Defect detection system and method for pattern to be inspected utilizing multiple-focus image signals

A defect detection system and method for a pattern to be inspected wherein multiple-focus images of the pattern to be inspected are obtained and a defect on the pattern to be inspected is detected utilizing the multiple-focus images.

BACKGROUND OF THE INVENTION 
The present invention relates to a defect detection method and the device 
for detecting a defect such as a foreign matter defect in the form of a 
contaminant, a discoloration defect, a deformation or pattern defect, etc. 
for LSI wafer patterns, etc. 
Among conventional visual inspection systems, for example, the one 
described in Technical Journal, Vol. 87, No. 132 (1987), pp. 31 to 38, The 
Institute of Electronics and Communication Engineers of Japan is well 
known. Such a system is shown in FIG. 23 where a circuit pattern on a 
wafer 1 lighted with a lamp 2 is enlarged and detected with an image 
sensor 4 through an object lens 3, whereby a variable density image such 
as a gray scale image of the circuit pattern is obtained. The detected 
variable density image is compared with the image of the preceding chip 7a 
(adjacent chip) stored in an image memory 5 in a defect judgment circuit 6 
for performing defect judgment. The detected image is simultaneously 
stored in the image memory 5 to be used for comparison with the next chip 
7b. 
An example of defect judgment is shown in FIG. 24. A detected image and a 
stored image are aligned in an alignment circuit 6a and a difference in 
images between the aligned detected image and stored image is detected 
with a difference or subtraction image detection circuit 6b. A defect is 
detected by binarizing the detected difference image in a binarization 
circuit 6c. With the above arrangement, a defect 8d existing in the 
detected image can be detected. As an example of devices of this type is 
described in SPIE Vol. 772, Optical Microlithography 6 (1987), pp. 247 to 
255. 
In the prior art systems, a defect is detected by finding an unmatched 
point between corresponding patterns, so that the detected defect has to 
be observed with other observation apparatus, for example, an optical 
microscope or a SEM to identify the kind of the defect such as deformation 
defect, a discoloration defect or a foreign matter defect. 
SUMMARY OF THE INVENTION 
It is therefore a object of the present invention to provide a defect 
detection method and system for a pattern to be inspected in which a 
defect such as a foreign matter defect, a discoloration defect and/or a 
deformation defect can be detected automatically in the pattern such as an 
LSI wafer pattern. 
Another object of the present invention is to provide a defect detection 
method and system for a pattern to be inspected in which a fatal defect 
and an insignificant detect can be discriminated in the pattern such as an 
LSI wafer pattern. 
In accordance with the present invention, a defect detection system and 
method for a pattern to be inspected comprises an image pickup for picking 
up multiple-focus images of the pattern and a defect detection arrangement 
for detecting a defect existing on the pattern by comparing signals of the 
multiple-focus images picked up with the image pickup and corresponding 
signals of a reference pattern. 
According to a feature of the present invention, a defect detection 
arrangement is provided for detecting the kind of defect existing on the 
pattern based on signals of the multiple-focus images picked up with the 
image pickup. 
According to another feature of the present invention, the signals of the 
multiple-focus images of the pattern to be tested and signals of 
multiple-focus images of a corresponding reference pattern having no 
defect are picked up by the image pickup, and a detection arrangement is 
provided for detecting the kind of defect existing on the pattern to be 
tested by comparing signals of the multiple-focus images with each other. 
The present invention also provides a defect detection system and method 
for a pattern to be tested which comprises an image pickup for picking up 
multiple-focus images of the pattern to be tested and multiple-focus 
images of a corresponding reference pattern having no defect, a defect 
detection arrangement for detecting a defect by comparing at least some 
image signals in the multiple-focus image signals picked up with the image 
pickup to determine an unmatched point, a foreign matter defect detection 
arrangement for detecting a defect detected with the defect detection 
arrangement as a foreign matter defect based on a difference or 
subtraction image signal obtained by comparing multiple-focus image 
signals with each other, and a discoloration defect detection arrangement 
for detecting a defect detected with the defect detection arrangement as a 
discoloration defect based on a difference signal obtained by comparing 
differentiated signals with each other obtained from differential 
processing of the image signals. 
