Method of inspecting reticles and apparatus therefor

Method of inspecting reticles and an apparatus therefor, where means for holding and transferring an inspected reticle and a standard reticle respectively, means for illuminating light with spatial coherency adjusted onto both reticles respectively, and an objective lens for collecting transmitted light or reflected light from the illuminated body produced by the illuminating, are installed on respective Fourier transformation surfaces of both reticle surfaces, and a light blocking plate for blocking light corresponding to the adjusted spatial coherency is installed, and electric signals obtained are compared thereby a defect or a foreign substance existing on the inspected reticle can be detected.

BACKGROUND OF THE INVENTION 
The present invention relates to method of inspecting reticles and an 
apparatus therefor, wherein in exposure process of a reticle and a mask to 
be used for manufacturing an LSI or a print board, foreign substance or 
defect on the reticle and the mask is detected before pattern on the 
reticle and the mask is transferred onto a wafer. 
In the exposure process used during manufacturing an LSI, a chromium 
pattern on a thick board called a reticle is printed and transferred to a 
semiconductor wafer. In this process, when foreign substance and defect 
exist on the reticle, since the pattern cannot be transferred to the 
semiconductor wafer exactly, all LSI chips become a defective unit. 
Consequently, inspection of foreign substance and defect before the 
exposure is inevitable in the control of the reticle. 
In addition to this, since the LSI is highly integrated in recent years and 
therefore the pattern becomes fine, a smaller foreign substance becomes a 
problem attendant upon this. Also foreign substance of a flat thin film is 
caused by residue of a resist during manufacturing the reticle, unfinished 
etching of chromium or chromium oxide for the pattern forming, and 
impurity melted in the reticle washing liquid and aggregated during the 
washing and drying. This foreign substance of the flat thin film becomes a 
problem, and the number of such foreign substance is apt to increase more 
and more. 
In the prior art, an apparatus for inspecting the foreign substance and 
defect is proposed, as disclosed in Japanese patent application laid-open 
No. 65428/1984 for example, comprising means for illuminating and scanning 
laser light to the substrate obliquely, a first lens installed to the 
upper side of the substrate so as to align the illuminated point of the 
laser light nearly to the focal plane for collecting the scattered light 
of the laser light, a light blocking plate installed to a Fourier 
transformation plane of the first lens for blocking regular scattered 
light from the substrate pattern, a slit installed to the focal point of a 
second lens to perform inverse Fourier transformation of the scattered 
light from foreign substance obtained through the light blocking plate for 
blocking the scattered light from position other than the illumination 
point of the laser light on the substrate, and a light receiving unit for 
receiving the scattered light coming from the foreign substance through 
the slit. 
In this proposal, paying attention to that pattern is generally constituted 
in the same direction or in combination of several different directions 
within the visual field, diffraction light by the pattern in this 
direction is removed by the space filter installed to the Fourier 
transformation plane, thereby only the reflected light from the foreign 
substance is emphasized and removed. 
Also in the prior art, as disclosed in Japanese patent application 
laid-open No. 139278/1983 for example, method for comparing data detected 
using an illuminating and detecting optical system similar to the exposure 
unit with data of a standard reticle or data in the design and for 
detecting the defect is proposed. 
This method is in that data detected using the detecting optical system is 
binarized and compared with the binary data of the pattern estimated from 
the design data. 
Further, the prior art is disclosed in U.S. Pat. No. 4,595,289 or U.S. Pat. 
No. 4,330,205. 
Among the above-mentioned prior art, Japanese patent application laid-open 
No. 65428/1984 is characterized in that reflected light from a foreign 
substance is separated by a light blocking plate from reflected light from 
a pattern and only the reflected light from the foreign substance is 
detected by the slit, and that since the foreign substance is detected by 
the binarization method, the detecting mechanism is simplified. On the 
other hand, however, since the foreign substance is detected by 
illumination of laser light from oblique upper direction being different 
from the original exposure unit, so to speak, by indirect illumination, 
only the reflected light from the chromium pattern of specific angle is 
blocked but the foreign substance from all chromium pattern cannot be 
discriminated. 
Also in the case of detecting by indirect means as above described, foreign 
substance without producing actual damage (hereinafter referred to as 
"false alarm") also may be detected. Particularly the pattern becomes fine 
and the number of foreign substances producing a problem is increased but 
the number of foreign substances producing no actual damage is also 
increased, thereby the number of false alarms is increased, and check 
regarding whether the detected foreign substance produces a problem or 
not, analysis and removing of the foreign substance and other works are 
increased and therefore the working efficiency is significantly 
deteriorated. 
Among the above-mentioned prior art, Japanese patent application laid-open 
No. 139278/1983 is characterized in that since the optical system similar 
to the exposure unit is provided, constitution of the optical system is 
simplified in comparison to the prior art. On the other hand, however, a 
problem exists in that the image signal processing system for comparing 
data is complicated in comparison to the prior art and much time is 
required for the inspection. 
The reference data is binary image, but since detection signal must be 
detected by tone image of multiple values due to limitation of the 
resolving power of the optical system, the detection signal is binarized 
and compared. During the binarization, even true pattern is binarized in 
the shape different from the reference pattern, which causes increase of 
the false alarms. In order to eliminate the false alarm, since algorithm 
is adopted where difference of several pixels is not made defect, a 
problem exists in that defect as large as several pixels may be 
overlooked. If the pixel size is made small in order to increase the 
resolving power as the measure for the overlooking, a problem exists in 
that much time is required for the inspection. 
