U.S. patent application number 12/624755 was filed with the patent office on 2010-04-01 for projection objective for microlithography.
This patent application is currently assigned to CARL ZEISS SMT AG. Invention is credited to Vladimer Kamenov, Daniel Kraehmer, Michael Totzeck.
Application Number | 20100079741 12/624755 |
Document ID | / |
Family ID | 39877324 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100079741 |
Kind Code |
A1 |
Kraehmer; Daniel ; et
al. |
April 1, 2010 |
PROJECTION OBJECTIVE FOR MICROLITHOGRAPHY
Abstract
A projection objective for use in microlithography, a
microlithography projection exposure apparatus with a projection
objective, a microlithographic manufacturing method for
microstructured components, and a component manufactured under the
manufacturing method are disclosed.
Inventors: |
Kraehmer; Daniel; (Essingen,
DE) ; Kamenov; Vladimer; (Essingen, DE) ;
Totzeck; Michael; (Schwaebisch Gmuend, DE) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
CARL ZEISS SMT AG
Oberkochen
DE
|
Family ID: |
39877324 |
Appl. No.: |
12/624755 |
Filed: |
November 24, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/EP2008/004081 |
May 24, 2008 |
|
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12624755 |
|
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60940117 |
May 25, 2007 |
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Current U.S.
Class: |
355/71 ; 355/67;
355/77 |
Current CPC
Class: |
G03F 7/70958 20130101;
G03F 7/70591 20130101; G03F 7/70341 20130101; G03F 7/70308
20130101; G03F 7/70941 20130101 |
Class at
Publication: |
355/71 ; 355/67;
355/77 |
International
Class: |
G03B 27/54 20060101
G03B027/54; G03B 27/72 20060101 G03B027/72; G03B 27/32 20060101
G03B027/32 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2007 |
DE |
102007024685.6 |
Claims
1. A projection objective configured to project an image in an
object plane into an exposure field in a field plane, the
projection objective comprising: a plurality of optical elements
including at least one optical element of polycrystalline material,
wherein: the exposure field extends along a scan direction in the
field plane; during use of the projection objective, light in the
exposure filed includes useful light and a stray light component;
the stray light component in the exposure field, averaged over the
scan direction, varies over the exposure field by less than 0.5%
relative to the useful light; and the projection objective is
configured to be used in microlithography.
2. The projection objective according to claim 1, wherein: the
plurality of optical elements are along a light ray path from the
object plane to the field plane; the optical element of
polycrystalline material is a last optical element before the field
plane in a ray direction of the light ray path from the object
plane to the field plane; the polycrystalline material comprises a
material selected from the group consisting of a fluoride, an oxide
of group II, an oxide of group III, an oxide of the rare earths,
garnet and spinel; and the optical element of polycrystalline
material has a stray light component which varies over the exposure
field by more than 0.2% relative to the useful light in the
exposure field.
3. The projection objective according to claim 1, wherein: a
maximum of the stray light component at the exposure field,
averaged over the scan direction, is less than 2% relative to the
useful light; and a maximum of the stray light component outside
the exposure field, averaged over the scan direction, is less than
2% relative to the useful light.
4. The projection objective according to claim 1, wherein at least
one surface of at least one field-proximate optical element has a
surface roughness configured to generate a stray light component
that complements the stray light component of the rest of the
projection objective so that in the exposure field the stray light
component of the projection objective, averaged over the scan
direction, varies over the exposure field by less than 0.5%
relative to the useful light.
5. The projection objective according to claim 4, wherein the at
least one surface includes an optically used area with a center and
a border, the surface roughness of the at least one surface
increases from the center of the optically used area to the border
of the optically used area, and the profile of the surface
roughness of the at least one surface as a function of a lateral
distance from the center of the optically used area corresponds to
a root function of a general polynomial function in which a lateral
distance represents a variable quantity.
6. The projection objective according to claim 5, wherein a
difference in surface roughness of the at least one surface from
the border of the optically used area to the center of the
optically used are is larger than 0.5 nm RMS.
7. The projection objective according to claim 4, wherein the
surface roughness of the at least one surface has a wavelength
range of local undulation between 10 mm and 10 .mu.m.
8. The projection objective according to claim 1, wherein: the
plurality of optical elements are along a light ray path from the
object plane to the field plane, including a last optical element
before the field plane in a ray direction of the light ray path
from the object plane to the field plane; a field aperture stop is
between the last optical element and the field plane; an optically
used area extends in a plane of the field aperture stop; and the
field aperture stop has an allowance for a lateral dimension of
less than 1 mm added to the optically used area in the plane of the
field aperture stop.
9. The projection objective according to claim 4, wherein: the
plurality of optical elements are along a light ray path from the
object plane to the field plane, including a last optical element
before the field plane in a ray direction of the light ray path
from the object plane to the field plane; the last optical element
has an upper side and an underside; relative to the light ray
direction from the object plane to the field plane, the upper side
is before the underside; the underside is before the field plane in
the light ray direction from the object plane to the field plane;
and the surface is the upper side of the last optical element.
10. The projection objective according to claim 1, wherein: the
plurality of optical elements are along a light ray path from the
mask plane to the field plane, including a last optical element
before the field plane in a ray direction of the light ray path
from the object plane to the field plane; the last optical element
has an upper side and an underside; relative to the light ray
direction from the object plane to the field plane, the upper side
is before the underside; the underside is before the field plane in
the light ray direction from the object plane to the field plane; a
field aperture stop is formed of masked off parts of the underside
of the last optical element; the masking-off comprises a coating of
an absorbent or reflective layer; an optically used area extends on
the underside of the last optical element; and the masking-off
includes an allowance for a lateral dimension of less than 0.5 mm
added to the optically used area.
11. A projection objective configured to project an image in an
object plane into an exposure field of a field plane, wherein
during use of the projection objective, an additional stray light
component with a non-constant profile over the exposure field is
present in the field plane, and wherein the projection is
configured to be used in microlithography.
12. A projection objective configured to project an image in an
object plane into an exposure field of a field plane, the
projection objective comprising: a mechanism configured to
introduce into the exposure field in the field plane an additional
stray light component with a non-constant profile over the exposure
field, wherein the projection objective is configured to be used in
microlithography.
13. The projection objective according to claim 12, wherein the
exposure field in the field plane includes a central area and a
border area, and the additional stray light component is lower in
the central area of the exposure field than in the border area of
the exposure field.
14. The projection objective according to claim 12, comprising a
plurality of optical elements arranged along a light ray path from
the object plane to the field plane, including at least one
field-proximate optical element before the field plane in a ray
direction along the light ray path from the object plane to the
field plane, or which in the ray direction from the object plane to
the field plane is arranged immediately before or after an
intermediate object plane that is conjugate to the field plane,
wherein at least one surface of the at least one optical element
has a surface roughness which produces the additional stray light
component with the non-constant profile over the exposure field,
the surface includes an optically used area with a center and a
border, and the surface roughness of the surface increases from the
center of the optically used area to the border of the optically
used area.
15. The projection objective according to claim 14, wherein: a
difference in surface roughness from the border of the optically
used area to the center of the optically used area is larger than
0.5 nm RMS; the surface roughness as a function of a lateral
distance from the center corresponds to a root function of a
general polynomial function in which a lateral distance represents
a variable quantity; and the surface roughness has a wavelength
range of local undulation of between 10 mm and 10 .mu.m.
16. The projection objective according to claim 12, comprising a
plurality of optical elements arranged along a light ray path from
the object plane to the field plane, including a last optical
element before the field plane in a ray direction along the ray
path from the object plane to the field plane is arranged, Wherein
a field aperture stop is between the last optical element and the
field plane, an optically used area extends in a plane of the field
aperture stop, and the field aperture stop has an added allowance
for a lateral dimension of less than 1 mm.
17. The projection objective according to claim 14, comprising a
plurality of optical elements arranged along a light ray path from
the mask plane to the field plane, including a last optical element
before the field plane in a ray direction along the light ray path
from the mask plane to the field plane is arranged, wherein: the
last optical element has an upper side and an underside; relative
to the light ray direction from the object plane to the field
plane, the upper side is before the underside; the underside is
before the field plane in the light ray direction from the object
plane to the field plane; and the surface is the upper side of the
last optical element.
18. The projection objective according to claim 17, wherein the
field aperture stop is formed of masked off parts of the underside
of the last optical element, the masking-off comprises a coating
with of absorbent or reflective layer, an optically used area
extends on the underside of the last optical element, and the
masking-off includes an allowance for a lateral dimension of less
than 0.5 mm added to the optically used area.
19. The projection objective according to claim 12, comprising a
plurality of optical elements including at least one optical
element comprising a polycrystalline material selected from the
group consisting of a fluoride, an oxide of group II, an oxide of
group III, an oxide of the rare earths, garnet or spinel, wherein
the optical element of polycrystalline material has a stray light
component which varies over the field by more than 0.2% relative to
useful light in the exposure field.
20. A method of introducing an additional stray light component of
a projection objective configured to be used in microlithography,
the projection objective configured to project an image in an
object plane into an exposure field of a field plane, the
projection objective comprising at least one field-proximate
surface having a surface roughness, the method comprising: prior to
introducing the additional stray light component, adapting or
altering the at least one field-proximate surface to obtain the
additional stray light component of the projection objective so
that the additional stray light component has a non-constant
profile over the exposure field.
21. The method according to claim 20, comprising: simulating or
measuring the stray light component within the exposure field of
the entire projection objective; and based on the simulation or
measurement, adapting or altering the surface roughness of the at
least one field-proximate surface to obtain the additional stray
light component.
22. The method according to claim 21, wherein the surface roughness
of the at least one field-proximate surface is determined from
measurements taken on a second projection objective that is equal
in design to the first projection objective, and the measurement
results of the second projection objective are carried over to the
first projection objective.
23. A method of generating a stray light component of a projection
objective configured to be used in microlithography according, the
projection objective being configured to project an image in an
object plane into an exposure field of a field plane, and the
projection objective including at least one field-proximate surface
having a surface roughness, the method comprising: prior to
generating the stray light component, adapting or alerting a
surface roughness of at least one field-proximate surface so that
an exposure field in the field plane receives the stray light
component of the projection objective, wherein: the exposure field
extends along a scan direction in the field plane; and the stray
light component, averaged over the scan direction, varies over the
exposure field by less than 0.5% relative to the useful light.
24. The method according to claim 23, comprising: simulating or
measuring the stray light component within the exposure field of
the entire projection objective; and based on the simulation or
measurement, adapting or altering the surface roughness of the at
least one field-proximate surface to obtain the stray light
component.
25. The method according to claim 24, wherein the required surface
roughness of the at least one field-proximate surface is determined
from measurements taken on a second projection objective that is
equal in design to the first projection objective, and wherein the
measurement results of the second projection objective are carried
over to the first projection objective.
26. The method according to claim 23, comprising: simulating or
measuring the stray light component of at least one lens; and based
on the simulation or measurement, adapting or altering the surface
roughness of the at least one field-proximate surface so that an
exposure field in the field plane receives the stray light
component of the projection objective.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of, and claims benefit
under 35 USC 120 to, international application PCT/EP2008/004081,
filed May 21, 2008, which claims benefit of German Application No.
10 2007 024 685.6, filed May 25, 2007 and U.S. Ser. No. 60/940,117,
filed May 25, 2007. International application PCT/EP2008/004081 is
hereby incorporated by reference in its entirety.
FIELD
[0002] The disclosure relates to a projection objective for use in
microlithography, a microlithography projection exposure apparatus
with a projection objective, a microlithographic manufacturing
method for microstructured components, and a component manufactured
under the manufacturing method.
BACKGROUND
[0003] The performance of projection exposure apparatus for the
microlithographic production of semiconductor elements and other
finely structured components is largely determined by the imaging
properties of the projection objectives. Examples for designs of
projection objectives of a projection exposure apparatus which
project an image of a mask into an exposure field can be found in
WO 2004/019128 A2, US 2005/0190435 A1, WO 2006/133801 A1 and US
2007/0024960. These references relate primarily to designs of
projection objectives for immersion lithography, as the technique
is called, wherein an immersion liquid is present between the last
optical element and the wafer which is located in the field plane
of the exposure field. The subject of WO 2004/019128 A2, US
2005/0190435 A1, WO 2006/133801 A1 and US 2007/0024960 in its
entirety, including the claims, is hereby incorporated by reference
in the content of the present application. Furthermore, there are
also designs of projection objectives of a projection exposure
apparatus for applications in so-called EUV (extreme ultraviolet)
lithography, which operate with an operating wavelength of less
than 100 nm and therefore generally cannot use lenses as optical
components, see US 2004/0051857 A1.
SUMMARY
[0004] The term "imaging properties" as commonly understood
encompasses besides the point-to-point imaging properties also
other kinds of imaging properties such as for example the amount of
stray light (hereinafter referred to as the stray light component)
contributed by the projection objective, because the contrast of
the image is affected by it.
