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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus for aligning a mask and a substrate
(particularly a semiconductor wafer) relative to each other, and in
particular to an alignment apparatus suitable for an apparatus for
exposing the pattern of a mask to a photosensitive substrate.
2. Related Background Art
In an apparatus for exposing the pattern of a mask to a photosensitive
substrate, it is requisite to highly accurately accomplish the work of
optically detecting a pattern (or a mark) preformed on the photosensitive
substrate and a pattern (or a mark) on the mask and positioning the mask
or the photosensitive substrate so that the two patterns (or marks) are
correctly superposed one upon the other, that is, the so-called alignment.
In recent years, in the field of the exposure apparatus used to make a
semiconductor element on a wafer, the stepper which is made into a system
for projecting the pattern of a mask onto a small area on a wafer through
a projection lens and in which the wafer is caused to effect stepping for
the exposure of the whole surface of the wafer has become the main
current. This stepper is such that a circuit pattern formed on a reticle
as a mask is projected onto a wafer by a projection lens of high resolving
power and high N.A. The alignment in the stepper is accomplished by
reversely projecting an alignment mark on the wafer onto the reticle side
through the projection lens, observing the alignment mark on the reticle
and the spatial image of the alignment mark of the wafer (which is formed
on the reticle surface) at a time, and detecting the positional deviation
between the two marks. In this case, it is usual to design the
illuminating light for observing (detecting) the marks so as to enter the
projection lens and the wafer through the reticle. The alignment optical
system (including the illuminating system) in such a conventional
alignment apparatus is described in detail, for example, in U.S. Pat. No.
4,402,596.
The construction of this conventional apparatus is such as shown, for
example, in FIG. 9 of the accompanying drawings wherein a projection lens
which is telecentric on the wafer W side (the image side) and
non-telecentric on the reticle R side (the object side) is used as a
projection lens PL. A pattern PA to be superposedly printed on the wafer W
and an alignment mark RM are formed on the reticle R. The wafer W is
placed on a two-dimensionally movable stage ST, and an alignment mark WM
matching the mark RM is formed on the wafer W. In FIG. 9, the light ray L
passing through the center of the pupil ep of the projection lens PL
represents the principal light ray of the illuminating system for
exposure. The alignment system is constituted by a light source 1 as
illuminating means, a beam splitter 2, a second objective lens 3, a first
objective lens 4 and a total reflection mirror 5 (and further, an
illuminating field stop, not shown). The image of the RM of the reticle R
is formed on an imaging surface 6 through the objective lenses 4 and 3,
and the image of the mark WM of the wafer W is once formed in the same
surface as the mark RM of the reticle R through the projection lens PL,
whereafter it is again formed on the imaging surface 6 by the objective
lenses 3 and 4. The light-receiving surface of an image pickup device such
as a television camera is positioned on the imaging surface 6, and the
images of both marks RM and WM are photoelectrically detected at a time.
In FIG. 9, the line l.sub.1 passing through the mark WM of the wafer W,
the center of the pupil ep and the mark RM of the reticle R represents the
principal light ray of this alignment optical system, and the objective
lenses 3 and 4 of this alignment optical system are used eccentrically.
The image of the light source 1 may be formed on the pupil ep of the
projectinn lens PL.
When, as shown in FIG. 9, the illuminating light for alignment is caused to
enter from the opposite side of the projection lens PL with respect to the
reticle R and illuminate the mark RM, the illuminating light passed
through the transparent portion around the mark RM travels along the
principal light ray l.sub.1 and illuminates a localized area including the
mark WM of the wafer W. Simultaneously therewith, the image of the mark RM
is formed on the wafer W. Usually, photoresist is applied to the surface
of the wafer W and this surface has reflectivity for the illuminating
light for alignment. Therefore, assuming that the localized area including
the mark WM is perpendicular to the optic axis AX of the projection lens
PL, the principal light ray l.sub.1 is parallel to the optic axis AX on
the wafer W side and thus, the image of the mark RM formed on the surface
of the wafer W is reflected by the wafer W and is reversely projected by
the projection lens PL so as to again overlap the mark RM. Of course, the
image of the mark WM on the wafer is also formed on the transparent
portion aoound the mark RM by the projection lens PL.
