U.S. patent application number 15/657624 was filed with the patent office on 2017-11-09 for microlithographic projection exposure apparatus.
The applicant listed for this patent is Carl Zeiss SMT GmbH. Invention is credited to Heiko Feldmann, Toralf Gruner, Vladimir Kamenov, Daniel Kraehmer, Alexandra Pazidis, Stephan Six, Bruno Thome, Karl-Stefan Weissenrieder, Achim Zirkel.
Application Number | 20170322343 15/657624 |
Document ID | / |
Family ID | 37421186 |
Filed Date | 2017-11-09 |
United States Patent
Application |
20170322343 |
Kind Code |
A1 |
Kamenov; Vladimir ; et
al. |
November 9, 2017 |
MICROLITHOGRAPHIC PROJECTION EXPOSURE APPARATUS
Abstract
The disclosure relates to a microlithographic projection
exposure apparatus, such as are used for the production of
large-scale integrated electrical circuits and other
microstructured components. The disclosure relates in particular to
coatings of optical elements in order to increase or reduce the
reflectivity.
Inventors: |
Kamenov; Vladimir;
(Essingen, DE) ; Kraehmer; Daniel; (Essingen,
DE) ; Gruner; Toralf; (Aalen-Hofen, DE) ;
Weissenrieder; Karl-Stefan; (Elchingen, DE) ;
Feldmann; Heiko; (Aalen, DE) ; Zirkel; Achim;
(Muenchen, DE) ; Pazidis; Alexandra;
(Essingen-Lauterburg, DE) ; Thome; Bruno; (Aalen,
DE) ; Six; Stephan; (Aalen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carl Zeiss SMT GmbH |
Oberkochen |
|
DE |
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|
Family ID: |
37421186 |
Appl. No.: |
15/657624 |
Filed: |
July 24, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13112357 |
May 20, 2011 |
9733395 |
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15657624 |
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12031595 |
Feb 14, 2008 |
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13112357 |
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PCT/EP2006/008605 |
Sep 4, 2006 |
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12031595 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 1/11 20130101; G03F
7/70308 20130101; G03F 7/70958 20130101; G03F 7/70191 20130101 |
International
Class: |
G02B 1/11 20060101
G02B001/11; G03F 7/20 20060101 G03F007/20; G03F 7/20 20060101
G03F007/20; G03F 7/20 20060101 G03F007/20 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 2005 |
DE |
10 2005 041 938.0 |
Claims
1. An optical system, comprising: a first optical element, a second
optical element, a first antireflection coating supported by the
first optical element, a second antireflection coating supported
either by the first optical element or by the second optical
element; wherein: within a first incidence angle range, the first
antireflection coating has a polarisation-dependent reflectivity
which is greater for s-polarised light than for p-polarised light;
within a second incidence angle range, the second antireflection
coating has a polarisation-dependent reflectivity which is less for
s-polarised light than for p-polarised light; the first and second
antireflection coatings are arranged in a beam path of projection
light so that their polarisation-dependent differences in
reflectivity at least partially compensate each other; and the
optical system is a microlithographic optical system.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of, and claims priority
to U.S. application Ser. No. 13/112,357, filed May 20, 2011, which
is a continuation of U.S. application Ser. No. 12/031,595, filed
Feb. 14, 2008, which claims priority under 35 U.S.C. .sctn.120 to,
International Application PCT/EP2006/008605, filed Sep. 4, 2006,
which claims benefit of German patent application serial number 10
2005 041 938.0, filed Sep. 3, 2005. The contents of these
applications are hereby incorporated by reference in its
entirety.
FIELD
[0002] The disclosure relates to a microlithographic projection
exposure apparatus, such as are used for the production of
large-scale integrated electrical circuits and other
microstructured components. The disclosure relates in particular to
coatings of optical elements in order to increase or reduce the
reflectivity.
BACKGROUND
[0003] Integrated electrical circuits and other microstructured
components are conventionally produced by applying a plurality of
structured layers onto a suitable substrate which, for example, may
be a silicon wafer. In order to structure the layers, they are
first covered with a photoresist which is sensitive to light of a
particular wavelength range, for example light in the deep
ultraviolet (DUV) spectral range. The wafer coated in this way is
subsequently exposed in a projection exposure apparatus. A mask,
which contains a pattern of structures, is thus illuminated by an
illumination system and imaged onto the photoresist with the aid of
a projection objective. Since the imaging scale is generally less
than one, such projection objectives are often also referred to as
reducing objectives.
[0004] After the photoresist has been developed, the wafer is
subjected to an etching process so that the layer becomes
structured according to the pattern on the mask. The photoresist
still remaining is then removed from the other parts of the layer.
