a value indicating the position θ1 is smaller than θ1max. The value θ1max is, for example, the maximum value of θ1 in the movable range of the first rotation stage  512 . 
     When the value indicating the position θ1 is smaller than θ1max (YES at S 137 ), the laser control processor  130  returns the processing to S 133 . 
     When the value indicating the position θ1 is equal to or larger than θ1max (NO at S 137 ), the laser control processor  130  advances the processing to S 138 . 
     At S 138 , the laser control processor  130  calculates an expression indicating the first relation λ1θ1 between the position θ1 of the first rotation stage  512  measured by the encoder  513  and the wavelength λ1 measured by using the optical meter  17 . 
     At S 139 , the laser control processor  130  stores the parameters of the first relation λ1θ1 in the memory  132 . The parameters of the first relation λ1θ1 are, for example, coefficients of the expression indicating the first relation λ1θ1. 
     After S 139 , the laser control processor  130  ends the processing of the present flowchart and returns to the processing illustrated in  FIG.  15   . 
       FIG.  22    illustrates an example of the first relation λ1θ1 acquired through the calibration of the wavelength λ1. As the position θ1 of the first rotation stage  512  is changed from θ1min to θ1max, the posture of the grating  51  changes and the wavelength λ1 changes. The laser control processor  130  may produce, at S 138  in  FIG.  16   , an approximate expression below indicating the relation between the position θ1 of the first rotation stage  512  measured by the encoder  513  and the wavelength λ1 measured by using the optical meter  17 . 
       λ1= a 1·θ1+ b 1
 
     Parameters such as the coefficients a1 and b1 included in the approximate expression may be stored in the memory  132  at S 139  in  FIG.  16   . 
     2.5.3 Wavelength λ2 Calibration 
       FIG.  17    is a flowchart illustrating calibration processing of the wavelength λ2 in the first embodiment. The processing illustrated in  FIG.  17    corresponds to a subroutine of S 14  in  FIG.  15   . 
     At S 141 , the laser control processor  130  sets the position Y of the linear stage  612  to a second position Ymax. Accordingly, the parallel plane substrate  61  moves to a position that the entire optical beam passes as illustrated in  FIG.  12   . Thus, the proportion of the wavelength component of the wavelength λ2 in a pulse laser beam becomes larger than the proportion of the wavelength component of the wavelength Xl. 
     Processing at S 142  to S 149  is the same as corresponding processing in  FIG.  16    with replacement of the wavelength λ1 and the position θ1 with the wavelength λ2 and the position θ2, respectively, and with any other accompanying replacement, and thus detailed description thereof is omitted. 
       FIG.  23    illustrates an example of the second relation λ2θ2 acquired through the calibration of the wavelength λ2. As the position θ2 of the second rotation stage  522  is changed from θ2 min to θ2max, the posture of the grating  52  changes and the wavelength λ2 changes. The laser control processor  130  may produce, at S 148  in  FIG.  17   , an approximate expression below indicating the relation between the position θ2 of the second rotation stage  522  measured by the encoder  523  and the wavelength λ2 measured by using the optical meter  17 . 
       λ2= a 2·θ2+ b 2
 
     Parameters such as the coefficients a2 and b2 included in the approximate expression may be stored in the memory  132  at S 149  in  FIG.  17   . 
     2.5.4 Intensity Ratio R Calibration 
       FIG.  18    is a flowchart illustrating calibration processing of the intensity ratio R in the first embodiment. The processing illustrated in  FIG.  18    corresponds to a subroutine of S 16  in  FIG.  15   . 
     At S 160 , the laser control processor  130  sets the position θ1 of the first rotation stage  512  so that the wavelength λ1 becomes a first measurement wavelength. For example, a value close to the wavelength of a pulse laser beam used as exposure light is selected as the first measurement wavelength. The position θ1 may be set by using the first relation λ1θ1 acquired through the calibration of the wavelength λ1 (refer to  FIG.  16   ). Alternatively, the position Y of the linear stage  612  may be set to the first position Ymin, laser oscillation may be actually performed, and the position θ1 may be set so that the wavelength λ1 measured by using the optical meter  17  becomes the first measurement wavelength. 
     At S 161 , the laser control processor  130  sets the position θ2 of the second rotation stage  522  so that the wavelength λ2 becomes a second measurement wavelength λ1+FSR/2. The second measurement wavelength is a value obtained by adding half of the free-spectral range to the first measurement wavelength. In other words, the wavelengths λ1 and λ2 are set so that the wavelength difference between the wavelengths λ1 and λ2 becomes equal to half of the free-spectral range. When the free-spectral range of the spectroscopic sensor  17   g  illustrated in  FIG.  6    is larger than the free-spectral range of the spectroscopic sensor  17   f , the wavelength difference between the wavelengths λ1 and λ2 may be set to half of the free-spectral range of the spectroscopic sensor  17   f.    
       FIG.  19    is a graph illustrating an exemplary result of interference fringe measurement when pulse laser beams of the wavelengths λ1 and λ2 having a wavelength difference equal to half of the free-spectral range are separately incident on the spectroscopic sensor  17   f . When the wavelength difference is set to half of the free-spectral range, peaks of the interference fringe of the wavelength λ1 and peaks of the interference fringe of the wavelength λ2 alternately appear at a large interval. 
       FIGS.  20  and  21    are graphs illustrating an exemplary result of interference fringe measurement when pulse laser beams of the wavelengths λ1 and λ2 having a wavelength difference equal to half of the free-spectral range are simultaneously incident on the spectroscopic sensor  17   f . The waveform illustrated in  FIG.  20    is substantially equivalent to a synthesis waveform of the waveform of the interference fringe of the wavelength λ1 and the waveform of the interference fringe of the wavelength λ2 in  FIG.  19   . 
     At S 162  in  FIG.  18   , the laser control processor  130  sets the position Y of the linear stage  612  to the first position Ymin. 
