Projection exposure apparatus and exposure method

A projection exposure apparatus and method for exposing a pattern on a mask onto a photosensitive substrate through a projection optical system comprises a two-dimensionally driven mask holding stage and a two-dimensionally driven substrate holding stage. Vibration sensors detect vibration of the mask and the projection optical system. A control device controls at least one of the mask holding stage and the substrate holding stage according to an output from at least one of the sensors so as to maintain a relative position between the mask and the substrate. The control device may memorize information regarding relative lateral shift and rotation of the mask and the substrate due to vibration, predict lateral shift and rotation on the basis of memorized information and information from vibration sensors, and employ the prediction in maintaining the relative position between the mask and the substrate. Exposure is performed so as to minimize the effects of vibration.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a projection exposure apparatus which is 
suitable to be applied to a projection exposure apparatus of a type in 
which a wafer stage is driven in a step-and-repeat method. 
2. Related Background Art 
A prior art projection exposure apparatus is shown in FIG. 4. 
A photo-mask (hereinafter referred to as a reticle) 5 is supported by a 
reticle support member 4 and kept in a conjugate positional relation with 
a photo-sensitive substrate (hereinafter referred to as a wafer) 2 mounted 
on a wafer stage 1, through a projection lens 3. 
A movable mirror 7 is attached to an end of the wafer stage 1 and it is 
moved as the wafer stage 1 is moved. A fixed mirror 6 is mounted in a lens 
barrel of the projection lens 3. 
The movable mirror 7 reflects a laser beam directed from a laser beam 
interference range finder (interferometer) 10 through a beam splitter 9, 
and the fixed mirror 6 reflects a laser beam directed from the 
interferometer 10 through the beam splitter 9 and a reflection mirror 8. 
The interferometer 10 measures the reflected light from the movable mirror 
7 and the fixed mirror 6 to detect a relative position of the movable 
mirror 7 and the fixed mirror 6. 
A control unit 11 controls the drive of the wafer stage 1 to a desired 
position by a drive unit 12 in accordance with the relative position of 
the movable mirror 7 and the fixed mirror 6 detected by the interferometer 
10. 
In such a type of projection exposure apparatus, the reticle support member 
4, the projection lens 3 and the wafer stage 1 are integrally coupled 
together by a predetermined support member (not shown). Thus, when the 
stage 1 is driven in X and Y directions, the entire apparatus is vibrated 
in such a direction that an optical axis AX is inclined around the wafer 
stage 1, as shown by a broken line in FIG. 4. 
In this case, it has been considered that there is no positional shift of 
the reticle 5 and the wafer 2 because the relative position of the reticle 
5, the projection lens 3 and the wafer 2 does not change like a pendulum. 
However, because the wafer stage 1 is movable in X and Y directions, a 
center of gravity changes by the movement. When a vibration is generated, 
a rotation error component to the reticle 5 and the wafer stage 1 is 
included in a vibration component due to a twist of the entire apparatus 
or stage yawing. Further, a shift component or a rotation component to the 
reticle 5 and the wafer stage 1 is included in the vibration component due 
to a delay in transmission of the vibration and a difference between 
lateral shifts by the vibration between the fixed mirror 6 and the reticle 
5. By the inclusion of the rotation error component and the shift 
component (or rotation component) in the vibration component, the relative 
position of the reticle 5 and the wafer 2 varies, which causes a stepping 
error. As a result, precise exposure is not attained. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to keep a constant relative 
relation between a reticle and a wafer by detecting a change of a relative 
position between the reticle and the wafer and controlling a drive system 
and exposure timing. 
It is another object of the present invention to provide a projection 
exposure apparatus and an exposure method which do not cause a lateral 
shift or a rotation error due to the vibration of the entire apparatus. 
In the projection exposure apparatus of the present invention for exposing 
a pattern on a photo-mask onto a photo-sensitive substrate through a 
projection optical system, vibration detection means for detecting 
vibration around the photo-mask is provided in a vicinity of a support 
member which supports the photo-mask. During the exposure, drive means for 
the photo-mask and/or drive means for the photo-sensitive substrate are 
controlled or exposure means is controlled in accordance with a result of 
detection by the vibration detection means. 
