U.S. patent application number 12/611999 was filed with the patent office on 2010-02-25 for microlithographic projection exposure apparatus.
This patent application is currently assigned to CARL ZEISS SMT AG. Invention is credited to Aurelian Dodoc, Joerg Mallmann, Hans-Juergen Rostalski, Karl Heinz Schuster, Wilhelm Ulrich.
Application Number | 20100045952 12/611999 |
Document ID | / |
Family ID | 33482288 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100045952 |
Kind Code |
A1 |
Dodoc; Aurelian ; et
al. |
February 25, 2010 |
MICROLITHOGRAPHIC PROJECTION EXPOSURE APPARATUS
Abstract
A microlithographic projection exposure apparatus contains an
illumination system (12) for generating projection light (13) and a
projection lens (20; 220; 320; 420; 520; 620; 720; 820; 920; 1020;
1120) with which a reticle (24) that is capable of being arranged
in an object plane (22) of the projection lens can be imaged onto a
light-sensitive layer (26) that is capable of being arranged in an
image plane (28) of the projection lens. The projection lens is
designed for immersion mode, in which a final lens element (L5;
L205; L605; L705; L805; L905; L1005; L1105) of the projection lens
on the image side is immersed in an immersion liquid (34; 334a;
434a; 534a). A terminating element (44; 244; 444; 544; 644; 744;
844; 944; 1044; 1144) that is transparent in respect of the
projection light (13) is fastened between the final lens element on
the image side and the light-sensitive layer.
Inventors: |
Dodoc; Aurelian;
(Heidenheim, DE) ; Schuster; Karl Heinz;
(Koenigsbronn, DE) ; Mallmann; Joerg; (Boppard,
DE) ; Ulrich; Wilhelm; (Aalen, DE) ;
Rostalski; Hans-Juergen; (Oberkochen, DE) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
CARL ZEISS SMT AG
Oberkochen
DE
|
Family ID: |
33482288 |
Appl. No.: |
12/611999 |
Filed: |
November 4, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12199998 |
Aug 28, 2008 |
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12611999 |
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10917371 |
Aug 13, 2004 |
7532306 |
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12199998 |
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PCT/EP04/05816 |
May 28, 2004 |
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10917371 |
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Current U.S.
Class: |
355/53 ;
355/67 |
Current CPC
Class: |
G03F 7/70341 20130101;
G03F 7/70833 20130101; G03F 7/70916 20130101; G03F 7/7015 20130101;
G03F 7/70241 20130101; G03F 7/70983 20130101; G03F 7/70975
20130101 |
Class at
Publication: |
355/53 ;
355/67 |
International
Class: |
G03B 27/42 20060101
G03B027/42; G03B 27/54 20060101 G03B027/54 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2003 |
DE |
103 24 477.8 |
Claims
1. An apparatus which, during operation, directs radiation along a
path from an object plane to an image plane in a microlithography
projection system, the apparatus comprising: a first optical
assembly comprising a plurality of first optical elements arranged
along the path; and a second optical assembly adjustably mounted
with respect to the first optical assembly, the second optical
assembly comprising one or more second optical elements positioned
in the path including a final optical element, where the final
optical element is the optical element closest to the image plane
along the path, the final optical element having a surface facing
the image plane, wherein the apparatus is a microlithography
projection objective configured to include a liquid between the
surface of the final optical element and the image plane, during
operation, and to image an object positioned at the object plane
onto a surface at the image plane using radiation having a
wavelength of 193 nm or less, the apparatus having a numerical
aperture at the image plane greater than 1.
2. The apparatus of claim 1, wherein the second optical assembly is
detachable from the first optical assembly without modification of
the first optical assembly.
3. The apparatus of claim 1, wherein the second optical assembly
comprises a plurality of fastening elements that are fastened to
the first optical assembly.
4. The apparatus of claim 3, wherein the first optical assembly
comprises a housing that contains the plurality of first optical
elements and the fastening elements are fastened to the
housing.
5. The apparatus of claim 4, wherein the fastening elements
comprise connectors accessible from outside the apparatus.
6. The apparatus of claim 5, wherein the connectors are screws.
7. The apparatus of claim 1, wherein the second optical assembly
comprises actuators configured to adjust the second optical
elements relative to the first optical elements.
8. The apparatus of claim 7, wherein the actuators are micrometer
screws.
9. The apparatus of claim 1, wherein the first optical assembly
comprises a housing that contains the plurality of first optical
elements and the second optical elements are positioned outside of
the housing.
10. The apparatus of claim 1, wherein the liquid is water.
11. The apparatus of claim 1, wherein the final optical element is
the only second optical element in the second optical assembly.
12. The apparatus of claim 1, wherein the second optical assembly
comprises a second optical element positioned in the path between
the final optical element and the first optical elements.
13. The apparatus of claim 1, wherein the final optical element is
a lens.
14. The apparatus of claim 13, wherein the lens comprises a curved
surface.
15. The apparatus of claim 13, wherein the lens is a plano-convex
lens.
16. The apparatus of claim 1, wherein the apparatus is configured
to include the liquid between a surface of a first optical element
closest to the image plane and a surface of a second optical
element furthest from the image plane, during operation.
17. A microlithography projection system which, during operation,
directs radiation along a path to a substrate at an image plane,
the microlithography projection system comprising: an illumination
system which, during operation, provides the radiation to an object
positioned at an object plane, the radiation having a wavelength of
193 nm or less; a projection objective comprising a first optical
assembly and a second optical assembly adjustably mounted with
respect to the first optical assembly, the first optical assembly
comprising a plurality of first optical elements arranged along the
path between the object plane and the image plane, and the second
optical assembly comprising one or more second optical elements
positioned in the path between the first optical assembly and the
image plane, the second optical assembly including a final optical
element, where the final optical element is the optical element
closest to the image plane along the path and the final optical
element has a surface facing the image plane; a stage configured to
position the substrate at the image plane; and a liquid supply
apparatus which, during operation, provides a liquid between the
surface of the final optical element and the substrate, wherein,
during operation, the projection objective forms an image of the
object at the image plane with the radiation, the projection
objective having a numerical aperture at the image plane greater
than 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International
Application PCT/EP04/005816, with an international filing date of
May 28, 2004, The full disclosure of this International Application
is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to microlithographic
projection exposure apparatuses such as are used for the production
of microstructured components. The invention relates, in
particular, to projection exposure apparatuses with a immersion
projection lens.
[0004] 2. Description of Related Art
[0005] Integrated electric circuits and other microstructured
components are conventionally produced by several structured layers
being applied onto a suitable substrate, which may be, for example,
a silicon wafer. In order to structure the layers, the latter are
firstly covered with a photoresist that is sensitive to light of a
particular wavelength range, for example light in the deep
ultraviolet spectral region (DUV). Subsequently the wafer that has
been coated in this way is exposed in a projection exposure
apparatus. In this process, a pattern of diffracting structures
contained in a mask is imaged onto the photoresist with the aid of
a projection lens. Since the lateral magnification in this case is
generally less than 1, projection lenses of such a type are
frequently also referred to as reduction lenses.
[0006] After the photoresist has been developed, the wafer is
subjected to an etching process, as a result of which the layer is
structured in accordance with the pattern on the mask. The
photoresist left behind is then removed from the remaining parts of
the layer. This process is repeated until all the layers on the
wafer have been applied.
[0007] One of the most prominent objects in the development of the
projection exposure apparatuses is to be able to define
lithographically structures having increasingly smaller dimensions
on the wafer. Small structures result in high integration
densities. This generally has a favorable effect on the performance
of the microstructured components produced with the aid of
apparatuses of such a type.
[0008] The size of the definable structures depends, above all, on
the resolving power of the projection lens that is being used.
Since the resolving power of the projection lenses is proportional
to the wavelength of the projection light, one approach for the
purpose of decreasing the resolving power consists in employing
projection light having shorter and shorter wavelengths. The
shortest wavelengths that are used at present are within the deep
ultraviolet spectral region (DUV) and amount to 193 nm or
occasionally even 157 nm.
