U.S. patent application number 11/257967 was filed with the patent office on 2006-04-13 for reflective optical element, optical system and euv lithography device.
Invention is credited to Marco Wedowski.
Application Number | 20060076516 11/257967 |
Document ID | / |
Family ID | 33393914 |
Filed Date | 2006-04-13 |
United States Patent
Application |
20060076516 |
Kind Code |
A1 |
Wedowski; Marco |
April 13, 2006 |
Reflective optical element, optical system and EUV lithography
device
Abstract
In order to obtain optimal reflectivity on optical elements for
the EUV and the soft X-ray range, multilayers constructed of a
number of layers are used. Contamination or degradation of the
surface leads to imaging defects and transmission losses. In the
prior art, it has been attempted to counter a negative change in
the surface by providing a cover layer system on the surface of the
reflective optical element that should protect the surface. The
invention renders the influence of the surface degradation
manageable by a targeted selection of the distribution of thickness
of the cover layer system, whereby at least one layer of the cover
layer system has a gradient that is not equal to zero.
Inventors: |
Wedowski; Marco; (Aalen,
DE) |
Correspondence
Address: |
HUDAK, SHUNK & FARINE, CO., L.P.A.
2020 FRONT STREET
SUITE 307
CUYAHOGA FALLS
OH
44221
US
|
Family ID: |
33393914 |
Appl. No.: |
11/257967 |
Filed: |
October 25, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP04/04368 |
Apr 26, 2004 |
|
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|
11257967 |
Oct 25, 2005 |
|
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Current U.S.
Class: |
250/503.1 |
Current CPC
Class: |
G03F 7/70958 20130101;
G21K 1/062 20130101; G02B 1/14 20150115; G02B 1/105 20130101; G02B
5/0891 20130101; B82Y 10/00 20130101; G02B 5/0883 20130101 |
Class at
Publication: |
250/503.1 |
International
Class: |
G01J 1/00 20060101
G01J001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2003 |
DE |
103 19 005.8 |
Claims
1. A reflective optical element for the extreme ultraviolet and/or
soft x-ray wavelength range comprising: a cover layer system having
at least one layer to protect the surface, whose spatial structure
can be described in a Cartesian system of coordinates (x, y, z)
with z=z(x, y), wherein a thickness distribution d=d(x, y) of at
least one layer of the cover layer system as a function of its
spatial coordinates has a gradient not equal to zero.
2. The reflective optical element according to claim 1, wherein a
maximum change in thickness of the cover layer system is at most 3
nm.
3. The reflective optical element according to claim 1, wherein an
overall thickness of the cover layer system is at most 8 nm.
4. A reflective optical element for the extreme ultraviolet and/or
soft x-ray range, comprising: a cover layer system having at least
one layer to protect the surface, whose spatial structure can be
described in a Cartesian system of coordinates (x, y, z) with
z=z(x, y), wherein the cover layer system has at least one outer
layer and a layer lying underneath, and a thickness distribution
d=d(x, y) of at least the layer of the cover layer system lying
underneath, as a function of its spatial coordinates, has a
gradient not equal to zero.
5. A reflective optical element, comprising: a multilayer system
for the extreme ultraviolet and/or soft x-ray range with a cover
layer system having at least one layer to protect the surface,
whose spatial structure can be described in a Cartesian system of
coordinates (x, y, z) with z=z(x, y), wherein the gradient of the
thickness distribution d=d(x, y) of at least one layer of the cover
layer system at the location (x, y) is not equal to the gradient of
the thickness distribution of at most ten individual layers of the
multilayer system immediately adjacent to the cover layer
system.
6. The reflective optical element according to claim 5, wherein a
maximum change in thickness of the cover layer system is at most 3
nm.
7. The reflective optical element according to claim 5, wherein an
overall thickness of the cover layer system is at most 8 nm.
8. A reflective optical element, comprising: a multilayer system
for the extreme ultraviolet and/or soft x-ray wavelength range with
a cover layer system having at least one layer to protect the
surface, whose spatial structure can be described in a Cartesian
system of coordinates (x, y, z) with z=z(x, y), wherein the cover
layer system has at least one outer layer and a layer lying
underneath, and the gradient of the thickness distribution d=d(x,
y) of at least the layer of the cover layer system lying underneath
at the location (x,y) is not equal to the gradient of the thickness
distribution of at most ten individual layers of the multilayer
system immediately adjacent to the cover layer system.
9. The reflective optical element according to claim 1, wherein at
least one layer of the cover layer system has a thickness variation
of .gtoreq.1 .ANG. over the surface of the optical element.
10. The reflective optical element according to claim 1, wherein
the reflective optical element has a shaper and the thickness
distribution of the cover layer system is such that the free
boundary surface of the reflective optical element reproduces the
shape of the shaper in at least one direction relative to the
xy-coordinates.
11. The reflective optical element according to claim 1, wherein
the thickness distribution of the cover layer system in at least
one direction relative to the xy-coordinates varies monotonically
with the intensity distribution along the beam cross section of the
operating incident radiation.
