U.S. patent number RE42,118 [Application Number 11/981,511] was granted by the patent office on 2011-02-08 for projection system for euv lithography.
This patent grant is currently assigned to Carl-Zeiss-SMT AG. Invention is credited to Udo Dinger, Russell Hudyma, Hans-Jurgen Mann.
United States Patent |
RE42,118 |
Hudyma , et al. |
February 8, 2011 |
Projection system for EUV lithography
Abstract
An EUV optical projection system includes at least six
reflecting surfaces for imaging an object (OB) on an image (IM).
The system is preferably configured to form an intermediate image
(IMI) along an optical path from the object (OB) to the image (IM)
between a secondary mirror (M2) and a tertiary mirror (M3), such
that a primary mirror (M1) and the secondary mirror (M2) form a
first optical group (G1) and the tertiary mirror (M3), a fourth
mirror (M4), a fifth mirror (M5) and a sixth mirror (M6) form a
second optical group (G2). The system also preferably includes an
aperture stop (APE) located along the optical path from the object
(OB) to the image (IM) between the primary mirror (M1) and the
secondary mirror (M2). The secondary mirror (M2) is preferably
concave, and the tertiary mirror (M3) is preferably convex. Each of
the six reflecting surfaces preferably receives a chief ray (CR)
from a central field point at an incidence angle of less than
substantially 15.degree.. The system preferably has a numerical
aperture greater than 0.18 at the image (IM). The system is
preferably configured such that a chief ray (CR) converges toward
the optical axis (OA) while propagating between the secondary
mirror (M2) and the tertiary mirror (M3).
Inventors: |
Hudyma; Russell (San Ramon,
CA), Mann; Hans-Jurgen (Oberkochen, DE), Dinger;
Udo (Oberkochen, DE) |
Assignee: |
Carl-Zeiss-SMT AG (Oberkochen,
DE)
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Family
ID: |
31999619 |
Appl.
No.: |
11/981,511 |
Filed: |
October 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/EP01/14301 |
Dec 6, 2001 |
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10004674 |
Dec 3, 2001 |
6600552 |
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09503640 |
Feb 14, 2000 |
6353470 |
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60255161 |
Dec 12, 2000 |
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Reissue of: |
10454831 |
Jun 4, 2003 |
06985210 |
Jan 10, 2006 |
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Foreign Application Priority Data
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Feb 15, 1999 [DE] |
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199 06 001 |
Oct 7, 1999 [DE] |
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199 48 240 |
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Current U.S.
Class: |
355/67;
355/53 |
Current CPC
Class: |
G03F
7/70275 (20130101); G03F 7/70233 (20130101); G02B
17/0657 (20130101) |
Current International
Class: |
G03B
27/54 (20060101); G03B 27/42 (20060101) |
Field of
Search: |
;355/53,55,67-71
;359/366,731,858,859 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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199 48 240 |
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Aug 2000 |
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DE |
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0 779 528 |
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Jun 1997 |
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EP |
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0 779 528 |
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Jun 1997 |
|
EP |
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0 816 892 |
|
Jul 1998 |
|
EP |
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1 178 356 |
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Feb 2002 |
|
EP |
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07283116 |
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Oct 1995 |
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JP |
|
WO99/57606 |
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Nov 1999 |
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WO |
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Other References
M F. Bal, "Next-Generation Extreme Ultraviolet Lithographic
Projection Systems," Delft Univ. of Technology, Feb. 2003, pp.
11-137. cited by other .
European Search Report for Application No. 99125783.3-2208. cited
by other .
Jewell, "Optical system design issues in development of projection
camera for EUV lithography," Proceedings of the SPIE 2437:340-346
(1995). cited by examiner .
"Design of Reflective Relay for Soft x-ray Lithography", Rodgers,
et al., SPIE vol.1364 International Lens Design Conference, 1990,
pp. 330-336. cited by examiner .
"Ring--Field EUVL Camera with Large Etendu", vol. 4 W.C. Sweatt,
OSA TOPS on Extreme Ultraviolent Lithography, pp. 1996, 178-180.
cited by examiner .
"Reflective Systems Design Study for Soft X-Ray Projection
Lithography", Jewell, et al., J. Vac Sci. Technol., B8 (6),
Nov./Dec. 1990, pp., 1519-1523. cited by examiner .
"Phase Shifting Diffraction Interferometry for Measuring Extreme
Ultraviolet Optics," Gary E. Sommargren, OSA TOPS on Extreme
Ultraviolet Lithography, 1996, pp. 108-112. cited by examiner .
"EUV Optical Design for a 100 nm CD Imaging System", Sweeney, et
al., SPIE vol. 3331, pp. 2-10. (undated). cited by
examiner.
|
Primary Examiner: Nguyen; Hung Henry
Attorney, Agent or Firm: Ohlandt, Greeley, Ruggiero &
APerle, LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is .Iadd.a reissue of U.S. patent
application Ser. No. 10/454,831, filed on Jun. 4, 2003, now U.S.
Pat. No. 6,985,210, which is .Iaddend.a continuation.Iadd.-in-part
.Iaddend.of International Application No. PCT/EP01/14301 and a
continuation-in-part of U.S. patent application Ser. No.
10/004,674. The PCT/EP01/14301 application was filed Dec. 6, 2001,
and claims priority of U.S. Provisional Patent Application Ser. No.
60/255,161, which was filed Dec. 12, 2000. The Ser. No. 10/004,674
application was filed Dec. 3, 2001 now U.S. Pat. No. 6,600,552 and
is a continuation-in-part of U.S. patent application Ser. No.
09/503,640. The Ser. No. 09/503,640 application was filed Feb. 14,
2000 and issued as U.S. Pat. No. 6,353,470. The present application
is also claiming priority of (a) German Patent Application No. 199
06 001 filed Feb. 15, 1999, and (b) German Patent Application No.
199 48 240 filed Oct. 7, 1999.
Claims
What is claimed is:
1. An EUV optical projection system, comprising: a primary mirror,
a secondary mirror, a tertiary mirror, a fourth mirror, a fifth
mirror and a sixth mirror, for imaging an object on an image,
wherein said system is configured to form an intermediate image
along an optical path from the object to the image between said
secondary mirror and said tertiary mirror, wherein said secondary
mirror is concave, wherein said tertiary mirror is convex, and
wherein said system has a numerical aperture greater than 0.18 at
the image.
2. The system according to claim 1, further comprising an aperture
stop located along said optical path from said object to said image
between said primary mirror and said secondary mirror.
3. The system according to claim 1, wherein said aperture stop is
not located on said primary mirror and said aperture stop is not
located on said secondary mirror.
4. The system according to claim 1, further comprising: an optical
axis between an object plane and an image plane, wherein said
system is further configured such that a chief ray from a central
field point converges toward said optical axis while propagating
between said secondary mirror and said tertiary mirror.
5. The system according to claim 1, further comprising: an optical
axis between an object plane and an image plane, wherein said
system is further configured such that a chief ray from a central
field point propagates approximately parallel to said optical axis
while propagating between said secondary mirror and said tertiary
mirror.
6. The system according to claim 1, wherein said tertiary mirror
along said optical path from said object to said image is
physically located closer to said object than said primary
mirror.
7. The system according to claim 1, further comprising: an optical
axis between a object plane and a image plane, wherein said system
is further configured such that a chief ray from a central field
point diverges away from said optical axis while propagating
between said secondary mirror and said tertiary mirror.
8. The system according to claim 1, wherein said primary mirror
along said optical path from said object to said image is
physically located closer to said object than said tertiary
mirror.
9. The system according to claim 1, wherein said primary mirror is
concave, said fourth mirror is concave, said fifth mirror is convex
and said sixth mirror is concave.
10. The system according to claim 1, wherein each of said primary
mirror, said secondary mirror, said tertiary mirror, said fourth
mirror, said fifth mirror, and said sixth mirror is disposed
between said object and said image, and wherein said object and
said image are separated from one another by a physical distance of
less than or equal to about 1500 mm.
11. The system according to claim 1, wherein each of said primary
mirror, said secondary mirror, said tertiary mirror, said fourth
mirror, said fifth mirror, and said sixth mirror is disposed
between said object and said image, and wherein said object and
said image are separated from one another by a physical distance of
less than or equal to about 1200 mm.
12. The system according to claim 1, wherein each of said primary
mirror, said secondary mirror, said tertiary mirror, said fourth
mirror, said fifth mirror, and said sixth mirror receives a chief
ray from a central field point at an incidence angle of less than
about 15.degree..
13. The system according to claim 1, wherein five of said primary
mirror, said secondary mirror, said tertiary mirror, said fourth
mirror, said fifth mirror, and said sixth mirror receive a chief
ray from a central field point at an incidence angle of less than
about 11.degree..
14. The system according to claim 1, wherein said system is
configured to have a RMS wavefront error of less than or equal to
about 0.017.lamda..
15. The system according to claim 1, wherein said system is
configured to have a RMS wavefront error of between 0.017.lamda.
and 0.011.lamda..
16. An EUV optical projection system, comprising: a primary mirror,
a secondary mirror, a tertiary mirror, a fourth mirror, a fifth
mirror and a sixth mirror, for imaging an object on an image,
wherein said system is configured to form an intermediate image
along an optical path from the object to the image between said
secondary mirror and said tertiary mirror, wherein each of said
primary mirror, said secondary mirror, said tertiary mirror, said
fourth mirror, said fifth mirror and said sixth mirror receives a
chief ray from a central field point at an incidence angle of less
than about 15.degree., and wherein said system has a numerical
aperture greater than 0.18 at the image.
17. The system according to claim 16, wherein five of said primary
mirror, said secondary mirror, said tertiary mirror, said fourth
mirror, said fifth mirror, and said sixth mirror receive a chief
ray from a central field point at an incidence angle of less than
about 11.degree..
18. The system according to claim 16, further comprising: an
optical axis between an object plane and an image plane, wherein
said system is further configured such that a chief ray from a
central field point converges toward said optical axis while
propagating between said secondary mirror and said tertiary
mirror.
19. The system according to claim 16, wherein said tertiary mirror
along said optical path from said object to said image is
physically located closer to said object than said primary
mirror.
20. The system according to claim 16, further comprising: an
optical axis between a object plane and a image plane, wherein said
system is further configured such that a chief ray from a central
field point diverges away from said optical axis while propagating
between said secondary mirror and said tertiary mirror.
21. The system according to claim 16, wherein said primary mirror
along said optical path from said object to said image is
physically located closer to said object than said tertiary
mirror.
22. The system according claim 16, wherein said secondary mirror is
concave, and wherein said tertiary mirror is convex.
23. The system according to claim 16, wherein each of said primary
mirror, said secondary mirror, said tertiary mirror, said fourth
mirror, said fifth mirror, and said sixth mirror is disposed
between said object and said image, and wherein said object and
said image are separated from one another by a physical distance of
less than or equal to about 1500 mm.
24. The system according to claim 16, wherein each of said primary
mirror, said secondary mirror, said tertiary mirror, said fourth
mirror, said fifth mirror, and said sixth mirror is disposed
between said object and said image, and wherein said object and
said image are separated from one another by a physical distance of
less than or equal to about 1200 mm.
