U.S. patent application number 13/109383 was filed with the patent office on 2011-09-08 for method for adjusting a projection objective.
This patent application is currently assigned to CARL ZEISS SMT GMBH. Invention is credited to Wolfgang EMER.
Application Number | 20110216303 13/109383 |
Document ID | / |
Family ID | 34911031 |
Filed Date | 2011-09-08 |
United States Patent
Application |
20110216303 |
Kind Code |
A1 |
EMER; Wolfgang |
September 8, 2011 |
METHOD FOR ADJUSTING A PROJECTION OBJECTIVE
Abstract
A projection objective having a number of adjustable optical
elements is optimized with respect to a number of aberrations by
specifying a set of parameters describing imaging properties of the
objective, each parameter in the set having an, absolute value at
each of a plurality of field points in an image plane of the
projection objective. At least one of the optical elements is
adjusted such that for each of the parameters in the set, the field
maximum of its absolute value is minimized.
Inventors: |
EMER; Wolfgang; (Aalen,
DE) |
Assignee: |
CARL ZEISS SMT GMBH
Oberkochen
DE
|
Family ID: |
34911031 |
Appl. No.: |
13/109383 |
Filed: |
May 17, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12060543 |
Apr 1, 2008 |
7965377 |
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13109383 |
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11102102 |
Apr 8, 2005 |
7372545 |
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12060543 |
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60560623 |
Apr 9, 2004 |
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Current U.S.
Class: |
355/77 |
Current CPC
Class: |
G03B 27/52 20130101;
G03F 7/70216 20130101; G03F 7/70191 20130101 |
Class at
Publication: |
355/77 |
International
Class: |
G03B 27/32 20060101
G03B027/32 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 2004 |
DE |
10 2004 035 595.9 |
Claims
1. A method for adjusting a projection objective of a projection
exposure machine of the type used for microlithography in the
fabrication of semiconductor components and having an illumination
device for providing an illumination beam and a plurality of
optical elements in the path of the illumination beam for
projecting an image onto a substrate, at least some of the optical
elements being capable of being adjusted for altering aberrations
of the projection objective, said method comprising the steps of:
(a) establishing an initial polarization of the illumination
device; (b) adjusting the projection objective with respect to at
least one of the aberrations under initial polarization; (c)
changing the polarization of the illumination device from said
initial polarization to a different polarization; and (d) adjusting
the projection objective with respect to said at least one
aberration based on said different polarization.
2. The method of claim 1 wherein at least one of said adjusting
steps comprises the step of satisfying a condition relating to an
absolute value of at least one parameter of a set of parameters,
each parameter in said set being a parameter representing
respective imaging property of the projection objective and having
an absolute value at each of a plurality of points in a field of an
image plane of the projection objective.
3. The method of claim 2 wherein said condition is a condition
requiring reduction of the absolute value of each parameter in said
set at the one of said plurality of points in said field at which
said absolute value is a maximum.
4. The method of claim 2 wherein said condition is a condition
requiring minimization of the absolute value of each parameter in
said set at the one of said plurality of points in said field at
which said absolute value is a maximum.
5. The method of claim 2 wherein said adjusting step is carried out
using a nonlinear numerical method.
6. The method of claim 2 wherein one of said absolute values is a
maximum of all of said absolute values of all of said parameters
and said condition is a condition requiring reduction of said
maximum of all of said absolute values of all of said
parameters.
7. The method of claim 2 wherein one of said absolute values is a
maximum of all of said values of all of said parameters and said
condition is a condition requiring minimization of said maximum of
all of said absolute values of all of said parameters.
8. The method of claim 2 wherein said adjusting step is carried out
using a nonlinear numerical method.
9. The method of claim 2 further comprising the steps of changing
an illumination setting of the illumination device after said
adjusting step has been carried out, and thereafter repeating said
adjusting step.
10. The method of claim 9 wherein said illumination setting
comprises a setting selected from the group consisting of a
coherence setting, a diameter setting, and a polarization
setting.
11. The method of claim 2 wherein at least one of said parameters
comprises individual Zernike coefficients describing wave
aberrations of an objective pupil of the projection objective.
12. The method of claim 2 wherein at least one of said parameters
comprises a linear combination of Zernike coefficients.
13. The method of claim 2 wherein at least one of said parameters
comprises an average of Zernike coefficients over a plurality of
field points lying within a portion of the image plane defined by a
scanner slit, said plurality of points lying along a line oriented
in a scanning direction.
