U.S. patent application number 10/091581 was filed with the patent office on 2002-07-11 for apparatus, system, and method for active compensation of aberrations in an optical system.
Invention is credited to Ivaldi, Jorge S., Meehan, Michael F., Taub, David G..
Application Number | 20020089734 10/091581 |
Document ID | / |
Family ID | 26894730 |
Filed Date | 2002-07-11 |
United States Patent
Application |
20020089734 |
Kind Code |
A1 |
Meehan, Michael F. ; et
al. |
July 11, 2002 |
Apparatus, system, and method for active compensation of
aberrations in an optical system
Abstract
Active compensation of aberrations in an optical system involves
attaching first and second force bars to a mirror. The first force
bar is bifurcated to form an opening near its longitudinal
midpoint. This opening defines first and second opposed surfaces. A
second force bar is substantially perpendicular to the first force
bar and extends through the opening of the first force bar so that
a medial portion of the second force bar is disposed in the opening
of the first force bar. The second force bar is connected to the
first surface by at least one actuator. Longitudinal movement of
the actuator causes a displacement of the mirror. A support
structure is used to support the weight of the force bars and
actuator. The force bars are connected to the support structure by
a plurality of flexures. A control module receives information from
a sensing module and controls the actuator. Other embodiments use
more than two force bars and are capable of more fully compensating
for any aberrations in the optical system.
Inventors: |
Meehan, Michael F.;
(Southbury, CT) ; Taub, David G.; (Stamford,
CT) ; Ivaldi, Jorge S.; (Trumbull, CT) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX PLLC
1100 NEW YORK AVENUE, N.W., SUITE 600
WASHINGTON
DC
20005-3934
US
|
Family ID: |
26894730 |
Appl. No.: |
10/091581 |
Filed: |
March 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10091581 |
Mar 7, 2002 |
|
|
|
09702854 |
Nov 1, 2000 |
|
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|
60199400 |
Apr 25, 2000 |
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Current U.S.
Class: |
359/291 ;
359/290 |
Current CPC
Class: |
G02B 7/1822 20130101;
G02B 7/185 20130101; G03F 7/70258 20130101; G03F 7/70233
20130101 |
Class at
Publication: |
359/291 ;
359/290 |
International
Class: |
G02B 026/00 |
Claims
What is claimed is:
1. An apparatus for active compensation of aberrations in an
optical system, comprising: a first force bar having at least two
ends configured for attachment to a first and second portion of a
mirror; a second force bar having at least two ends configured for
attachment to a third and forth portion of said mirror; and at
least one actuator, each of said actuators having a first end
connected to said first force bar and a second end connected to
said second force bar, such that longitudinal movement of said
actuator between said first and second force bars applies a force
to said mirror.
2. The apparatus of claim 1, wherein said first and second force
bars each have two ends and wherein said first force bar is
substantially perpendicular to said second force bar.
3. The apparatus of claim 1, wherein said first and second force
bars each have three ends.
4. The apparatus of claim 2, wherein said first force bar has an
opening near its longitudinal midpoint, and wherein said second
force bar partially resides in said opening.
5. The apparatus of claim 4, wherein said first force bar is
bifurcated to form said opening, and wherein said opening defines
first and second opposed surfaces, said second force bar having a
medial portion disposed within said opening and being connected to
said first surface by at least one said actuator.
6. The apparatus of claim 5, wherein said actuators are pneumatic
bellows.
7. The apparatus of claim 6, further comprising: a support
structure for supporting the weight of said first and second force
bars; and a plurality of flexures for connecting said first and
second force bars to said support structure, each of said flexures
having a first end connected to said support structure and a second
end connected to an end of said first or said second force bar.
8. The apparatus of claim 7, further comprising: a third force bar
having ends configured for attachment to a peripheral portion of a
mirror; a fourth force bar having ends configured for attachment to
said peripheral portion of said mirror; and at least one additional
actuator, each of said additional actuators having a first end
connected to said third force bar and a second end connected to
said fourth force bar, such that longitudinal movement of said
actuator between said third and fourth force bars applies a force
to said peripheral portion of said mirror.
9. The apparatus of claim 8, wherein said first and second force
bars are aligned to compensate for Z5 astigmatism, and wherein said
third and fourth force bars are aligned to compensate for Z6
astigmatism.
10. The apparatus of claim 9, wherein each of said force bars are
leaf springs.
11. A system for active compensation of aberrations in an optical
system, comprising: a mirror having a concave first surface and a
second surface; a compensation module attached to said second
surface of said mirror; a sensing module; and a control module in
communication with said sensing module to control said compensation
module to bend said mirror and compensate for an aberration in the
optical system based on information received from said sensing
module.
