U.S. patent application number 12/187152 was filed with the patent office on 2008-12-04 for projection objective for immersion lithography.
This patent application is currently assigned to Carl Zeiss SMT AG. Invention is credited to Markus BROTSACK, Toralf GRUNER, Alexander HIRNET, Michael LILL, Ulrich LOERING, Alexander PAZIDIS, Guenter SCHEIBLE, Patrick SCHEIBLE, Sigrid SCHEIBLE, Harald SCHINK, Karl-Heinz SCHUSTER, Karl-Stefan WEISSENRIEDER, Christoph ZACZEK.
Application Number | 20080297745 12/187152 |
Document ID | / |
Family ID | 35060194 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080297745 |
Kind Code |
A1 |
WEISSENRIEDER; Karl-Stefan ;
et al. |
December 4, 2008 |
PROJECTION OBJECTIVE FOR IMMERSION LITHOGRAPHY
Abstract
In a projection objective provided for imaging a pattern
arranged in an object plane of the projection objective into an
image plane of the projection objective with the aid of an
immersion medium arranged between a last optical element of the
projection objective in the light path and the image plane, the
last optical element has a transparent substrate and a protective
layer system that is fitted to the substrate, is provided for
contact with the immersion medium and serves for increasing the
resistance of the last optical element to degradation caused by the
immersion medium.
Inventors: |
WEISSENRIEDER; Karl-Stefan;
(Aalen, DE) ; HIRNET; Alexander; (Oberkochen,
DE) ; PAZIDIS; Alexander; (Aalen, DE) ;
SCHUSTER; Karl-Heinz; (Koenigsbronn, DE) ; ZACZEK;
Christoph; (Heubach, DE) ; LILL; Michael;
(Aalen, DE) ; SCHEIBLE; Patrick; (Aalen, DE)
; SCHEIBLE; Guenter; (Stuttgart, DE) ; SCHEIBLE;
Sigrid; (Stuttgart, DE) ; SCHINK; Harald;
(Aalen, DE) ; BROTSACK; Markus; (Aalen, DE)
; LOERING; Ulrich; (Oberkochen, DE) ; GRUNER;
Toralf; (Aalen-Hofen, DE) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
Carl Zeiss SMT AG
|
Family ID: |
35060194 |
Appl. No.: |
12/187152 |
Filed: |
August 6, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11015553 |
Dec 20, 2004 |
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12187152 |
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60530623 |
Dec 19, 2003 |
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60530978 |
Dec 22, 2003 |
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60544967 |
Feb 13, 2004 |
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60568006 |
May 4, 2004 |
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60591775 |
Jul 27, 2004 |
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60592208 |
Jul 29, 2004 |
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60612823 |
Sep 24, 2004 |
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Current U.S.
Class: |
355/30 ; 359/355;
427/160 |
Current CPC
Class: |
G03F 7/70983 20130101;
G03F 7/70341 20130101 |
Class at
Publication: |
355/30 ; 359/355;
427/160 |
International
Class: |
G03B 27/52 20060101
G03B027/52; G02B 13/14 20060101 G02B013/14; B05D 5/06 20060101
B05D005/06 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 2004 |
DE |
102004051730.4 |
Claims
1.-69. (canceled)
70. An optical element usable for a projection optical system which
exposes a substrate by projecting a predetermined pattern onto the
substrate, the optical element comprising: a base of the optical
element which is installed to an end of the projection optical
system on a side of the substrate and through which the exposure is
performed in a state that a liquid is maintained between the
optical element and the substrate; and a corrosion resistant film
which is formed on at least a part of a surface of the base of the
optical element to avoid corrosion of the base by the liquid.
71. The optical element according to claim 70, wherein the
corrosion resistant film is an oxide film.
72. The optical element according to claim 71, wherein the oxide
film is at least one of a silicon oxide film and an aluminum oxide
film.
73. The optical element according to claim 70, wherein the base of
the optical element is fluorite.
74. The optical element according to claim 70, wherein a light
beam, which is used for the exposure, is an ArF laser beam.
75. The optical element according to claim 70, wherein the liquid
is pure water.
76. An exposure apparatus which exposes a substrate by projecting
an image of a predetermined pattern onto the substrate through a
liquid, the exposure apparatus comprising: a projection optical
system which projects the image of the pattern onto the substrate;
an optical element which is installed to an end of the projection
optical system on a side of the substrate; and an apparatus which
supplies the liquid to a space between the optical element and the
substrate, wherein: the optical element includes a base, and a
corrosion resistant film which is formed on at least a part of a
surface of the base in order to avoid corrosion of the base.
77. The exposure apparatus according to claim 76, wherein the
corrosion resistant film is an oxide film.
78. The exposure apparatus according to claim 77, wherein the
corrosion resistant film is a multilayer oxide film.
79. The exposure apparatus according to claim 77, wherein the oxide
film is at least one of a silicon oxide film and an aluminum oxide
film.
80. The exposure apparatus according to claim 76, wherein the base
of the optical element is fluorite.
81. The exposure apparatus according to claim 80, wherein a light
beam, which is used for the exposure, is an ArF laser beam.
82. The exposure apparatus according to claim 76, wherein the
liquid is pure water.
83. An optical element used for a liquid immersion exposure
apparatus which exposes a substrate with an exposure beam via the
optical element and a liquid, the optical element comprising: a
base; and a member which prevents the base from contacting with the
liquid to avoid corrosion of the base by the liquid, wherein the
optical element makes contact with the liquid when the substrate is
exposed with the exposure beam via the optical element and the
liquid.
84. The optical element according to claim 83, wherein the member
includes a corrosion resistant film.
85. The optical element according to claim 84, wherein the
corrosion resistant film includes a multi-layer film.
86. The optical element according to claim 84, wherein the
corrosion resistant film includes an oxide coating layer.
87. The optical element according to claim 86, wherein the oxide
coating layer has a corrosion resistance.
88. The optical element according to claim 85, wherein the
multi-layer film has at least one layer which has a corrosion
resistance.
89. The optical element according to claim 85, wherein an outermost
layer of the multi-layer film has a corrosion resistance.
90. The optical element according to claim 85, wherein each of
layers included in the multi-layer film has a corrosion
resistance.
91. The optical element according to claim 84, wherein the
corrosion resistant film has a thickness of 50 to 2000
angstrom.
92. The optical element according to claim 85, wherein the
multi-layer film has an antireflection function as a whole.
93. The optical element according to claim 86, wherein the oxide
coating layer has a corrosion resistance against pure water.
94. The optical element according to claim 86, wherein the oxide
coating layer is made of silicon oxide.
95. The optical element according to claim 83, wherein the exposure
beam passes through the member.
96. The optical element according to claim 95, wherein the exposure
beam passes through a light-outgoing surface of the optical
element.
97. The optical element according to claim 96, wherein the exposure
beam passed through the base is incident on the member, and the
exposure beam passed through the member is incident on the
liquid.
98. The optical element according to claim 83, wherein the member
is provided on a liquid side with respect to the base.
99. The optical element according to claim 98, wherein the member
makes contact with the liquid.
100. The optical element according to claim 98, wherein the member
is provided on the base.
101. The optical element according to claim 83, wherein the member
makes contact with the liquid.
102. The optical element according to claim 101, wherein the
exposure beam passes through the member.
103. The optical element according to claim 101, wherein the
exposure beam passes through a light-outgoing surface of the
optical element.
104. The optical element according to claim 83, wherein the member
is provided on the base.
105. The optical element according to claim 83, wherein the member
is provided on a surface of the base which contacts with the
liquid.
106. The optical element according to claim 83, wherein the base is
a plate-shaped base.
107. An optical system used for a liquid immersion exposure
apparatus, comprising the optical element as defined in claim
83.
108. A liquid immersion exposure apparatus comprising an optical
element as defined in claim 83.
109. A liquid immersion exposure apparatus which exposes a
substrate with an exposure beam through a liquid, the apparatus
comprising: an optical element including a base; and a member which
prevents the base from contacting with the liquid to avoid
corrosion of the base by the liquid, wherein the optical element
contacts with the liquid when the substrate is exposed with the
exposure beam via the optical element and the liquid.
110. The liquid immersion exposure apparatus according to claim
109, further comprising a liquid supply member having a liquid
supply outlet for supplying the liquid, wherein the substrate is
moved relative to the liquid supply member.
111. The liquid immersion exposure apparatus according to claim
109, wherein the member includes a corrosion resistant film.
112. The liquid immersion exposure apparatus according to claim
111, wherein the corrosion resistant film includes a multi-layer
film.
113. The liquid immersion exposure apparatus according to claim
111, wherein the corrosion resistant film includes an oxide coating
layer.
114. The liquid immersion exposure apparatus according to claim
112, wherein the multi-layer film has at least one layer which has
a corrosion resistance.
115. The liquid immersion exposure apparatus according to claim
112, wherein an outermost layer of the multi-layer film has a
corrosion resistance.
116. The liquid immersion exposure apparatus according to claim
112, wherein the multi-layer film has at least one oxide layer
which has a corrosion resistance.
117. The liquid immersion exposure apparatus according to claim
109, wherein the beam passed through the base is incident on the
member, and the beam passed through the member is incident on the
liquid.
118. The liquid immersion exposure apparatus according to claim
109, wherein the member is provided on a liquid side with respect
to the base and makes contact with the liquid.
119. The liquid immersion exposure apparatus according to claim
109, wherein the member makes contact with the liquid.
120. The liquid immersion exposure apparatus according to claim
119, wherein the exposure beam passes through a light-outgoing
surface of the optical element.
121. The liquid immersion exposure apparatus according to claim
109, wherein the member is provided on the base.
122. A method for producing an optical element used for a liquid
immersion exposure apparatus, comprising: providing a base; and
providing a member on a surface of the base, the member prevents
any portion of the base from contacting with the liquid.
123. The method according to claim 122, wherein the member is
formed by film forming method which is one of a sputtering method,
an ion beam assist method, an ion plating method and a heating
vapor deposition method.
