U.S. patent application number 10/377737 was filed with the patent office on 2003-09-25 for mask-holding apparatus for a light exposure apparatus and related scanning-exposure method.
This patent application is currently assigned to NIKON CORPORATION. Invention is credited to Araki, Yasuo, Mizutani, Shinji, Narushima, Hiroaki, Tokuda, Noriaki, Yasuda, Masahiko.
Application Number | 20030179354 10/377737 |
Document ID | / |
Family ID | 28047049 |
Filed Date | 2003-09-25 |
United States Patent
Application |
20030179354 |
Kind Code |
A1 |
Araki, Yasuo ; et
al. |
September 25, 2003 |
Mask-holding apparatus for a light exposure apparatus and related
scanning-exposure method
Abstract
An exposure apparatus for exposing a substrate with an image of
a pattern formed on a mask, including an illumination system or
irradiation for illuminating or irradiating the mask with exposure
light. A projection optical system is included for projecting, onto
the substrate, an image of the pattern illuminated by the exposure
light. The mask is securely supported on a movable mask stage. In
one example, a mask holder is provided for supporting the mask from
below. A pressing member is also included for applying, from above,
a prescribed force to the mask, outside of the points supported by
the mask holder.
Inventors: |
Araki, Yasuo; (Yokohama-shi,
JP) ; Tokuda, Noriaki; (Kawasaki-shi, JP) ;
Yasuda, Masahiko; (Kawasaki-shi, JP) ; Mizutani,
Shinji; (Kawasaki-shi, JP) ; Narushima, Hiroaki;
(Tokyo, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
NIKON CORPORATION
Tokyo
JP
|
Family ID: |
28047049 |
Appl. No.: |
10/377737 |
Filed: |
March 4, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10377737 |
Mar 4, 2003 |
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09721758 |
Nov 27, 2000 |
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09721758 |
Nov 27, 2000 |
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09385963 |
Aug 30, 1999 |
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09385963 |
Aug 30, 1999 |
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08946735 |
Oct 9, 1997 |
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08946735 |
Oct 9, 1997 |
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08821529 |
Mar 21, 1997 |
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Current U.S.
Class: |
355/53 |
Current CPC
Class: |
G03B 27/42 20130101;
G03B 27/50 20130101; G03F 7/70866 20130101 |
Class at
Publication: |
355/53 |
International
Class: |
G03B 027/42 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 1996 |
JP |
08-093145 |
May 20, 1996 |
JP |
08-148585 |
Oct 29, 1996 |
JP |
08-303682 |
Oct 29, 1996 |
JP |
08-303683 |
Claims
What is claimed is:
1. A holding apparatus which holds a mask in a light exposure
apparatus, the mask containing a pattern to be transferred onto a
substrate by illuminating the pattern with an illuminating light,
and the light exposure apparatus being a scanning-type apparatus
that moves the mask in a direction of travel perpendicular to an
optical axis of the illumination light while the mask pattern is
illuminated by the illumination light, the holding apparatus
comprising: a mask holder which holds the mask by a holding force;
a mask driver, which is mechanically connected to the mask holder,
and which drives the mask holder in a direction of travel
perpendicular to the optical axis; a vacuum adhesion device, which
has a plurality of adhering portions, and which generates a vacuum
adhesion force between the mask and the mask holder, the adhering
portions generating the vacuum adhesion force to be used for
fastening the mask to the mask holder respectively so that the mask
is vacuum adhered to the mask holder by the plurality of adhering
portions; and an external force supplying system, which is
electrically connected to said mask driver, and which supplies an
external force to the mask in order to maintain a predetermined
positional relationship between the mask and the mask holder while
the mask driver drives the mask holder, wherein the external force
is different from the vacuum adhesion force, and wherein the mask
is held on the mask holder by the holding force including the
external force and the vacuum adhesion force generated from the
plurality of adhering portions.
2. The apparatus as set forth in claim 1, wherein the mask driver
drives the mask holder at an acceleration, a, less than the product
of the holding force, N, acting on the mask and a coefficient of
friction, .mu., between the mask and the mask holder divided by the
mass, m, of the mask as shown in the following
relationship:a<N.mu./m.
3. The apparatus as set forth in claim 1, further including a
plurality of support members to support the mask from below at a
plurality of support points.
4. The apparatus as set forth in claim 3, wherein the external
force supplying system applies the force, from above, to the mask
outside of the support points of the support members.
5. The apparatus as set forth in claim 4, wherein said external
force supplying system applies the force, from above, outside of
the support points of said mask holder, at two side portions of the
mask that are substantially parallel to the direction of
travel.
6. The apparatus as set forth in claim 1, further including an
airtight member to form an airtight space between the mask and said
mask holder, wherein said external force supplying system applies
the force to the mask by controlling air pressure within said
airtight member.
7. The apparatus as set forth in claim 1, wherein the force applied
by said external force supplying system is a magnetic force to
secure the mask to said mask holder.
8. The apparatus as set forth in claim 7, wherein said external
force supplying system further includes a clamp adapted to make and
break contact with the mask; an electromagnet attached to said mask
holder; a permanent magnet attached to said clamp and mounted
opposite to said mask holder; and a controller to control said
electromagnet to secure the mask to and release the mask from said
mask holder.
9. The apparatus as set forth in claim 1, wherein the force applied
by said external force supplying system is an electrostatic
attractive force to secure the mask to said mask holder.
10. The apparatus as set forth in claim 1, wherein the force
applied by said external force supplying system is generated by a
ringing phenomenon.
11. The apparatus as set forth in claim 1, wherein said external
force supplying system applies the force to the mask as a
compressing force by a fluid to secure the mask to said mask
holder.
12. A holding apparatus which holds a mask in a light exposure
apparatus, the mask containing a pattern to be transferred onto a
substrate by illuminating the pattern with an illumination light,
and the light exposure apparatus being a scanning-type apparatus
that moves the mask in a direction of travel perpendicular to an
optical axis of the illumination light while the mask pattern is
illuminated by the illumination light, the holding apparatus
comprising: a mask holder which holds the mask, wherein the mask
holder travels in a direction perpendicular to the optical axis; a
vacuum adhesion device which generates a vacuum adhesion force
between the mask and the mask holder, wherein the mask is adhered
to the mask holder by the vacuum adhesion force; and an external
force supplying system, which supplies an external force to the
mask in order to maintain a predetermined positional relationship
between the mask and the mask holder, wherein the external force is
different from the vacuum adhesion force, wherein the external
force effects to the mask in a direction perpendicular to a
formation surface of the mask pattern formed on the mask, and
wherein the mask is held on the mask holder by a holding force
including the vacuum adhesion force and the external force.
13. The apparatus as set forth in claim 12, wherein said external
force supplying system is electrically activated.
14. The apparatus as set forth in claim 13, wherein the force
applied by said external force supplying system is an electrostatic
attractive force that secures the mask to said mask holder by
applying a predetermined voltage there between.
15. The apparatus as set forth in claim 14, further including a
chrome portion provided on a surface of the mask and contacting
said mask holder.
16. The apparatus as set forth in claim 15, wherein said chrome
portion is provided on a side of the mask making contact with said
mask holder.
17. The apparatus as set forth in claim 15, further including an
electric portion provided on said mask holder making contact with
said chrome portion.
18. The apparatus as set forth in claim 12, wherein the force
applied by said external force supplying system is a compressive
force applied, from above, to an upper side portion of the
mask.
19. The apparatus as set forth in claim 18, further including an
airtight member to form an airtight space between the mask and said
mask holder.
20. The apparatus as set forth in claim 19, wherein said external
force supplying system applies the force to the mask by controlling
the air pressure within said airtight member.
21. The apparatus as set forth in claim 20, wherein said external
force supplying system applies the force to the mask as a
compressing force as a fluid to a portion of the mask other than a
portion in which the pattern is formed.
22. The apparatus as set forth in claim 21, wherein the fluid is a
gas and the compressive force is adjusted by controlling pressure
of the gas.
23. The apparatus as set forth in claim 12, wherein said external
force supplying system applies the force to the mask, wherein the
force is generated by a ringing phenomenon.
24. A scanning-exposure method for transferring a pattern formed on
a mask onto a substrate by illuminating the pattern with an
illumination light, and for moving the mask in a direction of
travel perpendicular to an optical axis of the illuminating light,
the scanning exposure method comprising: generating a vacuum
adhesion force between the mask and a mask holder that holds the
mask; applying an external force to the mask, wherein the external
force is different from the vacuum adhesion force, and wherein the
external force effects to the mask in a direction perpendicular to
a formation surface of the mask pattern formed on the mask; and
moving the mask holder in a direction of travel perpendicular to
the optical axis when the mask is held on the mask holder by a
holding force including the vacuum adhesion force and the external
force.
25. The method as set forth in claim 24, further including
generating the external force by electrical activation from an
external force generating member disposed on the mask holder.
26. The method as set forth in claim 24, wherein said applying an
external force applies the external force as an electrostatic
attraction force.
27. The method as set forth in claim 24, wherein said applying an
external force applies the external force as a compressive
force.
28. The method as set forth in claim 24, further including
generating the external force by a ringing phenomenon.
29. A method for manufacturing a device including transferring a
device-pattern formed on a mask onto a substrate of the device
using the scanning-exposure method as set forth in claim 24.
30. A scanning exposure method for transferring a pattern formed on
a mask onto a substrate by illuminating the pattern with an
illumination light, and for moving the mask in a direction of
travel perpendicular to an optical axis of the illumination light,
the scanning exposure method comprising: generating a vacuum
adhesion force between the mask and a mask holder that holds the
mask, wherein the vacuum adhesion force is obtained from a
plurality of adhering portions that generate the vacuum adhesion
force to be used for fastening the mask to the mask holder
respectively; applying an external force to the mask, wherein the
external force is different from the vacuum adhesion force; and
moving the mask holder in a direction of travel perpendicular to
the optical axis when the mask is held on the mask holder by a
holding force including the vacuum adhesion force and the external
force.
31. The method as set forth in claim 30, further including
generating the external force by electrical activation from an
external force generating member disposed on the mask holder.
32. The method as set forth in claim 30, wherein said applying an
external force applies the external force as an electrostatic
attraction force.
33. The method as set forth in claim 30, wherein said applying an
external force applies the external force as a compressive
force.
34. The method as set forth in claim 30, further including
generating the external force by a ringing phenomenon.
35. A method for manufacturing a device including transferring a
device-pattern formed on a mask onto a substrate of the device
using the scanning-exposure method as set forth in claim 30.
36. A holding apparatus which holds a mask in a light exposure
apparatus, the mask containing a pattern to be transferred onto a
substrate by illuminating the pattern with an illumination light,
and the light exposure apparatus being a scanning-type exposure
apparatus that moves the mask in a direction of travel
perpendicular to an optical axis of the illumination light while
the mask pattern is illuminated by the illumination light, the
holding apparatus comprising: a mask holder which holds the mask,
wherein the mask holder travels in a direction perpendicular to the
optical axis; a vacuum adhesion device which generates a vacuum
adhesion force between the mask and the mask holder, wherein the
mask is adhered to the mask holder by the vacuum adhesion force;
and an external force supplying system, which supplies an external
force to the mask in order to maintain a predetermined positional
relationship between the mask and the mask holder, wherein the
external force is different from the vacuum adhesion force, wherein
the external force is able to control based on information
regarding a mask thickness or information regarding a moving
velocity of the mask holder, and wherein the mask is held on the
mask holder by a holding force including the vacuum adhesion force
and the external force.
37. The apparatus as set forth in claim 36, wherein said external
force supplying system is electrically activated.
38. The apparatus as set forth in claim 37, wherein the force
applied by said external force supplying system is an electrostatic
attractive force that secures the mask to said mask holder by
applying a predetermined voltage there between.
39. The apparatus as set forth in claim 38, further including a
chrome portion provided on a surface of the mask and contacting
said mask holder.
40. The apparatus as set forth in claim 39, wherein said chrome
portion is provided on a side of the mask making contact with said
mask holder.
41. The apparatus as set forth in claim 39, further including an
electric portion provided on said mask holder making contact with
said chrome portion.
42. The apparatus as set forth in claim 36, wherein the force
applied by said external force supplying system is a compressive
force applied, from above, to an upper side portion of the
mask.
43. The apparatus as set forth in claim 42, further including an
airtight member to form an airtight space between the mask and said
mask holder.
44. The apparatus as set forth in claim 43, wherein said external
force supplying system applies the force to the mask by controlling
the air pressure within said airtight member.
45. The apparatus as set forth in claim 44, wherein said external
force supplying system applies the force to the mask as a
compressing force as a fluid to a portion of the mask other than a
portion in which the pattern is formed.
46. The apparatus as set forth in claim 45, wherein the fluid is a
gas and the compressive force is adjusted by controlling pressure
of the gas.
47. The apparatus as set forth in claim 36, wherein said external
force supplying system applies the force to the mask, wherein the
force is generated by a ringing phenomenon.
48. A scanning exposure method for transferring a pattern formed on
a mask onto a substrate by illuminating the pattern with an
illumination light, and for moving the mask in a direction of
travel perpendicular to an optical axis of the illumination light,
the scanning exposure method comprising: generating a vacuum
adhesion force between the mask and a mask holder that holds the
mask; applying an external force to the mask, wherein the external
force is different from the vacuum adhesion force, and wherein the
external force is able to control based on information regarding a
mask thickness or information regarding a moving velocity of the
mask holder; and moving the mask holder in a direction of travel
perpendicular to the optical axis when the mask is held on the mask
holder by a holding force including the vacuum adhesion force and
the external force.
49. The method according to claim 48, wherein the external force is
generated by electrically activating an external force generating
device that generates the external force.
50. The method as set forth in claim 48, wherein said applying an
external force applies the external force as an electrostatic
attraction force.
51. The method as set forth in claim 48, wherein said applying an
external force applies the external force as a compressive
force.
52. The method as set forth in claim 48, further including
generating the external force by a ringing phenomenon.
53. A method for manufacturing a device including transferring a
device-pattern formed on a mask onto a substrate of the device
using the scanning-exposure method as set forth in claim 48.
