Projection optical apparatus for projecting a mask pattern onto the surface of a target projection object and projection exposure apparatus using the same

A projection exposure apparatus projects the pattern of a reticle at a high resolution even when a wafer which is poor in flatness because of its warping or step is used. An illumination light beam emitted from a light source system illuminates a reticle through a condenser lens. The illumination light beam emerging from the reticle forms the intermediate image of the pattern of the reticle at a position near the reflecting surface of a reflecting mirror through a half prism and a projection lens. The illumination light beam reflected by the reflecting surface forms the pattern image of the reticle onto the surface of a wafer after passing through the projection lens and the half prism again. The shape of the reflecting surface of the reflecting mirror is changed to conform to the shape of the surface of the wafer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a projection optical apparatus for 
projecting a mask pattern onto the surface of a target projection object, 
which is suitable as an apparatus for projecting the pattern of a reticle 
or the like onto a photosensitive substrate in a projection exposure 
apparatus used in a photolithography process for manufacturing a 
semiconductor element, a liquid crystal display element, or the like. 
2. Related Background Art 
In a projection exposure apparatus for projecting and exposing the pattern 
of a reticle or photomask (to be represented by a "reticle" hereinafter) 
onto a semiconductor wafer (to be simply referred to as a "wafer" 
hereinafter) coated with a photoresist, or a glass plate, a projection 
optical system with a high resolution is used. Generally, to increase the 
resolution, the wavelength of illumination light (exposure light) must be 
shortened, or the numerical aperture (NA) of the projection optical system 
must be increased. However, the depth of focus (DOF) of a projected image 
decreases in proportion to the wavelength of the illumination light while 
it decreases in inverse proportion to the square of numerical aperture. 
In a semiconductor element, the flatness of a wafer used as an exposure 
target is increased such that the entire shot area on which the pattern of 
a reticle falls within the range of depth of focus. 
Since an exposed wafer is treated through various processes including 
development, warping occurs through these processes in some cases. To 
prevent the flatness of the warped wafer from falling out of the range of 
depth of focus, restrictions are conventionally provided to these 
processes, thereby suppressing the warping of the wafer within a 
predetermined allowance. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a projection optical 
apparatus capable of exposing, at a high resolution, the pattern of a 
reticle even onto a wafer which is entirely poor in flatness because of 
warping or a step. 
It is another object of the present invention to provide a projection 
optical apparatus capable of exposing the pattern of a reticle onto the 
exposure surface of a wafer at a high resolution even when a projection 
optical system having a large curvature of field is used. 
The above and other objects will become apparent from the following 
description. 
According to the present invention, there is provided a first projection 
optical apparatus for projecting an image of a mask pattern onto a surface 
of a target projection object, comprising intermediate image formation 
means for forming an intermediate image of the mask pattern, reflection 
means having a reflecting surface substantially arranged at a position 
where the intermediate image is formed, and re-imaging means for forming 
the intermediate image of the mask pattern onto the surface of the target 
projection object again on the basis of a light beam reflected by the 
reflection means. The reflection means imparts a wavefront aberration 
(optical path difference) according to a deformed state of the surface of 
the target projection object to the light beam from the intermediate image 
formation means. 
In this case, the intermediate image formation means may be commonly used 
as the re-imaging means. 
In addition, surface shape detection means for detecting a surface shape of 
the target projection object and surface shape changing means for changing 
a shape of the reflecting surface of the reflection means are provided. 
The surface shape changing means preferably controls the shape of the 
reflecting surface of the reflection means on the basis of the surface 
shape of the target projection object, which is detected by the surface 
shape detection means. 
According to the present invention, there is also provided a second 
projection optical apparatus for projecting an image of a mask pattern 
onto a surface of a target projection object, comprising intermediate 
image formation means for forming an intermediate image of the mask 
pattern onto first and second planes which are different from each other, 
first and second reflection means respectively having reflecting surfaces 
substantially arranged on the first and second planes, and re-imaging 
means for synthesizing light beams reflected by the first and second 
reflection means and forming the intermediate image onto the surface of 
the target projection object again on the basis of the synthesized light 
beam. The first and second reflection means respectively impart wavefront 
aberrations according to a deformed state of the surface of the target 
projection object to the light beams from the intermediate image formation 
means. 
According to the first projection optical apparatus of the present 
invention, the light beam from the mask pattern forms the intermediate 
image of the mask pattern at a position near the reflecting surface of the 
reflection means through the intermediate image formation means. The light 
beam reflected by the reflecting surface forms the intermediate image onto 
the surface of the target projection object through the re-imaging means. 
At this time, if the surface of the target projection object has a 
recessed point, a wavefront aberration (optical path difference) is 
imparted to the light beam from the intermediate image formation means 
while setting a point on the reflecting surface, which is conjugate with 
the recessed point, as a projecting portion. With this operation, the mask 
pattern is sharply projected even onto the recessed point on the surface. 
