U.S. patent application number 11/572394 was filed with the patent office on 2007-05-31 for extreme ultraviolet reticle protection using gas flow thermophoresis.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Michael Sogard.
Application Number | 20070121091 11/572394 |
Document ID | / |
Family ID | 35656764 |
Filed Date | 2007-05-31 |
United States Patent
Application |
20070121091 |
Kind Code |
A1 |
Sogard; Michael |
May 31, 2007 |
Extreme ultraviolet reticle protection using gas flow
thermophoresis
Abstract
Methods and apparatus for using a flow of a relatively cool gas
to establish a temperature gradient between a reticle and a reticle
shield to reduce particle contamination on the reticle are
disclosed. According to one aspect of the present invention, an
apparatus that reduces particle contamination on a surface of an
object includes a plate and a gas supply. The plate is positioned
in proximity to the object such that the plate, which has a second
temperature, and the object, which has a first temperature, are
substantially separated by a space. The gas supply supplies a gas
flow into the space. The gas has a third temperature that is lower
than both the first temperature and the second temperature. The gas
cooperates with the plate and the object to create a temperature
gradient and, hence, a thermophoretic force that conveys particles
in the space away from the object.
Inventors: |
Sogard; Michael; (Menlo
Park, CA) |
Correspondence
Address: |
AKA CHAN LLP
900 LAFAYETTE STREET
SUITE 710
SANTA CLARA
CA
95050
US
|
Assignee: |
Nikon Corporation
Tokyo
JP
100-8331
|
Family ID: |
35656764 |
Appl. No.: |
11/572394 |
Filed: |
January 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10898475 |
Jul 23, 2004 |
7030959 |
|
|
11572394 |
Jan 19, 2007 |
|
|
|
Current U.S.
Class: |
355/75 ; 355/30;
355/76 |
Current CPC
Class: |
G03F 7/70933 20130101;
G03F 7/70916 20130101; G03F 7/70875 20130101 |
Class at
Publication: |
355/075 ;
355/076; 355/030 |
International
Class: |
G03B 27/62 20060101
G03B027/62 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 21, 2005 |
US |
PCT/US05/25958 |
Claims
1. A lithography tool comprising: an optical surface: a reticle
chuck configured to hold a reticle defining a pattern, the reticle
chuck configured to position the reticle relative to the optical
surface; a reticle shield positioned between the optical surface
and the reticle; and a gas flow assembly, positioned adjacent the
reticle shield, and configured to provide a flow of gas to carry
contaminants substantially away from the optical surface, thereby
substantially preventing contaminants from contaminating the
optical surface.
2. The lithography tool of claim 1, further comprising a vacuum
created in the vicinity of the gas flow, the vacuum configured to
aid in the prevention of the contaminants from contaminating the
optical surface by substantially directing the gas flow toward the
vacuum.
3. The lithography tool of claim 2, wherein the vacuum is created
by a vacuum pump.
4. The lithography tool of claim 1, wherein the reticle is
configured to operate in a first environment having a first
pressure and the optical surface is configured to operate in a
second environment having a second pressure, wherein the first
pressure is higher than the second pressure.
5. The lithography tool of claim 1 further comprising a reticle
stage configured to move the reticle relative to the optical
surface.
6. The lithography tool of claim 1, wherein the contaminants are
the following types of contaminants: water vapor or
hydrocarbons.
7. The lithography tool of claim 1, wherein the reticle shield
includes an opening that is configured to allow the passage of
illumination radiation between the reticle and the optical
surface.
8. The lithography tool of claim 7, wherein the illumination
radiation is within one of the following ranges of wavelengths: a.
0.1 nm to 5 nm; b. 5 nm to 100 nm; or c. 100 nm to 250 nm.
9. The lithography tool of claim 1, wherein the optical surface is
part of a projection optical system configured to expose the
pattern defined by the reticle onto a substrate when illumination
radiation is projected onto the reticle and through the projection
optical system.
10. The lithography tool of claim 1, wherein the gas flow assembly
further comprises one or more nozzles configured to provide the
flow of gas.
11. The lithography tool of claim 1, wherein the gas flow assembly
is positioned between the reticle shield and the reticle chuck.
12. The lithography tool of claim 1, wherein the gas flow assembly
is positioned between the reticle shield and the optical
surface.
13. The lithography tool of claim 7, wherein the gas flow assembly
substantially surrounds the opening in the reticle shield and is
further configured to provide the flow of gas substantially away
from the opening.
14. The lithography tool of claim 7, wherein the gas flow assembly
substantially surrounds the opening in the reticle shield and is
further configured to provide a portion of the flow of gas through
the opening and away from the reticle.
15. The lithography tool of claim 10 wherein the gas passes through
particle filters associated with the nozzle opening.
16. The lithography tool of claim 15, wherein the particle filters
have an effective pore size of 1 millimeter or less.
17. The lithography tool of claim 10, wherein the one or more
nozzles have gas flow outlets that have an effective size of 1
millimeter or less.
18. The lithography tool of claim 10, wherein the one or more
nozzles are configured to provide the gas flow at a subsonic
velocity.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a national phase filing of
International Application No. PCT/US2005025958 (internationally
filed Jul. 21, 2005) which claims priority to U.S. patent
application Ser. No. 10/898,475, filed Jul. 23, 2004, now U.S. Pat.
No. 7,030,959, which are each incorporated herein by reference in
their entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of Invention
[0003] The present invention relates generally to equipment used in
semiconductor processing. More particularly, the present invention
relates to a mechanism which is arranged to reduce the amount of
particle contamination on a reticle used in an extreme ultraviolet
lithography system.
