Scanning projection optical device

A scanning projection optical device is disclosed which can be constructed using relatively small optical members, which employs a catadioptric system that is advantageous to attain a high NA, and which has arc-shaped object and image regions located out of an optical axis and a projecting magnification of 1. In the scanning projection optical device, the catadioptric system is an optical system telecentric in both object- and image- field sections, and has a first, positive power lens-system, a second, substantially non-power lens-system, and a concave mirror; the first, positive power lens-system and the concave mirror cooperatively satisfy Petzval condition to correct field curvature; and without impairing the Petzval condition satisfied by the first lens-system and the concave mirror, the second lens-system corrects aberration of the arc-shaped region centering around the optical axis.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a scanning projection optical device 
having a broad field and projecting with high resolving power. 
2. Description of the Prior Art 
As a representative example of conventional projection optical systems 
projecting with high resolving power, there has been known a stepper which 
adopts a step-exposure mode which comprises, by means of a dioptric 
system, dividing an object whose image is to be projected into a plurality 
of sections, and sequentially subjecting each of the sections to exposure 
to effect projection. Such a stepper is capable of scaling up and down and 
enables a high numerical aperture (NA) to be attained. The step-exposure 
mode is a mode which effects exposure by means of one dioptric projection 
lens. Such a dioptric projection lens is designed to have a magnifying 
power of projection of 1 so as to ensure a broad field, i.e., a broad 
exposure area. 
As regards the projection lens, however, an area acceptable in abaxial 
aberration is restricted to an extent of 150.times.150 mm.sup.2 around the 
optical axis. Accordingly, the projection lens is unsatisfactory when used 
to expose a field industrially desired, for example, a panel of 10.4 
inches. In this case, an exposure area having shorter sides of 10.4 inches 
is divided into quarters as shown in FIG. 14, or an exposure area having 
shorter sides of 6 inches is divided into halves as shown in FIG. 15, and 
each of the resulting divisions is sequentially subjected to exposure to 
effect projection. In FIGS. 14 and 15, the divisions A to D (or A to B) 
are masks (whose images are to be projected) which may be different from 
each other. Therefore, use of this mode enables a broad field to be 
exposed by employing a large scanning stroke of a scanning table. 
On the other hand, the step-exposure mode has problems in that masks must 
be replaced exposure to exposure, and that joints in the exposure area are 
unwantedly awkward. The joint-related problem can be solved to some extent 
by improving precision in junctional alignment between the exposure 
divisions during exposure. However, in view of prospective tendency of 
crystal panels to grow more precise and larger, joints in the exposure 
area will be likely to be unwantedly awkward, thereby making difficulty in 
production of large-sized liquid crystal panel displays. Further, exposure 
must be conducted mask-to-mask, leading to a prolonged exposure time, 
namely, lowered throughput. This is undesirable from the viewpoint of 
operational efficiency. 
As a mode to solve the drawback of the stepper mode, a scanning catoptric 
projection mode having high resolving power and a broad field has been 
known. An optical system employing a scanning catoptric projection mode is 
disclosed in Japanese Patent Publication No. 51083/1982 (U.S. Pat. No. 
3,748,015), and the optical system comprises a concave mirror 100 and a 
convex mirror 102 which are oppositely disposed as shown in FIG. 11. In 
this optical system, an object and its image are symmetrically positioned 
relative to an optical axis 104, and an annular aberration-corrected 
region 106 in which aberration is corrected is formed around the optical 
axis 104. A beam used for projection is only a beam which passes through 
the aberration-corrected region 106 limited by a field stop (not shown) in 
a lighting system located at a position conjugate to the position of the 
object. Scanning is conducted by synchronously moving a projective object 
located at the object-position and a sensitized material at the imaging 
position, relative to the projection system. 
The catoptric projection mode is characterized in that the 
aberration-corrected region 106 located at a certain level from the 
optical axis is used as shown in FIG. 11. The aberration-corrected region 
106 is an arc-shaped region as shown in FIG. 12, and the position thereof 
is univocally determined by curvature radii of the two mirrors 100, 102 
and configurational condition thereof. Practically, larger curvature radii 
of the concave and convex mirrors are rather employed to attain a lager 
arc-width of exposure. 
