Astigmatism-correcting optical system, projection exposure apparatus using the optical system and device manufacturing method

In a projection exposure apparatus, two plane-parallel plates, which are equal in thickness and refractive index, are interposed between a projection optical system and a wafer. By tilting these two plane-parallel plates at the same angle in opposite directions with respect to the optical axis of the optical system by means of an adjusting device, astigmatism caused in the optical system by exposure is corrected. The amount of astigmatism is calculated by a calculating device based on the amount of light incident on the optical system per unit of time which is obtained according to the output of a light amount sensor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an aberration-correcting optical system, a 
projection exposure apparatus, and a device manufacturing method, and more 
particularly, to an aberration-correcting optical system, a projection 
exposure apparatus used in manufacturing a semiconductor chip such as an 
IC and an LSI, an image pickup device such as a CCD and a display device 
such as a liquid crystal panel, and a device manufacturing method. 
2. Description of the Related Art 
In a scanning type exposure apparatus, since the cross section of 
illumination rays for illuminating a mask is shaped like a rectangle or an 
arc which extends in an orthogonal direction orthogonal to the scanning 
direction, the cross section of imaging rays produced from a mask pattern 
also extends in the orthogonal direction. If such imaging rays enter a 
projection optical system (a system composed of lenses or a combination 
system of lenses and mirrors), the lenses in the projection optical system 
absorb part of the imaging rays, and this causes the refractive index 
distribution and the deformation condition of a refracted plane in the 
scanning direction to differ from those in the orthogonal direction. 
Therefore, the best focal position (best imaging position) in a pattern in 
which the illumination rays are diffracted mainly in the scanning 
direction and the diffracted rays are made incident on the projection 
optical system differs from that in a pattern in which the illumination 
rays are diffracted mainly in the orthogonal direction and the diffracted 
rays are made incident on the projection optical system (this difference 
is hereinafter referred to as "astigmatism"). 
In order to solve the above problem, the applicant of this application 
proposes in Japanese Patent Laid-Open No. 8-008178 that a heating means 
for heating a projection optical system should be placed so that the 
refractive index distribution of the lens and the deformation condition of 
the refracted plane in the scanning direction is almost equal to those in 
the orthogonal direction. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved astigmatism-correcting optical system, an improved projection 
exposure apparatus, and an improved device manufacturing method. 
An astigmatism-correcting optical system of the present invention comprises 
two transparent plane-parallel plates which are arranged along an optical 
axis and are equal in refractive index and thickness, and means for 
turning the two plane-parallel plates so as to cause them to tilt at the 
same angle in opposite directions with respect to the optical axis. 
A projection exposure apparatus of the present invention comprises means 
for illuminating a mask, a projection optical system for projecting a 
pattern of the mask onto a substrate, the astigmatism of the projection 
optical system being variable, and a correction optical system for 
correcting the astigmatism. The correction optical system includes two 
transparent plane-parallel plates which are arranged along an optical axis 
and are equal in refractive index and thickness, and means for turning the 
two plane-parallel plates so as to cause them to tilt at the same angle in 
opposite directions with respect to the optical axis. 
A device manufacturing method of the present invention comprises a step of 
transferring a device pattern onto a substrate by using the above 
projection exposure apparatus. 
The present invention is intended for a so-called scanning type projection 
exposure apparatus for exposing a substrate while scanning a mask and the 
exposed substrate, on which a mask pattern is projected, with respect to a 
projection optical system and slit illumination rays, and a non-scan 
projection exposure apparatus for exposing a substrate while keeping a 
mask and the exposed substrate, on which a mask pattern is projected, 
almost stationary with respect to a projection optical system and 
illumination rays. The present invention is also intended for the nonscan 
projection exposure apparatus because this kind of apparatus has the same 
problem as that of the abovementioned scanning projection exposure 
apparatus when an illumination area on a mask is shaped like a rectangle 
whose length-to-width ratio is significantly deviated from 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a schematic view of a scanning type projection exposure apparatus 
according to a first embodiment of the present invention. Referring to 
FIG. 1, an illumination optical system 2 illuminates a predetermined area 
of a reticle R with slit-shaped illumination light from a light source 1. 