Now, the object side (the reticle side) of the projection lens PL is
non-telecentric, and by the objective lenses 3 and 4 being used
eccentrically, the regularly reflected light on the mark RM travels in the
direction of arrow CA and does not return toward the mirror 5 when the
llluminating light from the light source 1 is bent by the mirror 5 and
illuminates the mark RM of the reticle R. Such a construction can be
realized by obliquely disposing the mirror 5 at an angle of 45.degree.
with respect to the surface of the reticle (when the objective lenses 3
and 4 are disposed horizontally) with the fore end portion of the mirror 5
being made substantially coincident with the optic axes of the objective
lenses 3 and 4 so that the illuminating light (or the alignment light from
the mark may pass through the half area of the objective lenses 3 and 4.
Thus, the mark RM of the reticle R is illuminated by the reflected light
of the illuminating light for alignment reflected by the wafer W and, if
the pattern of the mark RM is of a light-intercepting property, the mark
RM is imaged as a dark portion on the image pickup device.
However, paying attention to the image of the mark RM formed on the imaging
surface 6, this image is in some cases formed by two images of different
properties (but of the same shape) being superposed one upon the other.
Assuming here that the off-axis aberration of the projection lens PL is
ideally zero and that the localized area including the mark WM of the
wafer W is an ideal reflecting plane perfectly perpendicular to the
principal light ray l.sub.1, the image of the mark RM formed on the wafer
W is reflected by the wafer W and is re-imaged at a position whereat it
accurately overlap the mark RM. Therefore, an image equal to the image
formed when the mark RM of the reticle R is simply illuminated is sharply
formed on the imaging surface 6 with good contrast, and thus, during the
alignmnnt thereof with the mark WM on the wafer, the pattern edge of the
mark RM can be detected precisely. Actually, however, the ideal conditions
as supposed above do not exist, and if the aberration of the projection
lens PL and the inclination of the surface of the wafer W with respect to
the optic axis of the projection lens PL deviate greatly from the ideal
conditions, the reversely projected image of the image of the mark WM of
the wafer W onto the reticle R will become bad in contrast and will not
accurately overlap the mark RM. Therefore, the pattern edge of the mark
will become unclear, and this has led to the problem that the mark
detection accuracy during alignment is reduced. The dual image as
described above has readily appeared particularly when the reticle R and
the wafer W are not accurately conjugate (in-focus) with respect to the
projection lens PL or when the telecentricity of the alignment system
including the projection lens goes slightly wrong. There has also been the
problem that the images of the mark RM and the mark WM overlap each other
and the two images cannot be discriminated from each other with a result
that much time is required for alignment.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a projection exposure
apparatus which is capable of accomplishing alignment of a substrate and a
reticle without being affected by the aberration of a projection lens, the
non-uniformity of the surface of the substrate, etc.
In accordance with the present invention, there is provided illuminating
means for illuminating a first mark formed on a substrate which is a first
plate and a second mark formed on a mask which is a second plate. There is
also provided an objective optical system for forming the image of the
first mark or the second mark on a predetermined imaging surface, and is
further provided an index mark member having a reflective
light-intercepting portion on the predetermined imaging surface or at a
position conjugate therewith. This reflective light-intercepting portion
transmits therethrough the image light from the first mark and reflects
the image light from the second mark.
The image of the first mark and the index mark member are optically
detected at a time, and further the image of the second mark and the index
mark member are optically detected at a time, whereby the positional
deviation between the mask and the substrate may be detected.