12. The reflective optical element according to claim 1, wherein
the thickness distribution is such that the convolution, in respect
of the xy-coordinates, of the intensity distribution along the beam
cross section of the operating incident radiation with the
electrical field intensity of the standing wave formed by
reflection at the free boundary surface and normalized to the
incident radiation intensity (the degradation profile) produces a
linear distribution.
13. The reflective optical element according to claim 1, wherein
the thickness distribution is such that the convolution, in respect
of the xy-coordinates, of the intensity distribution along the beam
cross section of the operating incident radiation with the
electrical field intensity of the standing wave formed by
reflection at the free boundary surface and normalized to the
incident radiation intensity (the degradation profile) produces a
rotationally symmetrical distribution.
14. The reflective optical element according to claim 1, wherein
the thickness distribution is such that the convolution, in respect
of the xy-coordinates, of the intensity distribution along the beam
cross section of the operating incident radiation with the
electrical field intensity of the standing wave formed by
reflection at the free boundary surface and normalized to the
incident radiation intensity (the degradation profile) produces a
superpositioning of a linear and a rotationally symmetrical
distribution.
15. The reflective optical element according to claim 12, wherein
the distribution resulting from the convolution in terms of the
xy-coordinates is such that the magnitude of this distribution
decreases at every point (x, y) of the surface with increasing
thickness of the cover layer system.
16. The reflective optical element according to claim 1, wherein
the thickness distribution is such that the weighted product, in
respect of the xy-coordinates, of the intensity distribution along
the beam cross section of the operating incident radiation and the
electrical field intensity of the standing wave formed by
reflection at the free boundary surface and normalized to the
incident radiation intensity (the degradation profile) yields a
linear distribution.
17. The reflective optical element according to claim 1, wherein
the thickness distribution is such that the weighted product, in
respect of the xy-coordinates, of the intensity distribution along
the beam cross section of the operating incident radiation and the
electrical field intensity of the standing wave formed by
reflection at the free boundary surface and normalized to the
incident radiation intensity (the degradation profile) yields a
rotationally symmetrical distribution.
18. The reflective optical element according to claim 1, wherein
the thickness distribution is such that the weighted product, in
respect of the xy-coordinates, of the intensity distribution along
the beam cross section of the operating incident radiation and the
electrical field intensity of the standing wave formed by
reflection at the free boundary surface and normalized to the
incident radiation intensity (the degradation profile) yields a
superpositioning of a linear and a rotationally symmetrical
distribution.
19. The reflective optical element according to claim 16, wherein
the distribution resulting from the weighted product in terms of
the xy-coordinates is such that the magnitude of this distribution
decreases at every point (x, y) of the surface with increasing
thickness of the cover layer system.
20. An optical system with at least two reflective optical elements
for the extreme ultraviolet and/or soft x-ray wavelength range, at
least one reflective optical element of which is an element
according to claim 1.
21. An optical system, comprising: at least two reflective optical
elements for the extreme ultraviolet and/or soft x-ray wavelength
range each with a cover layer system, wherein the cover layer
systems have different materials, or different thickness
distributions in the z-direction as a function of the x or
y-coordinates, or x and y-coordinates, or a combination thereof,
and the thickness distribution of at least one layer of one cover
layer system has a gradient not equal to zero.
22. The optical system according to claim 21, wherein a maximum
change in thickness of the cover layer system is at most 3 nm.
23. The optical system according to claim 21, wherein an overall
thickness of the cover layer system is at most 8 nm.
24. An optical system, comprising: at least two reflective optical
elements for the extreme ultraviolet and/or soft x-ray wavelength
range each with a cover layer system, wherein the cover layer
systems have different materials, or different thickness
distributions in the z-direction as a function of the x or
y-coordinates, or x and y-coordinates, or a combination thereof,
and the cover layer system has at least one outer layer and a layer
lying underneath, and the thickness distribution d=d(x, y) of at
least the layer of the cover layer system lying underneath has a
gradient not equal to zero.
25. An optical system comprising: at least two reflective optical
elements for the extreme ultraviolet and/or soft x-ray wavelength
range each with a cover layer system, wherein the cover layer
systems have different materials, or different thickness
distributions in the z-direction as a function of the x or
y-coordinates, or x and y-coordinates, or a combination thereof,
and the gradient of the thickness distribution d=d(x, y) of at
least one layer of one cover layer system at the location (x, y) is
unequal to all gradients of at most ten individual layers of the
multilayer system immediately adjacent to the cover layer
system.
26. The optical system according to claim 25, wherein a maximum
change in thickness of the cover layer system is at most 3 nm.
27. The optical system according to claim 25, wherein an overall
thickness of the cover layer system is at most 8 nm.