[0004] 2. Description of the Related Art
[0005] In photolithography systems, the accuracy with which
patterns on a reticle are projected off of or, in the case of
extreme ultraviolet (EUV) lithography, reflected off of the reticle
onto a wafer surface is critical. When a pattern is distorted, as
for example due to particle contamination on a surface of a
reticle, a lithography process which utilizes the reticle may be
compromised. Hence, the reduction of particle contamination on the
surface of a reticle is crucial.
[0006] Photolithography systems typically use pellicles to protect
reticles from particles. As will be appreciated by those skilled in
the art, a pellicle is a thin film on a frame which covers the
patterned surface of the reticle to prevent particles from becoming
attached to the patterned surface. Pellicles, however, are not used
to protect EUV reticles, since thin films generally are not
suitable for providing protection in the presence of EUV radiation.
Principles of thermophoresis may also be applied to protect
reticles from particle contamination by maintaining reticles at a
higher temperature than their surroundings, and, therefore, causing
the particles to move from the hotter reticle to the cooler
surroundings, e.g., cooler surfaces.
[0007] Since thermophoresis generally may not be used in a high
vacuum environment, in order for thermophoresis to be used in an
EUV system to protect a reticle mounted in a reticle chuck, gas at
a pressure of approximately fifty milliTorr (mTorr) or more may be
introduced to substantially flow around the reticle. With the gas
at a pressure of approximately fifty mTorr or more flowing around
the reticle, particles may be effectively pushed away from the
reticle towards a cooler surface. As will be appreciated by those
skilled in the art, at pressures close to zero, thermophoretic
forces are relatively insignificant. However, at low pressures of
approximately fifty mTorr, thermophoretic forces are generally
significant enough to convey particles from a hotter area to a
cooler area.
[0008] FIG. 1 is a diagrammatic side view representation of a
portion of an EUV lithography or exposure system. An EUV
lithography system 100 includes a chamber 104 which includes a
first region 108 and a second region 110. First region 108 is
arranged to house a reticle stage 114 which supports a reticle
chuck 118 that holds a reticle 122. Second region 110 is arranged
to house projection optics (not shown) and a wafer stage
arrangement (not shown). Sections 108, 110 are substantially
separated by a differential pumping barrier 126 through which an
opening 130 is defined.
[0009] Gas at a pressure of around fifty mTorr or more is supplied
to first region 108 through a gas supply opening 132 in chamber
104. In order for EUV radiation absorption losses in the gas to be
minimized, second region 110 is maintained at a lower pressure,
e.g., less than approximately one mTorr, than the pressure
maintained in first region 108. Hence, independent differential
pumping of first region 108 and second region 110 is maintained by
pump 134 and pump 136, respectively, so that the pressure in second
region 110 may be maintained at approximately one mTorr or less
while gas of a higher pressure is supplied through opening 130 into
first region 108.
[0010] In order for particles (not shown) located between reticle
122 and barrier 126 to be conveyed away from reticle 122 by the gas
using the principles of thermophoresis, a temperature differential
must be maintained between reticle 122 and the surroundings of
reticle 122. Typically, in order for thermophoresis to convey
particles away from reticle 122, reticle 122 is maintained at a
higher temperature than barrier 126. When reticle 122 is maintained
at a higher temperature than barrier 126, particles (not shown)
present between reticle 122 and barrier 126 may be attracted
towards barrier 126, as will be discussed below with respect to
FIG. 2. In come cases, particles (not shown) that are attracted
towards barrier 126 may pass into second region 110 through opening
130. The flow of gas from region 108 to region 110 will also convey
particles away from reticle 122, which helps in keeping particles
from coming into contact with reticle 122.
[0011] With reference to FIG. 2, the use of thermophoresis to
substantially repel particles away from the surface of a reticle
will be described. A reticle 222, which is maintained at a first
temperature, may be positioned in proximity to a cooler surface
226. Cooler surface 226 may be a differential pumping barrier in a
chamber used in EUV lithography, or may be a shield which is
arranged to protect reticle 222. A variation in gas temperature is
generally formed between reticle 222 and cooler surface 226 that
goes from being relatively warm near reticle 222 to being
relatively cool near cooler surface 226. This creates a temperature
gradient in the gas which is an essential condition for the
existence of thermophoresis. Particles 228 are generally repelled
from reticle 222 towards cooler surface 226. That is,
thermophoretic forces are such that particles are driven away from
the hotter reticle 222 towards cooler surface 226. Some particles
228 may become substantially attached to cooler surface 226.
[0012] While the positioning of a surface in proximity to a reticle
that is cooler than the reticle reduces particle contamination of
the reticle, maintaining surfaces of different temperatures within
an EUV apparatus is often problematic. For example, maintaining
surfaces at different temperatures may complicate temperature
control of critical systems. In addition, issues relating to
thermal expansion and distortion typically arise when a reticle and
adjacent components are maintained at different temperatures. When
there is thermal expansion or distortion within an EUV apparatus,
e.g., with respect to a reticle or a shield, the integrity of an
overall lithography process or, more generally, a semiconductor
fabrication process may be compromised. Also, the flow of gas from
region 108 of chamber 104 to region 110 may sweep particles
originating in region 108 into proximity with reticle 122, thereby
increasing the risk of contamination despite the protection
afforded by thermophoresis.
[0013] Therefore, what is desired is a system which allows an EUV
reticle to be efficiently and effectively protected from particle
contamination substantially without adversely affecting an overall
EUV lithography process. That is, what is needed is a system which
enables a reticle such as an EUV reticle to be protected from
particle contamination without a significant risk of thermal
expansion and distortion issues arising.
