Abstract:
An exposure apparatus for exposing a substrate to light includes a substrate stage adapted to hold and move the substrate, a scope adapted to measure a predetermined mark to align the substrate, and a controller adapted to control the position of the substrate stage and the operation of the scope, thereby executing first measurement and second measurement necessary for a single calibration item of the apparatus to align the substrate. The controller executes the first measurement and the second measurement at frequencies different from each other.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an exposure apparatus, exposure method, and device manufacturing method.  
       BACKGROUND OF THE INVENTION  
       [0002]     Conventionally, in order to attain precise relative alignment between a reticle and a wafer, the distance (base line amount) between a projection lens and an off-axis observation optical system needs to be precisely calculated.  
         [0003]      FIG. 2  is a view schematically showing a method of measuring the base line amount in a projection exposure apparatus. Reference numeral  1  denotes a light source;  2 , a reticle; and  3 , a position detection mark arranged on the reticle  2 . An illumination optical system  4  uniformly illuminates the reticle  2  with light from the light source  1 . A projection lens (projection optical system)  5  projects the pattern of a reticle onto a wafer. Reference numeral  6  denotes a wafer; and  7 , a wafer chuck to hold the wafer  6 . A wafer Z stage  8  can vertically drive the wafer chuck  7 . A wafer X-Y stage  9  can hold and drive the wafer Z stage  8  in the X and Y directions that are parallel to the wafer plane. A wafer stage includes the wafer chuck  7 , wafer Z stage  8 , and wafer X-Y stage  9 . A stage base  10  supports the X-Y stage  9 .  
         [0004]     A wafer stage position reference mark  11  is positioned to be almost flush with the surface of the wafer  6 . A TTL observation optical system  12  detects an image of the wafer stage position reference mark  11 , which has returned upon passing through the projection lens  5 . Reference numeral  13  denotes a TTL observation optical system arithmetic processor. Reference numeral  14  denotes an off-axis observation optical system provided separately from the projection lens  5 . Reference numeral  15  denotes an off-axis observation optical system arithmetic processor; and  16 , a controller to control the overall projection exposure apparatus.  
         [0005]     An image of the position detection mark  3  on the reticle  2  is projected onto the wafer stage position reference mark  11  through the projection lens  5 . The light reflected by the wafer stage position reference mark  11  is detected by the TTL position observation optical system  12 . This makes it possible to calculate the distance from the origin of the wafer stage  8  to the image of the reticle position detection mark  3 .  
         [0006]     The wafer Z stage  8  is activated to move the wafer stage position reference mark  11  under the off-axis observation optical system  14 . The off-axis position observation optical system  14  detects a position detection mark image of the wafer stage position reference mark  11 . This makes it possible to calculate the distance from the origin of the wafer stage  8  to the off-axis observation optical system  14 .  
         [0007]     On the basis of position information of a wafer stage position reference mark  11  measured by the projection lens  5  and that measured by the off-axis observation optical system  14 , the distance (base line amount) between the projection lens and the off-axis observation optical system can be calculated.  
         [0008]     For alignment precision improvement, it is very important to appropriately control the base line amount in such an exposure apparatus comprising the off-axis observation optical system  14  and TTL observation optical system  12  to control the base line amount, thereby indirectly executing alignment between the reticle and the wafer.  
         [0009]     In a best focus position detection method of detecting the position of the image plane of the projection lens  5  using the TTL observation optical system  12 , a reference mark on the wafer stage is observed through the projection lens  5 , a mark (reticle-side mark) on the reticle or reticle stage and the TTL observation optical system  12 . At this time, a focus adjustment process (to be also referred to as reticle-side focus measurement hereinafter) for the TTL observation optical system  12  on the reticle-side mark, and a process (to be also referred to as wafer-side focus measurement hereinafter) of moving the wafer stage in the direction of the optical axis of the projection lens  5  to match the image plane of the reticle-side mark with that of the wafer stage reference mark are executed. To improve the best focus position detection precision, it is very important to appropriately control the precisions of focus measurements on the reticle and wafer sides.  
