U.S. patent application number 09/915285 was filed with the patent office on 2002-01-31 for exposure method for overlaying one mask pattern on another.
This patent application is currently assigned to NIKON CORPORATION. Invention is credited to Kaneko, Ryoichi, Kawakubo, Masaharu.
Application Number | 20020012858 09/915285 |
Document ID | / |
Family ID | 27527227 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020012858 |
Kind Code |
A1 |
Kawakubo, Masaharu ; et
al. |
January 31, 2002 |
Exposure method for overlaying one mask pattern on another
Abstract
An exposure method in which mask patterns are overlaid on one
another on a substrate, which is an object to be exposed, by using
a first and second exposure apparatuses having respective exposure
fields of different sizes. The exposure method includes the steps
of: sequentially transferring a first mask pattern onto the
substrate in the form of a first array in units of a shot area of a
predetermined size by using the first exposure apparatus; detecting
at least either one of a perpendicularity error of the first array
from a design value and a mean value of rotation angles of the shot
areas in the first array when a second mask pattern is to be
sequentially transferred onto the substrate in the form of a second
array in units of a shot area different in size from the unit shot
area of a predetermined size by using the second exposure
apparatus; and rotating the second mask pattern and the substrate
relative to each other through an angle corresponding to a result
of the detection, and thereafter, sequentially transferring the
second mask pattern onto the substrate.
Inventors: |
Kawakubo, Masaharu;
(Kanagawa-ken, JP) ; Kaneko, Ryoichi;
(Kanagawa-ken, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. Box 19928
Alexandria
VA
22320
US
|
Assignee: |
NIKON CORPORATION
|
Family ID: |
27527227 |
Appl. No.: |
09/915285 |
Filed: |
July 27, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09915285 |
Jul 27, 2001 |
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09415500 |
Oct 12, 1999 |
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09415500 |
Oct 12, 1999 |
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09236090 |
Jan 25, 1999 |
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5989761 |
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09236090 |
Jan 25, 1999 |
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08654419 |
May 28, 1996 |
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Current U.S.
Class: |
430/22 ; 430/30;
430/311 |
Current CPC
Class: |
G03F 9/7046 20130101;
G03F 7/70358 20130101; G03F 7/70633 20130101; G03F 7/70458
20130101 |
Class at
Publication: |
430/22 ; 430/30;
430/311 |
International
Class: |
G03F 009/00; G03C
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 1995 |
JP |
130131/1995 |
Jun 20, 1995 |
JP |
152856/1995 |
Aug 4, 1995 |
JP |
199676/1995 |
Aug 9, 1995 |
JP |
203276/1995 |
Claims
1. An exposure method in which mask patterns are overlaid on one
another on a photosensitive substrate, which is an object to be
exposed, by using a first exposure apparatus having a first
exposure field of a predetermined size on said photosensitive
substrate, and a second exposure apparatus having a second exposure
field which is M.sub.1/N.sub.1 times (M.sub.1 and N.sub.1 are
integers; M.sub.1>N.sub.1) as large as said first exposure field
in a first direction and which is M.sub.2/N.sub.2 times (M.sub.2
and N.sub.2 are integers; M.sub.2.gtoreq.N.sub.2) as large as said
first exposure field in a second direction which is perpendicular
to said first direction, said exposure method comprising: a first
step of sequentially transferring an image of a first mask pattern,
which has an alignment mark and a first overlay accuracy measuring
mark, onto said photosensitive substrate in the form of a
two-dimensional array extending in said first and second directions
in units of said first exposure field by using said first exposure
apparatus; a second step of transferring an image of a second mask
pattern, which has a second overlay accuracy measuring mark, over a
plurality of images of said first mask pattern which have been
transferred onto said photosensitive substrate in said first step,
in a two-dimensional array extending in said first and second
directions on said photosensitive substrate in units of said second
exposure field with reference to a position of an image of said
alignment mark by using said second exposure apparatus; and a third
step of dividing an exposure area on said photosensitive substrate
into a plurality of reference measurement areas in units of an area
which is N.sub.1 times as large as a width of said second exposure
field in said first direction on said photosensitive substrate and
which is N.sub.2 times as large as a width of said second exposure
field in said second direction on said photosensitive substrate,
and measuring an amount of positional displacement between images
of said first and second overlay accuracy measuring marks lying at
mutually identical positions in a predetermined number of reference
measurement areas selected from among said plurality of reference
measurement areas, thereby obtaining a correction value which is
used when the position of the image of said alignment mark
transferred by said first exposure apparatus is detected by said
second exposure apparatus on the basis of said measured amount of
positional displacement; wherein an exposure position is corrected
by using said correction value obtained in said third step when
overlay exposure is to be carried out thereafter by using said
second exposure apparatus with respect to a surface of said
photosensitive substrate exposed by said first exposure
apparatus.
2. An exposure method according to claim 1, wherein, in said second
step, said second exposure apparatus calculates an exposure
position on the basis of the image of said alignment mark and by
use of a predetermined coordinate transformation parameter, and in
said third step, said second exposure apparatus obtains a
correction value for said coordinate transformation parameter.
3. An exposure method in which mask patterns are overlaid on one
another on a photosensitive substrate by using a first exposure
apparatus and a second exposure apparatus having respective
exposure fields of different sizes, said exposure method comprising
the steps of: sequentially transferring images of first and second
mask patterns containing overlay accuracy measuring marks onto a
photosensitive substrate for evaluation such that the images of
said first and second mask patterns are overlaid on one another by
using said first and second exposure apparatus; measuring an amount
of positional displacement between the overlaid images of said
overlay accuracy measuring marks at a predetermined measuring point
in a reference measurement area on said evaluation photosensitive
substrate in which a shot area formed in units of the exposure
field of said first exposure apparatus and a shot area formed in
units of the exposure field of said second exposure apparatus are
overlaid on one another such that neither of said overlaid shot
areas extends beyond a part of said reference measurement area; and
effecting alignment or correction of image-formation
characteristics on the basis of a result of said measurement when
exposure is to be carried out by said second exposure apparatus
with respect to a surface of said photosensitive substrate exposed
by said first exposure apparatus.
4. An exposure method in which mask patterns are overlaid on one
another on a photosensitive substrate, which is an object to be
exposed, by using a first exposure apparatus having a first
exposure field of a predetermined size on said photosensitive
substrate, and a second exposure apparatus having a second exposure
field which is M.sub.1/N.sub.1 times (M.sub.1 and N.sub.1 are
integers; M.sub.1.noteq.N.sub.1) as large as said first exposure
field in a first direction and which is M.sub.2/N.sub.2 times
(M.sub.2 and N.sub.2 are integers) as large as said first exposure
field in a second direction which is perpendicular to said first
direction, said exposure method comprising: a first step of
sequentially transferring an image of a first mask pattern, which
has an alignment mark and a first overlay accuracy measuring mark,
onto a plurality of first shot areas arrayed on said photosensitive
substrate in units of said first exposure field by using said first
exposure apparatus; a second step of sequentially transferring an
image of a second mask pattern, which has a second overlay accuracy
measuring mark, onto a plurality of second shot areas arrayed on
said photosensitive substrate, exposed in said first step, in units
of said second exposure field with reference to a position of an
image of said alignment mark by using said second exposure
apparatus; and a third step of defining a plurality of reference
measurement areas on said photosensitive substrate in each of which
any one of said first shot areas and any one of said second shot
areas are overlaid on one another such that neither of said
overlaid shot areas extends beyond a part of said reference
measurement area, and measuring an amount of positional
displacement between images of said first and second overlay
accuracy measuring marks lying at mutually identical positions in a
predetermined number of reference measurement areas selected from
among said plurality of reference measurement areas, thereby
obtaining a correction value which is used when the position of the
image of said alignment mark transferred by said first exposure
apparatus is detected by said second exposure apparatus on the
basis of said measured amount of positional displacement.
5. An exposure method according to claim 4, wherein the correction
value obtained in said third step is a correction value for a
parameter indicating a predetermined image-formation characteristic
calculated on the basis of the position of the image of said
alignment mark, said parameter being at least one parameter
selected from a parameter group consisting of shot magnification,
shot rotation, and shot perpendicularity, and wherein said
image-formation characteristic is corrected by using the correction
value obtained in said third step when overlay exposure is to be
carried out thereafter by using said second exposure apparatus with
respect to a surface of said photosensitive substrate exposed by
said first exposure apparatus.
6. An exposure method according to claim 4, wherein said first
exposure apparatus is a one-shot exposure type projection exposure
apparatus, and said second exposure apparatus is a scanning
exposure type projection exposure apparatus.
7. An exposure method in which a substrate is exposed with a second
pattern by using a second exposure apparatus, the method
comprising: providing the substrate on which a plurality of first
shot areas are formed, the plurality of first shot areas on the
substrate being formed by exposing the substrate with a first
pattern by using a first exposure apparatus before exposing the
substrate with the second pattern, each of the first shot areas
having M partial areas along a first direction (M is an integer),
and the first and second exposure apparatus having respective
exposure fields of different sizes; and performing a scanning
exposure in which each of a plurality of second shot areas is
exposed while moving the substrate along the first direction in the
second exposure apparatus, each of the second shot areas having N
partial areas along the first direction (N is an integer;
N.noteq.M), each partial area of the first shot areas substantially
overlapping with each partial area of the second shot areas.
8. An exposure method according to claim 7, wherein the substrate
is exposed by using the first exposure apparatus so that the first
shot areas formed into a plurality of rows parallel to the first
direction and the first shot areas adjacent to each other in the
first direction are arranged without positional deviation in a
second direction perpendicular to the first direction.
9. An exposure method according to claim 7, wherein a position of
the substrate in the second direction during scanning exposure for
one second shot area SC1 is different from a position of the
substrate in the second direction during scanning exposure for
another second shot area SC2 which is adjacent to the one second
shot area SCI in the first direction.
10. An exposure method according to claim 9, further comprising;
obtaining shot rotation information .theta. of each of the
plurality of first shot areas, wherein the substrate is moved in
the first direction based on the obtained shot rotation information
.theta..
11. An exposure method according to claim 7, further comprising:
obtaining shot rotation information .theta. of each of the
plurality of first shot areas, wherein the substrate is moved in
the first direction based on the obtained shot rotation information
.theta..
12. An exposure method according to claim 7, wherein M is an even
number and N is an odd number.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an exposure method for
transferring a mask pattern onto a photosensitive substrate during
photolithography processes in the manufacture of semiconductor
devices, liquid crystal display devices, imaging devices (e.g.
CCD), thin-film magnetic heads, etc. More particularly, the present
invention relates to an exposure method which is suitably applied
to a process in which exposure is sequentially carried out by the
mix-and-match method with respect to two layers, that is, a layer
called "middle layer", which requires no high resolution, such as
an ion-implanted layer used in production of a semiconductor memory
or the like, and a layer called "critical layer", which requires
high resolution.
[0003] 2. Related Background Art
[0004] Exposure apparatuses, e.g. step-and-repeat reduction
projection type exposure apparatuses (steppers), are used in
photolithography processes for producing semiconductor devices,
liquid crystal display devices, etc. Generally, a semiconductor
device such as a VLSI is formed by stacking a multiplicity of
pattern layers on a wafer while effecting alignment for each layer.
Among the pattern layers, a layer that needs the highest resolution
is called "critical layer", and a layer that needs no high
resolution, e.g. an ion-implanted layer used in production of a
semiconductor memory or the like, is called "middle layer". In
other words, the line width of a pattern which is exposed for the
middle layer is wider than the line width of a pattern exposed for
the critical layer.
[0005] There has been an increasing tendency for recent VLSI
manufacturing factories to carry out exposure operations for
different layers by using respective exposure apparatuses in a
process for producing a single type of VLSI in order to increase
the throughput (i.e. the number of wafers processed per unit time)
in the production process. Under these circumstances, it has become
common practice to carry out what is called "mix-and-match"
exposure. In the mix-and-match exposure process, exposure for the
critical layer is carried out by using a first stepper of high
resolution which performs one-shot exposure with a demagnification
ratio of 5:1, and exposure for the middle layer is carried out by
using a second stepper of intermediate resolution which performs
one-shot exposure with a demagnification ratio of 2.5:1. In this
case, the size of the exposure field of the second stepper is twice
as large as that of the first stepper in both lengthwise and
breadthwise directions, and the throughput of the second stepper in
the exposure process is approximately four times that of the first
stepper. This will be explained below with reference to FIG.
35.
[0006] Assuming that, as shown in FIG. 35, exposure units on a
wafer which are to be exposed by the first stepper are square shot
areas SA.sub.11, SA.sub.12, SA.sub.13, SA.sub.14, . . . each
surrounded by sides which are parallel to X- and Y-axes
perpendicularly intersecting each other, an exposure area which is
to be exposed by the second stepper is a shot area SB.sub.1 which
is so large as to substantially contain the four shot areas
SA.sub.11 to SA.sub.14. When exposure is to be carried out by the
second stepper over the four shot areas SA.sub.11, SA.sub.12,
SA.sub.13 and SA.sub.14 exposed by the first stepper, the second
stepper effects alignment of the shot area SB.sub.1, which
corresponds to the exposure field of the second stepper, on the
basis of alignment marks (wafer marks) attached to the shot areas
SA.sub.11 to SA.sub.14.
[0007] There is another conventional exposure method in which, for
example, a step-and-scan type scanning exposure apparatus with a
demagnification ratio of 4:1 is combined with either the
above-described first or second stepper. The step-and-scan exposure
is a process in which a shot area on a wafer which is to be exposed
is stepped to a scanning start position, and thereafter a reticle,
which serves as a mask, and the wafer are synchronously scanned
with respect to a projection optical system, thereby sequentially
transferring a pattern on the reticle onto the shot area. The
exposure field of the scanning exposure apparatus is equal, for
example, in the width of the non-scanning direction to the exposure
field of the first stepper, but the exposure field width in the
scanning direction of the scanning exposure apparatus is 1.5 times
that of the first stepper. It should be noted that there are
various combinations of different exposure field sizes of a
plurality of exposure apparatuses used in the mix-and-match
exposure method in addition to the above-described
combinations.
[0008] Thus, the throughput of an exposure process can be increased
by carrying out a mix-and-match exposure process using different
exposure apparatuses in combination according to the resolution
required for each layer on a wafer as described above. However,
when exposure apparatuses having respective exposure fields of
different sizes are used in combination, if a perpendicularity
error remains in the array of shot areas (i.e. shot array) of the
preceding layer, i.e. if the angle between the X- and Y-axes of the
shot array deviates from 90.degree., a given overlay error arises.
Such a perpendicularity error is due to the fact that the feed
directions of the wafer stage driven by motors are not accurately
perpendicular to each other.
[0009] For example, assuming that in FIG. 35 the imaginary straight
line 23A passing through the centers of the shot areas SA.sub.13
and SA.sub.14 in the four shot areas of the preceding layer is
parallel to the X-axis, if the angle between the X- and Y-axes of
the shot array deviates from 90.degree. by an angle
(perpendicularity error) W, the imaginary straight line 24 passing
through the centers of the shot areas SA.sub.11 and SA.sub.13 tilts
by the perpendicularity error W [rad] relative to the Y-axis. In
this case, if exposure is carried out with the center of the
subsequent shot area SB.sub.1 aligned with the center 25 of the
four shot areas SA.sub.11, SA.sub.12, SA.sub.13 and SA.sub.14,
which have the perpendicularity error W, a uniform overlay error
.DELTA.x arises in the direction X between the pattern in the shot
area SB.sub.1 and the pattern in each of the shot areas SA.sub.11
to SA.sub.14 of the preceding layer. Assuming that the length of
each side of the shot area SA.sub.11 is L, the overlay error
.DELTA.x is approximately L.multidot.W/2.
[0010] In a case where each shot area of the preceding layer has a
shot rotation (chip rotation) also, an overlay error arises which
is similar to that in a case where the angle between the X- and
Y-axes of the shot array deviates from 90.degree..
[0011] FIG. 36 shows the four shot areas SA.sub.11 to SA.sub.14 in
a situation where the perpendicularity error of the shot array is
zero, but the shot rotation is .theta. [rad]. Let us assume that
the shot rotation .theta. is of the same size as the
perpendicularity error W in FIG. 35. In the case of FIG. 36, even
if the subsequent shot area SB.sub.1 is exposed by rotating it
simply through an angle corresponding to the shot rotation .theta.,
a uniform overlay error .DELTA.x of the same size as that in the
case of FIG. 35 arises in the direction of the shot rotation
between the pattern in the shot area SB.sub.1 and the pattern in
each of the shot areas SA.sub.11 to SA.sub.14 of the preceding
layer.
[0012] That is, when exposure is sequentially carried out by using
exposure apparatuses having respective exposure fields of different
sizes, if the array of shot areas of the preceding layer has a
perpendicularity error or a shot rotation, a uniform overlay error
arises if the subsequent shot areas are simply aligned with respect
to the preceding shot areas.
[0013] On the other hand, in the above-described mix-and-match
method, in which after a layer on a wafer has been exposed by a
first exposure apparatus, overlay exposure is carried out on the
preceding layer by using a second exposure apparatus, the second
exposure apparatus may effect alignment by an enhanced global
alignment (hereinafter referred to as "EGA") method as disclosed,
for example, in Japanese Patent Application Unexamined Publication
(KOKAI) (hereinafter referred to as "JP(A)") No. 61-44429
(corresponding to U.S. Pat. No. 4,780,617). In this case, however,
some problems are experienced, which will be explained below with
reference to FIGS. 37(a) to 38(c).
[0014] FIGS. 37(a), 37(b) and 37(c) illustrate a related art in
which exposure is carried out by the mix-and-match method using two
exposure apparatuses having respective exposure fields of the same
size. First, a pattern image of a reticle RA shown in FIG. 37(b) is
transferred onto each of shot areas 129A, 129B, . . . , 129I of a
first layer, which are shown by the chain lines in FIG. 37(a), on a
wafer 20 by using a first exposure apparatus. In this case, it is
assumed that a coordinate system that defines each particular
travel position of a wafer stage of the first exposure apparatus
(i.e. stage coordinate system) comprises an X1-axis and a Y1-axis,
and that the Y1-axis is tilted by an angle W clockwise from an
ideal Y1*-axis which is perpendicular to the X1-axis. Further, the
reticle RA has two identical circuit patterns 112A and 112B (i.e.
two-chip pattern) formed in a pattern area 42A. The rotation angle
of the reticle RA has been set so that the circuit patterns 112A
and 112B are arrayed in a direction perpendicular to the X1-axis
when exposure is carried out.
[0015] As a result, the shot areas 129A to 129I of the first layer
are arrayed at a predetermined pitch along each of the X1- and
X1-axes, and the shot array has a perpendicularity error W.
Further, two identical circuit pattern images are transferred onto
each of the shot areas 129A to 129I in such a manner as to lie in
side-by-side relation to each other in a direction perpendicular to
the X1-axis.
[0016] Next, a pattern image of a reticle RC shown in FIG. 37(c) is
transferred onto each of shot areas of a second layer on the wafer
20 by using a second exposure apparatus. In this case, it is
assumed that a stage coordinate system of the second exposure
apparatus comprises an X2-axis and a Y2-axis, and that a direction
corresponding to the X1-axis of the first layer on the wafer 20 has
been set parallel to the X2-axis by pre-alignment carried out in
the second exposure apparatus. Although the origins of the
coordinate systems (X1,X1) and (X2,Y2) in FIG. 37(a) have been set
at the center of the wafer 20 for the sake of explanation, it
should be noted that the origins of these coordinate systems may be
set at any positions. The reticle RC also has two identical circuit
patterns 127A and 127B formed in a pattern area 42C, and the image
of the pattern area 42A of the reticle RA as projected on the wafer
20 (i.e. exposure field) and the projected image (exposure field)
of the pattern area 42C of the reticle RC are of the same size.
[0017] In this case, the second exposure apparatus effects
alignment by the above-described EGA method. That is, array
coordinates of wafer marks (not shown) provided for a predetermined
number of shot areas (sample shots) selected from the first layer
on the wafer 20 are measured to thereby calculate array coordinates
of all the shot areas in the stage coordinate system (X2,Y2). Thus,
the second exposure apparatus can recognize that the
perpendicularity error W is present in the shot array on the first
layer.
[0018] In the second exposure apparatus, therefore, the rotation
angle of the reticle RC is set so that the two circuit patterns
127A and 127B are arrayed in a direction perpendicular to the
X2-axis, as shown in FIG. 37(c), and thereafter, a shot array of a
second layer is set by taking into consideration the
perpendicularity error W. Then, exposure is carried out. As a
result, the circuit pattern images of the reticle RC are
transferred onto each of shot areas 130A, 130B, . . . , 130I of the
second layer, shown by the solid lines in FIG. 37(a), on the wafer
20. Thus, the shot array of the second layer is accurately overlaid
on the shot array of the first layer.
[0019] In a case where the exposure fields (shot areas) of two
exposure apparatuses have the same size as described above, even if
the shot array of the first layer has a perpendicularity error, the
overlay accuracy between the first and second layers can be
maintained at high level by effecting alignment according to the
EGA method, for example.
[0020] However, if the shot array of the first layer has a
perpendicularity error in a case where the exposure fields of the
two exposure apparatuses have different sizes, the overlay accuracy
between the two layers cannot be increased above a certain level by
an ordinary exposure method.
[0021] FIGS. 38(a), 38(b) and 38(c) illustrate a related art in
which exposure is carried out by the mix-and-match method using two
exposure apparatuses having respective exposure fields of different
sizes. First, a pattern image of a two-chip reticle RA, which has
two identical patterns 112A and 112B written in a pattern area 42A
as shown in FIG. 38(b), is transferred onto each of shot areas of a
first layer on a wafer 20 by using a first exposure apparatus.
Next, a pattern image of a three-chip reticle RB, which has three
identical circuit patterns 113A to 113C written in a pattern area
42B as shown in FIG. 38(c), is transferred onto each of shot areas
of a second layer on the wafer 20 by a second exposure apparatus.
The image of the reticle RB as projected on the wafer 20 has the
same horizontal width as that of the projected image of the reticle
RA, but the vertical width of the projected image of the reticle RB
is 3/2 times that of the reticle RA.
[0022] In this case also, the stage coordinate system of the first
exposure apparatus is denoted by (X1,Y1), and the stage coordinate
system of the second exposure apparatus is denoted by (X2,Y2), and
it is assumed that alignment and exposure are carried out with the
X2-axis aligned with the X1-axis. When exposure is carried out with
the first exposure apparatus by setting the reticle RA so that the
two circuit patterns of the reticle RA are arrayed in a direction
perpendicular to the X1-axis, the circuit patterns are transferred
onto each of shot areas 129A, 129B, . . . , 129I of the first
layer, shown by the chain lines in FIG. 38(a), on the wafer 20. In
this case also, the array of the shot areas 129A to 129I has a
perpendicularity error in the same way as in the example shown in
FIGS. 37(a) to 37(c).
[0023] Thereafter, the wafer 20 is aligned by the EGA method using
a second exposure apparatus, and then exposure is carried out in
such a manner that the three circuit patterns of the reticle RB are
arrayed in a direction perpendicular to the X2-axis. Consequently,
the three circuit patterns are transferred onto each of shot areas
131A to 131F of the second layer on the wafer 20, as shown by the
solid lines in FIG. 38(a). However, because each shot area of the
first layer has two circuit patterns transferred thereto, while
each shot area of the second layer has three circuit patterns
transferred thereto, the shot array of the first layer and the shot
array of the second layer undesirably differ from each other in the
number of rows in a direction approximately perpendicular to the
X1-axis. As a result, it becomes impossible to eliminate the effect
of a perpendicularity error, which is an error between the rows or
columns of a shot array. For example, in FIG. 38(a), if the shot
area 129A and the shot area 131A are aligned in the direction X1
(or X2), a large overlay error arises in the direction X1 between
the shot area 129B and the shot area 131A.
[0024] Meanwhile, if both a first and second exposure apparatuses
employ the EGA method, the following problems arise. The problems
will be explained below with reference to FIGS. 39(a) to 41(b).
[0025] In this EGA process, array coordinates of a predetermined
number of shot areas (sample shots), which have previously been
selected from among shot areas on a wafer, are measured to
determine, for example, six coordinate transformation parameters
for calculating array coordinates in a stage coordinate system, in
which the wafer stage is to be positioned, from the design array
coordinates of all the shot areas.
[0026] However, when a pattern for a middle layer is transferred
onto a critical layer by the mix-and-match exposure method, for
example, a given overlay error may remain if the coordinate
transformation parameters obtained by the EGA method (hereinafter
occasionally referred to as "EGA parameters") are used as they are
because different projection exposure apparatuses are used for the
critical and middle layers. This means that the EGA parameters may
have residual errors. In order to correct such residual errors, the
conventional practice is to measure overlay errors by conducting
test printing using marks for overlay accuracy measurement
(hereinafter referred to as "vernier marks"), as described
below.
