Scanning projection-exposure apparatus and methods

Apparatus and methods are disclosed for automatically performing detection of a possible exposure error before scanning exposure of a shot field on a substrate. Upon detection of a possible error, the apparatus will automatically continue an exposure sequence without stopping the apparatus. If the detected error is, e.g., a focusing error, the scanning direction for exposure of the shot field and all subsequent shot fields on the substrate are reversed. If the detected error is, e.g., a synchronization error between the mask and substrate, the prescan distance for the subject shot field is increased and the scanning direction for exposure of the shot field and all subsequent shot fields are reversed.

FIELD OF THE INVENTION 
This invention pertains to projection-exposure apparatus and methods used 
in the manufacture of semiconductor devices and/or liquid crystal display 
devices, and the like, by a lithographic process, and specifically 
pertains to a scanning projection-exposure apparatus. 
BACKGROUND OF THE INVENTION 
When manufacturing semiconductor devices and/or liquid crystal display 
devices, and the like, using lithography techniques, a projection-exposure 
apparatus is normally used. A projection-exposure apparatus uses an 
illumination-light flux (produced by a light source and passed through an 
illumination-optical system) to project and expose a pattern defined on a 
photomask or reticle (hereinafter referred to as a "mask") via a 
projection-optical system onto respective "shot fields" (individual 
exposure regions each representing one "die") on a photosensitive 
substrate such as a semiconductor wafer or glass plate, etc. (hereinafter 
referred to as a "substrate") to which a photosensitive agent, such as a 
photoresist, etc., has been applied. (A "die" represents the area occupied 
by one device on the surface of the substrate.) For exposure, the 
illumination-light flux is projected using a projection-optical system. 
The mask and substrate are each supported on a separate mask stage and 
substrate stage, respectively. Since the illumination-light source, 
illumination-optical system, and projection-optical system are normally 
stationary, at least the substrate stage is movable to permit exposure of 
various shot fields on the substrate wafer surface. 
One type of conventional projection-exposure apparatus is the so-called 
"step-and-repeat" type. With such an apparatus, a photosensitive substrate 
is mounted on a substrate stage that can move in two dimensions. An 
operation is sequentially repeated in which the photosensitive substrate 
is "stepped" (moved a predetermined amount in a lateral direction relative 
to the optical axis of the projection-exposure apparatus) by the substrate 
stage. After each step, the entire pattern defined by the mask is exposed 
onto the respective shot field on the photosensitive substrate. Such an 
apparatus is normally a "reducing" type in which the image of the pattern 
as formed on the substrate is smaller than the pattern defined by the 
mask. 
Another type of conventional projection-exposure apparatus is the so-called 
"step-and-scan" type, most of which are "reducing" (by which is meant that 
the image of the mask pattern formed on the substrate by the 
projection-optical system has a smaller surface area than the mask 
pattern). Such apparatus reducingly project the mask pattern onto each 
shot field on the photosensitive substrate by scanning, but "step" from 
one shot field to the next. In step-and-scan exposure apparatus, after the 
next shot field on the photosensitive substrate has been step-shifted into 
the exposure field of the projection-optical system, the mask and 
substrate must be synchronously moved in order to effect scanning exposure 
of the shot field. Such synchronous scanning requires that each of the 
mask and substrate stages accelerate from a stationary condition and move 
at a constant velocity. While the stages are moving at a constant 
velocity, a shutter in the illumination-optical system opens to allow the 
illumination-light flux to make the exposure. 
The period beginning the instant that acceleration of the stages begins up 
to the moment that the shutter opens for an exposure is termed the 
"prescan period". During the prescan period, a check is automatically made 
of whether or not the mask and substrate are being synchronously scanned. 
A check is also made using a focal-position detection system normally 
present in such apparatus. The focal-point detection system is operable to 
place the surface of the shot field at the best-focus plane of the 
projection-optical system. 
During the prescan period, errors are sometimes detected that are due, for 
example, to an error in acquiring focal-position data, or an imperfect 
synchronization of the positions or velocities of the mask stage and 
substrate stage. In the past, the incidence of such errors during the 
prescan period required that operation of the exposure apparatus be 
stopped, the error condition be evaluated and corrected, and scanning 
exposure subsequently restarted from the shot position at which the error 
occurred. However, since error evaluation and correction must be performed 
manually with such apparatus, such tasks are troublesome. Also, since 
operation of the exposure apparatus must be stopped in order to perform 
error evaluation and correction, the productivity of the exposure 
apparatus is decreased. 