         [0010]     When an exposure process is continuously performed, heat is generated upon driving the reticle and wafer stages and heat due to exposure light irradiation is also generated. A temperature change due to such heat varies the base line and best focus, resulting in deterioration of alignment precision and focus precision. To solve this problem, a method of executing measurement for every wafer exchange or predetermined wafer count is adopted.  
         [0011]     As shown in  FIG. 15 , this measurement method allows base line measurement by causing the off-axis observation optical system  14  and TTL observation optical system  12  to perform measurements at the same base line measurement timing. A measurement timing setting method shown in  FIG. 15  will be explained below.  
         [0012]     In step  1501 , an exposure process is started. The controller  16  determines in step  1502  whether the base line measurement timing is detected. If the base line measurement timing is detected (“YES” in step  1502 ), the flow advances to step  1503 . If no base line measurement timing is detected (“NO” in step  1502 ), the flow advances to step  1506 . In step  1503 , the TTL observation optical system  12  performs measurement. In step  1504 , the off-axis observation optical system  14  performs measurement. In step  1505 , the controller  16  combines the values measured by the observation optical systems to calculate the correction value of a correction item set in a job. The controller  16  controls to execute wafer alignment in step  1506  and exposure in step  1507 . The controller  16  determines in step  1508  whether exposure for all the shots on the wafer is complete. If exposure for all the shots is complete (“YES” in step  1508 ), the wafer exposure process is ended. If an unexposed shot remains (“NO” in step  1508 ), the flow returns to step  1502 .  
         [0013]     As described above, the conventional measurement method allows the TTL observation optical system  12  and off-axis observation optical system  14  to perform measurements at the same base line measurement timing.  
         [0014]     The conventional measurement method executes measurement at a preset design timing regardless of the state of the apparatus.  
         [0015]     For example, base line measurement is allowed by causing both the off-axis observation optical system and TTL observation optical system to perform measurements. However, parameters do not necessarily vary so greatly that corrections in both the optical systems are required. A variation on one side may be large and that on the other side may be small.  
         [0016]     Similarly, focus measurement systems also suffer focus variations on the reticle and wafer sides. A variation on one side may be large and that on the other side may be small.  
         [0017]     As a result, no measurement timing suitable for each observation optical system is set, so a further improvement in throughput is demanded.  
       SUMMARY OF THE INVENTION  
       [0018]     The present invention has been made in consideration of the above background, and has as its exemplary object to provide a novel technique for performing a process of a single apparatus calibration item including a plurality of measurement items.  
         [0019]     According to a first aspect of the present invention, there is provided an exposure apparatus for exposing a substrate to light, the apparatus comprising a substrate stage configured to hold the substrate and to move, a scope configured to measure a predetermined mark to align the substrate, and a controller configured to control a position of the substrate stage and an operation of the scope, and thereby to execute first measurement and second measurement necessary for a single calibration item of the apparatus to align the substrate, wherein the controller is configured to execute the first measurement and second measurement at respective frequencies different from each other.  
         [0020]     According to a second aspect of the present invention, there is provided an exposure method of exposing a substrate to light using an exposure apparatus including a substrate stage configured to hold the substrate and to move, and a scope configured to measure a predetermined mark to align the substrate, the method comprising steps of controlling a position of the substrate stage and an operation of the scope, thereby executing first measurement necessary for a single calibration item of the apparatus to align the substrate, and controlling the position of the substrate stage and the operation of the scope at a frequency different from a frequency of the first measurement, thereby executing second measurement necessary for the single calibration item.  
         [0021]     According to a third aspect of the present invention, there is provided a method of manufacturing a device comprising steps of exposing a substrate to a pattern using the above exposure apparatus, developing the exposed substrate, and processing the developed substrate to manufacture the device.  
         [0022]     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  
       [0023]     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.  