25. An EUV optical projection system, comprising: a primary mirror,
a secondary mirror, a tertiary mirror, a fourth mirror, a fifth
mirror and a sixth mirror, for imaging an object on an image; and
an aperture stop located along an optical path from said object to
said image between said primary mirror and said secondary mirror,
wherein said secondary mirror is concave, wherein said tertiary
mirror is convex, wherein said aperture stop is not located on said
primary mirror and said aperture stop is not located on said
secondary mirror, and wherein said system has a numerical aperture
greater than 0.18 at the image.
26. The system according to claim 25, wherein each of said primary
mirror, said secondary mirror, said tertiary mirror, said fourth
mirror, said fifth mirror, and said sixth mirror receives a chief
ray from a central field point at an incidence angle of less than
about 15.degree..
27. The system according to claim 25, wherein five of said primary
mirror, said secondary mirror, said tertiary mirror, said fourth
mirror, said fifth mirror, and said sixth mirror receive a chief
ray from a central field point at an incidence angle of less than
about 11.degree..
28. An EUV optical projection system, comprising: a primary mirror,
a secondary mirror, a tertiary mirror, a fourth mirror, a fifth
mirror and a sixth mirror, for imaging an object on an image, and
an aperture stop located along an optical path from said object to
said image between said primary mirror and said secondary mirror,
wherein said system is configured such that a chief ray from a
central field point converges toward said optical axis while
propagating between said secondary mirror and said tertiary mirror,
wherein said aperture stop is not located on said primary mirror,
and said aperture stop is not located on said secondary mirror, and
wherein said system has a numerical aperture greater than 0.18 at
the image.
29. The system according to claim 28, wherein said tertiary mirror
along said optical path from said object to said image is
physically located closer to said object than said primary
mirror.
30. The system of claim 1, further comprising at least one
additional mirror employed in cooperation with said primary mirror,
said secondary mirror, said tertiary mirror, said fourth mirror,
said fifth mirror, and said sixth mirror, for imaging said object
on said image.
31. The system of claim 1, wherein said primary mirror and said
secondary mirror form a first optical group, and wherein said
tertiary mirror, said fourth mirror, said fifth mirror and said
sixth mirror for a second optical group.
32. The system of claim 16, further comprising at least one
additional mirror employed in cooperation with said primary mirror,
said secondary mirror, said tertiary mirror, said fourth mirror,
said fifth mirror, and said sixth mirror, for imaging said object
on said image.
33. The system of claim 16, wherein said primary mirror and said
secondary mirror form a first optical group, and wherein said
tertiary mirror, said fourth mirror, said fifth mirror and said
sixth mirror for a second optical group.
34. The system of claim 25, further comprising at least one
additional mirror employed in cooperation with said primary mirror,
said secondary mirror, said tertiary mirror, said fourth mirror,
said fifth mirror, and said sixth mirror, for imaging said object
on said image.
35. The system of claim 28, further comprising at least one
additional mirror employed in cooperation with said primary mirror,
said secondary mirror, said tertiary mirror, said fourth mirror,
said fifth mirror, and said sixth mirror, for imaging said object
on said image.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microlithography objective, a
projection exposure apparatus containing the objective, and a
method of manufacturing an integrated circuit using the same. More
particularly, the present invention relates to an optical
projection system for extreme ultraviolet (EUV) lithography,
particularly including six mirrors arranged in two optical
groups.
2. Description of the Related Art
It is widely accepted that current deep ultraviolet (DUV)
projection printing systems used in a step and scan mode will be
able to address the needs of the semiconductor industry for the
next two or three device nodes. The next generation of
photolithographic printing systems will use exposure radiation
having soft x-ray or extreme ultraviolet wavelengths of
approximately 11 nm to 15 nm, also in a step and scan printing
architecture. To be economically viable, these next generation
systems will require a sufficiently large numerical aperture to
address sub 70 nm integrated circuit design rules. Further, these
photolithography systems will require large fields of view in the
scan direction to ensure that the throughput (defined in terms of
wafers per hour) is sufficiently great so that the process is
economically viable.
The theoretical resolution (R) of a lithographic printing system
can be expressed by the well-known relationship
R=k.sub.1.lamda./NA, where k.sub.1 is a process dependent constant,
.lamda. is the wavelength of light, and NA is the numerical
aperture of the projection system. Knowing that EUV resists support
a k.sub.1-factor of .about.0.5 and assuming a numerical aperture of
0.20, an EUV projection system can achieve a theoretical resolution
on the order of approximately 30 nm with .lamda.=13.4 nm. It is
recognized in the present invention that all reflective projection
systems for EUV lithography for use in a step and scan architecture
having both a large numerical aperture (0.20 to 0.30) and a large
field (2 to 3 mm) are desired to address the sub-50 nm linewidth
generations as defined by the International Sematech's
International Technology Roadmap for Semiconductors (1999).
Four-mirror projection systems, such as those described in U.S.
Pat. Nos. 5,315,629 and 6,226,346, issuing to Jewel and Hudyma,
respectively, lack the degrees of freedom necessary to correct
aberrations over a sufficiently large NA to achieve 30 nm design
rules. The '346 patent teaches that a four-mirror projection system
can be used to correct aberrations at a numerical aperture up to
0.14 (50 nm design rules). However, it is desired that the width of
the ring field be reduced to enable wavefront correction to the
desired level for lithography. The '346 patent demonstrates that
the ring field is reduced from 1.5 mm to 1.0 mm as a numerical
aperture is increased from 0.10 to 0.12. Further scaling of the
second embodiment in the '346 patent reveals that the ring field
must be reduced to 0.5 mm as a numerical aperture is increased
further to 0.14. This reduction in ring field width results
directly in reduced throughput of the entire projection apparatus.
Clearly, further advances are needed.
Five-mirror systems, such as that set forth in U.S. Pat. No.
6,072,852, issuing to Hudyma, have sufficient degrees of freedom to
correct both the pupil dependent and field dependent aberrations,
thus enabling numerical apertures in excess of 0.20 over meaningful
field widths (>1.5 mm). While minimizing the number of
reflections has several advantages particular to EUV lithography,
an odd number of reflections create a problem in that new stage
technology would need to be developed to enable unlimited parallel
scanning. To "unfold" the system to enable unlimited synchronous
parallel scanning of the mask and image with existing scanning
stage technologies, it is recognized herein that an additional
mirror should be incorporated into the projection system.
Optical systems for short wavelength projection lithog- raphy
utilizing six or more reflections have been disclosed in the patent
literature.
One early six mirror system is disclosed in U.S. Pat. No.
5,071,240, issuing to Ichihara and Higuchi entitled, "Reflecting
optical imaging apparatus using spherical reflectors and producing
an intermediate image." The '240 patent discloses a 6-mirror
catoptric or all-reflective reduction system utilizing spherical
mirrors. This particular embodiment is constructed with three
mirror pairs and uses positive/negative (P/N) and negative/positive
(N/P) combinations to achieve the flat field condition. Ichihara
and Higuchi also demonstrate that the flat field imaging condition
(zero Petzval sum) can be achieved with a system that utilizes an
intermediate image between the first mirror pair and last mirror
pair. The patent teaches the use of a convex secondary mirror with
an aperture stop that is co-located at this mirror. It is also
clear from examination of the embodiments that the '240 patent
teaches the use of low incidence angles at each of the mirror
surfaces to ensure compatibility with reflective coatings that
operate at wavelengths around 10 nm.
While the embodiments disclosed in the '240 patent appear to
achieve their stated purpose, these examples are not well suited
for contemporary lithography at extreme ultraviolet wavelengths.
First, the systems are very long (.about.3000 mm) and would suffer
mechanical stability problems. Second, the embodiments do not
support telecentric imaging at the image which is desired for
modern semiconductor lithography printing systems. Lastly, the
numerical aperture is rather small (.about.0.05) leaving the
systems unable to address 30 nm design rules.
Recently, optical projection production systems have been disclosed
that offer high numerical apertures with at least six reflections
designed specifically for EUV lithography. One such system is
disclosed in U.S. Pat. No. 5,815,310, entitled, "High numerical
aperture ring field optical projection system," issuing to
Williamson. In the '310 patent, Williamson describes a six-mirror
ring field projection system intended for use with EUV radiation.
Each of the mirrors is aspheric and share a common optical axis.
This particular embodiment has a numerical aperture of 0.25 and is
capable of 30 nm lithography using conservative (.about.0.6) values
for k.sub.1. The '310 patent suggests that both PNPPNP and PPPPNP
reimaging configurations are possible with a physically accessible
intermediate image located between the third and fourth mirrors.
This particular embodiment consists, from long conjugate to short
conjugate, of a concave, convex, concave, concave, convex and
concave mirror, or PNPPNP for short. The '310 patent suggests that
both PNPPNP and the PPPPNP power distributions can achieve 30 nm
design rules.
The preferred EUV embodiment disclosed in the '310 patent suffers
from several drawbacks, one of which is the high incidence angles
at each of the mirrored surfaces, particularly on mirrors M2 and
M3. In some instances, the angle of incidence exceeds 24.degree. at
a given location on the mirror. Both the mean angle and deviation
or spread of angles at a given point on a mirror surface is
sufficient to cause noticeable amplitude and phase effects due to
the EUV multilayer coatings that might adversely impact critical
dimension (CD control).
Two other catoptric or all-reflective projection systems for
lithography are disclosed in U.S. Pat. No. 5,686,728 entitled,
"Projection lithography system and method using all-reflective
optical elements," issuing to Shafer. The '728 patent describes an
eight mirror projection system with a numerical aperture of about
0.50 and a six-mirror projection system with a numerical aperture
of about 0.45 intended for use at wavelengths greater than 100 nm.
Both systems operate in reduction with a reduction ratio of
5.times.. Like the systems described in the '310 patent, these
systems have an annular zone of good optical correction yielding
lithography performance within an arcuate shaped field. While these
systems were designed for DUV lithography and are fine for that
purpose, these embodiments make very poor EUV projection systems.
Even after the numerical aperture is reduced from 0.50 to 0.25, the
incidence angles of the ray bundles are very large at every mirror
including the mask, making the system incompatible with either
Mo/Si or Mo/Be multilayers. In addition, both the aspheric
departure and aspheric gradients across the mirrors are rather
large compared to the EUV wavelength, calling into question whether
or not such aspheric mirrors can be measured to a desired accuracy
for EUV lithography. Recognizing these issues, the '728 patent
explicitly teaches away from using catoptric or all-reflective
projection systems at EUV wavelengths and instead restricts their
use to longer DUV wavelengths.
Another projection system intended for use with EUV lithography is
disclosed in U.S. Pat. No. 6,033,079, issuing to Hudyma. The '079
patent entitled, "High numerical aperture ring field projection
system for extreme ultraviolet lithography," describes two
preferred embodiments. The first embodiment that the '079 patent
describes is arranged with, from long to short conjugate, a
concave, concave, convex, concave, convex, and concave mirror
surfaces (PPNPNP). The second preferred embodiment from the '079
patent has, from long to short conjugate, a concave, convex,
convex, concave, convex, and concave mirror surfaces (PNNPNP). The
'079 patent teaches that both PPNPNP and PNNPNP reimaging
configurations are advantageous with a physically accessible
intermediate image located between the fourth and fifth mirror. In
a manner similar to the '240 and '310 patents, the '079 patent
teaches the use of an aperture stop at the secondary mirror and a
chief ray that diverges from the optical axis after the secondary
mirror.
