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Claims  |
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We claim:
1. An exposure apparatus comprising:
first light source means;
mask plate means having an exposure pattern area section and an
alignment/reflection area section adjacent to said exposure area section,
said alignment/reflection area section including a reflection portion and
a mask alignment mark portion and disposed on that surface of said mask
plate means which does not face said first light source means, said mask
plate means being arranged so that said exposure pattern area is
illuminated by said first light source means;
movable stage means for holding thereon a workpiece having a workpiece
alignment mark;
projection lens means interposed between said mask plate means and said
movable stage means and disposed across said mask plate means from said
first light source means so that said exposure pattern area section, when
illuminated, is projected through said projection lens onto said
workpiece;
alignment means for detecting the positional relation between said mask
alignment mark portion of said alignment/reflection area section in said
mask plate means and said workpiece alignment mark of said workpiece on
said movable stage means and producing a control signal for achieving
alignment of said mask alignment mark portion with said workpiece
alignment mark, said alignment means including
(a) second light source means arranged with respect to said mask plate
means such that light from said second light source means is directed to
illuminate said workpiece alignment mark so that the resulting reflected
light from said workpiece alignment mark is directed through said
projection lens means to said alignment/reflection area section and is
reflected at its reflection portion to provide a reflection image of said
workpiece alignment mark, and
(b) a detecting means for detecting said reflection image of said workpiece
alignment mark and an image of the mask alignment mark portion of said
alignment/reflection area to produce said control signal; and
means for adjusting relative position between said mask plate means and
said workpiece, said adjusting means being adapted to be responsive to
said control signal.
2. An apparatus according to claim 1, in which the wavelength of light from
said second light source means is substantially equal to that of light
from said first light source means, and said alignment/reflection area
section has a predetermined contour.
3. An apparatus according to claim 2, in which said alignment/reflection
area section includes a grating constituting said mask alignment mark
portion.
4. An apparatus according to claim 2, in which said contour of said
alignment/reflection area section constitutes said mask alignment mark
portion.
5. An apparatus according to claim 1, in which the wavelength of light from
said second light source means is different from that of said first light
source means, and said alignment/reflection area section includes a group
of substantially hyperbolic lines for forming a diffraction image
representative of said mask alignment mark portion.
6. An apparatus according to claim 5, in which said group of hyperbolic
lines are oriented to project toward said exposure mask pattern area
section in said mask plate means.
7. An apparatus according to claim 1, in which the wavelength of light from
said second light source means is different from that of said first light
source means, said mask alignment mark portion consists of first and
second alignment sub-portions with said reflective portion being
interposed therebetween, said first and second alignment sub-portions
carrying parts of a group of hyperbolic lines for providing, when
illuminated, an image representative of said mask alignment mark portion.
8. An apparatus according to claim 1, in which said alignment means further
includes
(c) means optically coupled with said second light source means for
changing the incident angle of light illuminating said workpiece alignment
mark in a plane perpendicular to said workpiece and parallel with the
lengthwise direction of said workpiece alignment mark, and
said detecting means including means for accumulating reflected light from
said workpiece alignment mark received via said reflection portion of said
alignment/reflection area section.
9. An apparatus according to claim 1, in which said second light source
means is positioned so that light from said second light source means is
able to illuminate said workpiece alignment mark obliquely without passing
said projection lens means.
10. An apparatus according to claim 1, in which said second light source
means is positioned so that light from said second light source means is
first directed to said alignment/reflection area section to be reflected
at the reflective portion of said alignment/reflection area section and to
be directed through said projection lens means to said workpiece alignment
mark to thereby illuminate said workpiece alignment mark.
11. A method of aligning an exposure mask plate with a workpiece having a
workpiece alignment mark in an exposure apparatus in which the exposure
mask plate is illuminated by first light source means to project images of
patterns contained in an exposure pattern area section of the exposure
mask plate through projection lens means onto the workpiece, the workpiece
being held on movable stage means, the method comprising the steps of:
arranging an alignment/reflection area section adjacent to said exposure
pattern area section on that surface of said exposure mask plate which
does not face said first light source means, said alignment/reflection
area section including a reflective portion and an alignment mark portion;
illuminating said workpiece alignment mark with second light source means
so that reflected light from said workpiece alignment mark passes through
said projection lens means and is reflected at said reflective portion of
said alignment/reflection area section;
detecting the positional relation between said alignment mark portion and
said workpiece alignment mark and producing a control signal
representative of misregistration between said alignment mark portion and
said workpiece alignment mark derived from the detected positional
relation; and
adjusting the relative position between said exposure mask plate and said
workpiece in accordance with said control signal.
