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BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a diversion detection which not only
detects a registration diversion between a reticle (a magnified mask) or a
full-size mask (hereinafter referred to as a reticle) and a wafer prior to
a main exposure but also detects a lithographic magnification error.
2. Description of the Prior Art
Recently, reduction projection exposure apparatus have been used as
apparatus for the lithography of microscopic circuit patterns in the
manufacture of semiconductor devices such as ICs and LSIs. This reduction
projection exposure apparatus is designed so that the circuit pattern
formed on a reticle is exposed and printed in a reduced form in the
photoresist (photosensitive material) formed on the wafer through a
projection lens. While the projection lens has a resolution on the order
of 1 micron, even if a projection lens of such high resolution is used,
there are cases where a diversion is caused and satisfactory accuracy is
not obtained in the registration exposure of the circuit pattern (chip)
already formed on the wafer and a projected image of the circuit pattern
on the reticle. It is considered that the causes of this diversion include
relative run-outs of the circuit pattern (chip) on the wafer and the
projected image of the circuit pattern on the reticle due to a run-out of
the wafer caused by the processing of the wafer (a process such as etching
or diffusion), variations in the reduction magnification of the projection
lens itself, a registration diversion between the wafer and the reticle
and the like. In the past, the detection of a registration diversion
between a wafer and a reticle has been effected by measuring with given
measuring means the circuit pattern formed on the wafer through its
exposure and development. However, there has been a disadvantage that even
if a diversion occurs during the registration exposure, not only it is
impossible to quantitatively detect the diversion but also even detection
of the occurrence of the diversion is not possible.
Also, there has been a problem that the detection of a registration
diversion between a wafer and a reticle is effected by forming reference
patterns (e.g., vernier marks) of a test reticle on the wafer and
measuring them with given measuring means, thus requiring much time and
labor.
SUMMARY OF THE INVENTION
The present invention has been made in view of these circumstances and it
is an object of the invention to provide a method and apparatus for
detecting any diversion that may be caused during a registration exposure,
an overall lithographic magnification, etc., without developing the
photo-sensitive substrate.
Thus, the present invention comprises aligning a substrate to be exposed
having first patterns formed at given positions and a photosensitive layer
formed on the surface and a mask having second patterns in such a manner
that given positional relations are established between the first and
second patterns, projecting an exposure energy beam onto the substrate to
be exposed through the mask to form latent images of the second patterns
on the photosensitive layer, photoelectrically detecting the latent images
by utilizing the fact that the exposed portions subjected to the
projection of the energy beam for the latent images and the unexposed
portion not subjected to the projection differ in optical characteristics
from each other, photoelectrically detecting the first patterns and
detecting positional relations between the first patterns and the latent
images, thereby detecting a diversion between the substrate to be exposed
and the mask without developing the substrate to be exposed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a reduction projection exposure apparatus
to which is applied a diversion detecting method according to the
invention,
FIGS. 2A and 2B are plan views of reticles used in the apparatus of FIG. 1,
FIG. 3 is a plan view of one of the chips on a wafer used in the apparatus
of FIG. 1,
FIGS. 4 and 8 show latent images formed by the apparatus of FIG. 1,
FIG. 5 is a waveform diagram of a photoelectric signal corresponding to the
latent images of FIG. 4,
FIG. 6 is a diagram showing the alignment marks on a wafer and the latent
images formed by the apparatus of FIG. 1,
FIG. 7 is a diagram showing a waveform of the photoelectric signal
corresponding to the alignment marks and the latent images in FIG. 6,
FIG. 9 is a schematic diagram of a reduction projection-type exposure
apparatus according to another embodiment to which is applied the
diversion detecting method according to the invention,
FIG. 10 is a plan view of a reticle used in the apparatus of FIG. 9,
FIG. 11 is a diagram showing the latent images formed by the apparatus of
FIG. 9,
FIG. 12 is a diagram showing the alignment marks on a wafer and the latent
images formed on the wafer by the apparatus of FIG. 9, and
FIGS. 13, 14A and 14B are diagrams showing waveforms of the photoelectric
signal and the image signals of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the invention will now be described in detail with
reference to the accompanying drawings.
