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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for detecting
patterns such as reticles and masks used for the fabrication of
semiconductor devices having a high degree of integration such as LSI's.
More specifically, the invention relates to a method and an apparatus for
detecting abnormal patterns suited for discriminating and detecting tiny
foreign matter (inclusive of pattern defects) adhered on the reticles and
masks from the pattern edges.
2. Description of the Prior Art
The prior art will now be described with reference to the case of detecting
tiny abnormal patterns on the reticles in the process for fabricating
LSI's which is a representative technology to which the present invention
is adapted.
In the exposure step used for the fabrication of LSI's, a chrome pattern on
a thick plate called a reticle is printed by baking onto a semiconductor
wafer. When foreign matter and defects exist on the reticle in this step,
the pattern is not correctly printed onto the semiconductor wafer, and all
of the LSI chips become defective. It is therefore essential to inspect
for foreign matter and defects prior to the exposure from the standpoint
of controlling the quality of reticles.
In recent years, smaller foreign matter is imposing another problem as the
wiring patterns become more fine accompanying the increase in the
integration degree of LSI's. In preparing the reticles, furthermore, the
resist remains, chromium or chromium oxide for pattern formation remains
after etching, and impurities dissolved in the reticle wash liquid
coagulate at the time of wash and dry, imposing a problem of formation of
foreign contaminant film which is on the increase.
A conventional apparatus for inspecting foreign matter and defects
consists, as disclosed, for example, in Japanese Patent Laid-Open No.
65428 /1984, of means for illuminating a substrate by a laser beam from a
tilted direction, a first lens which is provided above the substrate so
that an illuminating point of the laser beam and a plane of focal point
are nearly in agreement with each other, and which focuses the scattering
light of a laser beam, a shutter which is provided on a Fourier
transformed plane of the first lens and which shuts off the regularly
scattered light from the substrate pattern, a second lens which subjects
the scattered light from foreign matter obtained through the shutter to
the inverse Fourier transformation, a slit which is provided at the
imaging point of the second lens to shut off the scattered light from the
areas other than the laser beam illuminating point on the substrate, and a
light receiver which receives the scattering light from foreign matter
that has passed through the slit.
According to the above apparatus, attention is given to the fact that a
pattern is generally constituted in the same direction or by a combination
of two to three directions in a field. The light diffracted by the pattern
is removed by a space filter disposed on the Fourier transformed plane in
order to emphasize and detect only the light that is reflected by foreign
matter.
There has further been proposed a method for detecting defects by comparing
the data of an inspected reticle detected by using an optical system with
the data of a standard reticle detected using said system or with design
data, as disclosed, for example, in Japanese Patent Laid-Open No.
139278/1983.
According to Japanese Patent Laid-Open No. 65428/1984 that pertains to the
prior art as described above, the light reflected by foreign matter is
split by the shutter from the light reflected by the pattern, the light
reflected by foreign matter is detected by the slit, and the foreign
matter is detected by the binary method contributing to simplifying the
detecting mechanism. However, the foreign matter is detected by an
indirect illumination, i.e., illumination by laser beam from an upper
tilted direction, which is different from the traditional exposure system.
Namely, only the light reflected by the chrome pattern of a particular
angle is shut off, and foreign matter is not discriminated relying upon
the whole chrome pattern.
When foreign matter is detected by the indirect means as described above,
even foreign matter having no real problem (hereinafter referred to as lie
detecting) is detected. In particular, when the foreign matter increases
with the decrease in the size of the patterns, the foreign matter that has
problems increases, too, though the problems may not yet be regarded as
real problems. Therefore, the number of lie detections increases, and an
increased amount of work is required for checking, analyzing and removing
foreign matter, detrimental to operation efficiency.
Next, Japanese Patent Laid-Open No. 139728/1983 that also pertains to the
aforementioned prior art has an optical system similar to the exposure
system, making it possible to simply constitute the optical system
compared with that of the aforementioned prior art. However, the image
signal processing system for comparing the data is complex compared with
that of the aforementioned prior art and requires an extended period of
time for inspection.