When the intermediate image formed by the intermediate image formation 
means has a curvature of field while the surface of the target projection 
object is flat, the reflecting surface of the reflection means is curved 
in accordance with the curvature of field. With this operation, the image 
of the mask pattern is sharply projected onto the surface of the target 
projection object. 
When the intermediate image formation means is commonly used as the 
re-imaging means, the intermediate image formation means has light beam 
splitting means such as a half prism and a projection system. The 
intermediate image of the mask pattern is formed at a position near the 
reflecting surface of the reflection means through the light beam 
splitting means and the projection system. The intermediate image is 
formed onto the surface of the target projection object again through the 
projection system and the light beam splitting means. In this case, the 
projection magnification from the mask pattern to the surface is one. 
Surface shape detection means for detecting the shape of the surface of the 
target projection object and surface shape changing means for changing the 
shape of the reflecting surface of the reflection means are arranged. In 
this case, when the surface detected by the surface shape detection means 
is recessed at its center, the surface shape changing means imparts a 
wavefront aberration (optical path difference) to the light beam from the 
intermediate image formation means while setting the central portion of 
the reflecting surface as a projecting portion. Therefore, even when a 
large number of target projection objects with various surface shapes are 
sequentially to be exposed, the mask pattern can be exposed onto the 
surface at a high resolution. 
According to the second projection optical apparatus of the present 
invention, the intermediate image is formed at positions near the two 
reflecting surfaces. The light beams reflected by the reflecting surfaces 
are synthesized to form the intermediate onto the surface of the target 
projection object. At this time, when one of the reflecting surfaces is 
shifted to be close to the mask pattern with respect to the first plane 
while the other reflecting surface is shifted to be separated from the 
mask pattern with respect to the second plane, the image of the mask 
pattern is formed on first and second projection planes sandwiching the 
surface of the target projection object. That is, the depth of focus is 
increased by the double focus effect. 
The present invention will become more fully understood from the detailed 
description given hereinbelow and the accompanying drawings which are 
given by way of illustration only, and thus are not to be considered as 
limiting the present invention. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the prior art as described above, the flatness and warping of a wafer 
used as an exposure target are set to entirely fall within the range of 
depth of focus of an image projected by a projection optical system. 
Recently, however, along with a further increase in degree of integration 
of a VLSI or the like, a higher resolution has been required, resulting in 
a further decrease in depth of focus. For this reason, it becomes 
difficult to set the flatness of a wafer within the range of depth of 
focus. 
In addition, as the structure of a semiconductor element is complicated, 
the treatment processes for a wafer become complex accordingly. Therefore, 
the warping or step in a wafer tends to become larger through various 
processes. When the flatness of an entire wafer having such warping or a 
step falls outside the range of depth of focus, the yield of semiconductor 
elements to be finally obtained is undesirably decreased. 
Furthermore, in some cases, a slight curvature of field remains in a 
projection optical system. Conventionally, since the exposure field is 
small, the curvature of field in one shot area on a wafer is small as 
compared to the range of depth of focus and therefore poses no problem. 
Recently, however, a projection optical system having a large exposure 
field is sometimes used to manufacture a large chip pattern. As the 
exposure field becomes larger, the curvature of field in the periphery 
becomes larger accordingly. This causes a further decrease in substantial 
depth of focus when a flat exposure target is used, resulting in a 
difficulty in maintaining satisfactory imaging characteristics on the 
entire shot area. 
Various embodiments of the present invention will be described below with 
reference to the accompanying drawings. In the following embodiments, the 
present invention is applied to a projection exposure apparatus. 
First Embodiment! 
FIG. 1 is a view showing a projection exposure apparatus to which the first 
embodiment of the present invention is applied. Referring to FIG. 1, a 
light source system 1 is constituted by an exposure light source such as a 
mercury lamp or an excimer laser source for emitting illumination light 
(exposure light), and an optical integrator for forming a large number of 
secondary sources from the illumination light. An illumination light beam 
IL emitted from the light source system 1 illuminates the pattern area of 
a reticle 3 at an almost uniform illuminance distribution through a 
condenser lens 2. The illumination light beam IL emerging from the reticle 
3 is incident on a half prism 4. The illumination light beam transmitted 
through the half prism 4 forms the intermediate image of the pattern of 
the reticle 3 onto a first plane P1 near a reflecting surface 6a of a 
reflecting mirror 6 through a projection lens 5. The reticle 3 is 
supported by a reticle stage 26. 