This process is repeated until all the layers have been applied on
the wafer.
[0005] The mirrors used in projection exposure apparatus generally
include a reflection coating, which is made up of a plurality of
individual layers and whose reflection coefficient is often more
than 90%. Lenses and other refractive optical elements, on the
other hand, are provided with antireflection coatings in order to
reduce light losses and imaging errors due to undesired double
reflections at the interfaces of the refractive optical
elements.
SUMMARY
[0006] In some embodiments, the disclosure provides a projection
exposure apparatus, having optical elements whose (anti-)
reflection coatings on the one hand are economical and on the other
hand do not significantly compromise the imaging properties of the
projection objective.
[0007] In certain embodiments, the disclosure provides a
microlithographic projection exposure apparatus having an optical
element, on which there is an antireflection coating in order to
reduce the reflectivity. This is configured so that the
transmission coefficients of the antireflection coating for
mutually orthogonal polarization states differ from one another by
no more than 10% (e.g., by no more than 3%, by no more than 1%)
over an incidence angle in the range of from 0.degree. to
70.degree.. The projection exposure apparatus furthermore includes
a device for homogenizing an intensity distribution, which can be
arranged in or in the vicinity of a field or pupil plane.
[0008] The same applies for reflection coefficients of reflection
coatings.
[0009] The device for homogenizing the intensity distribution can
ensure that sizeable angle dependencies of the transmission
coefficient or the reflection coefficient, such as may occur in the
polarization optical optimisation, do not have an intolerable
effect on the imaging properties. Homogenizing the intensity
distribution in this context is intended to mean that undesired
variations of the intensity distribution in the image plane are
suppressed. The desired intensity distribution in the image plane
is an equidistribution such that, in the absence of a mask, all
points will be exposed to the same intensity.
[0010] Optionally, the coatings could be configured so that the
small difference in the transmission or reflection coefficient for
orthogonal polarization states is achieved not by a single coating,
but by the overall effect of a plurality or even all of the
coatings which are contained in the projection exposure apparatus.
The individual optimisation is in this way replaced by an overall
optimisation. Here again, the device for homogenizing the intensity
distribution can ensure that sizeable angle dependencies of the
transmission coefficient or the reflection coefficient do not have
an intolerable effect on the imaging properties.
[0011] The device for homogenizing the intensity distribution may
be a grey filter, such as is known in the prior art. It is
particularly favourable for one or more grey filters to be
specially tuned to the coating (or all the coatings as a whole).
The grey filter, which may for example be designed as a simple
transmission filter, may for example be designed so that only
variations of the intensity distribution due to the coating are
compensated for. Variations of the intensity distribution which are
attributable to different causes may, for example, be reduced by
other, filters (e.g., adjustable filters) or one or more different
devices.
[0012] Sizeable phase errors, which are due to the coatings, may be
corrected by a phase error correction device, for example
manipulators which are known per se or local non-axisymmetric
surface deformations.
[0013] The disclosure furthermore relates to antireflection
coatings having particularly advantageous polarization optical
properties. Thus, antireflection coatings are provided in which
both the reflection coefficients and the phase depend only slightly
on the polarization state. Other antireflection coatings have the
property that the reflection coefficient is greater for p-polarized
light than for s-polarized light over a particular incidence angle
range, or that p-polarized light passes through the antireflection
coating with a retardation relative to the s-polarized light within
a particular incidence angle range. This makes it possible to
combine a plurality of reflection or antireflection coatings so
that polarization-neutral behaviour is obtained overall. In some
embodiments, the coatings are distinguished in that they include
only layers with a packing density of more than 85%, and are
therefore very durable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Other features and advantages will be found in the following
description of an exemplary embodiment with reference to the
drawings, in which:
[0015] FIG. 1 shows a meridian section through a projection
exposure apparatus;
[0016] FIG. 2 shows a sectional representation (not to scale) of a
lens with an antireflection coating according to an exemplary
embodiment of the disclosure;
[0017] FIG. 3 shows a graph, in which the average transmissivity is
plotted as a function of the incidence angle for the exemplary
embodiment of an antireflection coating as shown in FIG. 2;
[0018] FIG. 4 shows a graph, in which the difference between the
transmission coefficients for s- and p-polarized light is plotted
as a function of the incidence angle for the exemplary embodiment
shown in FIG. 2;
[0019] FIG. 5 shows a graph, in which the phase difference between
s- and p-polarized light is plotted as a function of the incidence
angle for the exemplary embodiment shown in FIG. 2;
[0020] FIGS. 6 to 9 show graphs, in which the reflection
coefficients for s-polarized, p-polarized and unpolarized light are
respectively plotted as a function of the incidence angle for
antireflection coatings according to further exemplary
embodiments;
[0021] FIGS. 10 and 11 show graphs, in which the reflection
coefficients for s-polarized, p-polarized and unpolarized light, as
well as the phase difference, are respectively plotted as a
function of the incidence angle for antireflection coatings
according to two other exemplary embodiments.
