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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a defect inspection apparatus and, more
particularly, to an apparatus suitably used in inspection of defects such
as foreign matter attached to the surface of a reticle or a photomask used
as an original plate, or the surface of an anti-dust film (pellicle) of
the original plate in the manufacture of, e.g., a semiconductor element in
a photolithography process.
2. Related Background Art
In the manufacture of a semiconductor element, a liquid crystal display
element, or the like in the photolithography process, an exposure
apparatus for transferring a pattern formed on a reticle or a photomask
(to be referred to as a "reticle" hereinafter) as an original plate onto a
photosensitive substrate via a projection optical system is used. When
foreign matter larger than a prescribed size becomes attached to the
pattern formation surface or to a surface opposing the pattern formation
surface of the reticle, or when a pattern on the pattern formation surface
includes a defect, a pattern formed on the photosensitive substrate
becomes defective. For this reason, before the reticle is attached to the
exposure apparatus, the presence/absence of defects including foreign
matter, the position of a defect, the size of a defect, and the like must
be inspected.
In order to prevent foreign matter from becoming directly attached to the
reticle, an anti-dust film called a pellicle is often formed on the two
surfaces (or one surface) of the reticle. Even for a reticle formed with
such pellicle, the presence/absence of defects including foreign matter,
the position of a defect, the size of a defect, and the like on the
surface of the pellicle must be inspected.
FIG. 5 shows an example of a conventional defect inspection apparatus. For
the sake of explanation, a three-dimensional XYZ coordinate system is also
illustrated in FIG. 5.
Referring to FIG. 5, a reticle 1 to be inspected is placed on a table 2
facing its pattern formation surface down. The table 2 is movable in the Y
direction by a driving device 3. The moving amount, in the Y direction, of
the table 2 is measured by a distance measuring device 4 such as a linear
encoder. A light beam L1 emitted from a laser light source 5 is converted
into a sheet-like light beam L2 expanded in the X direction via a negative
cylindrical lens 6 and a positive cylindrical lens 7. The light beam L2 is
radiated onto the surface of the reticle 1, and forms a slit-like
illumination region 8 expanding in the X direction on an upper surface 1a
of the reticle 1.
If a defect 9 such as foreign matter is present in the illumination region
8 on the upper surface 1a of the reticle 1, scattered light L3 is
generated from the defect 9 upon radiation of the light beam L2. The
scattered light L3 is focused by a light-receiving lens 10, and an image
of the defect 9 is formed on the imaging surface of a one-dimensional
image pickup element 11 such as a one-dimensional CCD. The one-dimensional
image pickup element 11 has a plurality of light-receiving pixels, and
each light-receiving pixel receives light from a predetermined position in
the X direction. The coordinate in the X direction (X coordinate value) of
the defect 9 on the reticle 1 can be determined on the basis of the
position of the light-receiving pixel, which receives the image of the
defect, of the one-dimensional image pickup element 11, and the coordinate
in the Y direction (Y coordinate value) of the defect 9 can be determined
on the basis of the distance measurement output from the distance
measuring device 4 at that time. Furthermore, since the one-dimensional
image pickup element 11 outputs a pixel output signal having a magnitude
proportional to the received light amount, the size of the defect 9 can be
roughly determined based on the magnitude of the pixel output signal.
Therefore, the inspection result can be displayed as, e.g., a table which
shows the size of the defect in correspondence with the X and Y coordinate
values of the defect, or can be displayed as a defect map on the display
screen of a CRT display.
However, in the defect inspection apparatus shown in FIG. 5, when a defect
on the upper surface (a surface opposing the formation surface of a
circuit pattern) 1a of the reticle 1 is to be inspected, the light beam L2
is transmitted through the reticle 1, and is diffracted by a fine circuit
pattern on a lower surface (the formation surface of the circuit pattern)
1b of the reticle 1. The diffracted light is focused by the
light-receiving lens 10 as if it were scattered light from a defect, and
is undesirably detected by the one-dimensional image pickup element 11.
FIG. 6 shows another example of a conventional defect inspection apparatus.
For the sake of simplicity, a three-dimensional XYZ coordinate system is
also illustrated in FIG. 6.
