Abstract:
An exposure method, exposure apparatus and mask are suitable for manufacturing an active matrix liquid crystal display including, for example, a gate electrode layer and a source/drain electrode layer. A stitching portion between unit patterns in a second layer is offset from the stitching portion in a first layer by a predetermined distance. The stitching portions of the second layer are always positioned over unit patterns of the first layer. Accordingly, the contrast gap that occurs at the stitching portion as a boundary is defined only by an error in the exposure position of the second layer. The contrast gap is not affected by an error in the exposure position of the first layer, unlike the conventional method. Because the contrast gap caused by the error in the exposure position of the first layer is eliminated, the total contrast gap that occurs at the stitching portion as a boundary is significantly reduced.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to exposure apparatus and methods and, more particularly, to an exposure method, an exposure apparatus, and a mask that are suitable for, for example, manufacturing an active matrix liquid crystal display (liquid crystal panel) having a switching device. 
     In recent years, a high-resolution color liquid crystal display (LCD) with a wide screen has been used as a display for personal computers or television sets. The screen size (screen diagonal) of a current LCD is typically 10-12 inches; however, a wide screen LCD with a 16-inch screen, 20-inch screen, or still wider screen, is being developed. In response to the increased screen size, the resolution is also improved, and an LCD having a VGA (640X480) pixel matrix, XGA (1024X768) pixel matrix, or SXGA (1280X1024) pixel matrix is being manufactured. An active matrix LCD is superior in the response characteristic of image display, wide view angle characteristics, and multi-tone characteristics. In many applications, a thin film transistor LCD (TFT/LCD), which uses a thin film transistor as a switching device in each pixel, has been used. 
     FIG. 16 illustrates an example of the pixel structure of a TFT/LCD in a conventional display device as an enlarged plan view, which shows a portion of the array substrate (on which TFTs are formed). A plurality of gate lines  132  are formed on the glass substrates  142  along the horizontal direction of a display screen, and a plurality of data lines  130  are formed in the vertical direction. Areas defined by the gate lines  132  and the data lines  130  are display pixel areas, in which a transparent pixel electrode  134  made of ITO (indium tin oxide) is formed. A gate electrode  136 , which is derived from the gate line  132 , is formed in a corner of each display area. A channel layer  144  made of, for example, amorphous silicon (α-Si) is formed on the gate electrode  136  with a gate insulation film (not shown) therebetween. A drain electrode  140 , which is derived from the data line  130 , and a source electrode  138 , which is electrically connected to the transparent electrode  134 , are formed on the channel layer  144  simultaneously. A TFT is composed of a gate electrode  136 , a gate insulation film, a channel layer  144 , and source and drain electrodes  136 ,  140 . 
     The circuit pattern in each layer, which constitutes a TFT/LCD, is formed through a photolithographic process, in which a projection exposure apparatus is used to expose the circuit pattern formed on a photomask or reticle (collectively, referred to as a reticle) onto a resist layer (photosensitizer) formed on the glass substrate  142 . The resist layer is developed and used as a mask on which the circuit pattern has been transferred. Using the photoresist mask, a semiconductor layer made of, for example, α-Si is etched to form the channel layer  144 . The gate line  132 , gate electrode  136 , data line  130  and source/drain electrodes  138 ,  140  are formed by etching a metal interconnection layer. 
     There are two types of exposure apparatus, namely, a step-and-repeat-type and a scanning-type. In a scanning-type exposure apparatus, the reticle and the glass substrate are moved in synchronization with each other. 
     In a step-and-repeat-type apparatus, the photosensitive substrate (glass substrate) mounted on the movable stage is driven in a step-and-repeating manner to successively expose a portion of the reticle pattern onto a predetermined area on the photosensitive substrate in a section-by-section manner. In this type of exposure apparatus, a plurality of reticles are held in a reticle changer. The reticle changer and the stage are driven so as to successively expose a portion of the multiple reticle patterns onto one of the divided pattern areas on the photosensitive substrate, thereby forming a first layer. Other patterns on different reticles held in the reticle changer are subsequently exposed to form a second layer over the first layer. 
     FIG. 17 illustrates the first and second layers LY 1  and LY 2  exposed onto the photosensitive substrate P, which is mounted on the movable stage of an exposure apparatus. Unit patterns LY 1 A and LY 1 B of the first layer LY 1  are exposed successively onto the photosensitive substrate P, and LY 1 A and LY 1 B are combined through a stitching portion JN. Similarly, unit patterns LY 2 A and LY 2 B of the second layer LY 2  are successively exposed over the first layer, and LY 2 A and LY 2 B are combined through the stitching portion JN. 
     The movable stage (not shown in FIG. 17) that supports the photosensitive substrate P is moved within the X-Y plane in a controllable manner, and the position of the photosensitive substrate P mounted on the movable stage is controlled within the X-Y coordinate system. If, for example, the first unit pattern LY 1 A of the first layer LY 1  is exposed onto the substrate with an offset of −Δx from the target exposure position, and the first unit pattern LY 2 A of the second layer LY 2  is exposed with an offset of +Δx from the target exposure position, then the offset of the first pattern LY 2 A of the second layer LY 2  becomes +2Δx relative to the first unit pattern LY 1 A of the first layer LY 1 , which corresponds to the distance between the exposure positions of LY 1 A and LY 2 A. 
     If the second unit pattern LY 1 B of the first layer LY 1  is exposed with an offset of +Δx from the target exposure position, and the second unit pattern LY 2 B of the second layer LY 2  is exposed with an offset of −Δx from the target exposure position, then the offset of the second unit pattern LY 2 B of the second layer LY 2  becomes −2Δx relative to the second unit pattern LY 1 B of the first layer LY 1 , which is the distance between the exposure positions of LY 1 B and LY 2 B. Accordingly, the total offset of the second layer LY 2  relative to the first layer LY 1  becomes +4Δx with respect to the stitching JN, as shown in FIG.  18 . 
     If such an offset occurs during the exposure process, in a thin film transistor of the liquid crystal panel, the drain electrode DR and the source electrode SO formed in the second layer LY 2  are offset by +4Δx relative to the gate electrode GA formed in the first layer LY 1 , as shown in FIG.  19 . 
     The hatched areas PIL 1  and PIL 2  of the drain electrodes DR, which overlap the gate electrodes GA, define the capacitor capacitance generated between the gate electrode GA and the drain electrode DR. Change in the capacitance results in variation in the holding voltage of the thin film transistor. If the overlapping areas PIL 1  and PIL 2  differ in the left and right sides of the liquid crystal panel with the stitching portion JN as a boundary, the light-permeability of the liquid crystal panel varies from area to area. Consequently, the contrast differs between the left and right halves of the liquid crystal panel, separated at the stitching portion JN. 
