U.S. patent number RE38,320 [Application Number 09/809,224] was granted by the patent office on 2003-11-18 for projection exposure apparatus wherein focusing of the apparatus is changed by controlling the temperature of a lens element of the projection optical system.
This patent grant is currently assigned to Nikon Corporation. Invention is credited to Tohru Kiuchi, Seiro Murakami, Kenji Nishi, Yasuaki Tanaka, Kazuo Ushida.
United States Patent |
RE38,320 |
Nishi , et al. |
November 18, 2003 |
Projection exposure apparatus wherein focusing of the apparatus is
changed by controlling the temperature of a lens element of the
projection optical system
Abstract
A projection optical system of a projection exposure apparatus
according to the present invention has a plurality of optical
members made of glass materials at least one of which has a
temperature characteristic of index of refraction different from
that of the other glass material. Further, a temperature control
device for controlling a temperature of at least one of the optical
members is provided. An imaging characteristic of the projection
optical system is controlled. The imaging characteristic to be
controlled is a non-linear magnification or curvature of field. The
temperature control device sets the temperature to be controlled to
a variable target temperature determined in accordance with the
imaging characteristic of the projection optical system. An
exposing operation for transferring a mask pattern to a
photosensitive substrate is started after the temperature of the
optical member to be controlled reaches a predetermined allowable
range of the target temperature.
Inventors: |
Nishi; Kenji (Kanagawa-ken,
JP), Ushida; Kazuo (Tokyo, JP), Murakami;
Seiro (Chiba-ken, JP), Kiuchi; Tohru (Tokyo,
JP), Tanaka; Yasuaki (Kanagawa-ken, JP) |
Assignee: |
Nikon Corporation (Tokyo,
JP)
|
Family
ID: |
27529266 |
Appl.
No.: |
09/809,224 |
Filed: |
March 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
689233 |
Aug 6, 1996 |
05883704 |
Mar 16, 1999 |
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Foreign Application Priority Data
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Aug 7, 1995 [JP] |
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7-200800 |
Aug 9, 1995 [JP] |
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7-203277 |
Sep 11, 1995 [JP] |
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7-232274 |
Sep 12, 1995 [JP] |
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7-233851 |
Nov 10, 1995 [JP] |
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7-292362 |
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Current U.S.
Class: |
355/67; 355/53;
355/55; 359/820 |
Current CPC
Class: |
G03F
7/70883 (20130101); G03B 27/54 (20130101); G03B
27/26 (20130101); G03F 7/70891 (20130101) |
Current International
Class: |
G03B
27/54 (20060101); G03F 7/20 (20060101); G03B
027/54 (); G03B 027/42 (); G03B 027/52 () |
Field of
Search: |
;355/53,55,57,67,72,77
;359/820 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Adams; Russell
Assistant Examiner: Kim; Peter B.
Attorney, Agent or Firm: Oliff & Berridge PLC
Claims
What is claimed is:
1. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
.[.wherein a mask pattern is projected to a photosensitive
substrate,.]. comprising: a projection optical system having a
plurality of optical members made of glass materials, at least a
first optical member having an index of refraction responsive to
temperature in a different manner from that of a second optical
member; and a temperature control device which is provided in the
vicinity of at least one of said optical members and controls the
temperature of said at least one of said optical members.[.,
wherein, an imaging characteristic of said projection optical
system is controlled by using said temperature control device.].
.
2. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
according to claim 1, wherein .[.the.]. .Iadd.an .Iaddend.imaging
characteristic .[.to be.]. .Iadd.that is .Iaddend.controlled by
said temperature control device is a non-linear magnification
error.
3. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
according to claim 2, wherein the non-linear magnification error of
said projection optical system is corrected within a range of
.+-.50 nm by changing the temperature of the optical member to be
controlled by .+-.1.degree. C.
4. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
according to claim 2, further comprising a linear magnification
control device for correcting a linear magnification error of said
projection optical system, and wherein the linear magnification
error remaining after the non-linear magnification error of said
projection optical system is corrected to fall in a predetermined
allowable range by said temperature control device is reduced by
said linear magnification control device.
5. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
according to claim 1, further comprising a memory system for
storing a change amount of .[.the.]. .Iadd.an .Iaddend.imaging
characteristic of said projection optical system in accordance with
the change in an applied condition of said projection optical
system, and wherein the imaging characteristic of said projection
optical system is controlled by said temperature control device so
that the change amount of the imaging characteristic stored in said
memory system is canceled in accordance with the change in the
applied condition of said projection optical system.
6. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
according to claim .[.1.]. .Iadd.2.Iaddend., wherein the imaging
characteristic is a position of an image plane of said projection
optical system in the direction of an optical axis of said
projection optical system.
7. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
according to claim 6, wherein a curvature of field of said
projection optical system is controlled by said temperature control
device, and further comprising a magnification error control device
for controlling a magnification error of said projection optical
system, and wherein the magnification error generated when the
curvature of field of said projection optical system is controlled
by said temperature control device is reduced by said magnification
error control device.
8. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
according to claim 7, wherein the curvature of field of said
projection optical system is corrected to fall in a range of
.+-.0.3 .mu.m or less by controlling the temperature of said
optical member to be controlled within a range of .+-.1.degree. C.
or less by means of said temperature control device.
9. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
according to claim 6, further comprising a memory system for
storing a change amount of a position of the image plane of said
projection optical system in accordance with the change in an
applied condition of said projection optical system, and wherein
the position of the image plane of said projection optical system
is controlled by said temperature control device so that the change
amount of the position of the image plane stored in said memory
system is canceled in accordance with the change in the applied
condition of said projection optical system.
10. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
according to claim 6, further comprising a focus position control
device for shifting the image plane of said projection optical
system and said photosensitive substrate relative to each other in
the direction of the optical axis of said projection optical
system, and wherein offset of the focus position remaining after
the position of the image plane of said projection optical system
is controlled by said temperature control device is reduced by said
focus position control device.
11. A projection exposure apparatus wherein a mask pattern is
projected to a photosensitive substrate, comprising: a projection
optical system having a plurality of optical members made of glass
material, at least a first optical member having an index of
refraction responsive to temperature in a different manner from
that of a second optical member; a temperature control device which
is provided in the vicinity of at least one of said optical members
and sets a temperature of said at least one of said optical members
to a variable target temperature determined by a focusing feature
of said projection optical system; and an exposure control device
for starting an exposing operation regarding said photosensitive
substrate after the temperature of said optical member to be
controlled by said temperature control device falls in a
predetermined allowable range for the target temperature.
12. A projection exposure apparatus according to claim 11, further
comprising a temperature sensor for measuring the temperature of
said optical member to be controlled by said temperature control
device, and wherein said exposure control device compares a
measured value of said temperature sensor with the target
temperature and judges whether the exposing operation is to be
started.
13. A projection exposure apparatus according to claim 11, further
comprising a memory system for storing a time period during which
the temperature of said optical member to be controlled by said
temperature control device reaches a predetermined temperature, and
wherein said exposure control device starts the exposing operation
regarding said photosensitive substrate after a time period
corresponding to the time period stored in said memory system is
elapsed.
14. A projection exposure apparatus according to claim 11, wherein,
when the temperature of said optical member to be controlled by
said temperature control device is changed to the target
temperature, an intermediate temperature is set by said temperature
control device so that it is overshot or under shot with respect to
the target temperature.
15. A projection exposure apparatus wherein a mask pattern is
projected to a photosensitive substrate through a projection
optical system with predetermined exposure illumination light, and
projection optical system comprising: at least one set of lens
element, each set including two lens elements of glass materials
having opposite effects for contributing to the change in the focus
position in response to temperature change, said glass materials of
said set of the lens elements being selected so that at least one
of the focus position and a magnification error of the entire
projection optical system including a focus position and a
magnification error caused by expansion/contraction of a holding
member for holding said set of lens elements does not substantially
change in response to predetermined temperature change.
16. A projection exposure apparatus according to claim 15, wherein
there are two sets of lens elements each of which comprises said
lens elements made of the glass materials having opposite effects
for contributing to the change in the focus position in response to
the temperature change.
17. A projection exposure apparatus according to claim 15, wherein
the predetermined exposure illumination light is a laser beam
emitted from an excimer laser light source and having a wavelength
of 100-300 nm, and the glass materials are quartz and fluorite.
18. A projection exposure apparatus according to claim 17, wherein
said lens element made of fluorite in said set of lens elements is
isolated from the surrounding gas in said projection optical system
by means of at least one lens element made of other glass
material.
19. A projection exposure apparatus wherein a mask pattern is
transferred onto a photosensitive substrate through a projection
optical system with predetermined exposure illumination light,
comprising: a temperature control device for supplying
temperature-controlled fluid having no absorbing band for the
wavelength of the illumination light to the surroundings of a lens
element to be controlled in said projection optical system, and
wherein the temperature of said lens element to be controlled is
controlled by said temperature control device to control an imaging
characteristic of said projection optical system.
20. A projection exposure apparatus wherein a mask pattern is
transferred onto a photosensitive substrate through a projection
optical system with illumination light from an illumination optical
system, comprising: a temperature control device for supplying
temperature-controlled fluid having no absorption band for the
wavelength of the illumination light to an illumination light path
between said illumination optical system and said substrate, and
wherein an imaging characteristic of said projection optical system
is controlled by said temperature control device.
21. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
.[.wherein a pattern on a pattern forming surface of a mask is
transferred onto a photosensitive substrate through a projection
optical system with predetermined exposure illumination light,.].
comprising: a .[.light-permeable dust-proof films.]. .Iadd.light
transmitting member .Iaddend.disposed over .[.said.]. .Iadd.a
.Iaddend.pattern forming surface of .[.said.]. .Iadd.a
.Iaddend.mask with a predetermined gap and .[.fluid.]. .Iadd.a
medium .Iaddend.having .[.no.]. .Iadd.a low .Iaddend.absorption
band for the wavelength of the illumination light contained between
said mask and said .[.dust-proof films.]. .Iadd.light transmitting
member.Iaddend..
22. A projection exposure apparatus wherein a mask pattern is
transferred onto a photosensitive substrate through a projection
optical system with illumination light from an illumination optical
system, comprising: a correction device for correcting the change
in an imaging characteristic due to accumulation of heat of the
illumination light and environmental change, wherein said
correction device includes at least two different control systems
and a substrate lifting/lowering mechanism so that a non-linear
magnification error and curvature of field are corrected by
combination of said control system and said substrate lift/lower
mechanism.
23. A projection exposure apparatus according to claim 22, wherein
said control systems include at least two of: (a) a control system
for driving two lens elements independently in the direction of an
optical axis and for changing their inclination; (b) a control
system for changing the pressure in a space between two lens
elements; and (c) a control system which is provided in the
vicinity of at least one .[.of said two.]. lens .[.elements.].
.Iadd.element .Iaddend.and changes the temperature of said at least
one .[.of said two.]. lens .[.elements.].
.Iadd.element.Iaddend..
24. .[.A projection.]. .Iadd.An .Iaddend.exposure apparatus
.[.wherein a mask pattern is projected to a photosensitive
substrate,.]. comprising: a projection optical system having a
plurality of .Iadd.first .Iaddend.optical members made of .Iadd.a
first .Iaddend.glass .[.materials.]. .Iadd.material and at least
one second optical member made of a second glass material having an
index of refraction responsive to temperature in a different manner
from that of said first glass material.Iaddend.; at least one
temperature sensor .[.for sensing.]. .Iadd.which is provided in a
vicinity of said at least one second optical member and senses
.Iaddend.the temperature of .Iadd.said .Iaddend.at least .[.a
first.]. .Iadd.one second .Iaddend.optical member; and a
temperature control device which is provided in the vicinity of
.Iadd.said .Iaddend.at least .[.said first.]. .Iadd.one second
.Iaddend.optical member and controls the temperature of .Iadd.said
.Iaddend.at least .[.said first.]. .Iadd.one second
.Iaddend.optical member in response to said at least one
temperature sensor.[.; wherein, an imaging characteristic of said
projection optical system is controlled by using said temperature
control device.]. .
25. A projection exposure apparatus wherein a mask pattern is
projected to a photosensitive substrate, comprising: a projection
optical system having a plurality of optical members made of glass
materials, at least one of which has a temperature feature of index
of refraction different from that of the other glass material; and
a temperature control device which is provided in the vicinity of
said at least one optical member and controls the temperature of at
least one of said optical members; wherein, a focusing feature of
said projection optical system is controlled by using said
temperature control device; and the focusing feature to be
controlled by said temperature control device is a non-linear
magnification error.
26. A projection exposure apparatus wherein a mask pattern is
projected to a photosensitive substrate, comprising: a projection
optical system having a plurality of optical members made of glass
materials, at least one of which has a temperature feature of index
of refraction different from that of the other glass material; and
a temperature control device which is provided in the vicinity of
said at least one optical member and controls the temperature of at
least one of said optical members; wherein, a focusing feature of
said projection optical system is controlled by using said
temperature control device; and a memory system for storing a
change amount of the focusing feature of said projection optical
system in accordance with the change in an applied condition of
said projection optical system, and wherein the focusing feature of
said projection optical system is controlled by said temperature
control device so that the change amount of the focusing feature
stored in said memory system is cancelled in accordance with the
change in the applied condition of said projection optical
system.
27. A projection exposure apparatus wherein a mask pattern is
projected to a photosensitive substrate, comprising: projection
optical system having a plurality of optical members made of glass
materials, at least one of which has a temperature feature of index
of refraction different from that of the other glass material; and
a temperature control device which is provided in the vicinity of
said at least one optical member and controls the temperature of at
least one of said optical members; wherein, a focusing feature of
said projection optical system is controlled by using said
temperature control device; and the focusing feature is a position
of a focusing plane of said projection optical system in the
direction of an optical axis of said projection optical system.
28. A projection exposure apparatus wherein a mask pattern is
projected to a photosensitive substrate, comprising: a projection
optical system having a plurality of optical members made of glass
materials, at least one of which has a temperature feature of index
of refraction different from that of the other glass material; a
temperature control device which is provided in the vicinity of
said at least one optical member and sets a temperature of at least
one of said optical members to a variable target temperature
determined by a focusing feature of said projection optical system;
an exposure control device for starting an exposing operation
regarding said photosensitive substrate after the temperature of
said optical member to be controlled by said temperature control
device falls in a predetermined allowable range for the target
temperature; and a temperature sensor for measuring the temperature
of said optical member to be controlled by said temperature control
device, and wherein said exposure control device compares a
measured value of said temperature sensor with the target
temperature and judges whether the exposing operation is to be
started.
29. A projection exposure apparatus wherein a mask pattern is
projected to a photosensitive substrate, comprising: a projection
optical system having a plurality of optical members made of glass
materials at least one of which has a temperature feature of index
of refraction different from that of the other glass material; a
temperature control device which is provided in the vicinity of
said at least one optical member and sets a temperature of at least
one of said optical members to a variable target temperature
determined by a focusing feature of said projection optical system;
an exposure control device for starting an exposing operation
regarding said photosensitive substrate after the temperature of
said optical member to be controlled by said temperature control
device falls in a predetermined allowable range for the target
temperature; and a memory system for storing a time period during
which the temperature of said optical member to be controlled by
said temperature control device reaches a predetermined
temperature, and wherein said exposure control device starts the
exposing operation regarding said photosensitive substrate after a
time period corresponding to the time period stored in said memory
system has elapsed.
30. A projection exposure apparatus wherein a mask pattern is
projected to a photosensitive substrate, comprising: a projection
optical system having a plurality of optical members made of glass
materials at least one of which has a temperature feature of index
of refraction different from that of the other glass material; a
temperature control device which is provided in the vicinity of
said at least one optical member and sets a temperature of at least
one of said optical members to a variable target temperature
determined by a focusing feature of said projection optical system;
and an exposure control device for starting an exposing operation
regarding said photosensitive substrate after the temperature of
said optical member to be controlled by said temperature control
device falls in a predetermined allowable range for the target
temperature; wherein, when the temperature of said optical member
to be controlled by said temperature control device is changed to
the target temperature, an intermediate temperature is set by said
temperature control device so that it is over-shot or under-shot
with respect to the target temperature.
31. A projection exposure apparatus wherein a mask pattern is
projected to a substrate through a projection optical system
comprising: a temperature control device which is provided in the
vicinity of at least one optical member and controls the
temperature of at least one optical member of said projection
optical system .Iadd.to correct a first imaging characteristic of
said projection optical system.Iaddend.; and an imaging
characteristic control system provided separately from said
temperature control device .[.for correcting an.]. .Iadd.to correct
a second .Iaddend.imaging characteristic .Iadd.which is different
from said first imaging characteristic .Iaddend.of said projection
optical system.
32. A projection exposure apparatus according to claim 31, wherein
said imaging characteristic control system corrects an error in an
imaging characteristic which is caused as a result of controlling
of imaging characteristics of the projection optical system carried
out by said temperature control device.
33. A method of projecting a mask pattern to a substrate through a
projection optical system comprising: correcting a first imaging
characteristic of said projection optical system by controlling the
temperature of at least one optical member of the projection
optical system; and correcting an error in a second imaging
characteristic which is different from said first imaging
characteristics, said error being caused as a result of controlling
the temperature of said at least one optical member.
34. A method of projecting a mask pattern to a substrate through a
projection optical system .Iadd.which has a plurality of optical
members made of a first glass material and at least one optical
member made of a second glass material having an index of
refraction responsive to temperature in a different manner from
that of said first glass material, the method .Iaddend.comprising:
controlling the temperature of .Iadd.said .Iaddend.at least one
optical member .Iadd.of the second glass material .Iaddend.of said
projection optical system so that .[.the.]. .Iadd.said at least one
.Iaddend.optical member .Iadd.of the second glass material
.Iaddend.has a target temperature; and starting a projection
exposure operation after the temperature of .[.the.]. .Iadd.said at
least one .Iaddend.optical member .Iadd.of the second glass
material .Iaddend.has reached said target temperature.
35. A projection exposure apparatus according to claim 1, wherein
the optical member which is controlled by said temperature control
device is made of fluorite.
36. A projection exposure apparatus according to claim 7, wherein
the optical member which is controlled by said temperature control
device is made of fluorite.
37. A projection exposure apparatus according to claim 11, wherein
the optical member which is controlled by said temperature control
device is made of fluorite.
38. A method of projecting a mask pattern to a substrate through a
projection optical system so as to form a circuit element on the
substrate comprising: controlling the temperature of at least one
optical member of said projection optical system so that at least
one of a non-linear magnification error and a curvature of field of
the projection optical system is corrected; and projecting said
mask pattern to said substrate.
39. A method of projecting a mask pattern to a substrate through a
projection optical system so as to form a circuit element on the
substrate comprising: correcting a first imaging characteristic of
said projection optical system by controlling the temperature of at
least one optical member of the projection optical system.
correcting an error in a second imaging characteristic which is
caused as a result of controlling the temperature of said at least
one optical member; and projecting said mask pattern to said
substrate.
40. A method of projecting a mask pattern to a substrate through a
projection optical system so as to form a circuit element on the
substrate.Iadd., wherein said projection optical system has a
plurality of optical members made of a first glass material and at
least one optical member made of a second glass material having an
index of refraction responsive to temperature in a different manner
from that of said first glass material, said method
.Iaddend.comprising: controlling the temperature of .Iadd.said
.Iaddend.at least one optical member .Iadd.of the second glass
material .Iaddend.of said projection optical system so that
.[.the.]. .Iadd.said at least one .Iaddend.optical member .Iadd.of
the second glass material .Iaddend.has a target temperature; and
starting a projection of said mask pattern to said substrate after
the temperature of .[.the.]. .Iadd.said at least one
.Iaddend.optical member .Iadd.of the second glass material
.Iaddend.has reached said target temperature.
41. A projection exposure apparatus according to claim 1, wherein
said temperature control device comprises a temperature control
element provided so as to contact said at least one optical
member.
42. A projection exposure apparatus according to claim 1, wherein
said temperature control device controls the temperature of said at
least one optical member without contacting said at least one
optical member.
43. A projection exposure apparatus according to claim 11, wherein
said temperature control device comprises a temperature control
element provided so as to contact said at least one optical
member.
44. A projection exposure apparatus according to claim 11, wherein
said temperature control device controls the temperature of said at
least one optical member without contacting said at least one
optical member.
45. A projection exposure apparatus according to claim 22, further
comprising a stage on which a photosensitive substrate is mounted,
said stage having a light receiving portion, wherein said
non-linear magnification error and said curvature of field are
measured by shifting said mask pattern and said light receiving
portion relative to each other.
46. A method for making a lithographic system, comprising:
providing a projection optical system which has a plurality of
optical members made of glass materials, at least a first optical
member having an index of refraction responsive to temperature in a
different manner from that of a second optical member; and
providing a temperature control device which controls the
temperature of at least one of said optical members. .Iadd.
47. A method of projecting a mask pattern to a substrate through a
projection optical system comprising: controlling the temperature
of at least one optical member of said projection optical system so
that at least one of a non-linear magnification error and a
curvature of field of the projection optical system is
corrected..Iaddend..Iadd.
48. An exposure apparatus comprising: a mask having a surface on
which a pattern is formed; a light transmitting member disposed to
have a predetermined gap between itself and said surface having the
pattern; and a structure defining a chamber to accommodate said
mask and said light transmitting member, wherein a space between
said surface having the pattern of the mask and said light
transmitting member provides a gaseous atmosphere having an
absorption characteristic less than that of oxygen with respect to
a wavelength of an illumination light with which said mask is
irradiated..Iaddend..Iadd.
49. An exposure apparatus according to claim 48, wherein the
atmosphere provided in said chamber is the same gaseous atmosphere
provided in said space..Iaddend..Iadd.
50. An exposure apparatus according to claim 49, wherein said
gaseous atmosphere in said space and said chamber is provided by an
inert gas..Iaddend..Iadd.
51. An exposure apparatus according to claim 50, wherein said space
is enclosed by said surface having the pattern of the mask, said
light transmitting member, and a frame member disposed between said
surface and said light transmitting member..Iaddend..Iadd.
52. An exposure apparatus according to claim 51, further comprising
a supply device connected to said chamber to supply said gas to
said chamber, and wherein said frame member has a vent hole so that
the gas supplied to said chamber can enter said space through said
vent hole..Iaddend..Iadd.
53. A projection exposure method comprising: providing a gaseous
atmosphere in a space between a surface on which a pattern is
formed and a light transmitting member disposed to have a
predetermined gap between itself and said surface having the
pattern, said gaseous atmosphere having an absorption
characteristic less than that of oxygen with respect to a
wavelength of an illumination light with which said pattern is
irradiated; and transferring an image of said pattern onto a
substrate while said gaseous atmosphere between said surface having
the pattern and said light transmitting member is
maintained..Iaddend..Iadd.
54. A method according to claim 53, wherein a gas providing said
gaseous atmosphere is an inert gas..Iaddend..Iadd.
