U.S. patent application number 12/823403 was filed with the patent office on 2010-10-14 for oblique mirror-type normal-incidence collector system for light sources, particularly euv plasma discharge sources.
This patent application is currently assigned to CARL ZEISS SMT AG. Invention is credited to Wolfgang Singer.
Application Number | 20100259742 12/823403 |
Document ID | / |
Family ID | 34384517 |
Filed Date | 2010-10-14 |
United States Patent
Application |
20100259742 |
Kind Code |
A1 |
Singer; Wolfgang |
October 14, 2010 |
OBLIQUE MIRROR-TYPE NORMAL-INCIDENCE COLLECTOR SYSTEM FOR LIGHT
SOURCES, PARTICULARLY EUV PLASMA DISCHARGE SOURCES
Abstract
There is provided a collector system. The collector system
includes a first collector mirror and a second collector mirror.
The first collector mirror receives EUV light from a light source
at a first aperture angle via a first beam path, and reflects the
EUV light at a second aperture angle along a second beam path. The
first aperture angle is larger than or substantially equal to the
second aperture angle. The second mirror receives the EUV light
from the first mirror at the second aperture angle. The collector
is an oblique mirror type normal incidence mirror collector
system.
Inventors: |
Singer; Wolfgang; (Aalen,
DE) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
CARL ZEISS SMT AG
Oberkochen
DE
|
Family ID: |
34384517 |
Appl. No.: |
12/823403 |
Filed: |
June 25, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10569574 |
Sep 1, 2006 |
7781750 |
|
|
PCT/EP03/09466 |
Aug 27, 2003 |
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12823403 |
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Current U.S.
Class: |
355/67 ; 355/77;
359/850 |
Current CPC
Class: |
G03F 7/70075 20130101;
G03F 7/70116 20130101; G03F 7/70175 20130101 |
Class at
Publication: |
355/67 ; 355/77;
359/850 |
International
Class: |
G03B 27/54 20060101
G03B027/54; G03B 27/32 20060101 G03B027/32; G02B 5/08 20060101
G02B005/08 |
Claims
1. A system, comprising: a first mirror; and a second mirror,
wherein: the first and second mirrors are arranged so that, during
use of the system, light generated by a light source reflects from
the first and second mirrors and forms an intermediate image of the
light source; the second mirror is moveable between a first
orientation and a second orientation different from the first
orientation; when the second mirror is in the first orientation,
the intermediate image is at a first location; and when the second
mirror is in the second orientation, the intermediate image is at a
second location different from the first location.
2. The system of claim 1, wherein the first mirror has an
opening.
3. The system of claim 2, wherein: when the second mirror is in the
first orientation, light reflected from the second mirror passes
through the opening in the first mirror before forming the
intermediate image; and when the second mirror is in the second
orientation, light reflected from the second mirror passes through
the opening in the first mirror before forming the intermediate
image.
4. The system of claim 2, wherein the first mirror comprises a
plurality of segments, and the opening in the first mirror is
adaptable to the orientation of the second mirror.
5. The system of claim 1, wherein the first mirror comprises a
plurality of segments.
6. The system of claim 5, wherein the plurality of segments defines
an opening in the first mirror, and the opening is adaptable to the
orientation of the second mirror.
7. The system of claim 1, wherein the second mirror is configured
to tilt between the first and second orientations.
8. The system of claim 1, wherein: the second mirror is moveable
between the first orientation, the second orientation, and a third
orientation different from the first and second orientations; and
when the second mirror is in the third orientation, the
intermediate image is at a third location different from the first
and second locations.
9. The system of claim 8, wherein the first mirror has an
opening.
10. The system of claim 9, wherein: when the second mirror is in
the first orientation, light reflected from the second mirror
passes through the opening in the first mirror before forming the
intermediate image; when the second mirror is in the second
orientation, light reflected from the second mirror passes through
the opening in the first mirror before forming the intermediate
image; and when the second mirror is in the third orientation,
light reflected from the second mirror passes through the opening
in the first mirror before forming the intermediate image.
11. The system of claim 8, wherein the first mirror comprises
multiple segments, and the segments are adaptable to the
orientation of the second mirror.
12. The system of claim 1, further comprising the light source.
13. The system of claim 12, wherein the light source comprises a
plurality of point sources.
14. The system of claim 13, wherein the second mirror comprises a
plurality of segments.
15. The system of claim 1, wherein the second mirror comprises a
plurality of segments.
16. The system of claim 15, wherein, for at least some of the
plurality of segments of the second mirror, each segment is
individually moveable between different orientations.
17. The system of claim 1, wherein the system is configured to be
used an illumination system of a microlithography projection
exposure unit.