When a 10.4-inch panel as shown in FIG. 14 is exposed in the catoptric 
projection mode, a span l of the arc shown in FIG. 12 is selected in the 
direction of the shorter side of 10.4 inches. In this case, however, the 
panel is exposed only in the arc-width W in the longitudinal direction. 
Accordingly, the mask and the substrate (sensitized material) are 
synchronized to expose the entire area of the panel. 
According to this mode, since a mask need not be replaced and the problem 
in the stepper mode caused by the joints between the masks is eliminated, 
highly precise exposure can be attained, thereby enabling a large-sized 
liquid crystal panel to be produced. 
To deal with a prospective liquid crystal panel with high precision which 
has a large picture plane of 10.4 inches or more, the stepper mode has no 
choice but to increase number of divisions because of limited exposure 
area of up to about 150.times.150 mm2. It follows that joints between 
masks are likely to be more attractive unwantedly, and that throughput 
tends to decrease. 
On the other hand, the scanning catoptric projection mode is free from the 
two problems of "joints between masks" and "lowering in throughput due to 
replacement of masks" which are predictable with respect to the stepper 
mode, and is capable of coping with the demand for a broad field with high 
resolution. Thus, it is the practical point of the matter how to realize 
an optical system designed to have a longer arc-span l and a larger 
arc-width W. 
Among conventional techniques, a dioptric system has been used as a high 
resolution system, and a catoptric system has been used as a broad field 
system. 
The latter optical system to attain exposure in a broad field has a problem 
in that the system is large in size. The reason for the large size of the 
catoptric system to attain exposure in a broad field is as follows. The 
diameter of the concave mirror as a large-sized reflecting member shown in 
FIG. 11 is determined by the curvature radius of the concave surface of 
the concave mirror, height of an image, and numerical aperture (NA) of 
optical system per se. A beam directed into this catoptric system 
divergingly travels from the object-position 106 toward the concave mirror 
100 at NA. In this case, the distance L between the concave mirror 100 as 
a first optical member and object-position (image-position) 106 is the 
full length of the optical system. It follows as an inevitable consequence 
that the size of the optical system is large. Besides, when a larger 
arc-width W is intended, the curvature radius of the concave mirror tends 
to be large, thereby leading to a further large size. Therefore, when the 
catoptric system is employed, technical problem resides in how to produce 
such a large-sized concave mirror 100 with high precision and how to 
maintain the precision. 
On the other hand, Consideration will be given to influences of distortions 
of the catoptric member and the dioptric member on aberration. 
Aberrations caused by maximum distortions Pv of optical members are: 
2.times.Pv--with respect to distortion of the catoptric member, and 
(n-1).times.Pv--with respect to distortion of the dioptric member wherein n 
represents a refractive index of the lens and generally n=1.5. 
Therefore, aberration caused by the dioptric member is (0.5.times.Pv) which 
corresponds to 1/4 of the influence caused by the catoptric member. 
Consequently, it is advantageous for attaining broad field exposure with a 
large-sized optical member to use a dioptric member. 
Further, if a catoptric member were used to attain a high NA, it would be 
impossible to attain a high NA because the beam is shaded by the convex 
mirror 102 in the catoptric system shown in FIG. 11. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the above-mentioned 
problems, and therefore, it is an object of the present invention to 
provide a scanning projection optical device which can be constructed 
using relatively small optical members, which employs a catadioptric 
system that is advantageous to attain a high NA, and which has arc-shaped 
object and image regions located out of an optical axis and a projecting 
magnification of 1. 
It is a further object of the present invention to provide a scanning 
projection optical device which is capable of readily correcting Seidel's 
five aberrations and chromatic aberration, which can advantageously be 
used for exposure of, e.g., a 20-inch flat panel display that requires a 
resolving power of 278 mm, i.e., a resolving power as high as a several 
.mu.m. 