DETAILED DESCRIPTION
[0022] FIG. 1 shows a meridian section through a microlithographic
projection exposure apparatus, denoted overall by 10, in a highly
schematised representation which is not to scale. The projection
exposure apparatus 10 includes an illumination system 12 with a
light source 14 for generating a projection light beam 13. The
light source 14, which may for example be an excimer laser,
generates short-wave projection light. In the present exemplary
embodiment, the wavelength of the projection light is 193 nm. It is
likewise possible to use other wavelengths, for example 157 nm or
248 nm.
[0023] The illumination system 12 furthermore contains illumination
optics, indicated by 16, with a depolarizer 17 and a field aperture
18. The illumination optics 16 reshape the projection light beam
generated by the light source 14 in the desired way, and make it
possible to set up different illumination angle distributions. To
this end, the illumination optics 16 may for example contain
exchangeable diffractive optical elements and/or microlens arrays.
Since such illumination optics 16 are known in the prior art, see
for example U.S. Pat. No. 6,285,443 A, the explanation of further
details in this regard may be omitted.
[0024] An objective 19 of the illumination system 12 images the
field aperture 18 sharply onto a subsequent object plane of a
projection objective 20.
[0025] The projection objective 20 contains a multiplicity of
lenses and other optical elements, only a few of which (denoted by
L1 to L6) are indicated by way of example in FIG. 1 for the sake of
clarity. The projection objective 20 may also contain other optical
elements, for example imaging mirrors or mirrors used for folding
the beam path, or filter elements. In the case of extremely short
wavelengths, for example 13 nm, the projection objective 20
contains only mirrors as imaging elements, since sufficiently
transparent lens materials are not available for these short
wavelengths. The same applies for the illumination system 12.
[0026] The projection objective 20 is used to project a reduced
image of a mask 24, which can be arranged in an object plane 22 of
the projection objective 20 and is illuminated by the projection
light beam 13, onto a photosensitive layer 26 which, for example,
may be a photoresist. The layer 26 is located in an image plane 28
of the projection objective 20 and is applied onto a support 29,
for example a silicon wafer.
[0027] The lenses contained in the illumination system 12 and in
the projection objective 20 are provided with an antireflection
coating. The purpose of the antireflection coating is to reduce the
proportion of light which is reflected at the interfaces of the
lenses and is therefore lost for the projection, or leads to double
reflections. The coatings generally contain a multiplicity of a
thin individual layers, the refractive indices and thicknesses of
which are selected so that the desired properties are achieved for
the wavelength of the projection light 13.
[0028] In the case of antireflection coatings, these properties are
primarily a very high transmissivity of more than 98%. Such a high
transmissivity should be achieved for a large incidence angle
range. Especially in the case of very high-aperture projection
objectives 20, incidence angles of up to 70.degree. may occur, and
even more in particular cases. If the transmissivity depends too
strongly on the incidence angle, then this will lead to
field-dependent structure width variations with coatings close to
the pupil, and to angle-dependent structure width variations with
near-field coatings.
[0029] It is moreover expected of the antireflection coatings
applied on lenses that they have these optical properties
irrespective of the polarization state of the incident projection
light 13. If the transmissivity varies too greatly for orthogonal
polarization states in an antireflection coating, then this
polarization dependency may lead to undesired imaging errors. This
is related to the fact that, despite the use of a depolarizer 17 in
the illumination system 12, the projection light 13 does not remain
fully depolarized when it passes through the projection objective
20. Reasons for this may, for example, be intrinsically or
stress-birefringent lens materials, polarizing mask structures as
well as the polarization dependencies being discussed here in the
case of antireflection and reflection coatings.
[0030] If an antireflection coating is arranged in the vicinity of
a field plane, then the polarization dependency of its
transmissivity leads to intensities varying over the image field
when the projection light has a preferential polarization direction
that varies over the field. Such intensity variations in a field
plane become manifested as undesired field-dependent structure
width variations on the component. On the other hand, if an
antireflection coating with a polarization-dependent transmissivity
is arranged close to the pupil, then an already existing angle
dependency of the polarization state may likewise lead to undesired
structure width variations.
[0031] For this reason, when developing an antireflection coating,
attempts are made to keep the difference .DELTA.T between the
transmission coefficients for orthogonal polarization states less
than 10%, such as less than 3%.
