U.S. patent application number 09/801792 was filed with the patent office on 2001-08-23 for method for positioning substrate.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Kida, Yoshiki, Nishi, Kenji, Okumura, Masahiko.
Application Number | 20010016293 09/801792 |
Document ID | / |
Family ID | 27581893 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010016293 |
Kind Code |
A1 |
Nishi, Kenji ; et
al. |
August 23, 2001 |
Method for positioning substrate
Abstract
A method for positioning a wafer with respect to a reticle in a
projection exposure apparatus for a photolithographic process
capable of high speed search alignment of a wafer without any
limitation imposed on the arrangement of the search marks on the
wafer. For the first wafer in one lot, a first alignment sensor
system is used to detect the positions of first and second search
marks, and define a coordinate system with reference to the
positions of the search marks based on the detection results. Then,
while the first search mark is detected by the first alignment
sensor system, the position of a street-line is detected by a
second alignment sensor system and stored. For any of the second
and later wafers in the lot, while the first search mark is
detected by the first alignment sensor system, the position of a
street-line is detected by the second alignment sensor system, and
the offsets between the detected position and the stored position
are used to define a coordinate system which refers to the search
marks.
Inventors: |
Nishi, Kenji; (Tokyo,
JP) ; Kida, Yoshiki; (Tokyo, JP) ; Okumura,
Masahiko; (Tokyo, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN, HATTORI,
MCLELAND & NAUGHTON, LLP
1725 K STREET, NW, SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
Nikon Corporation
Tokyo
JP
|
Family ID: |
27581893 |
Appl. No.: |
09/801792 |
Filed: |
March 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09801792 |
Mar 9, 2001 |
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09500244 |
Feb 8, 2000 |
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6225012 |
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09500244 |
Feb 8, 2000 |
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09095023 |
Jun 9, 1998 |
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09095023 |
Jun 9, 1998 |
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08800390 |
Feb 14, 1997 |
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08800390 |
Feb 14, 1997 |
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08678788 |
Jul 11, 1996 |
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08800390 |
Feb 14, 1997 |
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08605787 |
Feb 22, 1996 |
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08605787 |
Feb 22, 1996 |
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08391648 |
Feb 21, 1995 |
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Current U.S.
Class: |
430/22 ;
430/30 |
Current CPC
Class: |
G03F 9/7011 20130101;
G03F 9/7046 20130101; G03F 7/70691 20130101 |
Class at
Publication: |
430/22 ;
430/30 |
International
Class: |
G03F 009/00; G03C
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 22, 1994 |
JP |
24536/1994 |
Feb 24, 1995 |
JP |
36432/1995 |
Jul 14, 1995 |
JP |
178630/1995 |
Dec 28, 1995 |
JP |
343247/1995 |
Mar 14, 1996 |
JP |
57893/1996 |
Feb 8, 1999 |
JP |
29918/1999 |
Claims
What is claimed is:
1. A method for positioning a substrate on a two-dimensionally
movable substrate stage, said substrate having a peripheral edge
with a cutout formed therein, said method comprising the steps of:
(a) transferring said substrate to a loading position above said
substrate stage; (b) measuring, at said loading position, positions
of a measurement point on said cutout formed in said peripheral
edge of said substrate and of another measurement point on said
peripheral edge of said substrate, by using a
noncontact-measurement technique; and (c) determining a rotational
error of said substrate at said loading position based on the
measurement results obtained through said step (b); and (c)
correcting said determined positional error while said substrate is
moving from said loading position above said substrate stage onto
said substrate stage.
2. A positioning method according to claim 1, further comprising
the step of: rotating said substrate to compensate for said
rotational error before said substrate is placed onto said
substrate stage.
3. A positioning method according to claim 1, wherein: the
positions of said measurement points are measured by using
two-dimensional image processing technique.
4. A method for positioning a substrate on a two-dimensionally
movable substrate stage, comprising the steps of: (a) forming on
each substrate first and second search marks each for indicating a
two-dimensional position; (b) detecting two-dimensional positions
of said first and second search marks on a first substrate; (c)
determining a rotational error of said first substrate based on the
two-dimensional positions detected through said step (b); (d)
detecting and storing an at least one-dimensional position of a
pattern spaced a predetermined distance from said first search mark
on said first substrate, while detecting a two-dimensional position
of said first search mark; (e) detecting an offset, from the
position stored through said step (d), of said pattern spaced said
predetermined distance from said first search mark, while detecting
a two-dimensional position of said first search mark on a second
substrate on said substrate stage, so as to determine from said
offset positional error of an offset positional error of said
second substrate.
5. A positioning method according to claim 4, wherein: said steps
(b), (c), (d) and (e) are performed on the first substrate in one
lot.
6. A method for positioning a substrate on a two-dimensionally
movable substrate stage, said substrate having a peripheral edge
with a cutout formed therein, said method comprising the steps of:
(a) transferring said substrate to a loading position above said
substrate stage; (b) measuring, at said loading position, positions
of a measurement point on said cutout formed in said peripheral
edge of said substrate and another measurement point on said
peripheral edge of said substrate, by using a two-dimensional image
processing system and a noncontact-measurement technique; (c)
determining, in said observation fields of said two-dimensional
image processing system, imaginary points corresponding to
reference points which would be used for positioning said substrate
on said substrate stage by using a contact-positioning technique;
and (d) using offsets, from said imaginary points, of the positions
of said measurement points measured by said two-dimensional image
processing system, to make a prediction of a position of said
substrate which will be found when said substrate has been placed
on said substrate stage.
7. A positioning method according to claim 6, wherein: said
prediction of the position of said substrate in said step (d) is
made not only using said offsets, from said imaginary points, of
the positions of said measurement points measured by said
two-dimensional image processing system, but also using measured
coordinates of a rotational center of a substrate lift means for
setting said substrate from said loading position onto said
substrate stage while rotating said substrate by a desired
angle.
8. A positioning method according to claim 6, further comprising
the step of; positioning said substrate, based on results of said
prediction, to a location on said substrate stage to which said
substrate would be positioned when said substrate is positioned by
using said contact-alignment technique.
9. A positioning method according to claim 6, wherein: said cutout
formed in said peripheral edge of said substrate comprises a
V-shaped cutout, and said measurement points whose positions are
measured by said two-dimensional image processing system include
one measurement point on said cutout and two measurement points on
other portions of said peripheral edge of said substrate.
10. A positioning method according to claim 6, wherein: said cutout
formed in said peripheral edge of said substrate comprises a flat
edge portion, and said measurement points whose positions are
measured by said two-dimensional image processing system include
one measurement point on said cutout and two measurement points on
other portions of said peripheral edge of said substrate.
11. A positioning method according to claim 6, further comprising
the steps of: in order to make said prediction of the position of
said substrate which will be found when said substrate has been
placed on said substrate stage, obtaining a rotational error and
offsets between a position of said substrate which will be found
when said substrate has been placed on said substrate stage through
said substrate lift means without any rotation effected thereby and
a position of said substrate which would be found when said
substrate had been positioned by using said contact-positioning
technique; and correcting said rotational error when said substrate
is placed onto said substrate stage through said substrate lift
means, and correcting said offsets through said substrate stage
after said substrate has been placed on said substrate stage.
12. A positioning method according to claim 7, further comprising
the steps of: in order to make said prediction of the position of
said substrate which will be found when said substrate has been
placed on said substrate stage, obtaining a rotational error and
offsets between a position of said substrate which will be found
when said substrate has been placed on said substrate stage through
said substrate lift means without any rotation effected thereby and
a position of said substrate which would be found when said
substrate had been positioned by using said contact-positioning
technique; and correcting said rotational error when said substrate
is placed onto said substrate stage through said substrate lift
means, and correcting said offsets through said substrate stage
after said substrate has been placed on said substrate stage.
13. A positioning method according to claim 6, wherein: said
imaginary points are determined by calculations.
14. A positioning method according to claim 6, further comprising
the step of: transferring an image of a pattern formed on a mask
onto said substrate placed on said substrate stage.
15. A positioning method according to claim 1, wherein said cutout
has a V-shaped configuration.
16. A positioning method according to claim 1, wherein said
substrate has a peripheral edge with a V-shaped cutout formed
therein.
17. A positioning method in which a substrate is positioned
relative to a substrate stage which is movable with said substrate
mounted thereon, said method comprising the steps of: transferring
said substrate to an off-position at which said substrate is not
mounted on said substrate stage; detecting positions of a V-shaped
cotout formed in said substrate and of another point of a
peripheral edge of said substrate at said off-position; and
transferring said substrate to an on-position at which said
substrate is placed on said substrate stage.
18. A positioning method according to claim 17, wherein said
detection of the position of said substrate is preformed by a
noncontact-measurement technique.
19. A positioning method according to claim 17, further comprising
a step of correcting the position of said substrate based on the
detected position of the substrate.
20. A positioning method according to claim 19, wherein said
correction of the position of said substrate is performed while
said substrate is transferred from said off-position to said
on-position.
21. A positioning method according to claim 18, wherein said
detection of the position by said noncontact-measurement technique
is performed by using a two-dimensional image processing
system.
22. A positioning method according to claim 21, wherein an
imaginary point corresponding to a reference point which would be
used to position said substrate on said substrate stage by using a
contact-positioning technique is set in said two-dimensional image
processing system.
23. A positioning method according to claim 17, wherein said
off-position is above said substrate stage.
24. An exposure method which exposes a substrate placed on a
movable substrate stage whit a pattern, said method comprising the
steps of: transferring a substrate to an off-position at which said
substrate is not placed on a substrate stage; detecting positions
of a V-shaped cutout formed in said substrate and of another point
on a peripheral edge of said substrate at said off-position;
transferring said substrate to an on-position at which said
substrate is placed on said substrate stage: and exposing said
substrate with said pattern.
25. An exposure method according to claim 24, wherein said
detection of the position of said substrate is performed by a
noncontact-measurement technique.
26. An exposure method according to claim 24, further comprising a
step of correcting the position of said substrate based on the
detected position of said substrate.
27. An exposure method according to claim 26, wherein said
correction of the position of said substrate is performed while
said substrate is transferred from said off-position to said
on-position.
28. An exposure method according to claim 25, wherein said
detection of the position by said noncontact-measurement technique
is performed by using a two-dimensional image processing
system.
29. An exposure method according to claim 24, wherein said
off-position is above said substrate stage.
30. A substrate which is exposed with a pattern by using the method
set forth in claim 24.
31. A method to make a positioning apparatus which determines a
position of a substrate stage, which holds and moves at least in a
first direction, said method comprising: providing a first transfer
device which transfers said substrate to an off-position at which
said substrate is not placed on said substrate stage; providing a
position detecting device which detects the position of said
substrate at said off-position by using a V-shaped cutout formed in
said substrate; and providing a second transfer device which
transfers said substrate to an on-position at which said substrate
is placed on said substrate stage.
32. A method according to claim 31, wherein said position detecting
device is a noncontact-measurement type detecting device which
detects the position of the substrate using a
noncontact-measurement technique.
33. A method according to claim 31, further comprising: Providing a
correction device which corrects the position of said substrate
based on the detected position of said substrate.
34. A method according to claim 33, wherein said correction device
corrects the position of said substrate while said substrate is
transferred from said off-position to said on-position.
35. A method according to claim 31, wherein said off-position is
above said substrate stage.
36. A method to make an exposure apparatus which exposes a
substrate held on a substrate stage with a pattern, comprising:
providing an exposure system which exposes said substrate with said
pattern; providing a first transfer device which transfers said
substrate to an off-position at which said substrate is not placed
on said substrate stage; providing a position detecting device
which detects the position of said substrate at said off-position
by using a V-shaped cutout formed in said substrate; and providing
a second transfer device which transfers said substrate to an
on-position at which said substrate is placed on said substrate
stage.
37. An method according to claim 36, wherein said detecting device
is a noncontact-measurement type detecting device which detects the
position of the substrate using a noncontact-measurement
technique.
38. A method according to claim 36, further comprising: Providing a
correction device which corrects the position of said substrate
based on the detected position of said substrate.
39. A method according to claim 36, wherein said correction device
corrects the position of said substrate while said substrate is
transferred from said off-position to said on-position.
40. A method according to claim 36, wherein said off-position is
above said substrate stage.
41. A substrate on which a pattern has been transferred by an
exposure apparatus manufactured by the method according to claim
36.
42. A method for positioning a substrate having a geometrical
feature, said method comprising the steps of: measuring said
geometrical feature of said substrate and determining a first
offset of said substrate with respect to a first reference based on
the measured result; correcting position of said substrate so as to
compensate said first offset; measuring an alignment mark formed on
said substrate and determining a second offset of said alignment
mark with respect to a second reference based on the measured
result; and correcting said first reference if said second offset
is determined as not falling within a predetermined allowable
range.
43. A method according to claim 42, wherein: each of said first and
second offsets is an offset in a rotational direction.
Description
[0001] This is a continuation-in-part application of U.S.
Continuation-in-Part appln. Ser. No. (095,023) filed on (Jun. 9,
1998) which is a continuation-in-part application of U.S.
Continuation-in-Part appln. Ser. No. (800,390) filed on (Feb. 14,
1997) of U.S. Pat. appln. Ser/ No. (678,788), filed on (Jul. 11,
1996) and U.S. Continuation-in-part appln. Ser. No. (605,787) filed
on (Feb. 22, 1996) of U.S. Pat. appln. Ser. No. (391,648), filed on
(Feb. 2, 1995).
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method for positioning a
photosensitized substrate for printing a pattern on the mask onto
the photosensitized substrate in an exposure apparatus used in a
photolithographic process for fabricating semiconductor devices,
image pick-up devices (such as charge-coupled devices), liquid
crystal displays, thin film magnetic heads or the like. In
particular, it relates to such a positioning method suitable for
performing a coarse alignment (or prealignment) operation of a
photosensitized substrate on a stage in an exposure apparatus.
[0003] In any of various exposure apparatuses used for fabrication
of semiconductor devices, liquid crystal displays or the like
including a projection exposure apparatus (such as a stepper) and a
proximity printing exposure apparatus, in order to transfer a
circuit pattern formed on a mask or reticle onto a photoresist film
formed on a photosensitized substrate such as a wafer (or a glass
plate, etc.) with high registration, it is required to establish
alignment between the reticle and the wafer with precision.
[0004] For this purpose, there have been proposed various types of
alignment sensor systems including: "laser step alignment (LSA)
type" in which a wafer has an alignment mark formed thereon which
comprises a linear array of dots, and a laser beam illuminates the
alignment mark to produce diffracted or scattered beams, which are
used to detect the position of the alignment mark (such as
disclosed in U.S. Pat. No. 5,243,195); "field image alignment (FIA)
type" in which an image-sensing device is used to take an image of
an alignment mark which is illuminated by illumination light having
a continuous spectrum of a wide wavelength range obtainable from a
halogen lamp, and the picture data of the image is subjected to an
image processing to measure the position of the alignment mark; and
"laser interferometric alignment (LIA) type" in which a wafer has
an alignment mark formed thereon which comprises a diffraction
grating, two laser beams having different frequencies with a small
difference between them illuminate the alignment mark from
different directions to produce two diffracted beams interfering
with each other, and the position of the alignment mark is
determined from the phase of the interference. There have been also
proposed various alignment techniques which may be categorized into
three types including: "through-the-lens (TTL) type" in which the
position of the wafer is measured through a projection optical
system; "through-the-reticle (TTR) type" in which the relative
position of a wafer with respect to a reticle is measured through
both a projection optical system and the reticle; and "off-axis
type" in which the position of the wafer is measured directly, or
not through a projection optical system.
[0005] By using any of these alignment sensor systems to detect
respective positions of two points on a wafer which is placed on a
wafer stage, the rotational position (or rotational angle) of the
wafer may be determined in addition to the position of the wafer
with respect to the translational displacement. Alignment sensor
systems usable for determining the rotational angle of a wafer
include LIA-type system using TTL-type technique, LSA-type system
using TTL-type technique, FIA-type system using off-axis-type
technique, and others.
[0006] The exposure apparatus is required not only to have the
ability of establishing alignment with precision between a reticle
and a wafer by using the detection results obtained from the
alignment sensor, but also to quickly establish such alignment so
as to keep high throughput (the number of wafers that can be
processed per unit of time). Thus, it is needed to realize highly
effective operations in all the steps performed in relation to the
exposure apparatus, from a transfer operation of a wafer onto the
wafer stage to an exposure operation. Here, we will describe with
reference to FIG. 1 the operations in the wafer loading step
preceding the final wafer alignment step, as performed in a typical
prior art exposure apparatus.
[0007] FIG. 1 shows a part of a wafer stage and the associated
elements for a typical prior art exposure apparatus, for
illustrating a wafer transfer mechanism. In FIG. 1, a lift device
19 is mounted on an X-stage 11 through a linear actuator 20. A
substrate or wafer 6 has been loaded onto the lift device 19 from a
wafer transfer unit (not shown). The lift device 19 has three
support pins (of which only two support pins 19a and 19b are shown
in FIG. 1) extending vertically through openings formed in a
material support 9, a .theta.-rotation correction mechanism 8 and a
wafer holder 7. The linear actuator 20 acts on the lift device 19
to lower/raise the three support pins, so as to lift down/up the
wafer 6 for placing it onto and removing it from the wafer holder
7. Each support pin has at its tip end a hole selectively
communicable with a vacuum source, which sucks the bottom surface
of the wafer 6 to hold it, so as to prevent the wafer 6 from
displacing horizontally upon vertical movements of the lift device
19.
[0008] Conventionally, a contact-prealignment process is used in
which the peripheral edge of the wafer 6 is pressed against and
engaged with a plurality of pins after the lift device 19 is
lowered to place the wafer 6 onto the wafer holder 7. This
prealignment process establish coarse alignment of the wafer 6 with
respect to the rotational and translational offsets before the
wafer 6 is held by vacuum suction to the wafer holder 7.
[0009] After the wafer 6 is held by vacuum suction and fixed to the
wafer holder 7 in this manner, an alignment sensor system of a
suitable type, such as the LSA type or the FIA type, is used to
detect the alignment marks (search marks) formed on the wafer 6 at
positions diametrically opposite to each other and produce
detection signals. A movable mirror 13 mounted on the material
support 9 and the associated laser interferometer disposed outside
of the material support 9 together serve to measure the coordinates
of the position of the material support 9. By determining such
coordinates when the detection signals is at a peak, the
translational errors and the rotational error of the wafer in terms
of the wafer stage coordinate system are determined. The
.theta.-rotation correction mechanism (or .theta.-table) is driven
to vanish the rotational error of the wafer 6, so that the
alignment in the rotational direction between the reticle and the
wafer 6 (search alignment) is performed.
[0010] In this prior art technique, the .theta.-rotation correction
mechanism 8 for rotating the wafer is disposed between material
support 9 and the wafer 6, while the wafer stage coordinate system
is defined with reference to the material support 9. Thus, there
have been many problems including unwanted horizontal displacements
of the wafer 6 which may occur due to insufficient suction for the
vacuum-holding of the wafer by the wafer holder 7, a poor rigidity
of the wafer stage due to the provision of various complicated
mechanisms on the material support 9, as well as a poor
controllability of the wafer stage due to the heavy weight thereof
imposed by such complicated mechanisms. These problems could not be
solved by disposing a .theta.-rotation correction mechanism under
the material support 9 because the incident angle of the laser beam
from the laser interferometer into the movable mirror 13 fixedly
mounted on the material support 9 would change when the rotation
angle of the wafer 6 is adjusted by driving the .theta.-rotation
correction mechanism, so that the range of rotation angle of the
.theta.-rotation correction mechanism would be limited, resulting
in a disadvantage that errors in prealignment could not be
corrected unless they are sufficiently small.