Referring to FIG. 6, a reticle 1 to be inspected is placed on a table (not
shown) facing its pattern formation surface down. A light beam L4 emitted
from a laser light source 12 is expanded in one direction by a negative
cylindrical lens 13 and a focusing lens 14, thus generating a sheet-like
light beam L5. The light beam L5 is obliquely radiated onto an upper
surface 1a of the reticle 1 at an angle .alpha., and forms a slit-like
illumination region 15 expanding in the X direction on the upper surface
1a of the reticle.
If a defect such as foreign matter is present in the illumination region 15
on the upper surface 1a of the reticle, scattered light is generated from
the defect upon radiation of the light beam L5. The scattered light from
the defect in the illumination region 15 is detected by a one-dimensional
image pickup element 11 via a light-receiving lens 10.
In this case, when the radiation angle .alpha. of the light beam L5 with
respect to the upper surface 1a of the reticle is set to be 5.degree. or
less, since the transmittance of the reticle 1 becomes considerably small,
most of the incident light beam L5 is reflected, and does not reach a
lower surface 1b of the reticle, on which a circuit pattern is formed.
Therefore, the diffracted light amount from the circuit pattern decreases,
and diffracted light from the circuit pattern can be prevented from being
erroneously detected as scattered light from a defect on the reticle 1.
In the above-mentioned prior art, the laser light source 5 or 12 is used as
a light source. Since the laser light source has a high luminance, even
when the slit-like illumination region 8 or 15 is formed, the light amount
per unit area (to be referred to as "illuminance" hereinafter) in the
illumination region is large, and scattered light from a very small defect
can be reliably detected.
However, a light beam emitted from the laser light source is called a
Gaussian beam, i.e., the luminance level is highest at the center of the
light beam, and is concentrically lowered toward the periphery. For this
reason, in both the illuminance distributions of the illumination regions
8 and 15 shown in FIGS. 5 and 6, the luminance level is lowered toward the
periphery.
FIG. 7A shows an illuminance distribution S(Y), in the Y direction, of a
certain section of the illumination region 8 shown in FIG. 5, and FIG. 7B
shows an illuminance distribution S(X), in the X direction, of the section
of the illumination region 8 shown in FIG. 5. As shown in FIG. 7B, the
illuminance levels at two end portions 8a and 8b in the X direction are
considerably lower than that at the central portion. The same applies to
the illuminance distribution of the illumination region 15 shown in FIG.
6.
In this case, when the moving speed of the table 2 by the driving device 3
in FIG. 5 is set to always detect scattered light based on a light beam
component corresponding to the peak of the illuminance distribution in the
Y direction shown in FIG. 7A, the light beam can always be radiated at an
almost uniform illuminance level in the Y direction of the reticle 1.
However, in the X direction of the reticle 1, since the luminance level
near the center of the reticle 1 is high, and the luminance levels at the
two end portions 8a and 8b in the X direction are low, even if the defect
size remains the same, the pixel output signal from the one-dimensional
image pickup element 11 assumes a different value depending on the
attached position, in the X direction, of the defect. Therefore, upon
estimation of the defect size from the value of the pixel output signal, a
large error may be generated depending on the attached position, in the X
direction, of the defect.
In order to eliminate such a drawback, each pixel output signal is
multiplied with the reciprocal number of an illuminance corresponding to
the position in the X direction as a predetermined correction coefficient
in accordance with the detected position, in the X direction, of the
scattered light, i.e., the address (a numerical value indicating the
position of the pixel) of the light-receiving pixel of the one-dimensional
image pickup element 11, thus obtaining an output independently of the
position in the X direction.
However, in the defect inspection apparatus shown in FIG. 6, when the
height of the reticle 1 changes even slightly, the illuminance
distribution with respect to the position in the X direction largely
changes. For this reason, even when the above-mentioned correction method
is adopted, a satisfactory correction effect cannot often be obtained.
More specifically, in the defect inspection apparatus shown in FIG. 6,
since the light beam L5 is obliquely incident on the reticle 1 at the
angle .alpha., if the height, in the Z direction, of the reticle 1,
changes by .DELTA.h, the illuminance center (the position, in the X
direction, on the reticle 1 where the maximum illuminance level is
obtained) of the illumination region 15 on the reticle 1 is decentered by
(.DELTA.h/tan.alpha.). For example, when .alpha.=5.degree., if the surface
of the reticle 1 is shifted by 1 mm in the Z direction, the illuminance
center is shifted by 1 mm/tan5.degree., i.e., 11 mm. When the illuminance
center of the illumination region 15 changes, the illuminance
distribution, in the X direction, of the illumination region 15 also
changes. In particular, the illuminance levels near the two ends, in the X
direction, of the illumination region 15 change by several fractions to
several times upon change in height of the reticle 1.