Method disclosed in U.S. Pat. No. 4,595,289 is according to comparison 
inspection using dark-field illumination, but even by this method, a 
problem exists in that a plurality of circuit corner portions enter one 
pixel of the detector, and when the scattered light signal from the 
circuit pattern becomes larger, it is difficult to decrease influence of 
the alignment error. That is, since the number of the circuit pattern 
corners detected by one pixel is varied due to misalignment, the signal 
level detected by corresponding one pixel of two detection systems 
comparing with each other is significantly varied. 
U.S. Pat. No. 4,330,205 relates to an apparatus for detecting optical 
defects using laser beam, but this also does not provide a simple and 
effective detector by the comparison detection as in the present 
invention. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide method of inspecting reticles and 
an apparatus therefor, wherein in order to solve respective problems in 
the prior art, only fine foreign substances or fine defects producing 
actual damage can be separated from a chromium pattern existing at 
arbitrary angle and detected. 
In order to attain the foregoing object, in the present invention, means 
for holding and transferring an inspected reticle and a standard reticle 
respectively, means for illuminating light with adjusted spatial coherency 
to both reticles respectively, and an objective lens for collecting 
transmission light or reflected light from the illuminated substance 
produced by illumination from respective illuminating means, are installed 
to a Fourier transformation surface of the respective reticle surfaces, 
and light corresponding to the adjusted spatial coherency is blocked by a 
light blocking plate and the obtained electrical signals relating to the 
inspected reticle and the standard reticle are compared respectively, 
thereby defects or foreign substances existing on the inspected reticle 
can be detected. 
Furthermore, the object can be attained by providing a standard reticle 
inspection data generating unit having equivalent constitution to the 
inspection apparatus of the reticle to be inspected or a standard reticle 
inspection data generating unit having function of converting the design 
data into inspection data inspected by the inspection unit of the reticle 
to be inspected, and a detecting means of defects or foreign substances 
for comparing electric signals detected from the detector of the inspected 
reticle of the inspection apparatus of the reticle to be inspected with 
electric signals outputted from the standard reticle inspection data 
generating unit and eliminating signals obtained from pattern on the 
reticle and actualizing and detecting the defects or the foreign 
substances existing on the reticle to be inspected.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention pays attention to that luminous flux contributing to 
imaging is diffracted and scattered by a foreign substance and defect 
thereby the transfer failure due to the foreign substance and the defect 
is generated. 
In general, numerical aperture (hereinafter referred to as "N.A.") at 
entrance side (body side) of a reduction projection lens is designed to 
value obtaining resolution being necessary and sufficient for imaging a 
pattern on a reticle. Consequently, luminous flux contributing to the 
imaging of the pattern passes through an aperture at entrance side of the 
reduction projection lens, but luminous flux passing through the outside 
of the aperture does not contribute to the imaging of the pattern. If a 
fine foreign substance exists, the luminous flux scattered and diffracted 
by the foreign substance passes through the outside from the entrance N.A. 
of the reduction projection lens and therefore obstructs the imaging of 
the pattern. 
This point can be further understood, for example, from description of 
"Response Function of Optical System having Space Filter" in "Wave Optics" 
by Kubota, pp. 387-389. That is, the reference describes that a 
disk-shaped space filter is held on a Fourier transformation surface of 
the imaging optical system, thereby a pattern having the space frequency 
determined by the diameter of the disk-shaped space filter, for example, 
the specific frequency determined by amount of radius d' when the inside 
of the lens is covered by circle of the radius d', cannot be resolved. 
Consequently, the description can be applied to the present object where 
difference of the space frequency between the pattern and the foreign 
substance, in other words, difference of size between the pattern and the 
foreign substance is utilized and only the foreign substance is detected. 
The present invention uses an illumination system equivalent to 
illumination system utilizing the above-mentioned principle and used in an 
exposure unit and an objective lens having larger N.A. than that of the 
reduction projection lens, and among luminous flux incident to the 
objective lens, the same area as the entrance N.A. of the reduction 
projection lens, i.e., the diffraction light is blocked by the blocking 
plate, thereby only the scattered light from the foreign substance can be 
taken. 
Consequently, in the invention, luminous flux scattered and diffracted by a 
foreign substance and a defect and passing through the inside of the 
aperture on the outside of the aperture at entrance side of the reduction 
projection lens of the exposure unit can be only selected and detected, 
thereby only the foreign substance producing actual damage can be 
discriminated from the pattern and detected. 
When the resolution of the foreign substance is reduced, information of the 
scattered light from the pattern corner spreads in the whole one pixel of 
the detector, thereby influence of the misalignment becomes small and the 
foreign substance can be detected stably. 
In the standard reticle detection data generating unit, using software or 
an electric circuit to the binary pattern image generated from the design 
data, space filter processing or convolution integral having the transfer 
function equivalent to the inspected reticle inspecting unit is applied 
("Introduction to Computer Image Processing" by H. Tamura, pp. 47-49) 
thereby the standard reticle inspection data of multiple values is 
generated. The standard reticle inspection data and the inspection data of 
the inspected reticle inspecting unit are compared in the data of multiple 
values, thereby error produced during the binarization of the inspection 
data of the inspected reticle inspecting unit (quantization error in the 
plane direction) can be eliminated and the plane alarm is eliminated and 
the fine defect of several pixels can be detected. 
