U.S. patent application number 12/212926 was filed with the patent office on 2009-01-15 for illumination system particularly for microlithography.
This patent application is currently assigned to Carl Zeiss SMT AG. Invention is credited to Martin Antoni, Udo Dinger, Joachim Hainz, Joerg Schultz, Karl-Heinz Schuster, Wolfgang Singer, Johannes Wangler, Joachim Wietzorrek.
Application Number | 20090015812 12/212926 |
Document ID | / |
Family ID | 7866645 |
Filed Date | 2009-01-15 |
United States Patent
Application |
20090015812 |
Kind Code |
A1 |
Schultz; Joerg ; et
al. |
January 15, 2009 |
ILLUMINATION SYSTEM PARTICULARLY FOR MICROLITHOGRAPHY
Abstract
There is provided an illumination system for scannertype
microlithography along a scanning direction with a light source
emitting a wavelength .ltoreq.193 nm. The illumination system
includes a plurality of raster elements. The plurality of raster
elements is imaged into an image plane of the illumination system
to produce a plurality of images being partially superimposed on a
field in the image plane. The field defines a non-rectangular
intensity profile in the scanning direction.
Inventors: |
Schultz; Joerg; (Aalen,
DE) ; Wangler; Johannes; (Koenigsbronn, DE) ;
Schuster; Karl-Heinz; (Koenigsbronn, DE) ; Dinger;
Udo; (Oberkochen, DE) ; Singer; Wolfgang;
(Aalen, DE) ; Antoni; Martin; (Aalen, DE) ;
Wietzorrek; Joachim; (Aalen, DE) ; Hainz;
Joachim; (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: |
7866645 |
Appl. No.: |
12/212926 |
Filed: |
September 18, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11345880 |
Feb 2, 2006 |
7443948 |
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12212926 |
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10150650 |
May 17, 2002 |
7006595 |
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11345880 |
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09679718 |
Sep 29, 2000 |
6438199 |
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10150650 |
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09305017 |
May 4, 1999 |
6198793 |
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09679718 |
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Current U.S.
Class: |
355/67 |
Current CPC
Class: |
G21K 1/06 20130101; G03F
7/70166 20130101; G03F 7/70358 20130101; G03F 7/70233 20130101;
B82Y 10/00 20130101; G03F 7/70083 20130101; G03F 7/702 20130101;
G03F 7/70108 20130101 |
Class at
Publication: |
355/67 |
International
Class: |
G03B 27/54 20060101
G03B027/54 |
Foreign Application Data
Date |
Code |
Application Number |
May 5, 1998 |
DE |
198 19 898 |
Feb 2, 1999 |
DE |
199 03 807 |
Feb 8, 1999 |
DE |
299 02 108 |
Jul 28, 2000 |
EP |
PCT/EP00/07258 |
Claims
1. (canceled)
2. An illumination system for illuminating a region of a surface
with illumination light emitted from a light source, the system
comprising: an incidence-side reflection-type fly-eye optical
system comprising multiple first reflective elements arranged in
rows; an emission-side reflection-type fly-eye optical system
comprising multiple second reflective elements arranged in rows
such that each second reflective element corresponds to and
receives illumination light from a respective first reflective
element of the incidence-side reflection-type fly-eye optical
system; and a condensing optical system comprising two reflective
mirrors that guide the illumination light, reflected by the
emission-side reflection-type fly-eye optical system, to the region
of the surface, wherein at least one of the reflective mirrors has
a center of curvature that is optically eccentric with respect to a
normal to the surface at the center of the illuminated region.
3. The system of claim 2, wherein the two reflective mirrors are
spherical mirrors.
4. The system of claim 2, wherein the two reflective mirrors are
aspherical mirrors.
5. The system of claim 2, wherein the two reflecting mirrors are a
spherical mirror and an aspherical mirror, respectively.
6. The system of claim 2, wherein: each of the first reflective
elements has a respective effective reflecting surface having a
respective center and a respective inclination; each of the second
reflective elements has a respective effective reflecting surface
having a respective center and a respective inclination; and the
inclinations of the second reflective elements are set such that
rays of the illumination light, propagating from the respective
centers of the respective effective reflecting surfaces of the
first reflective elements and reflecting from the respective
centers of the respective effective reflecting surfaces of the
second reflective elements, are converged by the condensing optical
system at a point in the region of the surface.
7. An exposure apparatus for transferring a pattern from a mask
onto a photosensitive substrate, the apparatus comprising an
illumination system as recited in claim 2 situated and configured
to illuminate the mask.
8. A method for manufacturing microdevices, comprising: exposing a
photosensitive substrate to a pattern defined on a reflective
reticle, using an exposure apparatus as recited in claim 7; and
developing the exposed photosensitive substrate.
9. An exposure apparatus, comprising: an illumination system
situated and configured to illuminate a region of a surface with
illumination light emitted from a light source, the illumination
system comprising (i) an incidence-side reflection-type fly-eye
optical system comprising multiple first reflective elements
arranged in rows, (ii) an emission-side reflection-type fly-eye
optical system comprising multiple second reflective elements
arranged in rows, wherein each second reflective element
corresponds to and receives illumination light from a respective
first reflective element of the incidence-side reflection-type
fly-eye optical system, and (iii) a condensing optical system
comprising two reflecting mirrors that guide the illumination
light, reflected by the emission-side reflection-type fly-eye
optical system, to the region of the surface; wherein (iv) the two
reflecting mirrors have respective centers of curvature, and (v)
the condensing optical system has an optical axis that passes
through the centers of curvature of the two reflecting mirrors but
is not parallel to a normal to the region of the surface at the
center of the region.
10. An exposure apparatus, comprising: an illumination system
situated and configured to illuminate a region of a surface with
illumination light emitted from a light source, the illumination
system comprising (i) an incidence-side reflection-type fly-eye
optical system comprising multiple first reflective elements
arranged in rows, (ii) an emission-side reflection-type fly-eye
optical system comprising multiple second reflective elements
arranged in rows, wherein each second reflective element
corresponds to and receives illumination light from a respective
first reflective element of the incidence-side reflection-type
fly-eye optical system, and (iii) a condensing optical system
comprising two reflecting mirrors that guide the illumination
light, reflected by the emission-side reflection-type fly-eye
optical system, to the region of the surface; wherein (iv) the
condensing optical system has an optical axis, the two reflecting
mirrors have respective centers of curvature, (v) the emission-side
reflection-type fly-eye optical system has an aperture plane, (vi)
a perpendicular line passing through a center of the aperture plane
is a virtual optical axis, and (vii) the optical axis of the
condensing optical system passing through the centers of curvature
of the two reflecting mirrors of the condensing optical system is
not parallel to the virtual optical axis.
11. An exposure apparatus, comprising an illumination system
situated and configured to illuminate a region of a surface with
illumination light emitted from a light source, the illumination
system comprising (i) an incidence-side reflection-type fly-eye
optical system comprising multiple first reflective elements
arranged in rows, (ii) an emission-side reflection-type fly-eye
optical system comprising multiple second reflective elements
arranged in rows, wherein each second reflective element
corresponds to and receives illumination light from a respective
first reflective element of the incidence-side reflection-type
fly-eye optical system, and (iii) a condensing optical system that
guides the illumination light, reflected by the emission-side
reflection-type fly-eye optical system, to the region of the
surface, the condensing optical system comprising an incidence-side
convex mirror and an emission-side concave mirror.
12. The exposure apparatus of claim 11, wherein: the emission-side
reflection-type fly-eye optical system has a periphery; the first
reflective elements of the emission-side reflection-type fly-eye
optical system have respective inclination angles; the closer to
the periphery, the greater the inclination angles; and the entire
emission-side reflection-type fly-eye optical system has a
convergent action.
13. The exposure apparatus of claim 11, wherein: the convex mirror
and concave mirror have respective centers of curvature; and at
least one of the centers of curvature is optically eccentric with
respect to a normal to the region of the surface at the center of
the region.
14. The exposure apparatus of claim 12, wherein: the convex mirror
and concave mirror have respective centers of curvature; the
emission-side reflection-side fly-eye optical system has an
aperture plane; a perpendicular line passing through a center of
the aperture plane is a virtual optical axis; and at least one of
the centers of curvature is eccentric with respect to the virtual
optical axis.
15. The exposure apparatus of claim 12, wherein: the condensing
optical system has an optical axis; the convex mirror and concave
mirror have respective centers of curvature; and the optical axis,
passing through the respective centers of curvature of the convex
mirror and the concave mirror, is not parallel to a normal to the
region of the surface at the center of the region.
16. The exposure apparatus of claim 12, wherein: the condensing
optical system has an optical axis; the convex mirror and concave
mirror have respective centers of curvature; the emission-side
reflection-side fly-eye optical system has an aperture plane; a
perpendicular line passing through a center of the aperture plane
is a virtual optical axis; and the optical axis, passing through
the respective centers of curvature of the convex mirror and the
concave mirror, is not parallel to the virtual optical axis.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention concerns an illumination system for
wavelengths .ltoreq.193 nm as well as a projection exposure
apparatus with such an illumination system.
[0003] In order to be able to further reduce the structural widths
of electronic components, particularly in the submicron range, it
is necessary to reduce the wavelengths of the light utilized for
microlithography. Lithography with very deep UV radiation, so
called VUV (Very deep UV) lithography or with soft x-ray radiation,
so-called EUV (extreme UV) lithography, is conceivable at
wavelengths smaller than 193 nm, for example.
[0004] 2. Description of the Prior Art
[0005] An illumination system for a lithographic device, which uses
EUV radiation, has been made known from U.S. Pat. No. 5,339,346.
For uniform illumination in the reticle plane and filling of the
pupil, U.S. Pat. No. 5,339,346 proposes a condenser, which is
constructed as a collector lens and comprises at least 4 pairs of
mirror facets, which are arranged symmetrically. A plasma light
source is used as the light source.
[0006] In U.S. Pat. No. 5,737,137, an illumination system with a
plasma light source comprising a condenser mirror is shown, in
which an illumination of a mask or a reticle to be illuminated is
achieved by means of spherical mirrors.
[0007] U.S. Pat. No. 5,361,292 shows an illumination system, in
which a plasma light source is provided, and the point plasma light
source is imaged in an annular illuminated surface by means of a
condenser, which has five aspherical mirrors arranged
off-center.
[0008] From U.S. Pat. No. 5,581,605, an illumination system has
been made known, in which a photon beam is split into a multiple
number of secondary light sources by means of a plate with concave
raster elements. In this way, a homogeneous or uniform illumination
is achieved in the reticle plane. The imaging of the reticle on the
wafer to be exposed is produced by means of conventional reduction
optics. A gridded mirror is precisely provided with equally curved
elements in the illumination beam path. The contents of the
above-mentioned patents are incorporated by reference.
SUMMARY OF THE INVENTION
[0009] The invention provides an illumination system for
microlithography that fulfills the requirements for advanced
lithography with wavelength less or equal to 193 nm. The system
illuminates a structured reticle arranged in the image plane of the
illumination system, which will be imaged by a projection objective
onto a light sensitive substrate. In stepper-type lithography
systems the reticle is illuminated with a rectangular field,
wherein a pregiven uniformity of the light intensity inside the
field is required, for example better than .+-.5%. In scanner-type
lithography systems the reticle is illuminated with a rectangular
or arc-shaped field, wherein a pregiven uniformity of the scanning
energy distribution inside the field is required, for example
better than .+-.5%. The scanning energy is defined as the line
integral over the light intensity in the scanning direction. The
shape of the field is dependent on the type of projection
objective. All reflective projection objectives typically have an
arc-shaped field, which is given by a segment of an annulus. A
further requirement is the illumination of the exit pupil of the
illumination system, which is located at the entrance pupil of the
projection objective. A nearly field-independent illumination of
the exit pupil is required.
[0010] One embodiment of the present invention is an illumination
system for scannertype microlithography along a scanning direction
with a light source emitting a wavelength .ltoreq.193 nm. The
illumination system includes a plurality of raster elements. The
plurality of raster elements is imaged into an image plane of the
illumination system to produce a plurality of images being
partially superimposed on a field in the image plane. The field
defines a non-rectangular intensity profile in the scanning
direction.
[0011] Another embodiment of the present invention is an
illumination system for scannertype microlithography along a
scanning direction with a light source emitting a wavelength
.ltoreq.193 nm. This embodiment includes a first optical component
having a plurality of first raster elements and a second optical
component having a plurality of second raster elements. A first
member of the plurality of first raster element deflects a first
member of a plurality of incoming ray bundles to a first member of
the plurality of second raster elements to provide an image of the
first member of the plurality of first raster elements on a field
in an image plane. A second member of the plurality of first raster
element deflects a second member of a plurality of incoming ray
bundles to a second member of the plurality of second raster
elements to provide an image of the second member of the plurality
of first raster elements on the field. The image of the first
member of the plurality of first raster element and the image of
the second member of the plurality of first raster elements are
partially superimposed, and the field defines a non-rectangular
intensity profile in the scanning direction.
[0012] Another embodiment of an illumination system for
microlithography with a wavelength .ltoreq.193 nm, in accordance
with the present invention, includes a primary light source, a
first optical component, a second optical component, an image
plane, and an exit pupil. The first optical component transforms
the primary light source into a plurality of secondary light
sources that are imaged by the second optical component in the exit
pupil. The first optical component includes a plurality of raster
elements that are imaged into the image plane, producing a
plurality of images being superimposed partially on a field in the
image plane. The field defines a non-rectangular intensity profile
in a scanning direction. The plurality of raster elements deflect a
plurality of incoming ray bundles to produce a plurality of
deflected ray bundles with deflection angles, and at least two of
the deflection angles are different from one another.
[0013] Another embodiment of an illumination system for
microlithography with a wavelength .ltoreq.193 nm includes a
primary light source, a first optical component, a second optical
component, an image plane, and an exit pupil. The first optical
component transforms the primary light source into a plurality of
secondary light sources that are imaged by the second optical
component in the exit pupil. The first optical component includes a
plurality of first raster elements that are imaged into the image
plane, producing a plurality of images being superimposed partially
on a field in the image plane. The plurality of first raster
elements deflect a plurality of incoming ray bundles to produce a
plurality of deflected ray bundles with first deflection angles,
and least two of the first deflection angles are different from one
another. The first optical component also includes a plurality of
second raster elements, where one of the plurality of first raster
elements corresponds to one of the plurality of second raster
elements. One of the plurality of first raster elements deflects
one of the plurality of incoming ray bundles to the corresponding
one of the plurality of second raster elements, and the plurality
of second raster elements deflects the plurality of deflected ray
bundles with second deflection angles to superimpose the plurality
of images partially on the field. The field defines a
non-rectangular intensity profile in a scanning direction.
[0014] In another embodiment of the present invention, an
illumination system for scannertype microlithography along a
scanning direction with a light source emitting a wavelength
.ltoreq.193 nm includes a first optical component with a first
optical element having a plurality of first raster elements. The
plurality of first raster elements deflect a plurality of incoming
ray bundles to produce a plurality of deflected ray bundles with
first deflection angles. The first optical component also has a
second optical element having a plurality of second raster
elements. One of the plurality of first raster elements corresponds
to one of the plurality of second raster elements, and one of the
plurality of first raster element deflects one of the plurality of
incoming ray bundles to the corresponding one of the plurality of
second raster elements. At least two of the first raster elements
are arranged symmetric to an axis of symmetry, and the at least two
of the first raster elements deflect the plurality of incoming ray
bundles with the first deflection angles to the corresponding one
of the plurality of second raster elements to fill an exit pupil of
the illumination system nearly point symmetric to a center of the
exit pupil.
[0015] Another embodiment of an illumination system for
microlithography with a wavelength .ltoreq.193 nm, in accordance
with the present invention, includes a primary light source, a
first optical component, a second optical component, an image
plane, and an exit pupil. The first optical component transforms
the primary light source into a plurality of secondary light
sources that are imaged by the second optical component in the exit
pupil. The first optical component includes a first optical element
having a plurality of first raster elements that are imaged into
the image plane, producing a plurality of images being superimposed
at least partially on a field in the image plane, where the
plurality of first raster elements deflect a plurality of incoming
ray bundles to produce a plurality of deflected ray bundles with
first deflection angles. At least two of the first deflection
angles are different from one another. The first optical component
also includes a second optical element having a plurality of second
raster elements, where one of the plurality of first raster
elements corresponds to one of the plurality of second raster
elements. One of the plurality of first raster element deflects one
of the plurality of incoming ray bundles to the corresponding one
of the plurality of second raster elements. At least two of the
plurality of first raster elements are adjacent to one another and
have two corresponding second raster elements, and at least another
one of the plurality of second raster elements is arranged between
the two corresponding raster elements.
[0016] An illumination system in accordance with the present
invention can also be employed in a projection exposure apparatus
for microlithography. Such a projection exposure apparatus
includes, in addition to the illumination system, a reticle located
at the image plane, a light-sensitive object on a support system,
and a projection objective to image the reticle onto the
light-sensitive object.
