Abstract:
A scanning exposure apparatus and method uses an optical member to change an intensity distribution of exposure light in an illumination region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of application Ser. No. 08/956,007 filed Oct. 22, 1997; which is a division of application Ser. No. 08/834,851 filed Apr. 10, 1997 (now U.S. Pat. No. 5,831,716 issued Nov. 3, 1998); which is a division of application Ser. No. 08/409,935 filed Mar. 23, 1995 (now U.S. Pat. No. 5,663,784 issued Sep. 2, 1997); which is a continuation of application Ser. No. 08/256,000 filed Jun. 8, 1994, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light exposure apparatus and method such as a light exposure apparatus and method used for manufacturing semiconductors, liquid crystal display elements or the like by means of a photolithographic process, and more particularly to a so-called slit-scanning type light exposure apparatus for successively exposing the pattern of a mask on light-sensitive substrates by scanning the mask and the photosensitive substrates in synchronism in a slit-like illuminating area. 
     2. Related Background Art 
     Upon manufacturing semiconductors or liquid crystal display elements by means of a photolithographic process, a projection light exposure apparatus is used which exposes the pattern of photomask or a reticle (hereinafter generally referred to as the “reticle”) on substrates (wafers, glass plates or the like) coated with photosensitive material through a projecting optical system. Conventionally, a stepper is mainly used as a projection light exposure apparatus to manufacture semiconductors. The stepper is a projection light exposure apparatus which successively totally transcribes the pattern image of a reticle pattern images in shot areas in a step and repeat way. 
     However, a so-called slit-scanning type projection light exposure apparatus has come to be developed, which will be described briefly. 
     In the slit scanning type projection light exposure apparatus, a reticle is scanned at a speed V in a predetermined scanning direction in an illuminating area having an elongated rectangular shape, a circular shape or a hexagonal shape (hereinafter generally referred to as a “slit shape”), wafers are scanned at a speed β·V (where β is the projection magnification of a projection optical system) in a direction conjugate with the scanning direction of the reticle in a light exposure area conjugate with the illuminating area and the projection optical system, and thus pattern images of the reticle are successively exposed and transcribed in exposure fields (shot areas) on the wafers. 
     The reasons why attention is paid to the slit scanning type light exposure apparatus are as follows: 
     First, as patterns of semiconductor devices have recently become finer and finer and the size of the devices have become larger and larger, much larger projection optical systems have been required. Thus, the manufacturing cost of the apparatus has become much higher and the size of the apparatus has also become much larger, reducing applicability of the apparatus to manufacture of semiconductors. 
     Secondly, as the device pattern has become finer and finer, distortions, flatness and astigmatism of the projection image of the projection optical system have come to be very near to their limits, and thus high operational performance required for the stepper cannot be obtained. 
     With the slit scanning light exposure method, light exposure is performed by the use of only a slit-like light exposure area in the projection optical system. When, therefore, the same projection optical system is used, the field size of the slit scanning type projection light exposure apparatus can be made larger by 2 1/2  times than that of the stepper. In other words, with the same field size as the stepper, the slit scanning type projection light exposure apparatus can employ an optical system ½ 1/2   times smaller than the stepper, improving a lens characteristic more than the stepper. 
     Thirdly, in a case where a reflecting and refracting optical system having a concave reflecting mirror is used as a projection optical system, areas in the light exposure field in which good optical characteristics can be obtained are limited to narrow circular arc portions. Thus, a slit scanning light exposure system is required for exposing the whole pattern of the reticle on wafers. 
     In the general projection light exposure apparatus, a suitable amount of exposure light (exposure light energy) is selected depending on light sensitivity of photoresists coated on wafers. Under the condition where continuous light is used as exposure light in the stepper, the amount of exposure light is controlled in accordance with the illuminating time length of the exposure light. When pulse light is used, on the other hand, the amount of the exposure light is controlled by the number of pulses of the illuminating light. 
     SUMMARY OF THE INVENTION 
     The control of the amount of exposure light is also necessary in the slit scanning type projection light exposure apparatus. When, for example, continuous light is used as exposure light in this type of prior apparatus, the width of the slit-shaped illuminating area in its scanning direction is made constant, and the amount of exposure light on every wafer is controlled to a moderate value by adjusting the scanning speeds of the reticle and the wafers and the exposure light reduction rate. 