The '079 patent teaches that the use of a convex tertiary mirror
enables a large reduction in low-order astigmatism. This particular
arrangement of optical power is advantageous for achieving a high
level of aberration correction without using high incidence angles
or extremely large aspheric departures. For both embodiments, all
aspheric departures are below 15 .mu.m and most are below 10 .mu.m.
Like the '240 patent, the '079 patent makes a significant teaching
related to EUV via the use of low incidence angles on each of the
reflective surfaces. The PPNPNP and PNNPNP power arrangements
promote low incidence angles thus enabling simple and efficient EUV
mirror coatings. The low incidence angles work to minimize
coating-induced amplitude variations in the exit pupil, minimize
coating-induced phase or optical path difference (OPD) variations
in the exit pupil, and generally lower the tolerance sensitivity of
the optical system. These factors combine to promote improved
transmittance and enhanced CD uniformity in the presence of
variations in focus and exposure.
While the prior art projection optical systems have proven adequate
for many applications, they're not without design compromises that
may not provide an optimum solution in all applications. Therefore,
there is a need for a projection optical system that can be used in
the extreme ultraviolet (EUV) or soft X-ray wavelength region that
has a relatively large image field with capable of sub 50 nm
resolution.
SUMMARY OF THE INVENTION
In view of the above, an EUV optical projection system is provided
including at least six reflecting surfaces for imaging an object on
an image. The system is configured to form an intermediate image
along an optical path from the object to the image between a
secondary mirror and a tertiary mirror, such that a primary mirror
and the secondary mirror form a first optical group and the
tertiary mirror and a fourth mirror, a fifth mirror and a sixth
mirror form a second optical group. The secondary mirror is
concave, and the tertiary mirror is convex.
The system may further include an aperture stop located along the
optical path from the object to the image between the primary
mirror and the secondary mirror. This aperture stop may be disposed
off each of the first mirror and the second mirror.
The system may be further configured such that a chief ray from a
central field point converges toward or propagates approximately
parallel to the optical axis while propagating between the
secondary mirror and the tertiary mirror. The primary mirror may be
physically located closer to the object than the tertiary
mirror.
The system may be further configured such that a chief ray from a
central field point diverges away from the optical axis while
propagating between the secondary mirror and the tertiary mirror.
The tertiary mirror may be physically located closer to the object
than the primary mirror.
The primary mirror is preferably concave, the fourth mirror is
preferably concave, the fifth mirror is preferably convex and the
sixth mirror is preferably concave.
The physical distance between the object and the image may be
substantially 1500 mm or less, and may further be substantially
1200 mm or less.
The system preferably has a numerical aperture at the image greater
than 0.18.
Each of the six reflecting surfaces preferably receives a chief ray
from a central field point at an incidence angle of less than
substantially 15.degree., preferably less than substantially
15.degree., and five of the six reflecting surfaces preferably
receives a chief ray from a central field point at an incidence
angle of less than substantially 11.degree., preferably less than
substantially 9.degree..
The system is preferably configured to have a RMS wavefront error
of 0.017.lamda. or less, and may be between 0.017.lamda. and
0.011.lamda..
In another embodiment, the shortcomings of the prior art are
overcome by a projection objective having an object plane and an
image plane and a light path for a bundle of light rays from the
object plane to the image plane. The six mirrors of the objective
are arranged in the light path from the object plane to the image
plane. In such an embodiment the mirror closest to the image plane
where e.g. an object to be illuminated such as a wafer is situated
is arranged in such a way that an image-side numerical aperture is
NA.gtoreq.0.15. In this application the image-side numerical
aperture is understood to be the numerical aperture of the bundle
of light rays impinging onto the image plane. Furthermore, the
mirror arranged closest to the image plane of the objective is
arranged in such a way that the image-side free working distance
corresponds at least to the used diameter of the mirror next to the
wafer. In a preferred embodiment the image-side free working
distance is at least the sum of one-third of the used diameter of
the mirror next to the image plane and a length between 20 and 30
mm. In an alternative embodiment the image-side free working
distance is at least 50 mm. In a particularly preferred embodiment,
the image-side free working distance is 60 mm. In this application
the free working distance is defined as the distance of the vertex
of the surface of the mirror next to the image plane and the image
plane. All surfaces of the six mirrors in this application are
rotational-symmetric about a principal axis (PA). The vertex of a
surface of a mirror is the intersection point of the surface of a
mirror with the principal axis (PA). Each mirror has a mirror
surface. The mirror surface is the physical mirror surface upon
which the bundle of light rays traveling through the objective from
the object plane to the image plane impinge. The physical mirror
surface or the used area of a mirror can be an off-axis or an
on-axis mirror segment relative to the principal axis (PA).
In another embodiment, a projection objective that comprises six
mirrors is characterized by an image-side numerical aperture, NA,
greater than 0.15 and an arc-shaped field width, W, at the wafer in
the range 1.0 mm.ltoreq.W. The peak-to-valley deviation, A, of the
aspheres are limited with respect to the best fitting sphere of the
physical mirror surface of all mirrors by: A.ltoreq.19
.mu.m.about.102 .mu.m (0.25-NA)-0.7 .mu.m/mm (2 mm-W). In a
preferred embodiment, the peak-to-valley distance A of the aspheres
is limited with respect to the best fitting sphere of the off-axis
segments of all mirrors by: A.ltoreq.12 .mu.m-64 .mu.m
(0.25-NA)-0.3 .mu.m/mm (2 mm-W).
According to yet another embodiment, a projection objective that
includes six mirrors is characterized by an image-side numerical
aperture NA.gtoreq.10.15 and an image-side width of the arc-shaped
field W.gtoreq.1 mm, and the angles of incidence AOI are limited
for all rays of the light bundle impinging a physical mirror
surface on all six mirrors S1, S2, S3, S4, S5, S6 by:
AOI.ltoreq.23.degree.-35.degree.(0.25-NA)-0.2.degree./mm (2 mm-W).
wherein the angles of incidence AOI refer to the angle between the
incident ray and the normal to the physical mirror surface at the
point of incidence. The largest angle of any incident bundle of
light rays occurring on any of the mirrors is always given by the
angle of a bundle-limiting ray.
Preferably, an embodiment of the invention would encompass all
three of the above aspects, e.g., an embodiment in which the free
optical working distance would be more than 50 mm at NA=0.20 and
the peak-to-valley deviation of the aspheres, as well as the angles
of incidence, would lie in the regions defined above.
The asphericities herein refer to the peak-to-valley (PV)
deviation, A, of the aspherical surfaces with respect to the best
fitting sphere of the physical mirror surface of an specific
mirror. The physical mirror surface of a specific mirror is also
denoted as the used area of this specific mirror. The aspherical
surfaces are approximated in the examples by using a sphere. The
sphere has a center on the figure axis vertex of the mirror. The
sphere intersects the asphere in the upper and lower endpoint of
the used area in the meridian section. The data regarding the
angles of incidence always refer to the angle between the incident
ray and the normal to the physical mirror surface at the point of
incidence. The largest angle of any incident bundle of light rays
occurring on any of the physical mirror surfaces is always given by
the angle of a bundle-limiting ray. The used diameter or the
diameter of the physical mirror surface will be defined here and
below as the envelope circle diameter of the physical mirror
surface or the used area of a mirror, which is generally not
circular.
In a preferred embodiment the free working distance is 60 mm.
The objective can be used not only in the EUV, but also at other
wavelengths, without deviating from the scope of the invention. In
any respect, however, to avoid degradation of image quality,
especially degradation due to central shading, the mirrors of the
projection objectives should be arranged so that the light path of
the bundle of light rays traveling from the object plane to the
image plane is obscuration-free. Furthermore, to provide easy
mounting and adjusting of the system, the physical mirror surfaces
have a rotational symmetry to a principal axis (PA). Moreover, to
have a compact design with an accessible aperture and to establish
an obscuration-free light path of the bundle of light rays
traveling from the object plane to the image plane, the projection
objective device is designed in such a way that an intermediate
image of the object situated in the object plane is formed after
the fourth mirror. In such systems, it is possible that the
aperture stop is situated in the front, low-aperture objective
part, with a pupil plane conjugated to the aperture stop imaged in
the focal plane of the last mirror. Such a system ensures
telecentricity in the image plane.
In an preferred embodiment of the invention, the aperture stop is
freely accessible and arranged in the light path from the object
plane to the image plane between the second and third mirror. Good
accessibility of the aperture stop is ensured when the ratio of the
distance between the first and third mirror to the distance between
the first and second mirror lies in the range of:
0.5<S1S3/S1S2<2. As defined for the free working distance in
general a distance between two mirrors is the distance of the
vertices of the surfaces of these mirrors.
Furthermore, in order to prevent vignetting of the light running
from the third to the fourth mirror, by the aperture stop arranged
between the second and third mirror, the ratio of the distance
between the second mirror and aperture stop to the distance between
the third mirror and the aperture stop lies in the range: 0.5<S2
aperture/(S3 aperture)<2. In such a system, the angles of
incidence on the physical mirror surfaces in the front part of the
objective are reduced.
An aperture stop which physically lies between the second mirror,
S2, and the first mirror, S1, must be formed at least partially as
a narrow ring in order to avoid clipping of light moving from S1 to
S2. In such a design, there is a danger that undesirable direct
light or light reflected on S1 and S2, will pass outside the
aperture ring and reach the image plane and thus the wafer.
However, if the aperture stop is placed in the light path between
the second and third mirror and physically close to the first
mirror (which can be easily achieved mechanically), an efficient
masking of this undesired light is possible. The aperture stop can
be designed both as an opening in the first mirror or an opening
which is arranged behind the first mirror.
In another embodiment of the invention, the aperture stop is
arranged on or near the second mirror. Arrangement of the aperture
on a mirror has the advantage that it is easier to manufacture.
In order to ensure an obscuration-free ray path with simultaneously
low angles of incidence, the ratio of the distance between the
first and third mirrors (S1S3) to the distance between the first
and second mirrors (S1S2) lies in the range:
0.3.ltoreq.S1S3/S1S2.ltoreq.2.0, while the ratio of the distance
between the second and third mirrors (S2S3) to the distance between
the third and fourth mirrors (S3S4) lies in the range:
0.7.ltoreq.S2S3/S3S4.ltoreq.1.4.
In order to be able to make the necessary corrections of imaging
errors in the six-mirror systems, in a preferred embodiment, all
six mirrors are designed to be aspherical. However, an alternative
embodiment whereby at most five mirrors are aspherical can simplify
the manufacturing, because it is then possible to design one
mirror, preferably the largest mirror, i.e., the quaternary mirror,
in the form of a spherical mirror. Moreover, it is preferred that
the second to sixth mirror be in a
concave-convex-concave-convex-concave sequence.
In order to achieve a resolution of at least 50 nm, the design part
of the rms wavefront section of the system should be at most
0.07.lamda. and preferably 0.03.lamda..
Advantageously, in the embodiments of the invention, the objectives
are always telecentric on the image-side.