Therefore, when the height of the reticle 1 changes, and the illuminance
center of the illumination region 15 is shifted, even if the pixel output
signal from the one-dimensional image pickup element 11 is multiplied with
a predetermined correction coefficient, nonuniformity, in the X direction,
of defect detection sensitivity still remains.
The above-mentioned drawback is caused by an upward peak pattern (Gaussian
distribution) of the illuminance distribution, in the X direction, of the
illumination region 8 or 15 on the reticle 1, as shown in FIG. 7B, and can
be eliminated by setting the illuminance distribution to have an almost
uniform illuminance level independently of the position in the X
direction.
As method of obtaining a uniform illuminance distribution, the enlargement
magnifications of the lenses 6 and 7 in FIG. 5 and the lenses 13 and 14 in
FIG. 6 are increased, so that only the central portion of the Gaussian
distribution is used.
However, if a light beam is simply expanded and is radiated onto the
reticle 1, the light beam is also radiated onto portions near side
surfaces 1c and 1d of the reticle 1 and the table 2, strong scattered
light is generated from these radiated portions, and such scattered light
cannot be distinguished from that from a defect. Thus, as still another
countermeasure, the two end portions of an expanded light beam may be
shielded by a light-shielding plate.
FIG. 8 shows main part of an apparatus in which such a countermeasure is
taken in the apparatus shown in FIG. 5. Note that a three-dimensional XYZ
coordinate system is also illustrated in FIG. 8 for the sake of
explanation. Referring to FIG. 8, a light beam L1 emitted from a laser
light source 5 and having a Gaussian distribution is expanded in the X
direction by a negative cylindrical lens 6A and a positive cylindrical
lens 7A, thus forming a light beam L6. The light beam L6 is radiated onto
a light-shielding plate 16 formed with a rectangular opening 17, which is
elongated in the X direction. A light beam L7 obtained by shielding the
two end portions, in the X direction, of the light beam L6 by the
light-shielding plate 16 is radiated onto the reticle 1, and forms a
slit-like illumination region 18 expanding in the X direction on the
reticle 1. Thus, illuminance nonuniformity, in the X direction, in the
illumination region 18 on the reticle 1 can be eliminated, a light beam
can be prevented from being radiated onto portions near the side surfaces
1c and 1d of the reticle 1, and no unnecessary scattered light is
generated.
However, when the light beam L6 is limited by the light-shielding plate 16,
as shown in FIG. 8, a diffraction effect when the light beam L6 passes
through the rectangular opening 17 poses a problem. Assume that, following
the general convention, the width of the Gaussian beam is defined by a
width corresponding to a point where the illuminance becomes 13.5% of the
maximum illuminance, the width, in the X direction, of the light beam L6
is represented by .DELTA.X, and the width, in the Y direction, of the beam
L6 is represented by .DELTA.Y. Furthermore, assuming that the length, in
the X direction (the length in the longitudinal direction), of the
rectangular opening 17 of the light-shielding plate 16 is represented by
a, and the width in the Y direction is represented by b, the following
relations are satisfied:
.DELTA.X>>a (1)
.DELTA.Y<<b (2)
Therefore, no diffraction occurs in the Y direction in the opening 17, and
diffraction occurs in only the X direction. For this reason, as is
apparent from the diffraction theory, even when the width, in the X
direction, of the slit-like illumination region 18 is almost a, the
illuminance distribution S(X), in the X direction, in the illuminance
region 18 has a fine structure, as shown in FIG. 9.
As shown in FIG. 9, the illuminance distribution S(X), in the X direction,
in the illumination region 18 largely changes in a sine-wave pattern at
especially two end portions, and has valleys corresponding to extremely
low illuminance levels at positions 18a and 18b in the X direction.
Therefore, even when the reticle 1 is moved in the Y direction, the
illuminance of the light beam is always lowered at the positions 18a and
18b in FIG. 9, and defect detection sensitivity is impaired as compared to
that for other regions.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above
situation, and has as its object to provide a defect inspection apparatus
for inspecting a defect by radiating a slit-like light beam onto an object
to be inspected, and receiving scattered light from a defect on the object
to be inspected, wherein the light beam can be prevented from being
radiated onto portions other than a region to be inspected on the object
to be inspected, and a variation in defect detection sensitivity caused by
a variation in illumination distribution in the slit-like illumination
region can be avoided.