In this case, even if the circular space filter within the inspected 
reticle inspecting unit is not used, the above-mentioned problem can be 
solved if the processing in the standard reticle inspection data 
generating unit is matched with the transfer function in the case of no 
space filter simultaneously. 
An embodiment of the present invention will now be described referring to 
FIGS. 1 through 4. 
As shown in FIG. 1, an apparatus for inspecting a defect or a foreign 
substance according to the invention comprises a sample holder unit 1, 
transmitted illumination units 2 and 102, reflected illumination units 3 
and 103, detecting units 4 and 104, and a data processing unit 5. 
The sample holder unit 1 comprises Z stage 9 for fixing a reticle 6 having 
a pelicle 7 by a fixing means 8 and scanning it in Z direction, X stage 10 
for scanning the reticle 6 through the Z stage 9 in X direction, Y stage 
11 for scanning the reticle 6 through the X stage 10 and the Z stage 9 in 
Y direction, a stage drive system 12 for driving each of the stages 9, 10, 
11, an auto-focusing unit 13 for detecting the position in Z direction of 
the reticle 6, and a processor 14 for driving the stage drive system 12 by 
command from the auto-focusing unit 13, wherein the reticle 6 during the 
inspection can be focused always with accuracy at necessary minimum. 
The stage 10 is constituted to perform periodic motion of maximum speed of 
about 1 mm/s and amplitude 200 mm in 1/2 period of uniform acceleration 
time of about 0.1 second, uniform motion of 0.1 second and uniform 
deceleration time of 0.1 second. 
The Y stage 11 is constituted to transfer the reticle 6 in Y direction in 
synchronization with the uniform acceleration time and the uniform 
deceleration time of the X stage 10 at step of every 0.15 mm. If the 
reticle 6 is transferred 670 times during one inspection time, it can be 
transferred 100 mm in about 130 seconds thereby the area of 100 mm square 
can be scanned in about 130 seconds. 
Although the X and Y stages 10, 11 are carried out in the embodiment, the 
invention is not limited to this. For example, X.theta. stage scanning the 
rotational direction and the X direction may be used. Also regarding the 
scanning speed, an example is shown in the above description and it may, 
of course, be arbitrarily set if necessary. 
The auto-focusing unit 13 may be that using an air micrometer or that 
detecting the position by laser interference method or that projecting the 
stripe pattern and detecting its contrast. 
Since the transmitted illumination units 2 and 102 are constituted by the 
same components, the transmitted illumination unit 2 will be described 
here. 
The transmitted illumination unit 2 is constituted so that g-ray 
(wavelength 436 mm) or i-ray (wavelength 365 mm) used in an exposure unit 
(not shown) is selected among luminous flux emitted from a mercury arc 
lamp 21 by a dichroic mirror 22, and when the light is concentrated by a 
condenser lens 23 onto a diffusing plate 24, the light diffused by the 
diffusing plate 24 is emitted from a portion limited by a circular 
diaphragm 25 and enters the reticle 6 and illuminates the reticle 6. 
The diaphragm 25 is installed nearly at the focal position of the 
collimator lens 26, and image of the diaphragm 25 is focused by the 
collimator lens 26 and an objective lens 41 of the detecting unit 4 to 
position 46 shown in dash-and-dot line. 
In order to attain the foregoing object of the invention, not only 
wavelength of the illumination light must be made the same as that of the 
illumination light used in the exposure unit, but also angle .theta. of 
luminous flux incident to one point 15 on the reticle 6 must be made the 
same. Where sin .theta. is defined as "spatial coherency". 
In the illumination of the exposure unit, since all area on the reticle 1 
must be illuminated uniformly, an optical element called an integrator 
assembling rod-shaped lenses is used in place of the diffusion plate 24. 
Since function of the integrator is basically the same as that of the 
diffusing plate 24 and the inspection range applied by the invention is 
from several hundred microns to 1.2 mm of the reticle 6, the diffusing 
plate 24 is sufficient. 
Since the incident angle .theta. of luminous flux being incident onto the 
reticle 6 is determined by size of the integrator, i.e., diameter of the 
diaphragm 25 installed at the rear side of the diffusing plate 24, 
aperture of the diaphragm 25 is set to have the same spatial coherency as 
that of the illumination used in the exposure unit using the reticle 1. 
Further in the exposure unit, since position of the integrator is not 
always set to the focal position of the collimator lens 26, the position 
of the diaphragm 25 need not be set always to the focal position of the 
collimator lens 26. 
However, if it is desired that the incident angle .theta. of luminous flux 
is made constant at arbitrary position within the light illumination area 
on the reticle 6, and the illumination condition of the luminous flux 
within the measuring area is made the same and the detecting condition of 
the foreign substance is made the same, the diaphragm 25 is preferably set 
to the focal position of the collimator lens 26. 
Since the reflected illumination units 3 and 103 are constituted by the 
same components, the reflected illumination unit 3 will be described. 
The reflected illumination unit 3 is constituted so that light emitted from 
a mercury arc lamp 31 and passing through a dichroic mirror 32, a 
condenser lens 33, a diffusing plate 34 and a diaphragm 35 passes through 
a relay lens 36 and illuminates the reticle 6 through a half mirror 42 and 
an objective lens 41 of the detecting unit 4. 