According to the present invention a defect detection system and method for 
a pattern to be inspected includes a first image pickup for picking up an 
image of a pattern and an image of a corresponding reference pattern 
having no defect, a defect detection arrangement for detecting a defect by 
comparing image signals picked up with the first image pickup to determine 
an unmatched point, a second image pickup for picking up multiple-focus 
images of the pattern to be inspected and multiple-focus images of a 
corresponding reference pattern having no defect, a foreign matter defect 
detection arrangement for detecting a defect as a foreign matter defect 
based on a difference image signal obtained by comparing the 
multiple-focus image signals with each other, and a discoloration defect 
detection arrangement for detecting a defect as a discoloration defect 
based on a difference signal obtained by comparing differentiated signals 
with each other obtained from differential processing of the 
above-mentioned images. 
The present invention according to a feature thereof, provides a defect 
detection system and method for a pattern to be inspected including an 
image pickup for picking up multiple-focus images of the pattern, 
multiple-focus images of a corresponding reference pattern having no 
defect, an optically differentiation-processed image of the pattern to be 
inspected, and an optically differentiation-processed image of the 
corresponding reference pattern having no defect; a defect detection 
arrangement for detecting a defect by comparing at least some image 
signals in the multiple-focus image signals to determine an unmatched 
portion; a foreign matter defect detection arrangement for detecting a 
defect as a foreign matter defect based on a difference image signal 
obtained by comparing multiple-focus image signals with each other; and a 
discoloration defect detection arrangement for detecting a defect as a 
discoloration defect based on a difference signal obtained by comparing 
differentiated signals with each other obtained. 
The present invention also provides a defect detection system and method 
for a pattern to be inspected which includes an image pickup for picking 
up an image of the pattern to be inspected, a defect detection arrangement 
for detecting a defect by comparing an image signal picked up by the image 
pickup and a reference pattern signal; a foreign matter defect detection 
arrangement for detecting a defect as a foreign matter defect based on the 
image signal picked up; a deformation defect detection arrangement or a 
discoloration defect detection arrangement for detecting a defect detected 
as a deformation defect or a discoloration defect based on the image 
signal picked up; a storage arrangement for storing the information on a 
foreign matter defect detected with the foreign matter defect detection 
arrangement and the information on a deformation defect or a discoloration 
defect detected with the deformation defect detection arrangement or 
discoloration defect detection means corresponding to the position on the 
pattern to be inspected of a defect detected; and a comparison arrangement 
for comparing the information on a foreign matter defect and the 
information o a deformation defect or discoloration defect at the same 
position on the pattern to be inspected in reading out the pieces of 
information indicative of the defects from the storage means. 
The present invention further provides a defect detection system and method 
for a pattern to be inspected including an image pickup for picking up a 
bright field image of the pattern to be inspected, a bright field image of 
a corresponding reference pattern having no defect, a dark field image of 
the pattern to be inspected, and a dark field image of the corresponding 
reference pattern having no defect; a defect detection arrangement for 
detecting a defect by comparing bright field image signals to determine an 
unmatched point; a deformation defect detection arrangement or a foreign 
matter defect detection arrangement for detecting a defect as a 
deformation defect or a foreign matter defect based on a difference signal 
obtained by comparing bright field images with each other; and a 
discoloration defect detection arrangement for detecting a defect as a 
discoloration defect based on a difference signal obtained by comparing 
dark field images with each other. 
The above and other objects, features, and advantages of the present 
invention will become more apparent from the following description when 
taken in connection with the accompanying drawings which show, for the 
purposes of illustration only, several embodiments in accordance with the 
present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention utilizes the following features for inspecting a 
pattern: 
(1) Multiple-focus images (variable density or gray scale images) are 
detected and compared with each other by moving a wafer to a detected 
defective portion and to a corresponding nondefective portion and moving 
the wafer up and down (Z direction) or utilizing multiple image pickups 
having different focal positions, a detection arrangement for detecting 
circuit pattern images in a plurality of Z positions and a storage 
arrangement for storing detected images. 