BRIEF SUMMARY OF THE INVENTION
[0014] The present invention relates to using a flow of a
relatively cool gas to establish a temperature gradient between a
reticle and a reticle shield such that particle contamination on
the reticle may be reduced. According to one aspect of the present
invention, an apparatus that reduces particle contamination on a
surface of an object includes a member having a surface proximate
to the object, e.g., a plate, and a gas supply. The plate is
arranged to be positioned in proximity to the object such that the
plate, which is of a second temperature, and the object, which is
of a first temperature, are substantially separated by a space. The
gas supply supplies a gas flow to the space. The gas is of a third
temperature that is lower than the first temperature and lower than
the second temperature. Heat flow between the gas, the plate, and
the object create a temperature gradient in the gas and, hence, a
thermophoretic force that is suitable for conveying particles in
the space away from the object.
[0015] In one embodiment, the plate includes at least a first
opening defined therein that enables the gas flow to pass
therethrough and into the space. In such an embodiment, the plate
may also include a second opening defined therein. The second
opening enables the gas flow to pass therethrough and out of the
space to convey the particles in the space away from the object and
away from the plate.
[0016] Allowing a reticle and a nearby surface, e.g., a reticle
shield, to remain at substantially the same temperature while
allowing for thermophoretic effects to convey particles away from
the reticle reduces particle contamination without causing
relatively significant thermal distortion effects and performance
issues. By maintaining a reticle and a nearby surface at
substantially the same temperature while providing a cooled or
chilled gas in a space between the reticle and the nearby surface,
a temperature gradient may be created between the reticle and the
nearby surface. The presence of the temperature gradient allows
thermophoretic forces to convey particles away from both the
reticle and the nearby surface. The source of the gas is local, and
the gas may be locally filtered, so the likelihood of the gas
sweeping additional particles into the vicinity of the reticle is
quite small.
[0017] According to another aspect of the present invention, a
method for reducing particle contamination on a surface of an
object includes providing a shield in proximity to the surface of
the object that is positioned such that there is a space defined
between the surface of the object and the shield. The shield has a
first opening defined therein, and the surface of the object is of
a first temperature while the shield is of a second temperature.
The method also includes providing a flow of a gas in the space
defined between the surface of the object and the shield, the gas
being of a third temperature that is lower than both the first
temperature and the second temperature. The flow of the gas is
provided through the first opening.
[0018] In one embodiment, the flow of the gas in the space creates
a temperature gradient in the space that enables the flow of the
gas to convey any particles in the space away from the surface of
the object. In another embodiment, providing the flow of the gas in
the space includes cooling the gas to the third temperature and
controlling the amount of the gas that flows through the first
opening.
[0019] According to still another aspect of the present invention,
an apparatus arranged to reduce particle contamination on a surface
of an object includes a chamber, a first scanning arrangement, and
a gas supply. The chamber has a first region and a second region
where the first region has a pressure of at least approximately 50
mTorr while the second region has a pressure that is less than the
pressure of the first region. The first scanning arrangement scans
the object, and is positioned in the first region. The first
scanning arrangement includes a plate that is arranged in proximity
to a first surface of the object such that a first surface of the
plate and the first surface of the object are substantially
separated by a space in the first region. The first surface of the
object is of a first temperature and the first surface of the plate
is of a second temperature. The gas supply supplies a gas flow to
the space. The gas is at a third temperature that is lower than the
first temperature and lower than the second temperature, and
cooperates with the plate and the object to create a thermophoretic
force to convey any particles in the space away from the
object.
[0020] In accordance with yet another aspect of the present
invention, an apparatus arranged to reduce contamination on a
surface of a first object includes a member having a first surface
proximate to the first object and a second surface proximate to the
second object. The member is in proximity to the second object such
that the member and the second object are substantially separated
by a space, and has a nozzle defined therethrough. The nozzle has
an associated aperture that is closer to the second object and an
opening, which is larger than the aperture, that is closer to the
first object. The nozzle also has a gas supply that supplies a gas
flow to the space. The apparatus also includes a pumping
arrangement that causes the gas flow to be conveyed through the
space substantially away from the aperture. In one embodiment, the
first object is a mirror associated with an optical arrangement and
the second object is a reticle mounted on a reticle stage assembly
and enclosed in a vacuum chamber.
[0021] These and other advantages of the present invention will
become apparent upon reading the following detailed descriptions
and studying the various figures of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention may best be understood by reference to the
following description taken in conjunction with the accompanying
drawings in which:
[0023] FIG. 1 is a diagrammatic side view representation of a
portion of an extreme ultraviolet lithography or exposure
system.
[0024] FIG. 2 is a diagrammatic representation of a reticle, a
nearby surface, and particles which are attracted away from the
reticle through the use of thermophoresis.
[0025] FIG. 3a is a diagrammatic representation of the layers of
gas flow between a reticle and a reticle shield in accordance with
an embodiment of the present invention.
[0026] FIG. 3b is a diagrammatic representation a temperature
gradient associated with a gas located between a reticle and a
reticle shield in accordance with an embodiment of the present
invention.
[0027] FIG. 4a is a diagrammatic cross-sectional side view
representation of a portion of an EUV lithography chamber which
uses a cooled gas to create thermophoretic forces in accordance
with an embodiment of the present invention.
[0028] FIG. 4b is a diagrammatic bottom view of one configuration
of openings, i.e., openings 432 of FIG. 4a, through which a gas may
flow between a reticle and a barrier in accordance with an
embodiment of the present invention.