55. A mask structure comprising: a surface having a pattern; a
light transmitting member disposed to have a predetermined gap
between itself and said surface; a frame member disposed between
said surface having the pattern and said light transmitting member;
and a fluid sealed in a space enclosed by said surface having the
pattern, said light transmitting member, and said frame member,
said fluid having an absorption characteristic less than that of
oxygen with respect to a wavelength of an illumination light with
which said pattern is irradiated..Iaddend..Iadd.
56. A mask structure according to claim 55, wherein said fluid is
an inert gas..Iaddend..Iadd.
57. An exposure method for transferring an image of a pattern
formed on a mask onto a substrate, wherein a mask structure
according to claim 55 is used as said mask..Iaddend..Iadd.
58. A method of manufacturing a mask structure according to claim
55, wherein the mask structure is manufactured in a fluidic
atmosphere having an absorption characteristic less than that of
oxygen with respect to a wavelength of an illumination light with
which said mask structure is irradiated..Iaddend..Iadd.
59. A method of manufacturing a mask structure comprising:
providing a space enclosed by a surface having a pattern, a light
transmitting member disposed to have a predetermined gap between
itself and said surface, and a frame member disposed between said
surface having the pattern and said light transmitting member;
applying vacuum to said space to evacuate said space; and supplying
a fluid to said space and sealing the fluid in said space after
said space has been evacuated, said fluid having an absorption
characteristic less than that of oxygen with respect to a
wavelength of an illumination light with which said mask structure
is irradiated..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection exposure apparatus
which is used in a lithography process for manufacturing
semiconductor elements, imaging elements (CCD), liquid crystal
display elements, thin film magnetic heads or the like, for example
and in which a mask pattern is transferred onto a photosensitive
substrate through a projection optical system.
2. Related Background Art
Projection exposure apparatuses for manufacturing a semiconductor
element such as IC, LSI and the like are generally categorized into
a projection exposure apparatus of step-and-repeat type in which a
reticle as a mask and shot areas of a wafer (or glass plate and the
like) as photosensitive substrate are positioned relative to each
other in a predetermined positional relation by using a projection
optical system and pattern images on the reticle are collectively
transferred to the entirety of each shot area, and a projection
exposure apparatus of step-and-scan type in which the pattern
images on the reticle are successively transferred to each of the
shot areas of the wafer by scan-moving the reticle and the shot
areas relative to the projection optical system. These two types
are in common in the point that the reticle pattern is projected
through the projection optical system. In this regard, it is
important how the reticle pattern is correctly projected onto the
wafer.
In general, although the projection optical system is designed so
that various optical aberrations become substantially "zero" under
a predetermined condition, if an atmospheric pressure and/or a
temperature around the projection optical system is changed due to
the change in the environment during the projection exposure or if
heat absorption occurs due to the illumination of exposure
illumination light, there will occur the change in index of
refraction of fluid existing between lens elements of the
projection optical system, expansion of the lens elements, the
change in index of refraction of the lens elements, and expansion
of lens barrel. As a result, when the reticle pattern is projected
onto the wafer, a distortion phenomenon (that the projected image
is deviated in a direction perpendicular to an optical axis of the
projection optical system) occurs. Such distortion is categorized
into a liter error (component that a focusing position is changed
with respect to an image height in a linear function manner) and a
non-linear error (components other than the linear error), and the
linear error is also called as "linear magnification error"
(component that the magnification is changed with respect to the
image height in a linear function manner). In the past, in order to
correct the linear magnification error, there has been utilized a
lens control system in which some of lens elements in the
projection optical system is driven or pressure of the fluid
between some lens elements is controlled.
However, if the non-linear error exists in the projection optical
system, in both projection optical systems of step-and-repeat type
of step-and-scan type, the projected image will be distorted,
thereby worsening the aligning accuracy.
The present invention aims to eliminate the above-mentioned
conventional drawback, and an object of the present invention is to
provide a projection exposure apparatus in which the distortion
that is worsened by the change in an environment condition such as
atmospheric pressure and a temperature around a projection optical
system and the absorption of exposure illumination light
(particularly, a non-linear magnification error (higher order
error)) can be corrected.
If the above-mentioned change in index of refraction of the fluid
existing between the lens elements of the projection optical
system, expansion of the lens elements, change in index of
refraction of the lens elements and expansion of lens barrel occur,
when the reticle pattern is projected onto the wafer, a position
(focus position) of an image plane (best focus plane) for the
projected image is devised or displaced in an optical axis of the
projection optical system, thereby causing a defocus phenomenon in
which the surface of the wafer is deviated out of the image plane.
Such defocus is categorized into linear defocus (component that a
defocus amount is changed with respect to the image height in a
linear function manner) and non-linear defocus (components other
than the linear defocus). In the past, in order to correct the
linear defocus, there have been proposed an auto focus mechanism in
which the focus position of the wafer is controlled along the
direction of the image plane and an auto levelling mechanism in
which an inclination angle of the wafer is controlled to be aligned
with the image plane.
However, if the non-linear defocus such as curvature of field
occurs in the image plane of the projection optical system, in both
projection optical systems of step-and-repeat type and of
step-and-scan type, focal depth of the eventually obtained image
plane will be entirely narrowed, thereby deteriorating the
resolving power.
The present invention aims to eliminate the above-mentioned
conventional drawback, and another object of the present invention
is to provide a projection exposure apparatus in which defocus of
an image plane of a projection optical system that is worsened by
the change in an environmental condition such as atmospheric
pressure around the projection optical system, the absorption of
exposure illumination light or flexure of a reticle (particularly,
non-linear defocus such as curvature of field) can be
corrected.
To solve the above problems, it is considered that a temperature of
at least one lens element of the projection optical system is
adjusted. When the lens elements is heated or cooled to achieve a
target temperature of the lens element, a predetermined time period
is elapsed until the target temperature of the lens element is
obtained. Accordingly, if the exposure is started before such time
period is elapsed, the exposure will be effected before the imaging
characteristic of the lens element is corrected. A further object
of the present invention is to provide a projection exposure
apparatus in which the exposure is effected after predetermined
temperature adjustment is completed.
Regarding the temperature adjustment, not only it takes a long time
to achieve the target temperature of the lens element, but also a
device for effecting the temperature adjustment is complicated.
Accordingly, it is desirable to provide a projection exposure
apparatus in which the imaging characteristic can be corrected
without the temperature adjustment and the corrected condition is
not substantially changed even if environmental conditions are
changed. A still further object of the present invention is to
provide such a projection exposure apparatus.
During the exposure, when the illumination light having high
illumination energy is directed, the imaging characteristic is
often gradually changed from an initial condition due to the
absorption of the illumination light of the projection optical
system. To eliminate this drawback, there have been proposed
various methods for keeping the entire temperature of the
projection optical system constant in order to maintain the initial
condition of the imaging characteristic of the projection optical
system. For example, a method in which temperature-adjusted air is
supplied around the projection optical system to keep the
temperature of the projection optical system constant has been
proposed.
In the above-mentioned conventional techniques, since the entire
projection optical system is uniformly cooled, the temperature of
the projection optical system as a whole can be kept substantially
constant. However, due to the difference in light paths for the
illumination light beam in the projection optical system caused by
the difference in illumination condition and/or the difference in
kind of patterns to be exposed, it is inevitable that temperature
distribution occurs in the projection optical system. And, as the
reduction of a line width achieved by remarkable progress of the
recent high density integrated circuits is advanced, the
fluctuation in the focusing ability of the projection optical
system becomes innegligible more and more.
Further, as the reduction of the line width is advanced,
illumination light having a short wavelength (for example,
ultraviolet light which can provide high resolving power,
far-ultraviolet light such as ArF excimer laser light (having a
wavelength of 193 cm) and the like) has been used. However, since
such short wavelength includes an oxygen absorption band, there
arises a problem that a part of illumination energy of the
illumination light which should be used to effect the exposure is
absorbed to the oxygen in air. Further, illumination light beams
having a wavelength shorter than the ultraviolet zone tend to cause
a photochemical reaction for changing the oxygen in air to ozone.
The illumination energy of the illumination light is also absorbed
to the ozone generated by such a photochemical reaction.
When the illumination light having the wavelength shorter than the
ultraviolet zone is used, for example, in an apparatus having a
pellicle (dust-proof film) for protecting the pattern area of the
reticle, the oxygen is changed to the ozone by the illumination
light within a closed space defined by a pellicle frame and the
reticle, and the generated ozone tends to be accumulated without
dispersion. Thus, there arises a problem that the energy loss due
to the absorption of the illumination light is gradually
increased.
The present invention aims to eliminate the above-mentioned
conventional drawback, and an object of the present invention is to
provide a projection exposure apparatus in which illumination
energy of exposure illumination light is hard to be absorbed.
SUMMARY OF THE INVENTION
The present invention provides a projection exposure apparatus
wherein a mask pattern is projected to a photosensitive substrate,
comprising a projection optical system having a plurality of
optical members (glass materials) at least one of which has a
temperature characteristic of index of refraction different from
the other glass material, and a temperature control device for
controlling a temperature of at least one of the optical members,
and further wherein an imaging characteristic of the projection
optical system is controlled by using the temperature control
device.
The imaging characteristic to be controlled may be a non-linear
magnification. The non-linear magnification error may be curvature
of field.
The present invention further provides a projection exposure
apparatus wherein a mask pattern is projected to a photosensitive
substrate, comprising a projection optical system having a
plurality of optical members (glass materials) at least one of
which has a temperature characteristic of index of refraction
different from those of the other glass materials, a temperature
control device for setting a temperature of at least one of the
optical members to a variable target temperature determined by an
imaging characteristic of the projection optical system, and an
exposure control device for starting an exposing operation
regarding the photosensitive substrate after the temperature of the
optical member to be controlled by the temperature control device
is contained within a predetermined allowable range for the target
temperature.
The present invention also provides a projection exposure apparatus
wherein a mask pattern is projected to a photosensitive substrate
through a projection optical system with predetermined exposure
illumination light and wherein the projection optical system has at
least one set of lens elements, each set comprising two glass
materials having opposite directions for changing a focus position,
and the glass materials of the set of the lens elements are
selected so that at least one of a focus position and a
magnification error of the entire projection optical system
(including the focus position and magnification error caused by
expansion/contraction of a holding member for holding the set of
lens elements) does not substantially change in response to
predetermined temperature change.
An example of the predetermined exposure illumination light is a
laser beam emitted from an excimer laser light source and having a
wavelength of 100-300 nm, and, in this case, an example of the two
different glass materials are quartz and fluorite.
Further, the present invention provides a projection exposure
apparatus wherein a mask pattern is transferred onto a
photosensitive substrate through a projection optical system with
predetermined exposure illumination light and wherein there is
provided a temperature control device for supplying
temperature-controlled fluid having no absorption band for a
wavelength of illumination light to the surroundings of lens
elements (to be controlled) in the projection optical system and
the temperature of the lens elements to be controlled is controlled
by the temperature control device to control an imaging
characteristic of the projection optical system.
The present invention further provides a projection exposure
apparatus wherein a mask pattern is transferred onto a
photosensitive substrate through a projection optical system with
illumination light from an illumination optical system and wherein
there is provided a temperature control device for supplying
temperature-controlled fluid having no absorption band for a
wavelength of the illumination light to an illumination lightpath
between the illumination optical system and the substrate and an
image characteristic of the projection optical system is controlled
by the temperature control device.
The present invention also provides a projection exposure apparatus
wherein a pattern on a pattern forming surface of a mask is
transferred onto a photosensitive substrate through a projection
optical system with predetermined exposure illumination light and
wherein a light permeable dust-proof film is disposed with respect
to the pattern forming surface of the mask with a predetermined gap
therebetween and fluid having no absorption band for a wavelength
of the illumination light is trapped between the mask and the
dust-proof films.
Lastly, the present invention also provides a projection exposure
surface wherein a mask pattern is transferred onto a photosensitive
substrate through a projection optical system with illumination
light from an illumination optical system and wherein there is
provided a correction device for correcting the change in an
imaging characteristic due to accumulation of heat of the
illumination light and environmental change and the correction
device includes two or more different control systems and substrate
lift/lower mechanisms so that a non-linear magnification error and
curvature of field are corrected by combination of the control
system and substrate lift/lower mechanisms.
According to the present invention, for example, when
far-ultraviolet light such as KrF excimer laser light (having a
wavelength of 248 nm) or ArF excimer laser light (having a
wavelength of 193 nm) is used as the exposure light, quartz and
fluorite are used as the plural glass materials having different
temperature characteristics of index of refraction. In this case,
since the quart has small expansion coefficient, when the quartz is
heated, it is not expanded, but, the index of refraction of the
quartz is increased. On the other hand, as the temperature is
increased, the fluorite is expanded, but, the index of refraction
of the fluorite is decreased. These features will be explained
later with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a constructural view showing a projection exposure
apparatus according to a first embodiment of the present
invention;
FIG. 2 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in the
first embodiment;
FIG. 3 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in a second
embodiment of the present invention;
FIG. 4 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in a third
embodiment of the present invention;
FIG. 5 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in a fourth
embodiment of the present invention;
FIG. 5A is a view showing an alteration of the fourth
embodiment;
FIG. 6 is a view showing light paths of two lens elements made of
glass materials having different features regarding the change in
temperature;
FIG. 7 is a graph showing the feature of the lens element, where
section (a) shows a non-linear magnification error of the lens
element made of quartz, section (b) shows a magnification error of
the lens element made of temperature-controlled fluorite, and
section (c) shows correction amounts regarding the non-linear
magnification error;
FIG. 8 is a graph showing a magnification error (distortion)
obtained by combination of a lens element made of quartz, a lens
element made of temperature-controlled fluorite and a lens control
device;
FIG. 9 is a constructional view showing a mechanism for measuring
the distortion, where section (a) is a plan view showing an opening
portion of a distortion measuring sensor 3 and projected images of
evaluating marks, and section (b) is a constructural view, in
partial section, showing a construction of the distortion measuring
sensor 3;
FIG. 10 is a graph showing the principle of the distortion
measurement, where section (a) shows a waveform of a detection
signal detected by the distortion measuring sensor 3, and section
(b) shows a waveform of a derivative signal of the detection
signal;
FIG. 11 is a graph showing a relation between an atmospheric
pressure and a temperature of a lens element made of fluorine;
FIG. 12 is a conceptional view showing distributions of zero-order
light from a reticle passing through a projection optical system
when illumination conditions are changed variously;
FIG. 13 is a graph showing a relation between the atmospheric
pressure and the temperature of the lens element made of fluorine
when the illumination condition is changed;
FIG. 14 is a graph showing a relation between illumination energy
passing through the projection optical system and the temperature
of the lens element made of fluorine;
FIG. 15 is a view showing non-linear errors in a conventional
projection exposure apparatus;
FIG. 16 is a view showing the influence of the non-linear error
upon the projection, where section (a) shows the influence of the
non-linear error during collective exposure, and section (b) shows
the influence of the non-linear error during scan exposure;
FIG. 17 is a constructional view of a projection exposure apparatus
according to a fifth embodiment of the present invention;
FIG. 18 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in the
fifth embodiment;
FIG. 19 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in a sixth
embodiment of the present invention;
FIG. 20 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in a
seventh embodiment of the present invention;
FIG. 21 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in an
eighth embodiment of the present invention;
FIG. 22 is a graph showing a feature of the lens element, where
section (a) shows curvature of field generated by the lens element
made of quartz, section (b) shows a distribution of focus positions
caused by the lens element made of temperature-controlled fluoride,
and section (c) shows a correction method for residual focus
positions;
FIG. 23 is a graph showing the distribution of focus positions
(curvature of field) obtained by combination of a lens element made
of quartz, a lens element made of temperature-controlled fluorine
an a focus position control mechanism (wafer stage);
FIG. 24 is a constructional view showing a mechanism for measuring
the imaging characteristic, where section (a) is a plan view
showing an opening portion of an imaging characteristic measuring
sensor 3 and projected images of evaluating marks, and section (b)
is a constructional view, in partial section, showing a
construction of the imaging characteristic measuring sensor 3;
FIG. 25 is a graph showing the principle of the imaging
characteristic measurement, where section (a) shows a waveform of a
detection signal detected by the imaging characteristic measuring
sensor 3, section (b) shows a waveform of a derivative signal of
the detection signal, and section (c) is an explanatory view for
explaining a focus position determining method;
FIG. 26 is a graph showing a relation between an atmospheric
pressure and a temperature of a lens element made of fluoride;
FIG. 27 is a graph showing a relation between the atmospheric
pressure and the temperature of the lens element made of fluorite
when the illumination condition is changed;
FIG. 28 is a graph showing a relation between illumination energy
passing through the projection optical system and the temperature
of the lens element made of fluorite;
FIG. 29 is a view showing curvature of field in a conventional
projection exposure apparatus;
FIG. 30 is a view showing the influence of the curvature of field
upon the projection, where section (a) shows the influence of the
curvature of field during collective exposure, and section (b)
shows the influence of the curvature of field during scan
exposure;
FIG. 31 is a perspective view showing the flexure of a reticle,
where section (a) is a perspective view showing the flexure of a
reticle having a small size, and section (b) is a perspective view
showing the flexure of a reticle having a large size;
FIG. 32 is a constructural view showing a projection exposure
apparatus according to a ninth embodiment of the present
invention;
FIG. 33 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in the
ninth embodiment;
FIG. 34 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in a tenth
embodiment of the present invention;
FIG. 35 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in an
eleventh embodiment of the present invention;
FIG. 36 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in a
twelfth embodiment of the present invention;
FIG. 37 is a constructural view showing a mechanism for measuring
the imaging characteristic, where section (a) is a plan view
showing an opening portion of an imaging characteristic measuring
sensor 3 and projected images of evaluating marks, and section (b)
is a constructional view, in partial section, showing a
construction of the imaging characteristic measuring sensor 3;
FIG. 38 is a graph showing a relation between a time and a
temperature of fluorite, where a section (a) is a graph showing a
relation between a control temperature T.sub.ij of a temperature
control medium and a temperature TF of fluorite, and section (b) is
an explanatory view for explaining the relation between the time
and the temperature of fluorite when the control temperature
T.sub.ij of the temperature control medium is overshot;
FIG. 39 is a flow chart showing a relation between a temperature
control sequence for a lens element made of fluorite and an
exposure sequence, according to an embodiment of the present
invention;
FIG. 40 is a view showing data stored in a common working area of
memories in a main control device of FIG. 2;
FIG. 41 is a constructural view showing a projection exposure
apparatus according to a thirteenth embodiment the present
invention;
FIG. 42 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in the
thirteenth embodiment;
FIG. 43 is an explanatory view showing a projecting optical system
and an imaging characteristic correcting mechanism used in a
fourteenth embodiment of the present invention;
FIG. 44 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism used in a
fifteenth embodiment of the present invention;
FIG. 45 is an explanatory view showing a projection optical system
and an imaging characteristic correcting mechanism use din a
sixteenth embodiment of the present invention;
FIG. 46 is a schematic constructural view, in partial section,
showing a projection exposure apparatus according to a seventeenth
embodiment of the present invention;
FIG. 47 is a schematic constructional view, in partial section,
showing a projection exposure apparatus according to an eighteenth
embodiment of the present invention; and
FIG. 48 is a view showing absorption spectrum of an ArF excimer
laser in gas, where section (a) shows the spectrum of the ArF
excimer laser in air, and section (b) shows the spectrum of the ArF
excimer laser in nitrogen gas.
DETAILED DESCRIPTION
First of all, a projection exposure apparatus according to a first
embodiment of the present invention will be explained with
reference to FIGS. 1, 2 and 6 to 14. In this embodiment, the
present invention is applied to a projection exposure apparatus of
step-and-scan type in which excimer laser light is used as exposure
illumination light. However, a projection optical system used in
this embodiment has such an ability as that it can also be used in
a projection exposure apparatus of step-and-repeat type (collective
exposure type).
Prior to the explanation of the first embodiment, in order to
enhance the understanding of a non-linear error of distortion
handled by the present invention, the non-linear error will be
generally described with reference to FIGS. 15 and 16.
In conventional projection exposure apparatuses, there arose a
problem that a non-linear error of distortion of the projection
optical system generated by the change in an environmental
condition cannot be corrected. A main portion of the non-linear
error is a non-linear magnification error (high order magnification
error) the magnification of which is changed in a secondary or more
function manner in accordance with a height of the projected image.
However, also in a condition as a reference condition in design,
the projection optical system includes the small non-linear
magnification error which is negligible in the practical use, and,
as shown in a section (a) of FIG. 15, the magnification .beta.
(projection magnification from a reticle to a wafer) of the
projection optical system is slightly changed from design value
.beta..sub.0 in a non-linear manner in accordance with the image
height. Regarding the reference condition, if the change in
atmospheric pressure and/or heat absorption due to illumination of
exposure illumination light occurs, the non-linear magnification
error shown in the section (a) of FIG. 15 becomes greater as shown
in a section (b) of FIG. 15. Incidentally, as shown in the section
(b) of FIG. 15, the non-linear magnification error in which the
magnification .beta. is changed in a negative (-) direction once
and then is changed in a positive (+) direction in accordance with
the image height is also called as "C-shaped distortion".
If the non-linear magnification error as shown in the section (b)
of FIG. 15 is generated in the projection exposure apparatus of
step-and-repeat type, as shown in a section (a) of FIG. 16,
original projected image 66 and 67 are changed in the non-linear
manner in accordance with the image height to form projected images
66A and 67A, thereby worsening the aligning accuracy between two
layers. On the other hand, if the non-linear magnification error as
shown in the section (b) of FIG. 15 is generated in the projection
exposure apparatus of step-and-scan type, as shown in a section (b)
of FIG. 16, the original projected images 66 and 67 are changed in
the non-linear manner in accordance with the image height to form
projected images 66B and 67B, thereby worsening the aligning
accuracy between two layers. In the section (b) of FIG. 16, a scan
direction on the wafer during the scan exposure is a direction
shown by the arrow Y. Thus, the projected images are not distorted
in the scan direction due to the averaging effect, but, the quality
of the images is deteriorated due to the averaging effect. Further,
it can be seen that the magnification error corresponding to the
image height is generated in a non-scan direction (X direction), as
in the collective exposure type.
FIG. 1 shows the projection exposure apparatus according to the
first embodiment. In FIG. 1, illumination light IL comprised of a
pulse laser beam emitted from an excimer laser light source 16 is
deflected by a mirror 15 to reach an illumination optical system
14. The excimer laser light source 16 may be a KrF excimer laser
light source (having oscillation wavelength of 248 nm) or an ArF
excimer laser light source (having oscillation wavelength of 193
nm). A YAG high harmonic laser generating device, a metal vapor
laser light source or a memory lamp may be used as an exposure
light source.