[0032] (Anti-) reflection coatings of lenses and mirrors may
furthermore cause the phase of the light passing through the
coatings to vary as a function of the polarization state. This
makes the coating optically birefringent, which has an unfavourable
effect on the imaging quality in the image plane. For this reason,
the permissible phase difference .DELTA..phi. between orthogonal
polarization states should be less than 1/10 of the wavelength
.lamda. of the projection light 13.
[0033] A high average transmissivity on the one hand, as well as a
low polarization dependency of the transmissivity and of the phase
on the other hand, cannot however be achieved over a sizeable
incidence angle range, or can be achieved at most with extremely
great outlay.
[0034] According to the disclosure, the coatings in the projection
exposure apparatus 10 therefore configured so that the polarization
dependency of the transmission coefficient and of the phase are
kept low over a large incidence angle range. The average
transmissivity and the average phases may however vary perceptibly
over the incidence angle range. The concomitant perturbations of
the imaging are corrected in a comparatively straightforward way,
for example with the aid of grey filters or--in the case of phase
errors--local non-axisymmetric surface deformations.
[0035] Substantial polarization independency, specifically in the
case of antireflection coatings, means that the transmission
coefficients for mutually orthogonal polarization states differ
from one another by no more than 10% (e.g., by no more than 3%, by
no more than 1%) over an incidence angle range of 70.degree.. The
same applies for the reflection coefficients in the case of
reflection coatings.
[0036] Layer systems configured in such a way can be developed and
produced with relatively little outlay. The way to do this in
detail may be found in standard textbooks, for example T. W.
Baumeister "Optical Coating Technology".
[0037] FIG. 2 shows a lateral section of a detail of an exemplary
embodiment of an antireflection coating 32, in which the
transmission coefficients for mutually orthogonal polarization
states differ from one another by no more than 1%. The
antireflection coating 32 consists of 6 thin individual layers L1
to L6, the materials and optical thicknesses of which are specified
in Table 1. The antireflection coating 32 is applied on a concave
surface 34 of a lens 36, which consists for example of quartz
glass, and it is configured for a wavelength of .lamda.=193 nm. The
quantity QWOT (quarter wave optical thickness) refers to the
optical thickness, i.e. the product of refractive index and the
geometrical thickness, in units of a quarter wavelength.
TABLE-US-00001 TABLE 1 Layer Specification Exemplary Embodiment 1
Layer 1 2 3 4 5 6 Material LaF.sub.3 MgF.sub.2 LaF.sub.3 MgF.sub.2
LaF.sub.3 MgF.sub.2 QWOT 1.37 0.44 1.41 0.75 0.60 0.87
[0038] Likewise suitable in principle, albeit less preferred owing
to the low durability, is the coating described as Exemplary
Embodiment 4 in JP 2004-302113 A, which is constructed from three
layers. EP 0 994 368 A2 describes a more durable coating which has
five layers but in which the transmission coefficients for
orthogonal polarization states differ from one another by about 5%
in the incidence angle range of from 0.degree. to 70.degree..
[0039] It will be assumed below that the light ray 30 contains both
a p-polarized component 38 indicated by double arrows and an
s-polarized component indicated by black circles 40. The majority
of the light striking the antireflection coating 32 will be
transmitted, with the transmission coefficients T.sub.s and T.sub.p
respectively for the s-polarized component 40 and for the
p-polarized component 38 differing slightly. In FIG. 2, this slight
difference is indicated by the arrow 42s for the transmitted
s-polarized component 40 being somewhat longer than the arrow 42p
for the transmitted p-polarized component 38.
[0040] In general, the reflectivity of the antireflection coating
32 also differs according to the polarization state of the incident
light, which is indicated in an exaggeratedly represented way at 44
in FIG. 2.
[0041] The average transmissivity T of the antireflection coating
32 is given by the following Equation (1):
T=(|T.sub.s|+|T.sub.p|)/2. (1)
[0042] The polarization dependency of the transmissivity is best
described by the difference between the transmission coefficients
T.sub.s and T.sub.p according to Equation (2)
.DELTA.T=|T.sub.s|-|T.sub.p|. (2)
[0043] For the average phase .phi. and the phase difference
.DELTA..phi., Equations (3) and (4) respectively apply:
.phi.=(arg(T.sub.s)+arg(T.sub.p))/2 (3)
.DELTA..phi.=arg(T.sub.s)-arg(T.sub.p). (4)
[0044] FIGS. 3, 4 and 5 show graphs in which the average
transmissivity T, the difference .DELTA.T between the transmission
coefficients according to Eq. (2) and the phase difference
according to Eq. (4) are respectively plotted as a function of the
incidence angle .alpha. for the antireflection coating 32. It can
be seen that .DELTA.T<1% and .DELTA..phi.<0.1.lamda. apply
over an angle range of 70.degree.. The average transmissivity T is
however not consistently higher than 98% over this incidence angle
range, rather it falls off to values below 92% for large incidence
angles. This may therefore lead to the aforementioned field- and/or
angle-dependent intensity variations.