     At S 163 , the laser control processor  130  transmits the oscillation trigger signal to the switch  13   a  of the pulse power module  13  so that the laser apparatus  100  performs laser oscillation. This processing is the same as the processing at S 133  (refer to  FIG.  16   ). 
     At S 164 , the laser control processor  130  measures the intensity ratio R by using the optical meter  17 . 
       FIG.  20    illustrates an exemplary result of interference fringe measurement when the intensity ratio R defined as Eλ1/(Eλ1+Eλ2) is 1/2. 
       FIG.  21    illustrates an exemplary result of interference fringe measurement when the intensity ratio R defined as Eλ1/(Eλ1+Eλ2) is 10/11. 
     As understood from  FIGS.  20  and  21   , when the wavelength difference between the wavelengths λ1 and λ2 is close to half of the free-spectral range, peaks of the interference fringe of the wavelength λ1 and peaks of the interference fringe of the wavelength λ2 can be distinguished and specified. When the first measurement wavelength and the second measurement wavelength are known, the interference fringes are potentially more easily distinguished. Once the interference fringes are distinguished, the intensity ratio R can be calculated from the light intensity I of each interference fringe. 
     At S 165  in  FIG.  18   , the laser control processor  130  stores the position Y and the intensity ratio R in association with each other in the memory  132 . 
     At S 166 , the laser control processor  130  sets the position Y to next position Y+ΔY. The value ΔY is a movement amount of the position Y for the calibration of the intensity ratio R. 
     At S 167 , the laser control processor  130  determines whether a value indicating the position Y is smaller than Ymax. 
     When the value indicating the position Y is smaller than Ymax (YES at S 167 ), the laser control processor  130  returns the processing to S 163 . 
     When the value indicating the position Y is equal to or larger than Ymax (NO at S 167 ), the laser control processor  130  advances the processing to S 168 . 
     At S 168 , the laser control processor  130  calculates an expression indicating the third relation RY between the position Y of the linear stage  612  measured by the encoder  613  and the intensity ratio R measured by using the optical meter  17 . 
     At S 169 , the laser control processor  130  stores the parameters of the third relation RY in the memory  132 . The parameters of the third relation RY are, for example, coefficients of the expression indicating the third relation RY. 
     After S 169 , the laser control processor  130  ends the processing of the present flowchart and returns to the processing illustrated in  FIG.  15   . 
       FIG.  24    illustrates an example of the third relation RY acquired through the calibration of the intensity ratio R. As the position Y of the linear stage  612  is changed from the first position Ymin to the second position Ymax, the position of the parallel plane substrate  61  changes and the intensity ratio R changes. However, the intensity ratio R is one between Ymin and Ya. Specifically, in the calibration of the wavelength λ1 (refer to  FIG.  16   ), sufficient clearance is ensured between the parallel plane substrate  61  and an optical beam so that the optical beam is not incident on the parallel plane substrate  61 . The intensity ratio R is zero between Ya+1/α and Ymax. Specifically, in the calibration of the wavelength λ2 (refer to  FIG.  17   ), the end face  616  of the parallel plane substrate  61  is disposed below the lower end position of an optical beam so that the entire optical beam is incident on the parallel plane substrate  61 . 
     The third relation RY is expressed by, for example, an expression as described below. 
         R= 1−α×( Y−Ya )
 
     However, R may be one when Y is equal to or smaller than Ya, and R may be zero when Y is equal to or larger than Ya+1/α. 
     The laser control processor  130  may produce the expression indicating the third relation RY at S 168  in  FIG.  18   . 
     Parameters such as the coefficients a and Ya included in the expression may be stored in the memory  132  at S 169  in  FIG.  18   . 
     2.5.5 Exposure Light Outputting 
       FIG.  25    is a flowchart illustrating outputting processing of exposure light in the first embodiment. The processing illustrated in  FIG.  25    corresponds to a subroutine of S 21  in  FIG.  15   . 
     At S 212 , the laser control processor  130  reads the target wavelengths λ1t and λ2t and the target intensity ratio Rt. The target wavelengths λ1t and λ2t and the target intensity ratio Rt may be received from the exposure control processor  210 . 
     Alternatively, the laser control processor  130  may receive a target average wavelength (λ1t+λ2t)/2 and a target wavelength difference λ1t−λ2t from the exposure control processor  210  in place of the target wavelengths λ1t and λ2t. Alternatively, the laser control processor  130  may receive the target wavelength λ1t and the target wavelength difference λ1t−λ2t from the exposure control processor  210 . The laser control processor  130  may calculate the target wavelengths λ1t and λ2t based on these values received from the exposure control processor  210  and may read results of the calculation. 
     The laser control processor  130  may receive target pulse energies Eλ1t and Eλ2t of the respective wavelength components from the exposure control processor  210  in place of the target intensity ratio Rt. The laser control processor  130  may calculate the target intensity ratio Rt based on these values received from the exposure control processor  210  and may read a result of the calculation. 
     At S 213 , the laser control processor  130  calculates a target position θ1t of the first rotation stage  512 , a target position θ2t of the second rotation stage  522 , and a target position Yt of the linear stage  612 . 
     The target position θ1t is calculated based on the first relation λ1θ1 acquired through the calibration of the wavelength λ1 and the target wavelength λ1t received from the exposure control processor  210 . 
     The target position θ2t is calculated based on the second relation λ2θ2 acquired through the calibration of the wavelength λ2 and the target wavelength λ2t received from the exposure control processor  210 . 
     The target position Yt is calculated based on the third relation RY acquired through the calibration of the intensity ratio R and the target intensity ratio Rt received from the exposure control processor  210 . 
     At S 214 , the laser control processor  130  sets the positions θ1, θ2, and Y of the various actuators to the respective target positions θ1t, θ2t, and Yt. Then, the laser control processor  130  controls the various actuators as described below. 