12. The system of claim 1, further comprising: a support structure
for supporting the weight of said compensation module; and a
plurality of flexures for connecting said compensation module to
said support structure, each of said flexures having a first end
connected to said support structure and a second end connected to
said compensation module.
13. The system of claim 12, wherein said compensation module
comprises pneumatic actuators.
14. The system of claim 13, further comprising: an air supply
module in fluid communication with compensation module.
15. The system of claim 14, wherein said pneumatic actuators are
pneumatic bellows.
16. The system of claim 11, wherein said compensation module
corrects for both Z5 and Z6 astigmatism.
17. The system of claim 11, wherein said sensing module measures
temperature and pressure.
18. The system of claim 11, wherein said sensing module measures
electromagnetic energy exiting the optical system.
19. The system of claim 11, wherein said control module comprises a
central processing unit and a memory unit.
20. A system for active compensation of aberrations in an optical
system, comprising: an mirror having a concave first surface and a
second surface; compensation means, attached to said second surface
of said mirror, for bending said mirror; sensor means for measuring
a parameter related to an aberration in the optical system; and a
control means in communication with said sensor means for receiving
data from said sensor means and for controlling said compensation
means to compensate for the aberration in the optical system based
on the data received from said sensor means.
21. The system of claim 20, further comprising: support means for
supporting the weight of said compensation module.
22. The system of claim 21, further comprising: air supply means
for supplying air to said compensation means.
23. A method for active compensation of aberrations in an optical
system, comprising the steps of: (1) predicting the presence of an
aberration in the optical system; (2) generating a corrective
signal related to the predicted aberration; and (3) adjusting at
least one actuator that controls a force applied to a mirror to
compensate for the predicted aberration.
24. The method of claim 23, further comprising the steps of: (4)
repeating steps (1) through (3) for additional correction of the
predicted aberration.
25. The method of claim 24, wherein step (1) comprises looking up a
value in a lookup table stored in a memory unit.
26. The method of claim 24, wherein step (1) comprises using an
output of a sensing module and an algorithm to predict the presence
of an aberration.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to and claims priority to U.S.
Provisional Application Ser. No. 60/199,400, filed Apr. 25, 2000,
titled "Apparatus, System and Method for Active Compensation of
Astigmatism in a Catadioptric Optical System," which is herein
incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to an optical system. More
particularly, it relates to an optical reduction system used in
semiconductor manufacturing.
[0004] 2. Related Art
[0005] Semiconductors are typically manufactured using various
photolithographic techniques, which are implemented using complex
optical systems. For example, one complex optical system used in
the manufacturing of semiconductors is a catadioptric optical
reduction system. While these complex optical systems perform
adequately for their intended purpose, controlling aberrations in
these optical systems is becoming increasingly important as
semiconductor manufacturers strive to make semiconductor components
smaller. Aberrations limit the minimum size of component features
that may be reproduced on a semiconductor chip.
[0006] Imaging in complex optical systems can be controlled using
multiple lenses and one or more mirrors. To reproduce small
component features on a semiconductor using such systems, a passive
device can be used to apply a force to a mirror of the optical
system and thereby compensate for some of the aberrations in the
optical system. For example, a passive device attached to a mirror
can be used to compensate for at least some of the astigmatism in
an optical system with relative success. Known passive devices have
limitations, however, and they do not provide an element of control
needed to compensate for changes in astigmatism and other
aberrations that result from environmental factors, such as changes
in temperature.
[0007] What is needed are new means for controlling aberrations in
complex optical systems that permit such systems to be used for
reproducing very small component features on a semiconductor.
SUMMARY OF THE INVENTION
[0008] The present invention provides an apparatus, system, and
method for active compensation of aberrations in an optical system.
In a preferred embodiment, first and second force bars are attached
to a mirror. The first force bar is bifurcated to form an opening
near its longitudinal midpoint. This opening defines first and
second opposed surfaces. The second force bar is substantially
perpendicular to the first force bar and extends through the
opening of the first force bar so that a medial portion of the
second force bar is disposed in the opening of the first force bar.
The second force bar is connected to the first surface by at least
one actuator. Longitudinal movement of the actuator applies a force
to the mirror causing a displacement or a change in the shape of
the mirror. A support structure is used to support the weight of
the force bars and actuators. The force bars are connected to the
support structure by a plurality of flexures.
[0009] In a preferred embodiment of the invention, the force bars
are leaf springs and the actuators are pneumatic bellows. A sensing
module for measuring a parameter used to control aberrations in the
optical system provides information to a control module. The
control module receives information from the sensing module and
controls an air supply module to operate the pneumatic bellows.
BRIEF DESCRIPTION OF THE FIGURES
[0010] The accompanying drawings, which are incorporated herein and
form part of the specification, illustrate the present invention
and, together with the description, further serve to explain the
principles of the invention and to enable a person skilled in the
relevant art to make and use the invention.
