U.S. patent number RE47,237 [Application Number 15/182,514] was granted by the patent office on 2019-02-12 for lithographic apparatus, fluid handling structure for use in a lithographic apparatus and device manufacturing method.
This patent grant is currently assigned to ASML Netherlands B.V.. The grantee listed for this patent is ASML NETHERLANDS B.V.. Invention is credited to Raymond Wilhelmus Louis Lafarre, Niek Jacobus Johannes Roset, Sergei Shulepov, Nicolaas Ten Kate.
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United States Patent |
RE47,237 |
Roset , et al. |
February 12, 2019 |
Lithographic apparatus, fluid handling structure for use in a
lithographic apparatus and device manufacturing method
Abstract
A lithographic apparatus including a fluid handling structure
configured to contain immersion fluid in a space adjacent to an
upper surface of the substrate table and/or a substrate located in
a recess of the substrate table, a cover having a planar main body
that, in use, extends around a substrate from the upper surface to
a peripheral section of an upper major face of the substrate in
order to cover a gap between an edge of the recess and an edge of
the substrate, and an immersion fluid film disruptor, configured to
disrupt the formation of a film of immersion fluid between an edge
of the cover and immersion fluid contained by the fluid handling
structure during movement of the substrate table relative to the
fluid handling structure.
Inventors: |
Roset; Niek Jacobus Johannes
(Eindhoven, NL), Ten Kate; Nicolaas (Almkerk,
NL), Shulepov; Sergei (Eindhoven, NL),
Lafarre; Raymond Wilhelmus Louis (Helmond, NL) |
Applicant: |
Name |
City |
State |
Country |
Type |
ASML NETHERLANDS B.V. |
Veldhoven |
N/A |
NL |
|
|
Assignee: |
ASML Netherlands B.V.
(Veldhoven, NL)
|
Family
ID: |
1000003474140 |
Appl.
No.: |
15/182,514 |
Filed: |
June 14, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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61346213 |
May 19, 2010 |
|
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61388972 |
Oct 1, 2010 |
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Reissue of: |
13109036 |
May 17, 2011 |
8767169 |
Jul 1, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03B
27/52 (20130101); G03F 7/70716 (20130101); G03F
7/70341 (20130101); G03F 7/70341 (20130101); G03F
7/70725 (20130101); G03F 7/2041 (20130101) |
Current International
Class: |
G03B
27/32 (20060101); G03B 27/58 (20060101); G03F
7/20 (20060101); G03B 27/52 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1501175 |
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Jun 2004 |
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CN |
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1550905 |
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Dec 2004 |
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CN |
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1637608 |
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Jul 2005 |
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CN |
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1 420 300 |
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May 2004 |
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EP |
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2002-036373 |
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Feb 2002 |
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JP |
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2003-324028 |
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Nov 2003 |
|
JP |
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2004-289127 |
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Oct 2004 |
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JP |
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2007-142168 |
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Jun 2007 |
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JP |
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2007-158343 |
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Jun 2007 |
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2007-201384 |
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2007-525007 |
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Aug 2007 |
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JP |
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2008-103703 |
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May 2008 |
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2008-153651 |
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Jul 2008 |
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JP |
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2009-188119 |
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Aug 2009 |
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JP |
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2009-231838 |
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Oct 2009 |
|
JP |
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2011-134776 |
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Jul 2011 |
|
JP |
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2011-192994 |
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Sep 2011 |
|
JP |
|
99/49504 |
|
Sep 1999 |
|
WO |
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2005/064405 |
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Jul 2005 |
|
WO |
|
Other References
Non-final Office Action issued for U.S. Appl. No. 14/973,421, dated
Mar. 27, 2017. cited by applicant .
Final Office Action dated Sep. 26, 2017 in corresponding U.S. Appl.
No. 14/973,421. cited by applicant .
Notice of Allowance issued in corresponding U.S. Appl. No.
14/973,421 dated Jan. 11, 2018. cited by applicant .
Non-final Office Action issued in corresponding U.S. Appl. No.
15/920,788 dated Oct. 1, 2018. cited by applicant .
Chinese Office Action dated Dec. 13, 2012 in corresponding Chinese
Patent Application No. 201110064190.X. cited by applicant .
U.S. Office Action dated Nov. 15, 2013 in corresponding U.S. Appl.
No. 13/047,165. cited by applicant .
U.S. Office Action dated Oct. 9, 2013 in corresponding U.S. Appl.
No. 13/047,251. cited by applicant.
|
Primary Examiner: Menefee; James
Attorney, Agent or Firm: Pillsbury Winthrop Shaw Pittman,
LLP
Parent Case Text
.Iadd.More than one reissue application has been filed for the
reissue of Pat. No. 8,767,169. The reissue applications are
continuation reissue application No. 16/203,799 and parent reissue
application No. 15/182,514 (the present application), both of which
are reissue applications of Pat. No. 8,767,169..Iaddend.
This application claims priority and benefit under 35 U.S.C. .sctn.
119(e) to U.S. Provisional Patent Application No. 61/346,213, filed
on May 19, 2010 and to U.S. Provisional Patent Application No.
61/388,972, filed on Oct, 1, 2010. The content of each of the
foregoing applications is incorporated herein in its entirety by
reference.
Claims
The invention claimed is:
1. A lithographic apparatus comprising: a substrate table having an
upper surface and a recess in the upper surface that is configured
to receive and support a substrate; a fluid handling structure
configured to contain immersion fluid in a space adjacent to the
upper surface of the substrate table and/or a substrate located in
the recess; .[.a cover comprising a planar main body that, in use,
extends around a substrate from the upper surface to a peripheral
section of an upper major face of the substrate in order to cover a
gap between an edge of the recess and an edge of the substrate;.].
an immersion fluid film disruptor, configured to disrupt the
formation of a film of immersion fluid between an edge of the
.[.cover.]. .Iadd.substrate .Iaddend.and immersion fluid contained
by the fluid handling structure during movement of the substrate
table relative to the fluid handling structure, the immersion fluid
film disruptor comprising a structure in or on a surface of the
fluid handling structure opposite the upper major surface of the
substrate and/or the upper surface of the substrate table, to form
a plurality of discrete gas jets; and a .[.controller.].
.Iadd.control system.Iaddend., configured to control supply of gas
to the structure .Iadd.of the immersion fluid film disruptor
.Iaddend.such that the jets of gas are provided when at least part
of the structure .Iadd.of the immersion fluid film disruptor
.Iaddend.is in the region of the .[.cover.]. .Iadd.edge of the
substrate .Iaddend.and the .[.lets.]. .Iadd.jets .Iaddend.of gas
are not provided when the at least part of the structure .Iadd.of
the immersion fluid film disruptor .Iaddend.is not in the region of
the .[.cover.]. .Iadd.edge of the substrate.Iaddend..
2. The lithographic apparatus according to claim 1, wherein the
structure .Iadd.of the immersion fluid film disruptor
.Iaddend.comprises a plurality of apertures, formed in the surface
of the fluid handling structure, to provide the gas jets.
3. The lithographic apparatus according to claim 2, wherein the
apertures are each configured to provide a jet of gas onto the film
of immersion liquid as it forms.
4. The lithographic apparatus according to claim 2, wherein the
surface of the fluid handling structure comprises a line of
openings, surrounding the space in which immersion fluid is
contained, configured to extract immersion fluid in order to
contain the immersion fluid in the space; and the plurality of gas
jets are provided outwardly of the line of openings.
5. The lithographic apparatus according to claim 4, wherein the
plurality of apertures .[.providing the gas jets.]. are arranged
along a second line, parallel to the line of openings.
6. The lithographic apparatus according to claim 2, wherein the
.[.controller.]. .Iadd.control system .Iaddend.is configured such
that it can control the supply of gas to a first group of one or
more apertures independently from a second group of one or more
apertures.
7. The lithographic apparatus according to claim 2, wherein the
.Iadd.control system is configured to control the supply the
.Iaddend.gas .[.is supplied.]. to the apertures such that the
maximum velocity of the gas is in the range of from approximately
Ma 0.3 to Ma 0.6.
8. The lithographic apparatus according to claim 2, wherein the
separation between the center of one aperture .Iadd.of the
plurality of apertures .Iaddend.and the center of an adjacent
aperture .Iadd.of the plurality of apertures .Iaddend.is in the
range of from approximately 100 .mu.m to 1 mm.
9. The lithographic apparatus according to claim 2, wherein the
width of the apertures is in the range of from approximately 50
.mu.m to 150 .mu.m.
10. The lithographic apparatus according to claim 2, wherein the
fluid handling structure comprises a plurality of openings, each
provided between two adjacent apertures, wherein the openings are
connected to an underpressure source.
11. A fluid handling structure for a lithographic apparatus that
includes a substrate table having a substantially planar upper
surface in which is formed a recess that is configured to receive
and support a substrate .[.and a cover comprising a substantially
planar main body that, in use, extends around the substrate from
the upper surface to a peripheral section of an upper major face of
the substrate in order to cover a gap between an edge of the recess
and an edge of the substrate.]., the fluid handling structure
comprising a plurality of apertures configured to provide a
corresponding plurality of discrete gas jets to disrupt the
formation of a film of immersion fluid between an edge of the
.[.cover.]. .Iadd.substrate .Iaddend.and immersion fluid contained
by the fluid handling structure during movement of the substrate
table relative to the fluid handling structure, and a
.[.controller.]. .Iadd.control system .Iaddend.configured to
control supply of gas to the apertures such that the jets of gas
are provided when the apertures are in the region of the
.[.cover.]. .Iadd.edge of the substrate .Iaddend.and the jets of
gas are not provided when the apertures are not in the region of
the .[.cover.]. .Iadd.edge of the substrate.Iaddend..
12. The fluid handling structure according to claim 11, wherein the
plurality of gas jets are provided by a corresponding plurality of
apertures formed in a surface of the fluid handling structure
opposite .[.the.]. .Iadd.an .Iaddend.upper major surface of the
substrate and/or the upper surface of the substrate table.
13. The fluid handling structure according to claim 12, wherein the
surface of the fluid handling structure comprises a line of
openings, surrounding the space in which immersion fluid is
contained, configured to extract immersion fluid in order to
contain the immersion fluid in the space; and the plurality of gas
jets are provided outwardly of the line of openings.
14. The fluid handling structure according to claim 11,
.[.wherein.]. .Iadd.further comprising a control system configured
to control supply of .Iaddend.the gas .[.is supplied.]. to the
apertures such that the maximum velocity of the gas is in the range
of from approximately Ma 0.3 to Ma 0.6.
15. The fluid handling structure according to claim 11, wherein the
separation between the center of one aperture .Iadd.of the
plurality of apertures .Iaddend.and the center of an adjacent
aperture .Iadd.of the plurality of apertures .Iaddend.is in the
range of from approximately 100 .mu.m to 1 mm.
16. The fluid handling structure according to claim 11, wherein the
width of the apertures is in the range of from approximately 50
.mu.m to 150 .mu.m.
17. The fluid handling structure according to claim 11, wherein the
fluid handling structure comprises a plurality of openings, each
provided between two adjacent apertures, wherein the openings are
connected to an underpressure source.
18. A device manufacturing method, comprising: providing a
substrate to a substrate table having an upper surface and a recess
in the upper surface that is configured to receive and support the
substrate; .[.providing a cover comprising a planar main body such
that it extends around the substrate from the upper surface to a
peripheral section of an upper major face of the substrate in order
to cover a gap between an edge of the recess and an edge of the
substrate;.]. providing an immersion fluid to a space between a
final element of a projection system and the substrate and/or a
substrate table using a fluid handling structure; providing a
plurality of discrete gas jets from a structure in or on a surface
of the fluid handling structure that is opposite .[.the.]. .Iadd.an
.Iaddend.upper major surface of the substrate and/or the upper
surface of the substrate table in order to disrupt the formation of
a film of immersion fluid between an edge of the .[.cover.].
.Iadd.substrate .Iaddend.and immersion fluid contained by the fluid
handling structure during movement of the substrate table relative
to the fluid handling structure; and controlling supply of gas to
the gas .[.lets.]. .Iadd.jets .Iaddend.such that the jets of gas
are provided when at least part of the structure .Iadd.of the
discrete gas jets .Iaddend.is in the region of the .[.cover.].
.Iadd.edge of the substrate .Iaddend.and the jets of gas are not
provided when the at least part of the structure .Iadd.of the
discrete gas jets .Iaddend.is not in the region of the .[.cover.].
.Iadd.edge of the substrate.Iaddend..
