Title: Collector for an illumination system with a wavelength of less than or equal to 193 nm
Abstract: There is provided a collector for guiding light with a wavelength of ≦193 nm onto a plane. The collector includes a first mirror shell for receiving a first ring aperture section of the light and irradiating a first planar ring section of the plane with a first irradiance, and a second mirror shell for receiving a second ring aperture section of the light and irradiating a second planar ring section of the plane with a second irradiance. The first and second mirror shells are rotationally symmetrical and concentrically arranged around a common axis of rotation, the first and second ring aperture sections do not overlap with one another, the first planar ring section substantially abuts the second planar ring section, and the first irradiance is approximately equal to the second irradiance.
Patent Number: 6,964,485 Issued on 11/15/2005 to Singer, et al.
| Inventors:
|
Singer; Wolfgang (Aalen, DE);
Wangler; Johannes (Königsbronn, DE)
|
| Assignee:
|
Carl Zeiss SMT AG (Oberkochen, DE)
|
| Appl. No.:
|
055608 |
| Filed:
|
January 23, 2002 |
Foreign Application Priority Data
| Jan 23, 2001[DE] | 101 02 934 |
| Jun 06, 2001[DE] | 101 27 298 |
| Aug 10, 2001[DE] | 101 38 313 |
| Current U.S. Class: |
359/850; 359/357; 359/857; 359/864 |
| Intern'l Class: |
G02B 007/18.2 |
| Field of Search: |
359/350,351,357,448,850,857,864
378/34,145,146,147
362/551
|
References Cited [Referenced By]
U.S. Patent Documents
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| 1865441 | Jul., 1932 | Mutscheller.
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| 2198014 | Apr., 1940 | Ott.
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| 3148834 | Sep., 1964 | Boehnke.
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| 3318184 | May., 1967 | Jackson.
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| 3501626 | Mar., 1970 | Benard.
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| 3689760 | Sep., 1972 | Stewart, Jr.
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| 4015120 | Mar., 1977 | Cole.
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| 5002379 | Mar., 1991 | Murtha.
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| 5369511 | Nov., 1994 | Amos.
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| 5745547 | Apr., 1998 | Xiao.
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| 5763930 | Jun., 1998 | Partlo.
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| 5768339 | Jun., 1998 | O'Hara.
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| 6064072 | May., 2000 | Partlo et al.
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| 6186632 | Feb., 2001 | Chapman et al.
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| 6198793 | Mar., 2001 | Schultz et al.
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| 6244717 | Jun., 2001 | Dinger.
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| 6285737 | Sep., 2001 | Sweatt et al.
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| 6389101 | May., 2002 | Levine et al.
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| 6469827 | Oct., 2002 | Sweatt et al.
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| 6624878 | Sep., 2003 | Sandstrom et al.
| |
| Foreign Patent Documents |
| 3001059 | Apr., 1983 | DE.
| |
| 19903807 | Nov., 1999 | DE.
| |
| WO99/27542 | Jun., 1999 | WO.
| |
| WO99/57732 | Nov., 1999 | WO.
| |
| WO00/63922 | Oct., 2000 | WO.
| |
| WO01/08162 | Feb., 2001 | WO.
| |
| WO01/09681 | Feb., 2001 | WO.
| |
| WO01/09684 | Feb., 2001 | WO.
| |
Primary Examiner: Dunn; Drew A.
Assistant Examiner: Pritchett; Joshua L.
Attorney, Agent or Firm: Ohlandt, Greeley, Ruggiero & Perle
Claims
1. A collector for guiding light with a wavelength of ≦193 nm onto a plane,
said collector comprising:
a first mirror shell for receiving a first ring aperture section of said light
and irradiating a first planar ring section of said plane with a first irradiance;
a second mirror shell for receiving a second ring aperture section of said light
and irradiating a second planar ring section of said plane with a second irradiance;
and
a central aperture obscuration with a numerical aperture≦0.30,
wherein said light impinges with an angle of incidence of less than 20°
to surface tangents of said first and second mirror shells,
wherein said first and second mirror shells are rotationally symmetrical and
concentrically arranged around a common axis of rotation,
wherein said first and second ring aperture sections do not overlap with one
another,
wherein said first planar ring section substantially abuts said second planar
ring section,
wherein said first irradiance is approximately equal to said second irradiance,
and
wherein said collector has a focal point.
2. A collector for guiding light with a wavelength of ≦193 nm onto a plane,
said collector comprising:
a first mirror shell for receiving a first ring aperture section of said light
and irradiating a first planar ring section of said plane with a first irradiance;
and
a second mirror shell for receiving a second ring aperture section of said light
and irradiating a second planar ring section of said plane with a second irradiance,
wherein said first and second mirror shells are rotationally symmetrical and
concentrically arranged around a common axis of rotation,
wherein said first and second ring aperture sections do not overlap with one
another,
wherein said first planar ring section substantially abuts said second planar
ring section,
wherein said first irradiance is approximately equal to said second irradiance,
and
wherein said first mirror shell includes a first segment with a first optical
surface and a second segment with a second optical surface.
3. The collector of claim 2, wherein said first segment is from a hyperboloid
and said second segment is from an ellipsoid.
4. The collector of claim 2, wherein said first segment is from a hyperboloid
and said second segment is from a paraboloid.