[0029] FIG. 4c is a diagrammatic bottom view of another
configuration of openings, i.e., openings 432 of FIG. 4a, through
which a gas may flow between a reticle and a barrier in accordance
with an embodiment of the present invention.
[0030] FIG. 5a is a diagrammatic representation of a reticle in a
first position with respect to a differential pumping barrier in
accordance with an embodiment of the present invention.
[0031] FIG. 5b is a diagrammatic representation of a reticle in a
second position with respect to a differential pumping barrier,
i.e., reticle 512 and differential pumping barrier 528 of FIG. 5a,
in accordance with an embodiment of the present invention.
[0032] FIG. 5c is a diagrammatic representation of a reticle in a
third position with respect to a differential pumping barrier,
i.e., reticle 512 and differential pumping barrier 528 of FIG. 5a,
in accordance with an embodiment of the present invention.
[0033] FIG. 5d is a diagrammatic representation of a reticle i.e.,
reticle 512 of FIG. 5a, in two extreme positions, illustrating the
application of an embodiment of the present invention.
[0034] FIG. 5e is a diagrammatic side view of a reticle with a
second differential pumping barrier in accordance with an
embodiment of the present invention.
[0035] FIG. 5f is a diagrammatic side view of yet another
embodiment of the present invention.
[0036] FIG. 6 is a block diagram side-view representation of an EUV
lithography system in accordance with an embodiment of the present
invention.
[0037] FIG. 7 is a process flow diagram which illustrates the steps
associated with fabricating a semiconductor device in accordance
with an embodiment of the present invention.
[0038] FIG. 8 is a process flow diagram which illustrates the steps
associated with processing a wafer, i.e., step 1304 of FIG. 7, in
accordance with an embodiment of the present invention.
[0039] FIG. 9 is a diagrammatic cross-sectional side-view
representation of a reticle stage assembly which utilizes a reticle
shield to protect a reticle in accordance with an embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] Particle contamination on critical surfaces of reticles such
as reticles used in extreme ultraviolet (EUV) lithography systems
may compromise the integrity of semiconductors created using the
reticles. Hence, protecting critical surfaces of reticles from
airborne contaminants is important to ensure the integrity of
lithography processes. Some reticles are protected from airborne
particles through the use of pellicles. However, pellicles are not
suitable for use in protecting surfaces of EUV reticles. While
thermophoresis is also effective in protecting reticle surfaces
from particle contamination when at least a slight gas pressure is
present, maintaining a surface that is in proximity to a reticle at
a lower temperature than that of the reticle to enable
thermophoretic forces to act often causes thermal expansion and
distortion within an overall EUV lithography system.
[0041] By introducing a gas that flows between a reticle and a
nearby surface, e.g., a reticle shield, that is at a cooler
temperature than those of the reticle and the nearby surface,
thermophoresis may be used to convey particles away from the
reticle while the reticle may be maintained at substantially the
same temperature as the nearby surface. The cooler gas will
typically establish local temperature gradients adjacent to both
the reticle and the nearby surface, thereby establishing
thermophoretic forces which will effectively sweep particles away
from both the reticle and the nearby surface. Since the reticle and
the nearby surface are maintained at substantially the same
temperature, particle contamination of the reticle may be reduced,
while the potential for thermal expansion and distortion effects is
also significantly reduced.
[0042] The introduction of a gas between a surface of a reticle and
a surface of a reticle shield at a temperature that is cooler than
the temperatures of the reticle and the reticle shield allows a
temperature gradient to be formed in the gas between the reticle
and the reticle shield. With reference to FIGS. 3a and 3b, the
formation of a temperature gradient between the reticle and the
reticle shield will be described in accordance with an embodiment
of the present invention. As shown in FIG. 3a, when a cooled gas
312 is substantially introduced between a reticle 304 and a surface
308 near reticle 304, as for example a reticle shield, a boundary
layer 316 is formed near a surface of reticle 304 and a boundary
layer 318 is formed near surface 308. Boundary layers 316, 318 are
generally warmer than the rest of cooled gas 312, as will be
understood by those skilled in the art, since the gas in boundary
layers 316, 318 may be partially heated by reticle 304 and surface
308, respectively.
[0043] Cooler gas 312 typically establishes local temperature
gradients 320, and cause thermophoretic forces to be established
which will generally cause particles to move away from reticle 304
and surface 308, and effectively be swept into the flow of cooled
gas 312. Hence, particle contamination of reticle 304 as well as
particle contamination of surface 308 may be reduced.
[0044] FIG. 3b is a diagrammatic representation of cooled gas
between a reticle and a nearby surface, e.g., cooled gas 312 of
FIG. 3a, and a temperature gradient in accordance with an
embodiment of the present invention. A temperature gradient 320
associated with cooled gas 312 may be such that the temperature
distribution is approximately gaussian, as indicated by
distributions 326, with the coolest temperature being substantially
midway between boundary layer 316 and boundary layer 318. More
generally, the temperature distribution is such that the coolest
temperature is approximately halfway between boundary layer 316 and
boundary layer 318, while the warmest temperatures are at boundary
layer 316 and boundary layer 318, as indicated at 322. It should be
appreciated that the actual profile of a temperature distribution
may vary widely.
