U.S. patent application number 12/951777 was filed with the patent office on 2011-03-17 for optical element module.
This patent application is currently assigned to CARL ZEISS SMT AG. Invention is credited to Thomas Bischoff, Hagen Federau, Willi Heintel, Jochen Wieland, Bernd Wuesthoff.
Application Number | 20110063590 12/951777 |
Document ID | / |
Family ID | 37076216 |
Filed Date | 2011-03-17 |
United States Patent
Application |
20110063590 |
Kind Code |
A1 |
Bischoff; Thomas ; et
al. |
March 17, 2011 |
OPTICAL ELEMENT MODULE
Abstract
An optical element module comprising a plurality of module
components is provided. The module components comprise an optical
element, an optical element holder and a contact element. The
optical element has a first coefficient of thermal expansion. The
optical element holder holds the optical element via the first
contact element and has a second coefficient of thermal expansion,
the second coefficient of thermal expansion being different from
the first coefficient of thermal expansion. At least one of the
module components is adapted to provide at least a reduction of
forces introduced into the optical element upon a thermally induced
position change in the relative position between the optical
element and the optical element holder, the position change
resulting from a temperature situation variation in a temperature
situation of the plurality of module components.
Inventors: |
Bischoff; Thomas; (Aalen,
DE) ; Federau; Hagen; (Meersburg, DE) ;
Heintel; Willi; (Aalen, DE) ; Wuesthoff; Bernd;
(Frankfurt am Main, DE) ; Wieland; Jochen; (Aalen,
DE) |
Assignee: |
CARL ZEISS SMT AG
Oberkochen
DE
|
Family ID: |
37076216 |
Appl. No.: |
12/951777 |
Filed: |
November 22, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12015894 |
Jan 17, 2008 |
7859641 |
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12951777 |
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PCT/EP2006/064427 |
Jul 19, 2006 |
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12015894 |
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60700517 |
Jul 19, 2005 |
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Current U.S.
Class: |
355/30 ;
355/77 |
Current CPC
Class: |
G02B 7/028 20130101;
G02B 7/181 20130101; G02B 7/008 20130101; G03F 7/70891 20130101;
G03F 7/70825 20130101 |
Class at
Publication: |
355/30 ;
355/77 |
International
Class: |
G03B 27/52 20060101
G03B027/52; G03B 27/32 20060101 G03B027/32 |
Claims
1. An optical element module comprising: an optical element, an
optical element holder and a first contact element; the optical
element having a first coefficient of thermal expansion; the
optical element holder holding the optical element via the first
contact element and having a second coefficient of thermal
expansion, the second coefficient of thermal expansion being
different from the first coefficient of thermal expansion; a first
contact point being formed on a first module component, the first
module component being one of the optical element and the optical
element holder; the first contact element having a second contact
point and a third coefficient of thermal expansion; the first
contact point, at a first temperature situation, contacting the
second contact point at a first location; the first contact element
contacting a second module component at a second location, the
second location, at the first temperature situation, being located
at a first contact location distance from the first location; the
second module component being different from the first module
component and being one of the optical element and the optical
element holder; at least one of the third coefficient of thermal
expansion and the contact location distance being selected such
that, at a given second temperature situation different from the
first temperature situation, a thermally induced modification in
the size of the first contact element with respect to the first
temperature situation compensates for the difference between the
first coefficient of thermal expansion and the second coefficient
of thermal expansion such that, at the second temperature
situation, there is substantially no shift between the first
contact point and the second contact point.
2. The optical element module according to claim 1, wherein the
optical element forms the first module component and the optical
element holder forms the second module component.
3. The optical element module according to claim 1, wherein the
optical element holder comprises a first material and the first
contact element comprises a second material different from the
first material.
4. The optical element module according to claim 1, wherein the
optical element comprises quartz (SiO.sub.2), the optical element
holder comprises Invar, and the first contact element comprises
steel.
5. The optical element module according to claim 1, wherein the
third coefficient of thermal expansion is considerably higher than
the second coefficient of thermal expansion.
6. The optical element module according to claim 1, wherein the
optical element mainly extends in a first plane and has a centrally
located optical element axis perpendicular to the first plane, and
the first contact point is located at a first contact surface of
the optical element, the first contact surface being perpendicular
to the optical element axis.
7. The optical element module according to claim 1, wherein the
optical element has an outer circumference and a plurality of first
contact elements contacting the optical element and the optical
element holder are distributed at the outer circumference.
8. The optical element module according to claim 7, wherein the
optical element holder comprises a ring shaped holder unit; the
holder unit extending along the outer circumference of the optical
element and contacting the first contact element.
9. The optical element module according to claim 8, wherein a ring
shaped frame unit is provided; the frame unit extending along an
outer circumference of the optical element holder and holding the
optical element holder via a plurality of deformation uncoupling
elements.
10. The optical element module according to claim 9, wherein the
frame unit comprises a material being, at least with respect to its
coefficient of thermal expansion, different from a material the
optical element holder comprises.
11. The optical element module according to claim 1, wherein a
second contact element is provided; the second contact element
contacting the optical element and the optical element holder and
having a fourth coefficient of thermal expansion.
12. The optical element module according to claim 11, wherein the
second contact element comprises a material being, at least with
respect to its coefficient of thermal expansion, different from a
material the optical element holder comprises.
13. The optical element module according to claim 11, wherein the
optical element comprises quartz (SiO.sub.2), the optical element
holder comprises Invar, and the second contact element comprises
steel.
14. The optical element module according to claim 11, wherein the
fourth coefficient of thermal expansion is considerably higher than
the second coefficient of thermal expansion.
15. The optical element module according to claim 11, wherein the
optical element has an outer circumference and a plurality of
second contact elements contacting the optical element and the
optical element holder are distributed at the outer
circumference.
16. An optical element module comprising: a plurality of module
components; the module components comprising an optical element, an
optical element holder and a contact element; the optical element
having a first coefficient of thermal expansion; the optical
element holder holding the optical element via the first contact
element and having a second coefficient of thermal expansion, the
second coefficient of thermal expansion being different from the
first coefficient of thermal expansion; at least one of the module
components being adapted to provide at least a reduction of forces
introduced into the optical element upon a thermally induced
position change in the relative position between the optical
element and the optical element holder; the position change
resulting from a temperature situation variation in a temperature
situation of the plurality of module components.
17. The optical element module according to claim 16, wherein at
least one of the first contact surface and the second contact
surface is formed by a low friction coefficient coating.
18. The optical element module according to claim 16, wherein the
contact element has a third coefficient of thermal expansion; the
third coefficient of thermal expansion being selected such that, at
the temperature situation variation, a thermally induced
modification in the size of the contact element compensates for the
position change.
19. The optical element module according to claim 16, wherein one
of the optical element and the optical element holder forms a first
module component; a first contact surface being formed on the first
module component; the contact element having a curved second
contact surface contacting the first contact surface; the first
contact element being adapted such that the second contact surface
executes a rolling motion with respect to the first contact surface
upon the position change.
20. An optical element unit comprising: a plurality of optical
element modules connected to each other and supporting a plurality
of optical elements, the plurality of optical element modules
comprising a first optical element module being an optical element
module according to claim 1.
21. An optical exposure apparatus for transferring an image of a
pattern formed on a mask onto a substrate comprising: a light path;
a mask location located within the light path and receiving the
mask; a substrate location located at an end of the light path and
receiving the substrate; an optical element unit within the light
path between the mask location and the substrate location, the
optical element unit comprising: a plurality of optical element
modules connected to each other and supporting a plurality of
optical elements, wherein the plurality of optical element modules
comprising a first optical element module being an optical element
module according to claim 1.
22. A method of holding an optical element comprising: in a first
step, providing a plurality of module components, the module
components comprising an optical element, an optical element holder
and a contact element, and, in a second step, holding the optical
element using the optical element holder, the optical element
holder holding the optical element via the contact element; the
optical element having a first coefficient of thermal expansion;
the optical element holder having a second coefficient of thermal
expansion, the second coefficient of thermal expansion being
different from the first coefficient of thermal expansion; at least
one of the module components being adapted to provide at least a
reduction of forces introduced into the optical element upon a
thermally induced position change in the relative position between
the optical element and the optical element holder; the position
change resulting from a temperature situation variation in a
temperature situation of the plurality of module components.
