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BACKGROUND OF THE INVENTION
This invention relates to an apparatus and a method for position detection
of high accuracy, to be used, for example, in relative alignment between a
photomask and a wafer in the manufacture of semiconductor circuit devices,
or in absolute alignment of a wafer in a wafer inspecting device.
The relative alignment is disclosed in U.S. Pat. No. 4,167,677, U.S. Pat.
No. 4,199,219 or the like. According to these disclosures, a photomask and
a wafer, for the manufacture of semiconductor circuits, are provided with
alignment mark patterns, respectively, the photomask and wafer being
disposed with a minute spacing or a projection optical system intervened
therebetween. The mark patterns of the photomask and the wafer are scanned
by a laser beam having spot-like or bar-like cross-section to detect the
relative position of the mark patterns to thereby measure the degree of
misalignment between the mask and the wafer.
FIG. 1 shows the relation between an automatic alignment mark pattern
(which will hereinafter be referred to simply as "AA pattern") formed on a
wafer and an automatic alignment signal (which will hereinafter be
referred to simply as "AA signal") obtainable therefrom by the laser beam
scanning and photoelectric conversion. In FIG. 1, a part (A) shows in an
enlarged cross-sectional view the AA pattern having edges 1 and 2. When
the wafer surface is scanned by a laser beam in the direction parallel to
the sheet of the drawing, the laser beam is diffracted and scattered by
the edges 1 and 2 of the AA pattern. The diffracted and scattered light
beams are received by photodetecting means, not shown in this Figure, so
that they are converted into an electric signal having a waveform such as
shown at a part (B) of FIG. 1. Pulses 1a and 2a of the waveform correspond
to the edges 1 and 2 of the AA pattern. These pulses are cut at a
predetermined threshold V and, by binarization, they are reformed into
pulses 1b and 2b shown at a part (C) of FIG. 1. The thus reformed signal
is compared with a similar signal obtained in respect to a mask (not
shown) or is compared with a reference established in the photodetecting
means, whereby an error signal for achieving the alignment is produced.
After completion of the alignment, an exposure step is effected wherein the
mask is illuminated by exposure light so that a photoresist material
applied onto the wafer is exposed to an actual element pattern (circuit
pattern) of the mask.
As is well known in the art, the photoresist layer of transparent material
has already been applied onto the wafer prior to the above-described
position detection of the wafer. It has been found that the presence of
such transparent material layer substantially influences the detection of
the AA pattern through the transparent material layer. More specifically,
it has been found that, when the wafer is coated with a photoresist layer,
the width of the pulse corresponding to the edge of the AA pattern becomes
greater than that obtainable from the same edge of the AA pattern when no
photoresist layer is formed on the wafer. This phenomenon degrades the
signal detecting accuracy.
FIG. 2 illustrates the principle of such phenomenon. In FIG. 2, a part (A)
shows the edge 1 of the AA pattern formed by a concavity defined in the
wafer surface. The wafer surface is coated with a photoresist layer 3 so
that, according to the direction of inclination of the stepped portion 1
of the AA pattern, the surface of the photoresist 3 is inclined downwardly
in the rightward direction as viewed in FIG. 2. According to the known
dark-field laser scanning technique, such as disclosed in the
above-described U.S. Patents, the scanning laser beam is perpendicularly
incident on the wafer surface. If the portion of the wafer surface on
which the beam is incident comprises a mirror surface, the incident light
is specularly reflected so that it goes back along the oncoming path such
as denoted by the hatched region 4. If on the other hand, the laser beam
is incident on the edge of the AA pattern, the laser beam is scattered and
diffracted by the edge so that the diffracted rays trace regions 5 and 6
as well as the region 4. At a pupil plane of the alignment optical system
(alignment scope), the reflected light from the wafer is subjected to the
spatial frequency filtering whereby the direct reflection light, tracing
the region 4, which has been specularly reflected from the wafer (i.e. the
reflected light which has not diffracted or scattered by the edge of the
AA pattern) is intercepted; whereas the scattered and diffracted light
tracing the regions 5 and 6 is transmitted so that it is directed to a
photoelectric detector (not shown in this drawing). In this manner, the
known dark-field scanning technique selects only the scattered light and
directs it to the detector to obtain an AA signal.