In projection systems which are operated with a reflection mask, a
telecentric light path on the object-side is not possible without
illumination through a beam splitter which reduces the transmission
strongly. One such device is known from JP 95 28 31 16.
In systems with transmission mask, the projection objective can be
telecentric on the object side. In these embodiments, the first
mirror is preferably concave.
The telecentericity error in the image plane, where the the wafer
is situated should not exceed 10 mrad and is typically between 5
mrad and 2 mrad, with 2 mrad being preferred. This ensures that
changes of the imaging ratio remain within tolerable limits over
the depth of focus.
In an preferred embodiments of the invention, the six mirror
objective could comprise a field mirror, a reducing three-mirror
subsystem and a two-mirror subsystem.
In addition to the projection objective also a projection exposure
apparatus is shown, that includes at least a projection objective
device. In a first embodiment, the projection exposure apparatus
has a reflection mask, while in an alternative embodiment, it has a
transmission mask. Preferably, the projection exposure apparatus
includes an illumination device for illuminating an off-axis
arc-shaped field and the system is designed as an arc-shaped field
scanner. Furthermore, the secant length of the scan slit is at
least 26 mm and the ring width is greater than 0.5 mm.
The invention will be described below with the aid of the drawings
as examples.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a plan view of an EUV optical projection system
according to a first preferred embodiment.
FIG. 2 schematically illustrates the geometry of the arcuate ring
field according to the preferred embodiments at the object.
FIG. 3 shows a plan view of an EUV optical projection system
according to a second preferred embodiment.
FIG. 4 shows a plan view of an EUV optical projection system
according to a third preferred embodiment.
FIG. 5 shows the ring field in the object plane of the
objective.
FIG. 6 shows an embodiment with an intermediate image, a freely
accessible aperture stop between a second and third mirror, and a
image side numerical aperture of 0.2.
FIG. 7 shows a prior art six-mirror objective arrangement for
wavelengths >100 nm as disclosed in U.S. Pat. No. 5,686,728.
FIG. 8 shows a second embodiment with an aperture stop between the
second and third mirror at the first mirror.
FIG. 9 shows a third embodiment with an aperture stop on the second
mirror and a working distance of 59 mm.
FIG. 10 shows a fourth embodiment with an intermediate image, a
image side numerical aperture NA of 0.28 as well as a free working
distance on the image-side which is at least the sum of one-third
of the useful diameter of the mirror nearest to the wafer and a
length which lies between 20 and 30 mm.
FIG. 11 shows a fifth embodiment of a system with an intermediate
image and a image side numerical aperture NA of 0.30.
FIGS. 12A and 12B show the used diameter for different physical
mirror surfaces or used areas of a mirror.
INCORPORATION BY REFERENCE
What follows is a cite list of references which, in addition to
that which is described in the background and brief summary of the
invention above, are hereby incorporated by reference into the
detailed description of the preferred embodiments, as disclosing
alternative embodiments of elements or features of the preferred
embodiment not otherwise set forth in detail below. A single one or
a combination of two or more of these references may be consulted
to obtain a variation of the preferred embodiments described below.
Further patent, patent application and non-patent references, and
discussion thereof, cited in the background and/or elsewhere herein
are also incorporated by reference into the detailed description of
the preferred embodiments with the same effect as just described
with respect to the following references:
U.S. Pat. Nos. 5,063,586, 5,071,240, 5,078,502, 5,153,898,
5,212,588, 5,220,590, 5,315,629, 5,353,322, 5,410,434, 5,686,728,
5,805,365, 5,815,310, 5,956,192, 5,973,826, 6,033,079, 6,014,252,
6,188,513, 6,183,095, 6,072,852, 6,142,641, 6,226,346, 6,255,661
and 6,262,836;
European patent applications no. 0 816 892 A1 and 0 779 528 A;
and
"Design of Reflective Relay for Soft X-Ray Lithography", J. M.
Rodgers, T. E. Jewell, International Lens Design Conference,
1990;
"Reflective Systems design Study for Soft X-ray Projection
Lithography", T. E. Jewell, J. M. Rodgers, and K. P. Thompson, J.
Vac. Sci. Technol., November/December 1990.
"Optical System Design Issues in Development of Projection Camera
for EUV Lithography", T. E. Jewell, SPIE Volume 2437, pages
340-347;
"Ring-Field EUVL Camera with Large Etendu", W. C. Sweatt, OSA TOPS
on Extreme Ultraviolet Lithography, 1996; and
"Phase Shifting Diffraction Interferometry for Measuring Extreme
Ultraviolet Optics", G. E. Sommargaren, OSA TOPS on Extreme
Ultraviolet Lithography, 1996;
"EUV Optical Design for a 100 nm CD Imaging System", D. W. Sweeney,
R. Hudyma, H. N. Chapman, and D. Shafer, SPIE Volume 3331, pages
2-10.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Three specific preferred embodiments relating to this optical
projection system are described.
First Preferred Embodiment:
FIG. 1 shows a plan view of a first preferred embodiment, and,
taking in conjunction with Table 1 and Table 2, provides an
illustrative, exemplary description of this embodiment. Light
impinges on an object, e.g. a reflective mask or reticle from an
illumination system and is directed to concave mirror M1 after
which it reflects from the mirror M1 and passes through a
physically accessible aperture stop APE that is located between
Mirror M1 and M2. This aperture stop APE is located a substantial
distance from the first concave mirror M1 and, likewise, this
aperture stop APE is located a substantial distance from concave
mirror M2. After the illumination reflects off concave mirror M2,
the light comes to a focus at an intermediate image IMI that is
located in close proximity to convex mirror M3. From mirror M3 the
illumination is directed toward concave mirror M4 where the light
is nearly collimated and directed toward convex mirror M5. Upon
reflection from mirror M5, the light impinges on concave mirror M6
where it is reflected in a telecentric manner (the chief rays are
parallel to the optical axis OA) and focused on the image IM. A
semiconductor wafer is typically arranged at the position of the
image IM. Since a concave optical surface has positive optical
power (P) and a convex optical surface has negative optical power
(N), this present embodiment may be characterized as a PPNPNP
configuration.
Although there are many ways to characterize this optical system,
one convenient way is to break the system into two groups G1 and
G2. Starting at the object OB, the first group G1 is comprised the
concave mirror pair M1 and M2. This group forms an intermediate
image IMI at a magnification of about -0.8.times. between mirror M2
and mirror M3. The remaining four mirrors (convex mirror M3,
concave mirror M4, convex mirror M5 and concave mirror M6) comprise
the second imaging or relay group G2. This second group G2 works at
a magnification of approximately -0.3.times., resulting in 4.times.
reduction (the reduction ratio is the inverse of the absolute value
of the optical magnification) of the object OB at the image IM.
The optical prescription of the first embodiment of FIG. 1 is
listed in Table 1 and Table 2. The aspheric mirror surfaces are
labeled A(1)-A(6) in the tables with A(1) corresponding to mirror
M1, A(2) corresponding to mirror M2, and so on. Four additional
surfaces complete the description of this illustrative and
exemplary embodiment with object OB and image IM representing the
planes, where in a lithographic apparatus the mask and the wafer
are arranged. A surface designation is also made for the location
of the aperture stop APE and intermediate image IMI. After each
surface designation, there are two additional entries listing the
vertex radius of curvature (R) and the vertex spacing between the
optical surfaces. In this particular embodiment, each of the
surfaces is rotationally symmetric conic surface with higher-order
polynomial deformations. The aspheric profile is uniquely
determined by its K, A, B, C, D, and E values. Each mirror uses
4th, 6th, 8th, 10th, and 12th order polynomial deformations. The
sag of the aspheric surface (through 12th order) in the direction
of the z-axis (z) is given by: .times. .times..times..times..times.
.times..times. .times..times. .times..times. .times. ##EQU00001##
where h is the radial coordinate; c is the vertex curvature of the
surface (1/R); and A, B, C, D, and E are the 4th, 6th, 8th, 10th,
and 12th order deformation coefficients, respectively. These
coefficients are listed in Table 2.
The optical system of this first preferred embodiment is designed
to project a ring field format that is illuminated with extremely
ultraviolet (EUV) or soft X-ray radiation. The numerical aperture
NAO at the object OB is 0.050 radians; at a 4.times. reduction this
corresponds to a numerical aperture NA of 0.20 at the image IM. The
ring field 21 at the object OB is shown with FIG. 2. It is centered
at 118 mm from the optical axis, which contains the vertex of each
of the aspheric mirrors. This annular field extends from 114 mm to
122 mm forming an arcuate slit with a width 23 of 8 mm. The extent
25 of the ring field 21 perpendicular to the scan direction 27
becomes 104 mm. The central field point is denoted with the
reference sign 29. At 4.times. reduction, this ring field becomes
2.0 mm wide in the scan direction at the image.
As a result of the distribution of optical power and location of
the aperture stop APE, the incidence angles are well controlled so
that the design is compatible with EUV or soft X-ray multilayer
coatings. As measured by the chief ray CR from the central field
point 29, this system exhibits very low incidence angles ranging
from 2.9.degree. to 12.5.degree.. The chief ray incidence angles
for the chief ray CR from the central field point 29 are: Object:
5.2.degree.; M1: 6.5.degree.; M2: 5.0.degree.; M3: 12.5.degree.;
M4: 5.6.degree., M5: 8.6.degree., and M6: 2.9.degree.. These low
incidence angles are a key enabling element for EUV lithography
since (1) they minimize the multilayer induced amplitude and phase
errors that have an adverse impact to lithographic performance and
(2) enable simplified coating designs that do not rely heavily on
the use of laterally graded coating profiles. With poor design
(i.e., failure to minimize these incidence angles), these
multilayer-induced amplitude and phase errors can lead to critical
dimension (CD) errors that are easily greater than 20% of the
nominal linewidth, making the system unusable for production
applications.
Besides the low incidence angles, a preferred system further
enables EUV lithography by utilizing mirrors with low peak aspheric
departure. The maximum peak departure, contained on mirror M1, is
25.0 .mu.m. The other mirrors have low-risk aspheres with
departures that range from 0.5 .mu.m to 14 .mu.m. The low aspheric
departures of the mirror surfaces facilitate visible light
metrology testing without a null lens or Computer Generated
Hologram CGH, resulting in surface figure testing to a high degree
of accuracy. An aspheric mirror with a very large peak departure is
unproducible because it cannot be measured to the required accuracy
to realize lithographic performance.
Table 3 summarizes the performance of the PPNPNP configuration of
FIG. 1. The table demonstrates that this first preferred embodiment
is able to achieve lithographic performance with a resolution on
the order of 30 nm (assuming a k1-factor of approximately 0.5). The
location of the aperture stop APE is selected so that the third
order astigmatism contribution from the strong concave secondary
mirror M2 is made very small. The strongly undercorrected
astigmatic contribution from the primary mirror M1 comes from the
aspheric departure on M1 and is balanced by the M3/M4 combination.