Furthermore, a variety of devices have been developed for detecting foreign
matter adhered onto the reticles and masks. According to the prior art
disclosed in Japanese Patent Laid-Open No. 65428/1984, the reticle is
directly illuminated by a polarized laser at a predetermined angle of
incidence, and the foreign matter is discriminated by utilizing the fact
that the foreign matter has a direction of polarization in the reflected
light different from those of the reticle substrate and the pattern.
According to the prior art disclosed in Japanese Patent Laid-Open No.
101390/1984, attention is given to the fact that the pattern edge on the
reticle is generally in the same direction or consists of a combination of
two to three directions in the field. Further, the diffracted light caused
by the pattern edge is removed by a space filter disposed on the Fourier
transformed plane, to emphasize and detect only the light that is
reflected by the foreign matter.
According to the prior apparatus disclosed in Japanese Patent Laid-Open No.
186324/1984, attention is given to the fact that the scattered light
caused by the pattern edge has directivity but the scattered light caused
by foreign matter has no directivity, and the foreign matter is
discriminated relying upon the logical product of light quantities
received by the light-receiving elements installed at a plurality of
places.
According to the prior apparatuses disclosed in Japanese Patent Laid-Open
Nos. 154634/1985 and 154635/1985, the foreign matter is discriminated by
arranging a plurality of detectors utilizing the phenomenon in that the
diffracted light caused by the pattern edge is focused in a predetermined
direction only whereas the scattered light caused by foreign matter is
scattered in all directions.
In the manufacture of semiconductor devices such as LSI's, a pattern on a
master plate called reticle is printed by baking onto a semiconductor
wafer in the step of exposure. In this case, if foreign matter exists on
the reticle, the pattern is not correctly printed and all of the chips
become defective. It is therefore necessary to detect foreign matter prior
to effecting the baking from the standpoint of controlling the quality of
reticles.
In producing the reticles and masks, however, the residue of resist, the
remnants of chromium or chromium oxide for pattern formation after the
etching, and impurities that are dissolved in the reticle wash liquid and
that coagulate at the time of washing and drying the reticle, adhere onto
the reticle. However, such foreign matters were so tiny and formed a thin
contaminant film that there seldom arouse any problem thus far. As the
technology for highly densely integrating the LSI's advances and as the
wiring patterns become more fine, however, the presence of foreign matter
of the order of submicrons becomes a serious problem though that was not
so far regarded as a problem.
According to the conventional apparatus disclosed in Japanese Patent
Laid-Open No. 65428/1979, however, the light reflected by tiny foreign
particles and thin contaminant films is so weak compared with the light
reflected by the pattern edge that it is not possible to distinguish the
pattern edge over the tiny foreign particles and thin contaminant films.
The light reflected by the tiny foreign particles can be emphasized
utilizing the conventional technology disclosed in Japanese Patent
Laid-Open No. 101390/1984. However, limitation is imposed on the light
reflected by the pattern edge that can be erased, and it is impossible to
remove all of the light reflected by the pattern edge using the same space
filter.
According to any other prior technology, it is not allowed to distinguish
tiny foreign particles of smaller than 1 .mu.m and thin contaminant films
over the pattern edges. This is because, the light is illuminated over a
wide area according to the conventional art, and the sum of strength of
the scattered light caused by the pattern edge so increases that the
signals of scattered light caused by tiny foreign matter become
undistinguishable. Even if the numerical aperture of the lens is increased
in order to avoid such adverse effects, it is not allowed to distinguish
the pattern edge over the foreign matter with the conventional apparatus.
This is because, if the numerical aperture of the lens is great, the light
is illuminated from many directions whereby the light scattered by the
pattern edge loses directivity making it difficult to distinguish the
light over the light scattered by foreign matter. That is, limitation is
imposed on reducing the illuminated region based upon the method which
illuminates one point.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a method and an
apparatus for detecting abnormal patterns, which are free from the
problems inherent in the aforementioned prior art, and which separate and
detect only those foreign matters that have problem from a chrome pattern
that exists maintaining a given angle.
