Patent Abstract:
An exposure apparatus including a pulse light source, an exposure unit which exposes a substrate to a pattern with light from the pulse light source, a determination unit which determines necessity of maintenance for the pulse light source based on a pulse rate of the pulse light source within a predetermined period of time, and a decision unit which decides a timing of the maintenance based on a determination result of the determination unit.

Full Description:
FIELD OF THE INVENTION 
   The present invention relates to an exposure apparatus, which exposes a substrate, such as a wafer or glass plate, to a pattern. 
   BACKGROUND OF THE INVENTION 
   In recent years, in the semiconductor device technology, the device integration degree has been increasing and the device feature size has been shrinking more and more. In a lithography system used in a lithography process, a pulse laser, such as a KrF or an ArF excimer laser, is used as a light source device in a far ultraviolet region. A lithography system using an excimer laser includes an excimer laser and an exposure device. The excimer laser is connected to the exposure device through an interface cable such as an optical fiber. An exposure operation in the lithography system is as follows. A reticle is illuminated with a pulse laser beam output from the excimer laser in accordance with an exposure sequence controlled by a main controller provided to the exposure device. A circuit pattern formed on the reticle is projected onto a resist coated wafer through an image forming optical system. Thus, the resist on the wafer is exposed with the pattern to form a latent image. 
   As the excimer laser, generally, a gas mixture of three types of gases, i.e., a halogen gas, such as fluorine, an inert gas, such as krypton or argon, and a rare gas, such as helium or neon, is sealed in a laser chamber. In the excimer laser, when electrical discharge occurs in the chamber, the halogen gas and inert gas react with each other to oscillate a pulse laser beam as exposure light. When the pulse laser beam is repeatedly oscillated, the halogen gas is bonded with impurities generated in the chamber or attracted to the inner side of the chamber. Then, the concentration of the halogen gas decreases to decrease the pulse energy of the laser beam, and the constituent components of the laser light source are degraded. 
   In a lithography system that uses the excimer laser as the light source, when the pulse energy fluctuates, inconveniences, such as a decrease in control accuracy of the exposure dose on a photosensitive substrate, occur. To obtain a high resolution and good line width reproducibility, an exposure dose control method of controlling the pulse energy for each exposure light pulse is necessary. Japanese Patent Laid Open No. 5-62876 proposes an exposure dose control method as a method which overcomes this issue. 
     FIG. 9  is a block diagram showing the structure of an exposure apparatus disclosed in Japanese Patent Laid Open No. 5-62876. Referring to  FIG. 9 , a gas such as KrF is sealed in a pulse laser light source  101 , which outputs a laser beam. An illumination optical system  102  includes a beam shaping optical system, an optical integrator, a collimator, and a mirror (not shown). A reticle  103  has the circuit pattern of a semiconductor device to be printed (transferred), and is illuminated by the illumination optical system  102 . The circuit pattern of the reticle  103  is reduced and projected onto a wafer  105  by a projection optical system (reduction projection lens)  104 . A half mirror  106  is arranged in the optical path between the illumination optical system  102  and reticle  103 . Part of the exposure light that illuminates the reticle  103  is extracted as it is reflected by the half mirror  106 . An ultraviolet photosensor  107  is arranged on the optical path of the light reflected by the half mirror  106 . The photosensor  107  generates an output corresponding to the intensity of the exposure light. 
   The output from the photosensor  107  is converted into an exposure energy per pulse by an integration circuit  108 , which integrates each pulse emission of the pulse laser light source  101 , and is provided to a controller  109 , which calculates an integrated exposure dose, of an exposure device. The controller  109  outputs an appropriate application voltage value and a laser emission command signal to a laser controller  110  on the basis of the calculation result. The energy of the exposure light generated by the pulse laser light source  101  is controlled in accordance with the application voltage value of the laser controller  110 . These operations are repeated to control the integrated exposure dose of the circuit pattern image of the reticle  103 , which is to be printed on the wafer  105 . 
   When the degradation of the light source, such as an excimer laser, exceeds an allowance, for example, the laser gas or constituent components must be exchanged. This maintenance is generally performed when the laser controller (light source controller) sends a maintenance request to the main controller of the exposure device in accordance with the number of emission pulses or use time. Upon reception of the maintenance request, the main controller can perform a process (e.g., an alarm to the operator) for the maintenance immediately after the exposure is ended if exposure operation is being performed, and immediately if a non exposure operation is being performed. 
