U.S. patent application number 11/434439 was filed with the patent office on 2006-11-23 for reflective optical element for ultraviolet radiation, projection optical system and projection exposure system therewith, and method for forming the same.
Invention is credited to Christoph Zaczek.
Application Number | 20060262389 11/434439 |
Document ID | / |
Family ID | 37448046 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060262389 |
Kind Code |
A1 |
Zaczek; Christoph |
November 23, 2006 |
Reflective optical element for ultraviolet radiation, projection
optical system and projection exposure system therewith, and method
for forming the same
Abstract
A reflective optical element for radiation with a wavelength A
in the ultraviolet wavelength range comprises a reflective surface,
and a dielectric multilayer system formed on said reflective
surface, said dielectric multilayer system comprising at least two
successive pairs of layers, each pair of layers consisting of a
high refractive index layer alternating with a low refractive index
layer, wherein optical thicknesses of said high refractive index
layers and optical thicknesses of said low refractive index layers
of each adjacent pair of layers are different from each other.
Inventors: |
Zaczek; Christoph; (Heubach,
DE) |
Correspondence
Address: |
WALTER A. HACKLER, Ph.D.;PATENT LAW OFFICE
SUITE B
2372 S.E. BRISTOL STREET
NEWPORT BEACH
CA
92660-0755
US
|
Family ID: |
37448046 |
Appl. No.: |
11/434439 |
Filed: |
May 15, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60683691 |
May 23, 2005 |
|
|
|
Current U.S.
Class: |
359/359 ;
359/587 |
Current CPC
Class: |
G02B 5/0891 20130101;
G02B 5/0875 20130101; G02B 5/0825 20130101 |
Class at
Publication: |
359/359 ;
359/587 |
International
Class: |
F21V 9/04 20060101
F21V009/04 |
Claims
1. Reflective optical element for radiation with a wavelength A in
the ultraviolet wavelength range comprising: a reflective surface,
and a dielectric multilayer system formed on said reflective
surface, said dielectric multilayer system comprising at least two
successive pairs of layers, each of said pairs of layers consisting
of a high refractive index layer alternating with a low refractive
index layer, wherein optical thicknesses of said high refractive
index layers and optical thicknesses of said low refractive index
layers of each adjacent pair of layers are different from each
other.
2. Reflective optical element according to claim 1, wherein said
optical thicknesses of all of said high and low refractive index
layers of said dielectric multilayer system are different from each
other.
3. Reflective optical element according to claim 1, wherein said
optical thickness of said high and low refractive index layers is
between 0.1 .lamda. and 0.35 .lamda..
4. Reflective optical element according to claim 1, wherein said
dielectric multilayer system further comprises a first low
refractive index layer formed contiguous to said reflective surface
having an optical thickness between 0.1 .lamda. and 0.2
.lamda..
5. Reflective optical element according to claim 1, wherein said
optical thickness of said high refractive index layers decreases
with increasing distance from said reflective surface.
6. Reflective optical element according to claim 5, wherein said
optical thickness of said low refractive index layers increases at
least on average with increasing distance from said reflective
surface.
7. Reflective optical element according to claim 1, wherein said
optical thickness of said low refractive index layers decreases
with increasing distance from said reflective surface.
8. Reflective optical element according to claim 7, wherein said
optical thickness of said high refractive index layers increases at
least on average with increasing distance from said reflective
surface.
9. Reflective optical element according to claim 1, wherein said
reflective surface is a surface of a metal film, in particular an
aluminium film being formed contiguous to a substrate, said metal
film preferably having a thickness smaller than 100 nm, in
particular between 55 nm and 100 nm.
10. Reflective optical element according to claim 1, wherein each
of said high refractive index layers is composed of one or more
materials selected from the group consisting of: lanthanum fluoride
(LaF.sub.3), gadolinium fluoride (GdF.sub.3), aluminium oxide
(Al.sub.2O.sub.3), neodymium fluoride (NdF.sub.3), dysprosium
fluoride (DyF.sub.3), lead fluoride (PbF.sub.2), hafnium oxide
(HfO.sub.2), and zirconium oxide (ZrO.sub.2).