     The laser control processor  130  controls the first rotation stage  512  so that the position θ1 of the first rotation stage  512  measured by the encoder  513  approaches the target position θ1t. 
     The laser control processor  130  controls the second rotation stage  522  so that the position θ2 of the second rotation stage  522  measured by the encoder  523  approaches the target position θ2t. 
     The laser control processor  130  controls the linear stage  612  so that the position Y of the linear stage  612  measured by the encoder  613  approaches the target position Yt. 
     At S 215 , the laser control processor  130  determines whether the trigger signal has been received from the exposure control processor  210 . 
     When the trigger signal has not been received (NO at S 215 ), the laser control processor  130  waits until the trigger signal is received. 
     When the trigger signal has been received (YES at S 215 ), the laser control processor  130  advances the processing to S 216 . 
     At S 216 , the laser control processor  130  transmits the oscillation trigger signal to the switch  13   a  of the pulse power module  13  and controls the laser apparatus  100  so that laser oscillation is performed. 
     After S 216 , the laser control processor  130  ends the processing of the present flowchart and returns to the processing illustrated in  FIG.  15   . 
     2.6 Effect 
     (1) According to the first embodiment, the laser apparatus  100  includes the gratings  51  and  52 , the first rotation stage  512  configured to change the wavelength λ1 of light included in a pulse laser beam by changing the posture of the grating  51 , the second rotation stage  522  configured to change the wavelength λ2 of light included in the pulse laser beam by changing the posture of the grating  52 , the encoder  513  configured to measure the position θ1 of the first rotation stage  512 , the encoder  523  configured to measure the position θ2 of the second rotation stage  522 , and the laser control processor  130 . 
     The laser control processor  130  reads the first relation λ1θ1 between the position θ1 of the first rotation stage  512  and the wavelength λ1 and the second relation λ2θ2 between the position θ2 of the second rotation stage  522  and the wavelength λ2 (S 20 ). 
     The laser control processor  130  performs control of the first rotation stage  512  based on the first relation λ1θ1 and the position θ1 of the first rotation stage  512  measured by the encoder  513  and control of the second rotation stage  522  based on the second relation λ2θ2 and the position θ2 of the second rotation stage  522  measured by the encoder  523  (S 213  and S 214 ). 
     Accordingly, the first rotation stage  512  is controlled based on the first relation λ1θ1 and the output from the encoder  513 , and the second rotation stage  522  is controlled based on the second relation λ2θ2 and the output from the encoder  523 . Thus, the wavelengths λ1 and λ2 can be adjusted without measurement thereof when exposure light is to be output. Wavelength calculation is typically performed by providing interference fringe data obtained by a spectroscopic sensor with deconvolution processing using a device function of the optical meter  17 . Normally, a time longer than the shortest oscillation period of the laser apparatus  100  is needed for wavelength calculation. Thus, it has been difficult to perform outputting while controlling two optional wavelengths for each pulse. However, according to the first embodiment, since the wavelengths λ1 and λ2 are not measured when exposure light is to be output, deconvolution processing along with wavelength calculation at exposure light outputting is unnecessary. Thus, it is possible to perform outputting while controlling two optional wavelengths for each pulse. 
     (2) According to the first embodiment, the laser apparatus  100  includes the optical meter  17  configured to measure the wavelengths λ1 and λ2. 
     The laser control processor  130  measures the wavelength λ1 by using the optical meter  17  (S 134 ) and performs processing of storing, as the first relation λ1θ1 in the memory  132 , the relation between the position θ1 of the first rotation stage  512  measured by the encoder  513  and the wavelength λ1 (S 138  and S 139 ). 
     The laser control processor  130  measures the wavelength λ2 by using the optical meter  17  (S 144 ) and performs processing of storing, as the second relation λ2θ2 in the memory  132 , the relation between the position θ2 of the second rotation stage  522  measured by the encoder  523  and the wavelength λ2 (S 148  and S 149 ). 
     Accordingly, the first relation λ1θ1 between the wavelength λ1 measured by using the optical meter  17  and the position θ1 of the first rotation stage  512  measured by the encoder  513  is stored. In addition, the second relation λ2θ2 between the wavelength λ2 measured by using the optical meter  17  and the position θ2 of the second rotation stage  522  measured by the encoder  523  is stored. Since the first relation λ1θ1 and the second relation λ2θ2 are measured and stored in advance, it is possible to accurately perform control of the wavelengths λ1 and λ2 using the encoders  513  and  523 . 
     (3) According to the first embodiment, the laser apparatus  100  includes the linear stage  612  configured to change the intensity ratio R between the wavelengths λ1 and λ2. 
     In a state in which the proportion of the wavelength component of the wavelength λ1 is set to be larger than the proportion of the wavelength component of the wavelength λ2 by the linear stage  612  (S 131 ), the laser control processor  130  measures the wavelength λ1 by using the optical meter  17  (S 134 ). In a state in which the proportion of the wavelength component of the wavelength λ2 is set to be larger than the proportion of the wavelength component of the wavelength λ1 by the linear stage  612  (S 141 ), the laser control processor  130  measures the wavelength λ2 by using the optical meter  17  (S 144 ). 
     Accordingly, since the intensity ratio R between the wavelengths λ1 and λ2 is changed and the wavelengths λ1 and λ2 are individually measured, it is possible to accurately measure these wavelengths. 
     (4) According to the first embodiment, the laser apparatus  100  includes the linear stage  612  configured to change the intensity ratio R between the wavelengths λ1 and λ2 and the encoder  613  configured to measure the position Y of the linear stage  612 . 
     The laser control processor  130  reads the third relation RY between the position Y of the linear stage  612  and the intensity ratio R (S 20 ) and performs control of the linear stage  612  based on the third relation RY and the position Y of the linear stage  612  measured by the encoder  613  (S 213  and S 214 ). 