54. The holding apparatus according to claim 36, wherein the
external force supplying system controls generation of the external
force.
55. The holding apparatus according to claim 36, wherein the
external force supplying system controls an amount of the external
force.
56. The method according to claim 48, further comprising
controlling generation of the external force.
57. The method according to claim 48, further comprising
controlling an amount of the external force.
58. A scanning exposure apparatus which exposes the substrate with
the pattern formed on the mask, wherein the scanning exposure
apparatus has the holding apparatus according to claim 1.
59. A scanning exposure apparatus which exposes the substrate with
the pattern formed on the mask, wherein the scanning exposure
apparatus has the holding apparatus according to claim 12.
60. A scanning exposure apparatus which exposes the substrate with
the pattern formed on the mask, wherein the scanning exposure
apparatus has the holding apparatus according to claim 36.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of U.S. Ser. No. 09/721,758 filed on
Nov. 27, 2000, now abandoned; which is a continuation of U.S. Ser.
No. 09/385,963 filed Aug. 30, 1999, now abandoned; which is a
continuation of U.S. Ser. No. 08/946,735 filed Oct. 9, 1997, now
abandoned; which is a continuation-in-part of U.S. Ser. No.
08/821,529 filed on Mar. 21, 1997, now abandoned. This application
claims the benefit of Japanese Patent Application No. 8-93145 filed
on Mar. 22, 1996, No. 8-148585 filed on May 20, 1996, No. 8-289305
filed on Oct. 11, 1996, No. 8-303682 filed on Oct. 29, 1996, No.
8-303683 filed on Oct. 29, 1996 and No. 8-303684 filed on Oct. 29,
1996, the disclosures of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to an exposure system for
projection-exposing the image of a pattern formed in a mask
(reticle) onto a substrate. More particularly, the present
invention relates to an exposure system and mask-holding apparatus
for fabrication of devices such as semiconductor integrated
circuits and liquid crystal display elements.
[0003] In exposure systems used to fabricate devices such as
semiconductor integrated circuits and liquid, crystal display
elements; a mask is illuminated or irradiated by illumination light
emitted from illumination optics to transfer the image of a pattern
formed in the mask, through projection optics, onto a
photosensitive substrate (wafer, etc.).
[0004] In recent years there have been substantial increases in the
levels of integration (circuit density) of semiconductor devices.
This has imposed increasingly strict requirements on the systems
used to fabricate these devices. These include requirements for
finer detail in the patterns formed on the wafers, and for more
layers of superimposed patterns exposed on one wafer. In other
words, these systems are required to form extremely fine-line width
patterns in the exposure mask, and to expose multiple superimposed
layers of different patterns on the wafer (semiconductor substrate)
in precise registration.
[0005] To accommodate these finer mask patterns, exposure systems
are now using projection optics with higher numerical apertures
(NA=0.55-0.6); and are using shorter-wavelength exposure light
(.lambda.=248 nm, etc.).
[0006] Both of these actions, however, decrease the depth of focus,
DF, at the wafer plane. The relationship between depth of focus,
DF, and light wavelength, .lambda., is expressed by the following
equation (A):
DF=k.sub.2.times..lambda./(NA)2 (A)
[0007] where k.sub.2 is a proportionality constant.
[0008] FIG. 29 shows a mask support mechanism in a conventional
type of exposure system. As shown in FIG. 29, a mask 112 is
supported in a horizontal state by a mask holder 111. Provided on
the mask holder 111 are fixtures (platens) 114 and 116, to which
the mask 112 is held fast by vacuum-induced suction.
[0009] In such prior mask support mechanisms, however, the mask 112
is supported only at its outer edge. This arrangement caused the
mask to sag from its own weight, as shown in FIG. 30(A).
[0010] When the mask pattern is exposed on a wafer, this sag causes
problems with lateral distortion of the pattern image, curvature of
the image plane, and reduced depth of focus.
[0011] The effects of such mask sag will be discussed further with
reference to FIG. 31. As shown in FIG. 31, the sagging of the mask
under its own weight produces a displacement .DELTA.Z in the
focusing direction (the Z direction of the optical axis of the
projection optics), and a displacement .DELTA.X (and .DELTA.Y) in a
plane perpendicular to the Z axis (a horizontal plane). The reduced
depth of focus due to today's high NA projection optics and short
wavelength exposure light, however, has made the tolerance range
for displacement in the focusing direction (.DELTA.Z) extremely
small. Also, in order to obtain the required overlay precision
between layers of patterns formed on the wafer, the .DELTA.X
(.DELTA.Y) tolerance, the tolerance for shifting in the horizontal
plane of the mask, must also be extremely tight.
[0012] The amount of sag in the mask 112 due to its own weight is
mainly a function of the thickness and size (area) of the mask.
FIG. 32 shows the results of calculations to determine how .DELTA.Z
and .DELTA.X (the displacements of the pattern of the mask 112 in
the Z and X directions) are influenced by mask thickness. For these
calculations, the mask 112 was assumed to be made of transparent
quartz and supported in two places at its outer edge. The exposure
light was assumed to be in the ultraviolet to deep ultraviolet
range.
[0013] The 6.4 mm-thick (0.25-inch) masks now widely used in
high-volume production, for example, would experience 0.6 .mu.m of
.DELTA.Z displacement (displacement in the focusing direction), and
35 nm of .DELTA.X displacement in the X direction. In comparison, a
2.3 mm-thick (0.09-inch) mask would have 4.5 .mu.m of displacement
in the focusing direction .DELTA.Z, and 95 nm of .DELTA.X
displacement. Thus if the projection optics has a reduction ratio
of 1/5, and a 2.3 mm-thick mask is used, the displacement in the
focusing direction (.DELTA.Z) would be 180 nm (4.5 gm/25) at the
wafer.
[0014] Thus from equation (A), with an Wine light source used for
exposure light (.lambda.=0.365 .mu.m), k.sub.z=0.7, and NA=0.6,
this 180 nm of displacement in the Z direction would take, up 1/4
of the depth-of-focus tolerance. Under the same conditions,
.DELTA.X, the mask displacement in the X direction (95 nm) would be
19 nm (95 nn/5). For a semiconductor device design using a 350-nm
line width, this amount of displacement would consume more than 15%
of the overlay tolerance usually required (1/3 of the line
width).
[0015] If semiconductor devices continue to become larger at the
rate they have been in recent years, it can be expected that the
current mainstream mask size of six inches square could soon
increase to seven or even nine inches square. A larger area mask
means a corresponding increase in the amount of sag in the wafer
due to its weight. A simple fix for this problem, then, might seem
to be to increase the thickness. There are limiting factors on the
thickness of the mask material (mask blank), however. These include
the difficulty in obtaining the required accuracy from the mask
generation system (electron beam exposure system) used to make the
mask.
[0016] Recently, expansion of the mask due to its irradiation by
the exposure or illumination light has also started to be seen as a
serious problem. When the mask is irradiated with exposure light,
it heats up and expands. When the outer edge of an expanding mask
is held tightly against the mask holder by suction, as shown in
FIG. 30(B), this sag becomes even worse. When a mask has thermal
displacement (distortion and shifting), its pattern will stretch,
and in some cases, the mask will even shift with respect to the
mask holder. When the exposure ends and the mask is no longer being
irradiated by exposure light, the mask temperature drops, and the
pattern starts to contract. External vibration can also cause
mispositioning. When these kinds of expanding, contracting,
slipping, and twisting displacements occur, they cause scaling,
offset, and rotational mispositioning of the projected pattern
image. Once these problems go beyond a certain point, satisfactory
exposures can no longer be performed.
[0017] Further increases in mask temperature can result in transfer
of heat to the mask holder 111, distorting it as shown in FIG.
30(C). When the holder becomes distorted, it puts stress on the
mask, distorting the mask pattern and the projected image. Mask
holder distortion can also cause mask mispositioning in the form of
offset and rotation of the projected pattern image.
[0018] Also used recently are "step-and-scan" systems, in which
both the mask and the photosensitive substrate are synchronously
scanned with respect to the exposure illumination light as the mask
pattern image is exposed on the substrate. In step-and-scan
exposure systems, the mask holder with the mask on it is mounted on
a movable mask stage, and the substrate is mounted on a movable
substrate stage. On the mask stage, as shown in FIG. 29, the mask
is held fast against the mask holder at a portion of its outer
edge, by vacuum suction. The mask stage and the substrate stage are
synchronously moved as the image of the transfer pattern formed on
the mask is exposed on the photosensitive substrate. To obtain the
desired throughput in step-and-scan systems, the stages must be
moved at high speeds and accelerations.
[0019] Driving the mask stage fast, however, can cause
mispositioning of the mask on the holder. (Rapid acceleration and
deceleration of the mask stage causes the mask to slip on its
holder with the force of inertia). Also, if an image reduction
optical system is used, mask slippage becomes even more of a
problem because the mask has to move farther than the substrate, by
a factor equal to the reduction ratio. (In a 1/5.times.image
reduction system the mask stage has to move 5 times as far as the
substrate stage).
[0020] If the system uses an interferometer movement mirror on the
mask stage to measure the mask position, for example, any
mispositioning of the mask on the mask holder will throw off the
position of the mask relative to the interferometer movement
mirror. This kind of mask mispositioning is not only a mask stage
problem, but will also degrade the overlay accuracy of the pattern
with respect to the substrate.
[0021] It is an objective of the present invention to provide a
mask-holding apparatus and exposure apparatus that can make good
projection exposures of mask patterns on substrates.
[0022] It is a further objective of the present invention to
provide a mask-holding apparatus and. exposure apparatus that can
reduce distortion and mispositioning of masks.
SUMMARY OF THE INVENTION
[0023] To accomplish the above objectives, a mask holding apparatus
incorporating the principles of the present invention provides
support members for supporting the mask from below, and pressing
members for applying, from above, a prescribed force to the mask,
outside of the support points of the support members.
[0024] Also, an exposure apparatus incorporating the principles of
the present invention provides an illumination system for shining
exposure light on the mask. A projection optical system is included
for projecting, onto the substrate, an image of the pattern
illuminated by this light. A mask holder is provided for supporting
the mask from below, as is a pressing member for applying, from
above, a prescribed force to the mask, outside of the points
supported by the mask holder.
[0025] In a scanning-type exposure apparatus incorporating the
principles of the present invention, the configuration places
counterweights on the sides of the mask that run substantially
parallel to the direction of travel of the mask.
[0026] According to a preferred embodiment of the present
invention, the sagging of the mask due to its own weight is
corrected by applying a prescribed force to the mask from above,
outside of the support points of the support members. That is, the
weight of the mask applies force having its moment in a direction
toward the center of the mask, with the support points of the
support members acting as fulcrums. The weight of a pressing member
on the other hand, applies force having its moment in a direction
away from the center of the mask, with the support points acting as
fulcrums. This cancels out the sag in the mask due to its
weight.
[0027] The exposure apparatus incorporating the principles of the
present invention is capable of reducing the positioning error of
the mask pattern image as it is projected onto the substrate. This
enables a highly precise imaging state to be achieved. Alignment
accuracy is also improved.
[0028] In the scanning-type exposure apparatus embodying the
present invention, the width of the illumination light in the
direction of travel (the scanning direction) is the width across
the narrower dimension of the slit-shaped illumination light. This
width is small enough to allow the mask sag in the scan direction
to be ignored. Thus for practical purposes, the sagging of the mask
due to its own weight can be corrected by placing counterweights
only at the edge of the mask on the sides along the direction of
travel. This minimizes the weight that must be added to the mask,
and thereby avoids adding any more inertial mass than necessary to
be overcome when the mask is moved.
[0029] Also, to accomplish the stated objectives, an exposure
apparatus incorporating the principles of the present invention
comprises a holding member on which the mask is loaded, an airtight
member for forming airtight space between the mask and the holding
member, and an adjustment system for adjusting the air pressure
within the airtight space such that the mask will be held fast to
the holding member.
[0030] An embodiment of the present invention provides a
configuration in which an airtight space is formed between the mask
and the holding member, and the mask is held fast to the holding
member by adjusting the pressure in. this airtight space. This
configuration allows that portion of the surface of the mask that
is being drawn against the holding member to be increased to a much
larger area, including the area in which the transfer pattern is
formed. That is, it can be enlarged to an area just slightly
smaller than the area of the mask itself, thus greatly increasing
the force with which the mask is held to the holder.
[0031] The pertinent relationships can be expressed by the
following equations (1) and (2):
F.sub.k=ma (1)
F.sub.h=N.mu. (2)
[0032] where
[0033] F.sub.k is the force of inertia when the mask is moved,
[0034] F.sub.h is the holding force exerted on the mask due to its
friction with the holding member,
[0035] m is the mass of the mask,
[0036] N is the vertical reactive force acting on the mask,
[0037] a is the acceleration of the mask when it is moved, and
[0038] .mu. is the coefficient of friction between the mask and the
holding member.
[0039] For the mask not to be shifted due to the force of inertia
when it is moved, the condition of the following equation (3) must
be met:
F.sub.k<F.sub.h (3)
[0040] From (1), (2), and. (3), the following equation (4) can be
derived:
a<N.mu.m (4)
[0041] Thus it becomes apparent that since m, the mass of the mask,
and .mu., the coefficient of friction between the mask and the
holding member are nearly constant, the value of acceleration (a)
can be increased (while still maintaining F.sub.k<F.sub.h) by
increasing N. Also, for the case in which the mask is held against
the holding member by vacuum-induced suction; N is given by the
following equation (5):
N=S(P.sub.o-P.sub.v) (5)
[0042] where
[0043] S is the area to which suction is applied,
[0044] P.sub.o is the outside pressure, and
[0045] P.sub.v is the vacuum pressure.
[0046] In the apparatus incorporating the principles of the present
invention, S (the mask's area of suction against the holding
member) can be considered the equivalent of an area enlarged to
just slightly smaller than the mask surface area. As a result, from
equations (4) and (5) above, a (the maximum permissible
acceleration during movement of the mask) can be increased over
that possible with other mask-holding systems.