11. Reflective optical element according to claim 1, wherein each
of said low refractive index layers is composed of one or more
materials selected from the group consisting of: aluminium fluoride
(AlF.sub.3), magnesium fluoride (MgF.sub.2), sodium fluoride (NaF),
lithium fluoride (LiF), thiolithe (Na.sub.5Al.sub.3Fl.sub.4),
cryolite (Na.sub.3AlF.sub.6), silicon oxide (SiO.sub.2), calcium
fluoride (CaF.sub.2), barium fluoride (BaF.sub.2), and strontium
fluoride (SrF.sub.2).
12. Projection optical system for forming an image of a pattern
arranged on a mask on a photosensitive substrate, comprising at
least one reflective optical element according to claim 1.
13. Projection optical system according to claim 12, wherein said
reflective optical element is a concave reflector.
14. Projection optical system according to claim 12, wherein said
reflective optical element is a deflecting mirror arranged in an
optical path before a concave reflector.
15. Projection optical system according to claim 12, wherein said
reflective optical element is a deflecting mirror arranged in an
optical path after a concave reflector.
16. Projection exposure apparatus for radiation in the ultraviolet
wavelength range, comprising: an illuminating system for
illuminating a mask, and a projection optical system for forming an
image of a pattern arranged on said mask on a photosensitive
substrate, said projection exposure apparatus further comprising at
least one reflective optical element according to claim 1.
17. Projection exposure apparatus according to claim 16, wherein
said at least one reflective optical element is arranged in said
illumination system.
18. Projection exposure apparatus according to claim 16, wherein
said at least one reflective optical element is arranged in said
projection optical system.
19. Method for forming a reflective optical element for radiation
with a wavelength .lamda. in the ultraviolet wavelength range, said
method comprising the steps of: forming a metal film, in particular
an aluminium film, on a substrate with a thickness preferably below
100 nm, in particular between 55 nm and 100 nm, a surface of the
metal film forming a reflective surface, forming a dielectric
multilayer system on said reflective surface starting with a first
low refractive index layer with an optical thickness between 0.1
.lamda. and 0.2 .lamda., superimposing over said first layer at
least two successive pairs of layers, each pair of layers
consisting of a high refractive index layer alternating with a low
refractive index layer, wherein optical thicknesses of said high
refractive index layers and optical thicknesses of said low
refractive index layers of each adjacent pair of layers are
different from each other.
20. Method according to claim 19, wherein said high and low
refractive index layers are formed on said substrate by a
deposition method selected from the group consisting of thermal
evaporation, ion assisted deposition, and sputtering.
Description
CLAIM OF PRIORITY
[0001] This application claims benefit under 35 U.S.C. 119(e)(1) of
U.S. Provisional Application No. 60/683,691, filed May 23, 2005.
The disclosure of U.S. Provisional Application No. 60/683,691 filed
May 23, 2005 is considered part of and is incorporated by reference
in the disclosure of this application.
TECHNICAL FIELD
[0002] The present invention relates to a reflective optical
element for radiation with a wavelength .lamda. in the ultraviolet
wavelength range comprising: a reflective surface, and a dielectric
multilayer system formed on said reflective surface, said
dielectric multilayer system comprising at least two successive
pairs of layers, each pair of layers consisting of a high
refractive index layer alternating with a low refractive index
layer, the invention further relates to a projection optical system
and a projection exposure apparatus comprising at least one such
reflective optical element, and to a method for forming such a
reflective optical element.
BACKGROUND
[0003] Reflective optical elements for radiation in the ultraviolet
wavelength range are used e.g. in microlithography projection
exposure systems for redirecting or bending of a laser beam with a
given center wavelength (e.g. 193 nm). For such reflective optical
elements, a high reflectance to the incident radiation over a wide
range of incident angles is desirable. Moreover, the difference in
amplitude and phase of the reflectivity of a polarization component
with an electrical field strength vector parallel to a plane formed
by the normal vector of the reflective surface and the direction of
the incident beam (p-polarized radiation) and a polarization
component perpendicular to that plane (s-polarized radiation)
should be as small as possible. This is because, if the
reflectivity of such a reflective optical element for s-polarized
radiation is significantly different from the reflectivity for
p-polarized radiation, the different intensity and phase of the two
polarization components in the reflected beam tend to degrade the
imaging performance of the projection exposure apparatus, if not
being compensated for.