18. A system, comprising: an illumination system, comprising: a
first mirror; and a second mirror, wherein: the first and second
mirrors are arranged so that, during use of the system, light
generated by a light source reflects from the first and second
mirrors and forms an intermediate image of the light source; the
second mirror is moveable between a first orientation and a second
orientation different from the first orientation; when the second
mirror is in the first orientation, the intermediate image is at a
first location; and when the second mirror is in the second
orientation, the intermediate image is at a second location
different from the first location; and a projection objective,
wherein the system is a microlithography projection exposure
unit.
19. The system of claim 18, wherein an exit pupil of the
illumination system coincides with an entrance pupil of the
projection objective.
20. A method, comprising: using the system of claim 17 to fabricate
a microelectronic component.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to an oblique mirror-type
normal-incidence collector system, in particular for light sources
which predominantly emit radiation with wavelengths of .ltoreq.193
nm. Most particularly preferred are EUV light sources, in
particular EUV plasma discharge sources. In addition, the invention
describes an illumination system comprising such a collector
system, which preferably is assigned to an EUV projection exposure
unit, in particular for EUV lithography.
[0003] 2. Description of the Prior Art
[0004] In order to be able to still further reduce pattern widths
for electronic components, in particular to the submicron range, it
is necessary to reduce the wavelengths of the light used for
microlithography. The use of light with wavelengths of .ltoreq.193
nm is conceivable, for example, lithography with soft x-rays,
so-called EUV lithography.
[0005] EUV lithography is one of the most promising lithography
techniques for the future. At the present time, wavelengths in the
range of 11-14 nm, in particular, 13.5 nm, are discussed as
wavelengths for EUV lithography, with a numerical aperture of
0.2-0.3. The image quality in EUV lithography is determined, on the
one hand, by the projection objective, and, on the other hand, by
the illumination system. The illumination system will provide an
illumination that is as uniform as possible of the field plane, in
which the pattern-bearing mask, the so-called reticle, is disposed.
The projection objective images the field plane into an image
plane, the so-called wafer plane, in which a light-sensitive object
is disposed. Projection exposure systems for EUV lithography are
designed with reflective optical elements. The shape of the field
of an EUV projection exposure unit is typically that of an annular
field with a high aspect ratio of 2 mm (width).times.22-26 mm (arc
length). The projection systems are usually operated in scanning
mode, wherein the reticle will be moved in the field plane and the
light-sensitive object, typically a wafer with a suitable
photoresist, will be moved in the image plane, synchronously
relative to one another. With respect to EUV projection exposure
units, reference is made to the following publications:
[0006] W. Ulrich, S. Beiersdorfer, H. J. Mann, "Trends in Optical
Design of Projection Lenses for UV- and EUV-Lithography" in
Soft-X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen
(editors), Proceedings of SPIE, Vol. 4146 (2000), pp. 13-24 and
[0007] M. Antoni, W. Singer, J. Schultz, J. Wangler, I.
Escudero-Sanz, B. Kruizinga, "Illumination Optics Design for
EUV-Lithography" in Soft X Ray and EUV Imaging Systems, W. M.
Kaiser, R. H. Stulen (editors), Proceedings of SPIE, Vol. 4146
(2000), pp. 25-34
the disclosure content of which is incorporated to the full extent
in the present Application.
[0008] For uptake of the radiation of EUV light sources, in
particular of laser-plasma sources and of discharge sources,
grazing-incidence collectors are utilized according to the prior
art, i.e, those in which the EUV radiation strikes the reflective
surfaces at a grazing incidence with the formation of a total
reflection. Such collectors can be designed, for example, as nested
systems, which are comprised of several collector shells and at
which two reflections occur each time. Such nested collectors are
also called Wolter systems. Mirror systems are utilized in such
collector systems, these mirror systems, for example, consisting of
a combination of hyperboloid-shaped and ellipsoid-shaped mirrors
and whose principle was described for the first time in the
literature in Annalen der Physik 10, 94-114, 1952, wherein the
disclosure content of this document is incorporated to the full
extent in the present Application.
[0009] The advantage of this type of collector consists of the fact
that even with large-scale light sources, such as, for example,
discharge sources, the power irradiated in the half-space is taken
up and bundled in the forward direction. A corresponding design is
known from U.S. Pat. No. 5,763,930. In this case it also must be
taken into consideration that it is not possible using this
principle to bend back the beam path in the vicinity of the light
source which would reduce the size of the illumination system, and
also the extension of the light source also runs counter to such
bending back according to the current prior art.
[0010] Another disadvantage of grazing-incidence collectors is
their shadowing effects, which arise due to the unavoidable
mechanical holders, particularly in the case of a nested structure.