[0045] In order to avoid intensity variations in the image plane
28, grey filters may be used which are likewise to be positioned
near the field. As an alternative to this, it is possible to
position filter elements with angle-dependent transmissivities near
the pupil. Such an angle-dependent grey filter is indicated by 50
in FIG. 1. Further designs of grey filters, which are suitable in
this context, may be found in US 2005/0018312 A1.
[0046] In a scanning projection exposure apparatus 10, it is also
feasible to use a field aperture, which includes a multiplicity of
individually displaceable aperture elements, in the illumination
system 12. Such field apertures which are known per se, as
described for example in EP 0 952 491 A2, make it possible to vary
the radiation dose in the image plane 28 as a function of the
longitudinal position of the slit-shaped light field.
[0047] If the antireflection coating 32 lies in the vicinity of a
pupil plane, however, then this will generate pupil apodisation.
Such pupil apodisations may be corrected by suitably configured
antireflection layers in the vicinity of a pupil plane. Tilting of
the pupil apodisation, which can be described by the Zernike
coefficients Z2/Z3, may be corrected by a mirror layer.
[0048] Stronger double reflections, which may occur owing to the
average transmissivity T being lower at particular angles, may be
absorbed by anti-scattering apertures.
[0049] Since the average phase .phi. is likewise not given priority
in the optimisation of the antireflection coating, phase errors due
to the antireflection coating 32 may lead to imaging errors.
[0050] Such imaging errors may be corrected, at least within
certain limits, by manipulators which are known per se.
Particularly good correction is achieved when interfaces of optical
elements, or plates separately provided here, are deformed locally
and non-axisymmetrically. The deformations, which may be generated
by adding or removing material, are in this case of the order of a
few nanometres, such as less than 50 nanometres.
[0051] Instead of respectively optimising the individual
antireflection coatings with a view to minimal polarization
dependency, it is also possible to carry out an overall
optimisation of a plurality or all of the antireflection coatings
contained in the projection objective 20, and optionally throughout
the projection exposure apparatus 10. The conditions mentioned
above may then be described as
.DELTA.T.sub.total<10%,
[0052] (e.g., <3%, <1%) and
.DELTA..phi..sub.total<.lamda./10.
[0053] Naturally, the above considerations also apply for
reflection coatings such as are used for curved imaging mirrors or
plane deviating mirrors in the projection exposure apparatus
10.
[0054] Several exemplary embodiments of antireflection coatings
will be described below, some of which likewise have a particularly
small difference between the transmission coefficients for
orthogonal polarization states. In other exemplary embodiments,
although this difference is greater, particularly high average
transmission coefficients and/or particularly small phase
splittings are nevertheless achieved over a sizeable incidence
angle range. It should furthermore be pointed out that the
transmission performance will now be described no longer by
specifying the transmission coefficients T, but by specifying the
reflection coefficients R. If the coatings have a negligible
absorption, then T=1-R applies. Small reflection coefficients
therefore correspond to large transmission coefficients, and vice
versa.
Exemplary Embodiment 2
[0055] Table 2 gives the layer specification for an exemplary
embodiment of an antireflection coating, which includes four layers
in total. FIG. 6 shows a graph in which the reflection coefficients
R.sub.s, R.sub.p and R.sub.a for s-polarized, p-polarized and
unpolarized light are respectively plotted as a function of the
incidence angle for this antireflection coating.
TABLE-US-00002 TABLE 2 Layer Specification Exemplary Embodiment 2
Layer 1 2 3 4 Material LaF.sub.3 MgF.sub.2 LaF.sub.3 MgF.sub.2 QWOT
2 1 1.25 1 Range 1.6-2.2 0.8-1.5 1.2-1.5 0.9-1.1
[0056] As in Exemplary Embodiment 1 described above, the layers are
counted starting from the support material which, for example, may
be a lens or a plane-parallel plate. CaF.sub.2, which has a
refractive index of about 1.56 at a wavelength of 193 nm, will be
assumed as the material of the support (substrate) in this
exemplary embodiment and the ones described below. It is however
also possible to use other support materials, for example synthetic
quartz glass (SiO.sub.2) or barium fluoride (BaF.sub.2); the
optical properties of the antireflection coating will only be
modified relatively slightly by this.