[0027] FIG. 39(a) shows a wafer 20 having vernier marks formed by a
projection exposure apparatus for exposure of a critical layer. In
FIG. 39(a), shot areas SE1, SE2, . . . , SEM (M is an integer of 12
or more, for example) are arrayed on the wafer 20 at a
predetermined pitch along each of the X- and Y-axes of an
orthogonal coordinate system (X,Y). In each shot area SEm (m=1 to
M), alignment marks (wafer marks) and overlay accuracy measuring
vernier marks have been formed.
[0028] FIG. 39(b) is an enlarged view showing the mark arrangement
in a shot area SEm. In FIG. 39(b), the shot area SEm has a wafer
mark 221X for the X-axis formed at an end in the direction +Y. The
wafer mark 221X comprises line-and-space patterns arranged at a
predetermined pitch in the direction X. The shot area SEm further
has a wafer mark 221Y for the Y-axis formed at an end in the
direction +X. The wafer mark 221Y comprises line-and-space patterns
arranged at a predetermined pitch in the direction Y. The wafer
marks 221X and 221Y are marks which are detected by an imaging
detection method (FIA method). Further, the shot area SEm has
vernier marks 222A to 222E formed therein at respective positions
which are distributed in a cross shape. The vernier marks 222A to
222E are, for example, box-in-box marks which are detected by an
imaging detection method (image processing detection method).
[0029] Next, predetermined vernier marks are overlaid on the wafer
20 shown in FIG. 39(a) by exposure using a projection exposure
apparatus for a middle layer. For the overlay exposure, it is
necessary to obtain array coordinates of each shot area SEm (m=1 to
M) on the wafer 20 in the stage coordinate system of the projection
exposure apparatus for a middle layer. Therefore, it is assumed
that the wafer marks 221A and 221Y of each shot area SEm (m=1 to M)
indicate the coordinates of the center of the corresponding shot
area. It is further assumed that the design array coordinates of
the center of each shot area SEm (m=1 to M) in the coordinate
system on the wafer 20 (i.e. the sample coordinate system) are
(Dxn,Dyn), and that the computational array coordinates of each
shot area SEm (m=1 to M) in the stage coordinate system of the
projection exposure apparatus for a middle layer are (Fxn,Fyn). In
this case, the X component Dxn and Y component Dyn of the design
array coordinates of the center of each shot area SEm are the X
coordinate of the corresponding wafer mark 221X and the Y
coordinate of the corresponding wafer mark 221Y, respectively,
which may be approximately expressed by the following equation (1):
1 [ Fxn Fyn ] = [ Rx - Rx ( W + ) Ry Ry ] [ Dxn Dyn ] + [ Ox Oy ] (
1 )
[0030] The transformation matrix in Eq. (1) has as elements six
coordinate transformation parameters (EGA parameters), including
scaling parameters Rx and Ry, rotation .THETA., perpendicularity W,
and offsets Ox and Oy. The scaling parameters Rx and Ry are linear
expansion and contraction quantities in the directions X and Y,
respectively. The rotation .THETA. is an angle of rotation of the
wafer 20. The perpendicularity W is a perpendicularity error, that
is, a deviation of the intersection angle between the X- and Y-axes
from 90.degree.. The offsets Ox and Oy are shift quantities in the
directions X and Y, respectively. Next, in order to determine
values of the six coordinate transformation parameters, the
projection exposure apparatus for a middle layer measures array
coordinates in the stage coordinate system of the wafer marks 221X
and 221Y provided for each of, for example, 10 shot areas (sample
shots) SEa, SEb, SEc, . . . , SEj selected from among the shot
areas on the wafer 20 shown in FIG. 39(a). The sample shots SEa to
SEj are disposed at the vertices of an approximately regular
polygon on the surface of the wafer 20 or at uniformly dispersed
random positions.
[0031] In this case, the measured values of the array coordinates
in the stage coordinate system of the wafer marks 221X and 221Y
obtained by the n-th measuring operation (n=1 to 10), that is, the
measured array coordinates of the center of the n-th sample shot,
are assumed to be (Mxn,Myn). Next, the design array coordinates
(Dxy,Dyn) of the wafer marks 221X and 221Y are substituted into the
right-hand side of Eq. (1) to obtain computational array coordinate
values (Fxn,Fyn). Then, deviations of the measured coordinate
values (Mxn,Myn) from the computational array coordinate values
(Fxn,Fyn), that is, alignment errors (Exn,Eyn)(=(Mxn-Fxn,Myn-Fyn)),
are obtained. Thereafter, values of the six EGA parameters are
determined so as to minimize the sum of the squares of the
alignment errors obtained for all the sample shots, that is, the
residual error component.
[0032] Assuming that the number of measured sample shots is K (K=10
in FIG. 39(a)), the residual error component is expressed by the
following equation (2). For example, values of the six EGA
parameters (scaling parameters Rx, Ry, wafer rotation .THETA.,
perpendicularity W, and offset quantities Ox, Oy) are obtained by
solving simultaneous equations established by setting the result of
partial differentiation of Eq. 2 with respect to each of the six
EGA parameters equal to zero. 2 Residual error component = n = 1 K
{ ( Mxn - FXn ) 2 + ( Myn - Fyn ) 2 } ( 2 )
[0033] Next, the six EGA parameter values thus obtained and the
design array coordinate values (Dxm,Dym) of each shot area SEm (m=1
to M) are sequentially substituted into the right-hand side of Eq.
(1), thereby obtaining array coordinate values in the stage
coordinate system of each shot area SEm of the critical layer on
the wafer 20. Assuming that the demagnification ratio for the
critical layer is 5:1, while the demagnification ratio for the
middle layer is 2.5:1, that is, the exposure field of the
projection exposure apparatus for the middle layer is 2 times as
large as the exposure field of the projection exposure apparatus
for the critical layer in both the directions X and Y, each middle
layer shot area contains four critical layer shot areas.
[0034] Therefore, when exposure is to be carried out by the middle
layer projection exposure apparatus, the critical layer shot areas
SEm (m=1 to M) shown in FIG. 39(a) are divided into a plurality of
blocks each comprising two shot areas in the direction X and two
shot areas in the direction Y, and array coordinates in the stage
coordinate system of the center of each block are obtained from the
computational array coordinates of the four shot areas in the
block. Thereafter, the array coordinates of the center of each
block on the wafer 20 are sequentially aligned with the center of
the exposure field of the middle layer projection exposure
apparatus, and a pattern image of a reticle for the middle layer,
which contains vernier marks, is transferred onto each block by
exposure. After the exposure process, the wafer 20 is subjected to
development process.
[0035] FIG. 40(a) shows the wafer 20 having overlaid vernier marks
formed by the middle layer projection exposure apparatus. In FIG.
40(a), shot areas SF1, SF2, . . . , SFN (N is an integer of 3 or
more, for example) of the middle layer are arrayed on the wafer 20
at a predetermined pitch along each of the X- and Y-axes, and each
shot area SFn (n=1 to N) contains four critical layer shot areas.
Further, the center 261 of each shot area SFn is approximately
coincident with the center of the corresponding block of four
critical layer shot areas. Each shot area SFn has 20 (=4.times.5)
vernier marks corresponding to a total of 20 vernier marks of the
critical layer, that is, four groups of five vernier marks 222A to
222E (see FIG. 40(b)).
[0036] Here, four shot areas SFa to SFd (shaded shot areas in FIG.
40(a)) are defined as objects to be measured, and amounts of
positional displacement of the middle layer vernier marks relative
to the critical layer vernier marks are measured, for example, at
measuring points 262 to 265 selected at random in the shot areas
SFa to SFd. FIG. 40(b) shows the shot area SFa among the four. In
FIG. 40(b), the middle layer shot area SFa has middle layer vernier
marks 224A to 224E, 226A to 226E, 228A to 228E, and 230A to 230E
formed to surround the vernier marks, respectively, which belong to
four critical layer shot areas SEp, SE(p+1), SEq and SE(q+1), which
underlie the shot area SFa. Accordingly, at the measuring point 262
in the shot area SFa, an amount of positional displacement in the
direction X or Y of the middle layer vernier mark 226C relative to
the critical layer vernier mark 222C in the shot area SE(p+1) is
measured. Similarly, an amount of positional displacement between
the two corresponding vernier marks is measured at each of the
measuring points 263 to 265.
[0037] Consequently, if all the critical layer vernier marks are
displaced, for example, by a predetermined amount .delta.X in the
direction X relative to the middle layer vernier marks at all the
measuring points 262 to 265, in FIG. 40(a), it is revealed that the
X-axis offset Ox in the EGA parameters has a residual error
.delta.X. Therefore, the residual error is previously stored in a
control system of the middle layer projection exposure apparatus as
a system constant to correct an alignment result, thereby making it
possible to form a middle layer pattern over the critical layer by
exposure with high overlay accuracy.
[0038] Thus, residual errors of the EGA parameters can be corrected
by measuring amounts of positional displacement between the
critical layer vernier marks and the middle layer vernier marks.
However, no particular consideration has heretofore been given to
the arrangement of measuring points for measuring amounts of
positional displacement between the critical layer vernier marks
and the middle layer vernier marks, as shown by the measuring
points 262 to 265 in FIG. 40(a). Accordingly, when the projected
image for the middle layer has a magnification error or a rotation
error, for example, the magnification or rotation error may be
erroneously judged to be a residual error of the EGA
parameters.
[0039] The above problem will be explained below with reference to
FIGS. 40(a) to 41(b). FIG. 41(a) shows a state where the middle
layer shot area SFa is slightly enlarged relative to a projected
image 266 obtained when there is no magnification error. As shown
in FIG. 41(a), in the central portion at the right end of the first
quadrant of the shot area SFa (i.e. the critical layer shot area
SE(p+1)), the middle layer vernier mark 226C is displaced relative
to the critical layer vernier mark 222C by .DELTA.x1 and .DELTA.y1
in the directions X and Y, respectively. In the center portion at
the right end of the second quadrant (i.e. the shot area SEp), the
middle layer vernier mark 224C is displaced relative to the
critical layer vernier mark 222C by approximately .DELTA.y 1 in the
direction Y, but the amount of displacement in the direction X of
the middle layer vernier mark 224C is so small as to be ignorable.
Similarly, in the third quadrant (i.e. the shot area SEq) and the
fourth quadrant (i.e. the shot area SE(q+1)), the two vernier marks
are displaced in symmetric relation to those in the second and
first quadrants, respectively.
[0040] When a projected image of the middle layer has such a
magnification error, if an amount of positional displacement in the
direction X between the two corresponding vernier marks is measured
at the measuring point 265 in the first quadrant of the shot area
SFd, shown in FIG. 40(a), and at the measuring point 263 in the
second quadrant of the shot area SFb, shown in FIG. 40(a), the
results of the measurement are .DELTA.x 1 and 0, respectively.
Accordingly, if residual errors of the EGA parameters of Eq. (1)
are obtained by simply processing these amounts of positional
displacement, predetermined errors remain in the scaling parameter
Rx and offset Ox in the direction X, respectively.
[0041] If an amount of positional displacement in the direction X
between the two corresponding vernier marks is measured at the
measuring point 262 in the first quadrant of the shot area SFa,
shown in FIG. 40(a), and at the measuring point 264 in the second
quadrant of the shot area SFc, shown in FIG. 40(a), the results of
the measurement are .DELTA.x 1 and 0, respectively. Accordingly, if
residual errors of the EGA parameters of Eq. (1) are obtained by
simply processing these amounts of positional displacement,
predetermined errors remain in the perpendicularity W and the
offset Ox in the direction X, respectively. That is, if an amount
of positional displacement in the direction X between two
corresponding vernier marks is measured at measuring points in
middle layer shot areas defined as objects to be measured, which
measuring points are in different columns on the critical layer,
the magnification error of the middle layer may be mistaken for a
residual error (linear error) in the EGA parameters. Such erroneous
recognition may also occur in the case of measuring an amount of
positional displacement in the direction Y between two
corresponding vernier marks.
[0042] FIG. 41(b) shows a state where the middle layer shot area
SFa has been rotated counterclockwise relative to the projected
image 266 obtained when there is no error (i.e. a state where the
shot area SFa has a shot rotation error). As shown in FIG. 41(b),
in the central portion at the right end of the first quadrant of
the shot area SFa, the middle layer vernier mark 226C is displaced
relative to the critical layer vernier mark 222C by -x2 and
.DELTA.y2 in the directions X and Y, respectively. In the central
portion at the right end of the second quadrant (i.e. the shot area
SEp), the middle layer vernier mark 224C is displaced relative to
the critical layer vernier mark 222C by approximately -.DELTA.x3 in
the direction X, but the amount of displacement in the direction Y
of the middle layer vernier mark 224C is so small as to be
ignorable. Similarly, in the third and fourth quadrants, the two
corresponding vernier marks are displaced in symmetric relation to
those in the second and first quadrants, respectively.
[0043] When a projected image of the middle layer has such a
rotation error, if an amount of positional displacement in the
direction X between two corresponding vernier marks is measured at
the measuring point 265 in the first quadrant of the shot area SFd,
shown in FIG. 40(a), and at the measuring point 263 in the second
quadrant of the shot area SFb, shown in FIG. 40(a), the results of
the measurement are -.DELTA.x2 and -.DELTA.x3, respectively.
Accordingly, if residual errors in the EGA parameters of Eq. (1)
are obtained by simply processing these amounts of positional
displacement, an error remains in a parameter other than the offset
Ox among the EGA parameters of Eq. (1). When an amount of
positional displacement in the direction Y between two
corresponding vernier marks is measured at each of the measuring
points 265 and 263, an error similarly remains in an EGA parameter
other than the offset Oy. Thus, it will be understood that, when
amounts of positional displacement between the critical layer
vernier marks and the middle layer vernier marks are measured to
correct residual errors of the EGA parameters, a mere magnification
error or rotation error of a middle layer shot area may be mistaken
for a residual error of an EGA parameter other than the offsets Ox
and Oy depending upon the selection of the positions of measuring
points in middle layer shot areas as objects to be measured.
[0044] When critical layer shot areas (chip patterns) have a
magnification error or a rotation error (chip rotation), such an
error may also be mistaken for a residual error of an EGA parameter
other than the offsets Ox and Oy depending upon the selection of
measuring points for measuring amounts of positional displacement
between the corresponding vernier marks.
[0045] As has been described above, residual errors of the EGA
parameters can be corrected by measuring amounts of positional
displacement between the critical layer vernier marks and the
middle layer vernier marks. However, there may be residual errors
not only in the above-described coordinate transformation
parameters related to the whole wafer but also in so-called in-shot
parameters comprising shot magnifications (i.e. linear expansion
and contraction of each chip pattern in the directions X and Y) rx
and ry, shot rotation (i.e. a rotation angle of each chip pattern)
.theta., and shot perpendicularity (i.e. a perpendicularity error
of the coordinate system in each chip pattern) w.
[0046] To obtain a correction value for the shot magnification rx,
for example, it is conceivable to measure an amount of positional
displacement between two corresponding vernier marks at each of the
two opposite measuring points 262 and 266 in the shot area SFa
shown in FIG. 40(a). A residual shot magnification error, i.e. a
correction value for the shot magnification rx, should be
calculable from the difference between the X components of the
amounts of positional displacement measured at the two measuring
points 262 and 266. Similarly, a residual shot rotation error
should be calculable.
[0047] In actual practice, however, the vernier mark positions on
the critical layer may have different stepping errors because the
measuring points 262 and 266 belong to different shot areas SE(p+1)
and SEp on the critical layer. That is, if no consideration is
given to the arrangement of measuring points at which vernier marks
are to be read, variation due to the stepping accuracy of the wafer
stage may be mistaken for a residual shot magnification error or a
residual shot rotation error. If an erroneous correction value is
used to correct the corresponding in-shot parameter, the alignment
accuracy reduces disadvantageously.
SUMMARY OF THE INVENTION
[0048] In view of the above-described problems, an object of the
present invention is to provide an exposure method capable of
minimizing an overlay error when a preceding layer has a
perpendicularity error in an array of shot areas or a shot rotation
in a case where exposure is carried out by the mix-and-match method
using a plurality of exposure apparatuses having respective
exposure fields of different sizes.
[0049] Another object of the present invention is to provide an
exposure method capable of minimizing an overlay error when a
perpendicularity error remains in a shot array on a first layer in
a case where exposure is carried out by the mix-and-match method
using a plurality of exposure apparatuses which are different from
each other in the size of exposure field (shot area) on a
photosensitive substrate.
[0050] Still another object of the present invention is to provide
an exposure method capable of increasing an overlay accuracy
between a critical layer pattern and a middle layer pattern in a
case where exposure is carried out by the mix-and-match method with
respect to a substrate where a critical layer and a middle layer
are mixedly present.
[0051] The present invention provides an exposure method in which
mask patterns are overlaid on one another on a substrate, which is
an object to be exposed, by using a first and second exposure
apparatuses having respective exposure fields of different sizes.
The exposure method includes the steps of: sequentially
transferring a first mask pattern onto the substrate in the form of
a first array in units of a shot area of a predetermined size by
using the first exposure apparatus; detecting at least either one
of a perpendicularity error of the first array from a design value
and a mean value of rotation angles of the shot areas in the first
array when a second mask pattern is to be sequentially transferred
onto the substrate in the form of a second array in units of a shot
area different in size from the unit shot area of a predetermined
size by using the second exposure apparatus; and rotating the
second mask pattern and the substrate relative to each other
through an angle corresponding to a result of the detection, and
thereafter, sequentially transferring the second mask pattern onto
the substrate.
[0052] The function of the above-described exposure method
according to the present invention will be explained below. Let us
assume that the perpendicularity of the first array, which is an
array of shot areas to which the first mask pattern is to be
transferred, has an angle W of deviation from a design value
(90.degree. in general). Further, it is assumed that each of these
shot areas has a square outer shape, and that one shot area in a
second shot array to which a second mask pattern is to be
transferred is laid over four shot areas in the first array. In
this case, according to the present invention, the second mask
pattern and the substrate are rotated relative to each other so
that the angle .delta. of rotation of the shot area in the second
array, which is defined about the center of the four shot areas, is
W/4. By doing so, the overlay error between the first mask pattern
image and the second mask pattern image on the substrate is reduced
to a minimum on the average.
[0053] On the other hand, when the perpendicularity error of the
first array is zero and the shot rotation of the four shot areas is
W, the rotation angle .delta. of the shot area over the four shot
areas is also set at W/4, whereby the overlay error between the
first mask pattern image and the second mask pattern image on the
substrate is reduced to a minimum on the average.
[0054] In addition, the present invention provides another exposure
method in which mask patterns are overlaid on one another on a
substrate, which is an object to be exposed, by using a first
exposure apparatus having a first exposure field of a predetermined
size, and a second exposure apparatus which scans a mask and the
substrate synchronously to sequentially transfer a pattern formed
on the mask onto the substrate, and which has a second exposure
field different in size from the first exposure field. The exposure
method includes the steps of: sequentially transferring a first
mask pattern onto the substrate in the form of a first array in
units of a shot area of a predetermined size by using the first
exposure apparatus; detecting at least either one of a
perpendicularity error of the first array from a design value and a
mean value of rotation angles of the shot areas in the first array
when a second mask pattern is to be sequentially transferred onto
the substrate in the form of a second array over the first array in
units of a shot area different in size from the unit shot area of a
predetermined size by using the second exposure apparatus; and
displacing the second mask pattern and the substrate relative to
each other in a direction perpendicular to a scanning direction of
the second exposure apparatus by a distance corresponding to a
result of the detection, and thereafter, sequentially transferring
the second mask pattern onto the substrate by a scanning exposure
method.
[0055] In this case, it is desirable to rotate the second mask
pattern and the substrate relative to each other through an angle
corresponding to the result of detection of at least either one of
a perpendicularity error of the first array from a design value and
a mean value of rotation angles of the shot areas in the first
array.
[0056] In the above-described exposure method according to the
present invention, a scanning type exposure apparatus such as a
step-and-scan exposure apparatus is used as the second exposure
apparatus. Let us assume that the width in one direction (e.g. a
direction Y) of each shot area in the first array is L, and that
the first array has a perpendicularity error W. Further, the width
in the direction Y of each shot area in the second array formed by
the second exposure apparatus is assumed to be (3/2)L. In this
case, if exposure is carried out with the centers of shot areas in
the second array being merely aligned with the center line of the
first array, an overlay error of a predetermined maximum width
occurs between the first and second mask pattern images.
[0057] Therefore, in the above-described method according to the
present invention, exposure is carried out with the centers of shot
areas in the second array being displaced relative to the center
line of the first array in a direction perpendicular to the
direction Y by a width d (.apprxeq.L.multidot.W/4). By doing so,
the overlay error between the first mask pattern image and the
second mask pattern image on the substrate is reduced to a minimum
on the average. In a case where the first array has a shot rotation
also, the overlay error can be minimized by displacing the position
of each shot area in the second array.
[0058] Further, if the rotation that is used in the first exposure
method is used in the second exposure method, the overlay error is
further reduced.
[0059] In addition, the present invention provides another exposure
method in which a first mask pattern is transferred onto a
photosensitive substrate in the form of a predetermined array by
using a first exposure apparatus having a first exposure field of a
predetermined shape, and a second mask pattern is transferred onto
the photosensitive substrate over the first mask pattern array by
using a second exposure apparatus having a second exposure field
different from the first exposure field in length in a
predetermined direction. In the exposure method, when the first
mask pattern is to be transferred onto the photosensitive substrate
by using the first exposure apparatus, an array of a plurality of
shot areas to each of which the first mask pattern is to be
transferred is set on the photosensitive substrate along a
direction (X1) corresponding to the direction in which the first
exposure field is different in length from the second exposure
field.
[0060] According to the above-described exposure method of the
present invention, the array of a plurality of shot areas to which
the first mask pattern is to be transferred is such that an
imaginary straight line passing through shot areas which are
adjacent to each other in the direction X1, which corresponds to
the direction in which the first exposure field is different in
length from the second exposure field, is parallel to the direction
X1. As a result, when shot areas of a second layer are arrayed over
the shot areas of the first layer by using the second exposure
apparatus, the overlay error is minimized even if the shot array of
the first layer has a perpendicularity error.
[0061] In this exposure method, when the first mask pattern is to
be transferred onto the photosensitive substrate by using the first
exposure apparatus, the photosensitive substrate and the first mask
pattern have previously been rotated through 90.degree. from their
ordinary positions. By doing so, even if a perpendicularity error
is present in the shot array of the first layer, the shot areas of
the first layer can be arrayed in a straight-line form along the
direction (X1) corresponding to the direction in which the first
exposure field is different in length from the second exposure
field.
[0062] One example of the second exposure apparatus is a scanning
exposure type exposure apparatus; in this case, it is desirable
that the above-described predetermined direction should be the
scanning direction. The reason for this is that the exposure field
of a scanning exposure type exposure apparatus can be readily
lengthened in the scanning direction.
[0063] In addition, the present invention provides another exposure
method in which mask patterns are overlaid on one another on a
photosensitive substrate, which is an object to be exposed, by
using a first exposure apparatus having a first exposure field of a
predetermined size on the photosensitive substrate, and a second
exposure apparatus having a second exposure field which is
M.sub.1/N.sub.1 times (M.sub.1 and N.sub.1 are integers;
M.sub.1>N.sub.1) as large as the first exposure field in a first
direction and which is M.sub.2/N.sub.2 times (M.sub.2 and N.sub.2
are integers; M.sub.2.gtoreq.N.sub.2) as large as the first
exposure field in a second direction which is perpendicular to the
first direction. The exposure method has the first step of
sequentially transferring an image of a first mask pattern, which
has an alignment mark and a first overlay accuracy measuring mark,
onto the photosensitive substrate in the form of a two-dimensional
array extending in the first and second directions in units of the
first exposure field by using the first exposure apparatus.
[0064] The exposure method according to the present invention
further has: the second step of transferring an image of a second
mask pattern, which has a second overlay accuracy measuring mark,
over a plurality of images of the first mask pattern, which have
been transferred onto the photosensitive substrate in the first
step, in a two-dimensional array extending in the first and second
directions on the photosensitive substrate in units of the second
exposure field with reference to the position of the image of the
alignment mark by using the second exposure apparatus; and the
third step of dividing an exposure area on the photosensitive
substrate into a plurality of reference measurement areas in units
of an area which is N.sub.1 times as large as the width of the
second exposure field in the first direction on the photosensitive
substrate and which is N.sub.2 times as large as the width of the
second exposure field in the second direction on the photosensitive
substrate, and measuring an amount of positional displacement
between the images of the first and second overlay accuracy
measuring marks lying at the mutually identical positions in a
predetermined number of reference measurement areas selected from
among the plurality of reference measurement areas, thereby
obtaining a correction value which is used when the position of the
alignment mark image transferred by the first exposure apparatus is
detected by the second exposure apparatus on the basis of the
amount of positional displacement measured as described above.
