U.S. patent application number 14/311767 was filed with the patent office on 2014-10-09 for arrangement for actuating an element in a microlithographic projection exposure apparatus.
The applicant listed for this patent is Carl Zeiss SMT GmbH. Invention is credited to Sascha Bleidistel, Juergen Fischer, Ulrich Schoenhoff.
Application Number | 20140300882 14/311767 |
Document ID | / |
Family ID | 50153484 |
Filed Date | 2014-10-09 |
United States Patent
Application |
20140300882 |
Kind Code |
A1 |
Bleidistel; Sascha ; et
al. |
October 9, 2014 |
ARRANGEMENT FOR ACTUATING AN ELEMENT IN A MICROLITHOGRAPHIC
PROJECTION EXPOSURE APPARATUS
Abstract
The invention relates to arrangements for actuating an element
in a microlithographic projection exposure apparatus. In accordance
with one aspect, an arrangement for actuating an element in a
microlithographic projection exposure apparatus comprises a first
number (n.sub.R) of degrees of freedom, wherein an adjustable force
can be transmitted to the optical element in each of the-degrees of
freedom, and a second number (n.sub.A) of actuators, which are
coupled to the optical element in each case via a mechanical
coupling for the purpose of transmitting force to the optical
element, wherein the second number (n.sub.A) is greater than the
first number (n.sub.R). In accordance with one aspect, at least one
of the actuators is arranged in a node of at least one natural
vibration mode of the optical element.
Inventors: |
Bleidistel; Sascha; (Aalen,
DE) ; Schoenhoff; Ulrich; (Neu-Ulm, DE) ;
Fischer; Juergen; (Heidenheim, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carl Zeiss SMT GmbH |
Oberkochen |
|
DE |
|
|
Family ID: |
50153484 |
Appl. No.: |
14/311767 |
Filed: |
June 23, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14157718 |
Jan 17, 2014 |
8786826 |
|
|
14311767 |
|
|
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|
61756086 |
Jan 24, 2013 |
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Current U.S.
Class: |
355/71 |
Current CPC
Class: |
G02B 27/0068 20130101;
G03F 7/7015 20130101; G03F 7/70266 20130101; G03F 7/70308 20130101;
G02B 7/181 20130101; G03F 7/709 20130101; G02B 5/0891 20130101;
G02B 26/0825 20130101; G02B 7/185 20130101; G03F 7/702 20130101;
G02B 7/005 20130101; G03F 7/70191 20130101; G03F 7/70825 20130101;
G03F 7/70233 20130101 |
Class at
Publication: |
355/71 |
International
Class: |
G03F 7/20 20060101
G03F007/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2013 |
DE |
102013201082.6 |
Claims
1.-20. (canceled)
21. An arrangement, comprising: an optical element configured so
that an adjustable force is transmittable to the optical element in
a first number of degrees of freedom; and a second number of sensor
elements configured to determine a parameter, wherein: the
parameter comprises at least one member selected from the group
consisting of a location of the optical element along the first
number of degrees of freedom and a position of the optical element
along the first number of degrees of freedom; and the second number
is greater than the first number.
22. The arrangement of claim 21, wherein the optical element
comprises a mirror.
23. The arrangement of claim 21, wherein the optical element
comprises an actively deformable mirror.
24. The arrangement of claim 23, wherein the actively deformable
mirror is configured to compensate an undesirable disturbance in
the projection exposure apparatus.
25. The arrangement of claim 21, wherein the optical element
comprises a non-actively deformable mirror.
26. The arrangement of claim 21, further comprising a plurality of
actuators, wherein each actuator is coupled to the optical element
via a mechanical coupling to transmit a force to the optical
element.
27. The arrangement of claim 26, wherein at least one of the
actuators is arranged at a node of at least one natural vibration
mode of the optical element.
28. The arrangement of claim 26, wherein the actuators are
configured so that actuation in the degrees of freedom is
substantially orthogonal to at least one natural vibration mode of
the optical element.
29. The arrangement of claim 26, wherein at least one of the
actuators comprises a Lorentz actuator.