     Accordingly, the linear stage  612  is controlled based on the third relation RY and the output from the encoder  613 . Thus, it is possible to adjust the intensity ratio R without measuring the intensity ratio R between the wavelengths λ1 and λ2 when exposure light is to be output. 
     (5) According to the first embodiment, the laser apparatus  100  includes the optical meter  17  configured to measure the intensity ratio R between the wavelengths λ1 and λ2. The laser control processor  130  measures the intensity ratio R by using the optical meter  17  (S 164 ) and stores, as the third relation RY in the memory  132 , the relation between the position Y of the linear stage  612  measured by the encoder  613  and the intensity ratio R (S 168  and S 169 ). 
     Accordingly, the third relation RY between the intensity ratio R measured by using the optical meter  17  and the position Y of the linear stage  612  measured by the encoder  613  is stored. Since the third relation RY is measured and stored in advance, it is possible to accurately perform control of the intensity ratio R using the encoder  613 . 
     (6) According to the first embodiment, the laser control processor  130  performs control of the first rotation stage  512  and the second rotation stage  522  so that the wavelength difference between the wavelengths λ1 and λ2 approaches half of the free-spectral range of the optical meter  17  (S 160  and S 161 ), and thereafter measures the intensity ratio R by using the optical meter  17  (S 164 ). 
     Accordingly, since adjustment is performed so that the wavelength difference between the wavelengths λ1 and λ2 approaches half of the free-spectral range, it is possible to measure the intensity ratio R of the wavelengths λ1 and λ2 even when light of the wavelength λ1 and light of the wavelength of λ2 are simultaneously output. 
     The other features of the first embodiment are the same as those of the comparative example. 
     3. LASER APPARATUS CONFIGURED TO SELECT THIRD RELATION RY IN ACCORDANCE WITH REPETITION FREQUENCY F AND CHARGING VOLTAGE V 
     3.1 Main Flow 
       FIG.  26    is a flowchart illustrating operation of the laser control processor  130  in a second embodiment. The configuration of the laser apparatus  100  according to the second embodiment is the same as in the first embodiment. The second embodiment is different from the first embodiment in that calibration of the third relation RY(f, V) (S 15   b ) is performed in place of the calibration of the intensity ratio R (S 16 ). In the second embodiment, exposure light outputting (S 21   b ) is performed in place of the exposure light outputting (S 21 ). 
     3.2 Third Relation RY(f, V) Calibration 
       FIG.  27    is a flowchart illustrating calibration processing of the third relation RY(f, V) in the second embodiment. The processing illustrated in  FIG.  27    corresponds to a subroutine of S 15   b  in  FIG.  26   . 
     At S 151 , the laser control processor  130  sets a repetition frequency f of a pulse laser beam to fmin. The value fmin is the minimum value of the repetition frequency f that can be requested by the exposure control processor  210 . 
     At S 152 , the laser control processor  130  sets the charging voltage V of the charger  12  (refer to  FIG.  2   ) to Vmin. The value Vmin is the minimum value of the charging voltage V in accordance with the range of pulse energy that can be requested by the exposure control processor  210 . 
     At S 16  following S 152 , the laser control processor  130  performs calibration of the intensity ratio R. The processing at S 16  is the same as the calibration of the intensity ratio R in the first embodiment. However, the calibration of the intensity ratio R in the second embodiment is different from the calibration of the intensity ratio R in the first embodiment in that the calibration of the intensity ratio R in the second embodiment is performed a larger number of times in accordance with loop processing of the present flowchart than the calibration of the wavelength λ1 and the calibration of the wavelength λ2. 
     At S 154  following S 16 , the laser control processor  130  sets the charging voltage V to the next value V+ΔV. The value ΔV is a change amount of the charging voltage V for the calibration of the third relation RY(f, V). 
     At S 155 , the laser control processor  130  determines whether the charging voltage V is lower than Vmax. 
     When the charging voltage V is lower than Vmax (YES at S 155 ), the laser control processor  130  returns the processing to S 16 . 
     When the charging voltage V is equal to or higher than Vmax (NO at S 155 ), the laser control processor  130  advances the processing to S 156 . 
     At S 156 , the laser control processor  130  sets the repetition frequency f to the next value f+Δf. The value Δf is a change amount of the repetition frequency f for the calibration of the third relation RY(f, V). 
     At S 157 , the laser control processor  130  determines whether the repetition frequency f is lower than fmax. 
     When the repetition frequency f is lower than fmax (YES at S 157 ), the laser control processor  130  returns the processing to S 152 . 
     When the repetition frequency f is equal to or higher than fmax (NO at S 157 ), the laser control processor  130  ends the processing of the present flowchart and returns to the processing illustrated in  FIG.  26   . 
       FIG.  28    illustrates exemplary data stored in the memory  132  through the calibration of the third relation RY(f, V). In the calibration of the third relation RY(f, V), the calibration of the intensity ratio R (S 16 ) is performed a plurality of times in accordance with the repetition frequency f and the charging voltage V. As a result, a plurality of third relations RY(f, V) in accordance with the repetition frequency f and the charging voltage V are stored in the memory  132 . The memory  132  may store the third relations RY(f, V) in a table format. This table is also referred to as a “third relation RY(f, V) table” in description below. 
     In  FIG.  28   , a third relation RY(fi, Vj) is the relation between the intensity ratio R and the position Y of the linear stage  612 , which is obtained by setting an i-th repetition frequency fi and a j-th charging voltage Vj and performing calibration of the intensity ratio R. The number of third relations RY(fi, Vj) is equal to a number obtained by multiplying the maximum value of i and the maximum value of j. 
     3.3 Exposure Light Outputting 
       FIG.  29    is a flowchart illustrating outputting processing of exposure light in the second embodiment. The processing illustrated in  FIG.  29    corresponds to a subroutine of S 21   b  in  FIG.  26   . 