The pattern formation surface of the reticle 3 is conjugate with the first 
plane P1. The reflecting surface 6a of the reflecting mirror 6 has a shape 
with three-dimensional patterns added to the first plane P1 in accordance 
with the surface shape of a wafer 7 used as an exposure target (to be 
described later). With this arrangement, a wavefront aberration (optical 
path difference) is imparted to the light beam from the projection lens 5 
at a predetermined distribution. The illumination light beam reflected by 
the reflecting surface 6a of the reflecting mirror 6 inversely passes 
through the projection lens 5 toward the half prism 4. The illumination 
light beam reflected by the half prism 4 forms the pattern image of the 
reticle 3 onto a shot area on a surface 7a of the wafer 7. A photoresist 
is coated on the surface 7a of the wafer 7. The wafer 7 is held on a wafer 
stage 28 for three-dimensionally positioning the wafer 7. 
In this case, when a plane conjugate with the first plane P1 with respect 
to the projection lens 5 and the half prism 4 is defined as a second plane 
P2, the surface 7a of the wafer 7 is partially shifted from the second 
plane P2 although the surface 7a of the wafer 7 is averagely set on the 
second plane P2. An axis Z1 is set parallel to the optical axis of the 
projection lens 5 while an optical axis obtained upon bending the above 
optical axis by the half prism 4 is defined as an axis Z2. At this time, 
assume that a point Q2 on the surface 7a is shifted from the second plane 
P2 by .DELTA.Z.sub.2 along the axis Z2 (direction to separate from the 
half prism 4 is defined as positive). The magnification of the projection 
lens 5 from the reticle 3 to the reflecting mirror 6 is defined as .beta., 
and a point on the reflecting surface 6a of the reflecting mirror 6, which 
is conjugate with the point Q2 on the wafer 7 is defined as a point Q1. 
The reflecting surface 6a is deformed such that the position of the point 
Q1 along the axis Z1 (direction to be close to the projection lens 5 is 
defined as positive) is shifted by .DELTA.Z.sub.1 which is determined in 
accordance with the following equations: 
EQU .DELTA.Z.sub.2 =2(1/.beta..sup.2).DELTA.Z.sub.1 (1) 
EQU .DELTA.Z.sub.1 =(.beta..sup.2 /2).DELTA.Z.sub.2 (2) 
More specifically, when the point Q2 on the surface 7a of the wafer 7 has a 
deformation amount .DELTA.Z.sub.2, the deformation amount .DELTA.Z.sub.1 
corresponding to a value obtained upon multiplying 1/2 the square of the 
magnification .beta. of the projection lens 5 and the deformation amount 
.DELTA.Z.sub.2 is added to the point Q1 on the reflecting surface 6a of 
the reflecting mirror 6, which is conjugate with the point Q2. If the 
projection lens 5 has a magnification of equal ratio (.beta.=-1), the 
deformation about .DELTA.Z.sub.1 of the reflecting surface 6a is 1/2 the 
deformation amount .DELTA.Z.sub.2 of the surface 7a of the wafer 7. 
Subsequently, in accordance with the deformation amounts of different 
points on the shot area on the surface 7a of the wafer 7, deformation 
amounts determined by equation (2) are added to the conjugate points on 
the surface 6a of the reflecting mirror 6. With this operation, an 
intermediate image formed on the first plane P1, i.e., the pattern image 
of the reticle 3 is formed again on the entire shot area on the surface 7a 
of the wafer 7 in an in-focus state. 
As described above, according to this embodiment, even when the wafer is 
warped, as shown in FIG. 1, the pattern image of the reticle 3 can be 
exposed onto the entire shot area on the surface 7a of the wafer 7 in an 
in-focus state (at a high resolution) by deforming the reflecting surface 
6a of the reflecting mirror 6 so as to conform to the warping of the 
wafer. As in this embodiment, when the intermediate image and the image on 
the surface of the wafer 7 are formed by the single projection lens 5, and 
the intermediate image is approximately flat, at least a side (reflecting 
mirror 6 side) where the intermediate image is formed is preferably 
telecentric. If the telecentricity is poor, the light beam reflected by 
the reflecting mirror 6 is eclipsed in some cases, resulting in a 
degradation in uniformity of the image formed on the wafer 7 again and 
uniformity of illuminance of the image. 
In this embodiment, the intermediate image and the image on the surface of 
the wafer 7 are formed by the single projection lens 5. Therefore, the 
arrangement of the optical system can be simplified although the 
projection magnification from the reticle 3 to the wafer 7 is one (equal 
ratio). 
In this embodiment, when the projection lens 5 has a curvature of field 
while the surface of the wafer 7 is flat, the reflecting surface 6a of the 
reflecting mirror 6 is curved in accordance with the curve of the image 
projected by the projection lens 5. In this case, when the lens for 
forming the intermediate image and the lens for forming the image on the 
wafer 7 are identical, and the curve of the image plane of the 
intermediate image is large, the principal ray out of the optical axis 
substantially coincides with the normal (of the corresponding place) of 
the image plane, preferably. When the surface of the wafer 7 is also 
warped, the reflecting surface 6a of the reflecting mirror 6 is deformed 
in accordance with the warping amount. With this operation, even when the 
projection lens 5 has a curvature of field, or even when the wafer is 
warped, the pattern image of the reticle 3 can be exposed onto the entire 
shot area on the surface of the wafer 7 at a high resolution (in an 
in-focus state). 