The objective lens 41 has the same function as that of the collimator lens 
26 of the transmitted illumination unit 2. 
The relay lens 36 is installed to generate an apparent diaphragm to a focal 
position 46 on upper side of the objective lens 41. More specifically, the 
real image of the diaphragm 35 is imaged to the focal position 46. 
Also in the reflected illumination unit 3, similar to the transmitted 
illumination unit 2, aperture angle of the diaphragm 35 is determined so 
that wavelength of the illumination light and angle .theta. of luminous 
flux incident to any one point 15 on the reticle 6 are made the same as 
that of the illumination light used in the exposure unit. 
Since the reflected illumination unit 3 is installed to detect a foreign 
substance on the chromium pattern of the reticle 6, it is unnecessary if 
the foreign substance on the chromium pattern need not be detected. 
When the reflected illumination unit 3 and the transmitted illumination 
unit 2 are used simultaneously, signal from the edge of the pattern 
becomes large. If this becomes a problem, both units must be used 
separately. 
Wavelength of the illumination light need not be necessarily made g-ray and 
i-ray, but may be other light of wide band including g-ray and i-ray. 
Because the foreign substance and the pattern are different in the 
diffraction state regarding lights of all wavelength ranges thereby the 
foreign substance can be discriminated from the pattern and detected even 
at the light of wide band. 
Since the detecting units 4 and 104 are constituted by the same components, 
the detecting unit 4 will be described. 
The detecting unit 4 comprises an objective lens 41, a half mirror 42, a 
field lens 43, a light blocking plate 44 and an imaging lens 45, and is 
constituted so that an inspection point 15 on the reticle 6 is imaged by 
the objective lens 41 and the imaging lens 45 to a detector 51. Also the 
detecting unit 4 has the field lens 43 in the vicinity of the imaging 
position of the objective lens 41. The field lens 43 has function that the 
focal position on upper side of the objective lens 41 is imaged on the 
circular light blocking plate 44. That is, light from the diaphragm 25 of 
the transmitted illumination unit 2 passes through the collimator lens 26 
and the objective lens 41 and is reflected on the reticle 6, and passes 
again through the objective lens 41 and the field lens 46 and is imaged on 
the light blocking plate 44. Then the position of the light blocking plate 
44 is made position of Fourier transformation of the position of the 
reticle 6 with respect to the position of the light source, i.e., the 
position of the diaphragm 25. 
In this case, N.A. at side of the reticle 6 of the reduction projection 
lens of the exposure unit is generally set larger than the spatial 
coherency of the illumination system of the exposure unit (equivalent to 
the spatial coherency of the transmitted illumination unit 2) by about 10% 
through 40%, and by about 10% in most cases. 
Since luminous flux passing through the outside of the aperture at entrance 
side of the reduction projection lens must pass through the inside of the 
aperture, N.A. of the objective lens 41 is made larger than N.A. of the 
reduction projection lens, and the light blocking plate 44 is installed so 
as to block luminous flux being incident within N.A. of the reduction 
projection lens. 
Consequently, in order to attain the foregoing object of the invention, the 
diameter dm of the light blocking plate 44 is calculated by following 
expression (1). 
##EQU1## 
where ds is diameter of the diaphragm 25, .alpha. is magnification of the 
imaging system of the diaphragm 25 and the light blocking plate 44, N.A. 
is value at side of the reticle 6 of the reduction projection lens, and 
sin is spatial coherency of the exposure unit. 
In this case, if .theta.=.theta.s, the detection condition of the foreign 
substance can be made the same. In this case, the expression (1) becomes 
##EQU2## 
It has been confirmed by the experiment that .delta. is clearance and may 
be several % (.delta.=0.02-0.08). 
When the reticle 6 is not moved and all inspection areas on the reticle 6 
are inspected simultaneously, size of the objective lens 41 is made large 
and the manufacturing becomes difficult in practice. In the invention, 
since the inspection area on the reticle 6 is limited and the reticle 6 is 
scanned by the sample holder unit 1 and all inspection areas can be 
inspected, the objective lens 41 having larger N.A. than that of the 
reduction projection lens in ordinary use can be used. 
In order to inspect the foreign substance irrespective of whether it 
produces actual damage or not, size of the light blocking plate 44 need 
not be necessarily matched with N.A. at entrance side of the reduction 
projection lens used in value of the expression (1), but may be made size 
calculated in that N.A./sin .theta.=1 is set in the expression (1) and 
arbitrary value .delta.' larger than .delta. by several % is used 
specifically. In this case, the expression (1) becomes 
EQU dm=ds..alpha..(1+.delta.') (1) 
Further, the spatial coherency of the illumination light need not be 
necessarily matched with the coherency of the exposure unit, but may be 
determined so that the diffraction light of 0-degree can be blocked by the 
light blocking plate 44. That is, size of the diaphragms 25, 35 and the 
light blocking plate 44 may be determined within range satisfying the 
expression (1). 
When the reflected illumination unit 3 is not installed, even if the half 
mirror 42, the field lens 43 and the imaging lens 45 are omitted and the 
light blocking plate 44 is installed to the focal position 46 and the 
detector 51 is installed to the position where the field lens 43 was 
installed respectively, effects of the invention can be obtained. In this 
case, the optical system of quite simple constitution can be obtained. 