[0011] Furthermore, in this prior art exposure apparatus, the
translational errors and the rotational error of the wafer 6 is
detected by measuring the positions of two alignment marks (search
marks) formed on the wafer 6 and distant from each other by means
of a single alignment sensor system of the LSA or the FIA type,
after the wafer 6 is held on the wafer holder 7. However, in order
to detect two distant alignment marks by means of a single
alignment sensor system, the wafer 6 has to be moved so as to
position the alignment marks sequentially into the detection area
of the alignment sensor system, and this operation has to be
repeated for each of the wafers in one lot, resulting in low
throughput of the exposure process. This problem could not be
conveniently solved by providing two alignment sensor systems for
simultaneous detection of the two alignment marks, because the
arrangement of the two alignment sensor systems on the exposure
apparatus imposes a limitation on the arrangement of the two
alignment marks on the wafer 6, so that it would be difficult for
such exposure apparatus to accommodate wafers of different sizes,
for example.
[0012] In this relation, the last problem could not be conveniently
solved by providing an adjustor mechanism for adjusting the
distance between the two alignment sensor systems because such
adjustor mechanism has to be inherently complicated, is difficult
to dispose in the space around the wafer stage where various
sensors and other components are closely disposed, and tends to
increase the manufacture costs.
[0013] Also, various contact-prealignment mechanisms have been used
to establish coarse alignment after the wafer has been placed on
the wafer holder 7. However, there is an disadvantage that high
throughput can not be obtained when the prealignment operation has
to be performed after the wafer has been placed on the wafer
holder. Nevertheless, it is desirable that any exposure apparatus
having a newly proposed prealignment mechanism which may provide
high throughput, may be capable of providing the matching with
another, existing exposure apparatus having a conventional
contact-prealignment mechanism.
[0014] The orientation of a wafer can be corrected to meet a
predefined reference by performing the prealignment process, in
which a noncontact-type prealignment mechanism is used to measure
the outer contour of the peripheral edge of the wafer and the
orientation of the wafer is so corrected as to coincide with the
reference. However, this prealignment process still suffers from a
problem: in the case that the alignment marks on the surface of a
wafer, which have been formed through a previous lithographic
process, offset (in a rotational direction) away from the desired
positions that are predefined relative to the geometrical features
(such as an orientation flat and a notch) defined by the
configuration of the peripheral edge of the wafer, the fine
alignment process following the prealignment process may not be
performed efficiently, or it may even be impossible at all to
perform the fine alignment process so the wafer has to be rejected
as a failure.
SUMMARY OF THE INVENTION
[0015] It is an object of the present invention to provide a method
for positioning a substrate which contributes to enhance the
rigidity of a wafer stage and reduce the weight of the wafer stage,
resulting in that the positioning operation of a wafer upon loading
of a wafer by, for example, a wafer loader system onto the wafer
stage, can be quickly performed with precision.
[0016] It is another object of the present invention to provide a
method for positioning a substrate by which when the positioning
operation of a wafer is performed using a wafer stage and with
reference to the positions of alignment marks formed on a wafer,
the positioning operation can be quickly performed without any
limitation imposed on the arrangement of the alignment marks.
[0017] It is a further object of the present invention to provide a
method for positioning a substrate which may achieve high matching
accuracy for the prealignment with other exposure apparatus in
which a contact-prealignment process is utilized.
[0018] It is further object of the present invention to provide a
method for positioning a substrate, such as a wafer, in which a
fine alignment process may be performed with high efficiency even
for such a wafer having its alignment marks the position of which
are displaced or offset from the desired positions represented by
the geometrical features of the wafer, so as to improve
productivity of the products to be fabricated on the wafer, such as
microdevices or others.
[0019] According to a first aspect of the present invention, there
is provided a method for positioning a substrate on a
two-dimensionally movable substrate stage, the substrate having a
peripheral edge with a cutout formed therein, the method comprising
the steps of: (a) transferring the substrate to a loading position
above the substrate stage; (b) measuring, at the loading position,
positions of a measurement point on the cutout formed in the
peripheral edge of the substrate and of another measurement point
on the peripheral edge of the substrate, by using a
noncontact-measurement technique; and (c) determining a rotational
error of the substrate based on the measurement results obtained
through the step (b).
[0020] In this alignment method, if the cutout formed in the
peripheral edge of the substrate comprises a notch as shown in FIG.
6(b), the measurement of the position at the measurement point on
the cutout may be preferably made by a two-dimensional image
processing unit. Further, for the substrate having a notch, the
two-dimensional, positional offsets (translational errors) and the
rotational error of the substrate may be detected by performing
one-dimensional position measurement at one additional measurement
point on the peripheral edge of the substrate other than the
measurement point on the notch.
[0021] On the other hand, if the cutout formed in the peripheral
edge of the substrate comprises an orientation flat, the position
measurement may be made at any of the measurement points on the
peripheral edge of the substrate by image processing units, and
one-dimensional position measurement may be sufficient for the
purpose. However, when one-dimensional position measurement is
used, the measurement are performed at measurement points including
one on the orientation flat and at least two other measurement
points (hence at least three measurement points in total) in order
to detect the two-dimensional offsets and the rotational error of
the substrate. In either case, the two-dimensional offsets may be
corrected by adding the offsets to the target position of the
alignment in the subsequent search alignment process. By virtue of
this method, a rotational correction mechanism on the substrate
stage may be eliminated and the accuracy is improved.
[0022] According to a second aspect of the present invention, there
is provided a method for positioning a substrate on a
two-dimensionally movable substrate stage, comprising the steps of:
(a) forming on each substrate first and second search marks each
for indicating a two-dimensional position; (b) detecting
two-dimensional positions of the first and second search marks on a
first substrate; (c) determining a rotational error of the first
substrate based on the two-dimensional positions detected through
the step (b); (d) detecting and storing an at least one-dimensional
position of a pattern spaced a predetermined distance from the
first search mark on the first substrate, while detecting a
two-dimensional position of the first search mark; (e) detecting an
offset, from the position stored through the step (d), of the
pattern spaced the predetermined distance from the first search
mark, while detecting a two-dimensional position of the first
search mark on a second substrate on the substrate stage, so as to
determine from the offset positional error an offset positional
error of the second substrate.
[0023] In this positioning method, for the second substrate, the
first search mark is positioned in the detection area of a
predetermined first alignment sensor system, and then the position
of a pattern (such as a street-line) in the detection area of a
second alignment sensor system spaced a predetermined distance form
the first alignment sensor is compared with the stored position for
the first substrate, and any alignment error may be determined form
the results of this comparison. Thus, for the second substrate, it
is unnecessary to perform the detection of the position of the
second search mark, and the detection of the position of the
pattern in the detection area under the second alignment sensor
system together with the detection of the first search mark by the
first alignment sensor system at the same time may be sufficient
for establishing alignment of the second substrate, resulting in a
reduce time required for the measurement.
[0024] According to a third aspect of the present invention, there
is provided a method for positioning a substrate on a
two-dimensionally movable substrate stage, the substrate having a
peripheral edge with a cutout formed therein, the method comprising
the steps of: (a) transferring the substrate to a loading position
above the substrate stage; (b) measuring, at the loading position,
positions of a measurement point on the cutout formed in the
peripheral edge of the substrate and of another measurement point
on the peripheral edge of the substrate, by using a two-dimensional
image processing system and a noncontact-measurement technique; (c)
determining, in the observation fields of the two-dimensional image
processing system, imaginary points corresponding to reference
points which would be used for positioning the substrate on the
substrate stage by using a contact-positioning technique; and (d)
using offsets, from the imaginary points, of the positions of the
measurement points measured by the two-dimensional image processing
systems, to make a prediction of a position of the substrate which
will be found when the substrate has been placed on the substrate
stage.
[0025] In this positioning method, the rotational error of the
substrate is detected at the loading position of the substrate
distant from the substrate stage, so that the rotational error may
be corrected through the substrate lift means while the substrate
is lowered from the loading position onto the substrate stage.
Therefore, it is unnecessary to provide a rotation correction
mechanism on the substrate stage side, and thereby the substrate
stage may have a relatively simple construction, an improved
rigidity and a reduced weight, resulting in that the alignment
operation of the substrate may be quickly performed with precision
upon loading of the substrate from a substrate transfer system
(such as a wafer loader system) onto the substrate stage.
[0026] Further, the imaginary points corresponding to reference
points which would be used for positioning the substrate on the
substrate stage by using a contact-positioning technique, are
determined in the observation fields of the two-dimensional image
processing system. Also, the offsets, from the imaginary points, of
the positions of the measurement points on the photosensitized
substrate measured by the two-dimensional image processing system
are used to establish alignment of the substrate. Therefore, a high
matching accuracy for the coarse positioning (prealignment)
process, with another exposure apparatus in which a
contact-positioning (prealignment) process is performed, may be
obtained.
[0027] Further, in this positioning method, if the cutout formed in
the peripheral edge of the substrate comprises a V-shaped notch,
the measurement points for the two-dimensional image processing
system may preferably include one on the cutout and two on
respective portions of the peripheral edge of the substrate other
than the cutout. In this case, the cutout is a recess such as a
notch, and thus the detection of the positions at the three
measurement points enables identification of the rotational angle
and the two-dimensional position of the substrate.
[0028] On the other hand, if the cutout formed in the peripheral
edge of the substrate comprises a flat edge portion, the
measurement points for the two-dimensional image processing system
may preferably include two on the cutout and one on a portion of
the peripheral edge of the substrate other than the cutout. In this
case, the cutout is a flat cutout such as an orientation flat, and
thus the detection of the positions at the three measurement points
enables identification of the rotational angle and the
two-dimensional position of the substrate.
[0029] Furthermore, it is preferable, in order to make the
prediction of the position of the substrate which will be found
when the substrate has been placed on the substrate stage, to
obtain a rotational error and offsets between a position of the
substrate which will be found when the substrate has been placed on
the substrate stage through the substrate lift means without any
rotation effected thereby and a position of the substrate which
would be found when the substrate had been positioned by using the
contact-positioning technique, to correct the rotational error when
the substrate is placed onto the substrate stage through the
substrate lift means, and to correct the offsets through the
substrate stage after the substrate has been placed on the
substrate stage. This enables simplification of the construction of
the substrate stage and a quick alignment of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The above and other objects, features and advantages of the
present invention will be apparent from the following detailed
description of preferred embodiments thereof, reference being made
to the accompanying drawings, in which:
[0031] FIG. 1 is a schematic representation of a prior art wafer
loading mechanism used in a projection exposure apparatus;
[0032] FIG. 2 is a flow chart illustrating a prealignment process
in a positioning method according to an embodiment of the present
invention;
[0033] FIG. 3 shows the relation between FIG. 3A and FIG. 3B: FIGS.
3A and 3B are flow charts illustrating a search alignment process
and a fine alignment process in the positioning method according to
the embodiment of the present invention;
[0034] FIG. 4 is a schematic representation of a projection
exposure apparatus which may be used for implementing the
positioning method of FIGS. 2 and 3;
[0035] FIGS. 5(a) through 5(d) show a wafer transfer unit, a wafer
loading mechanism, a turntable mechanism and other elements used in
the projection exposure apparatus of FIG. 4;
[0036] FIG. 6(a) is a plan view of a wafer having an orientation
flat, and FIG. 6(b) is a plan view of a wafer having a notch;
[0037] FIG. 7 is a schematic representation, partially cut out, of
an exemplified arrangement for a two-dimensional image processing
unit 50 for detecting the edge of a wafer;
[0038] FIGS. 8(a) and 8(b) illustrate an exemplified method of
detecting a notch of a wafer;
[0039] FIG. 9(a) is a plan view of a wafer with a first search mark
47A located in the observation field of the FIA-microscope 5A, and
FIG. 9(b) is p plan view of the wafer with a second search mark 47B
located in the observation field 5A of the FIA-microscope 5A;
[0040] FIG. 10(a) shows an image of the first search mark 47A which
may be observed for the first wafer in one lot in the course of the
process for the positioning method according to the embodiment of
the present invention, FIG. 10(b) shows a waveform of an
image-sensing signal obtained by scanning the image of FIG. 10(a)
in the Y-direction, and FIG. 10(c) shows a waveform of an
image-sensing signal obtained by scanning the image of FIG. 10(a)
in the X-direction;
[0041] FIG. 11(a) shows an image of the first search mark 47A which
may be observed for any of the second and later wafers in the lot,
and FIG. 11(b) shows an image which may be observed in the
observation field of the .theta.-microscope 5B at the same time
when the image of FIG. 11(a) is observed;
[0042] FIG. 12(a) shows a waveform of an image-sensing signal
corresponding to the image of FIG. 11(a), and FIG. 12(b) shows a
waveform of an image-sensing signal corresponding to the image of
FIG. 11(b);
[0043] FIG. 13 is a schematic representation, partially cut out, of
another exemplified arrangement for the image processing unit
50;
[0044] FIGS. 14(a) and 14(b) illustrate various notches and
orientation flats formed in wafers;
[0045] FIGS. 15(a) through 15(d) illustrate a method of detecting
the positions of the peripheral edge of an wafer 6 using line
sensors or two-dimensional image processing units.
[0046] FIGS. 16(a) through 16(d) illustrate a method of detecting
edge positions of an wafer 6N having a notch, while maintaining the
matching with a contact-prealignment mechanism;
[0047] FIGS. 17(a) through 17(d) illustrate a method of detecting
edge positions of an wafer 6 having an orientation flat, while
maintaining the matching with a contact-prealignment mechanism;
[0048] FIG. 18 shows the relation between FIG. 18A and FIG. 18B:
FIGS. 18A and 18B are flow charts illustrating exemplified
operations for determining whether a wafer replacement is to be
performed and for performing the wafer replacement;
[0049] FIG. 19 is a schematic illustrating the relationship among
the positions of three fixed pins used in a conventional
prealignment mechanism;
[0050] FIG. 20 is a schematic illustrating the relationship of the
positions of observation or viewing fields of image processing
units relative to a wafer;
[0051] FIG. 21 is a flow chart showing a process sequence for
establishing prealignment of a wafer;
[0052] FIG. 22 is a schematic plan view showing an arrangement of a
wafer useful for illustrating the process sequence;
[0053] FIG. 23 is a flow chart showing said another process
sequence for establishing prealignment of a wafer;
[0054] FIG. 24 is a schematic illustrating the relationship of the
positions of observation or viewing fields of image processing
units relative to a wafer;
[0055] FIG. 25 is a schematic plan view of a wafer useful for
illustrating a further process sequence;
[0056] FIG. 26 is a flow chart showing said further process
sequence for establishing prealignment of a wafer.
[0057] FIG. 27 is a flow chart showing further process sequence for
establishing prealignment of a wafer.
DETAILED DESCRIPTION OF THE INVENTION
[0058] Referring now to the accompanying drawings and in particular
to FIGS. 2 to 17 thereof, a positioning method according to an
embodiment of the present invention will be described in detail.
This embodiment is an exemplified application of the present
invention to a loading and alignment processes performed in a
stepper-type projection exposure apparatus in which a pattern on a
reticle is projected with demagnification through a projection
optical system onto each shot area on a wafer.
[0059] FIG. 4 shows a schematic representation of a projection
exposure apparatus which may be used for implementing the alignment
method of the embodiment. As shown in FIG. 4, the projection
exposure apparatus includes an illumination optical system IA
comprising a light source such as a mercury-vapor lamp, a fly-eye
lens and a condenser lens. The illumination optical system IA emits
an illumination light beam IL, with which an image of a pattern on
a reticle 1 is projected through a projection optical system 3 onto
a photoresist-coated wafer 6 within each of shot areas defined
thereon, by projection exposure with a given demagnification ration
of, for example, 4:1, 5:1, or others. In order to specify positions
and directions in this arrangement, we define here a
three-dimensional, rectangular, XYZ-coordinate system with the
Z-axis extending parallel to the optical axis AX of the projection
optical system 3 in FIG. 4, the X-axis perpendicular to the Z-axis
and parallel to the paper surface of FIG. 4 and the Y-axis
perpendicular to the paper surface of FIG. 4.
[0060] The reticle 1 is held on a reticle stage 32 which, in turn,
is carried on a reticle support 32. The reticle stage 32 is capable
of translation in the XY-plane and rotation in the XY-plane (or the
rotation in the .theta.-direction) by means of a reticle drive
system (not shown). Movable mirrors 33a and 33b for position
measurement in the X- and Y-directions, respectively, are fixedly
mounted on the reticle stage 32 on respective side edges thereof
and associated with laser interferometers 34a and 24b,
respectively, fixedly mounted on the reticle support 31. These
pairs of movable mirror and laser interferometer continuously serve
to detect the position of the reticle stage 32 in the X- and
Y-directions with a resolution on the order of 0.01 .mu.m, as well
as the rotational angle (or rotational position in the
.theta.-direction). The laser interferometers 34a and 34b produce
data signals indicative of the measured values and supply them to a
stage control system 16, which uses the supplied data signals so as
to control the reticle drive system mounted on the reticle support
31. The stage control system 16 also supplies the data signals
indicative of the measured values from the laser interferometers
34a and 34b to a central control system 18, which uses the supplied
data signals so as to control the stage control system 16.
[0061] A wafer 6 is held on a wafer holder 30 using vacuum suction,
and the wafer holder 30 is disposed on a material support 29 which,
in turn, is carried on an X-stage 11 through a Z/tilt drive unit
10. The Z/tilt drive unit comprises, in this embodiment, three
adjustable supports each independently extendible/contractible in
the Z-direction and serves to make any necessary corrections in the
position of the wafer 6 in the direction of the optical axis AX of
the projection optical system 3 (i.e., the Z-direction) as well as
in the tilt of the wafer 6. That is, the material support 29 is
supported by the Z/tilt drive unit 10 which, in turn, is mounted on
the X-stage 11. Further, the X-stage 11 is carried on a Y-stage 12,
which, in turn, is carried on a wafer base 14. The X-stage 11 and
the Y-stage 12 are arranged to be capable of displacement in the X-
and Y-directions, respectively, through a wafer stage drive system
(not shown). A movable mirror 13b is mounted on and fixed to the
material support 29 along a first one of four side edges thereof,
while a stationary laser interferometer 17b associated therewith is
disposed facing to the reflective surface of the movable mirror
13b. The movable mirror 13b and the laser interferometer 17b
cooperate to detect the X-coordinate and the rotational angle of
the material support 29. Similarly, a second movable mirror 13a is
mounted on and fixed to the material support 29 along a second one
of the side edges and extending perpendicular to the first movable
mirror 13b, while a second stationary laser interferometer 17a
associated therewith is disposed facing to the reflective surface
of the movable mirror 13a. The movable mirror 13a and the laser
interferometer 17a cooperate to detect the Y-coordinate of the
material support 29. The coordinate system defined by the
coordinates (X, Y) measured through the laser interferometers 17a
and 17b is called "the coordinate system of the wafer stage" (or
more simply "the stage coordinate system") and expressed by (X,
Y).
[0062] The interferometers 17a and 17b produce data signals
indicative of the measured values and supply them to the stage
control system 16, which uses the supplied data signals so as to
control the wafer stage drive system. The stage control system 16
also supplies the data signals indicative of the measured values
from the laser interferometers 17a and 17b to the central control
system 18, which uses the supplied data signals so as to control
the stage control system 16. A wafer transfer unit 39 (see FIG.
5(a)) is disposed adjacent to the wafer stage, for transferring a
wafer to and from the wafer stage. The wafer stage includes therein
a wafer transfer mechanism, which will be described later in
detail.
