U.S. patent application number 13/946516 was filed with the patent office on 2013-11-14 for substrate for an euv-lithography mirror.
The applicant listed for this patent is CARL ZEISS SMT GmbH. Invention is credited to Claudia EKSTEIN, Holger MALTOR.
Application Number | 20130301151 13/946516 |
Document ID | / |
Family ID | 46510655 |
Filed Date | 2013-11-14 |
United States Patent
Application |
20130301151 |
Kind Code |
A1 |
EKSTEIN; Claudia ; et
al. |
November 14, 2013 |
SUBSTRATE FOR AN EUV-LITHOGRAPHY MIRROR
Abstract
Substrates suitable for mirrors used at wavelengths in the EUV
wavelength range have substrates (1) including a base body (2) made
of a precipitation-hardened alloy, of an intermetallic phase of an
alloy system, of a particulate composite or of an alloy having a
composition which, in the phase diagram of the corresponding alloy
system, lies in a region which is bounded by phase stability lines.
Preferably, the base body (2) is made of a precipitation-hardened
copper or aluminum alloy. A highly reflective layer (6) is
preferably provided on a polishing layer (3) of the substrate (1)
of the EUV mirror (5).
Inventors: |
EKSTEIN; Claudia;
(Ellwangen, DE) ; MALTOR; Holger; (Aalen,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CARL ZEISS SMT GmbH |
Oberkochen |
|
DE |
|
|
Family ID: |
46510655 |
Appl. No.: |
13/946516 |
Filed: |
July 19, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2012/050533 |
Jan 14, 2012 |
|
|
|
13946516 |
|
|
|
|
Current U.S.
Class: |
359/838 ;
148/411; 148/415; 420/469; 420/528; 428/457; 501/1; 501/88;
501/90 |
Current CPC
Class: |
C22C 32/0084 20130101;
C22C 14/00 20130101; C22C 32/0015 20130101; G21K 1/062 20130101;
C22C 29/00 20130101; C22C 1/051 20130101; C22C 1/05 20130101; C22C
21/12 20130101; G02B 5/0891 20130101; C22C 9/00 20130101; C22C
21/00 20130101; C22C 32/0047 20130101; Y10T 428/31678 20150401 |
Class at
Publication: |
359/838 ; 501/1;
501/88; 501/90; 148/411; 148/415; 420/469; 420/528; 428/457 |
International
Class: |
G02B 5/08 20060101
G02B005/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 21, 2011 |
DE |
102011002953.2 |
Claims
1. A substrate for a mirror for extreme-ultraviolet (EUV)
lithography comprising: a base body consisting essentially of a
precipitation-hardened alloy.
2. The substrate according to claim 1, wherein the base body
consists essentially of a precipitation-hardened copper or aluminum
alloy.
3. A substrate for a mirror for EUV lithography comprising: a base
body consisting essentially of an alloy having a composition which,
in the phase diagram of the corresponding alloy system, lies in a
region which is bounded by phase stability lines.
4. The substrate according to claim 3, wherein the alloy is an
alloy with a substitution lattice.
5. The substrate according to claim 3, wherein the alloy is
precipitation-hardened.
6. The substrate according to claim 3, wherein the alloy is a
copper or aluminum alloy.
7. A substrate for a mirror for EUV lithography comprising: a base
body consisting essentially of a particulate composite.
8. The substrate according to claim 7, wherein the particulate
composite has dispersoids of an extent of between 1 nm and 20
nm.
9. The substrate according to claim 7 wherein the particulate
composite has a metallic matrix.
10. The substrate according to claim 9, wherein the particulate
composite has spheroidal dispersoids.
11. The substrate according to claim 9, wherein the metallic matrix
is a copper matrix or an aluminum matrix.
12. The substrate according to claim 7, wherein the particulate
composite has a ceramic matrix.
13. The substrate according to claim 12, wherein the ceramic matrix
is a silicon matrix or a carbon matrix with silicon carbide
dispersoids.