19. The method of claim 18, wherein the plurality of gas jets are
provided by a corresponding plurality of apertures formed in the
surface of the fluid handling structure and the controlling
comprises controlling supply of gas to a first group of one or more
apertures independently from a second group of one or more
apertures.
20. The method of claim 18, further comprising exhausting gas
through a plurality of openings, each opening provided between two
adjacent apertures providing a respective gas jet.
.Iadd.21. A liquid handling structure for a lithographic apparatus,
the liquid handling structure configured to contain liquid in a
space between a projection system of the lithographic apparatus and
an upper major surface of a substrate table of the lithographic
apparatus and/or of a substrate when located on the substrate
table, the liquid handling structure comprising: an outlet
configured to remove liquid from the space; a plurality of openings
located outward, relative to the space, of the outlet, the openings
configured to provide respective flows of gas incident on the upper
major surface that are spaced apart at the upper major surface; a
gas knife located outward, relative to the space, of the plurality
of openings, the gas knife configured to provide a flow of gas
incident on the upper major surface; and a control system,
configured to control supply of gas to the plurality of openings so
as to provide the respective flows of gas from the plurality of
openings when at least part of the plurality of openings is in the
region of an edge of the substrate and to provide the respective
flows of gas from the plurality of openings so as to divide liquid
on the upper major surface while intentionally allowing liquid to
pass between adjacent respective flows, and control supply of gas
to the gas knife to provide the flow of gas from the gas knife to
block liquid that passes between the adjacent respective flows from
the plurality of openings..Iaddend.
.Iadd.22. The liquid handling structure according to claim 21,
wherein the outlet comprises a line of apertures configured to
surround the space in which liquid is contained, the apertures
configured to extract liquid in order to contain liquid in the
space..Iaddend.
.Iadd.23. The liquid handling structure according to claim 22,
wherein the plurality of openings are arranged along a second line,
substantially parallel to the line of apertures of the
outlet..Iaddend.
.Iadd.24. The liquid handling structure according to claim 21,
wherein the control system is further configured to control the
supply of gas to a first group of the plurality of openings
independently from a second group of the plurality of
openings..Iaddend.
.Iadd.25. The liquid handling structure according to claim 21,
wherein the control system is further configured to control the
supply of gas to the plurality of openings such that the maximum
velocity of the gas is in the range of from about Ma 0.3 to about
Ma 0.6..Iaddend.
.Iadd.26. The liquid handling structure according to claim 21,
wherein a separation between a center of one opening of the
plurality of openings and a center of an adjacent opening of the
plurality of openings is selected from the range of about 100 .mu.m
to about 1 mm..Iaddend.
.Iadd.27. The liquid handling structure according to claim 21,
wherein a cross-sectional width of each of the plurality of
openings is selected from the range of about 50 .mu.m to about 150
.mu.m..Iaddend.
.Iadd.28. The liquid handling structure according to claim 21,
further comprising a plurality of apertures configured to exhaust
gas, each aperture provided between two adjacent openings of the
plurality of openings..Iaddend.
.Iadd.29. A lithographic apparatus comprising: the substrate table;
the projection system; and the liquid handling structure of claim
21..Iaddend.
.Iadd.30. The lithographic apparatus according to claim 29, wherein
the substrate table has a recess in the major upper surface of the
substrate, the recess configured to receive and support the
substrate..Iaddend.
.Iadd.31. The liquid handling structure according to claim 21,
wherein a gap between an edge of one opening of the plurality of
openings and an edge of an adjacent opening of the plurality of
openings is at least 50 .mu.m..Iaddend.
.Iadd.32. A device manufacturing method, comprising: providing a
liquid to a space between a projection system of a lithographic
apparatus and an upper major surface of a substrate table of the
lithographic apparatus and/or of a substrate when located on the
substrate table; containing liquid in the space by removing liquid
from the space using an outlet of a liquid handling structure of
the lithographic apparatus; providing a respective flow of gas
incident on the upper major surface from each opening of a
plurality of openings of the liquid handling structure, the
openings located outward, relative to the space, of the outlet;
providing a flow of gas incident on the upper major surface from a
gas knife of the liquid handling structure, the gas knife located
outward, relative to the space, of the plurality of openings; and
controlling supply of gas to the plurality of openings so as to
provide the respective flows of gas from the plurality of openings
when at least part of the plurality of openings is in the region of
an edge of the substrate and to provide the respective flows of gas
from the plurality of openings so as to divide liquid on the upper
major surface while intentionally allowing liquid to pass between
adjacent respective flows, and controlling supply of gas to the gas
knife such that the flow of gas therefrom blocks liquid that passes
between the adjacent respective flows from the plurality of
openings..Iaddend.
.Iadd.33. The method according to claim 32, wherein the outlet
comprises a line of apertures surrounding the space in which liquid
is contained, the apertures extracting liquid in order to contain
liquid in the space..Iaddend.
.Iadd.34. The method according to claim 33, wherein the plurality
of openings are arranged along a second line, substantially
parallel to the line of apertures of the outlet..Iaddend.
.Iadd.35. The method according to claim 32, further comprising
controlling the supply of gas to a first group of the plurality of
openings independently from a second group of the plurality of
openings..Iaddend.
.Iadd.36. The method according to claim 32, further comprising
controlling the supply of gas to the plurality of openings such
that the maximum velocity of the gas is in the range of from about
Ma 0.3 to about Ma 0.6..Iaddend.
.Iadd.37. The method according to claim 32, wherein a separation
between a center of one opening of the plurality of openings and a
center of an adjacent opening of the plurality of openings is
selected from the range of about 100 .mu.m to about 1
mm..Iaddend.
.Iadd.38. The method according to claim 32, wherein a
cross-sectional width of each of the plurality of openings is
selected from the range of about 50 .mu.m to about 150
.mu.m..Iaddend.
.Iadd.39. The method according to claim 32, further comprising
exhausting gas using a plurality of apertures, each aperture
provided between two adjacent openings of the plurality of
openings..Iaddend.
.Iadd.40. The method according to claim 32, wherein a gap between
an edge of one opening of the plurality of openings and an edge of
an adjacent opening of the plurality of openings is at least 50
.mu.m..Iaddend.
Description
FIELD
The present invention relates to a lithographic apparatus, a fluid
handling structure for use in a lithographic apparatus and a device
manufacturing method.
BACKGROUND
A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the
substrate. A lithographic apparatus can be used, for example, in
the manufacture of integrated circuits (ICs). In that instance, a
patterning device, which is alternatively referred to as a mask or
a reticle, may be used to generate a circuit pattern to be formed
on an individual layer of the IC. This pattern can be transferred
onto a target portion (e.g. comprising part of, one, or several
dies) on a substrate (e.g. a silicon wafer). Transfer of the
pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent
target portions that are successively patterned. Known lithographic
apparatus include so-called steppers, in which each target portion
is irradiated by exposing an entire pattern onto the target portion
at one time, and so-called scanners, in which each target portion
is irradiated by scanning the pattern through a radiation beam in a
given direction (the "scanning"-direction) while synchronously
scanning the substrate parallel or anti-parallel to this direction.
It is also possible to transfer the pattern from the patterning
device to the substrate by imprinting the pattern onto the
substrate.
It has been proposed to immerse the substrate in the lithographic
projection apparatus in a liquid having a relatively high
refractive index, e.g. water, so as to fill a space between the
final element of the projection system and the substrate. In an
embodiment, the liquid is distilled water, although another liquid
can be used. An embodiment of the present invention will be
described with reference to liquid. However, another liquid may be
suitable, particularly a wetting fluid, an incompressible fluid
and/or a fluid with higher refractive index than air, desirably a
higher refractive index than water. Fluids excluding gases are
particularly desirable. The point of this is to enable imaging of
smaller features since the exposure radiation will have a shorter
wavelength in the liquid. (The effect of the liquid may also be
regarded as increasing the effective numerical aperture (NA) of the
system and also increasing the depth of focus.) Other immersion
liquids have been proposed, including water with solid particles
(e.g. quartz) suspended therein, or a liquid with a nano-particle
suspension (e.g. particles with a maximum dimension of up to 10
nm). The suspended particles may or may not have a similar or the
same refractive index as the liquid in which they are suspended.
Other liquids which may be suitable include a hydrocarbon, such as
an aromatic, a fluorohydrocarbon, and/or an aqueous solution.
Submersing the substrate or substrate and substrate table in a bath
of liquid (see, for example, U.S. Pat. No. 4,509,852) means that
there is a large body of liquid that should be accelerated during a
scanning exposure. This may require additional or more powerful
motors and turbulence in the liquid may lead to undesirable and
unpredictable effects.
Other arrangements which have been proposed include a confined
immersion system and an all wet immersion system. In a confined
immersion system a liquid supply system provides liquid on only a
localized area of the substrate and in between the final element of
the projection system and the substrate using a liquid confinement
system (the substrate generally has a larger surface area than the
final element of the projection system). One way which has been
proposed to arrange for this is disclosed in PCT patent application
publication no. WO 99/49504.
In an all wet immersion system, as disclosed in PCT patent
application publication WO 2005/064405 the immersion liquid is
unconfined. In such a system the whole top surface of the substrate
is covered in liquid. This may be advantageous because then the
whole top surface of the substrate is exposed to the substantially
same conditions. This may have an advantage for temperature control
and processing of the substrate. In WO 2005/064405, a liquid supply
system provides liquid to the gap between the final element of the
projection system and the substrate. Liquid is allowed to leak over
the remainder of the substrate. A barrier at the edge of a
substrate table prevents the liquid from escaping so that it can be
removed from the top surface of the substrate table in a controlled
way.
The immersion system may be a fluid handling system or apparatus.
In an immersion system, immersion fluid is handled by a fluid
handling system, structure or apparatus. In an embodiment the fluid
handling system may supply immersion fluid and therefore be a fluid
supply system. In an embodiment the fluid handling system may at
least partly confine immersion fluid and thereby be a fluid
confinement system. In an embodiment the fluid handling system may
provide a barrier to immersion fluid and thereby be a barrier
member, such as a fluid confinement structure. In an embodiment the
fluid handling system may create or use a flow of gas, for example
to help in controlling the flow and/or the position of the
immersion fluid. The flow of gas may form a seal to confine the
immersion fluid so the fluid handling structure may be referred to
as a seal member; such a seal member may be a fluid confinement
structure. The fluid handling system may be located between the
projection system and the substrate table. In an embodiment,
immersion liquid is used as the immersion fluid. In that case the
fluid handling system may be a liquid handling system. In reference
to the aforementioned description, reference in this paragraph to a
feature defined with respect to fluid may be understood to include
a feature defined with respect to liquid.
In a fluid handling system or liquid confinement structure, liquid
is confined to a space, for example within a confinement structure.
The space may be defined by the body of the confinement structure,
the underlying surface (e.g. a substrate table, a substrate
supported on the substrate table, a shutter member and/or a
measurement table) and, in the case of a localized area immersion
system, a liquid meniscus between the fluid handling system or
liquid confinement structure and the underlying structure i.e. in
an immersion space. In the case of an all wet system, liquid is
allowed to flow out of the immersion space onto the top surface of
the substrate and/or substrate table.
In European patent application publication no. EP 1420300 and
United States patent application publication no. US 2004-0136494,
each hereby incorporated in their entirety by reference, the idea
of a twin or dual stage immersion lithography apparatus is
disclosed. Such an apparatus is provided with two tables for
supporting a substrate. Leveling measurements are carried out with
a table at a first position, without immersion liquid, and exposure
is carried out with a table at a second position, where immersion
liquid is present. Alternatively, the apparatus has only one
table.
After exposure of a substrate in an immersion lithographic
apparatus, the substrate table is moved away from its exposure
position to a position in which the substrate may be removed and
replaced by a different substrate. This is known as substrate swap.
In a two stage lithographic apparatus, for example ASML's
"Twinscan" lithographic apparatus, the substrate tables swap takes
place under the projection system.
SUMMARY
In a lithographic apparatus, a substrate is supported on a
substrate table by a substrate holder. The substrate holder may be
located in a recess of the substrate table. The recess may be
dimensioned so that when a substrate is supported by the substrate
holder the top surface of the substrate is generally in the same
plane as the surface of the substrate table surrounding the
substrate. Around the substrate, there may be a gap between an edge
of a substrate and an edge of the substrate table. Such a gap may
be undesirable in an immersion system of a lithographic apparatus.
As the gap moves under the immersion liquid in the space between
the final element of the projection system and the underlying
surface, the meniscus between the confinement structure and the
underlying surface crosses the gap. Crossing the gap may increase
the instability of the meniscus. The stability of the meniscus may
decrease with increased relative speed, e.g. scanning or stepping
speed, between the confinement structure and the substrate table.