30. The arrangement of claim 21, further comprising a third number
of actuators, wherein each actuator is coupled to the optical
element via a mechanical coupling to transmit a force to the
optical element, and third number is greater than the first
number.
31. The arrangement of claim 21, wherein the first number is at
least three.
32. The arrangement of claim 21, wherein the first number is
six.
33. The arrangement of claim 21, wherein the arrangement is
configured to be used in a microlithographic projection exposure
apparatus.
34. The arrangement of claim 21, wherein the arrangement is
configured to be used in an EUV microlithographic projection
exposure apparatus.
35. An apparatus, comprising: an arrangement, comprising: an
optical element configured so that an adjustable force is
transmittable to the optical element in a first number of degrees
of freedom; and a second number of sensor elements configured to
determine a parameter, wherein: the parameter comprises at least
one member selected from the group consisting of a location of the
optical element along the first number of degrees of freedom and a
position of the optical element along the first number of degrees
of freedom; the second number is greater than the first number; and
the apparatus is a microlithographic projection exposure
apparatus.
36. The apparatus of claim 35, wherein the microlithographic
projection exposure apparatus is an EUV microlithographic
projection exposure apparatus.
37. A method, comprising: transmitting a controllable force to an
optical element in a first number of degrees of freedom, the
optical element being a component of a microlithographic projection
exposure apparatus; and using a second number of sensor elements to
determine: a) a location of the optical element along the first
number of degrees of freedom; and/or b) a position of the optical
element along the first number of degrees of freedom, wherein the
second number is greater than the first number.
38. The method of claim 37, wherein the optical element comprises a
mirror.
39. The method of claim 37, wherein the optical element comprises a
mirror, and the method further comprises using adjustable forces to
actively deform the mirror.
40. The method of claim 37, wherein the optical element comprises a
mirror, and the method further comprises using adjustable forces to
position the mirror.
41. The method of claim 37, further comprising: determining an
imaging aberration in the projection exposure apparatus; and
positioning the optical element to at least partly compensate for
the imaging aberration and/or actively deforming the optical
element to at least partly compensate for the imaging
aberration.
42. The method of claim 37, further comprising using actuators to
transmit the controllable force to the optical element.
43. The method of claim 22, wherein at least one of the actuators
is arranged at a node of at least one natural vibration mode of the
optical element.
44. The method of claim 23, wherein the actuators are configured so
that actuation in the degrees of freedom is substantially
orthogonal to at least one natural vibration mode of the optical
element.
45. The method of claim 37, further comprising using a third number
actuators to transmit the controllable force to the optical
element, wherein the third number is greater than the first number.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of, and claims priority
under 35 USC 120 to, U.S. application Ser. No. 14/157,718, filed
Jan. 17, 2014, which claims priority under 35 U.S.C.
.sctn.119(e)(1) to U.S. Provisional Application No. 61/756,086
filed Jan. 24, 2013. U.S. application Ser. No. 14/157,718 also
claims benefit under 35 U.S.C. .sctn.119 to German Application No.
10 2013 201 082.6, filed Jan. 24, 2013. The contents of these
applications are hereby incorporated by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to arrangements for actuating an
element in a microlithographic projection exposure apparatus.
[0004] 2. Prior Art
[0005] Microlithography is used for producing microstructured
components such as, for example, integrated circuits or LCDs. The
microlithography process is carried out in a so-called projection
exposure apparatus having an illumination device and a projection
lens.
[0006] The image of a mask (=reticle) illuminated via the
illumination device is in this case projected via the projection
lens onto a substrate (e.g. a silicon wafer) coated with a
light-sensitive layer (photoresist) and arranged in the image plane
of the projection lens, in order to transfer the mask structure to
the light-sensitive coating of the substrate.