     When, however, a pulse light source 
     is used for providing pulse light e.g., an excimer laser or the like in a slit scanning type projection light exposure apparatus, energy variations of exposure light pulses are large. In consequence, it is difficult in the prior slit scanning type light exposure apparatus to set the accumulated values of the energies for respective pulses to a value within the allowed value. 
     It is accordingly an object of the present invention to obtain a required control accuracy for the amount of exposure light in spite of a small number of measurements of the generation of the pulse light when a pulse light source which produces light pulses having large energy variations is used. 
     In order to achieve this object of the present invention, a light exposure apparatus according to one aspect of the present invention including an illuminating optical system for projecting light pulses from a light source on a mask formed with a pattern, a mask stage for carrying the mask, a substrate stage for carrying a photosensitive substrate and a driving system for scanning the mask stage and the substrate stage in synchronism with each other so that the pattern is exposed on the photosensitive substrate, comprises: 
     a stop having two edges for limiting the shape of an illuminating area on the mask in the illuminating system to a predetermined shape; 
     an edge adjusting portion for adjusting position or positions of one or both of the two edges with respect to a scanning direction of the mask; 
     an exposure light energy detecting portion for detecting exposing energy per light pulse; 
     a calculating portion for calculating, in response to information from the exposure light energy detecting portion, accumulated amount of exposure light per each of accumulated exposure light calculating zones formed by dividing the light exposure area on the photosensitive substrate at predetermined intervals in a scanning direction of the photosensitive substrate; and 
     a control portion for controlling the edge 
     adjusting portion in response to a difference between a predetermined accumulated amount of exposure light and a calculated amount of exposure light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general view showing a projection light exposure apparatus of an embodiment according to the present invention; 
     FIG. 2A is a graph showing an ideal accumulated exposure light amount distribution on a wafer; 
     FIG. 2B shows a graph showing an exposure light amount distribution per pulse on a wafer; 
     FIG. 3 is a graph showing the accumulated exposure light amount distribution at the position X 21  in an enlarged scale; 
     FIG. 4 is a plan view of accumulated exposure light amount calculating zones per pulse light generation on the light exposure field on a wafer and a slit-shaped light exposure area per pulse light; 
     FIG. 5 is a graph of the accumulated amount of exposure light in an accumulated exposure light calculating zone; 
     FIG. 6 is a graph showing the constants memorized in a light exposure control system in correspondence to exposure light distributions per pulse; and 
     FIG. 7 is a flow chart showing operation of an example of the slit scanning type light exposure apparatus of FIG. 1 in a slit scanning exposure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a projection exposure apparatus will be described with reference to the accompanying drawings. This embodiment relates to a slit scanning type light projection apparatus to which the present invention is applied. 
     FIG. 1 shows a general structure of the projection exposure apparatus of this embodiment. An excimer laser light source  1  has a mechanism for generating KrF excimer laser beams (the wavelength: 248 nm) and narrowing the width of an oscillating spectrum to stabilize the oscillation spectrum so that its center is kept to a constant value. The excimer laser light source  1  produces pulse laser beams by the control signal of a light exposure control system  10  and sets the average value of output energies of pulses to a predetermined level. Pulse laser beams IL emitted from the excimer laser light source  1  are reflected by a mirror  2  and enter a beam reforming optical system  3  so as to be formed from an elongated rectangular shape into a nearly square shape in cross section. 
     The pulse laser beams IL emitted from the beam reforming optical system  3  are reflected by a mirror  4  and enter a beam uniforming optical system  5  which includes a 
     fly-eye-lens and an oscillation mirror and has functions to make uniform intensity distributions of the incident laser beams and to cancel out unevenness of illuminance produced by interference fringes due to equalization. The pulse laser beams IL leaving the beam uniforming optical system  5  enter a beam splitter  6  having a low reflectivity and then arrive at a pair of plate-like reticle blinds  8 A and  8 B at uniform intensity through a relay lens  7 . 
     A pair of other plate-like reticle blinds which will be described later are arranged in parallel with the paper surface of FIG. 1. A slit-shaped elongated image between the two reticle blinds  8 A and  8 B passes through a relay lens  11 , a mirror  12  and a main condenser lens  13  and is projected on a reticle R. A slit-shaped elongated illuminating area  27  defined by the reticle blinds  8 A and  8 B on the reticle R extends perpendicularly to the paper surface of FIG.  1 . The pulse laser beams IL are incident on the pattern of the reticle R in the illuminating area  27  at a uniform illumination distribution. 