[0057] Lanthanum fluoride (LaF.sub.3), which has a refractive index
of about 1.69 at a wavelength of 193 nm has been assumed for the
more highly refractive layers. Magnesium fluoride (MgF.sub.2),
which has a refractive index of about 1.43 at the same wavelength,
has been assumed for the less refractive layers. The known
production methods, for example PVD or CVD methods, may be employed
in order to produce the layers.
[0058] Of course, the materials mentioned for the more highly
refractive layers and the less refractive layers may also replaced
by other materials respectively with similar refractive indices.
Also suitable as more highly refractive materials, besides
LaF.sub.3, are in particular NdF.sub.3, Al.sub.2O.sub.3 and
ErF.sub.3. Besides MgF.sub.2 for the less refractive materials,
AlF.sub.3, chiolite or kryolite may for example also be envisaged.
Since these materials have somewhat different refractive indices
from the materials mentioned in Table 2, differences may arise for
the optical thicknesses specified there in units of QWOT (quarter
wave optical thickness). These are mentioned in the last row of
Table 2 in the form of range specifications. Even when employing
LaF.sub.3 and MgF.sub.2, it may be expedient to use optical
thicknesses within the value ranges in the table, for example in
order to carry out fine tuning.
[0059] A common feature of the more highly and less refractive
materials is that refractive indices in the range of between about
1.60 and 1.92, or in the range of between about 1.37 and 1.44, can
respectively be achieved by them without the packing density
thereby decreasing below a value of 85%. These layers are therefore
more durable and do not substantially change their optical
properties even after prolonged operating times and under different
environmental effects.
[0060] The graph shown in FIG. 6 reveals that with this
antireflection coating, consisting of only four layers, the
reflection coefficients R.sub.s and R.sub.p for s-polarized and
p-polarized light differ only very slightly from one another over
an incidence angle range of between 0.degree. and 60.degree.,
specifically by no more than 1%. For an incidence angle range of
between 0.degree. and 50.degree., not only the difference but also
the absolute value of the reflection coefficients R.sub.s and
R.sub.p are less than 1%.
[0061] A particular feature of this antireflection coating is that
the reflection coefficient R.sub.s for s-polarized light is less
than the reflection coefficient R.sub.p for p-polarized light for
incidence angles of between about 35.degree. and 55.degree.. Such
behaviour, which was described for the first time--albeit for an
incidence angle range above 55.degree.--in JP 2004-302113 is
unusual because p-polarized light is in principle transmitted
better than s-polarized light according to the Fresnel
equations.
[0062] This reversal of the reflection behaviour which is
conventional per se, over a particular angle range, can
advantageously be used to compensate for effects due to the
conventional polarization-dependent reflection behaviour at other
coatings. Even if the difference between the reflection
coefficients for the s-polarized and p-polarized light can be kept
very small, as shown by the first exemplary embodiment and also
some of the subsequent exemplary embodiments, this nevertheless
often involves more complex layer systems with six or more
individual layers, the production of which is correspondingly
elaborate. If however an antireflection coating having the
properties shown in FIG. 6 is combined with another simply
constructed antireflection coating, which has a higher reflectivity
for s-polarized light than for p-polarized light over an incidence
angle range, then polarization-neutral behaviour can be achieved
overall.
[0063] To this end, it is not categorically necessary that the
antireflection coatings, whose polarization dependencies are
intended to compensate for one another, should exhibit the
described behaviour in the same incidence angle range. Light rays
which strike one optical surface at large incidence angles may
strike another optical surface at small incidence angles, and vice
versa. If two identically constructed antireflection coatings,
which have ranges with R.sub.s>R.sub.p and R.sub.s<R.sub.p,
are applied onto optical surfaces selected in such a way, then
there polarization dependencies can neutralise one another. In
general, however, the situation is simplest when the compensating
antireflection coatings are applied on the entry and exit surfaces
of an optical element, for example a lens. This is because when
optical systems are being configured, attempts are often made to
make the incidence angles similar on the entry and exit surfaces of
the optical lenses. If however there are many other optical
elements between the antireflection coatings, then the incidence
angle distribution may be modified in a relatively complicated way
by the optical elements lying between them.
[0064] It is to be understood that the layer specification given in
Table 2 need not be identical over the entire surface of the
optical element. Since different regions on an optical element are
often exposed to different distributions of incidence angles, it
may be expedient for different antireflection coatings, which are
optimally adapted to the angle spectrum respectively encountered,
to be applied onto the different regions.
Exemplary Embodiment 3
[0065] Table 3 gives the layer specification for an exemplary
embodiment of an antireflection coating, which includes eight
layers in total. FIG. 7 shows a graph in which the reflection
coefficients R.sub.s, R.sub.p and R.sub.a for s-polarized,
p-polarized and unpolarized light are respectively plotted as a
function of the incidence angle for this antireflection
coating.