FIG. 1 is a schematic diagram of a reduction projection exposure apparatus
according to the invention. In FIG. 1, an exposure light l is of a
wavelength effective in sensitizing a photoresist (e.g., g-line or i-line)
and it is projected to a reticle R (mask) through reticle blinds 4a and 4b
which are slidable in an X-direction so that the circuit pattern formed on
the reticle R is imaged on a wafer W (substrate to be exposed) having
formed thereon a photoresist layer (photosensitive layer) through a
projection lens 7 having a design reduction magnification of 1/M (where M
is 1 or greater).
A light source 5 emits a laser beam l1 (helium-neon) having a wavelength
which does not sensitize the photoresist. The laser beam L1 is transmitted
through a beam splitter 11, bent by a mirror 12 toward the center of the
entrance pupil of the projection lens 7 and imaged as a beam spot SP on
the wafer W through the projection lens 7. When the laser beam l1 incident
to the wafer W illuminates a mark on the wafer W (e.g., the stepped edges
of the irregularities), it is diffracted or scattered so that the
diffracted light or the scattered light falls back on the projection lens
7 and it is then projected to a photo-detector 13 for detecting the
diffracted or scattered light through the mirror 12 and the beam splitter
11. The light source 5, the beam splitter 11, the mirror 12 and the
photodetector 13 form an alignment optical system of the through-the lens
(TTL) type which aligns the wafer W through the use of the projection lens
7. It is to be noted that the photodetector 13 also includes a
photoelectric element for receiving the amount of the regular reflected
light in addition to the detection of the diffracted or scattered light
and the diffracted or scattered light and the regular reflected light are
suitably separated and taken out by a spacial filter.
An off-axis microscope 6 is one known as a so-called laser scan
(oscillation) type photoelectric microscope so that it has an optical axis
parallel to the optical axis AX of the projection lens 7 and it is
disposed at a predetermined distance from the projection lens 7. This
off-axis microscope 6 is essentially used for the purpose of detecting the
alignment marks on the wafer W and aligning the wafer W with the
projection lens 7. In other words, the off-axis microscope 6 effects the
alignment by converging a laser beam (helium-neon) of a wavelength which
does not sensitize the photoresist into a spot of beam, causing the spot
to make small oscillations on the wafer W, photoelectrically detecting the
scattered or diffracted light from the stepped edges of the irregularities
on the wafer W and subjecting the resulting photo-electric signal to
synchronous detection. In this case, it is assumed that the beam spot SP
from the TTL type alignment optical system and the beam spot from the
off-axis microscope 6 are each shaped as an elongated ellipse (slit-like
shape) in the X direction or the Y direction (the direction vertical to
the plane of the drawing through a cylindrical lens which is not shown.
A stage 9 having the wafer W loaded thereon is movable two-dimensionally in
the X and Y directions, thereby altering the position of the wafer W as
desired. An actuator 8 moves the stage 9 in the X direction and also a
laser interferometer 4 projects a measuring laser beam vertically to the
reflecting plane of a movable mirror 14 fixedly mounted on the stage 9,
thereby detecting the X-direction position of the stage 9 and generating a
position signal corresponding to the detected position. It is assumed that
there are similarly provided an actuator for moving the stage 9 in the Y
direction and a laser interferometer for detecting the Y-direction
position of the stage 9 (which are not shown).
Operational control means 15 is responsive to the photo-electric signal
from the photodetector 13 and the position signal from the laser
interferometer 4 to compute a registration diversion (including an overall
lithographical magnification error) between the reticle R and the wafer W
and also to control the actuator 8, thereby effecting the positioning of
the stage 9.