12. A method according to claim 11, in which during the step of
illuminating said workpiece alignment mark, the incident angle of light
illuminating said workpiece alignment mark is varied in a plane
perpendicular to said exposure mask and parallel with the lengthwise
direction of said workpiece alignment mark.
13. A method according to claim 11, in which light for illuminating said
workpiece alignment mark is caused to pass through said projection lens
means.
14. A method according to claim 11, in which light for illuminating said
workpiece alignment mark is directed to said workpiece alignment mark
without passing through said projection lens means.
15. A method of aligning the positional relation between a mask and a
workpiece through a projection lens for projecting a circuit pattern on
the mask to the workpiece by projecting light, comprising the steps of:
providing a mask alignment mark adjacent to said circuit pattern on that
surface of said mask which faces said projection lens, said mask alignment
mark including a reflection portion and an alignment mark portion, said
alignment mark portion forming a grating pattern;
forming a reflecting and deflecting image obtained by illuminating a
substantially monochromatic light to said grating pattern;
providing a workpiece alignment mark on the workpiece;
forming a workpiece alignment mark image obtained by passing the reflected
light from said workpiece alignment mark through said projection lens and
reflecting at said reflection portion of mask alignment mark;
converting said reflecting and deflecting image and workpiece alignment
mark image to the image signals by a detecting means; and
aligning the positional relation between the mask and the workpiece
according to said image signals.
16. A method according to claim 15, in which said substantially
monochromatic light has a different wavelength from said projecting light.
17. A method according to claim 15, in which said grating pattern is formed
by a group of substantially hyperbolic lines.
18. A method according to claim 15, further comprising the step of
deforming said workpiece alignment mark, the incident angle of light
illuminating said workpiece alignment mark through the projection lens is
varied in a plane perpendicular to said mask and parallel with the
lengthwise direction of said workpiece alignment mark.
19. A projection alignment apparatus for projecting the circuit pattern on
a mask to a workpiece through a projection lens comprising:
(a) a light element arranged such that light from a light source means is
directed to illuminate a workpiece alignment mark, said mask including an
alignment/reflection area formed adjacent to said circuit pattern on that
surface of said mask which faces said projection lens, said
alignment/reflection area having a reflective portion, so that light
emanating from said workpiece alignment mark in response to the
illumination of said workpiece is directed through said projection lens to
said alignment/reflection area and is reflected at said reflective portion
to provide a reflection image of said workpiece alignment mark;
(b) a detecting mens for detecting said reflection image of said workpiece
alignment mark and a reflecting and deflecting image obtained by
illuminating a substantially monochromatic light to said mask
alignment/reflection area said mask alignment reflection area including a
grating pattern to produce an alignment signal; and
(c) aligning means for aligning relative position between said mask and
workpiece in accordance with said alignment signal.
20. A projection alignment apparatus for projecting the circuit pattern on
a mask to a workpiece through a projection lens comprising:
(a) illuminating means for directing a laser beam so that the incident
angle of light illuminating a workpiece alignment mark through the
projection lens is varied in a plane perpendicular to said workpiece and
parallel with the lengthwise direction of said workpiece alignment mark;
(b) detecting means for detecting said workpiece alignment mark by a
workpiece alignment mark image signal constituted by an accumulation of
picture elements of reflected images from said workpiece alignment mark
through the projection lens and for detecting an image reflected from a
mask alignment mark to produce a mask alignment mark image signal; and
(c) means for aligning said mask and workpiece in accordance with said
image signals.
21. A projection alignment apparatus according to claim 20, in which said
accumulation is processed in accordance with the following expression
##EQU8##
where O.sub..SIGMA. (i): workpiece alignment mark image signal for i-th
column on the image plane of a reflected image, and
t.sub.o, t.sub.n ; start and end time points of a variation of the incident
angle of the illuminating light.