[0047] Also, to accomplish the stated objectives, an exposure
apparatus incorporating the principles of the present invention
comprises a holding member for holding a mask, a clamping apparatus
for clamping the mask to the holding member by magnetic force, and
an exposure system for transferring an image of a pattern on a mask
being held on the holding member to a substrate.
[0048] In such an embodiment of the present invention, because the
mask is held fast to the holding member by magnetic force, the mask
can be held to the holding member more tightly than is possible
with conventional systems in which only a portion of the mask is
held by vacuum induced suction. This reduces problems with the mask
slippage, even when the mask is moved at high speeds. When changing
the mask, for example, a pressing member can be retracted upwardly
by causing an electromagnet to repel a permanent magnet. Once a new
mask has been loaded on the holding member, the electromagnet can
be caused to attract the permanent magnet, so that the pressing
member can be pulled in by the attractive force between the
electromagnet and the permanent magnet, thus clamping the mask onto
the holding member. When doing this, the amount of holding force
applied to the mask can be controlled by adjusting the amount of
current flowing in the electromagnet.
[0049] To accomplish the stated objectives, in a projection
exposure apparatus incorporating the principles of the present
invention, the mask holder that holds the mask is made of a
material having a coefficient of thermal expansion substantially
equal to that of the mask.
[0050] Thus, as the mask is distorted by thermal expansion, the
mask holder distorts a corresponding amount. This reduces the
thermal distortion, and thereby provides satisfactory projection of
the mask pattern.
[0051] To accomplish the stated objectives, a projection exposure
apparatus incorporating the principles of the present invention
comprises a holding apparatus for holding a mask in a prescribed
position without touching it, and an exposure system for exposing,
on a substrate, the image of the pattern of a mask held by the
holding apparatus. Since the mask is held without touching the
holder the flow of heat from the mask to the holder is cut off, and
thermal shape distortion of the holder is reduced. This enables
deformation of the mask due to shape distortion of the holder to be
controlled.
[0052] In an embodiment of the present invention, the projection
exposure apparatus comprises a displacement detection system for
detecting any displacement of the mask; a computer for computing
changes in the image of the pattern of the mask projected on the
substrate to be exposed, based on the detection results of the
displacement detection system; and a correction system for
correcting the pattern image changes computed by the computer. When
displacement of the mask from thermal expansion due to the exposure
process occurs, the corresponding displacement of the projected
mask image is computed, and the pattern image corrected based on
the results of these computations.
[0053] Accordingly, a projection exposure apparatus incorporating
the principles of the present invention can comprise a displacement
detection system for detecting displacement of a mask, and a
correction system for correcting the position of the mask to cancel
the displacement of the mask as detected by the displacement
detection system. When displacement of the mask from thermal
expansion due to the exposure process occurs, the position of the
mask is corrected to cancel this displacement, and the image of the
mask pattern can therefore be accurately exposed on the
substrate.
[0054] Further features of the present invention relate to mask
holding structures and methods which utilize electrostatic
attraction between the mask and the mask holder either in
conjunction with or separate from the above-noted pressure
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0055] The above and other objects, features, and advantages of the
present invention will become more apparent from the following
detailed description taken with the accompanying drawings, in
which:
[0056] FIG. 1 is a simplified schematic elevational view showing
the configuration of the projection exposure apparatus of a first
embodiment of the present invention;
[0057] FIG. 2 is a cross-sectional view of a reticle support
mechanism schematically shown in FIG. 1;
[0058] FIG. 3 is an exploded perspective view of the reticle
support mechanism of FIG. 2;
[0059] FIG. 4 is a plan view of the reticle support mechanism of
FIG. 1 and FIG. 2;
[0060] FIG. 5 is an enlarged view of a portion of FIG. 2;
[0061] FIG. 6 is a plan schematic view of the reticle support
mechanism of the first embodiment to explain the relationship
between weighting points and support points;
[0062] FIG. 7 is a schematic drawing to explain the operation of
the reticle support mechanism of the first embodiment;
[0063] FIG. 8 is a schematic elevational view of a scan-type
projection exposure apparatus of a second embodiment of the present
invention;
[0064] FIG. 9 is a plan view of a reticle support mechanism used in
the second embodiment;
[0065] FIG. 10 is a schematic diagram showing the configuration of
a reticle support mechanism of a projection exposure apparatus of a
third embodiment of the present invention;
[0066] FIG. 11 is a schematic elevational view of a projection
exposure apparatus of a fourth embodiment of the present
invention;
[0067] FIG. 12 is an elevational view, partially in cross-section,
showing a reticle stage used in the fourth embodiment of FIG.
11;
[0068] FIG. 13 is an elevational view, partially in cross-section,
showing the construction of a reticle stage used in a fifth
embodiment of the present invention;
[0069] FIG. 14 is an elevational view, partially in cross-section,
showing the construction of a reticle stage used in a sixth
embodiment of the present invention;
[0070] FIG. 15 is an elevational view, partially in cross-section,
showing the construction of a reticle stage used in a seventh
embodiment of the present invention;
[0071] FIG. 16 is a simplified schematic elevational view of a
projection exposure apparatus of an eighth embodiment of the
present invention;
[0072] FIG. 17, including FIGS. 17(A) and 17(B), shows enlarged
views of the mask holder portion of the eighth embodiment of the
present invention, where FIG. 17(A) is an oblique view, and FIG.
17(B) is a cross-sectional view taken along the line XVII-XVII of
FIG. 17(A);
[0073] FIG. 18 is a simplified schematic elevational view of a
projection exposure apparatus of a ninth embodiment of the present
invention;
[0074] FIG. 19 is a simplified schematic elevational view of a
projection exposure apparatus of a tenth embodiment of the present
invention;
[0075] FIG. 20 including FIGS. 20(A), 20(B), and 20(C), shows a
projection exposure apparatus of an eleventh embodiment of the
present invention, where FIG. 20(A) is an oblique view, FIG. 20(B)
is a cross-sectional view taken along the line XX-XX of FIG. 20(A),
and FIG. 20(C) is an oblique view;
[0076] FIG. 21 is a simplified schematic elevational view of a
projection exposure apparatus of a twelfth embodiment of the
present invention;
[0077] FIG. 22 is an exploded oblique view showing the mask holder
of the twelfth embodiment;
[0078] FIG. 23 is a cross-sectional view taken along the line
XXIII-XXIII of FIG. 22;
[0079] FIG. 24 is a simplified schematic elevational view of a
projection apparatus of a thirteenth embodiment of the present
invention;
[0080] FIG. 25 is a plan view showing the mask and mask
displacement detector of the thirteenth embodiment;
[0081] FIG. 26 including FIGS. 26(A), 26(B), and 26(C), is a plan
view showing the nature of mask mispositioning and piezoelectric
element distortion in the thirteenth embodiment;
[0082] FIG. 27 is a simplified schematic elevational view of a
projection apparatus of a fourteenth embodiment of the present
invention;
[0083] FIG. 28 includes FIG. 28(A) showing a side view partially in
cross-section; and FIG. 28(B) showing a plan view of a mask holder
of a fifteenth embodiment of the present invention;
[0084] FIG. 29 is a cross-sectional view of a prior art reticle
support mechanism;
[0085] FIG. 30, including FIGS. 30(A), 30(B), and 30(C), shows the
nature of reticle sag in a reticle support of a prior art reticle
support mechanism;
[0086] FIG. 31 is a schematic diagram used to explain the operation
of a prior art reticle support mechanism;
[0087] FIG. 32 is a table showing reticle sag due its own weight in
a prior art reticle support mechanism;
[0088] FIG. 33 is a simplified schematic elevational view showing
the configuration of the projection exposure apparatus used for the
sixteenth through nineteenth embodiments of the present
invention;
[0089] FIG. 34 is an oblique view showing the peripheral
configuration of the reticle stage of the exposure apparatus in the
sixteenth embodiment of the present invention;
[0090] FIG. 35 is an oblique view showing the construction of the
reticle used in the sixteenth embodiment;
[0091] FIG. 36 is an oblique view showing the construction of a
reticle holder used in the sixteenth embodiment;
[0092] FIG. 37 is an oblique view showing the peripheral
configuration of the reticle stage of the sixteenth embodiment
mounted in the exposure apparatus of FIG. 33;
[0093] FIG. 38 is an oblique view showing the peripheral
configuration of the reticle stage of the exposure apparatus in a
seventeenth embodiment of the present invention;
[0094] FIG. 39 is an oblique view showing the construction of the
reticle used in the seventeenth embodiment;
[0095] FIG. 40 is an oblique view showing the peripheral
configuration of the reticle stage of the seventeenth embodiment
mounted in the exposure apparatus of FIG. 33;
[0096] FIG. 41 is an oblique view showing the peripheral
configuration of the reticle stage of the exposure apparatus in an
eighteenth embodiment of the present invention;
[0097] FIG. 42 is an oblique view showing the construction of the
reticle used in the eighteenth embodiment;
[0098] FIG. 43 is an oblique view showing the peripheral
configuration of the reticle stage of the eighteenth embodiment
mounted in the exposure apparatus of FIG. 33;
[0099] FIG. 44 is an oblique view showing the peripheral
configuration of the reticle stage of the exposure apparatus in a
nineteenth embodiment of the present invention;
[0100] FIG. 45 is an oblique view showing the peripheral
configuration of the reticle stage of the nineteenth embodiment
mounted in the exposure apparatus of FIG. 33;
[0101] FIG. 46 is an oblique view showing the configuration of a
measurement system capable of being used in the sixteenth through
nineteenth embodiments;
[0102] FIG. 47 is a plan view showing an example of another reticle
specimen capable of being used in the apparatus incorporating the
principles of the present invention; and
[0103] FIG. 48 is a plan view showing the configuration of a
reticle specimen capable of being used in the apparatus
incorporating the principles of the present invention and also in
the past.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0104] Referring to the drawings, FIG. 1 shows projection exposure
apparatus 10 of a first embodiment of the present invention.
Apparatus 10 is a projection exposure apparatus (stepper) for
projection-exposing a pattern formed in a reticle or mask 12 onto a
substrate or a wafer 14.
[0105] In FIG. 1, the reticle 12 is held fast to a reticle holder
11 by vacuum-induced suction. An exposure light source included in
an illumination optical system 16 generates the exposure light,
which illumination optical system 16 emits so as to evenly
irradiate or illuminate the pattern surface of the reticle 12. The
reticle 12 is made of a material such as fused quartz, and has the
pattern to be exposed formed on its bottom surface. The reticle
holder 11, which is made of ceramic, etc., supports the reticle 12
in a horizontal position. The details of the support mechanism of
the reticle 12 are not shown in FIG. 1.
[0106] When the pattern formed on the reticle 12 is illuminated by
the exposure light, its image is projected through a projection
lens system 20 onto a wafer 14. The wafer 14 is held fast to a
wafer stage 32 by vacuum-induced suction. Interferometric
measurement reflector mirrors 34 and 36 are attached to the wafer
stage 32. The reflector mirror 34 and an interferometer 38 are set
up to measure the X-axis position of the wafer stage 32. That is,
the interferometer 38 directs a measurement laser beam at the
reflector mirror 34 and the fixed mirrors (not illustrated) located
in the lens barrel of the projection optical lens system 20. The X
position of the wafer stage 32 is then measured based on the
interference of the light reflected from these fixed mirrors and
the reflector mirror 34. The Y position (the position along the
axis perpendicular to the page) is measured by the reflector mirror
36 and another interferometer (not shown).
[0107] The wafer stage 32 is constructed so that it can be moved in
three dimensions (X, Y and Z) by a wafer drive system 42. An
alignment microscope 44 is located on the side of the projection
lens 20. The alignment microscope 44 is an off axis microscope (on
a different optical axis from that of the projection lens 20). It
is used for sensing alignment marks (not illustrated) formed on the
wafer 14. When the wafer 14 is being positioned relative to the
reticle 12, the coordinate position of the wafer 14 is determined
by performing a "least squares" approximation, for example, based
on the sensed positions of the alignment marks. The baseline
magnitude (the distance between the optical axes of the projection
lens 20 and the alignment microscope 44) is measured. This baseline
magnitude is the amount the wafer stage 32 must be moved to begin
an exposure. That is, the wafer is aligned on the alignment
microscope axis and then moved to the projection lens to perform
the exposure.
[0108] FIGS. 2, 3, and 4 show the detailed construction of the
support mechanism of the reticle 12. The reticle 12 is held fast to
the reticle holder 11, through four platens (support members) 46a,
46b, 46c, and 46d, by vacuum induced suction (FIGS. 2 and 3).
Platens 46a, 46b, 46c, and 46d are placed at the four corners of
the rectangular frame of the reticle holder 11 such that they
support the reticle 12 by its outer edge. An aperture 48 is formed
in the middle of the wafer holder 11, to allow light shining on an
illumination area (pattern area) 54 (see FIG. 4) of the reticle 12
to pass on through.
[0109] A counterweight 50 is a rectangular frame shaped like a
picture frame and is placed over the reticle 12 to exert a downward
force on the reticle 12, outside of the platens 46a, 46b, 46c, and
46d. An aperture 52 is formed in the middle of the counterweight 50
to allow exposure light to pass through to the illumination area 54
of the reticle 12. Preferably, the material used for the
counterweight 50 is a metal with a relatively high specific
gravity.
[0110] FIG. 5 is an enlarged view of a portion of FIG. 2 showing
how contact is made between the weighting points of the
counterweight 50 and the reticle 12. As can be seen from this
drawing, contact is made through tabs 56, formed on the
counterweight 50 so that the applied force will be concentrated in
small areas on the reticle 12.
[0111] As shown in FIG. 6, four tabs (56a, 56b, 56c, and 56d) are
provided on the counterweight 50, arranged so that the downward
force will be applied at the four corners of the reticle 12, as
shown in FIG. 6. For simplification, the innermost edges of the
counterweight 50 are not shown in FIG. 6 so that tabs 56a, 56b,
56c, and 56d can be clearly seen.