FIG. 2 shows examples of the reticle R used in the above-mentioned
reduction projection exposure apparatus. The reticle R shown in FIG. 2A
includes a pattern area 1 having a given circuit pattern formed with a
light shielding material such as chrome on a glass substrate and provided
on both sides of the pattern area 1 are light shielding portions 2 and 3
(second patterns) of rectangular shape which are made of chrome or the
like and arranged at equidistant positions from the center RC of the
pattern area 1 with a center distance L.sub.o. Also, if an XY rectangular
coordinate system is established with the origin being the center of the
pattern area 1 and the X axis passing through the centers of the light
shielding portions 2 and 3, of slit-like apertures (marks) 2a and 2b and
apertures (marks) 3a and 3b which are respectively formed in the light
shielding portions 2 and 3, the apertures 2a and 3a are positioned on the
X axis and the apertures 2b and 3b are each positioned on a straight line
parallel to the Y axis. FIG. 2B shows another example of the reticle R in
which the light shielding portions 2 and 3 are provided within the pattern
area 1.
FIG. 3 shows the circuit pattern formed on the wafer W used in the
previously described reduction projection exposure apparatus. Formed with
a center distance P in the chip are a cross alignment mark 10' (a first
pattern) consisting of linear marks 10a and 10b crossing at right angles
in the X and Y directions, respectively, and a cross alignment mark 11' (a
first pattern) consisting of linear marks 11a and 11b crossing at right
angles in the X and Y directions, respectively. If an XY rectangular
coordinate system is established with the center of the chip as its origin
in the like manner with those shown in FIGS. 2A and 2B, of the alignment
marks 10' and 11' the alignment marks 10a and 11a are positioned on the X
axis and the alignment marks 10b and 11b are each positioned on a straight
line parallel to the Y axis.
It is to be noted that the reason for arranging the pair of alignment marks
on the X axis of the XY coordinate system in FIGS. 2A, 2B and 3 is that
the center of the X-direction measuring beam of the laser interferometer
for measuring the position of the stage 9 is selected to coincide with the
X axis. Thus, there is an advantage that no Abbe error will be included in
the measurement of the X-direction position of each alignment mark.
Next, a description will be made of a method of detecting a registration
diversion between the reticle R and the wafer W and a lithographic
magnification error by the use of the reduction projection exposure
apparatus shown in FIG. 1. Here, it is to be assumed that the lithographic
magnification error includes not only a magnification error of the
projection lens 7 itself but also a change in the size of the chip on the
wafer W due to a run-out of the wafer W during the manufacturing processes
of the ICs or LSIs.
A method of detecting a magnification error of the projection lens 7 itself
will now be described first. According to this method, the surface of the
wafer W is first coated with a photoresist to a uniform thickness of about
0.5 to 2 .mu.m and the wafer W coated with the photoresist layer
(photosensitive material layer) is loaded on the stage 9.
Then, with the reticle blind 4a being retracted from above the light
shielding portion 2 of the reticle R, the pattern area 1, and the light
shielding portion 3 of the reticle R are shielded by the reticle blind 4b
and the reticle R is illuminated with the exposure light l thus forming
latent images corresponding to the apertures 2a and 2b of the light
shielding portion 2 on the photoresist layer of the wafer W. It is to be
noted that during the formation of these latent images, although it varies
depending on the thickness of the photoresist, the exposure light is
projected to the photoresist layer in an amount corresponding to the
saturation energy of the photoresist. The reason for this is that the
differences in optical characteristics, such as the reflectance and
refractive index, between the portions of the photoresist layer where the
latent images are formed and their surrounding portion are increased as
far as possible so as to facilitate the detection of the latent images
which will be described later.
Then, the stage 9 is moved in the X direction by L.sub.o /M+.alpha..sub.x
and in the Y direction by .alpha..sub.Y by means of the actuator 8 and the
laser interferometer 4. Here, if M' represents the actual reduction
magnification of the projection lens 7, the values of .alpha..sub.X and
.alpha..sub.Y respectively satisfy the relations L.sub.o /M>.alpha..sub.X
>L.sub.o .vertline.1/M-1/M'.vertline. and L.sub.o /M>.alpha..sub.Y
>L.sub.o .vertline.1/M-1/M'.vertline.. After the stage 9 has been moved,
the pattern area 1 and the light shielding portion 2 are shielded by the
reticle blind 4a and the reticle blind 4b is retracted from above the
light shielding portion 3, thus projecting the exposure light quantity
corresponding to the saturation energy of the photoresist as during the
formation of the latent images of the apertures 2a and 2b and thereby
forming latent images corresponding to the apertures 3a and 3b of the
light shielding portion 3 on the photoresist layer.
