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Description  |
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FIELD OF THE INVENTION AND RELATED ART
This invention relates generally to a projection exposure apparatus for
optically projecting and photoprinting a pattern of a first object upon a
second object by use of a projection optical system. More particularly,
the invention is concerned with a step-and-repeat type projection exposure
apparatus which is usable in the manufacture of microcircuits such as
integrated circuits and which includes a projection optical system as well
as an alignment system for aligning a reticle having a pattern and a wafer
having a surface film layer such as a photoresist layer, the apparatus
being operable to photoprint, after the reticle-to-wafer alignment, the
reticle pattern upon the wafer by the projection through the projection
optical system.
The present invention may be related to optical arrangements disclosed in
Japanese Laid-Open Patent Applications, Laid-Open Nos. 25638/1983,
53562/1979, 90955/1978 and 52021/1985 all of which were filed in Japan in
the name of the assignee of the subject application. Of these related
applications, Japanese Laid-Open Patent Application No. 25638/1983 shows
an exposure apparatus having a TTL (Through The Lens) alignment system for
aligning a reticle and a wafer by way of a projection optical system.
Japanese Laid-Open Patent Application No. 53562/1979 shows a basic
structure of a photoelectric detecting device for detecting alignment
marks provided on a reticle and a wafer. Japanese Laid-Open Patent
Applications Nos. 90955/1978 and 52021/1985 each shows a photoelectric
detecting device for detecting alignment marks provided on a mask or
reticle and a wafer while avoiding adverse effects of a photoresist layer,
formed on the surface of the wafer, upon detection of the mark.
In the projection exposure apparatus for use in the field of manufacture of
microcircuits, the resolving power and the superimposing accuracy are two
basic performances required. As for the resolving power, it only needs
very simple treatment because only a few parameters determine the
resolving power. In the projection exposure apparatuses, called
"steppers", used in the field of manufacture of semiconductor devices, it
is easy to estimate the resolving power of a projection lens system once
the numerical aperture (NA) and the wavelength to be used for the
photoprinting through the projection lens system are determined. Also, in
a case of X-ray exposure apparatuses, there are only a limited number of
parameters such as "half shade" which is determined by the size of an
X-ray source used.
In the field of semiconductor devices, the progress of the devices in the
points of high capacities and further miniaturization mainly owes to the
advance of the photolithography technique (i.e. the technique of printing
a pattern of a very narrow linewidth) and to the advance of the process
technique such as the etching process technique. As regards the resolving
power, the history of projection lens systems used in the steppers shows
the fact of steady advance of the resolving power. Recently, the minimum
resolvable linewidth has become less than 1 micron which was a deadlock in
the past, and a variety of lens systems having resolution of an order of
submicron linewidth have been developed.
On the side of the wafer process, there have recently been proposed various
ideas such as a trenching process, a low-step structure, a high-step
structure or the like. Briefly, these ideas are in the trend of developing
three-dimensional integrated-circuits.
The advance of the resolving power having been achieved on the exposure
apparatus side and the advance having been achieved on the process
technique side come into intimate contact with each other, on the same
stage of pattern superimposition. In this respect, the superimposing
accuracy has become more and more important in the exposure apparatus.
It is difficult to treat the superimposing accuracy in terms of simple
parameters such as those with which the resolving power can be treated.
This is just the implication of the variegation of the wafer process. On
the other hand, this is because of the multifariousness of the structures
of alignment systems used for the superimposition. What makes the factors
of the wafer process more complicated is the fact that the problem should
be discussed not only in the phase of a wafer substrate but also in the
phase of a photoresist coating applied to the wafer surface. One of the
targets which are currently and apparently aimed at in the field of
semiconductor devices is a three-dimensionally constructed
integrated-circuit. To go in such trend, it is inevitable to make the
surface step (recess/protrusion) of the wafer much deeper or higher.
Clearly, this adversely affects the state of resist coating. Also, there
is a tendency to further enlargement of the wafer size, i.e., from 6-inch
wafers to 8-inch or 10-inch wafers. Where a large-diameter wafer is coated
with a photoresist material in accordance with the drop-and-spin method,
apparently the state of coating is uneven between a central portion and a
peripheral portion of the wafer. Also, the unevenness grows with the
increase in the depth/height of the surface recess/protrusion of the
wafer. In fact, it is known that the state of alignment changes due to the
influence of the application of the resist material to the wafer. The
importance will be understood from the fact that studies have been made of
how to uniformly apply the resist material.