5. The collector of claim 2, wherein said first and second mirror shells have
dimensions that are different from one another in a direction of said axis of rotation.
6. The collector of claim 2,
wherein said first mirror shell is an inner mirror shell and said second mirror
shell is an outer mirror shell,
wherein said first mirror shell has a mean value of an initial point and an end
point with regard to said axis of rotation that indicates a position of said first
mirror shell,
wherein said second mirror shell has a mean value of an initial point and an
end point with regard to said axis of rotation that indicates a position of said
second mirror shell, and
wherein said position of said outer mirror shell is further distant from said
plane than said position of said inner mirror shell.
7. The collector of claim 2, wherein said first and second ring aperture segments
are separated by a gap.
8. The collector of claim 2, further comprising a central aperture obscuration
with a numerical aperture≦0.30.
9. The collector of claim 8, wherein said central aperture obscuration comprises
a diaphragm concentric to, and interior to, said first mirror shell.
10. The collector of claim 2, wherein said collector has an object-side maximum
numerical aperture NA
max≧0.4.
11. The collector of claim 2, wherein said first and second mirror shells are
two of a plurality of at least three mirror shells.
12. The collector of claim 2, wherein said light is from a light source that
emits rays that impinge with an angle of incidence of less than 20° to surface
tangents of said first and second mirror shells.
13. An illumination system, comprising the collector of claim 2.
14. The illumination system of claim 13, further comprising an optical element
having raster elements.
15. The illumination system of claim 14, wherein said raster elements are located
within said first and second planar ring section.
16. The illumination system of claim 14,
wherein said optical element is a first optical element, and
wherein said illumination system further comprises a second optical element for
imaging.
17. The illumination system of claim 14,
wherein said optical element is a first optical element, and
wherein said illumination system further comprises a second optical element for
field shaping.
18. The illumination system of claim 13,
wherein said plane is a first plane, and
wherein said illumination system has a second plane conjugated to a light source
for said light, between said collector and said first plane, in which an intermediate
image of said light source is formed.
19. The illumination system of claim 18, further comprising a diaphragm in or
near said intermediate image, wherein said diaphragm separates a space containing
said light source and said collector from a portion of said illumination system
downstream of said diaphragm.
20. An EUV projection exposure system comprising:
the illumination system of claim 13 for illuminating a mask; and
a projection objective for imaging said mask on a light-sensitive object.
21. A collector for guiding light with a wavelength of ≦193 nm onto a
plane, said collector comprising:
a first mirror shell for receiving a first ring aperture section of said light
and irradiating a first planar ring section of said plane with a first irradiance;
a second mirror shell for receiving a second ring aperture section of said light
and irradiating a second planar ring section of said plane with a second irradiance;
and
a central aperture obscuration with a numerical aperture≦0.30,
wherein said first and second mirror shells are rotationally symmetrical and
concentrically arranged around a common axis of rotation,
wherein said first and second ring aperture sections do not overlap with one
another,
wherein said first planar ring section substantially abuts said second planar
ring section, and
wherein said first irradiance is approximately equal to said second irradiance.
22. The collector of claim 21, wherein said central aperture obscuration comprises
a diaphragm concentric to, and interior to, said first mirror shell.
23. An illumination system for illuminating an object plane with radiation≦193
nm from a light source, comprising:
a collector, wherein said collector has a mirror shell and an optical system
arranged in a light path from the light source to the object plane behind said
collector,
a plane conjugated to said light source in said light path, situated between
said collector and said optical system, in which an intermediate image of said
light source is formed; and
a diaphragm in or near said intermediate image, wherein said diaphragm separates
a space containing said light source and said collector from a portion of said
illumination system downstream of said diaphragm.
24. A collector for guiding light with a wavelength≦193 nm comprising:
a first mirror shell; and
a second mirror shell,
wherein said first and second mirror shells are rotationally symmetrical and
concentrically arranged around a common axis of rotation, and
wherein said collector has a central aperture obscuration with a numerical aperture≦0.30.
25. The collector of claim 24, wherein said first and second mirror shells have
dimensions that are different from one another in a direction of said axis of rotation.
26. The collector of claim 24,
wherein said collector guides said light onto a plane,
wherein said first mirror shell is an inner mirror shell and said second mirror
shell is an outer mirror shell,
wherein said first mirror shell has a mean value of an initial point and an end
point with regard to said axis of rotation that indicates a position of said first
mirror shell,
wherein said second mirror shell has a mean value of an initial point and an
end point with regard to said axis of rotation that indicates a position of said
second mirror shell, and
wherein said position of said outer mirror shell is further distant from said
plane than said position of said inner mirror shell.
27. The collector of claim 24,
wherein said collector guides said light onto a plane,
wherein said first mirror shell is for receiving a first ring aperture section
of said light and irradiating a first planar ring section of said plane with a
first irradiance,
wherein said second mirror shell is for receiving a second ring aperture section
of said light and irradiating a second planar ring section of said plane with a
second irradiance,
wherein said collector has:
a first quotient of (i) a first ratio of a radial dimension of said first planar
ring section to an angular extension of said first ring aperture section and (ii)
a second ratio of a radial dimension of said second planar ring section to an angular
extent of said second ring aperture section; and
a second quotient of (i) a first radiant intensity, which is reduced by a loss
of reflectivity of said first mirror shell, which flows into said first ring aperture
section, and of (ii) a second radiant intensity, which is reduced by a loss of
reflectivity of said second mirror shell, which flows into said second ring aperture
section,
wherein said first quotient is substantially equal to said second quotient.