[0045] A cooled gas such as cooled gas 312 may be introduced into
an EUV lithography apparatus using a gas source or supply that is
substantially external to the apparatus. FIG. 4a is a diagrammatic
cross-sectional side view representation of a portion of an EUV
lithography chamber which uses a cooled gas to create
thermophoretic forces in accordance with an embodiment of the
present invention. An EUV lithography chamber 400 includes a first
region 410 and a second region 411 that are substantially separated
by a differential pumping barrier 428 or a reticle shield. A
pressure of approximately fifty milliTorr (mTorr) or more is
maintained in first region 410, while a pressure of less than
approximately 1 mTorr, i.e., a near vacuum, is maintained in second
region 411.
[0046] A reticle 412, which is held by a reticle chuck 408 that is
coupled to a reticle stage arrangement 404, and barrier 428 are
maintained at approximately the same temperature. A gas which is
supplied by gas supplies 416 and is cooled using coolers 424 may be
introduced through openings 432 into a space between reticle 412
and barrier 428. The flow of the gas is approximately laminar, and
may be controlled by gas flow controllers 420. In one embodiment,
filters 438 may be used to filter particles out of the gas as the
gas passes through openings 432 into the space between reticle 412
and barrier 428.
[0047] Openings 432 may generally be slots or orifices of various
shapes and sizes. As shown in FIG. 4b, openings 432 may be a series
of substantially round openings. Alternatively, openings 432' may
be slots as shown in FIG. 4c. It should be appreciated that the
number of openings 432, as well as the size and the shapes of
openings 432, may vary widely. In general, the shape and the
configuration of openings 432 may be chosen to enable an
approximately laminar flow of gas to be efficiently
established.
[0048] Gas that flows through openings 432 into the space between
reticle 412 and barrier 428 establishes local temperature gradients
adjacent to reticle 412 and barrier 428, and causes thermophoretic
forces to convey particles away from reticle 412 and barrier 428.
The particles may be conveyed through an opening, or differential
pumping aperture 436, defined within barrier 428 which is generally
arranged for an EUV beam to pass through. It should be appreciated
that although gas may escape from between reticle 412 and barrier
428 and into the remainder of first region 410 or into second
region 411, the amount of gas that escapes is typically not
excessive enough to significantly alter the pressure in first
region 410 or to compromise the vacuum in second region 411.
[0049] The gas introduced between reticle 412 and barrier 428 may
be a light gas such as helium or hydrogen. In general, the gas is a
pure gas that absorbs EUV radiation. In addition to being a light
gas such as helium or hydrogen, the gas may be argon or nitrogen.
Since nitrogen is relatively inexpensive, and is used in gas
bearings (not shown) which are typically a part of reticle stage
arrangement 404, nitrogen may often be used as the gas introduced
between reticle 412 and barrier 428.
[0050] During lithographic exposure, reticle 412 is scanned back
and forth above the opening 436 by means of reticle stage
arrangement 404. As reticle 412 scans, variations in temperature,
and therefore thermophoretic force, that are caused by the gas,
i.e., the cooled gas, warming up as the gas flows in contact with
reticle 412 and barrier 428 may generally be substantially averaged
out. Such a warming of the gas may be at least partially
compensated for by the thermodynamic cooling of the gas as the gas
approaches opening 436, which often results in a temperature drop
of the gas.
[0051] In order to maintain reticle 412 and barrier 428 at
substantially the same constant temperature, as heat is removed by
the cold flowing gas, a mechanism (not shown) for effectively
heating reticle 412 and barrier 428 may be provided. To facilitate
temperature control of barrier 428, thermal insulation 425 may be
used to thermally isolate barrier 428 from the surrounding
structures. The mechanism for effectively heating reticle 412 and
barrier 428 may generally be any suitable mechanism. By way of
example, reticle 412 may be sufficiently heated by EUV radiation
that passes through opening 436, and no other mechanism may need to
be used to heat reticle 412. The removal of heat by the flowing gas
is typically proportional to the heat capacity of the gas. Because
of the low pressure of the gas, the heat capacity is relatively
small, and the amount of heat removed from reticle 412 and barrier
428 is typically not excessive.
[0052] To reduce the amount of cooled gas that may effectively
escape from between a reticle and a barrier and into a surrounding
area, part of the flow of cooled gas may be shut down at times
depending upon the positioning of the reticle. For example, when a
reticle is near an extreme point of travel, gas flow through an
opening or openings which are not effectively covered by the
reticle may be shut off. As shown in FIG. 5a, when a reticle 512
that is supported by a reticle chuck 508 is scanned by a reticle
stage arrangement 504 over a barrier 528 or shield, reticle 512 may
be positioned such that openings 532a, 532b are both effectively
covered by reticle 512. However, when reticle 512 is at an extreme
point of travel such that opening 532b is not effectively covered
by reticle 512, as shown in FIG. 5b, a gas flow through opening
532b may be shut off. Alternatively, when reticle 512 is at another
extreme point of travel such that opening 532a is not effectively
covered by reticle 512, as shown in FIG. 5c, a gas flow through
openings 532a may be shut off. By shutting down the flow of gas
through one of openings 532a, 532b as appropriate, gas may be
substantially prevented from being directly pumped into a
surrounding environment.
[0053] FIG. 5d shows another embodiment which reduces the amount of
cooled gas escaping from between a reticle and a barrier. Skirts
540a and 540b, attached to stage arrangement 504'', effectively
extend the length of reticle 512, so that normal gas flow patterns
are maintained even when reticle 512 is at an extreme position of
travel. In one embodiment, a surface of skirts 540a and 540b which
opposes barrier 528 is at substantially the same level as a surface
of reticle 512 which opposes barrier 528. Such skirts 540a and 540b
experience no forces, save for the acceleration and deceleration of
the stage arrangement 504'' itself, nor does their location need to
be highly precise. Thus, skirts 540a and 540b may be constructed of
very light thin materials, so that their addition has no effect on
stage performance.