[0009] Another approach for the purpose of decreasing the resolving
power starts from the idea of introducing an immersion liquid
having a high refractive index into an immersion interspace that
remains between a final lens of the projection lens on the image
side and the photoresist or another light-sensitive layer to be
exposed. Projection lenses that are designed for immersion mode,
and that are therefore also designated as immersion lenses, may
attain numerical apertures (NA) of more than 1, for example 1.3 or
1.4. However, the immersion not only enables high numerical
apertures, and thereby an increased resolving power, but also has a
favourable effect on the depth of focus. The greater the depth of
focus, the less demanding are the requirements as regards an exact
positioning of the wafer in the image plane of the projection lens.
In the broader sense, one also speaks of immersion when the
light-sensitive layer is covered by an immersion liquid without the
final optical element of the projection lens on the image side
necessarily being immersed in the immersion liquid.
[0010] The implementation of immersion mode, however, requires
considerable additional efforts in terms of structure and process
engineering. For instance, it has to be ensured that the optical
properties of the immersion liquid are spatially homogeneous and
temporally constant--at least within the volume exposed to the
projection light--even when the wafer with the photoresist applied
thereon is moving relative to the projection lens. At the present
time the associated technological problems have not been solved
satisfactorily.
SUMMARY OF THE INVENTION
[0011] It is an object of the invention to specify a projection
exposure apparatus designed for immersion mode that, with a simple
structure, enables reliable and low-maintenance operation.
[0012] This object is achieved by means of a projection exposure
apparatus with an illumination system for generating projection
light. The apparatus further comprises a projection lens with which
a reticle can be imaged onto a light-sensitive layer. The
projection lens is designed for immersion mode, in which a final
lens of the projection lens on the image side is immersed in an
immersion liquid. According to the invention, a terminating element
is provided that is transparent in respect of the projection light
and is capable of being arranged between the final lens on the
image side and the light-sensitive layer in such a way that it is
immersed in the immersion liquid, at least with its image-side
boundary surface.
[0013] The provision of a terminating element in the interspace
between the final lens of the projection lens on the image side and
the light-sensitive layer has, inter alia, the advantage that
constituents issuing from the light-sensitive layer, or other
contaminants arising there, are able to accumulate on the final
lens on the image side at worst to a negligible extent, since the
terminating element, in particular the side thereof facing towards
the light-sensitive layer, acts like a protective shield for the
final lens on the image side. In this way the final image-side lens
of the projection lens does not have to be removed, but only the
terminating element occasionally has to be removed and mounted
again after cleaning or exchange. Particularly if the terminating
element is fastened to the projection lens from outside and can be
removed and mounted without disassembly of the projection lens, the
effort required for this remains comparatively low.
[0014] It is advantageous if an interspace that is capable of being
filled up at least partially with immersion liquid remains between
the final lens of the projection lens on the image side and the
terminating element. In this further embodiment the terminating
element is consequently immersed in immersion liquid on both sides,
so that a slight refraction of light occurs also on the boundary
surface of the terminating element on the object side. The
requirements as regards the adjustment and the manufacturing
accuracy of the terminating element are correspondingly low. For,
especially in the case of large numerical apertures, even a
terminating element with plane-parallel shape reacts very
sensitively to manufacturing defects, for example deviations from
the nominal thickness, from the parallelism of the boundary
surfaces and fitting errors.
[0015] If the interspace is not filled completely with immersion
liquid but only partially, a gas-filled region remains between the
immersion liquid and the final lens on the image side. This may be
advantageous, for example, when the projection lens is to be
capable of being converted between dry operation and immersion mode
with as little effort as possible. The fewer interspaces between
fixed optical elements such as lenses or terminating elements are
filled with immersion liquid, the less effect a change-over to
immersion mode generally has on the adjustment of the projection
lens. From this point of view it may even be advantageous not to
allow the projection lens to be immersed in the immersion liquid,
but to leave a gas-filled region between the terminating element
and the immersion liquid.
[0016] In an advantageous further embodiment of this configuration
the projection lens exhibits, for the purpose of introducing
immersion liquid into the interspace between the final lens on the
image side and the terminating element, a first immersion device
which is independent of a second immersion device for introducing
immersion liquid into the interspace between the final optical
element and the light-sensitive layer, so that no exchange of
immersion liquid between the interspaces is possible. In this way
it is ensured that contaminants issuing from the light-sensitive
layer are unable to reach the final lens of the projection lens on
the image side via the immersion liquid.
[0017] In another advantageous configuration the terminating
element has the same refractive index as the final lens of the
projection lens on the image side and is optically coupled onto
this lens with its object-side boundary surface in such a way that
projection light passing through the projection lens is not
refracted between the final lens on the image side and the
terminating element. The absence of any refraction between the
final lens and the terminating element results in a still smaller
adjustment effort in the course of exchanging the terminating
element. This can be realized, for example, by the final lens on
the image side, the terminating element and also the immersion
liquid located in between them having the same refractive
index.
[0018] A refraction of the projection light between the final lens
on the image side and the terminating element is also forestalled
when the terminating element is optically contacted with the final
lens on the image side and both elements have the same refractive
index. But optical contacting is also useful, in appropriate
circumstances, in the case of differing refractive indices, since
in this way the final lens on the image side is directly protected
against contaminants by the terminating element which is optically
contacted therewith. The terminating element can be optically
contacted with the final lens on the image side in particularly
simple manner if the two boundary surfaces facing towards one
another are flat. An adjustment is then superfluous, since the
position of the terminating element along the optical axis and also
the orientation in the plane perpendicular thereto are
predetermined by the flat boundary surfaces.
[0019] In an advantageous configuration a first interspace that is
capable of being filled up with a first immersion liquid remains
between the final lens of the projection lens on the image side and
the terminating element. A second interspace that is capable of
being filled with a second immersion liquid remains between the
terminating element and the light-sensitive layer. The first
interspace in this configuration is consequently capable of being
separated from the second interspace in fluidically sealing
manner.
[0020] The first immersion liquid and the second immersion liquid
do not necessarily have to be different. But the use of differing
immersion liquids has the advantage, inter alia, that the immersion
liquids can be optimally adapted to the specific conditions in the
two interspaces. The first immersion liquid, which is in contact
with the final lens of the projection lens on the image side, may
for example have a very low surface tension, which for the second
immersion liquid coming into contact with the light-sensitive layer
would no longer be acceptable. The first immersion liquid also does
not definitely have to be so easily cleanable as the second
immersion liquid, since it cannot be contaminated by the
light-sensitive layer. An adaptation of the two immersion liquids
from the point of view of chemical reactivity can furthermore be
undertaken. Since, for example, quartz glass and calcium-fluoride
crystals interact differently with adjoining liquids, the immersion
liquids may be selected in such a way that they react chemically as
little as possible with the adjoining optical faces.
[0021] The terminating element in this configuration may be
arranged in displaceable manner. This makes it possible to
distribute the shear forces acting on the immersion liquid, which
arise as a result of the relative movement between the fixed
projection lens and the light-sensitive layer, between the first
immersion liquid and the second immersion liquid in a practically
arbitrary ratio.
[0022] If during a scanning operation of the projection exposure
apparatus the terminating element is displaced synchronously with
the light-sensitive layer, constant distributions of force in the
immersion liquids can be achieved over the entire scanning
operation. This favors the formation of laminar flows of liquid,
counteracting the formation of bubbles. This applies, in
particular, if the terminating element is displaced in a plane
parallel to the light-sensitive layer.
[0023] With regard to minimal movements of the second immersion
liquid, the terminating element and the light-sensitive layer may
have like displacement speeds and displacement directions during a
projection. There is then no longer any relative movement between
the terminating element and the light-sensitive layer. The second
immersion liquid then remains with freedom of movement within the
second interspace, despite the common displacement movement of the
light-sensitive layer and of the terminating element. For this
reason it may also have a higher viscosity than the first immersion
liquid.
[0024] If flows within the second immersion liquid still arise,
these have their origin in inertial forces which appear in the
course of acceleration and deceleration during a scanning
operation. If these inertial forces are small enough, additional
measures, which are otherwise necessary in order to keep the
immersion liquid in the interspace between the final lens on the
image side and the light-sensitive layer, can be dispensed with.
Since these measures generally promote bubbling, this configuration
of the invention permits bubbles in the region of the second
immersion liquid to be very largely avoided.
