U.S. patent application number 11/546298 was filed with the patent office on 2007-02-08 for optical element and projection exposure apparatus based on use of the optical element.
This patent application is currently assigned to NIKON CORPORATION. Invention is credited to Takeshi Shirai.
Application Number | 20070030468 11/546298 |
Document ID | / |
Family ID | 32500864 |
Filed Date | 2007-02-08 |
United States Patent
Application |
20070030468 |
Kind Code |
A1 |
Shirai; Takeshi |
February 8, 2007 |
Optical element and projection exposure apparatus based on use of
the optical element
Abstract
A liquid immersion exposure apparatus includes a projection
optical system (PL) which projects an image of a pattern onto a
substrate (W) and a unit (5) which supplies a liquid (7) between an
optical element (4) at the end of the projection optical system
(PL) and the substrate (W). A corrosion-resistant film composed of
an oxide is formed on the surface of the optical element (4) to
prevent corrosion by the liquid (7). Consequently, a desired
performance of the projection optical system can be secured for a
long time even where a full field exposure in the step-and-repeat
manner or a scanning exposure in the step-and-scan manner is
performed in a liquid-immersion state.
Inventors: |
Shirai; Takeshi;
(Yokohama-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
NIKON CORPORATION
TOKYO
JP
|
Family ID: |
32500864 |
Appl. No.: |
11/546298 |
Filed: |
October 12, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11147284 |
Jun 8, 2005 |
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11546298 |
Oct 12, 2006 |
|
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PCT/JP03/15780 |
Dec 10, 2003 |
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11147284 |
Jun 8, 2005 |
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Current U.S.
Class: |
355/53 ;
355/30 |
Current CPC
Class: |
G03F 7/70316 20130101;
G03F 7/70875 20130101; G03F 7/70958 20130101; G03F 7/70983
20130101; G03F 7/70341 20130101 |
Class at
Publication: |
355/053 ;
355/030 |
International
Class: |
G03B 27/42 20060101
G03B027/42 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 10, 2002 |
JP |
2002-357641 |
Claims
1. A projection objective for imaging a pattern arranged in an
object plane of the projection objective into an image plane of the
projection objective with the aid of an immersion medium arranged
between an optical element of the projection objective and the
image plane, the optical element having a transparent substrate and
a protective layer system that is fitted to the substrate, is
provided for contact with the immersion medium and serves for
increasing the resistance of the optical element to degradation
caused by the immersion medium.
2. The projection objective as claimed in claim 1, wherein the
optical element is the last optical element of the projection
objective in the light path.
3. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer that
is essentially impermeable to the immersion medium.
4. The projection objective as claimed in claim 3, wherein the
barrier layer comprises at least one barrier layer material that is
essentially chemically resistant to the immersion medium, and is
essentially free of pores passing through from an outer side of the
barrier layer that is remote from the substrate to a side of the
barrier layer that faces the substrate.
5. The projection objective as claimed in claim 3, wherein the
barrier layer is a single layer.
6. The projection objective as claimed in claim 3, wherein the
barrier layer is formed as a multilayer layer.
7. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer that
contains at least one of the following fluoride materials or
essentially consists of such a material: actinium fluoride
(AcF.sub.3), bismuth fluoride (BiF.sub.3), erbium fluoride
(ErF.sub.3), europium fluoride (EuF.sub.3), gadolinium fluoride
(GdF.sub.3), holmium fluoride (HoF.sub.3), potassium magnesium
fluoride (KMgF.sub.3), lanthanum fluoride (LaF.sub.3), sodium
yttrium fluoride (NaYF.sub.4), neodymium fluoride (NdF.sub.3),
samarium fluoride (SmF.sub.3), terbium fluoride (TbF.sub.3),
titanium fluoride (TiF.sub.3), thulium fluoride (TmF.sub.3),
vanadium fluoride (VF.sub.3), ytterbium fluoride (YbF.sub.3),
yttrium fluoride (YF.sub.3).
8. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer that
contains at least one of the following oxide materials or
essentially consists of one of said materials: silicon dioxide
(SiO.sub.2), magnesium aluminum oxide (MgAl.sub.2O.sub.4), aluminum
oxide (Al.sub.2O.sub.3), tungsten dioxide (WO.sub.2), tungsten
trioxide (WO.sub.3).
9. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer made
of an oxidic material having a high packing density, the packing
density preferably being more than 95%, in particular more than 97%
or 98%, of the density of the bulk material and/or an average
refractive index of the oxidic material deviating less than 5%,
preferably less than 3%, in particular less than 2%, from the
refractive index of the bulk material.
10. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer that
essentially consists of an ion-sputtered oxide material, in
particular silicon dioxide.
11. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer that
essentially consists of an oxide material applied in a PECVD
method, in particular PECVD silicon dioxide.
12. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer that
has an optical layer thickness of between 0.15 .lamda. and 0.6
.lamda., in particular between approximately 0.2 .lamda. and 0.3
.lamda., or between approximately 0.4 .lamda. and 0.6 .lamda.,
where .lamda. is the operating wavelength of the projection
objective.
13. The projection objective as claimed in claim 12, wherein, for a
refractive index difference An between the refractive indices of
the barrier layer material and of the immersion medium,
.DELTA.n>0.04 holds true.
14. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer that
is designed as an antireflection layer.
15. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer, and
an optical layer thickness of the barrier layer is adapted to the
optical properties of a monolayer or multilayer dielectric layer
system adjoining the barrier layer in such a way that a
reflection-reducing effect results in conjunction with the layer
system.
16. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer that
is applied directly to an exit-side surface of the substrate.
17. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer, an
antireflection layer system being arranged between the substrate
and the barrier layer.
18. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer, an
antireflection layer system being applied to a surface of the
barrier layer that is remote from the substrate.
19. The projection objective as claimed in claim 17, wherein the
antireflection layer system is a magnesium fluoride/lanthanum
fluoride alternating layer system.
20. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one barrier layer made
of an organic material that is essentially impermeable to the
immersion medium, in particular made of a perfluorinated
fluorocarbon, preferably polytetrafluoroethylene.
21. The projection objective as claimed in claim 1, wherein the
protective layer system is designed as a graded index layer with a
continuous or discontinuous refractive index profile perpendicular
to a layer extent, a refractive index in a region near the
substrate essentially corresponding to the refractive index of the
substrate material and a refractive index in a region provided for
contact with the immersion medium essentially corresponding to the
refractive index of the immersion medium.
22. The projection objective as claimed in claim 1, wherein the
protective layer system is designed as a wear system that is
optimized in terms of refractive index in such a way that a gradual
material dissolution caused by contact with the immersion medium
does not lead to a substantial change in the optical properties of
the protective layer system.