The illumination optical system 14 includes a beam expander, a
light amount adjusting mechanism, a fly-eye lens, a relay lens, a
field stop (reticle blind), a movable blade for avoiding undesired
exposure before and after the scanning, and a condenser lens so
that the illumination light IL modified by the illumination optical
system 14 to have uniform illuminance distribution is directed onto
a predetermined-shaped illumination area of a pattern forming
surface (lower surface) of a reticle R. In this case, a main
control device 18 for totally controlling the entire operation of
the apparatus controls the pulse generating timing of the excimer
laser light source 16 and the beam attenuation ratio of the light
amount adjusting mechanism of the illumination optical system 14
through an illumination control system 17. The illumination light
passed through a pattern on the illumination area of the reticle R
is projected onto a wafer W (on which photoresist is coated)
through a projection optical system PL1, with the result that a
projected image obtained by contracting the reticle pattern with
the magnification .beta. (for example, .beta.=1/4, 1/5 or the like)
is transferred to the wafer W. Now, it is assumed that a direction
parallel to an optical axis AX of the projection optical system PL1
is referred to as "Z axis", a direction perpendicular to the Z axis
and parallel to the plane of FIG. 1 is referred to as "Y axis" and
a direction perpendicular to the plane of FIG. 1 is referred to as
"X axis".
The reticle R is held on a reticle stage 6, and the reticle stage 6
is rested on a reticle support 5 via air bearings for shifting
movement in the Y axis (Y direction). A Y-axis value of the reticle
stage 6 measured by a shiftable mirror 7 secured to an upper
surface of the reticle stage 6 and a laser interferrometer 8
secured to the reticle support 5 is supplied to a stage control
system 11. The stage control system 11 serves to control a position
and a shifting speed of the reticle stage 6 in response to command
from the main control device 18.
The wafer W is held on a wafer stage 2, and the wafer stage 2
serves to position the wafer in the X, Y and Z direction and in a
rotational direction and permits the scanning of the wafer W in the
Y direction. A shiftable mirror 9 is secured to an upper surface of
the wafer stage 2. By using the shiftable mirror 9 and an external
laser interferometer 10, X and Y values in the X-Y coordinate
system of the wafer stage 2 are always measured, and the measured
result is supplied to the stage control system 11. In response to
the command from the main control device 18, the stage control
system 11 controls a stepping operation of the wafer stage 2 and a
scanning operation in synchronous with the reticle stage 6. That is
to say, during the scan exposure, by using the magnification .beta.
(from the reticle side to the wafer side) of the projection optical
system PL1, under the control of the stage control system 11, the
reticle stage 6 is scanned in the -Y direction (or +Y direction) at
a speed of V.sub.R relative to the projection optical system PL1.
In synchronous with this scanning movement, the wafer stage 2 is
scanned at a speed of V.sub.W (=B.times.V.sub.R) in the +Y
direction (or -Y direction). In this way, the pattern images on the
reticle R are successively transferred to shot areas on the wafer
W.
A distortion measuring sensor 3 is secured on the wafer stage 2 in
the vicinity of the wafer W. As shown in a section (b) of FIG. 9,
the distortion measuring sensor 3 has a glass substrate 51 on which
a high light shield film 50 including a rectangular opening portion
50a in the same height as a surface of the wafer W is coated, a
photoelectric conversion element 52 for photoelectrically
converting the exposure illumination light passed through the
opening portion 50a into a detection signal, and a signal treatment
portion 53 for treating the detection signal S1 from the
photoelectric conversion element 52. The treated result from the
signal treatment portion 53 is supplied to the main control device
18 shown in FIG. 1. In the illustrated embodiment, as will be
described later, by driving the wafer stage 2, the projected image
of the pattern on the reticle R is scanned by the opening portion
50a of the distortion measuring sensor 3. In this case, on the
basis of the detection signal S1 outputted from the photoelectric
conversion element 52, the signal treatment portion 53 seeks or
determines the magnification errors (distortion) of the projection
optical system PL1 at various image heights.
In the illustrated embodiment, although the projection optical
system PL1 includes a plurality of lens elements, the major number
of lens elements are made of quartz, and the remaining lens
elements are made of fluorite. Further, there is provided a lens
control device 12 for controlling pressure of gas in a
predetermined gas chamber defined between two adjacent lens
elements made of quartz. If the imaging characteristics of the
projection optical system PL1 such as the magnification, focusing
position (best focus position) and curvature of field are changed
in accordance with the change in the environmental condition such
as the change in atmospheric pressure or the change in temperature,
or the history of the illumination amount of the exposure
illumination light regarding the projection optical system PL1, the
imaging characteristics are corrected by the lens control device 12
in response to the command from the main control device 18. The
lens control device 12 may be a lens position control device for
controlling a position (along the optical axis AX) and an
inclination angle of a predetermined lens element made of quartz or
fluorite, as well as a pressure control device. Alternatively, in
addition to the lens control device 12 or in place of the lens
control device 12, a reticle position control device for
controlling a position (along the optical axis AX) and an
inclination angle of the reticle R may be used.
Further, a lens temperature control device 13 for controlling a
temperature or temperatures of one or more lens elements made of
fluorite is connected to the projection optical system PL1. In the
illustrated embodiment, if the non-linear magnification error
(higher order magnification error) of the projection optical system
PL1 is worsened in accordance with the change in the environmental
condition or the history of the illumination amount, the non-linear
magnification error is corrected by the lens temperature control
device 13 on the basis of the command from the main control device
18. If the linear magnification error of the projection optical
system PL1 is worsened by correcting the non-linear magnification
error, the linear magnification error is corrected by the lens
control device 12.
Next, the projection optical system PL1 and the lens temperature
control device 13 according to the illustrated embodiment will be
fully explained with reference to FIG. 2 and some other
drawings.
FIG. 2 shows an inner construction of the projection optical system
PL1 and the imaging characteristic control system. In FIG. 2, as an
example, the projection optical system PL1 has a lens barrel 4
within which six lens elements 25-30, four lens elements 31-34 and
four lens elements 35A-38A are fixed mounted (in this order from
the wafer side). The lens elements 36A and 37A are made of
fluorite, and the other lens elements are made of quartz. The lens
elements 33 and 34 are secured to the lens barrel 4 via a lens
frame G1, the lens elements 35A-38A are secured to the lens barrel
4 via a lens frame G2, and the other lens elements are secured to
the lens barrel 4 via lens frames (not shown).
In this case, a gas chamber enclosed by the lens elements 33, 34
and the lens frame G1 is sealed, but, this gas chamber is connected
to a bellows mechanism 12b via a piping 12c, and an
expansion/contraction amount of the bellows mechanism 12b is
controlled by a control portion 12a. The lens control device 12 of
FIG. 1 is constituted by the control portion 12a, bellows mechanism
12b and piping 12c, and, by adjusting the expansion/contraction
amount of the bellows mechanism 12b, a pressure of the gas (for
example, air) in the gas chamber is controlled.
Gas having a temperature determined by the main control device 18
is supplied, from a temperature control device 13b, through a
piping 21, to a gas chamber enclosed by the lens elements 36A, 37A
and the lens frame G2, and the gas circulating in the gas chamber
is returned to the temperature control device 13b through a piping
22. When the gas supplied to the gas chamber has the same
constituents as those of the gas (air) surrounding the projection
optical system PL1, there is no need that the gas passed through
the gas chamber is returned to the temperature control device 13b
through the piping 22. However, for example, when gas (for example,
nitrogen gas) a temperature of which can easily be controlled is
used, it is necessary to use the piping 22 for effecting
closed-loop temperature control.
In the illustrated embodiment, since the temperatures of the lens
elements 36A and 37A are controlled by the temperature control
device 13b, it is necessary to prevent the temperatures of the lens
elements 36A and 37A from being transmitted to the adjacent lens
elements 35A and 38A via the lens frame G2, lens barrel 4 and gas.
To this end, gas having a constant temperature is supplied from the
temperature control device 13a to a gas chamber enclosed by the
lens elements 37A, 38A and the lens frame G2 and a gas chamber
enclosed by the lens elements 35A, 36A and the lens frame G2,
through pipings 19 and 20, respectively. Gas (for example, air)
having low heat conductivity is used as the constant temperature
gas, and the gases circulated in such gas chambers are returned to
the temperature control device 13a through pipings 23A and 24A,
respectively. The lens temperature control device 13 of FIG. 1 is
constituted by the temperature control devices 13a, 13b and the
pipings 19-22, 23A and 24A.
Further, in the projection exposure apparatus according to the
illustrated embodiment, in order to expose the various reticle
patterns with high resolving power, the illumination condition of
the illumination optical system 14 can be changed or switched
between a normal condition, a modulated-shape light source
condition, a ring illumination condition and a condition that a
.sigma. value (coherence factor) is small.
FIG. 12 shows conditions of the illumination light passing through
the projection optical system PL1 when the illumination optical
system 14 is switched or changed. In the normal condition, as shown
in a section (a) of FIG. 12, zero-order light passed through the
reticle R passes through a predetermined circular area at a pupil
plane (Fourier transform plane for the reticle R) of the projection
optical system PL1. On the other hand, in the modulated-shape light
source condition of the illumination optical system 14, as shown by
the sectional view in a section (b) of FIG. 12, the zero-order
light passed through the reticle R passes through a plurality of
spaced apart areas on the pupil plane of the projection optical
system PL1. In the ring illumination condition of the illumination
optical system 14, the zero-order light passed through the reticle
R passes through a substantially annular area on the pupil plane of
the projection optical system PL1. In the condition (of the
illumination optical system 14) that the .sigma. value is small, as
shown in a section (c) of FIG. 12, the zero-order light passed
through the reticle R passes through a substantially circular small
area on the pupil plane of the projection optical system PL1. Thus,
in dependence upon these illumination conditions, the imaging
characteristic of the projected image is changed.
Next, in the illustrated embodiment, an example of a method for
removing the non-linear magnification error of the projection
optical system PL1 will be explained.
The glass materials from which the lens elements of the projection
optical system PL1 are formed are quartz and fluorite. The quartz
has the property that, when it heated, it is not substantially
expanded because of its small expansion coefficient, but the index
of refraction thereof is increased. Accordingly, as shown in a
section (b) of FIG. 6, in a positive lens element 49B made of
quartz, when the temperature is increased, an image plane FB is
displaced to approach the lens element. On the other hand, the
fluorite has the property that, when it is heated, it is expanded
and the index of refraction thereof is decreased. Thus, as shown in
a section (a) of FIG. 6, in a positive lens element 49A made of
fluorite, when the temperature is increased, an image plane FA is
displaced to move away from the lens element. Incidentally, the
temperature/index of refraction features of these glass materials
are determined not only by the temperature characteristic of the
index of refraction itself but also by the direction toward which
the image plane is displaced in consideration of the thermal
expansion of the lens element made of the glass material, thermal
expansion of the lens barrel and thermal expansion of the lens
frames.
Now, the distortion of the projection optical system PL1 will be
described with reference to FIGS. 7 and 8. In FIGS. 7 and 8, the
ordinate indicates the image height H, and the abscissa indicates
the magnification .beta. of the projection optical system PL1 at a
given image height H. And, the magnification on the optical axis
(H-0) is represented by the design magnification .beta..sub.0. In
this case, under a certain environmental condition, after the
exposure is continued by a certain time period, the distortion
caused by the lens elements made of quartz in the projection
optical system PL1 becomes the non-linear error (higher order
magnification error) in which, as the image height H is increased,
the magnification .beta. is decreased below the design
magnification once and then is monotonously increased, as shown by
the curve 61 in a section (a) of FIG. 7.
In the illustrated embodiment, by adjusting the temperature of the
lens elements 36A, 37A made of fluorite in the projection optical
system PL1 by means of the lens temperature control device 13 of
FIG. 1, the distortion caused by the lens elements made of fluorite
in the projection optical system PL1 is set to have the non-linear
magnification error having a feature substantially opposite to that
shown by the curve 61 in the section (a) of FIG. 7. However, even
when the distortion caused by the lens elements made of fluorite is
set in this way, the distortion includes a concomitant linear
magnification error as offset. Accordingly, as shown by the curve
62A in a section (b) of FIG. 7, the actual distortion caused by the
lens elements made of fluorite in the projection optical system PL1
has a feature obtained by adding the linear magnification error
shown by the straight line 62B and the non-linear magnification
error having the feature substantially opposite to that shown by
the curve 61 in the section (a) of FIG. 7.
Therefore, if the image characteristic is not corrected by the lens
control device 12 of FIG. 1, the distortion caused by the entire
projection optical system PL1 will have a feature that can be
generally represented by the linear magnification error in which
the magnification .beta. is linearly increased from the design
magnification as the image height H is increased, as shown by the
straight line 63 in a section (c) of FIG. 7. Thus, the linear
magnification error (shown by the straight line 64) which can
substantially cancel the residual magnification error represented
the straight line 63 in the section (c) of FIG. 7 is applied to the
projection optical system PL1 by means of the lens control device
12 of FIG. 1. As a result, the distortion of the projection optical
system PL1 according to the illustrated embodiment has a feature as
shown by the curve 65 in FIG. 8 in which the non-linear
magnification error and the linear magnification error are both
removed substantially.
Next, an example of a method for measuring the distortion of the
projection optical system PL1 will be explained with reference to
FIGS. 9 and 10.
To this end, a test reticle having an illumination area on which a
plural pairs (for example, sixteen pairs) of evaluating marks are
equidistantly disposed is used as the reticle R of FIG. 1. In this
case, desirably, the plural pairs of evaluating marks include
various marks that, when projected, the image heights H thereof
have various values such as 100% of the maximum image height
H.sub.max (i.e., H.sub.max itself), 80% of the maximum image height
H.sub.max (i.e., 0.8 H.sub.max) and the like. Each pair of
evaluating marks are constituted by an X axis evaluating mark
formed by a line-and-space pattern (in which the lines are arranged
to be spaced in the X direction at predetermined constant
intervals) and a Y axis evaluating mark corresponding to what is
obtained by rotating the X axis evaluating mark by 90 degrees. A
projected image MY(i) of the i-th (i=1-16) Y axis evaluating mark
is shown in a section (a) of FIG. 9.
In the section (a) of FIG. 9, the projected image MY(i) has a
pattern in which bright portions P1-P5 are disposed at a
predetermined pitch in the Y direction, and the projected image
MY(i) is scanned in the Y direction by means of the rectangular
opening portion 50a of the distortion measuring sensor 3 by driving
the wafer stage 2 of FIG. 1. In this case, the detection signal S1
outputted from the photoelectric conversion element 52 is A/D
(analogue/digital)-converted in the signal treatment portion 53,
and the treated result is stored in correspondence to the Y axis
value of the wafer stage 2.
A section (a) of FIG. 10 shows the detection signal S1 obtained in
this example. In this example, since the opening portion 50a has
the rectangular shape having a Y direction width capable of
covering all of the bright portions P1-P5, as shown in the section
(a) of FIG. 10, the detection signal S1 is a signal which is
changed steppingly in accordance with the Y axis value. Thus, as an
example, in the signal treatment portion 53, the detection signal
S1 is differentiated by the Y axis values (difference calculation
in the actual treatment) to obtain a derivative signal dS1/dY as
shown in a section (b) of FIG. 10. Then, values Y.sub.1 -Y.sub.5 of
the Y axis where the derivative signal dS1/dY has peak values are
sought or determined. Since these values Y.sub.1 -Y.sub.5
correspond to the bright portions P1-P5 of the projected image
MY(i) of the evaluating marks shown in the section (a) of FIG. 9,
in the treatment portion 53, the Y axis value of the projected
image MY(i) is calculated from the following equation (1):
Regarding the projected image of the i-th axis evaluating mark, by
scanning the opening portion 50a of the distortion measuring sensor
3 in the X direction, X axis value X.sub.i of the projected image
can be determined. In this way, the X axis value X.sub.i and Y axis
value Y.sub.i of the projected images of each pair of evaluating
marks are determined, and the determined values in the coordinates
are supplied to the main control device. 18. The positions
(XD.sub.i, YD.sub.i) (design positions) of the projected images of
each pair of evaluating marks when the projection optical system
PL1 has no magnification error, are previously stored in the memory
in the main control device 18.
In this case, the actually measured position (X.sub.i, Y.sub.i) of
the projected image of the evaluating mark includes the rotational
error of the test reticle. To eliminate this rotational error, in
the main control device 18, the rotational error .DELTA..phi. is
determined by the method of least squares so that the square sum of
the difference between the position in calculation (XC.sub.i,
YC.sub.i) obtained by rotating the design position (XD.sub.i,
YD.sub.i) of the projected image of the evaluating mark by an
amount corresponding to the rotational error .DELTA..phi. and the
actually measured position (X.sub.i, Y.sub.i) becomes minimum.
Accordingly, the deviation amount of the projected images of each
pair of evaluating marks caused by the magnification error of the
projection optical systems PL1 becomes (X.sub.i -XC.sub.i, Y.sub.i
-YC.sub.i). In this case, i=1-16, for example.
In order to divide the deviation amount of the projected image into
the linear magnification error and the non-linear magnification
error, for example, the magnification .beta. is determined with
respect to the image height H on the basis of the following
equation (2) (k=predetermined coefficient):
.beta.=k.times.H+.beta..sub.0 (2)
A value of the coefficient k is determined by the method of squares
so that the square sum of the difference between a position
(XC.sub.i ', YC.sub.i ') obtained by correcting the position in
calculation (XC.sub.i, YC.sub.i) of the projected image of the
evaluating mark by using the magnification .beta. shown in the
above equation (2) and the actually measured position (X.sub.i,
Y.sub.i) of the projected image of the evaluating mark becomes
minimum. In this way, the linear magnification error is determined.
As a result, the deviation amount of the projected image of the
evaluating mark caused by the non-linear magnification error of the
projection optical system PL1 becomes (X.sub.i -XC.sub.i ', Y.sub.i
-YC.sub.i '), and, thus, the residual non-linear magnification
error (higher order magnification error) can be determined as a
function of the image height H, by dividing the deviation amount by
the image height in calculation (image height at position (XC.sub.i
', YC.sub.i ')). Since the magnification errors are measured by a
predetermined plural number of measuring points on the image height
H, the magnification error at any point between the measuring
points may be determined by interpolation on the basis of the
magnification errors at the adjacent measuring points.
Other than the above-mentioned error determining method, for
example, the linear magnification error may be defined as a
magnification error at a position where the image height H becomes
100% of a maximum radium H.sub.max of the exposure area obtained by
the projection optical system (i.e., H=H.sub.max), and the linear
magnification error may be determined by averaging the positional
deviation amounts of the projected images of the evaluating marks
near the image height H.sub.max. In this case, the linear
magnification error at an image height H smaller than H.sub.max is
determined to be proportional to the image height H.
After the linear magnification error determined in this way is
removed, the magnification error remaining at a position where the
image height H becomes 70% of the maximum radius H.sub.max of the
exposure area obtained by the projection optical system (i.e.,
H=0.7 H.sub.max) may be defined as the non-linear magnification
error, and the non-linear magnification error may be determined by
averaging the positional deviation amounts of the projected images
of the evaluating marks near the image height 0.7 H.sub.max.
Next, an example of a method for determining the value of the
temperature to be set for the lens elements 36A and 37A made of
fluorite (FIG. 2) will be explained. It is assumed that the
pressure (atmospheric pressure) of air surrounding the projection
optical system PL1 is "x". If the non-linear magnification error is
generated in the projection optical system PL1 by changing the
atmospheric pressure x from a reference value x.sub.0, the
non-linear magnification error must be canceled. Now, the value set
for the temperature y of the lens elements 36A, 37A for canceling
the non-linear magnification error will be described. In this case,
the non-linear magnification error of the projection optical system
PL1 caused by change in the atmospheric pressure x from the
reference value x.sub.0 is determined by optical calculation. And,
the temperature y of the lens elements 36A, 37A which can generate
non-linear magnification error having the feature opposite to that
of the optically determined non-linear magnification error is
determined by calculation. In this case, for the simplicity's sake,
as mentioned above, the magnification error at the 0.7 H.sub.max
image height position (after the linear magnification error based
on the magnification error at the maximum image height H.sub.max
was eliminated) may be defined as the non-linear magnification
error. However, the temperature of the lens elements 36A, 37A for
generating the non-linear magnification error having the opposite
feature may be determined by the method of least squares on the
basis of the magnification errors at plural image heights H.
The solid curve shown in FIG. 11 represents the temperature y
(y=F(x)) previously determined as a function F(x) of the
atmospheric pressure x. In FIG. 11, the abscissa indicates the
atmospheric pressure x and the ordinate indicates the temperature y
of the lens elements 36A, 37A made of fluorite, and "y.sub.0 "
represents a temperature of the lens elements 36A, 37A for
minimizing the non-linear magnification error at a reference
atmospheric pressure x.sub.0. In the practical use, the temperature
y may be set on the basis of the function F(x).
However, in actual, due to the manufacturing error of the optical
elements of the projection optical system PL1, the non-linear
magnification error cannot often be decreased adequately by using
the temperature y determined on the basis of the function F(x).
Further, as explained in connection with FIG. 12, the illumination
optical system 14 according to the illustrated embodiment shown in
FIG. 1 can be used under the various conditions such as the normal
condition, the modified-shape light source condition and the
condition that the coherent factor (.sigma. value) is small. Thus,
the above-mentioned function F(x) must be calculated for various
illumination conditions. However, the calculated result of the
function F(x) particularly regarding the modified-shape light
source condition and the small coherent factor condition sometimes
has less reliability. To avoid this, it is desirable that the
calibration of the function F(x) is performed in the following
manner in order to further reduce the non-linear magnification
error.
That is to say, in order to seek the optimum function to be well
applied, the non-linear magnification error of the projected image
from the projection optical system PL1 is determined by utilizing
the distortion measuring sensor 3 of FIG. 1 at any atmospheric
pressure x.sub.1 different from the reference atmospheric pressure
x.sub.0. Then, by controlling the temperature of the lens elements
36A, 37A made of fluorite by means of the lens temperature control
device 13, a temperature y.sub.1 at which the non-linear
magnification error of the projected image becomes zero (minimum)
is determined. In this case, the temperature of the lens elements
36A, 37A made of fluorite is made to be changed in the vicinity of
the temperature y determined by the theoretical function F(x).
Thereafter, a pressure in the gas chamber at which the residual
linear magnification error becomes zero is sought by using the lens
control device 12.
Further, regarding another atmospheric pressure x.sub.2 different
from the atmospheric pressure x.sub.1, a temperature y.sub.2 of the
lens elements made of fluorite at which the non-linear
magnification error of the projected image becomes zero and a
pressure in the gas chamber at which the residual linear
magnification error becomes zero are also determined. On the basis
of the actually measured temperatures of the lens elements made of
fluorite at the atmospheric pressures x.sub.0, x.sub.1, x.sub.2, as
shown by the broken line in FIG. 11, the function F(x)
representative of the temperature y of the lens elements made of
fluorite regarding the atmospheric pressure x can be determined,
for example, as a quadratic function. In this case, the pressure in
the gas chamber at which the residual linear magnification error
becomes zero can be determined as a function of the atmospheric
pressure. By increasing the number of the measuring points, the
temperature y of the lens elements made of fluorite may be
determined as a cubic function or function of higher degree of the
atmospheric pressure x. By using such function F(x), the non-linear
magnification error of the projected image can be further
decreased.