     It is preferred that the projection of the area defined between the reticle blinds  8 A and  8 B be carried out at a projection magnification of  1  or less. The positions of the reticle blinds  8 A and  8 B are defocussed by a predetermined amount from the conjugate pattern formed face (undersurface) of the reticle R so that the images of the edges of the reticle blinds  8 A and  8 B provide a predetermined trapezoidal distribution of the amounts of exposure light. The defocussed amount is changed by means of a defocus adjusting mechanism (not shown). The reticle blinds  8 A and  8 B can be moved toward and away from the optical axis of the illuminating optical system. In other words, the reticle blinds  8 A and  8 B are movable perpendicularly to the optical axis so as to adjust the narrow width of the slit-shaped elongated area. The light exposure control system  10  controls the operation of driving portions  9 A and  9 B. 
     The light beams reflected by the beam splitter  6  are collected on the light receiving surface of a photo-electric detector  26  by means of a condenser lens  25  and converted from light to electric signals. The output signals from the photo-electric detector  26  are supplied to the light exposure control system  10 . The output signals from the photo-electric detector  26  have a value proportional to the exposure light energy per laser pulse incident on a wafer W. In the light exposure control system  10 , the output signals from the photo-electric detector are accumulated and the accumulated exposure light amount on the wafer W can be calculated. 
     The reticle R is fixed to a reticle stage  14  by vacuum absorption means or by other mechanical means. The reticle stage  14  is scanned on a reticle guide  15  linearly in a direction X perpendicular to the optical axis AX of the projection optical system PL and parallel to the paper surface of FIG. 1. A movable mirror  16  is fixed to one end of the reticle stage  14 . Laser beams from a laser interferometer  17  disposed at the reticle side and externally of reticle stage  14  are reflected by the movable mirror  16 . The laser interferometer  17  always monitors the position of the reticle stage  14  in the direction X based on the reflected laser beams. The data of detected coordinates of the reticle stage  14  detected by the laser interferometer  17  is supplied to a stage control system  18 . The stage control system  18  controls the scanning speed and the scanning timing of the reticle stage  14 . The reticle blinds  8 A and  8 B form images of edges perpendicular to the scanning direction of the reticle R. In other words, the reticle blinds  8 A and  8 B have linear edges extending in a direction corresponding to a perpendicular direction to the scanning direction. 
     The image of the pattern in an illuminating area  27  on the reticle R is projected in the light exposure field (a shot area) on the wafer W. The area on the wafer W which area is conjugate with the slit-shaped illuminating area  27  and the projection optical system PL is a slit-shaped light exposure area  28 . 
     The wafer W is fixed to the wafer holder by vacuum adsorption means or the like. The wafer  19  is mounted on a Z stage  20  for positioning the wafer W in the Z direction parallel to the optical axis of the projection optical system PL. The Z stage  20  is mounted on an XY stage  21  for positioning the wafer W in a two-dimensional plane (the ordinate and the abscissa being taken on the X axis and the Y axis, respectively) perpendicular to the optical axis of the projection optical system PL. The XY stage  21  scans the wafer W in the direction X during the slit scanning light exposure, and the Z stage  20  performs the focussing of the wafer W. A leveling stage (not shown) for adjusting the angle of inclination of the wafer W and a sensor (not shown) for measuring the angle of inclination of the wafer W are provided. An alignment sensor for detecting the positional relationship between the reticle R and the wafer W is also provided but is not shown because this is not an essential element of the present embodiment. 
     A movable-mirror  22  is fixed to one end of the Z stage  20 . Laser beams from a laser interferometer  23  disposed at the wafer side and externally of the Z stage  20  are reflected by the movable mirror  22 . The laser interferometer  23  always monitors the positions in the directions X and Y of the Z stage  20  (the wafer W) in response to the thus reflected light. The data of the positions of the Z stage detected by the laser interferometer  23  is supplied to the stage control system  18 . The stage control system  18  controls the scanning speed and the scanning timing of the stage  21 . A main control system  24  for controlling the operation of the whole apparatus controls the light exposure control system  10  and the stage control system  18 . 