These collectors can be constructed in a filigree pattern, e.g., in
the form of a spoked wheel, but these lead to radiation losses
especially in the case of small sources. A similar problem results
with respect to the cooling equipment of the grazing-incidence
collector, which adds further mechanical structures and thus
contributes to the shading losses. Such cooling equipment cannot be
omitted, since for lithography at 13.5 nm, due to the necessity of
forming a vacuum, the heat transfer is insufficient and the thermal
stress would lead to an intolerable deformation of the mirror
shells.
[0011] Collectors with a single normal-incidence collector mirror,
such as have become known, for example, from EP 1,255,163, can in
fact be well cooled on the back side, but a shading which results
from the finite extent of the light source cannot be avoided. If a
discharge source is present instead of a laser plasma source, then
the space requirement which results due to the extended electrodes
leads to an intense reduction of the illumination power. In order
to avoid this problem, another bending back can be undertaken in
the vicinity of the source. Such collectors, which are composed of
two normal-incidence mirrors disposed in the form of a
Schwarzschild system, have become known from U.S. Pat. No.
5,737,137 and have also been proposed in EP 1,319,988 A2 for the
individual focusing of EUV light sources in an arrangement with a
plurality of light sources. Here, it is impossible, for reasons of
space, to arrange a particle filter after the light source, so that
there is a particularly rapid polluting of normal-incidence mirrors
in the vicinity of the source. In addition, the collectors with
Schwarzschild arrangement, which are known from the prior art, are
characterized in that the radiation emitted from the EUV light
source is taken up by the first normal-incidence collector mirror
with a small numerical aperture of typically NA.about.0.3. When
going to larger collection angles in such a system, the angles of
incidence on the second collector mirror increase in relation to
the surface normal line, which is accompanied by a reduced
reflectivity and an increased polarizing effect of the multi-layer
reflection coating.
SUMMARY OF THE INVENTION
[0012] The object of the invention consists of configuring a
collector system consisting of normal-incidence mirrors for a light
source, in particular, an EUV light source, whereby laser plasma or
discharge light sources are particularly preferred for this source,
in such a way that the problems of the prior art indicated above
are overcome. Thus, a particularly well-cooled collector system
should be provided which takes up the radiation of the light source
with a large numerical aperture and thus a high collection
aperture, as well as simultaneously makes possible small radiation
losses as well as angles of incidence that are as small as possible
on the multi-layer reflection films of the mirrors of the
collector. The collector system will also provide sufficient
structural space for the screening of the light source and
contribute to an overall length of the illumination system that is
as short as possible, as well as produce a spectrally pure
illumination radiation.
[0013] The inventors have recognized that with a collector system
comprised of two normal-incidence mirrors, when employing an
oblique mirror-type structure of a Schwarzschild system, a high
collection aperture can be combined with sufficiently small angles
of incidence. Collection aperture is understood in the present
Application as the numerical aperture taken up by the first
collector mirror; this is designated below as the first numerical
aperture, to which is assigned a first aperture angle. For this
purpose, a collector system according to the invention is
constructed in such a way that the numerical aperture bent back,
i.e., reflected, from a first normal-incidence mirror is smaller
than or equal to the collection aperture. Resulting from this is an
increase of the mirror size of the second normal-incidence mirror,
which in turn is positioned at a distance to the first
normal-incidence mirror, which approximates as much as possible the
distance to the source. According to the invention, this is
achieved by a non-symmetrical structure, i.e., an oblique
mirror-type construction of the collector. It is to be understood
by this that the axial symmetry of the collector is no longer
given. Here, the second normal-incidence mirror is essentially
positioned next to the light source. Sufficient structural space
for the inclusion of the light source in its own compartment can be
created thereby and at the same time, the second normal-incidence
mirror can be formed correspondingly enlarged and can be positioned
at a sufficient distance to the first normal-incidence mirror.
Also, with such an arrangement, the back side of the mirror
elements can be cooled in a simple manner.
[0014] In an advantageous configuration of the invention, at least
the first normal-incidence mirror is adapted to the off-axis
positioning of the second normal-incidence mirror by means of the
incorporation of conical parts. Conical parts in this configuration
may also be used for only one of these mirrors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention will be described below with examples on the
basis of the drawings.
[0016] Herein is shown:
[0017] FIG. 1 the structure of a lithography system with an oblique
mirror-type Schwarzschild collector system according to the
invention;
[0018] FIG. 2 the structure of a lithography system with an
axial-symmetric Schwarzschild collector system corresponding to the
prior art;
[0019] FIG. 3 an oblique mirror-type Schwarzschild collector system
with an adjustable second normal-incidence mirror;
[0020] FIG. 4 a-c the configuration of an oblique mirror-type
Schwarzschild collector with conical parts for shaping the mirror
surfaces.