The data processing unit 5 comprises a comparator 70, a binarization 
circuit 52, a micro computer 53 and a display means 54. 
The detector 51 is formed, for example, by one-dimensional solid image 
pickup element of charge moving type, and the X stage 10 is scanned and 
signal is detected in the detector 51. 
The detector 51 is not limited to one-dimensional solid image pickup 
element as above described, but that of two-dimensional element or single 
element may be used. 
The comparator 70 takes signals from the detectors 51 and 151, and outputs 
difference of two signals. 
The binarization circuit 52 previously sets the binarization threshold 
value, and judges whether a foreign substance exists or not. 
The micro computer 53 previously sets evaluation function. That is, since 
whether or not a foreign substance produces actual damage being transfer 
failure is function of intensity of scattered light due to the foreign 
substance and size of the foreign substance, function of a foreign 
substance producing actual damage is previously evaluated, and the micro 
computer 53 judges whether a foreign substance producing actual damage 
exists or not by the evaluation function and outputs the result to the 
display means 54. 
An apparatus for inspecting a pattern defect or a foreign substance 
according to the invention is constituted as above described. 
Next, inspection method and its operation will be described based on FIGS. 
2 through 6. 
A plan view of the reticle 6 as an inspection object is shown in FIG. 5a, 
and a sectional view in a line 80 is shown in FIG. 6a. 
Also, a plan view of a reticle as an inspection standard is shown in FIG. 
5b, and a sectional view thereof is shown in FIG. 6b. 
As shown in FIG. 5a, FIG. 5b, assuming the reticles 6 and 106, in an 
example of the case that foreign substances 81 and 82, a pattern defect 
83, edge portions 84, 184 of normal pattern, corner portions 86, 186 of 
normal pattern, and fine normal patterns 85, 185 existing on reticles 
respectively, operation of the invention will be described. The reticle 
106 is a standard reticle. 
Since the foreign substance 82 is fine, it scatters or diffracts the light 
much in comparison to the edge portion 84 of the normal pattern. That is, 
luminous flux 88 scattered to the outside from the range .theta. blocked 
by the light blocking plate 44 becomes more than luminous flux 90, 91 
scattered to the outside from .theta. of the edge portion 84 and the 
corner portion 86. Also regarding the foreign substance 81 and the pattern 
defect 83, since the foreign 81 has small size, the space frequency at the 
periphery of the foreign substance 81 becomes high, thereby luminous flux 
87 scattered to the outside from the range .theta. blocked by the light 
blocking plate 44 becomes more than luminous flux 90, 92 of the edge 
portion 84 and the corner portion 86. 
Detection signals in positions 80, 180 by the transmitted light are shown 
in FIG. 7a, FIG. 7b. The detection signal according to the invention has 
strong output at the corner portion of the foreign substance, and becomes 
as shown in FIG. 8a, FIG. 8b. 
Consequently, in this case, if binarization is performed at threshold value 
93, the foreign substances 81 and 82 can be separately detected with 
respect to the edge portion 84 and the corner portion 86 of the pattern. 
However, an LSI becomes fine and a fine normal pattern such as a pattern 85 
is used. In such pattern, since the space frequency is high, the luminous 
flux 92 scattered to the outside from the region .theta. blocked by the 
light blocking plate 44 becomes comparable to the foreign substances 81, 
82 and the pattern defect 83 or more. 
As a result, the binarization at the threshold value 93 cannot detect the 
pattern 92 separately from the foreign substances 81, 82 and the defect 
83. 
The detection signal in the detection position 180 of the inspection 
standard reticle in FIG. 6b is shown in FIG. 8b. 
The detection signal of the inspection object reticle 6 in FIG. 8a and the 
detection signal of the inspection standard reticle 106 in FIG. 8b are 
taken, and absolute value of difference of the two signals is obtained in 
the comparator 70. The result is shown in FIG. 9. 
In the case of FIG. 9, if threshold value 94 is set, the foreign substances 
81, 82 and the defect 83 can be discriminated from the patterns 84, 85, 86 
and detected. 
In this case, the output of the comparator 70 is outputted in absolute 
value of difference (.vertline.I.sub.1 -I.sub.2 .vertline.) as shown in 
FIG. 9, but need not be necessarily limited to this. For example, it may 
be outputted as difference of the two circuits shown in FIG. 1 (I.sub.1 
-I.sub.2). FIG. 10 shows value of the signal in FIG. 8b subtracted from 
the signal in FIG. 8a. When the signal in FIG. 8a is higher, the threshold 
value 95 detects it as a foreign substance or a defect. That is, the 
threshold value 95 represents that a foreign substance or a defect exists 
in the inspection object reticle 6 outputting the signal in FIG. 8a. On 
the other hand, the threshold value 96 represents that value of the signal 
in FIG. 8b is higher, i.e., a foreign substance or a defect exists in the 
standard reticle 106. 
Next, operation will be described. 
The inspection object reticle 6 and the standard reticle 106 drawing the 
same pattern are fixed respectively by fixing jigs 8 and 108 on the 
inspection stage 9. 
The two reticles 6 and 106 detect alignment marks by alignment units 71 and 
171. Based on the signals, the XY.theta. fine adjustment mechanisms 9, 10, 
11 are moved, and perform adjustment so that the inspection object 
positions of the two reticles 6 and 106 are imaged on positions 
corresponding to the detectors 45 and 145. 
