Single mirror projection optical system

A well corrected ring field catadioptric optical system utilizing refractive elements of the same glass type. A virtual relay corrected for axial and lateral primary color incorporates an air-surface doublet and two field lenses to correct for field aberrations to provide a well corrected ring field virtual image. A spherical mirror is used to make the virtual image accessible.

BACKGROUND OF THE INVENTION 
A ring field optical system is one which provides an annular zone centered 
about the optical axis wherein images formed in the annular zone are 
optically well corrected. Ring field systems have found important use in 
the field of microlithography wherein circuit patterns on a mask in the 
object plane may be projected without distortion onto the sensitized 
surface of a silicon wafer in the image plane. Ring field systems are 
disclosed in general by Scott in U.S. Pat. No. 3,821,763. Ring field 
systems of the type used in microlithography are disclosed by Offner in 
U.S. Pat. Nos. 3,748,015 and 4,293,186. All three patents are assigned to 
the assignee of the present application, and the disclosures of which are 
incorporated by reference herein. Both of the patents used in 
microlithography disclose an off-axis annular field optical system for 
forming in accurate micro detail an image of an object at unit 
magnification with high resolution. Each employs the concept of convex and 
concave mirrors in face-to-face relationship with their centers of 
curvature being more or less concentric. Both systems and particularly the 
latter were designed to function over a relatively wide radiation 
bandwidth, i.e., each system remains reasonably well corrected over a wide 
spectral band. 
As microlithography has developed more and more circuits are being packed 
into each integrated circuit. This requires that the lines of the circuit 
patterns used in forming such circuits be exceedingly small, i.e. of the 
order of one micron or less. As is well known this requires use of the 
lower end of the UV spectrum since it is at this end that image resolution 
or sharpness is greatest. Recognizing this the industry is now utilizing 
nearly monochromatic light in the deep UV as the radiation source in mask 
projection systems. Such sources may be provided by a single line of a 
mercury lamp or lasers such as the excimer laser. 
The present invention relates to a ring field optical system designed to 
function with such monochromatic radiation sources. In addition, it 
provides demagnification of the object at the image plane. 
BRIEF SUMMARY OF THE INVENTION 
The present invention relates to a ring field catadioptric optical system 
which utilizes refractive elements all of the same glass type to form a 
well corrected ring field virtual image. A spherical mirror is used to 
make the virtual image accessible. 
In carrying out the present invention an achromat of the Schupmann (Refer 
R. Kingslake "Lens Design Fundamentals", Academic Press 1978 Page 89) 
variety having a field lens positioned to correct for lateral color is 
compounded to provide a virtual relay which is configured in such a manner 
as to correct not only axial and lateral color but also the third and 
fifth order spherical aberration and coma as well as the chromatic 
variation of third and fifth order spherical aberration and coma. The 
virtual image of the relay is made accessible by a spherical mirror 
positioned to be nearly concentric with the virtual image. The virtual 
object is made accessible by the addition of refractive power in object 
space. The field aberrations of astigmatism, distortion, curvature of 
field and telecentricity in both object and image space are corrected by 
the incorporation of an airspaced doublet in object space and two field 
lenses, one located near the object and the other near the image. Object 
and image may be made accessible for use in a microlithograph projection 
system by the further addition of two fold mirrors. 
The design of the present invention is well corrected over a bandwidth of 
10A which is much wider than can be achieved with conventional refractive 
types such as the double gauss or Petzval of all the same glass type which 
would have a band width of 0.1A. In the deep UV only one practical glass 
type is available; that is fused silica. With this single glass type 
available the axial color can not be corrected thus resulting in a very 
narrow band width. In the herein described design the axial and lateral 
color are corrected even though a single glass type is used.

DESCRIPTION 
Referring now to FIG. 1 there is shown an achromat which essentially 
comprises a positive lens 11 and a negative lens 13 both of the same glass 
type. Such a system which corrects for axial color is known as the 
Schupmann achromat and is the only known acromat that utilizes a single 
glass type. Before the Shupmann design the only known refractive acromats 
required two different types of glass, i.e. a positive element of crown 
glass and a negative element of flint glass both of appropriate power. 