A reticle stage 3 holds the reticle (mask) R, and a projection optical 
system 4 projects a device pattern of the reticle R onto the surface of a 
wafer W. Two plane-parallel plates 5a and 5b, which are almost equal in 
refractive index and thickness, are placed on the side of the wafer W. The 
plates 5a and 5b are made of SiO.sub.2, or may be made of CaF.sub.2 or 
BK.sub.7 as needed. An XYZ stage 6 holds and moves the wafer W to an 
exposure range of the optical system 4 while sequentially stepping in the 
XY direction, and moves the surface of the wafer W in the Z direction in 
accordance with the result of measurement by a focusing system which is 
not shown. A half mirror 11 is placed in the illumination optical system 2 
to take out a part of the illumination light. The taken out part of 
illumination light enters a light amount sensor 7, and the exposure amount 
of the wafer W is detected based on the output of the sensor 7. An angle 
adjusting means 9 drives the plane-parallel plates 5a and 5b to change the 
angles thereof, and an illuminance measuring device 10 is placed on the 
XYZ stage 6. A calculation means 8 calculates the amount of light to be 
incident on the projection optical system 4 per unit of time, and predicts 
the change in optical characteristics of the projection optical system 4. 
When exposure is started, the projection optical system 4 absorbs part of 
the light, the temperature thereof changes, and the optical 
characteristics thereof also change. The amount of change in focus (focal 
position) and magnification is calculated and corrected by well-known 
methods. On the other hand, in a case in which an exposed area of the mask 
is rectangular, as shown in FIG. 4, the best focal position of a pattern V 
differs from that of a pattern H depending on exposure. In other words, 
astigmatism arises. 
When a focal position of the pattern V is FV and a focal position of the 
pattern H is FH, an astigmatism .DELTA.AS is given by the following 
expression: 
EQU .DELTA.AS=FV-FH. 
This value is calculated by the calculating means 8. 
In the calculating means 8, a change in astigmatism as one optical 
characteristic of the projection optical system 4 is predicted based on 
the amount Q of exposure light supplied to the reticle R per unit of time 
and monitored by the light amount sensor 7, the lengths x and y and area S 
of an exposed area determined by an unillustrated masking blade, and a 
transmittance T of the reticle R previously measured by the illuminance 
measuring device 10. 
The length-to-width ratio y/x of the exposed area is one parameter of 
astigmatism. Although astigmatism does not change when x and y in FIG. 4 
are almost equal, if the ratio y/x exceeds 2, the change in astigmatism 
owing to exposure is not negligible. The coefficient of astigmatism should 
be determined based on the relationship between the ratio y/x and the 
change of astigmatism previously found through experimentation. When the 
coefficient of astigmatism is .alpha., the astigmatism .DELTA.AS is given 
by the following expressions: 
EQU .DELTA.AS=.DELTA.AS1=.DELTA.AS2 
EQU .DELTA.AS1=.alpha..multidot.(y/x).multidot.T.multidot.Q.multidot.S.multidot 
.DT 
EQU .DELTA.AS2=-.DELTA.AS'exp(-k.multidot.t) 
where DT represents exposure time per unit of time, .DELTA.AS' represents a 
value of astigmatism calculated in a previous unit of time, and k is a 
parameter representing heat transfer of the projection optical system 4. A 
predicted value of astigmatism is obtained by predictive calculation using 
these expressions or an expression represented by a linear combination of 
these expressions, and changes along an envelope shown in FIG. 3 which can 
be represented by a function of a natural logarithm. 
FIG. 2 shows the adjusting means 9 for correcting astigmatism. In order to 
correct astigmatism caused in the optical system 4 according to the 
prediction result of astigmatism calculated by the calculating means 8, 
the adjusting means 9 turns and tilts the plane-parallel plates 5a and 5b 
at an angle .theta. in opposite directions with respect to a reference 
plane intersecting the optical axis of the optical system 4. For example, 
when NA is 0.6, .lambda. is 248 nm and the thickness of the plane-parallel 
plates 5a and 5b is 5 mm, the plane-parallel plates 5a and 5b are tilted 
at about .+-.30 minutes in order to correct astigmatism of 0.3 .mu.m for a 
pattern having a line width of 0.25 .mu.m. The material of the 
plane-parallel plates 5a and 5b is SiO.sub.2. 
In the scanning type projection exposure apparatus, exposure is performed 
while scanning the reticle R and the wafer W in synchronization with each 
other in the X direction of FIG. 1. In this case, the illumination area of 
the illumination optical system 2 is generally shaped like a slit, and the 
ratio of X to Y in FIG. 4 is, for example, more than 3. Therefore, it is 
essential to correct astigmatism. Since scanning is performed in the X 
direction, y is longer than x in the illumination area of FIG. 4. In this 
case, the change in focal position of the pattern V is larger than that of 
the pattern H, and the plane-parallel plates 5a and 5b shown in FIG. 2 are 
tilted in the XZ plane. When the plane-parallel plates 5a and 5b are thus 
tilted in the XZ plane, since there is little influence on the focal 
position of the pattern H, if the adjusting means 9 corrects astigmatisms 
of the pattern V and the pattern H so as to correct the focal position of 
the pattern H, the focal position of the pattern H is also corrected. 
