U.S. patent application number 15/917038 was filed with the patent office on 2018-10-18 for method for measuring an angularly resolved intensity distribution and projection exposure apparatus.
The applicant listed for this patent is Carl Zeiss SMT GmbH. Invention is credited to Wolfgang EMER, Dirk HELLWEG.
Application Number | 20180299787 15/917038 |
Document ID | / |
Family ID | 48783812 |
Filed Date | 2018-10-18 |
United States Patent
Application |
20180299787 |
Kind Code |
A1 |
EMER; Wolfgang ; et
al. |
October 18, 2018 |
METHOD FOR MEASURING AN ANGULARLY RESOLVED INTENSITY DISTRIBUTION
AND PROJECTION EXPOSURE APPARATUS
Abstract
A method for measuring an angularly resolved intensity
distribution in a reticle plane (24) of a projection exposure
apparatus (10). The apparatus includes an illumination system (16),
irradiating a reticle (22) arranged in the reticle plane (24) and
having a first pupil plane (20). All planes of the projection
exposure apparatus which are conjugate thereto are further pupil
planes, and the reticle plane (24) and all planes which are
conjugate thereto are field planes. The method includes: arranging
a spatially resolving detection module (44) in the region of one of
the field planes (24, 30) such that the detection module is at a
smaller distance from this field plane than from the closest pupil
plane (20), radiating electromagnetic radiation (21) onto an
optical module (42) from the illumination system, and determining
an angularly resolved intensity distribution of the radiation from
a signal recorded by the detection module.
Inventors: |
EMER; Wolfgang; (Aalen,
DE) ; HELLWEG; Dirk; (Aalen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carl Zeiss SMT GmbH |
Oberkochen |
|
DE |
|
|
Family ID: |
48783812 |
Appl. No.: |
15/917038 |
Filed: |
March 9, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14796328 |
Jul 10, 2015 |
9915871 |
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15917038 |
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13935972 |
Jul 5, 2013 |
9081294 |
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14796328 |
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61668554 |
Jul 6, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 7/70133 20130101;
G01J 1/4257 20130101; G03F 7/70191 20130101 |
International
Class: |
G03F 7/20 20060101
G03F007/20; G01J 1/42 20060101 G01J001/42 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 6, 2012 |
DE |
10 2012 211 846.2 |
Claims
1. A projection exposure apparatus for microlithography comprising
a radiation source for generating exposure radiation, and an
illumination system disposed downstream of the radiation source and
configured for radiating the exposure radiation into a reticle
plane of the projection exposure apparatus, wherein the
illumination system comprises: a beam expanding optical unit for
expanding a beam cross section of the exposure radiation from the
radiation source, a beam angle redistribution module for deflecting
partial beams of the exposure radiation from the beam expanding
optical unit, a divergence amplification module for amplifying a
divergence of the beam from the beam expanding optical unit, and an
exchanging device for exchanging at least one element of the beam
angle redistribution module for the divergence amplification
module.
2. The projection exposure apparatus according to claim 1, further
comprising a projection lens for imaging mask structures from the
reticle plane onto a wafer, a wafer stage for holding the wafer,
and a system for measuring an illumination angle distribution in
the reticle plane, wherein the system comprises a detector module
arranged at the wafer stage and the detector module is arranged in
a region of the conjugate pupil plane of the projection lens during
the measuring.
3. The projection exposure apparatus according to claim 1, wherein
the beam angle redistribution module comprises a diffractive
optical element.
4. The projection exposure apparatus according to claim 1, wherein
the divergence amplification module comprises a microlens element
array.
5. The projection exposure apparatus according to claim 1, wherein
the divergence amplification module comprises two microlens element
arrays having different focal lengths.
6. A method for measuring an angularly resolved intensity
distribution in a reticle plane of a projection exposure apparatus
for microlithography comprising an illumination system, which is
configured for irradiating a reticle arranged in the reticle plane
and has a first pupil plane, wherein all planes of the projection
exposure apparatus which are conjugate with respect to the first
pupil plane are further pupil planes, and the reticle plane and all
planes which are conjugate with respect to the reticle plane are
field planes, comprising: arranging an optical module in a beam
path of the projection exposure apparatus above the reticle plane,
arranging a spatially resolving detection module in a region of one
of the field planes such that the detection module is at a smaller
distance from the one field plane than from a closest one of the
pupil planes, radiating electromagnetic radiation onto the optical
module with the illumination system, and determining an angularly
resolved intensity distribution of the radiated radiation from a
signal recorded by the detection module.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 14/796,328, filed Jul. 10, 2015, which is a continuation
of U.S. patent application Ser. No. 13/935,972, filed Jul. 5, 2013,
now U.S. Pat. No. 9,081,294, which claims benefit under 35 U.S.C.
.sctn. 119(e) to U.S. Provisional Application No. 61/668,554, filed
Jul. 6, 2012, and which claims priority under 35 U.S.C. .sctn.
119(a) to German Patent Application No. 10 2012 211 846.2, also
filed on Jul. 6, 2012. The disclosures of these related
applications are hereby incorporated into the present application
by reference in their respective entireties.
FIELD OF AND BACKGROUND OF THE INVENTION
[0002] The invention relates to a method for measuring an angularly
resolved intensity distribution in a reticle plane of a projection
exposure apparatus for microlithography, and to a projection
exposure apparatus comprising a measurement system for measuring an
angularly resolved intensity distribution. Furthermore, the
invention relates to a method for measuring a beam divergence in an
illumination system of a projection exposure apparatus for
microlithography, and to a projection exposure apparatus.
[0003] Lenses of projection exposure apparatuses for
microlithography are being operated with ever smaller k1 factors.
This has the consequence that a precise measurement and
specification of the illumination system of such projection
exposure apparatuses is becoming more and more important. Such an
illumination system irradiates a reticle to be imaged with a
previously selected angular distribution. The angular distribution
chosen is also designated as an illumination setting. Examples of
such illumination settings include annular illumination, dipole
illumination and quadrupole illumination. In particular, it is
important to be able to characterize the illumination setting even
when the projection lens and the illumination system are integrated
in a projection exposure apparatus.