Although the function F(x) shown in FIG. 11 was determined under
the normal illumination condition (method shown in the section (a)
of FIG. 12), the similar calibration is effected under other two
illumination conditions (methods shown in the sections (b) and (c)
of FIG. 12). The results that the temperature y at which the
non-linear magnification error of the lens elements made of
fluorite becomes zero are determined as a function of the
atmospheric pressure x under the illumination conditions shown in
the section (b) and (c) of FIG. 12 are shown by the functions F1(x)
and F3(x) in FIG. 13. The function F2(x) shown in FIG. 13 is the
same as the function F(x) shown in FIG. 12, i.e., a function
determined under the normal illumination condition. When the
functions are determined in this way, three functions F1(x)-F3(x)
of FIG. 13 are stored in the memory in the main control device 18
of FIG. 1. In the main control device 18, in accordance with the
measured atmospheric pressure x, a target value of the temperature
y of the lens elements made of fluorite is determined in dependence
upon the function corresponding to the used illumination condition,
and the temperature of the lens elements is set to the target value
by using the lens temperature control device 13.
In the above example, while the non-linear magnification error
caused due to the difference in atmospheric pressure is corrected,
each lens element may be expanded or the index of refraction of
each lens element may be changed by the illumination energy when
the exposure illuminated light is passing through the projection
optical system, and, the non-linear magnification error is
generated by such expansion and/or the change in index of
refraction. To cope with this, it is necessary that the temperature
y of the lens elements made of fluorite at which the non-linear
magnification error is minimized is determined as a function of the
illumination energy e (e represents illumination energy passing
through the projection optical system PL1 per unit time). Further,
in this case, it is necessary that the function is determined for
each illumination condition.
FIG. 14 shows the temperature y of the lens elements made of
fluorite determined by effecting the calibration so that the
non-linear magnification error is minimized for the illumination
energy e. In FIG. 14, the abscissa indicates the illumination
energy e of the exposure illumination light passing through the
projection optical system PL1 and the ordinate indicates the
temperature y of the lens elements made of fluorite at which the
non-linear magnification error is minimized. In this case, the
illumination energy e can be determined by multiplying a signal
(for example, obtained by photoelectrically converting light flux
partly separated from the exposure illumination light in the
illumination optical system 14 of FIG. 1) by a predetermined
conversion coefficient. The functions g1(e), g2(e) and g3(e) of the
illumination energy e shown in FIG. 14 are functions which are
determined for the respective illumination conditions and which
represent the temperatures y at which the non-linear magnification
error is minimized.
In conclusion, ultimately, it is necessary that the temperatures y
of the lens elements made of fluorite at which the non-linear
magnification error is minimized are determined as a function Q (e,
x, I) including the illumination energy e, atmospheric pressure x
and illumination condition I as parameters. The function Q (e, x,
I) is also stored in the memory in the main control device 18 of
FIG. 1, and, in the main control device 18, it is desirable that a
target value of the temperature of the lens elements made of
fluorite is determined from the function Q (e, x, I) in dependence
upon the exposure illumination energy e, atmospheric pressure x and
illumination condition I.
In the above-mentioned embodiment, while an example that the
temperature of the lens elements 36A, 37A is controlled is
explained, even when a temperature of the lens barrel 4 or the lens
frame G2 is controlled, since a distance between the lens elements
is changed, the same technical advantage can be expected.
Accordingly, any mechanism for controlling the temperature of the
lens barrel 4 or the temperature of any lens frame may be provided.
Further, since the temperatures of the lens barrel 4 and the lens
frame G2 are changed (to change the distance between the lens
elements) even when the temperature of the lens elements 36A, 37A
is controlled in the above-mentioned manner, it is desirable that
the apparatus is designed in consideration of such temperature
change.
In the illustrated embodiment, while the excimer laser light source
was used as the exposure light source, regarding the illumination
energy of the exposure illumination light, when i-ray (having a
wavelength of 365 nm) of a memory lamp is used, the energy of the
i-ray is more absorbed than the energy of the excimer laser light
in the projection optical system, and, thus, the non-linear
magnification error of the projection optical system is more
increased that the excimer laser light. Accordingly, when the
method for controlling the temperature of the lens elements made of
fluorite in accordance with the illumination energy is applied to a
projection exposure apparatus (stepper and the like) using the
i-ray of the mercury lamp, the non-linear magnification error can
be reduced more effectively.
Next, a second embodiment of the present invention will be
explained with reference to FIG. 3. In FIG. 3, the same parts or
elements as those in the first embodiment (FIG. 2) are designated
by the same reference numerals and detailed explanation thereof
will be omitted. A projection optical system according to the
second embodiment is particularly suitable to be applied to a
projection exposure apparatus of step-and-scan type.
FIG. 3 shows a projection optical system PL2 according to the
second embodiment. In FIG. 3, the projection optical system PL2 has
a lens barrel 4 within which six lens elements 25-30, four lens
elements 31-34 and four lens elements 35B-38B are fixedly mounted
(in this order from the wafer side). The lens elements 35B to 37B
are secured to the lens barrel 4 via a lens frame G3, and the lens
element 38B is also secured to the lens barrel 4 via a lens frame
(not shown).
In this case, the lens element 36B alone is made of fluorite, and
the other lens elements are made of quartz. A pair of temperature
control elements 40A, 40B are secured to both surfaces of the lens
element 36B at one end thereof in a scan direction (i.e., Y
direction), and a pair of temperature control elements 40C, 40D are
secured to both surfaces of the lens element 36B at the other end
thereof in the Y direction. In this case, when it is assumed that a
width (in the Y direction) of a slit-shaped illumination area
generated by the projection optical system on a pattern forming
surface of a reticle is L, the temperature control elements 40A-40D
are disposed in zones through which the illumination light passing
through the illumination area having the width L does not pass.
The temperature control elements 40A-40D may be heaters, Peltier
effect elements and the like. The Peltier effect elements may be
used to effect the heating or the heat absorbing. Although not
shown, a temperature sensor is secured to the end of the lens
element 36B in the Y direction, and a detection signal from the
temperature sensor is supplied to an external temperature control
device 39. The temperature control device 39 serves to control the
heating or the heat absorbing of the temperature control elements
40A-40D so that the detected temperature becomes equal to a
temperature set by the main control device 18.
In this embodiment, there is also provided a heat exhausting
mechanism for preventing the heat (generated by controlling the
temperature of the lens element 36B as mentioned above) from
affecting an influence upon adjacent lens elements. That is to say,
gas having a predetermined temperature is supplied from a
temperature control device 13a, through a piping 19, to a gas
chamber enclosed by the lens elements 36B, 37B and the lens frame
G3, and the gas circulating in the gas chamber is returned to the
temperature control device 13a through a piping 23B. Similarly, gas
having a predetermined temperature is supplied from the temperature
control device 13a, through a piping 20, to a gas chamber enclosed
by the lens elements 35B, 36B and the lens frame G3, and the gas
circulating in the gas chamber is returned to the temperature
control device 13a through a piping 24B. In response to the command
from the main control device 18, the temperature control device 13a
keeps the temperature of the lens elements 35B, 37B adjacent to the
lens element 36B constant by the forced air conditioning. The other
arrangements are the same as those of the first embodiment shown in
FIG. 2.
As mentioned above, according to the second embodiment, since the
temperature of the lens element 36B made of fluorite is directly
controlled by the temperature control elements 40A-40D while
effectively utilizing the spaces (zones) which are not illuminated
by the illumination light passed through the slit-shaped
illumination area, the temperature of the lens element 36B can be
set to the desired target temperature quickly and accurately. With
this arrangement, the non-linear magnification error of the
projected image obtained by the projection optical system PL2 can
quickly be decreased with high accuracy.
Next, a third embodiment of the present invention will be explained
with reference to FIG. 4. In FIG. 4, the same parts or elements as
those in the first embodiment (FIG. 2) are designated by the same
reference numerals and detailed explanation thereof will be
omitted. A projection optical system according to the third
embodiment is suitable to be applied to both projection exposure
apparatuses of step-and-repeat type and step-and-scan type.
FIG. 4 shows a projection optical system PL3 according to the third
embodiment. In FIG. 4, the projection optical system PL3 has a lens
barrel 4 within which six lens elements 25-30, four lens elements
31, 32, 33A, 34A and four lens elements 35C-38C are fixedly mounted
(in this order from the wafer side). The lens elements 33A, 34A are
secured to the lens barrel 4 via a lens frame G4, the lens elements
35C, 36C are secured to the lens barrel 4 via a lens frame G5, and
the other lens elements are also secured via lens frames (not
shown).
The lens elements 33A, 36C are made of fluorite, and the other lens
elements are made of quartz. In this case, the feature of the
non-linear magnification error is mainly changed by the change in
temperature of the upper fluorite lens element 36C, and the feature
of the linear magnification error is mainly changed by the change
in temperature of the lower fluorite lens element 33A. Gas having a
variable temperature is supplied from a temperature control device
13c, through a piping 41A, to a gas chamber enclosed by the lens
elements 35C, 36C and the lens frame G5, and the gas circulating in
the gas chamber is returned to the temperature control device 13c
through a piping 41B. Similarly, gas having a variable temperature
is supplied from a temperature control device 13d, through a piping
42A, to a gas chamber enclosed by the lens elements 33A, 34A and
the lens frame G4, and the gas circulating in the gas chambers is
returned to the temperature control device 13d through a piping
42B. In response to the command from the main control device 18,
the temperature control devices 13c, 13d serve to set the
temperatures of the lens elements 36C, 33A to their target
temperatures, respectively.
In this embodiment, when the non-linear magnification error (higher
order magnification error) of the projected image obtained by the
projection optical system PL3 is to be corrected, the temperature
of the lens element 36C is controlled by the temperature control
device 13c, and, the linear magnification error caused at that time
is canceled by controlling the temperature of the lens element 33A
by means of the temperature control device 13d. When the feature of
the higher order magnification error due to the change in
atmospheric pressure differs from the feature of the higher order
magnification error due to the change in temperature during the
illumination of the exposure illumination light onto the projection
optical system, so that the third or higher order magnification
error is generated greatly, this method can also be utilized to
control these high order magnifications independently by using the
corresponding two fluorite lens elements. This is to say, for
example, the non-linear magnification error due to the change in
atmospheric pressure may be corrected by using the upper fluorite
lens element 36C, and the non-linear magnification error due to the
illumination of the illumination light may be corrected by using
the lower fluorite lens element 33A.
Next, a fourth embodiment of the present invention will be
explained with reference to FIG. 5. In FIG. 5, the same parts or
elements as those in the first embodiment (FIG. 2) are denoted by
the same reference numerals and detailed explanation thereof will
be omitted. A projection optical system according to the fourth
embodiments is suitable to be applied to both projection exposure
apparatuses of step-and-repeat type and step-and-scan type.
FIG. 5 shows a projection optical system PL4 according to the
fourth embodiment. In FIG. 5, the projection optical system PL4 has
a lens barrel 4A within which six lens elements 25, 26, 27A, 27A,
29A, 30, four lens elements 31, 32, 33B, 34B and two lens elements
35D, 36D are fixedly mounted (in this order from the wafer side). A
support 45 for holding a lens element 37D and a support 46 for
holding a lens element 38D are secured to an upper end of the lens
barrel 4A. The lens elements 28A, 29A are secured to the lens
barrel 4A via a lens frame G6 and the other lens elements are also
secured to the lens barrel 4A via lens frames (not shown).
In this fourth embodiment, the lens elements 28A, 29A are made of
fluorite and the other lens elements are made of quartz. Gas having
a variable temperature is supplied from a temperature control
device 13e, through a piping 43, to a gas chamber enclosed by the
lens elements 28A, 29A and the lens frame G6, and the gas
circulating in the gas chamber is returned to the temperature
control device 13e through a piping 44. In response to the command
from the main control device 18, the temperature control device 13e
serves to set the temperature of the lens elements 28A, 29A to the
target temperature. The supports 45 and 46 can be shifted in a
direction parallel to an optical axis AX of the projection optical
system PL4 and can be inclined by desired angles, independently, by
means of a drive device 47. The operation of the drive device 47 is
controlled by an imaging characteristic control device 48 on the
basis of the command from the main control device 18.
Also in this embodiment, although the non-linear magnification
error (higher order magnification error) of the projected image
generated by the projection optical system PL4 is corrected by
controlling the temperature of the fluorite lens elements 28A, 29A
by means of the temperature control device 13e, the linear
magnification error caused at that time is corrected by inclining
or lifting/lowering of the lens elements 37D, 38D via the supports
45, 46. Since not only the magnification error but also the defocus
of the focusing position and the curvature of field can be
corrected by combination of the movements of two supports 45, 46,
almost all of other aberrations generated during the correction of
the non-linear magnification error can be canceled by properly
controlling the temperature control device 13e and the drive device
47.
Next, referring to the actual numerical example of the projection
optical system, the numerical analysis as to how non-linear
magnification error is changed by the temperature control will be
explained.
In this case, for example, in consideration of the projection
optical system PL1 shown in FIG. 2, the projection optical system
PL1 includes nine lens elements by which the imaging characteristic
can be corrected when these lens elements are made of fluorite.
Now, it is assumed that the lens elements which can be made of
fluorite are lens elements L1 to L9. Regarding the case where no
lens element made of fluorite is used and the case where a single
lens element is made of fluorite and the other lens elements are
made of quartz, the magnification errors .beta.A [.mu.m] at the
position of 100% of the image height, the magnification errors
.beta.B [.mu.m] at the position of 70% of the image height, and the
magnification errors (non-linear magnification errors) .beta.C
[.mu.m] remaining at the position of 70% of the image height after
the magnification errors at the position of 100% of the image
height are corrected to become substantially zero were sought.
Further, to provide the case where the single lens element is made
of fluorite, the lens elements L1 to L9 are made to be of fluorite
in turn, and, the magnification errors .beta.A [.mu.m], the
magnification errors .beta.B [.mu.m] and the residual magnification
errors (non-linear magnification errors) .beta.C [.mu.m] obtained
when the temperature of the single fluorite lens element is changed
by 1.degree. C. are shown in the following Table 1:
TABLE 1 Fluorite .beta.A [.mu.m] .beta.B [.mu.m] .beta.C [.mu.m]
none -0.0144 -0.0204 +0.0100 L1 +0.0436 +0.0135 -0.0067 L2 -0.2797
-0.1862 +0.0199 L3 -0.2254 -0.1529 +0.0152 L4 -0.2576 -0.1739
+0.0167 L5 +0.1602 +0.0926 -0.0092 L6 +0.1045 +0.0572 -0.0056 L7
+0.2371 +0.1447 -0.0110 L8 +0.2911 +0.1793 -0.0142 L9 +0.3854
+0.2406 -0.0192
In the above Table 1, for example, when the lens element L4 is made
of fluorite, the residual magnification errors (non-linear
magnification errors) .beta.C becomes greater than that of none
fluorite case by 0.0067 .mu.m, i.e., about 7 nm. Further, when the
lens element L9 is made of fluorite, the residual magnification
errors (non-linear magnification errors) .beta.C is changed from
that of non fluorite case by 0.0292 .mu.m, i.e., about 29 nm. Thus,
it can be understood that, by changing the temperature of the
single fluorite lens element by .+-.1.degree. C., the non-linear
magnification error can be corrected by about .+-.20 nm at the
most.
In general, since the maximum resolving power of the temperature
control is about 0.01.degree. C., when the non-linear magnification
error is corrected by changing the temperature of the single
fluorite lens element by 1.degree. C., the resolving power of the
correction of the non-linear magnification error becomes about
.+-.0.2 nm (=.+-.20/100 [nm]). Further, from the Table 1, it can be
seen that the linear magnification error of about .+-.0.2 .mu.m is
generated by changing the temperature of the single fluorite lens
element by 1.degree. C. Accordingly, the resolving power of the
control of the linear magnification error when the temperature of
the single fluorite lens element is changed by 1.degree. C. becomes
a high level of about .+-.2 nm (=.+-.0.2/100 [.mu.m]).
When two or more lens elements are made of fluorite, the
temperature control range can be narrowed, possible additional
linear magnification error can be reduced, and/or a greater
non-linear magnification error can be corrected.
According to the first to fourth embodiment of the present
invention, the projection optical system includes a plurality of
sets of optical members (lens elements) made of glass elements
having different temperature characteristics of index of
refraction, and, the imaging characteristic of the projection
optical system is controlled by controlling the temperature of at
least one of the optical members. Accordingly, the distortion of
the projection optical system which is worsened by the change in
the environmental conditions such as the atmospheric pressure and
the temperature around the projection optical system, the change in
the illumination condition (for example, normal condition,
modified-shape light source illumination condition or the like) and
the absorption of the exposure illumination light can be corrected.
As a result, the aligning accuracy of the projected images on the
photosensitive substrate can be improved.
Further, when the imaging characteristic of the projection optical
system to be controlled by the temperature control device is the
non-linear magnification error, such non-linear magnification error
(which could not corrected by the conventional techniques) can also
be corrected.
In this case, when the non-linear magnification error of the
projection optical system is corrected within a range of .+-.50 nm
by changing the temperature of the optical member (to be
controlled) by .+-.1.degree. C. by means of the temperature control
device, as mentioned in the above numerical analysis, such
non-linear magnification error of the projection optical system is
corrected within such a range by controlling the temperature(s) of
one or two fluorite lens element(s). It is practical.
In the case where the linear magnification control means for
correcting the linear magnification error of the projection optical
system is provided, after the non-linear magnification error of the
projection optical system was corrected to be included within the
predetermined allowable range, and when the residual linear
magnification error is reduced by the linear magnification control
means, the distortion of the projection optical system can be
reduced to substantially zero.
In the case where the memory means for storing the change amount of
the imaging characteristic of the projection optical system in
dependence upon the change in the applied condition of the
projection optical system is provided, when the imaging
characteristic of the projection optical system is controlled by
the temperature control device to cancel the change amount of the
imaging characteristic stored in the memory means in response to
the change in the applied condition of the projection optical
system an advantage is brought about. Namely, if the applied
condition of the projection optical system is changed, the imaging
characteristics (such as the distortion and the like) of the
projection optical system can be returned to the good condition
without time lag.
FIG. 5A shows an alteration of the fourth embodiment shown in FIG.
5. In this alteration, although the arrangement of the lens
elements is the same as that of the fourth embodiment, the lens
elements may be of the same material. A spacer between the lens
elements 28A and 29A acts as a gas chamber which is connected to a
bellows mechanism 12b through a piping 12c, and an
extension/contraction amount of the bellows mechanism 12b is
controlled by a control portion 12a. The second portion 12a,
bellows mechanism 12b and piping 12c constitute the lens control
device 12 of FIG. 1, and a pressure of gas (for example, air) in
the gas chamber is controlled by adjusting the
expansion/contraction amount of the bellows mechanism 12b.
As mentioned above, in this alteration, although the temperature
control of the lens elements is not effected, also in this case,
the non-linear magnification error and the curvature of field can
be corrected by combination of independent control for adjusting
the shifting (along the optical axis) and the inclination of the
lens elements 37D, 38D by means of the drive device 47 and the
above-mentioned lens control device 12.
In this case, since the focus position is changed, it is necessary
that an autofocus mechanism is incorporated. However, the autofocus
mechanism itself is well-known, and, if necessary, an autofocus
mechanism shown in FIG. 17 may be referred to.
The following embodiments of the present invention show examples
that the present invention is applied to the correction of defocus
of the image plane of the projection optical system. Prior to the
explanation of these embodiments, the defocus of the image plane
will be explained.
In the conventional techniques, the non-linear defocus component of
the defocus of the image plane of the projection optical system
generated by the change in the environmental condition could not be
corrected. A main part of the non-linear defocus component is the
curvature of field in which the defocus amount is changed in
accordance with the image height of the projected image along a
curve similar to the quadratic function or functions of higher
degree. That is to say, in a reference condition for design, as
shown in a section (a) of FIG. 29, a focal position Z.sub.F of the
image plane of the projection optical system is situated in the
vicinity of a target position in design, regardless of the image
height.
However, if the change in atmospheric pressure and/or the heat
absorption due to the illumination of the exposure illumination
light is generated under the reference condition, the defocus
feature of the image plane will be changed to the curvature of
field as shown in a section (b) of FIG. 29. If such curvature of
field is generated, a width of the depth of focus (DOF) of the
entire image plane will be narrowed, with the result that it will
be difficult to obtain the desired resolving power through the
entire projected image of the reticle pattern.
If the curvature of field as shown in the section (b) of FIG. 29 is
generated in the projection exposure apparatus of step-and-repeat
type (collective exposure type), as shown in a section (a) of FIG.
30, the focus position of the original image plane 72 of the
projected image is changed in the non-linear manner in accordance
with the image height to an image plane 73A, thereby narrowing the
depth of focus totally. On the other hand, if the curvature of
field as shown in the section (b) of FIG. 29 is generated in the
projection exposure apparatus of step-and-scan type (scan exposure
type), as shown in a section (b) of FIG. 30, the defocus is not
generated in the original image plane 72 of the projected image
along the Y direction (scan direction) due to the averaging effect.
In this case, however, the image is deteriorated due to such
averaging effect. In the non-scan direction (X direction), as in
the collective exposure type, since the defocus is generated in
dependence upon the image height, the effective image plane after
the scan exposure becomes an image plane 73B as shown. Thus, a
so-called "U-shaped" curvature of field is generated.
Recently, since a size of the reticle has been increased, when the
large size reticle is used, there arises a problem that the
curvature of field is increased. For example, in the case where the
periphery of the reticle is held by vacuum as shown in a section
(a) of FIG. 31, when a small size reticle RA (about 5 in..times.5
in.) is supported by four support portions 74A, since the reticle
RA is not flexed, there is no problem. However, as shown in a
section (b) of FIG. 31, when a large size reticle RB (for example,
6-9 in..times.6-9 in.) is supported by four support portions 74B,
since the reticle RB is flexed by its own weight to a "U-shaped"
form in a direction (X direction) perpendicular to a longitudinal
direction (Y direction) of the support portion 74B, there arises a
problem that the curvature of field of the image plane is increased
accordingly. In particular, when the reticle RB shown in the
section (b) of FIG. 31 is used in the projection exposure apparatus
of step-and-scan type (scan exposure type) having the scan
direction along the Y direction, there is the danger of increasing
the "U-shaped" curvature of field shown in the section (b) of FIG.
30.
Now, fifth to eighth embodiments according to the present invention
will be explained with reference to FIGS. 17 to 21.
FIG. 17 shows a construction common to the fifth to eighth
embodiments, which corresponds to the construction shown in FIG. 1.
Thus, the elements corresponding to those shown in FIG. 1 are
designated by the same reference numerals and detailed explanation
thereof will be omitted.
