U.S. patent application number 11/216560 was filed with the patent office on 2006-03-30 for reflective optical element and euv lithography appliance.
Invention is credited to Hans-Jurgen Mann, Udo Nothelfer, Johann Trenkler.
Application Number | 20060066940 11/216560 |
Document ID | / |
Family ID | 32864042 |
Filed Date | 2006-03-30 |
United States Patent
Application |
20060066940 |
Kind Code |
A1 |
Trenkler; Johann ; et
al. |
March 30, 2006 |
Reflective optical element and EUV lithography appliance
Abstract
The invention relates to a reflective optical element and an EUV
lithography appliance containing one such element, said appliance
displaying a low propensity to contamination. According to the
invention, the reflective optical element has a protective layer
system consisting of at least one layer. The optical
characteristics of the protective layer system are between those of
a spacer and an absorber or correspond to those of a spacer. The
selection of a material with the smallest possible imaginary part
and a real part which is as close to 1 as possible in terms of the
refractive index leads to a plateau-type reflectivity course
according to the thickness of the protective layer system between
two thicknesses d1 and d2. The thickness of the protective layer
system is selected in such a way that it is less than d.sub.2.
Inventors: |
Trenkler; Johann;
(Schwabisch Gmund, DE) ; Mann; Hans-Jurgen;
(Oberkochen, DE) ; Nothelfer; Udo; (Radebeul,
DE) |
Correspondence
Address: |
HUDAK, SHUNK & FARINE, CO., L.P.A.
2020 FRONT STREET
SUITE 307
CUYAHOGA FALLS
OH
44221
US
|
Family ID: |
32864042 |
Appl. No.: |
11/216560 |
Filed: |
August 31, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP04/02014 |
Mar 1, 2004 |
|
|
|
11216560 |
Aug 31, 2005 |
|
|
|
Current U.S.
Class: |
359/359 ;
359/586 |
Current CPC
Class: |
G03F 7/70958 20130101;
G02B 5/0816 20130101; G02B 5/085 20130101; G21K 1/062 20130101;
G02B 5/0891 20130101; B82Y 10/00 20130101; G03F 7/70916 20130101;
G03F 7/7015 20130101 |
Class at
Publication: |
359/359 ;
359/586 |
International
Class: |
F21V 9/04 20060101
F21V009/04; G02B 1/10 20060101 G02B001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2003 |
DE |
103 09 084.3 |
Claims
1. A reflective optical element for the EUV and soft X-ray
wavelength region, comprising: a multilayer system and a protective
layer system, wherein a side of the multilayer system facing the
protective layer system terminates in an absorber layer, wherein
the protective layer system has at least one layer of a material
with a refractive index whose real part at an operating wavelength
between 12.5 nm and 15 nm is between 0.90 and 1.03, and whose
imaginary part at an operating wavelength between 12.5 nm and 15 nm
is between 0 and 0.025, so that the reflectivity plotted as a
function of the thickness of the protective layer system at first
drops, until a thickness d1 is reached, the reflectivity remains
essentially constant between thickness d1 and another thickness,
d2, d2>d1, and the reflectivity further drops for a
thickness>d2, and the thickness of the protective layer system
is smaller than d.sub.2.
2. The reflective optical element according to claim 1, wherein the
imaginary part is between 0 and 0.015 and the real part is between
0.95 and 1.02.
3. The reflective optical element according to claim 1, wherein the
protective layer system consists of one or more materials from the
group Ce, Be, SiO, SiC, SiO.sub.2, Si.sub.3N.sub.4, C, Y,
MoSi.sub.2, B, Y.sub.2O.sub.3, MoS.sub.2, B.sub.4C, BN,
Ru.sub.xSi.sub.y, Zr, Nb, MoC, ZrO.sub.2, Ru.sub.xMo.sub.y,
Rh.sub.xMo.sub.y, or Rh.sub.xSi.sub.y.
4. The reflective optical element according to claim 1, wherein the
multilayer system is a system that consists of molybdenum and
silicon layers, ending with a molybdenum layer on the side facing
the protective layer system.
5. The reflective optical element according to claim 1, wherein the
protective layer system ends on a side of a vacuum with a layer of
a material for which the build-up of carbon is suppressed.
6. The reflective optical element according to claim 1, wherein the
protective layer system ends toward a side of a vacuum with a layer
of a material that is inert to energy injection.
7. The reflective optical element according to claim 1, wherein the
protective layer system consists of two layers.
8. The reflective optical element according to claim 1, wherein the
protective layer system consists of three layers.