(2) A foreign matter defect is judged with a detection arrangement for 
detecting the density change in the Z direction on a difference image 
obtained by comparing variable density images at a defective portion and a 
nondefective portion in each Z position. 
(3) On a focal plane, detected variable density images in a defective 
portion and in a nondefective portion are compared after spatial 
differentiation, and a discoloration defect is judged with a detection 
arrangement which detects a density variation of a difference image 
depending on the differentiation order. 
(4) Foreign matter or contaminant defects and discoloration defects are 
judged successively in this order with the aforementioned detection 
arrangements, and the remainder of the defects are judged to be 
deformation or pattern defects. 
Referring to FIG. 1 of the drawings, a circuit pattern 11 on a wafer 1 to 
be inspected is illustrated, and multiple-focus images are detected at a 
plurality of points Z.sub.3, Z.sub.2, Z.sub.-3, which are located apart 
from each other on a straight line in Z direction through the focal point 
Z.sub.0 on the upper side and lower side of ZO. Therefore the image 
detected at Z.sub.0 is in focus for the circuit pattern 11 (pattern to be 
inspected), but the images at Z.sub.1, Z.sub.2 or Z.sub.3 are out of focus 
and the images of the circuit pattern 11 are blurred. A foreign matter or 
contaminant 10 such as a piece of dust, etc. is shown on the circuit 
pattern 11, so that the image of the foreign matter can be still in focus 
at Z.sub.1. This is the point to which attention is to be paid. When Z 
positions of focal planes are expressed on the axis of abscissas and 
density or gram scale values in gradation of difference images between the 
images at a defective portion and a nondefective portion are expressed on 
the axis of ordinates, a foreign matter or contaminant defect has a 
different waveform from that of a deformation or pattern defect or of a 
discoloration defect, so that a foreign matter defect can be discriminated 
as shown in FIG. 2. 
When a variable density image detected on a focal plane Z.sub.0 is 
spatially-differentiated, in the case of discoloration defect a DC 
component predominates, so that the density of a difference image of 
differentiated images becomes small as compared with that of a deformation 
defect or of a foreign matter defect. Assuming that the axis of abscissas 
expresses the order of differentiation, that is a first order 
differentiation and a second order differentiation (a zero order 
differentiation being indicative of no differentiation), and the axis of 
ordinates expresses the density values of difference images, then a 
discoloration defect takes a different waveform from that of a deformation 
defect or a foreign matter defect as shown in FIG. 3. Thereby a 
discoloration defect can be discriminated. 
Defects other than foreign matter defects and discoloration defects 
discriminated by waveform analysis are considered to have some defects in 
the shape of a circuit pattern and they are judged to be deformation or 
pattern defects. 
Tests or inspections are performed before and after the processing of a 
pattern to be inspected with a manufacturing device etc., and when a 
defect is detected in the same place on the pattern in both cases, i.e., 
before and after processing and if the defect is detected as a foreign 
matter defect in the before-process test and detected in the after-process 
test as a deformation defect or a discoloration defect the defect is 
discriminated (classified) as a fatal defect. These fatal defects are 
properly removed or patterns with them are not utilized so as to enable 
reliable manufacture of IC's and LSI'S. 
An embodiment of the present invention described with reference to FIG. 4 
wherein a circuit pattern to be inspected on the wafer 1 is illuminated by 
a Xe lamp 12 and is enlarged and detected with a TV camera 13 through an 
object lens 3. The output of the TV camera is converted to a digital 
signal with an A/D converter 14. Any photoelectric converter such as a TV 
camera or linear image sensor can be used. In the case of a linear image 
sensor, a two dimensional pattern on a wafer is detected with the 
self-scanning operation and an xy table which moves at a right angle to 
the direction of the self-scanning operation. The detected variable 
density or gray scale image is compared with the image of the preceding 
chip stored in an image memory 15b for the judgment of a defect. In other 
words a defect is detected, as shown in FIG. 5, by detecting a circuit 
pattern in a position 7d inside a chip 7 and comparing the detected 
circuit pattern image signals with a circuit pattern image signals in a 
position 7c, which is a position corresponding to that of the adjacent 
chip previously detected and stored in the image memory 15b. 