     As the glass substrate  142  is enlarged along with the increased size of TFT/LCDs, a scanning-type projection exposure apparatus with a plurality of projection lens systems has been preferably used to increase the projection exposure area of the apparatus. In such a scanning-type projection exposure apparatus, the circuit pattern on a reticle is divided into multiple trapezoid areas when exposed onto a glass substrate. The reticle and the glass substrate are synchronously scanned with respect to the projection lens systems. In this manner, the entire area of the reticle circuit pattern is transferred to the glass substrate. 
     FIG.  20 ( a ) shows a portion of the projection area formed on the glass substrate  142  by a scanning-type projection exposure apparatus. The trapezoid projection areas  150 ,  152  formed through individual projection lens systems overlap each other in the Y direction by a predetermined amount. This arrangement enables the circuit pattern to be illuminated uniformly. In the figure, the glass substrate  142  moves in the X direction relative to the projection areas  150 ,  152 . The range “b” (with a width of, for example, 5 mm) indicates the overlapping area of the projection areas  150  and  152  in the Y direction. The range “a” indicates the non-overlapping area of the projection area  150 , while the range “c” indicates the non-overlapping area of the projection area  152 . 
     In general, the imagery characteristics of a plurality of projection lens systems used in the scanning-type projection exposure system vary slightly. Suppose that the projection lens system used for image formation in the projection area  150  has an imagery characteristic that causes the image-forming position to shift ΔP in the −Y direction (as indicated by the left arrow in FIG.  20 ( a ) ), and further suppose that the projection system used for image formation in the projection areas  152  has an imagery characteristic that causes the image-forming position to shift ΔP in the +Y direction (as indicated by the right arrow), then overlay errors occur, as shown in FIG. 20, between the layers exposed by the projection lens systems that have characteristics different from each other. In FIG. 20, the horizontal axis represents a Y position, and the vertical axis represents an error with respect to the designated pattern-forming position in the layers. The positional errors in the areas “a” and “c” are ΔP with opposite signs, and therefore, the total offset between the areas “a” and “c” becomes 2ΔP. In the area “b”, the exposure ratio of the projection area  150  to the area  152  changes linearly, and the offset of the formed pattern also changes linearly from −ΔP to +ΔP. In this context, the area in which two projection areas are overlapped during exposure is called the “stitching portion”. 
     If the imagery characteristics of multiple projection lens systems of a scanning-type exposure apparatus vary slightly, the magnitudes and the directions of offset of the pattern images formed through these projection lens systems also vary with respect to the stitching portion. 
     Generally, a plurality of scanning-type projection exposure apparatus are used in the photolithographic process, each apparatus being used to expose one of the layers of a TFT. Accordingly, the accuracy in overlaying a plurality of layers may be adversely affected by variation in the imagery characteristics of the different scanning-type projection exposure apparatus. In addition, variation in the imagery characteristics of the multiple projection lens systems provided in a scanning-type projection exposure apparatus may also affect the overlay accuracy. 
     FIGS.  21 ( a )-( c ) show overlay errors in overlaid layers, which are caused when a layer of the gate line and gate electrode of a TFT and a layer of the data line and source/drain electrodes of the TFT are formed by separate scanning-type projection exposure apparatus. 
     FIG.  21 ( a ) is similar to FIG.  20 ( a ) and shows the overlapping area between the projection areas  150  and  152  in which a data line and source/drain electrodes (collectively referred to as source/drain electrodes) are formed as a first layer by the first scanning-type projection exposure apparatus. FIG.  21 ( b ) shows the overlapping area between projection areas  154  and  156  in which a gate line and a gate electrode (collectively referred to as a gate electrode) are formed as a second layer on the glass substrate  142  by the second scanning-type projection exposure apparatus. The imagery characteristic of the projection lens system that forms a pattern image in the projection area  154  causes the image-forming position to shift ΔP in the +Y direction, as indicated by the right arrow. On the other hand, the imagery characteristic of the projection lens system that forms a pattern image in the projection area  156  causes the image-forming position to shift ΔP in the −Y direction, as indicated by the left arrow. 
     For purposes of illustration, FIGS.  21 ( a ) and  21  ( b ) depict the case in which the possible overlay error becomes largest because the upper layer is exposed by a projection lens system that has an imagery characteristic opposite to that of the projection lens system for exposing the lower layer. 
     FIG.  21 ( c ) shows overlay errors that occur when the lower layer gate electrode is formed on the glass substrate through the second scanning-type projection exposure apparatus, and then the upper layer source/drain electrodes are formed over the lower layer through the first scanning-type projection exposure apparatus. The horizontal axis represents a Y position, and the vertical axis represents an error. 
     The dashed line A indicates the positional shift of the gate electrode formed in the lower layer, and the solid line B indicates the positional shift of the source/drain electrodes formed in the upper layer. The bold solid line C indicates the overlay error (C=B−A) between the gate electrode and the source/drain electrodes. The overlay error equals the offset of the upper source/drain electrodes relative to the lower gate electrode. Therefore, the overlay error of the source/drain electrodes with respect to the gate electrode becomes −2ΔP in the area a. The overlay error of the source/drain electrodes with respect to the gate electrode becomes 2ΔP in the area c. The overlay error in the area b, in which the gate electrode layer and the source/drain layer overlap each other, changes linearly from −ΔP to +ΔP, because the positional shifts of the gate electrode pattern and source/drain pattern change linearly, as mentioned above. Consequently, the largest possible overlay error in the area b is 4ΔP. 
     FIGS.  22 ( a )-( c ) illustrate the aforementioned overlay error more concretely, showing the positional shift of the source/drain electrodes  138 ,  140  that overlap the gate electrode  136  in each TFT area of a TFT/LCD formed by the projection exposure method. FIG.  22 ( a ) shows the overlay error of the source/drain electrodes  138 ,  140  with respect to the gate electrode  136  in the area “a”. The dashed line indicates the originally designed pattern-forming positions of the layers. Relative to the reference positions defined by the dashed line, the formed gate electrode  136  is offset in the +Y direction, while the formed source/drain electrodes  138 ,  140  are offset in the −Y direction. Similarly, FIG.  22 ( c ) shows the overlay error of the source/drain electrodes  138 ,  140  with respect to the gate electrode  136  in the area “c”. The gate electrode  136  is formed offset in the −Y direction, while the source/drain electrodes  138 ,  140  are formed offset in the +Y direction. Concerning the area “b” in which the patterns formed in the areas “a” and “c” overlap each other in a stitching portion, the overlay error of the upper layer with respect to the lower layer becomes small in the vicinity of the center. Near the edge portions of the area “b”, however, the overlay error in the area “b” comes close to the error rate in the area “a” or “c”. 
     In summary, a plurality of scanning-type projection exposure apparatus are used in the ordinary exposure process for manufacturing an LCD to expose and form patterns in the respective layers. Because each layer is formed through a different exposure apparatus, the overlaying accuracy of each layer is greatly affected by variations in the imagery characteristics of different projection lens systems used in the projection exposure apparatus, or by variations in the imagery characteristics of the plurality of projection exposure apparatus used in the exposure process. 