[0005] The stray light component of an objective has different
reasons, which are described in: Heinz Haferkorn, "Optik;
Physikalisch-technische Grundlagen and Anwendungen" (Optics,
Physical and Technical Theory and Applications), Fourth Revised and
Expanded Edition; Verlag WileY-VCH, Weinheim; pages 690-694. On the
one hand, there is the kind of stray light which is caused by the
scattering of light at inhomogeneities within a transparent optical
material, and on the other hand the kind of stray light which is
caused by the scattering of light at irregularities of the surfaces
of the optical elements. Besides these two primary causes of stray
light, there are also secondary causes such as for example double
reflections, scattering which takes place at parts of mounting
devices, at borders of aperture stops and at walls, or scattering
caused by undesirable dust particles. The foregoing secondary
causes of stray light are treated in the specialized literature
also under the term "false light". The secondary causes of stray
light can be reduced considerably through a careful layout of the
design, the mounts and aperture stops, as well as through increased
cleanness, blackening of the mount, and the development of
effective so-called anti-reflex coatings. In classic glass melts, a
term which herein is meant to also include the quartz glass for the
projection objectives used in microlithography, the inhomogeneities
inside a transparent optical material can be small enclosed
particles, minor variations of the refractive index, bubbles and
striations. New kinds of optical materials, in particular for
projection objectives used in immersion lithography, are
polycrystalline materials composed of a multitude of individual
crystals of different sizes with hollow spaces of different sizes
lying between them, which will hereinafter also be referred to as
bubbles (see WO 2006/061225 A1). The subject of WO 2006/061225 A1
in its entirety, including the claims, is hereby incorporated by
reference in the content of the present patent application. In the
polycrystalline materials, not only the inhomogeneities in the form
of bubbles are the reason for the stray light, but the base
material itself in the form of individual small crystals causes
stray light. This distinguishes the new materials from the classic
materials, since the basic material of the latter by itself
generally causes no stray light except for small variations of the
refractive index. This and the fact that significantly more bubbles
are present in the new materials than in the classic materials is
the reason why optical elements made of the new kinds of materials
can generate much more stray light than would be generated by
analogous elements made of conventional material. In addition, many
of the new materials consist of crystals that are birefringent, and
a light ray traversing the material therefore sees many changes of
the refractive index due to the different crystallographic
orientations, whereby stray light can be produced again due to the
refractive index variations themselves, as mentioned above. The
many refractive index variations themselves, in turn, have the
effect that the new kind of material itself hardly has a
birefringent effect despite the fact that it consists of many small
crystals of birefringent material.
[0006] The elastic scattering of light of the wavelength .lamda. at
the inhomogeneities inside a transparent optical material can be
treated according to three different cases based on the diameter D
of the scattering centers: [0007] cases where D is small in
comparison to .lamda. are referred to as Rayleigh scattering;
[0008] if D is about as large as .lamda., one speaks of Mie
scattering, and [0009] if D is significantly larger than .lamda.,
this is called geometric scattering.
[0010] In each of these three cases different models are used in
order to describe the elastic scattering of light. In classic
materials the Mie scattering and the geometric scattering occur
with predominance. In the new kinds of materials, none of the
aforementioned kinds of scattering can be disregarded because a
sufficient number of bubbles between the crystals can be very small
and a sufficient number of individual crystals may be very large as
a source of scattering.
[0011] The elastic scattering of light of the wavelength .lamda.
which takes place at irregularities of surfaces is described
through the theory of diffraction at gratings based on the
assumption of a grating whose height equals the quadratic mean
value of the height variation by which the irregularities deviate
from the ideal surface and whose grid period corresponds to the
mean local undulation wavelength of the irregularities. The
quadratic mean value of the height variation of the irregularity
from the ideal surface is also referred to as RMS value (root mean
square value) of the surface roughness.
[0012] When characterizing the measurable qualities of a projection
objective, an analysis as to which cause a measured stray light
component of the projection objective should be attributed to is a
priori impossible. However, a measurable property through which
stray light can be characterized is based on different lateral
penetrations into a shadow range (see WO 2005/015313 and the
references cited therein). Within the scope of conventional
measurement methods, this property is tested by using appropriate
test masks which have dark areas of different lateral diameters. In
images of such masks which are produced by the projection
objective, it is examined how large a portion of stray light is
found in the field of the projection objective at the center of the
shadow range of the respective images of the individual dark areas.
The diameters on the image side for the images of the individual
dark areas as measured in the field plane of the projection
objective are typically 10 .mu.m, 30 .mu.m, 60 .mu.m, 200 .mu.m,
400 .mu.m, 1 mm, and 2 mm. Such measurements are performed at
different field points in order to obtain the distribution of the
stray light component over the exposure field of the projection
objective.
[0013] Stray light which is still able to reach the center of a
shadow range of more than 400 .mu.m diameter has a range of more
than 200 .mu.m and is called long-range stray light, while stray
light which reaches the center of a shadow range of less than 200
.mu.m is referred to as short-range or medium-range stray light.
However, the transition between the terms is fluid so that an
amount of 500 .mu.m for the diameter of the shadow range can serve
equally well as borderline between the terms of long-range or
short/medium-range stray light.
[0014] The stray light stemming from secondary causes for stray
light is normally not very localized or focused in the field plane,
so that at a corresponding field point it normally extends
uniformly over a lateral range larger than for example 0.5 mm. This
stray light belongs accordingly to the long-range stray light and
is thus represented equally in each measurement regardless of the
diameters of the dark areas. This means that the long-range stray
light is always present as a background in a measurement of the
short-range or medium range stray light.
[0015] To quantify the proportion of the stray light which is due
to primary causes through a measure that is not falsified by a
stray light component that is due to secondary causes, the term
"stray light component" as used herein is understood to mean only
that part of the stray light which is obtained as the cumulative
result of the individual measurements of the short-range portion up
to a test diameter of 400 .mu.m, where in each of the individual
measurements of the short-range portion of the stray light the
measurement result is reduced by the value of the stray light
portion from the 1 mm measurement or an equivalent stray light
measurement of the long-range portion. By setting this rule for the
stray light component within the bounds of this application, the
short-range portion of the stray light due to primary causes is
thus set apart from the background of the long-range portion of the
stray light. This clear delineation of the stray light portion due
to primary causes is relevant because the long-range portion of the
stray light due to secondary causes contains the double reflections
which, in turn, depend on the way in which the mask that is to be
projected is illuminated.
[0016] It should also be noted here that as an alternative to the
measurement of the stray light via sensors, the stray light can
also be measured through an exposure method for photoresists, the
so-called Kirk test. In a first step of this test, one determines
the dose desired for the complete exposure of the photoresist, the
so-called clearing dose D.sub.c, and in a second step one
determines the dose D.sub.s involved in an over-exposure of
quadratic structures of different sizes, so that their image in the
photoresist completely disappears.
[0017] The ratio between D.sub.c and D.sub.s now represents a
measure for the relative stray light component of the square-shaped
structure being examined.
[0018] Current projection objectives generally have a stray light
component, according to the rule used herein, of about 1% in
relation to the useful portion of the light, wherein the stray
light component varies by about 0.2% over the image or over the
exposure field. Starting from this, a further reduction of the
stray light component can be achieved through a large development
effort in regard to the material and the surface finish of mirrors
and lenses. It should be noted, however, that projection objectives
using the aforementioned new kinds of optical materials will
according to predictions have a larger stray light component and a
higher variation of the stray light component.
[0019] In some embodiments, the disclosure ensures a good contrast
over the image or over the exposure field in projection objectives
with at least one optical element of polycrystalline material.
[0020] In certain embodiments, the disclosure provides a projection
objective that includes a multitude of optical elements and has at
least one optical element of polycrystalline material. The stray
light component of the projection objective, averaged over the scan
direction, has a variation over the exposure field of less than
0.5%, such as less than 0.2%, in relation to the useful light.
Accordingly, the projection objective has a constant stray light
component in the sense of the present application.
[0021] As used herein, a constant stray light component in the
exposure field, averaged over the scan direction, means a stray
light component for which the difference between the maximum value
in the exposure field and the minimum value in the exposure field
in relation to the useful light is less than 0.5% (e.g., less than
0.2%, less than 0.1%, less than 0.05%), or for which the difference
in relation to the maximum value in the exposure field is less than
60% (e.g., less than 25%, less than 12.5%, less than 6.75%).
[0022] The disclosure makes use of the observation that the
variation of the stray light component of a projection objective
over the exposure field often causes greater problems for the
manufacturers of semiconductor components than a somewhat greater
stray light component of the projection objective would cause by
itself, and that the variation of the stray light component over
the exposure field in a projection objective with at least one
optical element of polycrystalline material exceeds the variation
of the stray light component of many currently used projection
objectives.
[0023] This result of an increased variation of the stray light
component over the exposure field in comparison to many currently
used projection objectives is more pronounced if the last optical
element consists of polycrystalline material. It can make sense
especially in this case to generate a constant stray light
component of the projection objective over the exposure field in
the sense of this application.
[0024] Setting an upper limit for the stray light component of a
projection objective over the exposure field of 2% in relation to
the useful light, takes into account that unrestrictedly large
constant stray light components of a projection objective in the
sense of this application are not compatible with the fabrication
processes used by the manufacturers of semiconductor elements.
Large stray light components can still lead to loss of contrast,
and only small constant stray light components of a projection
objective in the sense of this application, representing a low
percentage of the useful light, are typically acceptable to the
manufacturers of semiconductor elements.
[0025] Of comparable importance is the stray light component of a
projection objective outside of the exposure field, because if it
is too large it leads to undesirable exposures outside of the
exposure field. Setting a maximum of 2% for the stray light
component of a projection objective outside of the exposure field
can represent an acceptable upper limit.
[0026] The concept to introduce additional stray light in
projection objectives with optical elements made of a fluoride, an
oxide of group II, an oxide of group III, rare earth oxides, garnet
or spinel leads to a compensation of the additional profile portion
which the crystals and the bubbles between the crystals contribute
to the profile of the stray light component in the exposure field,
so that the result is a constant stray light component of the
projection objective in the sense of this application over the
entire exposure field.
[0027] The concept of introducing additional stray light in
projection objectives with optical elements of a polycrystalline
material consisting of many crystals that are birefringent leads to
a compensation of the additional profile portion which the many
refractive index fluctuations that occur as a result of the
different orientations of the crystals contribute to the profile of
the stray light component in the exposure field, so that the result
is a constant stray light component of the projection objective in
the sense of this application over the exposure field.
[0028] The concept of introducing additional stray light in
projection objectives with at least one optical element of a
polycrystalline material, so that the result is a constant stray
light component in the sense of this application over the entire
exposure field, where the polycrystalline material exhibits a
lesser degree of birefringence than each of the individual
crystals, is especially important in projection objectives used for
immersion lithography, because in these projection objectives a
material that is nearly free of birefringence is used with
preference especially for the last optical element before the
exposure field.
[0029] The concept of introducing additional stray light in
projection objectives with at least one optical element of a
polycrystalline material, so that the result is a constant stray
light component in the sense of this application over the entire
exposure field, is of particular importance in cases where the
optical element itself already has a stray light component with a
profile variation of more than 0.1% over the exposure field,
because in this case the individual optical element itself exhibits
a variation of the stray light component over the exposure field
which equals about one-half the variation of the stray light
component over the exposure field that is seen in currently used
projection objectives.
[0030] In order to increase the resolution of future projection
objectives used for immersion lithography, it may become desirable
to further increase the numerical aperture NA, i.e. the aperture
angle. However, in order to accomplish this, materials with a
refractive index greater than 1.7 are needed for the last optical
element if the operating wavelength is for example 193 nm. In this
regard, the reader is referred to the discussion of the refractive
index of the last lens element in WO 2006/133,801. With other
operating wavelengths, too, such as for example 157 or 248 nm, it
is sensible to use a material with a high refractive index at the
respective operating wavelength for the last lens element in
projection objectives with a high aperture. The desired properties
for the imaging performance of such future systems, and likewise
the desired properties for the variation of the stray light
component over the exposure field, will probably be higher than for
present systems. The concept to introduce additional stray light in
projection objectives of this kind, so that the result is a
constant stray light component in the sense of this application
over the entire exposure field, takes this anticipated development
into account, as the disclosure also provides the capability to
meet increased future desired properties regarding the constancy of
the stray light component over the exposure field.
[0031] Applying a finishing treatment to at least one surface of at
least one field-proximate optical element represents a simple and
cost-effective way to introduce in a projection objective an
additional stray light component, so that the result is a constant
stray light component of the projection objective in the sense of
this application over the entire exposure field. The finishing
treatment can also be applied to several field-proximate surfaces,
so that the total additional stray light component comes out as the
sum of the stray light contributed by the individual surfaces. This
distribution of the desired surface roughness over several surfaces
can be advantageous if it results for the individual surface in a
roughness value which can be realized simply by omitting the last
polishing step on this surface or on parts of it. Field-proximate
in this context means that surfaces close to an intermediate image
rather than to the exposure field can also be selected for the
finishing treatment. This is particularly advantageous if these
surfaces are easier to work on in regard to their geometry, or if
based on their optical sensitivity in regard to image errors, they
are easier to install or uninstall than the last optical element
immediately before the exposure field. In particular a
planar-parallel plate is favored as an optical element under this
point of view, because the mechanical position tolerances that can
be allowed for a planar-parallel plate are much larger than for
lenses or mirrors. A planar-parallel plate has the additional
advantage that it can also be designed as an easily interchangeable
element and thus offers the possibility that this element can be
exchanged or reworked or altered according to customer
specifications at a later time when the system is in operation.
[0032] Increasing the roughness of a field-proximate surface at the
margin of the optically used area as compared to the center of the
optically used area of a surface is the simplest way of producing
in the exposure field an additional stray light component which has
a profile over the exposure field and is stronger in the border
area than in the central area of the exposure field, so that the
overall result is a constant stray light component in the sense of
this application over the entire exposure field for the projection
objective as a whole. An additional stray light component is
thereby produced which complements the otherwise existing stray
light component of the projection objective in an ideal way, so
that the result is a constant stray light component of the
projection objective in the sense of this application over the
entire exposure field.