28. The collector of claim 24,
wherein said collector guides said light onto a plane,
wherein said first mirror shell is for receiving a first ring aperture section
of said light and irradiating a first planar ring section of said plane with a
first irradiance,
wherein said second mirror shell is for receiving a second ring aperture section
of said light and irradiating a second planar ring section of said plane with a
second irradiance,
wherein said collector has:
a first ratio of a radial dimension of said first planar ring section to an angular
extent of said first ring aperture section; and
a second ratio of a radial dimension of said second planar ring section to an
angular extent of said second ring aperture section, and
wherein said first ratio is substantially equal to said second ratio.
29. The collector of claim 24,
wherein said collector guides said light onto a plane,
wherein said first mirror shell is for receiving a first ring aperture section
of said light and irradiating a first planar ring section of said plane with a
first irradiance,
wherein said second mirror shell is for receiving a second ring aperture section
of said light and irradiating a second planar ring section of said plane with a
second irradiance,
wherein said first and second planar ring sections have radial dimensions of
equal size,
wherein said first and second planar ring sections are concentric,
wherein said first planar ring section is an inner planar ring section and said
second planar ring section is an outer planar ring section,
wherein said first mirror shell has a dimension in a direction of said axis of
rotation,
wherein said second mirror shell has a second dimension in said direction of
said axis of rotation, and
wherein said dimension of said first mirror shell is larger than said dimension
of said second mirror shell.
30. The collector of claim 24, wherein said first and second mirror shells are
each a ring-shaped segment of an aspherical object.
31. The collector of claim 24, wherein said first and second mirror shells are
each a ring-shaped segment of a form selected from the group consisting of an ellipsoid,
a paraboloid and a hyperboloid.
32. The collector of claim 24, wherein said first mirror shell comprises a first
segment with a first optical surface and a second segment with a second optical surface.
33. The collector of claim 32, wherein said first segment is from a hyperboloid
and said second segment is from an ellipsoid.
34. The collector of claim 32, wherein said first segment is from a hyperboloid
and said second segment is from a paraboloid.
35. The collector of claim 24,
wherein said collector guides said light onto a plane,
wherein said first mirror shell is for receiving a first ring aperture section
of said light and irradiating a first planar ring section of said plane with a
first irradiance,
wherein said second mirror shell is for receiving a second ring aperture section
of said light and irradiating a second planar ring section of said plane with a
second irradiance,
wherein said first and second ring aperture segments are separated by a gap.
36. The collector of claim 24, wherein said central aperture obscuration comprises
a diaphragm concentric to, and interior to, said first mirror shell.
37. The collector of claim 24, wherein said collector has an object-side maximum
numerical aperture NA
max≧0.4.
38. The collector of claim 24, wherein said first and second mirror shells are
two of a plurality of at least three mirror shells.
39. An illumination system, comprising the collector of claim 24.
40. The illumination system of claim 39, further comprising an optical element
having raster elements.
41. The illumination system of claim 40,
wherein said collector guides said light onto a plane,
wherein said first mirror shell is for receiving a first ring aperture section
of said light and irradiating a first planar ring section of said plane with a
first irradiance,
wherein said second mirror shell is for receiving a second ring aperture section
of said light and irradiating a second planar ring section of said plane with a
second irradiance,
wherein said raster elements are located within said first and second planar
ring sections.
42. The illumination system of claim 40,
wherein said optical element is a first optical element, and
wherein said illumination system further comprises a second optical element for
imaging.
43. The illumination system of claim 40,
wherein said optical element is a first optical element, and
wherein said illumination system further comprises a second optical element for
field shaping.
44. The illumination system of claim 39,
wherein said collector guides said light onto a first plane, and
wherein said illumination system has a second plane conjugated to a light source
for said light, between said collector and said first plane, in which an intermediate
image of said light source is formed.
45. The illumination system of claim 44, further comprising a diaphragm in or
near said intermediate image, wherein said diaphragm separates a space containing
said light source and said collector from a portion of said illumination system
downstream of said diaphragm.
46. An EUV projection exposure system comprising:
the illumination system of claim 39 for illuminating a mask; and
a projection objective for imaging said mask on a light-sensitive object.
47. A process for producing a microelectronic device, comprising using the EUV
projection exposure system of claim 46.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is claiming priority of German Patent Application No.
101 02 934.9, filed on Jan. 23, 2001; German Patent Application No. 101 27 298.7,
filed on Jun. 6, 2001; and German Patent Application No. 101 38 313.4, filed on
Aug. 10, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a collector for illumination systems with a wavelength
of ≦193 nm, preferably ≦126 nm, and, particularly preferred, wavelengths
in the extreme ultra-violet (EUV) range. A plurality of rotationally symmetrical
mirror shells is arranged concentrically around a common axis of rotation. Light,
regarded as being partitioned into a plurality of ring aperture sections, is received
by the plurality of mirror shells, such that one of the ring aperture sections
is assigned to each mirror shell. Each of the mirror shells, in turn, irradiates
a planar ring section in a plane. Thus, there is an assignment or a correspondence
between a ring aperture section, a mirror shell and a planar ring section. In addition,
the invention also makes available an illumination system with such a collector,
a projection exposure system with an illumination system according to the invention,
as well as a method for the exposure of microstructures.