[0017] Typical light sources for wavelengths between 100 nm and 200
nm are excimer lasers, for example an ArF-Laser for 193 nm, an
F.sub.2-Laser for 157 nm, an Ar.sub.2-Laser for 126 nm and an
NeF-Laser for 109 nm. For systems in this wavelength region
refractive components of SiO.sub.2, CaF.sub.2, BaF.sub.2 or other
crystallites are used. Since the transmission of the optical
materials deteriorates with decreasing wavelength, the illumination
systems are designed with a combination of refractive and
reflective components. For wavelengths in the EUV wavelength
region, between 10 nm and 20 nm, the projection exposure apparatus
is designed as all-reflective. A typical EUV light source is a
Laser-Produced-Plasma-source, a Pinch-Plasma-Source, a
Wiggler-Source or an Undulator-Source.
[0018] The light of this primary light source is collected by a
collector unit and directed to a first optical element, wherein the
collector unit and the first optical element form a first optical
component. The first optical element is organized as a plurality of
first raster elements and transforms, together with the collector
unit, the primary light source into a plurality of secondary light
sources. Each first raster element corresponds to one secondary
light source and focuses an incoming ray bundle, defined by all
rays intersecting the first raster element, to the corresponding
secondary light source. The secondary light sources are arranged in
a pupil plane of the illumination system or nearby this plane. A
field lens forming a second optical component is arranged between
the pupil plane and the image plane of the illumination system to
image the secondary light sources into an exit pupil of the
illumination system, which corresponds to the entrance pupil of a
following projection objective. The images of the secondary light
sources in the exit pupil of the illumination system are therefore
called tertiary light sources.
[0019] The first raster elements are imaged into the image plane,
wherein their images are at least partially superimposed on a field
that must be illuminated. Therefore, they are known as field raster
elements or field honeycombs. If the light source is a point-like
source, the secondary light sources are also point-like. In this
case the imaging of each of the field raster elements can be
explained visually with the principle of a "camera obscura", with
the small hole of the camera obscura at the position of each
corresponding secondary light source, respectively.
[0020] To superimpose the images of the field raster elements in
the image plane of the illumination system the incoming ray bundles
are deflected by the field raster elements with first deflection
angles, which are not equal for each of the field raster elements
but at least different for two of the field raster elements.
Therefore individual deflection angles for the field raster
elements are designed. For each field raster element a plane of
incidence is defined by the incoming and deflected centroid ray
selected from the incoming ray bundle. Due to the individual
deflection angles, at least two of the incidence planes are not
parallel.
[0021] In advanced microlithography systems the light distribution
in the entrance pupil of a projection objective must fulfill
special requirements such as having an overall shape or uniformity.
Since the secondary light sources are imaged into the exit pupil,
their arrangement in the pupil plane of the illumination system
determines the light distribution in the exit pupil. With the
individual deflection angles of the field raster elements a
predetermined arrangement of the secondary light sources can be
achieved, independent of the directions of the incoming ray
bundles. For reflective field raster elements the deflection angles
are generated by the tilt angles of the field raster elements. The
tilt axes and the tilt angles are determined by the directions of
the incoming ray bundles and the positions of the secondary light
sources, to which the reflected ray bundles are directed.
[0022] For refractive field raster element the deflection angles
are generated by lenslets, which have a prismatic optical power.
The refractive field raster elements can be lenslets with an
optical power having a prismatic contribution or they can be a
combination of a single prism and a lenslet. The prismatic optical
power is determined by the directions of the incoming ray bundles
and the positions of the corresponding secondary light sources.
Given the individual deflection angles of the first raster
elements, the beam path to the plate with the raster elements can
be either convergent or divergent. The slope values of the field
raster elements at the centers of the field raster elements has
then to be similar to the slope values of a surface with negative
power to reduce the convergence of the beam path, or with positive
power to increase the divergence of the beam path. Finally the
field raster elements deflect the incoming ray bundles to the
corresponding secondary light sources having predetermined
positions depending on the illumination mode of the exit pupil.
[0023] The diameter of the beam path is preferably reduced after
the collector unit to arrange filters or transmission windows with
a small size. This is possible by imaging the light source with the
collector unit to an intermediate image. The intermediate image is
arranged between the collector unit and the plate with the field
raster elements. After the intermediate image of the light source,
the beam path diverges. An additional mirror to condense the
diverging rays is not necessary due to the field raster elements
having deflecting optical power
[0024] For contamination reasons there is a free working distance
between the light source and the collector unit, which results in
considerable diameters for the optical components of the collector
unit and also for the light beam. Therefore the collector unit has
positive optical power to generate a converging ray bundle to
reduce the beam diameter and the size of the plate with field
raster elements. The convergence of the light rays can be reduced
with the field raster elements, if the deflection angles are
designed to represent a negative optical power. For the centroid
rays impinging on the centers of the field raster elements, the
collector unit and the plate with the field raster elements form a
telescope system. The collector unit has positive optical power to
converge the centroid rays towards the optical axis, wherein the
field raster elements reduce the converging angles of the centroid
rays. With this telescope system the track length of the
illumination system can be reduced.
[0025] Preferably, the field raster elements are tilted planar
mirrors or prisms with planar surfaces, which are much easier to
produce and to qualify than curved surfaces. This is possible, if
the collector unit is designed to image the primary light source
into the pupil plane of the illumination system, which would result
in one secondary light source, if the field raster elements were
omitted. The plurality of secondary light sources is generated by
the plurality of field raster elements, which distribute the
secondary light sources in the pupil plane according to their
deflection angles. The positive optical power to focus the incoming
ray bundles to the secondary light sources is completely provided
by the collector unit.
[0026] Therefore the optical distance between the image-side
principal plane of the collector unit and the image plane of the
collector unit is nearly given by the sum of the optical distance
between the image-side principal plane of the collector unit and
the plate with the field raster elements, and the optical distance
between the plate with the field raster elements and the pupil
plane of the illumination system. Due to the planar surfaces, the
field raster elements do not influence the imaging of the primary
light source into one secondary light source, except for the
dividing of this one secondary light source into a plurality of
secondary light sources due to the deflection angles. For
point-like or spherical sources the collector unit has ellipsoidal
mirrors or conical lenses with a first or second focus, wherein the
primary light source is arranged in the first focus, and the
secondary light source is arranged in the second focus of the
collector unit.
[0027] Dependent on the focusing optical power of the collector
unit, the field raster elements can have a positive or negative
optical power. If the focusing power of the collector unit is too
low and the primary light source is imaged behind the pupil plane,
the field raster elements are preferably concave mirrors or
lenslets comprising positive optical power to generate the
secondary light sources in or nearby the pupil plane. If the
focusing power of the collector unit is too strong and the primary
light source is imaged in front of the pupil plane, the field
raster elements are preferably convex mirrors or lenslets
comprising negative optical power to generate the secondary light
sources in or nearby the pupil plane.
[0028] The field raster elements are preferably arranged in a
two-dimensional array on a plate without overlapping. For
reflective field raster elements the plate can be a planar plate or
a curved plate. To minimize the light losses between adjacent field
raster elements they are arranged only with intermediate spaces
between them, which are necessary for the mountings of the field
raster elements. Preferably, the field raster elements are arranged
in a plurality of rows having at least one field raster element and
being arranged among one another. In the rows the field raster
elements are put together at the smaller side of the field raster
elements. At least two of these rows are displaced relative to one
another in the direction of the rows. In one embodiment each row is
displaced relative to the adjacent row by a fraction of a length of
the field raster elements to achieve a regular distribution of the
centers of the field raster elements. The fraction is dependent on
the side aspect ratio and is preferably equal to the square root of
the length of one field raster element. In another embodiment the
rows are displaced in such a way that the field raster elements are
illuminated almost completely.
[0029] Preferably, only these field raster elements are imaged into
the image plane, which is completely illuminated. This can be
realized with a masking unit in front of the plate with the field
raster elements, or with an arrangement of the field raster
elements wherein 90% of the field raster elements are completely
illuminated.
[0030] It is advantageous to insert a second optical element with
second raster elements in the light path after the first optical
element with first raster elements, wherein one first raster
element corresponds to one of the second raster elements.
Therefore, the deflection angles of the first raster elements are
designed to deflect the ray bundles impinging on the first raster
elements to the corresponding second raster elements. The second
raster elements are preferably arranged at the secondary light
sources and are designed to image together with the field lens the
first raster elements or field raster elements into the image plane
of the illumination system, wherein the images of the field raster
elements are at least partially superimposed. The second raster
elements are called pupil raster elements or pupil honeycombs. To
avoid damaging the second raster elements due to the high intensity
at the secondary light sources, the second raster elements are
preferably arranged defocused of the secondary light sources, but
in a range from 0 mm to 10% of the distance between the first and
second raster elements.
[0031] For extended secondary light sources the pupil raster
elements preferably have a positive optical power to image the
corresponding field raster elements, which are arranged optically
conjugated to the image plane. The pupil raster elements are
concave mirrors or lenslets with positive optical power.
[0032] The pupil raster elements deflect incoming ray bundles
impinging on the pupil raster elements with second deflection
angles in such a way that the images of the field raster elements
in the image plane are at least partially superimposed. This is the
case if a ray intersecting the field raster element and the
corresponding pupil raster element in their centers intersects the
image plane in the center of the illuminated field or nearby the
center. Each pair of a field raster element and a corresponding
pupil raster element forms a light channel.
[0033] The second deflection angles are not equal for each pupil
raster element. They are preferably individually adapted to the
directions of the incoming ray bundles and the requirement to
superimpose the images of the field raster elements at least
partially in the image plane. With the tilt axis and the tilt angle
for a reflective pupil raster element or with the prismatic optical
power for a refractive pupil raster element the second deflection
angle can be individually adapted.
[0034] For point-like secondary light sources the pupil raster
elements only have to deflect the incoming ray bundles without
focusing the rays. Therefore the pupil raster elements are
preferably designed as tilted planar mirrors or prisms.
[0035] If both, the field raster elements and the pupil raster
elements deflect incoming ray bundles in predetermined directions,
the two-dimensional arrangement of the field raster elements can be
made different from the two-dimensional arrangement of the pupil
raster elements. Wherein the arrangement of the field raster
elements is adapted to the illuminated area on the plate with the
field raster elements, the arrangement of the pupil raster elements
is determined by the kind of illumination mode required in the exit
pupil of the illumination system. So the images of the secondary
light sources can be arranged in a circle, but also in an annulus
to get an annular illumination mode or in four decentered segments
to get a Quadrupol illumination mode. The aperture in the image
plane of the illumination system is approximately defined by the
quotient of the half diameter of the exit pupil of the illumination
system and the distance between the exit pupil and the image plane
of the illumination system. Typical apertures in the image plane of
the illumination system are in the range of 0.02 and 0.1. By
deflecting the incoming ray bundles with the field and pupil raster
elements a continuous light path can be achieved. It is also
possible to assign each field raster element to any of the pupil
raster elements. Therefore the light channels can be mixed to
minimize the deflection angles or to redistribute the intensity
distribution between the plate with the field raster elements and
the plate with the pupil raster elements.
[0036] Imaging errors such as distortion introduced by the field
lens can be compensated for with the pupil raster elements being
arranged at or nearby the secondary light sources. Therefore the
distances between the pupil raster elements are preferably
irregular. The distortion due to tilted field mirrors for example
is compensated for by increasing the distances between the pupil
raster elements in a direction perpendicular to the tilt axis of
the field mirrors. Also, the pupil raster elements are arranged on
curved lines to compensate for the distortion due to a field
mirror, which transforms the rectangular image field to a segment
of an annulus by conical reflection. By tilting the field raster
elements the secondary light sources can be positioned at or nearby
the distorted grid of the corresponding pupil raster elements.
[0037] For reflective field and pupil raster elements the beam path
has to be folded at the plate with the field raster elements and at
the plate with the pupil raster elements to avoid vignetting.
Typically, the folding axes of both plates are parallel. Another
requirement for the design of the illumination system is to
minimize the incidence angles on the reflective field and pupil
raster elements. Therefore the folding angles have to be as small
as possible. This can be achieved if the extent of the plate with
the field raster elements is approximately equal to the extent of
the plate with the pupil raster elements in a direction
perpendicular to the direction of the folding axes, or if it
differs less than .+-.10%.
[0038] Since the secondary light sources are imaged into the exit
pupil of the illumination system, their arrangement determines the
illumination mode of the pupil illumination. Typically the overall
shape of the illumination in the exit pupil is circular and the
diameter of the illuminated region is in the order of 60%-80% of
the diameter of the entrance pupil of the projection objective. The
diameters of the exit pupil of the illumination system and the
entrance pupil of the projection objective are in another
embodiment preferably equal. In such a system the illumination mode
can be changed in a wide range by inserting masking blades at the
plane with the secondary light sources to get a conventional, Dipol
or Quadrupol illumination of the exit pupil.
[0039] All-reflective projection objectives used in the EUV
wavelength region have typically an object field being a segment of
an annulus. Therefore the field in the image plane of the
illumination system in which the images of the field raster
elements are at least partially superimposed has preferably the
same shape. The shape of the illuminated field can be generated by
the optical design of the components or by masking blades that have
to be added nearby the image plane or in a plane conjugated to the
image plane.
[0040] The field raster elements are preferably rectangular.
Rectangular field raster elements have the advantage that they can
be arranged in rows being displaced against each other. Depending
on the field to be illuminated they have a side aspect ratio in the
range of 5:1 and 20:1. The length of the rectangular field raster
elements is typically between 15 mm and 50 mm, the width is between
1 mm and 4 mm.
[0041] To illuminate an arc-shaped field in the image plane with
rectangular field raster elements the field lens preferably
comprises a first field mirror for transforming the rectangular
images of the rectangular field raster elements to arc-shaped
images. The arc length is typically in the range of 80 mm to 105
mm, the radial width in the range of 5 mm to 9 mm. The
transformation of the rectangular images of the rectangular field
raster elements can be done by conical reflection with the first
field mirror being a grazing incidence mirror with negative optical
power. In other words, the imaging of the field raster elements is
distorted to get the arc-shaped images, wherein the radius of the
arc is determined by the shape of the object field of the
projection objective. The first field mirror is preferably arranged
in front of the image plane of the illumination system, wherein
there should be a free working distance. For a configuration with a
reflective reticle the free working distance has to be adapted to
the fact that the rays traveling from the reticle to the projection
objective are not vignetted by the first field mirror.
[0042] The surface of the first field mirror is preferably an
off-axis segment of a rotational symmetric reflective surface,
which can be designed aspherical or spherical. The axis of symmetry
of the supporting surface goes through the vertex of the surface.
Therefore a segment around the vertex is called on-axis, wherein
each segment of the surfaces which does not include the vertex is
called off-axis. The supporting surface can be manufactured more
easily due to the rotational symmetry. After producing the
supporting surface the segment can be cut out with well-known
techniques.
[0043] The surface of the first field mirror can also be designed
as an on-axis segment of a toroidal reflective surface. Therefore
the surface has to be processed locally, but has the advantage that
the surrounding shape can be produced before surface treatment.
[0044] The incidence angles of the incoming rays with respect to
the surface normals at the points of incidence of the incoming rays
on the first field mirror are preferably greater than 70.degree.,
which results in a reflectivity of the first field mirror of more
than 80%.
[0045] The field lens comprises preferably a second field mirror
with positive optical power. The first and second field mirror
together image the secondary light sources or the pupil plane
respectively into the exit pupil of the illumination system, which
is defined by the entrance pupil of the projection objective. The
second field mirror is arranged between the plane with the
secondary light sources and the first field mirror.
[0046] The second field mirror is preferably an off-axis segment of
a rotational symmetric reflective surface, which can be designed
aspherical or spherical, or an on-axis segment of a toroidal
reflective surface.
[0047] The incidence angles of the incoming rays with respect to
the surface normals at the points of incidence of the incoming rays
on the second field mirror are preferably lower than 25.degree..
Since the mirrors have to be coated with multilayers for the EUV
wavelength region, the divergence and the incidence angles of the
incoming rays are preferably as low as possible to increase the
reflectivity, which should be better than 65%. With the second
field mirror being arranged as a normal incidence mirror the beam
path is folded and the illumination system can be made more
compact.
[0048] To reduce the length of the illumination system the field
lens comprises preferably a third field mirror. The third field
mirror is preferably arranged between the plane with the secondary
light sources and the second field mirror.
[0049] The third field mirror has preferably negative optical power
and forms together with the second and first field mirror an
optical telescope system having a object plane at the secondary
light sources and an image plane at the exit pupil of the
illumination system to image the secondary light sources into the
exit pupil. The pupil plane of the telescope system is arranged at
the image plane of the illumination system. Therefore the ray
bundles coming from the secondary light sources are superimposed in
the pupil plane of the telescope system or in the image plane of
the illumination system accordingly. The first field mirror has
mainly the function of forming the arc-shaped field, wherein the
telescope system is mainly determined by the negative third field
mirror and the positive second field mirror.
[0050] In another embodiment the third field mirror has preferably
positive optical power to generate images of the secondary light
sources in a plane between the third and second field mirror,
forming tertiary light sources. The tertiary light sources are
imaged with the second field mirror and the first field mirror into
the exit pupil of the illumination system. The images of the
tertiary light sources in the exit pupil of the illumination system
are called in this case quaternary light sources.