[0004] The illumination setting in a projection exposure apparatus
is conventionally characterized by measuring a so-called
"pupilogram" with a sensor integrated in the wafer stage of the
projection exposure apparatus. A definition of a pupilogram is
included e.g. in the article by Joe Kirk and Christopher Progler
"Pinholes and pupil fills", Microlithography World, Autumn 1997,
pages 25 to 34. In order to generate the pupilogram, a special
measurement reticle with pinhole structures arranged thereon is
loaded into the reticle plane of the projection exposure apparatus.
The above-mentioned sensor is generally arranged for measurement
purposes below the wafer plane, to be precise in a conjugate pupil
plane of the projection lens. This has the effect that the
intensity distribution which is present in the pupil plane and
which corresponds to the angularly resolved intensity distribution
in the reticle plane is imaged on the sensor.
[0005] If this method is used during the projection operation of
the projection exposure apparatus, then generally a relatively
significant time delay arises since firstly the production reticle
has to be removed from the wafer stage and the pinhole reticle has
to be loaded for carrying out the measurement. Furthermore, in the
case of generally older projection exposure apparatuses, the
problem often arises that such apparatuses are not provided with a
suitable sensor arranged below the wafer plane for carrying out the
pupilogram measuremen.
OBJECTS AND SUMMARY OF THE INVENTION
[0006] It is an object of the invention to solve the problems
mentioned above and, in particular, to provide a measuring method
and a projection exposure apparatus whereby it is possible to
measure the angularly resolved intensity distribution in the
reticle plane during the projection operation of the projection
exposure apparatus without thereby creating a long time delay.
[0007] The above object is achieved according to one formulation of
the invention for example by the method mentioned below for
measuring an angularly resolved intensity distribution in a reticle
plane of a projection exposure apparatus for microlithography. The
projection exposure apparatus comprises an illumination system,
which is configured for irradiating a reticle arranged in the
reticle plane and has a first pupil plane. All planes of the
projection exposure apparatus which are conjugate with respect to
the first pupil plane are further pupil planes. The reticle plane
and all planes which are conjugate with respect thereto are field
planes. The method according to this formulation of the invention
comprises: arranging an optical module in the beam path of the
projection exposure apparatus, and arranging a spatially resolving
detection module in the region of one of the field planes in such a
way that the detection module is at a smaller distance from this
field plane than from the closest pupil plane. Furthermore, the
method comprises: radiating electromagnetic radiation onto the
optical module with the illumination system, and determining an
angularly resolved intensity distribution of the radiated radiation
from a signal recorded by the detection module.
[0008] In other words, according to this formulation of the
invention, the angularly resolved intensity distribution in the
reticle plane is measured with an optical module and a detection
module. The detection module is disposed downstream of the optical
module in the beam path of the projection exposure apparatus. The
detection module is arranged in the region of one of the field
planes of the projection exposure apparatus, for example in the
reticle plane or the wafer plane. In this case, an arrangement in
the region of one of the field planes is understood to mean that
the distance between the detection module, in particular a
detection area of the detection module, and the relevant field
plane is smaller than the distance between the detection module and
the closest pupil plane. In particular, the distance between the
detection module and the relevant field plane is a maximum of half,
in particular a maximum of one quarter, of the distance between the
detection module and the closest pupil plane. The detection module
is therefore situated substantially in the near field of the
corresponding field plane.
[0009] In accordance with the method according to this formulation
of the invention, for example, the optical module together with the
detection module can be inserted above the production reticle
loaded from the reticle stage. It is thus possible to carry out the
illumination setting during production operation, e.g. between the
exposure of two wafers, without further loss of time. Moreover,
according to the method according to the invention it is not
necessary to provide a sensor arranged below the wafer plane in the
wafer displacement stage, also called wafer stage. Therefore, the
method according to the invention can be carried out, in
particular, also in the case of older projection exposure
apparatuses which do not have a corresponding sensor incorporated
in their wafer displacement stage.
[0010] In accordance with one embodiment according to the
invention, the optical module is arranged in the region of one of
the field planes in such a way that the optical module, in
particular the upper edge of the optical module that is irradiated
during the measurement, is at a smaller distance from this field
plane than from the closest pupil plane. In particular, the optical
module is arranged in the region of one of the field planes in such
a way that the optical module is at a distance of a maximum of
half, in particular a maximum of one quarter, of the distance
between the optical module and the closest pupil plane.
[0011] In accordance with a further embodiment according to the
invention, the optical module of the detection module is arranged
in the region of the same field plane. The optical module and the
detection module can thus be integrated into a measurement system
with a compact construction.
[0012] In accordance with a further embodiment according to the
invention, the optical module is arranged in the region of the
reticle plane during the measurement. This makes it possible to
measure the angularly resolved intensity distribution particularly
precisely, since no corruption can occur for instance as a result
of imaging aberrations of interposed optical elements. In
accordance with a further embodiment according to the invention,
the detection module is likewise arranged in the region of the
reticle plane.
[0013] In accordance with a further embodiment according to the
invention, the optical module comprises two diffraction gratings
arranged successively in the beam path of the incoming radiation.
Preferably, the measurement system comprising the optical module
and the detection module is designed as a shearing interferometer.
In accordance with one embodiment, the relative situation and thus
the relative position and/or the relative orientation of the phase
gratings with respect to one another is varied during the
measurement of the intensity distribution.
[0014] In accordance with a further embodiment according to the
invention, a spatial coherence function is recorded by the
detection module and the spatial coherence function is thereupon
converted into the angularly resolved intensity distribution in the
reticle plane. The conversion can be effected using the van
cittert-zernike theorem.
[0015] In accordance with a further embodiment according to the
invention, the optical module comprises a focusing optical element,
with which the radiated radiation is focused onto the detection
module. The detection module is thus arranged in the region of a
focal plane of the focusing optical element.
[0016] In accordance with a further embodiment, already discussed
above, according to the invention, for the purpose of measuring the
angularly resolved intensity distribution, a measurement system
comprising the optical module and the detection module is inserted
into the beam path of the radiated radiation above a reticle
arranged in the reticle plane. In the case of this variant, the
angularly resolved intensity distribution can be measured with a
product reticle arranged in the reticle plane.