In order to achieve the above object, as shown in, e.g., FIGS. 1A and 1B,
there is provided a defect inspection apparatus according to the present
invention which comprises a light source (5) for emitting a light beam,
light beam expansion means (6A, 7A) for expanding the light beam in a
predetermined direction, and radiating the expanded light beam onto an
object (1) to be inspected, scanning means (2, 3) for scanning the light
beam to be radiated onto the object (1) to be inspected relative to the
object (1) to be inspected (i.e., relatively moving them), and
light-receiving means (10, 11) for photoelectrically converting scattered
light generated from a defect (including foreign matter) on the object (1)
to be inspected, and which inspects the defect on the basis of a
photoelectric conversion signal obtained from the light-receiving means,
wherein the apparatus further comprises light-shielding means (16) having a
plurality of edges (19a, 19b) for limiting the light beam (L6) expanded by
the light beam expansion means at two end portions, in 10 an expansion
direction (X direction) of the light beam (L6), and at least one of the
plurality of edges is formed to be transverse to the relative scanning
direction (Y direction).
As an example of the defect inspection apparatus according to the present
invention, the light-shielding means (16) may comprise an edge 19a for
limiting one end portion, in the expansion direction (X direction), of the
light beam (L6), and an edge 19b for limiting the other end portion
thereof, and the edges 19a and 19b may be formed to be substantially
parallel to each other.
According to the present invention with the above-mentioned arrangement, a
light beam (L1) emitted from the light source (5) and having a Gaussian
distribution is expanded in a predetermined direction (defined to be the X
direction) by the light beam expansion means (6A, 7A), thus forming a
light beam (L6). The light beam (L6) is radiated onto an opening (19) of
the light-shielding means (16), and its width in the X direction is
limited by the plurality of edges of the opening (19). A light beam (L8)
passing through the opening (19) is radiated onto the object (1) to be
inspected, and forms a slit-like illumination region (20) expanding in the
X direction on the object (1) to be inspected. The object (1) to be
inspected (1) can be moved by the scanning means (2, 3) in the Y direction
relative to the light beam (L8).
FIG. 1B is a view in the XY plane obtained when the light beam (L6)
incident on the opening (19) is viewed from the light source (5) side.
Referring to FIG. 1B, the left edge (19a) and the right edge (19b) of the
opening (19) are respectively inclined at almost 45.degree. with respect
to the Y axis as the scanning direction. When the light beam (L6) passes
through the opening (19), since the edges (19a, 19b) shield the light beam
(L6), a diffraction effect occurs by only these two edges, and illuminance
nonuniformity due to the diffraction effect is generated in the direction
parallel to the edges (19a, 19b). FIGS. 2A and 2B show this state.
FIGS. 2A and 2B show a simplified illuminance distribution in the
illumination region (20) formed by the light beam (L8) via the opening
(19) on the object (1) to be inspected. FIG. 2A shows, so to speak,
equi-illuminance lines obtained by connecting the same illuminance level
portions in the illumination region (20) by solid curves. As can be seen
from FIG. 2A, fringes of illuminance nonuniformity appear in the direction
parallel to the edges (19a, 19b) of the opening (19) (in the direction
crossing the Y axis at 45.degree.) due to the diffraction effect. An
illuminance distribution in the X direction obtained when the illumination
region (20) is cut along a line PP' parallel to the X axis corresponds to
a distribution SP indicated by a solid curve in FIG. 2B. Similarly, an
illuminance distribution in the X direction obtained when the illumination
region (20) is cut along a line QQ' parallel to the X axis corresponds to
a distribution SQ indicated by a broken curve in FIG. 2B. As can be
understood from a comparison between the distributions SP and SQ in FIG.
2B, an illuminance level at, e.g., a position X1 as a valley in the
distribution SP corresponds to a peak in the distribution SQ.
More specifically, when the object (1) is moved in the Y direction relative
to the light beam (L8), the illuminance level at the position X1 in the X
direction changes in accordance with the relative moving amount in the Y
direction, but always has a valley and a peak. In other words, the
illuminance level is high at some point although it is low at some other
point. Therefore, when the object (1) is scanned relative to the light
beam (L8), an almost uniform illuminance distribution can be obtained on
the surface to be inspected of the object (1) to be inspected, and defect
detection sensitivity can also be uniformed.