[0033] The difference of more than 0.5 nm between the respective
RMS values for the surface roughness at the margin of the optically
used area of a field-proximate surface and the surface roughness at
the center of the optically used area corresponds to an additional
stray light component of about 0.02% in proportion to the useful
light in the exposure field at an operating wavelength of e.g. 193
nm. The difference of 0.5 nm represents about the lower limit for a
value for which it makes sense to correct the stray light component
in the exposure field. The RMS value larger than 2 nm for the
difference in the surface roughness from the border to the center
fills the task of correcting projection objectives currently used
for microlithography with their variation of the stray light
component over the exposure field of 0.2% relative to the useful
light at a wavelength of e.g. 193 by introducing an additional
stray light component with a non-constant profile over the exposure
field in accordance with the disclosure, so that the result is a
constant stray light component of the projection objective in the
sense of this application over the entire exposure field.
Particularly in immersion objectives used for immersion
lithography, where the last lens immediately before the field is
strongly positive, an additional variation of the stray light
component over the exposure area from the border area to the
central area occurs, and to compensate for this variation it makes
sense to use a stronger differentiation of the RMS values of the
surface roughness from the border to the central area. Additionally
increased values for the difference in the RMS values of the
surface roughness are involved if a strongly diffusing material is
used for the last lens.
[0034] A surface roughness profile as a function of a lateral
distance from the center, expressed through a root function of a
general polynomial function in which the lateral distance
represents the independent variable offers the advantage of making
it easier to program the polishing machines, in particular the
polishing robots, because a system of functions is used which is
indigenous or familiar to the machines. Disclosed herein are
relatively simple and fast functions in this category, which allow
an increase of the RMS roughness value at the margin of a surface
to be accomplished in the simplest and fastest possible manner.
[0035] The local range of undulation wavelengths between 1 mm and
10 .mu.m has the advantage that it keeps the amount of so-called
out-of-field stray light small. The out-of-field stray light is
stray light which gets outside the exposure field into areas where
it can cause undesirable exposures. The local range of wavelengths
between 1 mm and 10 mm has the advantage that it not only has an
effect on the stray light but also influences the image-forming
wave front of a field point, so that it is possible with this local
wavelength range to make a simultaneous correction of the wave
front of an arbitrary field point.
[0036] A field aperture stop can help prevent the additional stray
light, which was introduced to achieve the result of a constant
stray light component of the projection objective in the sense of
this application over the entire exposure field, from getting into
areas outside of the exposure field and leading to undesirable
exposures of those areas.
[0037] The dimensional allowance between the field aperture stop
and the optically used area in the plane of the field aperture stop
represents an advantageous compromise between an overly tight
allowance which leads to a high cost due to the high precision
desired in the manufacturing process and an overly large allowance
which leads to too much undesirable stray light outside of the
exposure field.
[0038] An upper side, i.e. the object-facing surface of the last
optical element before the field plane, as seen in the direction of
the light rays from the mask plane to the field plane, is
advantageously suited for introducing the stray light by surface
roughness, as this surface is on the one hand located so close to
the exposure field that by a profile of the surface roughness over
the upper side a profile of the stray light component in the
exposure field can be produced, and that on the other hand the
sub-apertures of the individual field points on the upper side are
still wide enough that small irregularities in the finish of the
upper side have no effect on the image of the respective field
points. Particularly in projection objectives used for immersion
lithography, it is especially advantageous to finish the upper side
of the last optical element because, due to the small difference in
the refractive indices of the lens and the immersion liquid,
finishing or reworking of the underside would involve very large
values for the surface roughness which are difficult to achieve in
practice.
[0039] In projection objectives used for immersion lithography, the
design space between the last optical element and the wafer is too
narrow to allow the use of mechanical aperture stops. The concept
of masking therefore represents the best possible way of realizing
a field aperture stop in immersion systems, which prevents the
additional stray light, which was introduced to achieve the result
of a constant stray light component of the projection objective in
the sense of this application over the entire exposure field, from
getting into areas outside of the exposure field and leading to
undesirable exposures of those areas.
[0040] The masking is realized cost-effectively by a.
[0041] The dimensional allowance between the masking and the
optically used area in the plane of the field aperture stop
represents an advantageous compromise between an overly tight
dimensional allowance which leads to a high cost due to the high
manufacturing precision desired in particular for coating tools and
an overly large allowance which leads to too much undesirable stray
light outside of the exposure field.
[0042] Particularly in immersion objectives for use in immersion
lithography, where the refractive power of the last lens
immediately before the field is strongly positive, this strongly
curved lens alone has the effect that the path lengths traveled by
the light rays through the material differ by a few percent for
rays traversing the border area in comparison to rays passing
through the central area, which results in an additional variation
of the stray light component over the exposure field. This effect
is further increased if strongly diffusing material is used for the
last lens. The concept of introducing additional stray light, so
that the overall result is a constant stray light component of the
projection objective in the sense of this application over the
entire exposure field, is therefore advantageous to reduce the
variation of the stray light component over the exposure field in
projection objectives, which have a last lens of polycrystalline
material with positive refractive power.
[0043] Using a planar-parallel plate as the last optical element
has the advantage that the planar-parallel plate allows for large
mechanical position tolerances in comparison to lenses or mirrors
and that it is thus optically insensitive. This kind of optical
element is therefore advantageous in regard to reworking
operations, as it can be uninstalled from and reinstalled in the
projection objective without major problems. A refinishing
operation at the customer's location is thereby also made possible,
so that an adjustment of the stray light profile according to a
customer's wish becomes feasible. This customer request could be
connected for example with a specific illumination of the mask.
[0044] A surface roughness of a mirror surface has an approximately
16 times stronger effect than an equivalent surface roughness of a
lens in air with a refractive index of about 1.5. It is insofar
advantageous, if large variations of the stray light component over
the exposure field have to be corrected, to use for this purpose a
mirror surface so that the overall result is a constant stray light
component of the projection objective in the sense of this
application over the entire exposure field.
[0045] It is a further object of the disclosure to reduce the
variation of the stray light component over the image or over the
exposure field.
[0046] The disclosure makes use of the observation that the
variation of the stray light component over the exposure field
causes greater problems to the manufacturers of semiconductor
components than the stray light component itself.
[0047] This task can be solved by a projection objective with the
features in which an additional stray light component is introduced
with a non-constant profile over the exposure field, or that a
mechanism is provided in the projection objective for introducing
into the exposure field in the field plane an additional stray
light component with a non-constant profile over the exposure
field. The property of an additional stray light component as
having a non-constant profile over the exposure field in this
context is understood to mean a profile of the additional stray
light component wherein for at least two arbitrary field points
within the exposure field there is a difference of .gtoreq.0.02% in
the additional stray light component in relation to the useful
light portion. Thus, a projection objective is made available for
use in microlithography, serving to project an image of a mask
plane into a field plane and having an exposure field in the field
plane, which is characterized by the fact that besides the existing
stray light component of the projection objective an additional
stray light component is introduced with a non-constant profile
over the exposure field, and/or that the projection objective
includes a mechanism whereby besides the existing stray light
component of the projection objective an additional stray light
component with a non-constant profile over the exposure field is
introduced into the exposure field, so that the variation of the
stray light component over the exposure field is reduced.
[0048] It was further recognized that it makes sense for any
optical body if the stray light component in the border area of the
exposure field is increased in comparison to the central area of
the exposure field in order to equalize over the exposure field the
profile of the stray light component which stems from a homogeneous
light flow of the useful light even if the latter takes place only
in part of the optical body. This entails the precondition that the
optical body consists of a homogeneous material and has
homogeneously finished surfaces, as for example a lens or a
plurality of lenses of a projection objective. Particularly in
immersion objectives for use in immersion lithography, where the
refractive power of the last lens immediately before the field is
strongly positive, this strongly curved lens alone has the effect
that the path lengths traveled by the light rays through the
material differ by a few percent for rays traversing the border
area in comparison to rays passing through the central area, which
results in an additional variation of the stray light component,
with an increased proportion in the central area and a lower
proportion in the border area of the exposure field. This effect is
further increased if strongly diffusing material is used.
[0049] The finishing treatment of at least one surface of at least
one optical element close to the field (also referred to herein as
a field-proximate element) represents a simple and cost-effective
way to introduce in a projection objective an additional stray
light component with a non-constant profile over the exposure
field. The finishing treatment can also be applied to several
field-proximate surfaces, so that the total additional stray light
component comes out as the sum of the stray light contributed by
the individual surfaces. This distribution of the additional
surface roughness over several surfaces can be advantageous if it
results for the individual surface in a roughness value which can
be realized simply by omitting the last polishing step on this
surface or on parts of it. Close to a field (or field-proximate)
means in this context that surfaces close to an intermediate image
instead of close to the exposure field can also be selected for the
finishing treatment. This is particularly advantageous if these
surfaces are easier to work on in regard to their geometry, or if
based on their optical sensitivity in regard to image errors, they
are easier to install or uninstall than the last optical element
immediately before the exposure field. In particular a
planar-parallel plate is favored as an optical element under this
point of view, because the mechanical position tolerances that can
be allowed for a planar-parallel plate are much larger than for
lenses or mirrors. A planar-parallel plate has the additional
advantage that it can also be designed as an easily interchangeable
element and thus offers the possibility that this element can be
exchanged or reworked or altered according to customer
specifications at a later time when the system is in operation.
[0050] Increasing the surface roughness at the margin of the
optically used area as compared to the center of the optically used
area of a surface near a field (also referred to herein as a
field-proximate surface) is the simplest way of producing in the
exposure field an additional stray light component which has a
profile over the exposure field and is stronger in the border area
than in the central area of the exposure field. An additional stray
light component is thereby produced which complements the otherwise
existing stray light component of the projection objective in an
ideal way.
[0051] The difference of more than 0.5 nm between the respective
RMS values for the surface roughness at the margin of the optically
used area of a field-proximate surface and the surface roughness at
the center of the optically used area corresponds to an additional
stray light component of about 0.02% in proportion to the useful
light in the exposure field at an operating wavelength of e.g. 193
nm. The difference of 0.5 nm represents about the lower limit for a
value for which it makes sense to correct the stray light component
in the exposure field. The RMS value larger than 2 nm for the
difference in the surface roughness from the border to the center
fills the task of correcting projection objectives currently used
for microlithography with their variation of the stray light
component over the exposure field of 0.2% relative to the useful
light at a wavelength of e.g. 193 by introducing an additional
stray light component with a non-constant profile over the exposure
field in accordance with the disclosure.
[0052] Particularly in immersion objectives for use in immersion
lithography, where the refractive power of the last lens
immediately ahead of the field is strongly positive, a stronger
variation of the stray light component over the exposure field from
the border area to the central area occurs, as mentioned
previously, where it makes sense to compensate for the variation by
using larger values for the difference in RMS surface roughness
from the border to the center. Additionally increased values for
the difference in the RMS surface roughness are needed if strongly
diffusive material is used for a last lens in this kind of
arrangement.
[0053] The profile of the surface roughness as a function of a
lateral distance from the center according to a function
represented by the root of a general polynomial function in which
the lateral distance is the independent variable offers the
advantage of making it easier to program the polishing machines, in
particular the polishing robots, because a system of functions is
used which is indigenous or familiar to the machines. Relatively
simple and fast functions in this category are disclosed, which
allow an increase of the RMS roughness value at the border of a
surface to be accomplished in the simplest and fastest possible
manner.
[0054] The range of wavelengths of the local undulation between 1
mm and 10 .mu.m has the advantage that it keeps the amount of
so-called out-of-field stray light small. The out-of-field stray
light is stray light which gets outside the exposure field into
areas where it may cause undesirable exposure to light. The local
range of undulation wavelengths between 1 mm and 10 mm has the
advantage that it not only has an effect on the stray light but
also influences the image-forming wave front of a field point, so
that it is possible with this local wavelength range to make a
simultaneous correction of the wave front of an arbitrary field
point. As mentioned above, the local wave length range of a surface
roughness or irregularity is understood within the bounds of this
application to mean the range of the lateral grid periods of the
irregularities along the surface of an optical element.
[0055] A field aperture stop can help prevent the additionally
introduced stray light from getting into areas outside of the
exposure field and leading to undesirable exposures of those
areas.
[0056] The dimensional allowance between the field aperture stop
and the optically used area in the plane of the field aperture stop
represents an advantageous compromise between an overly tight
allowance which leads to a high cost due to the high precision
desired in the manufacturing process and an overly large allowance
which leads to too much undesirable stray light outside of the
exposure field.
[0057] An upper side, i.e. the object-facing surface of the last
optical element, is advantageously suited for introducing the stray
light by surface roughness, because this surface is on the one hand
located so close to the exposure field that by a profile of the
surface roughness over the upper side a profile of the stray light
component in the exposure field can be produced, and because on the
other hand the sub-apertures of the individual field points on the
upper side are still wide enough that small irregularities in the
finish of the upper side have no effect on the image of the
respective field point. Particularly in projection objectives used
for immersion lithography, the finish of the upper side of the last
optical element is especially important because, due to the small
difference in the refractive indices of the lens and the immersion
liquid, finishing or reworking of the underside would lead to large
values for the surface roughness, which would have a negative
effect on the imaging properties of the projection objective or on
the dynamics of the immersion liquid during the scanning
process.