Now, the influence of the presence of the photoresist coating on the
detection of the edge of the AA pattern will be considered. In the part
(A) of FIG. 2, the laser beams 7 and 8 are not directed to the edge of the
AA pattern. Since, however, the surface of the photoresist layer is
inclined as described in the foregoing, the laser beams 7 and 8 are
deflected by the prism action at the resist surface so that the reflected
rays of the laser beams 7 and 8 will travel in a region such as 5 or 6
along which the scattered light from the edge of the AA pattern travels.
Since the reflected rays of the laser beams 7 and 8 tracing the region 5
or 6 will also be detected by the photodetector, the resulting AA signal
contains a pulse of greater width such as shown in a part (C) of FIG. 2 as
compared with the AA signal shown in a part (B) of FIG. 2 which is
obtainable from the same edge 1 when no resist layer is formed on the
wafer surface. As will be easily understood, the increase in the pulse
width varies depending on the inclination or the like of the resist
coating. The variation of the pulse width of the signal corresponding to
each edge of the AA patterns is a serious factor of error in the position
detection of the edge.
In order to avoid this, it may be contemplated that the resist material on
the part of the AA pattern is removed. However, this requires an
additional step for removing the resist on the AA pattern.
SUMMARY OF THE INVENTION
It is accordingly a primary object of the present invention to provide new
and useful apparatus and method for position detection which are free from
the above-described problems and which ensure correct detection of a
pattern formed on an object, for the purpose of the detection of the
object, or a pattern similar to the former.
A second object of the present invention is to provide new and useful
apparatus and method for position detection which are effective to prevent
degradation of detection signals which may otherwise be caused by the
presence of a transparent layer on an object.
A third object of the present invention is to provide new and useful
apparatus and method for position detection which are effective to
receive, in a best mode, the information light from a detection pattern of
a predetermined formation.
In order to achieve these objects, the present invention in one aspect
thereof provides an apparatus for detecting the position of an object
having a detection pattern formed thereon by a concavity or convexity,
comprising means for scanning a light beam and the object relative to each
other, means for computing the position of the detection pattern, said
computing means having means for receiving the light emitted from the
object and containing information on the detection pattern, and means for
excluding an unwanted component of the information light to prevent the
unwanted component from participating in the computation by the computing
means.
Further, in accordance with one embodiment of the present invention which
will be described later, there is provided an apparatus for position
detection comprising a scanning system for scanning a detection pattern
formed on an object, along a scanning line with a scanning beam of
spot-like or bar-like cross-section, a light receiving system for
receiving the light reflected from the detection pattern and containing
the information on the detection pattern, and a control system for
controlling the light receiving system so that an unwanted portion of the
information light is excluded in accordance with whether the detection
pattern comprises a concavity or convexity formed on the object and/or in
accordance with the direction, with respect to the scanning line, of the
edge of the stepped portion defined at the boundary between the detection
pattern and the other area of the object. The control system is arranged
to select, after photoelectric conversion of the information light, only
the wanted signal component, or is adapted to prevent the unwanted portion
of the information light from being incident on a detector of the light
receiving system.