According to the present invention, there is provided scanning projection 
optical device, which projects an object by scanning with a predetermined 
scanning width, comprising: 
an illuminator for illuminating the object upon an arc-shaped region, 
a catadioptric system for forming on an image plane an image of the object 
illuminated on the arc-shaped region by the illuminator, 
photosensitive members located in the object plane and the image plane, and 
a driving device for relatively moving the illuminator and the catadioptric 
system in the direction perpendicular to the direction of the 
illumination, 
wherein said catadioptric system is an optical system telecentric in both 
object- and image- field sections, and has a first, positive power 
lens-system, a second, substantially non-power lens-system, and a concave 
mirror; 
said first, positive power lens-system and said concave mirror 
cooperatively satisfy Petzval condition to correct field curvature; and 
without impairing the Petzcal condition satisfied by the first lens-system 
and the concave mirror, said second lens-system corrects aberration of the 
arc-shaped region centering around the optical axis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the catadioptric shown in FIG. 2 which comprises a thin lens and a 
spherical mirror, it is required for forming a telecentric system that 
when the incident angle u0 of the principal ray entering into the thin 
lens 10 having a focal length of f1 is 0, the incident level of the 
principal ray h2 impinging upon the spherical mirror 12 having a focal 
length of f4 is 0. From this, the distance d1 between the thin lens 10 and 
the spherical mirror 12 is f1, i.e., d1=f1. 
On the other hand, to set the Petzval sum to be 0 to correct field 
curvature, the following relation is required to be satisfied. 
EQU .SIGMA.P.upsilon.=1f1n1+1/f4+1/f1n1=2/f1n1+1/f4=O (1) 
Taking into consideration the fact that the focal length f4 of the 
spherical mirror 12 is represented by the formula f4=r4/2 wherein r4 is 
the curvature radius of the spherical mirror 12, the curvature radius r4 
of the spherical mirror is as follows. 
EQU r4=-n1f1 (2) 
From the equation (2), the spherical mirror 12 is found to be a concave 
mirror. 
Then, the object distance s1 will be found. First, from the conjugate 
relationship between the object-position and the image-position of the 
spherical mirror 12, the following relation is satisfied. 
s2=s2'=r4 
From FIG. 2, the image distance s1 of the thin lens is as follows. 
s1'=r4+f1 
Taking the equation (2) into consideration, the following relation is 
obtained. 
EQU s1'=(1-n1)f1 (3) 
From the imagery relationship by the thin lens having a focal length of f1, 
the following relation is provided. 
EQU 1/s1=1/s1'-1/f1 (4) 
Finally, the following relation is obtained from the equations (3) and (4). 
EQU s1=(1-n1)f1/n1 (5) 
The equation (5) is obtained on the basis of the three conditions that the 
object field section and the image field section are telecentric, that the 
magnification is 1, and that no image curvature is caused. 
In the equation (5), s1 represents the working distance WD in the optical 
system. A working distance WD is a very important factor for an optical 
system and is required to exceed a given value according to the optical 
system. Therefore, once a working distance WD is given in designing of an 
optical system, then the focal length f1 of the thin lens 10 is determined 
from the equation (5). 
In the basic structure of the present invention, as shown in FIG. 3, the 
concave mirror 14 is miniaturized as compared with the concave mirror 16 
for a conventional broad field catoptric system. Attendantly, it is seen 
that the first lens system 1 and second lens system 6 of the optical 
system according to the present invention are markedly miniaturized as 
compared with the concave mirror 16 which is a main component of a 
conventional broad field catoptric system. 
According to the above description on the optical system shown in FIG. 2, 
it is found that only the field curvature can be corrected among 
aberrations in the optical system consisting of a lens and a spherical 
mirror. In the present invention, to attain a desired resolving power, 
Seidel's five aberrations and chromatic aberration are corrected as well 
as the field curvature in the aberration-free area of the optical system. 
To carry out this, there may be mentioned possible three methods which 
comprises, in an optical system which satisfies the abovementioned 
equations (2) and (5), 1 using an increased number of thin lenses instead 
of the thin lens 10 having a focal length of f1, and correcting 
aberrations of the optical system as a whole, 2 using a first lens system 
having substantially the same power as the thin lens 10 and a second lens 
system having a power instead of the thin lens 10 having a focal length of 
f1, and correcting aberrations of the optical system as a whole, and 3 
using a first lens system having substantially the same power as the thin 
lens 10 and a second lens system having no power instead of the thin lens 
10 having a focal length of f1, and correcting aberrations of the optical 
system as a whole. 