23. The method according to claim 22, wherein in the first step, a
contact element is provided, the contact element having a third
coefficient of thermal expansion; the third coefficient of thermal
expansion being selected such that, at the temperature situation
variation, a thermally induced modification in the size of the
contact element compensates for the position change.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 12/015,894, filed Jan. 17, 2008, which is a continuation under
35 U.S.C. .sctn.120 of international application PCT/EP2006/064427,
filed Jul. 19, 2006, which claims the benefit under 35 U.S.C.
119(e)(1) of provisional U.S. Patent Application Ser. No.
60/700,517 filed 19 Jul. 2005. The entire contents of these
applications are hereby incorporated herein by reference.
FIELD
[0002] The disclosure relates to optical element modules used in
exposure processes, in particular to optical element modules of
microlithography systems. It further relates to optical element
units comprising such optical element modules. It also relates to
optical exposure apparatuses comprising such optical element units.
Furthermore, it relates to a method of holding an optical element.
The disclosure may be used in the context of photolithography
processes for fabricating microelectronic devices, in particular
semiconductor devices, or in the context of fabricating devices,
such as masks or reticles, used during such photolithography
processes.
BACKGROUND
[0003] Typically, the optical systems used in the context of
fabricating microelectronic devices such as semiconductor devices
comprise a plurality of optical elements, such as lenses and
mirrors etc., in the light path of the optical system. Those
optical elements usually cooperate in an exposure process to
transfer an image formed on a mask, reticle or the like onto a
substrate such as a wafer. Said optical elements are usually
combined in one or more functionally distinct optical element
groups. These distinct optical element groups may be held by
distinct optical element units. Such optical element units are
often built from a stack of optical element modules holding one or
more optical elements. These optical element modules usually
comprise an external generally ring shaped support device
supporting one or more optical element holders each, in turn,
holding an optical element.
[0004] Optical element groups comprising at least mainly refractive
optical elements, such as lenses, mostly have a straight common
axis of symmetry of the optical elements usually referred to as the
optical axis. Moreover, the optical element units holding such
optical element groups often have an elongated substantially
tubular design due to which they are typically referred to as lens
barrels.
[0005] Due to the ongoing miniaturization of semiconductor devices
there is a permanent need for enhanced resolution of the optical
systems used for fabricating those semiconductor devices. This need
for enhanced resolution obviously pushes the need for an increased
numerical aperture and increased imaging accuracy of the optical
system.
[0006] Furthermore, to reliably obtain high-quality semiconductor
devices it is not only necessary to provide an optical system
showing a high degree of imaging accuracy. It is also necessary to
maintain such a high degree of accuracy throughout the entire
exposure process and over the lifetime of the system. As a
consequence, the optical elements of such an optical system is
desirably supported in a defined manner in order to maintain a
predetermined spatial relationship between said optical elements to
provide a high quality exposure process.
[0007] In this context there exist, among others, two general
requirements for the support of optical elements of the optical
system. One is that the rigidity of the support system of the
optical elements has to be as high as possible in certain
directions, in particular in the direction of the optical axis, to
keep the resonant frequencies of the system as high as possible.
Furthermore, deformations of the optical elements of the optical
system are to be avoided to the greatest possible extent in order
to keep imaging errors resulting from such deformations as low as
possible.
[0008] One such imaging error is for example stress induced
birefringence of refractive optical elements. Such stress induced
birefringence mainly results from stresses introduced into the
optical element via its peripheral support structure and radially
propagating through the optically used area of the optical element.
Such stresses are often thermally induced, resulting from
differences in the coefficient of thermal expansion (CTE) of the
optical element and its peripheral support structure. Variations in
the temperature situation of the optical element and its peripheral
support structure lead to relative movements between the optical
element and its peripheral support structure. These relative
movements are counteracted by the holding forces acting between the
optical element and its peripheral support structure leading to the
above undesired stress situations.
[0009] To avoid thermally induced stresses and deformations within
an optical element due to differences in the coefficient of thermal
expansion of the optical element and its optical element holder, it
is known to connect the optical element and its optical element
holder via deformation uncoupling elements. These deformation
uncoupling elements generally allow for relative movements between
the optical element and its optical element holder.
[0010] These deformation uncoupling elements may provide a
reduction of the stresses and, thus, the deformations introduced
into the optical element. However, they have the disadvantage that
they also reduce the rigidity of the support system. To deal with
this effect, the rigidity of the uncoupling elements might be
increased, but this would reduce their deformation uncoupling
abilities leading to increased stresses and, thus, the deformations
introduced into the optical element.
[0011] Another approach to deal with this problem is known from US
2001/0039126 A1 (to Ebinuma et al.). Here, it is provided for an
adaptation of the coefficients of thermal expansion between an
optical element and a support ring contacting the optical element
in order to reduce the introduction of thermally induced
deformations into the optical element resulting from differences in
the coefficients of thermal expansion. However, this solution my
have the disadvantage that, for certain optical elements with a
certain coefficient of thermal expansion, the adaptation of the
coefficient of thermal expansion may only be achieved with
comparatively expensive materials for such large parts as the
support ring.
SUMMARY
[0012] It is thus an object of the disclosure to, at least to some
extent, overcome the above disadvantages and to provide good and
long term reliable imaging properties of an optical system used in
an exposure process.
[0013] It is a further object of the disclosure to increase imaging
accuracy of an optical system used in an exposure process by
reducing thermally induced stresses introduced into an optical
element of the optical system.
[0014] It is a further object of the disclosure to increase imaging
accuracy of an optical system used in an exposure process by
reducing stress induced birefringence introduced into an optical
element of the optical system via its support structure.
[0015] These objects are achieved which is based on the teaching
that a reduction in the deformations introduced into an optical
element of the optical system via its support structure and a high
rigidity of the support mechanism for the optical element may be
achieved when at least one of the module components of an optical
element module is adapted to provide, compared to conventional
deformation uncoupling elements, at least a reduction of forces
introduced into the optical element upon a thermally induced
position change in the relative position between the optical
element and the optical element holder by maintaining, at the same
time, a high rigidity of the support mechanism. This reduction of
disturbing forces upon maintained support rigidity may be achieved
in several ways.
[0016] One solution is to provide a contact element that
compensates by its thermal expansion properties for the difference
in the coefficient of thermal expansion between the optical element
and the associated optical element holder such that, at least at a
given variation in the temperature situation, no relative shift
between the contact points of the module components occurs. With
this solution, the thermally induced introduction of disturbing
forces into the optical element may even be completely avoided.
This solution has the further advantage that, compared to the known
adaptation of the coefficients of thermal expansion of the optical
element and the optical element holder, with the contact element
only a relatively small part has to be adapted to the given
coefficient of thermal expansion situation. Furthermore, at a given
coefficient of thermal expansion situation, adaptation may be
provided easily by simply adapting the effective distance between
the contact points of the contact element with the optical element
and the optical element holder, respectively.
[0017] A second solution is to allow for a relative movement
between the optical element and the associated optical element
holder at a variation in the temperature situation, but to reduce
the disturbing forces introduced into the optical element as a
result of such a relative movement. Since these disturbing forces
predominantly result from frictional forces between the coupled
module components, a reduction of these frictional forces at the
interface of the coupled module components is provided. This may be
achieved by adapting the frictional properties of the module
components at the interface location to provide a low friction
contact. Furthermore, the relative motion between the module
components may be adapted to provide a type of motion with low
friction. In some embodiments, due to the low frictional forces
transmitted at such a motion type, a rolling motion is provided at
the interface location between the module components.
[0018] A third solution is to overall reduce, under normal
operating conditions, the holding forces exerted on the optical
element and, thus, also the disturbing forces introduced into the
optical element at a thermally induced relative movement between
the module components. This solution is based on the concept that
the holding forces usually counteract also the thermally induced
relative movement between the module components and, thus, have an
influence on the frictional forces introduced into the optical
element at such a relative movement between the module components.