DESCRIPTION OF THE INVENTION
[0021] FIG. 2 shows the typical structure of an EUV lithography
system for microlithography, which comprises a Schwarzschild
collector system 2' corresponding to the prior art.
[0022] The schematically simplified representation of the
Schwarzschild collector system 2' shows a first normal-incidence
collector mirror C1', which takes up light from light source 1 by
its concave, for example, parabolic or elliptical shape and bends
it back, i.e., reflects it, onto the second normal-incidence
collector mirror C2', which in turn can be of hyperbolic or
ellipsoid shape. This second normal-incidence collector mirror C2'
is thus disposed so that it is centered relative to the first
normal-incidence collector mirror C1', so that a symmetrical
collector system is formed, which images the light source 1 onto a
magnified intermediate image Z. A passage 8 is provided in the
first normal-incidence collector mirror C1' for the radiation
exiting from the second normal-incidence collector mirror C2'.
Accordingly, light from light source 1 is taken up by the first
normal-incidence collector mirror C1' at a first aperture angle and
irradiated to the second normal-incidence collector mirror C2' at a
second aperture angle. A third aperture angle is in turn assigned
to the light bundle exiting from the second normal-incidence
collector mirror C2'. In the present Application, aperture angle is
understood to be the angle between the edge ray of a light bundle
and the maximum beam angle relative to the optical axis. Under the
vacuum conditions prevailing in EUV illumination, the numerical
aperture then results from the sine of the aperture angle.
[0023] The task of the collector system is to take up a large first
aperture angle, for example, of 45.degree., from the source, and to
convert this into an essentially smaller third aperture angle, for
example, of 6.degree. after the collector system, whereby the third
aperture angle is adapted to the downstream illumination system.
For example, a half open-angle or first aperture angle of
45.degree. at the first normal-incidence collector mirror C1' leads
to a third aperture angle of 6.degree. at the second
normal-incidence collector mirror C2', and therefore to a reduction
in the numerical aperture from approximately NA.about.0.7 to
approximately NA.about.0.1, corresponding to an imaging scale
factor of the collector of approximately 7.
[0024] The second normal-incidence collector mirror C2' must
necessarily be made small in shape due to the bending, i.e.,
reflecting, back of the beam path and must be positioned at a
certain distance relative to the light source 1. This results in
the disadvantage that the second aperture angle of the beam path
after the first normal-incidence collector mirror C1' must be
larger than the first aperture angle taken up from the source by
means of the first normal-incidence collector mirror. In the
present example, the focal intercept, i.e., the intercept distance,
between the source and the first collector mirror C1' amounts to
450 mm; the focal intercept between the first collector mirror C1'
and the virtual intermediate image of the source (not depicted in
FIG. 2) amounts to 280 mm, from which the imaging scale factor can
be estimated at 280 mm/450 mm=0.622, i.e., the sine of the second
aperture angle is approximately 1/0.622=1.61 times greater than the
sine of the first aperture angle. Even a small first aperture angle
of only 38.degree. leads in this case to a second aperture angle of
90.degree.. This large second aperture angle must be converted by
means of the second normal-incidence collector mirror C2' into the
essentially smaller third aperture angle exiting from the second
normal-incidence collector mirror C2'. In an actual design of
normal-incidence collectors, it has been established that
deviations from the sine condition in the imaging of the light
source result due to the collector mirrors C1' and C2'.
Nevertheless, the above approximation correctly reproduces the
basic problem.
[0025] This disadvantageous effect is clear upon consideration of
an edge ray exiting from the first normal-incidence collector
mirror C1' in FIG. 2, which bounds the first aperture angle of
approximately 38.degree., which corresponds to a collection
aperture of approximately 0.6. For the second aperture angle, given
by the edge ray, which is irradiated by the first normal-incidence
collector mirror, there follows an angle of approximately
60.degree., which, as set forth above, is considerably larger than
the first aperture angle in the case of two-mirror normal-incidence
collectors according to the prior art. At the second
normal-incidence collector mirror C2', the edge ray at the second
aperture angle of 60.degree. is thus deviated by -6.degree.
relative to the edge ray at the third aperture angle, whereby each
time the angle refers to the beam angle relative to the axis of
symmetry. A larger angle of incidence of 33.degree. thus arises
relative to the mirror surface of the second normal-incidence
collector mirror C2', so that a reduction of the reflectivity of
the mirror surface occurs as well as undesired polarization
effects.