124. A method for producing an optical element used for a liquid
immersion exposure apparatus, comprising: providing a plate-shaped
base; and providing a member on a surface of the base where the
optical element makes contact with the liquid to prevent any
portion of the base from contacting with the liquid.
125. A method for producing a liquid immersion exposure apparatus
which exposes a substrate with an exposure beam through a liquid,
comprising: providing an optical element including a base and a
member which is provided on a surface of the base and which
prevents any portion of the base from contacting with the liquid;
and providing a projection optical system in which the optical
element is installed.
Description
[0001] The present application claims priority benefit to U.S.
Provisional 60/530,623, filed Dec. 19, 2003; U.S. Provisional
60/530,978, filed Dec. 22, 2003; U.S. Provisional 60/544,967, filed
Feb. 13, 2004; U.S. Provisional 60/568,006, filed May 4, 2004; U.S.
Provisional 60/591,775, filed Jul. 27, 2004; U.S. Provisional
60/592,208, filed Jul. 29, 2004; U.S. Provisional 60/612,823, filed
Sep. 24, 2004; and German Application 102004051730.4, filed Oct.
22, 2004. The disclosures of all of these foreign and domestic
applications are incorporated into this application by
reference.
[0002] The invention relates to a projection objective for imaging
a pattern arranged in an object plane of the projection objective
into an image plane of the projection objective with the aid of an
immersion medium arranged between an optical element of the
projection objective and the image plane.
[0003] Photolithographic projection objectives have been used for
several decades for fabricating semiconductor components and other
finely patterned devices. They serve for projecting patterns from
photomasks or reticles, also referred to hereinafter as masks or
reticles, onto an article coated with a light-sensitive layer with
very high resolution on a demagnifying scale.
[0004] Principally, three developments proceeding in parallel are
contributing to the production of ever finer structures of the
order of magnitude of 100 nm or less. Firstly, attempts are being
made to increase the image-side numerical aperture (NA) of the
projection objectives beyond the currently customary values into
the range of NA=0.8 or higher. Secondly, ever shorter wavelengths
of ultraviolet light are being used, preferably wavelengths of less
than 260 nm, for example 248 nm, 193 nm, 157 nm or less. Finally,
other measures for enhancing the resolution are also being used,
for example phase-shifting masks and/or oblique illumination.
[0005] There are also already approaches for improving the
resolution that can be obtained by introducing an immersion medium
having a high refractive index into the space between the last
optical element of the projection objective and the substrate. This
technique is referred to here as immersion lithography. The
projection objectives suitable for this are referred to as
immersion objectives. Introduction of the immersion medium results
in an effective wavelength .lamda..sub.eff=.lamda..sub.0/n.sub.I,
where .lamda..sub.0 is the vacuum operating wavelength and no is
the refractive index of the immersion medium. This results in a
resolution R=k.sub.1(.lamda..sub.eff/NA.sub.0) and a depth of focus
(DOF) DOF=.+-.k.sub.2(.lamda..sub.eff/NA.sub.0.sup.2), where
NA.sub.0=sin .THETA..sub.0 is the "dry" numerical aperture and
.THETA..sub.0 is half the aperture angle of the objective. The
empirical constants k.sub.1 and k.sub.2 are process-dependent.
[0006] The theoretical advantages of immersion lithography reside
in the reduction of the effective operating wavelength and the thus
improved resolution. This can be achieved with an unchanged vacuum
wavelength, so that technologies that have been established for the
corresponding wavelength with respect to the generation of light,
with respect to the selection of optical materials, with respect to
the coating technology, etc. can be adopted largely unchanged. The
use of immersion media is additionally a prerequisite for the use
of projection objectives having very high numerical apertures in
the range of NA=1 or higher.
[0007] For 193 nm, ultrapure water where n.sub.I.apprxeq.1.437 is
notable as a suitable immersion liquid.
[0008] The article "Immersion Lithography at 157 nm" by M. Switkes
and M. Rothschild, J. Vac. Sci. Technol. B19(6), November/December
2001, page 1 et seq., presents immersion liquids based on
perfluoropolyethers (PFPE), which are sufficiently transparent for
an operating wavelength of 157 nm and are compatible with some
photoresist materials that are currently used in microlithography.
One tested immersion liquid has a refractive index n.sub.I=1.37 at
157 nm. The publication also presents a lens-free optical system
for immersion interference lithography which operates with calcium
fluoride elements and silicon mirrors and is intended to enable the
imaging of 60 nm structures and below at a numerical aperture of
NA=0.86.
[0009] The applicant's patent applications WO 03/077036 and WO
03/077037 show refractive projection objectives for
microlithography which are suitable for immersion lithography on
account of high image-side numerical aperture.
[0010] The use of liquid immersion media represents a challenge not
only for the optical design of projection objectives; at other
points, too, modifications of known processes and devices are
necessary in order to obtain stable processes.
[0011] The patent specification U.S. Pat. No. 4,480,910 and U.S.
Pat. No. 5,610,683 (corresponding to EP 0 605 103) describe
projection exposure apparatuses provided for immersion lithography
and having devices for introducing immersion fluid between the
projection objective and the substrate.
[0012] The Japanese patent application JP 10-303114 A shows a
projection exposure apparatus provided for immersion lithography.
In order to reduce imaging problems on account of heating of the
immersion liquid, the use of an aqueous immersion liquid having
only a low temperature coefficient of the refractive index is
proposed. The intention is thereby to reduce temperature-dictated
variations in refractive index and losses in imaging quality that
are caused thereby.
[0013] One object of the invention is providing a projection
objective which is suitable for immersion lithography and whose
imaging quality is stable even in the event of prolonged contact
being made with immersion liquid.
[0014] Various formulations of the invention are reflected in the
independent claims. Advantageous developments are specified in the
dependent claims. The wording of all of the claims is made the
content of the description by reference.
[0015] One formulation of the invention provides a projection
objective for imaging a pattern arranged in an object plane of the
projection objective into an image plane of the projection
objective, which operates with the aid of an immersion medium
arranged between an optical element and the projection objective
and the image plane. The optical element has a substrate that is
transparent to the operating wavelength of the projection objective
and a protective layer system that is fitted to the substrate, is
provided for contact with the immersion medium and serves for
increasing the resistance of the optical element to degradation
caused by the immersion medium.
[0016] The optical element provided with the protective layer
system is preferably the last optical element in the light path,
which is followed directly by the image plane.
[0017] The protective layer system is designed in such a way that a
substantial lengthening of the service life of the projection
objective results compared with projection objectives without such
a protective layer system. This means, in particular, that the
optical properties of the optical element which comes into contact
with the immersion medium, or of the projection objective as a
whole, remain within a specification range for substantially longer
than without the protective layer system. The invention takes
account of the fact that, in the event of a prolonged contact
between a possibly aggressive immersion medium and the projection
objective, the optical properties of the projection objective which
are crucial for the imaging are impaired for example by virtue of
the fact that the immersion medium chemically and/or physically
attacks the optical element that comes into contact with the
immersion medium, that is to say the substrate and/or a coating
fitted to the substrate, and thus leads to an impairment of its
optical properties that are prescribed by the design. This effect
is avoided by the invention or at least significantly reduced in
such a way that the service life of the projection objective is not
limited by immersion-dictated degradation of the optical properties
of the optical element that comes into contact with the immersion
medium.
[0018] Since generally only the last optical element in the light
path comes into contact with the immersion medium, the optical
element that comes into contact with immersion medium is
hereinafter also referred to as "the last optical element".
However, the advantages of the invention can be obtained even when
the optical element that comes into contact with the immersion
medium is the first element in the light path or an optical element
arranged within the projection objective. Therefore, the term "the
last optical element" is representative in many cases of an optical
element which is provided for contact with an immersion medium.
[0019] A protective layer system in the sense of this application
may be formed by a single material layer. By way of example, an
essentially plane-parallel plate made of a transparent bulk
material or a thin single layer produced in a thin-film method may
be involved. A protective layer system may also comprise a
plurality of material layers lying one above the other and be
formed e.g. as a dielectric alternating layer system or as a coated
plate.
[0020] In one development, the substrate consists of a fluoride
crystal material, in particular of calcium fluoride. The use of
fluoride crystal materials for the last optical element is
practically mandatory in the case of systems for operating
wavelengths of 157 nm or less since other optical materials, for
example synthetic quartz glass, are generally not sufficiently
transparent to this wavelength. In the case of systems for higher
wavelengths, for example 193 nm, as well, the use of calcium
fluoride for the last optical element may be expedient since this
element arranged near to the image plane is exposed to high
radiation loads and calcium fluoride, in contrast to synthetic
quartz glass, has a lower tendency toward radiation-induced density
changes. On the other hand, the inventors have discovered that
calcium fluoride is sparingly soluble in water, with the result
that the substrate material would be chemically attacked when using
water or aqueous solutions as immersion liquid. This can be avoided
by means of the invention. In other embodiments, the last optical
element consists of synthetic quartz glass.
[0021] The invention can be used independently of the form of the
last optical element. In some embodiments, the last optical element
is a planoconvex lens having a spherically or aspherically curved
entry face and an essentially planar exit face to which the
protective layer system is fitted. In other embodiments, the last
optical element is an essentially plane-parallel plate, which is
exchangeable in some embodiments. The plane-parallel plate may, by
way of example, be wrung onto a penultimate optical element or be
connected thereto optically neutrally in a different way. An
exchangeable last optical element facilitates maintenance work
particularly if the optical properties of the last optical element
should deteriorate beyond a tolerable extent over the course of
time owing to the use of an immersion medium.
[0022] In many cases it may suffice for the protective layer system
to be essentially fitted only to the image-side exit face of the
last optical element. There are also embodiments in which the
protective layer system is fitted to the image-side exit face of
the substrate and also extends continuously over adjoining side
areas of the substrate. If appropriate, the protective layer system
may also extend right over the image-remote entry face of the
substrate. Consequently, depending on the requirement, it is
possible to create an "all round protection" for jeopardized
surfaces of the substrate which, if appropriate, might be wetted by
creeping immersion medium in the absence of such protection.
