U.S. patent number 5,212,588 [Application Number 07/682,780] was granted by the patent office on 1993-05-18 for reflective optical imaging system for extreme ultraviolet wavelengths.
This patent grant is currently assigned to The United States of America as represented by the United States. Invention is credited to Brian E. Newnam, Vriddhachalam K. Viswanathan.
United States Patent |
5,212,588 |
Viswanathan , et
al. |
May 18, 1993 |
Reflective optical imaging system for extreme ultraviolet
wavelengths
Abstract
A projection reflection optical system has two mirrors in a
coaxial, four reflection configuration to reproduce the image of an
object. The mirrors have spherical reflection surfaces to provide a
very high resolution of object feature wavelengths less than 200
.mu.m, and preferably less than 100 .mu.m. An image resolution of
features less than 0.05-0.1 .mu.m, is obtained over a large area
field; i.e., 25.4 mm .times.25.4 mm, with a distortion less than
0.1 of the resolution over the image field.
Inventors: |
Viswanathan; Vriddhachalam K.
(Los Alamos, NM), Newnam; Brian E. (Los Alamos, NM) |
Assignee: |
The United States of America as
represented by the United States (Washington, DC)
|
Family
ID: |
24741106 |
Appl.
No.: |
07/682,780 |
Filed: |
April 9, 1991 |
Current U.S.
Class: |
359/355;
359/859 |
Current CPC
Class: |
G02B
13/143 (20130101); G02B 17/061 (20130101); G03F
7/70233 (20130101) |
Current International
Class: |
G02B
13/14 (20060101); G02B 17/06 (20060101); G02B
17/00 (20060101); G03F 7/20 (20060101); G02B
005/10 () |
Field of
Search: |
;359/355,365,366,708,709,726,727,728,729,730,731,858,859,861,868 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
O R. Wood II et al., "Short-Wavelength Annular-Field Optical System
for Imaging Tenth Micron Features," 7 J. Vac. Sci. Technol., No. 6,
pp. 1613-1615, (1989). .
D. A. Markle, "The Future and Potential of Optical Scanning
Systems," Solid State Technology, pp. 159-166 (1984). .
T. E. Jewell et al., "20:1 Projection Soft X-Ray Lithography Using
Tri-Level Resist," 1263 SPIE Electron-Beam, X-Ray, and Ion-Beam
Technology: Submicrometer Lithographies IX, pp. 90-98 (1990). .
S. T. Yang et al., "Effect of Central Obscuration on Image
Formation in Projection Lithography," 1264 SPIE Optical/Laser
Microlithography III, pp. 477-485 (1990). .
J. B. Buckley et al., "Step and Scan: A System Overview of a New
Lithography Tool," 1088 SPIE Optical/Laser Microlithography II, pp.
424-433 (1990). .
B. E. Newnam, "Development of Free-Electron Lasers for XUV
Projection Lithography," 1227 SPIE Free-Electron Lasers and
Applications, pp. 116-133 (1990). .
Communication from Dave Shafer to Brian Newnam dated May 18,
1990..
|
Primary Examiner: Arnold; Bruce Y.
Assistant Examiner: Shafer; R. D.
Attorney, Agent or Firm: Wilson; Ray G. Gaetjens; Paul D.
Moser; William R.
Government Interests
This invention is the result of a contract with the Department of
Energy (Contract No. W-7405-ENG-36).
Claims
What is claimed is:
1. A reflecting optical system including illumination means for
projecting an image of an object, said optical system further
defining an image plane for receiving a reduced image
representation of said object, the improvement comprising:
first and second coaxial aspherical mirrors in partially obscured
arrangement and defining four reflecting surfaces between said
object and said image plane coaxial with said mirrors, said first
and second mirrors being defined by aspherical coefficients
effective to resolve image features of 0.05-0.25 .mu.m with a
telecentricity.ltoreq.1 mrad with distortions less than 0.1 of a
resolution value for said object over an image filed in said image
plane of at least 10 mm .times.10 mm when said illumination means
is a partially coherent photon beam having a wavelength less than
about 100 nm;
wherein said first aspherical mirror receives illumination from
said object and has a conic constant K<-1; and
said second mirror directs illumination onto said image plane and
has a conic constant 0>K>-1.
2. A reflecting optical systems including illumination means for
projecting an image of an object, said optical system further
defining an image plane for receiving a reduced image
representation of said object, the improvement comprising:
first and second coaxial aspherical mirrors in partially obscured
arrangement and defining four reflecting surfaces between said
object and said image plane coaxial with said mirrors, said first
and second mirrors being defined by aspherical coefficients
effective to resolve image features of 0.05-0.25 .mu.m with a
telecentricity.ltoreq.1 mrad with distortions less than 0.1 of a
resolution value for said object over an image field in said image
plane of at least 10 mm .times.10 mm when said illumination means
is a partially coherent photon beam having a wavelength less than
about 100 nm;
wherein said fist and second mirrors are separated at the
respective vertices by a distance defined by ##EQU3## where said
first mirror and said second mirror each satisfy the
relationship.