[0026] From the requirement of the above-named reduction of the
numerical aperture from the first to the third aperture angle,
there consequently results the disadvantage from the prior art that
the first aperture angle and thus the power that can be taken up
from the light source is limited, since the second aperture angle
must be larger than the first aperture angle. The second aperture
angle could in principle be larger than 90.degree., but, due to the
large angle spectrum, this leads all the more to a great loss of
reflectivity of the multi-layer system of the second
normal-incidence collector mirror, as well as to intense
polarization effects. Further, a second normal-incidence collector
mirror shaped in such a way would lead to problematical geometric
ratios.
[0027] Also shown in FIG. 2, proceeding in the light path from a
light source 1 to an illuminated plane, which is called the field
plane 13, are optical components of an illumination system as well
as the projection objective 126 of a projection exposure unit. All
components of the illumination system and of the projection
objective bear the same reference numbers as the corresponding
components in FIG. 1. Reference is made to the description in FIG.
1 relative to these components.
[0028] FIG. 1 shows a projection exposure unit, whose illumination
system comprises a collector system 2 according to the invention.
The collector system according to the invention is formed as an
oblique mirror system in a Schwarzschild arrangement, i.e., first
and second normal-incidence collector mirrors C1 and C2 are not
arranged or shaped in an axial-symmetric manner. In schematically
simplified representation, a concavely shaped first
normal-incidence collector mirror C1 is shown, which bends back the
light onto the second normal-incidence collector mirror C2. Here,
in the present Application, a normal-incidence mirror is understood
to be a mirror in which the angles of incidence relative to the
mirror normal line are smaller than 70.degree.. With this
condition, the concept of bending back the beam path is also
established by the first, concavely shaped normal-incidence
collector mirror C1.
[0029] According to the invention, the first normal-incidence
collector mirror C1 does not have a reducing imaging effect on the
image of the light source, so that the first aperture angle taken
up by the first collector mirror C1 is larger than or substantially
equal to the second aperture angle, which is emitted by the first
collector mirror C1 and is taken up by the second collector mirror
C2. Due to the effect of the first normal-incidence collector
mirror C1 according to the invention, as shown in FIG. 3, the light
source 1 is imaged into a virtual intermediate image Z.sub.v, which
has a distance to the first normal-incidence collector mirror C1
that is equal to or greater than the distance of the light source 1
to the normal-incidence collector mirror C1. Due to the oblique
mirror-type arrangement, a normal-incidence collector mirror C2,
which is preferably clearly larger when compared with the prior
art, is disposed extra-axially, i.e., it is positioned laterally
next to light source 1.
[0030] Proceeding from the selected positioning of the second
normal-incidence collector mirror C2, the latter is preferably
adjusted in its size such that it covers the entire second aperture
angle of the radiation exiting from the first normal-incidence
collector mirror C1. Further, the second normal-incidence collector
mirror C2 is preferably configured such that the intermediate image
Z of the light source 1 is substantially shaped by means of the
first normal-incidence collector mirror C1 in the region of a
passage 8.
[0031] In another configuration, the intermediate image Z of the
light source 1 is formed on the side of the first normal-incidence
collector mirror C1 that lies opposite the second normal-incidence
collector mirror C2. The radiation exiting from the second
normal-incidence collector mirror C2 is thus also advantageously
guided through a passage 8 in the first normal-incidence collector
mirror C1 and is imaged on the intermediate image Z outside the
region of the rear walls that may contain cooling devices of the
first normal-incidence collector mirror C1.
[0032] The formation of the intermediate image Z of the light
source 1 is of advantage, since it is thus possible to enclose the
light source 1 and the collector system 2 in a compartment that is
atmospherically separated from the downstream system. Such a
compartment is sketched in FIG. 1 and provided with the reference
number 10. It is also possible, based on the formation of the
intermediate image Z of the light source 1 to incorporate in the
beam path a diaphragm 12, which cooperates, for example, with a
raster spectral filter for the spectral filtering of the
illumination.
[0033] If a segment with an axis of symmetry is used for the first
normal-incidence collector mirror C1, then neither the light source
1 nor the second normal-incidence collector mirror C2 will lie on
this axis of symmetry, but these components are positioned lying
opposite and at a distance to this axis. Here, the distance is
defined as the distance between the central point of the second
normal-incidence collector mirror C2 and the light source 1. This
distance is at least long enough that the entire second
normal-incidence collector mirror C2 is situated outside the region
near the light source, whereby the region near the light source is
preferably understood as the region around the source point of the
EUV light source up to a distance of at least 100 mm and,
particularly preferred, of 200 mm.
[0034] If there is no axis of symmetry at the first
normal-incidence collector mirror C1, then the distanced
positioning of the second normal-incidence collector mirror C2 is
referred to the perpendicular distance relative to a straight line
that is established by the EUV light source 1 and the vertex S of
the first normal-incidence collector mirror C1. The vertex S for a
multi-segment first normal-incidence collector mirror C1 is defined
as the vertex of the envelope of the mirror surface.