     In manufacturing a TFT, if the overlapping area of the source electrode that covers the gate electrode changes, the parasitic capacitance between the source electrode and the gate electrode also changes, which further affects the characteristics of the TFT element. The change in the TFT element characteristics results in flickers or burning in the LCD screen. 
     It is clear from FIG. 21 that if the overlay error rate varies steeply in the stitching portion of area “b” in which two projection areas overlap each other, then the TFT element characteristics that are located on both sides of the stitching portion differ greatly from each other. Consequently, the difference will be visually recognized as unevenness or deterioration of the image quality with the stitching portion as a boundary. This phenomena is called “screen separation” or “uneven split”. 
     SUMMARY OF THE INVENTION 
     This invention was conceived in view of the problems in the prior art, and it is an object of the invention to provide an exposure method, an exposure apparatus, and a mask that can reduce the extent of an artificial contrast gap that occurs in stitching portions. 
     It is another object of the invention to provide a projection exposure method that can prevent screen separation, which causes deterioration of the LCD image quality. 
     These and other aspects and advantages of the invention are achieved by providing an exposure method according to one aspect of the invention. In the exposure method, a first layer exposure pattern having a first plurality of unit patterns is formed on a photosensitive substrate, and the first plurality of unit patterns are connected through at least one first layer stitching portion. A second layer exposure pattern having a second plurality of unit patterns is formed on the photosensitive substrate overlaying the first layer exposure pattern, and the second plurality of unit patterns are connected through at least one second layer stitching portion. In forming the second layer, the second layer stitching portion is formed offset from the first layer stitching portion. 
     In an exposure method according to another aspect of the invention, a first layer exposure pattern having a first plurality of unit patterns is formed on a photosensitive substrate, and the first plurality of unit patterns are connected through at least one first layer stitching portion. A second layer exposure pattern having a second plurality of unit patterns is formed on the photosensitive substrate overlaying the first layer exposure pattern, and the second plurality of unit patterns are connected through at least one second layer stitching portion. A contrast between the second plurality of unit patterns overlaying the first plurality of unit patterns is smoothed in accordance with the position of the second layer overlaying the first layer. In preferred forms, the smoothing is achieved by offsetting the second layer stitching portion from the first layer stitching portion. 
     In accordance with still another aspect of the invention, there is provided an exposure apparatus for forming a first layer and a second layer on a photosensitive substrate. The exposure apparatus includes an illumination optical system to illuminate a mask with luminous flux emitted from a light source. A plurality of divided patterns are formed on the mask through stitching portions, wherein a stitching portion in the first layer is disposed offset from a stitching portion in the second layer. A blind is disposed in an optical path of the luminous flux and changes a dimension of an illumination area on the mask illuminated by the illumination optical system. A projection lens system is disposed in the optical path on a side or the musk opposite from the illumination optical system. The projection lens system projects the luminous flux having passed through the mask onto the photosensitive substrate. A controller communicates with the blind to control a position of the blind in accordance with the divided patterns. 
     In accordance with yet another aspect of the invention, a mask set including a plurality of masks for forming a first layer and a second layer on a photosensitive substrate is provided. Each of the masks includes a plurality of unit patterns formed thereon, wherein the unit patterns of one mask have different dimensions from dimensions of the unit patterns of another mask. 
     In an exposure method according to yet a further aspect of the invention, a portion of a first pattern is projected into one of a plurality of first projection areas on a photosensitive substrate, the first projection areas being separate from each other with adjacent projection areas overlapping each other by a predetermined overlap amount in a direction perpendicular to a scanning direction of the substrate. The first pattern and the substrate are synchronously scanned in the scanning direction relative to the plurality of first projection areas to transfer the first pattern onto the substrate. A portion of a second pattern is projected into a plurality of second projection areas disposed offset from the plurality of first projection areas in the direction perpendicular to the scanning direction by a predetermined distance within a plane defined by the substrate. The second pattern and the substrate are also synchronously scanned in the scanning direction relative to the plurality of second projection areas to transfer the second pattern onto the substrate. 
     If the predetermined distance (positional shift) is set so as to be greater than the width of the stitching portion, then the stitching portions of upper and lower layers are prevented from adversely affecting each other. The maximum overlay error can be reduced by half, as compared with the prior ant method. By shifting the stitching portions, the overlay error changes gently at each stitching portion, thereby preventing screen separation from being conspicuous. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects and advantages of the present invention will be described in detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a side view of the exposure apparatus according to a first embodiment of the invention; 
     FIG. 2 is a schematic plan view showing the structure of the mask position detecting system according to the invention; 
     FIG. 3 is a schematic plan view used for explanation of the exposure operation for the first layer; 
     FIG. 4 is a schematic plan view used for explanation of the exposure operation for the second layer; 
     FIG. 5 is a plan view showing stitching portions in the first and second layers formed on the photosensitive substrate; 
     FIG. 6 is a cross-sectional view showing stitching portions in the first and second layers; 
     FIG. 7 illustrates an example of overlay error that varies throughout the stitching portion; 
     FIG. 8 is a schematic plan view showing variation in the overlapping area of the source or drain electrode, which is caused by the overlay error; 
     FIG. 9 is a plan view of a mask in which a plurality of unit patterns are formed; 
     FIG. 10 is a plan view of a modification of the mask in which a plurality of unit patterns are formed; 
     FIG. 11 is a schematic plan view showing stitching portions on a photosensitive substrate; 
     FIG. 12 is a perspective view of the scanning-type projection exposure apparatus according to a second embodiment of the invention; 
     FIG. 13 illustrates projection areas projected onto a glass substrate; 
     FIGS.  14 ( a )-( c ) show how projection areas overlap each other and the resultant overlay error according to the projection exposure method of a second embodiment; 
     FIGS.  15 ( a )-( c ) show how projection areas overlap each other and the resultant overlay error according to the projection exposure method of a third embodiment; 
     FIG. 16 illustrates the structure of a TFT/LCD; 
     FIG. 17 is a cross-sectional view showing a prior art method for stitching patterns in the first and second layers; 
     FIG. 18 shows how the overlaying accuracy changes with the prior art method; 
     FIG. 19 illustrates variation in the overlapping area of electrodes in the prior art; 
     FIGS.  20 ( a )-( b ) show how projection areas overlap each other and the resultant overlay error in the prior art; 
     FIGS.  21 ( a )-( c ) show how projection areas overlap each other and the resultant overlay error in the prior art; and 
     FIGS.  22 ( a )-( c ) illustrate overlay error in each TFT in the prior art. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows the overall structure of the step-and-repeat type exposure apparatus according to a first embodiment. An illumination light source  1 , such as a super-high pressure mercury-vapor lamp, excimer laser or the like, emits illumination light IL having a wavelength (exposure wavelength) that can expose a resist layer. Examples of illumination light IL include g-rays, i-rays, ultraviolet pulsed beam (e.g., FrF excimer laser beam), etc. The illumination light IL enters a fly-eye lens  2 . 