At first, a detected image, FIG. 6(a), and a stored image, FIG. 6(b), are 
aligned, FIG. 6(c) in an alignment circuit 16 shown in FIG. 4, and a 
difference image between the aligned detected image and stored image is 
detected. FIG. 6(d), with a difference image detection circuit 17 shown in 
FIG. 4. A binary image, FIG. 6(e), is obtained by binarizing the 
difference image signal in a binarization circuit 18 as shown in FIG. 4. 
In this way, pattern breakage 8b existing in the detected image, FIG. 
6(a), is detected as a defect. The image obtained by detecting a circuit 
pattern in a position 7d is newly stored in the image memory 15b and is 
used for the test in a position 7e, the position of the next chip. 
In FIG. 5, if a defect is in the position 7d the defect can be detected in 
the comparison between the patterns in the 7c and the 7d positions and 
also between patterns in the 7d and the 7e positions, so that in collating 
individual two-chips-comparison results with each other the location of 
the defect can be specified. The collation is performed with a CPU 31 
shown in FIG. 4. 
When a defect is judged to be in the 7d positions with the signal from the 
binarization circuit 18, the wafer is moved with a Z control circuit 19 
which moves the wafer up and down to detect the images of the defective 
portion 7d in individual Z positions and these images are stored in the 
image memory 15a. The movement of a wafer in xy direction is effected by a 
xy control circuit 20, and a wafer is moved to a nondefective portion 
corresponding to the defective portion, for example, to the position 7c 
and the images of the nondefective portion are detected at individual Z 
positions in the similar manner to the case of the defective portion 7d, 
and these images are stored in the image memory 15a. A defective portion 
image at Z=Z.sub.1 stored in the image memory 15a is aligned with a 
nondefective portion image at Z=Z.sub.1 stored in the image memory 15b in 
an alignment circuit 21a. A difference image between the aligned images is 
detected with a difference image detection circuit 22a and the density 
value at a defective portion of a difference image is detected with a 
maximum value detection circuit 23a. 
These processes are shown in FIG. 7. A difference image, FIG. 7(c) is 
obtained by aligning a defective portion image, FIG. 7 (a) and a 
nondefective portion image, FIG. 7(b). A maximum value of density in the 
difference image, FIG. 7(c) is detected with the maximum value detection 
circuit 23a, FIG. 7(d). The density difference between the defective 
portion 8b and the nondefective portion is so large that the density 
difference between the defective portion 8b and the non-defective portion 
can be detected with the maximum value detection circuit 23a. 
In similar manner, the images of a defective portion and a nondefective 
portion at Z.sub.2 to Z.sub.n positions stored in the image memories 15a 
and 15b are aligned in the alignment circuits 21b to 21n, and the 
difference images are detected with image detection circuits 22b to 22n, 
and the density values of the difference images at defective portions are 
detected with the maximum value detection circuits 23b to 23n. For 
example, 2n+1 sheets of variable density images having picture elements of 
1024.times.1024 are stored in each of these image memories 15a and 15b. 
The image memory 15a, in addition to storing multiple focus images, has a 
storage capacity for storing a sheet of variable density image to be used 
for ordinary defect judgment. Owing to this function, it is made possible 
to realize a sequence control in which defect judgment and defect 
classification can be made alternately as described later. 
A defective portion image and a nondefective image at Z=Z.sub.0 are aligned 
in a alignment circuit 24, and the first order derivatives of these images 
are obtained with a first order differentiation circuit 25a, and the 
difference image therebetween is detected with a difference image 
detection circuit 26a, and then the density value of the defective portion 
in the difference image is detected with a maximum value detection circuit 
27a. In a similar manner, the second order derivatives of a defect portion 
image and a nondefective portion image are obtained in a second order 
differentiation circuit 25b, and a difference image therebetween is 
detected with a difference image detection circuit 26b, and then the 
density value of the difference image is detected with a maximum value 
detection circuit 27b. 