When the light-shielding means (16) has the edge (19a) for limiting one end
portion, in the expansion direction (X direction), of the light beam (L6),
and the edge (19b) for limiting the other end portion thereof, and the
edges (19a, 19b) are formed to be substantially parallel to each other,
the manufacture of the light-shielding means (16) is easy, and fringes of
illuminance nonuniformity caused by the edge (19a) are formed to be
parallel to those of illuminance nonuniformity caused by the edge (19b),
thus uniforming the illuminance distribution on the surface to be
inspected more effectively. Note that the defect inspection apparatus
according to the present invention is preferably designed to perform
inspection on the object (21) to be inspected on the basis of the received
light intensity distribution of scattered light, which has been detected
for a predetermined time period by light-receiving means (17 to 20), as
shown in, e.g., FIG. 10. The predetermined time period is preferably
defined to be a time required for the received light intensity
distribution of scattered light to change by almost one cycle.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a perspective view showing an embodiment of a defect inspection
apparatus according to the present invention, and FIG. 1B is a plan view
showing a light-shielding plate 16 in FIG. 1A;
FIG. 2A is a view showing a kind of equi-illuminance lines of the
illuminance distribution of the illumination region on a reticle in FIG.
1A, and FIG. 2B is a graph showing the illuminance distributions, in the X
direction, of two sections of the illuminance distribution shown in FIG.
2A;
FIG. 3A is a view showing a kind of more detailed equi-illuminance lines of
the illuminance distribution of the illumination region on the reticle in
FIG. 1A, FIG. 3B is a graph showing the illuminance distributions, in the
X direction, of two sections of the illuminance distribution shown in FIG.
3A, and FIG. 3C is a graph showing the illuminance distributions, in the Y
direction, of two sections of the illuminance distribution shown in FIG.
3A;
FIGS. 4A to 4C are plan views showing modifications of an opening formed on
the light-shielding plates according to the embodiment shown in FIG. 1A;
FIG. 5 is a perspective view showing an example of a conventional defect
inspection apparatus;
FIG. 6 is a perspective view showing another example of a conventional
defect inspection apparatus;
FIG. 7A is a graph showing the illuminance distribution, in the Y
direction, of an illumination region 8 in FIG. 5, and FIG. 7B is a graph
showing the illuminance distribution, is the X direction, of the
illumination region 8 in FIG. 5;
FIG. 8 is a perspective view showing a case wherein a light beam is
expanded, and the expanded beam is limited by a light-shielding plate in
the defect inspection apparatus shown in FIG. 6;
FIG. 9 is a graph showing the illuminance distribution, in the X direction,
of an illumination region on a reticle shown in FIG. 8;
FIG. 10 is a perspective view for explaining the arrangement of a defect
inspection apparatus according to another embodiment of the present
invention;
FIGS. 11A to 11C are views for explaining oblique diffraction fringes on a
radiation region formed by partial light-shielding according to the
embodiment shown in FIG. 10, in which FIG. 11A shows a partial
light-shielding opening, FIG. 11B shows a pattern of oblique diffraction
fringes, and FIG. 11C shows the intensity distribution of radiated light;
FIG. 12 is a graph for explaining the received light intensity distribution
in the apparatus of the embodiment shown in FIG. 10;
FIG. 13 is a view showing the geometric relationship between a band-shaped
radiation region and oblique diffraction fringes; and
FIG. 14 is a perspective view for explaining the arrangement of a defect
inspection apparatus according to still another embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of a defect inspection apparatus according to the present
invention will be described below with reference to FIGS. 1A to 4C. In
this embodiment, the shape of the opening in the light-shielding plate 16
of the apparatus shown in FIG. 8 is improved. Thus, the same reference
numerals as in FIGS. 1A and 1B denote corresponding portions in FIGS. 5
and 8, and a detailed description thereof will be omitted. For the sake of
explanation, a three-dimensional XYZ coordinate system or a
two-dimensional XY coordinate system is illustrated in the drawings.