9. The reflective optical element according to claim 1, wherein the
thickness d1 of the protective layer system is such that a standing
wave formed by reflection at operating wavelength .lamda..sub.B has
a minimum at a distance from the surface of the reflective optical
element of 0.1 .lamda..sub.B or less.
10. The reflective optical element according to claim 1, wherein
the reflectivity curve as a function of the thickness of the
protective layer system is constant in the context of a
reflectivity decrease of 1% of the maximum reflectivity in the
protective layer thickness region between d.sub.1 and d.sub.2.
11. The reflective optical element according to claim 1, wherein
the reflectivity curve as a function of the protective layer
thickness between d1 and d2 goes through at least one inflection
point at the protective layer thickness d.sub.w and the thickness
of the protective layer system is less than d.sub.w.
12. The reflective optical element according to claim 1, wherein
the thickness of the protective layer system is equal to
d.sub.1.
13. A EUV lithography appliance with at least one reflective
optical element according to claim 1.
14. A reflective optical element for the EUV and soft X-ray
wavelength region, comprising a multilayer system and a protective
layer system, having at least one layer of a material with a
refractive index whose real part at an operating wavelength
.lamda.B between 12.5 nm and 15 nm is between 0.90 and 1.03, and
whose imaginary part at an operating wavelength of 12.5 nm to 15 nm
is between 0 and 0.025, so that the reflectivity plotted as a
function of the thickness of the protective layer system at first
drops, until a thickness d1 is reached, the reflectivity remains
essentially constant between thickness d1 and another thickness d2,
d2>d1, and the reflectivity further drops for a thickness>d2,
wherein a side of the multilayer system facing the protective layer
system ends in an absorber layer, wherein the thickness of the
protective layer system is smaller than d2, and wherein the
thickness d1 is chosen such that a standing wave formed upon
reflection at operating wavelength .lamda.B has a minimum at a
distance from the surface of the reflective optical element of 0.1
.lamda.B or less, and the minimum lies in the vacuum.
15. The reflective optical element according to claim 2, wherein
the protective layer system consists of one or more materials from
the group Ce, Be, SiO, SiC, SiO.sub.2, Si.sub.3N.sub.4, C, Y,
MoSi.sub.2, B, Y.sub.2O.sub.3, MoS.sub.2, B.sub.4C, BN,
Ru.sub.xSi.sub.y, Zr, Nb, MoC, ZrO.sub.2, Ru.sub.xMo.sub.y,
Rh.sub.xMo.sub.y, or Rh.sub.xSi.sub.y.
16. The reflective optical element according to claim 15, wherein
the multilayer system is a system that consists of molybdenum and
silicon layers, ending with a molybdenum layer on the side facing
the protective layer system.
17. The reflective optical element according to claim 16, wherein
the protective layer system ends on a side of a vacuum with a layer
of a material for which the build-up of carbon is suppressed.
18. The reflective optical element according to claim 17, wherein
the protective layer system ends toward the a side of a vacuum with
a layer of a material that is inert to energy injection.
19. The reflective optical element according to claim 18, wherein
the protective layer system consists of two layers.
20. The reflective optical element according to claim 18, wherein
the protective layer system consists of three layers.
21. The reflective optical element according to claim 20, wherein
the thickness d1 of the protective layer system is such that a
standing wave formed by reflection at operating wavelength
.lamda..sub.B has a minimum at a distance from the surface of the
reflective optical element of 0.1 .lamda..sub.B or less.
22. The reflective optical element according to claim 21, wherein
the reflectivity curve as a function of the thickness of the
protective layer system is constant in the context of a
reflectivity decrease of 1% of the maximum reflectivity in the
protective layer thickness region between d.sub.1 and d.sub.2.
23. The reflective optical element according to claim 22, wherein
the reflectivity curve as a function of the protective layer
thickness between d1 and d2 goes through at least one inflection
point at the protective layer thickness d.sub.w and the thickness
of the protective layer system is less than d.sub.w.
24. The reflective optical element according to claim 23, wherein
the thickness of the protective layer system is equal to
d.sub.1.
25. A EUV lithography appliance with at least one reflective
optical element according to claim 20.
Description
CROSS REFERENCE
[0001] This application is a continuation-in-part application of
International Application No. PCT/EP2004/002014, filed Mar. 1, 2004
and published as WO 2004/079753 on Sep. 16, 2004, which claims the
priority to German Application No. 103 09 084.3, filed Mar. 3,
2003.