[0058] In projection objectives used for immersion lithography, the
design space between the last optical element and the wafer is too
narrow to allow the use of mechanical aperture stops. The concept
of masking is therefore almost the only possible way in immersion
systems to realize a field aperture stop which prevents the
additionally introduced stray light from getting into areas outside
of the exposure field and leading to undesirable exposures of those
areas.
[0059] The masking is realized cost-effectively by a coating
[0060] The dimensional allowance between the masking and the
optically used area in the plane of the field aperture stop
represents an advantageous compromise between an overly tight
dimensional allowance which leads to a high cost due to the high
manufacturing precision desired in particular for coating tools and
an overly large allowance which leads to too much undesirable stray
light outside of the exposure field.
[0061] The concept of introducing additional stray light is
especially advantageous in projection objectives with optical
elements of polycrystalline material as the polycrystalline
material in these projection objectives causes a stronger variation
of the stray light component over the field than would be the case
in currently used projection objectives.
[0062] The concept of introducing additional stray light in
projection objectives with optical elements made of a fluoride, an
oxide of group II, an oxide of group III, rare earth oxides, garnet
or spinel leads to a compensation of the additional profile portion
which the crystals and the bubbles between the crystals contribute
to the profile of the stray light component in the exposure
field.
[0063] The concept of introducing additional stray light in
projection objectives with optical elements of a polycrystalline
material consisting of many crystals that are birefringent leads to
a compensation of the additional profile portion which the many
refractive index fluctuations that occur as a result of the
different orientations of the crystals contribute to the profile of
the stray light component in the exposure field.
[0064] The concept of introducing additional stray light in
projection objectives with at least one optical element of a
polycrystalline material which exhibits a lesser degree of
birefringence than each of the individual crystals is especially
important for projection objectives used for immersion lithography,
because in these projection objectives a material that is nearly
free of birefringence is used with preference especially for the
last optical element before the exposure field.
[0065] The concept of introducing additional stray light in
projection objectives with at least one optical element of a
polycrystalline material represents a sensible approach in
particular if the optical element itself already has a stray light
component with a profile variation of more than 0.1% over the
exposure field because in this case the individual optical element
itself exhibits a variation of the stray light component over the
exposure field which equals about one-half the variation of the
stray light component over the exposure field that is seen in
currently used projection objectives.
[0066] In particular a last optical element of polycrystalline
material located before the field plane, in reference to the
direction of a light ray from the mask plane to the field plane
leads to a stronger variation of the stray light component of a
projection objective over the exposure field, which needs to be
compensated in accordance with the disclosure, because downstream
of such a field-proximate optical element there is no further
possibility to place aperture stops immediately ahead of the field
plane with the exposure field in order to prevent the stray light
generated by this element from reaching the exposure field.
[0067] In order to increase the resolution of future projection
objectives used for immersion lithography, it will probably be
desirable to further increase the numerical aperture NA, i.e. the
aperture angle. However, in order to accomplish this, materials
with a refractive index greater than 1.7 are needed for the last
optical element if the operating wavelength is for example 193 nm.
In this regard, the reader is referred to the discussion of the
refractive index of the last lens element in WO 2006/133,801 A1.
With other operating wavelengths, too, such as for example 157 or
248 nm, it is sensible to use a material with a high refractive
index at the respective operating wavelength for the last lens
element in projection objectives with a high aperture. The desired
properties for the imaging performance of such future systems, and
likewise the desired properties for the variation of the stray
light component over the exposure field, will probably be higher
than for present systems. The concept to introduce additional stray
light in projection objectives of this kind takes this anticipated
development into account, as the disclosure also provides the
capability to meet increased future desired properties for the
variation of the stray light component over the exposure field.
[0068] Particularly in immersion objectives for use in immersion
lithography, where the refractive power of the last lens
immediately before the field is strongly positive, this strongly
curved lens alone has the effect that the path lengths traveled by
the light rays through the material differ by a few percent for
rays traversing the border area in comparison to rays passing
through the central area, which results in an additional variation
of the stray light component. This effect is further increased if
strongly diffusing material is used for the last lens. The concept
of introducing additional stray light in such projection objectives
is thus helpful in reducing the variation of the stray light
component over the exposure field in projection objectives with a
last lens of positive refractive power.
[0069] Using a planar-parallel plate as the last optical element
has the advantage that the planar-parallel plate allows for large
mechanical position tolerances in comparison to lenses or mirrors
and that it is thus optically insensitive. This kind of optical
element is therefore advantageous in regard to reworking
operations, as it can be uninstalled from and reinstalled in the
projection objective or exchanged for another planar-parallel plate
without major problems. A refinishing operation at the customer's
location is thereby also made possible, so that an adjustment of
the stray light profile according to a customer's wish becomes
feasible. This customer request could be connected for example with
a specific illumination of the mask.
[0070] A surface roughness of a mirror surface has an approximately
16 times stronger effect than an equivalent surface roughness of a
lens in air with a refractive index of about 1.5. It is insofar
advantageous, if large variations of the stray light component over
the exposure field have to be corrected, to use for this purpose a
mirror surface.
[0071] A further object of the disclosure is to provide a method of
reducing the variation of the stray light component of a projection
objective in the exposure field.
[0072] This task can be solved by a method in which additional
stray light with a non-constant profile over the exposure field is
introduced by an advance adaptation or an alteration of the surface
roughness of at least one field-proximate surface.
[0073] It was recognized in the disclosure that a method in which
the surface roughness of at least one field-proximate surface is
adapted in advance according to a specified profile over the
surface or altered in a way that is targeted to achieve the
specified profile represents a suitable way to produce an
additional stray light component which results in an overall
reduction of the variation of the stray light component over the
exposure field.
[0074] A method in which a simulation or a measurement is used to
determine the stray light component that is to be expected or is
present within the exposure field of the entire projection
objective, offers the possibility to determine the desired surface
roughness profile over the at least one field-proximate surface in
a way that is very specifically targeted and to realize the desired
profile in an equally target-oriented way by pre-adapting or
altering the surface roughness.
[0075] A method can offer the advantage of taking the measurements
on another projection objective of the same design, for example on
a prototype, instead of measuring the projection objective itself,
and to transfer the results to the projection objective for making
the correction. This saves expensive and risky corrective steps in
the manufacturing process, where the already completed projection
objective has to be disassembled again, i.e. the steps of measuring
the projection objective, uninstalling the surface that needs to be
changed, reworking the surface, and reinstalling the reworked
surface. By transferring the measurement results for example from a
prototype, the desired surface roughness can be preset or adapted
in advance already during the production of the optical elements of
the projection objective.
[0076] A method can have the advantage that measurements which were
already made in the production of the individual optical components
can be used for determining the stray light component to be
expected for the entire projection objective, so that the surface
roughness of the at least one field-proximate surface can be
adapted in advance already in the production of the respective
optical element, without having to disassemble the projection
objective again at a later stage of the production process.
[0077] If no measurements are performed on the optical components
in regard to the stray light component to be expected, the method
can offer the advantageous possibility to take such measurements on
the blanks of the lenses. The blanks can all be measured with one
and the same measurement setup, while lenses may in some cases
involve different measurement setups, depending on the geometry of
the lens. Performing a measurement regarding the existing stray
light component on the blanks offers insofar a significant cost
advantage over a measurement of the stray light component that is
performed in any of the subsequent production steps.
[0078] A method can be very cost-effective, because for the
determination of the stray light component of the projection
objective only at least one lens is measured or simulated for the
advance adaptation or subsequent alteration of the surface
roughness of the at least one field-proximate optical surface,
rather than making measurements on an entire projection objective
which would involve a more extensive measuring apparatus. This is
of particular interest for projection objectives used for immersion
lithography with a last lens of polycrystalline material, where
this individual lens alone already contributes a large portion of
the stray light component of the projection objective and where the
attention is focused on correcting this particular contribution to
the stray light component in accordance with the disclosure.
[0079] In comparison to certain methods it is advantageous to use
some methods because only a single measurement needs to be made,
e.g. on a lens prototype consisting, e.g., of polycrystalline
material, in order to make the correction in all projection
objectives that contain such a lens without the need to measure
each of the individual lenses by itself. It is also possible with
certain methods to perform a random sample examination within the
scope of a quality assurance program, wherein for the determination
of the desired surface roughness of the at least one
field-proximate surface a second lens is measured which is of
identical design as the first lens of the projection objective and
the results of the measurements from the second lens are applied to
the first lens.
[0080] A method is disclosed in which instead of measuring a lens,
the measurements are made on the blank from which the lens will be
made, in order to obtain from the measurement results of the blank
the data for the advance adaptation or subsequent alteration of the
surface roughness of the at least one field-proximate surface, so
that one obtains as a result the additional stray light component
with the non-constant profile over the exposure field of the
projection objective. This method is simple and cost-effective
because a suitable measurement setup for a blank can be realized in
a simpler and more cost-effective way than a corresponding
measurement setup for a completed lens or an entire objective.
[0081] It is a further object of this disclosure to provide a
method of introducing additional stray light by a preemptive
adaptation or subsequent alteration of the surface roughness of at
least one field-proximate surface, so that as a result the stray
light component of the projection objective, averaged over the scan
direction, varies over the exposure field by less than 0.5%, in
particular less than 0.2%, in relation to the useful light, and
accordingly a constant stray light component of the projection
objective in the sense of this application is achieved.
[0082] It was recognized in the disclosure that a method in which
the surface roughness of at least one field-proximate surface is
adapted in advance according to a specified profile over the
surface or changed in a way that is targeted to achieve the
specified profile represents a suitable way to produce an
additional stray light component which results in an overall
reduction of the variation of the stray light component over the
exposure field, so that a constant stray light component of the
projection objective in the sense of this application is
achieved.
[0083] A method is disclosed in which a simulation or a measurement
is used to determine the stray light component that is to be
expected or is present within the exposure field of the entire
projection objective, offers the possibility to determine the
desired surface roughness profile over the at least one
field-proximate surface in a way that is very specifically targeted
and to realize the desired profile in an equally target-oriented
way by adapting it in advance or changing it, so that a constant
stray light component of the projection objective in the sense of
this application is achieved.
[0084] A method is disclosed that offers the advantage of taking
the measurements on another projection objective of the same
design, for example on a prototype, instead of measuring the
projection objective itself, and to transfer the results to the
projection objective for making the correction. This saves
expensive and risky corrective steps in the manufacturing process,
where the already completed projection objective has to be
disassembled again, i.e. the steps of measuring the projection
objective, uninstalling the surface that needs to be changed,
reworking the surface, and reinstalling the reworked surface. By
transferring the measurement results for example from a prototype,
the desired surface roughness can be preset or adapted in advance
already during the production of the optical elements of the
projection objective, so that a constant stray light component of
the projection objective in the sense of this application is
achieved.
[0085] A method is disclosed that is very cost-effective, because
for the determination of the stray light component of the
projection objective only at least one lens is measured or
simulated for the advance adaptation or the alteration of the
surface roughness of the at least one field-proximate optical
surface in order to achieve a constant stray light component of the
projection objective in the sense of this application, rather than
making measurements on an entire projection objective which would
involve a more extensive measuring apparatus. This is of particular
interest for projection objectives used for immersion lithography
with a last lens of polycrystalline material, where this individual
lens alone already contributes a large portion of the stray light
component of the projection objective and where the attention is
focused on correcting this particular contribution to the stray
light component in accordance with the disclosure in order to
achieve a constant stray light component of the projection
objective in the sense of this application over the entire exposure
field.
[0086] In comparison to the method according to some methods it is
advantageous to use certain methods because only a single
measurement needs to be made, e.g. on a lens prototype consisting,
e.g., of polycrystalline material, in order to make the correction
in all projection objectives that contain such a lens so that a
constant stray light component of the projection objective in the
sense of this application is achieved without the need to measure
each of the individual lenses by itself. It is also possible with
certain methods to perform a random sample examination within the
scope of a quality assurance program, wherein for the determination
of the desired surface roughness of the at least one
field-proximate surface a second lens is measured which is of
identical design as the first lens of the projection objective and
the results of the measurements taken from the second lens are
applied to the first lens, so that a constant stray light component
of the projection objective in the sense of this application is
achieved.
[0087] A method is disclosed in which instead of measuring a lens,
the measurements are made on the blank from which the lens will be
made, in order to obtain from the measurement results of the blank
the data for the advance adaptation or the alteration of the
surface roughness of the at least one field-proximate surface, so
that one obtains as a result the additional stray light component
with the non-constant profile over the exposure field of the
projection objective. This method is simple and cost-effective
because a suitable measurement setup for a blank can be realized in
a simpler and more cost-effective way than a corresponding
measurement setup for a completed lens or an entire objective.
[0088] It is a further object of the disclosure to provide a
projection exposure apparatus with a projection objective, also to
provide a microlithographic manufacturing method which can be
performed with the projection exposure apparatus, and further to
describe a component which can be manufactured with the apparatus
and method.
[0089] The disclosure provides a projection exposure apparatus, a
manufacturing method, and a component.