When the flatness of the reticle 3 is poor, and the patten image of the 
reticle 3, which is projected by the projection lens 5, is curved, the 
reflecting surface 6a of the reflecting mirror 6 may be deformed to cancel 
the curvature. With this operation, the pattern image of the reticle 3 is 
exposed onto the wafer 7 in an in-focus state. 
In this embodiment, all elements from the reticle 3 to the intermediate 
image are arranged on the same optical axis, and the final image plane is 
formed on the optical axis bent by the half prism 4. To form the 
intermediate image and the final image on the same optical axis, the 
incident light beam from the reticle 3 may be deflected by the half prism 
4 and caused to be incident on the projection lens 5. 
Second Embodiment! 
FIG. 2 is a view showing a projection exposure apparatus to which the 
second embodiment of the present invention is applied. The same reference 
numerals as in FIG. 1 denote the same parts in FIG. 2, and a detailed 
description thereof will be omitted. Referring to FIG. 2, a reflecting 
mirror 6 which is thin to some extent and can be easily deformed is used. 
The reflecting mirror 6 is supported on a base 8 through extendible 
piezoelectric elements 9A to 9E (all piezoelectric elements are 
represented by the piezoelectric elements 9A to 9E) which are 
two-dimensionally distributed. The extension/contraction amounts of the 
piezoelectric elements 9A to 9E are individually controlled by a driving 
system 10. A reflecting surface 6a of the reflecting mirror 6 is entirely 
set to a desired shape by pushing/pulling a predetermined portion of the 
rear surface of the reflecting mirror 6. In place of the piezoelectric 
elements 9A to 9E, other electrostrictive elements or magnetostrictive 
elements, or set screw type expansion mechanism can also be used. 
A lot of projection optical systems 10A to 10C (all projection optical 
systems are represented by the projection optical systems 10A to 10C) are 
two-dimensionally distributed to be oblique with respect to the optical 
axis of a projection lens 5, which project slit pattern images onto the 
reflecting surface 6a of the reflecting mirror 6. Light beams reflected by 
the surface 6a are received by a lot of light-receiving optical systems 
11A to 11C (all light-receiving optical systems are represented by the 
light-receiving optical systems 11A to 11C) which are two-dimensionally 
distributed. Corresponding slit pattern images are formed again in the 
light-receiving optical systems 11A to 11C. Focus signals corresponding to 
the lateral shift amounts of the re-imaging positions are generated and 
supplied to a main control system 12 for controlling the operation of the 
entire apparatus. The projection optical systems 10A to 10C and the 
light-receiving optical systems 11A to 11C constitute an oblique incident 
type multipoint surface shape detection unit. 
Note that the projection optical systems 10A to 10C are not limited to 
those for projecting slit pattern images. 
An oblique incident type multipoint surface shape detection unit is 
described in, e.g., U.S. Ser. No. 07/964,954 in detail. U.S. Ser.No. 
07/964,954 is incorporated in this application as a reference as far as 
the oblique incident type multipoint surface shape detection unit is 
concerned. 
In the detection mechanism shown in FIG. 2, calibration is performed such 
that the focus signals are set to zero when the reflecting surface 6a of 
the reflecting mirror 6 matches the imaging plane of the projection lens 
5. When the reflecting surface 6a has a displacement along the direction 
of axis Z1 parallel to the optical axis of the projection lens 5, the 
positions of the slit pattern images formed in the light-receiving optical 
systems 11A to 11C are laterally shifted to change the focus signals. 
Therefore, the main control system 12 can calculate the distribution of 
the three-dimensional patterns on the reflecting surface 6a. 
Similarly, to measure the distribution of the three-dimensional patterns on 
the surface of the wafer 7, projection optical systems 13A to 13C and 
light-receiving optical systems 14A to 14C are arranged in correspondence 
with the projection optical systems 10A to 10C and the light-receiving 
optical systems 11A to 11C, and focus signals from the light-receiving 
optical systems 14A to 14C are supplied to the main control system 12. In 
this case, the focus signals change in correspondence with the 
displacement of the surface of the wafer 7, which is caused along an axis 
Z2 obtained upon bending the optical axis of the projection lens 5 by a 
half prism 4. In accordance with the focus signals, the main control 
system 12 calculates the distribution of the three-dimensional patterns on 
the surface of the wafer 7 with respect to the imaging plane by the 
projection lens 5 and the half prism 4. 