FIELD OF THE INVENTION
[0002] The invention concerns a reflective optical element for the
EUV and soft X-ray wavelength region, having a multilayer system
and a protective layer system, wherein the side of the multilayer
system facing the protective layer system terminates in an absorber
layer. Furthermore, the invention concerns an EUV lithography
appliance with a reflective optical element of this kind.
BACKGROUND OF THE INVENTION
[0003] Multilayers are composed of periodic repetitions, and in the
most simple case a period consists of two layers. The one layer
material should consist of a so-called spacer material, while the
other layer material should consist of a so-called absorber
material. Spacer material has a real part of the refractive index
close to 1, absorber material has a real part of the refractive
index significantly different from 1. The period thickness and the
thicknesses of the individual layers are chosen in dependence on
the operating wavelength, so that the reflectivity is generally
maximized at this operating wavelength.
[0004] Depending on the requirement of the reflective optical
element in regard to the reflection profile, various configuration
of the multilayer system are conceivable. Bandwidth and
reflectivity, for example, can be adjusted by having more than just
two materials in one period or by deviating from a constant layer
thickness or even from constant thickness ratios (so-called
depth-graded multilayers).
[0005] EUV lithography appliances are used in the production of
semiconductor components, such as integrated circuits. Lithography
appliances which are used in the extreme ultraviolet wavelength
region primarily have multilayer systems of molybdenum and silicon,
for example, as the optical reflective element. Although EUV
lithography appliances have a vacuum or a residual gas atmosphere
in their interior, it is not entirely possible to prevent
hydrocarbons and/or other carbon compounds from being inside the
appliance. These carbon compounds are split apart by the extreme
ultraviolet radiation or by secondary electrons, resulting in the
depositing of a carbon-containing contamination film on the optical
elements. This contamination with carbon compounds leads to
substantial reflection losses of the functional optical surfaces,
which can have a considerable influence on the economic
effectiveness of the EUV lithography process. This effect is
intensified in that typical EUV lithography appliances have eight
or more reflective optical elements. Their transmission is
proportional to the product of the reflectivities of the individual
optical reflective elements.
[0006] The contamination leads not only to reflectivity losses, but
also to imaging errors, which in the worst case make an imaging
impossible. Thus, cleaning cycles have to be provided when
operating an EUV lithography appliance or when using reflective
optical elements. These significantly increase the operating costs.
But the cleaning cycles not only increase the down time, but also
entail the risk of worsening of the homogeneity of the layer
thickness of the reflective optical elements and the risk of
increasing the surface relief, which leads to further reflectivity
losses.
[0007] One approach to controlling the contamination for Mo/Si
multilayer mirrors is found in M. Malinowski et al., Proceedings of
SPIE Vol. 4688 (2002), pages 442 to 453. A multilayer system of 40
pairs of molybdenum and silicon with pair thickness of 7 nm and a
.GAMMA.=(d.sub.Mo/(d.sub.Mo+d.sub.Si), with d.sub.Mo being the
thickness of the molybdenum layer and d.sub.Si the thickness of the
silicon layer, of around 0.4, was provided with an additional
silicon layer on the uppermost molybdenum layer. Multilayer systems
with different thickness of silicon protective layer were measured,
extending from 2 to 7 nm. Traditional Mo/Si multilayer systems have
a silicon protective layer of 4.3 mm, which helps protect against
contamination, although it very quickly becomes oxidized. The
measurements revealed that there is a reflectivity plateau for a
silicon protective layer of 3 nm, depending on the radiation dose.
It is therefore recommended to use silicon protective layers with a
thickness of 3 nm, instead of silicon protective layers with a
thickness of 4.3 nm. For a longer operating time can be achieved
with a silicon protective layer 3 nm in thickness, for the same
tolerance in the reflectivity loss.
SUMMARY OF THE INVENTION
[0008] The problem of the present invention is to provide a
reflective optical element for the EUV and soft X-ray wavelength
region that has the longest possible lifetime. Furthermore, the
problem of the invention is to provide an EUV lithography appliance
with the shortest possible down time.
[0009] The problem is solved by a reflective optical element, as
well as an EUV lithography appliance according to the claims.