[0017] In accordance with a further embodiment according to the
invention, the optical module is fixed to an edge region of a
reticle displacement stage of the projection exposure apparatus, in
particular integrated into the edge region. In this context, an
edge region should be understood to mean a region of the reticle
which, with the reticle loaded, is not concealed by the latter.
[0018] In accordance with a further embodiment according to the
invention, the angularly resolved intensity distribution is
measured for a plurality of field points in the reticle plane
simultaneously.
[0019] In accordance with a further embodiment according to the
invention, the detection module comprises an integral spatially
resolving area sensor. An appropriate spatially resolving area
sensor is a CCD sensor, for example.
[0020] In accordance with a further embodiment according to the
invention, the detection module comprises at least two point
sensors separated from one another. By way of example, a
four-quadrant diode can be used.
[0021] In accordance with a further embodiment according to the
invention, the optical module comprises a shadow casting element,
which is at least partly non-transmissive to the radiated
radiation, such that at least one shaded region is generated on a
detection module. For the purpose of determining the angularly
resolved intensity distribution, the position of a transition from
the shaded region to an unshaded region adjacent thereto is
determined using the detection module. In accordance with one
variant, the shadow casting element is designed as a grating.
Furthermore, the detection area of the detection module can be
provided with measurement markings in the form of a further
grating.
[0022] In accordance with a further embodiment according to the
invention, the optical module has a pole selection device, which
comprises a pinhole stop and a blocking element for blocking part
of the radiated radiation, said blocking element being offset with
respect to said pinhole stop in a direction transversely with
respect to the reticle plane. The blocking element can be
configured e.g. as a stop.
[0023] The abovementioned object can be achieved according to a
further formulation of the invention with a projection exposure
apparatus for microlithography. The projection exposure apparatus
according to the invention comprises an illumination system, which
is configured for irradiating a reticle arranged in a reticle plane
of the projection exposure apparatus with electromagnetic
radiation. The illumination system has a first pupil plane, wherein
all planes which are conjugate with respect to the first pupil
plane are further pupil planes of the illumination system. The
reticle plane and all planes which are conjugate with respect
thereto are field planes. The projection exposure apparatus
according to the invention furthermore comprises a measurement
system, which has an optical module and a detection module and is
configured for measuring an angularly resolved intensity
distribution of the radiated radiation in the reticle plane.
Furthermore, the projection exposure apparatus comprises a
positioning device for arranging the detection module in the region
of one of the field planes in such a way that the detection module
is at a smaller distance from this field plane than from the
closest pupil plane. The optical module is configured according to
the invention for generating a radiation distribution on the
detection module arranged in the region of the field plane, from
which radiation distribution it is possible to determine the
angularly resolved intensity distribution in the reticle plane.
[0024] The explanations and features given above with regard to the
method according to the invention can be applied to the projection
exposure apparatus according to the invention.
[0025] In the development of lithography processes for the
production of semiconductor chips, taking account of diffraction
effects that occur during the imaging of the mask structures onto
the wafer is often of great importance. Therefore, the mask
structures on the production reticles used are correspodningly
modified for the correction of said diffraction effects. This
correction is generally designated as optical proximity correction
(OPC). The optimization of the optical proximity correction
conventionally often takes place on a reference exposure apparatus.
However, the masks optimized via the reference exposure apparatus
are subsequently used on a plurality of projection exposure
apparatus. Therefore, there is a need to perform optical proximity
corrections separately in a tailored manner with regard to each
individual exposure apparatus. One parameter which significantly
influences the optical proximity correction is the beam divergence
of the exposure radiation in the input region of the illumination
system of the projection exposure apparatus. Such an illumination
system has a beam expanding optical unit for expanding the beam
cross section of the exposure radiation emerging from a radiation
source. The divergence of the beam directly downstream of the beam
expanding optical unit has a considerable influence on the optical
proximity correction.
[0026] In principle, it is conceivable to measure the beam
divergence directly at the output of the beam expanding optical
unit e.g. using suitably adapted sensors according to the
Shack-Hartmann principle. However, such a measurement requires a
service action at the projection exposure apparatus, which results
in a production stoppage of the apparatus. Therefore, it is an aim
to measure the beam divergence in an input region of the
illumination system of a projection exposure apparatus with a high
accuracy, without having to interrupt the operation of the exposure
apparatus for a long time.
[0027] For this purpose, in accordance with a further aspect of the
invention, a method for measuring a beam divergence in an
illumination system of a projection exposure apparatus, for
microlithography is provided, wherein the illumination system has a
beam expanding optical unit for expanding the beam cross section of
an exposure radiation emerging from a radiation source, and also a
beam angle redistribution module for deflecting partial beams of
the exposure radiation emerging from the beam expanding optical
unit. The method according to this aspect of the invention
comprises: exchanging at least one element of the beam angle
redistribution module for a divergence amplification module, which
is configured for amplifying the divergence of the beam emerging
from the beam expanding optical unit, and measuring the
illumination angle distribution in the reticle plane and
determining the divergence of the exposure radiation emerging from
the beam expanding optical unit from the measured illumination
angle distribution.
[0028] Depending on the embodiment, the beam angle redistribution
module can be formed by a diffractive optical element or else
comprise a plurality of optical elements. As a result of the
exchange according to the invention of an element of the beam angle
redistribution module, in particular of the diffractive optical
element, for a divergence amplification module, the divergence in
an input section of the illumination system, in particular directly
after the beam expanding optical unit, is increased in a
predetermined manner.
[0029] As mentioned above, for example for optimizing the optical
proximity correction of lithography masks there is great interest
in accurately knowing the divergence in said input section of the
illumination system, also designated hereinafter as input
divergence. The divergence amplification according to the invention
makes it possible to determine the input divergence with high
accuracy using an angularly resolved measurement of the intensity
distribution at a position disposed downstream of the illumination
system in the beam path of the projection exposure apparatus.