     At S 210   b , the laser control processor  130  sets the repetition frequency f and the charging voltage V based on a signal received from the exposure control processor  210 . 
     At S 211   b , the laser control processor  130  searches the third relation RY(f, V) table with the repetition frequency f and the charging voltage V thus set. The laser control processor  130  specifies the third relation RY(f, V) corresponding to the repetition frequency f and the charging voltage V thus set. 
     For example, when the repetition frequency f set at S 210   b  is equal to or higher than f1 and lower than f2 and the set charging voltage V is equal to or higher than V1 and lower than V2, a third relation RY(f1, V1) is specified in the third relation RY(f, V) table in  FIG.  28   . 
     Processing at S 212  or later is the same as corresponding processing in the first embodiment. Specifically, the first rotation stage  512 , the second rotation stage  522 , and the linear stage  612  are controlled based on the third relation RY(f1, V1) specified at S 211   b  so that the wavelengths λ1 and λ2 and the intensity ratio R approach respective target values. Thereafter, exposure light is output. 
     3.4 Effect 
     (7) According to the second embodiment, the third relation RY(f, V) is stored in the memory  132  in association with the repetition frequency f of a pulse laser beam. The laser control processor  130  performs control of the linear stage  612  based on the third relation RY(f, V) associated with the repetition frequency f of the pulse laser beam and based on the position Y of the linear stage  612  measured by the encoder  613 . 
     In the first and second embodiments, since an optical beam is spatially divided into the first part B1 and the second part B2 and the divided parts are separately subjected to wavelength control, gas density is preferably uniform on the optical path of the optical beam. However, when the repetition frequency f changes, fluctuation occurs to gas density on the optical path of the optical beam and affects the intensity ratio R in some cases. According to the second embodiment, since an appropriate third relation RY(f, V) is selected in accordance with the repetition frequency f, the accuracy of control of the intensity ratio R can be improved. 
     (8) According to the second embodiment, the laser apparatus  100  includes the laser chamber  10 , the pair of electrodes  11   a  and  11   b  disposed in the laser chamber  10 , and the charger  12  configured to store electric energy for applying voltage to the electrodes  11   a  and  11   b.    
     The third relation RY(f, V) is stored in the memory  132  in association with the charging voltage V of the charger  12 . The laser control processor  130  performs control of the linear stage  612  based on the third relation RY(f, V) associated with the charging voltage V of the charger  12  and based on the position Y of the linear stage  612  measured by the encoder  613 . 
     In the first and second embodiments, when the charging voltage V changes, fluctuation occurs to gas density on the optical path of an optical beam and affects the intensity ratio R in some cases. According to the second embodiment, since an appropriate third relation RY(f, V) is selected in accordance with the charging voltage V, the accuracy of control of the intensity ratio R can be improved. 
     3.5 Other Exemplary Configuration 
     The second embodiment is described above on the case in which the repetition frequency f and the charging voltage V in addition to the third relation RY between the position Y of the linear stage  612  and the intensity ratio R are stored in the third relation RY(f, V) table, but the present disclosure is not limited thereto. The relation among the position Y of the linear stage  612 , the intensity ratio R, the repetition frequency f, and the charging voltage V may be expressed by, for example, an expression as described below. 
         R=a 1· Y+a 2· Y   2   +b 1· f+b 2· f   2   +c 1· V+c 1· V   2   +d  
 
     The other features of the second embodiment are the same as those of the first embodiment. 
     4. LASER APPARATUS CONFIGURED TO ADJUST INTENSITY RATIO R WITHOUT USING THIRD RELATION RY 
     4.1 Main Flow 
       FIG.  30    is a flowchart illustrating operation of the laser control processor  130  in a third embodiment. The configuration of the laser apparatus  100  according to the third embodiment is the same as in the first embodiment. The third embodiment is different from the first embodiment in that setting of the position Y of the linear stage  612  (S 17   c ) is performed in place of the calibration of the intensity ratio R (S 16 ). In the third embodiment, exposure light outputting (S 21   c ) is performed in place of the exposure light outputting (S 21 ). The processing (S 12 ) of storing the positions of the various actuators and the processing (S 18 ) of returning the various actuators to the stored positions in the first embodiment are omitted in the third embodiment. 
     4.2 Setting of Position Y of Linear Stage  612   
       FIG.  31    is a flowchart illustrating setting processing of the position Y of the linear stage  612  in the third embodiment. The processing illustrated in  FIG.  31    corresponds to a subroutine of S 17   c  in  FIG.  30   . 
     Processing at S 170  and S 171  is the same as the processing at S 160  and S 161  in  FIG.  18   . Specifically, the laser control processor  130  controls the first rotation stage  512  and the second rotation stage  522  so that the wavelength difference between the wavelengths λ1 and λ2 becomes equal to half of the free-spectral range of the optical meter  17 . 
     At S 172 , the laser control processor  130  sets the repetition frequency f and the charging voltage V based on a signal received from the exposure control processor  210 . Since the position Y of the linear stage  612  is set after the repetition frequency f and the charging voltage V are set, the intensity ratio R can be accurately adjusted. 
     At S 173 , the laser control processor  130  reads the target intensity ratio Rt. The target intensity ratio Rt may be received from the exposure control processor  210 . 
     At S 174 , the laser control processor  130  transmits the oscillation trigger signal to the switch  13   a  of the pulse power module  13  (refer to  FIG.  2   ) so that the laser apparatus  100  performs laser oscillation. This processing is the same as the processing at S 133  (refer to  FIG.  16   ). 
     At S 175 , the laser control processor  130  measures the intensity ratio R by using the optical meter  17 . 
     At S 176 , the laser control processor  130  determines whether the difference between the intensity ratio R and the target intensity ratio Rt is in an allowable range. 