[0025] Although in the region of the first immersion liquid a
relative movement occurs between the projection lens and the
terminating element in the course of a scanning operation, and as a
result shear forces also arise that act on the first immersion
liquid, the terminating element may, unlike the wafer, be provided
with an edge which prevents an undesirable discharging of the first
immersion liquid during scanning operation. Therefore also for the
first immersion liquid no additional measures, such as, for
instance, an incident flow of gases, are necessary in order to
prevent a discharging of the immersion liquid. Consequently bubbles
cannot arise, or cannot arise to an appreciable extent, in the
first immersion liquid either.
[0026] The edge may be formed directly on the terminating element.
The edge may also be part of a tank which is open in the direction
towards the final lens of the projection lens on the image side and
in the bottom of which the terminating element is arranged. The
tank itself may then consist of an opaque material, for example a
metal or a crystal. Crystals such as crystalline silicon, for
example, have the advantage of being very dimensionally stable. On
account of their low specific weight, their stiffness and their low
chemical reactivity, ceramics, for example based on SiC, are also
highly suitable as material for the tank. The terminating element,
which, for example, may consist of quartz glass, then forms merely
a type of window at the bottom of the tank, through which the
projection light can pass.
[0027] In addition to having translational displaceability, the
terminating element may also be capable of being tilted about a
tilt axis parallel to the image plane. The tiltability constitutes
an additional degree of freedom, with which movements of the first
and second immersion liquids can be influenced.
[0028] If the terminating element is, for example, tilted during a
positioning movement in an exposure intermission in such a way that
the largest spacing between the terminating element and the
light-sensitive layer is situated at the front in the direction of
motion, a kind of wedge-shaped gap arises between the terminating
element and the light-sensitive layer. In this gap the second
immersion liquid is entrained in the course of a positioning
movement of the wafer over and beyond the light-sensitive layer.
The tilting movement in this case may be effected in such a way
that the shortest spacing between the terminating element and the
light-sensitive layer is so small that, as a consequence of
cohesive forces, the second immersion liquid is unable to pass
through this gap. By virtue of this it is possible to displace the
second immersion liquid over the surface of the wafer in very
simple manner, also over greater distances and at greater
speeds.
[0029] If an edge is provided on the terminating element, said edge
should be dimensioned in such a way that in the course of a tilting
movement of the terminating element the second immersion liquid is
nevertheless prevented by the edge from discharging. Hence also in
the course of positioning movements of the wafer, which are
generally carried out at greater speeds than the displacement
movements during an exposure, the necessity of entraining an
immersion liquid with the aid of incident flows of gases, or with
similar measures, also ceases to apply. Consequently no appreciable
formation of bubbles can occur, even in the course of positioning
movements.
[0030] Alternatively, or also in addition to tiltability of the
terminating element, there may be provision to displace the
terminating element perpendicular to the image plane. The spacing
between the light-sensitive layer and the terminating element may
then, for example, be reduced so far that the immersion liquid
remains in the second interspace solely by virtue of cohesive
forces.
[0031] If the second immersion liquid cannot be kept in the
vicinity of the final lens on the image side merely by virtue of
the cohesive forces or by virtue of a tilting of the terminating
element in the course of a movement of the wafer relative to the
projection lens, a holding device known as such may additionally be
provided, which holds the second immersion liquid in the second
interspace in non-contacting manner. For this purpose the holding
device may comprise, for example, at least one gas nozzle, the
discharge aperture of which can be directed towards the second
immersion liquid.
[0032] In another advantageous configuration of the invention, at
least the first interspace is arranged in a sealable container. The
container may be, for example, a type of housing which is
penetrated by the projection lens and which covers the entire
supporting structure for the wafer. As a result of evaporation of
the immersion liquid, after some time a saturation vapor pressure
arises within the container, which prevents more immersion liquid
from evaporating than condenses again simultaneously. In this way,
latent heat of evaporation may not arise at those places where the
immersion liquid comes into contact with a surrounding gas.
Heat-sinks of such a type bring about an inhomogeneous distribution
of temperature and hence also an inhomogeneous distribution of
refractive index of the immersion liquid, as a result of which the
imaging properties are impaired.
[0033] In general, however, it takes a very long time until the
saturation vapor pressure has arisen in the container merely as a
result of evaporation on the relatively small surface. Therefore
the projection exposure apparatus may comprise a supply device for
supplying a vapor phase of the first immersion liquid in the
container. Even if the first immersion liquid differs from the
second immersion liquid, in general it is not necessary also to
counteract an evaporation of the second immersion liquid, since as
a consequence of the terminating element the interface to a
surrounding gas is very small.
[0034] A minimal cooling of the first immersion liquid is obtained
when the vapor pressure of the vapor phase of the first immersion
liquid in the container is capable of being adjusted in such a way
that it is at least approximately equal to the saturation vapor
pressure of the vapor phase of the first immersion liquid at the
temperature prevailing in the container.
[0035] Another possibility for forestalling fluctuations of
temperature within the first immersion liquid as a consequence of
local evaporation consists in covering the first interspace in the
upward direction at least partially by means of a cover. The cover
reduces the size of the interface to a surrounding gas, on which
cooling can occur as a result of evaporation. This cover may, for
example, be constructed in such a way that only a small interspace
filled with a gas remains between the cover and the first immersion
liquid. The saturation vapor pressure arises relatively quickly
there as a result of evaporation.
[0036] But the gas in the interspace may also be a special
protective gas, the density of which is greater than the density of
a surrounding gas. As a consequence of the greater density, the
protective gas is kept in the interspace by the force of gravity.
The protective gas should furthermore be such that the solubility
thereof in the second immersion liquid is as low as possible,
preferably lower than 10.sup.-4 per cent by volume. In this way an
undesirable diminution of the transmitting power, or changes in
refractive index within the second immersion liquid that have their
origin in protective gas that has. gone into solution, can be
counteracted.
[0037] It is more favorable if the first immersion liquid directly
adjoins the cover, so that an interface to a surrounding gas or to
the aforementioned protective gas remains only where the cover is
interrupted.
[0038] This may be the case, for example, in the region of a recess
which is provided in the region of the final lens on the image
side. The cover does not then touch the projection lens; at the
same time, an equalisation of level may take place via the
peripheral gap between the lens and the cover, so that the volume
between the terminating element and the cover is always completely
filled with the first immersion liquid.
[0039] In this connection it is particularly favourable if the tank
has an edge that slides in sealing manner along the underside of
the cover during the displacement movement of the terminating
element. The edge prevents, on the one hand, a lateral discharging
of the first immersion liquid. At the same time, with its
upward-pointing lateral face it forms a seal acting in the
direction towards the cover.
[0040] A liquid film consisting of the first immersion liquid and
acting as a lubricating seal may always remain between an
upward-pointing lateral face of the edge and the cover. In order
that this liquid film does not break away during a movement of the
tank, a liquid reservoir may be sunk in the upward-pointing lateral
face of the edge, out of which immersion liquid can subsequently
flow. The immersion liquid may be under pressure in the liquid
reservoir, so that the liquid film does not break away even when
the relative movement between the tank and the cover generates an
underpressure in the marginal region of the edge. The pressure in
the liquid reservoir may, for example, be generated by first
immersion liquid being capable of being supplied under pressure to
the liquid reservoir from outside the tank.
[0041] In order to collect small amounts of the first immersion
liquid which pass through the narrow gap between the edge of the
tank and the cover and which display a lubricating action, on at
least one outer side of the edge an overflow channel may be
arranged which collects the overflowing first immersion liquid and
conducts it away.
[0042] In addition, in all the aforementioned configurations the
terminating element may have has at least approximately the same
refractive index as the first and second immersion liquids. In this
way it is ensured that small maladjustments of the terminating
element barely have any effect on the optical properties of said
liquids. For this reason the refractive index of the terminating
element may differ from the refractive index of the adjoining
immersion liquids by no more than 1% and preferably by no more than
0.5%.
[0043] This can be obtained, for example, by the adjoining
immersion liquids being water and by the terminating element
consisting of LiF.
[0044] It is particularly favorable if the terminating element is
without refractive power. "Without refractive power" here is to be
understood to mean the property of an optical element of having no
focusing or defocusing effect. An example of such an optical
element is a plane-parallel plate made of a homogeneous material.