An imaging characteristic measuring sensor 103 is secured on the
wafer stage 2 in the vicinity of the wafer W. As shown in a section
(b) of FIG. 24, the imaging characteristic measuring sensor 103 has
a glass substrate 51 on which a light shield film 50 including a
rectangular opening portion 50a in the same height as a surface of
the wafer W is coated, a photoelectric conversion element 52 for
photoelectrically converting the exposure illumination light passed
through the opening portion 50a into a detection signal, and a
signal treatment portion 153 for treating the detection signal S1
from the photoelectric conversion element 52. The treated result
from the signal treatment portion 153 is supplied to a main control
device 118 shown in FIG. 17. In the illustrated embodiment, as will
be described later, by driving the wafer stage 2, the projected
image of the pattern on the reticle R is scanned by the opening
portion 50a of the imaging characteristic measuring sensor 103. In
this case, on the basis of the detection signal S1 outputted from
the photoelectric conversion element 52, the signal treatment
portion 153 seeks or determines distribution of focus positions
(curvature of field) of the image plane of the projection optical
system PL5 at various image heights.
Further, a lens temperature control device 13 for controlling a
temperature or temperatures of one or more lens elements made of
fluorite is connected to a projection optical system PL5. In the
illustrated embodiment, if the non-linear defocus (curvature of
field and the like) of the projection optical system PL5 is
worsened in accordance with the change in the environmental
condition or the history of the illumination amount, the non-linear
defocus is corrected by the lens temperature control device 13 on
the basis of the command from the main control device 118. If the
linear magnification error of the projection optical system PL5 is
worsened by correcting the non-linear defocus, the linear
magnification error is corrected by the lens control device 12.
The projection exposure apparatus according to the illustrated
embodiment includes a focusing position detecting system comprising
a projection optical system 68 for obliquely directing a spot light
beam onto the wafer at a plurality of measuring points on the
exposure area of the projection optical system PL5 and a light
receiving optical system 69 for receiving a light beam reflected
from each measuring point to re-focus (re-form) a spot light beam
and for outputting a focus signal corresponding to a lateral
deviation amount of the re-formed spot light beam (relative to the
original spot light beam). A plurality of focus signals from the
light receiving optical system 69 are supplied to the main control
device 118. In this case, if the position of the surface of the
wafer W is changed (focus position) along the optical axis
direction (Z direction) of the projection optical system PL5, since
a level of the corresponding focus signal is also changed, the main
control device 118 can monitor the focus position of the wafer W.
And, in the main control device 118, by controlling the operation
of the drive mechanism for driving the wafer stage 2 in the Z
direction by means of a stage control system 11, the focus position
at a center of the average surface of the shot area (to be exposed)
on the wafer W is maintained in a predetermined focus position at a
center of the averaging surface of the image plane of the
projection optical system PL5.
In the illustrated embodiment, when the curvature of field of the
projection optical system PL5 is corrected, the focus position at
the center of the average surface of the image plane is often
offset. Thus, if such offset is generated, the focus position of
the wafer W is corrected to cancel the offset by driving the wafer
stage 2, thereby preventing the defocus from generating between the
image plane and the surface of the wafer W. Further, in the
illustrated embodiment, a mechanism (levelling stage) for
correcting an inclination angle of the wafer W is provided on the
wafer stage 2, so that the main control device 118 determines the
inclination angle of the wafer W on the basis of the focus
positions at the plurality of measuring points of the focusing
position detecting system, and the inclination angle of the wafer W
is aligned with the inclination angle of the image plane.
Further, in the illustrated embodiment, an alignment sensor 70 of
off-axis imaging type for detecting positions of alignment wafer
marks provided on the short areas of the wafer W is disposed at a
side of the projection optical system PL5.
Next, an example of a method for removing the curvature of field of
the projection optical system PL5 according to the illustrated
embodiment will be explained. Incidentally, regarding the following
description, FIG. 18 shows the fifth embodiment of the present
invention corresponding to FIG. 2, and, thus, the elements
corresponding to those shown in FIG. 2 are designated by the same
reference numerals and explanation thereof will be omitted.
First of all, the curvature field of the projection optical system
PL5 will be described with reference to FIGS. 22 and 23. In FIGS.
22 and 23, the abscissa indicates the image height H and the
ordinate indicates the focus position Z.sub.F of the image plane of
the projection optical system PL5 at various image heights H. The
focus position on the optical axis (H=0) is shown as a design focus
position. In this case, under a certain condition, after the
exposure is continued for a certain time period, the image plane
obtained by the quartz lens elements of the projection optical
system PL5 has downwardly-convex curvature of field as shown by the
curve 61A in a section (a) of FIG. 22.
To the contrary, in the illustrated embodiment, by adjusting the
temperature of the fluorite lens elements 36A, 37A of the
projection optical system PL5 by means of the lens temperature
control device 13 of FIG. 17, the curvature of field generated by
the fluorite lens elements of the projection optical system PL5 is
set to have a curvature substantially opposite to the curvature of
the curve 61A shown in the section (a) of FIG. 22. However, even
when the curvature of field generated by the fluorite lens elements
is set in this way, the offset which is uniform through the entire
range of the image height H is added to the focus position of the
image plane. Accordingly, the distribution of the focus positions
of the image plane obtained by the fluorite lens elements of the
projection optical system PL5 has a feature in which the curvature
of field substantially opposite to the curvature of the curve 61
shown in the section (a) of FIG. 22 includes the predetermined
offset, as shown by the curve 61C in a section (b) of FIG. 22.
Thus, as shown by the straight line 63A in a section (c) of FIG.
22, the focus position Z.sub.F of the image plane of the entire
projection optical system PL5 becomes substantially constant
through the entire range of the image height H, thereby generating
the defocus between the image plane and the surface of the wafer W.
To cancel the defocus, the focus on the surface of the wafer W is
corrected by an amount corresponding to the defocus by driving the
wafer stage 2 in the Z direction with respect to the projection
optical system PL5 by means of the drive mechanism. This means that
the offset of the defocus position as shown by the straight line
64A in the section (c) of FIG. 22 is added to the image plane of
the projection optical system PL5. As a result, the distribution of
the focus position Z.sub.F of the image plane of the projection
optical system PL5 according to the illustrated embodiment with
respect to the surface of the wafer W becomes to that shown by the
curve 64A in FIG. 23, thereby eliminating the curvature of field
and the predetermined defocus. Accordingly, the width of the depth
of focus of the entire image plane is widened in comparison with
the conventional techniques, with the result that the projected
image can be projected with a higher resolving power as a
whole.
By adjusting the temperature of the fluorite lens elements 36A,
37A, the linear magnification error of the image plane (error in
which the magnification is changed in proportion to the image
height) is often worsened. If such linear magnification error is
generated, a linear magnification error which can substantially
cancel the generated linear magnification error is added to the
image plane by the lens control device 12 of FIG. 17. As a result,
the imaging characteristic of the projection optical system PL5
according to the illustrated embodiment has a good feature having
no curvature of field and no distortion.
Now, an example of a method for measuring the distribution of the
focus positions of the image plane of the projection optical system
PL5 will be explained with reference to FIGS. 24 and 25.
To this end, a test reticle having a plural pairs (for example,
sixteen pairs) evaluating marks disposed equidistantly in the
illumination area is used as the reticle R of FIG. 17. Each pair of
evaluating marks are constituted by an X axis evaluating mark
formed by a line-and-space pattern (in which the lines are spaced
in the X direction at predetermined intervals) and a Y axis
evaluating mark corresponding what is obtained by rotating the X
axis evaluating mark by 90 degrees. A projected image MY(i) of the
i-th (i=1-16) Y axis evaluating mark is shown in a section (a) of
FIG. 24. In the following description, although the detection of
the focus position of the image plane (best focus plane) by using
the projected image of the Y axis evaluating mark is explained, the
X axis evaluating mark may also be used, or a difference between
the focus positions obtained by X axis and Y axis evaluating marks
may be determined for use.
In the section (a) of FIG. 24, the projected image MY(i) has a
pattern in which bright portions P1-P5 are disposed at a
predetermined pitch in the Y direction, and the projected image
MY(I) is scanned in the Y direction by means of the rectangular
opening portion 50a of the imaging characteristic measuring sensor
103 by driving the wafer stage 2 of FIG. 17. In this case, the
detection signal S1 outputted from the photoelectric conversion
element 52 shown in a section (b) of FIG. 24 is A/D
(analogue/digital)-converted in a signal treatment portion 153, and
the treated result is stored in correspondence to the Y axis value
of the wafer stage 2.
A section (a) of FIG. 25 shows the detection signal S1 obtained in
this example. In this example, since the opening portion 50a has
the rectangular shape, as shown in the section (a) of FIG. 24, the
detection signal S1 is a signal which is changed steppingly in
accordance with the Y axis value. Thus, as an example, in the
signal treatment portion 153, the detection signal S1 is
differentiated by the Y axis value (difference calculation in the
actual treatment) to obtain a derivative signal dS1/dY as shown in
a section (b) of FIG. 25. Then, positions Y.sub.1 -Y.sub.5 on the Y
axis where the derivative signal dS1/dY has peak values correspond
to the bright portions P1-P5 of the projected image MY(i) of the
evaluating mark shown in the section (a) of FIG. 24. When the
imaging plane of the imaging characteristic measuring sensor 103 is
positioned in the focus position (best focus position) of the image
plate, the contrast of the waveform of the derivative signal dS1/dY
corresponding to the bright portions P1-P5 becomes maximum, and an
average value .DELTA.S (corresponding to the inclination of the
detection signal S1 shown in a section (a) of FIG. 25) of heights
of the waveform corresponding to the bright portions P1-P5 becomes
maximum. Thus, in the signal treatment portion 153, the average
value .DELTA.S of the heights of the peak portions of the
derivative signal dS1/dY is determined. In this way, regarding all
of sixteen projected images, the average values .DELTA.S of the
heights of the peak positions are determined. That is to say, the
average value .DELTA.S of the heights of the peak portions
regarding the sixteen projected images are successively determined
while changing the Z axis position of the imaging characteristic
measuring sensor 103 by a predetermined step width by driving the
wafer stage 2 in the Z direction by means of the drive
mechanism.
As a result, regarding the projected images having a certain image
height among sixteen projected images, a measurement data as shown
by the plotted curve in a section (c) of FIG. 25 is obtained. In
the section (c) of FIG. 25, the abscissa indicates the Z axis
position of the imaging plane of the imaging characteristic
measuring sensor 103 and the ordinate indicates the average values
.DELTA.S of the heights of the peak portions. In the signal
treatment portion 153, the plotted curve shown in the section (c)
of FIG. 25 is closely resembled to a quadratic curve 71, for
example, by the method of least squares, and the Z axis position
Z.sub.F where the quadratic curve 71 has a peak is regarded as the
focus position of the imaging plane at that image height. By
determining the focus positions Z.sub.F of the image planes at
various image heights, the distribution of the focus positions
(curvature of field) as shown in FIG. 23 in the present
environmental condition can be measured. Since the focus positions
at the predetermined plural measuring points on the image height H
are measured, the focus position at any point between the measuring
points may be determined by interpolation on the basis of the focus
positions at the adjacent measuring points.
Next, an example of a method for determining the temperature to be
set for the lens elements 36A and 37A made of fluorite (FIG. 18)
will be explained. It is assumed that the pressure (atmospheric
pressure) of air surrounding the projection optical system PL5 is
"x". If the curvature of field exceeding the allowable value is
generated in the projection optical system PL5 by change in the
atmospheric pressure x from a reference value x.sub.0, such
curvature of field must be canceled. Now, the value of the
temperature y to be set for the lens elements 36A, 37A for
canceling the curvature of field will be described. In this case,
the curvature of field of the projection optical system PL5 caused
by changing the atmospheric pressure x from the reference value
x.sub.0 is determined by optical calculation. And, the temperature
y of the lens elements 36A, 37A which can generate curvature of
field having a feature opposite to that of the optically determined
curvature of field is determined by calculation.
The solid curve shown in FIG. 26 represents the temperature y
(y=FA(x)) previously determined as a function FA(x) of the
atmospheric pressure x. In FIG. 26, the abscissa indicates the
atmospheric pressure x and the ordinate indicates the temperature y
of the lens elements 36A, 37A made of fluorite, and "y.sub.0 "
represents a temperature of the lens elements 36A, 37A for
minimizing the curvature of field at a reference atmospheric
pressure x.sub.0. In the practical use, the temperature y may be
set on the basis of the function FA(x).
However, in actual, due to the manufacturing error of the optical
elements of the projection optical system PL5, the curvature of
field cannot often be decreased adequately by using the temperature
y determined on the basis of the function FA(x). Further, as
explained in connection with FIG. 12, the illumination optical
system 14 according to the illustrated embodiment shown in FIG. 17
can be used under the various conditions such as the normal
condition, the modified-shape light source condition and the
condition that the coherent factor (.sigma. value) is small. Thus,
the above-mentioned function FA(x) must be calculated for each of
various illumination conditions. However, the calculated result of
the function FA(x) particularly regarding the modified-shaped light
source condition and the small coherent factor condition sometimes
has less reliability. To avoid this, it is desirable that the
calibration of the function FA(x) is performed in the following
manner in order to further reduce the curvature of field.
That is to say, in order to seek the optimum function to be well
applied, the curvature of field of the projected image from the
projection optical system PL5 is obtained by utilizing the imaging
characteristic measuring sensor 103 of FIG. 17 at any atmospheric
pressure x.sub.1 different from the reference atmospheric pressure
x.sub.0. Then, by controlling the temperature of the lens elements
36A, 37A made of fluorite by means of the lens temperature control
device 13, a temperature y.sub.1 at which the curvature of field of
the projected image becomes zero (minimum) is determined. In this
case, the temperature of the lens elements 36A, 37A made of
fluorite is made to be changed in the vicinity of the temperature y
determined by the theoretical function FA(x). Thereafter, a
pressure in the gas chamber at which the residual linear
magnification error becomes zero is also determined by using the
lens control device 12. At the same time, the offset of the
residual focus positions is also sought.
Further, regarding another atmospheric pressure x.sub.2 different
from the atmospheric pressure x.sub.1, a temperature y.sub.2 of the
lens elements made of fluorite at which the curvature of field of
the projected image becomes zero and a pressure in the gas chamber
at which the residual linear magnification error becomes zero are
also determined. And, the offset of the residual focus positions is
also determined. On the basis of the actually measured temperatures
of the lens elements made of fluorite at the atmospheric pressures
x.sub.0, x.sub.1, x.sub.2, as shown by the broken line in FIG. 26,
the function FA'(x) representative of the temperature y of the lens
elements made of fluorite regarding the atmospheric pressure x can
be determined, for example, as a quadratic function. In this case,
the pressure in the gas chamber at which the residual linear
magnification error becomes zero can be determined as a function of
the atmospheric pressure x. At the same time, the residual offset
of the focus position can be determined as a function of the
atmospheric pressure x. By increasing the number of the measuring
points, the temperature y of the lens elements made of fluorite and
the like may be determined as a cubic function of the atmospheric
pressure x. By using such function FA(x), the curvature of field of
the projected image can be further decreased.
Although the function FA'(x) shown in FIG. 26 was determined under
the normal illumination condition (method shown in the section (a)
of FIG. 12), the similar calibration is effected under other two
illumination conditions (methods shown in the section (b) and (c)
of FIG. 12). The results that the temperatures y at which the
curvature of field of the lens elements made of fluorite becomes
zero are determined as a function of the atmospheric pressure x
under the illumination conditions of the sections (b) and (c) of
FIG. 12 are shown by the functions FA1(x) and FA3(x) in FIG. 27.
The function FA2(x) shown in FIG. 27 is the same as the function
FA'(x) shown in FIG. 26, i.e., a function determined under the
normal illumination condition. When the functions are determined in
this way, three functions FA1(x)-FA3(x) of FIG. 27 are stored in
the memory in the main control device 118 of FIG. 17. In the main
control device 118, in accordance with the measured atmospheric
pressure x, a target value of the temperature y of the lens
elements made of fluorite is determined in dependence upon the
function corresponding to the used illumination condition, and the
temperature of the lens elements is set to the target value by
using the lens temperature control device 13.
In the above example, while the curvature of field caused due to
the difference in atmospheric pressure is corrected, each lens
element may be expanded or the index of refraction of each lens
element may be changed by the illumination energy when the exposure
illumination light is passing through the projection optical
system, and, the curvature of field is generated by such expansion
and/or the change in index of refraction. To cope with this, it is
necessary that the temperature y of the lens elements made of
fluorite at which the curvature of field is minimized is determined
as a function of the illumination energy e (e represents
illumination energy passing through the projection optical system
PL5 per unit time). Further, in this case, it is necessary that the
function is determined for each illumination condition.
FIG. 28 shows the temperature y of the lens elements made of
fluorite determined by effecting the calibration so that the
curvature of field is minimized for the illumination energy e. In
FIG. 28, the abscissa indicates the illumination energy e of the
exposure illumination light passing through the projection optical
system PL5 and the ordinate indicates the temperature y of the lens
elements made of fluorite at which the curvature of field is
minimized. In this case, the illumination energy e can be
determined by multiplying a signal (for example, obtained by
photoelectrically converting light flux partly separated from the
exposure illumination light in the illumination optical system 14
of FIG. 17) by a predetermined conversion coefficient. The
functions gA1(e), gA2(e) and gA3(e) of the illumination energy e
shown in FIG. 28 are functions which are determined for the
respective illumination conditions shown in the sections (b), (a),
(c) of FIG. 12 and which represent the temperatures y at which the
curvature of field is minimized.
In conclusion, ultimately, it is necessary that the temperatures y
of the lens elements made of fluorite at which the curvature of
field is minimized are determined as a function Q (e, x, I)
including the illumination energy e, atmospheric pressure x and
illumination condition I as parameters. The function Q (e, x, I) is
also stored in the memory in the main control device 118 of FIG.
17, and, in the main control device 118, it is desirable that a
target value of the temperature of the lens elements made of
fluorite is determined from the function Q (e, x, I) in dependence
upon the exposure illumination energy e, atmospheric pressure x and
illumination condition I.
As explained in connection with FIG. 31, when the large size
reticle is used, the flexure of the reticle is increased, thereby
worsening the curvature of field of the image plane. To cope with
this, it is desirable that the curvature of fields of the image
planes of various reticles having different sizes and different
kinds are previously determined by using the imaging characteristic
measuring sensor 103, and the temperature of the fluorite lens
elements is controlled to minimize the curvature of field regarding
the used reticle. In this way, even when a reticle having a great
area is used, the flexure of the reticle and the curvature of field
can be canceled with each other.
In the illustrated embodiment, while the excimer laser light source
is used as the exposure light source, regarding the illumination
energy of the exposure illumination light, when i-ray (having a
wavelength of 365 nm) of a mercury lamp is used, the energy of the
i-ray is more absorbed than the energy of the excimer laser light
in the projection optical system, and, thus, the curvature of field
of the projection optical system is more increased than the excimer
laser light. Accordingly, when the method for controlling the
temperature of the lens elements made of fluorite in accordance
with the illumination energy is applied to a projection exposure
apparatus (stepper and the like) using the i-ray of the mercury
lamp, the curvature of field can be reduced more effectively.
Next, a sixth embodiment of the present invention will be explained
with reference to FIG. 19. Since FIG. 19 corresponds to FIG. 3, the
elements corresponding to those shown in FIG. 3 are designated by
the same reference numerals and explanation thereof will be
omitted. A projection optical system according to this embodiment
is particularly suitable for a projection exposure apparatus of
step-and-scan type.
According to this embodiment, since the temperature of the lens
element 36B made of fluorite is directly controlled by the
temperature control elements 40A-40D while effectively utilizing
the spaces (zones) which are not illuminated by the illumination
light passed through the slit-shaped illumination area, the
temperature of the lens elements 36B can be set to the desired
target temperature quickly and accurately. With this arrangement,
the curvature of field of the projected image obtained by a
projection optical system PL6 can quickly be corrected with high
accuracy.
Next, a seventh embodiment of the present invention will be
explained with reference to FIG. 20. Since FIG. 20 corresponds to
FIG. 4, the elements corresponding to those shown in FIG. 4 are
designated by the same reference numerals and explanation thereof
will be omitted. A projection optical system according to this
embodiment is suitable to be applied to both projection exposure
apparatuses of step-and-repeat type and step-and-scan type.
In this embodiment, as mentioned above, the lens elements 33A, 36C
are made of fluorite, and the other lens elements are made of
quartz. In this case, the feature of the curvature of field is
mainly changed by the change in temperature of the upper fluorite
lens element 36C, and the feature of the linear magnification error
is mainly changed by the change in temperature of the lower
fluorite lens element 33A. The gas having a variable temperature is
supplied from the temperature control device 13c, through the
piping 41A, to the gas chamber enclosed by the lens elements 35C,
36C and the lens frame G5, and the gas circulating in the gas
chamber is returned to the temperature control device 13c through
the piping 41B, the gas having a variable temperature is supplied
from the temperature control device 13d, through the piping 42A, to
the gas chamber enclosed by the lens elements 33A, 34A and the lens
frame G4, and the gas circulating in the gas chamber is returned to
the temperature control device 13d through the piping 42B. And, on
the basis of the command from the main control device 118, the
temperature control devices 13c, 13d set the temperature of the
lens elements 36C, 33A to their target temperature values.
In this embodiment, when the curvature of field of the projected
image obtained by a projection optical system PL7 is corrected, the
temperature of the lens element 36C is controlled by the
temperature control device 13c, and the generated linear
magnification error is canceled by controlling the temperature of
the lens element 33A by means of the lens temperature control
device 13d. When the feature of the curvature of field due to the
change in atmospheric pressure differs from the feature of the
curvature of field due to the change in temperature during the
illumination of the exposure illumination light onto the projection
optical system, so that the third or higher order error of the
focus position is generated greatly, this method can also be
utilized to control the curvature of fields independently by using
the corresponding two fluorite lens elements. That is to say, for
example, the curvature of field due to the change in atmospheric
pressure may be corrected by using the upper fluorite lens element
36C, and the curvature of field due to the illumination of the
illumination light may be corrected by using the lower fluorite
lens element 33A.
Next, an eighth embodiment of the present invention will be
explained with reference to FIG. 21. Since FIG. 21 corresponds to
FIG. 5, the elements corresponding to those shown in FIG. 5 are
designated by the same reference numerals and explanation thereof
will be omitted. A projection optical system according to the
eighth embodiment is suitable to be applied to both projection
exposure apparatuses of step-and-repeat type and step-and-scan
type.
Also in this embodiment, by controlling the temperature of the
fluorite lens elements 28A, 29A by means of the temperature control
device 13e, the curvature of field of the projected image obtained
by a projection optical system PL8 is corrected. And, the linear
magnification error generated by the correction of the curvature of
field is corrected by inclining and/or lifting/lowering the lens
elements 37D, 38D via the supports 45, 46. By combination of
movements of two supports 45, 46, since not only the magnification
error but also the defocus of the focusing position can be
corrected, almost all other aberrations generated during the
correction of the curvature of field can be canceled by properly
controlling the temperature control device 13e and the drive device
47.
Next, referring to the actual numerical example of the projection
optical systems according to fifth to eighth embodiments, the
numerical analysis as to how the curvature of field is changed by
the temperature control will be explained.