14. The method of claim 2 wherein said average is a weighted
average.
15. The method of claim 2 wherein said set of parameters is a set
which includes at least one parameter selected from the group
consisting of distortion and line width.
16. The method of claim 2 wherein said set of parameters includes
at least one parameter describing a centrable aberration, and said
adjusting step comprises the step of tilting a reticle of the
projection objective to adjust for said centrable aberration.
17. The method of claim 16 wherein said adjusting step comprises at
least one of the following steps: (i) displacing at least one of
the optical elements in a direction perpendicular to an optical
axis of the projection objective, and (ii) tilting at least one of
the optical elements in a direction perpendicular to said optical
axis of the projection objective.
18. The method of claim 16 wherein said set of parameters further
includes at least one parameter describing a tunable aberration,
and said adjusting step comprises the step of adjusting at least
one of the optical elements to adjust for said tunable aberration
and said centrable aberration jointly.
19. The method of claim 18 wherein said adjusting step comprises at
least one of the following steps: (i) displacing at least one of
the optical elements in a direction along the optical axis of the
projection objective; (ii) changing the wavelength of illumination
of the projection objective; (iii) changing a temperature within
the projection objective; (iv) changing an air pressure within the
projection objective; and (v) changing the composition of a purge
gas surrounding the optical elements.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of, and claims priority under 35
U.S.C. .sctn.120 to, U.S. application Ser. No. 11/102,102 filed
Apr. 8, 2005 which, in turn claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Provisional Application Ser. No. 60/560,623
filed Apr. 9, 2004 and under 35 U.S.C. .sctn.119 to German Patent
Application No. DE 10 2004 035 595.9 filed Jul. 22, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a method for adjusting a projection
objective of a projection exposure machine for microlithography for
the purpose of fabricating semiconductor components having a number
of optical elements, which can be set via manipulators, for
simultaneously minimizing a number of aberrations of the projection
objective, the minimization of the aberrations being carried out by
manipulating at least one portion of the optical elements with the
aid of their respective manipulators.
[0004] 2. Description of the Prior Art
[0005] EP 1 231 516 A2 discloses a method for specifying,
fabricating and adjusting a projection objective. For the
specification, that is to say the description of the optical
properties of the projection objective, use is made in this case of
a description of the transmission function of the objective pupil
for a number of field points. Field points represent a specific
position in the object or image plane of the projection objective.
The scalar transmission function of the objective pupil can be
specified for each field point in the form of a two-dimensional
complex variable. The phase of this complex variable is also
denoted wave aberration. EP 1 231 516 A2 describes these wave
aberrations for each field point by means of so-called Zernike
coefficients. Consequently, the image-forming properties of the
projection objective are likewise specified by the specification of
these Zernike coefficients.
[0006] In order to ensure optimum use of the projection objective
in a projection exposure machine for microlithography, for example
for the purpose of fabricating semiconductor components, the
above-described specification of the image-forming properties of
the projection objective is very important although, of course, in
addition to accurate knowledge relating to the lithographic process
to be carried out with the aid of the projection exposure machine
(illumination, precision of the structures to be exposed,
photo-resist process, etc.). In general, it is not only a single
lithographic process which is relevant, but rather it must even be
possible to carry out a multiplicity of various lithographic
processes with the aid of the projection objective. In order for it
to be possible to find a relationship between this multiplicity of
lithographic processes and the properties of the projection
objective, their most general description of the image-forming
properties of the projection objective is sensible.
[0007] The relationship between the objective properties and the
lithographic process is established in EP 1 231 516 A2 with the aid
of these Zernike coefficients. This can be accomplished in many
cases with the aid of a linear model, given the assumption of
sufficiently small aberrations.
[0008] Following on from the fabrication of a projection objective,
a concluding optimization by means of the manipulators
(xy-manipulation, tilt manipulation, z-manipulation, wavelength,
gas pressure or reticle tilt and reticle height) of the optical
elements located in the projection objective is important in
deciding the final image forming quality or the image-forming
properties of the projection objective.