[0051] Since the plane with the tertiary light sources is arranged
conjugated to the exit pupil, this plane can be used to arrange
masking blades to change the illumination mode or to add
transmission filters. This position in the beam path has the
advantage to be freely accessible.
[0052] The third field mirror is similar to the second field mirror
preferably an off-axis segment of a rotational symmetric reflective
surface, which can be designed aspherical or spherical, or an
on-axis segment of a toroidal reflective surface.
[0053] The incidence angles of the incoming rays with respect to
the surface normals at the points of incidence of the incoming rays
on the third field mirror are preferably lower than 25.degree..
With the third field mirror being arranged as a normal incidence
mirror the beam path can be folded again to reduce the overall size
of the illumination system.
[0054] To avoid vignetting of the beam path the first, second and
third field mirrors are preferably arranged in a non-centered
system. There is no axis of symmetry for the mirrors. An optical
axis can be defined as a connecting line between the centers of the
used areas on the field mirrors, wherein the optical axis is bent
at the field mirrors depending on the tilt angles of the field
mirrors.
[0055] With the tilt angles of the reflective components of the
illumination system the beam paths between the components can be
bent. Therefore the orientation of the beam cone emitted by the
source and the orientation of the image plane system can be
arranged according to the requirements of the overall system. A
preferable configuration has a source emitting a beam cone in one
direction and an image plane having a surface normal that is
oriented almost perpendicular to this direction. In one embodiment
the source emits horizontally and the image plane has a vertical
surface normal. Some light sources like undulator or wiggler
sources emit only in the horizontal plane. On the other hand the
reticle should be arranged horizontally for gravity reasons. The
beam path therefore has to be bent between the light source and the
image plane about almost 90.degree.. Since mirrors with incidence
angles between 30.degree. and 60.degree. lead to polarization
effects and therefore to light losses, the beam bending has to be
done only with grazing incidence or normal incidence mirrors. For
efficiency reasons the number of mirrors has to be as small as
possible.
[0056] A very compact configuration of the illumination system can
be designed, if the beam path from the plate with the pupil raster
elements to the field lens is crossing the beam path from the
collector unit to the plate with field raster elements. This is
only possible, if the field raster elements and the pupil raster
elements are reflective ones and are arranged on plates being
tilted to achieve the crossing of the two beam paths. The crossing
of the beam paths has the advantage that the beam path after the
plate with the pupil raster elements has an angle in the range of
35.degree. to 55.degree. with respect to the beam path in front of
the plate with the field raster elements. This was achieved with
only two normal incidence reflections.
[0057] By definition all rays intersecting the field in the image
plane have to go through the exit pupil of the illumination system.
The position of the field and the position of the exit pupil are
defined by the object field and the entrance pupil of the
projection objective. For some projection objectives being centered
systems the object field is arranged off-axis of an optical axis,
wherein the entrance pupil is arranged on-axis in a finite distance
to the object plane. For these projection objectives an angle
between a straight line from the center of the object field to the
center of the entrance pupil and the surface normal of the object
plane can be defined. This angle is in the range of 3.degree. to
10.degree. for EUV projection objectives. Therefore the components
of the illumination system have to be configured and arranged in
such a way that all rays intersecting the object field of the
projection objective are going through the entrance pupil of the
projection objective being decentered to the object field. For
projection exposure apparatus with a reflective reticle all rays
intersecting the reticle needs to have incidence angles greater
than 0.degree. to avoid vignetting of the reflected rays at
components of the illumination system.
[0058] In the EUV wavelength region all components are reflective
components, which are arranged preferably in such a way, that all
incidence angles on the components are lower than 25.degree. or
greater than 65.degree.. Therefore polarization effects arising for
incidence angles around an angle of 45.degree. are minimized. Since
grazing incidence mirrors have a reflectivity greater than 80%,
they are preferable in the optical design in comparison to normal
incidence mirrors with a reflectivity greater than 65%.
[0059] The illumination system is typically arranged in a
mechanical box. By folding the beam path with mirrors the overall
size of the box can be reduced. This box preferably does not
interfere with the image plane, in which the reticle and the
reticle supporting system are arranged. Therefore it is
advantageous to arrange and tilt the reflective components in such
a way that all components are completely arranged on one side of
the reticle. This can be achieved if the field lens comprises only
an even number of normal incidence mirrors.
[0060] The illumination system as described before can be used
preferably in a projection exposure apparatus comprising the
illumination system, a reticle arranged in the image plane of the
illumination system and a projection objective to image the reticle
onto a wafer arranged in the image plane of the projection
objective. Both, reticle and wafer are arranged on a support unit,
which allows the exchange or scan of the reticle or wafer.
[0061] The projection objective can be a catadioptric lens, as
known from U.S. Pat. No. 5,402,267 for wavelengths in the range
between 100 nm and 200 nm. These systems have typically a
transmission reticle. For the EUV wavelength range the projection
objectives are preferably all-reflective systems with four to eight
mirrors as known for example from U.S. Ser. No. 09/503,640 showing
a six mirror projection lens. These systems have typically a
reflective reticle.
[0062] For systems with a reflective reticle the illumination beam
path between the light source and the reticle and the projection
beam path between the reticle and the wafer preferably interfere
only nearby the reticle, where the incoming and reflected rays for
adjacent object points are traveling in the same region. If there
is no further crossing of the illumination and projection beam path
it is possible to separate the illumination system and the
projection objective except for the reticle region.
[0063] The projection objective has preferably a projection beam
path between the reticle and the first imaging element that is
convergent toward the optical axis of the projection objective.
Especially for a projection exposure apparatus with a reflective
reticle the separation of the illumination system and the
projection objective is easier to achieve.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] The invention will be described below on the basis of
drawings.
[0065] FIG. 1: Principle diagram of the beam path of a system with
two raster element plates.
[0066] FIGS. 2a, 2b: Imaging of the field and pupil raster
elements.
[0067] FIG. 3: Path of the light beam for a rectangular field
raster element in combination with a pupil raster element.
[0068] FIG. 4: Beam path according to FIG. 3 with field lens
introduced in the beam path.
[0069] FIG. 5: Beam path according to FIG. 3 with two field mirrors
introduced in the beam path.
[0070] FIG. 6: System with field and pupil raster elements.
[0071] FIGS. 7-14: Different arrangements of field raster elements
on a field raster element plate.
[0072] FIGS. 15-17: Raster of tertiary light sources in the
entrance pupil of the projection objective.
[0073] FIGS. 18-20: Relationship between illuminated surfaces of
field raster element plate and pupil raster element plate as well
as structural length and aperture in the reticle plane.
[0074] FIGS. 21-22: Illumination system with a collector unit,
field and pupil raster elements.
[0075] FIGS. 23-24: Beam path in a system with collector unit,
field and pupil raster elements.
[0076] FIG. 25-26: Illumination of the reticle of a system
according to FIGS. 23-24.
[0077] FIG. 27-28: Illumination of the reticle of a system
according to FIGS. 23-24 without pupil raster elements.
[0078] FIG. 29: Comparison of the intensity distribution of a
system according to FIGS. 23-24 with and without pupil raster
element plate.
[0079] FIG. 30: Integral scanning energy in the reticle plane of a
system according to FIGS. 23-24 with pupil raster element
plate.
[0080] FIG. 31: Pupil illumination for an object point in the
center of the illuminated field of a system according to FIGS.
23-24 with pupil raster element plate.
[0081] FIG. 32: Total energy of the tertiary light sources of a
system according to FIGS. 23-24 along the Y-axis.
[0082] FIGS. 33-39: Illumination system with a laser plasma source
as a light source as well as a collector unit and two mirror units,
which form a tele-system.
[0083] FIGS. 40-45: Course of the light beams in a system with
collector unit as well as two tele-mirrors according to FIGS.
37-39.
[0084] FIG. 46: Illumination of the reticle of an arrangement
according to FIGS. 44-45.
[0085] FIG. 47: Integral scanning energy of an arrangement
according to FIGS. 40-45.
[0086] FIG. 48: Pupil illumination of a system according to FIGS.
40-45.
[0087] FIGS. 48A-48C: System for a laser-plasma source with
diameter .ltoreq.50 .mu.m and without pupil raster element
plate.
[0088] FIGS. 49-52: System with a laser-plasma source, a collector
and a field raster element plate with planar field raster
elements.
[0089] FIGS. 53-58: Beam path in a system according to FIGS.
49-52.
[0090] FIG. 59: Illumination of the reticle with an illumination
arrangement according to FIGS. 52-58.
[0091] FIG. 60: Integral scanning energy in the reticle plane of a
system according to FIGS. 52-58.
[0092] FIG. 61: Pupil illumination of a system according to FIGS.
52-58.
[0093] FIG. 62: Intensity distribution in the scan direction of a
system according to FIGS. 52-58.
[0094] FIG. 63A: Raster element plate with individual raster
elements on a curved supporting surface.
[0095] FIG. 63B: Raster element plate with tilted raster elements
on a planar supporting plate.
[0096] FIG. 64: A configuration of the invention with lenslets and
prisms as raster elements in schematic presentation.
[0097] FIG. 65: A schematic view of a refractive embodiment with
prisms as field raster elements.
[0098] FIG. 66: A schematic view of a refractive embodiment with
field raster elements having positive and prismatic optical
power.
[0099] FIG. 67: A schematic view of a refractive embodiment with
field raster elements having negative and prismatic optical
power.
[0100] FIG. 68: A schematic view of a refractive embodiment with
field raster elements having positive and prismatic optical power
and prisms as pupil raster elements.
[0101] FIG. 69: A schematic view of a refractive embodiment having
an intermediate image of the primary light source.
[0102] FIG. 70: A schematic view of a reflective embodiment with
convex mirrors as field raster elements and planar mirrors as pupil
raster elements.
[0103] FIG. 71: A schematic view of a reflective embodiment with
convex mirrors as field raster elements and concave mirrors as
pupil raster elements.
[0104] FIG. 72: A schematic view of the principal setup of the
illumination system.
[0105] FIG. 73: An Arrangement of the field raster elements.
[0106] FIG. 74: An Arrangement of the pupil raster elements.
[0107] FIG. 75: A schematic view of a reflective embodiment with a
concave pupil-imaging field mirror and a convex field-forming field
mirror.
[0108] FIG. 76: A schematic view of a reflective embodiment with a
field lens comprising a telescope system and a convex field-forming
field mirror.
[0109] FIG. 77: A detailed view of the embodiment of FIG. 76.
[0110] FIG. 78: Intensity distribution of the embodiment of FIG.
77.
[0111] FIG. 79: Illumination of the exit pupil of the illumination
system of the embodiment of FIG. 77.
[0112] FIG. 80: A schematic view of a reflective embodiment with a
crossing of the beam paths.
[0113] FIG. 81: A detailed view of the embodiment of FIG. 80.
[0114] FIG. 82: A schematic view of a reflective embodiment with
two pupil planes.
[0115] FIG. 83: A schematic view of a reflective embodiment with an
intermediate image of the light source.
[0116] FIG. 84: A detailed view of a projection exposure
apparatus.
[0117] FIG. 85: Rectangular field in the field plane.
[0118] FIG. 86A: Superposition of a plurality of images of first
raster elements in the field plane providing for a rectangular
intensity profile.
[0119] FIG. 86B: Rectangular intensity profile.
[0120] FIG. 87A: Superposition of a plurality of images of first
raster elements in the field plane providing for a trapezoid
intensity profile.
[0121] FIG. 87B: Trapezoid intensity profile.
[0122] FIG. 88: First raster element plate with twelve first raster
elements.
[0123] FIG. 89: Assignment of axis symmetric first raster elements
to point-symmetric second raster elements.
[0124] FIG. 90: Field in an image plane and exit pupils for
different field points.
DESCRIPTION OF THE INVENTION
[0125] It shall be shown theoretically on the basis of FIGS. 1-20,
how a system can be provided for any desired illumination
distribution in a plane, which satisfies the requirements with
reference to uniformity and telecentricity.
[0126] In FIG. 1, a principle diagram of the beam path of a system
with two plates with raster elements is illustrated. The light of
the primary light source 1 is collected by means of a collector
lens 3 and converted into a parallel or convergent light beam. The
field raster elements 5 of the first raster element plate 7
decompose the light beam and produce secondary light sources at the
site of the pupil raster elements 9. At the position of the
secondary light sources the pupil plane of the illumination system
is arranged. The field lens 12 images these secondary sources in
the exit pupil of the illumination system or the entrance pupil of
the subsequent projection objective forming tertiary light sources.
The field raster elements 5 are imaged by the pupil raster elements
9 and the field lens 12 into the image plane of the illumination
system. In this plane the reticle 14 is arranged. Such an
arrangement is characterized by an interlinked beam path of field
and pupil planes from the source up to the entrance pupil of the
subsequent projection objective. For this, the designation "Kohler
illumination" is also often selected.
[0127] The illumination system according to FIG. 1 is considered
segmentally below. If the light intensity and aperture distribution
is known in the plane of the field raster elements, the system can
be described independent of source type and collector unit.
[0128] The field and pupil imaging are illustrated for the central
pair of field raster element 20 and pupil raster element 22 in
FIGS. 2A and 2B. The field raster element 20 is imaged on the
reticle 14 or the mask by means of the pupil raster element 22 and
the field lens 12. The geometric extension of the field raster
element 20 determines the shape of the illuminated field in the
reticle plane 14. The image scale is approximately given by the
ratio of the distance from pupil raster element 22 to reticle 14
and the distance from field raster element 20 to pupil raster
element 22. The field raster element 20 is designed such that an
image of primary light source 1, a secondary light source, is
formed at the site of pupil raster element 22. If the extension of
the primary light source 1 is small, for example, approximately
point-like, then all light rays run through the centers of the
pupil raster elements 22. In such a case, an illumination device
can be produced, in which the pupil raster element is dispensed
with.
[0129] As is shown in FIG. 2B, the task of field lens 12 consists
of imaging the secondary light sources in the entrance pupil 26 of
projection objective 24 forming tertiary light sources. With the
field lens the field imaging can be influenced in such a way that
it forms the arc-shaped field by control of the distortion. The
imaging scale of the field raster element image is thus almost not
changed.
[0130] A special geometrical form of a field raster element 20 and
a pupil raster element 22 is shown in FIG. 3.
[0131] In the form of embodiment represented in FIG. 3, the shape
of field raster element 20 is selected as a rectangle. Thus, the
aspect ratio of the field raster element 20 corresponds
approximately to the ratio of the arc length to the annular width
of the required arc-shaped field in the reticle plane. The
arc-shaped field is formed by the field lens 32, as shown in FIG.
4. Without the field lens 32, as shown in FIG. 3, a rectangular
field is formed in the reticle plane.
[0132] As shown in FIG. 4, one grazing-incidence field mirror 32 is
used for the shaping of arc-shaped field 30. Under the constraint
that the beam reflected by the reticle should not be directed back
into the illumination system, one or two field mirrors 32 are
required, depending on the position of the entrance pupil of the
objective.
[0133] If the principal rays run divergently into the objective
that is not shown, then one field mirror 32 is sufficient, as shown
in FIG. 4. In the case of principal rays entering the projection
objective convergently, two field mirrors are required. The second
field mirror must rotate the orientation of the ring 30. Such a
configuration is shown in FIG. 5.
[0134] In the case of an illumination system in the EUV wavelength
region, all components must be reflective ones.
[0135] Due to the high reflection losses at .lamda.=10 nm-14 nm, it
is advantageous that the number of reflections be kept as small as
possible.
[0136] In the construction of the reflective system, the mutual
vignetting of the beams must be taken into consideration. This can
occur due to construction of the system in a zigzag beam path or by
operating with obscurations.
[0137] The design process will be described below for the
preparation of a design for an EUV illumination system with any
illumination in a plane, as an example. The definitions necessary
for the design process are shown in FIG. 6.
[0138] First, the beam path is calculated for the central pair of
raster elements. In a first step, the size of field raster elements
5 of the field raster element plate 7 will be determined. As
indicated previously, the aspect ratio (x/y) results for
rectangular raster elements from the shape of the arc-shaped field
in the reticle plane. The size of the field raster elements is
determined by the illuminated area A of the intensity distribution
of the arbitrary light source in the plane of the field raster
elements and the number N of the field raster elements on the
raster element plate, which in turn is given by the number of
secondary light sources. The number of secondary light sources
results in turn from the uniformity of the field and pupil
illumination.
[0139] The raster element surface A.sub.FRE of a field raster
element can be expressed as follows with x.sub.FRE, y.sub.FRE:
A.sub.FRE=x.sub.FREy.sub.FRE=(x.sub.field/y.sub.field)y.sup.2.sub.FRE
whereby x.sub.field, y.sub.field describe the size of the
rectangle, which establishes the arc-shaped field. Further, the
following is valid for the number N of field raster elements:
N=A/A.sub.FRE=A/[y.sup.2.sub.FRE(x.sub.field/y.sub.field)].