         [0024]      FIG. 1  is a view for schematically explaining a projection exposure apparatus according to a preferred embodiment of the present invention;  
         [0025]      FIG. 2  is a view for schematically explaining the conventional projection exposure apparatus;  
         [0026]      FIG. 3  is a graph showing a method of determining the measurement timing as a function of the elapsed exposure time (wafer count);  
         [0027]      FIG. 4  is a graph showing an example in which a value measured by an observation optical system as a function of the air pressure variation varies;  
         [0028]      FIG. 5  is a graph showing a method of determining the next allowable air pressure variation on the basis of the variation amount measured when the allowable air pressure variation has been exceeded;  
         [0029]      FIG. 6  is a graph showing an example in which a value measured by an observation optical system as a function of the elapsed time varies;  
         [0030]      FIG. 7  is a graph showing a method of determining the next allowable elapsed time on the basis of the variation amount measured when the allowable elapsed time has been exceeded;  
         [0031]      FIG. 8  is a flowchart showing the flow when a measurement timing setting method is applied to an exposure apparatus;  
         [0032]      FIG. 9  is a flowchart showing the flow when the measurement timing setting method is applied to base line measurement;  
         [0033]      FIG. 10  is a view showing the base line measurement and correction timings;  
         [0034]      FIG. 11  is a flowchart showing the flow when the measurement timing setting method is applied to best focus measurement;  
         [0035]      FIG. 12  is a view showing the best focus measurement and correction timings;  
         [0036]      FIG. 13  is a flowchart showing the flow of a position measurement method according to the preferred embodiment of the present invention;  
         [0037]      FIG. 14  is a flowchart showing the flow of the overall semiconductor device manufacturing process; and  
         [0038]      FIG. 15  is a flowchart showing the flow of the conventional base line measurement process. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0039]     A preferred embodiment of the present invention will be described below with reference to the accompanying drawings.  FIG. 1  is a schematic view showing a projection exposure apparatus (projection aligner) according to the preferred embodiment of the present invention. Reference numeral  1  denotes a light source;  2 , a reticle; and  3 , a position detection mark arranged on the reticle  2 . An illumination optical system  4  uniformly illuminates the reticle  2  with light from the light source  1 . A projection lens (projection system)  5  projects the pattern of a reticle onto a wafer as an object. Reference numeral  6  denotes a wafer; and  7 , a wafer chuck to hold the wafer  6 . A wafer Z stage  8  can vertically drive the wafer chuck  7 . A wafer X-Y stage  9  can hold and drive the wafer Z stage  8  in the X and Y directions that are parallel to the wafer plane. A wafer stage includes the wafer chuck  7 , wafer Z stage  8 , and wafer X-Y stage  9 . A stage base  10  supports the X-Y stage  9 .  
         [0040]     A wafer stage position reference mark  11  is fixed to the surface of the wafer Z stage  8 . A TTL observation optical system  12  detects an image of the wafer stage position reference mark  11 , which has returned upon passing through the projection lens  5 . Reference numeral  13  denotes a TTL observation optical system arithmetic processor. Reference numeral  14  denotes an off-axis observation optical system provided separately from the projection lens  5 . Reference numeral  15  denotes an off-axis observation optical system arithmetic processor; and  16 , a controller to control the overall projection exposure apparatus. Reference numeral  17  denotes a measurement timing arithmetic processor;  31 , a measurement variation data file; and  32 , an allowable variation amount setting data file  32 . A storage unit  33  stores the measurement variation data file  31  and allowable variation amount setting data file  32 .  
         [0041]     A case wherein the projection exposure apparatus according to the preferred embodiment of the present invention is applied to base line measurement will be exemplified.  
         [0042]     For base line measurement, the TTL observation optical system  12  and off-axis observation optical system  14  measure the wafer stage position reference mark  11 . Conventionally, as shown in  FIG. 1 , measurements using a plurality of observation optical systems, i.e., the TTL observation optical system  12  and off-axis observation optical system  14  are done at the same measurement timing. In such measurements, rough position detection and precise position detection are executed after reticle-side focus measurement, wafer-side focus measurement, and dimming of a measurement target mark, thus calculating the position misalignment amount (position variation amount). The measurement timing in such measurements is set in a job. Measurement is done at a measurement timing corresponding to every wafer exchange in exposure or predetermined wafer count.  
         [0043]     A measurement timing according to the preferred embodiment of the present invention will be exemplified below. An example of setting of the measurement timings of the observation optical systems will be explained first.  