[0007] In a projection exposure apparatus designed for EUV (i.e.
for electromagnetic radiation having a wavelength of less than 15
nm), for lack of light-transmissive materials being available,
mirrors are used as optical components for the imaging process. The
mirrors can be fixed on a carrying frame and can be designed to be
at least partly manipulatable in order to enable the respective
mirror to be moved for example in six degrees of freedom (i.e. with
regard to displacements in the three spatial directions x, y and z
and with regard to rotations R.sub.x, R.sub.y and R.sub.z about the
corresponding axes). In this case, the position of the mirrors can
be determined via position sensors fixed to a sensor frame.
[0008] In a projection exposure apparatus designed for EUV, mirrors
designed to be manipulatable are used both as actively deformable
mirrors, in the case of which changes in the optical properties
that occur e.g. during the operation of the projection exposure
apparatus and resultant imaging aberrations, e.g. on account of
thermal influences, can be compensated for by active deformation,
and as non-actively deformable mirrors, in the case of which no
targeted deformation is effected.
[0009] The positional control of such mirrors serves, in
conjunction with a suitable actuator system (e.g. with Lorentz
actuators), to keep the mirrors in their position as stably as
possible, such that a deviation of the mirror positions that is
measured via the position sensors is as small as possible. One
approach that is possible in principle for this purpose consists in
increasing the controller gain and thus increasing the control
bandwidth. In this case, however, the problem occurs in practice
that the mirrors are not ideally rigid bodies, but rather each have
specific natural frequencies of the mechanical structures (e.g. of
a typical order of magnitude in the range of 2-3 kHz), wherein the
corresponding natural frequency spectra for the dimensions of the
mirrors and of the carrying and measuring structures, the
dimensions increasing with increasing numerical apertures, are
shifted further and further toward lower frequencies. This applies
all the more to actively deformable mirrors, which have to be
designed to be deformable and thus compliant in a targeted manner.
An excitation of the natural frequencies via the actuators can have
the effect, however, that on account of the relatively low damping
in the control loop comparatively large amplitudes are detected by
the respective position sensors, as a result of which the stability
of the control loop can be jeopardized and active positional
control can no longer be operated stably or can be operated only
with low control quality.
[0010] With regard to the prior art, reference is made for example
to U.S. Pat. No. 6,842,277 B2, US 2007/0284502 A1 and the
publication "Benefits of over-actuation in motion systems", by M.
G. E. Schneiders et al., Proceedings of the 2004 American Control
Conference (ACC 2004), Boston (2004).
SUMMARY OF THE INVENTION
[0011] It is an object of the present invention to provide
arrangements for actuating an element in a microlithographic
projection exposure apparatus which enable active positional
control of the element with higher control quality.
[0012] This object is achieved in accordance with the features of
the independent patent claims.
[0013] In accordance with one aspect, the invention relates to an
arrangement for actuating an element in a microlithographic
projection exposure apparatus, comprising: [0014] a first number
(n.sub.R) of degrees of freedom, wherein an adjustable force can be
transmitted to the optical element in each of the degrees of
freedom; and [0015] a second number (n.sub.A) of actuators, which
are coupled to the optical element in each case via a mechanical
coupling for the purpose of transmitting force to the optical
element; [0016] wherein the second number (n.sub.A) is greater than
the first number (n.sub.R); and [0017] wherein at least one of the
actuators is arranged in a node of at least one natural vibration
mode of the optical element.
[0018] The invention is based on the concept, in particular, of
performing, in an arrangement for actuating an element, in
particular a mirror, an "over-actuation" insofar as the number of
actuators exceeds the number of degrees of freedom (that is to say
for example for an actuation of the element or mirror in six
degrees of freedom, at least seven actuators are used). This
additional freedom with regard to the application of forces to the
optical element, the additional freedom being obtained as a result
of the surplus of actuators available (relative to the number of
degrees of freedom) in comparison with an unambiguously statically
determinate arrangement, can now be used further to the effect that
the forces are applied to the optical element in such a way that
the above-described excitation of natural vibrations of the
mechanical structures is reduced or even completely masked out. A
further advantage of this over-actuation is that a better
distribution of the forces is made possible on account of the
higher number of actuators.