[0030] Thus, by way of example, the measurement can be effected
with the pupilogram measurement already mentioned above, which is
known from the prior art and already provided in many projection
exposure apparatuses. In accordance with one embodiment, for this
purpose, a pinhole reticle is loaded into the reticle plane and the
pupilogram generated is recorded by a sensor integrated in the
wafer stage. The divergence at the location of the divergence
amplification module is calculated from the recorded pupilogram.
From this value in turn, the divergence amplification is calculated
and the divergence in the input section of the illumination system
during normal operation is thereby determined.
[0031] The divergence amplification according to the invention
increases the effective accuracy of the measurement of the input
divergence calculated back. Without the divergence amplification
according to the invention for the input divergence, the pupilogram
measuring method would only yield measurement results with low
accuracy. The accuracy achievable in this case would not suffice
for instance for optimizing the proximity correction of the reticle
structures.
[0032] The invention thus opens up the possibility of using
measuring systems already present in the projection exposure
apparatus for the divergence measurement and thereby determining
the required divergence measurement values in a cost-effective
manner. Furthermore, it is also possible to carry out the input
divergence measurement using a separate measurement system arranged
in the region of the reticle plane, for example. In accordance with
one embodiment according to the invention, for this purpose the
measurement system is used in one of the variants according to the
invention, that is to say that the illumination angle distribution
in the reticle plane is measured using the measurement system
described above.
[0033] In accordance with one embodiment--already discussed
above--of the method according to the further aspect of the
invention, the projection exposure apparatus has a projection lens
for imaging mask structures from the reticle plane onto a wafer,
and a wafer stage for holding the wafer, and the illumination angle
distribution is measured with a detector module arranged at the
wafer stage.
[0034] Furthermore, in accordance with the further aspect of the
invention, a projection exposure apparatus for microlithography
comprising a radiation source for generating exposure radiation,
and an illumination system disposed downstream of the radiation
source and serving for radiating the exposure radiation into a
reticle plane of the projection exposure apparatus, is provided.
The illumination system according to the invention comprises: a
beam expanding optical unit for expanding the beam cross section of
the exposure radiation emerging from the radiation source, a beam
angle redistribution module for deflecting partial beams of the
exposure radiation emerging from the beam expanding optical unit, a
divergence amplification module for amplifying the divergence of
the beam emerging from the beam expanding optical unit, and an
exchanging device for exchanging at least one element of the beam
angle redistribution module for the divergence amplification
module.
[0035] In accordance with one embodiment, the projection exposure
apparatus in accordance with the further aspect according to the
invention has a projection lens for imaging mask structures from
the reticle plane onto a wafer, a wafer stage for holding the
wafer, and a system for measuring the illumination angle
distribution in the reticle plane. The system comprises a detector
module arranged at the wafer stage, said detector module being
arranged in the region of the conjugate pupil plane of the
projection lens during the measurement process. Preferably, for
carrying out the measurement a measurement mask having punctiform
structures is arranged in the reticle plane.
[0036] In a further embodiment--already discussed above--in
accordance with the further aspect according to the invention, the
beam angle redistribution module comprises a diffractive optical
element. The exchanging device thus serves to replace the
diffractive optical element by the divergence amplification
module.
[0037] In a further embodiment in accordance with the further
aspect according to the invention, the divergence amplification
module comprises a microlens element array. In particular, the
divergence amplification module is designed as a microstructured
plate in which the microlens element array is formed by the
microstructuring.
[0038] In a further embodiment in accordance with the further
aspect according to the invention, the divergence amplification
module comprises two microlens element arrays having different
focal lengths. Preferably, the microlens element arrays are spaced
apart from one another in such a way that the divergence
amplification module acts as a telescope optical unit.
[0039] With regard to the further aspect of the invention, too, it
holds true that the features specified with regard to the
abovementioned embodiments of the method according to the invention
can correspondingly be applied to the projection exposure apparatus
according to the invention. Conversely, the features specified with
regard to the abovementioned embodiments of the projection exposure
apparatus according to the invention can correspondingly be applied
to the method according to the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] Exemplary embodiments according to the invention are
explained in greater detail below with reference to the
accompanying schematic drawings, in which:
[0041] FIG. 1 shows a schematic illustration of an embodiment
according to the invention of a projection exposure apparatus for
microlithography comprising a system for measuring an angularly
resolved intensity distribution in a reticle plane of the
projection exposure apparatus,
[0042] FIG. 2 shows a further embodiment according to the invention
of a projection exposure apparatus comprising a measurement system
of the abovementioned type,
[0043] FIG. 3 shows a further embodiment according to the invention
of a projection exposure apparatus comprising a measurement system
of the abovementioned type,
[0044] FIG. 4 shows a schematic sectional view of an embodiment
according to the invention of an optical module of the measurement
system in accordance with one of FIGS. 1 to 3,
[0045] FIG. 5 shows a schematic sectional view of the measurement
system in a further embodiment according to the invention,
[0046] FIG. 6 shows a schematic sectional view of the measurement
system in a further embodiment according to the invention,
[0047] FIG. 7 shows a plan view of a detection module of the
measurement system in an embodiment according to the invention,
[0048] FIG. 8 shows a schematic sectional view of the measurement
system in a further embodiment according to the invention,
[0049] FIG. 9 shows a schematic sectional view of a previously
known multimirror array that is optionally employed in the
illumination system of a projection exposure apparatus,
[0050] FIG. 10 shows a schematic sectional view of an illumination
system of a projection exposure apparatus for microlithography
comprising an exchanging device for the replacement according to
the invention of a diffractive optical element of the illumination
system by a microstructured plate,
[0051] FIG. 11 shows a schematic sectional view of the
microstructured plate in accordance with FIG. 10, and
[0052] FIG. 12 shows a schematic sectional view of a projection
exposure apparatus in a measuring configuration for measuring an
angularly resolved intensity distribution in the reticle plane of
the projection exposure apparatus.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS ACCORDING TO THE
INVENTION
[0053] In the exemplary embodiments described below, elements that
are functionally or structurally similar to one another are as far
as possible provided with the same or similar reference signs.
Therefore, for understanding the features of the individual
elements of a specific exemplary embodiment, reference should be
made to the description of other exemplary embodiments or the
general description of the invention.