The light source 1 may be an excimer laser or a lamp such as a mercury 
lamp. 
The number of plane-parallel plates may be more than two as long as the 
plates to be tilted in opposite directions are equal in number, thickness 
and refractive index. 
Furthermore, the adjusting means 9 also corrects coma by tilting the 
plane-parallel plates 5a and 5b integrally with respect to the optical 
axis in some cases. 
The projection optical system 4 is composed of only lenses or a combination 
of lenses and mirrors. 
In this embodiment, predictive calculation and correction of astigmatism 
.DELTA.AS may be performed for every shot (exposed area) on the wafer W, 
for every plural shots, for every wafer or every plural wafers, or at 
predetermined periods. 
FIG. 5 is a schematic view of a second embodiment of the present invention. 
In FIG. 5, numerals 51 and 52 respectively denote an illumination optical 
system, and a TTL focus measuring system for detecting the best focal 
plane of a projection optical system 4. A reticle R is held on a reticle 
stage 3, and a pattern image on the reticle R is formed onto the surface 
of a wafer W by the projection optical system 4. Two plane-parallel plates 
5a and 5b, which are almost equal in thickness and refractive index, are 
placed on the side of the projection optical system 4 close to the wafer 
W. An XYZ stage 6 holds and moves the wafer W to the exposure area while 
sequentially stepping in the XY direction, and moves the surface of the 
wafer W in the Z direction in accordance with the measurement result of an 
unillustrated focusing system. A reflecting surface 57 is placed on the 
XYZ stage 6. An astigmatism measuring means 58 calculates astigmatism of 
the projection optical system 4 based on the measurement result of the TTL 
focus measuring system 52, and the astigmatism of the projection optical 
system 4 is corrected by an adjusting means 9 in accordance with the 
calculated amount of astigmatism. The plane-parallel plates 5a and 5b are 
made of SiO.sub.2. 
Astigmatism is measured as follows. As shown in FIG. 6, a focus measuring 
mark 21 is put on the reticle R. The mark 21 is located in such an area as 
to be measured by the TTL focus measuring system 52 when the TTL focus 
measuring system 52 moves or the reticle R moves as in the scanning 
projection apparatus. If the reflecting surface 57 is moved to an imaging 
position of the mark 21 and the XYZ stage 6 is moved in the Z direction, 
as shown in FIG. 7, when the reflecting surface 57 is adjacent to the best 
focal position, the light reflected by the reflecting surface 57 transmits 
by a large amount through pattern transmitting portions 22 of the mark 21. 
If the reflecting surface 57 is not at the best focal position, since the 
light is blocked by Cr patterns of the mark 21, the amount of transmitting 
light decreases. The relationship between the amount of light and the 
position of the reflecting surface 57 in the Z direction is shown in a 
graph of FIG. 8. By performing such a measurement for the pattern V and 
the pattern H, respectively, and calculating the difference between the 
best focal positions of the patterns, astigmatism of the projection 
optical system 4 is calculated. The position of the reflecting surface 57 
is measured by the above-mentioned focusing system. Such a plane position 
measuring device as this focusing system is well known. 
Astigmatism is corrected by adjusting the angles of the plane-parallel 
plates 5a and 5b by means of the adjusting means 9 according to the 
measured astigmatism value. These structures and functions for astigmatism 
correction are just the same as those in the first embodiment. 
If a 45-direction mark and a 135-direction mark are added to the mark in 
FIG. 6 as marks for measurement, since the direction in which astigmatism 
of the projection optical system 4 is the biggest and the amount of 
astigmatism are known, it is possible to correct astigmatism in a desired 
direction. 
If three driving mechanisms such as piezo actuators are spaced at 
120.degree. outside an effective diameter of the plane-parallel plates 5a 
and 5b as a means for driving the adjusting means 9, an arbitrary 
astigmatism in a desired direction can be adjusted by adjusting the drive 
amount of the driving mechanisms. 
As another method of measuring the focus, a stage reference mark is formed 
on the reflecting surface 57 and the contrast between the stage reference 
mark and images of the marks in two directions as shown in FIG. 6 is 
measured by a CCD or the like located on the side of the mask. 
Furthermore, in order to calculate astigmatism, the best focal positions of 
the respective focus measuring marks in two directions as shown in FIG. 6 
may be found by exposing the marks onto the wafer W and measuring the line 
widths thereof after the wafer W is developed. 
In a further focus measuring system, an image of the focus measuring mark 
21 is projected onto a photo-detector located on the stage 6 and having a 
lattice pattern, and the amount of light passing through the lattice 
pattern is measured when the stage 6 is moved up and down. 
In a still further focus measuring system, an image of the focus measuring 
mark 21 on the reticle R is projected onto a CCD located on the stage 6, 
and the contrast of the image on the CCD is measured when the stage 6 is 
moved up and down. 