An increasingly unstable meniscus is a risk to increased
defectivity. For example an unstable meniscus may enclose gas as a
bubble in the immersion liquid, or may cause a droplet to escape
from the immersion space. Such a bubble may be drawn into the space
and result in imaging defects. A droplet may be a source of
contaminants and a heat load as it evaporates and it may later
collide with the meniscus causing a bubble to be drawn in to the
space.
One or more problems of crossing the gap may be reduced by the
provision of a two-phase extraction system. The two phase
extraction system extracts fluid such as immersion liquid and gas
(which may be present as a bubble in the liquid) from the gap.
Sources of defectivity, such as releasing a bubble into the space
or a droplet escaping from the space, may be reduced if not
eliminated. However, the provision of such an extraction system may
impart a heat load on the substrate table and the substrate. This
may have a negative impact on the overlay accuracy of patterns
formed on the substrate. The gap may implicitly limit the scan
speed that may be used to achieve reliable imaging of a
substrate.
It is therefore desirable to provide, for example, a system to
increase the stability of the meniscus and reduce defectivity, for
example the likelihood of creating a bubble or releasing a
droplet.
In an aspect of an invention, there is provided a lithographic
apparatus comprising: a substrate table having an upper surface and
a recess in the upper surface that is configured to receive and
support a substrate; a fluid handling structure configured to
contain immersion fluid in a space adjacent to the upper surface of
the substrate table and/or a substrate located in the recess; a
cover comprising a planar main body that, in use, extends around a
substrate from the upper surface to a peripheral section of an
upper major face of the substrate in order to cover a gap between
an edge of the recess and an edge of the substrate; and an
immersion fluid film disruptor, configured to disrupt the formation
of a film of immersion fluid between an edge of the cover and
immersion fluid contained by the fluid handling structure during
movement of the substrate table relative to the fluid handling
structure, the immersion fluid film disruptor comprising a
plurality of discrete gas jets formed on a surface of the fluid
handling structure opposite the upper surface of the substrate
and/or substrate table.
In an aspect of an invention, there is provided a fluid handling
structure for a lithographic apparatus that includes a substrate
table having a substantially planar upper surface in which is
formed a recess that is configured to receive and support a
substrate and a cover comprising a substantially planar main body
that, in use, extends around the substrate from the upper surface
to a peripheral section of an upper major face of the substrate in
order to cover a gap between an edge of the recess and an edge of
the substrate, the fluid handling structure comprising a plurality
of apertures configured to provide a corresponding plurality of
discrete gas jets to disrupt the formation of a film of immersion
fluid between an edge of the cover and immersion fluid contained by
the fluid handling structure during movement of the substrate table
relative to the fluid handling structure.
In an aspect of an invention, there is provided a device
manufacturing method, comprising: providing a substrate to a
substrate table having an upper surface and a recess in the upper
surface that is configured to receive and support the substrate;
providing a cover comprising a planar main body such that it
extends around the substrate from the upper surface to a peripheral
section of an upper major face of the substrate in order to cover a
gap between an edge of the recess and an edge of the substrate;
providing an immersion fluid to a space between a final element of
a projection system and the substrate and/or a substrate table
using a fluid handling structure; and providing a plurality of
discrete gas jets from a surface of the fluid handling structure
that is opposite the upper surface of the substrate and/or
substrate table in order to disrupt the formation of a film of
immersion fluid between an edge of the cover and immersion fluid
contained by the fluid handling structure during movement of the
substrate table relative to the fluid handling structure.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of
example only, with reference to the accompanying schematic drawings
in which corresponding reference symbols indicate corresponding
parts, and in which:
FIG. 1 depicts a lithographic apparatus according to an embodiment
of the invention;
FIGS. 2 and 3 depict a fluid handling structure as a liquid supply
system for use in a lithographic projection apparatus;
FIG. 4 depicts a further liquid supply system for use in a
lithographic projection apparatus;
FIG. 5 depicts, in cross-section, a liquid confinement structure
which may be used in an embodiment of the present invention as a
liquid supply system;
FIG. 6 depicts, in plan view, a substrate receiving section
according to an aspect of the invention;
FIGS. 7 and 8 depict, in plan view, a cover according to an aspect
of the invention in open and closed positions, respectively;
FIGS. 9 and 10 depict, in plan view, a cover according to an aspect
of the invention in closed and open positions, respectively;
FIGS. 11, 12 and 13 depict, in cross-section, an actuator system
for a cover according to an aspect of the invention in,
respectively, the closed position, an intermediate position and the
open position;
FIGS. 14 and 15 depict, in cross-section, an arrangement of
movement guides that may be used in an actuator system of an aspect
of the invention;
FIGS. 16, 17 and 18 depict an actuator system for a cover according
to an aspect of the invention in, respectively, a closed position,
an intermediate position and an open position;
FIGS. 19 and 20 depict, in cross-section, a cover according to an
aspect of the invention;
FIG. 21 depicts, in cross-section, an arrangement of a cover
according to an aspect of the invention;
FIGS. 22 to 25 schematically depict possible configurations of the
edge of a cover according to an aspect of the invention;
FIGS. 26 to 32 schematically depict, in cross section, arrangements
of covers according to an aspect of the invention;
FIG. 33 depicts an arrangement of a cover in use;
FIG. 34 depicts an arrangement of a cover in a system including an
immersion fluid film disruptor according to an aspect of the
invention;
FIG. 35 depicts an arrangement of a cover in a system including an
immersion fluid film disruptor according to an aspect of the
invention;
FIG. 36 depicts an arrangement of a cover in a system including an
immersion fluid film disruptor according to an aspect of the
invention;
FIG. 37 depicts in greater detail a portion of an arrangement as
depicted in FIG. 36;
FIG. 38 depicts a relationship between the jet pitch of an
arrangement as depicted in FIG. 36 and the resulting droplet
diameter;
FIG. 39 depicts, in cross-section, an arrangement of an immersion
fluid film disruptor as depicted in FIG. 36; and
FIG. 40 depicts a variation of the arrangement depicted in FIG.
39.
DETAILED DESCRIPTION
FIG. 1 schematically depicts a lithographic apparatus according to
one embodiment of the invention. The apparatus comprises: an
illumination system (illuminator) IL configured to condition a
radiation beam B (e.g. UV radiation or DUV radiation); a support
structure (e.g. a mask table) MT constructed to support a
patterning device (e.g. a mask) MA and connected to a first
positioner P.sup.M configured to accurately position the patterning
device MA in accordance with certain parameters; a substrate table
(e.g. a wafer table) WT constructed to hold a substrate (e.g. a
resist-coated wafer) W and connected to a second positioner PW
configured to accurately position the substrate W in accordance
with certain parameters; and a projection system (e.g. a refractive
projection lens system) PS configured to project a pattern imparted
to the radiation beam B by patterning device MA onto a target
portion C (e.g. comprising one or more dies) of the substrate
W.
The illumination system IL may include various types of optical
components, such as refractive, reflective, magnetic,
electromagnetic, electrostatic or other types of optical
components, or any combination thereof, for directing, shaping, or
controlling radiation.
The support structure MT holds the patterning device MA. The
support structure MT holds the patterning device MA in a manner
that depends on the orientation of the patterning device MA, the
design of the lithographic apparatus, and other conditions, such as
for example whether or not the patterning device is held in a
vacuum environment. The support structure MT can use mechanical,
vacuum, electrostatic or other clamping techniques to hold the
patterning device. The support structure MT may be a frame or a
table, for example, which may be fixed or movable as desired. The
support structure MT may ensure that the patterning device MA is at
a desired position, for example with respect to the projection
system PS. Any use of the terms "reticle" or "mask" herein may be
considered synonymous with the more general term "patterning
device."
The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to
create a pattern in a target portion of the substrate. It should be
noted that the pattern imparted to the radiation beam may not
exactly correspond to the desired pattern in the target portion of
the substrate, for example if the pattern includes phase-shifting
features or so called assist features. Generally, the pattern
imparted to the radiation beam will correspond to a particular
functional layer in a device being created in the target portion,
such as an integrated circuit.
The patterning device may be transmissive or reflective. Examples
of patterning devices include masks, programmable mirror arrays,
and programmable LCD panels. Masks are well known in lithography,
and include mask types such as binary, alternating phase-shift, and
attenuated phase-shift, as well as various hybrid mask types. An
example of a programmable mirror array employs a matrix arrangement
of small mirrors, each of which can be individually tilted so as to
reflect an incoming radiation beam in different directions. The
tilted mirrors impart a pattern in a radiation beam which is
reflected by the mirror matrix.
The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system. The
types of projection system may include: refractive, reflective,
catadioptric, magnetic, electromagnetic and electrostatic optical
systems, or any combination thereof. The selection or combination
of the projection system is as appropriate for the exposure
radiation being used, or for other factors such as the use of an
immersion liquid or the use of a vacuum. Any use of the term
"projection lens" herein may be considered as synonymous with the
more general term "projection system".
As here depicted, the apparatus is of a transmissive type (e.g.
employing a transmissive mask). Alternatively, the apparatus may be
of a reflective type (e.g. employing a programmable mirror array of
a type as referred to above, or employing a reflective mask).
The lithographic apparatus may be of a type having two (dual stage)
or more substrate tables (and/or two or more patterning device
tables). In such "multiple stage" machines the additional tables
may be used in parallel, or preparatory steps may be carried out on
one or more tables while one or more other tables are being used
for exposure.
Referring to FIG, 1, the illuminator IL receives a radiation beam
from a radiation source SO. The source SO and the lithographic
apparatus may be separate entities, for example when the source is
an excimer laser. In such cases, the source SO is not considered to
form part of the lithographic apparatus and the radiation beam is
passed from the source SO to the illuminator IL with the aid of a
beam delivery system BD comprising, for example, suitable directing
mirrors and/or a beam expander. In other cases the source SO may be
an integral part of the lithographic apparatus, for example when
the source is a mercury lamp. The source SO and the illuminator IL,
together with the beam delivery system BD if required, may be
referred to as a radiation system.
The illuminator IL may comprise an adjuster AD for adjusting the
angular intensity distribution of the radiation beam. Generally, at
least the outer and/or inner radial extent (commonly referred to as
.sigma.-outer and .sigma.-inner, respectively) of the intensity
distribution in a pupil plane of the illuminator IL can be
adjusted. In addition, the illuminator IL may comprise various
other components, such as an integrator IN and a condenser CO. The
illuminator IL may be used to condition the radiation beam, to have
a desired uniformity and intensity distribution in its
cross-section. Similar to the source SO, the illuminator IL may or
may not be considered to form part of the lithographic apparatus.
For example, the illuminator IL may be an integral part of the
lithographic apparatus or may be a separate entity from the
lithographic apparatus. In the latter case, the lithographic
apparatus may be configured to allow the illuminator IL to be
mounted thereon. Optionally, the illuminator IL is detachable and
may be separately provided (for example, by the lithographic
apparatus manufacturer or another supplier).
The radiation beam B is incident on the patterning device (e.g.,
mask) MA, which is held on the support structure (e.g., mask table)
MT, and is patterned by the patterning device MA. Having traversed
the patterning device MA, the radiation beam B passes through the
projection system PS. The projection system PS focuses the beam B
onto a target portion C of the substrate W. With the aid of the
second positioner PW and position sensor IF (e.g. an
interferometric device, linear encoder or capacitive sensor), the
substrate table WT can be moved accurately, e.g. so as to position
different target portions C in the path of the radiation beam B.
Similarly, the first positioner PM and another position sensor
(which is not explicitly depicted in FIG. 1) can be used to
accurately position the patterning device MA with respect to the
path of the radiation beam B, e.g. after mechanical retrieval from
a mask library, or during a scan. In general, movement of the
support structure MT may be realized with the aid of a long-stroke
module (coarse positioning) and a short-stroke module (fine
positioning), which form part of the first positioner PM.
Similarly, movement of the substrate table WT may be realized using
a long-stroke module and a short-stroke module, which form part of
the second positioner PW. In the case of a stepper (as opposed to a
scanner) the support structure MT may be connected to a
short-stroke actuator only, or may be fixed. Patterning device MA
and substrate W may be aligned using patterning device alignment
marks M1, M2 and substrate alignment marks P1, P2. Although the
substrate alignment marks as illustrated occupy dedicated target
portions, they may be located in spaces between target portions
(these are known as scribe-lane alignment marks). Similarly, in
situations in which more than one die is provided on the patterning
device MA, the patterning device alignment marks may be located
between the dies.