In the first method, due to the condition that both the object field 
section and the image field section of the optical system are telecentric, 
and the lens system consisting of the increased number of the lenses in 
the optical system, which corresponds to the thin lens 10, has an aperture 
larger than each of object and image regions which are aberration-free. 
Accordingly, this method is disadvantageous in that an increased number of 
lenses having a large aperture are used. 
In the second method, the following three cases may be mentioned depending 
upon signs (positive and negative) of powers of the first and second lens 
systems. 
______________________________________ 
first lens system 
second lens system 
______________________________________ 
case 1 positive positive 
case 2 positive negative 
case 3 negative positive 
______________________________________ 
In the case 3 where the first lens system has a negative power, the second 
lens system inevitably has a high power and an undesirably large aperture. 
The optical system according to the present invention includes the cases 1 
and 2, and in particular, provides a solution very close to one in the 
above third method 3. In other words, the optical system 
characteristically comprises a first lens-system having a positive power, 
a second lens-system having no substantial power, and a concave mirror. 
Then, explanation will be given on the fact that a case where the second 
lens-system has a low power is optimum. Criteria for evaluating 
performance of a lens include degree of geometrical aberration, modulation 
transfer function (MTF) which are well-known. However, evaluation based on 
the degree of wavefront aberration which is intelligible is employed for 
the convenience of explanation. In the evaluation, as generally known, RMS 
(root mean square) value of the degree of wavefront aberration is used. 
With respect to the evaluation using RMS value, a region in which Malatial 
condition: 
EQU RMS.ltoreq.0.07 .lambda.=(.lambda./14) (.lambda.: wavelength) 
is satisfied as a condition for a perfect lens is regarded as an 
aberration-free region. 
As a model for the evaluation, one as shown in FIG. 1 is used which 
comprises a convex lens 2 and a concave lens 3 as the second lens-system 6 
located away from the first lens-system to avoid use of any lens having an 
undesirably large aperture. 
A normal image height level of 220 mm is selected, and it is examined how 
broad extent can be regarded as a perfect lens around the normal level, 
namely, how large arc-width W can be attained. FIG. 4 shows curves of RMS 
values of wavefront aberrations, in which the focal length f23 of the 
second lens-system 6 is changed. The abscissa axis represents an image 
height and the ordinate axis represents RMS value of wavefront aberration. 
It is found that as the focal length f23 of the second lens-system 6 
becomes short from 10,000 mm to 2,500 mm, the RMS value of the wavefront 
aberration becomes large. The minimum of the RMS value of the wavefront 
aberration is 0.07 .lambda. when the focal length f23 is 3,000 mm. 
Accordingly, when the focal length f23 is positive, it must be 3,000 mm or 
more. FIG. 5 is a graphical representation showing curves similar to those 
in FIG. 4, in which each of the focal lengths f23 of the second 
lens-system 6 has a negative value. As the focal length f23 of the second 
lens-system 6 becomes large from -10,000 mm to -5,000 mm, the RMS value of 
the wavefront aberration becomes large. The minimum of the RMS value of 
the wavefront aberration is 0.07 .lambda. when the focal length f23 is 
-9,000 mm. Accordingly, when the focal length f23 is negative, it must be 
-9,000 mm or less. 
It is understood from the results of the above evaluation that in the 
optical system according to the present invention which comprises the 
first lens-system having a positive power, the second lens-system having 
no substantial power and the concave mirror; the power of the second 
lens-system, which is a reciprocal number of the focal length of the 
second lens-system, is required to satisfy the following condition: 
EQU -1.1.times.10.sup.-4 &lt;1/f23&lt;3.3.times.10.sup.-4 (6) 
In view of the fact that the second lens-system, which satisfies the above 
condition, exhibits no substantial power, the equation (1) should be 
satisfied substantially. In other words, the following relation: 
EQU f1/f4 s.apprxeq.-2/n1 
Should be satisfied between the focal lengths of the first lens-system and 
the concave mirror in accordance with the equation (1). 