22. A projection alignment apparatus according to claim 20, in which each
of said reflected image 0(t,i) is formed in accordance with the following
expression
##EQU9##
where I.sub.t (i, j): light intensity of a reflected image at time t for
address (i, j) of one picture element on an image plane of the reflected
image, and
j.sub.s, j.sub.e : start and end addresses for summation on image plane. |
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Claims |
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Description |
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A circuit pattern of a semiconductor device has been further miniatuarized
year by year and a requirement for a precision for alignment of the
pattern is becoming more and more severe. When the alignment precision is
0.3 .mu.m or larger, a wafer alignment method or a global alignment method
has been used. In this method, positions at several points on a periphery
of a wafer chip are measured by an alignment optical system and a laser
distance measurement device and chip exposure positions are calculated on
a presumption that other chips are accurately aligned and the wafer is
exposed by a step and repeat method. In this method, since only several
positions on the chip need be measured, an alignment detection time is
short, an exposure time to an entire area of the wafer is short and a
throughput is high. However, in this method, because of the presumption of
the alignment of the chips, a high alignment precision is not attained and
a specification required in a future alignment is hardly met. Accordingly,
it is necessary in future to align chip by chip.
A prior art chip-by-chip alignment method is shown in FIG. 1. When a
circuit pattern of a mask or reticle 1 is to be overexposed on a chip (or
circuit pattern) 21 on a workpiece 2 such as a wafer, if alignment target
marks 22A and 22A' are arranged adjacent to the circuit pattern 21 on the
wafer, a mirror 560' of alignment detection optical systems 500 and 560'
for detecting the target marks must be penetrated into an exposure light
flux 41. In order to avoid this, the mirror must be arranged outside of
the exposure light flux as shown by 560. In this arrangement, since images
of the wafer target marks 22A and 22A' through a reduction lens 3 do not
fall into a view field of the alignment detection optical system, the
wafer 2 is to be sequentially moved to positions 21' and 21". A distance
of movement is measured by a laser distance measurement device, the target
position is measured by the alignment detection optical system, an
alignment position of the reticle and the wafer circuit pattern is
calculated based on those measurements and the wafer is moved to that
position and overexposure is effected at that position. Such an extra
wafer movement step leads to reduction of the alignment precision and the
throughput. Numerals 12 and 12' denote alignment target marks on the
reticle. In order to avoid such an extra step, the wafer circuit pattern
21 and the target marks 22 and 22' may be arranged at positions
corresponding to the circuit pattern 11 of the reticle and the target
marks 12 and 12' of FIG. 1 as shown in FIG. 2 so that the alignment is
attained at the exposure position. In this method, however, since a
distance from the circuit pattern 21 and the wafer target mark 22' must be
larger than a predetermined length (1 mm), a width W.sub.S of an area
(scribe area) between adjacent circuit patterns (chips) increases and a
yield of the chip decreases and dusts are generated in a cutting step of
the chips.
FIG. 3 shows another prior art exposure position detection measure similar
to that disclosed in U.S. Pat. No. 4,357,100 issued on Nov. 2, 1982. A
protection glass 120 is mounted on a pattern surface 110 of a mask
(reticle) 1, a light reflected by a target pattern 22 of a wafer 2 and
transmitted through a reduction lens 3 is reflected by the protection
glass 120 and further transmitted through a light splitting layer 16, and
focused, together with an optical image of the alignment target pattern
12, on a predetermined focusing plane through an enlarging/focusing lens
3', as shown more clearly in the left-hand portion of FIG. 3 representing
an enlargement of the encircled part in the right-hand portion of FIG. 3.
In this prior art method, wavelengths of the exposure light and the
alignment light are different from each other and an axial chromatic
aberration due to the wavelength difference is compensated by fold-back of
light by the protection glass and the light splitting layer. By this
arrangement, alignment at the exposure position is attained. However, this
method involves the following problems. (1) Since the axial chromatic
aberration .DELTA. l (.DELTA.l=3-10 mm) is not large enough to facilitate
mounting of the optical system, a spacing between the protection glass and
the light splitting layer is approximately .DELTA.l/2 which makes the
mounting of the detection optical system difficult. (2) In this method,
the light from the wafer target mark is reflected by the protection glass
mounted on the lower surface of the reticle. However, such a protection
glass is not protected from dusts and a pellicle layer (which is a high
molecule layer having a thickness of several microns and which does not
influence an imaging characteristic of the reduction lens) is placed on
the reticle with a spacing of 6-10 mm therefrom. The larger the spacing
from the reticle surface is, the less is the influence by the dusts.