FIG. 4 shows the latent images 2a', 2b', 3a' and 3b' corresponding
respectively to the apertures 2a, 2b, 3a and 3b and formed on the
photoresist layer of the wafer W. As shown in FIG. 4, the center distance
between the latent images 2a' and 3a' becomes .DELTA.X and the center
distance between the latent images 2b' and 3b' becomes .DELTA.Y.
Then, the distance .DELTA.X between the latent images 2a' and 3a' becomes
L.sub.o /M if there is no change in the X-direction reduction
magnification of the production lens 7. However, if the actual reduction
magnification becomes 1/M' (where M' is 1 or greater) due to the
surrounding environment or the projection of the exposure light l, the
distance .DELTA.X becomes a length L.sub.o /M' or the sum of the offset
amount .alpha.X and an X-direction magnification error .delta.X of the
projection lens 7. Similarly, the distance .DELTA.Y between the latent
images 2b' and 3b' becomes a length representing the sum of the offset
amount .alpha.Y and the Y-direction magnification error .delta.Y of the
projection lens 7.
Then, the beam spot SP of the laser beam l1 by the previously mentioned TTL
type alignment optical system is positioned parallel to the left side of
the latent image 2a'. Then, the stage 9 is moved causing the beam spot SP
to make a relative scanning in the X direction so that the laser beam l1
reflected from the photoresist layer is projected through the projection
lens 7, the mirror 12 and the beam splitter 11 to the photodetector 13,
thereby generating a photoelectric signal I.sub.s. Also, during the spot
scanning by the beam spot SP, the position of the stage 9, that is, the
position of the beam spot SP, is detected by the laser interferometer 4,
thereby generating a position signal X.sub.s indicative of the position of
the beam spot SP.
FIG. 5 is a diagram showing the waveform of the photoelectric signal
I.sub.s resulting from the spot scanning by the beam spot SP. With the
waveform of the photoelectric signal I.sub.s, the exposed portions or the
portions corresponding to the latent images 2a' and 3a' and the unexposed
portions differ in waveform due to the production of the scattering or
diffraction of the laser beam l1 by the exposed portions. Therefore, the
operational control means 15 calculates the actual distance .DELTA.X
between the latent images 2a' and 3a' from the photoelectric signal
I.sub.s and also performs the calculation of .DELTA.X-.alpha.X, thus
determining an X-direction magnification error .delta.X of the projection
lens 7. In the like manner, the operational control means 15 calculates a
Y-direction magnification error .delta.Y of the projection lens 7 by
calculating the distance .DELTA.Y between the latent images 2b' and 3b' in
accordance with the photoelectric signal I.sub.s obtained by the relative
scanning of the beam spot SP in the Y direction and the position signal
Y.sub.s detected by the laser interferometer 4 and performing the
calculation of .DELTA.Y-.alpha.Y.
It is to be noted that the latent images 2a', 2b', 3a' and 3b' can be
detected by the off-axis microscope 6 in addition to the alignment optical
system of the TTL type. However, since the off-axis microscope 6 is a
laser oscillation-type photoelectric microscope, if it is desired to
obtain a photoelectric signal I.sub.s as shown in FIG. 5, it is necessary
to effect a synchronous detection at a frequency which is two times the
oscillation frequency of the laser beam. If the synchronous detection is
effected at the oscillation frequency of the laser beam, an S curve signal
is obtained which becomes zero at the peak of the photo-electric signal
I.sub.s and the zero point of the S curve signal corresponds to the edges
of the latent images 2a', 2b', 3a' and 3b'. Where the reticle-wafer
alignment is made in accordance with the detection by the off-axis
microscope 6, it is necessary to preliminarily determine the accurate
positional relation of the off-axis microscope 6 with the projection lens
7. This positional relation can also be determined by the same method as
in the case of the previously mentioned detection by the alignment optical
system.