Thereafter, the exposure position is corrected by using the
correction value obtained in the third step when overlay exposure
is carried out by using the second exposure apparatus with respect
to the surface of the photosensitive substrate exposed by the first
exposure apparatus.
[0065] In this case, it is desirable for the second exposure
apparatus to calculate the exposure position on the basis of the
alignment mark image and by use of a predetermined coordinate
transformation parameter (EGA parameter) and to obtain a correction
value for the coordinate transformation parameter in the third
step.
[0066] According to the above-described exposure method of the
present invention, the first exposure apparatus is used, for
example, for exposure of a critical layer, and the second exposure
apparatus is used, for example, for exposure of a middle layer
because the second exposure field is larger than the first exposure
field. Further, because the second exposure field is
M.sub.1/N.sub.1 times and M.sub.2/N.sub.2 times as large as the
first exposure field in the first and second directions,
respectively, if the widths in the first and second directions of
the first mask pattern image formed by the first exposure apparatus
are denoted by d and c, respectively, the widths in the first and
second directions of the second mask pattern image formed by the
second exposure apparatus are dM.sub.1/N.sub.1 and
cM.sub.2/N.sub.2, respectively.
[0067] Assuming that the integers M.sub.1 and N.sub.1 have no
common divider other than 1, and the integers M.sub.2 and N.sub.2
also have no common divider other than 1, an area on the
photosensitive substrate which has a size regarded as being the
least common multiple of the sizes of the first and second mask
pattern images is an area which has a width dM.sub.1 in the first
direction and a width cM.sub.2 in the second direction, that is, a
reference measurement area which is N.sub.1 times and N.sub.2 times
as large as the width of the second exposure field in the first and
second directions, respectively. Such a reference measurement area
contains an integer number of first and second mask pattern images
in each of the first and second directions.
[0068] If M.sub.1=2, N.sub.1=1, M.sub.2=2, and N.sub.2=1, for
example, the second mask pattern image itself is the reference
measurement area. In such a case, in the present invention, if an
amount of positional displacement between two corresponding
alignment mark images is measured at a measuring point in the top
right portion of the first reference measurement area, an amount of
positional displacement between two corresponding alignment mark
images is similarly measured at a measuring point in the top right
portion of each of the second to fourth reference measurement
areas. Thus, a magnification error or rotation error of the second
exposure field is approximately equally introduced into all the
measured amounts of positional displacement. Accordingly, there is
no likelihood that a magnification error or rotation error of the
second exposure field will be mistaken for an error component other
than an offset component in the amount of positional displacement
between the first and second mask pattern images. Thus, the overlay
accuracy improves.
[0069] In this case, if an alignment method in which coordinate
transformation parameters are employed, e.g. the EGA method, is
used for the exposure process carried out by the second exposure
apparatus, there is no likelihood that a magnification error or
rotation error of the second exposure field will be mistaken for a
coordinate transformation parameter other than an offset.
[0070] In addition, the present invention provides another exposure
method in which mask patterns are overlaid on one another on a
photosensitive substrate by using a first and second exposure
apparatuses having respective exposure fields of different sizes.
In the exposure method, images of a first and second mask patterns
containing overlay accuracy measuring marks are sequentially
transferred onto a photosensitive substrate for evaluation, being
overlaid on one another, by using the first and second exposure
apparatuses, and an amount of positional displacement between the
overlaid images of the overlay accuracy measuring marks is measured
at a predetermined measuring point in a reference measurement area
on the evaluation photosensitive substrate in which a shot area
formed in units of the exposure field of the first exposure
apparatus and a shot area formed in units of the exposure field of
the second exposure apparatus are overlaid on one another such that
neither of the overlaid shot areas extends over beyond a part of
the reference measurement area (or neither of the overlaid shot
areas extends over a plurality of shot areas). On the basis of the
result of the measurement, alignment or correction of
image-formation characteristics is effected when exposure is to be
carried out by the second exposure apparatus with respect to the
surface of the photosensitive substrate exposed by the first
exposure apparatus.
[0071] In the above-described exposure method according to the
present invention, the reference measurement area does not extend
over a plurality of shot areas in either of two layers. Therefore,
the amount of positional displacement measured at a measuring point
in the reference measurement area contains no effect of stepping
error of either of the exposure apparatuses. Accordingly, a high
overlay accuracy is obtained by carrying out exposure after a
parameter for alignment or a parameter indicating image-formation
characteristics has been corrected on the basis of the measured
amount of positional displacement.
[0072] In addition, the present invention provides another exposure
method in which mask patterns are overlaid on one another on a
photosensitive substrate, which is an object to be exposed, by
using a first exposure apparatus having a first exposure field of a
predetermined size on the photosensitive substrate, and a second
exposure apparatus having a second exposure field which is
M.sub.1/N.sub.1 times (M.sub.1 and N.sub.1 are integers;
M.sub.1.noteq.N.sub.1) as large as the first exposure field in a
first direction and which is M.sub.2/N.sub.2 times (M.sub.2 and
N.sub.2 are integers) as large as the first exposure field in a
second direction which is perpendicular to the first direction. The
exposure method has: the first step of sequentially transferring an
image of a first mask pattern, which has an alignment mark and a
first overlay accuracy measuring mark, onto a plurality of first
shot areas arrayed on the photosensitive substrate in units of the
first exposure field by using the first exposure apparatus; and the
second step of sequentially transferring an image of a second mask
pattern, which has a second overlay accuracy measuring mark, onto a
plurality of second shot areas arrayed on the photosensitive
substrate, exposed in the first step, in units of the second
exposure field with reference to the image of the alignment mark by
using the second exposure apparatus.
[0073] Further, the exposure method according to the present
invention has the third step of defining a plurality of reference
measurement areas on the photosensitive substrate in each of which
any one of the first shot areas and any one of the second shot
areas are overlaid on one another such that neither of the overlaid
shot areas extends over beyond a part of the reference measurement
area, and measuring an amount of positional displacement between
the images of the first and second overlay accuracy measuring marks
lying at the mutually identical positions in a predetermined number
of reference measurement areas selected from among the plurality of
reference measurement areas, thereby obtaining a correction value
which is used when the position of the alignment mark image
transferred by the first exposure apparatus is detected by the
second exposure apparatus on the basis of the amount of positional
displacement measured as described above.
[0074] In this case, the first exposure apparatus is used, for
example, for a critical layer, and the second exposure apparatus is
used, for example, for a middle layer. In the exposure method, if a
plurality of measuring points for measuring an amount of positional
displacement between a pair of overlay accuracy measuring marks are
set in each reference measurement area in order to obtain a linear
expansion and contraction error or the like in a shot area, the
distribution of the measuring points does not extend over a
plurality of first shot areas nor a plurality of second shot areas.
Accordingly, the linear expansion and contraction error or the like
can be accurately obtained without being affected by stepping
errors of the critical and middle layers, and the overlay accuracy
between the critical and middle layers is improved by correcting
the linear expansion and contraction error at the middle layer.
[0075] In this case, each alignment mark and each first overlay
accuracy measuring mark may be the same mark.
[0076] One example of the correction value obtained in the third
step is a correction value for a parameter indicating a
predetermined image-formation characteristic calculated on the
basis of the positions of the alignment mark images. One example of
such a parameter is at least one parameter selected from a
parameter group consisting of shot magnifications rx and ry, shot
rotation .theta., and shot perpendicularity w. In this case, it is
desirable to correct the image-formation characteristic by using
the correction value obtained in the third step when overlay
exposure is to be carried out thereafter by using the second
exposure apparatus with respect to the surface of the
photosensitive substrate exposed by the first exposure
apparatus.
[0077] Further, one example of the first exposure apparatus is a
one-shot exposure type projection exposure apparatus, and one
example of the second exposure apparatus is a scanning exposure
type projection exposure apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0078] FIG. 1 is a perspective view schematically showing an
exposure system used in a first example of a first embodiment of
the exposure method according to the present invention.
[0079] FIG. 2(a) illustrates the detection principle of a laser
step alignment type alignment system.
[0080] FIG. 2(b) is an enlarged view showing one example of a wafer
mark which is used in another type of alignment system.
[0081] FIG. 3 is a plan view showing a shot array of a critical
layer on a wafer in the first example.
[0082] FIG. 4 is an enlarged plan view showing a part of the shot
array shown in FIG. 3, together with a shot area of a middle layer
exposed over the shot array.
[0083] FIG. 5 is a plan view showing a shot array of a middle layer
exposed over the shot array shown in FIG. 3 in the first
example.
[0084] FIG. 6(a) is a plan view of a second example of the first
embodiment of the present invention, showing an array of shot areas
exposed on a first layer, together with one example of a shot array
in a case where shot areas exposed over the first-layer shot areas
have a large overlay error.
[0085] FIG. 6(b) is a plan view showing a shot area exposed by a
scanning type second exposure apparatus.
[0086] FIG. 7 is a plan view showing an array of shot areas in
which the overlay error reduces in the second example of the first
embodiment.
[0087] FIGS. 8(a), 8(b) and 8(c) illustrate an alignment method for
overlay exposure in which short shot areas are overlaid on an array
of long shot areas in the second example of the first
embodiment.
[0088] FIG. 9(a) is a plan view of a third example of the first
embodiment of the present invention, showing a shot array on a
wafer.
[0089] FIG. 9(b) is a plan view showing a shot area exposed by a
second exposure apparatus.
[0090] FIG. 10(a) is a plan view showing a part of the shot array
shown in FIG. 9(a), together with one example of an array in a case
where shot areas exposed over the shot array have a large overlay
error.
[0091] FIG. 10(b) is a plan view showing an array of shot areas in
which the overlay error reduces.
[0092] FIG. 10(c) illustrates an alignment method for overlay
exposure in which short shot areas are overlaid on an array of long
shot areas.
[0093] FIG. 11 is a perspective view schematically showing an
exposure system used in a first example of a second embodiment of
the exposure method according to the present invention.
[0094] FIG. 12(a) is a plan view showing the orientation of a
reticle when exposure is carried out for a first layer on a wafer
in the first example of the second embodiment.
[0095] FIG. 12(b) is a plan view showing the orientation of a wafer
when exposure is carried out for the first layer on the wafer.
[0096] FIG. 13 is a plan view for explaining an alignment method
executed before exposure is carried out for the second layer on the
wafer in the first example of the second embodiment.
[0097] FIG. 14(a) is a plan view showing a shot array when exposure
is carried out for a second layer on a wafer in the first example
of the second embodiment.
[0098] FIG. 14(b) is a plan view showing the orientation of a
reticle when exposure is carried out for the second layer.
[0099] FIG. 14(c) is a plan view showing first-layer shot areas on
the wafer.
[0100] FIG. 15 is a plan view illustrating a second example of the
second embodiment, showing a wafer which is placed on a wafer stage
of a scanning type exposure apparatus after it has been subjected
to exposure for a first layer.
[0101] FIG. 16 is a plan view showing a shot array of a second
layer on the wafer in the second example of the second
embodiment.
[0102] FIG. 17(a) is a plan view of a third example of the second
embodiment, showing shot arrays of first and second layers on a
wafer.
[0103] FIG. 17(b) is a plan view showing the orientation of a
reticle when exposure is carried out for the second layer on the
wafer.
[0104] FIG. 17(c) is a plan view showing the orientation of a
reticle when exposure is carried out for the first layer on the
wafer.
[0105] FIG. 18 is a perspective view schematically showing an
exposure system used in a third embodiment of the exposure method
according to the present invention.
[0106] FIG. 19(a) is a plan view showing a shot array of a critical
layer on a wafer in the third embodiment.
[0107] FIG. 19(b) is an enlarged plan view showing an arrangement
of vernier marks in a shot area of the critical layer.
[0108] FIG. 20(a) is a plan view showing a shot array and measuring
point arrangement on a middle layer exposed over the critical layer
shown in FIG. 19(a).
[0109] FIG. 20(b) is an enlarged plan view showing a part of the
vernier mark arrangement in a shot area of the middle layer.
[0110] FIGS. 21(a) and 21(b) show another example of the
arrangement of measuring points on the wafer.
[0111] FIG. 22(a) is an enlarged view showing another example of
the arrangement of measuring points in a shot area of the middle
layer.
[0112] FIG. 22(b) is an enlarged view showing still another example
of the arrangement of measuring points in a shot area of the middle
layer.
[0113] FIGS. 23(a) and 23(b) are plan views each showing one
example of a desirable arrangement of measuring points on a
wafer.
[0114] FIGS. 24(a) and 24(b) show one example of an arrangement of
measuring points in a case where measuring points are selected from
those which are at different positions in a plurality of reference
measurement areas on a wafer.
[0115] FIGS. 25(a), 25(b) and 25(c) show an example of reference
measurement areas used in a case where a plurality of chip patterns
fit in each shot area of a critical layer.
[0116] FIGS. 26(a), 26(b) and 26(c) show an example of reference
measurement areas used in a case where shot areas of a middle layer
are exposed by a scanning exposure method.
[0117] FIGS. 27(a) and 27(b) show that the expansion and
contraction quantity of shot areas of a middle layer differs
according to measuring points in the example shown in FIGS. 26(a)
to 26(c).
[0118] FIGS. 28(a), 28(b) and 28(c) show an example of reference
measurement areas used in a case where shot areas of a middle layer
are exposed by a scanning exposure method, and shot areas of a
critical layer are wider than the middle layer shot areas.
[0119] FIG. 29 is a perspective view schematically showing an
exposure system used in a fourth embodiment of the exposure method
according to the present invention.
[0120] FIG. 30(a) is a plan view showing a shot array of a critical
layer on a wafer in the fourth embodiment of the present
invention.
[0121] FIG. 30(b) is an enlarged plan view showing an arrangement
of vernier marks in a shot area of the critical layer shown in FIG.
30(a).
[0122] FIG. 31(a) is a plan view showing a shot array of a middle
layer exposed over the critical layer shown in FIG. 30(a).
[0123] FIG. 31(b) is an enlarged plan view showing vernier mark
arrangements on two layers in a shot area of the middle layer shown
in FIG. 31(a).
[0124] FIG. 32 is an enlarged plan view showing one example of
reference measurement areas and measuring points set on the wafer
shown in FIG. 31(a).
[0125] FIGS. 33(a), 33(b) and 33(c) show the way in which reference
measurement areas are determined in a case where a middle layer
shot area is twice as large as a critical layer shot area in each
of directions X and Y.
[0126] FIGS. 34(a), 34(b) and 34(c) show the way in which reference
measurement areas are determined in a case where first-layer shot
areas are exposed by a one-shot exposure method, while second-layer
shot areas are exposed by a scanning exposure method, and the
first-layer shot areas are wider than the second-layer shot
areas.
[0127] FIG. 35 is a view for explanation of a background art
related to the present invention, showing an overlay error due to a
perpendicularity error of the shot array on the preceding
layer.
[0128] FIG. 36 is a view for explanation of a background art
related to the present invention, showing an overlay error due to a
shot rotation of the shot array on the preceding layer.
[0129] FIGS. 37(a), 37(b) and 37(c) are views for explanation of a
background art related to the present invention, showing one
example of a mix-and-match exposure process.
[0130] FIGS. 38(a), 38(b) and 38(c) are views for explanation of a
background art related to the present invention, showing a case
where an overlay error arises in a mix-and-match exposure
process.
[0131] FIG. 39(a) is a plan view of a background art related to the
present invention, showing a shot array of a critical layer on a
wafer.
[0132] FIG. 39(b) is an enlarged plan view showing an arrangement
of vernier marks in a shot area of the critical layer shown in FIG.
39(a).
[0133] FIG. 40(a) is a plan view of a shot array and measuring
point arrangement on a middle layer exposed over the critical layer
shown in FIG. 39(a).
[0134] FIG. 40(b) is an enlarged plan view showing an arrangement
of vernier marks in a shot area of the middle layer shown in FIG.
40(a).
[0135] FIGS. 41(a) and 41(b) illustrate a background art related to
the present invention, in which FIG. 41(a) shows a shot area of a
middle layer which has a magnification error, and FIG. 41(b) shows
a middle layer shot area which has a rotation error.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0136] A first example of a first embodiment of the exposure method
according to the present invention will be described below with
reference to FIGS. 1 to 5. In this embodiment, two exposure
apparatuses are used: a first exposure apparatus of the stepper
type (one-shot exposure type) with a demagnification ratio of 5:1,
and a second exposure apparatus of the stepper type with a
demagnification ratio of 2.5:1. In this case, one shot area exposed
by the second exposure apparatus corresponds to four shot areas
exposed by the first exposure apparatus.
[0137] FIG. 1 shows an exposure system used in an exposure method
according to the first embodiment of the present invention. In the
exposure system shown in FIG. 1 are installed a first exposure
apparatus 1A of the stepper type which has a small exposure field,
and a second exposure apparatus 1B of the stepper type which has a
large exposure field. In this embodiment, the exposure apparatus 1A
is a high-resolution exposure apparatus, while the exposure
apparatus 1B is a low-resolution exposure apparatus. The
high-resolution exposure apparatus 1A is used to carry out exposure
for a critical layer on a wafer, and the low-resolution exposure
apparatus 1B is used to carry out exposure for a middle layer on
the wafer. However, the exposure apparatus 1A may be a
low-resolution exposure apparatus or the exposure apparatus 1B may
be a high-resolution exposure apparatus according to the kind of
semiconductor device to be produced.
[0138] First, in the exposure apparatus 1A, a pattern area 2A on a
reticle RA is illuminated by exposure light from an illumination
optical system (not shown), and an image of a pattern formed in the
pattern area 2A is formed on an exposure field 4A on a wafer 20 as
a projected image reduced to 1/5 by a projection optical system 3A.
A Z1-axis is taken in a direction parallel to an optical axis of
the projection optical system 3A, and two axes of an orthogonal
coordinate system set in a plane perpendicular to the Z1-axis are
defined as an X1-axis and a Y1-axis, respectively. The reticle RA
has an alignment mark 17X for the X1-axis formed at an end of the
pattern area 2A in the direction Y1 (e.g. within a masking frame)
and also has an alignment mark 17Y for the Y1-axis formed at an end
of the pattern area 2A in the direction X1.
[0139] The wafer 20 is held on a wafer stage 5A. The wafer stage 5A
comprises a Z-stage for moving the wafer 20 in the direction Z1 to
set an exposure surface of the wafer 20 which is to be exposed at
the best focus position, and an XY-stage for positioning the wafer
20 in both the directions of the X1- and Y1-axes. A pair of moving
mirrors 6A and 8A which are perpendicular to each other are fixed
on the wafer stage 5A. The coordinate in the direction X1 of the
wafer stage 5A is measured by a combination of the moving mirror 6A
and a laser interferometer 7A which is installed outside the wafer
stage 5A. The coordinate in the direction Y1 of the wafer stage 5A
is measured by a combination of the moving mirror 8A and a laser
interferometer 9A which is installed outside the wafer stage 5A.
The coordinates measured by the laser interferometers 7A and 9A are
supplied to a controller 10A which controls operations of the whole
apparatus. The controller 10A drives the wafer stage 5A to step in
both the directions X1 and Y1 through drive units (not shown),
thereby positioning the wafer 20. In this case, the stepping drive
of the wafer 20 is effected according to an array of shot areas
(i.e. unit areas to each of which a pattern image of the pattern
area 2A is to be projected by exposure) set on the exposure surface
of the wafer 20, that is, a shot map for a critical layer. The shot
map is generated by a map generating unit which comprises a
computer in the controller 10A.
[0140] The exposure apparatus 1A is provided with alignment systems
11A and 14A both of which are TTL (Through-The-Lens) and laser step
alignment type (hereinafter referred to as "LSA type") systems. An
LSA type alignment system is disclosed in detail, for example, in
JP(A) No. 60-130742. Therefore, only an outline of the alignment
systems 11A and 14A will be given below. A laser beam emitted from
the alignment system 11A for the X1-axis is reflected by a mirror
12A, which is disposed between the projection optical system 3A and
the reticle RA, and the reflected laser beam enters the projection
optical system 3A. The laser beam emanating from the projection
optical system 3A is converged onto an area near the exposure field
4A in the form of a slit-shaped light spot 13A elongated in the
direction Y1.
[0141] FIG. 2(a) shows a wafer mark MX for the X1-axis which serves
as an alignment mark on the wafer 20, which is an object to be
exposed. In FIG. 2(a), the wafer mark MX is a dot train pattern
comprising recesses and projections, which are arranged at a
predetermined pitch in a direction approximately parallel to the
slit-shaped light spot 13A. When the wafer mark MX is scanned in
the direction X1 relative to the slit-shaped light spot 13A by
driving the wafer stage 5A, shown in FIG. 1, diffracted light is
emitted in a predetermined direction as the wafer mark MX coincides
with the slit-shaped light spot 13A.
[0142] Referring to FIG. 1, the diffracted light returns to the
alignment system 11A via the projection optical system 3A and the
mirror 12A. In the alignment system 11A, the diffracted light is
photoelectrically converted by a light-receiving element to obtain
an alignment signal. The alignment signal is supplied to the
controller 10A. In the controller 10A, the X1 coordinate of the
wafer stage 5A measured when the alignment signal reaches a
maximum, for example, is sampled, thereby detecting the position of
the wafer mark MX in the direction of the X1-axis.
[0143] Similarly, a laser beam emitted from the LSA type alignment
system 14A for the Y1-axis enters the projection optical system 3A
via a mirror 15A and is converged onto the wafer 20 in the form of
a slit-shaped light spot 16A elongated in the direction of the
X1-axis. Diffracted light generated from the slit-shaped light spot
16A returns to the alignment system 14A via the projection optical
system 3A and the mirror 15A. The alignment system 14A supplies an
alignment signal to the controller 10A. Thus, the position in the
Y1-axis direction of a wafer mark for the Y1-axis on the wafer 20
is detected on the basis of the alignment signal.
[0144] It should be noted that, as each of the alignment systems
11A and 14A, it is also possible to use a TTL (Through-The-Lens)
type alignment system or an off-axis type alignment system which
detects the position of a wafer mark without passing a wafer mark
detecting light beam through the projection optical system 3A. As a
wafer mark detecting method, it is also possible to use an image
processing type detection method, or a so-called two-beam
interference type detection method in which two light beams are
applied to a diffraction grating-shaped wafer mark, and the
position of the wafer mark is detected from a signal obtained from
interference between a pair of diffracted light beams generated in
parallel from the illuminated wafer mark. When such an image
processing type or two-beam interference type alignment system is
used, a line-and-space pattern 22X as shown in FIG. 2(b) is used.
The line-and-space pattern 22X comprises recesses and projections,
which are arranged at a predetermined pitch in the measuring
direction, for example.
[0145] Next, the second exposure apparatus 1B will be explained.
The exposure apparatus 1B has an arrangement approximately similar
to that of the above-described first exposure apparatus 1A. An
image of a pattern formed in a pattern area 2B of a reticle RB is
projected through a projection optical system 3B onto an exposure
field 4B on a wafer 20 held on a wafer stage 5B as an image reduced
to 1/2.5. A Z2-axis is taken in a direction parallel to an optical
axis of the projection optical system 3B, and two axes of an
orthogonal coordinate system set in a plane perpendicular to the
Z2-axis are defined as an X2-axis and a Y2-axis, respectively. The
reticle RB has the pattern area divided into two columns in the
direction X2 and two rows in the direction Y2 to form partial
pattern areas 18A to 18D. The partial pattern areas 18A to 18D each
has the same circuit pattern formed therein. Further, the partial
pattern areas 18A to 18D are each provided with the same alignment
mark 19X for the X2-axis and the same alignment mark 19Y for the
Y2-axis. The X2 coordinate of the wafer stage 5B is measured by a
combination of a moving mirror 6B and a laser interferometer 7B.
The Y2 coordinate of the wafer stage 5B is measured by a
combination of a moving mirror 8B and a laser interferometer 9B.
The measured coordinates are supplied to a controller 10B. The
controller 10B controls the stepping drive of the wafer stage
5B.
[0146] The stepping drive of the wafer stage 5B is effected
according to an array of shot areas (i.e. areas to each of which a
pattern image of the pattern area 2B is to be projected by
exposure) set on the exposure surface of the wafer 20, that is, a
shot map for a middle layer. The shot map is generated by a map
generating unit which comprises a computer in the controller 10B.
In this case, the map generating unit in the controller 10A and the
map generating unit in the controller 10B have the function of
supplying shot map information prepared thereby to each other. When
exposure for a middle layer is to be carried out over a critical
layer, for example, shot map information for the critical layer
prepared by the map generating unit in the controller 10A of the
exposure apparatus 1A is transmitted from a communication unit in
the controller 10A to a communication unit in the controller 10B.