Taking into consideration the negligible power of the second lens-system, 
this relation is reformulated into the following relation: 
EQU -1.4&lt;f1/f4 &lt;-1.0 (7) 
When the second lens-system is composed of two lenses, it is apparent that 
the second lens-system is usually composed of one positive lens and one 
negative lens. Taking it into consideration that the second lens-system 
has no substantial power, it is readily understood that the following 
relation is satisfied. 
.vertline.f2.vertline..apprxeq..vertline.f3.vertline. 
EXAMPLE 1 
The first embodiment of the projection optical device of the present 
invention is a circular scanner which scans an arc-shaped image. An 
optical system of the projection optical device 1 comprises a light source 
30, a condenser lens 31 for collecting a beam from the light source 30, a 
diaphragm 32 having an arc-shaped aperture, a relay lens 33 for forming an 
image of the diaphragm aperture on an original mask sheet 34, and a first 
reflecting mirror 35, which are disposed on a first optical axis 36, as 
shown in FIG. 1. The beam from the light source 30 is collimated to 
illuminate the diaphragm 32 having the arc-shaped aperture around the 
first optical axis 36. The relay lens 33 forms an image of the diaphragm 
aperture on the original mask sheet 34. 
On a second optical axis 37 which is the optical axis of the beam reflected 
from the first reflecting mirror 35 disposed on the first optical axis 36, 
a convex lens 1, a convex lens 2, a concave lens 3 and a concave mirror 8 
comprising a catadioptric system 9 are disposed. The beam from the 
arc-shaped illuminated portion of he original mask sheet 34 is reflected 
by the first reflecting mirror 35, and caused to pass through a first 
lens-system composed of the convex lens 1 and a second lens-system 
composed of the convex lens 2 and concave lens 3, and then reflected by 
the concave mirror 8. The beam reflected from the concave mirror 8 is 
caused to repass through the second lens-system and the first lens-system, 
and then reflected by a second reflecting mirror 38. 
On a third optical axis 39 which is the optical axis of the beam reflected 
from the second reflecting mirror 38 disposed on the second optical axis 
37, a substrate 40 having photosensitivity is disposed. The beam reflected 
by the second reflecting mirror 38 forms an arc-shaped image around the 
third optical axis 39 on the substrate 40. 
The original mask sheet 34 and the substrate 40 are synchronously moved by 
a driving device 41 in the direction perpendicular to the first optical 
axis 36 and the third optical axis 39, thereby effecting scanning 
projection-exposure in the area of the span of the arc x the distance of 
the movement. 
In the above-described first embodiment of the scanning projection optical 
device, there are represented a focal length of the convex lens 1 by f1, a 
focal length of the convex lens 2 by f2, a focal length of the concave 
lens by f3, a focal length of the lens system 6 by f23, a focal length of 
the concave mirror 8 by f4, and the overall focal length by f. The 
catadioptric broad field exposure system is a symmetrical bilateral 
telecentric optical system in which the position of the concave mirror is 
a pupil position. 
A gamma ray-wavelength, a magnification of 1, a NA of 0.12, and an 
arc-diameter of 220 mm are used. 
When the following representations are made 
r1-r7: curvature radius (mm) 
d1-d6: lens thickness, or distance between lenses (mm) 
n1-n3: refractive index of lens at a g ray-wavelength 
.upsilon. e1-.upsilon. e3: Abbe constant of lens, these factors are 
specified as follows: 
______________________________________ 
r1 = -4013.12 d1 = 90 n1 = 1.526214 
r2 = -655.92 d2 = 700 .nu. el = 63.9 
r3 = -14455.47 d3 = 70 n2 = 1.729443 
r4 = -1406.74 d4 = 100 .nu. e2 = 53.6 
r5 = -895.39 d5 = 40 n3 = 1.599644 
r6 = -3079.84 d6 = 380 .nu. e3 = 40.5 
r7 = -2365.56 
______________________________________ 
wherein the surfaces of r1 and r2 define the convex lens 1, the surfaces of 
r3 and r4 define the convex lens 2, the surfaces of r5 and r6 define the 
concave lens 3, and the surface of r7 defines the refractive surface of 
the concave mirror 8. 