In the present invention, by the above-described construction, a reticle is
aligned with an index mark pattern, whereafter a wafer is aligned with the
index mark pattern, whereby alignment of the reticle and the wafer is
accomplished, and when the mark of the reticle is to be observed (or
detected), the transmitted illumination by the reflected light from the
wafer (or a reflective object replacing it) is not used, thereby enhancing
the mark detection accuracy.
BRIEF DESCRIPTION 0F THE DRAWINGS
FIG. 1 shows the general construction of an alignment apparatus according
to an embodiment of the present invention.
FIG. 2A is a plan view showing the shape of an index mark pattern.
FIG. 2B is a plan view showing the shape of the mark of a reticle.
FIG. 3 shows the manner of alignment of the index mark pattern and the
reticle mark and the waveform of a photoelectric signal.
FIG. 4 shows the manner of alignment of the index mark pattern and the mark
of a wafer.
FIG. 5 is a plan view showing the shape of the index mark according to
another embodiment.
FIG. 6 is a flow chart showing the operation of the apparatus according to
the embodiment of the present invention.
FIG. 7A is a plan view showing another shape of the reticle mark.
FIG. 7B is a plan view showing the shape of an index mark pattern
corresponding to the mark shape of FIG. 7A.
FIG. 8 illustrates the multiplex reflection on the reticle.
FIG. 9 illustrates the illuminating method during the alignment of the
index mark pattern and hhe wafer mark.
FIG. 10 shows the construction of an apparatus according to the prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows the construction of an alignment apparatus according to an
embodiment of the present invention as it is applied to a projection type
exposure apparatus. In FIG. 1, portions similar in function and effect to
the constiluent portions shown in FIG. 9 are given similar reference
numerals and symbols. In the present embodiment, a reference slit plate 10
provided with a slit is provided on a stage ST. The surface of the slit
plate 10 is defined so as to be substantially flush with the surface of a
wafer W. This slit plate 10 is illuminated from the back thereof by a
radiation source 12 through an optical fiber 11. In the present
embodiment, the wavelength of the illuminating light of the radiation
source 12 is the same as the wavelength of the light of the radiation
source 1, and is further the same as the wavelength of the illuminating
light for exposure. The stage ST is two-dimensionally moved by a motor 13
with the wafer W and the slit 10 being held thereon. The coordinates
position of the stage ST is detected by an interferometer 14 which
measures by a laser light. The interferometer 14 outputs a pulse signal
each time the stage ST is moved by a unit amount. Now, the alignment
system of the present embodiment is designed so as to form the image of
the exit end (secondary radiation source) of an optical fiber 20 on the
pupil ep of a projection lens PL by a lens system 21 and an objective lens
4. In the present embodiment, a beam splitter 2 is constituted by a
half-mirror, and lens systems 22 and 23 are disposed with the beam
splitter 2 interposed therebetween. The lens system 22 receives the image
light rays from marks RM and WM reflected by the beam splitter 2 through
the objective lens 4 and images those light rays on an index mark plate
24. That is, the index mark plate 24 is disposed so as to be conjugate
with a reticle R with respect to the composite system of the objective
lens 4 and the lens system 22. The index mark plate 24 is formed with an
index mark pattern 24a which is provided by a reflective
light-intercepting portion. An imaging lens 25 forms the image of the
index mark pattern 24a on the light-receiving surface of an image pickup
tube 26 such as ITV. The lens system 23 is disposed on the side opposite
to the lens system 22 with respect to the beam splitter 2, while the
system including the lens system 22, the beam splitter 2 and the lens
system 23 is defined so that a light-receiving element 27 and the index
mark pattern 24a are in a conjugate relation with each other. Now, the
image signal from the image pickup tube 26 is input to an operation
circuit 28, which electrially detects the positional relation between the
index mark pattern 24a and the mark WM of the wafer W and outputs to a
main control device 40 the information corresponding to the positional
deviation therebetween. Also, the photoelectric signal from the
light-receiving element 27 is input to an operation circuit 29, which
electrically detects the positional relation between the mark RM and the
index mark pattern 24a by moving the slit-like light-emitting portion of
the slit plate 10, detects the positional deviation information thereof on
the basis of the pulse signal from the interferometer 14 and outputs it to
the main control device 40. The main control device 40 controls the
driving of the motor 13 on the basis of the input of the coordinates
information from the interferometer 14 and moves the stage ST to a desired
position. The images of the index mark pattern 24a and the mark WM picked
up by the image pickup tube 26 are displayed by a Braun tube (CRT) 30 and
used for visual confirmation. Also, the radiation sources 1 and 12 may be
used with the radiation source for exposure and may be drawn around by the
fibers 11 and 20, respectively. Further, the light emission of the
radiation source 1 and of the radiation source 12 is alternatively
effected by the main control device 40.