[0011] FIG. 1 is a diagram of an example optical system in which
the present invention may be used.
[0012] FIG..2A is a diagram of a mirror having a Z5 Zernike circle
aberration.
[0013] FIG. 2B is a diagram of a mirror having a Z6 Zernike circle
aberration.
[0014] FIG. 3A is a side view of an example passive device for
compensation of astigmatism in an optical system.
[0015] FIG. 3B is a top view of the example passive device for
compensation of astigmatism in an optical system.
[0016] FIG. 4A is a side view of an example apparatus for active
compensation of astigmatism in an optical system according to an
embodiment of the present invention.
[0017] FIG. 4B is a top view of the example apparatus for active
compensation of astigmatism in an optical system according to an
embodiment of the present invention.
[0018] FIG. 5A is a side view of an example apparatus for active
compensation of astigmatism in an optical system according to a
second embodiment of the present invention.
[0019] FIG. 5B is a top view of the example apparatus for active
compensation of astigmatism in an optical system according to a
second embodiment of the present invention.
[0020] FIG. 6 is a diagram of an example system for active
compensation of astigmatism in an optical system according to an
embodiment of the present invention.
[0021] FIG. 7 is a flowchart of an example method for active
compensation of astigmatism in an optical system according to an
embodiment of the present invention.
[0022] The present invention is described with reference to the
accompanying drawings. In the drawings, like reference numbers
indicate identical or functionally similar elements. Additionally,
the leftmost digit of a reference number identifies the drawing in
which the reference number first appears.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] Overview of the Invention and Terminology
[0024] The present invention provides an apparatus, system, and
method for active compensation of aberrations in an optical system.
For example, the present invention can be used to actively
compensate for astigmatism and other aberrations in an optical
reduction system used in semiconductor manufacturing. The invention
uses force bars and actuators to bend a mirror's reflective surface
and thereby selectively alter incident electromagnetic wavefronts
relative to the reflective action of the unbent mirror. In an
embodiment, the force bars are attached to a peripheral portion of
the mirror by flexures.
[0025] To better describe the present invention, the following
terms are defined:
[0026] The term "aberration" means an optical system defect that
cause an image to deviate from the rules of paraxial imagery.
Aberrations can be described, for example, by Zernike circle
polynomials. Zernike circle polynomials, however, are not the only
means for describing aberrations as would be known to a person
skilled in the relevant art.
[0027] The term "active compensation" means deforming or bending
optical elements, such as the reflective surface of a mirror, by
various means for the purpose of correcting or controlling the
performance of an optical system and its associated imaging. Active
compensation can be used to compensate, for example, for
astigmatism due to changing environmental conditions such as
temperature. Active compensation systems can involve open-loop or
closed-loop control elements to control the deforming or bending of
an optical element without requiring manual intervention, as would
be known to a person skilled in the relevant art given the
description herein.
[0028] The term "actuator" means any apparatus that can be used to
apply force or displacement to a mechanical device. An actuator may
be operated, for example, electromechanically or pneumatically.
[0029] The term "aspheric mirror" means a mirror that has a
nonspherical surface. The aspheric surface of a mirror can be used,
for example, in a catadioptric optical system, to alter an incident
electromagnetic wavefront relative to the reflective action of a
spherical surface.
[0030] The term "astigmatism" means an aberration that results in
the tangential and radial image planes of an optical system being
separated axially.
[0031] The term "catadioptric optical system" means an optical
system whose focal power is achieved using both reflection and
refraction. While the relative powers of the lenses and mirrors in
a catadioptric optical system vary from system to system, such
systems are typically characterized by the use of reflective
surfaces to achieve a significant portion of the systems focal
power, in combination with refractive surfaces of little or zero
focal power. These systems produce an image that has improved
aberrational characteristics.
[0032] The term "flexure" means a device, which is stiff in
substantially only one dimension. Two ball joints connected by a
rod is an example of a flexure. The flexures used in a preferred
embodiment of the present invention comprise metal rods that have
eight notches cut out of them arranged as four opposed pairs. The
opposed pairs of notches are cut out of one end of the metal rod so
that the bottoms of the notches point to one another and are almost
touching. Two additional notches are cut out of the rod, just below
the first opposed pair of notches, oriented at a 90-degree angle
(perpendicular) to the first opposed pair of notches. The opposite
end of the metal rod has two pairs of notches cut in a similar
manner.
[0033] The term "force bar" means a longitudinal device capable of
receiving a force near its midpoint and transferring that force to
its ends. A leaf spring and a metal rod are examples of a force
bar.
[0034] The term "parameter related to an aberration" means any
parameter that can be monitored and that is useful for predicting
the presence of an aberration in an optical system and/or for
controlling an optical element to compensate for an aberration.