23. The projection objective as claimed in claim 1, wherein the
protective layer system has an effective refractive index n.sub.ss
at least in a region adjoining the immersion medium, such that
.DELTA.n<0.05, preferably .DELTA.n<0.01, in particular
.DELTA.n<0.005, holds true for a refractive index difference
.DELTA.n=|n.sub.I-n.sub.ss| with respect to the refractive index
n.sub.I of the immersion medium.
24. The projection objective as claimed in claim 1, wherein at
least one dielectric antireflection layer system having one or a
plurality of single layers is arranged between the substrate and
the wear system.
25. The projection objective as claimed in claim 1, wherein the
protective layer system consists of, at least in a region adjoining
the immersion medium, a mixed material having at least one material
having a low refractive index and at least one material having a
high refractive index, in which case, preferably, the material
having a low refractive index has a refractive index n.sub.L<
{square root over (n.sub.In.sub.S)} and the material having a high
refractive index has a refractive index n.sub.H> {square root
over (n.sub.In.sub.S)}, where n.sub.I is the refractive index of
the immersion medium and n.sub.s is the refractive index of the
substrate material, and a ratio of the material having a low
refractive index and the material having a high refractive index
being chosen such that an average refractive index n.sub.MIX of the
mixed material is present.
26. The projection objective as claimed in claim 25, wherein the
average refractive index n.sub.MIX is set such that it is in
proximity to the refractive index n.sub.I of the immersion medium,
in which case .DELTA.n=|n.sub.I-n.sub.MIX|<0.05 preferably holds
true, in particular .DELTA.n<0.01 holds true.
27. The projection objective as claimed in claim 25, wherein the
average refractive index n.sub.MIX.apprxeq. {square root over
(n.sub.In.sub.S)}.
28. The projection objective as claimed in claim 1, wherein the
protective layer system consists of a mixed material is a monolayer
antireflection layer having an average refractive index
n.sub.MIX.apprxeq. {square root over (n.sub.In.sub.S)}.+-.2% and
having an optical layer thickness of approximately Id.sub.QWOT,
where d.sub.QWOT is the layer thickness of a quarter wave layer of
the mixed material and I is an odd integer.
29. The projection objective as claimed in claim 25, wherein the
mixed material is constructed as a nanopatterned multilayer
material.
30. The projection objective as claimed in claim 25, wherein the
mixed material has a continuous mixture of two or more
components.
31. The projection objective as claimed in claim 1, wherein the
substrate consists of a fluoride crystal material, in particular
calcium fluoride.
32. The projection objective as claimed in claim 1, wherein the
optical element provided with the protective layer system, in
particular the last optical element, is a planoconvex lens having a
spherically or aspherically curved entry face and an essentially
planar exit face to which the protective layer system is
fitted.
33. The projection objective as claimed in claim 1, wherein the
optical element provided with the protective layer system is an
essentially plane-parallel plate.
34. The projection objective as claimed in claim 33, wherein the
plane-parallel plate is wrung onto an optical element or is
connected thereto optically neutrally in a different way.
35. The projection objective as claimed in claim 1, wherein the
optical element provided with the protective layer system is
exchangeable.
36. The projection objective as claimed in claim 1, wherein the
protective layer system comprises at least one layer that is made
of an immersion liquid and is arranged within the protective layer
system (double immersion).
37. The projection objective as claimed in claim 1, wherein the
protective layer system is essentially fitted only to an image-side
exit face of the optical element provided with the protective layer
system.
38. The projection objective as claimed in claim 1, wherein the
protective layer system is fitted to an image-side exit face of the
substrate and also extends continuously over adjoining side areas
of the substrate.
39. The projection objective as claimed in claim 1, wherein the
protective layer system essentially covers all outer areas of the
substrate.
40. The projection objective as claimed in claim 1, wherein the
protective layer system comprises a multilayer system having a
first layer with first defects and at least one second layer with
second defects, the first defects and the second defects being
distributed over different lateral positions in the layers in such
a way that a defect-free region exists in at least one of the
layers essentially at every location of the multilayer system.
41. The projection objective as claimed in claim 40, wherein an
interface between the first layer, which is nearer to the
substrate, and the second layer, which is further from the
substrate, is configured as a polishing interface produced by
polishing the first layer prior to coating with the second
layer.
42. The projection objective as claimed in claim 1, wherein the
projection objective has an image-side numerical aperture
NA.gtoreq.0.80, preferably NA.gtoreq.0.98, in particular
NA.gtoreq.1.
43. A method for protecting a projection objective, which is
designed for imaging a pattern arranged in an object plane of the
projection objective into an image plane of the projection
objective with the aid of an immersion medium arranged between an
optical element of the projection objective and the image plane,
against degradation of optical properties caused by the immersion
medium, comprising: fitting of a protective layer system provided
for contact with the immersion medium at least to an exit side of
the substrate.
44. The method as claimed in claim 43, wherein the protective layer
system is fitted to the last optical element of the projection
objective in the light path.
45. The method as claimed in claim 43, wherein the protective layer
system is designed as recited in claim 1.
46. The method as claimed in claim 43, wherein the fitting of the
protective layer system comprises the following: production of a
first layer of a multilayer system on the substrate or on a coating
applied on the substrate; removal of a portion of the first layer
by polishing in order to produce a polishing surface; production of
a second layer on the polishing surface of the first layer.
47. The method as claimed in claim 46, wherein the following are
carried out at least once: removal of a part of the second layer by
polishing in order to produce a polishing surface of the second
layer; production of a third layer on the polishing surface of the
second layer.
48. The method as claimed in claim 46, wherein, between the removal
of a part of a layer and the production of a further layer on the
removed layer, a cleaning is carried out for the removed layer.
49. An optical element having: a transparent substrate; and at
least one protective layer system that is fitted to the substrate,
is provided for contact with an immersion medium and serves for
increasing the resistance of the optical element to degradation
caused by the immersion medium.
50. The optical element as claimed in claim 49, wherein the
substrate consists of a fluoride crystal material, in particular
calcium fluoride.
51. The optical element as claimed in claim 49, which is designed
as a planoconvex lens having a spherically or aspherically curved
first face and an essentially planar second face to which the
protective layer system is fitted.
52. The optical element as claimed in claim 49, wherein the optical
element is an essentially plane-parallel plate.
53. The optical element as claimed in claim 49, wherein the
protective layer system is fitted to a first outer area of the
substrate and also extends continuously over adjoining side areas
of the substrate, the protective layer system preferably
essentially covering all outer areas of the substrate.
54. The optical element as claimed in claim 49, wherein the
protective layer system is designed as recited in claim 1.
55. An optical system having: at least one optical element provided
for contact with an immersion medium; the optical element having a
transparent substrate and a protective layer system that is fitted
to the substrate, is provided for contact with the immersion medium
and serves for increasing the resistance of the optical element to
degradation caused by the immersion medium.