Now, the shapes of the index mark pattern 24a of the index mark plate 24
and the mark RM of the reticle R are defined as shown, for example, in
FIGS. 2A and 2B. FIG. 2A is a plan view of the index mark pattern 24a as
seen from the image pickup tube 26 side, and the back side (the lens
system 22 side) of the hatching portion is a reflective light-intercepting
portion (a reflective chromium layer). In FIG. 2A, when the axis extending
toward the center of the reticle R (corresponding to the optic axis AX) is
the y-axis, the center line of the index mark pattern 24a lies on the
y-axis and a transparent portion 24b of a shape symmetrical with respect
to the center line is formed in the pattern 24a. The transparent portion
24b is for observing or detecting the bar-like mark WM on the wafer W
therethrough, and has edge portions stepwisely formed with slight amounts
of level differences so as to readily permit any positional deviation to
be visually confirmed when the mark WM is sandwiched therebetween. In
contrast, the mark RM of the reticle R comprises two parallel bar patterns
with the y-axis interposed therebetween, as shown in FIG. 2B, and the
spacing d therebetween is greater than the width of the transparent
portion 24b of the index mark pattern 24a in the x direction (the
direction orthogonal to the y-axis). Accordingly, in a state in which the
reticle R has been acculately aligned relative to the aparatus, the mark
RM is shielded from light by the index mark pattern 24a so that it cannot
be observed at all from the image pickup tube 26 side.
Description will now be made of the alignment operation of the apparatus of
the present embodiment, particularly, the reticle-wafer alignment
operation of the TTR (through-the-reticle) type. It is to be understood
here that the reticle R has been positioned relative to the apparatus by
the use, for example, of other microscope or the like exclusively for use
for reticle alignment. The radiation source 12 is first turned on, and
then the stage ST is moved so that the light-emitting slit of the slit
plate 10 comes close to the projected position of the mark RM of the
reticle R. Then, as shown in FIG. 3(a), the light-emitting slit 10a is
scanned in the x direction. FIG. 3(a) is a plan view of the index mark
plate 24 as seen from the lens system 22 side. In FIG. 3(a), the white
portion represents a portion in which the light reflected by the index
mark pattern 24a reaches the beam splitter 2, the lens system 23 and the
light-receiving element 27. When the light-emitting slit 10a is positioned
outside the index mark pattern 24a as shown, the image of the
light-emitting slit 10a is formed on the transparent portion around the
mark RM of the reticle R and also is transmitted therethrough and formed
on the transparent portion of the index mark plate 24. Therefore, the
image light from the light-emitting slit 10a does not reach the
light-receiving element 27. When the scanning of the light-emitting slit
10a progresses and the image of the light-emitting slit 10a is formed on
the transparent portion around the mark RM of the reticle R and also is
formed on the light-intercepting portion of the index mark pattern 24a,
the reflected image of the light-emitting slit 10a by the index mark
pattern 24a is formed on the light-receiving surface of the
light-receiving element 27 through the lens system 22, the beam splitter 2
and the lens system 23. Further, when the image of the light-emitting slit
10a overlaps the bar pattern of the mark RM, the quantity of the image
light transmitted through the reticle R and reaching the index mark
pattern 24a becomes smaller and therefore, the quantity of reflected light
is also reduced and the level of the photoelectric signal of the
light-receiving element 27 also becomes smaller.