In this case, for example, in consideration of the projection
optical system PL5 shown in FIG. 18, the projection optical system
PL5 includes two lens elements by which the imaging characteristic
can be corrected when these lens elements are made of fluorite.
Now, it is assumed that the lens elements which can be made of
fluorite are lens elements L1 and L2. Regarding the case where no
lens element made of fluorite is used and the case where a single
lens element L1 or L2 is made of fluorite and the other lens
elements are made of quartz, the magnification errors .beta.D
[.mu.m] at the position of 100% of the imaging height, and the
deviation amounts (i.e., curvature of field) .beta.E [.mu.m] of the
focus positions in the Z direction at the same image height were
sought. The magnification errors .beta.D [.mu.m] and the deviation
amounts .beta.E [.mu.m] of the focus positions when the temperature
of the single fluorite lens elements is changed by 1.degree. C.,
with the focus position at the image height of 0 (zero) being
considered as a reference, are shown in the following Table 2:
TABLE 2 Fluroite .beta.D [.mu.m] .beta.E [.mu.m] none 0 0 1.1
0.3041 0.1180 1.2 0.3971 0.2058
From the above Table 2, it can be understood that, by changing the
temperature of the single fluorite lens element by .+-.1.degree.
C., the curvature of field of about .+-.0.2 .mu.m can be corrected.
In general, since the maximum resolving power of the temperature
controls is about 0.01.degree. C., when the curvature of field is
corrected by changing the temperature of the single fluorite lens
element by 1.degree. C., the resolving power of the correction of
the curvature of field becomes about .+-.2 nm
(=.+-.0.2/100/[.mu.m]). Further, when two or more lens elements are
made of fluorite, the temperature control range can be narrowed,
possible additional linear magnification error can be reduced,
and/or greater curvature of field can be corrected.
According to the fifth to eighth embodiments of the present
invention, since the temperature control members for controlling
the temperature of at least one optical member is provided, the
defocus of the image plane of the projection optical system
(particularly, non-linear defocus such as the curvature of field)
which is worsened by the change in the environmental condition such
as the atmospheric pressure around the projection optical system,
the absorption of the exposure illumination light and the flexure
of the reticle can be corrected. As a result, the depth of focus of
the entire projected image can be widened, thereby improving the
resolving power of the entire projected image.
Further, when the imaging characteristic of the projection optical
system to be controlled by the temperature control device is the
curvature of field, and the magnification error generated upon the
correction of the curvature of field of the projection optical
system by means of the temperature control device is reduced by a
magnification error control means for controlling the magnification
error of the optical projection system, the magnification error
such as linear magnification error generated upon the correction of
the curvature of field can be effectively reduced, thereby
maintaining the good imaging characteristic as a whole.
When the curvature of field of the projection optical system is
corrected within a range of .+-.0.3 .mu.m or less by changing the
temperature of the optical member (to be controlled) by
.+-.1.degree. C. by means of the temperature control device, as
mentioned in the above numerical analysis, such curvature of field
can be corrected within such a range by controlling the temperature
of the single fluorite lens element, for example. It is
practical.
In the case where the memory means for storing the change amounts
of the position of the image plane of the projection optical system
in dependence upon the changes in the applied condition of the
projection optical system is provided, when the position of the
image plane of the projection optical system is controlled by the
temperature control device to cancel the change amount of the
position of the image plane stored in the memory means in response
to the change in the applied condition of the projection optical
system, even if the applied condition is changed, the curvature of
field can quickly be corrected with high accuracy.
In the case where the focus position control means for shifting the
image plane of the projection optical system and the photosensitive
substrate relative to each other along the optical axis of the
projection optical system, when the offset of the focus position
remaining after the position of the image plane of the projection
optical system is controlled by the temperature control device is
reduced by the focus position control means, the defocus generated
by the correction of the curvature of field is corrected, with the
result that the exposure can be effected in a condition that the
surface of the photosensitive substrate is always aligned with the
image plane.
The following embodiments of the present invention serve to control
both the non-linear magnification error and the non-linear defocus
and to carry out a desirable method for controlling the temperature
of the optical member to effect such correction. Regarding the
following ninth to twelfth embodiments, the non-linear
magnification error and the non-linear defocus are called
generically as "non-linear error".
The ninth to twelfth embodiments cope with a problem that it takes
a long time to bring the optical member (to be
temperature-controlled) to the desired target temperature. Prior to
the explanation of the ninth to twelfth embodiments, such problem
will be generally explained.
As in the above-mentioned embodiments, when the temperature of the
predetermined optical members (for example, lens elements 36A, 37A)
is controlled, there is time delay due to predetermined time
constant until the target temperature. For example, a section (a)
of FIG. 38 shows a case where the target temperature value of the
optical member having a temperature of T.sub.1 is changed to a
value T.sub.2. In the section (a) of FIG. 38, for example, even
when a set temperature (control temperature) T.sub.ij of gas
surrounding the optical member gradually approaches the target
temperature T.sub.2 along a curve 259 from a time point t.sub.1, a
temperature TF of the optical member is changed with delay along a
curve 258. Thus, when a certain time is elapsed (to reach a time
point t.sub.2) after the temperature T.sub.ij of the gas reaches
the target temperature T.sub.2, the temperature TF of the optical
member reaches an allowable range (.+-..DELTA.T) of the target
temperature T.sub.2. Since high accurate correction of the imaging
characteristic cannot be effected before the time point t.sub.2, in
the present invention, the exposure is started after the time point
t.sub.2 (at which the temperature TF of the optical member reaches
the allowable range (.+-..DELTA.T) of the target temperature
T.sub.2.
In this case, in order to judge whether the temperature TF of the
optical member reaches the allowable range (.+-..DELTA.T) of the
target temperature T.sub.2, for example, a method in which a
temperature change speed of the optical member is previously sought
and the temperature of the optical member left to be changed until
a calculated time period is elapsed, or a method in which a
temperature sensor is attached to the optical member and the actual
temperature of the optical member is measured can be used. In order
to make the temperature TF of the optical member approach to the
target temperature T.sub.2 for a short time, as shown in a section
(b) of FIG. 38, for example, the set temperature T.sub.ij of the
gas surrounding the optical member may be overshot over the target
temperature T.sub.2 along a curve 259A (or undershot when the
target temperature is low).
Now, the ninth to twelfth embodiments of the present invention will
be explained.
FIG. 32 shows a construction common to the ninth to twelfth
embodiments, which corresponds to the construction shown in FIG.
17. Thus, the elements corresponding to those shown in FIG. 17 are
designated by the same reference numerals and explanation thereof
will be omitted.
An imaging characteristic measuring sensor 203 is secured on the
wafer stage 2 in the vicinity of the wafer W. As shown in a section
(b) of FIG. 37, the imaging characteristic measuring sensor 203 has
a glass substrate 51 on which a light shield film 50 including a
rectangular opening portion 50a in the same height as a surface of
the wafer W is coated, a photoelectric conversion element 52 for
photoelectrically converting the exposure illumination light passed
through the opening portion 50a into a detection signal, and a
signal treatment portion 253 for treating the detection signal S1
from the photoelectric conversion element 52. The treated result
from the signal treatment portion 253 is supplied to a main control
device 218 shown in FIG. 32. In the illustrated embodiment, as will
be described later, by driving the wafer stage 2, the projected
image of the pattern on the reticle R is scanned by the opening
portion 50a of the imaging characteristic measuring sensor 203. In
this case, on the basis of the detection signal S1 outputted from
the photoelectric conversion element 52, the signal treatment
portion 253 seeks or determines the non-linear magnification error
and the curvature of field.
FIG. 33 shows the ninth embodiment of the present invention which
corresponds to FIG. 18. Thus, the elements corresponding to those
shown in FIG. 18 are designated by the same reference numerals and
explanation thereof will be omitted.
In FIG. 33, a lens temperature control device 213 for controlling
the temperature of one or more lens elements made of fluorite are
connected to a projection optical system PL9. In the illustrated
embodiment, when the non-linear magnification error (high order
magnification error) of the projection optical system PL9, or,
curvature of field, or both of them are worsened in accordance with
the change in environmental condition or the history of the
illumination amount, the imaging characteristic is corrected by the
lens control device 213 on the basis of the command from a main
control device 218. Further, when the linear magnification error of
the projection optical system PL9 is worsened upon the correction
of the non-linear magnification error and/or the curvature of
field, the linear magnification error is corrected by the lens
control device 12.
Temperature sensors 254B and 254A such as thermistors are mounted
on the lens elements 36A and 37A at areas through which the
exposure illumination light is not passed, and signal terminals of
the temperature sensors 254A, 254B are connected to a temperature
detection device 255. In the temperature detection device 255, the
average temperature of the lens elements 36A, 37A is determined on
the basis of the change in electric features of the temperature
sensors 254A, 254B. The determined temperature is supplied to the
temperature control device 213b and the main control device 218. In
the temperature control device 213b, a set temperature of gas
supplied through a piping 21 is minutely controlled so that the
detected temperature of the lens elements 36A, 37A quickly reaches
the target temperature (described later). In the main control
device 218, the exposure of the wafer W is started after the
detected temperature of the lens elements 36A, 37A reaches the
allowable range of the target temperature.
In this way, according to this embodiment, the temperature control
device 213 of FIG. 32 is constituted by the temperature control
devices 213a, 213b, temperature sensors 254A, 254B, temperature
detection device 255, and pipings 19-22, 23A and 24A.
Now, an example of a method for correcting the non-linear
magnification error (high order magnification error) of the
projection optical system PL9 or the curvature of field in this
embodiment will be explained. Incidentally, in the following
explanation, while an example that one of the non-linear
magnification error and the curvature of field is corrected by
controlling the temperature of the fluorite lens elements 36A, 37A
of the projection optical system PL9 of FIG. 33 will be explained,
this example is for a case where one of the non-linear
magnification error and the curvature of field is greatly worsened
in accordance with the applied condition of the projection optical
system PL9. If both of the non-linear magnification error and the
curvature of field are greatly worsened, for example, the
temperature of the lens elements 36A, 37A may be set to a value so
that the imaging characteristics of these lens elements are
included within their allowable ranges. If such set temperature
does not exist, the temperatures of the lens elements effective to
correct the non-linear magnification error and the curvature of
field may be controlled independently in the projection optical
system PL9.
Now, an example of a method for measuring the distortion of the
projection optical system PL9 and the curvature of field will be
explained with reference to FIG. 37.
To this end, a test reticle having a plural pairs (for example,
sixteen pairs) evaluating marks disposed equidistantly in the
illumination area is used as the reticle R of FIG. 32. In this
case, it is desirable that the pairs of evaluating marks include
marks at various image heights. Each pair of evaluating marks are
constituted by an X axis evaluating mark formed by a line-and-space
pattern (in which the lines are spaced in the X direction at
predetermined intervals) and a Y axis evaluating mark corresponding
to what is obtained by rotating the X axis evaluating mark by 90
degrees. A projected image MY(i) of the i-th (i=1-16) Y axis
evaluating mark is shown in a section (a) of FIG. 37.
In the section (a) of FIG. 37, the projected image MY(i) has a
pattern in which bright portions P1-P5 are disposed at a
predetermined pitch in the Y direction, and the projected image
MY(i) is scanned in the Y direction by means of the rectangular
opening portion 50a of the imaging characteristic measuring sensor
203 by driving the wafer stage 2 of FIG. 32. In this case, the
detection signal S1 outputted from the photoelectric conversion
element 52 shown in a section (b) of FIG. 37 is A/D
(analogue/digital)-converted in a signal treatment portion 253, and
the treated result is stored in correspondence to the Y axis value
of the wafer stage 2. And, for example, on the basis of a
derivative signal of the detection signal S1, the Y axis value and
the contrast of the mid point of the projected image MY(i) are
determined, and the magnification error at that image height is
determined on the basis of a deviation amount of the Y axis value
of the mid point from the design value. Further, the focus position
of the image plane at that image height is determined by seeking
the X axis value at which the contrast becomes highest by changing
the X axis value of the imaging characteristic measuring sensor
203. By determining the magnification errors and the focus
positions of the image planes of the projected images of all of the
evaluating marks, the distortion (including the non-linear
magnification error) and the curvature of field can be
determined.
Incidentally, regarding a method for determining the target
temperature of the fluorite lens elements 36A, 37A, the associated
explanations in connection with FIGS. 11 and 26 may be referred
to.
Next, an example of a method for actually setting the temperature
of fluorite lens elements to the target temperature will be
explained with reference to FIGS. 38 to 40.
First of all, a section (a) of FIG. 38 shows the change in
temperature of the fluorite lens elements as the time is elapsed.
In the section (a) of FIG. 38, the broken line curve 259 indicates
the set temperature T.sub.ij of the gas supplied from the lens
control device 213b of FIG. 33 to the gas chamber between the
fluorite lens elements 36A and 37A, and the solid line curve 258
indicates the temperature (fluorite temperature) TF of the lens
elements 36A, 37A measured by the temperature sensors 254A, 254B.
In the section (a) of FIG. 38, the target temperature of the
fluorite elements at an original time point t.sub.0 is T.sub.1. At
the time point t.sub.0, the temperature TF of the fluorite lens
elements is included within an allowable range (.+-..DELTA.T) of
the target temperature T.sub.1 having a predetermined allowable
width .DELTA.T. In this case, the target temperatures T.sub.i of
the fluorite lens elements and the allowable width .DELTA.T are
stored in a common working area 260 (FIG. 40) of the memory of the
main control device 218 of FIG. 33 so that the data stored in the
common working area 260 can be used by various control portions
independently.
In a further time point t.sub.1, as shown by a step 301 of a flow
chart shown in FIG. 39, it is assumed that the target temperature
T.sub.i of the fluorite lens element is changed (from T.sub.1) to
T.sub.2 (T.sub.2 >T.sub.1) in accordance with the applied
condition of the projection optical system according to the
illustrated embodiment by the temperature control portion in the
main control device 218 of FIG. 33. In this case, the target
temperature T.sub.2 is written or stored in the common working area
260 of FIG. 40 and is transferred to the temperature control device
213b of FIG. 33.
Thereafter, in a step 302 of the flow chart, the temperature
control device 213b of FIG. 33 gradually changes the temperature
T.sub.ij (referred to as "control temperature" hereinafter) of the
gas (temperature control medium in the illustrated embodiment) to
be supplied in the gas chamber between the lens elements 36A and
37A from the original target temperature T.sub.1 to the next target
temperature T.sub.2 with a predetermined time step, as shown by the
broken line curve 259 in the section (a) of FIG. 38. Further, the
temperature TF of the fluorite lens elements is also supplied to
the temperature control device 213b. Thus, in the temperature
control device 213b, the control temperature T.sub.ij of the
temperature control medium is continuously adjusted in a servo
manner so that the temperature TF of the fluorite lens elements
coincides with the target temperature T.sub.2. Accordingly, in
actual, the operation of the step 302 is continuously carried out
during the exposure. In this case, since the change in the
temperature of the fluorite lens elements 36A, 37A of FIG. 33
includes a predetermined time constant, the temperature TF of the
fluorite lens elements is changed with delay with respect to the
control temperature T.sub.ij, as shown by the solid line curve
258.
Thus, in a step 303 of the flow chart shown in FIG. 39, the
temperature control portion of the main control device 218 measures
the actual temperatures TF of the fluorite lens elements via the
temperature sensors 254A, 254B and the temperature detection device
255 of FIG. 33 and reads out the target temperature Ti (here,
Ti=T.sub.2) and the allowable width .DELTA.T from the common
working area 260 (FIG. 40) in the memory of the main control device
218. In a next step 304 of the flow chart, the temperature control
portion judges whether the temperature TF of the fluorite lens
elements is included within the allowable range of the next target
temperature. That is:
If the temperature TF of the fluorite lens elements is included
within the allowable range, the flow chart (program) goes to a step
306, where the temperature control portion sets a value of a
temperature change end flag (having an initial high level "1"
value) to a low level "0". Thereafter, the program is returned to
the step 303, thereby effecting the sampling of the temperature TF
of the fluorite lens elements. On the other hand, in the step 304,
if the temperature TF of the fluorite lens elements is not included
within the allowable range, in a next step 305, the temperature
control portion of the main control device 218 sets the value of
the temperature change end flag to the high level "1", and then,
the program is returned to the step 303. As a result, in the
example shown in the section (a) of FIG. 38, at a time point
t.sub.2, the temperature TF of the fluorite lens elements reaches
the allowable range of the target temperature T.sub.2, thereby
setting the temperature change end flag of FIG. 40 to the low level
"0".
In parallel with the operations in the steps 301 to 306 of the flow
chart in FIG. 39, the exposure operation is performed by an
exposure control portion of the main control device 218, as shown
in steps 311 to 316 of the flow chart. That is to say, after
replacement of wafer and alignment of the new wafer are executed in
a step 311, in a step 312, the exposure control portion of the main
control device 218 starts the count of an internal time.
Thereafter, in a step 313, the exposure control portion judges
whether the value of the temperature change end flag in the common
working area 260 of the main control device 218 has the low level
"0" (i.e., whether the temperature of the fluorite lens elements is
included within the allowable range of the target temperature). If
the temperature change end flag is in the low level "0", in a next
step 314, the exposure is performed. Then, the program is returned
to the step 311.
On the other hand, in the step 313, if the temperature change end
flag is in the high level "2", in a step 315, the exposure control
portion of the main control device 218 judges whether the count
time of the previously started timer exceeds a predetermined
allowable time (time out). If the time out does not occur, the
program is returned to the step 313, where the value of the
temperature change end flag is checked. On the other hand, in the
step 315, if the time out occurs, the program goes to a step 316,
where the exposure control portion performs error treatment in
which alarm information (indicating the fact that the temperature
of the fluorite lens elements does not reach the target
temperature) is transmitted to an operator.
As mentioned above, according to the illustrated embodiment, since
the temperature control portion and the exposure control portion of
the main control device 218 are operated in parallel through the
data stored in the common working area 260 in the memory of the
main control device 218, the setting of the temperature of the
fluorite lens elements of the projection optical system PL9 to the
target temperature and the exposure after the temperature was
included within the allowable range of the target temperature can
be performed smoothly.
From the actual numerical analysis of a model of the projection
optical system PL9, it was found that the non-linear magnification
error can be corrected by about .+-.20 nm at the most by changing
the temperature of the single fluorite lens elements by
.+-.1.degree. C. Since the maximum resolving power of the
temperature control of the fluorite lens element is about
0.01.degree. C., when the non-linear magnification error is
corrected by changing the temperature of the fluorite lens element
by 1.degree. C., the resolving power of the correction of the
non-linear magnification error becomes about .+-.0.2 nm
(=.+-.20/100 [nm]).
Similarly, it was found that the curvature of field of about
.+-.0.2 .mu.m can be corrected by changing the temperature of the
single fluorite lens element by .+-.1.degree. C. Further, since the
maximum resolving power of the temperature control of the fluorite
lens element is about 0.01.degree. C., when the curvature of field
is corrected by changing the temperature of the fluorite lens
element by 1.degree. C., the resolving power of the correction of
the curvature of field becomes about .+-.2 nm (=.+-.0.2/100
[.mu.m]). Accordingly, the allowable width .DELTA.T (refer to the
section (a) of FIG. 38) of the temperature of the fluorite lens
element may be set depending on the actually required correction
accuracy of the non-linear magnification error and the curvature of
field.
In the example shown in the section (a) of FIG. 38, while an
example that the control temperature T.sub.ij of the temperature
control medium (gas) is monotonously changed from the original
target temperature T.sub.1 to the next target temperature T.sub.2
is explained, in order to bring the temperature TF of the fluorite
lens element to the next target temperature T.sub.2, as shown by
the broken line curve 259A in the section (b) of FIG. 38, the
control temperature T.sub.ij of the temperature control medium may
be overshot over the next target temperature T.sub.2 once. Doing
so, as shown by the sold line curve 258A, the temperature TF of the
fluorite lens element reaches a time point t.sub.3 which is earlier
than the time point in case of the section (a) of FIG. 38, thereby
improving through-put (the number of wafers to be treated per unit
time) of the exposure process.
On the other hand, if the next target temperature is smaller than
the original target temperature, the control temperature may be
undershot below the next target temperature.
In the above-mentioned example, while the temperature TF of the
fluorite lens element was actually measured, for example, in the
section (a) of FIG. 38, the time duration of time period (t.sub.2
-t.sub.3) during which the temperature TF of the fluorite lens
element is changed from the original target temperature T.sub.1 to
the allowable range of the next target temperature T.sub.2 can be
previously be determined by tests as a function of a control width
of temperature (T.sub.2 -T.sub.1). In this case, the function of
the control width of temperature (T.sub.2 -T.sub.1) is stored in
the memory of the main control device 218 of FIG. 33, and, in the
temperature control portion of the main control device 218, when
the target temperature is changed, it may be judged that the
temperature of the fluorite lens element is included within the
allowable range of the changed target temperature after a time
period determined by the function and a predetermined offset time
period are elapsed. According to this method, although the exposure
time is increased by a time period corresponding to the
predetermined offset time period, since the temperature sensors can
be omitted, the entire apparatus can be simplified.
Next, a tenth embodiment of the present invention will be explained
with reference to FIG. 34. Since FIG. 34 corresponds to FIG. 19,
the elements corresponding to those shown in FIG. 19 are designated
by the same reference numerals and explanation thereof will be
omitted. A projection optical system according to this embodiment
is suitable to be applied to both projection exposure apparatuses
of step-and-repeat type and step-and-scan type.
As is in the sixth embodiment, also in this tenth embodiment, the
temperature control elements 40A-40D may be heaters. Peltier effect
elements and the like. The Peltier effect elements may be used to
effect the heating or the heat absorbing. The temperature sensors
254A, 254B are secured to one end of the lens element 36B in the Y
direction and are connected to the temperature detection device
255. The temperature detected by the temperature detection device
255 is supplied to a temperature control device 239. The
temperature control device 239 controls the heating or the heat
absorbing of the temperature control elements 40A-40D so that the
detected temperature becomes the set temperature instructed by the
main control device 218. Thus, in this embodiment, the temperature
control elements 40A-40D act as temperature control media.
Next, an eleventh embodiment of the present invention will be
explained with reference to FIG. 35. Since FIG. 35 corresponds to
FIG. 20, the elements corresponding to those shown in FIG. 20 are
designated by the same reference numerals and explanation thereof
will be omitted. A projection optical system according to this
embodiment is suitable to be applied to both projection exposure
apparatuses of step-and-repeat type and step-and-scan type.
In this embodiment, the temperature sensors 254A, 254B are mounted
on opposed ends of the lens elements 36C, 35C at areas through
which the exposure illumination light is not passed, and the
temperature sensors 254A, 254B are connected to the temperature
control device 255 so that the temperatures of the lens elements
36C, 35C detected by the temperature control device 255 are
supplied to the lens temperature control device 213c and the main
control device 218. Similarly, temperature sensors 256A, 256B are
mounted on opposed ends of the lens elements 33A, 34A at areas
through which the exposure illumination light is not passed, and
the temperature sensors 256A, 256B are connected to a temperature
control device 257 so that the temperatures of the lens elements
33A, 34A detected by the temperature control device 257 are
supplied to the lens temperature control device 213d and the main
control device 218. On the basis of the command from the main
control device 218, the lens temperature control devices 213c, 213d
set the temperatures of the lens elements 36C, 33A to their target
temperatures, respectively.