However, if the protection glass has a thickness corresponding to such a
spacing, the imaging characteristic of the reduction lens is influenced so
that a resolution power is lowered.
On the other hand, a device as shown in FIG. 4A is commercially available.
In this device, however, since an end of a detection system interrupts an
exposure light from a circuit pattern A as shown in FIG. 4B, an image of a
pattern is scratched off if it is to be imaged in a vicinity of a wafer
target. Accordingly, the pattern cannot be imaged in the vicinity of the
target and only an area inside of a pattern B can be exposed as the
circuit pattern.
It is an object of the present invention to provide a high throughput
exposure apparatus.
It is another object of the present invention to provide an exposure
apparatus having a high alignment precision.
It is another object of the present invention to provide a method for
aligning an exposure mask with a workpiece with a high precision in an
exposure apparatus.
In accordance with one aspect of the present invention, the exposure
apparatus comprises an exposure light source, mask plate means having an
exposure pattern area section and an alignment/reflection area section
adjacent thereto, movable stage means for holding a photosensitive
workpiece having a workpiece alignment mark, projection lens means
interposed between the mask plate means and the movable stage means and
disposed across the mask plate means from the exposure light source,
alignment means for detecting the positional relation between the mask
plate means and the workpiece to produce a control signal for aligning the
mask plate means with the workpiece, and drive means for driving the
movable stage means in accordance with the control signal. The
alignment/reflection area section is disposed on that surface of the mask
plate means which does not face the exposure light source. The scattered
or reflected light from the workpiece alignment mark illuminated by the
alignment light source is transmitted through the projection lens. The
alignment/reflection area section includes a reflection portion which
reflects the light transmitted through the projection lens to provide a
reflected image of the workpiece alignment mark and a mask alignment mark
portion for providing an image representative of the mask plate means.
Based on the detected positional relation between those images, the
control signal necessary to align the mask plate means (mask alignment
mark portion) with the workpiece (workpiece alignment mark) is generated.
In accordance with another aspect of the present invention, a method for
aligning an exposure mask with a workpiece in an exposure apparatus in
which the exposure mask is illuminated by an exposure light source to
project images of patterns contained in an exposure pattern area section
of the exposure mask through projection lens means onto the workpiece held
on a movable stage and having a workpiece alignment mark, the method
comprising the steps of:
arranging an alignment/reflection area section adjacent to the exposure
pattern area section on that surface of the exposure mask plate which does
not face the exposure light source means, the alignment/reflection area
section including a reflective portion and an alignment mark portion;
illuminating the workpiece alignment mark with alignment light source means
so that scattered or reflected light from the workpiece alignment mark
passes through the projection lens means and is reflected at the
reflection portion of the alignment/reflection area section;
detecting the positional relation between the alignment mark portion and
the workpiece alignment mark and producing a control signal representative
of misregistration between the alignment mark portion and the workpiece
alignment mark based on the detected positional relation; and
adjusting the relative position between the exposure mask and the workpiece
in accordance with the control signal.