     The fly-eye lens  2  makes the illumination light IL uniform and reduces the spectrum prior to guiding the illumination light IL to a first mirror  4 . The illumination light IL is reflected by the mirror  4 , passes through relay lenses  5   a ,  5   b  and is reflected by a second mirror  7 . The illumination light IL reaches a main condenser lens  8  and illuminates the pattern area on the mask M 1  with uniform illuminance. The fly-eye lens  2 , the mirrors  4 ,  7 , the relay lenses  5   a,    5   b  and the main condenser lens  8  constitute the illumination optical system. 
     A variable blind (field stop)  6  is positioned between the relay lens  5   a  and the relay lens  5   b  and is driven by a variable blind driving unit  6   a  so as to block an area outside of the pattern area on the mask M 1 , whereby illumination light IL illuminates only the pattern area of the mask M 1 . The blocked area is defined by the pattern areas formed on the respective masks M 1 -M 4 . 
     The illumination light IL, which has illuminated the pattern area of mask M 1 , penetrates the mask M 1 , passes through a projection lens system PL, and forms a pattern image of the mask M 1  on the photosensitive substrate P. A light beam reflected by the photosensitive substrate P passes through the mirror  4  and enters the photodetector (reflection monitor)  3 . The photodetector  3  photoelectrically detects the quantity of reflected light and outputs optical information (e.g., intensity) PS to a controller  12 . The optical information PS is used to obtain fluctuation in the imagery characteristic of the projection lens system PL. 
     The mask stage MS, which serves as a mask holder, is supported on a base  21  and is movable in the direction A on the base  21 . A plurality of mask tables MT are positioned on the mask sage MS, each of which supports one of the masks M 1 , M 2 , M 3  and M 4 , respectively. 
     A leveling holder  17   a  holds the photosensitive substrate P through a known adsorption mechanism (not shown). A Z-leveling stage  17   b  is positioned under the leveling holder  17   a  and moves in the Z direction. An XY stage  17 C is positioned under the Z stage  17   b  and moves in the X and Y directions. 
     One of the masks M 1 -M 4  on the mask tables MT is registered under the illumination light IL so as to cross the optical axis AX of illumination light IL. The height (vertical position) and the inclination of the registered mask are measured by sets of detection light emitting units  11   a  and light receiving units  11   b.  The detection light emitting unit  11   a  emits detection light (laser beam) AL to a reference surface of the registered mask (M 1 ). The light receiving unit  11   b  receives, through a parallel planar glass  20 , the reflected detection light AL that was reflected from the reference surface of the mask. 
     The detection light emitting unit  11   a  and the detection light receiving unit  11   b  are positioned so that the distance from the projection lens system is constant. The level of the detection light AL received by the detection light receiving unit  11   b  corresponds to the distance between the mask pattern and the projection lens system. 
     FIG. 2 illustrates an arrangement of the detection light emitting units and detection light receiving units for detecting the height and the inclination of the mask. A first detection system includes a detection light emitting unit  11   a  and a detection light receiving unit  11   b,  a second detection system includes a detection light emitting unit  11   a ′ and a detection light receiving unit  11   b ′, a third detection system includes a detection light emitting unit  18   a  and a detection light receiving unit  18   b , and a fourth detection system includes a detection light emitting unit  18   a  and a detection light receiving unit  18   b′.    
     The four pairs of the detection systems detect the height of four points P 1 -P 4  in the mask M 1  (or one of M 2 -M 4 ). Based on the detection result, displacement, if necessary, in the height and the inclination from the optical axis AX are determined with respect to a reference position. 
     To detect the height in the Z direction of the photosensitive substrate P mounted on the leveling holder  17   a , a horizontal position detection system ( 13   a,    13   b ) and a focal point detection system ( 14   a ,  14   b ) are provided. Light sources  13   a  and  14   a  emit illumination light that strikes the surface of the photosensitive substrate from an oblique direction with respect to the optical axis AX. Light receiving units  13   b  and  14   b  receive the light reflected from the surface of the photosensitive substrate P. Half mirrors  31  and  32  are positioned on the optical path, and a parallel planar glass  30  is positioned in front of the receiving unit  14   b . An image-forming luminous flux of the illumination light emitted from the light source  13   a  forms a pin-hole image or a slit image. 
     A plate controller  15  controls a leveling driving unit  16   a  and a Z-axis driving unit  16   b  based on photodetection signals S 1  and S 2  that are supplied from the receiving units  13   b  and  14   b . The driving units  16   a  and  16   b  drive the leveling holder  17   a  and the Z-axis stage  17   b , respectively, under the control of the plate controller  15  to adjust the position of the photosensitive substrate P by adjusting the height in the Z direction and the inclination with respect to the optical axis AX, thereby positioning the photosensitive substrate P in the optimum image-forming plane of the projection lens system PL. 
     The plate controller  15  also controls the XY stage  17   c  based on a direction from the controller  12 , which will be described below. 
     In this embodiment, the angle of the parallel planar glass  30  is adjusted in advance so that the optimum image-forming plane becomes the zero level in order to calibrate the focal point detection system. At the same time, the horizontal position detection system is also calibrated so that when the photosensitive substrate P is aligned with the image-forming plane, the parallel luminous flux from the light source  13   a  is focused on the center of the light-receiving element, which is divided into four sections and forms part of the light-receiving unit  13   b.    
     The controller  12  controls the overall exposure apparatus as well as the variable blind driving unit  6   a , the mask stage MS and the plate controller  15 . The controller  12  sets the illumination area by changing the size of the aperture of the variable blind  6  through the variable blind driving unit  6   a  corresponding to pattern data of the masks M 1 -M 4 . The controller  12  determines the position of the mask through the mask stage MS based on alignment data of the masks M 1 -M 4  detected by the alignment optical system (not shown). The controller  12  also directs the plate controller  15  to control the position of the XY stage  17   c  in a stepwise manner based on the mask pattern data. 
     The controller  12  supplies the detected displacement of the mask M 1  (or is one of M 2 -M 4 ) with respect to the height and the inclination relative to the optical axis AX to the plate controller  15 . The plate controller  15  then drives the leveling holder  17   a  and the Z stage  17   b  through the driving units based on the displacement so that the photosensitive substrate P is positioned in a conjugate position with respect to the mask M 1  (or one of M 2 -M 4 ). 
     FIG. 3 illustrates a plurality of masks M 1 -M 4  mounted on the mask stage MS, which are successively aligned with the optical axis AX by the controller  12 . The respective patterns (referred to as unit patterns) formed on the masks M 1 -M 4  are successively exposed onto the predetermined areas on the photosensitive substrate P. The controller  12  sets the aperture of the variable blind  6  through the variable blind driving unit  6   a  based on the mask pattern data. The controller  12  repeatedly moves and stops the photosensitive substrate P, through the plate controller  15 , to expose the unit pattern A 1  formed on the mask M 1 . The controller  12  then transfers the unit pattern B 1  formed on the mask M 2  onto the photosensitive substrate P adjacent the unit pattern A 1 . The unit patterns A 1  and B 1  transferred onto the photosensitive substrate P are connected through stitching JN 1 . 