56. The optical system as claimed in claim 55, which contains, in
addition to the optical element provided for contact with the
immersion medium, at least one further element that is preferably
not provided for contact with an immersion medium.
57. The optical system as claimed in claim 55, wherein the optical
element is designed as recited in claim 49.
58. A method for protecting a projection objective, which is
designed for imaging a pattern arranged in an object plane of the
projection objective into an image plane of the projection
objective with the aid of an immersion medium arranged between an
optical element of the projection objective and the image plane,
against degradation of optical properties caused by the immersion
medium, comprising: use of a cooled immersion medium with a media
temperature lying below an ambient temperature of the projection
objective.
59. The method as claimed in claim 58, wherein the media
temperature is set to less than 15.degree. C., the media
temperature preferably being set to a temperature in the range of
between 100.degree. C. and 50.degree. C.
60. The method as claimed in claim 58, wherein an immersion medium
which essentially consists of water is used.
Description
CROSS-REFERENCE
[0001] This is a Divisional of application Ser. No. 11/147,284
filed Jun. 8, 2005, which in turn is a Continuation of
International Application No. PCT/JP03/015780 filed Dec. 10, 2003
claiming the conventional priority of Japanese patent Application
No. 2002-357641 filed on Dec. 10, 2002. The disclosures of these
prior applications are incorporated herein by reference in their
entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a projection exposure
apparatus which is usable to transfer a mask pattern onto a
photosensitive substrate in the lithography step for producing
devices including, for example, semiconductor elements, image
pickup elements (CCD or the like), liquid crystal display elements,
and thin film magnetic heads. In particular, the present invention
relates to a projection exposure apparatus based on the use of the
liquid immersion method. The present invention also relates to an
optical element which is usable for the projection exposure
apparatus.
[0004] 2. Description of the Related Art
[0005] In the production of the semiconductor element or the like,
the projection exposure apparatus is used to transfer a pattern
image of a reticle as a mask via a projection optical system onto
each of shot areas on a wafer (or a glass plate or the like) coated
with a resist as a photosensitive substrate. Conventionally, the
reduction projection type exposure apparatus based on the
step-and-repeat system (stepper) has been used as the projection
exposure apparatus in many cases. However, recently, the attention
is also attracted to the projection exposure apparatus based on the
step-and-scan system in which the exposure is performed by
synchronously scanning the reticle and the wafer.
[0006] As for the resolution of the projection optical system
carried on the projection exposure apparatus, as the exposure
wavelength to be used is shorter, the resolution becomes higher.
Further, as the numerical aperture of the projection optical system
is larger, the resolution becomes higher. Therefore, the exposure
wavelength, which is used for the projection exposure apparatus, is
shortened year by year, and the numerical aperture of the
projection optical system is increased as well, as the integrated
circuit becomes fine and minute. The exposure wavelength, which is
dominantly used at present, is 248 nm based on the KrF excimer
laser. However, the exposure wavelength of 193 nm based on the ArF
excimer laser, which is shorter than the above, is also practically
used.
[0007] When the exposure is performed, the depth of focus (DOF) is
also important in the same manner as the resolution. The resolution
R and the depth of focus .delta. are represented by the following
expressions respectively. R=k1.lamda./NA (1)
.delta.=.+-.k2.lamda./NA.sup.2 (2)
[0008] In the expressions, .lamda. represents the exposure
wavelength, NA represents the numerical aperture of the projection
optical system, and k1 and k2 represent the process coefficients.
According to the expressions (1) and (2), the following fact is
appreciated. That is, when the exposure wavelength .lamda. is
shortened and the numerical aperture NA is increased in order to
enhance the resolution R, then the depth of focus .delta. is
narrowed. Conventionally, in the projection exposure apparatus, the
surface of the wafer is adjusted and matched with the image plane
of the projection optical system in the auto-focus manner. For this
purpose, it is desirable that the depth of focus .delta. is wide to
some extent. Accordingly, those having been suggested as the method
for substantially widening the depth of focus include, for example,
the phase shift reticle method, the modified illumination method,
and the multilayer resist method.
[0009] As described above, in the conventional projection exposure
apparatus, the depth of focus is narrowed as the wavelength of the
exposure light beam is shortened and the numerical aperture of the
projection optical system is increased. In order to respond to the
advance of higher integration of the semiconductor integrated
circuit, studies have been also made to further shorten the
exposure wavelength. However, if such a situation is continued as
it is, it is feared that the depth of focus may be too narrowed and
the margin may become insufficient during the exposure
operation.
[0010] In view of the above, the liquid immersion method has been
proposed as a method for substantially shortening the exposure
wavelength and deepening the depth of focus. In this method, the
space between the lower surface of the projection optical system
and the surface of the wafer is filled with a liquid such as water
or an organic solvent. The resolution is improved and the depth of
focus is magnified about n times by utilizing the fact that the
wavelength of the exposure light beam in the liquid is 1/n time
that in the air (n represents the refractive index of the liquid,
which is usually about 1.2 to 1.6).
[0011] If it is intended to apply the liquid immersion method to
the projection exposure apparatus based on the step-and-repeat
system as it is, the liquid leaks out from the space between the
projection optical system and the wafer when the wafer is subjected
to the stepping movement to the next shot area after the exposure
is completed for one shot area. Therefore, inconveniences arise
such that the liquid must be supplied again, and it is difficult to
recover the leaked liquid as well. If it is intended to apply the
liquid immersion method to the projection exposure apparatus based
on the step-and-scan system, it is necessary that the space between
the projection optical system and the wafer is filled with the
liquid during the period in which the wafer is moved as well,
because the exposure is performed while moving the wafer. The
projection optical system and the liquid make contact with each
other. Therefore, there is such a possibility that the end portion
of the projection optical system, which is in contact with the
liquid, may be corroded by the liquid. The objective lens is
installed to the end of the projection optical system. If the
objective lens is corroded, it is feared that any desired optical
performance cannot be obtained.
SUMMARY OF THE INVENTION
[0012] Taking the foregoing viewpoints into consideration, an
object of the present invention is to provide an optical element
which is preferably usable for a projection exposure system of an
exposure apparatus for performing the liquid immersion exposure.
Another object of the present invention is to provide a projection
exposure apparatus for the liquid immersion exposure which carries
such an optical element.
[0013] According to a first aspect of the present invention, there
is provided an optical element usable for a projection optical
system which exposes a substrate by projecting a predetermined
pattern onto the substrate, the optical element comprising: [0014]
a base of the optical element which is installed to an end of the
projection optical system on a side of the substrate and through
which the exposure is performed in a state that a liquid is
maintained between the optical element and the substrate; and
[0015] a corrosion resistant film which is formed on at least a
part of a surface of the base of the optical element to avoid
corrosion by the liquid.