[0009] It is known to introduce slight changes in the optical
properties by measuring parameters for which the effects of the
manipulation for the optical elements on these parameters are
known, whereupon optimization of the parameters is carried out. As
described at the beginning, Zernike coefficients which describe the
image-forming properties of the projection objective are determined
as a rule for this purpose from measured values. This is achieved,
for example, by means of measurements at a number of field points
in the field, relevant for lithographic imaging, in the image plane
of the projection objective. Zernike coefficients with designations
Z.sub.2 to Z.sub.37 are determined in this way (compare EP 1 231
516 A2), after which the optimization is performed. The average
root-mean-square deviation of all the measured field points from 0
is minimized for this purpose in the case of each Zernike
coefficient (so-called least square optimization).
SUMMARY OF THE INVENTION
[0010] The present invention is based on the object of further
improving a method of the type mentioned at the beginning; in
particular, the aim is to specify the image-forming properties of
the projection objective as faithfully as possible to reality.
[0011] This object is achieved according to the invention by virtue
of the fact that the adjustment is carried out by means of min-max
optimization of a number of parameters, suitable for describing the
imaging properties of the projection objective, at various field
points of an image plane of the projection objective, as a result
of which the individual parameters are optimized in such a way that
the parameter value of the field point which has the maximum
aberration is optimized, that is to say is generally minimized or
at least reduced.
[0012] The min-max optimization is also denoted synonymously as
minimax optimization.
The measures according to the invention provide greatly improved
concepts for adjusting lithography objectives and for optimizing
their manipulators. The use of the nonlinear min-max optimization
is advantageous in particular when the field maximum of the Zernike
coefficients is used to specify the image-forming quality of the
projection objective, since the min-max optimization optimizes
precisely this field maximum. A min-max optimization of a
projection objective is understood to be the optimization of a set
of parameters at various field points in the image plane of the
projection objective. Each individual parameter is optimized in
this case such that the worst value of all the field points is
optimal. Since it is not initially established at which field point
this worst value occurs, this optimization is a nonlinear method
for whose solution known numerical methods can be used. The various
parameters can feature in the optimization with different
weightings. Moreover, it is possible to introduce secondary
conditions in order, for example, to limit the maximum traverse
paths of the manipulators. It is conceivable, furthermore, to
combine a number of field points, in which case a parameter at a
field point is replaced by a function of this parameter at a number
of field points.
[0013] Possible as parameters are, for example, individual Zernike
coefficients to describe the wave aberrations in the objective
pupil.
[0014] Also conceivable as parameter is a linear combination of
Zernike coefficients which describes lithographically important
variables such as distortion or structure width.
[0015] Advantageous refinements and developments of the invention
follow from the dependent claims. Exemplary embodiments are
described in principle below with the aid of the drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 shows a schematic representation of a projection
exposure machine for microlithography which can be used to expose
structures on wafers coated with photosensitive materials;
[0017] FIG. 2 shows an illustration of a scanner slit within a full
image plane of a projection objective;
[0018] FIG. 3 shows a graph of a Zernike coefficient Z.sub.7 after
a tilt optimization with the aid of z-manipulators;
[0019] FIG. 4 shows a graph of a profile of an optimization of an
astigmatism aberration;
[0020] FIG. 5 shows a graph of a profile of a Zernike coefficient
Z.sub.2 with and without joint optimization of reticle tilt and
xy-manipulation;
[0021] FIG. 6 shows a graph of a profile of a Zernike coefficient
Z.sub.7 with and without joint optimization of reticle tilt and
xy-manipulation; and
[0022] FIG. 7 shows a graph of a distortion for annular and for
coherent illumination setting.
DETAILED DESCRIPTION
[0023] FIG. 1 illustrates a projection exposure machine 1 for
microlithography. This serves for exposing structures on a
substrate coated with photosensitive materials and which in general
overwhelmingly comprises silicon and is denoted as a wafer 2 for
fabricating semiconductor components such as, for example, computer
chips.
[0024] The projection exposure machine 1 essentially comprises in
this case an illumination device 3, a device 4 for accommodating
and exactly positioning a mask provided with a grid-like structure,
a so-called reticle 5 which is used to determine the later
structures on the wafer 2, a device 6 for holding, moving and
exactly positioning this very wafer 2, and an imaging device,
specifically a projection objective with a number of optical
elements such as, for example, lenses 8, which are supported by
mounts 9 and/or manipulators 9' in an objective housing 10 of the
projection objective 7.
[0025] The fundamental functional principle provides in this case
that the structures introduced into the reticle 5 are imaged in a
demagnified fashion onto the wafer 2.
[0026] After exposure has been performed, the wafer 2 is moved on
so that a multiplicity of individual fields each having the
structure prescribed by the reticle 5 are exposed on the same wafer
2.