2. Description of the Prior Art
Nested collectors for wavelengths of ≦193 nm, particularly wavelengths
in the range of x-rays have been made known from a plurality of publications.
Thus, U.S. Pat. No. 5,768,339 shows a collimator for x-rays, wherein the collimator
has several nested paraboloid-shaped reflectors. The collimator according to U.S.
Pat. No. 5,768,339 serves for the purpose of forming an isotropically emitted beam
bundle of an x-ray light source into a parallel beam.
A nested collector for x-rays has become known from U.S. Pat. No. 1,865,441,
which
serves for the purpose of collimating isotropic x-rays emitted by a source into
a parallel beam bundle, as in the case of U.S. Pat. No. 5,768,339.
U.S. Pat. No. 5,763,930 shows a nested collector for a pinch-plasma light source,
which serves for the purpose of collecting the radiation emitted by the light source
and bundling it in a light guide.
U.S. Pat. No. 5,745,547 shows several arrangements of multichannel optics, which
serve for the purpose of bundling into one point the radiation, particularly x-ray
radiation, due to multiple reflections coming from a source.
In order to achieve a particularly high transmission efficiency, the invention
according to U.S. Pat. No. 5,745,547 proposes elliptically shaped reflectors.
An arrangement has become known from DE 30 01 059 C2 for use in x-ray lithography
systems, and this arrangement has nested parabolic mirrors arranged between the
x-ray source and the mask. These mirrors are arranged in such a way that the divergent
x-ray radiation will be formed into a parallel-running output beam bundle.
The arrangement according to DE 30 01 059 in turn serves only for the purpose
of obtaining a good collimation for x-ray lithography.
The arrangement of nested reflectors, which has become known from WO 99/27542,
in an x-ray proximity lithography system serves for the purpose of refocusing the
light of a light source, so that a virtual light source is formed. The nested reflectors
may have an ellipsoid form.
A nested reflector for high-energy photon sources has become known from U.S.
Pat.
No. 6,064,072, which serves for the purpose of shaping the divergent x-ray radiation
into a parallel beam bundle.
WO 00/63922 shows a nested collector, which serves for the purpose of collimating
the neutron beam.
A nested collector for x-ray radiation has become known from WO 01/08162, which
is characterized by a surface roughness of the inner reflecting surface, of the
individual mirror shells, of less than 12 Å rms. The collectors shown in
WO 01/08162 also comprise systems with multiple reflections, particularly also
Wolter systems, and are characterized by a high resolution, as is required, for
example, for x-ray lithography.
For illumination optics to be used in EUV lithography, such as, for example,
shown in DE 199 03 807 or WO 99/57732, in addition to resolution, high requirements
are also placed on regularity or uniformity and telecentry. In such systems, the
light of the light source is collected by a collector for specific light sources.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a collector for an illumination
system for microlithography with wavelengths of ≦193 nm, preferably <126
nm, and particularly preferred, for wavelengths in the EUV range, which complies
with the high requirements for uniformity and telecentry that are required for
illumination optics. Especially in EUV-lithography, the illumination should be
as homogeneous as possible.
According to the invention, this object is solved by a collector with an
object-side aperture that receives light emitted from a light source and comprises
a plurality of rotationally symmetrical mirror shells that are arranged concentrically
around a common axis of rotation giving a so-called nested collector. An area to
be illuminated lies in a plane. The area is partitioned into a plurality of planar
ring sections, also denoted as ring elements. Ring aperture sections of the light,
also denoted as ring aperture elements, do not overlap and they may have spatial
gaps therebetween, whereas the planar ring sections substantially abut one another.
That is, given two adjacent planar ring sections, one being an inner section and
the other an outer section, the outer perimeter of inner planar ring section substantially
abuts the inner perimeter of outer planar ring section.
For example, consider a case of two such mirror shells. A first of the mirror
shells receives a first ring aperture section of the light and irradiates a first
of the planar ring sections, and a second of the mirror shells receives a second
ring aperture section of the light and irradiates a second of the planar ring sections.
The dimensions of the mirror shells in the direction of the axis of rotation as
well as the surface parameters and the positions of the mirror shells are selected
so that the irradiances of the individual planar ring sections are approximately
equal to one another.
The inventors have recognized that a uniform illumination to a great extent can
be achieved in a pregiven area of a plane by configuring a nested collector according
to the invention. It is particularly preferred that the mirror shells are an aspheric
annular segment, especially an ellipsoid, a paraboloid or a hyperboloid. A completely
parallel beam bundle and thus a light source lying in infinity results for a paraboloid.
For example, if one wishes to produce secondary light sources by means of a first
optical element with first raster elements, which is arranged in the plane to be
illuminated according to U.S. Pat. No. 6,198,793 B1, the disclosure content of
which is fully incorporated into the present application, then in the case of mirror
shells, which are shaped as ring-shaped segments of a paraboloid, the individual
raster elements must have a collecting or convergent effect.
The convergent effect may also be transferred to the collector. Such a collector
according to the invention would comprise shells, which are segments cut out from
ellipsoids, so that a convergent beam bundle is formed. By transferring the convergent
effect to a collector, which comprises shells that are segments cut out from ellipsoids,
the first raster elements of the first optical element can be formed, for example,
as planar facets.