Considering the system without any aspheres, the location of the
aperture stop APE also effectively balances the third-order coma
and distortion contributions from the primary mirror M1 and
secondary mirror M2. A hyperbolic profile is added to the primary
mirror M1 in such a way as to create a large undercorrected
spherical contribution, coma contribution, and astigmatism
contribution, thus promoting good aberration correction allowing
the residual wavefront error (departure from the ideal reference
sphere) to remain exceedingly small. In fact, aberration correction
and resulting aberration balance reduces the composite RMS
wavefront error is only 0.0125.lamda. (0.17 nm), with simultaneous
correction of the static distortion to less than 2 nm across the
field.
This optical projection system has further benefits in that the
system of FIG. 1 may be scaled in either numerical aperture or
field. For example, it is desirable to scale this concept to larger
numerical aperture to improve the modulation in the aerial image
thus allowing 30 nm resolution with a less aggressive k1-factor.
The results of a simple scaling experiment demonstrate that this
preferred embodiment easily supports such scaling to larger
numerical apertures. Without making any modifications, an analysis
of the composite root mean square (RMS) wavefront error was made at
a numerical aperture of 0.24, which represents a 20% increase to
the value shown in Table 2. The composite RMS wavefront error was
found to be 0.0287.lamda. (0.38 nm), a level that supports
lithographic quality imaging.
Referring to FIG. 2, it is desirable to increase the field of view
in the scan direction to increase the number of wafers per hour
(WPH) that the lithographic apparatus can process. The idea is that
more area can be printed per unit time with a wider arcuate slit.
The results of another simple scaling experiment demonstrate that
this preferred embodiment easily supports increases in field width.
Without making any modifications, an analysis of the composite RMS
wavefront error was made over a 3 mm wide arcuate slit, which
represents a 50% increase to the value shown in Table 2. The
composite RMS wavefront error was found to be 0.0285.lamda. (0.38
nm), again a level that supports lithographic quality imaging.
Second Preferred Embodiment:
In a second of these general embodiments, an optical projection
system for extreme ultraviolet (EUV) lithography including six
mirrors arranged in a PPNPNP configuration is disclosed. The plan
view of this second preferred embodiment is shown in FIG. 3, which
demonstrates a PPNPNP configuration designed for EUV lithography at
a wavelength of 13.4 nm. Like the first preferred embodiment, the
system is reimaging, and unlike the '310 and '079 embodiments,
locates the intermediate image IMI' before the second mirror pair.
In this example, the intermediate image IMI' is located between
mirror M2' and M3', helping to promote low incidence angle
variation across mirror M5'. This construction also enables low
mean incidence angles on mirror M1', M2', M4', and M6'. These low
incidence angles are advantageous for maintaining good multilayer
compatibility. The aperture stop APE' is located between M1' and
M2' and is significantly spaced from either mirror, e.g., more than
200 mm.
In addition to the features outlined by the first preferred
embodiment, this second preferred embodiment teaches that the
tertiary mirror M3' may be located on the object side of the
primary mirror M1' (i.e., closer to the object OB' than the primary
mirror M1'). This feature departs drastically from the teaches of
the prior art that show the tertiary mirror must be located either
in close proximity to the primary mirror ('079 patent) or on the
image side of the primary mirror('310 patent). This location of
mirror M3' enables a reduction in the overall length from object
plane OB to image plane IM (total track length) by some 250 mm.
This decrease in total track length is accomplished by shifting the
tertiary mirror from the image side of the primary mirror M1' to
the object side of the primary mirror M1' and then decreasing the
distance between mirror M1' and mirror M6'. This also allows the
parent diameter of the tertiary mirror M3' to be smaller than
either the primary mirror M1' or the secondary mirror M2'. These
changes affect the angular condition of the chief rays upon
reflection from the secondary mirror M2'. Prior art teaches that
the chief ray from the central field point must diverge from the
optical axis after reflection from the secondary mirror ('310
patent, '079 patent, etc.), but now the chief ray CR' assumes a
more parallel condition with respect to the optical axis OA'. In
this second embodiment, this chief ray CR' is made identically
parallel to the optical axis OA'. This change in chief ray angle
impacts the aberration balance in the design enough to form a
distinct local minima, so that the residual aberration set seen in
a Zernike decomposition of the wavefront differs from that of the
first preferred embodiment.
The optical prescription of this second preferred embodiment of
FIG. 3 is listed in Table 4 and Table 5. The aspheric mirror
surfaces are labeled A(1)-A(6) in the tables with A(1)
corresponding to mirror M1, A(2) corresponding to mirror M2, and so
on.
Like the first preferred embodiment, the object OB' will be
projected to the image IM' at 4.times. reduction in a ring field
format with a telecentric imaging bundle (chief rays parallel to
the optical axis OA' at the image IM'). Table 6 provides a
performance summary demonstrating that this preferred embodiment is
capable of lithographic performance at a wavelength of 13.4 nm. For
comparison to the first embodiment, this second preferred
embodiment also utilizes a numerical aperture NA of 0.20 at the
image IM' and projects a 2 mm wide field in the scan direction. The
system is compatible with reflective multilayer coatings since the
incidence angles at each mirror are relatively small. As measured
by the chief ray CR' from the central field point 29', the
incidence angles range from 3.9.degree. to 14.6.degree.. The exact
chief ray incidence angles for the chief ray CR' from the central
field point 29' are: Object OB': 5.6.degree.; M1: 7.2.degree.; M2:
4.4.degree.; M3: 14.6.degree.; M4: 8.8.degree., M5: 9.7.degree.,
and M6: 3.9.degree.. Again, these low incidence angles are a key
enabling element for EUV lithography since the low incidence angles
minimize the multiplayer induced amplitude and phase errors that
have an adverse impact to lithographic performance.
The composite RMS wavefront error across the field is 0.0131.lamda.
(0.18 nm), ranging from 0.0095.lamda. (0.13 nm) at the best field
point to 0.0157.lamda. (0.21 nm) at the worst. The distortion of
the chief ray has been reduced to less than 1 nm across the field.
Clearly this combination of telecentric imaging, a highly corrected
wavefront, and essentially no distortion demonstrates that this
system is suitable for modern lithography at soft x-ray or extreme
ultratviolet wavelengths.
This preferred embodiment has further advantages in that the system
of FIG. 3 may be scaled in either numerical aperture or field to
address even more advanced requirements. The results of a simple
numerical aperture scaling experiment demonstrate that this
preferred embodiment easily supports scaling to larger numerical
apertures. Without making any modifications, an analysis of the
composite root mean square (RMS) wavefront error was made at a
numerical aperture of 0.22, which represents a 10% increase to the
value shown in Table 4. The composite RMS wavefront error was found
to be 0.027.lamda. (0.36 nm), a level that supports lithographic
quality imaging.
The results of another simple scaling experiment demonstrate that
this preferred embodiment easily supports increases in field width.
Without making any modifications, an analysis of the composite RMS
wavefront error was made over a 3 mm wide arcuate slit, which
represents a 50% increase to the value shown in Table 6. The
composite RMS wavefront error was found to be 0.028.lamda. (0.38
nm), again a level that supports lithographic quality imaging.
Third Preferred Embodiment:
The third preferred embodiment is shown in FIG. 4. Like the first
and second preferred embodiments, this system utilizes a re-imaging
PPNPNP configuration with a physically accessible aperture stop
APE'' that is located between the primary mirror M1'' and secondary
mirror M2''. And like the first and second embodiments, the
intermediate image IMI'' is located between the secondary mirror
M2'' and the tertiary mirror M3''. Similar to the second
embodiment, the tertiary mirror M3'' is located on the object side
of the primary mirror M1''. This particular embodiment differs from
the second preferred embodiment in that the chief ray CR'' from the
central field point 29'' converges toward the optical axis OA''
after reflection from the secondary mirror M2'', thus forming
another advantageous projection system with distinct
characteristics.
The optical prescription for this third embodiment of FIG. 4 is
listed in Table 7 and Table 7. Table 7 lists the vertex radius of
curvature as well as the separation between these mirrors along the
optical axis. Each mirror is aspheric and labeled A(1)-A(6) in the
tables with A(1) corresponding to mirror M1'', A(2) corresponding
to mirror M2'', and so on. The prescription of the aspheric surface
deformation per equation (1) is listed in Table 8. Taken together
with the information provided in Table 9, an illustrative and
exempary description of this prefered embodiment is disclosed.
Like the first two preferred embodiments, the object OB'', e.g. a
pattern on mask or reticle, will be projected to the image IM'' at
4.times. reduction in a ring field format with a telecentric
imaging bundle (chief rays parallel to the optical axis at the
image). At the image'' typically a semiconductor wafer is arranged.
Table 6 provides a performance summary demonstrating that this
preferred embodiment is capable of lithographic performance at a
wavelength of 13.4 nm. For comparison purposes, this third
preferred embodiment also utilizes a numerical aperture NA of 0.20
at the image IM'' and projects a 2 mm wide field in the scan
direction. The system is compatible with reflective multilayer
coatings since the incidence angles at each mirror are relatively
small. As measured by the chief ray CR'' from the central field
point 29'', the incidence angles range from 3.9.degree. to
13.9.degree.. The exact chief ray incidence angles from the central
field point are: Object OB'': 6.6.degree.; M1: 8.0.degree.; M2:
4.4.degree.; M3: 13.9.degree.; M4: 8.6.degree., M5: 9.6.degree.,
and M6: 3.9.degree.. Again, these low incidence angles are a key
enabling element for EUV lithography since the low incidence angles
minimize the multiplayer induced amplitude and phase errors that
have an adverse impact to lithographic performance.
The composite wavefront error across the field is 0.0203.lamda.
(0.27 nm), ranging from 0.0148.lamda. (0.20 nm) at the best field
point to 0.0243.lamda. (0.33 nm) at the worst. The distortion of
the chief ray has been reduced to less than 1 nm across the field.
Clearly this combination of telecentric imaging, a highly corrected
wavefront, and essentially no distortion demonstrates that this
system is suitable for modern lithography at soft x-ray or extreme
ultratviolet wavelengths. The design can also be scaled in
numerical aperture or field like second preferred embodiment.
The optical design descriptions provided above for the first-third
embodiments herein demonstrate an advantageous catoptric projection
system concept for EUV lithography. While these embodiments have
been particularly described for use in a 13.4 nm tool, the basic
concept is not limited to use with lithographic exposure tools at
this wavelength, either shorter or longer, providing a suitable
coating material exists in the soft x-ray region of the
electromagnetic spectrum.
While exemplary drawings and specific embodiments of the present
invention have been described and illustrated, it is to be
understood that that the scope of the present invention is not to
be limited to the particular embodiments discussed. Thus, the
embodiments shall be regarded as illustrative rather than
restrictive, and it should be understood that variations may be
made in those embodiments by workers skilled in the arts without
departing from the scope of the present invention as set forth in
the claims that follow, and equivalents thereof. For example, one
skilled in the art may reconfigure the embodiments described herein
to expand the field of view, increase the numerical aperture, or
both, to achieved improvements in resolution or throughput.