Further, with regard to the photoresist, consideration has to be made of a
tendency to using a multilayer resist in accordance with the "age" of
submicron linewidth. Since, in a few manufacturing processes, some
measures such as the aforementioned multilayer resist process or the
contrast enhancement lithography (CEL) technique must be inevitably
adopted in order to improve the resolving power, it is also necessary to
arrange the exposure apparatus so as to meet them. It can be said that, in
the phase of the pattern superimposition, it is required to provide the
exposure apparatuses with effective measures for such newly proposed wafer
processes as described above.
On the other hand, the multifariousness of the alignment systems is an
evidence of the flexibility and hardness of constructing the system. Every
alignment system having been proposed or developed has a difference from
the others, and each system has its own advantage and disadvantage. An
example is found in an alignment system of a projection exposure apparatus
of the type disclosed in Japanese Laid-Open Patent Application, Laid-Open
No. 25638/1983. This is an example of excellent arrangement which
practically embodies a so-called TTL (through the lens) on-axis alignment
system with the aid of a projection lens optical system that is
telecentric both on the reticle side and on the wafer side. While the
projection lens system is arranged so that aberrations are corrected with
respect to g-line rays (436 nm in wavelength), it shows substantially the
same optical performance with respect to a He-Cd laser beam (442 nm in
wavelength). The proposed alignment system uses the laser beam scanning
method, using the He-Cd laser beam, for the detection of alignment
signals. For this reason, the exposure operation can be initiated just in
the state of completion of the alignment. Namely, the TTL on-axis
alignment is practically embodied. The TTL on-axis alignment system is
nearly idealistic in a sense that the error in the detection of the
alignment signals is the sole factor of the inaccurate operation of the
exposure apparatus. Only one weak point is that the signal detection is
not easy when the system is used with a resist material, such as the
multilayer resist, having an absorbency with respect to wavelengths near
that to be used for the photoprinting.
On the other hand, many proposals have been made of alignment systems using
a wavelength other than the photoprinting wavelength, more particularly
using a longer wavelength such as that of e-line rays (546 nm) or that of
a He-Ne laser beam (633 nm). Because of the use of the wavelength longer
than the photoprinting wavelength, these alignment systems are reliable,
for the wafer process using absorptive resist materials such as the
multilayer resist. However, due to various aberrations of the projection
lens system caused in relation to the "chromaticity", the position that
allows detection of a wafer alignment mark by way of the projection lens
system (in other words, the position in the image height direction related
to the projection lens system) is usually fixed. Therefore, while it
depends on the positional relation of the alignment mark with an
associated shot area of the wafer, it is necessary to move the wafer after
the mark is detected, so as to move the associated shot area to the
exposure position. Such movement leads to a factor of inaccuracy.
However, recent demands for the superimposing accuracy are very strict.
Even the above-described signal detection error which is the sole factor
of inaccuracy in the idealistic alignment system of the type described in
the aforementioned Japanese Patent Application, Laid-Open No. 25638/1983,
has to be treated as a problem.
The inventors of the subject application have made an analysis upon the
components of the error in the detection of the alignment signal and, from
the results, it has been found that almost all the error components result
chiefly from the application of the photoresist to the wafer surface.
While there are many factors of inaccuracy related to the resist coating,
it is considered that the most important factors are the following two:
The first is the effect of interference between the light reflected by the
surface of the resist layer and the light passed through the resist layer
and reflected backwardly from the substrate of the wafer. Particularly, as
described hereinbefore, the application of the photoresist material to the
wafer is not always uniform. In many cases, the resist coating is uneven
between the central portion and the peripheral portion. Also, the wafer
substrate itself involves a problem of unevenness resulting from the
working processes such as the etching, the sputtering, etc. As a
consequence, the structures of respective alignment marks associated with
different shot areas on the wafer, when they are considered on the
condition that the resist coating is existing on the wafer, are different
from each other (the variation occurring with the difference in the
location on the wafer). Accordingly, the effect of interference varies
with the location on the wafer. It appears therefrom that the interference
described above is the most striking one of the effects of the resist
coating that causes the alignment error.