SUMMARY OF THE INVENTION 
This invention addresses the shortcomings of convention projection-exposure 
apparatus and methods, as summarized above. An object of the invention is 
to provide a step-and-scan exposure apparatus that is able to continuously 
execute an exposure sequence while automatically performing error 
processing, without stopping the apparatus even if an error occurs during 
the prescan period. 
Focusing errors detected during prescan that involve the mask stage and 
substrate stage frequently arise because the shot field is located near 
the edge of the photosensitive substrate, which can cause the detection 
field of the autofocus system to move off the surface of the 
photosensitive substrate. According to the present invention, focusing 
errors caused by the position of the shot field near the edge of the 
photosensitive substrate can be resolved by setting the scanning direction 
relative to that shot field in the opposite direction. 
This invention was made in the course of research into the causes of the 
various types of focusing errors. The present invention provides a 
scanning projection-exposure apparatus that, while simultaneously scanning 
a mask and a photosensitive substrate, sequentially transfers the patterns 
on the mask via a projection-optical system onto multiple, respective shot 
fields arrayed on the photosensitive substrate. The scanning direction is 
changed to the opposite direction whenever a focusing error develops after 
initiation of scanning of the mask and photosensitive substrate but before 
actual exposure of the shot field begins. 
The scanning direction of one shot is reversed for the purpose of error 
processing. But, in order to avoid decreases in throughput that could 
affect subsequent scan sequences, it is preferable to change the scanning 
direction for all shot fields scanned after the focusing error occurred to 
the direction opposite the initially set direction. 
In addition, in step-and-scan projection-exposure apparatus, there are many 
cases in which errors due to imperfect synchronization in the positions or 
velocities of the mask stage on which the mask is mounted and the 
substrate stage on which the photosensitive substrate is mounted during 
prescanning of the mask stage and the substrate stage can be resolved by 
lengthening the prescanning distance to maintain a time margin for 
synchronization. 
This invention arose from, inter alia, my research on methods for resolving 
these kinds of synchronization errors between the mask and substrate. 
Thus, the invention provides a scanning-exposure apparatus that, while 
simultaneously scanning a mask and a photosensitive substrate, 
sequentially transfers the patterns on the mask via a projection optical 
system onto multiple, respective shot fields arrayed on the photosensitive 
substrate. If an error condition that could affect exposure of the shot 
field is detected, such as an error in synchronization, the prescanning 
distance preset by the apparatus can be lengthened. 
With this invention, error processing is performed automatically for 
focusing errors and synchronization errors (errors that have a high 
probability of resolution), even when errors occur during the prescan 
sequence, without having to stop the apparatus and perform error 
processing upon occurrence of every error, as in the past. Consequently, 
it is now possible in step-and-scan photolithography for the sequence of 
exposing multiple shot fields on a substrate to continue unaltered in 
cases where the cause of an error has been resolved by this automatic 
error processing. Thus, decreased throughput due to errors can be kept to 
a minimum. 
The foregoing and additional features and advantages of the invention will 
be more apparent from the following detailed description that proceeds 
with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of a step-and-scan projection-exposure apparatus 
according to the invention is shown in FIG. 1. Illumination light IL is 
produced by a light source 1. The illumination light IL reflects from a 
mirror 2, passes through a field aperture 3 and a relay lens 4, reflects 
from a mirror 5, passes through a condenser lens 6, and illuminates a mask 
7 at a uniform irradiance. The mirror 2, field aperture 3, relay lens 4, 
mirror 5, and condenser lens 6 comprise an "illumination-optical system". 
The light source 1 comprises a mercury lamp or laser light source as well 
as an optical integrator or analogous component. The plane in which the 
field aperture 3 is arranged is conjugate with the patterned surface 
(preferably the lower surface) of the mask 7. A slit-shaped illumination 
field on the mask 7 is defined by the field aperture 3. 