[0016] The corrosion resistant film is formed on the surface of the
base of the optical element of the present invention. Therefore,
even when the liquid immersion exposure is performed, it is
possible to avoid, for example, the corrosion, the erosion, and the
dissolution which would be otherwise cause by the contact between
the optical element and the liquid. Therefore, the desired
performance of the projection optical system can be maintained over
a long term even when the full field exposure such as those in the
step-and-repeat manner or the scanning type exposure such as those
in the step-and-scan manner, in which the optical element installed
to the end of the projection optical system is exposed to the
liquid repeatedly or continuously, is performed in the liquid
immersion state.
[0017] According to a second aspect of the present invention, there
is provided an exposure apparatus which exposes a substrate by
projecting an image of a predetermined pattern onto the substrate
through a liquid, the exposure apparatus comprising: [0018] a
projection optical system which projects the image of the pattern
onto the substrate; [0019] an optical element which is installed to
an end of the projection optical system on a side of the substrate;
and an apparatus which supplies the liquid to a space between the
optical element and the substrate, wherein: [0020] the optical
element includes a base, and a corrosion resistant film which is
formed on at least a part of a surface of the base to avoid
corrosion of the base.
[0021] The corrosion resistant film is formed on the surface of the
base of the optical element installed to the tip of the projection
optical system of the exposure apparatus of the present invention.
Therefore, even when the liquid immersion exposure is performed, it
is possible to avoid, for example, the corrosion, the erosion, and
the dissolution which would be otherwise cause by the contact
between the optical element and the liquid. Therefore, the desired
optical characteristics of the exposure apparatus can be maintained
over a long term even when the full field exposure such as those
based on the step-and-repeat system or the scanning type exposure
such as those based on the step-and-scan system, in which the
optical element installed to the end of the projection optical
system is exposed to the liquid repeatedly or continuously, is
performed in the liquid immersion state. Accordingly, it is
possible to realize the exposure in a state in which the wide depth
of focus is maintained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 shows a schematic arrangement of a projection
exposure apparatus according to the present invention.
[0023] FIG. 2 shows a positional relationship between an end
portion 4A of an optical element 4 of a projection optical system
PL shown in FIG. 1 and discharge nozzles and inflow nozzles for the
X direction.
[0024] FIG. 3 shows a positional relationship between the end
portion 4A of the optical element 4 of the projection optical
system PL shown in FIG. 1 and discharge nozzles and inflow nozzles
for supplying and recovering the liquid in the Y direction.
[0025] FIG. 4 shows a magnified view illustrating major parts to
depict a situation of the supply and the recovery of the liquid 7
with respect to the space between the optical element 4 and a wafer
W shown in FIG. 1.
[0026] FIG. 5 shows a front view illustrating, for example, a lower
end portion of a projection optical system PLA of a projection
exposure apparatus, a liquid supply unit 5, and a liquid recovery
unit 6 to be used in a second embodiment of the present
invention.
[0027] FIG. 6 shows a positional relationship between an end
portion 32A of an optical element 32 of the projection optical
system PLA shown in FIG. 5 and discharge nozzles and inflow nozzles
for the X direction.
[0028] FIG. 7 shows a positional relationship between the end
portion 32A of the optical element 32 of the projection optical
system PLA shown in FIG. 5 and discharge nozzles and inflow nozzles
for supplying and recovering the liquid in the Y direction.
[0029] FIG. 8 shows a schematic arrangement of the optical element
of the present invention.
[0030] FIG. 9 shows a relationship between the angle of incidence
and the reflectance of the ArF excimer laser (wavelength: 193 nm)
when an optical element is constructed by only fluorite.
[0031] FIG. 10 shows a relationship between the angle of incidence
and the reflectance of the ArF excimer laser (wavelength: 193 nm)
when an optical element 105 has respective layers formed on a
fluorite base.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
[0032] An explanation will be made below with reference to the
drawings about embodiments of the optical element of the present
invention and the projection exposure apparatus based on the use of
the optical element. However, the present invention is not limited
thereto.
[0033] At first, the optical element of the present invention will
be explained with reference to FIGS. 8 to 10. FIG. 8 shows a
cross-sectional structure of an optical element 105 of the present
invention. The optical element 105 includes an SiO.sub.2 layer 102,
an Al.sub.2O.sub.3 layer 103, and an SiO.sub.2 layer 104 which are
stacked in this order on a fluorite (CaF.sub.2) base 101. These
layers or the an SiO.sub.2 layer disposed on the outermost surface
functions as the corrosion resistant film (erosion resistant film).
The fluorite base 101 is formed to be lens-shaped, and it has a
thickness of 20 mm. The SiO.sub.2 layer 102, the Al.sub.2O.sub.3
layer 103, and the SiO.sub.2 layer 104 are formed so that the
optical thicknesses are 0.37.lamda., 0.05.lamda., and 0.37.lamda.
with respect to the designed main wavelength .lamda. (for example,
.lamda.=193 nm) respectively. A known sputtering method was used
for each of the layers so that the dense film was successfully
formed. The film formation method is not limited to the sputtering
method. It is also allowable to use, for example, the ion beam
assist method, the ion plating method, and the heating vapor
deposition method provided that the dense film can be formed. The
corrosion resistant films as described above may be provided on the
both sides of the base 101 respectively. Alternatively, the
corrosion resistant film as described above may be provided at only
the portion of the base 101 to be irradiated with the light
beam.
Investigation of Reflectance Characteristics
[0034] The reflectance characteristics were investigated in
relation to the angle of incidence of the light beam into the
optical element 105 obtained as described above. In order to make
comparison with the optical element 105 (hereinafter referred to as
"optical element A"), a fluorite base stacked with neither the
SiO.sub.2 layer nor the Al.sub.2O.sub.3 layer, i.e., an optical
element composed of only the fluorite base (hereinafter referred to
as "optical element B") was prepared. The ArF excimer laser beam
having a wavelength of 193 nm, which was used as the exposure light
beam for the exposure apparatus, was radiated onto the optical
element A and the optical element B respectively while changing the
angle of incidence to measure the reflectances. Obtained results
are shown in FIGS. 9 and 10. The reflected light beams were
measured for the S-polarized light and the P-polarized light which
were orthogonal to one another.
[0035] FIG. 9 shows a graph illustrating the reflectance
characteristic of the optical element B with respect to the angle
of incidence. As shown in FIG. 9, the average reflectance of the
S-polarized light and the P-polarized light in relation to the
optical element B was not more than about 0.04% in all regions
ranging to the maximum angle of incidence of 40 degrees in which
the optical element was to be used.
[0036] FIG. 10 shows a graph illustrating the reflectance
characteristic of the optical element A with respect to the angle
of incidence. As shown in FIG. 10, the average reflectance of the
S-polarized light and the P-polarized light was not more than about
0.04% in all regions ranging to the maximum angle of incidence of
40 degrees in which the optical element was to be used.
