Exposure apparatus

An exposure apparatus adapted for use in manufacture of semiconductor devices or substrates for liquid crystal display panels, comprises an illumination systems for irradiating a first object with a light beam from a light source; a projection optical system for projecting the image of a pattern on the first object, illuminated by the light beam, onto a second object; and a light attenuation device provided in the illumination system and adapted to gradually decrease the amount of light in the peripheral portion of the image of the pattern, projected onto the second object, as the distance from the center of the image increases.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an exposure apparatus adapted for use in 
manufacture of semiconductor devices, substrates for liquid crystal 
display panels or the like, and more particularly to an exposure apparatus 
for effecting so-called image area synthesis or composition for forming a 
pattern of a large area by partially superposing patterns of a unit area 
on a photosensitive substrate. 
2. Related Background Art 
In such exposure apparatus, in order to meet the requirement for a size 
increase of the photosensitive substrate to be exposed, there is employed 
an image area synthesis method, in which the exposure area of the 
photosensitive substrate is divided into plural unit areas and exposures 
are executed in repeated manner, respectively corresponding to such unit 
areas to finally synthesize a desired pattern. In such image area 
synthesis method, in order to avoid pattern discontinuity at the boundary 
of the unit exposure areas, possibly resulting from the error in 
patterning on the reticle for pattern projection, optical aberrations in 
the projection optical system and alignment error of the stage for 
positioning the photosensitive substrate, exposures of the unit areas are 
executed with mutual overlapping by a small amount at the boundary. 
However, such overlapping of the exposure areas gives a doubled exposure 
to the overlapping area, possibly resulting in a variation in the pattern 
line width in the pattern junction area, depending on the properties of 
the photosensitive material employed. Also such image area synthesis may 
generate a step difference at the junction of patterns, due to the 
positional aberration between the adjacent exposure areas, eventually 
deteriorating the properties of the device to be produced. Furthermore, if 
such pattern, obtained by the image area synthesis method, is overlaid in 
plural layers, with respectively different exposure apparatus, the error 
in overlapping of the exposure areas fluctuates in different manners at 
the pattern junctions in different layers, due to the difference in the 
lens aberrations and in the alignment precision in different exposure 
apparatus. Particularly in case of an active matrix liquid crystal display 
device, such phenomenon leads to a discontinuous variation of the image 
contrast at the junction of the patterns, thus significantly deteriorating 
the quality of the display device. 
For avoiding such drawback in the image area synthesis, the Japanese Patent 
Publication No. 63-49218 discloses a method of providing the reticle or a 
filter overlaid thereon, with light attenuating means for reducing the 
amount of transmitted light in a position corresponding to the pattern 
junction, thereby reducing the amount of exposure in the overlapping 
portion of the patterns substantially equal to that in other areas. 
However, such known method is associated with the following drawbacks. In 
case such light attenuating property is given to the reticle itself, it 
increases the burden to the preparation of the reticle, for example 
because of the increased number of steps required for the preparation and 
the increased probability of pattern defect generation in the course of 
preparation. On the other hand, in case such property is given to the 
filter to be overlaid on the reticle, the maintenance of the reticle 
becomes more difficult as the attaching and detaching of such filter 
increases the probability of damage and contamination of the reticle. Also 
as the reticle is usually provided, on both sides thereof, with pellicles 
of a certain thickness in order to prevent deposition of dust or the like 
onto the reticle pattern, the filter and the reticle pattern are 
inevitably separated by the thickness of the pellicle at minimum, so that 
the ideal light attenuating characteristics are difficult to attain on the 
reticle pattern. Besides an exclusive filter has to be prepared for each 
reticle, and considerable work will be inevitable for the preparation and 
maintenance of such filters. 
SUMMARY OF THE INVENTION 
In consideration of the foregoing, an object of the present invention is to 
provide an exposure apparatus capable of providing ideal light attenuating 
characteristics on the pattern transferring original such as a reticle, 
without providing such original itself with the light attenuating property 
or without requiring light attenuating means different for respective 
originals. 
The present invention is applicable to an exposure apparatus provided with 
illumination means; a projection optical system for guiding the 
illuminating light from said illumination means to a first object and 
projecting a pattern, formed on said first object, onto a second object; 
and light attenuation means for decreasing the amount of light in the 
peripheral area of an image of the pattern, projected onto the second 
object, as the distance from the center of said pattern image increases. 
The above-mentioned object can be attained by an exposure apparatus 
comprising imaging means provided between the light attenuation means and 
the first object and adapted to form an image of the light attenuation 
means on the first object, and attenuation position regulating means for 
regulating the position of light attenuation by the light attenuation 
means on the first object. 
The light attenuating characteristics of the light attenuation means are so 
determined that, when plural patterns are projected onto the second object 
by displacements of the respective projecting position with mutual 
overlapping in the peripheral area of the images of said patterns, the 
amount of light in the overlapping portion of the pattern images becomes 
approximately equal to that outside such overlapping portion. 
The light attenuating means can be composed, for example, of a filter which 
is provided at the periphery of an aperture transmitting the illuminating 
light from the illumination means and of which transmittance decreases as 
the distance from the center of the aperture increases. 
Consequently, the image of the light attenuating means is formed by the 
imaging means on the first object, and the amount of light in the 
peripheral portion of the illuminated area on the first object decreases 
precisely according to the light attenuating characteristics of the light 
attenuating means. The area of light attenuation can be regulated 
according to the size of the first object or the variation in the 
illuminated area on said first object. 
Consequently, when the exposure areas are made to mutually overlap in the 
peripheral portions, the amount of synthesized exposures in the 
overlapping portion becomes approximately equal to the amount of exposure 
in other areas. 
Also there is provided an exposure apparatus provided with an illuminating 
optical system for irradiating a reticle with a light beam from a light 
source; illumination area setting means provided in a position, 
substantially conjugate with the reticle, within the illumination optical 
system and adapted to arbitrarily set the area to be illuminated by the 
light beam on the reticle; and exposure means for exposing a 
photosensitive substrate to the image of a pattern formed on the reticle; 
and adapted to effect exposures on different areas on the photosensitive 
substrate by forming the images of the patterns in such a manner that said 
images partially overlap mutually, comprising control means for 
controlling the illumination area setting means in such a manner that the 
amount of light in a part of the image varies substantially continuously 
in the course of exposure. 
There is furthermore provided an exposure apparatus provided with an 
illumination optical system for irradiating a reticle with a light beam 
from a light source; a field diaphragm device provided in a position 
substantially conjugate with the reticle and adapted to arbitrarily set 
the reticle area to be illuminated by the light beam; and exposure means 
for exposing a photosensitive substrate to the image of a pattern formed 
on the reticle, and adapted to effect exposures on different areas on the 
photosensitive substrate by forming the images of the patterns in such a 
manner that said images partially overlap mutually, comprising field 
diaphragm control means for displacing the position of the edge of the 
field diaphragm device in synchronization with the exposure thereby 
varying the amount of light in the peripheral portion of the image on the 
photosensitive substrate. 