In FIG. 1, with respect to a plane parallel with the mask 7, a direction 
parallel with the surface of the page is the X direction and the direction 
perpendicular to the surface of the page is the Y direction. The 
length-wise direction of the slit-shaped illumination field extends in the 
Y direction, and the relative scanning direction of the mask 7 and the 
slit-shaped illumination field is the X direction. The direction of the 
optical axis of the projection-optical system 13, perpendicular to the X 
and Y directions, is the Z direction. 
The mask 7 is held on a mask stage 8 that moves and rotates in the X 
direction and Y direction. The mask stage 8 is supported by a mask-stage 
base 9 so that the mask stage 8 can move freely. The mask stage 8 
preferably comprises a "mask-scanning stage" (not detailed in FIG. 1) for 
scanning in the X direction and a mask "micro-movement stage" (not 
detailed in FIG. 1) situated above the mask-scanning stage that performs 
fine adjustments of the mask 7 both in the X direction and rotationally 
(.theta. direction). Movement of the mask stage 8, the mask-scanning 
stage, and the mask micro-movement stage is effected by a mask-stage drive 
system 45 controlled by a main control system 12. 
A movable mirror 10 is attached to an X-direction edge of the mask stage 8. 
A laser beam from an X-axis laser interferometer 11 is reflected by the 
movable mirror 10. The X-axis laser interferometer 11 detects the 
coordinate in the X direction of the mask stage 8 by photoelectrically 
converting an interference beam (produced by interference of a laser beam 
reflected from the movable mirror 10 and a laser beam reflected from a 
reference mirror) to positional information. The coordinate of the mask in 
the Y direction and the rotational angle of the mask stage 8 are measured 
by an electrostatic capacitance sensor (not shown in the figure). 
The measured coordinate from the X-axis laser interferometer 11 and the 
measurement results from the electrostatic capacitance sensor are supplied 
to the main control system 12. The main control system 12 sets the 
movement velocity, position, and rotational angle of the mask stage 8 
according to the exposure sequence. 
The image of the pattern on the mask 7 is projected and exposed onto the 
photosensitive substrate 14 by the illumination light IL via a 
projection-optical system 13. The conjugate image of the slit-shaped 
illumination field on the mask, i.e., the exposure field of the 
projection-optical system 13, is smaller than one shot field on the 
photosensitive substrate 14. Consequently, the photosensitive substrate 14 
is scanned in, e.g., the +X direction at a constant velocity in synchrony 
with scanning of the mask 7 in the -X direction. Such synchronous scanning 
achieves exposure of each complete shot field on the photosensitive 
substrate 14. To accomplish scanning, the photosensitive substrate 14 is 
held on a substrate stage 15 that moves freely in the X and Y directions 
relative to a base 16. The substrate stage 15 comprises an "X stage" (not 
detailed in FIG. 1) movable in the X direction and a "Y stage" (not 
detailed in FIG. 1) movable in the Y direction, as effected by 
substrate-stage drive system 46 controlled by the main control system 12. 
A movable mirror 17 is attached to an X-direction edge of the substrate 
stage 15. A laser beam from an X-axis laser interferometer 18 is reflected 
by the movable mirror 17. Although not shown in the figure, a movable 
mirror is also attached to the substrate stage 15 at a Y-direction edge; a 
laser beam from a Y-axis laser interferometer is reflected by the movable 
mirror. The X-axis laser interferometer 18 and the Y-axis laser 
interferometer detect the X coordinate and Y coordinate, respectively, of 
the substrate stage 15 by photoelectrically converting the respective 
interference beams to positional information. 
The detected X and Y coordinates of the substrate stage 15 are supplied to 
the main control system 12. The main control system 12 sets the movement 
velocity and position of the substrate stage 15 according to the exposure 
sequence. 
The FIG. 1 apparatus includes a focal-position detection system (AF sensor) 
20 for detecting the position, in the Z direction, of the surface of the 
photosensitive substrate 14 relative to the projection-optical system 13. 
A detection light that is not reactive with the photoresist is irradiated 
from a light source 21. The detection light is directed by a condenser 
lens 22 onto a slit in a transmission slit plate 23. The image of the slit 
is projected at an angle to the optical axis of the projection-optical 
system 13 by an objective lens 24 onto a measurement locus on the 
photosensitive substrate 14. The reflected light from the measurement 
locus is focused by a focusing lens 25 onto an oscillating slit plate 26, 
again forming an image of the slit image projected onto the measurement 
locus. 