[0019] The additional freedom obtained as a result of the
over-actuation according to the invention can be used firstly
during the positioning of the actuators and secondly also during
the driving of the actuators (i.e. the targeted configuration of
the generated line of force). As far as the positioning of the
actuators is concerned, in accordance with the abovementioned
approach of the invention, at least one of the actuators is
arranged in a node of a natural vibration mode, which has the
consequence that the respective undesired natural vibration mode is
not excited, independently of the excitation of the relevant
actuator.
[0020] In accordance with a further aspect, the invention relates
to an arrangement for actuating an optical element in a
microlithographic projection exposure apparatus, comprising [0021]
a first number (n.sub.R) of degrees of freedom, wherein an
adjustable force can be transmitted to the optical element in each
of the degrees of freedom; and [0022] a second number (n.sub.A) of
actuators, which are coupled to the optical element in each case
via a mechanical coupling for the purpose of transmitting force to
the optical element; [0023] wherein the second number (n.sub.A) is
greater than the first number (n.sub.R); and [0024] wherein the
actuators are arranged such that the actuation in the degrees of
freedom is substantially orthogonal to at least one natural
vibration mode of the optical element.
[0025] In accordance with the above approach, the actuators are
arranged such that the actuation in the degrees of freedom is
substantially orthogonal to at least one natural vibration mode.
Within the meaning of the present application, "substantially"
orthogonal should be understood such that the actuation of the
natural vibration modes, which becomes visible in the transfer
function of the open control loop as a weakly damped resonance, is
reduced in terms of its magnitude compared with a
non-over-actuation by at least 6 dB, in particular by at least 12
dB, more particularly by at least 20 dB.
[0026] In accordance with one embodiment, the optical element is a
mirror. Even if in the further embodiments the optical element is
in each case a mirror of a projection exposure apparatus designed
for EUV, the invention is not restricted thereto. In this regard,
the invention can also be realized in conjunction with other
optical elements, such as e.g. refractive or diffractive optical
elements. In further embodiments, the invention can also be
realized in a projection exposure apparatus designed for DUV (i.e.
for wavelengths of less than 200 nm, in particular less than 160
nm).
[0027] The mirror can be configured in particular in such a way
that it is actively deformable in order to compensate for an
undesirable disturbance in the projection exposure apparatus. Such
a disturbance can be, for example, a thermal expansion on account
of absorption of the radiation emitted by the (e.g. EUV) light
source, and also imaging aberrations (caused by such thermal
influences or in some other way).
[0028] For actively deforming a deformable mirror, use is typically
made of a comparatively high number of (deformation) actuators
(e.g. of the order of magnitude of 10-100), wherein in addition the
mirror is designed to be comparatively elastic in contrast to a
non-actively deformable mirror. According to the invention, these
(deformation) actuators, in particular, can be used for realizing
the over-actuation described above. In accordance with this
approach, therefore, the deformation actuators are used doubly
insofar as firstly they are used for deforming the relevant mirror
and secondly they are used for controlling the position of the
mirror and in this case for generating the requisite forces in such
a way that undesired natural vibration modes of the mirror are
excited to a lesser extent or are not excited at all. In other
words, therefore, the deformation actuators additionally perform
the function of the positioning actuators (exclusively present in
the case of a non-actively deformable mirror).
[0029] In accordance with a further embodiment, the mirror can also
be a non-actively deformable mirror.
[0030] In accordance with one embodiment, the arrangement
furthermore comprises a third number (n.sub.S) of sensor elements
for determining the location and/or position of the optical
element. In accordance with one embodiment, in this case the third
number (n.sub.S) of sensor elements is greater than the first
number (n.sub.R) of degrees of freedom.
[0031] In accordance with this aspect of the invention, in
particular in conjunction with a non-actively deformable mirror,
therefore, a surplus n.sub.S (i.e. at least n.sub.R+1) of sensors
can also be provided relative to the number of degrees of freedom
(n.sub.R) that exist in the positioning of the optical element.