3. An optical system according to claim 2, wherein the ratio of the
radius of curvature of said first mirror to said second mirror is
in the range 0.98 to 1.02.
4. An optical system according to claim 3, wherein said image field
is at least 25.4mm.times.25.4mm.
5. A reflecting optical system including illumination means or
projecting an image of an object, said optical system further
defining an image plane for receiving a reduced image
representation of said object, the improvement comprising:
first and second coaxial aspherical mirrors in partially obscured
arrangement and defining four reflecting surfaces between said
object and said image plane coaxial with said mirrors, said firs
and second mirrors being defined by aspherical coefficients
effective to resolve image features with 0.05-0.25 .mu.m with a
telecentricity .ltoreq.1 mrad with distortions less than 0.1 of a
resolution value for said object over an image field in said image
plane of at least 10 mm .times.10 mm when said illumination means
is a partially coherent photon beam having a wavelength less than
about 100 nm;
wherein said first and second mirrors are separated at the
respective vertices by a distance defined by ##EQU4## where said
first mirror and said second mirror each satisfy the relationship
and wherein the ratio of the radius of curvature of said first
mirror to said second mirror is in the range 0.98 to 1.02.
6. An optical system according to claim 5, wherein said image field
is at least 25.4 mm .times.25.4 mm.
Description
BACKGROUND OF THE INVENTION
This invention relates to optical projection exposure systems for
use in the manufacture of semiconductor devices and, more
particularly, to reflective, reducing projection exposure optics
having very high resolution under illumination of photons with
wavelengths less than 200 nm.
Advanced lithographic technologies capable of producing features of
.ltoreq.0.2 .mu.m and with high silicon-wafer throughput are needed
to meet the demand for larger, faster, and more complex integrated
circuits. The present technology for optical projection lithography
cannot obtain such a high resolution over a large image field,
e.g., an inch square, with a practical depth of focus, i.e., at
least 1 .mu.m. The main limitations have been: 1) lack of a source
having wavelengths <100 nm with sufficient average power and 2)
lack of a high-resolution, low-distortion optical system operating
at these short wavelengths.
The free-electron laser (FEL) is now being developed as a source of
short-wavelength photons; see, e.g., U.S. Pat. No. 4,917,447,
issued Apr. 17, 1990, to Newnam, and U.S. Pat. application Ser. No.
623,866, filed Dec. 7, 1990, now U.S. Pat. No. 5,144,193, issued
Sep. 1, 1992, both incorporated herein by reference. Operating at
wavelengths less than 200 nm and preferably at wavelengths less
than 20 nm, FEL's driven by rf linear accelerators will fulfill the
wavelength and average-power source requirements of projection
lithography to enable feature resolution of less than 0.1 .mu.m. As
used herein, wavelengths less than 200 nm will be referred to as
XUV wavelengths. It will be appreciated that a static or dynamic
random access memory (SRAM or DRAM) integrated circuit with 25
mm.times.25 mm dimensions and with 0.1 .mu.m features would have
about a 1 GByte memory capacity, the equivalent of a present
generation supercomputer. Indeed, such an extension of existing
optical lithographic technologies will enable the next generation
of electronics to be designed and built.
Existing projection optical systems for XUV cannot produce 0.1
.mu.m features over a field of view large enough for the
photolithographic production of practical integrated circuits.
Resolution of 0.1 .mu.m and 0.05 .mu.m features has been
demonstrated by AT&T Bell Laboratories, T. E. Jewell et al.,
"20:1 Projection Soft X-ray Lithography Using Tri-level Resist,"
1263 SPIE Electron-Beam, X-Ray, and Ion-Beam Technology:
Submicrometer Lithographies IX, pp. 90-95 (1990), incorporated
herein by reference. The magnet undulator in the National
Synchrotron Light Source VUV storage ring followed by a pinhole to
attain full spatial coherence was the light source. The projection
optics was a 20:1-reduction, Schwarzschild two-mirror,
two-reflection system. At an exposure wavelength of 36 nm, 0.2
.mu.m lines and spaces were produced in a trilayer resist.
Subsequent exposures at 14 nm produced 0.1 .mu.m and 0.05 .mu.m
features with small numerical aperture (NA) values of 0.08 and
0.12, respectively. The image field, however, was limited to
25.times.50 .mu.m.