[0004] For maximizing reflectance, it is well-known to superimpose
a dielectric multilayer system enhanced over the metal mirror
surface. The multi-layer system comprises alternating layers of
high refractive index layers and low refractive index layers, each
having an optical thickness of .lamda./4 for a given incident
angle. However, such a multilayer system does only yield high
reflectance and a small difference between polarization components
in a narrow range of incident angles.
[0005] In order to solve the above problems U.S. Pat. No. 6,310,905
discloses a reflective optical element with a dielectric multilayer
system consisting of an arrangement of high refractive index layers
alternating with low refractive index layers such that each high
refractive index layer follows a low refractive index layer shown
by the representation: L.sub.1/[H/L.sub.2].sup.x [1] wherein
L.sub.1, L.sub.2: represent the low refractive index layers
[0006] H: represents the high refractive index layers
[0007] X: defines an integer between 1 and 10
[0008] The above formula [1] defines a dielectric multilayer system
in which a succession of pairs of high refractive index layers H
alternating with low refractive index layers L.sub.2 is
superimposed over a first low refractive index layer L.sub.1. The
letters used for the high and low refractive index layers H,
L.sub.1, L.sub.2 are also representative for the optical thickness
of these layers, such that different optical thickness may be
expressed in terms of those letters, e.g. L.sub.1<L.sub.2. The
letter X defines the repetition index, i.e. the number of times
that the pair of layers HL.sub.2 is repeated in the multilayer
system.
[0009] A mirror with a dielectric multilayer system similar to the
one described above, albeit more complex, is disclosed in U.S. Pat.
No. 5,850,309. In this system, several pairs of layers consisting
of a high refractive index layer alternating with a low refractive
index layer--being separated by so-called bonding layers--are
repeated.
OBJECT OF THE INVENTION
[0010] It is the object of the invention to provide a reflective
optical element with a high reflectance as well as a small
separation in amplitude and phase of the polarization component(s)
of a reflected beam over a wide range of incident angles.
SUMMARY OF THE INVENTION
[0011] This object is achieved by a reflective optical element of
the above-mentioned kind in which the optical thicknesses of the
high refractive index layers and the optical thicknesses of the low
refractive index layers of each adjacent pair of layers are
different from each other.
[0012] The reflective optical element according to the invention
comprises a dielectric multilayer system wherein no adjacent pair
of layers has necessarily coinciding optical thicknesses of high
and low refractive index layers, as required in [1].
[0013] The invention is based on the insight that dielectric
multilayer system designs having periodic parts with pairs of
alternating high and low refractive index layers in most cases lead
to an inferior performance compared to multilayer systems with a
design not showing such a periodicity. This is in particular the
case when the dielectric layers absorb a portion of the incident
radiation and the portion absorbed by the high and low refractive
index layers is different.
[0014] Aperiodic designs are also advantageous in order to keep the
amplitude difference and, in particular, the phase difference of
the polarization components (s-, resp. p-polarization) in the
reflected beam as small as possible. Especially, the occurrence of
a phase shift between the two polarization components has not been
addressed in the above-mentioned prior art such that the reflective
optical elements described therein are not optimized in this
respect. However, if not compensated for, a phase shift of the
polarization components of a laser beam used in a microlithography
projection exposure apparatus may lead to a degradation of its
optical performance.
[0015] Although a design with two subsequent pairs of layers having
high and low refractive index layers with the same optical
thickness does not fall into the scope of the present invention,
dielectric multilayer systems represented by layer arrangements
with one layer thickness of subsequent pairs being identical, such
as e.g. H.sub.1L.sub.1H.sub.1L.sub.2 or
L.sub.1H.sub.1L.sub.1H.sub.2, H.sub.1.noteq.H.sub.2,
L.sub.1.noteq.L.sub.2, fall into the scope of the present
invention.