The oscillating slit plate 26 is oscillated in a specified direction by an 
oscillator 28 driven by a drive signal DS from the main control system 12. 
Light that has passed through the slit in the oscillating slit plate 26 is 
photoelectrically converted by a photoelectric conversion element on a 
photoelectric detector 27. The resulting photoelectric conversion signal 
is supplied to a signal-processing system 29. After processing, the signal 
is routed to the main controller 12. The AF sensor 20 is calibrated so 
that the focal-position signal is 0 [zero] when the measurement locus on 
the photosensitive substrate 14 is aligned with the best focus plane of 
the projection-optical system 13. 
During exposure of each shot field, the photosensitive substrate 14 is 
scanned at a constant velocity in the +X direction so that the amount of 
illumination received by each of various points within the shot field is 
constant. Velocity control is performed based on the measurement data 
obtained by the laser interferometer 18. Concretely, filtering is 
performed that is suited to the integrated coordinate data in the +X 
direction (as determined by the laser interferometer 18). The X stage of 
the substrate stage 15 is controllably driven so that the coordinate in 
the X direction remains constant. Meanwhile, where the reduction ratio of 
the projection-optical system 13 from the mask 7 to the photosensitive 
substrate 14 is .beta. (.beta.&lt;1), a difference is calculated between the 
value of the measurement results obtained by the laser interferometer 18 
multiplied by the magnification ratio 1/.beta. and the measurement results 
obtained by the laser interferometer 11. Scanning of the mask in the -X 
direction is performed under positional control such that this difference 
equals 0 [zero]. 
FIG. 2, shows an example of the scanning directions on various shot fields 
S1, S2, S3, . . . , as the photosensitive substrate 14 is scanned and 
exposed. The solid arrow within each shot field indicates the scanning 
direction for the respective shot field (i.e., the direction in which the 
exposure field is swept across the region occupied by the respective shot 
field). The dashed lines connecting the arrows indicate movement of the 
exposure field between the shot fields (actually, the exposure field is 
not formed between shots since the shutter in the illumination-optical 
system is closed during the time that exposure of a shot field is not 
occurring). 
Normally, to minimize movement of the mask, one shot field is scanned and 
exposed during a scanning movement of the mask in one direction, and the 
next shot field is scanned and exposed during a scanning movement of the 
mask in an opposite direction ("return direction"). Thus, two shot fields 
are exposed per "round-trip" scanning movement of the mask 7. In such a 
scheme, +X-direction scans and -X-direction scans are alternately repeated 
in successive shots; e.g., if the shot field S1 is scanned in the +X 
direction, as shown in FIG. 2, the next shot field S2 is scanned in the -X 
direction, the shot field S3 is scanned in the +X direction, and the shot 
field S4 is scanned in the -X direction, and so on. 
FIGS. 3 and 4 depict scanning of the mask and substrate in more detail. 
FIG. 3(a) shows the relative positional relationship between the mask 7 
and the slit-shaped illumination field 30; and FIG. 3(b) shows the 
positional relationship between the photosensitive substrate 14 and the 
slit-shaped exposure field 30P. FIG. 4 is a flow chart showing an 
exemplary method, according to the invention, for controlling scanning 
exposure. 
In a starting condition, the center of the illumination field 30 in FIG. 
3(a) is at the center position A of the mask 7, and the center of the 
exposure field 30P is at the center position AP of the first shot field 
40A on the photosensitive substrate 14. 
Upon initiation of scanning exposure (step 111 in FIG. 4), the mask 7 is 
scanned in the +X direction at a velocity V/.beta., and the photosensitive 
substrate 14 is scanned in the -X direction at velocity V. These motions 
of the substrate stage and mask stage are controlled by the main control 
system 12. In order to drive the X stage of the substrate stage 15 at the 
constant velocity V, the main control system 12 samples the integrated 
X-coordinate data (WS.sub.x) supplied from the laser interferometer 18 and 
generates an X-stage drive instruction to the substrate-stage drive system 
46 so that this integral value WS.sub.x remains at a constant value 
corresponding to the velocity V. Simultaneously, in order to drive the 
mask-scanning stage at a constant velocity V/.beta., the main control 
system 12 samples the integrated X-coordinate data (RS.sub.x /.beta.) 
supplied from the laser interferometer 11 and generates a drive 
instruction to the mask-stage drive system 45 so that this integral value 
remains at a constant value corresponding to the velocity V/.beta.. 