[0054] In order to facilitate the description, a Cartesian xyz
coordinate system is indicated in the drawing and reveals the
respective positional relationship of the components illustrated in
the figures. In FIG. 1, y-direction runs perpendicularly to the
plane of the drawing out of the latter, the x-direction runs toward
the right, and the z-direction runs upward.
[0055] FIG. 1 schematically illustrates a projection exposure
apparatus 10 for microlithography in a first embodiment according
to the invention. The projection exposure apparatus 10 comprises,
as is customary in the case of projection exposure apparatuses
known from the prior art, a radiation source 12 for generating
electromagnetic radiation 14 having a wavelength suitable for
lithography of e.g. 365 nm, 248 nm or 193 nm. The projection
exposure apparatus 10 can also be designed for radiation in the
extreme ultraviolet wavelength range (EUV). In this case, the
electromagnetic radiation 14 has a wavelength of <100 nm, e.g.
approximately 13.5 nm.
[0056] An illumination system 16 is disposed downstream of the
radiation source 12 and is configured for radiating the radiated
electromagnetic radiation 14 with a predefined angular distribution
onto a reticle 22 with mask structures arranged thereon. The
predefined angularly resolved intensity distribution generated by
the illumination system 16 is defined by the so-called illumination
setting. Examples of illumination settings include annular
illumination, quadrupole illumination or other, more complex
illumination configurations.
[0057] The illumination system 16 has an optical axis 18 and
generally comprises a plurality of optical elements. A pupil plane
20 is arranged within the illumination system 16, in which pupil
plane the radiation distribution is Fourier-transformed relative to
the reticle plane 24 of the projection exposure apparatus 10. The
reticle 22 is held in a displaceable manner by a reticle
displacement stage 25, also designated as "reticle stage", in the
reticle plane 24.
[0058] The projection exposure apparatus 10 furthermore comprises a
projection lens 26 for imaging the mask structures of the reticle
22 from the reticle plane 24 into a wafer plane 30. During an
imaging process, a wafer 28 is arranged on a wafer displacement
stage 32, also designated as "wafer stage", in the wafer plane
30.
[0059] According to the invention, the projection exposure
apparatus 10 comprises a measurement system 40 for measuring the
angularly resolved intensity distribution of the illumination
radiation 21 radiated into the reticle plane 24. The measurement
system 40 comprises an optical module 42 and a detection module 44.
In the embodiment shown, the optical module 42 is arranged in the
region of the reticle plane 24, to be precise in an edge region of
the reticle displacement stage 25 that is provided therefor. The
detection module 44 is situated directly below the optical module
42.
[0060] In accordance with an embodiment illustrated in FIG. 4, the
optical module 42 comprises two diffraction gratings 46 and 48
arranged successively in the beam path, as shown in FIG. 4. The
incoming illumination radiation 21 is split by the first
diffraction grating 46 into two partial beams 50 and 52 running
obliquely with respect to one another. The two partial beams are
pivoted by the second diffraction grating 48, such that they run
parallel to one another again, as illustrated by the reference
signs 50-1 and 52-1 in FIG. 4. Directly below the second
diffraction grating 48, the interference of the two partial beams
50-1 and 52-1 occurs. The detection module records the interference
pattern generated. The contrast of the recorded interference
pattern contains the information about the spatial coherence of the
illumination radiation 21.
[0061] The coherence function of the illumination radiation 21 is
determined by the evaluation of the recorded interference pattern
with an evaluation device. Preferably, a phase shifting technique
is employed in order to increase the accuracy of the evaluation.
For this purpose, e.g. the second diffraction grating 48 can be
rotated with respect to the first diffraction grating 46 for the
spatial phase shift. Alternatively, the two gratings can be moved
with respect to one another.
[0062] In accordance with a further embodiment the measurements are
carried out with different shear spacings, which are produced e.g.
by the provision of different grating constants or grating spacings
of the diffraction gratings. Furthermore, the measurements can be
effected for different orientation directions of the diffraction
gratings 46 and 48, e.g. for 0.degree. and 90.degree..
[0063] The spatial coherence function determined is thereupon
converted into the angularly resolved intensity distribution of the
illumination radiation 21 in the reticle plane 24 using the van
cittert-zernike theorem. The van cittert-zernike theorem is known
to the person skilled in the art and describes the relationship
between the extent of a light source and the spatial coherence of
the radiation generated thereby.
[0064] FIG. 2 shows a further embodiment of the projection exposure
apparatus 10 according to the invention. The latter differs from
the embodiment in accordance with FIG. 1 in that the measurement
system 40 is not integrated into the edge of the reticle
displacement stage 25. Rather, the optical module 42 and the
detection module 44 of the measurement system 40 in accordance with
FIG. 2 are arranged in different planes of the projection exposure
apparatus 10 during the measurement process. The optical module 42
is arranged in the reticle plane 24, while the detection module 44
is integrated into the wafer displacement stage 32 in such a way
that its detection area is arranged in the wafer plane 30 and thus
in a plane that is conjugate with respect to the reticle plane
24.
[0065] In accordance with a variant according to FIG. 2, the
optical module 42 is designed in the form of a reticle and is
loaded instead of a product reticle 22 by the reticle displacement
stage 25 in order to carry out the measurement process. The
measurement is effected analogously to the procedure described
above. In accordance with a further variant of the embodiment
according to FIG. 1, it is also possible to integrate the optical
module 42 in an edge region of the reticle displacement stage 25
and to use for the measurement a detection module 44 integrated
into the wafer displacement stage 32 in accordance with FIG. 2.
[0066] FIG. 3 illustrates a further embodiment of a projection
exposure apparatus 10 according to the invention. The latter
differs from the projection exposure apparatus 10 in accordance
with FIG. 1 in that the measurement system 40 is not integrated
directly into the reticle displacement stage 25, but rather is
inserted into the beam path of the illumination radiation 21 just
above the reticle plane 24 using a suitable displacement device 45
in order to carry out the measurement. This has the advantage that
for the measurement a reticle 22 arranged in the reticle
displacement stage 25 need not be removed and at the same time a
compact construction of the reticle displacement stage 25 is
possible. The arrangement of the measurement system 40 above the
reticle plane 24 is effected so close to the reticle plane 24 that
the measurement system 40 is situated closer to the reticle plane
24 than to the closest pupil plane 20 of the illumination system
16. Preferably, the distance from the closest pupil plane 20 is
more than double the magnitude of the distance from the reticle
plane 24.