Usually, due to the manufacture and mounting of the optical system
at a location different from the location of its later use, the
holding forces provided for the optical elements do not only
account for the forces occurring under normal operating conditions
of the optical system but also have to account for considerably
higher abnormal forces occurring during, for example, transport of
the optical system. Thus, in conventional systems, holding forces
are considerably higher than necessary in normal use. This
obviously leads to considerable disturbing forces introduced into
the optical element at a thermally induced relative movement
between the module components. These disturbing forces can be
reduced by providing a securing device which is only activated
under abnormal load conditions in order to hold the optical element
in place. Thus, under normal operating conditions, holding forces
which are considerably lower than in conventional systems may be
applied to the optical element leading, in turn, to reduced
disturbing forces.
[0019] It will be appreciated that arbitrary combinations of the
above solutions may be selected to combine their beneficial effects
and to further reduce the disturbing forces introduced into the
optical element at thermally induced relative movements between
some of the module components.
[0020] Thus, according to a first aspect of the disclosure there is
provided an optical element module comprising an optical element,
an optical element holder and a first contact element. The optical
element has a first coefficient of thermal expansion. The optical
element holder holds the optical element via the first contact
element and has a second coefficient of thermal expansion, the
second coefficient of thermal expansion being different from the
first coefficient of thermal expansion. A first contact point is
formed on a first module component, the first module component
being one of the optical element and the optical element holder.
The first contact element has a second contact point and a third
coefficient of thermal expansion. At a first temperature situation,
the first contact point contacts the second contact point at a
first location. Furthermore, the first contact element contacts a
second module component at a second location, the second location,
at the first temperature situation, being located at a first
contact location distance from the first location, and the second
module component being different from the first module component
and being one of the optical element and the optical element
holder. At least one of the third coefficient of thermal expansion
and the contact location distance is selected such that, at a given
second temperature situation different from the first temperature
situation, a thermally induced modification in the size of the
first contact element with respect to the first temperature
situation compensates for the difference between the first
coefficient of thermal expansion and the second coefficient of
thermal expansion such that, at the second temperature situation,
there is substantially no shift between the first contact point and
the second contact point.
[0021] According to a second aspect of the disclosure there is
provided an optical element module comprising an optical element,
an optical element holder and a first contact element. The optical
element has a first coefficient of thermal expansion. The optical
element holder holds the optical element via the first contact
element and has a second coefficient of thermal expansion, the
second coefficient of thermal expansion being different from the
first coefficient of thermal expansion. One of the optical element
and the optical element holder forms a first module component and
one of the optical element and the optical element holder forms a
second module component being different from the first module
component. A first contact surface is formed on the first module
component, the first contact element having a curved second contact
surface contacting the first contact surface. The first contact
element is adapted such that the second contact surface executes a
rolling motion with respect to the first contact surface upon a
thermally induced change in the relative position between the
optical element and the optical element holder.
[0022] According to a third aspect of the disclosure there is
provided an optical element module comprising a plurality of module
components. The module components comprise an optical element, an
optical element holder and a contact element. The optical element
has a first coefficient of thermal expansion. The optical element
holder holds the optical element via the first contact element and
has a second coefficient of thermal expansion, the second
coefficient of thermal expansion being different from the first
coefficient of thermal expansion. At least one of the module
components is adapted to provide at least a reduction of forces
introduced into the optical element upon a thermally induced
position change in the relative position between the optical
element and the optical element holder, the position change
resulting from a temperature situation variation in a temperature
situation of the plurality of module components.
[0023] According to a fourth aspect of the disclosure there is
provided an optical element unit comprising a plurality of optical
element modules connected to each other and supporting a plurality
of optical elements. The plurality of optical element modules
comprises a first optical element module being an optical element
module.
[0024] According to a fifth aspect of the disclosure there is
provided an optical exposure apparatus for transferring an image of
a pattern formed on a mask onto a substrate comprising a light
path, a mask location located within the light path and receiving
the mask, a substrate location located at an end of the light path
and receiving the substrate and an optical element unit located
within the light path between the mask location and the and the
substrate location.
[0025] According to a sixth aspect of the disclosure there is
provided a method of holding an optical element comprising, in a
first step, providing a plurality of module components, the module
components comprising an optical element, an optical element holder
and a contact element, and, in a second step, holding the optical
element using the optical element holder, the optical element
holder holding the optical element via the contact element. The
optical element having a first coefficient of thermal expansion.
The optical element holder has a second coefficient of thermal
expansion, the second coefficient of thermal expansion being
different from the first coefficient of thermal expansion. At least
one of the module components is adapted to provide at least a
reduction of forces introduced into the optical element upon a
thermally induced position change in the relative position between
the optical element and the optical element holder, the position
change resulting from a temperature situation variation in a
temperature situation of the plurality of module components.
[0026] Further aspects and embodiments of the disclosure will
become apparent from the dependent claims and the following
description of preferred embodiments which refers to the appended
figures. All combinations of the features disclosed, whether
explicitly recited in the claims or not, are within the scope of
the disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a schematic representation of an optical exposure
apparatus comprising preferred embodiments of an optical element
unit and optical element modules;
[0028] FIG. 2 is a perspective view of a schematic sectional
representation of a part of an optical element unit of the optical
exposure apparatus of FIG. 1;
[0029] FIG. 3 is a perspective view of schematic sectional
representation of another part of the optical element unit along
line III-Ill of FIG. 2;
[0030] FIG. 4 is a block diagram of a method of holding an optical
element;
[0031] FIG. 5 is a schematic sectional representation of a part of
a further optical element module used in the optical exposure
apparatus of FIG. 1;
[0032] FIG. 6A is a schematic perspective view of a further contact
element that may be used in the optical element module of FIG.
5;
[0033] FIG. 6B is another schematic view of the contact element of
FIG. 6A;
[0034] FIG. 7A is a schematic view of a part of a further optical
element module used in the optical exposure apparatus of FIG.
1;
[0035] FIG. 7B is a schematic view of the detail B of FIG. 7A;
[0036] FIG. 8A is a schematic view of a part of a further optical
element module used in the optical exposure apparatus of FIG.
1;
[0037] FIG. 8B is a schematic view of the detail B of FIG. 8A.
DETAILED DESCRIPTION OF THE DISCLOSURE
First Embodiment
[0038] In the following, a first preferred embodiment of an optical
exposure apparatus 1 comprising an optical projection system 2 with
an optical element unit 3 will be described with reference to FIGS.
1 and 2.
[0039] The optical exposure apparatus 1 is adapted to transfer an
image of a pattern formed on a mask 4 onto a substrate 5. To this
end, the optical exposure apparatus 1 comprises an illumination
system 6 illuminating said mask 4 and the optical element unit 3.
The optical element unit 3 projects the image of the pattern formed
on the mask 4 onto the substrate 5, e.g. a wafer or the like.
[0040] To this end, the optical element unit 3 holds an optical
element group 7. This optical element group 7 is held within a
housing 3.1 of the optical element unit 3. The optical element
group 7 comprises a number of optical elements 107 and 207 as well
as optical elements 407 and 507 such as lenses, mirrors or the
like. These optical elements 107, 207, 407, 507 are aligned along a
folded optical axis 3.2 of the optical element unit 3
[0041] The optical projection system 2 receives the part of the
light path between the mask 4 and the substrate 5. Its optical
elements 107, 207, 407, 507 cooperate to transfer the image of the
pattern formed on the mask 4 onto the substrate 5 located at the
end of the light path. To increase the numerical aperture NA of the
optical projection system 2, the optical projection system 2 may
comprise an immersion zone located between the lower end of the
optical element unit 3 and the substrate 5.
[0042] The optical element unit 3 is composed of a plurality of
optical element modules 3.3 and 3.4, optical element modules 3.5
and 3.6 as well as an optical element module 3.7 stacked and
tightly connected to form the optical element unit 3. Each optical
element module 3.3 to 3.6 holds one or more of the optical elements
107, 207, 407, 507, respectively. The optical element module 3.7 is
an interface module holding a reflecting optical element 12 used to
fold the optical axis 3.2. The optical element module 3.7 is an
interface module providing an interface for the respective module
stack.
[0043] FIG. 2 shows a schematic perspective view of a sectional
representation of a part of the optical element module 3.3 of the
optical element unit 3 at a first temperature situation T1 of the
optical element unit 3. The first temperature situation T1 is
characterized by a certain temperature profile within the
components of the optical element unit 3.