     When the difference between the intensity ratio R and the target intensity ratio Rt is in the allowable range (YES at S 176 ), the laser control processor  130  ends the processing of the present flowchart and returns to the processing illustrated in  FIG.  30   . 
     When the difference between the intensity ratio R and the target intensity ratio Rt is not in the allowable range (NO at S 176 ), the laser control processor  130  advances the processing to S 177 . 
     At S 177 , the laser control processor  130  controls the linear stage  612  so that the measured intensity ratio R approaches the target intensity ratio Rt. 
     After S 177 , the laser control processor  130  returns the processing to S 174 . 
     4.3 Exposure Light Outputting 
       FIG.  32    is a flowchart illustrating outputting processing of exposure light in the third embodiment. The processing illustrated in  FIG.  32    corresponds to a subroutine of S 21   c  in  FIG.  30   . 
     The processing illustrated in  FIG.  32    is different from the processing of the first embodiment illustrated in  FIG.  25    in that the processing illustrated in  FIG.  32    does not include control of the linear stage  612  based on the target intensity ratio Rt. 
     The other features of the processing illustrated in  FIG.  32    are the same as those of the processing illustrated in  FIG.  25   . Specifically, the target position θ1t of the first rotation stage  512  and the target position θ2t of the second rotation stage  522  are calculated based on the first relation λ1θ1 and the second relation λ2θ2 (S 213   c ). In addition, the first rotation stage  512  and the second rotation stage  522  are controlled so that the position θ1 of the first rotation stage  512  measured by the encoder  513  and the position θ2 of the second rotation stage  522  measured by the encoder  523  approach the target positions θ1t and θ2t, respectively (S 214   c ). Thereafter, exposure light is output. 
     4.4 Effect 
     (9) According to the third embodiment, the laser apparatus  100  includes the linear stage  612  configured to change the intensity ratio R between the wavelengths λ1 and λ2 and the optical meter  17  configured to measure the intensity ratio R between the wavelengths λ1 and λ2. 
     The laser control processor  130  reads the target intensity ratio Rt (S 173 ) and performs control of the linear stage  612  so that the intensity ratio R measured by using the optical meter  17  approaches the target intensity ratio Rt (S 177 ). Thereafter, the laser control processor  130  performs control of the first rotation stage  512  based on the first relation λ1θ1 and the position θ1 of the first rotation stage  512  measured by the encoder  513  and control of the second rotation stage  522  based on the second relation λ2θ2 and the position θ2 of the second rotation stage  522  measured by the encoder  523  (S 214   c ). 
     Accordingly, after controlling the linear stage  612  based on a measurement result of the intensity ratio R, the laser control processor  130  controls the first rotation stage  512  and the second rotation stage  522  based on the first relation λ1θ1, the second relation λ2θ2, and results of measurement by the encoders  513  and  523 . Thus, the intensity ratio R can be adjusted without measurement of the third relation RY in advance, and the wavelengths λ1 and λ2 can be adjusted based on the first relation λ1θ1 and the second relation λ2θ2 when exposure light is to be output. 
     (10) According to the third embodiment, the laser control processor  130  performs control of the first rotation stage  512  and the second rotation stage  522  so that the wavelength difference between the wavelengths λ1 and λ2 approaches half of the free-spectral range of the optical meter  17  (S 170  and S 171 ). Thereafter, the laser control processor  130  performs control of the linear stage  612  so that the intensity ratio R measured by using the optical meter  17  approaches the target intensity ratio Rt (S 175  and S 177 ). 
     Accordingly, since adjustment is performed so that the wavelength difference between the wavelengths λ1 and λ2 approaches half of the free-spectral range, it is possible to simultaneously output light of the wavelength λ1 and light of the wavelength λ2, measure the intensity ratio R thereof, and control the linear stage  612 . Thereafter, when exposure light is to be output, the wavelengths λ1 and λ2 can be adjusted to the target wavelengths λ1t and λ2t, respectively, by using the first relation λ1θ1 and the second relation λ2θ2. 
     The other features of the third embodiment are the same as those of the first embodiment. 
     5. LINE NARROWING DEVICE  14   D  IN WHICH A PLURALITY OF PRISMS  43  AND  44  ARE DISPOSED IN XZ PLANE 
     5.1 Configuration 
       FIGS.  33  and  34    schematically illustrate the configuration of a line narrowing device  14   d  in a fourth embodiment.  FIG.  33    illustrates the line narrowing device  14   d  viewed in the positive Y direction, and  FIG.  34    illustrates the line narrowing device  14   d  viewed in the negative X direction. The line narrowing device  14   d  includes prisms  43  and  44 , a grating  53 , and a parallel plane substrate  71  in place of the grating system  50   a  and the beam separation optical system  60   a  in the first embodiment. 
     The prisms  43  and  44  are disposed at positions different from each other in a wavelength dispersion direction DD of any of the prisms  43  and  44  on the optical path of the optical beam having passed through the prism  42 . The prism  43  is supported by a holder  431 , and the prism  44  is supported by a holder  441 . The positions of the prisms  43  and  44  are set such that the optical beam having passed through the prism  42  is incident across the prisms  43  and  44 . The wavelength dispersion direction DD of a prism is a direction in which the refraction angle of light at the surface of the prism disperses in accordance with the wavelength. In the example illustrated in  FIGS.  33  and  34   , the wavelength dispersion directions DD of the prisms  43  and  44  are identical. 
     The prisms  43  and  44  are disposed such that surfaces of the prisms  43  and  44  that the optical beam is incident on and output from are all parallel to the Y axis. 
     The prism  43  is rotatable about an axis parallel to the Y axis by a third rotation stage  432 , and the prism  44  is rotatable about an axis parallel to the Y axis by a fourth rotation stage  442 . 
     An encoder  433  is attached to the third rotation stage  432  of the prism  43 . An encoder  443  is attached to the fourth rotation stage  442  of the prism  44 . 