FIG. 6 shows astigmatism of the first embodiment of the scanning projection 
optical device. It is seen from FIG. 6 that, in the vicinity of the used 
image height of 220 mm, the astigmatism becomes 0 and hence the image 
plane is perpendicular to the optical axis without field tilt. FIG. 7 
shows a transverse aberration curve at the image height of 200 mm. It is 
seen from FIG. 7 that the transverse aberration is also corrected. 
To evaluate correction of the aberration, as shown in FIG. 8, the image 
height is plotted as abscissa and the RMS value as ordinate. In FIG. 8, 
the curve A shows the results on the present invention. When a region 
exhibiting an RMS value of wavefront aberration of 0.07 .lambda. or less 
is regarded as the aberration-free region, it corresponds to the range of 
207 mm to 234 mm in terms of the image height. Accordingly, an arc-width 
of up to 27 mm is attainable. 
On the other hand, in FIG. 8 by the curve B, there is shown aberration 
caused by an optical system using a conventional mode (catoptric mode) 
which is so constructed as to have the same full length as that of the 
first embodiment of the present invention, namely, one comprising two 
mirrors, as shown by a line B in FIG. 11, one of which is a concave mirror 
having a curvature radius of 2,000 mm. According to the conventional mode, 
when a region exhibiting an RMS value of wavefront aberration of 0.07 
.lambda. or less is regarded as the aberration-free region, it has a width 
of 6 mm. This is far inferior to the result of the first embodiment. As 
opposed to the optical system using the conventional mode, the scanning 
projection optical system comprises a convex lens 1 having a focal length 
of f1, a second lens-system 6 having a focal length of f23 which is 
composed of a convex lens having a focal length of f2 and a concave lens 
having a focal length of f3, and a concave mirror 8 having a focal length 
of f4, and the first embodiment has an overall focal length of f. The 
catadioptric broad field exposure system is a symmetrical bilateral 
telecentric optical system in which the position of the concave mirror is 
a pupil position. An arc-shaped region centering around the optical axis 
is illuminated as a projective object, and an arc-shaped region 
substantially centering around the optical axis is used as an image. 
It is, therefore, understood that if the optical system using a 
conventional mode were used to attain substantially the same width of an 
aberration-free region as that attained by the first embodiment, the 
concave mirror would be required to have a curvature radius of 3,000 to 
4,000 mm, and consequently, the conventional optical device would 
inevitably have an undesirably large size. 
Incidentally, in the first embodiment, 
1/f23=1.63.times.10.sup.-5 
f1/f4=-1.248, 
f1=1,476 mm, 
f2=2,132 mm, 
f3-2,120 mm, 
f23=61,456 mm, 
f4=-1,183 mm, and 
f=-5,043 mm. 
From this, the relations (6), (7) and (8) are found to be satisfied. 
EXAMPLE 2 
A gamma ray-wavelength, a magnification of 1, a NA of 0.12, and an 
arc-diameter of 220 mm are used. 
The same representations as in Example 1 are made. 
r1-r7: curvature radius (mm) 
d1-d6: lens thickness, or distance between lenses (mm) 
n1-n3: refractive index of lens at a g ray-wavelength 
.upsilon. e1-.upsilon. e3: Abbe constant of lens 
Then, these factors are specified as follows: 
______________________________________ 
r1 = -2142.44 d1 = 90 n1 = 1.526214 
r2 = -590.59 d2 = 720 .nu. e1 = 63.9 
r3 = .infin. d3 = 80 n2 = 1.526214 
r4 = -1663.10 d4 = 120 .nu. e2 = 63.9 
r5 = -825.35 d5 = 60 n3 = 1.584217 
r6 = -1535.12 d6 = 330 .nu. e3 = 42.5 
r7 = -2310.14. 