28. An optical system, comprising: at least two reflective optical
elements for the extreme ultraviolet and/or soft x-ray wavelength
range each with a cover layer system, wherein the cover layer
systems have different materials, or different thickness
distributions in the z-direction as a function of the x or
y-coordinates, or x and y-coordinates, or combinations thereof, and
the gradient of the thickness distribution d=d(x, y) of at least
the layer of one cover layer system lying underneath at the
location (x, y) is unequal to all gradients of at most ten
individual layers of the multilayer system immediately adjacent to
the cover layer system.
29. An EUV lithography appliance, comprising: at least two
reflective optical elements for the extreme ultraviolet and/or soft
x-ray wavelength range with at least one cover layer system, at
least one reflective optical element of which is an element
according to claim 1.
30. An EUV lithography appliance, comprising: at least two
reflective optical elements for the extreme ultraviolet and/or soft
x-ray wavelength range with at least one cover layer system,
wherein the cover layer systems have different materials, or
different thickness distributions in the z-direction as a function
of the x or y-coordinates, or x and y-coordinates, or a combination
thereof, and the thickness distribution of at least one layer of
one cover layer system has a gradient not equal to zero.
31. The EUV lithography appliance according to claim 30, wherein an
overall thickness of the cover layer system is at most 8 nm.
32. The EUV lithography appliance according to claim 30, wherein a
maximum change in thickness of the cover layer system is at most 3
nm.
33. An EUV lithography appliance, comprising: at least two
reflective optical elements for the extreme ultraviolet and/or soft
x-ray wavelength range with at least one cover layer system,
wherein the cover layer system has at least one outer layer and one
layer lying underneath, and the cover layer systems have different
materials, or different thickness distributions in the z-direction
as a function of the x or y-coordinates, or x and y-coordinates, or
a combination thereof, and the thickness distribution of at least
the layer of a cover layer system lying underneath has a gradient
not equal to zero.
34. A semiconductor component produced with an optical element
according to claim 1.
35. The reflective optical element according to claim 9, wherein at
least one layer of the cover layer system has a thickness variation
of .gtoreq.3 .ANG. over the surface of the optical element.
36. The reflective optical element according to claim 9, wherein at
least one layer of the cover layer system has a thickness variation
of .gtoreq.5 .ANG. over the surface of the optical element.
37. The reflective optical element according to claim 13, wherein
the distribution resulting from the convolution in terms of the
xy-coordinates is such that the magnitude of this distribution
decreases at every point (x, y) of the surface with increasing
thickness of the cover layer system.
38. The reflective optical element according to claim 14, wherein
the distribution resulting from the convolution in terms of the
xy-coordinates is such that the magnitude of this distribution
decreases at every point (x, y) of the surface with increasing
thickness of the cover layer system.
39. The reflective optical element according to claims 17, wherein
the distribution resulting from the weighted product in terms of
the xy-coordinates is such that the magnitude of this distribution
decreases at every point (x, y) of the surface with increasing
thickness of the cover layer system.
40. The reflective optical element according to claims 18, wherein
the distribution resulting from the weighted product in terms of
the xy-coordinates is such that the magnitude of this distribution
decreases at every point (x, y) of the surface with increasing
thickness of the cover layer system.
Description
CROSS REFERENCE
[0001] This application is a continuation-in-part application of
International Application No. PCT/EP2004/004368, filed Apr. 26,
2004 and published as WO 2004/097467 on Nov. 11, 2004, which claims
the priority to German Application No. 103 19 005.8, filed Apr. 25,
2003.
FIELD OF THE INVENTION
[0002] The invention concerns a reflective optical element for the
external ultraviolet and/or soft x-ray wavelength region with a
cover layer system having at least one layer, whose spatial
structure can be described in a Cartesian system of coordinates (x,
y, z) with z=z(x, y). Other systems of coordinates are possible for
describing the spatial structure.
[0003] Moreover, the invention concerns an optical system or an EUV
lithography device with at least two reflective optical elements
for the extreme ultraviolet and/or soft x-ray wavelength region
with at least one cover layer system.
BACKGROUND OF THE INVENTION
[0004] Multilayers composed of a plurality of layers are used to
achieve optimal reflectivity on optical elements for the EUV and
soft x-ray wavelength region. Such multilayers are composed from
periodic repetitions, a period consisting of two layers in the
elementary case. As a rule, one layer material should have the
highest possible index of refraction and slight absorption, while
the other layer material should have the lowest possible index of
refraction. The layer with the high index of refraction and slight
absorption is also known as a spacer, the layer with low index of
refraction is also called an absorber. The period thickness and the
thicknesses of the individual layers are chosen such, in dependence
on the operating wavelength, the mean angle of incidence, and the
angle bandwidth of the incident radiation, that the integrated
reflectivity over the illuminated surface is maximized. By a cover
layer system is meant the portion of a multilayer coating or an
optical element that is no longer periodic and forms a closure at
the free interface. In the elementary case, this is merely the last
individual layer.