Thus, the center of the illumination field 30 in FIG. 3(a) moves from 
position A to position B outside the mask 7, and the center of the 
exposure field 30P in FIG. 3(b) moves from position AP to position BP 
outside the first shot field 40A on the photosensitive substrate 14, 
thereby concluding the first scan and exposure. 
Next, in step 112, the main control system 12 drives the X stage of the 
substrate stage 15 to decelerate the substrate stage 15 and then to 
accelerate the substrate stage 15 in the +X direction, and to accelerate 
and then decelerate the Y stage of the substrate stage 15 in the +Y 
direction. Simultaneously, the main control system 12 decelerates the 
mask-scanning stage of the mask stage 8 and resets the position of the 
mask micro-movement stage to its reference position. Thus, the center of 
the illumination field 30 stops when it reaches position C in FIG. 3(a), 
which is more distal to the mask than position B; in FIG. 3(b), the center 
of the exposure field 30P moves from position BP to position CP outside 
the second shot field 40B on the photosensitive substrate 14. At this 
position CP, the substrate stage 15 has already started scanning in the +X 
direction. 
Next, in step 113, the main control system 12 drives the X stage of the 
substrate stage 15 in the +X direction at a constant velocity V, while the 
position of the Y stage in the Y direction is held stationary. This 
operation damps vibrations due to the acceleration and deceleration of the 
Y stage of the substrate stage 15. Simultaneously, the mask-scanning stage 
of the mask stage 8 accelerates in the -X direction. Thus, in FIG. 3(a), 
the center of the illumination field 30 moves from position C to position 
D nearer the mask 7; meanwhile, in FIG. 3(b), the center of the exposure 
field 30P moves from position CP to position DP near the second shot field 
40B. At position D, the mask stage starts to move at a constant velocity 
V/.beta. in the +X direction. At this point in time, the relative scanning 
velocity of the photosensitive substrate 14 should have reached the design 
value, but it is possible that the relative positions of the mask 7 and 
photosensitive substrate 14 have shifted. 
In step 114, the main control system 12 performs positional control of the 
Y stage of the substrate stage 15 and the micro-movement stage of the mask 
stage 8. In other words, the difference between the integrated 
X-coordinate WS.sub.x for the substrate side measured by the 
interferometer 18 and the X-coordinate data RS.sub.x /.beta. on the mask 
side measured by the laser interferometer 11 (WS.sub.x -RS.sub.x /.beta.), 
the difference between the Y-coordinate data WS.sub.y on the substrate 
side and the Y-coordinate data RS.sub.y /.beta. on the mask side (WS.sub.y 
-RS.sub.y /.beta.), and the difference between the rotational angle data 
WS.sub..theta. on the substrate side and the rotational angle data 
RS.sub..theta. on the mask side (WS.sub..theta. -RS.sub..theta.) are all 
sampled. The main control system 12 then issues a drive command to the 
substrate-stage drive system 46 that drives the Y stage of the substrate 
stage 15 so that the difference (WS.sub.y -RS.sub.y /.beta.) is a 
specified value, and issues drive commands to the mask-stage drive system 
45 (to actuate the mask micro-movement stage) so that the difference 
(WS.sub.x -RS.sub.x /.beta.) and the difference (WS.sub..theta. 
-RS.sub..theta.) are specified values. 
Thus, any unwanted positional shift of the mask 7 and the photosensitive 
substrate 14 relative to each other is corrected. At this time, the center 
of the illumination field 30 is in position E outside the pattern field on 
the mask 7, as shown in FIG. 3(a), and the center of the exposure field 
30P is in position EP outside the second shot field 40B, as shown in FIG. 
3(b). Then, when the center of the illumination field 30 is in position F 
immediately in front of the pattern field on the mask 7, as shown in FIG. 