All of these detected values are input to a defect classification circuit 
30. The defect kind judgment is made in the defect classification circuit 
30 based on FIG. 2 and FIG. 3 in accordance with the flow chart of FIG. 8, 
which processing is obtained by software. 
Referring to FIG. 4, a lens having shallow focus depth, a high resolution 
characteristic and a large value of NA such as 0.8 to 0.95 is selected for 
the object lens 3. When a defect is detected, utilizing the object lens of 
shallow focus depth, images are detected moving a wafer up and down, for 
example, at intervals of 0.2 .mu.m; among these images only one image on a 
certain plane is in focus. When the images are detected in the range of 
0.6 .mu.m, seven different focal plane images are detected, i.e., Z.sub.3, 
Z.sub.1 . . . Z.sub.-3. The alignment between a defective portion image 
and a nondefective portion image is performed in the alignment circuit 16, 
21a to 21n, and 24. This alignment can be effected in the manner described 
in The Technical Journal Vol. 87, No. 132, pp 31 to 38 of The Institute of 
Electronics and Communication Engineers of Japan. After the detection of 
difference images of seven pairs of aligned images, the density values of 
the difference images are detected; in the case where a defect is a 
foreign matter defect, the density values at Z.sub.1 and Z.sub.2 located 
on the upper side of a focal plane Z.sub.0 are still comparatively large 
as shown in FIG. 2, so that the defect can be easily judged if it is a 
foreign matter defect in the classification circuit 30, for example by 
following the flow chart shown in FIG. 8. 
Differentiation of an image is performed in a differential circuit 25; 
second order differentiation of an image is realized by processing the 
image with four kinds of edge operators (1, -2, 1) and by detecting the 
maximum value as shown in FIG. 9 and as described in U.S. Pat. No. 
4,791,586. 
A second order differentiation circuit 25b is shown in FIG. 10. In FIG. 
10(a), for example, an 8 bit digital image signal from the alignment 
circuit 24 is received in a three stage shift register 250; the output of 
the first stage and the third stage is supplied to an adder 251 and the 
output of the second stage is supplied to an amplifier 252 having a gain 
of two. The output of the adder 251 and the output of the amplifier 252 
are supplied to a subtracter 253. An operator of "1, -2, 1" is constituted 
by the shift register 250, the adder 251, the amplifier 252 and the 
subtracter 253. 
FIG. 10(b) shows a circuit for differentiation in three directions, a 
longitudinal direction, a horizontal direction and a diagonal direction; 
the output of the aligning circuit 24 is supplied to a 3.times.3 slicing 
circuit 254, and three picture elements of a longitudinal direction, a 
horizontal direction and a diagonal direction are selected and they are 
supplied to four operators OPI to OP4 to differentiate an image signal. 
Each operator can be similar to that shown in FIG. 10(a). The output of 
these four operators is supplied to a maximum value detection circuit 255 
and the maximum value among such outputs is selected. 
After differentiation a difference image is detected with the density value 
having a waveform as shown in FIG. 3. In the case of a discoloration 
defect the density value is small, so that it can be easily judged if the 
defect is a discoloration defect with the defect classification circuit 30 
following the flow chart shown in FIG. 8. It is recognized that such 
discoloration defect determination is independent of the illumination 
light wavelength. 
A process flow is shown in FIG. 11. The axis of abscissas expresses time. A 
test is made repeatedly in performing image detection and defect judgment 
as shown in part (a). Image detection is made with a TV camera 13, an A/D 
converter 14 and an image memory 15a as shown in FIG. 4, and defect 
judgment is made with an alignment circuit 16, a difference image 
detection circuit 17, a binarization circuit 18 and a CPU 31 which 
specifies a defect position. When a defect is detected, the 
above-mentioned testing is suspended and multiple-focus images of a defect 
portion and a corresponding nondefective portion are detected as shown in 
part (b). This image detection is made with the TV camera 13, A/D 
converter 14 and image memories 15a and 15b. In the next step, the 
difference images of these images and the maximum value are detected. 