[0005] Reflective optical elements are used, for example, in EUV
lithography instruments for the production of semiconductor
components. In use, they are exposed to both an irradiation of up
to 20 mW/mm.sup.2 EUV intensity or more, and to a residual gas
fraction of water, oxygen and hydrocarbons, as well as other
residual gas components. Residual gas components adsorbed onto
irradiated surfaces are split up into reactive cleavage products by
photoinduced electrons due to bombardment of the surface with EUV
photons. This generally leads to a degradation or contamination,
e.g., by oxidation, carbon deposits, interdiffusion, material
ablation, etc., of the multilayer surface. These effects lead to
imaging errors and transmission losses. In the worst case, the
desired imaging is totally impossible. Thus, regeneration cycles
must be provided during operation of the EUV lithography machine,
which not only significantly increase the operating costs, but also
in the extreme case lead to an irreversible damaging and, thus, may
entail a replacement of the affected reflective optical
elements.
[0006] Thus far, one has attempted to counter a negative alteration
of the surface by providing a cover layer system on the surface of
the reflective optical element, supposed to protect the surfaces.
The basic layout of traditional reflective optical elements is
sketched in FIGS. 1a to c. These show three different multilayer
systems 2. In FIG. 1a, a multilayer system 2 is shown in which the
layer thicknesses are constant both along the thickness of the
multilayer system 2 and also across the surface. FIG. 1b shows a
multilayer system 2 in which the thickness relations of a period
are constant along the entire depth, but there is a nonconstant
distribution of thicknesses in the surface, and so the multilayer
system 2 has a lateral gradient. In FIG. 1c, the multilayer system
2 does not have a lateral gradient, but the distribution of layer
thicknesses varies across the depth of the multilayer system
(so-called depth-graded multilayer). All multilayers are deposited
on a substrate 3. Beneath the multilayer system 2, a portion of the
substrate 3 is configured as an optically shaping region 5, also
known as a shaper. The shaper 5 is needed primarily to give the
optical element 1 a shape which leads to the desired optical
properties. The multilayer system 2 borders on a cover layer system
6, which in FIGS. 1a to c consists of two segments 7, 8, one
segment 7 generally serving to adapt the phase to the multilayer
system or to the interdiffusion protection and the second segment 8
generally serving for the actual contamination protection. The
boundary surface 4 of the cover layer system 6 next to the vacuum
is known as the free boundary surface 4.
[0007] Thus far in the prior art it has been attempted to
positively influence the contamination and degradation of the
reflective optical element by the choice of specific cover layer
materials. Thus, for example, U.S. Pat. No. 6,228,512 proposes
having a protective layer of SiO.sub.2, Zr.sub.2O or ZnO on a
MoRu/Be multilayer, which does not react with water. In particular,
ZnO is recommended, for when zinc is applied there is formed a ZnO
layer only 0.5 to 0.6 nm thick, which sufficiently protects the
multilayer against oxidation, without significantly impairing the
reflectivity--because of its slight thickness.
[0008] U.S. Pat. No. 5,958,605 proposes a special protection layer
system for EUV multilayers in which a lower layer of silicon or
beryllium is proposed, placed directly on the multilayer, and at
least one top layer is applied onto the lower layer, and this top
layer has a material which is resistant to oxidation and corrosion
and also protects the underlying layers against oxidation.
[0009] While the protective layers of U.S. Pat. No. 5,958,605 and
U.S. Pat. No. 6,228,512 provide a protection against degradation by
the influence of oxygen, there still occurs a contamination from
carbon-containing substances. These lead to uncontrolled losses of
reflectivity and changes in the wave front.
SUMMARY OF THE INVENTION
[0010] Against this background, the problem of the invention is to
provide a reflective optical element or a corresponding optical
system or an EUV lithography appliance in which the negative
influence of contamination and degradation is as low as
possible.
[0011] This problem is solved by a reflective optical element, an
optical system, and an EUV lithography machine according to the
claims. Furthermore, this problem is solved by a semiconductor
element according to the claims.
[0012] The present invention is based on the principle of using not
merely the selection of materials, but also the geometry of the
cover layer system to render the general degradation and especially
the contamination by carbon-containing substances as low as
possible, or to make their influence manageable. By specific choice
of the thickness distribution d(x, y) of the cover layer system as
a function of its spatial coordinates, wherein at least one layer
of the cover layer system should have a gradient not equal to zero,
one can control how much contamination occurs at which places of
the surface. In this way, the contamination can be calculated and
factored in when operating or designing of reflective optical
elements, optical systems, and EUV lithography appliances.
[0013] If the reflective optical element has a multilayer system,
the gradient of the two-dimensional thickness distribution of the
cover layer system that is different from zero in at least one
direction is not equal to the gradient of the two-dimensional
thickness distribution of the multilayer system.
[0014] The reflective optical element coated with a multilayer
system for the extreme ultraviolet and/or soft x-ray region with a
cover layer system containing at least one layer whose spatial
position can be described in a Cartesian system of coordinates (x,
y, z) by z=z(x, y) can be characterized in that the gradient of the
thickness distribution d(x, y) of at least one layer of the cover
layer system at location (x, y) is not equal to the gradient of the
at most ten individual layers of the multilayer system immediately
adjacent to the cover layer system.