In the accompanying drawings:
FIGS. 1 to 3, 4A and 4B show prior art exposure apparatus and alignment
methods;
FIGS. 5A to 5C show an exposure apparatus and an alignment method in
accordance with one embodiment of the present invention in which an
exposure light and an alignment light have substantially the same
wavelength;
FIGS. 6A, 6B, 7A, 7B, 8A and 8B show arrangements of an
alignment/reflection area sections which may be used in the apparatus of
FIG. 5 and signal waveforms representing a mask alignment mark image and a
wafer alignment mark image;
FIG. 9 shows images foussed by a projection lens for the exposure light and
the alignment light having a different wavelength from that of the
exposure light;
FIGS. 10A and 10B show an exposure apparatus and an alignment method in
accordance with another embodiment of the present invention in which the
exposure light and the alignment light have different wavelengths;
FIGS. 11A and 11B show an arrangement of the alignment/reflection area
sections which may be used in the apparatus of FIG. 10A;
FIGS. 12A to 12C and 13 illustrate alignment in the apparatus of FIG. 10A;
FIGS. 14A, 14B, 15A and 15B show exposure apparatuses and an alignment
methods in accordance with other embodiments of the present invention;
FIG. 16 shows an arrangement of the alignment/reflection area section for
the apparatus of FIG. 5A when the alignment light having a different
wavelength from the exposure light is used;
FIGS. 17A to 17C and 18 illustrate an operation of the apparatus of FIG. 5A
when the alignment/reflection area section of FIG. 16 is used;
FIGS. 19A to 19C show an improvement of the alignment/reflection area
section of FIG. 16;
FIGS. 20A, 20B and 21 show exposure apparatuses and alignment methods in
accordance with other embodiments of the present invention in which the
exposure light and the alignment light have different wavelengths;
FIGS. 22A to 22C, 23A and 23B show an incident angle of the alignment light
to a wafer alignment mark and an incident light to a projection lens from
the wafer alignment mark;
FIG. 24 shows a mode of change of the incident angle of the alignment light
to the wafer alignment mark;
FIGS. 25 and 26A to 26C illustrate detection of alignment mark images in
the apparatus of FIGS. 20A and 20B;
FIGS. 27A to 27C show another mode of change of the incident angle of the
alignment light to the wafer alignment mark;
FIGS. 28A and 28B illustrate multiple interference of alignment light
caused by lamination of the wafer, alignment mark and resist layer;
FIG. 29 shows a relation between a resist layer thickness and the multiple
interference, with the incident angle of the alignment light being a
parameter;
FIGS. 30A to 30C show signal waveforms when the alignment mark is detected
in an ideal manner;
FIG. 31 shows waveforms of detection signals for the alignment mark image
in the apparatus of FIG. 20A; and
FIG. 32 shows an exposure apparatus and an alignment method in accordance
with another embodiment of the present invention, in which the alignment
light has substantially the same wavelength as that of the exposure light.
Referring to FIGS. 5A to 5C which show one embodiment in which the
alignment is achieved by using an exposure light and an alignment light of
the same wavelength, an exposure apparatus includes an exposure light
source 4, a reticle (mask plate means) 1, a projection lens, e.g., a
reduction lens 3, a movable stage 7 for holding a photosensitive wafer
(workpiece) 2 having alignment marks (wafer alignment marks) 22 and 22'
and means (not shown) for driving the movable stage 7. The reticle 1
includes an exposure pattern area section including an area 11 and
alignment/reflection area section including areas 30 and 30' arranged
adjacent thereto. Before the exposure pattern area 11 is illuminated by
the light source 4 and an image thereof is projected on a chip 21 of a
wafer 2 through the reduction lens 3, the alignment of the reticle 1 and
the wafer 2 is carried out.
The alignment/reflection areas 30 and 30' are arranged on that surface of
the reticle 1 which does not face the light source 4 and each of the
alignment/reflection areas 30 and 30' has a reflection portion and a mask
alignment mark portion. Since the areas 30 and 30' are identical in
construction and arrangement, only the area 30 is explained below. The
reflection portion of the area 30 serves as a mirror when an alignment
illumination light 501' is projected to the wafer alignment mark 22
adjacent to the chip 21 of the wafer 2 through a semitransparent mirror
55, an enlarging/focussing lens 52 and a mirror 51 and also serves to
reflect scattered or reflected light transmitted through the reduction
lens 3 from the wafer target mark and direct it to an alignment detection
unit 5 disposed out of an exposure light path. The reflection portion has
a grating and a O-th order light (regular reflection light) is used to
illuminate the wafer alignment mark and detect its image. As shown in FIG.
