Abstract:
In a scanning exposure apparatus and method, first and second interferometers of an interferometer system monitor first and second stages of a stage system, one of which holds a mask and the other of which holds a substrate. The first stage is moved at a first scanning speed during scanning exposure, and the second stage is moved at a second scanning speed during the scanning exposure, the speeds differing in accordance with a projecting magnification of a projection unit. The first stage is moved in accordance with a deviation, obtained by using the interferometer system, between the first stage and the second stage in a direction other than the scanning direction during the scanning exposure.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a projection exposure apparatus for use in a lithography step in the course of manufacturing a semiconductor element, a liquid crystal display element, etc.  
           [0003]    The present invention relates to a projection exposure method and a projection exposure apparatus for use in transfer-exposure of a mask pattern onto a photosensitive substrate when, for example, a semiconductor element, a liquid crystal display element, or the like, is manufactured by a lithography process, and in particular, to a projection exposure method and apparatus for effecting an exposure by switching the step-and-repeat method with the step-and-scan method.  
           [0004]    2. Related Background Art  
           [0005]    This kind of projection exposure apparatus has hitherto been classified roughly into two types. One of them may involve the use of a method of exposing a photosensitive substrate such as a wafer, a plate, etc. by a step-and-repeat method through a projection optical system having an exposure field capable of including a whole pattern of a mask (reticle). The other type may involve the use of a scan method of effecting the exposure with a relative scan performed under mask illumination of arched slit illumination light, wherein the mask and the photosensitive substrate are disposed in a face-to-face relationship with the projection optical system interposed therebetween.  
           [0006]    A stepper adopting the former step-and-repeat exposure method is a dominant apparatus in the recent lithography process. The stepper exhibits a resolving power, an overlap accuracy and a throughput which are all higher than in an aligner adopting the latter scan exposure method. It is considered that the stepper will continue to be dominant for some period from now on into the future.  
           [0007]    By the way, a new scan exposure method for attaining a high resolving power has recently been proposed as a step-and-scan method on pp.424-433 of Optical/Laser Microlithography II (1989), SPIE Vol.1088. The step-and-scan method is a combined version of the scan method of one-dimensionally scanning the wafer at a speed synchronizing therewith while one-dimensionally scanning the mask (reticle) and a method of moving the wafer stepwise in a direction orthogonal to a scan-exposure direction.  
           [0008]    [0008]FIG. 1 is an explanatory view showing a concept of the step &amp; scan method. Herein, shot regions (one chip or multi-chips) arranged in an X-direction on a wafer W are scan-exposed by beams of arched slit illumination light RIL. The wafer W is stepped in a Y-direction. Referring to the same Figure, arrows indicated by broken lines represent a route of the step &amp; scan (hereinafter abbreviated to S &amp; S) exposure. The shot regions undergo the same S &amp; S exposure in the sequence such as SA 1 , SA 2 , . . . SA 6 . Subsequently, the same S &amp; S exposure is performed on the shot regions in the sequence such as SA 7 , SA 8 , . . . SA 12  arranged in the Y-direction at the center of the wafer W. In the aligner based on the S &amp; S method disclosed in the above-mentioned literature, an image of the reticle pattern illuminated with the arched slit illumination light RIL is formed on the wafer W via a ¼ reduction projection optical system. Hence, an X-directional scan velocity of the reticle stage is accurately controlled to a value that is four times the X-directional scan velocity of the wafer stage. Further, the reason why the arched slit illumination light RIL is employed is to obtain such advantages that a variety of aberrations become substantially zero in a narrow (zonal) range of an image height point spaced a given distance apart from the optical axis by using a reduction system consisting of a combination of a refractive element and a reflex element as a projection optical system. One example of such a reflex reduction projection system is disclosed in, e.g., U.S. Pat. No. 4,747,678.  
           [0009]    Proposed in, e.g., Japanese Patent Laid-open Application No. 2-229423 (U.S. Pat. No. 4,924,257) is an attempt to apply a typical projection optical system (full field type) having a circular image field to an S &amp; S exposure method other than the above-described S &amp; S exposure method which uses the arched slit illumination light. The following are particulars disclosed in this Patent Laid-open Application. Exposure light with which the reticle (mask) is illuminated takes a regular hexagon inscribed to a circular image field of a projection lens system. Two face-to-face edges of the regular hexagon extend in a direction orthogonal to the scan-exposure direction. It is thus attained the S &amp; S exposure exhibiting a more improved throughput. That is, this Patent Laid-open Application shows that the scan velocities of the reticle stage and of the wafer stage can be set much higher than by the S &amp; S exposure method using the arched slit illumination light by taking an as large reticle (mask) illumination region in the scan-exposure direction as possible.  
           [0010]    According to the above-described prior art disclosed in Japanese Patent Laid-open Application No. 2-229423, the mask illumination region is enlarged in the scan-exposure direction to the greatest possible degree. This is therefore advantageous in terms of the throughput.  
           [0011]    By the way, there is nothing but to take the zigzag S &amp; S method shown in FIG. 1 even in the apparatus disclosed in the above-mentioned Patent Laid-open Application in consideration of actual scan sequences of mask stage and the wafer stage.  
           [0012]    The reason for this is given as follows. A diameter of the wafer W is set to 150 mm (6 inch). When trying to complete the exposure of one-row shot regions corresponding to the wafer diameter by only one continuous X-directional scan, the premise is that a ⅕ projection lens system is employed. Based on this premise, a scan-directional (X-directional) length is as long as 750 mm (30 inch). It is extremely difficult to manufacture this kind of reticle. Even if such a reticle can be manufactured, a stroke of the reticle stage for scanning the reticle in the X-direction requires 750 mm or more. Therefore, the apparatus invariably highly increases in size. For this reason, there is no alternative but to perform the zig-zag scan even in the apparatus disclosed in the above-mentioned Patent Laid-open Application.  
           [0013]    It is therefore required that the periphery of the pattern region on the reticle be widely covered with a light shielding substance so as not to transfer the reticle pattern within an adjacent shot region with respect to, e.g., the shot regions SA 1 , SA 12  shown in FIG. 1.  
           [0014]    [0014]FIGS. 2A and 2B each illustrate a layout of a hexagonal illumination region HIL, a circular image field IF of the projection lens system and a reticle R during a scan exposure. FIG. 2A shows a state where the hexagonal illumination region HIL is set in a start-of-scan position on the reticle R. Only the reticle R one-dimensionally moves rightward in the same Figure from this state. FIG. 2B illustrates a state at the end of one scanning process.  
           [0015]    Referring to FIGS. 2A and 2B, the symbols CP 1 , CP 2 , . . . CP 6  represent chip patterns formed in row in the X-direction on the reticle R. A row of these six chip patterns correspond to the shot regions to be exposed by one scanning process in the X-direction. Note that in the same Figures, the central point of the hexagonal illumination region HIL coincides substantially with the center of the image field, i.e., an optical axis AX of the projection lens system.  
           [0016]    As obvious from FIGS. 2A and 2B, the light shielding substance equal to or larger than at least a scan-directional width dimension of the hexagonal illumination region HIL is needed for the exterior of the pattern region in the start- and end-of-scan areas on the reticle R. Simultaneously, a scan-directional dimension of the reticle R itself also increases. An X-directional moving stroke of the reticle stage is also needed corresponding to a total of an X-directional dimension of the entire patterns CP 1 -CP 6  and a scan-directional dimension of the hexagonal illumination region HIL. Those are thinkable problems in terms of shaping up an apparatus.  
           [0017]    Also, since being optimized for either the step-and-repeat method or the step-and-scan method, the prior-art projection exposure apparatus unavoidably has disadvantages of each of the methods. The disadvantages belonging to the two methods are described in the following.  
           [0018]    A. Step-And-Repeat Method  
           [0019]    1. In order to increase an area for patterns to be transferred on the reticle, it is necessary to increase a lens diameter of the projection optical system. Thus, the increase of the area is limited together when the manufacturing cost of the projection optical system increases.  
           [0020]    2. Since an exposure field to be effected by the projection optical system is in the shape of a square substantially inscribed to an effective exposure field, a distortion of said exposure field becomes larger and an overlap accuracy is deteriorated when the exposure is effected on a layer having a different wafer by use of a different projection exposure apparatus (matching).  
           [0021]    3. since the area of an exposure field to be exposed simultaneously is large and an exposure energy (a degree of illuminance) per unit area is small, it is necessary to prolong the exposure time when a resist having a low sensitivity is used, whereby a throughput is decreased.  
           [0022]    B. Step-And-Scan Method  
           [0023]    1. Though the projection optical system can be manufactured at low cost, the manufacturing cost of a stage mechanism becomes high since it is necessary to scan the reticle and the wafer in synchronization. Moreover, when a resist having a high sensitivity is used, it is necessary to shorten the exposure time. For this reason, the scan velocity of the reticle stage is required to be higher. As a result, the manufacturing cost increases.  
           [0024]    2. Due to vibration at the scan-exposure time and an averaging of the distortions in the projection optical system, the image forming performance is deteriorated.  
           [0025]    3. When an overlap exposure is effected on different layers on the wafer by use of a single projection exposure apparatus, a distortion becomes different for each exposure. As a result, the overlap accuracy is deteriorated.  
         SUMMARY OF THE INVENTION  
         [0026]    It is a primary object of the present invention, which has been devised in view of the foregoing problems, to provide a projection exposure apparatus by a scan method (or an S &amp; S method) exhibiting an increased throughput by minimizing a moving stroke of a reticle stage during a scan-exposure without providing a specially wide light shielding substance along the periphery of a pattern exposure region on a reticle (mask).  