[0063] The projection exposure apparatus is further provided with
three alignment sensor systems for establishing alignment between a
reticle 1 and a wafer 6. One is a TTL-type alignment sensor system
4, and the remaining two are FIA-type alignment sensor systems 5A
and 5B. The alignment sensor 4 incorporates an LSA-type alignment
sensor and an LIA-type alignment sensor disposed in parallel which
are selectively used depending on the required alignment accuracy.
When the alignment process is carried out, one of the three
alignment sensor systems 4, 5A and 5B is used to detect the
position of a certain feature formed on a wafer 6, which may be an
alignment mark or a selected pattern on the wafer, and the detected
position is used to establish alignment between the pattern formed
in each shot area on the wafer 6 during the previous layer forming
process and the pattern formed on the reticle. The detection
signals from any of the alignment sensor systems 4, 5 and 5B are
processed by an alignment control system 15, which is controlled by
the central control system 18. A reference mark bearing plate 43 is
fixedly mounted on the material support 29 and has a top surface
thereof which will be level with the surface of the wafer loaded on
the wafer holder. The reference mark bearing plate 43 has reference
marks formed on its top surface and providing the reference for
alignment operation.
[0064] In this manner, the stage control system 16 and the
alignment control system 15 are controlled by the central control
system 18, which also serves to generally control the operations in
the entire projection exposure apparatus in order to cause the
exposure operations to be carried out according to a given
sequence.
[0065] Further, there are provided three off-axis-type
two-dimensional image processing units 50, 51 and 52 disposed
adjacent to one end of the projection optical system 3 near the
wafer. The image processing units 50-52 serve to take an image of
the outer peripheral edges of a wafer when the wafer is transferred
to a loading position above the wafer holder 30 as described later.
The image processing units 50-52 produce picture signals to be
supplied to the alignment control system 15, which uses the
supplied picture signals to determine by calculation the lateral
shift errors and rotational position errors of the wafer in the
loading position. The positioning and the arrangement of the image
processing units 50-52 will be described later.
[0066] Referring next to FIGS. 5(a) and 5(b), the wafer transfer
system, as well as the wafer loading mechanism on the wafer stage
will be described. Here, the term "wafer stage" refers to the
arrangement comprising the wafer holder 30, the material support
29, the Z/tilt drive unit 10, the X-stage 11, the Y-stage 12 and
the wafer base 14.
[0067] FIG. 5(a) is a plan view showing the wafer transfer system
and a part of the wafer stage, and FIG. 5(b) is a front elevation
thereof. In FIGS. 5(a) and 5(b), a wafer transfer unit 39 is
disposed at a position higher than and to the -X-direction of the
wafer stage. The wafer transfer unit 39 comprises transfer arms 21
and 22 disposed along a line extending in the X-direction, a slider
23 for sliding the transfer arms 21 and 22 to a predetermined
position, and an arm drive system (not shown) for driving the
transfer arms 21 and 22. The slider 23 is separated form the
remaining portion of the projection exposure apparatus, so that any
vibrations introduced in the slider 23 for its operation may not be
transferred to the latter. Each of the transfer arms 21 and 22 has
a U-shaped flat plate having a top surface for receiving a wafer
thereon. The two transfer arms serve to unload an exposed wafer
from and load a new wafer on the wafer stage. Since the arrangement
of this type of wafer transfer unit is known, and will not be
described in more detail here for simplicity.
[0068] The transfer arms 21 and 22 are moved, in accordance with
instructions form a loader control unit 24, along the slider 23 to
a loading position, which is the position for loading a wafer onto
the wafer stage. Then, the transfer arm 22 unloads the previous,
exposed wafer 6A from the wafer stage. Then, the transfer arm 21
transfers the next-to-be-exposed wafer 6 onto the wafer stage and
place it on a lift support 38. FIG. 5(b) shows the exposed wafer 6A
carried on the transfer arm 22 on the slider 23 and the new wafer
just transferred from the transfer arm 21 onto the top end of the
lift support 38.
[0069] The lift support 38 is supported by a linear actuator 35 and
has three support pins 38a, 38b and 38c received in respective
through holes in the material support 29 and the wafer holder 30.
The linear actuator 35 is capable of producing a vertical
(Z-direction) displacement of the three support pins 38a-38c,
resulting in a vertical movement of the wafer placed thereon, so as
to enable transfer of a wafer to and from the transfer arms 21 and
22. More specifically, each of the support pins 38a-38c has at its
tip end a vacuum suction hole communicable with a vacuum source.
The support pins 38a-38c are lifted up/down such that their tip
ends are raised to an upper position in order to enable transfer of
a wafer to and from the transfer arms 21 and 22, while lowered to a
lower position below the top surface of the wafer holder 30 in
order to place a wafer on the wafer holder. The hold of a wafer by
vacuum suction provided by the tip ends of the support pins 38a-38c
effectively prevents any displacement of the wafer relative to the
tip ends upon lifting up/down of the lift support 38.
[0070] The linear actuator 35 is supported for rotation in the
XY-plane about its center axis 35Z, operationally connected with a
drive shaft 37 driven by a rotational drive system 36, and is
rotatable to any desired angular positions in accordance with
instructions from the central control system 18 controlling the
rotational drive system 36. The rotary system comprising the
rotation drive system 36, the drive shaft 37 and the linear
actuator 35 has a sufficient resolution in angular positioning of a
wafer. For example, it is capable of rotating the wafer 6 in either
of the clockwise and counterclockwise directions as viewed from
above with accuracy within 20 .mu.rad. The linear actuator 35, the
rotational drive unit 36 and the drive shaft 37 together compose
the drive mechanism for the lift support 38.
[0071] FIG. 5(c) shows a turntable 60 provided in the wafer
transfer system. In FIG. 5(c) the wafer 6 on the turntable 60 is
transferred to the lift support 38 through the wafer arm 21 of FIG.
5(b). There are disposed around the turntable 60 an eccentricity
sensor 61 comprising a light-emitting unit 61a and a
light-receiving unit 61b. The light-emitting unit 61a emits a light
beam having a slit-like cross section such that a part of the light
beam is blocked by the periphery of the wafer 6 and the remaining
part passes by the peripheral edge of the wafer 6. The
light-receiving unit 61b receives the passed-by part of light beam
and converting it to an electrical detection signal S1 to be
supplied to the central control system 18. The light-receiving unit
61b in this embodiment comprises a single photodiode.
Alternatively, the light-receiving unit 61b may comprise other
devices such as a line sensor to directly detect the position of
the peripheral edge of the wafer. It is assumed that the wafer 6 is
circular in shape with an flat region, or an orientation flat, FP
formed on its periphery as shown in FIG. 5(a).
[0072] In this arrangement, upon rotation of the turntable 60 with
the wafer 6 held thereon by vacuum suction, the amount of
protrusion of the peripheral edge of the wafer into the
eccentricity sensor 61 varies due to the eccentricity of the wafer
6 with respect to the turntable 60, as well as due to the existence
of the orientation flat. Accordingly, the detection signal S1 from
the light-receiving unit 61b, which is a function of the rotational
angle .phi. of the turn table 60, varies as a sinusoidal curve with
a local reduced level portion 62 corresponding to the orientation
flat. The central control system 18 determines, from the detection
signal S1 and the rotation angle .phi. of the turntable 60, i) the
rotation angle .phi..sub.F of the turntable 60 when the orientation
flat is centered to the eccentric sensor 61 and ii) the
eccentricity of the wafer 6 with respect to the turntable 60, and
then stop the rotation of the turntable 60 at a suitable position
so as to cause the orientation flat to face a desired direction.
Also, the central control system 18 uses the data indicative of the
determined eccentricity so as to adjust the position of the
material support 29 at which it can receive the wafer 6 which has
been transferred to the loading position.
[0073] Furthermore, the central control system 18 determines and
stores as digital data respective values of the detection signal S1
at four rotational angles .phi..sub.A, .phi..sub.B, .phi..sub.C and
.phi..sub.D, the first three corresponding to the measurement
points for the three image processing units 50, 51 and 52,
respectively, and the fourth to another predetermined angular
position. In this relation, it is possible that although it is
desired for the specifications of the exposure apparatus to measure
the position of the edge of the wafer 6 at measurement points
corresponding to the rotational angles .phi..sub.A, .phi..sub.B and
.phi..sub.C, it is difficult to dispose the image processing unit
52 at the position corresponding to the rotational angle
.phi..sub.C and it has to be disposed at the position corresponding
to the rotational angle .phi.0.sub.D. Further, it is even possible
that not three but only two image processing units can be disposed.
In the former case, the central control system 18 estimates the
measurement value at the measurement point corresponding to the
rotational angle .phi..sub.C, from i) the values of the detection
signal S1 at the rotational angles .phi..sub.A, .phi..sub.B,
.phi..sub.C and .phi..sub.D, and ii) the measurement values of the
positions of the peripheral edge of the wafer 6 at the measurement
points corresponding to the rotational angles .phi..sub.A,
.phi..sub.B and .phi..sub.D, and the estimated value is used to
determine the horizontal (translational) errors and the rotational
error of the wafer 6. In the latter case, the central control
system 18 estimates the measurement values at the measurement
points corresponding to the rotational angles .phi..sub.A and
.phi..sub.C, for example, from i) the values of the detection
signal S1 at the rotational angles .phi..sub.A, .phi..sub.B,
.phi..sub.C and .phi..sub.D and ii) the measurement values of the
positions of the peripheral edge of the wafer 6 at the measurement
points corresponding to the rotational angles .phi..sub.B and
.phi..sub.D, and the estimated values are used to determine the
hori-zontal (translational) errors and the rotational error of the
wafer 6.
[0074] In general, the above position determination method is used
to obtain the matching between such exposure apparatus using this
method and another exposure apparatus which is provided with a
mechanical prealignment arrangement using alignment pins and whose
mechanical reference positions corresponds to the rotational angles
.phi..sub.A, .phi..sub.B, .phi..sub.C and .phi..sub.D. However, the
exposure apparatus shown and described is capable of measuring the
positions of the edge of the wafer 6 by two-dimensional image
processing units 50-52 at the measurement points which correspond
to measurement points of a contact-prealignment mechanism in
another exposure apparatus, so that if the exposure apparatus has
to use the eccentricity sensor 61 shown in FIG. 5(c) in order to
obtain the matching with the contact-prealignment mechanism in that
another exposure apparatus, it is mainly under the situation that
the observation fields of the image processing units 50-52 can not
be located to cover the regions corresponding to regions about the
pins of the contact-prealignment mechanism.
[0075] Next, the arrangement and construction of the image
processing units 50-52 will be described in detail.
[0076] FIG. 6(a) shows a wafer 6 located at the loading position.
As shown, the three image processing units 50, 51 and 52 have
respective observation or viewing fields 50a, 51a and 52a located
at respective positions on the peripheral edge of the wafer 6. Note
that although the actual shape of the area which is actually
subjected to the image processing (i.e., observation field) is
rectangular, these observation fields are illustrated in FIGS. 6(a)
and 6(b) as circular areas for convenience of explanation. Among
these three observation fields, two observation fields 50a and 51a
are located on the orientation flat FP and the third observation
field 52a is located on the circular peripheral edge of the wafer.
By detecting the positions of the peripheral edge of the wafer 6 at
the three measurement points, the detection of the translational or
positional errors in the X- and Y-directions (horizontal offsets)
and the rotational error of the wafer 6 (i.e., the detection
required for the prealignment) is quickly performed when the wafer
6 is loaded.
[0077] Any detected translational errors in the X- and Y-directions
are corrected by adjusting the detection positions for a search
alignment operation described later, which is performed after the
wafer 6 has been placed and held on the wafer holder 30. Any
detected rotational error is corrected by rotating the lift support
38 through the rotational drive system 36 when the lift support 38
is lowered, and before the wafer 6 held on the lift support 38
contacts with the wafer holder 30.
[0078] Some types of wafers have no orientation flat, but instead
have a V-notch NP formed in their circular peripheral edge, such as
a wafer 6N shown in FIG. 6(b). For this wafer 6N, the three
observation fields 50a, 51a and 52a may be located such that one
observation field 51a covers the region of the V-notch NP and the
remaining two observation fields 50a and 52a cover respective
regions on the circular peripheral edge. By using this location of
the observation fields, the horizontal offsets (translational or
positional errors) and the rotational error of the wafer 6N having
the V-notch NP may be quickly detected when the wafer 6N is
loaded.
[0079] FIG. 7 shows an exemplified structure for the image
processing unit 50. As shown, the image processing unit 50 includes
a light source 58, such as an electric lamp or a light-emitting
diode, which emits light of a wavelength falling in a range within
which the photoresist coated on the wafer has little sensitivity.
The emitted light is collected into one end of a length of light
guide 57, emitted from the other end of the light guide 57 into a
collimator lens, reflected by a half-silvered prism 54, and
illuminated through an objective lens 53 onto an region on the
peripheral edge of the wafer 6 as placed on the tip ends of the
three support pins 38a-38c and located at the larding position. The
reflected light passes through the objective lens 53, the
half-silvered prism 54 and an image-forming lens 55 to strike a
photosensitive surface of an image pick-up device 59 such as a
two-dimensional charge-coupled device (CCD) to form an image of the
edge region on the photosensitive surface of the image pick-up
device 59. The image pick-up device 59 produces a picture signal to
be supplied to the alignment control system 15, which is arranged
to determine the position of the detected edge of the wafer 6 from
the picture signal.
[0080] FIG. 13 shows another exemplified structure for the image
processing unit 50. In the structure of FIG. 13, a light source
(not shown), such as an electric lamp or a light-emitting diode,
emits light of a wavelength falling in a range within which the
photoresist coated on the wafer has little sensitivity. The emitted
light is collected into one end of a length of light guide 72,
emitted from the other of the light guide 72 as an illumination
beam. The illumination beam is reflected by a deflection mirror 73
to pass through an aperture 75 formed in the top plate of a
material support 29A, and thereby directed upwardly. A wafer holder
80A mounted on the material support 29A has a cutout 74 allowing
the illumination beam passed through the aperture 75 to pass
through it. The illumination beam passed through the aperture 75
and the cutout 74 is illuminated onto a region on the peripheral
edge of the wafer 6 as placed on the tip ends of the three support
pins 38a-38c and located at the larding position. A part of the
illumination beam passes by the edge region and further passes
through an objective lens 53A and an image-forming lens 55A to
strike a photosensitive surface of an image pick-up device 59 such
as a two-dimensional charge-coupled device (CCD) to form an image
of the edge region on the photosensitive surface of the image
pick-up device 59. The image pick-up device 59 produces a picture
signal to be supplied to the alignment control system 15, which is
arranged to determine the position of the detected edge of the
wafer 6 from the picture signal.
[0081] In either of the structures of FIGS. 7 and 13 for the image
processing unit, the peripheral edge of the wafer 6 is slightly
deflected downwardly (i.e., to the -Z-direction) as indicated by
the phantom lines in FIG. 7 with exaggeration. Also, the inevitable
variation in the thickness of the wafers provides variation in the
deflection, so that the image-forming optical system comprising the
objective lens 53 and the image-forming lens 55 must be a
telecentric optical system having a numerical aperture (NA) small
enough to ensure a sufficient depth of field. Because the depth of
field is substantially proportional to .lambda./NA.sup.2 where
.lambda. is the wavelength of the light of the illumination beam,
smaller numerical aperture provides greater depth of field,
resulting in that even an edge region with a maximum possible
deflection may be detected with precision. For example, with the
wavelength .lambda. of the light of the illumination beam being
0.633 .mu.m, the numerical aperture NA of the order of 0.03
provides a depth of field greater than 0.5 mm and a resolution of
the order of 0.5 mm. Generally, detection limit is about one tenth
({fraction (1/10)}) of resolution, so that the above resolution
provides a detection limit of the order of 2 .mu.m, which enables
precision alignment.
[0082] In FIG. 6(a), if it is not needed to achieve close matching
with a prealignment mechanism of contact type in another exposure
apparatus, the two-dimensional image processing is not necessarily
required for detecting positions on the circular peripheral edge of
the wafer or on the edge of the orientation flat NP, but a
one-dimensional image pick-up device such as a line sensor disposed
such that its measuring direction is normal to the corresponding
edge portion of the wafer, or a camera tube for industrial
television (ITV) disposed such that its scanning direction is
normal to the corresponding edge portion of the wafer may be used,
and the image or picture signal from such device may be processed
to detect the position of the edge. This may be, because the
detected position of the circular peripheral edge or the edge of
the orientation flat NP is inherently of one-dimensional. For
example, for the wafer 6 shown in FIG. 6(a), the translational
errors in the X- and Y-directions and the rotational error of the
wafer may be determined from the detected one-dimensional positions
at the three measurement points.
[0083] On the other hand, if it is needed to achieve close matching
with a prealignment mechanism of contact type in another exposure
apparatus and line sensors are used in the image processing units
for detecting the peripheral edge of the wafer, if accuracy of
transfer of the wafer to the loading position is low, the matching
error becomes unacceptably large since the peripheral edge of the
wafer is out of the detection area of the line sensor. This will be
described in more detail below with reference to FIGS. 15(a)
through 15(d).
[0084] FIG. 15(a) shows a wafer 6 having an orientation flat FP and
respective linear observation or viewing fields (detection areas)
50a1, 51a1 and 52a1 of three line sensors, in which two observation
fields 50a1 and 51a1 are located on the orientation flat FP and the
third observation field 52a1 is located on the circular peripheral
edge of the wafer 6. FIG. 15(c) shows one of the observation
fields, 50a1, in an enlarged plan view. As shown in FIG. 15(c), it
is assumed that the observation field 50a1 located on the
peripheral edge 6a of the wafer 6 is at a position away from the
measurement point A to the non-measuring-direction (i.e., the
horizontal direction perpendicular to the measuring direction of
the line sensor). The measurement point A is a point at which a
positioning reference pin 81A of a prealignment mechanism of
contact type (which is pro-vided in another exposure apparatus used
for the previous exposure process) has once contacted with the edge
of the wafer in the course of the previous exposure process.
Accordingly, a broken line 82A passing through the measurement
point A and extending parallel to the measuring direction indicates
a matching position. When the observation field 50a1 is offset from
this matching position as shown in FIG. 15(c), a substantial error
(matching error) .DELTA.1 may possibly occur between the positions
of the measurement point 83A in the observation field 50a1 and the
measurement point A due to the irregularity in the shape of the
peripheral edge 6a of the wafer 6.
[0085] In such case, the matching error may be corrected, as
previously mentioned, by using values of eccentricity of the wafer
at predetermined rotational angles .phi..sub.A, .phi..sub.B,
.phi..sub.C and .phi..sub.D, which can be determined as shown in
FIG. 5(d) by means of the turntable 60 and the eccentricity sensor
61 of FIG. 5(c).
[0086] Alternatively, the matching error may be reduced by widening
the width (or dimension in the non-measuring-direction) of the
observation field (or detection area) 50a1 of the line sensor. For
this purpose, a line sensor is used whose observation field 50a1'
is wide enough to cover a range in the non-measuring-direction in
which the measurement point A (at which the positioning pin 81A has
once contacted with the edge of the wafer) may fall, as shown in
FIG. 15(c). Since the image in the observation field 50a1' is
optically or electrically integrated in the
non-measuring-direction, the detection signal from the line sensor
will indicate that the position of the peripheral edge 6a of the
wafer 6 for the observation field 50a1' is at the position
designated by the average line 84A. The distance between the
average line 84A and the measurement point A is smaller than the
distance between the measurement point 83A for the slit-like
observation field 50a1 and the measurement point A, so that the
matching error .DELTA.2 corresponding to the former distance is
smaller than the above-mentioned matching error .DELTA.1
corresponding to the latter distance. In summary, the matching
error between the alignment sensor system and the prealignment
mechanism of contact type may be reduced by widening the
observation field (or detection area) of the line sensor in the
non-measuring-direction to provide averaging effect.