     In order to successively project the pattern image of the reticle R on the photosensitive material on the wafer W, the Z stage  20  (i.e., the XY stage  21 ) is scanned at a constant speed β·V in a direction DW along the direction −X in synchronism with the scanning of the reticle stage  14  at the constant speed V in a direction DR (where β is the projection magnification of the projection optical system PL and is ¼, for example). When the reticle R is scanned in the direction −X, the wafer W is scanned in the direction X. As the reticle R and the wafer W are scanned in the opposite directions at the same constant speeds in complete synchronism with each other and laser beams are generated from the excimer laser beam source  1  at constant time intervals, the pattern images of the reticle R are successively projected on the photosensitive material on the wafers W. 
     Measured data from the laser interferometers  17  and  23  is used for facilitating the synchronized scanning of the reticle stage  14  and the Z stage  20  at the side of the wafer at the constant speeds. The driving mechanism of these stages comprises a linear motor and an air guide combined therewith, for example, which facilitates scanning at constant speeds. The positions and the speeds of the reticle stage  14  and the XY stage  21  at the side of the wafer are controlled by control signals produced by the main control system  24  and passing through the stage control system  18 . The light exposure control system  10  accumulates detected signals from the photo-electric detector  26 , and sets the edges of the reticle blinds  8 A and  8 B at the positions obtained by the calculation, and supplies to the laser source  1  the triggers of the pulse outputs and the average value of pulse energies under instructions of the main control system  24  (as will be detailed later). 
     A description will be given by assuming that the reticle R is scanned in the direction DR (the direction X) with respect to the illuminating area  27  and the wafer W is scanned in the direction DW (the direction −X) in a slit scanning exposure. 
     In this embodiment, the position of the lower reticle blind  8 B is moved within a predetermined range in a direction a perpendicular to the optical axis so that the edge at the rear side of the scanning direction is moved in a direction b along the direction X. The edge at the rear side in the scanning direction (the direction DW) is moved by ΔX in the direction X. By moving the edge at the rear side in the scanning direction of the light exposure area  28 , a small number of pulses allows the accumulated amount of exposure light to be set to the aimed amount of exposure light. Further, in this embodiment, the period of pulse generation is set so as to produce pulse light from the excimer laser beam source  1  every time the wafer W is moved by a distance d in the direction X. 
     FIG. 2A shows a distribution of the accumulated amount of exposure light E(X) at a position X when the wafer scanning direction is the direction X, and FIG. 2B shows a distribution of the amount of exposure light I(X) of the slit-shaped light exposure area at a position X in the scanning direction on the wafer when the distribution of the accumulated amount of exposure light E(X) is shown in FIG.  2 A. In FIGS. 2A and 2B it is assumed that the wafer is stationary and the slit-shaped light exposure area  28  as shown in FIG. 1 is moved on the wafer W in the positive direction X, for convenience. In FIG. 2B, the distribution of the amount of exposure light I(X) forms a trapezoidal shape in the scanning direction in which the portion in the region X 21 ≦X 22  is a rising line, the portion in the region X 22 ≦X 23  takes a constant value A 0 , and the portion in the region X 23 ≦X 24  is a descending line. In other words, between both sides of the trapezoid having widths W 1  and W 3  measured along the direction X is disposed a flat portion having a width W 2  measured along the direction X. The slit-shaped light exposure area is scanned from the position of the smallest value on the X axis toward the position of the largest value on the X axis in the positive direction. When the distribution of the amount of exposure light I(X) as shown in FIG. 2B, the distribution of the accumulated amount of exposure light E(X) as shown in FIG.  2 A. 
     In the rising range of the distribution of the amount of exposure light E(X), i.e., in the range X 21 ≦X 22 , the portion of the distribution of the accumulated amount of exposure light E(X) looks as if it were a continuous curve E 21 (X). However, it is not so when it is checked in detail. 