[0054] FIG. 5e shows an embodiment which allows less gas flow from
the region between a reticle 512' and a barrier 528' into a region
511' below barrier 528'. A nozzle 545 is attached to barrier 528',
and a gap 560 between the top surface of nozzle 545 and reticle
512' is reduced to a relatively small value, thereby limiting gas
flow into region 511'. Gap 560 may be approximately 1 mm or less,
for example. Gas inlets 550a and 550b installed on nozzle 545
provide a flow of gas largely parallel to the surface of reticle
512'. This flow is largely undisturbed as reticle 512' is scanned
back and forth by a stage arrangement 504'. Gas flow into region
510 will typically fluctuate as stage arrangement 504' scans, but
the EUV radiation does not pass through region 510, so the
fluctuations will not significantly affect the EUV intensity.
[0055] FIG. 5f shows another embodiment of the invention. Gas is
introduced into the region 521 between reticle 512' and barrier
528' through gas inlets 550a and 550b. The gas pressure at the
inlets is substantially higher than the ambient gas pressure in
region 521 and the ambient pressure in region 510'. Thus the gas
expands rapidly out of the inlets and cools significantly in the
process. The initial temperature of the gas at the inlets may be
adjusted to be warmer than, equal to, or cooler than the
temperature of reticle 512' or barrier 528', but as it expands into
region 521 a substantial fraction of it becomes cooler than reticle
512' and barrier 528'. Thus the desired temperature gradient in the
gas may be established under these conditions without the need to
initially cool the supply of gas with a cooler such as 424. In
addition the high gas pressure at inlets 550a, 550b causes the gas
flow to achieve a high velocity as it flows through region 521 into
region 510'. This imparts a substantial drag force on any particle
present which tends to quickly convey it out of region 521 and away
from reticle 512'. Thus in this embodiment reticle 512' is
protected by both a thermophoretic force arising from the
temperature gradient in the gas, and a drag force arising from the
high velocity of the gas flow in region 521.
[0056] In the embodiment described in FIG. 5f, the gas expanding
out of gas inlets 550a and 550b exits the inlets at subsonic
velocity. If the gas enters region 521 at supersonic velocity, it
will collide with the ambient gas, creating shock waves and heating
of the gas rather than the desired cooling. A subsonic flow into
region 521 may be substantially insured if gas inlets 550a and 550b
have openings whose dimensions are less than approximately the
molecular mean free path of the expanding gas. If gas inlets 550a
and 550b are each relatively large openings, they may be covered by
particle filters whose effective pore size is less than
approximately the molecular mean free path of the expanding
gas.
[0057] With reference to FIG. 6, an EUV lithography system will be
described in accordance with an embodiment of the present
invention. An EUV lithography system 900 includes a vacuum chamber
902 with pumps 906 which are arranged to enable desired pressure
levels to be maintained within vacuum chamber 902. For example,
pump 906b may be arranged to maintain a vacuum or a pressure level
of less than approximately 1 mTorr within a second region 908b of
chamber 902. Various components of EUV lithography system 900 are
not shown, for ease of discussion, although it should be
appreciated that EUV lithography system 900 may generally include
components such as a reaction frame, a vibration isolation
mechanism, various actuators, and various controllers.
[0058] An EUV reticle 916, which may be held by a reticle chuck 914
coupled to a reticle stage assembly 910 that allows the reticle to
scan, is positioned such that when an illumination source 924
provides beams which subsequently reflect off of a mirror 928, the
beams reflect off of a front surface of reticle 916. A reticle
shield assembly 920, or a differential barrier, is arranged to
protect reticle 916 such that contamination of reticle 916 by
particles may be reduced. In one embodiment, reticle shield
assembly 920 includes openings 950 through which a cooled gas,
which is supplied through a gas supply 954 with a temperature
controller 958, may flow.
[0059] As discussed above, reticle shield assembly 920 includes an
opening through which beams, e.g., EUV radiation, may illuminate a
portion of reticle 916. Incident beams on reticle 916 may be
reflected onto a surface of a wafer 932 held by a wafer chuck 936
on a wafer stage assembly 940 which allows wafer 932 to scan.
Hence, images on reticle 916 may be projected onto wafer 932.
[0060] Wafer stage assembly 940 may generally include a positioning
stage that may be driven by a planar motor, as well as a wafer
table that is magnetically coupled to the positioning stage by
utilizing an EI-core actuator. Wafer chuck 936 is typically coupled
to the wafer table of wafer stage assembly 940, which may be
levitated by any number of voice coil motors. The planar motor
which drives the positioning stage may use an electromagnetic force
generated by magnets and corresponding armature coils arranged in
two dimensions. The positioning stage is arranged to move in
multiple degrees of freedom, e.g., between three to six degrees of
freedom to allow wafer 932 to be positioned at a desired position
and orientation relative to a projection optical system reticle
916.
[0061] Movement of the wafer stage assembly 940 and reticle stage
assembly 910 generates reaction forces which may affect performance
of an overall EUV lithography system 900. Reaction forces generated
by the wafer (substrate) stage motion may be mechanically released
to the floor or ground by use of a frame member as described above,
as well as in U.S. Pat. No. 5,528,118 and published Japanese Patent
Application Disclosure No. 8-166475. Additionally, reaction forces
generated by motion of reticle stage assembly 910 may be
mechanically released to the floor (ground) by use of a frame
member as described in U.S. Pat. No. 5,874,820 and published
Japanese Patent Application Disclosure No. 8-330224, which are each
incorporated herein by reference in their entireties.