         [0044]     The measurement variation data file  31  to give a measurement timing independently of each observation optical system is created. More specifically, to create the measurement variation data file  31 , the exposure conditions (including the exposure amount and scan speed) are changed. An exposure process is performed upon an increase and decrease in exposure load, and the observation optical systems execute measurements at the same time. A detected position variation amount b is recorded in association with the elapsed exposure time (wafer count) for each exposure condition to create the measurement variation data file  31 . The measurement variation data file  31  makes it possible to calculate a measurement variation in each observation optical system upon an increase and decrease in exposure load.  
         [0045]     Position measurements are periodically performed for the observation optical systems when the exposure apparatus is inactive (downtime). The position variation amount b as a function of the elapsed time (other than the elapsed exposure time) is recorded in the measurement variation data file  31 . Similarly, the position variation amount b as a function of the air pressure variation is recorded in the measurement variation data file  31 .  
         [0046]     The allowable position variation amount with respect to the values of the measurement variation data file  31  is set for each observation optical system in consideration of the precision standard. Also, a boundary value beyond which the position variation amount b as a function of the elapsed time or air pressure variation exceeds the allowable position variation amount is set as an allowable elapsed time variation or allowable air pressure variation, and recorded in the allowable variation amount setting data file  32 . As described above, creating the data files  31  and  32  containing position variation amounts measured under the respective exposure conditions makes it possible to calculate a measurement timing suitable for each exposure condition.  
         [0047]     The flow of a position measurement method according to the preferred embodiment of the present invention will be explained below with reference to  FIG. 13 . In the position measurement method according to this embodiment, the wafer stage position reference mark  11  formed on the wafer  6  is sensed through the TTL observation optical system  12  and off-axis observation optical system  14 . Position information of the wafer stage position reference mark  11  is measured on the basis of the image sensing result.  
         [0048]     In step  1301 , the measurement timing arithmetic processor  17  calculates the measurement timings of the TTL observation optical system  12  and off-axis observation optical system  14 . More specifically, the measurement timing arithmetic processor  17  calculates the measurement timings of the TTL observation optical system  12  and off-axis observation optical system  14  on the basis of the amount of a variation in position information of the wafer stage position reference mark  11 .  
         [0049]     In step  1302 , the controller  16  independently controls the TTL observation optical system  12  and off-axis observation optical system  14  at the measurement timings calculated in step  1301 . The detailed flows of control operations in step  1302  are illustrated in  FIGS. 8, 9 , and  11 , as will be described later. However, the control operations are not limited to these methods and can be appropriately changed.  
         [0050]     A measurement timing determination method when various exposure conditions such as an exposure load variation, air pressure variation, and elapsed time vary in the observation optical systems will be exemplified. A case wherein the exposure load on the exposure apparatus increases and decreases will be explained first with reference to  FIG. 3 .  
         [0051]      FIG. 3  is a graph representing a result obtained when the position variation amount b recorded in the measurement variation data file  31  and a value measured by each observation optical system as functions of the elapsed exposure time (wafer count) vary. An allowable measurement variation B 0  is set in the allowable variation amount setting data file  32 . An elapsed exposure time t 1  beyond which the variation amount b exceeds the allowable measurement variation B 0  is calculated and determined as a first measurement timing t 1 . A corresponding observation optical system performs measurement at the measurement timing t 1 . The allowable measurement variation B 0  is compared with a variation amount C 1  measured at this time to calculate an increase/decrease coefficient H (H=a*C 1 /B 0  (a: adjustment coefficient)). The position variation amount b recorded in the measurement variation data file  31  is updated on the basis of the increase/decrease coefficient H. Similarly, an elapsed exposure time t 2  beyond which the updated position variation amount b exceeds the allowable measurement variation B 0  next is calculated and determined as a next measurement timing t 2 . In updating the measurement variation data file on the basis of the increase/decrease coefficient H, only the latest value may be used. However, the variation amount C 1  measured at this time may be calculated by averaging or time-weighting several previous position variation amounts b. By repeating this method in the same manner, the next measurement timing is determined.  
         [0052]     With this operation, if a measured variation amount C 1  is larger than the allowable measurement variation B 0 , the next measurement timing can be set earlier than in the schedule. If a measured variation amount C 1  is smaller than the allowable measurement variation B 0 , the next measurement timing can be set later than in the schedule.  
         [0053]     Setting of the measurement timing as a function of the air pressure variation will be explained below.  