[0112] FIG. 6 shows the typical top-view placement of the platens
and the tabs. As shown in FIG. 6, the tabs 56a, 56b, 56c, and 56d,
are placed on extensions of lines connecting the platens 46a, 46b,
46c, and 46d, respectively, to the center of the reticle 12 (radial
lines).
[0113] FIG. 7 shows the typical side-view placement of the platens
and the tabs. As shown in FIG. 7, the weighting points (tabs 56a,
56b, 56c, and 56d) are placed slightly outside of the platens 46a,
46b, 46c, and 46d, respectively, such that the weight of the
counterweight 50 will correct for the weight of the reticle 12 that
is causing it to sag. That is, the weight of the reticle 12 applies
force having its moment in a direction toward the center of the
reticle 12, with the platens 46a, 46b, 46c, and 46d as the
respective fulcrums. The weight of the counterweight 50, on the
other hand, applies force having its moment in a direction away
from the center of the reticle 12, with the platens 46a, 46b, 46c,
and 46d as the respective fulcrums. This cancels out the sag
flexure in the reticle 12 due to its own weight. In other words,
the tendency of the center of the reticle 12 to sag is corrected by
an opposing tendency to lift it.
[0114] The weight of the counterweight 50 and the locations of the
tabs 56a, 56b, 56c, and 56d are set based on the actual sag in the
reticle 12 due to its own weight.
[0115] The amount of sag in the reticle 12 can be determined by
calculating it from the reticle 12 material, its thickness and size
(area), and the placement of the platens 46a, 46b, 46c, and 46d.
The counterweight 50 can be set in place after the reticle 12 has
been mounted on the reticle holder 11; or the reticle 12 may be
loaded on the reticle holder 11 with the counterweight 50 already
on it. Also, it is not mandatory that there be four tabs on the
counterweight 50. There should be at least three, but the exact
number may be varied as desired, to suit conditions. Neither is it
mandatory that the counterweight 50 be made as a single piece: it
can be divided into as many pieces as there are weighting
points.
[0116] FIG. 8 shows the configuration of the scan-type projection
exposure apparatus of a second embodiment of the present invention.
Unlike the exposure apparatus of the first embodiment shown in FIG.
1, this projection exposure apparatus uses the step-and-scan
technique, in which exposures are performed by moving both the
wafer 14 and the reticle 12. The same or corresponding components
in this apparatus have the same numbers as they had in FIG. 1.
Explanation related to these parts will not be repeated.
[0117] As shown in FIG. 8, the reticle 12 is loaded on a reticle
stage 18 through the reticle holder 11. The reticle stage 18 can be
driven in translation and rotation in the XY plane by a reticle
drive system 22, with the optical axis of the projection optical
system 20 as the Z axis. The reflector mirrors 24 and 26 are
provided on the reticle stage 18 for interferometric measurement of
its position. The X position of the reticle stage 18 (its position
left to right on the page) is measured by an interferometer 28 and
a reflector mirror 24. That is, the measurement laser light from
the interferometer 28 is directed at the reflector mirror 24, and
the X position of the reticle stage 18 is measured based on light
reflected from the reflector mirror 24. Also, the Y position of the
reticle stage 18 (its position along the axis perpendicular to the
page) is measured by the reflector mirror 26 and another
interferometer (not shown in the drawing).
[0118] In the step-and-scan projection exposure apparatus shown in
FIG. 8, the pattern on the reticle 12 is gradually transferred to
the wafer 14 by shining a slit-shaped exposure light 58 (FIG. 9) on
the reticle 12 while synchronously moving the reticle stage 18 and
the wafer stage 32 in opposite directions. By so scanning the
reticle 12 and the wafer 14 in one stroke (one scan) across the
slit-shaped exposure light 58, the entire image of the reticle 12
pattern is transferred to the wafer 14 in one exposure.
[0119] FIG. 9 is analogous to FIG. 4 in that it shows the
relationship between the reticle 12 and the counterweights 60 and
62 of the second embodiment. That is, the counterweights 60 and 62
are loaded on the two opposing sides of the reticle 12 along the X
axis, the direction in which the reticle 12 is moved (the scanning
direction). In this embodiment, illumination light 58 is formed
into a slit on the reticle 12. This slit is narrow enough in the
scanning direction (X direction) so that the sagging of the
reticle, 12 (due to its own weight) can be ignored. Thus only the
sag in the Y direction (the direction perpendicular to the X
direction) needs to be accounted for. The counterweights 60 and 62,
then, can be placed as shown in FIG. 9, which allows the weight
applied to the reticle 12 to be held to a minimum, avoiding any
greater inertial mass than necessary when moving the reticle stage
18. Also, as means of weighting the reticle 12, it might be
preferable to apply the prescribed amount of force to the reticle
12 by a static means, such as a spring, or by a mechanically or
electrically controlled dynamic constraining mechanism.
[0120] FIG. 10 shows the support mechanism of the projection
exposure apparatus of a third embodiment of the present invention.
This embodiment comprises an interferometer 64 for measuring the
flatness of the surface of the reticle 12; piezoelectric elements
66 and 68 for applying a downward force on the outer edge of the
reticle 12; and a controller 70, for controlling the piezoelectric
elements 66 and 68 based on the output of the interferometer 64.
The interferometer 64 is set up so that in addition to measuring
the flatness of the surface of the reticle 12, it can also measure
how much the reticle 12 is sagging due to its own weight. The
piezoelectric elements 66 and 68 are set up so that they can apply
a controlled force just outside of the platens (not illustrated) of
the reticle holder 11, as was done in the first and second
embodiments described above. Using an internal CPU (not
illustrated), the controller 70 computes the weighting points and
weighting magnitude to be applied to the reticle 12 by the
piezoelectric elements 66 and 68, based on the reticle 12 surface
flatness data measured by the interferometer 64, and controls the
piezoelectric elements 66 and 68 accordingly. For example, multiple
piezoelectric elements can be placed around the outer edge of the
reticle 12 and the necessary piezoelectric elements selectively
driven, based on the actual state of the sag of the reticle 12.
[0121] This embodiment not only corrects for the sag in the reticle
12, it also effectively eliminates other problems that show up as
reticle 12 surface irregularities: problems, for example, such as
less than perfect flatness of the reticle holder 11. For
step-and-scan systems, the fact that the weights are not placed
directly on the reticle 12 in this embodiment provides another
advantage in that it avoids adding any more, inertial mass than
necessary.
[0122] Next, a fourth embodiment of the present invention will be
described. This embodiment applies the principles of the present
invention to a scanning projection exposure apparatus for
fabricating semiconductor devices.
[0123] FIG. 11 shows the overall configuration of the projection
exposure apparatus of this fourth embodiment of the present
invention, and FIG. 12 shows the reticle stage-related
configuration. In this embodiment, the illumination optics direct
exposure light through a condenser lens 210 to illuminate or
irradiate a reticle 212, in order to transfer the image of the
prescribed pattern formed in the reticle 212, through a projection
optical system 214, to a wafer 216. The projection optical system
214, which is held rigid with respect to the rest of the apparatus
by a frame 217, projects the image of the reticle 212 pattern, at
the prescribed reduction ratio (1/4, 1/5, etc.) onto the wafer
216.
[0124] The reticle 212 is held fast to a reticle holder 218 at four
locations around its outer edge (by vacuum suction), and loaded
onto a reticle stage 220 in this state. The reticle stage 220 can
be moved in the scan direction (Y direction) at the prescribed
speed by a reticle stage drive system 224. Attached to the reticle
stage 220 is a movement mirror 228, which reflects a laser beam
output by a reticle interferometer 226, to allow constant
monitoring of the position of the reticle stage 220 by the reticle
interferometer 226. In FIG. 11, only the reticle interferometer 226
and the movement mirror 228, which monitor the Y position (the
position in the scanning direction), are shown. In the actual
system, however, an interferometer and a movement mirror are also
provided for measuring the X position (the position in the
non-scanning direction).
[0125] As shown in FIG. 12, provided on the bottom of the reticle
holder 218 is an aperture glass 230, which, along with the reticle
212 and the reticle holder 218, forms an airtight space 232 to
isolate the bottom of the reticle 212 from the outside air. The
reason the aperture glass 230 is used to form the airtight space
232 is to make the space transparent to the illumination light. A
vacuum source 236, which is connected to the airtight space 232
through a vacuum conduit 234, reduces the pressure within the
airtight space 232 to less than the outside pressure to force the
reticle 212 tightly against the reticle holder 218. This difference
in pressure between the inside and outside of the airtight space
232 distorts the shape of the reticle 212. The resulting imaging
error in the projected image of the reticle pattern, however, is
corrected by a control section 238, which operates through a lens
controller 240 (as shown in FIG. 11) to adjust the imaging state of
the projection optical system 214 so as to cancel out the
error.
[0126] At the bottom end of the projection optics 214, the wafer
216 is held fast to a wafer stage 242 by vacuum suction, and loaded
onto a wafer stage 244 in this state. The wafer stage 244 is
movable in a plane perpendicular to the optical axis of the
projection optical system 214 (XY plane). The wafer stage 244 is
driven by a wafer stage drive system 246 such that the motion of
the wafer 216 is synchronized with the scanning of the reticle 212.
Attached to the wafer stage 244 is a movement mirror 250, which
reflects a laser beam output from a wafer interferometer 248. This
configuration is set up so that the interference light resulting
from the interference of the light reflected from the movement
mirror 250 with that reflected from a fixed mirror installed at the
bottom end of the projection optical system 214 (not illustrated)
can be used to constantly monitor the position of the wafer stage
244. In FIG. 11, only the wafer interferometer 248 and the movement
mirror 250, which monitor the Y position are shown. In the actual
system, however, an interferometer and a movement mirror are also
provided for measuring the X position.
[0127] Next, the principles and operation of the fourth embodiment,
configured as described above, will be described.
[0128] With the reticle 212 being uniformly illuminated by light
transmitted and converged by the condenser lens 210, the reticle
stage 220 is moved in the scanning direction (Y direction) of the
illumination area on the reticle 212. At the same time, the wafer
stage 244, with the wafer 216 loaded thereon, is moved in
synchronism with the motion of the reticle stage 220, and in the
opposite direction. During this time, the relative positions of the
reticle 212 and the wafer 216 are being monitored by the reticle
interferometer 226, and the wafer interferometer 248,
respectively.
[0129] The pertinent force relationships, as noted above, can be
expressed by the equations:
F.sub.k=ma (1)
F.sub.h=N.mu. (2)
[0130] where
[0131] F.sub.k is the force of inertia of the reticle 212,
[0132] F.sub.h is the holding force exerted on the reticle 212 by
its friction with the reticle holder 218,
[0133] m is the mass of the reticle 212,
[0134] N is the vertical reactive force acting on the reticle
212,
[0135] a is the acceleration of the reticle 212 motion, and
[0136] .mu. is the coefficient of friction between the reticle 212
and the reticle holder 218.
[0137] Then for the reticle 212 not to be shifted by the force of
inertia when the reticle stage 220 is moved, the following
condition must be met:
F.sub.k<F.sub.h (3)
[0138] From (1), (2), and (3), the following relation can be
derived:
a<N .mu./m (4)
[0139] Here, it becomes apparent that since m, the mass of the
reticle 212, and M, the coefficient of friction between the reticle
212 and the reticle holder 218, are nearly constant, the
acceleration, a, can be increased (while still maintaining
F.sub.k<F.sub.h) by increasing N. Also, if the reticle 212 is
held against the reticle 218 by vacuum induced suction, N is given
by the following equation (5):
N=S(P.sub.o-P.sub.v) (5)
[0140] where
[0141] S is the area to which suction is applied,
[0142] P.sub.o is the outside pressure, and
[0143] P.sub.v is the vacuum pressure.
[0144] In the present embodiment, the fact that the air pressure
inside the airtight space 232 under the reticle 212 is set lower
than the pressure on the outside, allows the surface of the reticle
212 being held against the reticle holder 218 by suction, including
the area in which the transfer pattern is formed, to be made
larger. That is, S, the suction surface area, can be considered the
equivalent of an area enlarged to a just slightly smaller area than
that of the entire surface of the reticle 212. As a result, from
equations (4) and (5) above, a, the maximum permissible
acceleration during movement of the reticle 212, can be increased
over that possible with other mask-holding systems. This embodiment
also provides an advantage in that isolating the pattern surface of
the reticle 212 from outside air protects it from
contamination.
[0145] FIG. 13 shows the construction associated with a reticle
stage 220 of the projection exposure apparatus of a fifth
embodiment of the present invention. The configuration of portions
of the projection exposure apparatus of this embodiment not
associated with the reticle stage 220 is as shown for the fourth
embodiment (FIG. 11). These portions are not illustrated here, and
will not be described. The components shown in FIG. 13 that are the
same components, or correspond to components in the fourth
embodiment, have the same numerals here as they had in FIG. 11.
[0146] In FIG. 13 the aperture glass 230 is installed at the bottom
of the reticle holder 218 in the floor area. As in the above
embodiment, the first airtight space 232 is thus formed between the
reticle 212, the reticle holder 218, and the aperture glass 230.
Provided on top of the reticle 212 is a bulkhead 254, on top of
which (in the ceiling area) is provided an aperture glass 256. A
second airtight space 258 is thus formed between the top surface of
the reticle 212, the bulkhead 254, and the aperture glass 256.
[0147] The first and second airtight spaces 232 and 258 are
interconnected by a vacuum conduit 260 such that the air pressure
inside the first and second airtight spaces 232 and 258 can be
adjusted to less than the outside air pressure by the vacuum source
236. This pressure differential enables the reticle 212 to be held
tightly against the reticle holder 218. When this is done, since
the first airtight space 232 below the reticle 212 and the second
airtight space 258 above the reticle 212 are interconnected by the
vacuum conduit 260, the pressure in the airtight space 232 is the
same as that in the airtight space 258. This prevents deformation
of the reticle 212 due to any difference in pressure above and
below it.