In FIG. 1 a field lens 12 is added and positioned to cause the chief ray 
through the center of the positive lens 11 to go through the center of the 
negative lens 13. This corrects for primary lateral color which the two 
element Schupmann achromat does not. 
FIG. 2 illustrates a virtual relay 14. In addition to correcting for axial 
and lateral color the virtual relay 14 of FIG. 2 corrects for third and 
fifth order spherical aberration and coma, as well as the chromatic 
variation of third and fifth order spherical aberration and coma. Virtual 
relay 14 is derived by compounding the elements of the positive lens 11 
and the field element 12 of the Schupmann design of FIG. 1. 
Thus, virtual relay 14 comprises three positive lenses 11A, 11B and 11C, 
the compounded positive lens, two field lenses 12A and 12B and the 
negative lens 13. A field stop is disposed just after lens 11C. These 
elements are so configured and positioned as to provide the correction 
described above. 
TABLE I is an example of construction data for the virtual relay 14 of FIG. 
2 which follows: 
TABLE I 
______________________________________ 
0.4X VIRTUAL RELAY 
N.A. AT OBJECT 0.1143 
N.A. AT IMAGE 0.2857 
SURFACE DISTANCE TO 
NO. FROM RADIUS NEXT SUR- 
OBJECT (MM) FACE (MM) MATERIAL 
______________________________________ 
0 PLANE -509.3784 AIR 
1 146.8335 40.0000 FUSED SILICA 
2 437.5290 3.0000 AIR 
3 97.8688 25.0000 FUSED SILICA 
4 74.8220 87.5000 AIR 
5 706.1405 5.0000 FUSED SILICA 
6 63.5693 113.9107 AIR 
7 849.0004 20.0000 FUSED SILICA 
8 -125.8509 8.1384 AIR 
9 110.5940 20.0000 FUSED SILICA 
10 -719.2014 276.6173 AIR 
11 -85.6669 10.0000 FUSED SILICA 
12 -197.8643 -150.0000 AIR 
13 INF 0.0 AIR 
______________________________________ 
STOP IS 4.0 MM AFTER SURFACE 6 
The virtual relay 14 while well corrected on axis and for lateral color, is 
not corrected for field aberrations. In addition, it has a virtual object 
and image. The system of FIG. 3 corrects the field aberrations and 
provides a real object and image. This is accomplished in the following 
way. 
By further compounding of the Schupmann positive element to five elements, 
11A, 11B, 11C, 11D, and 11E additional positive power is added thus 
resulting in the virtual object becoming real and accessible. 
A spherical mirror 18 is positioned within the design of FIG. 3 so that it 
is nearly concentric with the virtual image located on the optical axis in 
the image plane thus making the virtual image real and accessible. 
The spherical mirror 18 provides a ring field, i.e. an off axis real image 
of the object that is well corrected within the salt areas of an annular 
ring which is concentric about the optical axis of the system. The well 
corrected ring field is focused at the image plane as shown in FIG. 3. 
It should also be noted that the spherical mirror 18 provides a double of 
the axis color correction of the negative lens 13 since the rays pass 
through this element twice. The additional axial color correction thus 
provided balances the axial color introduced by the additional power of 
the Schupmann positive element. The Schupmann positive element 11 has been 
further compouned and increased in power in order to render the object 
real. 
In order to correct the field aberrations of astigmatism, distortion and 
curvature-of-field and the chromatic variations thereof, as well as the 
telecentricity of both the object and image, an air-spaced doublet 15 and 
field lenses 16 and 17 are added. 
The correction of these aberrations is interrelated such that the 
correction of any particular aberration can not be attributed completely 
to any particular element. It may be said, however, that the astigmatism 
and the chromatic variation thereof is esentially connected by air-spaced 
doublet 15. The power of the field lens 16 essentially corrects the 
curvature-of-field and telecentricity of the object. The bending of the 
field lens 16 essentially corrects the distortion. The field lens 17 
essentially corrects the telecentricity of the image. 