   It is not preferable to limit the maintenance timing to a time point immediately after the maintenance request is made or immediately after the exposure operation is ended. For example, during a process that does not require laser beam emission or in adjustment of the lithography system, if the exposure device receives a maintenance request signal and a maintenance process for the laser light source is performed immediately in response to the signal, the resources may be wasted. If a timing for checking the maintenance request signal is constantly set to a time point immediately before the emission timing of the excimer laser, waste of the resources can be avoided. When, however, a maintenance request is generated, the wafer exposure process is always delayed by a time corresponding to the maintenance process time. This decreases the throughput. 
   SUMMARY OF THE INVENTION 
   The present invention has been made on the basis of the recognition of the above problems, and an exemplary object of the present invention to make appropriate the maintenance timing of the light source, e.g., reducing or eliminating execution of unnecessary maintenance. 
   One aspect of the present invention is directed to an exposure apparatus, and the apparatus comprises a pulse light source, an exposure unit which exposes a substrate to a pattern with light from the pulse light source, and a determination unit which determines necessity of maintenance for the pulse light source based on a pulse rate of the pulse light source within a predetermined period of time. 
   According to a preferred embodiment of the present invention, the determination unit can determine that maintenance is necessary, if the pulse rate is not less than a predetermined value. 
   According to a preferred embodiment of the present invention, the determination unit can calculate the pulse rate from a number of pulses, which is weighted in accordance with time in the predetermined period of time. 
   According to a preferred embodiment of the present invention, the determination unit can determine the need for maintenance based on a total number of pulses from the pulse light source, as well as the pulse rate. 
   According to a preferred embodiment of the present invention, the apparatus can further comprise a decision unit, which decides a timing of the maintenance based on a determination result of the determination unit. 
   According to a preferred embodiment of the present invention, the decision unit can set the timing of the maintenance to a time point after oscillation of the pulse light source is completed. 
   According to a preferred embodiment of the present invention, the decision unit can decide whether the maintenance is to be performed prior to oscillation of the pulse light source. 
   According to a preferred embodiment of the present invention, the decision unit can decide whether the maintenance is to be performed prior to execution of an exposure job. 
   According to a preferred embodiment of the present invention, the apparatus can further comprise a management unit, which manages a process concerning the maintenance as a job queue together with the exposure job. 
   According to a preferred embodiment of the present invention, the pulse light source can comprise an excimer laser light source. 
   Another aspect of the present invention is directed to a method of maintaining a pulse light source, and the method can be applied to an exposure apparatus that exposes a substrate to a pattern with light from the pulse light source. The method comprises steps of determining necessity of maintenance for the pulse light source based on a pulse rate of the pulse light source within a predetermined period of time, and deciding a timing of the maintenance based on a determination result in the determining step. 
   According to the present invention, the maintenance timing of the light source device can be optimized. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a block diagram showing the schematic structure of a lithography system according to a preferred embodiment of the present invention; 
       FIG. 2  is a flowchart showing in detail an exposure control method in the lithography system shown in  FIG. 1 ; 
       FIG. 3  is a view showing control software installed in a main controller  109  and control software installed in a laser controller  110 , respectively, of an exposure device  150  shown in  FIG. 1 , and the relationship between the two software; 
       FIG. 4  is a table schematically showing an example of a maintenance executive control table  306 ; 
       FIGS. 5A and 5B  are graphs each showing the relationship among the number of actual oscillation pulses within a preset time, the number of weighted oscillation pulses, and Duty; 
       FIG. 6  is a flowchart showing the flow of a process that maintenance control software  303  executes upon reception of a maintenance request notification  307  from laser control software  301 ; 
       FIG. 7  is a flowchart showing the flow of a laser oscillation process and a maintenance process performed before laser oscillation; 
       FIG. 8  is a flowchart showing step S 801  (laser oscillation preprocess) of  FIG. 7  in detail; 
       FIG. 9  is a block diagram showing the structure of an exposure apparatus disclosed in Japanese Patent Laid Open No. 5-62876; and 
       FIG. 10  is a view showing a maintenance process for a laser light source  101  when a plurality of jobs are to be processed at once. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
   First Embodiment 
     FIG. 1  is a block diagram showing the schematic structure of a lithography system according to a preferred embodiment of the present invention. The lithography system can include an exposure device  150  and a light source device  100 . For example, the exposure device  150  includes an illumination optical system  102 , a projection optical system  104 , a wafer stage  111 , a main controller  109 , and the like. For example, the light source device  100  includes a light source  101 , such as a laser light source or a pulse laser light source, and a laser controller (light source controller). 