[0087] The matching error may be also reduced by using the
two-dimensional image processing units 50-52 in place of the line
sensors, as with the arrangement of FIG. 4.
[0088] FIG. 15(d) shows the observation or viewing field of the
two-dimensional image processing unit for the same wafer edge as
shown in FIG. 15(c). As shown in FIG. 15(d), the image processing
unit is arranged such that pixels constituting the image in the
observation field are scanned in the Y-direction (or
measuring-direction). The average line 84A' of the peripheral edge
6a of the wafer 6 in the observation field is determined, and the
position of the average line 84A' (or average position) is
determined. Similarly, an average position of the peripheral edge
6a of the wafer 6 is determined at each of the other two
measurement positions. Then, the position of the wafer 6 in the
X-direction, the position of the wafer 6 in the Y-direction and the
rotational angle of the wafer 6 are determined from the average
positions at the three measurement sites. Then, the positions and
the rotational angle of the wafer 6 are used to identify, from
among lines of pixels, one line 85 of pixels which includes a line
82A passing through the measurement point A (with which matching
must be established) and extending in the Y-direction. The sequence
of picture signals corresponding to the identified line 85 of
pixels are used to determine the position of the peripheral edge 6a
of the wafer 6 at a measurement point substantially coincident with
the measurement point A for the prealignment mechanism of contact
type. In this manner, the matching error may be reduced.
[0089] For wafers having one or more orientation flats and no
notch, such as the wafer 6 with the orientation flat FP as
described above, a linear (or one-dimensional) image pick-up device
may be used to conduct the position detection of the wafer edge
even at locations along the orientation flat FP. However, For
wafers having one or more notches, such as the wafer 6N with the
notch NP shown in FIG. 6(b), the position detection of the wafer
edge at notch NP has to be conducted in both the X- and
Y-directions, and thereby requires a two-dimensional image
processing unit. The position detection at the notch NP will be
described later with reference to FIGS. 8(a) and 8(b).
[0090] FIG. 8(a) is an enlarged plan view of the wafer 6N showing
the notch NP. As shown, as a conventional way of positioning the
wafer 6N with respect to the wafer holder, a cylindrical reference
pin with a predetermined diameter d is used and two edges in the
notch NP are engaged with the pin and pressed against the side
surface of the pin. Thus, the geometry of the notch NP is selected
in accordance with the geometry of the reference pin. For ensuring
compatibility with this method, we use the data indicative of the
image in the observation field 63, which is an area on the notch NP
and conjugate to the photosensitive surface of the image-sensing
device, so as to determine an imaginary reference pin 64 having a
diameter d and to be engaged with the two edges in the notch NP,
and then determine the X- and Y-coordinates of the center O of the
imaginary reference pin 64.
[0091] FIG. 8(b) illustrates alternative method of position
detection at the location of the notch NP, in which the data
indicative of the image in the observation field 63 is used to
determine the coordinates of the point P at which two edges 65A and
65B of the notch NP meet each other and the coordinates of the
point 65C at which one edge 65B and the circular peripheral edge of
the wafer meet each other. Then, the coordinates of the point 65D
at which the other edge 65A and the circular peripheral edge of the
wafer meet each other are determined based on the assumption that
the notch NP is symmetrical in shape. Now we have obtained the
triangle defined by the three points P, 65A and 65B. Then we
consider a line extending parallel to the line 65C-65D and
intersecting edges 65A and 65B, and the distance between the
intersections equal to the diameter d of the imaginary reference
pin. Then we take the midpoint between the intersections as a
center O, and the X- and Y-coordinates of the center O is
determined.
[0092] Referring next to FIGS. 14(a) and 14(b), still further forms
of wafers and detections systems suitable for them will be
described. With respect to variations of wafers with a notch, a
wafer 6M may have either a notch NP1 at a position of six o'clock
or a notch NP2 at a position of three o'clock as shown in FIG.
14(a), so that the detection system must be capable of precision
detection of each of these notches. For the purpose, as shown in
FIG. 14(a), a wafer holder 30B underlying the wafer 6M has two
cutouts 30Ba and 30Bb allowing illumination beams to pass
therethrough to illuminate the notches NP1 and NP2, respectively,
as well as three cutouts 30Bc, 30Bd and 30Be allowing illumination
beams to pass therethrough to illuminate locations of reference
pins generally used for mechanical prealignment process. The latter
three cutouts 30Bc-30Be are illuminated from the bottom side by
means of respective image processing units of FIG. 13 described
above. Where the wafer 6M has the notch NP1 at the position of six
o'clock, the position and the rotation angle of the wafer is
detected by means of a two-dimensional image processing system
having a circular observation field 51a2 and first and second line
sensors having linear observation fields 50a2 and 52a1,
respectively. On the other hand, where the wafer 6M has the notch
NP2 at the position of three o'clock, the position and the
rotational angle of the wafer is detected by means of another
two-dimensional image processing system having another circular
observation field 51a1 and second and third line sensors having
linear observation fields 52a1 and 50a1, respectively. That is,
this arrangement has five sensors including two-dimensional image
processing systems and three line sensors, and is capable of fine
alignment and prealignment for both of the above two types of
wafers with a notch.
[0093] With respect to variations of wafers with an orientation
flat, a wafer 6A may have either an orientation flat FP1 at the
position of six o'clock or an orientation flat FP2 at the position
of three o'clock as shown in FIG. 14(b), so that the detection
system must be capable of precision detection of each of these
orientation flats. For the purpose, as shown in FIG. 14(b), a wafer
holder 30C underlying the wafer 6A has three cutouts 30Ca, 30Cb and
30Cc corresponding to one orientation flat FP1 and three cutouts
30Cd, 30Ce and 30Cf corresponding to the other orientation flat FP2
formed therein. These six cutouts 30Ca-30Cf are used for
illumination from the bottom side by means of respective image
processing units shown in FIG. 13 described above. Where the wafer
6A has the orientation flat FP2 at the position of three o'clock,
the position and the rotation angle of the wafer is detected by
means of three line sensors having observation fields 52a2, 51a2
and 50a2, respectively. On the other hand, where the wafer 6A has
the orientation flat FP1 at the position of six o'clock, the
position and the rotational angle of the wafer is detected by means
of further three line sensors having linear observation fields
52a1, 51a1 and 50a1, respectively. This arrangement may be used in
the case where optical detection systems may be disposed above the
reference pins of a mechanical prealignment system on the wafer
stage. However it is difficult to use such arrangement, position
measurement results obtained by means of reference pins may be
replaced by the determination of the contour of the wafer using the
determination results on the turntable 60 shown in FIG. 5(c) as
described above.
[0094] Referring next to FIGS. 16(a)-16(d) and 17(a)-17(d), an
exemplified sequence of operations for conducting the detection of
the position and rotational angle of the wafer while making
matching with the prealignment mechanism of contact type, by using
the three two-dimensional image processing units 50-52. Each of the
image processing units 50-52 is of the transmitting illumination
type, such as shown in FIG. 13.
[0095] FIG. 16(a) shows a wafer 6N having a notch NP at a position
of six o'clock which has been just transferred to the loading
position. FIG. 16(a) shows the observation or viewing fields 86A,
86B and 86C of the three image processing units 50, 51 and 52,
respectively, (the images in the observation fields may be
subjected to the actual image processing). FIG. 16(a) also shows
imaginary pins 82A, 82B and 82C (indicated by imaginary line
circles) corresponding to the reference pins in the prealignment
mechanism of contact type which is equipped in an exposure
apparatus used for the previous exposure process, for example. It
is noted that, for clarity of the figure, the imaginary pins
82A-82C are illustrated enlarged and extracted to the outside of
the respective observation fields 86A-86C. Those points on the
sides of the imaginary pins 82A and 82B in the observation fields
86A and 86B on the circular peripheral edge of the wafer 6N, at
which the peripheral edge of a wafer should contact with the
imaginary pins 82A and 82B after prealignment, are referred to as
imaginary points A and B, respectively. Further, the center of the
imaginary pin 82C in the observation field 86C on the notch NP is
referred to as an imaginary point E.
[0096] In order to determine the positions of the imaginary points
A, B and E, a reference wafer having reference marks formed thereon
which indicate the imaginary points may be used, for example. The
reference wafer is loaded on the wafer holder 30 of the wafer stage
of FIG. 4, and the wafer stage is driven based on the measured
values provided from the laser interferometers 17a and 17b, with
the design positions of the three reference pins of the
contact-prealignment mechanism being used as the target values,
such that the reference marks may be sequentially moved into the
respective observation fields 86A-86C. Then, the positions of the
reference marks in the respective observation fields 86A-86C are
detected by the corresponding image processing units 50-52 and
stored. In this manner, the imaginary pints A, B and E have been
determined. The reference wafer may comprise a wafer with suitable
notches formed in the peripheral edge thereof, or a glass wafer
having suitable patterns formed thereon.
[0097] As previously described with reference to FIG. 5(b), the
lift support 38 is mounted for rotation about its center axis 35Z
by means of the rotational drive system 36. When the lift support
38 is rotated, the position of the imaginary points A, B and E,
which are the points at which the wafer edge should contact with
the imaginary pins 82A-82C, respectively, are moved within the
observation fields 86A-86C of FIG. 16(a). Therefore, we can
determine the coordinates of the rotation center 0' of the lift
support 38 in the stage coordinate system, by measuring the
positions of the imaginary points A, B and E on the reference wafer
which is held by vacuum suction and fixed to the lift support 38,
then rotating the lift support 38 by a predetermined angle, then
again measuring the positions of the imaginary points A, B and E,
and using differences in the measured positions for respective
imaginary points so as to determine the coordinates of the rotation
center O'. Thereafter, a new coordinate system (X, Y; O') is used
which is the same as the stage coordinate system (X, Y) (which is
the coordinate system fixed to the wafer stage and represented by
the measured values from the laser interferometers 17a and 17b)
translated such that the rotation center O' is taken as the origin
thereof. That is, the coordinates of the rotation center O' in the
new coordinate system (X, Y; 0') are (0, 0).
[0098] It is assumed that the imaginary points A and B lie on a
line parallel to the X-axis. Then, variable L is put equal to the
distance in the X-direction between the imaginary points A and B.
Further, the coordinates of the imaginary points A, B and E in the
new coordinate system (X, Y; O') are represented by (x.sub.1, a),
(x.sub.2, a) and ((x.sub.1+x.sub.2)/2, b), respectively, and the
relative coordinates of the imaginary points A, B and E in the
respective observation fields 86A, 86B and 86C are stored. Using y
as the value of the Y-coordinate in the new coordinate system (X,
Y; O), we can express that the imaginary points A and B lie on the
straight line y=a, as shown in FIG. 16(a). Further, the following
equation holds:
x.sub.2-x.sub.1=L
[0099] Because of the illumination by light beams from the bottom
side of the wafer 6N, the images observed in the observation fields
86A, 86B and 86C are seen as shown in FIGS. 16(b), 16(c) and 16(d),
respectively, wherein each image has a dark region (hatched in the
figure) corresponding to the peripheral edge portion of the wafer
and a bright region corresponding to the outside space of the
wafer, so that the peripheral edge of the wafer 6N can be detected
as the edge in the image dividing the dark and bright regions.
[0100] Next, we will describe how to detect the position and the
rotational angle of an actual wafer 6N when the wafer 6N has been
brought to the loading position as shown in FIG. 16(a). With
respect to an image in each of the two observation fields 86A and
86B on the circular peripheral edge of the wafer 6N, the peripheral
edge of the wafer corresponding to the edge in the image dividing
the bright region Bi and the dark region Dk has a shape of a
substantially straight line, as shown in each of FIGS. 16(b) and
17(c). Thus, using x and y for representing the values of the X-
and Y-coordinates in the new coordinate system (X, Y; O'), the
edges in the images in the observation fields 86A and 86B are
expressed by the functions y=f(x) and y=g(x), respectively. For the
purpose, the functions f(x) and g(x) may be approximated by
respective linear functions each including two factors, and
least-squares method may be applied to the data representing the
points on the detected edge in the observation fields 86A and 86B
so as to determine the two factors in each function. Alternatively,
the functions f(x) and g(x) may be approximated by respective
segments of a function of circle
(x-x.sub.0).sup.2+(y-y.sub.0).sup.2=r.sup.2, and least-squares
method may be used so as to determine the factors in this function,
since the peripheral edge of the wafer is substantially circular in
shape.
[0101] There is a measurement point C on the line represented by
one function y=f(x), whose coordinates in the new coordinate system
(X, Y; O') are represented by (x.sub.3, y.sub.3). There is a
measurement point D on the line represented by the other function
y=g(x), whose coordinates in the new coordinate system (X, Y; O')
are represented by (x.sub.4, y.sub.4). Then, the values of x.sub.3,
y.sub.3, x.sub.4 and y.sub.4 are determined such that they meet the
following two conditions: i) a straight line passing through both
the measurement points C and D should extend parallel to the
X-axis; and ii) the distance in the X-direction between the
measurement pints C and D should equal the above mentioned distance
L. This means that the values of x.sub.3 and x.sub.4 can be
determined by solving the following simultaneous equations:
g(x.sub.4)=f(x.sub.3),
x.sub.4-x.sub.3=L
[0102] Then, using the determined values of x.sub.3 and x.sub.4,
the values of the associated Y-coordinates y.sub.3 and y.sub.4 can
be determined as:
y.sub.3=f(x.sub.3)
y.sub.4=g(x.sub.4)
[0103] Further, we consider a straight line passing through a
midpoint of the line between the measurement points C and D and
extending parallel to the Y-axis. Since the midpoint has its
X-coordinate equal to (x.sub.4+x.sub.3)/2, the straight line is
represented by a function x=(x.sub.4+x.sub.3)/2. Because the
measurement points C and D are determined to meet the above
conditions, this straight line x=(x.sub.4+x.sub.3)/2 passes through
the center O of the wafer 6N. Therefore, the point of intersection
between the straight line x=(x.sub.4+x.sub.3)/2 and a straight line
passing through the measurement point C and perpendicular to the
straight line y=f(x) will coincide with the center O of the wafer.
Then, if we obtain the coordinates of the intersection (and hence
the center O of the wafer) and the distance between the center O
and the measurement point C (the radius r from the center O to the
measurement point C is obtained). Similarly, the point of
intersection between the straight line x=(x.sub.4+x.sub.3)/2 and a
straight line passing through the measurement point D and
perpendicular to the straight line y=g(x) will coincide with the
center O of the wafer. Then, if we obtain the coordinates of the
intersection (and hence the center O of the wafer) and the distance
between the center O and the measurement point D, the radius r'
from the center O to the measurement point D is obtained.
Alternatively, if the functions f(x) and g(x) are approximated not
by linear functions but by segments of a function of circle, the
straight lines which are used to intersect the straight line
x=(x.sub.4+x.sub.3)/2 at the center O would be lines perpendicular
to the tangents to the circle at the measurement points C and
D.
[0104] Further, the coordinates of the center of the reference pin
when the corresponding reference pin was pressed against and
engaged with the edges of the notch NP have to be determined from
the picture data obtained from the observation field 86C for the
notch NP. For the purpose, first, the distance LM between two
imaginary contact points of the imaginary pin 82C with the edges of
the notch NP is determined. Then, least-squares method is applied
to the picture data so as to obtain linear functions y=h(x) and
y=i(x) approximating the left and right edges of the notch NP,
respectively. The center of the imaginary pin 82C when it were
pressed against and engaged with the edges of the notch NP is
treated as a measurement point F, whose coordinates in the new
coordinate system (X, Y; O') are represented by (x.sub.5,
y.sub.5).
[0105] Then, the coordinate (x.sub.5, y.sub.5) is determined such
that the distance between tow points at which a straight line
passing through the measurement point F and parallel to the X-axis
intersects the straight lines represented by the linear functions
y=h(x) and y=i(x), respectively, is determined as LM; and the
measurement point F coincide the midpoint of the straight line
between the two points of intersection. For the purpose, first, the
values of the X-coordinates x.sub.51 and x.sub.52 of the two points
of intersection are determined by solving the following
simultaneous equations:
h(x.sub.51)=i(x.sub.52),
x.sub.52-x.sub.51=LM
[0106] Then, using the determined values of x.sub.51 and x.sub.52,
the values of x.sub.5 and y.sub.5 can be determined as: 1 x 5 = ( x
51 + x 21 ) / 2 , y 5 = h ( x 51 )
[0107] Next, in FIG. 16(a), the X-coordinate of a straight line
which i) is a segment of the straight line passing through the
midpoint of the line between the measuring points C and D (the line
x=(x.sub.3+x.sub.4)/2) and ii) crosses the observation field 86C,
is obtained as the X-coordinate of the imaginary point E (i.e.,
(x.sub.1+x.sub.2)/2), so that the rotational error .DELTA..theta.
of the wafer 6N with respect to the center O of the wafer 6N is
determined from the distance of the measurement point F on the
notch NP from the imaginary point E found in the observation field
86C, according to the following equation.
.DELTA..theta.={x.sub.5-(x.sub.1+x.sub.2)/2}/(r+r')
[0108] It is noted that this is the rotational error with respect
to the center O of the wafer 6N. Actually, the wafer 6N is rotated
about the center of rotation O' (0, 0) of the lift support 38.
Irrespective of the center of rotation, O or O', the amount of
rotational error .DELTA..theta. is common, so that it can be
accurately corrected by rotating the wafer 6N about the rotation
center O', which, however, introduces translational errors to the
measured values at each of the measurement points C, D and F.
Accordingly, calculations are made for the rotational error
.DELTA..theta. with respect to the new coordinate system (X, Y;
O'). Specifically, in FIG. 16(a), assuming that the wafer 6N has
been rotated about the rotation center O' by the angle to
compensate for the rotational error .DELTA..theta., and the
functions for the straight lines y=f(x) and y=g(x) have been
transformed by the rotation into functions y=f'(x) and y=g'(x),
respectively, calculations are made to determine the transformed
functions f'(x) and g'(x).
[0109] Since the rotational error .DELTA..theta. has been already
corrected, the calculations may be made in the manner similar to
that in the process for determining the functions f(x) and g(x).
That is, the coordinates of a measurement point on the straight
line y=f'(x) are represented by C' (x.sub.3', y.sub.3') and the
coordinates of a measurement point on the straight line y=g'(x) are
represented by D' (x.sub.4', y.sub.4'). Then, the coordinates
x.sub.3' and x.sub.4' are determined such that the measurement
points C' and D' are on a straight line parallel to the X-axis and
the distance between the measurement points C' and D' equals L.
Therefore, the coordinates x.sub.3' and x.sub.4' are determined by
resolving the following simultaneous equations: 2 g ' ( X 4 ' ) = f
' ( x 3 ' ) , x 4 ' - x 3 ' = L
[0110] Then, y.sub.3' and y.sub.4', the values of the
Y-coordinates, are represented as f'(x.sub.3') and g'(x.sub.4'),
respectively. Using these values, the average differences
(.DELTA.X, .DELTA.Y) between the imaginary points A (x.sub.1, a)
and B (x.sub.2, a) and the rotation-error-corrected measurement
points C' (x.sub.3', y.sub.3') and D' (x.sub.4', y.sub.4') are
expressed as: 3 X = ( x 3 ' + x 4 ' - x 1 - x 2 ) / 2 , Y = ( y 3 '
+ y 4 ' - 2 a ) / 2 = ( f ' ( x 3 ' ) + g ' ( x 4 ' ) - 2 a ) /
2
[0111] The differences (.DELTA.X, .DELTA.Y) equal the translational
or positional errors of the wafer 6N in the X- and Y-directions
from the position of the wafer which would be found if the wafer 6N
undergoes the prealignment process using the prealignment mechanism
of contact type. In this embodiment, these translational errors are
treated as offsets for the search alignment, and corrected by
adjusting the displacements of the wafer stage in the course of the
search alignment process.