         [0054]      FIG. 4  is a graph showing an example in which a value measured by each observation optical system as a function of the air pressure variation varies on the basis of the position variation amount b recorded in the measurement variation data file  31 . On the basis of the result shown in  FIG. 4 , each observation optical system determines an allowable air pressure variation e 1  beyond which the allowable measurement variation B 0  has been exceeded. The observation optical system sets the allowable air pressure variation e 1  in the allowable variation amount setting data file  32 .  
         [0055]      FIG. 5  is a graph showing an example in which a next allowable air pressure variation e 2  is determined on the basis of a variation amount C 1  measured when the allowable air pressure variation e 1  has been exceeded. If a variation which exceeds the allowable air pressure variation e 1  set as described above is detected, a corresponding observation optical system performs measurement. The variation amount C 1  measured at this time is compared with the allowable measurement variation B 0  to calculate an air pressure variation beyond which the allowable measurement variation B 0  has been exceeded, thereby determining the next allowable air pressure variation e 2 . At the next time, measurement is done when the changed allowable air pressure variation range e 2  has been exceeded.  
         [0056]     Also in this case, in updating the measurement variation data file on the basis of a measured variation amount C 1 , only the latest value may be used. However, the variation amount C 1  measured at this time may be calculated by averaging or time-weighting several previous position variation amounts b. By repeating this method in the same manner, the next measurement timing is determined.  
         [0057]     Setting of the measurement timing as a function of the elapsed time will be explained below.  
         [0058]      FIG. 6  is a graph showing an example in which a value measured by each observation optical system as a function of the elapsed time varies on the basis of the position variation amount b recorded in the measurement variation data file  31 . On the basis of the result shown in  FIG. 6 , each observation optical system determines an allowable elapsed time f 1  beyond which the allowable measurement variation B 0  has been exceeded. The observation optical system sets the allowable elapsed time f 1  in the allowable variation amount setting data file  32 . If the set allowable elapsed time f 1  has been exceeded, a corresponding observation optical system performs measurement.  
         [0059]      FIG. 7  is a graph showing an example in which a next allowable elapsed time f 2  is determined on the basis of a variation amount C 1  measured when the allowable elapsed time f 1  has been exceeded. If a variation which exceeds the allowable elapsed time f 1  set as described above is detected, a corresponding observation optical system performs measurement. The variation amount C 1  measured at this time is compared with the allowable measurement variation B 0  to calculate a time beyond which the allowable measurement variation B 0  has been exceeded, thereby determining the next allowable elapsed time f 2 . At the next time, measurement is done when the changed allowable elapsed time f 2  has been exceeded.  
         [0060]     The measurement timings of the observation optical systems are determined in the above manner.  
         [0061]     A case wherein a measurement timing setting method is applied to an exposure apparatus will be exemplified below with reference to  FIG. 8 .  FIG. 8  is a flowchart showing details of control in step  1302  of  FIG. 13 . An item to be corrected is preset in the job in the exposure process procedure. At the beginning of activation of the exposure apparatus, no values measured by the observation optical systems exist. Hence, the observation optical systems perform measurements at a measurement timing set in the job, as in the prior art.  
         [0062]     In step  101 , an exposure process is started.  
         [0063]     The controller  16  determines in step  102  whether the measurement timing of an observation optical system A is detected. If the measurement timing of the observation optical system A is detected (“YES” in step  102 ), the flow advances to step  105 . If no measurement timing of the observation optical system A is detected (“NO” in step  102 ), the flow advances to step  103 .  
         [0064]     The controller  16  determines in step  103  whether the measurement timing of an observation optical system B is detected. If the measurement timing of the observation optical system B is detected (“YES” in step  103 ), the flow advances to step  106 . If no measurement timing of the observation optical system B is detected (“NO” in step  103 ), the flow advances to step  104 .  
         [0065]     The controller  16  determines in step  104  whether the measurement timing of an observation optical system C is detected. If the measurement timing of the observation optical system C is detected (“YES” in step  104 ), the flow advances to step  107 . If no measurement timing of the observation optical system C is detected (“NO” in step  104 ), the flow advances to step  108 .  