   A gas, such as KrF, can be sealed in the laser light source  101  to emit light having a wavelength of 248 nm in a far ultraviolet region. For example, the laser light source  101  can be provided with a front mirror which forms a resonator, a band narrowing module formed of a diffraction grating, a prism, or the like, to narrow the band of the exposure wavelength, a monitor module formed of a spectroscope or a detector, to monitor the stability, spectrum, and width of the wavelength, and a shutter. The gas exchange operation of the laser light source  101 , wavelength stabilization, discharge application voltage, and the like, are controlled by a laser controller  110  (light source controller). In this lithography system, single control by only the laser controller  110  is not performed. The laser controller  110  operates in accordance with an instruction from the main controller  109  of the exposure device connected through an interface cable  130 . 
   For example, the illumination optical system  102  can include a beam shaping optical system, an optical integrator, a collimator, and a mirror (not shown). These members are formed of members that transmit or reflect light in the far ultraviolet region efficiently. The beam shaping optical system shapes a laser beam to have a desired spot shape. The optical integrator makes uniform the luminous intensity distribution characteristics of a beam. The circuit pattern of a semiconductor device to be printed is formed on a reticle (master)  103 , which can be aligned by a reticle stage (not shown). The reticle  103  is illuminated by the illumination optical system  102 . The circuit pattern of the reticle  103  is reduced and projected onto a wafer  105  by a projection optical system (reduction projection lens)  104 . 
   The wafer stage  111 , which aligns the wafer  105 , can move in a two-dimensional direction. A moving mirror  112  is fixed to the wafer stage  111 . A laser interferometer  113  detects the position of the wafer stage  111  by using the moving mirror  112 . 
   A stage control system  114  under the control of the main controller  109  of the exposure device controls a driver  115 , such as a motor, on the basis of the position information of the wafer stage  111  provided by the laser interferometer  113 , to move the wafer stage  111  to a predetermined position. 
   A half mirror  106  is arranged in the optical path between the illumination optical system  102  and reticle  103 . Part of the exposure light that illuminates the reticle  103  is extracted as it is reflected by the half mirror  106 . An ultraviolet photosensor  107  is arranged on the optical path of the light reflected by the half mirror  106 . The photosensor  107  generates an output corresponding to the intensity of the exposure light. The output from the photosensor  107  is converted into an exposure energy per pulse by an integration circuit  108 , which integrates each pulse emission of the laser light source  101 , and is provided to the main controller  109 , which calculates an integrated exposure dose, of the exposure device. 
   The main controller  109  outputs an application voltage value and a laser emission command signal to the laser controller  110  on the basis of the result calculated in accordance with a function V=f(E), which indicates the relationship between an application voltage V and pulse energy E. The energy of the exposure light generated by the pulse laser light source  101  is controlled in accordance with the application voltage value of the laser output controller  110 . These operations are repeated to control the integrated exposure dose of the circuit pattern image of the reticle  103 , which is to be printed on the wafer  105 . 
     FIG. 2  is a flowchart showing in detail an exposure control method in the lithography system shown in  FIG. 1 . In step S 201 , a total exposure dose Etotal as a target exposure dose is set. In step S 202 , a total number Ntotal of times of exposure (number of times of pulse emission) is set on the basis of the total exposure dose Etotal and a preset standard pulse energy Estd. In step S 203 , an integrated exposure dose SUM, a remaining number N of times of exposure, and an average pulse energy Eave are set. Note that the integrated exposure dose SUM is 0, that the average pulse energy Eave is a value obtained by Etotal÷Ntotal, and that the remaining number N of times of exposure is the total number Ntotal of times of exposure. 
   In step S 204 , the preset voltage V for initial exposure is calculated in accordance with the function V=f(E), which indicates the relationship between the voltage and energy. This function can be decided by measuring the pulse energy E with respect to the application voltage V. 