TABLE-US-00001 TABLE 1 Optical prescription first preferred
embodiment Vertex Element number radius of curvature Thickness (mm)
Glass Object OB INFINITY 806.775 A(1) -1997.63 -328.184 REFL
Aperture Stop APE INFINITY -399.404 A(2) 1148.069 649.7918 REFL
Intermediate INFINITY 132.9323 image IMI A(3) 486.7841 -277.569
REFL A(4) 660.9159 890.6587 REFL A(5) 393.8628 -476.472 REFL A(6)
580.3377 501.472 REFL Image IM
TABLE-US-00002 TABLE 2 Aspheric prescription Aspheric K A B C D E
A(1) -9.1388E+01 5.4676E-10 7.0301E-15 -1.4409E-19 2.1657E-25
5.5712E-30 A(2) -6.4930E-01 3.7924E-11 3.2952E-18 -1.1462E-21
8.4115E-26 -4.9020E-30 A(3) -2.3288E-01 3.3571E-10 1.8240E-14
-1.9218E-19 -4.2667E-23 2.9468E-23 A(4) -6.4180E-03 3.9345E-11
1.8257E-16 -6.9023E-22 1.3692E-26 -6.2042E-32 A(5) 1.5857E+00
-1.7764E-09 7.7970E-14 -1.2619E-18 5.4017E-22 -3.8012E-26 A(6)
8.9884E-02 -4.2455E-12 1.4898E-17 1.4824E-22 -7.0550E-28
6.6775E-32
TABLE-US-00003 TABLE 3 Performance summary first preferred
embodiment Metric Performance Wavelength 13.4 nm Numerical aperture
(image) 0.20 Ringfield format (image) i. Radius 30.0 mm ii. Width
2.0 mm iii. Chord 26.0 mm Reduction ratio (nominal) 4:1 Overall
length (mm) 1500 mm RMS wavefront error (waves @ .lamda. = 13.4 nm)
i. Composite 0.0125.lamda. ii. Variation
0.0076.lamda.-0.0167.lamda. Chief ray distortion (max) 1.9 nm Exit
pupil location Infinity Max. aspheric departure across
instantaneous clear aperture (ICA) i. M1 25.0 .mu.m ii. M2 0.5
.mu.m iii. M3 1.4 .mu.m iv. M4 14.0 .mu.m v. M5 3.0 .mu.m vi. M6
3.8 .mu.m
TABLE-US-00004 TABLE 4 Optical prescription second preferred
embodiment Vertex Element number radius of curvature Thickness (mm)
Glass Object Plane OB' INFINITY 786.7828 A(1) -1522.9647 -275.3849
REFL Aperture Stop APE' INFINITY -461.3979 A(2) 922.8035 452.3057
REFL Intermediate INFINITY 95.0000 image IMI' A(3) 273.0204
-218.5016 REFL A(4) 511.1320 834.1959 REFL A(5) 434.1472 -326.2172
REFL A(6) 440.9571 363.2172 REFL Image IM'
TABLE-US-00005 TABLE 5 Aspheric prescription second preferred
embodiment Aspheric K A B C D E A(1) -6.5661E+04 3.6028E-01
2.7656E-09 1.3237E-14 5.6475E-20 1.4711E-23 A(2) 1.0837E-03
-3.0142E+00 3.384E-18 -6.8499E-16 -1.8748E-20 1.0985E-24 A(3)
3.6627E-03 1.9328E+00 -1.6611E-08 -4.9082E-13 2.9169E-17
-3.8673E-27 A(4) 1.9564E-03 -1.2442E-01 -1.0927E-11 2.7712E-16
-2.0608E-21 3.6395E-26 A(5) 1.3034E-03 8.5377E+00 -6.9001E-09
-2.2929E-13 -8.9645E-18 -2.1791E-27- A(6) 2.2678E-03 1.4526E-01
3.2069E-11 3.3003E-16 5.1329E-21 -1.7296E-25
TABLE-US-00006 TABLE 6 Performance summary second preferred
embodiment Metric Performance Wavelength 13.4 nm Numerical aperture
(image) 0.20 Ringfield format (image) i. Radius 30.0 mm ii. Width
2.0 mm iii. Chord 26.0 mm Reduction ratio (nominal) 4:1 Overall
length (mm) 1250 RMS wavefront error (waves @ .lamda. = 13.4 nm) i.
Composite 0.0131.lamda. ii. Variation 0.0095.lamda.-0.0157.lamda.
Chief ray distortion (max) 0.9 nm Exit pupil location Infinity Max.
aspheric departure across instantaneous clear aperture (ICA) i. M1'
18.0 .mu.m ii. M2' 6.2 .mu.m iii. M3' 8.7 .mu.m iv. M4' 28.0 .mu.m
v. M5' 7.0 .mu.m vi. M6' 7.0 .mu.m
TABLE-US-00007 TABLE 7 Optical prescription third preferred
embodiment Vertex Element number radius of curvature Thickness (mm)
Glass Object Plane OB'' INFINITY 708.2875 A(1) -1351.9353 -222.3328
REFL Aperture Stop APE'' INFINITY -435.9047 A(2) 801.1198 389.5537
REFL Intermediate INFINITY 85.9324 image IMI'' A(3) 257.6903
-223.6826 REFL A(4) 508.9915 827.9429 REFL A(5) 434.7744 -321.5090
REFL A(6) 436.7586 358.5090 REFL Image IM''
TABLE-US-00008 TABLE 8 Aspheric prescription third preferred
embodiment Aspheric K A B C D E A(1) -7.3968E-04 1.8042E+00
2.2388E-09 4.0136E-15 6.8479E-19 -1.2865E-22 A(2) 1.2483E-03
-2.6267E+00 4.4819E-10 -1.7571E-15 5.8143E-20 -3.7874E-24 A(3)
3.8806E-03 -8.5604E-01 2.2165E-08 -6.7204E-12 1.1406E-15
-1.0131E-19 A(4) 1.9647E-03 -7.7387E-02 -3.8053E-11 -1.2483E-15
2.8880E-20 -3.4746E-25 A(5) 2.3000E-03 8.3687E+00 -6.1944E-09
-1.9683E-13 -1.6280E-17 4.8296E-21 A(6) 2.2896E-03 1.3269E-01
5.6594E-11 5.5533E-16 -1.1978E-21 7.3097E-25
TABLE-US-00009 TABLE 9 Performance summary third preferred
embodiment Metric Performance Wavelength 13.4 nm Numerical aperture
0.20 (image IM'') Ringfield format (image IM'') i. Radius 30.0 mm
ii. Width 2.0 mm iii. Chord 26.0 mm Overall length (mm) 1156
Reduction ratio (nominal) 4:1 RMS wavefront error (waves @ .lamda.
= 13.4 nm) i. Composite 0.0203.lamda. ii. Range
0.0148.lamda.-0.0243.lamda. Chief ray distortion (max) 1.5 nm Exit
pupil location Infinity Max. aspheric departure across
instantaneous clear aperture (ICA) i. M1'' 17.3 .mu.m ii. M2'' 6.4
.mu.m iii. M3'' 9.7 .mu.m iv. M4'' 32.2 .mu.m v. M5'' 6.7 .mu.m vi.
M6'' 6.7 .mu.m
In FIG. 5 the object field 1100 of a projection exposure apparatus
in the object plane of the projection objective according to the
invention is shown. The object plane is imaged by means of the
projection objective in an image plane, in which a light sensitive
object, for example a wafer with a light sensitive material is
arranged. The image field in the image plane has the same shape as
the object field. The object- or the image field 1100 has the
configuration of a segment of a ring field. The ring field has an
axis of symmetry 1200.
In addition the axis extending the object plane, i.e., the x-axis
and the y-axis are depicted. As can be seen from FIG. 5, the axis
of symmetry 1200 of the ring field runs in the direction of the
y-axis. At the same time the y-axis coincides with the scanning
direction of an projection exposure apparatus, which is designed as
a ring field scanner. The x-direction is thus the direction that
stands perpendicular to the scanning direction, within the object
plane. The ring field has a so called ring field radius R, which is
defined by the distance of the central field point 1500 of the
image field from the principal axis (PA) of the projection
objective. The arc-shaped field in the object plane as well as in
the image plane has a arc shaped field width W, which is the
extension of the field in scanning or in y-direction and a secant
length SL.
In FIGS. 6, 8 and 9, arrangements of the six-mirror projection
objectives are shown. Each embodiment has a free working distance
that corresponds at least to the used diameter of the physical
mirror surface or mirror segment next to the wafer. In contrast,
FIG. 7 shows a prior art system for use with wavelengths >100
nm, such as the system of U.S. Pat. No. 5,686,728. In all
embodiments shown in FIGS. 6, 8 and 9, the same reference numbers
will be used for the same components and the following nomenclature
will be employed: first mirror (S1), second mirror (S2), third
mirror (S3), fourth mirror (S4), fifth mirror (S5), and sixth
mirror (S6)
In particular, FIG. 6 shows a six-mirror projection objective with
a ray path from the object plane 2, i.e. reticle plane to the image
plane 4, i.e. wafer plane. The embodiment includes a field mirror
S1, which forms a virtual image of an object with an imaging ratio
.beta.>0. A three-mirror system formed from S2, S3 and S4 is
also provided and produces a real, reduced image of the virtual
image as the intermediate image, Z. Lastly, a two-mirror system S5,
S6, images the intermediate image Z in the wafer plane 4 while
maintaining the requirements of telecentricity. The aberrations of
the three-mirror and two-mirror subsystems are balanced against one
another so that the total system has a high optical quality
sufficient for integrated circuit fabrication applications.
The physical aperture stop B is arranged between the second mirror
S2 and the third mirror S3. And, as is clear from FIG. 6, the
aperture stop is accessible in the ray path between the second
mirror S2 and the third mirror S3. Furthermore, the distance
between the vertex V5 of the surface of the mirror next to the
wafer, i.e., the surface of the fifth mirror S5 in the present
embodiment, and the image plane is greater than the used diameter
of the physical mirror surface of mirror S5. The used diameter of a
physical mirror surface is explained in more detail in the
description of FIGS. 12A and 12B. In other words, the following
condition is fulfilled:
physical distance from the vertex V5 of the surface of mirror S5 to
the image plane 4>used diameter of mirror S5.
Other distance requirements are also possible and may be used, such
as the physical distance is (1) greater than the sum of one-third
of the used diameter of the mirror next to the wafer, S5, and 20
mm, or (2) greater than 50 mm. In the preferred embodiment, the
physical distance is 60 mm.
Such a physical distance guarantees a sufficiently free working
distance A, and allows the use of optical components compatible for
use with wavelengths<100 nm, and preferably wavelengths of 11 to
13 nm. Optical components in this range include, for example, Mo/Si
or Mo/Be multi-layer systems, where the typical multilayer systems
for .lamda.=13 nm is Mo/Si layer pairs and for .lamda.=11 nm, is
Mo/Be systems, both of approximately 70 layer pairs. Reflectivities
attainable in such systems are approximately 70%. In the
multi-layer layer systems, layer stresses of above 350 MPa may
occur. Stresses of such values may induce surface deformation,
especially in the edge regions of the mirror.
The systems according to the invention, as they are shown, for
example, in FIG. 5, have: RES=k.sub.1.lamda./NA. This results in a
nominal resolution of at least 50 nm and 35 nm at a minimum
numerical aperture of NA=0.2 for k.sub.1=0.77 and .lamda.=13 nm,
and for k.sub.1=0.64 and .lamda.=11 nm, respectively, where k.sub.1
is a parameter specific for the lithographic process.