In this embodiment, before exposure, the main control system 12 calculates 
the distribution of the shift amounts of the surface shape of the wafer 7 
from the imaging plane by using the focus signals from the light-receiving 
optical systems 14A to 14C. The extension/contraction amounts of the 
piezoelectric elements 9A to 9E are adjusted through the driving system 10 
in accordance with the distribution of the shift amounts. The distribution 
of the shift amounts of the shape of the reflecting surface 6a of the 
reflecting mirror 6 from the image formation plane, which is calculated 
using the focus signals from the light-receiving optical systems 11A to 
11C, is set to the relation of equation (2) with respect to the 
distribution of the shift amounts of the surface shape of the wafer 7. 
Thereafter, an illumination light beam IL is emitted from a light source 
system 1 to expose the pattern image of a reticle 3 onto the surface of 
the wafer 7. With this operation, even when the wafer 7 is warped, the 
pattern image of the reticle 3 can be exposed onto the entire shot area on 
the surface of the wafer 7 at a high resolution (in an in-focus state). 
When the projection lens 5 has a curvature of field, or when the flatness 
of the reticle 3 is poor, a displacement corresponding to the shape of the 
reflecting surface 6a of the reflecting mirror 6 is superposed. With this 
operation, the pattern image of the reticle 3 can be exposed onto the 
entire shot area on the surface of the wafer 7 in an in-focus state. 
The half prism 4 is used in the embodiments shown in FIGS. 1 and 2. Since 
the illumination light beam IL passes through the half prism 4 twice, 
about 75% of the illumination light beam IL transmitted through the 
reticle 3 is lost. To decrease such a loss in light amount, as shown in 
FIG. 2, the half prism 4 may be replaced with a polarization beam splitter 
16, and at the same time, a .lambda./4 plate 15 may be arranged between 
the projection lens 5 and the reflecting mirror 6. With this arrangement, 
when the illumination light beam IL for illuminating the reticle 3 is in a 
randomly polarized state, a P-polarized light component corresponding to 
about 50% of the illumination light beam IL passes through the 
polarization beam splitter 16. This P-polarized light component is 
returned to the polarization beam splitter 16 as S-polarized light through 
the projection lens 5, the .lambda./4 plate 15, the reflecting mirror 6, 
the .lambda./4 plate 15, and the projection lens 5. Almost 100% of this 
S-polarized light component is reflected toward the wafer 7. Therefore, 
the loss in light amount can be decreased to about 50%. 
In addition, ghost light (stray light) in the projection lens 5 can also be 
removed by the polarization beam splitter 16. The .lambda./4 plate 15 can 
be inserted anywhere in the optical path as far as it is located on the 
intermediate image side (reflecting mirror 6 side) with respect to the 
polarization beam splitter 16. However, to effectively remove the ghost 
light in the projection lens 5, the .lambda./4 plate 15 is preferably 
inserted on the intermediate image side with respect to the projection 
lens 5. 
In all embodiments, a back reflecting mirror having a rear surface as a 
reflecting surface is preferably used as the reflecting mirror 6 or 20. 
The reason for this is as follows. The reflecting mirror 6 or 20 is 
arranged at the image position. Therefore, if dust sticks to the 
reflecting surface of the reflecting mirror 6 or 20, the image quality is 
largely adversely affected. When a back reflecting mirror is used as the 
reflecting mirror 6 or 20, dust sticking to the surface is defocused, 
thereby minimizing the influence on the image quality. 
FIG. 3 is a side view showing a back reflecting mirror having a rear 
surface as a reflecting surface. A reflecting film 32 is formed on the 
rear surface of a transparent member 30 as of glass or the like, i.e., a 
surface opposite to the light beam incident side. The transparent member 
30 is attached to the piezoelectric elements 9A to 9E through a support 
member 34. The support member 34 is preferably formed of glass. Note that, 
when a back reflecting mirror is to be used as the reflecting mirror 6 or 
20, the projection lens 5 must be designed and manufactured in 
consideration of a spherical aberration and a coma according to a 
thickness d (FIG. 3) of the transparent member 30. 
Third Embodiment! 
FIG. 4 is a view showing a projection exposure apparatus to which the third 
embodiment of the present invention is applied. The same reference 
numerals as in FIG. 2 denote the same parts in FIG. 4, and a detailed 
description thereof will be omitted. Referring to FIG. 4, a half prism 4 
is arranged on a reflecting mirror 6 side (intermediate image side) with 
respect to a projection lens 5, and a projection lens 17 serving as a 
final imaging lens is arranged between the half prism 4 and a wafer 7. In 
this case, an illumination light beam passing through a reticle 3 forms 
the intermediate image of the reticle 3 to a portion near a reflecting 
surface 6a of the reflecting mirror 6 through the projection lens 5 and 
the half prism 4. The illumination light beam reflected by the reflecting 
mirror 6 is reflected by the half prism 4, and thereafter, forms the 
intermediate image onto a surface 7a of the wafer 7 again through the 
projection lens 17. The function of the reflecting mirror 6 is the same as 
that in the first and second embodiments. 