The second factor is the multiple reflection. The resist layer has, as one
character thereof, a function of optical waveguide. For this reason, a
portion of the light reflected by the wafer substrate is reflected at the
interface between the resist layer and the air. Such portion goes back to
the wafer substrate and is reflected thereby again. The higher the
reflection factor of the wafer substrate is, the more noticeable the
effect of multi-reflection is. Moreover, the multiple reflection finally
causes interference which results in further deterioration of the
alignment accuracy.
As another factor of inaccuracy resulting from the photoresist, there is
the shift of an image due to the refraction by the resist material.
However, such factor is merely a secondary factor. It has been confirmed
as a result of the analysis that the exclusion of the above-discussed two
factors, particularly the effect of interference, is contributive to the
improvement of the alignment accuracy, to a great extent.
SUMMARY OF THE INVENTION
Briefly, in the present invention, the error components in the detection of
alignment signals are analyzed and the factors causing such error
components are obviated. It is accordingly a primary object of the present
invention to provide a projection exposure apparatus for projecting a
pattern of a first object upon a second object by use of a projection
optical system, wherein the alignment detection accuracy is improved
significantly.
In accordance with one aspect of the present invention, to achieve the
above object, there is provided a projection exposure apparatus which
includes an imaging optical system for projecting an image of a pattern of
a reticle upon a wafer, and a mark detecting system for detecting an
alignment mark formed on the wafer, through a space between the imaging
optical system and the wafer.
The detection of the wafer alignment mark through the space between the
imaging optical system and the wafer permits a large extent of freedom of
geometrical design of the mark detecting system. Particularly, when the
mark detecting system is arranged to receive a mark detecting light
advancing with an angle larger than the angle of light caused by the
surface reflection at the wafer surface and/or the multiple reflection
within the surface layer provided on the wafer, the wafer alignment mark
can be detected without being affected by the surface layer (photoresist
layer) of the wafer, with the result that extraordinarily high alignment
accuracy is attained.
These and other objects, features and advantages of the present invention
will become more apparent upon a consideration of the following
description of the preferred embodiments of the present invention taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view schematically showing a major portion of a
step-and-repeat type projection exposure apparatus according to one
embodiment of the present invention.
FIG. 2 is a principle view explicating the reflection of a wafer
illuminating light at a wafer.
FIG. 3 is a schematic view showing signal waveforms obtainable, in the
apparatus of FIG. 1, from alignment marks formed on a reticle and a wafer.
FIG. 4 is similar to FIG. 1 but shows a projection exposure apparatus
according to another embodiment of the present invention.
FIG. 5 is a schematic side view showing a major portion of a projection
exposure apparatus according to a further embodiment of the present
invention.
FIGS. 6 and 7 are schematic views, respectively, showing further examples
of photoelectric detection of wafer alignment marks, in accordance with
the present invention.
FIG. 8 is a schematic view showing the directions of lights from the wafer
surface and to be detected by four mark detecting systems included in the
FIG. 1 apparatus.
FIG. 9 is a diagrammatic view of a control system included in the FIG. 1
apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1, there is shown a step-and-repeat type reduction
projection exposure apparatus, called "stepper", to which an embodiment of
the present invention is applied.
As shown in FIG. 1, the exposure apparatus includes a reduction projection
lens system 3 which is adapted to project, upon a wafer 4, a semiconductor
device manufacturing pattern formed in a pattern region 2 on a reticle 1
as the reticle 1 is illuminated with light supplied from an illumination
system 30. By this projection, the pattern of the reticle 1 is
photoprinted on the wafer 4, which is a workpiece, in a reduced scale. In
order to avoid an unpreferable change in the magnification for the pattern
projection due to any surface unevenness of the wafer 4 and/or to a focus
change resulting from an error in the focus detection or in the
focus-controlling position adjustment made in an automatic focusing system
(not shown) provided between the projection lens system 3 and the wafer 4,
usually the projection lens system 3 is arranged to be telecentric on the
wafer 4 side. The wafer 4 includes different surface portions onto which
images of the reticle 1 pattern are transferred in sequence, namely in the
step-and-repeat manner. In the state as illustrated in FIG. 1, the pattern
of the reticle 1 formed in the region 2 thereof is going to be projected
upon one, denoted at numeral 5, of the different surface portions (shot
areas) of the wafer 4.