[0037] That is, the optical element A exhibits the low values of
the average reflectance of the S-polarized light and the
P-polarized light in all regions of the angle of incidence assumed
for the use as the optical element, in the same manner as the
optical element B composed of only fluorite. It is appreciated that
the optical element A can be carried at the end portion of the
projection optical system of the projection exposure apparatus in
place of the optical element B.
Evaluation of Corrosion Resistance
[0038] Next, an acceleration test was carried out for the corrosion
resistance by immersing the optical element A and the optical
element B in pure water at 70.degree. C. for 3 hours respectively.
The surface roughnesses of the optical elements were measured by
using an AFM (atomic force microscope) and a contact type roughness
meter. It is assumed that the immersion in pure water at 70.degree.
C. for 3 hours corresponds to the immersion in pure water at room
temperature for about 10 days.
[0039] The surface roughness, which was obtained before immersing
the optical element B composed of only fluorite in pure water, was
3 angstrom RMS. The surface roughness, which was obtained after
immersing the optical element B in pure water, was about 3,000
angstrom RMS. Therefore, it is understood that the optical element
B was corroded to have the about 1,000 times surface roughness.
When the surface roughness of the surface of the optical element is
3,000 angstrom RMS as described above, the scattering of light is
caused to a large extent. Therefore, the transmittance of the
optical element is lowered, and any deviation arises from the
designed optical path. Therefore, it is impossible to exhibit any
intended desired optical performance.
[0040] On the other hand, the surface roughness of the optical
element A according to the present invention before being immersed
in pure water was 11 angstrom RMS. The surface roughness of the
optical element after being immersed in pure water was 14 angstrom
RMS. Therefore, it is appreciated that the surface roughness of the
optical element is scarcely changed before and after being immersed
in pure water. Therefore, the optical element A can maintain the
desired optical performance after being immersed in pure water,
i.e., even when the liquid immersion exposure is performed,
probably for the following reason. That is, it is considered that
the oxide coating film, which is formed on the surface of the
fluorite base, prevents the fluorite base from corrosion, because
the oxide coating film has the corrosion resistance against pure
water.
[0041] In the optical element A of this embodiment, the
three-layered multilayer film composed of the oxides is formed on
the fluorite base. However, it has been revealed that the same or
equivalent effect is obtained even when a single layer film of, for
example, SiO.sub.2 (optical film thickness: 0.55.lamda.) or
Al.sub.2O.sub.3 is formed. The film thickness of the multilayer
film or the single layer film having the corrosion resistance is
not specifically limited. However, it is desirable that the film
thickness is 50 angstroms to 2,000 angstroms in view of the fact
that the covering performance of the film on the surface of the
fluorite base is secured, and the angular reflection-preventive
performance is secured.
[0042] In this embodiment, the SiO.sub.2 layer and the
Al.sub.2O.sub.3 layer are formed on the fluorite base. However, in
place of these layers or together with these layers, a layer or
layers of fluoride or fluorides such as YF.sub.3, MgF.sub.2, and
LaF.sub.3 may be formed singly or in combination.
[0043] Next, an explanation will be made with reference to FIGS. 1
to 4 about a second embodiment illustrative of a projection
exposure apparatus to which the optical element of the embodiment
described above is applied. The projection exposure apparatus of
this embodiment is a projection exposure apparatus based on the
step-and-repeat system for performing the full field exposure for
the shot area on the substrate.
[0044] FIG. 1 shows a schematic arrangement of the projection
exposure apparatus of this embodiment. With reference to FIG. 1,
the exposure light beam IL, which is composed of an ultraviolet
pulse light beam having a wavelength of 193 nm, is radiated from an
illumination optical system 1 including, for example, an ArF
excimer laser light source as an exposure light source, an optical
integrator (homogenizer), a field diaphragm, and a condenser lens.
The exposure light beam IL illuminates a pattern provided on a
reticle R. The pattern of the reticle R is subjected to the
reduction projection onto an exposure area on a wafer W coated with
a photoresist at a predetermined projection magnification .beta.
(.beta. is, for example, 1/4 or 1/5) via a projection optical
system PL which is telecentric on the both sides (or on one side of
the wafer W). Those usable as the exposure light beam IL also
include, for example, the KrF excimer laser beam (wavelength: 248
nm), the F.sub.2 laser beam (wavelength: 157 nm), and the i-ray
(wavelength: 365 nm) of the mercury lamp.
[0045] In the following description, it is assumed that the Z axis
extends in the direction parallel to the optical axis AX of the
projection optical system PL, the Y axis extends in the direction
perpendicular to the sheet surface of FIG. 1 in the plane
perpendicular to the Z axis, and the X axis extends in parallel to
the sheet surface of FIG. 1.
[0046] The reticle R is held on a reticle stage RST. A mechanism,
which finely moves the reticle R in the X direction, the Y
direction, and the rotational direction, is incorporated in the
reticle stage RST. The two-dimensional position and the angle of
rotation of the reticle stage RST are measured in real-time by a
laser interferometer (not shown). A main control system 14
positions the reticle R on the basis of the value measured by the
laser interferometer.
[0047] On the other hand, the wafer W is fixed on a Z stage 9 which
controls the focus position (position in the Z direction) and the
angle of inclination of the wafer W by the aid of a wafer holder
(not shown). The Z stage 9 is fixed on an XY stage 10 which is
movable along the XY plane that is substantially parallel to the
image plane of the projection optical system PL. The XY stage 10 is
placed on a base 11. The Z stage 9 controls the focus position
(position in the Z direction) and the angle of inclination of the
wafer W to adjust and match the surface of the wafer W with respect
to the image plane of the projection optical system PL in the
auto-focus manner and the auto-leveling manner. The XY stage 10
positions the wafer W in the X direction and the Y direction. The
two-dimensional position and the angle of rotation of the Z stage 9
(wafer W) are measured in real-time as a position of a movement
mirror 12 by a laser interferometer 13. The control information is
fed from the main control system 14 to a wafer stage-driving system
15 on the basis of the measured result. The wafer stage-driving
system 15 controls the operation of the Z stage 9 and the XY stage
10 on the basis of the control information. During the exposure,
the operation, in which each of the shot areas on the wafer W is
successively subjected to the stepping movement to the exposure
position to perform the exposure with the pattern image of the
reticle R, is repeated in the step-and-repeat manner.
[0048] The liquid immersion method is applied to the projection
exposure apparatus of this embodiment in order that that the
resolution is improved by substantially shortening the exposure
wavelength, and the depth of focus is substantially widened.