[0023] In one development, the protective layer system comprises at
least one barrier layer that is essentially impermeable to the
immersion medium. The barrier layer may comprise of at least one
barrier layer material that is essentially chemically resistant to
the immersion medium, and be essentially free of pores passing
through from an outer side of the barrier layer that is remote from
the substrate to the side of the barrier layer that faces the
substrate. A barrier layer can be used to prevent immersion medium
from penetrating as far as the substrate to a significant degree.
The barrier layer may be provided by itself or in combination with
further material layers. A barrier layer may be formed as a single
layer or as a multi-layer.
[0024] If particles such as polishing residues, dust or the like
and/or mechanical damage such as scratches or the like are present
on the surface of the substrate to be protected prior to coating as
a consequence of preceding processing steps, there is the risk of a
coating applied to this non-ideal surface being disturbed directly
in the vicinity of these critical locations, with the result that
the substrate material cannot be totally sealed against immersion
liquid. As a result, the substrate material may be attacked at
points in the region of defects in a coating. In one development
problems resulting from this are avoided or reduced by means of a
particular method variant in the production of the protective layer
system. Firstly, a first layer of a multilayer system is applied on
the substrate or on a coating applied on the substrate. Afterward,
a portion of the first layer is removed again with the aid of an,
in particular, mechanical polishing method, as a result of which a
polishing surface arises on the partly removed first layer.
Afterward, a second layer is applied to said polishing surface of
the first layer. If appropriate, the steps of partial removal of an
underlying layer by polishing and subsequent coating with a
subsequent layer are repeated once or a number of times. Cleaning
of the polishing surface is preferably carried out between the
removal step and the subsequent coating. During the removal step,
generally some of the particles are sheared off or torn out from
the coated area. During the subsequent coating of the second layer,
generally particles will then admittedly be present again, but not
at the same locations. The method steps can be repeated as often as
until every particle has been removed at least once. If defects
remain within the individual layers during this multiple coating,
then the defects in the first and second layers are largely or
completely distributed at different lateral positions within the
layers, with the result that a defect-free region exists in at
least one of the layers of the multilayer system essentially at
every location of the latter. As a result, the multilayer system is
impermeable to the immersion medium over its entire lateral extent
in the direction perpendicular to the layer extent.
[0025] All of the layers of the multilayer system may consist of
the same material. It is also possible for layers made of different
materials to be produced, as a result of which e.g. an alternating
layer system may be created.
[0026] The protective layer system may contain at least one barrier
layer having at least one fluoride material which is essentially
transparent to the corresponding operating wavelength and is also
essentially insoluble with respect to the immersion medium. In
particular, depending on the operating wavelength, the barrier
layer may contain at least one of the following materials or
essentially consist of such a material: actinium fluoride
(AcF.sub.3), bismuth fluoride (BiF.sub.3), erbium fluoride
(ErF.sub.3), europium fluoride (EuF.sub.3), gadolinium fluoride
(GdF.sub.3), holmium fluoride (HoF.sub.3), potassium magnesium
fluoride (KMgF.sub.3), lanthanum fluoride (LaF.sub.3), sodium
yttrium fluoride (NaYF.sub.4), neodymium fluoride (NdF.sub.3),
samarium fluoride (SmF.sub.3), terbium fluoride (TbF.sub.3),
titanium fluoride (TiF.sub.3), thulium fluoride (TmF.sub.3),
vanadium fluoride (VF.sub.3), ytterbium fluoride (YbF.sub.3),
yttrium fluoride (YF.sub.3). All of the materials mentioned are
suitable down to 193 nm. In particular, the rare earth fluorides
ErF.sub.2, GdF.sub.3, LaF.sub.3 and also KMgF.sub.3 can also be
used at 157 nm.
[0027] It is also possible for the protective layer system to
comprise at least one barrier layer that contains at least one of
the following oxide materials or essentially consists of one of
said materials: silicon dioxide (SiO.sub.2), magnesium aluminum
oxide (MgAl.sub.2O.sub.4), aluminum oxide (Al.sub.2O.sub.3),
tungsten dioxide (WO.sub.2), tungsten trioxide (WO.sub.3). In this
case, all materials are suitable to 193 nm, and SiO.sub.2 can also
be used at 157 nm if small layer thicknesses are chosen.
[0028] In one development, the barrier layer essentially consists
of an oxidic material having a high packing density. The packing
density should be more than 95%, in particular more than 97%,
preferably more than 98%, of the density of the bulk material.
Refractive index differences with respect to the bulk material,
relative to the refractive index of isotropic substances or to the
average refractive index of anisotropic substances for the ordinary
and extraordinary rays, should be less than 5%, preferably less
than 3%, in particular less than 2%.
[0029] The use of silicon dioxide (SiO.sub.2) is particularly
favorable since this material can be used down to 193 nm as an
absorption-free material having a low refractive index in
interference layer systems and can be applied substantially in a
manner free of pores given suitable coating technology. It is
particularly expedient if the barrier layer essentially consists of
ion-sputtered oxide material, in particular silicon dioxide.
Experiments have shown that, in the case of ion enhanced deposition
of silicon dioxide or other oxide materials, the packing density of
the deposited material can be significantly increased, thereby
promoting the suitability as a barrier layer against an immersion
medium.
[0030] Another possibility consists in applying the barrier layer
material, in particular silicon dioxide, in a PECVD method. In the
case of plasma enhanced chemical vapor deposition, the constituents
are supplied in gaseous form as monomers and activated chemically
in a microwave plasma. Given suitable process conditions, a
hydrocarbon-free, only weakly absorbent and essentially pore-free
oxide layer, e.g. a quartz layer, forms on the substrate.
[0031] In one development, it has proved to be expedient if the
barrier layer has an optical layer thickness of between
approximately 0.15.lamda. and 0.6.lamda., in particular between
approximately 0.2.lamda. and 0.3.lamda., or between approximately
0.4.lamda. and 0.6.lamda., where .lamda. is the operating
wavelength of the projection objective. This is expedient
particularly when, for a refractive index difference .DELTA.n
between barrier layer material and immersion medium, the following
holds true: .DELTA.n.gtoreq.0.04. Such layers may be used by
themselves or in conjunction with further layers made of dielectric
material having a high refractive index or low refractive index as
an interference layer system with a reflection-reducing effect. By
way of example, the barrier layer itself may be designed as an
antireflection layer. For this purpose, its layer thickness may
essentially correspond to a quarter of the operating wavelength or
an odd multiple thereof. The optical layer thickness of the barrier
layer may also be adapted to the optical properties of a monolayer
or multilayer layer system adjoining the barrier layer in such a
way that a reflection-reducing effect is established in conjunction
with the layer system. Barrier layers having a geometrical layer
thickness of .ltoreq.15 nm are also possible.
[0032] In some embodiments, the barrier layer is applied directly
to an exit-side surface of the substrate. It is possible for an
antireflection layer system to be applied to a surface of the
barrier layer that is remote from the substrate. As an alternative
or in addition, it is also possible for an antireflection layer
system to be arranged between the substrate and the barrier layer.
In both cases, the antireflection layer system may be formed by a
single layer or by a multilayer system comprising a plurality of
single layers having dielectric material alternately having a high
refractive index and having a low refractive index.
[0033] The dielectric materials should be selected such that they
are essentially absorption-free at the operating wavelength
provided. Each of the layers having a low refractive index may
contain, depending on the operating wavelength, one of the
following materials exclusively or in combination with other
materials of this group: magnesium fluoride (MgF.sub.2), aluminum
fluoride (AlF.sub.3), chiolite (Na.sub.5Al.sub.3F.sub.14), cryolite
(Na.sub.3AlF.sub.6), silicon dioxide (SiO.sub.2). Each of the
layers having a high refractive index may contain one of the
following materials exclusively or in combination with other
materials of this group: lanthanum fluoride (LaF.sub.3), gadolinium
fluoride (GdF.sub.3), erbium fluoride (ErF.sub.3), aluminum oxide
(Al.sub.2O.sub.3), hafnium oxide (HfO.sub.2), zirconium dioxide
(ZrO.sub.2), holmium fluoride (HoF.sub.3), neodymium fluoride
(NdF.sub.2), samarium fluoride (SmF.sub.3), terbium fluoride
(TbF.sub.3), titanium fluoride (TiF.sub.3), yttrium fluoride
(YF.sub.3), ytterbium fluoride (YbF.sub.3), magnesium aluminum
oxide (MgAl.sub.2O.sub.4), tungsten trioxide (WO.sub.3) and
tungsten dioxide (WO.sub.2).
[0034] If an antireflection layer system is provided for contact
with the immersion medium, consideration should be given to a
lowest possible removal rate during operation. Therefore, from the
material groups mentioned, those materials are preferred in the
case of which the removal rate with respect to ultrapure water is
less than approximately 0.01 mg/(cm.sup.2day), in particular less
than 0.005 or 0.002 mg/(cm.sup.2day).
[0035] In one embodiment, a magnesium fluoride/lanthanum fluoride
alternating layer system is provided.
[0036] It has been found that, as an alternative to inorganic
materials, it is also possible to use other materials with a
hydrophobic effect for producing barrier layers. In one
development, the protective layer system comprises at least one
barrier layer made of a perfluorinated fluorocarbon that is
essentially water-impermeable to the immersion medium. By way of
example, perfluorinated alkanes and siloxanes can be used. Suitable
products are sold for example by the company Merck under the
designations WR1, WR2 or WR3 or WR4. It has been found that WR3 is
expedient particularly in conjunction with fluoridic materials,
while WR1 and WR2 are expedient particularly in combination with
oxidic materials. It is also possible to use
polytetrafluoroethylene (PTFE), which is known inter alia by the
trade name Teflon.RTM..