[0044] The optical element 107 of the optical element module 3.3 is
a rotationally symmetric lens having an optical axis 107.1. The
lens 107 is made of Quartz (SiO.sub.2) and has a first coefficient
of thermal expansion.
[0045] The lens 107 is usually positioned in space such that the
optical axis 107.1 of the lens 107 is substantially collinear with
the optical axis 3.2 of the optical element unit 3. It should be
noted that the position of the optical axis 107.1 of the lens 107
shown in FIG. 2 is not to scale. In reality, the optical axis 107.1
is located at a distance from the outer circumference of the lens
107 that by far exceeds the distance shown in FIG. 2.
[0046] The lens 107 is held by an optical element holder in the
form of a ring shaped lens holder 108 which, in turn, is held by a
ring shaped frame element 109. The lens holder 108 is made of Invar
that has a second coefficient of thermal expansion different from,
namely larger than the first coefficient of thermal expansion of
the lens 107. The lens holder 108 holds the lens 107 in place via a
plurality of first contact elements 110 and a plurality of second
contact elements 111. In the sectional view of FIG. 2, half of a
first contact element 110 and half of a second contact element 111
is shown, both contact elements 110 and 111 being symmetric with
respect to the sectional plane.
[0047] In the embodiment shown, three first contact elements 110
and three second contact elements 111 are evenly distributed at the
inner circumference of the lens holder 108. However, it will be
appreciated that, with other embodiments of the disclosure, a
different number of first and/or second contact elements may be
provided. Thus, for example, a large number of narrowly spaced
first and/or second contact elements may be formed to provide a
configuration similar to the one disclosed in the context of FIG. 2
of U.S. Pat. No. 6,392,825 B1 (Trunz et al.), the entire disclosure
of which is hereby incorporated herein by reference. In such a
configuration, the respective first and/or second contact elements
may be formed by a radially resilient element similar to the ones
disclosed in the context of FIG. 4 of U.S. Pat. No. 4,733,945
(Bacich), the entire disclosure of which is hereby incorporated
herein by reference. Furthermore, these first and/or second contact
elements may be formed as separate elements or monolithically
connected--in groups or altogether--via a contact element
connecting element, e.g. a connecting ring similar to the one shown
in FIG. 2 of U.S. Pat. No. 6,392,825 B1 (Trunz et al.), which is
then connected lens holder. Furthermore, only one type of contact
elements may be provided. For example, only the lower first contact
elements may be provided to support the lens from below.
[0048] At the first temperature situation T1, a first contact nose
110.1 formed on one end of the first contact element 110 contacts
the lens 107 at a first location 112. Thereby a first contact point
107.2 of a plane first contact surface 107.3 of the lens 107 is
contacted by a second contact point 110.2 on the first contact nose
110.1 of the first contact element 110. Furthermore, the first
contact element 110 contacts the lens holder 108 at a second
location 113, where the first contact element 110 is connected to
the lens holder 108 by means of screws or any other suitable
fastening technique.
[0049] Similarly a second contact nose 111.1 formed on one end of
the second contact element contacts the lens 107 at a third
location 114. Thereby a third contact point 107.4 of a plane third
contact surface 107.5 of the lens 107 is contacted by a fourth
contact point 111.2 on the second contact nose 111.1 of the second
contact element 111. Furthermore, the second contact element 111
contacts the lens holder 108 at a fourth location 115, where the
second contact element 111 is connected to the lens holder 108 by
means of screws or any other suitable fastening technique.
[0050] It will be appreciated that, with other embodiments of the
disclosure, the first and third contact surface may be curved
surfaces as well. Furthermore, the optical element may form contact
one or both noses as well, while one or both contact elements form
plane contact surfaces.
[0051] Along the optical axis 107.1, the first location 112 is
aligned with the third location 114 and the second location 113 is
aligned with the fourth location 115. Thus, both, the first
location 112 and the second location 113 as well as the third
location 114 and the fourth location 115 are spaced apart in a
radial direction 116 of the optical element module 3.3 by a contact
location distance D.
[0052] In general, in the radial direction 116, as a function of
the temperature situation T and the first coefficient of thermal
expansion .alpha..sub.L of the lens 107, the first and third
contact point 107.2 and 107.4 are located at a radius
L(T;.alpha..sub.L) from the optical axis 107.1. Furthermore, as a
function of the temperature situation T and the second coefficient
of thermal expansion .alpha..sub.R of the lens holder 108, the
second location 113 and the third location 115 are located at a
radius R(T;.alpha..sub.R) from the optical axis 107.1. Thus, as a
function of the temperature situation T, the first coefficient of
thermal expansion .alpha..sub.L and the second coefficient of
thermal expansion .alpha.R, the contact location distance
D(T;.alpha..sub.L;.alpha..sub.R) follows the equation:
D(T;.alpha..sub.L;.alpha..sub.R)=R(T;.alpha..sub.R)-L(T;.alpha..sub.L)
(1)
[0053] The first contact element 110 is designed such that, at the
first temperature situation T1 shown in FIG. 2, the distance E in
the radial direction 116 between its second contact point 110.2 and
the second location 113 (where the first contact element 110 is
fixedly connected to the lens holder 108) is equal to the contact
location distance D(T;.alpha..sub.L;.alpha.(.sub.R). Furthermore,
the second contact element 111 is designed such that, at the first
temperature situation T1 shown in FIG. 2, the distance E in the
radial direction 116 between its fourth contact point 111.2 and the
fourth location 115 (where the second contact element 111 is
fixedly connected to the lens holder 108) is also equal to the
contact location distance D(T;.alpha..sub.L;.alpha..sub.R).
[0054] In general, the distance E again is a function of the
temperature situation T and of the coefficient of thermal expansion
.alpha..sub.E of the respective contact element 110, 111, i.e.
E(T;.alpha..sub.E). Thus, at the first temperature situation T1 the
following equation is valid:
D(T1;.alpha..sub.L;.alpha..sub.R)=E(T1;.alpha..sub.E) (2)
[0055] It should be noted that, in the embodiment shown in FIG. 2,
the first and second contact element 110, 111 are made of the same
material having a third coefficient of thermal expansion. However
it will be appreciated that they may also be made of different
materials with different coefficients of thermal expansion, which
would then have to be accounted or in the above Equation (2).
[0056] While the first contact element 110 is a substantially rigid
element, the second contact element 111 is formed such that its
second contact nose 111.1 is supported in a manner to be resilient
in a direction parallel to the optical axis 107.1 of the optical
element 107. To this end, the second contact nose 111.1 is
connected to a base part 111.3 of the second contact element 111
via two leaf spring arms 111.4.
[0057] The second contact nose 111.1 protrudes from the second
contact element 111 in such a manner that the arms 111.4 are
elastically deflected when the base part 111.3 is screwed to the
lens holder 108. As a consequence, the second contact nose 111.1
exerts a clamping force F1 onto the second contact surface 107.5 of
the lens 107, said clamping force F1 being substantially
perpendicular to the second contact surface 107.5. The amount of
the clamping force may be adjusted by a spacer 111.5 of suitable
thickness placed between the base part 111.3 and the lens holder
108.
[0058] The first contact nose 110.1 protruding from the first
contact element 110 exerts a counteracting force F2 onto the first
contact surface 107.3 of the lens 107, said counteracting force F2
being substantially perpendicular to the first contact surface
107.3. The first contact nose 110.1 and the second contact nose
111.1 are arranged such that the counteracting force F2 is
collinear with the clamping force F1 and counteracts the clamping
force F1. In other words, the lens is clamped between the
respective first contact element 110 and the associated second
contact element 111.
[0059] The first contact surface 107.3 and the second contact
surface 107.5 are substantially perpendicular to the optical axis
107.1 of the lens. Thus, at the first temperature situation T1,
substantially no radial forces directed radially towards the center
of the lens 107 are introduced into the lens by its holding
mechanism.
[0060] At a second temperature situation T2 different from the
first temperature situation T1, the temperature within the
components of the optical element unit 3 is raised by a given
amount. As a consequence of the raised temperature, among others,
the lens 107 and the lens holder 108 expand in a radial direction
116. Since the second coefficient of thermal expansion of the lens
holder 108 is higher than the first coefficient of thermal
expansion of the lens 107, the raise in the temperature causes a
relative movement between the lens 107 and the lens holder 108 in
the radial direction 116 such that the lens holder 108 radially
moves away from the lens 107. In other words, the contact location
distance D(T;.alpha..sub.L;.alpha.(.sub.R) increases according to
Equation (1).