5B, elongated reticle target patterns (Cr or Cr.sub.2 O.sub.3 patterns or
clear patterns) are recorded on the area 30 in a transverse direction to
the grating to form the mask alignment mark portion. The wafer alignment
mark illumination light 501' is regularly reflected by the reflection
portion and directed to a center of entrance pupil of the reduction lens
3. When the incident light 501' to the wafer alignment marks 22 and 22' is
swung by a Galvanomirror 53' it is swung such that the illumination light
falls within the entrance pupil circle. The scattered (reflection) light
from the wafer alignment mark is focussed onto the alignment/reflection
area 30, and after it is regularly reflected, the image is refocused onto
a detection element unit (imaging element unit) 50 so that a signal shown
by a broken line in FIG. 5C is produced. On the other hand, the reticle
alignment mask portion is illuminated by a reticle target illumination
light 503 of another light path through the mirror 54. The incident angle
of the illumination light to the alignment/reflection area 30 is different
from that of the wafer illumination light and a direction of diffraction
of the light primarily diffracted by the reflection portion corresponds to
the direction of the light from the wafer alignment mark 22. Since there
is no grating in the reticle alignment mark, a signal shown by a solid
line in FIG. 5C is produced by the detection element unit 50. In order to
distinguish those signal patterns, the light paths are sequentially
switched so that the patterns are detected independently. The detected
signals are fed to the detecting and processing means 80, in which centers
of the waveforms of the detected signals are determined by known measures
such as symmetrical pattern matching technique, information on the
locations of the waveform centers are once stored in a memory at
particular addresses and the positional deviation between the detected
signals is determined from the address information. (See, for example,
U.S. Pat. No. 4,115,762 or Japanese Patent Publication No. 2284/81.) A
control signal is then produced on the basis of the determined positional
deviation and is used to drive the movable stage 7 or a reticle fine
adjusting mechanism 130 so that a high precision alignment is attained.
For the generation of the control signal, reference may be made to U.S.
Pat. No. 4,153,371 or U.S. Pat. No. 4,362,389.
FIGS. 6A and 6B show another arrangement 30-1 of the alignment/reflection
area section in the embodiment of the present invention in which the
exposure light and the alignment light have substantially the same
wavelength, and it is used as the area section 30 of the reticle shown in
FIG. 5A. The reflection portion 30-1a is mostly coated with Cr or Cr.sub.2
O.sub.3 except a reticle alignment pattern portion 30-1b which has a
grating. When the same detection unit as that in FIG. 5A is used, the
detection element unit 50 produces a signal as shown in FIG. 6B. In this
case, the reticle alignment mark signal is high only for the grated target
area.
FIGS. 7A and 7B show another arrangement 30-2 of the alignment/reflection
area section in the embodiment of the present invention in which the
exposure light and the alignment light have substantially the same
wavelength. In FIG. 7A, the reflection portion on the reticle has a
uniform grating and left and right areas thereof are clear. Again, the
same detection unit as that of FIG. 5A may be used. In the present
embodiment, a reticle signal and a wafer signal as shown in FIG. 7B are
produced.
FIGS. 8A and 8B show another arrangement 30-3 of the alignment/reflection
area section in the embodiment of the present invention in which the
exposure light and the alignment light have substantially the same
wavelength. A reflection portion 30-3a on the reticle is a uniform Cr or
Cr.sub.2 O.sub.3 surface. The reticle alignment mark detection
illumination light 503 (and the mirror 54) shown in FIG. 5A should not be
used and the wafer illumination light is also used to illuminate the
reticle. An outline or contour 30-3b of the reflection portion 30-3a forms
the reticle alignment mark portion, and a signal as shown in FIG. 8B is
produced. With this arrangement of the alignment/reflection area section,
the alignment mark signals of the wafer and the reticle are simultaneously
detected, unlike the embodiments shown in FIGS. 5A, 6A and 7A.
In the above embodiments, the wafer alignment mark illumination light is
directed to the wafer alignment mark through the reduction lens.
Alternatively, the light may be obliquely directed to the wafer alignment
mark from the above of the wafer without passing through the reduction
lens, provided that the scattered light from the wafer alignment mark is
detected through the reduction lens.