In this embodiment, the projection lenses 5 and 17 are respectively used 
for intermediate image formation and final image formation. For this 
reason, the ratio of the size of the pattern of the reticle 3 to that of 
the image projected onto the wafer 7 can be set to a desired value other 
than one. More specifically, when the magnification of the projection lens 
5 from the reticle 3 to the reflecting mirror 6 is defined as .beta., and 
the magnification of the projection lens 17 from the reflecting mirror 6 
to the wafer 7 is defined as .beta..sub.2, the magnification from the 
reticle 3 to the wafer 7 is represented by .beta..multidot..beta..sub.2. 
In this embodiment, the projection lenses 5 and 17 need not always be 
telecentric. In addition, as in the second embodiment shown in FIG. 2, a 
polarization beam splitter 16 may be used in place of the half prism 4, 
and a .lambda./4 plate 15 may be inserted in the optical path on the 
intermediate image side with respect to the polarization beam splitter 16. 
With this arrangement, the loss in light amount can be decreased. 
Fourth Embodiment! 
FIG. 5 is a view showing a projection exposure apparatus to which the 
fourth embodiment of the present invention is applied. The same reference 
numerals as in FIG. 2 denote the same parts in FIG. 5, and a detailed 
description thereof will be omitted. Referring to FIG. 5, the intermediate 
image of the pattern of a reticle 3 is formed onto an imaging plane 19B 
again by an illumination light beam reflected by a half prism 4. A 
variable magnification lens 18 and a wafer 7 are sequentially arranged 
below the imaging plane 19B, and the intermediate image on the imaging 
plane 19B is formed onto a surface 7a of the wafer 7 again through the 
projection lens 18. The variable magnification lens 18 is a projection 
lens capable of changing the magnification while fixing the object plane 
and the imaging plane. Note that a projection lens having a predetermined 
fixed magnification may be used in place of the variable magnification 
lens 18. 
In this embodiment, a reflecting mirror 6 has the same function as that in 
the above embodiments such that the shape of a reflecting surface 6a of 
the reflecting mirror 6 is changed in accordance with warping of the 
surface 7a of the wafer 7. With this arrangement, the first intermediate 
image of the reticle 3 is formed onto an imaging plane 19A consisting of a 
curved surface near the reflecting surface 6a. The second intermediate 
image is formed onto the imaging plane 19B consisting of a curved surface 
conjugate with the image formation plane 19A with respect to a projection 
lens 5 and the half prism 4. The final image is formed onto the surface 7a 
of the wafer 7, which matches a curved surface conjugate with the imaging 
plane 19B with respect to the variable magnification lens 18. Therefore, 
the pattern image of the reticle 3 is projected and exposed onto the 
surface of the wafer 7 in an in-focus state. 
In this embodiment, the pattern of the reticle 3 can be projected onto the 
wafer 7 at a desired magnification by the variable magnification lens 18. 
In this embodiment as well, as in the second embodiment shown in FIG. 2, a 
polarization beam splitter 16 may be used in place of the half prism 4, 
and a .lambda./4 plate 15 may be inserted in the optical path. The 
.lambda./4 plate 15 can be inserted anywhere in the optical path as far as 
it is located on the first intermediate image side (reflecting mirror 6 
side) with respect to the polarization beam splitter 16. However, to 
obtain the effect of removing ghost light in the projection lens 5, the 
.lambda./4 plate 15 is preferably inserted on the first intermediate image 
side with respect to the projection lens 5. 
Fifth Embodiment! 
FIG. 6 is a view showing a projection exposure apparatus to which the fifth 
embodiment of the present invention is applied. The same reference 
numerals as in FIG. 4 denote the same parts in FIG. 6, and a detailed 
description thereof will be omitted. In the embodiment shown in FIG. 4, 
the illumination light incident from the projection lens 5 onto the half 
prism 4 and reflected by the half prism 4 is lost. To prevent this, in 
this embodiment, a reflecting surface 20a of a reflecting mirror 20 is 
arranged bear a plane P4 on which the intermediate image of a reticle 3 is 
formed by an illumination light beam reflected by a half prism 4, as shown 
in FIG. 6. The reflecting mirror 20 is supported on a base 21 through 
piezoelectric elements 22A to 22E (all piezoelectric elements are 
represented by the piezoelectric elements 22A to 22E) which are 
two-dimensionally distributed. Three-dimensional patterns on the 
reflecting surface 20a of the reflecting mirror 20 are set in a desired 
distribution by pushing/pulling the rear surface of the reflecting mirror 
20 by the piezoelectric elements 22A. to 22E. A reflecting surface 6a of a 
reflecting mirror 6 is arranged near a plane P3 on which the intermediate 
image of the reticle 3 is formed by the illumination light beam 
transmitted through the half prism 4. 