           [0027]    To accomplish this object, according to one aspect of the present invention, there is provided a projection exposure apparatus by a scan-exposure method, including an illuminating means for illuminating a mask transfer region with illumination light for an exposure through an aperture of a variable field stop disposed in a position substantially conjugate to the mask. This apparatus also includes a driving means for configuring the aperture of the variable field stop in a rectangular shape (having edges orthogonal to a direction of the scan-exposure) and simultaneously making variable a width of the rectangular aperture of the stop in a widthwise direction (the scan-exposure direction) of the transfer region (pattern forming region) on the mask.  
           [0028]    The projection exposure apparatus further includes a control means for controlling the driving means to change a width of the rectangular aperture of the variable field stop in interlock with variations in position of the variable field stop on the mask transfer region which varies due to the one-dimensional movements of the mask stage.  
           [0029]    Based on the conventional scan-exposure method, the mask is irradiated with the illumination light via an aperture in a fixed shape (hexagon, arched illumination area, etc.). According to the present invention, however, the scan-directional width of the aperture (variable field stop) is varied interlocking with a scan of the mask or the photosensitive substrate. The same S &amp; S exposure method can be therefore realized simply by sequentially narrowing the aperture width without causing a large overrun of the mask in the start- and end-of-scan areas on the mask. Accordingly, the overrun of the mask stage is eliminated in terms of its necessity or extremely reduced, whereby the moving stroke of the mask stage can be minimized. At the same time, the width of the light shielding substance formed along the periphery of the pattern forming region on the mask may also be small to the same extent as that in the conventional mask. The advantage lies in a decrease in labor for inspecting a pin hole defect in the light shielding substance (normally, a chrome layer) during a manufacturing process of the mask.  
           [0030]    Further, the aperture of the variable field stop is set in a shape adapted to the pattern forming region on the mask, thereby making it possible to utilize the apparatus also as a stepper equal to the conventional one.  
           [0031]    Besides, an aperture position and a geometrical configuration of the variable field stop are set to cause variations one-dimensionally, two-dimensionally or in a rotational direction within the image field of the projection optical system. It is thus feasible to instantaneously correspond to mask patterns of a variety of chip sizes.  
           [0032]    As explained above, according to the present invention, it is possible to minimize the moving stroke of the mask (reticle) in accordance with the scan-exposure method. A dimension of the light shielding band on the mask can also be reduced.  
           [0033]    At the same time, the scan-directional illumination region on the mask can be taken large, and, therefore, the throughput can be remarkably enhanced in combination with a diminution in the moving stroke.  
           [0034]    It is another object of the present invention, which has been devised in view of the foregoing problems, to provide a projection exposure method capable of enjoying the advantages of the step-and-repeat method and the step-and-scan method and capable of compensating the disadvantages of the step-and-repeat method and the step-and-scan method, as well as a projection exposure apparatus which can be used in embodying such a projection exposure apparatus.  
           [0035]    To accomplish this object, according to the present invention, there is provided a projection exposure method which has a step-and-repeat mode and a step-and-scan mode, to effect an exposure in either the step-and-repeat mode or the step-and-scan by using at least one of information pieces on a layout of a plurality of shot regions on a photosensitive substrate, a quantity of integrated exposure required on the photosensitive substrate, configurations of these shot regions, a resolving power required for pattern images of a mask, and an allowance for distortions. Therefore, it is possible to realize an exposure method which can make the most of only the advantages of both the step-and-repeat mode aid the step-and-scan mode, and is excellent in terms of all the performances including the throughput (the number of wafers to be processed per unit time) and the image forming performance, etc.  
           [0036]    According to the projection exposure apparatus of the present invention, it is possible to use the above-mentioned exposure method.  
           [0037]    According to the present invention, one of the both exposure methods is selected in one of the following manners.  
           [0038]    1) An exposure time for one photosensitive substrate is calculated on the basis of a layout of the shot regions, required quantity of integrated exposure, etc. Then, an exposure method having the shorter exposure time is selected.  
           [0039]    2) When a configuration of the shot region exceeds the width of an effective exposure field of the projection optical system with respect to a scan direction in the step-and-scan mode, the step-and-scan mode is selected.  
           [0040]    3) An exposure mode which can satisfy both the resolving power required for an exposure of mask patterns and an allowance for distortions is selected.  
           [0041]    When, for example, an exposure is effected in the step-and-scan mode for each shot region on the photosensitive substrate, if movements among the shot regions are conducted in a direction orthogonal to the scan direction, as indicated by a locus, the exposure time is reduced. On the other hand, when the exposure for each shot region is effected in the step-and-repeat mode, the movements among the shot regions are conducted in the short-side direction, as indicated by a locus. Then, the exposure time is reduced. Therefore, a stepping direction of the photosensitive substrate is switched over in accordance with a selected exposure mode, whereby the exposure time is further reduced.  
           [0042]    Moreover, in order to make uniform a distribution of luminance on the mask, it is preferable to dispose an optical integrator in an illumination optical system. In this case, since a cross-sectional configuration of an optical element of the optical integrator is substantially the same as that of an illumination region on the mask, if the exposure mode is switched over to change the configuration of the illumination region on the mask, the optical integrator equipped with an optical element having a cross-sectional configuration substantially equal to the configuration of said illumination region is used to improve the illumination efficiency.  
           [0043]    According to the present invention, an exposure is effected in the step-and-scan mode or the step-and-repeat mode, whichever is optimal, in accordance with a layout of shot regions on the photosensitive substrate, or the like. Therefore, when, for example, mask patterns to be exposed (or shot regions on the photosensitive substrate) occupy an elongated area, the step-and-scan mode is adopted, while the step-and-repeat mode is adopted when the sensitivity of the photosensitive substrate is high and the exposure time is further reduced in the step-and-repeat mode. Thus, it is possible to make the most of the advantages of both the step-and-repeat method and the step-and-scan method fully.  
           [0044]    On the other hand, the step-and-repeat mode in which the distortion characteristics are substantially fixed is adopted when the overlap exposure is effected by use of a single projection exposure apparatus and a high overlap accuracy (high alignment accuracy) is intended to be kept, while the scan-exposure mode in which a degree of luminance is enhanced with a slit-like exposure region is adopted. Thus, it is possible to make up for the disadvantages of the step-and-scan method and the step-and-repeat method.  
           [0045]    When a direction of a stepping movement among the shot regions on the photosensitive substrate is switched over in accordance with a used exposure mode, the stepping is effected in a direction having a shorter moving distance, whereby there is an advantage that the stepping time can be shortened.  
           [0046]    When a plurality of optical integrators are replaceably provided in the illumination optical system and these optical integrators are switched to be used in accordance with the exposure mode, even if an exposure mode is changed and the size of the exposure region on the photosensitive substrate is changed, deterioration in the illumination efficiency can be prevented.  
           [0047]    It is still another object of the present invention to increase the size of an exposure field which can be transferred by one scan by using a projection optical system having a circular image field and utilizing a rectangular or slit-like region projecting along the diameter within said circular image field.  
           [0048]    It is still another object of the present invention to control one or both of a first light-shielding means (shutter) which is disposed between a light source and a secondary light source generating means and a second light-shielding means (movable blade) which is disposed between the secondary light source generating means and a condensing optical system in cooperation in accordance with a sequence of the scan-exposure.  
           [0049]    It is still another object of the present invention to accurately correct a relative rotational error (yawing) generated during a relative movement between the mask and the photosensitive substrate to improve the quality of an image which is projected onto the photosensitive substrate, as well as to improve an overlap accuracy between a pattern region formed on the photosensitive substrate and projected images of the mask patterns. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0050]    Other objects and advantages of the present invention will become apparent during the following discussion in conjunction with the accompanying drawings, in which:  
         [0051]    [0051]FIG. 1 is an explanatory view showing a concept of a conventional step &amp; scan exposure method using a beam of arched slit illumination light;  
         [0052]    [0052]FIGS. 2A and 2B are explanatory views showing a conventional scan-exposure method employing a beam of regular hexagonal illumination light;  
         [0053]    [0053]FIG. 3 is a view illustrating a construction of a projection exposure apparatus in an embodiment of this invention;  
         [0054]    [0054]FIG. 4 is a plan view illustrating shapes of blades of a blind mechanism;  
         [0055]    [0055]FIG. 5 is a plan view showing a pattern layout of a reticle which is suitable for the apparatus of FIG. 3;  
         [0056]    FIGS.  6 A- 6 E are explanatory views showing scan-exposure processes in the embodiment of this invention;  
         [0057]    [0057]FIG. 7 is a plan view illustrating another pattern layout of the reticle mountable on the apparatus of FIG. 3;  
         [0058]    [0058]FIG. 8 is a plan view illustrating shapes of the blades of the blind mechanism in a second embodiment;  
         [0059]    FIGS.  9 A- 9 F are explanatory views showing a sequence of a step &amp; scan exposure in the second embodiment;  
         [0060]    [0060]FIG. 10 is a plan view illustrating other shapes of the blades;  
         [0061]    [0061]FIG. 11 is a perspective view illustrating a structure of the blind mechanism in a third embodiment;  
         [0062]    [0062]FIG. 12 is an explanatory view showing how the blind mechanism operates;  
         [0063]    [0063]FIG. 13 is a view showing a construction of a projection exposure apparatus in a fourth embodiment of the present invention;  
         [0064]    [0064]FIG. 14 is a graph showing a relation of a throughput with respect to a sensitivity of a photoresist in the fourth embodiment;  
         [0065]    [0065]FIG. 15A is a graph showing a relation between a configuration of a shot region on a wafer and a usable exposure mode;  
         [0066]    [0066]FIG. 15B is a view showing a relation between an exposure region on the wafer and the shot regions;  
         [0067]    [0067]FIG. 16A is a plan view showing a relation between a reticle and an illumination region when an exposure is effected in the scan-exposure mode;  
         [0068]    [0068]FIG. 16B is a plan view showing a relation between the reticle and the illumination region when the exposure is effected in the collective-exposure mode; and  
         [0069]    [0069]FIGS. 17A and 17B are views for explaining a relation between an exposure mode and a stepping direction.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0070]    [0070]FIG. 3 illustrates a construction of a projection exposure apparatus in a first embodiment of this invention. This embodiment involves the use of a projection optical system (hereinafter simply termed a projection lens for simplicity) PL constructed of only a ⅕ reduction refractive element which is telecentric on both sides or of a combination of the refractive element and a reflex element.  