14. A substrate for a mirror for EUV lithography comprising: a base
body consisting essentially of an intermetallic phase of an alloy
system.
15. The substrate according to claim 14, wherein the base body
consists essentially of an intermetallic phase in which the
stoichiometric standard composition is observed.
16. The substrate according to claim 14, wherein the base body
consists essentially of an intermetallic phase having a composition
which corresponds to a phase stability line in the phase diagram of
the alloy system.
17. The substrate according to claim 14, wherein the alloy has a
composition which, in the phase diagram of the corresponding alloy
system, lies in a region which is bounded by phase stability
lines.
18. The substrate according to claim 14, wherein the base body
consists essentially of an intermetallic phase having the same
Bravais lattice as components of the base body in crystalline
form.
19. The substrate according to claim 14, wherein the alloy system
is a binary system, one component of which is copper.
20. The substrate according to claim 14, wherein the alloy system
is a binary aluminum-copper system.
21. The substrate according to claim 1, wherein the base body has a
face-centered cubic material structure.
22. The substrate according to claim 1, wherein the base body
experiences essentially no changes in microstructure in response to
changes in temperature from 20.degree. C. to 150.degree. C. over a
period of time of at least one year.
23. The substrate according to claim 1, further comprising a
polishing layer arranged on the base body.
24. The substrate according to claim 23, further comprising an
adhesion-promoter layer arranged between the base body and the
polishing layer.
25. A mirror for an EUV projection exposure apparatus, comprising:
a substrate according to claim 1, and a highly reflective layer on
the substrate.
26. A mirror for an EUV projection exposure apparatus, comprising:
a substrate according to claim 1, a polishing layer arranged on the
base body, and a highly reflective layer on the polishing layer.
Description
[0001] The present application claims priority from International
Application No. PCT/EP2012/050533, filed on Jan. 14, 2012, German
Patent Application No. 10 2011 002 953.2, filed on Jan. 21, 2011,
and U.S. Provisional Application No. 61/434,869, the entire
disclosures of which are incorporated herein by reference in their
entireties.
FIELD OF AND BACKGROUND OF THE INVENTION
[0002] The present invention relates to a substrate for a mirror
for Extreme-Ultraviolet (EUV) lithography comprising a base body
and also to a mirror for an EUV projection exposure apparatus
comprising such a substrate.
[0003] In order to make it possible to create ever finer structures
using lithographic methods during the production of semiconductor
components, for example, use is made of light having an ever
shorter wavelength. If light in the extreme ultraviolet (EUV)
wavelength range is used, for instance at wavelengths of between
about 5 nm and 20 nm, it is no longer possible to use lens-like
elements in transmission, but instead illumination and projection
objectives are fashioned from mirror elements with highly
reflective coatings which are adapted to the respective operating
wavelength. In contrast to mirrors in the visible and ultraviolet
wavelength ranges, it is also the case in theory that maximum
reflectivities only of less than 80% can be achieved per mirror.
Since EUV projective devices generally have a plurality of mirrors,
it is necessary for each of these to have the highest possible
reflectivity in order to ensure sufficiently high overall
reflectivity.
[0004] In order both to keep losses in intensity as a result of
stray radiation as low as possible and to avoid imaging
aberrations, mirror substrates or mirrors which are produced by
applying a highly reflective layer to the mirror substrate should
have the lowest possible microroughness. The root mean squared
(RMS) roughness is calculated from the mean value of the squares of
the deviation of the measured points over the surface with respect
to a central area, which is laid through the surface such that the
sum of the deviations with respect to the central area is minimal.
Particularly for optical elements for EUV lithography, the
roughness in a spatial frequency range of 0.1 .mu.m to 200 .mu.m is
particularly important for avoiding negative influences on the
optical properties of the optical elements.
OBJECTS AND SUMMARY OF THE INVENTION
[0005] It is an object of the present invention to provide mirror
substrates which are suitable as substrates for mirrors used at
wavelengths in the EUV wavelength range.