[0035] In this way, the radiation exiting from the light source can
reach the first normal-incidence collector mirror C1 without
shading losses and, at the same time, a sufficient distance can be
achieved between the first and the second normal-incidence
collector mirrors C1 and C2, whereby this distance is approximately
the same as the distance between the light source 1 and the first
normal-incidence collector mirror C1.
[0036] Due to the oblique mirror-type structure of the collector
according to the invention and the enlargement of the second
collector mirror C2 made possible thereby, as well as its extended
distance relative to the first collector mirror C1, the limitation
of the numerical aperture in the case of Schwarzschild collectors
for EUV light sources as is known from the prior art is
successfully overcome. Here, the angles of incidence at the second
collector mirror C2, even for a numerical aperture of NA>0.6 and
larger, which the first collector mirror C1 takes up from the light
source 1, are still small enough that the reflection losses and the
undesired polarization effects can be tolerated. Thus, if first
aperture angles that are as large as possible are preferred, i.e.,
for the numerical aperture with which the first normal-incidence
collector mirror C1 collects radiation from light source 1, then a
value of NA.gtoreq.0.5 and particularly of NA.gtoreq.0.6 and
especially of NA.gtoreq.0.7 is preferred. Due to the oblique
mirror-type structure of the collector according to the invention,
particularly high first aperture angles and thus numerical
apertures of NA.gtoreq.0.85 are also conceivable.
[0037] FIG. 1 sketches an advantageous configuration of the
collector according to the invention, in which the first beam path
exiting from light source 1 is shown, while that beam path which
travels to the second normal-incidence collector mirror C2 and is
designated the second beam path no longer penetrates up to a
certain distance from light source 1. This region through which the
second beam path does not penetrate preferably has a minimum radius
of 100 mm around the light source 1 and 200 mm is particularly
preferred. In general, it is preferred that in the case of a given
minimum distance d from the light source 1 to the mirror surface of
the first normal-incidence collector mirror C1, the second beam
path will not pass through a region around light source 1 with a
minimum radius of d/5 or, particularly preferred, of d/3. In
addition, it is preferred that in this region within the minimum
radius, no optical components, including the mechanical mounting
structures assigned to them will be found. As an additional
advantage according to the invention, it is achieved herewith that
sufficient structural space for the configuration of a compartment
6 enclosing the light source 1 is provided. In such a compartment
6, the atmosphere that surrounds light source 1 is separated from
the downstream region of the illumination system placed under
vacuum, so that any contamination coming from light source 1 cannot
reach this vacuum region. Here, a particle filter is used, for
example, which is constructed as a zirconium foil in one possible
design, in order to allow the EUV radiation to pass, but retains
the debris coming from light source 1. Further, additional or
alternative electrical or magnetic fields can be used for trapping
electrically charged dirt particles. Also, large-scale EUV light
sources, such as, e.g., discharge sources, in the case of an
oblique mirror-type Schwarzschild collector system according to the
invention with a lateral positioning of the second normal-incidence
collector mirror C2, do not prevent the bending back of the beam
path in the vicinity of the light source 1.
[0038] In a first preferred embodiment, the reflective surface of
the first normal-incidence collector mirror C1 is shaped as a
segment of an ellipsoid, whereas the extra-axially, i.e., off axis,
positioned second normal-incidence collector mirror C2 is shaped as
a segment of a hyperboloid. In particular, the first
normal-incidence collector mirror C1 has a large structural size in
order to achieve a sufficiently large aperture, so that it is
constructed of several individual segments in an advantageous
configuration of the invention. These can be mounted directly next
to one another on a common support structure or they can have
individual holders that can move relative to one another. The
second normal-incidence collector mirror C2 can also be constructed
in this way from individual segments. Due to the high thermal load
of the normal-incidence collector mirrors C1 and C2, these mirrors
are advantageously cooled. For this purpose, a cooling devices 9.1
and 9.2 for the first collector mirror C1 or the second collector
mirror C2 can be utilized for example, devices, which operate with
a cooling medium, e.g., with water, or by means of Peltier cooling.
Due to the bending back in the collector system according to the
invention by means of an oblique mirror-type arrangement,
sufficient structural space is present in back, both at the first
as well as the second normal-incidence collector mirrors C1 and C2,
in order to be able to accommodate such cooling devices.