______________________________________ 
The astigmatism curve of the second embodiment is substantially the same as 
that of the first embodiment shown in FIG. 6. In the vicinity of the used 
image height of 220 mm, the astigmatism becomes 0 and hence the image 
plane is perpendicular to the optical axis without field tilt. FIG. 9 is a 
graph showing a curve of the RMS value of the wavefront aberration caused 
by the second embodiment. It is understood from this graph that a 
sufficient aberration-free region (arc-width) can be attained. 
Incidentally, in the second embodiment, 
1/f23=6.81.times.10.sup.-6, 
f1/f4=-1.315, 
f1=1,519 mm, 
f2=3,161 mm, 
f3=-4,154 mm, 
f23=146,840 mm, 
f4=-1,155 nm, and 
f=-4,068 mm. 
Also in the second embodiment, the relations (6), (7) and (8) are found to 
be satisfied. 
In the above-described first and second embodiments, 
NA=0.12, 
image height=220 mm, and 
width of aberration-free region=27 mm. 
The resolving power is .pi./2NA=1.8 .mu.m. With respect to the field size, 
a span of the arc of 400 mm is attainable. Therefore, when a scanning 
length ranging over 600 mm is employed, it is possible to carry out 
continuous exposure covering a field as broad as 400 mm.times.600 mm. The 
field of 400 mm.times.600 mm enables 4 sheets, 2 sheets, and 1 sheet to be 
continuously exposed in terms of 10.4-inch mask, 16-inch mask, and 23-inch 
mask, respectively. 
EXAMPLE 3 
A gamma ray-wavelength, a magnification of 1, a NA of 0.12, and an 
arc-diameter of 220 mm are used. 
The same representations as in Example 1 are made. 
r1-r7: curvature radius (mm) 
d1-d6: lens thickness, or distance between lenses (mm) 
n1-n3: refractive index of lens at a g ray-wavelength 
e1-e3: Abbe constant of lens 
Then, these factors are specified as follows: 
______________________________________ 
r1 = -2949.42 d1 = 95 n1 = 1.526214 
r2 = -588.29 d2 = 608 .nu. el = 63.9 
r3 = -4822.14 d3 = 70 n2 = 1.526214 
r4 = -1471.79 d4 = 251 .nu. e2 = 63.9 
r5 = -744.07 d5 = 62 n3 = 1.534790 
r6 = -1173.27 d6 = 267 .nu. e3 = 40.5 
r7 = -2099.11. 
______________________________________ 
The astigmatism curve of the third embodiment is substantially the same as 
that of the first embodiment shown in FIG. 6. It is seen that in the 
vicinity of the used image height of 220 mm, the astigmatism becomes and 
hence the image plane is perpendicular to the optical axis without field 
tilt. FIG. 10 is a graph showing a curve of the RMS value of the wavefront 
aberration caused by the third embodiment. It is understood from this 
graph that a sufficient aberration-free region (arc-width) can be 
attained. 
Incidentally, in the third embodiment, 
1/f23=-1.72.times.10.sup.-5, 
f1/f4=-1.31, 
f1=1,376 mm, 
f2=3,992 mm, 
f3=-3,586 mm, 
f23=-58,165 mm, 
f4=-1,050 mm, and 
f=-6,274 mm. 
Also in the third embodiment, the relations (6), (7) and (8) are found to 
be satisfied. 
In the above-described third embodiment, 
NA=0.12, 
image height=220 mm, and 
width of aberration-free region=16.3 mm. 
The width of the aberration-free region is slightly narrow as compared with 
those in the first and second embodiment; however, other conditions are 
comparable. It is, therefore, possible to carry out continuous exposure 
covering substantially the same field size as previously mentioned with 
respect to the first and second embodiments. 
According to the present invention, there is provided a scanning projection 
optical device which can be constructed using relatively small optical 
members, which employs a catadioptric system that is advantageous to 
attain a high NA, and which has arc-shaped object and image regions 
located out of an optical axis and a projecting magnification of 1. 
According further to the present invention, there is provided a scanning 
projection optical device which is capable of readily correcting Seidel's 
five aberrations and chromatic aberration, which can advantageously be 
used for exposure of, e.g., a 20-inch flat panel display that requires a 
resolving power of 278 mm, i.e., a resolving power as high as a several 
.mu.m.