[0010] It has been found that reflective optical elements for the
EUV and soft X-ray wavelength region with long lifetime are
achieved if they are provided with a protective layer system that
has one or more layers of materials with a particular refractive
index, and in which the overall thickness of the protective layer
system is chosen according to particular criteria. The one or more
layers of the protective layer system should have a refractive
index at operating wavelengths between 12.5 nm and 15 nm whose real
part is between 0.90 and 1.03, preferably between 0.95 and 1.02,
and whose imaginary part is between 0 and 0.025, especially
preferably between 0 and 0.015. Thus, as compared to the layers of
the multilayer system situated underneath, the layers of the
protective layer system have the optical properties of a spacer or
lie between those of a spacer and an absorber. The choice of a
material with the smallest possible imaginary part and a real part
as close as possible to 1 results in a plateau-shaped reflexivity
curve, depending on the thickness of the protective layer system
between two thicknesses d.sub.1 and d.sub.2. This means that, with
these selected materials, the reflective optical element made of
multilayer system and protective layer system is insensitive to
fluctuations in the thickness of the protective layer system in
particular region. According to the invention, the reflective
optical element has a protective layer system with a thickness
smaller than d.sub.2.
[0011] The reflective optical elements of the invention have the
benefit that their relative insensitivity to thickness variations
in the protective layer system also translates into an
insensitivity to the build-up of a contamination layer. Without
substantial change in reflectivity, much thicker carbon layers can
be tolerated than with traditional reflective optical elements.
This also has a positive impact on the homogeneity of the imaging,
since even thickness fluctuations over the entire area are
negligible.
[0012] Basically, for a given operating wavelength, one will select
the material, the layer makeup of the protective layer system, and
the individual layer thicknesses so that a plateau in the
reflectivity is formed between two thicknesses d.sub.1 and d.sub.2
as a function of the thickness of the protective layer system. The
specific thickness of the protective layer system is then
advantageously chosen to be as small as possible, but still within
the reflectivity plateau. In practice, one must make sure that the
minimum layer thickness is always observed for each layer, so that
one can produce a closed layer.
[0013] It has been found that a standing wave field is formed by
reflection at the reflective optical element, whose minimum for a
protective layer thickness d1 lies in the vacuum at a distance of a
fraction of the operating wavelength. Now, if the layer thickness
of the protective layer system is increased, the minimum of the
standing wave field approaches the surface. Accordingly, the value
of the standing wave field at the surface increases until the
maximum is also achieved. Thus, the formation of the reflectivity
plateau in dependence on the thickness of the protective layer
system results because, with increasing layer thickness, the
additionally created absorption, i.e., the resulting decrease in
reflectivity, is compensated in that reflectivity gains are
produced by increasingly constructive interference after a certain
layer thickness.
[0014] As an additional effect, fewer photoelectrons are emitted
near the minimum of a standing wave field. Since the photoelectrons
also break down the hydrocarbons from the residual gas atmosphere
into carbon or carbon-containing particles, this has the result of
a noticeably slower build-up of the contamination.
[0015] A preferred embodiment is therefore characterized in that
the thickness d1 of the protective layer system is such that a
standing wave formed by reflection at operating wavelength
.lamda..sub.B has a minimum at a distance from the surface of the
reflective optical element of 0.1.lamda..sub.B or less. Thus, the
minimum lies in the vacuum. With increasing thickness, the surface
as it were migrates through the minimum until the thickness d.sub.2
is reached. This corresponds to a distance from the surface to the
minimum of at most 0.2.lamda..sub.B, and the minimum is located
inside the reflective optical element.
[0016] The specification of the essentially constant curve of the
reflectivity is to be understood as meaning that all reflectivity
fluctuations in a region that does not limit the functional
capability of the reflective optical element are considered to be
constant. In an especially preferred embodiment, a reflectivity
decrease of 1% of the maximum reflectivity in the protective layer
thickness region between d.sub.1 and d.sub.2 is considered harmless
and regarded as being a constant reflectivity curve in the context
of this invention.
[0017] It is to be assumed that the reflectivity curve as a
function of the protective layer thickness between d.sub.1 and
d.sub.2 goes through at least one inflection point at the
protective layer thickness d.sub.w. For due to the partial
compensation of the reflectivity loss in the protective layer
thickness region between d.sub.1 and d.sub.2, the slope of the
reflectivity curve changes in this protective layer thickness
region. Advantageously, the particular thickness of the protective
layer system is chosen to be .ltoreq.d.sub.w. This ensures that the
thickness of the protective layer corresponds to a reflectivity
which lies in the region of constant reflectivity in the sense of
this invention. As a result, the reflective optical element becomes
insensitive to an increase in the thickness of the protective
layer, for example, due to contamination.
[0018] In an especially preferred embodiment, the thickness of the
protective layer system is equal to d.sub.1.