From this, there results for the size of the individual field
raster element:
y.sub.FRE= {square root over (A/[N(x.sub.field/y.sub.field)])}
and
x.sub.FRE=(x.sub.field/y.sub.field)y.sub.FRE
[0140] The raster element size and the size of the rectangular
field in the reticle plane establish the imaging scale
.beta..sub.FRE of the field raster element imaging and thus the
ratio of the distances z.sub.1 and z.sub.2.
.beta..sub.FRE=x.sub.field/y.sub.field=z.sub.2/z.sub.1
The pregiven structural length L for the illumination system and
the imaging scale .beta..sub.FRE of the field raster element
imaging determine the absolute size of z.sub.1 and z.sub.2 and thus
the position of the pupil raster element plate. The following is
valid:
z.sub.1=L/(1+.beta..sub.FRE)
z.sub.2=z.sub.1.beta..sub.FRE
Then, z.sub.1 and z.sub.2 determine in turn the curvature of the
pupil raster elements. The following is valid:
R FRE = 2 z 1 z 2 z 1 + z 2 ##EQU00001##
[0141] In order to image the pupil raster elements in the entrance
pupil of the projection objective and to remodel the rectangular
field into an arc-shaped field, a field lens comprising one or more
field mirrors, preferably of toroidal form, are introduced between
the pupil raster element plate and the reticle. By introducing the
field mirrors, the previously given structural length is increased,
since among other things, the mirrors must maintain minimum
distances in order to avoid light vignetting.
[0142] The positioning of the field raster elements depends on the
intensity distribution in the plane of the field raster elements.
The number N of the field raster elements is pregiven by the number
of secondary light sources. The field raster elements will
preferably be arranged on the field raster element plate in such a
way that they cover the illuminated surfaces without mutually
vignetting.
[0143] In order to position the pupil raster elements, the raster
pattern of the tertiary light sources in the entrance pupil of the
projection objective will be given in advance. The tertiary light
sources are imaged by the field lens counter to the direction of
light into the secondary light sources. The aperture stop plane of
this imaging is in the reticle plane. The images of the tertiary
light sources give the (x, y, z) positions of the pupil raster
elements which are arranged at the positions of the secondary light
sources. The tilt and rotational angles remain as degrees of
freedom for producing the light path between the field and pupil
raster elements.
[0144] If a pupil raster element is assigned to each field raster
element in one configuration of the invention, then the light path
will be produced by tilting and rotating field and pupil raster
elements. Thereby the light beams, generated by the field raster
elements, are deviated in such a way that the center rays of the
light beams all intersect the optical axis in the reticle
plane.
[0145] The assignment of field and pupil raster elements can be
made freely. One possibility for arrangement would be to assign
spatially adjacent field and pupil raster elements. Thereby, the
deflecting angles become minimal. Another possibility consists of
homogenizing the intensity distribution in the pupil plane. This is
made, for example, if the intensity distribution has a
non-homogenous distribution in the plane of the field raster
elements. If the field and pupil raster elements have similar
positions, the distribution is transferred to the pupil
illumination. By intermixing the light beams the light distribution
in the pupil plane can be homogenized.
[0146] Advantageously, the individual components of field raster
element plate, pupil raster element plate and field mirrors of the
illumination system are arranged in the beam path such that a beam
path free of vignetting is possible. If such an arrangement has
effects on the imaging, then the individual light channels and the
field mirrors must be re-optimized.
[0147] With the design process described above, illumination
systems for EUV lithography are obtained for any light distribution
at the plate with the field raster elements with two
normal-incidence reflections for the field and pupil raster
elements and one to two normal or grazing-incidence reflections for
the field lens. These systems have the following properties: [0148]
a. An homogeneous illumination of an arc-shaped field [0149] b. An
homogeneous and field-independent pupil illumination [0150] c. The
combining of the exit pupil of the illumination system and the
entrance pupil of the projection objective [0151] d. The adjustment
of a pregiven structural length [0152] e. The collection of nearly
all light generated by the primary light source.
[0153] Arrangements of field raster elements and pupil raster
elements will be described below for one form of embodiment of the
invention with field and pupil raster element plates.
[0154] First, different arrangements of the field raster elements
on the field raster element plate will be considered. The intensity
distribution can be selected as desired.
[0155] The introduced examples are limited to simple geometric
shapes of the light distributions, such as circle, rectangle, or
the coupling of several circles or rectangles, but the present
invention is not limited on these shapes.
[0156] The intensity distribution will be homogeneous within the
illuminated region or have a slowly varying distribution. The
aperture distribution will be independent of the position inside
the light distribution.
[0157] In the case of circular illumination A of field raster
element plate 100, field raster elements 102 may be arranged, for
example, in columns and rows, as shown in FIG. 7. As an alternative
to this, the center points of the raster elements 102 can be
distributed uniformly by shifting the rows over the surface, as
shown in FIG. 8. The rows are displaced relatively to an adjacent
row. This arrangement is better adapted to a uniform distribution
of the secondary light sources in the pupil plane.
[0158] A rectangular illumination A with a arrangement of the field
raster elements 102 in rows and columns is shown in FIG. 9. A
displacement of the rows, as shown in FIG. 10, leads to a more
uniform distribution of the secondary light sources in the pupil
plane. However, without tilting the field raster elements 102 the
secondary light sources are arranged within a rectangle
corresponding to the arrangement of the field raster elements 102.
Since the pupil raster elements are typically arranged inside a
circle to get a circular illumination of the exit pupil of the
illumination system, it is necessary to tilt the field and pupil
raster elements to produce a continuous light path between the
corresponding field and pupil raster elements.
[0159] If illumination A of field raster element plate 100
comprises several circles, A1, A2, A3, A4, for example by coupling
several sources, then, intermixing is insufficient with an
arrangement of the raster elements 102 with a high (x/y)-aspect
ratio in rows and columns according to FIG. 11. A more uniform
illumination is obtained by shifting the raster element rows, as
shown in FIG. 12.
[0160] FIGS. 13 and 14 show the distribution of field raster
elements 102 in the case of combined illumination from the
individual rectangles A1, A2, A3, A4.
[0161] Now, for example, arrangements of the pupil raster elements
on the pupil raster element plate will be described. In the
arrangement of pupil raster elements, two points of view are to be
considered: [0162] 1. For minimizing the tilt angle of field and
pupil raster elements for producing the light path, it is
advantageous to maintain the arrangement of field raster elements.
This is particularly advantageous with an approximately circular
illumination of the field raster element plate. [0163] 2. For
homogeneous filling of the pupil, the tertiary light sources, which
are images of the secondary light sources, will be distributed
uniformly in the entrance pupil of the projection objective. This
can be achieved by providing a uniform raster pattern of tertiary
light sources in the entrance pupil of the projection objective.
These are imaged counter to the direction of light with the field
lens in the plane of the pupil raster elements and determine in
this way the ideal site of the pupil raster elements, which are
arranged nearby the secondary light sources.
[0164] If the field lens is free of distortion, then the
distribution of the pupil raster elements corresponds to the
distribution of the tertiary light sources. However, since the
field lens forms the arc-shaped field, distortion is purposely
introduced. This does not involve rotational-symmetric distortion,
but involves the bending of horizontal lines into arcs. In the
ideal case, the y distance of the arcs remains almost constant.
Real grazing-incidence field mirrors, however, also show an
additional distortion in the y-direction.
[0165] A raster 110 of tertiary light sources 112 in the entrance
pupil of the projection objective, which is also the exit pupil of
the illumination system, is shown in FIG. 15, as it had been
produced for distortion-free field lens imaging. The arrangement of
the tertiary light sources 112 corresponds precisely to the
pregiven arrangement of pupil raster elements.
[0166] If the field lenses are utilized for shaping the arc-shaped
field, as in FIG. 2016, then the tertiary light sources 112 lie on
arcs 114. If the pupil raster elements of individual rows are
placed on the arcs that compensate for the distortion, then one can
place the tertiary light sources again on a regular raster.
[0167] If the field lens also introduces distortion in the
y-direction, then the distribution of the tertiary light sources is
distorted in the y-direction, as shown in FIG. 17. This effect can
be compensated by arranging the pupil raster elements on a grid
that is distorted in y-direction.
[0168] The extent of the illuminated area onto the field raster
element plate is determined by design of the collector unit. The
extent of the illuminated area onto the pupil raster element plate
is determined by the structural length of the illumination system
and the aperture in the reticle plane.
[0169] As described above, the two surfaces must be fine-tuned to
one another by rotating and tilting the field and pupil raster
elements.
[0170] For illustration, the design of the illumination system will
be explained with refractive elements. The examples, however, can
be transferred directly to reflective systems. Various
configurations can be distinguished for a circular illumination of
field raster element plates, as presented below.
[0171] If a converging effect is introduced by tilting the field
raster elements, and a diverging effect is introduced by tilting
the pupil raster elements, then the beam cross section can be
reduced. The tilt angles of the individual raster elements are
determined by tracing the center rays for each pair of raster
elements. The system acts like a telescope-system for the central
rays, as shown in FIG. 18.
[0172] How far the field raster elements must be tilted, depends on
the convergence of the impinging beam. If the convergence is
adapted to the reduction of the beam cross section, the field
raster elements can be arranged onto a planar substrate without
tilting the field raster elements.
[0173] A special case results, if the convergence between the field
and the pupil raster element plate corresponds to the aperture
NA.sub.field at the reticle, as shown in FIG. 19.
[0174] No diverging effect must be introduced by the pupil raster
elements, so they can be utilized without tilting the pupil raster
elements. If the light source also has a very small etendue, the
pupil raster element can be completely dispensed with.
[0175] A magnification of the beam cross section is possible, if
diverging effect is introduced by tilting of the field raster
elements, and collecting effect is introduced by tilting the pupil
raster elements. The system operates like a retro-focus system for
the central rays, as shown in FIG. 20.
[0176] If the divergence of the impinging radiation corresponds to
the beam divergence between field and pupil raster elements, then
the field raster elements can be used without tilting the field
raster elements.
[0177] Instead of the circular shape that has been described,
rectangular or other shapes of illumination A of the field raster
element plate are possible.
[0178] The following drawings describe one form of embodiment of
the invention, in which a pinch-plasma source is used as the light
source of the EUV illumination system.
[0179] The principal construction without field lens of such a form
of embodiment is shown in FIG. 21; FIG. 22 shows the abbreviations
necessary for the system derivation, whereby for better
representation, the system was plotted linearly and mirrors were
indicated as lenses. An illumination system with pinch-plasma
source 200 as primary light source, as shown in FIG. 21, comprises
a light source 200, a collector mirror 202, which collects the
light and reflects it to the field raster element plate 204. By
reflection at the field raster elements, the light is directed to
the corresponding pupil raster elements of pupil raster element
plate 206 and from there to reticle 208. The pinch-plasma source is
an expanded light source (approximately 1 mm) with a directional
radiation in a relatively small steradian region of approximately
.OMEGA.=0.3 sr. Based on the etendue of the primary light source, a
pupil raster element plate 206 is used.
[0180] The following specifications are used, for example, for an
illumination system for EUV lithography: [0181] a. Arc-shaped
field: Radius R.sub.field=100 mm, segment--angle 60.degree., field
width .+-.3.0 mm, which corresponds to a rectangular field of 105
mm.times.6 mm [0182] b. Aperture at the reticle: NA.sub.field=0.025
[0183] c. Aperture at the source: NA.sub.source=0.3053 [0184] d.
Structural length L=1400.0 mm [0185] e. Number of field raster
elements, which find place in an x-row: 4 [0186] f. z.sub.1=330.0
mm
[0187] With the following equations the optical design of the
illumination system can be derived with the pregiven numbers:
NA field = D FRE 2 L D FRE = 2 L NA field D PRE x FRE = 4.0 x FRE =
D PRE 4.0 .beta. FRE = x field x FRE = z 4 z 3 .beta. FRE = x field
x FRE z 4 = z 3 .beta. FRE L = z 3 + z 4 z 3 = L 1 + .beta. FRE NA
' = D FRE 2 z 3 NA ' = D FRE 2 z 3 tan ( .theta. ) = - ( 1 - Ex )
sin ( .theta. ' ) 2 Ex - ( 1 - Ex ) cos ( .theta. ' ) Ex = f ( NA
source , NA ' ) Ex col = ( sk - s 1 sk + s 1 ) 2 = ( z 2 - z 1 z 2
+ z 1 ) 2 z 2 = z 1 1 + Ex col 1 - Ex col Ex col = 1 - R col a R
col = z 1 + z 2 2 ( 1 - Ex col ) 2 R PRE = 1 z 3 + 1 z 4 R PRE = 2
z 3 z 4 z 3 + z 4 ##EQU00002##
[0188] D.sub.FRE: diameter of the plate with the field raster
elements
[0189] x.sub.FRE: length of one field raster element
[0190] y.sub.FRE: width of one field raster element
[0191] .beta..sub.FRE: magnification ratio of the field raster
elements
[0192] D.sub.PRE: diameter of the plate with the pupil raster
elements
[0193] R.sub.col: Radius of the elliptical collector
[0194] Ex.sub.col: conical constant of the elliptical collector
[0195] NA': aperture after the collector mirror
[0196] With the pregiven specifications the following system
parameters can be calculated:
D FRE = 2 L NA field = 2 1400 mm 0.025 = 70.0 mm x FRE = D FRE 4.0
= 70.0 mm 4.0 = 17.5 mm y FRE = 1.0 mm .beta. FRE = x field x FRE =
105.0 mm 17.5 mm = 6.0 z 3 = L 1 + .beta. FRE = 1400.0 mm 1 + 6.0 =
200.0 mm z 4 = z 3 .beta. FRE = 200.0 mm 6.0 = 1200.0 mm NA ' = D
DRE 2 z 3 = 70.0 mm 2 200.0 mm = 0.175 Ex col = f ( NA source , NA
' ) = 0.078 z 2 = z 1 1 + Ex col 1 - Ex col = 100.0 mm 1 + 0.078 1
- 0.078 = 585.757 mm R col = z 1 + z 2 2 ( 1 - Ex col ) = 330.0 mm
+ 585.757 mm 2 ( 1 - 0.078 ) = 422.164 mm R PRE = 2 z 3 z 4 z 3 + z
4 = 2 200 1200 200 + 1200 = 342.857 mm ##EQU00003##
[0197] The total system with the previously indicated dimensions is
shown in FIG. 23 up to the reticle plane 208 in the yz section. The
central and the two marginal rays are drawn in. Secondary light
sources are produced at the plate with the pupil raster elements
206 by the field raster elements 204. The pupil plane of the
illumination system is arranged at the plate with the pupil raster
elements 206.
[0198] The total system is shown in FIG. 24 with an x-z fan of
rays, which impinge on the central field raster element.
[0199] FIGS. 25 and 26 show the illumination of the reticle with
the rectangular field (-52.5 mm<x.sub.field<+52.5 mm; -3.0
mm<x.sub.field<+3.0 mm). FIG. 25 shows a contour plot, FIG.
26a--3D presentation. The images of the field raster elements are
optimally superimposed in the reticle plane also in the case of the
extended secondary light sources, which are produced by the
pinch-plasma source, since a pupil raster element plate is
used.
[0200] In comparison to this, the illumination of the reticle
without pupil raster element plate is shown in contour lines and 3D
representation in FIGS. 27 and 28. The field raster elements are
not imaged sharply. This can be due for example to the extended
secondary light sources.
[0201] A similar illumination in the field plane can also be
achieved if not all of the first raster elements have the same
size, e.g. they have a different extension in y-direction, which is
also called the scanning direction. In case the second raster
elements have an optical effect another possibility to achieve such
an illumination in the filed plane is using second raster elements
having different optical power.
[0202] FIG. 29 shows an intensity profile parallel to the y-axis
for x=0.0 for ideally superimposed images of the first raster
elements in the image plane. This can be achieved in the case of
extended secondary light sources by introducing a pupil raster
element plate with second raster elements. In case the images of
the first raster elements are ideally superimposed a almost ideal
rectangular profile is formed in scanning direction. In case the
images are not ideally superimposed e.g. because the secondary
light sources are extended and no pupil facets or so called second
raster elements are used the profile decomposes and forms a
non-rectangular intensity profile in scanning direction. This is
shown in FIG. 29 as dotted lines. The gradient of the intensity
profile depicted in dashed lines is less than 100% per
millimeter.
[0203] FIG. 30 shows the scanning energy distribution as a function
of the field height. The scan energy is defined as the line
integral in scanning direction over the intensity distribution in
the reticle plane. The homogeneous scanning energy distribution can
be clearly recognized.
[0204] In FIG. 31, the illumination of the exit pupil is shown for
a object point in the center of the illuminated field. The x- and
y-axis represent not the extent in "mm", but in the sine of the ray
angles in the reticle plane. Corresponding to the arrangement of
the pupil raster elements, tertiary light sources 3101 are produced
in the exit pupil of the illumination system. The maximum aperture
amounts to NA.sub.field=0.025.