[0067] In the embodiment in accordance with FIG. 3, the optical
module 22 and the detection module 44 are arranged close to one
another, with the result that the measurement system 40 is present
with a compact design. According to an alternative variant, the
entire measurement system 40 in accordance with FIG. 3 can also be
integrated into the wafer displacement stage 32. In this case, it
is necessary to remove the reticle 22 and the wafer 28 before the
measurement.
[0068] FIG. 5 shows a further embodiment of the measurement system
40. This embodiment is likewise characterized by a compact design
and can be used in the projection exposure apparatus 10 for
instance in the variants in accordance with FIG. 1 and FIG. 3. The
system 40 in accordance with FIG. 5 comprises a focusing optical
element, e.g. in the form of an optical lens element or a focusing
mirror, as optical module 42. The detection module 44 is designed
as an areally resolving sensor and is arranged at the focal point
of the focusing optical element. In the embodiment in accordance
with FIG. 5, for measuring the entire illuminating field in the
reticle plane 24, the measurement system 40 is displaced
successively to different field points by a displacement device 49.
In the case of the arrangement of the measurement system 40 in an
edge region of the reticle displacement stage 25 analogously to
FIG. 1, the reticle displacement stage 25 can serve as displacement
device 49. In the case of the arrangement of the measurement system
40 above the reticle plane 24 in accordance with FIG. 3, the
displacement device 49 is designed as a separate system.
[0069] FIG. 6 illustrates a further embodiment of a measurement
system 40 according to the invention. It differs from the
measurement system 40 in accordance with FIG. 5 in that the optical
module 42 is formed by a two-dimensional arrangement of focusing
optical elements 54. The detection module 44 is designed as an
areal, spatially resolving sensor and comprises a CCD module, for
example. Preferably, the extent of the arrangement of focusing
optical elements 54 in the x-y plane extends over the entire
illuminated field in the reticle plane 24. It is thus possible to
measure the angular distribution of the illumination radiation 22
in the entire illuminated field in parallel.
[0070] FIG. 7 shows a further embodiment of the detection module 44
in accordance with FIG. 5. In accordance with this embodiment, the
detection module 44 is not embodied as an area sensor, but rather
comprises only a limited number of point sensors. Thus, for
instance, two or four point sensors can be provided. As illustrated
in FIG. 7, the detection module 44 can be embodied as a
four-quadrant diode, for instance. Under (a) in FIG. 7, such a
four-quadrant diode is illustrated for the case in which the
illumination radiation 21 is present as coherent illumination, and
for the case of quadrupole illumination under (b). Alternatively,
it is also possible to capture the intensity with optical fibers
and to carry out detection outside the measurement system 40.
[0071] The embodiment of the detection module 44 as illustrated in
FIG. 7 makes it possible to detect important information about the
illumination radiation 21. This includes, for instance, in the case
of (a) information about the energetic centering of the
illumination distribution and in the case of (b) information with
regard to the balance of the individual poles. The embodiment of
the detection module 44 as an arrangement of point sensors has the
advantage of low complexity, which facilitates integration and
accelerates the measurement process.
[0072] FIG. 8 illustrates a further embodiment of a measurement
system 40 according to the invention. This measurement system, too,
comprises an optical module 42 and a detection module 44 in the
form of an areally resolving sensor. The optical module 42
comprises a pole selection device 56 and a shadow casting element
61 in the form of a coarse grating, comparable with gratings used
in Moire methods. The pole selection device 56 comprises a first
stop 58 having a cutout 59, and a second stop 60, which is arranged
below the first stop 58 in such a way that it blocks one of the two
poles 21-1 or 21-2 when a dipole-type illumination distribution is
radiated in. It is thus possible to separately measure the
radiation distribution of one of the two poles 21-1 or 21-2.
[0073] The radiation of the illumination pole transmitted by the
pole selection device 56 thereupon passes through the shadow
casting element and is registered by the detection module 44. The
divergence of the radiated pole 21-1 is thereupon determined from
the position of the transition between a shaded region and an
illuminated region on the detector area of the detection module
44.
[0074] In accordance with one variant of the optical module 42 in
accordance with FIG. 8, a further shadow casting element is
arranged in proximity to the detection area of the detection module
44. In this case, a one-dimensionally resolving diode can be used
as detection module 44. In accordance with a further variant, the
intensity distribution measured by the measurement system 40
according to FIG. 8 is evaluated using phase shifting methods. Here
the stops 58 and 60 and/or the shadow casting element 61 and/or a
further shadow casting element arranged on the detection area are
moved or rotated with respect to one another.
[0075] The pole selection device 56 in accordance with FIG. 8 can
also be combined with the optical module 42 in accordance with FIG.
4 if the pole selection device 56 is arranged above the two
shearing gratings 46 and 48.
[0076] The measurement system 40 according to the invention in one
of the embodiments described above is particularly suitable for
measuring illumination settings generated by illumination systems
having a multimirror array. Such a multimirror array, also
designated as MMA, is illustrated in FIG. 9 and designated by the
reference sign 62. It comprises a two-dimensional array of
micromirrrors 64, at least some of which can be tilted at least
with respect to one axis with tilting devices 66. It is thus
possible to manipulate incoming partial beams 68 individually with
regard to their direction of propagation in reflection, such that
the corresponding emerging partial beams 70 acquire a direction of
propagation that can be set individually in each case. Embodiments
in which the illumination system of the projection exposure
apparatus comprises such a multimirror array are described for
example in WO 2009/080279 A1 and WO 2005/026843 A2.
[0077] FIGS. 10 to 12 illustrate a further aspect of the invention.
This aspect relates to the measurement of a beam divergence in an
illumination system of a projection exposure apparatus for
microlithography at a location in the beam path of the illumination
system, which location is designated in greater detail below.