[0016] In one embodiment, the optical thicknesses of all high and
low refractive index layers of the dielectric multilayer system are
different from each other. In this particular case, all layers of
the multilayer system have a different optical thickness, yielding
a completely aperiodical design.
[0017] In a preferred embodiment, the optical thickness of the high
and low refractive index layers is between 0.1 .lamda. and 0.35
.lamda.. It is advantageous when the optical thickness of the
dielectric layers fluctuates about the value of 0.25 .lamda..
[0018] In a highly preferred embodiment, the dielectric multilayer
system further comprises a first low refractive index layer formed
contiguous to the reflective surface having an optical thickness
between 0.1 .lamda. and 0.2 .lamda.. The first layer is used for
phase adaptation to the reflective surface and has a smaller
optical thickness compared to the other layers.
[0019] In a further highly preferred embodiment the optical
thickness of the more absorbing layer material, in this case the
high refractive index layers, decreases with increasing distance
from the reflective surface. The optical power of the incident
radiation (being proportional to the square of the field strength)
decreases exponentially from the topmost dielectric layer of the
multilayer system to the dielectric layer adjacent to the
reflective surface. The higher the field strength in the optical
material, the higher the absorption of radiation. It is therefore
advantageous when the optical thickness of the high refractive
index layers decreases with increasing field strength, such that
the absorption of radiation is reduced in comparison to a pure
periodic system.
[0020] In a preferred variant of this embodiment, the optical
thickness of the low refractive index layers increases at least on
average with increasing distance from the reflective surface. The
term "at least on average" means that the optical thickness of the
low refractive index layers increases, however not necessarily
monotonic, such that for some layers, a small decrease in optical
layer thickness may be tolerated. The increase of the optical
thickness of the low refractive index layers with increasing
distance from the reflective surface is advantageously combined
with the decrease of the optical thickness of the high refractive
index layers, particularly when the high refractive index layers
show higher absorption than the low refractive index layers.
Moreover, such a combination can also be advantageous for the
purpose of minimization of the phase difference of polarization
components.
[0021] In an alternative embodiment, the optical thickness of the
low refractive index layers decreases with increasing distance from
the reflective surface, which is particularly preferred for the
reasons set out above when the optical materials of the low
refractive index layers show higher absorption than the materials
used for the high refractive index layers. In a preferred variant
of this embodiment, the optical thickness of the high refractive
index layers increases at least on average with increasing distance
from the reflective surface.
[0022] In a highly preferred embodiment, the reflective surface is
a surface of a metal film, in particular an aluminium film being
formed contiguous to a substrate, the metal film preferably having
a thickness smaller than 100 nm, in particular between 55 nm and
100 nm. The surface roughness and consequently the scattering of
radiation at the aluminium film increase with increasing film
thickness due to the increasing surface roughness. A thickness
below 100 nm is therefore preferred. The substrate may be formed
e.g. of synthetic quartz glass, or any metal fluoride mentioned
above.
[0023] In a preferred embodiment, each high refractive index layer
is composed of one or more materials selected from the group
consisting of: lanthanum fluoride (LaF.sub.3), gadolinium fluoride
(GdF.sub.3), aluminium oxide (Al.sub.2O.sub.3), neodymium fluoride
(NdF.sub.3), dysprosium fluoride (DyF.sub.3), lead fluoride
(PbF.sub.2), hafnium oxide (HfO.sub.2), and zirconium oxide
(ZrO.sub.2). The high refractive index layers may also be composed
of compounds or mixtures of those materials.