BRIEF DESCRIPTION OF THE DRAWINGS
[0090] Examples of embodiments of the disclosure are hereinafter
presented in more detail with references to the drawing,
wherein
[0091] FIG. 1 is a schematic representation of an exposure field in
the field plane of a projection objective for microlithography
applications which is used as a scanner, including the distribution
of the useful light relative to two orthogonal axes (X- and
Y-axis);
[0092] FIG. 2 is a schematic representation of an exposure field in
the field plane of a projection objective for microlithography
applications which is used as a scanner and has a so-called
off-axis field of rectangular shape;
[0093] FIG. 3 is a schematic representation of an exposure field in
the field plane of a projection objective for microlithography
applications which is used as a scanner and has a so-called ring
field;
[0094] FIG. 4 is a schematic representation of an exposure field in
the field plane of a projection objective for microlithography
applications which is used as a stepper and has a square field;
[0095] FIG. 5 shows a schematically simplified sectional view of a
projection objective and a substitute model for the projection
objective in the form of a homogeneous glass cylinder serving to
explain the resultant natural stray light distribution;
[0096] FIG. 6 shows a schematic representation of an image-forming
light ray pattern of a projection objective according to geometric
optics to illustrate the concepts of field and pupil;
[0097] FIG. 7 represents a graph of the profile of the stray light
component in percent in relation to the useful light of a
projection objective for microlithography applications, averaged
over the scan direction Y, along the field in the X-direction;
[0098] FIG. 8 schematically represents the optical components of a
projection exposure apparatus for immersion lithography;
[0099] FIG. 9 represents a plan view of a polycrystalline material
with it microscopic structures;
[0100] FIG. 10 represents a graph of a model-dependent stray light
component, expressed in percent relative to the useful light, of a
polycrystalline material as a function of the average crystal
size;
[0101] FIG. 11 represents a graph of a model-dependent stray light
component, expressed in percent relative to the useful light, of a
polycrystalline material as a function of the average bubble
size;
[0102] FIG. 12 represents a sketch to illustrate principal concepts
in a lens and graphs to explain, respectively, the scattering at
inhomogeneities in a polycrystalline material of a last lens and
the concept of adapting the surface roughness of a last lens as
well as the resultant distribution of stray light over the
field;
[0103] FIG. 13 represent a graph of a corrected profile of the
stray light component, expressed in percent relative to the useful
light, of a projection objective for microlithography applications,
averaged over the scan direction Y along the field in the
X-direction;
[0104] FIG. 14a-b represents a sectional view in the Y-Z plane of
the optical components of a so-called two-mirror design of a
projection objective for immersion lithography with a numerical
aperture larger than 1;
[0105] FIG. 15 represents a sectional view in the Y-Z plane of the
optical components of a so-called four-mirror design of a
projection objective for immersion lithography with a numerical
aperture of 1.2;
[0106] FIG. 16 represents a sectional view in the Y-Z plane of the
optical components of a so-called RCR design of a projection
objective for immersion lithography with a numerical aperture of
1.25;
[0107] FIG. 17 represents a sectional view in the Y-Z plane of the
optical components of a further two-mirror design of a projection
objective for immersion lithography with a numerical aperture of
1.75;
[0108] FIG. 18 schematically illustrates the last lens element
before the field plane of the two-mirror design of FIG. 17;
[0109] FIG. 19 represents a sectional view of the optical
components of a so-called six-mirror design of a projection
objective for EUV lithography;
[0110] FIG. 20 represents a graph of a possible distribution of the
surface roughness over the optically used area of a surface of a
field-proximate optical element;
[0111] FIG. 21 represents a flowchart diagram of several possible
process steps for producing in a projection objective a corrected
stray light component;
[0112] FIG. 22 represents a flowchart diagram for a method of
producing microstructured semiconductor elements by a projection
exposure apparatus with a projection objective in accordance with
the present patent application.
DETAILED DESCRIPTION
[0113] FIG. 1 shows the exposure field 15 in the field plane of a
projection objective for microlithography applications which is
used as scanner, including the distribution of the useful light
along the X- and Y-axes. In FIG. 1, the field plane in which the
exposure field 15 is located is seen in plan view, meaning that the
plane of the paper coincides with the field plane. Further in FIG.
1, a coordinate system is defined in the field plane in accordance
with the rule that for so-called scanners the scan direction should
be oriented in the Y-direction. In so-called scanners, the mask
structure of a microstructured component is not transferred in its
entirety in one exposure step by the projection objective onto a
so-called wafer, because the image of the entire mask structure is
too large for the maximum image field 1 of a projection objective.
Instead, the mask structure is gradually moved through the object-
or mask plane of the projection objective in a scanning process,
while the wafer is moved at the same time in a synchronized
movement through the image- or field plane. In conventional
rotationally symmetric projection systems, which have refractive
elements exclusively, the maximum image field 1 in the field plane
is a circle whose center is defined by the optical axis 3 of the
projection system. By field aperture stops which are located in the
illumination system, the so-called REMA (reticle-masking) blades,
the maximum image field 1 is trimmed back to the rectangular
exposure field 15 whose center is defined by the optical axis 3 of
the objective. The REMA blades have the additional function at the
beginning and end of a scanning process, respectively, to retract
and deploy themselves over the exposure field 15. The center of the
exposure field 15 is formed by a central area 5 which is shaded in
FIG. 1. The border areas (also referred to herein as marginal
areas) 7 and 9 of the exposure field, which are likewise shaded in
FIG. 1, are those border areas 7 and 9 of the rectangular exposure
field which form the left and right margins of the exposure field
in the direction perpendicular to the scan direction. In the scan
direction, the front edge 11 and the rear edge 13 of the exposure
field 15 are the lines between which the exposure field 15 is
located and between which the light projected by the objective has
an intensity larger than zero. The respective intensity
distribution profiles of the useful light in the scan direction and
perpendicular to the scan direction are different from each other,
as shown in the two diagrams in FIG. 1. In the scan direction, the
intensity distribution profile of the useful light is adjusted so
that it takes on a value of zero at the front edge 11 and the rear
edge 13 and has its maximum within the central area 5. The exact
intensity distribution between these two points is selected so that
in the scanning process every partial area of a microstructured
component receives a nearly equal portion of the light. This would
be impossible to achieve with a so-called top hat profile, i.e. a
rectangular distribution profile in the scan direction, because a
pulsed laser is typically used as a light source and it could not
be ruled out in this case that one partial area of the
microstructured component would receive light from one more laser
pulse than another partial area, with 5 to 7 laser pulses per
partial area or per exposure field being typical. With an intensity
distribution in the scan direction which continuously increases
towards the central area 5 from a value of zero at the front edge
11 and at the rear edge 13, such intensity effects on the
microstructured components are suppressed.
[0114] In contrast, the intensity distribution perpendicular to the
scan direction is a so-called top hat distribution or rectangular
distribution over the exposure field 15, with the same intensity
value for the central area 5, the border areas 7 and 9 and all
field points lying in between along a line that is perpendicular to
the scan direction. Insofar, the shape of this intensity
distribution also does not change if it is averaged over the scan
direction. This intensity distribution, averaged over the scan
direction and expressed in percent relative to the useful light is
represented by the diagram in the bottom part of FIG. 1. This
averaged intensity distribution has the same value of 100% relative
to the useful light for the central area 5 as for the border area
7.
[0115] The stray light component defined according to the measuring
rule stated above is understood herein as a stray light component
that is averaged over the scan direction and expressed as a
relative amount in proportion to the useful light or, in other
words, as a relative amount in proportion to the 100% value of the
intensity distribution in the scan direction as illustrated in FIG.
1.
[0116] The exposure field 15 of a scanner typically measures 20 to
30 mm perpendicular to the scan direction and 5 to 10 mm in the
scan direction. Together with these dimensions, the central area 5
of the exposure field 15 should not exceed a diameter of 4 mm, and
the border areas 7 and 9 of the exposure field 15 should not exceed
a width of 2 mm perpendicular to the scan direction, as these areas
should only occupy small surface portions immediately at the center
and at the border of the exposure field 15 without spreading out
over major portions of the exposure field 15.
[0117] FIG. 2 shows the exposure field 45 in the field plane of a
projection objective for microlithography applications which is
used as a scanner and has a so-called off-axis field 45 of
rectangular shape as exposure field 45. The elements in FIG. 2
which are analogous to those in FIG. 1 have the same reference
numerals raised by 30. Such rectangular off-axis fields 45 as
exposure fields 45 of a projection objective are typical in
projection objectives which have at least one catadioptric partial
objective. The attribute "catadioptric" means here that besides
refractive elements such as for example lenses, there are also
reflective elements such as for example mirrors being used as
elements which contribute to the formation of the image and thus
carry refractive power. Due to the folded ray path of these
systems, the exposure field 45 is offset relative to the optical
axis 33 and the maximum image field 31 of these systems. When
referring to the optical axis 33 and the maximum image field 31 in
this context, this does not imply that the optical axis 33 as well
as the entire maximum image field 31 can be covered in the
projected image of these catadioptric projection objectives. It
only indicates that many of these catadioptric projection
objectives can still be described in terms of rotational symmetry
in regard to their design, even though the ray propagation pattern
used in the completed objective is not folded with rotational
symmetry relative to the optical axis 31 and the physical shapes of
some of the optical elements are no longer rotationally symmetric
relative to the optical axis 31. Examples for the design of a
catadioptric projection objective with a rectangular off-axis field
45 as exposure field 45 are presented in US 2005/0190435 A1, WO
2004/019128 A2 and WO 2006/133801 A1, as well as in FIGS. 14, 16
and 17 of the present patent application. What has been the above
in the context of FIG. 1 about the intensity distribution in the
scan direction and perpendicular to it is also directly applicable
to the rectangular off-axis field 45 and therefore needs no further
explanation. Rectangular off-axis fields 45 of catadioptric
projection objectives have about the same size as exposure fields
15 of purely refractive projection objectives. Catadioptric
projection objectives are used primarily for immersion lithography
because even with the large numerical aperture values (NA) of more
than 1 of an immersion objective, catadioptric projection
objectives allow the lens- and mirror diameters to be kept
relatively small in comparison to a purely refractive design.
[0118] FIG. 3 shows the exposure field 65 in the field plane of a
projection objective for microlithography applications which is
used as a scanner and has a so-called ring field 65 as exposure
field 65. The elements in FIG. 3 which are analogous to those in
FIG. 1 have the same reference numerals raised by 50. Such ring
fields 65 are typical for catadioptric objectives of a design that
does not allow for a folded light ray path that would lead to a
rectangular field. What has been the above in the context of FIG. 1
about the intensity distribution in the scan direction and
perpendicular to it is also directly applicable to the ring field
65 and therefore needs no further explanation. The intensity
distribution in the scan direction can differ from the intensity
distribution shown in FIG. 1 insofar as with different heights in
the X-direction the resultant distribution is not the same for all
intensity distributions in the scan direction. However, this is of
no consequence, and it would also be of no consequence if it
occurred in a system with a rectangular field 15, 45, as all
scanner systems are always designed so that regardless of the shape
of the intensity distribution along the scan direction, one always
obtains an intensity distribution perpendicular to the scan
direction which, when averaged over the scan direction, conforms to
a top-hat profile or rectangular profile of the type illustrated in
the lower part of FIG. 1. Ring fields 65 of catadioptric projection
objectives have about the same dimension perpendicular to the scan
direction as the dimension perpendicular to the scan direction of
exposure fields 15 of purely refractive projection objectives.
[0119] FIG. 4 shows the exposure field 85 in the field plane of a
projection objective for microlithography applications which is
used as a stepper and has a square-shaped field 85 as exposure
field 85. The elements in FIG. 4 which are analogous to those in
FIG. 1 have the same reference numerals raised by 70. In contrast
to a scanner, a stepper functions in such a way that the mask
structure for the semiconductor element to be produced, which is
located in the object- or mask plane of the projection objective,
is projected in its entirety, i.e. without a scanning process, into
the exposure field 85 in the field plane. However, this involves
the projection objective providing larger exposure fields 85 than
in the case of scanners. As an alternative for the large exposure
fields 85 in the case of steppers, the semiconductor element can be
exposed sequentially in a stepper in individual portions, using a
so-called stitching technique. In this case, it is also possible to
use smaller exposure fields 85 than in the case of scanners. The
exposure field 85 in steppers can arbitrarily be made larger and
smaller in the X-direction as well as the Y-direction by the REMA
blades in the illumination system. The intensity distribution over
the exposure field 85 in steppers is completely homogeneous, so
that the resultant distribution has a top-hat- or rectangular
profile in the X-direction as well as in the Y-direction. To ensure
that the steppers can be compared to the scanners within the scope
of this patent application, border areas 77 and 79, located to the
right and left at the borders of the stepper field perpendicular to
the Y-direction. Furthermore, analogous to the scanners described
herein, the intensity distribution in the X-direction is averaged
over the Y-direction, which results in a top-hat distribution of
the kind shown in the lower part of FIG. 1, with the same intensity
value of 100% of the useful light for the central areas 5 and 75,
respectively, as for the border areas 7 and 77, respectively. To
maintain the comparability with scanners, the stray light component
of steppers is likewise defined as being averaged along the
Y-direction.
[0120] FIG. 5 presents a schematic illustration of a projection
objective 103 and also a substitute model of a projection objective
as a homogeneous glass cylinder 111 serving to explain the natural
stray light distribution which occurs as a result in the field
plane 105. In the upper part of FIG. 5, a schematic representation
of a projection objective 103 is indicated by four lenses 109 along
an optical axis 113. This projection objective 103 has the function
of projecting an image of a mask 101 which is located in a mask
plane into a field plane 105. The mask to be projected is
homogeneously illuminated for this purpose by light 107 from an
illumination system which is not shown in the drawing. The
illumination system is capable of changing the angular distribution
of the incident light rays 107 falling homogeneously on the mask
101, without thereby changing the intensity distribution over the
mask. This makes it possible to have different so-called settings
available for the semiconductor manufacturer, which can be
described in terms of the theory of partially coherent images and
which have the purpose that certain structures on the mask 101 can
be projected into the smallest possible image size.