[0061] In conventional systems with a conventional clamping
mechanism, this would lead to a relative radial movement and a
residual elastic deformation at the interface between the
respective contact element and the lens, both leading to the
introduction of stresses into the optical element radially
propagating through the optically used area of the lens. As
previously explained, such radial stresses lead to imaging errors
such as stress induced birefringence.
[0062] However, the first contact element 110 and the second
contact element 111 compensate for this difference in the first and
second coefficient of thermal expansion such that, at the second
temperature situation T2, there is substantially no shift between
the first contact point 107.2 and the second contact point 110.2 as
well as substantially no shift between the third contact point
107.4 and the fourth contact point 111.2.
[0063] To this end, the first contact elements 110 and the second
contact elements 111 are made of a steel material having a third
coefficient of thermal expansion different from the first and
second coefficient of thermal expansion of the lens 107 and the
lens holder 108, respectively. The third coefficient of thermal
expansion is higher than the first and second coefficient of
thermal expansion.
[0064] For the first contact elements 110, at least one of the
second location 113 and the third coefficient of thermal expansion
.alpha..sub.E are selected such that
E(T2;.alpha..sub.E)=D(T2;.alpha..sub.L;.alpha..sub.R)=R(T2;.alpha..sub.R-
)-L(T2;.alpha..sub.L) (3)
[0065] The same applies for the second contact elements 111, i.e.
at least one of the fourth location 115 and the third coefficient
of thermal expansion .alpha..sub.E are selected such that Equation
(3) is valid.
[0066] In other words, due to the higher thermal expansion of the
first and second contact elements 110 and 111, the respective
contact element 110, 111, at the given temperature situation
variation between the first and second temperature situation T1 and
T2, spans the gap between the lens 107 and the lens holder 108 that
results from the difference in the first and second coefficient of
thermal expansion of the lens 107 and the lens holder 108,
respectively. Thus, at the second temperature situation T2 as well,
despite the thermal expansion of the module components,
substantially
[0067] The first and second contact elements do not necessarily
have to be fixedly mounted to the optical element holder. It will
be appreciated that, with other embodiments of the disclosure, in
the manner of a kinematic reversal, at least one of the respective
first and second contact element may be fixedly mounted to the
optical element and contact the optical element holder in the
manner as it has been described above for the contact between the
contact elements 110, 111 and the lens 107.
[0068] It will be appreciated that, with other embodiments of the
disclosure, other materials or combinations may be chosen. In any
case, the compensation of the difference in the coefficient of
thermal expansion between the lens holder and the lens by suitably
selecting the material (i.e. the coefficient of thermal expansion),
the size, and the location of the respective contact element.
[0069] It will be further appreciated that the compensation of the
difference in the first coefficient of thermal expansion of the
lens 107 and the second coefficient of thermal expansion of the
lens holder 108 provided by the first and second contact elements
110 and 111 may be effective during the entire temperature
situation variation, i.e. the transition between the first
temperature situation T1 and the second temperature situation T2.
However, depending on the change in the temperature profile in the
module components (lens 107, lens holder 108 and contact elements
110, 111) during the temperature situation variation, the first and
second contact elements 110 and 111 may not provide for a complete
compensation during the entire temperature situation variation.
[0070] Thus, it may be that, during the transition between the
first temperature situation T1 and the second temperature situation
T2, certain thermally induced radial disturbing forces are
introduced into the lens 107. However, the disclosure also provides
for a reduction of these thermally induced radial disturbing by the
following means.
[0071] First of all, as may be seen from FIG. 3, a gravity
compensation means 117 is provided. This gravity compensation means
117 is mounted to the lens holder 108 and located close to the
outer circumference of the lens 107. The gravity compensation means
117, in sum, exerts a force onto the lens that balances the
gravitational force acting onto the lens 107 due to its mass.
[0072] To this end, the gravity compensation means 117 comprises
force generating means 117.1 contacting the first contact surface
107.3 of the lens 107 over a certain fraction of the outer
circumference of the lens 107. In the embodiment of FIG. 3, three
force generating means 117.1 are extend over substantially the
entire part of the circumference of the lens 107 that is not taken
by the first contact elements 110.
[0073] Each force generating means 117.1 exerts a line force onto
the lens 107 that is parallel to the optical axis 107.1 of the
lens. The force generating means 117.1 is provided in the form of a
helical spring with elliptical coils that are inclined with respect
to the spring axis such as a so called BAL SPRING.RTM. manufactured
by Bal Seal Engineering Co. Inc., Pauling, Calif., U.S.A. However,
it will be appreciated that, with other embodiments of the
disclosure, another number and other types of force generating
means, e.g. leaf spring elements, magnetic or pneumatic elements
etc., may be used for the gravity compensation means.
[0074] The force generating means 117.1 is supported on a support
element 117.2 fixedly connected to the inner circumference of the
lens holder 108. Here again, similar to the first contact element
110, the support element 117.2 may be made of a material with a
coefficient of thermal expansion as well as mounted and designed
such that it compensates for the difference in the first
coefficient of thermal expansion of the lens 107 and the second
coefficient of thermal expansion of the lens holder 108. In other
words, the support element 117.2 may be designed such that, upon
the above temperature situation variation, there is substantially
no shift in the contact points between the force generating means
117.1 and the lens 107.
[0075] This avoids introduction of radial disturbing forces into
the lens via the gravity compensation means 117. However, it will
be appreciated that this complete compensation may also be omitted
to some extent. For example a radial relative movement may be
admitted between the force generating means 117.1 and the lens 107
upon thermal expansion since the force generating means 117.1, due
to its design, may execute a rolling movement with respect to the
first contact surface 107.3 of the lens 107. Such a rolling
movement is associated with a very low rolling friction acting onto
the lens 107 and, thus, leads to a considerably reduced
introduction of disturbing radial forces into the lens 107. To
further reduce the frictional forces introduced into the lens 107
at least one of the force generating means 117.1 and the first
contact surface 107.3 of the lens 107 may be provided with a low
friction coefficient contact surface, e.g. with a friction
coefficient coating at the respective contact surface.
[0076] An advantage of the gravity compensation means 117 lies
within the fact that the normal reaction force acting between the
first contact elements 110 and the lens 107 perpendicular to the
first contact surface 107.3 does not have to include a component
resulting from the balancing of the gravitational force acting onto
the lens 107. Thus, the first contact elements 110 only exert a
reduced normal contact force only balancing the clamping force
exerted by the associated second contact elements 111. This reduced
normal contact force has the advantage that upon any thermally
induced radial relative movement between the lens 107 and the first
contact elements 110 only a reduced frictional disturbing force
acts in the radial direction 116 onto the lens 107, said frictional
disturbing force being a function of the normal contact force and
the friction coefficient at the contact location.
[0077] A further reduction of the frictional disturbing force
acting onto the lens 107 upon any thermally induced radial relative
movement between the lens 107 and the contact elements 110 and 111
may be achieved by providing at least one of the lens 107 and the
first and second contact nose 110.1 and 111.1 with a low friction
coefficient contact surface, e.g. with a low friction coefficient
coating at the respective contact surface, i.e. the first and
second contact surface 107.3 and 107.5. and/or the contact surface
of the first and second contact nose 110.1 and 111.1. By this means
as well only a reduced frictional disturbing force acts in the
radial direction 116 onto the lens 107 upon such a thermally
induced radial relative movement, said frictional disturbing force
being a function of the normal contact force and the friction
coefficient at the respective contact location.
[0078] As mentioned above, the lens holder 108 is held by a ring
shaped frame element 109. The frame element 109 itself may form a
part of the housing 3.1 of the optical element unit 3 or may be
connected to a separate part, said separate part then forming a
part of the housing 3.1.
[0079] The lens holder 108 has a first axis of symmetry 108.1
substantially coinciding with the optical axis 107.1. The same
applies to the frame element 109, i.e. the frame element 109 has a
second axis of symmetry 109.1 substantially coinciding with the
optical axis 107.1 as well.