   In step S 205 , exposure is performed with a preset voltage Vave. In step S 206 , an actual exposure dose En is measured. In step S 207 , the measured exposure dose En is added to the present integrated exposure dose SUM to obtain a new integrated exposure dose SUM, and the remaining number N of times of exposure is decremented. In step S 208 , the integrated exposure dose SUM is subtracted from the total exposure dose Etotal, i.e., the sum of the remaining exposure doses is obtained, and the sum is divided by the remaining number N of times of exposure, thus obtaining a value Eave. Eave signifies the average energy per pulse of remaining exposure. 
   In step S 209 , the average energy Eave and the exposure energy En of one exposure pulse are compared. If the absolute value of the difference is smaller than a criterion Eε, the flow advances to step S 213 ; if NO, the flow advances to step S 210 . In step S 210 , whether Eave−En is larger or smaller than 0 is checked. If Eave−En&gt;0, in step S 211 , a preset change amount ΔV is added to the set voltage Vave to obtain a new Vave. If Eave−En&lt;0, in step S 212 , the change amount ΔV is subtracted from the set voltage Vave to obtain a new Vave, and the flow advances to step S 213 . In step S 213 , whether or not the remaining number N of times of exposure is zero is checked. If NO, the flow returns to step S 205 ; if YES, the process is ended. 
   In this manner, when the pulse energy that decreases along with a decrease in gas concentration of the laser light source  101  is detected and is fed back to the discharge application voltage to gradually increase the discharge application voltage, the pulse energy can be held at a constant value. 
     FIG. 3  is a view showing control software built in the main controller  109  and control software built in the laser controller  110 , respectively, of the exposure device  150  shown in  FIG. 1 , and the relationship between the two software. Information indicating the gas exchange time of the laser light source  101  and deterioration of the constituent components is usually obtained by counting the number of laser emission pulses and use time with laser control software  301  installed in the laser controller  110 . Maintenance control software  303  installed in the main controller  109  of the exposure device  150  can monitor the number of laser emission pulses and use time counted by the laser control software  301 . 
   When the count exceeds a preset upper limit value, the control software  301  of the laser controller  110  sends a maintenance request notification  307  to the maintenance control software  303  in the main controller  109  of the exposure device  150 . If the maintenance request notification  307  requests exchange of the constituent components of the laser light source  101 , the maintenance control software  303  sends a display request  310  to operation system software  304 , to display a predetermined message on the console screen to notify the operator. The operator can exchange the constituent components on the basis of the display content. 
   Main control software  302  supervises laser light source control other than the maintenance process. The exposure device  150  holds information indicating the current state of the lithography system as apparatus state information  305  in a storage (e.g., nonvolatile memory)  109   a . The apparatus state information  305  can include, e.g., information that means “oscillating laser” and “idling”. Also, time, the number of oscillation pulses (number of pulses of exposure light to be generated) commanded to the laser control software  301  (laser controller  110 ), and the maintenance state for each maintenance item are held as a maintenance executive control table  306  in the storage  109   a.    
     FIG. 4  is a table schematically showing an example of the maintenance executive control table  306 . Maintenance item  601  indicates a process or an operation for correcting a change over time in the characteristics of the laser light source, and includes, e.g., gas exchange, absolute wavelength correction, energy correction, and adjusted oscillation. The number  602  of oscillation command pulses is the integrated value of the number of oscillation pulses instructed from the main control software  302  built in the main controller  109  of the exposure device  150  to the laser control software  301  built in the laser controller  109 . The number  602  of oscillation command pulses is updated by the main control software  302  at each oscillation. When a maintenance process is performed, the number  602  of oscillation command pulses is cleared to zero by the maintenance control software  303 . Preset Duty (described as “Set Duty” as well in this specification)  604  indicates the proportion of laser oscillation time in preset time  603 . The operator can arbitrarily set the preset Duty  604  through the operation system software  304  for each maintenance item. A maintenance state  605  indicates the current state of a given maintenance item  601 , and can be “a normal state”, “executing maintenance”, or “maintenance requested”. 