[0027] The illumination device 3 provides a projection beam 11, for
example light or a similar electromagnetic radiation, required for
imaging the reticle 5 onto the wafer 2. A laser or the like can be
used as source for this radiation. The radiation is shaped in the
illumination device 3 via optical elements (not illustrated) such
that when impinging onto the reticle 5 the projection beam 11 has
the desired properties as regards diameter, polarization, coherence
and the like. The spatial coherence is in this case a measure of
the angular spectrum of the radiation in the reticle plane. This
parameter can be varied by the setting of various illumination
settings.
[0028] An image of the structures of the reticle 5 which are
introduced is produced via the projection beam 11 and transferred
onto the wafer 2 in an appropriately demagnified fashion by the
projection objective 7, as already explained above. The projection
objective 7 has a multiplicity of individual refractive,
diffractive and/or reflective optical elements such as, for
example, lenses 8, mirrors, prisms, plane-parallel plates and the
like, only the lens 8 being illustrated.
[0029] After the fabrication, a concluding optimization of the
manipulators of the optical elements, in particular the lenses 8,
the reticle tilt/reticle height and the wavelength is essential in
deciding the final image-forming quality of the projection
objective 7. In this case, the image-forming quality of the
projection objective is optimized, inter alfa, taking account of
the following aberrations: distortion, field curvature,
astigmatism, coma, spherical aberration and wavefront errors of
higher order.
[0030] It is known from the prior art to introduce slight changes
in the optical properties by measuring parameters in the case of
which the effects of the manipulation of the optical elements on
these parameters are known, whereupon optimization of the
parameters is carried out. As a rule, Zernike coefficients which
describe the image-forming properties of the projection objective
are determined for this purpose from measured values. This is
achieved, for example, by measurements at a number of field points
via an imaging scanner slit (=field in the image plane which is
relevant to lithographical imaging). As described, for example, in
EP 1 231 516 A2, Zernike coefficients with designations Z.sub.2 to
Z.sub.37 are determined in this way, after which the optimization
is performed. For this purpose, the average root-mean-square
deviation of all the measured field points from 0 is minimized for
each Zernike coefficient (so-called least square optimization).
Subsequently, the Zernike coefficients Z.sub.2 to Z.sub.9 are
represented by their corresponding function terms. Zernike
coefficients of higher order are described in EP 1 231 516 A2.
TABLE-US-00001 Zernike coefficient Z.sub.n Function term f.sub.n
Z.sub.2 .rho. .times. cos (.phi.) Z.sub.3 .rho. .times. sin (.phi.)
Z.sub.4 2 .times. .rho..sup.2 - 1 Z.sub.5 .rho..sup.2 .times. cos
(2.phi.) Z.sub.6 .rho..sup.2 .times. sin (2.phi.) Z.sub.7
(3.rho..sup.3 - 2.rho.) .times. cos (.phi.) Z.sub.8 (3.rho..sup.3 -
2.rho.) .times. sin (.phi.) Z.sub.9 6.rho..sup.4 - 6.rho..sup.2 +
1
[0031] It is likewise known to use these Zernike coefficients to
find the relationship between the objective properties and the
lithographic process. Assuming sufficiently small aberrations, this
can be accomplished in many cases with the aid of a linear
model:
L.sub.i=a.sub.2.times.Z.sub.2(i)+a.sub.3.times.Z.sub.3(i)+. . .
+a.sub.n.times.Z.sub.n(i)
[0032] The weighted sum can be truncated after a sufficient number
of terms, since in most cases the weighting factors become small
very rapidly with rising Zernike number n. Of course, it is also
possible to include square terms or terms of even higher order. The
weighting factors a.sub.n can be determined experimentally or by
simulation.
[0033] The variable L.sub.i describes a parameter of the
lithographic process at the field point i. L.sub.i can be, for
example, a horizontal offset of a structure relative to its ideal
position (distortion), or else the deviation from an ideal line
width.
[0034] The fabrication or optimization of a projection objective 7
firstly requires knowledge of the critical lithographic process for
which the projection objective 7 is later to be used. It is then
possible to calculate the appropriate weighting factors a.sub.n for
various Zernike coefficients from this information. Maximum
absolute values can then be derived for various Zernike
coefficients from the prescription as to how far various L.sub.i
may be maximized.