As an LSI circuit pattern is more and more miniaturized and a pattern width
is reduced to approximately 1 .mu.m, it is difficult to keep variation of
the pattern width within .+-.10%. When a resist layer is coated on an
underlying layer having a high reflection coefficient such as an Al
pattern and a pattern is exposed on such a resist layer, an interference
fringe (standing wave) having a high contrast is produced in parallel to
the resist layer by a reflection light from the underlying layer and the
incident light. If the resist exposed under this condition is developed,
fine uneveness is produced in a section of the resist layer. Thus, it is
difficult to reduce the variation of the pattern width. In recent years,
when a fine pattern is to be formed on an aluminum underlying layer, a
light-absorbing resist is used to prevent the reflection from the aluminum
surface. When such a light-absorbing resist is used, it is difficult to
align the exposure mask by using the alignment detection light having the
equal or nearly equal wavelength to that of the exposure light because no
substantial reflection light from a step of the wafer alignment mark is
obtained. In this case, it is necessary to use the alignment illumination
light having a wavelength which is longer than the wavelength of the
exposure light and which transmits the light to the light-absorbing
resist.
An embodiment in which the alignment is achieved by the alignment light
having a different wavelength from that of the exposure wavelength is now
explained.
In this embodiment, when the exposure pattern area section on the mask is
to be projected onto the wafer, the alignment/reflection area section
including a focusing pattern is arranged in adjacent to the exposure
pattern area section as will be explained later, and the focusing pattern
is illuminated by an essentially monochromatic light. The monochromatic
light used need not be a physically pure monochromatic light but a light
which can be practically considered as monochromatic may be sufficient for
the present purpose. A wavelength of the monochromatic light is longer
than the wavelength of the ordinary exposure light. The light reflected
from the focusing pattern illuminated by the monochromatic light is
diffracted and forms a condensed image. The condensed image is used as the
mask alignment mark image. On the other hand, the image of the alignment
mark on the chip of the wafer is focused in the vicinity of the mask
alignment mark by the exposure or projection lens as will be explained
later. Thus, based on those condensed images and the alignment image of
the chip, the accurate alignment is attained.
FIG. 9 shows a relation between image formation at the exposure wavelength
of a focusing lens 3 of a semiconductor exposure apparatus and image
formation at the alignment wavelength. When a wafer is at a position 2', a
point P of a reticle 1 is sharply formed on the wafer by the exposure
light (g-line) or dotted line light. However, where the alignment lights
521 and 522 (e-line) are obliquely directed to the wafer alignment mark
for aligning the wafer and the wafer is at the position 2', the wafer
alignment mark image is formed by the focusing lens 3 at a position P'
which is displaced from the position P by .DELTA.z. In order to resolve
this problem, it has been required to lower the level of the wafer from
the position 2' to the position 2 by .DELTA.z' to form the pattern of the
wafer near the point A on the mask. The distances .DELTA.z and .DELTA.z'
meet a relation of .DELTA.z=N.sup.2 .DELTA.z' where N is a magnification
of the lens, and .DELTA.z' is approximately 200 .mu.m and .DELTA.z is 20
mm when N=10. If the wafer must be moved up and down by 200 .mu.m for each
exposure, the total exposure time for the wafer will be further increased.
Referring to FIGS. 10A and 10B, a common principle to embodiments shown in
FIGS. 10A, 14A and 15A is now explained. An alignment/reflection area
section including focusing patterns 60 and 60' is formed in a scribe area
or bonding areas 60 and 60' around an exposure pattern area section 11, in
adjacent to the exposure pattern area section 11 on the mask 1. The
focusing pattern may comprise hyperbolas as shown in Fig. llA or
quasi-hyperbolas as shown in Fig. llB. The pattern comprises a number of
hyperbolas or quasi-hyperbolas drawn by shifting a hyperbola or
quasi-hyperbola having a width w in a direction x (direction of a line
connecting two focal points of the hyperbola) with a pitch p. A black or
hatched area represents a Cr surface and a white area represents a Cr-free
glass surface. The Cr surface forms the reflection portion. Further, in
FIG. 11B, the width w and the lengths l.sub.1, l.sub.2, l.sub.3, . . . of
the hatched strips are determined such that they form a group of
quasi-hyperbolas which will approximate a group of ideal hyperbolas as
much as possible that can be drawn with an electron beam lithography
apparatus. When a monochromatic light 502 of a wavelength .lambda. is
projected to this pattern at incident angle .theta..sub.0 which is near to
90.degree., a reflected and diffracted light is condensed into a slit
shape at a position 1122, as shown in FIG. 12A. On the other hand, the
alignment mark 22 (or 22'), in FIG. 10A, in the scribe area on the wafer
or the bonding area in the chip is obliquely illuminated by the e-lines
523 and 524 and the scattered light 503 is transmitted through the imaging
lens 3 and the image is focused at the position P' in FIG. 9, directed to
the alignment/reflection area section 60 along the light paths 503 and
503' of FIG. 12A, regularly reflected (without diffraction) and focused at
the position 1122. Thus, as shown in FIG. 12B, the focusing pattern is
formed such that the area 60 and the focusing position 1122 are spaced by
.DELTA.z from each other. FIG. 12C is a view looking in a normal direction
of the drawing in FIG. 12B. FIG. 13 shows the slit-shaped condensed image
(FIG. 13C) by the diffraction light from the alignment/reflection area
section formed in the vicinity of the position 1122 and the image (FIG.