[0006] This object is achieved, according to one aspect, by a
substrate for a mirror for EUV lithography comprising a base body,
characterized in that the base body is made of a
precipitation-hardened alloy, preferably a precipitation-hardened
copper or aluminum alloy.
[0007] During the precipitation hardening, an alloy is subjected to
heat treatment in order to increase the hardening strength thereof.
During the heat treatment, metastable phases are precipitated in
finely distributed form such that they form an effective obstacle
to dislocation movements. As a result, the long-term stability or
within certain limits the temperature stability of the structure of
the base body can be increased further. The precipitation hardening
is usually carried out in three steps. In a first step, which is
also referred to as solution annealing, the alloy is heated until
all the elements which are needed for precipitation are present in
solution. In order to obtain the purest possible distribution of
the mixed phase, the temperature should be chosen to be very high,
but not so high that individual constituents of the microstructure
melt. After the solution annealing, quenching can prevent fusion
and thus precipitation of coarse particles. The solid solution
remains in a metastable, supersaturated single-phase state. By
subsequent heating to temperatures which are low compared to the
solution annealing, the supersaturated single-phase solid solution
is converted into a two-phase alloy. The phase which is
predominantly cohesive and generally arises in a higher proportion
is called matrix, and the other phase is called precipitation.
Since many nuclei were formed during the preceding quenching, many
small precipitations which are distributed homogeneously in the
microstructure and increase the structural strength are formed. It
is advantageous for substrates and mirrors on the basis of a base
body made of precipitation-hardened alloys to be used at
temperatures which lie considerably below the solution annealing
temperature, preferably below the precipitation temperature.
[0008] In a further aspect, the object is achieved by a substrate
for a mirror for EUV lithography comprising a base body, wherein
the base body is made of an alloy having a composition which, in
the phase diagram, lies in a region which is bounded by phase
stability lines. Alloys having such compositions have the advantage
that any segregation processes can be stopped entirely by heat
treatments, and therefore said alloys then have an increased
high-temperature strength. This substrate has an increased
long-term stability, as a result of which it is possible to ensure
that the roughness values change as little as possible throughout
the service life of an EUV projection exposure apparatus comprising
mirrors based on this substrate. Particularly in the case of
mirrors which are arranged further to the rear in the beam path,
for example in the projection system, where they are exposed to
lower thermal loading, it is possible to ensure that the roughness
values remain constant over long periods of time.
[0009] The alloy is preferably an alloy with a substitution
lattice. In the case of substitution lattices, alloying components
having a relatively low concentration are incorporated into the
lattice structure of the component having the highest
concentration, such that the lattice strength is further increased.
This increases the structural stability given an increase in
temperature and in particular over long periods of time.
[0010] It is particularly preferable for the alloy to be
precipitation-hardened. During the precipitation hardening, an
alloy is subjected to heat treatment in order to increase the
hardening strength thereof. During the heat treatment, metastable
phases are precipitated in finely distributed form such that they
form an effective obstacle to dislocation movements. As a result,
the long-term stability or within certain limits the temperature
stability of the structure of the base body can be increased
further. The precipitation hardening is usually carried out in
three steps. In a first step, which is also referred to as solution
annealing, the alloy is heated until all the elements which are
needed for precipitation are present in solution. In order to
obtain the purest possible distribution of the mixed phase, the
temperature should be chosen to be very high, but not so high that
individual constituents of the microstructure melt. After the
solution annealing, quenching can prevent fusion and thus
precipitation of coarse particles. The solid solution remains in a
metastable, supersaturated single-phase state. By subsequent
heating to temperatures which are low compared to the solution
annealing, the supersaturated single-phase solid solution is
converted into a two-phase alloy. The phase which is predominantly
cohesive and generally arises in a higher proportion is called
matrix, and the other phase is called precipitation. Since many
nuclei were formed during the preceding quenching, many small
precipitations which are distributed homogeneously in the
microstructure and increase the structural strength are formed. It
is advantageous for substrates and mirrors on the basis of a base
body made of precipitation-hardened alloys to be used at
temperatures which lie considerably below the solution annealing
temperature, preferably below the precipitation temperature.