The map generating unit in the controller 10B generates a shot map
for the middle layer on the basis of the supplied shot map
information. Conversely, when exposure for a critical layer is to
be carried out over a middle layer, shot map information for the
middle layer prepared by the map generating unit in the controller
10B is supplied to the map generating unit in the controller
10A.
[0147] In the exposure apparatus 1B also, an alignment system 11B
for the X2-axis is a TTL and LSA type alignment system. A laser
beam from the alignment system 11B enters the projection optical
system 3B via a mirror 12B. The laser beam is converged through the
projection optical system 3B onto the wafer 20 in the form of a
slit-shaped light spot 13B elongated in the direction Y2. A laser
beam from an alignment system 14B for the Y2-axis enters the
projection optical system 3B via a mirror 15B, and the laser beam
is converged through the projection optical system 3B onto the
wafer 20 in the form of a slit-shaped light spot 16B elongated in
the direction X2. Diffracted light beams from the slit-shaped light
spots 13B and 16B are received by the corresponding alignment
systems 11B and 14B, thereby detecting the positions of the wafer
marks for the Y2- and X2-axes on the wafer 20.
[0148] Next, an exposure method in this embodiment will be
explained with reference to FIGS. 3 to 5. In this embodiment, the
exposure process will be explained by way of an example in which a
pattern image of a reticle for a middle layer is transferred by
using the second exposure apparatus 1B over a critical layer
transferred on the wafer 20 by using the first exposure apparatus
1A.
[0149] FIG. 3 shows a shot array of a critical layer on the wafer
20. In FIG. 3, the surface of the wafer 20 is divided into square
shot areas SA.sub.11, SA.sub.12, . . . , SA.sub.94 at a
predetermined pitch in each of first and second directions. Each
side of each square shot area SA has a length L. The shot areas
SA.sub.11 to SA.sub.94 have approximately the same size as that of
the exposure field 4A of the exposure apparatus 1A, shown in FIG.
1. An image of the circuit pattern in the pattern area 2A of the
reticle RA is projected onto each of the shot areas SA.sub.11 to
SA.sub.94 by using the exposure apparatus 1A, shown in FIG. 1. By
development and other processes carried out thereafter, the circuit
pattern images are made to appear as real circuit patterns.
Further, each of the shot areas SA.sub.ij (i=1 to 9; j=1 to 4) is
provided with images of the alignment marks 17X and 17Y formed on
the reticle RA, shown in FIG. 1, as a wafer mark MX.sub.ij for the
X-axis and a wafer mark MY.sub.ij for the Y-axis.
[0150] Next, a photoresist is coated over the wafer 20. The wafer
20 coated with the photoresist is loaded onto the wafer stage 5B in
the exposure apparatus 1B, shown in FIG. 1, and a circuit pattern
image of the reticle RB is projected onto each of shot areas of a
middle layer over the critical layer on the wafer 20. In this case,
each group of four critical layer shot areas arrayed in two rows
and two columns as shown in FIG. 3 corresponds to one middle layer
shot area. For example, a group of four shot areas SA.sub.11 to
SA.sub.14 in the top left corner corresponds to one middle layer
shot area SB.sub.1. To generate such a shot map for the middle
layer, the exposure apparatus 1B first effects EGA alignment. The
EGA alignment method is disclosed, for example, in JP(A) No.
4-277612 in addition to JP(A) No. 61-44429.
[0151] Here, the X2- and Y2-axes of the coordinate system that
define the travel position of the wafer stage 5B of the second
exposure apparatus 1B, shown in FIG. 1, are taken as X- and Y-axes,
respectively, in FIG. 3, and a coordinate system that is defined by
the X- and Y-axes is referred to as "stage coordinate system
(X,Y)". From among the critical layer shot areas SA.sub.11 to
SA.sub.94 on the wafer 20 shown in FIG. 3, a predetermined number N
(N is an integer of 3 or more) shot areas (i.e. shaded shot areas
in the figure) are selected as sample shots S.sub.1 to S.sub.9 (in
this case, N=9), and coordinate values in the stage coordinate
system (X,Y) of the wafer marks attached to the sample shots
S.sub.1 to S.sub.9 are measured by using the alignment systems 11B
and 14B, shown in FIG. 1. For the sake of simplicity, it is assumed
in the following description that the X coordinate of the X-axis
wafer mark MX.sub.ij attached to a shot area SA.sub.ij and the Y
coordinate of the Y-axis wafer mark MY.sub.ij attached to the shot
area SA.sub.ij represent the X and Y coordinates of the center of
the shot area SA.sub.ij.
[0152] Further, coordinate axes which constitute the coordinate
system on the wafer 20 (i.e. sample coordinate system) are assumed
to be an x-axis and a y-axis, respectively. It is further assumed
that design coordinate values of the centers of the shot areas
SA.sub.11 to SA.sub.94 on the critical layer in the sample
coordinate system (x,y) have already been supplied to the
controller 10B of the second exposure apparatus 1B as a part of
shot map data for the critical layer. Under these circumstances,
the transformation of array coordinates of an arbitrary point on
the wafer 20 in the sample coordinate system (x,y) into array
coordinates in the stage coordinate system (X,Y) is approximately
expressed by the following equation (3): 3 [ X Y ] = [ Rx - Rx ( W
+ ) Ry Ry ] [ X Y ] + [ Ox Oy ] ( 3 )
[0153] The transformation matrix in Eq. (3) has as elements six
coordinate transformation parameters, including scaling parameters
Rx and Ry of the wafer, a rotation .THETA. [rad] of the shot array,
a perpendicularity error W [rad] of the shot array, and offsets Ox
and Oy. The scaling parameters Rx and Ry are linear expansion and
contraction quantities of the wafer in the directions X and Y,
respectively. The rotation .THETA. is an angle of rotation of the
x-axis of the sample coordinate system relative to the X-axis. The
perpendicularity error W is an error of the intersection angle
between the x- and y-axes of the sample coordinate system from
90.degree.. The offsets Ox and Oy are shift quantities in the
directions X and Y, respectively.
[0154] Eq. (3) is usable in the present invention; in this
embodiment, however, Eq. (3) is approximated with the following
equation (4) using 1+.GAMMA.x and 1+.GAMMA.y for the scaling
parameters Rx and Ry and regarding the values of the new parameters
.GAMMA.x and .GAMMA.y as small in order to facilitate the
calculation: 4 [ X Y ] = [ 1 + x - ( W + ) 1 + y ] [ x y ] + [ Ox
Oy ] ( 4 )
[0155] To determine values of the six transformation parameters
(.GAMMA.x, .GAMMA.y, .THETA., W, Ox and Oy) in Eq. (4), the
controller 108 defines the array coordinate values of the centers
(wafer marks) of sample shots S.sub.i measured by the i-th (i=1 to
N) measuring operations as (XM.sub.i,YM.sub.i). Next, the design
array coordinates (x.sub.i,y.sub.i) of the centers of the sample
shots S.sub.i are substituted for the coordinates (x,y) on the
right-hand side of Eq. (4) to obtain computational array coordinate
values (X.sub.i,Y.sub.i). The sum of the squares of deviations of
the measured values (XM.sub.i,YM.sub.1) from the array coordinate
values (X.sub.i,Y.sub.i) is determined to be a residual error
component as expressed by the following equation (5): 5 Residual
error component = i = 1 N { ( X i - XM i ) 2 + ( Y i - YM i ) 2 } (
5 )
[0156] Then, the controller 10B determines values of the six
transformation parameters so that the residual error component is
minimized. For example, values of the six parameters are obtained
by solving simultaneous equations established by setting the result
of partial differentiation of the right-hand side of Eq. 5 with
respect to each of the six parameters equal to zero.
[0157] In this embodiment, it is assumed that among the
transformation parameters obtained as described above, the shot
array rotation .THETA. [rad] is regarded as zero, and the shot
array perpendicularity error W [rad] assumes a predetermined finite
value. This means that a perpendicularity error W exits in the
critical layer shot array. The other parameters, that is, scaling
parameters .GAMMA.x and .GAMMA.y and offsets Ox and Oy, may assume
any values, respectively. In this case, the array of the critical
layer shot areas SA.sub.11 to SA.sub.94 is as follows: As shown in
FIG. 3, for example, an imaginary straight line 23 connecting the
centers of shot areas which are adjacent to each other in the
direction X is parallel to the X-axis. An imaginary straight line
24 which passes through the center C.sub.11 of the first shot area
SA.sub.11 and which connects the centers of shot areas which are
successively adjacent to the shot area SA.sub.11 in the direction Y
has been rotated clockwise relative to the Y-axis by the
perpendicularity error W.
[0158] Next, in this embodiment, the reticle RB in the exposure
apparatus 1B, shown in FIG. 1, is rotated through a predetermined
angle .delta. [rad] to thereby rotate each shot area of the middle
layer by an angle .delta. in order to reduce an overlay error
between the critical and middle layers due to the perpendicularity
error W.
[0159] FIG. 4 shows the positional relationship between four shot
areas SA.sub.11 to SA.sub.14 of the critical layer and one middle
layer shot area SB.sub.1 over the four critical layer shot areas.
In FIG. 4, the shot area SB.sub.1 has its adjacent sides rotated
through an angle .delta. clockwise from the respective positions
which are parallel to the X- and Y-axes. Further, the controller
10B successively substitutes the design array coordinates of the
four shot areas SA.sub.11 to SA.sub.14 and the above-determined
transformation parameters into the right-hand side of Eq. (4),
thereby obtaining center coordinates of the four shot areas
SA.sub.11 to SA.sub.14 in the stage coordinate system (X,Y), and
further obtaining coordinates of the center 25 of the four sets of
center coordinates. Then, a circuit pattern image of the reticle RB
is projected onto the shot area SB.sub.1 with the center 25 made
coincident with the center of the exposure field 4B. As a result,
exposure is carried out in a state where the center 25 of the array
of the four shot areas SA.sub.11 to SA.sub.14 is coincident with
the center of the shot area SB.sub.1, and the shot area SB.sub.1
has been rotated through the angle .delta..
[0160] Similarly, as shown in FIG. 5, a circuit pattern image of
the reticle RB is sequentially projected onto middle layer shot
areas SB.sub.2, SB.sub.3, . . . , SB.sub.9 deployed over the
critical layer on the wafer 20.
[0161] Let us conduct evaluation of the overlay error in this
embodiment with reference to FIG. 4. In this embodiment, in an
array of four square shot areas SA.sub.11 to SA.sub.14, each side
of which has a length L, shot areas which are adjacent to each
other in the direction X lie such that an imaginary straight line
23A connecting the centers of these shot areas is parallel to the
X-axis, and shot areas which are adjacent to each other in the
direction Y lie such that an imaginary straight line 24 connecting
the centers of these shot areas intersects the Y-axis at an angle
(perpendicularity error) W. Accordingly, assuming that an overlay
error between the array of critical layer four shot areas SA.sub.11
to SA.sub.14 and the middle layer shot area SB.sub.1 is .DELTA.,
and that the perpendicularity error W and the rotation angle
.delta. are small, the ranges of X and Y components .DELTA..sub.x
and .DELTA..sub.y of the overlay error .DELTA. concerning the shot
area SA.sub.11 are approximately given by the following equation
(6):
(1/2)L.multidot.W-L.multidot..delta..ltoreq..DELTA..sub.X.ltoreq.(1/2)L.mu-
ltidot.W 0.ltoreq..DELTA..sub.Y.ltoreq.L.multidot..delta. (6)
[0162] In this case, if the rotation angle .delta. is assumed to be
zero as in the related art shown in FIG. 35, the X component
.DELTA..sub.x of the overlay error .DELTA. is uniformly
(1/2)L.multidot.W, and the Y component .DELTA..sub.y is uniformly
zero. Therefore, in order to reduce the overlay error to a lower
level than in the related art as a whole, the rotation angle
.delta. should be set within the range given by the following
equation (7):
0<.delta.<(1/2)W (7)
[0163] The value of the rotation angle .delta. at which the overlay
error reaches a minimum as a whole within the above range is
(1/4)W. That is, in this case, the ranges of the X and Y components
are obtained from Eq. (6) as follows:
(1/4)L.multidot.W.ltoreq..DELTA..sub.X.ltoreq.(1/2)L.multidot.W
0.ltoreq..DELTA..sub.Y.ltoreq.(1/4)L.multidot.W
[0164] Thus, it becomes possible to regard the overlay error as
minimum as a whole.
[0165] Although in the above-described embodiment the reticle-side
part in the exposure apparatus 1B is rotated through the rotation
angle .delta., the wafer-side part may be rotated through -.delta.
instead of rotating the reticle-side part. However, if the
wafer-side part is rotated, the shot area array on the critical
layer also changes, and it is therefore necessary to correct the
critical layer shot array. That is, rotating the reticle-side part
is advantageous because it is possible to omit a corrective
calculation which would otherwise be required.
[0166] Although in this embodiment the perpendicularity error W of
the shot array is assumed to be not zero, it should be noted that
an alignment method similar to that in the above-described
embodiment is also applicable in a case where the perpendicularity
error W is zero as in the related art shown in FIG. 36 and the shot
rotation .theta. of each of the critical layer shot areas SA.sub.11
to SA.sub.94, shown in FIG. 3, is not zero. That is, the overlay
error can be reduced as a whole by rotating the reticle-side part
of the second exposure apparatus 1B, for example, such that each
middle layer shot area is rotated through the angle
(.theta.+.delta.') relative to the corresponding array of four
critical layer shot areas, using the angle .delta.' in the range of
0<.delta.'<(1/2).theta.. In particular, if the angle .delta.'
is set at (1/4).theta., the overlay error is minimized as a whole.
The method of detecting the shot rotation .theta. will be explained
in a second example of the first embodiment.
[0167] Next, the second example of the first embodiment of the
present invention will be described with reference to FIGS. 6(a) to
8(c). In the second example, the stepper type exposure apparatus
1A, shown in FIG. 1, is used as a first exposure apparatus, and a
step-and-scan type scanning exposure apparatus with a
demagnification ratio of 4:1 is used as a second exposure
apparatus. In the first exposure apparatus, an image of two
identical circuit patterns (chip patterns) is transferred per shot
area; in the second exposure apparatus, an image of three identical
circuit patterns is transferred per shot area.
[0168] FIG. 6(a) shows a part of a shot array on a wafer loaded on
a wafer stage (not shown) of the scanning type second exposure
apparatus in this example. In FIG. 6(a), square shot areas
SA.sub.1, SA.sub.2 and SA.sub.3, each side of which has a length L,
are sequentially arrayed, lying adjacent to each other in the
direction Y. The shot areas SA.sub.1, SA.sub.2, and SA.sub.3 each
has two identical circuit patterns 26A and 26B formed side-by-side
in the direction Y by the first exposure apparatus, a developer,
etc. In FIG. 6(a), X- and Y-axes represent a stage coordinate
system of the second exposure apparatus. An imaginary straight line
27 passing through the centers of the shot areas SA.sub.1, SA.sub.2
and SA.sub.3 on the first layer is tilted by an angle W clockwise
relative to the Y-axis. The angle W is a perpendicularity error of
the shot array.
[0169] FIG. 6(b) shows a shot area SC which has a width L in the
direction X and a width (3/2)L in the direction Y on a wafer which
is to be exposed by the second exposure apparatus in this example.
In FIG. 6(b), directions +Y and -Y are scanning directions. That
is, the shot area SC is scanned in the direction +Y, for example,
relative to a slit-shaped illumination field 28, and a reticle
placed through a projection optical system is scanned in the
direction -Y in synchronism with the scanning of the shot area SC.
As a result, three identical circuit pattern images 29A to 29C are
formed on the shot area SC, lying side-by-side in the direction Y.
In this example, it is assumed that circuit pattern images for two
shot areas SC.sub.1 and SC.sub.2 each having the same size as that
of the shot area SC shown in FIG. 6(b) are overlaid on the shot
areas SA.sub.1, SA.sub.2 and SA.sub.3 shown in FIG. 6(a) by using
the second exposure apparatus.
[0170] In this case, it is conceivable to effect alignment such
that, as shown by the chain double-dashed lines in FIG. 6(a),
reference points 27a and 27b, which are at the centers of two
arrays of three circuit patterns arranged in the direction Y,
coincide with the centers of second-layer shot areas SC.sub.1 and
SC.sub.2, respectively, on an imaginary straight line 27 passing
through the centers of the first-layer shot areas SA.sub.1,
SA.sub.2 and SA.sub.3. However, this alignment method causes
overlay errors a and b in the direction X between the second-layer
shot area SC.sub.1 and the first-layer shot areas SA.sub.1 and
SA.sub.2. Similarly, overlay errors b and a in the direction X
arise between the second-layer shot area SC.sub.2 and the
first-layer shot areas SA.sub.2 and SA.sub.3. The overlay errors a
and b are expressed by the following equation (8):
a=(1/4)L.multidot.W b=(3/4)L.multidot.W (8)
[0171] Accordingly, it will be understood that the alignment method
shown in FIG. 6(a) causes a large overlay error b to arise
particularly in the second shot area SA.sub.2. In order to reduce
the overlay error, in this example, the center positions of the
second-layer shot areas SC.sub.1 and SC.sub.2 are shifted by a
predetermined distance in the direction X from the reference points
27a and 27b, respectively.
[0172] FIG. 7 shows the alignment method according to this example.
In FIG. 7, the center positions of the second-layer shot areas
SC.sub.1 and SC.sub.2 are shifted by a distance d in the respective
directions -X and +X relative to the reference points 27a and 27b
on the imaginary straight line 27 passing through the centers of
the first-layer shot areas SA.sub.1 to SA.sub.3. As a result, there
is a uniform overlay error c in the direction X between the
first-layer shot areas SA.sub.1 to SA.sub.3 and the second-layer
shot areas SC.sub.1 and SC.sub.2. The distance d and the overlay
error c are given by the following equation (9):
d=(1/4)L.multidot.W c=(1/2)L.multidot.W (9)
[0173] As a result, the overlay error c given by Eq. (9) is
(1/2)L.multidot.W in contrast to the overlay error b given by Eq.
(8). Accordingly, it will be understood that the alignment method
according to this example enables the maximum value of the overlay
error to reduce to (1/2)L.multidot.W, and thus the overlay error
reduces as a whole. It should be noted that during the alignment
shown in FIG. 7, the second-layer shot areas SC.sub.1 and SC.sub.2
may be rotated through a predetermined angle relative to the
first-layer shot areas SA.sub.1 to SA.sub.3 by additionally
applying the method according to the first example. By doing so,
the overlay error may be further reduced as a whole.
[0174] In the second example of the first embodiment, exposure may
be carried out by using the first exposure apparatus over shot
areas exposed by using the scanning type second exposure apparatus
in reverse relation to the above-described exposure operation. One
example of such an exposure operation will be explained below with
reference to FIGS. 8(a), 8(b) and 8(c).
[0175] FIG. 8(a) shows the first-layer shot areas SC.sub.1 and
SC.sub.2 on the wafer which have been formed with circuit patterns
by using the scanning second exposure apparatus. In FIG. 8(a), the
shot areas SC.sub.1 and SC.sub.2 each has three identical circuit
patterns arranged in the direction Y, and there is a predetermined
perpendicularity error in the shot array. Then, a reticle pattern
image including two identical circuit pattern images arranged in
the direction Y is formed over the shot areas SC.sub.1 and SC.sub.2
for each of the shot areas SA.sub.1, SA.sub.2 and SA.sub.3 by using
the first exposure apparatus. In this case, for the first and third
shot areas SA.sub.1 and SA.sub.3, it is only necessary to align
them with the first-layer shot areas SC.sub.1 and SC.sub.2,
respectively, in the direction X. For the second shot area
SA.sub.2, it is only necessary to align its position in the
direction X with an intermediate position between the first-layer
shot areas SC.sub.1 and SC.sub.2. Consequently, the overlay error
between the first and second layers is b only at the shot area
SA.sub.2.
[0176] However, in a case where a reticle pattern image is
transferred by using the first exposure apparatus, if a part of the
pattern image to be transferred can be selectively masked by using
a reticle blind (variable field stop), for example, the overlay
error can be reduced to approximately zero. In such a case, as
shown in FIG. 8(b), when the second shot area SA.sub.2 of the
second layer is to be exposed, first, the position in the direction
X of the shot area SA.sub.2 is aligned with the first-layer shot
area SC.sub.1. Thereafter, exposure is carried out with the lower
half of the shot area SA.sub.2 masked by controlling the reticle
blind. Consequently, exposure is effected only for the upper half
of the shot area SA.sub.2, which corresponds to the circuit pattern
26A.
[0177] Next, the position in the direction X of the shot area
SA.sub.2 is aligned with the first-layer shot area SC.sub.2, and
thereafter, exposure is carried out with the upper half of the shot
area SA.sub.2 masked by controlling the reticle blind.
Consequently, exposure is effected only for the lower half of the
shot area SA.sub.2, which corresponds to the circuit pattern 26B.
For the other shot areas SA.sub.1 and SA.sub.3, exposure similar to
that in the case of FIG. 8(a) is carried out. As a result, the
overlay error becomes zero at all the shot areas.
[0178] Next, a third example of the first embodiment of the present
invention will be described with reference to FIGS. 9(a) to 10(c).
In this example, the stepper type exposure apparatus 1A, shown in
FIG. 1, is used as a first exposure apparatus, and a step-and-scan
type scanning exposure apparatus with a demagnification ratio of
4:1 is used as a second exposure apparatus. In this case, shot
areas as exposure units which are to be exposed by the first
exposure apparatus each contains two identical square circuit
patterns, each side of which has a length L. Shot areas as exposure
units which are to be exposed by the second exposure apparatus each
contains three identical rectangular circuit patterns in which one
side has a length L, and the other side has a length 3L/2.
[0179] FIG. 9(a) shows a shot array on a wafer 20 loaded on a wafer
stage (not shown) of the scanning type second exposure apparatus.
In FIG. 9(a), square shot areas SA.sub.1, SA.sub.2, SA.sub.3, . . .
, SA.sub.66, each side of which has a length L, are arranged at a
predetermined pitch in each of the directions X and Y. The shot
areas SA.sub.1, SA.sub.2, . . . each has two identical circuit
patterns 26A and 26B formed to lie side-by-side in a direction
substantially parallel to the direction Y by the first exposure
apparatus, a developer, etc. In FIG. 9(a), X- and Y-axes represent
a stage coordinate system of the second exposure apparatus.
Further, the shot areas SA.sub.1, SA.sub.2, . . . have been each
formed with two wafer marks MYA.sub.i and MYB.sub.i for the Y-axis
and two wafer marks MXA.sub.i and MXB.sub.i for the X-axis, which
are detectable by an LSA type detection method. In FIG. 9(a), only
the four wafer marks MYA.sub.1, MYB.sub.1, MXA.sub.1 and MXB.sub.1
in the shot area SA.sub.1 are shown for the sake of simplicity.
[0180] In this example also, alignment is effected by the EGA
method in the same way as in the first example. In this example,
however, each shot area contains four one-dimensional wafer marks,
and therefore, two in-shot transformation parameters can be
obtained in addition to the above-described six transformation
parameters (i.e. scaling parameters Rx and Ry, shot array rotation
.THETA., shot array perpendicularity error W, and offsets Ox and
Oy).
[0181] Accordingly, in this example, a shot rotation (chip
rotation) .theta. [rad] and a shot perpendicularity error w [rad]
are obtained as in-shot transformation parameters. It should be
noted that shot magnifications rx and ry can also be obtained by
disposing two other one-dimensional wafer marks in each shot area.
However, this example is not particularly related to the
determination of shot magnifications rx and ry; therefore, wafer
marks for them are not provided in this example. A method in which
EGA alignment is effected by using three or more one-dimensional
wafer marks or two or more two-dimensional wafer marks, which are
disposed in each shot area, as described above, is also known as
"in-shot multipoint EGA alignment method".
[0182] More specifically, in this example, a predetermined number N
(N is an integer of 3 or more) of shot areas are selected from
among shot areas SA.sub.1 to SA.sub.66 on a wafer 20 as sample
shots S.sub.1 to S.sub.9 (in this case, N=9), and coordinate values
in the stage coordinate system (X,Y) of two pairs of wafer marks
attached to each of the sample shots S.sub.1 to S.sub.9 are
measured by using an LSA type alignment system. For example, a mean
value of the X coordinates of the two X-axis wafer marks of each
sample shot and a mean value of the Y coordinates of the two Y-axis
wafer marks of the sample shot are regarded as array coordinates of
the center of the sample shot, thereby obtaining parameters (i.e.
scaling parameters Rx and Ry, rotation .THETA., perpendicularity
error W, and offsets Ox and Oy) for transformation from the sample
coordinate system (x,y) into the stage coordinate system (X,Y) in
the same way as in the first example.