13b) formed by obliquely illuminating the chip 21, transmitting the light
therefrom through the focusing lens 3 and regularly reflecting the light
by the Cr surface of the alignment/reflection area section 60. As shown in
FIGS. 13a and 13b, the image from the chip 21 includes two peaks because
of the scattered light created by the edges of the mark 22 (22'). Since a
mid-point of the two peaks is a center of the mark 22 (22'), .DELTA.x
determined in a manner shown in FIG. 13d represents a positional deviation
of the pattern. Accordingly, the deviation is detected by focusing both
images onto the alignment detector 50 shown in FIG. 10A by the mirror 53
and the enlarging focusing lens 52 through the light path 504 shown in
FIGS. 10A and 10B so that an alignment control signal corresponding to the
deviation is produced. The wafer 2 or the mask 1 is finely moved in
accordance with the control signal so that the mask is perfectly aligned
with the wafer. Generation of an alignment control signal and attainment
of the alignment may be possible in the same manner as in FIG. 5A
embodiment. Immediately after the aignment is detected, the exposure light
(e.g. g-line) is emitted from the exposure light source means 4 so that
the alignment and the exposure are carried out without a time lag and the
total wafer exposure time is reduced.
To summarize the principle described above, when the exposure pattern area
section on the mask is to be projected onto the surface of the
semiconductor wafer which is the workpiece, the alignment/reflection area
section having the focusing pattern is arranged on that surface of the
mask which does not face the exposure light source, in adjacent to the
exposure pattern area section, the image of the reflected diffraction
light created by illuminating the focusing pattern with the monochromatic
light is detected as the reticle alignment mark image, the image of the
alignment mark on the workpiece formed by the scattered light transmitted
through the exposure or projection lens is detected and the alignment of
those images is attained by driving the means for moving the mask or
wafer.
Referring again to FIGS. 10A and 10B, an embodiment will be explained. As
shown in FIG. 10B, the mask (reticle) 1 has the exposure pattern area
section including the area 11 and has the alignment/reflection area
sections including areas 60 and 60' each carrying the focusing patterns,
the alignment/reflection area sections being arranged on that surface of
the mask 1 which does not face the exposure light source 4, at a position
corresponding to the scribe area. The focusing pattern comprises
hyperbolas or quasi-hyperbolas as shown in FIG. 11A or FIG. 11B. As shown
in FIG. 12B, the hyperbolas are oriented such that they project toward the
circuit pattern. The areas 60 and 61' may be rectangles having one side of
0.5-1 mm long and they are illuminated by a parallel beam of a He-Ne laser
beam source 51 through a mirror 54 (Since the illumination and detection
systems for the area 60' are identical to those for the area 60, they are
not shown.) The He-Ne laser beam diffracted by the focusing pattern is
condensed into a slit shape at the position 1122 shown in FIG. 10B. On the
other hand, the chip 21 to be exposed is on the wafer on the movable wafer
stage 7 and under the lens and at a position slightly deviated from the
exact wafer position. The chip already has had the alignment mark formed
in the scribe area, and if this mark aligns with the focusing pattern of
the alignment/reflection area 60 of the mask, the mask pattern can be
precisely overexposed on th | | |