     In the fourth embodiment, the prism  43  corresponds to the first optical element in the present disclosure, and the prism  44  corresponds to the second optical element in the present disclosure. The third rotation stage  432  corresponds to the first actuator in the present disclosure, and the fourth rotation stage  442  corresponds to the second actuator in the present disclosure. The encoder  433  corresponds to the first encoder in the present disclosure, and the encoder  443  corresponds to the second encoder in the present disclosure. 
     The grating  53  is disposed across the optical paths of the first part B1 of the optical beam having passed through the prism  43  and the second part B2 of the optical beam having passed through the prism  44 . The direction of grooves of the grating  53  matches the direction of the Y axis. The grating  53  is supported by a holder  531 . 
     The parallel plane substrate  71  is disposed on the optical path of the optical beam output through the window  10   a . For example, the parallel plane substrate  71  is disposed on the optical path of the optical beam between the window  10   a  and the prism  41 . The parallel plane substrate  71  is supported by a holder  711 . The parallel plane substrate  71  is disposed such that surfaces of the parallel plane substrate  71  that the optical beam is incident on and output from are both parallel to the Y axis. The parallel plane substrate  71  is rotatable about an axis parallel to the Y axis by a fifth rotation stage  712 . An encoder  713  is attached to the fifth rotation stage  712  of the parallel plane substrate  71 . 
     The fifth rotation stage  712  in the fourth embodiment corresponds to the third actuator in the present disclosure. The encoder  713  corresponds to the third encoder in the present disclosure. 
     5.2 Operation 
     The parallel plane substrate  71  transmits the optical beam with refraction at the same angle in directions opposite to each other at a surface on which the optical beam output through the window  10   a  is incident and a surface from which the optical beam is output toward the prism  41 . Thus, the optical beam output from the parallel plane substrate  71  has the same traveling direction as the optical beam incident on the parallel plane substrate  71  and has an optical path axis shifted from that of the incident optical beam in the direction of the X axis in accordance with the posture of the parallel plane substrate  71 . The optical path axis is the central axis of the optical path. 
     The optical beam having transmitted through the parallel plane substrate  71  is incident on the prism  41  and thereafter incident on the prism  42 . The prisms  41  and  42  transmit the optical beam while expanding the beam width of the optical beam in a plane parallel to the XZ plane. 
     The first part B1 of the optical beam having passed through the prism  42  is incident on the prism  43 , and the second part B2 thereof is incident on the prism  44 . The incident angles of the optical beams incident on the prisms  43  and  44  depend on the postures of the respective prisms  43  and  44 . The traveling directions of the optical beams incident on the prisms  43  and  44  are changed in accordance with the postures of the respective prisms  43  and  44 , and then the optical beams are output toward the grating  53 . 
     The light incident on the grating  53  from the prisms  43  and  44  is reflected by the grooves of the grating  53  and diffracted in a direction in accordance with the wavelength of the light. Accordingly, the light reflected by the grooves of the grating  53  is dispersed in a plane parallel to the XZ plane. The prism  43  is disposed in such a posture that the incident angle of the first part B1 of the optical beam incident on the grating  53  from the prism  43  matches the diffracting angle of light of the wavelength λ1 in the light diffracted by the grating  53 . The prism  44  is disposed in such a posture that the incident angle of the second part B2 of the optical beam incident on the grating  53  from the prism  44  matches the diffracting angle of light of the wavelength λ2 in the light diffracted by the grating  53 . When the incident angles of the optical beams incident on the grating  53  from the prisms  43  and  44  are different from each other, a wavelength difference occurs between the wavelength λ1 of the diffracted light returned from the grating  53  to the prism  43  and the wavelength λ2 of the diffracted light returned from the grating  53  to the prism  44 . 
     Although only optical beams in a direction from the prism  41  to the grating  53  are illustrated with dashed line arrows in  FIGS.  33  and  34   , an optical beam of a wavelength selected by the line narrowing device  14   d  travels from the grating  53  toward the prism  41  through paths opposite to the dashed line arrows. 
     The prisms  41  to  44  reduce the beam width of light returned from the grating  53  in a plane parallel to the XZ plane and return the light into the laser chamber  10  through the window  10   a.    
     The third rotation stage  432  and the fourth rotation stage  442  are controlled by the laser control processor  130 . 
     When the third rotation stage  432  slightly rotates the prism  43 , the traveling direction of the first part B1 of the optical beam output from the prism  43  toward the grating  53  slightly changes in a plane parallel to the XZ plane. Accordingly, the incident angle of the first part B1 of the optical beam incident on the grating  53  from the prism  43  slightly changes. As a result, the wavelength λ1 changes. 
     When the fourth rotation stage  442  slightly rotates the prism  44 , the traveling direction of the second part B2 of the optical beam output from the prism  44  toward the grating  53  slightly changes in a plane parallel to the XZ plane. Accordingly, the incident angle of the second part B2 of the optical beam incident on the grating  53  from the prism  44  slightly changes. As a result, the wavelength λ2 changes. 
     With the above-described configuration and operation, light of the wavelength λ1 and light of the wavelength λ2 in an optical beam output through the window  10   a  of the laser chamber  10  are selected and returned into the laser chamber  10 . Accordingly, the laser apparatus  100  can perform two-wavelength oscillation. The wavelengths λ1 and λ2 can be separately set by controlling the third rotation stage  432  and the fourth rotation stage  442 . 
     As the fifth rotation stage  712  changes the posture of the parallel plane substrate  71 , the shift amount of an optical beam in the direction of the X axis when the optical beam passes through the parallel plane substrate  71  changes and the positions of optical beams incident on the prisms  41  to  44  change in the direction of the X axis. Accordingly, the proportion between the first part B1 and the second part B2 changes. For example, the proportion of the first part B1 decreases as the parallel plane substrate  71  is rotated clockwise in  FIG.  33   , and the proportion of the first part B1 increases as the parallel plane substrate is rotated anticlockwise. Accordingly, the intensity ratio R between the wavelength component of the wavelength λ1 and the wavelength component of the wavelength λ2 included in a pulse laser beam can be adjusted. 