[0019] The advantageous properties of the invented reflective
optical element have especially positive impact when they are used
in an EUV lithography appliance. Especially when several reflective
optical elements are connected in succession, the more uniform
reflectivity and also more uniform field illumination for lengthy
periods of time have especially positive impact. It has been found
that even with increasing contamination the wavefront errors in the
complex optical systems of EUV lithography appliances can be kept
small. A major benefit consists in that fewer cleaning cycles are
required for the EUV lithography appliance, thanks to the longer
lifetime of the reflective optical elements. This not only reduces
the down time, but also the risk of degeneration of the layer
homogeneity, greater roughness of the surface, or partial
destruction of the uppermost protective layer from too intense
cleaning are significantly reduced. In particular, the cleaning
processes for the reflective optical elements of the invention can
be controlled such that the contamination layer is deliberately not
entirely removed, but rather a minimal contamination layer always
remains on the uppermost layer. This protects the reflective
optical element against being destroyed by too intense cleaning.
The thickness of the contamination layer can be measured in
traditional manner during its build-up or during the cleaning with
a suitable in situ monitoring system.
[0020] It has proven to be especially advantageous for the
protective layer system to consist of one or more materials from
the group Ce, Be, SiO, SiC, SiO2, Si.sub.3N.sub.4, C, Y,
MoSi.sub.2, B, Y.sub.2O.sub.3, MoS.sub.2, B4C, BN,
Ru.sub.xSi.sub.y, Zr, Nb, MoC, ZrO.sub.2, Ru.sub.xMo.sub.y,
Rh.sub.xMo.sub.y, Rh.sub.xSi.sub.y. The SiO.sub.2 should preferably
be amorphous or polycrystalline.
[0021] The best results are achieved with a multilayer system that
consists of Mo/Si layers and that ends with the molybdenum layer on
the side facing the protective layer system. Depending on the
operating wavelength, multilayer system, and requirement for the
reflective optical element, it can be advantageous for the cover
layer system to consist of precisely two or precisely three
layers.
[0022] In a preferred embodiment, the protective layer system ends
on the side of the vacuum with a layer of a material for which the
build-up of carbon-containing substances is suppressed. It has been
found that certain materials have a low affinity for
carbon-containing substances, in other words, carbon-containing
substances get stuck to them only with a low probability or they
have a slight adsorption rate. Thus, for these materials, the
build-up of carbon-containing substances is drastically reduced or
suppressed. It has been found that such materials can be used as a
protective layer for reflective optical elements for the EUV and
soft X-ray wavelength region, without showing significant negative
effects on the optical behavior of the reflective optical element.
Especially preferred as such are the materials ZrO.sub.2,
Y.sub.2O.sub.3, and silicon dioxide in various stoichiometric
relations. The silicon dioxide can be in the amorphous or
polycrystalline, or possibly even the crystalline state.
[0023] In another preferred embodiment, the protective layer system
ends toward the vacuum with a layer of a material that is inert to
energy injection, that is, to bombardment with EUV protons or to
external electric fields. This decreases the probability of
spontaneous electron emission, which in turn might split apart the
residual gases into reactive cleavage products. Hence, the
deposition of contamination on the protective layer system is
further reduced. One can influence the inertia to external
electromagnetic fields, for example, by giving the surface the
lowest possible relief and/or using materials that have a large gap
between the valency band and the conduction band. Especially
preferred for this are the materials Nb, BN, B.sub.4C, Y, amorphous
carbon, Si.sub.3N.sub.4, SiC, as well as silicon dioxide in various
stoichiometric relations. The silicon dioxide can be in the
amorphous or polycrystalline, or possibly even the crystalline
state.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The invention shall be explained more closely by means of
several examples and the figures. These show:
[0025] FIG. 1 shows the reflectivity of a multilayer with
protective layer system as a function of the thickness of the
protective layer system;
[0026] FIGS. 2a, b shows the position of the standing wave field
for different thicknesses of the protective layer system;
[0027] FIG. 3 shows the reflectivity of a first reflective optical
element as a function of the thickness of the contamination layer,
and
[0028] FIGS. 4a, b shows the dependency of the wavefront error on
the thickness of the contamination layer for a six-mirror system
for EUV lithography and
[0029] FIG. 5 shows the reflectivity of a second reflective optical
element as a function of the thickness of the contamination
layer.