Such means for continuously varying the illumination area on the reticle, 
within a range corresponding to the overlapping portion of the image of 
the reticle, in the course of exposure to the photosensitive substrate, 
causes a continuous variation in the amount of exposure in the overlapping 
portion of the images on the photosensitive substrate. 
The present invention is furthermore applicable to an exposure apparatus 
provided with an illumination optical system for guiding a light beam from 
a light source onto a reticle; a diaphragm member for defining the 
illumination area of the reticle by said light beam by regulating the area 
of the aperture transmitting the light beam; and a projection optical 
system for projecting a pattern on the reticle, illuminated by the light 
beam, onto a photosensitive substrate, and adapted to effect exposures in 
different areas on the photosensitive substrate with mutual overlapping of 
the peripheral portions of the images of the pattern. 
The aforementioned object can be attained by the presence of cumulative 
exposure detecting means for detecting the cumulative amount of exposures 
of the photosensitive substrate in respective exposures, and diaphragm 
control means for varying the position of a diaphragm member in the course 
of exposure, based on the cumulative amount of exposure detected by the 
cumulative exposure detecting means, in such a manner that the amount of 
exposure in the overlapping portion of the images of the patterns is 
attenuated according to predetermined light attenuating characteristics. 
Furthermore, the diaphragm control means is provided with target position 
designating means for designating the target position of the diaphragm 
member in the course of exposure corresponding to the cumulative amount of 
exposure detected by the cumulative exposure detecting means, and 
diaphragm driving means for moving the diaphragm member to the designated 
target position. 
When the position of the diaphragm member varies in the course of exposure, 
there results a variation in the illuminated area, defined by the 
diaphragm member, on the reticle, and the amount of exposure in the 
peripheral portion of the exposure area on the photosensitive substrate 
decreases corresponding to said variation. The present invention utilizes 
such effect and decreases the amount of exposure in the overlapping 
portion of the images of the patterns on the photosensitive substrate 
according to predetermined light attenuating characteristics, by varying 
the position of the diaphragm member in the course of exposure based on 
the cumulative amount of exposure. 
Since the target position of the diaphragm member is designated according 
to the cumulative amount of exposure and the diaphragm member is moved to 
such designated target position, the driving speed of the diaphragm member 
is regulated according to the eventual fluctuation in the intensity of the 
illuminating light on the reticle, whereby there can be prevented the 
variation in the light attenuating characteristics resulting from 
fluctuation in the illumination intensity. Also as the diaphragm member is 
moved to the target position designated according to the cumulative amount 
of exposure, there will not result a cumulative error in the speed of the 
drive control system for the diaphragm member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a schematic view of an exposure apparatus constituting a first 
embodiment of the to present invention, wherein the illuminating light 
from an ultra high pressure mercury lamp 1 constituting the exposure light 
source is condensed by an elliptical mirror 2, then reflected by a mirror 
3 and enters a wavelength selecting filter 4, which only transmits the 
light of a wavelength (generally that of g- or i-line) required for 
exposure. The illuminating light transmitted by the wavelength selecting 
filter 4 is formed as a light beam of uniform illumination intensity by a 
fly's eye integrator 5, and reaches a reticle blind 6, which regulates the 
irradiation area by the illuminating light, by the variation of the size 
of an aperture S, as will be explained later in detail. 
The illuminating light transmitted by the aperture S of the reticle blind 6 
is reflected by a mirror 7 and enters a lens system 8, which forms an 
image of the aperture S of the reticle blind 6 on a reticle (first object) 
9, thereby illuminating a desired area of the reticle 9 (by an 
illumination optical system 1-5, 7, 8. An image of a pattern present in 
the illuminated area of the reticle 9 is formed, by a projection lens 10, 
(projection optical system) onto a photosensitive substrate (second 
object) 11, whereby a specified area thereof is exposed to the pattern of 
the reticle 9. Said photosensitive substrate 11 is a wafer in case of a 
manufacturing process for a semiconductor integrated circuit, or a glass 
plate in case of a manufacturing process for a liquid crystal display 
device. 
The photosensitive substrate 11 is fixed on a stage 12, which is of a known 
structure composed of a pair of blocks (stepping means) respectively 
movable in mutually orthogonal directions and which adjust the position of 
the photosensitive substrate 11 in the horizontal plane. In case of the 
image area synthesis, after an exposure, the reticle 9 is replaced by 
another and the stage 12 is so driven as to set another position of the 
photosensitive substrate 11 for the next exposure area, and the entire 
area of the photosensitive substrate 11 is exposed by repeating the 
above-explained procedure after each exposure. The image area synthesis 
may also be attained by forming plural patterns on a reticle or reticles 
and suitably varying the illuminated area within the reticle in linkage 
with the alteration of the exposure area on the photosensitive substrate 
11. 
The position of the stage 12 is detected by an unrepresented laser 
interferometer which emits a laser beam 13 toward a movable mirror 14 
fixed on the stage 12 and measures the distance based on the interference 
between the incident light and the reflected light. 
As shown in FIGS. 1 to 4, the reticle blind is provided with a rectangular 
aperture S defined by a pair of L-shaped blades 60A, 60B combined in a 
direction perpendicular to the optical path of the illuminating light, and 
the size of said aperture S can be varied by the position adjustment of 
the blades 60A, 60B by driving mechanisms 61A, 61B (light attenuation 
position regulating means). As shown in FIGS. 2 and 3, the driving 
mechanism 61A or 61B is composed of a first block 610 on which the blade 
60A or 60B is fixed, and second and third blocks 611, 612 placed in 
superposed relationship therewith, and, by means of an unrepresented 
feeding mechanism consisting of a servo motor and a ball screw, the first 
block 610 is moved along guide grooves y1, y2 while the second block 611 
is moved along guide grooves x1, x2, whereby the blades 60A, 60B are moved 
in a plane perpendicular to the optical path (optical axis AX) of the 
illuminating light. As shown in FIG. 3, the driving mechanisms 61A, 61B 
are positioned in mutually opposite relationship with respect to the 
blades 60A, 60B, and the third blocks 612 of the above-mentioned 
mechanisms are integrally fixed, by means of an unrepresented frame, to 
the main body (not shown) of the exposure apparatus. Although FIG. 2 only 
illustrates the driving mechanism 61A for the blade 60A, the other 
mechanism 61B for the other blade 60B has the identical structure. 
As shown in FIGS. 1, 4 and 5, the blades 60A, 60B are composed of light 
shield plates 600A, 600B and ND filters 601A, 601B (light attenuation 
means) which are integrally fixed thereon and of which ends at the 
aperture S somewhat protrude from the light shield plates 600A, 600B. As 
shown in FIG. 5, the portions of the ND filters 601A, 601B (light 
absorbing members) protruding from the light shield plates 600A, 600B are 
formed triangularly in cross section, becoming thinner as the distance 
from the light shield plates 600A, 600B increases, whereby the 
transmittance of the reticle blind is 100% on the aperture S, then 
gradually decreases on the peripheral area of the aperture S as the 
distance from the center thereof increases, and reaches 0% at the position 
of the edges of the light shield plates 600A, 600B. 