Furthermore, the light path for a bundle of light rays running from
the object plane to the image plane of the objective shown in FIG.
6 is obscuration-free. For example, in order to provide image
formats of 26.times.34 mm.sup.2 or 26.times.52 mm.sup.2, the
projection objectives according to the invention are preferably
used in an arc-shaped field scan projection exposure apparatus,
wherein the secant length of the scan slit is at least 26 mm.
Numerous masks can be used in the projection exposure apparatus.
The masks or reticle are arranged in the object plane of the
projection objective. The masks include transmission masks, stencil
masks and reflection masks. The projection objective, which is
telecentric on the image side, i.e. in the image plane, can be
telecentric or non-telecentric on the object side, i.e. in the
object plane depending on which mask is used. For example, if the
bundle of light rays is telecentric on the object-side when using a
reflection mask a transmission-reducing beam splitter must be
employed. If the bundle of light rays is non-telecentric on the
object-side, unevennesses of the mask leads to dimensional errors
in the image. Therefore, the angle of incidence of the chief ray of
the bundle of light rays through the central field point 1500 in
the object plane is preferably below 10.degree., so that the
requirements for reticle evenness lies in an achievable range.
Moreover, the system of FIG. 6 which is telecentric on the image
side has an image-side error of telecentry at the wafer level of 1
mrad for a image side numerical aperture of 0.2.
Due to the high image-side telecentricity, the entrance pupil of
the last mirror S6 is at or near the focal plane of this mirror.
Therefore, in systems with an intermediate image as described
before, the aperture, B, is in the front, low-aperture objective
part preferably in the light path between the first and third
mirror S1, S3. Thus the pupil plane conjugated with the aperture
stop will be imaged in the focal plane of the last mirror.
All mirrors S1-S6 of FIG. 6 are designed to be aspherical, with a
maximum asphericity of approximately 7.3 .mu.m. The low asphericity
of the embodiment shown in FIG. 6 is advantageous from a
manufacturing point of view, since the technological difficulties
in processing the surfaces of the multilayer mirrors increases
proportionally with aspherical deviation and gradient of the
asphere.
The highest angle of incidence of a ray impinging a mirror surface
in the six-mirror objective shown in FIG. 6 occur on the fifth
mirror S5 and is approximately 18.4.degree.. The maximum variation
of the angles of incidence of the rays within a bundle of light
rays impinging onto a mirror surface occurs on mirror surface of
mirror S5 and is approximately 14.7.degree.. The wavefront error at
.lamda.=13 nm is better than 0.032.lamda.; the centroid distortion
of the point spread function is <3 mm; and the static,
dimension-corrected distortion lies at 4 nm.
A freely accessible aperture stop between the second and third
mirror as well as no vignetting of the bundle of light rays running
from S3 to S4 by the aperture stop is achieved with small angles of
incidence of the rays impinging onto the mirror surfaces when the
following distance conditions are fulfilled: 0.5<S1S3/S1S2<2
and 0.5<S2 aperture/(S3 aperture)<2. Here, the abbreviation
S1S3 means the mechanical distance or physical distance between the
vertices V1 and V3 of the surface of the mirrors S1 and S3. And,
"S2 aperture" means the mechanical distance between the vertex V2
of the surface of mirror S2 and the aperture. Furthermore, in order
to reduce the angles of incidence on the mirrors in any of the
embodiments of FIGS. 6, 8, and 9, the distance from the object
plane, where e.g. the reticle is situated to the vertex of the
surface of the mirror S1 is made smaller than the mechanical
distance from the vertex of the surface of mirror S2 to the vertex
of the surface of mirror S3, i.e., the following applies: reticle
S1<S2S3. To ensure a sufficient free working distance A not only
on the image side but also on the object side the reticle is
situated sufficiently far in front of the first mirror next to the
object plane, which is in the present case the surface of the
second mirror S2. In the present case, for example, the physical
distance between the reticle and the vertex V2 of the surface of
mirror S2 is 80 mm.
Furthermore, in the embodiments of FIGS. 6 and 8 to 10, the
physical distance between the mirrors S3 and S6 is chosen that
mirrors of sufficient thickness can be used. Thicker mirrors have
sufficient strength and stability properties that can withstand the
high layer tensions described above. In these systems, the
following relationship is preferred: 0.3 (used diameter S3+used
diameter S6)<S3S6. Here S3S6 denotes the physical distance
between the vertex V3 of the surface of mirror S3 and the vertex V6
of the surface of the mirror S6.
In the following table 10, the parameters of the system represented
in FIG. 6 are exemplarily shown in Code V(.TM.) nomenclature. The
objective is a 5.times. system with a 26.times.2 mm.sup.2
arc-shaped field in the image plane, wherein 26 mm is the secant
length of the arc-shaped field and 2 mm is the width W of the arc
shaped field. Furthermore the numerical aperture is 0.2 on the
image side. The mean image side radius of the system is
approximately 26 mm.
TABLE-US-00010 TABLE 10 element No. radius Thickness diameter Type
Object INF 80.9127 258.1723 413.0257 S1 A(1) -88.8251 197.5712 REFL
-324.2006 195.6194 0.0000 188.6170 S2 A(2) 324.2006 188.7078 REFL
aperture 67.1796 423.6214 183.2180 0.0000 S3 A(3) -423.6214
184.7062 REFL -74.9270 519.0546 S4 A(4) 498.5484 541.0453 REFL
109.8242 248.6244 281.5288 177.5488 S5 A(5) -281.5288 65.0842 REFL
S6 A(6) 281.5288 187.9549 REFL 78.3999 Image image width 59.202
53.9889 aspherical constants: Z = (CURV) Y.sup.2/[1 + (1 - (1 + K)
(CURV).sup.2Y.sup.2).sup.1/2] + (A)Y.sup.4 + (B)Y.sup.6 +
(C)Y.sup.8 + (D)Y.sup.10 asphere CURV K A B C D A(1) 0.00031800
-27.686599 0.00000E+00 1.32694E-15 2.00546E-20 -8.49471E-25 A(2)
0.00094928 -3.998204 0.00000E+00 4.03849E-15 -6.15047E-20
2.73303E-25- A(3) 0.00126752 0.424198 0.00000E+00 1.58766E-15
-8.27965E-20 2.80328E-24 A(4) 0.00123850 0.023155 0.00000E+00
2.46048E-17 -1.08266E-22 3.75259E-28 A(5) 0.00329892 2.902916
0.00000E+00 1.55628E-12 -6.71619E-17 -5.30379E-21 A(6) 0.00277563
0.072942 0.00000E+00 2.96285E-16 3.99125E-21 4.55007E-26 Reference
wavelength = 13 nm
FIG. 7 shows an arrangement of a projection objective for
microlithography with a wavelength of .lamda.<100 nm according
to U.S. Pat. No. 5,686,728. Components substantially similar to
those of FIG. 6 are provided with the same reference numbers. As is
clear, the physical distance between the vertex V5 of the surface
of the mirror next to the image plane S5 and the image plane, where
the wafer is situated is significantly smaller than the used
diameter of the fifth mirror S5, lying mainly in the range of
approximately 20 mm. This leads to strength and stability problems
for the optics in the EUV region because of the extreme tensions in
the layers. Furthermore, the system has asphericities of .+-.50
.mu.m and a maximum angle of incidence of 38.degree..
FIG. 8 is an alternative embodiment of a six-mirror system in which
the aperture stop is situated on the first mirror. The same
components as in FIG. 6 again receive the same reference number in
FIG. 8. The free working distance A to the wafer is 60 mm in this
embodiment, as it was in the embodiment of FIG. 6, and thus it is
greater than the used diameter of the mirror next to the wafer, S5.
Similarly, as with FIG. 6, the physical distance between the vertex
V2 of the surface of mirror S2 and the vertex V3 of the surface of
mirror S3 was increased significantly in comparison to that of U.S.
Pat. No. 5,686,728, so that large angles of incidence can be
avoided in the system.
One difference to the objective of FIG. 6, is that in FIG. 8 the
aperture stop B is placed on the first mirror S1. As a result of
this position, a reduction in vignetting from the light reflected
on S2 is possible, whereas with the physical aperture stop
positioned between S1 and S2 light of the bundle of light rays
running thorough the objective could pass above the aperture stop
which is designed as a narrow ring. In the embodiment shown in FIG.
4, the aperture can be either an opening in the S1 mirror or an
aperture disposed behind S1 close to this mirror.
Another advantage of this embodiment is the spherical design of
mirror S4, which presents advantages especially from the point of
view of manufacturing, because mirror S4 is the largest mirror of
the system. With such a design, the asphericity in the used range
is increased slightly to 10.5 .mu.m. The largest angle of incidence
occurs on mirror S5 and is approximately 18.6.degree.. The
wavefront error of the arrangement is 0.032.lamda., within a 1.7 mm
wide arc-shaped field at .lamda.=13 nm. Furthermore, if the mirror
S4 is designed to be slightly aspherical with 0.4 .mu.m, then the
wavefront error can be kept to 0.031.lamda. within a 1.8 mm wide
arc-shaped field at .lamda.=13 nm. Efficient masking of the
undesirable light is obtained not only when the aperture stop is
formed directly on mirror S1, but also when it is arranged behind,
i.e., after, mirror S1. Preferably, the aperture stop is positioned
such that the following relationship is obtained:
S2S1.ltoreq.0.9.times.S2 aperture. S2S1 denotes the mechanical
distance of the vertex V2 of the surface of mirror S2 and the
vertex V1 of the surface of the mirror S1.
Table 11 shows the constructional data of the 5.times. objective
according to FIG. 8 in Code V(.TM.) nomenclature, where the fourth
mirror S4 is spherical. The mean radius of the 26.times.1.7
mm.sup.2 image field is approximately 26 mm.
TABLE-US-00011 TABLE 11 element No. Radius Thickness diameter type
Object INF 85.2401 256.1389 358.4668 S1 A(1) 0.0024 203.8941 REFL
-358.4691 203.8845 0.0000 201.9677 S2 A(2) 358.4691 201.9942 REFL
aperture 60.7572 390.5456 187.2498 0.0000 S3 A(3) -390.5456
188.9474 REFL -104.1273 505.8686 S4 A(4) 494.6729 550.3686 REFL
114.3062 256.9217 281.6969 181.7337 S5 A(5) -281.6969 64.4286 REFL
S6 A(6) 281.6969 187.8549 REFL 78.1545 Image image width 60.0041
53.6996 aspherical constants: Z = (CURV) Y.sup.2/[1 + (1 - (1 + K)
(CURV).sup.2Y.sup.2).sup.1/2] + (A)Y.sup.4 + (B)Y.sup.6 +
(C)Y.sup.8 + (D)Y.sup.10 asphere CURV K A B C D A(1) 0.00035280
-58.238840 0.00000E+00 2.14093E-15 2.29498E-20 0.00000E+00- A(2)
0.00097971 -4.160335 0.00000E+00 1.54696E-15 8.15622E-21
0.00000E+00 A(3) 0.00117863 -2.136423 0.00000E+00 -1.78563E-16
3.45455E-20 0.00000E+00- A(4) 0.00124362 0.000000 0.00000E+00
0.00000E+00 0.00000E+00 0.00000E+00 A(5) 0.00338832 2.909987
0.00000E+00 7.90123E-13 7.04899E-17 0.00000E+00 A(6) 0.00278660
0.062534 0.00000E+00 2.79526E-16 7.00741E-21 0.00000E+00 Reference
wavelength = 13 nm
Another embodiment is shown in FIG. 9, where again the same
reference numbers are used for the same components as in the
previous figures. Here, the aperture stop B is placed optically and
physically on the secondary mirror or second mirror S2. The ability
to place the aperture stop on S2 makes manufacturing easier.