[0037] It is also possible to use liquid materials or materials
exhibiting lubricity for the formation of a protective layer
system. When using ultrapure water as the immersion medium, it is
possible to use, by way of example, oils or greases suitable for
vacuum technology, such as the vacuum oil available under the
designation Fomblin. Other greases based on fluoropolyethers (PFPE)
are also possible. During operation of the projection exposure
apparatus, the liquid protective medium or protective medium
exhibiting lubricity may be applied, if appropriate, progressively
again and again to the exit face of the projection objective and
adjoining areas in order to afford reliable protection against
attack by the immersion medium. In this case, care must be taken to
ensure that the protective film material does not contaminate the
immersion medium. In the case of water, therefore, the protective
film material should have no or only vanishingly sparing solubility
in water.
[0038] In the design of protective layer systems according to the
invention, the optical properties thereof should be adapted to the
optical properties of the immersion medium, in particular to the
refractive index n.sub.I thereof. The provision of protective layer
systems that are adapted in terms of refractive index is possible
in different ways within the scope of the invention.
[0039] In one embodiment, the protective layer system is designed
as a graded index layer with a continuous or discontinuous
refractive index profile perpendicular to the layer extent. In this
case, preferably, a refractive index in a region near the substrate
essentially corresponds to the refractive index of the substrate
material and a refractive index in a region provided for contact
with the immersion medium essentially corresponds to the refractive
index of the immersion medium. An ideal reflection-reducing effect
can be achieved, at least approximately, in this way.
[0040] A layer having a continuous gradient may be implemented for
example by joint deposition of two or more dielectric materials
having a different refractive index, the ratio of which changes in
the course of deposition. It is also possible to deposit two or
more different, dielectric materials alternately in order to
produce a gradient of the average refractive index in small steps.
A nano-layer-patterned mixed material can be produced given
sufficiently small layer thicknesses.
[0041] When designing a suitable protection for projection
objectives against immersion-dictated degradation of the optical
properties, it must be taken into account that some immersion
liquids, in particular with simultaneous irradiation with hard
ultraviolet radiation, may chemically/physically attack the
material that is in contact with the immersion liquid. Such wear
that is essentially caused by material removal and/or chemical
reactions can limit the service life of the projection objective
and necessitate exchange or repair. Some embodiments provide
measures that make it possible to render such more or less
unavoidable surface damage optically ineffective.
[0042] For this purpose, one embodiment provides for the protective
layer system to be designed as a wear system that is optimized in
terms of refractive index. The wear system may be optimized with
regard to its optical properties in such a way that a gradual
decrease in thickness and/or a gradual change in the optical
properties in the boundary region with respect to the immersion
medium do not lead to a substantial change in the optical
properties of the protective layer system and hence of the
projection objective. In other words: the optical effect of the
protective layer system becomes relatively insensitive to
immersion-dictated property changes.
[0043] In accordance with one development, the protective layer
system comprises a bulk material plate that is provided for contact
with the immersion medium and is made of a plate material whose
refractive index is less than the refractive index of the
substrate, in particular of calcium fluoride. The refractive index
of the plate material should preferably be in proximity to the
refractive index n.sub.I of the immersion medium. An example of a
plate material that is taken into consideration is lithium fluoride
(LiF) where n.apprxeq.1.443 at 193 nm. If, at 193 nm, ultrapure
water having a refractive index of n.sub.I.apprxeq.1.437 is used as
the immersion liquid, then the crucial refractive index difference
.DELTA.n with respect to the plate material is less than 0.01, as a
result of which an almost ideal refractive index adaptation is
possible. The bulk material plate may be exchangeable (exchangeable
plate).
[0044] Generally, when using an immersion medium having a
refractive index n.sub.I, it has proved to be advantageous if the
protective layer system has an effective refractive index n.sub.ss
at least in a region adjoining the immersion medium, such that
.DELTA.n<0.05 holds true for a refractive index difference
.DELTA.n=|n.sub.I-n.sub.ss|. Smaller refractive index differences
are preferred in accordance with .DELTA.n<0.01, in particular in
accordance with .DELTA.n<0.005. The optical properties of such
protective layer systems are relatively insensitive to gradual
material dissolution by the immersion medium, so that the
protective layer systems can be used as a wear layer.
[0045] In some embodiments, a dielectric antireflection layer
system having one or a plurality of single layers is arranged
between the substrate and the wear system, in particular the wear
plate.
[0046] It must be assumed that optically transparent materials are
not available for all immersion media in order to provide, from
bulk material, a wear system that is adapted in terms of refractive
index with a sufficiently small refractive index difference with
respect to the immersion medium and/or a reflection-reducing
system. In one development, the protective layer system consists
of, at least in a region adjoining the immersion medium, a mixed
material having at least one first material having a low refractive
index and at least one second material having a high refractive
index. Preferably, the first material has a refractive index
n.sub.L {square root over (n.sub.In.sub.S)} and the second material
has a refractive index n.sub.H> {square root over
(n.sub.In.sub.S)}, where n.sub.I is the refractive index of the
immersion medium and n.sub.S is the refractive index of the
substrate material, and a ratio of the first and second materials
being chosen such that an average refractive index n.sub.MIX of the
mixed material is present. In the selection of the material
combinations, care must be taken to ensure that the mixing partners
have essentially the same solubility in the immersion medium, so
that the immersion medium does not selectively attack one of the
components during operation. In this way, it is possible to avoid
gradual alterations of the optical properties and/or an evolving
porosity on account of preferred dissolution of a mixing
partner.
[0047] For wear systems, the average refractive index n.sub.MIX can
be set such that it is in proximity to the refractive index n.sub.I
of the immersion medium. Preferably, .DELTA.n<0.05 holds true,
in particular .DELTA.n<0.01 or .DELTA.n<0.005 holds true,
where .DELTA.n==|n.sub.I-n.sub.MIX|.
[0048] If the intention is to provide an essentially wear-free
protective layer system having optimized reflection-reducing
properties, then it may be expedient for n.sub.MIX.apprxeq. {square
root over (n.sub.In.sub.S)} to be set. An average refractive index
in the region of the geometric mean of the refractive indices of
the materials adjoining on both sides ensures that the reflected
amplitudes at the interfaces between substrate and protective layer
and also between protective layer and immersion medium have
essentially the same magnitude, with the result that an optimum
destructive interference becomes possible given suitable setting of
the relative phase of reflected partial beams. Preferably, at least
one of the following conditions should be met: n.sub.MIX= {square
root over (n.sub.In.sub.S)}.+-.2%, in particular .+-.1% and/or |
{square root over (n.sub.In.sub.S)}-n.sub.MIX|<0.02, in
particular <0.01. This enables optically effective monolayer
antireflection layers having optical thicknesses Id.sub.QWOT where
d.sub.QWOT is the layer thickness of a quarter wave layer of the
material and I is an odd integer.
[0049] The mixed material may be constructed as a nanopatterned
multilayer material or as a material with a continuous or
homogeneous mixture of two or more components. In an embodiment
designed for 193 nm, magnesium fluoride (MgF.sub.2) and lanthanum
fluoride (LaF.sub.3) are deposited jointly. This combination is
particularly resistant.
[0050] As an alternative to a layer system made of mixed material,
it is also possible to provide a single layer made of a single
material, for example a layer made of magnesium fluoride
(MgF.sub.2), the refractive index of which at 193 nm is
approximately n.apprxeq.1.44 and is thus well matched to the
refractive index n=1.443 of ultrapure water at 193 nm.
[0051] It may be complicated to find a suitable coating process
(for example vapor deposition, sputtering or the like) which
guarantees a hole-free and scratch-free layer. Furthermore, thin
layer systems produced by coating processes are generally sensitive
to cleaning during operating in the field, that is to say when
being operated by the customer, so that there is the risk of the
protective layer system subsequently being damaged during cleaning.
Some embodiments use protective layer systems which can be provided
without a coating process. In this case, the protective layer
system is formed by an essentially plane-parallel plate (protective
plate) which is wrung onto an essentially planar interface of the
substrate. During wringing, an optically neutral and, if
appropriate, rereleasable connection is produced between the
substrate and the wrung element. The substrate and the wrung plate
may be held jointly, that is to say as a combined optical element
of multipartite construction, in a common holder.
[0052] In one variant, the substrate consists of a fluoride crystal
material, in particular of calcium fluoride, and the protective
plate consists of synthetic quartz glass. Such embodiments are
beneficial particularly when using immersion media that essentially
consists of water, since the substrate material exhibiting
relatively high solubility in water can be permanently protected
against incipient dissolution by the chemically resistant and
water-insoluble quartz glass.
[0053] If use is made of a protective plate made of synthetic
quartz glass which is wrung onto a substrate made of fluoride
crystal material, temperature changes may be problematic since, on
account of the different coefficients of thermal expansion of plate
material and substrate material, said temperature changes may lead
to the production of mechanical stresses which, in unfavourable
cases, may lead to the detachment of the wrung plate from the
substrate and/or to damage in the interface region. In one
development, the substrate consists of calcium fluoride and the
wrung plate consists of barium fluoride. Barium fluoride, as
fluoridic material in comparison with synthetic quartz glass, has a
more favorable coefficient of thermal expansion than calcium
fluoride since the difference in coefficients of thermal expansion
between substrate material and plate material and thus the
thermally induced stresses turn out to be smaller in this case.
Moreover, the solubility of barium fluoride in water is
significantly lower than that of calcium fluoride (fluorspar). As a
result, it is possible to ensure a connection between substrate and
protective plate that is mechanically stable even in the event of
temperature fluctuations.
[0054] In one development, use is made of a protective plate having
a plate thickness of less than 5 mm. Preferably, the wrung
protective plate has a plate thickness of less than 3 mm, in
particular of less than 2 mm or less than 1 mm. For reasons of
handling, however, the plate thickness should not fall below values
of 50 .mu.m or 100 .mu.m. The lower limit of the plate thickness
prior to wringing may be chosen in such a way that the surfaces can
be postprocessed after the wringing process in order to be able
subsequently to correct surface deformations that are present, if
appropriate, by means of local material removal and/or to reduce
the plate thickness as a whole to a desired final thickness. As a
result, it is possible to create protective layer systems whose
thickness is greater than the layer thickness of protective layer
systems produced by vapor deposition or sputtering. Typically, the
latter are not thicker than 5 .mu.m or 2 .mu.m or 1 .mu.m or 500
nm.