[0183] Further, in this example, a shot rotation .theta., which is
a rotation angle .delta.f the x-axis in a shot, is calculated on
the basis of a mean value of Y-coordinate differences between the
pairs of Y-axis wafer marks of the sample shots, for example, and a
rotation angle .theta..sub.y of the y-axis in a shot is calculated
on the basis of a mean value of X-coordinate differences between
the pairs of X-axis wafer marks. The shot rotation .theta. is
subtracted from the rotation angle .theta..sub.y of the y-axis to
obtain an angle w, which is determined to be a shot
perpendicularity error.
[0184] It is assumed in this example that, as a result of the
alignment, as shown in FIG. 9(a), the shot array rotation .THETA.,
the shot array perpendicularity error W and the shot
perpendicularity error w have become capable of being regarded as
zero, but the shot rotation .theta. has become a predetermined
value other than zero. On such a shot array, overlay exposure is
effected by the scanning second exposure apparatus having a shot
area SC.sub.1 as shown in FIG. 9(b), which has a size (3L/2) that
is sufficiently large to contain three circuit patterns in the
direction Y as a scanning direction, and which has a width L in the
direction X. The method of alignment effected to carry out the
overlay exposure will be explained below.
[0185] FIG. 10(a) shows a part of the shot array on the wafer 20
shown in FIG. 9(a). In FIG. 10(a), square shot areas SA.sub.1,
SA.sub.2 and SA.sub.3, each side of which has a length L, have been
rotated counterclockwise through an angle corresponding to the shot
rotation .theta.. It is assumed that circuit pattern images for two
shot areas SC.sub.1 and SC.sub.2, each having the same size as the
shot area SC shown in FIG. 9(b), are overlaid on the shot areas
SA.sub.1 to SA.sub.3 by using the second exposure apparatus.
[0186] In this case, it is conceivable to effect alignment such
that, as shown by the chain double-dashed lines in FIG. 10(a),
reference points 31a and 31b, which are at the centers of two
arrays of three circuit patterns, coincide with the centers of the
second-layer shot areas SC.sub.1 and SC.sub.2, respectively, on an
imaginary straight line 31 passing through the centers of the
first-layer shot areas SA.sub.1, SA.sub.2 and SA.sub.3 in parallel
to the Y-axis. In this case, if the scanning direction in the
second exposure apparatus is restricted to the directions +Y and
-Y, it is necessary to rotate the wafer or the reticle clockwise
through the angle .theta., for example. Further, if the wafer is
rotated, it is necessary to correct the array coordinates of each
of the first-layer shot areas. In the following description, the
scanning direction is assumed to be a direction intersecting the
Y-axis at the angle .theta..
[0187] With this alignment method, however, X-direction overlay
errors a and b, which are given by the following equation (10),
arise between the second-layer shot area SC.sub.1 and the
first-layer shot areas SA.sub.1 and SA.sub.2, respectively, in the
same way as in a case where there is a shot array perpendicularity
error as shown in FIG. 6(a):
a=(1/4)L.multidot..theta. b=(3/4)L.multidot..theta. (10)
[0188] Accordingly, it will be understood that the alignment method
as shown in FIG. 10(a) causes a large overlay error b to arise
particularly at the second shot area SA.sub.2. In order to reduce
the overlay error, in this example, the center positions of the
second-layer shot areas SC.sub.1 and SC.sub.2 are shifted by a
predetermined distance from the reference points 31a and 31b in a
direction perpendicular to the scanning direction.
[0189] FIG. 10(b) shows an alignment method carried out in this
example. In FIG. 10(b), the center positions of the second-layer
shot areas SC.sub.1 and SC.sub.2 have been shifted by -d and +d
relative to the reference points 31a and 31b in a direction
intersecting the X-axis at the angle .theta. in the clockwise
direction. As a result, there is a uniform overlay error c in the
direction X between the first-layer shot areas SA.sub.1 to SA.sub.3
and the second-layer shot areas SC.sub.1 and SC.sub.2. The distance
d and the overlay error c are given by the following equation
(11):
d=(1/4).multidot..theta. c=(1/2)L.multidot..theta. (11)
[0190] As a result, the overlay error c given by Eq. (11) is
(1/2)L.multidot..theta. in contrast to the overlay error b given by
Eq. (10). Accordingly, it will be understood that the alignment
method according to this example enables the maximum value of the
overlay error to reduce to (1/2)L.multidot..theta., and thus the
overlay error reduces as a whole.
[0191] In the third example of the first embodiment, exposure may
be carried out by using the first exposure apparatus over shot
areas exposed by using the scanning type second exposure apparatus
in reverse relation to the above-described exposure operation. One
example of such an exposure operation will be explained below with
reference to FIG. 10(c).
[0192] FIG. 10(c) shows the first-layer shot areas SC.sub.1 and
SC.sub.2 on the wafer which have been formed with circuit patterns
by using the scanning second exposure apparatus. In a case where,
in FIG. 10(c), a reticle pattern image is to be formed for each of
the shot areas SA.sub.1, SA.sub.2 and SA.sub.3 over the shot areas
SC.sub.1 and SC.sub.2 by using the first exposure apparatus, for
the first and third shot areas SA.sub.1 and SA.sub.3, it is only
necessary to align them with the first-layer shot areas SC.sub.1
and SC.sub.2, respectively, in the direction X. For the second shot
area SA.sub.2, it is only necessary to align its position in a
direction perpendicular to the scanning direction with an
intermediate position between the first-layer shot areas SC.sub.1
and SC.sub.2. Consequently, the overlay error between the first and
second layers is b only at the shot area SA.sub.2.
[0193] Further, for the second shot area SA.sub.2, exposure may be
effected for the upper and lower halves separately in the same way
as in the method described with reference to FIGS. 8(b) and 8(c).
By doing so, the overlay error can be reduced to zero.
[0194] Although in the third example, only the shot rotation
.theta. is corrected, it should be noted that alignment may be
effected as follows: A mean value of the shot rotation .theta.
obtained by the in-shot multipoint EGA method and the shot
perpendicularity error w, i.e. (.theta.+w)/2, is regarded as pseudo
shot rotation, and alignment is effected on the basis of the pseudo
shot rotation.
[0195] Further, although in the third example the shot rotation
.theta. is obtained by measuring the positions of three or more
wafer marks in each sample shot, the shot rotation .theta. may be
obtained by using numerical values previously obtained by test
printing using the first exposure apparatus. In this case, in each
sample shot the ordinary EGA type alignment is effected by
measuring the positions of a pair of wafer marks as in the
conventional practice, and the shot rotation .theta. alone is
obtained by using the input numerical values.
[0196] Although in the above-described embodiment two steppers or a
combination of a stepper and a step-and-scan type projection
exposure apparatus is used, it should be noted that, for example,
two step-and-scan type projection exposure apparatuses may be used
as two exposure apparatuses having respective exposure fields of
different sizes.
[0197] The exposure method according to the first embodiment of the
present invention provides the following advantages. In a case
where exposure is carried out by the mix-and-match method using two
exposure apparatuses having respective exposure fields of different
sizes, a perpendicularity error in the shot area array on the
preceding layer or a mean value of rotation angles of the shot
areas is detected, and the shot areas of the subsequent layer are
rotated according to the result of the detection. Accordingly, the
overlay error between the two layers can be favorably reduced.
[0198] Further, in the exposure method according to the first
embodiment of the present invention, when exposure is to be carried
out by the mix-and-match method using two exposure apparatuses
having respective exposure fields of different sizes, a
perpendicularity error in the shot area array on the preceding
layer or a mean value of rotation angles of the shot areas is
detected, and exposure is carried out with the shot areas of the
subsequent layer shifted in a direction perpendicular to the
scanning direction of the second exposure apparatus according to
the result of the detection. Accordingly, the overlay error between
the two layers can be favorably reduced.
[0199] In this case, if the shot areas of the subsequent layer are
rotated in addition to the shifting of the shot areas, the overlay
error may be further reduced.
[0200] Next, a first example of a second embodiment of the exposure
method according to the present invention will be described with
reference to FIGS. 11 to 14(c). Two exposure apparatuses used in
this example are a one-shot exposure type projection exposure
apparatus (stepper) with a demagnification ratio of 5:1 and a
step-and-scan type projection exposure apparatus with a
demagnification ratio of 4:1. In this example, two chip patterns
are formed in each shot area exposed by the former projection
exposure apparatus (i.e. a two-chip reticle is used), and three
chip patterns are formed in each shot area scan-exposed by the
latter projection exposure apparatus (i.e. a three-chip reticle is
used). It should be noted that constituent elements in the second
embodiment which are similar to those in the first embodiment are
denoted by the same reference characters.
[0201] FIG. 11 shows an exposure system used in this example. In
FIG. 11, a stepper 1A, which is a one-shot exposure type projection
exposure apparatus, and a step-and-scan type projection exposure
apparatus (hereinafter referred to as "scanning exposure
apparatus") 1B are installed. In this example, the stepper 1A is a
high-resolution exposure apparatus, while the scanning exposure
apparatus 1B is a low-resolution exposure apparatus. The stepper 1A
is used to carry out exposure for a critical layer, which requires
high resolution, on a wafer, and the scanning exposure apparatus 1B
is used to carry out exposure for a middle layer, which does not
require high resolution, on the wafer. However, the stepper 1A may
be a low-resolution exposure apparatus or the scanning exposure
apparatus 1B may be a high-resolution exposure apparatus according
to the kind of semiconductor device to be produced.
[0202] First, in the stepper 1A, a pattern area 42A on a reticle RA
is illuminated by exposure light from an illumination optical
system (not shown), and an image of a pattern formed in the pattern
area 42A is formed on a rectangular exposure field 44A on a wafer
20 as a projected image reduced to 1/5 by a projection optical
system 3A. A Z1-axis is taken in a direction parallel to an optical
axis of the projection optical system 3A, and two axes of an
orthogonal coordinate system set in a plane perpendicular to the
Z1-axis are defined as an X1-axis and a Y1-axis, respectively. The
pattern area 42A on the reticle RA is divided into partial pattern
areas 112A and 112B of the same size in a predetermined direction
(in FIG. 11, in the direction Y1). The partial pattern areas 112A
and 112B each has original drawing patterns of circuit patterns and
alignment marks arranged according to the same layout.
[0203] The wafer 20 is held on a wafer stage 5A. The wafer stage 5A
comprises a Z-stage for moving the wafer 20 in the direction Z1 to
set an exposure surface of the wafer 20, which is to be exposed, at
the best focus position, and an XY-stage for positioning the wafer
20 in both the directions of the X1- and Y1-axes. A pair of moving
mirrors 6A and 8A which are perpendicular to each other are fixed
on the wafer stage 5A. The coordinate in the direction X1 of the
wafer stage 5A is measured by a combination of the moving mirror 6A
and a laser interferometer 7A which is installed outside the wafer
stage 5A. The coordinate in the direction Y1 of the wafer stage 5A
is measured by a combination of the moving mirror 8A and a laser
interferometer 9A which is installed outside the wafer stage 5A.
The coordinates measured by the laser interferometers 7A and 9A are
supplied to a controller 10A which controls operations of the whole
apparatus. The controller 10A drives the wafer stage 5A to step in
both the directions X1 and Y1 through drive units (not shown),
thereby positioning the wafer 20. In this case, the stepping drive
of the wafer 20 is effected according to an array of shot areas
(i.e. unit areas to each of which a pattern image of the pattern
area 42A is to be projected by exposure) set on the exposure
surface of the wafer 20, that is, a shot map for a critical layer.
The shot map is generated by a map generating unit which comprises
a computer in the controller 10A. It is assumed that a
predetermined perpendicularity error W remains in a coordinate
system (i.e. stage coordinate system) (X1,Y1) which defines the
travel position of the wafer stage 5A of the stepper 1A in this
example.
[0204] Further, the stepper 1A in this example is provided with an
off-axis imaging type (FIA type) alignment system 11A. The
alignment system 11A images an alignment mark (wafer mark) on the
wafer 20 and processes an imaging signal thus obtained to detect X1
and Y1 coordinates of the wafer mark. The detected coordinates are
supplied to the controller 10A.
[0205] It should be noted that, as the alignment system 11A, it is
also possible to use a TTR (Through-The-Reticle) type alignment
system or a TTL (Through-The-Lens) type alignment system in which
the position of a mark is detected through the projection optical
system 3A. As a mark detecting method, it is also possible to use a
laser step alignment (LSA) method in which a slit-shaped laser beam
and a mark are scanned relative to each other, or a so-called
two-beam interference method (LIA method) in which two light beams
are applied to a diffraction grating-shaped mark, and the position
of the mark is detected from a signal obtained from interference
between a pair of diffracted light beams generated in parallel from
the illuminated mark.
[0206] Next, in the scanning exposure apparatus 1B in this example,
a part of a pattern area 42B on a reticle RB is illuminated by
exposure light from an illumination optical system (not shown), and
an image of a part of the reticle pattern is formed in a
slit-shaped exposure area 144 on a wafer 20, which is held on a
wafer stage 5B, as a projected image reduced to 1/4 by a projection
optical system 3B. Here, a Z2-axis is taken in a direction parallel
to an optical axis of the projection optical system 3B, and two
axes of an orthogonal coordinate system set in a plane
perpendicular to the Z2-axis are defined as an X2-axis and a
Y2-axis, respectively. Under these circumstances, the reticle RB is
scanned in the direction -Y2 (or +Y2), and the wafer 20 is scanned
in the direction +Y2 (or -Y2) in synchronism with the scanning of
the reticle RB, thereby sequentially projecting an image of the
pattern formed in the pattern area 42B of the reticle RB onto the
exposure field 44B on the wafer 20.
[0207] The pattern area 42B of the reticle RB is divided into three
partial pattern areas 13A to 13C of the same size in the direction
Y2, which is the scanning direction. The size of the exposure field
44B is such that its dimension in the scanning direction is 3/2
times as large as the dimension of the exposure field 44A of the
stepper 1A, and the exposure field 44B is equal in size (1:1) to
the exposure field 44A in the non-scanning direction. That is, the
exposure field 44B is longer than the exposure field 44A in the
direction Y2.
[0208] The position of a reticle stage (not shown) for scanning the
reticle RB of the scanning exposure apparatus 1B is measured by a
laser interferometer (not shown). The X2 coordinate of the wafer
stage 5B is measured by a combination of a moving mirror 6B and a
laser interferometer 7B, and the Y2 coordinate of the wafer stage
5B is measured by a combination of a moving mirror 8B and a laser
interferometer 9B. The measured coordinates of the wafer stage 5B
are supplied to a controller 10B. In this example, the X2- and
Y2-axes are assumed to be perpendicular to each other. The
controller 10B controls synchronous drive of the reticle stage (not
shown) and the wafer stage 5B. Scanning exposure of the wafer stage
5B is effected according to a shot map for a middle layer set on an
exposure surface of the wafer 20, which is to be exposed. The shot
map is generated by a map generating unit which comprises a
computer in the controller 10B.
[0209] In this case, the map generating unit in the controller 10A
and the map generating unit in the controller 10B have the function
of supplying shot map information prepared thereby to each other.
When exposure for a middle layer is to be carried out over a
critical layer, for example; shot map information for the critical
layer prepared by the map generating unit in the controller 10A of
the stepper 1A is transmitted to the other controller 10B. The map
generating unit in the controller 10B generates a shot map for the
middle layer on the basis of the supplied shot map information.
Conversely, when exposure for a critical layer is to be carried out
over a middle layer, shot map information for the middle layer
prepared in the controller 10B is supplied to the controller
10A.
[0210] The scanning exposure apparatus 1B also has an off-axis
imaging type (FIA type) alignment system 11B provided at a side
surface of the projection optical system 3B. The alignment system
11B detects X2 and Y2 coordinates of a wafer mark on the wafer
20.
[0211] Next, one example of an exposure operation which is
performed in this example when exposure for a first-layer pattern
is first effected by using the stepper 1A and then exposure for a
second-layer pattern is effected by using the scanning exposure
apparatus 1B will be explained for each of the first and second
processing steps.
[0212] First, the first step will be explained.
[0213] In the first step, as shown in FIG. 12(a), the reticle RA is
fixed on the reticle stage (not shown) of the stepper 1A, shown in
FIG. 11, such that the reticle RA is rotated through 90.degree.
from its ordinary position. As a result, the two partial pattern
areas 112A and 112B in the pattern area 42A of the reticle RA lie
side-by-side in the direction X1. Next, as shown in FIG. 12(b), the
wafer 20 coated with a photoresist is fixed on the wafer stage 5A
of the stepper 1A, shown in FIG. 11, such that the wafer 20 is
rotated through 90.degree. from its ordinary position. As a result,
the wafer 20 is placed such that the cut portion (orientation flat)
of the outer periphery of the wafer 20 faces in the direction +X1.
Although in FIG. 12(b) the mutual origin of the X1 and Y1-axes is
set at the center of the wafer 20, in FIG. 12(a) the origin of the
two axes is set outside the reticle RA for the sake of explanation.
Further, in FIG. 12(a), the reticle RA is shown in the size of an
image thereof as projected on the wafer 20. In this example, as
shown in FIG. 12(b), the Y1-axis has rotated through an angle W
clockwise relative to an imaginary axis Y1* perpendicularly
intersecting the X1-axis; the angle W is a perpendicularity
error.
[0214] Next, a pattern image of the reticle RA is sequentially
projected onto shot areas 121A, 121B, . . . , 121I, which are
obtained by dividing a first-layer exposure area on the wafer 20 at
a predetermined pitch in each of the directions X1 and X1, by the
step-and-repeat method using the stepper 1A. For example, the
first-layer shot areas 121A to 121I are arranged in an array of 3
columns in the direction X1 and 3 rows in the direction Y1. In this
case, there is the perpendicularity error W between the X1- and
Y1-axes; therefore, the array of shot areas 121A to 121I also has
the perpendicularity error W.
[0215] However, in this example, exposure has been effected with
the reticle RA and the wafer 20 each rotated through 90.degree..
Therefore, in FIG. 12(b), the first shot area 121A on the wafer 20
has two partial shot areas 122A and 122B divided in the direction
X1. The-partial shot areas 122A and 122B have been exposed to
pattern images which are identical with each other. The same is the
case with the other shot areas 121B to 121I. When edges of the shot
areas 121A to 121C in the first row which are parallel to the array
direction of the partial shot areas 122A and 122B are connected
together, a straight line 125, which has no irregularity, is
obtained.
[0216] Thereafter, the wafer 20 is subjected to development,
thereby allowing the circuit pattern images and alignment mark
images in each shot area to appear as circuit patterns and wafer
marks, which comprise recess-and-projection patterns. In this
example, the first partial shot area 122A in the first shot area
121A is formed with an X-axis wafer mark 123X and a Y-axis wafer
mark 123Y, which comprise line-and-space patterns, respectively.
The second partial shot area 122B is also formed with an X-axis
wafer mark 124X and a Y-axis wafer mark 124Y. The wafer marks 123X,
123Y, 124X and 124Y are marks which are detectable with an imaging
type alignment sensor. It should be noted that the arrangement of
wafer marks is not necessarily limited to the example shown in FIG.
12(b). For example, the arrangement of wafer marks may be such that
a pair of wafer marks are disposed in each of the shot areas 121A
to 121I. It is also possible to dispose more than one pair of wafer
marks in each of the shot areas 121A to 121I. Further,
two-dimensional marks may be used as wafer marks.
[0217] Next, the second step will be explained.
[0218] A photoresist is coated over the wafer 20 having the circuit
patterns and wafer marks formed thereon in the first step. The
photoresist-coated wafer 20 is fixed at the ordinary rotation angle
on the wafer stage 5B of the scanning exposure apparatus 1B, shown
in FIG. 11. Thus, as shown in FIG. 13, the wafer 20 is disposed
such that its cut portion faces in the direction -Y2. As shown in
FIG. 11, the reticle RB for the second layer is also set at the
ordinary rotation angle, that is, at an angle at which the partial
pattern areas 113A to 113C are arranged in the direction Y2.
[0219] At this time, the first-layer shot array data is supplied
from the controller 10A of the stepper 1A to the controller 10B of
the scanning exposure apparatus 1B. The controller 10B determines a
second-layer shot array on the basis of the supplied shot array
data, together with alignment data (described later).
[0220] Thereafter, the wafer 20, which is an object to be exposed,
is subjected to alignment by the EGA method in the scanning
exposure apparatus 1B.
[0221] FIG. 13 shows the wafer 20 as an object to be exposed. In
FIG. 13, the origin of the stage coordinate system (X2,Y2) of the
scanning exposure apparatus 1B is set at the center of the wafer 20
for the sake of convenience. The origin of the stage coordinate
system (X1,Y1) of the stepper 1A used to expose the first layer is
also shown to be coincident with the center of the wafer 20. In
this case, because the wafer 20 is set at the ordinary rotation
angle, the two partial shot areas 122A and 122B in the shot area
121A, for example, are arranged in the direction Y2, and in each of
the other shot areas 121B to 121I, the two partial shot areas are
also arranged in the direction Y2. To effect EGA type alignment,
three or more shot areas are selected as sample shots from among
the nine shot areas 121A to 121I on the wafer 20, and the
coordinates in the stage coordinate system (X2,Y2) of the wafer
marks in the sample shots are measured by using the alignment
system 11B, shown in FIG. 11. When the shot area 121A, for example,
is selected as a sample shot, the coordinates of a pair of wafer
marks 123X and 123Y in the first partial shot area 122A of the shot
area 121A are measured, and for each of the other sample shots
also, the coordinates of a pair of wafer marks are measured.
[0222] Next, the measured values of the coordinates of the wafer
marks in the sample shots and the design array coordinates of these
wafer marks are statistically processed to determine values of EGA
parameters including a rotation (wafer rotation) .THETA..sub.1 of
the first-layer shot array, a perpendicularity error W.sub.1 of the
first-layer shot array, an offset Ox.sub.1 in the direction X2, and
an offset Oy.sub.1 in the direction Y2. In this example, the wafer
20 was rotated through 90.degree. at the time of exposure for the
first layer, and the X2- and Y2-axes of the second layer are
assumed to be perpendicular to each, other. Therefore, the rotation
.THETA..sub.1 is an angle between the Y1-axis of the first-layer
shot array and the X2-axis of the second-layer stage coordinate
system, and the perpendicularity error W.sub.1 is equal to an angle
obtained by subtracting .pi./2 (90.degree.) from the angle between
the Y1-axis and the axis (-X1-axis) which is in inverse relation to
the X1-axis, that is, the perpendicularity error W in FIG.
12(b).
[0223] After the EGA parameters have been obtained as described
above, the scanning exposure apparatus 1B, shown in FIG. 11, sets
the rotation angle of the wafer 20 such that the Y1-axis of the
first-layer shot array is rotated clockwise relative to the X2-axis
through the same angle as the perpendicularity error W.sub.1 (i.e.
W). This means that an offset of the same angle as the
perpendicularity error W.sub.1 is added to the desired value of the
shot array rotation (wafer rotation). As a result, a straight line
125 which connects together the right-hand edges of the shot areas
121A to 121C in the first column, which is parallel to the array
direction of the partial shot areas 122A and 122B of the shot area
121A, for example, becomes parallel to the direction Y2, which is
the scanning direction of the scanning exposure apparatus 1B. In
this state, a shot array of the second layer is determined by
taking into consideration the offsets Ox.sub.1 and Oy.sub.1 in the
EGA parameters.
[0224] FIG. 14(a) shows a second-layer shot array set over the
first layer as described above. In FIG. 14(a), for example,
second-layer shot areas 126A and 126B are set over the first-layer
shot areas 121A to 121C; similarly, other second-layer shot areas
126C to 126F are set. In this case, the shot area 126A, for
example, is divided into three partial shot areas 127A to 127C in
the direction Y2. The partial shot areas 127A to 127C are
respectively exposed to images of patterns in partial pattern areas
113C to 113A of the reticle RB, shown in FIG. 14(b). The partial
shot areas 127A to 127C in the second-layer shot area 126A each has
the same size as the size of each of the partial shot areas 122A
and 122B in the first-layer shot area 121A, shown in FIG. 14(c).
The other second-layer shot areas 126B to 126F also each has the
same configuration as that of the second-layer shot area 126A. The
shot areas 126A to 126F determined in this way are each exposed to
a pattern image of the reticle RB by the scanning exposure method.
Thereafter, development and other processing are carried out,
thereby allowing patterns to appear in the second-layer shot
areas.
[0225] In this example, as shown in FIG. 14(a), the straight line
125 connecting the right-hand edges of the first-layer shot areas
121A to 121C is parallel to the direction Y2, which is the scanning
direction for the second layer; therefore, the second-layer shot
areas 126A and 126B are overlaid on the first-layer shot areas 121A
to 121C substantially perfectly in both the directions X2 and Y2.