[0035] During optimization of the projection objective 7, various
Zernike coefficients are then minimized at various field points, it
being possible for these also to be various L.sub.i in a specific
instance. Projection objectives 7 are then also usually specified
such that various Zernike coefficients and/or various L.sub.i may
not exceed a maximum absolute value for a specific number of field
points. It is thereby ensured that the image-forming properties of
the projection objective 7 suffice for a representative selection
of lithographic processes.
[0036] FIG. 2 shows the round full image field 20 of the projection
objective 7. All the points on the reticle 5 which lie within this
region can be imaged onto the wafer 2 with the aid of the
projection objective 7. When the projection exposure machine 1 is
used as scanner, it is only a rectangular section, the so-called
scanner slit 21, from the full image field 20 that is used. During
a lithographic exposure with the aid of a scanner, the reticle 5
and the wafer 2 are moved simultaneously during imaging in a plane
perpendicular to the optical axis. The consequence of this is that
a point on the reticle 5 is imaged by various field points of the
projection objective 7. The aberrations relevant to this imaging
are therefore the aberrations of all the field points which lie on
a straight line in the scanner slit 21 which is orientated in the
scanning direction (indicated by arrow 22). In order to describe
the image-forming properties of the projection objective 7 in a
scanner, the relevant parameters such as, for example, Zernike
coefficients, are not specified for individual field points, but
averaged over all the field points in the scanning direction. It is
also possible to introduce a weighting during this averaging in
order to take account of different intensities of illumination in
the course of the scanning operation. The parameters averaged in
such a way are referred to as being scanner integrated.
[0037] FIGS. 3 to 7 illustrate profiles of Zernike coefficients
Z.sub.2 and Z.sub.7 over a number of field points which describe
the aberrations and which, for their part, are determined at
various field points in the scanner slit 21 of the projection
objective 7. Plotted respectively on the x-axis is the x-position
in the scanner slit 21, while the y-axis respectively specifies the
y-deviation of the respective Zernike coefficient from 0 in nm.
[0038] Various manipulators of the projection objective 7 are moved
during the optimization of the image-forming properties. These
manipulators can be subdivided into two classes with the aid of the
symmetry of the induced aberrations:
[0039] 1. Manipulators for optimizing tunable aberrations: Tunable
aberrations are changes in various Zernike coefficients at various
field points, the induced changes being invariant under an
arbitrary rotation about the optical axis (z-axis).
The following come into consideration in this case as
manipulators:
[0040] displacement of lenses 8 or reticle 5 along the optical
axis;
[0041] change in temperature and atmospheric pressure;
[0042] change in wavelength; and
[0043] change in the composition of the purge gas surrounding the
lenses 8.
[0044] 2. Manipulators for optimizing centrable aberrations:
Centrable aberrations are changes in various Zernike coefficients
at various field points, the induced changes in the plane
perpendicular to the optical axis having a marked axis of symmetry.
The following come into consideration in this case as
manipulators:
[0045] displacement of lenses 8 perpendicular to the optical axis;
and
[0046] tilting of lenses 8 or reticle 5 about an axis perpendicular
to the optical axis.
[0047] It is known from the Seidel aberration theory that small
changes in tunable aberrations always have the same field
distribution for a specific Zernike coefficient. This fundamental
"shape" of the aberrations is independent of the type of
manipulator. An equivalent theoretical model exists for centrable
aberrations.
[0048] The following table shows the tunable and centrable
aberrations of lowest order, the tunable aberrations presented here
corresponding to the third order Seidel aberrations. The centrable
aberrations refer in this case to an x-decentering, this
corresponding, however, to a displacement of the lens 8 along the
x-axis. For decenterings along another axis, it is necessary to
rotate the field profiles (with coordinates r, .phi. in the field
for lithographic imaging) correspondingly.
TABLE-US-00002 Zernike Type of Tunable Centrable coefficient
Z.sub.n aberration profile profile Z.sub.2 distortion r .times. cos
(.phi.) r.sup.2 Z.sub.3 distortion r .times. sin (.phi.) Z.sub.4
image surface r.sup.2 Z.sub.5 astigmatism r.sup.2 .times. cos
(.phi.) r .times. cos (.phi.) Z.sub.6 astigmatism r.sup.2 .times.
sin (.phi.) r .times. sin (.phi.) Z.sub.7 coma r .times. cos
(.phi.) r.sup.0 Z.sub.8 coma r .times. sin (.phi.) Z.sub.9
spherical r.sup.0 r .times. cos (.phi.) aberration
[0049] As an example, FIG. 3 shows a profile of a Zernike
coefficient Z.sub.7 of a projection objective 7. The tilt of this
profile can be set by z-manipulators. In accordance with the prior
art, the optimum tilt is achieved by means of a least square
optimization, that is to say the root-mean-square value of the
Zernike coefficient Z.sub.7 is minimized over all the field points.