[0148] In this embodiment as well, since the air pressure in the
airtight spaces 232 and 258 above and below the reticle 212 is set
to less than that outside the airtight spaces, the surface of the
reticle 212 held against the reticle holder 218 by suction,
including the area in which the transfer pattern is formed, can be
made larger. That is, S, the suction surface area, can be viewed as
the equivalent of an area enlarged to a just slightly smaller area
than that of the entire surface of the reticle 212. As a result,
from equations (4) and (5) above, a, the maximum permissible
acceleration during movement of the reticle 212, can be increased
over that possible with other mask holding systems.
[0149] FIG. 14 shows the construction in the vicinity of the
reticle stage 220 of the projection exposure apparatus of a sixth
embodiment of the present invention. The configuration of portions
of the projection exposure apparatus of this embodiment not
associated with the reticle stage 220 is as shown for the fourth
embodiment, in FIG. 11. These portions are not illustrated here,
and will not be described. The components shown in FIG. 14 that are
the same components, or correspond to components in the fourth and
fifth embodiments, have the same identifying numerals here as they
had in earlier figures.
[0150] In FIG. 14, a suction hole (not illustrated) is formed in
the top surface of a reticle holder 262, on which the reticle 212
is placed, and to which it is held fast by suction created by a
vacuum source 266 through a vacuum conduit 264.
[0151] The aperture glass 230 is placed on the bottom surface of
the reticle holder 262 (its floor area). As in the above fourth and
fifth embodiments, a first airtight space 267 is thus formed
between the reticle 212, the reticle holder 262, and the aperture
glass 230. Provided on top of the reticle 212 is a bulkhead 268, on
top of which (in the ceiling area) is provided an aperture glass
270. A second airtight space 272 is thus formed between the top
surface of the reticle 212, the bulkhead 268, and the aperture
glass 270. The second airtight space 272 is connected through an
air conduit 274 and a regulator 276 to a compressor 278. The
compressor 278 sets the air pressure in the airtight spaces 272 and
267, above and below the reticle 212, higher than atmospheric
pressure. This higher pressure presses the reticle 212 against the
reticle holder 262, increasing the holding force with which the
reticle 212 is held to the reticle holder 262. With this
configuration, then, this holding force can be controlled by using
the regulator 276 to control the pressure in the first and second
airtight spaces 267 and 272, below and above the reticle 212. For
example, the pressure in the second airtight space 272 above the
reticle 212 can be set slightly higher than that in the first
airtight space 267 below the reticle 212.
[0152] In the present embodiment, in addition to setting the air
pressure in the airtight spaces 267 and 272 below and above the
reticle 212 to a pressure greater than atmospheric pressure, the
pressure differential (P.sub.o-P.sub.v) at the contact surface
between the reticle 212 and the reticle holder 262 can also be
increased by vacuum suction. As a result, from equations (4) and
(5) above, a, the maximum permissible acceleration during movement
of the reticle 212, can be increased over that possible with other
mask-holding systems.
[0153] FIG. 15 shows the construction in the vicinity of the
reticle stage 220 in the projection exposure apparatus of a seventh
embodiment of the present invention. The configuration of portions
of the projection exposure apparatus of this embodiment not
associated with the reticle stage 220 is as shown for the fourth
embodiment, in FIG. 11. These portions are not illustrated here,
and will not be described. The components shown in FIG. 15 that are
the same components, or correspond to components in the fourth,
fifth, and sixth embodiments, have the same identifying numerals
here as they had in earlier figures.
[0154] In FIG. 15, electromagnets 282a and 282b are installed fixed
in position; one on either side of a reticle holder 280 on which
the reticle 212 is loaded. Installed on the sides of the
electromagnets 282a and 282b, respectively, are support blocks 284a
and 284b.
[0155] Installed on the support blocks 284a and 284b, respectively,
through hinges 286a and 286b, respectively, are reticle clamps 288a
and 288b. Installed on the bottoms of the reticle clamps 288a and
288b, respectively, are permanent magnets 290a and 290b. Connected
to the electromagnet 282a and 282b, respectively, are current
supplies 292a and 292b, which supply electrical current to generate
magnetic fields. These components are configured such that when,
under control of a current controller 294 electrical current is
supplied to the electromagnets 282a and 282b by the current
supplies 292a and 292b, respectively, magnetic fields are created
above the electromagnets 282a and 282b.
[0156] In the embodiment as described above, to remove or insert
the reticle 212, the current controller 294 controls the current
supplies 292a and 292b to generate magnetic fields in a direction
to cause the permanent magnets 290a and 290b to be repelled by the
electromagnets 282a and 282b. This causes the reticle clamps 288a
and 288b to be ejected upward by the repulsive force between the
electromagnets 282a and 282b and the permanent magnets 290a and
290b. With the reticle clamps 288a and 288b in the released
(retracted) state, a replacement reticle 212 can be loaded on the
reticle holder 280. Then, the current controller 294 controls the
current supplies 292a and 292b to generate magnetic fields in a
direction to cause the permanent electromagnets 290a and 290b to be
attracted to the electromagnets 282a and 282b. This causes the
reticle clamps 288a and 288b to be pulled down by the attractive
force between the electromagnets 282a and 282b and the permanent
magnets 290a, and 290b, clamping the reticle 212 on the reticle
holder 280. In this manner, the reticle 212 is secured to the
reticle holder 280. The amount of holding force applied to the
reticle 212 can be controlled by adjusting the amount of current
flowing in the electromagnets 282a and 282b. The current required
to develop a given amount of holding force can be reduced by
reducing the size of the gap between the electromagnets 282a and
282b and the permanent magnets 290a and 290b.
[0157] In FIG. 15, the reticle clamp 288a is shown in a closed
position while the reticle clamp 288b is shown in a released
position for illustrative purposes only. Normally both reticle
clamps would be in the same position.
[0158] In this embodiment, the vertical reactive force N can be
increased by increasing the magnetic force used to clamp the
reticle 212 on the reticle holder 280. Thus from the above equation
(4), a, the maximum permissible acceleration during movement of the
reticle 212, can be increased over that possible with other
mask-holding systems.
[0159] Next, an eighth embodiment of the present invention will be
described with reference to FIG. 16 and FIG. 17. FIG. 16 shows the
essential parts of the projection exposure apparatus of this
embodiment. In FIG. 16, a mask (reticle) 310 is held in place by a
mask holder (reticle holder) 312 of the present embodiment. The
mask holder 312 is connected to a vacuum pump 314 through a conduit
316. A light source 318, which emits the exposure light, is
provided, above the mask 310. An illumination optical system 320 is
provided, between the light source 318 and the mask 310. Placed on
the exposure-light-transparent side of the mask 310, with a
projection optical system 322 sandwiched therebetween, is a wafer
324 to be exposed.
[0160] The exposure operation of this embodiment is the same as
that of conventional projection exposure apparatus. That is,
exposure light emitted from the light source 318 passes through the
illumination optical system 320, illuminating or irradiating the
mask 310. The exposure light projects the pattern printed on the
mask 310 through the projection optical system 322 onto the wafer
324.
[0161] In this embodiment, however, the mask holder 312 has the
same physical properties as the mask 310. In particular, it is made
of a material that has the same coefficient of thermal expansion as
the mask. It is also shaped to reduce sagging due to its own
weight: This is done by making the mask holder 312 thicker, and by
performing finishing work to make its top and bottom surfaces
highly flat and plane-parallel.
[0162] FIG. 17(A) shows an enlarged view of the mask holder 312.
FIG. 17(B) is an end view looking in the direction of the arrows
along the line XVII-XVII of FIG. 17(A). As shown in FIGS. 17(A) and
17(B), the shape of the mask holder 312 is somewhere between that
of a cylinder and a thick disk. The mask 310 is positioned on a top
surface 312A of the holder 312 as indicated by the dotted lines.
The top surface 312A and bottom surface 312B of the holder 312 are
flat, plane-parallel surfaces. A groove (recess) 312C is formed in
the top surface 312A, forming a square corresponding to the
position of the mask 310. The groove 312C is connected to a suction
port 312E, which is formed in the side of the mask holder 312,
through an airway 312D, which is formed inside the mask holder 312.
A suction port 312E is in turn connected to the vacuum pump 314
through the conduit 316 (mentioned above). In addition, in the
present embodiment, the mask holder 312 is made of the same
material as the mask 310. Both the mask 310 and the mask holder
312, for example, may be made of quartz.
[0163] Next, the operation of this embodiment will be described.
The mask 310 is loaded on the top surface 312A of the mask holder
312 as shown in FIGS. 16 and 17 such that the groove 312C is
covered all the way around by the mask 312. As discussed above, the
top surface 312A of the mask holder 312 is flat. This means that
the entire bottom of the mask 310 is in contact with the top
surface 312A of the mask holder 312, except for the small area
directly over the groove 312C. With the system in this state, if a
vacuum is pulled in the slot 312C of the mask holder 312 by the
vacuum pump 314 [arrow F, FIG. 17(B)], the mask 310 will be drawn
against the mask holder 312 and held fast. This puts the entire
pattern area in the center portion of the mask 310 in close contact
with the top surface 312A of the mask holder 312. As stated
earlier, the mask holder 312 is made thick enough to prevent it
from sagging under its own weight. Accordingly, this reduces the
sagging of the mask 310 due its own weight to a satisfactory
level.
[0164] Once the mask 310 has been attached to the mask holder 312
as described above, and certain processes such as alignment have
been performed, exposure can begun. The temperature of the mask 310
now rises due to its being flooded with exposure light. As the
temperature rises, heat from the mask 310 is transferred to the
mask holder 212, and the temperature of the mask holder 312 also
rises. In this embodiment, as stated earlier, the mask holder 312
is made of the same material as the mask 310 (quartz, for example),
which means that they have the same coefficient of thermal
expansion. For this reason, there is good transfer of heat from the
mask 310 to the mask holder 312 through the portions of both that
are in tight contact, thus minimizing the difference in temperature
between them. In addition, the mask 310 and the mask holder 312
will experience about the same level of shape distortion due to
temperature change, and will therefore expand and contract
together. Accordingly, thermal deformation of the mask 310 is also
reduced to a satisfactory level.
[0165] Furthermore, exposure light passes through the pattern
portion of the mask 310 and the center of the mask holder 312.
Because the top and bottom surfaces 312A and 312B of the mask
holder 312 are plane-parallel, however, the presence of the mask
holder 312 has no detrimental effect on the exposure operation (the
imaging of the pattern).
[0166] As in the eighth embodiment described above, the sagging of
the mask due to its own weight as well as the thermal deformation
of the mask are reduced. This reduces image curvature and lateral
displacement of the pattern due to the mask sagging, as well as
image curvature and deformation due to thermal distortion, Precise
and stable images of the mask pattern can therefore be
obtained.
[0167] Next, a ninth embodiment of the present invention will be
explained, with reference to FIG. 18. Parts of this embodiment that
are equivalent to parts of the eighth embodiment will have the same
reference numerals. In the eighth embodiment, the mask was held to
the top surface of the mask holder by vacuum suction, whereas in
the ninth embodiment, it is drawn against the bottom.
[0168] In FIG. 18, the configuration of a mask holder 330 is
inverted top-to-bottom from the corresponding configuration of the
mask holder 312 of the eighth embodiment. Suction grooves (not
illustrated) are formed in the bottom surface 330A of the mask
holder 330. The mask 310 is drawn fast to the bottom end (the
projection optical system 322 end) of the mask holder 330 by the
vacuum pump 314. In addition, since in this embodiment, the mask
holder 330 is on the top end (the illumination optical system 320
end), a pellicle 332 is added over the mask 310 for protection.
[0169] In this embodiment, too, the mask holder 330 is made of a
material having the same coefficient of thermal expansion as the
mask 310. This provides the same reduction of heat-related
distortions and image curvature to satisfactory levels realized in
the eighth embodiment. Holding the perimeter of the mask 310 in
tight vacuum contact with the mask holder 330 also prevents
penetration of gas between the pattern surface of the mask 310 and
the contact surface of the mask holder 330, thereby reducing the
sagging of the mask 310 due to its own weight.
[0170] Next, a tenth embodiment of the present invention will be
described with reference to FIG. 19. In this embodiment, as in the
above, the mask-holder is made of the same material as the mask
(quartz, for example). Accordingly, the holder can be formed to
function as an optical element, such as a lens. That is the case in
this tenth embodiment, in which the mask holder performs a dual
function as part of the illumination optics.
[0171] In FIG. 19, a bottom surface 340A of a mask holder 340,
constructed the same as the corresponding element of the ninth
embodiment, described above, with the mask 3 10, with the pellicle
332 installed on it, held fast to it by vacuum suction. A top
surface 340B of the mask holder 340, however, is formed as a curved
surface, to make the mask holder 340 a lens that functions as part
of the illumination optical system 342. This configuration
simplifies the construction of the illumination optical system 342.
In addition, whereas the eighth and ninth embodiments actually
added one more optical element in the form of the mask holder, in
the tenth embodiment, due to the fact that the mask holder serves a
dual function, also serving as part of the illumination optical
system, the mask can be held without adding any optical elements to
the configuration.
[0172] Next, an eleventh embodiment of the present invention will
be described with reference to FIG. 20. In the eighth, ninth, and
tenth embodiments described above, the entire mask pattern surface
made contact with the mask holder. Reduction in the amount
o{circumflex over ( )} mask sag due to the weight of the mask,
however, can also be achieved in a configuration in which only a
portion of the pattern surface makes contact with the mask
holder.
[0173] FIG. 20(B), is an end view looking in the direction of the
arrows, along the line XX-XX of FIG. 20(A). In this embodiment, as
shown in FIG. 20(B), a mask holder 350 has a square through-hole
350A formed at its center. Formed around the lip of the
through-hole 350A is a vacuum suction groove (recess) 350B, which
is connected through an airway 350C to a suction port 350D formed
in the side of the mask holder 350. As in the embodiments described
above, the mask holder 350 is made of a material having the same
coefficient of thermal expansion as the mask 310.