A second object of the present invention is to provide a method and an
apparatus for detecting abnormal patterns, which are capable of
discriminating even those tiny foreign matters and thin contaminant films
of the order of submicrons.
To achieve these objects, the present invention deals with a system for
detecting abnormal patterns in a surface pattern on the surface of a
sample, wherein when the abnormal pattern has a portion smaller than the
surface pattern, the light for illuminating the sample is stopped down to
an opening for distinguishing the abnormal pattern over the surface
pattern, the surface pattern is limited by the illumination light to a
predetermined opening to form an image on a detector, and the surface
pattern detected by the detector is processed to detect the abnormal
pattern in the surface pattern.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a foreign matter inspecting apparatus
according to an embodiment of the present invention;
FIG. 2 is a diagram for explaining the scattering and diffraction caused by
foreign matter on the sample and by defects in the pattern;
FIG. 3 is a diagram of output curves of a detector which detects foreign
matter on the sample and defects in the pattern shown in FIG. 2;
FIG. 4 is a diagram which shows the output signals shown in FIG. 3 in a
binary form;
FIG. 5 is a diagram illustrating the structure of an apparatus for
discriminating a pattern and foreign matter according to a second
embodiment of the present invention;
FIG. 6 is a diagram illustrating the structure of a stage according to a
different embodiment;
FIG. 7 is a section view along the line III--III of FIG. ;
FIG. 8 is a view illustrating on an enlarged scale the surface of a sample
that is to be inspected;
FIG. 9 is a section view along the line V--V of FIG. 8;
FIG. 10 is a diagram for explaining the diffraction phenomenon of light;
FIGS. 11 and 12 are a plan view and a side view of FIG. 10, respectively;
FIG. 13 is a graph that illustrates a relationship between the pattern edge
angle and the intensity of the detected signal;
FIG. 14 is a diagram illustrating a relationship between the positions of
four laser diodes and the pattern edge directions detected by the light
emitted therefrom according to the second embodiment; and
FIGS. 15 and 16 are diagrams illustrating the structure of detect signal
separation means of the apparatus for discriminating patterns and foreign
matter according to the second embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The action that corresponds to the first object of the present invention
will now be described with reference to detecting tiny foreign matter and
defects on the surface pattern on the reticle.
The present invention is based on the discovery that the flux of light that
contributes to imaging undergoes the diffraction and scattering due to
foreign matter and defects; i.e., based on the discovery that defective
printing is caused by foreign matter and defects.
Generally, the numerical aperture (hereinafter referred to as N.A.) of the
incident side (object side) of a reduction projection lens has been
selected so as to provide sufficiently large resolving power for imaging a
pattern on the reticle. Therefore, the flux of light that contributes to
imaging the pattern passes through an opening on the incident side of the
reduction projection lens, but the flux of light that passes through the
outside of the opening does not contribute to imaging the pattern. When
there exists tiny foreign matter, the flux of light scattered and
diffracted by the foreign matter passes from the incident side N.A. to the
outside of the reduction projection lens to impair the imaging of the
pattern.
This point can be comprehended from the description of, for example, "Wave
Optics", written by Hiroshi Kubota, pp. 387-389, "Response Function of an
Optical System having Space Filter". That is, the above literature
describes that when a disc-like space filter is set on a Fourier
transformed plane of an imaging optical system and when the inside of, for
example, a lens of a space frequency determined by the diameter of the
disc-like space filter is covered by a circle having a diameter d', only
the pattern having particular space frequency determined by the radius d'
is not resolved. Therefore, this description can also be adapted to the
technology for detecting foreign matter only by utilizing the difference
in space frequency between the pattern and the foreign matter or, in other
words, by utilizing the difference in size between the pattern and the
foreign matter.