[0112] In the above example, it is assumed that there is relatively
large variations in the radius r (or r'); however, it may be
alternatively assumed that the radius r is constant because a
typical, maximum possible error in diameter of the wafers is as
small as within .+-.0.1 .mu.m.
[0113] If wafers have relatively great variations in diameter, the
contact points on the imaginary pins 82A and 82B at which a wafer
contact with them can no longer deemed to be constant, so that the
distance L between the imaginary points A and B can not be treated
as a constant distance. In such case, the contact point between the
edge of a particular wafer and the imaginary pins may be determined
from the slopes of the linear functions f(x) and g(x) (or the
tangents to a circle function if it is used) as determined from the
edges in the images in the observation fields 86A and 86B, and the
distance between the contact points may be used as the distance L.
Similarly, if wafers have relatively great variations in the shape
of the notch NP, the distance LM between the contact points between
the edges of a particular notch and the imaginary pin 82C may be
treated as a variable, while if such variations are relatively
small, the distance LM may be deemed as a constant. Further,
because the peripheral edge of a wafer is nominally circular in
shape, it may be convenient to approximate the peripheral edge by a
quadratic curve. Further, the above arithmetic operations are
usable when the light-receiving unit in the two-dimensional image
processing unit is disposed to be parallel to the line y=a. It may
be possible that the coordinate system on the light-receiving unit
has been rotated due to some reason, such as the error in the
positioning of the light-receiving unit or the limitations to the
space for the light-receiving unit. In such case, suitable
modifications may be made to the programs for the arithmetic
operations so as to make required corrections.
[0114] In this manner, the rotational error .DELTA..theta. and the
offsets .DELTA.X and .DELTA.Y of the wafer 6N may be determined
with precision with only one repetition of the sequence of
calculations for correction. Further, where the coordinates
x.sub.3' and y.sub.3' of the rotation-error-corrected measurement
points are determined following the determination of the
rotation-error-corrected functions f'(x) and g'(x), then, the
accuracy in the matching with the contact-prealignment mechanism
may be further improved by correcting the approximation errors
involved in the determination of the one-dimensional (or linear)
functions f'(x) and g'(x) such that the points at which the
imaginary pins are actually engaged with the wafer edge are used as
shown in FIG. 15(d).
[0115] FIG. 17(a) shows a wafer 6 having an orientation flat FP at
a position of six o'clock which has been just transferred to the
loading position. Again, FIG. 17(a) shows i) the observation or
viewing fields 86A, 86B and 86C of the three image processing units
50, 51 and 52, respectively, (the images in the observation fields
may be subjected to the actual image processing) and ii) imaginary
pins 82A, 82B and 82C (indicated by imaginary line circles)
corresponding to the reference pins in the prealignment mechanism
of contact type which is equipped in an exposure apparatus used for
the previous exposure process, for example. It is noted that, for
clarity of the figure, the imaginary pins 82A-82C are illustrated
enlarged and extracted to the outside of the respective observation
fields 86A-86C. Those points on the sides of the imaginary pins 82A
and 82B in the observation fields 86A and 86B located on the
orientation flat FP at which the peripheral edge of a wafer should
contact with the imaginary pins 82A and 82B after prealignment are
referred to as imaginary points A1 and B1, respectively. Similarly,
that point on the side of the imaginary pin 82C in the observation
field 86C on the circular peripheral edge at which the edge of a
wafer should contact with the imaginary pin 82C after prealignment
is referred to as an imaginary point E1.
[0116] In order to determine the positions of the imaginary points
A1, B1 and E1, a reference wafer having reference marks formed
thereon which indicate the imaginary points may be used as with the
previous example. In this example, again, the positions are
determined in terms of the coordinates in the stage coordinate
system of the rotation center O' of the lift support 38 of FIG.
5(b). Thereafter, a new coordinate system (X, Y; O') is used taking
the rotation center O' as its origin and having X-axis parallel to
the line passing through the imaginary points A1 and B1.
[0117] Then, variable L1 is put equal to the distance in the
X-direction between the imaginary points A1 and B1. Further, the
coordinates of the imaginary points A1, B1 and E1 in the new
coordinate system (X, Y; O') are represented by (-L1/2, c), (L1/2,
c) and (x.sub.8, y.sub.8), respectively, and the coordinates of the
imaginary points A1, B1 and E1 relative to the observation fields
86A, 86B and 86C are stored. Using y as the Y-coordinate in the new
coordinate system (X, Y; O'), it can be expressed that the
imaginary points A1 and B1 lie on the line y=c, as shown in FIG.
17(a).
[0118] Because of the illumination by light beams illuminated from
the bottom side of the wafer 6, the images observed in the
observation fields 86A, 86B and 86C are seen as shown in FIGS.
17(b), 17(c) and 17(d), respectively, wherein each image has a dark
region (hatched in the figure) corresponding to the peripheral
portion of the wafer and a bright region corresponding to the
outside space of the wafer, so that the peripheral edge of the
wafer 6 can be detected as the edge in the image dividing the dark
and bright regions.
[0119] Next, we will describe how to detect the position and the
rotational angle of an actual wafer 6 when the wafer 6 has been
brought to the loading position as shown in FIG. 17(a). With
respect to an image in each of the two observation fields on the
orientation flat FP, 86A and 86B, the peripheral edge of the wafer
corresponding to the edge in the image dividing the bright region
and the dark region has a shape of a substantially straight line
parallel to the X-axis. Then, measurement points C1 and D1 are
determined. The measurement point C1 is an intersection point at
which the edge of the orientation flat FP intersects a line passing
through the imaginary point A1 and perpendicular to that edge, and
which is represented by (-L1/2, y.sub.6). The measurement point D1
is an intersection point at which the edge of the orientation flat
FP intersects a line passing through the imaginary point B1 and
perpendicular to that edge, and which is represented by (L1/2,
y.sub.7). The detection accuracy may be improved by using the
average positions of the edge within predetermined ranges about the
measurement points C1 and D1, respectively, as the coordinates
y.sub.6 and y.sub.7.
[0120] Further, with respect to the observation field 86C, a
measurement point F1 is determined. The measurement point F1 is an
intersection point at which the circular peripheral edge of the
wafer 6 intersects a line passing through the imaginary point E1
and perpendicular to that peripheral edge, and which is represented
by (x.sub.9, y.sub.8). The coordinates (x.sub.9, y.sub.8) are
defined in the new coordinate system (X, Y; O'), and the peripheral
edge of the wafer 6 within the observation field 86C may be
represented by a straight line x=x.sub.9.
[0121] The distance between the two measurement points C1 and D1
will equal the distance L1, so that the rotational error
.DELTA..theta. of the wafer 6 is expressed as:
.DELTA..theta.=(y.sub.7-y.sub.6)/L1
[0122] Because the rotational error .DELTA..theta. of the wafer 6
is determined from the data obtained only the orientation flat FP,
the rotational error .DELTA..theta. can be corrected by making the
correction by rotating the lift support 38 about its rotation
center O', and thus it is unnecessary to know the center O of the
wafer 6. However, by rotating the wafer about the rotation axis O'
of the lift support 38, a corresponding offset is introduced in the
coordinates (x.sub.9, y.sub.8) of the measurement point F1.
Accordingly, the new coordinate system (X, Y; O') is rotated by
.DELTA..theta., and then, in the rotated coordinate system, points
C1', D1' and F1' are determined as the intersections between the
respective perpendiculars from the imaginary points A1, B1 and E1
to the corresponding edge portions of the wafer 6 and those edge
portions, and the coordinates (-L/2, y.sub.6'), (L/2, y.sub.7') and
(x.sub.9', y.sub.8) are determined by repeating the above
arithmetic operations. Thereafter, the translational or positional
errors (offsets) in the X- and Y-directions .DELTA.X and .DELTA.Y
are determined by calculations according to the following
equations: 4 X = x 9 ' - x 8 , Y = ( y 6 ' + y 7 ' ) / 2 - c
[0123] These offsets .DELTA.X and .DELTA.Y can be corrected in the
subsequent search alignment process by adjusting the wafer stage,
as with the example of FIGS. 16(a) to 16(d) described above.
Further, as described in connection with the detection of the
position for the wafer having a notch as shown in FIG. 16(a), if
there are errors between the positions of the edge of the wafer 6
as determined from the average value of the image data and the
positions of the actual contact points between the imaginary pins
and the edge of the wafer, such errors may be reduced by choosing
the data to be finally used, as shown in FIG. 15(d).
[0124] As described, in the embodiment described here, the
procedure includes the steps of detecting the peripheral edge of a
wafer, then correcting the rotational error of the wafer by means
of the rotatable lift support 38, and performing calculations in
order to obtain the offsets which will be introduced by the
rotation of the lift support 38, so as to eliminates another
detection process for checking the correction results. Thus, the
detection process need not be repeated, resulting in that the
noncontact-type wafer prealignment may be quickly performed.
[0125] Nevertheless, a limited positional offset and/or rotational
offset may be introduced in the position of the wafer when the lift
support 38 is rotated to place the wafer onto the wafer holder 30,
for some reason such as possible vibrations at starting the
vacuum-holding or possible failure of parallelism between the
surfaces of the wafer and the wafer holder 30. This may be
accommodated by detecting the difference between the edge position
of the wafer which is detected and stored with respect to the
reference wafer and the edge position of the actual wafer, by means
of an off-axis-type alignment sensor system, and the detected
difference is used as the system offset so as to add the system
offset to the determined results of the translational offsets
.DELTA.X and .DELTA.Y and the rotational offset .DELTA..theta.
during the search alignment process subsequent to the placement of
the wafer on the wafer holder 30.
[0126] Referring next to a flow chart divided into three pieces
shown in FIGS. 2, 3A and 3B, an exemplified sequence of operations
in the entire positioning process in the projection exposure
apparatus will be described.
[0127] To begin with step 101 in FIG. 2, a wafer 6 is held by
vacuum suction on and transferred by the transfer arm 21 along the
slider 23 of FIG. 5(b) onto the loading position, at which the
vacuum-holding by the transfer arm 21 is released and at the same
time the lift support 38 is raised by the linear actuator 35, and
the wafer is transferred onto the raised lift support 38. At the
same time, the vacuum-holding by the support pins 38a-38c of the
lift support 38 is activated (step 102). By this point of time, a
process for rough prealignment of the wafer 6 in the X-, Y- and
rotational (or .theta.-) directions has been established on an
outer contour basis by means of mechanisms including the turn table
60 of FIG. 5(c), resulting in that there may be errors in the X-
and Y-directions only within about 2 mm and a rotational error only
within about 5.degree. (5 degrees), in the positioning of the wafer
6 on the lift support 38.
[0128] For establishing this rough prealignment, the turntable 60
is used to rotate the wafer 6 to make a rough correction of any
rotational error of the wafer 6, and the loading position of the
transfer arm 21 for transferring the wafer 6 onto the lift support
38 is adjusted in the X- and Y-directions to make a rough
correction of any errors in the X- and Y-directions. Relatively
large translational and rotational errors may remain after this
rough prealignment because the mechanical unit including the
turntable 60, as well as the mechanical unit including the slider
23, are arranged to have no direct mechanical connection with the
remaining part (or main part) of the exposure apparatus in order to
prevent any vibrations from transmitting from the mechanical units
to the main part of the exposure apparatus. Thus, for example, the
main part of the exposure apparatus and the mechanical unit
including the slider 23 may shake independently with different
vibration modes resulting in a possible, large relative
displacement between them, which may cause a relatively large
positioning errors (transfer errors) upon transfer of the wafer
from the slider 23 to the lift support 38.
[0129] Thereafter, in step 103, the position detection of the
peripheral edge of the wafer 6 is carried out using the three
two-dimensional image processing units 50-52 of FIG. 4. In this
example, the position of the edge is measured within the two
observation fields 86A and 86B on the orientation flat FP of the
wafer 6 as well as within the observation field 86C on the circular
peripheral edge of the wafer 6. This position detection of the
wafer edge is carried out for the prealignment of the wafer. In a
typical, conventional prealignment process, three reference pins
mounted on a wafer holder have been used, and the edge of a wafer
is pressed against and engaged with the reference pins. Such
prealignment is referred to as the contact-type prealignment. In
contrast, the prealignment process of the present invention is of
noncontact-type.
[0130] The locations of the three observation fields 86A, 86B and
86C are selected to include therein the imaginary points A1, B1 and
E1, respectively, which are coincident with the contact points of
three reference pins at which they contact with a wafer, the three
reference pins being used in some other, particular exposure
apparatus for the contact-type prealignment. By virtue of this
selection of the locations of the observation field, an advantage
is obtained: when a previous photosensitive layer on a wafer (the
layer immediately below the layer which is going to be exposed) has
been exposed by using that particular exposure apparatus in which
contact-type prealignment is performed, a high degree of matching
between the prealignment processes may be obtained, and hence there
remain relatively small positioning errors after the prealignment.
As previously mentioned, when the locations of the observation
fields 86A-86C can not be selected to include the imaginary points
due to the interference by other sensors and devices disposed
around the projection optical system 3, the positions of the edge
of the wafer at the position of the reference pins may be
determined with precision from the measurement results of the
contour of the wafer obtained through the turntable 60 and the
eccentricity sensor 61 of FIG. 5(c).
[0131] Alternatively, if the wafer is of the type having a notch,
such as the wafer 6N with the notch NP as shown in FIG. 16(a),
matching with the contact-type prealignment may be obtained by
determining offsets between the peripheral edge of the wafer 6N and
each of the imaginary points A, B and E through the image
processing, as described with reference to FIGS. 16(a) to
16(d).
[0132] If the wafer has a notch and it in unnecessary to obtain
matching with any other exposure apparatus in which contact-type
prealignment is performed, then the method described above with
reference to FIG. 8(b) may be used, which includes obtaining
position data on all the points on the two edges 65A and 65B of the
notch NP and applying least-squares method to the position data so
as to obtain an imaginary notch shape (V-shape defined by two
approximated straight lines), and uses the point of intersection
between the two approximated straight lines as the notch detection
point. In this manner, the position detection of the wafer may be
achieved with precision irrespective of the error in the geometry
of the notch NP.
[0133] In the operation sequences described above with reference to
FIGS. 16 and 17, the positions of the imaginary points A, B and E,
as well as those of the imaginary points A1, B1 and E1, are
determined by using a reference wafer having reference marks formed
thereon. Alternatively, the positions of the imaginary points may
be determined by calculations, and several process sequences using
such determination method will be described below in detail.
[0134] [Process Sequence 1]
[0135] First, a process sequence for achieving prealignment of a
wafer 6 having an orientation flat FP will be described with
reference to schematic plan views of the wafer shown in FIGS. 19
and 20 as well as a flow chart shown in FIG. 21.
[0136] FIG. 19 is a schematic illustrating the relationship among
the positions of three fixed pins 91, 92 and 93 used in a typical,
conventional prealignment mechanism. As seen from FIG. 19, two of
the pins, 91 and 92, are in contact with the edge of the
orientation flat FP of the wafer 6 at contact points 91a and 92a,
respectively, while the remaining pin 93 is in contact with the
edge of the circular periphery of the wafer 6 at a contact point
93a. SL represents a straight line extending through the point 93a
and parallel to a line joining points 91a and 92a. CP represents a
point at which a line extending through the point 92a and
perpendicular to the line SL meets this line SL. Further, aa
represents the distance between the point 92a and the point CP, bb
represents the distance between the point 93a and the point CP, and
cc represents the distance between the point 91a and the point 92a.
The values of aa, bb and cc are design values which are
predetermined by the arrangement of the set of the three pins 91,
92 and 93.
[0137] FIG. 20 illustrates the relationship between (i) the
positions of the observation or viewing fields 86A, 86B and 86C
(which are fixed relative to the wafer stage) of the three image
processing units 50, 51 and 52, respectively, incorporated in the
projection exposure apparatus shown in FIG. 4 and (ii) the position
of the wafer 6 held on the lift support 38. As shown in FIG. 20,
two observation fields 86A and 86B associated with the image
processing units 50 and 51 are situated on the edge of the
orientation flat FP of the wafer 6, while the remaining one
observation field 86C associated with the image processing unit 52
is situated on the edge of the circular periphery of the wafer 6.
The positions of the observation fields 86A, 86B and 86C are known
constants predetermined for the particular exposure apparatus.
[0138] In FIG. 20, reference numerals 82A, 82B and 82C designate
imaginary pins. These imaginary pins 82A, 82B and 82C have the same
positional relationship among them as the fixed pins 91, 92 and 93
used in the above mentioned conventional prealignment mechanism;
however, as suggested by the term "imaginary", the pins 82A, 82B
and 82C do not actually exist on the wafer stage.
[0139] At the first step in this sequence, the image processing
units 50 and 51 having their observation fields situated on the
edge of the orientation flat FP of the wafer 6 are used in order to
determine an equation expressing a straight line representing the
edge of the orientation flat FP (S801). This equation may be
obtained by averaging Equations (1) and (2) below, wherein Equation
(1) expresses a straight line representing this edge as determined
based on the measurement results from the image processing unit 50
having the observation field 86A, while Equation (2) expresses a
straight line representing this edge as determined based on the
measurement results from the image processing unit 51 having the
observation field 86B. The averaged equation is given below as
Equation (3). Of course, either Equation (1) or (2) may be used in
place of Equation (3).
Y=A.sub.1X+B.sub.1 (1)
Y=A.sub.2X+B.sub.2 (2)
Y=AX+B
[0140] where
A=(A.sub.1+A.sub.2)/2, B=(B.sub.1+B.sub.2)/2 (3)
[0141] Next, an equation which expresses a circle representing the
edge of the circular periphery of the wafer 6 is determined based
on the measurement results from the image processing unit 52 having
its observation field 86C situated on this edge (S802). This
equation is formulated as Equation (4) below.
(X-X.sub.0).sup.2+(Y-Y.sub.0).sup.2=R.sub.0.sup.2 (4)
[0142] Thereafter, we determine a straight line LL.sub.3 which
extends parallel to the line expressed by Equation (3) above and is
spaced from that line by the distance aa toward the center of the
wafer 6 (S803). This line LL.sub.3 is determined according to
Equation (5) below.
Y=AX+B+aa(1+A.sup.2).sup.1/2 (5)
[0143] Then, we determine an intersection point 82C (X'.sub.3,
Y'.sub.3) between the circle expressed by Equation (4) above, which
represents the circular edge of the wafer 6, and the line LL.sub.3
expressed by Equation (5) above (S804). This intersection point 82C
is determined according to Equations (6) below.
X'.sub.3=[X.sub.0-AC+[(X.sub.0-AC).sup.2
-(1+A.sup.2)(X.sub.0.sup.2+C.sup.2-R.sub.0.sup.2)].sup.1/2]/(1+A.sup.2)
Y'.sub.3=AX.sub.3+B+aa(1+A.sup.2).sup.1/2
[0144] where
C=B+aa(1+A.sup.2).sup.1/2-Y.sub.0 (6)
[0145] Then, we determine a point 82D (X'.sub.4, Y'.sub.4) which
lies on the line LL.sub.3 and is distant from the intersection
point 82C by the distance bb, as well as a point 82E (X'.sub.5,
Y'.sub.5) which lies on the line LL.sub.3 and is distant from the
point 82D by the distance cc (S805). These points 82D and 82E are
determined according to Equations (7) below.