[0174] Unlike these other embodiments, however, in which the entire
pattern surface is in contact with the mask holder, in this
embodiment, only a portion of the pattern surface of the mask 310
touches the top surface 350E of the mask holder 350. While this
does allow the mask 310 to sag somewhat under its own weight, the
amount of sag is greatly reduced in comparison to that experienced
in the prior art arrangements. Also, because the materials used for
the mask 310 and the mask holder 350 have the same coefficient of
thermal expansion, any heat in the mask 310 is transferred to the
mask holder 350. For this reason, as in the embodiments described
earlier, thermal distortion and image curvature are reduced to
satisfactory levels.
[0175] FIG. 20(C) shows a modified version of the eleventh
embodiment. In this example, the mask holder 352 has a cylindrical
through-hole 352A. Also, a vacuum suction groove 3528 around the
lip of the through-hole 352A is not a continuous groove as it was
in the other version of this embodiment. Each groove 3528 in the
mask holder 352 is connected to the others, and to a suction port
352C. As in the other version, the material used to make the mask
holder 352 has the same coefficient of thermal expansion as the
mask 310.
[0176] In the above eighth, ninth, tenth, and eleventh embodiments,
the same material (quartz, for example) was used for the mask
holders and the masks. They need not necessarily be made of the
same material, however, as long as the materials are similar, and
in particular, as long as they have about the same coefficients of
thermal expansion.
[0177] In the tenth embodiment, shown in FIG. 19, the mask holder
was constructed so that it could also serve as part of the
illumination optics. The mask holder could also have been inverted,
bottom-to-top, however, and used as part of the projection optics.
This also applies to the eleventh embodiment.
[0178] The shape and dimensions of the mask holders in the eighth,
ninth, tenth, and eleventh embodiments are not confined to those
shown here. The mask holders in the eighth, ninth, tenth, and
eleventh embodiments, for example, are all cylindrical, but they
could also be other shapes, such as square blocks. The shape and
placement of the vacuum slots may also be changed as desired.
[0179] Next, a twelfth embodiment of the present invention will be
explained, with reference to the drawings. FIG. 21 shows the main
parts of the projection exposure apparatus of a twelfth embodiment
of the present invention. As shown in FIG. 21, a mask 400 is held
by a mask holder 410. Connected to the mask holder 410 through a
conduit 414 is an air conditioner 412. Provided above the mask 400
is a light source 416, which emits the exposure light. Provided
between the light source 416 and the mask 400 is an illumination
optical system 418. A wafer 422 is placed below the
exposure-light-transparent side of the mask 400, with a projection
optical system 420 sandwiched therebetween.
[0180] Exposure in this embodiment works the same as in most common
exposure systems. That is, exposure light from the light source 416
passes through the illumination optical system 418, and illuminates
the mask 400. The pattern printed on the mask 400 is projected
through the projection optical system 420 onto the wafer 422 by the
exposure light.
[0181] Next, the mask-holding means of this embodiment will be
described in detail, with reference to FIGS. 22 and 23. FIG. 22 is
an oblique exploded view of the mask holder 410, and FIG. 23 shows
a cross-sectional view taken along the line XXIII-XXIII of FIG. 22.
As shown in these drawings, the structure of the mask holder 410 is
divided into two holder frames, 450 and 452, formed so as to
enclose the outer edge of the mask 400. The holder frames 450 and
452 have slots 453, formed to have substantially U-shaped
cross-sections, as shown in FIG. 23. In other words, these holder
frames 450 and 452 are shaped so that they surround the mask 400
from three directions: top, bottom, and ends.
[0182] Also formed in the holder frames 450 and 452 are airjets for
blowing out air to push on the mask 400. More specifically,
multiple tiny exhaust ports 454 are symmetrically placed along the
surfaces of the slots 453 of the holder frames 450 and 452 for
supplying and, expelling air. Inside the holder frames 450 and 452,
all exhaust ports 454 are connected to each other and to intake
ports 458 (shown on only one side in FIG. 21) by airways 456. It is
these exhaust ports 454, airways 456, and intake ports 458, that
together form the air jets.
[0183] The above intake ports 458 are connected through the conduit
414 to the air conditioner 412. The purpose of the air conditioner
412 is to supply air at the prescribed temperature and flow rate.
This air is supplied through the conduit 414 to the mask holder
410.
[0184] Next, the operation of a holding means configured as
described above will be explained. The mask 400 is placed inside
the slots 453 of the holder frames 450 and 452 of the mask holder
410. During this step, the mask 400 is placed so that a slight gap
is formed between the edge of the mask 400 and the slots 453. In
this state, air is supplied by the air conditioner 412, at a
constant temperature and flow rate, flowing in the direction of the
arrows F shown in FIG. 22. This causes air to flow through the
conduit 414, the intake port 458, and the airway 456, in sequence,
to be blown at the mask 400 from the exhaust ports 454. As stated
above, the exhaust ports 454 are symmetrically placed along the
surfaces of the slots 453. This creates pressure that pushes
against the edges of the mask 400 symmetrically from three
directions, thus pushing the mask 400 away from the surfaces of the
slots 453. This enables the mask 400 to be held in place without
touching the holder frames 450 or 452. Also, because the air being
blown from the exhaust ports 454 is maintained at a constant
temperature and flow rate by the air conditioner 412; the mask 400
can beheld at a constant attitude.
[0185] Thus in this embodiment, the mask 400 is held in the mask
holder 410 without touching it. For this reason, even if the mask
400 heats up during exposure, this heat will not be transferred to
the mask holder 410. Accordingly, distortion changes caused by the
mask 400 warping due to thermal shape distortion of the mask holder
410, as well as offset and rotation due to mispositioning of the
mask 400 can be controlled to a satisfactory level, thus enabling
accurate and stable projection exposures to be performed. As an
additional advantage, it can also be expected that changes in the
image reduction ratio due to thermal expansion of the mask 400 can
be reduced by using the constant-temperature air to cool the mask
400.
[0186] In the twelfth embodiment described above, the present
invention is applied in the exposure apparatus for fabricating
integrated circuits. Of course, the invention could also be
applied, in other exposure apparatus, such as exposure apparatus
for liquid crystal display elements, or exposure apparatus of the
type that exposes by scanning the mask.
[0187] Although the above twelfth embodiment used air, other gases
could also be used. Magnetic fields could also be used to hold the
mask in place without touching it. The mask could be held without
touching it, for example by providing opposing magnetic poles
across from each other on the mask and the holder. Similarly,
opposing electrostatic fields could be used to hold the mask.
[0188] Next, a thirteenth embodiment of the present invention will
be described, with reference to the drawings.
[0189] FIG. 24 shows the main parts of the projection exposure
apparatus of a thirteenth embodiment of the present invention. In
this drawing, a mask 510 is held by a mask holder 512. Provided
above the mask 510 is a light source 514, which emits exposure
fight. Provided between the light source 514 and the mask 510, is
an illumination optical system 516. A wafer 520 is placed on the
side of the mask 510 that is transparent to the exposure light,
with the projection optical system 518 sandwiched therebetween. The
wafer 520 is set on a wafer stage 522. The above components are all
the same as in conventional projection exposure systems.
[0190] In this embodiment, a mask displacement detector 524 is
provided around the outer edge of the mask 510. FIG. 25 is a plan
view of the mask displacement detector 524. As shown in this
drawing, the mask displacement detector 524 comprises a fixed frame
526 and a plurality of piezoelectric elements 528A through 528H
inserted between the mask 510 and the fixed frame 526. These
piezoelectric elements 528A through 528H are inserted between the
fixed frame 526 and the mask 510 in a symmetrical arrangement with
respect to the mask 510. The configuration is such that if the mask
510 experiences displacement with respect to the fixed frame 526,
there will be a change in potential in the potential difference
output from each of the piezoelectric elements 528A through 528H,
due to the expansion and contraction of the mask 510 at the various
points at which the piezoelectric elements 528A through 528H are
provided. The voltage output electrodes (not illustrated) of each
of these piezoelectric elements 528A through 528H are independently
connected, through the fixed frame 526, to a potential difference
detector 530.
[0191] The detected output of the potential difference detector 530
is connected to a computation unit 532. The computation result
output from the computation unit 532 is connected to both a
projection optical system control unit 534 and a stage control unit
536. Of these, the potential difference detector 530 independently
monitors the potential difference of each of the piezoelectric
elements 528A through 528H. From the potential differences of the
piezoelectric elements 528A through 528H, the computation unit 532
computes (through a statistical process, for example) the extent of
the displacement from the initial state of the mask 510, from which
it derives the extent of change, with the displacement of the mask
510 seen in terms of changes in the pattern image. The system used
as the projection optical system control unit 534, for example,
could be one such as that disclosed in U.S. Pat. No. 5,117,255, in
which some of the lenses that make up the projection optical system
518 are shifted back and forth along the optical axis; or tilted
with respect to it, to adjust for distortion in the pattern image,
or to adjust the image reduction ratio. As an alternative, this
system could also be one such as disclosed in U.S. Pat. No.
4,666,273, in which the pressure in the airtight spaces formed
between some of the projection lenses is changed to adjust the
pattern image projection reduction ratio. When the presence of
linear expansion of the pattern image is indicated in the
computation results received from the computation unit 532, the
projection optical system control unit 534 controls the projection
reduction ratio of the projection optical system 518 to correct for
it. As for the stage control unit 536, when shifting or rotation of
the pattern image is indicated in the computation results received
from the computation unit 532, the stage control unit 536 controls
the wafer stage 522 to correct for it.
[0192] Next, the operation of this embodiment will be described.
Basic exposure operation is the same as in conventional exposure
apparatus. That is, exposure light emitted by the light source 514
passes through the illumination optics 516 to illuminate the mask
510. The pattern printed on the mask 510 is projected by the
illumination light, through the projection optical system 518 onto
the wafer 520. While this is going on, the potential difference
output of each of the piezoelectric elements 528A through 528H is
detected by the potential difference detector 530, and the
detection results fed to the computation unit 532.
[0193] At the start of the exposure process, in the initial mask
state, the potential difference values being output by the
piezoelectric elements 528A through 528H (the initial potential
difference values) are measured, and the measurements saved in the
computation unit 532. These initial values will serve as baseline
values for determining displacement of the mask 510. In other
words, subsequent correction operations will seek to restore the
mask 510 state represented by these initial values.
[0194] As exposure operation proceeds from this initial state,
displacement and distortion of the mask 510 will occur due to a
rise in the temperature of the mask 510. FIG. 26(A), for instance,
shows an example of mask expansion. The dotted line in this figure
indicates the initial state of the mask 510. As its temperature
rises, the mask 510 experiences equi-multiple expansion .DELTA.A in
all four directions, to the state indicated by the solid line. When
this happens, each of the piezoelectric elements 528A through 528H
of the mask displacement detector 524 will experience compression
between the mask 510 and the fixed frame 526, causing the potential
difference detector 530 to detect a potential difference
corresponding to this compression. From the data it receives from
the potential difference detector 530 and the above baseline
values, the computation unit 532 computes .DELTA.A, the extent of
mask 510 expansion.
[0195] For the purpose of making corrections, this kind of mask 510
expansion can be viewed as a magnification of the pattern image in
the projection optical system 518. Therefore, from data on pattern
expansion, the computation unit 532 computes .DELTA.a, the apparent
change in the pattern image reduction ratio for that amount of
expansion, and outputs this value to the projection optical system
control unit 534. The projection optical system control unit 534
then controls the reduction ratio of the projection optical system
518 so as to cancel out this apparent change in pattern image,
reduction ratio .DELTA.a. In other words it increases the reduction
ratio so as to reduce the pattern image size by enough to cancel
out the apparent change in the image reduction ratio .DELTA.a
corresponding to the expansion of the mask (.DELTA.A).
[0196] FIG. 26(B) shows an example of a mask mispositioning. In
this case, the mask 510 is shifted to the right, away from its
initial state (dotted line), by a displacement .DELTA.B (solid
line). When this happens, of the piezoelectric elements 528A
through 528H of the mask displacement detector 524, the potential
difference outputs of the elements 528A, 528B, 528E, and 528F will
not change. The piezoelectric elements 528C and 528D, however, will
be compressed, and the piezoelectric elements 528G and 528H will
expand, and their potential difference outputs will change from
their initial values by a corresponding amount. From these changes,
the computation unit 532 computes .DELTA.B, the lateral shift of
the mask 510.
[0197] For correction purposes, this kind of lateral shifting of
the mask 510 can be viewed as a lateral shift of .DELTA.b in the
position of the pattern image on the wafer 520. Accordingly, the
computation unit 532 outputs .DELTA.b, the lateral shift
computation result, to the stage control unit 536. The stage
control unit 536, in turn, controls the position of the wafer 520
so as to cancel out lateral shift .DELTA.B of the mask. In other
words, it controls the wafer stage 522 to move it just enough to
cancel out the .DELTA.b lateral shifting of the pattern image,
corresponding to the .DELTA.B lateral shift in the mask.
[0198] FIG. 26(C) shows an example of mask rotation, where the mask
510 has rotated out of its initial state, indicated by the dotted
lines, to the position indicated by the solid lines. When this
happens, of the piezoelectric elements 528A through 528H of the
mask displacement detector 524, the elements 528A, 528C, 5281, and
528G are compressed, while the elements 528B, 528D, 528F, and 528H
expand. Each element outputs a potential difference output change
corresponding to its compression or expansion. From these changes,
the computation unit 532 computes the rotation of the mask 510.
[0199] For correction purposes, this kind of mask 510 rotation can
be viewed as a .DELTA..theta. rotation of the projected pattern
image on the wafer 520. Accordingly, the computation unit 532
computes this .DELTA..theta. rotation, and outputs the result to
the stage control unit 536. The stage control unit 536 responds by
controlling the wafer stage 522 to rotate it just enough to cancel
out this .DELTA..theta. pattern image rotation.
[0200] In reality, displacement of the mask 510 is a combination of
expansion, contraction, shift, and rotation displacement, along
with nonuniform expansion of portions of the mask. The control
units 534 and 536 make corrections to cancel out image variations
that accompany these kinds of mask displacement, based on the
results of computations by the computation unit 532.