[0055] The invention also relates to a method for protecting a
projection objective, which is designed for imaging a pattern
arranged in an object plane of the projection objective into an
image plane of the projection objective with the aid of an
immersion medium arranged between an optical element of the
projection objective and the image plane, against degradations of
optical properties caused by the immersion medium, which method is
distinguished by the fact that use is made of a cooled immersion
medium having a media temperature that lies significantly below the
ambient temperature at the site of use. In this case, the media
temperature may be set for example to values of less than
15.degree. C. In particular, the media temperature may be kept in
the range of between approximately 10.degree. C. and approximately
5.degree. C. Active cooling of the immersion medium may be
advantageous for various reasons. If use is made of an immersion
medium which essentially consists of water, then the active cooling
may reduce the dissolution aggressiveness of the immersion medium
since, by way of example, the dissolution aggressiveness of water,
for example with respect to calcium fluoride, decreases with its
temperature. Active cooling can thus contribute to lengthening the
service life of immersion systems in which optical elements made of
calcium fluoride are in direct contact with water-containing
immersion media or immersion media that essentially consist of
water. Active cooling of the immersion medium may also be utilized
for minimizing temperature fluctuations in the region of the
immersion medium, even if the ambient temperature is subject to
relatively greater fluctuations. In this way, by way of example, in
the case of systems with a wrung plane-parallel plate as protective
layer system, the occurrence of thermal stresses on account of
different coefficients of thermal expansion of substrate and plate
material can be limited to harmless values.
[0056] The cooling of the immersion medium with respect to the
ambient temperature may also be provided in the case of immersion
systems in which optical elements provided for contact with an
immersion medium are present which do not have a protective layer
system for increasing the resistance of the optical element to
degradation caused by the immersion medium.
[0057] The problem of thermally induced stresses between a
substrate of an optical element and an element that is wrung onto
the substrate and may be, by way of example, a plane-parallel plate
is alleviated, in accordance with another aspect of the invention,
by virtue of the fact that the projection objective is assigned a
temperature regulation device that is set up for keeping the region
of the optical element provided with wringing at a largely constant
temperature independently of fluctuations in the ambient
temperature. The temperature regulation device may be activated
directly after installation of the optical element provided with
wringing, in order to ensure a largely constant temperature in the
region of the interface between substrate and wrung element during
transport between production side and envisaged site of use, during
demounting at the site of use and during later operation by the end
customer. The temperature regulation device may have components
that are constructionally connected to the projection objective.
The temperature regulation device may comprise, by way of example,
one or a plurality of Peltier elements that are fixed to the holder
of the optical element whose temperature is to be regulated and are
electrically driven in order to cool the holder. As an alternative
or in addition, it is also possible to provide on the holder fluid
channels for conducting through a cooling fluid, to which an
external cooling unit may be connected.
[0058] The invention has been illustrated hitherto in an exemplary
manner using the example of a projection objective for
microlithography in which the protective layer system may be fitted
e.g. to the last optical element (in the light propagation
direction). A refractive or catadioptric projection objective may
be involved in this case.
[0059] However, the invention is not restricted to optical imaging
systems of this type, nor is it restricted to optical imaging
systems generally, but rather can also be used in other optical
systems provided that the latter make use of optical elements which
can come into contact with an immersion medium. The optical system
may be, by way of example, a device for optically measuring another
optical system, at least one space that can be filled with an
immersion medium existing within the optical system for measurement
or between said system and the optical system to be measured.
[0060] The invention also relates to an optical element to which is
fitted a protective layer system that is provided for contact with
an immersion medium and serves for increasing the resistance of the
optical element to degradation caused by the immersion medium. Such
immersion-resistant optical elements can be incorporated instead of
conventional optical elements in conventional systems where contact
between an optical element and immersion medium is provided.
[0061] The above and further features emerge not only from the
claims but also from the description and from the drawings, in
which case the individual features may respectively be realized by
themselves or as a plurality in the form of subcombinations in an
embodiment of the invention and in other fields and may constitute
advantageous and inherently protectable embodiments.
[0062] FIG. 1 schematically shows a projection exposure apparatus
for immersion lithography in accordance with an embodiment of the
invention;
[0063] FIG. 2 schematically shows a last optical element in the
form of a planoconvex lens, in the case of which a monolayer
protective layer is applied on the exit side and to adjacent edge
regions;
[0064] FIG. 3 shows an embodiment with a plate-type last element,
in the case of which a two-layered protective layer is fitted;
[0065] FIG. 4 shows an embodiment with a planoconvex lens, to the
exit face of which a barrier layer and an outer multi-layer
antireflection coating are fitted;
[0066] FIG. 5 shows an embodiment with a planoconvex lens, which is
completely enclosed by a barrier layer;
[0067] FIG. 6 shows a diagram showing the dependence of the degree
of reflection R as a function of the angle of incidence I for the
systems in FIG. 4 (curve IV) and FIG. 5 (curve V);
[0068] FIG. 7 shows an embodiment of a last optical element with a
wear layer that is adapted in terms of refractive index and is made
of mixed materials;
[0069] FIG. 8 shows exemplary embodiments of exchangeable
protective layer systems, FIG. 8(a) schematically showing a
protective plate wrung onto a planoconvex lens and FIG. 8(b)
schematically showing a separately held, exchangeable protective
plate;
[0070] FIG. 9 shows a schematic diagram of the refractive index
profile of the refractive index n as a function of the layer
thickness d in a multilayer graded index layer;
[0071] FIG. 10 shows a diagram showing the dependence of the degree
of reflection R on the angle of incidence I, curve X showing the
reflection profile of an uncoated calcium fluoride substrate and
curve IX showing the reflection profile with an antireflection
graded index layer in the case of measurement in water with light
coming from the side of the water;
[0072] FIG. 11 shows a first embodiment using two layers with an
immersion medium (double immersion); and
[0073] FIG. 12 shows a second embodiment using two layers with an
immersion medium (double immersion).
[0074] FIG. 13 shows a schematic, enlarged sectional view through a
multilayer system serving as a protective layer system in
accordance with another embodiment of the invention; and
[0075] FIG. 14 shows a schematic section through the image-side end
region of a projection objective for immersion lithography with a
last optical element on the image side which has a substrate made
of calcium fluoride onto which a thin protective plate made of
quartz glass is wrung, temperature regulation devices for cooling
the last optical element and the immersion medium being
provided.
[0076] FIG. 1 schematically shows a microlithographic projection
exposure apparatus in the form of a wafer stepper 1, which is
provided for the fabrication of large scale integrated
semiconductor components by means of immersion lithography. The
projection exposure apparatus 1 comprises as light source an
Excimer laser 2 having an operating wavelength of 193 nm, other
operating wavelengths, for example 157 nm or 248 nm, also being
possible. A downstream illumination system 3 generates, in its exit
plane 4, a large, sharply delimited, very homogeneously illuminated
illumination field that is adapted to the telecentric requirements
of the down-stream projection objective 5. The illumination system
3 has devices for selection of the illumination mode and, in the
example, can be changed over between conventional illumination with
a variable degree of coherence, annular field illumination and
dipole or quadrupole illumination.
[0077] Arranged downstream of the illumination system is a device
(reticle stage) for holding and manipulating a mask 6 in such a way
that the latter lies in the object plane 4 of the projection
objective 5 and can be moved in this plane for scanning operation
in a travel direction 7.
[0078] The plane 4, also referred to as the mask plane, is followed
downstream by the reduction objective 5, which images an image of
the mask with a reduced scale of 4:1 onto a wafer 10 coated with a
photoresist layer. Other reduction scales, e.g. 5:1 or 10:1 or
100:1 or less, are likewise possible. The wafer 10 serving as a
light-sensitive substrate is arranged in such a way that the planar
substrate surface 11 with the photoresist layer essentially
coincides with the image plane 12 of the projection objective 5.
The wafer is held by a device 8 comprising a scanner drive in order
to move the wafer synchronously with the mask 6 in parallel with
the latter. The device 8 also comprises manipulators in order to
move the wafer both in the z direction parallel to the optical axis
13 of the projection objective and in the x and y directions
perpendicular to said axis. A tilting device having at least one
tilting axis running perpendicular to the optical axis 13 is
integrated.
[0079] The device 8 provided for holding the wafer 10 (wafer stage)
is constructed for use in immersion lithography. It comprises a
receptacle device 15, which can be moved by a scanner drive and the
bottom of which has a flat recess for receiving the wafer 10. A
peripheral edge 16 forms a flat, upwardly open, liquidtight
receptacle for a liquid immersion medium 20, which can be
introduced into the receptacle and discharged from the latter by
means of devices that are not shown. The height of the edge is
dimensioned in such a way that the immersion medium that has been
filled in can completely cover the surface 11 of the wafer 10 and
the exit-side end region of the projection objective 5 can dip into
the immersion liquid given a correctly set operating distance
between objective exit and wafer surface.
[0080] The projection objective 5 has an image-side numerical
aperture NA of at least NA=0.80, but preferably of more than 0.98,
and is thus particularly adapted to the use of immersion fluids
having a high refractive index. Suitable refractive projection
objectives are disclosed for example in the applicant's patent
applications WO 03/077036 and WO 03/077037, the disclosure content
of which is made the content of this description by reference.
[0081] The projection objective 5 has a planoconvex lens 25 as the
last optical element nearest the image plane 12, the planar exit
face 26 of said lens being the last optical face of the projection
objective 5. During operation of the projection exposure apparatus,
the exit side of the last optical element is completely immersed in
the immersion liquid 20 and is wetted by the latter. In the
exemplary case, ultrapure water having a refractive index
n.sub.I=1.437 (193 nm) is used as the immersion liquid.