Thus, the effect of the perpendicularity error in the first-layer
shot array is eliminated.
[0226] Although in the above-described example the X2- and Y2-axes
of the stage coordinate system in the scanning exposure apparatus
1B are assumed to be perpendicular to each other as shown in FIG.
13, it should be noted that the X2- and Y2-axes do not necessarily
need to be perpendicular to each other. In a case where the X2- and
Y2-axes are not perpendicular to each other, the wafer 20 should be
rotated such that the X1-axis of the first-layer shot array is
parallel to the direction Y2, which is the scanning direction.
[0227] Next, a second example of the second embodiment of the
present invention will be described with reference to FIGS. 15 and
16.
[0228] In this example also, the two projection exposure
apparatuses (i.e. stepper 1A and scanning exposure apparatus 1B)
shown in FIG. 11 are used. First, exposure is carried out with
respect to the wafer 20 in the stepper 1A in a state where the
reticle RA and the wafer 20 have been each rotated through
90.degree. relative to their ordinary positions, as shown in FIGS.
12(a) and 12(b). Next, the wafer 20 is restored to the ordinary
rotation angle in the scanning exposure apparatus 1B, as shown in
FIG. 13, and then, measurement for alignment is carried out. Up to
this point, the second example is approximately the same as the
first example. In this example, however, the rotation angle of each
sample shot is also measured during the measurement of coordinates
of the wafer marks in the sample shots. When the shot area 121A,
for example, is a sample shot, the rotation angle thereof is
measured as follows: For example; the coordinates of a pair of
wafer marks 123X and 123Y are measured, and at the same time, the
X2 coordinate of another X-axis wafer mark 124X is measured. A
difference between the X2 coordinates of the wafer marks 123X and
124X is divided by an approximate value of the distance in the
direction Y2 between the two wafer marks 123X and 124X to obtain a
rotation angle of the shot area 121A. Similarly, rotation angles of
the other sample shots are obtained, and a mean value of the
obtained rotation angles is defined as a shot rotation
.theta..sub.s. An alignment method in which the coordinates of the
number of wafer marks which exceeds two (one in the case of
two-dimensional wafer marks) are measured in each sample shot is
called "in-shot multipoint EGA alignment method".
[0229] Next, the rotation angle of the wafer 20 is set without
adding an offset to the desired value of the shot array rotation
(wafer rotation) in this example.
[0230] FIG. 15 shows the wafer 20 having a rotation angle set as
described above on the wafer stage 5B in the scanning exposure
apparatus 1B, shown in FIG. 11. In FIG. 15, a Y1-axis which
indicates one array direction of the first-layer shot array is set
parallel to the X2-axis of the stage coordinate system in the
scanning exposure apparatus 1B. As a result, the right-hand edges
of the shot areas 121A, 121B and 121C, which are parallel to the
array direction of the two partial shot areas in the first-layer
shot area 121A, for example, are slanted at the same angle as the
perpendicularity error W with respect to the direction Y2, which is
the scanning direction. The perpendicularity error W is a value
obtained by subtracting the shot rotation .theta..sub.s, obtained
by the in-shot multipoint EGA method, from the shot array rotation
.THETA..sub.1 obtained in the state shown in FIG. 13. If exposure
is simply carried out by the scanning exposure method with respect
to the wafer 20 which is in the state shown in FIG. 15,
second-layer shot areas would become shot areas 126A to 126F having
edges parallel to the directions X2 and Y2, as shown by the chain
double-dashed lines, resulting in an overlay error between the
first- and second-layer shot areas.
[0231] In order to avoid the above problem, in this example,
exposure is carried out with each of the second-layer shot areas
126A to 126F rotated counterclockwise through an angle equal to the
perpendicularity error W relative to the wafer 20. More
specifically, in a case where the scanning direction can be finely
adjusted during the scanning exposure in the scanning exposure
apparatus 1B, the reticle RB is rotated counterclockwise through an
angle equal to the perpendicularity error W in FIG. 11, and
thereafter, the reticle RB is scanned in a direction rotated from
the direction Y2 by the perpendicularity error W, and the wafer 20,
shown in FIG. 15, is scanned in a direction parallel to the
scanning direction of the reticle RB in synchronism with the
scanning of the reticle RB. To finely adjust the scanning direction
of the reticle RB in this way, the reticle RB should be gradually
shifted in the direction X2 according to the scanning position by
using, for example, a mechanism for finely adjusting the position
of the reticle RB. As a result, the second-layer shot areas 126A to
126F are each rotated counterclockwise by the perpendicularity
error W, as shown by the continuous lines in FIG. 16, and thus the
overlay error between the first- and second-layer shot areas is
minimized.
[0232] It should be noted that, when the exposure apparatus that
effects exposure for the second layer is a one-shot exposure type
projection exposure apparatus (e.g. stepper), the rotation angle
(shot rotation) of each of the shot areas 126A to 126F can be
corrected, as shown in FIG. 16, simply by rotating the reticle
RB.
[0233] Next, a third example of the second embodiment of the
present invention will be described with reference to FIGS. 17(a),
17(b) and 17(c).
[0234] In this example also, the two projection exposure
apparatuses (i.e. stepper 1A and scanning exposure apparatus 1B)
shown in FIG. 11 are used. First, as shown in FIGS. 12(a) and
12(b), exposure is carried out with respect to the wafer W in the
stepper 1A in a state where the reticle RA and the wafer 20 have
been each rotated through 90.degree. relative to their ordinary
positions. Up to this point, the third example is the same as the
first example. In this example, however, the subsequent exposure
for the second layer is carried out by the scanning exposure
apparatus 1B, shown in FIG. 11, with the reticle RB and the wafer
20 left rotated through 90.degree. from their ordinary positions.
The scanning exposure apparatus 1B in this example effects scanning
exposure in a direction parallel to the direction X2. For this
purpose, an exposure apparatus in which the scanning direction is
the direction X2 should be used as the scanning exposure apparatus
1B. Alternatively, an exposure apparatus in which the scanning
direction can be switched to either of the directions X2 and Y2
should be used as the scanning exposure apparatus 1B. The following
is a description of the exposure method for the second layer in
this example.
[0235] FIG. 17(a) shows the wafer 20 placed on the wafer stage 5B
in the scanning exposure apparatus 1B, shown in FIG. 11, after the
completion of exposure and development for the first layer. In FIG.
17(a), the X1-axis which indicates one array direction of
first-layer shot areas 121A, 121B, . . . , 121I is set
approximately parallel to the X2-axis of the stage coordinate
system in the scanning exposure apparatus 1B. In this case, at the
time of exposure for the first layer, as shown in FIG. 17(c), the
reticle RA is set such that the two partial pattern areas 112A and
112B lie side-by-side in the direction X1. The first-layer shot
array has a perpendicularity error W.
[0236] In this example also, the wafer 20 shown in FIG. 17(a) is
subjected to EGA alignment, thereby obtaining values of EGA
parameters including a shot array rotation (wafer rotation)
.THETA..sub.2, a shot array perpendicularity error W.sub.2, an
offset Ox.sub.2 in the direction X2, and an offset Oy.sub.2 in the
direction Y2. Thereafter, the rotation angle of the wafer 20 is set
such that the X1-axis is accurately parallel to the X2-axis on the
basis of the rotation .THETA..sub.2. Then, as shown in FIG. 17(b),
the rotation angle of the reticle RB is set such that the array
direction of the partial pattern areas 113A to 113C is parallel to
the direction X2. Then, the reticle RB is scanned in the direction
+X2 (or -X2) by the scanning exposure apparatus 1B, and the wafer
20 is scanned in the direction -X2 (or +X2) by the scanning
exposure apparatus 1B in synchronism with the scanning of the
reticle RB, thereby sequentially transferring a pattern image of
the reticle RB onto the second-layer shot areas 126A, 126B, . . . ,
126F, shown in FIG. 17(a), by the scanning exposure method. As a
result, the second-layer shot areas 126A and 126B are substantially
perfectly overlaid on the first-layer shot areas 121A, 121B and
121C, and thus the effect of the perpendicularity error W of the
first layer is eliminated.
[0237] Although in the above-described second embodiment exposure
is first carried out by the stepper 1A having a small exposure
field, and thereafter, exposure is carried out by the scanning
exposure apparatus 1B having a large exposure field, it should be
noted that the present invention is also applicable to an exposure
process in which exposure is carried out by the stepper 1A having a
small exposure field after exposure has been carried out by the
scanning exposure apparatus 1B having a large exposure field in
reverse relation to the above. In the latter case also, the effect
of the perpendicularity error of the first layer can be reduced.
Further, although in the above-described second embodiment the
stepper 1A and the scanning exposure apparatus 1B are used as a
combination of two exposure apparatuses, it should be noted that
both the two exposure apparatuses may be steppers. Alternatively,
both the two exposure apparatuses may be scanning exposure
apparatuses.
[0238] According to the second embodiment, when a first mask
pattern is to be transferred onto a photosensitive substrate by
using the first exposure apparatus, the array of a plurality of
shot areas on the photosensitive substrate to each of which the
first mask pattern is to be transferred by exposure is set in a
direction corresponding to a direction in which the exposure field
of the first exposure apparatus is different in length from the
exposure field of the second exposure apparatus (i.e. the second
exposure field). Therefore, a plurality of shot areas of a first
layer can be arranged in the form of a straight line in the
direction in which the exposure fields of the first and second
exposure apparatuses differ in length from each other. Accordingly,
even if the first-layer shot array has a perpendicularity error, an
overlay error between the first and second layers can be reduced by
overlaying the second-layer shot areas on the first-layer shot
areas along the direction in which the two exposure fields are
different in length from each other. Thus, it is possible to reduce
an overlay error between different layers in a case where exposure
is carried out by the mix-and-match method using a plurality of
exposure apparatuses having respective exposure fields (shot areas)
of different sizes because they are different from each other in
the length in a predetermined direction on a photosensitive
substrate.
[0239] In a case where the first mask pattern is transferred onto
the photosensitive substrate by using the first exposure apparatus
in a state where the photosensitive substrate and the first mask
pattern have previously been rotated through 90.degree. from their
ordinary positions, the effect of a perpendicularity error of the
first-layer shot array can be readily eliminated without providing
a special mechanism on the two exposure apparatuses, particularly
when the first exposure apparatus is a one-shot exposure type
exposure apparatus (e.g. stepper).
[0240] In a case where the second exposure apparatus is a scanning
exposure type exposure apparatus, and the above-described
predetermined direction (i.e. direction in which the two exposure
fields differ in length from each other) is the scanning direction,
the exposure field (i.e. second exposure field) of the second
exposure apparatus is likely to lengthen in the predetermined
direction in particular. Accordingly, the present invention is
particularly useful in such a case.
[0241] Although in the second embodiment the one-shot exposure type
exposure apparatus is used first and then the scanning exposure
type exposure apparatus is used, these two exposure apparatuses may
be used in the reverse order. That is, the scanning exposure type
exposure apparatus may be used first.
[0242] Next, a third embodiment of the exposure method according to
the present invention will be described with reference to FIGS. 18
to 23(b). In this embodiment, a projection exposure apparatus
(stepper) in which a reduced image of a pattern formed on a reticle
is projected onto each shot area on a wafer by the step-and-repeat
method is used as each of two exposure apparatuses. It should be
noted that constituent elements in this embodiment which are
similar to those in the first and second embodiments are denoted by
the same reference characters, and that arrangements similar to
those in the first and second embodiments will be briefly explained
in the following description.
[0243] FIG. 18 shows an exposure system used in this embodiment. In
the illustrated exposure system are installed a stepper 1A having a
small exposure field (hereinafter referred to as "fine stepper")
and a stepper 1B having a large exposure field (hereinafter
referred to as "middle stepper"). In this embodiment, the fine
stepper 1A is a high-resolution exposure apparatus, and the middle
stepper 1B is a low-resolution exposure apparatus. The fine stepper
1A is used to carry out exposure for a critical layer on a wafer,
and the middle stepper 1B is used to carry out exposure for a
middle layer on the wafer. However, the fine stepper 1A may be a
low-resolution exposure apparatus or the middle stepper 1B may be a
high-resolution exposure apparatus according to the kind of
semiconductor device to be produced.
[0244] First, in the stepper 1A, a pattern area 52A on a reticle RA
is illuminated by exposure light from an illumination optical
system (not shown), and an image of the original drawing patterns
of overlay accuracy measuring marks (vernier marks), which have
been written in the pattern area 52A according to a predetermined
layout, is projected onto a rectangular exposure field 54A on a
wafer 20 as an image reduced to 1/5 by a projection optical system
3A. A Z1-axis is taken in a direction parallel to an optical axis
of the projection optical system 3A, and two axes of an orthogonal
coordinate system set in a plane perpendicular to the Z1-axis are
defined as an X1-axis and a Y1-axis, respectively. The reticle RA
has an alignment mark 217X for the X1-axis formed at an end of the
pattern area 52A in the direction Y1 (e.g. within a masking frame)
and also has an alignment mark 217Y for the Y1-axis formed at an
end of the pattern area 52A in the direction X1.
[0245] A wafer stage 5A comprises a Z-stage, an XY-stage, etc. The
coordinate in the direction X1 of the wafer stage 5A is measured by
a combination of a moving mirror 6A and a laser interferometer 7A.
The coordinate in the direction Y1 of the wafer stage 5A is
measured by a combination of a moving mirror 8A and a laser
interferometer 9A. The coordinates measured by the laser
interferometers 7A and 9A are supplied to a controller 10A which
controls operations of the whole apparatus. The controller 10A
drives the wafer stage 5A to step, thereby positioning the wafer
20. In this case, the stepping drive of the wafer 20 is effected
according to a shot map for a critical layer. The shot map is
generated by a map generating unit which comprises a computer in
the controller 10A.
[0246] An off-axis imaging type (FIA type) alignment system 11A
images an alignment mark (wafer mark) or overlay accuracy measuring
vernier mark on the wafer 20 and processes an imaging signal thus
obtained to detect X1 and Y1 coordinates of the mark. The detected
coordinates are supplied to the controller 10A.
[0247] The middle stepper 1B in this embodiment has substantially
the same arrangement as that of the fine stepper 1A. In the middle
stepper 1B, however, an image of a pattern formed in a pattern area
52B of a reticle RB is projected onto a rectangular exposure field
54B on a wafer 20 held on a wafer stage 5B as an image reduced to
1/2.5 through a projection optical system 3B. Accordingly, the size
of the exposure field 54B is double that of the exposure field 54A
of the fine stepper 1A in both lengthwise and breadthwise
directions. A Z2-axis is taken in a direction parallel to an
optical axis of the projection optical system 3B, and two axes of
an orthogonal coordinate system set in a plane perpendicular to the
Z2-axis are defined as an X2-axis and a Y2-axis, respectively. The
pattern area 52B of the reticle RB is divided into two columns in
the direction X2 and two rows in the direction Y2 to form partial
pattern areas 218A to 218D. The partial pattern areas 218A to 218D
each has vernier mark original drawing patterns formed according to
the same layout.
[0248] The X2 coordinate of the wafer stage 5B in the middle
stepper 1B is measured by a combination of a moving mirror 6B and a
laser interferometer 7B. The Y2 coordinate of the wafer stage 5B is
measured by a combination of a moving mirror 8B and a laser
interferometer 9B. The measured coordinates of the wafer stage 5B
are supplied to a controller 10B. The controller 10B controls
stepping of the wafer stage 5B. Stepping drive of the wafer stage
5B is effected according to an array of shot areas (to each of
which the pattern image of the pattern area 52B is to be projected
by exposure) set on an exposure surface of the wafer 20, which is
to be exposed, that is, a shot map for a middle layer. The shot map
is generated by a map generating unit which comprises a computer in
the controller 10B.
[0249] In this case, the map generating unit in the controller 10A
and the map generating unit in the controller 10B have the function
of supplying shot map information prepared thereby to each other.
When exposure for a middle layer is to be carried out over a
critical layer, for example, shot map information for the critical
layer prepared by the map generating unit in the controller 10A of
the stepper 1A is transmitted from a communication unit in the
controller 10A to a communication unit in the controller 10B. The
map generating unit in the controller 10B generates a shot map for
the middle layer on the basis of the supplied shot map information.
Conversely, when exposure for a critical layer is to be carried out
over a middle layer, shot map information for the middle layer
prepared by the map generating unit in the controller 10B is
supplied to the map generating unit in the controller 10A.
[0250] The middle stepper 1B also has an off-axis imaging type (FIA
type) alignment system 11B provided at a side surface of the
projection optical system 3B. The alignment system 11B detects X2
and Y2 coordinates of a wafer mark or vernier mark on the wafer
20.
[0251] Next, one example of an operation of correcting coordinate
transformation parameters for alignment when exposure of the
pattern for the middle layer is to be effected by the middle
stepper 1B after exposure of the pattern for the critical layer has
been carried out by the fine stepper 1A in this embodiment will be
explained for each of the first to third processing steps. In this
embodiment also, the EGA (Enhanced Global Alignment) method is used
in which values of the six coordinate transformation parameters
(scaling parameters Rx and Ry, rotation .THETA., perpendicularity
W, and offsets Ox and Oy) in Eq. (1) are determined, and array
coordinates of each shot area are calculated from the coordinate
transformation parameters and design array coordinates.
[0252] First, the first step will be explained.
[0253] In the first step, an unexposed wafer 20 coated with a
photoresist is placed on the wafer stage 5A of the fine stepper 1A,
shown in FIG. 18, and a reduced image of the pattern on the reticle
RA is sequentially transferred by the step-and-repeat method onto a
multiplicity of shot areas arrayed on the wafer 20 in units of the
exposure field 54A. The reticle RA has the original drawing
patterns of a plurality of vernier marks formed according to a
predetermined layout in addition to a pair of alignment marks.
Thereafter, the wafer 20 is subjected to development, thereby
allowing the pair of alignment marks to appear as wafer marks
comprising recess-and-projection patterns, and also allowing the
vernier mark original drawing patterns to appear as vernier marks
comprising recess-and-projection patterns. The patterns obtained
after the development can be regarded as critical layer patterns on
the wafer 20. However, it is also possible to carry out the
following alignment and measurement of an amount of positional
displacement between two corresponding vernier marks with these
marks left in the form of latent images without effecting
development.
[0254] Next, the second step will be explained.
[0255] A photoresist is coated over the wafer 20 having the wafer
and vernier marks formed in the first step, and the
photoresist-coated wafer 20 is placed on the wafer stage 5B of the
middle stepper 1B, shown in FIG. 18. At this time, information
concerning the critical layer shot map used in the first step has
been supplied from the controller 10A of the fine stepper 1A to the
controller 10B of the middle stepper 1B. Thus, the controller 10B
can obtain design array coordinates of the critical layer wafer
marks on the wafer 20.
[0256] FIG. 19(a) shows the wafer 20 placed on the wafer stage 5B.
In FIG. 19(a), the X2- and Y2-axes of the middle stepper 1B are
shown as being X- and Y-axes, respectively. In this case, the wafer
20 has been roughly aligned by a pre-alignment mechanism (not
shown), and the surface of the wafer 20 has been divided into M (M
is an integer of 12 or more) critical layer shot areas SE1, SE2, .
. . , SEM in two directions which are approximately parallel to the
directions X and Y, respectively. In actual practice, a scribe line
area of a predetermined width lies between shot areas SEm (m=1 to
M); however, illustration of the scribe line area is omitted in
FIG. 19(a). The width (pitch) in the direction x of each shot area
SEm, including the scribe line area, is d, and the width (pitch) in
the direction Y is c. In this embodiment, each shot area SEm is
approximately square (d.apprxeq.c).
[0257] FIG. 19(b) shows a shot area SEm as a typical example of the
critical layer shot areas. In FIG. 19(b), the shot area SEm has a
wafer mark 221X for the X-axis formed at one end thereof, and also
has a wafer mark 221Y for the Y-axis formed at another end thereof.
Further, the shot area SEm has five vernier marks 222A to 222E
which are distributed in a cross shape, and also has four vernier
marks 223A to 223D which are formed at respective positions near
the four corners of the shot area SEm. The original drawing
patterns of marks distributed as shown in FIG. 19(b) have been
formed in the pattern area 52A of the reticle RA of the fine
stepper 1A, shown in FIG. 18.
[0258] It should be noted that the vernier marks 222A to 222E and
223A to 223D used in this embodiment are two-dimensional box-in-box
marks, which are detected by an imaging detection method with the
alignment system 11B, shown in FIG. 18. However, it is possible to
use other kinds of mark as vernier marks, for example, marks each
formed by a combination of two one-dimensional line-and-space
patterns which are crossed at right angles. It is also possible to
use the wafer marks 221X and 221Y as vernier marks. Further, marks
which are detected, for example, by the laser step alignment (LSA)
method may also be used as vernier marks. The distribution of
vernier marks is not necessarily limited to that shown in FIG.
19(b). That is, vernier marks used in this embodiment may be
distributed as desired.
[0259] Next, the controller 10B of the middle stepper 1B, shown in
FIG. 18, effects alignment by the EGA method. Accordingly, the
controller 10B drives the wafer stage 5B to move the field of view
of the alignment system 11B sequentially according to the critical
layer shot map, thereby measuring array coordinates (Mxn,Myn) in a
stage coordinate system (i.e. a coordinate system determined by
values measured with the laser interferometers 7B and 9B of the
middle stepper 1B) of each of the wafer marks 221X and 221Y
attached to 10 (for example) shot areas (sample shots) SEa, SEb,
SEc, . . . , SEj selected from among the shot areas on the wafer
20, as shown in FIG. 19(a). Then, values of the six EGA parameters
(scaling parameters Rx and Ry, rotation .THETA., perpendicularity
W, and offsets Ox and Oy) in Eq. (1) are determined so as to
minimize the residual error component (expressed by Eq. (2)) of the
alignment error, that is, the deviation of the measured values
(Mxn,Myn) of the wafer marks 221X and 221Y of each sample shot from
the array coordinate values, which are calculated from the design
array coordinates (Dxn,Dyn) of the wafer marks 221X and 221Y.
[0260] Next, the controller 10B sequentially substitutes the six
EGA parameters and the design array coordinate values (Dxm,Dym) of
the shot areas SEm (m=1 to M) into the right-hand side of Eq. (1),
thereby obtaining array coordinate values in the stage coordinate
system of each shot area Sem of the critical layer on the wafer 20.
At this time, because the exposure field 54B of the middle stepper
1B is twice as large as the exposure field 54A of the fine stepper
1A in both the directions X and Y, the controller 10B divides the
shot areas SEm (m=1 to M), shown in FIG. 19(a), into a plurality of
blocks each comprising an array of two shot areas in the direction
X and two shot areas in the direction Y, and obtains array
coordinates in the stage coordinate system of the center of each
block from the computational array coordinates of the four shot
areas in the block. Thereafter, the controller 10B sequentially
aligns the array coordinates of the center of each block on the
wafer 20 with the center of the exposure field 54B, and transfers
an image of the vernier mark original drawing patterns formed on
the reticle RB onto each block. Thereafter, development is carried
out, thereby allowing middle layer vernier marks to appear over the
critical layer vernier marks on the wafer 20. It should be noted
that the following measurement may be effected with the transferred
marks left in the form of latent images, as has already been
described above.
[0261] Next, a third step will be explained.
[0262] In the third step, an amount of positional displacement
between the critical and middle layer vernier marks is measured.
For this purpose, the wafer 20 having been subjected to development
in the second step is placed, for example, on the wafer stage 5B of
the middle stepper 1B, shown in FIG. 18, and an amount of
positional displacement between the critical and middle layer
vernier marks is measured by the alignment system 11B. However, the
measurement of a positional displacement between the critical and
middle layer vernier marks may be carried out by using another
measuring device of high accuracy.
[0263] FIG. 20(a) shows the wafer 20 having overlaid vernier marks
formed by the exposure process in the second step. In FIG. 20(a),
the wafer 20 has middle layer shot areas SF1, SF2, . . . , SFN (N
is an integer of 4 or more) arranged at a pitch 2d along the X-axis
and at a pitch 2c along the Y-axis. Each shot area SFn (n=1 to N)
contains four critical layer shot areas SEm. It should be noted
that, if there is a magnification error in each shot area SFn of
the middle layer, the widths of each shot area SFn in the
directions X and Y are slightly deviated from 2d and 2c,
respectively. Further, the center 261 of each shot area SFn is
substantially coincident with the center of the associated four
critical layer shot areas. Each shot area SFn has a total of 36
(=4.times.9) vernier marks corresponding to the nine vernier marks
222A to 222E and 223A to 223D (see FIG. 19(b)) in each critical
layer shot area SEm.