The protruding amount L.sub.1 of the ND filters 601A, 601B from the light 
shield plates 600A, 600B is maintained constant around the entire 
periphery of the aperture S, whereby the light attenuating characteristics 
in the peripheral portion of the aperture S are maintained constant along 
the entire periphery, except for the diagonal corner portions of the 
aperture S where the ND filters 601A, 601B mutually overlap. The 
appropriate dimension of the protruding amount L.sub.1 will be explained 
later. The above-mentioned protruding amount L.sub.1 needs only to be 
equal on the mutually opposed sides of the aperture S. 
FIGS. 6A to 6D show the exposed image and the distribution of the exposure 
amount on the photosensitive substrate 11, when exposure is conducted with 
a suitable size of the aperture S of the reticle blind 6 and without the 
reticle 9. When plural exposures are conducted with completely separate 
exposure positions as shown in FIG. 6A, the rectangular exposed images P1, 
P2 formed on the photosensitive substrate 11 are respectively associated, 
along the entire peripheries thereof, with light attenuated portions R1, 
R2 in which the amount of exposure is reduced by the ND filters 601A, 
601B. When the exposures are executed in such a manner that the light 
attenuated areas R1, R2 mutually overlap as shown in FIG. 6B, the exposure 
in one of such areas compensates the decrease in exposure of the other 
area as shown in FIG. 6C, so that the synthesized or composite exposure 
amount in the overlapping portion of the light attenuated areas R1, R2 
coincides, as shown in FIG. 6D, with the exposure amount in the area not 
affected by the ND filters 601A, 601B. 
In the following there will be explained the procedure of image area 
synthesis with the exposure apparatus of the present embodiment. 
FIG. 7 illustrates an example of synthesizing a pattern Pr by dividing the 
photosensitive substrate 11 into four rectangular exposure areas Ra-Rd, 
and for such purpose, there are employed, in succession, four reticles 
9a-9d, shown in FIGS. 8A to 8D, respectively corresponding to the exposure 
areas Ra-Rd. The reticles 9a-9d are so patterned that the overlapping 
portions only at the image area synthesis have identical patterns. 
Around each of the reticles 9a-9d, there is formed a light shield zone IB 
of a transmittance of 0%, capable of completely intercepting the incident 
light. Though not illustrated in FIG. 7, the exposure areas Ra-Rd are made 
to mutually overlap at the boundaries thereof by a desired amount (equal 
to the aforementioned protruding amount L.sub.1 or a width W to be 
explained later), and the blades 60A, 60B of the reticle blind 6 are so 
positioned at each exposure that the light attenuation areas obtained by 
the ND filters 601A, 601B coincide with the overlapping portions of the 
exposure areas Ra-R d. 
More specifically, for exposing the upper left exposure area Ra shown in 
FIG. 7, the blade 60A, positioned at the right and lower sides of the 
reticle 9a, is so positioned that the light attenuating areas thereof 
protrude from the light shield zone IB, while the blade 60B, positioned at 
the left and upper sides of the reticle 9a, is so positioned as to be 
completely hidden by the light shield zone IB, as shown in FIG. 8A. 
For exposing the upper right exposure area Rb, the blades 60A, 60B are so 
positioned, as shown in FIG. 8B, that the light attenuating areas thereof 
protrude from the light shield zone IB, at the left and lower sides of the 
reticle 9b. For exposing the lower left exposure area Rc, the blades 60A, 
60B are so positioned, as shown in FIG. 8C, that the light attenuating 
areas thereof protrude from the light shield zone IB, at the right and 
upper sides of the reticle 9c. Also for exposing the lower right exposure 
area Rd, the blades 60A, 60B are so positioned, as shown in FIG. 8D, that 
the light attenuating areas thereof protrude from the light shield zone IB 
at the left and upper side of the reticle 9d. 
Through these operations, in the overlapping portions of the exposure areas 
Ra-Rd, the light attenuating area of the preceding exposure overlaps with 
that of the succeeding exposure as shown in FIG. 6B, and the synthesized 
exposure in such overlapping portions becomes equal to the exposure in 
other areas as shown in FIG. 6D. Consequently the pattern Pr receives 
uniform exposure, and the line width does not vary even at the junctions 
of the pattern Pr. 
At the same time there is also eliminated the step difference at the 
pattern junction, as will be explained in the following with reference to 
FIGS. 9A to 9E. 
FIG. 9A shows the image area synthesis or composition by the conventional 
method, in which a positional aberration .delta. between the adjacent 
exposure areas results in a step difference of a same amount at the 
junction of patterns Pr1, Pr2. On the other hand, in the present 
embodiment, even in case of a positional aberration .delta. between the 
adjacent exposure areas as shown in FIG. 9B, the patterns Pr1, Pr2 are 
smoothly connected as indicated by thicker lines therein. 
As the junction portions of the patterns Pr1, Pr2 coincide with the light 
attenuating portions (of the width W) of the exposure areas, said junction 
portions receive gradually decreasing exposure toward the respective edges 
thereof, as indicated in FIG. 9C at cross sectional positions d1-d5. It is 
to be noted that the curves, indicating the exposure distribution of the 
patterns Pr1, Pr2 at the cross sections d1-d5, are mutually aberrated 
laterally in FIG. 9C, corresponding to the aberration .delta. of the 
exposure areas. 
Since the synthesized or composite exposure is constant in positions where 
the patterns Pr1, Pr2 completely overlap, the synthesized or composite 
exposures at the cross-sectional positions d1-d5 shown in FIG. 9B assume 
the forms as shown in FIG. 9D, and the maximum values of the synthesized 
exposure at said cross-sectional positions d1-d5 are mutually equal. In 
consideration of the above-mentioned distribution of exposure, if the 
property of photoresist is so selected that a portion thereof receiving at 
least a certain exposure Qc (50% of the maximum exposure in the 
illustrated case) remains on the photosensitive substrate 11 after the 
photoresist development, the pattern width at the cross-sectional 
positions d1-d5 becomes constant as shown in FIG. 9E so that the patterns 
Pr1, Pr2 are smoothly connected with a constant width as indicated by 
thicker lines in FIG. 9B. 
Even when the image area synthesis or composition is repeated in plural 
layers, such smooth connection of the junction of the patterns Pr1, Pr2 
suppresses the discontinuous variation in the alignment error in the 
junction of each layer. Consequently, in case of the liquid crystal 
device, the discontinuous variation in the image contrast at the junctions 
in the image field is eliminated and the quality of the displayed image is 
improved. 
The variation at the junction of patterns becomes smaller, as will be 
apparent from FIG. 9B, as the width W of the overlapping portion of the 
patterns is selected larger. However, an excessively large width W 
increases the number of synthesis of the image areas, leading to a lowered 
efficiency, so said width W should be selected, in case of the liquid 
crystal display device, at a value where the level of the aforementioned 
variation is not perceptible to the human eyes, generally in a range of 5 
to 10 mm. This width W is determined by the protruding amount L.sub.1 of 
the ND filters 601A, 601B shown in FIG. 5 and the projecting 
magnifications of the lens system 8 and of the projection lens 10. 