In this case, a light component of the illumination light beam reflected by 
the reflecting mirror 6, which is reflected by the half prism 4, and a 
light component of the illumination light beam reflected by the reflecting 
mirror 20, which is transmitted through the half prism 4, are synthesized 
to form the pattern image of the reticle 3 onto a surface 7a of a wafer 7 
through a projection lens 17. The function of the reflecting mirror 6 is 
the same as that in the above embodiments, and the function of the 
reflecting mirror 20 is the same as that of the reflecting mirror 6. In 
this embodiment, the illumination light beam reflected by the half prism 
4, i.e., the light beam lost in the third embodiment can be effectively 
used. As a result, in this embodiment, a light amount about twice that in 
the third embodiment can be obtained. 
In the third embodiment shown in FIG. 4, the final imaging plane is 
conjugate with the surface of the reflecting mirror 6. For this reason, 
dust or flaws on the reflecting surface of the reflecting mirror 6 tend to 
be transferred onto the final imaging plane (surface of the wafer 7). To 
the contrary, in this embodiment shown in FIG. 6, the two reflecting 
mirrors 6 and 20 are used. Therefore, the image of dust or flaws on one 
reflecting mirror can be canceled by the image of the other reflecting 
mirror, thereby minimizing the influence of the dust or flaws on the 
surfaces of the reflecting mirrors 6 and 20. 
In addition, as shown in FIG. 6, the reflecting surface 6a of the 
reflecting mirror 6 is set at a position to be separated from the 
projection lens 5 with respect to the imaging plane P3 along the optical 
axis of the projection lens 5 so as not to largely exceed the depth of 
focus. At this time, the reflecting surface 20a of the reflecting mirror 
20 may be set at a position to be close to the projection lens 5 with 
respect to the imaging plane P4 along the optical axis of the projection 
lens 5 so as not to largely exceed the depth of focus. In this manner, an 
offset may be provided between the positions of the two reflecting 
surfaces 6a and 20a. With this arrangement, the pattern image of the 
reticle 3 is formed on two planes 23A and 23B arranged to sandwich the 
surface 7a of the wafer 7 along the optical axis of the projection lens 
17. Therefore, the same effect as in projection through a double focus 
lens can be obtained on the surface 7a of the wafer 7, thereby 
substantially increasing the depth of focus of the projected image. 
Since the half prism 4 is used in the arrangement shown in FIG. 6, a loss 
of about 50% is caused in light amount by the half prism 4. To prevent 
this, the half prism 4 in FIG. 6 can be replaced with a polarization beam 
splitter 16, and .lambda./4 plates 15A and 15B can be respectively 
arranged between the polarization beam splitter 16 and the reflecting 
mirror 6 and between the polarization beam splitter 16 and the reflecting 
mirror 20. 
In this case, a P-polarized light component transmitted through the 
polarization beam splitter 16 is returned to the polarization beam 
splitter 16 as S-polarized light through the .lambda./4 plate 15A, the 
reflecting mirror 6, and the .lambda./4 plate 15A. Almost 100% of the 
S-polarized light component is reflected toward the projection lens 17. On 
the other hand, an S-polarized light component reflected by the 
polarization beam splitter 16 is returned to the polarization beam 
splitter 16 as P-polarized light through the .lambda./4 plate 15B, the 
reflecting mirror 20, and the .lambda./4 plate 15B. Almost 100% of the 
P-polarized light component is transmitted toward the projection lens 17. 
Therefore, the loss in light amount caused by optical path splitting in 
the polarization beam splitter 16 can be decreased to almost zero. 
Sixth Embodiment! 
FIG. 7 is a view showing a projection exposure apparatus to which the sixth 
embodiment of the present invention is applied. The same reference 
numerals as in FIGS. 2 and 6 denote the same parts in FIG. 7, and a 
detailed description thereof will be omitted. In the embodiment shown in 
FIG. 2, a light component of the illumination light incident on the half 
prism 4, which is reflected by the half prism 4, is lost. To prevent this, 
in this embodiment shown in FIG. 7, a second projection lens 24 identical 
to a projection lens 5 and a second reflecting mirror 20 are sequentially 
arranged to be symmetrical to a wafer 7 with respect to the half prism 4. 
A reflecting surface 20a of the reflecting mirror 20 is set near a 
position where the illumination light beam reflected by the half prism 4 
forms the intermediate image of a reticle 3 through the projection lens 
24. 
In this case, a light component of the illumination light beam reflected by 
the reflecting mirror 20, which is returned to the half prism 4 through 
the projection lens 24 again and transmitted through the half prism 4, and 
a light component reflected by a reflecting mirror 6, returned to the half 
prism 4 through the projection lens 5, and reflected by the half prism 4 
form the pattern image of the reticle 3 on a surface 7a of the wafer 7. 