These processes are realized with alignment circuits 21a, . . . , 21n, and 
24; difference image detection circuits 22a, . . . , 22n, 26a and 26b; 
differentiation circuits 25a and 25b; and maximum value detection 
circuits 23a, 23n, 27a and 27b. When all the maximum values of images and 
the maximum values of difference images of differentiated images at each Z 
position are detected, defects are classified into foreign matter defects, 
discoloration defects or deformation defects with the defect 
classification circuit 30. And again image detection and defect judgment 
ar repeatedly performed until a defect is detected as shown in part (a'). 
The defects existing on a test object pattern on a wafer are shown in FIG. 
12. Detection object defects to be found on a circuit pattern are 
deformation or pattern defects 8 such as swell out defects 8a, breakage 
defects 8b, short-circuit defects 8c or chipped off defects such as 
notches 8d, discoloration defects 9 or foreign matter or contaminant 
defects 10. 
The above explanation has been directed to a sequence is explained in which 
in each case when a defect is detected classification is made, but another 
sequence as shown in FIG. 13 can be used, for example, in which all the 
defects on a wafer are detected and the position coordinates thereof are 
stored in the CPU 31, and after the testing, defect classification is 
effected by reading out the defects successively from the memory. 
It is also possible to perform defect detection and defect classification 
in different optical system for performing defect detection at a high 
speed with low magnification and defect classification accurately with 
high magnification as shown in FIG. 14, the various parts being designated 
by like reference numerals as utilized in FIG. 4. Any type of illumination 
device may be utilized. Generally, inspection is carried out at high speed 
which necessitate an object lens 31 of lower magnification and which when 
utilized in an inspection system enables detection of existence of 
defects, but which has insufficient resolution to enable classification of 
defects. That is, when a defect is classified as described in connection 
with FIG. 4, the object lens 3 has high resolution. For example, if a high 
N.A. (Numerical Aperture) object lens, i.e., a high magnification lens is 
utilized, the multiple-focus images can be readily obtained due to the 
shallow focal depth thereof and such multiple-focus images are utilized 
for defect classification. By utilizing the two systems as shown in FIG. 
14, lower magnification with object lens 3' for defect detection at high 
speed and higher magnification with object lens 3 for defect 
classification at lower speed only a small amount of additional time is 
required since the classification is conducted only for the previously 
detected defects. Thus, the total inspection time for classification of 
defects is reduced. In FIG. 14, the object lens may be exchanged after 
defects are detected, and then the defects are classified. With exchange 
of the object lens, the lamp 12 and the TV camera are utilized in common. 
FIG. 15 shows another embodiment of multiple-focus image pickup. With this 
image pickup, it is possible to simultaneously obtain a plurality of 
images having focal planes at Z.sub.3, . . . , Z.sub.-3 by setting a 
plurality of TV cameras 13a, 13b, . . . in an optical path apart from each 
other and with different focal positions, thereby avoiding movement of 
wafer 1 up and down with Z control circuit 19. 
FIG. 9 shows the system utilizing the pickup of FIG. 8 wherein an image in 
focus is obtained with the camera 13a and is input to the image memory 15a 
to be used for defect judgment and also input to the alignment circuit 16 
through the A/D converter 14a. When defects are being classified, all 
images obtained with cameras 13a to 13n are input simultaneously to the 
image memories 15a and 15b. Therefore, the image memories have the 
capacity to be able to write n sheets of images simultaneously. In reading 
however, it is sufficient that an image can be read from the memory one 
sheet by one sheet. 
In FIG. 4 an image is differentiated for judging a discoloration defect, 
but it can be judged with other circuit constructions. As described in the 
book, "Methods of Image Pattern Recognition" pp. 17 and 18 published by 
Corona Inc., an edge of a circuit pattern can be emphasized when Fourier 
transformation is applied to an image signal and after filtering, an 
inverted Fourier transformation is applied. Utilizing the above method, a 
difference image between a defective portion image and a nondefective 
portion image is detected and based on the magnitude of a density value a 
defect can be judged as a discoloration defect. 