The depicted apparatus could be used in at least one of the
following modes:
In step mode, the support structure MT and the substrate table WT
are kept essentially stationary, while an entire pattern imparted
to the radiation beam B is projected onto a target portion C at one
time (i.e. a single static exposure). The substrate table WT is
then shifted in the X and/or Y direction so that a different target
portion C can be exposed. In step mode, the maximum size of the
exposure field limits the size of the target portion C imaged in a
single static exposure.
In scan mode, the support structure MT and the substrate table WT
are scanned synchronously while a pattern imparted to the radiation
beam B is projected onto a target portion C (i.e. a single dynamic
exposure). The velocity and direction of the substrate table WT
relative to the support structure MT may be determined by the
(de-)magnification and image reversal characteristics of the
projection system PS. In scan mode, the maximum size of the
exposure field limits the width (in the non-scanning direction) of
the target portion C in a single dynamic exposure, whereas the
length of the scanning motion determines the height (in the
scanning direction) of the target portion C.
In another mode, the support structure MT is kept essentially
stationary holding a programmable patterning device, and the
substrate table WT is moved or scanned while a pattern imparted to
the radiation beam B is projected onto a target portion C. In this
mode, generally a pulsed radiation source is employed and the
programmable patterning device is updated as desired after each
movement of the substrate table WT or in between successive
radiation pulses during a scan. This mode of operation can be
readily applied to mask-less lithography that utilizes programmable
patterning device, such as a programmable mirror array of a type as
referred to above.
Combinations and/or variations on the above described modes of use
or entirely different modes of use may also be employed.
An arrangement to provide liquid between a final element of the
projection system PS and the substrate is the so called localized
immersion system IH. In this system a liquid handling system is
used in which liquid is only provided to a localized area of the
substrate. The space filled by liquid is smaller in plan than the
top surface of the substrate and the area filled with liquid
remains substantially stationary relative to the projection system
PS while the substrate W moves underneath that area. Four different
types of localized liquid supply systems are illustrated in FIGS.
2-5.
As illustrated in FIGS. 2 and 3, liquid is supplied by at least one
inlet onto the substrate, preferably along the direction of
movement of the substrate relative to the final element. Liquid is
removed by at least one outlet after having passed under the
projection system. That is, as the substrate is scanned beneath the
element in a -X direction, liquid is supplied at the +X side of the
element and taken up at the -X side. FIG. 2 shows the arrangement
schematically in which liquid is supplied via inlet and is taken up
on the other side of the element by outlet which is connected to a
low pressure source. In the illustration of FIG. 2 the liquid is
supplied along the direction of movement of the substrate relative
to the final element, though this does not need to be the case.
Various orientations and numbers of in- and out-lets positioned
around the final element are possible; one example is illustrated
in FIG. 3 in which four sets of an inlet with an outlet on either
side are provided in a regular pattern around the final element.
Arrows in liquid supply and liquid recovery devices indicate the
direction of liquid flow.
A further immersion lithography solution with a localized liquid
supply system is shown in FIG. 4. Liquid is supplied by two groove
inlets on either side of the projection system PS and is removed by
a plurality of discrete outlets arranged radially outwardly of the
inlets. The inlets can be arranged in a plate with a hole in its
centre and through which the projection beam is projected. Liquid
is supplied by one groove inlet on one side of the projection
system PS and removed by a plurality of discrete outlets on the
other side of the projection system PS, causing a flow of a thin
film of liquid between the projection system PS and the substrate
W. The choice of which combination of inlet and outlets to use can
depend on the direction of movement of the substrate W (the other
combination of inlet and outlets being inactive). In the
cross-sectional view of FIG. 4, arrows illustrate the direction of
liquid flow in inlets and out of outlets.
Another arrangement which has been proposed is to provide the
liquid supply system with a liquid confinement member which extends
along at least a part of a boundary of the space between the final
element of the projection system and the substrate table. Such an
arrangement is illustrated in FIG. 5. The liquid confinement member
is substantially stationary relative to the projection system in
the XY plane though there may be some relative movement in the Z
direction (in the direction of the optical axis). A seal is formed
between the liquid confinement and the surface of the substrate. In
an embodiment, a seal is formed between the liquid confinement
structure and the surface of the substrate and may be a contactless
seal such as a gas seal. Such a system is disclosed in United
States patent application publication no. US 2004-0207824, hereby
incorporated in its entirety by reference.
FIG. 5 schematically depicts a localized liquid supply system with
a liquid confinement structure 12. The liquid confinement structure
12 extends along at least a part of a boundary of the space 11
between the final element of the projection system PS and the
substrate table WT or substrate W. (Please note that reference in
the following text to surface of the substrate W also refers in
addition or in the alternative to a surface of the substrate table
WT, unless expressly stated otherwise.) The liquid confinement
structure 12 is substantially stationary relative to the projection
system PS in the XY plane though there may be some relative
movement in the Z direction (in the direction of the optical axis).
In an embodiment, a seal is formed between the liquid confinement
structure 12 and the surface of the substrate W and may be a
contactless seal such as fluid seal, desirably a gas seal.
The liquid confinement structure 12 at least partly contains liquid
in the immersion space 11 between a final element of the projection
system PS and the substrate W. A contactless seal 16 to the
substrate W may be formed around the image field of the projection
system PS so that liquid is confined within the space 11 between
the substrate W surface and the final element of the projection
system PS. The immersion space 11 is at least partly formed by the
liquid confinement structure 12 positioned below and surrounding
the final element of the projection system PS. Liquid is brought
into the space 11 below the projection system PS and within the
liquid confinement structure 12 by liquid inlet 13. The liquid may
be removed by liquid outlet 13. The liquid confinement structure 12
may extend a little above the final element of the projection
system PS. The liquid level rises above the final element so that a
buffer of liquid is provided. In an embodiment, the liquid
confinement structure 12 has an inner periphery that at the upper
end closely conforms to the shape of the projection system PS or
the final element thereof and may, e.g., be round. At the bottom,
the inner periphery closely conforms to the shape of the image
field, e.g., rectangular, though this need not be the case.
In an embodiment, the liquid is contained in the immersion space 11
by a gas seal 16 which, during use, is formed between the bottom of
the liquid confinement structure 12 and the surface of the
substrate W. Other types of seal are possible, as is no seal (for
example in an all wet embodiment) or a seal achieved by capillary
forces between the undersurface of the liquid confinement structure
12 and a facing surface, such as the surface of a substrate W, a
substrate table WT or a combination of both.
The gas seal 16 is formed by gas, e.g. air or synthetic air but, in
an embodiment, N.sub.2 or another inert gas. The gas in the gas
seal 16 is provided under pressure via inlet 15 to the gap between
liquid confinement structure 12 and substrate W. The gas is
extracted via outlet 14. The overpressure on the gas inlet 15,
vacuum level on the outlet 14 and geometry of the gap are arranged
so that there is a high-velocity gas flow inwardly that confines
the liquid. The force of the gas on the liquid between the liquid
confinement structure 12 and the substrate W contains the liquid in
an immersion space 11. The inlets/outlets may be annular grooves
which surround the space 11. The annular grooves may be continuous
or discontinuous. The flow of gas is effective to contain the
liquid in the space 11. Such a system is disclosed in United States
patent application publication no. US 2004-0207824.
Other arrangements are possible and, as will be clear from the
description below, an embodiment of the present invention may use
any type of localized immersion system.
In a localized immersion system a seal is formed between a part of
the liquid confinement structure and an underlying surface, such as
a surface of a substrate W and/or substrate table WT. The seal may
be defined by a meniscus of liquid between the liquid confinement
structure and the underlying surface. Relative movement between the
underlying surface and the liquid confinement structure may lead to
breakdown of the seal, for example the meniscus, above a critical
speed. Above the critical speed, the seal may break down allowing
liquid, e.g. in the form of a droplet, to escape from the liquid
confinement structure, or gas, i.e. in the form of a bubble, to be
enclosed in the immersion liquid within the immersion space.
A droplet may be a source of defectivity. The droplet may apply a
thermal load on the surface which it is located as it evaporates.
The droplet may be a source of contamination, in leaving a drying
stain after it has evaporated. If the droplet lies in the path on
the underlying surface which moves under the projection system, the
droplet may contact the meniscus. The resulting collision between
the meniscus and the droplet may cause a bubble to form in the
liquid. A bubble may be a source of defectivity. A bubble in the
immersion liquid may be drawn in to the space between the
projection system and the substrate where it may interfere with an
imaging projection beam.
Critical speed may be determined by the properties of the
underlying surface. The critical speed of a gap relative to the
confinement structure may be less than the critical speed for the
surface of a relatively planar surface such as a substrate. On
increasing the scan velocity above the lowest critical speed for a
part of the undersurface, the scan velocity will exceed the
critical speed for more of the underlying surface. The problem may
be more significant at high scan velocities. However, an increased
scan velocity is desirable because throughput increases.
FIG. 6 depicts, in plan view, a substrate table WT that may be used
to support a substrate W. The substrate table may have a
substantially planar upper surface 21. In the upper surface 21 is a
recess 22 that is configured to receive and support a substrate
W.
In the recess may be a substrate support which may be a surface of
the recess. The surface of the recess 22 may include a plurality of
protrusions on which a lower surface of the substrate is supported.
The surface of the recess may include a barrier. In the surface of
the recess may be formed a plurality of openings. The barrier
surrounds the protrusions to define a space beneath the lower
surface of the substrate W. The openings are connected to an
under-pressure source. When a substrate is located above the
openings a space is formed beneath the substrate W. The space may
be evacuated by operation of the underpressure. This arrangement
may be used in order to secure the substrate W to the substrate
table WT.
In an arrangement, the recess may be configured such that the major
faces of the substrate, namely the upper face and the lower face,
are substantially parallel to the upper surface 21 of the substrate
table. In an arrangement, the upper face of the substrate W may be
arranged to be substantially coplanar with upper surface 21 of the
substrate table.
It should be appreciated that in the present application, terms
such as upper and lower may be used in order to define the relative
positions of components within the systems described. However,
these terms are used for convenience in order to describe the
relative positions of the components when the apparatus is used at
a particular orientation. They are not intended to specify the
orientation in which the apparatus may be used.
As depicted in FIG. 6, a gap 23 may be present between an edge of
the substrate W and an edge of the recess 22. According to an
aspect of the invention, a cover 25 is provided that extends around
the substrate W. The cover 25 extends from a peripheral section of
the upper surface of the substrate W (which in an embodiment may be
an edge of the substrate) to the upper surface 21 of the substrate
table WT. The cover 25 may entirely cover the gap 23 between the
edge of the substrate W and the edge of the recess 22. In addition,
an open central section 26 of the cover 25 may be defined by an
inner edge of the cover. The open central section 26 may be
arranged such that, in use, the cover 25 does not cover portions of
the substrate W on which it is intended to project a patterned beam
of radiation. The inner edge of the cover may cover portions of the
substrate which neighbor the surface of the substrate which is
imaged by the patterned projection beam. The cover is located away
from those portions of the substrate which are exposed by the
patterned projection beam.
As shown in FIG. 6, when the cover 25 is placed on the substrate W,
the size of the open central section 26 may be slightly smaller
than the size of the upper surface of the substrate W. As shown in
FIG. 6, if the substrate W is circular in shape, the cover 25 may
be generally annular in shape when viewed in plan view.
The cover 25 may be in the form of a thin cover plate. The cover
plate may, for example, be formed from stainless steel. Other
material may be used. The cover plate may be coated with Lipocer
coating of the type offered by Plasma Electronic GmbH. Lipocer is a
coating which may be lyophobic (e.g. hydrophobic) and is relatively
resistant to damage from exposure to radiation and immersion liquid
(which may be highly corrosive). More information on Lipocer may be
found in U.S. Patent Application Publication No. US 2009-0206304,
which is hereby incorporated by reference in its entirety.
As schematically depicted in FIG. 21, a lyophobic coating 141, such
as a layer of Lipocer, may be applied to the lower surface 25a of
the cover 25, namely the surface, that in use, may extend from a
peripheral section of the upper surface of the substrate W to the
upper surface 21 of the substrate table WT. The provision of such a
coating 141 on the lower surface 25a may minimize or reduce the
leakage of immersion liquid below the cover 25. For example, the
coating 141 may reduce the leakage of immersion liquid between the
cover 25 and the upper surface of the substrate W Minimizing or
reducing such immersion liquid leakage may in turn reduce the
likelihood of immersion liquid being passed to the underside of the
substrate W. This may reduce defects that may be introduced as a
result of the so-called back side contamination. Minimizing or
reducing immersion liquid leakage may reduce a thermal load on the
substrate W.