As explained in the foregoing, the exposure apparatus of the present 
embodiment, being provided with an imaging lens system 8 between the ND 
filters 601A, 601B having light attenuating property and the 
photosensitive substrate 11, is capable of obtaining ideal light 
attenuation characteristics on the photosensitive substrate 11 by forming 
the image of the ND filters 601A, 601B on said photosensitive substrate 
11, thereby precisely controlling the amount of exposure at the junction 
of patterns and fully exploiting the effect by such light attenuation. 
Also as the ND filters 601A, 601B are separated from the reticle 9, owing 
to the presence of the lens system 8, the reticle 9 no longer needs to 
have the light attenuating property and is freed from the danger of damage 
or contamination. Also as the light attenuating position on the reticle 9 
can be regulated by the driving mechanisms 61A, 61B, the light attenuating 
area can be modified according to the size of the reticle 9 or the 
variation in the exposure area. 
In the present embodiment, the ND filters 601A, 601B are mounted on the 
blades 60A, 60B of the reticle blind 6 for regulating the illuminated area 
of the reticle 9, but it is also possible, as shown in FIG. 10, to 
separate the ND filters 601A, 601B from the blades 60A, 60B, and to drive 
the blades 60A, 60B by driving mechanisms 61A, 61B while to drive the ND 
filters 601A, 601B by driving mechanisms 61C, 61D. In this case, on the 
sides of the reticle where the light attenuation is not required at the 
image area synthesis, the ND filters 601A, 601B are retracted from the 
illumination area defined by the blades 60A, 60B. Such configuration 
enables effective utilization of the area of the reticle, by forming the 
light shield zone, to be formed thereon, narrower than the width of the 
light attenuating areas by the ND filters 601A, 601B. On the other hand, 
when the ND filters 601A, 601B are mounted on the blades 60A, 60B, the 
light shield zone on the reticle has to be at least as wide as the ND 
filters 601A, 601B. The ND filters 601A, 601B, separated from the blades 
60A, 60B may further be provided with light shield plates 600A, 600B 
similar to the blades 60A, 60B. 
FIG. 11 shows a variation, in which the ND filters are removed from the 
light shield plates 600A, 600B of the reticle blind 6A (illumination area 
setting means), while there are newly provided an ND blind 70 composed of 
blades 700A, 700B consisting solely of ND filters and unrepresented 
driving mechanisms for said blades, and a lens system positioned between 
the ND blind 70 and the reticle blind 6A and adapted to form the image of 
the ND blind 70 onto the reticle blind 6A. The details of the blades 700A, 
700B will not be explained as they are similar to the ND filters 601A, 
601B shown in FIG. 1. Since the ND blind 70 is optically conjugate with 
the reticle blind 6A in this embodiment, the ideal light attenuation 
characteristics can be obtained on the reticle 9, despite that the light 
attenuating means is separated from the reticle blind 6A. 
FIGS. 12 and 13 illustrate a variation of the driving mechanism 61A shown 
in FIG. 2. This driving mechanism is provided with blocks 613x, 613y 
extending mutually perpendicularly from the corner of the blade 60A, and 
rollers 615x, 615y fitted in grooves 614x, 614y of the blocks 613x, 613y. 
As shown in FIG. 13, the roller 615x (or 615y) is rotatably to mounted on 
a nut 617 engaging with a ball screw 616. There are provided two ball 
screws respectively in the x- and y-directions in FIG. 12, linked 
respectively with the rollers 615y and 615x. The ball screw 616 is 
supported by bearings 618 on the main body F of the exposure apparatus and 
is rotated by a servo motor 619, whereby the nut 617 is axially moved to 
displace the roller 615x or 615y in a direction perpendicular to the 
groove 614x or 614y, thereby moving the blade 60A in a plane perpendicular 
to the optical path. 
The ND filter to be employed in this invention can be composed of chromium 
deposited on a transparent glass substrate so as to attenuate the 
transmittance to the illuminating light. In said filter, the chromium film 
is deposited in dots smaller than the resolution limit of the exposure 
apparatus and in such a manner that the density of the dots becomes larger 
as the distance from the optical axis of the exposure apparatus increases. 
The ND filter composed of such dot-shaped chromium film is preferably so 
positioned that the plane of the deposited chromium is aberrated from the 
position conjugate with the reticle 9. The amount of the aberration is so 
determined that the photosensitive substrate is not affected by a 
contaminant matter of a predetermined size or larger, eventually present 
on the above-mentioned ND filter utilizing the chromium film. 
The light attenuating means is not limited to an optical filter, but can 
also be any other element capable of reducing the amount of light, such as 
a liquid crystal element or an electrochromic element. The light 
attenuation characteristics of the ND filter are not limited to those of 
linear light attenuation as a function of the distance from the center of 
the aperture S, but can also be non-linear as long as the synthesized 
exposure in the overlapping portion of the exposure areas substantially 
coincides with the exposure in other areas. The light attenuation is also 
possible by blurring the image of the edge of aperture of the reticle 
blind, by displacing the focal position of the lens. It is furthermore 
possible to attain light attenuation by providing, in addition to the 
conventional reticle blind, a second reticle blind in another axial 
position and forming, on the reticle, a blurred image of the aperture of 
said second reticle blind. 
FIG. 14 is a schematic view of an exposure apparatus constituting a second 
embodiment of the present invention, wherein the illuminating light from 
an ultra high pressure mercury lamp 1, constituting the exposure light 
source, is condensed by an elliptical mirror 2, then guided through a 
shutter 15 (exposure means) and a mirror 3 and enters a wavelength 
selecting filter 4, which only transmits the light of a wavelength 
required for exposure (usually that of g- or i-line). The illuminating 
light transmitted by the wavelength selecting filter 4 is converted into a 
light beam of uniform intensity by a fly's eye integrator 5 and reaches a 
reticle blind 6 which regulates the illumination area of the illuminating 
light on the reticle, by varying the size of an aperture S. A part of the 
light beam emerging from the fly's eye integrator 5 is reflected by a half 
mirror 16 and enters a cumulative exposure meter (cumulative exposure 
detecting means) 17, of which information is used for controlling the 
opening period of the shutter 15. The above-mentioned cumulative exposure 
meter 17 detects the cumulative exposure from the opening of the shutter 
15 to the current time, thereby controlling the amount of exposure. 
The illuminating light transmitted by the aperture S of the reticle blind 6 
is reflected by a mirror 7 and enters a lens system 8, which forms an 
image of the aperture S of the reticle blind 6 on the reticle 9, thereby 
illuminating a desired area of the reticle 9. An image of a pattern, 
present in the illuminated area of the reticle 9, is formed by a 
projection lens 10 on a photosensitive substrate 11 such as a wafer or a 
glass plate, whereby a specified area of the photosensitive substrate 11 
is exposed to the image of the pattern of the reticle 9. 