         [0071]    Exposure illumination light emitted from a mercury lamp  2  is condensed at a second focal point through an elliptical mirror  4 . Disposed at this second focal point is a rotary shutter  6  for switching over a cutoff and a transmission of the illumination light with the aid of a motor  8 . The exposure the illumination light passing through the shutter  6  is reflected by a mirror  10 . The illumination light beam is then incident on a fly eye lens system  14  via an input lens  12 . A multiplicity of secondary light source images are formed on the outgoing side of the fly eye lens system  14 . The illumination light beam from each of the secondary light source images falls on a lens system (condenser lens)  18  via a beam splitter  16 . Movable blades BL 1 , BL 2 , BL 3 , BL 4  of a blind mechanism  20  are, as illustrated in FIG. 4, arranged on a rear focal plane of the lens system  18 . Four pieces of blades BL 1 , BL 2 , BL 3 , BL 4  are individually independently moved by a driving system  22 . In accordance with this embodiment, an X-directional (scan-exposure direction) width of an aperture AP is determined by edges of the blades BL 1 , BL 2 . A Y-directional (stepping direction) length of the aperture AP is determined by the edges of the blades BL 3 , BL 4 .  
         [0072]    Further, a shape of the aperture AP defined by the respective edges of the four blades BL 1 -BL 4  is so determined as to be embraced by a circular image field IF of the projection lens PL. Now, the illumination light has a uniform distribution of illuminance in a position of the blind mechanism  20 . A reticle R is irradiated with the illumination light via a lens system  24 , a mirror  26  and a main condenser lens  28  after passing through the aperture AP of the blind mechanism  20 . At this time, an image of the aperture AP defined by the four blades BL 1 -BL 4  of the blind mechanism  20  is formed on a pattern surface of the underside of the reticle R. Note that an arbitrary image forming magnification can be given by the lens system  24  in combination with-the condenser lens  28 . Herein, however, it is assumed that an approximately 2-fold enlarged image of the aperture AP of the blind mechanism  20  is projected on the reticle R. Hence, an X-directional moving velocity Vbl of the blades BL 1 , BL 2  may be set to Vrs/2 in order to make a scan velocity Vrs of the reticle R during a scan exposure coincident with a moving velocity of an edge image of the blades BL 1 , BL 2  of the blind mechanism  20  which is projected on the reticle R.  
         [0073]    Now, the reticle R undergoing the illumination light defined by the aperture AP is held on a reticle stage  30  movable at an equal velocity at least in the X-direction on a column  32 . The column  32  is integral with, though not illustrated, a column for fixing a lens barrel for the projection lens PL. The reticle stage  30  performs a microscopic rotational movement for a yawing correction and a one-dimensional scan movement in the X-direction with the aid of a driving system  34 . A movable mirror  36  for reflecting a length measuring beam emitted from a laser interferometer  38  is fixed to one end of the reticle stage  30 . An X-directional position of the reticle R and a yawing quantity are measured in real time by the laser interferometer  38 . Note that a fixed mirror (reference mirror)  40  for the laser interferometer  38  is fixed to an upper edge of the lens barrel for the projection lens PL. A pattern image formed on the reticle R is reduced by a factor of 5 through the projection lens PL and formed on a wafer W. The wafer W is held together with a fiducial mark plate FM by means of a wafer holder  44  capable of making a microscopic rotation. The holder  44  is installed on a Z stage  46  capable of effecting a micromotion in the (Z-) direction of an optical axis AX of the projection lens PL. Then, the Z stage  46  is installed on an XY stage  48  moving two-dimensionally in X- and Y-directions. This XY stage  48  is driven by a driving system  54 . Further, a yawing quantity and a coordinate position of the XY stage  48  are measured by a laser interferometer  50 . A fixed mirror  42  for the laser interferometer  50  is fixed to a lower edge of the lens barrel for the projection lens PL. A movable mirror  52  is fixed to one edge of the Z stage  46 .  
         [0074]    In accordance with this embodiment, the projection magnification is set to ⅕. Therefore, during the scan exposure an X-directional moving velocity Vws of the XY stage  48  is ⅕ of the velocity Vrs of the reticle stage  30 . Provided further in this embodiment is an alignment system  60 , based on a TTR (through the reticle) method, for detecting an alignment mark (or a fiducial mark FM) on the wafer W through the projection lens PL as well as through the reticle R. Provided also is an alignment system  62 , based on a TTL (through the lens) method, for detecting the alignment mark (or the fiducial mark FM) on the wafer W through the projection lens PL from a space under the reticle R. A relative alignment between the reticle R and the wafer W is conducted before a start of an S &amp; S exposure or during the scan exposure.  
         [0075]    Further, a photoelectric sensor  64  shown in FIG. 3, when the fiducial mark FM is formed as a luminescent type, receives the light from this luminescent mark via the projection lens PL, the reticle R, the condenser lens  28 , the lens systems  24 ,  18  and the beam splitter  16 . The photoelectric sensor  64  is employed when determining a position of the reticle R in a coordinate system of the XY stage  48  or when determining a detection central position of each of the alignment systems  60 ,  62 . By the way, the aperture AP of the blind mechanism  20  is elongated as much as possible in the Y-direction orthogonal to the scan direction (X-direction), thereby making it possible to decrease the number of the X-directional scanning actions, or in other terms, the number of Y-directional stepping actions of the wafer W. In some cases, however, the Y-directional length of the aperture AP may be varied by the respective edges of the blades BL 3 , BL 4  depending on sizes, shapes and an arrangement of chip patterns on the reticle R. An adjustment may be made so that the face-to-face edges of the blades, e.g., BL 3 , BL 4  are aligned with street lines for defining a shot region on the wafer W. With this adjustment, a correspondence to a variation in the Y-directional size of the shot region can be easily obtained.  
         [0076]    Further, if the Y-directional dimension of one shot region is not smaller than the Y-directional maximum dimension of the aperture AP, as disclosed in preceding Japanese Patent Laid-open Application No. 2-229423, it is required that an exposure quantity be brought into a seamless state by effecting an overlap exposure inwardly of the shot region. A method in this instance will be explained in greater detail.  
         [0077]    Next, the operation of the apparatus in this embodiment will be discussed. A sequence and control thereof are managed in a generalizable manner by a main control unit  100 . The basic action of the main control unit  100  lies in causing relative movements of the reticle stage  30  and the XY stage  48  keeping a predetermined velocity ratio during the scan exposure while retaining a relative positional relationship between the reticle pattern and the wafer pattern within a predetermined alignment error. These relative movements are effected based on inputting of velocity information given from tacho-generators in the driving systems  34 ,  54  as well as on inputting of yawing and positional information from the laser interferometers  38 ,  50 .  
         [0078]    Then, the main control unit  100  in this embodiment is remarkably characterized, in addition to its operation, by interlock-controlling the driving system  22  so that scan-directional edge positions of the blades BL 1 , BL 2  of the blind mechanism  20  are shifted in the X-direction in synchronization with scanning of the reticle stage  30 .  
         [0079]    Note that if the illumination quantity during the scan exposure is fixed, the absolute velocities of the reticle stage  30  and of the XY stage  48  have to be increased according as the scan-directional maximum opening width of the aperture AP becomes larger. In principle, when the same exposure quantity (dose amount) is given to a resist on the wafer, and if the width of the aperture AP is doubled, the velocities of the XY stage  48  and the reticle stage  30  have to also be doubled.  
         [0080]    [0080]FIG. 5 shows a relationship in layout between the reticle R mountable on the apparatus illustrated in FIG. 3 and the aperture AP of the blind mechanism  20 . It is herein assumed that four pieces of chip patterns CP 1 , CP 2 , CP 31  CP 4  are arranged in the scan direction on the reticle R. The respective chip patterns are sectioned by light shielding bands corresponding to the street lines. A periphery of an aggregated region (shot region) of the four chip patterns is surrounded with a light shielding band having a width Dsb larger than the street line.  
         [0081]    Let herein SBl, SBr be the right and left light shielding bands extending along the periphery of the shot region on the reticle R. It is also presumed that reticle alignment marks RM 1 , RM 2  be formed externally of these light shielding bands.  
         [0082]    The aperture AP of the blind mechanism  20  also includes edges E 1 , E 2  of the blades BL 1 , BL 2  which extend in parallel to the Y-direction orthogonal to the scan direction (X-direction). Let Dap be the scan-directional width of these edges E 1  E 2 . Further, a Y-directional length of the aperture AP is substantially equal to a Y-directional width of the shot region on the reticle R. The blades BL 3 , BL 4  are so set that the edges for defining the longitudinal direction of the aperture AP coincide with the center of the peripheral light shielding band extending in the X-direction.  