Next, a method of detecting a registration diversion between the reticle R
and the wafer W in accordance with the embodiment of the invention will be
described. Using the same method as the previously mentioned formation of
latent images, a photoresist layer is first formed on the wafer W having a
chip such as shown in FIG. 3 and latent images 2a' and 2b' respectively
corresponding to the apertures 2a and 2b of the light shielding portion 2
in the reticle R are formed as shown in FIG. 6. In this case, they are
formed so that the center distance between the latent image 2a' and the
alignment mark 11a preliminarily formed on the wafer W as shown in FIG. 3
becomes .DELTA.X' and the distance between the latent image 2b' and the
alignment mark 11b becomes .DELTA.Y'. The distance .DELTA.X' includes an
X-direction registration diversion .delta.X' between the reticle R and the
wafer W in addition to the offset amount .alpha.X' of the wafer W and the
distance .DELTA.Y' includes a Y-direction registration diversion .delta.Y'
between the reticle R and the wafer W in addition to the offset amount
.alpha.Y' of the wafer W.
Then, a relative scanning in the X direction is effected by using the beam
spot SP from the previously mentioned alignment optical system, thereby
generating a position signal X.sub.s of the beam spot SP and a
photoelectric signal I.sub.s.
FIG. 7 is a waveform diagram of the photoelectric signal I.sub.s obtained
as a result of the relative scanning by the beam spot SP. FIG. 7 shows the
photoelectric signal waveform by the regular reflected light. As shown in
FIG. 7, the waveform of the photoelectric signal I.sub.s differs between
the portion where the latent image 2a' is formed, i.e., the exposed
portion, and the unexposed portion. In other words, the portion of the
latent image 2a' absorbs the exposure light energy up to the saturation
and thus it is high in reflectance as compared with the unexposed portion.
Also, the waveform of the photoelectric signal I.sub.s differs between the
portion of the unexposed portion containing the alignment mark 11a and the
remainder of the unexposed portion. In other words, the reflected light is
reduced by scattering at both edges of the mark 11a. Thus, in accordance
with the photoelectric signal I.sub.s and the position signal X.sub.s, the
operational control means 15 determines the actual distance .DELTA.X'
between the latent image 2a' and the alignment mark 11a and it also
performs the calculation of .DELTA.X'-.alpha.X', thereby determining an
X-direction registration diversion .delta.X' between the reticle R and the
wafer W. Similarly, in accordance with a photoelectric signal I.sub.s
obtained as the result of a relative scanning by the beam spot SP and a
position signal Y.sub.s detected by the laser interferometer 4, the
operational control means 15 determines the actual distance .DELTA.Y'
between the latent image 2b' and the alignment mark 11b and it also
performs the calculation of .DELTA.Y'-.alpha.Y' thereby determining a
Y-direction registration diversion .delta.Y' between the reticle R and the
wafer W.
After the registration diversion between the reticle R and the wafer W has
been determined, the stage 9 is moved in the X direction by L.sub.o
/M-P+.delta.X' and in the Y direction by .delta.Y" by means of the
actuator 8 and the laser interferometer 4. Then, latent images 3a' and 3b'
respectively corresponding to the apertures 3a and 3b are formed by the
exposure light l at positions which are respectively apart from the center
of the alignment mark 10' by .alpha.X' in the X direction and by .alpha.Y'
in the Y direction. In this case, the center distance between the latent
image 3a' and the alignment mark 10a becomes .DELTA.X" and the center
distance between the latent iamge 3b' and the alignment mark 10b becomes
.DELTA.Y". Since these distances .DELTA.X" and .DELTA.Y" respectively
include, in addition to the offset amounts .alpha.X' and .alpha.Y', chip
size variations .delta.X" and .delta.Y" due to the run-out of the wafer
caused during the IC or LSI manufacturing processes, variations .alpha.X"
and .delta.Y" are determined by the above-mentioned method, thereby
determining variation of the chip size as an apparent magnification
variation. In other words, an overall lithographic magnification error
caused by the registration of the projected image of the pattern on the
reticle R and the chip on the wafer W is determined.