According to the present invention based upon the above-mentioned
principle, use is made of an illumination system equivalent to the
illumination system employed in the exposure system as well as an object
lens having an N.A. larger than the N.A. of the reduction projection lens.
As for the flux of light incident on the object lens, furthermore, the
same region as the N.A. of the incident side of the rejection projection
lens is shut off, i.e., the diffracted light is shut off by the shutter,
in order to receive only the light scattered by foreign matter.
According to the present invention, therefore, only the flux of light that
passes through an opening of the object lens is selectively detected, the
flux of light being scattered and diffracted by foreign matter and defects
and passing through the outside of opening of the incident side of the
reduction projection lens. Therefore, those foreign matters having problem
are enhanced and detected. And thus it is possible to distinguish those
foreign matters among the patterns only by binarizing the detected signal.
And even when comparing data of inspected reticle with those of standard,
by the art as are disclosed e.g. in the Japanese Patent Laid-Open No.
139278/1983, S/N ratio of the signal of the foreign matters increases,
since said signal of the foreign matters are enhanced.
The foregoing and other objects, advantages, manner of operation and novel
features of the present invention will be understood from the following
detailed description when read in connection with the accompanying
drawing.
[First Embodiment]
A first embodiment of the present invention will now be described in
conjunction with FIGS. 1 to 4.
The foreign matter inspection apparatus according to the present invention
consists, as shown in FIG. 1, of a sample stage system 1, a transmitting
illumination sytem 2, a falling illumination sytem 3, a detection system
4, and a processing system 5.
The sample stage system 1 consists of a Zstage 9 which fastens thereto a
reticle 6 having a pericle 7 by a fastening means 8 to scan it in the
Z-direction, an X-stage 10 which scans the reticle 6 in the X-direction
via the Z-stage 9, a Y-stage 11 which scans the reticle 6 in the
Y-direction via the X-stage 10 and Z-stage 9, a stage drive system 12 for
driving the stages 9, 10 and 11, a focal point position detect system 13
which detects the position of the reticle 6 in the Z-direction, and a
processing system 14 for driving the stage drive system 12 in response to
an instruction from the focal point position detect system 13. The sample
stage system 1 accomplishes the focusing maintaining a required minimum
precision while the reticle 6 is being inspected.
The X-stage 10 is designed to perform a periodical motion consisting of a
time of uniform acceleration of about 0.1 second, a uniform motion of 0.1
second and a time of uniform deceleration of 0.1 second at a one-half
period, at a maximum speed of about 1 mm/sec and an amplitude of 200 mm.
The Y-stage 11 is designed to move the reticle 6 in the Y-direction
stepwisely by 0.15 mm each time in synchronism with the time of uniform
acceleration and the time of uniform deceleration of the X-stage 10. If
the recticle 6 is moved 670 times during the one time of inspection, the
movement of 100 mm is accomplished in about 130 seconds. Namely, a square
region having a side of 100 mm can be scanned in about 130 seconds.
The X- and Y-stages 10 and 11 are put into practice in this embodiment.
However, the invention is in no way limited thereto only, but it is also
allowable to use an X.theta.-stage for effecting the scanning in the
rotational direction and in the X-direction. Further, the aforementioned
running speed is only an example which may be set to any value depending
upon the requirement.
Further, the aforementioned focal point position detect system 13 may
employ an air micrometer, or may detect the position relying upon the
laser interference method, or may project a fringe pattern to detect the
contrast.
The transmitting illumination system 2 is so constructed that a g-line
(wavelength, 436 mm) or an i-line (wavelength, 365 mm) to be used in the
exposure system (not diagrammed) is selected by a dichroic filter 22 from
the flux of light emitted from a mercury lamp 21, the selected line is
focused on a diffuser 24 by a focusing lens 23, the light diffused by the
diffuser 24 is emitted through a portion that is limited by a circular
opening to enter into a collimator lens 26, so that the reticle 6 is
illumianted.