[0062] As described above, a reticle may be protected from
particles using a reticle shield that covers the reticle except
where the reticle is illuminated by EUV, in conjunction with a
nozzle which generates a flow of gas substantially parallel to the
reticle surface. In one embodiment, the nozzle may be a part of a
fixed blind assembly. The gas flow drags particles with it, away
from the reticle surface. The dragging of particles away from the
reticle surface using gas flow may be referred to here as
viscophoresis. Gas also expands and cools from an inlet to provide
some thermophoretic protection.
[0063] FIG. 9 is a diagrammatic cross-sectional side-view
representation of a reticle stage assembly which utilizes a reticle
shield to protect a reticle in accordance with an embodiment of the
present invention. A reticle stage 1200 supports a reticle chuck
1204 which, in turn, supports a reticle 1208. Reticle 1208 is
shielded by a reticle shield 1220. A fixed blind aperture 1224 is
arranged substantially within reticle shield 1220, and reticle
shield 1220 is arranged to define a nozzle 1228. Nozzle 1128 opens
up into a projection optics environment 1216, while reticle stage
1200, reticle chuck 1204, and reticle 1208 are substantially within
a reticle stage environment 1212. It should be appreciated that, in
one embodiment, projection optics environment 1216 may be a
projection optics chamber and reticle stage environment 1212 may be
a reticle stage chamber. In general, projection optics environment
1216 is arranged to include components such as a mirror (not shown)
of an optical arrangement.
[0064] Gas flows between reticle 1208 and reticle shield 1220, as
represented by arrows 1230. The gas may be supplied by a gas supply
associated with or included in the nozzle. Some of the gas is
pumped from the reticle stage environment 1212, in one embodiment,
by means of vacuum pumps attached to a reticle stage environment
vacuum chamber (not shown). It should be appreciated that a reticle
stage environment vacuum chamber may be such that reticle 1208 is
substantially enclosed within the vacuum chamber. Some of the gas
exits through fixed blind aperture 1224 into projection optics
environment 1216. Projection optics environment 1216 is maintained
at a lower pressure than reticle stage environment 1212 and the
space between reticle 1208, and fixed blind aperture 1224
effectively serves as a differential pumping aperture. The higher
pressure in reticle stage environment 1212 allows for viscophoresis
and thermophoresis, and the lower pressure in projection optic
environment 1216 allows for a relatively high transmission of EUV
radiation through gas.
[0065] Projections optics mirror reflectivities are typically
sensitive to hydrocarbon and water vapor contamination. Less than a
monolayer absorbed on the surfaces of the mirrors may result in a
relatively significant reduction in reflectivity and, hence,
lithographic throughput. Outgassing of hydrocarbons or water vapor
from structures in the reticle stage environment 1212, such as
reticle stage 1200 or reticle chuck 1204 or cables or hoses
attached thereto, is substantially contained within reticle stage
environment 1212 by the flow of gas represented by arrows 1230.
Therefore, projection optics mirrors within projection optics
environment 1216 may be protected from contamination as a result of
outgassing. The containment of products and byproducts of
outgassing may be achieved in part through the use of differential
pumping between projection optics environment 1216 and reticle
stage environment 1212. However, the containment of products and
byproducts of outgassing is generally when the gas flows from
nozzle 1228 effectively prevent outgassing from parts of the
reticle stage environment 1212 from getting to fixed blind aperture
1224 and, hence, projection optics in projection optics environment
1216.
[0066] Gas flow enables outgassing of a hydrocarbon such as
methane, i.e., CH4, from the side of reticle stage 1200 or reticle
chuck 1204 to be substantially confined to the vicinity of reticle
stage 1200. The concentration of CH4 may be reduced by
approximately two orders of magnitude or more near the nozzle 1228
by the flow of gas.
[0067] When a situation arises in which reticle stage 1200 is moved
to one extreme end of motion such that the outgassing region is
closer to fixed blind aperture 1224 than it is in FIG. 9,
containment of CH4 outgassing due to gas flow and the pressure
differential between projection optics environment 1216 and reticle
stage environment 1212 is generally still in effect. This typically
assumes that the differential pumping condition between reticle
stage environment 1212 and projection optics environment 1216 is
maintained, which may involve the inclusion of reticle skirts such
as reticle skirts 540 of FIG. 5d, in one embodiment. The
concentration of CH4 in projection optics environment 1216 may
still be reduced, as for example by approximately two orders of
magnitude over the concentration if there were no pressure
differential or gas flow.
[0068] An EUV lithography system according to the above-described
embodiments, e.g., a lithography apparatus which may include a
reticle shield, may be built by assembling various subsystems in
such a manner that prescribed mechanical accuracy, electrical
accuracy, and optical accuracy are maintained. In order to maintain
the various accuracies, prior to and following assembly,
substantially every optical system may be adjusted to achieve its
optical accuracy. Similarly, substantially every mechanical system
and substantially every electrical system may be adjusted to
achieve their respective desired mechanical and electrical
accuracies. The process of assembling each subsystem into a
photolithography system includes, but is not limited to, developing
mechanical interfaces, electrical circuit wiring connections, and
air pressure plumbing connections between each subsystem. There is
also a process where each subsystem is assembled prior to
assembling a photolithography system from the various subsystems.
Once a photolithography system is assembled using the various
subsystems, an overall adjustment is generally performed to ensure
that substantially every desired accuracy is maintained within the
overall photolithography system. Additionally, it may be desirable
to manufacture an exposure system in a clean room where the
temperature and humidity are controlled.