     FIG. 6  is a flowchart showing the flow of a process that the maintenance control software  303  executes upon reception of the maintenance request notification  307  from the laser control software  301 . In step S 701 , it is checked whether the maintenance request is made because a specified number of pulses has been oscillated or a specified time has elapsed. This determination can be performed on the basis of the “number of oscillation command pulses” controlled by the maintenance executive control table  306 , or by acquiring information on the reason of the maintenance request from the laser control software  301 . If the maintenance request is made because the specified number of pulses has been oscillated, this means that the laser light source  101  is oscillating. In this case, the maintenance process must be performed, and accordingly, the flow advances to step S 704 . 
   If the maintenance request is made because the specified time has elapsed, in step S 702 , Duty is calculated to check whether or not the maintenance process should be executed at once. Duty indicates the proportion of the number of actual oscillation pulses to the maximum number of oscillation pulses that can be oscillated within a certain time. The Duty value can be calculated in accordance with the following equation (1):
 
Duty=(total number of weighted oscillation pulses)÷(maximum number of oscillation pulses)×100[%]  (1)
 
   The maximum number of oscillation pulses in equation (1) indicates the maximum number of oscillation pulses that can be oscillated within the preset time  603 , and can be calculated in accordance with the following equation (2):
 
(maximum number of oscillation pulses)=(maximum oscillation frequency of laser)×(preset time  603 )×60 [pulse]  (2)
 
   The total number of weighted oscillation pulses in equation (1) is the sum of values each obtained by multiplying the number of pulses (number of actual oscillation pulses) actually oscillated in a predetermined time segment (i.e., a time segment of t−Δt to t+ΔT) within the preset time  603  by the value of the function f(t) having as a variable typical time t in the predetermined time segment. The total number of weighted oscillation pulses can be calculated in accordance with the following equation (3):
 
(total number of weighted oscillation pulses)=Σ{(number of actual oscillation pulses)× f ( t )}  (3)
 
where t is 0.0 to the preset time  603  and f(t) is a weighting function.
 
     FIGS. 5A and 5B  are graphs each showing the relationship among the number of actual oscillation pulses within the preset time  603 , the number of weighted oscillation pulses, and Duty. In  FIGS. 5A and 5B , the axis of abscissa represents time, and the axis of ordinate represents the number of oscillation pulses. Each of numbers  404  and  504  of full-duty pulses indicates the maximum number of oscillation pulses in a certain time segment. Numbers  405  and  505  of set-duty pulses can be calculated in accordance with the following equation (4):
 (number of setduty pulses)=(number of full-duty pulses)×(Preset Duty 604)÷100 [pulse]  (4) 
   The numbers  402  and  502  of weighted oscillation pulses are respectively calculated by multiplying the numbers  401  and  501  of actual oscillation pulses by weighting functions (f(t))  403  and  503  of Exponential curves with which, e.g., the closer to a maintenance request reception time, the larger the weighting. The sum corresponds to equation (3). For example, the weighting functions  403  and  503  can be set by the operator by selecting from a plurality of polynomial functions or an arbitrary setting on the console screen. 
     FIG. 5A  shows a case wherein many laser oscillations are made in a time band close to the maintenance request reception time in the preset time. Due to the weighting function, the total number of weighted oscillation pulses becomes larger than the total number of actual oscillation pulses and larger than the value of “(numbers  405 ,  505  of set-duty pulses)×(preset time  603 )”. In this case, in step S 703 , the calculated Duty is larger than the preset Duty (Set Duty)  604 . Accordingly, it is determined that the maintenance should be executed immediately, and the flow advances to the process of step S 704 . 
   In step S 704 , the maintenance control software  303  loads the apparatus state information  305 . In step  706 , whether the laser is an “oscillating laser” or not is checked. If YES, in step S 707 , oscillation end is awaited. In step S 708 , execution of a desired maintenance process is started. In step S 709 , the maintenance state  605  is updated to “executing maintenance”. In step S 710 , execution of the maintenance process is ended. In step S 711 , the number  602  of oscillation command pulses is updated to zero. In step S 712 , the maintenance state  605  is updated to “normal”, and the process is ended. 