Such a plate does in fact have an effect on the position of the
image plane of the projection lens and on the correction of the
spherical aberration and must to this extent be taken into account
in the design of said projection lens. However, provided that a
difference in refractive index exists at the boundary surfaces,
such a plate offsets beams impinging at an angle merely in
parallel, the magnitude of the offset depending on the angle of
incidence. A terminating element without refractive power is
advantageous for the reason that, in this way, the requirements
regarding the adjustment thereof can be lowered further and
consequently the adjustment effort after a cleaning or an exchange
of the terminating element is again reduced.
[0045] Quartz glass, for example, enters into consideration by way
of material for the terminating element. Since in the case of very
short-wave projection light in the deep ultraviolet spectral
region, in particular at a wavelength of 157 nm, quartz glass and
other conventional optical materials are no longer sufficiently
transparent, the use of calcium-fluoride, barium-fluoride or
strontium-fluoride crystals or even of mixed crystals such as
calcium barium fluoride, for instance, has been proposed as a
substitute. These materials also enter into consideration for the
terminating element. However, these cubic crystals exhibit an
intrinsic birefringence which results in an impairment of the
imaging properties of the projection lens unless appropriate
countermeasures are taken.
[0046] For this reason the terminating element may comprise at
least two members consisting of one of the stated crystals, the
thicknesses of which are so chosen, and the crystal lattices of
which are so orientated relative to one another, that the influence
of intrinsic birefringence on projection light passing through is
at least approximately compensated.
[0047] The members may, for example, be joined to one another
seamlessly or may even be spaced from one another in the direction
of the optical axis. In the last-mentioned case an interspace
remaining between the members, which may be sealed all round, can
likewise be filled with a liquid that is transparent for the
projection light. The faces adjoining the interspace between the
members do not definitely have to be flat but may also exhibit a
curvature. If the face of the object-side partial element pointing
towards the interspace is concave and/or if the face of the
image-side members pointing towards the interspace is convex, then
a good compensation of the intrinsic birefringence can be achieved
also for beams that pass through the terminating element inclined
at large aperture angles.
[0048] In another advantageous configuration of the invention, at
least one face of the terminating element passed through by
projection light is reworked by local removal of material with a
view to correcting wavefront errors. This process, which is known
as such, of compensating for wavefront deformations by slight
removal of material, of the order of magnitude of a few nanometres,
can be employed particularly effectively in the case of the
terminating element, since the latter is located in the immediate
vicinity of the image plane. In this connection it is to be noted
that the quotient of the refractive indices of the terminating
element and of the immersion liquid is less than in dry systems
without immersion liquid, so that correspondingly more material has
to be removed in order to achieve the same effect as in dry
systems. Particularly when the terminating element is a
plane-parallel plate, the reworking of one or both faces turns out
to be particularly easy.
[0049] In another advantageous configuration of the invention a
protective layer that is impermeable in respect of immersion liquid
is applied onto at least one surface of the terminating element
that is able to come into contact with immersion liquid. Such a
protective layer is advantageous in particular when fluoride
crystals are used as material for the terminating element, since
these crystals exhibit a relatively high solubility in water. As a
result of the application of a layer of such a type, the material
can be prevented from being corroded if water or a substance
containing water is used by way of immersion liquid. The
application of such a protective layer is advantageous not only in
conjunction with a terminating element but quite generally in the
case of all optical elements consisting of cubic fluoride crystals,
particularly in the case of calcium-fluoride crystals that may come
into contact with the immersion liquid.
[0050] In the course of the application of a protective layer, care
should be taken to ensure that the latter completely covers the
face to be protected. Even extremely small openings in the
protective layer may result in a penetration of immersion liquid
and in the formation of underlayer corrosion. From this point of
view, an ion-assisted deposition of a protective layer of extremely
high compactness (preferably greater than 98%) is advantageous,
since as a result a local detachment of the protective layer in the
course of operation of the projection exposure apparatus is largely
prevented. The "compactness" of a material here is to be understood
to mean, for a given degree of crystallinity, the ratio of the
specific density of the material to a reference density at which
the material is perfectly free from cavities.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] Further advantages and features of the invention will become
apparent from the following description of the exemplary
embodiments on the basis of the drawing. Shown therein are:
[0052] FIG. 1 a meridional section through a projection exposure
apparatus according to a first exemplary embodiment of the
invention in greatly simplified, not-to-scale, schematic
representation;
[0053] FIG. 2 an enlarged detail from the image-side end of a
projection lens which is an integral part of the projection
exposure apparatus shown in FIG. 1;
[0054] FIG. 3 a projection exposure apparatus according to a second
exemplary embodiment of the invention in a representation
corresponding to FIG. 2, wherein the terminating element is
optically contacted with the final lens on the image side;
[0055] FIG. 4 a projection exposure apparatus with an additional
horizontal partition according to a third exemplary embodiment of
the invention in a detail representation corresponding to FIG.
1;
[0056] FIG. 5 a projection exposure apparatus with a displaced
terminating element according to a fourth exemplary embodiment of
the invention in a detail representation corresponding to FIG.
1;
[0057] FIG. 6 an enlarged detail from the image-side end of the
projection lens which is an integral part of the projection
exposure apparatus shown in FIG. 5, in a first displaced position
of the support and of the terminating element;
[0058] FIG. 7 the image-side end of the projection lens from FIG. 6
in a second displaced position of the support and of the
terminating element;
[0059] FIG. 8 the image-side end of the projection lens from FIG. 6
with tilted terminating element;
[0060] FIG. 9 an enlarged detail from the image-side end of a
projection lens according to a fifth exemplary embodiment of the
invention, wherein an additional cover covers the first
interspace;
[0061] FIG. 10 a further enlarged detail D from FIG. 9, in which
the transition between the cover and an edge of a tank receiving
the first immersion liquid is shown;
[0062] FIG. 11 the image-side end of a projection lens according to
a sixth exemplary embodiment of the invention, wherein the
image-side face of the final optical element on the image side is
curved;
[0063] FIG. 12 the image-side end of a projection lens according to
a seventh exemplary embodiment of the invention, wherein a
gas-filled interspace remains between the final optical element on
the image side and the immersion liquid;
[0064] FIG. 13 the image-side end of a projection lens according to
an eighth exemplary embodiment of the invention, wherein the
terminating element is divided up into two members along a curved
face;
[0065] FIG. 14 the image-side end of a projection lens according to
a ninth exemplary embodiment of the invention with terminating
element divided up in curved manner, which is completely received
in immersion liquid;
[0066] FIG. 15 the image-side end of a projection lens according to
a tenth exemplary embodiment of the invention, wherein a gas-filled
interspace remains between the terminating element and the
immersion liquid;
[0067] FIG. 16 the image-side end of a projection lens according to
an eleventh exemplary embodiment of the invention, wherein only the
interspace between the final optical element on the image side and
the terminating element is filled with immersion liquid.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0068] FIG. 1 shows, in greatly simplified schematic
representation, a meridional section through a microlithographic
projection exposure apparatus denoted overall by 10 according to a
first exemplary embodiment of the invention. The projection
exposure apparatus 10 exhibits an illumination system 12 for
generating projection light 13, which, inter alia, comprises a
light source 14, illuminating optics indicated by 16, and a
diaphragm 18. In the exemplary embodiment that is represented, the
projection light has a wavelength of 157 nm.
[0069] The projection exposure apparatus 10 further includes a
projection lens 20 which contains a plurality of lenses, only a few
of which, for the sake of clarity, are represented in exemplary
manner in FIG. 1 and denoted by L1 to L5. By reason of the short
wavelength of the projection light 13, the lenses L1 to L5 are
fabricated from calcium-fluoride crystals, which are still
sufficiently transparent even at these wavelengths. The projection
lens 20 serves to image, in reduced manner, a reticle 24 which is
arranged in an object plane 22 of the projection lens 20 onto a
light-sensitive layer 26 which is arranged in an image plane 28 of
the projection lens 20 and applied on a support 30.
[0070] The support 30 is fastened to the bottom of a tank-like
container 32 which is open in the upward direction and which is
capable of being displaced, in a manner not represented in any
detail, parallel to the image plane 28 with the aid of a displacing
device. The container 32 is filled up so far with an immersion
liquid 34 that during operation of the projection exposure
apparatus 10 the projection lens 20 is immersed in the immersion
liquid 34 with its final lens L5 on the image side. In the
exemplary embodiment that is represented, this lens L5 is a
large-aperture and comparatively thick lens; but here a
plane-parallel plate is also to be encompassed by the term
"lens".