         [0083]    The following is an explanation of how an S &amp; S exposure is conducted in this embodiment with reference to FIGS.  6 A- 6 E. The premise herein is such that the reticle R and the wafer W shown in FIG. 5 are relatively aligned by use of the alignment systems  60 ,  62  and the photoelectric sensor  64 . Incidentally, FIGS.  6 A- 6 E each sketch a profile of the reticle R shown in FIG. 5. For facilitating the understanding of motions of the blades BL 1 , BL 2  of the blind mechanism  20 , the blades BL 1 , BL 2  are herein illustrated just above the reticle R.  
         [0084]    To start with, as illustrated in FIG. 6A, the reticle R is set at a start-of-scan point in the X-direction. Similarly, one corresponding shot region on the wafer W is set at the start of the X-directional scan.  
         [0085]    At this time, an image of the aperture AP through which the reticle R is illuminated has a width Dap that is ideally zero. It is, however, difficult to make the width completely zero, depending on the conditions where the edges E 1 , E 2  of the blades BL 1 , BL 2  are configured. Then, in accordance with this embodiment, the width Dap of the image of the aperture AP on the reticle is smaller to some extent than the width Dsb of the light shielding band SBr on the right side of the reticle R. Generally, the width Dsb of the light shielding band SBr is on the order of 4-6 mm, while the width Dap of the image of the aperture AP on the reticle may be set to about 1 mm.  
         [0086]    Then, as shown in FIG. 6A, the X-directional center of the aperture AP is arranged to deviate by ΔXs from the optical axis AX in a direction (left side in the same Figure) opposite to the scan advancing direction of the reticle R. This distance ΔXs is set to approximately one-half of the maximum opening width Dap of the aperture AP with respect to this reticle R. Explaining it more specifically, the longitudinal dimension of the aperture AP is determined automatically by the Y-directional width of the shot region of the reticle R. Hence, a maximum value DAmax of the X-directional width Dap of the aperture AP is also determined by a diameter of the image field IF. The maximum value thereof is previously calculated by the main control unit  100 . Further, the distance ΔXs is determined to satisfy strictly a relationship such as DAmin+2·ΔXs=DAmax, where Dmin is the width (minimum) of the aperture AP at the start-of-scan point shown in FIG. 6A.  
         [0087]    Next, the reticle stage  30  and the XY stage  48  are moved in the directions reverse to each other at a velocity ratio proportional to the projection magnification. At this time, as illustrated in FIG. 6B, only the blade BL 2  located in the advancing direction of the reticle R in the blind mechanism  20  is moved in synchronization with the movement of the reticle R so that an image of the edge E 2  of the blade BL 2  exists on the light shielding band SBr.  
         [0088]    Then, the scan of the reticle R proceeds, and the edge E 2  of the blade BL 2  reaches, as shown in FIG. 6C, a position to determine the maximum opening width of the aperture AP. Thereafter, the movement of the blade BL 2  is halted. The driving system  22  for the blind mechanism  20  therefore incorporates a tacho-generator and an encoder for monitoring both a moving quantity and a moving velocity of each blade. Pieces of positional and velocity information given therefrom are transmitted to the main control unit  100  and employed for the synchronization with the scanning motion of the reticle stage  30 .  
         [0089]    Thus, the reticle R is sent in the X-direction at a constant velocity up to a position shown in FIG. 6D while being irradiated with the illumination light passing through the aperture AP having the maximum width. That is, the image of the edge E 1  of the blade BL 1  located in the direction opposite to the advancing direction of the reticle R is, as depicted in FIG. 6E, run in the same direction in synchronization with the moving velocity from the time when the image of the edge E 1  of the blade BL 1  reaches the light shielding band SBl on the left side of the shot region of the reticle R.  
         [0090]    Then, when the left light shielding band SBl is intercepted by the edge image of the right blade BL 2  (at this moment, the left blade BL 1  also comes, and the width Dap of the aperture AP becomes the minimum value DAmin), the movements of the reticle stage  30  and the blade BL 1  are stopped.  
         [0091]    With the actions described above, the exposure (for one shot) by one-scan of the reticle comes to an end, and the shutter is closed. However, if the width Dap of the aperture AP is well smaller than the width Dsb of the light shielding band SBl (or SBr) in that position, and when the illumination light leaking to the wafer W can be made zero, the shutter  6  may remain opened.  
         [0092]    Next, the XY stage  48  is stepped in the Y-direction by one row of the shot regions. Scanning on the XY stage  48  and the reticle stage  30  is effected in a direction reverse to the direction set so far. The same scan-exposure is performed on a different shot region on the wafer W.  
         [0093]    As discussed above, in accordance with this embodiment, the scan-directional stroke of the reticle stage  30  can be minimized. Besides, there is such an advantage that the light shielding bands SBl, SBr for defining the both sides of the shot region with respect to the scan direction may be small in terms of their widths Dsb. Note that an unevenness in the exposure quantity in the scan direction is caused on the wafer W until the reticle stage  30  is accelerated from the state shown in FIG. 6A enough to reach a constant velocity scan.  
         [0094]    For this reason, it is required that a prescan (prerunning) range be determined at the start of scanning until the state of FIG. 6A is obtained. In this case, it follows that the widths Dsb of the light shielding bands SBr, SBl are expanded corresponding to a length of the prescan. This is similarly applied to a case where an overscan is needed corresponding to the fact that the constant velocity motion of the reticle stage  30  (XY stage  48 ) can not be abruptly stopped when finishing one scan-exposure.  
         [0095]    Also in the case of performing the prescan and the overscan, however, the shutter  6  is set at a high speed. If an open response time (needed for bringing the shutter from a full closing state to a full opening state) and a close response time are considerably short, and just when the reticle stage  30  enters a main scan (position in FIG. 6A) after a completion of the prescan (acceleration) or shifting from the main scan to an overrun (deceleration), the shutter may be opened and closed interlocking therewith.  
         [0096]    A response time t s  of the shutter  6  may satisfy the following relationship under a condition such as Dsb&gt;DAmin:  
         ( Dsb−DA min)/ Vrs&gt;t   s    
         [0097]    where Vrs (mm/sec) is the constant scan velocity during the main scan on, e.g., the reticle stage  30 , Dsb (mm) is the width of each of the light shielding bands SBl, SBr, and DAmin (mm) is the minimum width of the aperture AP on the reticle R.  
         [0098]    Further, according to the apparatus in this embodiment, the yawing quantities of the reticle stage  30  and of the XY stage  48  are measured independently by the laser interferometers  38 ,  50 , respectively. A difference between the two yawing quantities is obtained by the main control unit  100 . A trace amount of rotation of the reticle stage  30  or the wafer holder  44  may be caused during the scan-exposure so that the difference therebetween becomes zero. In this instance, however, it is necessary that a center of the microscopic rotation be always identical with the center of the aperture AP. Taking a structure of the apparatus into consideration, it is possible to readily actualize a method of causing the microscopic rotation of an X-directional guide portion of the reticle stage  30  about an optical axis AX.  
         [0099]    [0099]FIG. 7 shows an example of another pattern layout of the reticle R mountable on the apparatus depicted in FIG. 3. The chip patterns CP 1 , CP 2 , CP 3  are employed for exposing the wafer by a step-and-scan (S &amp; S) method using the illumination light coming from the slit aperture AP as in the case of the reticle R shown in FIG. 5. Further, other chip patterns CP 4 , CP 5  formed on the same reticle R are employed for exposing the wafer by a step-and-repeat (S &amp; R) method. This kind of proper use can be easily attained by setting the aperture AP with the aid of the blades BL 1 -BL 4  of the blind mechanism  20 . When exposing, e.g., the chip pattern CP 4 , the reticle stage  30  is moved and set so that a center of the chip pattern CP 4  coincides with the optical axis AX. At the same time, the shape of the aperture AP may simply be matched with an external shape of the chip pattern CP 4 . Then, only the XY stage  48  may be moved in a stepping mode. As discussed above, if the reticle pattern is set as shown in FIG. 7, the S &amp; S exposure and the S &amp; R exposure can be executed selectively by the same apparatus and, besides, done without replacing the reticle.  
         [0100]    [0100]FIG. 8 illustrates one example of configurations of the blades BL 1 -BL 4  of the blind mechanism  20  that correspond to a case where a size of the on-the-reticle chip pattern to be exposed in the (Y-) direction orthogonal to the scan direction increases with respect to the image field IF of the projection optical system. The edges E 1 , E 2  for defining the scan-directional (X-directional) width of the aperture AP, as in the same way in FIG. 4 given above, extend in parallel in the Y-direction. The edges E 3 , E 4  for determining the longitudinal direction of the aperture AP are parallel to each other but inclined to the X-axis. The aperture AP assumes a parallelogram. In this case, four pieces of blades BL 1 -BL 4  move in the X- and Y-directions in interlock with the movement of the reticle during the scan exposure. An X-directional moving velocity Vbx of an image of each of the edges E 1 , E 2  of the blades BL 1 , BL 2  in the scan-exposure direction is, however, substantially the same as the scan velocity Vrs of the reticle. If there exists a necessity for moving the blades BL 3 , BL 4 , a Y-directional moving velocity Vby of each of the edges E 3 , E 4  is required to synchronize with a relationship such as Vby=Vbx·tan θe, where θe is the inclined angle of each of the edges E 3 , E 4  with respect to the X-axis.  