X'.sub.4=X'.sub.3-bb/(1+A).sup.1/2
Y'.sub.4=A.times.X'.sub.3-A.times.bb/(1+A)+B+aa(1+A.sup.2).sup.1/2
X'.sub.5=X'.sub.3-(bb+cc)/(1+A)
Y'.sub.5=A.times.X'.sub.3-A(bb+cc)/(1+A)+B+aa(1+A.sup.2).sup.1/2
(7)
[0146] Next, we determine an intersection point 82B (X'.sub.2,
Y'.sub.2) between (i) a line LL.sub.2 extending through the point
82D and perpendicular to the line LL.sub.3 and (ii) the edge of the
orientation flat FP, as well as an intersection point 82A
(X'.sub.1, Y'.sub.1) between (iii) a line LL.sub.1 extending
through the point 82E and perpendicular to the line LL.sub.3 and
(iv) the edge of the orientation flat FP (S806). These intersection
points 82B and 82A are determined according to Equations (8)
below.
X'.sub.2=X'.sub.4+aa.times.A/(1+A.sup.2).sup.1/2
Y'.sub.2=A.times.X'.sub.4-aa.times.A.sup.2/(1+A.sup.2).sup.1/2+B
X'.sub.1=X'.sub.5+aa.times.A/(1+A.sup.2).sup.1/2
Y'.sub.1=A.times.X'.sub.5-aa.times.A.sup.2/(1+A.sup.2).sup.1/2+B
(8)
[0147] These coordinates 82A (X'.sub.1, Y'.sub.1), 82B (X'.sub.2,
Y'.sub.2) and 82C (X'.sub.3, Y'.sub.3) thus determined represent
the coordinates of such points at which the pins 91, 92 and 93 used
in the above mentioned conventional prealignment mechanism should
be in contact with the wafer 6, respectively.
[0148] [Process Sequence 2]
[0149] Now, another process sequence for achieving prealignment of
a wafer 6 having an orientation flat FP will be described with
reference to a schematic plan view of the wafer shown in FIG. 22
and a flow chart shown in FIG. 23.
[0150] In FIG. 22, the wafer 6 outlined by a solid line represents
a wafer which is held on the lift support of the wafer stage but
has not yet undergone prealignment sequence, while the wafer
outlined by a phantom line represents a wafer which has undergone
prealignment through a conventional prealignment mechanism using
three fixed pins. The relationship among the positions of the fixed
pins 91, 92 and 93, as well as that among the positions of the
imaginary pins 82A, 82B and 82C, are identical to those described
with reference to FIGS. 19 and 20. In the situation shown in FIG.
22, the wafer 6 is held on the lift support with its orientation or
angular position rotated by an angle from the desired orientation
which should be established after prealignment, and with its
imaginary center located at a point P.sub.C having coordinates
P.sub.C (X.sub.C, Y.sub.C), or sifted by X.sub.C to the X-direction
and by Y.sub.C to the Y-direction.
[0151] At the first step in this sequence, initialization is
performed, in which each of the values for the rotation angle
.theta. of the wafer and for the coordinates X.sub.C and Y.sub.C of
the imaginary center of the wafer is set to "0" as the initial
value (i.e., .theta.=0, X.sub.C=0 and Y.sub.C=0), and the count n
of a counter is set to "1" (i.e, n=1) (S1001). Then, we increment
the count n by "1" (S1002) and determine points 82A (X'.sub.1,
Y'.sub.1) and 82B (X'.sub.2, Y'.sub.2) at which the imaginary pins
82A and 82B, respectively, should be in contact with the edge of
the orientation flat FP (S1003). These points 82A and 82B are
determined according to Equations (9) below.
X'.sub.1=-d.sub.0 cos .theta.+h sin .theta.+X.sub.C
Y'.sub.1=-d.sub.0 sin .theta.-h cos .theta.+Y.sub.C
X'.sub.2=d.sub.0 cos .theta.+h sin .theta.+X.sub.C
Y'.sub.2=d.sub.0 sin .theta.-hcos .theta.+Y.sub.C
[0152] where
2.times.d.sub.0=c, h=a (9)
[0153] Then, we determine a straight line LL.sub.1 extending
through the point 82A and having an inclination equal to .theta.
with respect to the Y-axis, as well as a straight line LL.sub.2
extending through the point 82B and having an inclination equal to
.theta. with respect to the Y-axis (S1004). These lines LL.sub.1
and LL.sub.2 are determined according to Equations (10) below.
Line LL.sub.1: (X-X'.sub.1)=-tan .theta. (Y-Y'.sub.1)
Line LL.sub.2: (X-X'.sub.2)=-tan .theta. (Y-Y'.sub.2) (10)
[0154] Next, we determine an intersection point 82A (X'.sub.1,
Y'.sub.1) between the line LL.sub.1 and the edge of the orientation
flat FP, as well as an intersection point 82B (X'.sub.2, Y'.sub.2)
between the line LL.sub.2 and the edge of the orientation flat FP,
by performing image processing with respect to the observation
fields 86A and 86B, respectively (S1005). Then, correction is made
to the value for the rotation angle .theta. according to Equation
(11) below, using the coordinates of the intersection points 82A
and 82B just determined by the image processing.
.theta.=tan.sup.-1[(Y'.sub.2-Y'.sub.1)/(X'.sub.2-X'.sub.1)]
(11)
[0155] Next, we determine a straight line LL.sub.3 which extends
parallel to a line joining points 82A and 82B and is spaced from
that line by the distance h toward the center of wafer (S1007).
This line LL.sub.3 is expressed by Equation (12) below.
Line LL.sub.3:
(Y'.sub.2-Y'.sub.1)X-(X'.sub.2-X'.sub.1)Y-X'.sub.1Y'.sub.2+-
X'.sub.2Y'.sub.1
+h[(Y'.sub.2-Y'.sub.1).sup.2-(X'.sub.2-X'.sub.1).sup.2].sup.1/2=0
(12)
[0156] Then, we determine an intersection point 82C (X'.sub.3,
Y'.sub.3) between the line LL.sub.3 and the edge of the circular
periphery of the wafer, by performing image processing (S1008).
Thereafter, the coordinates P.sub.C (X.sub.C, Y.sub.C) of the
imaginary center of the wafer are determined according to Equations
(13) below (S1009).
X.sub.C=X'.sub.3-R.sub.0 cos .theta.
Y.sub.C=Y'.sub.3-R.sub.0 sin .theta. (13)
[0157] [Process Sequence 3]
[0158] Now, a process sequence for achieving prealignment of a
wafer 6N having a notch will be described with reference to
schematic plan views of the wafer shown in FIGS. 24 and 25, a flow
chart shown in FIG. 26 and the illustrative view shown in FIGS.
8(a) and 8(b) illustrating the position of the notch.
[0159] FIG. 24 illustrates the relationship between (i) the
positions of the observation or viewing fields 86A, 86B and 86C
(which are fixed relative to the wafer stage) of the three image
processing units 50, 51 and 52, respectively, incorporated in the
projection exposure apparatus shown in FIG. 4 and (ii) the position
of the wafer 6N held on the lift support 38. As shown in FIG. 24,
the observation field 86C of the image processing unit 50 is
situated on the notch NP of the wafer 6N, while the remaining two
observation fields 86A and 86B of the image processing units 51 and
52 are situated on the edge of the circular periphery of the wafer
6N at positions opposite to the notch NP. The positions of the
observation fields 86A, 86B and 86C are known constants
predetermined for the particular exposure apparatus.
[0160] In FIG. 24, reference numerals 82A and 82B designate
imaginary fixed pins, and 82C designates an imaginary movable pin.
These imaginary pins 82A, 82B and 82C have the same positional
relationship among them as the set of two fixed pins and one
movable pin used in a typical, conventional prealignment mechanism;
however, as suggested by the term "imaginary", the pins 82A, 82B
and 82C do not actually exist on the wafer stage. The distance
between the imaginary pins 82A and 82B is
2.times.d.sub.0=2.sup.1/2.times.R.sub.0.
[0161] FIG. 25 is a schematic illustrating the wafer 6N held on the
lift support with its orientation or angular position rotated by an
angle with respect to the prealignment coordinate system, and
further illustrating the imaginary pins 82A, 82B and 82C in contact
with the edge of the wafer 6N. Here, imaginary center P.sub.C of
the wafer 6N is defined as a point which forms together with the
points of the imaginary pins 82A and 82B a rectangular equilateral
triangle having its right angle corner formed at the imaginary
center P.sub.C. Further, R represents the distance between the
imaginary center P.sub.C and the position 82C of the notch NP
(which is the position of the center O of the imaginary pin 82C in
contact with both edges of the notch NP).
[0162] At the first step in this sequence, initialization is
performed, in which the value for the rotation angle .theta. of the
wafer is set to "0" and the value for the radius R of the wafer is
set to "R.sub.0" as respective initial values (i.e., .theta.=0 and
R=R.sub.0), and the count n of a counter is set to "1" (i.e., n=1)
(S1301). Then, the position 82C (X.sub.N, Y.sub.N) of the notch NP
is determined based on the measurement results from the image
processing unit 50 (S1302).
[0163] Thereafter in this sequence, the count n of the counter is
incremented (S1303), and the coordinates P.sub.M (X.sub.M, Y.sub.M)
of P.sub.M are determined according to Equations (14) below
(S1304).
X.sub.M=X.sub.N-(R+d.sub.0)sin .theta.
Y.sub.M=Y.sub.N+(R+d.sub.0)cos .theta. (14)
[0164] Next, we determine two intersection points 82A (X'.sub.1,
Y'.sub.1) and 82B (X'.sub.2, Y'.sub.2) between (i) a straight line
LL extending through P.sub.M and perpendicular to a line joining
P.sub.M and 82C and (ii) the edge of the circular periphery of the
wafer 6N as determined based on the measurement results from the
image processing units 51 and 52 (S1305). The line LL is expressed
by Equation (15) below.
Line LL: (Y-Y.sub.M)=A(X-X.sub.M)
[0165] where
A=(X.sub.M-X.sub.N)/(Y.sub.M-Y.sub.N) (15)
[0166] Then, the midpoint P'.sub.M (X'.sub.M, Y'.sub.M) between the
intersection points 82A and 82B is determined according to
Equations (16) below (S1306).
X'.sub.M=(X'.sub.1+X'.sub.2)/2
Y'.sub.M=(Y'.sub.1+Y'.sub.2)/2 (16)
[0167] Then, the value for the rotation angle .theta. is corrected
according to Equation (17) below, using the values of the
coordinates of points P.sub.M and P'.sub.M (S1307).
.theta..fwdarw..theta.+(X.sub.M-X'.sub.M)/[R.sub.0(1+A.sup.2).sup.1/2]
(17)
[0168] Next, distance D between the points 82A and 82B is
determined, and the value for R is corrected according to Equation
(18) below, using the predetermined design value
D.sub.0=2.times.d.sub.0 (S1308).
R.fwdarw.R+(D-D.sub.0)/2 (18)
[0169] Then, coordinates P.sub.C (X.sub.C, Y.sub.C) of the
imaginary center of the wafer 6N are determined according to
Equations (19) below (S1309).
X.sub.C=X.sub.N-R sin .theta.
Y.sub.C=Y.sub.N+R cos .theta. (19)
[0170] Thereafter, steps S1303-S1309 are repeated (S1310).
[0171] Thereafter, in step 104, the positional errors in the X- and
Y-directions, .DELTA.X and .DELTA.Y, and the rotational error
.DELTA..theta. of the wafer 6 are determined by calculation from
the measurement results obtained in step 103. In this case the
rotational error is referred to as the positional errors of the
wafer in a broad sense. If the wafer has an orientation flat FP,
the rotational error .DELTA..theta. is determined from the
detection results obtained from the observation fields 86A and 86B
located on the orientation flat FP, and the translational errors in
the X- and Y-directions, .DELTA.X and .DELTA.Y, are determined from
the detection results obtained from the observation fields 86A and
86B, as described above with reference to FIGS. 17(a) to 17(d).
[0172] On the other hand, if the wafer undergoing the prealignment
process has a notch, such as the wafer 6N with the notch NP as
shown in FIGS. 16(a) to 16(d), the translational or positional
errors in the X- and Y-directions, .DELTA.X and .DELTA.Y, and the
rotational error .DELTA..theta. of the wafer are determined by
processing the detection results obtained from the observation
field 86C on the notch NP as well as the other two observation
fields 86A and 86B.
[0173] In the next step 105, the positional errors are examined as
to whether the determined rotational error .DELTA..theta. falls in
an allowable range within which it can be corrected by the rotation
of the lift support 38 and as to whether both the determined
positional errors .DELTA.X and .DELTA.Y fall in an allowable range
within which they allow the wafer to be vacuum-holding on the wafer
holder 30. If at least one of them is out of the corresponding
allowable range, then the procedure proceeds to step 109 where it
is determined whether this out-of-range event of the positional
errors .DELTA.X, .DELTA.Y and .DELTA..theta. has occurred at the
first trial of prealignment for that wafer. If so, in order to redo
the process for rough prealignment, the procedure proceeds to step
110 where the wafer 6 is transferred from the lift support 38 back
to the slider 23 (transfer arm 21) and thence to the turntable 60
of FIG. 5(c) to perform the process for rough prealignment. Then,
the procedure returns to step 101 to repeat the operations in steps
101 through 105.
[0174] Then, if step 105 again determines that at least one of the
positional errors .DELTA.X, .DELTA.Y and .DELTA..theta. is out of
the corresponding allowable range, then it is considered that there
may exist not only positional errors but also some other failure,
and the procedure proceeds to step 109 and thence to step 111,
where an error indication is given to a human operator and wait
state is held for his/her instructions.
[0175] On the other hand, if step 105 determines that all the
positional errors .DELTA.X, .DELTA.Y and .DELTA..theta. are within
the corresponding allowable ranges, then the lift support 38 is
lowered, during which the rotational error .DELTA..theta. of the
wafer is corrected (step 106). No sooner the wafer 6 is placed onto
the wafer holder than the vacuum-holding of wafer by the support
pins 38a-38c is deactivated and the vacuum-holding of the wafer by
the wafer holder 30 is activated, so that the wafer 6 is held on
the wafer holder 30 (step 107). Then, the positional errors
.DELTA.X and .DELTA.Y are added to the search alignment positions
(described later) as offsets, and the wafer stage is driven to move
the wafer (step 108), to complete the sequence of operations-for
the prealignment process. Then, the procedure proceeds to the
alignment sequence of FIG. 3A (including the search alignment
process and the fine alignment process).
[0176] The comparison of the prealignment process according to the
present invention with the prior art prealignment process described
with reference to FIG. 1 above has proven the following. In the
prior art process of FIG. 1, the rotational error of the wafer 6 is
measured by, for example, a contact-type measurement system, and
then the rotation of the wafer is corrected by the .theta.-rotation
correction mechanism 8 mounted on the material support 9. It takes
about 1 to 2 seconds to perform the above operation. In contrast,
according to the process of the present invention, since the
corrective rotation of the wafer for reducing the rotational error
to a value within an acceptable range is effected during lowering
the lift support 38, no additional time exclusively used for the
corrective rotation is required. However, when wafers in one lot
are sequentially processed, several wafers at the beginning of the
lot need to be replaced on the wafer holder, which consumes a
certain length of time. Nevertheless, learning effects are obtained
by making corrections with the errors averaged, so that the greater
number of wafers are included in one lot, the smaller number of
wafer replacement process are required and thus a shorter time is
required for performing wafer replacement process, so that more
effective results may be obtained.
[0177] Next, the wafer replacement process will be described.
Generally, in the case where different exposure apparatuses are
used to perform respective exposure processes for forming a level
of layer and the next level of layer on a wafer, if a matching
between the prealignment processes in the exposure apparatuses is
established, there would not occur an error of missing a search
mark in the search alignment process for forming the subsequent
level of layer. Therefore, the procedure may proceed from step 108
in FIG. 2 to step 112 in FIG. 3A.
[0178] However, under any of the following situations a) to c),
positional errors found in the prealignment process may be
relatively great, so that an error may possibly occur in the
subsequent search alignment process.
[0179] a) The parallelism between the top surfaces of the lift
support 38 and the wafer holder 30 is deteriorated for some reason,
so that rotational error and/or translational or positional errors
(or offset errors) may occur on a wafer when the wafer is
transferred onto the wafer holder 30.
[0180] b) The matching between the prealignment processes in the
exposure apparatuses is not established, so that a limited
rotational error and/or offset errors (such as relative rotation
and/or offsets between the peripheral edge of a wafer and a search
mark) may occur on the wafers in a particular lot.
[0181] c) The peripheral edge of a wafer chips at one of the
measurement points after the wafer has been undergone the exposure
process for a first level of layer, so that rotational error and/or
offset errors may occur on that particular wafer.
[0182] If any of these situations is possible, it is desirable that
the procedure proceeds from step 108 in FIG. 2 to step 130A in FIG.
18A where it is determined whether an error occur in the search
alignment process, and if so, the wafer replacement process is
performed wherein the wafer is transferred from the wafer holder 40
onto the lift support 38 to perform corrective rotation and return
back onto the wafer holder 30. The wafer replacement process of
FIGS. 18A and 18B will be described in the following.
[0183] To begin with step 130A in FIG. 18A, a high magnification
search alignment process is performed. Each wafer has first and
second search marks 47A and 14B formed thereon, as shown in FIG.
5(a). In the high magnification search alignment, the FIA-type
alignment sensor system 5A of FIG. 4 is used to detect the first
and second search marks 47A and 47B on the wafer, with the
magnification of the image-forming optical system in the alignment
sensor system 5A being set to a high magnification to process only
one frame of picture data. Then, in step 130B, it is determined
whether the offsets of the wafer in the X- and Y-directions,
.delta.X and .delta.Y, and the rotational error .delta..theta. of
the wafer can be measured. This is determined by examining the one
frame of picture data obtained in step 130A to determine whether
two search marks 47A and 47B are detectable. If the search marks
47A and 47B are detectable, the procedure proceeds to step 112 in
FIG. 3A. Therefore, in many cases, only the high magnification
search alignment process on a frame is performed before proceeding
to step 112, resulting in a minimum decrease in the throughput.
[0184] On the other hand, if the search marks 47A and 47B can not
be detected in step 130B, the procedure proceeds to one of step 131
(mode 1), step 132 (mode 2) and step 133 (mode 3) depending on the
mode previously selected. In step 131 for mode 1, the detection of
the search marks 47A and 47B is performed, with the magnification
of the alignment sensor system 5A being set to a low magnification
so as to make the view field thereof sufficiently wide. This is
called a low-magnification image-processing search alignment
process. In step 132 for mode 2, the detection of the search marks
47A and 47B is performed, with the magnification of the alignment
sensor system 5A being set to a high magnification, and with the
wafer stage being operated to make steps for frame continuation. In
step 133 for mode 3, the rotational error .delta..theta. is
determined with operator's assistance (manual assistance). This is
done, for example, by measuring the coordinates (F.sub.X1,
F.sub.Y1) of the first search mark 47A to determine the offsets
.delta.X and .delta.Y and measuring the Y-coordinate F.sub.Y2 of
the second search mark 47B to obtain the difference between the
Y-coordinates of the two search marks to determine the rotational
error .delta..theta.. From either of steps 131 and 132, the
procedure proceeds to step 134, where it is determined again
whether the offsets .delta.X and .delta.Y and the rotational error
.delta..theta. can be determined, i.e., whether the search marks
47A and 47 have been detected. If not, the procedure proceeds to
step 133 for the mode 3 operation. Otherwise if they have been
detected, then .delta.X, .delta.Y and .delta..theta. are determined
and the procedure proceeds to step 137.