[0201] As described above, in this thirteenth embodiment, mask
displacement can be detected through the use of a plurality of
piezoelectric elements. Then, based on these mask displacement
detection results, projection optics reduction ratio corrections
and wafer position corrections can be performed. Variations in the
projected pattern image, more specifically, scaling, offset, and
rotation, etc. of the projected image, can be effectively
corrected, to realize highly precise pattern overlay accuracy.
[0202] Next, a fourteenth embodiment of the present invention will
be described, with reference to FIG. 27. Elements of this
embodiment that have corresponding elements in the thirteenth
embodiment-will use the same reference numerals. In the thirteenth
embodiment, mask displacement is interpreted in terms of variations
in the projected pattern image, and these variations are then
effectively corrected to project a satisfactory image. In the
fourteenth embodiment, however, the mask displacement itself is
corrected.
[0203] In FIG. 27, the output side of the above potential
difference detector 530 is connected to a mask correction unit 550,
and the output side of the mask correction unit 550 is connected to
the respective output electrodes of the piezoelectric elements 528A
through 528H. It is commonly known that the piezoelectric elements
can be used as actuators. This embodiment utilizes this
feature.
[0204] As described earlier, the potential difference detector 530
detects the potential difference of each individual element of the
piezoelectric elements 528A through 528H, and supplies its
detection results to the mask correction unit 550. Like the above
computation unit 532, the mask correction unit 550 also stores the
initial potential difference values of the piezoelectric elements
528A through 528H (values obtained at the start of exposure). If,
as the exposure proceeds, mask 510 displacement starts to occur,
the potential difference values of the piezoelectric elements 528A
through 528H start to change from their initial values as described
above. In response, the mask correction unit 550 outputs correction
voltages to the applicable piezoelectric elements as necessary to
return their potential differences to the initial values.
[0205] If lateral shifting such as shown in FIG. 26(B) occurs,
correction voltages to cause the elements to expand will be applied
to the piezoelectric elements 528C and 528D, and correction
voltages to cause the elements to contract will be applied to the
piezoelectric elements 528G and 528H. The correction voltages will
be such as to cancel out the deviation of the potential differences
from, their initial values. Operation is the same for the cases
shown in FIGS. 26(A) and 26(C). Application of correction voltages
in this manner will correct for the displacement of the mask 510
thus restoring it to its initial state, and a satisfactory image of
the pattern will be projected onto the wafer 520.
[0206] Next, a fifteenth embodiment of the present invention will
be described with reference to FIG. 28. In the above thirteenth and
fourteenth embodiments, two-dimensional mask displacement in the
pattern plane, such as illustrated in FIG. 26, was detected and
corrected. This embodiment, however, detects mask displacement in
three dimensions, including the direction perpendicular to the mask
pattern plane.
[0207] FIG. 28(A) is a cross-section view of a mask displacement
detector 560 of the present embodiment. FIG. 28(B) is a plan view
of the detector shown in FIG. 28(A), showing the placement of the
piezoelectric elements with respect to the mask. As shown in these
drawings, a fixed frame 562 has an L-shaped cross section with the
short leg of the L fitting over the front of the mask 510 and the
long leg around its sides. The placement of the piezoelectric
elements 528A through 528H in the plane of the mask 510 is the same
as in the embodiments described earlier. In the present embodiment,
however, additional piezoelectric elements 564A through 564D are
symmetrically placed around the front of the pattern surface of the
mask 510, but may also be placed as indicated by the dotted lines
in FIG. 28(B).
[0208] Next, the operation of this embodiment will be described.
Its operation with respect to the piezoelectric elements 528A
through 528H is the same as that of the thirteenth and fourteenth
embodiments. The dotted line in FIG. 28(A) indicates the position
of the front surface of a mask 510 that has been tilted out of
position. If the mask 510 were tilted like this, the piezoelectric
elements 564A through 564D would output voltages corresponding to
the expansion and contraction corresponding to the tilting of the
mask 510. Thus the extent of the tilting of the mask 510 can be
determined from the voltages output by these piezoelectric elements
564A through 564D. This data on the tilt of the mask 510 can then
be viewed in terms of the tilt of the pattern image plane at the
wafer 520, and corrected as in the thirteenth embodiment. As an
alternative, the piezoelectric elements 564A through 564D can be
used as actuators to make the required corrections at the mask 510,
as in the fourteenth embodiment.
[0209] Also, the number of piezoelectric elements placed around the
edge of the mask can be varied as necessary. In general, increasing
the number of elements provides a more detailed picture of the mask
displacement, enabling finer displacement corrections to be made.
Also, in the fourteenth and fifteenth embodiments, separate
piezoelectric elements could be provided to detect mask
displacement and to correct it. Various kinds of actuators other
than piezoelectric elements could also be used.
[0210] In the above thirteenth, fourteenth, and fifteenth
embodiments, piezoelectric elements were used as the mask
displacement detection means. Any means could, however, be used, as
long, as it operates similarly. Instead of piezoelectric elements,
for example, capacitors or inductors could be used, and changes in
their capacitance or inductance detected.
[0211] Also, things such as the shape of the mask and configuration
of the exposure apparatus need not be limited to those of the above
embodiments. The present invention can also be applied, for
example, to scan-type exposure systems used for the fabrication of
liquid crystal display elements.
[0212] The principles of the present invention can be applied in
general to a specimen-holding technology for holding a
flat-panel-type specimen such as a mask on a movable specimen
table.
[0213] As noted above in conventional holding methods, the reticle,
mask, or specimen is held against the specimen table by
vacuum-induced suction. In exposure apparatus for transferring a
pattern formed on a mask onto a photosensitive substrate, the mask
is held fast to a mask stage by vacuum-induced suction. A vacuum
pad is provided on the upper surface of a mask stage, where the
mask is in contact with the stage. A vacuum pump or compressor is
used to evacuate the air from the upper part of the vacuum pad,
making the air pressure between the mask and mask stage less than
the outside air pressure. This causes the mask to be drawn tightly
against the mask-stage.
[0214] The sixteenth through nineteenth embodiments are directed
more specifically to the mask-holding problems.
[0215] FIG. 33 shows the projection exposure apparatus of the
sixteenth through nineteenth embodiments, through which the image
of a pattern formed on mask, specimen, or reticle 3' is transferred
to a wafer 10'. Pulsed light from pulsed laser light source 1' (an
excimer laser, etc.) is injected into illumination optical system
2'. The timing of light pulses emitted from pulsed laser light
source 1' can be set as desired through a trigger control section
(not illustrated). Illumination optical system 2' contains
components such as beam-forming optics, fight-reduction optics, an
optical integrator, a field stop, and a condenser lens. Light
pulses applied to illumination optical system 2' are transformed
thereby into pulsed exposure light IL', of substantially uniform
illumination, which illuminates reticle 3'.
[0216] Reticle 3' is held to reticle stage 4' by a reticle holder
to be described later (items 30' and 32' in FIG. 34). Reticle stage
4' is made so that it can be moved by reticle stage control unit
4'a, in a plane perpendicular to the optical axis of a projection
optical system 9'. In other words, reticle stage 3' is positioned
by moving it in the X (or -X) direction (a direction parallel to
the page in FIG. 33), and the Y direction (the direction
perpendicular to the page in FIG. 33, and perpendicular to the X
direction). During exposure, exposure light IL' scans the moving
reticle 3' in the X (and -X) direction. The peripheral
configuration of reticle stage 4' will be explained in detail
later.
[0217] Placed below reticle stage 4' is reticle blind 5', which has
a rectangular aperture or slit formed therein. This rectangular
aperture in reticle blind 5' effectively defines a rectangular
slit-shaped illumination area on reticle 3'. An interferometer
movement mirror 6', which is affixed to reticle stage 4', reflects
a laser beam emitted from the reticle-end interferometer 7', which
is installed external to the system. The X and Y coordinates of
reticle stage 4' are constantly measured using the light emitted
from reticle-end interferometer 7' and reflected by mirror 6'.
Coordinate data S1' measured in this manner is supplied to main
control system 8', which controls the operation of the entire
apparatus.
[0218] Projection optical system 9' is positioned under reticle
blind 5'. Through projection optical system 9', the image of that
portion of the pattern inscribed on reticle 3' that is within the
limits defined by the aperture of reticle blind 5', is projected
onto photoresist-coated wafer 10' (the photosensitive substrate).
Rectangular exposure area 11' is an area that, relative to
projection optical system 9', is conjugate to the area on reticle
3', the limits of which are defined by the aperture of reticle
blind 5'. Wafer 10' is held on Z leveling stage 12', and Z leveling
stage 12' is mounted on wafer-end XY stage 13'. Z leveling stage
12' is made up of components that include a Z stage and a leveling
stage. The Z stage positions wafer 10' in the Z direction (i.e.,
along the optical axis of projection optical system 9'), and the
leveling stage tilts wafer 10' as required to position its exposure
surface at the desired tilt angle. Wafer-end XY stage 13' is made
up of components such as an X stage, which positions wafer 10' in
the X direction, and a Y stage, which positions wafer 10' in the Y
direction.
[0219] An interferometer movement mirror 14' is installed on the
side of Z leveling stage 12', positioned to reflect a laser beam
emitted from wafer-end interferometer 15', installed external to
the system. These components are configured so that the X and Y
coordinates of wafer-end XY stage 13' are constantly being measured
by wafer-end interferometer 15', based on light reflected by mirror
14'. Coordinate data measured in this manner are supplied to the
main control system 8'.
[0220] In addition, the currently-set tilt and height (focus
position) on Z leveling stage 12' are sensed by Z leveling stage
position sensing unit 17', and the tilt and height data thus sensed
are supplied to computation unit 18'. Z leveling stage position
sensing unit 17' may be configured, for example, to have a rotary
encoder installed on the shaft of the drive motor (not shown) used
to drive Z leveling stage 12'. Heights may also be sensed directly,
using potentiometers.
[0221] Multipoint focus position sensor units 19' and 20' are
placed at either side, in the X direction, of projection optical
system 9'. Multipoint focus position sensor units 19' and 20'
measure the height of the wafer 10' surface. Focusing signals S2'
output by each of the multipoint focus position sensor units 19'
and 20' are supplied to computation unit 18'. From previously
acquired focus position data, computation unit 18' computes the
height and tilt angle to which Z leveling stage 12' needs to be set
(the target height and target tilt angle) for the next exposure
area within exposure area 11' to be exposed, and sends this target
height and target tilt data to main control system 8'. Through
wafer stage control, unit 16', main control system 8' then controls
the operation of Z leveling stage 12' in accordance with this data.
Through reticle stage control unit 4'a, main control system 8' also
causes reticle stage 4' to move, while at the same time controlling
the operation of wafer-end XY stage 13' (through wafer stage
control unit 16) to cause wafer-end XY stage 13' to scan in
synchronization with reticle stage 4'.
[0222] In the present embodiment, to perform an exposure by the
slit scan method, wafer 10' is scanned in scan direction RW' (the X
direction) by XY stage 13', in sync with the scanning of reticle 3'
in scan direction RR' (the X direction) by reticle stage 4'. At
this time, if the projection size-reduction ratio of projection
optical system 9' is .beta., and the scan rate of reticle 3' is VR,
then the scan rate of wafer 10' will be .beta..multidot.VR. Through
this scanning process, the entire pattern of reticle 3' will be
exposed portion-by-portion on wafer 10'. The scan may also be
performed in the reverse direction, in which case reticle 3' would
be scanned in the X direction, and wafer 10' would be scanned in
the -X direction in sync with the scanning of reticle 3'.
[0223] For slit scan exposure, the actual rate of travel of reticle
stage 4', and consequently, that of XY stage 13', on the wafer-end
of the system, are determined by the amount of light energy with
which reticle 3' is illuminated by pulsed exposure light IL', the
width of the reticle blind 5' aperture, and the sensitivity of the
photoresist applied to wafer 10'. That is, the scan rate is
determined by the amount of time required for the photoresist to be
adequately exposed as the pattern on reticle 3' is moved past the
aperture of reticle blind 5' by the movement of reticle stage
4'.
[0224] Next, the configuration of the reticle stage of the
sixteenth embodiment of the present invention will be described,
with reference to FIGS. 34, 35, 36, and 37. FIG. 34 shows the
peripheral configuration of reticle stage 4', and FIG. 35 shows the
construction of a reticle 3' that mounts on reticle stage 4'.
Reticle stage 4' is made in the shape of a flat rectangular panel.
Formed in the center portion of reticle stage 4' is a passage 28',
which passes light that has passed through pattern area 3'a, the
portion of reticle 3' in which the pattern is formed. The mirror 6'
has been omitted from FIG. 34. Reticle holders 30' and 32', which
hold reticle 3', are attached to reticle stage 4' at the rims of
the side edges of passage 28' (one holder at each edge of passage
28' in the X direction). Reticle holders 30' and 32' might be
formed, for example, of ceramic. Chrome layers 42', 44', 46', and
48' are formed by a vacuum deposition process on the bottom of
reticle 3' near the four corners, as shown in FIG. 35. Attraction
portions 34', 36', 38', and 40' are formed on the top surfaces of
reticle holders 30' and 32', where they make contact with chrome
layers 42', 44', 46', and 48', respectively, of reticle 3'.
Dielectric layers 34'; 36', 38', and 40' are formed on the top
surfaces of these attraction portions..
[0225] FIG. 36 shows the construction of reticle holder 32'. Formed
in the attraction portions on top of reticle holder 32' are
elliptical recesses 50'a and 50'b, in the centers of which are
formed suction holes 52'a and 52'b, respectively. Suction holes
52'a and 52'b are connected through conduits 54'a and 54'b to
pressure control unit 58', shown in FIG. 37. Pressure control unit
58', which contains a compressor (not illustrated), is configured
to draw air from suction holes 52'a and 52'b at a prescribed
pressure. Pressure control unit 58' is controlled by reticle stage
control unit 4'a. The drawing shows conduits 54'a and 54'b being
routed to the outside of reticle holder 32' as separate conduits;
but the two conduits may also be connected together inside reticle
holder 32', or inside reticle stage 4', and routed to the outside
as one conduit. The construction of reticle holder 30' is similar
to that of reticle holder 32', and will not be described.