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 illustrates the relation between an alignment mark having edges and
an alignment signal corresponding to the alignment mark;
FIG. 2 illustrates the influence of a photoresist layer at one of the edge
portions of the alignment mark upon the travel of the scanning beams and
upon the alignment signal;
FIG. 3 illustrates the influence of the photoresist layer at another edge
portion of the alignment mark upon the travel of the scanning beam;
FIG. 4 is an optical cross-section showing the major part of a position
detecting apparatus according to one embodiment of the present invention;
FIG. 5 is an enlarged plan view showing an example of an automatic
alignment pattern;
FIG. 6 is a view showing, in an enlarged scale, cross-sections of the
automatic alignment patterns formed by concavities and convexities;
FIG. 7 is a plan view showing a filter element according to the present
invention;
FIGS. 8 and 9 are views showing signal waveforms corresponding to the
automatic alignment patterns of FIG. 6 and obtained through the use of the
filter element of FIG. 7;
FIGS. 10A-10F are views showing various forms of the light receiving
sections according to the present invention, wherein FIGS. 10A, 10B, 10D
and 10F are cross-sectional views while FIGS. 10C and 10E are plan views;
FIGS. 10G and 10H illustrate the operation of shutter means, respectively;
FIG. 11 shows a modified form of the position detecting apparatus according
to the present invention;
FIG. 12 illustrates the manner of combining the signals produced from the
detecting system of FIG. 11;
FIG. 13 is an optical cross-section showing a position detecting apparatus
according to another embodiment of the present invention; and
FIG. 14 is a plan view showing an example of the arrangement of the
automatic alignment patterns.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring back to FIG. 2, the influence of the photoresist layer will now
be further considered. With regard to the scanning beams 7 and 8, the
influence of the photoresist layer 3 can be analyzed as follows: First,
the photoresist layer 3 allows transmission of a transmissively reflected
light beam (direct reflection light), such as denoted at 7a or 8a, through
the photoresist layer 3, which is reflected by the wafer surface and again
transmitted through the photoresist layer. Second, the photoresist layer 3
reflects surface reflection light, such as denoted at 7b or 8b, on the
surface of the photoresist layer. As is well known in the art, the
reflection factor at the interface between the photoresist and the air is
very small, of the order of 4%. Therefore, the surface reflection light
can be disregarded, so that the reflection light of the incident laser
beam 7 or 8 can be considered as being composed substantially only of the
transmissively reflected component 7a or 8a.
On the other hand, the laser beam incident on the edge 1 is scattered and
diffracted thereby and the scattered rays are directed to an expanded
region such as shown in FIG. 2, central ray being denoted by a reference
numeral 100. According to the known detecting technique as described in
the foregoing, the scattered rays from the edge 1 travelling in both the
regions such as 5 and 6 in FIG. 2 are taken up for the sake of the
detection of the edge.
It will be therefore understood that what causes the increase in the width
of the pulse as has been discussed with reference to the part (C) of FIG.
2 is chiefly the transmissively reflected components 7a and 8a. These
transmissively reflected components 7a and 8a travel in the direction of
the region 5. In view of this, the rays travelling in the region 6 are
selectively detected while the rays travelling in the region 5 are blocked
according to the present invention, whereby an alignment signal accurately
corresponding to the edge can be produced with a good S/N ratio.
While FIG. 2 shows a downwardly stepped portion in the rightward direction
as viewed in the figure, FIG. 3 shows another edge 2 which is defined by
an upwardly stepped portion in the rightward direction as viewed in the
figure. In FIG. 3, a transmissively reflected component 9a of a laser beam
9 which is the major factor of the increase in the pulse width travels in
the upward and rightward direction, i.e. in the direction of the region
6'. In this case, only the scattered and diffracted rays travelling in the
direction of the region 5' are selectively detected, as compared with the
FIG. 2 case, whereby an alignment signal accurately corresponding to the
edge 2 can be produced.
FIG. 4 shows an example of the optical arrangement for achieving the
selective detection of the rays. In FIG. 4, a scanning optical system
generally designated by a reference character AS is of a known type
comprising a laser beam source and an optical scanner. The source is not
limited to the laser beam source, and a light source supplying visible or
invisible light may be employed. The optical scanner may be a polygonal
mirror, galvano mirror, acousto-optic element or the like. A wafer W has
an AA pattern formed by a concavity defining left-hand and right-hand
edges 1 and 2. In a case where the wafer W is to be relatively aligned
with a mask, the latter is disposed in a plane which is slightly spaced
apart from the wafer W. Since the manner of detection of the mask is
substantially the same as in the conventional technique, illustration and
description thereof are omitted herein for the sake of simplicity.