[0121] The lower part of FIG. 5 represents, as a substitute model
for the projection objective 103, a homogeneous glass cylinder 111
which is homogeneously illuminated by the light rays 107 which fall
homogeneously on the mask 101. A glass cylinder 111 of this kind,
which is homogeneously illuminated over its cross-sectional area,
will generate equal amounts of stray light within equal-sized
surface elements of the cross-sectional area. If the glass cylinder
111 from the mask 101 to the field plane 105 along the optical axis
113 is looked at as a series of many such homogeneously illuminated
cross-sectional areas wherein the overall intensity of the
illumination decreases along the optical axis 113 from the mask 101
to the field plane 105 due to absorption and scattering, one
obtains a stray light component in the field plane 105, averaged
over the scan direction Y, which conforms to the diagram at the
lower right of FIG. 5. Due to the fact that each of the equal-sized
surface elements of each cross-sectional area generates an equal
amount of stray light, the proportion of stray light is higher in
the central area 115 of the exposure field of the field plane 105
than in the border area 117 of the exposure field (as illustrated
in the diagram at the lower right of FIG. 5), because the central
area 115 receives the stray light of more mutually adjacent surface
elements of each cross-sectional area than does the border area
117. This profile of the stray light component over the exposure
field as illustrated in the lower right-hand part of FIG. 5, which
results from the homogeneous illumination of a cylindrical glass
body, will be referred to hereinafter as the natural profile of the
stray light component.
[0122] FIG. 6 shows the image-forming light ray pattern of a
projection objective according to the principles of geometric
optics to illustrate the concepts of field and pupil. The
projection objective 123 in FIG. 6 is shown as a so-called 4f
system consisting in this schematically simplified representation
of two lenses 129, between the latter a pupil plane 133, and two
field-proximate planes 135, 137 in which the lenses 129 are
located. The projection objective projects an image of the mask
121, which is homogeneously illuminated by the light rays 127,
along the optical axis 131 into the field plane 125. To explain the
image-projecting light ray pattern, three specific ray paths are
shown for the axis point of the mask 121, i.e., the principal ray
139 along the optical axis 131, the upper aperture ray or coma ray
141, and the lower aperture ray or coma ray 143. These aperture
rays or coma rays are those rays which leave the axis point at the
maximum possible angle at which they can still be projected into an
image by the projection objective. Also shown is the path of the
principal ray 149 for the outermost field point to be projected by
the projection objective. The pupil is defined as the area at whose
center the principal rays 139, 149 of all field points intersect
each other and whose size is determined by the aperture rays 141,
143. Thus, the pupil does not necessarily always have to be in a
pupil plane 133 as shown in FIG. 6, but a representation like the
one in FIG. 6 facilitates the explanation of the optical concepts
of field and pupil. The pupil plane 133 according to FIG. 6 is
therefore the location relative to the light propagation direction
or Z-direction where the principal rays 139, 149 of the field
points meet each other. Since a principal ray 139 coincides with
the optical axis, the pupil in FIG. 6 also is the location where
all principal rays 139, 149 of the field points intersect the
optical axis. The principal rays 139, 149 of the field points thus
have no height, or distance from the optical axis, in the pupil.
The aperture rays 141, 143, on the other hand, define the border of
the pupil and thus have the maximum height, or maximum distance
from the optical axis, of all possible rays in the pupil. The
height, or distance from the optical axis, of the rays thus
represents a suitable criterion as to whether an optical element in
an objective can be referred to as being near a pupil
(pupil-proximate) or near a field (field-proximate). If the height
or distance of an aperture ray 141, 143 of the axis point, or
central field point, at a surface of an optical element is more
than six times the height of the principal ray of the outermost
projectable field point on the same surface, then the optical
element will be referred to herein as being near a pupil (or
pupil-proximate), otherwise it will be referred to herein as being
near a field (or field-proximate), wherein in so-called RCR designs
(refractive-catadioptric-refractive designs) the reference for the
distance of the rays in the elements of the Schupmann group G20
(see FIG. 16) is their optical axis. Based on this criterion, it is
clear that the two lenses 129 in FIG. 6 are located, respectively,
in field-proximate planes 135 and 137. Furthermore, field and pupil
are related to each other through a spatial Fourier transform
wherein the height, or distance from the optical axis 131, of an
image-forming ray 139, 141, 143, 149 in the field corresponds to
the angle between the image-forming ray 139, 141, 143, 149 and the
optical axis in the pupil. At the same time, the inverse
relationship also holds, i.e., the angle between the image-forming
ray 139, 141, 143, 149 and the optical axis in the field
corresponds to the height, or distance from the optical axis 131,
of the image-forming ray 139, 141, 143, 149 in the field. In other
words, the path of the principal ray 149 of the outermost field
point that can be projected has its maximum height, or greatest
distance from the optical axis 131, in the image plane of the mask
121, with an angle of zero relative to the optical axis 131. The
same ray path 149 crosses the optical axis 131 at the center of the
pupil plane 133 with the maximum angle of intersection, i.e. the
height of the ray from the optical axis 131 is minimal at this
point, while the angle relative to the optical axis 131 is maximal.
Conversely, the aperture rays have their smallest heights and
largest angles relative to the optical axis 131 in the image plane
of the mask 121 and the field plane 125, while their greatest
heights and smallest angles relative to the optical axis 131 occur
in the pupil plane 133. Based on this special relationship between
field and pupil, it is possible to perform interventions into the
light distribution in the pupil which have a uniform effect on
every field point of the field. The simplest possibility is for
example to constrict the pupil with an aperture stop, so that all
field points are lacking rays whose angle in the field is larger
than the maximum possible aperture angle allowed by the constricted
pupil.
[0123] By an illumination system, the light rays 127 which are
falling homogenously on the mask 121 are adapted in regard to their
angular distribution relative to the optical axis in order to meet
customer's desired properties that specify so-called illumination
settings, so that different areas with different intensities are
formed in the pupil of the projection objective, whereby lenses
near a pupil of the projection objective are illuminated
differently depending on the illumination setting. For example, an
annular setting in combination with a suitable mask structure has
the consequence that lenses near a pupil are receiving light only
in border areas of the optically usable part of the lens. For an
explanation of the working principle of the illumination settings
in combination with the mask structures, the reader is referred to
the pertinent literature concerning the theory of partially
coherent images of objects that are not self-luminous.
[0124] In the relationship between pupil, specifically lenses near
a pupil, and stray light it is important that due to the three
causes of Rayleigh scattering, Mie scattering and geometric
scattering, the elastic scattering of light of the wavelength
.lamda. which occurs at the inhomogeneities of the glass material
always produces an angular distribution that is symmetric around
the direction of the useful light ray. This means that for field
points at the border of the field, whose principal rays are
strongly angled in the pupil, and for a conventional setting with a
small sigma value (which is a setting in which only the central
area of the pupil, i.e. the area traversed by the principal rays,
is being used), the resultant angular distributions of the stray
light in pupil-proximate lenses are oriented outwards to the
housing of the objective and away from the optical axis, so that on
the way from the pupil to the field, stray light is absorbed by the
housing of the objective and by the lens mounts. The result of this
is a stray light component profile over the field which, due to the
stray light absorption, has a lower value in the border area 147 of
the exposure field than in the rest of the exposure field. For an
annular setting on the other hand, which uses the border area of
the pupil and thus the area traversed by the aperture rays, there
is overall only an insignificant difference in the angles of
inclination of the aperture rays between field points of the border
area and field points of the central area, but due to the proximity
of the border area of the pupil to the housing of the objective,
the part of the stray light that is scattered in the pupil under a
large angle is absorbed most strongly. Since large angles in the
pupil translate according to the Fourier transform into large
heights in the field, the stray light that is scattered in the
pupil under a large angle is subject to absorption in the housing
of the objective and therefore lacking in the border area 147 in
comparison to the central area 145 of the exposure field.
Accordingly, an annular illumination setting in particular (i.e. a
setting where the light rays 127 fall on the mask 121 with
rotational symmetry at angles of incidence within a narrowly
defined angular range) does not lead to a profile of the stray
light component that is qualitatively different from the profile
obtained with a conventional setting. Consequently, that part of
the variation of the stray light component averaged in the scan
direction which occurs as a result of different settings can
overall be considered negligible in relation to the amount by which
the stray light component, averaged in the scan direction,
according to the measurement rule used herein varies over the
field.
[0125] In projection objectives for immersion lithography, the last
lens with its strongly positive refractive power has the result
that the path lengths in the optical material are different for
different field points. The relative path length difference of all
image-forming rays of a field point in the border area of the
exposure field in comparison to all image-forming rays of the
central field point of the exposure field for such a lens alone can
amount to a few percent. Consequently, since the stray light
component due to inhomogeneities in the glass material depends
directly on the path length traveled in the glass material by the
useful light, this leads particularly in strongly scattering
material to a resultant stray light component profile over the
field with a lower value in the border area 147 of the exposure
field than in the central area 145.
[0126] In the context of FIGS. 5 and 6, a total of three different
effects have been discussed, all of which lead to a stray light
component, averaged over the scan direction, wherein the profile
over the exposure field has a stronger stray light component in the
central area 145 than in the border area 147 of the exposure field,
as illustrated in the right-hand part of FIG. 6. All of these three
effects result from the primary stray light due to elastic
scattering of light at inhomogeneities in the glass material and
are, respectively, the natural stray light profile of a
homogenously illuminated glass body, the stray light profile of the
lenses near a pupil, and the stray light profile due to the
differences in path length in strongly positive field lenses.
[0127] In addition to the effects just mentioned, which are due to
the primary cause of stray light, i.e. the elastic scattering of
light at inhomogeneities in the glass material, there is the
superimposed stray light which is due to the scattering of light at
surface irregularities which, as mentioned above, represents a
second primary cause of stray light. The lenses are usually
polished to a uniform finish quality on all parts of the surface
and consequently, the above train of reasoning that the
image-forming ray paths of field points from the border area of the
field are overall more strongly inclined relative to the optical
axis and relative to the refractive surfaces than the image-forming
ray paths of field points from the central area, in combination
with the fact that the angular distribution of the stray light is
rotationally symmetric to the direction of the useful light also in
the case of surface scattering, leads to the conclusion that the
scattering at the surface irregularities likewise results in an
average stray light component over the scan direction which is
stronger in the central area of the field than in the border area
of the field and is characterized by a profile over the field.
[0128] FIG. 7 shows a typical stray light component 151, averaged
over the scan direction, for a microlithography projection
objective of a current design as a profile graph along the
X-direction over the exposure field in accordance with the
measurement rule observed herein. As is evident, the stray light
component 151 is higher in the central area 155 of the field with a
value of 0.8% relative to the useful light than it is in the border
area 157 with a value of 0.6% relative to the useful light.
[0129] FIG. 8 schematically illustrates the optical part of a
projection exposure apparatus 201 for immersion lithography. The
projection exposure apparatus 201 has an excimer laser 203 as its
light source with a wavelength of 193 nm. As an alternative, it is
also possible to use other wavelengths such as 248 nm or 157 nm. An
illumination system 205 arranged in the light path downstream of
the light source produces a sharply delimited homogeneous
illumination field in its image plane 207 which is at the same time
the object plane 207 of the projection objective 211 which follows
in the light path. Normally in this arrangement the ray geometry at
the output side of the illumination system 205 is adapted to the
ray geometry at the input side of the projection system 211. As
mentioned above, the illumination system 205 includes a mechanism
for structuring the angular distribution of the light rays 207
falling on the object plane 207 and for controlling the state of
polarization of the incident light rays. A so-called reticle stage
holds the mask 213 in the object plane of the illumination system
and in accordance with the scanning process moves the mask along
the scan direction 215. After the object plane 207 which at the
same time represents the mask plane 207, the projection objective
211 follows next in the light path, projecting a reduced image of
the mask 213 onto a wafer 219. The wafer 219 carries a
light-sensitive so-called photoresist 221 and is positioned so that
the planar surface of the wafer 219 with the photoresist 221 is
located in the image plane 223, or field plane 223, of the
projection objective 211. The wafer 219 is held by a so-called
wafer stage 217 and advanced at a rate that is synchronized with
the movement of the mask 213. The wafer stage 217 also has
manipulators which can move the wafer 219 along the optical axis
225 or perpendicular to it. Likewise incorporated in the wafer
stage 217 is a tilting manipulator which can tilt the wafer 219
about an axis perpendicular to the optical axis 225. The wafer
stage 217 is designed specifically for immersion lithography and
includes a holder element 227 with a shallow recess for the
substrate 219 as well as a rim 229 to contain the immersion liquid
231.
[0130] The projection objective 211 for immersion lithography
applications has an image-side numerical aperture NA that is larger
than 1.0, preferably larger than 1.2, and with even higher
preference larger than 1.5. The projection objective 211 has as its
last optical element before the field plane 223 a planar-convex
lens 233 whose underside 235 is the last optical surface of the
projection objective 211 in the light path as seen in the direction
of the light rays propagating from the mask plane to the field
plane. This underside 235 is totally immersed in an immersion
liquid 231.
[0131] The hemispherical planar-convex lens 233 consists preferably
of polycrystalline material whose microscopic structure is
illustrated in FIG. 9. Conceivably, further lenses 237 of a
projection objective could also consist of polycrystalline
material.