     In FIG. 2A, the light exposure ends at the portion X&lt;X 21 , and the accumulated amount of exposure light becomes a constant value E 0  in an ideal case. The accumulated amounts of exposure light X 21 ≦X 22 , X 22 ≦X 23  and X 23 ≦X 24  are expressed by E 21 (X), E 22 (X) and E 23 (X), respectively. When the light exposure is performed at a constant scanning speed and the exposure light energy (pulse energy) per generated pulse can be set to a constant value every predetermined time interval, ideal accumulated amounts of exposure light expressed by distribution functions such as E 21 (X), E 22 (X) and E 23 (X) can be obtained and the accumulated amount of exposure light in the area in which the light exposure is terminated is the aimed exposure light E 0 . When, however, an excimer laser source is used as an actual light source, pulse energies are varied from pulse to pulse, and the predetermined accumulated amount of exposure light cannot be obtained from a small number of accumulated pulses. 
     More concretely, even if a generated pulse shows distributions of the amounts of exposure light I 21 (X), I 22 (X) and I 23 (X) and the flat portion shows an ideal value A 0 , the next pulse shows a different value from the amount of exposure light A 0 . At the variation of pulse energies of ±7%, the control accuracy of the amount of exposure light cannot be made ±1% or less, if the number of accumulated pulses is not 50 or more. Even if the exposure light energy density per pulse at the exposure surface of the wafer is increased by enhancing the resist sensitivity and improving light transmitting efficiency, the scanning speeds of the reticle and the wafer during the light exposure cannot be increased, because of the lower limit of the number of pulses. 
     At a constant scanning speed, the number of pulses cannot be reduced to a value smaller than its lower limit in order to reduce the maintenance cost for the excimer laser light source. 
     FIG. 3 shows the detail of the distribution of the accumulated amount of exposure light E(X) at and in the vicinity of the position X 21  in FIG.  2 A. Since pulse light exposure is carried out every time the XY stage  21  at the side of the wafer is moved by a distance d in the direction −X, i.e., the light exposure area  28  is moved by the distance d in the X direction, the curve of the distribution of the accumulated amount of exposure light E(X) is bent at every distance d. More concretely, the distribution of the accumulated amount of exposure light E(X) changes at every distance d as shown by E k (X), E k+1 (X), E k+2 (X), - - - . 
     FIG. 4 shows movements of the light exposure area  28  on the wafer W, in which at least one LSI is normally included in a light exposure field EA. In the coordinate system in which the wafer W is stationary, the light exposure area  28  advances in the positive direction of the X axis. In this connection, the first exposed calculating zone of the light exposure area  28  in the light exposure field EA is indicated by S 1 . As pulse light is generated every time the light exposure area is moved by the distance d in the direction X, the calculating zones S 1 , S 2 , S 3 , S 4 , - - - formed by dividing the light exposure area  28  at intervals of substantial distance of d are exposed by light in turn. 
     Let it be assumed that the width of the slit-shaped light exposure area  28  measured in the scanning direction is M times of the distance d (M being an integer such as 20 or so, for example). Then, the light exposure area  28  covers approximately M accumulated exposure light amount calculating zones S i−M  to S i−1  at the time of generation of pulse light, and covers the exposure light zone  32  consisting of almost all accumulated exposure light calculating zones S i−M+1  to S i  at the time of generation of the next pulse light. 
     After the light exposure, the distribution of the accumulated amount of exposure light in the light exposure field EA is made as even as possible. In doing so, a wider area defined by the reticle blinds  8 A and  8 B than the light exposure field EA in the scanning direction may be illuminated and the portions other than the circuit pattern on the reticle 
     to be transcribed may be shielded by a shielding plate (not shown) movable in synchronism with the reticle R. In other words, the light exposure portion defined by the light exposure area  28  may be controlled by other reticle blinds than the reticle blinds  8 A and  8 B. As the latter reticle blinds, there may be used a proximity blind provided closely to the upper or lower surface of the reticle R or a projection type blind employing a lens system. The projection type blinds are disposed closely to the reticle blinds  8 A and  8 B and set precisely in a plane conjugate with the pattern forming surface of the reticle R. The reticle blinds  8 A and  8 B are provided at such positions that they are displaced slightly from the conjugate surface and the projection image is blurred in the pattern forming face of the reticle R. In this embodiment, the reticle blind  8 A corresponding to the edge  28   a  of the light exposure area  28  disposed at the light exposure starting side (the front side of the scanning direction) does not move, but the reticle blind  8 B corresponding to the edge  28   b  of the light exposure area  28  disposed at the light exposure ending side (the rear side of the scanning direction) changes its position every time pulse light is generated, thereby making uniform the accumulated amount of exposure light in the scanning direction on the wafer W. 