         [0066]     In step  105 , the observation optical system A measures position information of the wafer stage position reference mark  11 . In step  106 , the observation optical system B measures position information of the wafer stage position reference mark  11 . In step  107 , the observation optical system C measures position information of the wafer stage position reference mark  11 . In this manner, for each of the observation optical systems A to C, it is determined whether a measurement timing is detected. The targeted one of the observation optical systems A to C executes measurement.  
         [0067]     The controller  16  determines in step  108  whether measurement is performed in any one of steps  105  to  107 . If measurement is performed in any one of steps  105  to  107  (“YES” in step  108 ), the flow advances to step  109 . If measurement is performed in none of steps  105  to  107  (“NO” in step  108 ), the flow advances to step  111 .  
         [0068]     In step  109 , the controller  16  combines the values measured by the observation optical systems to calculate the correction value of a correction item set in a job.  
         [0069]     In step  110 , the measurement timing arithmetic processor  17  updates the data in the allowable variation amount setting data file  32  as functions of the elapsed exposure time, air pressure variation, and elapsed time to calculate the next measurement timing on the basis of the latest data.  
         [0070]     The controller  16  controls to execute wafer alignment in step  111  and exposure in step  112 .  
         [0071]     The controller  16  determines in step  113  whether exposure for all the shots on the wafer is complete. If exposure for all the shots is complete (“YES” in step  113 ), the wafer exposure process is ended. If an unexposed shot remains (“NO” in step  113 ), the flow returns to step  102 .  
         [0072]     A case wherein a measurement timing setting method is applied to base line measurement will be exemplified with reference to  FIG. 9 .  FIG. 9  is a flowchart showing details of control in step  1302  of  FIG. 13 . In base line measurement, measurements are done using the TTL observation optical system  12  and off-axis observation optical system  14 . In this exposure process procedure, base line correction is preset in the job as an item to be corrected.  
         [0073]     In step  201 , an exposure process is started.  
         [0074]     The controller  16  determines in step  202  whether the measurement timing of the TTL observation optical system  12  is detected. If the measurement timing of the TTL observation optical system  12  is detected (“YES” in step  202 ), the flow advances to step  205 . If no measurement timing of the TTL observation optical system  12  is detected (“NO” in step  202 ), the flow advances to step  203 .  
         [0075]     The controller  16  determines in step  203  whether the measurement timing of the off-axis observation optical system  14  is detected. If the measurement timing of the off-axis observation optical system  14  is detected (“YES” in step  203 ), the flow advances to step  206 . If no measurement timing of the off-axis observation optical system  14  is detected (“NO” in step  203 ), the flow advances to step  207 . If it is determined that the TTL observation optical system  12  needs to execute measurement, the TTL observation optical system  12  does the measurement. If it is determined that the off-axis observation optical system  14  needs to execute measurement, the off-axis observation optical system  14  does the measurement.  
         [0076]     In step  205 , the TTL observation optical system  12  measures position information of the wafer stage position reference mark  11 . In step  206 , the off-axis observation optical system  14  measures position information of the wafer stage position reference mark  11 .  
         [0077]     The controller  16  determines in step  207  whether measurement is performed in at least one of steps  205  and  206 . If measurement is performed in at least one of steps  205  and  206  (“YES” in step  207 ), the flow advances to step  208 . If measurement is not performed in either of steps  205  and  206  (“NO” in step  207 ), the flow advances to step  210 .  
         [0078]     If position information is measured by the TTL observation optical system  12  and off-axis observation optical system  14 , the controller  16  combines in step  208  the values obtained by both the observation optical systems. If measurement is performed only by the off-axis observation optical system  14 , the measured value is combined with the latest value held in the TTL observation optical system  12 . If measurement is performed only by the TTL observation optical system  12 , the measured value is combined with the latest value held in the off-axis observation optical system  14 . In this manner, the controller  16  calculates the base line correction amount.  
         [0079]     In step  209 , the measurement timing arithmetic processor  17  updates the data in the allowable variation amount setting data file  32  as functions of the elapsed exposure time, air pressure variation, and elapsed time to calculate the next measurement timing on the basis of the latest data.  
         [0080]     The controller  16  controls to execute wafer alignment in step  210  and exposure in step  211 .  