[0185] In step 135 following step 133, it is determined that the
obtained rotational error .delta..theta. falls in the allowable
range within which it can be corrected by the rotation of the lift
support 38. If it can not be corrected (i.e., the rotational error
.delta..theta. exceeds the allowable limit), the procedure proceeds
to step 136 where an error indication is provided. Otherwise if it
can be corrected, the procedure proceeds to step 137, where the
vacuum-holding of the wafer by the lift support 38 and that by the
wafer holder 30 are activated and deactivated, respectively, and
thereafter the lift support 38 is raised with the wafer held
thereon, so as to perform the correction of the rotational error
.delta..theta. of the wafer. It is noted that the above operation
sequence is for the first wafer in one lot, i.e., the wafer which
is processed first among the wafers in the lot. For any of the
second and later wafers in the lot, the rotation of the lift
support 38 required for correcting the rotational error
.delta..theta. is added to the rotation of the lift support 38 for
correction performed in step 106. The operation in step 137 is
continued in which the vacuum-holding of the wafer by the wafer
holder 30 is activated, the lift support 38 is lowered, and the
vacuum-holding of the wafer by the lift support 38 is deactivated.
The wafer has been now replaced on the wafer holder 30. Then, in
step 138, the search alignment positions are corrected for the
offsets .delta.X and .delta.Y if the wafer is the first one in the
lot. Otherwise, if the wafer is any of the second and later ones,
the offsets .delta.X and .delta.Y are added to the search alignment
positions used in step 108 in FIG. 1, for example. Thereafter, the
procedure proceeds to step 112 in FIG. 3.
[0186] It is noted that each of the situations a) and c) varies
between wafers, so that the correction in step 106 is omitted
(error correction process for the second and later wafers is
performed instead). On the other hand, the situation b) above may
be identified when the first wafer is processed, so that the
process for the second and later wafers in step 137 in FIG. 18 is
performed in such case. However, if there exists a complex
situation including the situations a) to c), it is possible that
errors occur only on the first wafer, so that the search alignment
operation is inhibited by feeding back the measurement results to
the process for the second and later wafers. In such case, it is
preferable to perform the process utilizing the learning mechanism
as described above.
[0187] Further, the prior art projection exposure apparatus with a
part thereof shown in FIG. 1 includes the .theta.-rotation
correction mechanism 8 for corrective rotation of a wafer 6. This
mechanism 8 is a drive system and disposed between the material
support 9 having a movable mirror 13 mounted thereon and the wafer
6. In contrast, the projection exposure apparatus used with the
method of the present invention need not to have any drive system
disposed between the movable mirror 13 and a wafer 6, so that the
stability in stepping movement of the wafer is improved.
[0188] In the sequence of the alignment processes in FIGS. 3A and
3B, the search alignment process precedes the fine alignment
process. However, if the alignment sensor system used has a wide
detectable range (capture range) and the prealignment is performed
with good accuracy, the search alignment may be passed over, and
the procedure may go to the fine alignment process. For example,
the LSA-type and FIA-type alignment sensor systems have relatively
wide detectable ranges, such as up to .+-.25 .mu.m. In contrast,
the LIA-type alignment sensor system has a much smaller detectable
range, such as only about .+-.1 to .+-.2 .mu.m. Accordingly, if the
accuracy in the prealignment is within a range of .+-.25 .mu.m and
either the LSA-type or the FIA-type alignment sensor system is
used, the procedure may pass over the search alignment process to
go to the fine alignment process.
[0189] More specifically, in step 112 in FIG. 3A, the type of the
alignment sensor system used is determined. If it is of the LIA
type, the sequence of operations in step 103 and the subsequent
steps thereto are performed for the search alignment process. If it
is either of the LSA or the FIA type, the procedure proceeds to
step 121, where it is determined whether the accuracy in the
prealignment is within the detectable range of the alignment sensor
system used, i.e., whether the search alignment process has to be
performed. If it has, the procedure proceeds to step 113, otherwise
to step 126.
[0190] Now, the search alignment process will be described in
detail. Typically, a wafer has marks for the search alignment
formed thereon. The wafer 6 to be processed here has the first and
second search marks 47A and 47B formed thereon, which are search
marks for the FIA-type alignment and spaced a predetermined
distance nominally in the Y-direction. As shown in FIG. 5(a), the
first search mark 47A comprises an X-axis search mark 45X of a
line-and-space pattern consisting of parallel straight lines spaced
each other in the X-direction and an overlaid Y-axis search mark
45Y of a line-and-space pattern consisting of parallel straight
lines spaced from each other in the Y-direction. The second search
mark 47B is similar to the first search mark 47A and comprises an
X-axis search mark 44X and an overlaid Y-axis search mark 44Y.
Here, one FIA-type alignment sensor system 5A in FIG. 4 is used to
detect the positions of the two search marks 47A and 47B. Further,
as described later, another FIA-type alignment sensor system 5B in
FIG. 4 is used to detect the rotational angle of the wafer 6. In
order to more clearly distinguish these alignment sensor systems,
the alignment sensor system 5A is referred to as "FIA-microscope
5A" and the alignment sensor system 5B as ".theta.-microscope 5B"
hereinafter.
[0191] Each shot area defined on one wafer 6 has wafer marks for
the fine alignment associated therewith, which are referred to as
"fine marks" hereinafter. For example, FIG. 5(a) shows a shot area
SA as the representative of any of the shot areas on the wafer 6,
which has an X-axis fine mark 46X consisting of a liner array of
dots extending in the Y-direction and a Y-axis fine mark 46Y
consisting of a linear array of dots extending in the X-direction
associated therewith. These fine marks 46X and 46Y are detected by
the LSA-type alignment sensor system in the TTL-type alignment
sensor system 4 in FIG. 4. The usable fine marks include those for
the LIA-type alignment process and those for the FIA-type alignment
process.
[0192] FIGS. 9(a) and 9(b) show an exemplified arrangement of the
first and second search marks on the wafer 6. As shown, the first
search mark 47A is located within the street-line area at a
position surrounded by four shot areas 48A, 48B, 48C and 48D, and
the second search mark 47B is located also within the street-line
area at a position surrounded by four shot areas 49A, 49B, 49C and
49D. Also, two circular observation or viewing fields 5Aa and 5Ba
are shown. One circular observation field 5Aa is the effective
observation field of the FIA-microscope 5A in FIG. 4, and the other
circular observation field 5Ba, which is distant from the circular
observation field 5As to the +X-direction, is the effective
observation field of the .theta.-microscope 5B in FIG. 4.
[0193] In order to perform the search alignment process, in step
113, the wafer stage is driven to move the first search mark 47A
into the observation field 5Aa of the FIA-microscope 5A as shown in
FIG. 9(a), when the second search mark 47B is not found in the
observation field 5Ba of the .theta.-microscope 5B, and only edge
portions of the shot areas 49A and 49B and the street line area 70
are found there. Then, it is determined whether the wafer now
undergoing the exposure process is the first wafer in the lot (step
114). If so, the procedure proceeds to step 115, where the
FIA-microscope 5A is used to detect the X- and Y-coordinates
(F.sub.X1, F.sub.Y1) of the first search mark 47A.
[0194] Now, an example of detection method which may be used in
step 115 will be described with reference to FIGS. 10(a) to 10(c).
FIG. 10(a) shows a detection area 68 actually taken by the image
pick-up device in the observation field of the FIA-microscope. As
shown in FIG. 10(a), In the detection area 68 are found two
independent indicator marks 66X1 and 66X2 for the X-direction
position detection and two independent indicator marks 66Y1 and
66Y2 for the Y-direction position detection. These indicator marks
66X1, 66X2, 66Y1 and 66Y2 are disposed in such a plane in the
FIA-microscope in FIG. 4 which is conjugate to the surface of the
wafer, and illuminated by illumination beam independent from the
illumination beam for illuminating the marks formed on the wafer.
In the FIA-microscope 5A, an X-axis image pick-up device which
scans the detection area 68 in the direction corresponding to the
X-direction and a Y-axis image pick-up device which scans the
detection area 68 in the direction corresponding to the Y-direction
are provided in parallel. The X-axis image pick-up device scans the
detection area 68 in the direction traversing the indicator marks
66X1 and 66X2 to produce an image or picture signal SX1 such as
shown in FIG. 10(c). The signal segment 67X in FIG. 10(c)
corresponds to the X-axis search mark 45X. The image-sensing signal
SX1 is digitized and subjected to the image processing so as to
detect the X-coordinate of the first search mark 47A relative to
the indicator marks 66X1 and 66X2.
[0195] Similarly, the Y-axis image pick-up device scans the
detection area 68 in the direction traversing the indicator marks
66Y1 and 66Y2 to produce an image or picture signal SY1 such as
shown in FIG. 10(b). The signal segment 67Y in FIG. 10(b)
corresponds to the Y-axis search mark 45Y. The image signal SY1 is
subjected to the image processing so as to detect the Y-coordinate
of the first search mark 47A relative to the indicator marks 66Y1
and 66Y2. Instead of the indicator marks, a predetermined pixels in
the image pick-up device, or the scan starting point of an image
pickup tube (if it is used) may be used as the reference point for
the position detection.
[0196] Then, the wafer stage is driven to move the second search
mark 47B into the observation field 5Aa of the FIA-microscope 5A
(step 116) as shown in FIG. 9(b), and the FIA-microscope 5A is used
to detect the X- and Y-coordinates (F.sub.X2, F.sub.Y2) of the
second search mark 47B (step 117). Then, in step 118 (FIG. 3B), a
new coordinate system (X.sub.P, Y.sub.P) is defined which is
related to the wafer stage coordinate system (X, Y) by the
rotational angle .theta. and offsets ((F.sub.X.sub.1+F.sub.X2)/2,
(F.sub.Y1, F.sub.Y2)/2) (the coordinates in the new coordinate
system are referred to as "XY.theta.-transformed coordinates"
hereinafter). The rotational angle .theta. used for the
transformation is obtained as:
.theta.=arc tan ((F.sub.Y2-F.sub.Y1)/L) (20)
[0197] where L is the distance between the two search marks 47A and
47B.
[0198] Using the coordinates (X, Y) in the wafer stage coordinate
system, the XY.theta.-transformed coordinates (X.sub.P, Y.sub.P) in
the new coordinate system are expressed as: 5 [ X P Y P ] = [ cos -
sin sin cos ] [ X Y ] + [ ( F X1 + F X2 ) / 2 ( F Y1 + F Y2 ) / 2 ]
( 21 )
[0199] Then, in step 119, the wafer stage is driven according to
the XY.theta.-transformed coordinates (X.sub.P, Y.sub.P) to move
again the first search mark 47A into the observation fields 5Aa of
the FIA-microscope 5A as shown in FIG. 9(a). In the next step 120,
the image within the observation fields 5Ba of the
.theta.-microscope 5B (including street-lines and other patterns)
are taken under the condition existing when step 119 has been just
completed, and the image taken itself or some characterizing
features in the image are stored. This process will be described
with reference to FIGS. 11(a) and 11(b) in more detail.
[0200] FIG. 11(a) shows an image in the detectable area 68 of the
FIA-microscope 5A when step 119 has been just completed. FIG. 12(a)
shows the image signal SY1 obtained from the Y-axis image pick-up
device scanning the image of FIG. 11(a) in the Y-direction. As
seen, the center of the first search mark 47A is located to the
center of the detectable area 68 of the FIA-microscope 5A when the
wafer stage has been driven according to the XY.theta.-transformed
coordinates. Further, as seen, the image signal SY1 has three
negative peaks at positions Y.sub.1, Y.sub.2 and Y.sub.3
corresponding to the respective lines of the Y-axis search mark 45Y
in the first search mark 47A. Thus, the Y-coordinate YA obtained by
means of the average (Y.sub.1+Y.sub.2+Y.sub.3)/3 of the three
positions is determined as the position of the first search mark
47A in the Y-direction.
[0201] FIG. 11(b) shows an image in the detectable area 69 of the
.theta.-microscope 5B when step 119 has been just completed. As
shown, a pattern 71A in the shot area 49A and a pattern 71B in the
shot area 49B are found above and below, respectively, of the
street-line area 70 defined between the edges 70a and 70b of the
shot areas 49A and 49B. By virtue of the use of the
XY.theta.-transformed coordinates (X.sub.P, Y.sub.P) defined as
described above, the X.sub.P-axis for the XY.theta.-transformed
coordinates, or the straight line on which the coordinate
Y.sub.P=0, exists within the street-line area 70, and this straight
line is shown by a dash-and-dot line and called the imaginary line
70c. Here, the image of FIG. 11(b) is scanned in the direction
corresponding to the Y-direction (which may be deemed to be
substantially parallel to the Y.sub.P-direction) to produce the
image or picture signal SY2 shown in FIG. 12(b). As shown in FIG.
12(b), the image or picture signal SY2 has two negative peaks at
positions SR1 and SR2, which corresponds to the Y-coordinates of
the edges 70a and 70b in FIG. 11(b). Then, such position YB on the
image signal SY2 in FIG. 12(b) that corresponds to the imaginary
line 70c in FIG. 11(b) (i.e., the position at which Y.sub.P=0) is
determined.
[0202] Then, the distances .DELTA.SR1 and .DELTA.SR2 between the
position YB and the respective positions SR1 and SR2 are determined
to obtain the distances in the Y-direction between the imaginary
line 70c and the respective edges 70a and 70b defining the
street-line area 70 and store the obtained distances in the central
control system 18 of FIG. 4. Further, in order to clearly
distinguish the edges 70a and 70b from the other patterns (such as
patterns 71A and 71B), various useful characterizing features of
the image signal SY2, including its signal intensities at the
positions SR1 and SR2 as well as at other positions corresponding
to other patterns, and the distances from the positions SR1 and SR2
to such other patterns, are determined and stored in the central
control system 18. The positions of the search marks 47A and 47B
relative to the street-line area 70 are considered the same among
the wafers in the same lot, so that for any of the second and later
wafers the positions SR1 and SR2 are determined from the image
signal SY of FIG. 12(b) and from the positions SR1 and SR2 the
position YB at which the coordinate Y.sub.P equals zero is
determined.
[0203] As described, in this embodiment, the positions SR1 and SR2
of the negative peaks of the two edges are determined from the
image signal SY2 and from them the position YB is determined;
however, it is also contemplated that the image signal SY2 is
digitized and stored, and the correlation between the stored signal
SY2 and an image signal obtained for the next wafer is used to
determine the position YB at which the coordinate Y.sub.P equals
zero. Further, in this embodiment, the center of the first search
mark 47A is moved to the center of the detection area 68 of the
FIA-microscope 5A as shown in FIG. 11(a); however, such other new
coordinate system may be defined in that the imaginary line 70c on
which the coordinate Y.sub.P equals zero will be located at the
center of the detection area 69 of the .theta.-microscope 5B.
[0204] Then, the procedure proceeds to step 122, where the fine
alignment process is performed by detecting the positions of the
fine marks 46X and 46Y associated with a predetermined shot area on
the wafer 6. Here, the fine alignment process is performed using
enhanced global alignment (EGA) technique, such as disclosed in
Japanese Laid-Open Patent Publication No. Sho 61-44429
(44429/1986). More specifically, the wafer stage is driven
according to the XY.theta.-transformed coordinates to detect the
coordinates of the X-axis and Y-axis fine marks associated with a
predetermined number of shot areas (sample shot areas) selected
from among the shot areas on the wafer 6 by means of the alignment
sensor system 4, and the detection results are subjected to a
statistical processing so as to determine the coordinates of the
positions of all the shot areas on the wafer in terms of the
XY.theta.-transformed coordinates.
[0205] Then, in step 123, the wafer stage is driven according to
the coordinates of the positions of the shot areas determined by
the fine alignment process, so as to sequentially position the shot
areas to the exposure location to enable exposure operations to
print an image of the pattern on the reticle 1 onto each of the
shot areas by projection exposure. During this sequential
positioning of the shot areas, final position adjustments may be
made for each shot area by fixing the wafer stage positioned for
that shot area and moving the reticle stage to correct any residual
alignment error between the reticle and that shot area on the
wafer. After the exposure process for the wafer 6 is completed, the
wafer 6 is transferred out of the projection exposure apparatus,
and then steps 101 through 108 in FIG. 2 are performed for the
prealignment process for the next wafer to be exposed in the lot.
Then, for the wafer, steps 112 and 113 in FIG. 3 are performed and
the procedure proceeds to step 114.
[0206] This wafer is the second wafer to be processed in the lot,
so that the procedure proceeds from step 114 to step 124, where the
X- and Y-coordinates (F.sub.X1, F.sub.Y1) of the first search mark
47A are detected by the FIA-microscope 5A under the condition
similar to that shown in FIG. 9(a), while at the same time the
Y-coordinates SR1 and SR2 of the edges defining the street-line
area 70 are detected by the .theta.-microscope 5B. For the purpose,
the edges defining the street-line area 70 are distinguished from
the other patterns using the image data stored in step 120.
Further, from the Y-coordinates SR1 and SR2, the Y-coordinate YB
with which the value of Y.sub.P-axis in a new coordinate system
will be zero are determined.
[0207] Then, in step 125, a new XY.theta.-transformed coordinate
system (X.sub.P, Y.sub.P) is defined which is related to the wafer
stage coordinate system (X, Y) by the rotational angle .theta. and
offsets (Ox, Oy). The rotational angle .theta. used here is
obtained from the distance L' between the detection centers of the
FIA-microscope 5A and the .theta.-microscope 5B and the above
determined values as:
.theta.=arctan ((YB-F.sub.Y1)/L') (22)
[0208] Using the distance L between the two search marks 47A and
47B, the fist order approximation of the coordinates (F.sub.X2,
F.sub.Y2) are obtained as (F.sub.X1+L, F.sub.Y1+.theta..times.L).
Therefore, using the coordinates of the midpoint between the two
search marks 47A and 47B as the offsets (Ox, Oy), the new
XY.theta.-coordinates (X.sub.P, Y.sub.P) are expressed in terms of
the wafer stage coordinates (X, Y) as shown in Equation (21) above.
Thereafter, the wafer undergoes the alignment and exposure
processes in steps 122 and 123. In this manner, for any of the
second and later wafers in the lot, simultaneous measurements by
the FIA-microscope 5A and the .theta.-microscope 5B are performed
during the search alignment process to determine the positions in
the X- and Y-directions and the rotational angle of the wafer at
one time, resulting in a reduced measurement time and an improved
throughput.
[0209] Next will be described the procedure in the case where the
alignment sensor system used is either of the LSA-type or the
FIA-type, and hence no search alignment process is performed. In
such case, the procedure proceeds from step 121 to step 126 where
mode selection is performed. In this embodiment, options include
rough mode and fine mode. It is assumed that the accuracy
achievable by the prealignment process shown in FIG. 2 is such that
three times the standard deviation (3 .sigma.) of the prealignment
errors is of the order of 20 .mu.m. Thus, the achieved accuracy may
be sufficient for the initial accuracy for the fine alignment
process (rough mode) or may not sufficient for it (fine mode).
Accordingly, if rough mode is selected, the procedure proceeds to
step 122 where, with that prealignment accuracy achieved at this
point time, the EGA-type fine alignment process is performed by
measuring a predetermined number of sample shot areas for
example.
[0210] On the other hand, if fine mode is selected, the coordinates
of respective pairs of X-axis and Y-axis fine marks associated with
two spaced shot areas on the wafer, by means of selected one of the
alignment sensor systems (step 127), and from the results of this
measurement the XY.theta.-transformed coordinates are determined in
the same manner as in step 118. Then, the wafer stage is driven
according to the XY.theta.-transformed coordinates to locate the
centers of the fine marks associated with the third or later shot
areas (the first two shot areas may be included as well) to a
position about the center of the detection area of the alignment
sensor system so as to perform measurement (step 129). After the
measurement, exposure process is performed in step 123.