     The operation of this embodiment will be described with reference to a flow chart as shown in FIG.  7 . The pattern image of a reticle R is exposed in a light exposure field EA on the wafer W in a slit scanning light exposure as follows. First, in step  101  in FIG. 7, the reticle stage  14  and the XY stage  21  at the wafer side are placed at the positions separated by a distance corresponding to the amount of approach run for scanning from the positions corresponding to the predetermined light exposure area, then the approach run is initiated. Thereafter, pulse light is started to be generated from the excimer laser beam source  1  in step  102 . The pulse energies are stabilized and the average value of the pulse energies produced every time pulse light is generated is set in step  103  to the aimed average pulse energy. 
     In this case, the outputs of pulse energies from the excimer laser beam source  1  may be set to a constant value, or the average of the pulse energies may be set to the predetermined value. The set value of the pulse energy can be obtained from E 0 /M 0 , where M 0  is the number of the aimed pulses when a point on the wafer W is exposed and E 0  is a value converted from the aimed accumulated exposure light energy density on the photosensitive material on the wafer W into the accumulated amount of exposure light of the laser pulse energy. 
     In step  104 , the XY stage  21  at the wafer side and the reticle stage  14  are scanned at a stable speed in synchronism with each other in a predetermined relationship. The light exposure control system  10  is provided with memories having addresses corresponding to the accumulated exposure light calculating zones S 1  to S q  formed by dividing the light exposure field EA at the intervals of d at which the light exposure area  28  is relatively moved every time pulse light is generated. 
     The memories store the accumulated amounts of exposure light E(j) at every j-th calculating zone of the accumulated amount of exposure lights S j  (j being 1 to Q). The accumulated amount of exposure light E(j) is not precisely constant throughout the corresponding zone but the value of its central portion or of one end thereof or the average value is used as a representative value. Each memory has enough capacity which can memorize the aimed position a(j) when the light exposure in the j-th accumulated exposure light calculating zone S j  is terminated. The number Q of the accumulated exposure light calculating zones is expressed by the following equation: 
     
       
           Q=[L   1   /d]+ 1  (1)  
       
     
     where L 1  is the length L 1  of the light exposure field EA of FIG. 4 in the scanning direction, d is a scanning distance per pulse and [L 1 /d] shows the maximum integer which does not exceed the value L 1 /d. 
     Let it be considered that the data of the accumulated exposure light calculating zones is erased. Since the actually necessary accumulated exposure light calculating zones are contained in the light exposure area  28 , the number Q of the accumulated exposure light calculating zones may be expressed by: 
     
       
           Q=[L   2   /d]+ 1  (2)  
       
     
     where L 2  is the width of the light exposure area  28  of FIG. 4 in the scanning direction. 
     In this embodiment, however, Q is obtained from Equation (1). 
     The aimed position a(j) is a deviation from the standard value and the initial value is set to the standard value. Thus, in step  105  the variable i in the pulse counter is reset, i.e., i is made zero, all aimed positions a(j) are made zero. At the same time, the numbers 1 to Q are made equal to j and thus the initial values of all accumulated amounts of exposure light are also made zero. In step  106 , the light exposure area  28  enters the light exposure field EA on the wafer W. 
     In step  107 , a pulse of light is generated when the wafer W is moved by a distance d in the direction X from the position just prior to this position. A photo-electric conversion signal is supplied from the photo-electric detector  26  in FIG. 1 to light exposure control system  10 . A pulse energy p i  is calculated in the light exposure control system  10 . In the next step  108 , the value of the variable i is added to by 1. 