DETAILED DESCRIPTION OF THE INVENTION
EXAMPLE 1
[0030] On a Mo/Si multilayer system located on a substrate of
amorphous silicon dioxide, consisting of 50 pairs of 2.76 nm
molybdenum and 4.14 nm amorphous silicon (a-SiO.sub.2), a
three-layer protective layer system is deposited. The protective
layer system borders on the uppermost molybdenum layer of the
multilayer system with a Y-layer 1.2 nm thick. On the Y-layer is
placed a 1.5 nm Y.sub.2O.sub.3 layer. At the vacuum side, the
protective layer system is closed by a 1 nm thick amorphous silicon
dioxide layer. The choice of the materials and their thickness is
based on the criteria of the invention. In particular, the
materials are also selected so as to suppress carbon build-up
(Y.sub.2O.sub.3, a-SiO.sub.2) or to be inert to energy injection
(Y, a-SiO.sub.2).
[0031] Disregarding the interface and surface roughness, one
obtains a reflectivity of 70.2% at an operating wavelength of 13.5
nm for an angle of incidence of 0.degree. with the normal to the
surface. FIG. 1 shows the reflectivity of the entire reflective
optical element under these conditions as a function of the
thickness of the protective layer system, but holding constant the
thickness of 1.2 nm for Y and 1.5 nm for Y.sub.2O.sub.3. A distinct
reflectivity plateau is formed between a thickness d1=3.7 nm and a
thickness d2=6.68 nm of the protective layer system, or an
a-SiO.sub.2 layer of 1 nm and 2.98 nm. Accordingly, the thickness
of the silicon dioxide layer was selected to be 1.0 nm.
[0032] In FIGS. 2a and 2b, the resulting standing wave field is
shown for a protective layer system thickness of 3.7 nm (FIG. 2a)
and for a protective layer system thickness of 6.68 nm (FIG. 2b).
Segments a-c corresponding to the protective layer system of
amorphous SiO.sub.2 (a), Y.sub.2O.sub.3 (b), and Y (c) and segments
d, e corresponding to the multilayer system of molybdenum (d) and
amorphous silicon (e). As can be clearly seen, with increasing
thickness of the protective layer system the surface of the
reflective optical element is situated in the vicinity of the
minimum of the existing wave field, it migrates through the
minimum, so to speak. This would suggest a slight contamination due
to secondary electrons.
[0033] FIG. 3 shows the reflectivity of the reflective optical
element with the protective layer system of Y, Y.sub.2O.sub.3, and
a-SiO.sub.2 as a function of the built-up contamination layer. If
one selects a tolerance range of 1% for the fluctuation in
reflectivity, a carbon layer up to 4 nm thick can be tolerated
without significant change in reflectivity. The operating time is
therefore a multiple higher than for traditional reflective optical
elements.
[0034] In FIGS. 4a and b, these positive results are also shown by
means of an EUV lithography appliance with six reflective optical
elements (S1-S6) according to the invention as mirrors. The tested
mirror construction is shown in FIG. 4a. In FIG. 4b, the wavefront
error is shown as a function of the carbon thickness.
[0035] Although the wavefront varies periodically with the
increased carbon thickness, the absolute value of the wavefront
error does not exceed a value that would significantly impair the
imaging quality of the lithography system for any carbon
thickness.
[0036] Because of the insensitivity of the reflective optical
element discussed here with respect to the build-up of a carbon
contamination layer, it is possible to only remove the
contamination layer down to a layer of 0.5 nm when cleaning the
reflective optical element or when cleaning the entire EUV
lithography appliance. This will ensure, on the one hand, that the
cleaned optical element once again has a long lifetime. But it will
also make sure that the risk of degeneration of the layer
homogeneity or roughening of the surface or partial destruction of
the topmost layer by too intense cleaning is reduced.
EXAMPLE 2
[0037] On a multilayer system of 50 Mo/Si pairs located on an
amorphous silicon dioxide substrate, optimized for an operating
wavelength of 13.5 nm, a protective layer system of a 2.0 nm thick
cerium layer, which adjoins the topmost molybdenum layer of the
multilayer system, and a 1.5 nm thick silicon dioxide layer is
placed. The minimum of a standing wave produced by reflection on
the uncontaminated reflective optical element at operating
wavelength .lamda..sub.B lies in the vacuum, 0.05.lamda..sub.B from
its surface. For a maximum reflectivity of 70.9% at an operating
wavelength of 13.5 nm and a tolerated reflectivity decrease of 1%,
a carbon contamination layer can tolerate a thickness of up to 3.5
nm (see FIG. 5). This reflective optical element as well is
suitable for use in an EUV lithography appliance.
* * * * *