Examples include temperature, pressure, and electromagnetic
energy.
[0035] The term "corrective signal" means any signal or value used
in determining a force or displacement to be applied to an optical
element of an optical system in order to actively correct or
compensate for an aberration in the optical system.
[0036] Example Optical System in Which the Present Invention May be
Used
[0037] FIG. 1 shows an example optical system 100 in which the
present invention can be used. Example optical system 100 is
presented by way of example only, and it is not intended to limit
the scope of the present invention. As will be apparent to a person
skilled in the relevant art given the description herein, the
present invention can be used to compensative for aberrations in
many different optical systems.
[0038] Example optical system 100 is a catadioptric optical
reduction system. Example optical system 100 is used in the
manufacturing of semiconductors. As can be seen in FIG. 1, example
optical system 100 contains a reticle 110, a first lens group 120,
a folding mirror 130, a second lens group 140, a beamsplitter block
150, a quarter-waveplate 160, a concave mirror 170, an aberration
compensation module 175, and a third lens group 180. A
semiconductor or wafer is placed at an image plane 190.
[0039] An incident electromagnetic wavefront entering optical
system 100 at reticle 110 converges to a focal point at image plane
190. Optical system 100 can be used to reproduce the features of a
semiconductor mask located at reticle 110 on a wafer located at
image plane 190. An incident electromagnetic wavefront enters
optical system 100 at reticle 110 and passes through lens group
120. Folding mirror 130 is used to direct an incident
electromagnetic wavefront exiting lens group 120 into lens group
140 and beamsplitter block 150. Beamsplitter block 150 directs a
portion of the incoming electromagnetic wavefront through
quarter-waveplate 160 to mirror 170. Mirror 170 reflects an
incoming electromagnetic wavefront back through quarter-waveplate
160 and beamsplitter block 150 into lens group 180. When the
electromagnetic wavefront exits lens group 180, it converges to a
focal point at image plane 190.
[0040] As can be seen in FIG. 1, aberration compensation module 175
is attached to mirror 170. Aberration compensation module 175 is
used, for example, to compensate for astigmatism in both mirror 170
and optical system 100. Aberration compensation module 175
compensates for astigmatism, for example, by applying a force to
the periphery of mirror 170 and thereby bending and deforming the
reflective surface of mirror 170. In optical system 100, mirror 170
is preferably a concave, aspheric mirror that can be used to alter
incident electromagnetic wavefronts. The amount that an incident
electromagnetic wavefront is altered, relative to the normal
reflective action of mirror 170, is controlled by the bending or
displacing force applied by aberration compensation module 175 to
mirror 170.
[0041] Aberrations in an Optical System
[0042] Every optical system has aberrations. Aberrations are caused
by the inherent shortcomings of the lenses and mirrors used in an
optical system. For example, astigmatism is an example of a
monochromatic aberration, which causes image blurring. As described
below, the present invention can be used to correct or compensate
for astigmatism in optical 100. The invention is not limited to
correcting only astigmatism in optical system 100, however, as will
be apparent to a person skilled in the relevant art given the
description of the present invention herein.
[0043] FIGS. 2A and 2B are example diagrams of mirrors with
astigmatism. As would be known to a person skilled in the relevant
art, aberrations in an optical system can be described by Zernike
circle polynomials. FIG. 2A shows a mirror 170a having a Z5 Zernike
circle polynomial astigmatism. FIG. 2B shows a mirror 170b having a
Z6 Zernike circle polynomial astigmatism. As can be seen in FIGS.
2A and 2B, Z5 and Z6 astigmatism are offset from one another by
forty-five degrees.
[0044] As depicted in FIG. 2A, mirror 170a has a concave,
nonspherical reflective surface. The reflective surface of mirror
170a is deformed in a way that electromagnetic wavefronts incident
upon the peripheral surface of mirror 170a at 0.degree. and
180.degree. will converge sooner than will electromagnetic
wavefronts incident upon the peripheral surface of mirror 170a at
90.degree. and 270.degree.. The Z5 astigmatism of mirror 170a can
be induced by attaching an aberration compensation module 175 to
the non-reflecting surface of mirror 170a, as shown in FIG. 1. To
correct or compensate for Z5 astigmatism in mirror 170a and/or in
optical system 100, aberration compensation module 175 must apply a
pushing or pulling force on the periphery of the non-reflective
surface of the mirror 170a at 0.degree. and 180.degree. and an
opposite pushing or pulling force on the periphery of the
non-reflective surface of the mirror 170a at 90.degree. and
270.degree.. It will be understood by a person skilled in the
relevant art why applying forces in this manner corrects or
compensates for astigmatism.