[0071] Via a supply line 36 and a drainage line 38 the container 32
is connected to a conditioning unit 40 in which a circulating pump,
a filter for cleaning immersion liquid 34, and also a
temperature-control device are contained in a manner known as such
and therefore not represented in any detail. The conditioning unit
40, the supply line 36, the drainage line 38 and also the container
32 form overall an immersion device denoted by 42 in which the
immersion liquid 34 is circulated and in the process cleaned and
kept at constant temperature. The immersion device 42 serves, in a
manner known as such, for increasing the resolving power and/or the
depth of focus of the projection lens 20.
[0072] In an interspace 43 remaining between the final lens L5 on
the image side and the light-sensitive layer 26 an exchange element
44 is arranged, the details of which will be elucidated in the
following on the basis of FIG. 2.
[0073] FIG. 2 shows the image-side end face 45 of the projection
lens 20 in an enlarged detail from FIG. 1. In the enlarged
representation it can be discerned that the terminating element 44
has the shape of a plane-parallel plate with, for example, a
circular or rectangular base area and is separably and adjustably
attached to the image-side end face 45 of the projection lens 20
via two fastening elements indicated by 46 and 48. With a view to
illustrating the separability, a screw connection 52 is indicated
at the fastening element 46. For the purpose of adjustment,
fine-adjustment drives are provided which are indicated in FIG. 2
by micrometer screws 54, 55, 56 and 57.
[0074] The terminating element 44 comprises two plate-like members
44a and 44b connected to one another, which have the same
dimensions and which bear against one another seamlessly. By reason
of the short wavelength of the projection light 13, the two members
44a and 44b are also each fabricated from calcium-fluoride
crystals. The crystal lattices of the two members 44a, 44b are
orientated in such a way that a rotationally symmetrical
distribution of intrinsic birefringence results overall for the
terminating element 44. As an alternative to this, the terminating
element 44 may also comprise more than two members with differing
crystal orientations. With a total of four plane-parallel members
it is, for example, possible to compensate for the delay caused by
intrinsic birefringence to a very large extent for arbitrary
directions of incidence. Examples of the crystal orientations
considered here can be gathered, for example, from WO 02/093209 A2,
from WO 02/099450 A2 and also from US 2003/0011896 A1. The full
disclosure of these applications is incorporated herein by
reference.
[0075] In the case of the first exemplary embodiment shown in FIG.
2 the immersion liquid 34 flows around the exchange element 44 from
all sides and is located, in particular, in the two gap-like
interspaces 64 and 66 which remain between the terminating element
44, on the one hand, and the light-sensitive layer 26 or the final
lens L5 on the image side, on the other hand.
[0076] If emission of substances from the light-sensitive layer 26,
or a mechanical detachment of relatively small parts thereof,
occurs during the operation of the projection exposure apparatus
10, then the terminating element 44 prevents contaminants contained
in the immersion liquid 34 from being able to reach the flat
image-side boundary surface 68 of the final lens L5 of the
projection lens 20 in unhindered manner. Although such a contact is
also not totally ruled out, since the two gap-like interspaces 64
and 66 are not completely separated from one another, an exchange
of liquid between the gap-like interspaces 64 and 66 is at least
made considerably more difficult by the terminating element 44
situated in between. For this reason, contaminated immersion liquid
34 practically does not ascend to the final lens L5 but is
predominantly supplied via the drainage line 38 to the conditioning
unit 40 and cleaned therein.
[0077] By reason of the protective effect of the terminating
element 44 it is hardly still necessary to exchange the final lens
L5 on account of contamination by contaminated immersion liquid 34
and, in connection therewith, to adjust it in elaborate manner.
[0078] An exchange of the terminating element 44, on the other
hand, which is exposed in far higher measure to the contaminants
emanating from the light-sensitive layer 26, turns out to be
comparatively easy. For, in order to do this, the fastening
elements 46 and 48 merely have to be loosened from the housing of
the projection lens 20 with the aid of the screw connections 52.
The installation of the terminating element 44, which immediately
follows a cleaning or an exchange, also requires little adjustment
and is therefore easy. By reason of the design in the form of a
plane-parallel plate, the terminating element 44 is without
refractive power and therefore has only comparatively little effect
on the imaging properties. This holds, in particular, also for the
reason that the terminating element 44 floats in the immersion
liquid 34, so that, given suitable choice of the immersion liquid,
only a very slight or even infinitesimal refractive effect arises
at the boundary surfaces exposed to the projection light 13.
[0079] For all the optical elements that are fabricated from
fluoride crystals and that are able to come into contact with
immersion liquid a protective layer is applied, preferably at least
on the optically active faces, which protects the sensitive
crystals from the immersion liquid. In the first exemplary
embodiment represented in FIG. 2, protective layers 74, 76 and 78
of such a type are therefore applied onto the flat image-side
boundary surface 68 of the lens L5 and also onto the upper side 70
and onto the underside 72 of the terminating element 44.
[0080] The choice of the materials for the protective layers 74, 76
and 78 depends, above all, on the immersion liquid that is
employed, which in turn is chosen taking the wavelength of the
projection light that is being used into account. In the case of
light wavelengths of 193 nm, water, for example, enters into
consideration by way of immersion liquid, which rapidly corrodes
crystalline calcium fluoride on account of the relatively high
solubility thereof in water. In this case the protective layers 74,
76, 78 may consist of SiO.sub.2 or LaF.sub.3, since these materials
are not soluble in water.
[0081] In the case of a light wavelength of 157 nm, as used in the
exemplary embodiment described above, certain oils have a higher
transparency than water and are therefore better suited by way of
immersion liquid. The likewise highly transparent materials
MgF.sub.2 and LaF.sub.3, for example, then enter into consideration
as materials for the protective layers 74, 76, 78.
[0082] The protective layers 74, 76, 78 consisting of the stated
materials can be applied onto the boundary surfaces of the optical
elements in question by evaporation coating in a vacuum.
[0083] FIG. 3 shows, in a representation based upon FIG. 2, a
second exemplary embodiment of a microlithographic projection
exposure apparatus, wherein for like parts use is made of the same
reference numerals as in FIG. 2 and for parts corresponding to one
another use is made of reference numerals augmented by 200. In the
second exemplary embodiment the terminating element 244 is not
separated from the final lens L205 on the image side via an
interspace 66 but is optically contacted with said lens directly.
Provided that the lens L205 and the terminating element 244 are
fabricated from a material having the same refractive index, the
projection light 13 passes through the boundary surface between the
terminating element 244 and the final lens L205 without being
refracted. The fastening by optical contacting has the advantage
that no fastening elements 46, 48 are required. In addition, after
an exchange the terminating element 244 practically does not need
to be adjusted, since the two flat boundary surfaces 270 and 268
facing towards one another pertaining to the terminating element
244 and to the lens L205, respectively, guarantee the correct
arrangement by themselves in the course of optical contacting.
[0084] The image-side face 272 of the terminating element 244 is
reworked at some points 79a, 79b--represented in FIG. 3 on a
greatly exaggerated scale--by removal of material amounting to a
few nanometres in such a way that wavefront errors caused by the
projection lens 220 are corrected. Since reworking methods of such
a type are known as such, a more detailed elucidation will be
dispensed with.
[0085] FIG. 4 shows, in a representation based upon FIG. 1, a third
exemplary embodiment of a projection exposure apparatus, wherein
for like parts use is made of the same reference numerals as in
FIG. 1 and for parts corresponding to one another use is made of
reference numerals augmented by 300. The projection exposure
apparatus shown in FIG. 4 differs from that shown in FIG. 1 in that
it comprises not just one but two immersion devices 342a and 342b
which are independent of one another. The container 332 here is
subdivided horizontally by a partition 80 into two partial
containers 332a and 332b in such a way that the gap-like interspace
366 between the terminating element 44 and the final lens L5 on the
image side is arranged totally within the partial container 332a,
and the gap-like interspace 364 between the terminating element 44
and the light-sensitive layer 26 is arranged totally within the
partial container 332b. The terminating element 44 is sunk with
clearance in a suitably shaped cut-out 82 in the partition 80
between the partial containers 332a and 332b.