[0205] In FIG. 31, 18 tertiary light sources are shown with
sin(i.sub.x)=0. The total energy of the 18 tertiary light sources
with sin(i.sub.x)=0 is plotted in FIG. 32. The tertiary light
source 3101 has the number 1 in FIG. 32, the tertiary light source
3105 the number 18. The intensity distribution in the exit pupil
has a y-tilt due to the distortion errors introduced by the mirrors
tilted about the x-axis. The total energy of the individual
tertiary light sources can be adjusted via the reflectivity of the
individual raster elements, so that the energy of the tertiary
light sources can at least be controlled in a rotational symmetric
manner. Another possibility to get a rotational symmetric intensity
distribution in the exit pupil of the illumination system is a
collector mirror with a spatial dependent reflectivity.
[0206] The forms of embodiment of the invention, which use
different light sources, for example, are described below.
[0207] In FIGS. 33-39, another form of embodiment of the invention
is explained with a laser-plasma source as the primary light
source. If the field raster elements are not tilted, then the
aperture in the reticle plane (NA.sub.theoretical=0.025) is given
in advance by the ellipsoid or collector mirror. Since the distance
from the light source to the ellipsoid or collector mirror should
amount to at least 100 mm in order to avoid contaminations, a rigid
relationship between structural length and collection efficiency
results, as presented in the following table:
TABLE-US-00001 TABLE 1 Collection efficiency .pi.
(0.degree.-90.degree.): 0.degree.: Beam cone is emitted
horizontally Structural Collection 90.degree.: Rays are is emitted
in a length L angle .theta. torus with a mean angle of 90.degree..
1000 mm 14.3.degree. 2%-12% 2000 mm 28.1.degree. 6%-24% 3000 mm
41.1.degree. 12%-35% 4000 mm 53.1.degree. 20%-45% 5000 mm
90.0.degree. 50%-71%
[0208] As can be seen from this, the collection efficiency for a
structural length of 3000 mm is maximum 35%.
[0209] In order to achieve high collection efficiencies for
justifiable structural lengths, in the particularly advantageous
form of embodiment of the invention according to FIGS. 35-39, the
illumination system comprises a telescope system.
[0210] In the represented form of embodiment, a laser-plasma source
is used as the primary light source, whereby the field raster
element plate is arranged in the convergent beam path of a
collector mirror.
[0211] In order to reduce the structural length of the illumination
system, the illumination system is formed as a telescope system
(tele-system). One form of embodiment for forming such a telescope
system consists of arranging the field raster elements of the field
raster element plate on a collecting surface, and of arranging the
pupil raster elements of the pupil raster element plate on a
diverging surface. In this way, the surface normal lines of the
raster element centers are adapted to the surface normal lines of
the supporting surface. As an alternative to this, one can
superimpose prismatic components for the raster elements on a
planar plate. This would correspond to a Fresnel lens as a carrier
surface.
[0212] The above-described tele-raster element condenser thus
represents a superimposition of the classical telescope system and
the raster element condenser. The compression of the diameter of
the field raster element plate to the diameter of the pupil raster
element plates is possible until the secondary light sources
overlap.
[0213] In FIGS. 33 to 36, different arrangements are shown
schematically, from which the drastic reduction in structural
length, which can be achieved with a telescope system, becomes
apparent.
[0214] FIG. 33 shows an arrangement with collector mirror 300 and
laser-plasma light source 302.
[0215] With a arrangement of collector mirror, plate 304 with
non-tilted field raster elements and plate 306 with non-tilted
pupil raster elements, as shown in FIG. 34, the structural length
can be shortened only by the zigzag light path. Since the etendue
of a point-like source is approximately zero, the field raster
element plate 304, is, in fact, fully illuminated, but the pupil
raster element plate 306 is illuminated only with individual
intensity peaks.
[0216] However, now if the raster elements are introduced onto
curved supporting surfaces, i.e., the system is configured as a
telescope system with a collecting mirror and a diverging mirror,
as shown in FIG. 35, then the structural length can be
shortened.
[0217] In the case of the design according to FIG. 36, the
individual raster elements are arranged tilted on a planar carrier
surface.
[0218] The pupil raster elements of the pupil raster element plate
have the task of imaging the field raster elements into the reticle
in the case of expanded secondary light sources and to superimpose
these images. However, if a sufficiently good point-like light
source is present, then the pupil raster element plate is not
necessary. The field raster elements can then be introduced either
onto the collecting or onto the diverging tele-mirror. If the field
raster elements are arranged on the collecting tele-mirror, they
can be designed as either concave or planar mirrors. The field
raster elements on the diverging telescope mirror can be designed
as convex, concave or planar mirrors. Collecting raster elements
lead to a real pupil plane; diverging raster elements lead to a
virtual pupil plane.
[0219] Collector lens 300 and tele-raster element condenser or
tele-system 310 produce the pregiven rectangular field illumination
of 6 mm.times.105 mm with correct aperture NA.sub.field=0.025 in
the image plane of the illumination system. As in the previous
examples, with the help of one or more field lenses 314 arranged
between tele-raster element condenser 310 and reticle 316, the
arc-shaped field is formed and the exit pupil of the illumination
system is arranged at the entrance pupil of the projection
objective.
[0220] An interface plane for the design of the field lens 314 is
the plane of the secondary light sources. These secondary light
sources must be imaged by the field lens 314 in the entrance pupil
of the projection objective forming tertiary light sources. The
pupil plane of this imaging is in the reticle plane, in which the
arc-shaped field must be produced.
[0221] In FIG. 37, a form of embodiment of the invention with only
one field mirror 314 is shown. In the form of embodiment with one
field mirror, the arc-shaped field can be produced and the entrance
pupil of the illumination system can be arranged at the exit pupil
of the projection objective. Since reticle 316, however, is
illuminated with chief ray angles about 2.97.degree., there is the
danger that the light beam will run back into the illumination
system. It is provided in a particularly advantageous form of
embodiment to use as field mirrors two grazing-incidence mirrors as
shown in FIG. 38. This way, the orientation of the arc-shaped field
is inverted and the light beam leaves the illumination system
"behind" the field lens 314. With such a configuration the
illumination system can be well separated from the projection
objective. By using two field mirrors, one also has more degrees of
freedom in order to adjust telecentricity and uniformity of the
light distribution.
[0222] The design of the illumination systems will now be described
on the basis of examples of embodiment, whereby the numerical data
not will represent a limitation of the system according to the
invention.
[0223] In the first example of embodiment the illumination system
comprises a collector unit, a diverging mirror and a collecting
mirror forming a telescope system as well as field lenses, whereby
the raster elements are introduced only onto the collecting mirror.
All raster elements are identical and lie on a curved supporting
surface.
[0224] The parameters used are represented in FIG. 39 and are
selected as follows below: [0225] a. Arc-shaped field:
R.sub.field=100 mm, segment=60.degree., field height .+-.3.0 mm.
[0226] b. Position of the entrance pupil (Distance between reticle
plane and entrance pupil of the projection objective):
z.sub.EP=1927.4 mm. This corresponds to a principal ray angle of
i.sub.PB=2.97.degree. for y=100 mm. [0227] c. Aperture at the
reticle: NA.sub.field=0.025. [0228] d. Aperture at the source:
NA.sub.source=0.999. [0229] e. Distance between the source and the
collector mirror: d.sub.1=100.0 mm. [0230] f. Field raster element
size: y.sub.FRE=1, x.sub.FRE=17.5 mm. [0231] g. d.sub.3=100 mm.
[0232] h. Compression factor D.sub.FRE/D.sub.PRE=4:1. [0233] i.
Tilt angle .alpha. of the grazing-incidence mirrors,
.alpha.=80.degree.. [0234] j. Collector mirror is designed as an
ellipsoid with R.sub.col and Ex.sub.col. [0235] k. Curvatures of
the supporting surfaces R.sub.2 and R.sub.3: spherical. [0236] l.
Curvature R.sub.FRE of the field raster element: spherical. [0237]
m. The Field mirrors are torical mirrors without concical
contributions having the curvatures: R.sub.4x, R.sub.4y, R.sub.5x,
R.sub.5y.
[0238] FIG. 40 shows an arrangement of a illumination system with
collector mirror 300, whereby the first tele-mirror of the
telescope system 310 is not structured with field raster elements.
The two tele-mirrors of the telescope system 310 show a compression
factor of 4:1. The shortening of the structural length due to the
telescope system 310 is obvious. With the telescope system, the
structural length amounts to 852.3 mm, but without the telescope
system, it would amount to 8000.0 mm. In FIG. 41, a fan of rays is
shown in the x-z plane for the system according to FIG. 40. Since
there are no field raster elements the light source 302 is imaged
into the reticle plane.
[0239] FIG. 42 in turn represents a fan of rays in the x-z plane,
whereby the mirrors of the system according to FIG. 40 are now
structured and have field raster elements. Secondary light sources
are formed on the second mirror of the telescope system 310 due to
the field raster elements on the first mirror of the telescope
system 310. In the illuminated field, the light beams from the
several field raster elements are correctly overlaid, and a strip
with -52.5 mm<x.sub.field<+52.5 mm is homogeneously
illuminated.
[0240] In FIG. 43, the total system up to the entrance pupil 318 of
the projection objective is shown. The total system comprises:
primary light source 302, collector mirror 300, tele-raster element
condenser 310, field mirrors 314, reticle 316 and entrance pupil of
the projection objective 318. The drawn-in marginal rays 320, 322
impinge on the reticle and are drawn up to the entrance pupil 318
of the projection objective.
[0241] FIG. 44 shows an x-z fan of rays of a configuration
according to FIG. 43, which passes through the central field raster
element 323. This pencil is in fact physically not meaningful,
since it would be vignetted by the second tele-mirror, but shows
well the path of the light. One sees on field mirrors 314 how the
orientation of the arc-shaped field is rotated through the second
field mirror. The rays can run undisturbed into the projection
objective (not shown) after reflection at reticle 316.
[0242] FIG. 45 shows a fan of rays, which passes through the
central field raster element 323 as in FIG. 44, runs along the
optical axis and is focused in the center of the entrance
pupil.
[0243] FIG. 46 describes the illumination of the reticle field with
the arc-shaped field produced by the configuration according to
FIGS. 40 to 45 (R.sub.field=100 mm, segment=60.degree., field
height .+-.3.0 mm).
[0244] In FIG. 47, the scanning energy is shown for an arrangement
according to FIGS. 40 to 46. The scanning energy varies between 95%
and 100%. The uniformity thus amounts to .+-.2.5%.
[0245] In FIG. 48, the pupil illumination for an object point in
the center of the illuminated field is shown. The ray angles are
referred to the centroid ray.
[0246] Corresponding to the distribution of the field raster
elements, circular intensity peaks IP result in the pupil
illumination. The obscuration in the center M is caused by the
second tele-mirror.
[0247] The illumination system described in FIGS. 31 to 48 has the
advantage that the collecting angle can be increased to above
90.degree., since the ellipsoid can also enclose the source.
[0248] Further, the structural length can be adjusted by the
tele-system. A reduction of structural length is limited due to the
angular acceptance of the coating with multilayers and the imaging
errors of the surfaces with a high optical power.
[0249] For point-like light sources, for example, a laser-plasma
sources with a diameter .ltoreq.50 .mu.m, an arrangement can be
produced with only one plate with field raster elements. Pupil
raster elements are in this case not necessary. Then the field
raster elements can be introduced onto collecting mirror 350 of the
tele-system or onto the diverging second tele-mirror 352. This is
shown in FIGS. 48A-48C.
[0250] The introduction onto the second tele-mirror 352 has several
advantages: In the case of collecting field raster elements, a real
pupil plane is formed in "air", which is freely accessible, as
shown in FIG. 48A.
[0251] In the case of diverging field raster elements, in fact a
virtual pupil plane is formed, which is not accessible, as shown in
FIG. 48B. The negative focal length of the field raster elements,
however, can be increased.
[0252] In order to avoid an obscuration, as shown in FIG. 48C, the
mirrors of the tele-system 350, 352, can be tilted toward one
another, so that the light beam will be not vignetted by the
components.
[0253] A second example of embodiment for a illumination system
will be described below, which comprises a plate with planar raster
elements. The system is particularly characterized by the fact that
the collector unit and the plate with the field raster elements
form a telescope system. The converging effect of the telescope
system is then completely transferred onto the collector mirror,
wherein the diverging effect is caused by the tilt angles of the
field raster elements.
[0254] Such a system has a high system efficiency of 27% with two
normal-incidence mirrors (reflectivity.apprxeq.65%) for the
collector mirror and the plate with the field raster elements and
two grazing-incidence mirrors (reflectivity.apprxeq.80%) for the
two field mirrors.
[0255] Further, a large collecting efficiency can be realized,
whereby the collecting steradian amounts to 2.pi., but which can
still be increased.
[0256] Based on the zigzag beam path, there are no obscurations in
the pupil illumination. In addition, in the described form of
embodiment, the structural length can be easily adjusted.
[0257] The collector or ellipsoid mirror collects the light
radiated from the laser-plasma source and images the primary light
source on a secondary light source. A multiple number of individual
planar field raster elements are arranged in a tilted manner on a
supporting plate. The field raster elements divide the collimated
light beam into partial light beams and superimpose these in the
reticle plane. The shape of the field raster elements corresponds
to the rectangular field of the field to be illuminated. Further,
the illumination system has two grazing-incidence toroid mirrors,
which form the arc-shaped field, correctly illuminate the entrance
pupil of the projection objective, and assure the uniformity of the
light distribution in the reticle plane.
[0258] In contrast to the first example of embodiment of a
tele-system with collector unit as well as a telescope system
formed with two additional mirrors, in the presently described form
of embodiment, the laser-plasma source alone is imaged by the
ellipsoid mirror in the secondary light source. This saves one
normal-incidence mirror and permits the use of planar field raster
elements. Such a savings presupposes that no pupil raster elements
are necessary, i.e., the light source is essentially
point-like.
[0259] The design will be described in more detail on the basis of
FIGS. 49-51.
[0260] FIG. 49 shows the imaging of the laser-plasma source 400
through ellipsoid mirror 402. One secondary light source 410 is
formed. In the imaging of FIG. 50, a tilted planar mirror 404
deflects the light beam to the reticle plane 406.
[0261] In the imaging of FIG. 51, tilted field raster elements 408
are dividing the light beam and superimpose the partial light
bundles in the reticle plane 406. In this way, a multiple number of
secondary light sources 410 are produced, which are distributed
uniformly over the pupil plane. The tilt angles of the individual
field raster elements 408 correspond, at the center points of the
field raster elements, approximately to the curvatures of a
hyperboloid, which would image the laser-plasma source 400 in the
reticle plane 406, together with the ellipsoid mirror 402. The
diverging effect of the telescope system is thus introduced by the
tilt angles of the field raster elements.
[0262] In FIG. 52, the abbreviations are drawn in, as they are used
in the following system derivation. For better presentation, the
system was drawn linearly with refractive components.
[0263] The following values are used as a basis for the example of
embodiment described below, without the numerical data being seen
as a limitation: [0264] a. Arc-shaped field radius: R.sub.field=100
mm, segment angle 60.degree., field width .+-.3.0 mm, which
corresponds to a rectangular field of 105 mm.times.6 mm. [0265] b.
Aperture at the reticle: NA.sub.field=0.025. [0266] c. Aperture at
the source: NA.sub.source=0.999. [0267] d. z.sub.1=100.0 mm. [0268]
e. Structural length L=z.sub.3+z.sub.4=1400 mm. [0269] f. Number of
field raster elements within an x-row=4.
[0270] With the following equations the basic configuration of the
illumination system can be derived:
NA field = D FRE 2 L D FRe = 2 L NA field D PRE x FRE = 4.0 x FRE =
D PRE 4.0 .beta. FRE = x field x FRE = z 4 z 3 .beta. FRE = x field
x FRE z 4 = z 3 .beta. FRE L = z 3 + z 4 z 3 = L 1 + .beta. FRE NA
' = D FRE 2 z 3 NA ' = D FRE 2 z 3 tan ( .theta. ) = - ( 1 - Ex )
sin ( .theta. ' ) 2 Ex - ( 1 - Ex ) cos ( .theta. ' ) Ex = f ( NA
source , NA ' ) Ex col = ( sk - s 1 sk + s 1 ) 2 = ( z 2 - z 1 z 2
+ z 1 ) 2 z 2 = z 1 1 + Ex col 1 - Ex col Ex col = 1 - R col a R
col = z 1 + z 2 2 ( 1 - Ex col ) ##EQU00004##
[0271] D.sub.FRE: diameter of the plate with the field raster
elements
[0272] x.sub.FRE: length of one field raster element
[0273] y.sub.FRE: width of one field raster element
[0274] .beta..sub.FRE: magnification ratio of the imaging of field
raster elements
[0275] D.sub.PRE: diameter of the plate with the pupil raster
elements
[0276] R.sub.col: curvature of the elliptical collector
[0277] Ex.sub.col: conical constant of the elliptical collector
[0278] NA': aperture after the collector mirror
[0279] With the pregiven specifications the following system
parameters can be calculated:
D FRE = 2 L NA field = 2 1400 mm 0.025 = 70.0 mm x FRE = D FRE 4.0
= 70.0 mm 4.0 = 17.5 mm y FRE = 1.0 mm .beta. FRE = x field x FRE =
105.0 mm 17.5 mm = 6.0 z 3 = L 1 + .beta. FRE = 1400.0 mm 1 + 6.0 =
200.0 mm z 4 = z 3 .beta. FRE = 200.0 mm 6.0 = 1200.0 mm NA ' = D
DRE 2 z 3 = 70.0 mm 2 200.0 mm = 0.175 Ex col = f ( NA source , NA
' ) = 0.695 z 2 = z 1 1 + EX col 1 - Ex col = 100.0 mm 1 + 0.695 1
- 0.695 = 1101.678 mm R col = z 1 + z 2 2 ( 1 - Ex col ) = 100.0 mm
+ 1101.678 mm 2 ( 1 - 0.695 ) = 183.357 mm ##EQU00005##
[0280] The field mirrors are constructed similar to the case of the
first example of embodiment of a illumination system, i.e., two
toroid mirrors are again used as field mirrors.