[0264] Assuming that each middle layer shot area SFn is
M.sub.1/N.sub.1 times and M.sub.2/N.sub.2 times as large as the
critical layer shot area SEm in the directions X and Y,
respectively, in this embodiment M.sub.1/N.sub.1=2/1 and
M.sub.2/N.sub.2=2/1. Accordingly, a reference measurement area in
this embodiment is an area determined by multiplying the middle
layer shot area SFn by one in each of the directions X and Y, that
is, the shot area SFn itself. Therefore, four shot areas (reference
measurement areas) SFa to SFd which are substantially uniformly
distributed over the wafer 20, as shown by the hatching in the
figure, are defined as objects to be measured.
[0265] FIG. 20(b) shows the shot area SFa among the four reference
measurement areas. In FIG. 20(b), the shot area SFa has nine middle
layer vernier marks 224A to 224E and 225A to 225D formed to
surround the respective vernier marks which belong to the
second-quadrant shot area SEp of the four critical layer shot areas
underlying the middle layer shot area SFa. The shot area SFa
further has nine vernier marks (not shown) similarly formed to
surround the respective vernier marks which belong to each of the
other shot areas SE(p+1), SEq and SE(q+1) underlying the shot area
SFa. However, FIG. 20(b) shows the middle layer vernier mark 226C
corresponding to the vernier mark 222C formed in the intermediate
portion at the right end of the first-quadrant shot area SE(p+1)
among those middle layer vernier marks.
[0266] Next, in this embodiment, an amount of positional
displacement between the critical layer vernier mark 222C and the
middle layer vernier mark 226C is measured at each of measuring
points 232A to 232D lying at the mutually identical positions in
the shot areas (reference measurement areas) SFa to SFd, which are
objects to be measured, on the wafer 20. For example, the measuring
points 232A to 232D each lies in the intermediate portion at the
right end of the first quadrant [i.e. an area corresponding to the
shot area SE(p+1) in FIG. 20(b)] in the shot areas SFa to SFd.
Accordingly, at the measuring point 232A, positional displacements
(.DELTA.xa,.DELTA.ya) in the directions X and Y of the vernier mark
226C relative to the vernier mark 222C is measured. At the other
measuring points 232B to 232D, positional displacements
(.DELTA.xb,.DELTA.yb) to (.DELTA.xd,.DELTA.yd) are similarly
measured.
[0267] Thereafter, if there is a difference (.DELTA.xb-.DELTA.xd)
between the positional displacements in the direction X measured at
the two measuring points 232D and 232B in FIG. 20(a), for example,
the difference (.DELTA.xb-.DELTA.xd) is divided by the distance in
the direction X between the two measuring points 232D and 232B,
thereby obtaining a correction value (error) .DELTA.Rx for the
scaling parameter Rx in the direction X among the EGA parameters.
If there is a difference (.DELTA.yb-.DELTA.yd) between the
positional displacements in the direction Y measured at the
measuring points 232D and 232B, the difference
(.DELTA.yb-.DELTA.yd) is divided by the distance in the direction X
between the two measuring points, thereby obtaining a correction
value .DELTA..THETA. for the rotation .THETA. among the EGA
parameters. Further, mean values of the positional displacements in
the directions X and Y measured at the four measuring points are
defined as correction values .DELTA.Ox and .DELTA.Oy for the
offsets Ox and Oy among the EGA parameters. Similarly, correction
values .DELTA.Ry and .DELTA.W for the other EGA parameters, that
is, the scaling parameter Ry and the perpendicularity W, are also
obtained. These correction values are stored in a storage unit in
the controller 10B of the middle stepper 1B. It should be noted
that, if positional displacements between the corresponding vernier
marks are measured with another measuring device, and correction
values are obtained by using another computer or the like, the
operator inputs the correction values to the controller 10B through
an input device. Thus, the third step is terminated.
[0268] Thereafter, in a case where exposure is carried out by the
mix-and-match method using the fine stepper 1A and the middle
stepper 1B, shown in FIG. 18, a critical layer pattern is formed on
the wafer 20 by using the fine stepper 1A, and before a middle
layer pattern is formed by using the middle stepper 1B, coordinate
values of predetermined sample shots in the stage coordinate system
are measured, and values of the six EGA parameters in Eq. (1) are
determined on the basis of the measured coordinate values.
Thereafter, the controller 10B adds the EGA parameter correction
values (.DELTA.Rx, .DELTA.Ry, .DELTA..THETA., .DELTA.W, .DELTA.Ox,
and .DELTA.Oy), stored in the above-described third step, to the
determined EGA parameters (Rx, Ry, .THETA., w, Ox, and Oy) to
obtain corrected EGA parameters. Then, the controller 10B
calculates coordinate positions of the shot areas of the critical
layer by using the corrected EGA parameters, calculates exposure
positions of the shot areas of the middle layer on the basis of the
coordinate positions of the critical layer shot areas, and
sequentially transfers the reticle pattern for the middle layer
onto the middle layer shot areas on the basis of the exposure
positions.
[0269] In this embodiment, the measuring points used in the
above-described third step are at the mutually identical positions
in the shot areas (reference measurement areas) SFa to SFd, as
shown in FIG. 20(a). Accordingly, even when the middle layer shot
areas SFn have a magnification error or a rotation error, the same
offset value is superimposed at each measuring point, and the
magnification or rotation error affects only the offsets Ox and Oy
among the EGA parameters. Thus, the overlay accuracy between the
critical and middle layers improves because the values of other
influential linear parameters (Rx, Ry, .THETA., and W) are
accurate.
[0270] In the above-described embodiment, a magnification error or
rotation error in the middle layer shot areas affects the offsets
Ox and Oy among the EGA parameters. Therefore, the effect of the
magnification or rotation error is eliminated by averaging process.
The method of eliminating the magnification or rotation error will
be explained below with reference to FIGS. 21(a) to 23(b), in which
portions corresponding to those in FIGS. 20(a) and 20(b) are
denoted by the same reference characters.
[0271] FIG. 21(a) shows a first method of arranging measuring
points for eliminating an offset error. In FIG. 21(a), four middle
layer shot areas SFa to SFd are selected as reference measurement
areas on the wafer 20 in the same way as in FIG. 20(a). Then, an
amount of positional displacement between two corresponding vernier
marks is measured at each of four measuring points in the shot area
SFa, that is, a measuring point 233A at the bottom left in the
first quadrant, a measuring point 235A at the bottom right in the
second quadrant, a measuring point 234A at the top right in the
third quadrant, and a measuring point 236A at the top left in the
fourth quadrant. Mean values of the positional displacements in the
directions X and Y measured at the four measuring points are
assumed to be (.DELTA.xa',.DELTA.ya').
[0272] More specifically, FIG. 21(b) is an enlarged view of the
shot area SFa. As shown in FIG. 21(b), an amount of positional
displacement between the vernier mark 223D in the shot area SE(p+1)
and the middle layer vernier mark 227D is measured at the measuring
point 233A, and an amount of positional displacement between the
vernier mark 223C in the shot area SEp and the middle layer vernier
mark 225C is measured at the measuring point 235A. Similarly, an
amount of positional displacement between the vernier mark 223B (or
223A) and the vernier mark 229B (or 231A) is measured at the
measuring point 234A (or 236A).
[0273] Referring to FIG. 21(a), positional displacement is
similarly measured in each of the other shot areas SFb to SFd. That
is, in each shot area, an amount of positional displacement between
the two corresponding vernier marks is similarly measured at each
of the four measuring points lying at respective positions mutually
identical with the measuring points 233A to 236A in the shot area
SFa, and mean values of the measured amounts of positional
displacement are determined to be (.DELTA.xb',.DELTA.yb') to
(.DELTA.xd',.DELTA.yd'). Thereafter, EGA parameter correction
values are obtained from the amounts of positional displacement
measured in the four shot areas SFa to SFd. In this case, even if
the shot area SFa, for example, has a magnification error or
rotation error (shot rotation error), the effect of such an error
appears symmetrically at the four measuring points 233A to 236A;
therefore, the effect of the magnification or rotation error can be
eliminated by averaging the amounts of positional displacement at
the four measuring points. Accordingly, even if there is a
magnification or rotation error, no error will be introduced into
the offsets Ox and Oy in the EGA parameters.
[0274] Further, the exposure method according to this embodiment
provides the following advantageous effects: Since the measuring
points 233A to 236A lie in the center of the shot area SFa, the
distortion introduced by the projection optical system 3B of the
middle stepper 1B is small at the measuring points 233A to 236A,
and thus the distortion of the middle layer shot areas produces a
minimal effect on the measurement result. Further, in the shot area
SFa, for example, measurement is carried out in each of four
different corners of the four critical layer shot areas. Therefore,
the effects of the magnification or rotation errors in the critical
layer shot areas are canceled by the averaging process. Similarly,
the effect of the distortion of the critical layer shot areas is
reduced by the averaging process.
[0275] In this embodiment, it is only necessary to enable measuring
points to be symmetrically disposed in each middle layer shot area
used as a reference measurement area. Therefore, in FIG. 21(a),
only two measuring points shown by the black circles may be
selected from each of the shot areas SFa to SFd, for example, (i.e.
the measuring points 233A and 234A in the shot area SFa).
Alternatively, only two measuring points shown by the white circles
may be selected from each of the shot areas SFa to SFd (i.e. the
measuring points 235A and 236A in the shot area SFa).
[0276] Although in the above-described embodiment the measuring
points are concentrated on the center of each of the middle layer
shot areas used as reference measurement areas, the arrangement of
measuring points is not necessarily limited to it. As shown in FIG.
22(a), measuring points 237A to 240A may be set in the respective
centers of the four critical layer shot areas in the shot area SFa,
for example. It is also possible to select two measuring points
237A and 238A, shown by the black circles, or two measuring points
239A and 240A, shown by the white circles, from among the four
measuring points. In this case, the distortion of the critical
layer shot areas is minimized, and the effect of the distortion of
the middle layer shot areas is reduced by the averaging
process.
[0277] However, in a case where the distortion of the critical and
middle layer shot areas has previously been known to be small, as
shown for example in FIG. 22(b), four measuring points 241A to 244A
in the four corners of the shot area SFa may be selected.
Alternatively, only two measuring points 241A and 242A, shown by
the black circles, or only two measuring points 243A and 244A,
shown by the white circles, may be selected.
[0278] To sum up, an efficient measuring point layout which enables
the averaging effect to be obtained and which makes it possible to
minimize the number of measuring points and to shorten the time
required for measurement is such as that shown, for example, in
FIG. 23(a) or 23(b). In the layout shown in FIG. 23(a), measuring
points 233A to 233D and 234A to 234D are selected in four shot
areas (reference measurement areas) SFa to SFd on the wafer 20.
More specifically, the measuring points 233A to 233D are each on
the right side of the center of the associated shot area, toward
the top as viewed in the figure, and the measuring points 234A to
234D are each on the left side of the center of the associated shot
area, toward the bottom as viewed in the figure. In the layout
shown in FIG. 23(b), measuring points are selected as follows: In a
pair of mutually opposing shot areas SFa and SFc among the four
shot areas, measuring points 235A and 235C are selected which are
each on the left side of the center of the associated shot area,
toward the top, and measuring points 236A and 236C are selected
which are each on the right side of the center of the associated
shot area, toward the bottom; in the other pair of mutually
opposing shot areas SFb and SFd, measuring points 233B and 233D are
selected which are each on the right side of the center of the
associated shot area, toward the top, and measuring points 234B and
234D are selected which are each on the left side of the center of
the associated shot area, toward the bottom.
[0279] It is also possible to select from each reference
measurement area one measuring point which is at a symmetric
position with respect to the center of the area, as shown in FIGS.
24(a) and 24(b). That is, FIG. 24(a) shows middle layer shot areas
transferred over a critical layer on the wafer 20. From among the
shot areas, eight shot areas SFa to SFh, which are substantially
uniformly distributed, are selected as reference measurement areas.
The shot areas SFa to SFh each contains four critical layer shot
areas.
[0280] Then, from the two shot areas SFc and SFg, measuring points
233C and 233G are respectively selected which are each on the right
side of the center of associated shot area, toward the top, and
from the two shot areas SFa and SFe, measuring points 235A and 235E
are respectively selected which are each on the left side of the
center of the associated shot area, toward the top. From the two
shot areas SFd and SFh, measuring points 234C and 234H are
respectively selected which are each on the left side of the center
of the associated shot area, toward the bottom, and from the other
two shot areas SFb and SFf, measuring points 236B and 236F are
respectively selected which are each on the right side of the
center of the associated shot area, toward the bottom. Then, an
amount of positional displacement between the critical and middle
layer vernier marks is measured at each of the selected measuring
points. In this example, thereafter, the amounts of positional
displacement measured at two measuring points which are at
symmetric positions in a pair of middle layer shot areas (e.g. the
measuring points 235A and 236B) are averaged, thereby reducing the
effect of the magnification or rotation error of the middle layer.
The effects of the middle layer distortion, the reticle writing
error, etc. are also reduced by the averaging process.
[0281] Let us consider the measuring method in this example in
terms of one critical layer shot area SE, as shown in FIG. 24(b).
In this example, measurement is carried out twice at each of
measuring points 245A to 245D in the four corners of the shot area
SE. Therefore, assuming that the magnification or rotation error of
the critical layer shot areas is substantially uniform over the
wafer, it is possible to reduce the effects of magnification error,
rotation error, and distortion of the critical layer shot areas,
reticle writing error, etc. by averaging the amounts of positional
displacement measured, for example, at a pair of mutually opposing
measuring points (e.g. the measuring points 245A and 245C) among
the measuring points 245A to 245D in the four corners of the shot
area SE.
[0282] As has been described above, the third embodiment shows a
measuring point layout which is applicable in a case where the size
of each middle layer shot area is twice as large as each critical
layer shot area in each of the directions X and Y, and where one
chip pattern, for example, is formed in each critical layer shot
area. In actuality, however, two or more chip patterns may be
contained in each critical layer shot area; there are various size
ratios of the middle layer shot areas to the critical layer shot
areas. Further, projection exposure apparatuses usable in the third
embodiment are not necessarily limited to one-shot exposure type
projection exposure apparatuses such as steppers; it is also
possible to use scanning exposure type projection exposure
apparatuses, e.g. step-and-scan type projection exposure
apparatuses in which a pattern on a reticle is sequentially
transferred onto each shot area on a wafer by synchronously
scanning the reticle and the wafer with respect to a projection
optical system. Various other modifications of the third embodiment
will be explained below with reference to FIGS. 25(a) to 27(b).
[0283] In the modification shown in FIGS. 25(a) to 25(c), each
critical layer shot area SE has, as shown in FIG. 25(a), two
identical chip patterns 246A and 246B arranged in the direction Y.
As shown in FIG. 25(b), each middle layer shot area SF has
identical chip patterns arranged in two columns in the direction X
and four rows in the direction Y. In this case, assuming that each
chip pattern is a rectangular pattern having a width b in the
direction X and a width a in the direction Y, the width in the
direction X of the critical layer shot area SE is b, and the width
in the direction Y of the shot area SE is 2a. The width in the
direction X of the middle layer shot area SF is 2b, and the width
in the direction Y of the shot area SF is 4a. Accordingly, the shot
area SF is 2/1 times as large as the shot area SE in each of the
directions X and Y. Therefore, as shown in FIG. 25(c), a reference
measurement area SG, which has a size regarded as being the least
common multiple of the sizes of the shot areas SE and SF, has a
width 2b in the direction X and a width 4a in the direction Y. That
is, the reference measurement area SG has the same size as that of
the middle layer shot area SF. Accordingly, when a measuring point
247, for example, is selected in a certain reference measurement
area SG, in the other reference measurement areas also measuring
points which are at the identical positions with the measuring
point 247 are selected. By doing so, EGA parameter correction
values can be accurately obtained.
[0284] However, in order to reduce the effect of the magnification
error, rotation error, etc. of the middle layer shot areas, it is
desirable to select, for example, measuring points which are in
symmetric relation to the measuring point 247 with respect to the
center position in the reference measurement areas in the same way
as in the above-described third embodiment. The same is true of the
following modifications.
[0285] In the modification shown in FIGS. 26(a) to 26(c), a
critical layer shot area SE has, as shown in FIG. 26(a), two
identical chip patterns arranged in the direction Y. As shown in
FIG. 26(b), a middle layer shot area SH has three identical chip
patterns arranged in the direction Y. Further, the middle layer
projection exposure apparatus is of the scanning exposure type.
Thus, the shot area SH is exposed by scanning the wafer with
respect to a slit-shaped exposure area 248.
[0286] At this time, assuming that the critical layer shot area SE
has a width b in the direction X and a width 2a in the direction Y,
the width in the direction X of the middle layer shot area SH is b,
and the width in the direction Y of the shot area SH is 3a.
Accordingly, the shot area SH is 1/1 time as large as the shot area
SE in the direction X, and the former is 3/2 times as large as the
latter in the direction Y. Therefore, as shown in FIG. 26(c), a
reference measurement area SI, which has a size regarded as being
the least common multiple of the sizes of the shot areas SE and SH,
has a width b in the direction X and a width 6a in the direction Y.
In this modification also, when a measuring point 249, for example,
is selected in a certain reference measurement area SI, in the
other reference measurement areas also measuring points which are
at the identical positions with the measuring point 249 are
selected. By doing so, EGA parameter correction values can be
accurately obtained.
[0287] In this regard, FIG. 27(a) shows an enlarged view of one
example of the reference measurement area SI, shown in FIG. 26(c).
In FIG. 27(a), a pair of adjacent shot areas SH1 and SH2 exposed by
the scanning exposure method contain three critical layer shot
areas SEI, SE2 and SE3. FIG. 27(b) shows an expansion and
contraction quantity .DELTA.Y in the longitudinal direction
(direction Y) in the shot areas SHI and SH2, shown in FIG. 27(a),
due to a magnification error. The expansion and contraction
quantity .DELTA.Y in the direction Y changes at a period which is
equal to the length of each of the shot areas SH1 and SH2.
Accordingly, if the centers of the critical layer shot areas SE1 to
SE3 are defined as measuring points 250A to 250C, for example, the
expansion and contraction quantities of the middle layer shot areas
measured at the measuring points 250A to 250C show different values
as shown by the positions 251A to 251C in FIG. 27(b). Accordingly,
if a given measuring point 249 is selected in a certain reference
measurement area SI in FIG. 26(c), the measurement result is
affected by the magnification error of the middle layer shot areas
unless measuring points are selected at the identical positions
with the measuring point 249 in the other reference measurement
areas.
[0288] In the modification shown in FIGS. 28(a) to 28(c), a
critical layer shot area SF has, as shown in FIG. 28(a), identical
chip patterns arranged in three rows in the direction Y and two
columns in the direction X. As shown in FIG. 28(b), a middle layer
shot area SH exposed by the scanning exposure method has three
identical chip patterns arranged in the direction Y. In this case,
assuming that the width in the direction X of the critical layer
shot area SF is 2b, and the width in the direction Y of the shot
area SF is 3a, the width in the direction X of the middle layer
shot area SH is b, and the width in the direction Y of the shot
area SH is 3a. Accordingly, as shown in FIG. 28(c), a reference
measurement area SJ, which has a size regarded as being the least
common multiple of the sizes of the shot areas SF and SH, has a
width 2b in the direction X and a width 3a in the direction Y. That
is, the reference measurement area SJ has the same size as that of
the critical layer shot area SF. In this modification also, when a
measuring point 252, for example, is selected in a certain
reference measurement area SJ, in the other reference measurement
areas also measuring points which are at the identical positions
with the measuring point 252 are selected. By doing so, EGA
parameter correction values can be accurately obtained.
[0289] Although in the above-described third embodiment and
modifications thereof a combination of two steppers or a
combination of a stepper and a step-and-scan type projection
exposure apparatus is used, it should be noted that a combination
of usable projection exposure apparatuses is not necessarily
limited to the above. For example, it is also possible to use two
different step-and-scan type projection exposure apparatuses as an
exposure apparatus having a small exposure field and an exposure
apparatus having a large exposure field.
[0290] According to the exposure method in the third embodiment, an
area which is so large as to contain an integer number of first and
second exposure fields in each of two directions (i.e. an area
having a size regarded as being the least common multiple of the
sizes of the first and second exposure fields) is defined as a
reference measurement area, and an amount of positional
displacement between two corresponding overlay accuracy measuring
marks (i.e. vernier marks) is measured at each of measuring points
lying at the mutually identical positions in a predetermined number
of reference measurement areas. Therefore, there is no possibility
that the effect of a magnification or rotation error, for example,
of the second mask pattern will appear as a linear expansion and
contraction error or rotation error in alignment errors which may
arise during the exposure of the second mask pattern. Accordingly,
it is possible to increase the overlay accuracy between a critical
layer pattern and a middle layer pattern in a case where exposure
is carried out by the mix-and-match method with respect to a
substrate where a critical layer and a middle layer are mixedly
present.
[0291] In a case where the second exposure apparatus calculates an
exposure position by using coordinate transformation parameters and
obtains correction values for the parameters from results of
measurement carried out for each reference measurement area, the
overlay accuracy can be increased because a magnification or
rotation error of the second mask pattern has no effect on
parameters indicating linear expansion and contraction, rotation
and perpendicularity among the coordinate transformation
parameters.
[0292] Regarding offset parameters, the effect of a magnification
or rotation error of the second mask pattern can be reduced, for
example, by using mean values of results of measurement carried out
at measuring points disposed symmetrically with respect to the
center point in the reference measurement areas.
[0293] Next, one example of a fourth embodiment of the exposure
method according to the present invention will be described with
reference to FIGS. 29 to 32. Two exposure apparatuses used in this
example are a one-shot exposure type projection exposure apparatus
(stepper) with a demagnification ratio of 5:1 and a step-and-scan
type projection exposure apparatus with a demagnification ratio of
4:1. In this example, two chip patterns are formed in each shot
area exposed by the former projection exposure apparatus (i.e. a
two-chip reticle is used), and three chip patterns are formed in
each shot area scan-exposed by the latter projection exposure
apparatus (i.e. a three-chip reticle is used). It should be noted
that constituent elements in this example which are similar to
those in the first to third embodiments are denoted by the same
reference characters and will be briefly explained below.
[0294] FIG. 29 shows an exposure system used in this embodiment. In
FIG. 29, a one-shot exposure type projection exposure apparatus
(hereinafter referred to as "fine stepper") 1A, and a step-and-scan
type projection exposure apparatus (hereinafter referred to as
"scanning exposure apparatus") 1B are installed. In this
embodiment, the fine stepper 1A is a high-resolution exposure
apparatus, while the scanning exposure apparatus 1B is a
low-resolution exposure apparatus. The fine stepper 1A is used to
carry out exposure for a critical layer on a wafer, and the
scanning exposure apparatus 1B is used to carry out exposure for a
middle layer on the wafer.
[0295] First, in the fine stepper 1A, a pattern area 62A on a
reticle RA is illuminated by exposure light from an illumination
optical system (not shown), and an image of a pattern formed in the
pattern area 62A is formed on a rectangular exposure field 64A on a
wafer 20 as a projected image reduced to 1/5 by a projection
optical system 3A. A Z1-axis is taken in a direction parallel to an
optical axis of the projection optical system 3A, and two axes of
an orthogonal coordinate system set in a plane perpendicular to the
Z1-axis are defined as an X1-axis and a Y1-axis, respectively. The
pattern area 62A on the reticle RA is divided into partial pattern
areas 312A and 312B of the same size in the direction Y1. The
partial pattern areas 312A and 312B each has original drawing
patterns of alignment marks and overlay accuracy measuring marks
(vernier marks) written according to the same layout.
[0296] A wafer stage 5A comprises a Z-stage, an XY-stage, etc. The
coordinate in the direction X1 of the wafer stage 5A is measured by
a combination of a moving mirror 6A and a laser interferometer 7A.
The coordinate in the direction Y1 of the wafer stage 5A is
measured by a combination of a moving mirror 8A and a laser
interferometer 9A. The coordinates measured by the laser
interferometers 7A and 9A are supplied to a controller 10A which
controls operations of the whole apparatus. The controller 10A
drives the wafer stage 5A to step, thereby positioning the wafer
20. The stepping drive of the wafer 20 is effected according to a
shot map for a critical layer. The shot map is generated by a map
generating unit which comprises a computer in the controller
10A.
[0297] An off-axis imaging type (FIA type) alignment system 11A
images an alignment mark (wafer mark) on the wafer 20 to detect X1
and Y1 coordinates of the mark. The detected coordinates are
supplied to the controller 10A.
[0298] Next, in the scanning exposure apparatus 1B in this example,
a part of a pattern area 62B on a reticle RB is illuminated by
exposure light from an illumination optical system (not shown), and
an image of a part of the reticle pattern is formed in a
slit-shaped exposure area 314 on a wafer 20, which is held on a
wafer stage 5B, as a projected image reduced to 1/4 by a projection
optical system 3B. The reticle RB is scanned in the direction -Y2
(or +Y2), and the wafer 20 is scanned in the direction +Y2 (or -Y2)
in synchronism with the scanning of the reticle RB, thereby
sequentially projecting an image of the pattern formed in the
pattern area 62B of the reticle RB onto the exposure field 64B on
the wafer 20.