[0082] A particular feature of the projection objective 5 consists
in the fact that the planar exit face 26 of the planoconvex lens 25
produced from calcium fluoride has a protective layer system 30 for
increasing the resistance of the last optical element to
immersion-dictated degradation. The protective layer 30 can be used
to prevent the slightly water-soluble substrate material calcium
fluoride from being attacked and gradually dissolved by the
immersion liquid. Moreover, the protective layer system 30 is
optimized by a suitable selection of layer material and layer
thickness in such a way that it acts as an antireflection coating
for the interface between the optical arrangement and the immersion
liquid. In this case, the protective layer thickness is optimized
with regard to resolution behavior and the imaging errors induced
by the resolution.
[0083] The protective layer system 30 is designed as dielectric
single layer. Such a single layer acts as an antireflection layer
when its refractive index n.sub.ss lies between the refractive
index n.sub.s of the substrate and the refractive index n.sub.I of
the immersion medium. If the geometric mean of the refractive
indices of the immersion liquid and of the substrate material is
taken as the refractive index n.sub.ss for the single layer
(n.sub.SS= {square root over (n.sub.In.sub.S))}, then the
antireflection effect of the single layer becomes optimal in the
event of light incidence with an angle of incidence I.sub.0 and a
given operating wavelength .lamda. if the following holds true for
the single layer thickness:
D L 4 = 1 .lamda. 4 n SS cos ( I ) , ##EQU00001##
where I is odd. In this case, I is the refracted angle and the
relationship between the refracted angle I and the angle of
incidence I.sub.0 is given by Snell's law: n.sub.ssin
I.sub.0=n.sub.sssin I. Consequently, single layers whose optical
layer thickness corresponds to a quarter of the wavelength
(.lamda./4) or an odd multiple thereof are expedient. In the
exemplary system (calcium fluoride/water at 193 nm where
I=0.degree.) with n.sub.H.sub.2.sub.O=1.437 and
n.sub.CaF.sub.2=1.502, this is possible if the material of the
single layer has a refractive index n.sub.SS= {square root over
(n.sub.H.sub.2.sub.On.sub.CaF.sub.2)}=1.469 and the following holds
true for the geometric layer thickness: d.sub.L4=I32.8 nm, where
I=1, 3, 5, . . . . In other embodiments in which the substrate for
the last optical element consists of synthetic quartz glass
(n.sub.SiO.sub.2=1.552), the material of the single layer has an
optimized refractive index n.sub.SS= {square root over
(n.sub.H.sub.2.sub.On.sub.SiO.sub.2)}=1.493 and the geometric layer
thickness of the single layer is d.sub.L4=I32.3 nm
(I=0.degree.).
[0084] On account of the relatively small refractive index
differences between immersion medium and protective layer and
between protective layer and substrate, such single layers have a
good reflection-reducing effect even at high angles of incidence
such as typically occur in the case of projection objectives with
high numerical apertures. In a departure from the above exemplary
calculation, the thickness of the single layer should be optimized
for high angles of incidence in these cases.
[0085] A unary system made of a material whose refractive index
lies between n.sub.I and n.sub.s is preferably used for the single
layer. In the case of a unary system, it is advantageous, inter
alia, that no inhomogeneities can result on account of
material-dictated unequal removal.
[0086] For the cases in which the dielectric materials available
under the given conditions have refractive indices lying outside a
desired refractive index range, it is possible to use material
mixtures or mixed materials having two or more components which
have an effective refractive index n.sub.MIX in the desired range.
In this case, at least one material having a low refractive index
with a refractive index n.sub.L< {square root over
(n.sub.In.sub.S)} and at least one material having a high
refractive index with a refractive index n.sub.H> {square root
over (n.sub.In.sub.S)} should be used in a suitable mixture ratio.
For the immersion systems considered here, these are, by way of
example, for the materials having a low refractive index, aluminum
fluoride, chiolite, cryolite, magnesium fluoride, sodium fluoride,
lithium fluoride or mixtures of these materials and, for the
materials having a high refractive index, lanthanum fluoride,
erbium fluoride, gadolinium fluoride, neodymium fluoride, lead
fluoride, silicon dioxide, calcium fluoride, aluminum oxide,
thorium fluoride and mixtures of these materials. If materials of
comparable solubility in the immersion medium are chosen, then it
is possible largely to prevent material-dictated inhomogeneities
during removal and, if appropriate, a porosity evolving as a
result. Mixed systems comprising magnesium fluoride and lanthanum
fluoride may be advantageous on account of the sparing solubility
of these materials particularly in ultrapure water since such a
layer system can satisfy the optimum thickness conditions for a
long time.
[0087] When defining the initial geometric layer thickness of the
reflection-reducing protective layer 30, it should be taken into
account that the protective layer thickness may decrease due to
attack by the immersion medium over the lifetime of the projection
objective. The initial protective layer thickness should be chosen
such that a sufficiently thick residual protective layer still
remains after an expected lifetime of the projection objective for
a given rate of removal. The design may for example fix final layer
thicknesses such that, for an angle of incidence of 0.degree., they
at least still have a geometric layer thickness of .lamda./4 or
.lamda./8 and/or a geometric layer thickness of at least 15 nm. If
the immersion medium is lead past the last optical element in a
flowing manner during the exposure processes or between exposure
processes, care must be taken to ensure that the flow does not lead
to an inhomogeneous removal of the protective layer, rather that an
essentially isotropic removal is ensured. Depending on the system,
the initial geometric layer thicknesses may be in the micrometers
range, for example more than 10 .mu.m or more than 5 .mu.m or more
than 2 .mu.m. In this case, the maximum layer thickness may be
limited by the production of layer stresses.
[0088] Further exemplary embodiments, in which the layer system
acts as a barrier layer, are explained with reference to FIGS. 2 to
5. In the case of the embodiment in accordance with FIG. 2, the
last optical element 50 is a planoconvex lens having a spherical
entry face 51, a planar exit face 52 and a cylindrical edge 53
which adjoins the exit face and on which the holder 54 of the lens
engages. The protective layer system 60 consists of a single layer
made of essentially pore-free silicon dioxide which has been
deposited by plasma enhanced chemical vapor deposition (PECVD) on
the transparent substrate 55. This coating method, in which the
layer constituents are supplied in gaseous form as monomers and are
chemically activated in a microwave plasma, makes it possible to
produce on the substrate a hydrocarbon-free, only weakly absorbent
and essentially pore-free quartz layer 60 which not only completely
covers the planar exit side 52 but also extends across the
cylindrical edge region 53. As a result, the substrate is also
protected against immersion liquid which, promoted by capillary
forces, might penetrate between the holder 54 and the last optical
element. The coating method also enables an "all round protection"
of the last optical element, it being possible to coat not only the
exit side but also the edges and the entry side of the optical
substrate with a coating acting as a barrier layer (cf. FIG. 5).
Other coating methods, for example methods with physical vapor
deposition (PVD), can also be used for reducing such all round
coatings. If appropriate, through assistance by means of ion
irradiation (IAD), the packing density of the layer material can be
increased and the porosity can thus be reduced. Sputtering methods
such as ion beam sputtering (IBS) or magnetron sputtering with an
ion current that is less directional or concentrated in comparison
with IBS can also be used for producing protective layer systems
having a high packing density and low porosity.
[0089] In a method variant for producing a dense barrier layer, the
protecting layer system is constructed in multilayer fashion with
at least two partial coatings. In this respect, FIG. 3 shows, by
way of example, a protective layer system 80 which is constructed
from two single layers and is fitted to a last optical element in
the form of a plane-parallel plate 70. If the surface of the
coating is cleaned after application of the first layer 81 nearest
the substrate and prior to application of the subsequent outer
layer 82, then pores passing through from the outer side as far as
the substrate can largely be avoided. As a result, it is possible
to provide a liquidtight protective layer system even if the single
layers 81, 82 by themselves in each case have a porosity that is
insufficient for complete protection.
[0090] FIGS. 4 and 5 show variants of protective layer systems in
which the barrier effect blocking the ingress of immersion liquid
is in each case provided by a single layer made of silicon dioxide
applied in a PECVD method. In the case of the last optical element
90 in FIG. 4, a multilayer protective layer system 100 with a
barrier effect is fitted to the exit side thereof. This system
comprises a PECVD SiO.sub.2 layer 101 applied directly to the
transparent substrate, the thickness of which layer may lie between
approximately 0.1.lamda. and approximately 0.6.lamda., for example,
depending on the embodiment. An alternating layer system 102 made
of dielectric material alternately having a high refractive index
and having a low refractive index is applied to the outer side of
the barrier layer 101. The material having a high refractive index
essentially consists of lanthanum fluoride in the exemplary case,
while magnesium fluoride is used as the material having a low
refractive index. The layer thickness of the SiO.sub.2 barrier
layer 101 and the layer thickness of the single layers of the
alternating layer system 102 are adapted to one another in such a
way that the entire protective layer system 100 serves as an
antireflection coating.
[0091] An exemplary system is specified in Table 1; the
corresponding dependence of the degree of reflection R on the angle
of incidence I with respect to water (n.apprxeq.1.437) given a
numerical aperture NA=1.25 (corresponding to I=60.degree.) is shown
by curve IV in FIG. 6. The degree of reflection lies below 0.5% in
the entire angle of incidence range up to I=60.degree.. Table 1
specifies the optical layer thicknesses of the single layers in
fractions of the optical layer thickness d.sub.QWOT of a .lamda./4
layer of the corresponding material.
TABLE-US-00001 TABLE 1 Layer Material d.sub.QWOT Substrate
CaF.sub.2 1 SiO.sub.2 (PECVD) 1.000 2 MFg.sub.2 0.627 3 LaF.sub.3
0.244 4 MgF.sub.2 1.739
[0092] Such a coating may be produced as follows: Firstly, the exit
side of the substrate 91 which is to be coated is cleaned. The
barrier layer 101 is then applied in pore-free fashion by means of
PECVD. If appropriate, it is possible to carry out an optical
measurement of the SiO.sub.2 layer thickness and an adaptation of
the thickness of the first layer to the subsequent antireflection
design. Cleaning may optionally be carried out after the coating.