     In step  109 , a check is made whether the variable i is the limited value Q. If the variable i is not the upper limit Q, the process goes to step  110 . In step  110 , the accumulated amounts of exposure light E(j) of the first to i-th accumulated exposure light amount calculating zones S 1  to S i  are calculated. A set of constants A(k) corresponding to the divided zones of the distribution of the amounts of exposure light of the light exposure area  28  in the scanning direction are used here. The set of constants A(k) (k=1, 2, 3, - - - , M) express the values of the sections divided at intervals d from the trapezoidal distribution per pulse. As shown in FIG. 6, the curves  35  to  37  define a trapezoidal shape, and the number of its divided widths is M which corresponds to the width of the light exposure area  28  per generation of pulse exposure light in the scanning direction. The set of constants A(k) can be set by previously measuring the illuminance distribution in the light exposure area  28  or can be obtained by outputs from an intensity distribution monitor provided in the same plane as the wafer W or in another plane conjugate therewith. The actual distribution of the amounts of exposure light is proportional to the pulse energies p i  and is expressed by p i ·A(i+j−1). The new accumulated amount of exposure light E(j) is this value plus the just previous accumulated amount of exposure light E(j). A(i+j−1) is the trapezoidal intensity distribution corresponding to the i-th block of the light exposure area  28 , where i is 1 to Q. 
     In the next step  111 , a command for setting the reticle blind  8 B at the rear side in the scanning direction to the position a(i+1) is supplied from the light control system  10  to the driving portion  9 B, and the movement of the reticle blind  8 B stops by the time of generating the next pulse light. Since the reticle blind  8 B must be moved during the time interval between the generation of a pulse light and the generation of the next pulse light, the reticle blind  8 B is driven at a high speed by using a driving unit such as a piezo element or a motor and a position sensor such as a linear encoder. 
     In step  112 , a check is made as to whether the variable i is smaller than (M−m 2 ). The numbers M and m 2  will be explained. In FIG. 5 is shown the accumulated amounts of exposure light E(j). When light exposure is carried out on the i-th accumulated exposure light amount calculating zone on the wafer W, light exposure has been completed on the accumulated light exposure zone which is M zones prior to the presently exposed calculating zone (i.e., (i−M)th calculating zone) and the calculating zones preceding it. The calculating zone on which exposure is initiated and the calculating zones on which exposure has not yet been carried out are the (i−M)th to i-th calculating zones and the amounts of the accumulated amounts of exposure light E(j) are indicated in a bent line  34  descending toward the right side. 
     The position a(i+1) of the reticle blind  8 B is determined depending upon conditions as to how much the accumulated amounts of exposure light E(i−M+m 1 ), E(i−M+m 1 +1), - - - , E(i−M+m 2 ) of the m 1 -th to m 2 -th accumulated exposure light amount calculating zones counted toward the right side from the calculating zone on which exposure light has been exposed just now are deviated from the aimed accumulated amount of exposure light α 0 . 
     Returning to FIG. 7, when i&lt;(M−m 2 ) in step  112 , the number of the accumulated pulses is not sufficient. It shows that the position a(i+1) of the reticle blind  8 B cannot be changed from the initial value. The process goes to step  115 . When, on the other hand, it is determined that (M−m 2 )≦i, it means that pulses overlap so that the position of the reticle blind  8 B can be changed from the initial value to the position a(i+1) in accordance with the accumulated amounts of exposure light. In this condition, the process goes to step  113 . 
     In step  113 , an average α of the accumulated amounts of exposure light of the accumulated exposure light amount calculating zones is obtained from the accumulated amounts of the (i−M+m 1 )th to (i−M+m 2 )th calculating zones, and the aimed position a(i+m 1 ) is calculated from the value of the difference between α and α 0  multiplied by a proportional constant K. In other words, the following equation holds true: 
     
       
           a (1 +m   1 )= K (α−α 0 )  (3)  
       
     
     The constant K is determined by calculation or by experience so that light exposure blur becomes the minimum. When the amount of exposure light of the flat portion is lowered to the level A 1  as shown in FIG.  2 B and the accumulated amounts of the just previous light exposure is reduced to the curve  31 A smaller than the designed value as shown in FIG. 2A, the reticle blind  8 B is moved so that the edge at the rear side in the scanning direction of the distribution of the amount of exposure light per generation of the just previous exposure light is opened in the direction aA by the time of generation of the next exposure light and the light exposure is performed. In contrast, when the amount of the just previous light exposure of the flat portion increases to the level A 2  and the accumulated amount of exposure light is larger than the designed value as shown by the curve  31 B in FIG. 2A, the reticle blind  8 B is moved so that the edge at the rear side in the scanning direction of the distribution of the amount of exposure light per generation of the just previous exposure light is closed in the direction aB by the time of generation of the next exposure light and the light exposure are carried out. In this way, the accumulated amounts of exposure light is equalized even if pulse energies vary. 