This further concept according to the invention, which is
equivalent to the above-described over-actuation in terms of
control engineering, is also designated as "over-sensing"
hereinafter analogously to over-actuation. The additional freedom
obtained as a result of the surplus of sensors can be used for
choosing an arrangement of the sensors in such a way that specific
natural frequencies or natural vibration modes are not even
detected by the sensor system in the first place, with the
consequence that the positional control cannot react to such
natural frequencies. The concept of over-sensing has the further
advantage that forces are still applied to the optical element or
the mirror in a statically governed manner and inherent or
undesired deformations of the optical element or mirror are thus
avoided.
[0032] The above-described concept of "over-sensing" is also
advantageous independently of the concept of "over-actuation".
[0033] In accordance with a further aspect, therefore, the
invention also relates to an arrangement for actuating an optical
element in a microlithographic projection exposure apparatus,
comprising: [0034] a first number (n.sub.R) of degrees of freedom,
wherein an adjustable force can be transmitted to the optical
element in each of the degrees of freedom; and [0035] a third
number (n.sub.S) of the sensor elements for determining the
location and/or position of the optical element; [0036] wherein the
third number (n.sub.S) is greater than the first number
(n.sub.R).
[0037] The arrangement according to the invention can be designed,
in particular, for actuating an optical element in a
microlithographic projection exposure apparatus designed for
EUV.
[0038] The invention can furthermore be used both in the
illumination device and in the projection lens of a
microlithographic projection exposure apparatus.
[0039] In accordance with one embodiment, the first number
(n.sub.R) of degrees of freedom is at least three, in particular
six.
[0040] The invention furthermore relates to a microlithographic
projection exposure apparatus comprising an arrangement having the
features described above.
[0041] In accordance with a further aspect, the invention relates
to a method for actuating an optical element in a microlithographic
projection exposure apparatus, [0042] wherein adjustable forces are
transmitted to the optical element in a first number (n.sub.R) of
degrees of freedom; [0043] wherein the force transmission is
effected via a second number (n.sub.A) of actuators; [0044] wherein
the second number (n.sub.A) is greater than the first number
(n.sub.R); and [0045] wherein at least one of the actuators is
arranged in a node of at least one natural vibration mode of the
optical element.
[0046] In accordance with a further aspect, the invention also
relates to a method for actuating an optical element in a
microlithographic projection exposure apparatus, [0047] wherein
adjustable forces are transmitted to the optical element (100, 200)
in a first number (n.sub.R) of degrees of freedom; [0048] wherein
the force transmission is effected via a second number (n.sub.A) of
actuators; [0049] wherein the second number (n.sub.A) is greater
than the first number (n.sub.R); and [0050] wherein the actuation
in the degrees of freedom is substantially orthogonal to at least
one natural vibration mode of the optical element.
[0051] In accordance with one embodiment, the optical element is
actively deformed by the adjustable forces.
[0052] In accordance with one embodiment, the position of the
optical element is manipulated by the adjustable forces.
[0053] In accordance with one embodiment, a third number (n.sub.S)
of sensor elements are used to determine the location and/or
position of the optical element. In this case, in particular, the
third number (n.sub.S) can be greater than the first number
(n.sub.R).
[0054] The invention therefore also relates to a method for
positioning and/or actively deforming an optical element in a
microlithographic projection exposure apparatus, [0055] wherein a
controllable force is transmitted to the optical element in a first
number (n.sub.R) of degrees of freedom; and [0056] wherein a third
number (n.sub.S) of sensor elements are used to determine the
location and/or position of the optical element; [0057] wherein the
third number (n.sub.S) is greater than the first number
(n.sub.R).
[0058] In this case, the method can respectively comprise in
particular the following steps: [0059] determining at least one
imaging aberration in the projection exposure apparatus; and [0060]
positioning and/or actively deforming the optical element in such a
way that the imaging aberration is at least partly compensated
for.
[0061] Further configurations of the invention can be gathered from
the description and the dependent claims.