[0080] For reasons of reduced weight and good thermal conductivity,
the frame element 109 is made of aluminum. Thus, the frame element
109 has a fourth coefficient of thermal expansion different from
the second coefficient of thermal expansion of the lens holder 108.
To account for this fact, the lens holder 108 is connected to the
frame element 109 via a plurality of radial deformation uncoupling
elements 109.1 evenly distributed at the inner circumference of the
frame element 109.
[0081] The lens holder 108 is connected to the frame element 109
via one screw 118 per deformation uncoupling element 109.1. To
avoid distortion of the deformation uncoupling elements 109.1 when
tightening the screws 118, a protection ring 119 is placed between
the heads of the screws 118 and the deformation uncoupling elements
109.1.
[0082] In the following, a preferred embodiment of a method of
holding an optical element according to the present disclosure will
be described with reference to FIGS. 1 to 4.
[0083] FIG. 4 shows a block diagram of a preferred embodiment of a
method of holding an optical element.
[0084] In a first step 20, a plurality of module components 107,
108, 109, 110, 111, 117 of the optical element module 3.3 is
provided. At least one of these module components is adapted to
provide at least a reduction of forces introduced into the lens 107
upon a thermally induced position change in the relative position
between the lens 107 and the lens holder 108.
[0085] As mentioned above, the plurality of module components
comprises the lens 107 as it has been described above in the
context of FIGS. 2 and 3. The lens 107 is provided in a step
20.1.
[0086] The plurality of module components further comprises the
lens holder 108 and the frame element 109 as they have been
described above in the context of FIGS. 2 and 3. The lens holder
108 and the frame element 109 are provided in a step 20.2
[0087] The plurality of module components further comprises the
first and second contact elements 110 and 111 as they have been
described above in the context of FIGS. 2 and 3. The first and
second contact elements 110 and 111 are provided in a step 20.3. In
this step 20.3 the first and second contact elements 110 and 111
are designed such that they may compensate for the difference in
the coefficient of thermal expansion between the lens 107 and the
lens holder 108 at a temperature situation variation as it has been
described above in the context of FIGS. 2 and 3.
[0088] In a step 21.1 of a second step 21, the module components of
the optical element module 3.3 are mounted together such that the
lens 107, at a first temperature situation, is held by the lens
holder 108 via the first and second contact elements 110 and 111 to
provide a configuration as it has been described above in the
context of FIGS. 2 and 3.
[0089] In a step 21.2 a temperature situation variation is provided
wherein the temperature situation of the optical element module 3.3
changes from the first temperature situation T1 to the second
temperature situation T2 as it has been described above in the
context of FIGS. 2 and 3.
[0090] In a step 21.3 the lens 107, at said second temperature
situation T2, is held by the lens holder 108 via the first and
second contact elements 110 and 111 to provide a configuration as
it has been described above in the context of FIGS. 2 and 3. As it
has been described above in the context of FIGS. 2 and 3, the first
and second contact elements 110 and 111 are designed and mounted to
the lens holder 108 such that they compensate for the difference in
the coefficient of thermal expansion between the lens 107 and the
lens holder 108. Thus, at the second temperature situation T2 as
well, the lens 107 is held such that substantially not thermally
induced radial disturbing forces are introduced into the lens
107.
Second embodiment
[0091] In the following, a second preferred embodiment of an
optical element module 3.4 will be described with reference to
FIGS. 1 and 5. FIG. 5 shows a schematic sectional representation of
a part of the optical element module 3.4 of the optical element
unit 3 at a first temperature situation T1 of the optical element
unit 3. The first temperature situation T1 is characterized by a
certain temperature profile within the components of the optical
element unit 3.
[0092] The optical element 207 of the optical element module 3.4 is
a rotationally symmetric lens having an optical axis 207.1. The
lens 207 is made of Quartz (SiO.sub.2) and has a first coefficient
of thermal expansion.
[0093] The lens 207 is usually positioned in space such that the
optical axis 207.1 of the lens 207 is substantially collinear with
the optical axis 3.2 of the optical element unit 3. It should be
noted that the position of the optical axis 207.1 of the lens 207
shown in FIG. 5 is not to scale. In reality, the optical axis 207.1
is located at a distance from the outer circumference of the lens
207 that by far exceeds the distance shown in FIG. 5.
[0094] The lens 207 is held by an optical element holder in the
form of a ring shaped lens holder 208 which, in turn, may be held
by a ring shaped frame element similar to frame element 109 of FIG.
2. The lens holder has a first axis of symmetry 208.1 which
coincides with the optical axis 207.1.
[0095] The lens holder 208 is made of Invar that has a second
coefficient of thermal expansion different from, namely larger than
the first coefficient of thermal expansion of the lens 207. The
lens holder 208 holds the lens 207 in place via a plurality of
first contact elements 210 and a plurality of second contact
elements 211.
[0096] In the embodiment shown, three first contact elements 210
and three second contact elements 211 are evenly distributed at the
inner circumference of the lens holder 208. However, it will be
appreciated that, with other embodiments of the disclosure, a
different number of first and/or second contact elements may be
provided. Furthermore, only one type of contact elements may be
provided. For example, only the lower first contact elements may be
provided to support the lens from below.
[0097] The first contact element 210 is a cylindrical roller of
circular cross section. The first contact element 210 is supported
on a plane annular first platform 208.2 of the lens holder 208. The
plane of the platform 208.2 is substantially perpendicular to the
axis 208.1 and, thus, perpendicular to the optical axis 207.1. The
first contact element 210 contacts a plane first contact surface
207.3 of the lens 207, the first contact surface 207.3 being
perpendicular to the optical axis 207.1.
[0098] The second contact element 211 is also a cylindrical roller
of circular cross section. The second contact element 211 contacts
a plane second contact surface 207.5 of the lens 207, the second
contact surface 207.3 also being perpendicular to the optical axis
207.1. The second contact element 211 furthermore contacts a plane
annular second platform 208.3 formed on a contact ring 208.4 of the
lens holder 208. The plane of the second platform 208.3 is also
substantially perpendicular to the axis 208.1 and, thus,
perpendicular to the optical axis 207.1.
[0099] The first contact element 210 and the second contact element
211, in the situation shown in FIG. 5, are arranged such that they
are properly aligned in an axial direction parallel to the optical
axis 207.1. The cylindrical surface of the first contact element
210 forms a curved third contact surface 210.3 that contacts the
first contact surface 207.3 of the lens 207. Furthermore, the
cylindrical surface of the second contact element 211 forms a
curved fourth contact surface 211.3 that contacts the second
contact surface 207.5 of the lens 207.
[0100] To clamp the lens 207 between the first contact element 210
and the second contact element 211, a resilient clamping element
208.5 is connected to the lens holder 208. The clamping element is
designed in the manner of the second contact element 111 of FIG. 2.
Thus it has a clamping nose 208.6 connected via resilient arms
208.7 to a base part 208.8, which in turn is mounted to the lens
holder 208.
[0101] The clamping nose 208.6 protrudes in such a manner that the
arms 208.7 are elastically deflected when the base part 208.8 is
connected to the lens holder 208. As a consequence, the clamping
nose 208.6, via the contact ring 208.4 and the second contact
element 211, exerts a clamping force F1 onto the second contact
surface 207.5 of the lens 207, said clamping force F1 being
substantially perpendicular to the second contact surface 207.5.
The amount of the clamping force again may be adjusted by a spacer
of suitable thickness placed between the base part 208.8 and the
lens holder 208.
[0102] Since the first contact surface 207.3 and the second contact
surface 207.5 are substantially perpendicular to the optical axis
207.1 of the lens 207, at the first temperature situation T1,
substantially no radial forces directed radially towards the center
of the lens 207 are introduced into the lens 207 by its holding
mechanism.
[0103] At a transition to a second temperature situation T2
different from the first temperature situation T1, the temperature
within the components of the optical element unit 3 is raised by a
given amount. As a consequence of the rising temperature, among
others, the lens 207 and the lens holder 208 expand in a radial
direction 216. Since the second coefficient of thermal expansion of
the lens holder 208 is higher than the first coefficient of thermal
expansion of the lens 207, the raise in the temperature causes a
relative movement between the lens 207 and the lens holder 208 in
the radial direction 216 such that the lens holder 208 radially
moves away from the lens 207.