         [0101]    FIGS.  9 A- 9 F schematically illustrate a scan sequence during the S &amp; S exposure in the case of an aperture shape shown in FIG. 8. Throughout FIGS.  9 A- 9 F, it is assumed that the aperture AP is projected on the reticle R and defined by the respective edges E 1 -E 4  thereof. In accordance with a second embodiment shown in FIGS. 8 and 9A- 9 F, a chip pattern region CP on the reticle R which is to be projected on the wafer W has, it is also presumed, a size that is approximately twice the longitudinal dimension of the aperture AP. The second embodiment therefore takes such a structure that the reticle stage  30  is stepped precisely in the Y-direction orthogonal to the scan direction.  
         [0102]    At the first onset, the blades BL 1 , BL 2  shown in FIG. 8 are adjusted and set as illustrated in FIG. 9A at the start of scanning.  
         [0103]    More specifically, the aperture AP having a width narrowed most is positioned on the light shielding band SBr on the right side of the reticle R. Simultaneously, the left edge E 1  of the aperture AP is set in a position (edge position in which the aperture AP is expanded most in the X-direction) spaced most away from the optical axis AX. Further, throughout FIGS.  9 A- 9 F, the exposure quantity for one scan-exposure lacks in subregions Ad, and As each extending beltwise in the scan direction (X-direction). These subregions Ad, As are formed because of the fact that the upper and lower edges E 3 , E 4  of the aperture AP are inclined to the X-axis. A Y-directional width of each of the subregions Ad, As is univocally determined such as DAmax·tan θe, where θe is the inclined angle of each of the edges E 3 , E 4 , and DAmax is the maximum aperture width defined by the edges E 1 , E 2 . The scan-exposure is conducted while overlapping triangular areas shaped by the edges E 3 , E 4  of the aperture AP in the Y-direction with respect to the subregion Ad of the subregions Ad, As with this unevenness in terms of the exposure quantity that are set in the pattern region CP. An attempt to make the exposure quantity uniform is thus made. Further, in connection with the other subregion As, this subregion is matched exactly with the light shielding band on the reticle R.  
         [0104]    Now, the reticle R and the edge E 2  (blade BL 2 ) are made to run substantially at the same velocity in a +X-direction (right in the same Figure) from the state shown in FIG. 9A. Eventually, as depicted in FIG. 9B, the X-directional width of the aperture AP is maximized, and the movement of the edge E 2  is also halted. In this state shown in FIG. 9B, the center of the aperture AP substantially coincides with the optical axis AX.  
         [0105]    Thereafter, only the reticle R moves at the constant velocity in the +X-direction. As illustrated in FIG. 9C, the edge E 1  (blade BL 1 ) and the reticle R move rightward (in the +X-direction) substantially at the same velocity from the time when the left edge E 1  of the aperture AP enters the left light shielding band SBl. Approximately a lower half of the chip pattern region CP is thus exposed. The reticle R and the aperture AP are stopped in a state shown in FIG. 9D.  
         [0106]    Next, the reticle R is stepped precisely by a fixed quantity in a −Y-direction. The wafer W is similarly stepped in a +Y-direction. Then, a state shown in FIG. 9E is developed. At this time, a relative positional relationship in the Y-direction is so set that the overlapped subregion Ad undergoes an overlap exposure at the triangular area defined by the edge E 4 . Additionally, on this occasion, if it is required that the Y-directional length of the aperture AP be varied, a movement of the edge E 3  (blade BL 3 ) or E 4  (blade BL 4 ) is controlled in the Y-direction.  
         [0107]    Next, the reticle R is scan-moved in a −X-direction, and simultaneously the edge E 1  (blade BL 1 ) is moved in the −X-direction in interlock therewith. Then, as shown in FIG. 9F, when the aperture width defined by the edges E 1 , E 2  comes to the maximum, the movement of the edge E 1  is stopped. Only the reticle R continuously moves at the constant velocity in the −X-direction.  
         [0108]    With the actions described above, it is possible to expose, on the wafer W, the chip pattern region CP equal to or larger than the Y-directional dimension of the image field of the projection optical system. Besides, the overlapped subregion Ad is set. The two edge subregions (triangular areas) undergo the overlap exposure by two scan-exposing processes, wherein the exposure quantity lacks depending on the shape of the aperture AP by one scan-exposing process. The exposure quantity within the subregion Ad is also made uniform (seamless).  
         [0109]    [0109]FIG. 10 sketches other blade configurations of the blind mechanism  20 . The edges E 1 , E 2  of the blades BL 1 , BL 2  which determine the scan direction are conceived as straight lines parallel to each other. The edges of the blades BL 3 . BL 4  extending in the direction orthogonal to the scan-direction take triangles that are symmetric with respect to the Y-axis passing through the optical axis AX. Then, the edges of the blades BL 3 , BL 4  herein assume, when approaching each other in the Y-direction, complementary shapes capable of substantially completely intercepting the light. Accordingly, the aperture AP may take a so-called chevron shape. In the case of such a chevron shape also, the uniformness can be similarly attained by executing the overlap exposure on the triangular areas at both ends.  
         [0110]    Next, the blade configurations and the operation in accordance with a third embodiment of this invention will be explained with reference to FIGS. 11 and 12. Provided herein, as shown in FIG. 11, is a fixed blade BL 0  formed with a rectangular aperture ST having a fixed width in the scan-exposure direction (X-direction). The rectangular aperture ST includes two parallel edges E 1 ′, E 2 ′ for defining its X-directional width. The rectangular aperture ST is formed as a rectangle elongated in the Y-direction on the whole. The illumination region in the longitudinal direction is defined by the respective edges E 3 , E 4  of the blades BL 3 , BL 4  disposed on the lower surface of the fixed blade BL 0  and one-dimensionally moving in the Y-direction. Hence, the aperture AP is defined by the four edges E 1 ′, E 2 ′, E 3 , E 4 .  
         [0111]    Disposed further on the upper surface of the fixed blade BL 0  are the blades BL 1 , BL 2  moving in the X-direction in interlock (synchronization) therewith at the start or end of movement of the reticle R. In accordance with this embodiment the edges E 1 , E 2  of the blades BL 1 , BL 2  do not, unlike each of the preceding embodiments, serve as those for defining a range where an interior of the chip pattern region of the reticle R is illuminated during the scan-exposure. These edges E 1 , E 2  function simply as a shutter. Therefore, the blades BL 1 , BL 2  may be constructed to move at a high velocity with more reduction in weight than that of other blades BL 3 , BL 4 .  
         [0112]    [0112]FIG. 12 schematically showing a relationship of the reticle R versus the blades BL 1 , BL 2  and the fixed blade BL 0  that are shown in X-Z section. It is herein assumed that the reticle R moves from left to right, and the scan-exposure is then effected. Further, in this embodiment, the edges E 1 ′, E 2 ′ of the fixed blade BL 0  and the respective edges E 3 , E 4  of the blades BL 3 , BL 4  are located on a plane P 0  conjugate to the pattern surface of the reticle R. The blades BL 1 , BL 2  serving as a shutter are, it is presumed, located on a plane P 1  spaced by ΔZ away from the plane P 0  in the direction of the optical axis AX.  
         [0113]    Now, the edge E 1  of the left blade BL 1  is, as shown in FIG. 12, located more rightward than the edge E 2 ′ of the fixed blade BL 0  to completely shield the aperture AP from the light until the left light shielding band SBl on the reticle R reaches the right edge E 2 ′ partly configuring the aperture AP since the scan of the reticle R (wafer W) has been started. Then, a portion in the vicinity of the X-directional mid-point of the light shielding band SBl reaches a position corresponding to the edge E 1  of the blade BL 1 . Hereafter, the blade BL 1  moves leftward at a velocity corresponding to the moving velocity of the reticle R so that the edge E 1  follows up the light shielding band SBl. The edge E 1  of the blade BL 1  is thus located more leftward by a given quantity than the left edge E 1 ′ of the fixed blade BL 0  (the aperture AP is fully opened). Just at this moment, the movement of the blade BL 1  is halted. At the end of the scan-exposure, the right blade BL 2  moves so that the edge E 2  of the blade BL 2  tracks the light shielding band SBr on the right side of the reticle R. Eventually, the blade BL 2  completely shields the aperture AP from the light.  
         [0114]    In accordance with the embodiments discussed above, the edges E 1 , E 2  of the blades BL 1 , BL 2  functioning as the shutter are always projected in a defocus state on the light shielding band SBl or SBr but not projected within the chip pattern region at all.  
         [0115]    By the way, when effecting the scan-exposure by means of a pulse light source such as an excimer laser, etc., it follows that a pulse light emission takes place every time the reticle R (wafer W) moves a given distance (a value well smaller than the X-directional width of the aperture AP). In this instance, it is desirable that the projection image, on the reticle R, of the edges E 1 ′, E 2 ′ of the aperture AP which define the scan-directional width would rather lack in sharpness. Namely, it may be preferable that the edges E 1 ′, E 2 ′ deviate from the plane P 0 .  
         [0116]    On the other hand, the edges E 1 , E 2  of the blades BL 1 , BL 2  functioning as the shutter give an unnecessary light quantity to the exteriors of the light shielding bands, depending on the defocus quantity thereof, if the scan-directional width of each of the light shielding bands SBl, SBr is small. It is therefore desirable that the edges E 1 , E 2  of the blades BL 1 , BL 2  would rather be projected on the reticle R as a sharp image. Thereupon, paying attention to FIG. 12, the plane P 1  may be conjugate to the pattern surface of the reticle R, while the plane P 0  may be shifted to a defocus position.  
         [0117]    [0117]FIG. 13 is a view showing a construction of an apparatus for explaining a fourth embodiment of the present invention.  