[0024] In a further preferred embodiment, the low refractive index
layers are composed of one or more materials selected from the
group consisting of: aluminium fluoride (AlF.sub.3), magnesium
fluoride (MgF.sub.2), sodium fluoride (NaF), lithium fluoride
(LiF), thiolithe (Na5Al.sub.3F.sub.14), cryolite
(Na.sub.3AlF.sub.6), silicon oxide (SiO.sub.2), calcium fluoride
(CaF.sub.2), barium fluoride (BaF.sub.2), and strontium fluoride
(SrF.sub.2). The low refractive index layers may also be composed
of compounds or mixtures of those materials
[0025] The invention is further realized in a projection optical
system for forming an image of a pattern arranged on a mask on a
photosensitive substrate, comprising at least one reflective
optical element as described above. Preferably, the reflective
optical element is a concave reflector of a catadioptric projection
optical system, as the requirements regarding reflectance and
polarization conservation for such an element are generally very
high. Alternatively or in addition, the reflective optical element
can be realized as one of a first deflecting mirror arranged in an
optical path before the concave reflector and a second deflecting
mirror arranged in an optical path after the concave reflector. The
angles of incidence for these mirrors are relatively large, so that
the designs described above can be advantageously applied in this
case.
[0026] The invention further relates to a projection exposure
apparatus for radiation in the ultraviolet wavelength range,
comprising: an illuminating system for illuminating a mask, and a
projection optical system for forming an image of a pattern
arranged on said mask on a photosensitive substrate, said
projection exposure apparatus further comprising at least one
reflective optical element as described above. The at least one
reflective optical element is preferably arranged in the
illumination system and/or the projection optical system of the
apparatus. Thus, the improved properties of the multilayer designs
of the reflective optical elements described above can be
advantageously applied in microlithography systems.
[0027] The invention is also realized in a method for forming a
reflective optical element for radiation with a wavelength A in the
ultraviolet wavelength range comprising the steps of: forming a
metal film, in particular an aluminium film, on a substrate with a
thickness preferably below 100 nm, in particular between 55 nm and
100 nm, a surface of the metal film forming a reflective surface,
forming a dielectric multilayer system on the reflective surface
starting with a first low refractive index layer with an optical
thickness between 0.1 .lamda. and 0.2 .lamda., superimposing over
the first layer at least two successive pairs of layers, each pair
of layers consisting of a high refractive index layer alternating
with a low refractive index layer, wherein the optical thicknesses
of the high refractive index layers and the optical thicknesses of
the low refractive index layers of each adjacent pair of layers are
different from each other. The layers may be formed on the
substrate by a technique such as thermal evaporation, ion assisted
deposition, and sputtering.
[0028] Further features and advantages of the invention can be
extracted from the following description of an embodiment of the
invention, with reference to the figures of the drawing which show
inventive details, and from the claims. The individual features can
be realized individually or collectively in arbitrary combination
in a variant of the invention.
DRAWING
[0029] The schematic drawing shows embodiments of the invention
which are explained in the following description.
[0030] FIG. 1 shows an embodiment of a reflective optical element
for radiation with a wavelength in the ultraviolet wavelength range
according to the invention;
[0031] FIG. 2 shows a diagram of the optical thicknesses of the
layers of a first dielectric multilayer system design of the
embodiment of FIG. 1 compared to a state-of-the-art design;
[0032] FIG. 3 shows an analogous diagram for a second dielectric
multilayer system design of the embodiment of FIG. 1, being
optimized to minimize the phase shift between polarization
components of a reflected beam, and
[0033] FIG. 4 shows a catadioptric projection optical system having
three reflective optical elements according to the invention.
[0034] FIG. 1 shows a reflective optical element 1 for radiation
with a wavelength A in the ultraviolet wavelength range (in the
present case, .lamda.=193 nm) which may be used in a
microlithography projection exposure apparatus for redirecting or
bending a laser beam. The reflective optical element 1 comprises a
substrate 2 of synthetic quartz glass on which an aluminium film 3
with a thickness of between 55 nm and 100 nm is formed. On top of
the aluminium film 3, a dielectric multilayer system 4 is formed,
starting with a first low refractive index layer L.sub.0 adjacent
to a surface 6 of the aluminium film 3 and followed by a succession
of a number N (.ltoreq.50) of pairs of layers 5.1 to 5.N. Each pair
of layers 5.1 to 5.N consists of a high refractive index layer
H.sub.1 to H.sub.N alternating with a low refractive index layer
L.sub.1 to L.sub.N, the letters H.sub.i, L.sub.i
(1.ltoreq.i.ltoreq.N) also representing the optical thicknesses of
the layers in units of the incident wavelength: H i , L i = n Li ,
Hi .times. d Li , Hi .times. cos .function. ( .alpha. i ) .lamda. ,
##EQU1## wherein:
[0035] H.sub.i and L.sub.i define the optical thickness of the high
and low refractive index layers with layer number i,
1.ltoreq.i<N;
[0036] n.sub.Li,Hi is the refractive index of the low resp. high
refractive index layer with layer number i;
[0037] d.sub.Li, Hi is the thickness of the low resp. high
refractive index layer with layer number i
[0038] .alpha..sub.i is the angle of incidence of the beam inside
layer number i; and
[0039] .lamda. is the wavelength of the incident beam.