Collectors with shells, which are segments cut out from hyperboloids,
lead to a divergent beam bundle and are then particularly of interest, if the collector
is to be dimensioned as small as possible.
In contrast to the nested collectors according to the prior art, the collector
according to the invention is characterized in that the dimensions of the reflectors
of the different shells are different in the direction of the axis of rotation.
Thus, an extensively homogeneous illumination can be produced in an annular region
in the plane to be illuminated. If the dimensions and distances of the reflectors
are substantially the same as in the prior art cited in the introductory part of
this document, then, for example, a collimated beam or a focused beam can be achieved,
while a homogeneous illumination in an annular region, in contrast, cannot be provided.
In addition, the reflection losses that are dependent on angle of incidence can
be compensated for by a suitable design of the collector, so that a homogeneous
illumination is provided in a pregiven plane.
In a preferred embodiment of the collector according to the invention, the position
of an outer mirror shell has a longer distance to the plane to be illuminated than
the position of an inner mirror shell. In this application the mean value of the
initial point and the end point of a shell referred to the axis of rotation of
the collector is understood as the position of a mirror shell. Inner mirror shells
are understood in this application as those mirror shells that have the shorter
distance to the axis of rotation with regard to two mirror shells, an inner mirror
shell and an outer mirror shell. Since homogenization is also achieved with the
nested collectors only in a discrete approximation, it is of advantage if the collector
comprises as many shells as possible. Preferably, the collector according to the
invention has more than four, particularly preferred, more than seven, and most
particularly preferred, more than ten reflectors in a nested arrangement.
In case of an isotropically emitting light source, the collector according to
the invention assures that the same angular segments are imaged on the same surfaces.
In addition, the reflection losses that are dependent on angle of incidence can
be compensated for by a suitable design of the collector, so that a homogeneous
illumination is provided in the plane to be illuminated.
In a case of a non-isotropic source, the irradiation characteristic can be converted
by the collector into a homogeneous illumination.
In a preferred embodiment, the radial dimensions of at least two planar ring
sections
are of equal size, while the dimension in the direction of the axis of rotation
of the mirror shell of the collector that is assigned to the inner planar ring
section is larger than the dimension in the direction of the axis of rotation of
the mirror shell of the collector assigned to the outer planar ring section. The
inner planar ring section is understood as the planar ring section that has the
shorter distance to the axis of rotation of two planar ring sections, an inner
and an outer planar ring section.
Advantageously, the collector according to the invention is configured
such that the quotient of a first ratio of the radial dimension of a first planar
ring section to the angular extent of the assigned ring aperture section and a
second ratio of the radial dimension of a second planar ring section to the angular
extent of the assigned ring aperture section is of the same magnitude as the quotient
of a first radiant intensity, which flows into the first ring aperture section,
and of a second radiant intensity, which flows into the second ring aperture section,
i.e., the following equation applies:
##EQU1##
In an alternative embodiment of the invention, provision is made to form the
nested
mirror shells in such a way that multiple reflections occur at one mirror shell.
The reflection angles can be kept small by multiple reflections at one shell.
The reflectivity behaves nearly linearly with the angle of incidence relative
to the surface tangent in the case of reflection under grazing incidence with small
angles of incidence of less than 20° relative to the surface tangent in materials
such as molybdenum, niobium, ruthenium, rhodium, palladium or gold. This means
that the reflection losses for a reflection, for example, at 16° or for two
reflections at 8° are approximately the same. For the maximally achievable
aperture of the collector, however, it is advantageous to use more than one reflection.
Particularly preferred are systems with two reflections. Collectors
with two reflections can be formed, for example, as nested Wolter systems with
first mirror shells, which are annular segments cut out from hyperboloids, and
second mirror shells, which are annular segments cut out from ellipsoids.
Wolter systems are known from the literature, for example, from Wolter, Annalen
der Physik 10, 94-114, 1952. In the case of Wolter systems with a real intermediate
image of the source, which is formed by the combination of a hyperboloid surface
with an ellipsoid surface, reference is made to J. Optics, vol. 15, 270-280, 1984.
A particular advantage of Wolter systems is that a maximum collection aperture
of up to NA
max of approximately 0.985 corresponding to an aperture angle
of 80° can be selected in the case of a Wolter system with two reflections
with incidence angles smaller than 20° relative to the surface tangent. In
such a case one is still in the high-reflecting region of the reflection under
grazing incidence with a reflectivity>70%.
In a first embodiment of the invention, the first ring-shaped segment and the
second ring-shaped segment of a shell are not continuously fit together, but an
unused region of the mirror shell, a so-called gap, lies between the first and
the second ring-shaped segments.
In addition to the collector, the invention also makes available an illumination
system with such a collector. The illumination system is preferably a double faceted
illumination system with a first optical element with first raster elements and
a second optical element with second raster elements, as shown in U.S. Pat. No.
6,198,793 B1, the disclosure content of which is fully incorporated by reference
into the present document.
The first and/or second raster elements can be planar facets or facets with convergent
or divergent effect.
In one embodiment of the invention, only a ring-shaped area is illuminated on
the first optical element with first raster elements. The first raster elements
are then preferably arranged inside the ring-shaped area.