FIG. 1A shows the arrangement of a defect inspection apparatus according to
this embodiment. Referring to FIG. 1A, a reticle 1 as an object to be
inspected is placed on a table 2, and the table 2 is moved in the Y
direction by a driving device 3. The moving amount, in the Y direction, of
the table 2 is measured by a distance measuring device 4. A light beam L1
emitted from a laser light source 5 is converted into a sheet-like light
beam L6 expanded in almost the X direction via a negative cylindrical lens
6A and a positive cylindrical lens 7A. The light beam 6A is radiated onto
a parallelogrammic opening 19 of a light-shielding plate 16. A light beam
L8 passing through the opening 19 is radiated onto the surface of the
reticle 1, and forms a slit-like illumination region 20 expanding in
almost the X direction on the surface of the reticle 1. Scattered light
from a defect including foreign matter in the illumination region 20 is
focused by a light-receiving lens 10, and an image of the defect is formed
on the imaging surface of a one-dimensional image pickup element 11.
FIG. 1B is a plan view obtained when the light-shielding plate 16 of this
embodiment is viewed from the light source 5 side. As shown in FIG. 1B, if
the four vertices of the parallelogrammic opening 19 formed on the
light-shielding plate 16 are represented by A, B, C, and D, an edge 19a
defined by the side AD is parallel to an edge 19b defined by the side BC.
The edges 19a (side AB) and 19b (side BC) cross the Y direction as the
relative scanning direction at 45.degree., and the sides DC and AB are
parallel to the X direction. The two end portions, in the X direction, of
the light beam L6 are limited by the edges 19a and 19b of the opening 19.
Since the interval between the sides AB and DC of the opening 19 is larger
than the width, in the Y direction, of the light beam L6, the light beam
L6 is not limited by the sides AB and DC of the opening 19.
Therefore, on the slit-like illumination region 20 in FIG. 1A, a
diffraction pattern (fringes of illuminance nonuniformity) is formed in
the direction parallel to the edges 19a and 19b due to the diffraction
effect of the opening 19.
FIGS. 2A and 2B show the illuminance distribution of the light beam L8 on
the illumination region 20 on the reticle 1 under the diffraction effect
of the parallelogrammic opening 19 in FIG. 1B.
FIGS. 3A to 3C show in more detail the structure of the illuminance
distribution of the light beam L8 on the illumination region 20 shown in
FIGS. 2A and 2B. FIG. 3A shows a kind of equi-illuminance lines obtained
by connecting the same illuminance level portions in the illumination
region 20 by solid curves. Illuminance distributions S(X) in the X
direction obtained when the illuminance distribution shown in FIG. 3A is
cut along lines EE' and FF' parallel to the X axis respectively correspond
to a distribution SE indicated by a solid curve in FIG. 3B and a
distribution SF indicated by a broken curve in FIG. 3B. Also, illuminance
distributions S(Y) in the Y direction obtained when the illuminance
distribution shown in FIG. 3A is cut along lines GG' and HH' parallel to
the Y axis respectively correspond to a distribution SG indicated by a
solid curve in FIG. 3C and a distribution SH indicated by a broken curve
in FIG. 3C.
In this case, when the table 2 in FIG. 1A is moved in the Y direction,
since an arbitrary point on the reticle 1 is illuminated with the
illuminance distributions SE and SF shown in FIG. 3B, the illuminance
distribution on the entire surface of the reticle 1 becomes almost uniform
when these distributions are temporally integrated. Therefore, defect
detection sensitivity on the entire surface of the reticle 1 is also
almost uniform.
Of course, when the light-shielding plate 16 having the opening 19 is
applied to the conventional apparatus having an oblique incidence system
shown in FIG. 6, a change in detection sensitivity becomes considerably
small even when the height of the reticle 1 changes.
As indicated by an envelope 21 in the illuminance distribution shown in
FIG. 3B, even when a laser beam having a Gaussian distribution is expanded
and is radiated via the opening, illuminance nonuniformity inevitably
remains to some extent.
In this embodiment, illuminance nonuniformity in the X direction, which
still remains, as indicated by the envelope 21 in FIG. 3B, is corrected by
multiplying pixel output signals from the one-dimensional image pickup
element 11 in FIG. 1A with different correction coefficients in
correspondence with the addresses of light-receiving pixels. More
specifically, the reciprocal number of an illuminance level corresponding
to the address of each light-receiving pixel of the one-dimensional image
pickup element 11, which illuminance level is obtained from the envelope
21 shown in FIG. 3B, can be used as a gain of an amplifier for receiving
each pixel output signal.
The moving speed upon movement of the reticle 1 in the Y direction can be
set, so that a peak portion of the illuminance distribution in the
illumination region 20 is always irradiated upon detection of the
intensity of scattered light at an arbitrary position, in the X direction,
on the reticle 1.