During the exposure, whether a vibration component detected by the 
vibration sensor causes a change in the relative relation between the 
reticle and the wafer or not is determined, and if it does, the reticle 
drive means and/or the wafer drive means are controlled or the exposure 
means is controlled to maintain the relative relation of the reticle and 
the wafer. In this manner, the relative position of the reticle and the 
wafer is kept unchanged during the exposure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One embodiment of the present invention will now be explained in detail 
with reference to the drawing. FIG. 1 shows a projection exposure 
apparatus of the present invention in which illumination light emitted 
from an exposure device 30 which includes a shutter for controlling 
transmission and blocking of the illumination light illuminates a reticle 
5 held on a reticle holder 14 with a substantially uniform illumination 
through an illumination optical system 31. 
In FIG. 1, like elements to those shown in FIG. 4 are designated by like 
numerals. 
The exposure light transmitted through the reticle 5 is directed to a 
bi-telecentric (or mono-telecentric) projection lens 3, which focuses and 
projects a projection image of a circuit pattern formed on the reticle 5 
onto a wafer 2 having a resist layer formed on a surface thereof. 
The wafer 2 is mounted on a wafer stage 1 which is two-dimensionally driven 
in X and Y directions by a step-and-repeat method. When transfer exposure 
of the reticle 5 to one exposure area on the wafer 2 is completed, the 
wafer 2 is stepped to the next exposure position. The two-dimensional 
position of the wafer stage 1 is continuously detected by an 
interferometer 10 at a resolution of the order of 0.01 .mu.m. 
The exposure of the pattern is repeatedly done to the same wafer 2, which 
is then developed and etched so that reticle patterns are formed in a 
matrix on the wafer 2. 
The reticle 5 held on the reticle support member 4 through a reticle holder 
14 can be two-dimensionally driven in X and Y directions by a reticle 
drive unit (device) 23, and the two-dimensional position of the reticle 5 
is always detected by an interferometer 24. A movable mirror 20 which 
reflects a laser beam from the interferometer 24 is attached to an end of 
the reticle holder 14. 
The reticle drive system is light in weight, has a high controllability and 
is fully compatible to a fine movement because a magnification factor of 
the wafer 2 to the reticle 5 is usually 1/N (N.gtoreq.5). 
A vibration sensor 22 is mounted at an end of the reticle support member 4, 
and a vibration sensor 21 is mounted at substantially the same height 
level as the fixed mirror 6 which is fixed to a lens barrel of the 
projection lens 3. Information from the vibration sensors 21 and 22 is 
supplied to a vibration waveform processing unit (device) 13 which 
continuously detects a vibration status around the wafer stage 1 (around 
the fixed mirror 6) and a vibration status around the reticle 5. 
The control unit (device) 11 issues a drive command to the drive units 12 
and 23 in accordance with the position measurement signals from the 
interferometers 10 and 24, and controls the movement and the positioning 
of the wafer stage 1 and the reticle 5. The control unit 11 further 
controls an exposure device 30 in accordance with the information from the 
vibration waveform processing unit 13 to control the exposure operation. 
The exposure operation is controlled by controlling the opening and 
closing of a shutter in the exposure device 30. 
An exposure operation when the wafer stage 1 is stepped under a condition 
(at a wafer stage position) in which the vibration of the wafer stage 1 is 
hard to be propagated to the reticle 5, is explained below. 
FIG. 2(A) shows a speed signal S1 which represents a speed of the wafer 
stage 1 when the wafer stage 1 is driven. A pedestal-shaped rising portion 
of the signal level indicates the stepping of the wafer stage 1, and an 
area between pedestals indicates fine adjustment of the wafer stage 1. 
FIG. 2(B) shows an output S2 of the vibration sensor 21, and FIG. 2(C) 
shows an output S3 of the vibration sensor 22. FIG. 2(B) indicates that 
the vibration in the stepping of the wafer stage 1 is well propagated to 
the vibration sensor 21 because the vibration sensor 21 is mounted closely 
to the wafer stage 1. On the other hand, FIG. 2(C) indicates that the 
vibration is not linearly propagated to the vibration sensor 22. 
In the fine adjustment section of the wafer stage 1 between the pedestals 
of FIG. 2(A), when the wafer stage 1 is driven into a target exposure area 
(time t1, t2, t3 or t4), the vibration waveform processing unit 13 
determines the vibration status of the vibration sensors 21 and 22. When 
the vibration waveform processing unit 13 determines that vibration is not 
detected, it sends an exposure command to the control unit 11, which, in 
response to the exposure command, sends an exposure signal S4 shown in 
FIG. 2(D) to the exposure device 30 to carry out the exposure operation. 
In FIG. 2, the condition is such that the vibration of the wafer stage 1 is 
hard to be propagated to the reticle 5. Thus, at the time t1, t2, t3 or t4 
when the wafer stage 1 is driven into the target exposure area, the 
vibrations of the wafer stage 1 and the reticle 5 have been settled. 