[0011] In particularly preferred embodiments, the alloy is a copper
alloy or an aluminum alloy, very particularly preferably a
precipitation-hardened copper alloy. Copper alloys in particular
can be readily cooled, and it is therefore possible to ensure that
the operating temperature during the EUV lithography is
sufficiently low, in particular in the case of
precipitation-hardened alloys, in order to be able to prevent
structural changes. In addition, it is possible to obtain high
strengths both in the case of copper alloys and in the case of
aluminum alloys even at temperatures considerably above room
temperature.
[0012] In a further aspect, the object is achieved by a substrate
for a mirror for EUV lithography comprising a base body, wherein
the base body is made of a particulate composite. Particulate
composites likewise have a high strength or structural stability.
As a result, they are likewise highly suitable for use in mirror
substrates for EUV lithography, in particular for long-term
applications. Particulate composites have dispersoids which are
insoluble in a matrix. It is preferable for the dispersoids to be
made of ceramic material, in particular of oxides, carbides,
nitrides and/or borides. In a manner similar to the precipitations
in the precipitation hardening, the dispersoids form obstacles for
dislocation movements within a matrix, in particular when they are
present in finely distributed form.
[0013] It is preferable for the particulate composite to have
spheroidal dispersoids. It is thereby possible to reduce the stress
or distortion energy in the particulate composite, which can lead
to a higher high-temperature strength. Dispersoids having a
spheroidal geometry can be obtained by particular soft-annealing
processes. By way of example, it is possible to carry out
soft-annealing processes in which the material is held for one to
two hours at a temperature at which the basic phase of the matrix
of the particulate composite is stable, whereas other phases in
solutions go just into solution. Then, the temperature of the
material is fluctuated repeatedly around this temperature range,
and subsequently the material is slowly cooled at about 10.degree.
C. to 20.degree. C. per hour. Such temperature treatments can be
carried out with the alloys described above such that any
precipitations are spheroidized, in particular in the case of
precipitation-hardened alloys.
[0014] It has proved to be particularly advantageous for the
particulate composite to have dispersoids of an extent of between 1
nm and 20 nm. It is thereby possible to achieve particularly good
strengths and at the same time to minimize a negative influence on
microroughness values.
[0015] In preferred embodiments, the particulate composite has a
metallic matrix, this particularly preferably being a copper matrix
or an aluminum matrix. Examples of suitable dispersoids in this
case are titanium carbide, aluminum oxide, silicon carbide, silicon
oxide or carbon in a graphite or diamond modification.
[0016] In further preferred embodiments, the particulate composite
has a ceramic matrix, in particular a silicon or carbon matrix. In
this case, silicon carbide particles, in particular, have proved to
be suitable as dispersoids.
[0017] In a further aspect, the object is achieved by a substrate
for a mirror for EUV lithography comprising a base body, wherein
the base body is made of an intermetallic phase of an alloy
system.
[0018] Intermetallic phases are materials with a high strength and
a high melting temperature. By way of example, they are used in
aircraft engines or exhaust-gas turbochargers. In structural terms,
the elementary cells of these special alloys have a high valence
electron density. As a result, they have a covalent bond fraction
which is high for metals and thereby have a particularly high
lattice strength. It has been found that, in addition to a high
specific strength and high melting temperatures, intermetallic
phases overall have a high thermal stability with low diffusion
coefficients and a high creep strength. These properties can ensure
that, even under high thermal loading, as can occur for example in
the case of mirrors which are arranged further forward in the beam
path in an EUV projection exposure apparatus, in particular in the
illumination system of an EUV projection exposure apparatus, the
substrate experiences as little change as possible even over
relatively long periods of time, and as a result properties such as
the microroughness also remain as constant as possible.
[0019] It is advantageous for the base body to be made of an
intermetallic phase in which the stoichiometric standard
composition is observed. In other words, preference is given to
intermetallic phases with a composition having integer indices.