The opening 25 is positioned nearly at the position of focal point of the
collimator lens 26, so that an image is formed at a position of focal
point 46 indicated by a chain line above the collimator lens 26 and the
object lens 41 of the detection system 4.
To achieve the above-mentioned object of the present invention,
furthermore, it is necessary not only to set the wavelength of the
illuminating light to be equal to the wavelength of the illuminating light
used for the exposure system but also to maintain constant the angle
.theta. of the flux of light incident on a point 15 on the reticle 6.
Here, sin .theta. is defined to be "spatial coherency".
In the exposure system, furthermore, the whole area on the reticle 1 must
be uniformly illuminated. For this purpose, therefore, use is made of an
optical element called integrator which consists of a set of rod-like
lenses instead of using the diffuser 24. The function of the integrator is
basically the same as that of the diffuser 24. The range of inspection to
which the present invention is applied is from several hundred of microns
to 1.2 mm of the reticle 6. Therefore, the above-mentioned diffuser 24
suffices for the need.
Furthermore, since the angle of incidence .theta. of the flux of light
incident on the reticle 6 is determined by the size of the integrator,
i.e., by the diameter of opening 25 located at the back of the diffuser
24, the size of the opening 25 is so selected that the spatial coherency
becomes the same as that of illumination employed for the exposure system
that uses the reticle 1.
In the exposure system, the position of the integrator is not necessarily
set to the position of focal point of the collimator lens 26, and the
position of the opening 25 needs not necessarily be set to the position of
focal point of the collimator lens 26.
To maintain the angle of incidence .theta. of the flux of light at a given
position in a range illuminated with light on the reticle 6, however, it
is desired that the opening 25 is located at the position of focal point
of the collimator lens 26. This makes it possible to establish the same
condition of illumination by the flux of light in a measuring range to
detect foreign matter under the same condition.
The falling illumination system 3 is so constructed that the light emitted
from a mercury lamp 31 is permitted to pass through a dichroic filter 32,
a focusing lens 33, a diffuser 34 and an opening 35, and through a relay
lens 36 to illuminate the reticle 6 via a half mirror 42 and the object
lens 41 in the detection system.
The object lens 41 has the same function as the collimator lens 26 in the
transmitting illumination system 2.
The relay lens 36 is provided to form an apparent opening at the position
of focal point 46 over the object lens 41. Concretely speaking, a real
image of the opening 35 is formed at the position of focal point 46.
Even in the falling illumination system 3, the opening 35 is so determined
that the wavelength of the illuminating light and the angle .theta. of the
light flux incident on a given point 15 on the reticle 6 will become the
same as those of the illumination light used in the exposure system like
in the aforementioned transmitting illumination system 2.
Furthermore, the falling illumination system 3 is provided to detect
foreign matter on the chrome pattern on the reticle 6, but needs not be
provided when there is no need of detecting foreign matter on the chrome
pattern.
Moreover, when the falling illumination system 3 is to be used
simultaneously with the transmitting illumination system 2, the signal
from the edge of the pattern increases. When this becomes a problem,
therefore, they must be separately used.
The wavelength of the illumination light needs not necessarily be limited
to g-line and i-line only but may lie over a wide band that includes
g-line and i-line. This is because, the diffraction condition of light
differs over the whole wavelengths between the foreign matter and the
pattern, and the foreign matter can be detected being distinguished over
the pattern even with the light of a wide band.
The detection system 4 comprises an object lens 41, a half-mirror 42, a
field lens 43, a shutter 44 and an imaging lens 45, so that the image at a
point 15 of inspection on the reticle 6 is formed on a detector 51 through
the object lens 41 and the imaging lens 45. The detection system further
includes a field lens 43 near a position where the image is formed by the
object lens 41. The field lens 43 forms the focal point position 46 on the
upper side of the object lens 41 on the circular shutter 44. That is, the
image of the opening 25 of the transmitting illumination system 2 passes
through the collimator lens 26 and the object lens 41, reflected by the
reticle 6, passes through the object lens 41 again, passes through the
field lens 46, and is formed on the shutter 44. Here, the shutter 44 is
located at a Fourier transformed position of the reticle 6 with respect to
the position of the source of light, i.e., with respect to the position of
the opening 25.