         [0118]    [0118]FIG. 13 shows a projection exposure apparatus in the fourth embodiment. Referring to FIG. 13, a cross-sectional form of a laser beam emitted from an excimer laser source such as an ArF excimer laser, or a KrF excimer laser, or a pulse laser source  71  such as a harmonics generating apparatus of a YAG laser is expanded by a laser expander consisting of lenses  72 A and  72 B. This laser beam passes through a light quantity variable filter plate  73  and falls on a fly eye lens  74 A of a first group. The light quantity variable filter plate  73  is comprised of a plurality of light quantity attenuating filters which has a permeability varying stepwise on the circumference of a rotary plate thereof. It is possible to attenuate a quantity of the emitted laser beams by rotating the light quantity variable filter plate  73  through a driving motor  75 . The driving motor  75  is controlled by an exposure quantity control system  76 . The exposure quantity control system  76  performs continuous adjustment, etc., of a light emitting timing and a quantity of the light emitted from the pulse laser source  71  through a source system  82  for the laser source.  
         [0119]    The exposure quantity control system  76  is a control system for making a quantity of an integrated exposure for a wafer  105  to be an optimal exposure quantity. The main control  7  which manages an operation of the whole apparatus in a generalizable manner comprises the exposure quantity control system  76 , an exposure mode determination means  79  for determining a following exposure mode (the step-and-repeat mode or the step-and-scan mode) and a target integrated exposure mode, a memory  80  for storing various kinds of data, etc. Also, the operator supplies information on a kind of the reticle  96  to be exposed next, the photosensitivity of a photoresist on the wafer  105  (target integrated exposure quantity), a layout of shot regions on the wafer (a shot layout), and so on, to a control means  78  inside the main control system  77  through a keyboard  71 . An exposure mode to be used next is determined by the exposure mode determination means  79  on the basis of the above-mentioned information. Note that the control means  78  and the exposure mode determination means  79  in the present embodiment are software functions in the computer.  
         [0120]    The fly eye lens  74 B is disposed so as to be replaced with the fly eye lens  74 A of the first group through a fly eye lens replacing apparatus  83 . Pulse laser beams emitted from a multiplicity of secondary light sources on an ongoing plane of the fly eye lens  74 A (or  74 B) are, after passing through a collimator lens  84 , deflected by a vibration mirror  85 , and then fall on a fly eye lens  87 A of a second group. The vibration mirror  85  vibrates the laser beams to a predetermined direction by use of a vibrator  86 , whereby an unevenness in the illuminance generated on the wafer  105  under the influence of an interference fringe of the laser beams having strong coherency is attenuated. In this case, the ongoing plane of the fly eye lens  4 A of the first group is conjugate to the ongoing plane of the fly eye lens  87 A of the second group, as shown as an optical path indicated by a broken line. Also, the incident plane of the fly eye lens  87 A of the second group is conjugate to a pattern plane of a reticle  96 .  
         [0121]    Also, for the fly eye lens  87 A of the second group, the fly eye lens  87 B is disposed to be replaceable therewith through the fly eye lens replacing apparatus  83 .  
         [0122]    Since a configuration (size) of the illumination region on the reticle  96  in the step-and-repeat method is different from that in the step-and-scan method and an incident plane of each lens element of the fly eye lens  87 A of the second group is conjugate to the illumination region on the reticle  96 , it is required to optimize a length-to-breadth ratio (configuration) of the lens element of the fly eye lens in accordance with an exposure method (the illumination region on the reticle  96 ). That is, in this embodiment, the fly eye lenses  87 A and  87 B of the second group are used when the exposure is effected by the step-and-repeat method and the step-and-scan method, respectively. In the same way, the fly eye lenses  74 A and  74 B of the second group are used when the exposure is effected by the step-and-repeat method and the step-and-scan method, respectively.  
         [0123]    Pulse laser beams (hereinafter termed an illumination light) IL emitted from a tertiary light source of the ongoing plane of the fly eye lens  87 A (or  87 B) of the second group are condensed by a first relay lens  88  to reach a fixed blind (fixed field stop)  89 . The fixed blind  89  is arranged to retract from an optical path of the illumination light IL through a blind control apparatus  90  any time under a command of the exposure quantity control system  76 . The illumination light IL passing through an aperture of the fixed blind  89  falls on a movable blind which is comprises of four movable blinds (only two movable blinds  91 A and  91 B are illustrated in FIG. 13). The illumination light IL passing through the aperture of the movable blind further passes through a second relay lens  93 , a mirror  94  for bending the optical path, and a main condenser lens  95 , and then illuminates an illumination region  97  of the lower plane (a pattern plane) of the reticle  96  with uniformly distributed illuminance. Then, a pattern image within the illumination region  97  of the reticle  96  is projected onto an exposure region  106  on the wafer  105  through a projection optical system  104 .  
         [0124]    In this case, the movable blades  91 A,  91 B, and so on, are supported so as to go forward or backward in a direction perpendicular to the optical axis AX by use of opening/closing mechanisms  92 A,  92 B and so on, respectively. Operations of the opening/closing mechanisms  92 A,  92 B, and so on, are controlled by the blind control apparatus  90 . A plane on which the movable blind consisting of the movable blades  91 A,  91 B, etc., (hereinafter termed the movable blinds ( 91 A and  91 B)) are disposed is conjugate to the pattern plane of the reticle  96 . A plane on which the fixed blind  89  is disposed is at a position slightly deviating from the plane conjugate to the pattern plane of the reticle  96 . In this embodiment, the fixed blind  89  is used to determine the slit-like illumination region on the reticle  96  when the exposure is effected in the step-and-scan mode. Therefore, when the exposure is effected in the collective-exposure mode, the fixed blind  89  is retracted from the optical path for the illumination light IL through the blind control apparatus  70 .  
         [0125]    When the exposure is effected in the step-and-scan mode, it is feared that the pulse illumination light IL passing through the outer side of the light-shielding band which surrounds the patterns to be transferred of the reticle  96  upon start and completion of the scan-exposure may sensitize a photoresist on the wafer  105  when only the fixed blind  89  is used. Then, upon the start or completion of the scan-exposure, the movable blinds ( 91 A and  91 B) are gradually opened or closed in synchronization with scans of the reticle stage and the wafer stage, in respective scan directions. Thus, unnecessary pattern exposure can be prevented. When the scan-exposure is effected, since the movable blinds ( 91 A and  91 B) are used instead of a shutter, the pulse laser source  1  may effect pulse emission during a time from opening of the movable blinds ( 91 A and  91 B) to fully closing.  
         [0126]    On the other hand, when the exposure is effected in the step-and-repeat mode, the configuration and the size of the illumination region on the reticle  96  are determined by the movable blinds ( 91 A and  91 B). When the static-exposure for step-and-repeat type is effected, a quantity of integrated exposure is measured by, for example, an unillustrated photoelectric detector (integrator sensor). Light emission from the pulse laser source  71  is stopped at the time when this integrated exposure quantity reaches the target exposure quantity.  
         [0127]    Next, a stage mechanism, etc., in this embodiment will be described. Herein, the axis Z is taken in parallel to the optical axis AX of the projection optical system  104 , the axis X is taken on a plane perpendicular to the axis Z and in parallel to a sheet plane shown in FIG. 13, and the axis Y is taken perpendicular to the sheet plane shown in FIG. 13. First, the reticle  96  is held on the reticle stage  98 , and the reticle stage  98  is movably mounted on a reticle base  99  in the X direction via a linear motor  100 . A moving mirror  101  is fixed on one end of the reticle stage  99 , the X coordinate of the reticle stage  98  is measured by a laser interferometer  102  which is provided externally and emits laser beams to this moving mirror  101 , and the measured X coordinate is supplied to a stage control system  103 . The stage control system  103  controls an operation of the linear motor  100  on the basis of the supplied X coordinate. Note that there are provided between the reticle stage  98  and the reticle  96  an unillustrated micromotion stage for microscopically moving the reticle  96  in the X direction, the Y direction and the rotational direction (the θ direction).  
         [0128]    The wafer  105  is supported on a Z-levelling stage  108  which moves the wafer  105  in the Z direction and performs a levelling, and is mounted on a wafer base  110  via a Y stage  109 Y and an X stage  109 X. The X stage  109 X is X-directionally driven by the driving motor  113  with respect to the wafer base  110 , while the Y stage  109 Y is Y-directionally driven by an unillustrated driving motor with respect to the X stage  109 X. An L-shaped moving mirror  111  for the X axis and the Y axis is fixed on the Z-levelling stage  108 . An external laser interferometer  112  for irradiating laser beams onto this moving mirror  111  measures an X coordinate and a Y coordinate of the Z-levelling stage  108 , and supplies the measured coordinates to the stage control system  103 . The stage control system  103  controls operations of the X stage  109 X and the Y stage  109 Y through the driving motor  113 , etc., on the basis of the supplied X coordinate and the Y coordinate.  
         [0129]    More specifically, when the exposure is effected in the step-and-scan mode, if a projection magnification to be conducted by the projection optical system  104  is β (β is, for example, ¼ or ⅕), the wafer  105  is −X-directionally (or +X-directionally) scanned reticle  96  is scanned at a speed V W  (=β·V R ) via the X stage  109 X in synchronization with the +X-directional (or −X-directional) scanning of the reticle  96  at the speed V R  via the reticle stage  98 . On this occasion, a very small positional deviation, a deviation of a rotation angle, and a deviation of a speed are compensated by an unillustrated micromotion stage disposed on the reticle  96  side. Also, when the plurality of shot regions on the wafer  105  are successively exposed in the step-and-scan mode, a motion of the wafer  105  among the shot regions is effected by the stepping of the X stage  109 X and the Y stage  109 Y. That is, the exposure is effected by the step-and-scan method.  