The illumination system comprising the collector according to the invention preferably
is used in a projection exposure system for microlithography, wherein such a projection
exposure system is shown for example in PCT/EP 00/07258, the disclosure content
of which is fully incorporated in the present application. Projection exposure
systems comprise a projection objective arranged in the light path after the illumination
device, for example, a 4-mirror projection objective as shown in U.S. Pat. No.
6,244,717 B1, the disclosure content of which is fully incorporated in the present application.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described by example on the basis of the drawings, without
any restriction. Here:
FIG. 1 shows a schematic diagram of a collector;
FIG. 2 shows a diagram of a ring aperture section around a light source;
FIG. 3 shows a diagram of planar ring sections in a plane to be illuminated;
FIG. 4 shows a nested collector comprising ellipsoid segments;
FIG. 5 shows a nested collector comprising ellipsoid segments with a different
number of shells than in FIG. 4;
FIG. 6 shows a refractive nested collector;
FIG. 7 shows the i
th ellipse segment of a nested collector;
FIG. 8 shows the family of ellipses of a nested collector according to the embodiment
in Table 1;
FIG. 9 shows the reduction ratio β of the embodiment according to Table
1 as a function of the image-side aperture angle;
FIG. 10 shows the reduction ratio β of the embodiment according to Table
1 as a function of the radius r in plane 7 in the x-direction;
FIG. 11 shows a projection exposure system with a nested collector according
to the invention;
FIG. 12 shows an illumination distribution or irradiance of the planar ring
sections in the plane of the first raster elements of the projection exposure system
according to FIG. 11 as a function of the radial distance to the axis of rotation
z of the system;
FIG. 13 shows a projection exposure system with an intermediate image with a
nested collector;
FIG. 14 shows the reduction ratio β of an 8-shell nested Wolter system
according to FIG. 17;
FIG. 15 shows three shells of a nested Wolter system;
FIG. 16 shows two shells of a nested Wolter system;
FIG. 17 shows an 8-shell nested Wolter system;
FIG. 18 shows a diagram with the coordinates of a collector shell, designed
as a Wolter system with two reflections;
FIG. 19 shows the illumination distribution or irradiance of the planar ring
sections in the plane of the first raster elements of a system according to FIG.
20 with a collector according to FIG. 17;
FIG. 20 shows an EUV projection exposure system with a nested collector according
to FIG. 17;
FIG. 21 shows coordinate systems of all mirrors of the EUV projection exposure
system according to FIG. 20 with the nested collector according to FIG. 17;
FIG. 22 shows a first optical element of an illumination system according to
FIG. 20 with first raster elements; and
FIG. 23 shows a second optical element of an illumination system according to
FIG. 20 with second raster elements.
DESCRIPTION OF THE INVENTION
In the present document, the terms of radiometry, which are listed in the following
Table 1, are used according to Naumann/Schröder, "Bauelemente der Optik" (Components
of Optics), Hauser Publishers 1992, pp. 28-28.
| TABLE 1 |
| |
| Terms of radiometry |
| Physical quantity |
Formula |
Unit |
| |
| Radiant Flux Φe (Radiant Flux) |
##EQU2##
|
Watt [W] |
| Irradiance Ee (Irradiance or flux density) |
##EQU3##
|
Watts/cm2 |
| Radiant Intensity Ie (Radiant Intensity) |
##EQU4##
|
Watts/cm2/steradians |
| Radiance Le (Radiance) |
##EQU5##
|
Watts/cm2/steradians |
| |
A schematic diagram of a system with light source
1, collector
3,
source image
5 and intermediate plane
7 is shown in FIG.
1.
Light source
1 irradiates at a specified radiant intensity. The latter generally
depends on angles φ and φ (angles around the z-axis, not depicted):
1(φ φ). In FIG. 1, only φ is depicted, because of the following
equation for axially symmetrical light sources.
The following applies to axially symmetrical light sources:
The collector
3 collects the irradiated light and bundles it. The collector
3 forms an image of light source
1, whereby light source image
5
can either be real, as shown in FIG. 1, or virtual. Also, light source
1
may itself involve an image of a physical light source. In plane
7 behind
collector
3, in both cases, a specific illumination
9 is obtained,
which corresponds to the projection of the radiant intensity of the radiation cone
11, which is the solid angle element at angle φ′ in the image
space of the collector. If the illumination is homogenized in plane
7, then
it is also automatically homogenized in any other plane behind the collector, as
long as it lies sufficiently far away from the image plane, in which the image
5 of light source
1 lies. A radiation cone
13 that belongs
to the object space corresponds to a radiation cone
11 in the image space
and is filled with radiant intensity 1(φ) irradiated into the solid angle
element at angle φ.
According to the invention, any light source
1 is imaged into an
image of the source. The source image can be real (i.e., in the direction of light
to the right of collector
3) or virtual (i.e., in the direction of light
to the left of collector
3) or can lie in infinity.
In addition, the irradiation characteristic of any light source
1 is transformed
by the invention so that an extensively homogeneous illumination is produced in
a plane in front of or behind the intermediate image.
According to the invention, the following should apply:
##EQU6##
E: irradiance in plane 7
Φ: radiant flux
dA: surface element in plane 7
dΩ: angular element in the object-side aperture
I*(α): Radiant intensity of the source at angle α
R(α): attenuation or screening factor proportional to the light losses
due to the finite angle-dependent reflectivity of the collector (in the following
I(α)=R(α)·I*(α) is also used).