[0078] FIG. 10 illustrates an example of an illumination system 116
at which the beam divergence is measured with the method according
to the invention. Disposed upstream of the illumination system 116
is an irradiation source 112 for generating electromagnetic
radiation, for example having a wavelength in the UV wavelength
range, such as, for instance, 365 nm, 248 nm or 193 nm. The
illumination system 116 comprises a beam expanding optical unit
118, a beam angle redistribution module in the form of a first
diffractive optical element 120, a beam structuring element 123, a
second diffractive optical element 128, an input coupling optical
unit 129, a rod 134, a field stop 136 and a REMA lens 138.
[0079] The electromagnetic radiation 114 in the form of an optical
radiation beam firstly passes through a beam expanding optical unit
118. The latter expands the beam cross section of the beam. The
expanded beam subsequently passes through the first diffractive
optical element 120. The diffractive optical element 120 serves as
a beam angle redistribution module and has the function of
individually deflecting the partial beams of the expanded beam
which are parallel to one another upon impinging on the element
120, in accordance with a predefined angular distribution. The
function of the beam angle redistribution module therefore lies in
generating a desired illumination angle distribution. Instead of
the diffractive optical element 120 shown, the beam angle
redistribution module can e.g. also be formed with the aid of a
multimirror array, the individual mirrors of which are mounted in a
tiltable fashion. An illumination system comprising such a beam
angle redistribution module is described for example in WO
2005/026843 A2.
[0080] The beam of the electromagnetic radiation 114 is thereupon
transferred, with the illumination angle distribution impressed by
the first diffractive optical element 120 into a downstream pupil
plane by the beam structuring module 123. This pupil plane, which
is not illustrated in more specific detail, is situated in
proximity to the second diffractive optical element 128. The beam
structuring module 123 comprises, for the further structuring of
the radiation beam, a zoom system 125, schematically represented by
a moveable lens element, and a so-called axicon, schematically
represented by two optical elements. By moving the axicon elements
apart, it is possible to set the inner sigma of an illumination
setting, or the boundaries of the cross section of the beam of an
illumination setting. Secondly, it is possible to set the outer
sigma of the illumination setting, or generally the outer boundary
of the beam cross section, by the movement of the zoom or of the
lens element illustrated schematically.
[0081] With a suitable design of the diffractive optical element
120 and a suitable choice of the position of the axicon elements
and of the zoom, it is possible to generate any desired intensity
distribution at the output of the beam structuring module 123 in
proximity to the second diffractive optical element 128. A field
angle distribution is impressed on this intensity distribution in
the pupil plane by the second diffractive optical element 128, in
order to obtain a desired field shape in a field plane, such as
e.g. a rectangular field shape having an aspect ratio of 10:1. This
field angle distribution of the beam in the pupil plane is
transferred by the downstream input coupling optical unit 129 into
an illumination field 132 at the input of the rod 134.
[0082] In this case, the illumination field 132 at the input of the
rod 134 is situated in a field plane of the illumination optical
unit 116 and has an illumination angle distribution having a
maximum illumination angle value, which generally, but not
necessarily, corresponds to the numerical aperture of the preceding
input coupling optical unit 129. The illumination field 132 at the
input of the rod 134 is transferred into a field 135 at the output
of the rod 134. In this case, the maximum illumination angles in
the field 135 of the rod output correspond to those in the field
132 of the rod input. As a result of multiple total internal
reflections at the walls of the rod 134, secondary light sources
having the field shape of the field 132 at the rod entrance as the
shape of each individual secondary light source arise at the rod
exit in the exit pupils of the field points of the field 135. As a
result of this kaleidoscope effect of the rod 134, the field 132 is
homogenized with regard to the intensity distribution over the
field, since as it were the light of many secondary light sources
is superimposed in the field 132.
[0083] The field stop 136 delimits the field 135 in the lateral
extent thereof and provides for a sharp bright-dark transition of
the field. The downstream REMA lens 138, as it is called, images
the field 135 onto a reticle 122 arranged in the reticle plane 124.
In this case, the bright-dark edges of the field stop 136 are
transferred sharply into the reticle plane 124.
[0084] The illumination system 116 comprises an exchanging device,
which is illustrated with a double-headed arrow 171 in FIG. 10. The
exchanging device 171 is configured for exchanging the first
diffractive optical element 120 for a divergence amplification
module according to the invention in order to carry out the
divergence measurement. In other embodiments of illumination
systems, not explained in greater detail here, it may be necessary
to exchange a plurality of optical elements in order to carry out
the divergence measurement. In accordance with one embodiment, the
divergence amplification module is configured as 4f lens element
array. As an example of such a 4f lens element array, a
microstructured plate 170 is described in greater detail below with
reference to FIG. 11. A macroscopic telephoto lens is also
considered as a further example of a 4f lens element array.
[0085] The microstructured plate 170 illustrated schematically in
greater detail in FIG. 11 comprises two microlens element arrays
172 and 174 spaced apart from one another. The two microlens
element arrays 172 and 174 are in each case embodied
two-dimensionally, wherein the focal length f.sub.1 of the first
microlens element array 172 is greater than the focal length
f.sub.2 of the second microlens element array 174. The microlens
element arrays 172 and 174 are opposite one another at the distance
d=f1+f2. In order to ensure an optimum integration into the
projection exposure apparatus, the microstructured plate 170 is
produced with the dimensions of the first diffractive optical
element 120. When the microstructured plate 170 is arranged at the
location of the first diffractive optical element 120 in the beam
path of the illumination optical unit 116, the plate 170 fulfils
the function of a telescope optical unit and thus magnifies the
divergence of the incoming beam of the electromagnetic radiation
114. The beam size changes upon passing through the plate 170 in
accordance with m=f.sub.1:f.sub.1, while the divergence is
proportional to 1: m.
[0086] As a result of the dimensioning of f1>f2, as mentioned
above, the beam divergence can be magnified using the plate 170.