TABLE-US-00003 TABLE 3 Layer Specification Exemplary Embodiment 3
Layer 1 2 3 4 5 6 7 8 Material LaF.sub.3 MgF.sub.2 LaF.sub.3
MgF.sub.2 LaF.sub.3 MgF.sub.2 LaF.sub.3 MgF.sub.2 QWOT 1.8 1.9 1.1
1.8 1.55 1 1.25 1 Range 1.5- 1.7- 0.8- 1.6- 1.3- 0.8- 1.2- 0.9- 2.4
2.1 1.5 2.1 1.8 1.5 1.5 1.1
[0066] It can be seen in the graph of FIG. 7 that the reflection
behaviour differs from the per se conventional behaviour at
incidence angles of more than about 40.degree. here, because
s-polarized light is reflected much less than p-polarized light
there. The negative difference .DELTA.R=R.sub.s-R.sub.p of the
reflection coefficients R.sub.s and R.sub.p increases substantially
more strongly at the incidence angles of about 50.degree. than is
the case with the antireflection coating shown with the aid of FIG.
12 of JP 2004-302113. The antireflection coating with the layer
specification given in Table 3 can therefore be used even better to
compensate for polarization dependencies of other layers, as was
explained above in relation to Exemplary Embodiment 2.
[0067] A substantial advantage over the antireflection coating
described in JP 2004-302113 is, above all, that only layers which
have a packing density of more than 85% are used in the
antireflection coating described here. In the exemplary embodiment
described in JP 2004-302113, however, the packing density of the
lowermost layer is merely 49% in order to be able to achieve the
low refractive index of 1.21. A low packing density of this type is
disadvantageous because such an incompact layer is susceptible to
environmental effects and therefore modifies its optical properties
relatively quickly as a function of time.
Exemplary Embodiment 4
[0068] Table 4 gives the layer specification for another exemplary
embodiment of an antireflection coating, which includes seven
layers in total. FIG. 8 shows a graph in which the reflection
coefficients for s-polarized, p-polarized and unpolarized light are
respectively plotted as a function of the incidence angle for this
antireflection coating.
TABLE-US-00004 TABLE 4 Layer Specification Exemplary Embodiment 4
Layer 1 2 3 4 5 6 7 Material MgF.sub.2 LaF.sub.3 MgF.sub.2
LaF.sub.3 MgF.sub.2 LaF.sub.3 MgF.sub.2 QWOT 1.2 0.5 0.3 1.2 1.15
1.15 1.1 Range 0.96- 0.4- 0.2- 0.59- 1.06- 1.06- 1.0- 1.44 0.91 0.4
1.1 1.24 1.2 1.2
[0069] The reflection coefficients for s-polarized and p-polarized
light differ only very slightly between incidence angles of
0.degree. and 60.degree., specifically by no more than 0.1%. At 4%,
the absolute values R.sub.s and R.sub.p are likewise very similar
in an angle range of between about 20.degree. and 50.degree.. This
antireflection coating is therefore suitable in particular for such
optical elements which light strikes only obliquely with incidence
angles in the range, or at least predominantly obliquely.
[0070] The antireflection coating with the layer specification
given in Table 4 has also been optimised with a view to achieving a
minimal phase difference .DELTA..phi. between s-polarized and
p-polarized light after passing through the antireflection coating.
In order to obtain a small phase difference .DELTA..phi., it is
favourable for the coating to consist of as few layers as possible,
but at least for the thickness of the layers provided to be as
small as possible. Comparison of the layer specification given in
Table 4 with the layer specification given in Table 3, for
Exemplary Embodiment 3, shows that this rule can be satisfied
without thereby entailing intolerably large differences
.DELTA.R=R.sub.s-R.sub.p. In Exemplary Embodiment 4, a phase
difference is achieved which is less than 0.5.degree. for incidence
angles of between 0.degree. and 50.degree., and which does not
reach about 6.degree. until an incidence angle of 70.degree..
[0071] If all the layers are made about 7% thinner based on the
layer specification given in Table 4, then the range with
particularly small reflection coefficients will be shifted to
smaller incidence angles as revealed by the graph of FIG. 9. This
modification makes the antireflection coating particularly suitable
for incidence angles of between 0.degree. and about 40.degree.. In
this incidence angle range, the reflection coefficients R.sub.s and
R.sub.p for s-polarized and p-polarized light are both below about
0.2%; the differences .DELTA.R between the reflection coefficients
are an order of magnitude less. The phase difference .DELTA..phi.
is likewise shifted to smaller incidence angles here. The phase
difference .DELTA..phi. at incidence angles of 70.degree. is
therefore somewhat higher, specifically 10.degree..