The photosensitive substrate 11 is fixed on a stage 12, which is of a known 
structure composed of a pair of blocks respectively movable in mutually 
orthogonal directions. The position of the stage 12 is detected, based on 
a laser beam 13 emitted from an unrepresented laser interferometer and 
reflected by a movable mirror 14 provided on the stage 12, and the 
position of the photosensitive substrate 11 in the horizontal plane is 
thus adjusted. In case of the image area synthesis, after an exposure, the 
reticle 9 is replaced by another and the stage 12 is so driven as to align 
another exposure area of the photosensitive substrate 11 to the projection 
optical system, followed by an exposure, and the entire area of the 
photosensitive substrate 11 is exposed by repeating the above-explained 
procedure after each exposure. The image area synthesis may also be 
attained by forming plural patterns on a reticle and suitably varying the 
illuminated area within the reticle (thereby selecting a different 
pattern) in linkage with the alteration of the exposure area on the 
photosensitive substrate 11. 
The reticle blind 6 is similar, in structure, to that in the first 
embodiment, and is driven by similar driving mechanisms, but is different 
in that the blades defining the aperture S have a transmittance of 0% at 
the edge. More specifically, a rectangular aperture S is defined, as shown 
in FIG. 15, by L-shaped blades 62A, 62B mutually combined along a plane 
perpendicular to the optical axis AX of the illuminating light, and the 
size of the aperture S is varied by regulating the positions of the blades 
62A, 62B with driving mechanisms 61A, 61B shown in FIG. 14. 
The above-mentioned shutter 15, cumulative exposure meter 17 and driving 
mechanisms 61A, 61B are all controlled by a control system 18 (the driving 
mechanisms and the control system constituting movement control means). 
After the reticle blind 6 is set at a pattern area, to be exposed, of the 
reticle 9, the shutter 15 is opened under an instruction from the control 
system 18 to initiate the exposure to the photosensitive substrate 11. In 
synchronization with the function of the shutter 15, the reticle blind 6 
starts movement, and the cumulative exposure meter 17 measures the 
cumulative amount of exposure. When the signal from the cumulative 
exposure meter 17 reaches a predetermined exposure amount, the shutter 15 
is closed and the reticle blind 6 terminates the movement. The moving 
speed of the reticle blind 6 is so selected that the reticle blind moves 
over a distance corresponding to the area of desired image area synthesis, 
during the open period of the shutter 15. 
More specifically, in case of exposing two pattern areas A, B shown in 
FIGS. 16A and 16B with mutual overlapping of the dotted portions, the 
reticle blind 6 is at first set corresponding to the white portion of the 
pattern area A, as shown in FIG. 16C. Then, as shown in FIG. 16D, the 
blades 62A, 62B of the reticle blind 6 (blade 62A alone in the illustrated 
example) are so moved as to gradually expand the aperture S during the 
exposure, whereby the exposure amount in an area A1-A2 on the 
photosensitive substrate 11, corresponding to the expanded portion (dotted 
portion in FIG. 16D) of the aperture S decreases with a predetermined rate 
toward the end of the expanding direction of the aperture S. In the 
example illustrated in FIGS. 16A to 16E, the blade 62A alone is driven in 
one direction to cause reduction in exposure on only one side of the 
exposure area on the photosensitive substrate 11, but it is also possible 
to drive the blade 62A simultaneously in two directions to cause such 
reduction in exposure on two sides of the exposure area, or to drive the 
blades 62A and 62B simultaneously in two direction to cause such reduction 
in exposure on all the sides of the exposure area. Furthermore such 
reduction in exposure can be attained also by gradually reducing the 
aperture S in the course of the exposure. 
In such exposure apparatus, the time of exposure is generally determined 
for example by the sensitivity of the photosensitive material. Therefore, 
in the exploitation of the present invention, the moving speed of the 
reticle blind has to be adjusted according to the exposure time. Also the 
light source is generally composed of an ultra high pressure mercury lamp, 
of which illumination intensity is known to decrease with the time of use. 
Consequently, even for a constant amount of exposure, the exposure time 
becomes longer as the lamp is used for a longer time. The exposure time T 
(sec) is given by the following formula, as a function of the exposure 
amount D (mJ/cm.sup.2) and the illumination intensity I (mW/cm) at 
exposure: 
EQU T=D/I (1) 
In this equation (1), the exposure amount D can be given in advance, but 
the illumination intensity I is variable. As the reticle blind has to 
move, during the exposure time T, a distance L.sub.2 corresponding to the 
overlapping width W (mm) between the exposure areas (L.sub.2 =W/M.sub.1 
.multidot.M.sub.2 wherein M.sub.1 is the magnification of the reticle 
blind by the lens system 8 to the reticle, and M.sub.2 is the 
magnification of the reticle by the projection lens 10 to the 
photosensitive substrate), the moving speed V (mm/sec) of the reticle 
blind is given by the following equation (2): 
##EQU1## 
The width W is already determined at the designing of the reticle, and can 
therefore be given in advance. Consequently the moving speed of the 
reticle blind can be calculated if the exposing illumination intensity I 
is known. In the exposure apparatus shown in FIG. 14, the illumination 
intensity I can be obtained from the output of the cumulative exposure 
meter 17. More specifically, prior to the exposure operation, the shutter 
15 is opened, and the output of the cumulative exposure meter 17 is 
memorized when it becomes stabilized. The memorized value, multiplied by a 
certain coefficient, corresponds to the exposing illumination intensity I. 
The above-explained operation is executed at the replacement of the 
photosensitive substrate, or at the start of a lot consisting of plural 
photosensitive substrates. The moving speed of the reticle blind is 
determined from the equation (2), based on thus obtained illumination 
intensity I and is memorized in the control system 18. 
FIGS. 17A to 17D show the exposed images and the exposure distribution on 
the photosensitive substrate 11 when exposures are made with a suitable 
size of the aperture S of the reticle blind 6 and without the reticle 9. 
As shown in FIG. 17A, shots P1, P2 are respectively associated with light 
attenuation areas indicated by hatched areas R1, R2, in which the amount 
of exposure are gradually reduced by the movement of the reticle blind. 
When both shots are synthesized as shown in FIG. 17B, there are obtained 
respective distributions of the amounts of exposure as shown in FIG. 17C, 
and, in the overlapping portion of the light attenuation areas R1, R2, the 
amount of exposure of one of the areas compensates the variation in the 
amount of exposure in the other. Consequently, as shown in FIG. 17D, the 
synthesized or composite amount of exposure in the overlapping portion of 
the light attenuation areas R1, R2 becomes equal to the amount of exposure 
in the white areas shown in FIG. 17A which are not affected by the 
movement of the reticle blind. 
In the following there will be explained the procedure of image area 
synthesis with the exposure apparatus of the present embodiment. 
As in the example shown in FIG. 7, there will be considered a case of 
synthesizing a pattern Pr by dividing the photosensitive substrate 11 into 
four rectangular exposure areas Ra-Rd. 