This beam divergence magnification makes it possible to carry out
an angular distribution measurement for instance using a pupil
measuring device already known to the person skilled in the art
from the prior art, the functioning of said pupil measuring device
being illustrated in FIG. 12. In accordance with one embodiment
according to the invention, the angular distribution measurement is
carried out using one of the measurement systems 140 according to
the invention which are presented below and have already been
described above. This can be effected using a measurement system 40
which comprises the optical module 42 in accordance with FIG. 4 and
a detection module in one of the arrangements in accordance with
FIGS. 1 to 3. That is to say that at least the optical module 42 in
accordance with FIG. 4 is arranged in the reticle plane 124 or just
above the latter. Furthermore, the angular distribution measurement
can be effected using one of the measurement systems 40 shown in
FIG. 5 and FIG. 6 in an arrangement in accordance with FIG. 1 or
FIG. 3. In this case, the measurement system as a whole is arranged
in the reticle plane 124 or just above the latter.
[0087] Furthermore, in principle, a measurement of the beam
divergence at the location of the first diffractive optical element
120 can also be effected, for example by the measurement system 40
that results from the combination of the measurement module 42 in
accordance with FIG. 4 with a detection module 44 or the
measurement system in accordance with FIG. 5 or FIG. 6 being
inserted into the beam path of the illumination system 116 in the
place of the first diffractive optical element.
[0088] FIG. 12 schematically shows a projection exposure apparatus
110 having the radiation source 112 and the illumination system
116. The projection exposure apparatus 110 furthermore comprises a
projection lens 126 and an area-resolving detection module 144
integrated into a wafer displacement stage. For pupil measurement,
a special measurement reticle 122a is loaded into the reticle plane
124. The measurement reticle 122a has a multiplicity of punctiform
test structures 176, which can be embodied either as so-called
pinholes or else as opaque punctiform structures.
[0089] If the detection module 144 is then arranged in a conjugate
pupil plane at a position above or below the image plane 130 of the
projection lens 126, the intensity distribution in a pupil plane
178 of the projection lens 126 is generated on the detection area
144a of the detector module 144. Said intensity distribution is
also designated as "pupilogram" in the general part of the
description. The position of the detection module 144 above the
image plane 130 is also designated as intrafocal position and is
identified by the reference sign 180 in FIG. 12. The position below
the image plane 130 is analogously designated as extrafocal
position 182.
[0090] The intensity distribution measured by the detection module
144 when the latter is arranged in the position 180 or 182
corresponds to the angular distribution in the plane in which the
first diffractive optical element 120 is usually arranged. However,
the measurement accuracy of the method described with reference to
FIG. 12 does not suffice for the purposes of determining the
divergence of the electromagnetic radiation 114 at the output of
the beam expanding optical unit 118. At the output of the beam
expanding optical unit 118, the beam divergence is typically 1
mrad, which corresponds approximately to a divergence of 20 msigma
(FWHM) in the case of a long zoom focal length. In the case of the
pupilogram measurement using the arrangement from FIG. 12, 1 pixel
of the detector module corresponds to approximately 10 msigma, i.e.
changes in divergence of the relevant order of magnitude of
approximately 0.1 mrad are hardly measurable.
[0091] In order to solve this problem, as mentioned above, the
microstructured plate 170, instead of the diffractive optical
element 120, is inserted into the beam path directly at the output
of the beam expanding optical unit 118. By virtue of the beam
expansion effected thereby, using the arrangement from FIG. 12 the
beam divergence can now be measured with an accuracy which suffices
for carrying out the optimization of optical proximity corrections
on product reticles as described in the general part of the
description.
[0092] The above description of the preferred embodiments has been
given by way of example. From the disclosure given, those skilled
in the art will not only understand the present invention and its
attendant advantages, but will also find apparent various changes
and modifications to the structures and methods disclosed. The
applicant seeks, therefore, to cover all such changes and
modifications as fall within the spirit and scope of the invention,
as defined by the appended claims, and equivalents thereof.
LIST OF REFERENCE SIGNS
[0093] 10 Projection exposure apparatus [0094] 12 Radiation source
[0095] 14 Electromagnetic radiation [0096] 16 Illumination system
[0097] 18 Optical axis [0098] 20 Pupil plane [0099] 21 Illumination
radiation [0100] 21-1 First pole [0101] 21-2 Second pole [0102] 22
Reticle [0103] 24 Reticle plane [0104] 25 Reticle displacement
stage [0105] 26 Projection lens [0106] 28 Wafer [0107] 30 Wafer
plane [0108] 32 Wafer displacement stage [0109] 40 Measurement
system [0110] 42 Optical module [0111] 44 Detection module [0112]
45 Displacement device [0113] 46 First diffraction grating [0114]
48 Second diffraction grating [0115] 49 Displacement device [0116]
50 First partial beam [0117] 50-1 Pivoted partial beam [0118] 52
Second partial beam [0119] 52-2 Pivoted partial beam [0120] 54
Focusing optical element [0121] 56 Pole selection device [0122] 58
First stop [0123] 59 Cutout [0124] 60 Second stop [0125] 61 Shadow
casting element [0126] 62 Multimirror array [0127] 64 Micro mirror
[0128] 66 Tilting device [0129] 68 Incoming partial beam [0130] 70
Emerging partial beam [0131] 110 Projection exposure apparatus
[0132] 112 Radiation source [0133] 114 Electromagnetic radiation
[0134] 116 Illumination system [0135] 118 Beam expanding optical
unit [0136] 120 First diffractive optical element [0137] 121
Illumination radiation [0138] 122 Reticle [0139] 122a Measurement
reticle [0140] 123 Beam structuring module [0141] 124 Reticle plane
[0142] 125 Zoom system [0143] 126 Projection lens [0144] 127 Axicon
[0145] 128 Second diffractive optical element [0146] 129 Input
coupling optical unit [0147] 130 Image plane [0148] 132
Illumination field [0149] 134 Rod [0150] 135 Field [0151] 136 Field
stop [0152] 138 REMA lens [0153] 144 Detection module [0154] 144a
Detection area [0155] 170 Microstructured plate [0156] 171
Exchanging device [0157] 172 First microlens element array [0158]
174 Second microlens element array [0159] 176 Test structure [0160]
178 Pupil plane [0161] 180 Intrafocal position [0162] 182
Extrafocal position
* * * * *