Particular preference is given to intermetallic phases having the
smallest possible elementary cells. It is thereby possible to
further reduce the probability of mixed phases arising as the
temperature increases. As a result of the occurrence of appropriate
precipitations, for example at grain boundaries, mixed phases of
alloys having a differing structure could lead to an increase in
microroughness, which could impair the optical quality of a mirror
comprising such a substrate.
[0020] In particularly preferred embodiments, the base body is made
of an intermetallic phase having a composition which corresponds to
a phase stability line in the phase diagram of the corresponding
alloy system. In this context, a "phase stability line" is to be
understood as meaning a phase boundary line which runs parallel to
the temperature axis in the phase diagram. Such compositions have
the major advantage that no segregation occurs as the temperatures
increase. Particular preference is given to intermetallic phases on
a phase stability line which have no phase transition up to the
melting point. The fewer the phase transitions which lie in
particular in temperature ranges which can occur during use in EUV
projection exposure apparatuses, and the more parallel the phase
boundary line runs in relation to the temperature axis, the lesser
the probability of the microroughness being adversely affected
under the influence of thermal loading as a result of structural
changes in the base body of the substrate.
[0021] It is particularly preferable for the base body to be made
of an alloy having a composition which, in the phase diagram, lies
in a region which is bounded by phase stability lines. Alloys
having such compositions have the advantage that any segregation
processes can be stopped entirely by heat treatments, and therefore
said alloys then have an increased high-temperature strength.
[0022] It is advantageous that the intermetallic phase has the same
Bravais lattice as the components thereof in crystalline form. As a
result, it is possible to achieve a particularly stable crystalline
structure which can further reduce a structural change as the
temperature increases and/or over long periods of time, such that
the roughness values of a mirror for EUV lithography which is based
on such a substrate remain as unimpaired as possible throughout the
service life.
[0023] In particularly preferred embodiments, the alloy system is a
binary alloy system, preferably with copper as one of the two
components, particularly preferably a binary aluminum-copper
system. Copper, in particular, has a high thermal conductivity.
Substrates comprising a base body with a high copper fraction can
thus be cooled particularly readily in order to thereby
additionally prevent a structural change over the service life. On
the basis of aluminum, it is possible to obtain high-strength
materials which have a good dimensional stability. It should be
pointed out that intermetallic phases of other alloy systems may
also be suitable for mirror substrates for EUV lithography. In
particular, intermetallic phases of ternary or quaternary alloy
systems or alloy systems with five or more components may also be
involved. In this context, it should be pointed out that real
alloys always also have traces of impurities. Mention is made of
components of an alloy system here only if the respective component
has a marked influence on the phase diagram of the respective alloy
system.
[0024] As a whole, it has proved to be advantageous in the case of
the base body materials described here for the material of the base
body to have a face-centered cubic lattice structure. It is thereby
possible to further increase the structural strength compared to
body-centered cubic structures, for example, and therefore
face-centered cubic materials are particularly suitable for use
over long periods of time and, if appropriate, at elevated
temperatures.
[0025] It is particularly preferable that the material of the base
body experiences no changes in microstructure in the event of
changes in temperature from 20.degree. C. to 150.degree. C. over a
period of time of 1 year. This temperature range includes those
temperatures which are achieved when mirrors based on this
substrate are used in an EUV projection exposure apparatus. Since
the base body materials experience changes in structure only at
temperatures of above 150.degree. C., it is possible to reduce the
influence which the structure of the base body has on the roughness
values of the mirror substrate or of the mirror based thereon
practically to zero. The changes in structure may involve a very
wide variety of effects, for example the positional change of
dislocations, oscillations of the atoms, instances of roughening,
such as the so-called orange peel effect, or else segregation
processes.
[0026] In preferred embodiments, a polishing layer is arranged on
the base body. It is advantageous that an adhesion-promoter layer
is arranged between the base body and the polishing layer.