[0015] Preferably, at least one layer of the cover layer system (6)
has thickness variations .gtoreq.1 .ANG., preferably .gtoreq.3
.ANG., especially .gtoreq.5 .ANG., over the surface of the optical
element.
[0016] If the reflective optical element has a shaper, it can be
advantageous for the thickness distribution of the cover layer
system to be such that the free boundary surface of the reflective
optical element reproduces the shape of the shaper in at least one
direction relative to the xy-coordinates. The thickness variations
of the cover layer system can just compensate for the lateral
gradient of the multilayer system. This facilitates interferometric
checking of the reflective optical element. Furthermore, one can
thereby specifically shift the phase of the standing wave produced
upon reflection.
[0017] In an especially preferred embodiment, the thickness
distribution varies monotonically, preferably strictly
monotonically, in at least one direction relative to the
xy-coordinates with the intensity distribution across the beam
cross section of the operating incident radiation. This adapts the
thickness distribution of the cover layer system to the electrical
field intensity at the location of the free boundary surface of the
standing wave produced by reflection, so as to specifically modify
the number of photo-induced electrons.
[0018] Increasingly more photoelectrons are produced in places with
high radiation intensity and they split the residual gas components
adsorbed on the surface into reactive products, which results in
increased contamination or degradation of the surface. Assuming
that the free boundary surface is positioned relative to the
standing wave so that the electrical field intensity decreases with
larger cover layer thickness, an especially large thickness of the
cover layer system will now be provided in regions with high
radiation intensity. This will locally modify the electrical field
intensity of the standing wave produced by reflection at the
location of the free boundary surface next to the vacuum so that
fewer photoelectrons are induced. Thus, for example, one can adjust
a rate of photoemission which is constant over the entire surface,
regardless of the radiation intensity. This will diminish or
entirely prevent imaging errors due to contamination, especially
during permanent duty in lithography machines.
[0019] In another preferred embodiment, the thickness distribution
of the cover layer system is such that the convolution in regard to
the xy-coordinates of the intensity distribution across the beam
cross section of the operating incident radiation with the
electrical field intensity of the standing wave formed by
reflection at the free boundary surface, normalized to the incident
radiation intensity, yields a linear distribution, a rotationally
symmetrical distribution, or a superpositioning of a linear and a
rotationally symmetrical distribution (hereafter known as
degradation profile).
[0020] Instead of the convolution, one can also undertake a
weighted multiplication P=P(x, y) with respect to the
xy-coordinates of the intensity distribution across the radiation
cross section l(x, y) by the electrical field intensity E(x, y)
formed at the free boundary surface and normalized to the radiation
intensity, i.e., P(x, y)=A(x, y).times.I((x, y).times.E(x, y),
where A(x, y) is a dimensionless factor, dependent on the cover
layer material, preferably lying in the range of 1 to 4.
[0021] P(x, y) is "linear" precisely (disregarding the physical
dimensional units) when: "P(x=const., y) can be represented as
P(x=const., y)=y+b and P(x, y=const.) can be represented as P(x,
y=const.)=cy+d [with a, b, c, d .epsilon. R]."
[0022] P(x, y) is "rotationally symmetrical" precisely
(disregarding the physical dimensional units) when: "There is no
point (x0, y0) such that for all (x, y) on the mirror surface we
have: P( {square root over
((x-x.sub.0).sup.2+(y-y.sub.0).sup.2)})=const."
[0023] P(x, y) is "a superpositioning of a linear and a
rotationally symmetrical function" precisely (disregarding the
physical dimensional units) when: "P(x, y) can be represented as a
linear combination of a linear and a rotationally symmetrical
function per the above definitions."
[0024] The "normalized electrical field intensity at the free
boundary surface" denotes the gain factor of the actual electrical
field intensity at the free boundary surface (based on the
resulting standing wave) as compared to the time-averaged intensity
of the free traveling wave. The magnitude of the normalized
electrical field intensity at the free boundary surface is in the
range of 0 to approximately 4 and is calculated for each individual
point (x, y) of the surface from the relative position of free
boundary surface and standing wave field.
[0025] The degradation profile corresponds to the two-dimensional
photoemission profile across the irradiated surface of an optical
element with a cover layer system. Both the intensity distribution
of the operating incident radiation and the normalized electrical
field intensity of the standing wave formed by reflection at the
locus of the free boundary surface can be calculated on the basis
of a first thickness distribution of the cover layer system for
defined cover layer materials and defined operating wavelengths. By
varying both the intensity distribution of the operating incident
radiation and the normalized electrical field intensity of the
standing wave until the convolution of both quantities agrees with
the desired outcome, the ultimate thickness distribution of the
cover layer system can be found by calculating backward. In the
most simple case of degradation, which is carbon contamination, the
convolution is a point by point multiplication. In the general case
of degradation, this convolution is an integration over space and
time of the above product, multiplied by a function dependent on
the degradation process.