[0045] As depicted in FIG. 2B, mirror 170b also has a concave,
nonspherical reflective surface. The reflective surface of mirror
170b is deformed in a way that electromagnetic wavefronts incident
upon the peripheral surface of mirror 170b at 45.degree. and
225.degree. will converge sooner than will electromagnetic energy
wavefronts incident upon the peripheral surface of mirror 170b at
135.degree. and 315.degree.. The Z6 astigmatism of mirror 170b can
be induced by attaching an aberration compensation module to the
non-reflecting surface of mirror 170b and applying a pushing or
pulling force on the periphery of the non-reflective surface of the
mirror 170b at 45.degree. and 225.degree. and an opposite pushing
or pulling force on the periphery of the non-reflective surface of
the mirror 170b at 135.degree. and 315.degree.. As above, it will
be understood by a person skilled in the relevant art why applying
forces in this manner correct or compensate for Z6 astigmatism in
mirror 170b and/or optical system 100.
[0046] As would be known to a person skilled in the relevant art,
lenses and mirrors can have both Z5 and Z6 astigmatism. The
significance of classifying astigmatism as either Z5 or Z6
astigmatism is that the principle of superposition applies to
astigmatism in an optical system. As would be apparent to a person
skilled in the relevant art given the discussion herein, any
astigmatism in an optical system can be expressed as a combination
of Z5 and Z6 astigmatism. This is important because aberration
compensation module 175 can be designed to apply forces to the
periphery of mirror 170 that will compensate for any combination of
Z5 and Z6 astigmatism in optical system 100, as described
below.
[0047] Astigmatism in optical system 100, or any portion thereof,
is corrected or compensated for in a manner similar to that
described above. Astigmatism in the lenses of lens group 120 and
lens group 140 of optical system 100 will cause electromagnetic
wavefronts to arrive at certain locations on the reflective surface
of mirror 170 earlier than at other locations. To compensate for
the difference in arrival times of the wavefronts, aberration
compensation module 175 can be used to bend the reflective surface
of mirror 170 in a manner that will advance or retard wavefronts
arriving at a specific location on the reflective surface of mirror
170. Aberration compensation module 175 can also be used to
compensate for astigmatism in lens group 180, as would be apparent
to a person skilled in the relevant art given the discussion
herein.
[0048] How well astigmatism can be corrected or compensated for in
optical system 100 is dependent on the type of apparatus used to
bend mirror 170, as further described below.
[0049] Apparatus for Passive Compensation of Astigmatism
[0050] One means of compensating for astigmatism in optical system
100 is to attach a passive compensation device 300 to the
non-reflective surface of mirror 170, as shown in FIG. 3A. While
passive compensation devices, such as compensation device 300, work
well for their intended purposes, they have limitations in their
ability to compensate for astigmatism. For example, passive
compensation devises do not provide the element of control needed
to compensate for changes in astigmatism that result from
environmental factors like temperature. In order to more clearly
point out the features of the present invention, passive
compensation device 300, and its limitations, are described
below.
[0051] FIG. 3A (side view) and FIG. 3B (top view) illustrate
passive compensation device 300. Passive compensation device 300
comprises a metal bar 310 and two feet 330a and 330b attached to
the ends of metal bar 310. Passive compensation device 300 is
attached to mirror 170 using a washer 315, a thumb nut 320, and a
screw 350. Screw 350 is attached to the non-reflecting surface of
mirror 170 using a countersunk washer 340.
[0052] Passive compensation device 300 can be used to compensate
for astigmatism by aligning feet 330a and 330b with that portion of
mirror 170 where a pushing force is required. For example, if
passive compensation device 300 were attached to mirror 170a in
FIG. 2A, feet 330a and 330b would be located at 90.degree. and
270.degree., respectively. If passive compensation device 300 were
attached to mirror 170b in FIG. 2B, feet 330a and 330b would be
located at 135.degree. and 315.degree., respectively. The amount of
compensating force applied to mirror 170 is controlled by thumb nut
320.
[0053] As can be seen in FIGS. 3A and 3B, passive compensation
devise 300 has significant limitations. These limitations prevent
passive compensation device 300, and any similar passive
compensation devices, from being used in an optical system that is
required to reproduce very sharp images of fine features, such as
micron-sized component features in a semiconductor.
[0054] For example, passive compensation device 300 does not have a
control element for automatically adjusting the force applied to
mirror 170 based on changing environmental conditions. As would be
known to a person skilled in the relevant art, astigmatism in an
optical system is effected by changes in environmental conditions
such as temperature. Changes in temperature cause thermal stresses
that tend to expand or contract different components of an optical
system at different rates. Changes in temperature also cause
changes in the refractive index of optical materials. As the index
of refraction of optical materials change in a uniform temperature
environment, aberrations including astigmatism develop. As these
components' optical properties change with temperature, aberrations
such as astigmatism in an optical system change. Because there is
no means for actively varying the compensating force applied to
mirror 170 when using passive compensation device 300, some amount
of blurring will always occur at image plane 190 due to
aberrations, for example astigmatism.