[0039] In addition, the optical components of the illumination
system as well as the projection objective 126 disposed downstream
to the collector system 2 according to the invention in the light
path from the light source 1 to the illuminated field plane 13 are
shown in FIG. 1. The following are shown individually in FIG. 1: A
reticle or a mask 11 is positioned in the field plane 13 of a
projection exposure unit and imaged by means of a reduction optics
126 onto its image plane 130, in which a wafer 106 provided with a
light-sensitive material is typically found. FIG. 1 shows for this
purpose, as an example, a projection objective consisting of six
individual mirrors 128.1 to 128.6, which is derived, for example,
from U.S. Pat. No. 6,600,552, which is incorporated to the full
extent in the present Application. Also depicted is a telecentric
illumination of image plane 130 in the ideal case, i.e., the chief
ray of a beam bundle, which proceeds out from a field point of
field plane 13, and perpendicularly intersects the image plane 130.
In addition, the projection objective 126 has an entrance pupil
which in general coincides with the exit pupil of the illumination
system.
[0040] FIG. 1 also shows the typical structure of an EUV
illumination system, which is formed as a double-facetted
illumination system according to U.S. Pat. No. 6,198,793 B1,
whereby the content of this document is incorporated to the full
extent in the present Application. Such a system comprises a first
optical element with first raster elements 3, which is also
designated as a field facet mirror 3. A second optical element with
second raster elements 5, which is usually named a pupil facet
mirror 5, then follows in the beam path.
[0041] Field facet mirror 3 and pupil facet mirror 5 serve for the
illumination of a field in the field plane 13 as well as the
shaping of the illumination in the exit pupil of the illumination
system. The effect of each field raster element is such that it
forms an image of light source 1, wherein a plurality of so-called
secondary light sources is formed by the plurality of field facets
or field raster elements. The secondary light sources are formed in
or near the plane in which the pupil facet mirror 5 is disposed.
Thus, as shown in FIG. 1, if the secondary light sources come to
lie in the region of the pupil facet mirror 5, the field facets
themselves can have an optical effect, for example, a collecting
optical effect. These secondary light sources are imaged by the
downstream optical elements as tertiary light sources in the exit
pupil of the illumination system.
[0042] In addition, each field raster is imaged in the field plane
13 by the facets of the pupil facet mirror 5 and the downstream
optical elements of the second optical component 7, which, in the
example of FIG. 1, consists of the following three optical
elements: a first reflective optical element 19, a second
reflective optical element 21 and the grazing-incidence mirror 23.
The images of the field facets that are superimposed therein serve
for the illumination of a mask 11 in the field plane 13, whereby,
typically, starting from rectangular or arc-shaped field facets, an
illumination in the form of an annular field segment arises in the
field plane 13. In general, the microlithography system is formed
as a scanning system, so that the mask 11 in the field plane 13 and
the wafer 106 in the image plane 130 are moved synchronously in
order to effect an illumination or an exposure.
[0043] A configuration of the oblique mirror-type Schwarzschild
collector according to the invention, in which different tilting
angles can be adjusted for the second normal-incidence collector
mirror C2, is shown in FIG. 3. The position of the virtual
intermediate image Z.sub.y is established starting from the
established positions of the light source 1 and of the first
normal-incidence collector mirror C1. The position and the
orientation of the magnified intermediate image Z of the light
source 1 can be adjusted by the variation of adjustment of the
second normal-incidence collector mirror C2. For example, three
different tilting angles of the second normal-incidence collector
mirror C2 are sketched in FIG. 3. It is possible in this way to
comply with requirements for structural space resulting for the
downstream parts of the microlithography system, or if an EUV light
source with a plurality of source points is used, to select between
the individual source points. If the second normal-incidence
collector mirror C2 is constructed from individual segments, then
it is particularly advantageous when a plurality of source points
is superimposed, if the tilting angles assigned to the individual
segments can be individually adjusted. In general, for a second
normal-incidence collector mirror C2 with adjustable tilting angle,
either the passage opening 8 in the first normal-incidence
collector mirror C1 is configured sufficiently large or the passage
opening 8 is adapted to the respective adjustment of the second
normal-incidence collector mirror C2. If the first normal-incidence
collector mirror C1 is constructed in a modular manner, from
individual segments with individual support elements, then the
passage opening 8 can be shifted in a relatively simple manner.
[0044] Advantageous configurations of the oblique mirror-type
Schwarzschild collector are shown in FIGS. 4 a-c. FIG. 4a shows the
above-described oblique mirror-type structure of the two-mirror
collector with the second normal-incidence collector mirror C2
essentially disposed next to the source. The second
normal-incidence collector mirror C2 in FIG. 4b is disposed such
that the power irradiated from the source passes through a central
opening of the second normal-incidence collector mirror C2. In this
configuration, which can also have, for example, a
rotation-symmetric collector system, the collection aperture that
can be taken up in the second normal-incidence collector mirror C2
is limited, of course, due to the small opening. In FIG. 4c, the
first and second normal-incidence collector mirrors C1 and C2 each
have shapes which add a conical part to the shape of the mirror.