Therefore, the space between the surface of the wafer W and the tip
surface (lower surface) of the projection optical system PL is
filled with a predetermined liquid 7 at least during the period in
which the pattern image of the reticle R is transferred onto the
wafer W. The projection optical system PL has a plurality of
optical elements which include the optical element 4 as produced in
the embodiment described above, and a barrel 3 which accommodates
the optical elements. The optical element 4 is installed so that
the optical element 4 is exposed at the end (lower end) of the
barrel on the side of the wafer (see FIGS. 4 and 5). When the
optical element 4 is installed as described above, only the optical
element 4 makes contact with the liquid 7. Accordingly, the barrel
3, which is composed of metal, is prevented from the corrosion or
the like. In this embodiment, pure water is used as the liquid 7.
Pure water is advantageous in that pure water is available in a
large amount with ease, for example, in the semiconductor
production factory, and pure water exerts no harmful influence, for
example, on the optical lens and the photoresist on the wafer.
Further, pure water exerts no harmful influence on the environment,
and the content of impurity is extremely low. Therefore, it is also
expected to obtain the function to wash the surface of the
wafer.
[0049] It is approved that the refractive index n of pure water
(water) with respect to the exposure light beam having a wavelength
of about 200 nm is approximately in an extent of 1.44 to 1.47. The
wavelength of the ArF excimer laser beam of 193 nm is shortened on
the wafer W by 1/n, i.e., to about 131 to 134 nm, and a high
resolution is obtained. Further, the depth of focus is magnified
about n times, i.e., about 1.44 to 1.47 times as compared with the
value obtained in the air. Therefore, when it is enough to secure
an approximately equivalent depth of focus as compared with the
case of the use in the air, it is possible to further increase the
numerical aperture of the projection optical system PL. Also in
this viewpoint, the resolution is improved.
[0050] The liquid 7 is supplied in a temperature-controlled state
onto the wafer W by the aid of predetermined discharge nozzles or
the like by a liquid supply unit 5 including, for example, a tank
for accommodating the liquid, a pressurizing pump, and a
temperature control unit. The liquid 7, which has been supplied
onto the wafer W, is recovered by the aid of predetermined inflow
nozzles or the like by a liquid recovery unit 6 including, for
example, a tank for accommodating the liquid, and a suction pump.
The temperature of the liquid 7 is set, for example, to be
approximately equivalent to the temperature of a chamber in which
the projection exposure apparatus of this embodiment is
accommodated. The projection exposure apparatus of this embodiment
is arranged with a discharge nozzle 21a having a thin tip section
and two inflow nozzles 23a, 23b having wide tip sections so that
the end portion of the optical element 4 of the projection optical
system PL is interposed in the X direction (see FIG. 2). The
discharge nozzle 21a is connected to the liquid supply unit 5 via a
supply tube 21, and the inflow nozzles 23a, 23b are connected to
the liquid recovery unit 6 via a recovery tube 23. Further, another
set of discharge and recovery nozzles are arranged at positions
obtained by rotating the positions of the set of the discharge
nozzle 21a and the inflow nozzles 23a, 23b by substantially
180.degree. about the center of the end portion of the optical
element 4, and two sets of discharge and recovery nozzles are also
arranged so that the end portion of the optical element 4 is
interposed in the Y direction (see FIGS. 2 and 3).
[0051] FIG. 2 shows a positional relationship in relation to the
end portion 4A of the optical element 4 of the projection optical
system PL shown in FIG. 1, the wafer W, and the two sets of the
discharge nozzles and the inflow nozzles to interpose the end
portion 4A in the X direction. With reference to FIG. 2, the
discharge nozzle 21a is arranged on the side in the +X direction
with respect to the end portion 4A, and the inflow nozzles 23a, 23b
are arranged on the side in the -X direction. The inflow nozzles
23a, 23b are arranged in a sector-shaped open form with respect to
the axis which passes through the center of the end portion 4A and
which is parallel to the X axis. Another set of the discharge
nozzle 22a and the inflow nozzles 24a, 24b are arranged at the
positions obtained by rotating the positions of the set of the
discharge nozzle 21a and the inflow nozzles 23a, 23b by
substantially 180.degree. about the center of the end portion 4A.
The discharge nozzle 22a is connected to the liquid supply unit 5
via a supply tube 22, and the inflow nozzles 24a, 24b are connected
to the liquid recovery unit 6 via a recovery tube 24.
[0052] FIG. 3 shows a positional relationship in relation to the
end portion 4A of the optical element 4 of the projection optical
system PL shown in FIG. 1 and the two sets of the discharge nozzles
and the inflow nozzles to interpose the end portion 4A in the Y
direction. With reference to FIG. 3, the discharge nozzle 27a is
arranged on the side in the +Y direction with respect to the end
portion 4A, and the inflow nozzles 29a, 29b are arranged on the
side in the -Y direction. The discharge nozzle 27a is connected to
the liquid supply unit 5 via a supply tube 27, and the inflow
nozzles 29a, 29b are connected to the liquid recovery unit 6 via a
recovery tube 29. Another set of the discharge nozzle 28a and the
inflow nozzles 30a, 30b are arranged at the positions obtained by
rotating the positions of the set of the discharge nozzle 27a and
the inflow nozzles 29a, 29b by substantially 180.degree. about the
center of the end portion 4A. The discharge nozzle 28a is connected
to the liquid supply unit 5 via a supply tube 28, and the inflow
nozzles 30a, 30b are connected to the liquid recovery unit 6 via a
recovery tube 30. The liquid supply unit 5 supplies the
temperature-controlled liquid to the space between the wafer W and
the end portion 4A of the optical element 4 via at least one of the
supply tubes 21, 22, 27, 28. The liquid recovery unit 6 recovers
the liquid supplied onto the wafer W via at least one of the
recovery tubes 23, 24, 29, 30.
[0053] Next, an explanation will be made about a supply method and
a recovery method for the liquid 7.
[0054] With reference to FIG. 2, when the wafer W is subjected to
the stepping movement in the direction of the arrow 25A indicated
by the solid line (-X direction), the liquid supply unit 5 supplies
the liquid 7 to the space between the wafer W and the end portion
4A of the optical element 4 via the supply tube 21 and the
discharge nozzle 21a. The liquid recovery unit 6 recovers the
liquid 7 from the surface of the wafer W via the recovery nozzle 23
and the inflow nozzles 23a, 23b. In this situation, the liquid 7
flows in the direction of the arrow 25B (-X direction) on the wafer
W. The space between the wafer W and the optical element 4 is
filled with the liquid 7 in a stable state.
[0055] On the other hand, when the wafer W is subjected to the
stepping movement in the direction of the arrow 26A indicated by
the two-dot chain line (+X direction), then the liquid supply unit
5 supplies the liquid 7 to the space between the wafer W and the
end portion 4A of the optical element 4 by using the supply tube 22
and the discharge nozzle 22a, and the liquid recovery unit 6
recovers the liquid 7 by using the recovery tube 24 and the inflow
nozzles 24a, 24b. In this situation, the liquid 7 flows in the
direction of the arrow 26B (+X direction) on the wafer W. The space
between the wafer W and the optical element 4 is filled with the
liquid 7 in a stable state. As described above, the projection
exposure apparatus of this embodiment is provided with the two sets
of the discharge nozzles and the inflow nozzles which are inverted
to one another in the X direction. Therefore, even when the wafer W
is moved in any one of the +X direction and the -X direction, the
space between the wafer W and the optical element 4 can be filled
with the liquid 7 in the stable state.