Next, the X stage 10 and the Y stage 11 are scanned as above described. 
Then the reticles 6 and 106 are moved simultaneously. During the scanning, 
the focal points of the two reticles are aligned by the auto-focusing 
units 13 and 113 simultaneously. 
Then the alignment of the two reticles is always accompanied by error 
.beta.. 
When the error .beta. is produced, the detection signals in FIG. 8a and 
FIG. 8b are overlapped as shown in FIG. 11 by the comparator 70, thereby 
the output of the comparator 70 becomes as shown in FIG. 12. In this case, 
the foreign substances 81, 82, 83 cannot be discriminated from the normal 
patterns 84, 85, 86 and detected by the threshold value 95. 
The allowable alignment error .beta. will be estimated. Image of the 
pattern corner portion 86 by the detection optical system becomes as shown 
in FIG. 13. The total output signal of the pattern corner portion 8 is 
made Tc, and diameter of the image of the pattern corner 86 is made Dc, 
and also the total output signal of the foreign substance is made T.sub.f, 
and diameter thereof is made D.sub.f. Detection of the pattern corner 
portion 86 and the foreign substance 82 by the detection pixels W.times.W 
will be studied. 
Variation .DELTA.Ic of the detection signal of the corner portion due to 
the misalignment .delta. becomes following expression (2). The detection 
waveform approximates cone of diameter Dc and height h as shown in FIG. 
13, and the variation takes maximum value. 
##EQU3## 
When image of the foreign substance is imaged over adjacent detection 
pixels as shown in FIG. 14, the detection signal I.sub.f of the foreign 
substance becomes minimum as shown in following expression (3). 
EQU I.sub.f =1/4.multidot.T.sub.f (3) 
From expressions (2) and (3), the alignment error .beta. allowable to 
detect the foreign substance 82 by the comparator 70 must satisfy 
following expression (4). 
##EQU4## 
From expression (4)', if the diameter Dc of the pattern output signal is 
made large, the allowable error .beta. can be made large. On the other 
hand, in order to take the whole output signal Tc of the foreign substance 
into one pixel of the detector efficiently, following expression (5) must 
be satisfied. 
EQU W&gt;D.sub.f (.perspectiveto.Dc) (5) 
Consequently, state Dc.perspectiveto.W is the most efficient. 
On the other hand, in order to detect the foreign substance to degree of 
following expression 
EQU Tc.perspectiveto.5.multidot.T.sub.f (6) 
from expressions (6), (4)' 
##EQU5## 
In this case, in order that .beta.&lt;0.2 .mu.m, Dc becomes Dc=7.6 .mu.m. 
That is, the resolution of the image may be reduced to 7.6 .mu.m. 
Next, how to reduce the resolution of the image will be described. The 
resolution of the image is set by the numerical aperture of the objective 
lens. Consequently, the numerical aperture may be decreased. However, if 
the numerical aperture is decreased, the detection signal level is also 
lowered. Consequently, in order to reduce the resolution without 
decreasing the numerical aperture, a phase filter 72 having shape as shown 
in FIG. 15 and FIG. 16 is installed to position of Fourier transformation 
of the image of FIG. 1. 
The phase filter 72 is divided in ring band-shaped parts as shown in FIG. 
15, and phase of each part is varied in step of .pi.. Also FIG. 16 shows 
that divided linearly. Width l of each part in this case is made about 
value shown in following expression, thereby the image can be widened to 
following D.sub.f nearly intended value. 
##EQU6## 
where N. A. is numerical aperture of the objective lens, and L is size of 
the Fourier transformation surface. 
The invention can detect a foreign substance having specific size or more 
by only the detecting unit 4, i.e., by only the binarization without 
comparing. This principle and operation will be described. 
As shown in FIG. 2, description will be performed regarding the case that a 
pattern 17, two foreign substances 18a, 18b and a defect 19 exist on a 
glass substrate 16. Since one small foreign substance 18a is fine, it 
scatters or diffracts the light much in comparison to the edge 17a of the 
pattern 17. That is, luminous flux 56 scattered to the outside from the 
range .theta. blocked by the light blocking plate 44 becomes more than 
luminous flux 55 of the edge 17a of the pattern 17. 
Also regarding the defect 19 of the other large foreign substance 18b or 
the pattern 17, since the space frequency at the periphery is high, 
luminous flux 57, 58 scattered to the outside from the range .theta. 
blocked by the light blocking plate 44 becomes more than luminous flux 55 
the edge 17a of the pattern 17. 
Consequently, the output of the detector 51 generates output peak 59, 60, 
61, 62 by each of the luminous flux 55, 56, 57, 58. 
On the other hand, if threshold value 63 is set by the binarization circuit 
52 as shown in FIG. 3, the output peak 60, 61, 62 in three pieces project 
as output being the threshold value 63 or more, thereby only the two 
foreign substances 18a, 18b and the defect 19 of the pattern 17 can be 
detected. 
Coordinates of the X, Y stages 10, 11 and the level of the output peak 60, 
61 are stored in the memory controlled by the micro computer 53, and the 
storage content is processed and outputted to the CRT 54. 
Next, second and third embodiments of the invention will be described using 
FIGS. 17 through 18. 