3(a), and the center of the exposure field 30P has advanced to position FP 
immediately in front of the second shot field 40B, as shown in FIG. 3(b), 
the constant velocity driving and positional shift correction of the mask 
7 and photosensitive substrate 14 have been completed. 
The AF sensor 20 initiates acquisition of focal-position data for the 
surface of the photosensitive substrate from the time that the exposure 
field 30P reaches, e.g., position DP. Also, the main control system 12 
performs preparatory focus control according to the signal from the AF 
sensor 20. When the exposure field 30P has reached position EP immediately 
in front of the shot field 40B, in step 115, the main control system 12 
checks whether or not focus control has been properly accomplished. 
If focus control has been properly accomplished, a confirmation is made in 
the next step 116 of whether or not the mask 7 and the photosensitive 
substrate 14 are being synchronously driven. I.e., a check is made of 
whether or not the mask 7 and the photosensitive substrate 14 are being 
driven at respective constant velocities with correction of any unwanted 
positional shift. The check for whether or not positional shift correction 
has been completed is performed by ascertaining whether or not the three 
differences discussed above (WS.sub.x -RS.sub.x /.beta.), (WS.sub.y 
-RS.sub.y /.beta.), and (WS.sub..theta. -RS.sub..theta.) are at specified 
values. In addition, a velocity-synchronization check is performed by 
comparing the integrated X-coordinate data supplied from the laser 
interferometer 18 for the substrate side with the integrated X-coordinate 
data supplied from the laser interferometer 11 for the mask side. 
If the result at step 116 is "YES", the illumination field 30 relatively 
scans the mask 7 to the center position G, as shown in FIG. 3(a), and the 
exposure field 30P relatively scans the second shot field 40B to the 
center position GP, as shown in FIG. 3(b). Then, by repeating the 
operation from step 111 on, pattern exposure is performed for the second 
shot field 40B and subsequent shot fields on the photosensitive substrate 
14. 
Next, processing for when the result at step 115 is "NO", i.e., what 
happens when a focusing error occurs, is described with reference to FIG. 
5. Focusing errors can occur when attempting to scan and expose the shot 
fields S11, S12, S13, S14, . . . , situated along the edge of a 
photosensitive substrate 14. Initially, in the shot fields S11, S12, S13, 
. . . on the photosensitive substrate 14, the scanning direction is set 
for each shot field, and the main control system 12 scans and exposes each 
shot field according to that setting. The various scanning directions are 
set by this initial setting so that scanning in the +X direction and 
scanning in the -X direction are alternately repeated for consecutive 
shots, as explained in FIG. 2. 
However, a case is considered in which, e.g., scanning and exposure of the 
shot field S13 is complete and scanning and exposure of the next shot 
field S14 is upcoming; i.e., prescanning is approaching shot field S14. 
During the prescan period prior to reaching the shot field S14, the AF 
sensor 20 of the exposure device is already acquiring focal-position data 
and performing preparatory focus control, as explained in with respect to 
FIG. 3(b). However, since the shot field S14 is located at the edge of the 
photosensitive substrate 14, the normal prescan path would extend over the 
edge of the photosensitive substrate and focal position data would not be 
obtainable. 
In the past, an error would be detected in this kind of situation and the 
exposure apparatus would have stopped. However, in such a situation, 
processing according to the present invention would advance to step 117, 
where the scanning sequence passes through to the opposite side of the 
shot field S14 without exposing the shot field S14, as indicated by the 
dashed line L1, and all subsequent scanning directions that are set after 
that shot attempt are assigned reverse directions from the initial 
directions. Thereafter, processing returns to step 113 and prescanning to 
the shot field S14 is started from the opposite direction. 
When the scanning direction is reversed, focal position data acquisition 
and preparatory focusing become possible because the prescan portion is 
located inside the photosensitive substrate 14, making it highly probable 
that the error will be resolved. By progressing through step 117, the 
result at step 115 becomes "YES". If the result at step 116 is "YES", then 
the shot field S14 is scanned and exposed from the direction opposite the 
initially set direction, as indicated by the dashed line L2. The shot 
field S15 is then scanned and exposed from the direction opposite the 
initially set direction, as indicated by the dashed line L3. All 
subsequent shot fields S16, S17, . . . will also be scanned and exposed 
from directions opposite the directions initially set by the apparatus. 