The production of the alternating layer 102 is carried out at
elevated temperature in a manner coordinated with the system
calcium fluoride/PECVD SiO.sub.2 layer. One advantage of this
method implementation consists in the fact that the design of the
alternating layer system 102 can be simply coordinated with the
underlying system calcium fluoride/PECVD SiO.sub.2. In this way, a
highly reflection-reducing effect can be produced even when the
thickness reproducibility of the PECVD layer is not optimal. The
design adaptation may be effected, if appropriate, after
measurement of the actual thickness of the PECVD layer.
[0093] With a different method implementation, it is possible to
produce a last optical element which is protected against immersion
liquid and is fully antireflection-coated, said optical element
being shown by way of example in FIG. 5. In this case, after the
substrate 111 formed as a planoconvex lens had been cleaned,
firstly a multilayer alternating layer system 122 was
vapor-deposited onto its planar exit side, but without a last
sublayer with respect to the outer area. Afterward, an essentially
pore-free PECVD SiO.sub.2 layer 121 enclosing the entire element
and the alternating layer system and having an optical layer
thickness of approximately .lamda./4 was applied in the PECVD
method, said layer acting as a liquidtight barrier layer. In this
case, the optical layer thickness is adapted to the alternating
layer system 122 so as to result overall in a reflection-reducing
effect (cf. curve V in FIG. 6). The spherical entry side was
subsequently antireflection-coated with a single layer 123. One
advantage of this method implementation consists in the fact that,
on the outer side of the substrate 111 below the protective,
pore-free PECVD SiO.sub.2 layer, water-soluble or hydroscopic
substances can also be used for the alternating layer system 122.
An exemplary system having a degree of reflection R.ltoreq.0.2% in
the angle of incidence range of between 0.degree. and 60.degree. is
specified in Table 2.
TABLE-US-00002 TABLE 2 Layer Material d.sub.QWOT Substrate
CaF.sub.2 1 LaF.sub.3 0.302 2 MgF.sub.2 0.279 3 LaF.sub.3 2.1401 4
SiO.sub.2 (PECVD) 1.000
[0094] With reference to the embodiment shown schematically in FIG.
7, a description is given of another possibility for protecting a
last optical element 200 of an immersion objective with the aid of
a protective layer system against degradation of its optical
properties on account of contact with an immersion medium. The
protective layer system 210, which is fitted to the planar exit
side 201 of a calcium fluoride substrate 202, is designed as an
optically neutral wear layer or sacrificial layer which makes it
possible for the more or less unavoidable surface damage due to
attack by the immersion medium 220 to be rendered optically
ineffective. In the embodiment, this is achieved by virtue of the
fact that the protective layer 210 consists of a material whose
refractive index n.sub.MIX essentially corresponds to the
refractive index n.sub.I of the immersion medium. In such a case,
the space between the exit side 201 of the substrate and the
surface 230 of the wafer is filled with media having an identical
refractive index. Therefore, the location at which the solid/liquid
interface 211 between the exit side of the protective layer 210 and
the immersion medium is situated and the surface form of this
interface are unimportant for the optical effect. Therefore, a
gradual dissolution of the protective layer material under the
action of the immersion medium cannot impair the optical properties
of the system.
[0095] In the exemplary case, water having a refractive index
n.sub.I.apprxeq.1.437 at an operating wavelength of 193 nm is used
as the immersion liquid 220. The material of the protective layer
210 is a mixed material in which magnesium fluoride having a low
refractive index and lanthanum fluoride having a high refractive
index are present in a mixture ration such that an average
refractive index n.sub.MIX.apprxeq.1.437 is produced. The mixture
layer 210 was produced as an essentially homogeneous mixed layer by
simultaneous vaporization of magnesium fluoride and lanthanum
fluoride. In other embodiments, the materials were alternately
vapor-deposited in such a way as to produce a layer-patterned mixed
material, the single layers in each case having a thickness of only
a few nm, so that the transmitted light 240 "sees" only an average
refractive index.
[0096] In order that the wear layer 210 acquires a service life
that is as long as possible, the layer material was applied with
ion assistance (ion assisted deposition, IAD) in the exemplary
case, thereby producing a high packing density and low porosity.
When calculating the mixture ratio of the two or more components of
the mixed system, a possible residual porosity of the layer should
be taken into account, which gradually becomes saturated with
immersion liquid during operation, which contributes to the average
refractive index of the layer in the pores with its refractive
index n.sub.I.
[0097] In an alternative embodiment, pure magnesium fluoride
(refractive index n.apprxeq.1.44 to 193 nm) is vapor-deposited as
the wear layer. This also enables an adaptation of the refractive
indices of wear layer and immersion medium which suffices in many
cases. By controlling the compactness of the MgF.sub.2 layer, the
refractive index thereof can be controlled and adapted to the
refractive index of ultrapure water at 193 nm. This is an
alternative to a two-substance vapor deposition.
[0098] If required, a reflection-reducing antireflection layer 260
may additionally be inserted between the substrate 202 of the last
optical element and the wear layer 210 (left-hand partial figure of
FIG. 7).
[0099] FIG. 8(a) shows the exit-side end of another embodiment of
an immersion objective, in which the last optical element 300 is a
planoconvex lens. The latter is protected against attack by the
immersion liquid by a protective layer system 310 in the form of a
bulk material plate made of lithium fluoride (LiF). The
index-matched plane-parallel plate 310 is wrung onto the planar
exit side 302 of the calcium fluoride substrate 301, thereby
producing an optically neutral, releasable connection. Accordingly,
the plate 310 can be exchanged if required (detached plate depicted
by broken lines).
[0100] The plate material lithium fluoride has a refractive index
of approximately n=1.443 at an operating wavelength of 193 nm, with
the result that there is only a refractive index difference of
approximately .DELTA.n=0.006 with respect to the refractive index
n.sub.I of the water used as immersion liquid
(n.sub.I.apprxeq.1.437). The plate 310 may serve as a wear system
since, owing to the small refractive index difference with respect
to the immersion liquid, it is practically insignificant for the
optical properties of the system if the material slowly dissolves
during prolonged contact with the immersion liquid (cf.
explanations referring to FIG. 7). Should it be necessary to
exchange a wear plate after prolonged use, this is possible with
little effort on account of the releasable connection to the
substrate 310, without having to intervene in the objective
construction.
[0101] The embodiment in FIG. 8(b) is provided with a plate-type
exchangeable wear element 320, which has a separate holder 321 in
order to enable straightforward exchange. The protective layer
system formed by the plate 320 is fitted at a distance from the
last refractive optical element (planoconvex lens 322).
[0102] With the use of non-aqueous immersion fluids having a
refractive index that is comparable to the refractive index of
water but lower, an alternative material that is also appropriate
is sodium fluoride (NaF), which has a refractive index of
approximately n=1.385 at 193 nm. Sodium fluoride is readily soluble
in water, but it is sufficiently resistant for fluorinated liquids.
This approach may therefore be particularly expedient for 157 nm.
It is possible to adapt the refractive index of a fluorinated
medium to the refractive index of NaF or CaF.sub.2.
[0103] Embodiments of protective layer systems that have an
antireflection effect for the interface immersion liquid (in
particular water)/substrate material of the last optical element
(in particular calcium fluoride) have already been described in
connection with FIGS. 1 to 6. An embodiment with an outstanding
antireflection effect is explained in connection with FIGS. 9 and
10. In the case of the embodiments considered here, the protective
layer system essentially consists of a graded index layer with a
predeterminable refractive index profile perpendicular to the layer
extent, that is to say essentially parallel to the transmission
direction. In this case, it is possible to achieve a virtually
ideal antireflection effect if a continuous or approximately
continuous, virtually exponentially proceeding transition between
the refractive index of the substrate material of the last optical
element and the refractive index of the immersion medium takes
place within the graded index layer. Such graded index layers are
not practicable in conventional antiflection layer systems on
lenses that are in contact with air or a different gas, since a
refractive index in proximity to the refractive index of air (n=1)
cannot be achieved with solid materials. However, the inventors
have recognized that, in the case of immersion lithography, a
medium having a high refractive index that adjoins the optical
element, namely the immersion medium, can be used to achieve
optimum antireflection coating.
[0104] The basic concept will be explained in greater detail using
the exemplary case of water as immersion medium
(n.sub.I.apprxeq.1.437 at 193 nm). There are materials which are
suitable as layer material and have approximately this refractive
index, for example magnesium fluoride (n.sub.I=1.44). There are
also materials available which have a lower refractive index, for
example aluminum fluoride (AlF.sub.3) where n.apprxeq.1.42. The
refractive index of the substrate material calcium fluoride, where
n.apprxeq.1.51 is higher than n.sub.I. There are also materials
available whose refractive index is greater than that of the
substrate material, for example lanthanum fluoride (LAf.sub.3)
where approximately n=1.71. Given a suitable mixture ratio of
lanthanum fluoride and magnesium fluoride, it is possible to
produce a graded index layer which, in proximity to the substrate
material, essentially has the refractive index of the substrate
material and whose refractive index slowly decreases in the
direction of the refractive index of the immersion medium with
greater distance from the substrate by reducing the proportion of
lanthanum fluoride and increasing the proportion of magnesium
fluoride. In FIG. 9, by way of example, the refractive index
profile in a multilayer system is plotted against the thickness d
of the layer system, the average refractive index of successive
layers decreasing in small steps from the substrate (refractive
index n.sub.s) to the immersion medium (refractive index n.sub.I).
A similar refractive index profile may also be realized by means of
a sequence of very thin unalloyed lanthanum fluoride and magnesium
fluoride single layers a few nm thick whose thickness ratio varies
with distance from the substrate. In the selection of materials or
material mixtures, those materials which are not water-soluble or
only sparingly water-soluble should be selected.