[0026] The special advantage is that linear and rotationally
symmetrical gradients can be compensated by actuators on which the
optical elements can be mounted if necessary. The compensation for
geometrical errors of optical elements by means of actuators has
long been widely used. Imaging errors can be avoided by modifying
the rate of photoemission with different thicknesses of the cover
layer system and compensating for the remaining gradients with
actuators.
[0027] Especially preferred is a degradation profile such that its
value diminishes at each point of the surface with increasing
thickness of the cover layer system, i.e., at each point (x, y) of
the surface of the reflective optical element where the free
boundary surface emerges on account of contamination, the intensity
of the photoemission decreases, with the emergence of the free
boundary surface corresponding to an increase in the layer
thickness. Otherwise, one would have the effect of a further
increase in the likelihood of increased contamination when the
thickness is increased by contamination. This has to do with the
phase position of the standing wave relative to the free boundary
surface, which must always be chosen such that the intensity of the
standing wave decreases at the location of the free boundary
surface when the thickness of the cover layer increases, in order
to prevent excessive contamination. In the case of degradation due
to ablation of the layer, the phase location needs to be adjusted
in the opposite way.
[0028] The optical system with at least two reflective optical
elements for the extreme ultraviolet and/or soft x-ray wavelength
range with a cover layer system for each is characterized in that
the cover layer systems have different materials and/or different
thickness distributions in the z-direction as a function of the x
and/or y coordinate, and the thickness distribution of at least one
layer of a cover layer system has a gradient not equal to zero.
[0029] According to a particular embodiment, the cover layer
systems have different materials and/or have different thickness
distributions in the z-direction as a function of the x and/or y
coordinates, and the gradient of the thickness distribution d(x, y)
of at least one layer of the cover layer system at the location (x,
y) is unequal to all gradients of the maximum of 10 immediately
adjacent single layers of the multilayer system belonging to the
cover layer system.
[0030] In the case of optical systems with two or more reflective
optical elements or corresponding EUV lithography appliances, one
can use cover layer systems with thickness gradients and/or
different cover layer materials so that the necessary
two-dimensional profile of photoelectron emission and thus the
required degradation profile can be specifically adjusted for each
optical element. In particular, a differentiation in terms of
incident radiation intensity and wavelength band is advisable.
[0031] Mirrors at the start of the wave path are subject to a high
radiation intensity and exhibit a broad wavelength band. One should
optimize these reflective optical elements, under a certain loss of
reflectivity, so that the photocurrent is as low as possible and
constant across the surface, in order to prolong their
lifetime.
[0032] Reflective optical elements at the end of the wave path,
which are used for the actual imaging, are narrow-band and only
subject to a slight radiation intensity. These should be optimized
for a high reflectivity. For because of the overall lower
intensities, the emission of photoelectrons should have less effect
and result in relatively less contamination.
[0033] A special advantage is that one can adjust an approximately
identical lifetime for individual groups of reflective optical
elements of an optical system or an EUV lithography appliance.
[0034] Semiconductor components produced with the help of the
invented reflective optical elements or optical systems or EUV
lithography appliances have the advantage of being produced with
lower reject rates and lower cost. For on the one hand, the imaging
quality is better since the degradation is manageable, and on the
other hand longer lifetimes can be achieved for the devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] The invention shall now be explained more closely by means
of the following examples. These show:
[0036] FIG. 1a-c, reflective optical element of the prior art;
[0037] FIG. 2a, the two-dimensional intensity distribution of
incident EUV radiation for a first optical element;
[0038] FIG. 2b, the thickness distribution or the EUV radiation
intensity distribution on the cover layer system of a first optical
element from FIG. 2a in the y-direction;
[0039] FIG. 2c, the thickness distribution or the EUV radiation
intensity distribution on the cover layer system of a first
reflective optical element from FIG. 2a in the y-direction;
[0040] FIG. 3a, c, e, g, the reflectivity or the normalized
electrical field intensity at the location of the free boundary
surface of the standing wave formed by reflection at a fixed angle
of incidence as a function of the wavelength for different
positions of the free boundary surface on the first reflective
optical element;
[0041] FIG. 3b, d, f, h, the normalized electrical field intensity
of a standing wave formed by reflection at a fixed angle of
incidence and strongly influencing the photocurrent on the first
reflective optical element as a function of the cover layer system
depth for different positions of the free boundary surface on the
first reflective optical element;
[0042] FIG. 4a, the resulting two-dimensional photoemission profile
on the first reflective optical element;