The detecting system of FIG. 4 further includes a microscopic objective
lens of telecentric type, a condenser lens 11, a half mirror 16 for
dividing out a light receiving path from the projection optical path, a
stop D disposed in a focal plane of the objective lens 11 and a spatial
frequency filter 12 having transmitting sections 13, 14 and a
non-transmitting section 15. The filter 12 will be described later in
detail.
For the purpose of illustration, FIG. 4 shows the laser beam at two points
during the scanning period, the laser beams at these points being depicted
by hatched areas. When, during scanning of the wafer W coated with the
resist material 3, the laser beam is incident on the edges 1 and 2, the
rays diffracted by the edges 1 and 2 and travelling in the regions 5, 6,
5' and 6' pass through the transmitting sections 13 and 14 of the spatial
frequency filter 12. The specularly reflected rays from the wafer W or the
rays travelling in the regions 4 and 4' are blocked by the
non-transmitting section 15 of the filter 12. As is shown in this Figure,
the filter 12 is disposed in the Fourier transform plane (pupil plane)
with respect to the scan surface. Therefore, those of the reflected rays
from the surface scanned as having the same angular component (for
example, the diffracted rays 5 and 5'; the diffracted rays 6 and 6') pass
through the same section of the filter 12. In the case of the FIG. 4
arrangement, the diffracted rays 5 and 5' pass through the transmitting
section 13 while the diffracted rays 6 and 6' pass through the
transmitting section 14. As has already been discussed with reference to
FIGS. 2 and 3, the transmissively reflected rays from the stepped portions
caused by the presence of the photoresist layer 3 are mixedly included in
the diffracted rays 5 (with respect to the stepped portion 1) or in the
diffracted rays 6' (with respect to the stepped portion 2). In view of
this, only the rays transmitted through the transmitting section 14 are
taken up as the signal for the step 1 while only the rays transmitted
through the transmitting section 13 are detected as the signal for the
step 2, according to the present invention, whereby the edges can be
detected very accurately.
FIG. 5 shows an example of an AA pattern. The AA pattern is composed of
mark elements 17 and 18 which are inclined relative to the scanning line
SL at angles of 45 degrees and -45 degrees, respectively. The laser beam
having a spot-like or bar-like cross-section scans the AA pattern along
the scanning line SL. During this scanning, the laser beam is diffracted
at four edges 3, 32, 33 and 34 in the directions of arrows 21, 21', 22,
22', 23, 23', 24 and 24'. Since the AA pattern shown in FIG. 5 is inclined
relative to the scanning line, the diffracted rays are directed inclinedly
relative to the scanning line, i.e. in the directions perpendicular to the
direction of each edge.
As has been described with reference to FIGS. 2 and 3, the direction of
inclination of the resist surface differs depending on whether the AA
pattern is formed by concavities or convexities. This leads to that the
direction of the transmissively reflected component, which has been
refracted by the resist layer, from the wafer surface in the vicinity of
the edge also differs depending on whether the AA pattern is defined by
concavities or convexities. This is illustrated in FIG. 6. In FIG. 6, a
part (A) shows the AA pattern of FIG. 5 as being provided by concavities,
while a part (B) shows the AA pattern of FIG. 5 as being formed by
convexities. Arrows in FIG. 6 show the directions of travel of the
transmissively reflected rays (direct reflection rays) which have been
refracted by the resist layer. For example, in the part (A) the
transmissively reflected component at the left-hand edge of the left-hand
concavity are directed in the direction of arrow 21, while, in the part
(B) the transmissively reflected component at the left-hand edge of the
left-hand convexity is directed in the direction of arrow 21'.
It will be understood from the foregoing that, once during designing of the
AA pattern whether the AA pattern is to be made by concavities or
convexities is determined, the direction of advance of the transmissively
reflected component at each edge, that is, the region into which the
unwanted component is mixed can be forcasted.
FIG. 7 is a plan view showing the filter 12 disposed in the pupil plane.
The filter 12 has four transmitting sections 13a, 13b, 14a and 14b for
transmitting therethrough the diffracted rays as denoted by parenthesized
numerals, respectively, the numerals being corresponding to those in FIG.
5.