The following thus must apply to two planar ring sections with the same irradiance:
##EQU7##
from which follows the relation:
##EQU8##
In the case of anisotropic sources or large differences in the reflection losses
R(α), ring aperture sections and/or planar ring sections in plane
7
must be selected according to Eq. (2.3).
In general, the task of producing an intermediate image and at the same time
fitting
an irradiation characteristic cannot be fulfilled with simple optical elements,
such as, e.g., a mirror or a lens. In the case of rotationally symmetrical irradiation
characteristics around the z-axis, which is presently identical to the optical
axis of the system, an equal illumination can be achieved by means of a special
type of Fresnel optics, at least for discrete regions.
This is explained below in the example of a real intermediate image of source
1. Similar constructions result, and would be apparent to a person of average
skill in the art, for virtual intermediate images or a source image in infinity.
For example, three angular segments or ring aperture sections
20,
22,
24, as shown in FIG. 2, are selected around light source
1, and these
are arranged in such a way that an equivalent power is irradiated in the respective
angular segments or ring aperture sections in the radial direction from light source
1. In the case of an isotropically irradiating light source
1, such
as, for example, a dense plasma focus source, then identical angular increments
dα are selected, while in the case of anisotropically irradiating sources,
the angular distance is adapted correspondingly, so that the following applies:
##EQU9##
wherein
Φi: radiant flux
I(α): radiant intensity of the source at angle α
αi: inner angle of the ith angular segment
αi+1: external angle of the ith segment with
αi+1=αi+dαi
dαi: width of the ith angular segment
The generally different angular increments dα
i are determined
by means of Equation (2.4).
FIG. 2 shows a cross-section of an aperture having three ring aperture sections
20,
22,
24, where a light source is a point source. There
is a central shading. Ring aperture sections
20,
22,
24 lie
between NA
min and NA
max. Ring aperture sections
22
and
24 are continuously fit together; there is no gap or discontinuity between
ring aperture sections
22 and
24. That is, the outer perimeter of
ring aperture section
22 abuts the inner perimeter of ring aperture section
24. However, there is a small gap or discontinuity between the outer perimeter
of ring aperture section
20 and the inner perimeter of ring aperture section
22. Note that this configuration of the ring aperture sections is merely exemplary.
Referring to FIG. 3, planar ring sections
30,
32,
34
are assigned to the individual ring aperture segments or ring aperture sections
20,
22,
24. The planar ring sections
30,
32,
34 are selected so that distances dr of the same magnitude are achieved
between the edge or rim rays of the planar ring sections
30,
32,
34. The radial dimensions of at least two planar ring sections, e.g., planar
ring sections
30 and
32, are of equal size, i.e., dr. Thus, the following applies:
wherein
ri: distance of the ith planar ring section in plane
7 to be illuminated from the axis of rotation RA
dr: height increment=radial dimension
r1: any starting height (evident center-to-center shadowing in
the case of the nested collector).
FIG. 3 shows the illumination in plane
7 with planar ring sections
30,
32,
34. In plane
7, there is no discontinuity between planar
ring sections
30,
32 and
34. For example, the outer perimeter
of planar ring section
32 coincides with the inner perimeter of planar ring
section
34.
The respective elliptic shells of collector
3 are then determined by means
of the points of intersection of selected rays. In the case of a virtual intermediate
image, these shells are shaped like a hyperbola, and in the case of a source image
in infinity these shells are parabola-shaped. To determine the respective shells
a representative ray is selected for each ring aperture section
20,
22,
24.
For an ellipsoid-shaped or hyperbola-shaped or parabola-shaped shell, the indication
of object point and image point, here source
1 and source image
5,
and only one other point are thus sufficient to determine the shells. However,
presently two points are present, namely an initial point and an end point of the
collector shell, i.e., the problem is over-defined. However, since the imaging
quality for the source can usually almost be disregarded for illumination purposes,
one can add, for example, a conical component in the form of a wedge or a section
of a cone to the ellipses or hyperbolas or parabolas, which corresponds to a slight
defocusing, which does not matter. Alternatively, one can accept a slight shadowing,
since the gaps that occur can be selected to be very small. The size of the gaps
can be minimized by means of the layout and particularly the number of shells.
The gaps are selected, for example, so that they occur in front of the collector,
i.e., in the power taken up from the source, and not behind the collector, in the
surface to be illuminated.
It is also possible to construct the collector only from sections of cones, particularly
if the collector comprises many shells. This is advantageous in terms of manufacture.
Disregarding the reflection losses and shadowing, it is then assured
that the radiant flux Φ is almost the same in the angular segments or ring
aperture sections
20 to
24 as well as in surface segments or planar
ring sections
30 to
34.
In principle, however, it is also possible to compensate for losses of reflection
that are dependent on angle and thus on segment by suitable correction in the angular
increments α
i, whereby, since one would like to illuminate plane
7 in an extensively homogeneous manner according to the invention, the ring
aperture sections, which are assigned to planar ring sections with the same increments,
are not of the same size.
FIG. 4 shows a nested collector
3, comprising ellipsoid segments, which
are arranged in rotationally symmetrical manner around the z-axis, which assures
an extensively equally distributed illumination of plane
7. Only one half
of collector
3 is represented in section, based on the rotational symmetry
around the z-axis.