[0069] Further, semiconductor devices may be fabricated using
systems described above, as will be discussed with reference to
FIG. 7. The process begins at step 1301 in which the function and
performance characteristics of a semiconductor device are designed
or otherwise determined. Next, in step 1302, a reticle (mask) in
which has a pattern is designed based upon the design of the
semiconductor device. It should be appreciated that in a parallel
step 1303, a wafer is made from a silicon material. The mask
pattern designed in step 1302 is exposed onto the wafer fabricated
in step 1303 in step 1304 by a photolithography system. One process
of exposing a mask pattern onto a wafer will be described below
with respect to FIG. 8. In step 1305, the semiconductor device is
assembled. The assembly of the semiconductor device generally
includes, but is not limited to, wafer dicing processes, bonding
processes, and packaging processes. Finally, the completed device
is inspected in step 1306.
[0070] FIG. 8 is a process flow diagram which illustrates the steps
associated with wafer processing in the case of fabricating
semiconductor devices in accordance with an embodiment of the
present invention. In step 1311, the surface of a wafer is
oxidized. Then, in step 1312 which is a chemical vapor deposition
(CVD) step, an insulation film may be formed on the wafer surface.
Once the insulation film is formed, in step 1313, electrodes are
formed on the wafer by vapor deposition. Then, ions may be
implanted in the wafer using substantially any suitable method in
step 1314. As will be appreciated by those skilled in the art,
steps 1311-1314 are generally considered to be preprocessing steps
for wafers during wafer processing. Further, it should be
understood that selections made in each step, e.g., the
concentration of various chemicals to use in forming an insulation
film in step 1312, may be made based upon processing
requirements.
[0071] At each stage of wafer processing, when preprocessing steps
have been completed, post-processing steps may be implemented.
During post-processing, initially, in step 1315, photoresist is
applied to a wafer. Then, in step 1316, an exposure device may be
used to transfer the circuit pattern of a reticle to a wafer.
Transferring the circuit pattern of the reticle of the wafer
generally includes scanning a reticle scanning stage which may, in
one embodiment, include a force damper to dampen vibrations.
[0072] After the circuit pattern on a reticle is transferred to a
wafer, the exposed wafer is developed in step 1317. Once the
exposed wafer is developed, parts other than residual photoresist,
e.g., the exposed material surface, may be removed by etching.
Finally, in step 1319, any unnecessary photoresist that remains
after etching may be removed. As will be appreciated by those
skilled in the art, multiple circuit patterns may be formed through
the repetition of the preprocessing and post-processing steps.
[0073] Although only a few embodiments of the present invention
have been described, it should be understood that the present
invention may be embodied in many other specific forms without
departing from the spirit or the scope of the present invention. By
way of example, while the use of a cooled gas to establish
thermophoretic forces between a reticle and a reticle shield has
been described, a cooled gas may be used in proximity to a wafer
surface to establish thermophoretic forces to keep particles from
being attracted to the wafer surface. In addition, the introduction
of a cooled gas flow in proximity to a wafer surface may further
enable outgassing products of the wafer surface to be conveyed away
from the wafer surface.
[0074] A gas that is to be introduced into a space between a
reticle and a reticle shield has generally been described as being
cooled by coolers that are in proximity to openings in the reticle
shield. That is, a cooled gas has been described as being locally
cooled. It should be appreciated, however, that a gas may be cooled
by substantially any suitable mechanism in a suitable location. In
addition, the gas may be any suitable gas, as for example a light
gas such as helium or hydrogen.
[0075] Substantially any suitable mechanism may be used to maintain
the temperature of the reticle and the temperature of a reticle
shield at a temperature that is warmer than the temperature of a
cooled gas that is provided in the space defined between the
reticle and the reticle shield. The configuration of such suitable
mechanisms may generally vary widely.
[0076] A fixed blind aperture, e.g., fixed blind aperture 1224 of
FIG. 9, has generally been described as being the only channel
between a reticle stage environment or chamber and a projection
optics environment or chamber. It should be understood, however,
that there may be other channels between a reticle stage
environment and a projection optics environment. By way of example,
openings may exist in a reticle shield to accommodate alignment
microscopes and an interferometer fixed mirror. As gas flow is
arranged such that contamination or particles may be kept away from
the reticle shield, some contamination or particles may be
transported through any other openings in the reticle shield.
However, as the conductances between a reticle stage environment
and a projection optics environment is generally small compared to
the conductances which occur through the fixed blind aperture, any
contamination transported through other openings in the reticle
shield may likely be considered to be relatively negligible.
[0077] While the use of a gas flow in conjunction with a reticle
shield may be suitable for protecting projection optics, the use of
a gas flow in conjunction with a reticle shield may be suitable for
protecting other components of an overall system which utilizes an
EUV reticle. For instance, illumination optics may also be
protected using a gas flow and a reticle shield.
[0078] A reticle and a barrier or a reticle shield have been
described as having substantially the same temperature. In one
embodiment, the reticle and the barrier may have different
temperatures that are warmer than the temperature of a cooled gas
introduced into a space between the reticle and the barrier. That
is, the reticle and the barrier may have slightly different
temperatures as long as the different temperatures are both higher
than the temperature of the cooled gas provided between the reticle
and the barrier without departing from the spirit or the scope of
the present invention. Therefore, the present examples are to be
considered as illustrative and not restrictive, and the invention
is not to be limited to the details given herein, but may be
modified within the scope of the appended claims.
* * * * *