[0211] Generally, since the LSA-type and the FIA-type alignment
processes have relatively wide detectable areas, the rough mode
which is superior in throughput is selected. However, if it is
required to eliminate any distortions in the frame of the image
processing system of the FIA-type alignment system and/or the
effects of any magnification errors so as to achieve precision
measurement, it is desirable to measure and correct any distortions
in the frame, or to select the fine mode. Further, the mode
selection may be made depending on the implementation of the
processes in the procedure, or may be arranged to be automatically
made depending on the degree of the achieved accuracy of the
prealignment process. Regarding the step 124 in FIG. 3B, if no
pattern is found in the detection area of the .theta.-microscope
5B, it is determined that there exists no detectable pattern, and
the procedure automatically select the sequence in which steps 115
through 118, which have been performed for the first wafer, are
again performed and from step 118 the procedure proceeds to step
122.
[0212] In the embodiment described above, it is assumed that when
the prealignment process is completed the procedure can
unconditionally proceeds to the search alignment process and/or the
fine alignment process. In fact, however, it is possible that the
procedure may not do it. For example, if the exposure process for
the first layer has been done with a first exposure apparatus and
the exposure process for the second layer is to be done with a
second exposure apparatus, and the matching in the arrangement of
the alignment sensor between the first and second exposure
apparatus is not established, then the positions of the search
marks 47A and 47B prealigned in the second exposure apparatus may
be so different from those prealigned in the first one that the
search marks 47A and 47B may not found in the observation field in
the second exposure apparatus, even though the wafer is accurately
prealigned on a wafer contour basis. In such a case, when the first
wafer in the lot is to be processed, operator's instructions may be
waited after the prealignment process shown in FIG. 2 is completed,
in order to cause the operator to manually measure the positions of
the search marks 47A and 47B on the first wafer. Thereafter, the
measurement results may be used to determine and correct any offset
of the rotational angle of the rotational drive unit for the lift
support and the offsets of the search alignment position in the X-
and Y-directions. Thereafter, for any of the second and later
wafers in the lot, the procedure can automatically proceed from the
prealignment process in FIG. 2 to either the search alignment
process or the fine alignment process.
[0213] In the embodiment described above, the sensor systems for
the positions of the wafer edge, or the two-dimensional image
processing units, are disposed above the loading position of the
wafer; however, it may possibly difficult in some cases to disposes
the sensor systems for the positions of the wafer edge at such
locations due to some cause such as the size of the wafer. In such
a case, the wafer stage may be driven to move a wafer to such a
position that allows the measurement of the positions of the
peripheral edge of the wafer, with the wafer being held on the lift
support 38 or being placed on the wafer holder 30.
[0214] It is to be understood that the present invention may be
applied not only the step-and-repeat type of exposure apparatus but
also to other types of exposure apparatus including those of the
step-and-scan type.
[0215] Therefore, the present invention is not limited to the
embodiments and modifications described above, but may be embodied
in various other formes and arrangements without departing from the
spirit and the scope of the present invention.
[0216] In the positioning method according to the present
invention, the detection of the peripheral edge of the
photosensitized substrate (wafer) is performed by two-dimensional
image processing units and by using a noncontact-detection
technique just after the loading of the photosensitized substrate,
so that the correction (prealignment) of the rotational error of
the photosensitized substrate may be performed while the
photosensitized substrate is lowered to be placed onto the
substrate stage. Therefore, the time required for the prealignment
may be reduced. Further, it is unnecessary to provide a rotational
drive mechanism on the substrate stage side, so that the substrate
stage may have a relatively simple construction, an improved
rigidity and a reduced weight, resulting in an advantage that the
alignment operation of the photosensitized substrate may be quickly
performed with precision upon loading of the photosensitized
substrate from, for example, a substrate loader system onto the
substrate stage.
[0217] Further, the imaginary points corresponding to reference
points which would be used for positioning the photosensitized
substrate on the substrate stage by using a contact-positioning
technique, are determined in the observation fields of the
two-dimensional image processing systems. Also, the offsets, from
the imaginary points, of the positions of the measurement points on
the photosensitized substrate measured by the two-dimensional image
processing systems are used to position the photosensitized
substrate. Therefore, a high matching accuracy for the coarse
alignment (prealignment) process, with another exposure apparatus
in which contact-positioning (prealignment) process is performed,
may be obtained.
[0218] Further, in this positioning method, if the cutout formed in
the peripheral edge of the photosensitized substrate comprises a
wedge-shaped or V-shaped notch, and the measurement points for the
two-dimensional image processing systems are selected to include
one on the cutout and two on respective portions of the peripheral
edge of the photosensitized substrate other than the cutout, then
the detection of the positions of the photosensitized substrate at
the three measurement points enables identification of the
rotational angle and the two-dimensional position of the
photosensitized substrate.
[0219] On the other hand, if the cutout formed in the peripheral
edge of the photosensitized substrate comprises a flat edge
portion, and the measurement points for the two-dimensional image
processing systems are selected to include two on the cutout and
one on a portion of the peripheral edge of the substrate other than
the cutout, then the detection of the positions of the
photosensitized substrate at the three measurement points enables
identification of the rotational angle and the two-dimensional
position of the photosensitized substrate.
[0220] Furthermore, in order to make the prediction of the position
of the photosensitized substrate which will be found when the
substrate has been placed on the substrate stage, a rotational
error and offsets are obtained, which exist between a position of
the photosensitized substrate which will be found when the
substrate has been placed on the substrate stage through the
substrate lift means without any rotation effected thereby and a
position of the substrate which would be found when the substrate
had been aligned by using the contact-positioning technique. The
rotational error is corrected when the photosensitized substrate is
placed onto the substrate stage through the substrate lift means.
Also, the offsets is corrected through the substrate stage after
the substrate has been placed on the substrate stage. This enables
simplification of the construction of the substrate stage and a
quick positioning of the substrate.
[0221] Next, a process sequence for positioning a wafer,
illustrating an embodiment of the present invention, will be
described with reference to a flow diagram shown in FIG. 27. The
wafer has geometrical features, such as an orientation flat and a
notch, formed in its peripheral edge thereof. The process sequence
includes i) a prealignment process, ii) a search alignment process
and iii) a fine alignment process.
[0222] (1) Prealignment Process (Steps ST1 to ST3 in FIG. 27)
[0223] Briefly, the prealignment process according to this
embodiment is such a process as set forth below. In the
prealignment process, preselected portions of the peripheral edge
of a wafer 6 (such portions include the geometrical features, such
as an orientation flat and a notch) are measured. Then, any
displacement or offset of the wafer 6 (displacement or offset in a
rotational direction: a rotational offset hereinafter) represented
by the measurements of the geometrical features with respect to a
reference (referred to as the "first reference" hereinafter) is
determined. Then, the orientation of the wafer 6 is so corrected as
to be coincident with the desired orientation represented by the
first reference. The first reference is stored in predetermined
storage locations (referred to as the "prealignment reference
storage locations") in a storage device as a part of data for a
control program. Such a control program may be provided for the
central control system 18 or for the alignment control system
15.
[0224] More specifically, the wafer 6, which is undergoing the
prealignment procedure, is transferred by the transfer arm 21 onto
the lift support 38, which has been already set to the wafer
loading/unloading position. The wafer 6 thus transferred onto the
lift support 38 is then held on the lift support 38 by vacuum
suction. Then, the image processing units 50, 51 and 52 are
activated to take images of the preselected portions of the
peripheral edge of the wafer 6, which portions include the
geometrical features (step ST1). Thereafter, any rotational offset
of the wafer 6 (referred to as the "first offset" hereinafter)
relative to the orientation reference (the first reference) is
determined (step ST2). In the prealignment process of this
embodiment, only the rotational offset of the wafer is compensated;
however, it is contemplated that the prealignment process may be
modified such that both the rotational offset and the displacement
or offset in a translational direction (in x- and y-directions) of
the wafer are compensated. Following step ST2, the determined value
of the rotational offset is used to drive the lift support 38 for
rotation so as to compensate the detected rotational offset of the
wafer 6 (step ST3).
[0225] The first reference is a predefined reference, which is
determined in connection with the geometrical features of the
wafer. The wafer is rotated in a direction which is parallel to an
x-direction or a y-direction.
[0226] The prealignment procedure is completed with step ST3.
Thereafter, the lift support 38 is lowered and the wafer 6 having
been held on the lift support 38 by vacuum suction is released, so
that the wafer 6 is transferred onto the wafer holder 30 and then
held on the wafer holder 30 by vacuum suction.
[0227] (2) Search Alignment Procedure
[0228] Briefly, the search alignment process according to this
embodiment is such a process as set forth below. In the search
alignment process, the search alignment marks formed on the wafer 6
are measured. Then, any displacement or offset in a rotational
direction (rotational offset) of the wafer 6 as well as any
displacement or offset in translational direction, i.e. in x- and
y-directions of the wafer 6 (referred to as "the second reference"
hereinafter) with respect to another reference (a second reference)
are determined. The second reference is stored in predetermined
memory locations (referred to as "search alignment reference
storage locations" hereinafter,) which locations may be provided in
the storage device for the central control system 18 or in the
storage device for the alignment control system 15. Then, it is
determined whether the measured offset of the wafer 6 (the
rotational offset, in this embodiment) falls within a predetermined
allowable range. (This range represents an allowable angle for the
rotational offset and is recorded as data of the control program.)
Finally, the course of the process sequence to follow is selected
in accordance with the determination.
[0229] More specifically, The positions of the search alignment
marks formed on the wafer 6 held on the wafer holder 30 by vacuum
suction are measured (step ST4) by using appropriate image sensor
systems, such as the off-axis-type alignment sensor systems 5A and
5B or other alignment sensor systems suitable for the search
alignment process (not shown), as well as using a suitable
measurement method, such as LSA-method or FIA-method.
[0230] Then, any offsets of the wafer 6 with respect to the second
reference (such offsets include the translational displacement or
offsets X and Y in x- and y-directions, respectively, and the
rotational displacement or offset .theta.) are determined (step
ST5). The search alignment marks are formed through a previous
lithographic process, such that any offsets (including the
rotational offset .theta. and the translational offsets X and Y (in
x- and y-directions) of the wafer 6 with respect to the second
reference may be determined from the measurements of the marks.
[0231] Thereafter, it is determined whether the rotational offset
of the wafer 6 (referred to as the "second offset" hereinafter)
falls within the allowable range (step ST6). If so, the process
sequence proceeds to the fine alignment process.
[0232] Otherwise, i.e., if the rotational offset of the wafer 6
relative to the second reference is determined as not falling
within the allowable range, the first reference stored in the
prealignment reference storage locations is so corrected as to
compensate based on the second offset (the rotational offset) to
rewrite or update the above-mentioned data that defines the first
reference and is stored in the prealignment reference storage
locations (step ST7). (The new reference defined by the updated
data is referred to as the "third reference" hereinafter.) The data
defining the first reference may be backed up in other storage
locations before it is replaced with the data defining the third
reference.
[0233] From step ST7, the process sequence returns back to step ST1
to repeat the prealignment process. Specifically, in step ST1, the
vacuum suction holding the wafer 6 on the wafer holder 30 is
relieved to release the wafer 6 from the wafer holder 30, and the
lift support 38 is raised so that the wafer 6 is supported by the
lift support 38 instead of the wafer holder 30. Then, the wafer 6
is held on the lift support 38 by vacuum suction and raised to the
wafer loading/unloading position. Then, the image processing units
50, 51 and 52 are activated to take images of the preselected
portions of the peripheral edge of the wafer 6 (step ST1,) and any
rotational offset (the first offset) of the wafer 6 is determined
in connection with the geometrical features with respect to the
reference for wafer (which is the third reference at this point of
time) stored in the prealignment reference storage locations (step
ST2). Then, the determined value of the rotational offset is used
to drive the lift support 38 for rotation so as to compensate the
detected rotational offset of the wafer 6 (step ST3.)
[0234] Thereafter, the search alignment process (steps ST4 to ST6)
is again performed. Because i) the prealignment process has been
just completed using the third reference which is derived by
correcting the first reference so as to compensate the second
offset, little or no rotational offset will be detected in the
second-time search alignment process while some offsets in the
translational direction (in x- and/or y-directions) may be possibly
detected. Therefore, the value of the rotational offset (the second
offset) determined in the second-time fine alignment process will
fall within the allowable range, so that the process sequence will
proceed to the fine alignment process from the second-time search
alignment process. In the fine alignment process, any translational
offsets of the wafer 6 will be compensated by fine corrective
movement of the X-stage 11 and/or the Y-stage 12. In contrast, in a
typical, conventional, wafer positioning process, the prealignment
procedure is not repeated for the same wafer unlike the present
invention, when the second offset detected falls out of the
predetermined allowable range, the second offset is simply rejected
as a failure and the fine alignment process is not commenced.
[0235] (3) Fine Alignment Process
[0236] The fine alignment process is performed using
enhanced-global-alignment (EGA) method. In this process, if the
rotational offset of the wafer 6 is small enough to fall within the
allowable range, the EGA-method is performed by using LSA-method,
FIA-method and LIA-method, as well as the alignment sensor system 4
or any other alignment sensor system suitable for the fine
alignment process (not shown).
[0237] More specifically, the wafer 6 has an array of shot areas
defined on its top surface, some of which are preselected as the
sample shot areas. Each sample shot area has an EGA alignment mark
formed therein through a previous lithographic process. In the fine
alignment process, the positions of the EGA alignment marks are
sequentially measured. The measurements are mathematically
processed using a so-called statistical arithmetic operation, such
as least-squares-method operation, so as to derive position data
representing positions of all the shot areas defined in array on
the wafer 6. The position data thus derived is used in the
following exposure process, in which the wafer 6 is sequentially
aligned with accuracy to appropriate positions relative to the
position of the reticle 1.
[0238] (4) Thereafter, the exposure process in which a pattern
image on the reticle 1 is transferred on to the wafer through a
projection optical system 3 is carried out while each of the shot
area on the wafer is positioned with respect to an exposure
position based on the coordinate position for each shot area and a
baseline value measured in advance. The coordinate position of each
shot area is determined through EGA-method as set forth above.
Adjustment of translational offset of the wafer 6 is performed by
fine corrective movement of the X-stage and Y-stage or the reticle
stage 32 and adjustment of rotational offset of the wafer 6 is
performed by fine rotational movement of the reticle stage 32.
[0239] After exposures of all the shot areas on the wafer 6 have
been completed, the vacuum suction for holding the wafer 6 on the
wafer holder 30 is relieved to release the wafer 6 therefrom, and
the lift support 38 is raised to the wafer loading/unloading
position. Then, the wafer transfer arm 22 is activated to take the
exposed wafer 6 away from the lift support 38 and put a new wafer 6
on the lift support 38 for the next exposure process (step ST10).
Thereafter, the process sequence returns back to step ST1 and will
be repeated for the new wafer 6.
[0240] As described above, when the position of the alignment marks
formed on a wafer is displaced or offset with respect to the
geometrical features, such wafer can not undergo the fine alignment
process but will be rejected as a failure, resulting in poor
throughput of the lithography process. In contrast, according to
this embodiment of the present invention, for any such wafer, the
prealignment process will be repeated after the reference for the
prealignment process is so corrected as to compensate the
above-mentioned second displacement or offset, so that even such
wafer will undergo the fine alignment process without being
rejected as a failure.
[0241] In general, many wafers in the same lot are treated as a
batch in a lithographic process. Further, the relationship between
the set of positions of the alignment marks formed on a wafer and
the geometrical features defined by the peripheral edge of the
wafer tends to be substantially invariable among the wafers in the
same lot. As a result, in this embodiment, it must be determined in
step ST6 in the search alignment process that the offset of the
alignment marks with respect to the geometrical features of the
second and later wafers will be within an allowable range.
Therefore, while the first wafer in one lot may possibly has to
undergo the second-time prealignment process due to a possible
negative decision in step ST6 in the search alignment process, it
is quite improbable that any of the second and later wafers in that
lot has to undergo the prealignment process more than once, leading
to high throughput of the exposure process.
[0242] The embodiment described above merely presents an example
for providing clear understanding of the present invention, and is
not intended to limit the present invention. Indeed, any of the
elements and components shown in relation to the embodiment may be
modified to be various equivalents or may be embodied in various
other forms without departing from the scope and spirit of the
present invention.
[0243] For example, while in the prealignment process described
above the reference for the prealignment process may be so
corrected as to compensate only the rotational displacement or
offset of the wafer, it is contemplated that the prealignment
process is modified such that the correction of the reference for
the prealignment process is so effected as to compensate not only
the rotational offset but also the translational displacement or
offset in x- and y-directions.
[0244] The present invention may be applied to any of various types
of exposure apparatus including, for example, step-and-repeat
demagnification-projection steppers and step-and-scan
demagnification-projection steppers (also called "scanning
steppers"). As known, in a step-and-repeat
demagnification-projection stepper, the whole of the shot area of
the reticle pattern is exposed by one-shot exposure while both the
reticle and the wafer being held stationary and the whole of the
reticle pattern is illuminated at a time by the shot of
illumination light, so that the reticle pattern is printed and
transferred onto one area (shot area) through the exposure light
beam. In a step-and-scan demagnification-projection stepper, a
reticle and a wafer are synchronously moved and the reticle pattern
is serially transferred onto the aligned shot area by scanning and
exposing a slit-shaped exposure light beam which "moves" relative
to the shot area. Scanning and exposing of the exposure light are
repeated with respect to other shot areas on the wafer with the
wafer being moved in sequence.
[0245] The illumination light which forms the exposure light beam
may comprise any of various kinds of light, including light of one
of bright lines from a mercury-vapor lamp (such as g-line or
i-line), light from a krypton-fluorine (KrF) excimer laser (having
a wavelength of 248 nanometers (nm)), light from an argon-fluorine
(RrF) excimer laser (having a wavelength of 193 nm), light from a
fluorine (F.sub.2) laser (having a wavelength of 157 nm), light
from an argon (Ar.sub.2) laser (having a wavelength of 126 nm) and
a harmonic light such as a YAG laser. In addition, the illumination
light may also comprise a harmonic light. Such a harmonic light may
be made by amplifying an infrared or visible single-line laser
through an optical-fiber amplifier comprising an optical fiber
doped with erbium (or both erbium and yttrium) and converting
wavelength thereof into ultraviolet rays through a nonlinear
optical crystal. Such a single-line laser may be output from a
distributed-feedback (DFB) semiconductor laser or from an
optical-fiber laser.
[0246] Moreover, the present invention may be applied to an extreme
ultraviolet (EUV) exposure apparatus. In a typical EUV exposure
apparatus: the illumination light comprises EUV rays having its
wavelength spectrum extending from 5 nm to 15 nm (corresponding to
the region of soft X-rays); the illumination light beam may be so
defined as to provide an arc-shaped illumination field on a
reflection mask; the demagnification projection optical system used
is composed of only reflective elements (i.e., mirrors); and the
reflection mask and the wafer are moved in synchronism with their
velocity ratio depending on the demagnification ratio of the
demagnification optical system so as to transfer the pattern formed
on the reflection mask onto the wafer.
[0247] In addition, the present invention is applicable not only to
the projection exposure apparatus used for fabrication of
semiconductor devices, liquid crystal displays, thin-film magnetic
heads and image sensors (such as charge coupled devices), but also
to those used for transferring circuit patterns onto glass
substrates or silicon wafers in order to fabricate reticles or
masks.
[0248] As clearly understood from the above, it is one of
remarkable advantages of the present invention that even an
alignment mark formed on a wafer is displaced or offset with
respect to the geometrical features thereof, the wafer will not be
rejected as a failure so that the fine alignment process may be
surely performed for such wafer. In particular, in the case where
many wafers are sequentially processed and the position of the
alignment mark formed on a plurality of wafers to be processed is
similarly displaced or offset with respect to the geometrical
features of those wafers, all of the wafers except the first one to
be processed may be processed with a short process time, leading to
high throughput of the process.
* * * * *