         [0081]     The controller  16  determines in step  212  whether exposure for all the shots on the wafer is complete. If exposure for all the shots is complete (“YES” in step  212 ), the wafer exposure process is ended. If an unexposed shot remains (“NO” in step  212 ), the flow returns to step  202 .  
         [0082]     Even if a measurement timing calculated on the basis of the elapsed exposure time does not appear within a predetermined time, measurement is periodically performed at a measurement timing detected from the allowable elapsed time or allowable air pressure variation.  
         [0083]     Even when the apparatus is inactive (downtime) after exposure completion, a measurement timing may be determined on the basis of the allowable elapsed time, allowable air pressure variation, or a decrease in exposure load, and measurement and correction may be automatically executed for a required observation optical system.  
         [0084]     As described above, measurement timings are independently given for the respective observation optical systems to set measurement timings in consideration of variations unique to the observation optical systems, thereby allowing a required observation optical system to execute measurement. Correction is done on the basis of a result obtained by combining the measurement values.  
         [0085]     The measurement variation data file is updated using the measured data and the next measurement timing. is generated on the basis of the updated data. This makes it possible to shorten the times required for correction and measurement.  
         [0086]      FIG. 10  is a view illustrating the measurement timings of the off-axis observation optical system and TTL observation optical system when a position variation in the off-axis observation optical system  14  is large at the base line correction timing.  
         [0087]     A case wherein the measurement timing setting operations are applied to best focus measurement will be exemplified below.  
         [0088]     Best focus measurement is conventionally allowed by causing the TTL observation optical system  12  to perform focus measurements on the reticle-side mark  3  and wafer stage reference-side mark  11 .  
         [0089]     In such measurements, reticle-side and wafer-side focus measurements are executed at the same measurement timing, or measurement is done only on the wafer side to calculate the best focus position. The measurement timings are set in the job. Measurement is done at a measurement timing corresponding to every wafer exchange in exposure or predetermined wafer count.  
         [0090]     A measurement timing according to the preferred embodiment of the present invention will be exemplified below with reference to  FIG. 11 .  FIG. 9  is a flowchart showing details of control in step  1302  of  FIG. 13 . In this exposure process procedure, best focus correction is preset in the job as an item to be corrected.  
         [0091]     In step  301 , an exposure process is started.  
         [0092]     The controller  16  determines in step  302  whether the measurement timing at which the TTL observation optical system  12  is allowed to execute reticle-side focus measurement is detected. If the measurement timing at which the TTL observation optical system  12  is allowed to execute reticle-side focus measurement is detected (“YES” in step  302 ), the flow advances to step  305 . If no measurement timing at which the TTL observation optical system  12  is allowed to execute reticle-side focus measurement is detected (“NO” in step  302 ), the flow advances to step  303 .  
         [0093]     The controller  16  determines in step  303  whether the wafer-side focus measurement timing is detected. If the wafer-side focus measurement timing is detected (“YES” in step  303 ), the flow advances to step  306 . If no wafer-side focus measurement timing is detected (“NO” in step  303 ), the flow advances to step  307 .  
         [0094]     Reticle-side focus measurement is executed in step  305  and wafer-side focus measurement is executed in step  306 .  
         [0095]     In this manner, for reticle-side and wafer-side focus measurements to be executed by the TTL observation optical system  12 , it is determined whether measurement timings are detected. If reticle-side focus measurement is determined to be required, it is done. If wafer-side focus measurement is determined to be required, it is done.  
         [0096]     The controller  16  determines in step  307  whether measurement is performed in at least one of steps  305  and  306 . If measurement is performed in at least one of steps  305  and  306  (“YES” in step  307 ), the flow advances to step  308 . If measurement is not performed in either of steps  305  and  306  (“NO” in step  307 ), the flow advances to step  310 .  
         [0097]     If the reticle-side and wafer-side focus measurements are executed, the controller  16  combines both the focus measurement values in step  308 . If only the wafer-side focus measurement is executed, the measured value is combined with the held latest reticle-side focus measurement value to calculate the focus correction amount. If only the reticle-side focus measurement is executed, the measured value is combined with the held latest wafer-side focus measurement value to calculate the focus correction amount. As described above, the controller  16  independently gives reticle-side and wafer-side measurement timing data to the TTL observation optical system  12  to set measurement timings in consideration of variations unique to the respective observation targets, thereby allowing a required one to be measured. Correction is done using a result obtained by combining the measurement values.  