[0055] Passive compensation device 300 is also limited in its
ability to compensate for astigmatism because it can only apply a
pushing force to the periphery of mirror 170. As described above in
reference to FIGS. 2A and 2B, both a pulling force and a pushing
force must be applied to mirror 170 in order to accurately correct
or compensate for aberrations such as astigmatism in an optical
system.
[0056] As shown in FIG. 3A, passive device 300 applies a pulling
force to the central axis of mirror 170. This is due to the means
used to attach passive device 300 to mirror 170. This creates a
pulling force on the axis of mirror 170 that tends to distort the
reflecting surface of mirror 170 without compensating for
astigmatism. As would be apparent to a person skilled in the
relevant art, given the discussion herein, a pulling force applied
to the central axis of mirror 170 will not correct or compensate
for astigmatism because astigmatism is an off-axis aberration.
[0057] A further limitation of passive compensation device 300 is
that its weight is supported by mirror 170. The weight of passive
compensation device 300 thus tends to distort the reflective
surface of mirror 170, and can prevent optical system 100 from
reproducing very sharp images of fine features.
[0058] Additional limitations of passive compensation device 300,
and similar devices, will be apparent to a person skilled in the
relevant art given the discussion herein. These limitations are
overcome by the present invention.
[0059] Apparatus for Active Compensation of Aberrations in an
Optical System
[0060] The present invention provides an apparatus that can be used
for active compensation of aberrations in an optical system. For
example, the particular embodiments of the present invention
described in detail below are capable of simultaneously
compensating for both Z5 and Z6 astigmatism. As will be apparent to
a person skilled in the relevant art given the description herein,
embodiments of the present inventions are capable of compensation
for any aberration in an optical system.
[0061] FIG. 4A (side view) and FIG. 4B (top view) depict a
compensation module 400 for active compensation of astigmatism in
optical system 100 according to one embodiment of the present
invention. Compensation module 400, comprises a force bar 410
connected to a force bar 415 by actuators 420. Force bars 410 and
415 are attached to mirror 170 by a plurality of flexures 430. The
weight of force bars 410 and 415, as well as the wight of the other
components of compensation module 400, is supported by support
structure 450 rather than by mirror 170.
[0062] As can be seen in FIGS. 4A and 4B, force bar 410 is
substantially perpendicular to force bar 415. In an embodiment,
force bar 410 has an opening near its longitudinal midpoint. This
opening is formed in force bar 410 by bifurcating force bar 410
near its midpoint so that it has first and second opposed surfaces.
Force bar 415 is passed through the opening in force bar 410 and
partially resides in the opening. This arrangement permits
actuators 420a and 420b to connect force bar 415 to the first
opposed surface of force bar 410 and actuators 420c and 420d to
connect force bar 415 to the second opposed surface of force bar
410. As would be apparent to a person skilled in the relevant art,
only one actuator 420 is needed for the invention to function
properly and other means for connecting force bars 410 and 415
using an actuator 420 are possible. For example, force bar 410
might not have an opening. In that case, force bar 415 can be
connected to force bar 410 with only one actuator located at the
crossing point of force bars 410 and 415.
[0063] In the example embodiment show in FIG. 4A and 4B, the ends
of force bars 410 and 415 are attached to the periphery of mirror
170 by flexures 430. One end of force bar 410 is attached to mirror
170 by a flexure 430a. A second end of force bar 410 is attached to
mirror 170 by a flexure 430b. In an embodiment of the present
invention, force bars 410 and 415 are each made from metal. Force
bars 410 and 415 can also be leaf springs.
[0064] In the example embodiment show in FIGS. 4A and 4B, actuators
420 are pneumatic bellows. Control air is supplied to pneumatic
bellows 420 using air supply lines 422. As shown in FIGS. 4A and
4B, air supply line 422a supplies control air to pneumatic bellows
420a and air supply line 422b supplies control air to pneumatic
bellows 420b. Additional air supply lines (not shown) supply
control air to optional pneumatic bellows 420c and 420d if
required. The ends of force bar 410 will apply a pulling force on
the periphery of mirror 170 when pneumatic bellows 420a and 420b
are pressurized to a pressure greater than that of pneumatic
bellows 420c and 420d. Under these same conditions, the ends of
force bar 415 will apply a pushing force on the periphery of mirror
170. Venting pneumatic bellows 420c and 420d to the environment and
pressurizing pneumatic bellows 420a and 420b to a pressure greater
than atmospheric pressure will cause the ends of force bar 410 to
apply a pulling force to the periphery of mirror 170 and the ends
force bar 415 to apply a pushing force to the periphery of mirror
170.