The coating 141 on the lower surface 25a of the cover 25 may be
selected to be an anti-sticking layer. In other words, the coating
141 may be selected to prevent, minimize or reduce adhesion of the
cover 25 to the upper surface of the substrate W and/or the upper
surface 21 of the substrate table WT. This may prevent or reduce
damage to the cover 25, the substrate W and/or the substrate table
WT when the cover 25 is removed from the substrate W and the
substrate table WT.
The use of a coating 141 on the lower surface 25a of the cover 25
that is lyophobic and/or anti-sticking may prevent or reduce the
accumulation of contamination particles on the lower surface of the
cover 25. Such contaminant particles could result in damage to any
of the cover 25, the substrate W and/or the substrate table WT or
provide a source of subsequent defects on the substrate W.
Alternatively or additionally, such contaminant particles may
prevent a sufficient seal being formed between the cover 25 and the
upper surface of the substrate W and/or the upper surface 21 of the
substrate table WT, resulting in leakage of the immersion liquid,
which may be undesirable. Accordingly, it may be desirable to
prevent the accumulation of such contaminant particles.
The lower surface 25a of the cover 25 and/or the lower surface 141a
of a coating 141 applied to the lower surface 25a of the cover 25
may be configured to have low surface roughness. For example, for a
spray coating, the surface roughness R.sub.A may be less than 1
.mu.m. For a deposited coating the surface roughness R.sub.A may be
less than 200 nm. In general, reducing the surface roughness of the
lower surface 25a of the cover and/or the lower surface 141a of a
coating 141 applied to the lower surface 25a of the cover 25 may
reduce stress concentrations on the surface of the substrate W. The
surface roughness R.sub.A of parts of the cover 25 in contact with
the substrate in use may desirably be less than 200 nm, desirably
less than 50 nm, or desirably less than 10 nm.
Ensuring that the surface roughness of the lower surface 25a of the
cover 25 and/or the lower surface 141a of a coating 141 applied to
the lower surface 25a of the cover 25 is low may also assist in
reducing or minimizing leakage of immersion liquid below the cover
25. The cover 25 may be arranged such that the flatness of the
lower surface 25a of the cover 25 and/or the lower surface 141a of
a coating 141 applied to the lower surface 25a of the cover 25 is
maximized. This may provide optimized contact between the cover 25
and the substrate W and/or the substrate table WT, reducing or
minimizing immersion liquid leakage.
As schematically depicted in FIG. 21, a coating 142 may
alternatively or additionally be provided on the upper surface 25b
of the cover 25. The upper surface 25b of the cover 25 or the upper
surface 142b of a coating 142 on the upper surface 25b of the cover
may be selected for smoothness. This may reduce the likelihood of
the meniscus being pinned. For example, the upper surface 25b of
the cover 25 or the upper surface 142b of a coating 142 on the
upper suffice 25b of the cover may be smooth such that the peak to
valley distance of the surface is less than 10 .mu.m, desirably
less than 5 .mu.m.
The coating 142 on the upper surface 25b of the cover 25 may be
selected to be resistant to damage from exposure to radiation and
immersion liquid. This may help ensure that the working life of the
cover is sufficiently long to prevent unnecessary costs associated
with replacing the cover 25, including downtime for the
lithographic apparatus. The coating 142 on the upper surface 25b of
the cover 25 may be selected to be lyophobic, as discussed above.
Such a coating may provide a higher receding contact angle for the
immersion liquid. This in turn may permit a higher scan speed to be
used without, for example, the loss of immersion liquid from the
meniscus, as discussed above. As noted above, the coating 142 on
the upper surface 25b of the cover 25 may be formed from
Lipocer.
It should be appreciated that the coatings 141, 142 on the lower
and upper surface 25a, 25b of the cover 25 may be formed from the
single layer of material. Alternatively, one or both of the
coatings 141, 142 may be formed from a plurality of layers. For
example, the layers may be formed from different materials,
providing different benefits to the coating 141, 142. It should
also be appreciated that the coatings 141, 142 on the lower and
upper surfaces 25a, 25b of the cover 25 may be the same or
different from each other.
The cover plate may, for example, be 25 .mu.m thick. It may be
etched to be locally reduced in thickness, for example at one or
more of the edges. In a locally reduced area it may be 10 .mu.m
thick. The thickness of part of the cover may be reduced by other
processes such as laser ablation, milling and polishing.
As depicted in FIG. 21, the edges 25c, 25d of the cover 25, namely
the edges separating the lower and upper surfaces 25a, 25b of the
cover 25 may be substantially perpendicular to the lower and upper
surfaces 25a, 25b of the cover 25. Such an arrangement may be
relatively simple to manufacture.
However, in an arrangement as depicted in FIG. 21, the edges 25c,
25d of the cover 25 may form a step on the surface of the substrate
W and the upper surface 21 of the substrate table WT. Such a step
may be undesirable. In particular, as discussed above, when the
substrate W and substrate table WT move relative to the liquid
confinement structure, care must be taken to ensure that the seal
formed by a meniscus of liquid between the liquid confinement
structure and the substrate W and/or substrate table WT does not
break down. The introduction of a step on the surface may reduce
the critical speed between the liquid confinement structure and the
substrate W and/or substrate table WT up to which the liquid
confinement structure and/or the substrate W/substrate table WT may
move without the seal, for example the meniscus, breaking down.
One or more of the edges 25c, 25d of the cover 25 may be configured
to provide a reduced step. For example, as discussed above, the
thickness of the cover may be locally reduced at one or more of the
edges. For example, one or more of the edges 25c, 25d of the cover
may be configured to have a profile as schematically depicted in
any one of FIGS. 22 to 25.
As depicted in FIG. 22, an edge of the cover 25 may be configured
to have a section 143 in which the cover 25 tapers to a point.
Accordingly, such a cover may have no step. However, the extreme
edge of the cover 25 may be susceptible to damage.
In an alternative arrangement, as depicted in FIG. 23, the cover 25
may have an edge section 145 that includes a step 146 that is
smaller than the thickness of the cover 25 and a tapered section
between the step 146 and the main body of the cover 25 that has the
full thickness. For example, the main body of the cover 25 may be
25 .mu.m thick and the step 146 may be 10 .mu.m thick. Such an
arrangement has a smaller step than a cover that has a
perpendicular edge 25c, 25d but may be less susceptible to edge
damage than an arrangement as depicted in FIG. 22.
As depicted in FIGS. 22 and 23, the tapered section 143, 145 of the
edge of a cover 25 may be configured to linearly increase in
thickness relative to the distance from the edge. However, this is
not essential. As depicted in FIGS. 24 and 25, which correspond to
FIGS. 22 and 23, respectively, the tapered sections 147, 148 may be
curved instead. This may avoid the provision of sharp corners, for
example between the tapered section and the remainder of the cover
25 or between the tapered section and a reduced step at the edge of
the cover 25. Such sharp corners may be a source of instability in
the seal, for example the meniscus, between the liquid confinement
structure and the underlying surface. Accordingly, avoiding such
sharp corners may reduce the likelihood of droplets being lost from
the meniscus, reducing possible defectivity as discussed above.
As shown in FIGS. 22 to 25, the lower corner of the cover 25,
namely the corner that in use may be in contact with the upper
surface of the substrate W and/or the substrate table WT, may be a
relatively sharp corner. This may provide a relatively good seal
between the cover 25 and the substrate W and/or substrate table WT.
However, it should be appreciated that the lower corner may be
curved instead. This may reduce the likelihood of damage to the
substrate W.
In general avoiding sharp corners on the cover may facilitate the
provision of a coating on the cover 25, if desired.
It should be appreciated that, although FIGS. 22 to 25 depict one
edge of a cover 25 with a tapered section and/or a rounded, one or
more the edges of the cover 25 may be tapered and/or have one or
more rounded corners as discussed above. Furthermore, the edges of
a cover 25 may have a different respective arrangements of a
tapered section and/or rounded corners.
A further issue that may affect the selection of the arrangement of
one or both of the edges 25c, 25d of the cover 25 is the behavior
of the immersion liquid when the liquid confinement structure 12
crosses the cover 25. This behavior may also be affected by the
configuration of the liquid confinement structure 12 and one or
more materials used for the substrate table WT and the cover
25.
For example, when using a liquid confinement structure that does
not include a gas knife and a lyophilic (e.g., hydrophilic)
substrate table WT and cover 25, liquid film pulling may be
present. Liquid film pulling may also occur in other arrangements.
Liquid film pulling may lead to liquid loss on the substrate, which
is undesirable. In particular, such lost droplets may subsequently
collide with the meniscus during further relative movement between
the liquid confinement structure 12 and the substrate table WT.
These collisions may result in the formation of bubbles in the
immersion liquid.
The liquid film pulling may be caused by the movement of the edge
of the cover 25 under the meniscus (or vice versa). When a meniscus
crosses over a 90.degree. height step, the meniscus velocity would
have to be infinite during the step. Such a step occurs at a
perpendicular edge 25c, 25d of a cover 25 such as shown in FIG. 21.
Infinite meniscus velocity is not actually possible so the meniscus
is stretched, resulting in liquid film pulling as depicted in FIG.
33. Subsequently, the liquid film 201 will break up, resulting in
liquid loss on the substrate W and/or substrate table WT.
The liquid film pulling may be reduced by use of a tapered edge,
for example as depicted in FIG. 22. However, although the liquid
film is depinned earlier than in the case of a perpendicular edge,
a liquid film may still be pulled. Accordingly, although liquid
loss on the substrate and/or substrate table may be reduced, it may
remain above a desired level. This may be because it is not
possible to form an infinitely sharp tip at the edge of the cover
25. Therefore, there may remain a small edge that is effectively
perpendicular at the end of the tip of the edge of the cover
25.
In an embodiment, an immersion liquid film disruptor is provided to
stimulate fast break up of the liquid film. This may reduce the
liquid loss.
In an embodiment, the immersion liquid film disruptor includes one
or more structures that may be applied onto the upper surface
and/or edge of the cover 25. This structure causes film
instability, resulting in film break up. Such a surface profile may
be combined with a cover 25 having a perpendicular edge or any
arrangement of tapered edge, as described above.
The structure applied to the upper surface and/or edge of the cover
25 may be in the form of any kind of plural local variations of the
surface. For example, elongate features such as ridges and/or
channels may be provided. The cross section of such ridges and/or
channels may be any appropriate shape. Alternatively or
additionally, relatively short features such as pimples and/or
dimples may be provided. Such pimples and/or dimples may have any
appropriate shape.
FIG. 34 depicts an embodiment, in which a plurality of ridges 202
are provided on the upper surface of the cover 25. As shown, such
an arrangement promotes the break up of the liquid film 203. Such
an arrangement may reduce the likelihood of the meniscus being
pinned.
FIG. 35 depicts an embodiment, in which a plurality of channels 204
are provided on the edge of the cover 25. As shown, such an
arrangement promotes the break up of the liquid film 204. Such an
arrangement may reduce the likelihood of the meniscus being
pinned.
In an embodiment, depicted in FIG. 36 and, in more detail, FIG. 37,
the immersion liquid film disruptor includes a plurality of
discrete gas jets 210 provided by the liquid confinement structure
12. The gas jets 210 are directed onto the liquid film as it starts
to form, promoting fast break up of the film 212. Such an
arrangement may reduce the likelihood of the meniscus being
pinned.
The gas jets 210 may be provided, for example, by a row of
apertures 213 formed in the surface of the liquid confinement
structure 12 and connected to a gas supply. The apertures may be
provided outward of a line of openings 211 that are formed in the
surface of the liquid confinement structure and used to extract
immersion liquid in order to contain the immersion liquid. In other
words, the apertures 213 providing the gas jets 210 may be on the
opposite side of a line of openings 211 to the space in which the
immersion liquid is contained. For example, the row of apertures
213 may be provided in a line that is substantially parallel to the
line of openings 211 used to extract immersion liquid.
Although as depicted in, for example, FIG. 37, an equal number of
apertures 213 and openings 211 may be provided for a given section
of a line of openings 211, this may not necessarily be the case.
The factors determining the appropriate separation of the openings
211 will generally be different from the factors determining the
appropriate separation between the apertures 213, resulting in a
difference in the number of openings 211 and apertures 213.