A light source 9 produces a light beam which is deflectively reflected by a
polygonal mirror 8 rotatable in one direction. The light beam reflected
from the polygonal mirror 8 is directed to a triangular prism 31 by way of
an f-.theta. lens 41 and a reflecting block 32 in this order. By the prism
31, the range of scan of the scanningly deflected light beam is separated
into a first half and a second half. In one of the first and second
halves, the light beam is directed to a beam splitter 11. By this beam
splitter 11, the light beam is reflected toward a relay lens 42. The light
beam passed through the relay lens 42 is reflected by a reflecting block
33 and, then, passes through an objective lens 7. The light beam from the
objective lens 7 is reflected by a mirror 40 so that it is directed to an
alignment mark region 15, defined on the reticle 1, to thereby irradiate a
reticle alignment mark formed therein. The light beam passed through the
region 15 enters the projection lens system 3 by which it is projected
upon a mark region 6 defined on the wafer 4 thereby to irradiate a wafer
alignment mark formed therein. The light beam irradiating the reticle
alignment mark and the wafer alignment mark is scanningly deflected
relative to the regions 6 and 15 in the Y-axis direction. In the other
half of the scan range, the light beam from the prism 31 is guided by a
similar optical arrangement, not shown, to another alignment mark region
15' defined on the reticle 1 and to another alignment mark region 6'
defined on the wafer 4 so as to irradiate alignment marks formed in these
regions. Similarly, the light beam is scanningly deflected relative to the
regions 15' and 6' in the Y-axis direction. It will be understood that the
scan of the mark regions 6 and 15 and the scan of the mark region 6' and
15' are carried out in a time-sharing manner.
In each of the mark regions 15 and 15' of the reticle 1, there is formed an
alignment mark such as shown at 60 in an uppermost part (a) of FIG. 3. On
the other hand, in each of the mark regions 6 and 6' of the wafer 4, there
is provided an alignment mark such as shown at 61 in the same part of FIG.
3. The reference character S in the part (a) of FIG. 3 depicts the scan
direction with the scanningly deflected light beam, which direction
coincides with the Y-axis direction.
Referring back to FIG. 1, the light which is diffractively reflected by the
alignment mark 61 (FIG. 3) in the mark region 6 (6') and which advances
outwardly of the projection lens system 3 (generally rightwardly as viewed
in FIG. 1) is received by a photoelectric detector 14 with the aid of a
mirror 12a, an objective lens 13 and mirrors 12b and 12c. The mirrors
12a-12c, the objective lens 13 and the detector 14 cooperate with each
other to constitute a wafer mark signal detecting system 50. A similar
wafer mark signal detecting system is provided and illustrated in FIG. 1
so as to detect the light reflected by the wafer alignment mark and
advancing generally leftwardly as viewed in FIG. 1. While only two mark
signal detecting systems are illustrated, the exposure apparatus of the
FIG. 1 embodiment is actually provided with four wafer mark signal
detecting systems. This is because of the configuration of the alignment
mark provided in the mark region 6 (6'). In a case where the alignment
mark used has a single orientation (inclination) with respect to the scan
direction S, as compared with the alignment mark shown in FIG. 3, two of
the four detecting systems may be omitted. Also, while in FIG. 1 the mark
signal detecting system 50 is illustrated as having its optical axis O
extended in the X-axis direction as viewed from the above, this is only
for ease in illustration. Actually, as is best seen in FIG. 8, each
mark-signal detecting system is disposed so that its optical axis extends
with an inclination angle .theta.2 with respect to the X-axis direction as
viewed from the above. This inclination angle .theta.2 is determined in
accordance with the inclination .theta.1 of the alignment mark 61 (FIG. 3)
with respect to the scan direction S. As is best seen in FIG. 8, the four
mark-signal detecting systems are disposed in the four directions denoted
by characters A, B, C and D.