[0281] In FIGS. 53-58, the propagation of the light rays is shown
in a illumination system according to the previously given
parameters as an example.
[0282] In FIG. 53, the ray propagation is shown for an ellipsoid
mirror 402, which is designed for a source aperture NA=0.999 and
which images the primary light source 400 on a secondary light
source 410.
[0283] In the form of embodiment according to FIG. 54, a planar
mirror 404 is arranged at the position of the field raster element
plate, which reflects the light beam. The rays are propagated up to
the reticle plane 406.
[0284] Finally, in FIG. 55, the construction according to the
invention is shown, in which mirror 404 is replaced by the field
raster element plate 412. A fan of rays is depicted, wherein each
ray goes through the center of the individual field raster
elements. These rays intersect on the optical axis in the reticle
plane 406.
[0285] In this configuration the primary light source 400 is
arranged in the object plane of the collector mirror 402, wherein
the secondary light source 410 is arranged in the image plane of
the collector mirror 402. If the collector unit consists only of
one collector mirror 402 the image-side principal plane of the
collector unit is located at the vertex of the collector mirror
402. The optical distance between the vertex of the collector
mirror 402 and the secondary light source 410 is in this
configuration equal to the sum of the optical distance between the
vertex of the collector mirror 402 and the plate 412 with the field
raster elements and the optical distance between the plate 412 with
the field raster elements and the secondary light source 410. If
the refraction index is equal to 1.0, the optical distance is equal
to the geometrical distance.
[0286] FIG. 56 finally shows the total illumination system up to
entrance pupil 414 of the projection objective with two field
mirrors 416. The marginal rays 418, 420 strike on reticle 406 and
are further propagated up to the entrance pupil 414 of the
projection objective.
[0287] In FIG. 57, an x-z fan of rays is depicted for the system of
FIG. 56, and this fan strikes the central field raster element 422.
The rays illuminate the arc-shaped field on reticle 406 with the
correct orientation.
[0288] In FIG. 58, in addition, the entrance pupil 424 of the
projection objective is represented. The depicted rays are
propagated along the optical axis and are focused in the center of
the entrance pupil. The primary light source 400 is imaged into the
secondary light source 410 by the collector mirror 402, wherein the
field mirrors 416 image the secondary light source 410 into a
tertiary light source in the center of the entrance pupil 424 of
the projection objective.
[0289] In FIG. 59, the illumination of the reticle is shown with an
arc-shaped field (R.sub.field=100 mm, segment=60.degree., field
height .+-.3.0 mm), which is based on an illumination arrangement
according to FIGS. 52-58.
[0290] The integral scanning energy is shown in FIG. 60. The
integral scan energy varies between 100% and 105%. The uniformity
or homogeneity thus amounts to .+-.2.5%.
[0291] FIG. 61 represents the pupil illumination of the
above-described system for an object point in the center of the
illuminated field. The sines of the ray angles are referred to the
direction of the centroid ray. Corresponding to the field raster
element distribution, a distribution of tertiary light sources 6101
is produced in the pupil illumination. The tertiary light sources
6101 are uniformly distributed. There are no center obscurations,
since in the case of the described second form of embodiment, the
mirrors are arranged in zigzag configuration.
[0292] In FIG. 62, a profile of the intensity distribution at x=0
mm is shown in the scan direction with the use of two different
laser-plasma sources. Whereas without the pupil raster elements for
the 50-.mu.m source, the desired rectangular profile is obtained,
the 200-.mu.m source shows at the edges a clear blurring. This
source can no longer be considered point-like. The use of pupil
raster elements, such as, for example, in the case of the
pinch-plasma source, is necessary for the correct imaging of the
field raster elements into the reticle plane.
[0293] In FIGS. 63A+63B two possibilities are shown for the
formation of the field raster element plate. In FIG. 63A, the
raster elements 500 are arranged on a curved supporting surface
502. Thus the inclination of the raster elements corresponds to the
slope of the supporting surface. Such plates are described, for
example, in the case of the first form of embodiment with a
collector mirror and a telescope system comprising two mirrors.
[0294] If the field raster elements 500 are shaped in planar
manner, such as, for example, in the case of the second form of
embodiment that is described, in which collector unit and field
raster element plate are combined into a telescope system, then the
individual field raster elements are arranged under a pregiven tilt
angle on the raster element plate 504. Depending on the
distribution of the tilt angles on the plate, one obtains either
collecting or diverging effects. A plate with a diverging effect is
illustrated.
[0295] Of course, raster element plates with planar field raster
elements can be used also in systems according to the first example
of embodiment with a collector unit and two tele-mirrors. In the
case of such a system, the raster elements are then tilted onto one
of the mirrors such that a diverging effect is produced and onto
the other in such a way that a collecting effect is produced.
[0296] FIG. 64 shows a form of embodiment of the invention, which
is designed as a refractive system with lenses for wavelengths, for
example, of 193 nm or 157 nm. The system comprises a light source
600, a collector lens 602, as well as a field raster element plate
604 and a pupil raster element plate 606. Prisms 608 arranged in
front of the field raster elements serve for adjusting the light
path between the field raster element plate 604 and the pupil
raster element plate 606.
[0297] FIG. 65 shows another embodiment for a purely refractive
system in a schematically view. The beam cone of the light source
6501 is collected by the aspherical collector lens 6503 and is
directed to the plate with the field raster elements 6509. The
collector lens 6503 is designed to generate an image 6505 of the
light source 6501 at the plate with the pupil raster elements 6515
as shown with the dashed lines if the plate with the field raster
elements 6509 is not in the beam path. Therefore without the plate
with the field raster elements 6509 one secondary light source 6505
would be produced at the plate with the pupil raster elements. This
imaginary secondary light source 6505 is divided into a plurality
of secondary light sources 6507 by the field raster elements 6509
formed as field prisms 6511. The arrangement of the secondary light
sources 6507 at the plate with the pupil raster elements 6515 is
produced by the deflection angles of the field prisms 6511. These
field prisms 6511 have rectangular surfaces and generate
rectangular light bundles. However, they can have any other shape.
The pupil raster elements 6515 are arranged nearby each of the
secondary light sources 6507 to image the corresponding field
raster elements 6509 into the reticle plane 6529 and to superimpose
the rectangular images of the field raster elements 6509 in the
field 6531 to be illuminated. The pupil raster elements 6515 are
designed as combinations of a pupil prism 6517 and a pupil lenslet
6519 with positive optical power. The pupil prisms 6517 deflect the
incoming ray bundles to superimpose the images of the field raster
elements 6509 in the reticle plane 6529. The pupil lenslets 6519
are designed together with the field lens 6521 to image the field
raster elements 6509 into the reticle plane 6529. Therefore with
the prismatic deflection of the ray bundles at the field raster
elements 6509 and pupil raster elements 6515 an arbitrary
assignment between field raster elements 6509 and pupil raster
elements 6515 is possible. The pupil prisms 6517 and the pupil
lenslets 6519 can also be made integrally to form a pupil raster
element 6515 with positive and prismatic optical power. The field
lens 6521 images the secondary light sources 6507 into the exit
pupil 6533 of the illumination system forming tertiary light
sources 6535 there.
[0298] FIG. 66 shows another embodiment for a purely refractive
system in a schematically view. Corresponding elements have the
same reference numbers as those in FIG. 65 increased by 100.
Therefore, the description to these elements is found in the
description to FIG. 65. The aspherical collector lens 6603 is
designed to focus the light rays of the light source 6601 in a
plane 6605 which is arranged behind the plate with the pupil raster
elements 6615 as indicated by the dashed lines. Therefore the field
raster elements 6609 have a positive optical power to produce the
secondary light sources 6607 at the plate with the pupil raster
elements 6615. The field raster elements 6609 are designed as
combinations of a field prism 6611 and a field lenslet 6613. The
field prisms 6611 deflect the incoming ray bundles to the
corresponding secondary light sources 6607. The field lenslets 6613
are designed to generate the secondary light sources 6607 at the
corresponding pupil raster elements 6615. The field prisms 6611 and
the field lenslets 6613 can also be made integrally to form field
raster elements 6609 with positive and prismatic optical power.
[0299] FIG. 67 shows another embodiment for a purely refractive
system in a schematically view. Corresponding elements have the
same reference numbers as those in FIG. 66 increased by 100.
Therefore, the description to these elements is found in the
description to FIG. 66. The aspheric collector lens 6703 is
designed to focus the light rays of the light source 6701 in a
plane 6705 which is arranged between the plate with the field
raster elements 6709 and the plate with the pupil raster elements
6715 as indicated by the dashed lines. Therefore the field raster
elements 6709 have negative optical power to produce the secondary
light sources 6707 at the plate with the pupil raster elements
6715. The field raster elements 6709 are designed as combinations
of a field prism 6711 and a field lenslet 6713. The field prisms
6711 deflect the incoming ray bundles to the corresponding
secondary light sources 6707. The field lenslets 6713 are designed
to generate the secondary light sources 6707 at the corresponding
pupil raster elements 6715. The field prisms 6711 and the field
lenslets 6713 can also be made integrally to form field raster
elements 6709 with negative and prismatic optical power.
[0300] FIG. 68 shows another embodiment for a purely refractive
system in a schematically view. Corresponding elements have the
same reference numbers as those in FIG. 67 increased by 100.
Therefore, the description to these elements is found in the
description to FIG. 67. The aspheric collector lens 6803 is
designed to generate a parallel light bundle. Wherein in FIGS. 65
to 67 the plate with the field raster elements is arranged in a
convergent beam path, the plate with the field raster elements 6809
in FIG. 68 is arranged in a parallel beam path. The field raster
elements 6809 are designed as combinations of a field prism 6811
and a field lenslet 6813. The field prisms 6811 deflect the
incoming ray bundles to the corresponding secondary light sources
6807. The field lenslets 6813 are designed to generate the
secondary light sources 6807 at the corresponding pupil raster
elements 6815. They have positive optical power and a focal length
that corresponds to the distance between the field raster elements
6809 and the pupil raster elements 6815. Since the light source
6801 is a point-like source, also the secondary light sources 6807
are point-like. Therefore, the pupil raster elements 6815 are
designed as prisms 6817.
[0301] FIG. 69 shows another embodiment for a purely refractive
system in a schematically view. Corresponding elements have the
same reference numbers as those in FIG. 66 increased by 300.
Therefore, the description to these elements is found in the
description to FIG. 66. The aspheric collector lens 6903 is
designed to focus the light rays of the light source 6601 in a
plane 6905 which is arranged in front of the plate with the field
raster elements 6909 as indicated by with the dashed lines. Nearby
this image of the light source a transmissions filter 6937 is
arranged. This filter can also be used to select the used
wavelength range. In the plane 6905 also a shutter can be arranged.
The field raster elements 6909 have a positive optical power to
produce the secondary light sources 6907 at the plate with the
pupil raster elements 6915.
[0302] FIG. 70 shows an embodiment for a purely reflective system
in a schematically view. Corresponding elements have the same
reference numbers as those in FIG. 69 increased by 100. Therefore,
the description to these elements is found in the description to
FIG. 69. The beam cone of the light source 7001 is collected by the
ellipsoidal collector mirror 7003 and is directed to the plate with
the field raster elements 7009. The collector mirror 7003 is
designed to generate an image 7005 of the light source 7001 between
the plate with the field raster elements 7009 and the plate with
the pupil raster elements 7015 if the plate with the field raster
elements 7009 would be a planar mirror as indicated by the dashed
lines. The convex field raster elements 7009 are designed to
generate point-like secondary light sources 7007 at the pupil
raster elements 7015, since the light source 7001 is also
point-like. Therefore the pupil raster elements 7015 are designs as
planar mirrors. Since the intensity at the point-like secondary
light sources 7007 is very high, the planar pupil raster elements
7015 can alternatively be arranged defocused from the secondary
light sources 7007. The distance between the secondary light
sources 7007 and the pupil raster elements 7015 should not exceed
20% of the distance between the field raster elements and the pupil
raster elements. The pupil raster elements 7015 are tilted to
superimpose the images of the field raster elements 7009 together
with the field lens 7021 formed as the field mirrors 7023 and 7027
in the field 7031 to be illuminated. Both, the field raster
elements 7009 and the pupil raster elements 7015 are tilted.
Therefore the assignment between the field raster elements 7009 and
pupil raster elements 7015 is defined by the user. In the
embodiment of FIG. 70 the field raster elements 7009 at the center
of the plate with the field raster elements 7009 correspond to the
pupil raster elements 7015 at the border of the plate with the
pupil raster elements 7015 and vice versa. The tilt angles and the
tilt axes of the field raster elements are determined by the
directions of the incoming ray bundles and by the positions of the
corresponding pupil raster elements 7015. Since for each field
raster element 7009 the tilt angle and the tilt axis is different,
also the planes of incidence defined by the incoming and reflected
centroid rays are not parallel. The tilt angles and the tilt axes
of the pupil raster elements 7015 are determined by the positions
of the corresponding field raster elements 7009 and the requirement
that the images of the field raster elements 7009 has to be
superimposed in the field 7031 to be illuminated. The concave field
mirror 7023 images the secondary light sources 7007 into the exit
pupil 7033 of the illumination system forming tertiary light
sources 7035, wherein the convex field mirror 7027 being arranged
at grazing incidence transforms the rectangular images of the
rectangular field raster elements 7009 into arc-shaped images.
[0303] FIG. 71 shows another embodiment for a purely reflective
system in a schematically view. Corresponding elements have the
same reference numbers as those in FIG. 70 increased by 100.
Therefore, the description to these elements is found in the
description to FIG. 70. In this embodiment the light source 7101
and therefore also the secondary light sources 7107 are extended.
The pupil raster elements 7115 are designed as concave mirrors to
image the field raster elements 7109 into the image plane 7129. It
is also possible to arrange the pupil raster elements 7115 not at
the secondary light sources 7107, but defocused. The influence of
the defocus on the imaging of the field raster elements 7109 has to
be consider in the optical power of the pupil raster elements.
[0304] FIG. 72 shows in a schematic view the imaging of one field
raster element 7209 into the reticle plane 7229 forming an image
7231 and the imaging of the corresponding secondary light source
7207 into the exit pupil 7233 of the illumination system forming a
tertiary light source 7235. Corresponding elements have the same
reference numbers as those in FIG. 70 increased by 200. Therefore,
the description to these elements is found in the description to
FIG. 70.
[0305] The field raster elements 7209 are rectangular and have a
length X.sub.FRE and a width Y.sub.FRE. All field raster elements
7209 are arranged on a nearly circular plate with a diameter
D.sub.FRE. They are imaged into the image plane 7229 and
superimposed on a field 7231 with a length X.sub.field and a width
Y.sub.field, wherein the maximum aperture in the image plane 7229
is denoted by NA.sub.field. The field size corresponds to the size
of the object field of the projection objective, for which the
illumination system is adapted.
[0306] The plate with the pupil raster elements 7215 is arranged in
a distance of Z.sub.3 from the plate with the field raster elements
7209. The shape of the pupil raster elements 7215 depends on the
shape of the secondary light sources 7207. For circular secondary
light sources 7207 the pupil raster elements 7215 are circular or
hexagonal for a dense packaging of the pupil raster elements 7215.
The diameter of the plate with the pupil raster elements 7215 is
denoted by D.sub.PRE.
[0307] The pupil raster elements 7215 are imaged by the field lens
7221 into the exit pupil 7233 having a diameter of D.sub.EP. The
distance between the image plane 7229 of the illumination system
and the exit pupil 7233 is denoted with Z.sub.EP. Since the exit
pupil 7233 of the illumination system corresponds to the entrance
pupil of the projection objective, the distance Z.sub.EP and the
diameter D.sub.EP are predetermined values. The entrance pupil of
the projection objective is typically illuminated up to a
user-defined filling ratio .sigma..
[0308] The data for a preliminary design of the illumination system
can be calculated with the equations and data given below. The
values for the parameters are typical for a EUV projection exposure
apparatus. But there is no limitation to these values. Wherein the
schematic design is shown for a refractive linear system it can be
easily adapted for reflective systems by exchanging the lenses with
mirrors.
[0309] The field 7231 to be illuminated is defined by a segment of
an annulus. The Radius of the annulus is
R.sub.field=138 mm.
The length and the width of the segment are
X.sub.field=88 mm, Y.sub.field=8 mm
[0310] Without the field-forming field mirror, which transforms the
rectangular images of the field raster elements into arc-shaped
images, the field to be illuminated is rectangular with the length
and width defined by the segment of the annulus.