[0027] Preferred polishing layers are, inter alia, layers which
have been deposited without external current, for example
nickel-phosphorus or nickel-boron layers. In this case, they can be
present in a crystalline phase or in an X-ray-amorphous phase. In
the case of nickel-phosphorus layers, preference is given to layers
containing more than 11% by weight phosphorus. The layers can also
be nickel-phosphorus alloy layers which also comprise one or two
additional metals. The layers can likewise be nickel-phosphorus or
nickel-boron dispersion layers which, if appropriate, likewise
contain one or two additional metals. This also applies to
nickel-boron layers. Furthermore, copper layers, quartz glass
layers, amorphous or crystalline silicon layers, amorphous silicon
carbide layers or else indium-tin oxide (ITO) layers have proved to
be advantageous. All of these layers have the common feature that
they can be polished to roughnesses of an RMS value of 5 angstroms
or else considerably lower in particular in the spatial frequency
range of between 10 nm and 1 .mu.m. Using the base body materials
described here, it is possible to observe a stability of the
microroughness in the spatial frequency range of 10 nm to 250 .mu.m
even under thermal loading and in long-term operation, since base
body materials which have no morphological surface degradation
under these conditions are proposed. In particular, the
microroughness is obtained on an angstrom scale in RMS values. In
the spatial frequency range of 10 nm to 1 .mu.m, the changes in
roughness can lie in a region of less than 2.5 angstroms; in the
spatial frequency range of 1 .mu.m to 250 .mu.m, it is possible to
achieve a fluctuation of the roughness values of less than 3
angstroms.
[0028] Depending on the combination of the base body material and
the polishing layer material, it can be advantageous to provide an
adhesion-promoter layer between the base body and the polishing
layer in order to achieve a good bond between the base body and the
polishing layer.
[0029] In a further aspect, the object is achieved by a mirror for
an EUV projection exposure apparatus, comprising a substrate as
described above and a highly reflective layer on the substrate, in
particular on a polishing layer.
[0030] The mirrors for EUV projection exposure apparatuses are
distinguished by a structural strength which is high with regard to
long operating periods even at elevated temperatures and therefore
by approximately constant roughness values throughout the period of
use. In this case, it is possible to achieve service lives of a
number of years. The substrates mentioned here, in particular on
the basis of a base body made of an intermetallic phase, of a
precipitation-hardened copper alloy or of a particulate composite,
are suitable in particular but not only for use in the illumination
system of an EUV projection exposure apparatus, for example in the
form of facet mirrors.
[0031] The features mentioned above and further features are
apparent not only from the claims but also from the description and
the drawings, wherein the individual features can in each case be
realized by themselves or as a plurality in the form of
subcombinations in an embodiment of the invention and in other
fields and can constitute advantageous and inherently protectable
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] The present invention will be explained in more detail with
reference to exemplary embodiments. In this respect,
[0033] FIGS. 1a,b schematically show two variants of a substrate in
section;
[0034] FIGS. 2a,b schematically show two variants of a mirror in
section; and
[0035] FIG. 3 shows a phase diagram for a binary aluminum-copper
system.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0036] FIG. 1a schematically shows a first variant of an embodiment
of a substrate 1 comprising a base body 2 and a polishing layer 3
applied thereto. The base body 2 and the polishing layer 3 perform
different functions. Whereas a good dimensional stability is a
priority for the base body 2, good machining and polishing
properties are of primary importance for the polishing layer 3.
[0037] The polishing layer can be applied by conventional vacuum
coating processes, for example sputtering processes, electron beam
evaporation, molecular beam epitaxy or ion beam-assisted coating.
If the polishing layer is a metallic material, for example copper,
nickel-phosphorus or nickel-boron, it is preferably applied without
external current. Nickel-phosphorus or nickel-boron polishing
layers, in particular, can also be applied as dispersion layers, in
which case polytetrafluoroethylene can serve as the dispersant, for
example.
[0038] Nickel-phosphorus or nickel-boron polishing layers, in
particular, are preferably applied with relatively high
concentrations of phosphorus or boron, such that they are present
predominantly or even completely in amorphous form and thereby have
better polishing properties. They can then be hardened by, for
example, heat treatment, plasma treatment or ion bombardment.