[0040] For two adjacent pairs of layers 5.i, 5.i+1, the respective
optical thicknesses of the high refractive index layers H.sub.i,
H.sub.i+1 and the low refractive index layers L.sub.i, L.sub.i+1
are different, i.e. H.sub.i.noteq.H.sub.i+1 and
L.sub.i.noteq.L.sub.i+1 for all i.
[0041] In accordance with the above notation, the dielectric
multilayer system 4 shown in FIG. 1 can be represented by the
following formula: L.sub.0H.sub.1L.sub.1H.sub.2L.sub.2. . .
H.sub.iL.sub.iH.sub.i+1L.sub.i+1. . .
H.sub.N-1L.sub.N-1H.sub.NL.sub.N [2] wherein L.sub.0 lies in a
range of between 0.1 and 0.2 and H.sub.i, L.sub.i in a range
between 0.1 and 0.35 (in units of the wavelength .lamda.).
[0042] In the following, two examples for dielectric multilayer
system designs according to the formula [2] are described and
compared with the state of the art by numerical simulations of the
respective optical performance of these systems.
[0043] In the first example described in connection with FIG. 2,
the design target for the dielectric multilayer system is to
achieve an average reflectance R.sub.a of more than 96% at a
wavelength of 193 nm in a range of incident angles from 30.degree.
to 60.degree., while the average difference of amplitude
R.sub.s-R.sub.p and phase PR.sub.p-PR.sub.s of both polarization
components (s and p) should be as small as possible. The high and
low refractive index layers are defined by the (complex) indices of
refraction n.sub.H=1.778- i 0.0026, and n.sub.L=1.359-i 0.0004,
respectively, the imaginary part representing absorption occuring
in the optical media of these layers. The total number of layers of
the design must not exceed 15.
[0044] The optimized design with 15 layers and the above
constraints in accordance with the formula [1] of prior art
document U.S. Pat. No. 6,310,905 has been found to be
0.128/[0.253/0.257].sup.7, the overall thickness of this design
being 530 nm. The optical thicknesses of the low refractive index
layers are represented in FIG. 2 by the symbol .circle-solid., the
optical thicknesses of the high refractive index layers by the
symbol .tangle-solidup., both for a wavelength of .lamda.=193 nm
and an angle of incidence of .alpha.=49.1.degree.. With this
design, the merit function (representing the deviation of the
design result from the design target, i.e. the smaller the better)
is 79.02, the average difference of the amplitudes of the
polarization components in the reflected beam is
R.sub.s-R.sub.p=1.7%; their average difference of phase
PR.sub.s-PR.sub.p=2.1.degree., and the average reflectance
(including both polarization components) is R.sub.a=95.8%.
[0045] The performance of the design described above is compared to
an aperiodic design with 13 layers according to the formula [2],
having an overall thickness of 465 nm, wherein [0046] L.sub.0=0.153
[0047] H.sub.1=0.259 [0048] L.sub.1=0.243 [0049] H.sub.2=0.254
[0050] L.sub.2=0.244 [0051] H.sub.3=0.248 [0052] L.sub.3=0.250
[0053] H.sub.4=0.237 [0054] L.sub.4=0.265 [0055] H.sub.5=0.219
[0056] L.sub.5=0.310 [0057] H.sub.6=0.184 [0058] L.sub.6=0.276 are
the respective optical layer thicknesses, being represented in FIG.
2 by the symbol .diamond-solid. for the low refractive index layers
and by the symbol .box-solid. for the high refractive index
layers.