One of the most important features of the present invention resides in the
disposition of each wafer signal detecting system 50 provided generally
below the projection lens system 3 for detecting the light diffracted by
edges of the wafer alignment mark. Particularly, each wafer signal
detecting system is disposed so as to detect the mark-diffracted light
through a space between the projection lens system 3 and the wafer 4. In
the illustration of FIG. 1, alignment marks, such as at 61, provided in
relation to the current shot area 5 of the wafer 4 (the area which is
going to be exposed to the reticle pattern) are detected by the four wafer
signal detecting systems 50 which are disposed in the above-described
directional and angular relation with respect to the wafer.
As has been described hereinbefore, the most important factor of the error
in the detection of alignment signals is the interference between the
light reflected by the surface of the resist layer and the light reflected
from the wafer substrate. While the effect of such interference may be
removed in some ways, the most fundamental solution is to prevent the
light reflected by the resist surface from entering into the wafer signed
detecting system. The results of observation of the state of existence of
the resist coating on the wafer surface by use of a scan type electron
microscope (SEM) or an interference microscope have shown that, even for a
wafer having a very large surface step (recess/protrusion), the angle of
inclination of the surface of the resist material covering such surface
step is, at the maximum, of an order of 5 degrees more or less, and that
there is little possibility of existence of a steeper slope. From the
viewpoint of step coverage, usually a wafer having a large surface step is
coated with a resist layer of a thickness greater than the size (depth or
height) of the step (recess/protrusion). For this reason, the surface
inclination is held approximately at an order of 5 degrees or slightly
more or less.
In accordance with the present invention, in consideration of this, a
specific condition such as described below is set for each wafer signal
detecting system 50. Namely, in FIG. 2, reference character A denotes the
angle of expansion of the wafer mark illuminating light 17 emitted from
the projection optical system 3 (i.e. the angle of convergence of a light
directed from the projection optical system to an edge of the wafer mark);
reference character B denotes the angle of expansion of the light
reflected by the surface of the resist layer 40; and reference character C
denotes the angle of expansion of the light as reflected, twice or more,
by the surface 4' of the wafer substrate, these angles being taken with
respect to the optical axis of the projection lens system 3. Also,
reference character D denotes the inclination angle of the slope of the
resist layer 40 in the neighborhood of the mark region 6. Generally, the
angle C is larger than the angle B. For example, where the angle D is of
an order of 5 degrees and the angle B is approximately 10 degrees, then
the angle C is approximately 25 degrees. Further, the light having been
multi-reflected by the wafer substrate 4' (i.e. the light having been
reflected three or more times or more by the wafer substrate 4') has an
intensity which is very low and can be neglected. Thus, in accordance with
the present invention, each wafer signal detecting system 50 is arranged
so as to satisfy the following condition:
E.gtoreq.C+A (1)
wherein E is the angle set for detecting the light, denoted at 20, as
diffractively reflected from the edge of the alignment mark in the mark
region 6, namely the angle defined by the optical axis of the detection
system 50 with respect to the optical axis of the projection lens system
3.
By satisfying this condition, it is now possible to prevent both the light
caused by the reflection at the surface of the resist layer 40 and the
light caused by the multiple reflection within the resist layer 40 from
entering into the wafer signal detecting system 50. Therefore, each wafer
signal detecting system 50 can detect only the light reflected from the
wafer alignment mark.
According to the TTL (through the lens) alignment systems employed in
conventional step-and-repeat type projection exposure apparatuses, the
mark illuminating light is projected upon the wafer through the projection
lens system and the light reflected by the wafer is directed to a
photodetector by way of the projection lens system. As compared therewith,
the mark signal detecting system according to the present invention is
specifically arranged so as to detect the light reflected from the wafer
alignment mark and advancing outwardly of the projection lens system
without passing through the projection lens system. Moreover, a specific
limitation "E.gtoreq.C+A" is set. By this, the detection of the light
caused by the reflection at the resist surface and the light caused by the
multiple reflection can be avoided. This assures that only the light
diffracted by the wafer alignment mark can be separately extracted, and
this directly results in significant improvements in the alignment
accuracy.
With regard to a reticle signal detecting system, on the other hand, the
light beam from the light source 9 is scanningly deflected by the
rotatable polygonal mirror 8, whereby the mark region 15 (15') and thus
the alignment mark 60 formed in this region are scanned by this light
beam. The light reflected or diffracted by the edge of the reticle
alignment mark 60 is received by a photoelectric detector 10. Thus, the
reticle alignment mark is detected. As described hereinbefore, the
scanning beam as transmitted through the reticle 1 and projected by the
projection lens system 3 constitutes the light that illuminates the mark
region 6 (6') of the wafer 4. The reticle signal detecting system used in
the present invention is of a known type such as, typically, disclosed in
Japanese Laid-Open Patent Application, Laid-Open No. 53562/1979.