[0043] FIG. 4b, the photoemission profile from FIG. 4a in the
y-direction;
[0044] FIG. 4c, the photoemission profile from FIG. 4a in the
x-direction;
[0045] FIG. 5a, the two-dimensional intensity distribution of
incident EUV radiation for a second optical element;
[0046] FIG. 5b, the thickness distribution and the EUV radiation
intensity distribution on the cover layer system of a first
reflective optical element from FIG. 5a in the y-direction;
[0047] FIG. 5c, the thickness distribution or the EUV radiation
intensity distribution on the cover layer system of a first
reflective optical element from FIG. 5a in the x-direction;
[0048] FIG. 6a, c, e, g, the reflectivity or the normalized
electrical field intensity at the location of the free boundary
surface of the standing wave formed by reflection at a fixed angle
of incidence as a function of the wavelength for different
positions of the free boundary surface on the second reflective
optical element;
[0049] FIG. 6b, d, f, h, the normalized electrical field intensity
of a standing wave formed by reflection at a fixed angle of
incidence and strongly influencing the photocurrent on the first
reflective optical element as a function of the cover layer system
depth for different positions of the free boundary surface on the
second reflective optical element;
[0050] FIG. 7a, the resulting two-dimensional photoemission profile
on the second reflective optical element;
[0051] FIG. 7b, the photoemission profile from FIG. 7a in the
y-direction;
[0052] FIG. 7c, the photoemission profile from FIG. 7a in the
x-direction;
[0053] FIG. 8a, the two-dimensional intensity distribution of
incident EUV radiation for a third optical element;
[0054] FIG. 8b, the thickness distribution and the EUV radiation
intensity distribution on the cover layer system of a first
reflective optical element from FIG. 8a in the y-direction;
[0055] FIG. 8c, the thickness distribution or the EUV radiation
intensity distribution on the cover layer system of a first
reflective optical element from FIG. 8a in the x-direction;
[0056] FIG. 9a, c, e, g, the reflectivity or the normalized
electrical field intensity at the location of the free boundary
surface of the standing wave formed by reflection at a fixed angle
of incidence as a function of the wavelength for different
positions of the free boundary surface on the third reflective
optical element;
[0057] FIG. 9b, d, f, h, the normalized electrical field intensity
of a standing wave formed by reflection at a fixed angle of
incidence and strongly influencing the photocurrent on the first
reflective optical element as a function of the cover layer system
depth for different positions of the free boundary surface on the
third reflective optical element;
[0058] FIG. 10a, the resulting two-dimensional photoemission
profile on the third reflective optical element;
[0059] FIG. 10b, the photoemission profile from FIG. 10a in the
y-direction;
[0060] FIG. 10c, the photoemission profile from FIG. 10a in the
x-direction;
DETAILED DESCRIPTION OF THE INVENTION
[0061] FIGS. 1a to c have already been explained.
[0062] In FIGS. 2 to 4, a first preferred embodiment of the
invented optical element is described.
[0063] FIG. 2a shows the two-dimensional intensity distribution of
incident EUV radiation for a first optical element with which the
reflective optical element is to be utilized. The intensity
increases from the outside to the middle.
[0064] FIGS. 2b and 2c show the intensity curve in the x and y
direction corresponding to the broken lines drawn in FIG. 2a. This
intensity curve in the xy-direction corresponds in monotonic manner
to the thickness curve of the cover layer system. Where a high
radiation intensity impinges on the reflective optical element, the
cover layer system is also particularly thick. Where the intensity
is less, the cover layer system is also thinner. Corresponding to
FIGS. 1a-c, the thickness distribution designated as 7 pertains to
the lower segment of the cover layer system and the thickness
distribution designated as 8 corresponds to the upper segment of
the cover layer system.
[0065] As can be seen in FIGS. 3b, d, f, h, the specific reflective
optical element is a silicon-molybdenum multilayer with MoSi.sub.2
intermediate layers and a cover layer system 6 consisting of a
lower segment 7 of MoSi.sub.2, molybdenum, and ruthenium. The
carbon layer corresponds to the upper segment 8 of the cover layer
system.
[0066] FIGS. 3a, c, e, g plot the reflection curve as a function of
the wavelength of incoming radiation, as well as the normalized
electrical field intensity of the standing wave formed by
reflection at the site of the free boundary surface. Broken lines
show the position of the operating wavelength. In arbitrary units,
the normalized electrical field intensity at the site of the free
boundary surface is 10 units in FIG. 3a, 7 units in FIG. 3c, 4
units in FIG. 3e and 1 unit in FIG. 3g. In FIGS. 3b, d, f, h, the
normalized electrical field intensities are plotted at the
operating wavelength as a function of the position of the standing
wave relative to the individual layers and to the free boundary
surface at the vacuum. FIGS. 3a and b, FIGS. 3c and d, FIGS. 3e and
f, and FIGS. 3g and h go together.
[0067] As regards the intensity distribution of incident radiation
and the thickness distribution of the reflective optical element,
the indicated points lie on the line designated in FIG. 2a. FIG.
3a, b are located at the edge of the distribution, where the cover
layer thickness if 5 nm. In the case of FIGS. 3c, d, the cover
layer thickness is 6 nm, for FIGS. 3e, f it is 7 nm and for FIGS.
3g, h it is 8 nm. Here, both the radiation intensity and the layer
thickness are the highest.