[0132] FIG. 9 shows the microscopic structure of a polycrystalline
material schematically and not true to scale. The material 300
shown here is polycrystalline magnesium spinel (MgAl.sub.2O.sub.4)
and has a large number of differently oriented crystals 302
delimited by respective crystal boundaries 303. The mean crystal
dimension in this example is around 25 .mu.m. Interspersed between
the crystals 302 are hollow spaces, or bubbles 304, whose mean
dimension in this example is about 1 .mu.m. Other polycrystalline
materials are likewise conceivable for use as an optical material,
for example other polycrystalline spinels, polycrystalline YAG
[yttrium aluminum garnet (Y.sub.3Al.sub.5O.sub.12)],
polycrystalline LuAG [lutetium aluminum garnet
(Lu.sub.3Al.sub.5O.sub.12)], polycrystalline magnesium oxide (MgO),
polycrystalline beryllium oxide (BeO), polycrystalline aluminum
oxide (Al.sub.2O.sub.3), polycrystalline yttrium oxide
(Y.sub.2O.sub.3) or polycrystalline fluorides with a high
refractive index, such as for example BaLiF.sub.3 or LaF.sub.3.
[0133] FIG. 10 shows the stray light component in percent relative
to the useful light of a homogeneous polycrystalline material of
spinel with 40 mm thickness as a function of the mean crystal
dimension D according to the corresponding stray light model
presented in WO 2006/061225. This stray light model, besides taking
the stray light L.sub.ret into account which results from the
refractive index fluctuations due to the different orientations of
the crystals along a light path, also includes a stray light
component I.sub.scat which results from the total reflection taking
place at the crystal boundaries 303. This adds up to a total stray
light component for the stray light, which is represented as
I.sub.sum in FIG. 10 and has its minimum for the crystal size
marked by the arrow P. Furthermore, a model-dependent stray light
component of a polycrystalline material of spinel of 40 mm
thickness is represented in FIG. 11, expressed in percent relative
to the useful light as a function of the mean bubble diameter
according to the corresponding stray light model in WO
2006/061255.
[0134] Based on the stray light models in WO 2006/061255, or in
FIGS. 10 and 11, only specific parameter ranges for the mean
crystal size and the mean bubble diameter in polycrystalline
material are feasible for using this kind of material in projection
objectives for microlithography applications, as the stray light
component of the projection objective will otherwise become too
large. However, FIGS. 10 and 11 lead to the conclusion that even if
the parameter ranges that are optimal in regard to stray light are
adhered to in the production of the polycrystalline spinel
material, an optical element of spinel with a thickness of 40 mm
will still produce a stray light component of about 0.4% relative
to the useful light. By also considering the aforementioned natural
stray light distribution of a body carrying a homogeneous flow of
light, one arrives at the result that for a last, field-proximate
lens of polycrystalline material immediately before the field
plane, the profile of the stray light component, averaged over the
scan direction, has a variation over the entire field plane of 0.4%
relative to the useful light. The exact amount of variation over
the exposure field in the field plane for the stray light component
of such a field-proximate lens, averaged over the scan direction,
depends on the exact geometry of the lens and the exposure field as
well as on the distance of the lens from the field plane, and it is
entirely possible for the variation to be only half as large as the
aforementioned value. Insofar, a strongly positive single lens of
spinel, used as the last lens of the objective, has a variation of
the stray light over the exposure field that is about half as large
as the variation of an entire projection objective of current
design.
[0135] FIG. 12 represents a sketch to illustrate principal concepts
regarding the scattering at inhomogeneities 407 in the
polycrystalline material of a last lens 400 and regarding the
concept of adapting the surface roughness 403 of a last lens, as
well as the resultant stray light distributions 411, 413 over the
field. In FIG. 12 a last lens 400 of a projection objective is
located before the field plane 405 which extends perpendicular to
the optical axis 401 immediately after the last lens 400. The
inhomogeneities of the glass material are symbolically indicated in
the lens 400 as scatter lobes 407 which represent the angular
distribution of the stray light. The stray light component 411 of
the lens 400 due to the inhomogeneities of the glass material
(volume scatter), averaged over the scan direction and expressed as
a percentage relative to the useful light is shown in the
mid-portion of FIG. 12 as a profile graph over the field along the
X-direction. Current Monte Carlo simulations concerning the stray
light component 411 due to the volume scatter of a lens consisting
of polycrystalline material and arranged in last position before
the field plane in the ray direction from the mask plane to the
field plane lead to the result that the stray light component
averaged over the scan direction and expressed as a percentage of
the useful light is about 0.4% in the central area 415 of the
exposure field and about 0.2% in the border area 417 of the
exposure field, thus confirming the stray light values of WO
2006/061225 which have been discussed above. To compensate for the
stray light component 411 due to the volume scatter of the last
lens which consists of spinel, the surface roughness of the upper
side 402, i.e. the side of the last lens that faces away from the
field plane 405, is increased in the border zones 403, which
produces the result of an added stray light component 413. The
change of the surface roughness of the upper side 402 is selected
so that it results in an additional stray light component 413 whose
profile over the exposure field complements the stray light
component 411 due to the volume scatter, so as to add up to an
overall stray light component that is nearly constant. The added
stray light component 413 due to the surface roughness, expressed
as a percentage of the useful light and averaged over the scan
direction, is shown in the right hand portion of FIG. 12 as a
profile graph over the field along the X-direction. By changing the
surface roughness on the upper side 402 of the last lens, only a
very small amount of additional stray light 413 is introduced in
the central area of the exposure field 415, in contrast to the
border area 417 of the exposure field where the added amount of
stray light is about 0.5%, which compensates for the stray light
411 which comes from the volume scatter of the last lens. The
surface roughness of the upper side 402 does not necessarily have
to be produced in a reworking operation; it can also be adapted in
advance during the production process of the lens.
[0136] FIG. 13 shows the stray light component, expressed as a
percentage relative to the useful light, of a projection objective
for microlithography applications, which has been corrected in
accordance with the disclosure, averaged over the scan direction y
and represented as a profile graph 501 in the X-direction along the
field. The finely dotted line in FIG. 13 represents the stray light
component, averaged over the scan direction, of a projection
objective in which the last lens element does not consist of
polycrystalline material, in the form of a profile graph 503 along
the X-direction over the exposure field with a central area 505 and
a border area 507. The variation over the field is smaller than
0.2% for this stray light component, and the latter is therefore
considered a constant stray light component within the bounds of
this application. The horizontal grid lines and the bands 509 with
a height of 0.2% serve as a graphic background to indicate the
range within which a stray light component is considered constant
within this application. The stray light component of a comparable
projection objective in which the last lens consist of
polycrystalline material is represented by a broken line with the
reference symbol 502 in FIG. 13. The stray light component 502
exhibits a stronger variation over the field than would be
permissible for a constant stray light component 509. A solid and
heavier line 501 in FIG. 13 represents the stray light component of
a projection objective that has been corrected in accordance with
the disclosure, with a last lens of polycrystalline material. This
stray light component 501 of the projection objective which has
been corrected has a stray light component which in the central
area 505 and in the border area 507 as well as in all field points
in between amounts to about 1.3% relative to the useful light.
Accordingly, this represents a very constant stray light component,
averaged over the scan direction, with a variation over the
exposure field far below 0.2% relative to the useful light.
[0137] The disclosure is suited insofar not only for the correction
of projection objectives with a last lens of polycrystalline
material, but also for the improvement of current projection
objectives so that they will have a constant stray light component
with less than 0.2% variation over the exposure field.
[0138] FIG. 14 shows a so-called two-mirror design 2100 of a
projection objective for immersion lithography with an image-side
numerical aperture larger than 1. The design 2100 has been borrowed
from FIG. 38 of US 2005/0190435 A1, keeping the same reference
symbols. Only the reference symbols for the areas 2003 of increased
surface roughness are newly added in comparison to FIG. 38 of US
2005/0190435 A1. The design 2100 is drawn in FIG. 14a in an X-Y
sectional view and thus in a plane that is defined by the scan
direction y and the direction of the optical Z-axis, because the
folded configuration of the ray path could not be visualized
otherwise. The same form of representation is also used in all of
the catadioptric design discussed hereinafter. The mask plane 2101
is projected by the first refractive objective group 2110 onto an
extended intermediate image plane 2103. The first refractive group
has a pupil- or aperture plane A. The mirror group 2120 with the
mirrors 2121 and 2122 projects the extended intermediate image
plane 2103 into a further extended intermediate image plane 2104.
The second refractive objective group 2130 projects the extended
intermediate image plane 2104 into the field plane 2102. The last
lens before the field plane 2102 in the direction of the light rays
from the mask plane 2101 to the field plane 2102 carries the
reference symbol 2150. The surface areas of field-proximate optical
elements near the exposure field 2102 or near the intermediate
field planes 2103 and 2104, which are suitable for correcting the
variation of the stray light component over the exposure field by
increasing the surface roughness are indicated by a heavier
sawtooth line 2003. For better clarity, the lower part of the
second refractive group 2130 is shown in an enlarged view in FIG.
14b. Further indicated by the shaded bars in FIG. 14b is the area
2005 of the surface of the last optical element 2150 before the
field plane 2102 in the direction of the light rays from the mask
plane 2101 to the field plane 2102, where an aperture stop could be
suitably arranged to reduce stray light, in particular out-of-field
stray light. This aperture stop can be realized with mechanical
field aperture stops between the last optical element 2150 and the
field plane 2102. However, it is more advantageous to realize the
aperture stop by masking the surface parts 2005 of the last optical
element which are indicated by the shaded bars in FIG. 14b, because
this creates no spatial interferences and has no detrimental
influence on the flow dynamics of the immersion liquid. This
masking can be accomplished cost-effectively by placing an
absorbent or reflective coating on the areas 2005 that are shaded
in FIG. 14b.
[0139] However, in the representation of the design in FIGS. 14a
and 14b it should be noted that the design is shown in a Y-Z
sectional view and thus in the scanning direction, because the
structural concept of the design could not be represented in an X-Z
section, i.e. perpendicular to the scanning direction. The heavier
sawtooth lines 2003 in FIGS. 14a and 14b insofar indicate only the
field-proximate surfaces which can be considered for an adaptation
of the surface roughness, and on the other hand only illustrate the
principle that those areas 2003 of the field-proximate surfaces
which are met or traversed by rays of an outer field point of the
exposure field have a higher surface roughness. The areas 2003 of
the field-proximate surfaces with an increased surface roughness
that are suitable for reducing the amount by which a stray light
component, averaged over the scanning direction, varies
perpendicular to the scanning direction over the exposure field can
be illustrated better in an X-Z section of the design. Seen in an
X-Z sectional view, the areas 2003 with the increased surface
roughness are arranged on the optical elements in such a way that
they are located equally at the borders to the right and left
(relative to the x-direction) of the center of the optically used
area, so that they have an equal effect on the stray light
component, averaged over the scanning direction, in the border
areas to the right and left (relative to the x-direction) of the
central area.
[0140] FIG. 15 shows a so-called four-mirror design PL1 of a
projection objective for immersion lithography with an image-side
numerical aperture of 1.2. The design PL1 has been borrowed from
FIG. 9 of US 2007/0024960 A1, keeping the same reference symbols.
Only the reference symbol for the field plane W1 is newly added in
comparison to FIG. 9 of US 2007/0024960 A1. The mask plane R1 is
projected onto an intermediate image plane Q by the first
catadioptric objective group G1 consisting of the purely refractive
subgroup G11 with the lenses L1 to L4 and the catadioptric subgroup
consisting of the lens 5 and mirrors M1 and M2. The intermediate
image plane Q is projected into the field plane W1 immediately
after the lens 18 by the second catadioptric objective group G2
consisting of the two mirrors M3 and M4, the refractive subgroup
G21 with the lenses L6 and L7, the refractive subgroup G22 with the
lenses L8 to L12, and the refractive subgroup G23 with the lenses
L13 to L18. A pupil plane or aperture plane AS1 is located between
the subgroups G22 and G23. The broken lines extending the mirror
surfaces M2 and M3 illustrate the statement made above that
catadioptric designs can normally be described through the
terminology of rotationally symmetric designs, even if the real ray
path geometry or the real physical shapes of the optical elements
of such a design no longer exhibit this rotational symmetry. In
order to retrace this thought process, the design PL1 shown in FIG.
15 has to be rotated about the optical axis AX1. After this
rotation, all optical elements possess rotational symmetry relative
to the optical axis AX1, and the optical axis AX1 is now also the
optical axis of all optical elements within the design PL1.
[0141] The field-proximate surface areas near the field plane W1,
or near the intermediate image plane Q, in the direction of the
light path from the mask plane R1 to the field plane W1, which are
suitable for correcting the variation of the stray light component
over the exposure field by increasing the surface roughness are in
this design PL1 all of the mirror surfaces M1 to M4 and the
surfaces of the lenses L5, L6 and L18.