The inventors have recognized that for such a configuration of the
mirrors as non-imaging optical elements with conical parts, the
structure of the Schwarzschild collector according to the invention
with a large collection aperture can be particularly well realized.
Here, conical constants are known to the person skilled in the art,
for example, from standard software packages for optical
design.
[0045] By the addition of conical parts to the first
normal-incidence collector mirror C1, as shown in FIG. 4c, a
centered system can be provided, in which the light source 1 is
positioned essentially centrally in the second normal-incidence
collector mirror C2. With this measure according to the invention,
due to the conical parts on the first collector mirror C1, the
central opening in the second collector mirror C2 can be selected
larger, whereby a larger aperture angle is achieved according to
the invention. The conical part on the first normal-incidence
collector mirror C1 can be used not only for this in order to
enlarge the central shading in the second normal-incidence
collector mirror C2, but also to create in turn structural space
for the light source 1 and the components, such as e.g, a particle
filter, which surround it. This effect is shown in FIG. 4c.
[0046] In another advantageous configuration of the invention, a
reflective surface with a raster for spectral filtering is formed
on one of the two normal-incidence collector mirrors C1 or C2. Such
a raster spectral filter is known from US 2003/0043455 A1, wherein
the content of this document is incorporated to the full extent in
the present Application. If this spectral raster is provided at the
second normal-incidence collector mirror C2, then, by the combined
filtering effect from the multi-layer system which derives the
useful wavelength of 13.5 nm and wavelengths above 100 nm from the
irradiation of the light source, and the raster spectral filter
which is adjusted so that the wavelength region of 7 to
approximately 27 nm is selected directly, a particularly pure
spectral illumination can be achieved. For space reasons, it is
advantageous to form the diaphragm system for filtering the
undesired diffraction orders substantially in the region of passage
8 by means of the first normal-incidence collector mirror C1.
[0047] In an preferred embodiment of the invention, it is
conceivable to incorporate additional mirrors in the collector
system according to the invention. Such mirrors can contribute to
generating an image of the light source or serve for further
adaptation of the structural space of the collector. It is also
conceivable to form one of the mirrors, which follows the first
normal-incidence collector mirror C1, as a grazing-incidence
mirror. Under certain circumstances, a grazing-incidence mirror can
also be used for the second collector mirror directly following the
first normal-incidence collector mirror C1.
[0048] In an advantageous embodiment, at least one of the collector
mirrors of the invention according to EP 1,189,089, which is
incorporated to the full extent in the present Application, can be
mounted mechanically such that the optical properties are not
modified with changes in temperature. Further, at least one of the
mirrors can be designed as controllable, movable or tiltable, in
order to actively adjust the collector mirrors during mounting and
to keep them in an ideal state of adjustment during operation.
[0049] By providing the oblique mirror-type arrangement of the
normal-incidence mirrors of the Schwarzschild collector according
to the invention, a projection exposure unit is also disclosed,
which comprises an EUV illumination system with such a collector
system. Further, a method for exposure for the production of
microelectronic components is indicated, in which a projection
exposure unit characterized in such a way is used.
LIST OF REFERENCE NUMBERS
[0050] 1 Light source [0051] 2, 2' Collector system [0052] 3 First
optical element with first raster elements (field facet mirror)
[0053] 4 Particle filter [0054] 5 Second optical element with
second raster elements (pupil facet mirror) [0055] 6 Compartment
for the light source [0056] 7 Second optical component [0057] 8
Passage in the first normal-incidence mirror [0058] 9.1, 9.2
Devices for cooling [0059] 10 Compartment for incorporation of the
light source 1 and the collector system 2 [0060] 11 Pattern-bearing
mask [0061] 13 Field plane [0062] 19 First reflective optical
element [0063] 21 Second reflective optical element [0064] 23
Grazing-incidence mirror [0065] 27 Exit pupil of the illumination
system [0066] 104 Mask [0067] 106 Wafer provided with a
light-sensitive material [0068] 120 Projection objective [0069]
128.1, 128.2 Mirrors of the projection objective [0070] 128.3,
128.4, [0071] 128.5, 128.6 [0072] 130 Image plane [0073] d Minimum
distance of light source 1 to the mirror surface of the first
normal-incidence [0074] collector mirror C1 [0075] C1, C1' first
normal-incidence collector mirror [0076] C2, C2' second
normal-incidence collector mirror [0077] Z Intermediate image of
the light source [0078] Z.sub.v Virtual intermediate image of the
light source [0079] S Apical point or vertex of the first
normal-incidence collector mirror
* * * * *