It is to be noted that since, in this embodiment, the latent images 2a',
2b', 3a' 3b' and the alignment marks 10' and 11' are detected by means of
the laser beam l1 which does not sensitize the photoresist, it is possible
to enhance the detection accuracy by detecting the positions of such
latent images or the alignment marks a number of times and obtaining the
averages of the detected positions.
Also, the latent images 2' and 3' or the alignment marks 10' and 11' may
each be formed as a diffraction grating as shown in FIG. 8 so as to detect
the light diffracted in a direction perpendicular to the scanning
direction of the beam spot SP or any specified direction.
Referring now to FIG. 9, there is illustrated a schematic diagram of a
reduction projection exposure apparatus in accordance with another
embodiment. Note that in FIG. 9, the component parts performing the same
functions as in FIG. 1 are designated by the same reference numerals and
their explanation is omitted.
While, in the above-described embodiment, the TTL type alignment optical
system employs the laser beam of a wavelength which does not sensitize the
photoresist, this embodiment employs an illuminating light l' of the same
wavelength as the exposure light l for latent image forming purposes, that
is, the illuminating light l' of a wavelength which can sensitize the
photoresist.
The illuminating light l' having the same wavelength as the exposure light
l and emitted from a light source which is not shown, is directed through
a light guide 22 to a field stop 23, which in turn reshapes it in
conformity with the shape of a window 20 formed in a reticle R' (to will
be described later), passed through a beam splitter 24 and an objective
lens 25, caused to change its direction of travel by a mirror 26 and then
imaged on the wafer W through the window 20 of the reticle R' and the
projection lens 7. The illuminating light l' incident to the wafer W is
reflected by the wafer W, directed back to the projection lens 7 and then
projected to image pickup means 27 through the mirror 26, the objective
lens 25 and the beam splitter 24. The light guide 22, the field stop 23,
the beam splitter 24, the objective lens 25, the mirror 26 and the image
pickup means 27 form a TTL type alignment optical system for observing
both the patterns on the reticle R' and the patterns on the wafer W by the
use of the projection lens 7.
FIG. 10 shows the reticle R' used in the above-mentioned reduction
projection exposure apparatus. While the apertures 2a, 2b and 3a, 3b
respectively formed in the light shielding portions 2 and 3 on the sides
of the pattern area 1 of the reticle R' are cross-shaped as were their
counterparts shown in FIG. 2A, they are slightly displaced in the Y
direction from the X axis. The rectangular windows 20 and 21 respectively
formed at the right and lower ends of the reticle R' are provided for the
purposes of observing the latent images formed on the wafer W in
correspondence to the apertures 2a, 2b, 3a and 3b and the alignment marks
on the wafer W through the projection lens 7. Note that the cross-shaped
apertures 2a, 2b, 3a and 3b and the window 20 may be arranged on the X
axis. In this case, by arranging so that the apertures 3a and 3b and the
window 20 can be observed by the TTL type alignment optical system
simultaneously or through its forward and backward movements, it is
possible to prevent any Abbe error as in the case of the first embodiment.
Next, a method of detecting a magnification error of the projection lens 7
and a method of detecting a registration diversion between the reticle R'
and the wafer W by the use of the reduction projection exposure apparatus
shown in FIG. 9 will be described.