[0104] As mentioned above, in conventional systems with a
conventional clamping mechanism directly acting onto the lens, this
would lead to a relative radial movement and a residual elastic
deformation at the interface between the respective contact element
and the lens, both leading to the introduction of stresses into the
optical element radially propagating through the optically used
area of the lens. As previously explained, such radial stresses
lead to imaging errors such as stress induced birefringence.
[0105] However, the curved third contact surface 210.3 of the first
contact element 210 and the curved fourth contact surface 211.3 of
the second contact element 211, at this thermally induced relative
movement between the lens 207 and the lens holder 208, both execute
a rolling movement on the first contact surface 207.3 and the
second contact surface 207.5, respectively. The curved third
contact surface 210.3 and the curved fourth contact surface 211.3
also execute a rolling movement on the first platform 208.2 and the
second platform 208.3, respectively. Since both contact elements
210 and 211 have the same diameter, the contact elements 210 and
211 perform a synchronous rotation such that they keep being
aligned in a direction parallel to the optical axis 207.1.
[0106] This rolling movement is associated with very low frictional
forces introduced into the lens 207 and directed in the radial
direction 216. It will be appreciated that, in other words, the
rolling movement is a substantially pure rolling movement with
substantially no friction. The substantially negligible residual
friction that occurs here results from the deformation induced
deviation of the contact area from the ideal line contact of the
cylindrical contact element 210, 211 with its respective contact
partner. Thus, a considerable reduction of thermally induced radial
disturbing forces is achieved with the disclosure compared to
conventional systems without such rolling contact elements. Thus
disturbing radial stresses leading to imaging errors such as stress
induced birefringence may be reduced considerably with the
disclosure.
[0107] It will be appreciated that, with other embodiments of the
disclosure, the respective first and second contact element does
not necessarily have to contact the lens directly. For example,
intermediate elements may be connected to the lens and contact the
respective contact element. These intermediate elements may also be
a clamping element designed in the manner of the clamping element
208.5 as it has been described above.
[0108] As outlined above, certain considerably reduced thermally
induced radial disturbing forces may be introduced into the lens
207 at a temperature situation variation. However, the disclosure
also provides for a further reduction of these thermally induced
radial disturbing by the following means.
[0109] First of all, as had been explained in the context of FIG.
3, a gravity compensation means similar to the gravity compensation
means 117 is provided. This gravity compensation means has the
effect that the normal reaction force acting between the first
contact elements 210 and the lens 207 perpendicular to the first
contact surface 207.3 does not have to include a component
resulting from the balancing of the gravitational force acting onto
the lens 207. Thus, the first contact elements 210 only exert a
reduced normal contact force only balancing the clamping force
exerted by the associated second contact elements 211. This reduced
normal contact force has the advantage that upon any thermally
induced radial relative movement between the lens 207 and the first
contact elements 210 only an even further reduced frictional
disturbing force acts in the radial direction 216 onto the lens
207, said frictional disturbing force being a function of the
normal contact force and the friction coefficient at the contact
location.
[0110] A further reduction of the frictional disturbing force
acting onto the lens 207 upon any thermally induced radial relative
movement between the lens 207 and the contact elements 210 and 211
may be achieved by providing at least one of the lens 207 and the
first and second contact elements 210 and 211 with a low friction
coefficient contact surface, e.g. with a low friction coefficient
coating at the respective contact surface, i.e. the first and
second contact surface 207.3 and 207.5 and/or the third or fourth
contact surface 210.3 and 211.3. By this means as well, only an
even further reduced frictional disturbing force acts in the radial
direction 216 onto the lens 207 upon such a thermally induced
radial relative movement, said frictional disturbing force being a
function of the normal contact force and the friction coefficient
at the respective contact location.
[0111] Finally, a further reduction of the frictional disturbing
force acting onto the lens 207 upon any thermally induced radial
relative movement between the lens 207 and the contact elements 210
and 211 may be achieved by providing a securing device 222. This
securing device 222 overall allows to reduce, under normal
operating conditions, the holding forces exerted on the lens 207
and, thus, also the disturbing frictional forces introduced into
the lens 207 at a thermally induced relative movement between the
lens 207 and the lens holder 208 under normal operating
conditions.
[0112] This solution is based on the concept that the holding
forces usually counteract also the thermally induced relative
movement between the lens 207 and the lens holder 208 and, thus,
have an influence on the frictional forces introduced into the lens
207 at such a relative movement between the lens 207 and the lens
holder 208. Usually, due to the manufacture and mounting of the
optical element unit 3 at a location different from the location of
its later use, the holding forces provided for the lens 207 do not
only account for the forces occurring under normal operating
conditions of the optical system but also have to account for
considerably higher abnormal forces occurring during transport of
the optical element unit 3, for example. Thus, in conventional
systems, holding forces exerted onto the lens 207 are considerably
higher than necessary in normal use. Due to the correlation between
the holding forces and the disturbing forces outlined above, this
obviously leads to increased disturbing forces introduced into the
lens 207 at a thermally induced relative movement between the lens
207 and the lens holder 208.
[0113] The securing device 222, further reduces these disturbing
forces by allowing a reduction of the holding forces exerted on the
lens 207 under normal operating conditions. The securing device 222
is only activated under abnormal load conditions in order to hold
the lens 207. To this end, the securing device 222 is fixedly
mounted to the lens holder 208 and provides a stop element 222.1
that is spatially associated to the resiliently mounted clamping
nose 208.6 of the respective clamping element 208.5.
[0114] The clamping element 208.5, via the second contact element
211, under normal operating conditions of the optical element unit
3, i.e. under a normal load situation, exerts a first holding force
onto the lens 207. This first holding force ranges up to a holding
force limit is a maximum force that is necessary (together with the
holding forces of the other clamping elements 208.5) to hold the
lens 207 substantially in place against normal displacement forces
to be expected to act onto the lens 207 under said normal load
situation.
[0115] As long as the displacement forces acting onto the lens 207
do not require exertion of this holding force limit, a small gap
223 is formed between the stop element 222.1 and the clamping nose
208.6. As soon as the holding force limit is reached, the clamping
nose 208.6 comes into tight contact with the substantially rigid
stop element 222.1 such that the holding forces exerted in the lens
207 abruptly increase to hold the lens 207 in place against
abnormal displacement forces exceeding the displacement forces
acting under normal operating conditions of the optical element
unit 3.
[0116] It should be noted that the gap 223 shown in FIG. 5, for
reasons of better visibility, is way out of scale. In reality, the
gap 223 is sufficiently small to provide sufficient contact between
the lens 207 and the contact elements 210, 211 under any load
condition.
[0117] It will be appreciated that, with other embodiments of the
disclosure, the securing device may adapted to contact any other
movable part of the clamping element 208.5 or any other suitable
part of the lens 207 or any other movable component in mechanical
connection with the lens 207. Furthermore, the clamping element may
be of any other suitable design that is activated under abnormal
load conditions only. For example, it may be an active device, e.g.
a electrically, pneumatically or otherwise actuated device, that is
actively brought into contact with the lens 207, the clamping nose
208.6 or any other movable component in mechanical connection with
the lens 207 under abnormal load situations.
[0118] It will be further appreciated that the securing device 222
may also be used in combination with the embodiment shown in FIG. 2
leading to the above beneficial reduction in the necessary holding
forces under normal operating conditions.
[0119] It will be further appreciated that, with other embodiments
of the disclosure, the respective first and second contact element
does not necessarily have to be a cylindrical element. It is only
necessary that the respective contact element has a curved contact
surface that executes, upon a temperature situation variation, a
substantially purely rotational movement on an interface surface
between the lens and the lens holder where the relative motion
occurs. For example, the respective contact element may be a ball
shaped element. In this case, the contact partner of the contact
element does not necessarily have to provide one single planar
contact surface. For example, it is also possible that the ball
shaped contacts the two contact surfaces of a substantially
V-shaped groove extending in the radial direction. The ball shaped
contact element, upon a temperature situation variation, then may
execute the substantially frictionless rolling movement along this
V-shaped groove. Furthermore, the respective contact element may be
fixedly connected to one of the lens 207 and the lens holder
208.