In addition, although FIG. 36 depicts an arrangement in which the
liquid confinement structure 12 is generally square in plan view,
this need not be the case. Furthermore, although as depicted in
FIG. 36, the line of openings 211 may either be parallel to, or
perpendicular to, the scanning direction of the substrate table WT
(and therefore the substrate W and cover 25) relative to the liquid
confinement structure 12, this need not be the case. For example,
each of the lines of openings 211 may be arranged to be at an angle
of 45 degrees to the scanning direction, or any desirable angle.
Furthermore, although as depicted in FIG. 36, the lines of openings
211 may be straight, it will be appreciated that this need not be
the case. Accordingly, for example, the region of the liquid
confinement structure 12 bounded by the openings 211 may be bounded
by a plurality of concave lines of openings 211.
In an embodiment, the row of apertures 213 may be provided in a
shape that matches the shape of the lines of openings 211. However,
this need not be the case and it may be desirable for the line of
openings 211 to define a different shape from the line of apertures
213. In particular, at corners of the space bounded by the line of
openings 211, a different layout of apertures 213 may be provided
than the layout of openings 211.
Although the arrangement depicted in FIGS. 36 and 37 depicts a
system to extract the immersion liquid at the edge of the liquid
confinement structure 12 as being a plurality of discrete openings
211, in an embodiments, a different arrangement of liquid
extraction may be provided. For example, in place of a line of
openings 211, a continuous opening may be provided. Furthermore, a
continuous opening may be provided in which the surface of the
opening in contact with the immersion liquid is covered by a
micro-sieve, such as a porous material.
An advantage of using a plurality of discrete gas jets 210, rather
than a gas knife for example, is that liquid droplets may not be
collected between a line of gas jets 210 and a line of openings
211. Collection of liquid droplets in such an area may be a problem
during movement of the substrate table WT relative to the liquid
confinement structure. For example, liquid droplets collected in
such an area during movement may combine to form large droplets.
Such large droplets may, for example, collide with the meniscus,
for example when the movement of the substrate table WT relative to
the liquid confinement structure 12 reverses direction. Such
collisions of droplets with the meniscus may result in the
formation of bubbles within the immersion liquid, resulting in
image defects.
As explained above, the provision of the plurality of discrete gas
jets 210 may break up the film 212 into a plurality of relatively
small droplets. The droplets may be small enough not to create
bubbles when in collision with the meniscus. It will be appreciated
that the film of immersion liquid is drawn from the edge of the
cover 25 as the cover 25 relatively moves away from the liquid
confinement structure 12 and the meniscus formed between the liquid
confinement structure 12 and the top of the substrate W, cover 25
and substrate table WT.
The liquid contained in the film has a volume proportional to the
thickness of the film. This in turn may depend on various
parameters, including the "fly" height of the liquid confinement
structure 12 above the surface of the substrate W and/or substrate
table WT and the relative speed of the substrate and/or substrate
table WT relative to the liquid confinement structure 12. The film
thickness may depend on various settings of the liquid confinement
structure 12 such as one or more flow rates within the liquid
confinement structure 12. In an example, the film thickness may be
between approximately 10 .mu.m and 30 .mu.m.
The film 212 of liquid may then be broken up into rivulets 214 by
the gas jets 210. For example, a rivulet 214 may be formed between
adjacent gas jets 210 corresponding to adjacent apertures 213. As
depicted in FIG. 37, the rivulets 214 will subsequently break up to
form individual droplets.
As will be appreciated, the width of the rivulets 214 will be
related to the volume of the film 212 that corresponds to each
rivulet 214. In turn, this is dependent on the film thickness, as
discussed above, and the pitch of the gas jets 210, namely the
separation between the center of adjacent apertures 213.
The size of the resulting droplets 215 in turn is dependent on the
width of the rivulets 214. Accordingly, the size of the droplets
may be minimized by minimizing the width of the rivulet 214 which,
for a given film thickness, may in turn be minimized by minimizing
the pitch of the gas jets 210, namely the separation of the
apertures 213.
FIG. 38 depicts the results of a theoretical analysis of the
relationship between the jet pitch and the droplet diameter for
different film thicknesses, together with the results of two
experiments. The vertical axis represents the droplet diameter in
.mu.m and the horizontal axis represents the jet pitch in .mu.m.
The two experimental results are depicted by the large squares, the
results of the analysis for films with a thickness of 10 .mu.m are
depicted by the small diamond markers, the results of the analysis
for films with a thickness of 20 .mu.m are depicted by the small
square markers and the results of the analysis for films with a
thickness of 30 .mu.m are depicted by the small triangular markers.
This confirms that it is desirable to minimize the jet pitch.
However, one may not simply minimize the jet pitch to the smallest
size that may be machined. This is because it is desirable to
ensure that the gas jets 210 remain as discrete gas jets and may
therefore break up the liquid film 212.
As the gas jet pitch decreases, there is an increasing tendency for
the gas jets 210 to merge and, effectively, form a gas knife. The
minimum usable gas jet pitch, namely the gas jet pitch at which the
gas jets 210 remain sufficiently discrete that they may break up
the liquid film 212, may depend upon the "fly" height of the liquid
confinement structure 12, the diameter of the apertures 213 and the
velocity of the gas jets 210. In particular, the ratio of the "fly"
height to the diameter of the apertures may be a significant
factor.
As the "fly" height increases relative to the diameter of the
apertures 213, the greater the tendency for the gas jets 210 to
merge together for a given gas jet pitch. For an arrangement in
which the "fly" height is 1.5 to 2 times the diameter of the
apertures 213, the gas jets may remain sufficiently separate to
function to break up the liquid film 212 when the ratio of the gas
jet pitch to the diameter of the apertures 213 is approximately
1.5.
For example, the centers of adjacent apertures may be in the range
from approximately 100 .mu.m to 1 mm, for example approximately 750
.mu.m. The diameter of the apertures 213 may, for example, be in
the range of from approximately 50 .mu.m to 150 .mu.m, for example
100 .mu.m.
As noted above, the speed of gas flow in the gas jets 210 may
affect the performance of the gas jets in breaking up the liquid
film 212. In addition to affecting the gas jet pitch that may be
used without the gas jets 210 merging, the velocity of the gas flow
in the gas jets may also directly affect the size of the resulting
droplets.
In an embodiment, the gas speed may be in the range of from
approximately Ma 0.3 to Ma 0.6, for example approximately Ma
0.5.
Although as discussed above, it may be desirable to arrange the gas
jets 210 in order to prevent them from merging to form a gas knife,
it may be desirable to provide a gas knife in addition. In an
embodiment, a gas knife may be provided, for example along a line
parallel to the line of apertures 213 and on the opposite side of
line of apertures 213 from the line of openings 211.
In an embodiment, the separation between a line of openings 211,
that are formed in the surface of the liquid confinement structure
and used to extract immersion liquid, and a line of apertures 213
providing gas jets 210 may be, for example, approximately 0.5 mm to
3 mm, for example approximately 1.5 mm.
FIG. 39 schematically depicts in cross section a plurality of gas
jets 210 provided by a row of apertures 213. As shown, each
aperture 213 may be connected by a gas conduit 220 to a gas supply
221 including a gas source. The gas source may, for example,
provide a supply of clean air or nitrogen.
The gas supply may be controlled, for example, by a controller 222.
The controller 222, by means of the gas supply 221, may control the
gas flow to each of the apertures 213. The controller 222 may be
used to turn on the gas jets 210 only when required. For example,
during normal use of a lithographic apparatus, the gas jets 210 may
be not provided but when the liquid confinement structure 12
crosses the cover 25, when liquid pulling may occur, the gas jets
210 are provided. Alternatively or additionally, for example, the
controller 222 may be able, by use of one or more appropriate
control valves and/or control of the gas source, to control the gas
velocity in the gas jets 210.
Furthermore, the controller 222 may be able, by use of one or more
appropriate control valves and/or appropriate control of the gas
source, to select one or more apertures 213 to be used to form gas
jets 210. Accordingly, for example, in use, gas jets 210 may be
provided by one set of apertures 213 and not provided by a second
set of apertures 213. For example, when the liquid confinement
structure 12 crosses the cover 25, only the apertures 213 on one
side of the liquid confinement structure 12 may be used to provide
gas jets 210.
FIG. 40 depicts a variation of the embodiment depicted in FIG. 39.
Only the differences are discussed. As shown, the embodiment
depicted in FIG. 40 includes a plurality of openings 230, each
provided between an adjacent pair of apertures 213. The openings
230 may be connected by a suitable gas conduit 231 to an
underpressure source 232. The underpressure source may be
controlled by the controller 222, as shown, or may be provided with
a separate controller.
The underpressure source 232 may be controlled in order to provide
extraction through the openings 230 when the adjacent apertures 213
are used to provide gas jets 210. Provision of such an arrangement
may enable the gas jet pitch to be reduced without increasing the
likelihood of the gas jets 210 merging together.
In an embodiment, the upper surface 25b of the cover 25 or the
upper surface 142b of a coating 142 on the upper surface 25b of the
cover 25 may be configured to be as flat as possible. This may
further reduce any instabilities of the meniscus discussed above,
reducing the likelihood of droplets being lost from the meniscus
and subsequent defects as discussed above.
In an embodiment, the cover may be a part of the substrate table.
An actuator system may be provided to move the cover between at
least closed and open positions. In the closed position, the cover
25 may be in contact with the upper surface of a substrate W within
the recess 22. In the closed position, the cover 25 may be in
contact with the upper surface 21 of the substrate table WT. In the
closed position, the cover 25 may cover the gap 23 between the edge
of the substrate W and the edge of the recess 22.
The cover 25 may be configured so that, as the gap passes
underneath the immersion space 11, with respect to the immersion
liquid in the space, the gap is closed. By closing the gap, the
stability of the meniscus in crossing the gap may be improved. In
an embodiment, the cover forms a seal with one or both of the upper
surface of a substrate W within the recess 22 and the upper surface
21 of the substrate table WT. A cover 25 that provides a seal with
both the upper surface of the substrate W and the upper surface 21
of the substrate table WT may prevent immersion liquid from passing
into the gap 23. The cover may reduce the inflow of immersion
liquid into the gap 23. The cover may help reduce, if not prevent,
the flow of bubbles into the space 11 as a consequence of the gap
passing underneath the space 11.
In the open position, the cover 25 may be moved away from its
location at the closed position relative to the surface of the
recess 22. When a substrate is supported by the surface of the
recess 22, the cover 25 may be set apart from the substrate W. The
open position may be arranged such that, when the cover 25 is in
the open position, the substrate W may be unloaded from the
substrate table WT. If a substrate W is not present in the recess
22, a substrate W may be loaded onto the substrate table WT.
In an embodiment, the actuator system may be configured such that,
in moving the cover 25 from the closed position to the open
position, it enlarges the open central section 26 of the cover 25,
as depicted in FIG. 8. In such a process, the open central section
26 of the cover 25 may be enlarged sufficiently that the open
central section 26 is larger than the upper surface of the
substrate W in the open position. The open central section 26 of
the cover 25 may be enlarged sufficiently for the substrate W to be
able to pass through the central open section 26 of the cover
25.
In an embodiment, a substrate W may be loaded onto, or unloaded
from, a substrate table by moving the cover 25 to the open position
and passing the substrate W through the central open portion 26 of
the cover 25. In the case of loading a substrate W to a substrate
table WT, once the substrate W has passed through the open central
section 26 of the cover 25, the substrate W may be received in the
recess 22 of the substrate table WT. Subsequently, the cover 25 may
be moved by the actuator system to the closed position, in which it
covers the gap 23 between the edge of the substrate W and the edge
of the recess 22 in which the substrate W is supported.
The actuator system may be configured such that, in moving the
cover 25 to the open position, a plurality of portions of the cover
25 are moved in different respective directions relative to each
other. This arrangement may be used in order to enlarge the open
central section 26 of the cover 25 in moving to the open
position.
In an embodiment, the actuator system may be configured to
elastically deform at least a part of the cover 25. For example,
the actuator system may elastically deform at least part of the
cover 25 when the actuator moves the plurality of portions of the
cover 25 in respective different directions in order to enlarge the
open central section 26.
FIGS. 7 and 8 depict, in plan view, a cover 25 according to an
embodiment of the invention in the closed and open positions,
respectively. As shown, the cover 25 may be generally annular in
shape in plan view. The inner periphery 31, e.g. circumference, of
the cover 25 may define the open central section 26 of the cover 25
when it is in the closed position. A break in the generally annular
shape of the cover 25 may be provided between the inner periphery,
e.g., circumference, 31 and the outer periphery, e.g.,
circumference, 32 of the cover 25.