It is also possible to discriminate a discoloration defect with an optical 
arrangement. As shown in FIG. 17, an image detection system is constituted 
with a dark field illumination system comprising a lamp 32, a condenser 
lens 33, a narrow band filter 34 (wavelength .lambda..sub.1) for selecting 
the wave length for dark field illumination, a ring-shaped aperture slit 
35, a ring-shaped mirror 36, and a parabolic concave mirror 37; and with a 
bright field illumination system comprising a lamp 38, a condenser lens 
39, a wave length selecting filter 40 (wavelength .lambda..sub.2), a 
circular aperture slit 41, a half-mirror 42, an object lens 43, a 
wavelength separation mirror 44, a dark field image detection TV camera 
45, and a bright field image detection TV camera 46. In the 
above-mentioned image detection system, dark field illumination is limited 
to a wavelength .lambda..sub.1 with the filter 34 and the light is 
radiated in a slant direction from surroundings onto the pattern with the 
concave parabolic mirror 37, and bright field illumination is limited to a 
wavelength .lambda..sub.2 and the light is radiated onto the pattern from 
above. A dark field image of a defective portion and a dark field image of 
a nondefective portion are detected and these images are aligned in the 
alignment circuit 24a shown in FIG. 11 and then a difference image is 
detected with the difference image detection circuit 26a. 
In similar manner, a bright field image of a defective portion and a bright 
field image of a nondefective portion are detected and these images are 
aligned in the alignment circuit 24b and then a difference image is 
detected with a difference image detection circuit 26b. The density values 
of these difference images are detected in the maximum value detection 
circuits 27a and 27b. Then, as shown in FIG. 19, if the defect is a 
discoloration defect, the density value in the case of the dark field 
illumination is small in comparison with the case of a deformation defect 
or a foreign matter defect, so that the defect is easily judged to be a 
discoloration defect. As recognized, such discoloration defect 
determination is wavelength dependent. FIG. 18, the alignment circuit 16, 
the difference image detection circuit 17, and the binarization circuit 18 
are conventional circuits used for defect judgment as shown in FIG. 4. 
In FIG. 20, a detection system which can be utilized in place of the 
detection of dark field images is illustrated, wherein a wafer 1 is 
irradiated with an S polarized laser beam at an angle of .phi. with S 
polarization lasers 47a and 47b. The angle .phi. is about 1 degree. In 
this case, a laser beam which vibrates at a right angle to the plane 
formed with a normal line to the wafer and the radiating laser beam is 
called an S polarization laser beam and the beam which vibrates parallel 
to the plane is called a P polarization laser beam. When a circuit pattern 
on the wafer has low steps only, the polarization direction of the 
scattered light is not changed and proceeds towards an object lens 48 
keeping the S polarization plane as it is as shown in solid line, but in 
the case where there is a foreign matter or a high step in the pattern, 
the polarization plane of the laser beam which impinges on the pattern is 
changed, so that it contains many P polarization components shown in 
dotted line. Therefore, the scattered light from a circuit pattern edge 
having a foreign matter or a high step can be detected by providing a 
polar screen 49 which cuts off the S polarization beam behind the object 
lens 48 and by detecting the light which passes through the screen with an 
optical element 50 such as a photomultiplier. The scattered light signal 
is converted to a digital signal with the A/D converter 14. The detected 
signal is compared with the preceding chip signal stored in the image 
memory 15a. These signals are aligned in the alignment circuit 16 and the 
difference signal is detected with the difference signal detection circuit 
17. The scattered light signal from a circuit pattern edge with a high 
step is contained in both signals in common, so that only the scattered 
light signal from a foreign matter is contained in the difference signal. 
A defect can be detected by binarizing the difference signal with the 
binarization circuit 18, thereby defect detection can be performed. The 
aforedescribed arrangement is substantially disclosed in "The Japan 
Society of Applied Physics and Related Societies", March 1988, pg. 701 and 
is utilized to detect a defect such as a contaminant. 