Generally, the N.A. of the reduction projection lens of the exposure system
on the side of the reticle 6 has been set to be greater by 10 to 40% than
the spatial coherency of the illumination system of the exposure system
(which is equal to the spatial coherency of the transmitting illumination
system 2). In many cases, the N.A. has been set to be greater by about
10%.
The flux of light that passes through the outside of the opening on the
incident side of the reduction projection lens, must be caused to pass
through the opening. For this purpose, the N.A. of the object lens 41 is
set to be greater than the N.A. of the reduction projection lens.
Furthermore, the shutter 44 is provided to shut off the flux of light that
is incident on the N.A. of the reduction projection lens.
In order to achieve the object of the present invention, therefore, it is
desired to calculate the diameter dm of the shutter 44 according to the
following equation (1), i.e.,
##EQU1##
where d.sub.s denotes the diameter of the opening 25, .alpha. denotes the
magnification of the imaging system consisting of opening 25 and shutter
44, N.A. denotes a value of the reduction projection lens on the side of
reticle 6, and sin .theta..sub.s denotes the spatial coherency of the
exposure system.
It was mentioned already that the conditions for detecting the foreign
matter can be maintained constant if .theta.=.theta..sub.s. Symbol .delta.
denotes a margin which, the experiment proves, may be several percent.
When all of the regions on the reticle 6 are to be simultaneously
inspected, the object lens 41 becomes so bulky that that manufacture of it
involves great difficulty. According to the present invention, therefore,
the inspection region is limited on the reticle 6 which is scanned by the
sample stage system 1 so that all of the regions can be inspected. It is
therefore allowed to use the object lens 41 having N.A. greater than that
of the reduction production lens which is usually used.
To detect foreign matter inclusive of those foreign matter that may not
pose problem, the shutter 44 needs not necessarily be brought into
conformity with the N.A. of the reduction projection lens of the incident
side but may be brought into conformity with the spatial coherency of the
transmitting illumination system 2 and the falling illumination system 3.
That is, what is needed is to shut off the 0-th order diffraction light of
illumination light from the transmitting illumination system 2 and the
falling illumination system 3. Further, the size thereof may be greater
than the above size. Concretely speaking, N.A./sin .theta. is set to be 1
in the aforementioned equation (1), and .delta. is set to any value which
is several percent or greater.
The spatial coherency of the illumination light needs not necessarily be
brought into agreement with the coherency of the exposure system, but the
sizes of the openings 25, 35 and the shutter 44 may be so determined that
the .theta.-th order diffraction light is shut off by the shutter 44,
i.e., that the aforementioned equation (1) is satisfied.
When the falling illumination system 3 is not provided, none of the
half-mirror 42, field lens 43 and imaging lens 45 are needed, and the
shutter 44 should be installed at the position of focal point 46 and the
detector 51 should be installed at the position where the field lens 43
was located, to obtain the effects of the present invention. In this case,
the optical system features a very simple construction.
The signal processing system 5 consists of detector 51, binary processing
circuit 52, microcomputer 53 and CRT 54.
The detector 51 is comprised of, for example, a one-dimensional solid
imaging device of the charge coupled type to detect signals while scanning
on the X-stage 10. That is, when there exists foreign matter on the
reticle 6, the level and the intensity of light increase and the output of
the detector 51 increases.
The detector 51 needs not necessarily be comprised of the one-dimensional
solid imaging element but may be comprised of the two-dimensional element,
or of a single element.
The binary processing circuit 52 sets a binary threshold value in advance
to determined the presence or absence of foreign matter.
The microcomputer 53 evaluates in advance the function of foreign matter
that poses problem, since whether the printing is inhibited by the foreign
matter or not is determined by the function of the strength of the
scattered light caused by the foreign matter and the size of the foreign
matter. The microcomputer 53 further determines the presence or absence of
foreign matter that poses problem relying upon the evaluated function, and
sends the result to the CRT 54.