[0056] In the exposure apparatus of this embodiment, the liquid 7
flows on the wafer W. Therefore, even when any foreign matter is
adhered onto the wafer W, the foreign matter can be washed out with
the liquid 7. The liquid 7 is adjusted to have a predetermined
temperature by the liquid supply unit 5. Therefore, the surface of
the wafer W is temperature-adjusted, and it is possible to avoid
the decrease in the overlay accuracy or the like which would be
otherwise caused by the thermal expansion of the wafer brought
about by the heat generated during the exposure. Therefore, even
when a certain period of time is required from the alignment to the
exposure as in the EGA (enhanced global alignment) system, it is
possible to avoid the decrease in the overlay accuracy which would
be otherwise caused by the thermal expansion of the wafer possibly
brought about during such a period. In the projection exposure
apparatus of this embodiment, the liquid 7 flows in the same
direction as the direction in which the wafer W is moved.
Therefore, the liquid, which has absorbed the foreign matter and
the heat, can be recovered without allowing the liquid to stay on
the exposure area disposed just under the end portion 4A of the
optical element 4.
[0057] When the wafer W is subjected to the stepping movement in
the Y direction, the liquid 7 is supplied and recovered in the Y
direction. That is, when the wafer is subjected to the stepping
movement in the direction of the arrow 31A (-Y direction) indicated
by the solid line in FIG. 3, then the liquid supply unit 5 supplies
the liquid via the supply tube 27 and the discharge nozzle 27a, and
the liquid recovery unit 6 recovers the liquid by using the
recovery tube 29 and the inflow nozzles 29a, 29b. Accordingly, the
liquid flows in the direction of the arrow 31B (-Y direction) on
the exposure area disposed just under the end portion 4A of the
optical element 4. When the wafer is subjected to the stepping
movement in the direction of the arrow 33A (+Y direction) indicated
by the two-dot chain line, the liquid is supplied and recovered by
using the supply tube 28, the discharge nozzle 28a, the recovery
tube 30, and the inflow nozzles 30a, 30b. Accordingly, the liquid
flows in the direction of the arrow 33B (+Y direction) on the
exposure area disposed just under the end portion 4A. Therefore,
even when the wafer W is moved in any one of the +Y direction and
the -Y direction, the space between the wafer W and the end portion
4A of the optical element 4 can be filled with the liquid 7 in a
stable state, in the same manner as in the case in which the wafer
W is moved in the X direction.
[0058] There is no limitation to the provision of the nozzles for
supplying and recovering the liquid 7 in the X direction and/or the
Y direction. It is also allowable to provide nozzles for supplying
and recovering the liquid 7, for example, in oblique
directions.
[0059] Next, an explanation will be made about a method for
controlling the supply amount and the recovery amount of the liquid
7. FIG. 4 shows a situation in which the liquid is supplied to and
recovered from the space between the wafer W and the optical
element 4 of the projection optical system PL. With reference to
FIG. 4, the wafer W is moved in the direction of the arrow 25A (-X
direction). The liquid 7, which is supplied by the discharge nozzle
21a, flows in the direction of the arrow 25B (-X direction), and
the liquid 7 is recovered by the inflow nozzles 23a, 23b. In order
to maintain a constant amount of the liquid 7 existing between the
optical element 4 and the wafer W even during the movement of the
wafer W, the supply amount Vi (m.sup.3/s) of the liquid 7 is equal
to the recovery amount Vo (m.sup.3/s) thereof in this embodiment.
Further, the supply amount Vi and the recovery amount Vo of the
liquid 7 are adjusted to be proportional to the movement velocity v
of the XY stage (wafer W). That is, the main control system 14
determines the supply amount Vi and the recovery amount Vo of the
liquid 7 in accordance with the following expression. Vi=Vo=Dvd
(3)
[0060] In this expression, as shown in FIG. 1, D represents the
diameter (m) of the end portion of the optical element 4, v
represents the movement velocity (m/s) of the XY stage, and d
represents the working distance (distance between the lowermost
surface of the optical element 4 and the surface of the wafer W)
(m) of the projection optical system PL. The velocity v, at which
the XY stage 10 is subjected to the stepping movement, is set by
the main control system 14. D and d are previously inputted into
(stored in) the main control system 14. Therefore, when the supply
amount Vi of the liquid 7 and the recovery amount Vo thereof are
adjusted on the basis of the expression (3), a state is given, in
which the space between the wafer W and the optical element 4 shown
in FIG. 4 is always filled with the liquid 7.
[0061] It is desirable that the working distance d of the
projection optical system PL is made as narrow as possible in order
that the liquid 7 stably exists between the projection optical
system PL and the wafer W. However, if the working distance d is
too small, it is feared that the surface of the wafer W may make
contact with the optical element 4. Therefore, it is necessary to
provide a margin to some extent. Accordingly, the working distance
d is set to be, for example, about 2 mm.
[0062] Next, a third embodiment of the present invention will be
explained with reference to FIGS. 5 to 7. In this embodiment, the
optical element of the embodiment described above is applied to a
projection exposure apparatus based on the step-and-scan
system.
[0063] FIG. 5 shows a front view illustrating, for example, a lower
portion of a projection optical system PLA of the projection
exposure apparatus of this embodiment, a liquid supply unit 5, and
a liquid recovery unit 6. The same or equivalent constitutive
components as those shown in FIG. 4 are designated by the same
reference numerals. With reference to FIG. 5, an optical element
32, which is disposed at a lowermost end of a barrel 3A of the
projection optical system PLA, has an end portion 32A which is
formed so that the end portion has a rectangular shape which is
long in the Y direction (non-scanning direction) and which has a
necessary portion for the scanning exposure. The optical element 32
is such an optical element that a corrosion resistant film, which
is equivalent to that of the optical element produced in the first
embodiment, is provided on a fluorite base. During the scanning
exposure, a part of a pattern image of the reticle is projected
onto a rectangular exposure area disposed just under the end
portion 32A. The reticle (not shown) is moved at a velocity V in
the -X direction (or in the +X direction) with respect to the
projection optical system PLA, in synchronization with which the
wafer W is moved at a velocity .beta.V (.beta. represents the
projection magnification) in the +X direction (or in the -X
direction) by the aid of the XY stage 10. After the exposure is
completed for one shot area, the next shot area is moved to the
scanning start position in accordance with the stepping of the
wafer W. In the following procedure, the exposure is successively
performed for respective shot areas in the step-and-scan
manner.