In one prior art optical design (see Design 11, below), two coaxial
spherical mirrors are provided in a partially obscured system to
provide a 3.3.times.image reduction with four reflections. The
general configuration is similar to applicants' configuration shown
in FIG. 1. In one design a NA of 0.150 produces an image field of 5
mm.times.5 mm with a resolution of 0.1 .mu.m at a wavelength of 20
nm. In another design, a NA of 0.125 produces an image field of 10
mm.times.10 mm with a resolution of 0.1 .mu.m when illuminated with
an incoherent light source at a wavelength of 13 nm. Both designs
use a 40% central obscuration. However, even with these small field
sizes, the image distortion is as large as 0.25 .mu.m, whereas the
required distortion is .ltoreq.0.01 .mu.m for 0.1 .mu.m resolution
over the entire image field. It can be shown that the required
resolution can be obtained at these wavelengths over a field of
only 1.4 mm.times.1.4 mm. At practical field sizes, e.g.,
25.4mm.times.25.4mm, the maximum resolution is 0.15 .mu.m using 10
nm illumination with a distortion of 3.6 .mu.m. This is clearly
unacceptable performance for the advanced lithography system under
development. Accordingly, it is an object of the present invention
to resolve image features to .ltoreq.0.2 .mu.m using illuminated
wavelengths less than 100 nm.
It is another object of the present invention to provide the
desired resolution with a distortion less than 0.1.times.resolution
over image fields greater than 10mm.times.10mm.
Yet another object of the present invention is to provide an
optical system compatible with existing scanner and stepping
equipment for very large area projection.
Still another object of the present invention is to maintain a
telecentricity of less than 5 milliradians (mr) to provide an
acceptable depth of focus, e.g., about 1 .mu.m.
It is yet another object of the present invention to provide the
desired resolution and substantially distortionless image at
practical tolerances for optical systems, i.e., surface roughness,
dimensional tolerances, alignment tolerances, etc.
Additional objects, advantages and novel features of the invention
will be set forth in part in the description which follows, and in
part will become apparent to those skilled in the art upon
examination of the following or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, and in accordance with
the purposes of the present invention, as embodied and broadly
described herein, the apparatus of this invention may comprise
first and second coaxial four reflecting surfaces between an object
and an image field coaxial with the mirrors for resolving image
features of 0.05-0.25 .mu.m with a telecentricity of .ltoreq.5 mrad
and distortions less than 0.1 of the resolution over an image field
of at least 10mm.times.10mm when illuminated with a partially
coherent photon beam having a wavelength less than about 100
nm.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form a
part of the specification, illustrate the embodiments of the
present invention and, together with the description, serve to
explain the principles of the invention. In the drawings:
FIG. 1 is a schematic drawing in partial cross-section of a
projection reflection optical system according to the present
invention.
FIG. 2 illustrates the location of exemplary design dimensions.
FIG. 3 graphically depicts a diffraction intensity spread function
showing the aerial image resolution capability of an optical system
according to the embodiments of the invention presented herein.
FIG. 4 graphically depicts the resolution capability of the present
optical system in the image plane when the object is illuminated
with a beam having various partial coherence factors (RNA).
DETAILED DESCRIPTION OF THE INVENTION
Referring first to FIG. 1, there is shown in schematic
illustration, in partial cross-section, a projection reflection
optical system in accordance with the present invention. A photon
source 10 emits partially coherent photon beam 12 that is incident
on a target 14. In a preferred embodiment, photon source 10 is a
free electron laser (FEL) emitting photons with a wavelength less
than 200 nm, and preferably around 10-60 nm. Other photon sources
are available for emitting photons in the desired wavelength range,
e.g., a synchrotron light source such as the National Synchrotron
Light Source at Brookhaven National Laboratories. Target 14 is a
mask representing the desired circuit design to be printed onto a
semiconductor wafer. While FIG. 1 shows a reflection mask, it is
apparent that a transmission mask could be used by placing mask 14
between source 10 and target wafer 18.
The projection system comprises the reflecting optics 20 disposed
between mask 14 and semiconductor wafer 18. In one embodiment of
the present invention, wafer 18 is at least 10mm.times.10mm and is
to be illuminated through optics 20 without scanning or stepping of
an image over the surface of wafer 18. In a second embodiment,
wafer 18 may have an arbitrarily large diameter and is stepped by a
conventional precision translation mechanism to expose a plurality
of different image fields each having a dimension of at least
10mm.times.10mm. The optics may also be included in a scanning
system, e.g., a Micrascan system developed by The Perkin-Elmer
Corporation, to extend the image field size beyond that obtained
with nonscanning optics.
Features at least as small as 0.25 .mu.m, and preferably 0.1 .mu.m,
are printed on wafer 18 with essentially no distortion over the
image field occupied by wafer 18, assuming ideal reflection
surfaces in optics 20. Further, a maximum telecentric angle of 1 mr
is maintained from the image plane defined by wafer 18 in order to
provide an acceptable depth of focus, e.g., 1 .mu.m. Optics 20
maintain a distortion over the image field of wafer 18 of less than
0.1 of the resolution (see Table A) even when various manufacturing
tolerances are introduced into optics 20.