[0059] The aperiodic design as described above has a merit function
of 63.23 with an average difference of amplitude
R.sub.s-R.sub.p=1.7% and an average difference of phase
PR.sub.s-PR.sub.p=1.4.degree. of the two polarization components as
well as an average reflectance of R.sub.a=95.9%. The aperiodic
design is therefore clearly superior in its optical performance
compared to the design of the prior art. Moreover, such an improved
performance is achieved while the total thickness of the multilayer
system is reduced from 530 nm to 465 nm and the number of layers is
reduced from 15 to 13.
[0060] Such a result is possible as the optical thickness of the
more absorbing high refractive index layers H.sub.i of the
aperiodic design decreases with increasing distance from the
aluminium film 3, whereas the optical thickness of the less
absorbing low refractive index layers L.sub.i increases. As the
high refractive index material shows larger absorption than the low
refractive index material due to the larger imaginary part of the
refractive index Im(n.sub.H)=0.0026>Im(n.sub.L)=0.0004 and the
optical power of the incident radiation and consequently the
absorption increases with increasing distance from the aluminium
film 3, it is advantageous to reduce the optical thickness of the
high refractive index layers H.sub.i with increasing distance from
the aluminium film 3, respectively to increase the optical
thickness of the low refractive index layers L.sub.i, such that
most of the part of the multilayer system 4 being exposed to
radiation with a high optical power is covered by the
less-absorbing low refractive index layers L.sub.i. In contrast to
this, the state-of-the-art design has almost identical layer
thicknesses for all pairs of layers, see FIG. 2, such that
absorption effects cannot be taken into account.
[0061] With reference now to FIG. 3, a second comparison of an
aperiodic multilayer system design with a periodic multilayer
system is carried through. In this case, the design target is to
achieve an average reflectance R.sub.a of more than 98% (with a
wavelength of 193 nm and a range of incident angles from 0.degree.
to 45.degree.), the average difference of amplitude R.sub.s-R.sub.p
and phase PR.sub.p-PR.sub.s of both polarization components of the
reflected beam being as small as possible. The high and low
refractive index layers are defined by real indices of refraction
n.sub.H=1.745 and n.sub.L=1.359, so that no absorption in the
optical media of these layers is present. The total number of
layers is limited to 11.
[0062] The optimized design (best case) with 11 layers and the
above constraints in accordance with the formula [1] of prior art
document U.S. Pat. No. 6,310,905 is given by
0.125/[0.25/0.25].sup.5, the multilayer system having an overall
thickness of 362 nm. The optical thicknesses of the low refractive
index layers are represented in FIG. 3 for a wavelength of
.lamda.=193 nm and an angle of incidence of .alpha.=35.3.degree..
With this design, the merit function is 50.2, the average
difference of amplitude is R.sub.s-R.sub.p=0.5%, the average
difference of phase is PR.sub.s-PR.sub.p=0.2.degree., and the
average reflectance is R.sub.a=98.4%.
[0063] The aperiodic design optimized for this design target
consists of 11 layers with an overall thickness of 361 nm, wherein
[0064] L.sub.0=0.123 [0065] H.sub.1=0.273 [0066] L.sub.1=0.231
[0067] H.sub.2=0.271 [0068] L.sub.2=0.228 [0069] H.sub.3=0.269
[0070] L.sub.3=0.229 [0071] H.sub.4=0.264 [0072] L.sub.4=0.267
[0073] H.sub.5=0.169 [0074] L.sub.5=0.290 are the respective
optical layer thicknesses, being represented in FIG. 3.
[0075] The aperiodic design has a merit function of-45.3 with an
average difference of amplitudes of the reflected beam of
R.sub.s-R.sub.p=0.5%, an average difference of phase
PR.sub.s-PR.sub.p=0.1.degree., and an average reflectance of
R.sub.a=98.4%. The optical performance of the aperiodic design is
therefore still superior to the periodic design of the prior art
(although not as distinctively as in the first example). In both
cases, the number of layers is equal and the overall thickness of
the multilayer system is almost identical.