In this embodiment, the transmitting sections 13a, 14a, 13b and 14b of the
filter 12 are selectively used to selectively detect the diffracted rays,
in order to prevent the transmissively reflected component from being
taken up. More specifically, in a case where the AA pattern is formed by
concavities such as shown in the part (A) of FIG. 6, the transmitting
sections of the filter 12 are selectively used in the sequence of 13b,
13a, 14b and 14a in order to sequentially take up the diffracted rays
passing through the corresponding transmitting sections and to intercept
the rays as designated by the arrows in the part (A) of FIG. 6, whereby a
correct AA signal can be produced. If, on the other hand, the AA pattern
is formed by convexities such as shown in the part (B) of FIG. 6, the
transmitting sections of the filter 12 is sequentially used in the order
of 13a, 13b, 14a and 14b to sequentially take up the diffracted rays
passing through the corresponding transmitting sections, whereby a correct
AA signal can be produced. In this manner, the rays passing through the
pupil plane are selectively detected in accordance with the shape
(concavity or convexity) of the AA pattern, whereby a correct AA signal is
produced.
The selection of the rays can be achieved either by the provision of
separate photoreceptors for the respective transmitting sections wherein
the signals from the photoreceptors are selectively used, or by the
provision of a single photoreceptor wherein the rays passing through the
transmitting sections are selectively directed to the photoreceptor. When
separate photoreceptors are to be used, they may be provided by discrete
elements or may be provided by a four-division detector.
FIG. 8 shows the waveforms of the signals obtained through the use of the
filter shown in FIG. 7 in a case where the AA pattern of FIG. 5 is formed
by concavities, while FIG. 9 shows the waveforms of the signals in a case
where the AA pattern of FIG. 5 is provided by convexities. In FIG. 8, the
waveform (A) corresponds to the signal produced by the rays transmitted
through the transmitting section 13a; the waveform (B) corresponds to the
signal produced by the rays transmitted through the transmitting section
14a; the waveform (C) corresponds to the signal produced by the rays
transmitted through the transmittig section 13b; and the waveform (D)
corresponds to the signal produced by the rays transmitted through the
transmitting section 14b. Similarly in FIG. 9, the waveform (A)
corresponds to the signal from the transmitting section 13a; the waveform
(B) corresponds to the signal from the transmitting section 14a; the
waveform (C) corresponds to the signal from the transmitting section 13b;
and the waveform (D) corresponds to the signal from the transmitting
section 14b.
In the waveforms (A) and (C) of FIG. 8, the diffracted rays at the edges of
the left-hand mark element 17 of FIG. 5 are taken up as the pulses P21 and
P22; P21' and P22'. Since the direction of diffraction by the AA pattern
is determined according to the direction of edge, no ray is detected
through the transmitting sections 14a and 14b (FIG. 7). Subsequently, the
diffracted rays from the edges of the right-hand mark element 18 are taken
up through the transmitting sections 14a and 14b. This is shown in the
waveforms (B) and (D) of FIG. 8 as the pulses P23' and P24'; P23 and P24.
Among these pulse signals, the pulses P21', P22, P23 and P24' are combined
into a pulse train such as shown in the waveform (E) of FIG. 8, on the
basis of which, the computation is effected to produce an AA signal. In a
case where the AA pattern is formed by convexities, the transmissively
reflected components are eliminated in a similar manner as in FIG. 8 case,
and a combined signal including corresponding pulses p21, P22', P23' and
P24 can be obtained. This is self-explanatorily illustrated in FIG. 9. On
the basis of the combined signal thus obtained, the computation is
effected to produce an AA signal.
FIGS. 10A-10H show various forms of the element or elements to be disposed
downstream of the condenser lens 11 shown in FIG. 4. FIG. 10A shows prisms
35 and 36 for optically connecting the transmitting sections 13a and 14a
of the filter 12 to separate optical paths, respectively, on which
condenser lenses 37 and 38 are disposed respectively to direct the rays to
separate photoreceptors 39 and 40. In the FIG. 10A embodiment, similar two
prisms, two condensing lenses and two photoreceptors are additionally
disposed in a plane orthogonal to the sheet of the drawing, whereby all of
the four transmitting sections of the filter 12 are optically connected to
the respective detection systems.