More specifically, since the illuminance distribution of the light beam in
the illumination region 20 has a periodically two-dimensional distribution
in an oblique direction, as shown in FIG. 3A, a width .DELTA.y.sub.G, in
the Y direction, of the distribution SG shown in FIG. 3C, and a width
.DELTA.y.sub.H, in the Y direction, of the distribution SH (each of these
widths is smaller than .DELTA.y) shown in FIG. 3C are assumed to be beam
widths in the Y direction. Under this assumption, when the reticle 1 is
fed in the Y direction by a step amount of min(.DELTA.y.sub.g (X))/4 or
less or min(.DELTA.y.sub.H (X))/4 or less (min(.DELTA.y.sub.G (X)) or
min(.DELTA.y.sub.H (X)) is the minimum value of the width .DELTA.y.sub.G
or .DELTA.y.sub.H at an arbitrary position, in the X direction, on the
reticle 1), the peak portion of the illuminance distribution in the
illumination region 20 is radiated upon detection of the intensity of
scattered light at the arbitrary position, in the X direction, on the
reticle 1.
More specifically, in the defect inspection apparatus using the
light-shielding plate 16 on which the deformed opening 19 is formed, like
in this embodiment, the two-dimensional illuminance nonuniformity on the
reticle 1 in the X and Y directions is determined by the shape of the
opening in the light-shielding plate 16 and the moving speed (the relative
speed in the Y direction between the reticle 1 and the light beam).
As for the processing of a pixel output signal from the one-dimensional
image pickup element 11 in FIG. 1A, since the illuminance level of a light
beam on a defect changes upon movement of the reticle 1 in the Y
direction, and the value of a pixel output signal corresponding to the
defect also changes, the maximum value of the pixel output signal is
extracted. More specifically, when the value of the pixel output signal
from the one-dimensional image pickup element 11 obtained when the
illuminance distribution indicated by the line FF' in FIG. 3A is located
on the defect is larger that obtained when the illuminance distribution
indicated by the line EE' in FIG. 3A is located on the defect, the larger
value, the address of the light-receiving element of the one-dimensional
image pickup element 11 at that time, and the distance measurement output
from the distance measuring device 4 are stored as data in a storage unit
such as a memory.
Modifications of the opening formed in the light-shielding plate 16 in FIG.
1B will be described below with reference to FIGS. 4A to 4C.
FIG. 4A shows a case wherein a hexagonal opening 22 is formed in the
light-shielding plate 16. Referring to FIG. 4A, the light beam L6 emerging
from the cylindrical lens 7A in FIG. 1A is radiated onto the opening 22.
Two left edges 22a and 22b, in the X direction, of the opening 22 cross
the Y direction at almost 45.degree., and are perpendicular to each other.
Similarly, two right edges 22c and 22d of the opening 22 cross the Y
direction at almost 45.degree., and are perpendicular to each other. These
edges 22a, 22b, 22c, and 22d limit the width, in the X direction, of the
light beam L6. The light beam L6 is diffracted by these oblique edges 22a,
22b, 22c, and 22d, and is radiated onto the reticle as light having a
two-dimensional illuminance distribution in a checkerboard pattern.
Therefore, when the reticle is moved in the Y direction, illuminance
nonuniformity of the light beam in the X direction can be eliminated, as
in the case of the opening shown in FIG. 1B.
FIG. 4B shows a case wherein an opening 23 having two arcuated end portions
is formed in the light-shielding plate 16. Referring to FIG. 4B, the two
end portions, in the X direction, of the opening 23 respectively have
arcuated edges 23a and 23b. Therefore, the light beam L6 is diffracted by
these arcuated edges 23a and 23b, and is radiated onto the reticle as
light having a two-dimensional illuminance distribution in a concentric
pattern. Therefore, when the reticle is moved in the Y direction,
illuminance nonuniformity of the light beam in the X direction can be
eliminated, as in the case of the opening shown in FIG. 1B.
FIG. 4C shows a case wherein an isosceles-triangular opening 24 is formed
in the light-shielding plate 16. Referring to FIG. 4C, the light beam L6
emerging from the cylindrical lens 7A in FIG. 1A is radiated onto the
opening 24. A left edge 24a and a right edge 24b, in the X direction, of
the opening 24 cross the Y direction at almost 45.degree., and are
line-symmetrical with each other about the Y direction. These edges 24a
and 24b limit the width, in the X direction, of the li | | |