According to FIG. 4, a family or set of shells
40,
42,
44,
46 results, which are arranged so that the distances between adjacent shells
is approximately equal. The distances are taken with respect to the maximum shell
diameter, which is approximately proportional to the number of shells i. As is
apparent from FIG. 4, the dimensions of mirror shells
40,
42,
44
and
46 in the direction of the z-axis, i.e., the lengths of the mirror shells,
are different from one another. More specifically, for example, mirror shell
46
is shorter than mirror shell
40. FIG. 4 also shows light source
1,
plane
7 to be illuminated as well as source image
5. Three ring aperture
sections
20,
22,
24 correspond to those in the previous figures,
and in FIG. 4 a fourth ring aperture section
26 is also shown.
Alternatively, an arrangement is possible, in which the length of
the shells is reduced, as shown in FIG.
5. For example, the innermost angular
segment or ring aperture section
20 can be divided into two angular segments
or ring aperture sections
20.
1 and
20.
2. Correspondingly,
in plane
7, the assigned innermost planar ring section (not shown in FIG.
5) is also correspondingly divided into two planar ring sections (not shown in
FIG.
5). Then two shells
40.
1,
40.
2 result for
the two innermost segments, which are shorter than one shell
40, as can
be clearly seen from FIG.
5. The same components as in the preceding figures
are given the same reference numbers.
A similar arrangement can also be presented for refractive systems. In refractive
systems, the nested mirror shells
40,
42,
44,
46 are
replaced by ring-shaped off-axis segments of lenses
50,
52,
54,
56, as shown in FIG.
6. FIG. 6 also shows a fifth lens
58
for a fifth ring aperture section.
FIG. 6 shows schematically an arrangement of ring-shaped off-axis segments of
lenses
50,
52,
54,
56 and
58, which produces
an equally distributed illumination in plane
7 for a specific irradiation
characteristic of source
1. Only one-half of the system, which is rotationally
symmetrical around the z-axis, is shown schematically in section. Angular elements
of different sizes are deflected on height segments of equal sizes and thus a homogeneous
illumination is also achieved in the case of an anisotropic source irradiation.
Nested, reflective collectors necessarily have a central shadowing, i.e.,
below a specific aperture angle NA
min the radiation of the source cannot
be collected. This radiation must thus be blocked with a diaphragm, so that it
does not reach the illumination system behind the collector. The diaphragm can
be introduced, e.g., in the region of the collector.
The invention will be described below in more detail on the basis of a further embodiment.
The starting point is a point-to-point imaging with real source image in the
case of an isotropic source with a family or set of ellipses corresponding to the
invention, whereby the shell diameters are selected that the distance between adjacent
shells is approximately equal.
An ellipse is defined according to the equation:
##EQU10##
wherein
FIG. 7 shows as an example the i
th ellipse segment. Since the latter
is rotationally symmetrical around the z-axis, only one-half is shown in section.
Quantities used for a mirror shell for the calculation according to Table 1 are
shown in FIG.
7. The same reference numbers are used for the same segments
as in the preceding figures. The denotation is as follows:
v(i) the ith initial point of the ith mirror shell;
x(v(i)) the x-coordinate of the ith initial point;
z(v(i)) the z-coordinate of the ith initial point, i.e., the
initial point with respect to the axis of rotation RA;
h(i) the ith end point of the ith mirror shell;
x(h(i)) the x-coordinate of the ith end point;
z(h(i)) the z-coordinate of the ith end point, i.e., the end
point with reference to the axis of rotation RA;
m(i) the mean value of the initial and end points of the ith shell;
x(m(i)) the x-coordinate of the mean value;
z(m(i)) the z-coordinate of the mean value, i.e., the mean value of the
initial start and end points of the ith shell with respect to the axis
of rotation RA;
a, b parameters of the ellipse;
r(i) distance of the ith planar ring section of the ith
shell in the plane 7 to be illuminated from the axis of rotation RA; and
NA(i) sine of the angle of aperture of the inner edge ray of the ith
ring aperture section of the ith shell.
The mean value of the initial point and the end point of a mirror shell with
regard to the axis of rotation, indicates the position of the mirror shell. The
position of an outer mirror shell is further distant from plane
7 than is
the position of an inner mirror shell.
FIG. 8 shows the resulting family or set of ellipses of the shells
60,
62,
64,
66,
68,
70,
72,
74,
76,
80, for the embodiment calculated with the above-defined parameters. The
data are indicated in Table 2. All lengths in Table 2 are given in mm. All angles
of incidence relative to the surface tangents are at 19°. The angle of incidence
relative to the surface tangent of the maximum ray in the example of embodiment
according to FIG. 8 amounts to 18.54 degrees.
The following were selected as starting values:
Distance between plane 7 and source image 5:
z=900 mm
One-half the focal point distance:
e=1000 mm
Height increment on surface 7;
dr=7.5 mm
central obscuration in surface 7:
r
min˜22.5 mm (NA′
min˜0.025)
Minimum aperture NAmin for source 1:
NA
min=0.12
Maximum collected aperture, NAmax:
NA
max<0.55 corresponding to 33°
Angular increment at source 1:
dα
i=2.4°=const. (i.e.,
isotropic irra