[0226] FIG. 37 shows reticle 3' loaded on the reticle holders shown
in FIG. 34 (30' and 32'), along with its control system (55', 56',
57', 58', and 60'). Voltage control section 60' is connected to
reticle holders 30' and 32', and to reticle 3', so as to apply
prescribed voltages to the chrome layers on the back of reticle 3'
(42', 44', 46', and 48'), and to the dielectric layers (34', 36',
38', and 40'). Application of voltage in this way creates static
electricity between chrome layers 42', 44', 46', and 48' and
dielectric layers 34', 36', 38', and 40', in a manner so as `to
attract reticle 3' to reticle holders 30' and 32'. Reticle 3' is
additionally drawn to reticle holders 30' and 32' by suction
created by the vacuum system described earlier (50'a, 50'b, 52'a,
52'b, 54'a, 54'b, 58', etc.).
[0227] As shown in FIG. 37, mirror 56' is affixed to reticle stage
4' at the outer rim (m the Y direction) of passage 28', so as to
enable the Y-axis position of reticle 3', relative to reticle stage
4', to be measured by interferometer 55'. In interferometer 55',
laser light is divided into two beams prior to being emitted. One
of these beams (the upper one in FIG. 37) is directed at the side
surface of reticle 3', and the other beam (the lower one), is
directed at mirror 56'. The light reflected from the side of
reticle 3' and mirror 56' is combined in interferometer 55', and
the Y-axis position of reticle 3' relative to reticle stage 4' then
determined, based on the state of interference between the two
beams.
[0228] Similarly, an interferometer 57' is placed on the X side (-X
side) of reticle stage 4' to measure the X axis position of reticle
3' relative to reticle stage 4'. In interferometer 57', laser light
is divided into two beams and emitted. One of these beams (the
upper one in FIG. 37) is directed at the side surface of reticle
3', and the other beam (the lower one), is directed at the side
surface of reticle holder 32'. The light beams reflected from the
side surfaces of reticle 3' and reticle holder 32' are combined in
interferometer 57', and the X-axis position of reticle 3' relative
to reticle stage 4' (reticle holder 32') then determined, based on
the interference between the two beams. Because the applicable side
surfaces of reticle 3' and reticle holder 32' are made to reflect a
measurement laser, as described above, each of these surfaces must
be processed to give them an extremely high degree of flatness.
[0229] When a reticle 3' is loaded on the reticle holders 30' and
32' of the sixteenth embodiment of the present invention configured
as described above, reticle stage control unit 4'a controls
pressure control section 58' and voltage control section 60' so as
to cause reticle 3' to be held tightly against reticle holders 30'
and 32' by vacuum-induced suction as well as by electrostatic
attraction. That is, the compressor in pressure control section 58'
draws air from between reticle 3' and reticle holders 30' and 32',
into suction holes 52'a and 52'b of reticle holders 30' and 32',
and through conduits 54'a and 54'b.
[0230] Also, voltage control section 60' applies the prescribed
voltages to chrome layers 42', 44', 46', and 48', and dielectric
layers 34', 36', 38', and 40', to create static electricity between
chrome layers 42', 44', 46', and 48', and dielectric layers 34",
36', 38', and 40', such as to cause reticle 3' to be attracted to
reticle holders 30' and 32' by static electricity. Thus in the
present embodiment, because the conventional vacuum suction used to
hold reticle 3' is augmented by electrostatic attraction, the
holding force on reticle holders 30' and 32' is increased by that
amount, and reticle 3' can therefore be held more tightly. The
vacuum suction pressure can be reduced by the amount of the
electrostatic attraction holding force, to thereby reduce the
deformation of reticle 3' due to the force of the vacuum
suction.
[0231] Next, the configuration of the reticle stage of a
seventeenth embodiment of the present invention will be described,
with reference to FIGS. 38, 39, and 40. FIG. 38 shows the
peripheral configuration of reticle stage 4', and FIG. 39 shows the
construction of a reticle 3' to be loaded on reticle stage 4'. In
these drawings, the constituent elements of the configuration that
are the same as, or correspond to, elements of the sixteenth
embodiment, described above, will bear the same reference numbers,
and will not be described again.
[0232] Four attraction members, 62', 64', 66', and 68' are placed
on reticle stage 4' so as to surround passage 28'. Of these four,
attraction members 62' and 64' are installed directly on reticle
stage`4` in a manner so as to be movable in the Y-axis direction.
The other two attraction members 66' and 68' are installed on the
rear surfaces of reticle holders 30' and 32', respectively, in a
manner so as to be movable in the X-axis direction.
[0233] Formed on the inner wall portions of attraction members 62',
64', 66', and 68', in positions corresponding to the side surfaces
of reticle 3', are elongated dielectric layer strips. Two of these
four dielectric layer strips, 64'a and 66'a, are shown in FIG. 38.
Two similar dielectric strips, which are formed on attraction
members 62' and 68', are not shown in FIG. 38.
[0234] A chrome layer 70' is formed over the entire area of the
side surface of reticle 3', as shown in FIG. 39, for making contact
with the dielectric layer strips (64'a, 66'a, etc.) on attraction
members 62', 64', 66', and 68'. Formed on the bottom of, reticle
3', one in each corner, are contact areas 71' 72', 73', and 74',
for making contact with reticle holders 30' and 32'. FIG. 40 shows
a reticle 3' set in place on reticle holders 30' and 32'. A voltage
control section 75' is connected to reticle 3' and attraction
members 62', 64', 66", and 68' for applying prescribed voltages
thereto.
[0235] In this embodiment, in order to set reticle 3' in place on
reticle holders 30' and 32', first, with attractive members 62',
64', 66', and 68' retracted outward, reticle 3' is loaded onto
reticle holders 30' and 32' by a given transport system. Next,
attraction members 62', 64', 66', and 68' are moved inward until
they contact the sides of reticle 3'. Then, voltage from control
section 75' is applied to chrome layer 70' on the sides of reticle
3', and to the dielectric strips (including 64'a and 66'a) formed
on attraction members 62', 64', 66', and 68'. This creates static
electricity between contact areas 71', 72', 73' and 74' and
dielectric layers 34', 36', 38', and 40' (on reticle holders 30'
and 32'), thus holding reticle 3', through reticle holders 30' and
32', on reticle stage 4'. The output voltage of voltage control
section 75' can be adjusted according to the rate of movement of
reticle stage 4'.
[0236] In the present embodiment, because the side surfaces of
reticle 3' are held by electrostatic attraction, as described
above, slipping of reticle 3' during exposure, due to the effect of
its own inertia when reticle stage 4' is moved in the X-axis
direction, can effectively be prevented. Also, the prevention of
reticle 3' slippage during movement of reticle stage 4' can still
be quite effective, even if Y-axis attraction members 62' and 64'
are omitted, because the motion of reticle stage 4' is in the
X-axis direction. Note that this embodiment may also be used in
conjunction with the vacuum-suction mechanism of the above
sixteenth embodiment.
[0237] Next, the configuration of the reticle stage of an
eighteenth embodiment of the present invention will be described,
with reference to FIGS. 41, 42, and 43. FIG. 41 shows the
peripheral configuration of reticle stage 4', and FIG. 42 shows the
construction of a reticle 3' to be loaded on reticle stage 4'. In
these drawings, the constituent elements of the configuration that
are the same as, or correspond to, elements of the previous
embodiments described above will bear the same reference numbers,
and will not be described again.
[0238] Formed on reticle holders 30' and 32' installed on reticle
stage 4', are attraction surfaces 76'a, 76'b, 78'a, and 78'b, which
have been subjected to a prescribed surfacing process. Also, formed
on the bottom of reticle 3', in positions corresponding to those of
attraction surfaces 76'a, 76'b, 78'a, and 78'b, on reticle holders
30' and 32', are attraction portions 79'a, 79'b, 80'a, and 80'b,
which have also been subjected to a prescribed surfacing process. A
ringing phenomenon occurs between attraction surfaces 76'a, 76'b,
78'a, and 78'b of reticle holders 30' and 32' and attraction
portions 79'a, 79'b, 80'a, and 80'b, of reticle 3'. The prescribed
surfacing process is such that, for example, the coefficient of
friction between corresponding surfaces is at least 0.25.
[0239] When reticle 3' is set in place on reticle holders 30' and
32', as shown in FIG. 43, reticle 3' is pressed down on holders 30'
and 32' in a sliding motion, thus giving rise to a ringing
phenomenon between the reticle and the holders. This action affixes
reticle 3' to reticle holders 30' and 32'. According to this
embodiment, the holding force acting on reticle 3' can be increased
without adding anything in particular to the configuration. In this
embodiment, it is preferable for the areas and coefficient of
friction of attraction surfaces 76'a, 76'b, 78'a, and 78'b of
reticle holders 30' and 32', and of attraction portions 79'a, 79'b,
80'a, and 80'b, of reticle 3', to be adjusted as required.
[0240] Next, the configuration of the reticle stage of a nineteenth
embodiment of the present invention will be described, with
reference to FIGS. 44 and 45. FIG. 44 shows the peripheral
configuration of reticle stage 4' before it has a reticle 3' loaded
thereon, while FIG. 45 shows the peripheral configuration of
reticle stage 4' with. a reticle 3' already loaded. In these
drawings, the constituent elements of the configuration that are
the same as, or correspond to, elements of the previous embodiments
described above, will bear the same reference numbers, and will not
be described again.
[0241] Placed above reticle holders 30' and 32', are rubber
balloons 82'a, 82'b, 82'c and 82'd, which are used to hold reticle
3'. Of the four rubber balloons 82'a, 82'b, 82'c, and 82'd, rubber
balloons 82'a and 82'b are connected through conduit 84' to
pressure control section 86', while rubber balloons 82'c and 82'd
are connected through conduit 85' to pressure control section 87'.
Pressure control sections 86' and 87' adjust the pressure of the
air (gas) supplied to their respective rubber balloons (82'a, 82'b,
82'c, and 82'd), so as to control the gas pressure according to the
rate of movement of reticle stage 4'. Conduits 84' and 85' are
configured to be movable in the Z-axis direction.
[0242] As shown in FIG. 45, in order to set reticle 3' in place on
reticle holders 30' and 32', first, with conduits 84' and 85'
raised above reticle holders 30' and 32' in a standby state,
reticle 3' is loaded onto reticle holders 30' and 32'. Next,
conduits 84' and 85' are lowered until rubber balloons 82'a, 82'b,
82'c, and 82'd make good contact with reticle 3'. Then air, at a
prescribed pressure, is supplied to rubber balloons 82'a, 82'b,
82'c, and 82'd from pressure control sections 86' and 87', to form
rubber balloons 82'a, 82'b, 82'c, and 82'd into a condition in
which they are pressing down from above onto reticle 3'. In this
condition, control over the force being applied to reticle 3' is
performed by adjusting the pressure of the air being supplied by
pressure control sections 86' and 87', as well as the vertical
positioning of conduits 84' and 85'.
[0243] According to this embodiment as described above, it is
possible to control the reticle 3' holding force as a gradient
(rather than in steps) according to the thickness of reticle 3' and
the rate of movement of reticle stage 4'. Also, although rubber
balloons are used in this embodiment, other means such as air
cylinders could be substituted for the balloons. Also, instead of a
gas such as air, reticle 3' could also be held utilizing the
pressure of a fluid (liquid).
[0244] FIG. 46 shows a sensor arrangement, capable of being used in
any of the above embodiments, for measuring variations in the shape
of reticle 3'. Constituent elements of the configuration shown in
this drawing that are the same as, or correspond to, elements of
any of the above embodiments bear the same reference numbers, and
will not be described again. FIG. 46 shows a reticle 3' being held
to reticle stage 4' from below. In this drawing, items 88'a, 88'b,
88'c, and 88'd are sensors used to measure the position, in the
Z-axis direction, of prescribed locations on the rear surface of
reticle 3'.
[0245] Prior to performing an exposure, the reticle 3' is placed
and held in position on reticle holders 30' and 32' using any one
of the methods of the embodiments described above. Then sensors
88'a, 88'b, 88'c, and 88'd are placed on the rear surface of
reticle 3' and, as mentioned above, the distance between each of
the sensors and its corresponding location on the rear surface of
reticle 3' is measured by sensors 88'a, 88'b, 88'c and 88'd. Next,
based on these measurements, the change in position in the X, Y,
and Z directions of each reticle 3' measurement point is
computed.
[0246] Once the measurement is finished, sensors 88'a, 88'b, 88'c,
and 88'd are retracted outside the perimeter of reticle 3'. Then
after an alignment of reticle 3' is performed, based on the reticle
3' X, Y, and Z position data computed earlier, reticle 3' is
illuminated with exposure light IL', to start the exposure. Thus
the accuracy of reticle 3' alignment can be improved by measuring
the change in the position of reticle 3' in the loaded and held
state, using sensors 88'a, 88'b, 88'c, and 88'd, and then
performing the reticle 3' alignment based on those
measurements.
[0247] FIG. 47 shows the configuration of a circular reticle 90'
that may be used in the apparatus incorporating the principles of
the present invention. With circular reticle 90', the area of
contact between the reticle holder and the reticle can be much
larger than with the rectangular reticle 3' as shown in FIG. 48.
Provided in circular reticle 90', are a pattern area 92', the
center portion of the reticle, in which the pattern is formed, and
attraction area 93', the area outside of pattern area 92'. On the
other hand, in the rectangular reticle 3' of FIG. 48, it is
difficult to devote much area to attraction areas 94'a, 94'b, 94'c,
and 94d, because they are formed in the comers of reticle 3'. By
using the circular reticle 90' shown in FIG. 47 in the sixteenth
embodiment shown in FIGS. 34 through 37, for example, the
reticle-holding force can be increased by electrostatic attraction
alone, thus eliminating the need for the vacuum-suction
mechanism.
[0248] Although various embodiments of the present invention have
been described above, the invention is not limited to these
examples, but can be embodied in a variety of variations without
departing from the scope of the technical concepts that form the
gist of the invention as recited in the claims.
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