So, in relation to the position in which the light-emitting slit 10a scans,
the operation circuit 29 receives as an input the photoelectric signal I
from the light-receiving element 27 as shown in FIG. 3(b). In FIG. 3(b),
the ordinate represents the level of the photoelectric signal I and the
abscissa represents the position of the light-emitting slit 10a in the x
direction. At a position Xa, the image of the light-emitting slit 10a
crosses the outer edge 24c of the index mark pattern 24a and therefore the
level of the photoelectric signal I rises, whereafter at positions Xc and
Xd, said level assumes the bottom for the mark RM and at a position Xb,
said image crosses the outer edge 24d of the index mark pattern 24a and
therefore said level falls. The operation circuit 29 digitally samples the
photoelectric signal I in response to the pulse signal from the
interferometer 14, detects the positions Xa, Xb, Xc and Xd on the basis of
the waveform thereof and calculates the amount of offset .DELTA.R between
the mark RM and the index mark pattern 24a on the basis of the following
equation (1):
.DELTA.R=(Xa+Xb)/2-(Xc+Xd)/2 (1)
Subsequently, the light emission of the radiation source 12 is stopped and
the shot area on the wafer W to be exposed is positioned just beneath the
projection lens PL. At this time, the mark RM of the reticle R and the
mark WM of the wafer W are positioned with the accuracy within
approximately 1 .mu.m, but the radiation source 1 is turned on to effect
more precise alignment. Thereby, the localized area including the mark WM
of the wafer W is illuminated through the mark RM, and the inversely
projected image of the mark WM is formed between the two bar patterns of
the mark RM and further formed on the transparent portion 24b of the index
mark pattern 24a. Accordingly, the index mark pattern 24a picked up by the
image pickup tube 26 and the mark WM of the wafer assume such a relation
as shown in FIG. 4, and the mark RM is shielded from light by the index
mark pattern 24a and cannot be seen at all.
Since the image as shown in FIG. 4 is displayed on the CRT 30, the operator
finely moves the stage ST so that the mark WM comes to lie just at the
center of the transparent portion 24b while watching the displayed image.
At a point of time whereat the alignment of the mark WM and the index mark
pattern 24a has been completed, the main control device 40 temporarily
stores therein the then coordinates position of the stage ST. If the stage
ST is fed to this stored position by a distance corresponding to the
previously detected amount of offset .DELTA.R, the mark WM of the wafer W
and the mark RM of the reticle R are indirectly aligned.
As described above, in the present embodiment, manual alignment based on
visual observation is adopted with regard to the wafer W, but this portion
can also be automatized. For this purpose, the shape of the index mark
pattern 24a may be changed a little and the image signal from the image
pickup tube 26 may be electrically processed by the operation circuit 28,
whereby the positional deviation between the index mark pattern and the
mark WM may be detected. FIG. 5 is a plan view showing an example of the
then shape of the index mark pattern 24a. A rectangular transparent
portion 24e is provided below a transparent portion 24b for visual
observation. A straight edge portion orthogonal to the scanning line SL of
this transparent portion 24e and the edge portion of the mark WM are
utilized for alignment. The scanning line SL corresponds to the locus of
the photoelectric scanning of the image pickup tube 26, and the operation
circuit 28 receives as an input an image signal corresponding to the
scanning line SL and automatically measures the amount of deviation
.DELTA.W between the transparent portion 24e and the mark WM in the x
direction. When the position of the stage ST when this amount of deviation
.DELTA.W has been detected is the current position, if the stage ST is fed
in from the current position by an amount corresponding to the difference
between the previously found amount of offset .DELTA.R and the amount of
deviation .DELTA.W, the mark RM of the reticle R and the mark WM of the
wafer W are indirectly aligned. When the position of the mark WM is to be
thus automatically detected, the work of once driving the mark WM into the
center of the transparent portion 24b as during the visual observation
becomes unnecessary. The above-described operation is controlled by the
main control device 40 having a microcomputer contained therein, and such
operation is shown in the flow chart of FIG. 6.