     In step  115 , a check is made as to whether the time between adjacent generation of pulse light has elapsed. If the time has elapsed, the process returns to step  107 . If not, the process returns to step  115  after waiting in step  116 . 
     When it is determined that i=Q in step  109 , the process goes to step  117 . In step  117 , a command for stopping the Z stage  20  at the wafer side and the reticle stage  14  is supplied from the light exposure control system  10  to the stage control system  18  through the main control system  24 . In the succeeding step  118 , the scanning of the exposure light corresponding to one light exposure field on the wafer W ends. The data of the accumulated amounts of exposure light is supplied from the light exposure control system  10  to the main control system  24  and is stored. Differences between the actual accumulated amounts of exposure light and the aimed accumulated amount of exposure light are calculated. When the control accuracy exceeds the required value or the uniformity becomes poorer than required, a warning signal or an error signal is sent to a display unit. After the light exposure corresponding to one light exposure field is completed, the next process for light exposure on the next light exposure field starts. 
     In this embodiment, a description has been given in that scanning of slit scanning light exposure is performed in one direction. However, scanning in the other direction may also be carried out. In the latter case, the reticle blind  8 B is fixed at the standard position and the position of the reticle blind  8 A is controlled. In this embodiment, portions other than the pattern are covered with reticle blinds other than the reticle blinds  8 A and  8 B at the boundaries of the end portions of the light exposure field EA in FIG.  4 . When the reticle blinds  8 A and  8 B are moved in opposite directions to the scanning directions at the same speed as the scanning speed in synchronism with the scanning of the reticles at the boundaries of light exposure field EA, the image of the reticle blind  8 A or the reticle blind  8 B is stationary on the wafer W. The light exposure area may be limited by this mechanism. 
     In this embodiment, only the reticle blinds  8 A and  8 B for forming the images of the edges perpendicular to the scanning direction are described as reticle blinds, but it is preferred that reticle blinds for forming the images of the edges parallel with the scanning direction be provided. However, it is unnecessary that the latter reticle blinds be moved every time pulse light is generated. It is better that the positions of the latter reticle blinds can be adjusted only when the widths of the light exposure field perpendicular to the scanning direction and the positions of the light exposure field are changed. Accordingly, it is desirable that the reticle blinds for forming the images of the edges parallel with the scanning direction be provided at a position conjugate with the pattern forming face of the reticle R in the vicinity of the reticle blinds  8 A and  8 B as shown in FIG.  1 . The light shield band around the light exposure area of the reticle R is narrowed by using such reticle blinds. Thus, the light shield band of the reticle R is easily manufactured. 
     In this embodiment, the reticle blinds  8 A and  8 B of a projection type are used. However, reticle blinds of a proximity type comprising movable shield plates displaced closely to the upper or lower surface of the reticle R, for example, may be used instead of the reticle blinds of a projection type. 
     The present invention is most effective when it is applied to a light exposure apparatus of a slit scanning type using a pulse light source which emits pulse light whose energy is varied. However, the present invention can be applied to a projection light exposure apparatus of a slit scanning type using a continuous light emitting light source such as a hydrogen lamp when the light source generates light whose intensity varies greatly. In this case, no constant pulse interval is used as a parameter of light exposure, but the accumulated exposure light amount calculating zones for corresponding pulses of FIG. 4 S 1 , S 2 , - - - can be replaced by light exposure intervals divided at the scanning width per a constant time. 
     In this embodiment using a pulse light source, the exposure apparatus has memories for the accumulated amounts of exposure light corresponding to the accumulated exposure light calculating zones in each light exposure area which advance every time when pulse light is produced. However, each of the accumulated amounts of exposure light may be made to correspond to a zone in response to a plurality of successive pulses. 
     The present invention is not limited to the above-mentioned embodiments but is applicable to modifications within the scope of the present invention. 
     With the projection light exposure apparatus of a slit scanning light exposure type according to the present invention, the control accuracies of the accumulated amounts of exposure light can be enhanced by a small number of pulses, because the position in the scanning 
     direction of a variable field stop for limiting the light exposure area is controlled due to the accumulated amounts of exposure light during light exposure. 
     When changed with respect to the scanning direction of the mask in the illuminating area having a predetermined shape through the variable field stop, the position of the rear edge can be easily controlled.