     The controller  12  further controls the plate controller  15  to transfer the unit pattern C 1  formed on the mask M 3  onto the photosensitive substrate P adjacent the unit pattern B 1 , and the unit patterns B 1  and C 1  transferred onto the photosensitive substrate P are connected with each other through stitching JN 2 . Similarly, the unit pattern D 1  formed on the mask M 4  is transferred adjacent the unit pattern C 1 . The transferred unit patterns C 1  and D 1  are coupled through stitching JN 3 . The unit pattern D 1  is further connected to the unit pattern A 1  through stitching JN 4 . 
     In this manner, the unit patterns A 1 , B 1 , C 1  and D 1  of the masks M 1 -M 4  are successively exposed onto the photosensitive substrate P so that the unit patterns A 1 -D 1  are connected through stitching JN 1 -JN 4 . These divided areas form a layer (the first layer designated LY 11 ). Each of the unit patterns A 1 -D 1  has an overlap area. The projection image in the overlap area of a unit pattern is combined with the projection image in the overlap area of another unit pattern, thereby connecting the unit patterns at the stitching portions JN 1 -JN 4 . The width of the overlapped (double-exposed) area is preferably about 2 μm. 
     The controller  12  changes the masks on the mask stage MS and transfers the second layer on the photosensitive substrate P on which the first layer image has been formed, using masks M 11 , M 12 , M 13 , M 14  for the second layer, as shown in FIG.  4 . The controller  12  successively aligns the plurality of masks M 11 , M 12 , M 13 , M 14  mounted on the mask stage MS with the optical axis AX, and successively exposes the patterns formed on the masks M 11 -M 14  (unit patterns) onto the photosensitive substrate P on which the first layer LY 11  has been formed. The controller  12  controls the variable blind driving unit  6   a  and the plate controller  15  so as to expose the unit pattern A 2  formed on the mask M 11  onto the photosensitive substrate P. The unit pattern B 2  formed on the mask M 12  is then transferred onto the photosensitive substrate P adjacent the unit pattern A 2 , and the unit patterns A 2  and B 2  transferred on the photosensitive substrate P are connected with each other through stitching JN 11 . 
     The controller  12  then controls the variable blind driving unit  6   a  and the plate controller  15  so as to expose the unit pattern C 2  formed on the mask M 13  onto the photosensitive substrate P adjacent the unit pattern B 2 , and the unit patterns B 2  and C 2  transferred on the photosensitive substrate P are connected with each other through stitching JN 12 . The unit pattern D 4  formed on the mask M 14  is then transferred adjacent the unit pattern C 2 , and the unit pattern C 2  is connected to the unit pattern D 2  through stitching JN 13 . Finally, the unit pattern D 2  is connected to the unit pattern A 2  through stitching JN 14 . 
     In this manner, the unit patterns A 2 , B 2 , C 2  and D 2  of the masks M 11 -M 14  are successively exposed onto the photosensitive substrate P on which the first layer LY 11  has been formed, thereby forming the second layer (the second layer designated LY 12 ) including the unit patterns A 2 , B 2 , C 2  and D 2  connected to one another through stitching JN 11 , JN 12 , JN 13  and JN 14 . 
     The dimensions of the unit patterns A 2 , B 2 , C 2  and D 2  formed on the masks M 11 , M 12 , M 13  and M 14  for the second layer LY 12  are different from those of the unit patterns A 1 , B 1 , C 1  and D 1  formed on the masks M 1 , M 2 , M 3  and M 4  for first layer LY 11 . Consequently, the stitching portions JN 11 -JN 14  in the second layer LY 12  are offset from the stitching portions JN 1 -JN 4  in the first layer LY 11  by a displacement amount d (2 mm in this embodiment), as shown in FIG.  5 . 
     FIG. 6 shows the cross-sections of the first layer LY 11  and the second layer LY 12  formed on the photosensitive substrate P. The unit patterns D 1  and C 1  are connected with each other through stitching JN 3  in the first layer LY 11 . In the second layer LY 12 , which is formed over the first layer LY 11 , the unit patterns D 2  and C 2  are connected with each other through stitching JN 13 . The stitching JN 13  in the second layer LY 12  is offset from the stitching JN 3  in the first layer LY 11  by a distance “d”. 
     If the unit pattern D 1  in the first layer LY 11  is transferred with an offset-Δx from the target exposure position, and if the unit pattern D 2  in the second layer LY 12  is transferred with an offset +Δx from the target exposure position, then the overlay error of the unit pattern D 2  in the second layer LY 12  becomes +2Δx relative to the unit pattern D 1  in the first layer LY 11 . 
     If the unit pattern C 1  in the first layer LY 11  is transferred with an offset +Δx from the target exposure position, and if the unit pattern C 2  in the second layer LY 12  is transferred with an offset −Δx from the target exposure position, then the overlay error of the unit pattern C 2  in the second layer LY 12  becomes 2Δx relative to the unit pattern C 1  in the first layer LY 11 . Accordingly, the overlay error of the second layer LY 12  relative to the first layer LY 11  becomes 2Δx in the first area AR 1  in which the unit pattern D 2  of the second layer LY 12  covers the unit pattern D 1  of the first layer LY 11 , while it becomes −2Δx in the third area AR 3  in which the unit pattern C 2  of the second layer LY 12  covers the unit pattern C 1  of the first layer LY 11 , as shown in FIG.  7 . 
     Because the stitching portion JN 13  in the second layer is formed offset from the stitching portion JN 3  in the first layer by a distance d, the overlapped area (second area) AR 2  with a width d is formed between the first area AR 1  and the third area AR 3 , in which the unit pattern D 2  of the second layer LY 12  overlaps the unit pattern C 1  of the first layer LY 11 . In the second area AR 2 , the unit pattern C 1  in the first layer LY 11  is offset +Δx from the target position, and the unit pattern D 2  in the second layer LY 12  is offset +Δx from the target position. As a result, the overlay error between the first layer and second layer becomes zero in the second area AR 2 . 
     In the aforementioned case, the unit patterns D 1  and C 1  were exposed in the first layer LY 11  offset in opposite directions, and the unit patterns D 2  and C 2  were exposed in the second layer LY 12  offset in opposite directions, and an error occurs in the exposure position such that the difference between the overlay error in the first area AR 1  and the overlay error in the third area AR 3  is maximized. 
     However, since the second area AR 2  is defined by shifting the stitching portion JN 13  of the second layer LY 12  from the position of the stitching portion JN 3  of the first layer LY 11  by a distance d, the resultant patterns in the first layer LY 11  and the second layer LY 12  offset in the same direction in the second area AR 2 . As a result, the overlay error between the first and second layers LY 11  and LY 12  is canceled out in the second area AR. 