         [0130]    Next, when the exposure is effected in the step-and-repeat mode, the exposure is effected in a state where the reticle  96  and the wafer  105  are remain at a standstill. A motion among the plurality of shot regions on the wafer  105  is conducted by stepping of the X stage  109 X and the Y stage  109 Y. That is, the exposure is effected by the step-and-repeat method. However, a quantity of a positional deviation (a remaining error) between the reticle  96  and each shot region on the wafer  105  is compensated by an unillustrated micromotion stage on the reticle  96  side.  
         [0131]    The projection optical system  104  is provided with a lens controller  107 . The lens controller  107  compensates image forming characteristics such as a projection magnification of the projection optical system  104 , distortions, etc., by adjusting a pressure of a gas sealed up in a space between predetermined lenses inside the projection optical system  104 , or by adjusting a position in the direction of the optical axis AX of a predetermined lens which constitutes the projection optical system  104 , or by adjusting an angle of inclination of said lens. When the illumination light IL is continuously irradiated onto the projection optical system  104 , the image forming characteristics of the projection optical system  104  are changed due to heat accumulation. As a result, the main control system  77  offsets the changes of said image forming characteristics through a lens controller  107 . On this occasion, a distribution of luminance of light beams in the projection optical system  104  in the step-and-repeat mode is different from that in the step-and-scan mode so that a quantity of change of the image forming characteristics is also different. A quantity of change of the image forming characteristics in the step-and-repeat mode and that in the step-and-scan mode are stored in advance in a memory  80  of the main control system  77 , and the main control system  97  controls an operation of the lens controller  107  in accordance with an exposure mode.  
         [0132]    Further, though unillustrated, the projection exposure apparatus in the present embodiment is provided with alignment system of the TTR (through the reticle) method, the TTL (through the lens) method, or the off-axis method. A quantity of a positional deviation between the reticle  96  and each shot region on the wafer  105  is measured by this alignment system. Then, when the exposure is effected in the step-and-scan mode, the quantity of the positional deviation between the reticle  96  and said shot region is adjusted to be within a permissible value at the start of the scan, and a quantity of a positional deviation at a corresponding alignment mark is adjusted to be within the permissible value even during the scan if necessary. Moreover, when the exposure is effected in the step-and-repeat mode, said positional deviation is adjusted to be within the permissible range before the exposure, and the adjusted state is maintained during the exposure.  
         [0133]    Next, in the present embodiment, a specific description will be made on which mode, the step-and-repeat mode or the step-and-scan mode, the exposure mode determination means  79  in the main control system  77  selects.  
         [0134]    A. When a throughput (the number of wafers to be processed per unit time) is used as a criterion.  
         [0135]    First, if a time required for wafer replacement or wafer alignment is denoted by WA [sec], a time required for a stepping among the shot regions on the wafer by S [sec], and a time required for an exposure by EX [sec], a throughput T [the number of wafers per hour] can be obtained by the following expression.  
           T= 3600/( WA+S+EX )  (1)  
         [0136]    In this case, there is no substantial difference in the time WA required for the replacement and the alignment, and the stepping time S between the static-exposure and the scan exposure. On the contrary, there arises a large difference in the-exposure time EX.  
         [0137]    First, the exposure time EX scan  is obtained, by expressing the exposure EX in the step-and-scan mode as EX scan . For this reason, when the projection magnification by the projection optical system  104  is denoted by β [times], the sensitivity of the photoresist on the wafer  105  by P [mJ/cm 2 ], the X-directional width (the slit width) of the exposure region  106  on the wafer  105  by W [mm], the Y-directional length of said exposure region  106  by L [mm], the laser power of the pulse laser source  1  by E [mW], composite transmissivity from the pulse laser source  1  to the wafer  105  by μ, the frequency of the laser oscillation by F [Hz], and the minimum number of pulses in the exposure region  106  determined based on a quantity of light irradiation onto the wafer  105  by N m  [number of pulses], the following expression is given.  
           N   m   =μ·P·E /( W·L·F )  (2)  
         [0138]    Then, if the minimum number of pulses in the exposure region  106  which is determined by an unevenness in a light quantity for each pulse from the pulse laser source  1  is denoted by N e  [the number of pulses] and min (x, y) indicates x or y, whichever smaller, the minimum number of pulses N of the illumination light IL which is irradiated when one point on the wafer  105  finally crosses the exposure region  106  in the X direction [the number of pulses] is expressed as follows.  
           N =min ( N   e   , N   m )  (3)  
         [0139]    The scan velocity V R  [mm/sec] of the reticle  96  required for the above case, is expressed as follows.  
           V   R =( W·F )/( N·β )  (4)  
         [0140]    When the length to each shot region on the wafer  105  in the scan direction (the X direction) is denoted by a [mm] and the number of shot regions M, the exposure time EX scan  is expressed as follows.  
           EX   scan   =M ·( a+W )/( V   R ·β)  (5)  
         [0141]    This is an exposure time per wafer required for the scan-exposure, which time depends on the scan velocity V R  of the reticle. Therefore, if a high-speed reticle stage  98  is not developed, the exposure time becomes long and the throughput is lowered.  
         [0142]    On the other hand, when the exposure time EX in the expression (1) in the step-and-repeat mode is expressed as EX stil , the express time EX stil  is obtained. In this case, if the size (the field size) of the exposure region on the wafer  106  is denoted by a×b [mm 2 ], the minimum number of pulses N m  which is determined by a target exposure quantity onto the wafer  105  is expressed in the following expression.  
           N   m   =μ·P·E /( a·b·F )  (6)  
         [0143]    Then, if the minimum number of pulses which is determined based on an unevenness in a light quantity for each pulse emission is denoted by N e , the minimum number of pulses N [the number of pulses] of the illumination light IL finally irradiated onto the wafer  105  is expressed as follows.  
           N =min ( N   e   , N   m )  (7)  
         [0144]    Then, if the number of shots is M′, the exposure time EX stil  is expressed as follows.  
           EX   stil   =M′·N/F   (8)  
         [0145]    Therefore, the exposure time EX stil  is not restricted by the scan velocity V R  of the reticle stage  98 , as a matter of course.  
         [0146]    For example, when two chip patterns are included in each shot region, the minimum number of pulses N is a small value in a region where the photoresist has a high sensitivity according to the expressions (6) to (8). As a result, the exposure time is shorter in a case where a two-shot exposure is effected for each shot region in the step-and-repeat mode than in a case where each shot is exposed in the step-and-scan mode. Therefore, the throughput is higher in the step-and-repeat mode.  
         [0147]    On the contrary, in a region where the photoresist has a low sensitivity, an area of the exposure region on the wafer  105  in the step-and-repeat mode is wider than that in the step-and-scan mode so that the luminance in the exposure region is lowered to prolong the exposure time. Therefore, the throughput is higher in the step-and-scan mode.  
         [0148]    [0148]FIG. 14 shows a relationship between the sensitivity P of the photoresist and the throughput T. In FIG. 14, a curve  121  indicates the throughput in the step-and-scan mode, while a curve  122  the throughput in the step-and-repeat mode, respectively. As clearly seen from FIG. 14, the throughput is higher in the step-and-repeat mode when the sensitivity P of the photoresist is 0 to Po, and the throughput is higher in the step-and-scan mode thereafter. In the present embodiment, the exposure modes in which the throughput is higher is used in accordance with the sensitivity P of the photoresist.  
         [0149]    B. When the configuration of a shot region on the wafer  105  is used as a criterion.  
         [0150]    First, FIG. 15B shows a relationship between a shot region  123  on the wafer  105  and a circular effective exposure field  124  by the projection optical system  104 . Referring to FIG. 15B, the width of the shot region  123  in the Y direction is denoted by DY, and the width in the X direction (the scan direction in the step-and-scan mode) by DX. If the diameter of the effective exposure field  124  is denoted by L, an exposure region  106 A in the step-and-scan mode is a slit-like region having an X-directional width (slit width) W which is substantially inscribed to the effective exposure field  124 . An exposure region  16 B in the step-and-repeat mode is a region having an X-directional width smaller than the diameter L, and substantially inscribed to the effective exposure field  124 . The shot region  123  in FIG. 15A has a Y-directional width DY which is equal to the width of the exposure region  106 A and has an X-directional width DX which is longer than the diameter L. Therefore, in order to effect an exposure with one action, it is required to use the step-and-scan mode which effects a scan for the exposure region  106 A.  
         [0151]    [0151]FIG. 15A shows an exposure mode which can be used in accordance with the configuration of a shot region, in which the abscissa indicates the Y-directional width DY of the shot region, and the ordinate the X-directional width DX of the shot region. As seen-from FIG. 15B, a region which is disposed outside an arc  129 A having the radius L and which has a Y-directional width DY exceeding the Y-directional width of the exposure region having the slit width W when the Y-directional width of the shot region is W or more, that is, a hatched region  128  in FIG. 15A, becomes a region which can not be exposed. On the other hand, in FIG. 15A, a region  129 B which is surrounded by an arc  125  having the radius L, the axis DX and the axis DY is a region which can be exposed in the step-and-repeat mode. Further, a region  129 A surrounded by a straight line  126  which indicates the slit width W, a straight line  127  which passses through a point crossing the straight line  126  on the arc  125  and in parallel to the axis DX, and the axis DX is a region which can be exposed in the step-and-scan mode. Therefore, a region surrounded by the straight line  126 , the arc  125  and the axis DX (a region in which the region  129 B and the region  129 A overlap with each other) can be exposed either in the step-and-scan mode or in the step-and-repeat mode. Thus, one of these exposure modes, whichever has a higher throughput, can be selected, as mentioned above.  