[0299] The pattern area 62B of the reticle RB is divided into three
partial pattern areas 313A to 313C of the same size in the
direction Y2, which is the scanning direction. The size of the
exposure field 64B is such that its dimension in the scanning
direction is 3/2 times the dimension of the exposure field 64A of
the fine stepper 1A, and the exposure field 64B is equal in size
(1:1) to the exposure field 64A in the non-scanning direction. The
partial pattern areas 313A to 313C also each has original drawing
patterns of vernier marks formed according to the same layout.
[0300] The position of a reticle stage (not shown) for scanning the
reticle RB of the scanning exposure apparatus 1B and the X2 and Y2
coordinates of the wafer stage 5B are supplied to a controller 10B.
The controller 10B controls synchronous drive of the reticle stage
(not shown) and the wafer stage 5B. The scanning exposure operation
of the wafer stage 5B is effected according to a shot map for a
middle layer set on an exposure surface of the wafer 20, which is
to be exposed. The shot map is generated by a map generating unit
which comprises a computer in the controller 10B.
[0301] In this case, the map generating unit in the controller 10A
and the map generating unit in the controller 10B have the function
of supplying shot map information prepared thereby to each
other.
[0302] The scanning exposure apparatus 1B also has an off-axis
imaging type (FIA type) alignment system 11B provided at a side
surface of the projection optical system 3B. The alignment system
11B detects X2 and Y2 coordinates of a wafer mark or vernier mark
on the wafer 20.
[0303] Next, one example of an operation of correcting in-shot
parameters (i.e. shot magnifications rx and ry, shot rotation
.theta., and shot perpendicularity w) when exposure of the pattern
for the middle layer is to be effected by the scanning exposure
apparatus 1B after exposure of the pattern for the critical layer
has been carried out by the fine stepper 1A in this example will be
explained for each of the first to third processing steps.
[0304] First, the first step will be explained.
[0305] In the first step, an unexposed wafer 20 coated with a
photoresist is placed on the wafer stage 5A of the fine stepper 1A,
shown in FIG. 29, and a reduced image of the pattern on the reticle
RA is sequentially transferred by the step-and-repeat method onto a
multiplicity of shot areas arrayed on the wafer 20 in units of the
exposure field 64A. The reticle RA has original drawing patterns of
two sets of vernier marks formed according to a predetermined
layout in addition to two pairs of alignment marks. Thereafter, the
wafer 20 is subjected to development, thereby allowing the two
pairs of alignment marks to appear as wafer marks comprising
recess-and-projection patterns, and also allowing the two sets of
vernier mark original drawing patterns to appear as vernier marks
comprising recess-and-projection patterns. The patterns obtained
after the development can be regarded as critical layer patterns on
the wafer 20. However, it is also possible to carry out the
following alignment and measurement of an amount of positional
displacement between two corresponding vernier marks with these
marks left in the form of latent images without effecting
development.
[0306] Next, the second step will be explained.
[0307] A photoresist is coated over the wafer 20 having the wafer
and vernier marks formed in the first step, and the
photoresist-coated wafer 20 is placed on the wafer stage 5B of the
scanning exposure apparatus 1B, shown in FIG. 29. At this time,
information concerning the critical layer shot map used in the
first step has been supplied from the controller 10A of the fine
stepper 1A to the controller 10B of the scanning exposure apparatus
1B. Thus, the controller 10B can obtain design array coordinates of
the critical layer wafer and vernier marks on the wafer 20.
[0308] FIG. 30(a) shows the wafer 20 placed on the wafer stage 5B.
In FIG. 30(a), the X2- and Y2-axes of the scanning exposure
apparatus 1B are shown as being X- and Y-axes, respectively. In
this case, the wafer 20 has been roughly aligned by a pre-alignment
mechanism (not shown), and the surface of the wafer 20 has been
divided into Q (Q=32 in FIG. 30(a)) critical layer shot areas SM1,
SM2, . . . , SMQ in two directions which are approximately parallel
to the directions X and Y, respectively. In actual practice, a
scribe line area of a predetermined width lies between shot areas
SMq (q=1 to Q); however, illustration of the scribe line area is
omitted in FIG. 30(a). The width (pitch) in the direction X of each
shot area SMq, including the scribe line area, is b, and the width
(pitch) in the direction Y is 2a. In this embodiment, each shot
area SMq is approximately square (2a.apprxeq.b). Further, each shot
area SMq is divided into two partial shot areas of the same shape
in the direction Y, that is, first and second partial areas 315A
and 315B, in which circuit patterns identical with each other are
to be formed.
[0309] FIG. 30(b) shows a shot area SMq as a typical example of the
critical layer shot areas. In FIG. 30(b), the first partial shot
area in the shot area SMq is provided with a pair of wafer marks
321XA and 321YA for the X- and Y-axes, and also one set of four
vernier marks 331A, 331C, 331D and 331E which are distributed in a
cross shape. Similarly, the second partial shot area in the shot
area SMq is provided with a pair of wafer marks 321XB and 321YB and
four vernier marks 332A, 332C, 332D and 332E in symmetric relation
to the marks in the first partial shot area. In this case, the
original drawing patterns of marks distributed as shown in FIG.
30(b) have been formed in the pattern area 62A of the reticle RA in
the fine stepper 1A, shown in FIG. 29.
[0310] It should be noted that the wafer marks 321XA to 321YA, etc.
used in this example are one-dimensional line-and-space patterns
which are detected by an imaging detection method with the
alignment system 11B, shown in FIG. 29. The vernier marks 331A to
331E, etc. are two-dimensional box-in-box marks which are detected
by an imaging detection method with the alignment system 11B.
However, it is possible to use other kinds of mark as vernier
marks, for example, marks each formed by a combination of two
one-dimensional line-and-space patterns which are crossed at right
angles. It is also possible to use the wafer marks 321XA, 321YA,
etc. themselves as vernier marks. Conversely, vernier marks may be
used as wafer marks. In this embodiment, a part of the vernier
marks 331A to 331E and 332A to 332E are used as multipoint wafer
marks (alignment marks) in the shot area SMq as one example.
Further, marks which are detected by the laser step alignment (LEA)
method, for example, may also be used as vernier marks. The
distribution of wafer and vernier marks is not necessarily limited
to that shown in FIG. 30(b).
[0311] Next, the controller 10B of the scanning exposure apparatus
1B, shown in FIG. 29, effects alignment by the EGA method.
Accordingly, the controller 10B drives the wafer stage 5B to move
the field of view of the alignment system 11B sequentially
according to the critical layer shot map, thereby measuring array
coordinates in a stage coordinate system (i.e. a coordinate system
determined by values measured with the laser interferometers 7B and
9B of the scanning exposure apparatus 1B) of each of the wafer
marks 321XA and 321YA attached to nine (for example) shot areas
(sample shots) S310, S320, . . . , S390 selected from among the
shot areas on the wafer 20, as shown in FIG. 30(a). Then, values of
six EGA parameters (scaling parameters Rx and Ry, wafer rotation
.THETA., shot perpendicularity W, and offsets ox and Oy) on the
wafer 20 are determined so as to minimize the residual error
component, which is the sum of the squares of deviations of the
measured values of the wafer marks 321XA and 321YA of each sample
shot from array coordinate values calculated from the design array
coordinates of the wafer marks 321XA and 321YA.
[0312] Next, the controller 10B determines array coordinate values
of each shot area SMq (q=1 to Q) in the stage coordinate system
from the six EGA parameters and the design array coordinate values
of the critical layer shot area SMq. In this case, the exposure
field 64B of the scanning exposure apparatus 1B is equal in size
(1:1) to the exposure field 64A of the fine stepper 1A in the
direction X but 3/2 times as large as the exposure field 64A in the
direction Y. Therefore, the controller 10B divides perfect partial
shot areas (with no missing part) in the shot areas SMq (q=1 to Q),
shown in FIG. 30(a), into a plurality of blocks each comprising one
partial shot area in the direction X and three partial shot areas
in the direction Y, each block containing at least one shot area
SMq. Then, the controller 10B obtains array coordinates of the
center of each block in the stage coordinate system from the
computational array coordinates of the shot area SMq contained in
the block. Thus, an array (shot map) of middle layer shot areas is
determined.
[0313] For example, in the shot map for the middle layer to be
exposed by the scanning exposure apparatus 1B, as shown in FIG.
31(a), R (R=20 in FIG. 31(a)) shot areas SN1, SN2, . . . , SNR are
arranged in the directions X and Y over the critical layer on the
wafer 20. The width (pitch) in the direction X of each shot area
SNr (r=1 to R), including the scribe line area, is b, and the width
(pitch) in the direction Y is 3a. Accordingly, assuming that the
size of the middle layer shot area SNr in the direction X is
M.sub.1/N.sub.1 times as large as that of the critical layer shot
area SMq, and the size of the shot area SNr in the direction Y is
M.sub.2/N.sub.2 times as large as that of the shot area SMq, the
size ratios in this embodiment are M.sub.1/N.sub.1=1/1 and
M.sub.2/N.sub.2=3/2. Further, each shot area SNr is divided into
three partial shot areas 316A to 316C of the same size in the
direction Y (i.e. the scanning direction). The three partial shot
areas 316A to 316C are to be formed with identical patterns.
[0314] In this embodiment, it is assumed that six EGA parameters
(Rx, Ry, .THETA., W, Ox, and Oy) have no error, but four in-shot
parameters (shot magnifications rx and ry, shot rotation .theta.,
and shot perpendicularity w) have errors. Therefore, overlay
exposure is carried out in order to obtain errors (correction
values) of these in-shot parameters.
[0315] That is, the scanning exposure apparatus 1B sequentially
transfers an image of the vernier mark original drawing patterns
formed on the reticle RB onto each of the middle layer shot areas
SNr, shown in FIG. 31(a), by the scanning exposure method. Prior to
the exposure process, the projection magnification and scanning
speed of the projection optical system 3B have been adjusted
according to the calculated shot magnifications rx and ry, and the
reticle RB has been rotated according to the shot rotation .theta..
Further, the scanning direction has been adjusted according to the
shot perpendicularity w. Thus, the middle layer chip pattern has
previously been aligned with respect to the critical layer chip
pattern. After the exposure process, development is carried out,
thereby allowing middle layer vernier marks to appear over the
critical layer vernier marks on the wafer 20. It should be noted
that the following measurement may be carried out with the
transferred marks left in the form of latent images, as has already
been described above.
[0316] Next, the third step will be explained.
[0317] In the third step, measurement is carried out to determine
amounts of positional displacement between the corresponding
vernier marks in the critical layer shot areas SMq, shown in FIG.
30(a), and the middle layer shot areas SNr, shown in FIG. 31(a).
For this purpose, the wafer 20, shown in FIG. 31(a), which has been
subjected to the development in the second step, is placed, for
example, on the wafer stage 5B of the scanning exposure apparatus
1B, shown in FIG. 29, and amounts of positional displacement
between the corresponding vernier marks on the two layers are
measured by the alignment system 11B. However, the measurement of
positional displacement between the corresponding vernier marks may
be carried out by using another measuring device of high
accuracy.
[0318] In this case, it is assumed that, as shown in FIGS. 30(a)
and 31(a), the +Y direction end of the array of the critical layer
shot areas SMq is coincident with the +Y direction end of the array
of the middle layer shot areas SNr. The shot areas SN1 to SNR of
the middle layer M are each provided with 12 (=4.times.3) vernier
marks respectively corresponding to the vernier marks in the
critical layer shot areas SMq (each having 8 vernier marks).
[0319] FIG. 31(b) shows a middle layer shot area SNr. In FIG.
31(b), the first partial shot area in the shot area SNr has four
vernier marks 333A to 333E formed so as to surround the critical
layer vernier marks 331A to 331E (see FIG. 30(b)), respectively.
Similarly, the second and third partial shot areas in the shot area
SNr have four vernier marks 334A to 334E and four vernier marks
335A to 335E, respectively, formed so as to surround the
corresponding critical layer vernier marks. In this example, it is
assumed, for example, that the middle layer vernier mark 335E is
displaced by .DELTA.x and .DELTA.y in the directions X and Y
relative to the vernier mark 331E in a predetermined critical layer
shot area due to errors of the four in-shot parameters (rx, ry,
.theta., and w). Accordingly, errors (correction values) of the
in-shot parameters are obtained by detecting amounts of positional
displacement between the corresponding vernier marks on the two
layers at predetermined measuring points.
[0320] A method of setting measuring points in two shot areas SNr
and SN(r+1) which are contiguous with each other in the direction
Y, as shown for example by the hatching in FIG. 31(a), will be
explained below with reference to FIG. 32.
[0321] Referring to FIG. 32, areas in each of which one of the
middle layer shot areas SNr and SN(r+1) and one of the critical
layer shot areas SMq, SM(q+1) and SM(q+2) are perfectly overlaid on
one another such that neither of the overlaid shot areas extends
over a plurality of middle or critical layer shot areas, that is,
two hatched shot areas SMq and SM(q+2), are defined as reference
measurement areas, and four measuring points 336A to 336D are set
in the first reference measurement area SMq. Similarly, four
measuring points 336E to 336H are set in the second reference
measurement area SM(q+2) at respective positions corresponding to
the measuring points 336A to 336D.
[0322] At the measuring point 336A, amounts of positional
displacement in the directions X and Y between the critical layer
vernier mark 332A and the middle layer vernier mark 334A are
measured. Similarly, amounts of positional displacement between the
corresponding vernier marks of the two layers at each of the other
measuring points 336B to 336D and 336E to 336H. It should be noted
that other areas in FIG. 31(a) where any one of the critical layer
shot areas and any one of the middle layer shot areas are perfectly
overlaid on one another such that neither of the overlaid shot
areas extends over a plurality of middle or critical layer shot
areas may be used as reference measurement areas in addition to the
above.
[0323] Next, one example of a method obtaining errors of the four
in-shot parameters from the results of the measurement of amounts
of positional displacement between two corresponding vernier marks
at each of the measuring points will be explained. Here, amounts of
positional displacement between the two corresponding vernier marks
measured at two measuring points 336A and 336B, which are apart
from each other in the direction X in the first reference
measurement area SMq, are denoted by (.DELTA.xa,.DELTA.ya) and
(.DELTA.xb,.DELTA.yb), and amounts of positional displacement
between the two corresponding vernier marks measured at two
measuring points 336C and 336D, which are apart from each other in
the direction Y, are denoted by (.DELTA.xc,.DELTA.yc) and
(.DELTA.xd,.DELTA.yd). In this case, an error .DELTA.rx of the X
direction shot magnification rx is obtained from the difference
between the amounts of positional displacement .DELTA.xa and
.DELTA.xb, and an error .DELTA.ry of the Y direction shot
magnification ry is obtained from the difference between the
amounts of positional displacement .DELTA.yc and .DELTA.yd. An
error .DELTA..theta. of the shot rotation .theta. is obtained from
the difference between the amounts of positional displacement
.DELTA.ya and .DELTA.yb. Further, an error .DELTA.w of the shot
perpendicularity w is obtained from the difference between the
amounts of positional displacement .DELTA.xc and .DELTA.xd and the
shot rotation error .DELTA..theta..
[0324] Further, mean values of in-shot parameter errors .DELTA.rx,
.DELTA.ry, .DELTA..theta. and .DELTA.w, obtained in the other
reference measurement areas, are determined, and these mean values
are stored in the storage unit in the controller 10B of the
scanning exposure apparatus 1B as correction values .DELTA.rx',
.DELTA.ry', .DELTA..theta.' and .DELTA.w' for the in-shot
parameters. In this embodiment, none of the reference measurement
areas extend over two shot areas on either of the critical and
middle layers. Therefore, the in-shot parameter correction values
obtained as described above are accurate values which have got rid
of the effects of the stepping errors at the critical and middle
layers.
[0325] In this regard, let us consider a case where, in FIG. 32,
the central shot area SM(q+1), which extends over the two shot
areas SNr and SN(r+1), is used as a reference measurement area, and
measuring points 337B and 337A are set in the two shot areas SNr
and SN(r+1) within the reference measurement area. In this case,
amounts of positional displacement between the two corresponding
vernier marks measured at the two measuring points 337B and 337A
contain the middle layer stepping error independently of each
other. Therefore, even when an error of the shot magnification ry
in the direction Y, for example, is calculated from the sum of the
amounts of positional displacement measured at the two points 337B
and 337A, the calculated error contains the stepping error. In
other words, even if a reference measurement area extends over a
plurality of shot areas of either layer, if a plurality of
measuring points in the reference measurement area are set so that
the distribution of the measuring points does not extend over a
plurality of shot areas, the mixing of the stepping error can be
prevented. In a case where amounts of positional displacement
between the corresponding vernier marks are measured by another
measuring device, and correction values for the in-shot parameters
are determined by another computer, for example, the operator
inputs the correction values to the controller 10B through an input
unit or by on-line communication from that computer. Thus, the
third step is terminated.
[0326] In a case where exposure is carried out by the mix-and-match
method using the fine stepper 1A and the scanning exposure
apparatus 1B, shown in FIG. 29, after the above-described third
step, first, a critical layer pattern is formed on the wafer 20 by
using the fine stepper 1A. Thereafter, before exposure for a middle
layer pattern is carried out by using the scanning exposure
apparatus 1B, coordinate positions of a multiplicity of wafer marks
in predetermined sample shots are measured, and values of six wafer
EGA parameters and four in-shot parameters are determined from the
result of the measurement. Thereafter, the controller 10B adds the
correction values (.DELTA.rx', .DELTA.ry', .DELTA..theta.', and
.DELTA.w'), stored in the above-described third step, to the
determined in-shot parameters (rx, ry, .theta., and w), thereby
obtaining corrected in-shot parameters. Then, the controller 10B
calculates the coordinate position of each shot area of the
critical layer by using the six wafer EGA parameters, calculates
the exposure position for each shot area of the middle layer on the
basis of the calculated coordinate positions, and sequentially
effects positioning (e.g. setting of the scanning start position)
of the middle layer shot areas on the basis of the calculated
exposure positions. Then, the scanning exposure apparatus 1B
transfers an image of the reticle pattern onto each shot area by
the scanning exposure method while correcting the image-formation
characteristics according to the corrected in-shot parameter
values. In this embodiment, the corrected in-shot parameter values
are accurate; therefore, the overlay accuracy between the critical
and middle layers is higher than in the conventional exposure
process.
[0327] Next, other examples of the fourth embodiment of the present
invention will be explained with reference to FIGS. 33(a) to 34(c).
In the example shown in FIGS. 33(a) to 33(c), a critical layer shot
area SK shown in FIG. 33(a) is allotted one chip pattern, and a
middle layer shot area SL shown in FIG. 33(b) is allotted a total
of four identical chip patterns arranged in an array of two columns
in the direction X and two rows in the direction Y. The exposure
apparatus for the critical layer is a one-shot exposure type
projection exposure apparatus (stepper) having a demagnification
ratio of 5:1, and the exposure apparatus for the middle layer is a
stepper having a demagnification ratio of 2.5:1.
[0328] Assuming that the width in the direction X of the critical
layer shot area SK is d, and the width in the direction Y of the
shot area SK is c, the width in the direction X of the middle layer
shot area SL is 2d, and the width in the direction Y of the shot
area SL is 2c. Therefore, the shot area SL is twice as large as the
shot area SK in each of the directions X and Y. Accordingly, as
shown in FIG. 33(c), in an area 339 where an array of four critical
layer shot areas and one middle layer shot area are overlaid on one
another, an area where a shot area SK and a shot area SL are
overlaid on one another without extending over a plurality of
critical or middle layer shot areas, that is, each critical layer
shot area SK itself, is used as a reference measurement area.
Therefore, two measuring points 340A and 340B are set in one
reference measurement area 339a, for example, and an amount of
positional displacement between the corresponding vernier marks of
the two layers is measured at each of the measuring points 340A and
340B. By doing so, correction values for in-shot parameters, e.g.
the shot magnification rx and the shot rotation .theta., can be
accurately obtained.
[0329] In the example shown in FIGS. 34(a) to 34(c), a first-layer
shot area SO shown in FIG. 34(a) is allotted a total of six
identical chip patterns arranged in an array of two columns in the
direction X and three rows in the direction Y, and a second-layer
shot area SP shown in FIG. 34(b) is allotted three identical chip
patterns arranged in the direction Y. The exposure apparatus for
the first layer comprising the shot areas SO is a stepper, and the
exposure apparatus for the second layer comprising the shot areas
SP is a step-and-scan type projection exposure apparatus.
[0330] Assuming that the width in the direction X of the shot area
SO is 2b, and the width in the direction Y of the shot area SO is
3a, the width in the direction X of the shot area SP is b, and the
width in the direction Y of the shot area SP is 3a. That is, the
shot area SP is 1/2times as large as the shot area SO in the
direction X, and the former is equal in size (1:1) to the latter in
the direction Y. According, as shown in FIG. 34(c), in an area 341
where one first-layer shot area and two second-layer shot areas are
overlaid on one another, an area where a shot area SO and a shot
area SP are overlaid on one another without extending over a
plurality of first- or second-layer shot areas, that is, each
second-layer shot area SP itself, is used as a reference
measurement area. Therefore, two measuring points 342A and 342B are
set in one reference measurement area 341a, for example, and an
amount of positional displacement between the corresponding vernier
marks of the two layers at each of the measuring points 342A and
342B. By doing so, a correction value for an in-shot parameter,
e.g. the shot magnification ry, can be accurately obtained.
[0331] Although in the above-described embodiment a combination of
two steppers or a combination of a stepper and a step-and-scan type
projection exposure apparatus is used, it should be noted that the
combination of exposure apparatuses is not necessarily limited to
those described above. For example, step-and-scan type projection
exposure apparatuses which are different from each other may be
used as two exposure apparatuses having respective exposure fields
of different sizes.
[0332] According to the exposure method of the fourth embodiment,
none of the set reference measurement areas extend over a plurality
of shot areas on either of two layers (e.g. critical and middle
layers). Therefore, no stepping error is contained in an amount of
positional displacement between two corresponding overlay accuracy
measuring marks measured at any of the measuring points in the
reference measurement areas. Accordingly, the overlay accuracy
between the two layers can be improved by correcting the
coordinates during alignment or the image-formation characteristics
on the basis of the measured amounts of positional displacement
between the corresponding overlay accuracy measuring marks.
[0333] Further, according to the fourth embodiment, an area where
any one of a plurality of first shot areas and any one of a
plurality of second shot areas are overlaid on one another such
that neither of the overlaid shot areas extends over beyond a part
of that area (or neither of them extends over a plurality of first
or second shot areas) is used as a reference measurement area, and
an amount of positional displacement between the corresponding
overlay accuracy measuring marks (vernier marks) of the two layers
is measured at each of measuring points set in predetermined
reference measurement areas. Accordingly, correction values used in
detection of the image positions of alignment marks (wafer marks)
can be accurately obtained without being affected by stepping
errors in the first and second shot areas. As a result, it is
possible to increase the overlay accuracy between a critical layer
pattern and a middle layer pattern in a case where exposure is
carried out by the mix-and-match method with respect to a substrate
where a critical layer and a middle layer are mixedly present. It
is also possible to eliminate the effects of so-called seam errors
between the first-layer shot areas and between the second-layer
shot areas in addition to the stepping error.
[0334] Further, it is possible to obtain a correction value for an
in-shot parameter with high accuracy in a case where a correction
value obtained in the third step is a correction value for a
parameter indicating a predetermined image-formation
characteristic, which is calculated on the basis of the positions
of alignment mark images, and the parameter indicating the
predetermined image-formation characteristic is at least one
parameter selected from the parameter group consisting of shot
magnification, shot rotation, and shot perpendicularity.
Accordingly, the image-formation characteristics can be corrected
with high accuracy by using the corrected in-shot parameter.
[0335] In a case where the first exposure apparatus is a one-shot
exposure type projection exposure apparatus, and the second
exposure apparatus is a scanning exposure type projection exposure
apparatus, the exposure method according to the present invention
is particularly effective because in such a case the exposure
fields of the two exposure apparatuses are likely to differ in size
from each other. In the case of a scanning exposure type projection
exposure apparatus, the shot magnification, shot rotation and shot
perpendicularity can be readily corrected at the time of exposure;
therefore the overlay accuracy between the two layers can be
further increased by using parameters corrected by the method
according to the present invention.
[0336] It should be noted that the present invention is not
necessarily limited to the above-described first to fourth
embodiments, but may adopt various arrangements without departing
from the gist of the present invention.
* * * * *