     The difference between the overlay error in the second area AR 2  and the overlay error in the first area AR 1  becomes 2Δx, and the difference between the overlay error in the second area AR 2  and the overlay error in the third area AR 3  also becomes 2Δx. FIG. 8 illustrates the overlapping areas (PIL 1 , PIL 2 , PIL 3 ) of the drains DR formed in the second layer LY 12  over the gates GA formed in the first layer LY 11  in thin film transistors. The overlapping area is minimized in the first area AR 1  (PIL 1 ), while being maximized in the third area AR 3  (PIL 3 ). The overlapping area PIL 2  in the second area AR 2  is in-between. In other words, an area having all intermediate contrast level (the second area AR 2 ) is formed between the first area AR 1  and third area AR 3 , which have a large amount of contrast gap. 
     In contrast with the case in which the first area AR 1  is directly connected to the third area AR 3 , the change in the contrast becomes smooth, and the contrast gap is prevented from being conspicuous in each unit pattern. 
     In this embodiment, the maximum differential of overlay (i.e., the maximum difference between the overlay errors of adjacent unit patterns on the photosensitive substrate) becomes almost half of that with the conventional method. 
     In this embodiment, a unit pattern is formed in a mask, and a plurality of masks are successively aligned with the optical axis AX of the projection lens system PL for exposure; however, the invention is not limited to this arrangement, and a plurality of unit patterns may be formed in a mask. If this is the case, a necessary unit pattern is defined by the variable blind  6  and separately exposed. 
     FIG. 9 illustrates a mask M 21  on which first layer unit patterns A 1 , B 1 , C 1 , D 1 , E 1 , F 1 , G 1 , H 1  and I 1  are formed. The first pattern layer is exposed onto a photosensitive layer using the mask M 21 , then the second pattern layer is exposed over the first layer using a mask M 22  (shown in FIG.  10 ). The dimensions and arrangement of the unit patterns A 2 , B 2 , C 2 , D 2 , E 2 , F 2 , G 2 , H 2  and I 2  of the second layer mask M 22  are slightly different from those or the unit patterns A 1 , B 1 , C 1 , D 1 , E 1 , F 1 , G 1 , H 1  and I 1  of the first layer mask M 21 . Consequently, the stitching portion JN 22  of the unit patterns in the second layer is offset from the position of the stitching portion JN 21  of the first layer by a distance d, as shown in FIG.  11 . 
     Similar to the embodiment shown in FIG. 5, the contrast gap that occurs at the stitching portions JN 21  and JN 22  between two adjacent unit patterns can be reduced. In this embodiment, an exposure apparatus that aligns a single mask with the optical axis AX and illuminates a necessary unit pattern using a variable blind  6  may be used in place of the exposure apparatus of FIG. 1, which has a mask stage MS. 
     Although the offset of the stitching portions between the first and second layers is set to 2 mm, the offset amount is not limited to this value. According to experimental data, an offset d of at least 1.5 mm can sufficiently reduce the contrast gap in practical use. 
     In the above-described embodiment, a single projection lens system PL is used in the exposure apparatus; however, a scanning-type exposure apparatus, which has a plurality of projection lens systems, may be used. 
     Although two layers (LY 11 , LY 12 ) are formed photosensitive substrate, the invention is not limited to two-layer exposure. The invention can be broadly applied to cases in which three, four or more layers are formed, as long as the stitching portions of different layers offset from one another. 
     A second embodiment of the invention will be described with reference to FIGS. 12-14. 
     FIG. 12 illustrates an example of a projection exposure apparatus used in the projection exposure method according to the second embodiment. FIG. 12 is a perspective view of a scanning-type projection exposure apparatus  100  having a plurality of projection lens systems and forming an erecting positive image with a magnification of one as a whole. In FIG. 12, the coordinate system is defined such that the X axis extends along the direction that a reticle  102 , on which a predetermined circuit pattern is formed, and a glass substrate  104  coated with resist are driven, the Y axis extends perpendicular to the X axis within the plane of the reticle  102 , and the Z axis extends vertical to the reticle  102 . An illumination optical system  103  uniformly illuminates a reticle  102  positioned in the XY plane. The illumination optical system  103  has a trapezoid field stop (not shown) so that adjacent optical pattern images overlap each other by a predetermined amount. The illumination optical system  103  makes luminous flux emitted by the light source (not shown) uniform through the lens system, which includes a fly-eye lens. The luminous flux is then shaped by the field stop so as to have a trapezoid profile and illuminates the circuit pattern on the reticle. The projection exposure area on the reticle  102  becomes trapezoid-shaped. 
     The reticle  102  is mounted on the reticle stage (not shown) and moves in the X and Y directions along with the movement of the reticle stage. A plurality of projection lens systems  105   a - 105   g  are positioned under the reticle  102 , each of the projection lens systems being located so as to correspond to one of the apertures of the field stops. Each of the projection lens systems  105   a - 105   g  is preferably constituted by pairs of Dyson optical systems. The projection optical systems  105   a - 105   g , each having two Dyson optical systems, are arranged in two rows (upper row and lower row) so that the projection lens systems  105 A,  105 B,  105 C and the projection lens systems  105 D,  105 E,  105 F,  105 G are alternately positioned. 
     When illumination light  106   a - 106   g  having a trapezoid profile is guided onto the reticle  102 , the patterns on the reticle  102  are exposed in the trapezoid projection areas  107   a - 107   g  through the projection lens systems  105   a - 105   g.  The glass substrate  104  is mounted on the X-Y stage (not shown) and moves in the X and Y directions along with the movement of the X-Y stage. 
     The reticle stage and the X-Y stage are synchronously moved in the X direction relative to the projection lens system, thereby transferring the pattern of the reticle  102  onto the glass substrate  104  with uniform exposure distribution over the entire area Because the scanning-type projection exposure apparatus  100  has a plurality of projection lens systems  105   a - 105   g , a large exposure area can be ensured without increasing the exposure area of each projection lens system. 
     FIG. 13 is a plan view of the glass substrate  104  in which the exposure projection areas  107   a - 107   g  are formed through the projection lens systems  105   a - 105   g . The projection areas  107   a - 107   g  are formed in a trapezoid shape so that the sum of the widths of the projection areas  107   a - 107   g  along the scanning direction (X direction) becomes constant at any Y position. The projection areas  107   a - 107   g  are arranged alternately in two columns so that the top of the trapezoids of one column are disposed facing a direction opposite to that of the trapezoids of the other columns. The trapezoids in the two columns are also arranged so that the Y positions of the trapezoids in one column overlap the Y positions of the trapezoids in the other column by a predetermined amount (e.g., 5 mm). When the glass substrate  104  is exposed, the total exposure amount of the overlapped portions of the projection areas  107   a - 107   g  becomes equal to the exposure amount of the other portions, which do not overlap in the Y direction. Accordingly, the exposure distribution becomes uniform over the entire area of the glass substrate  104 . Although, in this embodiment, the shape of the projection areas  107   a - 107   g  is trapezoid, it is not limited to a trapezoid. For example, the projection area may be hexagonal. A plurality of the scanning-type projection exposure apparatus  100  are used for exposure, each being used to form one of the pattern layers of a TFT. 