Photon beam 12 is a partially coherent source, i.e., having an
adjustable partial coherence factor, typically 0.4-1.0, where the
partial coherence factor (.sigma. or RNA) is the ratio of the
numerical aperture (NA) of the condensing system (output optics of
laser 10) to that of the imaging system (optics 20), both NA's
being measured at the mask plane. This value affects how close to a
step function in intensity the optical system actually reproduces
in the wafer resist plane.
FIG. 4 graphically depicts the relative beam intensity as a
function of displacement on the image surface for a system imaging
over 25.4mm.times.25.4mm at 13 nm with NA 0.11. A 0.1 .mu.m wide
line is shown to illustrate the required resolution. A totally
coherent beam (RNA=0) has a number of high peaks outside the
required resolution that would provide an undesirable broadening of
the image. At RNA=0.70, the number of peaks is reduced, but some
undesirable peaks still exist outside the 0.1 .mu.m lines. An
acceptable resolution is obtained for RNA 0.88 and 1.00. The beam
displacement begins to broaden as the beam incoherence increases
and the resolution becomes unacceptable for totally incoherent
light. The optimum partial coherence factor must be computed for
each particular system. The present system design provides the
desired resolution and distortion for RNA values in the range
0.8-1.
Optical system 20 is a partially obscured reflecting system
comprising a first aspherical convex reflecting surface 22 defining
a hole 24 therethrough for exiting beam 32 to wafer 18. a second
aspherical concave reflecting surface 28 defining a hole
therethrough for admitting photons 16 from mask 14. The central
portion of beam 16 is blocked by obscurator 26 from illuminating
wafer 18. A preferred obscuration is 0.4, i.e., 40% of the incident
photons are blocked by obscuration 26.
It will be understood that photons in a beam 16 originating from
mask 14 will reflect first from surface 22, second from surface 28,
third from surface 22, and fourth from surface 28 before forming
beam 32 incident on wafer 18. Thus, four reflections occur from two
reflecting surfaces with the first and third surfaces 22 and the
second and fourth surfaces 28 being located on the same aspherical
mirrors. Reflecting surfaces 22. 28 are formed from coatings on
aspheric substrates that are appropriate to reflect the selected
wavelength of beam 16. See, e.g., O. R. Wood II et al.,
"Short-Wavelength Annular-Field Optical System For Imaging
Tenth-Micron Features," 7 J. Vac Sci. Technol. B, No. 6, pp.
1613-1615 (1989), incorporated herein by reference.
In accordance with the present invention, it has been found that an
image of mask 14 can be reduced by at least 3.3.times.and projected
with negligible distortion, i.e., less than 0.1 of the resolution,
onto image frames greater that 10mm.times.10mm when the following
design conditions are met:
1. The ratio of the radius of curvature of surface 22 to surface 28
is in the range 0.98 to 1.02. This condition is needed to make the
Petzval sum sufficiently close to zero that the image is in an
acceptably flat plane.
2. The distance between surface 22 and surface 28 has to obey the
inequality ##EQU1## where the relationship must be satisfied by
both surface 22 and surface 28 in order for the higher aberrations
to cancel before the image is formed.
3. The conic constant and the actual conic shape determined by
Equation 1, below, are selected to balance the higher-order field
curvature without degrading image quality.
4. The aspheric coefficients are selected to reduce the distortion
to levels equal to or less than 0.1 of the resolution.
The optical performance of aspheric surfaces may be determined by
suitable computer software, e.g. Code V available from Optical
Research Associates, Pasadena, CA, to optimize the resolution and
distortion. When all of the above conditions are simultaneously
satisfied according to the present invention, it is found that an
image resolution of at least 0.1 .mu.m can be obtained over large
image areas with a distortion of less than 0.1 of the
resolution.
This surprising result provides a reflective reducing projection
system that is relatively simple to align, i.e., two coaxial
reflecting surfaces, and is tolerant of conventional manufacturing
processes.
As used herein, the aspheric surfaces are characterized by the
conventional formula: ##EQU2## where Z is the sag of the surface
parallel to the Z axis;
c is the curvature at the vertex of the surface;
K is the conic constant, where
______________________________________ K = 0 sphere 0 > K >
-1 ellipsoid with major axis on the optical axis K = -1 paraboloid
K < -1 hyperboloid K > 0 oblate spheroid
______________________________________
A,B,C,D are the 4th, 6th, 8th, 10th order deformation coefficients
(A=B=C=D for a pure conic).
h.sup.2 =X.sup.2 +Y.sup.2 ; i.e. h is a height above the Z axis X
and Y are dimensions on the other two orthogonal axes.