[0086] By virtue of the separation of the immersion liquids 334a
and 334b in separate immersion devices 342a and 342b, contaminated
immersion liquid from the partial container 332b is prevented from
getting into the gap-like interspace 366 between the terminating
element 44 and the lens L5 and from being able to contaminate the
latter in this way.
[0087] In the following a fourth exemplary embodiment will be
described on the basis of FIGS. 5 and 6, which show schematically a
detail from the image-side end of a projection lens, and an
enlarged detailed representation thereof, respectively. Parts
similar to those in FIGS. 1 to 4 are denoted by like reference
numerals, and parts corresponding to one another are denoted by
reference numerals augmented by 400.
[0088] In the fourth exemplary embodiment a terminating element 444
is provided parallel to the image plane 28. The terminating element
444 is likewise constructed in the form of a plane-parallel plate
which, however, is considerably larger than in the exemplary
embodiments described above. In the exemplary embodiment that is
represented, the terminating element 444 has a rectangular basic
shape and is sunk into a bottom 486 of a tank 488. The tank 488,
which, for example, may be fabricated from metal, a ceramic or a
crystal, serves for receiving a first immersion liquid 434a, which
in the exemplary embodiment that is represented is de-ionised
water. An edge 490 of the tank 488 is so high that, given an
appropriate filling height of the first immersion liquid 434a, a
first interspace 492 remaining between the final lens L5 on the
image side and the terminating element 444 is filled out completely
with the first immersion liquid 434a.
[0089] Between the terminating element 444 and the light-sensitive
layer 26 there remains a flatter second interspace 494 which is
filled with a second immersion liquid 434b. In the exemplary
embodiment that is represented, the second immersion liquid 434b is
likewise de-ionised water. The second interspace 494 is so flat
that the second immersion liquid 434b is hindered solely by
cohesive forces from discharging laterally out of the second
interspace 494. The smaller the spacing between the terminating
element 444 and the light-sensitive layer 26, the better do the
cohesive forces hold the second immersion liquid 434b in the second
interspace 494.
[0090] In order to reduce the requirements as regards the
parallelism of the terminating element 444 relative to the image
plane 28, a material having a refractive index that is as equal as
possible to the refractive index of the surrounding immersion
liquids 434a, 434b can be chosen for the terminating element 444.
In the case where use is made of water by way of immersion liquid,
LiF, which is still highly transparent at least at a wavelength of
193 nm, is suitable, for example, by way of material for the
terminating element 444. The difference in the refractive indices
then amounts to only 0.0066.
[0091] If the projection light has a particularly short wavelength,
for example 157 nm, the first immersion liquid 434a may also
consist of a fluorinated hydrocarbon that has a higher transmission
than water at these wavelengths. For the second immersion liquid
434b the somewhat lower transmission is not too disadvantageous,
inasmuch as the height of the second interspace 494 will generally
be very low. In addition, water has the advantage that it corrodes
the light-sensitive layer 26 less severely than is the case, for
instance, with fluorinated hydrocarbons.
[0092] In the fourth exemplary embodiment the projection exposure
apparatus is designed for scanning operation. This means that the
reticle 24 is displaced in the object plane 22 during the
projection. Synchronously with this, the support 30 with the
light-sensitive layer 26 applied thereon is also displaced parallel
to the image plane 28. The lateral magnification of the projection
lens 420 determines the ratio of the displacement speeds and the
displacement directions of the reticle 24 and of the support
30.
[0093] For this purpose, the support 30 is clamped, with the aid of
clamping elements 31a, 31b which are discernible in FIG. 5, on a
displaceable table 33 which is ordinarily designated as a wafer
stage. The table 33 can be displaced with great accuracy parallel
to the image plane 28 in a manner known as such with the aid of
actuating drives. The actuating drives are represented in
simplified manner in FIG. 5 and are denoted by 35a and 35b.
[0094] Manipulators 497a, 497b are fastened to the table 33, so
that said manipulators jointly execute displacement movements of
the table 33. The manipulators 497a, 497b are connected to the tank
488 via actuating arms 498a, 498b. The manipulators 497a, 497b are
constructed in such a way that they are able to move the tank 488
parallel to the image plane 28 and relative to the table 33, to
displace it perpendicular thereto, i.e. parallel to an axis OA, and
also to tilt it relative to the image plane 28. In the exemplary
embodiment that is represented, tilting movements in particular are
possible about two horizontal axes which extend perpendicular to
directions of motion of the table 33 and perpendicular to the
optical axis OA.
[0095] Furthermore, in FIG. 5 optional gas-discharge nozzles 499a,
499b are discernible, with which a stream of gas can be directed
onto a peripheral gap which is formed between the edge 490 of the
tank 488 and the light-sensitive layer 26.
[0096] The projection exposure apparatus show in FIGS. 5 and 6
operates as follows:
[0097] During a scanning operation the table 33 is displaced
together with the manipulators 497a, 497b in the direction of the
arrow 496b (see FIG. 6) with the aid of the actuating drives 35a,
35b. The manipulators 497a, 497b execute no actuating movements
during this process, so that the tank 488 with the terminating
element 444 sunk therein moves synchronously and at the same
displacement speed and in the same displacement direction with the
table 33 and consequently also with the light-sensitive layer 26.
In FIG. 6 this is indicated by an arrow 496a which has the same
direction and length as the arrow 496b. The tank 488 consequently
moves away beneath the projection lens 420 during the scanning
operation together with the light-sensitive layer 26.
[0098] FIG. 7 shows, in a detail corresponding to FIG. 6, the
relative position of the tank 488 and of the light-sensitive layer
26, on the one hand, and of the projection lens 420, on the other
hand, at the end of the scanning operation. Since the tank 488
moves synchronously, in parallel and at the same displacement speed
as the light-sensitive layer 26 during the scanning operation, no
shear forces act on the second immersion liquid 434b in the second
interspace 494. The second immersion liquid 434b therefore remains
in the second interspace 494 also during the displacement movements
of the support 30. An incident flow on the second immersion liquid
434b with gases emerging from the discharge nozzles 499a, 499b may
therefore be reduced or may even become superfluous. Hence one of
the significant causes of the formation of bubbles in the second
immersion liquid 434b ceases to apply, either entirely or
partially.
[0099] Since the first immersion liquid 434a remains in the tank
488 solely by reason of the force of gravity, here too no incident
flow with gases is required in order to prevent an escape of the
immersion liquid during the scanning operation. Bubbles are also
unable to arise to an appreciable extent as a result of the
intermixing which the fixed projection lens 420 brings about in the
first immersion liquid 434a passing by. Such an intermixing is
entirely desirable, since in this way the formation of relatively
large temperature gradients is counteracted.
[0100] Overall it is possible for considerably diminished numbers
of rejects to be achieved in this way, since neither in the first
immersion liquid 434a nor in the second immersion liquid 434b can
bubbles arise to an appreciable extent as a consequence of the
displacement movements during a scanning operation.
[0101] Between consecutive exposure cycles it is frequently
necessary to reposition the support 30, with the light-sensitive
layer 26 applied thereon, with respect to the projection lens 420.
The displacement speeds in the course of these positioning
movements are generally distinctly higher than in the course of the
movements during exposure.
[0102] If the tank 488 is exactly the same size as the
light-sensitive layer 26 applied on the support 30, during
positioning movements of such a type the tank 488 can be displaced
just as synchronously and at the same speed as has been described
above in connection with the scanning operations. In general,
however, for various reasons it will be expedient if the tank 488
has smaller dimensions parallel to the image plane 28 than the
light-sensitive layer 26 applied on the support 30. For example,
the smaller the tank 488, the smaller also is the interface of the
first immersion liquid 434a to a surrounding gas. Accordingly, less
heat of evaporation is also withdrawn from the first immersion
liquid 434a. This in turn has a favorable effect on a homogeneous
distribution of temperature, and hence of refractive index, within
the first immersion liquid 434a. From this point of view it would
be ideal if the tank 488 is only slightly larger than the region on
the light-sensitive layer 26 that is exposed overall during a
scanning operation.