     FIG. 5B  shows a case wherein many laser oscillations are made in a time zone close to (maintenance request reception time−preset time) in the preset time  603 . Due to the weighting function, the total number of weighted oscillation pulses becomes smaller than the total number of actual oscillation pulses and smaller than the value of “(numbers  405 ,  505  of set-duty pulses)×(preset time  603 )”. In this case, in step S 703 , the calculated Duty is smaller than the preset Duty (Set Duty)  604 . Accordingly, it is determined that the maintenance need not be executed immediately, and the flow advances to the process of step S 705 . In step S 705 , the maintenance state  605  is updated to “maintenance requested”, and the process is ended. 
   As described above, the maintenance timing can be decided on the basis of a change in time (e.g.,  FIGS. 5A and 5B ) of the use status of the laser light source  101  within a predetermined time before reception of a maintenance request. 
     FIG. 7  is a flowchart showing the flow of a laser oscillation process and a maintenance process performed before laser oscillation. During the laser oscillation process, in the exposure device  150 , the main control software  302  requests the maintenance control software  303  to execute an oscillation preprocess. In step S 801 , the maintenance control software  303  executes a laser oscillation preprocess. When the laser oscillation preprocess is ended, the main control software  302  executes the laser oscillation process in step S 802 . 
     FIG. 8  is a flowchart showing step S 801  (laser oscillation preprocess) of  FIG. 7  in detail. In step S 901 , the maintenance control software  303  reads out the maintenance executive control table  306 . In steps S 902  to S 908 , the maintenance control software  303  processes respective maintenance items registered on the maintenance executive control table  306 . 
   More specifically, in step S 902 , the maintenance control software  303  checks whether or not the maintenance state  605  corresponding to a given maintenance item  601  is a “maintenance requested” state. If the maintenance state  605  is “normal” or “executing maintenance”, the flow advances to step S 908 . If the maintenance state  605  is “maintenance requested”, in step S 903 , execution of the maintenance process is started (for example, a message prompting execution of the maintenance is displayed on the console and a necessary process is executed). In step S 904 , the maintenance state  605  is updated to “executing maintenance”. 
   When execution of the maintenance process is ended in step S 905 , the number  602  of oscillation command pulses is updated to zero in step S 906 . In step S 907 , the maintenance state  605  is updated to the “normal” state. The flow then advances to step S 908 . In step S 908 , whether or not the next maintenance item (non processed maintenance item) is present is checked. If YES, the flow returns to step S 902 ; if NO, the series of laser oscillation preprocess are ended. 
   Second Embodiment 
   In the following description, the difference between the first and second embodiments of the present invention will be described. Except for the items to be described herein, the second embodiment can be the same as the first embodiment. 
     FIG. 10  is a view showing the maintenance process of a laser light source  101  when a plurality of jobs are to be processed at once. Japanese Patent Laid Open No. 8-167562 discloses a method of storing a file including job information in a job queue  1001  in advance, when a plurality of jobs are to be processed, and continuously processing them, as shown in  FIG. 10 . According to this embodiment, in the job queue  1001 , a maintenance process  1002  for the laser light source  101  is placed before exposure jobs A, B, C, and D, to maintain the laser light source  101  at an optimal state during execution of the exposure jobs A, B, C, and D. Therefore, errors such as an exposure dose control error can be decreased. 
   Alternatively, the job file can include flag information indicating whether or not the maintenance process for the laser light source  101  is to be executed, and whether or not the main control software  302  is to execute the maintenance process before the job can be decided. A job managing portion for managing the jobs can be typically formed in a software manner, and can be installed in the main control software  302 . 
   In the first embodiment, flag information indicting whether or not a laser oscillation preprocess is to be executed may be managed, and the maintenance process is to be performed at which timing, i.e., before the start of each exposure job or before the start of each laser oscillation, can be selected. For example, in an exposure job concerning a rough layer, an emphasis can be put on the throughput. As shown in  FIG. 10 , the process flow shown in  FIG. 8  is performed before the start of the exposure job. In an exposure jog for a critical relayer, an emphasis can be put on the exposure accuracy. The process flow shown in  FIG. 8  can be performed for each laser oscillation, as in the first embodiment. On which one an emphasis is to be put, the quality of the laser beam or the throughput, can be selected for each job. 
   As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the claims. 
   CLAIM OF PRIORITY 
   This application claims priority from Japanese Patent Application No. 2004-020379 filed on Jan. 28, 2004, the entire contents of which are hereby incorporated by reference herein.

Technology Classification (CPC): 6