     The projection exposure method according to the second embodiment will be described in conjunction with FIGS.  14 ( a )-( c ), using an example in which a gate electrode layer and a source/drain electrode layer are formed. The scanning-type projection apparatus and the associated elements used for forming the gate electrode layer bear the same symbols as in FIG. 12, and another scanning-type projection exposure apparatus and the associated elements used for forming the source/drain electrode layer bear symbols with the designation (′) to clarify the explanation. The same applies to the projection areas  107   a - 107   g  shown in FIG.  13 . 
     A reticle  102  on which a gate electrode pattern is formed is mounted on the reticle stage (not shown) in the scanning-type exposure apparatus  100  used for forming a lower layer (gate electrode). The pattern image is divided into a plurality of sections by the projection lens systems  105   a - 105   g , which are then projected into the projection areas  107   a - 107   g  (FIG. 13) formed on the resist layer (not shown), which covers the glass substrate  104 . 
     The reticle  102  and the glass substrate  104  are synchronously moved relative to the projection lens systems  105   a - 105   g , so that the entire area of the gate electrode pattern is uniformly exposed onto the resist layer of the glass substrate  104 . 
     FIG.  14 ( b ) shows the projection areas  107   a  and  107   e  formed on the glass substrate  104  by the scanning-type projection exposure apparatus  100 , which overlap each other in the Y direction. In FIG.  14 ( b ), the projection lens system  105   a,  which forms a pattern image in the projection area  107   a , has an imagery characteristic that causes the image-forming position to shift ΔP in the +Y direction. The projection lens system  105   e , which forms a pattern image in the projection area  107   e , has an imagery characteristic that causes the image-forming position to shift ΔP in the −Y direction. 
     The resist layer is developed after the exposure and is then used as a mask for patterning the lower metal interconnect layer, thereby forming a gate electrode. 
     Subsequently, a gate insulation film, a channel layer and the like are formed, and another scanning-type projection exposure apparatus  100 ′ (different from the exposure apparatus  100  used for forming the gate electrode) is used to form source/drain electrodes defining an upper layer. A reticle  102 ′ on which a source/drain electrode pattern is formed is mounted on the reticle stage (not shown). The source/drain electrode pattern formed in the reticle  102 ′ and the glass substrate  104  are shifted by a predetermined distance in the Y direction relative to the multiple apertures of the field stop (not shown) of the illumination optical system  103 ′ and the multiple projection lens systems  105   a ′- 105   g ,′ which are provided corresponding to the apertures. The Y direction is perpendicular to the optical axes of the projection lens systems  105   a ′- 105   g ′ and to the moving direction of the reticle  102 ′ and the glass substrate  104 . Assuming that the source/drain electrode pattern is formed in the reticle  102 ′ at substantially the same position as that of the gate electrode pattern formed in the reticle  102 , the reticle stage of the scanning-type projection exposure apparatus  100 ′, which supports the reticle  102 ′, is moved so that the position of the reticle  102 ′ shifts in the Y direction by the predetermined distance from the position of the reticle  102  that was mounted on the reticle stage of the scanning-type projection exposure apparatus  100 . At the same time, the X-Y stage that supports the glass substrate  104  is also moved in the Y direction by the predetermined distance. 
     The pattern image of the reticle  102 ′ is divided into a plurality of sections by the projection lens systems  105   a ′- 105   g ′, which are then projected into the projection areas  107   a ′- 107   g ′ on the resist layer (not shown) covering the glass substrate  104 . 
     FIG.  14 ( a ) shows the projection areas  107   a ′ and  107   e ′ formed on the glass substrate  104  by the scanning-type projection exposure apparatus  100 ′, which overlap each other in the Y direction. 
     As shown in FIG.  14 ( a ), the projection lens system  105   a ′, which forms a pattern image in the projection area  107   a ′, has an imagery characteristic that causes the image-forming position to shift ΔP in the −Y direction, and the projection lens system  105   e ′, which forms a pattern image in the projection area  107   e ′, has an imagery characteristic that causes the image-forming position to shift ΔP in the +Y direction. 
     Because the reticle  102 ′ and the glass substrate  104  are shifted a predetermined distance relative to the projection lens systems  105   a ′- 105   g ′, the projection areas  107   a ′ and  107   e ′ for the source/drain electrode overlap each other in the Y direction in the area b′. The overlapping area b′ shifts in the Y direction from the overlapping area b, in which the gate electrode projection areas  107   a  and  107   e  overlap each other by the predetermined distance. In this embodiment, the predetermined distance is equal to the width of the overlapping area b′ for the projection areas  107   a ′ and  107   e′.    
     The reticle  102 ′ and the glass substrate  104  are then synchronously moved in the X direction relative to the projection lens systems  105   a ′- 105   g ′ to expose the entire area of the source/drain electrode pattern onto the resist layer of the glass substrate  104 . The resist layer is developed after the exposure and is then used as a mask for patterning the metal interconnect layer to form source/drain electrodes. 
     FIG.  14 ( c ) shows the overlay error between the gate electrode layer and the source/drain electrode layer. The horizontal axis represents a Y position, and the vertical axis represents an error. 
     The dashed line A indicates the positional shift of the gate electrode formed in the lower layer, and the solid line B indicates the positional shift of the source/drain electrodes formed in the upper layer. The bold solid line C indicates the overlay error (C=B−A) between the gate electrode and the source/drain electrodes. The overlay error equals the offset of the upper source/drain electrodes relative to the lower gate electrode. Therefore, the overlay error of the source/drain electrodes with respect to the gate electrode becomes −2ΔP in the area a. The overlay error of the source/drain electrodes with respect to the gate electrode becomes 2ΔP in the area c. 
     The stitching portion (area b) of the gate electrode layer and the stitching portion (area b′) of the source/drain electrode layer are adjacent each other, but do not overlap each other. Accordingly, the overlay error changes from −2ΔP to zero (0) in the area b, corresponding to the positional shift of the gate electrode. Similarly, the overlay error changes from zero (0) to +2ΔP in the area b′, corresponding to the positional shift of the source/drain electrode. The total change of the overlay error in the areas b+b′ becomes 4ΔP. In this embodiment, the position of the stitching portion in tie upper layer (i.e., the source/drain layer) is offset from the position of the stitching portion in the lower layer (i.e., the gate electrode layer) by a distance equal to the width of the stitching portion. Accordingly, the change of the overlay error in the stitching portion can coincide with the change in the positional error of the stitching portion. Although the largest possible error is 4ΔP, which is the same as in the prior art, the rate of change (i.e., the slope of the bold line C) in the TFT characteristic at the stitching portion becomes one half (½) of the prior art method, because the width of the area in which the overlay error changes is doubled. Several TFTs are formed between two TFTs that have different characteristics, so that the TFT characteristics change gradually. As a result, the screen separation caused by the variation in the imagery characteristics of the projection lens systems can be considerably reduced. 