[0065] In another embodiment of the present invention, actuators
420 can be electro-mechanical devices, such as solenoids, rather
than pneumatic devices. Using actuators other than pneumatic
bellows are contemplated and can be advantageous in optical systems
where a source of compressed air is not readily available. In other
embodiments, actuators 420 can be hydraulic or piezoelectric
devices. The number of actuators 420 can be varied.
[0066] As can be seen in FIGS. 4A and 4B, the weight of force bars
410 and 415, as well as the weight of the other components of
compensation module 400, is supported by support structure 450. The
ends of force bars 410 and 415 are connected to support structure
450 by flexures 455. In a preferred embodiment, support structure
450 is a ring. Support structure 450 is connected to a weight
bearing portion 460 of optical system 100 by connecting devices
465. Support structure 450 can be connected to any weight bearing
portion 460 of optical system 100 that can support the weight of
compensation module 400 without affecting the performance of
optical system 100. Support structure 450 can be connected to
weight bearing portion 460 by any connecting devices 465 that will
perform the function of connecting support structure 450 to weight
bearing portion 460.
[0067] FIG. 5A (side view) and FIG. 5B (top view) depict a
compensation module 500 that is capable of simultaneously
compensating for combinations of both Z5 and Z6 astigmatism in an
optical system. As can be seen in FIGS. 5A and 5B, compensation
module 500 comprises two adjustable force modules 501A and 501B,
which operate in a manner similar to compensation module 400. Each
adjustable force module 501 comprises two substantially
perpendicular force bars connected by actuators. In FIG. 5A,
adjustable force modules 501B is shown rotated 45 degrees from its
actual position in order to more clearly show the openings in force
bar 540. The actual orientation of force module 501B is that shown
in FIG. 5B. The geometry of the attachment flexures 430 and force
bar comprising force modules 501 are selected along with other
hardware to allow independent operation without mechanical
interference.
[0068] As shown in FIG. 5B, adjustable force module 501A is aligned
to compensate for Z5 astigmatism and comprises force bars 510 and
520. Adjustable force module 501B is aligned to compensate for Z6
astigmatism and comprises force bars 530 and 540. As will be
apparent to a person skilled in the relevant art(s), given the
discussion herein, compensation module 500 can be adjusted to
actively compensate for any astigmatism in optical system 100.
[0069] Although compensation module 500 is shown as comprising only
two adjustable force modules, there are situations where more than
two adjustable force modules may be used. For example, more than
two adjustable force modules may be used in applications where
warranted due to the size of mirror 170 and the maximum force that
can be applied to mirror 170 by a single adjustable force module.
Other applications where it may be desirable to use more than two
adjustable force modules will be apparent to a person skilled in
the relevant art given the discussion herein. For example, when
forces are to be applied at different distances from the central
axis.
[0070] System and Method for Active Compensation of Aberrations in
an Optical System
[0071] FIG. 6 illustrates a preferred embodiment of a system 600
for actively controlling aberrations such as astigmatism in optical
system 100 according to the present invention. System 600 comprises
a compensation module 500 having a pneumatic actuator 420, a
control module 610, a sensing module 620, an air supply module 630,
and a three-way valve 640. Pneumatic compensation module 500
operates as described above. The operation of compensation module
500 is controlled by control module 610 using information received
from sensing module 620. Compensation module 500 is connected to
air supply module 630 using three-way valve 640 and air supply
lines 422 and 638. Control module 610 controls three-way valve 640
to control the pressure in pneumatic actuator 420.
[0072] In an embodiment of the present invention, system 600
controls aberrations such as astigmatism in optical system 100
using a method 700 as depicted in FIG. 7. Method 700 starts with
system 600 in a state of equilibrium.
[0073] In step 710 of method 700, senor module 620 is used to
monitor a parameter related to an aberration in optical system 100
and used to predict the presence of an aberration, such as
astigmatism, in optical system 100. Sensing module 620 can monitor
any parameter that is measurable and that can be related to the
control of an aberration, such as astigmatism, in optical system
100. For example, sensing module 620 can monitor temperature at
various locations in optical system 100. Temperature is one
parameter that can be used to determine thermally induced optical
variations in optical system 100 that affect astigmatism. The
temperature monitored by sensing module 620 can be used, for
example, in an algorithm that predicts the presents of astigmatism
in optical system 100, or alternatively, the temperature monitored
by sensing module 620 can be used to lookup a predetermined value
in a lookup table stored, for example, in a memory unit 614, which
is used to predict the presents of astigmatism in optical system
100. Sensing module 620 can also monitor other parameters such as,
for example, electromagnetic energy exiting optical system 100 at
beamsplitter block 150. Electromagnetic energy exiting optical
system 100 can be used, for example, to determine whether
astigmatism in optical system 100 is being corrected. Other
parameters that can be monitored by sensing module 620 would be
apparent to a person skilled in the relevant arts given the
discussion herein.