To illustrate the capabilities of the aspherical mirror design,
design examples 1-11 are presented below, where design 11 is a
prior art design using spherical mirrors. Referring now to FIG. 2,
there is defined the design dimensions. It will be understood that
surfaces 1 and 3 are the same and surfaces 2 and 4 are the same.
All linear dimensions are in millimeters unless otherwise noted,
and are not separately labeled. The numerical values are presented
as computer output. In practical application, the dimensions will
be rounded to two decimal places and the aspherical parameters to
two significant figures.
FIG. 3 illustrates a typical diffraction intensity spread function
for the exemplary designs showing the diffraction intensity
variation in the image plane. The bottom portion of the function
must be below the resolution sensitivity of a selected photoresist
so that the resist responds only to the narrow peak portion of the
function. The width of the peak function defines the aerial image
resolution available from a given design. A design with high level
peripheral peaks would be unacceptable.
FIG. 4 illustrates the effect of the partial coherence factor on
the diffraction intensity profile. The effective resolution is
again defined by the spread of the curve at a relative intensity
that affects the resist. A suitable partial coherence factor is
selected to provide a diffraction intensity within the selected
profile above the resist sensitivity and with sufficient power
within the profile to expose the resist. The following exemplary
designs were evaluated with a partial coherence factor in the range
0.8 to 1.0.
Each design includes the following items: basic hyperbolic with
higher order deformations, for use
______________________________________ DESIGN PARAMETERS
______________________________________ NA Numerical aperture in
image space WL Wavelength used in nanometers XOB Object point in
mask field (for evaluation) YOB Object point in mask field (for
evaluation) RDY Radius of curvature THI Separation between surfaces
RMD-GLA Describes the nature of material between surfaces and event
on surface (air, lens, mirror, etc.) CIR Radius of surface CIR OBS
Radius of aperture STO Surface 3; location of the aperture stop K,
A, B Values for aspherical surfaces; C = D = 0, unless otherwise
noted. ______________________________________ DESIGN 1 NA = 0.11 WL
= 13 Image Size = 25.4 .times. 25.4 Field 1 Field 2 Field 3
______________________________________ XOB 0.00000 0.00000 0.00000
YOB 0.00000 42.33850 59.87568
______________________________________ RDY THI RMD GLA
______________________________________ OBJ: Infinity 2112.445332
AIR 1 1777.57409 -913.342638 REFL AIR 2 1779.26774 913.342638 REFL
AIR STO: 1777.57409 -913.342683 REFL AIR 4 1779.26774 1116.395075
REFL AIR IMG: Infinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 99.900000 CIR OBS 1,3 45.000000 CIR S2,4
220.000000 CIR OBS 2,4 75.000000
______________________________________ ASPHERICAL PARAMETERS S1,3 K
= -1.0792218744 A = -0.44886622E-10 B = -0.58712169E-16 S2,4 K =
-0.120004294299 A = -0.44502823E-11 B = -0.47581115E-17
______________________________________ DESIGN 2 NA = 0.11 WL = 13
Image Size = 40 .times. 0.25 Field 1 Field 2 Field 3 Field 4
______________________________________ XOB 0.00000 46.84620
47.14620 47.44620 YOB 0.00000 47.44620 47.14620 46.84620
______________________________________ RDY THI RMD GLA
______________________________________ OBJ: Infinity 2112.461005
AIR 1: 1777.59247 -913.336954 REFL AIR 2: 1779.26508 913.336954
REFL AIR STO: 1777.59247 -913.336954 REFL AIR 4: 1779.26508
1116.382811 REFL AIR IMG: Infinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS ClR S1,3 100.000000 CIR OBS 1,3 41.115000 CIR S2,4
200.000000 CIR OBS 2,4 80.000000
______________________________________ ASPHERICAL PARAMETERS S1,3 K
= -1.07851182545 A = -0.44869423E-10 B = -0.59169516E-16 S2,4 K =
-0.113293055237 A = -0.45937664E-11 B = -0.4862S754E-17
______________________________________ DESIGN 3 NA = 0.2 WL = 60
Image Size = 30 .times. 30 Field 1 Field 2
______________________________________ XOB 0.00000 0.00000 YOB
0.00000 72.56542 ______________________________________ RDY THI RMD