     The scanning-type exposure apparatus according to a third embodiment of the invention will now be described referring to FIGS.  15 ( a )-( c ). The structure of the exposure apparatus of this embodiment is the same as that of the second embodiment, and the explanation thereof will be omitted. The imagery characteristic of each projection lens system used in the exposure apparatus is also the same as that shown in FIG.  14 . 
     FIGS.  15 ( a ) and  15 ( b ) show the positional relationship among the projection areas  107   a ′,  107   e ′ for source/drain electrodes and the projection areas  107   a ,  107   e  for gate electrodes. In this embodiment, the positional shift of the projection areas  107   a ′ and  107   e ′ in the Y direction, with respect to the position of the projection area  107   a  and  107   e , is set to b′+b″, which is greater than that of the second embodiment. Each of the widths of the area b′ and the area b″ is equal to that of the stitching portion b. 
     FIG.  15 ( c ) shows overlay errors that occur in the projection exposure method of the third embodiment. The gate electrode (lower layer) is formed on the glass substrate  104  using the scanning-type projection exposure apparatus  100 , and the source/drain electrodes (upper layer) are formed using the scanning-type projection exposure apparatus  100 ′. The horizontal axis represents a Y position, and the vertical axis represents an error. 
     The dashed line A indicates the positional shift of the gate electrode formed in the lower layer, and the solid line B indicates the positional shift of the source/drain electrodes formed in the upper layer. The bold solid line C indicates the overlay error (C=B−A) between the gate electrode and the source/drain electrodes. The overlay error equals the offset of the upper source/drain electrodes relative to the lower gate electrode. Therefore, the overlay error of the source/drain electrodes with respect to the gate electrode becomes −2ΔP in the area a. The overlay error of the source/drain electrodes with respect to the gate electrode becomes 2ΔP in the area c. 
     The stitching portion (area b) of the gate electrode layer and the stitching portion (area b″) of the source/drain electrode layer do not overlap each other. Accordingly, the overlay error changes from −2ΔP to zero (0) in the area b, corresponding to the positional shift of the gate electrode. 
     The overlay error in the area b′ located between the lower layer stitching portion (area b) and the upper layer stitching portion (area b″) becomes zero, because the imagery characteristics of the projection lens systems  105   e  and  105   a ′ that form the projection areas  107  and  107   a ′, respectively, are the same, and the gate electrode formed in the projection area  107   e  and the source/drain electrode formed in the projection area  107   a ′ contain a positional error of the same direction and the same magnitude. 
     The overlay error changes from zero (0) to +2ΔP in the area b″ corresponding to the positional shift of the source/drain electrode. 
     In this embodiment, the position of the stitching portion in the upper layer (i.e., the source/drain layer) is offset from the position of the stitching portion in the lower layer (i.e., the gate electrode layer) by a distance equal to twice the width of the stitching portion. Accordingly, the change of the overlay error in the stitching portion can coincide with the change of the positional error of the stitching portion. The positional shift of the stitching portion is set to be greater than the width of the stitching portion, so that the stitching portions of the upper and lower layers do not adversely affect each other. Consequently, the change of the overlay error is reduced to 2ΔP from 4ΔP which is the conventional maximum overlay error. Moreover, the overlay error changes in two steps because of the extra area b″, and the rate of change (i.e., the slope of the bold line C) can be made more gradually than in the second embodiment. Several TFTs are formed between two TFTs, which have different characteristics, so that the TFT characteristics change gradually in this area, thereby reducing the screen separation caused by the variation in the imagery characteristics of the projection lens systems. 
     The maximum overlay error can be reduced by half, as compared with the conventional method, and thus, screen separation is not recognizable even if the stitching portions increase because of shifting the position of the stitching portions. 
     The pattern and the glass substrate are shifted in the Y direction relative to the projection lens system of the scanning-type projection exposure apparatus by a distance twice the width of the stitching portion b. This arrangement can prevent screen separation from being conspicuous, which is caused by a change of the TFT characteristic in stitching portions due to the overlay error between the gate electrode and source/drain electrodes. 
     Screen separation is a phenomenon wherein differences in the image quality of the left and right halves of the screen become visible because of abrupt changes of the overlay accuracy in the stitching portion. According to the projection exposure method of this embodiment, the overlay error that occurs in the stitching portion changes gradually, as compared with the conventional method, and screen separation can be sufficiently suppressed. 
     The present invention is not meant to be limited to the embodiments described above, and those of ordinary skill in the art will contemplate many modifications and substitutions that fall within the scope of the invention. 
     For example, the positional shift of the stitching portion is set equal to the width of the stitching portion (area b) in the second embodiment, and it is set to about twice the width of the stitching portion in the third embodiment. The positional shift of the stitching portion, however, may be less than the width of the stitching portion, as that amount of position shift can also change the overlay error in the stitching portions so as to suppress screen separation. 
     Although a gate electrode layer and a source/drain electrode layer are exposed, as an example of layers that affect screen separation, the invention can be applied to the case in which an accumulated capacitive line and a display electrode are exposed and layered. 
     In the second embodiment, the position of the upper layer reticle  102 ′ in the reticle stage is shifted in the Y direction with respect to the position of the lower layer reticle  102  by a predetermined distance in order to shift the stitching portion of the upper layer. The invention, however, is not limited to this method, and any method can be used as long as the stitching portion of the upper layer is offset from the stitching portion of the lower layer. For example, the position of a source/drain electrode pattern formed on a reticle may be shifted a predetermined distance in the Y direction, with respect to the position of a gate electrode pattern formed on another reticle. The position of the apertures of the field stops and the position of the projection lens systems  105   a ′- 105   g ′ of the scanning-type projection exposure apparatus  102 ′ can be shifted a predetermined distance in the Y direction with respect to the positions of the field stop aperture and the projection lens systems  105   a - 105   g  of the scanning-type projection exposure apparatus  100 . 
     Alternatively, one of the layers that may affect screen separation may be shifted in the +Y direction, while the other layer may be shifted in the −Y direction during exposure so that the patterns on the layers relatively offset from each other by a predetermined amount. 
     In the second embodiment, the reticle  102  and the glass substrate  104  are held within a horizontal plane, as shown in FIG.  12 . However, a scanning-type projection exposure apparatus with a vertical stage may be used, in which the reticle  102  and the glass substrate  104  may be held within a vertical plane (along the Z axis). 
     Thus, according to the invention, the overlay error can be sufficiently suppressed, and screen separation that deteriorates the image quality of a TFT/LCD can be reduced.