As shown in FIG. 17, an apparatus for inspecting a defect or a foreign 
substance according to the invention comprises a sample holder unit 1, a 
transmitted illumination unit 2, a reflected illumination unit 3, a 
detecting unit 4, a standard data generating unit 201 and a data 
processing unit 5. 
Since the transmitted illumination unit 2, the reflected illumination unit 
3, the detecting unit 4 and the data processing unit 5 are similar to 
those in constitution of the first embodiment shown in FIG. 1, the 
detailed description shall be omitted here. Also regarding the inspection 
method and its action and operation similar to that already described, the 
description shall be omitted here. 
The second embodiment is as shown in FIG. 17, and the standard data 
generating unit 201 will be first described. 
The standard data generating unit 201 comprises a prescribed design 
standards reading means 202, a bit-image generating means 203, a bit-image 
memorizing means 204 and a transfer function convoluting means 205. 
The prescribed design data standards reading means 202 reads the design 
data during drawing the pattern to the reticle from an MT (magnetic tape) 
or a photo disk 209. The bit-image generating means 203 generates a binary 
pattern image from the design data, and the result is stored in the 
bit-image memorizing means 204. Since the bit image memorizing means 204 
must read the data at high speed, a semiconductor memory such as SRAM to 
enable the high-speed processing is preferable therefor. In the transfer 
function convoluting means 205, convolution integral of the transfer 
function equivalent to the detecting unit 4 is performed and outputted to 
the data processing unit 5. In this case, for comparison with signal from 
the detecting unit 4 at real time, the transfer function convoluting means 
205 is preferably a pipeline type as disclosed in "Automatization of 
Appearance Inspection" edited by the Institute of Electrical Engineers of 
Japan, Research Expert Committee for Automatization of Inspection, pp. 
267-268, published by OHMSHA Company. 
Operation action regarding the standard data generating unit 201 is as 
follows. In FIG. 17, the standard data generating unit 201 introduces 
signal equivalent to the detection signal of the standard reticle from the 
design data of the pattern. 
The transfer function convoluting method to generate the standard reticle 
inspection data will be described. In "Image Optics:Corona Company" by S. 
Hasegawa (pp. 49, 56), regarding output image when an image passes through 
the optical system, gradation of image of the optical system, i.e., 
deterioration of the image due to the point spread function is described 
as follows. 
When the input image to the optical system is made f (x, y), the point 
spread function is made h (x, y), and output is made g (x, y), 
##EQU7## 
On the other hand, between spectrums 
EQU G(u,v)=F(u,v).multidot.H(u,v) (10) 
where G (u, v), F (u, v), H (u, v) are Fourier transformation of g (x, y), 
f (x, y), h (x, y) respectively. 
Consequently, the function h (x, y) to be convoluted by expression (9) by 
the input image and the output image from expression (10) in inverse 
Fourier transformation by following expression (11). 
##EQU8## 
The above relation, of course, applies also when the g (x, y) is not point 
spread function. 
Next, h (x, y) will be calculated specifically. 
In the optical system, if N. A.=0.5 and use wavelength .lambda.=0.5 .mu.m, 
convolution function becomes Fourier inverse transformation of the optical 
system of N. A.=0.5, i.e., a circle. This function becomes Sinc function. 
Thinking the case of use to component of first order, size Wc of the 
convolution function becomes 
EQU Wc=4.times..lambda./N. A..perspectiveto.4(.mu.m) (12) 
Next, the pixel size .DELTA.Wc of the convolution filter will be thought. 
.DELTA.Wc is preferably made the same as the pixel size during the pattern 
drawing onto the reticle. 
In this case, since the binary image data generated from the design data 
can be subjected to covolution integral as it is, the edge portion of the 
pattern does not spread between the pixels of the convolution filter and 
the quantization error can be eliminated. 
However, .DELTA.Wc is not necessarily limited to this. In order to decrease 
the pixel number in the whole filter area, .DELTA.Wc may be made large, 
and in order to raise the accuracy, .DELTA.Wc may be made small. 
The pixel number N.sub.2 of the convolution filter becomes 
EQU N.sup.2 =(W.sub.c /.DELTA.W.sub.c).sup.2 (13) 
Also when the circular space filter 44 as shown in FIG. 1 is used, part of 
the space filter is subjected to inverse Fourier transformation and the 
convolution function may be calculated. Size of the convolution function 
may be estimated according to the expression (11). 
Further, although not described here, in the case of a space filter and a 
phase filter as disclosed in Japanese patent application No. 149516/1986, 
the convolution function h (X, Y) may be calculated by expression (11). 
Next, the detection signal of the inspection object reticle 6 in FIG. 8a 
and the output signal of the standard data generating unit 201 in FIG. 8b 
are taken, and absolute value of difference of the two signals is obtained 
in the comparator 70. The result is shown in FIG. 9. 
In the case of FIG. 9, if the threshold value 94 is set, the foreign 
substances 81, 82 and the defect 83 can be discriminated from the patterns 
84, 85, 86 and detected. 
In this case, the output of the comparator 70 is outputted in absolute 
value as shown in FIG. 9, but need not be necessarily limited to this. As 
shown in FIG. 10, it may be outputted as difference of two circuits. FIG. 
10 shows value of the signal in FIG. 8b subtracted from the signal in FIG. 