At step 100 in FIG. 6, the reticle R is positioned at a predetermined
position with respect to the apparatus of the present embodiment by
conventional means. At step 101, the main control device 40 causes
electric power to be supplied to the radiation source 12, and at step 102,
the main control device 40 samples the output of the detector 27 on the
basis of the signal of the interferometer 14 while moving the stage ST and
stores it in the internal memory in the device 40. At seep 103, the
aforementioned .DELTA.R is calculated on the basis of the sampled output
of the detector in the internal memory. At step 104, the stage ST is moved
and placed at a predetermined position at which the mark WM can be
observed through the transparent portion 24a as shown in FIG. 4. At step
105, electric power is supplied to the radiation source 1 instead of to
the radiation source 12. At step 106, the ITV 26 is operated and at step
107, the aforementioned .DELTA.W is calculated. At step 108, the stage ST
is moved by (.DELTA.W-.DELTA.R). By the above-described operations, the
optical alignment of the mark RM of the reticle R and the mark WM of the
wafer W is completed.
In the present embodiment, the light-receiving element for detecting the
alignment of the index mark pattern 24a and the mark RM of the reticle R
is disposed conjugately with the index mark pattern 24a and the mark RM,
but alternatively, it may be disposed conjugately with respect to the
pupil of the lens system 23 (or the projection lens PL). Further, an image
pickup tube may be disposed instead of the light-receiving element being
disposed conjugately with the index mark pattern 24a. In such case, two
image pickup tubes for alignment will become necessary, but if the image
pickup tube 26 in FIG. 1 is changed over or used in a composite view
field, only one such tube will be required. To this end, the image light
from the lens system 23 can be directed into the optical path between the
index mark plate 24 and the image pickup tube 26 by the use of a
reflecting mirror or a relay lens so that it may be observed by means of
the image pickup tube 26. In such case, the alignment of the index mark
pattern 24a and the mark RM of the reticle will be accomplished by the
photoelectric scanning of the image pickup tube itself. Therefore, it will
not be necessary to scan by the light-emitting slit 10a, but a surface
light-emitting portion of a size and shape capable of illuminating the
whole of the mark RM and index mark pattern 24a can be provided on the
stage ST.
Reference is now had to FIGS. 7A and 7B to describe the shapes of the index
mark pattern and reticle mark according to another embodiment of the
present invention. FIG. 7A is a plan view of the mark RM on the reticle R,
and this mark RM is constituted by a long bar-like reticle alignment mark
RM-R for aligning the reticle R relative to the apparatus, and a mark RM-S
to be used in the same manner as the mark shown in FIG. 2B. FIG. 7B is a
plan view showing the index mark pattern 24a corresponding to these marks
RM. The index mark pattern 24a is formed by portion 240A for shielding the
mark RM-R, a portion 240B for shielding the mark RM-S, and a nipping mark
portion 240C used during the alignment with the mark WM of the wafer W.