FIG. 10B shows a four-division detector (a unit detector comprising four
independently operable detecting or sensitive regions) which is disposed
just behind the filter 12. While only two detecting regions 43a and 43b
are shown in FIG. 10B, the detector includes, as is best shown in FIG.
10C, four detecting regions 43a, 44a, 43b and 44b which cover the four
transmitting sections 13a, 14a, 13b and 14b, respectively.
If the four-division detector is modified so that each sensitive region has
a shape such as that of each transmitting section of the filter 12 shown
in FIG. 7, the filter can be omitted. FIGS. 10D and 10E show such modified
form of four-division detector 45, wherein FIG. 10D is a side view and
FIG. 10E is a front view, the sensitive regions being denoted by reference
numerals 45a-45d.
FIG. 10F shows a single-photoreceptor arrangement. This modification
employs a shutter element 46 such as a liquid crystal shutter or a
mechanical shutter to selectively open/close the transmitting sections of
the filter 12. The shutter 46 is disposed just behind the filter 12, and
the light beam passed through the shutter element is directed by a
condensing lens 47 to a photoreceptor 48.
FIG. 10G shows the operation of the shutter element 46. During the laser
beam scanning and immediately after the detection of signals of the edges,
portions of the shutter element 46 are alternately opened or made
transmissible to permit alternate or sequential transmission of the rays
from the transmitting sections 13a, 14a, 13b and 14b of the filter 12
(FIG. 7). With this arrangement, the same selecting function as of the
foregoing embodiments is achieved. If, for example, the transmitting
sections of the filter 12 are to be opened in the sequence of 13b, 13a,
14b and 14a, the sections of the liquid crystal shutter are sequentially
made transmissible, as shown in FIG. 10G, in synchronism with the beam
scanning. If the scanning speed is relatively high as compared with the
shutter releasing operation, only one section of the shutter element may
be opened or made transmissible per one scanning operation, so that all
the pulse signals are taken up through four scanning operations. In such
case, the signal waveforms such as (A)-(D) shown in FIG. 8 or 9 are
sequentially memorized and, after all the four signals are detected, they
are combined into a signal waveform such as (E) shown in FIG. 8 or 9.
In a case where four pulse signals are detected independently from each
other, the sequence of the selection of the transmitting sections is not
essential. Only the interrelation between a particular edge and an
associated one of the transmitting sections is necessary. Further, the
alternation of the transmitting sections is not required to be made so
quickly. Accordingly, the shutter element may be formed by a disk with a
notch which is rotated to block the unnecessary transmitting sections,
such as shown in FIG. 10H.
In the foregoing, the invention has been described with reference to the
arrangements for use with a very fine automatic alignment pattern and for
achieving alignment with a tolerance of the order of microns or
submicrons. If, however, a slightly larger error is allowable,
satisfactory signal detection can be achieved without employing precise
filtering at the pupil plane. This will now be described with reference to
FIG. 11.
FIG. 11 shows a detection system having a polygonal mirror PM, a scanning
lens system L and photoreceptors P1 and P2. Each of the photoreceptors P1
and P2 is disposed so that it directly receives the diffracted rays.
Reference characters E1 and E2 designate edges of an automatic alignment
pattern. In operation, the polygonal mirror is rotated to scanningly
deflect the laser beam. When the laser beam is incident on each edge, it
is diffracted and the diffracted rays travel in determined directions. In
the FIG. 11 case, each edge of the AA pattern extends orthogonally to the
sheet of the drawing and, therefore, two photoreceptors are disposed along
the scanning direction. If, on the other hand, the AA pattern is formed by
two oppositely inclined pattern elements such as shown in FIG. 5, four
photoreceptors are disposed inclinedly relative to the scanning line.