GLA ______________________________________ OBJ: Infinity
2179.315832 AIR 1: 1762.59624 -917.387909 REFL AIR 2: 1779.78410
917.387909 REFL AIR STO: 1762.59624 -917.387909 REFL AIR 4:
1779.78410 1116.395648 REFL AIR IMG: Infinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 180.000000 CIR OBS 1,3 65.000000 CIR S2,4
334.000000 CIR OBS 2,4 76.000000
______________________________________ ASPHERICAL PARAMETERS S1,3 K
= -1.05917603497 A = -0.44866691E-10 B = -0.59681694E-16 C =
0.76701423E-23 S2,4 K = -0.111128812476 A = -0.44963481E-11 B =
-0.45136SSIE-17 C = 0.28646191E-23
______________________________________ DESIGN 4 NA = 0.25 WL = 60
Image Size = 25.4 .times. 25.4 Field 1 Field 2 Field 3
______________________________________ XOB 0.00000 0.00000 43.85237
YOB 0.00000 43.85237 43.85237
______________________________________ RDY THI RMD GLA
______________________________________ OBJ: Infinity 2204.396014
AIR 1: 1757.45553 -918.922730 REFL AIR 2: 1780.00067 918.922730
REFL AIR STO: 1757.45553 -918.922730 REFL AIR 4: 1780.00067
1116.173808 REFL AIR IMG lnfinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 224.000000 CIR OBS 1,3 130.000000 CIR S2,4
406.000000 CIR OBS 2,4 250.000000
______________________________________ ASPHERICAL PARAMETERS S1,3 K
= -1.05356782398 A = -0.44825057E-10 B = -0.56374571E-16 C =
-0.1825881E-22 S2,4 K = -1.109800068715 A = -0.447177E-11 B =
-0.43106911E-17 C = -0.32960971E-23
______________________________________ DESIGN 5 NA = 0.125 WL = 60
Image Size = 45 .times. 15 Field 1 Field 2 Field 3
______________________________________ XOB 0.00000 0.00000 0.00000
YOB 0.00000 55.34662 79.06660
______________________________________ RDY THI RMD GLA
______________________________________ OBJ: Infinity 2112.461005
AIR 1: 1777.59247 -913.336954 REFL AIR 2: 1779.26508 913.336954
REFL AIR STO: 1777.59247 -913.336954 REFL AIR 4: 1779.26508
1116.382811 REFL AIR IMG: Infinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 110.800000 CIR OBS 1,3 48.000000 CIR S2,4
218.400000 CIR OBS 2,4 80.000000 ASPHERICAL PARAMETERS S1,3 K =
-1.0785118245 A = -0.44869423E-10 B = -0.59169516E-16 S2,4 K =
-0.113293055237 A = -0.45937664E-11 B = -0.48628754E-17 DESIGN 6 NA
= 0.3 WL = 193 Image Size = 20 .times. 20 Field 1 Field 2
______________________________________ XOB 0.00000 0.00000 YOB
0.00000 48.37697 ______________________________________ RDY THI RMD
GLA ______________________________________ OBJ: Infinity
2181.511134 AIR 1: 1764.53736 -917.308729 REFL AIR 2: 1780.03913
917.308729 REFL AIR STO: 1764.53736 -917.308729 REFL AIR 4:
1780.03913 1115.459595 REFL AIR IMG: Infinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 270.000000 CIR OBS 1,3 180.000000 CIR S2,4
476.000000 CIR OBS 2,4 320.000000
______________________________________ ASPHERICAL PARAMETERS S1,3 K
= -1.01121952725 A = -0.43782954E-10 B = -0.53282998E-16 C =
-0.26252808E-22 S2,4 K = -0.106613504476 A = -0.43876219E-11 B =
-0.40585805E-17 C = -0.36094611E-23
______________________________________ DESIGN 7 NA = 0.08 WL = 13
Image Size = 10 .times. 10 Field 1 Field 2 Field 3
______________________________________ XOB 0.00000 0.00000 0.00000
YOB 0.00000 16.66869 23.57310
______________________________________ RDY THI RMD GLA
______________________________________ OBJ: Infinity 2112.445332
AIR 1: 1777.57409 -913.342638 REFL AIR 2: 1779.26774 913.342638
REFL AIR STO: 1777.57409 -913.342683 REFL AIR 4: 1779.26774
1116.395075 REFL AIR IMG: Infinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 99.900000
CIR OBS 1,3 35.000000 CIR S2,4 178.400000 CIR OBS 2,4 75.000000
______________________________________ ASPHERICAL PARAMETERS S1,3 K
= -1.0792218744 A = -0.44886622E-10 B = -0.58712169E-16 S2,4 K =
-0.120004294299 A = -0.44502823E-11 B = -0.47581115E-17
______________________________________ DESIGN 8 NA = 0.08 WL = 10
Image Size 10 .times. 10 Field 1 Field 2
______________________________________ XOB 0.00000 0.00000 YOB
0.00000 31.78014 ______________________________________ RDY THI RMD
GLA ______________________________________ OBJ: Infinity
3395.578764 AIR 1: 1908.95435 -995.239908 REFL AIR 2: 1944.11819
995.239908 REFL AIR STO: 1908.95435 995.239908 REFL AIR 4:
1944.11819 1177.992696 REFL AIR IMG: Infinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 77.500000 CIR OBS 1,3 28.000000 CIR S2,4
150.000000 CIR OBS 2,4 85.000000
______________________________________ ASPHERICAL PARAMETERS S1,3 K
= -0.199386899695 A = -0.31621472E-10 B = -0.1886711E-15 C =
0.34375037-20 S2,4 K = -0.021487096778 A = -0.356I6796E-11 B =
-0.50684015E-17 C = 0.30046828E-22
______________________________________ DESIGN 9 NA = 0.15 WL = 10
Image Size = 25.4 .times. scan length Field 1 Field 2
______________________________________ XOB 0.00000 0.00000 YOB
42.00000 43.00000 ______________________________________ RDY THI
RMD GLA ______________________________________ OBJ: Infinity
2113.682611 AIR 1: 1777.52674 -913.375814 REFL AIR 2: 1779.29040
913.375814 REFL AIR STO: 1777.52674 -913.375814 REFL AIR 4:
1779.29040 1116.317708 REFL AIR IMG: Infinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 134.900000 CIR OBS 1,3 43.000000 CIR S2,4
350.000000 CIR OBS 2,4 78.000000
______________________________________ ASPHERICAL PARAMETERS S2,4 K
= 0.008275366943 A = -0.20575388E-10 B = -0.34017633E-16 C =
0.38413684E-22 S3,5 K = -0.011823781455 A = -0.18384562E-11 B =
-0.19656686 C = -0.145S0091E-23
______________________________________ DESIGN 10 NA = 0.15 WL = 10
Image Size = 14 .times. 14 Field 1 Field 2
______________________________________ XOB 0.00000 23.33619 YOB
0.00000 23.33619 ______________________________________ RDY THI RMD
GLA ______________________________________ OBJ: Infinity
2112.078151 AIR 1: 1777.13023 -913.459893 REFL AIR 2: 1779.28237
913.459893 REFL AIR STO: 1777.13023 -913.459893 REFL AIR 4:
1779.28237 1116.602503 IMG: lnfinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 134.000000 CIR OBS 1.3 40.000000 CIR S2,4
350.000000 CIR OBS 2,4 60.000000 ASPHERICAL PARAMETERS S2,4 K =
-1.07953019409 A = -0.44951395E-10 B = -0.57691058E-16 C =
-0.22368228E-22 S2,4 K = -0.122809 A = -0.439687E-11 B =
-0.454033E-17 C = -0.307562E-23
______________________________________ DESIGN 11 NA = 0.125 WL = 13
Image Size = 10 .times. 10 Field 1 Field 2 Field 3
______________________________________ XOB 0.00000 0.00000 0.00000
YOB 0.00000 16.67357 23.58018
______________________________________ RDY THI RMD GLA
______________________________________ OBJ: Infinity 2110.181200
AIR 1: 1772.51360 -915.197560 REFL AIR 2: 1779.35380 915.197560
REFL AIR STO: 1772.51360 -915.197560 REFL AIR 4: 1779.35380
1117.554340 REFL AIR IMG: Infinity 0.000000 AIR
______________________________________ APERTURE DATA/EDGE
DEFINITIONS CIR S1,3 110.490600 CIR OBS 1,3 32.385000 CIR S2,4
203.200000 CIR OBS 2,4 59.055000
______________________________________
The system performance for each of the above designs has been
determined and is summarized in Table A. The headings used in Table
A are as follows:
TABLE A
__________________________________________________________________________
PERFORMANCE VALUES
__________________________________________________________________________
RED reduction factor from object to image. WL wavelength range for
performance values - nm. NA numerical aperture range for
performance. RES resolution available over entire field in .mu.m.
FIELD field size side dimension in mm. TEL telecentricity in
milliradians in image space. DIST maximum distortion over entire
field in .mu.m/% of RES.
__________________________________________________________________________
DESIGN RED WL NA RES FIELD TEL DIST
__________________________________________________________________________
1 3.3 10-20 .08-.17 0.1 25.4X 0.4 .002/2 2 3.3 10-20 .08-.17 0.1 40
.times. 0.25 0.4 .008/8 3 3.3 40-60 .2-.25 0.15 30X 0.45 .004/3 4
3.5 40-60 .25-.3 0.15 25.4X 0.5 .004/3 5 3.3 40-60 .12-.2 0.17 45
.times. 15 0.7 .005/5 6* 3.3 190-200 .30 0.25 20X 0.3 .05/20 7 3.3
10-20 .08-.13 0.05 10X 0.15 .0007/1.4 8* 4.5 10-20 .08 0.1 10X 0.1
.01/10 9 3.3 10-20 .15-.16 0.1 25.4 .times. 0.4 .001/1 scan 10 3.3
10- 20 .15-.17 0.1 14X 0.2 .001/1 11* 3.3 10-20 .125 0.1 10X 0.2
.01/10
__________________________________________________________________________
* Design, as evaluated, does not meet photolithography requirements
as noted below.