The method of detecting a magnification error of the projection lens 7
itself will be described first. Firstly, in accordance with the same
method as used with the apparatus shown in FIG. 1, latent images 2a', 2b',
3a' and 3b' respectively corresponding to the apertures 2a, 2b, 3a and 3b
of the reticle R' are formed as shown in FIG. 11. In this case, it is so
designed that the center distance between the latent images 2a' and 3a'
and the center distance between the latent images 2b' and 3b' become
.DELTA.X and .DELTA.Y, respectively. Note that since the apparatus shown
in FIG. 9 includes the TTL type alignment optical system employing the
illuminating light l' of the same wavelength as the exposure light 1,
these latent images may be formed by the illuminating light l'. In the
case, two optical systems for illuminating the apertures 2a and 2b and the
apertures 3a and 3b, respectively, are necessary.
Then, the illuminating light l' from the alignment optical system of the
TTL type is projected onto the wafer W so that the regular reflected light
from the wafer W is received by the image pickup means 27 and its
photosensitive surface 27a is photoelectrically scanned in the X
direction, thus generating an image signal IP corresponding to the
resulting scanning line SL.
Where the thickness of the photoresist is selected to be 1 .mu.m and its
exposure light saturation energy (sensitivity) is selected to be 90
mJ/cm.sup.2, if the illuminance of the latent image forming exposure light
l on the wafer W is 300 mw/cm.sup.2, then the time required for projecting
to the photoresist the amount of exposure light corresponding to the
saturation energy of the photoresist is 0.3 second (90/300). On the other
hand, if the illuminance of the illuminating light l' for producing the
image signal IP is set to 30 mw/cm.sup.2, the time required for saturating
the photoresist is 3 seconds (90/30). As a result, if the image pickup
means 27 is operated at the same scanning speed as a television camera or
the like so as to generate an image signal IP corresponding to a scanning
line SL in about 1/30 second even if the unexposed area of the photoresist
is sensitized by the projection of the illuminating light l', the
resulting image signal IP has a satisfactorily distinguishable contrast
between the latent images 2a' and 3a' or the exposed portions by the
exposure light l and the unexposed portions.
In accordance with the same method as used with the apparatus shown in FIG.
1, an X-direction magnification error of the projection lens 7 can be
detected on the basis of the image signal IP obtained in the
above-mentioned manner.
Also, a Y-direction magnification error of the projection lens 7 can be
easily detected by detecting the picture elements corresponding to a
virtual scanning line SL parallel to the latent image 3a' from the image
signal IP produced by the image pickup means 27 and arranging the picture
elements in time sequence to obtain an image signal similar to the image
signal IP of the scanning line SL and its description will be omitted.
Also, since the image pickup means of the alignment optical system
arranged above the window 21 of the reticle R' has a scanning line in a
direction perpendicular to the scanning line SL of the image pickup means
for observing the window 20, a Y-direction image signal may be obtained by
aligning the wafer W in such a manner that the pair of latent images are
observed through the window 21. While, in this embodiment, the illuminance
of the illuminating light l' is selected to be lower than that of the
exposure light l, the illuminance of the illuminating light l' may be
selected to be the same or higher than that of the exposure light l
depending on the sensitivity of the photo-resist. In this case, however, a
high-speed image processing circuit is provided so that the extraction
(data sampling) of the image signal IP from the image pick-up means 27 is
completed in a very short period of time, e.g., several milliseconds,
after the beginning of the illumination by the illuminating light l'.
Next, a description will be made of the method of detecting a registration
diversion between the reticle R and the wafer W in accordance with the
second embodiment of the invention. Firstly, in accordance with the same
method used with the apparatus shown in FIG. 1, latent images 2a' and 2b'
respectively corresponding to the apertures 2a and 2b of the reticle R'
are formed on the wafer W having the alignment marks 11a and 11b as shown
in FIG. 12. In this case, it is so arranged that the center distance
between the latent image 2a' and the alignment mark 11a and the center
distance between the latent image 2b' and the alignment mark 11b become
.DELTA.X' and .DELTA.X', respectively.
Then, the illuminating light l' from the TTL type alignment optical system
is projected to the latent images on the wafer W through the window 20 and
an image signal IP corresponding to a scanning line SL resulting from an
X-direction scanning of a photosensitive surface 27a by the image pickup
means 27 is generated.