By using the defect detection system described above, it is also possible 
to realize a more effective processing system which is described in the 
following. As shown in FIG. 21, a semiconductor manufacturing line is 
provided wherein a pattern on a wafer is successively formed or 
manufactured by successive processing, for example, in a plurality of 
devices A, B . . . F, wherein the devices may be identical or different. A 
wafer is inspected utilizing a detection system having the construction 
described in the above, and in each of these devices A, B, . . . , F 
inspection and defect classification are performed prior to and after 
processing in the respective device by a test or inspection apparatus as 
represented by the double headed arrows. The occurrence of a defect can be 
examined as the wafer passes through each of these devices by checking the 
coordinates of the defect. For example, a defect detected in the 
inspection of a wafer processed in device B can be a defect originating in 
the preceding device A or a defect produced or occurring in the device B. 
It can be determined whether the defect is produced in the device B or 
occurred in the preceding device A by referring to the defect data 
obtained in the inspection when the wafer is processed in the device A. 
Among the defects introduced from the preceding device A, some of the 
foreign matter defects do not cause deformation defects or discoloration 
defects in the device B. That is, a pattern with a fatal foreign matter 
defect is without fail detected as a pattern with a deformation defect or 
a discoloration defect in the following device, but a pattern having no 
fatal foreign matter defect is not detected as a deformation defect in the 
following devices and is accepted as a good article. On a defect 
classified as a foreign matter defect by a inspection apparatus having a 
construction as described above, its coordinates are stored in a memory, 
and its fatalness can be judged by performing detection and classification 
of the defect again after the wafer passes through the next device as 
shown in FIG. 21. Thereby, it becomes possible to obtain more accurate 
information of the condition of a manufacturing and line accurately. 
For example, if a large number of foreign matter defects occur in device A, 
whereas a small number of foreign matter defects occur in device C, but 
more fatal foreign matter defects are caused in device C than in device A, 
then device C is primarily responsible for reducing yield and problems in 
device C must be corrected. Such correction may entail cleaning or tuning 
of device C. That is, for manufacturing purposes, the number of defects 
occurring in a particular device is not important so long as the defects 
are not fatal foreign matter defects and with the disclosed system, the 
count of fatal foreign matter defects is detected and the appropriate 
device corrected so as to improve yield. 
FIG. 22 shows a system for judging the fatalness of a foreign matter defect 
by using a visual inspection apparatus. In the figure, a defect is 
detected with a defect judgment section 40 and the coordinates are stored 
in a defect attribute storage section 42. For a detected defective 
portion, the defects are classified into foreign matter defects and other 
defects with the defect classification section 41 and are stored in the 
defect attribute storage section 42 coupled with their coordinates. After 
the same wafer is processed in the next process or device the wafer is 
inspected and defect judgment and classification are performed in a 
similar manner and the defect coordinates and the kind thereof are stored 
in the defect attribute storage section 42. For a defect which is judged 
to be a foreign matter defect in the preceding process or device its 
coordinates are examined at the fatalness judgment section 43 and if the 
defect is judged in the next process or device to be a deformation defect 
or a discoloration defect, the defect is judged to be a fatal defect. 
Several embodiments have been described above, and in every case an object 
wafer images are detected and their positions are aligned to perform 
defect judgment and defect classification. At a place where the density of 
a circuit pattern on a wafer is small, it can occur that there is a 
pattern in the corner of one of the two images to be aligned and no 
pattern in the corresponding corner of the other image, which makes the 
accurate alignment of images impossible. Therefore, a dummy pattern is 
inserted into the area where no pattern is found so that in every detected 
image a pattern may be found and accurate alignment may be possible. A 
pattern of any shape can be used. 
In the above embodiments, wafers have been referred to for explanatory 
purposes but the present invention can be utilized for semiconductor 
products such as TFT's or thin film magnetic heads. 
According to the present invention, the defects detected with the visual 
inspection system can be automatically classified and visual observation 
is not needed. Foreign matter defects can be judged whether they are fatal 
or not. Thus the present invention can greatly contribute much to the 
development of the yield management of manufacturing process and 
facilities. 
While we have shown and described several embodiments in accordance with 
the present invention, it is understood that the same is not limited 
thereto but is susceptible to numerous changes and modifications as known 
to one of ordinary skill in the art, and we therefore do not wish to be 
limited to the details shown and described herein but intend to cover all 
such modifications as are encompassed by the scope of the appended claims.