The apparatus for inspecting foreign matter according to the present
invention is constructed as described above. The method of inspection will
now be described in conjunction with FIGS. 2 to 4.
FIG. 2 illustrates the case where there exist a pattern 17, two foreign
matters 18a, 18b and a defect 19 on a glass substrate 16. The smaller
foreign matter 18a is so tiny that it causes the light to be more
scattered or diffracted than by the edge 17a of the pattern 17. That is,
the flux of light 56 scattered on the outside of the range .theta. shut
off by the shutter 44 becomes greater than the flux of light 55 scattered
by the edge 17a of the pattern 17.
Moreover, since the surrounding space frequency is high, the fluxes of
light 57 and 58 scattered by the larger foreign matter 18b and the defect
19 of the pattern 17 on the outside of the range .theta. shut off by the
shutter 44 become greater than the flux of light 55 scattered by the edge
17a of the pattern 17.
Therefore, the outputs of the detector 51 generate output peaks 59, 60, 61
and 62 due to the fluxes of light 55, 56, 57 and 58 as shown in FIG. 3.
Furthermore, a threshold value 63 is set by the binary processing circuit
52 as shown in FIG. 3. In this case, the three output peaks 60, 61 and 62
are greater than the threshold value 63, from which it is made possible to
detect two foreign matters 18a, 18b and the defect 19 of pattern 17.
The coordinates of the X- and Y-stages 10 and 11, and the levels of the
output peaks 60, 61 are stored in a memory that is controlled by the
microcomputer 53, and the stored contents are processed and are produced
to the CRT 54.
According to the present invention, use is made of illumination which is
optically equivalent to the illumination of the exposure system, and the
light is selectively detected which is scattered and diffracted by foreign
matter and defects and which is no more incident on the reduction
projection lens of the exposure system. Therefore, only those foreign
matters that impose problem are detected being distinguished over the
pattern.
Furthermore, the inspection region is limited on the reticle, and the
reticle is scanned to inspect all of the regions. Therefore, there can be
used an object lens having N.A. which is greater than that of the
reduction projection lens that is usually used.
Moreover, the structure of the illumination system can be simplified, and
employment of the binary processing circuit helps simplify the structure
of the detection system.
[Second Embodiment]
The second embodiment comprises means for illuminating a plurality of light
rays of good directivity on a point on the surface of a sample maintaining
a predetermined angle of incidence from different directions, an optical
detection system having a predetermined numerical perture for forming the
image of light reflected by the sample, a light detector installed at the
position where the image is formed, separation means which is illuminated
by the plurality of said illumination means and which separate the light
reflected by the surface of the sample or separate the detect signals of
said light detector so that they are corresponded to the light rays, and
means for processing the detect signals that are obtained through the
separation.
In the above-mentioned optical detection system in which the angle of
incidence of the plurality of light rays is all i and the number of light
rays is m, it is recommended that lenses having the above-mentioned
numerical aperture N.A. are arranged for the positions where the sources
of illumination of the number m are arranged so as to satisfy the
following relation,
N.A.<sin i.multidot.sin (.pi./2m) (2)
Next, action of the second embodiment that corresponds to the second object
of the present invention will be described.
According to this embodiment, the light having high directivity reflected
by the pattern edge is almost all removed by a spatial filter defined by
the plural sources of illumination and reflected light detect lenses that
are arranged to satisfy the above relation (2). Moreover, since a laser
beam having high directivity is used as illumination light, the region
being illuminated increases and whereby it is allowed to use a lens of a
large numerical aperture N.A. Therefore, the region to be detected can be
squeezed to a tiny region. Concretely speaking, the region to be detected
can be squeezed to a square region each side measuring from 0.5 .mu.m to 3
.mu.m, that is suited for detecting foreign matter of 0.1 .mu.m in size.
Moreover, the embodim | | |