The alignment signal obtainable by the mark detecting systems of the FIG. 1
apparatus will now be described, taken in conjunction with FIG. 3.
As a result of scan of the reticle mark 60 and the wafer mark 61 with the
light beam along the scan direction S, a signal such as shown in the part
(b) of FIG. 3 is produced by the above-described reticle signal detecting
systems (photodetectors 10), and a signal such as shown in the part (c) of
FIG. 3 is produced by the wafer signal detecting systems (photodetectors
14). These signals are combined by a signal combining circuit 70 (FIG. 8)
and, thereafter, the resultant signal is processed in a known manner by a
control circuit 71 (FIG. 8), whereby an alignment signal such as shown in
the lowermost part (d) of FIG. 3 is obtainable. In accordance with the
thus obtained alignment signal, a driving means (not shown) is actuated so
as to relatively move the reticle 1 and the wafer 4 to thereby align them.
According to the present embodiment, as described, a reticle mark signal
obtained by receiving the light reflected from the reticle and a wafer
mark signal obtained by receiving the light reflected from the wafer
without intervention of the projection lens system 3 are combined into an
alignment signal. This assures remarkably high alignment accuracy.
In the present embodiment, the light source 9 shown in FIG. 1 is preferably
of the type producing a light of a wavelength substantially equal to that
of the light emitted from the illumination system 30 for the photoprinting
purpose. However, any wavelength of light different from the photoprinting
wavelength may of course be used and, in such case, a correcting optical
system 16 (FIG. 1) may be used to correct optical aberrations caused in
relation to the scanning beam projected upon the wafer 4 by the projection
optical system. The correcting optical system 16 may be retracted, if
necessary, at the time of projection of the reticle 1 pattern (i.e. the
photoprinting). Also, the light source 9 may preferably comprise a laser.
However, any one of other types of high-luminance light sources may be
used. For example, spectral lines of an Hg lamp may be used.
Referring now to FIG. 4, another embodiment of the present invention will
be described. In the FIG. 4 embodiment, the reticle mark signal and the
wafer mark signal are detected by use of lights having different
wavelengths. For this purpose, there are provided, in addition to the
optical arrangement shown in FIG. 1, an additional light source 37 which
is adapted to produce light of a wavelength different from that of the
light emitted from the light source 9; photoelectric detectors 35 for
detecting the light emitted from the additional light source 37 and
reflected by the wafer alignment mark; and beam splitters 34 and 36. An
aberration correction optical system 16 is made movable so as to be
retractably introduced in accordance with the wavelength used. Of course,
three or more kinds of wavelengths may be used with a similar optical
arrangement. In the present embodiment, one or more wavelengths can be
used as the alignment light in accordance with the state of the wafer 4
surface. Therefore, excellent alignment signals are constantly obtainable,
independently of the wafer process conditions.
FIG. 5 shows a further embodiment wherein optical fibers 17 and 17' are
used in place of the mirrors 12a-12c and the lenses 13 of the FIG. 1
embodiment, in order to guidingly direct, to the photodetectors 14 and
14', the light diffracted by the edges of the mark formed in the region 6
of the wafer 4. This arrangement facilitates the incorporation of the
wafer detecting optical systems into the exposure apparatus as well as
reduces the manufacturing cost of the detecting optical systems.
FIGS. 6 and 7 show further embodiments of the present invention,
respectively, wherein the mark detecting position on the wafer 4 surface
is made changeable. Generally, the reticle patterns used for the
manufacture of semiconductor devices do not have the same configuration
but have different shapes and sizes. For this reason, the position of the
mark region 6 (6') is changeable with the reticle used. In order to meet
this, it will be desirable to make the mark detecting position changeable.
In the FIG. 1 embodiment, the wafer signal detecting system may be made
mechanically movable. However, each of the embodiments shown in FIGS. 6
and 7 easily assures the change of the mark detecting position.
In the FIG. 6 example, the wafer alignment mark may be formed at | | |