The function of the reflecting mirror 6 is the same as that in the second 
embodiment, and the function of the reflecting mirror 20 is the same as 
that of the reflecting mirror 6. 
In this embodiment, the second projection lens 24 is arranged on the 
optical axis of the light beam reflected and split by the half prism 4 
while the second reflecting mirror 20 is arranged near the image plane. 
For this reason, the light beam lost in the second embodiment can be 
effectively used. As a result, in this embodiment, a light amount about 
twice that in the second embodiment can be obtained on the surface of the 
wafer 7. In addition, with this arrangement, the influence of dust or 
flaws on the reflecting mirrors 6 and 20 can be minimized, as in the 
embodiment shown in FIG. 6. Furthermore, when an offset is provided along 
the optical axis of the projection lens in correspondence with the 
position of the reflecting mirror 6 or 20, the depth of focus can be 
substantially increased. 
Since the half prism 4 is used in the embodiment shown in FIG. 7, a loss of 
about 50% is caused in light amount by the half prism 4. To prevent this, 
a polarization beam splitter 16 may be arranged in place of the half prism 
4, a first .lambda./4 plate 15A may be arranged between the projection 
lens 5 and the reflecting mirror 6, and a second .lambda./4 plate 15B may 
be arranged between the projection lens 24 and the reflecting mirror 20. 
With this arrangement, the loss in light amount, which is caused by 
optical path branching in the polarization beam splitter 16 can be 
decreased to almost zero. 
The .lambda./4 plates 15A and 15B can be inserted anywhere in the optical 
path as far as they are located on the intermediate image side with 
respect to the polarization beam splitter 16. However, to obtain the 
effect of removing ghost light in the projection lenses 5 and 24, the 
.lambda./4 plates 15A and 15B are preferably inserted on the intermediate 
image side with respect to the projection lenses 5 and 24, respectively. 
In each of the above embodiments, the final imaging plane is almost 
conjugate with the reflecting mirror. For this reason, in, e.g., FIG. 1, 
dust or flaws on the reflecting surface of the reflecting mirror 6 tend to 
be transferred onto the surface of the wafer 7, which corresponds to the 
final image formation plane. The reflecting mirror 6 may be arranged with 
an offset with respect to the final imaging plane along the optical axis 
(direction of axis Z1) within a range not to adversely affect the 
aberrations and the field variable effect. With this arrangement, the 
influence of dust or flaws can be minimized. 
In addition, in the embodiment shown in FIG. 2, the oblique incident type 
multipoint surface shape detection mechanism is used to control the shape 
of the surface 6a of the reflecting mirror 6 in a closed-loop system. 
However, since the extension/contraction amounts of the piezoelectric 
elements 9A to 9E are almost the same as the displacement amount of the 
surface 6a, the shape of the surface 6a may be controlled in an open-loop 
system. 
Furthermore, the illumination light beam passing through the reticle 3 may 
be detected by an image point detection unit arranged at the position of 
the final imaging plane (surface of the wafer 7), thereby obtaining the 
shape of the image formation plane. With this arrangement, the shape of 
the reflecting surface of the reflecting mirror 6 may be controlled by 
closed-loop control in which a signal detected by the image point 
detection unit is fed back to a surface shape control unit. 
In the embodiment shown in FIG. 1, the shape of the imaging plane by the 
projection lens 5 and the half prism 4 can be set to an arbitrary curved 
surface in accordance with the shape of the reflecting surface 6a of the 
reflecting mirror 6. Therefore, when the pattern of the reticle 3 is to be 
exposed onto not the wafer 7 but a target transfer object having a surface 
shape other than a planar shape, e.g., a wafer having a spherical surface 
(spherical wafer), or a spherical liquid crystal substrate having a 
spherical surface, the shape of the reflecting surface 6a is set to a 
corresponding shape. With this operation, the pattern of the reticle 3 can 
be exposed onto the entire surface of the target transfer object at a high 
resolution. 
As has been described above, the projection optical apparatus of the 
present invention can be applied to a projection exposure apparatus, and 
particularly, to a so-called stepper for performing projection and 
exposure by a step-and-repeat method. A projection exposure apparatus is 
described in, e.g., U.S. Ser. No. 08/397,506 in detail. The U.S. Ser. No. 
08/397,506 is incorporated in this application as a reference as far as 
the projection exposure apparatus is concerned. 
From the invention thus described, it will be obvious that the invention 
may be varied in many ways. Such variations are not to be regarded as a 
departure from the spirit and scope of the invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
to be included within the scope of the following claims. 
The basic Japanese Application No.127480/1994 filed on Jun. 9, 1994 is 
hereby incorporated by reference.