FIG. 13 is a waveform diagram of the image signal produced as a result of
the illumination by the illuminating light l'. As shown in FIG. 13, there
is a difference in exposure time between the portion including the latent
image 2a' and the surrounding portion and the image signal IP differs in
waveform between these portions, thus making it possible to easily detect
the position of the latent image 2a'. Also, if the edges of the alignment
marks 11a have a large fall, the regular reflected light from the
photoresist corresponding to the edges is not projected to the image
pickup means 27 so that the level of the image signal IP decreases in its
portion corresponding to the edges and its position is detected easily. If
the edges of the alignment mark 11a have a small fall, the difference in
thickness between the photoresist layer corresponding to the alignment
mark 11a and the photoresist layer corresponding to the surrounding
portion becomes the difference in level between the portions in the image
signal IP (the difference between the light and dark portions) and this
permits detection of the position of the alignment mark 11a.
Then, where an image signal IP is produced by projecting onto the wafer W
the illuminating light l' of a wavelength capable of sensitizing the
photoresist, the illumination by the illuminating light l' changes the
absorptance of the photoresist on the alignment mark which has not been
sensitized by the projection of the exposure light l and this in turn
slightly varies the waveform of the portion of the image signal IP
corresponding to the mark 11a; thus, if the image signal IP is obtained in
a very short period of time, there is the danger that the detection of the
position of the alignment mark 11a will be inaccurate. Thus, while it is
better to obtain the image signal IP after the sensitization of the
photoresist has stabilized, this means that the unexposed portions around
the latent image 2a' and the alignment mark 11a are sensitized
unnecessarily and the image signal IP loses its contrast.
As a result, the present embodiment is so designed that the latent image
2a' is formed without exposure by the exposure light l as shown in FIG.
14A, that is, the latent image 2a' is formed by exposing the photoresist
layer including the photoresist on the alignment mark 11a but excluding
the portion where the latent image 2a' is to be formed and then the
illuminating light l' is projected. However, this is effective only in
cases where the observation is made by means of light having a wavelength
to which the photoresist is sensitive and either of the two methods may be
used in cases where the latent images are detected with light which cannot
sensitize the photoresist. Also, even in cases where the photoresist layer
excluding the portion of the latent image 2a' is exposed to the exposure
light 1 as shown in FIG. 14A, there is the danger of deteriorating the
signal-to-raise ratio (S/N) of the image signal IP due to the effect of an
interference caused by any variation in the wavelength of the illuminating
light l', the thickness of the photoresist layer or the like, the
refractive index of the photoresist or the like. While, with the method of
making the observation with the regular reflected light, as shown in FIG.
14B, the contrast of the image signal IP corresponding to the latent image
2a' is dependent on the absorptance (or the reflectance) of the
photoresist with respect to the wavelength of the illuminating light l'
and thus its waveform is relatively simple, there is the danger of
deteriorating the S/N of the image signal IP in the portion corresponding
to the mark 11a due to the effect of an interference fringe. Also, where
the diffracted or scattered light due to the scanning by the laser beam is
detected as shown in FIG. 1, there is the possibility that the S/N of the
photoelectric signal I.sub.s is improved in the portion corresponding to
the stepped portions of the alignment mark 11a and the S/N of the
photoelectric signal I.sub.s corresponding to the portion of the latent
image 2a' is deteriorated. As a result, where an apparatus comprising both
of an alignment optical system as shown in FIG. 1 and an alignment optical
system as shown in FIG. 9 is used, it is possible to improve the detection
accuracy of the latent image 2a' and the alignment mark 11a by detecting
the latent image 2a' from the regular reflected light of the light l'
capable of sensitizing the photoresist layer and detecting the alignment
mark 11a from the scattered or diffracted light of another light incapable
of sensitizing the photoresist layer.
On the basis of the image signal IP or the photoelectric signal I.sub.s
shown in FIGS. 13, 14A or 14B and obtained as described above, an
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