Exemplary Embodiment 5
[0072] In Exemplary Embodiments 2 and 3, the phase splitting may
also be reduced if it is feasible for the thicker layers, in
particular, to be made thinner.
[0073] Table 5 shows the layer specification for an antireflection
coating which is based on the layer specification shown in Table 3
for Exemplary Embodiment 3. The thicker layers 2, 4 and 5 provided
there are now much thinner.
TABLE-US-00005 TABLE 5 Layer Specification Exemplary Embodiment 5
Layer 1 2 3 4 5 6 7 8 Material LaF.sub.3 MgF.sub.2 LaF.sub.3
MgF.sub.2 LaF.sub.3 MgF.sub.2 LaF.sub.3 MgF.sub.2 QWOT 2.4 0.2 1.1
0.4 0.3 1 1.25 1 Range 1.8- 0.05- 0.3- 0.05- 0.05- 0.8- 1.2- 0.9-
2.6 0.4 1.4 0.4 0.4 1.5 1.5 1.1
[0074] FIG. 10 shows a graph in which the reflection coefficients
for s-polarized, p-polarized and unpolarized light are respectively
plotted as a function of the incidence angle for this
antireflection coating. The phase difference .DELTA..phi. for
Exemplary Embodiment 5 is plotted with a line of dashes, and for
Exemplary Embodiment 3 with thin dots and dashes for comparison. It
may be seen clearly that much smaller phase differences
.DELTA..phi. are entailed for incidence angles of more than about
30.degree. owing to the reduction of the layer thicknesses. On the
other hand, the reflection behaviour has not been compromised
significantly by the modification carried out, as shown by a
comparison of FIGS. 10 and 7.
Exemplary Embodiment 6
[0075] Table 6 gives the layer specification for another exemplary
embodiment of an antireflection coating, which includes eight
layers in total. FIG. 11 shows a graph corresponding to FIG. 10, in
which the reflection coefficients for p-polarized, s-polarized and
unpolarized light as well as the phase difference .DELTA..phi. are
plotted as a function of the incidence angle.
TABLE-US-00006 TABLE 6 Layer Specification Exemplary Embodiment 6
Layer 1 2 3 4 5 6 7 8 Material LaF.sub.3 chiolite LaF.sub.3
chiolite LaF.sub.3 chiolite LaF.sub.3 chiolite QWOT 0.82 0.49 0.15
1.19 2.15 1.77 0.64 1.410 Range 0.4- 0.3- 0.3- 1.15- 1.9- 1.7- 0.5-
1.15- 0.85 0.55 0.55 1.7 2.15 1.8 0.95 1.4
[0076] The antireflection coating according to this exemplary
embodiment is distinguished by a particularly small phase
difference, the absolute value of which does not exceed 5.degree.
throughout the incidence angle range of between 0.degree. and
70.degree.. With this antireflection, it is furthermore noteworthy
coating that the phase difference .DELTA..phi. is negative in an
angle range of between 0.degree. and about 65.degree.. This means
that p-polarized light passes through the antireflection coating
with a retardation relative to the s-polarized light within this
incidence angle range. This unusual behaviour may be used to
compensate for a positive phase difference, in a similar way as was
explained above in connection with Exemplary Embodiment 2 for the
reflection coefficients R.sub.s, R.sub.p. Here again, it is true
that the combination of at least one antireflection coating having
a positive phase difference with another antireflection coating
having negative phase splitting can achieve the effect that
s-polarized and p-polarized light no longer have a significant
phase difference after passing through the two antireflection
coatings.
[0077] In this case, for example, it is also possible that the
contributions of a multiplicity of antireflection coatings to a
sizeable positive phase difference may be compensated for by a
single antireflection coating or a few antireflection coatings with
a negative phase difference. Here again, the angle ranges of the
antireflection coatings with a positive phase difference and those
with a negative phase difference need not necessarily coincide.
[0078] Methods of computer-assisted optimisation, for example the
variation method, may be employed in order to achieve substantially
polarization-neutral behaviour in respect of reflectivity and phase
by combining different antireflection coatings.
[0079] In general, it will be simplest to optimise the
antireflection coatings in a first step such that a minimal
difference in the reflectivity for orthogonal polarization states
is obtained overall. In a second step, phase differences still
existing on one or a few, for example 4, antireflection coatings
may then be reduced. The reverse procedure may of course also be
adopted, by starting with reduction of the phase differences and
subsequently optimising the reflectivity. Simultaneous optimisation
in respect of both the reflectivity and the phase difference is
also possible in principle.
* * * * *