[0103] For larger positioning movements, however, a smaller tank
488 means that the tank 488 cannot jointly execute this movement,
at least not fully. In this case a relative movement between the
light-sensitive layer 26 and the terminating element 444 is
unavoidable. This relative movement is generated by the
manipulators 497a, 497b which are fastened to the table 33.
[0104] In order to prevent formation of bubbles in the second
immersion liquid 434b also during a faster positioning movement of
the support 30, in the fourth exemplary embodiment represented in
FIGS. 5 to 7 the entire tank 488 can be additionally tilted with
the aid of the manipulators.
[0105] In FIG. 8 the image-side end of the projection lens 420 is
shown in a representation based upon FIGS. 6 and 7, the tank 488
having been tilted by 2.degree.. The tilt axis, which is denoted in
FIG. 8 by TA, extends perpendicular both to the optical axis OA and
to the direction of motion 496b of the support 30. By virtue of the
tilting movement of the tank 488 about the axis TA, the second
interspace 494, which has a constant height during a scanning
operation, is given the shape of a wedge-like prism. In the tilted
position of the tank 488 the end of the tank 488 situated at the
rear in the direction of motion 496b is removed just so far from
the light-sensitive layer 26 that damage thereto is avoided. The
cohesive forces now acting more strongly prevent, even at higher
positioning speeds, the second immersion liquid 434b from emerging
from the second interspace 494, whereas the support 30 moves away
in the direction of the arrow 496b beneath the fixed or at worst
slowly moving tank 488.
[0106] If during the scanning operations the spacing between the
light-sensitive layer 26 and the terminating element 444 is so
small that a tilting movement about the tilt axis TA could involve
damage to the layer 26, alternatively a tilt axis may be chosen
that extends through the end of the tank 488 situated at the rear
in the direction of motion 496b. For the manipulators 497a, 497b
this means that the manipulator 497b raises the side of the tank
488 situated at the front in the direction of motion by the
requisite distance.
[0107] In order in the third exemplary embodiment shown in FIGS. 5
to 8 to prevent undesirable losses of heat of the first immersion
liquid 434a and of the second immersion liquid 434b, the projection
exposure apparatus exhibits a container 90 which tightly seals the
space surrounding the immersion liquids 434a, 434b outwardly. Via
an inlet 92, water vapor which is generated by an evaporator 94 can
be introduced into the space surrounded by the container 90. The
water vapor is introduced until such time as the saturation vapor
pressure applying at the existing temperature is attained, at least
approximately, within the space surrounded by the container 90. In
this way the immersion liquids, which here each consist of water,
are prevented from gradually evaporating, which would result in a
cooling of the liquid at the interfaces to the surrounding
atmosphere. It will be understood that in the event of a
change-over to other immersion liquids other liquids also have to
be evaporated correspondingly in the evaporator 94.
[0108] A fifth exemplary embodiment will be described in the
following on the basis of FIGS. 9 and 10, which show schematically
a detail from the image-side end of a projection lens and an
enlarged detailed representation D thereof, respectively. Parts
similar to those in FIGS. 1 to 4 are denoted by identical reference
numerals; parts that have counterparts in the fourth exemplary
embodiment bear reference numerals augmented by 100.
[0109] In the fifth exemplary embodiment, in contrast with the
third exemplary embodiment shown in FIGS. 5 to 8, an additional
cover plate 500 is provided which almost totally covers the tank
588 in the upward direction. The cover plate 500, which does not
have to be transparent, exhibits an opening 502, through which the
image-side end of the projection lens 520 is immersed in the first
immersion liquid 534a. The edge 590 of the tank 588 slides along
the underside of the cover plate 500 in the course of a
displacement movement of the tank 588 indicated by an arrow
596a.
[0110] The space between the cover plate 500 and the tank 588 is
filled out completely with the first immersion liquid 534a. For
this purpose the opening 502 is so dimensioned that a peripheral
gap 504 remains around the image-side end of the projection lens
520, in which a liquid level can be adjusted.
[0111] FIG. 10 shows an enlarged detail D from the region of the
edge 590. In the detail D it can be discerned that the edge 590 of
the tank 588 is provided on its upward-pointing lateral face with a
peripheral wedge-shaped groove 506 and with a likewise peripheral
rectangular groove of larger cross-section, which constitutes a
reservoir 508 for the immersion liquid 534a. The reservoir 508 and
the wedge-shaped groove 506, which is connected to the reservoir
508 via a duct 510, are always filled with the first immersion
liquid 534a, so that a thin liquid film is formed on the
upward-pointing lateral face of the edge 590. This liquid film acts
as a lubrication and in this way enables low-friction and
vibration-free sliding of the tank 588 along the underside of the
cover plate 500.
[0112] In order to ensure that the liquid film does not break away
in the course of a movement of the tank 588 away beneath the cover
plate 500, with gas bubbles thereby being introduced into the first
immersion liquid 534a, the first immersion liquid 534a in the
reservoir 508 and in the wedge-shaped groove 506 is under a slight
overpressure. This overpressure is generated by first immersion
liquid 534a being constantly supplied under pressure to the
reservoir 508 via a supply line 512. At the same time, excess first
immersion liquid 534a is able to flow away via a drainage line 514.
If the contact pressure generated by the dead weight of the cover
plate 500 does not suffice by way of counterpressure, the cover
plate 500 can be additionally loaded, for example with the aid of
springs.
[0113] For the case where first immersion liquid 534a emerges
through a somewhat wider gap 516 on the outside of the edge 590, a
peripheral overflow channel 518 is provided which collects emergent
first immersion liquid 534a and conducts it away in a manner not
represented in any detail. A protective gas 519 that is heavier
than air and that, for example, may have the property of having
only very low solubility in respect of the first immersion liquid
534a may be charged into the overflow channel 518. As a result,
molecules from a gas surrounding the entire arrangement, which
impair the optical properties of the first immersion liquid 534a in
undesirable manner, are prevented from going into solution. The
protective gas 519 in the overflow channel 518 is may be renewed
continuously, in order to counteract a gradual intermixing with the
surrounding gas.
[0114] The seal in the region of the edge 590 is of importance to
the extent that the support 30 of the light-sensitive layer 26 is
not only frequently displaced in a plane parallel to the image
plane 28 but, with a view to diminishing imaging errors, can also
be tilted slightly about a horizontal axis. If the cover plate 500
is then not to be tilted together with it, the seal in the
direction towards the edge 590 must be so constructed that it
guarantees sufficient imperviousness in relation to the cover plate
500 even in the case of relatively small tilting movements of the
tank 588.
[0115] If the case may arise that the lubrication by the first
immersion liquid 534a is not sufficient for a short time, for the
cover plate 500 and the edge 590 it is advisable to choose
materials or coatings of these parts that minimize or even entirely
avoid a contamination of the first immersion liquid 534a as a
consequence of abrasion in the course of start-up. Aluminum oxide
or diamond, for example, enter into consideration here as
coatings.
[0116] The cover plate 500 has, on the one hand, the advantage that
the occurrence of waves in the tank 588 is prevented. On the other
hand, the cover plate 500 limits the interface of the first
immersion liquid 534a relative to a surrounding atmosphere to the
narrow peripheral gap 504 which remains between the image-side end
of the projection lens 520 and the cover plate 500. In this way
only very little heat is withdrawn from the first immersion liquid
534a as a consequence of evaporation. This in turn diminishes the
temperature gradient and hence the refractive-index gradient within
the first immersion liquid 534a, which is formed in the course of
heating by the projection light 13. In the case of the second
immersion liquid 534b the problem of evaporation does not exist to
an appreciable extent, inasmuch as the interface between the second
immersion liquid 534b and a surrounding atmosphere is in any case
very small.
[0117] In order to introduce the immersion liquids 534a, 534b into
the first and second interspaces 592 and 594, respectively, a
relatively small amount of the second immersion liquid 534b may
firstly applied onto the light-sensitive layer 26. Subsequently the
underside of the tank 588 is mounted on one side or parallel, and
the second immersion liquid 534b is expressed in bubble-free
manner. The spacing between the tank 588 and the light-sensitive
layer 26 can later be adjusted precisely with the aid of the
manipulators 497a, 497b.
[0118] Subsequently the cover plate 500 is laid over the tank 588.