         [0098]     The controller  16  controls to execute wafer alignment in step  310  and exposure in step  311 .  
         [0099]     The controller  16  determines in step  312  whether exposure for all the shots on the wafer is complete. If exposure for all the shots is complete (“YES” in step  312 ), the wafer exposure process is ended. If an unexposed shot remains (“NO” in step  312 ), the flow returns to step  302 .  
         [0100]      FIG. 12  is a view illustrating the reticle-side and wafer-side focus measurement timings when a variation in reticle-side focus is small.  
         [0101]     Assume that the respective observation optical systems execute measurements not at the same timing but at different times to combine the measurement values, thus correcting the focus position. In case of best focus correction, it is possible to exactly correct the focus position because a best focus result is calculated by wafer-side focus correction.  
         [0102]     As for base line correction, the base line amount is calculated by measuring a variation in distance between the projection optical system and the off-axis optical system. Therefore, the observation optical systems are allowed to execute measurements at the same measurement timing. However, measurement values obtained by the observation optical systems are basically kept unchanged in the exposure apparatus after it is activated. Accordingly, a variation in state in each observation optical system is monitored after apparatus activation. For example, assume that the TTL observation optical system  12  suffers position misalignment (position variation), the off-axis observation optical system  14  also suffers position misalignment (position variation), and the base line amount is unchanged. In this case, the TTL observation optical system  12  and off-axis observation optical system  14  record similar variations in the measurement variation data file obtained by pre-measurement by the corresponding exposure apparatus. As a result, both the observation optical systems can execute measurements at the same measurement timing, thus achieving exact correction.  
         [0103]     Even when the apparatus is inactive after exposure completion, a measurement timing is determined on the basis of the allowable elapsed time or allowable air pressure variation, and measurement and correction are automatically done for a required observation optical system. Hence, correction and measurement need not necessarily be executed immediately after the job starts.  
         [0000]     [Semiconductor Device Manufacturing Process] 
         [0104]     A case wherein an exposure apparatus according to a preferred embodiment of the present invention is applied to a semiconductor device manufacturing process will be exemplified below.  FIG. 14  is a flowchart showing the flow of the overall semiconductor device manufacturing process. In step S 1  (circuit design), the circuit of a semiconductor device is designed. In step S 2  (mask fabrication), a mask (also called an original or reticle) is fabricated on the basis of the designed circuit pattern. In step S 3  (wafer manufacture), a wafer (also called a substrate) is manufactured using a material such as silicon. In step S 4  (wafer process) called a pre-process, a projection exposure apparatus according to the preferred embodiment of the present invention is caused to form an actual circuit on the wafer by lithography using the mask and wafer. In step S 5  (assembly) called a post-process, a semiconductor chip is formed using the wafer manufactured in step S 4 . This step includes an assembly step (dicing and bonding) and packaging step (chip encapsulation). In step S 6  (inspection), the semiconductor device manufactured in step S 5  undergoes inspections such as an operation confirmation test and durability test. After these steps, the semiconductor device is completed and shipped in step S 7 .  
         [0105]     The semiconductor device process in step S 4  includes the following steps: an oxidation step of oxidizing the wafer surface; a CVD step of forming an insulating film on the wafer surface; an electrode formation step of forming an electrode on the wafer by vapor deposition; an ion implantation step of implanting ions in the wafer; a resist processing step of applying a photosensitive agent to the wafer; an exposure step of causing the above-described exposure apparatus to expose the wafer, which has been subjected to the resist processing step, through the mask on which the circuit pattern is formed; a development step of developing the latent image pattern of the wafer exposed in the exposure step; an etching step of etching portions other than the resist image developed in the development step; and a resist removal step of removing any unnecessary resist remaining after etching. By repeating these steps, a multilayered structure of circuit patterns is formed on the wafer.  
         [0106]     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 appended claims.  
         [0107]     This application claims the benefit of Japanese Patent Application No. 2005-168407 filed on Jun. 8, 2005, which is hereby incorporated by reference herein in its entirety.