[0062] The invention is explained in greater detail below on the
basis of exemplary embodiments illustrated in the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] In the figures:
[0064] FIGS. 1a-b show schematic illustrations for elucidating one
approach according to the invention in conjunction with a
non-actively deformable mirror;
[0065] FIG. 2 shows a schematic illustration for elucidating one
approach according to the invention in conjunction with an actively
deformable mirror;
[0066] FIGS. 3-4 show schematic illustrations for elucidating one
embodiment on the basis of the example of positional control of a
vibratory body;
[0067] FIG. 5 shows a diagram for elucidating a control loop on the
basis of the example of an actively deformable mirror with
realization of the over-actuation according to the invention;
[0068] FIG. 6 shows a diagram for elucidating a control loop on the
basis of the example of an actively deformable mirror with
realization of the over-sensing according to the invention;
[0069] FIG. 7 shows a diagram for elucidating a control loop on the
basis of the example of an actively deformable mirror with
realization of a statically determinate I-controller; and
[0070] FIG. 8 shows a schematic illustration of an exemplary
construction of a microlithographic projection exposure apparatus
which is designed for operation in the EUV and in which the present
invention can be realized.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0071] FIGS. 1a-b firstly show schematic illustrations for
elucidating one approach according to the invention in conjunction
with a non-actively deformable mirror.
[0072] In accordance with FIG. 1a, a mirror 100 to be held in a
defined position is conventionally mounted isostatically by virtue
of the fact that three actuators 111, 112 and 113 having a force
direction or drive direction perpendicular to the mirror 100 are
used to position the mirror 100 in the three degrees of freedom z,
R.sub.x and R.sub.y (i.e. with regard to displacement in the
spatial direction z and rotation about the x- and y-axis,
respectively). With exactly these three actuators 111, 112 and 113,
the three degrees of freedom z, R.sub.x and R.sub.y are statically
determinate. As likewise explained here with reference to FIGS. 3
and 4, however, these three actuators 111, 112 and 113 can excite
elastic natural frequencies or natural vibration modes of the
mirror 100.
[0073] As indicated in FIG. 1b, a higher number (in the example
n.sub.A=4) of actuators 111, 112, 113 and 114 relative to the
number of degrees of freedom (in the example the three degrees of
freedom z, R.sub.x and R.sub.y) is now used according to the
invention, as likewise explained in even greater detail with
reference to FIGS. 3 and 4, the actuators 111-114 being positioned
in such a way that no undesired excitation or associated
disturbance of the positional control takes place for some natural
frequencies or natural vibration modes of the mirror 100.
[0074] FIG. 2 serves for clarifying the concept according to the
invention in conjunction with an actively deformable mirror 200,
likewise merely indicated schematically. In accordance with FIG. 2,
a comparatively high number (e.g. 10, 100 or more) of deformation
actuators 211, 212, . . . serve for actively deforming the
deformable mirror 200, wherein the mirror 200 is simultaneously
designed to be comparatively elastic in order to enable an active
deformation. According to the invention, the deformation actuators
211, 212, . . . are used doubly insofar as firstly they serve for
deforming the mirror 200 and secondly they serve, by way of the
over-actuation described above, to configure the positional control
of the mirror 200 in such a way that an undesired excitation of
natural frequencies or natural frequency modes of the mirror 200 as
far as possible does not occur.
[0075] The principle and the functioning of the over-actuation
applied according to the invention to an optical element such as a
mirror, for example, are explained below on the basis of a specific
exemplary embodiment with reference to the schematic illustrates in
FIGS. 3 and 4. In this case, the movement of the optical element in
FIGS. 3 and 4 is restricted to a translational and a rotational
degree of freedom, for the sake of simplicity, and the system is
subdivided or discretized into three nodes 310, 320, 330 for
describing the vibration capability, wherein each of the nodes 310,
320, 330 has a respective translational degree of freedom q.sub.1,
q.sub.2 and q.sub.3 and a respective rotational degree of freedom
.phi..sub.1, .phi..sub.2 and .phi..sub.3. Furthermore, in
accordance with FIG. 3, the same mass m is assigned to each node
310, 320, 330, wherein the nodes 310, 320, 330 are associated with
the same stiffness k.