[0142] FIG. 16 shows a so-called RCR design
(refractive-catadioptric-refractive design) of a projection
objective for immersion lithography with an image-side numerical
aperture of 1.25. The design has been borrowed from FIG. 19 of WO
2004/019128 A2, wherein the reference symbols have been maintained
to the largest extent, except that each of the reference symbols of
the groups and lenses has been expanded with an added zero, while
the reference symbol W1 for the field plane, the reference symbol
M10 for the first direction-changing mirror, and the reference
symbol M20 for the second direction-changing mirror have been newly
added in comparison to FIG. 19 of WO 2004/019128 A2. The first
refractive objective group G10 with the lenses L110 to L1100
projects the mask plane R1 into a first extended intermediate image
area after the first direction-changing mirror M10. The
catadioptric group G20 consisting of the lenses L210, L220 and a
spherical mirror CM forms a so-called Schupmann achromat for the
correction of the longitudinal chromatic aberration and projects
the first extended intermediate image area into a second extended
intermediate image area before the second direction-changing mirror
M20. The second intermediate image plane is projected into the
field plane W1 immediately below the lens L3150 by the second
refractive objective group G30 with the lenses L310 to L3150. The
second refractive objective group has a pupil plane or aperture
plane identified as AS. As has already been mentioned above, the
optical axis of the Schupmann achromat, or group G20, represents
the reference axis for the definition of the concepts of field and
pupil as used herein in regard to all elements after the first
direction-changing mirror M10 and before the second
direction-changing mirror M20, because in contrast to all other
designs presented herein, the rotational symmetry of the design
about the optical axis is broken by these direction-changing
mirrors. The field-proximate surfaces near the field plane W1, or
near the intermediate image plane Q, in the direction of the light
path from the mask plane R1 to the field plane W1, which are
suitable for correcting the variation of the stray light component
over the exposure field by increasing the surface roughness are in
this RCR design the direction-changing changing mirror surfaces M10
and M20 as well as the surfaces of the lenses L100, L310 and
L3150.
[0143] FIG. 17 shows a further two-mirror design 800 of a
projection objective for immersion lithography with an image-side
numerical aperture of 1.75. The design 800 has been borrowed from
FIG. 8 of WO 2006/133801 A1, wherein the reference symbols have to
the largest extent been maintained. Only the reference symbols of
the objective groups G100 to G900 have been expanded in comparison
to FIG. 8 of WO 2006/133801 A1 by adding double zeroes. The first
refractive objective group ROP1 projects the mask plane OP into an
extended intermediate image plane IMI1. The first refractive group
has a pupil plane or aperture plane identified as AS. The extended
intermediate image plane IMI1 is projected into a further extended
intermediate image plane IMI2 by the mirror group COP2 with the
mirrors CM1 and CM2. The second refractive objective group ROP3
projects the extended intermediate image plane IMI2 into the field
plane IP. The last lens before the field plane IP in the direction
of the light rays from the mask plane OP to the field plane IP
carries the reference symbol LOE and consists of two partial lenses
LOE1 and LOE2 with an immersion liquid IL between the partial
lenses (see description of FIG. 18).
[0144] The field-proximate surfaces near the field plane IP, or
near the extended intermediate image planes IMI1 and IMI2, in the
direction of the light path from the mask plane OP to the field
plane IP, which are suitable for correcting the variation of the
stray light component over the exposure field by increasing the
surface roughness are in this design 800 the mirror surfaces CM1
and CM2 as well as the surfaces of the lenses B800, LOE and the
lens before CM1 in the direction of the light rays from the mask
plane OP to the image plane IP.
[0145] FIG. 18 shows as a detail of the design 800 of FIG. 17 the
last lens element LOE before the field plane IP in the direction of
the light rays from the mask plane OP to the image plane IP. This
lens element consists of quartz glass for the partial lens LOE1 and
sapphire for the partial lens LOE2, wherein the crystallographic
axis in the latter is oriented in the direction CA parallel to the
optical axis AX. Between the two partial lenses LOE1 and LOE2 there
is an immersion liquid. Other crystalline materials with a high
index of refraction are also mentioned in WO 2005/133801 A1 for the
second partial lens LOE2, such as for example spinel
(MgAl.sub.2O.sub.4), YAG [yttrium aluminum garnet
(Y.sub.3Al.sub.5O.sub.12)], magnesium oxide (MgO), beryllium oxide
(BeO), aluminum oxide (Al.sub.20.sub.3), yttrium oxide
(Y.sub.2O.sub.3) or lanthanum fluoride (LaF.sub.3). In the context
of immersion lithography, it is important to note the teaching of
WO 2006/133801 A1 that when a high image-side numerical aperture is
specified as a desired property in a design, the value of the
image-side numerical aperture should not exceed the refractive
index of the last optical element before the exposure field. It is
insofar important for designs with a numerical aperture larger than
1.7, as in the case of the design 800, for the last lens element to
have a refractive index larger than 1.7 at the applicable operating
wavelength. Sapphire, which is used as the material of a second
partial lens LOE2 in FIG. 18, has a refractive index of 1.92 at an
operating wavelength of 193 nm and thus has according to the
teachings of WO 2006/133801 A1 enough of a numerical distance from
the image-side numerical aperture of 1.75 of the design 800.
However, it would also not involve a major task to adapt the design
800 to a design in which the last lens before the exposure field
consists of polycrystalline material with a refractive index larger
than 1.7 at an operating wavelength of e.g. 193 nm and to
simultaneously realize high numerical aperture values around
1.7.
[0146] FIG. 19 shows a six-mirror design of a projection objective
for applications in so-called EUV (extreme ultraviolet)
lithography. The design has been borrowed from FIG. 1 of US
2004/0051857 A1, keeping to a large extent the same reference
symbols to which only the numeral 5 has been added. The first
catoptric objective group G15 projects the mask plane OB5 into the
intermediate image IMI5 by the mirrors M15 and M25. The objective
group includes the pupil plane or aperture plane APE5. The second
catoptric objective group G25 projects the intermediate image IMI5
into the field plane IM5 by the mirrors M35, M45, M55, and M65.
Projection objectives for EUV lithography normally consist of
mirrors, as there are no materials in existence that are
sufficiently transparent for wavelengths below 100 nm. Insofar, the
task of equalizing the profile over the exposure field for the
portion of the stray light component that results from
inhomogeneities in the glass material does not present itself in
these projection objectives. However, mirrors with the same surface
finish scatter the light about 16 times as strongly as lenses with
a refractive index of about 1.5 in air. Consequently, EUV
projection objectives are much more critical than conventional
refractive systems in regard to stray light that is due to the
surface properties of the optical elements. As an additional
factor, not only the polish of the optical element itself but also
the highly reflective coatings play a big part in EUV objectives as
a source of stray light. Insofar, it is also of practical benefit
in projection objectives used for EUV lithography to reduce the
stray light component, averaged over the scan direction, in its
profile over the exposure field in accordance with the disclosure,
or to take measures to ensure in accordance with this patent
application that the stray light component, averaged over the scan
direction, has a constant profile over the exposure field. The
field-proximate surfaces near the intermediate image plane IMI5, in
the direction of the light path from the mask plane OB5 to the
field plane IM5, which are suitable for correcting the variation of
the stray light component over the exposure field by increasing the
surface roughness are in this design the mirror surfaces M25, M35
and M45.
[0147] As the optically used areas on the mirrors of the projection
objective are in many cases located at a considerable distance from
the optical axis OA5 of the projection objective, the optical axis
can no longer serve as reference axis for the distance under the
definition that was given above for distinguishing close-to-pupil
and field-proximate elements in projection objectives for EUV
lithography. Rather, the normal vector at the geometric center
point of an optically used area of a surface is chosen to serve as
new reference axis for the distance according to which
pupil-proximate and field-proximate elements in projection
objectives for EUV lithography are distinguished. If an aperture
ray of the central field point of the exposure field on the surface
of an optical element has a distance from the thus defined normal
vector that is six times as large as the distance that the
principal ray of a border point of the exposure field on the same
surface of the optical element has from the normal vector, the
optical element is referred to as pupil-proximate, otherwise it
will be referred to as field-proximate.
[0148] As a possible example, FIG. 20 shows a distribution of the
increase in surface roughness as an RMS value over the optically
used area of the upper side of a last lens before the field plane
in the direction of the light rays from the mask plane to the field
plane, which correlates to the additional stray light component,
averaged in the scan direction, in regard to its profile over the
exposure field, with a smaller stray light component in the central
area of the exposure field and a high stray light component in the
border area of the exposure field, so that as a result the stray
light component, averaged over the scan direction, will have a
smaller variation over the exposure field of the projection
objective, or more specifically, that a stray light component of
the projection objective, averaged over the scan direction, is
obtained which is constant in the sense of this patent application.
The scale on the X-axis of the diagram is normalized so that the
height of the border of the optically used area in the positive
X-direction on the upper side of the last lens has a value of 1,
and the height of the center of the optically used area has a value
of zero. The maximum amount for the increase of the RMS value in
this diagram of slightly more than 2 nm at the left and right
borders of the optically used area in comparison to the RMS value
at the center of the optically used area is sufficient at an
operating wavelength of e.g. 193 nm in order to correct the
variation of the stray light component of a projection objective,
averaged over the scan direction, which amounts to about 0.2% over
the exposure field. This is based on the assumption of typical
geometric relationships of the last lens, distances between the
last lens and the exposure field, aspect ratios of the exposure
field, as well as the refractive indices of the last lens element
according to the designs of FIGS. 14, 15, 16 and 17. Depending on
the different parameters, it is also possible that different values
of about 0.1% to about 0.4% are obtained for the amount by which
the stray light component, averaged over the scan direction, varies
over the exposure field. If the amount by which the stray light
component, averaged over the scan direction, varies over the
exposure field is to be corrected by more than 0.2%, the desired
value for the surface roughness is obtained by normalizing the
diagram of FIG. 20 accordingly. The profile of the surface
roughness value in the diagram of FIG. 20 can be described by a
function in the form of a root of a general polynomial, wherein the
lateral distance from the center represents the independent
variable. This description has the advantage that the coefficients
obtained from it are advantageously suited for the programming of
polishing machines such as for example polishing robots. However,
the profiles that can be realized with the polishing machines are
not open to an arbitrary choice, as the polishing heads have a
finite dimension which imposes limits on the curvatures of the
curves that represent the profile of the surface roughness in the
diagrams exemplified by FIG. 20. It is for example not possible for
polishing machines to realize the break at height 0 in the diagram
curve of FIG. 20, as the finite dimension of the polishing head
will always have the consequence that a surface roughness value
different from zero will remain at the height 0. This would for
example have the result of a residual value of the additional stray
light component 413 in the central area 415, as shown in FIG.
12.
[0149] FIG. 21 schematically illustrates the different methods
whereby it is possible to provide a projection objective for
applications in the field of microlithography with an additional
stray light component, averaged over the scan direction, whose
profile over the exposure field is such that the stray light
component of the projection objective, averaged in the scan
direction, has a reduced variation over the exposure field or, more
specifically, that a stray light component of a projection
objective, averaged in the scan direction, is obtained which is
constant in the sense of this patent application. In a first step
A, the stray light component of the projection objective is either
simulated or determined from data of the components or data of the
respective blanks As an alternative first step B, it is possible to
take measurements on the projection objective itself or on a
projection objective of identical design and thereby determine the
variation of the stray light component over the exposure field of
the projection objective. In a second step, the surface roughness
of a surface of a field-proximate optical element or the surface
roughness properties of several surfaces of a plurality of
field-proximate optical elements are either appropriately adapted
in advance during production, prior to installation in the
projection objective, or subsequently altered by the appropriate
amount, so that the stray light component, averaged in the scan
direction, has a reduced variation over the exposure field or, more
specifically, that a stray light component of the projection
objective, averaged in the scan direction, is obtained which in the
sense of this patent application is constant over the exposure
field. The success of the measures taken in the second step is
verified in a third step by a measurement which is taken as part of
a qualifying examination of the projection objective. Depending on
the result of the third step, the projection objective is either
accepted as having a sufficiently good correction, or the process
loops back to the second step, wherein the surface roughness of the
surface of the field-proximate element or of the surfaces of the
field-proximate elements is changed from its previous value. These
process steps two and three are repeated until the correction is
found to be sufficient.
[0150] As an alternative to the foregoing method, it can be
reasonable for projection objectives in which one individual lens
contributes a major portion of the stray light component, to
determine only the contribution of the individual lens in a first
step of the method and to compensate the contribution in a second
step by an advance adaptation or subsequent alteration of the
surface roughness, so that the qualification test of the projection
objective can be performed in a third step. Under this alternative
procedure, the measurements can be performed on the lens itself in
a first process step B, or the contribution of the lens is
determined from measurements taken in a first process step B on a
lens of the same design. As an alternative, the individual lens can
be simulated as part of a first process step A, or the contribution
from this lens can be determined from data that are obtained from
the blank of the lens.
[0151] FIG. 22 schematically illustrates the process steps for
producing microstructures on a wafer by using a projection exposure
apparatus with a projection objective according to this patent
application. In a first step, a thin metal film is vapor-deposited
on the wafer. Next, in a second step, the wafer with the metal film
is overlaid with a photosensitive coating, the so-called
photoresist. In a third step, the projection exposure apparatus
with a projection objective according to the present patent
application transfers the structures of a mask in the mask plane in
a scanning process to the currently addressed surface of a
semiconductor element on the wafer by photographic exposure of the
photoresist. This step is repeated until all surfaces of all
semiconductor elements on the wafer have been exposed.
Subsequently, the wafer with the exposed photoresist is developed,
whereby the photoresist is removed from the wafer at those
locations on the wafer that received a sufficient exposure. This
makes it possible to remove the metal film at the locations where
the photoresist was removed in the preceding process step. This
process step is called etching. In a next step, the wafer is ready
for further treatment for which the wafer returns to the starting
point of the process of FIG. 22 or is directed to the starting
point of another process in another apparatus.
[0152] Even though the disclosure has been described through the
presentation of specific embodiments, those skilled in the
pertinent art will recognize numerous possibilities for variations
and alternative embodiments, for example by combining and/or
exchanging features of individual embodiments. Accordingly, it will
be understood by those skilled in the pertinent art that such
variations and alternative embodiments are considered as being
included in the present disclosure and that the scope of the
disclosure is limited only by the attached patent claims and their
equivalents.
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