Signal waveforms obtained by the scanning of the edges E1 and E2 in this
order are shown in FIG. 12. In FIG. 12, the waveforms shown at a part (B)
are those when no photoresist layer is formed while the waveforms shown at
a part (C) are those when a photoresist layer is formed. In the part (B)
the signal produced by the photoreceptor P1 contains a pulse E1 and a
pulse E2 which is smaller than the pulse E1 and the signal detected by the
photoreceptor P2 contains a pulse E1 and a pulse E2 which is greater than
the pulse E1.
However, in the part (C) which shows signals when a photoresist layer is
formed, the transmissively reflected rays (direct reflection rays) which
have been refracted by the resist layer enter into the photoreceptor P2
with respect to the edge E1, and enter into the photoreceptor P1 with
respect to the edge E2. Therefore, the pulse E2 would be expanded in the
signal P1, while the pulse E1 would be expanded in the signal P2. In view
of this, the output from the photoreceptor P1 is taken up with respect to
the edge E1 while the output from the photoreceptor P2 is taken up with
respect to the edge E2 in accordance with the present invention, to
provide a combined detection signal having a waveform shown in a part (D)
of FIG. 12. In this manner, correct detection is achieved.
FIG. 13 shows an alignment and exposure apparatus of stepper type for
manufacturing semiconductor circuit elements, according to one aspect of
the present invention. The apparatus includes a projection lens 52 for
projecting the image of a mask 50 onto a wafer 51 at a one-to-one
magnification or a reduced scale. In a case where the alignment light and
the exposure light have different wavelengths, a quarter wave plate 52a is
detachably disposed in the lens system during the alignment operation
which is replaced by a lens 52b during exposure. The lens 52b compensates
for any focus deviation (defocus) due to the difference in the wavelength.
The quarter wave plate 52a is provided for the purpose of separation of
the mask reflection and the wafer reflection according to the direction of
polarization. Where the alignment light and the exposure light have the
same wavelength, or where the projection lens is corrected relative to
both the two wavelengths, the lens 52b can be omitted and the quarter wave
plate 52a may be fixedly disposed.
On each of the mask 50 and the wafer 51, there are provided two AA
patterns, one of which is shown in FIG. 14. For example, the mark elements
depicted by solid lines are provided on the wafer, while the mark element
depicted by a broken line is formed on the mask.
The alignment and exposure apparatus shown in FIG. 13 further includes a
laser beam source 53 which provides a laser beam linearly polarized in the
direction orthogonal to the sheet of the drawing; a polygonal mirror 54
rotatable at a constant speed; an f-.theta. lens 55 which co-operates in
the constant speed scanning of the laser beam; an observation system 56; a
beam splitter 57; and a scanning field dividing prism 58 by which the
laser beam is directed to one of the two AA patterns during the first half
of one scanning operation and is directed to the other AA pattern during
the second half of the one scanning operation. As shown in FIG. 13, the
left-hand and right-hand systems following the scanning field dividing
prism 58 are symmetrical, so that correspsonding elements are designated
by the same reference numerals. For the sake of simplicity of description,
only the right-hand system will now be described. The system comprises a
polarization beam splitter 59 for reflecting/transmitting the laser beam
in accordance with the state of polarization thereof; a reflector 60 for
deflecting the optical path; a condenser lens 61; a filter 62 which has an
arrangement such as shown in FIG. 7; and a division-type detector 63. The
transmitting sections of the filter 62 are arranged in accordance with the
direction of the AA pattern. The sensitive regions of the four-division
detector 63 are arranged in compliance with the transmitting sections of
the filter 62 to receive the rays from the wafer 51. The system further
includes a half mirror 64 having a small reflection factor; a polarization
beam splitter 65; a condenser lens 66; a light source 67 for the
observation; a relay lens 68; a reflector 69; a spatial frequency filter
70; a condenser lens 71; a photoreceptor 72 for receiving the light from
the mask 50; and a microscopic objective lens 73 disposed to view the AA
patterns on the mask 50 and wafer 51.
In operation, the laser beam supplied from the laser beam source 53 is
incident on the polygonal mirror 54 whereat it is scanningly deflected.