[0076] The system discretized in a simplified manner in accordance
with FIG. 3 shows, as illustrated schematically in FIGS. 4a-c,
three vibration modes, wherein a first vibration mode is the
translation of the rigid body (FIG. 4a), a second vibration mode is
the rotation of the rigid body (FIG. 4b) and a third vibration mode
is a first bending vibration of the rigid body (FIG. 4c).
( q 1 q 2 q 3 ) = ( 1 1 1 ) = q m 1 ( 1 ) ( q 1 q 2 q 3 ) = ( + 1 0
- 1 ) = q m 2 ( 2 ) ( q 1 q 2 q 3 ) = ( + 1 - 1 + 1 ) = q m 3 ( 3 )
##EQU00001##
[0077] Conventionally, two actuators could then be chosen for a
statically determinate actuation, via which actuators the
rigid-body translation and the rigid-body rotation can be actuated,
for which purpose, in the specific case, one actuator (for applying
the force F.sub.1) can be arranged at the node 310 and the other
actuator (for applying the force F.sub.3) can be arranged at the
node 330. For the control of the translation and respectively
rotation by a controller, a transformation matrix T.sub.a can
usually be used which generates a desired translational force f and
a desired torque M, via these two actuators:
( f 1 f 2 f 3 ) = T a ( f M ) , where T a = ( 1 2 1 2 l 0 0 1 2 1 2
l ) ( 4 ) ##EQU00002##
[0078] wherein the following holds true:
q m 1 T T a = ( 1 0 ) , q m 2 T T a = ( 0 1 l ) , q m 3 T T a = ( 1
0 ) ( 5 ) ##EQU00003##
[0079] Upon checking how the vibration modes of the system are
excited in the case of such a statically determinate actuation via
the chosen actuators and using the abovementioned transformation
matrix, it is then evident that the force f excites the
translational rigid-body mode (mode 1) as desired and the torque M
excites the rotational rigid-bodied mode (mode 2), but the force f
also additionally excites the bending mode (mode 3) (since, as can
be seen from (5), the bending mode (=mode 3) is visible in the
translational axis). Consequently, the bending mode is also visible
in the transfer function of the control loop for the translational
movement and may possibly lead undesirably to a limitation of the
bandwidth that can be set.
[0080] The problem described above can now be rectified via the
over-actuation according to the invention as follows. For this
purpose, an additional actuator is provided in the exemplary
embodiment, the additional actuator being arranged at the node 320
for applying the force F.sub.2 in accordance with FIG. 3.
Consequently, three actuators are available for generating the
forces for translation and rotation, such that compared with the
above-described statically determinate actuation via two actuators,
additional freedom is obtained with regard to the design of the
transformation matrix Ta, since the transformation matrix Ta is now
no longer uniquely determinate. In order to use the freedom
additionally obtained as a result, the elements of the
transformation matrix Ta are preferably chosen such that the force
f and the torque M still actuate only the corresponding
(translational or rotational) rigid-body degree of freedom, but the
force f can no longer excite the bending mode.
[0081] In the specific exemplary embodiment, the transformation
matrix Ta can be chosen as follows:
( f 1 f 2 f 3 ) = T a ( f M ) , where T a = ( 1 3 1 2 l 1 3 0 1 3 1
2 l ) ( 6 ) ##EQU00004##
[0082] wherein the following holds true:
q m 1 T T a = ( 1 0 ) , q m 2 T T a = ( 0 1 l ) , q m 3 T T a = ( 0
0 ) ( 7 ) ##EQU00005##
[0083] As can be seen from (7), the bending mode (=mode 3) is no
longer visible in the translational axis.
[0084] FIG. 5 shows a diagram for elucidating the construction and
function of a control loop for the case of an actively deformable
mirror with the realization of the above-explained concept of
over-actuation according to the invention. In this case, n.sub.R
denotes the number of positionally controlled rigid-body degrees of
freedom and n.sub.A denotes the number of positionally controlled
actuators, wherein the number of actuators exceeds the number of
degrees of freedom, that is to say n.sub.A>n.sub.R holds
true.