         [0152]    In regions other than those, only one of the step-and-repeat mode and the step-and-scan mode can be used. Therefore, the exposure is effected in the usable exposure mode. When the Y-directional width of a shot region is too wide, like that of the region  128 , the exposure can be effected neither in the step-and-repeat mode nor in the step-and-scan mode so that the exposure mode determination means  79  of the main control system  77  gives a warning indication on an unillustrated display apparatus, indicating that the exposure can not be effected.  
         [0153]    C. When resolution and the distortion are used as criteria.  
         [0154]    In the step-and-repeat method, in general, an uneven distortion in an image projected by the projection optical system depending on each of projection exposure apparatuses becomes an overlap error when the exposure is effected onto different layers on the wafer by use of these projection exposure apparatuses (mix-and-match). On the contrary, in step-and-scan method, since the wafer is X-directionally scanned with respect to the slit-like exposure region  106 A (see FIG. 15B) so that the averaging effect can be obtained in said exposure region  106 A, a quantity of distortion in the X direction which is a scan direction decreases. Therefore, it is considered that the overlap accuracy among the layers on the wafer is higher, compared with that in the step-and-repeat method. However, in an exposure by use of a single projection exposure apparatus, the overlap accuracy becomes higher in the step-and-repeat method in which there is no influenced of a distortion of the projection optical system.  
         [0155]    With respect to a degree of resolution, with a wafer having a deteriorated flatness of the surface, a larger portion is liable to deviate from the range of depth of focus within an exposure region by the step-and-repeat method in which a wide shot region are exposed en bloc. On the other hand, it is possible to divide one shot region for an exposure and successively effect auto-focusing on each of divided regions by the step-and-scan method, whereby the entire shot region can be contained within the range of the focus depth easily.  
         [0156]    However, by the step-and-scan method, a projection image is deteriorated by a distortion averaged within the slit-like exposure region. The projection image is also deteriorated by a vibration at the step-and-scan time. As a result, the depth of focus becomes substantially shallow, compared with that in the step-and-repeat method. Therefore, the exposure method in which an allowance (margin) of distribution of the focal positions on the entire shot region is larger for said depth of focus of the focal positions on the entire shot region for said depth of focus is larger when the substantial depth of focus is used as a criterion is selected.  
         [0157]    When the exposure is effected on the second and subsequent layers on the wafer, a permissible value for an alignment accuracy between each shot region on the wafer and the reticle may be different in the step-and-repeat method and in the step-and-scan method, depending on the distortion, etc. Then, it is preferable to use the exposure mode with a larger permissible value for the alignment accuracy. However, as a matter of fact, an exposure mode is selected taking a substantial margin for the depth of focus and the permissible value for the alignment accuracy into consideration.  
         [0158]    Next, description will be made of an operation after an exposure mode is determined by using the criteria mentioned above. First, when the step-and-scan method is selected, the reticle  96  is scanned in the −X direction (or the +X direction) with respect to the illumination region  97 A, as shown in FIG. 16A. This illumination region  97 A is inscribed to a circular region  131  which is conjugate to the effective field  124  in FIG. 15B. Herein, assuming that two identical circuit patterns  130 A and  130 B are formed on a pattern region on the reticle  96 , the X-directional length of the circuit pattern  130 A is smaller than the Y-directional width thereof, and the X-directional length of the both circuit patterns  130 A and  130 B as a whole is larger than the Y-directional width thereof.  
         [0159]    In synchronization with the scan of the reticle  96 , the shot regions  134 A,  134 B,  134 C, . . . on the wafer  105  are respectively scanned in the +X direction (or the −X direction) with respect to the slit-like exposure region  106 A, as shown in FIG. 17A. Each of these shot regions  134 A,  134 B,  134 C, . . . is X-directionally divided into two sub-shot regions  132 A,  133 A,  132 B,  133 B, . . . . An image of the circuit pattern  130 A in FIG. 16A is projected onto the first two sub-shot regions  132 A and  132 B, while an image of the circuit pattern  130 B in FIG. 16A is projected onto the second two sub-shot regions  133 A,  134 B, . . . .  
         [0160]    Also, though the exposure region  106 A stands still actually and the wafer  105  is scanned, a locus of the exposure region  106 A for the wafer  105  is denoted by the loci  135 A,  135 B,  135 C, . . . in FIG. 17A. As seen from FIG. 17A, when an exposure is effected for a lot of shot regions on the wafer  105  successively in the step-and-scan mode, the shot regions  134 A,  134 B,  134 C, and  134 D in the first row which are disposed in the Y direction serving as a non-scanning direction are successively exposed first. Thereafter, the exposure action is shifted to the shot region  134 E in the second row. At this time, a stepping among the shot regions in the first row is conducted in the non-scanning direction, as indicated by the loci  135 B,  135 D and  135 F, and the stepping from the first row to the second row is conducted in a slanting manner, as indicated by the locus  135 H.  
         [0161]    In adjacent shot regions, as indicated by the loci  135 A,  135 C,  135 E,  135 G and  135 I, the wafer  105  is scanned in the +X direction and the −X direction alternately for the exposure region  106 A. In this manner, when the exposure is to be effected for the adjacent shot regions, the reticle  96  may be scanned in the −X direction and the +X direction alternately for the illumination region  97 A, thereby avoiding a useless motion (idle returning) of the reticle stage  98 . As described above, when the exposure is to be effected in the step-and-scan mode, it is possible to improve a throughput of the exposure process (productivity) by conducting the stepping in the non-scanning direction, thereby decreasing a useless motion of the stage mechanism.  
         [0162]    Next, in order to successively effect an exposure of the two identical circuit patterns  130 A and  130 B on the reticle  96  shown in FIG. 16A onto a multiplicity of shot regions on the wafer  105  in the step-and-repeat mode, a taking-two method by which the two circuit patterns  130 A and  130 B are static exposed and a taking-one method by which one of the two circuit patterns  130 A and  130 B is used to effect the exposure can be considered.  
         [0163]    When the taking-two method is employed, images of the two circuit patterns on the reticle  96  in FIG. 16A are exposed with respect to the shot regions  134 A,  134 B,  134 C, . . . in the first row disposed, for example, in the Y direction, as shown in FIG. 17A by the step-and-repeat method, and the stepping is conducted in the Y direction. In this case, since the Y-directional length of the shot region  134 A is smaller than the Y-directional width thereof, a distance to be stepped can be reduced by conducting the stepping in the Y direction, whereby the throughput of the exposure process can be improved, compared with the case in which the stepping is conducted in the X direction.  
         [0164]    On the other hand, when the taking-one method is employed, the circuit pattern  130 A, one on the reticle  96 , is illuminated by the illumination region  97 B at the exposure, as shown in FIG. 16B. Then, as shown in FIG. 17B, the exposure is effected by the step-and-repeat method for shot regions  134 J,  134 K,  134 V, . . . , in the first row disposed in the X direction on the wafer  35 . In this case, each of the shot regions  134 J,  134 K,  134 V, . . . , is X-directionally divided into two sub-shot regions  132 J,  133 J,  132 K,  133 K,  132 V,  133 V, . . . , respectively, so that an image of the circuit pattern  130 A in FIG. 16B is exposed two times on each of the shot regions  134 J,  134 K,  134 V, . . . , respectively.  
         [0165]    For this reason, the exposure field by the projection optical system is shifted to the X direction on the wafer  105 , as indicated by the loci  136 A,  136 B,  136 C, . . . , in FIG. 17B. These loci  136 A,  136 B,  136 C, . . . also indicate an orbit of the stepping on the wafer  105 . In this case, since the X-directional width of the sub-shot region  132 J, for example, is smaller than the Y-directional width thereof so that the distance to be stepped is shorter, compared with the Y-directional stepping, thereby improving the throughput of the exposure process.  
         [0166]    When the taking-one method is employed, compared with the case of the taking-two method mentioned above, the size of the illumination region on the reticle  94  becomes ½. Therefore, the illumination efficiency is deteriorated if the same fly eye lenses  74 A and  87 A are used. Then, a third fly eye lens is disposed replaceable with the fly eye lenses  74 A and  74 B of the first group, and another third fly eye lens is disposed replaceable with the fly eye lenses  87 A and  87 B of the second group. Then, the configurations of the lens elements of these two third fly eye lenses may be optimized for the step-and-repeat method and the taking-one method. Thereby, a degree of illuminance in the case of the taking-one method can be improved and the exposure time can be shortened.  
         [0167]    Furthermore, according to this embodiment, since the configurations of the lens elements which constitute the fly eye lens of the first and second groups are switched over in FIG. 13, an angle of vibration of the vibration mirror  85  which removes spacial coherence may be changed. In this manner, it is possible to set the number of minimum exposure pulses required for limiting the influence of interference fringe caused by the pulse laser beams to a predetermined permissible value to be smaller, whereby the throughput of the exposure process can be improved.  
         [0168]    In the foregoing embodiments, a pulse laser source is employed as the light source of the exposure light beams. However, the present invention can be applied to a case where continuous light beams such as i beam, g beam, etc., of a mercury lamp are used as the exposure light beams.  
         [0169]    As described above, the present invention is not limited to the foregoing embodiments, and can be modified and altered within the scope of the claims of the present invention.