[0074] In step 720 of method 700, control module 610 uses the
output of sensing module 620 to generate a corrective signal
related to an aberration, such as astigmatism. In an embodiment of
the present invention, control module 610 communicates with senor
module 620 using communications link 616. As shown in FIG. 6,
control module 610 comprises a central processing unit (CPU) 612
and memory unit 614. In an embodiment, several parameters can be
monitored simultaneously by senor module 620. Alternatively,
several senor modules 620 can be used to monitor several
parameters.
[0075] Data relating the output of sensing module 620 to an
aberration, such as astigmatism, in optical system 100 can be
determined empirically or analytically and stored in memory unit
614. One technique for storing the data in memory unit 614 is to
store the data in a lookup table. For example, if sensing module
620 monitors both temperature and pressure, the various
temperatures that can be measured by sensing module 620 can be
stored in memory unit 614 in a lookup table that relates a
temperature of optical system 100 to a pressure in pneumatic
actuator 420. In this embodiment, CPU 612 receives temperature and
pressure data from sensing module 620. CPU 612 then looks up the
pressure for pneumatic actuator 420 that corresponds to the
temperature data received form sensing module 620 in the lookup
table stored in memory unit 614. CPU 612 generates a corrective
signal based on the difference between the pressure data retrieved
from the lookup table and the pressure data received from the
output of sensing module 620. In another embodiment, control module
610 produces a corrective signal based solely on the combined
outputs of sensing module 620 without retrieving data from a lookup
table stored in memory. A person skilled in the relevant art will
know that other methods and techniques of generating a corrective
signal based on the output of sensing module 620 are contemplated
and considered to be part of the present invention. These methods
and techniques include both open-loop and closed-loop control
means. These methods and techniques can also include feedback
loops.
[0076] In step 730 of method 700, the corrective signal generated
in step 720 is used to adjust the air pressure in pneumatic
actuator 420 and thereby compensate for an aberration, such as
astigmatism, in optical system 100. As shown in FIG. 6, system 600
has an air supply module 630. Air supply module 630 can be used to
adjust the air pressure in pneumatic actuator 420. Adjusting the
air pressure in pneumatic actuator 420 changes the bending or
displacing force applied by compensation module 500 to the
non-reflecting surface of mirror 170. In an embodiment, compressed
air is stored in an air tank 635. The pressure of the compressed
air in air tank 635 is determined and regulated by well-known
mechanisms (not shown) for regulating and maintaining air pressure
in a tank. These mechanisms form a part of air supply module 630.
When air pressure in pneumatic actuator 420 needs to be increased
based on the corrective signal, control module 610 sends a signal
over a communications link 618 to three-way valve 640. The signal
changes the position of three-way valve 640 and allows compressed
air from air tank 635 to flow into pneumatic actuator 420 until the
desired air pressure has been established in pneumatic actuator
420. When air pressure in pneumatic actuator 420 needs to be
decreased based on the corrective signal, control module 610 sends
a signal to three-way valve 640 that vents air in pneumatic
actuator 420 to the environment until the desired air pressure in
actuator 420 has been established.
[0077] In step 740 of method 700, steps 710 through 730 are
continuously repeated in a loop to achieve additional correction or
compensation of aberrations in optical system 100. Active
compensation of aberrations in optical system 100 ends when control
passes to step 750 of method 700.
[0078] Various embodiments of the present invention have been
described above, which can be used to actively compensate for
aberrations in an optical system. It should be understood that
these embodiments have been presented by way of example only, and
not limitation.
[0079] In particular, it will be understood by those skilled in the
relevant art that embodiments of the present invention can be used
to correct or compensate for aberrations in an optical system other
than astigmatism, and that the present invention is not limited to
only correcting or compensating for astigmatism. For example, the
force modules described herein can be tailored to correct for
different aberrations by varying the number of force bars per
module, by varying the number attachment points between a mirror
and a force bar, and/or by varying the distance of the attachment
points from the central axis of the mirror. Additionally, the
number of ends per force bar can be varied, e.g., a force bar can
have three, four, five, or more ends depending on the shape and
construction of a force bar. Higher order aberrations can be
corrected, for example, by using multiple combinations of force
modules at different orientations, as will be apparent to those
skilled in the relevant art given the description herein.
[0080] It will also be understood by those skilled in the relevant
art that various changes in form and details of the embodiments
described above may be made without departing from the spirit and
scope of the present invention as defined in the claims. Thus, the
breadth and scope of the present invention should not be limited by
any of the above-described exemplary embodiments, but should be
defined only in accordance with the following claims and their
equivalents.
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