[0085] In accordance with FIG. 5, the desired values for the mirror
position are fed to a position controller 510, which generates a
static transformation matrix T.sub.a for the n.sub.R positionally
controlled rigid-body degrees of freedom. On the basis of the
transformation matrix T.sub.a and a driving signal for the mirror
deformation, actuators 520 for actuating the mirror 530 are driven
with position determination via the position sensors 540. The
resultant static transformation matrix T.sub.a is in turn fed to
the position controller 510, etc.
[0086] FIG. 6 shows an analogous diagram for elucidating a control
loop for the case of an actively deformable mirror with the
realization of the concept of "over-sensing" according to the
invention, likewise explained above. In this case, n.sub.R denotes
the number of positionally controlled rigid-body degrees of freedom
and n.sub.S denotes the number of sensors, wherein the number of
sensors exceeds the number of degrees of freedom;
n.sub.S>n.sub.R holds true.
[0087] FIG. 7 shows a further exemplary embodiment of the
invention, wherein components that are analogous or substantially
functionally identical to FIG. 5 are designated by reference
numerals increased by "200". In this case, once again n.sub.R
denotes the number of positionally controlled rigid-body degrees of
freedom and n.sub.A denotes the number of positionally controlled
actuators, wherein the following holds true:
n.sub.A>n.sub.R.
[0088] The exemplary embodiment in FIG. 7 takes account of the
circumstance that the over-actuation applied according to the
invention can lead to undesired deformations of the optical
element. The cause of the undesired deformations is that the
position controller generally exerts both dynamic and small static
forces in order to keep the optical element stably in position. The
static forces can be position- and time-dependent. The
overdeterminate application of the variable static forces to an
overdeterminate number of force application points (actuators) can
then lead to undesired deformations of the optical element.
[0089] This problem can be solved as follows by the concept
described with reference to FIG. 7: The position controller is
typically a PID-like controller, i.e. a controller whose dynamic
behavior has a proportional component (P component), a derivative
component (D component) and an integral component (I component).
The I component generates the static forces, whereas the P
component and the D component generate the dynamic forces. If the I
component is then separated from the PD component and applied
statically determinately to a smaller statically determinate subset
(n.sub.R) of actuators, the static forces are always applied
statically determinately to a statically determinate number of
force application points, with the result that the undesired
deformations described above are avoided.
[0090] FIG. 8 shows a schematic illustration of a microlithographic
projection exposure apparatus which is designed for operation in
the EUV and in which the present invention can be realized, for
example.
[0091] The projection exposure apparatus in accordance with FIG. 8
comprises an illumination device 6 and a projection lens 31. The
illumination device 6 comprises, in the light propagation direction
of the illumination light 3 emitted by a light source 2, a
collector 26, a spectral filter 27, a field facet mirror 28 and a
pupil facet mirror 29, from which the light impinges on an object
field 4 arranged in an object plane 5. The light emerging from the
object field 4 enters into the projection lens 31 with an entrance
pupil 30. The projection lens 31 has an intermediate image plane
17, a first pupil plane 16 and a further pupil plane with a stop 20
arranged therein. The projection lens 31 comprises a total of 6
mirrors M1-M6. M6 denotes the last mirror relative to the optical
beam path, the mirror having a through-hole 18. M5 denotes the
penultimate mirror relative to the optical beam path, the mirror
having a through-hole 19. A beam emerging from the object field 4
or reticle arranged in the object plane passes onto a wafer,
arranged in the image plane 9, after reflection at the mirrors
M1-M6 in order to generate an image of the reticle structure to be
imaged.
[0092] The arrangement according to the invention can be used for
positioning and/or actively deforming one or a plurality of mirrors
in the projection lens 31 and/or in the illumination device 6.
[0093] Even though the invention has been described on the basis of
specific embodiments, numerous variations and alternative
embodiments are evident to a person skilled in the art, e.g. via
combination and/or exchange of features of individual embodiments.
Accordingly, it goes without saying for a person skilled in the art
that such variations and alternative embodiments are concomitantly
encompassed by the present invention, and the scope of the
invention is restricted only within the meaning of the accompanying
patent claims and the equivalents thereof.
* * * * *