Designs 1-10 are all aspheric designs according to the present
invention. In all cases the first mirror 22 (FIG. 1) has a conic
constant 0>K>-1, i.e., a basic ellipsoid with higher order
deformations. The second mirror 28 (FIG. 1,) has a conic constant
K<-1, i.e., a basic hyperbolic with higher order deformations,
for use in nonscanning systems (Designs 1-9). Design 10, a scanning
system, illustrates mirror 28 with a conic constant K>0, i.e., a
basic oblate spheroid with higher order deformations.
It will be seen that Designs 6, 8, and 11 do not appear suitable
for high-resolution, low-distortion lithography systems. Design 6
was evaluated at a wavelength of about 200 nm and has an
unacceptable distortion for lithographic application, but other
imaging applications may exist. Design 8 was evaluated at a
reduction factor of 4.5.times.and can meet both the resolution and
distortion requirements over an image field size of only
2mm.times.2mm. Design 11 is the prior art design and can likewise
meet both the resolution and distortion requirements over an image
field size of only 2mm.times.2mm.
The optical performance of most of the above acceptable designs is
also shown in Table B by the image quality as determined by the
R.M.S. value for the square root of wave aberration given in units
of wavelength.
TABLE B ______________________________________ System .lambda. nm
Center Edge ______________________________________ 1 13 .012 .056 2
13 .018 .068 3 60 .020 .053 5 60 .006 .040 7 13 .014 .007 9 10 .009
.009 10 10 .078 .043 ______________________________________
Calculated quality values .ltoreq.0.071.lambda. represent
diffraction limited systems.
Design 1 has been further examined to determine the sensitivity of
the image quality, defined above, to tolerances of critical
dimensions. Tolerances related to mirror radii, mirror spacing, and
tilt angle were varied until image quality was no longer
diffraction limited. The resulting tolerances and field image
effects for Design 1 are shown in Table C.
TABLE C ______________________________________ R (Radius)
.+-.0.0005 mm = .+-.0.5 .mu.m S (Spacing) .+-.0.0001 mm = .+-.0.1
.mu.m T (Tilt Angle) 4 micro-radians
______________________________________ Image Field Nominal RMS
Toleranced RMS (across diagonal) (.lambda. units) (.lambda. units)
______________________________________ Axis 0.012 0.025 .7 to edge
0.024 0.040 Edge of Image 0.056 0.071
______________________________________
These tolerances are well within the state-of-the-art for precision
mirror fabrication and the dimensional changes under expected
operating conditions. The distortion remained less than 0.1 of the
resolution value. By way of comparison, a tilted, de-centered,
four-mirror system, similar to that shown by European Patent
Application No. 87306037.0, published Jan. 1, 1988, requires the
following tolerances to remain a diffraction limited system:
______________________________________ Radius .+-.0.001 .mu.m
Spacing .+-.0.001 .mu.m Tilt .+-.0.1 micro-radians
______________________________________
Even with these very tight tolerances, the distortion increases to
0.2 of the resolution values.
The optical projection systems according to the present invention
have other significant performance characteristics. A standard
telecentricity value is 5 mr, whereas the subject designs provide a
telecentricity of .ltoreq.0.7 mr over the large field area. This
provides a large axial distance within which the image plane may be
located. The aspherical surfaces are only slight departures from
best-fit spheres as shown in Table D. The small deviations
introduced by the aspherecity herein are well within existing
manufacturing capabilities.
TABLE D ______________________________________ Design Deviation
from Best Fit Sphere Mirror 22 Mirror 28 Design (.mu.m) (.mu.m)
______________________________________ 1 1.1 1.5 2 1.2 1.7 5 1.7
3.2 7 1.3 1.3 8 0.2 0.2 ______________________________________
Thus, an optical projection system is provided herein that is
capable of photolithographic exposures with 0.1 .mu.m resolution
and distortions less than 0.1 of the resolution value. The image
fields can be at least 25.4mm.times.25.4mm for a single projection,
nonscanning exposure, or can have a field width of 25.4mm and
scanned in small steps over any desired length. In addition to
lithography, these high resolution optical systems are also
expected to have significant application in other technical fields,
i.e., biotechnology.
The foregoing description of embodiments of the invention have been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and obviously many modifications and variations are
possible in light of the above teaching. The embodiments were
chosen and described in order to best explain the principles of the
invention and its practical application to thereby enable others
skilled in the art to best utilize the invention in various
embodiments and with various modifications as are suited to the
particular use contemplated. It is intended that the scope of the
invention be defined by the claims appended hereto.
* * * * *