Silicon as polishing layer material can also be deposited in
amorphous or crystalline form in a manner controlled by the coating
process. Amorphous silicon can be polished more effectively than
crystalline silicon and, if required, can likewise be hardened by
heat treatment, plasma treatment or ion bombardment. Polishing
layers made of silicon or silicon dioxide can also be smoothed
through use of ion beams. The polishing layer can also be made of
silicon carbide or of indium-tin oxide.
[0039] Preferred thicknesses of the polishing layer 3 can be about
5 .mu.m to 10 .mu.m for metal-based, polished polishing layers. In
the case of non-metallic polishing layers 3, preferred layer
thicknesses are about 1.5 .mu.m to 3 .mu.m. Using conventional
polishing processes, metallic polishing layers can be polished to
root mean squared roughnesses of less than 0.3 nm in the spatial
frequency range of 1 .mu.m to 200 .mu.m and to root mean squared
roughnesses of less than 0.25 nm in the spatial frequency range of
0.01 .mu.m to 1 .mu.m. Using conventional polishing processes,
non-metallic polishing layers can be polished to root mean squared
roughnesses of less than 0.3 nm over the entire spatial frequency
range of 0.01 .mu.m to 200 .mu.m.
[0040] FIG. 1b schematically shows a variant of the substrate 1
shown in FIG. 1a, in which an adhesion-promoter layer 4 is arranged
between the base body 2 and the polishing layer 3. It is preferable
that the adhesion-promoter layer 4 can have a thickness of up to 1
.mu.m, preferably of between 100 nm and 500 nm. By way of example,
it can be applied using CVD (chemical vapor deposition) or PVD
(physical vapor deposition) processes.
[0041] Such substrates 1 can be further processed to form EUV
mirrors 5, as is shown schematically in FIG. 2a in a first variant
of an embodiment, by applying a highly reflective layer 6 to the
polishing layer 3. For use in the case of EUV radiation in the
wavelength range of about 5 nm to 20 nm and with normal incidence
of radiation, the highly reflective layer 6 is particularly
preferably a multilayer system of alternating layers of material
with a differing real part of the complex refractive index via
which a crystal with network planes at which Bragg diffraction
takes place is simulated to some extent. A multilayer system of
alternating layers of silicon and molybdenum can be applied, for
example, for use at 13 nm to 14 nm. Particularly if the highly
reflective layer 6 is configured as a multilayer system, it is
preferably applied using conventional vacuum coating processes such
as, for example, sputtering processes, electron beam evaporation,
molecular beam epitaxy or ion-beam-assisted coating. For use in the
case of EUV radiation in the wavelength range of about 5 nm to 20
nm and with grazing incidence of radiation, preference is given to
mirrors with an uppermost layer of metal, for example of
ruthenium.
[0042] FIG. 2b schematically shows a further variant of the mirror
5 shown in FIG. 2a, in which an adhesion-promoter layer 4 is
arranged between the base body 2 and the polishing layer 3 of the
substrate 1 of the mirror 5.
[0043] In a first example, the base body 2 of the mirror 5 or of
the substrate 1 can be made of a particulate composite. In
particular, the base body 2 can be made of a particulate composite
having a metallic matrix. By way of example, the latter can be a
2000 to 7000 series aluminum alloy, preferably a 5000 to 7000
series aluminum alloy, copper, a low-alloy copper alloy or copper
niobate. The preferably spheroidal dispersoids of an extent in the
range of 1 nm to 20 nm are advantageously titanium carbide,
titanium oxide, aluminum oxide, silicon carbide, silicon oxide,
graphite or diamond-like carbon, it also being possible for
dispersoids of differing materials to be provided in the matrix.
These materials can be produced by powder metallurgy, for example.
The base body 2 can also be made of a particulate composite having
a ceramic matrix. By way of example, particulate composites having
a silicon or carbon matrix and silicon carbide dispersoids are
particularly suitable. As a result of their covalent bond, they
have a particularly high lattice rigidity. It is particularly
preferable for the dispersoids to be distributed as homogeneously
as possible in the matrix, for said dispersoids to be as small as
possible and for the composite to have the smallest possible
dispersoid spacings.