The reason that the scanning directions for the shot field S14 and for all 
subsequent shot fields S15, etc., are changed to an opposite direction in 
step 117 is described with reference to FIG. 6. FIG. 6 depicts an instance 
in which only the scanning direction for the shot field S14 in which the 
error occurred was changed, while the scanning sequence for the subsequent 
shot fields S15, . . . , were not changed. In such an instance, the 
scanning direction L2 for shot field S14 and the scanning direction L5 for 
the next scan field S15 are the same. Consequently, once the shot field 
S14 has been scanned and exposed along the direction shown by path L2, the 
scanning sequence must pass through shot field S15 along path L4 and then 
reverse direction and scan and expose shot field S15 from the opposite 
side along path L5. 
As can be seen when FIG. 6 is compared with FIG. 5, the scanning sequence 
in FIG. 6 requires an additional scan of the shot field S15, expending 
unnecessary time. In addition, since the scanning directions are the same 
for shot field S14 and shot field S15, which are consecutively scanned and 
exposed, the scanning directions for the mask 7 must also be the same. In 
order to accomplish this, after completion of scanning of shot field S14, 
the mask scanning stage must be returned to the opposite side of the mask 
without performing an actual exposure scan, which is also a time-wasting 
operation. In order to improve the throughput of the exposure apparatus by 
eliminating such wasted time, it is preferable that, if the scanning 
direction of one shot field (shot field S14, here) is changed, to change 
the scanning directions of all subsequent shot fields as well. 
Processing in an instance in which a synchronization error occurs and the 
result at step 116 is "NO" is described with reference to FIG. 7. In FIG. 
7, it is assumed that: (a) scanning and exposure of the shot field S21 is 
completed, (b) prescanning of the prescan distance d1 from the prescan 
start point P22 has been performed for the next shot field S22, and (c) 
the mask 7 and the photosensitive substrate 14 are not being driven in 
complete synchrony even immediately in front of the shot field S22. In 
such a case, the result at step 116 would be "NO". At this time, in the 
past, the projection-exposure apparatus would have been stopped due to the 
detected error. However, with this invention, even if the result at step 
116 were "NO," processing would immediately advance to step 118 without 
stopping the exposure apparatus. 
At step 118, the exposure sequence passes through the shot field S22 
without exposing it. At the same time, the prescan distance d1 set by the 
exposure device is changed to d2 (d2&gt;d1), and the scanning directions for 
all subsequent shot fields are changed to opposite directions. When the 
processing in step 118 is completed, processing returns to step 113, the 
shot field S22 is prescanned for a prescan distance d2 from the prescan 
start point Q22 in the opposite direction to immediately in front of shot 
field S22, during which time velocity control and position control of the 
substrate stage and mask stage are performed according to steps 113 and 
114. 
Synchronization errors between the mask 7 and the photosensitive substrate 
14 are frequently remedied by thus lengthening the prescan distance and 
thereby extending the time so that synchronization can be accomplished. If 
the result at step 115 and the result at step 116 are both "YES" as a 
result of the processing in step 118, scanning and exposure of the shot 
field S22 will be executed from the opposite direction. Scanning and 
exposure of the subsequent shot fields S23, S24, . . . will be performed 
with the mask and photosensitive substrate in synchrony by performing the 
prescanning with the modified prescan distance d2 from their respective 
prescan start points Q23, Q24, . . . . 
An exposure apparatus according to this invention, as described above, 
provides improved operability in cases where an error occurs after the 
start of prescanning and before the start of exposure. Thus, error 
processing can be automatically performed according to the cause of the 
error, without stopping the exposure apparatus. By changing the scanning 
sequence subsequent to the error, decreases in throughput due to changing 
the scanning direction to the opposite direction for the shot field for 
which the error occurred can be kept to a minimum. 
Whereas the invention has been described in connection with a preferred 
embodiment, it will be understood that the invention is not limited to 
that embodiment. On the contrary, the invention is intended to encompass 
all modifications, alternatives, and equivalents as may be included within 
the spirit and scope of the invention as defined by the appended claims.