In an arrangement such as that depicted in FIGS. 7 and 8, the cover
25 has a plurality of portions 35 each of which are moveable by the
actuator system in respective different directions. In moving the
plurality of portions 35 the open central section 26 of the cover
25 may be enlarged or reduced. The plurality of portions may be
combined together to form a single integral cover. However, as
depicted in FIG, 8, the provision of the break 30 across the
periphery, e.g., circumference, of the cover 25 may facilitate the
elastic deformation of the cover 25 in order to enlarge the central
open section 26.
Although the arrangement of FIGS. 7 and 8 includes a break 30 from
the inner periphery 31 to the outer periphery 32 of the cover 25,
this is not essential.
Additional breaks may be provided in order to facilitate the
enlargement, e.g., by elastic deformation, of the cover 25 in order
to enlarge the open central section 26 of the cover 25 according to
this aspect of the invention.
The provision of any of the covers disclosed herein may have a
variety of additional benefits for a substrate table within a
lithographic apparatus in addition to the reduction of defects
caused by bubbles and/or the reduction of bubbles, as described
above.
The cleaning of the substrate table WT and the immersion system may
be reduced. This, in turn, may reduce the down time of the
lithographic apparatus.
A cover may reduce the transfer of contaminants from the upper
surface of the substrate W to the lower surface of the substrate W.
This may reduce defects that may be introduced as a result of the
so-called back side contamination.
The provision of a cover covering the gap between the edge of the
substrate W and the edge of the recess 22 may enable the edge of
the substrate W to traverse the projection system and immersion
system at a higher speed than is otherwise possible. This may
increase the throughput of the lithographic apparatus.
The provision of a cover may obviate the need for an extraction
system in order to remove immersion liquid and bubbles from the gap
between the edge of the substrate W and the edge of the recess 22.
This may reduce the heat load applied to the substrate table WT.
The thermal stability of the substrate table WT may improve. The
overlay accuracy of patterns formed on the substrate W may
consequently improve.
An extraction system for the gap between the edge of the substrate
W and the edge of the recess 22 may be a two-phase extractor. This
type of extractor may produce flow induced vibrations. Therefore,
the provision of a cover, which may result in such an extractor
being obsolete (not being required), may reduce the vibrations
within the substrate table WT.
The provision of a cover may result in a simpler system overall
than a system that uses an extractor for the gap between the edge
of the substrate W and the edge of the recess 22, as disclosed
above. The provision of a cover over the gap 23 may reduce the cost
of goods of the apparatus as a whole.
It should be appreciated that the provision of a cover according to
an aspect of the invention may eliminate the need for an extraction
system at the gap between the edge of the substrate W and the edge
of the recess 22, as discussed above. However, a cover according to
an aspect of the present invention may be used in conjunction with
an extraction system. The benefits discussed above may still apply
because the requirements of the extraction system may be
reduced.
FIGS. 9 and 10 depict, in plan view, an arrangement of a cover 25
according to an embodiment of the invention. The cover depicted in
FIGS. 9 and 10 is similar to the cover depicted in FIGS. 7 and 8
and, for brevity, only the differences will be discussed in
detail.
As shown, the cover 25 is formed from a plurality of discrete
sections 40. In the closed position, the sections 40 are arranged
to abut adjacent sections 40 of the cover 25 in order to form a
single cover 25. For example, as shown in FIG, 9, for a circular
substrate W, when each of the discrete sections 40 of the cover 25
abut each other in the closed position, the combination of the
discrete sections 40 provides a cover 25 having a generally annular
shape.
The actuator system is configured such that it can move portions of
the cover 25 in different directions in order to move the cover
from the closed position to the open position. In the case of a
cover 25 such as that depicted in FIGS. 9 and 10, each such portion
of the cover 25 is one of the discrete sections 40. The actuator
system moves each of the discrete sections 40 of the cover 25 in a
respective different direction.
When the cover 25 is in the open position, the discrete sections 40
of the cover 25 may be set apart from each other, providing the
enlarged open central section 26 through which the substrate W may
pass as described above.
FIGS. 11, 12 and 13 depict, in cross-section, an actuator system
that may be used in an aspect of the invention in, respectively,
the closed position, an intermediate position and the open
position.
As shown FIG. 11, in the closed position, each portion of the cover
25 is positioned on, and extends between, a peripheral portion 45
of the upper surface of the substrate W and the upper surface 21 of
the substrate table WT. In moving the cover 25 from the closed
position to the open position, the actuator system 50 may be
configured such that each portion of the cover 25 is first moved in
a direction substantially perpendicular to the upper surface of the
substrate W and the upper surface 21 of the substrate table WT.
FIG. 12 depicts a portion of the cover 25 in an intermediate
position between the closed and open positions after an initial
movement, as described above.
In moving from the open position to the closed position, the cover
25 may be moved to the intermediate position shown in FIG. 12 such
that the cover 25 may subsequently be moved to the closed position
only by a movement in a direction substantially perpendicular to
the upper surface of the substrate W and the upper surface 21 of
the substrate table WT.
Such an arrangement may beneficially ensure that, when the cover 25
is in contact with the substrate W or close to the substrate W, the
relative movement of the cover 25 to the substrate W is only in a
direction that is substantially perpendicular to the upper surface
of the substrate W. This may prevent or reduce the generation of
contaminant particles at the edge of the substrate W. This may
prevent or reduce the movement of pre-existing contaminant
particles at the edge of the substrate W towards the upper surface
of the substrate W on which a pattern is to be formed. On
contacting the substrate by moving the cover in a direction
substantially perpendicular to the surface of the substrate W, a
force applied to the substrate W is applied in a direction
substantially perpendicular to the substrate W. As the force is
applied around the periphery of the substrate W, the force applied
is substantially uniform. Distortions in the substrate W caused by
the application of the force are thereby reduced, if not minimized.
Forces in the plane of the substrate W by application of the cover
25 are reduced or minimized, limiting the movement of the substrate
W in the recess. Position errors by applying the cover 25 to the
edge of the substrate W may be reduced, if not prevented.
The actuator system 50 may be configured such that it can move each
of the portions of the cover 25 between the intermediate position
depicted in FIG. 12 and the open position depicted in FIG. 13 by
moving each of the portions of the cover 25 in a direction that is
substantially parallel to the upper surface of the substrate W and
the upper surface 21 of the substrate table WT.
As shown in FIGS. 11, 12 and 13, the actuator system 50 may include
an actuator stage 51 that is configured to provide movement of the
cover 25 in a direction substantially perpendicular to the upper
surface of the substrate W and the upper surface 21 of the
substrate table WT, for example in a vertical direction. The
actuator stage 51 may be referred to as a transverse actuator
stage.
The actuator system 50 may include an actuator stage 52 configured
to provide movement of the cover 25 in a direction substantially
parallel to the upper surface of the substrate W and the upper
surface 21 of the substrate table WT, for example in a horizontal
direction. The actuator stage 52 may be referred to as a lateral
actuator stage.
It will be appreciated that, although the use of pneumatic
actuators as depicted may be beneficial, alternative actuators may
be used for one or both of the actuator stages 51, 52. For example,
an electrostatic actuator and/or an electromagnetic actuator may be
used.
The actuator stage 51 may be configured in order to ensure that
substantially the only movement provided is in the direction
substantially perpendicular to the upper surface of the substrate W
and the upper surface 21 of the substrate table WT. The actuator
stage 51 may include one or more movement guides. The one or more
movement guides are configured to permit relative movement of the
components of the actuator stage 51 in the direction substantially
perpendicular to the upper surface of the substrate W and the upper
surface 21 of the substrate table WT. However, the movement guide
reduces or minimizes the movement of the component of the actuator
stage 51 in a direction substantially parallel to the upper surface
of the substrate W and the upper surface 21 of the substrate table
WT.
FIGS. 14 and 15 depict, in cross-section, an arrangement of
movement guides that may be used in order to help ensure that the
actuator stage 51 only provides movement in a particular direction.
Such a direction may be a direction substantially perpendicular to
the upper surface of the substrate W and the upper surface 21 of
the substrate table WT. FIG. 14 depicts a movement guide 60 when
the cover 25 is in the closed position. FIG. 15 depicts the
movement guide 60 when the cover 25 is in the open position.
As shown, the actuator stage 51 includes first and second
components 61,62. The first and second components 61,62 may be
moved relative to one another in the direction substantially
perpendicular to the upper surface of the substrate W and the upper
surface 21 of the substrate table WT by means of the actuator
provided as described above. Elastic hinges 63 are provided between
the first and second components 61,62 of the actuator stage 51. The
elastic hinges permit movement of the first and second components
61,62 in a direction substantially perpendicular to the upper
surface of the substrate W and the upper surface 21 of the
substrate table WT. The elastic hinges are configured to restrict
movement in a direction substantially perpendicular to this desired
direction of movement.
It will be appreciated that an alternative or additional movement
guide may be used. However, the use of one or more such elastic
hinges as described above may be beneficial because this form of
movement guide does not have, or desirably minimizes, frictional
forces. Frictional forces may reduce the reproducibility of the
force that is applied on the upper surface of the substrate W when
the cover 25 is moved to the closed position.
FIGS. 16, 17 and 18 depict a further actuator system that may be
used with an aspect of the present invention. FIG. 16 depicts the
actuator system 70 when the cover 25 is in the closed position.
FIG. 17 depicts the actuator system 70 in an intermediate position.
FIG. 18 depicts the actuator system 70 when the cover 25 is in the
open position.
The actuator system 70 depicted in FIGS. 16, 17 and 18 may provide
a simpler actuation system than that depicted in FIGS. 11, 12 and
13. Separate actuator stages are not required. Instead, each
portion of the cover 25 is connected to a piston 71 that is mounted
within a system of movement guides 72,73 within the substrate table
WT.
A movement guide 72 may, in cooperation with the piston 71, be used
to move the cover 25 from the closed position in a direction
substantially perpendicular to the upper surface of the substrate W
and the upper surface 21 of the substrate table WT to the
intermediate position. A movement guide 73 may be arranged such
that, in conjunction with the piston 71, it moves the cover 25 in a
direction substantially parallel to the upper surface of the
substrate W and the upper surface 21 of the substrate table WT. In
order to move the cover 25 between the closed and open positions,
the gas pressure on one or both sides of the piston 71 may be
changed by connecting one or both of the movement guides 72,73 to
an appropriate under-pressure or over-pressure source 74,75.
The cover 25 may be configured such that in the closed position it
not only covers the gap 23 between the edge of the substrate W and
the edge of the recess 22 in the substrate table but it covers a
further gap 77. For example, an additional gap may exist between
the actuator system and a part of the substrate table further away
from the substrate holder such as an additional component 78. The
additional component 78 may be a component of a sensor system used
in order to monitor the position and/or displacement of the
substrate table WT relative to the projection system.
FIGS. 19 and 20 depict, in cross-section, an embodiment of the
invention in which a different arrangement of cover 125 is provided
to cover the gap 23 between the edge of the substrate W and the
edge of the recess 22 in the substrate table WT in which the
substrate W is supported. In particular, a cover 125 of an
embodiment of the invention may be configured to be moved away from
the substrate table WT to permit loading/unloading a substrate W
to/from the recess 22 in the substrate table WT. In such an
arrangement it is not necessary to enlarge an open central section
of the cover 125 in moving the cover 125 to the open position.
In common with the arrangements discussed above, the cover 125 is
arranged in the form of a thin plate of material that surrounds the
edge of the substrate W. The cover 125 extends from a peripheral
area 45 of the upper surface of the substrate W to the upper
surface 21 of the substrate receiving section. Openings 127 for gas
outlets may be provided that are connected to an under-pressure
source 128. The pressure in a space on the lower side 125a of the
cover 125 may be lower than the gas pressure on the upper side 125b
of the cover 125. The pressure difference may be used in order to
secure the cover 125 and substantially prevent any movement of the
cover 125 during use.
In order to prevent or reduce deformation of the cover 125, the
cover may include one or more supports 126 that extend from the
lower surface 125a of the cover 125 to the bottom of the recess 22
when the cover 125 is located on top of a substrate W in the recess
22.
In order to move the cover 125 in order to permit loading and
unloading of a substrate W, a cover handling system 130 such as a
robot arm may be provided. The cover handling system 130 may be
specifically configured such that the movement of the cover 125
when the cover 125 is in contact with the substrate W or close to
the substrate W is only in a direction that is substantially
perpendicular to the upper surface of the substrate W and the upper
surface 21 of the substrate table WT.