[0044] In a second example, the base body 2 can be made of an alloy
having components which have similar atomic radii and have a
structure with a substitution lattice. By way of example, this may
be the alloy system copper-nickel or silicon-aluminum.
[0045] In a third example, the base body 2 can be made of a
precipitation-hardened alloy. By way of example, it can be made of
precipitation-hardened copper or aluminum alloys such as AlCu4Mg1,
CuCr, CuNi1Si, CuCr1Zr, CuZr, CuCoBe, CuNiSi. In specific
embodiments, the alloys were subjected to a further heat treatment
after the precipitation hardening, this having the effect that the
precipitations assume a spheroidal form in order to reduce stress
or distortion energies in the material so as to further increase
the high-temperature strength. To this end, the material is held
for one to two hours at a temperature at which the basic phase of
the matrix of the particulate composite is stable, whereas other
phases in solutions go indeed into solution. Then, the temperature
of the material is fluctuated repeatedly around this temperature
range, and subsequently the material is slowly cooled at about
10.degree. C. to 20.degree. C. per hour.
[0046] In a fourth example, the base body 2 can be made of an
intermetallic phase. FIG. 3 shows the phase diagram of a binary
aluminum-copper system, the intermetallic phases of which are
particularly suitable as the material of the base body 2. At
300.degree. C., sixteen intermetallic phases of Al.sub.xCu.sub.y,
where x, y are integers, are stable. Of these, ten intermetallic
phases stay stable upon cooling to room temperature (not shown
here). The most important phases are indicated in FIG. 3 with the
stoichiometric composition thereof. All of them lie at phase
boundary lines which run parallel to the temperature axis over a
certain temperature range. As a result, the microstructure thereof
remains completely unchanged in these respective temperature
ranges. Particular preference is given to Al.sub.2Cu,
Al.sub.2Cu.sub.3 or Al.sub.3Cu.sub.5, inter alia, as the material
of a base body of a mirror substrate for EUV lithography. In
variations, it is also possible to use other binary alloy systems,
one component of which is copper, for example binary systems of
copper and zinc, tin, lanthanum, cerium, silicon or titanium.
[0047] In a fifth example, the base body 2 can also be made of an
alloy having a composition which lies between two phase stability
lines. These regions are shaded gray in FIG. 3. Since the
precipitation processes have been stopped by heat treatments, these
alloys are present in a thermally stable phase. In this respect,
preference is given to compositions from particularly wide ranges,
for instance between Al.sub.2Cu and AlCu.
[0048] The substrates of the examples mentioned here have
particularly high strengths of 300 MPa or more, even at
temperatures of up to 150.degree. C., and also have a good
long-term stability. The substrates, which comprise copper in the
base body thereof, additionally have high thermal conductivities,
and therefore they can be readily cooled. On account of their
special base body, the substrates do not experience any changes in
microstructure in temperature ranges which arise in long-term
operation of mirrors in EUV projection exposure apparatuses. As a
result, EUV mirrors having such a substrate have the advantage that
the roughness values thereof remain substantially constant over
their service life, in particular in the spatial frequency range of
0.1 .mu.m to 200 .mu.m. The EUV mirrors described here are suitable
both for use in the illumination system, with which a mask or a
reticle is illuminated with EUV radiation, and in the projection
system, with which the structure of the mask or of the reticle is
projected onto an object to be exposed, for example a semiconductor
wafer, of an EUV projection exposure apparatus. Owing to their
high-temperature strength and resilience, they are particularly
suitable for mirrors arranged further forward in the beam path,
where the thermal loading is higher, for instance in the
illumination system. They are particularly suitable for use as
facets of pupil facet mirrors and particularly of field facet
mirrors.
[0049] The above description of the specific 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 disclosed. The applicant seeks,
therefore, to cover all such changes and modifications as fall
within the spirit and scope of the invention, as defined by the
appended claims, and equivalents thereof.
* * * * *