[0311] The distance from the image plane to the exit pupil is
Z.sub.EP=1320 mm.
[0312] The object field of the projection objective is an off-axis
field. The distance between the center of the field and the optical
axis of the projection objective is given by the radius
R.sub.field. Therefore the incidence angle of the centroid ray in
the center of the field is 6.degree..
[0313] The aperture at the image plane of the projection objective
is NA.sub.wafer=0.25. For a reduction projection objective with a
magnification ratio of .beta..sub.proj=-0.25 and a filling ratio of
.sigma.=0.8 the aperture at the image plane of the illumination
system is
NA field = .sigma. NA wafer 4 = 0.05 ##EQU00006## D EP = 2 tan [
arcsin ( NA field ) ] Z EP .apprxeq. 2 NA EP Z EP .apprxeq. 132 mm
##EQU00006.2##
[0314] The distance Z.sub.3 between the field raster elements and
the pupil raster elements is related to the distance Z.sub.EP
between the image plane and the exit pupil by the depth
magnification .alpha.:
Z.sub.EP=.alpha.Z.sub.3
[0315] The size of the field raster elements is related to the
field size by the lateral magnification .beta..sub.field:
X.sub.field=.beta..sub.fieldX.sub.FRE
Y.sub.field=.beta..sub.fieldY.sub.FRE
[0316] The diameter D.sub.PRE of the plate with the pupil raster
elements and the diameter D.sub.EP of the exit pupil are related by
the lateral magnification .beta..sub.pupil:
D.sub.EP=.beta..sub.pupilD.sub.PRE
[0317] The depth magnification ox is defined by the product of the
lateral magnifications .beta..sub.field and .beta..sub.pupil:
.alpha.=.beta..sub.field.beta..sub.pupil
[0318] The number of raster elements being superimposed at the
field is set to 200. With this high number of superimposed images
the required field illumination uniformity can be achieved.
[0319] Another requirement is to minimize the incidence angles on
the components. For a reflective system the beam path is bent at
the plate with the field raster elements and at the plate with the
pupil raster elements. The bending angles and therefore the
incidence angles are minimum for equal diameters of the two
plates:
D PRE = D FRE ##EQU00007## 200 X PRE Y PRE = 200 X field Y field
.beta. field 2 = D EP 2 .beta. pupil 2 = .beta. field 2 .alpha. 2 D
EP 2 ##EQU00007.2##
[0320] The distance Z.sub.3 is set to Z.sub.3=900 mm. This distance
is a compromise between low incidence angles and a reduced overall
length of the illumination system.
.alpha. = Z EP Z 3 = 1.47 ##EQU00008##
Therefore
[0321] .beta. field .apprxeq. 200 X field Y field D EP 2 .alpha. 2
4 .apprxeq. 2.05 .beta. pupil .apprxeq. .alpha. .beta. field
.apprxeq. 0.7 D FRE = D PRE = .beta. field .alpha. D EP .apprxeq.
200 mm X FRE = X field .beta. field .apprxeq. 43 mm Y FRE = Y field
.beta. field .apprxeq. 4 mm ##EQU00009##
[0322] With these values the principal layout of the illumination
system is known.
[0323] In a next step the field raster elements 7309 have to be
distributed on the plate as shown in FIG. 73. The two-dimensional
arrangement of the field raster elements 7309 is optimized for
efficiency. Therefore the distance between the field raster
elements 7309 is as small as possible. Field raster elements 7309,
which are only partially illuminated, will lead to uniformity
errors of the intensity distribution in the image plane, especially
in the case of a restricted number of field raster elements 7309.
Therefore only these field raster elements 7309 are imaged into the
image plane which are illuminated almost completely. FIG. 73 shows
a possible arrangement of 216 field raster elements 7309. The solid
line 7339 represents the border of the circular illumination of the
plate with the field raster elements 7309. Therefore the filling
efficiency is approximately 90%. The rectangular field raster
elements 7309 have a length X.sub.FRE=46.0 mm and a width
Y.sub.FRE=2.8 mm. All field raster elements 7309 are inside the
circle 7339 with a diameter of 200 mm. The field raster elements
7309 are arranged in 69 rows 7341 being arranged one among another.
The field raster elements 7309 in the rows 7341 are attached at the
smaller y-side of the field raster elements 7309. The rows 7341
consist of one, two, three or four field raster elements 7309. Some
rows 7341 are displaced relative to the adjacent rows 7341 to
distribute the field raster elements 7309 inside the circle 7339.
The distribution is symmetrical to the y-axis.
[0324] FIG. 74 shows the arrangement of the pupil raster elements
7415. They are arranged on a distorted grid to compensate for
distortion errors of the field lens. If this distorted grid of
pupil raster elements 7415 is imaged into the exit pupil of the
illumination system by the field lens a undistorted regular grid of
tertiary light sources will be generated. The pupil raster elements
7415 are arranged on curved lines 7443 to compensate the distortion
introduced by the field-forming field mirror. The distance between
adjacent pupil raster elements 7415 is increased in y-direction to
compensate the distortion introduced by field mirrors being tilted
about the x-axis. Therefore the pupil raster elements 7415 are not
arranged inside a circle. The size of the pupil raster elements
7415 depends on the source size or source etendue. If the source
etendue is much smaller than the required etendue in the image
plane, the secondary light sources will not fill the plate with the
pupil raster elements 7415 completely. In this case the pupil
raster elements 7415 need only to cover the area of the secondary
light sources plus some overlay to compensate for source movements
and imaging aberrations of the collector-field raster element unit.
In FIG. 74 circular pupil raster elements 7415 are shown.
[0325] Each field raster element 7309 corresponds to one of the
pupil raster elements 7415 according to an assignment table and is
tilted to deflect an incoming ray bundle to the corresponding pupil
raster element 7415. A ray coming from the center of the light
source and intersecting the field raster element 7309 at its center
is deflected to intersect the center of the corresponding pupil
raster element 7415. The tilt angle and tilt axis of the pupil
raster element 7415 is designed to deflect this ray in such a way,
that the ray intersects the field in its center.
[0326] The field lens images the plate with the pupil raster
elements into the exit pupil and generates the arc-shaped field
with the desired radius R.sub.field. For R.sub.field=138 mm, the
field forming gracing incidence field mirror has only low negative
optical power. The optical power of the field-forming field mirror
has to be negative to get the correct orientation of the arc-shaped
field. Since the magnification ratio of the field lens has to be
positive, another field mirror with positive optical power is
required. Wherein for apertures NA.sub.field lower than 0.025 the
field mirror with positive optical power can be a grazing incidence
mirror, for higher apertures the field mirror with positive optical
power should be a normal incidence mirror.
[0327] FIG. 75 shows a schematic view of a embodiment comprising a
light source 7501, a collector mirror 7503, a plate with the field
raster elements 7509, a plate with the pupil raster elements 7515,
a field lens 7521, an image plane 7529 and an exit pupil 7533. The
field lens 7521 has one normal-incidence mirror 7523 with positive
optical power for pupil imaging and one grazing-incidence mirror
7527 with negative optical power for field shaping. Exemplary for
the imaging of all secondary light sources, the imaging of one
secondary light source 7507 into the exit pupil 7533 forming a
tertiary light source 7535 is shown. The optical axis 7545 of the
illumination system is not a straight line but is defined by the
connection lines between the single components being intersected by
the optical axis 7545 at the centers of the components. Therefore,
the illumination system is a non-centered system having an optical
axis 7545 being bent at each component to get a beam path free of
vignetting. There is no common axis of symmetry for the optical
components. Projection objectives for EUV exposure apparatus are
typically centered systems with a straight optical axis and with an
off-axis object field. The optical axis 7547 of the projection
objective is shown as a dashed line. The distance between the
center of the field 7531 and the optical axis 7547 of the
projection objective is equal to the field radius R.sub.field. The
pupil imaging field mirror 7523 and the field-forming field mirror
7527 are designed as on-axis toroidal mirrors, which means that the
optical axis 7545 paths through the vertices of the on-axis
toroidal mirrors 7523 and 7527.
[0328] In another embodiment as shown in FIG. 76, a telescope
objective in the field lens 7621 comprising the field mirror 7623
with positive optical power, the field mirror 7625 with negative
optical power and the field mirror 7627 is applied to reduce the
track length. Corresponding elements have the same reference
numbers as those in FIG. 75 increased by 100. Therefore, the
description to these elements is found in the description to FIG.
75. The field mirror 7625 and the field mirror 7623 of the
telescope objective in FIG. 74 are formed as an off-axis
Cassegrainian configuration. The telescope objective has an object
plane at the secondary light sources 7607 and an image plane at the
exit pupil 7633 of the illumination system. The pupil plane of the
telescope objective is arranged at the image plane 7629 of the
illumination system. In this configuration, having five
normal-incidence reflections at the mirrors 7603, 7609, 7615, 7625
and 7623 and one grazing-incidence reflection at the mirror 7627,
all mirrors are arranged below the image plane 7629 of the
illumination system. Therefore, there is enough space to install
the reticle and the reticle support system.
[0329] In FIG. 77 a detailed view of the embodiment of FIG. 76 is
shown. Corresponding elements have the same reference numbers as
those in FIG. 76 increased by 100. Therefore, the description to
these elements is found in the description to FIG. 76. The
components are shown in a y-z-sectional view, wherein for each
component the local co-ordinate system with the y- and z-axis is
shown. For the collector mirror 7703 and the field mirrors 7723,
7725 and 7727 the local co-ordinate systems are defined at the
vertices of the mirrors. For the two plates with the raster
elements the local co-ordinate systems are defined at the centers
of the plates. In table 2 the arrangement of the local co-ordinate
systems with respect to the local co-ordinate system of the light
source 7701 is given. The tilt angles .alpha., .beta. and .gamma.
about the x-, y- and z-axis are defined in a right-handed
system.
TABLE-US-00002 TABLE 2 Co-ordinate systems of vertices of mirrors X
[mm] Y[mm] Z[mm] .alpha. [.degree.] .beta. [.degree.]
.gamma.[.degree.] Light source 7701 0.0 0.0 0.0 0.0 0.0 0.0
Collector mirror 0.0 0.0 125.0 0.0 0.0 0.0 7703 Plate with field
0.0 0.0 -975.0 10.5 180.0 0.0 raster elements 7709 Plate with pupil
0.0 -322.5 -134.8 13.5 0.0 180.0 raster elements 7715 Field mirror
7725 0.0 508.4 -1836.1 -67.8 0.0 180.0 Field mirror 7723 0.0 204.8
-989.7 -19.7 0.0 180.0 Field mirror 7727 0.0 -163.2 -2106.2 49.4
180.0 0.0 Image plane 7731 0.0 -132.1 -1820.2 45.0 0.0 0.0 Exit
pupil 7733 0.0 -1158.1 -989.4 45.0 0.0 0.0
[0330] The surface data are given in table 3. The radius R and the
conical constant K define the surface shape of the mirrors
according to the formula
z = 1 R h 2 1 + 1 - ( 1 + K ) ( 1 R ) 2 h 2 , ##EQU00010##
wherein h is the radial distance of a surface point from the
z-axis.
TABLE-US-00003 TABLE 3 Optical data of the components Field Pupil
Collector raster raster Field Field Field mirror element element
mirror mirror mirror 7703 7709 7715 7725 7723 7727 R [mm] -235.3
.infin. -1239.7 -534.7 -937.7 -65.5 K -0.77855 0.0 0.0 -0.0435
-0.0378 -1.1186 Focal -- .infin. 617.6 -279.4 477.0 -757.1 length f
[mm]
[0331] The light source 7701 in this embodiment is a
Laser-Produced-Plasma source having a diameter of approximately 0.3
mm generating a beam cone with an opening angle of 83.degree.. To
decrease the contamination of the collector mirror 7703 by debris
of the source 7701 the distance to the collector mirror 7703 is set
to 125 mm.
[0332] The collector mirror 7703 is an elliptical mirror, wherein
the light source 7701 is arranged in the first focal point of the
ellipsoid and wherein the plate with the pupil raster elements 7715
is arranged in the second focal point of the ellipsoid.
[0333] Therefore the field raster elements 7709 can be designed as
planar mirrors. The distance between the vertex of the collector
mirror 7703 and the center of the plate with the field raster
elements 7709 is 1100 mm. The field raster elements 7709 are
rectangular with a length X.sub.FRE=46.0 mm and a width
Y.sub.FRE=2.8 mm. The arrangement of the field raster elements is
shown in FIG. 73. The tilt angles and tilt axis are different for
each field raster element 7709, wherein the field raster elements
are tilted to direct the incoming ray bundles to the corresponding
pupil raster elements 7715. The tilt angles are in the range of
-4.degree. to 4.degree.. The mean incidence angle of the rays on
the field raster elements is 10.5.degree.. Therefore the field
raster elements 7709 are used at normal incidence.
[0334] The plate with the pupil raster elements 7715 is arranged in
a distance of 900 mm from the plate with the field raster elements
7709. The pupil raster elements 7715 are concave mirrors. The
arrangement of the pupil raster elements 7715 is shown in FIG. 72.
The tilt angles and tilt axis are different for each pupil raster
element 7715, wherein the pupil raster elements 7715 are tilted to
superimpose the images of the field raster elements 7709 in the
image plane 7731. The tilt angles are in the range of -4.degree. to
4.degree.. The mean incidence angle of the rays on the pupil raster
elements 7715 is 7.5.degree.. Therefore the pupil raster elements
7715 are used at normal incidence.
[0335] The field mirror 7725 is a convex mirror. The used area of
this mirror defined by the incoming rays is an off-axis segment of
a rotational symmetric conic surface. The mirror surface is drawn
in FIG. 77 from the vertex up to the used area as dashed line. The
distance between the center of the plate with the pupil raster
elements 7715 and the center of the used area on the field mirror
7725 is 1400 mm. The mean incidence angle of the rays on the field
mirror 7725 is 12.degree.. Therefore the field mirror 7725 is used
at normal incidence.
[0336] The field mirror 7723 is a concave mirror. The used area of
this mirror defined by the incoming rays is an off-axis segment of
a rotational symmetric conical surface. The mirror surface is drawn
in FIG. 77 from the vertex up to the used area as dashed line. The
distance between the center of the used area on the field mirror
7725 and the center of the used area on the field mirror 7723 is
600 mm. The mean incidence angle of the rays on the field mirror
7723 is 7.5.degree.. Therefore the field mirror 7723 is used at
normal incidence.
[0337] The field mirror 7727 is a convex mirror. The used area of
this mirror defined by the incoming rays is an off-axis segment of
a rotational symmetric conic surface. The mirror surface is drawn
in FIG. 77 from the vertex up to the used area as dashed line. The
distance between the center of the used area on the field mirror
7723 and the center of the used area on the field mirror 7727 is
600 mm. The mean incidence angle of the rays on the field mirror
7727 is 78.degree.. Therefore the field mirror 7727 is used at
grazing incidence. The distance between the field mirror 7727 and
the image plane 7731 is 300 mm.
[0338] In another embodiment the field mirror and the field mirror
are replaced with on-axis toroidal mirrors. The vertices of these
mirrors are arranged in the centers of the used areas. The convex
field mirror has a radius R.sub.y=571.3 mm in the y-z-section and a
radius R.sub.x=546.6 mm in the x-z-section. This mirror is tilted
about the local x-axis about 12.degree. to the local optical axis
7745 defined as the connection lines between the centers of the
used areas of the mirrors. The concave field mirror has a radius
R.sub.y=-962. 14 mm in the y-z-section and a radius R.sub.x=-945.
75 mm in the x-z-section. This mirror is tilted about the local
x-axis about 7.5.degree. to the local optical axis 7745.
[0339] FIG. 78 shows the illuminated arc-shaped area in the image
plane 7731 of the illumination system presented in FIG. 77. The
orientation of the y-axis is defined in FIG. 77. The solid line
7849 represents the 50%-value of the intensity distribution, the
dashed line 7851 the 10%-value. The width of the illuminated area
in y-direction is constant over the field. The intensity
distribution is the result of a simulation done with the optical
system given in table 2 and table 3.
[0340] FIG. 79 shows the illumination of the exit pupil 7733 for an
object point in the center (x=0 mm; y=0 mm) of the illuminated
field in the image plane 7731. The arrangement of the tertiary
light sources 7935 corresponds to the arrangement of the pupil
raster elements 7715, which is presented in FIG. 74. Wherein the
pupil raster elements in FIG. 74 are arranged on a distorted grid,
the tertiary light sources 7935 are arranged on a undistorted
regular grid. It is obvious in FIG. 79, that the distortion errors
of the imaging of the secondary light sources due to the tilted
field mirrors and the field-shaping field mirror are compensated.
The shape of the tertiary light sources 7935 is not circular, since
the light distribution in the exit pupil 7733 is the result of a
simulation with a Laser-Plasma-Source which was not spherical but
ellipsoidal. The source ellipsoid was oriented in the direction of
the local optical axis. Therefore also the tertiary light sources
are not circular, but elliptical.