Though not illustrated in FIG. 7, the exposure areas Ra-Rd are made to 
mutually overlap at the boundaries thereof by a desired amount 
(corresponding to L.sub.2 or W mentioned before), and the blades 62A, 62B 
of the reticle blind 6 are so positioned at each exposure that the light 
attenuation area obtained by the movement of the reticle blind coincides 
with the overlapping portions of the exposure areas Ra-Rd. 
More specifically, for exposing the upper left exposure area Ra shown in 
FIG. 7, the blade 62A, positioned corresponding to the right and lower 
sides of the reticle 9a, is so positioned that the light attenuation area 
of the blade 62A protrudes from the light shield zone IB, while the blade 
62B, positioned corresponding to the left and upper sides of the reticle 
9a, is so positioned as to be completely retracted in the light shield 
zone IB, as shown in FIG. 8A. 
For exposing the upper right exposure area Rb, the blades 62A, 62B are so 
positioned that the light attenuation areas thereof protrude at the left 
and lower sides of the reticle 9b from the light shield zone IB, as shown 
in FIG. 8B. For exposing the lower left exposure area Rc, the blades 62A, 
62B are so positioned that the light attenuation areas thereof protrude 
from the light shield zone IB at the right and upper sides of the reticle 
9c, as shown in FIG. 8C. Also for exposing the lower right exposure area 
Rd, the blades 62A, 62B are so positioned that the light attenuation areas 
thereof protrude from the light shield zone IB at the left and upper sides 
of the reticle 9d, as shown in FIG. 8D. 
Through these operations, in the overlapping portions of the exposure areas 
Ra-Rd (namely in the one-dimensionally overlapping portions, excluding a 
portion where four pattern areas mutually overlap), the light attenuation 
area of the preceding exposure overlaps with that of the succeeding 
exposure as shown in FIG. 17B, and the synthesized amount of exposure in 
such overlapping portions becomes equal to the amount of exposure in other 
areas as shown in FIG. 17D. Consequently the pattern Pr receives uniform 
exposure, and the line width does not vary even at the junctions of the 
pattern Pr. 
At the same time there is also eliminated the step difference at the 
pattern junction, as already explained in relation to FIGS. 9A to 9E. 
As explained in the foregoing, the exposure apparatus of the present 
embodiment, being provided with a control system 18 for moving the reticle 
blind 6 within the overlapping range of the images of the reticle 9 in the 
course of exposure to the photosensitive substrate 11, is capable of 
obtaining ideal light attenuation characteristics in the peripheral areas 
R1, R2 of the image on the photosensitive substrate, thereby precisely 
controlling the amount of exposure at the junction of patterns and fully 
exploiting the benefit of such light attenuation. 
In the following there will be explained a variation in the control on the 
reticle blind movement, with reference to FIGS. 18A and 18B, which 
respectively show the output of the cumulative exposure meter 17 shown in 
FIG. 14, corresponding to a single shutter opening operation, and the 
variation in the moving speed of the reticle blind as a function of time. 
Thus, in this variation, the voltage obtained by multiplying the output of 
the cumulative exposure meter with a suitable coefficient is directly used 
as the speed instructing voltage for the movement of the reticle blind. In 
this case, the output of the cumulative exposure meter need not be 
measured in advance, because the speed of the reticle blind becomes 
automatically lower, following the eventual decrease in illumination 
intensity of the light source. Also the movement of the reticle blind can 
be completely synchronized with the open/closing operations of the 
shutter, as the output of the cumulative exposure meter responds to the 
opening and closing of the shutter. Consequently the calculation according 
to the aforementioned equation (2) can be dispensed with. The suitable 
coefficient mentioned above is to cause the reticle blind to travel over 
the distance L.sub.2, when it is moved according to the wave form 
indicating the variation in the moving speed, as shown in FIG. 18B. This 
coefficient is also inversely proportional to the exposure amount D. The 
control system for the embodiment shown in FIGS. 18A and 18B is 
illustrated, as a block diagram, in FIG. 19. 
The output of the cumulative exposure meter 17 is amplified by an amplifier 
21, and is supplied to an integrating circuit 22 and a multiplier 25. On 
the other hand, the above-mentioned coefficient is released from an output 
port of a microprocessor 23, and is supplied, through a D/A converter 24, 
to the other input terminal of the multiplier 25, of which output is 
supplied to a servo circuit 26 for driving a motor 27 for moving the 
reticle blind. 
In the above-explained second embodiment, the movement of the reticle blind 
is controlled according to the output of the cumulative exposure meter, 
but, if the variation in the intensity of the light source is negligibly 
small, the movement of the reticle blind may be simply controlled in 
synchronization with the opening and closing of the shutter. 
In the second embodiment explained above, the blades 62A, 62B of the 
reticle blind 6 for defining the illumination area on the reticle 9 are 
moved in the course of exposure, and such method is perfectly acceptable 
in the image area synthesis in one-dimensional direction. However, in case 
of two-dimensional image area synthesis as shown in FIG. 7, there is 
formed, in the exposure area of the photosensitive substrate, a portion 
where four pattern areas mutually overlap, and the amount of exposure in 
such overlapping portion becomes larger than in the remaining area. The 
countermeasure for such phenomenon will be explained in the following. 
FIG. 20 is a schematic view of an exposure apparatus constituting a third 
embodiment of the present invention. It is same in the base structure as 
the apparatus shown in FIG. 14, but is different in that, among four edges 
of the blades of the reticle blind 6, at least two in the mutually opposed 
relationship are provided with an ND blind 30 composed of ND filters 30A, 
30b having gradually varying transmittance in the edge portion as shown in 
FIG. 22. In this embodiment, among the plural blades corresponding to the 
area for effecting the image area synthesis, a part effects the movement 
of the reticle blind as in the preceding embodiment, while the remaining 
part forms the light attenuation area in the overlapping portions by the 
ND filters. More specifically, in case of the image area synthesis as 
shown in FIG. 7, the light attenuation areas for the overlapping for 
example between the exposure areas Ra and Rb or between Rc and Rd are 
formed by the reticle blind movement explained above, while those for the 
overlapping between the exposure areas Ra and Rc or between Rb and Rd are 
formed by the ND blind. In such configuration, in the overlapping portion 
where four pattern areas mutually overlap, the amount of exposure per 
pattern area decreases and the synthesized amount of exposure in said 
overlapping portion becomes comparable to that in the remaining area. 
Consequently satisfactory image area synthesis can be achieved also in 
two-dimensional manner. 
In this embodiment, with respect to the blades not requiring light 
attenuation at the image area synthesis, the ND filter 30A and/or 30B is 
retracted from the illumination area defined by the blades 62A, 62B. This 
configuration allows effective utilization of the area of the reticle, as 
the light shield zone to be formed thereon can be made narrower than the 
light attenuation areas obtained by the ND filters 30A, 30B. In case the 
ND filters 30A, 30B are integrally formed with the blades 62A, 62B, the 
light shield zone on the reticle has to be at least as wide as the light 
attenuation areas of the ND filters 30A, 30B. The ND filters 30A, 30B, 
formed separate from the blades 62A, 62B, may be provided with light 
shield plates 31A, 31B similar to the blades 62A, 62B. 