Accordingly, the control unit 11 can carry out the exposure operation 
without controlling the exposure device in a special manner (that is, 
without delaying an exposure timing). 
An exposure operation in a bad equilibrium of the wafer stage 1 and the 
reticle 5 is now explained. 
In FIG. 3, the vibration is detected by the vibration sensor 22 even at a 
time t2 or t4 at which the speed signal S1 (FIG. 3(A)) of the wafer stage 
1 is within a predetermined window (that is, the wafer stage 1 has been 
driven into the target exposure area). This is due to the bad equilibrium 
of the wafer stage 1 and the reticle 5. At the time point t2 or t4, the 
vibration at the stepping is propagated to the vicinity of the reticle 5 
in delay, or the vibration of the reticle 5 continues. 
In this case, the neighborhood of the reticle 5 vibrates in spite of the 
fact that the neighbourhood of the wafer stage 1 (neighbourhood of the 
fixed mirror 6) does not vibrate (or in spite of the fact that the output 
of the vibration sensor 21 is zero). Thus, there is a possibility of 
lateral shift or rotation error during the exposure. Accordingly, the 
vibration waveform processing unit 13 does not send the exposure command 
to the control unit 11 at the time point t2 or t4, but suspends until the 
output representing the vibration from the vibration sensor 22 reaches 
zero. 
Thus, the exposure is not carried out at the time point t2 or t4, and when 
the output from the vibration sensor 22 reaches zero at a subsequent time 
point t2' or t4', the vibration waveform processing unit 13 sends the 
exposure command to the control unit 11, which sends the exposure signal 
S4 shown in FIG. 3(D) to the exposure device 30 in response to the 
exposure command to carry out the exposure. 
Through the control to suppress the exposure until the outputs of the 
vibration sensors 21 and 22 reach zero even after the wafer stage 1 has 
been driven into the exposure area, the lateral shift or the rotation 
error during the exposure can be prevented. Since the output of the sensor 
21 is zero when the output of the sensor 22 is zero, the exposure may be 
controlled based on only the output of the sensor 22, or it may be 
controlled based on both outputs of the sensor 21 and the sensor 22. 
In this manner, problems in the exposure are prevented by controlling the 
exposure based on the vibration information from at least one of the 
vibration sensor 21 and the vibration sensor 22. 
Further, problems in the exposure due to the vibration can also be 
prevented even for a first layer wafer which has no alignment mark 
thereon. 
In the present embodiment, the exposure is suspended until the vibration 
ceases. Alternatively, a lateral shift and a rotation may be predicted 
from the vibrations detected by the vibration sensors 21 and 22, and the 
drive of at least one of the reticle support member 4 and the wafer stage 
1 may be controlled based on the prediction. 
For example, the wafer stage 1 is stepped to correspond to the wafers of a 
predetermined shot arrangement. The vibrations of the vibration sensor 21 
and the vibration sensor 22 are detected. 
A relation between the vibrations (magnitudes, phase differences of the 
vibration waves, etc.) from the vibration sensors, and a relative shift of 
the reticle and the wafer and a rotation when the vibration is zero is 
previously determined and stored in a memory in the vibration waveform 
processing unit 13. 
In the exposure operation, the lateral shift and the rotation of the 
reticle are predicted based on the stored relation and the outputs from 
the sensors, and the control unit 11 controls the drive of at least one of 
the wafer stage 1 and the reticle support member 4 based on the 
prediction. 
In this manner, the reduction of the throughput due to the waiting time 
until the cease of the vibration can be prevented. 
In the present embodiment, it is assumed that the reticle 5 is vibrated by 
the stepping of the wafer stage 1. In a system in which the wafer stage is 
rotated around one point in the projection exposure apparatus, the 
information of the vibration sensors 21 and 22 may be analyzed to devise a 
condition to maintain the relative position of the reticle 5 and the wafer 
2. 
In the present embodiment, the vibration sensor 22 is mounted at one point. 
Alternatively, the vibration sensors 22 may be mounted at several points 
to measure a rotation component. 
In the apparatus of the present type, the positional relation of the 
reticle 5 and the wafer stage 1 is normally fixed by servo control based 
on the position information from the interferometer 24. In detecting the 
vibration, it is desirable to stop the servo control of the position of 
the reticle 5 based on the position information from the interferometer 
24. By doing so, only the vibration due to the movement of the wafer stage 
1 can be precisely detected. The projection optical system may be a 
refraction system, a reflection system or a combination thereof.