In this embodiment, when the imaging characteristics (non-linear
magnification error and curvature of field) of the projected image
of a projection optical system PL11 are to be corrected, the
temperature of the lens element 36C is controlled by means of the
temperature control device 213c, and the linear magnification error
generated upon the correction of the imaging characteristics is
canceled by controlling the temperature of the lens element 33A by
means of the temperature control device 213d. When the imaging
characteristic due to the change in atmospheric pressure differs
from the imaging characteristic due to the change in temperature
during the illumination of the exposure illumination light onto the
projection optical system, so that the third or higher order
magnification error is generated greatly, this method can also be
utilized to control the imaging characteristics independently by
using the corresponding two fluorite lens element. That is to say,
for example, the non-linear magnification error or the curvature of
field due to the change in atmospheric pressure may be corrected by
using the upper fluorite lens element 36C, and the non-linear
magnification error or the curvature of field due to the change in
temperature during the illumination of the exposure illumination
light may be corrected by using the lower fluorite lens element
33A.
Next, an twelfth embodiment of the present invention will be
explained with reference to FIG. 36. Since FIG. 36 corresponds to
FIG. 21, the elements corresponding to those shown in FIG. 21 are
designated by the same reference numerals and explanation thereof
will be omitted. A projection optical system according to this
embodiment is suitable to be applied to both projection exposure
apparatuses of step-and-repeat type and step-and-scan type.
Also in this embodiment, the lens elements 28A, 29A are made of
fluorite and the other lens elements are made of quartz. Gas having
a variable temperature is supplied from the temperature control
device 213e, through the piping 43, to the gas changer enclosed by
the lens elements 28A, 29A and the lens frame G6, and the gas
circulating the gas chamber is returned to the temperature control
device 213e through the piping 44. The temperature sensors 254A,
254B are mounted on opposed ends of the lens elements 28A, 29A at
areas through which the exposure illumination light is not passed,
and the temperature sensors 254A, 254B are connected to the
temperature control device 255 so that the temperatures of the lens
elements 28A, 29A detected by the temperature control device 255
are supplied to the lens temperature control device 231e and the
main control device 218.
On the basis of the command from the main control device 218, the
lens temperature control device 213e sets the temperature of the
lens elements 28A, 29A to its target temperature. The supports 45
and 46 can be shifted in a direction parallel to an optical axis AX
of a projection optical system PL12 and can be inclined by desired
angles, independently, by means of the drive device 47.
Also in this embodiment, the non-linear magnification error and/or
the curvature of field of the projected image of the projection
optical system PL12 is corrected by controlling the temperature of
the fluorite lens elements 28A, 29A by means of the temperature
control device 213e, and the linear magnification error generated
upon the correction of the non-linear magnification error and/or
the curvature of field is corrected by inclining or
lifting/lowering the lens elements 37D, 38D via the supports 45,
46. Since not only the magnification error but also the defocus of
the focusing position and the trapezoidal distortion can be
corrected by combination of the movements of two supports 45, 46,
almost all of other aberrations generated during the correction of
the non-linear magnification error and/or the curvature of field
can be canceled by properly controlling the temperature control
device 213e and the drive device 47.
According to the ninth to twelfth embodiments of the present
invention, the temperature(s) of the predetermined optical members
are controlled, the imaging characteristics of the projection
optical system (particularly, non-linear magnification error and
the non-linear error such as the curvature of field) which are
worsened by the change in the environmental condition such as the
atmospheric pressure around the projection optical system, the
absorption of the exposure illumination light and the change in the
illumination condition can be corrected. Further, since the
exposure is started after the temperature of the optical member to
be controlled falls in the predetermined allowable range of the
target temperature, the imaging characteristic of the projection
optical system is maintained in the desired condition during the
exposure, thereby always maintaining the imaging characteristic of
the projected image projected on the photosensitive substrate in
the good condition.
The aligning accuracy of the projected images can be improved by
correcting the non-linear magnification error, for example, and the
depth of focus of the entire projected image can be widened by
correcting the curvature of field.
Further, when the imaging characteristics to be controlled are the
non-linear magnification error (high order magnification error) and
the curvature of field, and one of the non-linear magnification
error and the curvature of field is greater than the other, the
temperature of the optical elements (lens elements) effective to
the correction of the imaging characteristic having the greater
error may be controlled. In projection optical systems in which the
non-linear magnification error and the curvature of field are
generated simultaneously, the non-linear magnification error and
the curvature of field may be corrected independently by
controlling the temperatures of the optical members independently,
or, the non-linear magnification error and the curvature of field
may be corrected simultaneously by controlling the temperature of
the optical member effective to correct both of the imaging
characteristics.
In the case where the temperature sensors for measuring the
temperatures of the optical members to be controlled by the
temperature control devices are provided and the exposure control
means compares the temperatures measured by the temperature sensors
and the target temperatures and judges whether the exposure is to
be started, the exposure can be started quickly on the basis of the
actually measured temperatures of the optical members to be
controlled, and the imaging characteristic is always maintained in
the desired condition during the exposure.
On the other hand, in the case where the memory means for storing
the time period during which the temperature of the optical member
to be controlled by the temperature control means reaches the
predetermined temperature is provided, when the exposure control
means starts the exposure regarding the photosensitive substrate
after a time period corresponding to the time period stored in the
memory means is elapsed, since the temperature sensors can be
omitted, the start timing of the exposure can be determined by a
simple mechanism.
In the case where the temperature of the optical member to be
controlled is set to the target temperature by the temperature
control means, when an intermediate set temperature (control
temperature) obtained by the temperature control means is overshot
or undershot with respect to the target temperature, the
temperature of the optical member can be set to the target
temperature more quickly.
Next, embodiments of the present invention in which the temperature
control of the lens elements is not utilized to correct the
non-linear error will be explained with reference to FIGS. 41 to
45. In the following thirteenth to sixteenth embodiments,
combination of glass materials having opposite effects for
contributing to the change in the non-linear error is used.
Explaining more specifically, the relations shown in FIG. 7 can be
applied to a relation of a quartz lens element and a fluorite lens
element. For example, when two kinds of glass materials, one of
which (first glass material) is made of quartz and the other of
which (second glass material) is made of fluorite are used and
almost all of the lens elements in a projection optical system are
formed from the first glass material, regarding the distortion of
the imaging characteristics of the projection optical system, the
distortion generated by the lens elements made of the first glass
material becomes the non-linear magnification error as shown by the
curve 61 in the section (a) of FIG. 7. On the other hand, the
distortion generated by the lens elements made of the second glass
material has the tendency that, as shown by the curve 62A in the
section (b) of FIG. 7, the offset of the linear magnification error
(shown by the straight line 62B) is added to the non-linear
magnification error having the property opposite to that of the
curve 61. Thus, by combining the lens elements made of two kinds of
glass materials to cancel the non-linear magnification error (curve
61) by the non-linear magnification error (curve 62A), the
non-linear magnification error of the projection optical system can
be reduced. Accordingly, by such combination, it is possible to
provide a projection optical system in which the non-linear
magnification error is almost not generated by the change in the
environmental condition and/or the absorption of the illumination
light.
However, if such combination leaves as it is, the linear
magnification error exists. Thus, by reducing such residual linear
magnification error by using a linear magnification control means
such as a gas pressure control device and a lens drive mechanism,
the distortion of the projection optical system can be removed
substantially completely.
Further, the relations shown in FIG. 22 can be applied to the
relation of the quartz lens element and the fluorite lens
element.
The distribution of the focus positions Z.sub.F associated with the
lens elements made of the first glass material and depending upon
the image height H under a certain condition causes the
downwardly-convex curvature of field, as shown by the curve 61A in
the section (a) of FIG. 22. On the other hand, the distribution of
the focus positions associated with the lens elements made of the
second glass material has the tendency that, as shown by the curve
62C in the section (b) of FIG. 22, the offset common to the all of
the image heights H is added to the upwardly-convex curvature of
field having a property opposite to that of the curve 61A.
Thus, by combining the lens elements made of two kinds of glass
materials and by considering the temperature of the lens frame, the
temperature of the lens barrel, and the temperature difference
between the lens elements to cancel the curvature of field (curve
61A) by the curvature of field (62C), the curvature of field
(non-linear defocus) of the projection optical system can be
reduced. In this way, it is possible to provide a projection
optical system in which the curvature of field is almost not
generated by the change in the environmental condition and/or the
absorption of the illumination light.
If such combination leaves as it is, the predetermined offset is
remaining in the distribution of the focus positions of the
projected image, as shown by the straight line 63A in the section
(c) of FIG. 22. However, such offset of the focus positions can be
removed substantially completely, for example, by adjusting the
height of the substrate by means of a focusing position control
means such as a stage for controlling the height of the
substrate.
FIG. 41 shows a construction common to the thirteenth to sixteenth
embodiments, which corresponds to the construction shown in FIG.
32. Thus, the elements corresponding to those shown in FIG. 32 are
designated by the same reference numerals and explanation thereof
will be omitted.
In FIG. 41, among a plurality of lens elements constituting a
projection optical system PL13, a large number of lens elements are
made of quartz, and the remaining small number of lens elements are
made of fluorite. There is provided a lens control device 412 for
controlling a gas pressure in a predetermined gas chamber between a
predetermined pair of fluorite lens elements. When the imaging
characteristics such as the magnification of the projection optical
system PL13, the focus position (best focus position) at a center
of an average plane of the image plane and the curvature of field
are changed in accordance with the change in the environmental
condition such as atmospheric pressure and temperature or the
history of the illumination amount of the exposure illumination
light onto the projection optical system PL13, the imaging
characteristics are corrected by the lens control device 412 on the
basis of the command from a main control device 418. As the lens
control device 412, a lens position control device for controlling
the position (along the optical axis AX direction) and inclination
of the predetermined fluorite lens element or a reticle position
control device for controlling the direction (along the optical
axis AX direction) and inclination of the reticle R may be used
besides the pressure control device.
In the illustrated embodiment, when the curvature of field of the
projection optical system PL13 is canceled by using the lens
elements formed from two different glass materials, the offset
sometimes occurs in the focus position at the center of the average
plane of the image plane. If such offset is generated, the focus
position of the wafer W is corrected by an amount corresponding to
the offset amount by driving the wafer stage 2, thereby preventing
the defocus from being generated between the image plane and the
surface of the wafer W.
Next, the projection optical system PL13 according to the
thirteenth embodiment will be explained with reference to FIG. 42
and the like.
Since FIG. 42 corresponds to FIG. 33, the elements corresponding to
those shown in FIG. 33 are designated by the same reference
numerals and explanation thereof will be omitted. As can be seen
from FIG. 42, the kinds of the lens elements shown in FIG. 42 are
the same as those shown in FIG. 33.
Now, the non-linear magnification error (high order magnification
error) and the curvature of field of the projection optical system
PL13 will be described. In this example, a case where one of the
non-linear magnification error and the curvature of field is
canceled by using the fluorite lens elements 36A, 37A in the
projection optical system PL13 will be explained. This deals with
the case where one of the non-linear magnification error and the
curvature of field is greatly worsened in accordance with the
applied condition of the projection optical system PL13. Thus, if
both of the non-linear magnification error and the curvature of
field are greatly worsened, one feasible approach may be, for
example, that lens elements effective to correct the non-linear
magnification error and the curvature of field, respectively, are
made from fluorite.
The glass materials from which the lens elements of the projection
optical system PL13 are formed are quartz and fluorite. As the
temperature is increased, the quartz is almost not expanded due to
its small expansion coefficient, but the index of refraction is
increased. Accordingly, as shown in the section (b) of FIG. 6, in
the positive quartz lens element 49B, when the temperature is
increased, the image plane FB is displaced to approach to the lens
element. On the other hand, as the temperature is increased, the
fluorite is expanded and the index of refraction thereof is
decreased. Accordingly, as shown in the section (a) of FIG. 6, in
the positive fluorite lens element 49A, when the temperature is
increased, the image plane FA is displaced to move away from the
lens element. In other words, the quartz lens element and the
fluorite lens element have opposite effects for contributing to the
change in the focus position in response to the temperature
change.
Now, the distortion of the imaging characteristics of the
projection optical system PL13 will be explained with reference to
FIGS. 7 and 8. In FIGS. 7 and 8, the ordinate indicates the image
height H, and the abscissa indicates the magnification .beta. of
the projection optical system PL13 at that image height H. And, the
magnification on the optical axis (H=0) is represented by the
design magnification .beta..sub.0. In this case, under a certain
environmental condition, after the exposure is continued by a
certain time period, the distortion caused by the lens elements
made of quartz in the projection optical system PL13 becomes the
non-linear error (high order magnification error) in which, as the
image height H is increased, the magnification .beta. is decreased
below the design magnification once and then is monotonously
increased, as shown by the curve 61 in the section (a) of FIG.
7.
To the contrary, in the illustrated embodiment, the distortion
caused by the lens elements made of fluorite in the projection
optical system PL13 is set to have the non-linear magnification
error having a feature substantially opposite to that shown by the
curve 61 in the section (a) of FIG. 7. However, even when the
distortion caused by the lens elements made of fluorite is set in
this way, the distortion includes the predetermined linear
magnification error as offset. Accordingly, as shown by the curve
62A in the section (b) of FIG. 7, the actual distortion caused by
the lens elements made of fluorite in the projection optical system
PL13 has a feature obtained by adding the linear magnification
error shown by the straight line 62B to the non-linear
magnification error having the feature substantially opposite to
that shown by the curve 61 in the section (a) of FIG. 7.
Therefore, if the imaging characteristic is not corrected by the
lens control device 412 of FIG. 41, the distortion caused by the
entire projection optical system PL13 has a feature that is
represented by the linear magnification error in which the
magnification .beta. is linearly increased from the design
magnification as the image height H is increased, as shown by the
straight line 63 in the section (c) of FIG. 7. Thus, the linear
magnification error (shown by the straight line 64) which can
substantially cancel the residual magnification error represented
by the straight line 63 in the section (c) of FIG. 7 is applied to
the projection optical system PL13 by means of the lens control
device 412 of FIG. 41. As a result, the distortion of the
projection optical system PL13 according to the illustrated
embodiment has a feature as shown by the curve 65 in FIG. 8 in
which the non-linear magnification error and the linear
magnification error are both eliminated.
Next, the curvature of field of the imaging characteristics of the
projection optical system PL13 will be explained with reference to
FIGS. 22 and 23. In FIGS. 22 and 23, the abscissa indicates the
image height H and the ordinate indicates the focus positions
Z.sub.F of the image plane of the projection optical system PL13 at
various image heights H. The focus position on the optical axis
(H=0) is shown as a design focus position. In this case, under a
certain condition, after the exposure is continued for a certain
time period, the image plane obtained by the quartz lens elements
of the projection optical system PL13 has downwardly-convex
curvature of field as shown by the curve 61A in the section (a) of
FIG. 22.
To the contrary, in the illustrated embodiment, the curvature of
field caused by the fluorite lens elements 36A, 37A of the
projection optical system PL13 is set to have curvature of field
having a property substantially opposite to that of the curve 61A
shown in the section (a) of FIG. 22. However, even when the
curvature of field generated by the fluorite lens elements is set
in this way, the offset which is uniform through the entire range
of the image height H is added to the focus positions of the image
plane. Accordingly, the distribution of the focus positions of the
image plane obtained by the fluorite lens elements of the
projection optical system PL13 has a feature in which the curvature
of field substantially opposite to the curvature of the curve 61A
shown in the section (a) of FIG. 22 is added to the predetermined
offset, as shown by the curve 61C in the section (b) of FIG.
22.
Thus, as shown by the straight line 63A in the section (c) of FIG.
22, the focus positions Z.sub.F of the image plane of the entire
projection optical system PL13 become substantially constant
through the entire range of the image height H, thereby generating
the defocus between the image plane and the surface of the wafer W.
To cancel the defocus, the focus position on the surface of the
wafer W is corrected by an amount corresponding to the defocus by
driving the wafer stage 2 in the Z direction with respect to the
projection optical system PL13 by means of the drive mechanism.
This means that the offset of the focus position as shown by the
straight line 64A in the section (c) of FIG. 22 is added to the
image plane of the projection optical system PL13. As a result, the
distribution of the focus positions Z.sub.F of the image plane of
the projection optical system PL13 according to the illustrated
embodiment with respect to the surface of the wafer W becomes to
that as shown by the curve 65A in FIG. 23, thereby eliminating the
curvature of field and the predetermined defocus. Accordingly, the
degree of the depth of focus of the entire image plane is made
larger in comparison with the conventional techniques, with the
result that the projected image can be projected with a higher
resolving power as a whole. Even if the environmental condition is
changed to some extent, since the curve 61A shown in the section
(a) of FIG. 22 and the curve 62C shown in the section (b) of FIG.
22 are changed in the substantially opposite directions, the great
curvature of field does not appear.
By canceling the curvature of field by the fluorite lens elements
36A, 37A, the linear magnification error of the image plane (error
in which the magnification is changed in proportion to the image
height) is often worsened. If such linear magnification error is
generated, a linear magnification error which can substantially
cancel the generated linear magnification error is added to the
image plane by the lens control device 412 of FIG. 41. As a result,
the imaging characteristic of the projection optical system PL13
according to the illustrated embodiment has a good feature having
no curvature of field and no distortion.
The imaging characteristic is changed, besides the change in
environmental conditions, such as atmospheric pressure, by the
illumination energy of the exposure illumination light passing
through the projection optical system. The imaging characteristic
is also changed when the illumination condition of the illumination
optical system 14 is switched between the normal condition, the
ring illumination condition, the so-called modulated-shape light
source condition and the small coherent factor (.sigma. value)
condition. Thus, most standard conditions are previously
determined, for example, regarding the illumination energy,
atmospheric pressure and illumination condition, and, the
configurations of the fluorite lens elements 36A, 37A are set so
that the non-linear magnification error and the curvature of field
become minimum under these standard conditions. By doing so, even
if the applied condition of the projection exposure apparatus is
changed to some extent, the non-linear magnification error and the
curvature of field will not become excessively great.
In the illustrated embodiment, while the excimer laser light source
was used as the exposure light source, regarding the illumination
energy of the exposure illumination light, when i-ray (having a
wavelength of 365 nm) of a mercury lamp is used, the energy of the
i-ray is more absorbed than the energy of the excimer laser light
in the projection optical system, and, thus, the non-linear
magnification error and the curvature of field of the projection
optical system are more increased than the excimer laser light.
Accordingly, when the method for correcting the imaging
characteristic regarding the illumination energy by the fluorite
lens elements is applied to a projection exposure apparatus
(steppers and the like) using the i-ray of the mercury lamp, the
non-linear magnification error and the curvature of field can be
reduced more effectively.
When the i-ray of the mercury lamp is used as the exposure
illumination light, in place of the glass material (fluorite)
associated with the excimer laser light, magnesium fluoride
(MaF.sub.2) or lithium fluoride (LiF.sub.2) may be used as the
glass material.
Next, a fourteenth embodiment of the present invention will be
explained with reference to FIG. 43. Since FIG. 43 corresponds to
FIG. 34, the elements corresponding to those shown in FIG. 34 are
designated by the same reference numerals and explanation thereof
will be omitted. A projection optical system according to this
embodiment is suitable to be applied to both projection exposure
apparatus of step-and-repeat type and of step-and-scan type.
In this case, the lens element 36B alone is made of fluorite and
the other lens elements are made of quartz. The other constructions
are the same as those of the embodiment shown in FIG. 42. According
to this embodiment, since the non-linear magnification error or the
curvature of field caused by the other lens elements is canceled by
the single fluorite lens element 36B, the non-linear magnification
error and/or the curvature of field of a projection optical system
PL14 can be suppressed with a simple construction.
Next, a fifteenth embodiment of the present invention will be
explained with reference to FIG. 44. Since FIG. 44 corresponds to
FIG. 35, the elements corresponding to those shown in FIG. 35 are
designated by the same reference numerals and explanation thereof
will be omitted. A projection optical system according to this
embodiment is suitable to be applied to both projection exposure
apparatus of step-and-repeat type and of step-and-scan type.
In this case, the lens elements 33A, 36C are made of fluorite and
the other lens elements are made of quartz. In this example, the
non-linear magnification error or the curvature of field is mainly
corrected by the upper fluorite lens element 36C, and the feature
of the linear magnification error is mainly changed by the change
in temperature of the lower fluorite lens element 33A. Gas having a
variable temperature is supplied from a temperature control device
413d, through the piping 42A, to the gas chamber enclosed by the
lens elements 33A, 34A and the lens frame G4, and the gas
circulating in the gas chamber is returned to the temperature
control device 413d through the piping 42B. And, on the basis of
the command from a main control device 418, the temperature control
device 413d sets the temperature of the lens element 33A to the
target temperature value.
In this embodiment, when the imaging characteristics (non-linear
magnification error and the curvature of field) of the projected
image obtained by a projection optical system PL15 is corrected by
the lens element 36C, and the linear magnification error generated
upon the correction of the imaging characteristics is canceled by
controlling the temperature of the lens element 33A by means of the
temperature control device 413d. When the imaging characteristic
due to the change in atmospheric pressure differs from the imaging
characteristic due to the change in temperature during the
illumination of the exposure illumination light onto the projection
optical system, so that the third or higher order magnification
error or the defocus is generated greatly, this method can also be
utilized to control the imaging characteristics independently by
using the corresponding two fluorite lens elements. That is to say,
for example, the non-linear magnification error or the curvature of
field due to the change in atmospheric pressure may be corrected by
using the upper fluorite lens element 36C, and the non-linear
magnification error or the curvature of field due to the
illumination of the illumination light may be corrected by using
the lower fluorite lens element 33A.
Next, a sixteenth embodiment of the present invention will be
explained with reference to FIG. 45. Since FIG. 45 corresponds to
FIG. 36, the elements corresponding to those shown in FIG. 36 are
designated by the same reference numerals and explanation thereof
will be omitted. A projection optical system according to this
embodiment is suitable to be applied to both projection exposure
apparatuses of step-and-repeat type and step-and-scan type.
In this embodiment, the lens elements 28A, 29A are made of fluorite
and the other lens elements are made of quartz. The supports 45, 46
can independently be shifted along a direction parallel to an
optical axis AX of a projection optical system PL16 and be inclined
by desired angles by means of the drive device 47. The operation of
the drive device 47 is controlled by the imaging characteristic
control device 48 on the basis of the command from the main control
device 418.