The complete design comprised of the compounded Schupmann positive 
elements, 11A, 11B, 11C, 11D and 11E; both elements of the Schupmann field 
lens 12A and 12B; the Schupmann negative element 13; the sphericl mirror 
18, the air-spaced doublet 15, and the two field lenses 16 and 17 were 
reoptimized as a complete system to achieve the best overall balance of 
the aberrations and the chromatic variations thereof. 
FIG. 4 illustrates an optical system identical to that of FIG. 3 save for 
the addition of two folding mirrors 19 and 20 to make the image more 
accessible and adaptable for use in a microlithography main projection 
system. In addition, a more compact arrangement results. One fold 19 is 
located between the air-spaced doublet 15 and the compounded Schupmann 
positive element 11. The other fold 20 is located between the Schupmann 
field lens 12 and the Schupmann negative lens 13. Thus in this system the 
object in the object plane is reflected by folding mirror 19 and the image 
by fold 20 before undergoing the same reflection from the spherical mirror 
18 as in the system of FIG. 3. The image then appears as a well corrected 
off axis image at image plane 30. 
The surfaces of the elements of the optical systems are numbered from 1 to 
30 in FIG. 4 to ease in understanding prescriptions set forth in TABLES II 
and III for the situations of N.A=0.33 and N.A.=0.40, respectively. FIG. 4 
illustrates the optical system where N.A.=0.33 and the parameters 
therefore shown in more detail in TABLE II. TABLE III illustrates the 
required parameters for the case N.A.=0.40. Naturally, other parameters 
would be required for various other values of the normal aperture N.A. and 
TABLES II and III are given by way of example only. 
TABLE II 
______________________________________ 
SINGLE MIRROR PROJECTION SYSTEM 
N.A. = 0.33 AT IMAGE 
DISTANCE 
SURFACE TO NEXT 
NO. FROM RADIUS SURFACE 
OBJECT (MM) (MM) MATERIAL 
______________________________________ 
0 PLANE 15.0106 AIR 
1 250.7601 20.0000 FUSED SILICA 
2 495.7365 220.1050 AIR 
3 1316.0222 7.0000 FUSED SILICA 
4 256.8690 17.0000 AIR 
5 2094.2528 20.0000 FUSED SILICA 
6 -440.3327 300.0000 AIR 
7 PLANE -316.9289 AIR 
8 3427.7484 -25.0000 FUSED SILICA 
9 388.2743 -1.0000 AIR 
10 -381.6655 -25.0000 FUSED SILICA 
11 -1439.2312 -136.0369 AIR 
12 -219.7194 -30.0000 FUSED SILICA 
13 -553.6858 -50.0369 AIR 
14 -125.2624 -10.0000 FUSED SILICA 
15 -103.1942 -25.0000 AIR 
16 190.2020 -30.0000 FUSED SILICA 
17 205.2266 -103.7988 AIR 
18 -145.9760 -20.0000 FUSED SILICA 
19 1300.6540 -50.6144 AIR 
20 -101.4453 -20.0000 FUSED SILICA 
21 -561.9992 -72.5311 AIR 
22 PLANE 180.0000 AIR 
23 -111.2179 10.0000 FUSED SILICA 
24 -238.5001 250.0000 AIR 
25 -400.0000 -250.0000 AIR 
26 -238.5001 -10.0000 FUSED SILICA 
27 -111.2179 -185.0000 AIR 
28 479.8622 -10.0000 FUSED SILICA 
29 182.4330 -6.9982 AIR 
30 PLANE AIR 
______________________________________ 
STOP IS 3.5 MM AFTER SURFACE 13 
TABLE III 
______________________________________ 
SINGLE MIRROR PROJECTION SYSTEM 
N.A. = 0.40 AT IMAGE 
DISTANCE 
SURFACE TO NEXT 
NO. FROM RADIUS SURFACE 
OBJECT (MM) (MM) MATERIAL 
______________________________________ 
0 PLANE 15.0034 AIR 