     The laser control processor  130  controls the third rotation stage  432  based on the target wavelength λ1t received from the exposure control processor  210 . Accordingly, the third rotation stage  432  changes the posture of the prism  43 . 
     The laser control processor  130  controls the fourth rotation stage  442  based on the target wavelength λ2t received from the exposure control processor  210 . Accordingly, the fourth rotation stage  442  changes the posture of the prism  44 . 
     The laser control processor  130  controls the fifth rotation stage  712  based on the target intensity ratio Rt received from the exposure control processor  210 . Accordingly, the fifth rotation stage  712  adjusts the posture of the parallel plane substrate  71 . 
     The other features of the fourth embodiment are the same as those of any one of the first to third embodiments. However, in the flowcharts of the first to third embodiments, a constituent component of the first embodiment may be replaced with a corresponding constituent component of the fourth embodiment. For example, the first rotation stage  512 , the second rotation stage  522 , and the linear stage  612  in the first embodiment may be replaced with the third rotation stage  432 , the fourth rotation stage  442 , and the fifth rotation stage  712  in the fourth embodiment. 
     5.3 First Modification 
       FIGS.  35  and  36    schematically illustrate the configuration of a line narrowing device  14   e  in a first modification of the fourth embodiment.  FIG.  35    illustrates the line narrowing device  14   e  viewed in the positive Y direction, and  FIG.  36    illustrates the line narrowing device  14   e  viewed in the negative X direction. 
     In the line narrowing device  14   e , the prism  42  is rotatable about an axis parallel to the Y axis by a sixth rotation stage  422 . An encoder  423  is attached to the sixth rotation stage  422  of the prism  42 . 
     The prism  42  in the first modification corresponds to the first optical element in the present disclosure. The sixth rotation stage  422  corresponds to the first actuator in the present disclosure. The encoder  423  corresponds to the first encoder in the present disclosure. 
     In the line narrowing device  14   e , the prism  43  is supported by the holder  431  such that the prism  43  maintains a constant posture. 
     In the first modification, the sixth rotation stage  422  rotates the prism  42 . Accordingly, the incident angle of an optical beam incident on the grating  53  from the prism  42  through the prisms  43  and  44  changes. Thus, the wavelengths λ1 and λ2 both change. Moreover, the wavelength λ2 can be changed independently from the wavelength λ1 by rotating the prism  44  by using the fourth rotation stage  442 , thereby changing the wavelength difference between the wavelengths λ1 and λ2. Thus, the wavelengths λ1 and λ2 can be made close to the target wavelengths λ1t and λ2t, respectively, by controlling both the sixth rotation stage  422  and the fourth rotation stage  442 . 
     As the prism  42  is rotated, positions at which optical beams are incident on the prisms  43  and  44  from the prism  42  change. Accordingly, as the prism  42  is rotated, the proportion between the first part B1 and the second part B2 changes and the intensity ratio R between the wavelengths λ1 and λ2 changes. When the intensity ratio R between the wavelengths λ1 and λ2 becomes out of the allowable range as the prism  42  is rotated, the intensity ratio R of the wavelengths λ1 and λ2 may be adjusted by controlling the posture of the parallel plane substrate  71 . 
     The other features of the first modification are the same as those of the fourth embodiment described above with reference to  FIGS.  33  and  34   . However, in each flowchart, the third rotation stage  432  of the fourth embodiment may be replaced with the sixth rotation stage  422  of the first modification. 
     5.4 Second Modification 
       FIGS.  37  and  38    schematically illustrate the configuration of a line narrowing device  14   f  in a second modification of the fourth embodiment.  FIG.  37    illustrates the line narrowing device  14   f  viewed in the positive Y direction, and  FIG.  38    illustrates the line narrowing device  14   f  viewed in the negative X direction. 
     The line narrowing device  14   f  includes a linear stage  452 . An encoder  453  is attached to the linear stage  452 . 
     The linear stage  452  moves the prisms  43  and  44  integrally with the holders  431  and  441 , the third and fourth rotation stages  432  and  442 , and the encoders  433  and  443 , respectively. A direction in which the prisms  43  and  44  are moved by the linear stage  452  is a direction intersecting the YZ plane. The direction intersecting the YZ plane is a direction intersecting a plane parallel to both the optical path axis of optical beams incident on the prisms  43  and  44  from the prism  42  and the grooves of the grating  53  and is, for example, the wavelength dispersion direction DD (refer to  FIG.  33   ) of any of the prisms  43  and  44 . 
     In the second modification, the linear stage  452  corresponds to the third actuator in the present disclosure. The encoder  453  corresponds to the third encoder in the present disclosure. In the second modification, the parallel plane substrate  71  (refer to  FIGS.  33  and  34   ) may be omitted. 
     As the linear stage  452  moves the prisms  43  and  44 , the proportion between the first part B1 incident on the prism  43  and the second part B2 incident on the prism  44  in the optical beam output from the prism  42  changes. For example, the proportion of the first part B1 decreases as the prisms  43  and  44  are moved in the negative H direction, and the proportion of the first part B1 increases as the prisms  43  and  44  are moved in the positive H direction. 
     The other features of the second modification are the same as those of the fourth embodiment described above with reference to  FIGS.  33  and  34   . However, the fifth rotation stage  712  of the fourth embodiment may be replaced with the linear stage  452  of the second modification. 
     6. OTHER 
     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 scope of the appended claims. Further, it would be also obvious for those skilled in the art that embodiments of the present disclosure would be appropriately combined and used. 
     The terms used throughout the present specification and the appended claims should be interpreted as “non-limiting” terms unless otherwise stated. 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, indefinite articles “a/an” 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”.