As discussed above, in embodiments of the invention, for example
those depicted in FIGS. 11 to 18, an actuator system may be
provided that moves the cover 25 from an open position, in which a
substrate W may be loaded to the substrate table WT and/or a
substrate W may be unloaded from a substrate WT, to a closed
position, in which the cover 25 extends from the upper surface 21
of the substrate table WT to the periphery of the substrate W. In
the closed position, the cover 25 may be in physical contact with
the peripheral section of the substrate W and the upper surface 21
of the substrate table WT, in particular if the cover 25 is to form
a seal. Such physical contact could result in damage of one or more
of the cover 25, the substrate W and/or the upper surface 21 of the
substrate table WT. Accordingly, an appropriate control system for
the actuator system may be provided.
In an embodiment of the invention, a controller is provided in
order to control the actuator system that positions the cover 25.
In order to help ensure that the cover is accurately moved relative
to the substrate W and/or substrate table WT, the controller may
use data that represents the height of the upper surface of at
least the peripheral section of the substrate W relative to the
upper surface 21 of the substrate table WT (or vice versa). Such
data may, for example, be previously acquired in a metrology
station, which may be part of the lithographic apparatus or part of
a lithography system including the lithographic apparatus.
Based on the data representing the height of the upper surface of
the peripheral section of the substrate W relative to the upper
surface 21 of the substrate table WT (or vice versa), the
controller may determine the position to which the cover 25 should
be moved in order to provide a desired contact between the cover 25
and the upper surface of the substrate W and the upper surface 21
of the substrate table WT.
An appropriate feedback mechanism may be provided for the
controller to control the actuator system to move the cover 25 to
the desired position determined by the controller.
The controller may be configured, for example, to help ensure that
the position of the cover 25 in the closed position is sufficiently
close to or in contact with the upper surface of the peripheral
section of the substrate W to prevent, reduce or minimize leakage
of the immersion liquid. Alternatively or additionally, the
controller may be configured to help ensure that, when the cover 25
is in the closed position, the force exerted on the upper surface
of the peripheral section of the substrate W by the lower surface
of the cover 25 is within a given range. For example, it may be
desirable to ensure that the force is less than a certain value in
order to prevent or reduce the likelihood of damage to the
substrate W. Alternatively or additionally, it may be desirable to
ensure that the force exerted on the upper surface of the
peripheral section of the substrate W by the lower surface of the
cover 25 exceeds a certain value in order to ensure that sufficient
contact is made in order to control the leakage of immersion liquid
below the cover 25.
In an embodiment of the invention, the data representing the height
of the upper surface of the peripheral section of the substrate W
relative to the upper surface 21 of the substrate table WT (or vice
versa) may provide data for the relative height at a plurality of
locations around the peripheral section of the substrate W. From
such data, the controller may be able to determine the desired
position of respective portions of the cover 25 at a plurality of
locations around the edge of the substrate W.
In an embodiment of the invention, the actuator system may be
correspondingly configured to be able to adjust the height of the
cover 25 independently at a plurality of locations around the cover
25. In such an arrangement, local variations of the height of the
upper surface of the substrate W and/or substrate table WT may be
taken into account in controlling the positioning of the cover 25
in the closed position. This may in turn assist in preventing or
reducing immersion liquid leakage and/or damage to the substrate W,
the substrate table WT and/or the cover 25.
As identified above, when the cover 25 is moved to the closed
position, it may exert a force on the upper surface of the
peripheral section of the substrate W. It should be appreciated
that this force may be exerted regardless of the arrangement of the
control system for the actuator system used to move the cover 25.
The force exerted on the substrate W may be sufficient to cause a
movement of the upper surface of the substrate W, for example due
to deformation of the substrate and/or due to deformation of the
support section of the substrate table WT that supports the
substrate W. Such movement of the upper surface of the substrate W
may be undesirable because it may result in errors in the pattern
formed on the substrate W.
In an embodiment of the invention, the cover 25 may be provided
with a region that is relatively flexible, namely has a lower
stiffness than the remainder of the cover. Such a relatively
flexible section may be configured such that, when the cover 25 is
moved to the closed position, any forces exerted on the cover
and/or any inaccuracies in the positioning of the cover relative to
the substrate W and/or substrate table WT results in a deformation
of the relatively flexible section of the cover rather than a
deformation of the substrate W or the support section of the
substrate table WT that supports the substrate.
FIGS. 26 to 30 depict schematically, in cross-section, arrangements
of covers 25 of an embodiment of the present invention having
relatively flexible sections. As shown in FIGS. 26 to 28, a cover
25 may be formed from a single section of material and one or more
relatively flexible sections may be provided in which the thickness
of the cover is reduced. For example, as depicted in FIG. 26, one
or more of the edge sections 161,162 of the cover 25 may have
smaller thickness than the thickness of the remainder 163 of the
main body of the cover 25. The sections 161,162 of reduced
thickness will accordingly be less stiff than the remainder 163 of
the main body of the cover 25.
The sections 161,162 of reduced thickness of the cover 25 may
extend around the cover 25, for example along the entirety of the
inside and/or outside edge of the cover 25. It will also be
appreciated that in some arrangements, only one edge of the cover
25 will have a section of reduced thickness in order to provide a
relatively flexible section.
It will be appreciated that such arrangements may be combined with
embodiments discussed above in which the edge of the cover 25 is
tapered. In this case, it will be appreciated that the edges 161a,
162a of the reduced thickness sections 161, 162 may be tapered.
Similarly, the edges of the covers depicted in FIGS. 27 to 32,
described below may be tapered. However, for brevity, this is not
discussed in detail for each embodiment discussed below.
As depicted in FIG. 27, a relatively flexible section of the cover
25 may be provided by the formation of a groove 165 in the lower
surface of the cover 25. The groove 165 results in an associated
portion 166 of the cover 25 that has a reduced thickness and
therefore reduced stiffness. It will be appreciated that the groove
165 may extend around the cover 25. Accordingly, in use, the groove
165 may be positioned above the gap between the edge of the
substrate W and the edge of the recess in the substrate table WT,
extending around the full periphery of the substrate.
Although FIG. 27 depicts an arrangement in which a single groove
165 is provided in the lower surface of the cover 25, it will be
appreciated that a plurality of grooves may be provided in order to
increase the flexibility of a section of the cover 25. However, in
general, it is desirable to retain sufficient sections of the main
body of the cover 25 with relatively high stiffness, namely
sections of the cover having the full thickness, in order to help
ensure that the cover 25 does not deform excessively in use.
As depicted in FIG. 28, the arrangements depicted in FIGS. 26 and
27 may be combined. In other words, a cover 25 may have a section
of reduced thickness 161,162 at one or more of the edges of the
cover 25 and may also be provided with one or more grooves 165 on
the lower surface of the cover 25.
In corresponding further arrangements, as depicted in FIGS. 29 to
31 respectively, the main body of the cover 25 may be formed from a
planar section of material 170 attached to at least one support
section of material 171. The combination of the planar section of
material 170 and the support section of material 171 provides
sections of the main body of the cover 25 having full thickness
and, accordingly, relatively high stiffness. Sections of the main
body of the cover 25 that are formed from the planar material 170
that is not supported by support sections of material 171 provide
sections of the cover 25 of reduced thickness 161,162 that have
relatively low stiffness. Likewise, gaps between two support
sections of material 171 provide grooves 165 that provide
relatively flexible sections of the cover 25.
Although not depicted in FIGS. 20 to 31, it will be appreciated
that the covers 25 of this aspect of the invention may include
supports such as those discussed above, including supports to
connect the cover 25 to an actuator system.
FIG. 32 depicts an embodiment of the present invention, in which
the main body of the cover 25 is supported by one or more supports
172. As shown, the cover 25 has a reduced thickness section 161,162
at either edge of the cover 25, providing relatively flexible
sections of the cover 25. In addition, grooves 165 are provided in
the lower surface of the cover 25. The grooves 165 are positioned
such that they each extend around the cover 25 in a position
between a respective edge of the cover 25 and the position of the
one or more supports 172. Accordingly, the grooves 165 provide
additional relatively flexible sections of the cover 25. It will be
appreciated that in variations of this arrangement, one or more of
the relatively flexible sections of the cover 25 may be
omitted.
Although specific reference may be made in this text to the use of
lithographic apparatus in the manufacture of ICs, it should be
understood that the lithographic apparatus described herein may
have other applications in manufacturing components with
microscale, or even nanoscale features, such as the manufacture of
integrated optical systems, guidance and detection patterns for
magnetic domain memories, flat-panel displays, liquid-crystal
displays (LCDs), thin-film magnetic heads, etc. The skilled artisan
will appreciate that, in the context of such alternative
applications, any use of the terms "wafer" or "die" herein may be
considered as synonymous with the more general terms "substrate" or
"target portion", respectively. The substrate referred to herein
may be processed, before or after exposure, in for example a track
(a tool that typically applies a layer of resist to a substrate and
develops the exposed resist), a metrology tool and/or an inspection
tool. Where applicable, the disclosure herein may be applied to
such and other substrate processing tools. Further, the substrate
may be processed more than once, for example in order to create a
multi-layer IC, so that the term substrate used herein may refer to
a substrate that already contains multiple processed layers.
The terms "radiation" and "beam" used herein encompass all types of
electromagnetic radiation, including ultraviolet (UV) radiation
(e.g. having a wavelength of or about 365, 248, 193, 157 or 126
nm).
The term "lens", where the context allows, may refer to any one or
combination of various types of optical components, including
refractive and reflective optical components.
To operate one or more movements of a component of the present
invention, such as an actuator, there may be one or controllers.
The controllers may have any suitable configuration for receiving,
processing, and sending signals. For example, each controller may
include one or more processors for executing the computer programs
that include machine-readable instructions for the methods
described above. The controllers may include data storage medium
for storing such computer programs, and/or hardware to receive such
medium.
While specific embodiments of the invention have been described
above, it will be appreciated that the invention may be practiced
otherwise than as explicitly described. For example, the
embodiments of the invention may take the form of a computer
program containing one or more sequences of machine-readable
instructions describing a method as disclosed above, or a data
storage medium (e.g. semiconductor memory, magnetic or optical
disk) having such a computer program stored therein. Further, the
machine readable instruction may be embodied in two or more
computer programs. The two or more computer programs may be stored
on one or more different memories and/or data storage media. The
computer programs may be suitable for controlling a controller
referred to herein.
One or more embodiments of the invention may be applied to any
immersion lithography apparatus, in particular, but not
exclusively, those types mentioned above, whether the immersion
liquid is provided in the form of a bath, only on a localized
surface area of the substrate, or is unconfined on the substrate
and/or substrate table. In an unconfined arrangement, the immersion
liquid may flow over the surface of the substrate and/or substrate
table so that substantially the entire uncovered surface of the
substrate table and/or substrate is wetted. In such an unconfined
immersion system, the liquid supply system may not confine the
immersion liquid or it may provide a proportion of immersion liquid
confinement, but not substantially complete confinement of the
immersion liquid.
A liquid supply system as contemplated herein should be broadly
construed. In certain embodiments, it may be a mechanism or
combination of structures that provides a liquid to a space between
the projection system and the substrate and/or substrate table. It
may comprise a combination of one or more structures, one or more
liquid inlets, one or more gas inlets, one or more gas outlets,
and/or one or more liquid outlets that provide liquid to the space.
In an embodiment, a surface of the space may be a portion of the
substrate and/or substrate table, or a surface of the space may
completely cover a surface of the substrate and/or substrate table,
or the space may envelop the substrate and/or substrate table. The
liquid supply system may optionally further include one or more
elements to control the position, quantity, quality, shape, flow
rate or any other features of the liquid.
Moreover, although this invention has been disclosed in the context
of certain embodiments and examples, it will be understood by those
skilled in the art that the present invention extends beyond the
specifically disclosed embodiments to other alternative embodiments
and/or uses of the invention and obvious modifications and
equivalents thereof. In addition, while a number of variations of
the invention have been shown and described in detail, other
modifications, which are within the scope of this invention, will
be readily apparent to those of skill in the art based upon this
disclosure. For example, it is contemplated that various
combination or sub-combinations of the specific features and
aspects of the embodiments may be made and still fall within the
scope of the invention. Accordingly, it should be understood that
various features and aspects of the disclosed embodiments can be
combined with or substituted for one another in order to form
varying modes of the disclosed invention. Thus, it is intended that
the scope of the present invention herein disclosed should not be
limited by the particular disclosed embodiments described above,
but should be determined only by a fair reading of the claims that
follow.
The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the claims set out below.
* * * * *