In the above-explained embodiment, the ND blind is positioned in the 
vicinity of the reticle blind, but it is also possible, as shown in FIG. 
21, to provide an optical system 32 thereby maintaining the reticle blind 
6 and the ND blind 30 in mutually conjugate relationship. The ND blind 30 
in this case is to be provided with blades 30A, 30B consisting solely of 
ND filters, and unrepresented driving mechanisms for driving the blades 
30A, 30B. Such configuration makes it possible to obtain ideal light 
attenuation characteristics on the reticle 9, as the ND blind 30 is in 
conjugate relationship to the reticle blind 6. 
The ND blind to be employed in the two-dimensional image area synthesis 
need not necessarily be composed of the ordinary optical light attenuating 
filters, but, as in the first embodiment, may also be composed, for 
example, of other means such as liquid crystal elements or electrochromic 
elements. 
Also the light attenuating characteristic achieved by the reticle blind 
movement or by the ND blind need not be limited to the linear reduction of 
the transmitted light as a function of the distance from the center 
(optical axis AX) of the aperture S, but may also include non-linear 
variation in the amount of transmitted light, as long as the synthesized 
amount of exposure in the overlapping portion of the exposure areas 
substantially coincides with the amount of exposure in other areas. The 
light attenuation can also be achieved by blurring the image of the edge 
of aperture of the reticle blind, by defocusing the lens. More 
specifically, the light attenuation can also be achieved by providing, in 
addition to the conventional reticle blind, a second reticle blind in an 
axially different position, thereby forming, on the reticle, a blurred 
image of the aperture of said second reticle blind. 
Another example of the control system 18 for the above-explained second (or 
third) embodiment is shown, as a fourth embodiment, in FIG. 23, and FIG. 
24 shows a flow chart of the control sequence of the above-mentioned 
control system. 
As shown in FIG. 23, the control system for achieving the above-explained 
function of the reticle blind 6 is provided with a controller 33 composed 
of a microcomputer and peripheral devices thereof. The controller 33 
provides a shutter driver circuit 34 with a shutter driving signal 
according to the cumulative exposure Q detected by a cumulative exposure 
meter 17, and also provides a blind driver circuit 36 with a blind driving 
signal, based on the cumulative exposure Q and the current position Xa, Ya 
of the blades 62A, 62B of the reticle blind 6 in the X and Y directions, 
detected by a blind position sensor 35. The shutter driver circuit 34 
drives an actuator 37 for opening and closing the shutter 15, in response 
to the above-mentioned shutter driving signal. Also the blind driver 
circuit 36 drives a blind actuator (a servo motor in this embodiment) 38 
for driving the reticle blind 6, with a speed corresponding to the blind 
driving signal from the controller 33. 
The cumulative exposure meter 17 can be so constructed as to convert the 
output of an integrating sensor, releasing a voltage corresponding to the 
intensity of the illuminating light, by V/F convertion into a pulse train 
of a frequency corresponding to said voltage and cumulatively counting the 
number of the pulses by a counter, but it may also be so constructed as to 
fetch the output signal of the integrating sensor into the controller 33 
after A/D conversion and to cumulatively count the pulses by the software. 
Also the blind position sensor 35 can be so constructed as to effect V/F 
conversion on the output signal of a potentiometer, releasing a voltage 
corresponding to the position of the reticle blind 6, into a pulse train 
of a frequency corresponding to the output voltage and to cumulatively 
count the number of pulses by a counter, or to fetch the output signal of 
the potentiometer into the controller 33 after A/D conversion and to 
cumulatively count the number of pulses by the software. 
FIG. 23 illustrates only one set of the blind driver circuit 36 and the 
blind actuator 38, but, as will be apparent from FIGS. 2 and 3, there are 
provided, in total, four sets of the blind driver circuit 36 and the blind 
actuator 38, for independently drive the blades 62A, 62B respectively in 
the X and Y directions. There is also provided a memory 39 storing, in 
advance, data for effecting exposure according to the kind of the 
photosensitive substrate 11. The data stored in the memory 39 include the 
appropriate exposure D of the photosensitive substrate 11, the initial 
position X.sub.0, Y.sub.0 of the blades 62A, 62B of the reticle blind 6 in 
the X and Y directions at the start of exposure, and the moving distances 
Lx, Ly of the blades 62A, 62B in the X and Y directions in the course of 
exposure. The moving distances Lx, Ly can be determined from the following 
equations (3): 
EQU Lx=Wx/(M.sub.8 .multidot.M.sub.10) 
EQU Ly=Wy/(M.sub.8 .multidot.M.sub.10) (3) 
wherein Wx and Wy are overlapping widths in directions corresponding to the 
X and Y directions of the exposure area on the photosensitive substrate 
11, and M.sub.8 and M.sub.10 are magnifications of the lens system 8 and 
the projection lens 10. The appropriate exposure D, the overlapping widths 
Wx, Wy and the initial position X.sub.0, Y.sub.0 are determined at the 
designing stage of the reticle. 
In the following there will be explained the sequence of an exposure 
operation in the exposure apparatus of the fourth embodiment, with 
reference to a flow chart shown in FIG. 24. 
The controller 33 starts the sequence shown in FIG. 24, when a 
predetermined exposure area of the photosensitive substrate 11 is 
positioned, by the function of the stage 12, with respect to the 
projection lens 10. At first a step S1 reads the appropriate exposure D, 
the initial position X.sub.0, Y.sub.0 of the blades 62A, 62B of the 
reticle blind 6, and the moving distances Lx, Ly of the blades 62A, 62B 
from the memory 39, and a next step S2 moves the blades 62A, 62B to the 
initial position X.sub.0, Y.sub.0. Then a step S3 initiates the exposure 
by opening the shutter 15. After the start of exposure, a step S4 reads 
the cumulative exposure Q from the cumulative exposure meter 17, and a 
step S5 calculates the target position Xn, Yn of the blades 62A, 62B 
corresponding to the cumulative exposure Q, according to the following 
equations (4): 
EQU Xn=X.sub.0 +(Q/D).multidot.Lx 
EQU Yn=Y.sub.0 +(Q/D).multidot.Ly (4) 
A next step S6 fetches the current position Xa, Ya of the blades 62A, 62B 
detected by the blind position sensor 35, and a next step S7 drives the 
blades 62A, 62B with a speed corresponding to the positional differences 
.DELTA.X (=Xn-Xa) and .DELTA.Y (=Yn-Ya), in order promptly to move the 
blades to the target position Xn, Yn. Stated differently, the moving speed 
is made larger as .DELTA.X and .DELTA.Y become larger. 
Subsequently a step S8 discriminates whether the cumulative exposure Q has 
reached the appropriate exposure D, and, if not, the sequence returns to 
the step S4 to repeat the calculation of the target position Xn, Yn 
corresponding to the cumulative exposure Q and the setting of the driving 
speed. On the other hand, if the step S8 identifies that the appropriate 
exposure D has been reached, a step S9 closes the shutter 15 to terminate 
the exposure, thereby terminating the sequence. 