Also in this embodiment, the non-linear magnification error and/or
the curvature of field of the projected image obtained by the
projection optical system PL16 are corrected by the fluorite lens
elements 28A, 29A, and the linear magnification error generated by
the correction of the non-linear magnification error and/or the
curvature of field is corrected by inclining and/or
lifting/lowering the lens elements 37D, 38D via the supports 45,
46. By combination of movements of two supports 45, 46, since not
only the magnification error but also the defocus of the focusing
position and the trapezoidal distortion can be corrected, almost
all of other aberrations generated during the correction of the
non-linear magnification error or the curvature of field can be
canceled by properly controlling the drive device 47.
In the thirteenth to sixteenth embodiments of the present
invention, by combining the lens elements made of two kinds of
glass materials having different temperature characteristics, even
if the environmental condition is changed, the generation of the
focus position error and the magnification error can be prevented.
However, in actual, the lens elements are supported by the lens
frames G1-G6 and the lens barrel 4, 4A of the projection optical
system. Therefore, regarding expansion/contraction caused by the
change in the environmental condition, the apparatus must be
designed in consideration of not only the expansion/contraction of
the lens elements but also the expansion/contraction of the lens
frames and the lens barrel. Further, when the fluorite is used as
the glass material from which the lens elements are formed, it must
be realized that the feature of the fluorite is particularly
changed greatly in accordance with the change in the environmental
condition. To the contrary, the quartz is less influenced upon the
change in the environmental condition. Therefore, it is desirable
that the fluorite lens elements are disposed at areas where the
temperature control must be effected with high accuracy.
According to the thirteenth to sixteenth embodiments of the present
invention, since two sets of lens elements made of two kinds of
glass materials having different effects for contributing to the
change in the focus position in dependence upon the temperature
change are used, even if the change in the environmental condition
such as the atmospheric pressure surrounding the projection optical
system or the absorption of the exposure illumination light occurs
to some extent, the imaging characteristics of the projection
optical system such as the focus position error, non-linear
magnification error (high order magnification error) or curvature
of field are not greatly worsened.
Regarding the aberrations including not only the above-mentioned
aberrations but also coma and astigmatism, even if the change in
the environmental condition such as the atmospheric pressure
surrounding the projection optical system or the absorption of the
exposure illumination light occurs to some extent or even if the
illumination condition is changed, such aberrations are not greatly
worsened.
By providing a plurality of lens elements formed from one of two
glass materials and by correcting different imaging characteristics
for respective ones of these lens elements, various imaging
characteristics can be corrected effectively.
In the case where the exposure illumination light is a laser beam
emitted from the excimer laser light source and having a wavelength
of 100-300 nm and two glass materials are quartz and fluorite,
since the non-linear magnification error and the curvature of field
of the fluorite lens element has a feature opposite to that of the
non-linear magnification error and the curvature of field of the
quartz lens element, the non-linear magnification error and the
curvature of field of the projection optical system can be
suppressed.
In this case, when a set of fluorite lens elements are disposed
within the projection optical system in such a manner that the
fluorite lens elements are isolated from the environmental gas with
the interposition of one or more quartz lens elements, even if the
temperature of the environmental gas is changed, since the fluorite
lens elements are less influenced upon such temperature change, the
imaging characteristics can be stabilized.
Next, a seventeenth embodiment of the present invention will be
explained referring to the accompanying drawings. In this
embodiment, the present invention is applied to a projection
exposure apparatus of stepper type in which pattern images on a
reticle are collectively transferred to shot areas on a wafer.
FIG. 46 schematically shows, partially in section, a construction
of the projection exposure apparatus according to this embodiment.
In FIG. 46, illumination light IL emitted from an illumination
optical system EL including an ArF excimer laser light source
having a narrow band having an oscillation wavelength of about 193
nm and an fly-eye lens for making illumination distribution of
illumination light on a reticle 501 uniform is directed onto the
reticle 501, so that images of patterns formed in a pattern area PA
on a lower surface of the reticle 501 are reduction-projected onto
shot areas on a wafer 502 through a projection optical system PL.
The illumination light IL may be KrF excimer laser light, high
harmonics of copper vapor laser or YAG laser, or bright-line
(g-ray, i-ray or the like) in a ultraviolet zone of a high pressure
mercury lamp, as well as the ArF excimer laser light. It is assumed
that a Z axis is parallel with an optical axis AX of the projection
optical system PL, an X axis is parallel with the plane of FIG. 46
and a Y axis is perpendicular to the plane of FIG. 46. The details
of the projection optical system PL will be described later.
The reticle 501 is rested on a reticle stage 501a via a reticle
holder (not shown). The reticle stage 501a serves to position the
reticle 501 in the X direction, the Y direction and a rotational
direction.
The wafer 502 is vacuum-absorbed to a wafer holder 509 which is
secured to a wafer stage 508. The wafer stage 508 serves to drive
the wafer in the X and Y directions in a so-called step-and-repeat
manner and to incline the surface of the wafer 502 in any direction
and to minutely shift the wafer 502 in the optical axis AX
direction (Z direction). The position of the wafer stage 508 on the
X-Y plane can always be detected by a laser interferometer (not
shown) with a resolving power of about 0.01 .mu.m, for example. The
positional information (or velocity information) of the wafer stage
508 is sent to a main control system (not shown). The main control
system serves to control the position of the wafer stage 508 on the
basis of the positional information (or velocity information).
Now, the construction of the projection optical system PL will be
described. In this embodiment, among the lens elements constituting
the projection optical system PL, one or more lens elements
(referred to as "controllable lens elements" hereinafter) are
formed from a glass material having optical features such as focal
length, index of refraction and the like (which are changed by the
temperature difference) which are different from those of other
glass materials of the other lens elements in terms of amount of
change (temperature characteristic). And, among these controllable
lens elements, a lens element in which the imaging characteristics
(to be controlled) of the projection optical system is changed by
the change in the optical feature of that lens element is selected
as a lens element to be controlled. By controlling the temperature
of the lens element (to be controlled) by fluid (referred to as
"control fluid" hereinafter) directly contacting with the lens
element to change the optical feature (for example, index of
refraction), the imaging characteristic of the projection optical
system is controlled. At the same time, a material having small
illumination light absorption is selected as such control fluid,
thereby reducing loss of the illumination energy. The projection
optical system PL according to this embodiment is constituted by
quartz lens elements and fluorite lens elements, and the fluorite
lens elements are used as the controllable lens elements and
nitrogen gas is used as the control fluid.
In FIG. 46, a group of lens elements of the projection optical
system PL comprise lens elements 503a, 503b, 503c, . . . , 503n
from a reticle side, and the plurality of lens elements 503a-503n
are disposed within a space 514 defined by a lens barrel 504
forming a housing of the projection optical system PL in such a
manner that peripheries of the lens elements are secured to the
lens barrel 504 via lens frames (not shown). Although almost all of
the lens elements are made of quartz, at least the lens element
503b is made of fluorite. When the illumination light of
far-ultraviolet zone is used as is in this embodiment, the glass
materials from which the lens elements of the projection optical
system are formed are quartz and fluorite. These glass materials
have different coefficients of thermal expansion. For example, the
index of refraction of the fluorite is changed more than that of
the quartz, and, thus, the fluorite lens element can be used as the
controllable lens element.
A closed space 512 is defined by the lens element 503b, an adjacent
lens element 503a and the lens barrel 504, and a closed space 513
is defined by the lens element 503b, an adjacent lens element 503c
and the lens barrel 504. A part of the lens barrel 504 defining the
space 512 has an introduction port 510a for introducing the
temperature-adjusted nitrogen (N.sub.2) gas from an external
temperature adjustment device 507 and a discharge port 511A for
discharging the nitrogen gas in the space 512. As is in the space
512, a part of the lens barrel 504 defining the space 513 has an
introduction port 510B for introducing the temperature-adjusted
nitrogen gas from the external temperature adjustment device 507
and a discharge port 511B for discharging the nitrogen gas in the
space 513. The nitrogen gas for adjusting the temperatures in the
spaces 512, 513 is introduced into the spaces 512, 513 through the
introduction ports 510A, 510B via a supply conduit 515 from the
external temperature adjustment device 507. After the temperature
in the spaces 512, 513 were adjusted, the nitrogen gas is
discharged through the discharge ports 511A, 511B and is returned
to the temperature adjustment device 507 through a discharge
conduit 516.
A temperature sensor 505 is protruded from a side wall of the lens
barrel toward the interior of the space 513, which temperature
sensor 505 serves to measure the temperature in the space 513. The
temperature sensor 505 is disposed so that a tip end of the sensor
is situated out of the exposure area of the projection optical
system PL. A measurement signal from the temperature sensor 505 is
supplied to an external temperature measuring device 506, so that
the temperature adjustment device 507 can compare the temperature
measured by the temperature measuring device 506 with a target
temperature, thereby controlling the temperature of the nitrogen
gas supplied to the spaces 512, 513.
In the illustrated embodiment, while an example that the nitrogen
gas is used as the control fluid supplied to the spaces 512, 513 is
explained, the control fluid is not limited to the nitrogen gas,
but, any gas may be used so long as the following conditions are
satisfied. That is to say, the control fluid is used to adjust the
temperature of the lens element (i.e., lens element 503b).
Accordingly, although both gas and liquid can be used as the
control fluid, it is preferable that the control fluid is selected
among gases and liquids which does not cause erosion of the
materials of the lens elements and the lens barrel defining the
spaces 512, 513, is inert, is not harmful and is easy to handle.
Further, in the illustrated embodiment, gas or liquid which is
photochemically stable with respect to the exposure illumination
light and has no wavelength band absorbing the illumination light
is selected as the control fluid.
FIG. 48 shows absorbing spectrum of the ArF excimer laser light,
where sections (a) and (b) of FIG. 48 show conditions of the
absorbing spectrum in air and in nitrogen gas, respectively. In the
sections (a) and (b) of FIG. 48, the abscissa indicates the
wavelength .lambda. [nm] and the ordinate indicates the spectrum
intensity I. As shown by the curve 521 in the section (a) of FIG.
48, the ArF excimer laser light has spectrum having a width of a
wavelength from about 192.8 nm to about 193.7 nm and a center of
the wavelength positioned at about 193.2 nm position. In the
wavelength band, there are a plurality of narrow bands
(Schumann-Runge bands) for absorbing oxygen (O.sub.2). In the
section (a) of FIG. 48, such oxygen absorbing bands are shown by a1
(having the longest wavelength) to a6 (having the shortest
wavelength).
When the ArF excimer laser light is used, the band narrowing
treatment is effected by inserting a band-pass filter in a
resonator. However, when the band narrowing treatment is effected,
as shown in the section (a) of FIG. 48, it is very difficult to
effect the band narrowing treatment for complete elimination of the
oxygen absorbing bands, and, thus, it is not avoidable that the
narrowed wavelength band overlaps with the oxygen absorbing
band(s). Accordingly, it is not preferable that air including
oxygen is used as the control fluid.
Ozone (O.sub.3) in the air has a strong absorbing band or zone
called as "Hartley zone" extending from a wavelength of about 320
nm to a short wavelength zone. In particular, the ozone has a
stronger absorbing force than the oxygen. Thus, by merely removing
the ozone from the air, the absorbing rate of the illumination
energy of the laser light is greatly reduced. Accordingly, when the
air from which the ozone is removed is used as the control fluid,
the absorption of the laser energy can be greatly reduced in
comparison with the case where the air is used as the control
fluid.
On the other hand, as shown by the curve 522 in the section (b) of
FIG. 48, the nitrogen gas used in this embodiment has no wavelength
band (zone) absorbing the ArF excimer laser light, thereby not
absorbing the laser energy. For example, when the air is used as
the control fluid, impurities such as ammonium ion
(NH.sub.4.sup.+), sulfate ion (SO.sub.4.sup.2-) and organic silanol
slightly remaining in the air (even if the air is cleaned
substantially completely) are activated by the laser energy,
thereby producing the fog on the surfaces of the lens elements.
However, when the nitrogen gas is used, since the nitrogen gas does
not include such impurities and is inert, the fog is not produced
on the surfaces of the lens elements, and, thus, the lens elements
are maintained in a clean condition. The nitrogen gas can be easily
handled, is inert, and particularly has good stability. Thus, the
nitrogen gas can be used as the control fluid for almost all of
kinds of illumination lights. Incidentally, other than the nitrogen
gas and the air from which the ozone is removed, various gas such
as carbon dioxide (CO.sub.2) can be used as the control fluid. For
example, since the carbon dioxide has absorbing bands near the
wavelengths of 190 nm and 254 nm, when the ArF excimer laser light
(having a wavelength of 193 nm) and the KrF excimer laser light
(having a wavelength of 248 nm) are used, the carbon dioxide is not
suitable; whereas, the g-ray (having a wavelength of 436 nm) and
i-ray (having a wavelength of 365 nm) of the mercury lamp are used
as the illumination light, the carbon dioxide can be used as the
control fluid.
Now, an operation of the projection exposure apparatus having the
above-mentioned construction will be described.
First of all, a relation between the temperature T and the index R
of refraction of the lens element to be controlled is previously
determined, and the determined relation is stored as a function
having the index R of refraction as the variable. Now, the
determined function for representing the temperature T is referred
to as f(R). Further, a relation between a change amount .DELTA.R of
the index of refraction of the lens element 503b and a change
amount .DELTA.M of the imaging characteristics of the projection
optical system such as magnification, distortion and the like
(magnification, in this embodiment) is previously determined, and
the determined relation is stored as a function having the change
amount .DELTA.M of the magnification as the variable. Now, the
function for representing the determined change amount .DELTA.R of
the index of refraction is referred to as g(.DELTA.M). Then, a
relation between a change amount of environmental data such as
atmospheric pressure, humidity and atmospheric temperature and a
changed amount of illumination amount of the illumination light
during the exposure, and a change amount of the imaging
characteristic of the projection optical system PL is checked. When
there is provided a measuring means for measuring the imaging
characteristic of the projection optical system PL for a short
time, the changed amount the imaging characteristic of the
projection optical system PL may be measured by using such a
measuring means.
Next, a method for correcting the imaging characteristic of the
projection optical system PL during the actual exposure will be
explained. The present imaging characteristic of the projection
optical system PL is calculated on the basis of the environmental
data and the illumination amount of the illumination light. Then, a
difference between the calculated imaging characteristic and the
design imaging characteristic is determined. The target temperature
of the lens element 503b is determined to cancel such difference.
In this case, the change amount .DELTA.R of the index of refraction
of the lens element 503b corresponding to the change amount of the
imaging characteristic of the projection optical system PL is
determined on the basis of the previously determined function g
(.DELTA.M), and then, the target temperature of the lens element
503b is determined on the basis of the function f(R) and the change
amount .DELTA.R of the index of refraction.
Then, the temperature-adjusted nitrogen gas is supplied from the
temperature adjustment device 507 of FIG. 46 and is circulated
through the spaces 512, 513. In this case, the temperature and flow
rate of the nitrogen gas are adjusted so that the temperature of
the lens element 503b reaches the target temperature. In the
illustrated embodiment, while the temperature (average temperature)
of the lens element 503b is not directly measured, it is controlled
so that the measurement temperature measured by the temperature
sensor 505 becomes substantially equal to the target temperature.
However, if further accurate control is required, it may be
achieved that after the control of temperature adjustment device
507 so that the temperature of the temperature sensor 505 of FIG.
46 becomes the target temperature, a value of the measurement
temperature of the temperature sensor 505 at which the imaging
characteristic of the projection optical system PL becomes closest
to the design value is determined in minute adjustment manner while
the imaging characteristic is being actually measured. Thereafter,
the control is effected by using the determined temperature value
as a target value to be set in the temperature adjustment device
507.
The above-mentioned operation is continued during the actual
exposure. Accordingly, the imaging characteristic is kept constant
through the entire exposing process of the projection optical
system PL. Since the nitrogen gas is used as the control fluid, the
spaces (filled with the air in the conventional techniques) are
filled with the nitrogen gas. As a result, when the short
wavelength light such as the ArF excimer laser light is used as the
exposure light source as is in the illustrated embodiment, it can
be prevented that the illumination energy is decreased due to the
absorption of light energy, unlike the case of the oxygen. In place
of air, since the fluid such as nitrogen gas which is chemically
and optically inert and is hard to be reacted is used, the
disadvantage due to the generation of ozone can be avoided. As is
in the illustrated embodiment, since not only the ArF excimer laser
light is used as the illumination light but also the fluid having
no band or zone absorbing the wavelength of the used illumination
light is selected as the control fluid, even when any illumination
light is used, the illumination energy of the illumination light is
not absorbed by the control fluid, with the result that the
illumination light can be directed onto the wafer W
efficiently.
Next, an eighteenth embodiment of the present invention will be
explained with reference to FIG. 47. In this embodiment, a pellicle
(dust-proof film) for preventing foreign matters from adhering to a
pattern surface of a reticle is attached to a lower surface of the
reticle to form a sealed space between the pellicle and the pattern
surface of the reticle. The sealed space is filled with nitrogen
gas. Further, a space between an illumination optical system and a
projection optical system is also sealed, and this sealed space is
filled with nitrogen gas. Since the other constructions of this
embodiment are the same as those of the embodiment shown in FIG.
46, the elements same as those shown in FIG. 46 are designated by
the same reference numerals and explanation thereof will be
omitted.
FIG. 47 schematically shows a construction of the projection
exposure apparatus according to this embodiment. In FIG. 47, the
pellicle 531 is mounted on the pattern surface of the reticle 501
via a pellicle frame 532 so that the pellicle becomes in parallel
with the reticle and a predetermined space is formed between the
pellicle and the reticle. The pellicle 531 is formed from a
substantially square transparent thin film, and a closed space 533
covering the entire pattern area PA on the reticle 501 is defined
by the pellicle 531, reticle 501 and pellicle frame 532. The space
533 is filled with the nitrogen gas. In order to fill the closed
space 533 with the nitrogen gas, the pellicle may be attached to
the reticle in a nitrogen gas atmosphere condition, or, when the
pellicle film has sufficient strength, after the pellicle is
attached to the reticle, a space defined by the reticle, pellicle
frame and pellicle may be made in a vacuum condition, and then the
space may be filled with the nitrogen gas.
A lower portion of the illumination optical system EL and an upper
portion of the lens barrel 504 of the projection optical system PL
are secured to a cylindrical frame 534, and a substantially sealed
space 535 is defined by a lower surface of the illumination optical
system EL, frame 534, lens barrel 504 of the projection optical
system PL and the uppermost lens element 503a of the projection
optical system PL. The reticle 501 and a reticle stage 501a are
disposed within this space 535. The nitrogen gas for adjusting the
temperature is circulated through the space 535. The nitrogen gas
is supplied to the space 535 from an external temperature
adjustment device 536 through a supply conduit 537 and an
introduction port 353a formed in the frame 534. After the
temperature in the space 535 is adjusted by the nitrogen gas, the
nitrogen gas is returned to the temperature adjustment device 536
through a discharge port 535b formed in the frame 534 and a
discharge conduit 538. A temperature sensor 539 for measuring the
temperature in the space 535 is mounted on the frame 534. A
measured value from the temperature sensor 539 is supplied to a
temperature measuring device 540 so that the temperature adjustment
device 536 compares the measured value from the temperature sensor
539 with the target temperature and controls the temperature and
flow rate of the nitrogen gas supplied to the space 535.
In the illustrated embodiment, while an example that two spaces
533, 535 between the pellicle 531 and the reticle 501 and between
the illumination optical system EL and the projection optical
system PL are provided is explained, one of these spaces may be
provided. As is in the embodiment shown in FIG. 46, the gas
supplied to the spaces 533, 535 is not limited to the nitrogen gas,
but, various gases or liquids can be used in accordance with the
kind of the illumination light.
According to the projection exposure apparatus of this embodiment
having the above-mentioned construction, the illumination light IL
emitted from the illumination optical system EL reaches the
projection optical system PL without contacting with the air.
Accordingly, in addition to the advantage obtained by the
embodiment shown in FIG. 46, the amount of the laser energy
absorbed to the oxygen in the air is further decreased while the
illumination light from the illumination optical system EL reaches
the wafer W, thereby utilizing the energy from the light source
effectively. Further, since the space 533 between the pellicle 531
and the reticle 501 is filled with the nitrogen gas, the ozone is
not generated, thereby preventing the absorption of the
illumination energy due to accumulation of ozone.
Since the temperature in the space 535 is controlled, the
temperature of the reticle 501 is also adjusted. Accordingly, for
example, the distortion of the pattern and the change in the
imaging characteristic caused by expansion of the reticle can be
suppressed.
As mentioned above, when the space 533 between the pellicle 531 and
the reticle 501 is formed independently from the space 535
enclosing the reticle 501, the nitrogen gas contained in the space
533 does not absorb the ArF excimer laser light and prevents the
production of the impurity such as fog due to the laser energy,
thereby preventing the smudge of the pattern surface.
Since the pressure in the space defined by the reticle 501,
pellicle 531 and pellicle frame 532 is adjusted, it can be designed
so that a certain amount of gas is introduced into the space 533
filled with the nitrogen gas, i.e., a vent hole is formed in the
pellicle frame 532. With this arrangement, it is considered that
the oxygen is gradually entered into the space 533, thereby
decreasing the effect of the nitrogen gas. In such a case, as is in
the illustrated embodiment, by bringing the entire space enclosing
the reticle 501 to the nitrogen gas atmosphere, the desired purpose
can be achieved. If the reticle is stored in the place exposed to
the oxygen, since the oxygen is gradually entered into the space
533 and/or the space 535, the desired effect cannot be achieved.
Accordingly, it is desirable that the reticle is stored in the
nitrogen gas atmosphere.
The projection exposure apparatus according to the seventeenth and
eighteenth embodiments is not limited to the projection exposure
apparatus of stepper type, but, can be applied to a scan-exposure
type projection exposure apparatus of step-and-scan type in which a
reticle and a wafer are simultaneously scanned so that pattern
images of the reticle are successively transferred to shot areas on
the wafer.
According to the projection exposure apparatus of the seventeenth
embodiment of the present invention, by controlling the
temperature(s) of the lens element(s) in the projection optical
system by using the temperature-controlled fluid, the imaging
characteristics of the projection optical system are corrected.
Accordingly, for example, it is possible to suppress the change in
the imaging characteristics of the projection optical system caused
by the change in the environmental condition and the illumination
amount of the illumination light. Further, since the fluid having
no absorbing band absorbing the wavelength(s) of the illumination
light is used, the loss of the illumination energy of the
illumination light can be reduced.
In addition, merely by controlling the temperature of the fluid,
the change in the imaging characteristics of the projection optical
system can be suppressed. Since the fluid has no absorbing band
absorbing the wavelength(s) of the illumination light, the light
energy of the illumination light is not absorbed to the temperature
control fluid but can reaches the photosensitive substrate.
According to the projection exposure apparatus of the eighteenth
embodiment of the present invention, the dust and other foreign
matters can be prevented from adhering to the pattern surface of
the reticle by the dust-proof film (pellicle). Further, since the
fluid has no absorbing band absorbing the wavelength(s) of the
illumination light is used, the loss of the illumination energy of
the illumination light can be reduced.
* * * * *