8a. When the signal in FIG. 8a is higher, the threshold value 95 detects 
the signal as a foreign substance or a defect. That is, the threshold 
value 95 represents that a foreign substance or a defect exists in the 
inspection object reticle 6 outputting the signal in FIG. 8a. 
Next, operation will be described. 
The inspection object reticle 6 is fixed by the fixing jigs 8 and 108 on 
the inspection stage 9. The reticle 6 detects alignment marks by the 
alignment mark detecting means 71 and 171. Based on the signals, the 
XY.THETA. fine adjustment mechanisms 9, 10, 11 are moved, and performs 
adjustment so that the inspect object position of the reticle 6 is imaged 
on position corresponding to the detector 45. 
On the other hand, in the standard data generating unit 201, the design 
data of the inspected reticle 6 is set to the prescribed design standards 
reading means 202, and bit image is generated in the bit-image generating 
means 203 and stored in the bit-image memorizing means 204. 
The X stage 9 and the Y stage 10 are subjected to automatic focusing and 
scanned as above described. At the same time, bit image of corresponding 
pattern is read in sequence from the bit image memorizing means 204 and 
processed in the transfer function convoluting means 205. 
The transfer function convoluting means 205 is shown in detail in FIG. 19. 
The transfer function convoluting means 205 comprises a shift register 
211, a weight data memory 212, a window circuit 213, a multiplying and 
accumulating circuit 214, a shift register 215, a weight data memory 216, 
a window circuit 217 and a multiplying and accumulating circuit 218. 
The bit image read in sequence from the bit image memorizing means 204 is 
cut out through the shift register 211 by amount N.sup.2 of the 
convolution filter, and transmitted to the window circuit 213 having 
function as the convolution filter calculated by expression (11). The bit 
image is weighted in the window circuit 213 in accordance with weight data 
transmitted through the weight data memory 212 from the micro computer 53, 
and added in the multiplying and accumulating circuit 214. Thus the 
weighted integral is finished. 
Next, in order to calculate the signal level detected by the detector 51 
within the inspected reticle inspecting unit 4, calculated value by the 
multiplying and accumulating circuit 214 is added in area of the same size 
as the pixel size of the detector 51. 
Consequently, Ni of size to be cut out becomes following expression (14). 
EQU Ni=W/.DELTA.W (14) 
Signal added in the multiplying and accumulating circuit 214 is cut out 
through the shift register by amount of Ni.times.Ni. 
The cutting-out signal is weighted by the window circuit 217, and added in 
the multiplying and accumulating circuit 218, and outputted as the 
standard reticle inspection data to the comparator 70. 
In this case, weight of the window circuit 217 calculates characteristics 
of the filter produced since the CCD is scanned and characteristics of 
phenomenon called crosstalk that the detected optical signal leaks out of 
the CCD element. 
Then alignment of the reticle and the standard reticle inspection data 
converting the design data is necessarily accompanied by error .beta.. 
Regarding treatment of the error .beta., in similar manner to that 
described in the first embodiment, processing using expressions 
(2).about.(7) as described in detail may be performed. Also in this 
example, the operation principle in the case of detecting a foreign 
substance having specific size or more only by the detecting system, i.e., 
only by the binarization is as described in the first embodiment, and the 
description shall be omitted here. 
In place of the standard data generating unit 201 of the second embodiment 
shown in FIG. 17, in the third embodiment, a standard reticle inspection 
data memorizing unit 206 shown in FIG. 18 may be used. This is means for 
detecting the standard reticle in the detecting unit 4 and storing the 
detected data, and constituted by a mass storage memory 207 such as an 
optical disk, a hard disk, a magnetic tape, and a high speed memory 208 
such as SRAM, DRAM. In the invention, the standard reticle is previously 
held on the sample holder unit 1, and detection data from the detecting 
unit 4 is stored through a switch 210 to the standard reticle inspection 
data memorizing unit 206. 
Next, the inspection reticle is held thereon, and detection data from the 
detecting unit 4 is transmitted through the switch 210 (FIG. 18) to the 
data processing unit 5 and compared with signal from the standard reticle 
inspection data memorizing unit 206. During operation of the invention, 
the standard reticle inspection data memorizing unit 206 is used in place 
of the standard data generating unit 201. The invention is technology 
enabled by that the memory of high speed and large capacity can be 
supplied at low cost. 
In any of the embodiments, one detecting unit is used. In this 
constitution, since distortion of image due to aberration of the detection 
optical system need not be corrected, false alarm due to misalignment 
caused by distortion of the image can be decreased. 
According to the invention having the above-mentioned constitution, since 
standard signal equivalent to detection signal of the inspection object by 
inspection means can be generated from the design data and compared with 
the detection signal, the detection accuracy, the detection speed and the 
detection reliability can be improved. 
Also according to the present invention, since illumination equivalent 
optically to illumination of an exposure unit is used and light scattered 
and diffracted by a foreign substance and a defect and being not incident 
to a reduction projection lens of the exposure unit can be selectively 
detected, the detection signal is eliminated from the pattern and the 
detection signal from the defect or the foreign substance producing actual 
damage can be actualized and the defect or the foreign substance can be 
detected. 
Since the inspection area on the reticle is limited and the reticle is 
scanned and the whole inspection area can be inspected, the objective lens 
having N. A. larger than that of the reduction projection lens in ordinary 
use can be used. 
Further constitution of the illumination system can be simplified, and 
constitution of the detection system can be also simplified.