[0341] Due to the mixing of the light channels and the user-defined
assignment between the field raster elements and the pupil raster
elements, the orientation of the tertiary light sources 7935 is
different for nearby each tertiary light source 7935. Therefore,
the planes of incidence of at least two field raster elements have
to intersect each other. The plane of incidence of a field raster
element is defined by the centroid ray of the incoming bundle and
its corresponding deflected ray.
[0342] FIG. 80 shows another embodiment in a schematic view.
Corresponding elements have the same reference numbers as those in
FIG. 76 increased by 400. Therefore, the description to these
elements is found in the description to FIG. 76. In this embodiment
the beam path between the plate with the pupil raster elements 8015
and the field mirror 8025 is crossing the beam path from the
collector mirror 8003 to the plate with the field raster elements
8009. With this arrangement it is possible to have light sources
8001 emitting a beam cone horizontally and to arrange the reticle
horizontally in the image plane 8029 simultaneously.
[0343] FIG. 81 shows a similar embodiment to the one of FIG. 80 in
a detailed view. Corresponding elements have the same reference
numbers as those in FIG. 80 increased by 100. Therefore, the
description to these elements is found in the description to FIG.
80. The definition of the local co-ordinate systems is the same as
in FIG. 77. The positions of the local co-ordinate systems are
given in table 4.
TABLE-US-00004 TABLE 4 Co-ordinate systems of vertices of mirrors X
[mm] Y[mm] Z[mm] .alpha. [.degree.] .beta. [.degree.]
.gamma.[.degree.] Light source 8101 0.0 0.0 0.0 0.0 0.0 0.0
Collector mirror 0.0 0.0 100.0 0.0 0.0 0.0 8103 Plate with field
0.0 0.0 -10.0 10.5 180.0 0.0 raster elements 8109 Plate with pupil
0.0 -322.5 -159.8 31.0 0.0 180.0 raster elements 8115 Field mirror
8125 0.0 1395.9 -1110.3 -20.3 0.0 180.0 Field mirror 8123 0.0 746.5
-645.4 13.6 0.0 180.0 Field mirror8127 0.0 1053.2 -1784.2 86.3
180.0 0.0 Image plane 8131 0.0 906.0 -1537.1 82.0 0.0 0.0 Exit
pupil 8135 0.0 -413.5 -1491.0 82.0 0.0 0.0
The surface data are given in table 5.
TABLE-US-00005 TABLE 5 Optical data of the components Field Pupil
Collector raster raster Field Field Field mirror element element
mirror mirror mirror 8103 8109 8115 8125 8123 8127 R [mm] -200.00
-1800.0 -1279.7 -588.9 -957.1 -65.5 K -1.0 0.0 0.0 -0.0541 -0.0330
-1.1186 Focal -- 900.0 639.8 -317.5 486.8 -757.1 length f [mm]
[0344] The light source 8101 in this embodiment is also a
Laser-Produced-Plasma source. The distance to the collector mirror
8103 is set to 100 mm. The collector mirror 8103 is a parabolic
mirror generating a parallel ray bundle, wherein the light source
8101 is arranged in the focal point of the parabola. Therefore the
field raster elements 8109 are concave mirrors to generate the
secondary light sources at the corresponding pupil raster elements
8115. The focal length of the field raster elements 8109 is equal
to the distance between the field raster elements 8109 and the
corresponding pupil raster elements 8115. The distance between the
vertex of the collector mirror 8103 and the center of the plate
with the field raster elements 8109 is 1100 mm. The field raster
elements 8109 are rectangular with a length X.sub.FRE=46.0 mm and a
width Y.sub.FRE=2.8 mm. The arrangement of the field raster
elements 8109 is shown in FIG. 73. The mean incident angle of the
rays intersecting the field raster elements 8109 is 10.5.degree.,
the range of the incidence angles is from 8.degree. up to
13.degree.. Therefore the field raster elements 8109 are used at
normal incidence.
[0345] The plate with the pupil raster elements 8115 is arranged in
the focal plane of the field raster elements 8109. The pupil raster
elements 8115 are concave mirrors. The arrangement of the pupil
raster elements 8115 is similar to the arrangement shown in FIG.
74. The mean incidence angle of the rays intersecting the pupil
raster elements 8115 is 10.0.degree., the range of the incidence
angles is from 7.degree. up to 13.degree.. Therefore the pupil
raster elements 8115 are used at normal incidence. Between the
plate with the pupil raster elements 8115 and the field mirror 8125
the beam path is crossing the beam path between the collector
mirror 8103 and the plate with the field raster elements 8109. The
field mirror 8125 is a convex mirror. The distance between the
center of the plate with the pupil raster elements 8115 and the
center of the used area on the field mirror 8125 is 1550 mm. The
mean incidence angle of the rays intersecting the field mirror 8125
is 13.degree., the range of the incidence angles is from 11.degree.
up to 15.degree.. Therefore the field mirror 8125 is used at normal
incidence.
[0346] The field mirror 8123 is a concave mirror. The distance
between the center of the used area on the field mirror 8125 and
the center of the used area on the field mirror 8123 is 600 mm. The
mean incidence angle of the rays intersecting the field mirror 8123
is 7.5.degree., the range of the incidence angles is from 6.degree.
up to 9.degree.. Therefore the field mirror 8123 is used at normal
incidence.
[0347] The field mirror 8127 is a convex mirror. The distance
between the center of the used area on the field mirror 8123 and
the center of the used area on the field mirror 8127 is 600 mm. The
mean incidence angle of the rays intersecting the field mirror 8127
is 78.degree., the range of the incidence angles is from 73.degree.
up to 82.degree.. Therefore the field mirror 8127 is used at
grazing incidence.
[0348] FIG. 82 shows another embodiment in a schematic view.
Corresponding elements have the same reference numbers as those in
FIG. 76 increased by 600. Therefore, the description to these
elements is found in the description to FIG. 76. In this embodiment
the field mirror 8225 and the field mirror 8223 are both concave
mirrors forming an off-axis Gregorian telescope configuration. The
field mirror 8225 images the secondary light sources 8207 in the
plane between the field mirror 8225 and the field mirror 8223
forming tertiary light sources 8259. In FIG. 82 only the imaging of
the central secondary light source 8207 is shown. At the plane with
the tertiary light sources 8259 a masking unit 8261 is arranged to
change the illumination mode of the exit pupil 8233. With stop
blades it is possible to mask the tertiary light sources 8259 and
therefore to change the illumination of the exit pupil 8233 of the
illumination system. Possible stop blades has circular shapes or
for example two or four circular openings. The field mirror 8223
and the field mirror 8227 image the tertiary light sources 8259
into the exit pupil 8233 of the illumination system forming
quaternary light sources 8235.
[0349] FIG. 83 shows another embodiment in a schematic view.
Corresponding elements have the same reference numbers as those in
FIG. 82 increased by 100. Therefore, the description to these
elements is found in the description to FIG. 82. In this embodiment
the collector mirror 8303 is designed to generate an intermediate
image 8363 of the light source 8301 in front of the plate with the
field raster elements 8309. Nearby this intermediate image 8363 a
transmission plate 8365 is arranged. The distance between the
intermediate image 8363 and the transmission plate 8365 is so large
that the plate 8365 will not be destroyed by the high intensity
near the intermediate focus. The distance is limited by the maximum
diameter of the transmission plate 8365, which is in the order of
200 mm. The maximum diameter is determined by the possibility to
manufacture a plate being transparent at EUV. The transmission
plate 8365 can also be used as a spectral purity filter to select
the used wavelength range. Instead of the absorptive transmission
plate 8365 also a reflective grating filter can be used. The plate
with the field raster elements 8309 is illuminated with a diverging
ray bundle. Since the tilt angles of the field raster elements 8309
are adjusted according to a collecting surface the diverging beam
path can be transformed to a nearly parallel one. Additionally, the
field raster elements 8309 are tilted to deflect the incoming ray
bundles to the corresponding pupil raster elements 8315.
[0350] FIG. 84 shows an EUV projection exposure apparatus in a
detailed view. The illumination system is the same as shown in
detail in FIG. 77. Corresponding elements have the same reference
numbers as those in FIG. 77 increased by 700. Therefore, the
description to these elements is found in the description to FIG.
77. In the image plane 8429 of the illumination system the reticle
8467 is arranged. The reticle 8467 is positioned by a support
system 8469. The projection objective 8471 having six mirrors
images the reticle 8467 onto the wafer 8473, which is also
positioned by a support system 8475. The mirrors of the projection
objective 8471 are centered on a common straight optical axis 8447.
The arc-shaped object field is arranged off-axis. The direction of
the beam path between the reticle 8467 and the first mirror 8477 of
the projection objective 8471 is convergent to the optical axis
8447 of the projection objective 8471. The angles of the chief rays
8445 with respect to the normal of the reticle 8467 are between
5.degree. and 7.degree.. As shown in FIG. 84, the illumination
system 8479 is well separated from the projection objective 8471.
The illumination and the projection beam path interfere only nearby
the reticle 8467. The beam path of the illumination system is
folded with reflection angles lower than 25.degree. or higher than
75.degree. in such a way that the components of the illumination
system are arranged between the plane 8481 with the reticle 8467
and the plane 8483 with the wafer 8473.
[0351] In a scannertype lithography projection exposure apparatus
equipped with a pulsed light source the dose at a specific point
within the object to be illuminated depends on the number of light
pulses hitting the reticle plane. Assuming a stable pulse frequency
of the light source and a rectangular intensity profile this number
fluctuates by one count. To obtain a high dose uniformity it is
advantageous that the fluctuation of the dose due to the discrete
pulse sequence is minimized. This can be achieved if the first
pulse and the last pulse of a pulse sequence do not contribute to
the dose of an object point as much as the pulses in the middle of
a pulse sequence do. A trapezoid intensity profile as shown in FIG.
87B can provide for such a behavior. Also other intensity profiles
except an perfect rectangular profile are possible, e.g. a
Lorentz-profile or a Gaussian profile.
[0352] FIGS. 85 to 87 show the influence of superposition of the
images of different first raster elements in the image plane on the
intensity profile of the illumination in scanning direction, here
in y-direction. To describe the effect the illumination system it
is assumed to have no field mirrors for forming the arc-shaped
field. Therefore images of the rectangular first raster elements
are also rectangular in the image plane having the same
aspect-ratio as the field raster elements. Such a rectangular field
in the image plane is shown in FIG. 85 and denoted with reference
number 8500. If the images 8600, 8602, 8604 of the first raster
elements are superimposed almost congruently, as shown in FIG. 86A,
an almost rectangular intensity profile 8606 as shown in FIG. 86B
results in scanning direction.
[0353] In FIG. 87A a case is shown where the images of the first
raster element are not superimposed congruently in the image plane.
In this application non-ideal superposition means that the images
of the first raster elements are not fully congruent in the field
plane. This can be achieved in that the first raster elements of
the first raster element plate have a different size, e.g. a
different extension in y-direction, which coincides with the
scanning direction in a scannertype lithography projection exposure
apparatus.
[0354] To superimpose three different images 8700, 8702, 8704 not
congruently in the image plane in a first embodiment the raster
element plate comprises three different raster elements with a
different extension in y-direction and therefore different aspect
ratios. The intensity profile in y-direction resulting from the
field in the image plane as shown in FIG. 87A is depicted in FIG.
87B. As it is apparent from FIG. 87B a nearly trapezoid intensity
profile results from the field as shown in FIG. 87 A.
[0355] A non-congruent superposition of the images of the first
raster elements in the image plane can also be achieved if all
first raster elements have identical size, i.e. a identical aspect
ratio but the corresponding second raster elements have different
optical power. In such a case the images of the first raster
elements have a different size in the image plane and thus are not
superimposed congruently.
[0356] To achieve a non-congruent superposition of the images of
the first raster elements in the image plane it is possible to
combine the two aforementioned methods, i.e. first raster elements
of different size and different optical power of the second raster
elements.
[0357] A raster element plate with first raster elements as shown
in FIG. 73 having raster elements of different size, i.e. extension
in y-direction and therefore different aspect ratio is shown in
FIG. 88. FIG. 88 shows a raster element plate with four first
raster elements with a first extension in y-direction 8800.1,
8800.2, 8800.3 8800.4, four first raster elements with a second
extension in y-direction 8802.1, 8802.2, 8802.3, 8802.4 and four
first raster elements with a third extension in y-direction 8804.1,
8804.2, 8804.3, 8804.4. The raster elements are arranged symmetric
on the raster element plate in respect to the x- and the
y-axis.
[0358] For obtaining also a sufficient telecentricity during the
scan process it is necessary to fill the exit pupil for each field
point for the different first raster elements of different size
with tertiary light sources in a uniform manner. This can be
achieved if the deflection angles of the deflected ray bundle of
the plurality of the first raster elements is chosen in such a
manner that the corresponding plurality of second raster elements
are nearly point symmetric to the center of the pupil raster
element plate shown, for example, in FIG. 74. In this application
nearly point symmetric means that the telecentricity error in the
exit pupil for each field point is less than 1 mrad, preferably
less than 0.1 mrad. Since the tertiary light sources in the exit
pupil for each field point of the object field corresponds to the
arrangement of the second raster elements on the pupil raster
element plate, the exit pupil of each field point is also filled
point symmetric with tertiary light sources as shown in FIG. 89.
FIG. 89 shows schematically the principle of arrangement of first
and second raster elements. Two first raster elements 8900.1 and
8900.2 of identical size, which are arranged symmetrically with
respect to an axis of symmetry 8910 in the first raster element
plate 8950. In this case the axis of symmetry is the x-axis, which
is perpendicular to the scanning direction. The deflection angles
of the first raster elements 8900.1 and 8900.2 are chosen such that
the corresponding pupil facets 8980.1 and 8980.2 are arranged point
symmetrically with respect to the center of the second raster
element plate 8990.
[0359] As discussed in the examples before e.g. in FIGS. 73-79 the
light source, which illuminates the first raster element plate is
denoted as primary light source. The plurality of first raster
elements forms a plurality of secondary light sources. The second
raster element plate is arranged in or near the site of the
secondary light sources.
[0360] The exit pupil for seven field points is shown in FIG. 90.
Point 9000 lies outside the field in the image plane. Therefore no
illumination occurs in the exit pupil 9050 for this point. Point
9002 lies within the filed. The images of the first raster elements
8804.1, 8804.2, 8804.3, 8804.4 of the filed raster element plate
shown in FIG. 88 are superimposed in this field point. Therefore
four tertiary light sources 9010.1, 9010.2, 9010.3, 9010.4
illuminate the exit pupil 9052. The four tertiary light sources
9010.1, 9010.2, 9010.3, 9010.4 are symmetric to the center C of the
exit pupil.
[0361] In field point 9003 the images of eight first raster
elements 8804.1, 8804.2, 8804.3, 8804.4, 8802.1, 8802.2, 8802.3,
8802.4 of the raster element plate shown in FIG. 88 are
superimposed. In the exit pupil 9054 eight uniformly distributed
tertiary light sources 9010.1, 9010.2, 9010.3, 9010.4, 9012.1,
9012.2, 9012.3, 9012.4 are depicted which are point symmetric to
the center of the exit pupil.
[0362] In field point 9004 the images of all twelve first raster
elements 8804.1, 8804.2, 8804.3, 8804.4, 8802.1, 8802.2, 8802.3,
8802.4, 8800.1, 8800.2, 8800.3, 8800.4 of the raster element plate
in FIG. 88 are superimposed. In the exit pupil 9056 twelve
uniformly distributed tertiary light sources 9010.1, 9010.2,
9010.3, 9010.4, 9012.1, 9012.2, 9012.3, 9012.4, 9014.1, 9014.2,
9014.3, 9014.4 are depicted which are point symmetric to the center
of the exit pupil.
[0363] For field point 9005 the images of eight first raster
elements are superimposed. The situation corresponds to the
situation in filed point 9003. The exit pupil 9058 is illuminated
by eight tertiary light sources.
[0364] For field point 9006 the images of four first raster
elements are superimposed. The situation corresponds to the
situation in filed point 9002. The exit pupil 9060 is illuminated
by four tertiary light sources.
[0365] Point 9007 lies outside the field, therefore the exit pupil
9062 is not illuminated.
[0366] If one scans an object in y-direction at the beginning 4
tertiary light sources are turned on then 8 and at last 12 light
sources are turned on. Then four light sources to a total of eight
light sources are turned off, in the next step further four light
sources to a total of four light sources are turned off and outside
the field in the image plane the exit pupil is not illuminated.
[0367] As a result of the special assignment of first raster
elements and second raster elements the center of gravity of the
illumination of the exit pupil is located in the center of the exit
pupil for each field point. Thus the telecentricity of the
illumination system does not depend on the field position, a
prerequisite for telecentric wafer exposure. The described feature
of the exit pupil holds for any axially symmetric illumination of
the first raster elements and is purely based on the assignment of
first and second raster elements.
[0368] According to the invention an illumination system is
provided which is insensitive to fluctuations of the pulse sequence
of the primary light source. Moreover the illumination system
according to the invention is characterized by a optimal
telecentricity during all phases of the scan process. In contrast
to that illumination systems of the state of the art consider only
scanning integrated telecentricity.
* * * * *