[0064] The liquid immersion method is also applied in this
embodiment, and thus the space between the optical element 32 and
the surface of the wafer W is filled with the liquid 7 during the
scanning exposure. The liquid 7 is supplied and recovered by using
the liquid supply unit 5 and the liquid recovery unit 6
respectively in the same manner as in the second embodiment.
[0065] FIG. 6 shows a positional relationship between the end
portion 32A of the optical element 32 of the projection optical
system PLA and the discharge nozzles and the inflow nozzles for
supplying and recovering the liquid 7 in the X direction. With
reference to FIG. 6, the end portion 32A of the optical element 32
has a rectangular shape which is long in the Y direction. The three
discharge nozzles 21a to 21c are arranged on the side in the +X
direction, and the two inflow nozzles 23a, 23b are arranged on the
side in the -X direction so that the end portion 32A of the optical
element 32 of the projection optical system PLA is interposed in
the X direction.
[0066] The discharge nozzles 21a to 21c are connected to the liquid
supply unit 5 via a supply tube 21, and the inflow nozzles 23a, 23b
are connected to the liquid recovery unit 6 via a recovery tube 23.
The discharge nozzles 22a to 22c and the recovery nozzles 24a, 24b
are arranged at positions obtained by rotating the positions of the
discharge nozzles 21a to 21c and the recovery nozzles 23a, 23b by
substantially 180.degree. about the center of the end portion 32A.
The discharge nozzles 21a to 21c and the inflow nozzles 24a, 24b
are arranged alternately in the Y direction, and the discharge
nozzles 22a to 22c and the inflow nozzles 23a, 23b are arranged
alternately in the Y direction. The discharge nozzles 22a to 22c
are connected to the liquid supply unit 5 via a supply tube 22, and
the inflow nozzles 24a, 24b are connected to the liquid recovery
unit 6 via a recovery tube 24.
[0067] When the wafer W is moved in the scanning direction (-X
direction) indicated by the solid line arrow to perform the
scanning exposure, the liquid 7 is supplied and recovered by the
liquid supply unit 5 and the liquid recovery unit 6 by using the
supply tube 21, the discharge nozzles 21a to 21c, the recovery tube
23, and the inflow nozzles 23a, 23b. The liquid 7 is allowed to
flow in the -X direction so that the space between the optical
element 32 and the wafer W is filled therewith. When the wafer W is
moved in the direction (+X direction) indicated by the two-dot
chain line arrow to perform the scanning exposure, the liquid 7 is
supplied and recovered by using the supply tube 22, the discharge
nozzles 22a to 22c, the recovery tube 24, and the inflow nozzles
24a, 24b. The liquid 7 is allowed to flow in the +X direction so
that the space between the optical element 32 and the wafer W is
filled therewith. When the direction, in which the liquid 7 is
allowed to flow, is switched depending on the scanning direction,
the space between the wafer W and the end portion 32A of the
optical element 32 can be filled with the liquid 7, even when the
wafer W is subjected to the scanning exposure in any one of the +X
direction and the -X direction. Accordingly, the exposure can be
performed at a high resolution and a wide depth of focus.
[0068] The supply amount Vi (m.sup.3/s) of the liquid 7 and the
recovery amount Vo (m.sup.3/s) thereof are determined in accordance
with the following expression. Vi=Vo=DSYvd (4)
[0069] In this expression, DSY represents the length (m) of the end
portion 32A of the optical element 32 in the X direction.
Accordingly, the space between the optical element 32 and the wafer
W can be filled with the liquid 7 in a stable state even during the
scanning exposure.
[0070] The number and the shapes of the nozzles are not
specifically limited. For example, the liquid 7 may be supplied and
recovered by using two pairs of nozzles for the long side of the
end portion 32A. In this case, the discharge nozzles and the inflow
nozzles may be arranged while being aligned vertically in order
that the liquid can be supplied and recovered in any one of the +X
direction and the -X direction.
[0071] When the wafer W is subjected to the stepping movement in
the Y direction, the liquid 7 is supplied and recovered in the Y
direction in the same manner as in the second embodiment.
[0072] FIG. 7 shows a positional relationship between the end
portion 32A of the optical element 32 of the projection optical
system PLA and the discharge nozzles and the inflow nozzles for the
Y direction. With reference to FIG. 7, when the wafer is subjected
to the stepping movement in the non-scanning direction (-Y
direction) perpendicular to the scanning direction, the liquid 7 is
supplied and recovered by using the discharge nozzle 27a and the
inflow nozzles 29a, 29b arranged in the Y direction. When the wafer
is subjected to the stepping movement in the +Y direction, the
liquid 7 is supplied and recovered by using the discharge nozzle
28a and the inflow nozzles 30a, 30b arranged in the Y direction.
The supply amount Vi (m.sup.3/s) of the liquid 7 and the recovery
amount Vo (m.sup.3/s) thereof are determined in accordance with the
following expression. Vi=Vo=DSXvd (5)
[0073] In this expression, DSX represents the length (m) of the end
portion 32A of the optical element 32 in the Y direction. The space
between the optical element 32 and the wafer W can be continuously
filled with the liquid 7 by adjusting the supply amount of the
liquid 7 depending on the movement velocity v of the wafer W when
the stepping movement is performed in the Y direction as well, in
the same manner as in the second embodiment.
[0074] As described above, when the wafer W is moved, the liquid is
allowed to flow in the direction corresponding to the direction of
the movement. Accordingly, the space between the wafer W and the
end portion of the projection optical system PL can be continuously
filled with the liquid 7.
[0075] The liquid, which is usable as the liquid 7 in the
embodiments described above, is not specifically limited to pure
water. It is possible to use liquids (for example, cedar oil) which
have the transmittance with respect to the exposure light beam,
which have the refractive index as high as possible, and which are
stable against the photoresist coated to the surface of the wafer
and the projection optical system.
[0076] It is a matter of course that the present invention is not
limited to the embodiments described above, which may be embodied
in other various forms without deviating from the gist or essential
characteristics of the present invention.
[0077] The base of the optical element of the present invention is
lens-shaped. However, there is no limitation thereto. It is also
allowable to use those each of which is formed as a film on a
fluorite plate-shaped base as a cover glass to be disposed between
the liquid and the conventional fluorite lens.
[0078] According to the projection exposure apparatus of the
present invention, the end portion of the projection optical system
is not corroded by the liquid. Therefore, the operation of the
apparatus is not stopped in order to exchange the corroded optical
element. Accordingly, it is possible to efficiently produce final
products having fine patterns. Further, the optical characteristics
of the optical element of the present invention are stable, because
the optical element is not corroded. When the projection exposure
apparatus, which carries the optical element of the present
invention, is used, it is possible to produce final products having
stable qualities.
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