In a still further focus measuring system, a focus measuring mark as shown 
in FIG. 6 is put on the stage 6, an image of the mark is projected onto 
the reticle R, and imaging rays are reflected by the reflecting surface of 
the reticle R and made incident on the projection optical system 4. The 
image of the mark is projected by the projection optical system 4 onto a 
photodetector having a lattice pattern on the stage 6, and the amount of 
light passing through the lattice pattern is measured when the stage 6 is 
moved up and down. 
In a still further focus measuring system, a focus measuring mark shown in 
FIG. 6 is put on the stage 6, an image of the mark is projected onto a 
photodetector having a lattice pattern on the side of the reticle R (the 
lattice pattern is not formed on the reticle R in some cases), and the 
amount of light passing through the lattice pattern is measured when the 
stage 6 is moved up and down. 
The illumination optical system 51 is provided with a laser such as excimer 
laser or a lamp such as a mercury lamp, as a light source. 
The projection optical system 4 is composed of only lenses or a combination 
of lenses and mirrors. 
In this embodiment, measurement and correction of astigmatism .DELTA.AS may 
be performed for every plural shots (exposed areas) on the wafer W, for 
every wafer or every plural wafers, or at predetermined periods. 
In the scanning type exposure apparatus, exposure is performed while 
scanning the reticle R and the wafer W in synchronization with each other 
in the X direction of FIG. 5. The main points of astigmatism correction at 
this time are similar to those mentioned in the first embodiment. 
The number of paired plane-parallel plates in this embodiment may be an 
arbitrary even number more than two as long as the plates are equal in the 
number of plates to be tilted in opposite directions, thickness and 
refractive index. Furthermore, coma also can be corrected by turning and 
tilting more than two plane-parallel plates together in the same direction 
of the arrow (.theta. or -.theta. direction) in FIG. 2 by means of the 
adjusting means 9. 
The material of the plane-parallel plates in this embodiment may be not 
only SiO.sub.2, but also CaF.sub.2, and BK.sub.7 or the like also may be 
used as the material in a case in which the exposure light takes a form of 
an i line (having a wavelength of 365 nm). 
In the above embodiments, the amount of astigmatism correction is adjusted 
by turning two paired plane-parallel plates in opposite directions through 
the same angle. 
In the above embodiments, the plane-parallel plates for astigmatism 
correction may be interposed between the reticle R and the projection 
optical system 4. 
In the above embodiments, the aberration-correcting optical system 
disclosed in Japanese Patent Laid-Open No. 7-92424 by the applicant of 
this application may be used to adjust the astigmatism. 
An embodiment of a device manufacturing method using the above-mentioned 
exposure apparatus will be next described. FIG. 9 is a flowchart showing 
processes of manufacturing a semiconductor device (a semiconductor chip 
such as an IC or an LSI, a liquid crystal panel, a CCD and the like). In 
Step 1 (circuit design), circuit design of the semiconductor device is 
performed. In Step 2 (mask fabrication), a mask on which the designed 
circuit pattern is formed is fabricated. On the other hand, in Step 3 
(wafer fabrication), a wafer is fabricated by using a material such as 
silicon. In Step 4 (wafer process) called a preprocess, an actual circuit 
is formed on the wafer by lithography using the above prepared mask and 
wafer. In the next Step 5 (assembly), called a postprocess, a 
semiconductor chip is manufactured by using the wafer fabricated in Step 
4, and an assembly process (dicing, bonding), a packaging process (chip 
sealing) and the like are included in this step. In step 6 (inspection), 
the semiconductor device manufactured in Step 5 is subjected to a 
performance test, an endurance test, and the like. The semiconductor 
device is completed through these steps, and shipped (Step 7). 
FIG. 10 is a detailed flowchart of the above wafer process. In Step 11 
(oxidation), the surface of the wafer is oxidized. In Step 12 (CVD), an 
insulating film is formed on the surface of the wafer. In Step 13 
(electrode formation), electrodes are formed on the wafer by evaporation. 
In Step 14 (ion implantation), ions are implanted into the wafer. In Step 
15 (resist process), a photosensitive material is applied on the wafer. In 
Step 16 (exposure), the circuit pattern of the mask is printed on the 
wafer though exposure by the above-mentioned exposure apparatus. In Step 
17 (development), the exposed wafer is developed. In Step 18 (etching), 
parts other than the developed resist image are cut away. In Step 19 
(resist stripping), the portion of the resist which is unnecessary after 
etching is removed. By repeating these steps, multiple circuit patterns 
are formed on the wafer. 
The use of the manufacturing method in this embodiment makes it possible to 
manufacture a highly integrated device which has been difficult to 
manufacture.