1 191.1886 20.0000 FUSED SILICA 
2 380.6867 156.2513 AIR 
3 -7886.6829 7.0000 FUSED SILICA 
4 193.8456 19.0000 AIR 
5 -1841.8397 20.0000 FUSED SILICA 
6 -288.5379 300.0000 AIR 
7 PLANE -219.4054 AIR 
8 1910.4091 -25.0000 FUSED SILICA 
9 380.8600 -1.0000 AIR 
10 -372.7529 -25.0000 FUSED SILICA 
11 -1055.4615 -259.5532 AIR 
12 -261.0379 -30.0000 FUSED SILICA 
13 -34966.4670 -29.9486 AIR 
14 -113.1974 -10.0000 FUSED SILICA 
15 -100.0095 -32.0000 AIR 
16 229.8248 -30.0000 FUSED SILICA 
17 275.3900 -49.9961 AIR 
18 -190.6421 -20.0000 FUSED SILICA 
19 1336.8166 -91.1971 AIR 
20 -85.2476 -20.0000 FUSED SILICA 
21 -1424.8770 -65.7494 AIR 
22 PLANE 180.0000 AIR 
23 -113.1155 10.0000 FUSED SILICA 
24 -244.2444 150.0000 AIR 
25 -300.0000 - 150.0000 AIR 
26 -244.2444 -10.0000 FUSED SILICA 
27 -113.1155 -185.0000 AIR 
28 -627.9968 -10.0000 FUSED SILICA 
29 165.7158 -6.9995 AIR 
30 PLANE AIR 
______________________________________ 
STOP IS 17.0 MM AFTER SURFACE 13 
TABLES IV and V are also included below. TABLE IV illustrates the 
performance of the ring field optical system of FIG. 4 for the conditions 
N.A.=0.33 and TABLE V illustrates the performance of the ring field 
optical system of FIG. 4 for the condition N.A.=0.40. 
TABLE IV 
______________________________________ 
N.A. AT OBJECT 0.083 
N.A. AT IMAGE 0.33 
RADIUS OF RMS WAVE 
ANNULUS ABERRATION (WAVELENGTH UNITS) 
(MM) AT WAVELENGTH (ANGSTROM UNITS) 
IMAGE 3075A 3080A 3085A 
______________________________________ 
18.75 0.092 0.101 0.114 
19.00 0.076 0.082 0.096 
19.50 0.048 0.048 0.061 
20.00 0.038 0.026 0.035 
20.50 0.059 0.045 0.044 
21.00 0.092 0.081 0.079 
21.25 0.114 0.102 0.100 
______________________________________ 
TABLE V 
______________________________________ 
N.A. AT OBJECT 0.1 
N.A. AT IMAGE 0.40 
RADIUS OF RMS WAVE 
ANNULUS ABERRATION (WAVELENGTH UNITS) 
(MM) AT WAVELENGTH (ANGSTROM UNITS) 
IMAGE 3075A 3080A 3085A 
______________________________________ 
19.0 0.125 0.136 0.149 
19.5 0.074 0.082 0.087 
20.0 0.033 0.052 0.055 
20.5 0.085 0.080 0.076 
21.0 0.142 0.137 0.134 
______________________________________ 
While not limited to such the ring field optical system of FIGS. 3 and 4 
have a magnification of 0.25. However, other magnifications are possible 
and are considered to form a part of this invention. A magnification of 
0.25 is described by way of example only. 
For purposes of clarification, FIG. 5 is a detailed showing of the image 
plane shown in FIG. 3 which illustrates the truncation of element 17 such 
that the rays directed toward mirror 18 do not pass through element 17. 
There has been disclosed a ring field system utilizing refractive elements 
of a single material and a single spherical mirror. The ring field system 
of the present invention which is adaptable for use in mask projection 
systems for the fabrication of integrated circuits and is well connected 
over a bandwidth of 10A based on a single line mercury or excimer laser 
source. 
Other modifications of the present invention are possible in light of the 
above description which should not be construed as placing limitations on 
the invention other than those set forth in the claims which follow.