In order to fill the tank 588 with the first immersion liquid 534a,
the latter may, for example, be charged via the peripheral gap 504
which remains between the projection lens 520 and the cover plate
500. However, it will be easier if the edge 590 of the tank 588 is
provided with an inlet and with an outlet, via which the first
immersion liquid 534a can be charged into the tank 588 and removed
therefrom. During operation of the projection exposure apparatus
the first immersion liquid 534a may also be circulated continuously
in a circuit, as elucidated in connection with the first three
exemplary embodiments.
[0119] Of course, the arrangement shown in FIGS. 9 and 10 may
likewise be accommodated in a container 90, as shown in FIG. 5 with
reference to the fourth exemplary embodiment. In this way the
evaporation of the immersion liquids is further reduced.
[0120] FIGS. 11 to 16 each show, in schematic representations based
upon FIG. 2, the image-side end of projection lenses according to
further exemplary embodiments of the invention. In these Figures,
for parts similar to those in FIG. 2 use is made of the same
reference numerals, and for parts corresponding to one another use
is made of reference numerals that are augmented by 600, 700, 800,
900, 1000 and 1100, respectively.
[0121] In the sixth exemplary embodiment shown in FIG. 11 the
image-side boundary surface 668 of the final lens L605 on the image
side is not flat but curved in aspherically concave manner. For it
has been shown that, in the case of immersion objectives in
particular, an aspherically curved face in the immediate vicinity
of the image plane 28 is particularly well suited for the
correction of higher-order imaging errors. The prerequisite for
this, however, is that the refractive indices of the final lens
L605 on the image side and of the immersion liquid 34 differ
sufficiently from one another.
[0122] In the case of the projection lens 620 the terminating
element 644 also comprises two members 644a, 644b which are
fabricated from calcium-fluoride crystals or from similar cubically
crystalline crystals with suitably chosen crystal orientations. In
the exemplary embodiment that is represented, the final lens L605
on the image side consists of quartz glass. As an alternative to
this, the final lens L605 on the image side may likewise consist of
a cubically crystalline material. The crystal orientations of the
crystals that the final lens L605 on the image side and the members
644a, 644b consist of may then likewise be so aligned that a very
extensive correction of the intrinsic birefringence is achieved.
The way in which a reciprocal birefringence compensation can be
achieved with three crystal orientations rotated relative to one
another about the optical axis is described in detail in the
printed publications WO 02/093209 A2, WO 02/099450 A2 and US
2003/0011896 A1 already mentioned above.
[0123] In the seventh exemplary embodiment represented in FIG. 12
the first interspace 792 remaining between the final lens L705 on
the image side and the terminating element 744 is not filled with
immersion liquid 34 completely but only partially. Therefore a
gap-like interspace 793 filled with a surrounding gas remains
between the final lens L705 on the image side and the immersion
liquid 34.
[0124] This variant is particularly advantageous in the case of
projection lenses that are provided both for dry operation and for
immersion mode. In order to have to perform as few modifications as
possible to the projection lens in the event of a change from dry
operation to immersion mode, and vice versa, the optical conditions
should change at as few boundary surfaces as possible. In the case
of the projection lens 720, for this reason the image-side face 768
of the final lens L705 on the image side still adjoins a
surrounding gas and not, for instance, immersion liquid 34.
[0125] On the other hand, also in the case of the projection lens
720 it is guaranteed that the terminating element 744 is surrounded
by the immersion liquid 34 on both sides. On account of the lower
refractive-index quotient at the boundary surfaces of the
terminating element 744, positional tolerances and manufacturing
tolerances of the terminating element 744 may consequently only
have a slight effect on the imaging properties of the projection
lens 720.
[0126] In the eighth exemplary embodiment shown in FIG. 13 the
terminating element 844, which is plane-parallel overall, likewise
comprises two members 844a, 844b which may consist of cubically
crystalline materials with differing crystal orientations. In
contrast with the exemplary embodiments described above, the
boundary surface between the two members 844a, 844b in the case of
the projection lens 820 is not flat but curved. Furthermore, the
two members 844a, 844b are not optically contacted with one another
directly but are spaced from one another, so that a narrow gap 899
remains between the members 844a, 844b, which in the case of the
projection lens 820 is filled with a surrounding gas.
[0127] In the case of the projection lens 820 only the image-side
partial element 844b comes into contact with the immersion liquid
34. Therefore it will generally be sufficient to exchange only the
partial element 844b when required. The object-side partial element
844a, on the other hand, may be mounted on or in the projection
lens 820 in such a manner that an exchange can only be carried out
with major effort. Consequently only the partial element 844b
constitutes an exchange element in the real sense of the word.
[0128] The partitioning of the terminating element 844 into two
members 844a, 844b along a curved parting surface has the advantage
that the image-side partial element 844b is likewise relatively
insensitive to manufacturing tolerances and positional tolerances.
For, on the one hand, the image-side face is immersed in immersion
liquid 34, so that the refractive-index quotient there is small. On
the other hand, on the object-side face of the partial element 844b
only relatively small entrance angles arise by reason of the convex
curvature thereof in the case of light beams that pass through the
terminating element 844 at large angles relative to the optical
axis, so that manufacturing tolerances and positional tolerances
are able to have less effect there.
[0129] The projection lens 920 shown in FIG. 14 according to a
ninth exemplary embodiment differs from the projection lens 820
merely by virtue of the fact that the immersion liquid 34 comes
directly up against the final lens L905 on the image side.
Therefore, unlike in the case of the projection lens 820 shown in
FIG. 13, both the gap 999 remaining between the two members 944a,
944b and the first interspace 992 between the final lens L905 on
the image side and the terminating element 944 are filled up with
immersion liquid 34. Positional tolerances and manufacturing
tolerances of the terminating element 944 have even less effect on
the imaging properties of the projection lens in this variant.
[0130] Since, although the object-side partial element 944a is
exposed to the immersion liquid 34, it is relatively well protected
by the final lens L905 on the image side or by the image-side
partial element 944b, also in the case of the projection lens 920
an exchange of optical elements on account of contamination may be
restricted to the image-side partial element 944b. However, the
immersion liquid 34 reaching as far as the final lens L905 on the
image side necessitates more extensive modifications if a
change-over from dry operation to immersion mode is desired.
[0131] FIG. 15 shows an image-side end of a projection lens 1020
according to a tenth exemplary embodiment of the invention. Unlike
in the case of the exemplary embodiments described previously, the
terminating element 1044 is not immersed in the immersion liquid
34. The faces of the terminating element 1044 that are permeated by
projection light consequently adjoin a surrounding gas both on the
object side and on the image side. This arrangement is also
advantageous, in particular, in the case of projection lenses that
are to be suitable both for dry operation and for immersion mode.
This is because the arrangement shown in FIG. 15 requires
particularly few modifications in the event of a change-over
between the operating modes. On the other hand, the optical
conditions at the two boundary surfaces of the terminating element
1044 are largely identical. This is advantageous to the extent
that, in particular, imaging errors that are generated by
positional tolerances, for example tilting movements, at the
object-side boundary surface, are compensated really well by
imaging errors acting in the opposite sense on the image-side
boundary surface.
[0132] The projection lens 1120 shown in FIG. 16 differs from the
projection lens 1020 shown in FIG. 15 by virtue of the fact that
the first interspace 1192 between the final lens L1105 on the image
side and the terminating element 1144 is filled out not with a
surrounding gas but with a liquid 1134. The image-side face of the
terminating element 1114 is therefore comparatively insensitive to
manufacturing tolerances, in particular fitting errors, of the
terminating element 1144.
[0133] The second interspace 1194 between the terminating element
1144 and the light-sensitive layer 26 can be filled up either
partially with immersion liquid 34, as is the case with the
projection lens 1020 shown in FIG. 15. During dry operation, as
shown by FIG. 16, the light-sensitive layer 26 is not covered by
immersion liquid.
[0134] The above description of the preferred embodiments has been
given by way of example. From the disclosure given, those skilled
in the art will not only understand the present invention and its
attendant advantages, but will also find apparent various changes
and modifications to the structures and methods disclosed. The
applicant seeks, therefore, to cover all such changes and
modifications as fall within the spirit and scope of the invention,
as defined by the appended claims, and equivalents thereof.
* * * * *