[0120] FIGS. 6A and 6B show different views of an example of a
contact element 310 that may replace the first contact element 210
and/or the second contact element 211 of FIG. 5. The contact
element 310 has a movable contact part 310. 4 with a spherical
contact surface 310.3. Via a flexure 310.5, the spherical contact
part 310.4 is monolithically connected to a base part 310.6. The
base part 310.6 may be connected to the lens holder 208 or the lens
207, such that the flexural axis 310.7 of the flexure 310.5 runs
perpendicular to the radial direction 216 in a plane perpendicular
to the optical axis 207.1 of the lens 207.
[0121] It will be appreciated that the optical element unit 3.4 of
FIG. 5 may as well be used to perform a method of holding an
optical element similar to the one as it has been described above
with reference to FIG. 4.
[0122] The difference with respect to the method performed with the
embodiment of FIG. 2 lies within the fact that, upon any change in
the temperature situation, i.e. in step 21.2 of FIG. 4, a relative
motion takes place at the mechanical interface between the lens 207
and the lens holder 208. However, this relative motion is a low
friction motion, namely a rolling motion.
Third Embodiment
[0123] In the following, a third preferred embodiment of an optical
element module 3.5 will be described with reference to FIGS. 1, 7A
and 7B. FIGS. 7A and 7B show representations of a part of the
optical element module 3.5 of the optical element unit 3.
[0124] The optical element 407 of the optical element module 3.5 is
a rotationally symmetric lens having an optical axis which, when
mounted in the optical element unit 3, lies in a substantially
horizontal plane.
[0125] The lens 407 is made of Quartz (SiO.sub.2) and is held by an
optical element holder in the form of a ring shaped lens holder
408. The lens holder has a first axis of symmetry which coincides
with the optical axis of the lens 407. The lens holder 408 is made
of Invar that has a second coefficient of thermal expansion
different from, namely larger than the first coefficient of thermal
expansion of the lens 407. The lens holder 408 holds the lens 407
in place via a plurality of first contact elements 410 and a
plurality of second contact elements 411 in a manner as it has been
described above in the context of FIG. 2, such that it is here
mainly referred to the above explanations.
[0126] The optical element module 3.5 also has a gravity
compensation device 417. This gravity compensation device 417--as
the gravity compensation device 117--is adapted to exert a support
force onto the lens 407 that substantially balances the
gravitational force acting on the lens 407 due to its mass.
[0127] Due to the so called standing arrangement of the lens 407,
the gravity compensation device 417 comprises a flexible tension
element 417.1 in the form of a rope or strap. The tension element
417.1 has a middle section 417.3 and two end sections 417.4 and
417.5. Both end sections 417.4 and 417.5 are hung to the lens
holder 408 at a location located above the center of gravity of the
lens 407 such that the middle section 417.3 is wrapped around a
lower part of the lens 407.
[0128] The force exerted by the gravity compensation device 417
onto the lens 407 may be adjusted by adjusting the pretension of
springs 417.6 acting via nuts 417.7 onto threaded bolts 417.8
connected to the tension element 417.1 at its respective end
section 417.4 and 417.5. The angle of wrap is about 170.degree.
such that the forces exerted by the gravity compensation device 417
onto the lens 407 are distributed over a wide area avoiding local
stress concentrations.
[0129] The lens 407 may be located in a seat within the lens holder
408 precisely defining the position of the lens in the radial
direction, i.e. in the vertical plane. Anyway, it will be
appreciated that the forces exerted by the gravity compensation
device 417 may slightly exceed the gravitational force acting on
the lens 407 such that the lens 407 is pulled against a plurality
of stops--preferably two stops--provided on the lens holder 408 at
the upper circumference of the lens 407 to secure the position of
the lens in the vertical plane.
[0130] Furthermore, one or several further stops may be provided at
the lower part of the lens holder 408 which the lens 407 may
contact in case of abnormal vertical loads acting onto the lens
407, e.g. during transport of the optical element unit 3.
Fourth Embodiment
[0131] In the following, a fourth preferred embodiment of an
optical element module 3.6 will be described with reference to
FIGS. 1, 8A and 8B. FIGS. 8A and 8B show representations of a part
of the optical element module 3.6 of the optical element unit
3.
[0132] The optical element 507 of the optical element module 3.6 is
a rotationally symmetric mirror having an optical axis which, when
mounted in the optical element unit 3, lies in a substantially
horizontal plane.
[0133] The mirror 507 is held by an optical element holder in the
form of a ring shaped mirror holder 508. The mirror holder has a
first axis of symmetry which coincides with the optical axis of the
mirror 507. The mirror holder 508 is made of a material that has a
second coefficient of thermal expansion different from, namely
larger than the first coefficient of thermal expansion of the
mirror 507. The mirror holder 508 holds the mirror 507 in place via
a plurality of first contact elements 510 and a plurality of second
contact elements 511 in a manner as it has been described above in
the context of FIG. 2, such that it is here mainly referred to the
above explanations.
[0134] The optical element module 3.6 also has a gravity
compensation device 517. This gravity compensation device 517--as
the gravity compensation devices 117 and 417--is adapted to exert a
support force onto the mirror 507 that substantially balances the
gravitational force acting on the mirror 507 due to its mass.
[0135] Due to the so called standing arrangement of the mirror 507,
the gravity compensation device 517 comprises a plurality of
resilient force exerting elements 517.1 in the form of leaf spring
elements. The leaf spring elements 517.1 are formed by slots in an
arc shaped base element 517.9 fixedly connected to the mirror
holder 508 at a location located below the center of gravity of the
mirror 507. The base element 517.9 is arranges symmetrically with
respect to the vertical axis of symmetry 507.6 of the mirror
507.
[0136] The free ends of the leaf spring elements 517.1 contact the
outer circumference of the mirror 507 over an angle of about
90.degree. in a lower part of the mirror 507. However, it will be
appreciated that other angles may be chosen, where appropriate. The
leaf spring elements 517.1 are adapted such that the force exerted
onto the mirror by the respective leaf spring element 517.1
decreases with increasing distance from the vertical axis 507.6 of
the mirror 507. Thus, proper support corresponding to the mass
distribution of the mirror is achieved.
[0137] Again, the mirror 507 may be located in a seat within the
mirror holder 508 precisely defining the position of the mirror in
the radial direction, i.e. in the vertical plane. Anyway, it will
be appreciated that the forces exerted by the gravity compensation
device 517 may slightly exceed the gravitational force acting on
the mirror 507 such that the mirror 507 is pushed against a
plurality of stops--preferably two stops--provided on the mirror
holder 508 at the upper circumference of the mirror 507 to secure
the position of the mirror in the vertical plane.
[0138] Furthermore, one or several further stops may be provided at
the lower part of the mirror holder 508 which the mirror 507 or the
leaf springs 517.1 may contact in case of abnormal vertical loads
acting onto the mirror 507, e.g. during transport of the optical
element unit 3.
[0139] Although, in the foregoing, embodiments of the present
disclosure have been described where the optical element has a
circular shape, it will be appreciated that, with other embodiments
of the present disclosure, the optical element may have any other
shape. The same applies for the optical element holder.
[0140] Furthermore, the present disclosure has been described
mostly in the context of embodiments where refractive optical
elements such as lenses and plane parallel plates are held by
respective optical element holders. However, it will be appreciated
that, with other embodiments of the present disclosure, other types
of optical elements, such as reflective and/or diffractive optical
elements, e.g. mirrors or gratings or the like, may be held by a
corresponding optical element holder as it has been described
above.
[0141] Furthermore, the present disclosure has been described in
the context of an optical element unit incorporating different
designs of optical element modules. However, it will be appreciated
that the disclosure may also be used in the context of optical
element units incorporating one single design or type of optical
element module.
[0142] Furthermore, the present disclosure has been described in
the context of an optical element unit having a folded optical
axis. However, it will be appreciated that the disclosure may also
be used in the context of optical element units having a straight
optical axis or an arbitrarily often folded optical axis.
[0143] Finally, the present disclosure has been described in the
context of embodiments for optical exposure processes. However, it
will be appreciated that the disclosure may also be used in the
context of any other optical application, where a relief of an
optical element from stresses resulting from thermal expansion in
the region of the respective optical element is required.
* * * * *