In the present embodiment, since the exposure in the overlapping portion of 
the exposure areas on the photosensitive substrate 11 is reduced by the 
movement, in the course of exposure, of the reticle blind 6 for defining 
the illumination area of the reticle 9, it is not necessary to provide the 
reticle 9 itself with the light attenuating property, or to employ 
exclusive light attenuating filters according to the kinds of the reticle 
9 or the variation in the overlapping portion of the exposure areas. Since 
the reticle blind 6 is positioned, for the function thereof, in conjugate 
relationship with the reticle 9, it is rendered possible to precisely 
control the illumination area of the reticle 9 by the positions of the 
blades 62A, 62B and to attenuate the exposure in the overlapping portion 
of the exposure areas on the photosensitive substrate 11 according to the 
desired light attenuation characteristics. 
Also in the present embodiment, as the target position of the blades 62A, 
62B is calculated in successive manner according to the cumulative 
exposure Q and the driving speed of the reticle blind 6 is regulated 
according to the difference between the calculated target position and the 
current position of the blades 62A, 62B, it is rendered possible to 
precisely control the exposure, overcoming the variation in the 
illumination intensity on the reticle 9 and the eventual error in the 
speed of the control system. 
More detailedly, in case the exposure is varied by the movement of the 
blades 62A, 62B as shown in FIG. 16D, the exposure amount can be 
theoretically varied at a constant rate as shown in FIG. 16E by simply 
driving the blades 62A, 62B with a constant speed V=L.sub.2 .multidot.I/D, 
wherein L.sub.2 is the moving distance of the blade, I is the illumination 
intensity (light amount per unit time) of the exposing light source, and D 
is the appropriate exposure. In practice, however, such method results in 
an error in the exposure, since the reticle blind 6 does not respond to 
the eventual fluctuation in the illumination intensity I during the 
exposure or the transient variation in the light amount at the opening 
and/or closing of the shutter 15. In order to resolve such drawback, there 
can be conceived a method of detecting the intensity of the illuminating 
light with a sensor during the exposure and to regulate the driving speed 
of the reticle blind 6 according to the variation in the illumination 
intensity. However, such method generates an error in the position of the 
reticle blind 6, because of an error in the speed of the control system, 
and such error is gradually accumulated and becomes largest at the end of 
exposure. For this reason, the synthesized amount of exposure at both ends 
of the overlapping portion of the exposure areas may be significantly 
aberrated from the amount of exposure in other areas. 
On the other hand, in the present embodiment, since the driving speed is 
regulated according to the difference between the target position of the 
blades 62A, 62B calculated in succession and the current position, there 
will not result accumulation of the positional error, and the blades 62A, 
62B can even follow the eventual variation in the illumination intensity 
in the course of exposure, thereby exactly controlling the amount of 
exposure. 
In the exposure apparatus of the present embodiment, if the control system 
for the reticle blind 6 involves a delay in response, the blades 62A, 62B 
move with an aberration from the target position, corresponding to such 
delay in response. Such drawback can be resolved by adding an offset 
value, corresponding to the delay, to the target position, in determining 
the driving speed. The interval of clock signals for the controller 33, 
governing the cycles of calculation of the target position, should be 
shorter for obtaining a higher precision, and should be at least 
sufficiently shorter than the time required for opening or closing the 
shutter 15. 
Now reference is made to FIG. 25 for explaining a fifth embodiment of the 
present invention, representing a modification in the drive control system 
for the reticle blind. Since the configuration of the optical systems is 
the same as that of the second embodiment, the following description will 
be concentrated on said drive control system. 
In the present embodiment, as shown in FIG. 25, a voltage signal 
corresponding to the intensity of the illuminating light on the reticle is 
released from an integrating sensor 40 and is converted, by a V/F 
converter 41, into a pulse train of a frequency corresponding to the 
voltage. Then the difference in the number of pulses, between thus 
converted pulse train and a pulse train released from a blind position 
sensor 42 corresponding to the moving distance of the reticle blind 6, is 
calculated by an up-down counter 43, and a D/A converter 44 generates an 
analog voltage, corresponding to the calculated difference in the number 
of pulses. The output voltage of the D/A converter 44 is supplied to a 
blind driver circuit 45, which rotates a servo motor 46 for driving the 
reticle blind, with a speed corresponding to the output voltage mentioned 
above. 
In this embodiment, the cumulative count of the output pulses of the V/F 
converter 41, corresponding to the appropriate exposure for the 
photosensitive substrate, is selected the same as the precalculated 
cumulative count of the output pulses of the blind position sensor 42 
corresponding to the moving distance of the reticle blind at the exposure. 
Consequently the output of the up-down counter 43 corresponds to the 
aberration between the target position of the reticle blind and the 
current position thereof, so that the reticle blind is constantly driven 
with a speed corresponding to the aberration from the target position, and 
there can be attained highly precise light attenuation characteristics as 
in the fourth embodiment. 
In the following there will be explained a sixth embodiment of the present 
invention, with reference to FIGS. 26, 27A and 27B. 
The present embodiment is the same as the fourth embodiment in that the 
blades 62A, 62B (62A only in the illustrated example) of the reticle blind 
6 are so moved as to gradually expand the aperture S in the course of 
exposure as shown in FIG. 26, but it differs from the fourth embodiment in 
that the blades 62A, 62B are so controlled that the amount of exposure in 
the range A1-A2 on the photosensitive substrate 11, corresponding to the 
expanded portion (dotted area in FIG. 26) of the aperture S, decreases 
according to a higher-order function, toward the end of the expanding 
direction of the aperture S. More specifically, in the present embodiment, 
the equations (4) employed in the step S5 of the flow chart shown in FIG. 
24, for determining the target position Xn, Yn, are replaced by the 
following equations (5): 
EQU Xn=X.sub.0 +.alpha..sub.x (Q/D).multidot.Lx 
EQU Yn=Y.sub.0 +.alpha..sub.y (Q/D).multidot.Ly (5) 
wherein .alpha..sub.x and .alpha..sub.y are equations or data tables for 
obtaining desired variations according to higher-order functions. 
In the present embodiment, when two patterns Pr1, Pr2 to be exposed on the 
photosensitive substrate are mutually aberrated, within the overlapping 
portion A1-A2, in a direction along the edges of the exposure areas 
(vertical direction in FIG. 27A), the patterns Pr1, Pr2 are mutually 
connected with a pattern variation according to a high-order function in 
the direction of positional aberration, as indicated by a hatched area in 
FIG. 27A. On the other hand, with the first-order light attenuating 
characteristics as in the fourth embodiment, the patterns Pr1, Pr2 are 
mutually connected with a constant inclination relative to the direction 
of positional aberration, as illustrated by the hatched area in FIG. 27B. 
Consequently, in case of forming the pattern of the liquid crystal display 
device by the image area synthesis, the present embodiment is superior 
because the pattern junctions are less conspicuous to the human eyes.