Therefore this arrangement is advantageous. The system of FIG. 9 is
a 4.times. reduction system with a wavefront error of 0.021.lamda.
within a 2 mm wide image side arc-shaped field at .lamda.=13 nm.
The maximum asphericity in the used range lies at 11.2 .mu.m, and
the largest angle of incidence, which occurs at S5, is
approximately 18.3.degree.. The ring field radius R as defined in
FIG. 1 of the arc-shaped field in the image plane is approximately
26 mm, as with the previous two embodiments. Furthermore, the
distance between the image plane and the vertex V5 of the surface
of the mirror next to the image plane, S5, is greater than the used
diameter of the mirror next to the wafer, S5, and lies at around 59
mm in this embodiment.
Table 12 shows the optical parameters of the embodiment of FIG. 9
in Code V(.TM.) nomenclature.
TABLE-US-00012 TABLE 12 element No. Radius thickness diameter Type
Object INF 84.0595 205.6642 473.5521 S1 A(1) -145.8261 147.3830
REFL -327.7260 136.4700 aperture 112.0176 0.0000 S2 A(2) 473.5521
112.1228 REFL 190.4830 163.5236 0.0000 184.4783 S3 A(3) -190.4830
185.3828 REFL -399.1713 358.6720 S4 A(4) 589.6560 654.5228 REFL
207.5220 310.1977 276.2668 175.3066 S5 A(5) -276.2668 65.2138 REFL
S6 A(6) 276.2668 182.8159 REFL 77.5085 Image image width 59.0000
53.9968 aspherical constants: Z = (CURV) Y.sup.2/[1 + (1 - (1 + K)
(CURV).sup.2Y.sup.2).sup.1/2] + (A)Y.sup.4 + (B)Y.sup.6 +
(C)Y.sup.8 + (D)Y.sup.10 asphere CURV K A B C D A(1) 0.00015851
441.008070 0.00000E+00 -3.49916E-16 1.27478E-19 -3.37021E-- 25 A(2)
0.00089932 -5.032907 0.00000E+00 -6.95852E-15 -7.53236E-20
-2.74751E-- 24 A(3) 0.00188578 0.913039 0.00000E+00 -1.60100E-15
-9.53850E-20 1.30729E-26- A(4) 0.00108147 0.038602 0.00000E+00
2.48925E-18 -5.29046E-24 -4.37117E-31- A(5) 0.00269068 7.253316
0.00000E+00 -5.70008E-13 -9.32236E-17 -6.09046E-2- 1 A(6)
0.00281036 0.150957 0.00000E+00 1.30822E-15 1.86627E-20 5.08158E-25
Reference wavelength = 13 nm
FIG. 10 shows an embodiment of the invention which includes a field
mirror S1, a first subsystem with the second to fourth mirror S2-S4
and a second subsystem with the fifth and sixth mirror, S5, S6. The
field mirror S1 with imaging ratio, .beta., .beta.>0 produces a
virtual image of the object in the object plane 2. The virtual
image is then imaged by the first subsystem consisting of the
second, third and fourth mirrors, S2, S3, S4, which has
.beta.<0, producing a real intermediate image Z in a plane
conjugate to the object plane 2. The real intermediate image Z is
imaged as a real image into image plane 4 by the second subsystem
which consists of the fifth and sixth mirrors, S5, S6. The image
side numerical aperture of the system is NA=0.28. The optical free
working distance A between the vertex of the surface of the last
mirror S5 and the image plane 4 corresponds to at least the sum of
one-third of the used diameter of the mirror nearest to the image
plane and a length which lies between 20 and 30 mm. The aperture
stop B is situated on the second mirror S2.
Table 13 shows the optical parameters of the embodiment of FIG. 10
in Code V(.TM.) nomenclature.
TABLE-US-00013 TABLE 13 element No. Radius thickness Diameter Type
Object INF 151.2625 194.7605 229.0820 S1 A(1) -39.4068 162.9862
REFL -189.6752 147.1426 aperture 65.0637 0.0000 S2 A(2) 229.0820
65.1650 REFL 137.5708 168.3504 0.0000 230.5128 S3 A(3) -137.5708
234.0072 REFL -300.3445 386.2567 S4 A(4) 437.9153 630.7784 REFL
133.0981 343.1578 353.0840 257.0225 S5 A(5) -353.0840 79.9521 REFL
S6 A(6) 353.0840 264.2853 REFL 78.6376 image image width 44.0000
54.0051 aspherical constants: Z = (CURV) Y.sup.2/[1 + (1 - (1 + K)
(CURV).sup.2Y.sup.2).sup.1/2] + (A)Y.sup.4 + (B)Y.sup.6 +
(C)Y.sup.8 + (D)Y.sup.10 + (E)Y.sup.12 + (F)Y.sup.14 + (G)Y.sup.16
+ (H)Y.sup.18 + (J)Y.sup.20 K A B C D asphere CURV E F G H J A(1)
-0.00080028 0.000000 -3.35378E-09 5.36841E-14 -7.86902E-19
-5.07886E-- 24 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00
0.00000E+00 A(2) 0.00040002 0.000000 1.68178E+08 2.05570E-12
2.42710E-16 5.69764E-20 0.00000E+00 0.00000E+00 0.00000E+00
0.00000E+00 0.00000E+00 A(3) 0.00113964 -2.760663 0.00000E+00
-3.55779E-15 1.03881E-19 -3.64996E-2- 4 0.00000E+00 0.00000E+00
0.00000E+00 0.00000E+00 0.00000E+00 A(4) 0.00128753 0.019273
0.00000E+00 5.82746E-18 -1.77496E-22 1.64954E-27 -6.20361E-33
0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 A(5) 0.00373007
11.6888968 0.00000E+00 -5.53902E-12 -4.32712E-16 -1.54425E- -19
0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 A(6)
0.00240387 -0.002567 0.00000E+00 -6.78955E-16 -8.39621E-21
-2.95854E-- 25 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00
0.00000E+00 Reference wavelength = 13 nm
FIG. 11 shows a similar, yet alternative, embodiment to that of
FIG. 10 with a six-mirror objective with field mirror S1 as well as
first and second subsystems as shown in FIG. 10. The embodiment
shown in FIG. 11 comprises as the embodiment in FIG. 10 an
intermediate image Z. Furthermore the aperture B is formed on the
second mirror S2 similar and the numerical aperture on the image
side is NA=0.30. The optical parameters of this alternative
embodiment are shown in Table 14 in Code V(.TM.) nomenclature.
TABLE-US-00014 TABLE 14 element No. radius thickness Diameter type
Object INF 103.2808 197.1874 219.3042 S1 A(1) -39.2890 157.6222
REFL -180.0152 142.1492 aperture 67.2659 0.0000 S2 A(2) 219.3042
67.4347 REFL 131.2051 167.6895 0.0000 228.0182 S3 A(3) -131.2051
232.3162 REFL -247.5850 401.4441 S4 A(4) 378.7901 613.5493 REFL
134.4001 355.7774 348.5086 268.3735 S5 A(5) -348.5086 81.5255 REFL
S6 A(6) 348.5086 269.2435 REFL 75.4983 image image width 36.1195
53.9942 aspherical constants: Z = (CURV) Y.sup.2/[1 + (1 - (1 + K)
(CURV).sup.2Y.sup.2).sup.1/2] + (A)Y.sup.4 + (B)Y.sup.6 +
(C)Y.sup.8 + (D)Y.sup.10 + (E)Y.sup.12 + (F)Y.sup.14 + (G)Y.sup.16
+ (H)Y.sup.18 + (J)Y.sup.20 K A B C D asphere CURV E F G H J A(1)
-0.00061615 0.000000 -5.19402E-09 1.09614E-13 -3.44621E-18
1.58573E-2- 2 -7.07209E-27 0.00000E+00 0.00000E+00 0.00000E+00
0.00000E+00 A(2) 0.00066911 0.000000 1.69112E-08 2.39908E-12
2.89763E-16 1.00572E-19 1.84514E-29 0.00000E+00 0.00000E+00
0.00000E+00 0.00000E+00 A(3) 0.00140031 0.000000 -8.71271E-10
-1.47622E-15 -3.40869E-20 4.32196E-2- 4 -2.23484E-28 0.00000E+00
0.00000E+00 0.00000E+00 0.00000E+00 A(4) 0.00143731 0.000000
2.18165E+12 2.65405E-17 -2.01757E-22 1.14856E-28 1.49857E-32
-8.61043E-38 0.00000E+00 0.00000E+00 0.00000E+00 A(5) 0.00378996
0.000000 8.54406E-08 2.25929E-12 3.36372E-16 1.92565E-20
5.75469E-24 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 A(6)
0.00246680 0.000000 -3.61754E-12 -8.29704E-16 -1.53440E-20
-2.24433E-- 25 5.91279E-30 0.00000E+00 0.00000E+00 0.00000E+00
0.00000E+00 Reference wavelength = 13 nm
FIGS. 12A and 12B define the used diameter D as used in the
description of the above embodiments. As a first example, the
illuminated field 100 on a mirror in FIG. 12A is a rectangular
field. The illuminated field corresponds to the area on a mirror
onto which a bundle of light rays running through the objective
from the object side to the image side impinge. The used diameter D
according to FIG. 12A is then the diameter of the envelope circle
102, which encompasses the rectangle 100, where the corners 104 of
the rectangle 100 lie on the envelope circle 102. A more realistic
example is shown in FIG. 12B. The illuminated field 100 has a
kidney shape, which is expected for the physical mirror surfaces of
the mirrors S1-S6 or the so called used areas of the mirrors S1-S6,
when the field in the image plane as well as the field in the
object plane is an arc shaped field as depicted in FIG. 5. The
envelope circle 102 encompasses the kidney shape fully and it
coincides with the edge 110 of the kidney shape at two points, 106,
108. The used diameter D of the physical mirror surface or the used
area of the mirrors S1-S6 is then given by the diameter of the
envelope circle 102.
Thus, the invention provides a six-mirror projection objective with
an imaging scale of preferably 4.times., 5.times. or 6.times. for
use in an EUV projection system. Other uses may be employed,
however. The six-mirror projection objective has the resolution
required for the image field, which is e.g. arc-shaped and has a
advantageous structural design, since the aspheres of the mirror
surfaces are relatively low, the angles of incidence of the rays of
the bundle of light rays impinging the mirror surfaces are small,
and there is enough room for mounting the mirrors.
It should be understood by a person skilled in the art, that the
disclosure content of this application comprises all possible
combinations of any element(s) of any claims with any element(s) of
any other claim, as well as combinations of all claims amongst each
other.
* * * * *