[0068] The resulting photoemission profile can be estimated by
multiplying the normalized field intensity value of the standing
wave at the location of the free boundary surface by the value of
the radiation intensity at this place. In FIGS. 3a, b, the
intensity of the EUV radiation is 1 unit, in FIGS. 3c, d it is 1.5
units, in FIGS. 3e, f it is 2.5 units and in FIGS. 3g, h it is 10
units. Thus, one gets a constant photoemission profile of 10 units
over the entire surface. This profile is depicted in two dimensions
in FIG. 4a and along the broken line in the y-direction in FIG. 4b
and in the x-direction in FIG. 4c.
[0069] The constant photoemission profile over the entire surface
is achieved thanks to a decrease in the relative reflectivity
toward the middle. The reflectivity decreases from 68.8% at the
margin to 65% at the intensity maximum, which should be properly
factored into the optical design of the overall system. A
reflectivity loss varying in time due to the buildup of a thick and
inhomogeneous contamination would be much less favorable to the
illumination of the wafer and the wave front properties.
[0070] Moreover, one must properly select the phase location of the
standing wave relative to the free boundary surface of the
reflective optical element. Contrary to the example above (FIGS.
3b, d, f, h), a phase shift by half a period with increasing
thickness of the protective layer would also increase the
normalized electric field intensity of the standing wave at the
site of the free boundary surface and thus the contamination would
further build up over time.
[0071] In FIGS. 5 to 7, a second sample embodiment of the
reflective optical element is explained. Here, there is a
homogeneous intensity distribution 9 of the incident radiation
(FIG. 5a). The reflective optical element, however, has a lateral
gradient in the multilayer system. On top of this, as shown by
FIGS. 5b, c, there is placed a cover layer system with a thickness
distribution. The multilayer system with lateral gradient is placed
on a substrate which has a flat shaper. The coating with the cover
layer system shown in FIG. 5b, c has the effect that the free
boundary surface reproduces the shaper. Thus, the thickness
variation of the multilayer system is exactly compensated by the
thickness variation of the cover layers.
[0072] The situation regarding the positions 1, 2, 3, 4 of FIG. 5a
is represented in FIGS. 6a, b, 6c, d, 6e, f and 6g, h. The
intensity of the radiation is 1 unit throughout, the normalized
electrical field intensity value of the standing wave is 10 units
in FIG. 6a, 7 units in FIG. 6c, 4 units in FIG. 6c [sic!] and 1
unit in FIG. 6g.
[0073] The resulting photoemission profile is depicted in FIGS. 7a
to c and has a value of 10 units at position 1, 7 units at position
2, 4 units at position 3, and 1 unit at position 4. Even though the
contamination profile is no longer constant for this special
optical element, it can still be calculated and therefore factored
into the configuration of optical systems. Furthermore, this
reflective optical element is especially suitable for optical
interferometric inspection of its construction. Moreover, it can be
specifically used as a phase shifter in an optical system.
[0074] In FIGS. 8 to 10, a third embodiment of the reflective
optical element is presented. This reflective optical element is to
be used with a radiation whose intensity distribution is neither
linear nor rotationally symmetrical, but rather elliptical, for
example. Since a constant thickness of cover layer is preserved in
the y-direction (FIG. 8b, broken line) and the layer thickness
distribution varies monotonically with the intensity distribution
in the x-direction (FIG. 8c, broken line), the end result is a
rotationally symmetrical carbon contamination profile (FIGS. 10a to
c).
[0075] In FIGS. 9a to h, the situation is shown at positions 1 to 4
in FIG. 8a. At a layer thickness of 5 nm (FIG. 9a, b), the standing
wave has a normalized electrical field intensity of 10 units and
the intensity is likewise 10 units. This gives a value of 100 units
for the photoemission. At a layer thickness of 6 nm, the normalized
electrical field intensity of the standing wave is 7 units and the
intensity is 7 units. This gives a value of 49 units for the
photoemission. At a layer thickness of 7 nm, the normalized
electrical field intensity of the standing wave is 4 units and the
intensity is likewise 4 units. This gives a photoemission of 16
units. At a thickness of 8 nm, the normalized electrical field
intensity of the standing wave is 1, the value of the intensity is
1 unit, and the overall value for the photoemission is likewise 1
unit. Thus, thanks to the special thickness distribution of the
cover layer system, the contamination profile is flattened in the
y-direction, while it is preserved in the x-direction, so that the
profile can be converted from an elliptical shape to a rotationally
symmetrical shape, as shown in FIG. 10a-c. The rotationally
symmetrical profile can be compensated by means of actuators
through a movement of the reflective optical element in the
direction of the surface normals, so that there are no effects on
the imaging properties.
[0076] A similar technique is possible with suitable adjustment of
the thickness distribution of the cover layer system in order to
achieve linear gradients.
[0077] Furthermore, it is conceivable by suitable adjustment of the
thickness distribution of the cover layer system to achieve a
cleaning action which leaves behind liner or rotationally
symmetrical (and thus correctable by actuator) degradation
everywhere.
* * * * *