The surroundings of these portions 240A, 240B and 240C are all
transparent. Where such a reticle is used, in an apparatus wherein a field
stop is provided in an illuminating optical system so that the
illuminating light from the radiation source 1 may illuminate only the
localized portion including the mark RM-S, the mark WM on the wafer W and
the mark RM-S are observed at a time. If at this time, the projection lens
PL is non-telecentric on the object side, there has arisen the problem as
shown in FIG. 8. Usually, the mark RM formed on the reticle R is deposited
by evaporation on the side opposite to the incidence side of the
illuminating light IL. If the object side of the projection lens PL is
non-telecentric, the principal light ray will become oblique and thus,
part of the illuminating light regularly reflected by the mark RM is
re-reflected by the upper surface of the interior of the reticle R and it
passes through the marginal transparent portion CP of the mark RM near the
optic axis AX and reaches the wafer W. Therefore, it has heretofore
happened that unexpected stray light is applied to the vicinity of the
mark WM on the wafer to be aligned with the mark RM. So, as in the present
embodiment, the mark portion 240c of the index mark pattern 24a is defined
so as to lie in the transparent portion on the reticle R and, when the
mark WM on the wafer W is to be observed, the range of irradiation can be
defined so as to be limited to the area AP including only the mark portion
240c as shown in FIG. 9 by the field stop in the illuminating optical
system. Thus, it does not happen that as in the previous first embodiment,
the illuminating light for observing the mark WM of the wafer illuminates
the mark RM of the reticle R and therefore, unexpected stray light is
prevented from being applied onto the wafer. Again in the present
embodiment, the relation between the reticle R and the index mark pattern
24a is detected by scanning the light-emitting slit 10a, but as is
apparent from the shape of the pattern of FIGS. 7A and 7B, such detection
is accomplished by detecting the positional relation in the x direction
between the mark RM-R and the reflective light-intercepting portion 240A
as the amount of offset.
Of the marks RM of the reticle R shown in FIG. 7A, the mark RM-S may be
deleted to provide an entirely transparent portion. In that case, the mark
portion 240c of the index mark pattern 24a can be provided centrally of
the shielding portion 240B, and the mark RM-R and the wafer mark WM when
the alignment of the reticle and the wafer has been accomplished are
substantially at the same position and the error during the detection of
each mark becomes minimum.
Also, as the exposure apparatus, in an apparatus wherein only the mark on
the wafer is detected through the projection lens PL or an apparatus
provided with an off-axis type wafer alignment sensor for detecting the
mark of the wafer independently of the projection lens PL, it is requisite
to measure the relative distance between the mark detection center
position by the wafer alignment sensor and the pattern projection center
point of the reticle R, the so-called base line. In the measurement of
this base line, a reference mark of the same shape as the mark WM of the
wafer may be provided on the surface of the slit plate 10, and the
relative distance between the position of the stage ST when the reference
mark and the index mark pattern 24a have been aligned and the position of
the stage ST when the reference mark has been detected by the off-axis
type wafer alignment sensor may be measured. When the off-axis type wafer
alignment sensor is designed so as to be capable of photoelectrically
detect the light-emitting slit 10a, the reference mark need not be
provided in particular. In such case, however, it is necessary to observe
the index mark pattern 24a and the light-emitting slit 10a at a time.
Now, the radiation sources 1 and 12 shown in FIG. 1 may be ones which
produce lights of different wavelengths from the exposure light.
Particularly, if the illuminating light from the radiation source 1 is of
a wavelength which will not sensitize the photoresist of the wafer and the
illuminating light from the radiation source 12 is of the same wavelength
as the exposure light, it will be advantageous in that the mark WM on the
wafer is not sensitized and is therefore protected. In this case, the
objective lens 4, the lens system 22, etc. in the alignment optical system
may preferably be achromatized. Also, a transmitting type linear Fresnel
pattern may be provided instead of the slit 10a so that a convergent light
beam of linear cross-section (a sheet-like spot light) may be formed in a
space distant from the surface of the linear Fresnel pattern by the
inherent focal length thereof and this spot light may be imaged on the
pattern surface of the reticle R or on the index mark pattern, whereby a
similar effect may be obtained. In such case, if the inherent focal length
of the linear Fresnel pattern is made to correspond to the amount of
chromatic aberration of the projection lens, it will be advantageous when
use is made of an alignment light of different wavelength from the
exposure light.
As described above, according to the present invention, in the TTR
(through-the-reticle) type alignment system, the influence of the
reflected image of the mark of the mask (reticle) caused when the image
side of the projection optical system is telecentric is eliminated and the
visibility of the mask mark, especially the contrast of the edge, is
enhanced with a result that improved alignment accuracy can be expected.
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