The deflected laser beam is converted by the f-.theta. lens 55 into a
parallel-scan beam, and it passes through the beam splitter 57 and is
incident on the prism 58. The laser beam is reflected by the left-hand
inclined surface, for example, of the prism 58 during the first half of
one scanning operation, and is reflected by the right-hand inclined
surface of the prism during the second half of the scanning operation. The
laser beam reflected from the prism 58 is again reflected by the
polarization beam splitter 59 and transmitted through the half mirror 64.
Subsequently, the laser beam enters into the objective lens 73 by which it
is focused onto the mask 50 and then is focused onto the wafer 51 through
the projection lens 52, to thereby scan both the mask and wafer. The light
beam reflected by the AA pattern on the mask 50 enters into the objective
lens 73 and is reflected by the half mirror 64. At the same time, a part
of the reflected beam from the mask 50 passes through the half mirror 64
so that it is directed to the four-division detector 63 for the wafer
detection. Since, however, this beam has been linearly polarized in the
direction orthogonal to the sheet of the drawing, it is intercepted by the
polarization beam splitter 59. The light beam reflected by the half mirror
64 enters into the polarization beam splitter 65 at which the light beam
reflected by the mask 50 and linearly polarized in the direction
perpendicular to the sheet of the drawing is reflected, whereas any noise
(the light beam reflected by the wafer 51 and linearly polarized in the
direction parallel to the sheet of the drawing, as will be described
hereinafter) is blocked. The reflected light beam from the polarization
beam splitter 65 is incident on the filter 70 by way of the relay lens 68
and the reflector 69. At the filter 70, the transmissively reflected
component (direct reflection component) is intercepted while the scattered
component from the AA pattern is transmitted therethrough so that it is
condensed by the condensing lens 71 to be incident on the photoreceptor
72, whereby an AA signal for the mask is detected.
The scanning beam transmitted through the mask 50 enters into the
projection lens 52 so that it is refractively transmitted therethrough.
During transmission through the projection lens 52, the scanning beam is
converted into a circularly polarized beam by the quarter wave plate 52a.
Then, the scanning beam scans the wafer 51. The light beam reflected by
the AA pattern of the wafer 51 again passes through the quarter wave plate
52a in the opposite direction, so that it is converted into a linearly
polarized light with the phase thereof rotated through 90 degrees.
Subsequently, the reflected laser beam enters into the polarization beam
splitter 59 by way of the half mirror 64. Since the beam is linearly
polarized in the direction parallel to the sheet of the drawing by the
quarter wave plate 52a, the reflection light from the wafer 51 passes
through the polarization beam splitter 59 and through the transmitting
sections of the filter 62 by way of the reflector 60 and the condensing
lens 61 so that it is incident on the four-division detector 63.
The alignment and exposure apparatus further includes a control processing
unit 80 for selecting the output signals from the four-division detector
63 in accordance with the shape (concavity or convexity) of the AA pattern
and on the basis of the time schedule of scanning operation, to establish
an AA signal with respect to the wafer 51. This can be achieved either by
sequentially operating the sensitive regions of the four-division detector
63 according to the rules as has been described in the foregoing; by
memorizing all the output signals from all of the sensitive regions of the
detector 63 and thereafter selecting the signals; or by a combination of
these processes. In any event, on the basis of the AA signal thus obtained
and the AA signal with reference to the mask detected by the photoreceptor
72, computation is effected in the unit 80. In accordance with the results
of computation (deviation in the directions x, y and .theta.), a
correcting mechanism 81 of the exposure apparatus is driven to move a mask
chuck 82 to achieve alignment between the mask 50 and wafer 51. The wafer
may, of course, be moved in place of the mask 50.
The arrangements of the AA pattern and the filter are not limited to the
above-described ones. For example, if the number of the transmitting
sections of the filter is twice as of the non-parallel pattern elements
constituting the AA pattern, it is possible to have a variety of shapes.
Further, if it is not necessary to severely restrict the S/N characteristic
of the signal, a desired signal can be obtained without employing
four-division of the signal even for the AA pattern shown in FIG. 5. For
example, the signals (A) and (B) shown in FIG. 8 may b | | |