[0105] For the system illustrated in FIG. 9, FIG. 10 shows the
dependence of the degree of reflection on the angle of incidence I
(curve IX). In comparison with this, the curve X shows the
reflection profile of an uncoated calcium fluoride substrate. The
intense reflection-reducing effect of the graded index layer, which
enables degrees of reflection of distinctly less than 0.1 for
angles of incidence of up to almost 80.degree., is clearly
discernible. The reflection-reducing effect becomes clear
particularly at high angles of incidence above 50.degree. or
60.degree..
[0106] An explanation has been given here, on the basis of a few
examples, of how it is possible, by refractive index adaptation
between the immersion liquid and the material adjoining the latter,
to create protective layer systems whose optical properties are
relatively insensitive to a chemical/physical attack by the
immersion medium. In this case, a vanishing refractive index
difference between the adjoining media at the solid/liquid
interface is to be sought in the ideal case. If a sufficiently
small refractive index difference cannot be obtained solely on the
basis of the choice of material for the solid, it is also possible
to modify the refractive index n.sub.I of the immersion medium by
suitable additives. A practically vanishing refractive index
difference is to be sought since very small irregularities in the
rate of removal over the area cross section may impermissibly
impair the imaging wavefront. Owing to the through-flow of the
immersion medium, a nonuniform, nonequidistant removal can be
avoided only with difficulty.
[0107] With reference to FIGS. 11 and 12, an explanation is given
of embodiments of the invention in which two layers of an immersion
medium are situated between the last optical element and the
surface to be exposed, in order to enable the projection objective
to be optically coupled to the light-sensitive wafer surface and,
at the same time, a simple exchange of parts which are possibly
susceptible to wear. FIG. 11 shows a last optical element 400, in
the case of which the substrate 401 is a planoconvex lens made of
calcium fluoride (silicon dioxide in other embodiments). A
plane-parallel plate 410 made of lithium fluoride is wrung onto the
planar exit side 402 of the lens. In order to minimize reflections
on account of the refractive index difference between calcium
fluoride (n.sub.s=1.502 at 193 nm) and lithium fluoride
(n.sub.s=1.443 at 193 nm), a .lamda./4 antireflection layer 412 is
interposed between the substrate and the plate 410. A
plane-parallel plate 418 made of lithium fluoride and serving as an
exchangeable plate is optically and mechanically coupled to the
plate 410 with the aid of a thin immersion layer 415 made of
ultrapure water. The exchangeable plate 418 is in contact with the
immersion medium 420 (ultrapure water), which fills the interspace
between the projection objective and the wafer 425 to be exposed.
The layers 412, 410, 415 and 418 form a protective layer system
comprising a layer made of an immersion liquid. By virtue of the
very small refractive index differences between the plates 410 and
418 and the intervening immersion layer 415, the plates and the
layer act approximately like a single plane-parallel optical
element. However, the intervening water film 415 permits the wear
plate 418 to be exchanged in a manner that is simple and does not
burden the rest of the system, should this be necessary after a
certain operational duration. In other embodiments, the immersion
layers 415 and 420 are formed on the opposite sides of the
terminating plate 418 by means of different immersion media.
[0108] Another embodiment of a last optical element 500 with double
immersion is shown with reference to FIG. 12. A planoconvex lens
501 made of calcium fluoride has optically coupled to it a
plane-parallel exchangeable plate 518, likewise made of calcium
fluoride. The optical coupling is effected by means of an immersion
layer 515 made of water or heavy water which is kept in a closed
system or a closed circuit in order to fill the space between the
substrate 501 and the exchangeable plate 518. Fitted between the
substrate 501 and the immersion layer 515 are a .lamda./4
antireflection layer 512 and a wear layer 513, which adjoins the
immersion medium and has a comparable refractive index. Toward the
exchangeable plate 518 there follow a wear layer 506 and a
.lamda./4 antireflection layer 517 adjoining the plate 518. The
antireflection layers 512, 517 ensure a high transmission between
the calcium fluoride (element 501 and 518) having a high refractive
index (n=1.502) and the water film having a low refractive index
(n=1.437). For this purpose, the refractive index of the
antireflection layers may be approximately n=1.468. A further
.lamda./4 antireflection layer and a wear layer 521 provided for
contact with the immersion medium 520 are fitted to the wafer-side
exit side of the plate 518. The exchangeable plate 518 which is
antireflection-coated on both sides and protected against attack by
water can easily be exchanged should this be necessary after
prolonged use. The protective layer system 530 in this case
comprises eight layers, also including one immersion layer 515. The
immersion layer 515 may be formed by a different immersion medium
than the immersion layer 520 on the opposite side of the coated
termination plate 518.
[0109] When using customary standard coating techniques in the
production of protective layer systems, it is not possible to
preclude the presence of particles, for example polishing residues
or dust, and/or scratches on the substrate surface to be protected
prior to coating. In this case, there is the risk that a layer
applied thereto, directly in the vicinity of these critical points,
will not be able to totally seal the underlying substrate. By way
of example, if calcium fluoride is used as substrate material and
water is used as immersion liquid, then the calcium fluoride may be
attacked at these locations and be dissolved in the water. As a
result, these locations may be enlarged by the formation of etching
pits, as a result of which wavefront deformations, scattered light
and other faults that disturb the imaging may arise. The formation
of such faults is avoided in the case of a particular method
variant by virtue of the fact that after a first coating by which a
first layer is applied to the substrate (or a coating applied
thereto), a part of said first layer is polished away by means of a
mechanical method. In this case, some of the particles are sheared
off or torn out from the coated area and a polishing interface
arises, which is smoother, that is to say has less roughness, than
the surface of a conventional coating. After this step, the
polishing surface is cleaned and coated a second time. In the
course of this second coating, generally particles are admittedly
present again on the surface, but they will not be at exactly the
same positions as the particles present prior to the first coating.
These method steps (coating-polishing-coating) are preferably
repeated until it is ensured that each particle has been removed at
least once. What can thereby be achieved is that the substrate is
covered with an impermeable protective layer at every point, with
the result that an attack by the immersion medium cannot occur and
the long-term stability of the substrate material with respect to
the immersion medium is thus ensured.
[0110] FIG. 13 shows a highly schematic, enlarged illustration of a
last optical element 600 with a calcium fluoride substrate 601, the
planar substrate surface 602 of which has been coated with a
protective layer system 630 produced in this way. In the case of
the example, the protective layer system 630 comprises a first
layer 611 applied directly on the substrate and a second layer 612
applied to said first layer. Both layers consist of the same
coating material. The interface 615 between the first and second
layers is formed as an optically smooth polishing interface on
account of the polishing step, the second layer 612 bearing on said
polishing interface. Since the coating process was carried out in
an atmosphere not totally free of particles, the particles have
resulted in the formation of defects 621 within the first layer.
Defects 622 may arise again during the production of the second
layer 612 as well. However, since the first defects 621 of the
first layer and the second defects 622 of the second layer are
distributed at different lateral positions within the layers, a
defect-free region exists in at least one layer of the protective
multilayer system 630 at every location of the coated surface, so
that overall the entire coated surface is covered with defect-free
regions adjoining one another (which may lie in different layers of
the multilayer system. With the use of a layer material that is
chemically resistant to the immersion medium, it is thereby
possible to produce a protective layer which effects complete
sealing against the immersion medium and is essentially homogeneous
from a chemical standpoint.
[0111] FIG. 14 shows a highly schematic, enlarged illustration of
the image-side end region of a lithography objective with a last
optical element 700 on the image side which has a calcium fluoride
substrate 701 in the form of a planoconvex lens held in a holder
element 750. A thin plate 730 made of synthetic quartz glass is
wrung onto the planar substrate surface 702 facing the image plane.
The thin quartz glass plate 730 acts as a protective layer system
in order to protect the water-soluble substrate material calcium
fluoride against chemical attack by the immersion liquid 720, which
essentially consists of water and fills a narrow interspace between
the image-side exit face of the protective plate 730 and the
surface of a semiconductor wafer 725 that is coated with a
light-sensitive layer. The uncoated protective plate 730 adheres to
the planar substrate surface 702 solely by adhesion forces, thereby
ensuring a large-area optical contact along a single interface. The
thin quartz glass plate 730 (plate thickness approximately 1 mm)
was postprocessed after wringing on the uncovered surface by ion
beam sputtering in order to correct surface deformations that are
possibly present by means of targeted local material removal.
[0112] In the holder element 750, fluid channels 751 are introduced
in direct proximity to the optical element 700 and, during
operation of the projection system, are connected to a temperature
regulation device 760 which comprises a cooling unit in order to
provide a cooling liquid having a temperature of approximately
10.degree. C. which is conducted in a closed circuit through the
cooling channels 751. A temperature regulation device 770 for the
immersion medium 720 ensures that the latter is likewise kept at a
temperature of approximately 10.degree. C. The active cooling both
of the last optical element and of the immersion medium makes it
possible to reduce temperature fluctuations to values significantly
below 1 K, so that thermally induced stresses between substrate 701
and wrung protection plate 730 can be reduced to harmless values.
The temperature regulation device 760 is activated as required
immediately after the wringing of the protective plate 730 and will
also be operational during the transport of the projection
objective between production site and site of use and also during
the construction of the projection exposure apparatus in order to
maintain a largely constant temperature of the lower region of the
objective and especially of the last optical element in order to
avoid detachment of the two parts on account of thermally induced
stresses.
[0113] In another embodiment (not shown), the wrung plate 730
consists of barium fluoride, which, in comparison with the
substrate 701 which likewise consists of fluoride crystal material,
has a significantly smaller difference in the coefficient of
thermal expansion than quartz glass, so that the risk of thermally
induced stresses and associated detachment of the protective plate
is reduced or avoided already on account of the very similar
coefficients of thermal expansion of substrate material and plate
material.
[0114] 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. It is
sought, therefore, to cover all changes and modifications as fall
within the spirit and scope of the invention, as defined by the
appended claims, and equivalents thereof.
* * * * *