[0076] However, the main difference in performance of the two
designs of FIG. 3 is that the phase difference of the polarization
components is much smaller in the aperiodic design than in the
periodic design of the state of the art. Consequently, even in case
that no absorption in the layers of the dielectric layer system
occurs (which is only a theoretically assumption, as absorption is
always present in real systems), the aperiodic design can be used
to reduce the phase difference between polarization components.
[0077] The optical materials of the high refractive index layers
H.sub.i of FIG. 1 can be selected from a multitude of materials,
including lanthanum fluoride (LaF.sub.3), gadolinium fluoride
(GdF.sub.3), aluminium oxide (Al.sub.2O.sub.3), neodymium fluoride
(NdF.sub.3), dysprosium fluoride (DyF.sub.3), lead fluoride
(PbF.sub.2), hafnium oxide (HfO.sub.2), and zirconium oxide
(ZrO.sub.2). Likewise, the optical materials for the low refractive
index layers L.sub.i can be chosen from the group of materials
including aluminium fluoride (AlF.sub.3), magnesium fluoride
(MgF.sub.2), sodium fluoride (NaF), lithium fluoride (LiF),
thiolithe (Na.sub.5Al.sub.3F.sub.14), cryolite (Na.sub.3AlF.sub.6),
silicon oxide (SiO.sub.2), calcium fluoride (CaF.sub.2), barium
fluoride (BaF.sub.2), and strontium fluoride (SrF.sub.2). It is
also not necessary to use the same optical material for all of the
high, respectively low refractive index layers H.sub.i, L.sub.i of
the dielectric layer system 4.
[0078] Also, the reflective optical element 1 as shown in FIG. 1
represents only one of a plurality of possible realizations of the
inventive concept. It is e.g. also possible that the dielectric
layer system starts with a high reflective index layer at the
aluminium layer film, as is the case e.g. with the (periodic)
designs described in U.S. Pat. No. 5,850,309.
[0079] Reflective optical elements as described above may be
advantageously applied e.g. in projection exposure apparatuses for
microlithography. Such an apparatus generally comprises an
illuminating system and a projection optical system. FIG. 4 shows a
projection optical system 10 of such an apparatus which forms an
image of a pattern on a mask (reticle) being arranged in a reticle
plane R on a photosensitive substrate arranged in a wafer plane W.
The projection optical system 10 is a catadioptric system having a
concave reflector M2 which is arranged in a beam path 11 between a
first deflecting mirror M1 and a second deflecting mirror M3.
[0080] The projection optical system 10 comprises three image
forming systems G1 to G3, each of which with a plurality of
transmissive optical elements, i.e. lens elements, the arrangement
and optical function of which is beyond the scope of the present
invention and will therefore not be described herein; for a
detailed description of the image forming systems G1 to G3,
reference is made to WO 2004/019128. In the following, we will
focus on the general properties of the image forming systems G1 to
G3 and especially the reflective optical elements arranged
therein.
[0081] The first, dioptric image forming system G1 comprises only
transmissive elements and images the pattern on the reticle plane R
on a first intermediate image (not shown) which is located before
the first deflecting mirror M1. The second, catadioptric image
forming system G2 comprises the first deflecting mirror M1 and the
concave reflector mirror M2 and is used for generating a second
intermediate image on the basis of the first intermediate image.
The second intermediate image is imaged on the waver plane W by the
third, catadioptric image forming system G3 via the second
deflecting mirror M3. It is understood by the person skilled in the
art that for the purpose of imaging, each of the image forming
systems G1 to G3 comprises a pupil plane, the concave reflector M2
being located in the pupil plane of the second image forming system
G2.
[0082] The first and second deflecting mirrors M1, M3 as well as
the concave reflector M2 are designed as reflective optical
elements having a dielectric multilayer system as described above.
In this way, the advantageous properties of the layer designs
described herein, i.e. high reflectance and small separation of
polarization components, can be applied for the purpose of UV and
VUV microlithography. The skilled person will appreciate that the
inventive reflective elements described above may equally well be
applied in illuminating systems of projection exposure apparatuses
for microlithography and in other optical systems for the
ultraviolet wavelength range, respectively.
* * * * *