When, however, the field maximum (=maximum absolute value of a
Zernike coefficient at all the field points in the scanner slit 21)
is used to specify the projection objective 7, it is advantageous
for precisely this field maximum to be set as small as possible.
This is achieved by means of a nonlinear min-max optimization.
[0050] As may be seen from FIG. 3, the field maximum in accordance
with a least square optimization (curve 12a) can be substantially
larger than in accordance with a min-max optimization (curve 12b).
The inventor has established that performance with regard to the
Zernike coefficients Z.sub.7 and Z.sub.9 can be improved by more
than half a nanometer in a significant number of cases for the
projection objectives 7 solely by changing from least square
optimization to min-max optimization. It was possible here for the
field maximum to be lowered from 4.06 nm (least square
optimization) to 2.92 nm (min-max optimization).
[0051] FIG. 4 shows a joint optimization of centrable and tunable
aberrations. The joint optimization of centrable and tunable
aberrations on the projection objective 7 instead of a sequential
procedure additionally permits a more accurate and speedy
optimization of the optical image-forming properties of the
projection objective 7. A profile of a Zernike coefficient Z.sub.7
(coma) whose tunable component (tilt) and centrable component
(offset) are minimized with the aid of a min-max optimization was
selected as an example. The simultaneous optimization of tilt and
offset here delivers a substantially better result than the
sequential min-max optimization of tunable aberrations with the aid
of z-manipulators, and subsequent optimization of centrable
aberrations with the aid of xy-manipulators. A curve 13a shows the
uncorrected Z.sub.7 profile here. A curve 13b is the Z.sub.7
profile with optimized tilt. A curve 13c shows the Z.sub.7 profile
with optimized tilt and offset (optimized sequentially one after
another). As may further be seen from FIG. 4, a simultaneous
optimization of the field maximum with the aid of z-manipulators
(tilt) and xy-manipulators (offset) is the most advantageous (curve
13d), in which case the field maximum of 8.2 nm can be lowered to
5.6 nm in contrast with the sequential method.
[0052] FIG. 5 shows the effect of a reticle tilt (curve 14a) or of
a movement of the xy manipulator (curve 14b) on a profile of the
Zernike coefficient Z.sub.2. FIG. 6 shows the effect of a reticle
tilt (curve 15b) or of a movement of the xy manipulator (curve 15a)
on a Z.sub.7 profile. A Z.sub.2 offset, which corresponds to the
curve 14a, could be removed in the case of the above-described
scenario by means of a reticle tilt. However, when the Z.sub.7
offset corresponding to the curve 15a is removed by the
xy-manipulator in a second step, a Z.sub.2 error is introduced
again into the objective (curve 14b). In the case of a 10 nm
Z.sub.7 offset, this would result nevertheless in an additional
Z.sub.2 error of more than 3 nm. This could be avoided by means of
an orthogonalized concept of xy-manipulators and reticle tilt, for
which purpose it would be necessary to treat the reticle tilt like
any other xy-manipulator.
[0053] It is advantageous to apply a distortion optimization
dependent on the illumination setting of the projection objective
7. The distortion values can be substantially improved in specific
cases by tracking the manipulators during changing of the
illumination setting (for example from annular to coherent). A
geometrical distortion (Z.sub.2) is usually not optimized, but a
combination of geometrical distortion and coma-induced distortion.
All the scanner-integrated Zernike coefficients vanish here, with
the exception of Z.sub.7, the Z.sub.7 profile including higher
tunable components. In the case of the present optimization with
the aid of z-manipulators, a Z.sub.2- component in the projection
objective 7 is then increased so that the resulting distortion
results in an annular illumination setting of 0 (curve 16a in FIG.
7). In the event of a change in setting from annular to coherent,
however, the result in this case is distortion values of up to
approximately 15 nm (curve 16b). It is therefore proposed according
to the invention to track the xy-and z-manipulators (including
wavelength and reticle) during each change in illumination
setting.
* * * * *